National Institute Of Materials Physics - Romania

The temperature behavior of the main piezoelectric parameters, such as the electromechanical coupling factor k(p), the mechanical quality factor Qm, the dielectric permittivity epsilon (real and imaginary part), the loss tangent tan delta, the piezoelectric charge constants d(33) and d(31) and the voltage constants 933 and 931 of a soft type piezoelectric material was investigated. The results showed that temperatures under 150 degrees C do not influence these parameters, which indicate that every transducer made from this type of material may be successfully used up to this temperature. Between 150 degrees C and 250 degrees C, the piezoelectric properties undergo more or less important changes, mainly due to the depoling effect and above that, at temperatures over 250 degrees C, they degrade very rapidly, tending to zero.

Accelerate ageing of lead zirconate-titanate ferroceramics (PZT) by fast neutron irradiation is investigated. The ferroelectric behavior of Pb1-wMw(Zr,Ti)O-3, where w < 0.5 wt%, M = Li, Nb, Cr, Bi, Mn, Sb, La for Zr/Ti ratio = 51.5/48.5, prepared by solid state reaction, is strongly affected by fast neutron irradiation at Phi(i) = (10(13) divided by 10(18)) n/cm(2) integrated fluxes. Structural investigations (XRD and SEM) reveal modifications of the microstructure, porosity and elementary cell parameters. High irradiation levels induce tetragonal-cubic at the same time with a ferro-paraelectric transition. The micrographs show the evolution of 90 degrees and 180 degrees walls as well as herringbone structure typical for tetragonal distortions with irradiation level. A relation between macroscopic characteristics (piezoelectric and dielectric) and microscopic properties (crystallographic parameters, porous structure and grain morphology) as function of irradiation doses can be established using a model based on microstructural parameters. These results offer opportunities to understand the fundamentals of radiation effects upon piezoelectric ceramics. (c) 2007 Elsevier Ltd. All rights reserved.

A study was made to determine the effect of sample shape, compacting pressure and green density gradients on the final shape and density of sintered bodies. Experiments were carried out on a PZT type powder, pressed with a large range of shapes from long cylinders up to thin discs, at pressures ranging between 40 and 180 MPa. Important shape distorsion were found for both long cylinders and thin discs but surprisingly the final density was not essentially changed even if the green compacted density showed important changes within the samples. Differences in shrinkage of diameter and thickness brought about by sintering for samples with different shapes were found.

The new advanced procedure of mechanochemical synthesis was used to prepare barium titanate from barium and titanium oxides. The X-ray examination of the powders milled for different times showed the phase evolution during synthesis. The perovskite phase was initiated after 5 hours of milling and was completed after 50 hours. Sintered samples from these nanopowders show different dielectric properties depending on their morphostructure. Thus for samples with an average grain size of 2 mu m we obtained the highest dielectric constant of 5 800 at room temperature. A sharp maximum of the dielectric constant with values of about 16 000 was recorded at the Curie temperature.

Accuracies of three methods used to determine the mechanical qualily-factors of-piezoceramic resonators were evaluated and discussed-for the specific case of five materials with different mechanical quality factors (between 20 and 3000) and electromechanical coupling coefficients (between 2.5% and 60%). The mechanical quality factor Q(m) was calculatedfor both radial and thickness modes. The three methods are: the equivalent circuit, the band width and the iterative method. The last two methods proved to be very accurate for all materials (the error of Q(m) is less than 0.1%), but for low Q(m) materials the second method requires to redefine the involved frequencies. The first method is less accurate, especially for materials with low mechanical quality and electromechanical coupling factors or large difference between the mechanical quality factors of the resonance and antiresonance bands (the error of Q(m) is more than 10-20%).

Barium strontium titanate (BST) bulk ceramic was used as target for PLD thin film deposition. In order to avoid stoichiometry modification during the deposition, a more reactive oxidant ambient in the chamber was produced by rising a radiofrequency discharge 13.56 MHz, 150 W. Thin films of stoichiometric BST was deposited on alumina substrate with the thickness between 400 and 500 nm heated at 650 degrees C. An aditional anealing was made at 800 degrees C for 6 hours. XRD and SEM were used for sample characterization. Capacity measurements at 100 kHz were performed versus temperature.

In the present investigation we studied the effect of lead content on a hard type ceramics starting from lack to excess PbO. Specimens of Pb-x(Mn0.017Sb0.033Zr0.48Ti0.47)O-3 with 0.96 <= x <= 1.06 were prepared by conventional oxide mixing technique and sintered at temperatures between 1280 and 1350 degrees C. In samples with lower Ph concentration the pyrochlore phase appeared, while in those with higher Pb an excess of PbO phase was detected. These phases proved detrimental for the properties of sintered ceramics. The highest values of piezoelectric properties were observed only for samples with an excess lead of about 2% sintered at 1340 degrees C, indicating that it represents the optimum amount of PbO. For such samples, the density p was 7.77 g/cm (3). the remnant polarization P, was 37 mu C/cm(3) the planar coupling coefficient k(P) was 0.57 and the charge constant d(33) was 340 C-P/N. (c) 2007 Elsevier Ltd. All rights reserved.

A comparative study of photodetector performance for nano- and micro-crystalline PbS thin layers is presented Both types of PbS films were obtained using the Chemical Bath Deposition method. To prepare nano-crystalline thin films we used a short reaction time with no doping Bi added At low levels of irradiance (0.1 mW/cm(2)) and room temperature (RT), the macro-crystalline PbS outperforms its nano-crystalline counterpart, by a factor of 6 for the value of Delta R/R-dark. However the nano-crystalline photodetectors show significantly lower optical quenching for large values of irradiance (approximate to 500 mW/cm(2)) at RT and better Delta R/R-dark performance at lower temperature, with an optimum temperature Of 150 K. The low frequency noise behavior is a superposition of 1/f flicker noise at low frequency and generation-recombination and thermal noise at higher frequency. The proportionaliiy factor between the 1/f noise and the DC dark current for the series resistor is 1.5 times larger for the nano-crystalline photodetector at room temperature.

Three types of materials for dielectric resonators are presented in this paper. The Pb-BNT materials show the highest dielectric constant (similar to 85). A very small controllable temperature coefficient if in the range (-2 divided by +4) ppm/degrees C can be achieved by using dielectric resonators Mg-(Zr0.8Sn0.2)TiO4. On the other hand, the resonators made of Ba(Zn1/3Ta2/3)O-3 exhibit the highest values for the Qxf product, ranging from 50000 to 130000 GHz, which make them attractive for millimetre wave applications.

Diodes processed on n-type epitaxial silicon with a thickness of 25, 50 and 75 mu m had been irradiated with reactor neutrons and high-energy protons (24 GeV/c) up to integrated fluences of Phi(eq) = 10(16) cm(-2). Systematic experiments on radiation-induced damage effects revealed the following results: in contrast to standard and oxygen-enriched float zone (FZ) silicon devices no space charge sign inversion was observed after irradiation. It is shown that the radiation-generated concentration of deep acceptors, dominating the behavior of n-type FZ diodes, is compensated by creation of shallow donors. Thus a positive space charge is maintained throughout the irradiation up to the highest fluence and even during prolonged elevated-temperature annealing cycles. Defect analysis studies using thermally stimulated current measurements attribute the effect to a damage-induced shallow donor at E-C-0.23 eV. It is argued that, as in the case of thermal donors, oxygen dimers, out diffusing from the Cz substrate during the diode processing, are responsible precursers. Results from extensive annealing experiments at elevated temperatures are verified by comparison with prolonged room-temperature annealing. These results showed that in contrast to FZ detectors, which always have to be cooled, room-temperature storage during beam off periods of future elementary particle physics experiments would even be beneficial for n-type epi-silicon detectors. A dedicated experiment at CERN-PS had successfully proven this expectation. It was verified, that in such a scenario the depletion voltage for the epi-detector could always be kept at a moderate level throughout the full S-LHC operation (foreseen upgrade of the large hadron collider). Practically no difference with respect to FZ-silicon devices was found in the damage-induced bulk generation current. The charge trapping measured with Sr-90 electrons (mip's) is also almost identical to what was expected. A charge collection efficiency of 60% (2500 e) in 50 mu m n-type epi-diodes after 24 GeV/c proton irradiation with Phi(eq) = 6.2 x 10(15) cm(-2) was reported recently, independent of the operating temperature down to -20 degrees C. (c) 2006 Elsevier B.V. All rights reserved.