Surface morphology and dielectric properties of alkoxy-derived Sr2Ta2O7 and Sr2Ta, Nb2O7 thin films

Sr₂Ta₂O₇ and Sr₂(Ta, Nb)₂O₇ thin films were prepared from molecular structure-controled alkoxide solutions. The Sr₂Ta₂O₇ thin films initiated to crystallize at around 650 °C and showed random orientation. The surface topography developed was dependent on the solutions. In the thin films prepared from the ethanolic solution, grains formed at low temperatures. However, many small pores still remained and no grains were observed in the thin films prepared from the methoxyethanolic solution. In contrast, the 800 °C-annealed Sr₂(Ta, Nb)₂O₇ thin films showed the (0 k 0) orientation as in Sr₂Nb₂O₇ thin films. The dielectric constants and loss factors of the 750 °C-annealed Sr₂Ta₂O₇ and Sr₂(Ta_0.7Nb_0.3)₂O₇ thin films were around 90 and less than 0.05 at 100 kHz, respectively.     

Grain orientation and electrical properties of Sr2Nb2O7 ceramics

Sr₂Nb₂O₇ ceramics sintered at 1753 K and the hot-pressed compacts annealed at 1073 K possess low density because of the formation of voids by large expansion. The expansion is attributed to cleavage and due to the preferential grain growth along the <010> direction. An increase in dielectric constant at temperatures over 973 K and a parabolic decrease of resistivity with increasing temperature are observed. The constant variation corresponds well to the decrease in thermal strain and the grain orientation factor along the <0K0> direction.     

Abnormal grain growth and liquid-phase sintering in Sr0.6Ba0.4Nb2O6 (SBN40) ceramics

Ceramics of Sr0.6Ba0.4Nb2O6 (SBN40) were prepared by the conventional mixed oxide route. Sintering at temperatures 1260 °C led to rapid, non-uniform grain growth and a duplex microstructure. Presintering at 1250 °C followed by higher temperature sintering (1350–1450 °C) controlled grain growth. Rapid cooling from 1450 °C froze-in second phases at grain boundaries. Scanning electron microscopy and transmission electron microscopy showed that the resulting grain-boundary phases were Nb2O5-rich and BaO-deficient, having low liquid-formation temperatures. In contrast, SBN40 ceramics prepared with excess BaO and a deficiency of Nb2O5 showed no enhancement of grain growth at the highest temperature. Sintering behaviour and microstructural development provide evidence for the existence of a liquid phase which assists abnormal grain growth. The effect of presintering in controlling grain growth is discussed, and a mechanism for abnormal grain growth in Sr0.6Ba0.4Nb2O6 (SBN40) ceramics is proposed. © 1998 Chapman & Hall     

Densification and phase formation in seeded, reactively sintered Sr0.53Ba0.47Nb2O6 ceramics

Sr_0.53Ba_0.47Nb₂O₆ (SBN53) ceramics were reactively sintered from SrNb₂O₆ and BaNb₂O₆ powders. The formation temperature decreased from 1260°C for unseeded samples to 1130°C for the samples with 15.4 wt% seeds plus 0.85 mol% V₂O₅ at a heating rate of 4°C/min. For the V₂O₅-free samples, the activation energy was lowered from 554 ± 15 kJ/mol for unseeded samples to 241 ± 17 kJ/mol for the samples with 15.4 wt% seeds. In the V₂O₅-containing samples, densification and phase formation occurred simultaneously. In the V₂O₅-free samples, however, phase formation was completed before densification. In both cases, ceramics with 95% relative density were obtained. In all cases, SBN53 formed directly, rather than via a variety of intermediate SBN solid solutions. The microstructure evolution was also studied.     

模板晶粒生长技术制备SrBi2Nb2O9织构陶瓷

以熔盐法制备的片状SrBi2Nb2O9晶体为模板剂,采用模板晶粒生长技术和流延法制备了Sr-Bi2Nb2O9织构陶瓷,研究了模板含量对SrBi2Nb2O9织构陶瓷烧结行为、织构度、显微结构的影响。结果表明:模板含量为10wt%时,1200℃保温2h烧结可获得体积密度最大的SrBi2Nb2O9织构陶瓷,模板含量继续增加,体积密度降低;织构陶瓷的晶粒尺寸随模板含量的增加而逐渐增大,且晶粒取向性生长趋于明显,当模板含量为10wt%时,织构化SrBi2Nb2O9陶瓷的晶粒取向率f达到最大值,为0.81;织构化陶瓷的压电常数d33达到13pC/N,高于固相法制备的陶瓷的压电常数。     

Sintering of MgO: Densification and grain growth

The densification rate and grain growth rate of pure MgO powder compacts are measured between 1450 and 1650° C in air. Densification rate in a semilog plot appears to be linear up to about 94% of the theoretical, followed by marked nonlinearity. The time dependence of grain growth is 1/2 at the beginning and then decreases considerably with further sintering. Application of lattice diffusion model to the densification and grain growth data gives calculated diffusion coefficients in fair agreement with the directly measured diffusion coefficients for magnesium; they are also in fair agreement with the literature value of the diffusion coefficients calculated from various other kinetic processes in MgO.     

2D Perovskite Sr2Nb3O10 for High-Performance UV Photodetectors

2D perovskites, due to their unique properties and reduced dimension, are promising candidates for future optoelectronic devices. However, the development of stable and nontoxic 2D wide-bandgap perovskites remains a challenge. 2D all-inorganic perovskite Sr₂Nb₃O₁₀ (SNO) nanosheets with thicknesses down to 1.8 nm are synthesized by liquid exfoliation, and for the first time, UV photodetectors (PDs) based on individual few-layer SNO sheets are investigated. The SNO sheet-based PDs exhibit excellent UV detecting performance (narrowband responsivity = 1214 A W⁻¹, external quantum efficiency = 5.6 × 10⁵%, detectivity = 1.4 × 10¹⁴ Jones @270 nm, 1 V bias), and fast response speed (t_rise ≈ 0.4 ms, t_decay ≈ 40 ms), outperforming most reported individual 2D sheet-based UV PDs. Furthermore, the carrier transport properties of SNO and the performance of SNO-based phototransistors are successfully controlled by gate voltage. More intriguingly, the photodetecting performance and carrier transport properties of SNO sheets are dependent on their thickness. In addition, flexible and transparent PDs with high mechanical stability are easily fabricated based on SNO nanosheet film. This work sheds light on the development of high-performance optoelectronics based on low-dimensional wide-bandgap perovskites in the future.     

Densification and grain growth during pressureless sintering of TiO2 nanoceramics

Anatase titania nanopowders with mean particle sizes of 7, 15, 26 and 38 nm synthesized by sol–gel method were used to sinter bulk TiO₂ nanoceramics. The relative densities and average grain sizes of the TiO₂ nanoceramics were studied as a function of the compaction pressure on green sheet, sintering temperature, and mean particle size of the starting TiO₂ nanopowders. The relative density of the TiO₂ nanoceramics increases rapidly and average grain size increases slowly with increasing sintering temperature below 800 °C. Sintering at higher temperatures above 800 °C enhances the densification of the TiO₂ nanoceramics and leads to a increase of the grain size. Bulk TiO₂ nanoceramics with an average grain size of less than 60 nm and relative density over 95% was obtained by a phase-transformation-assisted pressureless sintering at a relatively low temperature (800 °C).     

Single-crystal growth of La2−2x Sr1+2x Mn2O7 under pressure

Large single crystals of La_2–2x Sr_1+2x Mn₂O₇ (x=0.3 to 0.525) have been prepared under controlled atmospheric conditions. The crystals were grown by the floating-zone technique in an image furnace under a mixed oxygen/argon atmosphere pressurized to 6–8×10⁵ Pa. Rectangular single crystals with sizes up to 50×9×4 mm³ have been obtained. The phase-purity, composition, and quality of the crystals were analyzed by X-ray diffraction and electron probe microanalysis. The magnetic behavior is found to be sensitive to the composition of the atmosphere during growth.     

Synthesis of Sr0.5Ba0.5Nb2O6 by urea method

A precipitate was formed when an aqueous solution of SrCl₂, BaCl₂, NbF₅ and urea in stoichiometric ratio (total metal cations to urea ratio is 5) is heated on a water bath.. This precipitate on decomposition at 750 °C yielded the nanocrystallites of ternary oxide, Sr_0.5Ba_0.5Nb₂O₆ (SBN) as confirmed by X-ray diffraction study (XRD). This is much lower temperature as compared to that prepared by traditional solid state method (1000 °C) as reported for the formation of SBN ceramics. Transmission electron microscope (TEM) investigations revealed that the average particle size is 100 nm for the calcined powders. The room temperature dielectric constant at 1 kHz is found to be 1050. The ferroelectric hysteresis loop parameters of these samples were also studied.     

Real structure of Cd2Nb2O7 pyrochlore single crystals

X-ray topography and diffractometry are used to study the crystal structure of cadmium pyroniobate Cd₂Nb₂O₇ at room temperature. Structural quality parameters were determined for crystals grown with different degrees of impurity doping. The nature of the crystal lattice damage is analyzed as a function of the type and concentration of impurities. Pis’ma Zh. Tekh. Fiz. 25, 26–33 (July 26, 1999)     

Formation and electrical characterization of Ti-modified Sr0.3Ba0.7Nb2O6 ceramic system

Titanium modified Sr_0.3Ba_0.7Nb₂O₆ ceramic system has been studied in a wide range of compositions. As the sintering temperature exceeds the 1250 °C, the substitution of niobium by titanium induces liquid phase formation, which enhances the densities of the samples with compositions in the monophasic range. X-ray diffraction analysis shows a linear titanium incorporation into the Sr_0.3Ba_0.7Nb_2−yTi_yO_6−y/2 system up to a solubility limit 0.07 < y_m < 0.1, which it is also confirmed by the ferroelectric–paraelectric temperature transition. Both, the diffuse character of the system and the dielectric constant at room temperature increase as the Ti content increase. The dielectric constant (ɛ_r = 420) of the sample with a titanium content of 0.07 are two times higher than the reported for the SBN (30/70) system.     

Microstructure and grain growth in Li2O-Al2O3-SiO2 glass ceramics

Microstructure and grain growth were studied in two glass ceramics of the Li₂O-Al₂O₃-SiO₂ system, one an experimental material of basic composition Li₂O-Al₂O₃-4SiO₂ and the second a commercial material of approximately 0.7Li₂O-Al₂O₃-6SiO₂ composition with small amounts of other oxides. There was evidence from transmission electron microscopy that the commercial material contained residual glass at grain-boundary triple points and glass layers at some but not all grain boundaries. No definite evidence was found for residual glass in the experimental material. The composition of secondphase regions in the commercial material was studied by STEM microanalysis. Al-rich regions of irregular morphology were found but there was no evidence that residual glass was SiO₂-rich, as has been previously suggested for this type of glass-ceramic. Grain growth showed a fairly similar time dependence in the two materials with growth more rapid in the commercial material at a given temperature. It is suggested that grain growth is controlled by precipitate particles rather than by residual glass phase.     

SrO-Nb2O5-TiO2系压敏陶瓷中Nb5+和Sr2+的研究

用显微观察分析、I-V特性及复阻抗频谱的测量,研究了不同Nb5+和Sr2+掺量的SrO-Nb2O5-TiO2系半导体压敏陶瓷材料的微观结构和相关电学性质;讨论了掺杂Nb5+和Sr2+的分布和作用;Nb5+固溶在TiO2中取代Ti4+并使晶粒成为半导化,同时也有助于晶粒生长;而Sr2+主要分布在晶粒边界处,对表面受主态及材料相关电学性能有重要影响;在大气气氛中热处理后的实验结果表明:处理温度在800°C以上时,能显著提高压敏电压,但只有适当的热处理温度,才能使非线性系数有所改善.     

Sr0.4Ba0.6Nb2O6物相形成过程的XRD分析

采用X射线衍射技术(Guinier-Hagg相机)分析了Sr0.4Ba0.6Nb2O6(SBN40)原始混合粉料经不同温度煅烧后的相组成,并利用PIRUM程序对不同反应温度下形成铌酸锶钡相的晶胞尺寸进行了计算.结果表明:在Sr0.4Ba0.6Nb2O6的形成过程中将出现中间相Na5Nb4O15、Sr5Nb4O15、SrNb2O6与BaNb2O6,而SrNb2O6与BaNb2O6最终反应生成铌酸锶钡.反应生成的铌酸锶钡的晶胞参数随反应温度的升高而变小.依据这些实验结果,文中提出了Sr0.4Ba0 6Nb2O6相的形成机制.     

Magnetic and Transport Properties of Bilayered Perovskite Sr3Ir2O7

We report magnetic and transport properties of polycrystalline samples of bilayered perovskite Sr₃Ir₂O₇, which is synthesized by using high pressure technique. Magnetic susceptibility shows weak ferromagnetism below 290K with small size of remanent moment (10^–3B/Ir). Resistivity shows insulating behavior below 290K. We discuss the magnetism and conductivity in Sr₃Ir₂O₇ comparing with those of Sr₃Ru₂O₇, which has a quite similar crystal structure to Sr₃Ir₂O₇, from the view point of rotation angle of MO₆ (M=Ir, Ru) octahedra.