Transition Metal Diborides Research Articles

Effects of transition metal (TM = Zr, Hf, Nb, Ta, Mo, W) elements on the shear properties of TMB2s: A first-principles investigation

Transition metal diborides (TMB2s) exhibit unique combination of excellent properties makes them promising candidates for extreme environmental applications. It is believed that activation of dislocation plasticity is beneficial to retain fracture strength of TMB2s based materials at high temperature, since plastic deformation can release elastic energy accumulated in the material and then fracture is suppressed. In this paper, shear properties of TMB2s (TM=Zr, Hf, Nb, Ta, Mo, W) for both (0001)[2¯110]/3 and (011¯0)[2¯110]/3 shear modes are evaluated by first-principles and the dependence of shear properties on TM elements is discussed by electron redistribution during the deformation. The results show that properties of the (0001)[2¯110]/3 shear are solely determined by TM–B bond and exhibit substantial dependence on TM elements, while the (011¯0)[2¯110]/3 shear are controlled by both TM–B bond and B–B σ bond, which makes the dependence of shear properties on TM elements not that obvious. The simulated ideal shear strengths demonstrate that dislocation nucleation become easier in an order of ZrB2 (≈HfB2), NbB2, TaB2, MoB2, WB2, which reasonably explains some experimental observations and provides useful information for the design of TMB2 based materials by alloying.

Metal-Rich Transition Metal Diborides as Electrocatalysts for Hydrogen Evolution Reactions in a Wide Range of pH

Solid solutions of HfB2-ZrB2 mixtures were prepared by high-energy ball milling of diboride and additive powders followed by spark plasma sintering (SPS). A mixture of stoichiometric 1:1 HfB2-ZrB2 borides was the base composition to which Hf, Zr, Ta, LaB6 or Gd2O3 was added. Hf, Zr and Ta were added in order to bring the boron-to-metal ratio down to 1.86, rendering the boride as MeB1.86. In the case of LaB6 and Gd2O3, 1.8 mol% was added. Electroanalytical behavior of hydrogen evolution reactions was evaluated in 1 M H2SO4 and 1 M NaOH solutions. The LaB6 additive material showed Tafel slopes of 125 and 90 mV/decade in acidic and alkaline solutions respectively. The Hf and Zr rich samples showed Tafel slopes of about 120 mV/decade in both electrolytes. The overpotentials of hydrogen evolution reactions (at 10 mA/cm2) in the alkaline solution were about 100 mV lower than those in acidic solution. The metal-rich diborides and addition of LaB6 showed better hydrogen evolution reaction (HER) activities than the base 1:1 HfB2-ZrB2 stoichiometric diboride solid solution. The higher activity of metal-rich borides could be attributed to the increased electron population at the d-orbitals of the metal shown by band structure modeling calculations using the Density Functional Theory approach.

Enthalpies of Formation of Transition Metal Diborides: A First Principles Study

The enthalpies of formation of transition metals diborides in various structures have been obtained from density functional theory (DFT) calculations in order to determine the ground state at T = 0 K and p = 0. The evolution of the enthalpies of formation along the 3D, 4D, and 5D series has been correlated to the considered crystal structures. In the whole, the calculated values of the enthalpies of formation of the diborides in their ground state are in good agreement with the experimental ones when available. The calculated values of the lattice parameters at T = 0 K of the ground state agree well with the experimental values. The total and partial electronic densities of states have been computed. Special features of the transition metal electronic partial density of states have been evidenced and correlated to the local environment of the atoms.

Structure, composition, and mechanical properties of thin films of transition metals diborides

The effect of the deposition conditions on the structure, composition, and mechanical properties of thin films of diborides of transition metals that have been produced by high frequency magnetron sputtering. It has been shown that depending on the applied bias voltage and substrate temperature coatings of various structures are formed: from amorphous-like to nanocrystalline. Under the optimal energy conditions (bias voltage 50 V and substrate temperature 500°C) superstoichiometric thin films of transition metals diborides of grain sizes 20–30 nm, hardness 44 GPa, and anomalously high recovering of the imprint depth have been produced.

Influence of boron vacancies on phase stability, bonding and structure of MB2 (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W) with AlB2 type structure

Transition metal diborides in hexagonal AlB2 type structure typically form stable MB2 phases for group IV elements (M = Ti, Zr, Hf). For group V (M = V, Nb, Ta) and group VI (M = Cr, Mo, W) the stability is reduced and an alternative hexagonal rhombohedral MB2 structure becomes more stable. In this work we investigate the effect of vacancies on the B-site in hexagonal MB2 and its influence on the phase stability and the structure for TiB2, ZrB2, HfB2, VB2, NbB2, TaB2, CrB2, MoB2, and WB2 using first-principles calculations. Selected phases are also analyzed with respect to electronic and bonding properties. We identify trends showing that MB2 with M from group V and IV are stabilized when introducing B-vacancies, consistent with a decrease in the number of states at the Fermi level and by strengthening of the B–M interaction. The stabilization upon vacancy formation also increases when going from M in period 4 to period 6. For TiB2, ZrB2, and HfB2, introduction of B-vacancies have a destabilizing effect due to occupation of B–B antibonding orbitals close to the Fermi level and an increase in states at the Fermi level.

Mechanical strength and electronic instabilities in ultra-incompressible platinum dinitrides

The mechanical properties and electronic structure of recently synthesized $\mathrm{Pt}{\mathrm{N}}_{2}$, proposed as a potential candidate for superhard materials, have been investigated by means of density functional theory. Although it shows a clear band gap indicating a covalent bonding nature, the calculated shear moduli and ideal strengths of both proposed $\mathrm{Pt}{\mathrm{N}}_{2}$ polymorphs are much lower than those of $\mathrm{Re}{\mathrm{B}}_{2}$, suggesting that it should be weaker than $\mathrm{Re}{\mathrm{B}}_{2}$, whose load-invariant hardness is less than 30 GPa. The anisotropic strength of the pyrite $\mathrm{Pt}{\mathrm{N}}_{2}$ polymorph is significantly higher than that of the fluorite polymorph due to a larger covalent contribution. The shear instability for both polymorphs occurs in a cleavagelike mode between the weakly bonded crystal planes. This behavior is different from transition-metal (TM) diborides where the TM-TM or TM-B bonds are the carriers of the shear instability.

Hot Pressing and Characterization of ZrB<sub>2</sub>-SiC-MoSi<sub>2</sub> Composite

Transition metal diborides, especially zirconium and hafnium diboride are potential ceramic material for ultra high temperature applications above 1800°C. These borides are characterized by high melting point, formation of high melting point oxides, good oxidation resistance and excellent thermo-mechanical properties. In this present exploration, zirconium diboride (ZrB2) has been selected for its moderate density (6.09 gm/cc) and better oxidation resistance compared to high density hafnium diboride (11.2 gm/cc). The developed ZrB2 composite in the present study contains 10 wt. % SiC and 10 wt. % MoSi2 as sintering additives. SiC and MoSi2 were added to improve the thermal shock resistance and sinterability of the ultra high temperature ceramics (UHTCs). Vacuum hot pressing was carried out at 1800°C for a holding period of 30 minutes and applied pressure of 30 MPa. Attractive feature of this ZrB2 composite is good machinability due to better electrical conductivity and complicated shapes can be realized easily through electro discharge machining (EDM) process. Detailed XRD phase analysis and microstructural investigation of the polished and fractured composites was carried out using SEM. Mechanical and thermal properties tests have been carried out for the optimized ZrB2 composite material.

Debye Temperatures of Transition Metal Diborides TMB2 (TM= Ti, Zr, Os, Nb, Ru) Under Pressure

The modern scientific and technical revolution is responsible for increasing interest and impetus in the search for materials possessing specific and desired properties. Today, the transition metal diborides (TMB2) are of great interest because they have been found to possess unique physical and chemical properties. As an important physical quantity, the Debye temperature is closely related to the elastic constants, specific heat and melting point. Through the quasi harmonic Debye model, the dependency of Debye temperature on pressure P for various transition metal diborides is successfully obtained. The obtained results are consistent with the available theoretical results.

General Trends in Electronic Structure, Stability, Chemical Bonding and Mechanical Properties of Ultrahigh Temperature Ceramics TMB2 (TM = transition metal)

The electronic structure, stability, chemical bonding and mechanical properties of 3d, 4d and 5d transition metal diboride TMB2 were investigated using first-principles calculations based on density functional theory. All the primary chemical bonds, i.e., metallic, ionic and covalent have contributions to the bonding of TMB2. The number of valence electrons of transition metals or the valence electron concentration (VEC) of TMB2 has strong effects on the lattice parameters, stability and mechanical properties of TMB2. Both lattice constants a and c decrease with VEC, but c decreases faster than a, which is attributed to the enhanced TM d–B p (sp2) bonding. Bulk modulus B of TMB2 increases continuously with VEC due to the enhanced TM d–B p (sp2) and TM dd bonding. Shear modulus G increases with VEC, reaching a maximum at VEC = 3.33, and then decreases with further increase of VEC. YB2 and MnB2 have low Young's modulus and are predicted to have good thermal shock resistance. According to Pugh's criterion (G/B < 0.571), MnB2, MoB2 and WB2 are predicted as ductile or damage tolerant ultrahigh temperature ceramics (UHTCs).

Layered titanium diboride: towards exfoliation and electrochemical applications.

Layered transition metal diborides (TMDB), amongst other refractory metal borides, are commonly employed for material fabrication such as wear- and corrosion-resistant coatings due to their impressive chemical stability and thermal conductivity. In spite of the wide scope of studies carried out on TMDB in the physical field, investigations on its electrochemistry remain limited. Since the physical properties play a vital role in any material's electrochemical behaviour, we explore the viability of the most popular form of titanium boride, layered TiB2, as catalysts for electrochemical energy reactions, including hydrogen evolution and oxygen reduction. Three types of TiB2 were compared in this work - TiB2 separately modified with sodium naphtalenide and butyllithium in an attempt to exfoliate TiB2 and unmodified TiB2. The electrocatalytic activity displayed by all three TiB2 materials provides a wider range of opportunities for the application of TiB2 in material studies.

Nb-B-C thin films for electrical contact applications deposited by magnetron sputtering

The high wear resistance, high chemical inertness, and high electrical conductivity of magnetron-sputtered transition metal diborides make them a candidate material for sliding electrical contacts. However, their high hardness makes it difficult to penetrate surface oxides, resulting in a high electrical contact resistance. In this study, the authors have investigated how the contact resistance can be improved by the formation of softer Nb-B-C films. The Nb-B-C films were deposited by magnetron sputtering and shown to exhibit a nanocomposite microstructure consisting of nanocrystalline NbB2−x grains with a solid solution of C separated by an amorphous BCx phase. The formation of the BCx phase reduces the hardness from 41 GPa for the NbB2−x film to 19 GPa at 36 at. % C. As a consequence the contact resistance is drastically reduced and the lowest contact resistance of 35 mΩ (contact force 5 N) is achieved for a film containing 30 at. % C. However, crack formation and subsequent delamination and fragmentation is observed for the C-containing Nb-B-C films in tribology tests resulting in high friction values for these films.

Microstructural Evolution of (Ti,W,Cr)B2 Coatings Deposited on Steel Substrates during Annealing

The topic of the present experiments are transition metal diboride coatings of composition (Ti0.49W0.51)B2 and (Ti0.44W0.30Cr0.26)B2. The coatings were deposited on steel substrates using dc magnetron sputtering. We investigated how annealing in argon at elevated temperatures modifies microstructure. The as-deposited films are amorphous. Annealing between 700 and 1100 °C results in the formation of nano-crystalline precipitates with average grain diameters of about 10–50 nm. A TiC phase (Fm-3m; a ≈ 4.3 Å) is observed as the dominating precipitate phase. In addition, small amounts (10%–20%) of a Cr23C6 phase (Fm-3m; a ≈ 10.6 Å) are observed. In contrast to literature data on the same coatings deposited on silicon substrates, the formation of boride precipitate phases is strongly suppressed here. From investigations with X-ray diffractometry, electron microscopy and secondary ion mass spectrometry we conclude that the nanostructure of the coatings is formed by reactive phase formation of the boride coating with the carbon containing steel substrate.

Grain refining aluminium foundry alloys with commercial Al–B master alloys

In Al–3B master alloy, a higher fraction of borides is of the AlB2 variety, while in Al–8B alloy, the predominant species is the AlB12 phase. AlB12 is less stable than AlB2 and is engaged in exchange reactions, leading to the formation of transition metal diborides that subsequently settle at the bottom of the melt. Hence, AlB12 is involved more in precipitating transition elements than in refining the grain structure. With predominantly AlB12 particles, Al–8B master alloys are better suited for the removal of transition metal impurities in the manufacture of aluminium conductors. Al–3B, on the other hand, is a better grain refiner as the majority of its borides are of the AlB2 variety. Which of the two master alloys is used in grain refinement does not make a difference once the transition metal impurities have been precipitated. B is dedicated to grain refinement in an impurity free aluminium melt and produces exceptionally small equiaxed grains across the section of the samples.

Effect of Elastic Constants on the Ultrasonic Properties of Group VIB Transition Metal Diborides

The ultrasonic properties like ultrasonic attenuation, sound velocity in the group VIB transition metal diborides like CrB2 and MoB2 have been studied along unique axis at room temperature. The second- and third order elastic constants (SOEC & TOEC) have been calculated for these diborides using Lennard- Jones potential. The velocities VL and VS2 increases with the angle from the unique axis and VS1 have maximum at 45 0 with unique axis of the crystal. The inconsistent behaviour of angle dependent velocities is associated to the action of second order elastic constants. Debye average sound velocities of these compounds are increasing with the angle and has maximum at 55 0 with unique axis at room temperature. Hence when a sound wave travels at 55 0 with unique axis of these materials, then the average sound velocity is found to be maximum. The mechanical properties of CrB2 are better than MoB2, because CrB2 has low ultrasonic attenuation comparison t h a n MoB2. The comparison of calculated ultrasonic parameters with available theoretical/experimental physical parameters gives information about classification of these compounds.

Phase Stability, Physical Properties, and Hardness of Transition-Metal Diborides MB2 (M = Tc, W, Re, and Os): First-Principles Investigations

Using first-principles calculations, the structural stability, elastic strength, and formation enthalpies of four diborides MB2 (M = Tc, W, Re, and Os) are investigated by means of the pseudopotential plane-waves method, as well as the roles of covalency and bond topology in the phase incompressibility. Three candidate structures of known transition-metal diborides are chosen to probe. The calculated lattice parameters, elastic properties, Poisson’s ratio, and B/G ratio are derived. It is observed that the ReB2-type structure containing well-defined zigzag covalent chains exhibits an unusual incompressibility along the c axis comparable to that of diamond. Formation enthalpy calculations demonstrate that the ground-state phase is synthesizable at low pressure, whereas the other phase can be achieved through the phase transformation. Moreover, according to Mulliken overlap population analysis, a semiempirical method to evaluate the hardness of multicomponent crystals with a partial metallic bond is presented. The predicted hardness of WB2–WB2, ReB2–ReB2, and OsB2–OsB2 is in reasonable agreement with experiment data. Both strong covalency and a zigzag topology of interconnected bonds underlie the ultraincompressibilities. In addition, the superior performance and largest hardness of ReB2–ReB2 indicate that it is a superhard material. This work provides a useful guide for designing novel borides materials having excellent mechanical performances.

ChemInform Abstract: A Review of Transition Metals Diborides: from Wettability Studies to Joining

AbstractReview: 128 refs.

A review of transition metals diborides: from wettability studies to joining

The renewed interest in transition metals diborides, classified as ultra high temperature ceramics, for applications at high temperature or in highly aggressive environments has given rise in recent years to an increasing number of research projects particularly devoted to the study of their interfacial characteristics when placed in contact with liquid metals and alloys, which complement and upgrade the limited number of older investigations. This paper critically reviews these systematic studies, addressing both basic (wettability, interfacial tension and phase equilibria determination) and application (joining by brazing) aspects. Particular reference is made to studies aimed at elucidating the role that dissolution, chemical reactions, additions of active metal elements to the molten matrix have in the wetting process and on the solid–liquid adhesion in relation to the mechanical characteristics of brazed joints.

Structure, Elastic Stiffness, and Hardness of Os1–xRuxB2 Solid Solution Transition-Metal Diborides

On the basis of recent experiments, the solid solution transition-metal diborides were proposed to be new ultra-incompressible hard materials. We investigate using density functional theory based methods the structural and mechanical properties, electronic structure, and hardness of Os1–xRuxB2 solid solutions. A difference in chemical bonding occurs between OsB2 and RuB2 diborides, leading to significantly different elastic properties: a large bulk, shear moduli, and hardness for Os-rich diborides and relatively small bulk, shear moduli, and hardness for Ru-rich diborides. The electronic structure and bonding characterization are also analyzed as a function of Ru–dopant concentration in the OsB2 lattice.

A Common Regularity of Stoichiometry-Induced Morphology Evolution of Transition Metal Carbides, Nitrides, and Diborides during Self-Propagating High-Temperature Synthesis

The morphologies of the transition metal carbide (TMC) (ZrCx, NbCx, and TaCx), transition metal nitride (TMN) (TiNx), and transition metal diboride (TMD) (NbB2x and TaB2x) particles formed during the self-propagating high-temperature synthesis (SHS) were investigated. The results indicate that the ceramics with wide stoichiometric ranges all show a stoichiometry-induced morphology evolution, i.e., octahedron → truncated-octahedron → spherelike → sphere, for TMCs and TMNs, and hexagonal prism → polyhedron → spherelike, for TMDs. For TMCs and TMNs, the increase in the stoichiometry leads to the increase in the growth rate in the ⟨111⟩ crystalline direction. Hence, their morphologies show an evolution process of gradual exposure of the {100} surfaces and shrinkage of the {111} surfaces. When the exposed {100} surfaces are roughed because of the extremely high combustion temperatures during the SHS and thus turn round, the growth shapes of the TMC and TMN crystals change to spherelike. On the other hand, when...

Control of Interfacial Reactivity Between ZrB2 and Ni-Based Brazing Alloys

Transition metals diborides (Ti,Zr,Hf)B2 play a key role in applications where stability at extremely high temperatures and damage tolerance are required; however, much research has still to be done to optimize the joining of these materials to themselves or to other high-temperature materials. In this study, the reactivity at the solid-liquid interface between ZrB2 ceramics and Ni-based brazing alloys has been addressed; it is shown how the reactivity and the dissolution of the solid phase can be controlled and even suppressed by adjusting the brazing alloy composition on the basis of thermodynamic calculations. Wetting experiments on ZrB2 ceramics by Ni, Ni-B 17 at.%, and Ni-B 50 at.% were performed at 1500 and 1200 °C by the sessile drop technique. The obtained interfaces were characterized by optical microscopy and SEM-EDS, and interpreted by means of the ad hoc-calculated B-Ni-Zr ternary diagram. A correlation among microstructures, substrate dissolution, shape of the drops, spreading kinetics, and the phase diagram was found. The effect on the interfacial reactivity of Si3Ni4 used as a sintering aid and issues related to Si diffusion into the brazing alloy are discussed as well.