XU Xiaohong, WANG Dongbin,2*, RAO Zhenggang, WU Jianfeng, ZHOU Yang

(1.State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, China; 2.School of Material Science and Engineering, Henan Polytechnic University, Jiaozuo 454000, China)

Abstract: Sialon/SiC composites were synthesized in situ from SiC, α-Si3N4, AlN, calcined bauxite,quartz and Y2O3 via layered buried sintering at different temperatures (1 540-1 640 ℃) . The results showed that the O’-sialon/SiC sample with 60 wt% silicon carbide sintered at 1 600 ℃ exhibited excellent mechanical properties, with apparent porosity of 16.01%, bulk density of 2.06 g·cm-3, bending strength of 52.63 MPa, and thermal expansion coefficient of 5.83×10-6 ℃-1 . The oxide film formed on the surface was linked closely to O'-sialon, so the oxide film was not easily broken. After 100 h oxidization, the sample surface was smoother and denser, with oxidation weight gain rate 23.6 mg/cm2 and oxidation rate constant 2.0 mg2·cm-4·h-1. Therefore,the sample had the excellent high-temperature oxidation resistance. It was confirmed that the in-situ sialon/SiC composites could be a promising candidate for solar absorber owing to its high-temperature oxidation resistance.

Key words: sialon/SiC composites; high-temperature oxidation resistance; solar absorber

1 Introduction

As one of the most important components of a solar thermal tower plant (STTP) system, solar absorber materials are designed to receive concentrated solar energy, which determine the power generation efficiency of the system. The high-temperature resistance (instantaneous temperature up to 1 000 ℃)is essential for the solar thermal power generation[1,2].As reported by Hoffschmidtet al[3,4], metal fiber mesh,siliconized silicon carbide (SSiC) foam, clay bound silicon carbide (CBSiC) foam and cordierite foam are candidate materials for solar absorber.

Silicon carbide (SiC) is an important hightemperature structural material. SiC has excellent mechanical properties and thermal performance, such as high strength, high hardness, high temperature resistance, low thermal expansion coefficient, high heat conductivity and good chemical stability. Thus SiC can be considered as an ideal candidate for solar absorber[4].However, the strong covalent bond and high sintering temperature (> 2 100 ℃) of SiC hamper the application of SiC as solar absorber[5].

Sialon ceramics is a kind of Si-Al-O-N hightemperature structural material, which was reported by Riedel R[6]and Oynamaet al[7]in the early 1970s.Sialon retains the excellent properties of Si3N4with high strength, hardness and good heat resistance as well as good toughness, chemical stability and oxidation resistance.

In recent years, sialon and SiC composites have attracted attention.The improvement of the addition of highly conductive SiC particles in the thermal diffusivity behaviour of sialon ceramics was investigated[8]. Mullite and corundum co-bonded SiC ceramics, Al2O3/SiC composite ceramics and SiC-bonded cordierite-mullite were prepared for solar thermal storage[9-11]. Mullite/SiC ceramics, cordierite/SiC ceramics and mullite-bonded Si3N4/SiC ceramics were prepared for solar heat absorber[12-14].

The oxidation resistance behavior ofβ-sialon/SiC composites refractory manufactured by carbonthermal-reduction reaction and nitridation reaction of clay mineral with the addition of SiC particle was investigated[15]. However, the thermal shock resistance,high-temperature oxidation resistance, softening temperature, thermal conductivity and solar radiation absorption rate of composites should be improved for the requirements of solar absorber materials.

Compared with other composite materials, sialon/SiC composites have good mechanical properties,thermal shock resistance and high-temperature oxidation resistance. In this work, sialon/SiC composite ceramics for using as solar absorber were fabricated from SiC,α-Si3N4, AlN, calcined bauxite, quartz and Y2O3. The microstructure, physical properties and oxidation resistance of the composites were investigated.

2 Experimental

Commercially available SiC (about 61 μm,Danjiangkou Hongyuan Silicon Carbide Co., LTD,China, 99.48%), SiC (about 20 μm, purity 99%,Dongguan Baiter Grinding Materials Co., Ltd, China),α-Si3N4(about 5-7 μm, purity 99.5%, Gaoyi Fudang Special Refractory Materials Co., Ltd, China), AlN(about 5-7 μm, purity 99.5%, Qingzhou Maite Materials Co., Ltd, China), Y2O3(purity 99.9%, Sinopharm Chemical Reagent Co., Ltd, China), calcined bauxite(about 50 μm, Xinmi, China), and quartz(about 40 μm, Yingde, China) were used as raw materials. The corresponding chemical compositions are listed in Table 1. The batch formulaes are shown in Table 2. The target product of C1, C2 and C3 areβ-sialon/SiC, O’-sialon/SiC, and β/O’-sialon/SiC, respectively.

The starting materials powders were dry-mixed by ball milling for 0.5 h (ball-to-powder mass ratio is 2:1). The mixed powders with 5 wt% polyvinyl alcohol solutions as binder were pressed under 60 kN at room temperature to form cylindrical green bodies (Φ30×5 mm) or pressed under 50 kN to form cubical green bodies (36× 6.5×6.5 mm) . Thereafter, the green bodies were dried at 100 ℃ for 24 h. The green bodies were layered buried sintered at different temperature (1 540-1 40 ℃) with an interval of 20 ℃ with a hold period of 3 h. The schematic diagram is shown in Fig.1, in which the samples were buried in graphite and Si3N4powders to avoid oxidation.

Fig.1 Schematic diagram of the layered buried sintering

Apparent porosity and bulk density were tested using a digital display ceramic water absorption apparatus (Model TXY, Xiangyi Machine Co.,Ltd., Xiangtan, China) and an electronic analytical balance (Model AUY120, Japan). Bending strength was measured by an electronic universal testing machine (Model RGM-4100, Reger Instrument Co.,Ltd., Shenzhen, China) through three-point bending method. Phase compositions of sintered samples were characterized by X-ray diffraction (XRD) (Model D/ Max-IIIA, Rigaku., Japan). Microstructures were observed using a scanning electron microscope (SEM)(Model JSM-5610LV, Jeol., Japan). The microstructures and chemical compositions were characterized using an electron probe microanalysis (EPMA) (Model JXA 8230/INCAX-ACT, Jeol., Japan).

Oxidation resistance tests were carried out with the cylindrical samples. Firstly, the samples were weighed and measured to calculate the surface area.Afterwards, the samples were heated to 1 300 ℃ and held for scheduled time in the furnace. The specific weight gain was examined to evaluate the hightemperature oxidation resistance of the samples.

Table 1 Chemical compositions of raw materials /wt%

Table 2 Batch formulaes of the sample

3 Results and discussion

3.1 Phase identification

Figs.2-4 shows the XRD patterns of C1-C3 samples sintered at different temperatures.The phase compositions of C1 samples were SiC,α-Si3N4,β-Si3N4, corundum,β-sialon and O’-sialon.The phase compositions of C2 samples were SiC,β-Si3N4, corundum, O’-sialon andβ-sialon. The phase compositions of C3 samples were SiC,α-Si3N4,β-Si3N4, corundum,β-sialon, O’-sialon and X-sialon.

Fig.2 XRD patterns of C1 samples sintered at different temperatures

Fig.3 XRD patterns of C2 samples sintered at different temperatures

There were characteristic peak of O’-Sialon in the XRD patterns of C2 samples sintered at different temperatures, which showed that it is feasible to fabricate O’-sialon/SiC composites. As shown in Fig.3, a lot of O’-sialon generated at 1 540 ℃ and the contents slightly changed with the increase of sintering temperature. O’-sialon is the main phase and the content ofβ-Sialon is always very small. The synthesis temperature of O’-Sialon is lower than that ofβ-Sialon and O’-Sialon can be generated at 1 300 ℃[16].

Fig.4 XRD patterns of C3 samples sintered at different temperatures

Fig.5 shows the XRD patterns of O’-sialon and C2 samples sintered at 1 600 ℃. The main phases of O’-sialon samples were O’-sialon, Si2N2O, Si3N4,α-SiO2and mullite. With the addition of SiC, Si2N2O and mullite disappeard, andβ-sialon was generated. It is because the change of reaction system may cause the decomposition of Si2N2O[17]:

The mullite can be nitrided intoβ-sialon by N2. The escape of gas generated by the reaction will increase the porosity. The synthesis of O’-sialon is inhibited, which reduces the load capacity of the materials. This two reasons lead to the decrease of bending strength.

3.2 Microstructure

Fig.5 XRD patterns of samples O’-sialon and C2 sintered at 1 600 ℃

Fig.6 shows the microstructures of fractured surfaces of samples O’-sialon and C2 sintered at 1 600 ℃ for 3 h. In Fig.6(a) and 6(c), there was no significant change between the number and size of pores in O’-sialon/SiC (C2 samples) and in O’-Sialon.The apparent porosities of O’-Sialon and C2 samples were 31.60% and 33.00%, respectively. As shown in Fig.6(b), there were many hexagonal O’-sialon grains in O’-sialon samples, which were grown from spherical blossom-shaped grains and benefited for the bending strength of sample. The SiC particles during sintering does not interfere with the chemical composition of the main phase (α-sialon)[18]. In other words, the presence of SiC does not change the solubility of theα-Si3N4phase in the liquid phase. However, the SiC addition to Si3N4–AlN–Y2O3inhibited the elongated grain growth of the sialon phase. In C2 samples, there were blossomshaped O’-sialon grains adhering to the surface of the particles, indicating that the SiC addition inhibited the grain growth of the O’-sialon phase. Therefore, SiC addition increases the apparent porosity and inhibits the grain growth of the O’-sialon phase. The combined effects of two aspects decrease the bending strength from 89.51 to 52.36 MPa.

Fig.6 Fracture morphologies of samples sintered at 1 600 ℃: (a,b) sample O’-sialon; (c, d) sample C2(corroded by 5 wt%HF solutions for 90 seconds)

The apparent porosity(Pa), bulk density(D)and bending strength are the important parameters to evaluate the densification and mechanical property of sintered compacts. Fig.7 shows the relationships betweenPa,D, bending strength and sintering temperatures of the samples. At 1 600 ℃ sintering temperature, C1-C3 samples possessed the highest bulk density of 2.21, 2.06 and 2.19 g·cm-3, respectively.With the increasing of sintering temperature, more liquid phase is generated, which fills the pores.Meanwhile, the liquid phase accelerates the mass transfer and deposition, crystal nucleation and growth. ThusPareduces quickly andDincreases evidently(Fig.7(a), 7(b)).

Fig.7 Relationships between (a) apparent porosity, (b) bulk density, (c) bending strength and sintering temperature of series C

At 1 640 ℃, the bulk density of samples C1-C3 decreased to 2.16, 2.00 and 2.10 g·cm-3, respectively.If the sintering temperature was further increased,abnormal grains overgrowth increased apparent porosity and large amounts of high-temperature lowviscosity liquid phase formed excess glass phase,resulting in the decrease of volume density.

3.3 High-temperature oxidation resistance

3.3.1 Appearance of oxidized samples

Fig.8 shows the photograph of series C after different oxidation time. After oxidized at 1 300 ℃ for 30 h, brown glass appeared and covered the surface of samples C1 and C3. With the increase of oxidation time, brown glass arised and some bubbles appeared.This phenomenon was due to incomplete oxidation of SiC and Si3N4. After oxidation tests, the surface of C2 samples were smooth and there was no bubbles on the surface. Therefore, O’-Sialon/SiC possesses better high-temperature oxidation resistance than other 2 samples.

Fig.8 Photograph of series C after different oxidation time

In the initial oxidation stage, oxide film was formed on the sample surface. With increasing oxidation time, oxide film was gradually thickened and inhibited the diffusion of oxygen. Due to continuous consumption of oxygen, the internal oxygen partial pressure becomes smaller. At high temperature and low oxygen partial pressure, SiC and Si3N4are incompletely oxidized into SiO (g), CO (g) and NO (g). When NO (g) escapes from the interior of the sample, it is oxidized to NO2(g), so the samples surface become brown.

3.3.2 Oxidation rate constant

Oxidation rate constant can be revealed by Eq.(2)19]:

where∆m/Sis weight gain per unit area(mg/cm2),Kis oxidation rate constant(the value ofKis smaller, the slower oxidation rate),tis oxidation time(h), andnis constant (n=1, linear rule;n=2, parabolic law;n=3,Cubic law).

Table 3 Rate constant (K), fitted coeffiecient of oxidation reaction (r) of series C sintered at 1 640

Table 3 Rate constant (K), fitted coeffiecient of oxidation reaction (r) of series C sintered at 1 640

Sample No. K/(mg2·cm-4·h-1) r C1 1.854 5 0.966 4 C2 2.011 9 0.853 9 C3 1.404 1 0.898 2

Table 4 Element compositions at the given point in Fig.10/wt%

Fig.9 Relationships of weight change of series C and oxidation time

Fig.9 and Table 3 show the results of weight gain rate and rate constant (K)of series C sintered at 1 600℃ after different oxidation time. After oxidizing 100 h, sample C1 had minimum oxidation weight gain rate of 16.3 mg/cm2and sample C3 had minimum oxidation rate constant of 1.4 mg2·cm-4·h-1. During oxidation process, a large number of gas escaped from samples C1 and C3 and some bubbles appeared.Thus the oxidation weight gain rate and oxidation rate constant was smaller than the actual value. Therefore,sample C2 has the optical high-temperature oxidation resistance, with oxidation weight gain rate 23.6 mg/cm2and oxidation rate constant 2.0 mg2·cm-4·h-1.

Fig.10 Back-scattered electron images, elements distribution and EDS spectra of given points of sample C1 before oxidation

Fig.11 Back-scattered electron images, elements distribution and EDS spectra of given points of sample C1 after 100 h oxidization

As Fig.9 shows, the samples mass increased sharply in early stage of oxidation. As the oxidation continued, the weight gain reduced gradually. In this stage, oxidation reaction is controlled by the diffusion of oxygen. Among O’-sialon,β-sialon and SiC, the oxygen content of O’-sialon is the highest. So the oxygen partial pressure of O’-sialon is the highest and it is difficult for oxygen to diffuse. In addition, the oxide film formed on the surface is linked closely to O’-sialon, so the oxide film is not easily broken[20].The oxidation resistance of O’-sialon is better thanβ-sialon.

3.3.3 Microstructure

Figs.10 and Fig.11 show the back-scattered electron images, elements distribution and EDS spectra of given points of sample C1 before and after oxidation.In Fig.10, there are a lot of Si, Al, O and C elements in sample section. N and Y elements are distributed evenly. Large particles of SiC grains are wrapped by sialon. Table 4 shows the element compositions at the given point in Fig.10. The main crystal phase are SiC andβ-sialon before oxidation.

In Fig.11, there is an approximate 30 μm oxide layer on the surface of the sample. Si and Al are enriched in the oxide layer. Table 5 shows the element compositions at the given point in Fig.11. So, the products of oxidation reaction are mullite and quartz.In the oxidation tests, the following reactions may occur[21]:

Table 5 Element compositions at given point in Fig.11/wt%

After oxidation, the sample surface was smoother and denser. Antioxidant mechanistic studies suggest that in the initial oxidation SiC, Sialon and Si3N4in surface are oxidated into mullite, quartz and glass phase, which form the oxidation film and restrain the interfacial reaction. There is remarkable weight gain in the interfacial reaction stage. As the experiment goes on, the oxidation film becomes thick and the internal oxygen partial pressure is reduced. So Si3N4is incompletely oxidated into NO(g). When NO(g)escapes from the interior of the sample, it is oxidized to NO2(g).The escape of large amounts of gas generates bubbles on the surface of samples C1and C3. The high oxygen partial pressure in sample C2 inhibits oxygen diffusion.Meanwhile, O’-sialon is linked closely to the oxide film formed on the surface. So sample C2 has the optical oxidation resistance.

4 Conclusions

Sialon/SiC composite ceramics were fabricated by layered buried sintering process. The results showed that sample O’-sialon/SiC with the ratio of 4:6 sintered at 1 600 ℃ had better high-temperature oxidation resistance than other samples. The sample had apparent porosity of 16.01%, bulk density of 2.06 g·cm-3,bending strength of 52.63 MPa and thermal expansion coefficient of 5.83×10-6℃-1. In active oxidation stage, SiC, Sialon and Si3N4in surface were oxidated into mullite, quartz and glass phase, which formed the oxidation film and restrained the interfacial reaction.There was remarkable weight gain in the interfacial reaction stage. In passive oxidation stage, the oxidation film inhibited the diffusion of oxygen and decreased oxygen partial pressure, resulting in the incomplete oxidation of Si3N4into NO(g). When NO(g)escape from the interior of the sample, it is oxidized to NO2(g). The escape of large amounts of gas generates bubbles on the surface of samples C1 and C3. The oxygen content in O’-sialon is high, and the oxidation film will inhibit the diffusion of oxygen more effectively. Therefore,sample C2 had the optimal high-temperature oxidation resistance.