JOURNAL OF MATERIALS SCIENCE LETTERS19 (2000 ) 787– 789

The structure of ceramic foams produced using polymeric precursors

M. R. NANGREJO, X. BAO, M. J. EDIRISINGHEDepartment of Materials, Queen Mary and Westfield College, University of London, Mile End Road,London E1 4NS, UKE-mail: M.J.Edirisinghe@qmw.ac.uk

During the last decade porous ceramic materials havebeen finding increasing applications due to their fa-vorable properties such as high temperature stability,high permeability, low mass, low specific heat capac-ity and low thermal conductivity. These characteristicsare essential for many technological applications suchas catalyst supports, filters for molten metals and hotgases, refractory linings, thermal and fire insulators andporous implants [1, 2].

Ceramic foams can be produced by different meth-ods, principally impregnation of polymer foams withslurries containing appropriate binders and ceramic par-ticles followed by pressureless sintering at elevatedtemperatures [2–5]. This involves coating an open-cellpolymeric sponge with a ceramic slurry several times,pyrolysis of the polymer to form a ceramic skeletonfollowed by sintering. Ceramic foams produced by thismethod are generally of low strength as their struts arethin and can contain a hole in the center [2, 6–8].

Recently, a new method to produce silicon carbide(SiC) foams using polymeric precursor solutions wasdeveloped by Baoet al. [9] where a polyurethane foamwas immersed in a polymeric precursor solution to forma pre-foam which was pyrolyzed in nitrogen. The mainadvantages of this new approach are the simplicity andease of control of structure of the final product. Thisnew process was exploited further to prepare siliconcarbide-silicon nitride (SiC-Si3N4) composite foams[10]. In this letter we provide microstructural evidenceof the improvements in structure of the ceramic foamsproduced by our method.

The polysilane precursor discussed in this study wassynthesized by the alkali dechlorination of a combi-nation of chlorinated silane monomers in refluxingtoluene/tetrahydrofuran with molten sodium as de-scribed previously [11, 12]. The structure of the SiCpolysilane precursor synthesized is given below. Ph in-dicates a phenyl group.

A polyurethane sponge with open cells of size 500–800 µm was used. 0.8 g of the polysilane precursorwas dissolved in 2000 mm3 dichloromethane to form apolymeric precursor solution. The polyurethane spongewas first cut into cubes of side 10 mm and then im-

mersed in the precursor solution for about 2 h. Thesamples were air dried overnight at room temperature toobtain pre-foams. These pre-foams were subsequentlypyrolyzed in nitrogen. This procedure was also usedto prepare SiC-Si3N4 composite foams. Here 0.4 g ofthe polysilane precursor was dissolved in 2000 mm3 ofdichloromethane and then 0.1 g of Si3N4 powder (withparticles in the size range 0.1–4.0µm and supplied bythe AME Division of Morgan Matroc Ltd., UK) wasadded into the precursor solution before forming thepre-foam.

The pre-foam was placed in an alumina boat andheated from the ambient temperature to 900◦C at 1◦Cmin−1 in a tube furnace (Lenton Thermal Designs Ltd.,Market Harbrough, UK) in the presence of flowingnitrogen gas (flow rate approximately 2.5× 105 mm3

min−1) followed by soaking at this temperature for 2 h.Subsequently, the furnace was switched off and allowedto cool to the ambient temperature. The structures of thecross-sections of the foams produced were investigatedusing a Cambridge S360 scanning electron microscope(SEM). Samples studied using the SEM were coatedwith gold prior to examination.

Micrographs in Figs 1a and 2a show the cross-sections of SiC and SiC-Si3N4 foams. The foams con-sist of a three-dimensional array of struts and a well de-fined open cell structure with cell sizes between 400µmand 900µm. The cell window size varies from 200µmto 600µm. Some of the cell windows are covered witha thin ceramic membrane.

The struts of the SiC and SiC-Si3N4 foams (Figs 1band 2b) do not show any strength lowering surfacecracks usually present in foams prepared from ceramicslurries [13]. Such cracks are likely to be caused mainlyby the non-uniform coating of the polymeric foam bythe ceramic slurry [13, 14].

Micrographs in Figs 1c and 2c show the triangular-shape strut cross-sections of the pyrolyzed SiC and SiC-Si3N4 foams prepared and it is noteworthy that there isno hole at the center of strut. In contrast ceramic foamsmade by ceramic slurry coating method can containsuch defects after pyrolysis [6–8] and this must be detri-mental to their mechanical properties. The eliminationof the hole by our processing method is probably dueto the better penetration of the precursor solution intothe polyurethane web structure during coating and theinward mobility of the polymeric precursor during py-rolysis. The polysilane precursor used does not fullycross-link until about 600◦C and therefore such move-ment is possible during heating. The elimination of

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Figure 1 Scanning electron micrographs of a pyrolyzed SiC foam showing (a) cell structure (b) strut surface and (c) strut cross-section.

Figure 2 Scanning electron micrographs of a pyrolyzed SiC-Si3N4 composite foam showing (a) cell structure (b) strut surface and (c) strut cross-section.

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cracks and the center hole in the struts in the ceramicfoam prepared by our process can only lead to a higherstrength.

AcknowledgmentThe authors wish to thank the Government of Pakistanfor partial support of this work via a Ph.D. scholarshipto Mr. Nangrejo. The experimental work for this letterwas done when we were at Loughborough Universityand Mr. F. Page and Mr. J. S. Bates are thanked for helpin electron microscopy. The help of Dr. D. H. Rossand Mr. T. J. Atkinson in resolving technical matters atLoughborough is acknowledged.

References1. P. S E P U L V E D A andJ. G. P. B I N N E R, J. Euro. Ceram. Soc.

19 (1999) 2059.2. J. S A G G I O-W O Y A N S K Y , C. E. S C O T T and W. P.

M I N N E A R , Amer. Ceram. Soc. Bull. 71 (1992) 1674.

3. L . M . S H E P P A R D, Ceram. Trans. 31 (1993) 3.4. F. F. L A N G E andK . T. M I L L E R , Adv. Cerm. Mater. 2 (1987)

827.5. S. B. B H A D U R I andZ. B. Q I A N , J. Mater. Synth. Proc. 3

(1995) 361.6. S. B. B H A D U R I , Adv. Performance Mater. 1 (1994) 205.7. D. A . H I R S C H F E L D, T . K . L I andD. M . L I U , Key Engi-

neering Mater. 115(1996) 65.8. J.-M . T U L L I A N I , L . M O N T A N A R O, T. J. B E L L and

M . V . S W A I N , J. Amer. Ceram. Soc. 82 (1999) 961.9. X . B A O, M . R. N A N G R E J O and M . J. E D I R I S I N G H E,

J. Mater. Sci., in press.10. M . R. N A N G R E J O. X . B A O and M . J. E D I R I S I N G H E,

J. Euro. Ceram. Soc., submitted.11. X . B A O, M . J. E D I R I S I N G H E, G. F. F E R N A N D O and

M . J. F O L K E S, ibid. 18 (1998) 915.12. X . Z H A N G andR. W E S T, J. Polym. Sci. Polym. Chem. 17(1979)

2833.13. V . R. V E D U L A , D. J. G R E E N and J. R. H E L L M A N ,

J. Amer. Ceram. Soc. 82 (1999) 649.14. D. D. B R O W N andD. J. G R E E N, ibid. 77 (1994) 1467.

Received 13 Octoberand accepted 2 November 1999

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