Adsorption-chemical activity and interaction in the boron nitride-titanium hydride system

Abstract

The paper examines the effect of physicochemical properties of starting boron nitride and titanium hydride on their reactive activity in the synthesis of the TiN-TiB x composite. There are kinetic data on thermal desorption of the active adsorbate from the BN surface, which precipitated from the gas mixture (20 vol.% O 2 -80 vol.% N 2 under the relative humidity of 80%). Kinetics of hydrogen release from titanium hydride in isothermal conditions and controlled rate of temperature increase in the reactive system is researched. A possible mechanism of chemical transformations of starting compounds and synthesis of the TiN-TiB x composite is considered.Materials with high hardness and wear resistance that are based on titanium nitride and transient-metal borides are of great interest for contemporary technology. Such ceramics are produced by reactive sintering under pressure or electric discharge sintering [1, 2] in self-propagating high-temperature synthesis. Physicochemcal transformations at the interface of hard and gaseous phases play an important role in the above processes; therefore, adsorption and desorption substantially contribute to their kinetics. The surface activation of reagents resulting from desorption of admixtures and chemical adsorption of a gaseous reagent on the surface of fine particles of the hard-phase reagent are inseparable stages of the synthesis to be considered to study the reaction mechanism.This study is focused on the effect of starting BN and TiH 2 on the kinetics of synthesizing titanium boride and nitride and on the mechanism of chemical transformations. Of interest is reactive sintering in the BN-TiH 2 system in which refractory TiN and TiB 2 phases are final products of the synthesis. This raises the question of the role of hydrogen in this interaction. Titanium hydride is known to release active atomic hydrogen when heated. The fine boron nitride powder is capable of enveloping hard particles of titanium hydride. This means that atomic hydrogen will react with BN mixed with TiH 2 when the former decomposes. As a result, reactive surfaces of hard particles of starting TiH 2 and BN and intermediate products from the synthesis of the TiN-TiB 2 composite are formed.Assuming that splitting the hard boron nitride with atomic hydrogen is the most probable synthesis mechanism, we performed preliminary thermodynamic calculations of the following reactions: BN + 2H ↔ HB = NH + 2H → H 2 B−NH 2 +2H → BH 3 + NH 3 , (1) BN + 3H ↔ BH 3 + N,(2) BN + 3H ↔ B + NH 3 ,(3) 2BN + 6H ↔ B 2 H 6 + 2N, (4) BN + 3H ↔ BH 3 + N, (5) BN + 1.5H 2 ↔ BH 3 + 0.5 NH 3 .(6) Figure 1 shows that molecular hydrogen does not react with boron nitride, which is confirmed by the calculations (curve 6). Boron nitride is intensively oxidized by dry oxygen and CO 2 at 700 to 800°C, B 2 O 3 and N 2 being

Interaction of titanium hydride and boron nitride at temperature gradient

“…Surface heating is peculiar in that the temperature gradient is formed in depth of the reactive volume depending on the surface energy density and heating time. Two experimental modes were used: (i) rapid heating (at a selected rate) and subsequent temperature leveling in depth of the sample; (ii) unchanged temperature gradient in depth of the pressed sample for revealing the kinetics of transformations of the initial compounds, considering the BN-TiH 2 interaction kinetics [5]. The second mode involved preheating of the reacting mixture to 500-600°C and holding at temperatures corresponding to intensive decomposition of titanium hydride and subsequent temperature rise to reach the gradient parameters (1150°C) with the cylinder end being isolated and open to exposure.…”

“…1 and Table 1. A mixture of refractory TiN-TiB 2 compounds may be synthesized in SHS conditions close to adiabatic. The paper [5] examines physical and chemical features and thermodynamic parameters in the transformation of the initial TiH 2 and BN compounds and possible mechanism of their interaction and formation of the final TiB 2 and TiN products. (2); Ti + NH 3 ↔ TiN + 1.5H 2 (3); Ti + 0.5N 2 ↔ TiN (4) and 1.5Ti + BN ↔ 0.5TiB 2 + TiN (5) Note that the interaction between TiH 2 and BN by the overall reaction…”

“…Figure 2 shows temperature dependences of logarithmic equilibrium constants of possible reactions. The paper [5] considers different options of the synthesis mechanism including the interaction of atomic hydrogen with BN, formation of active boron and atomic nitrogen, and subsequent synthesis of TiN and TiB 2 . Below is a synthesis scheme considering the probability of parallel reactions in contact of metal titanium with crystal boron nitride and active (probably, atomic) boron and nitrogen:…”

“…The processes took two stages. At the first stage, titanium hydride decomposed, which caused the temperature to increase over the reactive volume to 500-600°C [5].…”

“…Titanium nitride and borides form by exothermic chemical reactions. In developing a ceramic composite, one should consider the synthesis stages and their kinetic parameters [5], which include surface treatment of the initial compounds to remove chemically active constituents of the adsorbate, decomposition of titanium hydride with a controlled rate of hydrogen release resulting from changes in temperature in the reactive area or H 2 partial pressure, formation of intermediate products and their involvement in the TiN-TiB 2 synthesis, and sintering of the synthesized titanium boride and nitride. The kinetics and mechanism of this multistage process and structurization of the ceramic material (composite) are still poorly understood.…”