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1.
A comprehensive study was performed in order to find the effect of different calcination and sintering conditions on the physical properties of calcium manganite ceramics in dependence of temperature T and partial pressure of oxygen p(O2). The eventual formation of oxygen vacancies during sintering was investigated and the results were confirmed by monitoring the release of oxygen using a ZrO2 oxygen sensor. The phase transition behavior was studied using thermogravimetric analysis (TGA) in a wide range of p(O2) ≈ 10?1 MPa down to 10-19 MPa at high temperatures accompanied by dilatometry- and XRD-measurements. Furthermore, the present study reveals for the first time a way for reducing and preventing crack formation that may occur during sintering. The present systematic research provides essential fundamental information before performing electrical measurements necessary in order to understand important factors about charge carriers and electrical transport mechanism.  相似文献   
2.
Defects, in particular vacancies, play a crucial role in substituted perovskite systems, influencing the structural features that underpin ferroelectricity. B-site vacancies introduce cation disorder in the perovskite lattice and are in fact one of the main driving forces for relaxor behaviour in barium titanate (BaTiO3, BT) based ferroelectrics. In this work, material systems are carefully selected to qualitatively study the change in B-site vacancy concentration for heterovalent substituted BT-based ferroelectric polycrystals. Raman spectroscopy was used to investigate those systems, and B-site vacancy specific Raman modes were identified unambiguously by comparison with charge-compensated BT, where B-site vacancies are absent. This study validates the hypothesis that vacancies induce Raman scattering because of symmetry breaking in the BT lattice, establishing this method as a vital tool to study substitutional defects in ceramic materials.  相似文献   
3.
Titanium-rich (Sr/Ti?=?0.995) strontium titanate (ST) ceramics, air-sintered in a temperature range of 1400–1625?°C, were reported to possess anomalies in the grain growth and analogous anomalies in the grain boundary (GB) conductivity activation energy. However, these two interface-related phenomena, occurring at GBs, could not be associated with each other using a simple “brick-layer” model. In this work we revise the topic and advocate that the deviation from the model comes from the oxygen vacancies localized at GBs of the rapidly-cooled ST ceramics. To verify this, we annealed the ceramics in oxygen and performed their systematic and comparative analysis using impedance spectroscopy. A levelling-off in the GB conductivity activation energy, which increases for ≤1.24?eV, and a four-fold decrease in the GB permittivity are observed after annealing. Thus, we confirm a key role of oxygen vacancies in relation between the grain growth and GB conductivity anomalies of as-sintered Ti-rich ST ceramics.  相似文献   
4.
Designing the core-shell structure and controlling defect engineering are desirable for improving the performance and stability of semiconductor photocatalysts. Herein, CdS nanorods covered with ultra-thin ZnIn2S4 nanosheets, named as CdS@ZnIn2S4-SV (CdS@ZIS-SV), was synthesized through the strategy of constructing core-shell structure and regulating vacancies. The core-shell structure can confine Cd2+ and S2? locally around CdS instead of rapidly diffusing into the solution, thereby inhibiting photo-corrosion. The abundant S vacancies can capture photogenerated electrons and promote the separation of electron-hole pairs, thereby preventing the oxidation of S2? by the holes. In addition, Z-Scheme heterojunction structure helps the effective separation of electron-hole pairs. Notably, the hydrogen production rate of CdS@ZIS-SV reached 18.06 mmol g?1 h?1, which was 16.9 and 19.6 times than pristine CdS (1.16 mmol g?1 h?1) and ZIS (0.92 mmol g?1 h?1), respectively. Photoelectric Characterization (PEC), Scanning Kelvin Probe (SKP), UV–vis diffuse reflectance spectra (UV–Vis DRS), Finite-Difference Time-Domain (FDTD) explain the electron transfer mechanism and the reason for the enhanced photocatalytic activity. This work has guiding significance for the preparation of photo-catalysts with high activity and inhibiting photo-corrosion by adjusting S vacancies.  相似文献   
5.
《Ceramics International》2022,48(22):33135-33142
This investigation highlights the significant influences of alkaline earth ion substitution for Gd on the structure, vacancy defects, magnetic and dielectric properties in GdMnO3 (GMO) ceramics synthesized using the solid-state reaction method. The structure measurements indicate that all of the Gd0.90A0.10MnO3 (A = Ca, Sr, Ba) samples show a single phase structure, and the introduction of A2+ ion induces structure distortion. A2+ ions substitution increases the Mn4+ ion concentration in GMOs, but has no significant effect on the oxygen vacancy concentration. All samples with irregular grain shapes have dense microstructures, and A2+ ion substitution inhibits grain growth. Positron annihilation experimental results indicate that A2+ ion substitution can increase the vacancy size and concentration, while the vacancy concentration increases first and then decreases with increasing A2+ ion radius. The evolution of the temperature- and magnetic field-dependent magnetization curves shows that A2+ ion substitution could obviously affect the magnetic state of GMOs, and improve the magnetic transition temperature and magnetization of Gd0.90A0.10MnO3. The dielectric measurements reveal that the A2+ ion substituted samples exhibit giant dielectric constant characteristics over a broad frequency range. It is found that the enhanced magnetization of Gd0.90A0.10MnO3 has a close relationship with the vacancy concentration, and the giant dielectric constant behaviors in Gd0.90A0.10MnO3 ceramics can be associated with the mixed-valent structure of Mn3+/Mn4+.  相似文献   
6.
Novel continuous freestanding β-SiC/SiOxCy/Cfree nanocomposite films, namely, β-SiC nano-crystals in amorphous SiOxCy and free C cluster matrix material, were fabricated by melt spinning the polycarbosilane (PCS) precursor. Effects of oxidation curing time and sintering temperatures on the photoluminescence (PL) properties of nanocomposite films were investigated. The PL spectra show two strong blue emissions at 416 nm and 435 nm, which are unchanged neither with oxygen content nor with β-SiC crystallite size. The PL intensity of the films is enhanced by increasing curing time when sintered at 1200 °C. However, a reversed trend is identified after the films were sintered at 1300 °C. Spectroscopy and microscopy studies indicate that the radiative recombination of carriers is ascribed to the oxygen mono- and di-vacancy from SiOxCy at the surfaces of β-SiC nano-crystals, whereas the photogeneration of carriers occurs in the β-SiC nano-crystals cores. The obtained results are expected to have important applications in advanced optoelectronic devices.  相似文献   
7.
Radiation damage may modify the segregation state and phase separation conditions in Fe-Cr alloys with compositions and temperatures of technological interest. We use Metropolis Monte Carlo simulations to study segregation effects at the best stable variant of the Σ = 5 (2 1 0) grain boundary, in the 5-10 at.% range of Cr composition. The role of irradiation induced vacancies in segregation is discussed. At a low Cr composition, an oscillatory Cr segregation profile is evidenced in the vicinity of the boundary. Under specific conditions, Cr ordering is observed close to the boundary. The correlations between such ordering and local stress is discussed. The binding energy of vacancies to specific sites of the grain boundary is found to be positive and the interplay between Cr segregation and the clustering of vacancies at the boundary is discussed.  相似文献   
8.
We developed a new method to realize enhancement-mode zinc oxide (ZnO) thin-film transistors (TFT) by metalorganic chemical vapor deposition (MOCVD). We used growth interruptions during MOCVD to encourage complete oxidation of deposited ZnO film, where diethylzinc and oxygen were used as sources. With this method, turn-off characteristics were significantly improved, and threshold voltage was shifted to positive voltages. ZnO TFTs grown at 450 °C showed 107 on/off ratio with 18 cm2/V s mobility, and + 5 V threshold voltage. Our data support that the surface layer is also important in determining ZnO TFT characteristics.  相似文献   
9.
利用气相沉积法,在低氧气氛下制备高缺陷的ZnO晶体。分别将样品在800℃、900℃、1000℃下通氧退火1h,对ZnO晶体做表面修饰。PL光谱实验和光催化降解亚甲基蓝实验表明ZnO晶体的氧空位、表面态和光催化活性间存在内在联系。1000℃下退火的样品表面缺陷程度多于800℃和900℃下退火样品,光催化活性也优于后两者。  相似文献   
10.
Band gaps and defect levels in functional oxides   总被引:2,自引:0,他引:2  
Most ab-initio calculations of the electronic structure use the local density approximation, which gives good structural data but severely underestimates the band gaps of semiconductors and insulators. This paper presents calculations of the band structures and oxygen vacancy levels of some important oxide semiconductors and insulators, using density functional methods which do give more accurate band gaps. The materials SnO2, Cu2O, SrCu2O2, CuAlO2, SrTiO3, HfO2, ZrO2, La2O3, ZrSiO4, and SiO2 are covered.  相似文献   
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