全文获取类型
收费全文 | 121152篇 |
免费 | 3452篇 |
国内免费 | 441篇 |
专业分类
电工技术 | 1067篇 |
综合类 | 2334篇 |
化学工业 | 19624篇 |
金属工艺 | 5475篇 |
机械仪表 | 3865篇 |
建筑科学 | 3325篇 |
矿业工程 | 644篇 |
能源动力 | 2051篇 |
轻工业 | 9799篇 |
水利工程 | 1586篇 |
石油天然气 | 499篇 |
武器工业 | 5篇 |
无线电 | 10678篇 |
一般工业技术 | 22393篇 |
冶金工业 | 10903篇 |
原子能技术 | 436篇 |
自动化技术 | 30361篇 |
出版年
2024年 | 70篇 |
2023年 | 348篇 |
2022年 | 552篇 |
2021年 | 1082篇 |
2020年 | 872篇 |
2019年 | 996篇 |
2018年 | 15568篇 |
2017年 | 14482篇 |
2016年 | 11309篇 |
2015年 | 1812篇 |
2014年 | 1718篇 |
2013年 | 3290篇 |
2012年 | 5378篇 |
2011年 | 11477篇 |
2010年 | 9912篇 |
2009年 | 7102篇 |
2008年 | 8208篇 |
2007年 | 9072篇 |
2006年 | 1213篇 |
2005年 | 2123篇 |
2004年 | 1935篇 |
2003年 | 1907篇 |
2002年 | 1253篇 |
2001年 | 635篇 |
2000年 | 668篇 |
1999年 | 595篇 |
1998年 | 2514篇 |
1997年 | 1725篇 |
1996年 | 1161篇 |
1995年 | 691篇 |
1994年 | 532篇 |
1993年 | 604篇 |
1992年 | 234篇 |
1991年 | 251篇 |
1990年 | 174篇 |
1989年 | 182篇 |
1988年 | 184篇 |
1987年 | 153篇 |
1986年 | 133篇 |
1985年 | 158篇 |
1984年 | 134篇 |
1983年 | 102篇 |
1982年 | 145篇 |
1981年 | 151篇 |
1980年 | 165篇 |
1979年 | 87篇 |
1978年 | 98篇 |
1977年 | 282篇 |
1976年 | 630篇 |
1973年 | 69篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
122.
The purpose of this paper is to evaluate two methods of assessing the productivity and quality impact of Computer Aided Software Engineering (CASE) and Fourth Generation Language (4GL) technologies: (1) by the retrospective method; and (2) the cross-sectional method. Both methods involve the use of questionnaire surveys. Developers' perceptions depend on the context in which they are expressed and this includes expectations about the effectiveness of a given software product. Consequently, it is generally not reliable to base inferences about the relative merits of CASE and 4GLs on a cross-sectional comparison of two separate samples of users. The retrospective method that requires each respondent to directly compare different products is shown to be more reliable. However, there may be scope to employ cross-sectional comparisons of the findings from different samples where both sets of respondents use the same reference point for their judgements, and where numerical rather than verbal rating scales are used to measure perceptions. 相似文献
123.
J Nesvera M Pátek J Hochmannová Z Abrhámová V Becvárová M Jelínkova J Vohradsky 《Canadian Metallurgical Quarterly》1997,179(5):1525-1532
The complete nucleotide sequence (4,826 bp) of the cryptic plasmid pGA1 from Corynebacterium glutamicum was determined. DNA sequence analysis revealed four putative coding regions (open reading frame A [ORFA], ORFA2, ORFB, and ORFC). ORFC was identified as a rep gene coding for an initiator of plasmid replication (Rep) according to the high level of homology of its deduced amino acid sequence with the Rep proteins of plasmids pSR1 (from C. glutamicum) and pNG2 (from Corynebacterium diphtheriae). This function was confirmed by deletion mapping of the minimal replicon of pGA1 (1.7 kb) which contains only ORFC. Deletion derivatives of pGA1 devoid of ORFA exhibited significant decreases in the copy number in C. glutamicum cells and displayed segregational instability. Introduction of ORFA in trans into the cells harboring these deletion plasmids dramatically increased their copy number and segregational stability. The ORFA gene product thus positively influences plasmid copy number. This is the first report on such activity associated with a nonintegrating bacterial plasmid. The related plasmids pGA1, pSR1, and pNG2 lacking significant homology with any other plasmid seem to be representatives of a new group of plasmids replicating in the rolling-circle mode. 相似文献
124.
Jan van Eijck 《Formal Aspects of Computing》1994,6(1):766-787
Presuppositions of utterances are the pieces of information you convey with an utterance no matter whether your utterance is true or not. We first study presupposition in a very simple framework of updating propositional information, with examples of how presuppositions of complex propositional updates can be calculated. Next we move on to presuppositions and quantification, in the context of a dynamic version of predicate logic, suitably modified to allow for presupposition failure. In both the propositional and the quantificational case, presupposition failure can be viewed as error abortion of procedures. Thus, a dynamic assertion logic which describes the preconditions for error abortion is the suitable tool for analysing presupposition. 相似文献
125.
126.
The neuronal cytoskeleton is one of the most profoundly altered organelles in late life neuro-degenerative disorders that are characterized by progressive impairments in cognitive abilities. The elucidation of the protein building blocks of these organelles as well as advances in understanding how these proteins become altered in Alzheimer's disease (AD) and other less common dementing illnesses, i.e., diffuse Lewy body disease (DLBD) or the Lewy body variant of AD (LBVAD), will provide insights into the molecular basis of these disorders. Within, we review evidence that normal adult human tau is abnormally phosphorylated and converted into the subunits of AD paired helical filaments (PHFs), and that Lewy bodies (LBs) represent accumulation of altered neurofilament (NF) triplet subunits. Although the precise biological consequences of PHF and LB formation in neurons is unknown, growing evidence suggests that the formation of PHFs and LBs from normal neuronal cytoskeletal proteins could have deleterious effects on neuronal function and survival. Finally, insights into the composition of PHFs and LBs could lead to the development of novel strategies for the timely and accurate diagnosis of AD, DLBD and the LBVAD. 相似文献
127.
Immunofluorescence microscopic observations indicated that a monoclonal antibody, Vmp 18, raised against the peptide 199-208 of murine interleukin 1 alpha, cross-reacted with an antigenic determinant of Drosophila thorax muscles. Immunoelectron microscopic analysis showed that the gold particles were mainly localized in the Z-line which is the attachment site of thin filaments from adjacent sarcomeres. On the contrary, the antibody failed to mark the Z-line in vertebrate skeletal muscle. A Western blot of total protein extract from Drosophila thorax muscles bound a protein of 43 kDa. Our observations suggest that the Vmp 18 antibody could contribute to clarify the composition of the Z-line in insect's flight muscles. 相似文献
128.
129.
R Herruzo-Cabrera L Malo-González ME Calle Purón MJ Vizcaíno-Alcaide J Del Rey-Calero 《Canadian Metallurgical Quarterly》1993,9(4):442-446
A cross-sectional study was performed to obtain risk factors for hepatitis B disease, HBsAg carriers and immunised personnel, among 2470 workers in a general hospital in Madrid, Spain. The data obtained were analyzed with multiple logistic regression to obtain beta coefficients for variables. The results of the analysis show that being a nurse or being regularly exposed to blood are the most important risk factors for hepatitis B acquisition. The length of time working at the same job activity was also a risk factor. The resulting beta coefficients allow the construction for a hepatitis non-immunised, HBsAg carrier and immunised HBV status, which can select subjects for a hepatitis B vaccination program. 相似文献
130.
Certain new sulfide-smelting processes and coal-gasification processes generate high-strength sulfur dioxide streams, for
which a new process for converting sulfur dioxide to elemental sulfur needs to be developed because no process exists that
is generally and economically applicable to the treatment of such streams. A thermodynamic and experimental investigation
to develop a new process for converting sulfur dioxide to elemental sulfur by a cyclic process involving calcium sulfide and
calcium sulfate without generating secondary pollutants was carried out. In this process, the starting raw material, calcium
sulfate, is reduced by a suitable reducing agent, such as hydrogen, to produce calcium sulfide, which is used to reduce sulfur
dioxide to elemental sulfur vapor and calcium sulfate. The latter is, in turn, reduced to regenerate calcium sulfide. In this
Part I, detailed experimental results are presented on the kinetics of the reaction between sulfur dioxide and calcium-sulfide
pellets, which produces elemental sulfur and calcium sulfate. The experiments were carried out at temperatures between 1023
and 1088 K and sulfur-dioxide partial pressures between 9 and 60 kPa by the use of a thermogravimetric analysis (TGA) technique.
The rate of this reaction was demonstrated by the conversion of 40 pct calcium-sulfide pellets obtained from the hydrogen
reduction of fresh calcium sulfate in 10 minutes at 1073 K under a sulfur-dioxide partial pressure of 43 kPa. The reactivity
decreased somewhat during the first three cycles but remained largely unchanged thereafter up to the tenth cycle. This characteristic
of the pellets is important because the solids must be reusable for repeated cycles to avoid generating secondary pollutants.
A pore-blocking model was found to fit the reaction rate. The reaction is first order with respect to sulfur-dioxide partial
pressure and has an activation energy of 101 to 134 kJ/mol (24 to 32 kcal/mol) for calcium-sulfide pellets reacted and regenerated
several different times. Sulfur dioxide-containing streams from certain sources, such as the regenerator off-gas from an integrated-gasification,
combined-cycle, desulfurization unit and new sulfide-smelting plants, contain much higher partial pressures of SO2. In these cases, the rate of the first reaction is expected to be proportionally higher than in the test conditions reported
in this article. The reduction kinetics of calcium-sulfate pellets with hydrogen gas is reported in the accompanying Part
II. 相似文献