Efficient Synthesis of WB_5–x–WB₂ Powders with Selectivity for WB_5–x Content

Abstract

We proposed an efficient method toward the synthesis of higher tungsten boride WB5–x in the vacuumless direct current atmospheric arc discharge plasma. The crystal structure of the synthesized samples of boron-rich tungsten boride was determined using computational techniques, showing a two-phase system. The ab initio calculations of the energies of various structures with similar X-ray diffraction (XRD) patterns allowed us to determine the composition of the formed higher tungsten boride. We determined the optimal parameters of synthesis to obtain samples with 61.5% WB5–x by volume. The transmission electron microscopy measurements showed that 90% of the particles have sizes of up to 100 nm, whereas the rest of them may have sizes from 125 to 225 nm. Our study shows the possibility of using the proposed vacuumless method as an efficient and inexpensive way to synthesize superhard WB5–x without employing resource-consuming vacuum techniques.

Theoretical Study of Adsorption Properties and CO Oxidation Reaction on Surfaces of Higher Tungsten Boride

“…We used the crystal structure of WB 5 model proposed in 11,73 to construct slabs with surfaces of different crystallographic orientations. The calculated lattice constants for bulk WB 5 (Pmmn space group) are a = 5.209 Å, b = 6.349 Å, c = 8.893 Å, which are consistent with experimental measurements 21,19,74 .…”

“…It should be noted, that crystal structure of higher tungsten boride WB 5-x is disordered with respect to location of B-triangles 19,21 . This leads to composition variation (0 < x < 2) of WB 5-x .…”

“…Furthermore, based on the strong adsorption of NO and CO, as well as the weak adsorption of N 2 and CO 2 , we suggest that WB 5 could be used as active material for an automotive catalytic converter. In addition to its e cient production 19 , the advantage of WB 5-x over currently used catalysts is its resistance to poisoning by sulfur gases, particularly SO 2 46 , whose adsorption energy is calculated to be positive on this surface, see Figure 2.…”

“…Higher metal borides with structure type of MeB 5-x (Me = W, Mo) were actively studied in recent years, along with the diborides 13,[19][20][21] . According to 19 , the powder of higher tungsten boride can be e ciently obtained by vacuumless atmospheric arc plasma method, not using high pressure or inert atmosphere. Such technology could signi cantly reduce the cost of production of nanosized higher TMBs powders, namely tungsten borides, for use in catalysis.…”

Synthesis, Phase Evolutions, and Stabilities of Boron-Rich Tungsten Borides at High Pressure

“…Besides, due to the strong TM–B bonds, the metal borides often have relatively large metastability (i.e., with a large number of metastable phases, see refs – ), and there may exist many unexplored metastable structures yet technological interest, − awaiting experimental discovery. Traditional methods for the synthesis of W–B compounds include self-propagating high-temperature ( T ) reactions, , molten-salt growth, direct-current arc discharge plasma technique, arc melting, , and solid-state reaction methods at ambient pressure. − However, most of those syntheses often involve excess boron, especially for the boron-rich borides, which prevents obtaining phase-pure samples, although alloying with Sc and Ta is seemingly much favorable for the formation of boron-rich tungsten borides at ambient pressure. , In addition, the reactions of starting reactants by traditional methods are usually incomplete and leave behind unreacted reactants or competing boron-poor boride byproducts in the final samples, leading to many contentious issues regarding their structures, compositions, and properties. ,− Many possible structures and compositions, for example, have been proposed for previously assigned hexagonal-“WB 4 ” including WB 3 , WB 3+ x , WB 4.2 , and W 1– x B 3 . Besides, the intrinsic hardness of WB 2+ x and WB 3+ x is still under debate due to the low-quality samples synthesized based on traditional methods. ,,− Thus, the methodological advancement toward synthesizing high-quality tungsten borides is highly desired to address the aforementioned issues in this important class of materials.…”