carbothermal reduction

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Preparation and characterization of ZrB2-SiC composite powders from zircon via microwave-assisted boro/carbothermal reduction Xiangong Deng, Shuang Du, Haijun Zhang, Faliang Li, Junkai Wang, Wanguo Zhao, Feng Liang, Zhong Huang, Shaowei Zhang

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Cite this article as: Xiangong Deng, Shuang Du, Haijun Zhang, Faliang Li, Junkai Wang, Wanguo Zhao, Feng Liang, Zhong Huang, Shaowei Zhang, Preparation and characterization of ZrB2-SiC composite powders from zircon via microwave-assisted boro/carbothermal reduction, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2015.07.077 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Preparation and characterization of ZrB2-SiC composite powders from zircon via microwave-assisted boro/carbothermal reduction Xiangong Deng1, Shuang Du1,2, HaijunZhang1*, Faliang Li1, Junkai Wang1, Wanguo Zhao1, Feng Liang1, Zhong Huang1, Shaowei Zhang1* 1 The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China 2 Refractory Co. Ltd., Wuhan Iron and Steel (Group) Corp., Wuhan 430081, China

Abstract: ZrB2-SiC composite powders were successfully synthesized via microwave-assisted boro/ carbothermal reduction technique using zircon (ZrSiO4), activated carbon (C) and boron oxide (B2O3) as raw materials. They were characterized by using X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), energy-dispersive spectroscopy (EDS) and transmission electron microscopy (TEM). Thermodynamic analysis on the synthesis process was carried out, and the effects of SiC bedding powder, reaction temperature, soaking time and n(B2O3):n(ZrSiO4) molar ratio on the formation of ZrB2-SiC composite powders were examined. The results showed that phase pure ZrB2-SiC composite powders were successfully synthesized at 1573 K which was 200 K lower than that required by using the conventional boro/carbothermal reduction method. In the final composite powders, fibrous SiC phases with 0.1~0.5 μm in diameter and 1.4~4.2 μm in length were homogeneously distributed among spherical ZrB2 particles with the average size of 0.1~1 μm. Keywords: ZrB2-SiC; Composite powders; Boro/carbothermal reduction; Microwave; Zircon

*Corresponding author: Prof. Haijun Zhang, E-mail: [email protected]; Prof. Shaowei Zhang, E-mail: [email protected]

1

1. Introduction Because of their high melting point, high chemical stability, moderate thermal and electrical conductivities, good thermal shock resistance and excellent mechanical properties [1,2], ZrB 2-SiC based ultra-high temperature ceramics (UHTCs) are potentially applicable to many important areas such as in plasma-arc electrodes, hypersonic flights, rocket propulsion systems, and many other atmospheric re-entry vehicles [3-5]. During the past few years, several techniques and methodologies have been developed to prepare ZrB2-SiC composites, including mechanical mixing[6,7], sol-gel processing [8-10], self-propagating high-temperature synthesis (SHS) [3,11], spark plasma sintering (SPS) [12], and boro/carbothermal reduction method [12-14]. Of these, the boro/carbothermal reduction method is the most extensively used [12-17]. Nevertheless, it still suffered several disadvantages, e.g., 1) requirements of high heating temperature (1773~1973K) and long heating time [13,15,16], 2) use of expensive starting materials (such as ZrO2, B4C) and thus high cost of the final products [13,16], and 3) poor purity of the final product powders [3,13]. To address these problems it is necessary to develop an alternative technique. Microwave heating technique exhibits several main advantages over conventional heating techniques/processes, including: 1) a relatively low formation temperature for a target product [18], 2) more uniform heating (at molecular level) on starting materials [19], 3) higher heating and cooling rates, shorter reaction time and higher energy efficiency, as a result of heat generated inside the sample itself owing to the intrinsic nature of microwaves, and 4) improved homogeneity in the final product and relatively high yield [20,21]. In the past few years, microwave heating has been widely used for preparing a range of materials [22-26]. In our group, several types of ultrafine non-oxide powders, e.g., phase pure ZrB2-SiC [24], TiB2 [25] and TiC [26] ultrafine powders, have been successfully synthesized by using the microwave heating 2

technique at a much lowered temperature (at least 200 K lower than required by the conventional heating method). Zircon (ZrSiO4), as a relatively cheap natural raw material containing both zirconium and silicon, is widely used by the refractory and ceramic industries. Also, it has been used to synthesize ZrB2-SiC composite powders [3, 12, 27-28]. For example, Jalaly et al. [27] synthesized ZrB2-based composite powders from ZrSiO4 via a high-energy ball milling process. Their results showed that a considerable amount of ZrB 2 and a small amount of Mg2B2O5 and Mg2SiO4 were formed at 1673 K, whereas carbon did not lead to the formation of any carbide phase. Oh et al. [12] prepared ZrB2-SiC powders at 1773 K via a two-step carbo-borothermal reduction process using ZrSiO4, B4C and C as raw materials. Ryu et al. [3] synthesized ZrB2-based composite powders via a combustion synthesis process using ZrSiO 4, Mg, C, B and NaCl as raw materials. ZrB2-SiC-ZrC and ZrB2-SiC-ZrC-ZrSi composite powders were obtained respectively at 1763K and 1943K. Although ZrB2-SiC composite powders could be prepared by using the above mentioned studies, two important issues still remained, i.e., remaining of high levels of impurities in the final products and requirement of a high firing temperature (1673~1943 K). In this work, ZrB2-SiC composite powders were prepared from zircon, activated carbon and boron oxide by using a microwave boro/carbothermal reduction technique, taking advantage of the unique property of microwave heating and the relatively low cost of natural zircon. The effects of processing parameters such as bedding powder, heating temperature, soaking time and the molar ratio of n(B2O3):n(ZrSiO4) on the synthesis of ZrB2-SiC composite powders were investigated systematically. To the best of our knowledge, this is the first report on the preparation of pure ZrB2-SiC composite powders at a relatively low temperature by using such a simple method.

2. Experimental procedure 2.1 Sample preparation 3

Commercial zircon powder (ZrSiO4, Australian, d50≈ 21.4 μm, the chemical compositions were given in Table 1), B2O3 (≥98%, Bodi Chem. Co. Ltd., Tianjin, China) and activated carbon (C, Guoyao Chem. Co. Ltd., Shanghai, China, d50≈37.0 μm) were used as the starting materials. These starting materials were mixed in the stoichiometric ratios indicated by reaction (1) in a planetary ball mill containing Al2O3 balls for 4 h at 300 rpm. The resultant powders were pressed under 150 MPa to form pellets with 20 mm in diameter. The pellets were embedded in SiC powders placed in a alumina crucible (covered by alumina hollow sphere and polycrystalline mullite fiber insulation materials) since it is well known that SiC has excellent microwave absorbing property and high thermal conductivity [29,30], and an argon-protected microwave furnace (Model: MW-L0316V, 3 kW, 2.45 GHz, size of rotating platform: Φ340 mm, size of furnace cavity: Φ500 ×560 mm, by Changsha Longtech Co. Ltd, Hunan province, China) was used for ZrB 2-SiC composite powder preparation. The schematic diagram of the microwave heating system is shown in Fig. 1. The power supply of microwave furnace can be varied from 0.3 to 2.85 kW and the water-cooled magnetron operated at a frequency of 2.45 GHz. The surface of SiC bedding powders measured by an infra-red temperature measurement system was regarded as the synthesis temperature of sample. The furnace was heated at 20 K/min to a temperature between 1373 and 1573 K and held for 3 h before being cooled to room temperature. The fired samples were dispersed in deionized water and subjected to centrifugation to remove impurity phases such as B2O3 and C. Finally, the ZrB2-SiC composite powders were obtained upon drying the powders collected from the centrifuge at 383 K for 24 h. ZrSiO4(s)+B2O3(s)+8C(g)=ZrB2(s)+SiC(s)+7CO(g)

(1)

Δ r G1θ =205.750－1.14T (kJ  mol1 )

2.2 Characterization Crystalline phases in as-prepared samples were identified by powder X-ray diffraction (XRD) analysis using a Philips X’Pert PRO diffractometer (PANalytical, NETHER-LANDS, 40 kV, 40 4

mA). Spectra between 15 and 80 (2) were recorded at 40 mA and 40 kV using Cu Kα radiation ( = 0.1542 nm). The scan rate was 2/min with a step of 0.05. ICDD cards No. 89-3930, 73-1708, 65-8837, 83-0944, 79-1771, and 83-1378 were used for the identification of ZrB2, SiC, ZrC, m-ZrO2, t-ZrO2 and ZrSiO4, respectively. The Rietveld refinement method was used to calculate the contents of the crystalline phases in the final samples. Microstructures and morphologies of as-prepared samples were observed by using a field emission scanning electron microscope (FE-SEM, Nova400NanoSEM, PHILIPS, NETHER LANDS, 15 kV) and a transmission electron microscope (TEM, JEM–2100UHRSTEM, JEOL, JAPAN, 200 kV). The samples for SEM were coated with gold, and those for TEM were prepared by ultra-sonic dispersion of the powdered samples in EtOH, followed by dropping and drying the suspension onto a holey carbon film. Energy-dispersive spectroscopy (EDS) was used for assisting phase identifications in the samples.

3. Results and discussion 3.1 Thermodynamic analysis on the ZrB2-SiC formation According to the ZrO2-SiO2 phase diagram [31], pure ZrSiO4 is stable up to 1960 K above which it decomposes into ZrO2 and SiO2. However, when impurity phases (e.g., Fe2O3, K2O, Na2O and B2O3) were present, the actual decomposition temperature of ZrSiO4 would be reduced to various extents [31]. Based on these and the general mechanism of a boro/carbothermal reduction process, the main possible reactions in the present case can be described as follows. ZrSiO4(s)=ZrO2(s)+SiO2(s)

(2)

Δ r G2θ =26.80－0.013T (kJ  mol1 ) ZrO2(s)+B2O3(l)+5C(s)=ZrB2(s)+5CO(g)

(3)

Δ r G3θ =1420.80－0.80T (kJ  mol1 )

SiO2(s)+3C(s)=SiC(s)+2CO(g)

(4) 5

Δ r G4θ =603.15－0.33T (kJ  mol1 )

ZrO2(s)+3C(s)=ZrC(s)+2CO(g)

(5)

Δ r G5θ =666.55－0.35T (kJ  mol1 ) The values of lg(PCO/Pθ) (PCO and Pθ represent the pressure of CO in reaction system and standard atmospheric pressure, respectively) at different temperatures could be obtained from the ΔrG values in equations 3-5. To show the equilibrium relationship in the Zr-B-Si-C-O system, log(PCO/Pθ) vs. T was plotted based on the thermodynamic data corresponding reactions 3-5. The obtained stable region diagram of condensed phases (Fig. 2) indicates that at a given PCO with increasing the temperature, ZrSiO4 is initially changed to ZrB2 and SiO2, then to ZrB2 and SiC, and finally to ZrC and SiC. On the other hand, at a given temperature, with decreasing PCO the stable phases are changed from ZrO2-SiO2-C-B2O3 to ZrB2-SiO2-C, then to ZrB2-SiC-C, and finally to ZrC-SiC-C. These results predict theoretically that ZrB2-SiC composite powders could be synthesized using zircon, B2O3 and C as starting materials at appropriate temperatures and PCO. To further predict the reaction products at different temperatures in the ZrSiO4-B2O3-C system, thermodynamic calculations were also performed by using the FactSage 6.2 software (Center for Research in Computational Thermochemistry, Montreal, Canada). In this case, thermodynamically most stable phases and their relative contents at given temperature can be predicted. Fig. 3 shows the predicted equilibrium phases at different temperatures for the ZrSiO4-B2O3-C system with C:B2O3:ZrSiO4=8:1:1(molar ratio). B2O3 starts to appear as a liquid phase and a gas phase at 675 K and 1680 K, respectively (Fig. 3b). Under the effect of liquid B2O3, ZrSiO4 starts to decompose into monoclinic ZrO2 (m-ZrO2) and SiO2 at about 725 K, and finally completely decomposes upon increasing the temperature to 1675 K (Fig. 3c and Fig. 3d). The formed m-ZrO2 transforms into tetragonal ZrO2 (t-ZrO2) at 1475-1575 K. Upon increasing the temperature to 1675 K, ZrB2 and SiC are formed presumably from reactions 3 and 4, i.e., the boro/carbothermal reduction occurs (Fig. 3c). On increasing the temperature to 1775 K, ZrB2 and SiC remain as the stable crystalline phases, 6

suggesting the completion of the boro/carbothermal reduction at this stage (Fig. 3c-3d). On the other hand, a certain amount of SiO (Fig. 3d) is also formed from the partial oxidation of SiC by CO. On further increasing the temperature to 1780 K, ZrC also appears as a stable phase (Fig. 3c). The thermodynamic calculation results via FactSage further confirm that ZrB2-SiC composite powders can be synthesized at appropriate temperatures by using ZrSiO4, B2O3 and C as raw materials.

3.2 Effect of SiC bedding powder on the synthesis of ZrB2-SiC composite powders Fig. 4 illustrates XRD patterns of samples fired at 1573 K for 3 h with or without SiC bedding powder. For the sample without SiC bedding powder, ZrB2 was the primary crystalline phase, along with minor ZrO2 and ZrSiO4, but no SiC was detected, indicating that the boro/carbothermal reduction had occurred but not been completed. In comparison, only ZrB2 and SiC peaks were detected in the sample with SiC bedding powder, indicating that their formation reactions had been completed. These results indicated that the SiC bedding powder greatly promoted the synthesis of ZrB2-SiC composite powders. This can be attributed to the excellent microwave absorbing property of SiC bedding powder, which led to a uniform heating on the starting materials and thus the accelerated formation of ZrB2 and SiC [30].

3.3 Effect of heating temperature on the synthesis of ZrB2-SiC composite powders Firing temperature is an important parameter for the preparation of ZrB2-SiC composite powders. A series of ZrB2-SiC composite powders were prepared at various temperatures by using the present microwave boro/carbothermal reduction method. Fig. 5a shows XRD patterns of samples with the molar ratios of n(C):n(B2O3):n(ZrSiO4)=10:1.5:1 after 3 h firing at 1373~1573 K. At 1373 K, ZrSiO4 remained as the main crystalline phase, along with minor ZrO2 and ZrB2, indicating that the microwave boro/carbothermal reduction had occurred. However, no SiO2 peaks 7

were detected, suggesting that SiO2 might be present in a borosilicate glass or as an amorphous phase before the formation of SiC [12]. At 1473K, ZrB2 peaks evidently increased whereas ZrSiO4 peaks decreased. However, it was still hard to confirm the formation of SiC in the final product due to the overlapping between the strongest peak ((111) reflection) of SiC and that of ZrSiO4 (2θ=35.6°) [32]. On increasing the temperature to 1523 K, ZrB2 became the primary phase with the disappearance of ZrSiO4. Meanwhile, minor t-ZrO2 and SiC peaks were still present in the sample, suggesting the completion of the phase transformation of ZrO2 and the occurrence of the carbothermal reduction between SiO2 and C. On further increasing the temperature to 1573 K, only ZrB2 and SiC phases were detected, indicating the microwave boro/carbothermal reduction has been completed at this temperature and phase pure ZrB2-SiC composite powders were successfully obtained. In comparison, Krishnarao et al. [13] synthesized ZrB2-SiC composite powders by a boron carbide reduction method using ZrO2, B4C and Si as raw materials. Their results showed that ZrB2-SiC-B4C and ZrB2-SiC-ZrO2 composite powders were obtained at 1773 K and 1973 K, respectively [13]. On the other hand, we also prepared ZrB2-SiC composite powders from ZrSiO4, B2O3 and C starting materials by using the conventional heating method. Our results showed that phase pure ZrB2-SiC composite powders were synthesized at 1773 K [15]. Therefore, it can be concluded that the present microwave-assisted boro/carbothermal reduction technique can greatly lower the formation temperature of ZrB2 and SiC. The contents of crystalline phases in the samples fired at various temperatures for 3 h were calculated by using the Rietveld refinement method. As shown in Fig. 5b, the contents of ZrB2-SiC increased whereas those of ZrSiO4 and ZrO2 decreased with increasing the firing temperature. At 1373 K, the relative contents of ZrSiO4, ZrB2 and ZrO2 were about 81 wt%, 5 wt% and 14 wt%, respectively. On increasing the temperature from 1473 to 1523 K, the content of ZrB2 increased rapidly from about 58 to 81 wt% and that of SiC increased from 0 to 15 wt%. No ZrSiO4 was left in the sample. On further increasing the temperature to 1573 K, the contents of ZrB2 and SiC increased 8

to 72 wt% and 28 wt%, respectively. The weight ratio of ZrB2 to SiC was close to the theoretical value (74/26) calculated based on reaction (1). These results further confirmed that phase pure ZrB2-SiC composite powders were successfully prepared from natural ZrSiO4 raw material at 1573 K for 3 h by the present microwave boro/carbothermal reduction method.

3.4 Effect of soaking time on the synthesis of ZrB2-SiC composite powders Fig. 6a presents XRD patterns of samples fired at 1573 K for various hours. When the soaking time was 1 h, ZrB2 was formed as the main crystalline phase, along with some ZrSiO4 and ZrO2, but no SiC was detected, indicating that the boro/carbothermal reduction had occurred. Up increasing the soaking time to 3 h, only ZrB2 and SiC were detected, indicating that the formation process of phase pure ZrB2-SiC composite powders had been completed. Further increasing the soaking time to 4 h, the crystalline phases in final sample remained unchanged, suggesting that a soaking time longer than 3 h is unnecessary for the preparation of ZrB2-SiC composite powders by using the present method. The relative contents of crystalline phases in the samples prepared with various soaking times were also calculated by the Rietveld refinement method. As shown in Fig. 6b, when the soaking time was 1 h, the content of ZrB2 was only 47 wt%, and no SiC was formed. Upon increasing the soaking time to 3 h, the contents of ZrB2 and SiC were about 72 and 28 wt%, respectively. On further increasing the soaking time from 3 h to 4 h, the content of ZrB2 increased from 72 to 80 wt%, whereas that of SiC decreased from 28 to 20 wt%. The decrease in the SiC content might arise from the loss in Si associated with the SiO volatilization [33], which agreed well with the thermodynamic prediction shown in Fig. 4.

3.5 Effect of B2O3 content on the synthesis of ZrB2-SiC composite powders The content of B2O3 is another important parameter greatly affecting the preparation of 9

ZrB2-SiC composite powders considering its high vapor pressure and rapid vaporization at a high temperature. XRD patterns and relative contents of crystalline phases of the final products prepared at 1573 K for 3 h with various content of B2O3 are shown in Fig. 7. When the molar ratio of n(B2O3):n(ZrSiO4) was 1 (i.e. theoretically ratio), ZrB2 and SiC were formed as the main crystalline phases, along with some ZrC. The formation of ZrC was related to the insufficient B2O3 due to the evaporation loss of B2O3 at the firing temperature. On increasing the molar ratio of n(B2O3):n(ZrSiO4) to 1.5, only ZrB2 and SiC peaks were identified, indicating that the boro/carbothermal reduction process had been completed. On further increasing the molar ratio of n(B2O3):n(ZrSiO4) to >1.5, there were no obvious changes in the ZrB2 and SiC phases, indicating that it was not necessary to use so much B2O3. . The contents of crystalline phases in the samples with various molar ratios of n(B2O3):n(ZrSiO4) were also calculated by the Rietveld refinement method. As shown in Fig. 7b, in the sample with the molar ratio of n(B2O3):n(ZrSiO4)=1, 62 wt% ZrB2 and 27 wt% SiC were formed along with about 11 wt% ZrC. When the molar ratio of n(B2O3)/n(ZrSiO4) was 1.5, the contents of ZrB2 and SiC increased respectively to about 72 wt% and 28 wt%. The weight ratio between ZrB2 and SiC (72/28) was very close to the theoretical one (about 74/26) calculated based on the composition of starting ZrSiO4. On further increasing the molar ratio of n(B2O3):n(ZrSiO4) from 1.5 to 2.5, there were no obvious changes in the contents of ZrB2 and SiC, indicating that the molar ratio of n(B2O3):n(ZrSiO4)=1.5 was sufficient for the synthesis of phase pure ZrB2-SiC composite powders.

3.6 Microstructure of as-prepared ZrB2-SiC composite powders Detailed microstructure and phase morphologies of the as-prepared ZrB2-SiC composite powders were examined by using FE-SEM (Fig. 8a). A number of fibrous phases (particle 1) were homogeneously distributed among the spherical particles (particle 2). Most of the fibrous phases 10

were 0.1~0.5 μm in diameter and 1.4~4.2 μm in length, whereas the average size of the spherical particles was 0.1~1.0 μm. Point-and-shoot EDS analysis (Fig. 8b and Table 2) was further performed to identify the elemental compositions of these two types of phases. The results revealed that the spherical phases contained Zr and B, while the fibrous phases contained Si and C. On the basis of XRD pattern (Fig. 5a) and the EDS results in Table 2, it can be reasonably concluded that the spherical and fibrous phases were ZrB2 and SiC, respectively. TEM (Fig. 9) further revealed the presence of the two types of phases in the as-prepared composite powders. The size of the spherical one (particle 2 and 3 in Fig. 9a) was about 160 nm and the average diameter of the fibrous one (particle 1 in Fig. 9a) was about 100 nm. Both dot-EDS in Table 3 and EDS mapping in Fig. 9b showed that the former contained mainly Zr and B, whereas the latter contained mainly Si and C. These results confirmed again that the former was ZrB2 and the latter SiC. Furthermore, Fig. 9 also reveals that the fibrous SiC phases were homogeneously distributed among the spherical ZrB2 particles.

3.7 Effect of heating method on the preparation of ZrB2-SiC composite powders Table 4 shows the effect of heating method on the preparation of ZrB2-SiC composite powders. It can be seen that the microwave heating not only reduced the reaction temperature from 1773 K (conventional heating) to 1573 K and overall reaction time from 10 h (conventional heating) to 4.5 h , but also resulted in relatively smaller particle size. These can be explained based on the following: 1) the SiC bedding powder was an excellent microwave absorber [30]; 2) homogeneous heating induced by the microwave was directly delivered to the reactants on molecular levels under the electromagnetic field [22]; 3) the coexistence of thermal and non-thermal effects arose from the microwave heating [34].

4. Conclusions 11

ZrB2-SiC composite powders were successfully synthesized via a microwave-assisted boro/ carbothermal reduction process using ZrSiO4, B2O3, and C as the starting materials. This method can not only lower the reaction temperature and reduce the overall heating time, but also reduce the grain size in the prepared composite powders. The SiC bedding powder, heating temperature, soaking time, as well as the B2O3 content showed evident effects on the synthesis of ZrB2-SiC composite powders. The optimal molar ratio of n(B2O3):n(ZrSiO4) was 1.5:1 and the optimal firing conditions were 1573 K for 3h. In the as-synthesized ZrB2-SiC composite powders, fibrous SiC phases with 0.1~0.5 μm in diameter and 1.4~4.2 μm in length were homogeneously distributed among 0.1~1 μm spherical ZrB2 grains.

Acknowledgements This work was financially supported by National Natural Science Foundation of China (General program, 51272188, 51472184, 51472185), State Basic Research Development Program of China (973 Program, 2014CB660802), Natural Science Foundation of Hubei Province, China (Contract No.2013CFA086) and Foreign cooperation projects in Science and Technology of Hubei Province, China (Contract No.2013BHE002).

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materials by microwave heating and their microstructure and properties, Ceram. Int. 40 (2014) 16761- 16766. [22] J.J. Ru, Y.X. Hua, C.Y. Xu, J. Li, D. Wang, K. Gong, R. Wang, Z.R. Hou, Microwave-assisted preparation of submicron-sized FeTiO3 powders, Ceram. Int. 40 (2014) 6799-6805. [23] H.B. Sun, Y.J. Zhang, H.Y. Gong, T. Li, Q.S. Li, Microwave sintering and kinetic analysis of Y2O3-MgO composites, Ceram. Int. 40 (2014 ) 10211-10215. [24] Y.N. Cao, H.J. Zhang, F.L. Li, L.L. Lu, S.W. Zhang, Preparation and characterization of ultrafine ZrB2-SiC composite Powders by a combined sol-gel and microwave boro/carbothermal reduction method, Ceram. Int. 41 (2015 ) 7823-7829. [25] H.J. Zhang, F.L. Li, Preparation and microstructure evolution of diboride ultrafine powder by sol-gel and microwave carbothermal reduction method, J. Sol-gel Soc.Technol. 45 (2008) 205-211. [26] H.J. Zhang, F.L. Li, Q.L. Jia, G.T. Ye, Preparation of titanium carbide powders by sol-gel and microwave carbothermal reduction methods at low temperature, J. Sol-gel Sci. Technol. 46 (2008) 217-222. [27] M. Jalaly, M.S. Bafghi, M. Tamizifar, F.J. Gotor, In situ synthesis of a ZrB2-based composite powder using a mechanochemical reaction for the zircon/magnesium/boron oxide/graphite system, Int. J. Appl. Ceram. Techn. (2013) 1-9. [28] M. Jalaly, M. Tamizifar, M.S. Bafghi, F.J. Gotor, Mechanochemical synthesis of ZrB2-SiCZrC nanocomposite powder by metallothermic reduction of zircon, J. Alloy. Compd. 581 (2013) 782-787. [29] H. Nakano, K. Watari, Y. Kinemuchi, K. Ishizaki, K. Urabe, Microstructural characterization of high-thermal-conductivity SiC ceramics, J. Eur. Ceram. Soc. 24 (2004) 3685-3690. [30] Y. Fang, D. Agrawai, M. Lanagan, T. Shrout, C. Randall, Ceramic materials and multilayer electronic devices, Ceramic Transaction 150 (2004) 359-366. [31] A. Kaiser, M. Lobert, R. Telle, Thermal stability of zircon (ZrSiO4), J. Eur. Ceram. Soc. 28 15

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Table captions Table 1 Chemical compositions of the starting materials of zircon. Table 2 Elemental compositions of the selected particles in Fig. 8 determined by EDS. Table 3 Elemental compositions of the selected particles in Fig. 9a determined by EDS. Table 4 Effect of heating method on the preparation of ZrB2-SiC composite powders

17

Figure captions Figure 1. Schematic diagram of the microwave heating furnace. Figure 2. Stability domain of condensed phases in the Zr-B-Si-C-O system. Figure 3. Equilibrium diagrams for the ZrSiO4-B2O3-C system with molar ratios of C:B2O3: ZrSiO4=8:1:1 (the equilibrium phase compositions in (a) the whole system, (b) B2O3-C-CO, (c) ZrSiO4-ZrO2-ZrB2-ZrC and (d) ZrSiO4-SiO2-SiC-SiO). Figure 4. XRD patterns of samples fired at 1573 K for 3 h with/without SiC bedding powders, n(C):n(B2O3):n(ZrSiO4)=10:1.5:1. Figure 5. XRD patterns of samples fired at various temperatures for 3 h (a), and relative contents of crystalline phases in the fired samples (b), n(C):n(B2O3):n(ZrSiO4)=10:1.5:1. Figure 6. XRD patterns of final products prepared at 1573 K for 3h with various soaking times (a), and relative contents of the crystalline phases in final products (b), n(C):n(B2O3):n(ZrSiO4)= 10:1.5:1. Figure 7. XRD patterns of final product samples synthesized at 1573 K for 3 h with different B2O3 contents (a), and relative contents of crystalline phases in the final product samples (b), n(C):n(ZrSiO4)=10:1. Figure 8. FE-SEM (a) and EDS (b) of final products prepared at 1573 K for 3h, n(C):n(B2O3):n(ZrSiO4)=10:1.5:1. Figure 9. TEM photographs (a) and EDS mapping (b) of final products prepared at 1573 K for 3 h, n(C):n(B2O3):n(ZrSiO4) =10:1.5:1.

18

Table 1 Composition

ZrO2

SiO2

Al2O3

CaO

Fe2O3

TiO2

MgO

K2O

Na2O

Content (wt/%)

65.83

32.30

0.98

0.16

0.13

0.11

0.07

0.02

0.01

19

Table 2 Elemental composition (atomic / %) Preparation conditions n(C):n(B2O3):n(ZrSiO4) =10:1.5:1, 1573 K /3 h

Selected points Zr

B

Si

C

Total

Spectrum 1

0

0

43

57

100

Spectrum 2

29

71

0

0

100

20

Table 3 Elemental composition (atomic / %) Preparation conditions

n(C):n(B2O3):n(ZrSiO4)=10:1.5:1, 1573 K/3 h

Selected points Zr

B

Si

C

Total

Spectrum 1

0

0

61

39

100

Spectrum 2

32

68

0

0

100

Spectrum3

0

0

60

40

100

21

Table 4 Microwave heating Boro/carbothermal reduction temperature / K

Conventional heating [15]

1573

1773

1.5

7

3

3

Power of heating furnace / kW

2.5

8

Contents of ZrB2 and SiC / wt%

100

100

Overall heating time / h Soaking time / h

Particle morphology

Particle size / μm

ZrB2

Spherical particles

SiC

Fibrous particles

Granular particles

ZrB2

0.1~1.0

1.5~5.5

SiC

0.1~0.5 μm in diameter 1.4~4.2 μm in length

1.9~3.8

22

Hexagonal columnar particle

Figure 1

23

Figure 2

2

ZrO2(s)+SiO2(s)+C(s)+B2O3(l)

ZrO2(s)+SiO2(s)+C(s)+B2O3(g) (3)

θ

lg(PCO/P )

1

1777 K

0

1926 K

1817 K ZrB2(s)+SiO2(s)+C(s) 1725 K (5) ) s ( C -1 s)+ ZrC(s)+SiC(s)+C(s) ( C Si s )+ ( (4) ZrB 2 -2 1523 1573 1673 1773 1873 Temperature/K

24

1973

Figure 3

25

26

1 ZrB2 2 SiC 3 m-ZrO2 4 t-ZrO2 5 ZrSiO4

1 (002)

1 (110) 2 (220) 1 (102) 1 (111) 1 (200) 2 (311) 1 (201)

With SiC bedding powders 2 (111)

Intensity/a.u.

1 (001)

1 (100)

1 (101)

Figure 4

Without SiC bedding powders 3 3 34 33 33 35

20

30

40 50 60 2 Theta / degree

27

70

80

Figure 5 2 SiC 3 m-ZrO2

4 t-ZrO2 5 ZrSiO4 1 (110) 2 (220) 1 (102) 1 (111) 1 (200) 2 (311) 1 (201)

1 (101)

1 ZrB2

1 (002)

1573 K

1 (100) 2 (111)

1 (001)

Intensity/a.u.

(a)

1523 K 1473 K 5 5 343 5 5 1373 K

5 5 4 53

53

3 20

30

40 50 60 2 Theta / degree

Relative contents /%

(b) 100

ZrB2 m-ZrO2

80

SiC

70

ZrSiO4

t-ZrO2

60 40 20 0 1373

1423

1473 1523 Temperature / K

28

1573

80

Figure 6

1 (110) 2 (220) 1 (102) 1 (111) 1 (200) 2 (311) 1 (201)

4 t-ZrO2 5 ZrSiO4

3h 1h 5 20

5 5

5 5 5 43 5 5 3 5 5 4 5

3 43

5

30

40 50 60 2 Theta / degree

(b) 100 Crystal phase contents /%

1 ZrB2 2 SiC 3 m-ZrO2 1 (002)

1 (001)

1 (100) 2 (111)

Intensity/a.u.

4h

1 (101)

(a)

ZrB2

5

70

80

SiC

80 60 40 20 0 1

2 3 Soaking time / h

29

4

Figure 7 1 ZrB2 2 SiC 3 ZrC B2O3:ZrSiO4=2.5 1 (002)

1 (110) 2 (220) 1 (102) 1 (111) 1 (200) 2 (311) 1 (201)

1 (101)

1 (100) 2 (111)

Intensity/a.u.

1 (001)

(a)

B2O3:ZrSiO4=2 B2O3:ZrSiO4=1.5

3 3 20

30

B2O3:ZrSiO4=1 3 3 3

40 50 60 2 Theta / degree

(b) 100

ZrB2

70

SiC

80

ZrC

Relative contents /%

80 60 40 20 0 1.0

1.5 2.0 n(B2O3)/n(ZrSiO4)

30

2.5

Figure 8

31

Figure 9

32