全文获取类型
收费全文 | 74345篇 |
免费 | 1855篇 |
国内免费 | 1728篇 |
专业分类
电工技术 | 1776篇 |
综合类 | 779篇 |
化学工业 | 10754篇 |
金属工艺 | 5308篇 |
机械仪表 | 2924篇 |
建筑科学 | 2147篇 |
矿业工程 | 526篇 |
能源动力 | 2490篇 |
轻工业 | 3989篇 |
水利工程 | 665篇 |
石油天然气 | 2510篇 |
武器工业 | 77篇 |
无线电 | 8191篇 |
一般工业技术 | 17253篇 |
冶金工业 | 11382篇 |
原子能技术 | 1112篇 |
自动化技术 | 6045篇 |
出版年
2023年 | 335篇 |
2022年 | 671篇 |
2021年 | 992篇 |
2020年 | 715篇 |
2019年 | 864篇 |
2018年 | 1314篇 |
2017年 | 1329篇 |
2016年 | 1430篇 |
2015年 | 1267篇 |
2014年 | 1894篇 |
2013年 | 4349篇 |
2012年 | 2857篇 |
2011年 | 3863篇 |
2010年 | 3088篇 |
2009年 | 3473篇 |
2008年 | 3369篇 |
2007年 | 3446篇 |
2006年 | 3038篇 |
2005年 | 2715篇 |
2004年 | 2280篇 |
2003年 | 2076篇 |
2002年 | 1977篇 |
2001年 | 1990篇 |
2000年 | 1805篇 |
1999年 | 2000篇 |
1998年 | 3802篇 |
1997年 | 2847篇 |
1996年 | 2495篇 |
1995年 | 1667篇 |
1994年 | 1335篇 |
1993年 | 1220篇 |
1992年 | 909篇 |
1991年 | 858篇 |
1990年 | 728篇 |
1989年 | 673篇 |
1988年 | 510篇 |
1987年 | 491篇 |
1986年 | 438篇 |
1985年 | 442篇 |
1984年 | 353篇 |
1983年 | 335篇 |
1982年 | 341篇 |
1981年 | 333篇 |
1980年 | 371篇 |
1979年 | 354篇 |
1978年 | 303篇 |
1977年 | 427篇 |
1976年 | 653篇 |
1975年 | 322篇 |
1973年 | 321篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
We report a case of pulmonary carcinosarcoma. Surgery is required for this rare mixed type, biphasic tumor. It is generally considered to be a malignant formation issuing from a single cell line but with a two-way differentiation into epithelial and conjunctive components. There is a differentiation continuum between spindle-cell carcinomas (also called monophasic sarcomatoid carcinoma) and carcinosarcoma (or biphasic sarcomatoid carcinoma) leading to debate concerning the immunohistochemical and ultrastructural features. 相似文献
122.
Sverak J.G. Wang W. Gervais Y. Do X.-D. Mukhedkar D. 《Power Delivery, IEEE Transactions on》1992,7(3):1196-1206
An improved multivariable, probabilistic method for the safety analysis of power grounding systems is presented. The application of this method is demonstrated in four examples. The first three examples illustrate the practical evaluation of one new and two existing substation grounding grids when the substation is connected to an overhead distribution system. The fourth example ascertains the relative impact of a difference between the S curve of IEC Publication 479 and Dalziel's equation for the allowable body current used in ANSI/IEEE Std 80-1986 on the validity of an exposure model based on the requirements of Std 80. In this example, the statistical data on ground faults have been taken from actual system records. Further possible refinements of the exposure model are discussed 相似文献
123.
124.
Time variation of drinking is substantial and has an effect on aggregate estimates of consumption. In this article it is shown that because of a considerable seasonal variation in consumption (+/- 20%) a serious bias in annual consumption estimates can be expected in surveys with a limited time frame. The present study analyzes drinking data collected in the general population of the Netherlands from March 1985 through December 1985 (including Christmas and New Year's Eve). Since it was expected that sensitivity to temporal fluctuations might not be equal for different methods of measurement, several indices of consumption were compared. Although the assessed seasonal effect varies indeed across types of measurement, across male and female subsample and across types of alcoholic beverage, the general tendency is for consumption to be highest in the spring season and lowest in the autumn. Sales figures fluctuate accordingly. It is evident that the risk of biased estimates is larger the shorter the time frame of the survey. Seasonal variation was highest in the frequency domain. Furthermore, exclusion from the time frame of collective holidays, during which people drink more often and more per occasion (viz., Christmas), increases the risk of biased estimates. Even estimates of abstention, but also regular heavy drinking among women, appear to vary considerably over the three seasons in this study. The main conclusion is that results of comparisons of survey data on drinking, particularly those over time, are more or less invalid if the respective time frames of the surveys do not correspond. 相似文献
125.
Han-Bin Lin Jung-Young Su Yu-Pin Liao Way-Seen Wang 《Quantum Electronics, IEEE Journal of》1995,31(6):1131-1139
By using micro-prisms, improved three-dimensional (3-D) bends of the embedded and buried waveguides of step-index profile are proposed. A simple phase compensation rule for the optimal design of the micro-prism is also presented. Through the simulation of 3-D semivectorial finite-difference beam propagation method, the transmission characteristics of the improved bends are shown to have been enhanced dramatically as compared with those of the conventional ones. Even for a bend angle of as large as 10°, the normalized transmitted power can still be greater than 95%. These results of 3-D bends are then compared with those of the two-dimensional (2-D) ones which are simplified from 3-D structures by the effective index method, and physical explanation of the discrepancy between the 3-D and 2-D results is introduced. The influences of waveguide structures and prism parameters on the transmission characteristics are discussed in detail. Some criteria for the design of large-angle low-loss 3-D improved bends are also accessed 相似文献
126.
Kollipara R.T. Arodzero A. Bashindzhagyan G. Brau J.E. Frey R. Gao D. Mason D. Sinev N. Strom D. Yang X. 《IEEE transactions on nuclear science》1995,42(2):92-101
The SSC GEM silicon Central Tracker design incorporated 18-cm long single-sided AC-coupled silicon microstrip ladders. Compared to the 12-cm long ladders considered in the preliminary stages of the tracker design, the 18-cm long ladders have the advantage of reduced cost, channel count and overall power consumption, and led to a simplified tracker assembly. However, such long ships also present the challenge of maintaining satisfactory performance. The increased capacitance and series resistance contribute to lower signal-to-noise ratios, longer time walk, higher power consumption per channel and increased probability of crosstalk to neighboring channels. In this paper, an accurate method to calculate the geometric capacitance of the AC-coupled microstrips is presented and the calculated results are compared with measurements, SPICE simulations are performed to predict the noise, the extent of interstrip capacitive coupling and the dispersion of the detector signal due to the finite series resistance of the metal strips and the long length of the detector. The influence of the preamplifier current and the shaping time on the signal and noise levels is also presented. The study concludes that the 18-cm long ladders can successfully satisfy the performance goals of the GEM silicon Central Tracker 相似文献
127.
Recent clinical and laboratory evidence indicates that Meniere's disease is an immune-mediated disease. Dexamethasone perfusion of the inner ear through the round window plus intravenous dexamethasone often will stop the dizzy spells, reduce the fullness and low-frequency tinnitus, and sometimes improve the hearing in patients with Meniere's disease. The dexamethasone must act mostly on the endolymphatic sac and, to a lesser extent, on the stria vascularis and spiral ligament, the known targets of immune response in the inner ear, to reduce the endolymphatic hydrops and restore the fluid dynamics of the endolymph. Despite the good results with streptomycin perfusion, the number of patients with further hearing loss is large, so dexamethasone perfusion with intravenous dexamethasone should be tried first. The initial response to dexamethasone perfusion plus intravenous dexamethasone has been very good, with very little risk of further hearing loss, and it holds great promise for the future. 相似文献
128.
1.引言 在人工智能领域中,产生式系统是一种比较有效的知识表示方法,并得到了广泛应用。目前许多较为成功的专家系统都是用产生式系统实现的~[1]。随着应用领域的不断扩士其知识座的抓橄称渐增 相似文献
129.
Singular-value-decomposition (SVD)-based moving-average (MA) order determination of non-Gaussian processes using higher-order statistics is addressed. It is shown that the MA order determination of autoregressive moving-average (ARMA) models is equivalent to the rank determination of a certain error matrix, and a SVD approach is proposed. Its simplified form is applied to pure MA models. To improve the robustness of the order selection, a combination of the SVD and the product of diagonal entries (PODE) test is proposed. Some interesting applications of the two SVD approaches are presented, and simulations verify their performance 相似文献
130.
Performance model of interactive video-on-demand systems 总被引:5,自引:0,他引:5
Li V.O.K. Wanjiun Liao Xiaoxin Qiu Wong E.W.M. 《Selected Areas in Communications, IEEE Journal on》1996,14(6):1099-1109
An interactive video-on-demand (VoD) system allows users to access video services, such as movies, electronic encyclopedia, interactive games, and educational videos from video servers on a broadband network. This paper develops a performance evaluation tool for the system design. In particular, a user activity model is developed to describe the usage of system resources, i.e., network bandwidth and video server usage, by a user as it interacts with the service. In addition, we allow batching of user requests, and the effect of such batching is captured in a batching model. Our proposed queueing model integrates both the user activity and the batching model. This model can be used to determine the requirements of network bandwidth and video server and, hence, the trade-off in communication and storage costs for different system resource configurations 相似文献