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   4347篇
  免费   80篇
  国内免费   12篇
电工技术   40篇
综合类   7篇
化学工业   389篇
金属工艺   59篇
机械仪表   57篇
建筑科学   126篇
矿业工程   10篇
能源动力   106篇
轻工业   337篇
水利工程   16篇
石油天然气   19篇
无线电   238篇
一般工业技术   358篇
冶金工业   2236篇
原子能技术   24篇
自动化技术   417篇
  2022年   28篇
  2021年   35篇
  2020年   27篇
  2019年   19篇
  2018年   45篇
  2017年   34篇
  2016年   41篇
  2015年   44篇
  2014年   79篇
  2013年   132篇
  2012年   92篇
  2011年   114篇
  2010年   92篇
  2009年   83篇
  2008年   103篇
  2007年   110篇
  2006年   100篇
  2005年   63篇
  2004年   67篇
  2003年   83篇
  2002年   60篇
  2001年   62篇
  2000年   60篇
  1999年   109篇
  1998年   728篇
  1997年   417篇
  1996年   254篇
  1995年   162篇
  1994年   170篇
  1993年   167篇
  1992年   59篇
  1991年   53篇
  1990年   50篇
  1989年   48篇
  1988年   43篇
  1987年   45篇
  1986年   28篇
  1985年   40篇
  1984年   20篇
  1983年   26篇
  1982年   16篇
  1981年   25篇
  1980年   39篇
  1978年   21篇
  1977年   65篇
  1976年   128篇
  1975年   15篇
  1974年   15篇
  1973年   18篇
  1972年   13篇
排序方式: 共有4439条查询结果,搜索用时 15 毫秒
21.
22.
GH is synthesized at multiple extrapituitary sites suggestive of an autocrine/paracrine mechanism of action. We have investigated a possible autocrine/paracrine mechanism of GH action, compared the cellular response to exogenous versus endogenously produced GH, and determined the nature of the interaction between external stimuli and endogenously produced GH. BRL cells expressing the GH receptor were transiently transfected with expression plasmids containing either the hGH or the bGH gene and the response of the cell was measured by CAT reporter plasmids requiring either STATs 1 and 3 or STAT5 for their response. Transient transfection of the hGH gene resulted in hGH accumulation in the cell and secretion into the media. The functional response through STATs 1 and 3 and STAT5 obtained with endogenously produced hGH was comparable or greater in magnitude to that obtained with the maximal stimulatory dose of exogenous hGH. Similar results were obtained with an expression plasmid containing the bGH gene. Endogenously produced hGH interacted in an additive manner when combined with submaximal doses of both exogenous hGH and serum. Such results were also observed in a more physiologically relevant mammary carcinoma cell line (MCF-7). The nonreceptor-dimerizing hGH antagonist, hGH-G120R, used in cells expressing the homologous receptor extracellular domain was able to only partially inhibit the response of the cell to endogenously produced hGH, in contrast to full inhibition of exogenous hGH. We therefore conclude that GH can function in an autocrine/paracrine manner, additive in effect to external stimuli.  相似文献   
23.
BACKGROUND: Serum creatinine has been reported in previous studies to be a prognostic indicator for overall mortality, in particular in a hypertensive population. METHODS: The Program on the Surgical Control of the Hyperlipidemias (POSCH) was a randomized, controlled clinical trial. All patients had survived a single myocardial infarction, were normotensive, were not obese, were not having heart failure, and were free of diabetes mellitus and renal disease at entry into the study. POSCH had followed its control group patients (N = 417) for a minimum of 7.0 years. In this group, a prospective post hoc analysis of the relationship of baseline serum creatinine with subsequent overall and atherosclerotic coronary heart disease mortality was performed. RESULTS: The baseline serum creatinine values in the control group patients ranged from 0.7 to 1.9 mg/dL (60 to 170 mumol/L), and were found to be independent predictors (P < .01) of both overall mortality and atherosclerotic coronary heart disease mortality. Each 0.1 mg/dL (9 mumol/L) increment in the baseline serum creatinine increased the relative risk for subsequent overall mortality by 36% and the relative risk for subsequent atherosclerotic coronary heart disease mortality by 47%. CONCLUSIONS: These results demonstrate that a serum creatinine value, obtained in normotensive, nonobese, normoglycemic survivors of a myocardial infarction without preexistent renal disease or heart failure, provides independent prognostic information regarding subsequent overall and atherosclerotic coronary heart disease mortality.  相似文献   
24.
Wood has long traditions as a building material, and is often used in construction elements, and as interior and exterior surfaces in the Nordic countries. In most applications, there are reaction to fire requirements to products used as surfaces, e.g. in escape routes and larger public spaces. Most wood products will therefore have to be treated with fire retardant (FR) agents to fulfil the strict requirements to properties connected to heat release and flame spread. Unfortunately, FR agents usually also increase the smoke production, as they cause a more incomplete combustion of the wood. The wood product manufacturers seek to find the optimal amount of FR additives where both heat release and smoke production in the classifying test are within the requirements given in the building regulations. This paper describes models for prediction of the European reaction to fire classes of wood products. The models are based on multivariate statistical analysis, and use test results from the cone calorimeter test as input. The presented models are, with very good precision, able to predict which Euroclass and additional smoke class a wood based product would obtain if it were to be tested in the single burning item test. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   
25.
This paper presents a Knowledge-based system (KBS) developed to allow users, who may not be knowledgeable about sensors, to select sensors suitable for their specific needs. The KBS runs on a micro-computer. The selection criteria are user specified and are based on the desired measurement parameters. The system output includes all of the operational and dimensional parameters of the recommended sensor, price, and vendor information.  相似文献   
26.
27.
28.
Response to selection for milk yield in Holsteins was examined by a controlled selection experiment initiated in 1964. Foundation cows were paired by sire and divided randomly into two breeding groups, selection and control. Selection group was mated to four sires each year highest for Predicted Difference milk. Twenty bulls selected in 1964 as near breed average for milk sired all control cows. Milk yield was recorded for each lactation, and height of udder from ground, distances between teats, and perimeter and area bound by the four teats were measured at 30 to 75 days postpartum on 153 selection and 202 control cows over 14 yr. Most estimates of repeatability and heritability of udder dimensions before and after milking were .45 or larger. Correlations with milk yield were negative for udder height and positive for other udder measurements. Selection cows exceeded controls for lactational milk yield. Daughters of high milk bulls had greater distances between teats, greater perimeters, and larger areas of udder floor. Selection cows did not differ from controls in udder height for first parity but were below controls in udder height and had udders that collapsed more for all parities.  相似文献   
29.
1. The mechanism of renal protection by angiotensin-converting enzyme inhibition is still the subject of debate. Inhibition of proteinuria might play a role. If so, a good antiproteinuric response to angiotensin-converting enzyme inhibition should predict subsequent protection against renal structural damage. This hypothesis has not been tested in models where treatment is started after the renal disease is well established, i.e. models that mimic the clinical situation. 2. We therefore investigated this hypothesis in 96 male Wistar rats with established adriamycin nephrosis. Reduction of proteinuria was achieved by lisinopril (0, 2, 5 and 10 mg day-1 kg-1) on two different sodium diets (0.3% and 0.05% NaCl). Therapy started 6 weeks after adriamycin (at stable proteinuria) and was continued for 6 weeks. 3. Lisinopril reduced blood pressure by 32 +/- 4% and proteinuria by an average of 72 +/- 7%, with stabilization after 2 weeks. Considerable interindividual differences in antiproteinuric response was found. Glomerulosclerosis score was reduced by 15 +/- 5%. All the effects of angiotensin-converting enzyme inhibitors were enhanced by sodium depletion, but sodium depletion in itself did not affect blood pressure (124 +/- 4 mmHg), proteinuria (664 +/- 68 mg/day) or glomerulosclerosis score (30 +/- 5%). Interestingly, the more proteinuria was reduced initially in an individual rat, the less sclerosis was found in the long term in that rat. 4. In conclusion, angiotensin-converting enzyme inhibition lowers proteinuria and prevents glomerulosclerosis in established adriamycin nephrosis. These effects are enhanced by sodium depletion. The individual short-term antiproteinuric effect predicts the protection against ultimate glomerular damage. This is consistent with the hypothesis that reduction of proteinuria is a mechanism by which angiotensin-converting enzyme inhibitors exert renoprotection.  相似文献   
30.
With the object of assessing the long-term prognosis and the frequencies of recurrence and remission in women chronic low abdominal pain without laparoscopically visible cause, questionnaire were sent in 1985 and 1991 to 55 women who had been submitted to laparoscopy in 1982-1984 for this reason. These women had been told that there was no demonstrable explanation of the pain experienced and were then discharged. 65% and 55% respectively had experienced and unfavourable course with considerable and continued symptoms. Only 22% stated in both investigations that they had experienced a favourable course and that they were, by and large, free from pain. 36% changed from an unfavourable to a favourable course or the reverse. The assessment made by the women was confirmed by a series of subordinate questions and this demonstrated a marked difference between the favourable and unfavourable courses of the condition. It is concluded that laparoscopy with exclusion of significant pathology is not, in itself, satisfactory as treatment of this patient group and that no improvement occurs in the course of time. The condition varies greatly with many recurrences and remissions and, for this reason, uncontrolled reports of the therapeutic effects are of no significance. When compared with the literature, it is suggested that this patient group should be referred early in the course of the condition to a therapist with specialist psychological/sexological insight and/or to a physiotherapist with interest in this patient group.  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