Advanced Ceramics 4th year Lecture notesDr. Saad B. H. Farid 2014-2015

Part 1: Functional and Engineering CeramicsPart 2: Bio-Ceramics

1. Insulating/ Thermal Conductive Ceramics1. Introduction to Bio-Ceramics

2. Semi-conductive Ceramics2. Alumina and Zirconia in Surgical Implants

3. Piezoelectric Ceramics3. Bioactive Glasses - Materials

4. Dielectric Ceramics4. Bioactive Glasses - Clinical Applications

5. Magnetic Ceramics5. A/W Glass-Ceramics

6. Opto-electro-ceramics6. Machinable and Phosphate Glass-Ceramics

7. High-Temperature High-Strength Ceramics7. Porous Hydroxyapatite

8. Porous Ceramics for Filtration8. Hydroxyapatite Coatings

9. Ceramic Bearing9. Pyrolytic Carbon Coatings

10. Cutting Tools10. Bioceramic Composites

11. Ceramic-Matrix Composites11. Calcium Phosphate Cements

12. Ceramic Materials for Energy Systems12. Radiotherapy Glasses

13. Functionally Graded Materials13. Dental Glass-Ceramics and ZrO2-Ceramics

1. Insulating Ceramics/High Thermal Conductive CeramicsMaterials for PKGBecause of its high thermal conductivity, high mechanical strength, good insulation characteristics, moderate dielectric properties, and high chemical durability, Alumina (HTCC: high-temperature co-fired ceramics) is the most popular ceramics material for semiconductor packages.However, for power devices like power amplifier for base station or for satellites, higher thermal conductivity ceramic materials is required to dissipate the heat generated in the devices. To meet this requirement, aluminum nitride (AlN), which has high thermal conductivity (TC) and a low thermal expansion coefficient comparable to that of Si, has been adopted for packages requiring high thermal dissipation.Another market trend, toward higher power, higher working frequencies, and lower power consumption, requires reduction of the resistivity of conductors in co-fired packages. To meet this requirement, glass ceramics (LTCC: low-temperature co-fired ceramics) with silver or copper conductors have been developed.New materials have been developed such that a novel AlN that can co-fired at low temperature to reduce cost. The second is a novel LTCC that has a high thermal coefficient of expansion. The third is also a novel LTCC that has low permittivity and low loss tangent at high frequency.

Process FlowFigure 1 shows the process flow for a co-fired multilayer ceramic package. There are many steps to produce a multilayer package; however, there are only a few differences among different materials. Of course, material composition, metallize composition and process condition is different for each. Among these steps, metallizing is an especially critical technology for package production.

Aluminum Nitride AlN: Material PropertiesAlN has a characteristic dielectric dispersion at high frequencies. Figure 2 shows the frequency dependence of the dielectric loss (tan ) of AN75W and AN242. The dielectric loss of AlN shows a maximum at a few gigahertz. This phenomenon is due to the piezoelectricity of AlN, and the peak frequency inversely depends on the crystallite size. As the crystallite size of AN75W is smaller than that of AN242, the peak (dispersion) frequency of AN75W is correspondingly higher.

LTCC with High Thermal Coefficient of ExpansionDue to the ever-increasing I/O counts for the IC devices, packaging trends have been changing to surface mountable area array second-level interconnection, namely ball grid array (BGA) and chip scale package (CSP).

BGAThe motivations for these types of second-level mounting are described as follows:1. Higher wiring density: smaller packages, thinner packages, lighter packages,2. Higher performance: electrical performance, thermal performance, higher I/O counts,3. Lower cost.A surface mounting technology (SMT) package, such as BGA, has low height interconnection between the substrate and the printed wiring board PWB (also called printed circuit board PCB).

Figure 1: Process flow for multilayer ceramic package fabrication.When we have a big difference of TCE between the substrate and the PWB, BGA, and CSP packages, they receive more severe shear strain, damaging the reliability of solder joints, compared with Pin grid array PGA. This shear strain is a big problem, since alumina ceramics has TCE of 7 ppm/C while the TCE of a typical PWB is 1216 ppm/C.The observations suggest that the reliability largely depends on the TCE mismatch between the substrate and the potting compound.A new ceramics is developed with TCE of 13 ppm/C, which is in the range of PWBs (1216 ppm/C) and that of the potting compounds (1030 ppm/C).Hint: it is done by incorporating glass in a composite.The dielectric constant is 5.3 at 1 MHz, which is lower than 9.8 of alumina. The Youngs modulus is 110 GPa, approximately one-third that of alumina. Also, copper conductor is co-firable.

Figure 2: Frequency dependence of dielectric loss of AlN.It is found that the equivalent plastic strain generated by the TCE mismatch among Si-die, substrate, potting compound and PWB drastically decreases as TCE of the substrate increases from 11.5 to 13 ppm/C.

LTCC with Low Permittivity and Low Loss Tangent at High FrequencyThe ceramic package used for microwave applications requires following properties;(a) Lower dielectric constant and lower loss tangent in the radio frequency range;(b) Lower resistivity conductor;(c) Thermal expansion coefficient of the ceramic material close to that of semiconductor chips;(d) High reliability of hermeticity (airtight). A new LTCC material was designed to be able to sinter at less than 1000C because of co-firing with copper conductor. The LTCC is composed of lead-free, SiO2Al2O3MgOZnOB2O3 system glass and ceramic fillers. In order to satisfy electrical and thermal properties, the amount of crystalline phases precipitated after sintering is adjusted.The coefficient of thermal expansion is 7.5 ppm/C in the range of 40300C. This value is close to that of GaAs chips that are mainly used for microwave applications. Thermal conductivity and flexural strength and volume resistivity are as good as conventional LTCC material. The dielectric constant is 6.0, which is lower than that of alumina in the range of 260 GHz. The loss tangent increases as frequency increases and close to that of alumina, which is good for microwave applications.Copper metallization

Squares are the unit-less dimension of length divided by width. RH: Relative HumidityCopper was used for metallization material because of excellent migration resistance. It is important for co-firing process to match the shrinkage behavior of copper metallization to that of LTCC material. Since the shrinkage of copper starts at lower temperature than that of LTCC material, glass and ceramic fillers are added to copper paste to control the shrinkage behavior of copper metallization. After co-firing, copper metallization, the LTCC material is plated with nickelgold or coppergold.Adhesion strength of the metallization was measured, and no change was observed after 1000 h aging at 150C.Sheet resistance of the metallization was 2.5m/ (12m thickness), where and no increase was observed after 1000 h aging at 150C. The insulation resistance between lines separated by 100m space was more than 1012 after 1000 h of High Humidity Biased Test HHBT (semiconductor component reliability test) (85C, 85% RH, 5.5 V).

2. Semiconductive Ceramicsa- PTC ThermistorsBarium titanate (BaTiO3) is a ferroelectric material with a high dielectric constant and high insulation resistance. Therefore, it has been widely used in the electrical industry for ceramic capacitors since its discovery in 1943. The insulating BaTiO3 ceramic is converted into a semiconductor by adding a small amount of rare earth metal oxide such as Sm2O3, CeO2, Y2O3, and La2O3. In 1955, unusual temperature dependence of resistance above the Curie temperature of semiconductive BaTiO3 ceramics was discovered.The resistance of this semiconductor called the positive temperature coefficient (PTC) thermistor drastically increases above the Curie temperature (TC), up to the temperature (Tn) where the resistance reaches its maximum value. The characterized temperature is divided into three regions (I, II, and III in Figure 4) according to the resistance behavior.

Conduction Mechanisms

Figure 3: Characterized temperature regions of PTC ceramicsThe conduction mechanisms in the regions IIII are explained as follows. In the temperature region I(T < TC), the resistivity of PTC thermistor is in the range of 10106 cm. To produce semiconductive BaTiO3, a small amount of rare earth metal ions (e.g. Sm3+ or La3+) are substituted at the Ba2+ site, or Nb5+ and Ta5+ ions are substituted at the Ti4+ site. These ions provide conductive electrons.In the region II above the Curie temperature, resistance across the grain boundary increases exponentially with increasing temperature. The increase of resistance corresponds to the decrease of spontaneous polarization (Ps) of BaTiO3 due to the phase transition from the ferroelectric tetragonal phase to the paraelectric cubic phase. The gradual decrease of Ps and dielectric constant cause the potential barrier height to recover. This recovery results in an increase of resistance in region II.In the temperature region III (T > Tn), the electrons that overcome the double Schottky barrier increase with temperature, and resistance decreases from maximum resistance.

Manufacturing ProcessAdditives and their effects on PTC characteristics are listed in Table 1. The transition temperature TC can be lowered or elevated from its original value (120C) by substitution of Sr2+ and Pb2+ at the Ba2+ site.BaCO3, SrCO3, Pb3O4, TiO2 and donor dopant (e.g. La2O3, Sm2O3, Y2O3 and Nb2O5) are used as starting materials. Manufacturing processes of PTC thermistor are almost the same as those in the electronic ceramics industry.

To control the resistance and the temperature coefficient within the exact range, much attention is paid to the firing temperature and ensuing cooling rate. The impurity of rare materials and contamination in manufacturing process must be decreased, because these increase the resistance at room temperature. In particular, Fe and Al strongly affect the resistivity of PTC thermistor.Non-precious metal electrodes such as Ni, Zn, and Al provide ohmic contact with PTC thermistor, which is n-type semiconductor ceramics. InGa alloy also makes ohmic contact, and is used for experimental samples.

ApplicationsTable 2 shows three basic functions and applications of PTC. PTC thermistors are used in a lot of electric products, such as color televisions, refrigerators, hot-wind heaters, and personal computers.

b- NTC ThermistorsThe negative temperature coefficient (NTC) thermistors are semiconductive materials whose resistance decreases with increasing temperature as shown in Figure 4 with other thermistors.R = A exp(B/T), B = E/k, where A is a constant, B a thermistor constant, E the activation energy, and k Boltzmanns constant.

Conduction MechanismNTC thermistor usually consists of transition metals (Cu, Fe, Co, Ni, etc.) spinel manganites. The conductivity is due to the transfer of electrons between Mn3+ and Mn4+ ions. The resistance and thermistor constant is dependent on the composition, purity, cation distribution, and crystal structures.

Manufacturing Process

Figure 4 Temperature dependence of different types of thermistors in contrast to a platinumMn3O4, NiO, Co2O3, and Fe2O3 are used as starting materials. NTC thermistors are produced by the general method of the manufacturing of electroceramics. Precious metals such as Ag, Pd, and Pt are used for electrodes of NTC thermistor, which is mainly p-type semiconductor ceramics.

ApplicationsNTC thermistors are used as temperature compensation, temperature sensing, and surge current suppression devices. All of these applications are based on the resistancetemperature characteristics of NTC thermistors. Although various thermistor constant B and resistivity are required for many applications, these values are obtained within certain limits, because the thermistor constant B is dependent on the resistivity.

Ceramic VaristorsMetal oxide varistors are ceramic semiconductive devices having highly nonlinear currentvoltage characteristics, as shown in Figure 5, expressed as I = (V/C).

is the nonlinear exponent, C the constant corresponding to the resistance, and V1 and V2 are the voltages at the currents of I1 and I2, respectively. C is convenient1y given by Vc called varistor voltage, that is, a voltage per unit length (V/mm) when 1 mA/cm2 of current flows through the body. Thus, the ceramic varistor is characterized by the non-linear exponent and varistor voltage Vc.

Figure 5 Typical VI characteristic of ceramic varistorTwo types of ceramic varistors are manufactured. Zinc oxide based ceramic varistors were developed in 1970. They exhibit a high non-linearity on voltagecurrent characteristics. Their value is in the range of 4050, and the Vc adjustable to values in the range from 50 to 250 V/mm. Strontium titanate based varistors were developed in 1980. The feature of these varistors is their larger electrostatic capacitance compared with ZnO varistors. The SrTiO3 ceramics are essentially dielectrics with a die1ectric constant of 320, which is much higher than that of ZnO.

Manufacturing ProcessThe effect of additives on varistor properties of ZnO varistors are listed in Table 3. The varistor voltage Vc is dependent on the number of grain boundaries between a couple of electrodes, because the varistor voltage across a single grain boundary is constant value (3V) at each boundary. To obtain the varistors with the various voltages Vc, the grain size are controlled by firing temperature or additives, such as B and Sb.Strontium titanate based varistors are manufactured by firing in a reducing atmosphere and re-oxidized on only grain boundary, like a boundary-layered ceramic capacitor.

Applications

Figure 6 Typical application of ZnO varistor as a transient protective device.Metal oxide varistors are mainly used in circuits for protection against inductive surges, very short spike noise, or power surges. They result to protect circuit simply by inserting between surge entrance line and ground lines shown in Figure 6. A varistor should be chosen that have a varistor voltage Vc slightly higher than the signal voltage applied to the load to be protected. The varistor is insulator in normal operation where the applied voltage is lower than Vc. If a transient pulse, whose voltage is higher than Vc, is incident, the current through varistor rapidly increases, resulting in a conducting shunt path for the incident pulse.ZnO-based varistor have become popular because of the high non-linearity on voltagecurrent characteristics.

3. Piezoelectric CeramicsCertain materials produce electric charges on their surfaces as a consequence of applying mechanical stress. The induced charges are proportional to the mechanical stress. This is called the direct piezoelectric effect and it was discovered in quartz by Piere and Jacques Curie in 1880. Materials showing this phenomenon also conversely have a geometric strain proportional to an applied electric field. This is the converse piezoelectric effect. The root of the word piezo means pressure; hence the original meaning of the word piezoelectricity implied pressure electricity.Piezoelectricity is extensively utilized in the fabrication of various devices such as transducers, actuators, surface acoustic wave devices, frequency control and so on. In this chapter, we describe the piezoelectric materials that are used, and various potential applications of piezoelectric materials.

Piezoelectric MaterialsThis section summarizes the current status of piezoelectric materials: single-crystal materials, piezoceramics, piezopolymers, piezocomposites and piezofilms.

i- Single CrystalsAlthough piezoelectric ceramics are widely used for a large number of applications, single-crystal materials retain their utility, being essential for applications such as frequency stabilized oscillators and surface acoustic devices. The most popular single-crystal piezoelectric materials are quartz, lithium niobate (LiNbO3), and lithium tantalate (LiTaO3). The single crystals are anisotropic, exhibiting different material properties depending on the cut of the materials and the direction of bulk or surface wave propagation. Quartz has a cut with a zero temperature coefficient.

ii- Polycrystalline MaterialsBarium titanate (BaTiO3) is one of the most thoroughly studied and most widely used piezoelectric materials. Just below the Curie temperature (120C), the vector of the spontaneous polarization points in the [001] direction (tetragonal phase), below 5C it reorients in the [011] (orthorhombic phase) and below 90C in the [111] direction (rhombohedral phase). The dielectric and piezoelectric properties of ferroelectric ceramic BaTiO3 can be affected by its own stoichiometry, microstructure, and by dopants entering onto the A or B site in solid solution. Modified ceramic BaTiO3 with dopants such as Pb or Ca ions have been developed to stabilize the tetragonal phase over a wider temperature range and are used as commercial piezoelectric materials. The initial application was for Langevin-type piezoelectric vibrators. Piezoelectric Pb(Ti,Zr)O3 solid solutions (PZT) ceramics have been widely used because of their superior piezoelectric properties.

iii- PolymersPolyvinylidene difluoride, PVDF or PVF2, is piezoelectric when stretched during fabrication. Thin sheets of the cast polymer are then drawn and stretched in the plane of the sheet, in at least one direction, and frequently also in the perpendicular direction, to transform the material to its microscopically polar phase. Crystallization from the melt forms the non-polar -phase, which can be converted into the polar -phase by a uniaxial or biaxial drawing operation; the resulting dipoles are then reoriented through electric poling.

iv- CompositesPiezo-composites, comprised of a piezoelectric ceramic and a polymer phase, are promising materials because of their excellent and readily tailored properties. A piezo-composite, such as the PZTrod/polymer composite is a most promising candidate. The advantages of this composite are high coupling factors, low acoustic impedance, good matching to water or human tissue, mechanical flexibility, broad bandwidth, and low mechanical quality factor. Piezoelectric composite materials are made by forming a composite structure, that is, by replacing some of the heavy, stiff ceramic with a light, soft polymer. Piezoelectric composite materials are especially useful for underwater sonar and medical diagnostic ultrasonic transducer applications.

v- Thin FilmsBoth zinc oxide (ZnO) and aluminum nitride (AlN) are simple binary compounds with a Wurtzite-type structure(hexagonal crystal system), which can be sputter-deposited as a c-axis oriented thin film on a variety of substrates. ZnO has large piezoelectric coupling and thin films of this material are widely used in bulk acoustic and surface acoustic wave devices. The fabrication of highly oriented (along the c-axis) ZnO films have been studied and developed extensively. The performance of ZnO devices is limited, however, due to their low piezoelectric coupling (2030%). PZT thin films are expected to exhibit higher piezoelectric properties. At present the growth of PZT thin films is being carried out for use in microtransducers and microactuators.Application:

a- Pressure Sensors/Accelerometers/GyroscopesOne of the very basic applications of piezoelectric ceramics is a gas igniter. The very high voltage generated in a piezoelectric ceramic under applied mechanical stress can cause sparking and ignite the gas. There are two means to apply the mechanical force, either by a rapid, pulsed application or by a more gradual, continuous increase.Piezoelectric ceramics can be employed as stress sensors and acceleration sensors, because of the direct piezoelectric effect. A three-dimensional (3D) stress sensor can be designed by combining an appropriate number of quartz crystal plates (extensional and shear types), the multilayer device can detect 3D stresses.

b- Piezoelectric Vibrators/Ultrasonic Transducers

Piezoelectric ResonanceWhen an electric field is applied to a piezoelectric material, deformation (L) or strain (L/L) arises. When the field is alternating, mechanical vibration is caused, and if the drive frequency is adjusted to a mechanical resonance frequency of the device, large resonating strain is generated. This phenomenon can be understood as a strain magnification due to accumulating input energy, and is called piezoelectric resonance. Piezoelectric resonance is very useful for realizing energy trap devices, actuators, etc. Electromechanical Coupling Factor, k: corresponds to the rate of electromechanical transduction.k2 = (stored mechanical energy/input electrical energy)k2 = (stored electrical energy/input mechanical energy) = d2/0 sThe general processes for calculating the electromechanical parameters (k31, d31, sE11, and X33) are:1. The sound velocity in the specimen is obtained from the resonance frequency fR2. Knowing the density , the elastic compliance sE11 can be calculated.3. The electromechanical coupling factor k31 is calculated from the value and the antiresonance frequency fA4. Knowing the permittivity X33, the d31 is calculated

Piezoelectric Vibrators

Figure 7 Piezoelectric buzzer.In the use of mechanical vibration devices such as filters or oscillators, the size and shape of a device are very important, and both the vibrational mode and the ceramic material must be considered.The resonance frequency of the bending mode in a centimeter-size sample ranges from 100 to 1000 Hz, which is much lower than that of the thickness mode (100 kHz). For these vibrator applications, the piezoceramic should have a high mechanical quality factor (QM) rather than a large piezoelectric coefficient d; that is, hard piezoelectric ceramics are preferable. For speakers or buzzers, audible by humans, devices with a rather low resonance frequency are used (kilohertz range).

c- Ultrasonic TransducersUltrasonic waves are now used in various fields. The sound source is made from piezoelectric ceramics as well as magnetostrictive materials. Piezoceramics are generally superior in efficiency and in size to magnetostrictive materials. In particular, hard piezoelectric materials with a high QM are preferable. A liquid medium is usually used for sound energy transfer. Typical applications are ultrasonic washers, ultrasonic microphones for short-distance remote control and underwater detection, such as sonar and fish finding, and non-destructive testing. Ultrasonic scanning detectors are useful in medical electronics for clinical applications ranging from diagnosis to therapy and surgery.One of the most important applications is based on ultrasonic echo field. Ultrasonic transducers convert electrical energy into mechanical form when generating an acoustic pulse and convert mechanical energy into an electrical signal when detecting its echo. The transmitted waves propagate into a body and echoes are generated which travel back to be received by the same transducer. These echoes vary in intensity according to the type of tissue or body structure, thereby creating images. An ultrasonic image represents the mechanical properties of the tissue, such as density and elasticity. We can recognize anatomical structures in an ultrasonic image since the organ boundaries and fluid-to-tissue interfaces are easily distinguished. The ultrasonic imaging process can also be done in real time. This means we can follow rapidly moving structures such as the heart without motion distortion. In addition, ultrasound is one of the safest diagnostic imaging techniques. It does not use ionizing radiation like X-rays, thus, it is routinely used for fetal and obstetrical imaging. Useful areas for ultrasonic imaging include cardiac structures, the vascular systems, the fetus, and abdominal organs such as liver and kidney. In brief, it is possible to see inside the human body without breaking the skin by using a beam of ultrasound.

Figure 8 Basic transducer geometry for acoustic imaging applications.d- Resonators/FiltersWhen a piezoelectric body vibrates at its resonant frequency, it absorbs considerably more energy than at other frequencies resulting in a dramatic decrease in the impedance. This phenomenon enables piezoelectric materials to be used as a wave filter. A filter is required to pass a certain selected frequency band or to block a given band. The band width of a filter fabricated from a piezoelectric material is determined by the square of the coupling coefficient k, that is, it is nearly proportional to k2. Quartz crystals with a very low k value of about 0.1 can pass very narrow frequency bands of approximately 1% of the center resonance frequency. On the other hand, PZT ceramics with a planar coupling coefficient of about 0.5 can easily pass a band of 10% of the center resonance frequency. The sharpness of the passband is dependent on the mechanical quality factor QM of the materials. Quartz also has a very high QM of about 106, which results in a sharp cut-off to the passband and a well-defined oscillation frequency.

e- Surface Acoustic Wave DevicesA surface acoustic wave (SAW), also called a Rayleigh wave, is essentially a coupling between longitudinal and shear waves. The energy carried by the SAW is confined near the surface. An associated electrostatic wave exists for a SAW on a piezoelectric substrate, which allows electro-acoustic coupling via a transducer. There is a very broad range of commercial system applications which include front-end and intermediate frequency (IF) filters, community antenna television (CATV) and video cassette recorder (VCR) components, synthesizers, analyzers and navigators.

f- Piezoelectric TransformersWhen input and output terminals are fabricated on a piezo device and input/output voltage is changed through the vibration energy transfer, the device is called a piezoelectric transformer. Piezoelectric transformers were used in color TVs because of their compact size in comparison with the conventional electromagnetic coil-type transformers. Recent lap-top computers with a liquid crystal display require a very thin, no electromagnetic-noise transformer to start the glow of a fluorescent back-lamp.

g- Piezoelectric ActuatorsPiezoelectric and electrostrictive devices have become key components in smart actuator systems such as precision positioners, miniature ultrasonic motors and adaptive mechanical dampers.Piezoelectric actuators are forming a new field between electronic and structural ceramics. Application fields are classified into three categories: positioners, motors and vibration suppressors.The manufacturing precision of optical instruments such as lasers and cameras, and the positioning accuracy for fabricating semiconductor chips, which must be adjusted using solid-state actuators, are generally in the order of 0.1m. Regarding conventional electromagnetic motors, tiny motors smaller than 1 cm3 are often required in office or factory automation equipment and are rather difficult to produce with sufficient energy efficiency. Ultrasonic motors whose efficiency is insensitive to size are considered superior in the mini-motor area. Vibration suppression in space structures and military vehicles using piezoelectric actuators is another promising field of application.

4. Dielectric CeramicsA material with high electric resistivity is categorized as an insulator material. When we pay attention to their dielectric polarization and apply the materials to the electronics circuits, we usually call them "dielectrics". Dielectric ceramics are essential electrical materials for today's advanced electronics devices. Production quantity of the dielectric ceramic is the largest among the other electronics ceramics such as magnetic, semiconductors, insulators, resistors and piezoelectric, and electro-optic materials. Main applications are for ceramic capacitors and microwave resonators. The dielectric ceramics is classified into two groups based on their dielectric properties.Classification of the dielectric ceramics1- High-Q materialsThe dielectrics of this group are called temperature compensating dielectrics, because they can compensate the temperature dependence of other components. Dielectric constant changes linearly with temperature. Ceramic capacitors with these materials stabilized the resonant circuits in which a high quality factor (Q value) and a resonant frequency are extremely important. In some cases, the ceramics also called linear dielectrics, because the polarization changes linearly with an applied electric field.Dielectric constant of this group ranges from about 4 to 400. The temperature coefficient is in the range from +120 to 4700 ppm/C. Q value (defined as a reciprocal number of a dissipation factor tan ) is in the range from 1000 to 100 000. These characterized values are intrinsically given by the compositions, and are modified with kinds and contents of the composed elements. Typical dielectrics are TiO2, MgTiO3, and CaTiO3.Titanate-based materials are dominant compositions, which sinter at normally higher than 1100C. Some of the high Q materials for microwave application need very high soaking temperature (>1400C). Today, glass ceramics are widely used for ceramic multilayer substrate with Ag and Cu as an inner conductor. About 4050% of the basic oxides such as Al2O3, SiO2, MgO and alkali-earth oxides compose the dielectrics, which can sinter at relatively lower temperature ( 105. The Parameter R = (1 ) / E. is the transverse rupture strength, the thermal conductivity, is the Poissons ratio, is the thermal expansion coefficient and E is the Youngs modulus.

11- Ceramic-Matrix CompositesIt is widely known that controlling the microstructure of materials can modify their physical properties. In this section, a brief illustration of ceramic-matrix composites is presented.Toughening Mechanisms in Ceramics1. Ceramic materials are in general brittle. So, according to the fracture mechanics, the strength is governed by the flaw size and the fracture toughness.2. General approaches for producing strong ceramics are to reduce the maximum size of processing flaws or to enhance the fracture toughness.3. The former approach is limited by the nature of the microstructure of ceramics, since a grain boundary itself can be a flaw responsible for brittle fracture, and surface flaws generated in use may reduce the strength of ceramics.4. The fracture toughness of ceramics is improved by introduction of secondary phases into matrix materials when the secondary phases are chosen to act as barriers to crack propagation.5. As an example, Whiskers introduced into a ceramic matrix, for example, can retard the crack propagation because the stresses in a whisker spanning the crack plane will tend to pull the crack shut "crack bridging".Ceramic matrix composites may be classified into two categories:1. One is a group of toughened ceramics reinforced with particulates and whiskers. These materials exhibit brittle behavior in spite of considerable improvements in fracture toughness and strength. The maximum in fracture toughness is around 10MPa.m1/2 or more.2. The second consists of continuous-fiber composites exhibiting quasi-ductile fracture behavior accompanied by extensive fiber pull out. The fracture toughness of this class of materials can be higher than 20MPa.m1/2 when produced with weak interfaces between the fibers and matrix.Toughening mechanisms in ceramics explored in recent years may be summarized as follows:1. Crack Bowing (Bend): Brittle materials containing secondary phase dispersions can possess higher strength than those of homogeneous materials. The strength increases with an increase in the volume fraction of dispersed particles and decreases with dispersed particle size. The strength therefore depends on the particle spacing. the strengthening is achieved in a manner analogous to the bowing of a dislocation between pinning points.2. Crack Deflection: The crack deflection can enhance the fracture toughness. The most effective morphology for deflecting a propagating crack has been found to be rod-shaped grains having a high aspect ratio, and this can account for fourfold increase in fracture toughness. This was demonstrated in rod-shaped grains of silicon nitride and lithium-alumino-silicate glass ceramic.3. Microcracking: In composites containing secondary fine particles, the local residual stress due to mismatch of thermal expansion coefficients is generated during cooling from the processing temperature. The residual stresses cause spontaneous microcracking. However, when the residual stress is less than the local strength of the material, the internal stress remains in the material. The volume expansion of the cracked layers leads to the generation of compressive stresses to the crack interface, resulting in an increase in the fracture toughness.4. Transformation Toughening: The microstructure of partially stabilized zirconia (PSZ) in a system such as CaO-ZrO2 consists of metastable tetragonal ZrO2. The stress-induced transformation of tetragonal zirconia to monoclinic contributes to the toughening because all particles within several microns of the cracks become monoclinic while all other particles remain tetragonal. As a result, a compressive stress is generated at the crack interface due to the volume expansion accompanying phase transformation, in a similar way to microcracking. High fracture toughness is achieved by controlling the size of the precipitates, e.g. ZrO2 ceramics doped with around 3 mol% Y2O3 exhibit high strength when fabricated from an ultrafine powder to produce fine-grained materials.5. Bridging: The frictional stress between the fiber and the matrix resists crack propagation in the pullout process. The bridging stresses are generated by a variety of micro-processes, such as frictional interlocking, fiber bridging and sliding pullout. A weak interface leading to de-bonding of fibers from the matrix is essential for producing toughened composites. Example is the toughening of alumina and silicon nitride with silicon carbide whiskers.Carbon FibersSic FibersOxide FibersWhiskersProcessing of Ceramic CompositesHot PressingChemical Vapor ImpregnationLiquid Impregnation and PyrolysisNovel TechniquesOther advanced ceramics12- Ceramic Materials for Energy SystemsCeramic materials also play an important role in the field of battery technology. The Li-ion battery is a typical case in which ceramic materials are applied. Here, only the case of Li-ion battery is presented. Other examples of ceramics in energy systems such as solid oxide fuel cell SOFC may be found in literature.Li-ion batteryIn Li-ion batteries, lithium oxides are used for a positive active material, and carbons for a negative active material. Both of the active materials are considered to be ceramics prepared by normal ceramic production processes. They are used in powder form in Li-ion batteries.One of the important parameters in evaluating a battery is its capacity to store electric energy. Coulomb capacity is normally expressed as a unit of ampere-hour. One ampere-hour capacity delivers 1 A current for 1 h (3600 s); that is, it has energy of 3600 C. Therefore, 1 molar active materials can give 26.8 A h capacity (= 96 500/3600), if they have mono-valence. When the mean voltage of a battery is in volts V, energy capacity is expressed to be V A h, which is Wh. Therefore, a higher voltage gives higher energy, and also higher power because power is proportional to V squared.Another important parameter for battery is C. C-rate can normalize current density for a battery. One C-rate current for a battery with 1 A h capacity is 1 A. Battery engineers often compare performance of batteries with C-rate when the batteries have the same chemistry of electrode active materials.Positive Electrode (Cathode)The most popular positive electrode material for Li-ion batteries is LiCoO2. This material is a kind of ceramic (discovered by Mizushima et al. in 1988) that delivers 4 V potential. During charging, Li-ion/electron pairs are extracted electrochemically from LiCoO2 ceramic particles. Electrons come to the negative electrode through an external circuit, while Li-ions also come to the negative electrode through the electrolyte inside the battery. The valence of trivalent Co ions in the particles increases tetra-valence to maintain electrostatic neutrality after the extraction of Li-ions and electrons. The equation of this reaction can be expressed as follows:Li+Co+3O2 nLi+ + ne + Li+(1n)Co+3(1n)Co+4n O2, where, n is the number of ions and electrons.This valence change in the positive electrode is so-called solid-state reductionoxidation (redox) reaction. For this reaction, positive electrode active material must contain a transition element. This reaction is characteristic for Li-ion batteries. Oxygen deficiency in the LiCoO2 affects the Co valence and keeps electrical neutrality. Therefore, control of oxygen partial pressure is very important for producing LiCoO2 ceramic material.Negative Electrode (Anode)Carbon is normally used as the negative electrode active material of Li-ion batteries. It is widely known that the use of carbon for negative electrode generated great advances in Li-based battery technologies. Generally, lithium metal is too reactive for practical use.During charging, electrons extracted from positive electrode transport to the negative carbon electrode through an external circuit. At the same time, Li-ions also extracted from positive electrode transport to the negative through liquid electrolyte. These two particles form a pair and exist in the carbon material in charged state. The equation of this reaction can be expressed as follows:C + nLi+ + ne CLinNegative electrode carbon is normally powder with particle size range of 520m. As carbon particles must receive electrons from external circuit and Li-ions from liquid electrolyte, the carbon particles have to contact both the metal current collector and the electrolyte liquid. Carbon powder is mixed with PVDF binder and then coated by a machine onto a thin metal copper foil.Battery ReactionBattery reaction of Li-ion battery is illustrated in the next figure. Electrochemical reactions in the positive and negative electrodes during charging are expressed by the following equations:Positive electrode: LiCoO2 nLi + ne + Li(1n)CoO2Negative electrode: C + nLi + ne CLinOverall reaction is as follows.LiCoO2 + C Li(1n)CoO2 + CLin (charging reaction)Where is the discharging reaction?

Chargedischarge reaction of Li-ion battery13- Functionally Graded MaterialsFunctionally graded materials (FGM's) are a new generation of engineered materials wherein the microstructural details are spatially varied through non-uniform distribution of the reinforcement phase(s).

Phase A particles with phase B matrixPhase B particles with phase A matrixTransition zoneSchematic representation of FGM

Schematic of a ceramic/metal FGM for the relaxation of thermal stress

1. The volume fraction of the reinforcement phase is increased gradually replacing the matrix phases.2. FGM's is accomplished using reinforcements with different properties, sizes, and shapes.3. The produced microstructure has varying thermal and mechanical properties continuously or discretely at the macroscopic scale.4. Many techniques are involved in the fabrication of FGM's. E.g. mechanical mixing or powder deposition then compaction, tape casting, chemical vapor deposition CVD, physical vapor deposition PVD, and electrophoretic deposition EP.5. Functionally graded materials are ideal for applications involving severe thermal gradients, e.g. thermal structures in advanced aircraft and aerospace engines to computer circuit boards.

The FGM concept was originated in an effort to reduce thermal stress for thermal barrier materials. Subsequently, much research has been done focusing on the thermal and mechanical aspects of FGMs. Recent research has revealed that the concept of FGM can also be advantageously extended to electronic, optical, biomedical and other fields. Many interesting and unique concepts and experimental results have been forthcoming to appear not only in structural materials but also in other various fields.Generally, the most popular applications of the FGM concept are based on combining two or more incompatible functions into a given material (such as high thermal resistivity and excellent mechanical properties) in order to suit a particular circumstance. Example is High order step wise functionally graded materials Al2O3-Ti. These materials combine the high fracture toughness of Ti phase and the relatively low density of Al2O3 and stand for candidate materials for harsh mechanical and thermal environments.However, many phenomena and experimental results, such as the control of electrical properties of perovskite oxide and the optical filter characteristics of graded TiO2/SiO2 film mentioned above, cannot be explained by such simple combination of two functions. In addition, recent research has revealed that natural plants and living bodies have many intelligent properties due to their graded structures. This encourage us to create new functions through gradation techniques.Part 2: Bio-Ceramics1- Introduction to Bio-Ceramics Chp1Most clinical applications of bioceramics relate to the repair of the skeletal system, composed of bones, joints and teeth, and to augment (enhance) both hard and soft tissues. Ceramics are also used to replace parts of the cardiovascular system, especially heart valves. Special formulations of glasses are also used therapeutically for the treatment of tumors.Bioceramics are produced in a variety of forms and phases and serve many different functions in the repair of the body, which are summarized in Fig. 1.1 and Table 1.1. In many applications ceramics are used in the form of bulk materials of a specific shape, called implants, prostheses, or prosthetic devices. Bioceramics are also used to fill space while the natural repair processes restore function. In other situations the ceramic is used as a coating on a substrate, or as a second phase in a composite, combining the characteristics of both into a new material with enhanced mechanical and biochemical properties.Bioceramics are made in many different phases. They can be single crystals (sapphire), polycrystalline (alumina or hydroxyapatite), glass (Bioglass), glassceramics (apatite-wollastonite A/W glass-ceramic) or composites (polyethylene-hydroxyapatite). The phase or phases used depend on the properties and function required. For example, single crystal sapphire is used as a dental implant because of its high strength. A/W glass-ceramic is used to replace vertebrae because it has high strength and bonds to bone. Bioactive glasses have low strength but bond rapidly to bone, so are used to augment the repair of boney defects.Ceramics and glasses have been used for a long time outside the body for a variety of applications in the health care industry. Eye glasses, diagnostic instruments, chemical ware, thermometers, tissue culture flasks, chromatography columns, lasers, and fiber optics for endoscopy are commonplace products in the multi-billion dollar industry.Ceramics are widely used in dentistry as restorative materials: gold porcelain crowns, glass-filled ionomer cements, endodontic treatments, dentures etc. However, use of ceramics inside the body as implants is relatively new: alumina hip implants have been used for just over 40 years.Many compositions of ceramics have been tested for potential use in the body but few have reached human clinical application. Clinical success requires the simultaneous achievement of a stable interface with connective tissue and an appropriate, functional match of the mechanical behavior of the implant with the tissue to be replaced. Few materials satisfy this severe dual requirement for clinical use.Types of BioceramicsTissue InterfacesThere are four general types of implanttissue response, as summarized in Table 1.3. It is critical that any implant material avoids a toxic response that kills cells in the surrounding tissues or releases chemicals that can migrate within tissue fluids and cause systemic damage to the patient. One of the main reasons for the interest in ceramic implants is their lack of toxicity. The most common response of tissues to an implant is formation of a nonadherent fibrous capsule. The fibrous tissue is formed in order to wall off or isolate the implant from the host. It is a protective mechanism and with time can lead to complete encapsulation of an implant within the fibrous layer. Metals and most polymers produce this type of interfacial response; the cellular mechanisms which influence this response are described in a later section.The second type of response is that for biologically inactive, nearly inert ceramics. Alumina or zirconia, also develop fibrous capsules at their interface. The chemical inertness of alumina and zirconia results in a very thin fibrous layer under optimal conditions. More chemically reactive metallic implants elicit thicker interfacial layers. However, it is important to remember that the thickness of an interfacial fibrous layer also depends upon motion and fit at the interface.The third type of interfacial responseis when a bond forms across the interface between implant and the tissue. This is termed a "bioactive" interface. The interfacial bond prevents motion between the two materials and mimics the type of interface that is formed when natural tissues repair themselves. This type of interface requires the material to have a controlled rate of chemical reactivity. An important characteristic of a bioactive interface is that it changes with time, as do natural tissues, which are in a state of dynamic equilibrium.The fourth type of interfacial responseis is as follows: When the rate of change of a bioactive interface is sufficiently rapid the material "dissolves" or "resorbs" and is replaced by the surrounding tissues. Thus, a resorbable biomaterial must be of a composition that can be degraded chemically by body fluids or digested easily by macrophages. The degradation products must be chemical compounds that are not toxic and can be easily disposed of without damage to cells.Types of BioceramicTissue AttachmentsThe mechanism of attachment of tissue to an implant is directly related to the tissue response at the implant interface. There are four types of bioceramics, each with a different type of tissue attachment, summarized in Table 1.4 with examples. The relative level of reactivity of an implant also influences the thickness of the interfacial layer between the material and the tissue.Nearly inert, implant forms a non-adherent fibrous layer at the interface, while a bioactive material undergoes chemical reactions in the body, but only at its surface.2- Alumina and Zirconia in Surgical Implants Chp2The potential of ceramics as biomaterials relies upon their compatibility with the physiological environment. Bioceramics are compatible because they are composed of ions commonly found in the physiological environment (calcium, potassium, magnesium, sodium, etc.) and of ions showing limited toxicity to body tissue (zirconium and titanium). This section deals with the two nearly inert ceramics most used in surgical implants: alumina and zirconia.Nearly inert bioceramics undergo little or no chemical change during long-term exposure to the physiological environment. Even in those cases where these bioceramics may undergo some long-term chemical or mechanical degradation, the concentration of degradation product in adjacent tissue is easily controlled by the bodys natural regulatory mechanisms. Tissue response to immobilized inert bioceramics involves the formation of a very thin, several micrometers or less, fibrous membrane surrounding the implant material. Inert bioceramics may be attached to the physiological system through mechanical interlocking, by tissue ingrowth into undulating (wavy) surfaces, or by cement fixation. The nearly inert ceramic most used for surgical implants is alumina.Alumina Ceramics as Implant MaterialsHigh- density, high purity (> 99.5%) Al2O3 is used in load-bearing hip prostheses and dental implants because of its combination of excellent corrosion resistance, good biocompatibility, high wear resistance, and high strength. Although some dental implants are single-crystal sapphire, most Al2O3 devices are very fine-grained polycrystalline -Al2O3, produced by pressing and sintering at temperatures ranging from 16001800C, depending upon the properties of the raw material. A very small amount of MgO (7 m can decrease mechanical properties by about 20%. High concentration of sintering aids must be avoided because they remain in the grain boundaries and degrade fatigue resistance.These and other physical properties are summarized in Table 2.1 for a commercially available implant material, along with the International Standards Organization (ISO) requirements and the proposed new standards for alumina implants. Other typical properties of commercially available alumina implant materials are listed in Table 2.2. Methods exist for lifetime predictions and statistical design of proof tests for load bearing ceramics. Applications of these techniques show that specific prosthesis load limits can be set for an Al2O3 device based upon the flexural strength of the material and its use environment.12 Load bearing lifetimes of 30 years at 12,000 N loads or 200 MPa stresses have been predicted. Results from aging and fatigue studies show that it is essential that Al2O3 implants be produced with the highest possible standards of quality assurance, especially if they are to be used as orthopedic prostheses in younger patients.Use of Zirconia Ceramics in Surgical ImplantsMedical grade alumina has outstanding biocompatibility and wear resistance. However, it exhibits moderate flexural strength and toughness. For this reason, the diameter of most alumina femoral head prostheses has been limited to 32 mm. Zirconia is also exceptionally inert in the physiological environment, and zirconia ceramics have an advantage over alumina ceramics of higher fracture toughness and higher flexural strength and lower Young's modulus.Zirconia ceramics suggested for surgical implants fall into two basic types: tetragonal zirconia stabilized with yttria (TZP) and magnesium oxide partially stabilized zirconia (Mg-PCZ). Properties of zirconia are compared with alumina ceramic implant materials in Table 2.4. Zirconia may be suitable for bearing surfaces in total hip prostheses. However, there are three major drawbacks (disadvantages) regarding zirconia. One is the reported strength reduction with time in physiological fluids. The second is its wear properties, and third is the potential radioactivity of the material.The deleterious (harmful) martensitic (non-diffusional) transformation from tetragonal to monoclinic phase in yttria-doped zirconia is due to aging in water and the accompanying reduction in toughness. However, tests in simulated body fluids and in animals have shown only slight decreases in fracture strength and toughness. The observed strength after two years is still much higher than the strength of alumina tested under similar conditions.Zirconia is often accompanied by radioactive elements with a very long half-life, such as thorium and uranium. These elements are difficult and expensive to separate from zirconia. There are two types of radiation of concern in zirconia ceramics: gamma and alpha. The gamma radioactivity of alumina, zirconia and Co-Cr-alloy femoral head prostheses has been measured. Alumina was found to have the lowest gamma radioactivity and zirconia and the Co-Cr-alloy were found to be approximately the same. The gamma radioactivity for the Co-Cr-alloy and zirconia were found to be of the same order of magnitude as the national ambient radioactivity in France. The data suggest that the level of gamma radiation in commercially available zirconia bioceramics is not a major concern. However, significant amounts of alpha radiation have been observed with zirconia ceramics intended for surgical implants.Alpha particles, because of their high ionization capacity, destroy soft and hard tissue cells. The alpha emission observed from zirconia ceramic femoral head prostheses is a concern. Although the activity is small, questions concerning the long-term effects of alpha radiation emission from zirconia ceramics must be answered.SummaryAlumina and zirconia ceramics are both exceptionally biocompatible, due to their chemical stability in the physiological environment. Zirconia ceramics have a higher fracture strength, toughness and lower modulus of elasticity than alumina ceramics. The phenomena of slow crack growth, static and cyclic fracture, low toughness, stress corrosion, deterioration of toughness with time, and sensitivity to tensile stresses are all serious concerns for both ceramics in high load bearing applications. Both alumina and zirconia ceramics undergo slight reduction in fracture strength with time in the physiological environment. Alumina and zirconia bioceramics should be restricted to designs involving compressive loading or limited tensile loads.As a consequence of a mismatch in elastic modulus is that a bioceramic implant will shield a bone from mechanical loading, allowing nearly all the mechanical load to be carried by the implant. Living bone must be under a certain amount of tensile load in order to remain healthy; if it is unloaded or is loaded in compression it will undergo biological changes which lead to resorption, weakening of the bone, and deterioration of the implantbone interface. The high modulus of elasticity of alumina and zirconia limit their effectiveness as load bearing bone interface materials. Metal alloys suffer the same limitation. The high modulus of elasticity of alumina does not limit its ability to serve as an articulating surface.Alumina is an excellent material for certain orthopedic applications, such as the ball in an artificial hip joint, because of its excellent biocompatability, low friction, high wear resistance, and high compressive strength. The tribological properties of alumina ceramics are far superior to zirconia ceramics. Alumina or alumina-articulating surfaces in total joint replacements have the best tribological properties and the best clinical results.3- Bioactive Glasses - Materials Chp3It was discovered by Hench and colleagues in 1969 that bone can bond chemically to certain glass compositions. This group of glasses has become known as bioactive glasses, based upon the following definition:"A bioactive material is one that elicits (obtains) a specific biological response at the interface of the material which results in the formation of a bond between the tissues and the material"Bioactive glasses have numerous applications in the repair and reconstruction of diseased and damaged tissue, especially hard tissue (bone). One aspect that makes bioactive glasses different from other bioactive ceramics and glass-ceramics is the possibility of controlling a range of chemical properties and rate of bonding to tissues. The most reactive glass compositions develop a stable, bonded interface with soft tissues. It is possible to design glasses with properties specific to a particular clinical application. This is also possible with some glass-ceramics, but their heterogeneous microstructure restricts their versatility.Compositions of Bioactive GlassesThe base components in most bioactive glasses are SiO2, Na2O, CaO, and P2O5. The first, and most well-studied composition, termed Bioglass 45S5 contains 45% SiO2, 24.5% Na2O, 24.4% CaO and 6% P2O5, all in weight percent. The Bioglass 45S5 (Registered trademark University of Florida, Gainesville, FL),composition in mole percent is given in Table 3.1, along with many other compositions investigated for surface reaction kinetics. Hench and coworkers have studied a series of glasses in this four-component system with a constant six weight percent P2O5 content.Properties of Bioactive GlassesThe primary advantage of bioactive glasses is their rapid rate of surface reaction, which leads to fast tissue bonding. Their primary disadvantage is mechanical weakness and low fracture toughness due to an amorphous two dimensional glass network. The tensile bending strength of most of the compositions is in the range of 4060 MPa, which make them unsuitable for load-bearing applications. For some applications low strength is offset (balanced) by the glasses low modulus of elasticity of 3035 GPa. The importance of this value is that it is close to that of cortical bone.The low strength does not influence the utility of bioactive glasses as a coating, where interfacial strength between metal and the coating is the limiting factor. Low strength also has no effect on use of bioactive glasses as buried (masked) implants, in low-loaded or compressively loaded devices, in the form of powders or as the bioactive phase in composites. A new generation of highly bioactive glass-ceramics that also have high strength is thus studied.

Notes on Preparation Processing of Bioactive Glasses1. Bioactive glasses are produced by conventional glass manufacturing methods.2. Contamination of the glass must be avoided in order to retain the chemical reactivity of the material.3. Purity of raw materials must be assured.4. Analytical grade compounds are typically used for most components.5. Silica can be added in the form of high purity (flint quality) glass sand, since chemically prepared silicas are difficult to handle without adsorption of water and agglomeration.6. The choice of raw materials can affect the properties of the glass.7. Crystal-water free compounds are used. This is due to the dissolution of OH ions in the glass structure and the associated decrease of viscosity.8. Preferential vaporization of fluxes will also affect glass viscosity and tendency to crystallize or phase separate, as well as alter the final glass composition.9. Weighing, mixing, melting, homogenizing and forming of the glass must be done without introducing impurities or losing volatile constituents, such as Na2O or P2O5.10. Melting is usually done in the range of 13001450 C, depending on composition.Notes on Shaping Processing of Bioactive Glasses1. Bulk specimens can be formed by casting or injection molding in graphite or steel molds.2. Annealing is crucial, 450550 C, because of the high coefficient of thermal expansion of the bioactive glass compositions. Each type of device (composition) must have its own annealing schedule established.3. Bioactive glasses are soft glasses and final shapes can be easily made by machining. Standard machine tools or dental handpieces can be used.4. Diamond-cutting tools are preferred with copious irrigation (with plentiful water washing), although dry grinding is also possible.5. If a granulated or powdered material is required, the melt can be rapidly quenched in water or air before grinding and sieving into the desired particle sizes. The glass frit should be rapidly dried to avoid corrosion while in contact with water.Other processing methods used for bioactive glass coatings are exists4- Bioactive Glasses - Clinical Applications Chp6The bioactive glasses had valuable properties. When in contact with body fluids and tissues, they would develop a reactive layer at their surfaces. Due to the gel-like structure of the bioactive glass, it provided a compliant interface between bulk glass and tissue. It was found that the 45S5 formulation of Bioglass and related compositions did not break-up when drilled with standard surgical drilling equipment. The glass was relatively soft and extremely suitable for microsurgical drilling techniques, opening up its use in certain otolaryngological applications.Particles possibly will appear after implantation of a bioactive glass if it were to become damaged in some way; however, cells found in blood and tissues are programmed to pick up and digest all unusual particles. Tests showed that these bioactive glasses in solid or particulate form were not toxic to any of the tissues or systems with which they were in contact.Applications of bioactive glass have required several different forms of the material:1. Solid shapes.The glasses can be fabricated by casting into shaped implants for specific purposes where mechanical strength is of secondary importance. E.g. middle ear device.2. Particulates of various size ranges.Bioactive glass in the form of particulate has found application in the treatment of e.g. periodontal disease; where it is essential to replace lost bone and soft tissue connections if teeth are to be saved.3. Particulates combined with autologous bone particles.The success of particulate material in restoring bone in the periodontal area, where the surgical site preparation releases bone fragments and associated growth factors to combine with the bioactive glass, has led to a series of investigations in which the glass has been mixed with autologous bone before implanting.4. Particulates delivered via an injectable system.A group of applications that depend on the soft tissue adhesion of bioactive glass; which are introduced by injection. The particle size and shape, characteristics of the vehicle and needle size and length provide many problems which need to be solved in advance. Preclinical experiments in animals have concentrated on the treatment of two urological conditions which occur in children and some adults.5- A/W Glass-Ceramic Chp13Glass can be converted by heat treatment into glass-crystal composites containing various kinds of crystalline phases with controlled sizes and contents. The resultant glass-ceramic can exhibit superior properties to the parent glass and to sintered crystalline ceramics. Generally, monophase bioactive ceramics such as Bioglass-type glasses and sintered HA (hydroxyapatite Ca5(PO4)3OH) do not show as high a mechanical strength as human cortical bone. Natural bone is a composite in which an assembly of HA small crystal particles is effectively reinforced by organic collagen fibers. Kokubo et al. attempted to prepare a similar composite by a process of crystallization of glass in 1982. In this attempt, -wollastonite (CaOSiO2), consisting of a silicate chain structure, was chosen as the reinforcing phase. This type of glass ceramics is called A/W glass ceramic (apatite/wollastonite glass ceramic).Advantages of A/W Glass Ceramics 1. A/W glass-ceramic can easily be machined into various shapes, even into screws, by diamond tools.2. The mechanical properties is superior to both glass and glass ceramics, see table 13.1. When loaded in an aqueous body environment, this glass-ceramic shows a decrease in mechanical strength, i.e. fatigue, by slow crack growth due to stress corrosion, similar to other ceramics. The magnitude of its fatigue is, however, much lower than those of glass and glass-ceramic.3. The parameter, n, of slow crack growth, which is derived from the dependence of the bending strength upon the stressing rate, is 33 for A/W glass-ceramic, whereas it is 9 and 18 for glass and glassceramic, respectively.4. When a bending stress of 65 MPa is continuously applied in the body, A/W glass-ceramic should withstand it for over ten years, whereas glass, glass-ceramic, and dense sintered HA would survive only one minute.5. The magnitude of the fatigue of A/W glass-ceramic can be further decreased by a surface modification such as Zr+ ion implantation (a type of coating).Summary (AP formation)The mechanism of apatite formation on the surfaces of CaOSiO2-based glasses and glass-ceramics, including A/W glass-ceramic, in the body can be interpreted as follows. The calcium ion dissolved from the glasses and glass-ceramics increases the ion activity product of the apatite in the surrounding body fluid, and the hydrated silica on the surfaces of the glasses and glass-ceramics provides favorable sites for apatite nucleation.Consequently, the apatite nuclei are rapidly formed on their surfaces. Once the apatite nuclei are formed, they spontaneously grow by consuming calcium and phosphate ions from the surrounding body fluid.In the case of A/W glass-ceramic, although the presence of the silica gel layer on its surface could not be detected even under the high resolution transmission electron micrographs, the dissolution of an appreciable amount of the silicate ion from the glass-ceramic into the simulated body fluid indicates the formation of a large number of silanol groups (SiOH) at the surface of the glass-ceramic in the body.It is expected that the bone-like apatite layer could be formed on the surfaces of various kinds of materials including metals, ceramics and organic polymers by the following biomimetic method at ordinary temperature and pressure. When a material as a substrate is placed on or in granular particles of a CaOSiO2-based glass soaked in the simulated body fluid for a certain period, a large number of apatite nuclei can be formed on the surface of the substrate, as well as on the surfaces of the glass particles, by the calcium and silicate ions dissolved from the glass particles. When the substrate is then soaked in another solution supersaturated with respect to the apatite, the apatite nuclei grow spontaneously in situ on the substrate by consuming the calcium and phosphate ions from the surrounding solution to form the apatite layer.A dense and uniform layer of bone-like apatite was formed on various kinds of materials, including stainless steel, titanium metal, platinum, gold, silicon, carbon, alumina, zirconia, polymethylmethacrylate, polyethylene, polyethylene terephthalate (PET), and polyethersulfone (PES), by this method. The thickness of the apatite layer continued to increase with increased soaking time in the second solution. The rate of growth of the apatite layer increased with increasing temperature and the degree of the supersaturation of the second solution. 6- Machinable and Phosphate Glass-Ceramics Chp16In a living tissue, the bioactive materials are not regarded as foreign bodies and encapsulated by fibrotic tissue; instead, direct bonding takes place. The special combinations of properties required for medical indications can be adjusted and varied in glasses and glass-ceramics.Glass-ceramics consist of at least one glassy and at least one crystalline phase. The processing to develop a glass-ceramic is characterized by a formation of a base glass and an additional heat treatment of the glass. During this heattreatment process, nucleation and crystallization has to be controlled to form the crystals in the base glass.Biomaterials for bone substitution are called bioactive if a stable bond to the bone is formed. Hench et al showed that Bioglass can bond to bone in animals. The interface reactions between Bioglass and bone, the formation of different Ca-, P-, and SiO2-rich layers, the dependence on the composition of the implant, the environment, and reaction time were studied. It was shown that the preferred bonding of bone and biomaterial (glass-ceramic or sintered ceramic) can be achieved if the biomaterial contains apatite crystals in the basic material or develops an apatite layer. Mica crystals will permit predictable mechanical machining properties. It is believed that for a material to be machinable and bioactive, it should contain both mica and apatite crystals; thus, the glass-ceramics developed for medicine is derived from different base glasses.Compositions and ProcessingBIOVERIT products is taken here as case study.BIOVERIT I is a mica-apatite glass-ceramic with a chemical composition from the SiO2-(Al2O3)-MgO-Na2O-K2O-F-CaO-P2O5 base glass system. These glasses are from the silico-phosphate type. The key to the development of BIOVERIT I was to form a phase-separated base glass consisting of three glassy phases and to control the nucleation and crystallization by heat treating the glass. BIOVERIT II glass-ceramic contains mica as the main crystal phase and secondary crystals, e.g. cordierite crystals.The base glass is derived from the SiO2-Al2O3-MgO-Na2O-K2O-F system, called silicate glasses. The base glass is phase separated into two glassy phases and micas were formed during heat treatment of the glass. Because of the high mica content, BIOVERIT I and II are machinable glass-ceramics.The chemical composition of BIOVERIT III glass-ceramic is characterized by glasses from the CaO-Al2O3-P2O5-Na2O (ZrO2-FeO/Fe2O3) system, so called "invert" glasses of a phosphate type. In comparison to BIOVERIT I and II, the base glass of BIOVERIT III does not show phase separation. Apatite, AlPO4 crystals, and other phosphate crystals grow via a special process in the base glass during heat treatment. Machinable mica-based glass-ceramics were developed for technical applications by Beall et al. The composition of the base glasses are characterized by the alkali-fluoroboro-silicate system. The glass-ceramic implants known as Dicor have been used for dental restorations. Typical chemical compositions of BIOVERIT-type glass-ceramics are shown in Tables 16.116.3, with the range of possible chemical compositions given. 7- Porous Hydroxyapatite Chp19The natural bone is approximately 70% HA by weight and 50% HA by volume. The basis for making the material macroporous is not so obvious, and require an approval for the architecture of tissues and its effect on regeneration and repair, e.g. throught SEM photos.ProcessingSintered HAThe most widely-used process to fabricate porous HA implants utilizes isostatic compaction and sintering of calcium phosphate powders that contain naphthalene particles. Volatilization of the naphthalene particles leaves a porosity which consists of spherical voids communicating by a narrow-necked aperture wherever the particles were in contact. To permit bone in-growth of any depth, these apertures must exceed 100 m or they will represent blind ends and discontinuities in bone. Another sintering process for creating a macroporous structure utilizes pretreatment with hydrogen peroxide. Other techniques to produce porous ceramis exists.HA CementMore recently, water-setting HA cements have been employed to create HA materials with various porosities. The most completely-characterized HA cement in this group is made by reacting tetra-calcium phosphate and calcium hydrogen phosphate in an aqueous environment.Ca4(PO4)2 + CaHPO4 Ca5(PO4)3OHUnder in vitro conditions at 37C, the HA cement sets in approximately 15 minutes and the isothermal chemical reaction is completed in 4 hours. Porosity is obtained by mixing the cement prior to set with sucrose granules and then dissolving the granules in water.CompositionThe chemical compositions of the different HA preparations are traditionally evaluated with X-ray diffraction. X-ray diffraction typically show a crystalline structure which is essentially pure HA with trace levels of beta tri-calcium phosphate. This beta form of tricalcium phosphate is also present within human bone at similar low concentrations.8- Hydroxyapatite Coatings Chp21Ceramic coatings are used on metallic substrates in a variety of applications, including enhancement of corrosion resistance of a metal or the creation of a more refractory surface for high temperature service. In the biomedical field, coatings have been used to modify the surface of implants, and in some cases to create an entirely new surface, which gives the implant properties which are quite different from the uncoated device.Because of its similarity to the inorganic component of bone and tooth structure, synthetic hydroxyapatite [Ca10(PO4)6(OH)2] was one of the first materials considered for coating metallic implants. As bulk HA is brittle and relatively weak when compared to common implant metals and alloys and high strength ceramics like aluminum and zirconium oxides, the best use of HA in load-bearing implant applications is as a coating on one of these stronger implant materials. In spite of the relatively good tissue response to metallic implant surfaces, such as the passive titanium oxide layer present on titanium, with the use of calcium phosphate materials as coatings it is possible to present a surface which is conducive to bone formation.One reason for the use of HA or a similar calcium phosphate surface is to cause earlier stabilization of the implant in surrounding bone. This is the case, for example, in a dental implant, where the healing time is reduced and the prosthetic attachment can be placed earlier. Another reason to use an HA coating is to extend the functional life of the prosthesis, as in the case of a cementless hip prosthesis, stabilized by the HA coating in the surrounding femur without the use of polymethylmethacrylate bone cement. Under the proper conditions a cementless prosthesis should remain functional longer than a cemented device in which stability is threatened by fracture of the bone cement after a limited number of years in service.ProcessingIndustrial and laboratory techniques used for coating HA and other ceramic materials onto metallic substrates include plasma spraying, electrophoretic deposition, sputtering and hot isostatic pressing. The plasma spray method is currently the most widely-used method for coating. There exist a number of alternative ways of HA coating.Plasma Spraying: Plasma spraying, the most common means of applying HA coatings to implant devices, employs a plasma, or ionized gas, partially to melt and carry the ceramic particulate onto the surface of the substrate. In flame spraying, another thermal spraying technique, the carrier gasses are not ionized and the temperatures generated are considerably lower than in plasma spraying. (Details can be found in the course of powder technology)Pure, 100% crystalline HA particles in the 2040 m range are typically used as the starting material for plasma spraying. When the softened particulate impinges on the substrate surface, individual particles deform into characteristic shapes called "splats". Three passes of the HA spray are typically made for any given area of the implant surface. Deformation and spreading of individual particles takes place on impact with the substrate, and the final coating thickness typically averages 4060 m. The flow of the softened HA is usually sufficient to form a dense coating with less than 2% residual porosity.HA coatings produced by the plasma-spraying process typically contain considerable amorphous calcium phosphate material and small amounts of crystalline phases other than HA. It is possible to increase the crystallinity and in some cases the bond strength of plasma-sprayed HA coatings by a post-deposition heat treatment. However, this extra step is usually not feasible commercially because of factors such as the adverse effects of the annealing temperature on the mechanical properties of the substrate metal or alloy, the time and expense of the additional operation and the contamination of the HA surface.Other Coating Techniques: Because of the difficulty in producing highly-crystalline coatings and reproducibly high bond strength using the plasma-spraying technique, other coating methods have been investigated for commercial applications as follows:1. An electrophoretic deposition process, in which HA particulate is suspended in an alcohol or other suitable solution and then subjected to an electric field, is a method which deposits the HA on an implant surface with minimal alteration of the starting material. This is a useful technique for placing HA on porous surfaces which cannot be completely coated other techniques such as plasma spraying. However, as the HA is only weakly deposited and the individual particles are not bonded together, high temperature sintering is necessary after deposition. Because of low bond strength, electrophoretically-deposited coatings are perhaps best used for porous implant designs where the presence of an HA coating is only necessary for a limited time period.2. Hot isostatic pressing (HIP) can be used to densify HA powder placed on the surface of a metallic implant. In order to achieve a uniform application of pressure on the particulate mass, an encapsulation material (e.g., a noble metal foil) is necessary. The advantage of the method is that lower sintering temperatures (less than 900C) are required to attain densification and bonding of the HA coating, thus the chances of altering the microstructure or mechanical properties of the metal substrate are reduced.3. Ion beam sputtering and radio frequency (RF) sputtering are thin film deposition techniques in which a target material is bombarded with an ion beam in a vacuum chamber, and atomic-sized fragments of sputtered material form coatings on suitably placed substrates. The typical coatings sputtered from an HA target are amorphous on deposition, as the sputtered components from the HA target (Ca, P, O and H) do not possess enough energy to recombine into HA. A heat treatment in the order of 500C is usually sufficient to provide enough thermal energy to form a crystalline coating that is predominantly HA. Although sputter-deposited coatings generally have better bond strength and mechanical properties than thick coatings, the durability of thin 1 m coatings in the body has not yet been demonstrated.4. Thermal spray techniques other than plasma spraying are also potential candidates for the production of commercial HA coatings on implants. One approach involves the use of the high velocity oxy-fuel (HVOF) technique. In this method the much higher velocity of the particles causes them to fuse and flow into irregularities on the metal surface more easily, in spite of being subjected to much lower temperatures than plasma spraying, thus the initially high crystallinity of the HA can be maintained.Composition: The composition of plasma-sprayed HA coatings is somewhat different to that of the starting material, as might be expected because of the high degree of melting experienced by the ceramic particulate.710 There are several analytical techniques which are useful for evaluation and analysis of HA coatings. Observation of the coating surface by scanning electron microscopy (SEM) or light microscopy (LM) can be useful prior to the use of other techniques which determine the composition and physical properties.XRD has been widely used as a means of determining the composition and structure of plasma-sprayed HA coatings as well as for estimating the percentage of crystallinity and identifying secondary crystalline phases generated as a result of the high temperature spraying process. Several changes in the diffraction pattern can be noted, including an increase in the background area under the peaks. A slight bulge (hump) in combination with peaks indicates a material which is partly crystalline and partly amorphous. A second change in the pattern would be the appearance of new peaks, as indicated by arrows to the left. These new peaks are indicative of one or more crystalline phases which have been generated by the plasma-spraying process, such as and -tri-calcium phosphate, calcium oxyphosphate, calcium pyrophosphate, or calcium oxide. Finally, lattice parameters may also alters depending on impurities.9- Pyrolytic Carbon Coatings Chp24The carbon can exist in a number of forms. The advantages of using carbon are, some of its forms, offer the most outstanding biocompatibility, chemical inertness and thrombo-resistance compared with any of the ceramics used in biomedical applications. These properties have, for example, made various forms of carbon the preferred material where interface to blood flow is required. Alternatively, they can be attached to both soft and hard tissue, which is a prerequisite for a wide range of biomedical devices.Three types of carbon are commonly used for biomedical devices: the low temperature isotropic (LTI) form of pyrolytic carbon, glassy (vitreous) carbon and the ultralow-temperature isotropic (ULTI) form of vapor-deposited carbon. These three forms of carbon have a disordered lattice structure and are collectively referred to as turbostratic carbons. While the microstructure of turbostratic carbon might seem very complicated due to its disordered nature, it is in fact quite closely related to the structure of graphite. It can be described as randomly oriented hexagonal graphite layersAlthough pyrolytic carbons were developed originally for elevated temperature applications (e.g., as coatings for nuclear fuel particles), LTI pyrolytic carbon has found wide appeal in the biomedical materials industry, in particular for mechanical cardiac valve prosthetic devices, as it has been shown to be highly thromboresistant and to have inherent cellular biocompatibility with blood and soft tissue. Moreover, it displays excellent durability, strength and resistance to wear, and had been thought to be immune to cyclic fatigue failure.ProcessingDense and high strength LTI pyrolytic carbon components are typically made by co-depositing carbon and silicon carbide on a polycrystalline graphite substrate via a chemical vapor-deposition, fluidized-bed process (in vaccum) using a gas mixture of a silicon containing carrier gas with a hydrocarbon (e.g., propane, methyltrichlorosilane and helium gas mixtures) at elevated temperatures. The resulting material contains typically 10 wt% silicon, often in the form of discrete submicron -SiC particles randomly dispersed in a matrix of roughly spherical micron-size subgrains of pyrolytic carbon; the carbon itself has a subcrystalline turbostratic structure, with a crystallite size of typically less than 10 nm.The solid products of these pyrolysis reactions are carbon and silicon carbide, which deposit as a coating on everything in the bed, including the fluidizing particles and the graphite components to be coated. The fluidizing bed system is equipped with metering devices so that the quantity of carrier gas, hydrocarbon and silicon-containing hydrocarbon can be controlled to give the proper concentrations for deposition.The ULTI vapor-deposited carbons can be produced with much higher densities and strengths but only as a thin coating, typically in the range of 0.11.0 mm in thickness. The coating is formed by a hybrid vacuum process by using a catalyst to deposit carbon from a carbon-bearing precursor. With variations in processing parameters, the density, crystallite size and isotropy of the coating can be varied within quite wide ranges.The adhesion of the ULTI coating to the substrate is naturally an important design consideration, particularly when used on flexible substrates. When used as a coating on certain clinical stainless steels and titanium ( Ti-6Al-4V) alloys, a bond strength exceeding 70 MPa is achieved. This excellent bond strength is, in part, due to the formation of carbides at the carbon/metal interface. The bond strength with other materials that do not form carbides is typically lower.Mechanical PropertiesThe mechanical properties of the various turbostratic carbons are closely related to their microstructures, with a strong correlation of most of these properties with coating density. Other microstructural features such as crystallite size, structure and orientation, grain size and composition are also important in determining the resulting properties. Some of the more important mechanical properties of the clinically useful turbostratic carbons, together with a typical polycrystalline graphite substrate onto which coatings might be deposited, are presented in Table 24.1. 10- Bioceramic Composites Chp26Numerous studies were conducted to develop reliable composites of bioactive glasses and ceramics reinforced with a metallic phase. Structural failures occurred due to attack of the metalglass phase boundaries in the composite material and the materials never became clinically important and therefore only an abbreviation is presented here. Seer table 26.1. It seems that Functionally graded materials FGM is the solution. In our department, there is a current PhD research on Alumina-Zirconia FGM to produce all- ceramic prosthesis components for Hip Joint by electrophoretic deposition.11- Calcium Phosphate Cements Chp28Bone grafts are frequently required for surgical procedures in bone regeneration. A large proportion of the bone grafts are derived from autologous bone, which remains the gold standard despite the numerous synthetic bone substitutes that are commercially available because autologous bone has all the prerequisites for new bone formation. However, it has its own limitations, such as the amount of autogenous bone available, increased morbidity, increased anaesthesia time, blood loss and postoperative donor site complications. For this reasons, the majority of bone substitutes are based on calcium phosphates ceramics, calcium sulphates and bioactive glasses, presented in the form of granules, blocks, pastes and putties.Calcium phosphate cements comprise a promising group that are mouldable and undergo in situ setting, which may or may not produce a resorbable material depending on the composition. Current synthetic bone grafts are primarily made either from calcium phosphate ceramics or selfsetting calcium phosphate cements and composite materials. Calcium phosphates are applied for bone tissue engineering and bone substitute materials.Current strategies in developing calcium phosphate bone substitutes are focused towards in situ setting of the cements under physiological conditions with adequate mechanical properties, which can be tailored to resorb by controlling structure and composition. Calcium phosphates show excellent biocompatibility and are able to integrate biologically with bone in the living body due to their similarity to poorly crystalline carbonated hydroxyapatite (HA), which forms the mineral component of bone and teeth. This is one of the main reasons for their widespread use as a bone substitute or bone repair material and thus has been widely researched and used in orthopaedics, dental applications and increasingly they are being applied in spinal surgery.Calcium phosphate biomaterials can be divided in two major categories: the ceramic calcium phosphates and the calcium phosphate cements. The ceramic calcium phosphates are clinically applied in the form of blocks or granules, which exhibit excellent biocompatibility that is able to support bone growth and osseointegrate mainly at the surface. Ceramic calcium phosphates are thus limited by poor resorbability, lack of adhesiveness, require pre-shaping and are brittle in nature. These limitations led to the development of self-setting calcium phosphate cements (CPCs), which was a significant step towards the clinical application of calcium phosphates. The self-setting cements confer the ability to mould the material into any desired shape, such that it can be easily introduced in irregularlyshaped bone cavities. Excellent cytocompatibility, bioactivity, mouldability and resorbability make calcium phosphate cements a unique group of materials which have made a significant impact in the field of bone regeneration.Calcium phosphate cement is a generic term that describes hydraulic cements formed from a mixture of precursors that constitute one or several calcium phosphate powders and a liquid or aqueous component that sets to a hardened mass, with the end product being a calcium phosphate material. CPCs generally consist of a concentrated mixture of one or several calcium phosphate powders and an aqueous solution and only those transformations that can occur at physiological temperature under aqueous conditions can be considered for in vivo applications.Calcium phosphate (or orthophosphate) cement is a generic term to describe chemical formulations in the ternary system Ca(OH)2 H3PO4 H2O which can experience a transformation from a liquid or pasty state to a solid state and in which the end-product is a calcium orthophosphate (contains Ca2+ & orthophosphates PO43)12- Radiotherapy Glasses Chp291. Radiotherapy glasses are defined as radioactive glasses used for in situ irradiation, beta or gamma radiation, of targeted organs inside the body.2. Glasses used for this purpose must not only be biocompatible, but also chemically insoluble in the body during the time that the glass is radioactive, to prevent the unwanted release of the radioisotopes from the targeted site.3. The development of radiotherapy glasses was motivated by the need to deliver large (>10,000 rads), localized doses of beta radiation to diseased organs in the body in such a way as to minimize, and ideally avoid, damage to adjacent healthy tissue.4. Irradiating malignant tumors inside the body by external beam radiation is limited in several important ways.a. A major limitation is that the maximum dose which can be safely delivered is constrained by the need to protect surrounding healthy tissue and is usu