全文获取类型
收费全文 | 7488篇 |
免费 | 355篇 |
国内免费 | 9篇 |
专业分类
电工技术 | 83篇 |
综合类 | 9篇 |
化学工业 | 1397篇 |
金属工艺 | 96篇 |
机械仪表 | 125篇 |
建筑科学 | 237篇 |
矿业工程 | 14篇 |
能源动力 | 235篇 |
轻工业 | 458篇 |
水利工程 | 66篇 |
石油天然气 | 54篇 |
无线电 | 604篇 |
一般工业技术 | 1565篇 |
冶金工业 | 1623篇 |
原子能技术 | 40篇 |
自动化技术 | 1246篇 |
出版年
2023年 | 73篇 |
2022年 | 113篇 |
2021年 | 185篇 |
2020年 | 144篇 |
2019年 | 153篇 |
2018年 | 158篇 |
2017年 | 170篇 |
2016年 | 183篇 |
2015年 | 162篇 |
2014年 | 236篇 |
2013年 | 409篇 |
2012年 | 373篇 |
2011年 | 449篇 |
2010年 | 360篇 |
2009年 | 343篇 |
2008年 | 386篇 |
2007年 | 319篇 |
2006年 | 285篇 |
2005年 | 243篇 |
2004年 | 209篇 |
2003年 | 199篇 |
2002年 | 175篇 |
2001年 | 125篇 |
2000年 | 112篇 |
1999年 | 123篇 |
1998年 | 299篇 |
1997年 | 200篇 |
1996年 | 193篇 |
1995年 | 140篇 |
1994年 | 133篇 |
1993年 | 106篇 |
1992年 | 73篇 |
1991年 | 67篇 |
1990年 | 68篇 |
1989年 | 55篇 |
1988年 | 60篇 |
1987年 | 64篇 |
1986年 | 42篇 |
1985年 | 79篇 |
1984年 | 47篇 |
1983年 | 30篇 |
1982年 | 46篇 |
1981年 | 50篇 |
1979年 | 27篇 |
1978年 | 27篇 |
1977年 | 47篇 |
1976年 | 59篇 |
1975年 | 24篇 |
1974年 | 27篇 |
1973年 | 24篇 |
排序方式: 共有7852条查询结果,搜索用时 15 毫秒
91.
A sensing strategy for the reverse engineering of machined parts 总被引:1,自引:0,他引:1
The reverse engineering of machined parts requires sensing an existing part and producing a design (and perhaps a manufacturing process) for it. We have developed a reverse engineering system that has proven effective with a set of machined parts. This paper describes the system, presents some results, and discusses strategy for a new system.This work was supported by ARPA under ARO grant number DAAH04-93-G-0420, DARPA grant N00014-91-J-4123, NSF grant CDA 9024721, and a University of Utah Research Committee grant. All opinions, findings, conclusions or recommendations expressed in this document are those of the authors and do not necessarily reflect the views of the sponsoring agencies. 相似文献
92.
The Rules of Beeping: Exchanging Messages Via Intentional "Missed Calls" on Mobile Phones 总被引:1,自引:0,他引:1
Jonathan Donner 《Journal of Computer-Mediated Communication》2007,13(1):1-22
This article explores the practice of "beeping" or "missed calling" between mobile phone users, or calling a number and hanging up before the mobile's owner can pick up the call. Most beeps are requests to call back immediately, but they can also send a pre-negotiated instrumental message such as "pick me up now" or a relational sign, such as "I'm thinking of you." The practice itself is old, with roots in landline behaviors, but it has grown tremendously, particularly in the developing world. Based on interviews with small business owners and university students in Rwanda, the article identifies three kinds of beeps (callback, pre-negotiated instrumental, and relational) and the norms governing their use. It then assesses the significance of the practice using adaptive structuration theory. In concluding, the article contrasts beeping with SMS/text messaging, discusses its implications for increasing access to telecommunications services, and suggests paths for future research. 相似文献
93.
THE IMPORTANCE OF NEUTRAL EXAMPLES FOR LEARNING SENTIMENT 总被引:2,自引:0,他引:2
Most research on learning to identify sentiment ignores "neutral" examples, learning only from examples of significant (positive or negative) polarity. We show that it is crucial to use neutral examples in learning polarity for a variety of reasons. Learning from negative and positive examples alone will not permit accurate classification of neutral examples. Moreover, the use of neutral training examples in learning facilitates better distinction between positive and negative examples. 相似文献
94.
Abstract. Knowledge systems development and use have been significantly encumbered by the difficulties of eliciting and formalizing the expertise upon which knowledge workers rely. This paper approaches the problem from an examination of the knowledge competencies of knowledge workers in order to define a universe of discourse for knowledge elicitation. It outlines two categories and several types of knowledge that could serve as the foundations for the development of a theory of expertise. 相似文献
95.
96.
The reactions of two sets of lignin model compounds over a sulfided CoOMoO3/γ-Al2O3 catalyst were studied. The first set mimicked lignin methoxyphenol residues and comprised 4-methyl-guaiacol, 4-methyl catechol, eugenol and vanillin. Deoxygenation and hydrogenation were facile and led to ultimate molar yields of single-ring products as high as 0.70. The selectivity to single-ring products increased with increases in temperature, o-hydroxydiphenylmethane, phenyl ether and o,o'-biphenol constituted the second set that mimicked thermally stable lignin bonds.Fragmentation of o-hydroxydiphenylmethane and phenyl ether occurred readily; o,o'-biphenol reacted to dibenzofuran.
These results suggest that the single-ring products obtained from catalytic liquefaction of lignins should be more desirable than those obtained from pyrolysis because of both higher yields and reduced complexity. 相似文献
These results suggest that the single-ring products obtained from catalytic liquefaction of lignins should be more desirable than those obtained from pyrolysis because of both higher yields and reduced complexity. 相似文献
97.
Barch Deanna M.; Cohen Jonathan D.; Servan-Schreiber David; Steingard Sandra; Steinhauer Stuart S.; van Kammen Daniel P. 《Canadian Metallurgical Quarterly》1996,105(4):592
Semantic priming in word pronunciation was examined at 5 stimulus onset asynchronies (SOAs) in 75 medicated and 25 unmedicated people with schizophrenia (SCZ) and in 10 depressed and 28 normal controls. At SOAs 相似文献
98.
The role of the proinflammatory cytokines interleukin-1 alpha (IL-1 alpha), interleukin-1 beta (IL-1 beta), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF alpha) in THC-induced catalepsy in mice was examined. Recombinant IL-1 beta (400 ng/mouse, IV) and TNF alpha (500 ng/mouse, IV) were effective in potentiating the cataleptic effect of low-dose THC (10 micrograms/mouse, IV). Recombinant IL-1 alpha and IL-6 did not potentiate catalepsy at any dose tested. Anti-IL-1 beta and anti-TNF alpha antibodies were effective in attenuating high-dose (75 micrograms/mouse) THC-induced catalepsy. Antibodies to IL-1 alpha and IL-6 had no effect on catalepsy. Early onset catalepsy (10 min postinjection) was potentiated by exogenous recombinant IL-1 beta and TNF alpha but only later catalepsy (2 h postinjection) was attenuated by antibodies to endogenous IL-1 beta or TNF alpha. This divergence of the cytokine effect suggests that these substances regulate, by different mechanisms, the early and late THC-induced cataleptic response. 相似文献
99.
S Nagl H Tichy WE Mayer N Takahata J Klein 《Canadian Metallurgical Quarterly》1998,95(24):14238-14243
Phylogenetic trees for groups of closely related species often have different topologies, depending on the genes used. One explanation for the discordant topologies is the persistence of polymorphisms through the speciation phase, followed by differential fixation of alleles in the resulting species. The existence of transspecies polymorphisms has been documented for alleles maintained by balancing selection but not for neutral alleles. In the present study, transspecific persistence of neutral polymorphisms was tested in the endemic haplochromine species flock of Lake Victoria cichlid fish. Putative noncoding region polymorphisms were identified at four randomly selected nuclear loci and tested on a collection of 12 Lake Victoria species and their putative riverine ancestors. At all loci, the same polymorphism was found to be present in nearly all the tested species, both lacustrine and riverine. Different polymorphisms at these loci were found in cichlids of other East African lakes (Malawi and Tanganyika). The Lake Victoria polymorphisms must have therefore arisen after the flocks now inhabiting the three great lakes diverged from one another, but before the riverine ancestors of the Lake Victoria flock colonized the Lake. Calculations based on the mtDNA clock suggest that the polymorphisms have persisted for about 1.4 million years. To maintain neutral polymorphisms for such a long time, the population size must have remained large throughout the entire period. 相似文献
100.
SA Klein JM Dobmeyer TS Dobmeyer M Pape OG Ottmann EB Helm D Hoelzer R Rossol 《Canadian Metallurgical Quarterly》1997,11(9):1111-1118
OBJECTIVE: To characterize changes of Th1/Th2 cytokine production by peripheral blood mononuclear cells (PBMC) that occur during the course of HIV infection by cytoplasmic cytokine staining on single cell level. DESIGN AND METHODS: Mitogen-stimulated PBMC from 16 healthy donors, 18 HIV-1-infected individuals without AIDS and 14 patients with AIDS were stained intracellularly with fluorescein-labelled MAb against interleukin (IL)-2, IL-4, IL-10 and interferon (IFN)-gamma. Additionally, co-staining of CD4+ T-cell, CD8+ T-cell, natural killer (NK) cell, B-cell and monocytic markers was performed. Fluorescence staining was analysed by three-colour flow-cytometry. RESULTS: A reduced percentage of IL-2 and IFN-gamma (Th1 type)-producing cells among CD4+ T cells from HIV-1-infected individuals could be demonstrated. There was a continuous decrease of IFN-gamma-producing CD4+ T cells in the course of HIV infection and a dramatic reduction of IL-2-expressing cells among CD4+ T cells in patients with AIDS. In contrast to Th1 cytokines, the frequency of Th2 cytokine expressing cells among CD4+ T cells increased in HIV-infected individuals. The maximum frequency of IL-4-expressing cells among CD4+ T cells was seen in HIV-infected individuals without AIDS, whereas the rate of IL-10-producing cells was highest in patients with AIDS. In HIV-infected individuals no significant proportion of Th0 cells expressing both Th1 and Th2 cytokines was detectable. In CD8+ T cells the percentage of IL-2 was expressing cells decreased continuously accompanied by a strong increase of the frequency of IFN-gamma-producing cells. CONCLUSION: The decreased percentage of cells expressing IL-2 and IFN-gamma in conjunction with an increased proportion of IL-4- and IL-10-producing cells among the CD4+ T cells in HIV-1-infected individuals demonstrate a Th1 to Th2 cytokine shift in the course of HIV infection on a single cell level. There was no evidence of a Th1 to Th0 cytokine shift. In addition to the loss of CD4+ T cells in HIV infection, the qualitative changes of Th1/Th2 cytokine expression may serve as a marker for progressive failure of cell-mediated immunity. 相似文献