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   507231篇
  免费   5869篇
  国内免费   1100篇
电工技术   9601篇
综合类   809篇
化学工业   79045篇
金属工艺   18244篇
机械仪表   14881篇
建筑科学   12456篇
矿业工程   2171篇
能源动力   12645篇
轻工业   49646篇
水利工程   4614篇
石油天然气   8112篇
武器工业   39篇
无线电   59072篇
一般工业技术   94971篇
冶金工业   95512篇
原子能技术   10356篇
自动化技术   42026篇
  2021年   4060篇
  2019年   3843篇
  2018年   6545篇
  2017年   6331篇
  2016年   6659篇
  2015年   4463篇
  2014年   7669篇
  2013年   22770篇
  2012年   12377篇
  2011年   17006篇
  2010年   13332篇
  2009年   15123篇
  2008年   15808篇
  2007年   15831篇
  2006年   14106篇
  2005年   12767篇
  2004年   12405篇
  2003年   12048篇
  2002年   11938篇
  2001年   11921篇
  2000年   11483篇
  1999年   11760篇
  1998年   27914篇
  1997年   20131篇
  1996年   15694篇
  1995年   12176篇
  1994年   11018篇
  1993年   10669篇
  1992年   8221篇
  1991年   7701篇
  1990年   7821篇
  1989年   7586篇
  1988年   7303篇
  1987年   6231篇
  1986年   6403篇
  1985年   7413篇
  1984年   6848篇
  1983年   6364篇
  1982年   5849篇
  1981年   6031篇
  1980年   5727篇
  1979年   5568篇
  1978年   5418篇
  1977年   6268篇
  1976年   8088篇
  1975年   4763篇
  1974年   4530篇
  1973年   4640篇
  1972年   3822篇
  1971年   3484篇
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
101.
High division of the brachial artery was observed in two cadavers, during routine dissection of upper extremities. In the first case, the brachial artery of the right upper extremity was divided into its two terminal branches immediately after passing between the lateral and medial roots of the median nerve and just below the origin of the profunda brachii artery. The lateral branch was the radial artery, located in the space normally occupied by the brachial artery and the medial one was the ulnar artery. In the second case, the brachial artery was divided into its two terminal branches just below the origin of the profunda brachii artery. Accurate knowledge of the relationships and course of these major arterial conduits and particularly of their variational patterns, is of considerable practical importance in the conduct of reperative surgery in the arm, forearm and hand.  相似文献   
102.
Code-division multiple-access (CDMA) implemented with direct-sequence spread spectrum (DS/SS) signaling is a promising multiplexing technique for cellular telecommunications services. The efficiency of a direct-sequence spread-spectrum code-division multiple-access (DS-CDMA) system depends heavily on the shape of the spectrum of the spread signal. Maximum efficiency is obtained with an ideal brick-wall bandpass spectrum. There are two approaches toward achieving such a spectrum. One is to use a simple spreader that produces a broad spectrum and then follow it with a precise, high order filter to band limit the spectrum. A second approach, which is the approach taken in this paper, is to use a spreader that produces a spectrum close to the ideal spectrum and then employ a simple filter to control the out-of-band power. The proposed spreader/despreader is based on a simple hybrid function and can be easily implemented. An analysis provides a compact expression for the signal-to-noise ratio (SNR) of a RAKE receiver. The expression includes the effects of baseband, intermediate frequency (IF) and RF filtering as well as the effects of the spectral densities of the spreading/despreading functions. The analysis shows that the proposed spreader/despreader yields superior performance over a conventional pseudo noise (PN) spreading/despreading mechanism  相似文献   
103.
The transverse resonance approach to guided wave analysis is applied to shear horizontal (SH) wave propagation in periodically layered composites. It is found for SH waves that at high values of the guided wavevector β, the wave energy is trapped in the slower of the two media and propagates accordingly at the slower wavespeed. At low values of β, however, the modes demonstrate a clustering behavior, indicative of the underlying Floquet wave structure. The number of modes in a cluster is observed to correlate with the number of unit cells in the layered plate. New physical insights into the behavior of these systems are obtained by analyzing the partial waves of the guided SH modes in terms of Floquet waves. We show that the fast and slow shear waves in the periodically layered composite play an analogous role to the longitudinal and shear partial waves comprising Lamb waves in a homogeneous plate  相似文献   
104.
105.
106.
The biotransformation of oxazepam by Bifidobacterium bifidum was studied. The major metabolite was purified by chromatographic methods and found to be desmethyldiazepam using NMR, IR and other physicochemical data.  相似文献   
107.
Data from the past few years have shown that as caffeine metabolizes solely in the liver, caffeine elimination can serve as a liver function test. We have collected data by monitoring 40 persons with liver diseases (11 chronic alcoholic hepatitis, 24 liver cirrhosis, 5 non-cirrhotic liver disease). Eight subjects served as controls. The patients with liver cirrhosis were classified according to the Child--Pugh scoring system. To determine caffeine elimination blood samples were collected before and at 3, 6, 9 and 12 hours after oral administration of 0.2 g caffeine. Fasting serum caffeine concentration and concentration 12 hours after administration, serum clearance, half life, peak concentration and volume of distribution have been compared. The respective values measured in patients with non-cirrhotic liver diseases did not differ significantly from the controls. The disappearance of caffeine was significantly decreased in cirrhotics. Our results demonstrated a good correlation between impairment of caffeine elimination and assessment of severity of liver disease by the Child--Pugh classification. Measuring serum levels in samples taken 12 hours after caffeine administration is a simple and useful method in the diagnosis of liver diseases at cirrhotic stage.  相似文献   
108.
A review of isolates of Neisseria meningitidis and other laboratory evidence of meningococcal infection was carried out for the period 1990-1995. Thirty-three patients with evidence of meningococcal disease were identified over the six year period. Group B organisms were the most commonly identified, but an increase in Group C has occurred and is almost equal in number. Data from clinical, laboratory and public health sources should be centralized in a National Surveillance Centre.  相似文献   
109.
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