全文获取类型
收费全文 | 37756篇 |
免费 | 4181篇 |
国内免费 | 2758篇 |
学科分类
工业技术 | 44695篇 |
出版年
2024年 | 119篇 |
2023年 | 641篇 |
2022年 | 843篇 |
2021年 | 1107篇 |
2020年 | 1269篇 |
2019年 | 1144篇 |
2018年 | 1156篇 |
2017年 | 1300篇 |
2016年 | 1412篇 |
2015年 | 1454篇 |
2014年 | 2226篇 |
2013年 | 2625篇 |
2012年 | 2446篇 |
2011年 | 2756篇 |
2010年 | 2208篇 |
2009年 | 2325篇 |
2008年 | 2394篇 |
2007年 | 2501篇 |
2006年 | 2314篇 |
2005年 | 1983篇 |
2004年 | 1590篇 |
2003年 | 1310篇 |
2002年 | 1131篇 |
2001年 | 903篇 |
2000年 | 778篇 |
1999年 | 680篇 |
1998年 | 536篇 |
1997年 | 478篇 |
1996年 | 407篇 |
1995年 | 386篇 |
1994年 | 316篇 |
1993年 | 278篇 |
1992年 | 252篇 |
1991年 | 179篇 |
1990年 | 147篇 |
1989年 | 166篇 |
1988年 | 127篇 |
1987年 | 68篇 |
1986年 | 90篇 |
1985年 | 67篇 |
1984年 | 67篇 |
1983年 | 48篇 |
1982年 | 54篇 |
1981年 | 42篇 |
1980年 | 33篇 |
1979年 | 31篇 |
1978年 | 29篇 |
1977年 | 26篇 |
1964年 | 28篇 |
1955年 | 35篇 |
排序方式: 共有10000条查询结果,搜索用时 203 毫秒
121.
122.
E. GLASER 《Journal of microscopy》2005,218(1):1-5
It is well known that the estimation of an object's volume by means of serial cross-sections, the so-called Cavalieri method, yields an unbiased estimate. But by itself it provides no means by which to estimate how precise this estimate is unless the shape of the volume is fully known beforehand. This knowledge can only be partially determined from the serial section information that is collected. Methods have been developed that claim to surmount this difficulty by using the serial section data to create a mathematical model of the volume's shape properties. The model then is used to estimate (predict) the precision of the volume estimate (its CE) from the single set of data available. Unfortunately, the theory underlying the model is flawed and so the model itself amounts to no more than an unsubstantiated guess about the shape of the volume. Therefore, the precision of the volume estimates that one obtains from the method is only as good as the model and this cannot be ascertained from the single set of acquired data. In this letter I explain the inadequacies of the modelling method. I suggest that it be used only with caution, if at all. Instead I suggest two alternative ways to predict the CE, one that is based upon a rule-of-thumb approach to the object's shape, and another that is based upon spectral analysis of the measurement function and that is easy to implement with available computer software. 相似文献
123.
针对水环境系统的随机不确定性和模糊不确定性,分析了影响水环境中水质风险的不完善性和模糊性因素,将水环境中水质风险作为一个模糊事件,利用模糊事件概率理论,提出了水环境水质超标的模糊随机风险率计算模型。通过引入模糊事件信息熵来评价隶属函数特征值取值的可靠程度,定量描述了隶属函数特征值与风险率的关系,进而计算出风险率。实例计算结果证明了方法的有效性。 相似文献
124.
125.
126.
Virtual teams often face tight schedules and a need to start quickly and perform instantly. The goal of our study was to enhance understanding of the challenges faced by such teams. We used time–interaction–performance theory as the framework for following the processes and functions within virtual teams working on a systems development task. Our study provided a detailed examination of the group process, applied to virtual teams working under time pressure. The challenges faced by virtual teams in such settings showed that teams must work to enhance their effectiveness in multiple dimensions. 相似文献
127.
The mirror effect for word frequency refers to the finding that low-frequency words have higher hit rates and lower false alarm rates than high-frequency words. This result is typically interpreted in terms of conventional signal detection theory (SDT), in which case it indicates that the order of the underlying old item distributions mirrors the order of the new item distributions. However, when viewed in terms of a mixture version of SDT, the order of hits and false alarms does not necessarily imply the same order in the underlying distributions because of possible effects of mixing. A reversal in underlying distributions did not appear for fits of mixture SDT models to data from 4 experiments. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
128.
Gregory S. Ho Chen Huang Emily A. Carter 《Current Opinion in Solid State & Materials Science》2007,11(5-6):57-61
Orbital-free density functional theory (OF-DFT) can be made to scale linearly with sample size, allowing thousands of atoms to be treated explicitly with quantum mechanics. State-of-the-art kinetic energy density functionals and ion–electron pseudopotentials are used to obtain accurate structural property predictions for nanoparticles, nanowires, extended surfaces, and nanoindentation of simple metals. 相似文献
129.
Mobile robots can be used in many applications, such as exploration, search and rescue, reconnaissance, security, and cleaning.
Mobile robots usually carry batteries as their energy source and their operational time is restricted by the finite energy
available from the batteries. Therefore, energy constraints are critical to the service time of mobile robots. This paper
investigates the minimum-energy control problem for translational trajectory generation, which minimizes the energy drawn
from the batteries. Optimal control theory is used to find the optimal velocity trajectory in analytic form. To demonstrate
energy efficiency obtainable, we performed simulations of minimum-energy velocity control and compared the results with loss-minimization
control and energy-optimal trapezoidal velocity profiles. Simulation results showed that significant energy savings can be
achieved, of up to 9% compared with loss-minimization control and up to 10% compared with energy-optimal trapezoidal velocity
profile. We also performed an actual robot experiment using Pioneer 3-AT platform to show the validity of the proposed minimum-energy
velocity control. The experimental results revealed that the proposed minimum-energy velocity control can save the battery
energy up to 10% compared with loss-minimization control.
Categories (3): Robot control, (5): RobotMotion Planning 相似文献
130.
阐述了时空效应规律在软土深基坑工程中的运用情况,实践证明,运用时空效应规律,能可靠而合理地利用土体自身在基坑开挖过程中控制土体位移的潜力而达到保护环境的目的。 相似文献