全文获取类型
收费全文 | 10426篇 |
免费 | 96篇 |
国内免费 | 58篇 |
专业分类
系统科学 | 87篇 |
丛书文集 | 90篇 |
教育与普及 | 13篇 |
理论与方法论 | 73篇 |
现状及发展 | 4996篇 |
研究方法 | 607篇 |
综合类 | 4571篇 |
自然研究 | 143篇 |
出版年
2018年 | 87篇 |
2017年 | 79篇 |
2016年 | 91篇 |
2014年 | 79篇 |
2013年 | 78篇 |
2012年 | 216篇 |
2011年 | 355篇 |
2010年 | 117篇 |
2009年 | 75篇 |
2008年 | 248篇 |
2007年 | 251篇 |
2006年 | 233篇 |
2005年 | 231篇 |
2004年 | 323篇 |
2003年 | 198篇 |
2002年 | 203篇 |
2001年 | 337篇 |
2000年 | 323篇 |
1999年 | 241篇 |
1992年 | 138篇 |
1991年 | 142篇 |
1990年 | 132篇 |
1989年 | 115篇 |
1988年 | 119篇 |
1987年 | 150篇 |
1986年 | 131篇 |
1985年 | 184篇 |
1984年 | 145篇 |
1983年 | 105篇 |
1982年 | 109篇 |
1981年 | 134篇 |
1980年 | 141篇 |
1979年 | 331篇 |
1978年 | 263篇 |
1977年 | 267篇 |
1976年 | 158篇 |
1975年 | 192篇 |
1974年 | 292篇 |
1973年 | 251篇 |
1972年 | 286篇 |
1971年 | 286篇 |
1970年 | 363篇 |
1969年 | 301篇 |
1968年 | 255篇 |
1967年 | 291篇 |
1966年 | 251篇 |
1965年 | 199篇 |
1958年 | 113篇 |
1957年 | 82篇 |
1954年 | 80篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
Vitamin B12 (methylcobalamin) was administered orally (3 mg/day) to 9 healthy subjects for 4 weeks. Nocturnal melatonin levels after exposure to bright light (ca. 2500 lx) were determined, as well as the levels of plasma melatonin over 24 h. The timing of sleep was also recorded. Vitamin B12 was given blind to the subjects and crossed over with placebo. We found that the 24-h melatonin rhythm was significantly phase-advanced (1.1 h) in the vitamin B12 trial as compared with that in the placebo trial. In addition, the 24-h mean of plasma melatonin level was much lower in the vitamin B12 trial than with the placebo. Furthermore, the nocturnal melatonin levels during bright light exposure were significantly lower in the vitamin B12 trial than with the placebo. On the other hand, vitamin B12 did not affect the timing of sleep. These findings raise the possibility that vitamin B12 phase-advances the human circadian rhythm by increasing the light sensitivity of the circadian clock. 相似文献
12.
13.
The timing of sleep and sleep EEG parameters in 10 healthy male subjects were investigated in four seasons under controlled conditions. The phase of nocturnal sleep was delayed about one and a half hours in winter as compared to that in summer. The duration of stage 4 sleep decreased and REM sleep increased significantly in winter compared with summer. The seasonality in the timing of sleep can be explained by photoperiodic time cues, but the changes in sleep EEG parameters are difficult to explain in terms of photoperiod. 相似文献
14.
DNA fingerprinting transforms the art of cell authentication. 总被引:4,自引:0,他引:4
The increasing diversity of new cell cultures is seriously stretching the capabilities of traditional methods of identification. DNA fingerprinting is set to play an important role in increasing confidence in the authenticity of cultures in research and industry. 相似文献
15.
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
16.
The failing heart. 总被引:19,自引:0,他引:19
Cardiomyopathies are disorders affecting heart muscle that usually result in inadequate pumping of the heart. They are the most common cause of heart failure and each year kill more than 10,000 people in the United States. In recent years, there have been breakthroughs in understanding the molecular mechanisms involved in this group of conditions, with knowledge of the genetic basis for cardiomyopathies perhaps seeing the largest advance, enabling clinicians to devise improved diagnostic strategies and preparing the stage for new therapies. 相似文献
17.
N. Ait-Langomazino R. Sellier G. Jouquet M. Trescinski 《Cellular and molecular life sciences : CMLS》1991,47(6):533-539
Summary A blown bitumen Mexphalte R 90/40 with a high content of saturated hydrocarbons was degraded by several microorganisms to the same extent. In batch cultures ofSaccharomycopsis lipolytica, maximal biodegradation was estimated to be about 9% w/w, 3.2·10–3 g/cm2 and 3.1·10–3 cm of degraded bitumen. The Mexphalte R 90/40 degradation rate was closely coupled to biofilm formation. The microbial activity concerned predominantly the oxidation of saturated hydrocarbons. A direct distillation bitumen 80/100 with a low content of saturated hydrocarbons and a high content of aromatic hydrocarbons and resins was more resistant to biodegradation. 相似文献
18.
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
19.
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
20.
就近几年图书馆界关于新世纪馆员意识转换及社会角色转换等方面的研究进行了综述,指出新时期的馆员要实现自我社会角色的转换,首先是要明确转换的必要性,然后采取有效的策略逐步地、系统地推进转换,才能取得较好的效果。 相似文献