全文获取类型
收费全文 | 204392篇 |
免费 | 2439篇 |
国内免费 | 553篇 |
专业分类
电工技术 | 3505篇 |
综合类 | 137篇 |
化学工业 | 30825篇 |
金属工艺 | 10263篇 |
机械仪表 | 6976篇 |
建筑科学 | 3940篇 |
矿业工程 | 2157篇 |
能源动力 | 4608篇 |
轻工业 | 12576篇 |
水利工程 | 2913篇 |
石油天然气 | 7722篇 |
武器工业 | 22篇 |
无线电 | 21491篇 |
一般工业技术 | 44978篇 |
冶金工业 | 34513篇 |
原子能技术 | 6627篇 |
自动化技术 | 14131篇 |
出版年
2021年 | 1992篇 |
2019年 | 2001篇 |
2018年 | 3687篇 |
2017年 | 3722篇 |
2016年 | 4008篇 |
2015年 | 2240篇 |
2014年 | 3792篇 |
2013年 | 8755篇 |
2012年 | 5704篇 |
2011年 | 7245篇 |
2010年 | 5882篇 |
2009年 | 6515篇 |
2008年 | 6629篇 |
2007年 | 6520篇 |
2006年 | 5591篇 |
2005年 | 5137篇 |
2004年 | 4644篇 |
2003年 | 4553篇 |
2002年 | 4252篇 |
2001年 | 4298篇 |
2000年 | 3968篇 |
1999年 | 4045篇 |
1998年 | 10052篇 |
1997年 | 7043篇 |
1996年 | 5352篇 |
1995年 | 4120篇 |
1994年 | 3597篇 |
1993年 | 3839篇 |
1992年 | 2962篇 |
1991年 | 2982篇 |
1990年 | 2890篇 |
1989年 | 2840篇 |
1988年 | 2851篇 |
1987年 | 2486篇 |
1986年 | 2557篇 |
1985年 | 2837篇 |
1984年 | 2684篇 |
1983年 | 2537篇 |
1982年 | 2287篇 |
1981年 | 2258篇 |
1980年 | 2336篇 |
1979年 | 2350篇 |
1978年 | 2369篇 |
1977年 | 2482篇 |
1976年 | 2879篇 |
1975年 | 2185篇 |
1974年 | 2067篇 |
1973年 | 2164篇 |
1972年 | 1935篇 |
1971年 | 1745篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
N.A. Zhuk M.G. Krzhizhanovskaya N.A. Sekushin V.V. Kharton B.A. Makeev V.A. Belyy R.I. Korolev 《Ceramics International》2021,47(14):19424-19433
The solid solutions based on the pyrochlore-type system Bi2MgNb2-xTaxO9 were formed in the compositional range х = 0–2.0 (Bi1·6Mg0·8Nb1.6-tTatO7.2, t = 0–1.6). The Rietveld method was used to refine the structure for Bi2MgNb2-xTaxO9 (x = 0, 1.0, 2.0). The increasing tantalum content led to the slight decrease in the cubic unit cell parameters from 10.56934 (4) Å for x = 0 and 10.54607 (3) Å for x = 2 (sp.gr. Fd-3m:2). At the same time, tantalum additions suppressed grain growth in the pyrochlore ceramics during sintering and made it possible to obtain materials with an average grain size of 1–2 μm (Bi1·6Mg0·8Ta1·6O7.2). The increase in the Ta5+ concentration led to the decrease in the dielectric permeability from 104 (Bi1·6Mg0·8Nb1·6O7.2) to 20 (Bi1·6Mg0·8Ta1·6O7.2) at room temperature, while the dielectric loss tangent remained lower than 0.002, which is due to the small grain size and the high porosity of the samples. An increase in temperature has practically no effect on the values of the dielectric permittivity in the entire frequency range. The samples have weak through conductivity. The activation energies of electrical conductivity varied in the range of 0.84–1.00 eV, and the less tantalum, the lower the activation energy. The electrical properties of the samples at 200 Hz to 1 MHz are described by the simplest parallel scheme. 相似文献
12.
In the present work, two types of shear thickening fluids have been synthesized by using neat and aminosilane functionalized silica nanoparticles and their viscosity curves have been obtained by the rheometer. Based on the values of peak viscosity of synthesized shear thickening fluids, the surface functionalized nanosilica based shear thickening fluid has been chosen as a best candidate due to the high viscosity for impregnation into the neat Kevlar of different layers viz. four (04) and eight (08) layers for velocity impact study. The experimental investigations reveal high energy absorption of shear thickening fluid impregnated Kevlar as compared to the neat Kevlar. The maximum energy absorption 62 J is achieved corresponding to the initial velocity 154 m∙s−1 for 08 layers shear thickening fluid impregnated Kevlar specimen. The data have also been analytically determined and validated with the experimental data. The experimental data have good agreement with the analytical data within the accuracy of around 15 to 20%. The present findings can have significant inferences towards the fabrication of shear thickening fluids using nanomaterials for numerous applications such as soft armors, dampers, nanofinishing and so forth. 相似文献
13.
N. Clament Sagaya Selvam Lijie Du Bao Yu Xia Pil J. Yoo Bo You 《Advanced functional materials》2021,31(12):2008190
Electroreduction of small molecules such as H2O, CO2, and N2 for producing clean fuels or valuable chemicals provides a sustainable approach to meet the increasing global energy demands and to alleviate the concern on climate change resulting from fossil fuel consumption. On the path to implement this purpose, however, several scientific hurdles remain, one of which is the low energy efficiency due to the sluggish kinetics of the paired oxygen evolution reaction (OER). In response, it is highly desirable to synthesize high-performance and cost-effective OER electrocatalysts. Recent advances have witnessed surface reconstruction engineering as a salient tool to significantly improve the catalytic performance of OER electrocatalysts. In this review, recent progress on the reconstructed OER electrocatalysts and future opportunities are discussed. A brief introduction of the fundamentals of OER and the experimental approaches for generating and characterizing the reconstructed active sites in OER nanocatalysts are given first, followed by an expanded discussion of recent advances on the reconstructed OER electrocatalysts with improved activities, with a particular emphasis on understanding the correlation between surface dynamics and activities. Finally, a prospect for clean future energy communities harnessing surface reconstruction-promoted electrochemical water oxidation will be provided. 相似文献
14.
Steblevskaya N. I. Medkov M. A. Belobeletskaya M. V. 《Theoretical Foundations of Chemical Engineering》2021,55(5):1086-1090
Theoretical Foundations of Chemical Engineering - A method for producing lanthanum–cerium phosphates activated by terbium of the compositions La0.8Ce0.15Tb0.05PO4 and La0.8Ce0.15Tb0.05(PO3)3... 相似文献
15.
Glass and Ceramics - The effect of diopside on the sintering temperature, structure, and properties of low-temperature electrotechnical porcelain was investigated. Diopside concentrate in the... 相似文献
16.
Madyshev I. N. Dmitrieva O. S. Mayasova A. O. Dmitriev A. V. 《Chemical and Petroleum Engineering》2021,57(1-2):136-142
Chemical and Petroleum Engineering - A contact device for heat and mass transfer devices with the ability to maintain the uniformity of flow distribution over the cross section of the device is... 相似文献
17.
In this communication, the structural, micro-structural, dielectric, electrical, magnetic, and leakage-current characteristics of a double perovskite (Y2CoMnO6) ceramic material have been reported. The material was synthesized via a high-temperature mixed-oxide route. The compound crystallizes in a monoclinic structure which is confirmed from preliminary X-ray structural study. The morphological study by using scanning electron micrograph reveals the almost homogeneous distribution of grains throughout the surface of the sample. The nature of frequency-dependence of dielectric constant has been described by the Maxwell-Wagner model. The occurrence of a dielectric anomaly in the temperature dependence of dielectric permittivity study demonstrates the ferroelectric-paraelectric phase transition in the material. From the Nyquist plots, we found the existence of both grain and grain boundary effects. The frequency dependence of conductivity was studied by the Jonscher’s Power law, and the conduction phenomenon obeys the large overlapping polaron tunneling model. By using the Arrhenius equation, the activation energy has been calculated which is nearly equal to the energy required for the hoping of the electron. Both impedance and conductivity analysis demonstrate that the sample exhibits negative temperature coefficient of resistance (NTCR) properties indicating the semiconducting type of material at high temperatures. The anti-ferromagnetic character of the material is observed from the nature of magnetic hysteresis loop. The leakage current analysis suggests that the conduction process in the material follows the space charge limited conduction phenomenon. Such material will be helpful for modern electronic devices and spintronic applications. 相似文献
18.
Theoretical Foundations of Chemical Engineering - The main approaches to developing a reactor block for the hydrodesulfurization of diesel fuels are considered taking into account the reactivity of... 相似文献
19.
E. Sebastián A. Murciano R. Madrigal P.N. De Aza P. Velasquez 《Ceramics International》2021,47(15):21466-21475
The influence of surface topography on cellular behaviour and its importance for the development of three-dimensional scaffolds for bone tissue engineering are a topic of growing interest. To date, the introduction of topographical patterns into the surface of 3D porous ceramic scaffolds has proven difficult, due partly to the brittle nature of ceramic materials as well as the currently available fabrication technologies. In this study, a grooved pattern was introduced into the surface of 3D multilayer porous ceramic scaffolds by the chemical etching technique. The patterned scaffolds were characterised by X-Ray Diffraction (XRD), Scanning Electron Microscopy with Energy Dispersive X-Ray Spectroscopy (SEM-EDX) and Digital Holographic Microscopy (DHM). Their bioactivity was also evaluated in vitro by immersion in simulated body fluid (SBF) for 12 h, 1, 7, 14 and 21 days. Scaffolds were constituted mainly with a mixture of the calcium pyrophosphate (Ca2O7P2) and β-tricalcium phosphate (Ca?(PO?)?) phases. The pyrophosphate on the external layer was dissolved as a result of the etching process, leaving grooves on the surface. Ridges and grooves were nano-/micrometric, with dimensions of around 900 nm–1.5 μm in width and 200 nm–300 nm in depth. Moreover, the mechanical properties and bioactive capacity of the patterned scaffolds were not affected by chemical etching, making them suitable to be used in bone tissue engineering. 相似文献
20.
Semiconductors - In conductors with a very low defect density, electrons at low temperatures collide mainly with the sample edges; therefore, the ballistic transport of charge and heat is... 相似文献