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   4854篇
  免费   4篇
  国内免费   5篇
电工技术   7篇
综合类   1篇
化学工业   53篇
金属工艺   11篇
机械仪表   13篇
建筑科学   12篇
能源动力   10篇
轻工业   30篇
水利工程   3篇
无线电   24篇
一般工业技术   68篇
冶金工业   4602篇
原子能技术   1篇
自动化技术   28篇
  2022年   3篇
  2021年   4篇
  2020年   2篇
  2019年   4篇
  2018年   4篇
  2017年   4篇
  2016年   5篇
  2014年   9篇
  2013年   9篇
  2012年   10篇
  2011年   10篇
  2010年   16篇
  2009年   13篇
  2008年   17篇
  2007年   25篇
  2006年   19篇
  2005年   7篇
  2004年   10篇
  2003年   17篇
  2002年   7篇
  2001年   9篇
  2000年   7篇
  1999年   135篇
  1998年   1421篇
  1997年   873篇
  1996年   507篇
  1995年   278篇
  1994年   258篇
  1993年   286篇
  1992年   43篇
  1991年   54篇
  1990年   76篇
  1989年   68篇
  1988年   70篇
  1987年   67篇
  1986年   48篇
  1985年   38篇
  1984年   4篇
  1983年   10篇
  1982年   12篇
  1981年   32篇
  1980年   46篇
  1979年   5篇
  1978年   8篇
  1977年   92篇
  1976年   206篇
  1975年   6篇
  1970年   2篇
  1964年   1篇
  1955年   3篇
排序方式: 共有4863条查询结果,搜索用时 15 毫秒
981.
OBJECTIVE: To study the blood levels of a group of lipids and lipoproteins in patients with ischemic cerebrovascular disease which was not secondary to cardiac embolic disease. PATIENTS AND METHODS: We assessed 40 patients of an average age of 64.5 years. Of these, 26 had cerebral infarcts due to disease of the great vessels and 14 had lacunar infarcts. Forty persons with no cerebrovascular disease were used as controls. We excluded patients diagnosed as having renal failure, liver, hematological, neoplastic and acute febrile disorders. Plasma was analyzed for total cholesterol, triglycerides, lipoproteins (a), HDL, LDL, VLDL and apoprotein B between 3 weeks and 6 months after the initial stroke. RESULTS AND CONCLUSIONS: Levels of triglycerides and of lipoprotein (a) were significantly greater in patients than in controls. We found no differences between the levels of total cholesterol, HDL, LDL, VLDL in the three groups studied. However, apoprotein B was greater in the controls than in the patients. There were no significant differences between the groups with lacunar ictus and those with infarcts secondary to disease of the great vessels.  相似文献   
982.
Genital human papillomavirus infection is one of the most common sexually transmitted diseases. Imiquimod is a new agent, an immune-response modifier, that has been demonstrated to have potent in vivo antiviral and antitumor effects in animal models. The present prospective, multicenter, double-blind, randomized, vehicle-controlled trial evaluated the efficacy and safety of daily patient-applied imiquimod for up to 16 weeks for the treatment of external genital warts. Wart recurrence was investigated during a 12-week treatment-free follow-up period. In the intent-to-treat analysis, baseline warts cleared from 49 of 94 (52%) patients treated with 5% imiquimod cream, 13 of 90 (14%) patients treated with 1% imiquimod cream, and 3 of 95 (4%) vehicle-treated patients; the differences between the groups treated with vehicle and imiquimod were significant (P < 0.0001). For subjects who completed the follow-up period, recurrence rates after a complete response were 19% (9 of 48 patients) in the 5% imiquimod cream group, 17% (2 of 12) in the 1% imiquimod cream group, and 0% (0 of 3) in the vehicle-treated group. There were no systemic reactions, although local skin reactions (generally of mild or moderate severity) were common, particularly in the 5% imiquimod cream group. Local reactions caused two patients to discontinue treatment. The most frequently reported local skin reactions were erythema, excoriation or flaking, and erosion. Patient-applied 5% imiquimod cream is effective for the treatment of external genital warts and has a favorable safety profile.  相似文献   
983.
The electrophysiological responses to (-)-CGP 12177 ((-)-4-(3-tertiarybutylamino-2-hydroxypropoxy) benzimidazol-2-one), an agonist for the putative beta4-adrenoceptor, were investigated on isolated perfused ferret hearts paced at 100 min(-1) and compared to those of (-)-noradrenaline and (-)-adrenaline, mediated through beta1- and beta2-adrenoceptors respectively. The three agonists decreased ventricular monophasic action potential duration but prolonged the action potential plateau; beta3-adrenoceptor-selective agonists had no effect. (-)-CGP 12177 was the most potent, but (-)-noradrenaline the most efficacious; both agonists caused ventricular extra-systoles. Because only (-)-noradrenaline but not (-)-CGP 12177 elicited shortening of the refractory period, the mechanism of arrhythmias mediated through beta1- and putative beta4-adrenoceptors may be different.  相似文献   
984.
UDP-N-acetylglucosamine-3-O-acyltransferase (UDP-GlcNAc acyltransferase) catalyzes the first step of lipid A biosynthesis (M. S. Anderson and C. R. H. Raetz, J. Biol. Chem. 262:5159-5169, 1987). We here report the isolation of the lpxA gene of Pseudomonas aeruginosa from a library of Pseudomonas strain PAO1 expressed in Escherichia coli LE392 (J. Lightfoot and J. S. Lam, J. Bacteriol. 173:5624-5630, 1991). Pseudomonas lpxA encodes a 10-carbon-specific UDP-GlcNAc acyltransferase, whereas the E. coli transferase is selective for a 14-carbon acyl chain. Recombinant cosmid 1137 enabled production of a 3-hydroxydecanoyl-specific UDP-GlcNAc acyltransferase in E. coli. It was identified by assaying lysozyme-EDTA lysates of individual members of the library with 3-hydroxydecanoyl-acyl carrier protein (ACP) as the substrate. Cosmid 1137 contained a 20-kb insert of P. aeruginosa DNA. The lpxA gene region was localized to a 1.3-kb SalI-PstI fragment. Sequencing revealed that it contains one complete open reading frame (777 bp) encoding a new lpxA homolog. The predicted Pseudomonas LpxA is 258 amino acids long and contains 21 complete hexapeptide repeating units, spaced in approximately the same manner as the 24 repeats of E. coli LpxA. The P. aeruginosa UDP-GlcNAc acyltransferase is 54% identical and 67% similar to the E. coli enzyme. A plasmid (pGD3) containing the 1.3-kb SalI-PstI fragment complemented E. coli RO138, a temperature-sensitive mutant harboring lpxA2. LpxA assays of extracts of this construct indicated that it is > 1,000-fold more selective for 3-hydroxydecanoyl-ACP than for 3-hydroxymyristoyl-ACP. Mass spectrometry of lipid A isolated from this strain by hydrolysis at pH 4.5 revealed [M-H]- 1,684.5 (versus 1,796.5 for wild-type lipid A), consistent with 3-hydroxydecanoate rather than 3-hydroxymyristate at positions 3 and 3'.  相似文献   
985.
Stepped-heating experiments on martian meteorites Allan Hills 84001 (ALH84001) and Elephant Moraine 79001 (EETA79001) revealed low-temperature (200 to 430 degrees Celsius) fractions with a carbon isotopic composition delta13C between -22 and -33 per mil and a carbon-14 content that is 40 to 60 percent of that of modern terrestrial carbon, consistent with a terrestrial origin for most of the organic material. Intermediate-temperature (400 to 600 degrees Celsius) carbonate-rich fractions of ALH84001 have delta13C of +32 to +40 per mil with a low carbon-14 content, consistent with an extraterrestrial origin, whereas some of the carbonate fraction of EETA79001 is terrestrial. In addition, ALH84001 contains a small preterrestrial carbon component of unknown origin that combusts at intermediate temperatures. This component is likely a residual acid-insoluble carbonate or a more refractory organic phase.  相似文献   
986.
The nonsteroidal antioestrogen tamoxifen has been shown to block a number of voltage-gated cation-selective channels but its effect on ligand-gated cation-selective channels has not been studied. We have investigated the action of tamoxifen and the related derivative toremifene on ligand-gated cationic nicotinic acetylcholine and 5-HT3 receptor channels. Tamoxifen and toremifene both inhibited cationic currents of adult-type human muscle nicotinic acetylcholine receptors expressed in Xenopus oocytes with similar IC50 values of 1.2 +/- 0.03 microM (nH = 0.84 +/- 0.02) and 1.2 +/- 0.1 microM (nH = 1.1 +/- 0.1), respectively. Tamoxifen could also block native 5-HT3 receptors in NG108-15 neuroblastoma/glioma hybrid cells with IC50 = 0.81 +/- 0.15 microM and nH of 1.3 +/- 0.3. The characteristics of block by tamoxifen at the 5-HT3 receptor were voltage- and use-independent. The inhibition of the 5-HT-evoked currents were not overcome by increasing concentrations of 5-HT consistent with a noncompetitive mechanism of block.  相似文献   
987.
988.
GADD45 is a ubiquitously expressed mammalian gene that is induced by DNA damage and certain other stresses. Like another p53-regulated gene, p21WAF1/CIP1, whose product binds to cyclin-dependent kinases (Cdk's) and proliferating cell nuclear antigen (PCNA), GADD45 has been associated with growth suppression. Gadd45 was found to bind to PCNA, a normal component of Cdk complexes and a protein involved in DNA replication and repair. Gadd45 stimulated DNA excision repair in vitro and inhibited entry of cells into S phase. These results establish GADD45 as a link between the p53-dependent cell cycle checkpoint and DNA repair.  相似文献   
989.
Furosemide, in combination with breast binding and milk fluid restriction, seems to inhibit postpartum lactation effectively, but has no effect in reducing serum prolactin levels. Inhibition of milk secretion must therefore be mediated by other mechanisms.  相似文献   
990.
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