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   8068篇
  免费   439篇
  国内免费   25篇
电工技术   145篇
综合类   21篇
化学工业   2012篇
金属工艺   114篇
机械仪表   223篇
建筑科学   293篇
矿业工程   34篇
能源动力   241篇
轻工业   757篇
水利工程   74篇
石油天然气   41篇
无线电   613篇
一般工业技术   1272篇
冶金工业   831篇
原子能技术   58篇
自动化技术   1803篇
  2023年   97篇
  2022年   264篇
  2021年   345篇
  2020年   199篇
  2019年   239篇
  2018年   267篇
  2017年   270篇
  2016年   310篇
  2015年   262篇
  2014年   337篇
  2013年   610篇
  2012年   516篇
  2011年   571篇
  2010年   423篇
  2009年   425篇
  2008年   410篇
  2007年   353篇
  2006年   277篇
  2005年   222篇
  2004年   196篇
  2003年   143篇
  2002年   142篇
  2001年   107篇
  2000年   90篇
  1999年   95篇
  1998年   241篇
  1997年   183篇
  1996年   126篇
  1995年   89篇
  1994年   87篇
  1993年   67篇
  1992年   41篇
  1991年   32篇
  1990年   31篇
  1989年   26篇
  1988年   21篇
  1987年   32篇
  1986年   26篇
  1985年   30篇
  1984年   27篇
  1983年   32篇
  1982年   23篇
  1981年   15篇
  1980年   21篇
  1979年   23篇
  1978年   16篇
  1977年   27篇
  1976年   20篇
  1975年   15篇
  1973年   19篇
排序方式: 共有8532条查询结果,搜索用时 15 毫秒
21.
We consider patches that contain any given 3D polynomial curve as a pregeodesic (i.e. geodesic up to reparametrization). A curve is a pregeodesic if and only if its rectifying plane coincides with the tangent plane to the surface, we use this fact to construct ruled cubic patches through pregeodesics and bicubic patches through pairs of pregeodesics. We also discuss the G1 connection of (1,k) patches with abutting pregeodesics.  相似文献   
22.
With reference to the kinematic wave theory coupled with the hypothesis of constant linear velocity for the rating curve, rising limb analytical solutions have been calculated for overland flow, over an Hortonian-infiltrating surface, and sediment discharge. These analytical solutions are certainly easier to use than the numerical integration of the basic equations and they may be used to obtain an initial evaluation of the parameters of more complex models generally devised for complicated cases.Notation a exponent of the Horton law [T–1] - b exponent of the rill erosion equation - B inter-rill erosion coefficient [MLm–2T m–1] - c sediment concentration [ML–3] - c o reference sediment concentration [ML–3] - E I inter-rill erosion [ML–2T–1] - E R rill erosion [ML–2T–1] - f c final infiltration rate of the soil [LT–1] - f o initial infiltration rate of the soil [LT–1] - h flow depth [L] - h o reference flow depth [L] - i infiltration rate [LT–1] - k rill erosion coefficient [ML–1–b T–1] - K integration constant - L() Laplace transformation - m exponent of the inter-rill erosion equation - n Manning's coefficient [L–1/3T] - p rainfall intensity [LT–1] - q water discharge per unit width [L2T–1] - q s sediment discharge per unit width [ML–1T–1] - t time [T] - t p ponding time [T] - x distance along the flow direction [L] Greek Letters coefficient of the stage-discharge equation [L2–T–1] - exponent of the stage-discharge equation - rill erosion coefficient [L–1]  相似文献   
23.
Sensitivity analysis studies how the variation in model outputs can be due to different sources of variation. This issue is addressed, in this study, as an application of sensitivity analysis techniques to a crop model in the Mediterranean region. In particular, an application of Morris and Sobol' sensitivity analysis methods to the rice model WARM is presented. The output considered is aboveground biomass at maturity, simulated at five rice districts of different countries (France, Greece, Italy, Portugal, and Spain) for years characterized by low, intermediate, and high continentality. The total effect index of Sobol' (that accounts for the total contribution to the output variation due a given parameter) and two Morris indices (mean μ and standard deviation σ of the ratios output changes/parameter variations) were used as sensitivity metrics. Radiation use efficiency (RUE), optimum temperature (Topt), and leaf area index at emergence (LAIini) ranked in most of the combinations site × year as first, second and third most relevant parameters. Exceptions were observed, depending on the sensitivity method (e.g. LAIini resulted not relevant by the Morris method), or site-continentality pattern (e.g. with intermediate continentality in Spain, LAIini and Topt were second and third ranked; with low continentality in Portugal, RUE was outranked by Topt). Low σ values associated with the most relevant parameters indicated limited parameter interactions. The importance of sensitivity analyses by exploring site × climate combinations is discussed as pre-requisite to evaluate either novel crop-modelling approaches or the application of known modelling solutions to conditions not explored previously. The need of developing tools for sensitivity analysis within the modelling environment is also emphasized.  相似文献   
24.
In this paper we propose a heuristic approach for the problem of packing equal rectangles within a convex region. The approach is based on an Iterated Local Search scheme, in which the key step is the perturbation move. Different perturbation moves, both combinatorial and continuous ones, are proposed and compared through extensive computational experiments on a set of test instances. The overall results are quite encouraging.  相似文献   
25.
International Journal on Software Tools for Technology Transfer - System designers make trade-offs between metrics of interest such as execution time, functional quality and cost to create a...  相似文献   
26.
We report on the electrochromic behavior of a dinuclear ruthenium complex, which provides high environmental stability together with an intense absorption maximum at 1237 nm in solution in its +3 oxidation state. To verify its optoelectronic properties, we used this complex as the photoactive material in a planar photodetector prototype for near-infrared light pulses, succeeding in the detection of a train of light pulses at a wavelength of 1280 nm.  相似文献   
27.
The VLDL receptor has been described as a new member of the LDL receptor supergene family that specifically binds VLDL in vitro via apolipoprotein E and lipoprotein lipase. Both apolipoprotein E and lipoprotein lipase are constituents of chylomicron remnants, another triglyceride-rich lipoprotein which has been proposed as a physiological ligand for the VLDL receptor. We used human chylomicron remnants to study their uptake into LDL, receptor-deficient Chinese hamster ovary cells overexpressing the human VLDL receptor. The uptake into these cells was compared to that into cells transfected with an empty transfection vector. Human chylomicron remnants were produced in vitro by hydrolysis with lipoprotein lipase, and were labeled with 125I. The uptake of these remnants into the cells overexpressing the VLDL receptor was found to be about 3-fold higher than the uptake into the control cells. The addition of a surplus of either apolipoprotein E or inactivated lipoprotein lipase to the remnants led to an increase in particle uptake. The chylomicron remnant uptake was inhibited by addition of the 39 kDa receptor associated protein These in vitro experiments strongly support the idea that the VLDL receptor is a physiological receptor for chylomicron remnants. The increase of receptor-mediated uptake induced by the addition of apoE or lipoprotein lipase underlines the role of these two proteins in this process.  相似文献   
28.
Liu J  White I  DeVoe DL 《Analytical chemistry》2011,83(6):2119-2124
The use of porous polymer monoliths functionalized with silver nanoparticles is introduced in this work for high-sensitivity surface-enhanced Raman scattering (SERS) detection. Preparation of the SERS detection elements is a simple process comprising the synthesis of a discrete polymer monolith section within a silica capillary, followed by physically trapping silver nanoparticle aggregates within the monolith matrix. A SERS detection limit of 220 fmol for Rhodamine 6G is demonstrated, with excellent signal stability over a 24 h period. The capability of the SERS-active monolith for label-free detection of biomolecules was demonstrated by measurements of bradykinin and cytochrome c. The SERS-active monoliths can be readily integrated into miniaturized micrototal-analysis systems for online and label-free detection for a variety of biosensing, bioanalytical, and biomedical applications.  相似文献   
29.
The surfaces of carbon-carbon (C/C) aircraft brakes are usually coated with anti-oxidant to protect them from oxidation. These surfaces do not include the friction surfaces since it is known that when anti-oxidant get onto the friction surface, the friction coefficient decreases. The anti-oxidant migration (AOM), however, happens during processing, heat treatment and application. In this study, phosphorus based anti-oxidants inhibited 3-D C/C aircraft brake system was investigated. The effects of their migration on friction and wear in the 3-D C/C brakes were revealed by sub-scale dynamometer tests and microscopic analysis. Dynamometer results showed that when AOM occurred, both landing and taxi coefficients decreased in humid environment and the wear was slightly lowered. Microscopic study showed that under high humidity conditions there was no formation of the friction film.  相似文献   
30.
Future climate scenarios suggest that crop plants will experience environmental changes capable of affecting their productivity. Among the most harmful environmental stresses is drought, defined as a total or partial lack of water availability. It is essential to study and understand both the damage caused by drought on crop plants and the mechanisms implemented to tolerate the stress. In this study, we focused on four cultivars of tomato, an economically important crop in the Mediterranean basin. We investigated the biochemical mechanisms of plant defense against drought by focusing on proteins specifically involved in this stress, such as osmotin, dehydrin, and aquaporin, and on proteins involved in the general stress response, such as HSP70 and cyclophilins. Since sugars are also known to act as osmoprotectants in plant cells, proteins involved in sugar metabolism (such as RuBisCO and sucrose synthase) were also analyzed. The results show crucial differences in biochemical behavior among the selected cultivars and highlight that the most tolerant tomato cultivars adopt quite specific biochemical strategies such as different accumulations of aquaporins and osmotins. The data set also suggests that RuBisCO isoforms and aquaporins can be used as markers of tolerance/susceptibility to drought stress and be used to select tomato cultivars within breeding programs.  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