全文获取类型
收费全文 | 749962篇 |
免费 | 8469篇 |
国内免费 | 1682篇 |
专业分类
电工技术 | 13675篇 |
综合类 | 604篇 |
化学工业 | 118290篇 |
金属工艺 | 30621篇 |
机械仪表 | 23454篇 |
建筑科学 | 17509篇 |
矿业工程 | 5252篇 |
能源动力 | 19179篇 |
轻工业 | 62488篇 |
水利工程 | 8742篇 |
石油天然气 | 18165篇 |
武器工业 | 48篇 |
无线电 | 80506篇 |
一般工业技术 | 151149篇 |
冶金工业 | 130729篇 |
原子能技术 | 18224篇 |
自动化技术 | 61478篇 |
出版年
2021年 | 6712篇 |
2019年 | 6364篇 |
2018年 | 11167篇 |
2017年 | 11264篇 |
2016年 | 11913篇 |
2015年 | 7543篇 |
2014年 | 12710篇 |
2013年 | 34157篇 |
2012年 | 19777篇 |
2011年 | 26688篇 |
2010年 | 21577篇 |
2009年 | 24075篇 |
2008年 | 24550篇 |
2007年 | 24169篇 |
2006年 | 21046篇 |
2005年 | 19302篇 |
2004年 | 18287篇 |
2003年 | 17846篇 |
2002年 | 17451篇 |
2001年 | 16977篇 |
2000年 | 16232篇 |
1999年 | 16108篇 |
1998年 | 37780篇 |
1997年 | 27596篇 |
1996年 | 21482篇 |
1995年 | 16497篇 |
1994年 | 14922篇 |
1993年 | 14556篇 |
1992年 | 11246篇 |
1991年 | 10973篇 |
1990年 | 10781篇 |
1989年 | 10578篇 |
1988年 | 10167篇 |
1987年 | 9072篇 |
1986年 | 8851篇 |
1985年 | 10133篇 |
1984年 | 9355篇 |
1983年 | 8901篇 |
1982年 | 8036篇 |
1981年 | 8279篇 |
1980年 | 7872篇 |
1979年 | 7926篇 |
1978年 | 7817篇 |
1977年 | 8869篇 |
1976年 | 11245篇 |
1975年 | 7082篇 |
1974年 | 6791篇 |
1973年 | 6877篇 |
1972年 | 5903篇 |
1971年 | 5523篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
102.
103.
Robben J.; der Schueren J.Van; Verhasselt P.; Aert R.; Volckaert G. 《Protein engineering, design & selection : PEDS》1995,8(2):159-165
The deletion of nine residues from the C-terminus of the bacterialchloramphenicol acetyltransferase (CAT) results in depositionof the mutant protein in cytoplasmic inclusion bodies and lossof chloramphenicol resistance in Escherichia coli. This foldingdefect is relieved by C-terminal fusion of the polypeptide withas few as two residues. Based on these observations, efficientpositive selection for the cloning of DNA fragments has beendemonstrated. The cloning vector encodes a C-terminally truncatedCAT protein. Restriction sites in front of the stop codon allowthe insertion of target DNA, resulting in the production ofproperly folded CAT fusion proteins and regained chloramphenicolresistance. The positive selection of recombinants is accomplishedby growth of transformants on chloramphenicol-containing agarplates. The method appears particularly convenient for the cloningof DNA fragments amplified by the PCR because minimal informationto restore CAT folding can be included in the primers. The cloningof random sequences shows that the folding defect can be relievedby fusion to a wide variety of peptides, providing great flexibilityto the positive selection system. This vector may also contributeto the determination of the role of the C-terminus in CAT folding. 相似文献
104.
A systematic and straightforward procedure is developed for the synthesis and analysis of transformer-isolated power converters. The procedure can be used to determine the ranges of duty-ratio over which the transformer-isolated power converters of a given class can be operated without transformer saturation. The procedure can also be used to study the dependence of the power converter switch stresses on duty-ratios. This information is useful in the selection of the transformer-isolated power converter most suitable for a given application and in the design of this power converter with minimum switch stresses, high power density, and low cost 相似文献
105.
106.
Several in vivo and in vitro methods for monitoring immunological properties of two allergoids obtained by formaldehyde treatment of ovalbumin (OA) were developed. The calculated molecular weight of allergoids was 80 kD (OA-F1) and 165 kD (OA-F2), respectively. The allergenic activity in vitro of allergoids in mast-cell histamine release assay was 1000 times lower than of OA. Both allergoids showed reduced ability to induce passive cutaneous anaphylaxis in the Sprague-Dawley rats or systemic anaphylaxis in Dunkin-Harley guinea-pigs. The ability of OA and allergoids to bind to the OA-specific IgE antibodies was measured in vivo by the inhibition of passive cutaneous anaphylaxis (PCA-inhibition). Allergoid binding to IgE was 51-66% lower than the native allergen. Moreover, the avidity of OA-specific IgG antibodies, measured by ELISA-inhibition, for allergoids and allergen was of the same order. Allergoids induced a different pattern of humoral immune response from that, induced by the native allergen. Thus, after immunization of BALB/c mouse, both allergoids induced a higher production of IgG and a lower production of IgE than OA, only OA-F2 induced a lower production of IgG1. The differences in the IgA response to the immunogens was not significant. Delayed hypersensitivity studies in the BALB/c mouse showed that allergoids were 5- to 12-times less effective in inducing a cell-mediated immune response than OA. The present study provides a battery of immunological methods for preclinical testing of modified allergens. 相似文献
107.
Carboxyl-terminated butadiene-acrylonitrile-rubber decreases modulus and yield stress of the studied epoxy but increases fracture toughness. The addition of glass bead compensates for the loss in modulus but has little effect on yield stress. However, it significantly contributes to the fracture toughness by providing additional mechanisms for toughening of both the unmodified and rubber-modified epoxy. For the toughened epoxies studied, fracture surfaces gave only limited information on fracture mechanisms since significant energy absorption also occurs in the material below the fracture surface. Suggestions for suitable material compositions for fiber composite matrices are given. 相似文献
108.
V. A. Arzamastsev N. L. Sardaev A. S. Kochergin 《Metal Science and Heat Treatment》1996,38(11):477-479
The technological process of cold forging applied for the first time in the production of the casing of the internal joint with races is described. The process operations of cold forging and the annealing and carburizing regimes for this part me described. 相似文献
109.
Nitric oxide reduction and carbon monoxide oxidation over carbon-supported copper-chromium catalysts 总被引:1,自引:0,他引:1
S. Stegenga R. van Soest F. Kapteijn J. A. Moulijn 《Applied catalysis. B, Environmental》1993,2(4):257-275
Carbon supported copper-chromium catalysts are shown to be very active for both the reduction of nitric oxide with carbon monoxide and the oxidation of carbon monoxide with oxygen. Mixed copper-chromium oxide active phases have good activity in the simultaneous removal of nitric oxide and carbon monoxide from exhaust gases. The influence of several catalyst variables has been investigated. The activity per volume of catalyst increases with increasing loading, while the intrinsic activity shows a maximum around C/M=100−50. An optimum catalyst for nitric oxide reduction and carbon monoxide oxidation has a copper/chromium ratio of 2/1. The apparent activation energy for the carbon monoxide oxidation over carbon supported copper-chromium catalysts is 77 kJ/mol, suggesting that the Cu---O bond rupture is the rate-limiting process. The reduction of nitric oxide takes place at higher temperatures. Since all catalysts have a low selectivity for molecular nitrogen formation at lower temperatures, the dissociation of nitric oxide is probably rate determining, resulting in a slightly reduced catalyst system. In an excess of carbon monoxide the reaction is first-order in nitric oxide and zero-order in carbon monoxide. Moisture inhibits the reaction by reversible competitive adsorption, whereas carbon dioxide does not. Oxygen completely inhibits the reduction of nitric oxide due to the more rapid reoxidation of the catalytic sites compared to nitric oxide. Therefore, the reduction of nitric oxide takes place only when all oxygen has been converted and, hence, is shifted to higher temperatures. As a possible consequence, the production of nitrous oxide is reduced. Nitric oxide and molecular oxygen react preferentially with carbon monoxide, so, in an excess of oxidizing component, gasification of the carbon support occurs at higher temperatures after carbon monoxide has been completely consumed. 相似文献
110.
Summary The synthesis of new high optical abrasion resistance coating materials has been undertaken by functionalizing melamine and
tris(m-aminophenyl)phosphine oxide with a triethoxysilane containing reagent. These highly functionalized compounds have been
used for coating a bis-phenol-A polycarbonate substrate by the sol-gel method. Preliminary data shows the abrasion resistance
of the polymer substrate is considerably improved when it is coated by these optically clear materials. 相似文献