全文获取类型
收费全文 | 29909篇 |
免费 | 1294篇 |
国内免费 | 112篇 |
专业分类
电工技术 | 393篇 |
综合类 | 229篇 |
化学工业 | 5507篇 |
金属工艺 | 678篇 |
机械仪表 | 613篇 |
建筑科学 | 1030篇 |
矿业工程 | 94篇 |
能源动力 | 986篇 |
轻工业 | 4059篇 |
水利工程 | 245篇 |
石油天然气 | 150篇 |
武器工业 | 6篇 |
无线电 | 2401篇 |
一般工业技术 | 4401篇 |
冶金工业 | 6325篇 |
原子能技术 | 252篇 |
自动化技术 | 3946篇 |
出版年
2024年 | 55篇 |
2023年 | 162篇 |
2022年 | 525篇 |
2021年 | 828篇 |
2020年 | 552篇 |
2019年 | 624篇 |
2018年 | 715篇 |
2017年 | 790篇 |
2016年 | 756篇 |
2015年 | 587篇 |
2014年 | 917篇 |
2013年 | 1528篇 |
2012年 | 1340篇 |
2011年 | 1719篇 |
2010年 | 1251篇 |
2009年 | 1298篇 |
2008年 | 1186篇 |
2007年 | 1034篇 |
2006年 | 844篇 |
2005年 | 895篇 |
2004年 | 947篇 |
2003年 | 853篇 |
2002年 | 827篇 |
2001年 | 712篇 |
2000年 | 563篇 |
1999年 | 544篇 |
1998年 | 2059篇 |
1997年 | 1365篇 |
1996年 | 940篇 |
1995年 | 629篇 |
1994年 | 489篇 |
1993年 | 542篇 |
1992年 | 225篇 |
1991年 | 295篇 |
1990年 | 229篇 |
1989年 | 205篇 |
1988年 | 211篇 |
1987年 | 176篇 |
1986年 | 165篇 |
1985年 | 189篇 |
1984年 | 135篇 |
1983年 | 94篇 |
1982年 | 123篇 |
1981年 | 124篇 |
1980年 | 121篇 |
1979年 | 98篇 |
1978年 | 84篇 |
1977年 | 141篇 |
1976年 | 233篇 |
1975年 | 86篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
A chromosomally integrated Bradyrhizobium japonicum hoxA mutant is unable to oxidize hydrogen in free-living conditions. Derepressing conditions that induce hydrogenase activity in free-living, wild-type B. japonicum cells cannot induce expression of the hydrogenase structural genes in the hoxA mutant. The DNA-binding capacity of HoxA at the hup promoter region was studied by means of gel retardation. Both heterotrophically growing cells and cells induced to express hydrogenase activity contain a protein that specifically binds to the hup promoter region. Crude protein extracts isolated from a B. japonicum hoxA mutant do not contain this binding compound. The HoxA protein was overexpressed in E. coli and isolated in the form of a maltose-binding protein (MBP)-HoxA fusion. The MBP-HoxA hybrid protein specifically bound to a 50 bp region of the hupSL promoter known to be important for regulation of hupSL expression. 相似文献
122.
MJ de Gortari BA Freking SM Kappes KA Leymaster AM Crawford RT Stone CW Beattie 《Canadian Metallurgical Quarterly》1997,28(4):274-290
We report the evaluation of 1036 bovine microsatellite primer pairs for their suitability as linkage markers in sheep. Approximately 58% (605/1036) of bovine primer pairs amplified a locus in sheep. Sixty-seven per cent (409/605) of amplified loci were detected as polymorphic. Marker heterozygosity, allele number and range of allele sizes were significantly lower in sheep than cattle sampled in this study. However, median fragment size was similar. These data suggest that high-resolution comparative linkage maps between closely related species can be constructed relatively efficiently. 相似文献
123.
W. Sjoerd Kijlstra Joop C.M.L. Daamen Jolinde M. van de Graaf Bart van der Linden Eduard K. Poels Alfred Bliek 《Applied catalysis. B, Environmental》1996,7(3-4):337-357
The effect of water on the selective catalytic reduction (SCR) of nitric oxide with ammonia over alumina supported with 2–15 wt.-% manganese oxide was investigated in the temperature range 385–600 K, with the emphasis on the low side of this temperature window. Studies on the effect of 1–5 vol.-% water vapour on the SCR reaction rate and selectivity were combined with TPD experiments to reveal the influence of water on the adsorption of the single SCR reactants. It turned out that the activity decrease due to water addition can be divided into a reversible inhibition and an irreversible deactivation. Inhibition is caused by molecular adsorption of water. TPD studies showed that water can adsorb competitively with both ammonia and nitric oxide. Additional kinetic experiments revealed that adsorbed ammonia is present in excess on the catalyst surface, even in the presence of water. Reduced nitric oxide adsorption is responsible for the observed reversible decrease in the reaction rate; the fractional reaction order changes from 0.79 in the absence of water to 1.07 in its presence. Deactivation is probably due to the dissociative adsorption of water, resulting in the formation of additional surface hydroxyls. As the amount of surface hydroxyls formed is limited to a saturation level, the deactivating effect on the catalyst is limited too. The additional hydroxyls condense and desorb in the temperature range 525–775 K, resulting in a lower degree of deactivation at higher temperature. A high temperature treatment at 775 K results in a complete regeneration. The amount of surface hydroxyls formed per unit surface area decreases at increasing MnOx-loading. The selectivity to the production of nitrogen is enhanced significantly by the presence of gas phase water. 相似文献
124.
OD de Rada AC Villaro LM Montuenga A Martínez DR Springall JM Polak 《Canadian Metallurgical Quarterly》1993,162(1-2):121-124
The presence of nitric oxide synthase (NO-synthase), the enzyme responsible for the production of nitric oxide (NO) from L-arginine, is shown immunocytochemically in the intrinsic neurons of the human and porcine respiratory tract. NO-synthase immunoreactivity is demonstrated in a subpopulation of neurons of the microganglia present in the wall of the extra- and intrapulmonary bronchi as well as in the hilar region of the lung in relation to blood vessels. The immunostaining was also found in some nerve fibers of the respiratory nervous system. Human and porcine lung gave similar results. The possible involvement of NO in the nonadrenergic noncholinergic (NANC) nervous regulation of the lung is discussed. 相似文献
125.
R de Franchis 《Canadian Metallurgical Quarterly》1998,10(12):1041-1044
Bleeding oesophageal varices (BOV), resulting from portal hypertension, can prove fatal. Not only is it important to stop the initial bleeding, which may lead to hypovolaemic shock, but also to treat this condition in the longer term, and, consequently, the prevention of rebleeding needs to be addressed. This review highlights the current findings on the haemostatic drug, terlipressin, focusing particular attention on the potential for longer-term treatment strategies in the prevention of rebleeding. The efficacy of terlipressin in treating acute BOV, its low incidence of severe side-effects (comparable to those of somatostatin) and its favourable comparison with sclerotherapy in the prevention of early rebleeds, all indicate the potential for terlipressin administration to be extended to 5 days in the longer-term treatment of BOV. In addition, terlipressin administration, in conjunction with sclerotherapy, can significantly reduce the likelihood of rebleeding compared with sclerotherapy alone and further supports its potential use in the longer-term treatment of BOV. 相似文献
126.
J Gálvez R García-Domenech C de Gregorio Alapont JV de Julián-Ortiz L Popa 《Canadian Metallurgical Quarterly》1996,14(5):272-276
BACKGROUND: There is no empirical data available on attitudes concerning AIDS and habits towards HIV infected patients of physicians in general or private practice. In this study results of a self-evaluation are presented. METHODS: 178 physicians working with out-patients in different medical fields were randomly selected for a cross sectional study and interviewed using a standardised questionnaire. RESULTS: 89% think that they are sufficiently informed about AIDS (in the USA 20%). They regarded the risk of infection to be lower than the Anglo-American physicians. They believed there is a lack of interchange of information between colleagues regarding the degree of infectiousness of referred patients. A third of the physicians fear that other patients will go elsewhere if they find out that their physician is treating AIDS patients. 54% would hold special clinic sessions for HIV-patients outside the normal schedule for practice times. 89% believed that HIV patients were partly to blame for their illness. CONCLUSIONS: Although the physicians recognise the problem of HIV-infection, they partly deny the real necessities and facts. A reason for this could be the emotions underlying the general attitude to everything pertaining to HIV-disease. Attitudes to HIV-disease and the dealing with it in daily practice must be considered on the basis of individual emotional motives. 相似文献
127.
128.
G. M. Gusev X. Kleber U. Gennser D. K. Maude J. C. Portal D. I. Lubyshev P. Basmaji M. de P. A. Silva J. C. Rossi Yu. V. Nastaushev M. R. Baklanov 《Solid-state electronics》1996,40(1-8):441-446
Electron scattering by a single barrier is predicted to reveal singularities as the magnetic field is changed, because the number of electron collisions with the barrier dramatically increases as chaotic orbits around the barrier are changed into periodic orbits. To test this experimentally we have measured the magnetoresistance of AlGaAs/GaAs heterostructures with a two-dimensional electron gas and a lateral lattice containing a macroscopic number of oval-shaped antidots fabricated using electron lithography. Reproducible fluctuations in the magnetoresistance are observed at low field, which are due to the oscillations of the number of electron collisions with the antidots. The number of collisions N before the electron escapes from the antidot has been calculated as a function of B in an electric field. The position of the maxima in N(B) obtained from calculations and experiment are in reasonable agreement. 相似文献
129.
E Pittenauer ER Schmid G Allmaier B Pfanzagl W L?ffelhardt CQ Fernández MA de Pedro W Stanek 《Canadian Metallurgical Quarterly》1993,22(9):524-536
A strategy for the structural characterization of the four major NaBH4-reduced peptidoglycan monomers derived from muramidase-digested peptidoglycan from the cyanelles of the flagellate Cyanophora paradoxa Korschikoff is described. Initial molecular weight determination of these glycopeptides was performed by positive and negative ion plasma desorption mass spectrometry. Due to the presence of two pairs of disaccharide tripeptide and disaccharide tetrapeptide monomers differing in mass by 112 units, respectively, an as yet unknown peptidoglycan modification either at the carbohydrate or at the peptide moiety was assumed. beta-Elimination of the disaccharide unit from the unreduced peptidoglycan monomers yielded the corresponding (modified) N1-lactyltripeptides and -tetrapeptides, respectively. These peptides, N-terminally blocked with lactic acid, unambiguously showed the modification to be located on the peptide moiety. By positive ion fast atom bombardment/hybrid tandem mass spectrometry of the reduced peptidoglycan monomers as well as of the corresponding deglycosylated monomers (= N1-lactylpeptides) the modification was determined to be linked to the glutamic acid moiety. Based on combined data from plasma desorption mass spectrometry, tandem mass spectrometry, accurate mass measurement and amino acid analysis of the acid hydrolysate after derivatization with o-phthaldialdehyde by high-performance liquid chromatography we could establish the structure of the modification as N-acetylputrescine. Finally, the confirmation of the linkage of the glutamic acid to diaminopimelic acid via the gamma-COOH was based on the presence of a-type peptide backbone fragment ions in the positive ion plasma desorption mass spectra of the modified N1-lactylpeptides. 相似文献
130.
A method of positive identification of the specificity of polymerase chain reaction (PCR) product using internal oligonucleotide probe is introduced. The hybridization was done on the agarose gel which was dried after electrophoresis. Detection of the expression of T cell receptor a chain variable (TCR V alpha) genes on mRNA level was used as the experimental model. Twenty nine TCR V alpha gene subfamilies could be distinguished clearly in healthy human peripheral blood lymphocytes by this method. Positive identification of PCR product on dried agarose gel by internal oligonucleotide probe is relatively simple and less time consuming. 相似文献