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   98139篇
  免费   6916篇
  国内免费   3121篇
电工技术   4412篇
技术理论   6篇
综合类   4832篇
化学工业   13730篇
金属工艺   3578篇
机械仪表   4433篇
建筑科学   5597篇
矿业工程   2090篇
能源动力   2284篇
轻工业   6216篇
水利工程   1774篇
石油天然气   2725篇
武器工业   590篇
无线电   8331篇
一般工业技术   9722篇
冶金工业   23671篇
原子能技术   871篇
自动化技术   13314篇
  2024年   375篇
  2023年   1337篇
  2022年   2292篇
  2021年   2968篇
  2020年   2050篇
  2019年   1773篇
  2018年   2316篇
  2017年   2724篇
  2016年   2833篇
  2015年   3239篇
  2014年   3395篇
  2013年   3911篇
  2012年   6017篇
  2011年   6334篇
  2010年   4371篇
  2009年   4265篇
  2008年   3972篇
  2007年   3579篇
  2006年   3513篇
  2005年   5646篇
  2004年   4223篇
  2003年   3417篇
  2002年   2326篇
  2001年   2026篇
  2000年   1441篇
  1999年   1792篇
  1998年   7001篇
  1997年   4534篇
  1996年   3188篇
  1995年   1975篇
  1994年   1537篇
  1993年   1423篇
  1992年   527篇
  1991年   507篇
  1990年   450篇
  1989年   409篇
  1988年   372篇
  1987年   278篇
  1986年   240篇
  1985年   196篇
  1984年   90篇
  1983年   96篇
  1982年   146篇
  1981年   188篇
  1980年   207篇
  1979年   68篇
  1978年   97篇
  1977年   609篇
  1976年   1321篇
  1975年   99篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
131.
Head and eye movements were simultaneously recorded during locomotory and pecking behavior of 4 pigeons, which were trained to traverse a conditioning chamber, with a pecking key and a food dispenser at each end. Each trial involved key pecking, walking, and feeding. Head movements were registered with a skull-mounted miniature accelerometer, and eye movements were recorded with implanted electrooculogram (EOG) electrodes. An almost perfect temporal coordination between head and eye movements was observed during both walking and feeding bouts. During walking, head movements primarily provide retinal image stability, and eye movements support visual scanning. During feeding, head movements mainly subserve the grasping of food items, and eye movements maintain visual fixation on them. Because the eyes are reflexively closed during the middle phase of pecks, the head and eye movements are then under ballistic control. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
132.
Indices of fluid balance were evaluated during and after aortic surgery in 16 consecutive patients. Thoracic electrical impedance (TI), heart rate (HR), central venous (CVP), pulmonary artery mean (PAMP), pulmonary wedge (PWP) and mean arterial (MAP) pressures as well as fourteen arterial and venous blood gas variables were followed. Consistent with a reduction of TI during the operation, fluid balance was in excess, and it remained elevated on the first postoperative morning. The HR, MAP and PWP remained stable, while CVP and PAMP decreased. Of the determined variables only TI revealed a meaningful correlation to fluid balance (rho = -0.41; p < 0.01). The results indicate that while central venous and pulmonary artery mean pressures gave the impression of a volume deficit, the positive fluid balance was mirrored by thoracic electrical impedance.  相似文献   
133.
134.
135.
A modified natural surfactant was administered to a patient with life-threatening adult respiratory distress syndrome caused by viral pneumonia. Subsequently, there was a marked improvement in gas exchange. In order to assess the mechanism for improved oxygenation, computed tomography of the lungs was done. Quantitative analysis of the scans taken before and after surfactant administration indicates that improvement in gas exchange was largely due to the expansion of underinflated and collapsed lung areas. Although this is a single case report, it provides insight into the possible beneficial effect of instilled surfactant in severe respiratory distress from viral pneumonia.  相似文献   
136.
137.
138.
Overproduction of proteinase A by recombinant Saccharomyces cerevisiae was investigated by cultivations in a cell-recycling bioreactor. Membrane filtration was used to separate cells from the broth. Recycling ratios and dilution rates were varied and the effect on enzyme production was studied both experimentally and by computer simulations. Experiments and simulations showed that cell mass and product concentration were enhanced by high ratios of recycling. Additional simulations showed that the proteinase A concentration decreased drastically at high dilution rates and the optimal volumetric productivities were at high dilution rates just below washout and at high ratios of recycling. Cell-recycling fermentation gave much higher volumetric productivities and stable product concentrations in contrast to simple continuous fermentation.  相似文献   
139.
Organochlorines are persistent and highly lipophilic environmental contaminants which bioaccumulate in the food chain. Some of these chemicals, 2,2-bis(p-chlorophenyl)-1,1,1-trichloroethane (DDT) and polychlorinated biphenyls (PCBs), have been suggested to be of significance in the aetiology of breast cancer. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is an anti-oestrogen in animal studies and should be thus lower the risk of breast cancer. The other isomers of polychlorinated dibenzo-p-dioxins (PCDDs) or the chemically related polychlorinated dibenzofurans (PCDFs) have not been tested regarding carcinogenesis of the breast. The purpose of this study was to investigate whether PCDDs or PCDFs influence the risk for breast cancer. Consecutive patients who underwent surgery for a breast disease between 1993 and 1995 were recruited for the study. Cases were 22 patients with infiltrative breast cancer and controls were 19 patients operated for a benign breast disease during the same time period. Approximately 10 g of breast tissue free from tumour was taken from the specimen and frozen until analysis. Fat was extracted, cleaned and analysed with a high-resolution gas chromatograph coupled to a high-resolution mass spectrometer. Median concentrations of octachlorinated dibenzo-p-dioxin (OCDD) were 598 (170-14,880) and 396 (103-1,847) pg/g lipid in the cases and in the controls, respectively. In a multivariate logistic regression analysis controlling for other risk factors for breast cancer increased odds ratio (OR) was obtained for OCDD: 401-1000 pg/g lipid yielded OR 3.8, 95% confidence interval (CI) 0.4-39, > 1000 pg/g lipid gave OR 5.2, CI 0.4-72. When the lipid OCDD variable was examined as a continuous risk factor there was a 1.09 (9%), CI 0.95-1.25, increase in the adjusted OR for breast cancer per 100 unit (pg/g lipid) increase in OCDD. No differences were found between cases and controls for the other six tested PCDDs. Mean concentration of TCDD was in the cases 3.6 (1.0-7.9) and in the controls 3.3 (1.1-6.3) pg/g lipid. For PCDFs no significant differences were found between cases and controls. The results were not changed if oestrogen or progesterone receptor status, S-phase fraction and DNA ploidy were considered. Breast tissue concentration of OCDD was increased in cancer patients, whereas the concentrations of other PCDDs and PCDFs were equal in cases and controls.  相似文献   
140.
In rhodopsin, the 11-cis-retinal chromophore forms a complex with Lys296 of opsin via a protonated Schiff base. Absorption of light initiates the activation of rhodopsin by cis/trans photoisomerization of retinal. Thermal relaxation through different intermediates leads into the metarhodopsin states which bind and activate transducin (Gt) and rhodopsin kinase (RK). all-trans-Retinal also recombines with opsin independent of light, forming activating species of the receptor. In this study, we examined the mechanism by which all-trans-retinal activates opsin. To exclude other amines except active site Lys296 from formation of Schiff bases, we reductively methylated rhodopsin (PM-rhodopsin), which we then bleached to generate PM-opsin. Using spectroscopic methods and a Gt activation assay, we found that all-trans-retinal interacted with PM-opsin, producing a noncovalent complex that activated Gt. The residual nucleotide exchange in Gt catalyzed by opsin was approximately 1/250 lower relative to that of photoactivated rhodopsin (pH 8.0, 23 degrees C). Addition of equimolar all-trans-retinal led to an occupancy of one-tenth of the putative retinal binding site(s) of opsin and enhanced the Gt activation rate 2-fold. When the concentration of all-trans-retinal was increased to saturation, the Gt activation rate of the opsin/all-trans-retinal complex was approximately 1/33 lower compared to that of photoactivated rhodopsin. We conclude that all-trans-retinal can form a noncovalent complex with opsin that activates Gt by different mechanisms than photolyzed rhodopsin.  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