Some nitrides, BN, AIN and Si
3N
4, are used as electric insulators up to moderate temperature. The difficult problems occured when they were to be employed at higher temperatures than 2000°C. The reactions between those insulators and conductors of refractory metals, tungsten and tantalum, were investigated up to 2100°C in a tungsten furnace under argon atmosphere.
In all of the power reactions of W with BN with various compositions at 1100° to 2000°C for 2 hrs., W
2B was first formed, followed by WB, W
2B
5 and WB
4 stepwisely, and nitrogen gas was released. Between Ta and BN (50-50 molar composition), TaN, TaB and small amount of TaB
2 were formed. In the reactions between metals and AIN (50-50 molar composition) for 2 hrs., W did not react, but Ta formed TaN over 1600°C. Vaporisation Al and AlN were observed considerably with elevating temperature. In the reactions between metals and Si
3N
4 (50-50 molar composition) for 2 hrs., W
3Si
2 and WSi
2, and TaN and TaSi
2 were formed, but vaporisation of Si
3N
4 was observed over 1400°C.
In the reaction between sintered pellets of W and BN, reaction layers of W
2B and WB were confirmed on the original W by means of XMA. Here kinetics of formation of W
2B from W and BN were investigated at the temperatures from 1270° to 1360°C by means of the TGA. By the method of J. H. Sharp et al., the curve of a reaction yield α with a reduced time
t/
t0.5 showed that the reaction rate was controlled by the phase-boundary reaction. An activation energy of this reaction was determined to be 87.0±17.0kcal/mole from the Arrhenius plots, and this value is larger than the activation energy 64.0±6.3kcal/mole of diffusion of B in W
2B given by A. P. Epik.
Insulating properties of BN for W leads dropped at higher temperature than 1500°C owing to the formation of conducting borides. Tungsten borides were easily oxidized in air in the range of 400° to 600°C. Their resistivities have a order of 10
-5 ohm-cm as metallic conductor. W
2B was weak paramagnetic, and WB and W
2B
5 were diamagnetic.
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