全文获取类型
收费全文 | 11425篇 |
免费 | 695篇 |
国内免费 | 307篇 |
专业分类
电工技术 | 361篇 |
综合类 | 381篇 |
化学工业 | 1583篇 |
金属工艺 | 523篇 |
机械仪表 | 614篇 |
建筑科学 | 501篇 |
矿业工程 | 172篇 |
能源动力 | 313篇 |
轻工业 | 461篇 |
水利工程 | 149篇 |
石油天然气 | 298篇 |
武器工业 | 30篇 |
无线电 | 1435篇 |
一般工业技术 | 1566篇 |
冶金工业 | 2672篇 |
原子能技术 | 92篇 |
自动化技术 | 1276篇 |
出版年
2024年 | 42篇 |
2023年 | 131篇 |
2022年 | 250篇 |
2021年 | 332篇 |
2020年 | 268篇 |
2019年 | 273篇 |
2018年 | 265篇 |
2017年 | 261篇 |
2016年 | 256篇 |
2015年 | 343篇 |
2014年 | 407篇 |
2013年 | 592篇 |
2012年 | 546篇 |
2011年 | 592篇 |
2010年 | 492篇 |
2009年 | 517篇 |
2008年 | 459篇 |
2007年 | 455篇 |
2006年 | 413篇 |
2005年 | 399篇 |
2004年 | 274篇 |
2003年 | 271篇 |
2002年 | 258篇 |
2001年 | 249篇 |
2000年 | 211篇 |
1999年 | 309篇 |
1998年 | 857篇 |
1997年 | 527篇 |
1996年 | 400篇 |
1995年 | 262篇 |
1994年 | 228篇 |
1993年 | 208篇 |
1992年 | 105篇 |
1991年 | 101篇 |
1990年 | 85篇 |
1989年 | 103篇 |
1988年 | 86篇 |
1987年 | 63篇 |
1986年 | 62篇 |
1985年 | 74篇 |
1984年 | 31篇 |
1983年 | 24篇 |
1982年 | 40篇 |
1981年 | 30篇 |
1980年 | 32篇 |
1979年 | 14篇 |
1977年 | 53篇 |
1976年 | 86篇 |
1974年 | 19篇 |
1973年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
52.
Access path deployment is a critical issue in physical database design. Access paths typically include a clustered index as the primary access path and a set of secondary indexes as auxiliary access paths. To deploy the right access paths requires an effective algorithm and accurate estimation of the parameters used by the algorithm. One parameter central to any index-selection algorithm is the block selectivity of a query. Existing methods for estimating block selectivities are limited by restrictive assumptions. Furthermore, most existing methods produce estimates useful for aiding the selection of secondary indexes only. Little research has been done in the area of estimating block selectivities for supporting the selection of the clustered index. The paper presents a set of methods that do not depend on any specific assumption, produce accurate estimates, and can be used to aid in selecting the clustered index as well as secondary indexes 相似文献
53.
Very high-order microring resonator filters for WDM applications 总被引:6,自引:0,他引:6
B.E. Little S.T. Chu P.P. Absil J.V. Hryniewicz F.G. Johnson F. Seiferth D. Gill V. Van O. King M. Trakalo 《Photonics Technology Letters, IEEE》2004,16(10):2263-2265
High-order microring resonators having from 1 to 11 coupled cavities are demonstrated. These filters exhibit low loss, flat tops, and out-of-band rejection ratios that can exceed 80 dB. They achieve performance that is suitable for commercial applications. 相似文献
54.
55.
We report the construction of a synthetic flavo-heme protein that incorporates two major physiological activities of flavoproteins: light activation of flavin analogous to DNA photolyase and rapid intramolecular electron transfer between the flavin and heme cofactors as in several oxidoreductases. The functional tetra-alpha-helix protein comprises two 62-aa helix-loop-helix subunits. Each subunit contains a single cysteine to which flavin (7-acetyl-10-methylisoalloxazine) is covalently attached and two histidines appropriately positioned for bis-his coordination of heme cofactors. Both flavins and hemes are situated within the hydrophobic core of the protein. Intramolecular electron transfer from flavosemiquinone generated by photoreduction from a sacrificial electron donor in solution was examined between protoporphyrin IX and 1-methyl-2-oxomesoheme XIII. Laser pulse-activated electron transfer from flavin to meso heme occurs on a 100-ns time scale, with a favorable free energy of approximately -100 meV. Electron transfer from flavin to the lower potential protoporphyrin IX, with an unfavorable free energy, can be induced after a lag phase under continuous light illumination. Thus, the supporting peptide matrix provides an excellent framework for the positioning of closely juxtaposed redox groups capable of facilitating intramolecular electron transfer and begins to clarify in a simplified and malleable system the natural engineering of flavoproteins. 相似文献
56.
We studied the long-term clinical course of five patients with chronic manganese intoxication. The mean scores of the King's College Hospital Rating Scale for Parkinson's disease increased from 15.0 +/- 4.2 in 1987 to 28.3 +/- 6.70 in 1991 and then to 38.1 +/- 12.9 in 1995. The deterioration was most prominent in gait, rigidity, speed of foot tapping, and writing. Tissue concentrations of manganese in blood, urine, scalp hair, and pubic hair returned to normal. Follow-up MRIs did not show paramagnetic high-signal intensity on T1-weighted images. The data indicate that clinical progression in patients with manganese parkinsonism continues even 10 years after cessation of exposure. 相似文献
57.
58.
CC O'Shea AP Thornell IR Rosewell B Hayes MJ Owen 《Canadian Metallurgical Quarterly》1997,7(5):591-599
A major issue is whether surface expression of the pre-TCR is necessary for signaling the development of immature thymocytes. To address this question, we generated transgenic mice expressing a TCRbeta chain that had a strong endoplasmic reticulum (ER) retrieval signal (TCRbetaER) and that was expressed intracellularly but failed to reach the cell surface. In TCRbetaER transgenic mice, there was a failure of allelic exclusion. Also, the transgene failed to rescue the developmental defects observed in TCRbeta-null mice. In contrast, TCRbeta transgenes with a mutant ER retrieval sequence or lacking this sequence signaled efficient allelic exclusion and suppressed the TCRbeta-/- defect. These data show that exit of the pre-TCR from the ER/cis-Golgi is required for progression through the double-negative thymocyte checkpoint. 相似文献
59.
60.
Ismail K. Meyerson B.S. Rishton S. Chu J. Nelson S. Nocera J. 《Electron Device Letters, IEEE》1992,13(5):229-231
The authors report on the fabrication and the resultant device characteristics of the first 0.25-μm gate-length field-effect transistor based on n-type modulation-doped Si/SiGe. Prepared using ultrahigh vacuum/chemical vapor deposition (UHV/CVD), the mobility and electron sheet charge density in the strained Si channel are 1500 (9500) cm2/V-s and 2.5×1012 (1.5×1012 ) cm-2 at 300 K (77 K). At 77 K, the devices have a current and transconductance of 325 mA/mm and 600 mS/mm, respectively. These values far exceed those found in Si MESFETs and are comparable to the best results achieved in GaAs/AlGaAs modulation-doped transistors 相似文献