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   251406篇
  免费   4320篇
  国内免费   1031篇
电工技术   4325篇
综合类   200篇
化学工业   36828篇
金属工艺   8224篇
机械仪表   7269篇
建筑科学   6788篇
矿业工程   540篇
能源动力   7246篇
轻工业   28561篇
水利工程   2120篇
石油天然气   1851篇
武器工业   5篇
无线电   33407篇
一般工业技术   46262篇
冶金工业   46535篇
原子能技术   3368篇
自动化技术   23228篇
  2021年   1812篇
  2020年   1355篇
  2019年   1666篇
  2018年   2583篇
  2017年   2601篇
  2016年   2889篇
  2015年   2166篇
  2014年   3660篇
  2013年   11970篇
  2012年   6768篇
  2011年   9682篇
  2010年   7462篇
  2009年   8410篇
  2008年   8928篇
  2007年   8947篇
  2006年   8299篇
  2005年   7497篇
  2004年   7214篇
  2003年   7099篇
  2002年   6595篇
  2001年   7027篇
  2000年   6327篇
  1999年   6827篇
  1998年   16406篇
  1997年   11383篇
  1996年   8592篇
  1995年   6552篇
  1994年   5716篇
  1993年   5570篇
  1992年   3999篇
  1991年   3774篇
  1990年   3551篇
  1989年   3372篇
  1988年   3278篇
  1987年   2594篇
  1986年   2487篇
  1985年   3157篇
  1984年   2798篇
  1983年   2561篇
  1982年   2347篇
  1981年   2394篇
  1980年   2203篇
  1979年   2049篇
  1978年   1879篇
  1977年   2178篇
  1976年   2729篇
  1975年   1589篇
  1974年   1468篇
  1973年   1538篇
  1972年   1130篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
Business process redesign (BPR) has been widely touted as a means for leveraging the power of information technology to change business processes radically, resulting in substantial improvements in organizational effectiveness and efficiency. This study represents an early attempt at examining the strategic nature of the phenomenon. Because BPR is radical change and has the ability to alter both the competitive and operational nature of the corporation, it is critical that BPR be coordinated with corporate strategy. Through a survey of information system executives, the relationships between corporate competitive strategy and three types of BPR – intrafunctional, interfunctional and interorganizational – were examined. It was found that organizations following a cost strategy tended to do more BPR projects on interfunctional processes. Furthermore, the positive effect of a cost strategy on interfunctional BPR was found to be strengthened by the degree of integration of IS and business planning. It was also found that while interorganizational BPR was not related to a specific strategy, it was strongly related directly with IS–business planning integration.  相似文献   
972.
Human C4b-binding protein (C4BP) functions as a cofactor to factor I in the degradation of C4b and accelerates the decay rate of the C4b2a complex. In this study we describe a monoclonal antibody directed against the alpha-chain of C4BP that inhibits the binding of C4b to C4BP. In order to identify the structural domain of the alpha-chain of C4BP that interacts with C4b, tryptic fragments of C4BP were generated. Amino acid sequence analysis of the fragments revealed that the residues Ser333-Arg356 of the alpha-chain of C4BP contain the epitope of this antibody, and as a consequence, that this part of the alpha-chain of C4BP is likely to be involved in the interaction with C4b.  相似文献   
973.
The objective of this study was to explore women's attitudes towards prenatal diagnosis of trisomy 21 and to examine some of the factors possibly responsible for these attitudes before implementing in real practice serological screening of pregnant women at risk for trisomy 21. We carried out a telephone survey on a representative sample of women who had recently had a normal livebirth delivery in the Marseille district in 1990. The participation rate was 80 per cent and the average age of the mothers was 28.9 years. Among the 514 women interviewed, 78 per cent stated that they would ask for an amniocentesis for a 1 per cent risk of trisomy 21 at their next pregnancy. When adjusting for confounding factors, the decision to have or not to have an amniocentesis was found to depend not only on the women's attitude towards induced abortion, but also on their understanding of the risk involved and on the social context (knowing a handicapped child, discussion with the father). It also depended on the women's age and on what they knew about amniocentesis from the medical point of view. The risk of miscarriage can influence a woman's choice but this objection was not found to affect the women's decisions significantly in our survey. The data showed the existence of a high potential demand for fetal karyotyping.  相似文献   
974.
Theophylline pellets were coated with Eudragit RS 30 D in a miniature fluid-bed pan coater called MiniWiD developed recently. The dispersions were plasticized with varying amounts of triethyl citrate (TEC), dibutyl phthalate (DBP), and polyethylene glycol 6000 (PEG) and applied at different temperatures ranging from 25 to 45 °C. Theophylline release was tested by dissolution using the USP Apparatus 2 (paddle) in 0.1 N hydrochloric acid under sink conditions over 6 hours.

At a coating level of 4 % (0.7 mg/cm2) sustained-release profiles were obtained from dispersions plasticized with TEC or DBP. By reducing the amount of plasticizer from 20 to 10%, films with higher permeabilities were obtained. This effect was compensated by tempering the pellets at 50 deg;C for 24 hours. The coating temperature had little effect on the dissolution profiles of TEC-plasticized films and no effect on films with DBP.

Coatings plasticized with 20% PEG were applied at temperatures ranging from 25 to 45 °C. These films required a coating level of about 18 % (3.3 mg/cm2) to provide comparable sustained-release properties. In contrast to DBP and TEC, a strong influence of the coating temperature on the release rates was observed in which higher temperatures led to slower release rates. This behavior can be explained by the minimum film-forming temperature (MFT). Since PEG does not lower the MFT of Eudragit RS 30 D, the application of these films below the MFT of 45 °C is associated with a lower degree of film formation.  相似文献   
975.
976.
Bladder psoas hitch is an surgical technique which, in very complicated cases, like repeated failures of vesico-ureteral re-implants or undiversions, allow us to bridge the shortness of the ureter and obtain a good vesico-ureteral reimplant. The surgical maneuver is described and several of the 11 cases operated by this technique are commented. The results are presented.  相似文献   
977.
OBJECTIVE: To compare the efficacy and toxicity of three patient-controlled analgesia (PCA) morphine regimens. DESIGN: A prospective, randomized, pilot study of three PCA morphine regimens: (1) 1 mg with 6-minute lockout (n = 10), (2) 2 mg with 12-minute lockout (n = 12), and (3) 2 mg with 20-minute lockout (n = 12). SETTING: Large teaching institution. PARTICIPANTS: Thirty-four patients undergoing cholecystectomy or hysterectomy. MAIN OUTCOME MEASURES: Pain scores (0 = no pain, 1 = mild pain, 2 = moderate pain, 3 = severe pain), sedation scores, analgesic consumption, and patient attempts (patient activation of PCA device) and injections (doses actually delivered) were evaluated using analysis of covariance. Distribution of pain and sedation scores and adverse effects were assessed using Fisher's exact test. RESULTS: Data on 24 patients were evaluable. Six patients withdrew for poor pain control (2 in group 1, 1 in group 2, and 3 in group 3). Three other patients withdrew because of adverse effects and 1 withdrew because of pump problems. Mean morphine consumption did not differ significantly among the groups. Distribution of pain and sedation scores and the number of patients with nausea were similar across treatment groups. The mean injection to attempt ratio was significantly smaller in group 3 (0.71 +/- 0.11) compared with groups 1 and 2 (0.9 +/- 0.06 and 0.83 +/- 0.09, respectively; p = 0.001). Adverse events occurred similarly among treatment groups. CONCLUSIONS: No significant differences in the efficacy or toxicity of the three morphine PCA regimens were identified.  相似文献   
978.
Behavior change remains the only means for primary prevention of HIV disease. Psychology should take a leading role in efforts to curtail the epidemic, but has not contributed to HIV prevention at a level proportionate to the urgency of the crisis. The authors propose an updated agenda for behavioral research on AIDS-HIV prevention implementing accelerated community trials of promising behavior change models, conducting trials of community-level interventions on a large scale and focused on populations most vulnerable to HIV infections, establishing partnerships between HIV research and community service organizations, integrating efforts from across psychology disciplines to advance and refine HIV prevention interventions, and mobilizing interdisciplinary HIV prevention resources and communication mechanisms to rapidly translate research findings to community and public policy arenas. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
979.
The treatment of tracheo-bronchial stenosis or tracheomalacia is mainly carried out by means of resection or tracheoplastic operative strategies. Since the introduction of metal-mesh stents, a definitive endoluminal therapy has to be considered under new aspects. Six patients with malignant stenosis or tracheomalacia due to compression were treated by implantation of Palmaz- or Wallstents. Immediately after the implantation, patients were relieved from dyspnoea, the forced inspiratory volume-1 (FIV1) was normalized. All implanted stents were well tolerated, even in the long-time follow-up (19 months). Bronchoscopic control showed overgrowth of the metal meshes by respiratory epithelium. The implantation of metal-mesh stents is an adequate alternative in the treatment of malignant stenosis and tracheomalacia.  相似文献   
980.
BACKGROUND: There is no agreement on the management of patients with cirrhosis and recurrent variceal bleeding after failure of medical or endoscopic treatments or both. Portal systemic shunts are highly effective in preventing rebleeding but are associated with a high incidence of chronic encephalopathy. This study compared the results of a slightly modified Sugiura procedure (esophageal transection plus esophagogastric devascularization plus splenectomy) with those of nonselective portal systemic shunts in patients with previous variceal bleeding. METHODS: Fifty-four patients were included in this randomized controlled study between January 1984 and April 1989. The major end point was chronic encephalopathy. Secondary end points were recurrent variceal bleeding, survival, ascites, and hepatocellular carcinoma. RESULTS: Twenty-seven patients were assigned to each group. The rate of chronic encephalopathy was significantly (p = 0.002) lower after modified Sugiura procedure than after portal systemic shunt. Recurrent variceal bleeding was more frequent after modified Sugiura procedure than after portal systemic shunt, but the difference is not significant. One-, two-, and three-year survival rates were 93%, 81%, and 67%, respectively, in the modified Sugiura group and 78%, 66%, and 39%, respectively, in the portal systemic shunt group (p = 0.044). CONCLUSIONS: These results suggest that the modified Sugiura procedure is better overall than the nonselective portal systemic shunt in the management of patients with cirrhosis and recurrent variceal bleeding. Although the rebleeding rate is higher after the modified Sugiura procedure, this does not seem to affect mortality in these patients.  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