全文获取类型
收费全文 | 863425篇 |
免费 | 9989篇 |
国内免费 | 2045篇 |
专业分类
电工技术 | 15658篇 |
综合类 | 3348篇 |
化学工业 | 132718篇 |
金属工艺 | 34916篇 |
机械仪表 | 27552篇 |
建筑科学 | 20669篇 |
矿业工程 | 5518篇 |
能源动力 | 21383篇 |
轻工业 | 70120篇 |
水利工程 | 9952篇 |
石油天然气 | 17291篇 |
武器工业 | 54篇 |
无线电 | 96255篇 |
一般工业技术 | 170371篇 |
冶金工业 | 141106篇 |
原子能技术 | 18189篇 |
自动化技术 | 90359篇 |
出版年
2021年 | 7120篇 |
2019年 | 6825篇 |
2018年 | 24957篇 |
2017年 | 23950篇 |
2016年 | 21535篇 |
2015年 | 8586篇 |
2014年 | 13587篇 |
2013年 | 36712篇 |
2012年 | 23720篇 |
2011年 | 36377篇 |
2010年 | 29824篇 |
2009年 | 30076篇 |
2008年 | 31702篇 |
2007年 | 32348篇 |
2006年 | 22207篇 |
2005年 | 21161篇 |
2004年 | 20299篇 |
2003年 | 19816篇 |
2002年 | 18625篇 |
2001年 | 18032篇 |
2000年 | 17477篇 |
1999年 | 17429篇 |
1998年 | 40661篇 |
1997年 | 29473篇 |
1996年 | 22985篇 |
1995年 | 17657篇 |
1994年 | 16043篇 |
1993年 | 15690篇 |
1992年 | 12135篇 |
1991年 | 11620篇 |
1990年 | 11513篇 |
1989年 | 11239篇 |
1988年 | 10848篇 |
1987年 | 9574篇 |
1986年 | 9473篇 |
1985年 | 10901篇 |
1984年 | 9849篇 |
1983年 | 9363篇 |
1982年 | 8436篇 |
1981年 | 8689篇 |
1980年 | 8196篇 |
1979年 | 8278篇 |
1978年 | 8051篇 |
1977年 | 9070篇 |
1976年 | 11512篇 |
1975年 | 7167篇 |
1974年 | 6849篇 |
1973年 | 6992篇 |
1972年 | 5931篇 |
1971年 | 5473篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
We propose a standardization procedure that provides a convenient, quantitative and reproducible laboratory-based method for measuring the state of polarization (SOP) fluctuations produced by polarization varying devices. This method is based on the SOP distributions generated by commercial polarization scramblers. We show that these devices generate distributions of the maximum change of the SOP (in a given sample time) that follow Rayleigh statistics, which scale linearly with scrambling frequency and the sample time. We use this procedure to measure the SOP fluctuations in a short length of coiled fiber subject to mechanical perturbations. 相似文献
32.
Epoxy novolac/anhydride cure kinetics has been studied by differential scanning calorimetry under isothermal conditions. The system used in this study was an epoxy novolac resin (DEN431), with nadic methyl anhydride as hardener and benzyldimethylamine as accelerator. Kinetic parameters including the reaction order, activation energy and kinetic rate constants, were investigated. The cure reaction was described with the catalyst concentration, and a normalized kinetic model developed for it. It is shown that the cure reaction is dependent on the cure temperature and catalyst concentration, and that it proceeds through an autocatalytic kinetic mechanism. The curing kinetic constants and the cure activation energies were obtained using the Arrhenius kinetic model. A suggested kinetic model with a diffusion term was successfully used to describe and predict the cure kinetics of epoxy novolac resin compositions as a function of the catalyst content and temperature. Copyright © 2003 Society of Chemical Industry 相似文献
33.
A set of poly[N‐oligo(ethylene oxide)yl 4‐vinylpyridinium tosylate] (P4VOEOOTs) has been prepared by spontaneous polymerization of 4‐vinylpyridine. This method gives a grafted polyelectrolyte having a positive charge on every backbone pyridinic moiety. The P4VP15Ts, P4VP164Ts, P4VP350Ts and P4VP750Ts aqueous solution conductivities were determined in the concentration range from 6 × 10?4 to 10?2 M at 25 °C. The variation of the conductivity versus concentration of the investigated system exhibits typical polyelectrolyte behaviour. The polyelectrolyte mobility was found to be dependent on the oligo(ethylene oxide) (OEO) side‐chain length. Manning's rod‐like model fails to describe these results. A simple steric effect is proposed to explain the influence of the OEO length. Copyright © 2003 Society of Chemical Industry 相似文献
34.
The activation of safety valves causes the development of flow reaction forces that have to be transferred in an adequate way via the piping to the steel structure or via the connected vessel into the foundation. If the safety valve outlet piping is connected to a blowdown system or, in case of blowing off into the atmosphere, are equipped with a T‐piece at the outlet, the stationary reaction forces are compensated completely. The transient opening process, however, develops flow reaction forces which culminate in peaks of short duration. In this article, a simple method will be proposed for the estimation of the resulting reaction forces as a function of the length of the pipe at the safety valve outlet. CFD calculations and blowdown tests executed with a full‐lift safety valve have confirmed this method on principle. Special importance is attributed to the short duration of the effect of the reaction forces which seems to have only a negligible impact on the supporting steel structure. 相似文献
35.
Ferrite particles coated with biocompatible phases can be used for hyperthermia treatment of cancer. We have synthesized substituted
calcium hexaferrite, which is not stable on its own but is stabilized with small substitution of La. Hexaferrite of chemical
composition (CaO)0.75(La203)0.20(Fe2O3)6 was prepared using citrate gel method. Hydroxyapatite was prepared by precipitating it from aqueous solution of Ca(NO3)2 and (NH4)2HPO4 maintaining pH above 11. Four different methods were used for coating of hydroxyapatite on ferrite particles. SEM with EDX
and X-ray diffraction analysis shows clear evidence of coating of hydroxyapatite on ferrite particles. These coated ferrite
particles exhibited coercive field up to 2 kOe, which could be made useful for hysteresis heating in hyperthermia. Studies
by culturing BHK-21 cells and WBC over the samples show evidence of biocompatibility. SEM micrographs and cell counts give
clear indication of cell growth on the surface of the sample. Finally coated ferrite particle was implanted in Kasaulli mouse
to test its biocompatibility. The magnetic properties and biocompatibility studies show that these hydroxyapatite coated ferrites
could be useful for hyperthermia. 相似文献
36.
Korolev I. A. Alekseenko N. N. Porodnov B. T. Sapunov V. A. Savel'ev D. V. 《Measurement Techniques》2003,46(9):865-871
The design of a sylphon bellows sensor and the basic circuits of an LC-generator and of a microprocessor unit are presented. An analytical pressure–frequency conversion function and a special method of adjusting the sensor ensure an error of less than 0.05%. The dynamic range is up to 105. The instruments developed cover the ranges 103, 104, and 105 Pa. 相似文献
37.
The tensile elastic modulus (E), yield stress (σY) and microhardness (MH) of neat and binary and ternary blends of glassy semicrystalline ethylene–vinyl alcohol copolymer (EVOH), a glassy amorphous polyamide and a semicrystalline nylon‐containing ionomer covering a broad range of properties were examined. The tests were carried out on dry and water‐equilibrated samples to produce stiffer and softer materials, respectively. From the results, more accurate linear correlations were found to describe adequately the microhardness, modulus and yield stress of these strongly self‐associated polymers through hydrogen bonding. Copyright © 2003 Society of Chemical Industry 相似文献
38.
39.
Barbara M. Johnston Peter R. Johnston 《International journal for numerical methods in engineering》2003,56(4):589-607
Accurate numerical evaluation of integrals arising in the boundary element method is fundamental to achieving useful results via this solution technique. In this paper, a number of techniques are considered to evaluate the weakly singular integrals which arise in the solution of Laplace's equation in three dimensions and Poisson's equation in two dimensions. Both are two‐dimensional weakly singular integrals and are evaluated using (in a product fashion) methods which have recently been used for evaluating one‐dimensional weakly singular integrals arising in the boundary element method. The methods used are based on various polynomial transformations of conventional Gaussian quadrature points where the transformation polynomial has zero Jacobian at the singular point. Methods which split the region of integration into sub‐regions are considered as well as non‐splitting methods. In particular, the newly introduced and highly accurate generalized composite subtraction of singularity and non‐linear transformation approach (GSSNT) is applied to various two‐dimensional weakly singular integrals. A study of the different methods reveals complex relationships between transformation orders, position of the singular point, integration kernel and basis function. It is concluded that the GSSNT method gives the best overall results for the two‐dimensional weakly singular integrals studied. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
40.
Chloride doped polyaniline conducting polymer films have been prepared in a protic acid medium (HCl) by potentiodynamic method
in an electrochemical cell and studied by cyclic voltammetry and FTIR techniques. The FTIR spectra confirmed Cl- ion doping in the polymers. The polymerization rate was found to increase with increasing concentration of aniline monomer.
But the films obtained at high monomer concentration were rough having a nonuniform flaky polyaniline distribution. Results
showed that the polymerization rate did not increase beyond a critical HCl concentration. Cyclic voltammetry suggested that,
the oxidation-reduction current increased with an increase in scan rate and that the undoped polyaniline films were not hygroscopic
whereas chloride doped polyaniline films were found to be highly hygroscopic. 相似文献