Materials and Design 33 (2012) 98–103

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Materials and Design

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Short Communication

Microstructure and mechanical properties of porous Si3N4 ceramicsprepared by freeze-casting

Yongfeng Xia, Yu-Ping Zeng ⇑, Dongliang JiangState Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 DingxiRoad, Shanghai 200050, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 16 December 2010Accepted 14 June 2011Available online 30 June 2011

0261-3069/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.matdes.2011.06.023

⇑ Corresponding author. Tel.: +86 21 52415203; faxE-mail address: yuping-zeng@mail.sic.ac.cn (Y.-P.

Porous silicon nitride (Si3N4) ceramics were prepared from water-based Si3N4 slurries via a freeze-castingprocess. The green body shows good replica of ice dendrites with uniform pore channels. The microstruc-ture of porous Si3N4 ceramics could be controlled by the freezing temperature. After sintering, porousSi3N4 ceramics with uniform microstructure was obtained. At low freezing temperature, the pore size dis-tribution was narrower and concentrated with pore morphology from tree-like to vimineous channels.Porous Si3N4 ceramics with a porosity of 51%, flexural strength of 77 MPa, and aspect ratio (AR) of 5.20was obtained at 1850 �C with 3% sintering additives.

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1. Introduction

Porous silicon nitride (Si3N4) ceramics have been used in manyfields, such as high-temperature gas filters and separation mem-branes because of its high-temperature strength, good oxidationresistance, thermal–chemical corrosion resistance, thermal shockresistance, low thermal expansion coefficient [1]. For practicalapplications, these properties are related to the pore structure(shape, morphology, orientation, porosity, etc.). It was found thatthe amount of the sintering additive substantially influenced thesintering behavior of Si3N4, including densification, grain growth,phase transformation rate as well as aspect ratio of b-Si3N4 grains[2].

Many processes, including adding fugitive substance [3], carbo-thermal nitridation , combustion synthesis [4], in situ reactionbonding [5], have been used to fabricate porous Si3N4 ceramicswith controlled pore structure. Specially, freeze casting [6] hasbeen used to prepare porous ceramics. Water [7,8] and camphene[9] have been successfully used as freezing vehicles. The porousceramics from freeze casting have not only ultra-high porosityand large interconnection but also dense ceramic networks to en-hance its mechanical properties. Besides, freeze casting can easilycontrol the pore morphology, porosity, microstructure as well asthe mechanical properties of porous ceramics by adjusting theconcentration of ceramic slurry and freezing temperature [10,11].

Usually, the strength of porous ceramics is inferior to that of thecorresponding dense materials. In order to improve the strength ofSi3N4 ceramics with high porosity, it is important to control the

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characteristics of b-Si3N4 grains, such as grain size, aspect ratioand bonding energy of the grain/amorphous intergranular phaseinterface [12]. Therefore, b-Si3N4 was introduced into porousceramics by phase transformation. Besides, it is well known thatuniform pore with small size can also improve the mechanicalproperties.

In this work, porous silicon nitride ceramics with enhancedmechanical properties were fabricated by freeze-casting processat different freezing temperatures to obtain different pore morphol-ogy. The microstructure, open porosity and mechanical strength ofporous Si3N4 were investigated.

2. Experimental procedures

2.1. Materials

a-Si3N4 (SN-E10, purity >99.5%, a ratio >95%, average particlesize of 0.5 lm; UBE Industries, Ltd., Tokyo, Japan), Y2O3 (purityP99.99 wt%; Yuelong Company, Shanghai, China) and Al2O3 (purity>99.9%, a ratio >95%, average particle size 0.5 lm; Wusong Fertil-izer Factory, Shanghai, China) were used as the starting materials.Y2O3 and Al2O3 in the mole ratio of 3:5 were used as sintering addi-tives with content 1–10 wt%. A small amount (2 wt%) of dispersant(NH4PAA, BK Guiulini Chemic Representative Office, Ladenburg,Germany) was used to raise the dispersibility of the slurry.

2.2. Fabrication procedures

a-Si3N4, Y2O3, Al2O3 and ammonium polyacrylate (NH4PAA)were mixed and were then ball-milled in distilled water for 24 hto obtain a humongous slurry. The resultant slurries with 50 wt%

Fig. 1. Schematic illustration of freezing process.

Y. Xia et al. / Materials and Design 33 (2012) 98–103 99

solid loading were poured into a sealed metal container(95 � 50 � 10 mm), which was immerged in a refrigerant in a freez-ing bath as shown in Fig. 1. In the experiment, samples were frozenat �18 �C for 12 h and �198 �C (liquid nitrogen) for 10 min. Afterfreezing completely, the specimens were immediately transferredto a lyophilizer (Shanghai Zhongke Biomedicine High-tech DevelopCo. Ltd., Shanghai, China) to avoid the melting of ice. And all speci-mens were then subjected to a 5 Pa vacuum to remove ice com-pletely with heating rate of 10 �C/h. The specimens were thenheated at 600 �C in air with a heating rate of 2 �C/min for 3 h to burnout the organic additives and sintered at 1750–1850 �C in 0.5 MPanitrogen atmosphere for 2 h at a heating and cooling rates of 3 �C/min.

2.3. Characterizations

Specimens were machined into a rectangle bar with dimensionsof 3.0 � 4.0 � 36.0 mm to measure the flexural strength via thethree point bending test (Model AUTOGRAPH AG-I, Shimadzu, Ja-pan). A support distance of 30 mm and a cross-head speed of

Fig. 2. The SEM images of porous Si3N4 ceramics frozen at different temperature: (a) grefrozen at �18 �C; and (d) 1800 �C, 5 wt% frozen at �198 �C.

0.5 mm/min were used, and the direction of pore channels wasparallel to the pressing direction. The open porosity and bulk den-sity were determined by the Archimedes method using distilledwater as medium. Phase analysis was conducted by X-ray diffrac-tion (XRD) via a computer-controlled diffractometer (Model RAX-10, Rigaku, Japan) with Cu Ka radiation (wavelength of1.5418 Å). Morphology of porous Si3N4 ceramics was observed byscanning electron microscopy (SEM) (Model JXA-8100, JEOL, To-kyo, Japan). The aspect ratio R95 was defined as the mean aspect ra-tio of the 10% largest grains based on the assumption that all the b-Si3N4 grains in a microstructure have approximately the same as-pect ratio [13]. The pore size distribution was determined by mer-cury porosimetry (Model pore sizer 9320, Micrometritics Co. Ltd.,USA).

3. Results and discussion

3.1. Effect of freezing temperature on the microstructure

Fig. 2 shows the SEM images of green samples and sinteredceramics frozen at �18 �C and �198 �C. For green samples, thepores in both samples show a good replica of ice dendrites, butthe microstructure is quiet different at different freezing tempera-tures. First, the pore size of sample frozen at �18 �C is about fivetimes than that of sample frozen at �198 �C. Second, the pore mor-phology obtained at �18 �C is typical tree-like, while the pore mor-phology of sample frozen at �198 �C consists of vimineouschannels. These different microstructures were generated by dif-ferent morphological ices. During freezing process, the tempera-ture of metal module is lower owing to its higher thermalconductivity coefficient than that of the ceramic slurry, resultingin a temperature gradient between metal module and slurry. Thusthe ice crystal generates on the top of metal and grows along thegradient orientation. The lower cooling temperature results in lar-ger amount of crystal nucleus, which induces ice crystals compet-itively growth and restrain ice crystals from further coarsening.Therefore, the pores from replica of ice crystals are smaller. After

en sample frozen at �18 �C; (b) green sample frozen at �198 �C; (c) 1800 �C, 5 wt%

Fig. 3. The pore size distribution of porous Si3N4 ceramics frozen at different temperatures: (a) �18 �C; and (b) �198 �C.

100 Y. Xia et al. / Materials and Design 33 (2012) 98–103

sintering, the microstructure is well preserved except for pore sizedecreasing because of shrinkage. Thus, the pore size and morphol-ogy of porous Si3N4 ceramics are controllable by changing thefreezing temperature.

Fig. 3 shows the pore size distribution of porous Si3N4 ceramicsfrozen at �18 �C and �198 �C. At �18 �C, the pore size distributioncomprises distinct regions. For green sample, the large pore maycorrespond to the aligned open pores, the intermediate-size poresto the open pore in the dendrites and the small size pores <1 lm tothe pores between particles in the pore tendon. After sintering, thevolume of small pores decreases because of liquid phase sintering.However, the large and intermediate-size pores are preserved be-cause the pore size is too large and the amount of liquid phase isnot enough. For the sample frozen at �198 �C, the pore size distri-bution is defined by aligned open pores and some small pores.After sintering, the small pores disappear and the size of large porebecomes smaller, which might be caused by shrinkage and graingrowth.

Above analyses indicated that sample frozen at �198 �C has amore uniform pore size distribution. Therefore, the sample frozenat �198 �C has been further characterized.

3.2. Effect of sintering temperature and additives on themicrostructure of samples frozen at �198 �C

Fig. 4 shows the SEM images of the porous Si3N4 ceramics fro-zen at �198 �C. All of the specimens have a uniform microstruc-ture. The microstructure, which is different from that reported byFukasawa et al. [7], consists of pore channels surrounded by b-Si3N4 grains. With low sintering additive content (3 wt%), the as-pect ratio (AR) of b-Si3N4 grain is 1.14, 3.14 and 5.20 with sinteringtemperature of 1750, 1800 and 1850 �C in Fig. 4a–c. Although alarge proportion of equiaxed grains is observed in Fig. 4a, theXRD diffraction patterns indicated that the phase transformationfrom a-Si3N4 to b-Si3N4 is completely accomplished in Fig. 5. Thisresult is well accorded with previous work, which reported thatthe phase transformation started at 1450 �C and finished at1750 �C [14]. Meanwhile, with increasing additives content, thepore channels become narrower and more glass phase is visible(Fig. 4d and e).

In sintering process, the a-Si3N4 transform to b-Si3N4 and thenb-Si3N4 grow within the favorable direction of (0 0 0 1) direction.Temperature is a main driving force for grain growth and high tem-perature results in large grain. Lai and Tien [15] have found that

the relationship between sintering temperature and aspect ratio(AR) of b-Si3N4 can be expressed as the following equation:

ln AR ¼ lnK1=3

L

K1=5W

!þ 2

15ln t � ðQ L=3� QW=5Þ

RTð1Þ

where QL and QW are activation energies, and KL and KW are rateconstants in the length and width directions, respectively. In ourexperimental results, the aspect ratios (AR) appear to increase withincreasing sintering temperature.

Furthermore, the microstructure of porous Si3N4 is greatly influ-enced by sintering additives. Because Si3N4 is a covalent compoundand it’s self-diffusion coefficient is low, the liquid phase generatedby sintering additive is necessary for densification of Si3N4 ceram-ics. The three stages of densification: particles rearrangement,solution-reprecipitation and solid state diffusion [16], are all re-lated to liquid phase. Thus, more liquid phase sintering additive re-sults in smaller pore channels.

3.3. Porosity and mechanical properties of samples frozen at �198 �C

Fig. 6 shows the porosity of sintered porous Si3N4 ceramics. Theporosities of Si3N4 ceramics decrease when sintering temperatureincreases. Likewise, with increasing the content of sintering addi-tives, the porosities of the porous Si3N4 ceramics decrease. Espe-cially the sample with 10 wt% sintering additives sintered at1850 �C, presents porosity of only 13% due to the higher contentof sintering additives and higher sintering temperature. Therefore,the porosity can be controlled by adjusting sintering temperatureand the amount of sintering additives.

Higher sintering temperature usually results in lower viscosityof liquid phase, which is better to rearrange the ceramic particles.At the same time, the mass transport is easier to take place at high-er temperature. More sintering additives increase the content of li-quid phase, which acts on the solid particles to pack pores andeliminate pores. The densification of Si3N4 ceramic is chieflydependent on particle rearrangement and solution-reprecipitation,both of which are enhanced by liquid phase formation [17].

Fig. 7 shows the flexural strength of porous Si3N4 ceramics.With increasing sintering temperature and additive content, theflexural strength increases. That is opposite to the relationship ofporosity with sintering temperature and additive.

According to Griffith theory, the mechanical strength is depen-dent on the critical crack size. And the cracks originate from de-fects, such as impurity, pores. In porous ceramics, pores are the

Fig. 4. The SEM images of porous Si3N4 ceramics frozen at -198oC : (a)1750oC, �198 �C: (a) 1750 �C, 3 wt%; (b) 1800 �C, 3 wt%; (c) 1850 �C, 3 wt%; (d) 1800 �C, 10 wt%; and(e) 1850 �C, 10 wt%.

Y. Xia et al. / Materials and Design 33 (2012) 98–103 101

Fig. 5. The XRD pattern of porous Si3N4 ceramics sintered at different temperature.

Fig. 6. The porosity of porous Si3N4 ceramics frozen at �198 �C.

Fig. 7. The flexural strength of porous Si3N4 ceramics frozen at �198 �C.

102 Y. Xia et al. / Materials and Design 33 (2012) 98–103

main source of cracks. Pores not only decrease the load area, butalso weaken the load capability because the stress concentrateson the surrounding region of pores. That is why the flexural

strength is inversely proportional to the porosity. The relation be-tween flexural strength (r) and porosity (P) can be expressed asfollowing equation [18]:

r ¼ r0 expð�bPÞ ð2Þ

where r0 is the flexural strength at a porosity of 0 and b is the struc-tural factor. It can be seen from Figs. 6 and 7 that the flexuralstrength is decreasing with porosity increasing, indicating thatporosity is a influence factor of flexural strength.

In addition, the mechanical properties of porous Si3N4 ceramicsare closely related to their microstructure, especially the b-Si3N4

grain morphology. The high aspect ratio of b-Si3N4 grains is favor-able to the mechanical property of porous Si3N4 ceramics.

4. Conclusions

Porous Si3N4 ceramics with unidirectionally aligned pore chan-nels were prepared by a freeze casting process. After sintered innitrogen atmosphere, a-Si3N4 particles were completely trans-formed into fibrous b-Si3N4 grains, resulting in the porous Si3N4

ceramics with a unique microstructure replicated from ice den-drites. The microstructure is remarkably different at differentfreezing temperatures. The pore size is smaller and the pore mor-phology is from tree-like to vimineous channels at deeper coolingtemperatures. At the same time, the aspect ratio of b-Si3N4 in-creases with sintering temperature increasing and pore channelsbecome narrower with increasing additive amount. Furthermore,the porosity of samples decreased with higher temperature andhigher content of additives because of particle rearrangementand solution-reprecipitation of densification in liquid phase. Andwith the porosity increasing from 32.97% to 53.67%, the flexuralstrength of porous Si3N4 ceramics decreased from 224.6 MPa to65.8 MPa.

Acknowledgment

The authors would like to thank the financial support from theNational Natural Science Foundation of China (Project Nos.50902140 and 50872142).

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