Characterization And Morphology Of ZrB₂Synthesized At High Pressure And High Temperature

Part of the hard ceramic compounds, a kind of materials with high hardness,consist of the transition metal elements (Ti, Zr, W, Mo) and light elements (B, C, N, O)and are usually with high valence electron density, strong covalent bond and3Dspatial arrangement of atomic. In addition to the property of high hardness, thesecompounds has the properties of high temperature resistance, corrosion resistance andgood electrical conductivity and been widely used in many fields. In this thesis, theZirconium diborides (ZrB2) were prepared by two kinds of chemical reactions atHigh-temperature and High-pressure (HPHT). The physical character includingrelative density, hardness and morphology of the ZrB2synthesized at HPHT wereanalyzed. Furthermore, single crystal of ZrB2was prepared by high temperature andhigh pressure condition with liquid B2O3and ZrB2powder. The growth mechanismand hardness of single crystal ZrB2has been studied.In this work, the high purity powder of Zr and B were used as the start materialsand the high-pressure synthetic experiments are carried out on the cubic anvilapparatus (SPD6×600T) to provide the sample with temperature of1700℃~2000℃and pressure of5.0GPa. With the holding time of30min, the block materials of pureZrB2were obtained. Both the relative density and the hardness of ZrB2prepared at thecondition of5.0GPa,1900℃increased firstly and decreased with the extension ofholding time. Based on the results of the Scanning Electron Microscope (SEM)images of the section of ZrB2block materials, it is found that the internal grains grewup with the increase of holding time. The block material density and hardness of ZrB2are affected by the change in particle size directly.The high purity powder of ZrO2and B were used as the raw materials. The ZrB2was prepared by borothermal reduction reaction at the condition of5.0GPa and 1300℃~2050℃. It is found that the rate of reaction is speeded up with thetemperature increasing. According to the results of composition, the chemical reactionwas determined as follows: ZrO2(s)+B(s)→ZrB2(s)+B2O3(l). At the same time, thediffusion of B atomic in the borothermal reduction reaction process were analyzedand the results showed the liquid B2O3exacerbated the diffusion.The commercial available high purity powder of ZrB2(99.95%,10μm) and B2O3were used as raw material to produce the single crystal particles (up to80μm) at thecondition of5.0GPa,1900℃. These single crystal particles have differentmorphology compared with the six prisms or hexagonal flaky morphology as reportedpreviously. It is worthy noted that the single crystalline particles does not growpreferentially in one direction, but grow in all directions with a relatively uniformspeed. The growth model of stack by layers of the morphology was analyzed and theinfluence of pressure, temperature, the content of B2O3on the morphology of ZrB2prepared at high pressure was discussed.According to the hardness test of the large ZrB2single crystal, the Vickershardness value of (100) or (001) lattice plane of ZrB2single crystal was firstlyobtained. With the load of0.098N and0.245N, the hardness of ZrB2single crystalsshown44.1GPa and39.8GPa. The hardness testing of ZrB2single crystal makes thecharacterization of hardness of ZrB2more accurate.