全文获取类型
收费全文 | 38965篇 |
免费 | 2984篇 |
国内免费 | 2904篇 |
专业分类
44853篇 |
出版年
2024年 | 97篇 |
2023年 | 514篇 |
2022年 | 1175篇 |
2021年 | 2153篇 |
2020年 | 1358篇 |
2019年 | 1714篇 |
2018年 | 1724篇 |
2017年 | 1175篇 |
2016年 | 1642篇 |
2015年 | 2398篇 |
2014年 | 2827篇 |
2013年 | 3072篇 |
2012年 | 3575篇 |
2011年 | 3164篇 |
2010年 | 1987篇 |
2009年 | 1614篇 |
2008年 | 1968篇 |
2007年 | 1718篇 |
2006年 | 1586篇 |
2005年 | 1285篇 |
2004年 | 1052篇 |
2003年 | 909篇 |
2002年 | 758篇 |
2001年 | 664篇 |
2000年 | 588篇 |
1999年 | 628篇 |
1998年 | 351篇 |
1997年 | 364篇 |
1996年 | 344篇 |
1995年 | 316篇 |
1994年 | 332篇 |
1993年 | 263篇 |
1992年 | 311篇 |
1991年 | 242篇 |
1990年 | 213篇 |
1989年 | 189篇 |
1988年 | 127篇 |
1987年 | 101篇 |
1986年 | 92篇 |
1985年 | 86篇 |
1984年 | 59篇 |
1983年 | 53篇 |
1982年 | 34篇 |
1981年 | 9篇 |
1980年 | 9篇 |
1979年 | 11篇 |
1976年 | 1篇 |
1965年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 18 毫秒
31.
Xu Yuping Wang Lu Tang Quehui Naselli-Flores Luigi Jeppesen Erik Han Bo-Ping 《Ecosystems》2023,26(4):752-767
Ecosystems - The relationship between species diversity and ecosystem functioning is one of central topics in modern ecology, but variable and controversial patterns have been found depending on... 相似文献
32.
33.
Haoran Xu Shuai Chen Xue Song Jingying Wang 《Molecular & cellular biomechanics : MCB》2023,20(1):15-22
Clinically, it is common for Class III patients with maxillary skeletal deficiency, which may result in a variety of
adverse consequences. Protraction headgear and rapid maxillary expansion (PE) is an effective treatment, but its
effect on upper airway hydrodynamics has not been reported. The main purpose of this study was to evaluate the
changes of the flow in the upper airway after PE by computational fluid dynamics (CFD). The sample includes
fifteen patients (6 males, 9 females, age 11.00 ± 1.00) and the paired T-test was used to analyze the differences
between the measured data before and after treatment. The maximum flow velocity decreased from 8.42 ±
0.16 m/s to 6.98 ± 0.36 m/s (p < 0.05), and the maximum shear force decreased from 3.72 ± 1.48 Pa to 2.13 ±
0.18 Pa. The maximum negative pressure decreased from −101.78 ± 33.60 Pa to 58.15 ± 9.16 Pa, only the changes
of velopharynx and glossopharynx were statistically significant; while the maximum resistance decreased from
140.88 ± 68.68 Pa/mL/s to 45.95 ± 22.96 Pa/mL/s. PE can effectively reduce the airflow resistance of the upper
airway and the probability of airway collapse, thus improving the patient’s ventilation function. 相似文献
34.
35.
36.
沙地云杉苗期生长与干物质生产关系的研究 总被引:21,自引:4,他引:17
本文用不同_的模型定量地研究了沙地云杉苗期的生长规律、季节动态及不同生长时期干物质在各器官之间的分配规律。1)1年生幼苗一直保持较高的生长速度,根生长尤为迅速。5年生幼苗在接近生长上限时,增长越来越慢。2)根、茎、叶干物质的生产符合理查德模型;根、茎、叶干重与全株干重之百分比表现出不同的变化趋势,反映了于物质在各器官中的分配规律。3)不同年度各器官的干重变化反映了由于自疏造成的叶的脱落和部分枝的脱落情况。4)各模型的相关系数几乎都达到了极显著的水平。 相似文献
37.
In recent years, black ginseng, a new type of processed ginseng product, has attracted the attention of scholars globally. Ginsenoside and ginseng polysaccharide, the main active substances of black ginseng, have been shown to carry curative effects for many diseases. This article focuses on the mechanism of their action in anti-inflammatory response, which is mainly divided into three aspects: activation of immune cells to exert immune regulatory response; participation in inflammatory response-related pathways and regulation of the expression level of inflammatory factors; effect on the metabolic activity of intestinal flora. This study identifies active anti-inflammatory components and an action mechanism of black ginseng showing multi-component, multi-target, and multi-channel characteristics, providing ideas and a basis for a follow-up in-depth study of its specific mechanism. 相似文献
38.
Shumin Guo Jie Wu Zhaoqiang Han Zhutao Li Pinshang Xu Shuwei Liu Jinyang Wang Jianwen Zou 《Global Change Biology Bioenergy》2023,15(4):478-493
Existing studies suggest that biochar application can reduce soil nitrous oxide (N2O) emissions, mainly based on short-term results. However, it remains unclear what the effects (i.e., legacy effects) and underlying mechanisms are on N2O emissions after many years of a single application of biochar. Here, we collected intact soil columns from plots without and with biochar application in a subtropical tea plantation 7 years ago for an incubation experiment. We used the N2O isotopocule analysis combined with ammonia oxidizer-specific inhibitors and molecular biology approaches to investigate how the legacy effect of biochar affected soil N2O emissions. Results showed that the soil in the presence of biochar had lower N2O emissions than the control albeit statistically insignificant. The legacy effect of biochar in decreasing N2O emissions may be attributed to the reduced effectiveness of the soil substrate, nitrification and denitrification activities, and the promotion of the further reduction of N2O. The legacy effect of biochar reduced the relative contribution of nitrifier denitrification/bacterial denitrification, nitrification-related N2O production, and the relative abundance of several microorganisms involved in the nitrogen cycle. Our global meta-analysis also showed that the reduction of N2O by biochar increased with increasing application rate but diminished and possibly even reversed with increasing experimental time. In conclusion, our findings suggest that the abatement capacity of biochar on soil N2O emissions may weaken over time after biochar application, but this remains under further investigation. 相似文献
39.
40.
Mu-jin Tang Shao-ling Zeng Jian-wu Chen Yong-xia Shi Wei Xu Mei-jin Yuan Yi Pang 《Insect Science》2003,10(4):221-229
A series of plasmids were constructed to examine the effects of p19 and orf1‐orf2 genes from Bacillus thuringiensis on Cyt1Aa synthesis and inclusion formation. The plasmids expressed the cyt1Aa gene along with either p19 or orf1‐orf2, or each of them coordinatively with p20 in the acrystalliferous strain of B. thuringiensis subsp. israelensis 4Q7. No effect on the expression of Cyt1Aa protein was found when P19 or Orf1‐Orf2 co‐expressed with Cyt1Aa. However, when including p20 gene, the constructs with p19 or orf1‐orf2 gene produced lower yield of Cyt1Aa proteins than without p19 or orf1‐orf2 gene. Electron microscopy observation and bioassay showed that P19 and Orf1‐Orf2 have no influence on the crystal size and toxicity of Cyt1Aa protein. It is presumed that P19 and Orf1‐Orf2 might have negative effects on Cyt1Aa synthesis in B. thuringiensis. 相似文献