全文获取类型
收费全文 | 21816篇 |
免费 | 1868篇 |
国内免费 | 1399篇 |
专业分类
25083篇 |
出版年
2024年 | 52篇 |
2023年 | 236篇 |
2022年 | 577篇 |
2021年 | 852篇 |
2020年 | 625篇 |
2019年 | 784篇 |
2018年 | 836篇 |
2017年 | 627篇 |
2016年 | 932篇 |
2015年 | 1401篇 |
2014年 | 1632篇 |
2013年 | 1702篇 |
2012年 | 2010篇 |
2011年 | 1932篇 |
2010年 | 1144篇 |
2009年 | 1090篇 |
2008年 | 1261篇 |
2007年 | 1151篇 |
2006年 | 1041篇 |
2005年 | 882篇 |
2004年 | 860篇 |
2003年 | 687篇 |
2002年 | 553篇 |
2001年 | 361篇 |
2000年 | 289篇 |
1999年 | 258篇 |
1998年 | 191篇 |
1997年 | 152篇 |
1996年 | 151篇 |
1995年 | 118篇 |
1994年 | 109篇 |
1993年 | 64篇 |
1992年 | 88篇 |
1991年 | 70篇 |
1990年 | 82篇 |
1989年 | 62篇 |
1988年 | 45篇 |
1987年 | 46篇 |
1986年 | 27篇 |
1985年 | 27篇 |
1984年 | 33篇 |
1983年 | 13篇 |
1982年 | 10篇 |
1981年 | 7篇 |
1980年 | 2篇 |
1978年 | 2篇 |
1967年 | 2篇 |
1965年 | 1篇 |
1964年 | 1篇 |
1962年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
72.
用鲜花循环式动态顶空技术采集红厚壳鲜花的香气成分,TCT-GC-MS联用技术分析鉴定。4-羟基-2-丁酮,双烯酮,1、2-环氧-2-甲基丁烷,1、2-环氧-3-甲基丁烷,2-乙基戊烷,3-己醇,甲基环戊烷,2-丁醇等24个成分被检测出,占总离子流出峰面积的90.28%。并对结果进行了分析讨论。 相似文献
73.
XF Wang XT Tian E Ohkoshi B Qin YN Liu PC Wu MJ Hour HY Hung K Qian R Huang KF Bastow WP Janzen J Jin SL Morris-Natschke KH Lee L Xie 《Bioorganic & medicinal chemistry letters》2012,22(19):6224-6228
Based on a shared structural core of diarylamine in several known anticancer drugs as well as a new cytotoxic hit 6-chloro-2-(4-cyanophenyl)amino-3-nitropyridine (7), 30 diarylamines and diarylethers were designed, synthesized, and evaluated for cytotoxic activity against A549, KB, KB-vin, and DU145 human tumor cell lines (HTCL). Four new leads 11e, 12, 13a, and 13b were discovered with GI(50) values ranging from 0.33 to 3.45μM. Preliminary SAR results revealed that a diarylamine or diarylether could serve as an active structural core, meta-chloro and ortho-nitro groups on the A-ring (either pyridine or phenyl ring) were necessary and crucial for cytotoxic activity, and the para-substituents on the other phenyl ring (B-ring) were related to inhibitory selectivity for different tumor cells. In an investigation of potential biological targets of the new leads, high thoughput kinase screening discovered that new leads 11e, 12 and 13b especially inhibit Mer tyrosine kinase, a proto-oncogene associated with munerous tumor types, with IC(50) values of 2.2-3.0μM. Therefore, these findings provide a good starting point to optimize a new class of compounds as potential anticancer agents, particularly targeting Mer tyrosine kinase. 相似文献
74.
iQPR技术处理污水是一项新型尖端的技术,此技术可以成功降低污水乃至受到污染的地下水中的各种污染指标。但是,iQPR技术处理污水尤其是地下水是否存在潜在的生物安全性问题有待于进一步研究。因此,为评估iQPR技术对生物安全性的影响,本研究首先分析了三种不同iQPR法处理水的水质成分;其次系统研究了iQPR水对SD鼠在个体水平、组织水平和病理形态学损伤的研究。研究表明:iQPR处理的水质成分较对照组普通饮用水好,在个体组织水平检测未见异常,尽管其中一组iQPR处理水造成了SD鼠的脾小体增大,但是可能的原因是水处理环节存在微生物污染现象,因此,初步认定此技术未造成SD大鼠的个体损伤。本研究为揭示iQPR处理的水对生物体的安全性评价提供一个理论依据。 相似文献
75.
Different positions of males approaching towards females and male choices between webs with and without stabilimentum in a sexually cannibalistic orb‐web spider,Argiope bruennichi 下载免费PDF全文
Males of the orb‐weaving spider species Argiope bruennichi (Araneidae) are frequently victims of sexual cannibalism. Therefore, a male spider approaching a female should have strategies to avoid being killed before copulation. Our present field study detected six types of A. bruennichi male positions vis‐à‐vis the female web. In 78% of situations (39/50), only one male attached to a female. Two males attached to the same female in 11 cases. We observed no cases of three or more males on the same female web. We most commonly observed the situation of a male staying in its own web and connecting to a female's web with its silk thread (46% of cases). Of the female webs chosen by males, 68% were decorated with both an upper and lower portion of stabilimentum – a conspicuous white silk structure that reflects much more ultraviolet light than other spider silks in the web. Only 14% (7/50) of the selected webs lacked stabilimentum. Therefore, we conducted an experiment to investigate the males' choice between females' webs with and without stabilimentum. Of the 24 males used in the experiment, 10 chose webs with stabilimentum. This result did not show a strong preference of the male for stabilimentum between equally sized webs, and thus did not support an earlier suggestion that stabilimentum in A. bruennichi might function to guide males to females for mating. 相似文献
76.
77.
Shi Yang Li Wei Yu Yao Yong Jin Yuan Wen Shu Tay Nian-Lin Reena Han Rehena Sultana Pryseley N. Assam Alex Tiong-Heng Sia Ban Leong Sng 《BMC anesthesiology》2017,17(1):169
Background
The Supreme? laryngeal mask airway (SLMA) is a single-use LMA with double lumen design that allows separation of the respiratory and the alimentary tract, hence potentially reducing the gastric volume and risk of aspiration. The purpose of this prospective cohort study is to evaluate the the role of the SLMA as an airway technique for women undergoing category 2 and 3 Cesarean delivery under general anesthesia.Methods
We recruited 584 parturients who underwent category 2 or 3 Cesarean delivery under general anesthesia, in which 193 parturients underwent category 2 and 391 parturients underwent category 3 Cesarean delivery. The primary outcome was insertion success rate at 1st attempt in SLMA insertion. The secondary outcomes included anaesthetic, obstetric outcomes and maternal side effects associated with airway device.Results
The 1st attempt insertion success rate was 98.3%, while the overall insertion success rate was 100%. The mean (Standard deviation) time to effective ventilation was 15.6 (4.4) seconds. Orogastric tube insertion was successful at the 1st attempt in all parturients. There was no clinical evidence of aspiration or regurgitation. No episodes of hypoxemia, laryngospasm or bronchospasm were observed intra-operatively. The incidence of complications was low and with good maternal satisfaction reported.Conclusions
The SLMA could be an alternative effective airway in category 2 and 3 parturients emergency Cesarean Delivery under general anesthesia in a carefully-selected obstetric population.Trial registration
Clinical Trials Registration: Clinicaltrials.gov Registration NCT02026882. Registered on December 31, 2013.78.
Ouyang G Chen Z Cai XJ Song BA Bhadury PS Yang S Jin LH Xue W Hu DY Zeng S 《Bioorganic & medicinal chemistry》2008,16(22):9699-9707
Fourteen title compounds, 1-substituted-5-substitutedphenylthio-4-pyrazolaldoxime ester derivatives 4a-4n, were synthesized from the starting material 1-substitutedphenyl-3-methyl-5-substitutedphenylthio-4-pyrazolaldoximes 3 by treatment with acyl chloride. The synthesized compounds were characterized by physical constants, and the structures of the title compounds were further confirmed by IR, 1H NMR, 13C NMR and elemental analysis. The bioassay results showed that title compounds possessed weak to good anti-TMV bioactivity with 4l showing significant enhancement of disease resistance in tobacco leaves with high affinity for TMV CP. 相似文献
79.
It is commonly accepted that the climate records in Chinese loess can be correlated well with that in marine sediments. However, discrepancies for the stratigraphic position of paleomagnetic polarity reversal boundary seriously restrict an accurate teleconnection between these two archives. For example, the exact stratigraphic position of the whole Matuyama–Brunhes (MB) transitional polarity zone remains uncertain in Chinese loess. In this study, an accurate transitional zone of the MB reversal from the Mangshan profile, southeast margin of the Chinese Loess Plateau (CLP), was statistically determined using multiple subsets of parallel samples. By integrating results from the central CLP, the whole MB transitional zone is consistently located across the pedostratigraphic and climatostratigraphic transitional zones between S8 and L8 over a wide region of the CLP. This conclusion further supports that the paleosol unit S8 should be teleconnected to the marine oxygen isotope stage 19 rather than 21, and thus unambiguously supplies an accurate age control in constructing a new loess time scale. 相似文献
80.
Julie Hoover-Fong Nara Sobreira Julie Jurgens Peggy Modaff Carrie Blout Ann Moser Ok-Hwa Kim Tae-Joon Cho Sung?Yoon Cho Sang?Jin Kim Dong-Kyu Jin Hiroshi Kitoh Woong-Yang Park Hua Ling Kurt?N. Hetrick Kimberly?F. Doheny David Valle Richard?M. Pauli 《American journal of human genetics》2014,94(1):105-112