2016 International Conference on Applied Mechanics, Mechanical and Materials Engineering (AMMME 2016) ISBN: 978-1-60595-409-7

Preparation of ZrB 2 Powders by a Combined Sol-Gel and Carbothermal Reduction Method Zheng-guo YAN 1,*, Lei YUAN 1, Chun WANG 2 and Jing-kun YU1 1School of Metallurgy, Northeastern University, 3-11, Wenhua Road, Heping District, , , P. R. 2Yinkou Qinghua Refractory Material Co., Ltd, Qinghuali Panlong Street, Dashiqiao, Liaoning, P. R. China *Corresponding author

Keywords: Powder, Sol-gel, Carbothermal reduction, Zirconium diboride.

Abstract. Zirconium diboride (ZrB 2) powders were synthesized by a combined sol-gel and carbothermal reduction method in which zirconium oxychloride, boric acid and phenolic resin were used as source of zirconia, boron oxide, and carbon. The infuences of molar ratio of raw materials and calcination temperatures on the phase composition and morphology of powders were investigated. The results showed that the powders with smaller particle size and higher purity were synthesized at 1500 oC with the molar ratio of Zr, B and C was 2.4: 1: 6.

Introduction In recent years, more attention have been paid to ultra high temperature ceramics (UTHTC) due to their excellent physical and chemical properties. As one of important ultra-high temperature ceramics, zirconium diboride (ZrB 2) is characterized by high melting point, high hardness, good thermal and electric conductivity, high thermal shock resistance, and chemical stability. These unique properties make ZrB 2 candidate material for high-temperature electrodes, wear parts, cutting tool, advanced hypersonic re-entry vehicles, and molten- metal crucibles, et al [1,2]. Characteristics of powders have significant impact on the properties of end products. Therefore, powders preparation method is the key part of processing. Methods used for synthesis of ZrB 2 include boro/carbothermal reduction, mechanochemical processing, self-propagation, and chemical solution [3-6]. Boro/carbothermal reduction needs high temperature and a long processing duration, which makes the synthesized powders with a relatively large crystallite size and poor sinterability. Mechanochemical processing needs a long processing time, and the purity of the synthesized powders is not very high. Self-propagating synthesis has low cost and low consumption of energy, but it is difficult to control the purity of the synthesized powder. Chemical solution is very effective way for synthesizing homogenous powders with high purity. All of components such as carbon, boron and the metal-containing sources are dissolved in liquid medium, and subject to condensation and hydrolysis to form precursor. Finally, ceramic powders are obtained via calcination of the mixture at a moderate temperature. In the present study, a combined sol-gel and carbothermal reduction process was practiced to synthesize zirconium diboride. The phase composition and morphology of synthesized ZrB 2 powder were characterized, and the influences of the raw material ratio and temperatures on the powders synthesized were studied.

Experimental

Zirconium oxychloride (ZrOCl 2·8H 2O, Sinopharm Chemical Reagent Co., Ltd, Shenyang, China) and boric acid (H3BO 3, Sinopharm Chemical Reagent Co., Ltd, Shenyang, China) used as source of zirconia and boron oxide, were of analytical grade. Phenolic resin ( mingyan adhesive material co., Ltd, Wuxi, China) was used as the carbon source which char yield was about 60wt%. In view of the vaporization of B 2O3 and the burning loss of carbon, excess of H 3BO 3 and phenolic resin were added to promote synthesis of ZrB 2. The raw materials molar ratio of reactants is listed in Table 1. Table 1. Raw material molar ratio of samples/mol.

Sample ZrOCl 2·8H 2O H3BO 3 C A1 1 2 5 A2 1 2.2 5.5 A3 1 2.4 6 A4 1 3 7.5

Boric acid and Zirconium oxychloride were dissolved in distilled water with continuous stirring and prepared into a solution of 1 mol·l -1 and 2 mol·l -1, respectively. The phenolic resin was dissolved in anhydrous ethanol. The boric acid solution was slowly dropped into the zirconium oxychloride solution to make the mixture evenly mixed. The pH value of the hybrid solution was adjusted to 4 using dilute ammonia under stirring. The phenolic resin dissolved in anhydrous ethanol was then added into the mixture of solution. The hybrid sol-gel was aged in air for 2h followed by drying at 110 oC for 24h in an oven and crushed to powder in a mortar with a pestle to get precursor powders. Finally, the precursor powders were calcined at temperatures in the range of 1400-1600 oC for 2h in an alumina tube furnace under the flowing argon atmosphere. The phase purity and homogeneity of prepared powders were characterized by the D/MAX-RB type X - ray diffraction (XRD with Cu K α radiation at 30 kV and 30 mA). The morphology of powders was observed by SSX-550 scanning electron microscope (SEM).

Results and Discussion

★ ★—ZrB ◇—ZrO A1 2 2 ★ ●—ZrC ◇ ★ ● ◇★

● ● ★ ◇ ◇◇ ★ ◇ ● ★ ★ ◇ ◇ ◇ ◇ ● ●

A2

Intensity / a.u. A3

A4

10 20 30 40 50 60 70 80 90 o 2 theta / ( ) o Figure 1. XRD patterns of ZrB 2 powder synthesized at 1600 C with different molar ratio. Figure 1 shows the XRD patterns and morphology of the powders synthesized at 1600 oC with different molar ratios of raw material. In all samples, ZrO 2 and ZrC were observed, well defined ZrB 2 XRD peaks appeared, and ZrB 2 was dominant phase. The intensity of ZrB 2 phase increased with higher molar ratio of B and C in the raw materials, while ZrO 2 and ZrC phase decreased gradually. Considering the vaporization of the intermediate product B 2O3 and oxidation burning loss of C, it is helpful to synthesis of ZrB 2 powders as more boric acid and phenolic resin were added. The morphology of powders synthesized at 1600 oC with different molar ratios of raw material is shown in in Figure 2. The particle size of the powders increased with increase molar ratio of B and C in the raw materials, the trend was more obvious when the molar content of B and C was excess of 20%. The results indicate that more carbon can promote more B2O3 participation in carbothermal reduction, but not exist under molten state, which can promote the ZrB 2 grains growth. Comprehensive consideration of results above, the appropriate molar ratio of Zr, B and C was 1:2.4:6.

A1 A2

A3 A4

Figure 2. SEM images of powder synthesized at 1600 oC with different molar ratio. The XRD patterns of the powders (sample A3) synthesized at different temperatures are shown in o Figure 3. The strong ZrB 2 crystalline peaks were observed in powders calcined at 1400 C, which is lower than the temperature required for conventional solid state synthesis of ZrB 2. In addition, a small amount of generated ZrC and some unreacted ZrO 2 were detected. With increasing of o temperature to 1500 C, the ZrO 2 and ZrC phase reduced to trace which implied that the carbothermal reduction had been completed. While the calcined temperature was increased to 1600 o C, ZrO 2 phase was detected. As intermediate product, B 2O3 played important role in the synthesis process of ZrB 2, although B 2O3 was not detected in the XRD patterns of powders owing to its amorphous state. It’s well known that B 2O3 has a low melting point and high vapor pressure. So it can be easily vaporized at higher temperature. And the loss of B 2O3 by vaporization increased with increasing of calcined temperature, Excessive loss of B 2O3 would likely lead to the lack of B elements in the reaction, thus affected the yield of ZrB 2.

★ ★— ◇— ZrB 2 ZrO 2 ★ ●—ZrC

★◇ ★ o ◇ ★ 1400 C ◇ ◇◇ ★ ◇ ★ ★ ◇ ◇ ● ◇◇ ★● ★ ●

o

1500 C Intensity/a.u.

o 1600 C

10 20 30 40 50 60 70 80 90 o 2 theta / ( )

Figure 3. XRD patterns of powder synthesized at different temperatures with molar ratio of Zr: B: C 1:2.4:6.

1400 ℃ 1500 ℃

1600 ℃

Figure 4. SEM images of powders synthesized at different temperatures with molar ratio of Zr: B: C 1:2.4:6. The SEM images of powders synthesized at different temperatures with molar ratio of Zr, B and C 1:2.4:6 are shown in Figure 4. It can be seen that there are two kinds particles in the powders calcined at low temperature, i.e. nearly spherical morphology particles and cylindrical morphology particles. The microstructure of the powders was changed from nearly spherical to cylindrical, and the size of grains increased with increasing of calcination temperature. The results indicated that the appropriate calcination was 1500 oC.

Conclusions Chemical solution is very effective way for synthesizing homogenous powders with high purity. In this study, using zirconium oxychloride, boric acid and phenolic resin as raw materials, the ZrB 2 powders were synthesized by a combined sol-gel and carbothermal method at lower temperature. Comprehensive consideration of experimental results, the optimum calcination temperature was 1500 oC, the appropriate molar ratio of Zr, B and C was 1:2.4:6.

Acknowledgement This research was financially supported by the Fundamental Research Funds for the Central Universities (Project No. N140204012 ).

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