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   403711篇
  免费   5133篇
  国内免费   1364篇
电工技术   7565篇
综合类   316篇
化学工业   59923篇
金属工艺   16434篇
机械仪表   13499篇
建筑科学   8357篇
矿业工程   2155篇
能源动力   11557篇
轻工业   28477篇
水利工程   4270篇
石油天然气   7921篇
武器工业   17篇
无线电   49310篇
一般工业技术   83077篇
冶金工业   74970篇
原子能技术   9183篇
自动化技术   33177篇
  2022年   2560篇
  2021年   3907篇
  2020年   2960篇
  2019年   3809篇
  2018年   6490篇
  2017年   6489篇
  2016年   6937篇
  2015年   4326篇
  2014年   7164篇
  2013年   20057篇
  2012年   11438篇
  2011年   15297篇
  2010年   12168篇
  2009年   13729篇
  2008年   14067篇
  2007年   13841篇
  2006年   12436篇
  2005年   11127篇
  2004年   10513篇
  2003年   10425篇
  2002年   9839篇
  2001年   9734篇
  2000年   9142篇
  1999年   9527篇
  1998年   24330篇
  1997年   16715篇
  1996年   12801篇
  1995年   9517篇
  1994年   8365篇
  1993年   8385篇
  1992年   6055篇
  1991年   5697篇
  1990年   5681篇
  1989年   5317篇
  1988年   5045篇
  1987年   4424篇
  1986年   4300篇
  1985年   4814篇
  1984年   4425篇
  1983年   3995篇
  1982年   3655篇
  1981年   3730篇
  1980年   3430篇
  1979年   3327篇
  1978年   3334篇
  1977年   3750篇
  1976年   4847篇
  1975年   2850篇
  1974年   2663篇
  1973年   2730篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
992.
The literature on infants' perception of facial and vocal expressions, combined with data from studies on infant-directed speech, mother-infant interaction, and social referencing, supports the view that infants come to recognize the affective expressions of others through a perceptual differentiation process. Recognition of affective expressions changes from a reliance on multimodally presented information to the recognition of vocal expressions and then of facial expressions alone. Face or voice properties become differentiated and discriminated from the whole, standing for the entire emotional expression. Initially, infants detect information that potentially carries the meaning of emotional expressions; only later do infants discriminate and then recognize those expressions. The author reviews data supporting this view and draws parallels between the perceptions of affective expressions and of speech. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
993.
Insect chitinases: molecular biology and potential use as biopesticides   总被引:2,自引:0,他引:2  
Chitin, an insoluble structural polysaccharide that occurs in the exoskeletal and gut linings of insects, is a metabolic target of selective pest control agents. One potential biopesticide is the insect molting enzyme, chitinase, which degrades chitin to low molecular weight, soluble and insoluble oligosaccharides. For several years, our laboratories have been characterizing this enzyme and its gene. Most recently, we have been developing chitinase for use as a biopesticide to control insect and also fungal pests. Chitinases have been isolated from the tobacco hornworm, Manduca sexta, and several other insect species, and some of their chemical, physical, and kinetic properties have been determined. Also, cDNA and genomic clones for the chitinase from the hornworm have been isolated and characterized. Transgenic plants that express hornworm chitinase constitutively have been generated and found to exhibit host plant resistance. A transformed entomopathogenic virus that produces the enzyme displayed enhanced insecticidal activity. Chitinase also potentiated the efficacy of the toxin from the microbial insecticide, Bacillus thuringiensis. Insect chitinase and its gene are now available for biopesticidal applications in integrated pest management programs. Current knowledge regarding the molecular biology and biopesticidal action of insect and several other types of chitinases is described in this mini-review.  相似文献   
994.
An additional noise component is observed in the noise spectrum of transistors in a partially-depleted (PD) medium-thickness SOI-CMOS technology. We identify the origin of this additional noise in the noisy resistance of the body film. This resistance, coupled to the gate capacitance, forms an RC filter and generates the hump-shape of the additional noise component. Several experimental observations that support this model are presented  相似文献   
995.
We examined the associations between cerebral infarction (CI), asymptomatic arteriosclerosis obliterans (ASO), and known risk factors for these diseases. The subjects were 67 elderly patients (11 men and 56 women, mean +/- SD age of 79.6 +/- 8.5 years). in 44 patients CI was diagnosed by CT scan; 23 were classified as having cortical infarction and 21 as having lacunar infarction. In 41 patients asymptomatic ASO was diagnosed by an ankle-pressure index (API) of less than 0.9. To identify risk factors for these diseases, we examined the association among these diseases and hypertension (blood pressure > or = 140/90 mmHg), hypercholesterolemia (total cholesterol concentration > or = 220 mg/dl), hypertriglyceridemia (triglyceride concentration > or = 150 mg/dl), low HDL-cholesterolemia (HDL-C concentration < 40 mg/dl), high LDL-cholesterolemia (LDL-C concentration > or = 150 mg/dl), and glucose intolerance (fasting blood sugar concentration > or = 110 mg/dl). The incidence of asymptomatic ASO in the subjects with CI was significantly higher than that in the subjects without CI (chi 2 test; p < 0.05, odds ratio 6.4), including cortical infarction (p < 0.05, odds ratio 8.9) and lacunar infarction (p < 0.05, odds ratio 3.8). Patients with lacunar infarction were more likely to have hypertension than were controls (p < 0.05). Cortical infarction was not associated with these risk factors. Both low HDL-C and high LDL-G were more common in patients with asymptomatic ASO than patients without asymptomatic ASO (p < 0.05). These results indicate that CI and asymptomatic ASO are strongly associated in the elderly, especially in subjects with cortical infarction, and that aging itself contributes to cortical infarction.  相似文献   
996.
997.
The cDNA encoding PH-20 hyaluronidase from human sperm has been mutated at five positions by in vitro mutagenesis. We have changed three acidic amino acids and two arginine residues that are conserved in the sequence of mammalian PH-20 polypeptides as well as in the hyaluronidases from bee and hornet venom. Of the former, the mutants [Gln113]PH-20 and [Gln249]PH-20 had no detectable enzymatic activity; the mutant [Asn111]PH-20 had about 3% activity. The mutant [Thr252]PH-20 was also inactive, while [Gly176]PH-20 had only about 1% activity. This indicates that the PH-20 hyaluronidases, like numerous enzymes that hydrolyze glycosidic bonds, have acidic amino acids in their active site. Moreover, for the binding of the substrate, the polyanion hyaluronan, arginine residues appear to be essential.  相似文献   
998.
Set out is a brief account of the two major accomplishments by the Russian Research Center ‘Kurchatov Institute’ in creating the full-scope simulators and mathematical modeling technologies. Presented are the basic specifications of one of the world's largest simulators—the full-scope simulator for the Leningrad NPP which is the new-generation one. Owing to the extended modeling scope accomplished is the possibility of training personnel to act in terms of not only the design-basis but rather beyond the design-basis accidents. To minimize the expenditures for creating the simulators, analyzers and other modeling and control means, the RRC ‘Kurchatov Institute' has created the unique technology of mathmodeling automation. Thanks to its versatility and application at its creation of the ELUD philosophy (easy to learn, use and develop) good use is made of this technology both in nuclear and thermal power engineering, as well as in gas industry.  相似文献   
999.
The structure of SiC–AlN powders is investigated by x-ray diffraction and transmission electron microscopy methods. The powders were produced by joint carbon reduction and nitriding of silicon and aluminum oxide mixtures. The results show that a mixture of solid solutions forms during joint SiC and AlN synthesis at 1700°C, with SiC forming β (3C) and α (2H) modifications with different grain morphology. The fiber form is characteristic of β-SiC, whereas the grains of the solid solution based on SiC have a predominantly equiaxed form. α-SiC grain dimensions are considerablys smaller than those of AlN. Institute of Materials Science, National Academy of Sciences of Ukraine. Kiev Translated from Poroshkovaya Metallurgiya, Nos. 7–8, pp. 81–86, July–August, 1997.  相似文献   
1000.
Quality improvement techniques provide a scientific approach that allows nurses and other health care professionals to improve patient satisfaction and outcomes. Continuous quality improvement (CQI) encourages the health care team to move beyond minimum standards of care and create an environment in which all team members are continuously working to improve services. This article reviews the principles of CQI and discusses the nurses' role in implementing and maintaining a successful CQI program. Anemia management is used as an example to illustrate how CQI principles and tools can lead to improvements in patient outcomes.  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