全文获取类型
收费全文 | 24439篇 |
免费 | 1111篇 |
国内免费 | 154篇 |
专业分类
电工技术 | 392篇 |
综合类 | 39篇 |
化学工业 | 5287篇 |
金属工艺 | 781篇 |
机械仪表 | 793篇 |
建筑科学 | 485篇 |
矿业工程 | 53篇 |
能源动力 | 1756篇 |
轻工业 | 1609篇 |
水利工程 | 210篇 |
石油天然气 | 171篇 |
武器工业 | 3篇 |
无线电 | 2984篇 |
一般工业技术 | 5503篇 |
冶金工业 | 2167篇 |
原子能技术 | 226篇 |
自动化技术 | 3245篇 |
出版年
2024年 | 117篇 |
2023年 | 493篇 |
2022年 | 1141篇 |
2021年 | 1323篇 |
2020年 | 1057篇 |
2019年 | 1076篇 |
2018年 | 1432篇 |
2017年 | 1129篇 |
2016年 | 1109篇 |
2015年 | 723篇 |
2014年 | 1016篇 |
2013年 | 1909篇 |
2012年 | 1115篇 |
2011年 | 1361篇 |
2010年 | 1052篇 |
2009年 | 1014篇 |
2008年 | 933篇 |
2007年 | 734篇 |
2006年 | 626篇 |
2005年 | 465篇 |
2004年 | 354篇 |
2003年 | 330篇 |
2002年 | 275篇 |
2001年 | 228篇 |
2000年 | 241篇 |
1999年 | 254篇 |
1998年 | 501篇 |
1997年 | 374篇 |
1996年 | 348篇 |
1995年 | 262篇 |
1994年 | 241篇 |
1993年 | 227篇 |
1992年 | 169篇 |
1991年 | 193篇 |
1990年 | 152篇 |
1989年 | 154篇 |
1988年 | 127篇 |
1987年 | 131篇 |
1986年 | 125篇 |
1985年 | 152篇 |
1984年 | 121篇 |
1983年 | 125篇 |
1982年 | 114篇 |
1981年 | 123篇 |
1980年 | 97篇 |
1979年 | 75篇 |
1978年 | 54篇 |
1977年 | 66篇 |
1976年 | 82篇 |
1974年 | 35篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
A. Sharma A. Agrawal 《Photonics Technology Letters, IEEE》2006,18(8):944-946
A novel split-step finite-difference method for wide-angle beam propagation is presented. The formulation allows solution of the second-order scalar wave equation without having to make the slowly varying envelope and one-way propagation approximations. The method is highly accurate and numerically efficient requiring only simple matrix multiplication for propagation. 相似文献
102.
103.
Time-series model for wireless fading channels in isotropic and nonisotropic scattering environments
An autoregressive (AR) model is presented for isotropic and nonisotropic scattering environments characterized by Rice factor 0/spl les/K相似文献
104.
M. Kangas J. Villegas N. Kumar T. Salmi D.Yu. Murzin F. Sandelin E. Harlin 《Catalysis Today》2005,100(3-4):363-366
An investigation of the effect of reaction conditions on product distribution in the skeletal isomerisation reaction of linear butenes has been carried out. The main reaction routes over ferrierite have been identified. Beside the main product isobutene, major by-product formation occurs. The unwanted reactions include dimerisation of butene to form octenes, hydrogen transfer yielding small amounts of saturated C3 and C4 hydrocarbons and disproportionation producing propene and pentenes. The most abundant by-products were pentene and propene, though these were not formed in equimolar amounts as could be expected. Oligomerisation experiments of propene over ferrierite produced large amounts of butene and pentene, revealing the presence of adsorbed nonene. The cracking of this surface species to hexene and propene is the most likely reaction route for the excess propene formation. This additional path to propene formation operates mainly at temperatures above 623 K. 相似文献
105.
V Kumar PM Carabateas JA Dority WG Earley JP Mallamo C Subramanyam LD Aimone B Ault DL DeHaven Hudkins MS Miller 《Canadian Metallurgical Quarterly》1995,38(10):1826-1830
Replacement of the pyridinium ring of 6,11-ethanobenzo[b]quinolizinium cations with thiazolium (4a and 4b) and N-methylimidazolium (4c and 4d) resulted in equipotent compounds in the [3H]TCP binding assay. The corresponding N-methyl-1,2,4-triazolium analogs were less potent in this assay. The thiazolium derivative 4b, with a Ki = 2.9 nM, is being evaluated as a possible neuroprotective N-methyl-D-aspartic acid (NMDA) antagonist. 相似文献
106.
A Notghi R Hutchinson D Kumar NB Smith LK Harding 《Canadian Metallurgical Quarterly》1994,35(7):976-981
Segmental colonic transit has been measured in 101 patients. Two MBq of 111Indium absorbed on resin pellets and encapsulated in an enteric coated capsule was given at 7 00 am. Hourly images during the first day, and three images during each subsequent day were acquired for up to three days. Using all scan and patient data the scans were categorised in one of the five patterns of colonic transit: normal, rapid, right delay, left delay, or generalised delay. The geometric centres and per cent activity at each time point was compared between the five groups of colonic transit patients to find the best time for imaging and so to distinguish the five groups. During the first day, early images did not help in diagnosis of patterns of transit, however, in the later images (six hours onwards after the ingestion of the activity) the rapid transit groups could be identified. Images at 27 and 51 hours were both required to distinguish all five groups of patients from each other. Only in the 'normal' transit patients was there some excretion of the activity during the course of the second day, otherwise there was no difference in the images taken in the course of a day (second or third day). A simplified protocol requires a minimum of three images to distinguish all five patterns of colonic transit. The activity should be ingested in the morning (7 00 am) and the first image taken at the end of the working day (8-10 hours after ingestion), the second image on the morning of the second day, and the third image during the course of the third day. This simple protocol would provide all the clinically relevant information necessary for correct classification of the colonic transit. 相似文献
107.
108.
JF DeFrance S Sands FC Schweitzer L Ginsberg JC Sharma 《Canadian Metallurgical Quarterly》1997,9(4):283-293
The purpose of this article is to deliberate the moral and legal dilemma entailed in the weapon of the labour strike as a pressure tactic on the Israeli Finance Ministry regarding job slots, budgets and, in effect, violating the collective agreement signed by the nurses and impairing patients' treatment, as opposed to refraining from striking and suffering the heavy burden of work, the lack of trained personnel, low wages, and the inability to give patients proper, high quality treatment. 相似文献
109.
Proximal duodenal obstruction due to tuberculosis can masquarade as duodenal ulcer. Although commonest cause of duodenal obstruction is ulcer, other causes must be considered, particularly tuberculosis which is common in tropics. 相似文献
110.
F Liao G Alkhatib KW Peden G Sharma EA Berger JM Farber 《Canadian Metallurgical Quarterly》1997,185(11):2015-2023
The chemokine receptors CXCR4, CCR2B, CCR3, and CCR5 have recently been shown to serve along with CD4 as coreceptors for HIV-1. The tropisms of HIV-1 strains for subgroups of CD4(+) cells can be explained, at least partly, by the selective use of G protein-coupled receptors (GPCRs). We have identified a novel human gene, STRL33, located on chromosome 3 that encodes a GPCR with sequence similarity to chemokine receptors and to chemokine receptor-like orphan receptors. STRL33 is expressed in lymphoid tissues and activated T cells, and is induced in activated peripheral blood lymphocytes. When transfected into nonhuman NIH 3T3 cells expressing human CD4, the STRL33 cDNA rendered these cells competent to fuse with cells expressing HIV-1 envelope glycoproteins (Envs). Of greatest interest, STRL33, in contrast with CXCR4 or CCR5, was able to function as a cofactor for fusion mediated by Envs from both T cell line-tropic and macrophage-tropic HIV-1 strains. STRL33-transfected Jurkat cell lines also supported enhanced productive infection with HIV-1 compared with control Jurkat cells. Despite the sequence similarities between STRL33 and chemokine receptors, STRL33-transfected cell lines did not respond to any in a panel of chemokines. Based on the pattern of tissue expression of the STRL33 mRNA, and given the ability of STRL33 to function with Envs of differing tropisms, STRL33 may play a role in the establishment and/or progression of HIV-1 infection. 相似文献