全文获取类型
收费全文 | 1203篇 |
免费 | 129篇 |
国内免费 | 101篇 |
专业分类
化学 | 811篇 |
晶体学 | 19篇 |
力学 | 58篇 |
综合类 | 8篇 |
数学 | 238篇 |
物理学 | 299篇 |
出版年
2023年 | 22篇 |
2022年 | 25篇 |
2021年 | 41篇 |
2020年 | 52篇 |
2019年 | 47篇 |
2018年 | 49篇 |
2017年 | 42篇 |
2016年 | 69篇 |
2015年 | 56篇 |
2014年 | 64篇 |
2013年 | 84篇 |
2012年 | 72篇 |
2011年 | 98篇 |
2010年 | 62篇 |
2009年 | 55篇 |
2008年 | 61篇 |
2007年 | 56篇 |
2006年 | 62篇 |
2005年 | 54篇 |
2004年 | 36篇 |
2003年 | 24篇 |
2002年 | 19篇 |
2001年 | 15篇 |
2000年 | 22篇 |
1999年 | 18篇 |
1998年 | 14篇 |
1997年 | 13篇 |
1996年 | 18篇 |
1995年 | 24篇 |
1994年 | 11篇 |
1993年 | 16篇 |
1992年 | 16篇 |
1991年 | 17篇 |
1990年 | 9篇 |
1989年 | 5篇 |
1988年 | 4篇 |
1987年 | 5篇 |
1986年 | 6篇 |
1985年 | 8篇 |
1984年 | 5篇 |
1983年 | 8篇 |
1982年 | 6篇 |
1981年 | 4篇 |
1980年 | 5篇 |
1978年 | 7篇 |
1977年 | 6篇 |
1976年 | 6篇 |
1975年 | 2篇 |
1974年 | 3篇 |
1973年 | 4篇 |
排序方式: 共有1433条查询结果,搜索用时 15 毫秒
11.
本文介绍了一种以氟气和惰性气体为原料合成 Kr F2 和 Xe F2 的热催化合成方法并对合成机理和条件作了简单的讨论 ,利用合成的 Kr F2 作为氟化剂合成了含氟高能氧化剂 NF4BF4,纯度约为 93%,其红外光谱和拉曼光谱基本与文献值吻合。元素分析结果表明产品 NF4BF4中含 B5.8%(mass) ,自由氟原子含量为 9.95%(mass) ,与理论值基本一致 ,此结果也间接证明了 Kr F2 质量的可靠性。粉末状的 Xe F2 经升华 -再结晶纯化后可得纯净晶体产品。 相似文献
12.
ZheLi DangLi WeiHuang KechangXie 《天然气化学杂志》2005,14(2):115-118
The Cu-Mo/ZSM-5 catalysts with different Cu/Mo ratios were prepared by wet impregnation method, and their catalytic performance for selective catalytic reduction of NOx was studied. The results showed that Cu-Mo/ZSM-5 is a very effective catalyst for NOx catalytic reduction with ammonia, especially when Cu/Mo molar ratio is about 1.5. It not only exhibited the extremely high catalytic activity, but also showed good stability for 02. The bulk phase structure of Cu-Mo/ZSM-5 catalysts was determined by XRD technique, and the results indicated that there is a maximum dispersion for Cu species when Cu/Mo molar ratio is 1.5, and an interaction between Cu and Mo along with HZSM-5 may be present in Cu-Mo/ZSM-5, which may possibly result in a special structure favorable for the catalytic reduction of NOx over Cu-Mo/ZSM-5 catalyst. 相似文献
13.
Rearrangement of Tertiary Allyl Alcohols Induced by Bromination: The Effect of a Fluorine Substituent on the Rate and the Outcome of the Reaction The allyl alcohol bearing a methyl and a t-butyl group at the hydroxylated position was found to undergo a rearrangement when treated with bromine (or N-bromosuccinimide) in an aqueous medium and to afford a product mixture containing two regioisomeric ketones and one oxirane. Introduction of an additional methyl group or a fluorine atom at the non-terminal olefinic center led to a more selective discrimination between potential migratory groups. As the result of an exclusive t-butyl shift only one product, a ketone, was formed in both cases. Whereas the reaction rate was only slightly affected by the additional methyl group, it was substantially decreased by the fluorine atom. 相似文献
14.
The thermal decomposition characteristics of1,7-diazido-2,4,6-trinitrazaheptane (DATH) and multi-component systems containing
DATH were studied by using DSC, TG and DTG techniques. Three –NO2 groups in the DATH molecule break away first from the main chain when DATH is heated up to 200°C. Following this process,
the azido groups and the residual molecule decompose rapidly to release a great deal of heat within a short time. In the multi-component
systems, DATH undergoes a strong interaction with the binder of the double-base propellant and a weak interaction with RDX.
The burning rates of the two propellants were determined by using a Crawford bomb. The results showed that the burning rate
rises by about 19–66% when 23.5%DATH is substituted for RDX in a minimum smoke propellant. Meanwhile, the N2 level in the combustion gases is enhanced, which is valuable for a reduction of the signal level of the solid propellant.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
15.
Simple and sensitive electrochemical method for the determination of metronidazole, based on a nanostructured film coated glassy carbon electrode (GCE), is described. Multi-walled carbon nanotubes (MWNT) was dispersed into water in the presence of a hydrophobic surfactant to give very stable and homogeneous MWNT suspension, and a MWNT-film coated GCE was achieved via evaporating solvent. Metronidazole yields a well-defined reduction peak whose potential is −0.71 V at the MWNT-film coated GCE in pH 9.0 Britton-Robinson buffer. Compared with bare GCE, the MWNT-film modified GCE significantly enhances the reduction peak current of metronidazole. All the experimental parameters were optimized for the determination of metronidazole. The detection limit is 6×10−9 mol/l at 2 min accumulation. This method has been successfully used to determine metronidazole in the drugs. Furthermore, results obtained by the proposed method have been compared with spectrophotometric method. 相似文献
16.
淫羊藿根与叶活性成分的分析和比较 总被引:8,自引:0,他引:8
采用细胞膜色谱法(CMC)筛选,并结合离体药理实验确定主要活性成分,利用高效液相色谱法分析比较了淫羊藿根与叶中的活性成分及其差异性.色谱条件为Kromasil ODS 柱(150 mm× 4.6 mm.I.D )流动相甲醇-水(70:30,V:V);检测波长270nm。筛选发现淫羊藿根中的两个有效成分YYH-214和YYH-216对血管有较强的舒张作用,表明活性成分在CMC模型体系中的保留特性与药理作用之间存在良好的相关性。在此色谱条件下淫羊藿叶未检测到这两种活性成分. 相似文献
17.
18.
Klumpp DA Rendy R Zhang Y McElrea A Gomez A Dang H 《The Journal of organic chemistry》2004,69(23):8108-8110
A variety of aminoalkynes and related heterocycles are reacted in the Bronsted superacid CF(3)SO(3)H (triflic acid), and products are obtained in generally good yields (69-99%) from Friedel-Crafts-type reactions. The reactions are consistent with the formation of novel dicationic intermediates having a vinyl cationic site and an adjacent protonated N-heterocycle or ammonium cation. 相似文献
19.
Journal of Optimization Theory and Applications - In this paper, we study a general minimization vector problem which is expressed in terms of a perturbation mapping defined on a product of locally... 相似文献
20.
We provide a direct computational proof of the known inclusion
where
is the product Hardy space defined for example by R. Fefferman and
is the classical Hardy space used, for example, by E.M. Stein. We
introduce a third space
of Hardy type and analyze the interrelations among these spaces. We give simple sufficient conditions for a given function
of two variables to be the double Fourier transform of a function in
and
respectively. In particular, we obtain a broad class of multipliers on
and
respectively. We also present analogous sufficient conditions in the case of double trigonometric series and, as a by-product,
obtain new multipliers on
and
respectively. 相似文献