全文获取类型
收费全文 | 249215篇 |
免费 | 1671篇 |
国内免费 | 267篇 |
专业分类
电工技术 | 4039篇 |
综合类 | 91篇 |
化学工业 | 40421篇 |
金属工艺 | 15158篇 |
机械仪表 | 10066篇 |
建筑科学 | 4177篇 |
矿业工程 | 3759篇 |
能源动力 | 3879篇 |
轻工业 | 9705篇 |
水利工程 | 4087篇 |
石油天然气 | 13007篇 |
武器工业 | 28篇 |
无线电 | 20201篇 |
一般工业技术 | 62806篇 |
冶金工业 | 32145篇 |
原子能技术 | 11139篇 |
自动化技术 | 16445篇 |
出版年
2021年 | 2759篇 |
2019年 | 2732篇 |
2018年 | 5435篇 |
2017年 | 5718篇 |
2016年 | 6050篇 |
2015年 | 3057篇 |
2014年 | 5335篇 |
2013年 | 10112篇 |
2012年 | 7325篇 |
2011年 | 8941篇 |
2010年 | 7304篇 |
2009年 | 8122篇 |
2008年 | 8184篇 |
2007年 | 7862篇 |
2006年 | 6362篇 |
2005年 | 5564篇 |
2004年 | 5481篇 |
2003年 | 5303篇 |
2002年 | 5134篇 |
2001年 | 4952篇 |
2000年 | 4942篇 |
1999年 | 4257篇 |
1998年 | 7177篇 |
1997年 | 5838篇 |
1996年 | 4426篇 |
1995年 | 3656篇 |
1994年 | 3422篇 |
1993年 | 3628篇 |
1992年 | 3214篇 |
1991年 | 3359篇 |
1990年 | 3483篇 |
1989年 | 3371篇 |
1988年 | 3268篇 |
1987年 | 3261篇 |
1986年 | 3158篇 |
1985年 | 3316篇 |
1984年 | 3254篇 |
1983年 | 3146篇 |
1982年 | 2993篇 |
1981年 | 3047篇 |
1980年 | 3072篇 |
1979年 | 3257篇 |
1978年 | 3490篇 |
1977年 | 3461篇 |
1976年 | 3912篇 |
1975年 | 3210篇 |
1974年 | 3293篇 |
1973年 | 3341篇 |
1972年 | 2987篇 |
1971年 | 2754篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
881.
Calcitonin, the serum calcium-lowering hormone, has been used in the treatment of hypercalcemia of malignancy and postmenopausal osteoporosis in humans for several years without any adverse effects. Recent studies in rats have indicated that calcitonin may be associated with morphologic effects on the pituitary. A large study was performed on 2 strains of rats, Sprague-Dawley (SD) and Fischer-344 (F-344), with 2 types of calcitonin, salmon-derived (sCT) and porcine-derived (pCT) calcitonin to evaluate possible effects on the pituitary. Sixteen groups of 42 male and 42 female SD or F-344 rats were given 0 (vehicle control), 1.25, 5.0, or 80.0 IU/kg/day of sCT or pCT, once daily, subcutaneously, for 1 yr. An increased incidence of adenomas of the adenohypophysis was observed in male SD rats at all dose levels of sCT, female SD rats given 80 IU/kg/day of sCT, male SD rats at the high dose level of pCT, and male F-344 rats at the high dose level of sCT. Also, an increased incidence of total proliferative lesions, due mostly to an increased incidence of focal hyperplasia of the pars distalis, occurred in female F-344 rats given the high dose of sCT. These pituitary proliferations were histologically similar to those that occur spontaneously, and the incidences observed were comparable to those that could occur in rats on 2-yr or lifetime studies, indicating that the injection of calcitonin had decreased the latency period. 相似文献
882.
883.
Ferrara K.W. Algazi V.R. 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》1994,41(2):185-198
In order to determine the limiting velocity resolution that can be achieved using ultrasound, and to provide a model which can be generalized for the analysis of disturbed flow, a theoretical and experimental evaluation of the statistics of the received signal from laminar flow following the transmission of a train of short pulses is presented. The authors derive the autocorrelation function and determine the length of the correlated signal for various flow rates, comparing experimental measurements to theoretical predictions. High resolution experimental RF M-mode images are used to verify the theoretical model. Using a fluid with a density, viscosity, volume concentration, particle size, and speed of sound which is similar to that of blood, the authors show that the signal remains correlated for a long interval under many conditions of clinical interest. Including a comparison with experimental data, the effect of the lateral transit time through the sample volume and the axial velocity spread within the sample volume on the correlation of the received signal is evaluated. When a significant range of velocity components is present within the sample volume, this range is the limiting factor in the length of the correlated signal interval. Therefore, the use of a wideband signal, which reduces the sample volume size, produces a returned signal that may be correlated for a larger number of pulses, or for a longer time 相似文献
884.
V. Carelli G. Di Colo E. Nannipieri M. F. Serafini 《Drug development and industrial pharmacy》1994,20(9):1673-1681
The enhancement effects of some chemicals on the percutaneous absorption of alprazolam through human skin was investigated in vitro. Linoleic acid, oleic acid, Comperlan F® (linoleic acid diethanolamide), Comperlan KD® (coconut fatty acid diethanolamide) and Ethomeen C12® (bis-(2-hydroxyethyl)cocamine) were evaluated for their enhancing effect either as neat solvents or combined with propylene glycol. The effects of skin pretreatment with the enhancers on the percutaneous absorption of alprazolam from a drug suspension in an aqueous gel were also investigated: skin pretreatment with some enhancers has shown potentially interesting aspects. The determination of alprazolam partitioning into untreated and pretreated horny layer supported the idea that, with oleic and linoleic acid, a contribution to the modification of the diffusional resistance of the skin was given by an increase of the drug solubility in the stratum coraeum. 相似文献
885.
A finite element partitioning scheme has been developed to reduce the computational costs of modeling electrically large geometries. In the partitioning scheme, the cylinder is divided into many sections. The finite element method is applied to each section independent of the other sections, and then the solutions in each section are coupled through the use of the tangential field continuity conditions between adjacent sections. Since the coupling matrix is significantly smaller than the original finite element matrix, it is expected that both the CPU time and memory costs can be significantly reduced. The partitioning scheme is coupled to the bymoment method to account for the boundary truncation. Numerical results are presented to demonstrate the efficiency and accuracy of the method 相似文献
886.
Bareikis V. Liberis J. Matulioniene I. Matulionis A. Sakalas P. 《Electron Devices, IEEE Transactions on》1994,41(11):2050-2060
Experimental results on noise temperature and spectral density of current fluctuations (electron diffusion) at high electric fields in silicon, gallium arsenide, and indium phosphide are presented. The dominant noise sources are discussed in their relation to electron scattering mechanisms. Physical backgrounds of high speed-low noise performance (noise-speed tradeoff) are considered. Suppression in short samples of the fluctuations having long correlation time constant and (or) high threshold energy is discussed 相似文献
887.
888.
889.
Leung V.C.M. Qian N. Malyan A.D. Donaldson R.W. 《Selected Areas in Communications, IEEE Journal on》1994,12(8):1376-1388
A distributed circuit-switched approach for supporting the deployment of high speed wireless personal communication services in urban areas through the interconnection of base stations via metropolitan area networks (MANs) is proposed. Broadband MANs minimize traffic congestion by dynamic sharing of link capacity and by serving as distributed switches for partitioning call control functions. While the DQDB protocol readily supports distributed packet-switching over the IEEE 802.6 MAN, isochronous traffic such as voice and video is best supported by circuit-switched connections. The authors present an enhanced bi-state pre-arbitrated (PA) transport mechanism, and associated call control and handoff management techniques, which enable distributed circuit-switching over the MAN. These capabilities are not currently addressed in the 802.6 standards. The bi-state PA transport mechanism facilitates statistical multiplexing of variable rate isochronous traffic sources. The network capacity is constrained by the call setup delay performance, and is analyzed by simulations. Alternative signaling architectures, involving different placements of call control network elements, are evaluated. The effects of erasure nodes, and close bus versus open bus architectures, are considered. The overlap inter-MAN call setup procedure is proposed to reduce delays. Different call handoff procedures are formulated according to the type of handoff and the resulting change in call connectivity. Most handoffs are intra-MAN, requiring simple procedures with short delays 相似文献
890.
Tsong-Ho Wu Kobrinski H. Ghosal D. Lakshman T.V. 《Selected Areas in Communications, IEEE Journal on》1994,12(1):79-87
The viability of distributed control restoration using digital cross-connect systems (DCS) depends on its capability for restoring services within specified time requirements, and its economics for providing restoration compared to other alternatives. The authors report a Bellcore study for the impact of the DCS architecture on distributed restoration. This study concludes that currently proposed distributed control DCS self-healing schemes may not meet the 2 second restoration objective for large metropolitan local exchange carrier's networks, regardless of the distributed algorithm used, if the present DCS system architecture which uses serial message processing and serial path cross-connection remains unchanged. They also discuss several DCS architecture enhancement options, including a parallel processing/cross-connect DCS architecture, which may improve the service restoration time 相似文献