全文获取类型
收费全文 | 1616806篇 |
免费 | 25167篇 |
国内免费 | 7137篇 |
专业分类
电工技术 | 35013篇 |
综合类 | 6660篇 |
化学工业 | 276743篇 |
金属工艺 | 65819篇 |
机械仪表 | 46181篇 |
建筑科学 | 48498篇 |
矿业工程 | 11738篇 |
能源动力 | 50541篇 |
轻工业 | 126398篇 |
水利工程 | 16314篇 |
石油天然气 | 38037篇 |
武器工业 | 207篇 |
无线电 | 199931篇 |
一般工业技术 | 301298篇 |
冶金工业 | 216791篇 |
原子能技术 | 34393篇 |
自动化技术 | 174548篇 |
出版年
2021年 | 15720篇 |
2020年 | 12090篇 |
2019年 | 14822篇 |
2018年 | 18274篇 |
2017年 | 17711篇 |
2016年 | 22712篇 |
2015年 | 17749篇 |
2014年 | 28981篇 |
2013年 | 88398篇 |
2012年 | 38484篇 |
2011年 | 53074篇 |
2010年 | 45161篇 |
2009年 | 52753篇 |
2008年 | 48845篇 |
2007年 | 46518篇 |
2006年 | 47178篇 |
2005年 | 42378篇 |
2004年 | 43929篇 |
2003年 | 43652篇 |
2002年 | 42271篇 |
2001年 | 39621篇 |
2000年 | 37685篇 |
1999年 | 37765篇 |
1998年 | 62056篇 |
1997年 | 49514篇 |
1996年 | 41979篇 |
1995年 | 34658篇 |
1994年 | 31897篇 |
1993年 | 31879篇 |
1992年 | 27211篇 |
1991年 | 24439篇 |
1990年 | 24618篇 |
1989年 | 23685篇 |
1988年 | 22238篇 |
1987年 | 20445篇 |
1986年 | 19861篇 |
1985年 | 23118篇 |
1984年 | 22840篇 |
1983年 | 20799篇 |
1982年 | 19661篇 |
1981年 | 19848篇 |
1980年 | 18435篇 |
1979年 | 18848篇 |
1978年 | 18108篇 |
1977年 | 18607篇 |
1976年 | 20839篇 |
1975年 | 16240篇 |
1974年 | 15725篇 |
1973年 | 15831篇 |
1972年 | 13272篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
942.
C Llanes P Gabant M Couturier Y Michel-Briand 《Canadian Metallurgical Quarterly》1994,176(11):3403-3407
The Inc A/C plasmids, like Inc P and Inc Q plasmids, have a broad host range. However, their maintenance functions remain to be studied. An autoreplicative region of 2.79 kb named RepA/C, able to replicate both in the family Enterobacteriaceae and in Pseudomonas spp., was isolated and sequenced. The stability, copy number, and incompatibility expression of this replicon were determined. RepA/C and a nonautoreplicative fragment of 16 kb of this replicon were used as probes and showed specific hybridizations with the Inc P3-A/C plasmids from Pseudomonas spp. and members of the Enterobacteriaceae. These probes could be used as tools for identification of the plasmids of this epidemiologically important Inc group. 相似文献
943.
The expression patterns of laminin A, B1, B2, and nidogen genes were identified by in situ hybridization in postimplantation mouse extraembryonic tissues and maternal decidua during the period when the chorioallantoic placenta is established. Laminin and nidogen genes were not coordinately expressed either in the decidua or in trophoblast cells, indicating that these genes are regulated independently in these cell types during the establishment of the placenta. Laminin A mRNA was absent from the decidua except in the outer layer of cells adjacent to the myometrium and in the central decidual zone adjacent to the remnant of the uterine epithelium on Day 9. At this stage laminin B1, B2, and nidogen genes were strongly expressed in these cells and also in other regions of the decidua. Laminin B1 mRNA was present at higher levels in the decidua capsularis than in the decidua basalis, while nidogen mRNA showed highest expression in the decidua basalis. Laminin B2 mRNA was produced uniformly throughout the decidua at very high levels, suggesting that laminin B2 chains may be an important component of the decidual matrix. By Day 11, the nidogen gene was expressed only in endothelial cells lining the maternal blood spaces within the decidua. Laminin B1 and nidogen mRNAs were found at high levels within trophoblast giant cells at all stages, while laminin A mRNA was detected in trophoblast giant cells at later stages and laminin B2 mRNA was not produced in high levels by these cells. The patterns of gene expression show a very high degree of regional specialization, suggesting that the extracellular matrices in different regions of the decidua and extraembryonic membranes are likely to be composed of quite different ratios of laminin and nidogen polypeptides.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
944.
945.
Several methodological and ethical issues are addressed in the context of 3 related school-based studies of the primary and targeted prevention of depressive symptoms and disorder in high school adolescents. These issues include obtaining S consent and the protection of confidentiality, minimizing attrition over long-term follow-up periods, the "unit of assignment" issue common to most school-based research, and ensuring therapist fidelity to the intervention protocol. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
946.
Garner H.R. Balog R.P. Luebke K.J. 《IEEE engineering in medicine and biology magazine》2002,21(4):123-125
Given the enormous size of the genome and that there are potentially many other types of measurements we need to do to understand it, it has become necessary to pick and choose one's targets to measure because it is still impossible to evaluate the entire genome all at once. What has emerged is a need to have rapidly customizable microarrays. There are two dominant methods to accomplish custom microarray synthesis, Affymetrix-like microarrays manufactured using light projection rather than semiconductor-like masks used by Affymetrix to mass manufacture their GeneChip/sup TM/ arrays now, or the ink-jet printing method employed by Agilent. The manufacture of these custom Affymetrix-like microarrays can now be done on a digital optical chemistry (DOC) machine developed at the University of Texas Southwestern Medical Center, and this method offers much higher feature numbers and feature density than is possible with ink-jet printed arrays. On a microarray, each feature contains a single genetic measurement. The initial DOC prototype has been described in several publications, but that has now led to a second-generation machine. This machine reliably produces a number of arrays daily, has been deployed against a number of biomedical questions, is being used in new ways and has also led to a number of spin-off technologies. 相似文献
947.
The state of the surface of n-GaAs crystals upon high-vacuum microwave plasmachemical (HVMWPC) etching in various gas mixtures and the influence of the semiconductor surface condition on the photoelectric characteristics of related metal-semiconductor-metal structures with double Schottky barrier (MSMDSB structures) are investigated. Dependence of the HVMWPC etching rate of the GaAs surface on the gas mixture composition and substrate temperature is determined. It is shown that the HVMWPC etching regime strongly influences the photoelectric properties of MSMDSB structures: the treatment can lead to either growth or drop in photosensitivity of the samples. Optimum etching regimes are established for which good semiconductor surface quality and high photosensitivity of the MSMDSB structures are retained at a high etching rate. 相似文献
948.
María P. Albano Liliana B. Garrido 《Journal of Materials Synthesis and Processing》2002,10(4):211-218
Si3N4powders coated with 6 wt% Y2O3and 4 wt% Al2O3were prepared by coprecipitation. The resulting powders were dispersed in water at different pH values and with addition of various amounts of ammonium polyacrylate (NH4PA) to produce 32 vol% slips. The influence of the amount of NH4PA solution added and pH on the rheological properties of 32 vol% coated Si3N4slips were studied. In addition, the sintered density of cast samples was determined and related to the degree of slip dispersion. The adsorption of the NH4PA on the coated particle surface was rather high and the surface became saturated near 0.86 mg/m2at pH 9.2. High NH4PA concentrations (1.7–3 wt%) were necessary to obtain well dispersed 32 vol% coated Si3N4slips at pH 9.2. The best stabilization was obtained with the addition of 2.3 wt% NH4PA; in this condition, the viscosity reached a minimum value of 35 mPa.s at 100 s–1. The slip viscosity increased with increasing pH from 9.2 to 10.2. Slips with low viscosities gave a more dense packing of cast samples and consequently higher sintered density values. 相似文献
949.
A new system based on the harnessing of solar and wind power is presented for heat dissipation in air conditioning facilities. The innovation offered by the new system is the integration of two well-known subsystems: a cooling tower and a solar chimney which increases the air flow without the use of electric power. The system is described and presented together with a model for its study. The field data acquired from the prototype built in southern Spain establishes the actual potential of the system, which provides a new approach to a sustainable technological development. 相似文献
950.