首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1003篇
  免费   17篇
  国内免费   5篇
电工技术   102篇
综合类   1篇
化学工业   184篇
金属工艺   15篇
机械仪表   11篇
建筑科学   41篇
矿业工程   2篇
能源动力   7篇
轻工业   161篇
水利工程   6篇
无线电   50篇
一般工业技术   158篇
冶金工业   202篇
原子能技术   22篇
自动化技术   63篇
  2023年   8篇
  2022年   7篇
  2021年   21篇
  2020年   10篇
  2019年   7篇
  2018年   10篇
  2017年   8篇
  2016年   9篇
  2015年   9篇
  2014年   22篇
  2013年   60篇
  2012年   32篇
  2011年   46篇
  2010年   32篇
  2009年   37篇
  2008年   77篇
  2007年   79篇
  2006年   60篇
  2005年   29篇
  2004年   22篇
  2003年   27篇
  2002年   29篇
  2001年   26篇
  2000年   19篇
  1999年   27篇
  1998年   58篇
  1997年   38篇
  1996年   27篇
  1995年   10篇
  1994年   19篇
  1993年   23篇
  1992年   16篇
  1991年   8篇
  1990年   12篇
  1989年   7篇
  1988年   9篇
  1987年   12篇
  1986年   9篇
  1985年   4篇
  1984年   4篇
  1983年   6篇
  1982年   5篇
  1981年   6篇
  1980年   2篇
  1979年   5篇
  1978年   16篇
  1977年   2篇
  1976年   4篇
  1971年   2篇
  1969年   2篇
排序方式: 共有1025条查询结果,搜索用时 31 毫秒
41.
BACKGROUND & AIMS: Interleukin 12 (IL-12) is a heterodimeric, macrophage-derived cytokine that is elevated in Crohn's disease (CD). Epstein-Barr virus-induced gene 3 (EBI3) is a recently characterized human glycoprotein that is homologous to the 40-kilodalton chain of IL-12 and forms a heterodimer with the 35-kilodalton chain of IL-12. We investigated the expression of EBI3 in colonic mucosa of normal control subjects, patients with ulcerative colitis (UC), and patients with CD. METHODS: Colonic tissue was analyzed for messenger RNA (mRNA) expression by quantitative polymerase chain reaction and for protein expression by immunohistology and Western blotting. RESULTS: EBI3 mRNA was present in intestinal biopsy specimens from healthy subjects and patients with CD but was elevated only in active UC. EBI3 levels in UC specimens correlated with histological scores of activity and T-cell infiltration. EBI3-positive cells that had a shape consistent with that of macrophages were identified in the lamina propria, and protein was detected by Western blotting. CONCLUSIONS: EBI3 is a novel IL-12-related cytokine that is expressed by macrophage-like cells in normal intestine and CD and has enhanced expression in active UC but not in active CD.  相似文献   
42.
A mar-resistant coating system was obtained by screening combinations of silicone and vinyl compounds. It was found that binary systems, such as the γ-glycidoxypropyltrimethoxysilane–glycidyl methacrylate system, were excellent not only in mar resistance but also in adhesion to base resin polymers. Haze values of these coatings, after the sand-falling test, reached less than 10%. The coating process consisted of three steps: the preparation of prepolymer, the coating to plastic surface, and the curing of the coating by heating. It was found that the prepolymerization step could be performed by irradiation very conveniently; the control of viscosity for coating application and thickness control could be possible without any gel formation. These coatings were also good in weather resistance due perhaps to the good adhesion to base resin and could be applicable to plastic spectacles, glazing materials, and a variety of other products.  相似文献   
43.
Electrophoretic karyotype studies have shown that clinical isolates of Candida albicans have extensive chromosome length polymorphisms. Chromosome translocation is one of the causes of karyotypic variation. Chromosome translocation events have been shown to occur very frequently at or near the major repeat sequence (MRS) on chromosomes. The MRS consists of the repeated sequences RB2, RPS and HOK, and the repeated sequences are considered to be the template for recombination. To investigate which element of the MRS is important for chromosome translocation, we constructed three cassettes, each containing a URA blaster and sequences homologous to one of the repeats, for insertion into the MRS region on the chromosomes. The ura3 strain STN22u2, which shows a stable, standard karyotype, was transformed with each construct. Insertion events with each cassette occurred at almost all chromosomes. Insertion into the RB2 repeat, but not into the RPS repeat, was accompanied by chromosome translocation in some transformants: chromosome translocations between chromosomes R and 7 and chromosomes 1 and 7 were found, as well as deletions of 7A and 7C from chromosome 7. We conclude that the insertion at the RB2 region may initiate chromosome translocation in C. albicans.  相似文献   
44.
The buffy-headed marmoset (Callithrix flaviceps) is apparently the only predispersal herbivore of the seeds of Siparuna guianensis at the Caratinga Biological Station in southeastern Brazil. Both the fruit receptacles and the frutioles (seeds) of S. guianensis are relatively rich in nutrients such as carbohydrates, proteins, and lipids, but the receptacles contain high concentrations of benzylisoquinoline alkaloids. The latter presumably act as a qualitative chemical defense, impeding the access of potential predators to the alkaloid-poor frutioles. However, on ripening, the receptacle splits open, exposing the frutioles, which enables C. flaviceps to avoid the plant's chemical defenses effectively. Taking care to avoid contact with the receptacle, the marmosets pluck out the frutioles and ingest the seeds. Qualitative and quantitative changes (in particular a significant reduction in daily ranging) in the marmosets' behavior during the period when S. guianensis frutioles were accessible indicate that this was a preferred plant resource. This is the first record of the consumption of S. guianensis seeds by callitrichine monkeys (which are not known to be systematic seed eaters), despite the fact that both are widely distributed in the Neotropics. It is thus possible that the behavioral strategy observed here is a unique phenomenon resulting from a specific combination of factors, including the abundance of S. guianensis within the study area. The lack of other records may nevertheless be a result of insufficient sampling effort, in which case, the feeding strategies of callitrichines may have an important influence on the reproductive biology of S. guianensis in many areas.  相似文献   
45.
Large-sized β-FeSi2 substrates were successfully prepared for the first time from the silicide bulk crystal grown by the molten salt method. The structural, electrical and optical properties of the as-grown β-FeSi2 bulk crystals were also investigated. The crystal is single phase β-FeSi2, and polycrystalline with no preferable growth crystallographic orientations. It was also determined that the β-FeSi2 shows a p-type conduction, and the hole concentration and the Hall mobility at room temperature were about 1017 cm− 3 and 10 cm2/Vs, respectively. In addition, the PL emission around 0.8 eV was realized from the β-FeSi2 bulk crystal. This simple vacuum-free growth technique of β-FeSi2 and the large-sized substrate preparation procedure encourage us to develop future silicide-based electronics.  相似文献   
46.
From the viewpoint of mitigating global warming by SF6 gas, this paper discusses breakdown (BD) characteristics of different electronegative gas mixtures with N2O gas as SF6 gas substitutes for quasi-uniform electric field under lightning impulse voltage applications. Experimental results revealed the positive synergism in breakdown strength of binary N2O / CO2 and ternary N2O / CO2 / O2 gas mixtures, respectively. Furthermore, N2 gas as a retardant gas was also mixed with the electronegative gas mixtures in order to reduce the electron energy into the effective levels of electron attachment ability by the electronegative gas mixtures. As the result, ternary N2O / CO2 / N2 and quaternary N2O / CO2 / O2 / N2 gas mixtures could exhibit the significant synergistic effect in breakdown strength. The optimum mixture rate of quaternary N2O / CO2 / O2 / N2 gas mixtures was consistent with that estimated by assuming the independent contribution of component gases to the improvement of impulse BD characteristics.  相似文献   
47.
A network of biological databases is reviewed, supplying a framework for studies of human genes and the association of their genomic variations with human phenotypes. The network is composed of GeneCards, the human gene compendium, which provides comprehensive information on all known and predicted human genes, along with its suite members GeneDecks and GeneLoc. Two databases are shown that address genes and variations focusing on olfactory reception (HORDE) and transduction (GOSdb). In the realm of disease scrutiny, we portray MalaCards, a novel comprehensive database of human diseases and their annotations. Also shown is GeneKid, a tool aimed at generating novel kidney disease biomarkers using systems biology, as well as Xome, a database for whole-exome next-generation DNA sequences for human diseases in the Israeli population. Finally, we show LifeMap Discovery, a database of embryonic development, stem cell research and regenerative medicine, which links to both GeneCards and MalaCards.  相似文献   
48.
Enhancement of mass transfer between two phases and improvement of extraction efficiency are expected if features of micro spaces are adopted for a liquid-liquid two-phase process. In this work, three extraction operations, contact flow in a Y-shaped microchannel, segmented flow and emulsification, have been investigated. Extraction using segmented flow is dependent on mass transfer by molecular diffusion and its rate is enhanced by the internal circulation flow generated inside slugs. The extraction rate can be controlled precisely by adjusting operational parameters such as the flow rate and the flow rate ratio. Liquid-liquid two-phase separation can be achieved instantly. Thus, continuous operation of a process involving liquid-liquid extraction and then two-phase separation is expected to be possible. For extraction based on emulsification, rapid extraction is made possible owing to significant increase in the interfacial area. Though extraction itself is rapid, liquid-liquid separation may take a long time because coalescence of stable emulsions requires time. It is suitable for systems strongly requiring only rapid extraction. Advanced reaction-separation systems can be developed by combining the extraction operations, making full use of the characteristics of segmented flow or emulsification and the reaction.  相似文献   
49.
50.
Where is here?     
Taking John Willinsky's observation that in Canadian identity politics, the question 'who am I?' often necessitates the additional question 'where is here?' (Willinsky, 1998), this article examines excerpts from interviews with a group of teacher education students in order to explore how they used family (hi)stories about 'country of origin' in the construction of their identities. Making 'country of origin' problematic, the article also examines the limits and possibilities of being and becoming Canadian. 'Where are you from?' complicates and challenges the entitlement to belong. Drawing on critiques by Canadian educators, the article challenges the widely held significations given to Canadian immigration and to official policies of multiculturalism. By juxtaposing data from eight interviews, the issue of racial identity is explored, in terms of how it appears or does not appear in the stories the students tell. These juxtapositions highlight the unofficial intersections between Canadian identity, race and ethnicity.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号