全文获取类型
收费全文 | 1555266篇 |
免费 | 26237篇 |
国内免费 | 7025篇 |
专业分类
电工技术 | 34431篇 |
综合类 | 6439篇 |
化学工业 | 272391篇 |
金属工艺 | 65025篇 |
机械仪表 | 43617篇 |
建筑科学 | 47122篇 |
矿业工程 | 11411篇 |
能源动力 | 50377篇 |
轻工业 | 118948篇 |
水利工程 | 15760篇 |
石油天然气 | 37551篇 |
武器工业 | 131篇 |
无线电 | 197269篇 |
一般工业技术 | 291008篇 |
冶金工业 | 191834篇 |
原子能技术 | 33992篇 |
自动化技术 | 171222篇 |
出版年
2021年 | 15584篇 |
2019年 | 14673篇 |
2018年 | 15810篇 |
2017年 | 14982篇 |
2016年 | 21396篇 |
2015年 | 17395篇 |
2014年 | 28759篇 |
2013年 | 88010篇 |
2012年 | 36374篇 |
2011年 | 48984篇 |
2010年 | 42776篇 |
2009年 | 50960篇 |
2008年 | 45783篇 |
2007年 | 43074篇 |
2006年 | 45601篇 |
2005年 | 40193篇 |
2004年 | 42450篇 |
2003年 | 42248篇 |
2002年 | 41343篇 |
2001年 | 38546篇 |
2000年 | 36737篇 |
1999年 | 36347篇 |
1998年 | 53502篇 |
1997年 | 44337篇 |
1996年 | 38971篇 |
1995年 | 32895篇 |
1994年 | 30621篇 |
1993年 | 30369篇 |
1992年 | 26758篇 |
1991年 | 24098篇 |
1990年 | 24223篇 |
1989年 | 23362篇 |
1988年 | 21983篇 |
1987年 | 20088篇 |
1986年 | 19555篇 |
1985年 | 22934篇 |
1984年 | 22750篇 |
1983年 | 20760篇 |
1982年 | 19562篇 |
1981年 | 19692篇 |
1980年 | 18329篇 |
1979年 | 18866篇 |
1978年 | 18058篇 |
1977年 | 18469篇 |
1976年 | 20735篇 |
1975年 | 16251篇 |
1974年 | 15763篇 |
1973年 | 15827篇 |
1972年 | 13313篇 |
1971年 | 11921篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
K. N. Subramanian J. G. Lee 《Journal of Materials Science: Materials in Electronics》2004,15(4):235-240
Properties of body centered tetragonal tin are highly anisotropic. As a consequence large stresses can develop at the tin grain boundaries due to coefficient of thermal expansion mismatch during temperature excursions. A modeling approach to evaluate the 3D stress states that develop at grain boundaries during thermomechanical fatigue in tin-based solder is presented. Development of significant amounts of stresses in the plane of the grain boundary can cause grain-boundary sliding and surface-relief effects, while those normal to the grain boundary can cause grain-boundary decohesion and cracking. 相似文献
992.
Hambleton P.J. Ng B.K. Plimmer S.A. David J.P.R. Rees G.J. 《Electron Devices, IEEE Transactions on》2003,50(2):347-351
The nonlocal enhancement in the velocities of charge carriers to ionization is shown to outweigh the opposing effects of dead space, increasing the avalanche speed of short avalanche photodiodes (APDs) over the predictions of a conventional local model which ignores both of these effects. The trends in the measured gain-bandwidth product of two short InAlAs APDs reported in the literature support this result. Relatively large speed benefits are predicted to result from further small reductions in the lengths of short multiplication regions. 相似文献
993.
The problem of operating freeze drying of pharmaceutical products in vials placed in trays of a freeze dryer to remove free water (in frozen state) at a minimum time was formulated as an optimal control problem. Two different types of freeze dryer designs were considered. In type I freeze dryer design, upper and lower plate temperatures were controlled together, while in type II freeze dryer design, upper and lower plate temperatures were controlled independently. The heat input to the material being dried and the drying chamber pressure were considered as control variables. Constraints were placed on the system state variables by the melting and scorch temperatures during primary drying stage. Necessary conditions of optimality for the primary drying stage of freeze drying process in vials are derived and presented. Furthermore, an approach for constructing the optimal control policies that would minimize the drying time for the primary drying stage was given. In order to analyze optimal control policy for the primary drying stage of the freeze-drying process in vials, a rigorous multi-dimensional unsteady state mathematical model was used. The theoretical approach presented in this work was applied in the freeze drying of skim milk. Significant reductions in the drying times of primary drying stage of freeze drying process in vials were obtained, as compared to the drying times obtained from conventional operational policies. 相似文献
994.
995.
必须利用水库多蓄水,另外再通过回灌把水蓄在含水层中供缺水时使用。城市废水也是一种重要水资源,但.必须加强规划和管理以防止造成对健康的负面影响,用于灌溉时则应防止污染地下水。长期以来地下水不仅受到非点源污染,还因对其开采超过回灌导致含水层减少。在灌区,地下水位必须通过排水或抽水系统加以控制,以防止水涝和土壤盐碱化。含盐的水排出后必须妥善处置。缺水的国家可以通过从其他国家进口大部分粮食和电力来节水,这往往比开发本国的水资源划算。本地水源就可用于更具社会、生态或经济回报的用途.或以备不时之需。 相似文献
996.
T. Choi J.‐H. Jang C.K. Ullal M.C. LeMieux V.V. Tsukruk E.L. Thomas 《Advanced functional materials》2006,16(10)
The probing of the micromechanical properties within a two‐dimensional polymer structure with sixfold symmetry fabricated via interference lithography reveals a nonuniform spatial distribution in the elastic modulus “imprinted” with an interference pattern in work reported by Tsukruk, Thomas, and co‐workers on p. 1324. The image prepared by M. Lemieux and T. Gorishnyy shows how the interference pattern is formed by three laser beams and is transferred to the solid polymer structure. The elastic and plastic properties within a two‐dimensional polymer (SU8) structure with sixfold symmetry fabricated via interference lithography are presented. There is a nonuniform spatial distribution in the elastic modulus, with a higher elastic modulus obtained for nodes (brightest regions in the laser interference pattern) and a lower elastic modulus for beams (darkest regions in the laser interference pattern) of the photopatterned films. We suggest that such a nonuniformity and unusual plastic behavior are related to the variable material properties “imprinted” by the interference pattern. 相似文献
997.
S. V. Doronin 《Chemical and Petroleum Engineering》2006,42(7-8):461-464
Current features are considered in the calculation of carrying capacities for constructions in engineering plant (EP). Methods
and algorithms are described for EP calculations with comprehensive incorporation of the effects from technological and working
defects on the behavior of structures under standard and emergency conditions.
__________
Translated from Khimicheskoe i Neftegazovoe Mashinostroenie, No. 8, pp. 38–40, August, 2006. 相似文献
998.
Suetsugu T. Kazimierczuk M.K. 《IEEE transactions on circuits and systems. I, Regular papers》2006,53(7):1468-1476
Design equations for satisfying the off-nominal operating condition [i.e., only the zero-voltage switching (ZVS) condition] of the Class-E amplifier with a linear shunt capacitance at a duty ratio D=0.5 are derived. A new parameter s (V/s), called the slope of switch voltage when the switch turns on is introduced to obtain an image of the distance from the nominal conditions. By examining off-nominal Class-E operation degree of the design freedom of the Class-E amplifier increases by one. In addition various amplifier parameters such as operating frequency, output power, and load resistance range can be set as design specifications. For example, the peak switch voltage and switch current can be taken into account in the design procedure. Examples of a design procedure of the Class-E amplifier for off-nominal operation are given. The theoretical results were verified with PSpice simulation and experiments. 相似文献
999.
Models that link ecological responses to hydrologic changes are important for assessing the effects of flow regulation on aquatic and riparian ecosystems. Based on the Recruitment Box Model, a graphical model used to prescribe environmental flows for cottonwood (Populus spp.) recruitment, we designed a simulation model to represent the influence of river flow dynamics on seedling recruitment of riparian pioneer woody plants. The model simulates the influence of temporal patterns of river stage on dispersal, germination, initial recruitment and over‐winter survival of first‐year seedlings of riparian pioneer shrubs and trees. We used the model to simulate seedling recruitment patterns for five species (Acer saccharinum, Betula nigra, Populus deltoides, Salix nigra and Salix exigua) on the Wisconsin River (Wisconsin, USA) under three flow scenarios: historic (1935–2002), simulated natural (1915–1975) and simulated regulated flows (1915–1975). Simulation results agreed well with field‐observed relative differences among years (1997–2000) in seedling densities for the five focal species. Simulated successful recruitment years were highly synchronous among species, but species differed in their sensitivity to flows at different times during the growing season, consistent with among‐species differences in seed dispersal timing. Comparison of simulated natural and regulated flows for 1915–1975 showed that flow regulation decreased monthly flow variability, increased late summer to winter baseflow and reduced the magnitude of spring peaks. Simulated recruitment and over‐winter survival of tree seedlings of all species was enhanced under the regulated flow scenario, likely due to increased summer baseflow and reductions in peak flood magnitude. Our analyses show the utility of extending the Recruitment Box Model to include multiple species of riparian shrubs and trees, and the effects of post‐colonization flows on their recruitment success. However, some key functional relationships between flow patterns and woody seedling demography (e.g. shear stress thresholds for seedling mortality) have not been adequately quantified and merit further study. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
1000.