National Institute Of Materials Physics - Romania

The effect of Fe and Ni doping on piezoelectric properties of a soft type piezoelectric material was investigated. The materials composition was as follows: Pb0.95Bi0.03Nb0.02Zr0.51Ti0.49-xMxO3, where M stands for the transitional metals Fe or Ni and x = 0, 0.02, 0.04, 0.06, 0.08 and 0.10. The materials were prepared by the conventional ceramic technique. X-ray diffractograms showed that compounds were completely formed and they were situated in the nanometric range with an average crystallite size of about 95 urn. The optimum amounts of doping for both types of transitional elements were situated somewhere around x = 0.06 with better results for nickel doped samples. The maximum density for 0.06 nickel doped material was 7.87 and 7.80 g/cm(3) for iron doped one. The electromechanical coupling factor k(p) for 0.06 nickel doped samples was 0.665 while for the correspondingly iron doped ones it was 0.638. The relative dielectric constant was about 4050 for nickel doped samples and 3400 for iron doped ones. The corresponding values for the charge constant d(33) were 625 and 530 pm/V, respectively. These results were discussed in terms of the positions occupied by Ni and Fe into the lattice, the type of vacancies created by this and the shift of the morphotropic phase boundary. (c) 2005 Published by Elsevier Ltd.

The influence of preliminary treatment in hydrogen plasma on elimination of radiation defects and formation of thermal donors has been studied in detector structures made of standard and oxygenated float zone silicon has been studied. A new type of thermal donors has been found in as-treated diodes. These thermal donors are unstable and can be eliminated by heat-treatment at 200-250 degrees C. After irradiation with 3.5 MeV electrons the detectors had been annealed at temperatures of 50-350 degrees C. It has been found that preliminary hydrogenation at 300 degrees C leads to disappearance of main vacancy-type radiation defects at lower annealing temperatures, The annealing of hydrogenated and irradiated crystals is accompanied by hydrogen redistribution and formation of hydrogen-related donors. Preliminary irradiation influences on both these processes.

Effect of Bi additives on the piezoelectric properties of PT ceramics having the formula (Pb1-3x/2Bix(Ti0.98Mn0.02)O3, with x = 0,04, 0.06, 0.08 was investigated. The samples were prepared by conventional ceramic technique, using p.a. purity raw materials. The Mixed powders were sintered at temperatures between 1050-1300 degrees C. Poling was done in fields of about 70 kV/cm. Density and coupling factors of the samples were determined as a function of sintering temperatures, and doping level. Temperature dependence of the main piezoelectric characteristics was also investigated for temperatures as high as 500 degrees C. A Curie point of 450 degrees C, a low dielectric constant (< 140) and a large electromechanical anisotropy), (the radial mode is nearly inexistent, at room temperature and the thickness coupling factor is about 0.4) were found. These enhanced properties make such ceramics very attractive for high temperature and high frequency transducers applications.

The paper describes the investigations of Ba1-xSrxTiO3 ceramic materials prepared by solid-state reaction techniques. The addition of MgO and MnO2 improves sintering process and decreases the dielectric loss. The materials exhibit a dielectric constant epsilon(r) similar to 1000 and low loss tan delta similar to 10(-3) at microwave frequencies.

Wireless communications systems require new materials with special properties in specific frequency bands. The investigations on ZST type ceramics, (Zr0.8Sn0.2)TiO4, presented in this paper, recommend this materials for applications in microwaves and millimeter waves. The ZST materials were prepared using a standard solid-state reaction technology. The samples morphology, phase-composition and microstructure investigations were performed by using the scanning electron microscopy (SEM), and energy-disperse X-ray spectrometry (EDX). The crystalline phases were identified by X-ray diffractometry (XRD). The electromagnetic properties were investigated on ZST resonators by using a Computer Aided Measurement (CAM) in microwaves, combining a HP 8757C network analyzer and a HP 8350B sweep oscillator. The dielectric characteristics at millimeter waves were analyzed by investigation of the Whispering Gallery Modes on ZST disks. The low level NiO doping provides ZST materials with temperature coefficient tau(f) in the range (-2 divided by +4) ppm/degrees C and decreases the dielectric loss. Materials with high values of the Q f product up to 50,000 and a dielectric constant about 36 at microwave frequencies were obtained. ZST dielectric resonators and substrates for hybrid integrated circuits with dimensions 1" x 1" and thickness in the range 0.6 divided by 1 mm were manufactured.

Lead zirconate-titanate (PZT) epitaxial thin films are considered wide-gap p-type semiconductors with standard Schottky contacts. Capacitance-voltage, current-voltage, as well as the dielectric hysteresis are analyzed using a model based on the conventional metal-semiconductor Schottky contact in which the effect of the ferroelectric polarization on band-bending was considered. In addition, presence of the deep traps and its effect on the measured quantities were considered and discussed. For a single-crystal like PZT films with a remnant polarization of 40 mu C/cm(2) the free carrier concentration is estimated to about 3 x 10(18) cm(-3), whereas the effective density of the fixed charges in the depletion region is of about 1.8 x 10(19) cm(-3) .

Powders of barium titanate (BT) and lead zirconate-titanate (PZT) were mixed together to form solid solutions in the ceramic system (1 - x)BT (.) xPZT, where 0 <= x <= 1. The mixed compounds were synthesized by sintering pressed samples of the mixed powders at high temperatures between 1100-1400 degrees C, for 3 hours. Dielectric and piezoelectric properties of the sintered samples were investigated at room temperature. X-ray diffractograms showed that monophase ceramics with perovskite structure were formed along the whole compositional range. The dielectric permittivity epsilon(r) increased with increasing amount of PZT, reaching a maximum value of about 6900 for composition with x = 0.6, and then suddenly decreases up to lower values of less than 2000. The piezoelectric properties (electromechanical coupling coefficient k(p), voltage and charge constants g and d respectively) behave as follows: at first there is a slight decrease of their values for all compositions with x <= 0.6 and then suddenly, and rather constantly, their values increase by more than 100 % for compositions with x >= 0.6. Such a behavior could possible be explained in terms of the structural changes brought about by the incorporation of PZT phase into BT one. The mechanical mixture of BT and PZT particles of similar dimensions gives rise to both new compositions as well as to distinct particles of BT or PZT of different dimensions under the influence of temperature, whose properties may differ from the initial ones so that their properties can be either the result of a size effect or the compositional one, but most probably the combination of both.

Nonmonotonic current transients at constant temperature, following a pulsed charge injection, have been observed in silicon diodes heavily irradiated with gamma rays. The features of the transients have been studied at different temperatures and using various reverse biases. The particular shape of the measured transients is interpreted, with the help of numeric simulations, in terms of a strong time dependency of the space charge density due to the discharge of radiation induced deep traps. In particular, a nonmonotonic transient is generated if the discharge of a dominant trap determines the change of the space charge sign (type inversion). In this case the active volume of the sample, from which the current is collected, is increased during the discharge and causes the sample to become fully depleted for a short time. During this time interval the active volume reaches its maximum value and a peak is produced in the transient shape. The measurement of this peak provides a sensitive way to detect type inversion in semiconductor devices and to determine the responsible energy levels. The correlation of nonmonotonic transients with thermally stimulated current spectra and the distortion produced in deep level transient spectroscopy spectra are discussed in detail.

Pyroelectric current spectroscopy (PCS) was used to investigate the homogeneity of the Nb doped lead zirconate-titanate Pb(Zr-0.92 Ti-0.08)O-3, (PZT) ceramics intended to be used in the infrared (IR) pyroelectric detectors manufacturing. The non-homogeneous composition in the ceramic volume is reflected by the occurrence of several peaks in the PCS spectra, suggesting the presence of PZT phases with different Zr/Ti ratio, thus having different transition temperatures. The non-homogeneity was confirmed by compositional analysis performed using an Energy Dispersive X-ray Spectrometer (EDAX) attached to a Scanning Electron Microscope (SEM). PCS can be a useful tool in optimizing the calcination and sintering parameters (temperature and time) during ceramic preparation. (C) 2004 Elsevier B.V. All rights reserved.

In this work, it is shown that the analysis of thermally stimulated currents (TSC) and current transients (CT) at constant temperature can be a suitable tool to study the influence of deep levels on space charge density N(T) in irradiated silicon diodes. In particular, the occurrence of space charge sign inversion (SCSI) can be related to signal discontinuities in TSC and CT measurements. This approach has been adopted in this work to study devices made of standard Float Zone (FZ) and Diffusion Oxygenated Float Zone silicon, irradiated by gamma-rays up to a dose of 300 Mrad. Our study shows that all the samples are inverted at 50 K after a low-temperature excitation. Several space charge sign inversions, from positive to negative and vice versa, have been observed between cryogenic and room temperature, and have been related to carriers emission from dominating deep traps. Only standard FZ silicon remains inverted at room temperature after a dose of 300 Mrad. (C) 2004 Elsevier B.V. All rights reserved.