Novel borothermal process for the synthesis of nanocrystalline oxides and borides of niobium

Abstract

A new process has been developed for the synthesis of nanocrystalline niobium oxide and niobium diboride using an amorphous niobium precursor obtained via the solvothermal route. On varying the ratio of niobium precursor to boron and the reaction conditions, pure phases of nanostructured niobium oxides (Nb(2)O(5), NbO(2)), niobium diboride (NbB(2)) and core-shell nanostructures of NbB(2)@Nb(2)O(5) could be obtained at normal pressure and low temperature of 1300 °C compared to a temperature of 1650 °C normally used. The above borothermal process involves the in situ generation of B(2)O(2) to yield either oxide or diboride. The niobium oxides and borides have been characterized in detail by XRD, HRTEM and EDX studies. The core-shell structure has been investigated by XPS depth profiling, EFTEM and EELS (especially to characterize the presence of boron and the shell thickness). The niobium diboride nanorods (with high aspect ratio) show a superconducting transition with the T(c) of 6.4 K. In the core-shell of NbB(2)@Nb(2)O(5), the superconductivity of NbB(2) is masked by the niobium oxide shell and hence no superconductivity was observed. The above methodology has the benefits of realizing both oxides and borides of niobium in nanocrystalline form, in high purity and at much lower temperatures.

Low‐Temperature Synthesis of Nanocrystalline NbB2 Powders by Borothermal Reduction in Molten Salt

“…Until now, various methods including solid‐state reaction, molten salt method, mechanochemically assisted preparation, self‐propagating high‐temperature synthesis, carbothermal reduction, and borothermal reduction have been used to synthesize NbB 2 powders . Table summarizes the results reported in the literatures.…”

“…The purpose of this communication is to report on the preparation of nanocrystalline NbB 2 powders at a temperature of 800°C–1000°C by the borothermal reduction of Nb 2 O 5 . As a common method, borothermal synthesis of NbB 2 powders has been reported in previous literatures . The difference in this paper is that the reducing process proceeded in the bath of molten salt, which greatly decreased the synthesis temperature.…”

In situ synthesis of NbB2–NbC composite powders by milling-assisted carbothermal reduction of oxide raw materials

“…Many processes are available for the fabrication of niobium boride or niobium boride-based ceramic powders including high temperature methods using solid state reactions, borothermal and carbothermal reductions [14][15][16][17][18][19][20][21], low-temperature synthesis in an autoclave [22,23], self-propagating high-temperature synthesis (SHS) [3,[24][25][26][27], chemical vapour deposition (CVD) [8] and ball milling and mechanochemical synthesis [28,29]. Borothermal and carbothermal reductions require temperatures above 1600°C to obtain the niobium boride phase if Nb 2 O 5 and B/B 2 O 3 are used as starting materials [14,15,21].…”

Synthesis of group IV and V metal diboride nanocrystals via borothermal reduction with sodium borohydride

“…Synthesis of metal diborides via reduction in metal oxides using polyborane was proposed by Forsthoefel and Sneddon to produce group IV and V MB 2 crystals with size <1.0 μm. Few groups focused their activity on NbB 2 : Jha et al used NbO 2 with elemental boron via solid‐state reaction to obtain nanorods (40 × 800 nm 2 ), Ran and co‐worker combined Nb 2 O 5 , B and NaCl/KCl to produce nanocrystals (61 nm) . Ma et al obtained nanocrystals of NbB 2 via thermal reduction in Nb 2 O 5 , H 3 BO 3 , at 650°C for 10 hours using NaCl/MgCl 2 as molten salt.…”