Influences of Composition of Starting Powders and Sintering Temperature on the Pore Size Distribution of Porous Corundum-Mullite Ceramics

Porous corundum-mullite ceramics were prepared by an in-situ decomposition poreforming technique. Starting powders were mixtures of milled Al(OH)3 and microsilica and were formed into oblong samples with a length of 100mm and a square cross-section with edge size of 20mm. The samples were heated at 1300°C, 1400°C, 1500°C or 1600°C for 3h in air atmosphere, respectively. Apparent porosity was detected by Archimedes’ Principle with water as a medium. Pore size distribution and the volume percentage of micropores were measured by mercury intrusion porosimetry. The results show that the pore morphology parameters in the samples depend on four factors: particle size distribution of starting powders, decomposition of Al(OH)3, the expansion caused by mullite and sintering. The optimum mode which has a higher apparent porosity up to 42.3%, well-distributed pores and more microsize pores up to 16.3% is sample No.3 and the most apposite sintering temperature of this sample is 1500°C.

Porous ceramics can be made by adding pore-forming agents such as sawdust, starch, graphite or organic particulates [7] into the starting powers, or by injection molding [8], or by gelcasting [1].Zhen-Yan Deng, et al. made porous alumina ceramics by the decomposition of Al(OH) 3 [9,10].In their study, the pores in the samples were formed in-situ by the decomposition of Al(OH) 3 powder.This pore forming in-situ technique exploiting the decomposition of starting powders is a good way to prepare porous ceramics with welldistributed pores.
Mullite has a lower thermal-expansion coefficient 11,12 than alumina.Introducing mullite to corundum ceramics can increase their thermal-shock resistance as well as their mechanical and chemical stability [13,14].This article described the preparation of porous corundum-mullite ceramics, using Al(OH) 3 powder and microsilica.The decomposition of Al(OH) 3 creates a series of transitional Al 2 O 3 phases, which ultimately transform to a stable α-Al 2 O 3 phase at ~1200°C.Because Al(OH) 3 experiences a 60% volume contraction during decomposition [9], porous corundum-mullite ceramics with high porosity should be obtainable.
When the in-situ decomposition pore-forming technique was used to prepare porous corundum, the pore morphology and size distribution were found to be related to the shape and size of the original Al(OH) 3 particles [10].However, when this technique was used to prepare porous corundum-mullite ceramics the effects of the expansion resulting from mullite formation should be considered.In this paper the open porosity and pore size distribution of porous corundum-mullite ceramics were investigated as a function of the composition of the starting powders and the sintering temperature.

Experimental Procedure
Al(OH) 3 powder was wet milled for 3h in a planetary mill using alumina balls.The particle size distribution of milled Al(OH) 3 powder(Dv50=5.2µm) is given in Fig. 1.The starting powders were mixtures of milled Al(OH) 3 and microsilica (Elkem Microsilica, Grade 983, whose compositions are shown in Table I).The oblong samples with a length of 100mm and a square cross-section with edge size of 20mm were dried at 110°C for 24h, and then heated at 300°C for 3h to decompose Al(OH) 3 , then raised to an elevated temperature with a heating rate of 3°C/min to 1300°C, 1400°C, 1500°C or 1600°C for 3h, respectively .The particle size distribution was measured by a laser particle size analyzer (Matersizer 2000).X-ray diffractometry (Philips X'pert TMP) was used to analyze the phase composition in the sintered specimens prepared at different temperatures.The apparent porosity was detected by Archimedes' Principle with water as a medium.The pore size distribution was measured by mercury intrusion porosimetry (AutoPore IV 9500, Micromeritics Instrument Corporation).

Effect of the composition of the starting powders on the porosity and the pore size distribution of the specimen
As shown in Fig. 2, the apparent porosity of the sintered specimens increases with the Al(OH) 3 content in the starting powders.Pores in the samples are formed by the decomposition of the Al(OH) 3 particles producing 60% volume contraction and leaving space around Al 2 O 3 particles.The pore size distributions of the specimens prepared from different starting powders sintered at 1500°C are shown in Fig. 3.They are bimodal.One pore group is representative of micropores whose size is less than 200nm and the other is formed by bigger pores whose size is in the range from 400nm to 1000nm.It was found from Fig. 3 that with increasing Al(OH) 3 content in the start powder mixtures, the pore size increased, but when the calculated mullite content in the samples was more than 70% the influence of the Al(OH) 3 content became smaller.Fig. 4 shows the percentage of the volume of pores in the first range (<200nm) of the total pore volume.It was found that with decreasing Al(OH) 3 content the ratio of micropore volume to total pore volume decreased, but sample No.3 had the maximum micropore volume.Based on the results we may consider that sample No.3 is the best in our study because it has the highest porosity and the most micropores.On the other hand sample No.3 has a mullite content of 70% which is very close to the apposite content of mullite in mullitecorundum ceramics[14-16], so that sample No.3 is well-suited to prepare porous corundummullite ceramics.

Effect of Sintering Temperature on the porosity and the pore size distributions of the specimen
Fig. 5 shows the apparent porosities of corundum-mullite ceramics sintered at different temperatures.As shown, the porosity decreased slightly from 49.1% to 42.4% with sintering temperature from 1300°C to 1400°C, and maintained stably with sintering temperature from 1400°C to 1500°C, but decreased abruptly to 18.5% with further increase in sintering temperature to 1600°C.When the sintering temperature rose from 1400°C to 1500°C, the curve of pore size distribution moved toward larger pore size, and the percentage of microsize pores decreased.
In the sample sintered at 1600°C, the pore size increased and the microsize pores disappeared.

Discussion
The porosity and pore size distribution in the samples depended on four factors: 1) Porosity and pore size distribution of green compacts.They depended on the particle size distribution of Al(OH) 3 and microsilica.2) Pores formed by the decomposition of Al(OH) 3.
3) Influence of expansion caused by the formation of mullite.4) Sintering.During reaction sintering of corundum-mullite materials the expansion caused by mullite formation plays important roles on porosity and pore size distribution in the samples.The roles can be divided into two kinds.As shown in Fig. 9, a pore is surrounded by grains and reaction areas marked RA in Fig. 9.During heating the expansion resulting from mullite formation reaction may give two different kinds of influences on the pore size.When the expansion is larger and the bond between the grains is weaker the expansion makes the grains separate with pore growth, as shown in Fig. 9A.However, in the case when expansion is smaller and the bonds between grains are very strong, the grains can not be separated by expansion and the expansion volume will enter into the pore to make the pore size and volume decrease.10 XRD analyses of mullite for the specimen sintered from 3# powders at different sintering temperatures With rising sintering temperature the mullite formation reaction becomes fast, the mullite content and resulting expansion are larger.In this case the expansion increases the size and volume of pores in the samples as shown in Fig. 9B.It makes the porosities of the samples heated at 1400°C and 1500°C almost the same (Fig. 5) because the expansion cancels out of the shrinkage of sintering, but the pore size increases when the sintering temperature rises from 1400°C to 1500°C.When the sintering temperature rises to 1600°C, sintering pays an important role because the mullite formation reaction has almost finished at lower temperatures.The porosity of the samples is very low.
The same reason may explain the results shown in Fig. 4 Sample No.3 heated at 1500°C has the most apposite expansion volume to enter into pores to increase the percent of microsize pore volume.

Conclusions
The in-situ decomposition pore-forming technique has been successfully employed to fabricate porous corundum-mullite ceramics.The porosity and the size distribution of pores are determined by the content of Al(OH) 3 , the particle size of starting powders, the expansion of mullite formation and the sintering temperature.The optimum mode is sample No. 3

Fig. 2 Fig. 3
Fig.2Variation of the apparent porosities in sintered specimens sintered at 1500°C for 3h with the composition of starting powders Fig.3Variation of the pore size distributions of the porous specimens with different starting powders sintered at 1500°C for 3h

Fig. 4
Fig.4 Variation in the volume percentage of micropores in total pore volume with different starting powders sintered at 1500°C for 3h

Fig. 9
Fig. 9 Schematics of the change of pore morphology during reaction sintering