Rare-earth Metal Boride Research Articles

Bromides of Rare Earth Metal Boride Carbides—A System of Building Blocks

Bromides of Rare Earth Metal Boride Carbides—A System of Building Blocks

The possibility of obtaining pure borides of rare-earth metals

The possibility of obtaining pure borides of rare-earth metals

Thermal electromotive force of cermet films based on borides of rare-earth metals

The thermal emf and electrical conductivity of cermet films comprised of conducting solid solutions of europium and praseodymium borides and a dielectric glass crystalline binder were studied using a combination of temperature and voltage measurements. The dependence of these properties on temperature and the Seebeck coefficient were also determined. Results indicate that the mechanism of charge transfer in these films is ambiguous.

Standard entropy for borides of non-transition metals, rare-earth metals and actinides

Using as initial data the most reliable values of standard entropy for 10 compounds, the entropies for 40 compounds of non-transition metals, rare-earth metals and actinides have been evaluated by the method of comparative calculation. Taking into account the features of boride structures, two methods, i.e. additive and proportional, have been selected for the entropy calculations. For the range of borides the entropies were calculated from the linear relation of the latter to the number of boron atoms in the boride. For borides of rare-earth metals allowance has been made for magnetic contributions in conformity with the multiplicity of the corresponding ions. Insignificant differences in the electronic contributions to the entropy for borides and metals have been neglected. For dodecaborides only the additive method has been used. This is specified by the most rigid network that provides the same contribution to compound entropy.

The micromechanical properties of melted borides of rare earth metals

The compounds of boron with metals are characterized by a high strength of the interatomic bonds and a low relaxation ability of the internal stress. As a result they are very fragile materials. The value of the microfragility of these compounds may be used for a comparative estimation of their strength and of their possible practical application. The structure of the surface disturbance and the relative microfragility (microelasticity) of a number of alkaline and rare earth metal hexaborides obtained by melting (sometimes accompanied by zone purification) were determined by electron microscopy and measurements of the microhardness and microfragility. The modulus of elasticity and the effective surface energy of a number of materials were determined by the method of continuous indentation. It was established that, if we place these compounds in the sequence in which their microelasticity decreases, we have the following order: SmB 6→BaB 6→EuB 6→CaB 6→ YbB 6→LaB 6→PrB 6→NdB 6→GdB 6. In this order there is a noticeable increase in the length of the metal-metal and metal-B bonds which have a metallic character and are weakened when passing from samarium to gadolinium. The ability of the metal atoms to transfer some of their electrons to the boron atoms increases in the same direction. This also increases the fragility of the compounds because of the strong acceptor ability of the boron atoms and their tendency to form a stable sp 3 electron state.

Boride Cathodes

The thermionic emission properties of the borides of the alkaline-earth and rare-earth metals and thorium have been investigated. These compounds all have the same formula MB6 and the same crystal structure consisting of a three-dimensional boron framework in whose interlattice spaces the metal atoms are embedded. The valence electrons of the metal atoms are not accepted by the B6 complex, thus giving rise to the presence of free electrons which impart a metallic character to these compounds. This, together with the strong bonds between the boron atoms in the framework, produces a series of compounds which have high electrical conductivities and high thermal and chemical stabilities—ideal properties for a cathode material. When this structure is heated to a sufficiently high temperature, the metal atoms at the surface evaporate away. They are, however, immediately replaced by diffusion of metal atoms from the underlying cells. The boron frame work does not evaporate but remains intact. This process gives a mechanism for constantly maintaining an active cathode surface. Thermionic emission measurements made on these materials show the rare-earth metal borides to be superior to the others. The highest emission was obtained from lanthanum boride. Its emission constants for the Dushman equation were φ=2.66 volts and A=29 amps/cm2/degK2. This is higher than the emission normally obtained from thoria. Lanthanum boride has a relatively low evaporation rate corresponding to a latent heat of evaporation of 169 kilocalories per mole. If the hexaborides are operated at high temperature in contact with the refractory metals, boron diffuses into their metal lattices forming interstitial boron alloys with them. When this occurs, the boron framework which holds the alkaline-earth or rare-earth metal atoms collapses, permitting the latter to evaporate. However, the hexaboride cathodes may be operated at high temperatures in contact with tantalum carbide or graphite. Lanthanum boride cathodes are especially useful in applications where high current densities are required. They are also suitable for high voltage applications because they stand up well under positive ion bombardment. Since they are atmospherically stable and activate easily, they have found wide use in experimental demountable systems.