全文获取类型
收费全文 | 424371篇 |
免费 | 6431篇 |
国内免费 | 1925篇 |
专业分类
电工技术 | 7336篇 |
综合类 | 542篇 |
化学工业 | 64175篇 |
金属工艺 | 17458篇 |
机械仪表 | 12315篇 |
建筑科学 | 10124篇 |
矿业工程 | 2134篇 |
能源动力 | 10630篇 |
轻工业 | 38384篇 |
水利工程 | 4139篇 |
石油天然气 | 7917篇 |
武器工业 | 42篇 |
无线电 | 47914篇 |
一般工业技术 | 82874篇 |
冶金工业 | 83096篇 |
原子能技术 | 8960篇 |
自动化技术 | 34687篇 |
出版年
2021年 | 3962篇 |
2020年 | 2970篇 |
2019年 | 3494篇 |
2018年 | 6107篇 |
2017年 | 5919篇 |
2016年 | 6467篇 |
2015年 | 4550篇 |
2014年 | 7136篇 |
2013年 | 19144篇 |
2012年 | 11377篇 |
2011年 | 15184篇 |
2010年 | 12374篇 |
2009年 | 13940篇 |
2008年 | 14358篇 |
2007年 | 14244篇 |
2006年 | 12477篇 |
2005年 | 11357篇 |
2004年 | 10568篇 |
2003年 | 10086篇 |
2002年 | 9770篇 |
2001年 | 9671篇 |
2000年 | 9100篇 |
1999年 | 9663篇 |
1998年 | 26197篇 |
1997年 | 18363篇 |
1996年 | 14122篇 |
1995年 | 10103篇 |
1994年 | 9072篇 |
1993年 | 8874篇 |
1992年 | 6153篇 |
1991年 | 6091篇 |
1990年 | 5817篇 |
1989年 | 5706篇 |
1988年 | 5464篇 |
1987年 | 4542篇 |
1986年 | 4569篇 |
1985年 | 5264篇 |
1984年 | 4896篇 |
1983年 | 4438篇 |
1982年 | 4198篇 |
1981年 | 4353篇 |
1980年 | 4116篇 |
1979年 | 3916篇 |
1978年 | 3924篇 |
1977年 | 4667篇 |
1976年 | 6446篇 |
1975年 | 3498篇 |
1974年 | 3333篇 |
1973年 | 3364篇 |
1972年 | 2866篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
This paper presents the evolution of low-frequency, high-power transcontinental transmission, starting from the first successful transatlantic link from Poldhu up to the Italian transcontinental station of Coltano. The trend toward higher and higher transmitting power, and the dead end into which wireless was heading if it were not for research in the higher frequency ranges, are pointed out. 相似文献
52.
53.
The aim of this study was to evaluate the use of total coliforms (TC) and faecal coliforms (FC) using a membrane filtration method for precise monitoring of faecal pollution in Korean surface water. The samples were collected in Korea from both main rivers and their tributaries. Presumptive TC * FC were enumerated. The ratios of presumptive FC to TC were not constant, but varied widely, and TC were difficult to enumerate because of overgrowth by background colonies. For FC this was not the case. Seven hundred and three purified strains of presumptive TC * FC and their background colonies were biotyped using API 20E. Among 272 presumptive TC, non-faecal related species, Aeromonas hydrophila dominated (34.6%) and E. coli accounted for only 5.1%. In contrast, E. coli made up 89% of the 209 presumptive FC. Furthermore, of 164 background colonies on Endo Agar LES, 54.9% was A. hydrophila, while background colonies on m-FC Agar were few (58 strains), and despite their atypical colony appearance, most of them were biotyped as enteric bacteria. These results reveal that the detection of FC rather than TC using m-FC Agar is more appropriate for faecal pollution monitoring in eutrophicated surface water located in a temperate region. 相似文献
54.
G. Pfaff 《Inorganic Materials》2003,39(2):123-126
A new class of special effect pigments based on synthetically manufactured silica flakes coated with metal oxides was developed and introduced into the market. Silica flake pigments show extraordinary color effects such as improved interference and strong angle-dependent behavior. They can be used in automotive paints, industrial paints, plastics, and printing inks. 相似文献
55.
56.
Using a corrected sum rule and a generalized virial identity, we study the analytical expression for entire modes of the collective elementary excitation spectrum in a trapped Bose–Einstein condensate at any atom number. Explicit analytical formulas for the spectrum are obtained for the harmonic traps with both spherical symmetry and axial symmetry using the gaussian approximation for the N-body ground-state wave function of the condensate. These formulas give the simple dependence of all energy levels on the atom numbers, their interaction strength and trap geometry parameters. 相似文献
57.
Removal of gold from basic solutions containing [Au(CN)2]? has been demonstrated using the inherently conducting polymer polypyrrole. Polymers containing sulfonated aromatic dopants have been found to display a significant ability to remove gold from such solutions. Experiments performed in solutions containing both gold and copper cyanide complexes indicate that the recovery process is not highly selective. However, the polypyrroles used display significantly faster rates of gold recovery than activated carbon. © 2003 Society of Chemical Industry 相似文献
58.
G Garrote J
M Cruz H Domínguez J
C Paraj 《Journal of chemical technology and biotechnology (Oxford, Oxfordshire : 1986)》2003,78(4):392-398
Non‐isothermal processing of lignocellulosic materials in aqueous media (autohydrolysis reaction) under mild conditions leads to solutions containing valuable chemicals (oligosaccharides, sugars and acetic acid) and other, undesired, compounds (belonging to the extractive and acid‐soluble lignin fractions) which have to be removed in further purification treatments. Liquors obtained by non‐isothermal autohydrolysis of Eucalyptus globulus wood and corncobs under a variety of operational conditions were extracted with ethyl acetate in order to remove non‐saccharide components, and the suitability of the fraction dissolved in the organic phase was assayed for possible utilisation as an antioxidant. The yield and antioxidant activity of ethyl acetate extracts (measured by the α,α‐diphenyl‐β‐picrylhydrazyl (DPPH) radical scavenging capacity) showed a strong dependence on the autohydrolysis conditions. The antioxidant activity of extracts obtained under selected operational conditions compared well with synthetic antioxidants. © 2003 Society of Chemical Industry 相似文献
59.
We consider the problem of estimating an unknown deterministic parameter vector in a linear model with a random model matrix, with known second-order statistics. We first seek the linear estimator that minimizes the worst-case mean-squared error (MSE) across all parameter vectors whose (possibly weighted) norm is bounded above. We show that the minimax MSE estimator can be found by solving a semidefinite programming problem and develop necessary and sufficient optimality conditions on the minimax MSE estimator. Using these conditions, we derive closed-form expressions for the minimax MSE estimator in some special cases. We then demonstrate, through examples, that the minimax MSE estimator can improve the performance over both a Baysian approach and a least-squares method. We then consider the case in which the norm of the parameter vector is also bounded below. Since the minimax MSE approach cannot account for a nonzero lower bound, we consider, in this case, a minimax regret method in which we seek the estimator that minimizes the worst-case difference between the MSE attainable using a linear estimator that does not know the parameter vector, and the optimal MSE attained using a linear estimator that knows the parameter vector. For analytical tractability, we restrict our attention to the scalar case and develop a closed-form expression for the minimax regret estimator. 相似文献
60.
R. Murgan F. Razak D. R. Tilley T. Y. Tan J. Osman M. N. A. Halif 《Computational Materials Science》2004,30(3-4):468-473
We derive an expression for transmittivity (TSHG) of second harmonic generation (SHG) signals from a ferroelectric (FE) film. Intensities of up and down fields in the medium are investigated in relation to TSHG. The derivations are made based on undepletion of input fields and nonlinear wave equation derived from the Maxwell equations. We present two cases: film without mirrors and with partial mirrors. Expressions for the newly derived nonlinear susceptibility coefficients of SHG for real crystal symmetry [J. Opt. Soc. Am. B 19 (2002) 2007] are used to get more realistic results. Variations in TSHG with respect to film thickness are illustrated. 相似文献