National Institute Of Materials Physics - Romania

Ceramics (Ba1-xSrx)TiO3, (BST) with x=0.25; 0.35; 0.40; 0.50; 0.60; 0.75; 0.90 were prepared by sintering method from BaCO3, SrCO3 and TiO2 of high purity (99.98%). The solid-state reaction was employed to obtain pure and doped BST ceramic samples. In order to improve the sintering process the following additives were added: 1.0 wt.% MgO and 1 wt.% MnO2. The sintering treatment was performed in die temperature range 1200 to 1260 degrees C, for 2 h. Archimedean method was used for ceramic density measurements, which was found to depend linearly on strontium content: rho(x)=5.4(2)-0.8(8).x, (g/cm(3)). SEM, EDX and XRD methods were used for sample characterization. Morphology, pores and grain size distribution of both pure and doped ceramics were investigated by SEM analysis. A bimodal grain distribution could be observed in pure BST for x <= 0.25, whereas a uniform one was noticed for doped samples of x = 0.25. Well faceted grains of similar to 5 pm size, Could be seen for x=0.75 samples. Pores size has grown with the increase of Sr content. X-ray diffraction patterns have shown a dominant perovskite structure and some other minor phases. Dielectric permittivity and loss were investigated at 1kHz, in the temperature range - 150 to + 150 degrees C, at a pace of similar to 2 degrees C /min. Sharp transitions, with higher peaks values of permittivity were noticed with the increase of the sintering temperature. A shift Of the Curie points to lower values was noticed with the increase of Sr content, according to the equation: Tc = 127.4-331.x. Measured at room temperature, in the frequency range of similar to 1 GHz, permittivity was found to increase and losses to decrease with the sintering temperature. For x <= 0.35 permittivity of the samples, being in the ferroelectric phase could not be measured in microwave domain at room temperature. Our results point to important application of these BST compositions for microwave devices. (C) 2008 Elsevier B.V. All rights reserved.

Calcium phosphate compounds have been studied for biomedical applications due to chemical and structural similarity to the mineral phase of bone and tooth. The composition, physico-chemical properties, crystal size and morphology of synthetic apatite are extremely sensitive to preparative conditions and sometimes it resulted into non-stoichiometric calcium deficient hydroxyapatite (HAp) powders. The present paper describes the synthesis of calcium phosphate ceramics powders via a sol-gel method, The powders were sintered at 600 and 800 degrees C. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDXS) were used for characterization and evaluation of the phase composition, morphology and particle size of samples, The functional group of the coatings were analyzed using Fourier transform infrared (FTIR). The XRD analysis revealed a well crystallized HA structure at both temperatures. At 800 degrees C a small amount of CaO (about 0.4%) was detected. The mean crystallite size, determined from the breadth of the diffraction lines, increases from 30 nm at 600 degrees C to about 110 nm at 800 degrees C. FTIR spectra showed the presence of various PO43- and OH- groups present in the powders. Osteoblast cells were used to determine cell proliferation, viability and citotoxicity after interaction with the prepared bioceramics, by MTT (3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide, a tetrazole) assay. We note that the viability of HAp-600 starts to decrease after 12h, while the viability of HAp-800 decreases after 6h.

Polymer-like organic thin films obtained by plasma techniques are of wide interest for various applications. Polythiophene-like thin films were deposited by plasma polymerization in a parallel plate radiofrequency reactor starting from thiophene vapors. The optical properties of the films were obtained from spectroscopic ellipsometry, in the 250-1250 nm range. The calculated band gap is in agreement with a polythiophene-like material consisting of cross-linking of oligomers. The presence of functional groups and film stability against oxidation is analyzed by FTIR spectrometry. The monitoring of functional groups incorporation during the film deposition was done by analyzing the integral intensities of specific absorption bands and led to the conclusion that homogeneous films are obtained whatever the process duration.

In the present work an X-band electron magnetic resonance (EMR) investigation in the paramagnetic regime of colossal magnetoresistive manganite La2/3Ca1/3 MnO3 (LCMO) nanowires was carried out. The temperature dependence of the EMR line width has been analyzed in terms of the small polaron hopping scenario. From this analysis, the polaron activation energy E-a in LCMO nanowires was evaluated. A discussion is given concerning the factors which could explain the smaller value of E-a as compared with the corresponding ones for LCMO nanoparticles.

The influence of the gate voltage on photoconductivity in P-Si/SiO2/(nano)crystalline PbS, Metal-Oxide Semiconductor Field-Effect Transistor (MOSFET)-like structures is investigated. The effect on the photo-signal in the wavelength range 500-1180 nm is opposite compared to the effect in the wavelength range 1600-2500 nm: a negative gate voltage increases the signal in the short wavelength range and decreases it for the long wavelength range. The opposite is valid for a positive gate bias. An on-off ratio better than 85 times is obtained for the spectral range 500-1180 nm, corresponding to the absorption in p-Si and better than 3.5 times for the spectral range 1600-2500 nm corresponding to the absorption in the (nano)crystal line PbS region, respectively. (c) 2007 Elsevier B.V. All rights reserved.

Leakage currents determine bumps in the capacitance-voltage characteristics of metal-oxide-semiconductor-type heterostructures with capacitance values larger than the insulator capacitance. A nearly parallel shift with frequency is also observed. These aspects are connected with the presence of a high density of interface states. We illustrate these effects for the case of nano-PbS/SiO2/Si heterostructures. The phenomena were simulated and we propose a method to quickly estimate the interface state density level of a heterostructure when leakage channels are present. (C) 2008 American Institute of Physics.

The nano-crystalline PbS/n-Si heterostructure was studied using photocurrent spectra, I-V and C-V characteristics and admittance spectroscopy. The frequency dependent junction capacitance and conductance measurements show the presence of two contributions: first, a defect related mechanism which we attribute to a deep trap level with a large cross-section in nano-crystals with smaller sizes around 5 nm at the interface with Si and a second mechanism with an activation energy of about 250 meV, attributed to carrier transport in relatively large PbS grains of about 15 nm in size. (c) 2007 Elsevier B.V. All fights reserved.

High dielectric materials have gained an important position in microwave electronics by reducing the size and cost of components for a wide range of applications from mobile telephony to spatial communications. Ba(Zn1/3Ta2/3)O-3 (BZT) is an A(B'B '')O-3 type perovskite material, showing ultra high values of the quality factor Q. Ceramic-based BZT dielectric materials were prepared by solid state reaction. The samples were sintered at temperatures in the range 1400 divided by 1600 degrees C for 4 h. Compositional, structural and morphological characterization were performed by using XRD, SEM and EDX analysis. The dielectric properties were measured in the microwave range (6 divided by 7 GHz). An additional annealing at 1400 degrees C for 10 h has improved some dielectric parameters. For samples sintered at temperatures higher than 1500 degrees C, the permittivity values were obtained in the interval 30 divided by 35 and almost do not change the value after the annealing. The Q x f product substantially increases up to about 135,000 GHz, exhibiting a low temperature coefficient of the resonant frequency (tau(f)) in microwaves. The best parameters were obtained for the samples sintered at 1600 degrees C with additional annealing. The achieved high values of the Q x f product recommend these materials for microwave and millimeter wave applications. (C) 2007 Elsevier B.V. All rights reserved.

In hydrogenated high-purity Si, the vacancy-oxygen (VO) center is shown to anneal already at temperatures below 200 degrees C and is replaced by a center, identified as a vacancy-oxygen-hydrogen complex, with an energy level 0.37 eV below the conduction-band edge and a rather low thermal stability. At long annealing times, the process is reversed and the concentration of the latter defect is reduced, while the VO center partly recovers. The divacancy (V-2) center anneals in parallel with the initial annealing of the VO center, and the loss in V-2 exhibits a one-to-one proportionality with the appearance of a hole trap 0.23 eV above the valence-band edge attributed to a divacancy-hydrogen (V2H) center.

Self assembled InGaAs/GaAs quantum dots (QD) have a great potential for high performance optoelectronic devices such as low threshold laser diodes, infrared detectors, modulators, memories. In order to characterize the behavior of the QD system, we use two p(+)-n structures grown epitaxially on GaAs under similar conditions. The first structure acts as reference while in the second structure a single QD self-assembled layer is introduced in the middle of the n-GaAs matrix layer. The structure is designed such that for OV applied bias the QD layer lies outside the depleted region. When the reverse bias is increased, the charge from the QD system is removed and the depletion layer moves further into the GaAs matrix material. The electronic structure of the QD is investigated using two methods: photoluminescence, in order to characterize the transition energies between electron and hole levels in the QD system and capacitance spectroscopy in order to study the electron levels in the conduction band only. In addition, admittance spectroscopy spectra are measured in order to characterize the carrier transport mechanism. There is no evident step in the capacitance versus frequency behavior at room temperature in the range 1 Hz-1 MHz, indicating a large carrier cross section caption and/or a low activation energy for the carrier transport between the dot system and the wetting layer and GaAs barriers. The plateau in the C-V behavior, due to charging or discharging of the QD system is modeled using the solution of a Poisson equation and the resulting energy of the electron states within the conduction band and QD size distribution are correlated with results from photoluminescence studies, which involve transitions between energy levels both from the conduction and valence bands.