Transition Metal Diborides Research Articles

X‐ray Photoelectron Spectroscopy Studies of Metallic Diborides

The binding energies of core electrons in d‐type stoichiometric diborides of several transition metals were measured by X‐ray photoelectron spectroscopy. Metal core lines in the diborides shifted either to higher or to lower binding energies compared with those appertaining to corresponding elemental states and varied with the metal. The shifts were < 0.6 eV. Carbides, oxides, and oxycarbides of variable stoichiometries were the major impurities detected in the diborides. The binding energy shifts showed a progressive change as the electronegativity difference between the and the metalloid (boron, carbon, nitrogen, oxygen) increased. This indicated that the bond in boro‐oxycarbides may be partially ionic.

各種ＰＶＤ法による遷移金属２ほう化物薄膜の作製

各種ＰＶＤ法による遷移金属２ほう化物薄膜の作製

Bonding properties and vibrational entropy of transition metal MeB 2(AlB 2) diborides

The transition metal diborides MeB 2 were chosen as an example to illustrate an approach introduced by us, by which existing thermodynamic information can be systematized and new information generated. We consider trends in the melting temperature, the enthalpy of formation and lattice vibration properties as the metal Me varies along the 3d, 4d and 5d transition metal series. The trends show a consistent picture which can be explained from the electronic density of states by a rigid-band like argument. This fact gives us confidence to apply the regularities and obtain new information by interpolation and extrapolation procedures. In particular, we estimate the standard entropy S⊖ at 298.15 K for stable AlB 2 structure compounds where there are no previous measurements and for some metastable compounds ( Me ≡ Sc, V, Cr, Mn, Y, Mo, Tc, W and Re). The results are compared with estimates based on various empirical methods. In the Me-B systems, the MeB 2 phase may be stable at all temperatures, or be a high-temperature phase or a metastable phase. We explain the observed trend. Through the use of thermodynamic phase diagram calculations (the so-called CALPHAD method), we study the thermodynamic properties of the metastable AlB 2 structure phase FeB 2.

Thermal expansion studies on the group IV–VII transition metal diborides

The thermal expansions of the group IV–VII transition metal diborides were studied with the aid of X-ray powder diffraction. The diborides were studied over the temperature range 298–1500 K. All the diborides except for CrB 2 display larger thermal expansion coefficients in the c direction than in the a direction. The expansion coefficients in the c direction decrease with increasing radius of the metal atom, a fact which can be correlated to an increase in metal-boron bond strength. The thermal expansion coefficient in the a direction changes very little with the size of the metal radius, owing to the fact that the bonding strength in the basal plane is determined by the strong B-B bonds within the boron layer. The mean thermal expansion coefficients of the diborides increase linearly with their reciprocal melting points.

ChemInform Abstract: Enthalpies of Formation of First‐Row Transition‐Metal Diborides by a New Calorimetric Method (determination of ΔfH°m of ScB2, TiB2, and VB2 at 1400 K).

ChemInform Abstract: Enthalpies of Formation of First‐Row Transition‐Metal Diborides by a New Calorimetric Method (determination of Δ_fH°_m of ScB₂, TiB₂, and VB₂ at 1400 K).

TiB2 and ZrB2 diffusion barriers in GaAs Ohmic contact technology

The transition metal diboride compounds, ZrB2 and TiB2, interposed between Ni/Ge/Au Ohmic contact metallization on n-type GaAs wafers and an overlying thick Au contact layer, have been investigated to evaluate their effectiveness in stabilizing the Ohmic contact by limiting the in-diffusion of Au. All of the metal layers were e-beam deposited except the ZrB2 which was rf-diode sputtered. The barrier layer thicknesses were 50 and 100 nm for the TiB2 and the ZrB2, respectively. Postdeposition alloying of the contacts was performed at 400, 425, or 450 °C. Auger electron spectroscopy depth profiling of the resultant Ohmic contacts demonstrates that the barrier layers effectively preclude penetration of Au to the Ohmic contact structure. Specific contact resistivities for such contacts are in the low 10−7 Ω cm2 range; although some degradation of the contact resistivity is observed after long term annealing, the values of resistivities do not exceed 1.5×10−6 Ω cm2 after 92 h at 350 °C.

Enthalpies of formation of first-row transition-metal diborides by a new calorimetric method

The standard enthalpies of formation of the diborides of Sc, Ti, and V have been determined calorimetrically at (1400 ± 2) K. Each of these borides was dropped together with solid (platinum or palladium + boron) from room temperature into the calorimeter to generate liquid alloys (platinum or palladium + scandium or titanium or vanadium + boron). In separate experiments alloys of the same composition were formed by dropping the elements. The following values of Δ f H m o are reported: ScB 2, −(307.0 ± 14.8) kJ · mol −1; TiB 2, −(328.4 ± 10.2) kJ · mol −1; VB 1.925, −(206.9 ± 12.2) kJ · mol −1. The results are compared with earlier literature values where they exist. However, for ScB 2 there are no earlier values and for VB 2 only equilibrium values. The systematic variation of the enthalpy of formation of the diborides from ScB 2 to MnB 2 is discussed in relation to the structural parameters and to the change in MeMe, BB, and MeB bonding.

遷移金属非酸化物の材料開発

The preparations (thin film, whisker, fine powder, sintered body and single crystal) of transition metal carbides, nitrides and diborides, electrical, superconductive, mechanical and oxidation resistance characteristics, and practical applications of these compounds are reviewed from the standpoints of material science. These refractory compounds are promising candidate materials for the following areas.(1) Super-LSI technology and electronic device: field and thermal emitters, TiN as diffusion barrier in metallization to Si semiconductor, resistive thermoconductive humidity sensor with TaN film and Josephson turnel junction with NbN film.(2) Energy related materials: solar-selective surface with ZrC thin film, emitter for thermionic generator, electrodes (ZrB2 and HfB2) for MHD generator and first wall coated with TiC or TiB2 film for tokamak.

Synthesis and properties of some refractory transition metal diboride thin films

Thin films of ZrB 2 and TiB 2 were prepared by r.f. diode sputtering and successfully recrystallized by rapid annealing methods. The properties of the as-deposited and annealed films, which will be described, were evaluated by four-point probe resistivity measurements, Auger electron spectroscopy, Rutherford back- scattering, X-ray diffractometry and transmission and scanning electron microscopy. The resultant annealed thin film resistivities are shown to approach those of the refractory metal silicides; the materials thus appear to be promising candidates for integrated circuit metallization application.

The electronic structure of the first-row transition-metal diborides

The electronic structures of ScB2, TiB2, VB2, CrB2 and MnB2 have been examined by theoretical investigations. The band structures and accompanying density-of-states plots are presented. The calculated Fermi Levels are, −5.6 eV (ScB2), −5.7 eV (TiB2), −6.3 eV (VB2), −7.1 eV (CrB2), and −7.8 eV (MnB2). The valence bands at the Fermi Edge are localised about the metal 3d orbitals. The charge distributions of the diborides are obtained from the density-of-states plots and show that the metals possess the following positive charges: Sc (+2.28), Ti (+1.99), V (+1.85), Cr (+1.52), and Mn (+1.08). The bonding within the diborides is explained with the help of solid-state calculations at a Special Point and quasi-molecular cluster calculations.

The electronic structure of the first-row transition metal diborides

The electronic structures of ScB2, TiB2, VB2, CrB2 and MnB2 have been examined by theoretical investigations. The band structures and accompanying density-of-states plots are presented. The calculated Fermi Levels are, −5.6 eV (ScB2), −5.7 eV (TiB2), −6.3 eV (VB2), −7.1 eV (CrB2), and −7.8 eV (MnB2). The valence bands at the Fermi Edge are localised about the metal 3d orbitals. The charge distributions of the diborides are obtained from the density-of-states plots and show that the metals possess the following positive charges: Sc (+2.28), Ti (+1.99), V (+1.85), Cr (+1.52), and Mn (+1.08). The bonding within the diborides is explained with the help of solid-state calculations at a Special Point and quasi-molecular cluster calculations.

Reactions of transition metal diborides with aluminum

Reactions of transition metal diborides with aluminum

Phase Identification in TiB2-Ni Composites by Analytical Electron Microscopy

The transition metal diboride TiB2 is characterized by high hardness and high melting point (3253 K) . These properties make this material attractive for applications such as valve components in coal liquefaction plants and cutting tools. Liquid phase hot pressing using nickel as the fluidizing medium allows densification at lower temperatures than when using TiB2 powders alone, but the nickel and TiB2 react to form a complex multiphase microstructure. The purpose of this investigation was to identify the nickel-rich binder phase. The material examined was taken from a cylindrical compact hot pressed at ∼1720 K. During pressing most of the original 15 mol % Ni exuded from the initial mixtures. Specimens 3 mm dia were prepared for analytical electron microscopy (AEM) examination by mechanical lapping followed by ion milling.A typical microstructure of the TiB2-Ni composite examined at 120 kv by conventional transmission electron microscopy (TEM) is shown in Fig. 1. The microstructure is characterized by TiB2 grains bonded by a second phase which was observed at multiple grain intersections. X-ray energy dispersive spectroscopy (EDS) measurements were made using a Philips EM400T/FEG. probe sizes of ∼10 nm dia and probe currents of ∼5 nA were used so that measurements could be made in thin regions of the binder phase, where beam broadening was small. Typical x-ray spectra from an intergranular region and an adjacent TiB2 grain are shown in Fig. 2. The results of standardless quantitative analyses of binder phase spectra indicated a composition (for Z > 11) of at least 95% Ni.

Electron transfer and bonding in the transition metal diborides the heats of formation of the metal diborides derived from a theoretical bonding model

If interatomic electron transfer changes the bond orders from n o 1 , n o 2 , n o 3 …, to 1, n 2, n 3…, and z 1, z 2, z 3…, are the numbers of bonds of each type, the stability of a given compound is accounted for by the number of transfer electrons Δϵ = z 1( n o 1 − n 1) + z 2( n o 2 − n 2) + z 3( n o 3 − n 3)… occupying the lowest energy bands arising from the interaction of various atomic states. Δϵ is proportional to the heat of formation ΔH f . ΔH f = ΔϵE b where E b is the average energy for all valence bands. This approach is used to correlate the heats of formation of the transition metal diborides with Δϵ values obtained from electron population analyses. The consistency of the results lends support to the electron transfer bonding model and to the hypothesis that boron atoms in a metal diboride form a graphite-type layer structure after increasing their valence with the transfer electrons to a value of four. The delocalization of the transfer electrons between the π orbitals of boron and the empty d orbitals of the metal ensures electroneutrality and the stability of the metal diboride.

Golcao calculation of electronic spectra of transition-metal compounds

1. Basic concepts are presented on electron-spectrum calculation for crystalline substances using the GOLCAO method. 2. Detailed calculations are presented on the electron spectra of transition-metal diborides. 3. The calculations have been used to estimate electron structure parameters such as dissociation energy, atomic charge, electron-state filling, and statistical weights of atoms with stable electronic configurations. 4. There is a relationship between the trends in the properties of these compounds and the filling of the orbitals in the stable sp configurations of the boron atom and the d configurations of the metal atom.

Some physical characteristics of the diborides of transition metals of groups IV and V

Some physical characteristics of the diborides of transition metals of groups IV and V

Nuclear magnetic resonance knight shifts and quadrupole effects in transition metal diborides

Nuclear magnetic resonance data for ScB 2, YB 2 and MnB 2 are presented. These data together with earlier NMR data provide a complete picture of the 3 d and 4 d transition metal diborides: ScB 2 through MnB 2 and YB 2 through MoB 2. The Knight shifts, electric quadrupole coupling constants, and linewidths of the central resonance of the 45Sc and 11B nuclei in ScB 2 are essentially temperature independent in the range 4–300°K. The boron Knight shifts. K(B) are 0·00(1)%. −0·004(10)% and −0·05(2)% in ScB 2, YB 2 and MnB 2, respectively. K(Sc)=0·07(1)% and K(Y)=0·20(3)% in the respective diborides. The difference between these values and the higher values for metallic Sc and Y is attributed primarily to reduced orbital and Pauli contributions. For the 3 d series ScB 2 to MnB 2, the Knight shifts, electronic specific heats, magnetic susceptibilities and transport properties indicate increasing d-electron character with increasing group number. The quadrupole coupling constant of 11B across both 3 d and 4 d series does not correlate with the c/a ratio or unit cell volume. This may indicate deviation of the boron atoms from a planar configuration.

Gaseous Metal Borides. II. Mass-Spectrometric Evidence for the Molecules UB2, UB, and CeB and Predicted Stability of Gaseous Diborides of Electropositive Transition Metals

A combination of Knudsen effusion and mass-spectrometric techniques has been used for the investigation of the reactions (1) UB2(g) + 2C(g) = UC2(g) + 2B(g), (2) UB(g) + C(g) = UC(g) + B(g), and (3) CeB(g) = Ce(g) + B(g). The experimental third-law enthalpies have been combined with appropriate literature data to obtain the following dissociation energies or heats of atomization, D0°, in kilocalories/mole: UB, 76 ± 8; CeB, 72 ± 5; and UB2, 227 ± 10. The Pauling model of a strongly polar M–B2 bond has been used to estimate the heats of atomization of gaseous diborides of selected electropositive transition metals.

Electronic structure of transition metal diborides

Electronic specific heat, magnetic susceptibility and V51 Knight shift measurements on the transition metal diborides of the first series (ScMn) show that the values of the density of states at the Fermi level are high and typical for a dominantly d-character band. Very high values of the density of states for CrB2, (VCr)B2 and (MnCr)B2 are related to strong electron-electron correlation effects.

Legierungsverhalten von HfB 2 mit uran- und übergangsmetalldiboriden

Phase boundaries in the ternary hafnium-uranium-boron system are delineated for the 1100°C section on the basis of X-ray examination in the temperature range 1100–1700°C. The ternary equilibria are: HfB 2UB 12UB 4; HfB 2UB 4UB 2; HfB 2UB 2(U, Hf) and HfB 2HfB(U, Hf), where the brackets represent saturated solid solutions. The mutual solubility of the diborides of the transition metals titanium, zirconium and hafnium, and uranium diboride is slight because of the considerable disparity in c a . The formation of mixed crystals between hafnium diboride and the other transition metal diborides, and the mutual solubility of all these diborides, can be understood in terms of the size differences of the transition metal atoms. All binary systems with radius differences <14% have complete mutual solubility. The systems HfB 2MoB 2 and ZrB 2MoB 2, for which the radius difference = 14%, have partial and complete mutual solubility, respectively. All systems with a greater radius difference have a solubility gap.