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   5865篇
  免费   17篇
  国内免费   2篇
电工技术   32篇
综合类   1篇
化学工业   234篇
金属工艺   26篇
机械仪表   31篇
建筑科学   53篇
矿业工程   2篇
能源动力   32篇
轻工业   87篇
水利工程   10篇
石油天然气   2篇
无线电   150篇
一般工业技术   198篇
冶金工业   4878篇
原子能技术   10篇
自动化技术   138篇
  2021年   9篇
  2020年   9篇
  2019年   8篇
  2018年   11篇
  2017年   16篇
  2015年   12篇
  2014年   12篇
  2013年   47篇
  2012年   23篇
  2011年   38篇
  2010年   25篇
  2009年   49篇
  2008年   49篇
  2007年   47篇
  2006年   41篇
  2005年   39篇
  2004年   29篇
  2003年   31篇
  2002年   35篇
  2001年   27篇
  2000年   19篇
  1999年   173篇
  1998年   1453篇
  1997年   811篇
  1996年   547篇
  1995年   314篇
  1994年   265篇
  1993年   287篇
  1992年   54篇
  1991年   89篇
  1990年   80篇
  1989年   97篇
  1988年   91篇
  1987年   88篇
  1986年   89篇
  1985年   60篇
  1984年   12篇
  1983年   20篇
  1982年   25篇
  1981年   35篇
  1980年   57篇
  1979年   12篇
  1978年   30篇
  1977年   137篇
  1976年   286篇
  1975年   14篇
  1974年   13篇
  1971年   9篇
  1955年   7篇
  1954年   7篇
排序方式: 共有5884条查询结果,搜索用时 0 毫秒
41.
42.
43.
44.
45.
This article is a discussion of the use of narrow-band light sources coupled with cameras equipped with band-pass filters to document patterned injuries on human skin. Several case reports are included.  相似文献   
46.
Mineralocorticoid action is facilitated by 11 beta-hydroxysteroid dehydrogenase type II (11 beta HSD2), which metabolizes glucocorticoids and allows aldosterone to bind to the nonselective mineralocorticoid receptor. We have recently demonstrated the presence of the 11 beta HSD2 protein in a wide range of human epithelia, suggesting that it is the sole isoform endowing specificity in man. In the present study we have used an immunopurified polyclonal antibody (RAH23) raised against a C-terminal peptide derived from the cloned rat 11 beta HSD2 protein to perform immunohistochemical and molecular analysis in rat tissues. In frozen sections of rat kidney, strong staining was seen with the RAH23 antibody in the distal tubule; weaker staining was observed in the thick ascending loop of Henle and the medullary and papillary collecting ducts. Punctate cortical staining was observed in the fetus at 20 days gestation and in 8-day-old rats, with a noticeable increase in the staining pattern at 16 days of age. The kidney did not attain the adult pattern of staining until 28 days of age. Epithelia of ileum and colon also stained with RAH23, as did excretory ducts of the submandibular gland. Intrahepatic and excretory bile ducts displayed strong immunoreactivity in the epithelial lining. Rat adrenal glands showed evidence of the 11 beta HSD2 antigen in the zona fasciculata and zona reticularis, but not in the zona glomerulosa or medulla. Western blot analysis with the RAH23 antibody revealed strong bands in the kidney, colon, adrenal gland, and submandibular gland at 40 kDa, colinear with the migration of the cloned 11 beta HSD2 enzyme. A band of medium intensity was also seen at this size in the pancreas, whereas a band of moderate intensity was seen in the bile duct, and weaker bands were noticed in the stomach, small intestine, and liver, with a diffuse band at 36-42 kDa in the prostate. Strong bands were seen in the pancreas and prostate at 78 kDa, with weaker signals in the colon, adrenal, stomach, and bile duct. A number of tissues also displayed multiple bands at about 30 kDa. Enzymatic assays on tissue homogenates showed extensive conversion of corticosterone to its 11-dehydro product in an NAD-dependent manner in the submandibular gland, adrenal gland, and kidney, but not in the pancreas or prostate. This study confirms the ubiquitous presence of 11 beta HSD2 in sodium-transporting epithelia, demonstrates the high level of 11 beta HSD2 protein and enzyme activity in the rat adrenal, and suggests a possible role for the enzyme in the biliary system. Further studies are required to determine the relevance of the various molecular species to the activity, latency, and processing of the enzyme.  相似文献   
47.
We have isolated a putative serine/threonine receptor kinase gene with an expression pattern indicating that it may play a role in the stylar response to pollination. Differential display PCR was used to select tobacco mRNAs with increased accumulation following pollination. NTS16, a cDNA identified by this method, is homologous to a ca. 2.4 kb mRNA primarily expressed in pistil tissues. Levels of this mRNA increase during floral development and are further increased by pollination reaching maximal accumulation 12-18 hours after pollination and then declining. mRNA levels can also be increased by the application of ethylene to unpollinated flowers. A polypeptide encoded by the NTS 16 open reading frame has sequence similarity to the catalytic domain of several receptor protein kinases from plants including the S-receptor kinases implicated in the rejection of self-pollen in Brassica species and the Pto gene product of tomato which confers resistance to a bacterial pathogen.  相似文献   
48.
This study assessed the cost-effectiveness of two low-intensity programs for quitting smokeless tobacco, based on results of a randomized trial with 1,069 volunteer participants. Cost data were collected for two levels of intervention: manual only (a self-help manual) and assisted self-help (the manual plus a videotape and two supportive phone calls from tobacco cessation counselors). Incremental cost-effectiveness ratios were calculated for assisted self-help vs. quitting on one's own, using the manual-only quit rate and data from another study as alternative proxies for no intervention. A threshold analysis was conducted to determine the spontaneous quit rate at which the manual-only intervention becomes more cost-effective than assisted self-help. The cost to provide and receive the assisted self-help intervention averaged US $56 per participant vs. $20 for the manual-only intervention (societal perspective, Year 2000 dollars). Estimates for incremental cost per quit for the assisted self-help intervention ranged from $922 to $1,758, depending on the proxy used for no intervention. The manual-only intervention was more cost-effective than assisted self-help if quitting among motivated chewers who do not receive treatment does not exceed 3.4%. Support from a wife or partner added little cost to a quit attempt for male chewers ($3-$4). Providing a manual, video, and brief phone counseling to smokeless tobacco users who want to quit is a reasonable use of health care resources. The self-help quitting guide also may be a cost-effective treatment, but it remains to be demonstrated whether it is more effective than quitting on one's own.  相似文献   
49.
This paper presents long-term outcomes of the largest clinical trial of smokeless tobacco (SLT) cessation reported to date. SLT users in five northwestern states were recruited to call a toll-free number, and 1,069 users were randomized to one of two self-help conditions: either a manual-only condition or an assisted self-help condition, which included the manual, a targeted video, and two support phone calls. Significant between-group differences were not found for either the 12- or 18-month point-prevalence measure of abstinence from either SLT only or all tobacco products using outcomes based on either the responder or intention-to-treat outcomes. However, using a repeated point-prevalence measure across all three assessment points, we found that significantly more assisted self-help participants reported abstinence, compared with manual-only participants. Compared with manual-only participants, those in the assisted self-help condition were significantly more likely to use recommended cessation techniques. Results demonstrate that low-cost, minimal interventions delivered by mail and phone can help a sizable proportion of individuals quit using SLT.  相似文献   
50.
The British otter Lutra lutra L. requires good water quality, an adequate supply of fish and certain types of vegetation or cavities (holts) adjacent to water bodies for resting and breeding sites. In the U.K. otters and their holts are legally protected. Trained surveyors are needed to confirm the presence of otters on a river as the animals are rarely seen, but their tracks and droppings (spraints) may be detected. To evaluate a river for otters, information is required on water quality, fish species and biomass, as well as available habitat. Pioneer evaluation work based on the River Corridor Survey techniques has been done on Welsh rivers with good otter populations. Riparian vegetation, especially trees, is shown to be particularly important. This work is now being extended to areas on the margins of otter distribution such as the lower river Wye in England and Wales. Such surveys provide important management information for water authorities and give opportunities for enhancement work to benefit this animal.  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