全文获取类型
收费全文 | 10841篇 |
免费 | 779篇 |
国内免费 | 764篇 |
专业分类
12384篇 |
出版年
2024年 | 22篇 |
2023年 | 147篇 |
2022年 | 379篇 |
2021年 | 660篇 |
2020年 | 381篇 |
2019年 | 472篇 |
2018年 | 417篇 |
2017年 | 310篇 |
2016年 | 457篇 |
2015年 | 718篇 |
2014年 | 823篇 |
2013年 | 809篇 |
2012年 | 1028篇 |
2011年 | 821篇 |
2010年 | 495篇 |
2009年 | 474篇 |
2008年 | 511篇 |
2007年 | 480篇 |
2006年 | 398篇 |
2005年 | 368篇 |
2004年 | 286篇 |
2003年 | 258篇 |
2002年 | 194篇 |
2001年 | 207篇 |
2000年 | 150篇 |
1999年 | 163篇 |
1998年 | 98篇 |
1997年 | 109篇 |
1996年 | 111篇 |
1995年 | 101篇 |
1994年 | 110篇 |
1993年 | 66篇 |
1992年 | 65篇 |
1991年 | 84篇 |
1990年 | 54篇 |
1989年 | 43篇 |
1988年 | 25篇 |
1987年 | 16篇 |
1986年 | 14篇 |
1985年 | 23篇 |
1984年 | 14篇 |
1983年 | 17篇 |
1982年 | 3篇 |
1981年 | 2篇 |
1980年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
以芦苇浆为原料,采用超声辅助硫酸水解法制备了纳米纤维素.在单因素实验的基础上,响应面法优化纳米纤维素制备工艺条件,结果表明最佳制备工艺条件为超声时间32 min,硫酸浓度52%,反应温度54℃,纳米纤维素得率最高(78.67%);通过傅里叶变换红外(FTIR)、X射线衍射和透射电子显微镜(TEM)对最佳工艺条件制备的纳米纤维素进行性能表征,分析表明最佳工艺条件制备的纳米纤维素聚集态结构为纤维素Ⅰ型,呈棒状. 相似文献
982.
Molecular characterization and mRNA transcript profile of vitellogenin in Chinese shrimp, Fenneropenaeus chinensis 总被引:1,自引:0,他引:1
A full-length cDNA encoding vitellogenin (Vg) was cloned from Chinese shrimp, Fenneropenaeus
chinensis using RACE method. The full-length cDNA consist of 7,942 nucleotides including a 7,761 bp open reading frame, which encodes
2,587 amino acid residues. The deduced amino acid sequence showed high (from 94% to 37%) identity with other known crustacean
Vgs. In addition, a consensus cleavage site (R-X-K/R-R) recognized by an endopeptidase and a member of subtilisin family of
serine protease were identified in the deduced Vg precursor. RT-PCR analysis shown that Vg mRNA can be detected in both ovary
and hepatopancreas of vitellogenic females but not in other experimental tissues including muscle, heart, lymph organ, gill,
haemocytes and intestine. These results suggest that the Vg gene may be expressed exclusively in mature females, and both
ovary and hepatopancreas are the possible tissues for Vg synthesis in F. chinensis. In addition, Vg gene is detected in genomic DNA of both females and males. 相似文献
983.
984.
Chunping Ma Jiyin He Yancheng Wu Junlang Li Jiayi Chen Yangfan Zhang Jinpeng Mo Haibo Xie Zhenguo Chi Yang Li Yongcan Jin 《Luminescence》2023,38(10):1720-1728
Multistimuli-responsive fluorescent materials have garnered great research interest benefited from their practical applications. Two twisted-structure compounds containing tetraphenylethylene (TPE) as the aggregation-induced emission (AIE) group and a pyridine unit as the acid reaction site to obtain new multistimuli-responsive fluorescent compounds (namely, TPECNPy: TPECNPy-2 and TPECNPy-3) were successfully synthesized through a one-step Knoevenagel condensation reaction. The multiple-stimuli response process of TPECNPy was investigated by means of photoluminescence (PL) spectra and emission colour. The results showed that both TPECNPy compounds with excellent AIE abilities displayed reversible emission wavelength and colour changes in response to multiple external stimuli, including grinding–fuming by CH2Cl2 or annealing and HCl-NH3 vapour fuming. More importantly, fluorescent nanofibre films were prepared by electrospinning a solution of TPECNPy mixed with cellulose acetate (CA), and these exhibited reversible acid-induced discolouration, even with only 1 wt% TPECNPy. The results of this study may inspire strategies for designing multistimuli-responsive materials and preparing fluorescent sensing nanofibre films. 相似文献
985.
玉米秸秆分批补料获得高还原糖浓度酶解液的条件优化 总被引:2,自引:1,他引:2
木质纤维素高浓度还原糖水解液的获得是纤维乙醇产业化发展的方向。在发酵工业领域,分批补料法是实现这一目标的重要研究途径。本研究采用分批补料法对获得高浓度玉米秸秆酶解还原糖的条件进行了优化。以稀硫酸预处理的玉米秸秆为原料,考察了液固比、补加量与补加时间对分批补料糖化的影响。结果表明,秸秆高浓度酶解液条件的初始物料为20% (重量/体积),木聚糖酶220 U/g (底物),纤维素酶6 FPU/g (底物),果胶酶50 U/g (底物),在24 h、48 h后分批补加8%预处理后的物料,同时添加与补料量相应的木聚糖酶20 U/g (底物),纤维素酶2 FPU/g (底物),72 h后,最终糖化结果与非补料法相比,还原糖浓度从48.5 g/L提高到138.5 g/L,原料的酶解率最终达到理论值的62.5%。试验结果表明补料法可以显著提高秸秆水解液还原糖浓度。 相似文献
986.
水稻器官干物质运转特性的因子分析 总被引:3,自引:0,他引:3
本文对33个水稻品种(组合)5个器官的干物质转运率和移动率(共10个性状)进行了因子分析,结果表明,5个器官的干物质运转特性均可自成主因子,均具有重要作用,主因子1为茎杆运转因子,主因子2为叶片运转因子,主因子3为功能叶片运转因子,主因子4为功能叶外其它叶片运转因子,主因子5为叶鞘运转因子.除主因子1具有较大的方差贡献外,其余主因子方差贡献接近.杂交F1比常规品种具有更大的主因子l得分,部分常规品种也具有较高的主因子l得分,可作为亲本加以利用. 相似文献
987.
988.
乙肝病毒PreSl片段与乙肝表面抗原羧端的融合表达 总被引:4,自引:0,他引:4
我们利用聚合酶链反应法(PCR)得到了编码乙肝病毒肝细胞受体结合位点PreSl(21 ̄47)的基因片段,并将它分别融合到S基因中相应于第175,188和223位氨基酸残基处。所得到的融合基因插入痘苗病毒表达载体pGJP-5后,在哺乳动物细胞CV-1中进行了暂时表达,对融合蛋白的表达、分泌和抗原性的研究表明,3种融合基因均能表达具有S和PreSl双重抗原性的融合蛋白,但融合位点对表达水平和分泌性质有 相似文献
989.
Wen-Ming Xie Bing-Jie Ni Guo-Ping Sheng Han-Qing Yu Min Yang 《Applied microbiology and biotechnology》2010,85(6):2001-2008
A mathematical model with a consideration of energy spilling is developed to describe the activated sludge in the presence
of different levels of metabolic uncouplers. The consumption of substrate and oxygen via energy spilling process is modeled
with a Monod term, which is dependent on substrate and inhibitor. The sensitivity of the developed model is analyzed. Three
parameters, maximum specific growth rate (μ
max), energy spilling coefficient (q
max), and sludge yield coefficient (Y
H) are estimated with experimental data of different studies. The values of μ
max, q
max, and Y
H are found to be 6.72 day-1, 5.52 day-1, and 0.60 mg COD mg-1 COD for 2, 4-dinitrophenol and 7.20 day-1, 1.58 day-1, and 0.62 mg COD mg-1 COD for 2, 4-dichlorophenol. Substrate degradation and sludge yield could be predicted with this model. The activated sludge
process in the presence of uncouplers that is described more reasonably by the new model with a consideration of energy spilling.
The effects of uncouplers on substrate consumption inhibition and excess sludge reduction in activated sludge are quantified
with this model. 相似文献
990.
Rui Bai Cheng Yuan Wenjie Sun Jianguo Zhang Yuan Luo Yanping Gao Yangyi Li Yan Gong Conghua Xie 《International journal of biological sciences》2021,17(8):1995
Abnormal expression and dysfunction of Never-in-mitosis-A-related kinase 2 (NEK2) result in tumorigenesis. High levels of NEK2 are related to malignant progression, drug resistance, and poor prognosis. However, the relationship between NEK2 levels and the occurrence of non-small cell lung cancer (NSCLC) remains unknown. This study aimed to explore the impacts of NEK2 on the oncogenesis of NSCLC and the tumor microenvironment. Downregulation of NEK2 inhibited A549 and H1299 cell proliferation, migration, and invasion, blocking cell cycle at the G0/G1 phase. Loss of NEK2 inhibited the release of IL-10 from tumor cells, M2-like polarization of macrophages, angiogenesis, and vascular endothelial cell migration. Furthermore, NEK2 deficiency inhibited tumor growth in vivo. Taken together, NEK2 knockdown inhibited the occurrence and development of NSCLC, M2 polarization of macrophages, and angiogenesis. The abnormal expression of NEK2 might not only indicate tumor progression and patient prognosis but also serve as a potential molecular therapeutic target with great development prospects. 相似文献