Deposition of boron nitride on graphite at 1300 K

Abstract

Abstract.A method for the deposition of BN onto graphite and other substrates is described. Boron tricbloride (BC13) and ammonia (NH3) diluted with Ar were used as reacting gases. The deposition process was carried out at 1300 K as well as lower temperatures in an open system at pressures of 1 arm. The consequences of the introduction of hydrogen to the system were considered. It was demonstrated that the replacement of argon with hydrogen increases the efficiency of the process as well as the theoretical rate of BN deposition. However, the acceleration of the deposition seems to be unprofitable, because the resulting supersaturation leads to the formation of an amorphous phase. The modification of the experimental conditions were proposed. In the previous paper [5-1 we discussed conditions for BN deposition by thermal decomposition of a gas mixture from boron trichloride (BC13) and ammonia (NH3) diluted in argon without hydrogen. Theoretical analysis of the CVD process depicted the optimal conditions under which this process could be carried out. In this part of our study we report the preparation of BN deposit by chemical vapour deposition onto * Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday ** To whom correspondence should be addressed graphite from a BC13 + NH 3 + Ar gas mixture in an open system at 1 atm total pressure and at a substrate temperature of 1300 K. ExperimentalProcedure Figure 1 illustrates the apparatus and gas delivery system used for CVD growth of the BN deposit. The BC13 (99.9%) obtained from Merck (Germany) was admitted into the evaporation vessel (8) and then diluted in the mixing chamber (7) with argon (14). Both of these gases as well as NH 3 (99.9%) obtained from Fertilisers Factory, Pulawy (Poland), were introduced into the system through electronic flowmeters (9, 10, 11) delivered by Brooks Instrument (The Netherlands). The NH 3 gas was introduced directly into the silica reaction tube, while the BC13 + Ar gas mixture was introduced there through another tube placed in the center of the reactor in order to avoid the premature reaction of BC13 with ammonia. The ends of these tubes were narrowed to form nozzles. The reaction chamber enclosed the graphite substrate (4) on which the deposition process takes place. The substrate was placed on the supporting silica tube just underneath the gas distributing nozzles. The temperature of the substrate is measured by Pt-PtRhl0 thermocouple which is kept inside the silica tube supporting the substrate (5). Outlet gases were absorbed in the indicator solution and in water. The quartz reactor was kept inside the movable resistance furnace (2) where temperature was controlled by Eurotherm 815 temperature controller (6).After the substrate was placed inside the reactor tube the whole system was flushed with argon dried in silica gel and MgC104 columns. Then the temperature was raised and when it reached a constant value ammonia gas was passed through the system. NH 3 gas was cleaned by passing it th...

Gibbs free energy of formation of the LnMnO3 (Ln=Yb, Ho, and Er) phases determined by the e.m.f. method

“…X-ray measurements after the experiments showed a (MnO + Ln 2 O 3 + LnMnO 3 ) three-phase mixture present in the working electrode. It confirmed that the cell worked under three-phase equilibrium condition described by the reaction (6), and no side reaction occurred during the cell operation. Our results derived for the stability of LnMnO 3 phases (with Ln = Yb, Ho, and Er) can be compared with those of Atsumi et al A very good agreement is obtained between previous results and those derived in this work.…”

“…The change in the standard Gibbs free energy of the oxidation of MnO to Mn 2 O 3 based on the data of Robie and Hemingway [13] was given in our previous paper [7]. When it is added to equation (7), and then (8) and (9), one can obtain for the reactions: The results derived in our earlier studies [4][5][6][7] are also shown in this figure. The values of the enthalpy change are almost identical for phases with Yb, Ho, and Er, but higher by about 10 kJ than enthalpies obtained for reaction (10) in systems with Nd, Sm, Gd which, by the way, are also almost identical.…”

“…cells of the type were used to determine the equilibrium oxygen potential over respective working electrodes. The cell arrangement was the same as described in our previous papers [5,6]. The cell was hooked up to the measuring set: a Keithley multimeter model 2000 and a PC computer with IEEE 488 interface while the software written in Delphi 4 was implemented for continuous data gathering.…”

“…In our previous papers [4][5][6][7] we determined thermodynamic properties of phases existing in (Ln, Mn, O) systems for Ln = Nd, Sm, Gd. In this paper, thermodynamic properties of LnMnO 3 phases for Ln = Yb, Ho, and Er have been investigated.…”

Investigation of Boron Nitride Prepared by Low Pressure Chemical Vapor Deposition at 650~1200 °C

“…The flux of reactants to the deposition substrate is proportional to the concentration of the limiting precursor, which has the further advantage of allowing simple control of the magnitude of the growth rate [13]. At higher temperatures (region III, 1000~1200 °C), the growth rate decreases because of an increased desorption rate, depletion of reactants on reactor walls, and especially the enhanced gas phase nucleation, which can drastically reduce the permeability and uniformity of BN deposited in fiber preform [14]. Besides, the intensive decomposition of NH 3 at over high temperatures can also lead to a decline in the growth rate.…”