V. N. Paderno

Some peculiarities of structure formation in eutectic d‐ and f‐transition metals boride alloys

There are developed the physical principles of creation of the composite materials based on transitions and rare earth metals boride phases, in which are combined the advantages of ceramics (high hardness, strength) simultaneously with very high fracture toughness and heat resistance that is not peculiar to ceramics materials.The basis of constructuion of such compositions is the combination of phases having one common element boron and others with strong difference in electron structure (for instance, d‐ and f‐ metals). Due to the fundamental differences in electron structure, electron cores symmetry, and symmetry of crystal structure of d‐ and f‐ metals borides the mutual solubility of such materials is negligible.It was shown that quasibinary boride systems, particularly lanthanum hexaboride with d‐transition metals diborides are characterized by eutectic type of constitutional diagram.There were determined the phases relations in series pseudobinary systems of borides, corresponding to the eutectic co...

The peculiarities of the structure formation in directionally crystallized eutectics EuB6-MeB2

Abstract The possibility of producing fiber-strengthened eutectic composites in situ in the quasibinary EuB6–MeB2 (Me–Zr, Hf, Sc) alloys is shown. By directional crystallization of the eutectic composition perfect real structures on the base of uniformly distributed MeB2 fibers in the EuB6 matrix may be formed.

The micromechanical properties of melted borides of rare earth metals

Abstract 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.

Features of the real structure of lanthanum hexaboride single crystals

Abstract The real structure of lanthanum hexaboride single crystals was investigated using transmission electron microscopy (thin foil) and diffraction methods. Two structural defects were found, flat segregations and dislocations, having Burgers vector b ‖〈100〉. Most of the dislocations are arranged in {100} planes, which may be slide planes for a simple cubic structure, but the dislocation loop configuration showed that during single-crystal growth their movement is not characterized by slide but by diffusion creep. It is shown that the main sourse of dislocations arising during growth of single crystals is generation by flat segregation which according to diffraction investigations has a different structure from both lanthanum hexaboride and tetraboride.

MgB2: Synthesis, sound velocity, and dynamics of the vortex phase

The sound velocity is measured in polycrystalline MgB2 synthesized from the elements, and the bulk modulus and the Debye temperature are calculated. The conversion of an elastic wave into electromagnetic radiation is investigated in the mixed state.The dynamic parameters of the vortex lattice are estimated. This paper was published in Low Temp. Rhys., 2002, v.28, N3, p.270-274The sound velocity is measured in polycrystalline MgB2 synthesized from the constituent elements, and the bulk modulus in compression and the Debye temperature are calculated. The conversion of an elastic wave into electromagnetic radiation is investigated in the mixed state. The dynamic parameters of the vortex lattice are estimated.

Zirconium Dodecaboride-based cutting material

Samples of zirconium dodecaboride are obtained by means of arc and induction melting of a mixture of zirconium and boron powders. A solder for binding the zirconium dodecaboride to the steel holder of the tool is selected. It is established that cutting tools supplied with zirconium dodecaboride may be used in operations of finish turning of adhesive-active titanium alloys.

Some physical properties of alloys of the systems ZrC-NbC and TaC-HfC

Hall effect and electric resistance of zirconium carbide niobium carbide and tantalum carbide hafnium carbide alloys

Structural and high-frequency (0–110 MHz) resistive characteristics of MgB2 in the temperature range 5–300 K

The structure and resistivity of polycrystalline MgB2 and its impedance at frequencies of 9–110 MHz are determined in the temperature range 5–300 K. It is shown that the lattice type and symmetry of the superconducting phase of MgB2 remain unchanged over this temperature range. At the superconducting transition temperature Tc=39.5 K a structural instability is observed which is accompanied by an overshoot of the measured lattice parameters. It is concluded that the strain of the crystals upon a change in temperature is of a substantially anisotropic character. Measurements of the temperature and frequency dependence of the surface resistance Rs(T,f ) in the superconducting state reveal a transition from the Pippard nonlocal limit at T≪Tc to the London local limit near Tc. At T/Tc<0.76 the value of Rs(T) is well described by an exponential dependence exp(−Δ(T)/kT) in accordance with the BCS theory.

Directional crystallization of MoSi2 and some compositions based on it

Abstract It has been shown that by directional crystallization of pseudobinary compositions of molybdenum disilicide (MoSi 2 ) with some other refractory compounds, particularly with diborides of transition metals (composite materials having a disilicide matrix and extended in crystallization direction diboride inclusions) can be obtained that may improve the mechanical properties of the matrix material.

Kinetics of the densification of lanthanum hexaboride powder with and without a nickel addition during hot pressing

ConclusionsAn investigation was carried out into the kinetics of the densification of LaB6 powder with and without a nickel addition. The effects of temperature, applied pressure, and holding time on the relative densities of the alloyed and unalloyed powders were determined. The results obtained are analyzed within the framework of the theory of volume viscous flow of a porous crystalline solid. It has been established that porous lanthanum hexaboride with and without a nickel alloying addition experiences densification as a result of creep of material controlled by dislocation slip. The energies of activation for creep of these materials and their Laplacian pressures were estimated.