全文获取类型
收费全文 | 252355篇 |
免费 | 3777篇 |
国内免费 | 1045篇 |
专业分类
电工技术 | 4339篇 |
综合类 | 201篇 |
化学工业 | 37163篇 |
金属工艺 | 8221篇 |
机械仪表 | 7269篇 |
建筑科学 | 6805篇 |
矿业工程 | 543篇 |
能源动力 | 7248篇 |
轻工业 | 28666篇 |
水利工程 | 2124篇 |
石油天然气 | 1852篇 |
武器工业 | 5篇 |
无线电 | 33414篇 |
一般工业技术 | 46232篇 |
冶金工业 | 46522篇 |
原子能技术 | 3367篇 |
自动化技术 | 23206篇 |
出版年
2021年 | 1807篇 |
2020年 | 1348篇 |
2019年 | 1667篇 |
2018年 | 2576篇 |
2017年 | 2599篇 |
2016年 | 2881篇 |
2015年 | 2169篇 |
2014年 | 3660篇 |
2013年 | 11969篇 |
2012年 | 6769篇 |
2011年 | 9681篇 |
2010年 | 7450篇 |
2009年 | 8408篇 |
2008年 | 8929篇 |
2007年 | 8945篇 |
2006年 | 8292篇 |
2005年 | 7494篇 |
2004年 | 7216篇 |
2003年 | 7106篇 |
2002年 | 6600篇 |
2001年 | 7025篇 |
2000年 | 6330篇 |
1999年 | 6828篇 |
1998年 | 16413篇 |
1997年 | 11381篇 |
1996年 | 8595篇 |
1995年 | 6546篇 |
1994年 | 5711篇 |
1993年 | 5571篇 |
1992年 | 3997篇 |
1991年 | 3775篇 |
1990年 | 3550篇 |
1989年 | 3371篇 |
1988年 | 3281篇 |
1987年 | 2590篇 |
1986年 | 2491篇 |
1985年 | 3159篇 |
1984年 | 2799篇 |
1983年 | 2561篇 |
1982年 | 2346篇 |
1981年 | 2391篇 |
1980年 | 2201篇 |
1979年 | 2050篇 |
1978年 | 1880篇 |
1977年 | 2179篇 |
1976年 | 2731篇 |
1975年 | 1584篇 |
1974年 | 1492篇 |
1973年 | 1558篇 |
1972年 | 1145篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
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
102.
R. Oliva A. L. Gemal A. W. N brega A. C. P. Araú jo 《Food Additives & Contaminants》2003,20(8):758-763
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
103.
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
104.
105.
106.
107.
108.
Seo K. Heiblum M. Knoedler C.M. Oh J.E. Pamulapati J. Bhattacharya P. 《Electron Device Letters, IEEE》1989,10(2):73-75
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
109.
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
110.
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