A comparative study on combustion synthesis of Nb–B compounds

Abstract

A comparative study on the preparation of specific niobium borides (including Nb 3 B 2 , NbB, Nb 5 B 6 , Nb 3 B 4 , and NbB 2 ) in the Nb-B system was experimentally conducted by self-propagating high-temperature synthesis (SHS) from elemental powder compacts of their corresponding stoichiometries. Effects of the sample green density, preheating temperature, and starting stoichiometry on combustion characteristics, as well as on product composition were studied. Experimental evidence indicated that except for the samples of Nb:B = 3:2, upon ignition a planar and self-sustained combustion front was established and proceeded throughout the entire sample in a steady manner. However, combustion of the samples with Nb:B = 3:2 was characterized by a localized reaction zone propagating along a spiral trajectory, due largely to the low combustion temperatures which further resulted in a poor degree of phase conversion with a significant amount of Nb left unreacted. The incomplete reaction in the Nb:B = 3:2 powder compact produced boride compounds Nb 3 B 4 and NbB in minor quantities. Reactant compacts of Nb:B = 1:1 and 1:2 were shown to yield practically single-phase monoboride NbB and diboride NbB 2 , respectively. In contrast, multiphase compounds consisting of Nb 3 B 4 , NbB, and NbB 2 were synthesized from the powder compacts with starting stoichiometries Nb:B = 3:4 and 5:6. However, it was found that two boride phases Nb 3 B 2 and Nb 5 B 6 did not appear in the end products from any of the reactant compacts. Based upon the temperature dependence of combustion wave velocity, the activation energies associated with combustion synthesis of NbB and NbB 2 were determined to be 151.8 and 132.4 kJ/mol, respectively.

Niobium boride layers deposition on the surface AISI D2 steel by a duplex treatment

“…These are due to the presence of hard niobium boride phases such as NbB, NbB 2 and Nb 3 B 4 detected by XRD analysis as shown in Figure 2. This kind of niobium boride phases were also obtained at earlier studies [8][9][10][11][12]. The depth of the niobium boride layers changes between 0.42 μm to 2.43 μm, depending on the treatment temperatures and times as shown in Figure 3.…”

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].…”

“…NbB 2 nanocrystals were obtained from chloride-based raw materials at low temperatures (500-600°C) using an autoclave [22,23]. SHS processes were conducted using Nb/B and Nb 2 O 5 /B powder mixtures, and non-stoichiometric phases of niobium boride were obtained instead of a single NbB 2 phase [24][25][26][27]. NbB 2 was prepared by chemical vapour deposition using gas mixtures of NbCl 5 , BCl 3 and H 2 on a quartz substrate between the temperatures of 950 and 1200°C, resulting in the production of homogeneous films between 950 and 1050°C and pillar crystals between 1050 and 1200°C during the precipitation step of the process [8].…”

Combustion Synthesis of UHTC Composites from Ti–B4C Solid State Reaction with Addition of VIb Transition Metals

“…Combustion synthesis in the mode of self-propagating high-temperature synthesis (SHS) has been a promising alternative, which takes advantage of highly exothermic reactions, and hence, has merits of low energy requirement, short processing time, simplicity, and high-purity products [10][11][12]. A number of borides and carbides of transition metals (mostly the groups IVb and Vb) have been produced by the SHS process from the elemental powder compacts of their corresponding stoichiometries [13][14][15][16][17]. Due to the low reaction enthalpy, however, solid state combustion between the transition metals of the group-VIb (Cr, Mo, and W) and boron or carbon is not feasible [12].…”