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   200508篇
  免费   969篇
  国内免费   487篇
电工技术   4103篇
综合类   154篇
化学工业   26913篇
金属工艺   7534篇
机械仪表   5485篇
建筑科学   4056篇
矿业工程   747篇
能源动力   4728篇
轻工业   14574篇
水利工程   1786篇
石油天然气   2889篇
武器工业   7篇
无线电   27419篇
一般工业技术   37634篇
冶金工业   45407篇
原子能技术   3729篇
自动化技术   14799篇
  2021年   1309篇
  2020年   1074篇
  2019年   1405篇
  2018年   2132篇
  2017年   2113篇
  2016年   2241篇
  2015年   1537篇
  2014年   2641篇
  2013年   7986篇
  2012年   4499篇
  2011年   6167篇
  2010年   4955篇
  2009年   5799篇
  2008年   6176篇
  2007年   6226篇
  2006年   5658篇
  2005年   5317篇
  2004年   5265篇
  2003年   5110篇
  2002年   4931篇
  2001年   5319篇
  2000年   4933篇
  1999年   5455篇
  1998年   15635篇
  1997年   10328篇
  1996年   7943篇
  1995年   5796篇
  1994年   5020篇
  1993年   5027篇
  1992年   3394篇
  1991年   3269篇
  1990年   3226篇
  1989年   3058篇
  1988年   2787篇
  1987年   2240篇
  1986年   2305篇
  1985年   2611篇
  1984年   2319篇
  1983年   2065篇
  1982年   1904篇
  1981年   2044篇
  1980年   1793篇
  1979年   1662篇
  1978年   1659篇
  1977年   2024篇
  1976年   2703篇
  1975年   1422篇
  1974年   1369篇
  1973年   1316篇
  1972年   1124篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
Continuous real-time estimating of compaction quality during the construction of a hot mix asphalt (HMA) pavement is addressed in this paper. The densification of asphalt pavements during construction usually is accomplished by using vibratory compactors. During compaction, the compactor and the asphalt mat form a coupled system whose dynamics are influenced by the changing stiffness of the mat. The measured vibrations of the compactor along with process parameters such as lift thickness, mix type, mix temperature, and compaction pressure can be used to predict the asphalt mat density. Contrary to existing techniques in the literature in which a model is developed to fit experimental data and to predict mat density, a neural network-based approach is adopted that is model-free and uses pattern-recognition techniques to estimate density. The neural network is designed to read the entire frequency spectrum of roller vibrations and to classify these vibrations into different levels. The intelligent asphalt compaction analyzer (IACA) is then trained to convert these vibration levels into a “number” indicative of the asphalt mat density at a given location. This two-step process eliminates the need for regression analysis and produces more accurate density measurements than those reported elsewhere in the literature. Compaction studies of HMA mixes on a stiff subgrade indicate that the changes in the vibration characteristics of the roller are attributable to an increased compaction of the HMA base. The results also show that, with the neural network working as a classifier, the IACA can estimate the density continuously, and in real time, with accuracy levels adequate for quality control in the field.  相似文献   
952.
953.
954.
A new AlGaN/GaN-based high electron mobility transistor (HEMT) is proposed and its micro-wave characteristics are discussed by introducing a nanoscale AlN or InN layer to study the potential improvement in their high frequency performance. The 2DEG transport mechanism including various sub-band calculations for both (Al,In) N-based HEMTs are also discussed in the paper. Apart from direct current characteristics of the proposed HEMT, various microwave parameters such as transconductance, unit current gain (h 21 = 1) cut-off frequency (f t ), high power-gain frequency (f max). Masons available/stable gain and masons unilateral gain are also discussed for both devices to understand its suitable deployment in microwave frequency range.  相似文献   
955.
The results of a complex investigation into dark-current relaxation in the long-time region of an MIM structure based on an As-Se thin-film chalcogenide system are presented. The values of parameters describing the electronic processes ocurring in the contact layers of the investigated compounds are estimated. The coincidence of the nature of conductivity and charge-accumulation mechanisms is revealed. The relaxation-time distribution function is calculated, and its structural sensitivity to such technological factors as the change in the composition stoichiometry and the method for manufacturing experimental samples is established.  相似文献   
956.
The effect of ultraviolet radiation of polycrystalline zinc oxide films (with a thickness of 200 nm) on their resistivity, transparency, and luminescence in the visible and violet spectral regions is studied. It is shown that, under irradiation of the films in air and vacuum, the conductivity, transmittance, and edge luminescence intensity increase with characteristic times of about 100 min. It is established that the corresponding processes controlled by desorption of oxygen atoms and molecules from the surface of nanocrytals in the ZnO films are reversible.  相似文献   
957.
Mechanical properties of cellular solids depend on the ratio of the sample size to the cell size at length scales where the two are of the same order of magnitude. Considering that the cell size of many cellular solids used in engineering applications is between 1 and 10 mm, it is not uncommon to have components with dimensions of only a few cell sizes. Therefore, both for mechanical testing and for design, it is important to understand the link between the cellular morphology and size effects, which is the aim of this study. In order to represent random foams, two-dimensional (2D) Voronoi tessellations are used, and four representative boundary value problems - compression, shear, indentation, and bending - are solved by the finite element (FE) method. Effective elastic and plastic mechanical properties of Voronoi samples are calculated as a function of the sample size, and deformation mechanisms triggering the size effects are traced through strain maps. The modeling results are systematically compared with experimental results from the literature. As a rule, with decreasing sample size, the effective macroscopic stiffness and strength of Voronoi samples decrease under compression and bending, and increase under shear and indentation. The physical mechanisms responsible for these trends are identified.  相似文献   
958.
This article describes the processing and characterisation of lanthanum-doped lead zirconate titanate (PLZT)-based ferroelectric fibres for composite transducer applications. X-ray diffraction of the extruded and sintered fibres indicated some lead loss during sintering; however, the fibres exhibited low porosity (1.54%), high maximum piezoelectric strain (4041 ppm) and relatively low coercive field (0.77 kV/mm). The low coercive field of the lanthanum-doped fibres may be advantageous in terms of facilitating polarization of the fibres in composite architectures.  相似文献   
959.
The procedure of analysis of wastewaters of mining-and-processing integrated works on the content of Se, As, Sb, Te, and Bi with the use of an atomic absorption method with electrothermal atomizer, which involves the preliminary sample preparation and choice of temperature program, modifier, and corrector of nonselective absorption, was described. The conditions were matched for the determination of low concentrations of these elements in natural waters.  相似文献   
960.
The specific volume of the diluted magnetic semiconductor Cd1 − x Mn x GeAs2 (x = 0.06–0.3) has been determined for the first time by strain measurements at pressures of up to 7 GPa. From the pressure dependences of the relative specific volume, we evaluated the volume magnetostriction (spontaneous magnetization coefficient). A scaling relation was used to estimate the bulk modulus of the magnetically ordered and disordered phases.  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