Journal of Materials Processing Technology 175 (2006) 358–363

Porous titanium foil by tape casting technique

Zbigniew S. Rak a,∗, Janusz Walter b

a Energy Research Centre of the Netherlands ECN, Westerduingweg 3, 1755 ZG Petten, The Netherlandsb Cracow University of Technology, Al. Jana Pawla II 37, 31-864 Cracow, Poland

Abstract

A novel method of manufacturing titanium porous sheets used as a support for photo- and electro-catalysts is presented. The titanium foilsare shaped by the tape casting technique from titanium powder and organic binder and then sintered in vacuum. The unique combination of areducing chemical agent and vacuum in one processing step was applied to improve sinterability of titanium powder that results in decreasingof the temperature of the process to even 1000 ◦C. The manufacturing technique allows manufacturing of strong titanium products, porous orfully dense.© 2005 Elsevier B.V. All rights reserved.

Keywords: Porous Ti sheets; Tape casting; Sintering conditions; Titanium hydride; Vacuum; Microstructure

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. Introduction

Titanium metal is the ninth most abundant element in thearth’s crust. It is also present in meteorites and in the sun.ure titanium (Ti) is a silvery-white, lustrous metal with

ow density (4.51 g/cm3) and good flexural strength (above00 MPa). Titanium is as strong as steel with only 45% ofts weight, so when combined with other metals, it greatlymproves the strength and the ability to withstand extremes ofemperature. The ultimate tensile strength of some titaniumowder metallurgy (P/M) manufactured all (i.e. Ti–6Al–4V)ay exaggerate even 1000 MPa, depending on the compact

rocessing. Ti has low thermal and electrical conductivitynd is one of the most corrosion-resistant structural metalsTable 1). In powder form, it is dark grey [1]. Titanium haseen recognized as an element for over 200 years. However,i has gained strategic importance in the last 60–70 yearsue to the development of a relatively safe, economicalethod to produce titanium metal in the late 1930s made by.J. Kroll. Also, the titanium alloys development progressed

apidly, due to the beneficial effects of aluminium addition.

turbine engines for both military and commercial aircrafts,airframes, and in various applications in missiles and spacevehicles. In most aircraft engines, Ti-based alloys partsaccount for 20–30% of engine weight.

Non-aerospace applications include the use of Ti inspecialty chemical, pulp and paper, oil and gas, marine,biomedical, and consumer goods industries. A very thinlayer of titanium dioxide formed on the surface of Tiproducts makes it so corrosion resistive. The thickness ofthis protective layer is in the range of few to tens angstroms.Titanium dioxide formed on the surface of the Ti products ischaracterized by very high melting point (1640 ◦C) and, ingeneral, its sinterability is low [2].

Therefore, the application of the typical powder met-allurgy processes for processing of the Ti powder into thedense or porous objects is very limited. The process formanufacture of Ti powders is slow and costly, and this hasresulted in slow growth of powder metallurgy as a meansof manufacturing titanium parts. One of the most importantconsiderations in manufacture of a Ti powder metallurgyproduct is control of oxygen content. Powders must be han-

ecause of its high strength-to-weight ratio, titanium and itslloys are widely used in both aerospace and non-aerospacepplications. Aerospace applications include the use in gas

∗ Corresponding author.

dled very carefully because they have a very high affinity foroxygen the porous 3-D Ti components are manufactured atthis moment by reduction of sodium fluorotitanate (Na2TiF5)by an aluminium-zinc alloy to produce a molten Ti–Zn alloy.The zinc is then reduced from this by evaporation. Another

924-0136/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2005.04.066

Z.S. Rak, J. Walter / Journal of Materials Processing Technology 175 (2006) 358–363 359

Table 1Selected properties of pure titanium [1]

Properties Value

Density at 25 ◦C (g/cm3) 4.5Melting point (◦C) 1668 ± 50Boiling point (◦C) 3500Mean coefficient of linear expansion,

20–700 ◦C (K−1)(9.0–10.1) × 10−6

Thermal conductivity at 20–25 ◦C(W m−1 K−1)

22.1

Modulus of elasticity at 25 ◦C (GPa) 100–110Modulus of rigidity at 25 ◦C (GPa) 38.6Ultimate tensile strength at 25 ◦C

(grade 3) (MPa)450–517

Fracture toughness at 25 ◦C (MPa) 70–100Electrical resistivity at 25 ◦C (� cm) 42 × 10−6

process uses electrolysis to reduce either TiCl4 or titaniumdioxide (TiO2) to titanium metal [1].

The application of titanium objects made by P/M tech-nique is quite broad, from titanium filters produced forelectro-chemical and other corrosion resistant applicationto dense Ti parts, which are commercially made for thechemical-processing industry. More sophisticated shapesproducts are made for the military, airplanes and automotiveindustries, such as impellers, missile housings, keel spliceformers, engine mount supports, etc.

Efforts are in progress to evaluate the possibility of usingadvanced techniques, such as rapid consolidation, mechani-cal alloying, and nanostructures to enhance the behaviour oftitanium-based materials.

Also, the attraction of the porous titanium sheets as asupport for catalyst used in the electro-chemical processes(Ti-based electrodes), photo-electrochemical reactors (sup-port for titania photo-catalyst), organically modified solarcells (current collector), chemical processes, etc., is contin-uously growing [3–6]. However, the availability of porousfoils or sheets with a well defined porosity and mechanicalstrength on the market is very limited. Therefore, a cheapand easy method of manufacturing of the titanium porousfoils/sheets was the main objective of the presented work.

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of manufacturing. Especially, the metal injection moldinggained a lot of interest in the last time due to a great precision,fine surface of details, low machining operations and oppor-tunity of mass production [2]. The mechanical properties oftitanium P/M products depend on alloy composition and thedensity and final microstructure of the compact. However,specially prepared metal powders are required, of roundshapes and sizes of 10–30 �m. These powders are manufac-tured by a number of techniques such mechanical fabrication,chemical fabrication or atomisation of the melted metal.Especially, the latter is interested for P/M because by thistechnique a nearly perfectly round shaped powder particlesare formed what results in a very good packing and goodflow properties during PIM or pressing processes. The twoformer techniques are mainly used for fabrication of ceramicpowders.

The blended elemental method, which is basically apressing and sintering P/M technique, involves cold pressingand CIP a blend of fine elemental titanium and master alloypowders that have been sintered. Titanium sponge fines arethe common elemental powder used in this process; theseparticles are obtained as a product of the Hunter or Krollreduction processes and they are available at a relatively lowcost [1]. This powder has an irregular shape, which makesit easy to cold pressing into green shapes. The powder issintered at temperatures in the range of 1150–1315 ◦C invc

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. General information

.1. Manufacturing and processing of titanium powder

The processing of the Ti powder to the 2-D or 3-D objectsy powder metallurgy was of the scientists and engineersnterest for many years, but in reality, Ti P/M has beeneveloped as a net-shape technique very recently, in the last5–30 years. The powder metallurgy allows to produce smalletal objects, simple or of complicated shapes, or layers by

niaxial pressing, cold isostatic pressing (CIP), hot isostaticressing (HIP), hot pressing (HP), plasma spraying deposi-ion or metal injection molding in cheap, fast way resulting innearly net-shape and by this, significantly reducing the cost

acuum to prevent gas contamination that severely degradeompact properties.

Atomisation involves the formation of powder from theolten state using a spray of droplets. This technique is

ypically applied to metal powders, especially, using plasma-elting techniques. The molten material is fed into a gas

xpansion nozzle to form droplets that solidify prior to impactith the chamber wall. The fluids that can be used to break up

he molten stream include air, nitrogen, helium, argon, waternd oil. The molten stream is disintegrated by rapid expansionut of the fluid nozzle. The approach is ideal for fabricat-ng highly alloyed materials. The flexibility of the approach

akes it attractive for producing the small particles used in/M. Gas atomisation of the titanium is performed totallynder inert conditions (argon or helium), thereby avoidinghe rapid oxidation of the Ti powder with oxygen from airr nitrification in the nitrogen inert atmosphere. The par-icle shape is spherical with a wide size distribution. Theain advantage of gas atomisation is the product homogene-

ty and good packing properties available with the resultingpherical powder. High gas pressures (4–10 MPa) are usedo produce the small particles sizes needed for PIM. Atomi-ation pressures near 100 MPa have produced powders withean particle sizes in the range of 5–20 �m [2].The shaped metal objects usually contain a smaller

uniaxial pressing) or bigger (PIM) quantity of organicinders. These binders have to be removed a priori a properintering process. Usually, this process is performed in lowemperatures up to 250–500 ◦C necessary for evaporationf solvents and thermal pyrolysis of organic compounds, in

360 Z.S. Rak, J. Walter / Journal of Materials Processing Technology 175 (2006) 358–363

air or in neutral atmosphere. The proper sintering process isapplied mainly in neutral atmosphere (nitrogen) or vacuumin the temperature range between 1100 and 1300 ◦C whenthe majority or all organic additives are removed. Themetal objects are sintered usually to the full density in themaximum temperatures or to a porous form, by applying alower sintering temperature. Ti powders are recommendedto sinter in vacuum or in air atmosphere to avoid oxidationor nitrification processes.

2.1.1. Titanium hydrideTitanium is a very reactive metal and reacts with nitrogen,

oxygen and water to form nitride or oxide at temperature aslow as 700 ◦C according to the following reactions:

2Ti + N2 → 2TiN (1)

Ti + O2 → TiO2 (2)

To prevent these reactions, it is necessary to use during theprocessing of and sintering the nitrogen and oxygen-freeatmosphere. Therefore, the sintering process of titaniumobjects should be provided in argon or hydrogen atmosphereor in vacuum. However, even the use of these neutral orreducing atmospheres do not prevent completely fromthe formation of the very thin, angstrom thickness of thetitanium oxide or titanium nitride layers. The layers preventatftoqabobtEnatHaqpmsahnapaair

Ti objects and increased mechanical strength of the finalobjects [6].

2.1.2. Tape casting technologyThe tape casting is a low-cost process for making high-

quality ceramic sheets used as a supports and multilayerstructures for integrated circuits [13,14]. The ceramic sheet isobtained by spreading slurry on a sliding tape, using a deviceknown as a doctor-blade. The slurry is composed of ceramicpowders dispersed in an aqueous or organic medium contain-ing binders and deflocculating agents. The sheet is dried suchthat the liquid medium is completely removed. Then, firingconsolidates the sheet. Between the drying and firing steps,the sheet can be as punched, laminated, or rolled. Moreover,it is possible to produce 3-D objects by cutting and superim-posing layers of sheets before firing them.

The tape casting slurry consists of inorganic and organiccompounds. The organic compounds of the slip are solvents,dispersants, binders, plasticizers, and others (wetting agents,homogenisers, deformers) [15,16]. The typical solventsused in the tape casting technology are: water, methylalcohol, ethyl alcohol, isopropyl alcohol, toluene, acetone,ethyl acetate, and others. The stability of the suspension(to avoid the sedimentation and agglomeration processes) isprotected by the use of dispersants. Typical materials used, asdispersants are inorganic solvents are phosphate esters, Men-hcaodotstootptpt

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good sintering of the titanium powder. Even the use ofhe hydrogen atmosphere is not a sufficient protection of theormation of these compounds that usually are formed inhe processing step. Only the use of any substance capablef evolving hydrogen in its nascent condition, in sufficientuantity and at sufficient rate to prevent oxidation of the met-ls over the range of sintering temperatures employed, maye utilized for the purpose of the prevention. Metal hydridesr organic compounds, such as paraffin hydrocarbons maye advantageously employed for this purpose. Hydrides ofitanium or zirconium may be employed for this purpose.specially, for the processing of titanium objects, the tita-ium hydride, capable of dissociation at temperatures as lows 300 ◦C, therefore, evolving hydrogen is very attractive forhis purpose [6–12]. This documented in a number of patents.owever, the use of the titanium hydride was not as efficient

s it was considered. The quantity of the used hydride wasuite high, from 10 to 50 wt.%, and mainly without anyrotective atmosphere. Therefore, the final effects were alsoodest. Finally, the application of titanium hydride with

intering in vacuum resulted in the proper action of the useddditive [6,12]. The use of fraction percent of the titaniumydride works very effectively with respects to oxide anditrogen contaminants. As a result, the metal hydride articlesct as a scavenger and react with these contaminants underyrolysis and sintering conditions, so that the metal particlesre protected against undesired nitriding or oxidation. Inddition, the fusion of the metal articles during the sinterings enhanced by the absence of the metal nitrides and oxides,esulting in a decreased temperature of sintering of the

aden fish oil, glyceryl trioleate, etc. [15]. Typical organicompounds used as binders are polysaccharides, polyvinyllcohol, polyvinyl butyral, polymethyl methacrylates, andthers. Typical plastizers are polyethylene glycol (PEG),ioctyl phthalate, triethylene glycol, diethyl oxalate, andthers [16]. Sometimes, this same compound works as oneype of component of the organic binder mixture, in anotherlurry composition is added for another purpose. In general,he different organic additives are used for the aqueous andrganic tape casting suspensions. The organic componentsf the casting suspension are completely burnout duringhe sintering process. They are not present in the finalroduct but they have a strong influence on the quality ofhe final product. Therefore, the good knowledge of theirroperties is essential for the preparation of a good-qualityape suspension for tape casting process.

. Experimental

.1. Raw materials

Two types of the Ti powders, coarse – 60 + 100 mesh andne – 325 meshes from Alfa Aesar (Figs. 1 and 2) andommercial binder B-33305 from Ferro were used in the per-ormed experiments.

The organic binder is a mixture of polymer polyvinylutyral (PVB) with solvents (toluene/ethanol), surface-activegent and plasticizer. The binder solids was 22.4 wt.%,esin/plasticizer ratio −1.7/1, and viscosity of 450 cPs.

Z.S. Rak, J. Walter / Journal of Materials Processing Technology 175 (2006) 358–363 361

Fig. 1. SEM image of the coarse Ti powder.

PVB resin is a thermoplastic polymer, soluble in specificsolvents, i.e. a mixture of ethanol and toluene. The prop-erties of PVB are determined by the butyral, hydroxyl, andacetyl group they contain (it is a mixture of polyvinyl butyral,polyvinyl alcohol, and polyvinyl acetone). Their commercialname is Movital. PVB starts to decompose above tempera-tures of 350 ◦C and burns out completely in air.

PEG 2000 plasticizer {HO(CH2CH2)xOH}: mp 44–48 ◦C; toluene solvent (C6H5CH3): boiling point −110.8 ◦C;ethanol solvent (C2H5OH): boiling point −78.5 ◦C; titaniumdihydride (TiH2): dissociation above 288 ◦C.

TiH2 is added to the paste composition in the quantity of0.2 % or lower. TiH2 decomposes to the Ti metal form and tothe very highly reactive hydrogen radicals at the temperature288 ◦C or even lower (in vacuum).

3.2. Processing

The composition of the paste for tape casting was:55 vol.% Ti powder and 45 vol.% of binder. Additionally,0.15 wt.% TiH2 was added to the mixture. All components of

the paste was mixed by shaking in a Turbula mixer for 45 minand then the tape was cast on the glass plate coated with theTeflon foil. Doctor-blade tape casting technique was used forshaping a tape with the thickness of 1.0 mm and width of30 cm. The tape was dried in ambient atmosphere for 24 h toremove the majority of the solvents and then was kept for 1 hin an electric oven at the temperature of 60 ◦C. The elastictape was cut for square samples of sizes 12 cm × 12 cm orround shape samples of diameter 30.0 mm. The morphologyof the green tape before the sintering process is showed inFig. 3.

The sintering technology of the manufactured titaniumobjects in the shape of sheets and foils was based on aunique combination of a reducing chemical agent andvacuum applied in one processing step. The use of a neutralatmosphere (Ar), reducing atmosphere (Ar + H2) or vacuumseparately during the sintering process was not successful fora good sintering of the compacted Ti powder to a dense com-ponent. It was necessary to clean off effectively the surfaceof the Ti particles from the passivated titanium dioxide layer

Fig. 3. (A) The morphology of the green tape after the drying process: topview and (B) the morphology of the green tape after the drying process:cross-section.

Fig. 2. SEM image of the fine Ti powder.

362 Z.S. Rak, J. Walter / Journal of Materials Processing Technology 175 (2006) 358–363

Fig. 4. Sintering curve of the titanium tape cast sheet.

before the process of sintering will take place. This wasachieved by using the well know reductant, titanium hydrideTiH2, which decompose at 288 ◦C to very high reactivehydrogen H+ ion [6], form of very fine Ti that promoteseffectively the sintering process of the Ti components.

The in situ formed hydrogen ions react with the layer ofpassivating layer of TiO2, decompose it to the Ti metal andwater vapour. The water vapour is removed by vacuum andthe active reaction goes in two steps:

2TiH2 → Ti + 4H+ (3)

TiO2 + 4H+ → Ti + H2O ↑ (4)

Combined reaction can be written in such way:

2TiH2 + TiO2 → 3Ti + 2H2O ↑ (5)

The sintering curve of Ti objects in a vacuum furnace(10−4 bar vacuum) is presented in Fig. 4.

The samples were placed between the molybdenum platescoated with BN thin layer and then sintered in an electricoven under vacuum (10−6 Hg). The heating rate was 3 K/minbetween 20 and 600 ◦C, then the heating rate was increasedto 10 K/min to maximum temperature and the dwelling wasperformed at the maximum temperature for 1 h. The sampleswere cooled with a furnace in a natural way keeping the wholetime vacuum in the oven. The samples were unloaded at ther

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Table 2Properties of the sintered Ti sheet

No. Properties Value

1 Linear shrinkage 15.9%x–y-axis 2.2%z-axis 15.9%

2 Thickness 370 �m3 Porosity 36.2%4 Average pore size 22 �m5 Thermal expansion coefficient

between 20 and 1000 ◦C8.3 × 10−6 ◦K−1

The morphology of the Ti particles, green tape and sinteredsamples was investigated by SEM. The average pores sizewas evaluated by planimetric technique from SEM picture.

3.4. Results

The properties of the porous tapes used as a support forthe electro-chemical devices (support for electro- and photo-catalysts) are summarized in Table 2. The sintered tape was

Fig. 5. (A) The SEM image of the top view of the Ti porous foil and (B) theSEM image of the cross-section of the Ti porous foil.

oom temperature and tested.

.3. Characterization

The sintered samples were characterized according theirhrinkage, porosity and density, thermal expansion coeffi-ient and microstructure.

The thickness of the samples was measured before andfter the sintering process and the shrinkage of sintering wasalculated from these data.

The porosity was measured by the Archimedes watermmersion technique.

Thermal expansion coefficient (TEC) was measured onintered samples between 20 and 1000 ◦C in argon atmo-phere.

Z.S. Rak, J. Walter / Journal of Materials Processing Technology 175 (2006) 358–363 363

very flexible and strong. Average pore size measured fromthe SEM photograph is approximately 22 �m (Fig. 5). Coeffi-cient of thermal expansion measured in the temperature range20–1000 ◦C is 8.3 × 10−6 ◦K−1.

4. Discussion and concluding remarks

The developed technology allows for the manufacture ofstrong titanium products, fully dense or porous. The tapecasting technology, commonly used in the ceramic indus-try, was successfully applied for shaping the green Ti foil.The organic binder system was used to protect the Ti par-ticles from the long contact with surrounding air or watervapour. Thus, lowering the possibility for further growingof the titanium oxide on the surface of Ti particles. Twotypes of Ti powder were used in the performed experiments,fine and course with the average particle size, respectively,25 and 100 �m. The highest flexibility and strength wasachieved for the tape manufactured from fine powder. Themaximum sintering temperature was so low as 1000 ◦C. Thecrucial innovation was the use of the reducing agent, titaniumhydride, and a vacuum, in one sintering step. The organiccomponents of the tape were first evaporated (toluene, ethy-lene) in the temperatures below 150 ◦C and then the organicbinder, plasticizer and surfactant were pyrolysed to an amor-prTovt(sidismetde

solar cells, solar reactors, chemical reactors, batteries, Ti elec-trodes and other devices. The manufactured samples of Tiporous sheets successfully performed their task in electroly-sis processes, in PEC reactor and in organically modified solarcells.

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[2] R.M. German, Powder Injection Moulding, MPIF, Princeton, NJ,1990.

[3] S. Ichikawa, R. Doi, Hydrogen production from water and conversionof carbon dioxide to useful chemicals by room-temperature photo-electrolysis, Catal. Today 27 (1996) 271–277.

[4] Z.S. Rak, L. Berkeveld, Improvement of the coating technology forTi-based anodes, ECN Unpublished Work No. ECN-CX-03-012, Jan-uary 2003.

[5] EU Project NANOMAX, 2001, Contract No. ENK6-CT-2001-00575,private communication.

[6] Z.S. Rak, L. Berkeveld, G. Snijders, Method for producing a poroustitanium material article, Dutch Patent No. 1,020,534, 23 August2002.

[7] L. Small, Sintered metal composition, US Patent No. 2,254,549, 2September, 1941.

[8] Improvements in porous material and t method of making such mate-rial, General Electric Co., US Patent No. 1,004,352, 3 November,

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hous carbon phase in the temperatures below 250 ◦C. Theesidual carbon and products of decomposition of TiH2 andiO2 reacted together forming gaseous products, those evap-rate very easy from the Ti surface and was removed by theacuum pomp from the system. Therefore, the surface ofhe sintered Ti was completely clean from all contaminantsEDX analysis of tested samples) and allowed for a rapidintering process between the neighboured Ti particles form-ng a strong neck between separate particles (Fig. 5B). Theeveloped technique of sintering allows also for a perform-ng of a good sintering of the Ti particles compacted by otherhaping techniques, such as uniaxial pressing, CIP, injectionoulding, impregnation of a polyurethane foam/squeezing,

tc. The Ti components were prepared by other shapingechniques were also successfully sintered according to theescribed procedure [6,12]. The porous Ti foils are of inter-st for application in PEM fuel cells, organically modified

1961.[9] Y. Tomohiro, Production of high density titanium sintered compact,

Japan Patent No. 10,183,151, 30 June 1998.10] R.M. Pilliar, Surgical prosthetic device or implant having pure metal

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assemblies and method for making same, US Patent No. 5,863,398,26 January 1999.

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14] R.E. Mistler, Tape casting: past, present, potential, Am. Ceram. Soc.Bull. 67 (10) (1988) 82.

15] R. Moreno, The role of slip additives in tape-casting technology. PartI: solvents and dispersants, Am. Ceram. Soc. Bull. 71 (10) (1992)1521–1530.

16] R. Moreno, The role of slip additives in tape-casting technology. PartII: binders and plasticizers, Am. Ceram. Soc. Bull. 71 (11) (1992)1647–1657.