全文获取类型
收费全文 | 6208篇 |
免费 | 29篇 |
国内免费 | 17篇 |
专业分类
系统科学 | 119篇 |
丛书文集 | 24篇 |
教育与普及 | 6篇 |
理论与方法论 | 61篇 |
现状及发展 | 3368篇 |
研究方法 | 357篇 |
综合类 | 2222篇 |
自然研究 | 97篇 |
出版年
2018年 | 59篇 |
2017年 | 51篇 |
2016年 | 64篇 |
2015年 | 44篇 |
2014年 | 49篇 |
2013年 | 51篇 |
2012年 | 155篇 |
2011年 | 238篇 |
2010年 | 87篇 |
2008年 | 164篇 |
2007年 | 147篇 |
2006年 | 161篇 |
2005年 | 161篇 |
2004年 | 182篇 |
2003年 | 132篇 |
2002年 | 131篇 |
2001年 | 146篇 |
2000年 | 132篇 |
1999年 | 100篇 |
1992年 | 104篇 |
1991年 | 69篇 |
1990年 | 74篇 |
1989年 | 53篇 |
1987年 | 70篇 |
1986年 | 63篇 |
1985年 | 76篇 |
1984年 | 86篇 |
1982年 | 63篇 |
1981年 | 46篇 |
1980年 | 78篇 |
1979年 | 179篇 |
1978年 | 132篇 |
1977年 | 175篇 |
1976年 | 75篇 |
1975年 | 96篇 |
1974年 | 193篇 |
1973年 | 165篇 |
1972年 | 177篇 |
1971年 | 155篇 |
1970年 | 208篇 |
1969年 | 204篇 |
1968年 | 186篇 |
1967年 | 195篇 |
1966年 | 185篇 |
1965年 | 133篇 |
1964年 | 48篇 |
1958年 | 60篇 |
1957年 | 48篇 |
1956年 | 47篇 |
1955年 | 46篇 |
排序方式: 共有6254条查询结果,搜索用时 15 毫秒
1.
董文伟 《科技情报开发与经济》2004,14(6):296-297
以具体工程为例,介绍了建筑物地基处理的设计、施工中遵循的原则,施工中遇到特殊情况时采取的处理方法和必要的检测方法,对几种桩基的设计要求、施工工艺及过程、质量控制等情况进行了简单阐述。 相似文献
2.
关于21世纪图书馆员继续教育问题的思考 总被引:4,自引:1,他引:3
罗雪晴 《科技情报开发与经济》2004,14(6):14-15
论述了21世纪对图书馆员进行继续教育的必要性,介绍了图书馆员继续教育的内容,针对我国图书馆员继续教育的状况,提出了应采取加大投入、进行针对性培训、运用激励机制等措施来加强对图书馆员的继续教育。 相似文献
3.
KERMACK WO 《Science progress》1948,36(142):288-292
4.
Y Y He T McNally I Manfield O Navratil I G Old S E Phillips I Saint-Girons P G Stockley 《Nature》1992,359(6394):431-433
The three-dimensional crystal structure of the Escherichia coli methionine repressor, MetJ, complexed with a DNA operator fragment is described in an accompanying article. The complex exhibits several novel features of DNA-protein interaction. DNA sequence recognition is achieved largely by hydrogen-bond contacts between the bases and amino-acid side chains located on a beta-ribbon, a mode of recognition previously hypothesized on the basis of modelling of idealized beta-strands and DNA, and mutagenesis of the Salmonella phage P22 repressors Arc and Mnt. The complex comprises a pair of MetJ repressor dimers which bind to adjacent met-box sites on the DNA, and contact each other by means of a pair of antiparallel alpha-helices. Here we assess the importance of these contacts, and also of contacts that would be made between the C-helices of the protein and DNA in a previous model of the complex, by studying mutations aimed at disrupting them. The role of the carboxy-terminal helix face in operator binding was unclear, but we demonstrate that recognition of operator sequences occurs through side chains in the beta-strand motif and that dimer-dimer interactions are required for effective repression. 相似文献
5.
Although theoretical studies show that overcompensatory density-dependent mechanisms can potentially generate regular or chaotic fluctuations in animal numbers, the majority of realistic single-species models of invertebrate populations are not overcompensatory enough to cause sustained population cycles. The possibility that overcompensation may generate cycles or chaos in vertebrate populations has seldom been considered. Here we show that highly overcompensatng density-dependent mortality can generate recurrent population crashes consistent with those observed in a naturally limited population of Soay sheep. The observed interval of three or more years between crashes points to sharp 'focusing' of mortality over a narrow range of population density. 相似文献
6.
孙智宏 《淮阴师范学院学报(自然科学版)》2002,1(1):13-15
设{Bn}为Bernoulli数,m、n为自然数,本文证明了同余式(2-22n)B2n≡1-4n ∑mk=1(2n)/(2k)24kB2k (mod 24m 3)与(3-32n)B2n≡2-6n 2∑mk=1(2n)/(2k)32kB2k (mod 32m 1).取m=1,2,得到[5]中宣布的(2-22n)B2n(mod 27)与(3-32n)B2n(mod 35)的简单同余式. 相似文献
7.
煤矸石空心砖是一种新型环保墙体材料,符合国家及山西省出台的限制生产和使用实心黏土砖,鼓励发展新型建筑材料的政策。利用煤矸石制造的空心砖可以作为保温材料,既可节约建筑能耗,又可节约土地资源,减少环境污染。煤矸石空心砖同黏土实心砖相比,具有高强、轻质、隔音、隔热、保温、防震等优点,市场前景良好。 相似文献
8.
A physically based model for ground‐level ozone forecasting is evaluated for Santiago, Chile. The model predicts the daily peak ozone concentration, with the daily rise of air temperature as input variable; weekends and rainy days appear as interventions. This model was used to analyse historical data, using the Linear Transfer Function/Finite Impulse Response (LTF/FIR) formalism; the Simultaneous Transfer Function (STF) method was used to analyse several monitoring stations together. Model evaluation showed a good forecasting performance across stations—for low and high ozone impacts—with power of detection (POD) values between 70 and 100%, Heidke's Skill Scores between 40% and 70% and low false alarm rates (FAR). The model consistently outperforms a pure persistence forecast. Model performance was not sensitive to different implementation options. The model performance degrades for two‐ and three‐days ahead forecast, but is still acceptable for the purpose of developing an environmental warning system at Santiago. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
9.
Abnormal pattern detected in fragile-X patients by pulsed-field gel electrophoresis. 总被引:26,自引:0,他引:26
The fragile-X syndrome is the most frequent inherited form of mental retardation, with an incidence of 1 in 1,500 males. It is characterized by the presence of a fragile site at Xq27.3 induced in vitro by folate deprivation or by inhibitors of deoxynucleotide synthesis. Its mode of inheritance is unusual for an X-linked trait, with incomplete penetrance in both males and females. Some phenotypically normal males transmit the mutation to all their daughters who rarely express any symptoms, but penetrance is high in sons and daughters of these carrier women. Genetic and physical mapping of the Xq27-q28 region has confirmed that the disease locus is located at or very near the fragile site. Hypotheses proposed to account for the abnormalities in the inheritance of the disease include sequence rearrangements by meiotic recombination or a mutation that affects reactivation of an inactive X chromosome during differentiation of female germ cells. To detect such rearrangements, or methylation changes that may reflect a locally inactive X chromosome, we used pulsed-field gel analysis of DNA from fragile-X patients with probes close to the fragile-X locus. The probe Do33 (DXS465) detected abnormal patterns in fragile-X patients, but not in normal controls or in non-expressing male transmitters. 相似文献
10.
Summary Host cell reactivation capacity for ozone T3 phage was determined for differentE. coli strains deficient in one or more of the DNA repair mechanisms. The results indicate that DNA polymerase I appears to play a key role in the repair of damage produced on the DNA by ozone while thelexA gene product seems to play a minor one.This research was sponsored by the National Sciences and Engineering Council of Canada. One of us (PLH) acknowledges a scholarship from the NSERCof Canada. 相似文献