全文获取类型
收费全文 | 97109篇 |
免费 | 3925篇 |
国内免费 | 3307篇 |
专业分类
化学 | 38160篇 |
晶体学 | 979篇 |
力学 | 7876篇 |
综合类 | 234篇 |
数学 | 34230篇 |
物理学 | 22862篇 |
出版年
2023年 | 461篇 |
2022年 | 700篇 |
2021年 | 800篇 |
2020年 | 891篇 |
2019年 | 869篇 |
2018年 | 10992篇 |
2017年 | 10827篇 |
2016年 | 7008篇 |
2015年 | 1835篇 |
2014年 | 1409篇 |
2013年 | 1854篇 |
2012年 | 5378篇 |
2011年 | 12237篇 |
2010年 | 6828篇 |
2009年 | 7221篇 |
2008年 | 7870篇 |
2007年 | 9807篇 |
2006年 | 1253篇 |
2005年 | 2208篇 |
2004年 | 2248篇 |
2003年 | 2570篇 |
2002年 | 1645篇 |
2001年 | 762篇 |
2000年 | 771篇 |
1999年 | 582篇 |
1998年 | 577篇 |
1997年 | 459篇 |
1996年 | 488篇 |
1995年 | 415篇 |
1994年 | 369篇 |
1993年 | 315篇 |
1992年 | 225篇 |
1991年 | 241篇 |
1990年 | 229篇 |
1989年 | 163篇 |
1988年 | 139篇 |
1987年 | 119篇 |
1986年 | 126篇 |
1985年 | 103篇 |
1984年 | 75篇 |
1983年 | 75篇 |
1982年 | 61篇 |
1981年 | 51篇 |
1980年 | 60篇 |
1979年 | 48篇 |
1914年 | 45篇 |
1913年 | 40篇 |
1912年 | 40篇 |
1909年 | 41篇 |
1908年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
151.
Qi Wang Huaxiang Yang Zhiqiang Fan Hong Xu 《Journal of polymer science. Part A, Polymer chemistry》2004,42(5):1093-1099
Ethylisobutylaluminoxane (EBAO) and its analogues were synthesized by a reaction between an triethylaluminum (Et3Al)/triisobutylaluminum (i‐Bu3Al) mixture and 4‐fluorobenzeneboronic acid, phenylboronic acid, or n‐butaneboronic acid and subsequent hydrolysis with water. They were used as cocatalysts in ethylene polymerization catalyzed by an iron complex {[(ArN?C(Me))2C5H3N]FeCl2, where Ar is 2,6‐diisopropylphenyl}. Polyethylene with a high molecular weight and a narrow molecular weight distribution was prepared with modified EBAOs, and the performance of the iron complex at high polymerization temperatures was greatly improved. The activators for the iron complex also affected the polymerization activity and the molecular weight of the resultant polyethylene. It was suggested that the stereo and electronic effects of the substitute groups of aluminoxane contributed to the improved performance of the new activators. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1093–1099, 2004 相似文献
152.
A Nafion-modified glassy carbon electrode incorporated with tobramycin for the voltammetric stripping determination of Cu2+ has been explored. The electrode was fabricated by tobramycin containing Nafion on the glassy carbon electrode surface. The modified electrode exhibited a significantly increased sensitivity and selectivity for Cu2+ compared with a bare glassy carbon electrode and the Nafion modified electrode. Cu2+ was accumulated in HAc-NaAc buffer (pH 4.6) at a potential of -0.6 V (vs. SCE) for 300 s and then determined by differential pulse anodic stripping voltammetry. The effects of various parameters, such as the mass of Nafion, the concentration of tobramycin, the pH of the medium, the accumulation potential, the accumulation time and the scan rate, were investigated. Under the optimum conditions, a linear calibration graph was obtained in the concentration range of 1.0 x 10(-9) to 5.0 x 10(-7) mol l(-1) with a correlation coefficient of 0.9971. The relative standard deviations for eight successive determinations were 4.3 and 2.9% for 1.0 x 10(-8) and 2.0 x 10(-7) mol l(-1) Cu2+, respectively. The detection limit (three times signal to noise) was 5.0 x 10(-10) mol l(-1). A study of interfering substances was also performed, and the method was applied to the direct determination of copper in water samples, and also in analytical reagent-grade salts with satisfactory results. 相似文献
153.
In the present work, a new method was established by applying solid-phase extraction (SPE) to preconcentrate and separate polymeric aluminum (Al) and using ICP-AES to determine the polymeric Al, the total monomeric Al, and the total Al in soil extracts, respectively. A modified resin was prepared with impregnated 8-hydroxyquinoline-5-sulfoxinate (HQS) on the anion-exchange resin. It has good recognition ability for Al fractions, compared to the commonly used cation ion-exchange resin, which has a better ability to adsorb cations and a weak ability to recognize detailed Al species. The optimum conditions for Al fractionation sorption, elution and separation and the interference of foreign ions were studied with the prepared resin by continuous column and batch procedures. Monomeric Al was bound to Pyrocathecol Violet (PCV) at pH 6.2, whereas the polymeric Al species did not react with PCV for at least 15 min. Because a stable complex of Al-PCV was not absorbed on the HQS modified resin, the polymeric Al could be preconcentrated on-line by the HQS-modified resin. The adsorbed polymeric Al was eluted with 3 mL of 3 mol L(-1) of HCl, and then detected by ICP-AES. The method has been applied to directly determine polymeric Al in soil extracts with high selectivity as well as a high preconcentration factor. It gives a limit of detection of 0.6 ng mL(-1) with a relative standard deviation of less than 5.7% (n = 5, 0.24 microg mL(-1) Al). 相似文献
154.
Raymond Mackay Jie Zhang Qi Wu Yuzhuo Li 《Colloids and surfaces. A, Physicochemical and engineering aspects》2004,250(1-3):343-348
The longitudinal relaxation times (T1) of water in concentrated silica and alumina slurries were measured as a function of solids content. It was shown that the results could be fit very well with a two-phase fast-exchange model between free and surface-bound water. As expected, values of T1 for bound water were in the order of 20–2000 times lower than that for free water, indicating a higher effective viscosity of the surface-bound water. The strength of the interaction depended on the particular surface, and all of the aluminas examined interacted more strongly with water than the two silicas studied, which themselves differed considerably. The chemical mechanical polishing (CMP) removal rate of tantalum by silica slurries was shown to be directly correlated with the interaction parameters, derived from the NMR relation times rather than with total surface hydroxyl group concentration. 相似文献
155.
156.
157.
158.
159.
A New Triterpene from the Orchid Pholidota yunnanensis 总被引:2,自引:0,他引:2
ZhiMingBI ZhengTaoWANG LuoShanXU 《中国化学快报》2004,15(10):1179-1181
A new triterpene, 25-methylenecyclopholidonyl p-hydroxy-trans-cinnamate, was isolated from a orchid Pholidota yunnanensis. The structure elucidation and ^1H, ^13C-NMR assignments were achieved by spectral and chemical method. 相似文献
160.
We shall answer several questions concerning additive or multiplicative commutators in a division ring which are algebraic of bounded degree over its center. 相似文献