全文获取类型
收费全文 | 261595篇 |
免费 | 4758篇 |
国内免费 | 1817篇 |
专业分类
电工技术 | 5441篇 |
技术理论 | 5篇 |
综合类 | 1164篇 |
化学工业 | 42235篇 |
金属工艺 | 11041篇 |
机械仪表 | 7762篇 |
建筑科学 | 6610篇 |
矿业工程 | 1660篇 |
能源动力 | 5619篇 |
轻工业 | 25844篇 |
水利工程 | 2892篇 |
石油天然气 | 5998篇 |
武器工业 | 243篇 |
无线电 | 27899篇 |
一般工业技术 | 49368篇 |
冶金工业 | 47698篇 |
原子能技术 | 5629篇 |
自动化技术 | 21062篇 |
出版年
2021年 | 2484篇 |
2019年 | 2133篇 |
2018年 | 3398篇 |
2017年 | 3376篇 |
2016年 | 3705篇 |
2015年 | 2809篇 |
2014年 | 4604篇 |
2013年 | 11385篇 |
2012年 | 7707篇 |
2011年 | 10232篇 |
2010年 | 8144篇 |
2009年 | 8693篇 |
2008年 | 9504篇 |
2007年 | 9630篇 |
2006年 | 8462篇 |
2005年 | 7419篇 |
2004年 | 6679篇 |
2003年 | 6226篇 |
2002年 | 6227篇 |
2001年 | 6337篇 |
2000年 | 5925篇 |
1999年 | 5900篇 |
1998年 | 13012篇 |
1997年 | 9617篇 |
1996年 | 7330篇 |
1995年 | 5563篇 |
1994年 | 5114篇 |
1993年 | 4981篇 |
1992年 | 3957篇 |
1991年 | 3766篇 |
1990年 | 3840篇 |
1989年 | 3793篇 |
1988年 | 3549篇 |
1987年 | 3040篇 |
1986年 | 3068篇 |
1985年 | 3423篇 |
1984年 | 3331篇 |
1983年 | 3088篇 |
1982年 | 2709篇 |
1981年 | 2927篇 |
1980年 | 2656篇 |
1979年 | 2849篇 |
1978年 | 2756篇 |
1977年 | 2863篇 |
1976年 | 3731篇 |
1975年 | 2467篇 |
1974年 | 2305篇 |
1973年 | 2327篇 |
1972年 | 1985篇 |
1971年 | 1788篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
D.L Orphal C.E Anderson Jr. R.R Franzen J.D Walker P.N Schneidewind M.E Majerus 《International Journal of Impact Engineering》1993,14(1-4):551-560
Calculations of steel target penetration by L/D ≤ 1 tungsten and tungsten alloy projectiles have been extended to L/D = 1/32 over the velocity range 1.5 to 5 km/s. The ratio of crater to projectile diameter tends to 1 as L/D decreases over this entire velocity range. For impact velocities of 1.5 and 3 km/s, penetration depth normalized by projectile length, P/L, increases with decreasing projectile L/D up to a maximum value and then decreases for still lower L/D. Experiments at impact velocities of 2 and 3 km/s confirm these results. For 5 km/s impact velocity, the calculations show P/L increasing with decreasing projectile L/D over the entire range 1/32 ≤ L/D ≤ 1. The projectile L/D for which the maximum P/L occurs appears to depend on the impact velocity. P/L generally scales with impact velocity as P/L vf(L/D) where f(L/D) ranges from 0 for a long rod to, we believe, 2 in the limit as projectile L/D approaches zero. The calculations show for 1/8 ≤ L/D ≤ 1/2, P/L v0.9; for L/D = 1/16, P/L v1.5; and for L/D = 1/32, the new results give P/L v1.9. 相似文献
993.
994.
Mass media--friend or foe? Radio, television, and/or the printed word regularly enter most of the world's households. With a better understanding between health workers and those who produce what we hear, see and read, positive messages could do much to change the habits the jeopardize health. Clear, accurate information is the first step towards improvement. 相似文献
995.
996.
An improved, combined staining method for myofibrillar ATPase (m-ATPase) and for acetylcholinesterase activity is described. This method allows the observations, on the same slide, of the classical histochemical m-ATPase profile following the Brooke and Kaiser technique and the neuromuscular junction morphology. Thus the pattern of innervation, nerve ending structure and number of nerve endings along the fibres is shown simultaneously for the basic differentiation between slow and fast fibres. The use of acidic and alkaline preincubation allows better visualization of endplate morphology and avoids the masking effect of a positive m-ATPase reaction. The technique has been validated on skeletal muscles from avian and mammalian species. 相似文献
997.
Inhibiting effects of serotonin and serotonin antagonists on the migration of mononuclear leucocytes
The effect of serotonin and the serotonin antagonists ketanserin, methiotepine and ICS-205-930 on the migration of leucocytes was studied by using the sealed capillary migration technique. The migration of mononuclear leucocytes was inhibited by serotonin at 10(-4) and 10(-6)-10(-10)mol/l. An inhibition of the mononuclear leucocyte migration was also caused by ICS-205-930 at 10(-4)mol/l, ketanserin at 10(-4) and 10(-8)-10(-10)mol/l and methiotepine at 10(-4) and 10(-6)-10(-8)mol/l. No inhibiting effects of serotonin or the serotonin antagonists were found on the migration of polymorphonuclear leucocytes. Thus, both serotonin and serotonin antagonists may inhibit mononuclear leucocyte migration. 相似文献
998.
C Sissi L Bolgan S Moro G Zagotto C Bailly E Menta G Capranico M Palumbo 《Canadian Metallurgical Quarterly》1998,54(6):1036-1045
To elucidate structure-activity relationships for drugs that are able to poison or inhibit topoisomerase II, we investigated the thermodynamics and stereochemistry of the DNA binding of a number of anthracene derivatives bearing one or two 4, 5-dihydro-1H-imidazol-2-yl-hydrazone side chains (characteristic of bisantrene) at different positions of the planar aromatic system. An aza-bioisostere, which can be considered a bisantrene-amsacrine hybrid, was also tested. The affinity for nucleic acids in different sequence contexts was evaluated by spectroscopic techniques, using various experimental conditions. DNA-melting and DNase I footprinting experiments were also performed. The location and number of the otherwise identical side chains dramatically affected the affinity of the test compounds for the nucleic acid. In addition, the new compounds exhibited different DNA sequence preferences, depending on the locations of the dihydroimidazolyl-hydrazone groups, which indicates a major role for the side-chain position in generating specific contacts with the nucleic acid. Molecular modeling studies of the intercalative binding of the 1- or 9-substituted isomers to DNA fully supported the experimental data, because a substantially more favorable recognition of A-T steps, compared with G-C steps, was found for the 9-substituted derivative, whereas a much closer energy balance was found for the 1-substituted isomer. These results compare well with the alteration of base specificity found for the topoisomerase II-mediated DNA cleavage stimulated by the isomeric drugs. Therefore, DNA-binding specificity appears to represent an important determinant for the recognition of the topoisomerase-DNA cleavable complex by the drug, at least for poisons belonging to the amsacrine-bisantrene family. 相似文献
999.
Medeiro F. Perez-Verdu B. Rodriguez-Vazquez A. Huertas J.L. 《Solid-State Circuits, IEEE Journal of》1995,30(7):762-772
We present a tool that starting from high-level specifications of switched-capacitor (SC) /spl Sigma//spl Delta/ modulators calculates optimum specifications for their building blocks and then optimum sizes for the block schematics. At both design levels, optimization is performed using statistical techniques to enable global design and innovative heuristics for increased computer efficiency as compared with conventional statistical optimization. The tool uses an equation-based approach at the modulator level, a simulation-based approach at the cell level, and incorporates an advanced /spl Sigma//spl Delta/ behavioral simulator for monitoring and design space exploration. We include measurements taken from two silicon prototypes: (1) a 16 b @ 16 kHz output rate second-order /spl Sigma//spl Delta/ modulator; and (2) a 17 b @ 40 kHz output rate fourth-order /spl Sigma//spl Delta/ modulator. Both use SC fully differential circuits and were designed using the proposed tool and manufactured in a 1.2 /spl mu/m CMOS double-metal double-poly technology.<> 相似文献
1000.
L D''Hondt C Chatelain C Doyen JC Osselaer A Ferrant A Bosly 《Canadian Metallurgical Quarterly》1996,38(4):353-354
We report the case of a young patient with refractory acute lymphoblastic leukemia relapse, after allogeneic bone marrow transplantation, who was treated by donor leukocyte infusions. We observed potent adoptive immunotherapy which produced a cytologic complete remission and total chimeric state. This was of short duration and the patient died of severe graft-versus-host disease. We present a short summary of the literature concerning acute lymphoblastic leukemia and donor leukocyte infusions. 相似文献