The grain size influence on the dielectric relaxation and nonlinear dielectric properties of BaTiO 3 ceramics with grain size in the range of (92Ä936) nm densified by Spark Plasma Sintering (SPS) from ultrafine powders were investigated. The progressive reduction of the Curie temperature and of the effective permittivity results from a combination of intrinsic size effects and low-permittivity grain boundary layer. A model of dielectric cylindrical cavities was employed in order to calculate intrinsic effective permittivity values in GHz range. An interesting feature is the presence of a thermally activated Debyelike relaxation in the ferroelectric state of ceramics with grain size above 300 nm, with activation energies of 0.45-0.49 eV, which seems to be related to the domain walls forced motion under the applied field. By diminishing grain size, a progressive reduction of the ferroelectric nonlinear character was obtained, reaching a macroscopic non-switching character and a linear permittivity versus field dependence for the finest ceramics (grain size of 90-100 nm) until very high values of the applied field. The observed behavior supports the idea of frozen polarization induced by pinning centers as due to a large number of grain boundaries and charged defects in the fine structures.

An impedance spectroscopy method, using an Agilent E4991A RF Impedance/Material Analyzer, was employed for the investigation of radiofrequency and microwave dielectric characteristics for some ceramic materials with high dielectric permittivity. Experimental observations recorded single, double and even triple resonances in the permittivity spectra, while for the selected samples significantly different values of permittivity from the ones found in existing literature were directly measured and observed in the frequency range of 100 MHz-1 GHz. The specified material analyzer is designed to work within certain permittivity-frequency domains; working outside these limits leads to significant errors when carrying out direct measurements of permittivity. Having observed that the resonance measurements are not associated with the intrinsic material properties but with the sample resonances, we proposed a dielectric resonance cavity model for sample purposes and we determined the dielectric permittivity at different resonance frequencies. A scaling procedure, based on the computed values of permittivity, was applied in order to correct the directly measured permittivity spectra where the frequency was outside the resonance domains. The corrected data are in good agreement with those measured using other techniques and suggest that the proposed procedure extends the spectroscopy method currently used for high permittivity measurements. The sources of error for the proposed method were investigated.

In many laboratories, offices and home rooms exists devices and equipment working in the radio frequency (RF) and microwave (MW) bands. People working with these equipments may be exposed to electromagnetic fields of high density that can produce adverse effects on human health. It is suitable to perform measurements on the scattered fields, especially in the near-field regions (Fresnel' radiating and reactive regions), where the incident power density can reach dangerous levels for occupational personnel and for people living in the neighbourhood. The devices subjected to measurements are: a microwave oven, a microwave generator for supply a TEM (Transversal ElectroMagnetic) exposure cell; wireless communication devices, different mobile phones, a power microwave generator that produces cold plasma. In this paper besides a general introduction, many experimental results concerning the power density, electric and magnetic intensities of the scattered fields of the mentioned particular devices are reported; also, a comparison of these results with the most recent international protection standards is performed. The goal of the introduction is to define the radiation regions of different sources, thus permitting a better interpretation of the experimental data.

As a consequence of the increasing number and the transmitting power of radiofrequency (RF) and microwave sources, there is an increase of incident power density on human body. This paper presents the significant results of measurements concerning the power density of incident electromagnetic fields in the 75 -3000 MHz band. Also an interpretation of experimental results and comparisons with recent protection standards are given. Measurements are performed in the area of the "Alexandru I. Cuza" University including offices, laboratories, classrooms, and student dormitories; also, some buildings situated at different distances from the main building were subjected to measurements, such as: the Faculty of Orthodox Theology, the Akademos building, the Department of Interdisciplinary Research. The experimental data are obtained by measurements on intensities of electric field E (V/m), magnetic field H (A/m) and power density S (W/m 2 ); the results are displayed as graphics of spectral curves (peaks), or numerically, where the values refer to individual frequency or on a narrow frequency band. After comparisons with recent protection standards, the most important conclusion is that, generally, in the area subjected to measurements the values of power density (integrated over a frequency band) do not surpass the safety values established in the recent protection standards for the general public; however there are two places, a plasma laboratory and an adjacent office where the power densities are higher than the safety limits.