Dielectric behaviour of hot pressed AIN ceramic is studied before and after exposing the samples to inorganic acid (HCl and HNO3) vapours with a specific aim to study the effect of these vapours on the dielectric constant () and dissipation factor (tan ). Four samples having different volume percentage of porosity (0.2 to 15%) are selected for this study. Dielectric dispersion increases after exposing the samples to the above acid vapours. Tan also increases quite appreciably; the increase being more at higher porosity. Recovery studies show that the exposure effect is reversible. The exposure time dependence of and tan indicates that these parameters show a maxima at a particular exposure time. However, no such maxima is observed in the gravimetric measurements. The increase in dielectric parameters after exposure to acid vapours is explained in terms of the ionic conduction due to the dissociation of these vapours in the presence of moisture. The porosity dependence of this effect is discussed in terms of closed and open porosity reported by other workers. 相似文献
The kinetics and mechanism of absorption/desorption of nitrogen in liquid Nb were investigated in the temperature range of
2470 °C to 2670 °C in samples levitated in a N2/Ar stream with various nitrogen partial pressures. The nitrogen solution reaction in liquid Nb was found to be exothermic,
with the standard enthalpy and entropy of solution of −236.4 ± 23.3 kJ/mol and −-5.3 ± 8.3 J/K · mol, respectively. Above
the threshold flow rate of the N2/Ar stream, the absorption process was determined to be second order with respect to nitrogen concentration, indicating that
the rate-controlling step is either the adsorption of nitrogen molecules on the liquid surface or dissociation of adsorbed
nitrogen molecules into surface-adsorbed atoms. The desorption process was found to be second order as well. At lower flow
rates, however, the absorption rate was found to depend on the gas-phase mass transfer rate. The rate equation for nitrogen
absorption in the range of 2470 °C to 2670 °C is given by
with the value ofQ calculated to be −327.2 ± 20.6 kJ/mol, while nitrogen desorption at 2670 °C follows the relation
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