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   6551篇
  免费   157篇
  国内免费   14篇
电工技术   131篇
综合类   7篇
化学工业   1085篇
金属工艺   138篇
机械仪表   97篇
建筑科学   140篇
矿业工程   27篇
能源动力   239篇
轻工业   290篇
水利工程   20篇
石油天然气   18篇
无线电   727篇
一般工业技术   1073篇
冶金工业   1903篇
原子能技术   39篇
自动化技术   788篇
  2023年   46篇
  2022年   72篇
  2021年   123篇
  2020年   107篇
  2019年   104篇
  2018年   138篇
  2017年   134篇
  2016年   113篇
  2015年   91篇
  2014年   129篇
  2013年   354篇
  2012年   195篇
  2011年   264篇
  2010年   212篇
  2009年   212篇
  2008年   229篇
  2007年   191篇
  2006年   188篇
  2005年   187篇
  2004年   147篇
  2003年   130篇
  2002年   112篇
  2001年   112篇
  2000年   87篇
  1999年   123篇
  1998年   560篇
  1997年   343篇
  1996年   226篇
  1995年   138篇
  1994年   181篇
  1993年   156篇
  1992年   95篇
  1991年   83篇
  1990年   65篇
  1989年   62篇
  1988年   71篇
  1987年   63篇
  1986年   62篇
  1985年   85篇
  1984年   58篇
  1983年   51篇
  1982年   43篇
  1981年   52篇
  1980年   47篇
  1979年   38篇
  1978年   39篇
  1977年   69篇
  1976年   119篇
  1974年   33篇
  1973年   36篇
排序方式: 共有6722条查询结果,搜索用时 156 毫秒
21.
Two mobilizable cloning vectors, designated pABW1 and pAWB2, were constructed basing on the E. coli vector pBGS18 and oriT originating from RK2. In pABW2 the kanamycin resistance gene was replaced by a novel tetracycline resistance cassette derived from Tn1721. Both vectors, specific for E. coli, allow to perform the cloning steps in E. coli and then to efficiently transfer the constructs by conjugation to the host of choice. A vector which cannot propagate in the given host can be applied for identification of the host specific plasmid replicator regions. With the use of pABW2 we defined the minimal replicator region of pTAV202-a mini-derivative of the large pTAV1 plasmid of P. versutus. We also proved that RepC' encoded on this fragment is the principal initiator replication protein and that oriV is located along its coding sequence.  相似文献   
22.
23.
24.
In 1895 Trendelenburg described his sign to determine the integrity of hip function. We found the sign to be positive in a patient whose hip was clinically and radiologically normal, and therefore investigated this in other patients. We confirmed that a medial shift of the mechanical axis of the leg below the hip may cause a positive Trendelenburg sign. This has not been previously described.  相似文献   
25.
Cigarette smoking continues to be the single, most preventable cause of death and disability in the United States. For individuals who have cancer, continuing to smoke negatively impacts their treatment, survival, and risk for second primary tumors. This review of behavioral and pharmacological approaches to smoking cessation focuses on the recent comprehensive review of cessation interventions by the Agency for Health Care Policy and Research (AHCPR), as well as on new developments in the field. An intervention model is outlined that provides oncologists with a brief and easily implemented method of systematically treating patients who smoke. By assessing patient smoking status, advising smoking patients to quit, and proactively assisting their patients in quitting, oncologists can significantly influence patient health and fulfill their professional and ethical responsibility to address this life-threatening behavior.  相似文献   
26.
27.
Identification of a specific biomolecular target appropriately sensitive to a wide array of anesthetics has been elusive. At concentrations close to their respective ED50's for anesthesia in man or other species, 18 compounds, differing in potencies up to 66,000 fold, inhibited cytochrome P450 mediated metabolism of aminopyrine, a synthetic substrate, and arachidonic acid (AA), an endogenous substrate, in isolated liver microsomes. There was a highly significant correlation for both substrates between the absolute concentrations required for anesthesia (EC50) and for inhibition of P450 activity (Ki or IC50). The mean Ki/EC50 ratio was 0.97 for inhibition of aminopyrine demethylase. The mean IC50/EC50 ratios were 0.42 and 0.64 for inhibition of two AA-derived products and 2.8 for a third; a mean ratio of 1.4 for inhibition of overall AA metabolism suggests interaction of general anesthetics with a composite of P450 isozymes. The universal cytochrome P450 monooxygenases, in conjunction with other lipid oxygenases (cyclooxygenases and lipoxygenases) participate in the second messenger AA cascade. In nerve cells the sensitivity of these enzymes to hydrophobic neurodepressant drugs may underlie the state of general anesthesia: reversible disruption of intracellular and intercellular signalling without impairment of enzymes vital to cell respiration.  相似文献   
28.
Islet amyloid polypeptide forms islet amyloid deposits in non-insulin-dependent diabetes mellitus. We have generated transgenic mice which express human islet amyloid polypeptide in their pancreatic beta cells yet do not develop islet amyloid deposits despite producing levels of the amyloidogenic human peptide 2 - 3 fold higher than the native (mouse) peptide. To determine whether marked overproduction of islet amyloid polypeptide is a potential cause of islet amyloid formation, we increased expression of this transgene by producing homozygous transgenic animals and by making heterozygous mice experimentally insulin resistant with nicotinic acid. Pancreatic content of islet amyloid polypeptide-like immunoreactivity in homozygous and nicotinic acid-treated mice was 2-fold (25 +/- 7 fmol/microg; n = 6) and 3.5-fold (47 +/- 20 fmol/microg; n = 3) higher, respectively, than that of untreated heterozygous animals (13+/-2 fmol/microg; n = 11; both p < 0.05). Despite this marked increase in production of islet amyloid polypeptide, neither group of mice developed gross islet amyloid deposits even after 16 months of age. We conclude that overproduction of islet amyloid polypeptide, even as produced by extreme insulin resistance, is not in itself sufficient for islet amyloid formation.  相似文献   
29.
30.
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