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   4994篇
  免费   79篇
  国内免费   72篇
电工技术   34篇
综合类   52篇
化学工业   123篇
金属工艺   22篇
机械仪表   46篇
建筑科学   57篇
矿业工程   14篇
能源动力   7篇
轻工业   102篇
水利工程   18篇
石油天然气   49篇
武器工业   6篇
无线电   112篇
一般工业技术   82篇
冶金工业   4276篇
原子能技术   2篇
自动化技术   143篇
  2024年   4篇
  2023年   6篇
  2022年   10篇
  2021年   23篇
  2020年   17篇
  2019年   14篇
  2018年   26篇
  2017年   19篇
  2016年   19篇
  2015年   30篇
  2014年   29篇
  2013年   45篇
  2012年   58篇
  2011年   64篇
  2010年   57篇
  2009年   51篇
  2008年   50篇
  2007年   47篇
  2006年   53篇
  2005年   52篇
  2004年   29篇
  2003年   32篇
  2002年   19篇
  2001年   17篇
  2000年   18篇
  1999年   153篇
  1998年   1398篇
  1997年   739篇
  1996年   470篇
  1995年   266篇
  1994年   227篇
  1993年   269篇
  1992年   43篇
  1991年   55篇
  1990年   64篇
  1989年   54篇
  1988年   56篇
  1987年   63篇
  1986年   49篇
  1985年   61篇
  1983年   6篇
  1982年   17篇
  1981年   24篇
  1980年   28篇
  1979年   3篇
  1978年   7篇
  1977年   82篇
  1976年   199篇
  1975年   6篇
  1955年   3篇
排序方式: 共有5145条查询结果,搜索用时 31 毫秒
961.
Framing as a theory of media effects   总被引:10,自引:0,他引:10  
Research on framing is characterized by theoretical and empirical vagueness. This is due, in part, to the lack of a commonly shared theoretical model underlying framing research. Conceptual problems translate into operational problems, limiting the comparability of instruments and results. In this paper I systematize the fragmented approaches to framing in political communication and integrate them into a comprehensive model. I classify previous approaches to framing research along two dimensions: the type of frame examined (media frames vs. audience frames) and the way frames are operationalized (independent variable or dependent variable). I develop a process model of framing, identifying four key processes that should be addressed in future research: frame building, frame setting, individual-level processes of framing, and a feedback loop from audiences to journalists.  相似文献   
962.
The outer membrane of pathogenic Leptospira species grown in culture media contains lipopolysaccharide (LPS), a porin (OmpL1), and several lipoproteins, including LipL36 and LipL41. The purpose of this study was to characterize the expression and distribution of these outer membrane antigens during renal infection. Hamsters were challenged with host-derived Leptospira kirschneri to generate sera which contained antibodies to antigens expressed in vivo. Immunoblotting performed with sera from animals challenged with these host-derived organisms demonstrated reactivity with OmpL1, LipL41, and several other proteins but not with LipL36. Although LipL36 is a prominent outer membrane antigen of cultivated L. kirschneri, its expression also could not be detected in infected hamster kidney tissue by immunohistochemistry, indicating that expression of this protein is down-regulated in vivo. In contrast, LPS, OmpL1, and LipL41 were demonstrated on organisms colonizing the lumen of proximal convoluted renal tubules at both 10 and 28 days postinfection. Tubular epithelial cells around the luminal colonies had fine granular cytoplasmic LPS. When the cellular inflammatory response was present in the renal interstitium at 28 days postinfection, LPS and OmpL1 were also detectable within interstitial phagocytes. These data establish that outer membrane components expressed during infection have roles in the induction and persistence of leptospiral interstitial nephritis.  相似文献   
963.
OBJECTIVES: To determine the effect of massive intracerebral hemorrhage (ICH) on regional cerebral blood flow (rCBF) and metabolism, and to test the hypothesis that there is persistent ischemia in the perihematoma region after ICH. BACKGROUND: Cerebral ischemia is postulated to be one of the mechanisms of neural injury after ICH. Presumably the hematoma induces ischemia by mechanical compression of the surrounding microvasculature. METHODS: The authors induced ICH in eight anesthetized mongrel dogs by autologous blood injection (7.5 mL) under arterial pressure in the deep white matter adjacent to the left basal ganglia. They measured serial rCBF using radiolabeled microspheres in regions around and distant to the hematoma, as well as cerebral oxygen extraction, oxygen consumption (CMRO2), glucose utilization, and lactate production by serial sampling of cerebral venous blood from the sagittal sinus. Mean arterial pressure (MAP) and intracranial pressure (ICP) were monitored continuously. All measurements were recorded at 0.5, 1.0, 2.0, 3.5, and 5.0 hours after induction of ICH and compared with prehematoma values. Evans Blue dye was injected at the end of the experiment, and intensity of staining was compared with three control animals. RESULTS: Compared with prehematoma ICP (12.5+/-2.0 mm Hg, mean+/-standard error), significant elevation in ICP was observed after ICH peaking at 5 hours (34.4+/-5.2 mm Hg). Compared with prehematoma MAP (125.8+/-7.0 mm Hg), significant elevation in MAP was observed at 120 minutes after onset of hematoma (139.1+/-4.6 mm Hg), with return to the prehematoma value by 5 hours. There were no significant changes observed in cerebral oxygen extraction (51.4+/-4.3% versus 44.8+/-4.9%) and CMRO2 (1.8+/-0.3 versus 1.64+/-0.2 mL O2/100 g/min) at 5 hours posthematoma (or any other posthematoma measurement) compared with prehematoma values. There were no significant differences observed in rCBF in the perihematoma gray (18.2+/-0.9 mL/100 g/min versus 20.1+/-1.5 mL/100 g/min) or white matter (15.6+/-1.4 mL/100 g/min versus 15.3+/-1.1 mL/100 g/min) at 5 hours posthematoma (or any other posthematoma measurement) compared with prehematoma values. No changes were observed in cerebral glucose utilization, lactate production, and rCBF in other regions after introduction of ICH. Permeability of the blood-brain barrier was more prominent in the ipsilateral hemisphere in animals with ICH compared with control animals. CONCLUSIONS: Despite a prominent increase in ICP and MAP after ICH, the authors found no evidence to support the presence of an ischemic penumbra in the first 5 hours after ICH. Thus, other mechanisms for acute neural injury and late rCBF changes after ICH must be investigated.  相似文献   
964.
Tuberculous infection of the esophagus is a rare disease and usually occurs secondary to tuberculous mediastinal lymphadenopathy. We report a 74-year-old woman presenting with dysphagia and weight loss. The chest radiograph showed punctuated calcifications lining from the right hilar region to the paracardiac region. Upper gastrointestinal endoscopy revealed a 0.5 x 5 cm lesion localized in the 25th cm of the esophagus covered partly with normal mucosa, partly demonstrating ulcerous areas. Biopsy revealed granulomatous infiltrates being tuberculoid in type. Thorax CT showed a calcified lymph node at the right hilum. The patient did not give consent to therapy and died 1 month later.  相似文献   
965.
In recent years new curing agent technology has been developed which allows water-based curing agents to be introduced into high performance coating, these are classified according to the type of epoxy resin used. Type I utilises low molecular weight, liquid epoxy resins and Type II pre-formed dispersions of higher molecular weight solid epoxy resins. The chemical composition of the binder of water-borne epoxy coatings plays a critical role in their performance properties, hence some of the structural properties of the amine hardeners will be reviewed. New curing agents will be discussed which demonstrate the versatility these products can offer to the coating formulator. Performance data of clear coats and anti-corrosive primers confirm the applicability of the new systems for the long term protection of steel and concrete surfaces.  相似文献   
966.
目的研究雅江雪牛与牦牛肉食用品质及营养成分含量的差异。方法选择健康成年(5~7岁)的西藏公牦牛、公雅江雪牛和母雅江雪牛各6头,对其体尺、背最长肌基本营养成分和氨基酸、脂肪酸以及食用品质指标进行分析。结果雅江雪牛肉的蛋白质含量较丰富,属于高蛋白性食品。公雅江雪牛和母雅江雪牛肉中必需氨基酸与总氨基酸的比值(essential amino acid/total amino acid,EAA/TAA)分别为44.7%和44.8%,必需氨基酸与非必需氨基酸的比值(essential amino acid/nonessential amino acids,EAA/NEAA)分别为80.7%和80.8%,表明雅江雪牛肉为优质蛋白;雅江雪牛的脂肪含量较高,相对于普通牦牛,具有更好的风味品质;雅江雪牛蒸煮损失率较低,具有较好的加工性能;雅江雪牛颜色鲜红,更符合消费者喜好。结论雅江雪牛生长速度快,脂肪含量相较于传统牦牛明显升高,能够在西藏有限的牧草资源环境中产生相对于牦牛更高的经济利用价值,对西藏地区的肉牛产业起到良好的促进作用。  相似文献   
967.
针对当前多参数加权分数阶傅里叶变换(Multiple Parameters Weighted-type FRactional Fourier Transform, MP-WFRFT)星座特征缺乏定量研究的缺限,该文提出一种新型的星座叠加数学模型。通过分别研究信号的时域及频域分量,给出了星座模糊及裂变的定性及定量分析结论,并搭建了基于MP-WFRFT的星座预编码系统。通过构建相关混合整数优化模型,并利用遗传算法进行模型优化,进而设计出符合预期通信需求的预编码系统。经仿真分析,验证了所提出的星座叠加数学模型及优化方案的正确性;同时通过USRP硬件平台测试表明了所搭建的基于MP-WFRFT星座预编码系统的可行性。  相似文献   
968.
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   
969.
Cashew apple and guava residues from fruit juice processing industry were prepared as dehydrated fruit powders. Differences in compositional and physicochemical characteristics between both fruit powders were significant at the 5% probability level, with exception to ash content. It is important to point out that despite the dehydration process, both samples can still be considered good sources of vitamin C. In addition, the results show that the guava and cashew apple powders may be useful in the food industry as high dietary "fiber ingredients." Higher lipid (14.05%) content was observed in guava powder. The guava powder could be an alternative source of lipid. The data also indicated that these materials are good sources of ash. The results suggest further utilization of these residues as valuable food ingredients or as a resource for nutraceuticals.

PRACTICAL APPLICATIONS


Large quantity of agro-industrial residues formed by fruit juice extraction represents a major disposal problem. An alternative to recycling the fruit-industrial residue is to submit it to drying processes and transform into value-added products as dehydrated fruit powder. In this way, this process contributes to diminish the problem and to recover valuable biomass and nutrients. The purpose of this study was to develop a technology to obtain guava and cashew apple powders with the objective of finding alternatives to the exploitation of these residues as valuable sources of vitamin C, as ashes and lipid that should also be regarded as potential nutraceutical resources in future or functional ingredients.  相似文献   
970.
以推型金属带为基体的无级变速箱(CVT)的动力传递效率低于有级变速箱,大家已往认识到采用CVT改进发动机/负荷匹配新取得的燃料经济性的好处有被取消的趋势.本组三篇论文详细阐述了发生在作为第一级用来取得效率改进的带传动中损耗机理的研究.已经进行试验研究了涉及推型金属V-带CVT空载和低载的转矩损失.本第一篇论文述及发生在金属带CVT由于带推块和带间相对运动造成主要转矩损失的新分析方法.本研究考虑到其他研究中的新发现并改进了金属V-带的设计.本文中采用几种不同的试验方法提供的实验数据推荐了转矩损失的数字模型.本组第二篇论文阐述了由于带轮变形的许多附加转矩损失机理.在第三篇论文中阐述了根据本文的发现提出带滑动损失的分析.  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