Synthesis of Rod-Like High-Purity b-Sialon Powder by a Novel CarbothermalReduction–Nitridation Method with a Nanocasting Procedure

Qiang Yan,z Qian Liu,w,y and Qingfeng Liuy

zGraduate School of the Chinese Academy of Sciences, Beijing 100049, China

yThe State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute ofCeramics, Chinese Academy of Sciences, Shanghai 200050, China

Rod-like high-purity b-Sialon(Si6�zAlzOzN8�z, z5 1,2,3) pow-ders were synthesized from the mesoporous SiO2/C/Al2O3 com-posites, that were prepared using a nanocasting procedure,through a carbothermal reduction–nitridation method at14201C. The XRD analysis provided considerable evidence re-garding the high purity of resultant b-Sialon powders with var-ious z-values. In SEM observation, it was also found that theparticle morphology of the resultant product is nearly the sameas that of the parent SBA-15 (SiO2 source). The resultant rod-like b-Sialon powders may provide the ceramic composites withan ideal enhancement material.

I. Introduction

SIALON ceramics have been recognized as a potential candidatefor engineering materials because of their excellent properties,e.g. high fracture toughness, high strength even at an elevatedtemperature, good corrosion resistance, and outstanding ther-mal shock resistance.1 b-Sialon, one of the most importantforms of SiAlON, is a solid solution of b-Si3N4 and has a gen-eral formula: Si6�zAlzOzN8�z (0ozo4.2).The b-Sialon ceramicswere typically prepared by the reaction sintering of mixtures ofSi3N4, AlN, and Al2O3 at temperatures higher than 15001C.2

But it is difficult to avoid forming a glassy phase in the grainboundary, which is generally not beneficial to the corrosion re-sistances and high-temperature strength of the SiAlON ceram-ics, by liquid-phase sintering.3 Therefore, the preparation ofpure b-Sialon powder and its sintering has received considerableattention, as it avoids the formation of a glassy phase in thegrain boundary.4

At present, b-Sialon powders mostly prepared mainly usingthe carbothermal reduction–nitridation (CRN) method. Becauseof the low cost of raw materials, the natural aluminosilicateminerals as raw materials for this process have been widely in-vestigated.5–7 But due to the problems of exactly controlling thecompositions, for example the Si/Al ratio, in the raw materials,it is difficult to obtain the products with tunable z-values. Sev-eral reports have described the synthesis of b-Sialon powders byheating carbon-containing powder mixtures of SiO2–Al2O3,

8

SiO2–Al2O3 � 2H2O,9 or powder mixtures of zeolite–carbonblack.4 In all these reports, the raw materials mixed by mechan-ical is insufficient for obtaining the expected pure b-Sialon.Therefore, the products always contained mullite, a-Si3N4,or Si2N2O as impurity phases. In order to synthesize high-

purity b-Sialon powder with tunable z-values, new methodsare still required.

The mesoporous silica SBA-15 with a larger pore size (nor-mally 4–6 nm) and a high surface area has been widely studied.10

Because of SBA-15’s larger pore size, carbon and aluminumspecies can be introduced easily into its nanosized channels.Hence, we can obtain good powder mixtures of SiO2–Al2O3–Cthat are well-proportioned in nanoscale by impregnating thecarbon and aluminum species into the channels of SBA-15.Moreover, it is possible to accurately adjust the Si/Al ratios inSiO2–Al2O3–C composites by controlling the nanocasting pro-cedure. In addition, it is well known that the morphology ofmesoporous silica SBA-15 is tunable by changing the reactionconditions.11 Thereby, it is possible to obtain a controllablemorphology of b-Sialon powder. So here we report the synthesisof highly pure b-Sialon powder using SBA-15 as the startingmaterial by a CRN method.

II. Experimental Procedure

According to the general formula of b-Sialon (Si6�zAlzOzN8�z,0ozo4.2), here we design three compositions (that is z5 1,2,3)of b-Sialon for preparation and designate the resultant productsas samples z-1, z-2, and z-3, respectively. A flow chart of thesynthesis procedure for b-Sialon powders is shown in Fig. 1.Detailed experimental procedures for z-3 powder as an exampleare described in the following:

Firstly, a high-quality SBA-15 sample was prepared followingthe synthesis procedure reported by Zhao et al.10

Secondly, nanocasting of SBA-15: Calcined SBA-15(1 g) wasadded to an aqueous solution obtained by dissolving anhydrousaluminum chloride (2.225 g), sucrose (1.5 g), and H2SO4 (0.17 g)in deionized water (5 mL). The resulting slurry was placed in adrying oven at 801C for 6 h, followed by a further treatment at1601C for 6 h. In order to further fill the pores of the SBA-15,additional sucrose (0.9 g), H2SO4 (0.1 g), and deionized water(5 mL) were added to the pretreated sample and the mixtureswere again subjected to the drying treatment described above.Then the carbonization was completed by pyrolysis with heatingtypically to 8001C in nitrogen flow, yielding the SBA-15/C/Al2O3 composite powders.

Finally, CRN: the process was carried out in a carbon tubefurnace by heating the SBA-15/C/Al2O3 composite powders to13701–14501C for 6 h in a nitrogen flow of 0.6 L/min, with aheating rate of 101C/min. The residual carbon was removed byburning the resultant powders at 7001C for 4 h.

Nitrogen adsorption–desorption isotherms were performedon a Micromeritics ASAP2010 system (Micromeritics, Nor-cross, GA) at �1961C. X-ray diffraction(XRD) patterns werecollected on a Rigaku D/MAX-g b instrument using Cu-Ka1(l5 0.15406 nm) radiation at 40 kV and 60 mA. Scanning

D. Thompson—contributing editor

wAuthor to whom correspondence should be addressed. e-mail: qianliu@sunm.shcnc.ac.cn

Manuscript No. 27329. Received January 5, 2010; approved March 19, 2010.

Journal

J. Am. Ceram. Soc., 93 [9] 2470–2472 (2010)

DOI: 10.1111/j.1551-2916.2010.03806.x

r 2010 The American Ceramic Society

2470

electron microscope (SEM) observations were carried out onJeol JSM-6700F (Sahnghai, China) working at 10.0 kV.

III. Results and Discussion

In the entire synthesis process, the nanocasting of SBA-15 is sig-nificant. So nitrogen adsorption–desorption isotherms of the par-ent SBA-15 and SBA-15/C/Al2O3 composite powders for all threesamples z-1,z-2,and z-3 were recorded to confirm the impregna-tion of carbon and aluminum species into the channels of SBA-15. The textural parameters of z-3 powder as an example aresummarized in Table I. As shown in Table I, the BET surfacearea(577 m2/g) and pore size (6.1 nm) of parent SBA-15 decreaseafter the nanocasting procedure by introducing carbon and alu-minum species into the nanosized channels, resulting in a lowerSBET value (359 m2/g) and a smaller pore size(2.6 nm) for theSBA-15/C/Al2O3 composite. If the silica template is further re-moved from SBA-15/C/Al2O3, the BET surface area and the poresize of SBA-15/C/Al2O3 composite increase rapidly to a highervalue for the resultant C/Al2O3 (721 m2/g). Such a phenomenonprovides considerable evidence of a successful impregnation ofcarbon and aluminum species into the SBA-15 channels. Fur-thermore, it is also found that the SBA-15/C/Al2O3 composite

powders still have an enough higher surface area and an efficientordered porous structure, which provides a high reaction activityfor the final formation of Sialon. The porosity in SBA-15/C/Al2O3 composites plays an important role in the transfer of ni-trogen gas to the reaction sites. Moreover, we could obtain SBA-15/C/Al2O3 composites with different component ratios by con-trolling the nanocasting procedure. All these results suggest thatthe composite powders prepared are highly suitable as raw ma-terials for the synthesis of high-purity b-Sialon.

Besides the nanocasting process, the calcination temperatureand time are also important parameters for the preparation ofhigh-purity b-Sialon powders. Figure 2 shows the XRD patternsof the products synthesized by firing SBA-15/C/Al2O3 compos-ite powder for the final sample z-3 at 13701, 14001, 14201, and14501C (all for 6 h), respectively. It has been found that, amongthe different batches of b-Sialon powders heat-treated at 13701–14501C for 6 h, the best one was that which underwent calcina-tion at 14201C, with the highest purity of b-Sialon phase. Hence,the temperature of 14201C was fixed as an optimal heat-treat-ment condition.

Figure 3 shows the XRD patterns of the as-prepared b-Sialonpowder samples calcined at 14201C with different z-values(z5 1, 2, 3). It is found that high-purity b-Sialon with differentz-values from SBA-15/C/Al2O3 composites was obtained suc-cessfully by the CRN process. As shown in Fig. 3, it can be seenthat the peaks of (a),(b), and(c) spectra can be assigned to thestandard diffraction peaks of b-Si5AlON7 (JCPDS:77-0755), b-Si4Al2O2N6 (JCPDS:76-0599), and b-Si3Al3O3N5 (JCPDS: 79-0483), respectively. It is also confirmed that z-values prospectedfrom raw materials are in accordance with the phase analysisresults from XRD patterns. That is, the SBA-15/C/Al2O3 com-posites are ideal raw materials for the synthesis of high-purityb-Sialon powders with various z-values. Comparatively, inprevious research reported by other authors, the b-Sialon

Table I. Textural Parameters of Parent SBA-15, SBA-15/C/Al2O3 Composite Powder for Final Sample z-3, and z-3 Powder

Samples SBET (m2/g) VBJH (cm3/g) Dpore (nm)

SBA-15 577 0.88 6.1SBA-15/C/Al2O3 359 0.23 2.6z-3 15.8 0.03 7.7

The average pore diameter(Dpore) coming from N2 sorption isotherms.

Fig. 1. Schematic drawing showing the synthesis of b-Sialon powder: (a) nanocasting of SBA-15, (b) carbothermal reduction–nitridation, (c) removingthe residual carbon.

Fig. 2. X-ray diffraction patterns of the products synthesized by firingSBA-15/C/Al2O3 composite powder for the final sample z-3 at(a) 13701C/6 h, (b) 14001C/6 h, (c) 14201C/6 h, and (d) 14501C/6 h.

Fig. 3. X-ray diffraction patterns of b-Sialon powder samples preparedwith different z-values (a) z-1, (b) z-2, and (c) z-3; all samples were heat-treated at 14201C.

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products, prepared either from carbon-containing powder mix-tures of SiO2–Al2O3

8 or from powder mixtures of zeolite–carbonblack4 as starting powder mixtures, generally contain mullite, a-Si3N4, or Si2N2O as impurity phases. The rather high contentsof impurity phases result mainly from two aspects. Because theSi, Al, and C species in these raw materials mixed mechanicallyare not distributed uniformly, SiO gas can be formed easily inthe area of Si,C-rich at around 14001C.2 And then, typically a-Si3N4 or Si2N2O phases are developed by the reaction betweenN2 and the as-generated SiO gas in the presence of carbon, asindicated in the following chemical reaction equations4:

SiO2 þ C! SiOðgÞ þ COðgÞ (1)

2SiOðgÞ þN2ðgÞ þ C! Si2N2Oþ COðgÞ (2)

3SiOðgÞ þ 2N2ðgÞ þ 3C! Si3N4 þ 3COðgÞ (3)

As contrasted with others, in our experiments, the Si, Al, andC components in the SBA-15/C/Al2O3 composites prepared bythe nanocasting procedure are distributed uniformly, and in ad-dition, the higher surface area and porosity of SBA-15/C/Al2O3

composites aid the transfer of nitrogen to the reaction sites, sothat the Si, Al, O, C, and N sources can react synchronously at amicrolevel. It can be understood reasonably that the well-dis-persed configuration prevents the formation of SiO gas, andhence in our process, the formation of a-Si3N4 or Si2N2O phasesare suppressed, yielding the high-purity b-Sialon powder.

The morphology of the powder products can be seen in theSEM observation, which are shown in Fig. 4, with images of theparent SBA-15 powders as the reference, the SBA-15/C/Al2O3

composite powders, and their resulting products. Figure 4 alsoprovides a rough illustration of the evolution process from theparent SBA-15 powders to the resultant b-Sialon powders. Fig-ure 4(a) shows the morphology of the SBA-15 powders, which isa bundle of rod-like particles with a diameter of around 1 mm.Figure 4(b) shows the morphology of the SBA-15/C/Al2O3 com-posite powders, which is similar to that of its parent SBA-15,whereas Fig. 4(c) demonstrates the morphology of the resultantsample z-3. It is found that the particle morphology of the re-sultant product is nearly the same as that for the parent SBA-15.This can be attributed to the fact that reactions in the innerchannels and outer surfaces occur simultaneously in the SBA-15/C/Al2O3 configuration. It is known that we can synthesizethe mesoporous silica SBA-15 with tunable morphologies bychanging the reaction conditions.11 In short, we can also controlthe morphology of b-Sialon powders by changing that of theparent SBA-15.

IV. Conclusion

In conclusion, a new kind of rod-like high-purity b-Sialon pow-ders was prepared by a CRN method under an optimal heat-treatment condition at 14201C for 6h, using mesoporous silicaSBA-15 as the starting materials. It was found that the incor-porated mesoporous SBA-15/C/Al2O3 composites prepared by ananocasting procedure not only have a higher surface area andordered porous structure but are also mixed uniformly, indicat-ing that the composites are ideal raw materials for the CRNsynthesis of high-purity b-Sialon powder with various z-values.Furthermore, it was also found that the particle morphology ofthe resultant product is nearly the same as that of the parentSBA-15, indicating that we can control the morphology of b-Sialon powder by changing that of the template SBA-15. Theresultant b-Sialon powders can be expected to find their use inpreparing high-performance Sialon ceramics or ceramic com-posites as an enhancement phase.

References

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Fig. 4. Scanning electron microscopic images of (a) SBA-15 powders, (b) SBA-15/C/Al2O3 composite powders for sample z-3, and (c) final b-Sialonpowders z-3.

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