P. Peshev

On the borothermic preparation of titanium, zirconium and hafnium diborides

Abstract 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 TiB2 is obtained after a heattreatment of the reactants at 1700 °C for I h; HfB2 is obtained after heat treatment at 1750 °C for 2 h. Experimental results show that higher temperatures are necessary for the preparation of stoichiometric ZrB2 by the borothermic method. The structure of HfB2, 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 borothermic preparation of some vanadium, niobium and tantalum borides

Abstract The formation of vanadium, niobium and tantalum diborides by the reactions of V2O3, Nb2O5 and Ta2O5 with elemental boron under vacuum in the temperature range 1000 °–1750 °C has been investigated. It has been established that niobium and tantalum diborides with a composition close to that of stoichiometric NbB2 and TaB2 are obtained after a heat treatment of the reactants at 1650 °C for 1 h. Borothermic reduction of V2O3 does not give a single-phase product but a mixture of VB2 and VB. A method for the chemical analysis of NbB2− and TaB2 samples has been proposed.

ON THE PREPARATION OF SOME CHROMIUM, MOLYBDENUM, AND TUNGSTEN BORIDES.

Abstract The formation of CrB2, Mo2B5 and W2B5 by the reactions of Cr2O3, MoO2 and WO2 with elemental boron in vacuum has been investigated. It has been established that the borothermic reduction of Cr2O3 is complete at temperatures above 1600°C, CrB2 containing CrB being obtained; the amount of CrB increases with increase in temperature. The reaction between MoO2 and B at low temperatures leads to a partial reduction of the dioxide to Mo2O3. Formation of Mo2B5 begins above 1200° C and at 1600°C the pure compound only is obtained. Mo2B5 has a defect structure. The optimum temperature for the borothermic preparation of W2B5 is also 1600°C. The product obtained displays a certain deficiency of boron. At low temperatures W2B-phase is obtained, while above 1600°C, monoboride, WB, is formed simultaneously with W2B5.

Multiphase alloys La2−xCaxMg17 for hydrogen storage

Abstract Hydrogen storage by alloys with composition La2−xCaxMg17 (1.4 ⩽ x ⩽ 1.8) representing a mixture of the phases La2Mg17, CaMg2 and magnesium in various ratios has been investigated. It is found that these alloys have good absorption-desorption characteristics and exhibit the highest hydrogen capacity at the lowest hydriding pressure (5 bar). Prolonged cycling with the alloy La0.4Ca1.6Mg17 has shown an initial decrease in its absorption capacity till the 80–100th cycle, after which it remained constant and fairly high till the 250th hydriding-dehydriding cycle. The desorption characteristics of the alloys under investigation are close to those of single-phase alloys with the same general formula and a high lanthanum content (x

Calcium- and nickel-substituted lanthanum-magnesium alloys for hydrogen storage

Abstract The effect of the substitution of a transition metal for magnesium in La 2 Mg 17 , which had already had part of the lanthanum replaced by calcium, on the hydrogen storage capacity of the alloys is studied. The composition La 2− x Ca x Mg 16 Ni is studied. It was found that the partial substitution of nickel for magnesium in La 2− x Ca x Mg alloys does not significantly affect their hydrogen storage capacity but considerably accelerates the desorption of hydrogen because of the formation of Mg 2 NiH 4 .

The elemental and phase composition of boride coatings deposited by diffusion on a Wc-Co alloy

Abstract The elemental and phase compositions of diffusion boride coatings on a 94 wt.% WC-6 wt.% Co alloy were studied. The coatings were obtained by interaction between alloy plates and powdery TiB2 in an argon atmosphere. The distributions of tungsten, cobalt and titanium were studied by X-ray microprobe analysis. The phase distribution in the direction perpendicular to the coating surface was investigated by obtaining X-ray patterns of the coating. The upper layer was found to consist of orthorhombic CoWB with traces of TiC. The concentrations of both phases decrease in the depth of the coatings.

The composition and oxidation resistance of coatings deposited on hard carbide alloys using TiB2

Abstract The phase composition of coatings obtained on hard 94 wt.% WC-6 wt.% Co alloys by thermal diffusion using TiB 2 was studied and the changes in phase composition of the parent alloy were traced. It was found that CoWB and TiC are the major phases in the coating. The resistance to high temperature oxidation of 94 wt.% WC-6 wt.% Co and 79 wt.% WC-15 wt.% TiC-6 wt.% Co alloys, both with boron-containing coatings, was established and compared with the resistance of the same alloys without coatings. The coatings were shown to increase the oxidation resistance of the alloys considerably.

On the preparation of crystalline aluminium borides by a vapour deposition process

The possibility of the formation of crystalline aluminium borides rich in boron, by a vapour-deposition process has been investigated. The gas phase contained pure AlBr3, BCl3 and H2. Deposition on tungsten leads to a complex mixture of tungsten and aluminium borides. Crystals of C4AlB24 were deposited on a graphite surface at 1400°–1600°C, carbon being supplied at the beginning of the process by diffusion from the graphite. Subsequently, to make further crystal growth possible, a compound containing carbon was introduced into the gas phase; otherwise β-rhombohedral boron appears on the C4AlB24 crystals already formed. A binary compound of aluminium and boron (α-AlB12) has been prepared at temperatures above 1600°C.

Preparation of titanium disilicide single crystals by chemical vapour transport with halogens

Abstract The preparation of TiSi 2 single crystals by chemical transport in the presence of halogens is studied. It is shown that the tetrahalides formed during the interaction of TiSi 2 with halogens, and above all the tetrahalides of titanium, TiX 4 , are the transporting agents. The gas phase composition is complex in all cases, but according to thermodynamic analysis silicon should be transported in the form of a dihalide, SiX 2 , whereas titanium transport should occur through TiX 3 and TiX 2 , the contribution of the latter increasing in the sequence chlorine → bromine → iodine. Single crystals of TiSi 2 are obtained in the presence of the three halogens, but transport with chlorine is found to be the most efficient. The conditions for the preparation of TiSi 2 single crystals in the form of needles, polyhedra or plates with maximum dimensions of 10 mm, 4 mm and 5 mm, respectively, and a composition close to the stoichiometric are established.

ON THE PREPARATION AND SOME PHYSICAL PROPERTIES OF GADOLINIUM SESQUISULPHIDE AND GADOLINIUM MONOSULPHIDE.

Abstract The optimum conditions for the preparation of stoichiometric gadolinium sesquisulphide and gadolinium monosulphide according to the reactions Gd 2 O 3 +3 H 2 S = Gd 2 S 3 +3 H 2 O (1) and Gd 2 O 3 +2 Gd 2 S 3 + 3 C = 6 GdS +3 CO (2) have been investigated. It has been established that stoichiometric Gd 2 S 3 is obtained at temperatures above 750 °C. Gadolinium monosulphide was prepared by carrying out reaction (2) in vacuo in two stages : i.e. , the stoichiometric mixture of the three components was heat-treated at 1600 °C for 2–3 h, ground and pressed into pellets, then heat-treatment of the pellets was carried out at 1700 °C for 2 h. The magnetic susceptibilities of Gd 2 S 3 and GdS have been measured and the effective magnetic moments have been determined. The latter are close to the magnetic moment of the Gd 3+ ion which indicates the prevalence of ionic bonds in both sulphides. The dependence on temperature of the work function, ϑ, for cathodes of Gd 2 S 3 and GdS has been investigated. The conductivities of both sulphides have been determined theoretically; Gd 2 S 3 is deduced to possess a semiconducting, and GdS a metal, conductivity.