全文获取类型
收费全文 | 50769篇 |
免费 | 8713篇 |
国内免费 | 7022篇 |
专业分类
化学 | 37504篇 |
晶体学 | 754篇 |
力学 | 2787篇 |
综合类 | 518篇 |
数学 | 6260篇 |
物理学 | 18681篇 |
出版年
2024年 | 139篇 |
2023年 | 980篇 |
2022年 | 1555篇 |
2021年 | 1812篇 |
2020年 | 2198篇 |
2019年 | 2125篇 |
2018年 | 1811篇 |
2017年 | 1827篇 |
2016年 | 2564篇 |
2015年 | 2529篇 |
2014年 | 3069篇 |
2013年 | 3965篇 |
2012年 | 4635篇 |
2011年 | 4850篇 |
2010年 | 3547篇 |
2009年 | 3257篇 |
2008年 | 3533篇 |
2007年 | 3180篇 |
2006年 | 2900篇 |
2005年 | 2444篇 |
2004年 | 1836篇 |
2003年 | 1520篇 |
2002年 | 1547篇 |
2001年 | 1292篇 |
2000年 | 1077篇 |
1999年 | 961篇 |
1998年 | 737篇 |
1997年 | 643篇 |
1996年 | 627篇 |
1995年 | 555篇 |
1994年 | 483篇 |
1993年 | 352篇 |
1992年 | 331篇 |
1991年 | 309篇 |
1990年 | 262篇 |
1989年 | 198篇 |
1988年 | 140篇 |
1987年 | 120篇 |
1986年 | 140篇 |
1985年 | 110篇 |
1984年 | 64篇 |
1983年 | 63篇 |
1982年 | 45篇 |
1981年 | 33篇 |
1980年 | 22篇 |
1979年 | 24篇 |
1978年 | 13篇 |
1977年 | 10篇 |
1974年 | 11篇 |
1957年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
991.
A new method for indirect determination of captopril (CPT) with NaNO3 and NH4SCN by floatation and separation of copper has been studied. In the weak acid, a small amount of Cu(II) can be reduced to Cu(I) by CPT, then Cu(I) reacted with the SCN, which can float on the surface of the liquid phase with NaNO3. A good linear relationship is observed between the floatation yield (E%) of Cu(II) and the amount of captopril. The linear range is 2?32 mg/L. On the ground, captopril can be indirectly determined by determining E(%) of Cu(II). The method is simple, rapid, reliable and has good selectivity. The developed method can be applied to indirect determination of captopril with satisfactory results. 相似文献
992.
993.
GuoHongLI YueMaoSHEN KeQinZHANG 《中国化学快报》2005,16(3):359-361
A novel saponin was isolated from the transformed products of ginsenoside Rh1 by Bacillus subtilis. It‘s structure was determined to be 3-O-β-D-glucopyranosyl-6-O-β-D-glucopyranosyl-20 (S)-protopanaxatriol on the basis of the spectral data. 相似文献
994.
995.
996.
研究应用双波长K系数—标准加入吸光光度法同时测定锰和锌二组分混合物.试验确定了锰和锌测定中K_(Mn)与K_(Zn)的回归方程.对合成试样、人发试样测定,结果满意. 相似文献
997.
A New Reaction-controlled Phase-transfer Catalyst System 总被引:1,自引:0,他引:1
MingQiangLI XiGaoJIAN GuiMeiWANG YanYU 《中国化学快报》2004,15(3):350-352
A new reaction-controlled phase-transfer catalyst system was designed and synthesized.In this system, heteropolytungstate [C7H7N(CH3)3]9PW9O34 was used for catalytic epoxidation of cyclohexene with H2O2 as the oxidant. The conversion of H2O2 was 100% and the yield of cyclohexene oxide was 87.1% based on cyclohexene. Infrared spectra showed that both fresh catalyst and the recovered catalyst do have completely same absorption peak, indicating the structure of catalyst is very stability and can be recycled. 相似文献
998.
999.
Hexagonally structured mesoporous carbons C15 and CMK-5 and cubically structured carbon C48 were synthesized using ordered silica SBA-15 and MCM-48 as templates and carbon precursors of different structures. The surfaces of these ordered carbons were chemically functionalized by employing an approach, in which the selected diazonium compounds were in situ generated and reacted with the carbon frameworks of the mesoporous carbons. The aromatic organic molecules containing chlorine, ester, and alkyl groups were covalently attached to the surface of these ordered mesoporous carbons. The presence of functional groups on the modified carbons was confirmed with Fourier transform infrared spectroscopy, thermogravimetric analysis, and nitrogen adsorption. The BET-specific surface area and the pore width of ordered carbons were significantly reduced, whereas the primary structure of these ordered carbons and their unit cells were intact. Basically, the density of grafted functional groups is related to the specific surface area of the sample, particularly the surface area of mesopores. The surface functionalization reaction takes place only on the external surface of carbon C15, while it occurs on both of the internal and external surface of CMK-5 carbon with the nanopipe structure. The presence of the micropores in CMK-5 carbon should be responsible for its lower grafting density because the small micropores are inaccessible in the reaction. It was also proposed that the preferred adsorption/reaction in C48 may be related to the observed unsymmetrical degradation of the XRD patterns for the functionalized C48 samples. The chemical modification process considerably reduced the primary mesopores in these ordered carbons by approximately 1-1.5 nm, affording carbons with micropores in the cases of C15 and C48, and mixed micropores and small mesopores in the case of CMK-5. A grafting density of approximately 0.9-1.5 micromol/m(2) was achieved under current research. 相似文献
1000.
采用水/CTAB/正丁醇/正辛烷体系微乳液法及水热技术制备了BaLiF3∶Er3+纳米微粒.利用X射线衍射(XRD)、环境扫描电镜(ESEM)和红外荧光光谱等手段对所制备产物进行了表征.X射线衍射数据表明, 所制备微粒与JCPDS 标准卡片18-715吻合很好, 利用谢乐公式计算所制备产物平均粒径在98.45 nm左右, 与环境扫描电镜观察结果基本相同.BaLiF3∶Er3+纳米微粒的红外发射图谱由4个峰构成, 最强峰位于1540 nm处, 属于Er3+的f→f跃迁. 相似文献