全文获取类型
收费全文 | 495945篇 |
免费 | 16863篇 |
国内免费 | 8213篇 |
专业分类
电工技术 | 14644篇 |
技术理论 | 14篇 |
综合类 | 11992篇 |
化学工业 | 77951篇 |
金属工艺 | 24663篇 |
机械仪表 | 20166篇 |
建筑科学 | 18279篇 |
矿业工程 | 6022篇 |
能源动力 | 10205篇 |
轻工业 | 38334篇 |
水利工程 | 7105篇 |
石油天然气 | 15001篇 |
武器工业 | 1289篇 |
无线电 | 53240篇 |
一般工业技术 | 88529篇 |
冶金工业 | 59491篇 |
原子能技术 | 9969篇 |
自动化技术 | 64127篇 |
出版年
2022年 | 5982篇 |
2021年 | 8556篇 |
2020年 | 6226篇 |
2019年 | 6006篇 |
2018年 | 21871篇 |
2017年 | 21282篇 |
2016年 | 17942篇 |
2015年 | 8861篇 |
2014年 | 11806篇 |
2013年 | 20024篇 |
2012年 | 19837篇 |
2011年 | 28834篇 |
2010年 | 24257篇 |
2009年 | 22027篇 |
2008年 | 23862篇 |
2007年 | 24414篇 |
2006年 | 15563篇 |
2005年 | 14500篇 |
2004年 | 12264篇 |
2003年 | 11825篇 |
2002年 | 11275篇 |
2001年 | 10557篇 |
2000年 | 9636篇 |
1999年 | 9107篇 |
1998年 | 15376篇 |
1997年 | 11618篇 |
1996年 | 9156篇 |
1995年 | 7160篇 |
1994年 | 6458篇 |
1993年 | 6075篇 |
1992年 | 5055篇 |
1991年 | 4713篇 |
1990年 | 4752篇 |
1989年 | 4607篇 |
1988年 | 4286篇 |
1987年 | 3733篇 |
1986年 | 3645篇 |
1985年 | 3980篇 |
1984年 | 3894篇 |
1983年 | 3603篇 |
1982年 | 3237篇 |
1981年 | 3448篇 |
1980年 | 3164篇 |
1979年 | 3430篇 |
1978年 | 3337篇 |
1977年 | 3413篇 |
1976年 | 4285篇 |
1975年 | 3022篇 |
1974年 | 2846篇 |
1973年 | 2881篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
Sheldon H. Jacobson Julie L. Virta Jon M. Bowman John E. Kobza John J. Nestor 《IIE Transactions》2003,35(3):259-269
Aviation security protects vital national interests, as well as passengers and aircraft. Key components of an aviation security system include baggage and passenger screening devices and operations. Determining how and where to assign (deploy) such devices can be quite challenging. Moreover, even after such systems are in place, it can be difficult to measure their effectiveness. This paper describes how discrete optimization models can be used to address these questions, based on three performance measures that quantify the effectiveness of airport baggage screening security device systems. These models are used to solve for optimal airport baggage screening security device deployments considering the number of passengers on a set of flights who have not been cleared using a security risk assessment system in use by the Federal Aviation Administration (i.e., passengers whose baggage is subjected to screening), the number of flights in this set, and the size of the aircraft for such flights. Several examples are provided to illustrate these results, including an example that uses data available from the Official Airline Guide. 相似文献
42.
A.L. Pan H.G. ZhengZ.P. Yang F.X. Liu Z.J. Ding Y.T. Qian 《Materials Research Bulletin》2003,38(5):789-796
Small Ag particles or clusters dispersed mesoporous SiO2 composite films were prepared by a new method: First the matrix SiO2 films were prepared by sol-gel process combined with the dip-coating technique, then they were soaked in AgNO3 solutions followed by irradiation of γ-ray at room temperature and in ambient pressure. The structures of these films were examined by X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM), and optical absorption spectroscopy. It has been shown that the Ag particles grown within the porous SiO2 films are very small, and they are isolated and dispersed from each other with very narrow size distributions. With increasing the soaking concentration and an additional annealing, an opposite peakshift effect of the surface plasmon resonance (SPR) was observed in the optical absorption measurements. 相似文献
43.
L. B. Knayz'kov 《Journal of Infrared, Millimeter and Terahertz Waves》2003,24(11):1931-1938
Variants are considered of palarizational frequency-selective multipath interference devices for millimetric- and submillimetric- wave receivers. Main expressions are presented that describe their characteristics. Advantages are pointed out of the devices as compared with Fabry-Perot interferometers when solving problems of received frequency-band shaping, heterodyne noise rejection, and signal- and heterodyne-radiation transfer to mixer. 相似文献
44.
低温脂肪酶产生菌的筛选、鉴定及其部分酶学性质 总被引:15,自引:1,他引:14
从南极乔治王岛冻土来源的76株低温细菌中筛选到13株低温脂肪酶产生菌,对其中的BTsl0022菌株进行鉴定。通过生理生化特征、16s rDNA基因序列的同源性和系统发育分析发现,菌株RTsl0022属于假单胞菌属(Pseudomonas),但与已定名的假单胞菌有一定的差异,与未定名的Pseudomonas sp.PsB的亲缘关系最接近,故将其暂定名为Pseudomonas sp.BTsl0022。对该菌脂肪酶的酶学性质初步研究表明,酶的最适作用温度为24℃,对热敏感,60℃处理30min仅残留25%酶活性,酶的适宜作用pH范围在7.0~9.0,最适pH为8.0。 相似文献
45.
D. Vignolles D. Smirnov G. Rikken B. Raquet H. Rakoto C. Proust M. Nardone J. Léotin F. Lecouturier M. Goiran O. Drachenko J. M. Broto L. Brossard A. Audouard 《Journal of Low Temperature Physics》2003,133(1-2):97-120
An overview over past and present activities and future developments at the Toulouse pulsed magnetic field facility is given, both as far as technical developments of the infrastructure, as well as low temperature physics performed at the LNCMP are concerned. 相似文献
46.
Beebe-Frankenberger Margaret; Bocian Kathleen M.; MacMillan Donald L.; Gresham Frank M. 《Canadian Metallurgical Quarterly》2004,96(2):204
A cohort of 2nd-grade students provided comparisons of academic and social competence based on school retention/promotion decisions. Sample groups were (a) retained, (b) at risk for retention, (c) special education, and (d) promoted. Findings suggested most children with academic deficiencies are identified by schools early and are sorted into educational treatments differing in intensity that represent a continuum of competence. The authors provide empirical evidence counter to the assumptions that retained students have the requisite ability to catch up and have more problem behaviors than other low-achieving students. The relevance of high-stakes test scores for promotion/retention decisions and the parallels between schools' implementation of retention policy and implementation of regulations for identifying children with disabilities are included in the discussion. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
47.
48.
The hydrocarbon-group composition and distribution of naphthenoaromatic hydrocarbons of West Siberian oils were studied by the technique of mass spectrometry. Naphthenoaromatic hydrocarbons of the general formula CnH2n?z , where z is 8, 10, 12, 14, 16, or 18, containing from one to six saturated cycles are present in oils in significant concentrations. Naphthenomonoarenes with one saturated cycle dominate in all oils. An increase in the amount of compounds containing 3–6 saturated cycles in their molecule with the occurrence depth was noted. Naphthenobiarenes of the general formula CnH2n?z , where z is 14, 16, 18, 20, 22, or 24 containing from one to six saturated cycles are present in oils in concentrations from 20.0 to 54.0 wt % of the biaromatic fraction. Most oils are characterized by the prevalence of structures with one and two saturated cycles in their molecule. The dependence of the number of cycles in naphthenobiarenes on the nature of original organic matter (OM) was not traced. The lack of correlation between the number of cycles and the OM type is presumably due to the fact that, for the most part, fused polycyclic naphthenobiarenes reflect the degree of catagenetic alteration of organic matter. 相似文献
49.
The selector activated sludge (SAS) systems are known to prevent excessive growth of filamentous microorganisms responsible for bulking sludge, but these systems were hardly ever modelled. This study aimed to develop a model capable of predicting rapid substrate removal in the SAS systems. For this purpose, the Activated Sludge Model No. 3 (ASM3) was extended with three processes (adsorption, direct growth on the adsorbed substrate under aerobic or anoxic conditions). The modified ASM3 was tested against the results of batch experiments with the biomass originating from two full-scale SAS systems in Germany. The endogenous biomass was mixed with various readily biodegradable substrates (acetate, peptone, glucose and wastewater) and the utilisation of substrate (expresses as COD) and oxygen uptake rates (OURs) were measured during the experiments. In general, model predictions fitted to the experimental data, but a considerable number of kinetic (5) and stoichiometric (2) parameters needed to be adjusted during model calibration. The simulation results revealed that storage was generally a dominating process compared to direct growth in terms of the adsorbed substrate utilisation. The contribution of storage ranged from 65-71% (Plant A) and 69-92% (Plant B). 相似文献
50.
Extraction of Cs, Sr, Eu, U, Np, and Am with mixtures of dipicolinic acid diamides with chlorinated cobalt dicarbollide was studied. A synergistic effect was revealed. 相似文献