Capacitance Measurements and AC Conductivity of Nickel ...

168 S. Darwishtan δ (Eq. (2)) predicts a decease in tan δobserved in Fig.1. The expression of−1with increasing ω where the term ω is dominant for lower frequencies, followed1/2by a loss minimum at ω min ≈1/[C ′(rR)] [1,6] and finally to increase withω above ω min where the term in ω is dominant.The variation of tan δ with frequency at various temperatures is representedin Fig. 2. The decrease of tan δ with frequency is clearly evident as predicted forlow frequencies and no indication of a minimum is observed over the frequency10 2 T= 295K10 1tan δ10 010 -1T= 318KT= 325KT= 360KT= 385K10 -210 3 10 4 10 5f (Hz)Figure 2: Dependence of loss tangent on frequency at different temperatures.range investigated. However, further work is aimed at extending the measurementsto higher frequencies and temperatures employed in an attempt to observe welldefinedminima in tan δ as predicted by the model of Goswami and Goswami [19].It is also hoped to investigate dielectric losses at very low frequencies in otherorganic materials. On the other hand, it can be seen from Fig. 2 that tan δ increaseswith the increase of temperature at all frequencies. The increase in tan δ with−1temperature is consistent with Eq. (2) as the ω term becomes dominant because ofthe decreasing value of R with temperature.The frequency dependence of the AC conductivity ( σ AC = σ − σDC) fordifferent values of temperatures is shown in Fig. 3. The conductivity obeyed theempirical law of frequency dependence given by the power law of the form [8,20]:sσ AC( ω)= Aω(3)the variation of the exponent s with temperature gives information on the specific