R 784 Philips Res. Repts 27,28-37, '1972

PRODUCT PROPERTIES: A NEW APPLICATIONOF COMPOSITE MATERIALS

by J. van SUCHTELEN

AbstractA new class of physical properties of composite materials is that of"product properties" in which the phases or submaterials of the com-posite are selected in such a way that an effect in one of the phases orsubmaterials leads to a second effect in the other phase. A typicalexample is the magneto-electric effect in a composite material with onemagnetostrictive and one piezoelectric phase: a magnetic field inducesa distortion of the magnetostrictive phase, which in turn distorts thepiezoelectric phase in which art electric field is generated. The com-posite as a whole can be considered macroscopically as a new, homo-geneous material with a magneto-electric effect not exhibited by anyof the composing phases on their own. The coupling, in this case, isof the mechanical kind. The entire class of product properties can besearched systematically for interesting properties by a kind of matrixscanning procedure. Typical examples will be given in the present paper.

1. Introduction; composites versus devices

An essential characteristic of a' composite material is that on a microscopiescale it is built up of two or more submaterials that are in intimate contact.Macroscopically, however, it can be considered as a homogeneous material inthe sense that any piece of it (provided that its size is large as compared to themicrostructure period) has always the same physical properties. These prop-erties are intrinsic and cannot be affected by minute damages. Compositescan be considered as real "materials".

Some devices, e.g. integrated circuits, also consist of microscopie arrange-ments of submaterials. However, such devices are not comparable to com-posites: the submaterials (p- and n-type semiconductors, insulators, etc.) aredeliberately arranged in a very special pattern to form the elements of the cir--cuit. The IC as a whole can only function properly when all its elements do so.This requires a high degree of perfection and close tolerances as far as the shapeof the submaterials is concerned. Fabrication of such devices involves much

, skill and labour, and their vulnerability is high. For example, a tiny scratchon the surface of an IC represents a catastrophic damage.Composite structures can be prepared by the embedding of one submaterial

in the other (artificial composites) but also by spontaneous decomposition orphase rearrangement of a suitable starting material under the influence ofmacroscopie manipulations (natural composites). In many cases, anisotropiccomposite materials can be obtained in a one-step process such as unidirectionalsolidification of eutectics 1), unidirectional decomposition of eutectoids and

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PRODUCT PROPERTIES: A NEW APPLICATIO~ OF COMPOSITE MATERIALS 29.

supersaturate'! solid solutions 2), spinodal phase separation under the influenc~of a magnetic field (Alnico alloys) 3), etc.

2. Sum and product properties in composites

Any physical property of a material can be considered as the action of aphysical quantity X in the material, resulting in a physical quantity Y. Thisproperty can be defined as an X-Y effect (X = input parameter, Y = outputparameter). Phenomenologically, the behaviour of the material can be charac-terized by a proportionality tensor or factor bY/bX = A (e.g. resistivity, sus-ceptibility, elasticity). This proportionality factor, A, may be a constant, or itmayalso depend on X or Y.In a composite material the submaterials have their own set of X-Y effects

for all possible physical quantities. The combination of these effects determinesthe properties of the composite materials. We can distinguish two classes ofcomposite properties:(a) Those which are the result of an X-Y effect in submaterial I and an X-Y

effect in submaterial H (with a proportionality factor different from thatin 1). Together these will give an X-Yeffect ofthe composite. These prop-erties will be named as "sum properties".

(b) Those resulting from an X-Y effect in submaterial I and a Y-Z effect insubmaterial H. If by whatever mechanism the Y quantity is transferredfrom I to H, then the two effects are coupled and the composite exhibitsan X-Z effect, defined as a "product property".

These two classes do not exhaust the specific properties of composite mate-rials. Other classes can be discerned which are related to the anisotropy of cer-tain composite structures, to the periodicity (resonance effects), or to the small-ness of the dimensions of the submaterial particles (size effects,' typical surfacephenomena, phenomena depending on relaxation of diffusion processes betweenthe submaterials). These other classes will be left out of consideration here.

2.1. Sum properties

Sum properties are very common. For instance, the resistivity tensor of acomposite (diagonalized form) has elements which are essentially series andparallel resistances of the two submaterials with some geometrical factor.When measured in various directions, the resistivity changes between geometricand arithmetic mean values. When the ratio of the proportionàlity factors inthe two phases deviates considerably from unity, the perfection of the structurecan be of great importance. For example, in the case of an insulating matrixcontaining aligned metallic needles the electrical resistivity in the needle direc-tion will strongly depend on discontinuities in the needles.The averaging of factors allows the designer to combine favourable proper-

ties of different materials in one new material. A well-known example of this

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30 J. van SUCHTELEN

kind is the application of this concept to light and strong materials. By embed-ding a volume fraction x of strong paraIlel fibres with a high Young's modulusEl but also a high density Dl' in a weaker matrix (Young's modulus E2;E2 « El) which is however much lighter (density D2; D2 « Dl) one obtainsa composite which, in the direction of the fibres exhibits the Young's modu-lus 4)

Ecomp = XE1 + (l-x)E2

and the densityDcomp= XD1 + (l-x)D2.

Because of El ~ E2 and Dl ~ D2 the combination of Dcomp and Ecomp maybe more favourable for certain applications than any single-phase material._ Another example is the Siemens "Feldplatte" 5). Essentially this materialconsists of a semiconductor matrix with large HaIl angle, and highly con-ductive (metallic) needles with a smaIl Hall .angle and a diameter of about1 [Lm.This eutectic composite exhibits a strong magnetoresistive effect in anarrangement such that the (macroscopie) electric current, the needles and theapplied magnetic field are mutually perpendicular. This effect is brought aboutby the short-circuiting of the Hall voltage by the metallic needles, allowing thecurrent carriers in the semiconductor to follow the direction of the Hall angle,so that the current is zigzagging through the structure and the path of the chargecarriers between the electrodes is increased. This increase manifests itself macro-scopically as a magnetoresistive effect.

2.2. Product properties

An essentially different class of composite properties is based on the fact thatthe constituent submaterials I and II are coupled with respect to the Yparam-eter, so that the output Y of the X-Y effect in submaterial I can act as theinput parameter for a Y-Z effect in submaterial Il. The transfer of the Yparam-eter from submaterial I to submaterial II can be brought about by couplingmechanisms of several kinds. A typical example is the magneto-electric effectin a composite material having one magnetostrictive and one piezoelectricphase. A magnetic field induces a change in the shape of the magnetostrictivephase, which in turn stresses the piezoelectric phase in which an electric fieldis generated. In this case the coupling is mechanical. But electrical, optical,magnetic, thermal and chemical coupling is possible too. Some examples aretabulated below.The magnitude of the Y parameter in material II in the composite is not

necessarily equal to that in bulk material I. In the magnetostrictive-piezoelectriccase the distortion of the piezoelectric phase (due to an external magnetic field)is smaIler than that of the magnetostrictive phase as it would be in the bulkmagnetostrictive material. Firstly because the stress generated in the magneto-

PRODUCT PROPERTIES: A NEW APPLICATION OF COMPOSITE MATERIALS 31

strictive material alone has to deform both the magnetostrictive and the piezo-electric phase, so that this stress is divided over the two phases. The resultingstrain over the composite as a whole is therefore smaller than in the magneto-strictive phase in the bulk (structural coupling factor kl). Secondly, becausethe two materials may make a poor mechanical contact, so that slip may occur'at the interfaces and energy may be dissipated by friction (coupling efficiencyfactor k2). The product of these dimensionless coupling factors gives the frac-tion of the quantity Y in material II as compared to the value that it wouldhave in bulk material I for the same quantity X. For the analysis of ac proc-esses, complex k factors can be used (in general a function of frequency) so asto allow for the description ofphase lag and dissipation. The value kl dependson the microstructure of the composite. For anisotropic materials, such as uni-directionally solidified eutectics, kl should be considered as a tensor. In generalthe k2 factors of in-situ grown composites are higher than those of artificialcomposites or sintered materials; in the extreme case those of powder mix-tures are zero. Ikll and Ik21 can never exceed unity.As a whole the composite can be characterized by its X-Z proportionality

factor or tensor kl k2 A B, with

material I (bulk) X-Y effect:()Y-=A()X '

material II (bulk) Y-Z effect:()Z-=B()Y ,

composite: X-Y-Z effect:

2.3. Direct and parametrie coupling

Product properties can be divided into the subclasses of "direct" and of"parametric" product properties. In the direct ones, the X-Z effect is consid-ered to be the essential property: there is an X input and a Z output, witha transfer of energy between the phases. The magnetostrictive-piezoelectricexample is typical.In the parametrie product properties, the quantity Z is not the output param-

eter; instead it is itself a coupling factor between an input P and an output Q(e.g. a resistance). An example is the combination of a magnetostrictive and apiezoresistive phase. The resistance (Z) of the second phase can be varied bypressure variations (Y), generated in the first phase by magnetie-field varia-tions (X). The relevant quantities in an application, however, are voltage andcurrent (Pand Q). Such a coupling can be used in modulators (of electric cur-rent, light intensity, etc.) and amplifiers of several kinds.

32 J. van SUCHTELEN

TABLE I

Matrix of propertiesThe table classifies physical properties or phenomena in materials according to the kindof input parameter (colums) a,nd output parameter (rows).

C chemical composition, cencentration T temperatureE electric field e dielectric constantH magnetic field X magnetic susceptibilityi electric current A thermal conductivityK (mechanical) force p, chemical potentialLll deformation e resistivityM magnetic polarization FM ferromagnetic11 refractive index FE ferroelectricP dielectric polarization NE Nernst-Ettinghausen

{+i}, etc. bracketed symbols indicate that this symbol is essential as a second inputparameter (e.g., i and H in the Hall effect)

1 2 3____ -+X mechanical magnetic electrical

+y ------- KILll HIM ElP, i

1 mechanical KILll elasticity 1) magnetostriction 1) electrostriction(M, P, T) 2) magnetoviscosity 2) KirkendalI effect

(suspensions) 3) electroviscosity(suspensions)

4) indirect: thermalexpansion

2 magnetic HIM ) piezomagnetism X 1) superconductors2) p,(LlI)esp. at (Lll, T, light flux) i FI:I ie

TRj Te 2) galvanic deposition (FM layer

3) direct generation ofmagnetic field

3 electrical ElP, i ) piezoelectricity 1) {+i} magneto- e, e2) piezoresistivity resistance (Lll, M, T,

2) {+i} Hall effect light flux)3) ac resonance effec {+H} Hall effect4) induction of

voltage4 optical and light or stress birefringence ) Faraday effect 11) electroluminescence

particles particle triboluminescence 2) magneto-optic 2) laser junctionsflux Kerr effect 3) neE)

3) deflection of 4) Kerr effectcharged particles 5) absorption by

galvanic deposits

I 6) cold emission ofelectrons

5 thermal T, 1) heat of transition 1) adiabatic de- I 1) dissipation ingrad T of pressure-induc- magnetization resistanceheat ed phase 2) {+i} grad T 2) Pel tier effect

current transition NE effect 3) {+H} grad T- 2) piezoresistivity + 3) {+E} magneto- NE effect

Joule heating resistance effect +Joule heating

6 chemical I. (grad) C, pressure-induced I1) electromigration- (grad) p, I (grad) p, phase transition 2) galvanic deposition

I

"PRODUCT PROPERTIES: A NEW APPLICATION OF COMPOSITE MATERIALS 33

TABLE I (continued)

4 5 6optical and particle radiation thermal

Ichemical

light or particle flux T, grad Tjheat current (grad) C, (grad) p.

thermal expansion osmotic pressure

-photomagnetic effect {+H} ferromagnetic mate- dependence of Tc ~>nC(FM)

rial at T Rj Tc

1) photoconductivity 1) thermoelectric effects dependence of Tc on C(FE)2) photo-emission 2) {+ E} ferroelectrics at chemical potential (grad C)3) {+H} PEM effect TRj Tc4) ionization 3) {+i} e(T)

n thermoluminescence chemoluminescence(LIl, M, P, T, E)fluorescence scintillationcolour-centre activation

Iabsorption Ä reaction heat

\light- or particle-stimulated 1) Soret effect (grad T)reactions (photosensitive layers) 2) phase transition (T)

3) change of chemical equi-librium (T)

34 J. van SUCHTÈLEN

2.4. Tabulation of sum andproduct propertiesIn principle one might investigate all X-Y-Z combinations systematically by

considering all X-Z combinations arid then trying an intermediate Y's. TheX-Y-Z combination is rejected (1) if either of the X-Y or Y-Z effects does notexist in a particular material, or (2) if the X-Z effects are of no interest. Such ananalysis is equivalent to the consideration of all terms in a matrix product.Such a term in a matrix product can be expressed by three digits, correspondingto the rows and columns. Thus the term ijk represents the property in whichan i-j effect in phase I is coupled to a j-k effect in phase Il. ""It should be mentioned that in practice several parallel cross-effects may be

present in one composite:Ca) an X-Z effect can be obtained via more than one Yparameter:

(b) 'with an X-Y-Z combination it is possible that not onlyX-Y occurs in sub-material I and Y-Z in submaterial 11, but also X-Z and/or Y-Z in sub-material I and X-Z and/or X-Y in submaterial 11.

The analysis has been carried out by combining all X and Y into six headings,corresponding to the"type of physical parameters: 1. mechanical; 2. magnetic;3. electrical; 4. opticalor particle radiation (including y-rays, X-rays, etc.);5. thermal; 6. chemical. In a 6X 6 matrix (table I) the most obvious physicaleffects have been collected, combining an action, output, or result (Y) with aforce, an input, or a cause (X). With the aid of this matrix all X-Y-Z terms(including complications due to anisotropic structure and ac or de properties)have been examined. In this way both sum and product properties are indicatedautomatically. A selection of product properties is given in table Il.

3. ExperimentalExperimental results with items 213, 312 and 444 (table II) have demonstrated

the feasibility of the concept of product properties in composites. The 213 and312 examples (magneto-electric and electromagnetic effects)have been realizedin specimens of BaTi03-CoFe204 eutectic. The eutectic composition was foundto be about 38 moles per cent CoFe204; the eutectic temperature is about1350 °C. The growth rate was 20 cm/h. The constituent phases of the eutecticcomposite exhibit a faceted structure, the smallest dimension being a few !Lm(fig. I). In rod-shaped specimens of this composite both the integral (X-Z) andthe separate (X-Yand Y-Z) coupling constants were measured by means ofinput and output connections for all three of the quantities involved (electricfield, length variation and magnetic field). Preliminary results have shown that"the coupling-factor product k1 k2 is near to the~value to be expected theoreti-

PRODUCT PROPERTIES: A NEW APPLICATION OF COMPOSITE MATERIALS 35

TABLE II

Product properties of composite materials

X-Y-Z proper~y phase I property phase 11 result(table I) ,X-y lit Y-Z X-Z

123 piezomagnetism magnetoresistance piezoresistance;phonon drag

124 piezomagnetism Faraday effect rotation of polari-zation by mechani-cal deformation

134 piezoelectricity e1ectroluminescence piezoluminescence134 piezoelectricity Kerr effect rotation of polari-

zation by mechani-cal deformation

213 magnetostriction piezoelectricity magneto-electriceffect

213 magnetostriction piezoresistance magnetoresistance;spin-wave inter-action

253 N ernst - Ettings- Seebeck effect quasi-Hall effecthausen effect

214 magnetostriction stress- induced magnetically induc-birefringence ed birefringence

312 electrostriction piezomagnetism electromagneticeffect

313 electrostriction piezoresistivity ! coupling between(J and E (negative

343 electro1uminescence photoconductivity diff. resistance,quasi-Gunneffect)

314 electrostriction stress-induced electrically induc-birefringence ed birefringence

light modulation421 photomagnetic effect magnetostriction

~photostriction431 photoconductivity electrostriction

434 photoconductivity electroluminescence wavelength changer(IR-visible, etc.)

443 scintillation photoconductivity radiation-inducedconductivity(detectors)

444 scintillation, fluorescence radiation detectors,fluorescence 2-stage fluorescence

36 J. van SUCHTELEN

Fig. 1. Transverse section of the BaTi03-CoFe204 eutectic composite. Magnification 1580 x.

cally on the basis of the measured sub-effects, volume fraction and relativeYoung's moduli.

An example of the 444 item concerns an X-ray fluorescence screen consistingof a homogeneous mixture of fine (Roj 1 [J.m)PbCl2 particles in an anthracenematrix. The X-rays (wavelength about 0·1 A) release secundary electrons bothin the anthracene and the PbCI2, but with much higher efficiency in the PbCl2because of the heavy Pb atoms (X-Y; X = X-rays, Y = secondary electrons).The electrons have a range of the order of a few [J.m; as a consequence theyspend most of their range in the anthracene matrix, which is a good scintillator(Y-Z; Y = secondary electrons, Z = visible light). The composite has anX-ray to visible-light conversion efficiency exceeding that of anthracene (satu-rated with PbCI2) by at least one order of magnitude in a layer thickness ofd < 0·05 cm (fig. 2, curves 2 and 3).

Similar results were obtained with the NaCI-PbS eutectic composite (eutectic

-,

PRODUCT PROPERTIES: A NEW APPLICATION OF COMPOSITE MATERIALS 37

..."1

11111111

10 II

3

~----~o~------~o 0----1O.~=- L_ ~ ~ ~ _

o 0'1 0'2 0-3 0'4__,____ d (cm)

Fig. 2. Conversion of X-rays (wavelength FI:I 0'1 Á) into visible light (vertical axis); d =thickness of the sample. 1: PbCI2; 2: anthracene saturated with PbCI2; 3: 'the anthracene-PbCl2 composite.

__----0--2

composition about 0·5 mole per cent PbS; eutectic temperature R:::i 8000C)and with a composite material that has been obtained by solidification of anNaCl meIt containing 3 mole per cent Bi203• The smallest dimension of thedispersed phase was of the order of magnitude of 1 [.Lm.

Acknowledgement

The optical experiments were carried out by L. A. H. van Hoof and themagnetic and electrical experiments by A. M. J. G. van Run, D. R. Terrell,H. F. J. I. Giller and J. H. Scholing.

Eindhoven, November 1971

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R. W. Kraft and D. L. Albright, Trans. met. Soc. AIME 221,95, 1961.J. D. Hunt and K. A. Jackson, Trans. met. Soc. AIME 236, 843, 1966.K. A. Jackson and J. D. Hunt, Trans. met. Soc. AIME 236, 1129, 1966.

2) F. M. A. Carpay, Acta met. 18; 747, 1970.3) J. W. Cahn, Trans. met. Soc. AIME 242, 166, 1968.

K. J. de Vos, Thesis, Eindhoven, Netherlands.4) J. L. Broutman and R. H. Krock, Modern composite materials, Addison-Wesley

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