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   586214篇
  免费   8513篇
  国内免费   2070篇
电工技术   10598篇
综合类   527篇
化学工业   87399篇
金属工艺   19600篇
机械仪表   16609篇
建筑科学   16284篇
矿业工程   1275篇
能源动力   16244篇
轻工业   62028篇
水利工程   4656篇
石油天然气   3384篇
武器工业   16篇
无线电   76635篇
一般工业技术   106308篇
冶金工业   111541篇
原子能技术   7684篇
自动化技术   56009篇
  2021年   3968篇
  2020年   2855篇
  2019年   3622篇
  2018年   5622篇
  2017年   5707篇
  2016年   6203篇
  2015年   4928篇
  2014年   8249篇
  2013年   27695篇
  2012年   14702篇
  2011年   21225篇
  2010年   16348篇
  2009年   18618篇
  2008年   19828篇
  2007年   20154篇
  2006年   18318篇
  2005年   16864篇
  2004年   16306篇
  2003年   15981篇
  2002年   15254篇
  2001年   15590篇
  2000年   14320篇
  1999年   15325篇
  1998年   36185篇
  1997年   25906篇
  1996年   20005篇
  1995年   15499篇
  1994年   13691篇
  1993年   13314篇
  1992年   9720篇
  1991年   9258篇
  1990年   8747篇
  1989年   8378篇
  1988年   8087篇
  1987年   6708篇
  1986年   6558篇
  1985年   8082篇
  1984年   7473篇
  1983年   6627篇
  1982年   6100篇
  1981年   6164篇
  1980年   5742篇
  1979年   5455篇
  1978年   5063篇
  1977年   6151篇
  1976年   8129篇
  1975年   4267篇
  1974年   4044篇
  1973年   3969篇
  1972年   3121篇
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
921.
922.
As Australia is one of the most multicultural societies in the world, acculturation of migrants and changes in migrants' health status should be an important focus of public health research. The absence of an accepted measure of acculturation is one barrier to exploring the relationship between acculturation and health. This paper presents data from a study of 851 Arabic-speaking adults attending 20 Arabic-speaking general practitioners in Canterbury, Sydney. An eight-item scale assessing acculturation was developed with a structural equation modelling program (LISREL). This acculturation scale was based on similar scales used with Hispanic populations, was theoretically grounded and had high internal consistency and criterion-related validity. To show the application of a scale of acculturation, patients' preferences for participation in medical decision making, according to level of acculturation, were examined. After adjustment for age, sex and highest level of formal education, significant inverse associations between acculturation and preferences for patient (versus family) involvement in medical decision making were found. Mechanisms for how acculturation affects health need to be explored.  相似文献   
923.
Sampled-data techniques are used to examine the dynamic characteristics and closed-loop performance of the quasi-square-wave boost converter. Direct duty-ratio control and current-mode control are considered. With direct duty-ratio control, the system poles remain well damped under all load conditions, and there is no zero in the control-to-output transfer function. Good closed-loop performance is therefore achieved. Under current-mode control, the requirement for a stabilizing ramp is seen to depend on load conditions, and the achievable voltage control-loop bandwidth is smaller than that using direct duty-ratio control; however, there is a significant reduction in the magnitude of the DC source to output-voltage frequency response  相似文献   
924.
J. Stickforth 《Acta Mechanica》1997,123(1-4):195-201
Summary The differential equation of the Kepler orbit is formulated within the framework of a relativistic Hamiltonian multi-time formalism. The calculation takes into consideration terms of the order 1/c 2. The integration of the differential equation yields an orbit with backward movement of the perihelion.  相似文献   
925.
Three cases are presented of labyrinthine fistula which suddenly arose in patients who had undergone stapedectomy interposing a vein graft from the back of the hand. These patients had all enjoyed 12-15 years apparently problem free period before the fistula occurred. In all three cases the fistula was accompanied by objective dizziness and varying degrees of sensorineural or mixed hearing loss. The patients underwent revision surgery which identified the fistula and the endolymph oozing out of the graft from the previously installed prosthesis. The authors present the cases, describe how the symptoms arose and the results obtained with revision surgery. Finally, they discuss the need to first perform stapedotomy with a platinar hole; they indicate that the number of cases of total or partial stapedectomy should be limited only to those cases characterized by anatomical variations and/or whenever complications arise during surgery.  相似文献   
926.
The epithelium of the digestive system mucosa consists of a highly dynamic cell population. The conditions under which mitotic activity in the gastrointestinal epithelium is regulated is as yet poorly understood. Nevertheless, it is assumed that some biogenic amines might be involved. Having demonstrated that dopaminergic cells occur in the stomach of gerbils (Meriones unguiculatus), in the present study we examined the influence of dopamine antagonist haloperidol on the proliferation of epithelial cells in the mucosa of the stomach. Proliferating cells were detected immunocytochemically and quantified after in-vivo labeling with 5-bromo-2'-desoxyuridine in both haloperidol- and saline-treated animals. The results show that acute doses of haloperidol significantly increases the proliferation rate in the pyloric mucosa, suggesting that dopamine plays a probable modulatory role in the regulation of mitotic activity. These findings are discussed with regard to the role of paraneurons in regulating epithelial mitosis.  相似文献   
927.
The MnO activities in (MnO-CrO x -CaO-SiO2)-containing melts, which were saturated with the (Mn, Cr)3O4 spinel phase, were determined at 1500 °C under an oxygen partial pressure of 10−8.99 atm. This was done by equilibrating the samples with platinum. The activity of MnO in the melt was then calculated from the activity coefficient of manganese in the resultant Pt-Cr-Mn alloy. Darken’s quadratic formalism for ternary metallic solutions was used to calculate the activity coefficient of manganese in the Pt-Cr-Mn system, in which platinum was considered to be the solvent. It was found that an increase in the concentration of MnO in the melt increases both the MnO activity and the activity coefficient of MnO. For a constant MnO concentration in the (MnO-CrO x -CaO-SiO2)-containing melts, the activity of MnO can be increased by increasing the basicity of the melt. In order to obtain high-manganese recoveries from (MnO-CrO x -CaO-SiO2)-containing melts into an alloy phase, basic slags in which the activity coefficient of MnO is high should therefore be used.  相似文献   
928.
BACKGROUND AND AIMS OF THE STUDY: Surgical treatment of functional tricuspid regurgitation associated with advanced valvular disease remains controversial, mainly due to the difficulty in choosing between valve replacement or reconstructive surgery. Failure to correct tricuspid regurgitation during valvular surgery carries a poor prognosis, as reoperation may represent a significant risk to the patient. Thus correct tricuspid valve surgery is vital to achieve improved early and long-term clinical results. METHODS: A total of 142 patients underwent concomitant tricuspid annuloplasty with mitral valve replacement and their clinical outcome was assessed. RESULTS: Overall hospital mortality rate was 11.3%. Seven patients died during follow up. The overall actuarial survival rate for 10 years was 74.1 +/- 14.2%. Postoperatively, 75.8% of the patients had no residual tricuspid insufficiency, while 24.2% had first- or second-degree tricuspid insufficiency and were treated medically. All patients were in NHYA functional class I or II postoperatively. CONCLUSION: Clinically, it is important to assess the severity of functional tricuspid insufficiency before and/or during the operation. Bicuspidalization annuloplasty for functional tricuspid insufficiency has provided good results, with a low incidence of reoperation and significant clinical improvement. The technique could be used in most patients with functional tricuspid regurgitation.  相似文献   
929.
Much time and effort are spent on designing primary research studies. Similar care must be given to planning systematic reviews. The review should be based on an important, well-focused question that is relevant to patient care. By formulating the question properly, the criteria that primary studies must meet to be included in the review become clear. These criteria, which comprise the types of persons involved, exposure, control group, outcomes, and study designs of interest, can then be refined so that they are clinically relevant, sensible, and workable. Inclusion criteria that are too narrow will limit the amount of data in the review, thereby increasing the risk for chance results and making the review less useful for the reader. Reviews should include studies whose designs offer the least biased answer to the question being asked. To maximize available data and reduce the risk for bias, as many relevant studies as possible need to be identified, regardless of publication status or language. Multiple overlapping search strategies should therefore be used and must be carefully planned. Strategies include searching the many electronic databases available (after careful consideration of which terms to enter), manually searching journals and conference proceedings, searching bibliographies of articles, searching existing registers of studies, and contacting companies or researchers. The time taken to formulate the question adequately and develop appropriate searches will increase the chance of producing a high-quality, meaningful review.  相似文献   
930.
PURPOSE: The objectives of this study were to quantify the immunohistochemical stainings of collagen types I, III and IV, and investigate the value of glycohistochemical staining with 3 lectin types specific to a particular glycan structure, Arachis hypogaea, Triticum vulgare and concanavalin A, as a method of defining possible changes in the collagen structure of the tunica albuginea in potent and impotent patients. MATERIALS AND METHODS: The study involved 4 normal men, 4 with pure venous leakage and 4 with pure arterial disease. Collagen types I, III and IV, and lectins Arachis hypogaea, Triticum vulgare and concanavalin A were studied using a cell image processor. The labeling index relates to the percentage of staining and mean optical density relates to the staining intensity. RESULTS: Mean labeling index values for the 3 types of collagen and lectins were similar (p > 0.05). Mean optical density value relating to collagen type I was significantly higher in the arteriogenic group than in the other groups (p < 0.05), while mean optical density value of collagen type IV was significantly higher in the venogenic group than in the 2 other groups (p < 0.05). Mean optical density values relating to the 3 lectin types were similar in the 3 clinical groups (p > 0.05). CONCLUSIONS: An alteration in the distribution and structure of the various collagen types and lectins in the tunica albuginea of impotent patients has been shown that may interfere with normal function and lead to impotence.  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