This paper reviews the crystal chemistry, synthesis, densification, microstructure, mechanical properties, and oxidation behavior of zirconium diboride (ZrB2) and hafnium diboride (HfB2) ceramics. The refractory diborides exhibit partial or complete solid solution with other transition metal diborides, which allows compositional tailoring of properties such as thermal expansion coefficient and hardness. Carbothermal reduction is the typical synthesis route, but reactive processes, solution methods, and pre-ceramic polymers can also be used. Typically, diborides are densified by hot pressing, but recently solid state and liquid phase sintering routes have been developed. Fine-grained ZrB2 and HfB2 have strengths of a few hundred MPa, which can increase to over 1 GPa with the addition of SiC. Pure diborides exhibit parabolic oxidation kinetics at temperatures below 1100°C, but B2O3 volatility leads to rapid, linear oxidation kinetics above that temperature. The addition of silica scale formers such as SiC or MoSi2 improves the oxidation behavior above 1100°C. Based on their unique combination of properties, ZrB2 and HfB2 ceramics are candidates for use in the extreme environments associated with hypersonic flight, atmospheric re-entry, and rocket propulsion.

Refractory Diborides of Zirconium and Hafnium

This review paper illustrates the main normal and superconducting state properties of magnesium diboride, a material known since the early 1950s but only recently discovered to be superconductive at a remarkably high critical temperature Tc = 40 K for a binary compound. What makes MgB2 so special? Its high Tc, simple crystal structure, large coherence lengths, high critical current densities and fields, and transparency of grain boundaries to current promise that MgB2 will be a good material for both large-scale applications and electronic devices. During the last seven months, MgB2 has been fabricated in various forms: bulk, single crystals, thin films, tapes and wires. The largest critical current densities, greater than 10 MA cm−2, and critical fields, 40 T, are achieved for thin films. The anisotropy ratio inferred from upper critical field measurements is yet to be resolved as a wide range of values have been reported, γ = 1.2–9. Also, there is no consensus on the existence of a single anisotropic or double energy gap. One central issue is whether or not MgB2 represents a new class of superconductors, which is the tip of an iceberg awaiting to be discovered. To date MgB2 holds the record for the highest Tc among simple binary compounds. However, the discovery of superconductivity in MgB2 revived the interest in non-oxides and initiated a search for superconductivity in related materialss; several compounds have since been announced to be superconductive: TaB2, BeB2.75, C–S composites, and the elemental B under pressure.

https://iopscience.iop.org/article/10.1088/0953-2048/14/11/201/pdf

Review of the superconducting properties of MgB2

This review paper illustrates the main normal and superconducting state properties of magnesium diboride, a material known since early 1950's, but recently discovered to be superconductive at a remarkably high critical temperature Tc=40K for a binary compound. What makes MgB2 so special? Its high Tc, simple crystal structure, large coherence lengths, high critical current densities and fields, transparency of grain boundaries to current promises that MgB2 will be a good material for both large scale applications and electronic devices. During the last seven month, MgB2 has been fabricated in various forms, bulk, single crystals, thin films, tapes and wires. The largest critical current densities >10MA/cm2 and critical fields 40T are achieved for thin films. The anisotropy ratio inferred from upper critical field measurements is still to be resolved, a wide range of values being reported, between 1.2 and 9. Also there is no consensus about the existence of a single anisotropic or double energy gap. One central issue is whether or not MgB2 represents a new class of superconductors, being the tip of an iceberg who awaits to be discovered. Up to date MgB2 holds the record of the highest Tc in its class. However, the discovery of superconductivity in MgB2 revived the interest in non-oxides and initiated a search for superconductivity in related materials, several compounds being already announced to become superconductive: TaB2, BeB2.75, C-S composites, and the elemental B under pressure.

Review of superconducting properties of MgB2

and Perspectives Sophie Carenco,†,‡,§,∥,⊥ David Portehault,*,†,‡,§ Ced́ric Boissier̀e,†,‡,§ Nicolas Meźailles, and Cleḿent Sanchez*,†,‡,§ †Chimie de la Matier̀e Condenseé de Paris, UPMC Univ Paris 06, UMR 7574, Colleg̀e de France, 11 Place Marcelin Berthelot, 75231 Paris Cedex 05, France ‡Chimie de la Matier̀e Condenseé de Paris, CNRS, UMR 77574, Colleg̀e de France, 11 Place Marcellin Berthelot, 75231 Paris Cedex 05, France Chimie de la Matier̀e Condenseé de Paris, Colleg̀e de France, 11 Place Marcellin Berthelot, 75231 Paris Cedex 05, France Laboratory Heteroelements and Coordination, Chemistry Department, Ecole Polytechnique, CNRS-UMR 7653, Palaiseau, France

Nanoscaled metal borides and phosphides: recent developments and perspectives

Microstructural design of hard coatings

The electronic structure and ground state properties of ${\mathrm{AlB}}_{2}$ type transition metal diborides ${\mathrm{TMB}}_{2}$ (TM=Sc, Ti, V, Cr, Mn, Fe, Y, Zr, Nb, Mo, Hf, Ta) have been calculated using the self consistent tight-binding linear muffin-tin orbital method. The equilibrium volume, bulk moduli ${(B}_{0}),$ pressure derivative of bulk moduli ${(B}_{0}^{\ensuremath{'}}),$ cohesive energy ${(E}_{\mathrm{coh}}),$ heat of formation $(\ensuremath{\Delta}H),$ and electronic specific heat coefficient $(\ensuremath{\gamma})$ are calculated for these systems and compared with the available experimental and other theoretical results. The bonding nature of these diborides is analyzed via the density of states (DOS) histogram as well as the charge density plots, and the chemical stability is analyzed using the band filling principle. The variation in the calculated cohesive properties of these materials is correlated with the band filling effect. The existence of a pseudogap in the total density of states is found to be a common feature for all these compounds. The reason for the creation of the pseudogap is found to be due to the strong covalent interaction between boron p states. We have made spin polarized calculations for ${\mathrm{CrB}}_{2},$ ${\mathrm{MnB}}_{2},$ and ${\mathrm{FeB}}_{2}$ and found that finite magnetic moments exist for ${\mathrm{MnB}}_{2}$ and ${\mathrm{CrB}}_{2}$ whereas ${\mathrm{FeB}}_{2}$ is nonmagnetic.

Electronic structure, bonding, and ground-state properties of AlB 2 -type transition-metal diborides

This paper brings out one of the striking features exhibited by all oxide superconductors. It is found the irrespective of the value of ${T}_{c}$, the average electronegativity \ensuremath{\chi} of all the oxide superconductors found up until now lie between 2.5 and 2.65 in the Pauling's scale and this is considered to be one of the criteria for oxide materials to be considered superconductors. This narrow range is in contrast to the elemental superconductors whose values lie in a broad range from 1.3 to 1.9. Further, the pressures required to drive some of the elemental metals to become superconductors have been calculated on the basis of their electronegativity values. The pressures so obtained agree well with the experimental observations and those calculated on the basis of band theory. The average electronegativity of A\char21{}15 Chevrel-phase superconductors are also given. Based on the concept of electronegativity the other oxide systems which are likely to become superconductors have been suggested.

Correlation between electronegativity and superconductivity.

Correlation between electronegativity and superconductivity

The self-consistent tight-binding linear muffin-tin orbital method was employed to calculate the electronic structure and the total energy of ${\mathrm{Ti}}_{2}X\mathrm{Al}$ $(X=\mathrm{N}\mathrm{b},\mathrm{}\mathrm{V},\mathrm{}\mathrm{Zr})$ in $B2,$ ${D0}_{19},$ and O (orthorhombic) phases and the results were used to study the phase stability and cohesive properties of these intermetallic compounds. Our theoretical calculation shows that the $B2$ phase is the most stable phase of ${\mathrm{Ti}}_{2}\mathrm{NbAl}$ as observed by experimentalists. However, the three phases are close in energy indicating the possibility of the presence of all these phases in equilibrium over a range of temperatures, which is in accordance with experimental observations. Our calculations predict that ${\mathrm{Ti}}_{2}\mathrm{VAl}$ is more stable in the $B2$ phase whereas ${\mathrm{Ti}}_{2}\mathrm{ZrAl}$ is more stable in the ${D0}_{19}$ phase. We also report the calculated equilibrium lattice parameters, cohesive energies, heats of formation, and bulk modulii of these systems, and a possible comparison of the calculated quantities with the available experimental data is made. From our studies we are made to conclude that the concept of pseudogaps which has been very much emphasized for binary intermetallics does not carry so much significance with respect to ternary systems.

Electronic structure, phase stability, and cohesive properties of Ti 2 X Al ( X = N b , V , Zr )

We report here the effect of pressure on the band structure and the superconducting behavior of the hcp metal lutetium. The present work shows that the average of the square of the phonon frequency is considerably increased under pressure and this effect is taken into account while calculating ${T}_{c}$. The calculated ${T}_{c}$ values are in fairly good agreement with the experimental trends. The question of s\ensuremath{\rightarrow}d valence transfer under pressure as well as Heine's fifth-power law are discussed. We also report the variation of other parameters such as the electronic specific heat and conduction bandwidth with pressure.

R. Asokamani

Papers

Electronic structure, bonding, and ground-state properties of AlB 2 -type transition-metal diborides

Correlation between electronegativity and superconductivity.

Correlation between electronegativity and superconductivity

Electronic structure, phase stability, and cohesive properties of Ti 2 X Al ( X = N b , V , Zr )

Band structure and superconductivity in Lu at high pressures.