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   382243篇
  免费   4424篇
  国内免费   1422篇
电工技术   7434篇
综合类   364篇
化学工业   57144篇
金属工艺   14295篇
机械仪表   11692篇
建筑科学   9860篇
矿业工程   1231篇
能源动力   10574篇
轻工业   35718篇
水利工程   3164篇
石油天然气   3786篇
武器工业   20篇
无线电   50775篇
一般工业技术   74320篇
冶金工业   69509篇
原子能技术   6979篇
自动化技术   31224篇
  2021年   2835篇
  2019年   2642篇
  2018年   4203篇
  2017年   4264篇
  2016年   4492篇
  2015年   3134篇
  2014年   5469篇
  2013年   17222篇
  2012年   8846篇
  2011年   12416篇
  2010年   9906篇
  2009年   11308篇
  2008年   12235篇
  2007年   12314篇
  2006年   10950篇
  2005年   9894篇
  2004年   9617篇
  2003年   9765篇
  2002年   9313篇
  2001年   9761篇
  2000年   9047篇
  1999年   9401篇
  1998年   22023篇
  1997年   15814篇
  1996年   12521篇
  1995年   9730篇
  1994年   8756篇
  1993年   8534篇
  1992年   6477篇
  1991年   6303篇
  1990年   6106篇
  1989年   5869篇
  1988年   5612篇
  1987年   4901篇
  1986年   4788篇
  1985年   5434篇
  1984年   4968篇
  1983年   4746篇
  1982年   4427篇
  1981年   4305篇
  1980年   4099篇
  1979年   3978篇
  1978年   3693篇
  1977年   4465篇
  1976年   5858篇
  1975年   3181篇
  1974年   3022篇
  1973年   2993篇
  1972年   2549篇
  1971年   2228篇
排序方式: 共有10000条查询结果,搜索用时 12 毫秒
101.
Green fluorescent protein (GFP) is increasingly being used in plant biology from the cellular level to whole plant level. At the cellular level, GFP is being used as an in vivo reporter to assess frequency of transient and stable transformation. GFP has also proven to be an invaluable tool in monitoring trafficking and subcellular localization of protein. At the organ level and up, many exciting applications are rapidly emerging. The development of brighter GFP mutants with more robust folding properties has enabled better macroscopic visualization of GFP in whole leaves and plants. One interesting example has been the use of GFP to monitor virus movement in and among whole plants. GFP is also emerging as a powerful tool to monitor transgene movement and transgenic plants in the field. In a proof-of-concept study, tobacco was transformed with a modified version of the GFP gene controlled by a constitutive (35S) promoter. GFP expression in progeny plants ranged from 0% to 0.5%, and approximately 0.1% GFP was the minimal amount needed for unambiguous macroscopic detection. GFP is the first truly in vivo reporter system useful in whole plants, and we project its usefulness will increase even further as better forms of GFP genes become available.  相似文献   
102.
Neurobiological research in schizophrenia has been hampered by several confounding factors such as the heterogeneity of the illness and the paucity of biological markers. Recent progress in research methods, however, has enabled the improvement in our understanding its pathophysiology. This paper reviews recent neurochemical investigations of schizophrenia and its animal models which were conducted in Japan in the last decade. The research areas reviewed are (i) monoamine and their metabolites in body fluids, (ii) phospholipids and prostaglandins, (iii) neurochemistry in autopsy brains, (iv) immunological measures, (v) magnetic resonance spectroscopy, (vi) regional cerebral blood flows (rCBF), (vii) molecular genetics, and (viii) animal models. It is worth noting that there exist abnormalities of amino acidergic (glutamatergic and GABAergic) neurotransmission as well as monoaminergic (dopaminergic and serotonergic) one in postmortem schizophrenic brains. These abnormalities and also the findings of altered rCBF indicate the existence of disturbed neuronal circuits that contribute to the diverse symptoms of schizophrenia. Also, dysfunction of membrane phospholipids derived from studies on magnetic resonance spectroscopy may underlie negative symptoms in schizophrenia. Given that schizophrenia is considered to comprise a group of disorders with a diverse heterogeneity of etiologies, research in the next decade is expected to identify putative genes that are involved in vulnerability to schizophrenic phenotype.  相似文献   
103.
As we uncover the physiological and molecular mechanisms behind human reproduction, we gain the potential to exert more control over our reproductive capabilities. In the past two decades, the prospects for 'infertile' women to bear children, or 'sterile' men to father them, have improved dramatically; recently, women have given birth in their sixties, well beyond their natural menopause. Thanks to developments in contraception, couples can now enjoy an active sex-life without a significant risk of pregnancy, and more accurately control the size and timing of their families. But these new freedoms have a price: they also have the potential to cause demographic distortions, medical harm to individuals and abuse of human rights.  相似文献   
104.
105.
Two classes of glial cells are found in the embryonic Drosophila CNS, midline glial cells and lateral glial cells. Midline glial development is triggered by EGF-receptor signalling, whereas lateral glial development is controlled by the gcm gene. Subsequent glial cell differentiation depends partly on the pointed gene. Here we describe a novel component required for all CNS glia development. The tramtrack gene encodes two zinc-finger proteins, one of which, ttkp69, is expressed in all non-neuronal CNS cells. We show that ttkp69 is downstream of gcm and can repress neuronal differentiation. Double mutant analysis and coexpression experiments indicate that glial cell differentiation may depend on a dual process, requiring the activation of glial differentiation by pointed and the concomitant repression of neuronal development by tramtrack.  相似文献   
106.
A study was undertaken to determine fertility in a group of females who as children had been operated on for appendicitis. The 134 women operated on for appendicitis were reviewed. Their ages ranged from 2-18 years at the time of appendectomy. Our data show that perforated appendicitis before puberty has little if any role in the etiology of tubal infertility.  相似文献   
107.
The effect of high oxide field stress is studied using capacitance-time (C-t) measurements of MOS capacitors. The stress results in parallel shifts of the C-t curve along the time axis. The flatband voltage shift ΔVFB obtained from the initial deep depletion capacitance C(t=0+) follows the same trend as that from the high-frequency C-V characteristics. However, the discrepancy between the two flatband voltages becomes larger as the stress increases due to the effect of interface charges on C-t characteristics. The flatband voltage difference is converted to interface trap density, showing a steady increase of interface trap density with stress, similar to that from low-frequency C-V measurements  相似文献   
108.
This paper presents the design criteria, procedure, and implementation of a soft-switched power-factor-correction (PFC) circuit based on the extended-period quasi-resonant (EPQR) principles. All power electronic devices including switches and diodes in the circuit are fully soft switched. The design method is demonstrated in a prototype circuit. The operating principles are confirmed with computer simulation and experimental results. A comparison of the EP-QR operation and zero-voltage-transition (ZVT) pulse-width modulation (PWM) method  相似文献   
109.
A comprehensive survey of photosensitivity in silica glasses and optical fiber is reviewed. Recent work on understanding the mechanisms contributing to germanium or aluminum doped fiber photosensitivity is discussed within the framework of photoelastic densification models  相似文献   
110.
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