全文获取类型
收费全文 | 613181篇 |
免费 | 7966篇 |
国内免费 | 2062篇 |
专业分类
电工技术 | 11481篇 |
综合类 | 524篇 |
化学工业 | 91259篇 |
金属工艺 | 21894篇 |
机械仪表 | 17722篇 |
建筑科学 | 16498篇 |
矿业工程 | 1666篇 |
能源动力 | 16966篇 |
轻工业 | 58453篇 |
水利工程 | 5083篇 |
石油天然气 | 4880篇 |
武器工业 | 27篇 |
无线电 | 80767篇 |
一般工业技术 | 115913篇 |
冶金工业 | 115604篇 |
原子能技术 | 9733篇 |
自动化技术 | 54739篇 |
出版年
2021年 | 4503篇 |
2020年 | 3188篇 |
2019年 | 4045篇 |
2018年 | 6573篇 |
2017年 | 6672篇 |
2016年 | 7081篇 |
2015年 | 5285篇 |
2014年 | 8890篇 |
2013年 | 28581篇 |
2012年 | 15053篇 |
2011年 | 21358篇 |
2010年 | 16723篇 |
2009年 | 19049篇 |
2008年 | 20359篇 |
2007年 | 20577篇 |
2006年 | 18349篇 |
2005年 | 16775篇 |
2004年 | 16294篇 |
2003年 | 16177篇 |
2002年 | 15431篇 |
2001年 | 15873篇 |
2000年 | 14713篇 |
1999年 | 15612篇 |
1998年 | 37308篇 |
1997年 | 26689篇 |
1996年 | 20839篇 |
1995年 | 16062篇 |
1994年 | 14412篇 |
1993年 | 13946篇 |
1992年 | 10300篇 |
1991年 | 9858篇 |
1990年 | 9403篇 |
1989年 | 8942篇 |
1988年 | 8694篇 |
1987年 | 7334篇 |
1986年 | 7239篇 |
1985年 | 8532篇 |
1984年 | 7956篇 |
1983年 | 7135篇 |
1982年 | 6665篇 |
1981年 | 6565篇 |
1980年 | 6222篇 |
1979年 | 5962篇 |
1978年 | 5527篇 |
1977年 | 6751篇 |
1976年 | 9132篇 |
1975年 | 4608篇 |
1974年 | 4455篇 |
1973年 | 4282篇 |
1972年 | 3587篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
931.
Here, we report investigations about the direct effect of glycosaminoglycans, such as dermatan sulfate, chondroitin 4- and 6-sulfate upon cAMP-dependent protein kinase activity. The results indicate that glycosaminoglycans strongly influence the phosphorylation activity of this enzyme against histone type IIa and [Val6, Ala7]-kemptide. While chondroitin 4-sulfate and dermatan sulfate exhibit inhibitory effects, chondroitin 6-sulfate shows a stimulating effect. In addition, the chondroitin 6-sulfate is also able to reduce the chondroitin 4-sulfate and dermatan sulfate specific inhibition. 相似文献
932.
C. A. Pineda P. Wenzl J. Mayer J. Mesjasz-Przybylowicz W. J. Przybylowicz V. M. Prozesky 《Nuclear instruments & methods in physics research. Section B, Beam interactions with materials and atoms》1997,130(1-4):362-367
A Brachiaria breeding project is being conducted at CIAT, Colombia to combine acid soil adaptation of B. decumbens with other favourable traits of Brachiaria species that are less adapted to acid soils. Micro-PIXE was applied to investigate nutrient uptake and distribution in root tips of different species grown in hydroponic culture under control and simulated acid soil stress conditions. Different sample measurement approaches were evaluated, including: (1) linear scans with single point measurements along the root axis; (2) mapping of whole root tips; and (3) mapping of root cross sections. Different tissue types could be distinguished on the base of differences in nutrient concentrations and/or Al stress. Al, if supplied under nutrient stress conditions, increased P accumulation in the central vascular tissue of the meristematic and elongation zone of B. decumbens and B. brizantha. Furthermore, a negative correlation was found between Al and Cl accumulation in the root cap of B. decumbens. 相似文献
933.
Nitric oxide has been shown to decrease myocardial contractility and O2 consumption. This study was designed to evaluate the hypothesis that nitric oxide-mediated increases in cyclic GMP require elevated cyclic AMP to produce cardiac depression. Using isolated, Langendorff-perfused rat hearts, we determined the effects of intracoronary nitroprusside (NP, 1 and 10 mM) in the absence and presence of isoproterenol (ISO, 10(-8) M) on cardiac function, O2 consumption, cyclic GMP and cyclic AMP. ISO, with and without NP, increased cyclic AMP (from 287 +/- 21 to 477 +/- 33 pmol/g) without altering cyclic GMP. Left-ventricular pressure increased from 97 +/- 12 to 178 +/- 9 mm Hg and dP/dtmax from 1,786 +/- 275 to 4,049 +/- 354 mm Hg/s. NP increased cyclic GMP (from 4 to 30 pmol/g) in both the absence and presence of ISO, but NP did not alter cyclic AMP. Without ISO, NP insignificantly altered left-ventricular pressure; however, in the presence of ISO, NP significantly decreased left-ventricular pressure by -25 +/- 4 mm Hg and decreased dP/dtmax by -619 +/- 142 mm Hg/s. Isoproterenol increased O2 consumption, but the changes with NP were not significant. When this study was repeated in the presence of LY83583, a guanylate cyclase inhibitor, NP still produced cardiac depression in the presence of ISO. Therefore, cardiodepressant effects of NP were only observed against a background of inotropic stimulation with ISO. However, effects of NP on contractility were unrelated to increases in cyclic GMP or cyclic GMP-induced changes in cyclic AMP. 相似文献
934.
I Tamai T Nakanishi K Hayashi T Terao Y Sai T Shiraga K Miyamoto E Takeda H Higashida A Tsuji 《Canadian Metallurgical Quarterly》1997,49(8):796-801
Although recent evidence suggests that certain beta-lactam antibiotics are absorbed via a specific transport mechanism, its nature is unclear. To confirm whether peptide transport in the rat can be largely ascribed to the intestinal oligopeptide transporter PepT1, the transporter has been functionally characterized and its significance in the intestinal absorption of beta-lactam antibiotics was evaluated. For evaluation of transport activity complementary RNA (cRNA) of rat PepT1 was synthesized in-vitro and expressed in Xenopus laevis oocytes. cRNA induced uptake of several beta-lactam antibiotics and the dipeptide [14C]glycylsarcosine; this was specifically inhibited by various dipeptides and tripeptides but not by their constituent amino acids or by tetra- or pentapeptides. The transport activity of PepT1 for beta-lactam antibiotics correlated well with their in-vivo intestinal transport and absorption. Furthermore, mutual inhibitory effects on uptake were observed between glyclsarcosine and beta-lactam antibiotics. Hybrid depletion of the functional expression of rat PepT1 in oocytes injected with rat intestinal epithelial total mRNA was studied using an antisense oligonucleotide corresponding to the 5'-coding region of PepT1. In oocytes injected with rat mRNA pre-hybridized with the antisense oligonucleotide against rat PepT1, the uptake of [14C]glycylsarcosine was almost completely abolished, whereas its uptake was not influenced by a sense oligonucleotide for the same region of PepT1. Similarly, the uptake of beta-lactam antibiotics was also reduced by the antisense oligonucleotide against rat PepT1. These results demonstrate that the intestinal proton-coupled oligopeptide transporter PepT1 plays a predominant role in the carrier-mediated intestinal absorption of beta-lactam antibiotics and native oligopeptides in the rat. 相似文献
935.
Scale selection using three different representations for images 总被引:1,自引:0,他引:1
J. Martinez-Baena J. A. García J. Fdez-Valdivia Rosa Rodriguez-Sanchez 《Pattern recognition letters》1997,18(14):1453-1467
936.
937.
Mehra A. Indiresan A. Shin K.G. 《IEEE transactions on pattern analysis and machine intelligence》1997,23(10):616-634
We propose architectural mechanisms for structuring host communication software to provide QoS guarantees. We present and evaluate a QoS sensitive communication subsystem architecture for end hosts that provides real time communication support for generic network hardware. This architecture provides services for managing communication resources for guaranteed QoS (real time) connections, such as admission control, traffic enforcement, buffer management, and CPU and link scheduling. The architecture design is based on three key goals: maintenance of QoS guarantees on a per connection basis, overload protection between established connections, and fairness in delivered performance to best effort traffic. Using this architecture we implement real time channels, a paradigm for real time communication services in packet switched networks. The proposed architecture features a process per channel model that associates a channel handler with each established channel. The model employed for handler execution is one of “cooperative” preemption, where an executing handler yields the CPU to a waiting higher priority handler at well defined preemption points. The architecture provides several configurable policies for protocol processing and overload protection. We present extensions to the admission control procedure for real time channels to account for cooperative preemption and overlap between protocol processing and link transmission at a sending host. We evaluate the implementation to demonstrate the efficacy with which the architecture maintains QoS guarantees on outgoing traffic while adhering to the stated design goals 相似文献
938.
939.
RF Speck K Wehrly EJ Platt RE Atchison IF Charo D Kabat B Chesebro MA Goldsmith 《Canadian Metallurgical Quarterly》1997,71(9):7136-7139
The chemokine receptor CCR5 acts as an essential cofactor for cell entry by macrophage-tropic human immunodeficiency virus type 1 (HIV-1) strains, whereas CXCR4 acts as an essential cofactor for T-cell-line-adapted strains. We demonstrated that the specific amino acids in the V3 loop of the HIV-1 envelope protein that determine cellular tropism also regulate chemokine coreceptor preference for cell entry by the virus. Further, a strong correlation was found between HIV-1 strains classified as syncytium inducing in standard assays and those using CXCR4 as a coreceptor. These data support the hypothesis that progressive adaptation to additional coreceptors is a key molecular basis for HIV-1 phenotypic evolution in vivo. 相似文献
940.
In the present investigation, the process model developed in Part I has been applied to evaluate the microstructure and strength
evolution during active brazing of ceramics. As a starting point, reaction-layer growth is assumed to occur isothermally with
no restrictions in the supply of reactive element. Different kinds of diagrams are then constructed to show how specific process
variables (e.g. the heating and cooling period, the limiting layer thickness, and the diffusion mechanism) affect the growth
kinetics. It is concluded that the key to improved joint properties lies in control of the reaction-layer thickness through
optimization of the brazing conditions, and an illustration of this is given.
This revised version was published online in November 2006 with corrections to the Cover Date. 相似文献