全文获取类型
收费全文 | 2949篇 |
免费 | 292篇 |
国内免费 | 109篇 |
专业分类
化学 | 965篇 |
晶体学 | 1篇 |
力学 | 175篇 |
综合类 | 121篇 |
数学 | 1273篇 |
物理学 | 815篇 |
出版年
2024年 | 10篇 |
2023年 | 31篇 |
2022年 | 132篇 |
2021年 | 135篇 |
2020年 | 92篇 |
2019年 | 75篇 |
2018年 | 86篇 |
2017年 | 148篇 |
2016年 | 159篇 |
2015年 | 100篇 |
2014年 | 212篇 |
2013年 | 166篇 |
2012年 | 187篇 |
2011年 | 185篇 |
2010年 | 134篇 |
2009年 | 153篇 |
2008年 | 160篇 |
2007年 | 162篇 |
2006年 | 159篇 |
2005年 | 163篇 |
2004年 | 124篇 |
2003年 | 78篇 |
2002年 | 64篇 |
2001年 | 53篇 |
2000年 | 49篇 |
1999年 | 41篇 |
1998年 | 47篇 |
1997年 | 36篇 |
1996年 | 27篇 |
1995年 | 25篇 |
1994年 | 23篇 |
1993年 | 21篇 |
1992年 | 10篇 |
1991年 | 16篇 |
1990年 | 8篇 |
1989年 | 8篇 |
1988年 | 12篇 |
1987年 | 8篇 |
1986年 | 8篇 |
1985年 | 6篇 |
1984年 | 5篇 |
1982年 | 8篇 |
1981年 | 8篇 |
1980年 | 3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3350条查询结果,搜索用时 31 毫秒
991.
992.
993.
Bhavna A. Shah Alpesh V. Patel Maryam I. Bagia Olutayo A. Oluyinka 《Journal of Dispersion Science and Technology》2013,34(12):1827-1841
AbstractEDCC-MCM-41, a novel ethylenediamine derivative of MCM-41 was synthesized from a sugar industry waste, bagasse fly ash. Studies were carried out on its ability to remove Cr(VI) from aqueous solution. Its performance was also compared with that of unmodified MCM-41 and an already known derivative NH2-MCM-41 (both also synthesized from bagasse fly ash) at the same experimental conditions. Results show that the novel adsorbent produced a better sorption performance (adsorption capacity of 49.04?mg g?1) than MCM-41 and NH2-MCM-41 which produced adsorption capacities of 13.78?mg g?1 and 25.29?mg g?1 respectively. Thermodynamics, kinetics and isotherm studies confirm that the sorption process involving the novel adsorbent is favorable for Cr(VI) and can best be represented by pseudo-second-order kinetics model. The sorption process was also found to be multistep, involving both film diffusion and intraparticle diffusion into the pores on the adsorbent’s surface.Abbreviations: BFA: Bagasse Fly Ash; MCM-41: Mobil Composition of matter no. 41; NH2-MCM-41: mine functionalized MCM-41; EDCC-MCM-41: Ethylenediamine grafted MCM-41; qe: Mass of material adsorbed at equilibrium per mass of adsorbent; qt: Mass of material adsorbed at any time t per mass of adsorbent; C0: The initial concentration; Ce: The residual concentration at equilibrium; Ct: Residual concentration at any time t; W: The weight of adsorbents; rRMSD: Root mean square deviation; R: Gas constant 相似文献
994.
2-(2-Cyano-acetylamino)-4,5,6,7-tetrahydro-benzo[b]thiophene-3-carboxamide(3) was used as starting material for synthesis of 4-thiazolidinone, thiazolidine, and thiophene derivatives 6, 7a, b, and 8a, b, respectively. Thiocarbomyl derivative 5, 4-thizolidinone 9, and thioxothiazolidine 10 were obtained from reaction of 3 with thioglycolic acid and phenyl isothiocyanate/sulfur, respectively. Condensation of 3 with selected cyclic ketones and aromatic aldehydes yielded the arylidine derivatives 11a, b and 13, respectively. Refluxing of 11a, b with sulfur and morpholine yielded the thiophene derivatives 12a, b, respectively. Diazocoupling of compound 3 withp-tolyl diazonium chloride yielded the hydrazone derivative 14. The newly synthesized compounds were characterized by infrared, 1H NMR, and mass spectral studies. Representative compounds of the synthesized product were tested and evaluated as antimicrobial agents. Compound 12b gives very high antimicrobial activity against Ampicillin. 相似文献
995.
996.
Kevin Buckley Jemma G. Kerns Jacqueline Vinton Panagiotis D. Gikas Christian Smith Anthony W. Parker Pavel Matousek Allen E. Goodship 《Journal of Raman spectroscopy : JRS》2015,46(7):610-618
Fragility fractures, those fractures which result from low level trauma, have a large and growing socio‐economic cost in countries with aging populations. Bone‐density‐based assessment techniques are vital for identifying populations that are at higher risk of fracture, but do not have high sensitivity when it comes to identifying individuals who will go on to have their first fragility fracture. We are developing Spatially Offset Raman Spectroscopy (SORS) as a tool for retrieving chemical information from bone non‐invasively in vivo. Unlike X‐ray‐based techniques SORS can retrieve chemical information from both the mineral and protein phases of the bone. This may enable better discrimination between those who will or will not go on to have a fragility fracture because both phases contribute to bone's mechanical properties. In this study we analyse excised bone with Raman spectroscopy and multivariate analysis, and then attempt to look for similar Raman signals in vivo using SORS. We show in the excised work that on average, bone fragments from the necks of fractured femora are more mineralised (by 5–10%) than (cadaveric) non‐fractured controls, but the mineralisation distributions of the two cohorts are largely overlapped. In our in vivo measurements, we observe similar, but as yet statistically underpowered, differences. After the SORS data (the first SORS measurements reported of healthy and diseased human cohorts), we identify methodological developments which will be used to improve the statistical significance of future experiments and may eventually lead to more sensitive prediction of fragility fractures. © 2015 The Authors. Journal of Raman Spectroscopy Published by John Wiley & Sons, Ltd. 相似文献
997.
Susana S. Ramos Sandra S. Almeida Paula M. Leite Renato E.F. Boto Samuel Silvestre Paulo Almeida 《Tetrahedron》2014
Several novel atropisomeric N-alkyl-N-[(2-alkylchalcogen)phenyl)]acetamides have been synthesized and fully characterized by 1H and 13C NMR, FTIR, and HRMS (FAB). The barriers of the restricted rotation about the N-aryl bond between the two atropisomeric forms were measured by accurate lineshape simulation of variable temperature NMR (VT-NMR) spectra obtained in DMSO-d6 solution and ranged from 17.0 to 20.5 kcal/mol. The relationship between the structure of the different acetanilide moieties and both coalescence temperature and energy of rotation are herein discussed. Taking in mind the acetamide structural resemblances with nimesulide related compounds known by their anticancer activity, the in vitro cytotoxicity of 20 representative acetanilides, against human breast (MCF-7) and prostate (LNCaP) cancer cell lines as well as normal human dermal fibroblasts (NHDF) was also preliminary evaluated. Interestingly a selective antiproliferative activity was observed for cancerous cells with prominence to LNCaP within the most potent O- and/or N-benzylic and -hexyl acetanilides. 相似文献
998.
999.
《Operations Research Letters》2021,49(5):708-714
We extend the approach of Koole et al. (2012) [15] and Legros et al. (2018) [20] for the G/M/1 queue. The idea is to provide a Markovian approximation where a state represents the oldest customer's wait. This modeling is made possible by creating states with negative wait, representing an estimate of the time at which a new customer would arrive when the system is empty. We apply this method for performance evaluation and routing optimization. Finally, we further extend the model to the G/M/1+G queue. 相似文献
1000.
针对电子商务网站中涌现的大量在线评价信息,本文提出一种基于在线评价信息的概率语言多属性变权决策方法,指导消费者的购买决策。在本文方法中,首先利用概率语言描述在线评价信息,构建概率语言决策矩阵,并重新定义了区间概率语言的得分函数;其次,利用变权方法处理决策矩阵,得出不同方案的各属性权重;再次,依据得出的属性权重,基于后悔理论,考虑决策者风险规避系数,求出各方案的综合感知效用值,并据此排序。最后,利用汽车之家网站提供的顾客在线评价信息,以一个汽车选择的实例说明了本文所提方法的合理性与实用性。 相似文献