Boride Carbide Research Articles

Soft phonons in superconducting LuNi2B2C.

Inelastic-neutron-scattering techniques have been used to measure the low-lying phonon-dispersion curves of superconducting ${\mathrm{LuNi}}_{2}$${\mathrm{B}}_{2}$C along the [\ensuremath{\xi}00] and [00\ensuremath{\xi}] symmetry directions. The most important result of this experiment is that the phonon frequencies of the acoustic and first optical ${\mathrm{\ensuremath{\Delta}}}_{4}$ [\ensuremath{\xi}00] branches in the vicinity of the zone-boundary point ${\mathit{G}}_{1}$ decrease with decreasing temperature. Actually these two branches exhibit pronounced dips at low temperatures, close to ${\mathit{G}}_{1}$. This shows that the electron-phonon interaction is quite strong and causes an incipient lattice instability, a behavior typical of strongly coupled conventional superconductors. Furthermore, the phonon anomalies occur at wave vectors close to those of the incommensurate magnetically ordered structures observed in the magnetic compounds of this family, which suggests that both the magnetic ordering and the incipient lattice instabilities are influenced by common nesting features of the Fermi surfaces of the rare-earth nickel boride carbides.

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

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

Competition between magnetism and superconductivity inTmNi2B2C observed by muon-spin rotation

We report muon-spin-rotation measurements of the internal field in the rare-earth nickel boride carbide superconductor ${\mathrm{TmNi}}_{2}$${\mathrm{B}}_{2}$C from 100 mK up to well above the superconducting transition temperature (${\mathit{T}}_{\mathit{c}}$=9.5 K). An oscillatory muon response indicates that the muon is affected by a quasistatic local field that follows a ${\mathit{T}}^{\mathrm{\ensuremath{-}}1}$ dependence over a wide temperature range and without interruption at the superconducting transition. The corresponding relaxation rate remains constant in the normal state, but begins to rise very sharply with decreasing temperature below ${\mathit{T}}_{\mathit{c}}$ scaling approximately with the local field down to its maximum at 2.5 K. The quasistatic internal field may be attributed to a spiral structure or slow three-dimensional correlations of the Tm moments. Decoupling experiments reveal a dynamic depolarization mechanism which may tentatively be ascribed to fast two-dimensional correlations of the Ni moments, slowed by the onset of superconductivity.

Generalized susceptibility and magnetic ordering in rare-earth nickel boride carbides.

The generalized susceptibility for rare-earth nickel boride carbides has been calculated using the normal-state electronic structure. Peaks in the susceptibility occur near wave vectors corresponding to those observed for the incommensurate spin density waves in ${\mathrm{HoNi}}_{2}$${\mathrm{B}}_{2}$C between approximately 4.7 and 6.0 K [A. I. Goldman et al., Phys. Rev. B 50, 9668 (1994)].

Microstructure and properties of boride carbide and aluminide powders for spraying : S. Stolarz et al (Inst. of Non-Ferrous Metals, Sowinskiego, Poland)

Microstructure and properties of boride carbide and aluminide powders for spraying : S. Stolarz et al (Inst. of Non-Ferrous Metals, Sowinskiego, Poland)

Tunneling spectroscopic measurements on boride carbide superconductors

The new boride carbide superconductor LuNi 2B 2C has been investigated by electron tunneling technique using cryogenic temperature scanning tunneling microscopy. A gap parameter of Δ(4.7K)=2.1−2.5 meV (2 Δ/ k B T c =2.9−3.5) was obtained by fitting the Dynes' function to the experimental conductance curves. These values are consistent with the BCS weak coupling superconductivity.

Competition between magnetism and superconductivity in the new rare earth nickel boride carbides

Competition between magnetism and superconductivity was investigated for the newly discovered RNi 2B 2C superconductors with the magnetic rare earth elements R=Tm, Er, Ho, Dy, and Tb. Superconductivity is also observed for the compounds with magnetic R ions and their T c 's are approximately scaled by the de Gennes factor, which implies a very weak coupling between the rare earth magnetic moments and the conduction electrons due to a small conduction electron density at the rare earth site. Associated with the antiferromagnetic order of the rare earth moments, characteristic resistive transition under magnetic field was observed.

An investigation of the Boron isotope effect in Ni and Pd based boride carbide superconductors

The boron isotope effect ( 10B- 11B) was investigated for the boride carbide superconductors YNi 2B 2C and YPd 2B 2C. A sizable isotope effect was observed for both systems and values for the boron isotope exponent were similar in the two systsems ( α B ≈0.27).

The nature of the superconducting phase in yttrium palladium boride carbide

The superconducting multiphase material of nominal composition YPd 5B 3C 0.3 is investigated by means of light-optical metallography, high-resolution electron microscopy, electron diffraction and energy dispersive X-ray spectroscopy. The phase morphology is found to depend sensitively on the conditions adopted during cooling from the melt. Superconductivity is linked to the occurrence of a needle-shaped phase. The structure of this phase is based on a body-centered tetragonal unit cell with a=0.375 nm and c=1.06 nm. It is concluded that the superconducting phase is based on a structure and stoichiometry equivalent to that of LuNi 2B 2C where Lu is replaced by Y and Ni by Pd.

Superconductivity to 21 K in intermetallic thorium-based boride carbides

We report the observation of superconductivity in intermetallic thorium-based boride carbides. For ThM 2B 2C, superconducting T c's of 6 K (M=Ni), 6.5 K (M=Pt), and 14.5 K (M=Pd) have been measured. These compounds are isostructural with the recently reported LnNi 2B 2C superconductors. We have also observed bulk superconductivity in an as-yet-undetermined thorium-palladium-boron-carbon quaternary phase at 21.5 K. Critical-field measurements on the two palladium-based superconductors, as well as the pressure dependence of T c for the 21.5 K superconductor, are reported.

Competition between magnetism and superconductivity in rare-earth nickel boride carbides.

The competition between magnetism and superconductivity was investigated for the recently discovered R${\mathrm{Ni}}_{2}$${\mathrm{B}}_{2}$C superconductors with the magnetic rare-earth elements R=Tm,Er,Ho,Dy. The systematic decrease of ${\mathit{T}}_{\mathit{c}}$, approximately scaled by the de Gennes factor, implies a very weak coupling between the rare-earth magnetic moments and the conduction electrons due to a small conduction-electron density at the rare-earth site. Associated with the antiferromagnetic order of the rare-earth moments, a pronounced dip structure in the upper critical field ${\mathit{H}}_{\mathit{c}2}$(T) was observed; a similar structure has been seen in the previously known magnetic superconductors R${\mathrm{Rh}}_{4}$${\mathrm{B}}_{4}$ and R${\mathrm{Mo}}_{6}$${\mathrm{S}}_{8}$. For ${\mathrm{HoNi}}_{2}$${\mathrm{B}}_{2}$C, the pair-breaking associated with the magnetic transition is strong enough to bring about a resistive reentrant behavior even under zero field, an effect which, to our knowledge, has not previously been observed in antiferromagnetic superconductors.

Electronic properties of superconducting LuNi2B2C and related boride carbide phases.

Band properties of the quaternary superconductor ${\mathrm{LuNi}}_{2}$${\mathrm{B}}_{2}$C (${\mathit{T}}_{\mathit{c}}$\ensuremath{\approxeq}17 K) and related intermetallic phases (including, for example, LuNiBC and ${\mathrm{YNi}}_{2}$${\mathrm{B}}_{2}$C) have been calculated in the local-density approximation via the linear augmented-plane-wave method. The results feature a density-of-states peak near the top of nearly filled Ni(3d) bands, with only modest B and C orbital admixture. The calculated ${\mathrm{LuNi}}_{2}$${\mathrm{B}}_{2}$C bands suggest that these interesting boride carbide phases represent a family of conventional (rather than high-${\mathit{T}}_{\mathit{c}}$) superconductors.

Superconductivity at 23 K in yttrium palladium boride carbide

COPPER oxide compounds have dominated superconductivity research since 1986 because of their very high transition temperatures (Tcs). In contrast, no new families of high-Tc intermetallic compounds have been discovered since the A15-type Nb3X compounds were first reported in 19531. The intermetallies with highest Jcs have all been based on niobium, with the highest Tcs being 20.7 K for bulk Nb3Ga and 23.2 K for sputtered films of Nb3Ge (refs 2, 3). Here we report the observation of superconductivity at 23 K in a multiple-phase bulk sample of a quaternary intermetallic, yttrium palladium boride carbide. This is higher than any Tc reported previously for a bulk intermetallic compound. Although the materials are not yet single-phase, the superconducting volume fraction is large. We propose that this compound may represent the first of a new family of superconducting intermetallics with relatively high Tcs.

Synthesis, characterization, and ceramic conversion reactions of borazine-modified hydridopolysilazanes: new polymeric precursors to silicon nitride carbide boride (SiNCB) ceramic composites

The high-yield synthesis of borazine-substituted hydridopolysilazanes has been achieved by the reaction of liquid borazine with hydridopolysilazane (HPZ) at temperatures between 60 and 90 [degrees]C. The spectroscopic data for the polymers and a model study of the reaction of hexamethyldisilazane with borazine indicate that the new polymers have retained their hydridopolysilazane backbones but are substituted with pendant borazines by means of a borazine-boron to polymer-nitrogen linkage. Such linkages may have been formed by either hydrogen or trimethylsilane elimination reactions. Molecular weight and distribution studies suggest an inhomogeneous distribution of borazine throughout the molecular weight distribution. A decrease in molecular weight with increasing borazine content was observed that apparently results from chain scission reactions of Si-N bonds in the polymer backbone. Studies of the thermolytic reactions of the polymers demonstrated that they are converted in high ceramic and chemical yields to SiNCB ceramic composites. The ceramic yield is increased over that observed for the unmodified hydridopolysilazane due to both the retention of borazine-boron and-nitrogen in the ceramic and a borazine cross-linking reaction which retards the loss of polymer backbone components. Significant differences in the onset, phases and extent of crystallization in the ceramic products obtained from HPZ and the modified polymersmore » were observed. In contrast to the ceramic materials obtained from HPZ, the borazine-modified polymers yield ceramics that are amorphous to 1400[degrees]C. 21 refs., 15 figs., 3 tabs.« less

Possibilities of the formation of carbides and borides by ion implantation

Abstract In the system of boron and carbon, the formation of boron carbide was investigated after ion implantation of 25 keV B ions into carbon or of 25 keV C ions into boron and subsequent annealing. TEM and electron diffraction studies showed that the crystallization of boron carbide begins only at temperatures above 1050°C. By implantation of 20 keV C ions into iron (ion dose 1017 C ions/cm2) only the metastable e-Fe2O will be generated, which at above 220°C transforms into the stable cementite Fe3C. After implantation of 20 keV B ions into iron, no formation of iron boride could be found. These experimental facts can be understood qualitatively with the help of the thermal-spike model. The energy density or the temperature in the thermal spikes is not sufficient for the generation of cementite iron boride or boron carbide.

1: Production and properties of master alloys of Al–Ti–B type and their ability to grain refine aluminium

AbstractA great number of Al–Ti and Al–Ti–B master alloys have been synthesized, varying parameters such as the order of addition of the alloying elements, the relative and absolute amounts of titanium and boron, the temperature ofaddition, the holding time before cooling, and the cooling rate. The efficiencies of all the master alloys as grain refiners of aluminium were tested and the time characteristics, i.e. the change in grain-refining potency with contact time, determined. The Al3Ti phase found in the master alloys can develop three different crystal morphologies, depending on the production parameters chosen. The morphology of the Al3Ti particles seems to influence the time characteristic of the master alloy. The results from the grain-refinement test can be explained neither by boride–carbide theory nor by the peritectic theory.

Electron spin resonance study of EuB 6- xC x: Correlation between electronic localization and transport properties

EuB 6− x C x carbide borides with a CaB 6-type structure were prepared in the range 0 ⩽ x ⩽ 0.20. Magnetic and transport measurements performed on single crystals show that pure EuB 6 is ferromagnetic below T c = 13.7 K. It is semiconducting in the paramagnetic region with an intrinsic small band gap estimated to be 0.05–0.1 eV; an electronic transition to a semimetal occurs below T c . The carbon-doped EuB 6 samples are degenerate semiconductors in which both the carrier concentration and the antiferromagnetic interactions increase with carbon content. These carbide borides were studied using electron spin resonance in the X band at temperatures above the magnetic ordering temperature. A comparison at 300 K of the peak-to-peak linewidth ΔH p- p (the first derivative of the Eu 2+ resonance line) in both the Eu 2+B 6− x C x and the Eu 2+ 1− x Gd 3+ x B 6 systems shows that only x 2 electrons are delocalized in the conduction band for x carbon atoms present in the unit cell. This observation is well confirmed by Hall effect measurements. Furthermore a weak and extremely narrow line which appears (superimposed on the Eu 2+ resonance line) only when x ≠ 0 involves an electronic localization for the remaining x 2 electrons introduced. They are likely to form pairs.

Characteristics of structure formation during deposition of Ni-Cr-B-Si-C alloys

1. The structure of Ni−Cr−B−Si−C alloys used for hard facings is multiphase and, depending on the facing temperature-time conditions, consists of the γ solid solution based on nickel, complex borocarbide (Ni, Fe)3(B, C), chromium boride CrB, carbides Cr23C6 and Cr7C3 and eutectics based on them, and also Ni6Si2B+γ eutectics in various ratios. 2. The most sensitive to changes in the facing temperature are eutectics based on Cr23C6 and (Ni, Fe)3(B, C). Structural changes with rising facing temperatures lead to a lower hardness of the coating. For the PG-SR4-OM alloy it decreases by 1–1.5 units HRC for each 10°C (heating rate 20 deg/sec).

A survey of present knowledge of thermionic emitters

In Sections 1 and 2, after an account of factors influencing the work function of metals, values are quoted and the usefulness of a number of metals as practical thermionic emitters is discussed. The effect of adsorbed films is described in Section 3 and illustrated in connection with emitters with monatomic layers of thorium, caesium and barium. The emission from compounds, and in particular from semiconductors, is considered in Section 4. Sections 5 and 6 deal with the behaviour of certain metallic borides and carbides. Some of these, such as lanthanum boride and thorium carbide, are likely to be valuable as practical emitters. The general properties of oxides are discussed in Sections 8 and 9, where the groups in the periodic table are compared. The barium-strontium-oxide type of cathode is treated in Section 10. This is the most important practical cathode, and the behaviour and limitations are dealt with in some detail. Section 11 describes the performance of thoria as a thermionic emitter.