The radio frequency and microwave dielectric properties of doped magnesium oxide

Abstract

A Study of the dielectric properties of pure magnesium oxide and MgO doped with iron or chromium is presented. Measurements were carried out in the frequency range between 5 x 10(^2) Hz and 9.3 x 10(^9) Hz room temperature using a.c. bridge, Q-meter, slotted-line and cavity methods. The basic ideas and principles of each technique have been discussed individually in relation to their suitability for dielectric measurements on materials having particular characteristics. A major attempt has been made to develop the slotted-line technique (500 MHz - 7.5 GH) and the cavity resonator (9,3 GHz) method. Sources of possible error in these techniques have been investigated in detail and some suggestions made to minimize them. A new method for measuring the dielectric properties of low loss solids has been examined theoretically and some suggestions for its practical development pointed out. The dielectric data obtained at room temperature has been analysed and interpreted in terms of the hopping theory involved in the "Universal Law" of dielectric response. The frequency variation of conductivity, followed σ(_ac) (w) (_ɑ) w(^n) law with n = 0.98 ± 0.02 for pure MgO. The dielectric constant, ɛ', loss factor, ɛ'' and loss tangent, Tanδ, decrease slightly over the frequency range. Similar behaviour was also observed for Fe doped MgO and Cr doped MgO samples. The variation of ɛ' and ɛ'' both agree with the "Universal Law" showing that ɛ'(w) (_ɑ) w(^n-1) and ɛ''(w)(^x) w(^n-1); the magnitude of n obtained was also 0.98. The data of ɛ' and ɛ'' approximately fit the Kramers-Kronig relation i.e constant when 0.5 <n < 1. Measurements were extended to higher temperatures (up to 700º C)/using special apparatus which was designed for this purpose. The data obtained fit the Jonscher theory very well ; the exponent n decreases with increasing temperature and falls to a value of n = 0.5 at 700ºC. In the high temperature range the Having a proposal for the temperature dependence of dielectric constant was tested and the change of dielectric constant with temperature was found to agree well with the relation for MgO single crystal. A detailed comparison is made of the effect of doping MgO with the trivalent ions Fe(^3+) and Cr(^3+). The addition of either significantly increases the conductivity by an amount proportional to the concentration and this effect is explained in terms of an increase in the number of hopping sites available.