全文获取类型
收费全文 | 15576篇 |
免费 | 2253篇 |
国内免费 | 1741篇 |
学科分类
工业技术 | 19570篇 |
出版年
2024年 | 40篇 |
2023年 | 288篇 |
2022年 | 384篇 |
2021年 | 516篇 |
2020年 | 570篇 |
2019年 | 508篇 |
2018年 | 486篇 |
2017年 | 569篇 |
2016年 | 680篇 |
2015年 | 720篇 |
2014年 | 1037篇 |
2013年 | 1208篇 |
2012年 | 1323篇 |
2011年 | 1210篇 |
2010年 | 954篇 |
2009年 | 943篇 |
2008年 | 972篇 |
2007年 | 1141篇 |
2006年 | 976篇 |
2005年 | 833篇 |
2004年 | 710篇 |
2003年 | 586篇 |
2002年 | 437篇 |
2001年 | 420篇 |
2000年 | 376篇 |
1999年 | 315篇 |
1998年 | 226篇 |
1997年 | 215篇 |
1996年 | 151篇 |
1995年 | 123篇 |
1994年 | 160篇 |
1993年 | 90篇 |
1992年 | 67篇 |
1991年 | 61篇 |
1990年 | 45篇 |
1989年 | 41篇 |
1988年 | 27篇 |
1987年 | 15篇 |
1986年 | 19篇 |
1985年 | 25篇 |
1984年 | 23篇 |
1983年 | 17篇 |
1982年 | 11篇 |
1981年 | 7篇 |
1980年 | 8篇 |
1964年 | 2篇 |
1962年 | 2篇 |
1959年 | 20篇 |
1956年 | 2篇 |
1955年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 374 毫秒
81.
Quality assessment of SRTM C- and X-band interferometric data: Implications for the retrieval of vegetation canopy height 总被引:1,自引:0,他引:1
The Shuttle Radar Topography Mission distinguished itself as the first near-global spaceborne mission to demonstrate direct sensitivity to vertical vegetation structure. Whether this sensitivity is viewed as exploitable signal or unwanted bias, a great deal of interest exists in retrieving vegetation canopy height information from the SRTM data. This study presents a comprehensive application-specific assessment of SRTM data quality, focusing on the characterization and mitigation of two primary sources of relative vertical error: uncompensated Shuttle mast motion and random phase noise. The assessment spans four test sites located in the upper Midwestern United States and examines the dependence of data quality on both frequency, i.e., C-band vs. X-band, and the number of acquired datatakes. The results indicate that the quality of SRTM data may be higher than previously thought. Novel mitigation strategies include a knowledge-based approach to sample averaging, which has the potential to reduce phase noise error by 43 to 80%. The strategies presented here are being implemented as part of an ongoing effort to produce regional- to continental-scale estimates of vegetation canopy height within the conterminous U.S. 相似文献
82.
根据桃园矿单翼相邻工作面倒装的实践,从加快综采设备的搬家速度、提高劳动效率方面,简述桃园矿综采快速倒装的几点主要做法。 相似文献
83.
We have calculated thermal conductivity of alumina nanofluids (with water and ethylene glycol as base fluids) using temperature
as well as concentration-dependent viscosity, η. The temperature profile of η is obtained using Gaussian fit to the available experimental data. In the model, the interfacial resistance effects are incorporated
through a phenomenological parameter α. The micro-convection of the alumina nanoparticle (diameter less than 100 nm) is included through Reynolds and Prandtl numbers.
The model is further improved by explicitly incorporating the thermal conductivity of the nanolayer surrounding the nanoparticles.
Using this improved model, thermal conductivity of copper nanofluid is calculated. These calculations capture the particle
concentration-dependent thermal conductivity and predict the dependence of the thermal conductivity on the size of the nanoparticle.
These studies are significant to understand the underlying processes of heat transport in nanofluids and are crucial to design
superior coolants of next generation. 相似文献
84.
Twenty-five years of natural coordinates 总被引:3,自引:0,他引:3
Javier García de Jalón 《Multibody System Dynamics》2007,18(1):15-33
In the early eighties, the author and co-workers created and further developed the natural coordinates to describe the motion
of 2-D and 3-D multibody systems. Natural coordinates do not need angles or angular parameters to define orientation, leading
to constant inertia matrices and to the simplest form of the constraint equations. Natural coordinates are composed by the
Cartesian coordinates of some points and the Cartesian components of some unit vectors distributed on the different bodies
of the system. The points and vectors can be located in the joints, being shared by contiguous bodies, decreasing or even
eliminating the need to set joint constraints and reducing the total number of variables. However, other authors prefer not
to share variables in order to get even simpler equations and to keep a bigger decoupling of equations, which is preferable
in some cases.
In this paper the history of natural coordinates is reviewed, as well as the main contributions coming from other research
groups. In the second part of the paper some application areas in which natural coordinates can be particularly advantageous
are examined.
Commemorative Contribution. 相似文献
85.
基于CH365型接口和MCX314As型运动控制器的PCI总线运动控制卡设计 总被引:2,自引:0,他引:2
以CH365型PCI总线接口和MCX314As型运动控制器为核心硬件,自主研发了基于运动控制器的PCI总线运动控制卡,该控制卡能够实现4轴位置、速度和S曲线的加减速控制,具有直线、圆弧、位模式插补功能及自动原位搜寻功能,同时具有4路信号输入和8路通用输出. 相似文献
86.
本文是2004年陈典发工作([1])的继续,我们研究随机限制市场中欧式不定权益的对冲问题,获得了它们的上对冲成本以及对冲交易策略的风险溢价表达式。 相似文献
87.
Kirk C. Valanis 《Canadian Metallurgical Quarterly》2003,129(9):1039-1047
In this paper we derive Newton’s 2nd Law of Motion from the constitutive equation of elasticity of a space-time continuum in four dimensions. This we do by introducing a four-dimensional material continuum with a Minkowskian metric, in analogy with Einstein’s general theory of relativity. The four-continuum is deformable in both space and time. The physics of the deformation is embedded in a variational principle, which is a form-invariant extension of its classical mechanical counterpart in three dimensions, but with the acceleration term absent. General dynamic equations of elasticity in four dimensions are thereby derived. When the constraint of temporal inextensibility (universal time) is introduced, these equations yield readily the dynamic equations of elasticity in three dimensions. The presence of the inertia term in these equations, is a direct consequence of the temporal curvature induced by the deformation of the four-continuum. Newton’s law of motion for rigid bodies follows when the additional constraint of spatial inextensibility is introduced. 相似文献
88.
89.
The effect of FEF carbon black as filler on the thermal capacity c, diffusivity a, and thermal conductivity λ, of styrene butadiene rubber (SBR) composites in the temperature range 300–420 K was studied. The filler strongly increases the thermal diffusivity, whilst strongly decreasing the thermal capacity and the thermal conductivity (except at high FEF content ≥80 phr). The influence of the filler on the thermoelastic behaviour of the same composites was also investigated. It was found that the thermoelastic temperature change (ΔT) increased with carbon black concentration as well as the entropy change per unit extension. 相似文献
90.
本文主要完成了液态金属钠高温沸腾实验回路的应力分析与计算,确定了回路高温区连接管系的布置型式和支吊架系统,是该回路总体设计的重要组成部分。 相似文献