全文获取类型
收费全文 | 22408篇 |
免费 | 641篇 |
国内免费 | 25篇 |
专业分类
电工技术 | 233篇 |
综合类 | 18篇 |
化学工业 | 4570篇 |
金属工艺 | 515篇 |
机械仪表 | 392篇 |
建筑科学 | 979篇 |
矿业工程 | 95篇 |
能源动力 | 551篇 |
轻工业 | 1566篇 |
水利工程 | 275篇 |
石油天然气 | 208篇 |
武器工业 | 1篇 |
无线电 | 1568篇 |
一般工业技术 | 3543篇 |
冶金工业 | 5072篇 |
原子能技术 | 175篇 |
自动化技术 | 3313篇 |
出版年
2022年 | 126篇 |
2021年 | 246篇 |
2020年 | 204篇 |
2019年 | 272篇 |
2018年 | 310篇 |
2017年 | 319篇 |
2016年 | 378篇 |
2015年 | 308篇 |
2014年 | 480篇 |
2013年 | 1298篇 |
2012年 | 787篇 |
2011年 | 1005篇 |
2010年 | 809篇 |
2009年 | 819篇 |
2008年 | 940篇 |
2007年 | 1013篇 |
2006年 | 829篇 |
2005年 | 790篇 |
2004年 | 692篇 |
2003年 | 640篇 |
2002年 | 607篇 |
2001年 | 401篇 |
2000年 | 398篇 |
1999年 | 402篇 |
1998年 | 598篇 |
1997年 | 471篇 |
1996年 | 439篇 |
1995年 | 409篇 |
1994年 | 350篇 |
1993年 | 401篇 |
1992年 | 341篇 |
1991年 | 252篇 |
1990年 | 305篇 |
1989年 | 306篇 |
1988年 | 294篇 |
1987年 | 277篇 |
1986年 | 301篇 |
1985年 | 316篇 |
1984年 | 311篇 |
1983年 | 303篇 |
1982年 | 242篇 |
1981年 | 270篇 |
1980年 | 271篇 |
1979年 | 261篇 |
1978年 | 235篇 |
1977年 | 240篇 |
1976年 | 242篇 |
1975年 | 205篇 |
1974年 | 179篇 |
1973年 | 187篇 |
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
121.
Modern bipolar junction transistors (BJTs) tend to operate with saturated carrier velocity in the collector space-charge region. A physical analysis of the effects of velocity saturation on the output resistance of BJTs, especially for high current levels is presented. Physical analyses show that when the collector current density approaches a critical value, the Early voltage can increase significantly due to carrier-velocity saturation as the resulting base push-out partially offsets base-width modulation. This effect is also demonstrated in simulations with PISCES and MMSPICE 相似文献
122.
Chlorisondamine is a charged molecule that acts as long-acting nicotinic antagonist in many species, including pigeon. Evidence indicates that, despite the charged nature of chlorisondamine, it blocks some central effects of nicotine. The present study examined the time course of chlorisondamine's blockade of nicotine-induced c-fos expression in the pigeon brain. Chlorisondamine's central blockade was examined from 1 hr to 28 days prior to nicotine administration. Nicotine stimulated increases in c-fos mRNA in the hippocampus, hyperstriatum accessorium, hyperstriatum ventrale, nucleus accumbens, bulbus olfactorius, paleostriatum augmentatum, and stratum griseum et fibrosum superficiale. Nicotinic receptors labeled by [(125)I]-epibatidine were not always found in the same regions as nicotine-induced increases in c-fos expression. Acute chlorisondamine increased the level of c-fos mRNA in the cerebellum, hippocampus, hyperstriatum accessorium, locus parolfactorius, nucleus accumbens, tectum opticum, paleostriatum augmentatum, and stratum griseum et fibrosum superficiale but had no effect on its own 24 hr after administration. Chlorisondamine blocked nicotine-induced increases in c-fos RNA for 4 days in the nucleus accumbens, a week in the bulbus olfactorius, and 2 weeks in the stratum griseum et fibrosum superficiale. The time course of chlorisondamine's blockade of nicotine-induced c-fos expression is consistent with the time course of the ability of chlorisondamine to block behavioral and physiological responses to nicotine. 相似文献
123.
This is the last part of a four-part survey of optimization models and solution algorithms for winter road maintenance planning. The two first parts of the survey address system design problems for winter road maintenance. The third part concentrates mainly on vehicle routing problems for spreading operations. The aim of this paper is to provide a comprehensive survey of optimization models and solution methodologies for the routing of vehicles for plowing and snow disposal operations. We also review models for the fleet sizing and fleet replacement problems. 相似文献
124.
The pressure-flow (P-F) relationship in intravenous catheters is shown to be represented by the predictive model P = RLF + RTF2. The pressure loss due to F, calculated from experimental data, was found to equal the expected Poiseuille laminar flow loss. Pressure loss due to F2 was found to be the sum of turbulent P losses due to entrance length flow disturbance, kinetic energy at the outflow, and minor shape change losses. Analysis of the flow capabilities of intravenous catheters can be used to evaluate suitability for specific intravenous infusion needs. 相似文献
125.
Argues that J. Langenbucher and P. E. Nathan (see record 1984-12497-001) misinterpreted an established legal standard in their study of the reliability of blood-alcohol judgments by hosts, bartenders, and police officers. Their error was in the assumption that dramshop laws and related Alcohol Beverage Control laws assume that any service to a person whose blood-alcohol level is over .10 is prohibited. Implications for social policy and the establishment of standards of intoxication are discussed. (19 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
126.
127.
Large diameter slender cylindrical shells used as buoyancy chambers and support legs for compliant marine structures experience a variety of impact loadings which could induce significant levels of dynamic response. By examining the response of a typical range of cylindrical shell geometries to a simulation of the impact loading typical of wave slamming, the paper examines the likely levels and practical significance of the transient dynamic response. It shows that the levels of dynamic response are sufficiently severe to have important implications for the triggering of buckling and fatigue collapse characteristics. It is suggested that this phenomenon requires additional more-detailed consideration, including the provision of reliable predictions of transient slam pressure distributions and the levels and forms of hydrodynamic damping, as well as the effects of more practical shell and stiffening geometries, and ambient stress levels. 相似文献
128.
We recently reported the presence of reliable asymmetries in frontal-brain electrical activity in infants that distinguished between certain positive- and negative-affect elicitors. In order to explore the degree to which these asymmetries in brain activity are associated with individual differences in affective response, 35 ten-month-old female infants were presented with a stranger-approach, mother-approach, and maternal-separation experience while an electroencephalogram (EEG) from the left- and right-frontal and left- and right-parietal scalp regions was recorded and facial and other behavioral responses were videotaped. Changes in frontal-EEG asymmetry reflected behavioral changes between conditions. In addition, individual differences in affective response to separation were related to differences in frontal-brain asymmetries. These findings indicate that lawful changes exist in asymmetries of frontal-brain activation during the expression of certain emotions in the first year of life and that individual differences in emotional responsivity are related to these measures of brain activity. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
129.
Mann-Fu Rau David Rieck James W. Evans 《Metallurgical and Materials Transactions B》1987,18(1):257-278
An “environmental cell” located in a high voltage transmission electron microscope has been used to study the reduction of
single crystal iron oxides by hydrogen and hydrogen-argon mixtures. The cell enables a direct observation of the solid during
reaction, thus permitting the nucleation and growth of solid reaction products to be observed. Hematite was reduced at temperatures
in the range 387 to 610°C with gas pressures up to 5.3 kP. Reduction with pure hydrogen was considerably faster than when
argon was present. Lath magnetite which rapidly transforms to porous magnetite and thence (more slowly) to porous iron was
observed. The reduction of magnetite and of wustite single crystals was observed in the temperature range 300 to 514°C using
both hydrogen and hydrogen-argon mixtures at gas pressures up to 6.6 kP. Incubation periods were found for magnetite reduction;
during these periods faceted pits formed in the oxide. Iron formed in the early stages was epitaxial with the host magnetite;
at later stages the epitaxy was lost and fissures frequently formed in the metal. The morphology of the iron differed between
the gas mixtures. Disproportionation accompanied the reduction of wustite, producing intermediate polycrystalline magnetite
despite reducing conditions. The disproportionation appeared to be promoted by the reduction reaction. For both oxides, reduction
in the hydrogen-argon mixture was slower than in pure hydrogen. 相似文献
130.
R. James Maguire Richard J. Tkacz David L. Sartor 《Journal of Great Lakes research》1985,11(3):320-327
Water and sediment samples from 29 locations in the Detroit and St. Clair rivers were analyzed for the highly toxic tri-n-butyltin (Bu3Sn+) species and for the less toxic di-n-butyltin (Bu2Sn2+) and n-butyltin (BuSn3+) species and inorganic tin. In general, locations sampled in the St. Clair River were less contaminated with butyltin species than those in the Detroit River. Inorganic tin and BuSn3+ were detected in over 90% of all subsurface water samples, while Bu2Sn2+ and Bu3Sn+ were detected in 45 and 28% of the same samples, respectively. The highest concentration ofBu3Sn+ in subsurface water, 5.9 × 10?10 mol Sn/L, was at the mouth of the Ecorse River, a tributary of the Detroit River. The three butyltin species and inorganic tin were also detected in 23–46% of all sediment samples. The highest concentrations of Bu3Sn+ in sediment were found close to the mouths of the River Rouge, another tributary of the Detroit River, and the Ecorse River, and were 6.2 × I0–7 and 1.7 × I0–7 mol Sn/kg dry weight, respectively, for the top 2 cm of sediment. 相似文献