全文获取类型
收费全文 | 831篇 |
免费 | 193篇 |
国内免费 | 19篇 |
专业分类
化学 | 1017篇 |
晶体学 | 2篇 |
综合类 | 2篇 |
物理学 | 22篇 |
出版年
2023年 | 4篇 |
2022年 | 8篇 |
2021年 | 38篇 |
2020年 | 48篇 |
2019年 | 47篇 |
2018年 | 20篇 |
2017年 | 18篇 |
2016年 | 61篇 |
2015年 | 60篇 |
2014年 | 65篇 |
2013年 | 73篇 |
2012年 | 65篇 |
2011年 | 53篇 |
2010年 | 45篇 |
2009年 | 45篇 |
2008年 | 70篇 |
2007年 | 49篇 |
2006年 | 44篇 |
2005年 | 57篇 |
2004年 | 38篇 |
2003年 | 35篇 |
2002年 | 15篇 |
2001年 | 9篇 |
2000年 | 12篇 |
1999年 | 6篇 |
1998年 | 9篇 |
1997年 | 15篇 |
1996年 | 10篇 |
1995年 | 3篇 |
1994年 | 5篇 |
1993年 | 6篇 |
1992年 | 2篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1987年 | 3篇 |
1986年 | 1篇 |
1984年 | 1篇 |
1978年 | 1篇 |
排序方式: 共有1043条查询结果,搜索用时 15 毫秒
991.
Csajbók E Bányai I Vander Elst L Muller RN Zhou W Peters JA 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(16):4799-4807
The effects of dealumination, pore size, and calcination on the efficiency (as expressed in the relaxivity) of Gd3+-loaded zeolites for potential application as magnetic resonance imaging (MRI) contrast agents were studied. Partial dealumination of zeolites NaY or NaA by treatment with (NH4)2SiF6 or diluted HCl resulted in materials that, upon loading with Gd3+, had a much higher relaxivity than the corresponding non-dealuminated materials. Analysis of the 1H NMR dispersion profiles of the various zeolites showed that this can be mainly ascribed to an increase of the amount of water inside the zeolite cavities as a result of the destruction of walls between cavities. However, the average residence time of water inside the Gd3+-loaded cavities did not change significantly, which suggests that the windows of the Gd3+-loaded cavities are not affected by the dealumination. Upon calcination, the Gd3+ ions moved to the small sodalite cavities and became less accessible for water, resulting in a decrease in relaxivity. The important role of diffusion for the relaxivity was demonstrated by a comparison of the relaxivity of Gd3+-loaded zeolite NaY and NaA samples. NaA had much lower relaxivities due to the smaller pore sizes. The transversal relaxivities of the Gd3+-doped zeolites are comparable in magnitude to the longitudinal ones at low magnetic fields (<60 MHz). However at higher fields, the transversal relaxivities steeply increased, whereas the longitudinal relaxivities decreased as field strength increased. Therefore, these materials have potential as T1 MRI contrast agents at low field, and as T2 agents at higher fields. 相似文献
992.
Coprecipitation of yttrium (Y) and rare earth elements (REEs) with phosphate and arsenate removes these elements from solution in variable proportions. During both phosphate and arsenate Coprecipitation, middle REEs (Sm and Eu) are progressively depleted in solution relative to heavier and lighter elements. Solution complexation by oxalate (Ox
2-) influences Y and REE removal patterns by strongly enhancing the retention of Y and the heaviest REEs in solution. The extent of this enhancement is well described by a quantitative account of the comparative solution complexation of Y and REEs as M(Ox)+ and M(Ox)
. The comparative behavior of phosphate and arsenate coprecipitation exhibits both similarities and differences. During arsenate coprecipitation the light REEs are retained in solution, relative to the heavy REEs, to a greater extent than is the case for phosphate coprecipitation. Notable irregularities are observed in the comparative coprecipitation behavior of nearest-neighbor elements (e.g., Eu–Gd–Tb and Tm–Yb–Lu). Such irregularities are very similar for phosphate and arsenate coprecipitation in the absence and in the presence of solution complexation. 相似文献
993.
994.
995.
Liwu Qian Dr. Jun Zhu Dr. Zhu Chen Yicai Gui Qiang Gong Dr. Yanping Yuan Dr. Jiantao Zai Dr. Xuefeng Qian Prof. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(5):1233-1240
Nearly monodisperse YVO4 architectures with persimmon-like, cube-like and nanoparticle shapes have been synthesised on a large scale by means of a complexing-agent-assisted solution route. The shape and size of these as-prepared architectures can be tuned effectively by controlling the reaction conditions, such as reaction time, the molar ratio of complexing agent/Y3+ and different complexing agents. As a typical morphology, the growth process of monodisperse nanopersimmons has been examined. To extend this method, other LnVO4 (Ln=Ce, Gd, Dy, Er) complexes with well-defined shape and dimensionality can also be achieved by adjusting different rare earth precursors. Further studies reveal that the morphology of the as-synthesised lanthanide orthovanadate is determined mainly by the interaction between rare earth ion and the complexing agent. Ultraviolet (UV) absorption and photoluminescence spectra show that the optical properties of YVO4 nanopersimmons are relevant to their size and shape. This work sheds some light on the design of well-defined complex nanostructures, and explores the potential applications of the as-synthesised architectures. 相似文献
996.
997.
Jian‐Qin Gu Dr. Ling‐Dong Sun Prof. Dr. Zheng‐Guang Yan Dr. Chun‐Hua Yan Prof. Dr. 《化学:亚洲杂志》2008,3(10):1739-1739
998.
999.
The optical properties of SrSi2AlO2N3 doped with Eu2+ and Yb2+ are investigated towards their applicability in LEDs. The Eu2+-doped material shows emission in the green, peaking around 500 nm. The emission is ascribed to the 4f65d1–4f7 transition on Eu2+. In view of the too low quantum efficiency and the considerable thermal quenching of the emission at the operation temperature of high power LED (>1W/mm2) this phosphor is only suitable for application in low power LEDs. The Yb2+ emission shows an anomalously red-shifted emission compared to Eu2+, which is characterized by a larger FWHM, a larger Stokes shift and lower thermal quenching temperature. The emission is ascribed to self-trapped exciton emission. The Yb2+ activated phosphor is found to be unsuitable for the use in any phosphor-converted LEDs. 相似文献
1000.
Xuemiao Yin Yaxing Wang Xiaoyan Li Jian Xie Mark A. Silver Lanhua Chen Daopeng Sheng Guoxun Ji Zhifang Chai Shuao Wang 《中国化学》2019,37(1):53-57
We present a new method for generating europium oxalate compounds by decomposing of N‐methyl 2‐pyrrolidone in HNO3 media. The reaction exhibits high selectivity towards Eu3+ over UO22+, which suggests that these conditions could be applied to a selective crystallization based separation system for trivalent lanthanides and hexavalent actinides. 相似文献