Tungsten Tetraboride Research Articles

Reactivity of tungsten tetraboride powders with nickel alloys: Effect of TiAl3 and Zr additions

Sintering of WB4-B-TaB2 powders is significantly activated by Ni additions. Porosity removal is achieved at 1100 °C-150 MPa-1 h, that is, 250 °C below the temperature needed without nickel. However, there is strong chemical reactivity between Ni and WB4-B-TaB2 powders leading to the formation of W2B5, NiB, Ni4B3 and M2B5 phases. Since no metallic nickel remains after sintering, these ceramic composites are very brittle. Strength and toughness of WB4-B-TaB2-Ni alloys are notably improved by TiAl3 and Zr additions. Although Ni containing borides are still present in these materials, there are also Ni-rich regions free of boron after sintering, which provide a significant strengthening effect (reaching TRS values near 1 GPa). However, when Zr is added to the mixtures, WB4 grains are fully decomposed into a combination of mixed borides.

Theoretical prediction of stable WB4 monolayer as a high-capacity anode material for alkali-metal ion batteries

As a rapidly evolving 2D monolayer material, tungsten tetraboride MBene exhibits innate advantages in electrochemical applications because of its unique graphene-like structure and metallic properties. In this study, we utilized first-principles calculations to identify the potential applications of WB4 as an anode material in rechargeable alkali-metal-ion batteries. The findings imply that the Li, Na, and K adsorption on the WB4 surface results in exceptionally high conductivity, and the WB4 monolayer can effectively adsorb these ions with significant adsorption energies of −2.516 eV, −2.356 eV, and −2.941 eV for Li, Na, and K ions, respectively. The results indicate that WB4 exhibits excellent stability during lithiation, sodiation, and potassiation. Notably, we propose high storage capacities for LIBs (708mAh/g), SIBs (472mAh/g), and KIBs (177 mAh/g), with maintained structural stability for the adsorption of these metal ions. The measured average open circuit voltages for WB4 were found to be 0.77 V (Li), 0.73 V (Na), and 0.80 V (K), and metallicity remain good maintained within the entire adsorption process. The calculated migration energy barriers for Li, Na, and K were 0.47 eV,0.21 eV and 0.13 eV respectively. The notable properties of WB4 make it an advantageous electrode material for alkali-metal ion batteries.

Stability and mechanical properties of W1−xMoxB4.2 ( x=0.0–1.0 ) from first principles

Heavy transition-metal tetraborides (e.g., tungsten tetraboride, molybdenum tetraboride, and molybdenum-doped tungsten tetraboride) exhibit superior mechanical properties, but solving their complex crystal structures has been a long-standing challenge. Recent experimental x-ray and neutron diffraction measurements combined with first-principles structural searches have identified a complex structure model for tungsten tetraboride that contains a boron trimer as an unusual structural unit with a stoichiometry of 1:4.2. In this paper, we expand the study to binary ${\mathrm{MoB}}_{4.2}$ and ternary ${\mathrm{W}}_{1\ensuremath{-}x}{\mathrm{Mo}}_{x}{\mathrm{B}}_{4.2}$ ($x=0.0--1.0$) compounds to assess their thermodynamic stability and mechanical properties using a tailor-designed crystal structure search method in conjunction with first-principles energetic calculations. Our results reveal that an orthorhombic ${\mathrm{MoB}}_{4.2}$ structure in $Cmcm$ symmetry matches well the experimental x-ray diffraction patterns. For the synthesized ternary Mo-doped tungsten tetraborides, a series of ${\mathrm{W}}_{1\ensuremath{-}x}{\mathrm{Mo}}_{x}{\mathrm{B}}_{4.2}$ structures are theoretically designed using a random substitution approach by replacing the W to Mo atoms in the $Cmcm$ binary crystal structure. This approach leads to the discovery of several ${\mathrm{W}}_{1\ensuremath{-}x}{\mathrm{Mo}}_{x}{\mathrm{B}}_{4.2}$ structures that are energetically superior and stable against decomposition into binary ${\mathrm{WB}}_{4.2}$ and ${\mathrm{MoB}}_{4.2}$. The structural and mechanical properties of these low-energy ${\mathrm{W}}_{1\ensuremath{-}x}{\mathrm{Mo}}_{x}{\mathrm{B}}_{4.2}$ structures largely follow the Vegard's law. Under changing composition parameter $x=0.0--1.0$, the superior mechanical properties of ${\mathrm{W}}_{1\ensuremath{-}x}{\mathrm{Mo}}_{x}{\mathrm{B}}_{4.2}$ stay in a narrow range. This unusual phenomenon stems from the strong covalent network with directional bonding configurations formed by boron atoms to resist elastic deformation. The findings offer insights into the fundamental structural and physical properties of ternary ${\mathrm{W}}_{1\ensuremath{-}x}{\mathrm{Mo}}_{x}{\mathrm{B}}_{4.2}$ in relation to the binary ${\mathrm{WB}}_{4.2}/{\mathrm{MoB}}_{4.2}$ compounds, which open a promising avenue for further rational optimization of the functional performance of transition-metal borides that can be synthesized under favorable experimental conditions for wide applications.

Unusual suppression of tungsten 5d electron depletion in superhard tungsten tetraboride solid solution with chromium under compression

The lattice compressibility and deformation in superhard tungsten tetraboride (WB4) solid solution with chromium (Cr) are investigated by high-pressure x-ray diffraction and x-ray absorption fine structure (XAFS) spectroscopy up to 54 GPa. In contrast to pure WB4, the c-axis softening is effectively suppressed in W0.9Cr0.1B4, and less compressibility is shown for the a- and c-axes in the entire pressure range. Meanwhile, the white-line peak of W L3-edge XAFS in W0.9Cr0.1B4 shows an absence of the sudden intensity drop as previously observed in WB4 at ∼21 GPa, suggesting a strong inhibition of W 5d electron depletion. This phenomenon is followed by an initial increase and then decrease for the W–B bond disorder, with the magnitude greatly lower than that of WB4. Besides the apparent atomic size mismatch effect, these results imply that addition of Cr, which has the same number of valence electrons as W, can introduce an unexpected electronic structure change to strengthen the W-B bond via a modification of W vacancies and B trimers distribution in WB4 lattice. Our findings point out the great significance to precise manipulation of the intrinsic W vacancies and B trimers through different solute atoms to rational optimization of WB4 hardness.

Synthesis and sintering of tungsten tetraboride and tantalum-bearing tungsten tetraboride under ultra high temperature and high pressure

As a highly controversial candidate superhard material, tungsten tetraboride (WB4) has attracted widespread attention due to its desirable mechanical properties. Here, single phase WB4 ceramics and WB4-based materials containing tantalum (Ta) have been successfully synthesized by a high-pressure reaction at 5.0 GPa and 1400 °C - 3000 °C. We systematically investigated the effect of processing temperature on the phase stability of WB4 and doping with different amounts of Ta on mechanical properties and thermal stability. The results confirm that increasing treatment temperature is beneficial to the synthesis of the WB4 phase, and high pressure can effectively inhibit the decomposition of WB4. Additionally, the hardness value of the WB4-based ceramic is found to decrease with the increase of the doped tantalum content. However, the hardness of WB4 is less than W0.9Ta0.1B4 (10 at. % Ta), indicating that a small amount of Ta doping is positive for the mechanical properties of WB4. The results show that the W0.9Ta0.1B4 ceramic synthesized at a temperature of 2800 °C and 5.0 GPa has excellent overall properties. Its Vickers hardness is 42 GPa (0.49 N), and its oxidation resistance temperature is 542 °C, which is much higher than the hardness and thermal stability of tungsten carbide (WC). This work can provide practical guide for the design of new transition metal boride superhard materials, and point out the visual direction for exploring the next generation of superhard materials.

Effect of tantalum on phase transition and thermal stability of metastable tungsten tetra-boride

Decomposition of WB4 into WB2 at temperature higher than 1200 °C makes the preparation of pure bulk WB4 ceramic difficult. Doping of the third element was reported to be an effective way for thermal stability improvement of ceramic materials. In the present work, the nearest element of W, tantalum (Ta), was introduced into tungsten boride synthesized by a combination of high energy ball milling and thermal treatment with (W:Ta):B= (2/3:1/3):4.5 & 5. The phase composition, chemical state of each element, and thermal stability of the as-synthesized samples were investigated. Results show that, with W:Ta molar ratio of 2/3:1/3, ei, 2:1, when the (W + Ta):B = 1: 4.5, about 49.5% mass fraction of WB4 phase was maintained successfully after being thermal treated at 1500 °C for 1 h, compared with pure WB22 phase obatined by the Ta-free sample. Increasing B content in raw materials to (W + Ta):B = 1:5, the mass fraction of WB4 phase was increased to 97%, while the only phase can be detected in the Ta-free samples is also WB2. The significantly improved thermal stability of WB4 can be mainly attributed to the completely dissolve of Ta into the W-B system. It was also found that the oxidation resistance of the W-B system was increased with the introduction of Ta.

Anomalous lattice stiffening in tungsten tetraboride solid solutions with manganese under compression

Tungsten tetraboride (WB4)-based solid solutions represent one of the most promising superhard metal candidates; however, their underlying hardening mechanisms have not yet been fully understood. Here, we explore the lattice compressibility of WB4 binary solid solutions with different manganese (Mn) concentrations using high-pressure x-ray diffraction (XRD) up to 52 GPa. Under initial compression, the lattices of low and high Mn-doped WB4 alloys (i.e. W0.96Mn0.04B4 and W0.84Mn0.16B4) are shown to be more and less compressible than pure WB4, respectively. Then, a c-axis softening is found to occur above 39 GPa in WB4, consistent with previous results. However, an anomalous sudden a-axis stiffening is revealed at ~36 GPa in W0.96Mn0.04B4, along with suppression of c-axis softening observed in WB4. Furthermore, upon Mn addition, a simultaneous stiffening of a- and c-axes is demonstrated in W0.84Mn0.16B4 at ~37 GPa. Speculation on the possible relationship between this anomalous stiffening and the combined effects of valence-electron concentration (VEC) and atomic size mismatch is also included to understand the origin of the nearly identical hardness enhancement in those two solid solutions compared to WB4. Our findings emphasize the importance of accurate bonding and structure manipulation via solute atoms to best optimize the hardness of WB4 solid solutions.

Unravelling the structure and strength of the highest boride of tungsten WB4.2

Tungsten tetraboride is outstanding among transition-metal light-element compounds for its easy synthesis and superior mechanical properties. Its crystal structure, however, has eluded scientists for over half a century, impeding fundamental understanding and rational property optimization. Recent x-ray and neutron diffraction studies suggest rare boron trimers occupying vacant metal sites in the highest boride of tungsten ${\mathrm{WB}}_{4.2}$, but a viable crystal structure and key mechanical properties remain unresolved. Here we identify a ${\mathrm{WB}}_{4.2}$ phase in orthorhombic symmetry with a very large 104-atom unit cell using a tailored search algorithm treating boron trimer as a coherent unit. First-principles studies establish phase stability and unveil the mechanism for strength enhancement by newly identified bonding features. These findings solve a challenging crystal structure and elucidate its benchmark mechanical behaviors. This work offers powerful insights for exploring novel structures and properties of materials containing intricate multiatomic constituent structural units.

Nonrandomly Distributed Tungsten Vacancies and Interstitial Boron Trimers in Tungsten Tetraboride

Tungsten tetraboride (WB4) and its solid solutions represent one of the most promising candidates for superhard metals; however, the structural and bonding uncertainties regarding the fractionally ...

Microscopic investigation of local structural and electronic properties of tungsten tetraboride: a superhard metallic material

Tungsten borides, such as tungsten tetraboride ( WB4) exhibit a wide range of appealing physical properties, including superhardness, chemical inertness and electronic conductivity. Among the various tungsten borides, the most puzzling remains WB4, with its crystal structure to linger in question for over half a century ( Lech et al. in Proc Natl Acad Sci USA 112: 3223- 3228, 2015). In the present investigation, polycrystalline WB4 samples have been synthesized with two different methods and characterized at the atomic level by combining X- ray diffraction, scanning electron microscopy and nuclear magnetic resonance spectroscopy. The 11 B multiple quantum MAS experiment revealed a range of boron sites that were not resolved within the experiment. This result is in contrast to the 11 B MAS spectrum of WB2 with four resolved, discernible boron resonances. However, despite the structural complexity and boron- site variety in WB4, the detection of a single exponential of 11 B spin- lattice relaxation recovery suggested that all of the boron sites relaxed with a single time constant. The Knight shift ( K) was found to be independent of temperature while the T 1 1 was governed by the Korringa law with a Korringa product T1T = 350 sK across the entire temperature range ( 168- 437 K) of this study. The measured Korringa product was small, indicating substantial spin- lattice relaxation resulting from coupling with the conduction carriers. The abovementioned experimental results not only clearly rule out structures, such as the `` MoB4- type phase'' of WB4, with the resulting Fermi level in the pseudo- gap as has previously been predicted theoretically; but they also provide a comprehensible and valuable insight into the structural and electronic properties of WB4 at the atomic level.

In situ synthesis, mechanical properties, and microstructure of reactively hot pressed WB4 ceramic with Ni as a sintering additive

In this study, tungsten tetraboride (WB4) ceramics were synthesized in situ from powder mixtures of W and amorphous B with Ni as a sintering aid by reactive hot pressing method. The as-synthesized ceramics exhibited porosity as low as 0.375% and ultra-high Vickers hardness (Hv), as much as 49.808 ± 1.683 GPa (for the low load of 0.49 N). It was seen that the addition of Ni greatly improved the sinterability of WB4 ceramic. Besides, the flexural strength and fracture toughness of WB4 ceramic were measured for the first time to be 332.857 ± 36.763 MPa and 4.136 ± 0.259 MPa m1/2, respectively, suggesting that the ceramic has good mechanical properties. The effects of sintering temperature and holding time on the densification, Vickers hardness, and mechanical properties of WB4 ceramics were also investigated systematically as part of our study. The results indicated that increasing the sintering temperature can obviously improve the densification and mechanical properties of the ceramics. The bulk density and Vickers hardness of WB4 ceramic sintered at 1650 °C for 60 min under 30 MPa revealed the highest values of 6.366 g cm−3 and 27.948 ± 0.686 GPa (for the high load of 9.8 N), respectively. The flexural strength increased to the highest value of 332.857 ± 36.763 MPa for sintering temperature up to 1550 °C, but decreased slightly as the sintering temperature further increased to 1650 °C. On the other hand, the fracture toughness increased gradually with increasing temperature. It was also found that Vickers hardness showed a similar trend as the densification of the samples with increasing temperature and holding time. Besides, no obvious improvements in the densification, mechanical properties, and Vickers hardness of the samples with sintering time were observed in this study. The microstructure and fracture behaviours of the as-synthesized WB4 ceramic were also revealed, and the toughening mechanism has been discussed.

Effects of Dodecaboride-Forming Metals on the Properties of Superhard Tungsten Tetraboride

Tungsten tetraboride alloys with Y, Sc, Gd, Tb, Dy, Ho, and Er were prepared by arc-melting and investigated for their thermal stability and mechanical properties. The phase composition and purity were confirmed by powder X-ray diffraction (PXRD) and energy dispersive X-ray spectroscopy (EDS). In all cases, except for Sc, a change to a “dendritic” morphology was observed. For alloys with Sc, the metal boride and boron grains became smaller; however, no patterning was observed. This shows that tuning the composition of the alloys of WB4 with transition metals and lanthanides results in a modification of the morphology leading to patterning and smaller grains that enhance mechanical and thermal properties. For alloys of WB4 with Y, Sc, Gd, Dy, Tb, Ho, and Er, an increase in hardness to 50.2 ± 2.4, 48.9 ± 2.5, 48.0 ± 2.1, 46.3 ± 2.6, 48.5 ± 2.3, 46.4 ± 3.4, and 47.2 ± 2.9 GPa at low load, respectively, compared to 41.2 ± 1.4 GPa for WB4 is observed. Moreover, the alloys of WB4 with Y, Sc, and Gd showed an in...

Effects of Variable Boron Concentration on the Properties of Superhard Tungsten Tetraboride.

Tungsten tetraboride is an inexpensive, superhard material easily prepared at ambient pressure. Unfortunately, there are relatively few compounds in existence that crystallize in the same structure as tungsten tetraboride. Furthermore, the lack of data in the tetraboride phase space limits the discovery of any new superhard compounds that also possess high incompressibility and a three-dimensional boron network that withstands shear. Thus, the focus of the work here is to chemically probe the range of thermodynamically stable tetraboride compounds with respect to both the transition metal and the boron content. Tungsten tetraboride alloys with a variable concentration of boron were prepared by arc-melting and investigated for their mechanical properties and thermal stability. The purity and phase composition were confirmed by energy dispersive X-ray spectroscopy and powder X-ray diffraction. For variable boron WBx, it was found that samples prepared with a metal to boron ratio of 1:11.6 to 1:9 have similar hardness values (∼40 GPa at 0.49 N loading) as well as having a similar thermal oxidation temperature of ∼455 °C. A nearly single phase compound was successfully stabilized with tantalum and prepared with a nearly stoichiometric amount of boron (4.5) as W0.668Ta0.332B4.5. Therefore, the cost of production of WB4 can be decreased while maintaining its remarkable properties. Insights from this work will help design future compounds stable in the adaptable tungsten tetraboride structure.

Formation of metastable tungsten tetraboride by reactive hot-pressing

Abstract The synthesis of tungsten tetraboride (WB 4 ) powders prepared by reactive hot-pressing in a vacuum from elemental tungsten (W) and amorphous boron (B) powders is reported in this work. This study systematically investigates the effects of the synthesizing temperature, B/W molar ratio, pressure and time on phase formation. The XRD pattern for the as-synthesized powders was analysed. Metastable tungsten tetraboride was synthesized in a vacuum at 1350 °C with a B/W molar ratio of 8.0 under a low uniaxial pressure (30 MPa) for 1 h. The scanning electron microscopy and transmission electron microscopy results showed that the particles had a tendency to form agglomerates. Amorphous boron was found to exist in the final product, and formation of the crystal phase WB 4 was confirmed.

Extrinsic Hardening of Superhard Tungsten Tetraboride Alloys with Group 4 Transition Metals.

Alloys of tungsten tetraboride (WB4) with the group 4 transition metals, titanium (Ti), zirconium (Zr), and hafnium (Hf), of different concentrations (0-50 at. % on a metals basis) were synthesized by arc-melting in order to study their mechanical properties. The phase composition and purity of the as-synthesized samples were confirmed using powder X-ray diffraction (PXRD) and energy dispersive X-ray spectroscopy (EDS). The solubility limit as determined by PXRD is 20 at. % for Ti, 10 at. % for Zr, and 8 at. % for Hf. Vickers indentation measurements of WB4 alloys with 8 at. % Ti, 8 at. % Zr, and 6 at. % Hf gave hardness values, Hv, of 50.9 ± 2.2, 55.9 ± 2.7 and 51.6 ± 2.8 GPa, respectively, compared to 43.3 GPa for pure WB4 under an applied load of 0.49 N. Each of the aforementioned compositions are considered superhard (Hv > 40 GPa), likely due to extrinsic hardening that plays a key role in these superhard metal borides. Furthermore, these materials exhibit a significantly reduced indentation size effect, which can be seen in the plateauing hardness values for the W1-xZrxB4 alloy. In addition, W0.92Zr0.08B4, a product of spinoidal decomposition, possesses nanostructured grains and enhanced grain hardening. The hardness of W0.92Zr0.08B4 is 34.7 ± 0.65 GPa under an applied load of 4.9 N, the highest value obtained for any superhard metal at this relatively high loading. In addition, the WB4 alloys with Ti, Zr, and Hf showed a substantially increased oxidation resistance up to ∼460 °C, ∼510 °C, and ∼490 °C, respectively, compared to ∼400 °C for pure WB4.

Enhancing the Hardness of Superhard Transition-Metal Borides: Molybdenum-Doped Tungsten Tetraboride

By creation of solid solutions of the recently explored low-cost superhard boride, tungsten tetraboride (WB4), the hardness can be increased. To illustrate this concept, various concentrations of molybdenum (Mo) in WB4, that is, W1–xMoxB4 (x = 0.00–0.50), were systematically synthesized by arc melting from the pure elements. The as-synthesized samples were characterized using energy-dispersive X-ray spectroscopy (EDS) for elemental analysis, powder X-ray diffraction (XRD) for phase identification, Vickers microindentation for hardness testing, and thermal gravimetric analysis for determining the thermal stability limit. While the EDS analysis confirmed the elemental purity of the samples, the XRD results indicated that Mo is completely soluble in WB4 over the entire concentration range studied (0–50 at. %) without forming a second phase. When 3 at. % Mo is added to WB4, Vickers hardness values increased by about 15% from 28.1 ± 1.4 to 33.4 ± 0.9 GPa under an applied load of 4.90 N and from 43.3 ± 2.9 to 5...

Stoner-enhanced paramagnetism in tungsten tetraboride

We demonstrate that tungsten tetraboride (WB4), a heavy transition metallic compound without magnetic atoms, is an exchange-enhanced paramagnet revealed by the magnetization and specific heat measurements. WB4 has a small effective magnetic moment of 0.53 μB/fu. The high magnetic susceptibility in the magnitude of 1 memu (mol·Oe)−1 below 10 K obeys quadratic temperature dependence. The upturn behavior of CP/T versus T2 at low temperatures is attributed to the electron-paramagnon interactions. A high Stoner enhancement parameter, Z = 0.93, was derived to explain the enhanced paramagnetism based on the Stoner model.

Structural stability and elastic properties of WB4 under high pressure

A comparative study on the structure stability and elastic properties for various types of tungsten tetraboride ( WB 4) has been carried out with the generalized gradient approximation (GGA) and local density approximation (LDA) in the framework of density functional theory (DFT). Five types of WB 4 are considered i.e., orthorhombic Immm, Pnnm, Pmmn, monoclinic C2/m and hexagonal P63/mmc structure. Our calculations indicate that the P63/mmc-4u structure of WB 4 is unstable at both ambient and pressure conditions, but the other four types of WB 4 are stable, in agreement with recent theoretical results. By eliminating mechanical calculations, we find that the four types (C2/m, Immm, Pnnm and Pmmn) of WB 4 are potential candidates to be ultra-incompressible and hard materials. Moreover, the WB 4 in C2/m type is the most ultra-incompressible among the considered structures due to its superior mechanical properties, and the P63/mmc-2u structure of WB 4 is not considered to be hard material because of its low hardness. In addition, the calculated B/G ratio exhibits the positive pressure dependence, and four types show brittle nature within 100 GPa.

Structure of superhard tungsten tetraboride: A missing link between MB2 and MB12 higher borides

Superhard metals are of interest as possible replacements with enhanced properties over the metal carbides commonly used in cutting, drilling, and wear-resistant tooling. Of the superhard metals, the highest boride of tungsten--often referred to as WB4 and sometimes as W(1-x)B3--is one of the most promising candidates. The structure of this boride, however, has never been fully resolved, despite the fact that it was discovered in 1961--a fact that severely limits our understanding of its structure-property relationships and has generated increasing controversy in the literature. Here, we present a new crystallographic model of this compound based on refinement against time-of-flight neutron diffraction data. Contrary to previous X-ray-only structural refinements, there is strong evidence for the presence of interstitial arrangements of boron atoms and polyhedral bonding. The formation of these polyhedral--slightly distorted boron cuboctahedra--appears to be dependent upon the defective nature of the tungsten-deficient metal sublattice. This previously unidentified structure type has an intermediary relationship between MB2 and MB12 type boride polymorphs. Manipulation of the fractionally occupied metal and boron sites may provide insight for the rational design of new superhard metals.

Lattice stress states of superhard tungsten tetraboride from radial x-ray diffraction under nonhydrostatic compression

In this work, we examine the lattice behavior of the economically interesting superhard material, tungsten tetraboride (${\mathrm{WB}}_{4}$), in a diamond anvil cell under nonhydrostatic compression up to 48.5 GPa. From the measurements of lattice-supported differential stress, significant strength anisotropy is observed in ${\mathrm{WB}}_{4}$. The (002) planes are found to support the highest differential stress of 19.7 GPa within the applied pressure range. This result is in contrast to ${\mathrm{ReB}}_{2}$, one of the hardest transition metal borides known to date, where the same planes support the least differential stress. A discontinuous change in the slope of c/a ratio is seen at 15 GPa, suggesting a structural phase transition that has also been observed under hydrostatic compression. Speculations on the possible relationship between the observed structural changes, the strength anisotropy, and the orientation of boron-boron bonds along the $c$ direction within the ${\mathrm{WB}}_{4}$ structure are included.