全文获取类型
收费全文 | 2513篇 |
免费 | 256篇 |
国内免费 | 84篇 |
专业分类
化学 | 771篇 |
力学 | 165篇 |
综合类 | 98篇 |
数学 | 1213篇 |
物理学 | 606篇 |
出版年
2024年 | 7篇 |
2023年 | 27篇 |
2022年 | 92篇 |
2021年 | 109篇 |
2020年 | 104篇 |
2019年 | 58篇 |
2018年 | 68篇 |
2017年 | 134篇 |
2016年 | 141篇 |
2015年 | 82篇 |
2014年 | 188篇 |
2013年 | 147篇 |
2012年 | 168篇 |
2011年 | 164篇 |
2010年 | 122篇 |
2009年 | 121篇 |
2008年 | 135篇 |
2007年 | 120篇 |
2006年 | 125篇 |
2005年 | 138篇 |
2004年 | 105篇 |
2003年 | 62篇 |
2002年 | 54篇 |
2001年 | 47篇 |
2000年 | 40篇 |
1999年 | 35篇 |
1998年 | 40篇 |
1997年 | 33篇 |
1996年 | 24篇 |
1995年 | 23篇 |
1994年 | 21篇 |
1993年 | 19篇 |
1992年 | 9篇 |
1991年 | 16篇 |
1990年 | 8篇 |
1989年 | 8篇 |
1988年 | 9篇 |
1987年 | 7篇 |
1986年 | 6篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1982年 | 5篇 |
1981年 | 8篇 |
1980年 | 2篇 |
1979年 | 5篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1972年 | 1篇 |
1971年 | 1篇 |
1959年 | 1篇 |
排序方式: 共有2853条查询结果,搜索用时 296 毫秒
31.
Liu Xianlong 《数理统计与管理》1998,(6)
本文从理论上讨论了费歇判别与回归模型的等价,指出了利用费歇判别值对企业的综合经济效益进行排序是不合理的,并提出了改进的综合评价方法 相似文献
32.
Polyethers, which are produced by alcoholysis of oxiranes, are often used as solvents in chemical plants. The spectra of the products can be influenced by the choice of the catalyst (acid or base). The results of the base-catalyzed ethanolysis of 1,2-epoxybutane are displayed. The reaction was investigated calorimetrically by varying the molar ratio between ethanol and 1,2-epoxybutane, the starting temperature and the concentration of the catalyst sodium hydroxide. Additionally, the influence of the used alcohol will be shown in one example. To determine the activation parameters, the experiments were evaluated by the nonlinear software package TA-kin v3.3. 相似文献
33.
34.
中国的百岁老人研究Ⅲ.百岁老人聚居区——中国长寿之乡的成因和评定 总被引:4,自引:2,他引:4
秦俊法 《广东微量元素科学》2007,14(11):23-39
探讨了中国长寿之乡的成因,提出了评定中国长寿之乡的四项指标和六项衡量标准。主张中国长寿之乡应由有资质的学术团体经过认真调查和核实后认定,被命名的长寿之乡应是相当于县(市)或大城市区一级以上的地域,总人口应在10万人以上;该地区每百万人口中应有70位以上百岁老人;并应有反映后备力量和发展速度的指标。 相似文献
35.
介绍了用原子吸收分光光度法等测定湛江港湾海产动物的污染残毒水平,并对其残毒水平及海产品质量进行了评价。 相似文献
36.
洛阳市郊区叶菜中重金属含量抽样分析及评价 总被引:1,自引:0,他引:1
为了研究洛阳市郊区蔬菜中重金属污染状况,采用火焰原子吸收法对郊区李楼乡和孙旗屯乡抽取的叶菜样品中重金属Cu、Zn、Pb、Cd含量进行了测定。结果表明,8个蔬菜样品中Cu含量在0.393 9~0.970 7 mg/kg之间,Zn含量在1.395 0~10.080 0 mg/kg之间,均未超过国家蔬菜卫生限量标准;Pb含量在0.100 0~0.248 6 mg/kg之间,其中有3个样品Pb含量超出国家蔬菜卫生限量标准;Cd含量有5个样品未检出,其余3个样品也不超国标。经用单因子和综合因子污染指数评价,洛阳市郊区叶菜类蔬菜重金属污染大部分已处于警戒级到轻度污染,因此,应加强蔬菜中重金属污染的预防与治理工作。 相似文献
37.
Simple two-parameter Hückel and Pitzer equations were used for the calculation of the activity coefficients of aqueous hydrochloric acid at 25°C up to a molality of 2.0 mol-kg–1. The data obtained by Harned and Ehlers(12,13) from galvanic cells without liquid junction were used in the analysis and the parameters obtained were compared to those obtained from all reliable data presented in the literature for HCl at this temperature. A good agreement between the parameters was always observed. The activity coefficients obtained by the new equations were compared to those suggested by Robinson and Stokes,(8) Hamer and Wu,(1) and Pitzer and Mayorga,(9) and good agreement was also found. The data from the most important literature data sets for HCl were also predicted using the new activity coefficient equations, and the magnitude of the resulting errors was close to the precision of the measurements, the errors forming an almost random pattern for all data sets. 相似文献
38.
Summary In the preceding paper we reported on a docking study with the SYSDOC program for predicting the binding sites of huperzine A in acetylcholinesterase (AChE) [Pang, Y.-P. and Kozikowski, A.P., J. Comput.-Aided Mol. Design, 8 (1994) 669]. Here we present a prediction of the binding sites of 1-benzyl-4-[(5,6-dimethoxy-1-indanon-2-yl)methyl]piperidine (E2020) in AChE by the same method. E2020 is one of the most potent and selective reversible inhibitors of AChE, and this molecule has puzzled researchers, partly due to its flexible structure, in understanding how it binds to AChE. Based on the results of docking 1320 different conformers of E2020 into 69 different conformers of AChE and on the pharmacological data reported for E2020 and its analogs, we predict that both the R- and the S-isomer of E2020 span the whole binding cavity of AChE, with the ammonium group interacting mainly with Trp84, Phe330 and Asp72, the phenyl group interacting mainly with Trp84 and Phe330, and the indanone moiety interacting mainly with Tyr70 and Trp279. The topography of the calculated E2020 binding sites provides insights into understanding the high potency of E2020 in the inhibition of AChE and provides hints as to possible structural modifications for identifying improved AChE inhibitors as potential therapeutics for the palliative treatment of Alzheimer's disease. 相似文献
39.
40.