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   260979篇
  免费   4282篇
  国内免费   1820篇
电工技术   5413篇
技术理论   5篇
综合类   1164篇
化学工业   42071篇
金属工艺   11031篇
机械仪表   7754篇
建筑科学   6601篇
矿业工程   1660篇
能源动力   5594篇
轻工业   25840篇
水利工程   2891篇
石油天然气   5998篇
武器工业   243篇
无线电   27797篇
一般工业技术   49215篇
冶金工业   47258篇
原子能技术   5618篇
自动化技术   20928篇
  2021年   2479篇
  2019年   2123篇
  2018年   3394篇
  2017年   3364篇
  2016年   3687篇
  2015年   2804篇
  2014年   4585篇
  2013年   11359篇
  2012年   7690篇
  2011年   10211篇
  2010年   8129篇
  2009年   8672篇
  2008年   9487篇
  2007年   9594篇
  2006年   8442篇
  2005年   7403篇
  2004年   6659篇
  2003年   6206篇
  2002年   6213篇
  2001年   6319篇
  2000年   5908篇
  1999年   5877篇
  1998年   12863篇
  1997年   9540篇
  1996年   7278篇
  1995年   5527篇
  1994年   5086篇
  1993年   4951篇
  1992年   3945篇
  1991年   3752篇
  1990年   3826篇
  1989年   3774篇
  1988年   3532篇
  1987年   3019篇
  1986年   3051篇
  1985年   3410篇
  1984年   3325篇
  1983年   3079篇
  1982年   2699篇
  1981年   2904篇
  1980年   2644篇
  1979年   2842篇
  1978年   2736篇
  1977年   2842篇
  1976年   3694篇
  1975年   2457篇
  1974年   2291篇
  1973年   2322篇
  1972年   1982篇
  1971年   1785篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
We report the results of studies which have been made on heteroepitaxial layers of GaAs and AlGaAs grown by metalorganic chemical vapor deposition on composite substrates that consist of four different types of heteroepitaxial layered structures of Ge and Ge-Si grown by molecular beam epitaxy on (100)-oriented Si substrates. It is found that of the four structures studied, the preferred composite substrate is a single layer of Ge ∼1 μm thick grown directly on a Si buffer layer. The double-crystal X-ray rocking curves of 2 μm thick GaAs films grown on such substrates have FWHM values as small as 168 arc sec. Transmission electron micrographs of these Ge/Si composite substrates has shown that the number of dislocations in the Ge heteroepitaxial layer can be greatly reduced by an anneal at about 750° C for 30 min which is simultaneously carried out during the growth of the GaAs layer. The quality of the GaAs layers grown on these composite substrates can be greatly improved by the use of a five-period GaAs-GaAsP strained-layer superlattice (SLS). Using the results of these studies, low-threshold optically pumped AlGaAs-GaAs DH laser structures have been grown by MOCVD on MBE Ge/Si composite substrates.  相似文献   
992.
Short staffing and insufficient technical and business knowledge among staff members can hinder information centers from meeting the needs of end users. Establishing groups of selected users with special skills improves end-user computing by opening communication channels, influencing training objectives, and fostering better user morale. This article explains the types of user groups that an information center can establish and the benefits to be derived from them.  相似文献   
993.
The characteristics of optical pumping dynamics in laser-pumped, rare-earth-doped, solid-state laser materials are investigated by using a tunable alexandrite laser to pump Y3Al5O12 :Nd3+ in an optical cavity. It is found that the slope efficiency of the Nd laser operation depends strongly on the wavelength of the pump laser. For pump wavelengths resulting in low slope efficiencies, intense fluorescence emission is observed from the sample in the blue-green spectral region. This is attributed to the excited-state absorption of pump photons which occurs during radiationless relaxation from the pump band to the metastable state. This type of process is an important loss mechanism for monochromatic pumping of laser systems at specific pump wavelengths  相似文献   
994.
Plasmid deoxyribonucleic acid of group N streptococci codes for a wide variety of important metabolic properties associated with dairy fermentations. The increasing knowledge of the genetics of these bacteria has created exciting possibilities for genetically constructing improved starter strains. A review of current literature is presented on the genetics of group N streptococci, especially the development and implications of gene transfer and molecular cloning systems for further genetic studies and for strain improvement. Functional properties of plasmid deoxyribonucleic acid and gene transfer systems, which include transduction, conjugation, transformation, and protoplast fusion, are reviewed and followed by a discussion of recent advances in gene cloning.  相似文献   
995.
Direct conversion receiver (DCR) architecture, which has introduced the zero intermediate frequency (IF) approach, supports efficient wireless handset designs with a high level of integration. Transmission over wireless channels is subject to time dispersion due to multipath propagation and frequency dispersion due to Doppler effect, the design of wireless receivers are tremendously important in supporting reliable communication links. With wireless technology growing, the choice of optimal wireless receiver architecture that supports monolithic integration without performance degradation becomes an important dimension in modern handset design. The increasing demand for mobile terminals with smaller physical dimensions has led to the investigation of DCR, which supports single chip and multinode designs. Since DCR is a promising architecture for fourth generation mobile terminals, it is important to note that implementation of a certain dc offset technique does not compromise the benefits of low complexity receiver design.  相似文献   
996.
A new mathematical method to analyse the performance of guard-channel-based strategies is proposed. The analysis is based on the iterative calculation of the average effective channel holding time. The method presented provides higher accuracy than alternative solutions previously reported in the literature  相似文献   
997.
The results of block model calculations for multiple discretizations of a dielectric sphere and circular cylinder suggest that it is essential that the cells must be arranged for a best-fit of the body being modeled, the matrix elements must be reasonably accurate, and the cells must be small enough so that the pulse-function basis approximation is not blatantly unreasonable. When these criteria are approximately satisfied the remaining errors appear to be mainly due to imperfect representation of the shape of the object being modeled. It appears that the accuracy can be improved by using discretizations having cells of reduced size near the surface of the object. Geometric factors are defined which allow testing the potential accuracy of a solution without dimensioning or inverting a large matrix. Several unique procedures for discretization are also described that have the potential of partially mitigating the errors due to inaccurate representation of the shape of a scatterer.  相似文献   
998.
Germination and growth from spores of toxigenic B. cereus strain USDA 201 in BHI broth containing Na2EDTA were studied. Reduced growth was effected by low EDTA concentrations (< 300 ppm). Little or no growth was observed after incubation for 48 hr in broth containing from 300–1,000 ppm of EDTA EDTA did not affect percent spore germination, or release of Ca following heat activation and subsequent incubation. However, delayed colony development was observed as EDTA concentrations increased in the broth, and atypically small colonies formed on Plate Count Agar. Growth inhibition by 500 ppm EDTA was evident over the pH range 5–9, with the highest spore resistance observed at pH 7. Addition of Fe, Zn, and Ca to the media reversed the growth inhibitory action, whereas Mg was less effective.  相似文献   
999.
The minimum inhibitory concentrations (MICs) of several naphthoquinones (NQ) were determined in the presence or absence of nitrite (NO2?) against various strains of Clostridium perfringens. In fluid thioglycollate medium, MICs ranged from 70–100 ppm for 2-methyl-1,4-NQ (menadione), 200–280 ppm for 1,4-NQ, 180–250 ppm for 1,2-NQ, >500 ppm for several water-soluble derivatives and 100–300 ppm for NO2?. Using a type B strain in homogenized meat medium, MICs were 670 ppm for menadione, 620 ppm for 1,4-NQ and 770 ppm for NO2?. Nitrite, menadione and 1,4-NQ exhibited comparable and additive rather than synergistic inhibition. Some NQ compounds may have potential as partial nitrite substitutes subject to safety evaluation.  相似文献   
1000.
Immunoglobulin absorption by the calf has been the subject of considerable research. Despite these efforts little is known about the cytological events that occur at the level of the intestinal epithelial cell. These events have been studied extensively and characterized in the laboratory rodent; however, there have been few attempts to make corollaries between the two species. All neonatal animals display certain similarities in their intestinal morphology that may be correlated, with immunoglobulin absorption. Selectivity in absorption appears to be variable among neonatal animal species; however, all demonstrate some selectivity. Selectivity in absorption implies that receptors are a necessary component in the transport of immunoglobulins. Selectivity further requires binding of immunoglobulins to an endocytic vesicle membrane to ensure transport through the cell, circumvention of intracellular digestion, and release at the basolateral cell membrane. A decrease of immunoglobulin absorption may be accomplished in a variety of ways such as competition between intestinal microbes and immunoglobulins for a common receptor on the intestinal epithelial cell. An additional consideration is aberrant synthesis or recycling of the cell membrane receptor, as induced by metabolic decelerators such as cortisol. Failure to recycle immunoglobulin receptors also would decrease efficiency of absorption.  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