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Ice-mould freeze casting of porous ceramic components

Journal of the European Ceramic Society27(2007)

4595–4601

Tassilo Moritz?,Hans-J¨u rgen Richter

Fraunhofer Institute for Ceramic Technologies and Systems,Winterbergstrasse28,01277Dresden,Germany

Received13December2006;received in revised form5April2007;accepted14April2007

Available online29June2007

Abstract

Porous,hollow ceramic components were produced by freeze casting technique.For this purpose aqueous slurries with high solid contents were prepared which were stable against freezing down to at least?5?C.Ice cores were made by coating steel components with freezing water which were subsequently dip-coated with the ceramic suspensions.After freeze drying which removes both,the ice core and the frozen suspension liquid, and sintering,ceramic components with a high amount of open porosity including steel parts could be achieved.As an example hydroxyapatite was used for showing the opportunities of the freeze casting technology among others for applications in the?eld of bone replacement.The in?uence of the solid content of the hydroxyapatite slurries on the ice crystal growth has been investigated by means of compact hydroxyapatite bodies which were prepared by freeze casting using ice moulds with cylindrical cavities.

Keywords:Shaping;Porosity;Freeze casting;Apatite

1.Introduction

Freeze casting which is known as a shaping technique for refractories for about60years1did not lose its attractiveness for shaping advanced ceramic materials till this day.The freeze casting technique involves the preparation of a ceramic slip that is poured into a mould which is then frozen and subjected to sublimation drying of the solvent.The frozen solvent acts tem-porarily as a binder holding the part together for demoulding.2,3 A variation of the freeze casting technology4–7uses sols which form gels when frozen below0?C as a form of chemical bonds between the powder particles.The ceramic powders and sol are mixed into a paste which is fed into a mould.This is then frozen to temperatures below?20?C.The frozen component is released from the mould and allowed to warm back up to ambient temper-ature afterwards.Although it still has the original water content the component is solid and now simply needs to be dried out.

The advantages of this shaping method can be seen in its environmental friendliness,8because of the minimized organic additive concentration and the use of water as suspension liquid,in the elimination of drying stresses such as in the

?Corresponding author.Tel.:+493512553747;fax:+493512554197.

E-mail address:Tassilo.Moritz@ikts.fraunhofer.de(T.Moritz).exclusion of shrinkage during freeze drying.Because of the almost zero shrinkage the authors described,this variation of the freeze casting process can be called a near net shape pro-cess.Furthermore,this shaping technique offers the possibility to adjust a certain amount of open porosity with interconnect-ing pore channels9or pore size gradients in ceramic bodies.3 In several publications9–12freeze casting is used for achiev-ing porous ceramic structures for applications as biocatalysts or biosensors,separation?lters,catalyst supports,gas distributors, preforms for metal-impregnated ceramic-metal composites,and implantable bone scaffolds.Sintered bodies of Al2O3and Si3N4 with aligned channels have been fabricated by unidirectional freeze casting of conventional aqueous ceramic slurries11,13or silica-sol containing ceramic slurries.14For attaining pore struc-tures in ceramic bodies,slips with lower solid contents are ?rst frozen to obtain vehicle ice crystals which are often con-nected with each other in dendritic shapes,surrounded by frozen concentrated ceramic slurry.After freeze drying channels are created replicating the shape of the interconnected crystals.9The greatest in?uence on the porosity and the pore size distribution of the ceramic body can be exerted by the solid content of the suspension,the temperature gradient and the ice crystal growth rate over the thickness of the sample.With increasing freez-ing rate and larger temperature gradients the pore sizes become smaller.15Thereby,the thermal isolation of the already frozen

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part of a sample causes a decrease of the crystal growth rate. Since large pores decrease the mechanical strength of the freeze cast ceramic bodies drastically,the freezing behaviour of aque-ous slurries was controlled by using hydrogen bond forming compounds,such as glycerol,to reduce the size of ice crystals during solidi?cation of water.16In most cases silicon rubber,2,7 polyurethane,9?uorocarbon polymer11or metals are used as mould materials for freeze casting.In1991a casting technology which was patented by Yodice17has demonstrated the possi-bility and advantages of freeze casting with ice patterns.This technology,called Freeze Cast Process(FCP),was developed by DURAMAX Co.It started with the building of solid master and silicone moulds.Then ice patterns were made with the mould and dipped into refrigerated ethyl silicate slurry and stuccoed. After repeating the dipping and drying processes,a ceramic shell was made and then it was put in room temperature and allowed the ice pattern to melt,drain,and dry.18

Due to its chemical and crystallographic structure simi-lar to that of bone mineral hydroxyapatite has been widely used as a bone replacement material in restorative dental and orthopaedic implants.19It shows excellent biocompatibility with hard tissues,skin and muscle tissues,because it does not exhibit any cytotoxic effects and can be bonded to the bone directly. Hydroxyapatite as the basic mineral in mature bone is the most used material for bioactive?xation.20Once hydroxyapatite is implanted,it is bonded to the bone directly,enhancing the?xa-tion in a shorter time and reducing healing time.19A synthetic scaffold for bone tissue engineering requires an inner structure with interconnecting pores.Pore sizes with diameters above 300?m are recommended to promote good vascularisation and attachment of bone cells to guide their growth into all three dimensions.21As critical minimum pore size needed for the for-mation of a vital new bone100?m are necessary.20Duan et al.22produced biphasic porous hydroxyapatite/tricalcium phos-phate ceramics with interconnecting pores.The porosity of the materials was50–60%.

The goal of this article is to show the opportunities of ice-mould freeze casting for shaping of porous ceramic components with individual and complex geometries without complicate demoulding.Moreover,this novel technique allows producing closed porous ceramic shells with encapsulated components. The material combination hydroxyapatite/steel was chosen as an example for encapsulation of two completely different kinds of materials by this shaping technique.Furthermore,the com-ponents made by ice-mould freeze casting shall emphasize the potential of this method for producing porous ceramic parts for a variety of applications,for instance as?lters,supports for catalysts,sensors or bone scaffolds.Beside hydroxyapatite also alumina and zirconia were used as materials for ice-mould freeze casting,23,24but the results of these investigations are not described in this article.

2.Experimental procedure

In this work ice was used as mould and as core material for preparation of compact hydroxyapatite parts such as of hydrox-yapatite shells including steel components.The steel parts acted as a demonstrator showing the feasibility of encapsulating dif-ferent materials in porous ceramic shells.Hydroxyapatite has been chosen as a suitable material for which freeze casting could be used to adjust a desired porosity of50–60%with pore sizes up to100?m.As a component with individual and com-plex shape for demonstrating the opportunities of ice-mould freeze casting3D data of a human jaw-bone had been used for producing a stereolithographic model which served as initial part.

In contrast to the state of the art,the ice cores were removed by sublimation drying without melting.No sol–gel transition was used for mechanical strengthening of the ceramic scaffold in the green state,but ice was the only binding phase in the frozen slip.

2.1.Materials

Hydroxyapatite powder(Merck KGaA,Darmstadt,Ger-many)with a median particle size(d50)of2.04?m and a speci?c surface of72.7m2/g has been used.For adjusting the neces-sarily high solid contents in the slurries of at least70wt.%, a commercially available dispersant(Dolapix CE64,Zschim-mer&Schwarz,Lahnstein,Germany)was used.Furthermore, glycerol(Sigma–Aldrich,Seelze,Germany)was added to the suspensions as a cryoprotectant for lowering the freezing point and for in?uencing the ice crystal growth.

As steel parts which should be encapsulated in a hydroxya-patite shell,cylindrical pro?les(5mm in diameter,30mm in length)made of ferritic stainless steel type430were used.

2.2.Fabrication procedure

For producing hollow hydroxyapatite bodies ice cylinders (diameter40mm,height5–7mm)were made by freezing deion-ized water in silicon rubber moulds.The steel parts which should be encapsulated in hydroxyapatite shells must be encased in ice previously.Initially,the steel rods were equipped with thin textile?laments.Afterwards,the steel parts were cooled in liq-uid nitrogen and dipped into water which was at a temperature close to its freezing point.For achieving a certain thickness of the ice coating,the steel rods were dipped alternately into liq-uid nitrogen and into water.In this way a dense and thick ice shell with adjustable thickness could be fabricated.It must be taken into account that the hydroxyapatite shell shrinks dur-ing sintering,whereas the steel rod elongates simultaneously. For preventing cracking of the hydroxyapatite shell the thick-ness of the ice coating must act as a spacer to enable both, shrinkage of the ceramic powder and thermal expansion of the steel rod.The ice cylinders and the ice-coated steel rods were stored in a refrigerator at?20?C before use.Furthermore,ice moulds had been prepared showing a cylindrical cavity(40mm in diameter and10mm in height)for achieving compact discs of hydroxyapatite.

For stabilising the surface charge the initial hydroxyapatite powder was thermally treated at800?C for1h previous to the slip preparation.The zeta potential of the initial powder and of the heat treated powder was measured in dependence

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on the pH of the suspension liquid by means of Zetamaster S (Malvern Instruments GmbH,Worcestershire,UK).The slips for the freeze casting process were produced by stirring the pow-ders into water in an ultrasonic bath.Since hydroxyapatite is soluble in acidic milieu the pH of the suspension media must be in the basic range.According to the results of the zeta potential measurements the pH was adjusted to 11to achieve a suf?-cient surface charge of the powder particles for electrostatic stabilisation.The electrostatic stabilising effect was additionally improved by adding the dispersant Dolapix CE 64.In a last step 8–10vol.%glycerol were added to the suspensions as a cryopro-tectant.For achieving high solid contents and for maintaining a viscosity which was suitable for the casting process,the slips were treated meanwhile by an ultrasonic sonotrode (UW 2200,Bandelin,Germany).Alternatively,the slips were treated in a planetary ball mill (Pulverisette,Fritsch,Germany)for 4h.To investigate the in?uence of the solid content on the pore size dis-tribution of the freeze cast parts suspensions with solid contents of 25.5,32,and 42.5vol.%had been prepared.The prepared slips were cooled down to ?2?C for avoiding melting of the ice core when getting into contact with a relatively warm slip.This cooling step was done in a double wall glass vessel.A sodium chloride/ice water mixture with a temperature of approx.?19?C was used as cooling liquid for this purpose.The initially prepared ice cores were dipped into the cold suspensions which enclosed the ice completely.For avoiding an excessive ice crystal growth in the hydroxyapatite shell due to a low temperature gradient,the ice cores were dipped into liquid nitrogen previous to the contact with the suspension.Moreover,cooled suspension was poured into the ice mould with the cylindrical cavities for pro-ducing cylindrical discs.After the instantaneous freezing step of the hydroxyapatite shell,the formed bodies were put into a freeze drier (Gamma 20,Martin Christ GmbH,Osterode,Ger-many)and dried at 25–40?C at a vacuum pressure of 2.5mbar for 48h.

The shrinkage of the samples during drying could be neglected.After complete removal of the suspension liquid,the ice cores,and the ice moulds,the green hydroxyapatite bodies could be handled carefully.The residual organic material was burnt out at 500?C for 1h.Sintering was done at 1350?C for 2h.The sintering step was carried out in hydrogen atmosphere for protecting the steel against oxidation.In the case of the compact hydroxyapatite discs sintering was done in air.2.3.Characterisation

Rheometric measurements were carried out for character-izing the viscosity of the slips in dependence on the solid content with an annular gap system (Rheolab MC 10,Phys-ica,Germany).The solid content of the slips was measured by drying at 110?C (Halogen Moisture Analyzer HR 73,Mettler-Toledo,Germany).The pore size distribution of the ceramic parts in the green state was estimated by mercury intrusion (AutoPore IV 9500,micromeritics).Archimedes prin-ciple was used for the density measurements.Sintered samples were cut and prepared for light microscopic and FESEM

investigations.

Fig.1.Dependency of the zeta potential of the initial hydroxyapatite powder ( )and of the powder after a heat treatment at 800?C for 1h ( )on the pH value of the dispersion media.

3.Results and discussion

Fig.1shows the dependency of the zeta potential of the ini-tial hydroxyapatite powder ( )and of the powder after a heat treatment at 800?C for 1h ( )on the pH value of the disper-sion media.The diagramme emphasized the necessity of this thermal treatment for achieving a suf?cient surface charge and a high zeta potential which are prerequisites for the preparation of a suspension with suf?ciently high solid content.After the ther-mal treatment the dispersing behaviour of the hydroxyapatite powder was improved remarkably.Simultaneously,the speci?c surface of the powder was decreased from 72to 25m 2/g by this heat treatment.

Fig.2shows the dependence of the viscosity in ceramic sus-pensions with different solid contents on the shear rate.All suspensions of hydroxyapatite showed shear thinning.Solid con-tents larger than 28vol.%caused a remarkable increase in the dynamic viscosity.However,for the freeze casting experiments with hydroxyapatite also suspensions with solid contents up to 45vol.%had been

used.

Fig.2.Dependency of the suspension viscosity from the shear rate measured at suspensions with different solid contents of hydroxyapatite powder.

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Fig.3.Steel rod coated with ice (the textile ?lament was ?xed to the steel rod for convenient handling during the coating step).

Encapsulation of a steel rod in an ice core with adjustable thickness is quite simple,especially if the temperature of the water is not far from the freezing point.An encapsulated steel rod is shown in Fig.3.This ice core was dipped into the cold hydroxyapatite suspension for coating.The temperature of the ice core and the duration of the coating step in?uenced the thick-ness of the frozen slip layer.Fig.4shows a cylindrical ice core after coating with hydroxyapatite slip.The textile ?lament stick-ing out of the frozen slip shell enabled a convenient handling of the ice core during dip-coating.The ?lament remained in the sample after removal of the ice core by sublimation,but it disappeared during the burning out of the organic additives at 500?C.The remaining small hole in the shell was closed during the sintering step.

Fig.5allows looking into a hydroxyapatite shell enclosing a steel rod after removing the ice core by sublimation drying.By adjusting the ice core thickness the distance between the ceramic shell and the encapsulated part can be varied.After sintering a closed shell enclosing a component of another material can

Fig.4.Hydroxyapatite shell after sublimation of the ice core and the suspension

liquid.Fig.5.Opened hydroxyapatite shell encapsulating a steel rod after freeze drying.

achieved in this way.An optical microscopic image of a cross-section of a hydroxyapatite shell with a steel rod inside can be seen in Fig.6.It is necessary to emphasize here that the chosen material combination only served as a demonstrator.The goal of this experiment was to show that a component of a certain mate-rial can be encapsulated in a shell of another material in this way.In future applications such porous ceramic shells could act for instance as thermal or mechanical protection of encapsu-lated components which simultaneously enable an exchange of gaseous components.

The in?uence of the solid content of the suspension on the ice crystal growth and the resulting pore structure of cylindri-cal hydroxyapatite bodies which were produced by shaping in ice moulds can be seen in Figs.7–9.With increasing solid con-tent the volume of ice in the frozen part such as the size of the ice crystals was reduced.The direction of the ice crystal growth follows the temperature gradient in the freezing suspen-sion.A strong gradient in temperature at the interface between ice mould and suspension caused the formation of many small ice crystals (Fig.7),whereas a weak gradient in the centre

part

Fig.6.Optical microscopic image of the cross-section of a steel rod (a)encap-sulated in a hydroxyapatite shell (b)after sintering.

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Fig.7.Optical microscopic image of a polished cross-section of a sintered hydroxyapatite part produced from a suspension with 42.5vol.%solid

content.

Fig.8.Optical microscopic image of a polished cross-section of a sintered hydroxyapatite part produced from a suspension with 32vol.%solid

content.

Fig.9.Optical microscopic image of a polished cross-section of a sintered hydroxyapatite part produced from a suspension with 25.5vol.%solid

content.

Fig.10.Pore size distribution and cumulative pore volume of a hydroxyap-atite part in the green state produced from a suspension with a solid content of 25.5vol.%.

of the shaped bodies allows few ice crystals to grow very strong (Fig.9).

Fig.10shows the multi-modal pore size distribution of a hydroxyapatite part in the green,i.e.,unsintered state which was prepared using a suspension with a solid content of 25.5vol.%.Whereas the peak at 70nm can be attributed to the pores between the powder particles in a relatively dense package,the peak at 400nm and the peak between 20and 100?m are caused by ice crystals.The porosity of the sample was 76%.After sintering the smaller pores disappeared and the open porosity decreased to 25%.For comparison,Fig.11shows the pore size distribution of a green,freeze cast hydroxyapatite part made of a suspension with a solid content of 42.5vol.%.In contrast to the pore size distribution shown in Fig.10at higher solid contents only the pores attributed to the particle package could be detected.The larger pores arising from the ice crystal growth are distributed over a large size range and consume only a quarter of the entire pore volume,whereas in the case of the part made of the suspen-sion with 25.5vol.%almost 60%of the pore volume is caused by ice crystals.The porosity of the part made of the

suspen-

Fig.11.Pore size distribution and cumulative pore volume of a hydroxyap-atite part in the green state produced from a suspension with a solid content of 42.5vol.%.

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Fig.12.Human jaw-bone made of hydroxyapatite by freeze casting in ice moulds.

sion with42.5vol.%was51%in the green state and13%after sintering.

The component shown in Fig.12is a human jaw-bone which was made from a model produced by stereolithography,subse-quent reshaping of the component in an ice mould and freeze casting of a hydroxyapatite suspension.This part demonstrates the opportunity of producing complex and individually shaped components made of ceramic materials by using ice as mould material.

4.Conclusions

Freeze casting of components from aqueous suspensions by means of ice moulds and ice cores combines a number of advan-tageous aspects arising from the ice itself.Ice can act as a very strong binder between the powder particles in the frozen compo-nent.Ice can be used as mould material and can be machined for this purpose.It may encapsulate components and transfect them into closed shells,because the ice core can be removed by sub-limation.Moreover,ice can be used as a pore forming agent by adjusting the solid content of the powder suspension.The shape of the ice crystals and their aspect ratio can be in?uenced by so-called cryoprotectants.Ice is a non-expensive,available and environmental-friendly material.The experiments have shown that completely closed hydroxyapatite shells can be produced using ice cores which were removed subsequently by freeze drying.Furthermore,other materials,for instance steel,can be transfected into such closed shells by encapsulating them in ice cores.The distance between the core material and the shell can be adjusted simply by the thickness of the ice core.The sublima-tion process causes an open pore structure of the cast material, because the shape and the size of the ice crystals determine the shape and the size of the resulting pores.Ice volume and the size of the ice crystals can be in?uenced by the solid content of the initial suspension.Moreover,the size of the ice crystals depends on the freezing rate,caused by the temperature gradient. The direction of the ice crystal growth follows the temperature gradient.

Ice mould freeze casting may be a suitable shaping tech-nique for producing individually shaped ceramic components with complex geometry,like dental parts or bone scaffold.

Acknowledgement

Financial support of the German Research Foundation(DFG) is gratefully acknowledged.

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