全文获取类型
收费全文 | 123037篇 |
免费 | 9576篇 |
国内免费 | 5529篇 |
专业分类
电工技术 | 6237篇 |
技术理论 | 1篇 |
综合类 | 9104篇 |
化学工业 | 21259篇 |
金属工艺 | 5928篇 |
机械仪表 | 7122篇 |
建筑科学 | 8824篇 |
矿业工程 | 2347篇 |
能源动力 | 4078篇 |
轻工业 | 10114篇 |
水利工程 | 2207篇 |
石油天然气 | 4958篇 |
武器工业 | 767篇 |
无线电 | 14420篇 |
一般工业技术 | 14990篇 |
冶金工业 | 6147篇 |
原子能技术 | 1560篇 |
自动化技术 | 18079篇 |
出版年
2024年 | 458篇 |
2023年 | 1573篇 |
2022年 | 3016篇 |
2021年 | 4321篇 |
2020年 | 3406篇 |
2019年 | 3128篇 |
2018年 | 3441篇 |
2017年 | 3730篇 |
2016年 | 3603篇 |
2015年 | 4243篇 |
2014年 | 5779篇 |
2013年 | 7579篇 |
2012年 | 7987篇 |
2011年 | 8501篇 |
2010年 | 7566篇 |
2009年 | 7419篇 |
2008年 | 7200篇 |
2007年 | 6920篇 |
2006年 | 6778篇 |
2005年 | 5722篇 |
2004年 | 4525篇 |
2003年 | 4058篇 |
2002年 | 4579篇 |
2001年 | 3966篇 |
2000年 | 3060篇 |
1999年 | 2557篇 |
1998年 | 2244篇 |
1997年 | 1806篇 |
1996年 | 1672篇 |
1995年 | 1379篇 |
1994年 | 1071篇 |
1993年 | 902篇 |
1992年 | 661篇 |
1991年 | 508篇 |
1990年 | 474篇 |
1989年 | 334篇 |
1988年 | 301篇 |
1987年 | 223篇 |
1986年 | 179篇 |
1985年 | 164篇 |
1984年 | 135篇 |
1983年 | 129篇 |
1982年 | 109篇 |
1981年 | 91篇 |
1980年 | 78篇 |
1979年 | 60篇 |
1978年 | 50篇 |
1977年 | 64篇 |
1976年 | 103篇 |
1974年 | 46篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
151.
152.
MA Rott VC Witthuhn BA Schilke M Soranno A Ali TJ Donohue 《Canadian Metallurgical Quarterly》1993,175(2):358-366
In Rhodobacter sphaeroides, cytochrome c2 (cyt c2)-deficient mutants are photosynthetically incompetent (PS-). However, mutations which suppress the photosynthetic deficiency (spd mutations) of cyt c2 mutants increase the levels of a cyt c2 isoform, isocyt c2. To determine whether isocyt c2 was required for photosynthetic growth of Spd mutants, we used Tn5 mutagenesis to generate a PS- mutant (TP39) that lacks both cyt c2 and isocyt c2. DNA sequence analysis of wild-type DNA that restores isocyt c2 production and photosynthetic growth to TP39 indicates that it encodes the isocyt c2 structural gene, cycI. The Tn5 insertion in TP39 is approximately 1.5 kb upstream of cycI, and our results show that it is polar onto cycI. The cycI gene has been physically mapped to a region of chromosome I that is approximately 700 kb from the R. sphaeroides photosynthetic gene cluster. Construction of a defined cycI null mutant and complementation of several mutants with the cycI gene under the control of the cyt c2 promoter region indicate that an increase in the levels of isocyt c2 alone is necessary and sufficient for photosynthetic growth in the absence of cyt c2. The data are discussed in terms of the obligate role of isocyt c2 in cyt c2-independent photosynthesis of R. sphaeroides. 相似文献
153.
Lossless compression techniques are essential in archival and communication of medical images. Here, a new segmentation-based lossless image coding (SLIC) method is proposed, which is based on a simple but efficient region growing procedure. The embedded region growing procedure produces an adaptive scanning pattern for the image with the help of a very-few-bits-needed discontinuity index map. Along with this scanning pattern, an error image data part with a very small dynamic range is generated. Both the error image data and the discontinuity index map data parts are then encoded by the Joint Bi-level Image Experts Group (JBIG) method. The SLIC method resulted in, on the average, lossless compression to about 1.6 b/pixel from 8 b, and to about 2.9 b/pixel from 10 b with a database of ten high-resolution digitized chest and breast images. In comparison with direct coding by JBIG, Joint Photographic Experts Group (JPEG), hierarchical interpolation (HINT), and two-dimensional Burg prediction plus Huffman error coding methods, the SLIC method performed better by 4% to 28% on the database used 相似文献
154.
155.
This paper contributes to extend the minimax disparity to determine the ordered weighted averaging (OWA) model based on linear programming. It introduces the minimax disparity approach between any distinct pairs of the weights and uses the duality of linear programming to prove the feasibility of the extended OWA operator weights model. The paper finishes with an open problem. 相似文献
156.
WF Shen JC Montgomery S Rozenfeld JJ Moskow HJ Lawrence AM Buchberg C Largman 《Canadian Metallurgical Quarterly》1997,17(11):6448-6458
Recent studies show that Hox homeodomain proteins from paralog groups 1 to 10 gain DNA binding specificity and affinity through cooperative binding with the divergent homeodomain protein Pbx1. However, the AbdB-like Hox proteins from paralogs 11, 12, and 13 do not interact with Pbx1a, raising the possibility of different protein partners. The Meis1 homeobox gene has 44% identity to Pbx within the homeodomain and was identified as a common site of viral integration in myeloid leukemias arising in BXH-2 mice. These integrations result in constitutive activation of Meis1. Furthermore, the Hoxa-9 gene is frequently activated by viral integration in the same BXH-2 leukemias, suggesting a biological synergy between these two distinct classes of homeodomain proteins in causing malignant transformation. We now show that the Hoxa-9 protein physically interacts with Meis1 proteins by forming heterodimeric binding complexes on a DNA target containing a Meis1 site (TGACAG) and an AbdB-like Hox site (TTTTACGAC). Hox proteins from the other AbdB-like paralogs, Hoxa-10, Hoxa-11, Hoxd-12, and Hoxb-13, also form DNA binding complexes with Meis1b, while Hox proteins from other paralogs do not appear to interact with Meis1 proteins. DNA binding complexes formed by Meis1 with Hox proteins dissociate much more slowly than DNA complexes with Meis1 alone, suggesting that Hox proteins stabilize the interactions of Meis1 proteins with their DNA targets. 相似文献
157.
In this work, we designed, fabricated and tested a disposable, flow-through amperometric sensor for free chlorine determination in water. The sensor is based on the principle of an electrochemical cell. The substrate, as well as the top microfluidic layer, is made up of a polymer material. The advantages include; (a) disposability from low cost; (b) stable operation range from three-electrode design; (c) fluidic interconnections that provide on line testing capabilities; and (d) transparent substrate which provides for future integration of on-chip optics. The sensor showed a good response and linearity in the chlorine concentration ranging from 0.3 to 1.6 ppm, which applies to common chlorination process for drinking water purification. 相似文献
158.
159.
A direct projection from the retina to the dorsal raphe nucleus at the pontomesencephalic junction was demonstrated with both antero- and retrograde tracing techniques in the rat. Following intravitreous injections of choleratoxin subunit B (CTB), horseradish peroxidase (HRP) and CTB-conjugated HRP, varicose fibers were labeled in the lateral region of the dorsal raphe nucleus, predominantly contralateral to the injection. Many of these labeled fibers were intermingled with serotonin-immunoreactive neurons, but some fibers were also found further laterally, beyond the boundary of dorsal raphe nucleus but within the periaqueductal gray. Following injections of the retrograde tracers Fluoro-Gold and CTB into the dorsal raphe nucleus and adjacent periaqueductal gray (without contamination of previously known targets of retinal projections), a small population of ganglion cells was labeled in the retina. These data provide evidence for the existence of a direct retinal projection to the lateral region of the dorsal raphe nucleus and the adjacent mesopontine periaqueductal gray in the rat. This projection may have a role in sensorimotor coordination and the regulation of circadian rhythm as well as sleep and wakefulness. 相似文献
160.
HM Chaung CH Hong CP Chiang SK Lin YS Kuo WH Lan CC Hsieh 《Canadian Metallurgical Quarterly》1996,95(7):545-550
This review reports the different genetic factors that have been identified either as risk factor for Alzheimer's disease (AD) or directly causing the disease. First are reviewed epidemiological data and biological mechanisms about the apoplipoprotein E gene allele epsilon 4 that is a major risk factor for Alzheimer's disease. The second part describes the mutations responsible for early-onset autosomal dominant AD found in three different genes. The gene located on chromosome 21 encodes the amyloid precusor protein (APP). The presenilin 1 and presenilin 2 genes, located on chromosome 14 and 1 respectively, encode not yet known membrane proteins. 相似文献