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   3960篇
  免费   93篇
  国内免费   5篇
电工技术   156篇
综合类   3篇
化学工业   826篇
金属工艺   88篇
机械仪表   89篇
建筑科学   64篇
矿业工程   1篇
能源动力   138篇
轻工业   342篇
水利工程   13篇
石油天然气   16篇
无线电   395篇
一般工业技术   625篇
冶金工业   965篇
原子能技术   101篇
自动化技术   236篇
  2023年   23篇
  2022年   46篇
  2021年   56篇
  2020年   31篇
  2019年   43篇
  2018年   52篇
  2017年   61篇
  2016年   66篇
  2015年   41篇
  2014年   87篇
  2013年   176篇
  2012年   149篇
  2011年   153篇
  2010年   136篇
  2009年   132篇
  2008年   145篇
  2007年   148篇
  2006年   115篇
  2005年   102篇
  2004年   96篇
  2003年   115篇
  2002年   84篇
  2001年   84篇
  2000年   82篇
  1999年   134篇
  1998年   395篇
  1997年   212篇
  1996年   166篇
  1995年   103篇
  1994年   109篇
  1993年   110篇
  1992年   62篇
  1991年   49篇
  1990年   40篇
  1989年   29篇
  1988年   33篇
  1987年   30篇
  1986年   27篇
  1985年   32篇
  1984年   31篇
  1983年   35篇
  1982年   20篇
  1981年   29篇
  1980年   22篇
  1979年   21篇
  1978年   23篇
  1977年   22篇
  1976年   48篇
  1973年   11篇
  1972年   10篇
排序方式: 共有4058条查询结果,搜索用时 15 毫秒
991.
A code of FEM for an elastic and plastic material has been developed to calculate a residual stress distribution in D-I can by both-sided ironing. It is possible for the code to simulate a forming process of deep drawing from a sheet to a can, an ironing process of both-sided of the can wall, and a deformation of the whole of the can under perfect unloading when the can is removed from an equipment of deep drawing and ironing. From these simulations, quite different and contrary distributions of residual stress are obtained according to mutual positions of dies for the ironing, which are considered to expand the wall of can as an ellipsoidal shape or shrink it to a hyperbolic shape.  相似文献   
992.
A qualitative design was used to explore Italo-Canadian beliefs about the cause and treatment/cure of childhood disabilities. A convenience sample of eight second-generation Italo-Canadian mothers of children with moderate to severe disabilities was recruited through a pediatric rehabilitation centre in Southern Ontario. The mothers were interviewed using open-ended questions that served as probes to stimulate their thoughts and feelings. Data analysis proceeded concomitantly with data collection, and consisted of content analysis. Three major categories of traditional causation beliefs were identified: (a) punishment for past wrongdoing, (b) "bad blood" caused by intermarriage or a family curse, and (c) supernatural forces such as mal'occhio (the evil eye). The major category identified for cure of childhood disability was magico-religious healing. These traditional beliefs were described by the mothers to be strongest among first-generation Italo-Canadians, especially grandparents. The mothers themselves expressed their acceptance of some of these beliefs, but only minimal acceptance of or resistance to others. Based on this study, it would be premature to formulate implications for practice; however, several directions for future research are identified.  相似文献   
993.
Previous work suggests that slowly adapting (SA) periodontal afferents have different synaptic arrangements in the principal (Vp) and oral trigeminal nuclei and that the synaptic structure associated with transmitter release may be related directly to bouton size. The present study examined the ultrastructures of SA and fast adapting (FA) vibrissa afferents and their associated unlabeled axonal endings in the cat Vp by using intra-axonal labeling with horseradish peroxidase and a morphometric analysis. All SA and FA afferent boutons contained clear, round, synaptic vesicles. All the FA and most SA boutons were presynaptic to dendrites, but a few SA boutons were axosomatic. Both types of bouton were frequently postsynaptic to unlabeled axonal ending(s) containing pleomorphic, synaptic vesicles (P-ending). The size of labeled boutons was larger in FA than SA afferents, but the size of dendrites postsynaptic to labeled boutons was larger for SA than FA afferents. Large-sized FA and SA boutons made synaptic contacts with small-diameter dendrites. The size of FA and SA boutons was larger than that of their associated P-endings. A morphometric analysis made on the pooled data of SA and FA boutons indicated that apposed surface area, active zone number, total active zone area, vesicle number, and mitochondrial volume were highly correlated in a positive linear manner with labeled bouton volume. These relationships were also applicable to unlabeled P-endings, but the range of each parameter was smaller than that of the labeled boutons. These observations provide evidence that the two functionally distinct types of vibrissa afferent manifest unique differences but share certain structural features in the synaptic organization and that the ultrastructural "size principle" proposed by Pierce and Mendell ([1993] J. Neurosci. 13:4748-4763) for Ia-motoneuron synapses is applicable to the somatosensory system.  相似文献   
994.
RATIONALE AND OBJECTIVES: The authors compared the embolic effect of radiolucent absolute ethanol (AE) with that of a radiopaque iohexol-ethanol (IES) solution for renal ablation in dogs and for the destruction of human aldosteronomas by the technique of transcatheter arterial embolization (TAE) to test whether IES can be an alternative to AE. METHODS: The embolic agents were infused through a balloon catheter into the renal arteries of 17 dogs (9 infused with 0.3 mL/kg AE; 8 infused with IES). The immediate and parenchyma were compared between the two groups. Transcatheter arterial embolization with IES also was performed in three humans with unilateral aldosteronoma. RESULTS: The IES was visualized faintly under fluoroscopy in all dogs. There were no significant differences in embolic effects between the AE and IES. Three patients with aldosteronoma were treated successfully by TAE with IES. CONCLUSIONS: The IES can be used as a "visible ethanol" to improve the safety and ease of ethanol embolization.  相似文献   
995.
Multilayer mirrors with a system wavelength resolution (lambda/Dlambda) as high as 30-50 are required for the diagnostics of cosmic plasmas with temperatures of 1-20 MK. Such a high wavelength resolution can be realized by increasing the number of layer pairs contributing to the reflectance, by selecting less-absorbing materials for both the reflector and the spacer, and by decreasing the thickness of the reflector. We have fabricated a multilayer mirror tuned to 284 A with a silicon carbide reflector (20% thickness of the layer period) and an aluminum spacer and achieved lambda/Dlambda ~26.8 with a peak reflectivity of ~13.0%. This wavelength resolution is close to the value obtained with a numerical simulation and is considerably higher than the value obtained with the conventional Mo/Si multilayer.  相似文献   
996.
The general behavior of two-photon absorption-enhanced refractive-index change in a third-order nonlinear optical medium is briefly described. The nonlinear medium was the solution of a new dye, trans-4-[p-(N-hydroxyethyl-N-methylamino)styryl]-N-methyl pyridinium iodide (ASPI) in dimethyl sulfoxide as the solvent, that was poured into a 20-cm-long quartz hollow fiber of 100-mum internal diameter. This dye solution has a strong two-photon absorption and subsequent upconversion fluorescence emission when excited with 1064-nm laser radiation. When the input peak intensity reached 500-1500-MW/cm(2) levels, obvious changes in beam profiles of the output IR laser beam were observed due to a self-focusing or self-trapping process occurring inside the fiber system. As a result of this process, highly directional frequency-upconverted superradiant lasing output was obtained with a beam size ~5 times smaller than that of a linearly transmitted He-Ne probe laser beam. The demonstrated mechanism can be useful for fiber laser-amplifier and fiber-integrated optics devices.  相似文献   
997.
998.
Microsystem Technologies - An optical sensor, such as a light waveguide implemented total analysis system (TAS), is one of the functional components that will be needed to realize a...  相似文献   
999.
DNA origami methods enable the fabrication of various nanostructures and nanodevices, but their effective use depends on an understanding of their structural and mechanical properties and the effects of basic structural features. Frequency‐modulation atomic force microscopy is introduced to directly characterize, in aqueous solution, the crossover regions of sets of 2D DNA origami based on different crossover/nick designs. Rhombic‐shaped nanostructures formed under the influence of flexible crossovers placed between DNA helices are observed in DNA origami incorporating crossovers every 3, 4, or 6 DNA turns. The bending rigidity of crossovers is determined to be only one‐third of that of the DNA helix, based on interhelical electrostatic forces reported elsewhere, and the measured pitches of the 3‐turn crossover design rhombic‐shaped nanostructures undergoing negligible bending. To evaluate the robustness of their structural integrity, they are intentionally and simultaneously stressed using force‐controlled atomic force microscopy. DNA crossovers are verified to have a stabilizing effect on the structural robustness, while the nicks have an opposite effect. The structural and mechanical properties of DNA origami and the effects of crossovers and nicks revealed in this paper can provide information essential for the design of versatile DNA origami structures that exhibit specified and desirable properties.  相似文献   
1000.
The aim of this research is to construct a shape optimization method based on the convected level set method, in which the level set function is defined as a truncated smooth function obtained by using a sinus filter based on a hyperbolic tangent function. The local property of the hyperbolic tangent function dramatically reduces the generation of red the error between the specified profile of the hyperbolic tangent function and the level set function that is updated using a time evolution equation. In addition, the small size of the error facilitates the use of convective reinitialization, whose basic idea is that the reinitialization is embedded in the time evolution equation, whereas such treatment is typically conducted in a separate calculation in conventional level set methods. The convected level set method can completely avoid the need for additional calculations when performing reinitialization. The validity and effectiveness of our presented method are tested with a mean compliance minimization problem and a problem for the design of a compliant mechanism.  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