首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   414727篇
  免费   5859篇
  国内免费   1383篇
电工技术   7726篇
综合类   352篇
化学工业   62227篇
金属工艺   14334篇
机械仪表   11908篇
建筑科学   11693篇
矿业工程   877篇
能源动力   11149篇
轻工业   43895篇
水利工程   3325篇
石油天然气   1836篇
武器工业   13篇
无线电   55563篇
一般工业技术   76798篇
冶金工业   75872篇
原子能技术   5575篇
自动化技术   38826篇
  2021年   2678篇
  2019年   2443篇
  2018年   3856篇
  2017年   3922篇
  2016年   4165篇
  2015年   3441篇
  2014年   5735篇
  2013年   19346篇
  2012年   10232篇
  2011年   14723篇
  2010年   11323篇
  2009年   12921篇
  2008年   13964篇
  2007年   14336篇
  2006年   12836篇
  2005年   12011篇
  2004年   11618篇
  2003年   11314篇
  2002年   10949篇
  2001年   11170篇
  2000年   10303篇
  1999年   10821篇
  1998年   24223篇
  1997年   17726篇
  1996年   14001篇
  1995年   11042篇
  1994年   9833篇
  1993年   9410篇
  1992年   7150篇
  1991年   6809篇
  1990年   6430篇
  1989年   6144篇
  1988年   5896篇
  1987年   4913篇
  1986年   4862篇
  1985年   6007篇
  1984年   5626篇
  1983年   4858篇
  1982年   4525篇
  1981年   4470篇
  1980年   4228篇
  1979年   4100篇
  1978年   3784篇
  1977年   4578篇
  1976年   6161篇
  1975年   3130篇
  1974年   3020篇
  1973年   2906篇
  1972年   2328篇
  1971年   2021篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
We present the results of an evaluation of the performance characteristics of a composite multivariate quality control (CMQC) system that incorporates quality control rules for univariate, multivariate, and correlation conditions. The CMQC system evaluated is designed to help analysts detect unacceptable trends and systematic error in one or more variables, unacceptable random error in one or more variables, and unacceptable changes in the correlation structure of any pair of variables. It is also designed to be tolerant of missing data, to allow analysts to reject as few as one or as many as all variables in a run, and to provide analysts with control statistics and graphics that logically relate to sources of analytical error. We show that the various components of the CMQC system have adequate statistical power to detect systematic errors, random errors, and correlation changes under the conditions likely to be encountered with multivariate analytical measurement systems: (1) a single variable with increased systematic or random error; (2) all variables or a subgroup of variables affected by a common problem that increases systematic or random error; and (3) missing data for one or more variables in a run. We also show that the power of the multivariate component of the CMQC system to detect systematic and random errors is higher than the power of an alternative multivariate test criterion.  相似文献   
992.
A procedure that facilitates at least-squares synthesis of periodic, discrete-time signals from Wigner quasi-distributions is proposed. The scheme is based on expanding the desired time sequence on a generally nonorthogonal, Gabor-type basis whose associated biorthogonal function presumably exists. The specific basis selection may crucially affect the efficiency and quality of the ensuing synthesis procedure. The cited basis type constitutes a considerable generalization over the standardly used orthogonal variety, thus creating previously unavailable degrees of freedom. Of primary significance is the acquired capability of generating time-frequency basis functions that are well localized. Localization is a highly desirable property that can advantageously serve in various applications. It is shown and numerically demonstrated that benefits of localization as well as the fact that achieving effective time-frequency basis localization renders a certain degree of oversampling unavoidable  相似文献   
993.
A probabilistic approach to fracture mechanics in the form of a typical case study is described wherein the integrity of a high-pressure water pipeline is assessed. An analysis methodology is discussed incorporating the probability density functions of defect sizes, the statistics of defect occurrences and the statistical distribution of material properties. This method enables the analyst to supply a very simple assessment of safety, based on the probability of failure (a single number) which may be compared to accepted industrial standards (e.g. 10−6 for nuclear applications). It is argued that this method often offers the only way to scientifically and economically assess the integrity of fracture prone structures.  相似文献   
994.
This paper presents the results of laboratory measurements of relative permeability for two phase flow in partially saturated samples of unconsolidated sands. Relative permeabilities for tap water and air, de-aired water and nitrogen were measured using a steady—state technique. Three different sand samples (fine sand, heterogeneous sand and coarse sand) were tested.  相似文献   
995.
Eight countries collaborated and shared technical information on the harvesting of small trees and forest residues in a three year program. Proceedings and reports from workshops and reviews are summarized in a review of activities and harvesting systems of the participating countries. Four databases were developed for harvesting and transportation of these materials.  相似文献   
996.
The authors have developed a 2-D device simulator for heterostructure metal-semiconductor-metal (MSM) photodetectors. They have incorporated a model of multilayer optics into the simulator and used it to analyze the temporal response of a resonant-cavity enhanced heterostructure with a confining buffer layer and a distributed Bragg reflector (DBR). The authors show that through fine tuning the layer thicknesses, optical resonance enhancement of the light absorption can be obtained  相似文献   
997.
An efficient hypersingular boundary integral equation method for three-dimensional fracture mechanics was presented in a previous paper. The details of the numerical implementation of this method are further discussed herein. In particular, an algorithm for achieving the required differentiability of the crack surface displacement function is discussed. To illustrate the utility of the method, computational results for several strongly interacting multiple-crack geometries are presented. The calculated stress intensity factors are in excellent agreement with those obtained by an approximate analytical method due to Kachanov and Laures.  相似文献   
998.
999.
This paper considers the evolution of the velocity and temperature fields within a jet of fluid on a thermally insulated wall. Numerical solutions are obtained for a range of Prandtl numbers and for a class of initial velocity and temperature profiles relevant to intrusion jets observed in certain thermal cavity flows. Numerical results are compared with asymptotic solutions which describe the initial structure of the jet and its subsequent diffusion at large distances downstream.  相似文献   
1000.
A coupled-mode analysis of superimposed holographic transmission gratings in single-mode planar optical waveguides is presented. The diffraction characteristics are shown to be sensitive to the polarization of the incident wave, the angular separation of the output waves and the relative index modulation between the gratings. The coupling between gratings affects the diffraction efficiencies and decreases the angular and wavelength selectivities. Nevertheless, good channel separation and high fanout can be obtained together  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号