National Institute Of Materials Physics - Romania

Pyroelectric measurements were performed on pure and on L or D alanine doped TCS. Crystals were grown in paraelectric phase (similar to 52 degreesC) from solutions by slow evaporation of the solvent. Pyroelectric current was measured by, crossing the Curie point, both on heating and cooling. Spontaneous polarization P-S(T) was also recorded using the charge integration method. The pyroelectric current of pure TGS samples along with the spontaneous polarization were detected only on heating, using a poling procedure similar to those applied to ceramic ferroelectrics. The remarkable asymmetry of measured pyroelectric coefficient of D and I. alanine doped samples suggests a non-equivalent substitution of glycine in the host lattice.

We propose an improved method of thermally stimulated currents (TSC) spectra analysis in the case of diodes having a concentration of traps higher than that of doping impurities. Beside the calculation of trap concentrations from TSC peaks analysis, the method allows us to evaluate the density and the type of the very deep trapping level which, due to the contribution of leakage current, can not be detected in a real TSC experiment. The proposed method is applied to a p(+)-n Silicon diode irradiated with 1.82x10(13)neutrons/cm(2). (C) 2001 American Institute of Physics.

An experimental study was made of the mechanical and electromechanical nonlinearities of a PZT trimorph type actuator. The actuator consisted of two piezoceramic poled plates, sandwiched on a thin metallic sheet, forming the usual trimorph structure, with one end fixed and the other one free. The experiments were conducted in two steps. In the first step a force was applied to the free end of the trimorph and its displacement was measured as a function of the applied force either for increasing and decreasing force. The nonlinearities in this case must be pure mechanical and they give information about the structural arrangement of the piezoceramic elements, glue and metallic sheet. For the second step the actuator was electrically activated with increasing and decreasing DC voltages and the displacement of the free end was determined as a function of applied DC voltage. The nonlinearities in this case are of electromechanical nature and they were the result of the combined effect of intrinsic material nonlinearities and structural arrangement. The C, experimental results showed that pure mechanical nonlinearities are practically inexistent even for forces up to 0.7 N. The displacement of the free end follows the theoretical predictions. When the actuator was electrically activated the displacement of the free end was entirely hysteretic and the electromechanical nonlinearities were different from the theoretical predictions. The results were discussed in terms of the displacement of ferroelastic-ferroelectric domain walls.

Conventional and mechanical activation techniques were used to prepare a soft PZT type piezoceramic material. The resulting powders exhibit micro and nanostructures, respectively as indicated by X-ray and electron microscopy. Average crystallite sizes of 1 mu and 50 nm respectively were estimated for the powders. The sintered behavior of pressed samples from the two types of powders was investigated within the 8001300 degreesC temperature range and the densities, structural morphology and dielectric and piezoelectric properties were investigated. Optimum sintering temperature were about 100 degreesC lower for samples made from mechanical activated powder compared to the conventionally ones. Sintered samples from this powder exhibited also enhanced properties compared to those made from the conventionaly prepared powder. It is concluded that the mechanical activation technique is a reliable and simple method to prepare high quality PZT type materials.

A detailed formula for the equivalent pyroelectric coefficient of a bimorph structure acting as a pyroelectric detector was deduced. It is shown that electric fields occur in the structure when it is subjected to a temperature variation, due to the unequal charges generated by the two components. The field dependence of the ferroelectric polarization has to be considered in this case and the (partial derivativeP/partial derivativeE) derivative appears explicitly in the deduced formula. The effect is that the equivalent pyroelectric coefficient has different values for heating and cooling. This could lead to a redressing effect on the pyroelectric signal if the temperature difference varies periodically. The equivalent pyroelectric coefficient is, also, thickness dependent. This fact offers the possibility of designing structures with high values of pyroelectric coefficient compared with the values of the component phases. (C) 2000 American Institute of Physics. [S0021- 8979(00)04701-6].

Graded ferroelectric (GF) structures were obtained by successive deposition of Pb1-3y/2Lay(Zr0.4Ti0.6)O-3 films with different La contents on Pt/TiO2Si substrates. The method used for deposition was sol gel. The final structure consists of four layers, starting from the Pt electrode to the top gold electrode: (1) no La; (2) 4% La; (3) 8% La; and (4) 12% La. Ferroelectric properties of the GF structure were investigated, and an anomalous shift of the hysteresis loop along the polarization axis was found. The sense of this shift depends on the initial direction of the ferroelectric polarization. The presence of hysteresis displacement "up" or "down" can be explained if the presence of a built-in charge in the GF structure is admitted. The built-in potential produced by this charge is computed using a simple theoretical model. (C) 2000 American Institute of Physics. [S0003-6951(00)01040-8].

p-type lead sulphide (PbS) thin films were chemically deposited onto Si3N4/n-Si substrates. A three-contact pseudo-metal-oxide-semiconductor structure was designed in order to investigate the possibility of enhancing the signal-to-noise ratio in the PbS film by field effect. It was proved that on the entire sensitivity domain of PbS (1-3 mu m at room temperature), the magnitude of the photoconductive signal generated by the PbS film depends on the polarity and value of the gate voltage. It is shown that the signal-to-noise ratio can be enhanced in this way with by at least 25% compared with the case when the gate electrode is in air (standard configuration for photoconductive measurements). (C) 2000 American Institute of Physics. [S0003-6951(00)01514-X].

The trapping levels induced in p-n Si junctions by irradiation with 24 GeV proton were investigated using Thermally Stimulated Currents (TSC) methods in the 90-300 K temperature range. Several trapping levels, with activation energies between 0.27 and 0.57 eV, were put into evidence. The spatial distribution of the traps was investigated using different wavelengths for the light used to fill the traps. The results lead to the conclusion that the deepest trapping levels are not uniformly distributed in the volume of the sample. For the most important trapping level the average introduction rate was estimated to 0.9-1.2 cm(-1). The activation energy and the capture cross-section of this trapping level seems to depend on the impurity element introduced in Si. (C) 2000 Elsevier Science B.V. All rights reserved.

Optical charging spectroscopy (OCS) is first time reported as applied to p-n junctions. The existence of one deep trapping level for electrons and two deep trapping levels for holes was put into evidence, using this method, in proton irradiated p(+)-n-n(+) silicon structures. An analytical formula for the OCS discharging current for this type of structures was deduced. (C) 2000 Elsevier Science B.V. All rights reserved.

Lead titanate (PT) is well known as a good piezoelectric material with a high Curie temperature (similar to 490 degreesC) and a low dielectric constant (similar to 200). But pure lead titanate cannot be sintered as ceramic bodies, for practical purposes, due to its large crystal anisotropy (tetragonality c/a=1.064). However, dense PT ceramic bodies can be obtained if small additions of other elements are used. Ca and Sm modified PT were succesfully tried, but such dopings proved rather expensive for mass production. Therefore some other doping elements must be tried. In this regard we investigated the effect of Bi and Y additives on the piezoelectric properties of PT ceramics. The general formula of our Bi, Y doped lead titanate ceramics was: (Pb1-3x/2Mex)(Ti0,98Mn0,02)O-3, with Me=Bi,Y and x=0.04, 0.06, 0.08. The modified PT ceramics were prepared by conventional ceramic technique, using p.a.purity raw materials. The mixed powders were sintered at temperatures between 1000-1300 degreesC. Poling was done in fields of about 70kV/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 degreesC. It was found that Bi,Y modified lead titanate ceramics have a very high Curie point (T-c>450 degreesC), 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), which make them competitive to the rare-earth doped PT ceramics, for high temperature and high frequency transducers applications.