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   27843篇
  免费   1686篇
  国内免费   69篇
电工技术   370篇
综合类   37篇
化学工业   6682篇
金属工艺   1062篇
机械仪表   1598篇
建筑科学   530篇
矿业工程   8篇
能源动力   1148篇
轻工业   2431篇
水利工程   101篇
石油天然气   21篇
武器工业   1篇
无线电   4608篇
一般工业技术   5848篇
冶金工业   1817篇
原子能技术   333篇
自动化技术   3003篇
  2024年   26篇
  2023年   334篇
  2022年   510篇
  2021年   849篇
  2020年   598篇
  2019年   670篇
  2018年   889篇
  2017年   868篇
  2016年   996篇
  2015年   796篇
  2014年   1260篇
  2013年   1775篇
  2012年   1894篇
  2011年   2258篇
  2010年   1699篇
  2009年   1711篇
  2008年   1564篇
  2007年   1198篇
  2006年   1045篇
  2005年   924篇
  2004年   821篇
  2003年   796篇
  2002年   750篇
  2001年   629篇
  2000年   555篇
  1999年   537篇
  1998年   839篇
  1997年   499篇
  1996年   464篇
  1995年   308篇
  1994年   218篇
  1993年   183篇
  1992年   158篇
  1991年   121篇
  1990年   103篇
  1989年   117篇
  1988年   87篇
  1987年   79篇
  1986年   66篇
  1985年   51篇
  1984年   49篇
  1983年   39篇
  1982年   34篇
  1981年   33篇
  1980年   33篇
  1979年   14篇
  1977年   30篇
  1976年   44篇
  1975年   20篇
  1974年   13篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
62.
A new compact millimeter-wave distance-measurement sensor prototype has been developed. The sensor is a step-frequency radar implemented using coherent heterodyne technique. It operates in Ka-band (26.5-40 GHz) and is realized using MICs and MMICs. The sensor transmits sinusoidal signals of incremental frequencies and demodulates the received signals into base-band I/Q signals for processing. Experimental results show that the sensor is capable of measuring distance with less than 0.2 inch of absolute error and a low transmitted power of only -20±3 dBm  相似文献   
63.
A two-dimensional finite element model was constructed to analyze the simulated mechanical behavior of a cantilevered fixed partial denture. The variations of the models were made by altering the degree of bone support, the number of splinted abutments, and the length of the pontics. High stress concentrations were observed around the connectors of the fixed prosthesis and the tooth closest to the cantilever. Reduced bone support increased the deflection and stress concentrations. There was reduction in displacement and stress concentration when the teeth were splinted together. To improve the prognosis of the fixed partial denture cantilever, the number of abutments should be increased and the number of pontics decreased.  相似文献   
64.
To specify manufacturing tolerances of a reflector antenna, various errors such as random surface errors and misalignment errors must be considered at one time because superposition of the effects of those errors may not hold. Based on the Rahmat-Samii's formulation (1983), a method for computing efficiently the average power pattern of a reflector antenna with those errors is presented. Simulation results show that superposition of the effects of errors does not generally hold and demonstrate how those errors degrade the peak-gain and sidelobe levels  相似文献   
65.
66.
67.
We address the problem of determining the topology and bridge-capacity assignments for a network connecting a number of token rings via source-routing bridges. The objective is to minimize the cost of bridge installations while meeting the network users' performance requirements. The problem is modeled as a mixed 0–1 integer program. A comparison is given between two solution algorithms: a simulated annealing algorithm using the flow-deviation algorithm for each routing subproblem, and a drop algorithm using the simplex method for the same subproblems to provide benchmark solutions. In the former algorithm, the routing subproblem is formulated as a nonlinear program with penalty functions to model node and link capacity constraints, and in the latter as a multicommodity flow model with the same capacity constraints. Computational results show that the simulated-annealing/flow-deviation algorithm produced substantially better solutions than the LP-based drop algorithm.  相似文献   
68.
The plug loading effect occurring during the etchback of tungsten was investigated in a magnetically enhanced reactive ion etcher using SF 6/Ar mixtures. It was found that while the plug loading effect is independent of varying SF6/Ar flow rate ratio and magnetic field intensity, it is reduced under the condition of high selectivity of tungsten relative to TiN which was achieved at high chamber pressure and low RF power. It is proposed that when TiN is used as a glue layer, the W etch rate enhancement in the plug is mainly controlled by a local loading effect. Under the optimized etchback conditions the plug loss was successfully controlled without the tungsten residue left on severe topology  相似文献   
69.
To determine the mechanism of measles virus-induced cell death, we studied the infection of Vero cells and monocytic cell lines with wild-type (Chicago-1) and vaccine (Edmonston) strains of measles virus. DNA fragmentation indicative of apoptosis was apparent by flow cytometry, agarose gel electrophoresis, and electron microscopy. Within syncytia, DNA strand breaks were demonstrated by end labeling with terminal transferase and then by visualization.  相似文献   
70.
This paper presents theoretical results on mode characteristics of surface-emitting (SE) lasers utilizing an active second-order grating section. Based on a coupled-mode approach, longitudinal modes and the associated space-harmonic transverse modes are calculated via a numerical technique. From these, the lasing-mode spectrum, near- and far-field patterns of the radiation mode, and the surface-emission power efficiency are obtained. Effects of the substrate reflector and the grating parameters are also investigated. Finally, comparisons are made with conventional, edge-emitting DFB lasers. The results indicate that with a suitable choice of structural parameter values, DFB SE lasers can be made to possess both the spectral discrimination of the conventional DFB lasers and the advantages of SE lasers at the same time and also that the second lowest longitudinal mode may be preferred over the fundamental longitudinal mode for many applications due to its symmetric field distribution  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