全文获取类型
收费全文 | 445955篇 |
免费 | 4963篇 |
国内免费 | 1806篇 |
专业分类
电工技术 | 7710篇 |
综合类 | 640篇 |
化学工业 | 65844篇 |
金属工艺 | 19381篇 |
机械仪表 | 13968篇 |
建筑科学 | 9736篇 |
矿业工程 | 3317篇 |
能源动力 | 10840篇 |
轻工业 | 32996篇 |
水利工程 | 5497篇 |
石油天然气 | 11939篇 |
武器工业 | 64篇 |
无线电 | 50623篇 |
一般工业技术 | 92661篇 |
冶金工业 | 80178篇 |
原子能技术 | 12246篇 |
自动化技术 | 35084篇 |
出版年
2021年 | 3857篇 |
2019年 | 3707篇 |
2018年 | 6530篇 |
2017年 | 6686篇 |
2016年 | 7073篇 |
2015年 | 4336篇 |
2014年 | 7326篇 |
2013年 | 19072篇 |
2012年 | 11600篇 |
2011年 | 15553篇 |
2010年 | 12261篇 |
2009年 | 13971篇 |
2008年 | 14338篇 |
2007年 | 14283篇 |
2006年 | 12300篇 |
2005年 | 11355篇 |
2004年 | 10939篇 |
2003年 | 10614篇 |
2002年 | 10276篇 |
2001年 | 10443篇 |
2000年 | 9837篇 |
1999年 | 10184篇 |
1998年 | 25067篇 |
1997年 | 17687篇 |
1996年 | 13559篇 |
1995年 | 10336篇 |
1994年 | 9073篇 |
1993年 | 9304篇 |
1992年 | 6855篇 |
1991年 | 6651篇 |
1990年 | 6390篇 |
1989年 | 6377篇 |
1988年 | 6091篇 |
1987年 | 5428篇 |
1986年 | 5482篇 |
1985年 | 6060篇 |
1984年 | 5661篇 |
1983年 | 5312篇 |
1982年 | 4834篇 |
1981年 | 4884篇 |
1980年 | 4908篇 |
1979年 | 4731篇 |
1978年 | 4758篇 |
1977年 | 5126篇 |
1976年 | 6466篇 |
1975年 | 4163篇 |
1974年 | 4052篇 |
1973年 | 4111篇 |
1972年 | 3548篇 |
1971年 | 3186篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
To prevent post-transfusional hepatitis B and C, two epidemiologic studies were performed. The first, based on the frequencies distribution of hepatitis B virus serological markers versus sex and classes of age, has permitted the assessment of the profile of the infection in a population composed of 573 north vietnamese blood donors. There is no significant difference between men and women frequencies of HBs antigen (11.5%), anti-HBs antibody (70.2%) and anti-HBc antibody alone (3.8%), but a significant difference of no-marker frequencies: 7.8% and 17.9% in men and women respectively (X2 = 9.11; p = 0.010). The percentage of no-marker decreases when the mean age of each class increases. The second, using the increase of the serum alanine aminotransferase (ALAT) activity as an indirect marker of non-A, non-B hepatitis for determining in a population of more than 25,000 parisian blood donors, the percentage of donors eliminated. They are between 0.70 and 0.76 in women and 2.26 and 2.46 in men. These investigations can be applied to prevent the hepatitis B transmission in a population of 102 south vietnamese women in age to procreate or to determine the percentage of blood donors eliminated (3.12%) in a population of 2,950 Parisians composed in majority (50.9%) of new donors. The hemobiologist will have an important role to elaborate strategies for orientation of blood gifts with hepatitis B and C virus markers. 相似文献
62.
The three-dimensional structure of glutathione S-transferase from Arabidopsis thaliana has been solved at 2.2 A resolution (Reinemer et al., 1996). The enzyme forms a dimer of two identical subunits. The structure shows a new G-site architecture and a novel and unique dimer interface. Each monomer of the protein forms a separate G-site. Therefore, the requirements on the dimer interface are reduced. As a consequence, the interactions between the monomers are weaker and residues at the dimer interface are more variable. Thus, the dimer interface looses its relevance for a classification of plant glutathione S-transferases and the formation of heterodimers becomes even more difficult to predict. 相似文献
63.
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
64.
65.
Cresswell M.W. Allen R.A. Guthrie W.F. Sniegowski J.J. Ghoshtagore R.N. Linholm L.W. 《Semiconductor Manufacturing, IEEE Transactions on》1998,11(2):182-193
The physical widths of reference features incorporated into electrical linewidth test structures patterned in films of monocrystalline silicon have been determined from Kelvin voltage measurements. The films in which the test structures are patterned are electrically insulated from the bulk-silicon substrate by a layer of silicon dioxide provided by SIMOX (Separation by the IMplantation of OXygen) processing. The motivation is to facilitate the development of linewidth reference materials for critical-dimension (CD) metrology-instrument calibration. The selection of the (110) orientation of the starting silicon and the orientation of the structures' features relative to the crystal lattice enable a lattice-plane-selective etch to generate reference-feature properties of rectangular cross section and atomically planar sidewalls. These properties are highly desirable for CD applications in which feature widths are certified with nanometer-level uncertainty for use by a diverse range of CD instruments. End applications include the development and calibration of new generations of CD instruments directed at controlling processes for manufacturing devices having sub-quarter-micrometer features 相似文献
66.
The value of flexible videoendoscopy as a biofeedback tool in conservative swallowing rehabilitation of patients suffering from aspiration after head and neck surgery was examined. For this purpose, the outcomes of conventional swallowing therapy and of swallowing therapy with support of videoendoscopic biofeedback were compared. In the first 40 days of swallowing therapy, videoendoscopic biofeedback significantly increased the chance of therapeutic success, shortening the period of functional rehabilitation in comparison to conventional swallowing therapy. Limitations of this visual biofeedback procedure are poor cognitive skills of the patient, rejection of the procedure by the patient and local factors that make the positioning of the endoscope difficult. 相似文献
67.
Moser L.E. Melliar-Smith P.M. Agrawala V. 《Parallel and Distributed Systems, IEEE Transactions on》1994,5(5):459-473
Presents protocols for determining processor membership in asynchronous distributed systems that are subject to processor and communication faults. These protocols depend on the placement of a total order on broadcast messages. The types of systems for which each of these protocols is applicable are characterized by the properties of the communication mechanisms and by the availability of stable storage. In the absence of stable storage or of a mechanism for distinguishing promptly delivered messages, the authors show that no membership protocol can exist. They also discuss their experience in implementing these membership protocols 相似文献
68.
Considers the applicability of algorithm based fault tolerance (ABET) to massively parallel scientific computation. Existing ABET schemes can provide effective fault tolerance at a low cost For computation on matrices of moderate size; however, the methods do not scale well to floating-point operations on large systems. This short note proposes the use of a partitioned linear encoding scheme to provide scalability. Matrix algorithms employing this scheme are presented and compared to current ABET schemes. It is shown that the partitioned scheme provides scalable linear codes with improved numerical properties with only a small increase in hardware and time overhead 相似文献
69.
Ju D.-C.R. Chuan-Lin Wu Carini P. 《Parallel and Distributed Systems, IEEE Transactions on》1994,5(10):1113-1120
We present a classification scheme for array language primitives that quantifies the variation in parallelism and data locality that results from the fusion of any two primitives. We also present an algorithm based on this scheme that efficiently determines when it is beneficial to fuse any two primitives. Experimental results show that five LINPACK routines report 50% performance improvement from the fusion of array operators 相似文献
70.
McKinley P.K. Xu H. Esfahanian A.-H. Ni L.M. 《Parallel and Distributed Systems, IEEE Transactions on》1994,5(12):1252-1265
Multicast communication, in which the same message is delivered from a source node to an arbitrary number of destination nodes, is being increasingly demanded in parallel computing. System supported multicast services can potentially offer improved performance, increased functionality, and simplified programming, and may in turn be used to support various higher-level operations for data movement and global process control. This paper presents efficient algorithms to implement multicast communication in wormhole-routed direct networks, in the absence of hardware multicast support, by exploiting the properties of the switching technology. Minimum-time multicast algorithms are presented for n-dimensional meshes and hypercubes that use deterministic, dimension-ordered routing of unicast messages. Both algorithms can deliver a multicast message to m-1 destinations in [log 2 m] message passing steps, while avoiding contention among the constituent unicast messages. Performance results of implementations on a 64-node nCUBE-2 hypercube and a 168-node Symult 2010 2-D mesh are given 相似文献