Temperature-dependent Redistribution Research Articles

Morphological Characteristics of W/Cu Composite Nanoparticles with Complex Phase Structure Synthesized via Reactive Radio Frequency (RF) Thermal Plasma

The W/Cu binary system is characterized by its mutual insolubility and excellent wettability, making W/Cu composite materials ideal for managing thermal and electrical properties in electronic components. To optimize material properties, control over the microstructure is crucial, and nanocomposites with uniform dispersion offer significant advantages. In this study, W/Cu composite nanoparticles were synthesized by feeding a blended feedstock of tungsten trioxide (WO3) micro-powder and cupric oxide (CuO) micro-powder into a reactive radio frequency (RF) argon–hydrogen thermal plasma system. Cu-coated W nanocomposite particles were obtained through the vaporization, reduction, and condensation processes. The resulting nanocomposite particles were composed of body-centered cubic (BCC) α-W, A15 β-W, and face-centered cubic (FCC) Cu phases, with a chemical composition closely matching theoretical calculations. The phase evolution and morphological changes of the synthesized particles were analyzed as a function of heat treatment temperatures up to 1000 °C in a reducing atmosphere. Up to 600 °C, the phase composition and morphology remained stable. At 800 °C, localized diffusion and coalescence of Cu led to the formation of particulate Cu, and a significant phase transformation from metastable β-W to α-W was observed. Additionally, extensive Cu segregation due to long-range diffusion resulted in distinct Cu-rich and Cu-depleted regions. In these regions, notable sintering of W particles and the complete disappearance of β-W occurred. The results showed that the temperature-dependent redistribution of Cu plays a crucial role in the phase transformation of W and the morphology of W/Cu composite particles.

Temperature dependence of the Auger recombination coefficient in InGaN/GaN multiple-quantum-well light-emitting diodes.

This study investigated the temperature dependence of the Auger recombination coefficient (C) in an InGaN/GaN blue multiple-quantum-well (MQW) light-emitting diode structure at temperatures between 20 and 100°C. The temperature dependence of C was determined by fitting the measured external quantum efficiency (EQE) data using an analytical model or numerical simulation. In the analytical model, the carrier density in InGaN MQWs was assumed to be constant and independent of temperature. In contrast, the inhomogeneous carrier distribution in MQWs and its temperature-dependent redistribution were included in the numerical simulation. When the analytical model was employed to fit the EQE curve, C decreased with increasing temperature. On the other hand, when the numerical simulation was employed, C increased steadily by ∼31% as the temperature was increased from 20 to 100°C. We found that the temperature-dependent carrier distribution is important to consider when determining the temperature dependence of the Auger recombination coefficient in InGaN MQW structures.

Anomalous temperature-dependent photoluminescence peak energy in InAlN alloys

InAlN has been studied by means of temperature-dependent time-integrated photoluminescence and time-resolved photoluminescence. The variation of PL peak energy did not follow the behavior predicted by Varshni formula, and a faster redshift with increasing temperature was observed. We used a model that took account of the thermal activation and thermal transfer of localized excitons to describe and explain the observed behavior. A good fitting to the experiment result is obtained. We believe the anomalous temperature dependence of PL peak energy shift can be attributed to the temperature-dependent redistribution of localized excitons induced by thermal activation and thermal transfer in the strongly localized states. V-shaped defects are thought to be a major factor causing the strong localized states in our In0.153Al0.847N sample.

Coexistence of the Pseudogap and the Superconducting Gap Revealed by thec-Axis Optical Study of YBa2(Cu1-xZnx)3O7-δ

To address the issue of the pseudogap in the superconducting state of the high temperature superconducting cuprates, we studied the temperature dependent redistribution of the spectral weight in the \textit{c}-axis polarized optical spectra for underdoped YBa$_{2}$(Cu$_{1-x}$Zn$_{x}$)$_{3}$O$_{7-\delta}$ single crystals with $x=0,0.007,$ 0.012, and 0.04. It was found that for Zn substituted samples, the spectral weight transfer to the high energy region, which characterizes the pseudogap state, continues even below the superconducting transition temperature (\textit{T}$_{c}$), indicating the coexistence of the pseudogap and the superconducting gap. Moreover, the observation of the pseudogap in the Zn-doped non-superconducting sample gives evidence that the pseudogap is not a precursor of superconductivity.

Ion beam synthesis of Mn/Sb clusters in silicon

In order to investigate the formation of Mn/Sb clusters embedded in crystalline silicon, sequential ion implantation with fluences of 1 × 1016 at cm−2 and 2 × 1016 at cm−2, respectively, was used to incorporate Mn and Sb ions at high concentrations into Si(0 0 1). Based on investigations with Rutherford backscattering spectroscopy (RBS) and corresponding channelling measurements (RBS/c), we report on a temperature dependent redistribution of the implanted species during the rapid thermal annealing process governed by the radiation-induced defects. Additionally performed cross-sectional TEM analyses, including EDX measurements, clearly show the presence of hexagonal shaped elementary Sb precipitates as well as compound clusters consisting of Mn and Sb, which are aligned to the crystal structure of the host silicon. In electron magnetic resonance measurements many samples exhibit broad resonance bands persisting up to approximately 60 K. For out-of-plane rotations, the bands show a weak angular dependence of the resonance field but a strong angular dependence of the intensity. Zero-field-cooled and field-cooled magnetization curves were measured on selected samples with a SQUID magnetometer between 10 and 400 K at different applied fields. The curves show a weak magnetic signal generated by different magnetic phases while at least one can be ascribed to superparamagnetic nanoparticles of MnSb.

Correlation of defect structure and ionic conductivity in δ-phase solid solutions in the Bi 3NbO 7–Bi 3YO 6 system

Defect structure and conductivity behaviour are discussed in the solid solution Bi 3Nb 1− x Y x O 7− x (0.0 ≤ x ≤ 1.0). Investigations were carried out using a combination of ac impedance spectroscopy and powder X-ray and neutron diffraction. Low temperature conductivity and activation energy both increase as a function of x. The former is attributed to an increase in oxide ion vacancy concentration, whereas the latter is due a redistribution of oxide ion vacancies as determined by neutron diffraction measurements. The defect structures at room temperature and 800 °C are presented. Curvature in Arrhenius plots of conductivity throughout the composition range is associated with a temperature dependent redistribution of oxide ions.

In-Plane Spectral Weight Shift of Charge Carriers in YBa 2 Cu 3 O 6.9

The temperature-dependent redistribution of the spectral weight of the CuO2 plane-derived conduction band of the YBa2Cu3O6.9 high-temperature superconductor (superconducting transition temperature = 92.7 kelvin) was studied with wide-band (0.01- to 5.6-electron volt) spectroscopic ellipsometry. A superconductivity-induced transfer of the spectral weight involving a high-energy scale in excess of 1 electron volt was observed. Correspondingly, the charge carrier spectral weight was shown to decrease in the superconducting state. The ellipsometric data also provide detailed information about the evolution of the optical self-energy in the normal and superconducting states.

Thickness dependent phase separation in La0.7Ca0.3MnO3 films

The complex dielectric function of La0.7Ca0.3MnO3 films with various thicknesses and on different substrates was determined by the analysis of measured ellipsometric parameters. The temperature-dependent redistribution of low-energy spectral weight shows that the metallicity is developed gradually below an onset temperature that is found to depend strongly on global and local strain properties, not on film thickness alone. Importantly, the 2 nm film shows no metallic conduction but an optical charge-carrier response below 240 K. This suggests that ultrathin films are characterized by phase separation and a percolation-type transition.

Structural and physical properties of La2/3Ca1/3MnO3 prepared via a modified sol-gel method

High purity powder of manganese perovskite, La2/3Ca1/3MnO3, is prepared using a modified sol-gel synthesis based on the dissolution and homogenisation of metal salts in ethanol-acetic acid mixture without using any complexing aid (e.g., polyol or polyhydroxy acid, etc.), which is essentially used in the polymeric precursor routes. This modification minimises the organic contamination in the resulting ceramic that is formed as single perovskite at 650°C, after short calcination time periods. The formation of a monophasic material and absence of second phases or phase segregation was confirmed by powder X-ray diffraction, energy dispersive X-ray and electron microscopy of the ceramic calcined at higher temperatures (800–1400°C). The calcined samples are nanocrystalline up to 1000°C (average particle size, ∼44 nm) however, significant particle growth is observed at higher temperatures with micron-sized grains present in the sample sintered at 1400°C. The sample exhibits the characteristic colossal magnetoresistance behaviour. Owing to the high chemical and structural purity of the obtained ceramic, the intrinsic bulk features like metal-insulator transition and ferromagnet-antiferromagnet behaviour of the polycrystalline sample are comparable to those observed in the single crystal La2/3Ca1/3MnO3 specimen, used as a reference. The single-crystal-like properties are also corroborated by the observation of a sharp metallic fermi edge in the UPS measurements. The variable temperature photoemission spectra reveal a temperature dependent redistribution of spectral weight close to the fermi level corroborating the temperature dependent resistance and magnetoresistance of the sample. The material shows a homogeneous grain size and a high sinterability as shown by TEM and SEM studies, respectively. XPS study indicates a charge carrier hopping between Mn3+(3d4) and Mn4+(3d3) sites.

Aminopeptidase A: A nephritogenic target antigen of nephrotoxic serum

We investigated potential targets of antibody-mediated glomerular injury induced with a noncomplement binding fraction of sheep anti-rat nephrotoxic serum (NTS). This model is characterized by severe complement- and leukocyte-independent proteinuria within 24 hours of NTS injection into rats. NTS-reactive glomerular cell and matrix proteins were identified by immunoprecipitation, Western blot analysis, protein sequencing, cDNA library screening, and enzyme-linked immunosorbent assay. Proteinuria was measured in rats injected with NTS from which reactivity against type IV collagen had been removed by immunoadsorption, and antibodies were eluted from the glomeruli of proteinuric rats that had been injected with unabsorbed NTS. Having identified aminopeptidase A (APA) as a major target of NTS, we studied the effect of NTS and anti-APA on mouse glomerular epithelial cells in culture. NTS identified several podocyte and matrix proteins; however, APA was the only cell surface protein reactive with antibodies eluted from the glomeruli of rats injected with NTS. Although the eluate also contained reactivity to the noncollagenous domains of alpha1 and alpha3 chains of type IV collagen, immunodepletion of these antibodies did not diminish the ability of NTS to cause proteinuria. We also documented the surface expression of APA on mouse glomerular epithelial cells in culture, and found that NTS and specific anti-APA antibodies induce a time- and temperature-dependent redistribution of the antigen. APA, a type II integral membrane metallopeptidase, is a major target of NTS in vivo and is known to be present on the surface of podocytes. NTS-induced proteinuria is independent of reactivity to known nephritogenic matrix proteins. These findings, in combination with previous studies showing that monoclonal anti-APA antibodies induce severe proteinuria in mice, suggest that anti-APA antibodies are responsible for complement-independent proteinuria in this model.

Cathodoluminescence Deep Level Spectroscopy of Etched and In-Situ Annealed 6H-SiC

ABSTRACTThrough variable-energy electron beam excitation (0.5 keV-2.0 keV), we observe depth dependent differences in the cathodoluminescence spectra of 6H-SiC (0001) Si-terminated surfaces. The etched SiC exhibits three deep level defects in the near-surface region, including a defect peak observed at 0.92 eV, known to be associated with vanadium. In-situ annealing produces a dramatic relative decrease in the luminescence from vanadium impurities near the surface after annealing to 500 °C, and then the subsequent re-emergence of vanadium in the nearsurface regime after annealing to 810 °C. This temperature-dependent redistribution suggests either diffusion or segregation of vanadium from the bulk up toward the near-surface region.

Temperature-dependent photoemission spectral weight in La0.6Sr0.4MnO3.

We report temperature-dependent redistribution of spectral weight in photoemission spectra of ${\mathrm{La}}_{0.6}$${\mathrm{Sr}}_{0.4}$${\mathrm{MnO}}_{3}$ over a wide energy range of several eV. We attribute this to a change in the electron correlation strength induced by the changing degree of ferromagnetic order with temperature. We discuss the relationship between the temperature-dependent spectral weight close to the Fermi level and the unusual transport properties. \textcopyright{} 1996 The American Physical Society.

Dynamics of small heat shock protein distribution within the chloroplasts of higher plants.

Accumulation of the small heat shock proteins (sHSPs) in response to high temperature stress is thought to contribute to the development of thermotolerance in eukaryotic organisms, but the mechanism of action is unknown. We are investigating the chloroplast-localized sHSP, HSP21, with the goal of defining its contribution to the acquisition of thermotolerance in plants. Following an initial heat stress and period of recovery, HSP21 is localized primarily in the soluble fraction of the chloroplast. During an additional stress, HSP21 undergoes a temperature-dependent redistribution from the soluble to the insoluble chloroplast fraction in both isolated organelles and intact plants. The change in HSP21 partitioning is accompanied by depletion of the 10-11 S HSP21-containing complexes from the soluble chloroplast fraction. HSP21 in the insoluble fraction cannot be solubilized by nonionic detergent under conditions that release essentially all the pigments and proteins from the thylakoid membranes, indicating that HSP21 in its insoluble state is not dependent for its insolubility on attachment to an intact membrane. The temperature-dependent redistribution of HSP21 is affected by light intensity but occurs in both leaf and root plastids, suggesting that the function of this activity is not strictly related to the presence of the photosynthetic apparatus. Our study indicates that the chloroplast sHSP has dynamic properties similar to those of cytoplasmic sHSPs from plants and other organisms and suggests that the ability to partition between a soluble and an insoluble state reflects a functionally important property of all sHSPs.

Internalization and degradation of monoclonal antibody chCE7 by human neuroblastoma cells

Internalization and cellular degradation of a chimeric monoclonal anti-neuroblastoma antibody (MAb chCE7) is described. Immunofluorescence localization showed temperature-dependent redistribution of MAb chCE7 from the cell surface at 0 degrees C to vesicular structures after heating to 37 degrees C. SKN-AS cells which were pulse-labelled at 0 degrees C with radioiodinated chCE7 released 50% of the initial cell-bound radioactivity into the medium after 20 hr at 37 degrees C. Low-molecular-weight radioactivity in the medium was identified as iodotyrosine and the major intracellular degradation product was found to be an antibody fragment of 43 kDa. Degradation was blocked by chloroquine, an inhibitor of lysosomal function. MAb chCE7 binds to a carbohydrate epitope, as treatment with tunicamycin abolishes cell binding. In adherent cells in culture, inhibition of protein synthesis by cycloheximide affected cell-surface binding sites for chCE7 in a biphasic manner, with an initial increase followed by long-term decrease. Expression of binding sites was also found to be inhibited by sodium azide, by the lysosomotropic drug chloroquine and by Brefeldin A, a drug which prevents export of newly synthetized proteins to the cell surface. When cells were treated with high doses of chCE7 up to 24 hr and cell-surface binding sites were then measured after an acidic buffer wash, no loss of surface binding sites was found, indicating that pretreatment with MAb chCE7 does not induce down-regulation of its binding sites.

Anomalies in the temperature behaviour of absorption in the optical 1.5–3.0 eV range for the high-T c superconductor YBa 2Cu 3O 7-δ

Spectral and temperature measurements of the absorption coefficient, K ( E, T), have been performed within a dielectric transparency window (from 1.5 to 3 eV) which persists in the metal phase. An unusual temperature-dependent increase in absorption was observed above T c for the metal phase. In combination with the measured absorption spectra and their concentration dependences it indicates an important role for the metal-type intraband transitions which is enhanced with oxygen concentration. It is concluded that there occurs a noticeable temperature-dependent redistribution of electrons between the CuO 2 plane and the reservoir of charges. The K( T) curve has a peculiarity at T c and follows the superconducting gap width below T c. All the peculiar features of K( T) are conditioned by the narrowness of the conduction hole band and provide information on the electron structure near the Fermi level.

Effect of oxygen stoichiometry on electronic Raman scattering in YBa 2Cu 3O x (7.0 > x > 6.8)

The polarized electronic Raman response of YBaCuO (Y-123) single crystals and polycrystals with oxygen stoichiometries of 7.0, 6.9, and 6.8 and its temperature dependent redistribution in the normal and superconducting states has been investigated for frequency shifts between 70cm−1 and 2000cm−1. Polarized spectra of polycrystalline samples were found to yield a predominantly zz polarized (i.e.∥ c-axis) response. In the normal state temperature dependencies differ displaying Bose-Einstcin and quasi-Fermi-liquid behaviors for the zz dominated polycrystal and single crystal xx polarizations respectively but are found to be essentially independent of oxygen stoichiometry. In the superconducting (SC) state partial gap formation by condensation of charge density fluctuations below 200cm−1 and pair breaking peak enhancement of the electronic Raman response between 200cm−1 and 330cmt−1 appears independent of oxygen stoichiometry in the zz dominated polycrystals. Comparison to the known xx SC response yields similar gap values but smaller pairbreaking peak widths indicating greater quasiparticle lifetimes or smaller momentum transfer. Enhanced subgap zz polarized electronic excitations observed at 10K only in the oxygen depleted sample (O = 6.8) and similar xx excitations in Y-123 and Y-124 possibly suggest that they originate from the Cu(1)-O(1) chain bands.

Temperature variations of the energy spectrum of high temperature superconductors near the fermi level seen by optical absorption

Abstract The temperature dependence of optical absorption spectra in the visible range has been measured between 60 and 180 K on superconducting, single-crystalline films of YBa2Cu3O7−δ and Bi2Sr2CaCu2O8. The analysis of the experimental results was performed under the assumption of a high ratio between the total valence band width and the width of the narrow hole band. The linear increase of the absorption above Tc is attributed to a variation of the number density of holes, which is caused by a temperature dependent redistribution of electrons between the conducting CuO2 plane and a second electron subsystem that acts as a charge carrier reservoir. The occurrence of superconductivity at Tc reflected in a peculiar behaviour of the optical absorption at T ≤ Tc. It is shown that optical spectroscopy in the visible range provides valuable information about temperature dependent changes of the electronic structure of HTSC near the Fermi level.

Temperature dependent redistribution of organic nitrogen during ‘dry' storage of tulip bulbs cv. Apeldoorn

Tulip bulbs cv. Apeldoorn are dry stored at 5°C for 12 weeks to ensure subsequent optimal flowering when planted in the greenhouse at higher temperatures of 17–20°C. Both temperature and duration of the cold treatment determine the subsequent rate of the shoot elongation, the time until anthesis and the flower size, pigmentation and water content. In search for cold‐specific physiological changes, possibly related to the development of the potential of proper flowering (flowering preparation), we studied the redistribution of organic nitrogen in both cooled (5°C) and non‐cooled (17°C) bulbs.During 12 weeks of dry storage, the total protein‐ and free amino acid‐nitrogen content decreased in the scales, whereas the opposite was found in the basal plate (with root primordia) and the shoot. In the shoot, this occurred significantly more at 17°C than at 5°C. At the same time, there was a tissue‐specific change in the free amino acid composition in both cooled and non‐cooled bulbs. Changes specific for the 5°C treatment were only found for the alanine content, in both the basal plate (with root primordia) and the shoot, and for the proline, asparagine, threonine, glycine and isoleucine content, in the shoot only. These changes are, for the greater part, completed within the first 6–8 weeks of dry storage. Bulbs stored for such a short period of time at 5°C still show flowering disorders. Thus, flowering preparation is only partly accompanied by changes in free amino acid contents.

Temperature dependent redistribution of organic nitrogen during 'dry' storage of tulip bulbs cv. Apeldoorn

Temperature dependent redistribution of organic nitrogen during 'dry' storage of tulip bulbs cv. Apeldoorn

Temperature dependent redistribution among the multiple forms of rat Y b-glutathione-S-transferase

Anionic (Yb) rat liver glutathione-S-transferases are susceptible to temperature or pH dependent transitions to more basic forms of this class of proteins. At elevated temperatures (25-30 degrees) or at pH values above 9.0 the protein is rapidly and irreversibly converted to forms that are no longer retained by anion exchange resins and display basic components in chromatofocusing systems, because bound glutathione is removed at the higher temperatures or pH. Sharp increases in enzymatic activity with 1,2-dichloro-4-nitrobenzene as a substrate, accompany the temperature induced changes. Microheterogeneity patterns for this protein are contingent upon these interconversions, and the results explain apparent variations in relative amounts of the multiple forms under different conditions in terms of glutathione binding.