全文获取类型
收费全文 | 537071篇 |
免费 | 27171篇 |
国内免费 | 13160篇 |
专业分类
电工技术 | 21638篇 |
技术理论 | 58篇 |
综合类 | 20105篇 |
化学工业 | 87950篇 |
金属工艺 | 28790篇 |
机械仪表 | 28366篇 |
建筑科学 | 29748篇 |
矿业工程 | 13125篇 |
能源动力 | 14395篇 |
轻工业 | 31915篇 |
水利工程 | 8161篇 |
石油天然气 | 30778篇 |
武器工业 | 2722篇 |
无线电 | 57813篇 |
一般工业技术 | 84250篇 |
冶金工业 | 52261篇 |
原子能技术 | 9825篇 |
自动化技术 | 55502篇 |
出版年
2023年 | 5336篇 |
2022年 | 9476篇 |
2021年 | 14494篇 |
2020年 | 11190篇 |
2019年 | 9811篇 |
2018年 | 12742篇 |
2017年 | 13921篇 |
2016年 | 13197篇 |
2015年 | 14549篇 |
2014年 | 19694篇 |
2013年 | 28020篇 |
2012年 | 26231篇 |
2011年 | 29883篇 |
2010年 | 25287篇 |
2009年 | 24953篇 |
2008年 | 24784篇 |
2007年 | 24104篇 |
2006年 | 24115篇 |
2005年 | 21478篇 |
2004年 | 14944篇 |
2003年 | 13642篇 |
2002年 | 12487篇 |
2001年 | 11955篇 |
2000年 | 12299篇 |
1999年 | 13832篇 |
1998年 | 18194篇 |
1997年 | 13829篇 |
1996年 | 11751篇 |
1995年 | 9394篇 |
1994年 | 7888篇 |
1993年 | 6875篇 |
1992年 | 5435篇 |
1991年 | 4912篇 |
1990年 | 4357篇 |
1989年 | 4043篇 |
1988年 | 3847篇 |
1987年 | 3140篇 |
1986年 | 3075篇 |
1985年 | 3169篇 |
1984年 | 2902篇 |
1983年 | 2739篇 |
1982年 | 2489篇 |
1981年 | 2393篇 |
1980年 | 2476篇 |
1979年 | 2423篇 |
1978年 | 2414篇 |
1977年 | 2524篇 |
1976年 | 2930篇 |
1975年 | 2207篇 |
1973年 | 2177篇 |
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
991.
Oracle9i数据库安全管理机制剖析 总被引:4,自引:0,他引:4
数据库安全和数据安全机制是数据库开发设计者目前必须考虑的问题之一。Oracle9i数据库管理系统在数据库安全方面为数据库管理员和应用开发者提供了很好的安全机制。文章从不同侧面和角度讨论Oracle9i数据库系统的安全管理机制,为实施更安全更灵活的数据库安全策略提出了全方位的技术思路。 相似文献
992.
钢坯在炉内的横向跑偏和纵向跑偏位移量,是步进式加热炉设备的重要性能之一.重点介绍了鞍钢1780热连轧生产线步进式加热炉钢坯跑偏测试的应用技术. 相似文献
993.
J. C. Williams R. G. Baggerly N. E. Paton 《Metallurgical and Materials Transactions A》2002,33(3):837-850
Single crystals of Ti-Al alloys containing 1.4, 2.9, 5, and 6.6 pct Al (by weight) were oriented for 〈a〉 slip on either basal or prism planes or loaded parallel along the c-axis to enforce a nonbasal deformation mode. Most of the tests were conducted in compression and at temperatures between
77 and 1000 K. Trace analysis of prepolished surfaces enabled identification of the twin or slip systems primarily responsible
for deformation. Increasing the deformation temperature, Al content, or both, acted to inhibit secondary twin and slip systems,
thereby increasing the tendency toward strain accommodation by a single slip system having the highest resolved stress. In
the crystals oriented for basal slip, transitions from twinning to multiple slip and, finally, to basal slip occurred with
increasing temperature in the lower-Al-content alloys, whereas for Ti-6.6 pct Al, only basal slip was observed at all temperatures
tested. A comparison of the critically resolved shear stress (CRSS) values for basal and prism slip as a function of Al content
shows that prism slip is favored at room temperature in pure Ti, but the stress to activate these two systems becomes essentially
equal in the Ti-6.6 pct Al crystals over a wide range of temperatures.
Compression tests on crystals oriented so that the load was applied parallel to the c-axis showed extensive twinning in lower Al concentrations and 〈c+a〉 slip at higher Al concentrations, with a mixture of 〈c+a〉 slip and twinning at intermediate compositions. A few tests also were conducted in tension, with the load applied parallel
to the c-axis. In these cases, twinning was observed, and the resolved shear for plastic deformation by twinning was much lower that
that for 〈c+a〉 slip observed in compression loading.
This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals
and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following
ASM committees: Materials Science and Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic
Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee. 相似文献
994.
995.
996.
An experimental arrangement has been developed for measuring the transient temperature responses and the thermal diffusivities of foil materials in the range of 10 to 300K by using the optical reflectivity technique. The cryogenic system with optical windows is designed to provide temperatures from 10 to 300 K. The front surface of a foil specimen is heated by a pulsed Nd:YAG laser. In situ measurement of the reflectivity of a continuous-wave He–Ne laser at the rear surface is conducted on the microsecond time scale. Using the temperature dependence of reflectivity, the transient temperature response is deduced. The thermal diffusivity is obtained by fitting Parker's formulae to the experimental data on temperature rise. Stainless-steel foils are chosen as samples and are studied in the region from 10 to 300 K. The accuracy is examined by comparing the present results with the theoretical temperature responses and thermal diffusivity data from the literature. Good agreement is observed. 相似文献
997.
We consider a space-time coded (STC) orthogonal frequency-division multiplexing (OFDM) system with multiple transmitter and receiver antennas over correlated frequency- and time-selective fading channels. It is shown that the product of the time-selectivity order and the frequency-selectivity order is a key parameter to characterize the outage capacity of the correlated fading channel. It is also observed that STCs with large effective lengths and ideal built-in interleavers are more effective in exploiting the natural diversity in multiple-antenna correlated fading channels. We then propose a low-density parity-check (LDPC)-code-based STC-OFDM system. Compared with the conventional space-time trellis code (STTC), the LDPC-based STC can significantly improve the system performance by exploiting both the spatial diversity and the selective-fading diversity in wireless channels. Compared with the previously proposed turbo-code-based STC scheme, LDPC-based STC exhibits lower receiver complexity and more flexible scalability. We also consider receiver design for LDPC-based STC-OFDM systems in unknown fast fading channels and propose a novel turbo receiver employing a maximum a posteriori expectation-maximization (MAP-EM) demodulator and a soft LDPC decoder, which can significantly reduce the error floor in fast fading channels with a modest computational complexity. With such a turbo receiver, the proposed LDPC-based STC-OFDM system is a promising solution to highly efficient data transmission over selective-fading mobile wireless channels 相似文献
998.
999.
1000.
S N Maitra 《Sadhana》1985,8(4):373-385
The burn time and burnout velocity of a multistage rocket flown vertically in vacuum with constant thrust tangential to the
flight path and a prescribed initial/final thrust-to-weight ratio in an arbitrary stage have been determined.
The present paper also deals with optimal staging under given conditions of flight. 相似文献