全文获取类型
收费全文 | 241682篇 |
免费 | 3433篇 |
国内免费 | 630篇 |
专业分类
电工技术 | 3876篇 |
综合类 | 170篇 |
化学工业 | 38827篇 |
金属工艺 | 10969篇 |
机械仪表 | 7729篇 |
建筑科学 | 5940篇 |
矿业工程 | 2274篇 |
能源动力 | 5667篇 |
轻工业 | 15720篇 |
水利工程 | 3342篇 |
石油天然气 | 7840篇 |
武器工业 | 27篇 |
无线电 | 23914篇 |
一般工业技术 | 51057篇 |
冶金工业 | 40913篇 |
原子能技术 | 6870篇 |
自动化技术 | 20610篇 |
出版年
2021年 | 2713篇 |
2020年 | 2006篇 |
2019年 | 2603篇 |
2018年 | 4423篇 |
2017年 | 4393篇 |
2016年 | 4792篇 |
2015年 | 2954篇 |
2014年 | 4796篇 |
2013年 | 11065篇 |
2012年 | 7293篇 |
2011年 | 9234篇 |
2010年 | 7451篇 |
2009年 | 7992篇 |
2008年 | 8366篇 |
2007年 | 8211篇 |
2006年 | 7115篇 |
2005年 | 6508篇 |
2004年 | 5776篇 |
2003年 | 5654篇 |
2002年 | 5391篇 |
2001年 | 5170篇 |
2000年 | 4725篇 |
1999年 | 4787篇 |
1998年 | 10790篇 |
1997年 | 7764篇 |
1996年 | 6039篇 |
1995年 | 4702篇 |
1994年 | 4104篇 |
1993年 | 4345篇 |
1992年 | 3464篇 |
1991年 | 3279篇 |
1990年 | 3299篇 |
1989年 | 3240篇 |
1988年 | 3186篇 |
1987年 | 2859篇 |
1986年 | 2882篇 |
1985年 | 3272篇 |
1984年 | 3099篇 |
1983年 | 2858篇 |
1982年 | 2595篇 |
1981年 | 2550篇 |
1980年 | 2624篇 |
1979年 | 2629篇 |
1978年 | 2622篇 |
1977年 | 2713篇 |
1976年 | 3082篇 |
1975年 | 2384篇 |
1974年 | 2244篇 |
1973年 | 2331篇 |
1972年 | 2061篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
171.
Brian M. Louie Thomas Franaszek Tina Pho Wen Yen Chiu David S. Soong 《应用聚合物科学杂志》1985,30(9):3841-3856
Oxygen injection has been proposed and experimentally demonstrated as an effective control measure for limiting the rate of heat release and altering the rate of polymerization in emulsion processes. A detailed mathematical model has been previously developed to describe the system behavior with and without oxygen injection. A simple lab scale apparatus was constructed and run extensively. Only trace quantities of oxygen are needed to inhibit the reaction completely. The facile response makes this method attractive for fast temperature control. However, because of the rapid penetration of dissolved oxygen into the polymer particles, growing radical chains are terminated prematurely, lowering product molecular weights. To minimize this detrimental effect, pulsed oxygen control is used and extensive experimental work was performed to determine the effects of controller set points on molecular weight. Moderate oxygen flows and moderate set point temperatures are found to give the optimal response without significant lowering of the final molecular weight. Injected quantities agree well with the order-of-magnitude sparging calculations needed to completely stop initiation. However, complete agreement between model simulations and experimental results was not reached due to an unmeasured loss of monomer from the reactor. Chain transfer to monomer is found to be important in modeling the polymer molecular weight. It has, however, a negligible effect on the reaction rate. 相似文献
172.
Intense irradiation (25–40 A/cm2) at intermediate energy (400 keV) has been found to cause “metallisation” of tungsten oxide and titanium niobate crystal surfaces. The possible mechanisms and the likely consequences for high-resolution imaging of clean oxide surfaces are briefly discussed. 相似文献
173.
New fundamental solutions which automatically satisfy boundary conditions at the interfaces of an elastic plate perfectly bonded to two elastic halfspaces are implemented in a 3-D boundary element method (BEM) for crack problems. The BEM features a new integration scheme for highly singular kernels. The capability is achieved through a part analytic and part numerical integration procedure, such that the analytic part of the integration is similar for all slip/opening variations, ‘Part-through’ elliptic cracks in an elastic plate with traction-free surfaces are analysed and the stress intensity factor (SIF) values along the crack front are found to compare favourably with widely accepted numerically obtained SIF results by Raju and Newman.1 相似文献
174.
175.
176.
Self-similarity in diffraction by a self-similar fractal screen 总被引:1,自引:0,他引:1
Lakhtakia A. Holter N. Varadan V. 《Antennas and Propagation, IEEE Transactions on》1987,35(2):236-239
Using the Sierpinski gasket as an example, it has been shown here that the paraxial Fraunhoffer-zone diffracted field of a self-similar fractal screen also exhibits self-similarity. This also establishes that fractal structures can be used with great profit in problems involving array syntheses. Recently, it has been shown that the Sierpinski gasket is itself a member of a much wider class of gaskets, and the potential for the use of fractal structures in electromagnetic (EM) problems may be vast indeed. 相似文献
177.
178.
Synthesis of 2-[18F]fluoro-2-deoxy-D-glucose ([18F]FDG) involving base hydrolysis was optimized. Fluorine-18 was isolated from irradiated water to more than 90% by sorption of [1
8F]fluoride on QMA anion-exchange resin, which was followed by elution with a 96 : 4 (by volume) acetonitrile-water mixture containing Kryptofix 2.2.2 and potassium carbonate (molar ratio 2 : 1). This composition is the best for preparing the complex [K/K2.2.2]+
18F- used in nucleophilic fluorinations. No additional azeotropic drying is required. Base hydrolysis under optimized conditions (40-45°C), followed by neutralization with HCl, removal of traces of the solvent, and purification of the final product on a combined SCX/Alumina N column, yielded [1
8F]FDG of high radiochemical (>99%) and chemical purity with minimal product loss. With an RB-86 robotic system (Anatech, Sweden), the synthesis time was 38 min. The procedure is used in the Institute of Human Brain, Russian Academy of Sciences for routine synthesis of FDG; the radiochemical yield of the product by the end of synthesis (EOS) is reproducibly high: 63±3% (n = 40). 相似文献
179.
D. C. Peets R. Liang C. Stock W. J. L. Buyers Z. Tun L. Taillefer R. J. Birgeneau D. A. Bonn W. N. Hardy 《Journal of Superconductivity and Novel Magnetism》2002,15(6):531-534
We have grown cubic centimeter-size crystals of YBa2Cu3O x suitable for neutron studies, by a top-seeded melt-growth technique. Growth conditions were optimized with an eye toward maximizing phase purity. It was found that the addition of 2% Y2BaCuO5 and 0.5% Pt (by mass) were required to prevent melt loss and to obtain the highest crystallinity. A neutron diffraction study on a mosaic of six such crystals found that the final Y2BaCuO5 concentration was 5%, while other impurity phases comprised less than 1% by volume. The oxygen content was set to x = 6.5, the crystals were detwinned, and then carefully annealed to give the well-ordered ortho-II phase. The neutron study determined that 70% of the mosaic's volume was in the majority orthorhomic domain. The neutron (006) and (110) rocking curve widths were ~1° per crystal and ~2.2° for the mosaic, and the oxygen chain correlation lengths were >100 Å in the a- and b-directions and ~50 Å in the c-direction. 相似文献
180.
The crystal structure of [NpO2{OC(NH2)2}5]ClO4·H2O and [NpO2Cl{OC(NH2)2}4] was studied by X-ray diffraction. The electronic absorption spectra of these complexes were recorded. The coordination polyhedron of Np in both complexes is a distorted pentagonal bipyramid. There is no cation-cation interaction of neptunyl cations in these complexes. 相似文献