The Chemistry of Boron and Titanium Diboride Formation: Decomposition of TiCl 4 and BCl 3 in Hydrogen and Helium
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In general, what is known or proposed about the chemistry in these systems has arisen from equilibrium calculations and modeling of the deposition process [2-7]. Two of these reports suggest that BCI 2H may form in the gas phase but not BCIH 2 [2, 5], and that BC12 H may play a role in the deposition process. In this work, we used a high-temperature flow reactor (HTFR) equipped with a mass spectrometer to monitor the concentrations of gas-phase species during the reaction of BC13 and/or
TiC14 as a function of temperature, residence time, and mixture composition. The data obtained suggest that a surface reaction involving BC13 and H2 is responsible for BC13 removal from the gas phase and for accelerated rates of TiCI 4 reaction in the presence of these two gases. The experiments also demonstrate that BHC12 forms, confirming the earlier prediction. The results are used to identify several reactions that occur during the deposition process. EXPERIMENTAL METHODS A schematic of the HTFR is shown in Figure 1. Reactions occur within a 100-cm long graphite tube (5.0 cm ID) enclosed within a water-cooled, insulated vacuum chamber. Three independently controlled graphite heating elements surround the tube and heat the gases flowing within it to temperatures up to 1500 K. Reactor pressure is measured and controlled (to ± 0.1 torr) by a pressure transducer coupled to a throttle valve in the vacuum line. BC13 and TiCI4 enter the HTFR through a water-cooled injector and mix with the preheated carrier gas. The injector is movable, allowing the reactant residence time to be varied with respect to a quartz probe used for sampling. Delivery of gases to the reactor is controlled by calibrated mass flow controllers. Initial concentrations of 2-7% for BC13 and TiC14 were used. For TiC14 , the vapor pressure above a temperature-controlled liquid reservoir was used to drive a mass flow controller.
* This work was supported by the DoD Advanced Research Projects Agency.
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Mat. Res. Soc. Symp. Proc. Vol. 410 01996 Materials Research Society
Gases are extracted from the HTFR by a quartz sampling probe inserted into the center of the flow in the diagnostic region. The pressure inside the probe was maintained at 2.0 ± 0.1 torr by a pressure transducer/throttle valve combination. Once extracted by the probe, the gases flow past a 200-jim orifice attached to an Extrel EXM-500 quadrupole mass spectrometer system where a small portion is expanded into the mass spectrometer for analysis using 70 eV ionization energy. The accuracy of the mass and peak height calibration was verified by measuring the mass spectra of perfluorotributylamine and boron trichloride.
BCI3/TiC14
To Mass Spec t
H2/He
w decrease residence time increase residence time
insert injector withdraw injector o
Figure 1: Schematic of high-temperature flow reactor experiment. In a typical experiment, a carrier gas consisting of helium or hydrogen (or a mixture of the two) entered the reaction tube and was preheated to the reactor temperature by the first heating
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