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   318724篇
  免费   5729篇
  国内免费   867篇
电工技术   5240篇
综合类   282篇
化学工业   53075篇
金属工艺   12473篇
机械仪表   9264篇
建筑科学   7865篇
矿业工程   1914篇
能源动力   7558篇
轻工业   32317篇
水利工程   3510篇
石油天然气   7293篇
武器工业   18篇
无线电   32772篇
一般工业技术   61683篇
冶金工业   56486篇
原子能技术   7650篇
自动化技术   25920篇
  2021年   3272篇
  2019年   2855篇
  2018年   4943篇
  2017年   4896篇
  2016年   5255篇
  2015年   3453篇
  2014年   5529篇
  2013年   14597篇
  2012年   8941篇
  2011年   11266篇
  2010年   9203篇
  2009年   10123篇
  2008年   10328篇
  2007年   10013篇
  2006年   8736篇
  2005年   8063篇
  2004年   7486篇
  2003年   7236篇
  2002年   7171篇
  2001年   6926篇
  2000年   6528篇
  1999年   6612篇
  1998年   16573篇
  1997年   12197篇
  1996年   9325篇
  1995年   6943篇
  1994年   6262篇
  1993年   6108篇
  1992年   4603篇
  1991年   4536篇
  1990年   4357篇
  1989年   4379篇
  1988年   4347篇
  1987年   3690篇
  1986年   3667篇
  1985年   4236篇
  1984年   4033篇
  1983年   3665篇
  1982年   3468篇
  1981年   3621篇
  1980年   3494篇
  1979年   3398篇
  1978年   3482篇
  1977年   4029篇
  1976年   5243篇
  1975年   3187篇
  1974年   3032篇
  1973年   3064篇
  1972年   2659篇
  1971年   2493篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Four hundred forty-six patients with Tourette Syndrome (TS) and/or their parents completed a 52-item self-report survey about vocal and motor tics, and the frequency of associated co-morbid conditions of aggression, obsessions and compulsions, attentional problems, sleep disturbance, mood disturbance, anxiety, and self-mutilative behaviours which have been frequently reported in the literature on TS. Respondents also responded to an open-ended question regarding the most disabling aspects of TS. Results were analyzed within two age groups; under 18 years of age (N = 245) and 18 years of age or older (N = 177). Tics and associated conditions were the most frequently reported disabling aspect by both age groups. Subjects under age 18, however, reported significantly more frequent problems with hyperactivity, temper control, aggressive behaviours and sleepwalking than adults with TS.  相似文献   
992.
993.
994.
BACKGROUND: Hypertension and hypercholesterolemia are frequently associated with this leading to considerable cardiovascular risk. METHODS: An open parallel randomized study was performed in which the effects of doxazosin, an alpha-adrenergic blocker and enalapril, an inhibitor of the angiotensin converting enzyme were compared in 70 patients with essential high blood pressure and plasma cholesterol levels greater than 240 mg/dl. Following 2-4 weeks of placebo administration the patients were randomly treated with one of the two drugs. When required doses were increased and hydrochlorothiazide added until blood pressure lower than 160/95 mmHg was achieved. After this period the patients were observed for a minimum of 8 weeks. The mean length of the study was of 22 weeks. RESULTS: Both drugs significantly reduced blood pressure without modifying cardiac frequency. Doxazosin tended to favorably modify the lipid profile of the plasma while enalapril significantly reduced the levels of cholesterol, lipids and high density lipoproteins (HDL). Upon termination of the study the total HDL/cholesterol index increased 8.6% in those treated with doxazosin and decreased 5.5% in those receiving enalapril (p < 0.05). CONCLUSIONS: Although doxazosin and enalapril are potent antihypertensive drugs, the effects on plasma lipid obtained with doxazosin indicate that a reduction in cardiovascular risk was achieved with this drug in the patients included in this study.  相似文献   
995.
The importance of maximal versus submaximal exercise testing and the significance of heart failure on the prognostic value of exercise-provoked ST-segment depression > or = 0.1 mV was studied in 143 patients recovering from acute myocardial infarction. Patients were exercise tested prior to discharge and follow up lasted for up to 18 months (mean 17 months). End-point was first major event (i.e. first non-fatal reinfarction or death). A symptom-limited exercise test was superior to a heart-rate-limited test in detecting ST-segment depressions (27% vs. 20%: P < 0.5), and patients with ST-segment depression at lower heart rates did not have an increased risk of subsequent events compared with patients with ST-segment depression at higher heart rates (14% vs. 27%; NS). Heart failure surpassed ST-segment depression as a risk predictor (34% vs. 18%). Based on a meta-analysis including 13 studies (1987 patients) exercise-provoked ST-segment depression possessed an increased risk of subsequent major events (P < 0.0001; risk ratio = 1.90; 95% confidence limits 1.43,2.51). Thus, ST-segment depression provoked by a symptom-limited test selects patients with an increased risk of subsequent major events. In patients with a history of heart failure exercise-provoked ST-segment depression is of limited value.  相似文献   
996.
X-ray diffraction and ab initio MO theoretical calculations were used in order to investigate the structural and electronic properties of sarmazenil, a weak inverse agonist at the omega modulatory sites (benzodiazepine receptors). This compound was compared to bretazenil, a partial agonist, and to the antagonist flumazenil on the basis of structural and electronic data. The conformational and theoretical properties (interatomic pi overlap populations, molecular electrostatic potential (MEP), the topology of frontier orbitals, and proton affinity) of these three imidazobenzodiazepinones were determined in order to analyse the stereoelectronic properties in relation with their distinct intrinsic efficacies at the omega modulatory sites.  相似文献   
997.
The evolution of Drosophila subobscura mitochondrial DNA has been studied in experimental populations, founded with flies from a natural population from Esporles (Majorca, Balearic Islands, Spain). This population, like other European ones, is characterized by the presence of two very common (>96%) mitochondrial haplotypes (called I and II) and rare and endemic haplotypes that appear at very low frequencies. There is no statistical evidence of positive Darwinian selection acting on the mitochondrial DNA variants according to Tajima's neutrality test. Two experimental populations, with one replicate each, were established with flies having a heterogeneous nuclear genetic background, which was representative of the composition of the natural population. Both populations were started with the two most frequent mitochondrial haplotypes, but at different initial frequencies. After 13 to 16 generations, haplotype II reached fixation in three cages and its frequency was 0.89 by generation 25 in the fourth cage. Random drift can be rejected as the force responsible for the observed changes in haplotype frequencies. There is not only statistical evidence of a linear trend favoring a mtDNA (haploid) fitness effect, but also of a significant nonlinear deviation that could be due to a nuclear component.  相似文献   
998.
999.
The SSC GEM silicon Central Tracker design incorporated 18-cm long single-sided AC-coupled silicon microstrip ladders. Compared to the 12-cm long ladders considered in the preliminary stages of the tracker design, the 18-cm long ladders have the advantage of reduced cost, channel count and overall power consumption, and led to a simplified tracker assembly. However, such long ships also present the challenge of maintaining satisfactory performance. The increased capacitance and series resistance contribute to lower signal-to-noise ratios, longer time walk, higher power consumption per channel and increased probability of crosstalk to neighboring channels. In this paper, an accurate method to calculate the geometric capacitance of the AC-coupled microstrips is presented and the calculated results are compared with measurements, SPICE simulations are performed to predict the noise, the extent of interstrip capacitive coupling and the dispersion of the detector signal due to the finite series resistance of the metal strips and the long length of the detector. The influence of the preamplifier current and the shaping time on the signal and noise levels is also presented. The study concludes that the 18-cm long ladders can successfully satisfy the performance goals of the GEM silicon Central Tracker  相似文献   
1000.
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