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   5076篇
  免费   58篇
  国内免费   3篇
电工技术   23篇
综合类   2篇
化学工业   294篇
金属工艺   39篇
机械仪表   34篇
建筑科学   77篇
矿业工程   6篇
能源动力   61篇
轻工业   267篇
水利工程   21篇
石油天然气   2篇
无线电   233篇
一般工业技术   277篇
冶金工业   3573篇
原子能技术   23篇
自动化技术   205篇
  2023年   12篇
  2022年   16篇
  2021年   22篇
  2020年   10篇
  2019年   18篇
  2018年   25篇
  2017年   25篇
  2016年   37篇
  2015年   30篇
  2014年   23篇
  2013年   80篇
  2012年   51篇
  2011年   100篇
  2010年   63篇
  2009年   68篇
  2008年   91篇
  2007年   93篇
  2006年   98篇
  2005年   76篇
  2004年   56篇
  2003年   57篇
  2002年   68篇
  2001年   60篇
  2000年   35篇
  1999年   139篇
  1998年   1126篇
  1997年   672篇
  1996年   365篇
  1995年   238篇
  1994年   215篇
  1993年   230篇
  1992年   48篇
  1991年   52篇
  1990年   51篇
  1989年   70篇
  1988年   54篇
  1987年   49篇
  1986年   41篇
  1985年   46篇
  1984年   26篇
  1983年   25篇
  1982年   35篇
  1981年   26篇
  1980年   42篇
  1979年   12篇
  1978年   15篇
  1977年   84篇
  1976年   183篇
  1975年   13篇
  1973年   10篇
排序方式: 共有5137条查询结果,搜索用时 31 毫秒
61.
Seven hundred three members of the Society for Industrial and Organizational Psychology indicated agreement or disagreement with 49 propositions regarding cognitive ability tests in organizations. There was consensus that cognitive ability tests are valid and fair, that they provide good but incomplete measures, that different abilities are necessary for different jobs, and that diversity is valuable. Items dealing with the unique status of cognitive ability were most likely to generate polarized opinions. A 2-factor model, classifying items as those reflecting societal concerns over the consequences of ability testing and those reflecting an emphasis on the unique status of "g," fit the data well, and these factors proved especially important for predicting responses to the more controversial items. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
62.
63.
64.
65.
66.
67.
Fast atom bombardment ionization with tandem mass spectrometry of both positive and negative ions is a useful technique for the identification of intact glycerophosphoethanolamine (GPE) phospholipids, providing information as to polar head group and fatty acyl substituents. In the identification of GPE molecular species, positive ion neutral loss scanning for 141 units was attempted to confirm the presence of the phosphoethanolamine polar head group. This scan was found to discriminate against the abundant subclass of phospholipids having an 1-O-alk-1'-enyl linkage, termed plasmalogens, as well as 1-O-alkyl ether species. The neutral loss process is suggested to involve attack of a carbonyl oxygen from either sn-1 or sn-2 on the sn-3 methylene carbon with loss of neutral phosphoethanolamine. Using FAB/MS/MS alone, it is not possible to differentiate between plasmalogens and other 1-O-alkyl ether molecular species having the same molecular weight. The combination of mild acid hydrolysis, which selectively hydrolyzes the labile 1-O-alk-1'-enyl bond, with subsequent FAB/MS/MS distinguished species of these distinct subclasses. Using these techniques and precursor ion scans for the arachidonoyl carboxylate anion, m/z 303, the arachidonic acid containing glycerophosphoethanolamine molecular species were identified and the relative abundance of arachidonoyl plasmalogen, alkylacyl, and 1,2-diacyl GPE molecular species in the human polymorphonuclear leukocyte (neutrophil) was determined to be 75.4%, 12.1%, and 12.5%, respectively. These values were not significantly different from that reported in the literature using conventional methodology.  相似文献   
68.
The tropical marine spongeAmphimedon terpenensis (family Niphatidae, order Haplosclerida) has previously been shown to possess unusual lipids, including unusual fatty acids. The biosynthetic origin of these fatty acids is of interest as the sponge supports a significant population of eubacterial and cyanobacterial symbionts. The total fatty acid composition of the sponge was analyzed by gas chromatography/mass spectrometry of the methyl esters. Among the most abundant of the fatty acids in intact tissue were 16∶0, 18∶0 and 3,7,11,15-tetramethylhexadecanoic (phytanic) acid. In addition, three brominated fatty acids, (5E,9Z)-6-bromo-5,9-tetracosadienoic acid (24∶2Br), (5E,9Z)-6-bromo-5,9-pentacosadienoic acid (25∶2Br) and (5E,9Z)-6-bromo-5,9-hexacosadienoic acid (26∶2Br) were also present. The three brominated fatty acids, together with phytanic acid, were isolated from both ectosomoal (superficial) and choanosomal (internal) regions of the sponge. Analysis of extracts prepared from sponge/symbiont cells, partitioned by density gradient centrifugation on Ficoll, indicated that phytanic acid and the three brominated fatty acids were associated with sponge cells only. Further, a fatty acid methyl ester sample from intact tissue ofA. terpenensis was partitioned according to phospholipid class, and the brominated fatty acids were shown to be associated with the phosphatidylserine and phosphatidylethanolamine fractions that are commonly present in marine sponge lipids. The phosphatidylcholine and phosphatidylglycerol fractions were rich in the relatively shorter chain fatty acids (16∶0 and 18∶0). The association of brominated long-chain fatty acids (LCFA) with sponge cells has been confirmed. The findings allow comment on the use of fatty acid profiles in chemotaxonomy and permit further interpretation of LCFA biosynthetic pathways in sponges. The assignment of the sponge studied, which is currently placed asA. terpenensis, is being supported to some extent, but the species is unusual in having C25 fatty acids as the major constituent in this group. Other factors, such as season or microenvironmental conditions, may influence observed fatty acid composition which tends to reduce the usefulness of fatty acid profiles as markers in sponge chemotaxonomy.  相似文献   
69.
70.
Interleukin (IL)-1 alpha and beta are cytokines which can mediate inflammatory, bone resorbing, and reparative effects in the periodontium, but few longitudinal data exist exploring their role following periodontal therapy. This study examined gingival crevicular fluid (GCF) concentrations of IL-1 alpha and IL-1 beta at sites with shallow sulci (SS) or inflamed moderate/advanced pockets (M/AP) before and 6 months after treatment with closed scaling/root planing (SC/RP) or papillary flap debridement (PFD), all in the same subject (n = 14 patients). No significant differences were noted in IL-1 alpha or beta concentrations (determined with two-site enzyme-linked immunosorbent assays) between SS and M/AP sites at baseline. While both therapies improved clinical parameters of periodontal disease, IL-1 alpha concentration increased significantly (p < 0.05) in M/AP-PFD sites 6 months after treatment, but were unchanged in other groups. IL-1 beta concentrations were numerically lower after therapy, except for a significant increase (p < 0.05) in M/AP-PFD sites. These data suggest that surgical wound healing in an inflamed, plaque-infected site (M/AP-PFD) results in prolonged production of IL-1, which may be a reflection of the extent of tissue trauma and delayed wound healing. In spite of increased IL-1 levels, these sites demonstrated significant short-term improvement in clinical attachment level (+ 1.8 mm, p < or = 0.001) postoperatively.  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