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   218231篇
  免费   16575篇
  国内免费   8464篇
电工技术   12050篇
技术理论   14篇
综合类   12354篇
化学工业   37795篇
金属工艺   11143篇
机械仪表   12683篇
建筑科学   17422篇
矿业工程   5291篇
能源动力   6166篇
轻工业   14123篇
水利工程   3474篇
石油天然气   11662篇
武器工业   1433篇
无线电   26756篇
一般工业技术   27985篇
冶金工业   11477篇
原子能技术   2337篇
自动化技术   29105篇
  2024年   938篇
  2023年   3490篇
  2022年   6223篇
  2021年   8419篇
  2020年   6210篇
  2019年   5318篇
  2018年   5748篇
  2017年   6604篇
  2016年   6014篇
  2015年   7791篇
  2014年   10185篇
  2013年   13039篇
  2012年   13046篇
  2011年   14801篇
  2010年   12475篇
  2009年   12319篇
  2008年   11685篇
  2007年   11221篇
  2006年   11637篇
  2005年   10363篇
  2004年   7041篇
  2003年   6111篇
  2002年   5650篇
  2001年   5057篇
  2000年   5153篇
  1999年   5855篇
  1998年   5433篇
  1997年   4489篇
  1996年   4067篇
  1995年   3406篇
  1994年   2851篇
  1993年   2234篇
  1992年   1742篇
  1991年   1319篇
  1990年   1047篇
  1989年   903篇
  1988年   703篇
  1987年   509篇
  1986年   409篇
  1985年   339篇
  1984年   220篇
  1983年   203篇
  1982年   167篇
  1981年   152篇
  1980年   139篇
  1979年   101篇
  1978年   64篇
  1977年   65篇
  1976年   78篇
  1975年   39篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
沈长发  陈宝珍 《核动力工程》1989,10(3):13-17,39
文中详细介绍了秦山核电厂蒸汽发生器带钩波形板蒸汽干燥器的设计观点。  相似文献   
92.
Thermal maturation of Swedish Alum Shale kerogen and bitumen has been determined from core samples from Eastern and Central Sweden. In samples from Eastern Sweden (Öland and Gotland), the kerogen and bitumen are thermally immature with respect to petroleum generation. In some areas of Central Sweden (Närke, ÖstergÖtland, and Kinnekulle in VästergÖtland), the kerogen is immature, whereas the bitumen is marginally mature to mature. In other areas of Central Sweden (Halleberg-Hunneberg in VästergÖtland), the kerogen is supermature and the bitumen mature. This suggests that bitumen from a mature source-rock has migrated into the Alum Shales of Central Sweden. Migration in Central Sweden is further evidenced by the occurrence of obviously-migrated bituments in vugs and voids in the organicpoor Ordovician limestone overlying the Alum Shale in Central Sweden, and in concretions within the Alum Shale itself. Based on biomarker distributions of extracted bitumen, Alum Shale kerogen pyrolysate and obviously-migrated oils, and the fact that the Alum Shale in most of the areas studied is the only petroleum source-rock extant, it is suggested that the migrated bitumen in Central Sweden is from the Alum Shale itself. Bitumen has migrated from areas where the Alum Shale is in close proxmity to Permo-Carboniferous intrusions, such as Halleberg-Hunneberg, into nearby areas such as Närke and ÖstergÖtland, where there is no evidence of intrusion and the indigenous organic matter is thermally immature. Other areas, where Alum Shales were associated with intrusions and consequently sourced oil, may have been eroded away. There are producing wells on the island of Gotland, where the Alum Shale is also thermally immature. It is therefore assumed that heating which was responsible for generating Gotland's oil was very localized (such as by an intrusion) or that the oil has migrated from a thermally moremature, distant area. On the basis of reservoir rock porosity, and the fact that the Alum Shale of Gotland contains no migrated component, localized heating is favored.  相似文献   
93.
本文引入L-,l-连通度的概念,把Rosenfeld等人建立的三维数字拓扑模糊化;并在三维地震处理中找到了它的应用模型.  相似文献   
94.
The microscopic structures of PLZT(7.9/70/30 and x/65/35, x = 7 or 8) ceramics were studied by means of transmission electron microscopy. The presence of micro-regions in PLZTs was first verfied.  相似文献   
95.
An empirical formula for calculating the extinction cross section (ECS) by raindrops over a broad frequency range is first derived based on extensive calculations made on a widely varying in mean radius of modified Pruppacher and Pitter (MPP) raindrop models ranging from 0.25 to 3.5 mm. The expansion coefficients in the empirical formula are determined by least-squares curve fitting of numerical data obtained by the volume integral equation formulation (VIEF). The formula satisfies the frequency and raindrop size dependence. Numerical results obtained from the empirical formula for calculating the ECS are generally in good agreement with those calculated by the VIEF for raindrops with mean radius varying from 0.25 to 3.5 mm in the frequency range from 0.6 to 100 GHz. The average error in the ECS is less than 10%. The formula thus provides a simple and inexpensive method for calculating the ECS of raindrops, which otherwise requires complicated and expensive methods of calculation. By implementing this empirical formula of ECS into the rain attenuation equation, a new numerically empirical formula for calculating the specific rain attenuation is also proposed. The validity of the empirical formula for calculating the specific rain attenuation is also checked by comparing the obtained results of specific rain attenuation with those obtained from Li et al.'s (1995) solution, Yeo et al.'s (1993) measurement, and Olsen et al.'s (1978) power-law equation  相似文献   
96.
For batchwise radical chain solution multicomponent copolymerization, the minimum end time problem for the predetermined monomer conversion and number average molecular weight has been studied by considering the initiator concentration (or feed rate) and temperature as the two control variables. Applying the maximum principle and “unit segment treatment” (by which the co-polymerization can be treated as homo-polymerization in the kinetic analysis), it is found that for initiator addition processes the optimal initiator addition policy is to make the rate of initiation constant for the optimal temperature variation case, and the initiator concentration constant for the isothermal case as in homo-polymerization. For single charge of initiator processes, the optimal initial initiator concentration is such that it should be maintained at the lowest possible value (by which the polymerization is strongly dead-end) for the optimal temperature variation case without chain transfer steps, and at some optimal value (by which the polymerization is near dead-end) for the best isothermal case.  相似文献   
97.
A study on time schemes for DRBEM analysis of elastic impact wave   总被引:1,自引:0,他引:1  
 The precise integration and differential quadrature methods are two new unconditionally stable numerical schemes to approximate time derivative with more than the second order accuracy. Recent studies showed that compared with the Houbolt and Newmark methods, they produced more accurate solutions with large time step for the problems where response is primarily dominated by low and intermediate frequency modes. This paper aims to investigate these time schemes in the context of the dual reciprocity BEM (DRBEM) formulation of various shock-excited scalar elastic wave problems, where high modes have important affect on traction response. The Houbolt method was widely recommended in many literatures for such DRBEM dynamic formulations. However, this study found that the damped Newmark algorithm was the most efficient and accurate for impact traction analysis in conjunction with the DRBEM. The precise integration and differential quadrature methods are shown inapplicable for such shock-excited problems due to the absence of numerical damping. On the other hand, we also found that to achieve the same order of accuracy, the differential quadrature method required much less computing effort than the precise integration method due to the use of the Bartels–Stewart algorithm solving the resulting Lyapunov matrix analogization equation. Received 6 November 2000  相似文献   
98.
秦山核电厂气载放射性释放的环境影响   总被引:3,自引:1,他引:2  
本文评价了秦山核电厂气载放射性流出物对环境的影响。应用现场及风洞大气扩释实验结果和厂址周围的人口与食谱调查资料,估算了秦山核电厂在正常运行和事故条件下释放的气载放射性流出物对公众产生的个人有效剂量当量和集体有效剂量当量。计算结果表明,正常运行时厂址边界(0.5km)处的最大个人有效剂量当量为2.7×10~(-2)mSv/a,该剂量的大部分来自~(137)Cs 的食入(主要由地表湿沉积引起);80km 范围内的集体有效剂量当量为1.1人·Sv/a,归一化集体有效剂量当量为3.7人·Sv/GW(e)·a。文中还给出了事故情况下剂量估算结果。  相似文献   
99.
A direct method for transient stability analysis of a multimachine power system is presented. Emphasis is placed on the detailed development and evaluation of the proposed method in transient stability assessment. An energy-type Lyapunov function, taking into account the effect of transfer conductance, is introduced first. The potential energy boundary surface method is then adopted to determine the critical energy value quickly. The main computational burden in the direct method lies in the reduction of the admittance matrices. In this paper, an efficient reduction technique incorporating the sparsity technique and compensation theory is developed. Finally, the proposed method was tested on the Taipower system, which is the only power system in Taiwan. The results obtained confirm the validity and accuracy of the proposed method, and hence demonstrate the superiority and potential of the direct method in real power system applications.  相似文献   
100.
This paper presents a design method of optical frequency modulation (FM) subcarrier (with super carrier) transmission modem. The nonideal link characteristics, including laser chirping, fiber dispersion, voltage controlled oscillator (VCO) phase noise, relative intensity noise (RIN), and equivalent network model of laser diode of such a system that may bring about signal distortion are discussed first. We then propose a hierarchical methods to establish the system equivalent model. Finally this FM modem is applied to a GSM wireless system, in which multichannel signals are transmitted over fiber between radio port and basestation, and system performance is appraised by its dynamic range. It is found that optical FM subcarrier (with super carrier) transmission technique can improve the system dynamic range, compared with the intensity modulated direct detection (IMDD) method, and this is a economical and efficient method  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