全文获取类型
收费全文 | 27754篇 |
免费 | 2848篇 |
国内免费 | 3156篇 |
出版年
2024年 | 17篇 |
2023年 | 437篇 |
2022年 | 540篇 |
2021年 | 1482篇 |
2020年 | 1194篇 |
2019年 | 1368篇 |
2018年 | 1358篇 |
2017年 | 960篇 |
2016年 | 1301篇 |
2015年 | 1880篇 |
2014年 | 2324篇 |
2013年 | 2367篇 |
2012年 | 2829篇 |
2011年 | 2503篇 |
2010年 | 1538篇 |
2009年 | 1501篇 |
2008年 | 1588篇 |
2007年 | 1466篇 |
2006年 | 1172篇 |
2005年 | 976篇 |
2004年 | 871篇 |
2003年 | 695篇 |
2002年 | 632篇 |
2001年 | 385篇 |
2000年 | 376篇 |
1999年 | 333篇 |
1998年 | 233篇 |
1997年 | 196篇 |
1996年 | 175篇 |
1995年 | 137篇 |
1994年 | 159篇 |
1993年 | 88篇 |
1992年 | 115篇 |
1991年 | 98篇 |
1990年 | 91篇 |
1989年 | 73篇 |
1988年 | 48篇 |
1987年 | 40篇 |
1986年 | 27篇 |
1985年 | 34篇 |
1984年 | 15篇 |
1983年 | 27篇 |
1982年 | 23篇 |
1981年 | 7篇 |
1980年 | 6篇 |
1979年 | 5篇 |
1978年 | 7篇 |
1977年 | 7篇 |
1973年 | 8篇 |
1965年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
42.
The goal of this study is to evaluate the effects of intermediate megavoltage (3-MV) photon beams on SBRT lung cancer treatments. To start with, a 3-MV virtual beam was commissioned on a commercial treatment planning system based on Monte Carlo simulations. Three optimized plans (6-MV, 3-MV and dual energy of 3- and 6-MV) were generated for 31 lung cancer patients with identical beam configuration and optimization constraints for each patient. Dosimetric metrics were evaluated and compared among the three plans. Overall, planned dose conformity was comparable among three plans for all 31 patients. For 21 thin patients with average short effective path length (< 10 cm), the 3-MV plans showed better target coverage and homogeneity with dose spillage index R50% = 4.68±0.83 and homogeneity index = 1.26±0.06, as compared to 4.95±1.01 and 1.31±0.08 in the 6-MV plans (p < 0.001). Correspondingly, the average/maximum reductions of lung volumes receiving 20 Gy (V20Gy), 5 Gy (V5Gy), and mean lung dose (MLD) were 7%/20%, 9%/30% and 5%/10%, respectively in the 3-MV plans (p < 0.05). The doses to 5% volumes of the cord, esophagus, trachea and heart were reduced by 9.0%, 10.6%, 11.4% and 7.4%, respectively (p < 0.05). For 10 thick patients, dual energy plans can bring dosimetric benefits with comparable target coverage, integral dose and reduced dose to the critical structures, as compared to the 6-MV plans. In conclusion, our study indicated that 3-MV photon beams have potential dosimetric benefits in treating lung tumors in terms of improved tumor coverage and reduced doses to the adjacent critical structures, in comparison to 6-MV photon beams. Intermediate megavoltage photon beams (< 6-MV) may be considered and added into current treatment approaches to reduce the adjacent normal tissue doses while maintaining sufficient tumor dose coverage in lung cancer radiotherapy. 相似文献
43.
This study applies two variables in the measurement of company patent deployment strategies: patent family depth and earn plan ratio. Patent family depth represents the degree to which certain fields and markets are valued by the patent owner. Earn plan ratio defined as the ratio of the number of patent forward citations to patent family size. Earn plan ratio indicates the degree to which a patent family could be cited by later innovators and competitors. This study applies a logistic regression model in the analysis LED industry data. The results demonstrate that patent value has a positive relationship with the patent family depth, and earn plan ratio. 相似文献
44.
45.
Wei Guo Qiulan Huang Yuhui Feng Taicong Tan Suhao Niu Shaoli Hou Zhigang Chen Zhi-Qiang Du Yu Shen Xu Fang 《Biotechnology and bioengineering》2020,117(8):2410-2419
Metabolic engineering of Saccharomyces cerevisiae for high-level production of aromatic chemicals has received increasing attention in recent years. Tyrosol production from glucose by S. cerevisiae is considered an environmentally sustainable and safe approach. However, the production of tyrosol and salidroside by engineered S. cerevisiae has been reported to be lower than 2 g/L to date. In this study, S. cerevisiae was engineered with a push-pull-restrain strategy to efficiently produce tyrosol and salidroside from glucose. The biosynthetic pathways of ethanol, phenylalanine, and tryptophan were restrained by disrupting PDC1, PHA2, and TRP3. Subsequently, tyrosol biosynthesis was enhanced with a metabolic pull strategy of introducing PcAAS and EcTyrAM53I/A354V. Moreover, a metabolic push strategy was implemented with the heterologous expression of phosphoketolase (Xfpk), and then erythrose 4-phosphate was synthesized simultaneously by two pathways, the Xfpk-based pathway and the pentose phosphate pathway, in S. cerevisiae. Furthermore, the heterologous expression of Xfpk alone in S. cerevisiae efficiently improved tyrosol production compared with the coexpression of Xfpk and phosphotransacetylase. Finally, the tyrosol yield increased by approximately 135-folds, compared with that of parent strain. The total amount of tyrosol and salidroside with glucose fed-batch fermentation was over 10 g/L and reached levels suitable for large-scale production. 相似文献
46.
A theoretical two-mode binding model for porphyrin binding to natural DNA is presented. One of the binding modes is assumed to be base sequence specific with binding sites n base-pairs long. The other binding mode has binding sites which consist of only one base-pair and can involve cooperativity. The model fits satisfactorily to data for H2TMPyP-4, Cu(II)TMPyP-3 and Cu(II)TMPyP-4 binding to calf thymus DNA in both a high (mu congruent to 1.0 M) and a low (mu congruent to 0.2 M) ionic strength buffer. The results show that the fraction of porphyrin bound in the non-specific mode reaches a maximum at certain input DNA to porphyrin concentrations ratios. The value of this maximum decreased, and its position shifted to higher DNA to porphyrin concentration ratios for binding in the high ionic strength buffer. The value of the cooperativity parameter obtained through the fitting process suggests that the non-specific binding is positively cooperative. The results are compared with the data analysed using other techniques. 相似文献
47.
48.
Mutations in the conserved C-terminal sequence in thyroid hormone receptor dissociate hormone-dependent activation from interference with AP-1 activity. 总被引:4,自引:2,他引:2 下载免费PDF全文
49.
50.