全文获取类型
收费全文 | 428篇 |
免费 | 26篇 |
国内免费 | 1篇 |
专业分类
化学 | 291篇 |
晶体学 | 11篇 |
力学 | 4篇 |
数学 | 67篇 |
物理学 | 82篇 |
出版年
2022年 | 3篇 |
2021年 | 11篇 |
2020年 | 9篇 |
2019年 | 14篇 |
2018年 | 4篇 |
2017年 | 7篇 |
2016年 | 18篇 |
2015年 | 18篇 |
2014年 | 17篇 |
2013年 | 22篇 |
2012年 | 18篇 |
2011年 | 22篇 |
2010年 | 20篇 |
2009年 | 21篇 |
2008年 | 34篇 |
2007年 | 14篇 |
2006年 | 20篇 |
2005年 | 12篇 |
2004年 | 13篇 |
2003年 | 11篇 |
2002年 | 15篇 |
2001年 | 7篇 |
2000年 | 8篇 |
1999年 | 7篇 |
1998年 | 3篇 |
1997年 | 7篇 |
1996年 | 6篇 |
1995年 | 5篇 |
1994年 | 8篇 |
1993年 | 8篇 |
1992年 | 6篇 |
1991年 | 8篇 |
1990年 | 5篇 |
1989年 | 4篇 |
1988年 | 6篇 |
1987年 | 7篇 |
1986年 | 9篇 |
1985年 | 6篇 |
1984年 | 4篇 |
1983年 | 3篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1975年 | 3篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1961年 | 1篇 |
1888年 | 1篇 |
1887年 | 1篇 |
排序方式: 共有455条查询结果,搜索用时 0 毫秒
451.
Ventura GT Hall GJ Nelson RK Frysinger GS Raghuraman B Pomerantz AE Mullins OC Reddy CM 《Journal of chromatography. A》2011,1218(18):2584-2592
The accurate establishment of oil similarity is a longstanding problem in petroleum geochemistry and a necessary component for resolving the architecture of an oil reservoir. Past limitations have included the excessive reliance on a relatively small number of biomarkers to characterize such complex fluids as crude oils. Here we use multiway principal components analysis (MPCA) on large numbers of specific chemical components resolved with comprehensive two-dimensional gas chromatography-flame ionization detection (GC×GC-FID) to determine the molecular relatedness of eight different maltene fractions of crude oils. MPCA works such that every compound eluting within the same first and second dimension retention time is quantitatively compared with what elutes at that same retention times within the other maltene fractions. Each maltene fraction and corresponding MPCA analysis contains upwards of 3500 quantified components. Reservoir analysis included crude oil sample pairs from around the world that were collected sequentially at depth within a single well, collected from multiple depths in the same well, and from different depths and different wells but thought to be intersected by the same permeable strata. Furthermore, three different regions of each GC×GC-FID chromatograms were analysed to evaluate the effectiveness of MPCA to resolve compositional changes related to the source of the oil generating sediments and its exposure to biological and/or physical weathering processes. Compositional and instrumental artefacts introduced during sampling and processing were also quantitatively evaluated. We demonstrate that MPCA can resolve multi-molecular differences between oil samples as well as provide insight into the overall molecular relatedness between various crude oils. 相似文献
452.
Silmar A. do Monte Elizete Ventura Tamires F. da Costa Sidney R. de Santana 《Structural chemistry》2011,22(3):497-507
Abstract
Ab initio and DFT calculations have been performed to characterize some ground state structures of the title molecules. Relative energies, rotational barriers, NBO charges, and dipole moments (μ) have been calculated and analyzed. It has been confirmed that only highly correlated methods (e.g., CCSD) are able to yield the non-planar structure as a minimum, for the H2NNO molecule. On the other hand, all computational levels here employed are able to yield a planar C2NNO frame for the (CH3)2NNO as a minimum. Important correlations between atomic charges and bond distances are discussed. Replacement of H by methyl group increases the rotational barrier and μ values by at least 3 kcal/mol and 0.4 D, respectively. The largest μ values are obtained for the structures in which the nitrogen lone pair is parallel to the NO group π system, and are consistent with a larger contribution of a dipolar resonance structure. 相似文献453.
D.C. Leitao C.T. Sousa J. Ventura J.S. Amaral F. Carpinteiro K.R. Pirota M. Vazquez J.B. Sousa J.P. Araujo 《Journal of Non》2008,354(47-51):5241-5243
In this work we use nanoporous alumina substrates as templates for the growth of Ni and Ni80Fe20 nanowires. Our membranes were obtained by a two-step anodization process of high-purity aluminum foils: first anodizations were performed at 40 V for 15 h in 0.3 M oxalic acid, at 2–6 °C; the second anodization was carried out using the same conditions for 30 min, resulting in pore lengths of ~1.3 μm. After the second anodization, the pore bottom barrier-layer was thinned, to allow the current to flow through electron tunneling. A pulsed electrodeposition method was then used to grow Ni and Ni80Fe20 nanowires. Transport characterization and isothermal magnetization measurements performed on the produced arrays are presented. 相似文献
454.
Ventura Reyes y Prósper 《Mathematische Annalen》1887,29(1):154-156
Sans résumé 相似文献
455.
A. Martínez Cutillas D. Sanz-Serrano S. Oh F. Ventura A. Martínez de Ilarduya 《Macromolecular bioscience》2023,23(7):2300066
Synthetic and functional grafts are a great alternative to conventional grafts. They can provide a physical support and the precise signaling for cells to heal damaged tissues. In this study, a novel RGD peptide end-functionalized poly(ethylene glycol)-b-poly(lactic acid)-b-poly(globalide)-b-poly(lactic acid)-b-poly(ethylene glycol) (RGD-PEG-PLA-PGl-PLA-PEG-RGD) is synthetized and used to prepare functional scaffolds. The PGl inner block is obtained by enzymatic ring-opening polymerization of globalide. The outer PLA blocks are obtained by ring-opening polymerization of both, l -lactide or a racemic mixture, initiated by the α-ω-telechelic polymacrolactone. The presence of PGl inner block enhances the toughness of PLA-based scaffolds, with an increase of the elongation at break up to 300% when the longer block of PGl is used. PLA-PGl-PLA copolymer is coupled with α-ω-telechelic PEG diacids by esterification reaction. PEGylation provides hydrophilic scaffolds as the contact angle is reduced from 114° to 74.8°. That difference improves the contact between the scaffolds and the culture media. Moreover, the scaffolds are functionalized with RGD peptides at the surface significantly enhancing the adhesion and proliferation of bone marrow-derived primary mesenchymal stem cells and MC3T3-E1 cell lines in vitro. These results place this multifunctional polymer as a great candidate for the preparation of temporary grafts. 相似文献