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排序方式: 共有948条查询结果,搜索用时 10 毫秒
921.
Jiten Ghosh 《Phase Transitions》2013,86(8):694-707
Microstructural characterization of ball milled perovskite BaTiO3 powders has been done by the modeling of X-ray diffraction profiles. The study reveals that on size reduction, BaTiO3 powders undergo a continuous, displacive, and diffusionless dynamic phase transitions involving tetragonal (T), monoclinic (M), and orthorhombic (O) symmetry via the second-order type [T?→?(T?+?M)?→?(M?+?O)?→?O] when stimulated by a high-power pulse of pressure in a planetary mill. The order parameter, a phenomenological quantity to describe the general behavior of a system going through phase transitions has been estimated using spontaneous strain calculated from lattice parameters or physical distortions derived from atomic coordinates or both. At room temperature, BaTiO3 nanoparticle achieved an orthorhombic phase when a critical size (<15?nm) has been reached at later stage of milling (≥70?h). Raman's study reveals similar structural phase transitions sequence on size reduction and TEM study reveals the corresponding particle diameter. 相似文献
922.
Păcurariu P. Liťă D. Lazău R. I. Kovacs G. Lazău I. 《Journal of Thermal Analysis and Calorimetry》2003,72(3):823-830
The paper presents the kinetic study of the crystallization processes which take place in basalt glasses containing variable
amount of nucleation agent (CaF2, 310%). The activation energies have been calculated using Kissinger's equation and verified with the Ozawa-Flynn-Wall equation.
In this order, the DTA curves have been registered with different heating rates, between 4 and 20 degrees/min. A correlation
equation between the activation energy and the amount of nucleation agent (% of CaF2) was established. By X-ray diffraction it was proved that the crystalline phase formed in the crystallization process represents
a pyroxenic solid solution, Ca(Mg,Fe)SiO3.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
923.
Tingting Jing Yuepeng Xin Liang Zhang Xing Sun Chunwang He Jiachen Wang Fei Qin 《Chemical record (New York, N.Y.)》2023,23(4):e202300022
Thermal protection is one of the crucial issues for the advanced propulsion systems of Reusable Launch Vehicles. New service requirements for materials, such as high strength, low density, low thermal expansion, high thermal stability, etc., are raised for the thermal structure with the increasing demand of flight Mach numbers and thrust-to-weight ratio. Carbon fiber-reinforced ceramic composites, which generally meet the aforementioned requirements, show great potential for various applications and they have been widely applied in the thermal protection for hypersonic vehicles. This paper gives a comprehensive and systematic review of current research status for carbon fiber-reinforced ceramic composites applied in the thermal structure of advanced propulsion systems. Three aspects are presented and discussed: the ceramic composites fabrication and the property characterization, the thermal performance of composite thermal structure used in practical engines, and the numerical methods for predicting mechanical and thermal properties of composites as well as the physicochemical phenomenon in the cooling channels. Firstly, the main manufacturing processes for the carbon-reinforced ceramic composites are presented and the corresponding advantages and disadvantages are analyzed. The high-temperature oxidation and ablation behaviors of composites are demonstrated and the improvement of oxidation and ablation resistance by introducing the ultra-high-temperature ceramics into C/C composites is discussed in detail. Then, several typical applications of carbon fiber-reinforced ceramic composites (mainly C/SiC), including the work of RCI, JAXA and NASA, have been reviewed and analyzed. After that, the current research status of macroscale equivalent and multiscale numerical methods for predicting the mechanical and thermal properties of composites as well as the complex physicochemical phenomenon occurring in hydrocarbon fuels are sorted out. Finally, several potential prospects are pointed out for the future research on the thermal protection of advanced propulsion systems based on the carbon fiber-reinforced ceramic composites. 相似文献
924.
锂硫电池具有理论能量密度高、成本低廉和环境友好等优点,是最有前途的下一代高比能二次电池系统之一。当前,基于有机电解液的液态锂硫电池存在多硫化锂穿梭效应、电解液易燃以及锂枝晶等问题,致使电池的库仑效率低、循环性能差,且存在严重的安全隐患。采用固态电解质(如凝胶聚合物、固态聚合物、陶瓷、复合电解质等)替代有机电解液是解决上述问题的有效途径。本文总结了近年来固态锂硫电池电解质的研究现状,评述了各类固态电解质的优缺点及改性策略,重点介绍了陶瓷固态电解质的研究进展。最后,对固态锂硫电池的未来发展趋势进行预测与展望。 相似文献
925.
Henryk Matusiewicz 《Mikrochimica acta》1993,111(1-3):71-82
A commercial laboratory microwave acid digestion system was evaluated for the acid dissolution of ceramic powders (Al2O3, AlN, BN and Si3N4) prior to the determination of their trace element content by microwave induced plasma atomic emission spectrometry. Newly designed vessels, capable of withstanding internal pressures of over 110 bar, provide rapid and satisfactory results for sample dissolution. Sample preparation time was approximately 30 min (including the subsequent cooling time and preparation of the final solution). Results from conventional stainless-steel acid digestion vessel (Teflon bomb) dissolution are compared with the microwave bomb results of microwave plasma atomic emission spectrometry. 相似文献
926.
927.
研究了非化学计量和掺杂对无铅压电陶瓷(Na1/2Bi1/2)0.92Ba0.08TiO3的压电性能及去极化温度的影响.研究发现A位非化学计量可以提高陶瓷的压电性能;B位掺杂对材料电学性能的影响规律类似于Pb(Ti,Zr)O3系压电陶瓷的相关规律;由于非化学计量和掺杂会影响到A位离子对B位离子与氧离子形成的BO6八面体的耦合作用,影响到畴的稳定性,从而影响
关键词:
无铅压电陶瓷
非化学计量
掺杂
电性能 相似文献
928.
929.
The glasses with composition (50−X)PbO–XSrO-25TiO2-25B2O3 (where X=0, 5, 10 and 15 mol%) were prepared using conventional quenching technique. The Tg, Tc, density and CTE of the glass samples were measured. The Tg observed to increase with increasing concentration of SrO, while the Tc first decreased and then increased. The glass samples were converted to glass ceramics by following two stage heat treatment schedule. The glass ceramic samples were characterized by XRD, SEM and dielectric measurements. The XRD results revealed the formation of ferroelectric lead titanate as a major crystalline phase in glass ceramics. Additional phases observed include Sr2B2O5 and PbB2O4. The room temperature (RT) dielectric constant of glass and glass ceramics are in the range of 100–120 which is promising for capacitor application. 相似文献
930.
Mn+1AXn phases (MAX phases for short with M: transition metal, A: A group elements, X: C or N, and n = 1–3) have attracted considerable attention due to the unique combination of the ceramic- and metal-like properties. The density functional theory (DFT) has emerged as a powerful theoretical approach that complements experimental testing and serves as a predictive tool in the identification and characterization of MAX phases. After the beginning with a brief introduction of the MAX phase and DFT, we review the DFT study on this class of materials, including crystal structure, electronic structure, point defects, lattice dynamics, and related properties, phase stability, compressibility, and elastic properties. Comparison between the theoretical values and available experimental ones shows that they are in decent agreement for most part, especially in the lattice constants, elastic properties, and compressibility. This article is concluded with an outlook of future research on DFT study of MAX phases, major challenges to be met and possible solutions in some cases. 相似文献