Boron In Vacuum Research Articles

Synthesis of a high-boron aluminum boride via borothermic reduction of alumina

The reaction between alumina and boron in vacuum was studied between 800 and 1500‡C at different compositions of the starting mixture. The reaction products were analyzed chemically and by x-ray diffraction, optical, and microstructural analysis techniques. The compositions of the reaction intermediates and products were determined, which made it possible to propose a reaction scheme which differs significantly from the commonly accepted one and assumes that not only B2O3 but also AlO are removed in the course of the reaction. It is shown that only 60% of the starting Al is incorporated into the forming phase, AlB18-31 (Β-B hexagonal structure,a = 1.0970 nm,c = 2.3780 nm). No aluminum dior dodecaboride was detected in the reaction products. The proposed mechanism of the process explains why the borothermic reduction of various metal oxides typically yields only the highest boron phases.

High-temperature reaction of aluminum oxide with boron in vacuum

The reaction of aluminum oxide with boron in vacuum was studied over a wide range of temperatures. Analysis of the reaction products was carried out by chemical, x-ray diffraction, optical, and metallographic methods. The temperature dependence of the reaction was determined. A high boron phase of the composition AlB18–31 is produced by the borothermic reduction of aluminum oxide. The crystalline phase possesses a hexagonal structure with the elementary cell parameters a = 1.097, c = 2.378 nm. Aluminum dodecaboride and diboride phases were not detected in the reaction products.

Factors influencing the morphologies of boron deposited by chemical vapour deposition

The factors influencing the morphology of boron prepared by chemical vapour deposition in a closed system were investigated. The formation of crystalline or amorphous deposits is confined to well-separated regions defined by temperature and deposition rate or supersaturation. The experimental conditions for simultaneous growth of crystalline and amorphous boron in the temperature range 1390–1640 K are given by k = 1.59 × 10 8 × exp(−20 070/ T) where k is the deposition rate ( μg cm −2 s −1) and T the temperature (K). At deposition rates higher than that given by this relation, amorphous boron is formed. At temperatures lower than 1390 K, amorphous boron is formed at considerably lower deposition rates than those given by the relation. In the temperature range 1390−1640 K the transition of amorphous boron to crystalline boron is controlled by grain boundary self-diffusion with an activation energy of 172 kJ mol −1. Heat treatment of amorphous boron in vacuum in the temperature range 1270–1520 K results in the formation of β-rhombohedral boron via the formation and subsequent decomposition of α-rhombohedral boron. The crystals of α-rhombohedral boron are probably formed by a sublimation/ condensation mechanism.

On the borothermic preparation of titanium, zirconium and hafnium diborides

The formation of titanium, zirconium and hafnium diborides by reaction between metal dioxides and elemental boron in vacuum in the temperature range 1000 °–1750 °C has been investigated. It has been established that titanium diboride with a composition close to that of stoichiometric TiB 2 is obtained after a heattreatment of the reactants at 1700 °C for I h; HfB 2 is obtained after heat treatment at 1750 °C for 2 h. Experimental results show that higher temperatures are necessary for the preparation of stoichiometric ZrB 2 by the borothermic method. The structure of HfB 2, and its stability at boiling point and at room temperature in the presence of acids, mixtures of acids, mixtures of acids with oxidizing agents, and alkaline solutions has been studied.

On the preparation of some chromium, molybdenum and tungsten borides

The formation of CrB 2, Mo 2B 5 and W 2B 5 by the reactions of Cr 2O 3, MoO 2 and WO 2 with elemental boron in vacuum has been investigated. It has been established that the borothermic reduction of Cr 2O 3 is complete at temperatures above 1600°C, CrB 2 containing CrB being obtained; the amount of CrB increases with increase in temperature. The reaction between MoO 2 and B at low temperatures leads to a partial reduction of the dioxide to Mo 2O 3. Formation of Mo 2B 5 begins above 1200° C and at 1600°C the pure compound only is obtained. Mo 2B 5 has a defect structure. The optimum temperature for the borothermic preparation of W 2B 5 is also 1600°C. The product obtained displays a certain deficiency of boron. At low temperatures W 2B-phase is obtained, while above 1600°C, monoboride, WB, is formed simultaneously with W 2B 5.