全文获取类型
收费全文 | 111782篇 |
免费 | 8131篇 |
国内免费 | 4192篇 |
专业分类
电工技术 | 5854篇 |
技术理论 | 11篇 |
综合类 | 6580篇 |
化学工业 | 18945篇 |
金属工艺 | 5815篇 |
机械仪表 | 6670篇 |
建筑科学 | 8626篇 |
矿业工程 | 3081篇 |
能源动力 | 3098篇 |
轻工业 | 6628篇 |
水利工程 | 1717篇 |
石油天然气 | 6216篇 |
武器工业 | 691篇 |
无线电 | 13958篇 |
一般工业技术 | 14243篇 |
冶金工业 | 5895篇 |
原子能技术 | 1238篇 |
自动化技术 | 14839篇 |
出版年
2024年 | 431篇 |
2023年 | 1749篇 |
2022年 | 2883篇 |
2021年 | 3985篇 |
2020年 | 3044篇 |
2019年 | 2584篇 |
2018年 | 2983篇 |
2017年 | 3369篇 |
2016年 | 2936篇 |
2015年 | 3855篇 |
2014年 | 4885篇 |
2013年 | 6252篇 |
2012年 | 6558篇 |
2011年 | 7180篇 |
2010年 | 6251篇 |
2009年 | 6166篇 |
2008年 | 6163篇 |
2007年 | 5754篇 |
2006年 | 6038篇 |
2005年 | 5409篇 |
2004年 | 3692篇 |
2003年 | 3196篇 |
2002年 | 2849篇 |
2001年 | 2756篇 |
2000年 | 2872篇 |
1999年 | 3379篇 |
1998年 | 2911篇 |
1997年 | 2547篇 |
1996年 | 2287篇 |
1995年 | 1921篇 |
1994年 | 1551篇 |
1993年 | 1219篇 |
1992年 | 964篇 |
1991年 | 738篇 |
1990年 | 600篇 |
1989年 | 500篇 |
1988年 | 390篇 |
1987年 | 294篇 |
1986年 | 206篇 |
1985年 | 176篇 |
1984年 | 108篇 |
1983年 | 81篇 |
1982年 | 87篇 |
1981年 | 57篇 |
1980年 | 46篇 |
1979年 | 32篇 |
1978年 | 25篇 |
1977年 | 25篇 |
1976年 | 40篇 |
1973年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
Tzong-Lin Wu Hung-Chun Chang 《Lightwave Technology, Journal of》1995,13(9):1926-1934
Based on vectorial formulations which combine the surface integral equation method and the finite-element method, a novel numerical approach is proposed for calculating the dispersion coefficients of dual-mode elliptical-core fibers with arbitrary refractive index profiles. By differentiating the original formulations involving the propagation constant β and the guided mode fields Hx and Hy once and twice with respect to the normalized frequency V, the new formulations for {dβ/dV, dHx/dV, dHy/dV} and for {d2β/dV2, d2 Hx/dV2, d2Hy/dV2 } are obtained respectively. Once {β, Hx, Hy } is solved through the eigenvalue procedure which dominates the computing time, only a few matrix manipulations are required to obtain {dβ/dV, dHx/dV, dHy/dV} and {d2β/dV2, d2Hx/dV2 , d2Hy/dV2}. Some numerical examples are examined to see the influence of different refractive index distributions with dips on the dispersions of the four nondegenerate LP 11 modes for elliptical-core fibers 相似文献
102.
103.
104.
RJ Bergeron WR Weimar Q Wu Y Feng JS McManis 《Canadian Metallurgical Quarterly》1996,39(26):5257-5266
A series of analogues and homologues of spermine were synthesized, and their impact on MK-801 binding to the N-methyl-D-aspartate (NMDA) receptor was evaluated. These tetraamines encompass both linear and cyclic compounds. The linear molecules include norspermine, N1, N11-diethylnorspermine, N1,N12-bis(2,2,2-trifluoroethyl)spermine, homospermine, and N1,N14-diethylhomospermine. The cyclic tetraamines consist of the piperidine analogues N1,N3-bis(4-piperidinyl)-1,3-diaminopropane, N1,N4-bis(4-piperidinyl)-1,4-diaminobutane, N1,N4-bis(4-piperidinylmethyl)-1,4-diaminobutane, and N1,N4-bis[2-(4-piperidinyl)ethyl]-1,4-diaminobutane and the pyridine analogues N1,N3-bis(4-pyridyl)-1,3-diaminopropane, N1,N4-bis(4-pyridyl)-1,4-diaminobutane, N1,N4-bis(4-pyridylmethyl)-1,4-diaminobutane, and N1,N4-bis[2-(4-pyridyl)-ethyl]-1,4-diaminobutane. This structure-activity set makes it possible to establish the importance of charge, intercharge distance, and terminal nitrogen substitution on polyamine-regulated MK-801 binding in the NMDA channel. Four families of tetraamines are included in this set: norspermines, spermines, homospermines, and tetraazaoctadecanes. Calculations employing a SYBYL modeling program revealed that the distance between terminal nitrogens ranges between 12.62 and 19.61 A. The tetraamines are constructed such that within families cyclics and acyclics have similar lengths but different nitrogen pKa's and thus different protonation, or charge, states at physiological pH. The pKa values for all nitrogens of each molecule and its protonation state at physiological pH are described. The modifications at the terminal nitrogens include introduction of ethyl and beta,beta,beta-trifluoroethyl groups and incorporation into piperidinyl or pyridyl systems. The studies clearly indicate that polyamine length, charge, and terminal nitrogen substitution have a significant effect on how the tetraamine regulates MK-801 binding to the NMDA receptor. Thus a structure-activity basis set on which future design of MK-801 agonists and antagonists can be based is now available. 相似文献
105.
106.
107.
We generated plasmid expression vectors encoding ubiquitin and beta-galactosidase (beta-gal) with different intervening amino acids, allowing for the production of processed protein products that have either stabilizing or destabilizing residues at their N-termini. P815 cells transfected with plasmids encoding beta-gal with a destabilizing N-terminus did not have detectable expression beta-gal unless they were treated with inhibitors specific for the proteasome. Inhibitors of other proteolysis pathways had no such effect. Nevertheless, transfectants expressing beta-gal with different amino acid residues were equally sensitive to cytolysis by a CTL clone specific for a beta-gal peptide presented in the context of H-2Ld. In contrast to vectors encoding native beta-gal, plasmid vectors encoding beta-gal with a destabilizing residue did not induce detectable anti-beta-gal Abs when injected into skeletal muscle of BALB/c mice. However, such vectors were significantly more effective than vectors encoding native beta-gal or beta-gal with a stabilizing residue in stimulating CTL specific for P13.2, a lacZ transfectant of P815. We conclude that incorporation of strategies that enhance proteasome-dependent degradation may generate DNA vaccines that are more effective in inducing cellular immunity against targeted Ags. 相似文献
108.
对数型产量衰减曲线方程的建立与应用 总被引:4,自引:0,他引:4
在产量衰减曲线方程的基础上,通过数学推导,得到了一种对数型产量减曲线方程。方程既可以用来预测油气田整个开发阶段的指标,又可以比较准确的测算油气田的可采集量。对国内外一些油田的实际应用表明,对数型产量衰减曲线方程是十分实用和有效的。 相似文献
109.
聚苯硫醚酰胺的合成与表征 总被引:7,自引:1,他引:6
用4-氯苯甲酰4′-氯苯胺和硫化钠为原料在常压下合成了聚苯硫醚酰胺,并对其进行了表征。结果表明所合成的产物为结晶性聚合物,并且有较高的热稳定性。 相似文献
110.