全文获取类型
收费全文 | 906903篇 |
免费 | 16970篇 |
国内免费 | 5110篇 |
专业分类
电工技术 | 18971篇 |
技术理论 | 10篇 |
综合类 | 4845篇 |
化学工业 | 135707篇 |
金属工艺 | 38476篇 |
机械仪表 | 31478篇 |
建筑科学 | 25338篇 |
矿业工程 | 6320篇 |
能源动力 | 24537篇 |
轻工业 | 69512篇 |
水利工程 | 9921篇 |
石油天然气 | 19253篇 |
武器工业 | 582篇 |
无线电 | 104398篇 |
一般工业技术 | 175751篇 |
冶金工业 | 162279篇 |
原子能技术 | 17487篇 |
自动化技术 | 84118篇 |
出版年
2022年 | 5862篇 |
2021年 | 9743篇 |
2020年 | 7410篇 |
2019年 | 8634篇 |
2018年 | 18067篇 |
2017年 | 18475篇 |
2016年 | 16894篇 |
2015年 | 11206篇 |
2014年 | 17016篇 |
2013年 | 41720篇 |
2012年 | 27429篇 |
2011年 | 38907篇 |
2010年 | 32003篇 |
2009年 | 34228篇 |
2008年 | 34550篇 |
2007年 | 35432篇 |
2006年 | 27534篇 |
2005年 | 26743篇 |
2004年 | 23897篇 |
2003年 | 23316篇 |
2002年 | 21574篇 |
2001年 | 20987篇 |
2000年 | 19777篇 |
1999年 | 21009篇 |
1998年 | 50655篇 |
1997年 | 35694篇 |
1996年 | 27667篇 |
1995年 | 20720篇 |
1994年 | 18149篇 |
1993年 | 17659篇 |
1992年 | 12754篇 |
1991年 | 11831篇 |
1990年 | 11626篇 |
1989年 | 11110篇 |
1988年 | 10285篇 |
1987年 | 9162篇 |
1986年 | 8923篇 |
1985年 | 9893篇 |
1984年 | 9118篇 |
1983年 | 8163篇 |
1982年 | 7610篇 |
1981年 | 7709篇 |
1980年 | 7241篇 |
1979年 | 7078篇 |
1978年 | 6878篇 |
1977年 | 8154篇 |
1976年 | 10692篇 |
1975年 | 5935篇 |
1974年 | 5553篇 |
1973年 | 5718篇 |
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
981.
Single-polarization single-mode photonic crystal fibers 总被引:5,自引:0,他引:5
K. Saitoh M. Koshiba 《Photonics Technology Letters, IEEE》2003,15(10):1384-1386
A new structure of single-polarization single-mode (SPSM) photonic crystal fiber (PCF) is proposed and analyzed by using a full-vector finite element method with anisotropic perfectly matched layers. From the numerical results it is confirmed that the proposed fiber is low-loss SPSM-PCF within the wavelengths ranging from 1.48 to 1.6 /spl mu/m, where only the slow-axis mode exists and the confinement loss is less than 0.1 dB/km. 相似文献
982.
A mismatch between demand and supply for bandwidth is common in transport carrier networks. This mismatch is generally a result of the disparity between a carrier's capacity buildout and its anticipated customer demand. A carrier with temporary bandwidth deficit or lack of presence in a geographical region and a carrier with surplus capacity in the right locations can be brought together by the emerging bandwidth exchange technology. Bandwidth exchange offers a win-win solution, in which the carrier with a deficit avoids losing revenue by buying capacity from the carrier with surplus, and the latter makes additional revenue by retail sale of its excess capacity. While the concept of real-time purchase and exchange of bandwidth has attracted a lot of attention, many technical challenges stand in the way of making it a reality. The purpose of this article is to provide an engineering framework for enabling real-time bandwidth exchange with committed quality of service and service level agreement among transport carriers. Special emphasis is given to identifying the architectural requirements and the enabling infrastructure necessary for building a viable bandwidth exchange that can be used for creating revenue out of surplus stranded capacity. Indepth analysis of cross-carrier service level agreement specification, capacity publication, route design, and service provisioning are also provided in the article. 相似文献
983.
984.
The scale along the coordinate axes in the equations of motion hitherto used for an H-maser (in relative units) is shown to depend on Q as the quality factor for an H-maser. This hinders both analysis of the operation of the H-maser and optimization of its parameters. The modified equations of motion proposed here do not have that shortcoming. We determine the atomic flux into the bulb that gives the H-maser the best metrological characteristics. 相似文献
985.
Examination of the wreckage of a light aircraft revealed that approximately 20 cm was missing from one tip of the aluminum
alloy propeller. Fractographic and metallographic examination of the remaining portion of the propeller revealed extensive
grain-boundary separation in the vicinity of the fracture, and grain edges and corners rounded by corrosion on the fracture
surface. Energy dispersive X-ray analysis (EDXA) revealed fluorine on, and in the vicinity of, the fracture surface. In the
ensuing litigation, it was asserted that the crash occurred because the propeller fractured in flight as the result of intergranular
attack caused by the use of a fluorine-bearing cleaner. 相似文献
986.
M. Kawasaki T. Tsukamoto Y. Kimura T. Iwasaki H. Yamane 《Journal of Materials Science》2003,38(4):739-745
The damage imposed on SiO
x
deposited nylon 6 films as a result of abrasion with a cotton cloth and Gelboflex testing was examined by evaluating the rate at which copper plates, which were enveloped by the damaged films, were corroded by H2S. Abrasion with a cotton cloth caused some micro-cracking of the SiO
x
layer and the permeation rate of H2S approached that of the uncoated nylon 6 film. Damage to the SiO
x
layer by twisting and crushing progressed gradually with the number of Gelboflex test cycles and correspondingly the corrosion rate of the copper plates increased. Comparison of the corrosion rates of the copper plates kept in the pouches made of various commercial films with those obtained for the damaged SiO
x
deposited nylon 6 films showed a clear relationship between the H2 permeation rate of the films and the corrosion rate of the copper plates by H2S. 相似文献
987.
O. M. Jadaan N. N. Nemeth J. Bagdahn W. N. Sharpe Jr 《Journal of Materials Science》2003,38(20):4087-4113
The objective of this work is to present a brief overview of a probabilistic design methodology for brittle structures, review the literature for evidence of probabilistic behavior in the mechanical properties of MEMS (especially strength), and to investigate whether evidence exists that a probabilistic Weibull effect exists at the structural microscale. Since many MEMS devices are fabricated from brittle materials, that raises the question whether these miniature structures behave similar to bulk ceramics. For bulk ceramics, the term Weibull effect is used to indicate that significant scatter in fracture strength exists, hence requiring probabilistic rather than deterministic treatment. In addition, the material's strength behavior can be described in terms of the Weakest Link Theory (WLT) leading to strength dependence on the component's size (average strength decreases as size increases), and geometry/loading configuration (stress distribution). Test methods used to assess the mechanical properties of MEMS, especially strength, are reviewed. Four materials commonly used to fabricate MEMS devices are reviewed in this report. These materials are polysilicon, single crystal silicon (SCS), silicon nitride, and silicon carbide. 相似文献
988.
A three-dimensional FE analysis of large deformations for impact loadings using tetrahedral elements
A three-dimensional dynamic program for the anaysis of large deformations in contact-penetration problems is developed using
the finite element Lagrangian method with explicit time integration. By incorporating a tetrahedral element, which allows
a single-point integration without a special hourglass control scheme, this program can be more effective to the present problem.
The position code algorithm is used to search contact surface. Eroding surfaces are also considered. The defense node algorithm
was slightly modified for the calculation of contact forces. A study of obliquity effects on metallic plate perforation and
ricochet processes in thin plates impacted by a sphere was conducted. It is well simulated that on separation of two parts
of the sphere, the portion still within the crater tends to perforate, while the portion in contact with the plate surface
ricochets. This deformation pattern is observed in experiments, especially at high obliquities. A long rod that impacts an
oblique steel plate at high impact velocity was also simulated in order to study the dynamics of the rod caused by the three
dimensional asymmetric contact. The agreement between simulated and experimental results is quite good. Fracture phenomena
occuring at high obliquity deserves further investigations.
Received: 20 February 2002 / Accepted: 20 September 2002 相似文献
989.
Accidents in different complex sociotechnical systems are rarely compared using the same theoretical framework for risk management. We conducted a comparative analysis of two Canadian public health disasters involving drinking water distribution systems, the North Battleford Cryptosporidium parvum outbreak in April 2001 and the Walkerton E. coli outbreak in May 2000. Both accidents resulted from a complex interaction between all levels of a complex sociotechnical system. However, the low-level physical and individual factors differed in the two cases, whereas, the high-level governmental and regulatory factors tended to be the same. These findings may have implications for the design of public policies to minimize risk in complex sociotechnical systems. 相似文献
990.
1 引言石磨水洗牛仔布的成本是非常高的 ,但由于客户喜欢陈旧、磨损的外观 ,所以这种高成本又是十分必要的。那么 ,如何将这么高的成本降下来呢 ?通常的做法是把工厂转移到劳动力低廉的发展中国家。如果有着悠久历史的纺织品公司因为劳动力成本较高而不再具有竞争力 ,他们将不得不寻找另外一种可替代石磨水洗而同样能获得满意外观的方法。这是一种似乎对于我们具有实际意义的想法 ,它似乎具有逻辑性又很合理 ,但是这可能吗 ?当然 ,这种方法必须工艺简单 ,保证附加成本较低 ;必须可连续生产 ,以保证产量较高 ;必须可在常规设备上采用现有工… 相似文献