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   540927篇
  免费   7026篇
  国内免费   1859篇
电工技术   10156篇
综合类   503篇
化学工业   82762篇
金属工艺   21139篇
机械仪表   17590篇
建筑科学   12481篇
矿业工程   2525篇
能源动力   14717篇
轻工业   45533篇
水利工程   5301篇
石油天然气   8478篇
武器工业   24篇
无线电   65212篇
一般工业技术   107491篇
冶金工业   98851篇
原子能技术   11035篇
自动化技术   46014篇
  2021年   4842篇
  2020年   3705篇
  2019年   4813篇
  2018年   7965篇
  2017年   7957篇
  2016年   8514篇
  2015年   5647篇
  2014年   9126篇
  2013年   26415篇
  2012年   14716篇
  2011年   19500篇
  2010年   15513篇
  2009年   17451篇
  2008年   18107篇
  2007年   18066篇
  2006年   16045篇
  2005年   14479篇
  2004年   13834篇
  2003年   13615篇
  2002年   13049篇
  2001年   12845篇
  2000年   12105篇
  1999年   12570篇
  1998年   30732篇
  1997年   21567篇
  1996年   16853篇
  1995年   12714篇
  1994年   11240篇
  1993年   11183篇
  1992年   8353篇
  1991年   7902篇
  1990年   7859篇
  1989年   7553篇
  1988年   7122篇
  1987年   6321篇
  1986年   6185篇
  1985年   7046篇
  1984年   6502篇
  1983年   5969篇
  1982年   5509篇
  1981年   5517篇
  1980年   5106篇
  1979年   4991篇
  1978年   4900篇
  1977年   5680篇
  1976年   7340篇
  1975年   4274篇
  1974年   3952篇
  1973年   4066篇
  1972年   3381篇
排序方式: 共有10000条查询结果,搜索用时 796 毫秒
971.
The LP-BM5 murine leukemia virus causes acquired immunodeficiency syndrome in C57BL/6J mice (MAIDS), similar to that of AIDS in humans. The objective of this study was to determine the effect of LP-BM5 viral infection on cellular activation and membrane integrity of splenocytes. Oxidative burst in splenocytes in response to exposure to PMA (20 microg/ml) was significantly higher (p<.02) in infected than in control mice at two weeks post-infection using luminol-enhanced chemiluminescence. By 13 weeks post-infection superoxide anion production in infected mice was significantly lower when compared to controls coinciding with decreased proliferative response to mitogens. The extent of cell membrane damage as indicated by lactic dehydrogenase (LDH) activity in serum was significantly higher in infected than in control mice (p<.001). The results from this study suggests that LP-BM5 virus causes an initial stimulation of cellular activity followed by a decreased cell activation characterized by decreased proliferation of splenocytes and decreased oxygen radical production. Decreased cell membrane integrity indicated by increased LDH activity may partly be responsible for these changes.  相似文献   
972.
1. Supramedullary structures including the ventral medial prefrontal cortex (MPFC) and the midbrain cuneiform nucleus (CnF) project directly and indirectly to premotor sympatho-excitatory neurons of the rostral ventrolateral medulla (RVLM) that are critically involved in the generation of sympathetic vasomotor tone. 2. Electrophysiological studies have demonstrated that activation of depressor sites within the MPFC is associated with splanchnic sympathetic vasomotor inhibition and inhibition of the activity of RVLM sympathoexcitatory neurons. 3. Antidromic mapping and anatomical studies support the notion that a relay in the nucleus tractus solitarius is involved in the cardiovascular response to MPFC stimulation. 4. The midbrain CnF, which lies adjacent to the midbrain periaqueductal grey, is a sympathoexcitatory region of the midbrain reticular formation. Sympathoexcitatory responses evoked from the CnF are associated with short-latency excitation of RVLM neurons. 5. Cuneiform nucleus stimulation induces the expression of mRNA for the immediate early genes c-fos and NGFI-A in mid-brain, pontine and hypothalamic structures. 6. The MPFC and CnF are supramedullary structures with opposing modulatory influences on sympathetic vasomotor drive, whose roles in cardiovascular control mechanisms warrant further investigation.  相似文献   
973.
We determine the automorphism group of various Goppa codes 𝒞 L(D,G) associated with certain function fields F/F q of genus g>0. It is well known that, for deg D=n>2g+2, the automorphism group {AutD,G(F/Fq) can be embedded into Aut(𝒞L(D,G)) as a subgroup. We show that, under certain conditions on the divisors D and G AutD,G(F/Fq ) is actually isomorphic to Aut(𝒞L(D,G))  相似文献   
974.
Conventional standard processors do not correspond well to the characteristics of multimedia signal processor algorithms. Therefore, special architectural approaches are necessary for multimedia processors to deliver the required high processing power with efficient use of hardware resources. Programmable approaches offer a high degree of flexibility. In order to attain multimedia signal processor performance, architectural strategies for programmable processors are based on parallelization and adaptation principles. The future multimedia signal processor implementation hinges upon an optimal trade-off between the two design spaces, which can be effectively addressed by a codesign approach  相似文献   
975.
976.
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   
977.
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   
978.
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   
979.
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   
980.
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