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   4964篇
  免费   74篇
  国内免费   72篇
电工技术   30篇
综合类   52篇
化学工业   102篇
金属工艺   25篇
机械仪表   43篇
建筑科学   56篇
矿业工程   13篇
能源动力   4篇
轻工业   102篇
水利工程   16篇
石油天然气   49篇
武器工业   6篇
无线电   89篇
一般工业技术   132篇
冶金工业   4260篇
原子能技术   2篇
自动化技术   129篇
  2024年   4篇
  2023年   7篇
  2022年   11篇
  2021年   23篇
  2020年   16篇
  2019年   14篇
  2018年   25篇
  2017年   22篇
  2016年   21篇
  2015年   27篇
  2014年   32篇
  2013年   47篇
  2012年   54篇
  2011年   60篇
  2010年   47篇
  2009年   53篇
  2008年   62篇
  2007年   51篇
  2006年   58篇
  2005年   53篇
  2004年   30篇
  2003年   27篇
  2002年   17篇
  2001年   17篇
  2000年   15篇
  1999年   144篇
  1998年   1386篇
  1997年   736篇
  1996年   472篇
  1995年   263篇
  1994年   221篇
  1993年   259篇
  1992年   40篇
  1991年   54篇
  1990年   65篇
  1989年   52篇
  1988年   61篇
  1987年   62篇
  1986年   50篇
  1985年   58篇
  1983年   6篇
  1982年   19篇
  1981年   26篇
  1980年   28篇
  1979年   3篇
  1978年   10篇
  1977年   83篇
  1976年   199篇
  1975年   6篇
  1955年   3篇
排序方式: 共有5110条查询结果,搜索用时 15 毫秒
991.
A set of eleven biallelic and three multiallelic molecular markers have been developed to analyze populations of Histoplasma capsulatum. All markers are amplified by polymerase chain reaction (PCR) and can be readily scored using minimal amounts of template DNA. The 11 biallelic loci have polymorphic restriction endonuclease sites or small insertions or deletions which may be assessed by agarose gel electrophoresis. These markers are inherited in an unambiguous manner and are ideal for assessing structure and gene flow within US populations of H. capsulatum, but are monomorphic in non-US populations. Both length and sequence variation are present in the multiallelic loci, which can be scored by direct sequencing, polyacrylamide gel electrophoresis, or single-strand conformation polymorphism (SSCP): As they are hypervariable, the multiallelic loci can be used to type isolates and to assess the level of genetic variation within populations. Preliminary results indicate that the three multiallelic markers presented are sufficient to distinguish isolates at the individual level and are polymorphic in both US and non-US populations. This collection of molecular markers will be a useful tool in population and epidemiology studies of H. capsulatum.  相似文献   
992.
An age greater than 60 and diuretic use have been implicated as risk factors for nonsteroidal anti-inflammatory drug (NSAID)-induced decreases in renal function. Misoprostol, a prostaglandin E1 analog, was studied in nine elderly osteoarthritic patients at risk for NSAID-induced renal dysfunction to determine whether it could prevent NSAID-induced renal dysfunction. Subjects received ibuprofen 2400 mg/day and either misoprostol 800 mcg/day or placebo for 14 days in a randomized, double-blinded, crossover trial. Glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) studies using inulin and PAH plasma clearance, urinary prostaglandin E2 (PGE2) and protein excretion, and serum electrolytes were obtained at baseline, after the first dose, and on day 7 and 14 of each treatment period. Prostaglandin E2 excretion was significantly reduced after the first dose of ibuprofen and throughout the 14 days in both the misoprostol and placebo treatment groups. No statistically significant differences in GFR, ERPF, protein excretion, serum potassium, or serum sodium were detected between misoprostol and placebo treatment during the 14 days of ibuprofen treatment. However, a subset of two patients who exhibited a decrease of greater than 20% in GFR during placebo treatment, appeared to demonstrate an attenuation of this decline when treated with misoprostol. Effect of time, independent of treatment group, indicated that ERPF was significantly decreased from baseline after the first dose of ibuprofen (P < or = 0.05), whereas GFR was notably diminished from baseline on day 14 only (P < or = 0.05). Misoprostol does not influence GFR and ERPF in unselected subjects purportedly at risk for NSAID-induced renal dysfunction.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
993.
994.
Correlative echocardiographic and pathological findings in a thoracopagus with conjoined hearts are reported. One twin had tricuspid atresia with discordant atrioventricular connections and concordant ventriculoarterial connections. The morphologic right ventricle was hypoplastic and there was a large muscular ventricular septal defect. The other twin had hypoplasia of the mitral valve anulus and left ventricle with double-outlet right ventricle and pulmonary valve atresia. The tricuspid valve was severely insufficient in part because of a large orifice and redundant, elongated leaflets with abnormal chordal attachments. The left ventricles of these two twins shared a perforated common "free wall" with at least two large defects allowing mixing of the circulations at that level. Not all anatomic details were established conclusively by fetal echocardiography; however, sufficient diagnostic information was obtained to support a decision not to aggressively resuscitate these twins after elective cesarean delivery at 31 weeks' gestation.  相似文献   
995.
The result of prompt squatting is an increase of the venous return and of the arterial resistance. Consequences are increasing of the end-diastolic and systolic chamber pressure, stroke volume, average arterial blood pressure as well as decreasing of the frequency of the pulse controlled by baro-receptors. The regulatory mechanism both change the intensity of heart murmurs of various cardiopathies and of functional heart murmurs in children. Only the systolic murmur of muscular subvalvular aortic stenosis softens. The experiment is most valuable for the recognition of mild insufficiency of semilunar valves.  相似文献   
996.
Primary duodenal adenocarcinoma not involving the ampullary region is rare. Our aim was to review the outcome of these patients and determine the factors that affect survival. We performed a retrospective review of all patients with primary, nonampullary duodenal adenocarcinoma at the Cleveland Clinic Foundation from January 1986 through December 1996. Twenty-six patients with primary, nonampullary duodenal malignancies were identified. There were 16 adenocarcinomas, 3 gastrinomas, 3 stromal tumors, 3 leiomyosarcomas, and 1 carcinoid tumor. Patients with adenocarcinoma had symptoms present an average of 6.1 months. Tumors were identified by upper gastrointestinal contrast study and esophagogastroduodenoscopy in 90 per cent and 87 per cent of patients, respectively. Twelve of 13 (93%) cancers found in the third or fourth portion of the duodenum were adenocarcinomas. Seven of the 16 adenocarcinomas were resectable on exploration. Those that were contained within the serosa have not recurred (mean, 6 years); one of the two patients with locally invasive adenocarcinoma remains disease free. The average survival for patients with unresectable disease was 6.7 months. The 5-year survival rates were: all adenocarcinoma, 38 per cent; resectable, 86 per cent; and unresectable, 0 per cent. All patients presenting with weight loss or obstructive symptoms died of disease; those with melena survived long term. Patients with tumors other than adenocarcinoma had a 90 per cent 5-year survival. We conclude that patients typically present with a long history of symptoms. Distal duodenal malignancies are most frequently adenocarcinomas. Upper gastrointestinal contrast study or endoscopy is often diagnostic. Patients with weight loss and/or obstructive symptoms had invasive disease and a morbid prognosis. Aggressive surgery is warranted, and most with resectable disease (86%) had long-term survival.  相似文献   
997.
Burkholderia pseudomallei causes melioidosis and is believed to be an intracellular pathogen in human and animal disease. The uptake of B. pseudomallei by mouse peritoneal macrophages and cells in tissue culture was examined by electron microscopy. In all the systems studied B. pseudomallei were phagocytosed and apparently inhibited the normal processes of intracellular killing. Destruction of the phagosome membrane occurred and the bacteria escaped into the cytoplasm.  相似文献   
998.
基于Gabor、Fisher脸多特征提取及集成SVM的人脸表情识别*   总被引:1,自引:1,他引:1  
针对于静态的灰度图像表情库,提出了基于多种脸部表情特征多级分类的表情识别算法。首先在选取的人脸特征点上做局部的Gabor小波变换,为了提高特征提取速度,利用改进的弹性图匹配算法来提取图像中的人脸有效区域,在提取的人脸区域中提取几何特征,并通过Fisher脸法提取统计特征,利用几何特征与建立的相应一级集成SVM来进行初次分类,最后利用Fisher特征与建立的相应二级集成SVM进行最终分类。通过在JAFFE与Cohn-Kanade表情库中实验,证明本文方法同单个特征相比较,具有更高的表情识别率以及更强的鲁棒性  相似文献   
999.
快速城市化伴随各类环境污染、生物多样性质量下降、生态系统服务功能退化、疾病蔓延等城市问题,直接威胁着城市生态安全乃至人类生命健康.基于对"生态"一词源流与内涵的梳理,倡导汲取传统生态智慧,运用"生命观、动态观、系统观"辨析和破解城市生态建设及管理方法中存在的问题与误区.结合上海生态实践,通过营建以乡土植物和土著动物为主...  相似文献   
1000.
在传统渗碳、渗氮及表面淬火工艺的汽车零部件表面强化技术基础上,简单综述了传统工艺在汽车零部件表面强化上应用的工艺优缺点及研究现状,重点阐述了新型表面形变强化、高能束流表面强化、表面冶金强化、表面复合强化及表面纳米强化的汽车零部件表面强化技术,详细介绍了目前广泛使用的新型表面形变强化中的喷丸强化工艺、高能束流表面强化中的激光表面淬火与电子束表面淬火工艺及表面冶金强化中等离子喷涂工艺的技术特点和应用现状,分析了国内外汽车零部件表面复合强化技术和表面纳米强化技术的工艺特点和研究现状,尤其是将表面复合强化技术中的传统工艺与传统工艺、传统工艺与新型工艺、新型工艺与新型工艺的多种复合工艺的相互结合及QPQ盐浴复合处理技术在汽车零部件表面强化上的应用,最后展望了未来汽车零部件表面强化技术的发展方向。  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