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   250381篇
  免费   2557篇
  国内免费   758篇
电工技术   4954篇
综合类   180篇
化学工业   37084篇
金属工艺   14238篇
机械仪表   10766篇
建筑科学   5746篇
矿业工程   1621篇
能源动力   5322篇
轻工业   17473篇
水利工程   2795篇
石油天然气   4513篇
武器工业   2篇
无线电   26889篇
一般工业技术   52634篇
冶金工业   38997篇
原子能技术   5287篇
自动化技术   25195篇
  2021年   1310篇
  2019年   1246篇
  2018年   10635篇
  2017年   11276篇
  2016年   6942篇
  2015年   2108篇
  2014年   2808篇
  2013年   7827篇
  2012年   7163篇
  2011年   14768篇
  2010年   13095篇
  2009年   12671篇
  2008年   12474篇
  2007年   14408篇
  2006年   5085篇
  2005年   8316篇
  2004年   6506篇
  2003年   5804篇
  2002年   4771篇
  2001年   4194篇
  2000年   4061篇
  1999年   4072篇
  1998年   10391篇
  1997年   7393篇
  1996年   5670篇
  1995年   4293篇
  1994年   3647篇
  1993年   3591篇
  1992年   2621篇
  1991年   2560篇
  1990年   2416篇
  1989年   2436篇
  1988年   2384篇
  1987年   2127篇
  1986年   2068篇
  1985年   2370篇
  1984年   2187篇
  1983年   2014篇
  1982年   1881篇
  1981年   1944篇
  1980年   1798篇
  1979年   1817篇
  1978年   1779篇
  1977年   2109篇
  1976年   2688篇
  1975年   1557篇
  1974年   1549篇
  1973年   1607篇
  1972年   1350篇
  1971年   1262篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Mathematical models are helpful in the understanding of diseases through the use of dynamical indicators. A previous study has shown that brain activity can be characterized by a decrease of dynamical complexity in depressive subjects. The present paper confirms and extends these conclusions through the use of recent methodological advances: first episode and recurrent patients strongly differ in their dynamical response to therapeutic interventions. These results emphasize the need for clinical follow-ups to avoid recurrence and the necessity of specific therapeutic intervention in the case of recurrent patients.  相似文献   
992.
BACKGROUND: A diaphragmatic phonomyogram (PMG) evoked by maximal phrenic nerve stimulation at end expiratory lung volume (FRC) has been previously described as a good index of changes in diaphragmatic contractility with fatigue. A study was undertaken to assess whether this conclusion could be extended to different lung volumes. METHODS: Diaphragmatic compound motor action potentials (CMAPs) were recorded on each side of the chest by the means of surface electrodes placed over the eight intercostal spaces in five healthy subjects. Diaphragmatic PMGs from both sides were recorded with condenser microphones fixed to the skin close to the CMAP recording electrodes. Oesophageal and gastric balloon tipped catheters were employed to measure transdiaphragmatic pressure twitches (TwPDI) which served as the standard measure of changes in diaphragmatic contractility. PMG and TwPDI responses were compared at different lung volumes over inspiratory capacity both before and after fatiguing inspiratory resistive loading. RESULTS: No consistent relationship was found in different subjects or on different days in the same subject between PMG and lung volume or between PMG and TwPDI. However, the PMG:CMAP ratio from both sides at any given lung volume decreased after fatigue in roughly the same proportion as the TwPDI. CONCLUSIONS: These results show that, although PMG can detect changes in diaphragmatic contractility caused by fatigue in normal subjects, lung volume changes need to be controlled and each subject should serve as his or her own control.  相似文献   
993.
Experiments were carried out in a two pack system; polyvinyl butyral, zinc tetroxychromate in one pack, and phosphoric acid in the other with an isopropanol solvent in both packs. This study examines the functions of the phosphoric acid and the chromate containing pigment separately by utilising the feature that the alcohol dispersed components have sufficient conductivity to carry out potential-time measurements in the wet components before and during mixing.  相似文献   
994.
Fistulas secondary to Crohn's disease occur in about 33% of patients. A colobronchial fistula complicating Crohn's disease is very rare, having been reported only twice previously. We present an unusual fistula secondary to Crohn's colitis that originated from the splenic flexure and crossed the diaphragm to involve the bronchial tree.  相似文献   
995.
996.
For the first time, a direct optically injection-locked oscillator based on the Esaki-Tsu effect in superlattices is reported. The microwave output power is -7 dBm at 20 GHz with an optical input power of -10 dBm at a wavelength of 1.3 μm  相似文献   
997.
A nonlinear dynamic model is developed for a process system, namely a heat exchanger, using the recurrent multilayer perceptron network as the underlying model structure. The perceptron is a dynamic neural network, which appears effective in the input-output modeling of complex process systems. Dynamic gradient descent learning is used to train the recurrent multilayer perceptron, resulting in an order of magnitude improvement in convergence speed over a static learning algorithm used to train the same network. In developing the empirical process model the effects of actuator, process, and sensor noise on the training and testing sets are investigated. Learning and prediction both appear very effective, despite the presence of training and testing set noise, respectively. The recurrent multilayer perceptron appears to learn the deterministic part of a stochastic training set, and it predicts approximately a moving average response of various testing sets. Extensive model validation studies with signals that are encountered in the operation of the process system modeled, that is steps and ramps, indicate that the empirical model can substantially generalize operational transients, including accurate prediction of instabilities not in the training set. However, the accuracy of the model beyond these operational transients has not been investigated. Furthermore, online learning is necessary during some transients and for tracking slowly varying process dynamics. Neural networks based empirical models in some cases appear to provide a serious alternative to first principles models.  相似文献   
998.
We have examined the effects of a 5-HT1A receptor agonist (8-hydroxy-2-(di-n-propylamino)tetralin, 8-OH-DPAT) on food preference in ageing rats that had been given a 'palatable' meal 15 min before administration of the drug. Ageing rats consumed a greater amount of the 'palatable' pre-meal than the young rats. In young rats lipids were the predominant source of calories, but in old animals lipid and protein consumption was similar. Administration of 8-OH-DPAT resulted in an increase in total caloric intake in both groups. Concomitant with this there was a significant increase in protein intake in both groups, which was most important in ageing rats, where proteins became the predominant source of calories.  相似文献   
999.
HIV induces a multi-organ infection with a dual tropism for both lymphocytes and monocytes/macrophages. The lung is a target both for HIV infection and HIV-related opportunistic infections. The SCID mouse has provided the opportunity to develop a small animal model for HIV infection. However, HIV-1 infection of the human fetal thymus and liver (SCID Liv/Thy) implanted in these mice occurred only after direct intraimplant injection of HIV-1 and the resultant HIV-1 infection was restricted to the human thymus. Here we report that human foetal lung can develop in SCID Liv/Thy mice resulting in the development of normal human alveolar and bronchiolar lung compartments which can be productively infected with cell-free HIV-1 virus, leading to a systemic and bifocal infection. This SCID-Hu model should be useful for studying AIDS physiopathology, human viruses with lung tropism and for helping to define gene therapy protocols in lung human cells in vivo.  相似文献   
1000.
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