Macroporous Alumina Ceramics with Aligned Microporous Walls byUnidirectionally Freezing Foamed Aqueous Ceramic Suspensions

He-Jin Yoon, Uoong-Chul Kim, Ji-Hwan Kim, and Young-Hag Koh*,w

Department of Dental Laboratory Science and Engineering, Korea University, Seoul 136-703, Korea

Won-Young Choi and Hyoun-Ee Kim**

Department of Materials Science and Engineering, Seoul National University, Seoul 151-742, Korea

This paper reports a novel way of producing macroporous ce-ramics with aligned microporous walls by unidirectionally freez-ing foamed aqueous ceramic suspensions. The stability of the airbubbles introduced by magnetic stirring was improved remark-ably using polyvinyl alcohol (PVA) as both a binder and emul-sifying agent. Furthermore, the unidirectional freeze castingmethod could not only trap the air bubbles in the solidified sam-ple successively but also induce the preferential growth of icedendrites. This endowed the samples with uniform macroporessurrounded by aligned microporous alumina walls after freezedrying and sintering. The sample produced with a PVA contentof 8 wt% showed a high compressive strength of 4.671.6 MPaat a high porosity of 91 vol%.

I. Introduction

POROUS materials with closed or open pores have been usedextensively in a range of fields, such as ceramic filters, sup-

ports for catalysts, bone scaffolds, porous piezoelectric ceramics,electrodes in fuel cells, etc.1 Thus far, a variety of manufacturingmethods has been developed to produce these materials, includ-ing the use of pore-forming fugitive phases,2 sponge replicationmethod,3 pyrolysis of preceramic precursors,5 foaming of ce-ramic suspensions,5–10 freeze casting,11–15 and electric field-as-sisted processing.16 Fundamentally, the functions of porousmaterials should be affected significantly by their pore struc-ture, such as porosity, pore size, pore morphology, and inter-connections between pores, as well as the microstructure of theceramic walls.17 Therefore, it is still of scientific and technicalimportance to develop new manufacturing methods that cancontrol the pore structure of porous materials in a tightly con-trolled manner.

We herein propose a novel and versatile way of producingmacroporous alumina ceramics with aligned micropores in al-umina walls by unidirectionally freezing foamed aqueous ce-ramic suspensions. In particular, air bubbles were introducedphysically by magnetic stirring with the assistance of polyvinylalcohol (PVA) polymer as the binder and emulsifying agent.18

Various amounts of PVA, ranging from 0 to 8 wt% in relationto DI water, were added to the suspensions, in order to evaluatethe effect of PVA addition on the stability of air bubbles. There-after, the foamed alumina suspensions were frozen unidirection-

ally at B�701C to induce unidirectional solidification of thesuspension from the bottom to top,11,12 followed by freeze dry-ing and sintering at 14501C for 2 h. This new approach gave thesample uniform macropores with aligned microporous walls.The pore structures (e.g., porosity, pore size, and degree of poreorientation) of the fabricated samples were characterized. Inaddition, the compressive strength test was measured to deter-mine the structural integrity of the sample.

II. Experimental Procedure

In this study, PVA was used as a binder and emulsifying agentto stabilize air bubbles generated in aqueous alumina suspen-sions. First, the predetermined amounts of PVA (Sigma Aldrich,St. Louis, MO), ranging from 0 to 8 wt%, were dissolved in DIwater using a magnetic stirrer for 1 h. Commercially availablealumina powders (Al2O3, Kojundo Chemical Lab. Co. Ltd.,Saitama, Japan) with an average particle size of 0.3 mm werethen added to the PVA solution with the assistance of a nonionicsurfactant (Hypermer KD-6, UniQema, Everburg, Belgium) asa dispersant, and stirred for 1 h. The amounts of alumina pow-der, DI water, and dispersant in the suspension were 5, 5, and0.2 g, respectively. This simple process resulted in the vigorousformation of air bubbles throughout the suspension. In addi-tion, the stirring speed was adjusted from 800 to 1400 rpm tocontrol the size of the air bubbles.

The prepared suspensions containing the air bubbles werepoured into 20 mm diameter polyethylene molds and then fro-zen unidirectionally by immersing them in a cooled ethanolbatch at B�701C at a constant dipping speed of 0.5 mm/s. Thegreen samples were then freeze-dried to remove the ice dendritesand heat-treated at 4001C for 2 h to remove the PVA and poly-meric dispersant, followed by heat-treatment at 14501C for 2 hto densify the alumina walls.

The pore structures (e.g., porosity, pore size, and degree ofpore orientation) of the fabricated samples were characterizedby scanning electron microscopy (SEM, JSM-6360, JEOL Tech-niques, Tokyo, Japan). The total porosity of the sample wascalculated by measuring the dimensions and weight. The poros-ity of the macropores was measured from the SEM images ofthe samples prepared by infiltrating the porous alumina ceram-ics with an epoxy resin (Spurrs epoxy, Polysciences Inc., War-rington, PA). The porosity of the micropores formed in thesintered alumina walls was estimated by considering the totalporosity and porosity of the macropores.

In addition, compressive strength tests were carried out toevaluate the structural integrity of the samples. The sampleswith a diameter and height of B16 and B30 mm, respectively,were loaded at a crosshead speed of 5 mm/min using a screw-driven load frame (Instron 5565, Instron Corp., Canton, MA).The stress and strain responses of the samples during the com-

M. Edirisinghe—contributing editor

This work was supported by the Korea Science and Engineering Foundation (KOSEF)grant funded by the Korea government (MEST) (No. 2009-0073415).

*Member, The American Ceramic Society.**Fellow, The American Ceramic Society.wAuthor to whom correspondence should be addressed. e-mail: kohyh@korea.ac.kr

Manuscript No. 27017. Received November 3, 2009; approved December 21, 2009.

Journal

J. Am. Ceram. Soc., 93 [6] 1580–1582 (2010)

DOI: 10.1111/j.1551-2916.2010.03627.x

r 2010 The American Ceramic Society

1580

pressive strength tests were monitored. Five samples were testedto obtain an average value along with its standard deviation.

III. Results and Discussion

This study demonstrates the utility of unidirectional freeze cast-ing of foamed aqueous suspensions for the production of mac-roporous alumina ceramics with aligned microporous walls.Figure 1 shows the samples prepared with various PVA con-tents (0, 2, 4, 6, and 8 wt%) used as both a binder and emul-sifying agent. Without the addition of PVA, macropores wereformed only near the top of the sample, due to the extensiveflotation of air bubbles.19,20 However, the extent of the macro-porous region marked by the arrows increased remarkably withan increasing PVA content. The sample prepared with a PVA

content of 8 wt% demonstrated that the macropores wereformed uniformly throughout the sample. It is reasonable tosuppose that the PVA polymer will not only increase the vis-cosity of the suspension but also function as an emulsifyingagent, thereby retarding the flotation of air bubbles.18

The sizes of macropores formed in the samples were con-trolled by adjusting the stirring speed, ranging from 800 to 1400rpm, during the foaming process. All of the prepared samplesshowed a very uniform macroporous structure with small inter-connections between the macropores, as shown in Figs. 2(A)–(D), presumably due to the lack of the Ostwald ripening andcoalescence processes of the air bubbles caused by the rapid so-lidification of the foamed suspensions.20 The size of the macro-pores measured by considering the digitally colored imagesbased on the SEM images of the epoxy-filled samples decreasedsignificantly from 4607164 to 222750 mm with increasing stir-ring speed from 800 to 1400 rpm, as summarized in Table I.

One of the striking features of the present method, namely theunidirectional freeze casting as a new setting technique, is theability to create aligned micropores in sintered alumina wallsobtained by the preferential growth of ice dendrites.11,12 Re-gardless of the stirring speeds, all of the prepared samplesshowed aligned micropores formed in the sintered aluminawalls. A typical SEM micrograph of the sintered alumina wallis shown in Fig. 3, in which the micropores were aligned towardthe center of the macropores, because heat would dissipatethrough the air bubbles during freezing.

The total porosity of the sample produced at a stirring speedof 800 rpm, which was calculated by measuring its dimensions

Fig. 1. Optical photograph of the samples produced using a range ofPVA contents (0, 2, 4, 6, and 8 wt%) after sintering at 14501C for 2 h.

Fig. 2. Scanning electron micrographs of the samples produced at various stirring speeds of (A) 800, (B) 1000, (C) 1200, and (D) 1400 rpm, showing themacroporous structures.

Table I. Sizes of the Macropores in the Samples Produced atVarious Stirring Speeds (800, 1000, 1200, and 1400 rpm)

Stirring Speed (rpm) 800 1000 1200 1400

Size (mm) 4607164 3907112 253751 222750

June 2010 Rapid Communications of the American Ceramic Society 1581

and weight, was as high as 91 vol%. The porosity of themacropores was 80 vol%, as calculated from a digitally coloredimage of the epoxy-filled sample. In addition, the porosity ofthe micropores formed in the sintered alumina walls was alsocalculated by considering the total porosity and porosity ofthe macropores. The measured porosity was 55 vol%, whichindicates that the alumina walls were quite porous.

Compressive strength tests were carried out to evaluate thestructural integrity of the sample, which would be expected tofind useful applications as the bone scaffold. The sample pro-duced at a stirring speed of 800 rpm exhibited the typical frac-ture behavior of a highly porous ceramic foam, as shown inFig. 4.17 In other words, the stress increased with an elastic re-sponse and reached a maximum, at which partial fracture of thealumina walls would occur, resulting in a decrease in the ap-parent stress, followed by densification of the fractured sample.The measured compressive was as high as 4.671.6 MPa at aporosity of 91 vol%, which was attributed to the construction ofaligned micropores surrounded by dense alumina walls withoutnoticeable defects, such as cracks or flaws.

It is worth mentioning that the unidirectional freeze castingmethod as a new setting technique of a foamed ceramic suspen-sion cannot only create aligned micropores in ceramics walls

formed as the replica of ice dendrites but also preserve air bub-bles without extensive floatation due to the rapid solidificationof the suspension. In addition, this technique would be expectedto eliminate the possible problems often caused by the dryingshrinkage of a wet ceramic foam,20 because the ice dendritesformed in the alumina walls could be sublimed by freeze drying,which would allow the sample to experience negligible shrinkageduring drying process. It is believed that this technique can beapplied to a variety of materials, including ceramics and metals.

IV. Conclusions

We fabricated macroporous alumina ceramics with aligned mi-croporous walls by unidirectionally freezing foamed aqueousceramic suspensions. PVA polymer was used to stabilize the airbubbles introduced by magnetic stirring. The unidirectionalfreeze casting method as a new setting technique allowed thesamples produced with a PVA content of 8 wt% to have uni-form macropores formed by air bubbles, as well as aligned mi-cropores in sintered alumina walls formed as the replica of thepreferentially grown ice dendrites. It was possible to control thesize of the macropores, ranging from 4607164 to 222750 mm,by simply adjusting the stirring speed during the foaming pro-cess. The compressive strength of the sample was as high as4.671.6 MPa at a high porosity of 91 vol%.

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Fig. 3. Typical Scanning electron micrograph of the samples producedusing a stirring speed of 800 rpm, showing the preferentially orientedaligned micropores formed in the alumina walls. The arrow indicates thedirection of the preferential orientation of the micropores.

Fig. 4. A typical stress versus strain response of the sample during thecompressive strength test. (stirring speed5 800 rpm).

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