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Lattice Defects (lattice + defect)
Selected AbstractsElectrical Conductivity and Lattice Defects in Nanocrystalline Cerium Oxide Thin FilmsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2001Toshio Suzuki The results of the electrical conductivity and Raman scattering measurements of CeO2 thin films obtained by a polymeric precursor spin-coating technique are presented. The electrical conductivity has been studied as a function of temperature and oxygen activity and correlated with the grain size. When compared with microcrystalline samples, nanocrystalline materials show enhanced electronic conductivity. The transition from extrinsic to intrinsic type of conductivity has been observed as the grain size decreases to <100 nm, which appears to be related to a decrease in the enthalpy of oxygen vacancy formation in CeO2. Raman spectroscopy has been used to analyze the crystalline quality as a function of grain size. A direct comparison has been made between the defect concentration calculated from coherence length and nonstoichiometry determined from electrical measurements. [source] On the Design of High-Efficiency Thermoelectric Clathrates through a Systematic Cross-Substitution of Framework ElementsADVANCED FUNCTIONAL MATERIALS, Issue 5 2010Xun Shi Abstract Type I clathrates have recently been identified as prospective thermoelectric materials for power generation purposes due to their very low lattice thermal conductivity values. The maximum thermoelectric figure of merit of almost all type I clathrates is, however, less than 1 and occurs at, or above, 1000,K, making them unfavorable especially for intermediate temperature applications. In this report, the Zintl,Klemm rule is demonstrated to be valid for Ni, Cu, and Zn transition metal substitution in the framework of type I clathrates and offers many degrees of freedom for material modification, design, and optimization. The cross-substitution of framework elements introduces ionized impurities and lattice defects into these materials, which optimize the scattering of charge carriers by the substitution-induced ionized impurities and the scattering of heat-carrying lattice phonons by point defects, respectively, leading to an enhanced power factor, reduced lattice thermal conductivity, and therefore improved thermoelectric figure of merit. Most importantly, the bandgap of these materials can be tuned between 0.1 and 0.5,eV by adjusting the cross-substitution ratio of framework elements, making it possible to design clathrates with excellent thermoelectric properties between 500 and 1000,K. [source] Plane-wave X-ray topography and its application at SPring-8JOURNAL OF SYNCHROTRON RADIATION, Issue 3 2002Satoshi Iida Plane-wave X-ray topography experiments were carried out at a 200,m-long beamline, BL20B2, at SPring-8. Relatively high-energy X-rays of 30,keV with an angular divergence of about 0.01,arcsec were produced by using only one collimator crystal. FZ-Si and CZ-Si wafers were characterized in transmission geometry (Laue case). Clear oscillatory profiles in rocking curves of the FZ-Si crystal were observed. Plane-wave topographic images of dislocations, growth striations and grown-in microdefects in the CZ-Si crystals were obtained. The dependence of the topographic images of the lattice defects on the sample,photoplate distance was also studied. [source] Microstructures of metal grains in ordinary chondrites: Implications for their thermal historiesMETEORITICS & PLANETARY SCIENCE, Issue 3 2000Hugues LEROUX Three ordinary chondrites, Saint Séverin (LL6), Agen (H5), and Tsarev (L6) were selected because they display contrasting microstructures, which reflects different thermal histories. In Saint Séverin, the microstructure of the Ni-rich metal grains is due to slow cooling. It consists of a two-phase assemblage with a honeycomb structure resulting from spinodal decomposition similar to the cloudy zone of iron meteorites. Microanalyses show that the Ni-rich phase is tetrataenite (Ni = 47 wt%) and the Ni-poor phase, with a composition of ,25% Ni, is either martensite or taenite, these two occurring adjacent to each other. The observation that the Ni-poor phase is partly fcc resolves the disagreement between previous transmission electron microscopy (TEM) and Mössbauer studies on iron meteorites and ordinary chondrite metal. The Ni content of the honeycomb phase is much higher than in mesosiderites, confirming that mesosiderites cooled much more slowly. The high-Ni tetrataenite rim in contact with the cloudy zone displays high-Ni compositional variability on a very fine scale, which suggests that the corresponding area was destabilized and partially decomposed at low temperature. Both Agen and Tsarev display evidence of reheating and subsequent fast cooling obviously related to shock events. Their metallic particles mostly consist of martensite, the microstructure of which depends on local Ni content. Microstructures are controlled by both the temperature at which martensite forms and that at which it possibly decomposes. In high-Ni zones (>15 wt%), martensitic transformation started at low temperature (<300 °C). Because no further recovery occurred, these zones contain a high density of lattice defects. In low-Ni zones (<15 wt%), martensite grains formed at higher temperature and their lattice defects recovered. These martensite grains present a lath texture with numerous tiny precipitates of Ni-rich taenite (Ni = 50 wt%) at lath boundaries. Nickel composition profiles across precipitate-matrix interfaces show that the growth of these precipitates was controlled by preferential diffusion of Ni along lattice defects. The cooling rates deduced from Ni concentration profiles and precipitate sizes are within the range 1,10 °C/year for Tsarev and 10,100 °C/year for Agen. [source] Thermo- and galvanomagnetic measurements of semiconductors at ultrahigh pressurePHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2003Vladimir V. Shchennikov Abstract The thermoelectric power, magnetoresistance, and thermomagnetic effects were measured for Te and Se micro-samples in the vicinity of semiconductor,metal phase transitions at high pressure. From longitudinal and transverse Nernst-Ettingshausen effects the scattering parameter r of holes was estimated and a decrease of the effective mass of holes was found during the closing of semiconductor gap. After the high pressure treatment an inversion of the sign of parameter r corresponding to a change of scattering mechanism was observed for the Te sample due to an increase of the density of lattice defects. [source] Role of excitons in the excitation of deep-level emission in ZnO crystalsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2010V. I. Kushnirenko Abstract In undoped ZnO crystals, photoluminescence excitation (PLE) spectra of deep-level green and orange PL bands as well as photocurrent (PC) spectra were measured at 77 and 290 K. Some PLE and PC peaks whose energy positions coincided with that of free exciton and its excited states were observed, the intensity of PL and PC excited in these peaks being higher than under excitation by band-to-band light. At the same time, a sharp minimum was found in PLE and PC spectra at the position of donor-bound exciton. Obtained results were accounted for by the interaction of excitons with lattice defects. A number of longer wavelength PLE and PC peaks whose energy positions coincided in fact with that of the first, second and third phonon replicas of free exciton were also observed. The origin of these peaks is discussed. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Positron annihilation studies of defects in Si1-xGex/SOI heterostructuresPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 11 2009A. Calloni Abstract The strain of a SiGe alloy epitaxially grown on Si can be released by heating at 750°C. However the strain relaxation is accompanied by the generation of threading dislocations and associated point defects. The attempted remedy is to let SiGe grow on a very thin Si substrate on top of a SOI heterostructure, with the purpose of concentrating the defects at the Si-SiO2 interface of the SOI substrate. A slow positron beam was used to investigate the effectiveness of the remedy. The positron diffusion length in SiGe decreases after the thermal treatment. This result shows the generation of lattice defects still occurs in the SiGe layer, in spite of the deposition on SOI. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Ionic and electronic processes in non-linear optical crystalsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2005Igor N. Ogorodnikov Abstract The paper presents the results of a study of the formation and decay of lattice defects in nonlinear optical crystals of NH4H2PO4 (ADP), KH2PO4 (KDP), Li2B4O7 (LTB) and LiB3O5 (LBO) with a sublattice of mobile hydrogen (ADP, KDP) and lithium (LTB, LBO) cations. By means of the luminescent and absorption optical spectroscopy with (the) a nanosecond time resolution under excitation with an electron beam, it was revealed that the optical absorption of these crystals in the visible and UV spectral ranges is produced by optical hole-transitions from the local defect level to the valence band states. The valence band density of the states determines the optical absorption spectral profile, and the relaxation kinetics is rated by the interdefect radiationless tunnel recombination between the trapped hole center and the H0 and Li0 electron trapped centers. At 290 K, the H0 and Li0 centers are subject to thermally stimulated migration. All manifestations of a radiative recombination observed in these crystals are accounted for by the involvement of additional electronic and hole centers of a different nature in the recombination process. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Effects of synthesis conditions on crystal morphological structures and thermal degradation behavior of hydrotalcites and flame retardant and mechanical properties of EVA/hydrotalcite blendsPOLYMER COMPOSITES, Issue 2 2007Longchao Du The effects of synthesis methods and reaction conditions on the crystal morphological structures and thermal degradation behavior of hydrotalcites have been studied by X-ray diffraction (XRD), transmission electron microscopy (TEM), particle size analysis (PSA), and differential thermal analysis (DTA). The flame retardant and mechanical properties of ethylene,vinyl acetate (EVA) blends with the corresponding hydrotalcites have been estimated by limiting oxygen index (LOI), UL-94, and mechanical measurements. The results from the XRD, TEM, and PSA demonstrate that the hydrotalcites synthesized by ultrasound method have larger crystal sizes and particle size distribution than those by mechanical stirring method. Higher reaction temperature, longer dripping time, and lower solution concentration can increase the crystal and particle sizes of ultrasound-synthesized hydrotalcites, whereas the longer ultrasound aging time can increase the crystal sizes and decrease the particle sizes of hydrotalcites because of the smashing conglomeration. The DTA data give a positive evidence that the hydrotalcite samples prepared by mechanical stirring method with longer alkaline dripping time have higher thermal degradation temperature than those by ultrasound method, since the ultrasound-synthesized hydrotalcites have more lattice defects than stirring-prepared hydrotalcites. The data from LOI, UL-94, and mechanical tests show that the ultrasonic-synthesized hydrotalcites have better flame retardant properties, whereas the stirring-synthesized hydrotalcites have better tensile strength in the EVA/hydrotalcite blends. POLYM. COMPOS., 28:131,138, 2007. © 2007 Society of Plastics Engineers [source] | |||||||||||||