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   280947篇
  免费   3177篇
  国内免费   1309篇
电工技术   4818篇
综合类   521篇
化学工业   40860篇
金属工艺   11189篇
机械仪表   8484篇
建筑科学   6549篇
矿业工程   1579篇
能源动力   7066篇
轻工业   22587篇
水利工程   3074篇
石油天然气   5807篇
武器工业   43篇
无线电   33700篇
一般工业技术   57067篇
冶金工业   51299篇
原子能技术   7503篇
自动化技术   23287篇
  2021年   2244篇
  2019年   2136篇
  2018年   3653篇
  2017年   3739篇
  2016年   3978篇
  2015年   2555篇
  2014年   4448篇
  2013年   12149篇
  2012年   7217篇
  2011年   9910篇
  2010年   7671篇
  2009年   8871篇
  2008年   9241篇
  2007年   9259篇
  2006年   7912篇
  2005年   7332篇
  2004年   7270篇
  2003年   7052篇
  2002年   6764篇
  2001年   6884篇
  2000年   6514篇
  1999年   6791篇
  1998年   16335篇
  1997年   11625篇
  1996年   8949篇
  1995年   6865篇
  1994年   6047篇
  1993年   6077篇
  1992年   4424篇
  1991年   4200篇
  1990年   4099篇
  1989年   4080篇
  1988年   3773篇
  1987年   3418篇
  1986年   3404篇
  1985年   3755篇
  1984年   3476篇
  1983年   3238篇
  1982年   3045篇
  1981年   3049篇
  1980年   3040篇
  1979年   2846篇
  1978年   2887篇
  1977年   3156篇
  1976年   4111篇
  1975年   2429篇
  1974年   2388篇
  1973年   2407篇
  1972年   2024篇
  1971年   1785篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
992.
An analytical expression for both band-to-band and band-trap-band indirect tunnelings is used to study the gate-induced drain leakage (GIDL) current of MOSFETs measured before and after hot-carrier stress. The voltage and temperature dependence of GIDL are characterized. Both results show that interface traps situated near the midgap participate in the conduction of GIDL, and band-trap-band indirect tunneling could be the major mechanism. This is further supported by the fact that the percentage increase in GIDL induced by hot-carrier stress is about the same as the corresponding increase in interface-trap density. On the other hand, under low-field conditions, trap-assisted Poole–Frenkle emission dominates over tunneling for temperatures even well below room temperature.  相似文献   
993.
In a placebo-controlled, double-blind randomized design, we investigated the cardiovascular interaction between caffeine (250 mg intravenously) and nicotine (4 mg chewing gum) in 10 healthy volunteers, both under baseline conditions and during physical and mental stress (standing up and mental arithmetic). Caffeine alone induced a significant increase in blood pressure associated with a decrease in heart rate, whereas nicotine alone increased both blood pressure and heart rate. The combination of caffeine and nicotine increased systolic and diastolic blood pressure by 10.8 +/- 2.0 and 12.4 +/- 1.9 mm Hg, respectively. This pressor response did not differ significantly from the calculated additive effects of caffeine and nicotine on blood pressure, measuring 12.9 +/- 2.0 and 14.2 +/- 2.1 mm Hg, respectively. Heart rate and forearm blood flow also showed a similar response when the combination of caffeine and nicotine was compared with the calculated sum. During physical stress (standing up), blood pressure, heart rate, and plasma catecholamines increased in the placebo test. The pressor response to standing up was less pronounced after the combination of caffeine and nicotine compared with the sum of the separate effects (combination versus sum: delta diastolic blood pressure, 24.7 +/- 1.9 versus 35.2 +/- 2.6 mm Hg [p < 0.01]; delta mean arterial pressure, 22.1 +/- 2.0 mm Hg versus 28.6 +/- 1.6 mm Hg [p < 0.05]). The plasma catecholamine response did not differ between the combination and the sum of both drugs. During mental arithmetic, blood pressure, heart rate, and forearm blood flow increased in the placebo test. The forearm vasodilator response to mental stress was attenuated by the combination of caffeine and nicotine compared with the sum of both drugs (combination versus sum: delta forearm blood flow, -0.1 +/- 0.3 versus 1.4 +/- 0.5 ml/100 ml/min [p < 0.05]). We conclude that the combined administration of caffeine and nicotine shows additive effects on cardiovascular parameters during baseline conditions but less than additive effects during sympathoadrenal stimulation.  相似文献   
994.
Bacterial collagenase was used to study the susceptibility of dermal sheep collagen (DSC) cross-linked with a mixture of the water-soluble carbodiimide 1-ethyl-3-(3-dimethyl aminopropyl)-carbodiimide hydrochloride and N-hydroxysuccinimide (E/N-DSC) towards enzymatic degradation. Contrary to non-cross-linked DSC (N-DSC), which had a rate of weight-loss of 18.1% per hour upon degradation, no weight loss was observed for E/N-DSC during a 24 h degradation period. The tensile strength of the E/N-DSC samples decreased during this time period, resulting in partially degraded samples having 80% of the initial tensile strength remaining. The susceptibility of E/N-DSC samples towards enzymatic degradation could be controlled by varying the degree of cross-linking of the samples. Ethylene oxide sterilization of E/N-DSC samples made the material more resistant against degradation compared with non-sterilized E/N-DSC samples. This may be explained by a decrease of the adsorption of bacterial collagenase onto the collagen owing to reaction of ethylene oxide with remaining free amine groups in the collagen matrix.  相似文献   
995.
We report an unusual case of a 1 cm papillary renal cell carcinoma arising from the outer surface of the free wall of a radiographically simple renal cyst. Most malignancies arising from the wall of a renal cyst are present along the inner surface, usually adjacent to underlying renal parenchyma. While this is a rare occurrence, it does reinforce the need to explore even small solid renal masses.  相似文献   
996.
997.
This prospective study evaluates the results of extracorporal shock-wave piezoelectric lithotripsy in 52 patients, 41 with radiolucent and 11 with calcified shell gallstones. The fragmentation rate was above 90% and the complication rate 31% particularly with pains or intolerance to the adjuvant dissolution therapy. At the end of the treatment, seven cholecystectomies were carried out among the 31 patients for who lithotripsy failed. After 18 months, the success rate was 23% and 12 patients only remained stone free. Nowadays, thanks to the new laparoscopic therapy, extracorporal shock-wave lithotripsy is no longer judged appropriate, especially because of its poor cost-effectiveness and high recurrence ratio.  相似文献   
998.
Studies done on adults reveal a significant role of fibrinogen (FG) in cardiovascular disease. The main purpose of our study was to determine the level of plasma FG in children, its distribution according to age and sex, and its relationship to other cardiovascular risk factors such as cigarette smoking, plasma lipid disturbances, and a positive family history for other cardiovascular risk factors. We studied a population of 2224 children, aged 2 to 18 years, from five different schools located in Madrid. We found a significantly higher level of FG in the groups of females aged 6 to 8 and 16 to 18 years than in males of the same ages (p < 0.001). The study population was distributed into two groups according to the plasma fibrinogen level: > 394 mg/dl and < 394 mg/dl. In both groups we determined plasma levels of total cholesterol, triglycerides, and very low-density lipoprotein (VLDL) cholesterol and found these parameters significantly higher in the group with an FG level > 394 mg/dl (p < 0.05).  相似文献   
999.
A specimen fromt he fossil forest of Dunarobba (Central Italy) attributed toTaxodioxylon gypsaceum was chemically analyzed particularly for its terpene content. Main chemical constituents show variations related to ageing and carbonization process, while terpene content exhibits a drop of mono- and sesquiterpenes in favour of diterpenes and higher MW compounds. All components are exclusively hydrocarbons and represent alicyclic or partially aromatic structures. The identified families include abietane, phyllocladane, norabietane and norpimarane among diterpenes and traces of cadinane and selinane among sesquiterpenes.  相似文献   
1000.
Mathematical models describing dynamics of crust formation and kinetics of crust color and firmness changes were developed for the deep-fat frying of beef meatballs. Good agreement (R2 ranged between 0.815 and 0.987) was observed between experimental and predicted data. Crust color lightness, redness, and yellowness decreased exponentially with frying time while total color change increased. All color parameters followed first order reaction kinetics. Meatball firmness was measured by peak force obtained from a puncture test whose kinetics model had a reaction rate constant of 5.39E-3 1/(s.Nn-1) and a reaction order of 0.0013.  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