全文获取类型
收费全文 | 314337篇 |
免费 | 41517篇 |
国内免费 | 35487篇 |
专业分类
电工技术 | 28954篇 |
技术理论 | 16篇 |
综合类 | 30071篇 |
化学工业 | 49951篇 |
金属工艺 | 11780篇 |
机械仪表 | 21057篇 |
建筑科学 | 21652篇 |
矿业工程 | 6450篇 |
能源动力 | 11972篇 |
轻工业 | 17852篇 |
水利工程 | 10598篇 |
石油天然气 | 11367篇 |
武器工业 | 3869篇 |
无线电 | 38612篇 |
一般工业技术 | 31835篇 |
冶金工业 | 8364篇 |
原子能技术 | 4591篇 |
自动化技术 | 82350篇 |
出版年
2024年 | 1126篇 |
2023年 | 4801篇 |
2022年 | 8557篇 |
2021年 | 11232篇 |
2020年 | 11016篇 |
2019年 | 9857篇 |
2018年 | 9221篇 |
2017年 | 12203篇 |
2016年 | 13751篇 |
2015年 | 15291篇 |
2014年 | 17191篇 |
2013年 | 20824篇 |
2012年 | 23569篇 |
2011年 | 26327篇 |
2010年 | 19574篇 |
2009年 | 19768篇 |
2008年 | 20956篇 |
2007年 | 23511篇 |
2006年 | 21565篇 |
2005年 | 18696篇 |
2004年 | 15502篇 |
2003年 | 12755篇 |
2002年 | 9873篇 |
2001年 | 7808篇 |
2000年 | 6643篇 |
1999年 | 5351篇 |
1998年 | 4449篇 |
1997年 | 3647篇 |
1996年 | 3100篇 |
1995年 | 2637篇 |
1994年 | 2231篇 |
1993年 | 1622篇 |
1992年 | 1339篇 |
1991年 | 1067篇 |
1990年 | 880篇 |
1989年 | 681篇 |
1988年 | 483篇 |
1987年 | 293篇 |
1986年 | 270篇 |
1985年 | 332篇 |
1984年 | 303篇 |
1983年 | 188篇 |
1982年 | 261篇 |
1981年 | 129篇 |
1980年 | 143篇 |
1979年 | 72篇 |
1978年 | 32篇 |
1977年 | 30篇 |
1959年 | 35篇 |
1951年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 218 毫秒
991.
Claus Bahlmann Author Vitae 《Pattern recognition》2006,39(1):115-125
The selection of valuable features is crucial in pattern recognition. In this paper we deal with the issue that part of features originate from directional instead of common linear data. Both for directional and linear data a theory for a statistical modeling exists. However, none of these theories gives an integrated solution to problems, where linear and directional variables are to be combined in a single, multivariate probability density function. We describe a general approach for a unified statistical modeling, given the constraint that variances of the circular variables are small. The method is practically evaluated in the context of our online handwriting recognition system frog on hand and the so-called tangent slope angle feature. Recognition results are compared with two alternative modeling approaches. The proposed solution gives significant improvements in recognition accuracy, computational speed and memory requirements. 相似文献
992.
993.
Bastnasiteisamainsourceofrareearthproducts inwhichCeO2/REOisabout50%.Atpresent,a cidity leachingcombinedwithalkali conversion method[1]iscommonlyusedinthebastnasitetreat ment.Thismethodisunfriendlytotheenvironment becausetheradioactiveelementofthoriuman… 相似文献
994.
针对日益激烈的市场竞争,探讨了引发大规模商战的原因,建立了两企业集团间的商战模型,并分析了模型的均衡点,同时在此基础上建立了多个企业集团问商战模型,分析了模型的稳定性,最后根据模型的结论给出了企业间避免恶性商战的相关建议与对策。 相似文献
995.
A scheme to randomly generate dust points on the surface of a three-dimensional (3D) object is proposed. This scheme will potentially eliminate the bias of a density of points which are approximately equally spaced on the surface, as well as eliminate possible weighting requirements for areas of higher density. This method has the potential to reduce computational effort, as fewer points might be required to define the surface area. The global trial function vector can be assigned to these dust points and then entered into the complex variable boundary element method (CVBEM) commercial code. This scheme can be applied to each piece of a system that is comprised of a connected multiple geometry. A noticeable benefit of this approach is that the same problem can be studied several times, using a new random distribution of surface points each time. The smallest error can then be determined and that run selected. The Mathematica code used to generate the scheme is attached as an appendix. 相似文献
996.
997.
Christopher Small 《Remote sensing of environment》2006,104(2):168-189
Urban environmental conditions are strongly dependent on the biophysical properties and radiant thermal field of the land cover elements in the urban mosaic. Observations of urban reflectance and surface temperature provide valuable constraints on the physical properties that are determinants of mass and energy fluxes in the urban environment. Consistencies in the covariation of surface temperature with reflectance properties can be parameterized to represent characteristics of the surface energy flux associated with different land covers and physical conditions. Linear mixture models can accurately represent Landsat ETM+ reflectances as fractions of generic spectral endmembers that correspond to land surface materials with distinct physical properties. Modeling heterogeneous land cover as mixtures of rock and/or soil Substrate, Vegetation and non-reflective Dark surface (SVD) generic endmembers makes it possible to quantify the dependence of aggregate surface temperature on the relative abundance of each physical component of the land cover, thereby distinguishing the effects of vegetation abundance, soil exposure, albedo and shadowing. Comparing these covariations in a wide variety of urban settings and physical environments provides a more robust indication of the global variability in these parameter spaces than could be inferred from a single study area. A comparative analysis of 24 urban areas and their non-urban peripheries illustrates the variability in the urban thermal fields and its dependence on biophysical land surface components. Contrary to expectation, moderate resolution intra-urban variations in surface temperature are generally as large as regional surface heat island signatures in these urban areas. Many of the non-temperate urban areas did not have surface heat island signatures at all. However, the multivariate distributions of surface temperature and generic endmember fractions reveal consistent patterns of thermal fraction covariation resulting from land cover characteristics. The Thermal-Vegetation (TV) fraction space illustrates the considerable variability in the well-known inverse correlation between surface temperature and vegetation fraction at moderate (< 100 m) spatial resolutions. The Thermal-Substrate (TS) fraction space reveals energetic thresholds where competing effects of albedo, illumination and soil moisture determine the covariation of maximum and minimum temperature with illuminated substrate fraction. The dark surface endmember fraction represents a fundamental ambiguity in the radiance signal because it can correspond to either absorptive (e.g. low albedo asphalt), transmissive (e.g. deep clear water) or shadowed (e.g. tree canopy shadow) surfaces. However, in areas where dark surface composition can be inferred from spatial context, the different responses of these surfaces may still allow them to be distinguished in the thermal fraction space. 相似文献
998.
以壬基酚和三氯化磷为原料,芳烃为溶剂,采用直接酯化法,可制备三壬基苯基亚磷酸酯。结果表明,借助正交设计方法,确定了最佳工艺条件:壬基酚/三氯化磷(摩尔比)3.05/1.00,溶剂质量分数为壬基酚的40%~50%,三氯化磷的滴加时间1 h,滴加温度20℃,反应温度140℃,反应时间2 h,真空度(绝压)0.01 MPa,抽真空时间4 h;在此条件下制备的试样质量达到美国GE公司的质量指标,即酸值(KOH)小于0.10 mg/g,含磷质量分数大于4.20%,折光指数为1.525 5~1.528 0,黏度(60℃)大于250 mPa.s。 相似文献
999.
1000.
Giorgio Vittadini Marco Fattore Pietro G. Lovaglio 《Computational statistics & data analysis》2007,51(12):5828-5846
A new approach for the estimation and the validation of a structural equation model with a formative-reflective scheme is presented. The basis of the paper is a proposal for overcoming a potential deficiency of PLS path modeling. In the PLS approach the reflective scheme assumed for the endogenous latent variables (LVs) is inverted; moreover, the model errors are not explicitly taken into account for the estimation of the endogenous LVs. The proposed approach utilizes all the relevant information in the formative manifest variables (MVs) providing solutions which respect the causal structure of the model. The estimation procedure is based on the optimization of the redundancy criterion. The new approach, entitled redundancy analysis approach to path modeling (RA-PM) is compared with both traditional PLS Path Modeling and LISREL methodology, on the basis of real and simulated data. 相似文献