全文获取类型
收费全文 | 2580篇 |
免费 | 151篇 |
国内免费 | 10篇 |
专业分类
电工技术 | 22篇 |
综合类 | 4篇 |
化学工业 | 617篇 |
金属工艺 | 58篇 |
机械仪表 | 37篇 |
建筑科学 | 95篇 |
矿业工程 | 6篇 |
能源动力 | 96篇 |
轻工业 | 299篇 |
水利工程 | 18篇 |
石油天然气 | 18篇 |
无线电 | 413篇 |
一般工业技术 | 446篇 |
冶金工业 | 230篇 |
原子能技术 | 16篇 |
自动化技术 | 366篇 |
出版年
2024年 | 5篇 |
2023年 | 57篇 |
2022年 | 101篇 |
2021年 | 187篇 |
2020年 | 113篇 |
2019年 | 120篇 |
2018年 | 127篇 |
2017年 | 108篇 |
2016年 | 179篇 |
2015年 | 78篇 |
2014年 | 115篇 |
2013年 | 165篇 |
2012年 | 137篇 |
2011年 | 158篇 |
2010年 | 120篇 |
2009年 | 100篇 |
2008年 | 102篇 |
2007年 | 77篇 |
2006年 | 53篇 |
2005年 | 53篇 |
2004年 | 39篇 |
2003年 | 35篇 |
2002年 | 42篇 |
2001年 | 32篇 |
2000年 | 32篇 |
1999年 | 24篇 |
1998年 | 77篇 |
1997年 | 51篇 |
1996年 | 31篇 |
1995年 | 23篇 |
1994年 | 17篇 |
1993年 | 19篇 |
1992年 | 19篇 |
1991年 | 12篇 |
1990年 | 15篇 |
1989年 | 7篇 |
1988年 | 8篇 |
1987年 | 10篇 |
1986年 | 9篇 |
1985年 | 6篇 |
1984年 | 6篇 |
1983年 | 9篇 |
1982年 | 12篇 |
1981年 | 9篇 |
1980年 | 6篇 |
1979年 | 7篇 |
1978年 | 6篇 |
1977年 | 7篇 |
1976年 | 6篇 |
1972年 | 2篇 |
排序方式: 共有2741条查询结果,搜索用时 15 毫秒
991.
An extended version of the spectral domain approach (SDA) is developed to analyze discontinuities in open coplanar waveguide with finite metallization thickness. By making use of the exact Green's function in the spectral domain, the effects of surface wave and radiation phenomena are accurately accounted for. Both longitudinal and transverse components of the aperture electric fields are used in the analysis to allow modelling of structures with large transverse dimensions at high frequencies. The procedure also includes mode conversion near the discontinuities. As an illustration of the method, analytical steps and computed scattering parameters of the coplanar waveguide short-circuits and transitions are provided and compared against measured data 相似文献
992.
Peptide metabolites of angiotensin I and II are active components of the renin-angiotensin system. One such peptide is angiotensin-(1-7), which has been shown to be present in various tissues and has properties distinct from those of angiotensin II. We examined the effects of angiotensin-(1-7) on endothelium-intact and denuded rat aorta. Second, we evaluated whether an interaction occurred between angiotensin-(1-7) and angiotensin peptides, as well as noradrenaline. Finally, we addressed whether the responses to angiotensin-(1-7) were mediated by an AT1 receptor. Angiotensin-(1-7) produced concentration-dependent relaxations of the rat aorta that were significantly greater in endothelium-intact preparations (81.1 +/- 18.9% and 29.6 +/- 2.9% for intact and denuded, respectively). Angiotensin-(1-7) inhibited responses generated to angiotensin I, II, III, and noradrenaline. In endothelium-denuded preparations, angiotensin-(1-7) produced a rightward shift of the concentration-effect curves to angiotensin II and noradrenaline. In addition, the inhibition against angiotensin I and II was significantly greater in endothelium-intact preparations [mean median inhibitory concentration (IC50) values for endothelium-intact preparations, 1.25 x 10(-9) M and 1.57 x 10(-9) M for angiotensin I and II, respectively; and for endothelium-denuded preparations, 1.77 x 10(-8) M and 1.17 x 10(-8) M for angiotensin I and II, respectively). Losartan did not affect relaxations in endothelium-intact preparations but caused a significant potentiation of the relaxation by angiotensin-(1-7) in denuded preparations. We conclude that angiotensin-(1-7) is a component of the renin-angiotensin system that acts to modulate the pressor effects of angiotensin II and noradrenaline. 相似文献
993.
The energetic parameters of Escherichia coli were analyzed for the aerobic/anaerobic transition. The electrochemical proton potential (delta p) across the cytoplasmic membrane was determined in the steady state of respiration with O2, nitrate, fumarate, dimethylsulfoxide (Me2SO), and for fermentation. With O2, a proton potential of -160 mV was obtained. For anaerobic respiration with nitrate, fumarate or Me2SO, delta p decreased only slightly by about 20 mV in contrast to earlier assumptions, whereas delta p dropped by approximately 40 mV during fermentation. Under all conditions, the membrane potential (delta psi) contributed the major portion to delta p. The cellular ATP levels were highest for aerobic growth (about 13 micromol/g dry cells) and decreased to 3-6 micromol/g in anaerobic metabolism. Delta G'Phos, however, was constant due to equivalent changes of the ADP contents. Transition to the stationary growth phase caused a massive drop in the ATP content. It is concluded that, during anaerobic respiration, the energetic situation for the bacteria is very similar to that for aerobic growth with respect to delta G'Phos and delta p whereas, for fermentation, a significant decrease in delta p was observed. The consequences for the cellular energetics and for the regulation of the aerobic/anaerobic transition are discussed. 相似文献
994.
Thin films of SnSe2 were prepared by vacuum evaporation. A two-source evaporation technique was used to prepare doped thin films.The variation in the resistivity with the preparation conditions and with temperature (100–500 K) was studied. Thin films of SnSe2 were shown to be stable below 130°C. By doping with tin impurities we were able to reduce the resistivity to a few ohm centimetres.The high resistivity after annealing or heat treatments nd in the case of high substrate temperatures was interpreted as due to ionization and redistribution of impurities and to a partial transformation of SnSe2 to SnSe. 相似文献
995.
The production capacity of a solar still which converts saline water to fresh water can be increased by introducing hot feed water into the unit at night. A waste heat source, such as cooling water from a power plant, can be used to preheat the feed. The nocturnal production, i.e. the distilled water produced at night, seems to be influenced by several parameters. However, a simplified mathematical model suggests that the distillate depends only on the initial brine temperature, the drop in brine temperature and the brine depth. This was experimentally verified for different brine depths and for initial brine temperatures up to 150°F. 相似文献
996.
This paper presents two new amplifier placement methods to minimize the number of amplifiers in metropolitan wavelength-division-multiplexing (WDM) rings based on integer programming techniques. The first method describes the amplifier placement problem exactly and uses a nonlinear programming solver to obtain a solution. The second method approximates some requirements in the problem and employs a linear programming solver to derive the amplifier placement solution. A new amplifier placement method for self-healing WDM rings is also reported in this paper. The new method is based on iteratively solving an amplifier placement problem for a ring network under different link failure scenarios. The solution provides a minimum number of amplifiers required to operate the self-healing ring under a normal or any single-link or single-node failure conditions. 相似文献
997.
The atmospheric corrosion of copper by hydrogen sulphide has been followed during field tests using different sulphide concentrations, for 77 days, and during exposure to a well-defined synthetic atmosphere in a test chamber. The main components formed at the surface of the samples are copper oxides and sulphide. We show that exposure tests performed for short times in synthetic atmospheres cannot be extrapolated to long time exposure in real conditions, since three successive phases of film growth are observed. The mechanism of film growth is discussed, assuming that cationic diffusion through the cuprous oxides and sulphide is the rate-determining step. 相似文献
998.
Milk quotas, based on an average fat content, severely limit milk production on UK farms. Predictions of the time-course of lactation are incorporated into most computerized herd management programs but these models take no account of food inputs, body weight change or milk composition. Dynamic models are generally used to simulate metabolic pathways and, as such, have little direct relevance to commercial milk production. Dynamic models can be converted to an adaptive-predictive model that partitions food energy into milk and non-milk constituents. This paper reports the development of an adaptive-predictive model to partition food into milk and non-milk components. Additional functions further partition milk energy into the principal constituents, fat, protein and lactose. 相似文献
999.
Hwa-Joon Oh Mueller S.M. Jacobi C. Tran K.D. Cottier S.R. Michael B.W. Nishikawa H. Totsuka Y. Namatame T. Yano N. Machida T. Dhong S.H. 《Solid-State Circuits, IEEE Journal of》2006,41(4):759-771
The floating-point unit (FPU) in the synergistic processor element (SPE) of a CELL processor is a fully pipelined 4-way single-instruction multiple-data (SIMD) unit designed to accelerate media and data streaming with 128-bit operands. It supports 32-bit single-precision floating-point and 16-bit integer operands with two different latencies, six-cycle and seven-cycle, with 11 FO4 delay per stage. The FPU optimizes the performance of critical single-precision multiply-add operations. Since exact rounding, exceptions, and de-norm number handling are not important to multimedia applications, IEEE correctness on the single-precision floating-point numbers is sacrificed for performance and simple design. It employs fine-grained clock gating for power saving. The design has 768K transistors in 1.3 mm/sup 2/, fabricated SOI in 90-nm technology. Correct operations have been observed up to 5.6 GHz with 1.4 V and 56/spl deg/C, delivering 44.8 GFlops. Architecture, logic, circuits, and integration are codesigned to meet the performance, power, and area goals. 相似文献
1000.
The photocrystallization of amorphous GexSe1?x films (0 ? x < 0.2) was investigated using electrical conductivity and optical microscopy measurements. For Ge0.05Se0.95 films the crystallization effect is enhanced slightly and the nucleation for crystallization is found to be heterogeneous. For other compositions the crystallization effect is suppressed and nucleation is found to be homogeneous. 相似文献