M. A. Ahmed

Dielectric behavior of hexaferrites BaCo2−xZnxFe16O27

Abstract The dielectric constant (e′) and dielectric loss (tan δ) for hexaferrites BaCo2−xZnxFe16O27 have been studied as a function of frequency (f), temperature (T) and composition (x). The experimental results indicate that e′ and tan δ above the relaxation frequency only decrease as the frequency increases and as the temperature decreases. Tan δ shows the dielectric relaxation at certain critical frequencies which rise as temperature increases. The activation energy for the dielectric relaxation (ED), e′, and tan δ are found to be minimum for x = 0.8.

Dielectric behaviour of CuCr ferrites

Abstract Samples of the system CuFe 2− x Cr x O 4 where x = 0.0, 0.2, 0.4, 0.6 and 0.8 were prepared. The dielectric constant and dielectric loss were measured at different frequencies and temperatures from room temperature to 600 K. The dielectric constant was measured using the phase detector technique (lock-in amplifier). The results showed that the dielectric loss decreases with increasing frequency and Cr substitution. The dielectric constant decreases with both increasing frequency and Cr substitution at room temperature. At moderate temperatures, the dielectric constant shows a dispersion peak e ′ max , and this peak shifted to higher frequency with increasing temperature. The results are explained in the light of the fact that the dielectric polarization process is similar to that of conduction. The appearance of the dispersion peak is related to the contribution of two types of charge carriers.

Ac conductivity in CuCr ferrites

Abstract Samples of the type CuFe 2− x Cr x O 4 with x = 0.0, 0.2, 0.4, 0.6 and 0.8 were prepared. Ac conductivity measurements were carried out at different frequencies up to 10 5 Hz and at various temperatures from room temperature up to about 550 K. The ac conductivity was measured using the phase detector technique (lock-in amplifier). The results showed that the conductivity increases with both temperature and frequency, and the activation energy in the paramagnetic region is higher than that in the ferrimagnetic region. At high temperatures the conductivity becomes frequency independent. We conclude that the conductivity in these ferrites consists of two kinds of conduction mechanisms.

Dielectric behaviour in Ni-Al ferrites at low frequencies

Abstract The dielectric constant (ϵr) and the dielectric loss (tan σ) have been measured for samples of the type NiAlxFe2−xO4 (x = 0, 0.2, 0.4, 0.6, 0.8 and 1). The measurements were carried out in the frequency range from 100 up to 105 Hz. The dielectric behaviour is found to be abnormal, giving a relaxation peak at a certain frequency. The peak shifts to lower frequency due to increasing aluminium substitution. The abnormal behaviour of the dielectric is explained due to two types of charge carriers. The temperature dependence of the dielectric relaxation frequency is measured and the activation energy of the dielectric relaxation is calculated.

The ac electrical conductivity for Co-substituted SbNi ferrites

Abstract The real ac electrical conductivity σ′ac has been studied for the ferrite system Co x Ni 5 3 −x Sb 1 3 Fe 1 O 4 as a function of frequency, temperature and composition. The experimental results indicate that σ′ac increases with increasing frequency, temperature and Co ion content. σ′ac becomes frequency independent at higher temperatures. The activation energies for electrical conduction and the Curie temperatures decrease with increasing Co ion content. The activation energy for electrical conduction in the ferrimagnetic region decreases as frequency increases. In the power law for ac electrical conductivity (σ2(ω) = Bωn), the constant B increases, while the power n of the frequency decreases with increasing Co ion substitution and temperature. An empirical formula for the temperature dependence of n is suggested.

Dielectric behaviour in Co-substituted NiSb ferrites

Abstracts are not published in this journal

Semiconductive properties of CuCr ferrites

Abstract Samples of type CuFe 2- x Cr x O 4 , where x = 0.0, 0.2, 0.4, 0.6, and 0.8, were prepared by the usual ceramic technique. The X-ray measurements confirmed that all samples were of spinel type of tetragonal crystal structure. The lattice parameter, density, theoretical density, and porosity were determined at different Cr substitutions. Dc resistivity (ρ) and thermoelectric power (α) were measured at temperatures differing from room temperature up to 580 K. The drift mobility μ n for n-carriers within the sample was calculated at various temperatures. The results indicated that the samples have semiconducting behavior. At room temperature the majority carriers are electrons, except in case x = 0.8. The Curie temperature T c shifts to higher temperature with increasing Cr substitution. The drift mobility μ n increases exponentially with temperature. The results are in good agreement with previous results.

Electrical properties of thin metal zinc films

Thin metal zinc films 40 to 200 nm thick are deposited by thermal evaporation at room temperature onto glass substrates with a deposition rate of 0.2 to 0.7 nm sec−1. The electrical resistivity is measured as a function of film thickness, deposition rate and annealing temperature. The experimental results show that electrical resistivity decreases as the film thickness, deposition rate and annealing temperature increase, while the temperature coefficient of resistivity increases with the increase in the film thickness. The calculated values of the activation energy for the conduction electrons increases as the film thickness and deposition rate increase. The well known Fuchs-Sondheimer model is applied for zinc films. The theoretically calculated values for the electrical resistivity and the temperature coefficient of resistivity are in good agreement with the experimental results.

Structural study of NiXMg1–XFe2O4 ferrites

Abstract The ferrite system Ni x Mg1-x Fe2O4 with 0≤x≤1 was prepared using the usual ceramic technique. The prepared samples were studied by X-ray diffraction and IR spectroscopy. X-ray diffraction analysis proved that all the samples were single-phase with the cubic spinel structure. The lattice constant, radius of the tetrahedral ion, unshared octahedral edge, tetrahedral bond and tetrahedral edge decrease while the bulk and theoretical densities, radius of octahedral ion, octahedral bond and shared octahedral edge increase as nickel ion substitution increases. The positions and intensities of the four bands of IR absorption spectra characterizing ferrites are composition dependent.

The effect of linear thermal expansion on the temperature coefficient of resistance of double-layer thin metallic films

A general theoretical expression for the temperature coefficient of resistance of double-layer thin metallic films, based on the well known Fuchs-Sondheimer model, is derived. This expression includes the linear thermal expansion coefficients and Poissons ratios of the double layers and the substrate, also the film dimensions and temperature coefficient of resistance of the double-layer thin film, with and without the thermal expansion of both the film layers and the substrate. Numerical calculations are carried out for gold-silver double-layer films deposited on a glass substrate, where variations in the temperature coefficient of resistance depending on thermal expansion are studied as a function of reduced film thickness. The computed numerical results, using the derived new expression for the temperature coefficient of resistance of the double-layer thin metallic films, show that the thermal expansion decreases the value of the temperature coefficient of resistance.