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   255142篇
  免费   4062篇
  国内免费   1047篇
电工技术   4357篇
综合类   206篇
化学工业   38046篇
金属工艺   8291篇
机械仪表   7481篇
建筑科学   6831篇
矿业工程   542篇
能源动力   7362篇
轻工业   29007篇
水利工程   2129篇
石油天然气   1852篇
武器工业   5篇
无线电   33953篇
一般工业技术   46833篇
冶金工业   46543篇
原子能技术   3395篇
自动化技术   23418篇
  2021年   1902篇
  2020年   1430篇
  2019年   1755篇
  2018年   2676篇
  2017年   2713篇
  2016年   3029篇
  2015年   2291篇
  2014年   3834篇
  2013年   12176篇
  2012年   6995篇
  2011年   9924篇
  2010年   7636篇
  2009年   8575篇
  2008年   9091篇
  2007年   9056篇
  2006年   8405篇
  2005年   7585篇
  2004年   7307篇
  2003年   7169篇
  2002年   6689篇
  2001年   7085篇
  2000年   6401篇
  1999年   6873篇
  1998年   16438篇
  1997年   11396篇
  1996年   8611篇
  1995年   6567篇
  1994年   5717篇
  1993年   5575篇
  1992年   4003篇
  1991年   3778篇
  1990年   3559篇
  1989年   3377篇
  1988年   3282篇
  1987年   2598篇
  1986年   2491篇
  1985年   3164篇
  1984年   2800篇
  1983年   2562篇
  1982年   2344篇
  1981年   2391篇
  1980年   2203篇
  1979年   2049篇
  1978年   1880篇
  1977年   2177篇
  1976年   2727篇
  1975年   1586篇
  1974年   1490篇
  1973年   1558篇
  1972年   1144篇
排序方式: 共有10000条查询结果,搜索用时 14 毫秒
91.
There are some foods that contain mutagenic or carcinogenic agents, some of which occur naturally and others that may be formed during preparation or cooking. Several foods such as legumes, also contain natural antimutagens and/or anticarcinogens. Lupine is one such legume that contains high amounts of protein (40%) and oils (14%). About 90 species of lupine have been reported throughout Mexico. However, the use of this crop as a source of food has been limited by the presence of antinutritional agents such as phenolic compounds (PC), carbohydrates (CH) and quinolizidinic alkaloids (Qas). It has also been suggested that consuming these compounds can affect human health and may even reduce the risk of disease. The objective of this work was to determine the effect of PC, CH and Qas, isolated and quantified from Lupinus campestris on the mutagenicity of 1-nitropyrene (1-NP) as a model mutagen and we used the Salmonella typhimurium tester strain YG1024 by the Kado microsuspension method. The results indicate that L. campestris seeds have 11 mg (+)catechin equivalent g(-1) seed coat; 120.3 mg g(-1) seeds and 2.13 mg g(-1) seeds of PC, CH and Qas, respectively. 1-NP mutagenicity was inhibited by 86% for PC, 76% for CH and 75% for Qas at concentrations of 200, 512 and 13.6 microg/tube, respectively.  相似文献   
92.
The structure of the recently introduced Brazilian government programme for monitoring pesticide residues in six major agricultural products is described. Residues found from the analysis of samples of tomatoes and strawberries are discussed in detail. The much greater frequency of detection of pesticide residues, many of which are not approved for use in Brazil, in produce being sold on the domestic market, as opposed to that produced for export, is of major concern. It is perceived that this new monitoring programme will become permanent and be used to underpin enforcement actions that will need to be taken to reduce the numbers of incidents of illegal residues occurring, particularly in fresh fruits and vegetables.  相似文献   
93.
Neutron-irradiated quartz crystal is one of the interesting materials for the study of the glassy dynamics. The temperature and frequency dependence of the sound velocity in amorphous materials at temperatures above the low-temperature maximum is not well understood. In this paper we present the results of our measurements of the variation of the ultrasound velocity (270 – 650 MHz) in neutron-irradiated quartz crystals with different irradiation doses in temperature range 5 - 35 K. We have found that the temperature dependence of the ultrasound velocity is linear in the sample with a dose < 10 19 n/cm2 and is sublinear in samples with a dose > 10 19 n/cm2 . We discuss our results in the framework of the different theoretical approaches. PACS numbers: 61.43. j, 62.65.+k, 63.  相似文献   
94.
95.
96.
97.
98.
A high-gain ballistic hot-electron device is described. The GaAs-AlGaAs heterostructure device, with a 21-mm-thick pseudomorphic In 0.12Ga0.88As base, had a current gain of 27 at 77 K and 41 at 4.2 K. As characteristically seen in ballistic devices, transfer into the L valley limited the maximum gain. The Γ-L valley separation in the strained In0.12Ga0.88As was estimated to be about 380 meV  相似文献   
99.
A combination of concept (the product portfolio and the product life cycle) and empirical research (the PIMS study) can aid in clarifying the key issues involved in the selection of a firm's core strategies. Strategy control is multistage: periodic re-evaluation of strategy alternatives by use of the marketing audit; an analytical framework for control of chosen strategies by breaking down components and reporting their performance variances; and 'marketing segment' analysis for control of the detailed marketing plan.?  相似文献   
100.
Polystyrene packaging material taint was sensorily evaluated in cocoa powder for drinks and chocolate flakes using short-cut signal detection measures on differences between control and test samples and on recognition of styrene. No differences were observed in cocoa powder for drinks and plain chocolate flakes treated with 0.5 dm2 polystyrene of 1 mm thickness. However, differences were detected in milk chocolate flakes and plain chocolate flakes, which were in contact with a larger area or thicker polystyrene packaging material. The latter results were confirmed by the styrene recognition test, so polystyrene is a potential source of off-flavour for chocolate products. The amount of residual styrene in the polystyrene used was about 320 ppm, while the amounts of styrene ranged from 7 to 132 ppb in cocoa drinks and from 414 to 1447 ppb in chocolate flakes.  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