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Abstract

ZrB₂ powders were synthesised by microwave heating using the sol–gel method for precursor with eight C₆H₁₄O₆ (ZrOCl₂·8H₂O), boric acid (H₃BO₃), citric acid (I), and mannitol (as raw materials). The results show that the temperature at which the ZrB₂ powder is completely synthesised by microwave heating is 1400°C, which is 300°C lower than the traditional way. XRD and SEM were used to characterise the phase composition and morphology of the powders. In addition, the effects of raw material ratios and synthesis temperature on the synthesis of powder were investigated. The effect of temperature on the purity of the synthesised zirconium boride was considerable. The nearly spherical ZrB₂ ultra-fine powders of high purity were synthesised successfully at 1400°C by microwave, with the ratios of C:Zr is 7.44:1, and B:Zr is 2.77:1.

Keywords

ZrB2 powder sol–gel precursor microwave sintering

Introduction

Zirconium diboride (ZrB₂) ceramics is known for its ultra-high temperature properties. It has high melting temperature, high hardness, outstanding thermal conductivity and electrical conductivity, and neutron capture performance [1 -3]. Such a series of characteristics determine its potential value for use in the high-temperature field and some other limited fields. Ultra-high temperature ceramics (UHTCs) are ceramic materials that have melting point higher than 2000°C [4]. In recent years, as major military powers are striving to seize the strategic technological commanding heights of hypersonic vehicles and propulsion technologies (such as supersonic combustion ramjet), interest in UHTCs with better performance has been rekindled. This is mainly due to the fact that with the increase of flight speed, the materials need to be able to withstand higher temperature than (2000–2400)°C, work in oxidative or corrosive environments, have a long working life, and be reusable [5]. UHTCs may be the only candidate materials to meet these requirements. Therefore, ZrB₂, as one of the ultra-high temperature materials with the most potential, has attracted more and more attention from researchers. However, the high melting point of ZrB₂ has limited its application. In the past few years, many ways have been proposed to improve the sintering performance of ZrB₂ to improve its densification. These include the introduction of sintering aid (e.g. B [6], HfN [7], Mo [8], La [9]), and forming composites with other materials (e.g. SiC [10], ZrC [3,9]). But those methods would to some degree introduce a secondary phase into the grain boundaries, which would weaken some of the mechanical properties of ZrB₂. Consequently, the problem of the sintering activity of ZrB₂ should be addressed at its roots. Ultra-fine powders have a larger specific surface area, which can offer a driving force for sintering, improve the relative density, and enhance the mechanical behaviour of the ZrB₂ ceramics. Therefore, finding a simple and effective way to synthesise ZrB₂ powders with fine and homogeneous particle size is definitely important.

At present, various traditional methods have been used to synthesise ZrB₂ powders, such as solid-state reaction [11], electrochemical [12], and self-propagating high-temperature synthesis [13]. However, those routes all need high temperature and a long production period, and the powders that were synthesised have a relatively large crystallite size and poor sinterability. Therefore in recent years, the powders synthesised by wet chemistry method have attracted great attention; these techniques include Inorganic–Organic Hybrid Precursors [2], sol–gel method [14], and Solution-Based method [15]. Using those techniques, the ZrB₂ powders can be synthesised at low temperature, and submicron particles can be obtained. The advantage of using the sol–gel method is that high chemical and phase homogeneity can be achieved by mixing the starting components at the molecular or colloidal level [14]: This paper discusses a wet chemistry method, the sol–gel method, to synthesise ZrB₂ powders. First, a mixed solution of Zr source, B source, and C source should be prepared, and then the ZrB₂ should be synthesised by carbothermal reduction method according to the reaction [16]. Zirconium oxychloride, boric acid, citric acid, and mannitol were used as raw materials to prepare the precursor, and microwave sintering was used to prepare ZrB₂ powders. This work focuses on the effect of the C:Zr and B:Zr ratios and the temperature system on the purity of the synthesised zirconium boride.

Experimental procedure

Zirconyl chloride octahydrate (ZCO, ZrOCl₂·8H₂O, 98%, Aladdin) and boric acid (H₃BO₃, GR, Aladdin) were used as the sources of zirconia and boron. Mannitol (C₆H₁₄O₆, GR, Aladdin) and citric acid (C₆H₈O₇, GR, Aladdin) were used as carbon source. The sol–gel method was used with a new composite system of zirconium ions and citric acid, mannitol, and boric acid. Figure 1 describes the synthesis of ZrB₂ precursor powders. First, a mixed solution was prepared by dissolving citric acid, boric acid, mannitol, and ZCO. The mixed solution was water bath heated and stirred at 80°C for gel state. The gel was dried at 120°C and milled into powder. Then, the precursor powders were subjected to pyrolysis by tubular furnace with argon at 600°C. Finally, the powder was microwaved for high-temperature calcination at (1100–1400)°C for 1 h.

Figure 1

Flowchart for the synthesis of zirconium diboride precursor powders using the sol–gel method.

Results and discussion

Figure 2 shows the XRD spectra of the precursor powders, which were prepared in different ways, then pyrolysed by tube furnace at 600°C. C₆H₁₂O₆ indicates that the precursor is prepared from glucose, zirconium oxychloride, boric acid, and citric acid, while C₆H₁₄O₆ indicates that the precursor is prepared from mannitol, zirconium oxychloride, boric acid, and citric acid. XRD results show that the precursor prepared from glucose contains the characteristic peak of boron oxide after being treated at 600°C, while the precursor prepared from mannitol has no characteristic peak of boron oxide. This proves that boron oxide is precipitated in the precursor prepared from glucose, and the precipitated boron oxide is easy to volatilise in the later high-temperature calcination. The results show that there is no precipitation of boron oxide in the precursor of mannitol, which is more conducive to participate in the later carbothermal reduction reaction.

Figure 2

The XRD spectra of the pyrolysis precursor powders at 600°C by tube furnace.

The influence of temperature factors

In order to study the effect of calcination temperature on the phase of synthesised powders, excess boron(B/Zr = 5.78) and carbon (C/Zr = 25.72) were used in the experiment (at high temperature, excess B will volatilise at high temperature, and excess carbon can be removed by subsequent heat treatment). Figure 3 shows the X-ray diffraction patterns of the precursor powders at different heat-treatment temperatures by microwave from (1100 to 1400)°C. The XRD spectra indicate that the ZrB₂ can be synthesised at 1100°C, which is below the theoretical generating temperature. The m-ZrO₂, t-ZrO₂, and ZrB₂ phases were observed from (1100 to 1300)°C. As the temperature rose from (1100 to 1300)°C, the relative content of m-ZrO₂ phase gradually reduced, while the t-ZrO₂ phase increased. This is because, with the increase of temperature, part of m-ZrO₂ reacts further to form ZrB₂, while part of m-ZrO₂ quasi becomes t-ZrO₂. And, as the result shown the dominant phase was m-ZrO₂ at 1100°C, while at the other temperature, the dominant phase was ZrB₂. Until no other phase appears, ZrB₂ is treated at 1400°C. It's not hard to infer from the results that ZrB₂ can be synthesised at 1100°C by microwave calcination under argon atmosphere. As the calcination temperature rises, the borothermal and carbothermal reductions occur more and more deeply; at 1300°C, the reduction is almost complete, while at 1400°C, it is finished. Meanwhile, the m-ZrO₂ is gradually converted to the t-ZrO₂, and at 1400°C, the ZrO₂ phases are fully reacted.

Figure 3

The XRD patterns of the precursor powders at different heat-treatment temperatures by microwave from (1100 to 1400)°C.

The influence of the ratio of B and Zr

The high volatility of boron oxide at high temperature, therefore, means that the ratio of B and Zr should be adjusted to obtain the pure powder. In order to further optimise the experimental parameters and study the effect of B on the phase of the synthesised powders, the powders with different B content were calcined at high temperature (1400°C). Figure 4 shows the XRD of samples prepared with different molar ratios of B:Zr. The results show that the main crystal phase of the sample is ZrO₂ when B: Zr = 2.44:1. This is because B₂O₃ is easy to volatilise in the high-temperature reaction process. Although the B/Zr ratio of 2.44 is obviously higher than that of ZrB₂ (B/Zr = 2), it cannot participate in the reaction completely. Therefore, XRD results show that there is a large amount of ZrO₂ remaining. When B/Zr = 2.6, the results show that there is a very weak characteristic peak of ZrC, which is still caused by the insufficient content of B, so when the content of B is increased to B/Zr = 2.77, pure ZrB₂ phase is obtained. When the raw materials were used with B:Zr = 2.77:1, the main crystal phase of the sample was ZrB₂. The ZrO₂ was totally converted to ZrB₂. The result shows that as the B content increased, the ZrO₂ gradually transformed into ZrB₂. The results show that B/Zr = 2.77:1 is the best ratio.

Figure 4

The X-ray diffraction patterns of the precursor powders with different molar ratio values of B:Zr.

The influence of the ratio of C and Zr

The carbon content was adjusted under the optimum B:Zr ratio to study the effect of carbon content on the powder. Carbon plays an important role in the process of reducing ZrO₂. The content of carbon was adjusted by adjusting the content of Mannitol (C₆H₁₄O₆) and citric acid. Figure 5 shows the XRD spectra of samples prepared with different molar ratio of C:Zr. When the raw material C:Zr ratio was 7.13:1, there was a small amount of unreacted ZrO₂. Although the C/Zr ratio in the raw material is 7.13, which is significantly higher than the stoichiometric ratio of carbothermal reaction (C/Zr = 5), the results show that a large amount of ZrO₂ is still not involved in the reaction. This is because the carbon source in the raw powder is provided by organic matter. During the pyrolysis at high temperature, some carbon will react with the oxygen in the organic matter. By increasing the content of C, when the C:Zr ratio was 7.44:1, the ZrO₂ peak disappeared. The results show that with sufficient Boron, ZrO₂ is gradually reduced to ZrB₂ with the increase of carbon content.

Figure 5

The XRD spectra of samples prepared with different molar ratio of C:Zr.

The microstructure of powders

In order to observe the micro morphology of the powder. Pure ZrB₂ powder was prepared by calcining at 1400°C for 1 h with the best raw material parameters, B/Zr = 2.77 and C/Zr = 7.44. XRD is shown in Figure 6. The results show that the ZrB₂ powder prepared with the best parameters has high purity. Then the powder prepared with the best parameters was observed by scanning electron microscope (SEM). Figure 6 shows the SEM imagery of sample prepared from the optimal formulation. The high-purity ZrB₂ powders are highly uniform, with the size of the nearly spherical particle in the range (1–2) µm (Figure 7).

Figure 6

The XRD of ZrB2 powder was prepared by calcining at 1400°C for 1 h with B/Zr = 2.77 and C/Zr = 7.44.

Figure 7

SEM imagesof samples prepared from the optimal formulation by microwave at 1400°C.

Conclusions

ZrB2 powders were prepared by the sol–gel method and microwave sintering method. Research suggests that the optimal Zr:B:C ratios are 1:2.77:7.44. High-purity ZrB₂ was prepared by microwave at 1400°C from the raw material with the optimal ratios of Zr:B:C. The high-purity ZrB₂ powders are highly uniform, while the size of the nearly spherical particle is in the range (1–2) µm. The pure ZrB₂ powder has high sintering activity, which is beneficial to the sintering of high-density ceramics.

Footnotes

Acknowledgements

This work was financially supported by the Guangzhou Postdoctoral Research Fund.

Disclosure statement

No potential conflict of interest was reported by the author(s).

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