全文获取类型
收费全文 | 93457篇 |
免费 | 284篇 |
国内免费 | 812篇 |
出版年
2023年 | 4篇 |
2022年 | 2篇 |
2021年 | 9篇 |
2020年 | 9篇 |
2019年 | 14篇 |
2018年 | 11846篇 |
2017年 | 10677篇 |
2016年 | 7464篇 |
2015年 | 608篇 |
2014年 | 297篇 |
2013年 | 325篇 |
2012年 | 4222篇 |
2011年 | 12821篇 |
2010年 | 12010篇 |
2009年 | 8239篇 |
2008年 | 9792篇 |
2007年 | 11368篇 |
2006年 | 275篇 |
2005年 | 509篇 |
2004年 | 977篇 |
2003年 | 1023篇 |
2002年 | 780篇 |
2001年 | 258篇 |
2000年 | 164篇 |
1999年 | 31篇 |
1998年 | 9篇 |
1997年 | 24篇 |
1996年 | 10篇 |
1995年 | 2篇 |
1994年 | 7篇 |
1993年 | 30篇 |
1992年 | 20篇 |
1991年 | 38篇 |
1990年 | 8篇 |
1989年 | 10篇 |
1988年 | 20篇 |
1987年 | 14篇 |
1984年 | 7篇 |
1983年 | 20篇 |
1982年 | 2篇 |
1975年 | 6篇 |
1974年 | 2篇 |
1972年 | 246篇 |
1971年 | 275篇 |
1970年 | 5篇 |
1965年 | 13篇 |
1962年 | 24篇 |
1956年 | 5篇 |
1944年 | 12篇 |
1940年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 187 毫秒
91.
Vertical ionization energies (VIEs) of medazepam, nordazepam and their molecular subunits have been calculated using the electron
propagator method in the P3/CEP-31G* approximation. Vertical electron affinities (VEAs) have been obtained with a ∆SCF procedure
at the DFT-B3LYP/6-31+G* level of theory. Excellent correlations have been achieved between IEcalc and IEexp, allowing reliable assignment of the ionization processes. Our proposed assignment differs in many instances from that previously
reported in the literature. The electronic structure of the frontier Dyson orbitals shows that the IE and EA values of the
benzodiazepines can be modulated by substitution at the benzene rings. Hardness values, evaluated as (IE − EA)/2, follow the
trend of the experimental singlet transition energies. Medazepam is a less hard (i.e., less stable) compound than nordazepam. 相似文献
92.
Background
Plant lipoxygenases (LOXs) have been proposed to form biologically active compounds both during normal developmental stages such as germination or growth as well as during responses to environmental stress such as wounding or pathogen attack. In our previous study, we found that enzyme activity of endogenous 9-LOX in Nicotiana benthamiana was highly induced by agroinfiltration using a tobacco mosaic virus (TMV) based vector system. 相似文献93.
Background
Since the first well-known electrocardiogram (ECG) delineator based on Wavelet Transform (WT) presented by Li et al. in 1995, a significant research effort has been devoted to the exploitation of this promising method. Its ability to reliably delineate the major waveform components (mono- or bi-phasic P wave, QRS, and mono- or bi-phasic T wave) would make it a suitable candidate for efficient online processing of ambulatory ECG signals. Unfortunately, previous implementations of this method adopt non-linear operators such as root mean square (RMS) or floating point algebra, which are computationally demanding. 相似文献94.
Recent studies suggest that evergreen plants may maintain their photosynthetic capacity through the winter. Since mild winters
are predicted to be more frequent in the future, the metabolic activity of plants is also likely to increase. The aim of the
present study was to assess how various environmental factors, such as temperature, photoperiod and preceding frost, affect
the recovery of photosynthesis during a mild spell in winter. The recovery of photosynthesis was studied in a series of growth
chamber experiments where the overwintering of lingonberry (Vaccinium vitis-idaea) was interrupted by an intermittent warm spell of 1 week during different phases of winter. Rapid activation was observed
in all the experiments during the first 3–4 days. No obvious effects of the phase of winter or photoperiod on the recovery
of photosynthesis were observed, but a severe freezing treatment prior to the warm spell retarded the recovery significantly.
Once recovered, however, lingonberry was able to maintain high rates of photosynthesis even at near-freezing temperatures,
which prevail in their natural sub-nivean environment. The apparent quantum yield of photosynthesis remained high through
the winter for lingonberry. This may prove advantageous for evergreen dwarf shrubs which overwinter in dim environments under
snow. 相似文献
95.
HUMAN serum contains several protease inhibitors, one of which is identified as α1-globulin (α1-antitrypsin) and another as α2-macroglobulin. Serum also contains inhibitors against the activation of plasminogen, which may differ from the protease inhibitors. Thus, inhibition of plasminogen activation by urokinase is selectively increased during the last trimester of pregnancy1,2. Likewise, increased inhibition of streptokinase-activated human euglobulin or of a tissue plasminogen activator was noticed in some groups of patients with thrombotic disease3,4. This observation is of particular interest because the tissue activator, which is present in venous endothelium5, is believed to assist in the removal of fibrinous deposits caused by tissue injury6. 相似文献
96.
Inhibition of RNA Polymerase by Streptolydigin 总被引:23,自引:0,他引:23
GIOVANNI CASSANI RICHARD R. BURGESS HOWARD M. GOODMAN LAWRENCE GOLD 《Nature: New biology》1971,230(15):197-200
97.
98.
Javier Mar Arantzazu Arrospide José María Begiristain Isabel Larrañaga Elena Elosegui Juan Oliva-Moreno 《BMC neurology》2011,11(1):46
Background
Patients with acquired brain damage (ABD) have suffered a brain lesion that interrupts vital development in the physical, psychological and social spheres. Stroke and traumatic brain injury (TBI) are the two main causes. The objectives of this study were to estimate the incidence and prevalence of ABD in the population of the Basque Country and Navarre in 2008, to calculate the associated cost of the care required and finally to assess the loss in health-related quality of life. 相似文献99.
J Theodore Phillips Edward Fox William Grainger Dianne Tuccillo Shifang Liu Aaron Deykin 《BMC neurology》2011,11(1):126
Background
The ability to self-inject in patients with multiple sclerosis (MS) has been associated with a reduced risk of missed injections and drug discontinuation, and a beneficial effect on patients' independence. However, injection anxiety, needle phobia and disease-related disability are major barriers to a patient's ability to self-administer treatment. Use of an autoinjector may improve patients' ability to self-inject. This study evaluated the safe and effective use of Avonex Pen™ (prefilled pen), a single use autoinjector, for intramuscular delivery of interferon beta-1a (IM IFNβ-1a, Avonex) in MS patients. 相似文献100.
Although seedling herbivory is an important selective filter in many plant communities, how and why seedlings are selected
is poorly understood. Here, we examined the putative role of herbivore olfaction in dictating seedling selection. Using a
Y-tube olfactometer we compared snail (Helix aspersa) preference for pellets derived from 14-day-old macerated seedlings of nine European grassland (‘Test’) species against standard
(‘Control’) pellets derived from lettuce. Snail movement towards ‘Test’ pellets was strongly correlated with seedling acceptability
(Pearson’s r
2 = 0.86, P > 0.01) and where snails exhibited a positive choice for the ‘Test’ species, the choice was made more quickly for highly
acceptable species (r
2 = 0.86, P > 0.01). In elucidating a link between seedling acceptability and olfactory response to macerated seedlings, our study suggests
that even from an early ontogenetic stage plant selection by snails may be governed by olfactory cues. This finding highlights
the need for research on the role of plant volatiles in plant–herbivore interactions to consider more fully interactions operating
at the seedling stage. 相似文献