全文获取类型
收费全文 | 186171篇 |
免费 | 18267篇 |
国内免费 | 9906篇 |
专业分类
电工技术 | 15182篇 |
技术理论 | 13篇 |
综合类 | 19373篇 |
化学工业 | 17938篇 |
金属工艺 | 7235篇 |
机械仪表 | 15963篇 |
建筑科学 | 22647篇 |
矿业工程 | 7277篇 |
能源动力 | 8156篇 |
轻工业 | 14759篇 |
水利工程 | 8423篇 |
石油天然气 | 8687篇 |
武器工业 | 2067篇 |
无线电 | 10410篇 |
一般工业技术 | 17899篇 |
冶金工业 | 8023篇 |
原子能技术 | 2638篇 |
自动化技术 | 27654篇 |
出版年
2024年 | 813篇 |
2023年 | 2355篇 |
2022年 | 4817篇 |
2021年 | 5649篇 |
2020年 | 5967篇 |
2019年 | 4920篇 |
2018年 | 4896篇 |
2017年 | 5812篇 |
2016年 | 7125篇 |
2015年 | 7382篇 |
2014年 | 12179篇 |
2013年 | 12244篇 |
2012年 | 13954篇 |
2011年 | 15268篇 |
2010年 | 11003篇 |
2009年 | 11230篇 |
2008年 | 10511篇 |
2007年 | 12538篇 |
2006年 | 10952篇 |
2005年 | 9069篇 |
2004年 | 7660篇 |
2003年 | 6518篇 |
2002年 | 5220篇 |
2001年 | 4261篇 |
2000年 | 3616篇 |
1999年 | 3008篇 |
1998年 | 2580篇 |
1997年 | 2163篇 |
1996年 | 1780篇 |
1995年 | 1458篇 |
1994年 | 1310篇 |
1993年 | 977篇 |
1992年 | 908篇 |
1991年 | 664篇 |
1990年 | 563篇 |
1989年 | 480篇 |
1988年 | 402篇 |
1987年 | 271篇 |
1986年 | 233篇 |
1985年 | 219篇 |
1984年 | 256篇 |
1983年 | 244篇 |
1982年 | 199篇 |
1981年 | 99篇 |
1980年 | 95篇 |
1979年 | 69篇 |
1978年 | 54篇 |
1977年 | 46篇 |
1976年 | 43篇 |
1959年 | 31篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
The three-dimensional packing problem can be stated as follows. Given a list of boxes, each with a given length, width, and
height, the problem is to pack these boxes into a rectangular box of fixed-size bottom and unbounded height, so that the height
of this packing is minimized. The boxes have to be packed orthogonally and oriented in all three dimensions. We present an
approximation algorithm for this problem and show that its asymptotic performance bound is between 2.5 and 2.67. This result
answers a question raised by Li and Cheng [5] about the existence of an algorithm for this problem with an asymptotic performance
bound less than 2.89.
This research was partially supported by FAPESP (proc. 93/0603-1) and by CNPq/ProTeM-CC, project ProComb (proc. 680065/94-6). 相似文献
962.
无源软开关电路拓扑的研究 总被引:1,自引:0,他引:1
提出一个研究无源软开关电路的新技术方法;其特征是对基本的单管隔离型PWM DC/DC变换器进行类比分析,由此直接导出可行的缓冲能量再生复位电路.该新方法能使DC/DC PWM硬开关变换器转变为软开关变换器.以新型Boost无源软开关变换器为例,进行了电路理论分析与中功率样机的实验测试;结果表明此新变换器具有较宽的软开关工作范围、较低的电应力、较小的缓冲元件量值等特点.由此认为,这种研究方法推导简单、物理意义清晰,还能深入地研究无源软开关的新电路拓扑. 相似文献
963.
在对比分析了传统辅助水刹车与气控盘式辅助刹车的特性后,重点对气控盘式辅助刹车进行了数学分析,得到了该辅助刹车的制动扭矩取决于施加的气体压力,与钻具质量无关,冷却液的流量应同钻柱质量保持正比关系,可精确控制下钻速度并易于实现自动送钻功能。得出了气控盘式辅助刹车更适用于车载钻机的结论。 相似文献
964.
Xu Qianjun Li Xu Chen Zuyu 《Frontiers of Architecture and Civil Engineering in China》2007,1(2):217-221
It is usually difficult to determine the actual safety factors of rock masses in an ordinary two-dimensional stability analysis
if the safety factors of the different cross sections in the rock mass vary significantly. In addition to the actual slope,
arch dam abutment, and the actual foundation of a high building, another example is that the different cross sections of the
foundation in the monolith of a gravity dam vary significantly, just like the condition at the overflow dam in the Baise project.
A three-dimensional stability analysis method based on the upper-bound theorem was employed to solve this problem. The parameters
used in the analysis were obtained from geomechanics tests, as well as continuity simulations of the randomly distributed
joints. Two failure patterns against sliding are analyzed. One pattern is the foundation slide along deep-seated planes which
were determined by calculations. The other pattern is the foundation slide along the planes across the bottom of the high
steps in the foundation pit. The results indicate that a special overflow dam monolith can be considered to be safe in case
of considering the three dimensional effect. However, a key wall with a depth of 5m must be constructed at the upper side
of this monolith in order to ensure the safety of the foundation.
Translated from Chinese Journal of Rock Mechanics and Engineering, 2006, 25(3): 533–538 [译自: 岩土力学与工程学报] 相似文献
965.
赵波 《水资源与水工程学报》2006,17(5):58-60
塌方是隧洞施工过程中经常遇到的难题,本文针对引红济石工程1#支洞在施工过程中出现的两次塌方原因进行分析,总结出隧洞施工过程中避免塌方的主要方法和策略,具有普遍的借鉴意义。 相似文献
966.
S. Kumar Author Vitae Author Vitae S. Ranganath Author Vitae Author Vitae F.T. Chew Author Vitae 《Pattern recognition》2006,39(6):1088-1098
This paper presents a robust rule-based approach for the splitting of binary clumps that are formed by objects of diverse shapes and sizes. First, the deepest boundary pixels, i.e., the concavity pixels in a clump, are detected using a fast and accurate scheme. Next, concavity-based rules are applied to generate the candidate split lines that join pairs of concavity pixels. A figure of merit is used to determine the best split line from the set of candidate lines. Experimental results show that the proposed approach is robust and accurate. 相似文献
967.
Stability of large-scale coal-fired MHD channels is studied by (1) linearized stability analysis, and (2) time-dependent 1-D analysis. The channel length is 15 m with 600 electrode pairs, and the output power ranges from 220.6 MW through 258.7 MW. Linearized stability analyses show that the Faraday channels operated with fixed loading resistance are stable, whereas the two waves of u and u–a (u, a: gas and sound velocity) become unstable in the Faraday channel with fixed loading factor. Two waves of u and u–a are unstable in the diagonal channel with fixed loading current and the u + a wave becomes unstable in the diagonal channel with fixed electrode current. Time-dependent one-dimensional analyses indicate that the Faraday channels with fixed load resistance are smooth without growth of fluctuation. The diagonal channels with fixed electrode current are smooth with no fluctuation, though the linear theory indicates that the u + a wave is unstable. The diagonal channel with fixed load current suffers large disturbance along the latter half of the channel, being consistent with the linearized analysis which indicates that the u - a and u waves are unstable. 相似文献
968.
T. LUNDGREN E. U. ENGSTR
M R. LEVI-SETTI A. LINDE J. G. NORN 《Journal of microscopy》1994,173(2):149-154
The incorporation into rat incisor dentin of two calcium isotopes, the stable 44Ca and the radioactive 45Ca, was studied using secondary ion mass spectrometry (SIMS) stepscanning and imaging, and autoradiography, respectively. The results demonstrated a time-dependent incorporation of the calcium isotopes into the mineral phase of dentin. With the SIMS step-scanning, detecting 44Ca, the ion yield was high in the odontoblasts 2 min after intravenous injection. After 10 min a marked increase in signal intensity was found at the dentin mineralization front. This result was consistent with those obtained by 45Ca autoradiography; a peak of incorporation occurred 10 min after injection of the isotope. Likewise, localization of 44Ca to the mineralization front could be demonstrated 10 min after injection by SIMS imaging. In images obtained at earlier intervals, no such increase in ion yield could be detected. The results show that the nonradioactive, stable isotope 44Ca can be used as a marker for biomineralization in a similar way to radioactive 45Ca. 相似文献
969.
万安水利枢纽底孔坝段中隔墙流激振动响应分析 总被引:3,自引:0,他引:3
研究了万安水利枢纽底孔坝段中隔墙在水流脉动壁压作用下的结构随机振动问题,进行了一种泄流工况下的隔墙结构流激振动响应统计量的理论计算,模拟计算了隔墙-水流-基础耦合系统固有频率对流激振动响应的影响,并对水流脉动壁压随机荷载作用下的结构动力可靠度进行了理论分析。 相似文献
970.
Exact solution of elasto-plastic stresses in a metal-matrix composite beam of arbitrary orientation subjected to transverse loads 总被引:1,自引:0,他引:1
An analytical elasto-plastic stress analysis is presented for a metal-matrix composite beam of arbitrary orientation subjected to a single transverse force applied to the free end of the beam and a uniformly distributed load. The material is assumed to be perfectly plastic in the elasto-plastic solution. A composite consisting of stainless-steel-reinforced aluminum was produced for this work. Sample problems are given for various orientation angles. Elastic, elastoplastic and residual normal and shear stresses are calculated. The location of the elasto-plastic boundary of the beam is obtained according to the x coordinates of the beam. 相似文献