National Institute Of Materials Physics - Romania

The present study is focused on the wet chemical synthesis and the characterization of ZnO-CdS composites. The X-ray diffraction shows that the composites contain ZnO in hexagonal wurtzite structure and CdS in cubic phase. The scanning/transmission electron microscopy images reveal flower-like structures with different sizes depending on the CdS content. The optical investigations on composites reveal that the reflectance spectra disclose two thresholds of similar to 370 nm and similar to 460 nm associated with the ZnO and CdS, respectively. The photocatalytic activity measurements evidenced that the degradation efficiency of RhB in the presence of composites is higher comparatively with pristine ZnO, depending on the catalyst morphology, which varies with CdS content and the pH value of RhB solution. The electron paramagnetic resonance revealed the presence of the paramagnetic point defects in the samples. Thus, the wet chemical approaches are suitable for a large scale production of such ZnO-CdS composites having enhanced photocatalytic activity.

By platinum deposition on a 150 nm thick film of lead zirco-titanate oriented PZT(001), grown on strontium titanate (001) single crystals with a strontium ruthenate buffer layer, which did not show initial preferential out-of-plane orientation of its ferroelectric polarization, a band bending near the interface towards lower energies is observed using photoelectron spectroscopy, by following all core levels from the substrate (Pb 4f, Zr 3d, Ti 2p, O 1s). This is unexpected given the fact that platinum has a larger work function than PZT and a rectifying contact for electrons is expected to be built at the interface. This observation may have two explanations: (i) platinum forms an alloy with elements from PZT yielding a metal with considerable lower work function; (ii) platinum provides electrons to the substrate which are able to compensate the depolarization field generated by the outwards polarization state. Several arguments are brought in favor of the second hypothesis, especially the attenuation of core levels from the substrate which is well described by exponential functions with reasonable values of the photoelectron inelastic mean free path, suggesting the formation of a sharp interface. High resolution transmission electron microscopy confirmed the sharpness of the interface. (C) 2017 Elsevier B.V. All rights reserved.

Samples of MgB2 with relative density above 95% were obtained by spark plasma sintering (SPS) at 1150 A degrees C for a heating time of 100 A degrees C/min and a maximum pressure of 95 MPa. Dwell time was of 1, 4, 7, 10, and 20 min. Samples show that dwell time has a low influence on superconducting characteristics. However, small differences were observed and they are discussed from the viewpoint of density, structural, microstructural, critical current density, irreversibility field, and pinning type. The dominant contribution for all investigated samples is given by point and delta T (c) pinning.

Un-doped ZnO (UDZO) and Li-doped ZnO (LZO) polycrystalline thin films were grown on platinized silicon by pulsed laser deposition (PLD). The electrical properties were investigated on as-grown and annealed UDZO and LZO films with capacitor configuration, using top and bottom platinum electrodes. In the case of the as-grown films it was found that the introduction of Li increases the resistivity of ZnO and induces butterfly shape in the C-V characteristic, suggesting ferroelectric-like behavior in LZO films. The properties of LZO samples does not significantly changes after thermal annealing while the properties of UDZO samples show significant changes upon annealing, manifested in a butterfly shape of the C-V characteristic and resistive-like switching. However, the butterfly shape disappears if long delay time is used in the C-V measurement, the characteristic remaining non-linear. Pyroelectric signal could be measured only on annealed films. Comparing the UDZO results with those obtained in the case of Li: ZnO, it was found that the pyroelectric properties are considerably enhanced by Li doping, leading to pyroelectric signal with about one order of magnitude larger at low modulation frequencies than for un-doped samples. Although the results of this study hint towards a ferroelectric-like behavior of Li doped ZnO, the presence of real ferroelectricity in this material remains controversial. (C) 2017 Elsevier B.V. All rights reserved.

Hydroxyapatite-starch composites solidify rapidly via jellification, making them suitable candidates for robocasting. However, many aspects related to hydroxyapatite powder characteristics, hydroxyapatite-starch interaction, and composites composition and properties need to be aligned with robocasting requirements to achieve a notable improvement in the functionality of printed scaffolds intended for bone regeneration. This article presents a preliminary evaluation of hydroxyapatite-starch microcomposites. Thermal analysis of the starting powders was performed for predicting composites' behavior during heat-induced densification. Also, morphology, mechanical properties, and hydroxyapatite-starch interaction were evaluated for the jellified composites and the porous bodies obtained after conventional sintering, for different starch additions, and for ceramic particle size distributions. The results indicate that starch could be used for hydroxyapatite consolidation in limited quantities, whereas the composites shall be processed under controlled temperature. Due to a different mechanical behavior induced by particle size and geometry, a wide particle size distribution of hydroxyapatite powder is recommended for further robocasting ink development.

We investigate the dynamic behavior of perovskite solar cells by focusing on the relaxation time tau, which corresponds to the slow de-polarization process from an initial bias pre-poled state. The dynamic electrical model (DEM) is employed for simulating the J-V characteristics for different bias scan rates and pre-poling conditions. Depending on the sign of the initial polarization normal or inverted hysteresis may be induced. For fixed pre-poling conditions, the relaxation time, in relation to the bias scan rate, governs the magnitude of the dynamic hysteresis. In the limit of large tau the hysteretic effects are vanishing for the typical range of bias scan rates considered, while for very small tau the hysteresis is significant only when it is comparable with the measurement time interval.

Low-loss dielectrics with various microstructure and, consequently, different extrinsic losses were investigated by using terahertz time-domain spectroscopy. The losses have been extrapolated and compared with those measured at microwave frequencies. The results showed that extrinsic factors have a considerably contribution in microwave domain. The intrinsic limit of dielectric loss of low absorption polycrystalline materials have been estimated by terahertz spectroscopy.

Ceramic compounds of ferroelectric lead zirconate titanate (PZT) and ferroelectric barium titanate (BT) were prepared from nanopowders corresponding to the formula (1-x)PZT.xBT with x = 0; 0.1; 0.2 ... 0.9; 1. The samples were investigated by X-ray diffraction and electron microscopy and the piezoelectric properties were determined by means of an impedance analyzer. The sintering process is complex since samples densify at different temperatures depending on the composition resulting in different morphostructures. Accordingly, the piezoelectric properties show unusual behavior discussed in terms of different grain size and composition. A remarkably high enhancement of the relative dielectric constant, epsilon(r) coupled with maxima in both, piezoelectric charge constant d(33) and the piezoelectric voltage constant g(33), is observed for x = 0.4 while the values of the coupling coefficient kp are still high similar to those of pure PZT.

Investigations of barium strontium titanate (BST) layers deposited on MgO and Si substrates are presented. Since the Sr content determines the dielectric and optical properties of the BST layers at room temperature, accurate compositional analysis was performed by using Rutherford Backscattering technique at 3.041 Mev.

In the current work we present AlN/Si based Surface Acoustic Wave (SAW) devices exploited as high sensitivity temperature sensors. The experimental results show that the resonance frequency varies between 3.2 and 5.5 GHz, depending on the finger/Interdigit spacing width and metal thickness. The temperature measurements were performed between -250 degrees C and +150 degrees C in a cryostat. The sensitivities were investigated between 24-150 degrees C and the values were compared to the results obtained for similar GaN/Si based SAW structures.