Transition Metal Diborides Research Articles

Refractory Monocarbides and Diborides of Transition Metals: Promising Components of High-Temperature Composite Materials

Abstract—Development of innovative composite materials capable of operating under extreme conditions requires a comprehensive engineering approach. One of the components of such materials can be represented by monocarbides and diborides of transition metals of groups IV–VI of the periodic table: titanium, zirconium, hafnium, vanadium, niobium, tantalum, as well as their binary systems. Most isomorphic pairs of monocarbides and ddiborides are characterized by unlimited mutual solubility. The HfC–TaC system is of particular interest. The melting point of TaC–4HfC exceeds 4000°C, which is the highest refractoriness among all known synthesized substances.

General trends in surface stability and oxygen adsorption behavior of transition metal diborides (TMB2)

The potential applications of transition metal diborides (TMB2) in extreme environments are particularly attractive but still blocked by some intrinsic properties such as poor resistances to thermal shock and oxidation. Since surface plays a key role during grain growth and oxygen adsorption, an insight into the surface properties of TMB2 is essential for understanding the materials performance and accelerating the development of ultra-high temperature ceramics. By employing two-region modeling method, the stability and oxygen adsorption behavior of TMB2 surfaces were investigated by first-principles calculations based on density functional theory. The effects of valance electron concentration on the surface stability and oxygen adsorption were studied and the general trends were summarized. After analyzing the anisotropy in surface stability and oxygen adsorption, the observed grain morphology of TMB2 were well explained, and it was also predicted that YB2, HfB2 and TaB2 may have better initial oxidation resistance than ZrB2.

The affection of spin-orbit coupling on the electronic and optical properties of AlB2-like transition metal diborides: A first principle study

Abstract Based on the density functional theory and using GGA and GGA + so approximations, the electronic and optical properties of bulk TMB2 (TM = Cr, Mn, and Fe) are calculated. We calculate the electronic properties of these compounds in the presence of spin-orbit interaction. The highest and lowest spin-orbit effects are observed in MnB2 and FeB2 compounds respectively; this effect also causes degeneration disappear in these compounds and its greatest effect is on the orbital p of transition metals at low energy levels. In the up spin state and near the Fermi level, the energy levels are shifted to higher energies, which is not commonly observed in down spin state. Moreover, the optical properties of TMB2 including the real and imaginary parts of e(ω), energy loss function, absorption coefficient, reflectivity, and refraction are calculated for two directions. The spin-orbit effect has significant influence on the optical coefficient of these compounds. Energy loss of the emitted photons is significantly reduced in these compounds at both directions in the presence of spin-orbit effects. Also, the spin-orbit effect in both directions increases the plasmon frequency and eliminates the transparency of the material at higher energies than the plasmon frequency.

Ab initio inspired design of ternary boride thin films

The demand to discover new materials is scientifically as well as industrially a continuously present topic, covering all different fields of application. The recent scientific work on thin film materials has shown, that especially for nitride-based protective coatings, computationally-driven understanding and modelling serves as a reliable trend-giver and can be used for target-oriented experiments. In this study, semi-automated density functional theory (DFT) calculations were used, to sweep across transition metal diborides in order to characterize their structure, phase stability and mechanical properties. We show that early transition metal diborides (TiB2, VB2, etc.) tend to be chemically more stable in the AlB2 structure type, whereas late transition metal diborides (WB2, ReB2, etc.) are preferably stabilized in the W2B5−x structure type. Closely related, we could prove that point defects such as vacancies significantly influence the phase stability and even can reverse the preference for the AlB2 or W2B5−x structure. Furthermore, investigations on the brittle-ductile behavior of the various diborides reveal, that the metastable structures are more ductile than their stable counterparts (WB2, TcB2, etc.). To design thin film materials, e.g. ternary or layered systems, this study is important for application oriented coating development to focus experimental studies on the most perspective systems.

Substoichiometry and tantalum dependent thermal stability of α-structured W-Ta-B thin films

Physical vapor deposited (PVD) WB2 thin films crystallize in the α-AlB2-prototype structure rather than in their thermodynamically stable ω-W2B5-z-prototype structure. Contrary to the majority of α-AlB2-type transition metal diborides (TMB2), α-WB2 exhibits a more ductile character. Combining density functional theory and sophisticated experiments, we show that the stability of α-WB2 thin films is basically influenced by point defects such as vacancies present in PVD materials. With the help of α-TaB2 (one of the most ductile TMB2 with high preference for α-AlB2-type), the thermally driven decomposition and phase transformation of α-W1-xTaxB2-z to the ω-W2B5-z-type can be shifted to temperatures above 1200 °C.

First‐principles investigation of the thermodynamic stability of MB2 materials surfaces (M = Ti/Zr/Hf)

AbstractSome of the renewed interest in transition metal diborides (MB2, M = Ti/Zr/Hf) arises from their potential use as matrices in ultrahigh‐temperature ceramic matrix composites (UHTCMCs). Crucial to the understanding of such composites is the study of the fiber/matrix interfaces, which in turn requires a deep knowledge of the surface structures and the thermodynamics of the matrix material. Here we investigate the surface stability of MB2 compounds by first‐principles calculations. Five surfaces are stabilized when going from a M‐rich to a B‐rich environment, respectively (0001)M, (100)M, (101)B(M), (113)M and (0001)B, with the highly stable (100)M, (101)B(M) and (113)M surfaces being discussed here for the first time. The mechanism behind the surface stability is analyzed in terms of cleavage energy, surface strain and surface bonding states. Our results provide important information for a better understanding of the most likely surfaces exposed to the fibers in UHTCMCs, thereby for the construction of reliable interfaces and ultimately UHTCMCs models.

Selenidation of epitaxial silicene on ZrB2

Abstract The deposition of elemental Se on epitaxial silicene on ZrB 2 thin films was investigated with synchrotron-based core-level photoelectron spectroscopy and low-energy electron diffraction. The deposition of Se at room temperature caused the appearance of Si 2 p peaks with chemical shifts of n × 0.51 ± 0.04 eV (n = 1-4), suggesting the formation of SiSe 2 . This shows that capping the silicene monolayer, without affecting its structural and electronic properties, is not possible with Se. The annealing treatments that followed caused the desorption of Se and Si, resulting in the etching of the Si atoms formerly part of the silicene layer, and the formation of bare ZrB 2 (0001) surface area. In addition, a ZrB 2 (0001)-(√7 × 3)R40.9° surface reconstruction was observed, attributed to a Se-termination of the surface of the transition metal diboride thin film.

Hot corrosion behavior of ZrB2-HfB2 solid solutions in KCl and K2SO4 at 1500 °C

Novel electrode coating materials are needed for coal-fired magnetohydrodynamic (MHD) direct-current extraction power plants. Addition of potassium salt to increase the conductivity of coal plasma combined with high temperature render the service conditions extremely arduous. Therefore, the electrode coating materials must possess certain properties including high-temperature corrosion resistance, high-temperature oxidation resistance, and adequate electrical conductivity. In this work, those very properties of three different compositions of transition metal diboride solid solutions of HfB2-ZrB2 are studied. Two main approaches of oxidation mitigation methods were employed: adding chemical dopants, such as Hf and La, and performing pre-anodization on the samples to create a protective oxide coating. Thermogravimetric experiments were run at different partial pressures of oxygen to determine the high-temperature oxidation resistance of these boride ceramics. In addition, two different salt coatings (KCl and K2SO4) were applied to the surface of the samples to test their hot corrosion resistance. High-temperature polarization resistance tests were also conducted to measure the electrical resistivity. The addition of lanthanum aided in long term oxidation and corrosion protection, with a parabolic rate constant, Kp = 1.33 × 10−5kg2/m4s, a 40% improvement over the sample without the addition of lanthanum under the same conditions. Anodization, however, proved disadvantageous when combined with either lanthanum or a corrosive environment.

Mechanical properties of single crystals of transition metals diborides TMB2 (TM = Sc, Hf, Zr, Ti). Experiment and theory

The mechanical behaviour of TMB2 whiskers (TMB2 = Sc, Hf, Zr, Ti) of 10–20 μm in diameter of the directionally reinforced ceramics LaB6–TMB2 was experimentally studied by the nanoindentation. The pop-in (the abrupt elastic–plastic transition in the indenter penetration) caused by the nucleation of dislocations in the previously dislocations-free region under the imprint was observed in all samples under study. For the first time the experimental estimations of the theoretical shear strength of these materials are obtained. Zone structures and types of interatomic bonds for TMB2 (TM= Sc, Zr, Hf, Ti) are theoretically calculated. The effects of the electronic structures on mechanical properties of diborides of transition metals were analyzed.

Theoretical calculations of hardness and metallicity for multibond hexagonal 5d transition metal diborides with ReB2 structure

The hardness, electronic, and elastic properties of 5d transition metal diborides with ReB2 structure are studied theoretically by using the first principles calculations. The calculated results are in good agreement with the previous experimental and theoretical results. Empirical formulas for estimating the hardness and partial number of effective free electrons for each bond in multibond compounds with metallicity are presented. Based on the formulas, IrB2 has the largest hardness of 21.8 GPa, followed by OsB2 (21.0 GPa) and ReB2 (19.7 GPa), indicating that they are good candidates as hard materials.

Study of electronic structure and Compton profiles of transition metal diborides

We report Compton profiles (CPs) of transition metal diborides (MB2; M= Ti and Zr) using a 740GBq 137Cs Compton spectrometer measured at an intermediate resolution of 0.34a.u. To validate the experimental momentum densities, we have employed the linear combination of atomic orbitals (LCAO) method to compute the theoretical CPs along with the energy bands, density of states (DOS) and Mulliken's population response. The LCAO computations have been performed in the frame work of density functional theory (DFT) and hybridization of Hartree-Fock and DFT (namely B3LYP and PBE0). For both the diborides, the CPs based on revised Perdew-Burke-Ernzerhof exchange and correlation functions (DFT-PBESol) lead to a better agreement with the experimental momentum densities than other reported approximations. Energy bands, DOS and real space analysis of CPs confirm a metallic-like character of both the borides. Further, a comparison of DFT-PBESol and experimental data on equal-valence-electron-density scale shows more ionicity in ZrB2 than that in TiB2, which is also supported by the Mulliken's population based charge transfer data.

Doping-induced superconductivity of ZrB2 and HfB2

Unlike the widely studied $s$-type two-gap superconductor MgB$_2$, the chemically similar compounds ZrB$_2$ and HfB$_2$ do not superconduct above 1 K. Yet, it has been shown that small amounts of self- or extrinsic doping (in particular with vanadium), can induce superconductivity in these materials. Based on results of different macro- and microscopic measurements, including magnetometry, nuclear magnetic resonance (NMR), resistivity, and muon-spin rotation ($\mu$SR), we present a comparative study of Zr$_{0.96}$V$_{0.04}$B$_2$ and Hf$_{0.97}$V$_{0.03}$B$_2$. Their key magnetic and superconducting features are determined and the results are considered within the theoretical framework of multiband superconductivity proposed for MgB$_2$. Detailed Fermi surface (FS) and electronic structure calculations reveal the difference between MgB$_2$ and transition-metal diborides.

Сравнительный анализ модуля Юнга нанотвёрдости в вискерах диборидов переходных металлов

Сравнительный анализ модуля Юнга нанотвёрдости в вискерах диборидов переходных металлов

ZrB2-HfB2 solid solutions as electrode materials for hydrogen reaction in acidic and basic solutions

Spark plasma sintered transition metal diborides such as HfB2, ZrB2 and their solid solutions were investigated as electrode materials for electrochemical hydrogen evolutions reactions (HER) in 1M H2SO4 and 1M NaOH electrolytes. HfB2 and ZrB2 formed complete solid solutions when mixed in 1:1, 1:4, and 4:1 ratios and they were stable in both electrolytes. The HER kinetics of the diborides were slower in the basic solution than in the acidic solutions. The Tafel slopes in 1M H2SO4 were in the range of 0.15–0.18V/decade except for pure HfB2 which showed a Tafel slope of 0.38V/decade. In 1M NaOH the Tafel slopes were in the range of 0.12–0.27V/decade. The composition of HfxZr1−xB2 solid solutions with x=0.2–0.8, influenced the exchange current densities, overpotentials and Tafel slopes of the HER. The EIS data were fitted with a porous film equivalent circuit model in order to better understand the HER behavior. In addition, modeling calculations, using density functional theory approach, were carried out to estimate the density of states and band structure of the boride solid solutions.

Quantum confinement induced band gaps in MgB2 nanosheets

The discovery of two-dimensional semiconducting materials, a decade ago, spawned an entire sub-field within solid-state physics that is focused on the development of nanoelectronics. Here we present a new class of semiconducting two-dimensional material based on hexagonal MgB2. Although MgB2 is a semimetal, similar to the other well-studied transition metal diborides, we demonstrate that, unlike the transition metal diborides, thinning MgB2, to create nanosheets, opens a band gap in the density of states. We predict that a 7 Å thick MgB2 nanosheet will have a band gap of 0.51 eV. MgB2 nanosheets differ from other two-dimensional semiconductors in that the band gap is introduced by (001) surfaces and is opened by the quantum confinement effect. The implications of these findings are that nanostructured MgB2 is not merely a new composition, but also has intrinsic mechanisms for tuning its electronic properties, which may facilitate the development of nanoelectronics.

Synthesis of ultra-refractory transition metal diboride compounds

Abstract

Superhardness effect in transition-metal diborides films

The structure, composition, and properties of transition-metal diboride films have been studied. It was shown that they are characterized by a wide range of structural states, namely from amorphous-like to nanocrystalline with crystallite sizes of 1–50 nm. The characteristic peculiarity of the structure of film transition-metal diborides with high physical and mechanical properties is the formation of a nanocrystalline (columnar) structure with the growth texture in plane [00.1] and a nanocrystallite size of 20–50 nm. The element composition of a superhard highly textured film transition-metal diborides was studied by ion mass spectrometry and Auger electron spectroscopy. The overstoichiometry effect in nanocrystalline transitionmetal diboride films is explained. It was shown that this effect is related to the formation of an additional B⎯B covalent bond, which is realized at subgrain boundaries and leads to the appearance of superhardness in the formed coatings.

First-Principles Analysis on the Catalytic Role of Additives in Low-Temperature Synthesis of Transition Metal Diborides Using Nanolaminates.

Carbon is known to significantly accelerate the formation reaction of zirconium diboride in reactive nanolaminates, although the detailed mechanism remains unclear. Here we investigate the catalytic effect of both C and N on the synthesis of ZrB2 using a first-principles theoretical approach. We show that the strong interactions of C and N with Zr at the B/Zr interfaces of the nanolaminate enhance the solid-state amorphization of the Zr lattice. Amorphization of the Zr, in turn, accelerates intermixing of the constituent layers of the reactive nanolaminate. On the basis of these results we propose that the addition of elements with strong binding energies to transition metals may facilitate low-temperature synthesis of transition metal diborides using reactive nanolaminates.

Transition Metal Diborides as Electrode Material for MHD Direct Power Extraction: High-temperature Oxidation of ZrB2-HfB2 Solid Solution with LaB6 Addition

Transition metal borides are being considered for use as potential electrode coating materials in magnetohydrodynamic direct power extraction plants from coal-fired plasma. These electrode materials will be exposed to aggressive service conditions at high temperatures. Therefore, high-temperature oxidation resistance is an important property. Consolidated samples containing an equimolar solid solution of ZrB2-HfB2 with and without the addition of 1.8 mol pct LaB6 were prepared by ball milling of commercial boride material followed by spark plasma sintering. These samples were oxidized at 1773 K (1500 °C) in two different conditions: (1) as-sintered and (2) anodized (10 V in 0.1 M KOH electrolyte). Oxidation studies were carried out in 0.3 × 105 and 0.1 Pa oxygen partial pressures. The anodic oxide layers showed hafnium enrichment on the surface of the samples, whereas the high-temperature oxides showed zirconium enrichment. The anodized samples without LaB6 addition showed about 2.5 times higher oxidation resistance in high-oxygen partial pressures than the as-sintered samples. Addition of LaB6 improved the oxidation resistance in the as-sintered condition by about 30 pct in the high-oxygen partial pressure tests.

Critical Issues for Producing UHTC-Brazed Joints: Wetting and Reactivity

A brief survey is presented of the most important interaction phenomena occurring at the solid-liquid interfaces in metal-ceramic systems at high temperatures, with special attention to the most recent developments concerning wetting and joining transition metals diborides. These phenomena are described and discussed from both the experimental and theoretical points of view in relation to joining ceramic and metal-ceramic systems by means of processes in the presence of a liquid phase (brazing, TLPB etc.). It is shown that wetting and the formation of interfacial dissolution regions are the results of the competition between different phenomena: dissolution of the ceramic in the liquid phase, reaction and formation of new phases at the solid-liquid interface, and drop spreading along the substrate surface. We emphasize the role of phase diagrams to support both the design of the experiments and the choice of active alloying elements, and to interpret the evolution of the system in relation to temperature and composition. In this respect, the sessile-drop technique has been shown to be helpful in assessing critical points of newly calculated phase diagrams. These studies are essential for the design of joining processes, for the creation of composite materials, and are of a particular relevance when applied to UHTC materials.