全文获取类型
收费全文 | 109307篇 |
免费 | 5852篇 |
国内免费 | 15498篇 |
专业分类
化学 | 92955篇 |
晶体学 | 1375篇 |
力学 | 2387篇 |
综合类 | 1036篇 |
数学 | 12705篇 |
物理学 | 20199篇 |
出版年
2024年 | 107篇 |
2023年 | 827篇 |
2022年 | 2076篇 |
2021年 | 2390篇 |
2020年 | 2767篇 |
2019年 | 2745篇 |
2018年 | 2354篇 |
2017年 | 3292篇 |
2016年 | 3549篇 |
2015年 | 3050篇 |
2014年 | 4099篇 |
2013年 | 8320篇 |
2012年 | 7504篇 |
2011年 | 6021篇 |
2010年 | 5124篇 |
2009年 | 6889篇 |
2008年 | 7110篇 |
2007年 | 7470篇 |
2006年 | 6805篇 |
2005年 | 5875篇 |
2004年 | 5452篇 |
2003年 | 4531篇 |
2002年 | 5566篇 |
2001年 | 3369篇 |
2000年 | 3140篇 |
1999年 | 2864篇 |
1998年 | 2484篇 |
1997年 | 2036篇 |
1996年 | 1765篇 |
1995年 | 1661篇 |
1994年 | 1475篇 |
1993年 | 1211篇 |
1992年 | 1182篇 |
1991年 | 802篇 |
1990年 | 674篇 |
1989年 | 662篇 |
1988年 | 476篇 |
1987年 | 360篇 |
1986年 | 328篇 |
1985年 | 265篇 |
1984年 | 279篇 |
1983年 | 159篇 |
1982年 | 245篇 |
1981年 | 199篇 |
1980年 | 213篇 |
1979年 | 201篇 |
1978年 | 188篇 |
1977年 | 126篇 |
1976年 | 111篇 |
1973年 | 71篇 |
排序方式: 共有10000条查询结果,搜索用时 125 毫秒
271.
提出了一种新的高功率宽带放大器,它由速调管和自由电子脉塞组成,用永久周期磁铁聚焦。研究了这种器件的电子束聚焦、群聚和辐射。在速调管提供一阶调制系数为30%,波导管的半径1.51cm,摇摆器的磁场振幅0.16T,周期7.92cm,电子束的电压490kV,电流50A和半径0.368cm的条件下,预估这种器件的辐射波频率为11.4GHz,输出辐射功率为18MW,输出辐射带宽为44%。 相似文献
272.
室温或近室温固相反应要求绿色化、清洁化[1,2]。我们以KMnO4和MnCl2·4H2O为原料,用室温固相氧化还原反应制备氧化锰粉体时,得到了一种对H2O2分解具有较高催化活性的纳米KMn8O16粉体,用XRD、SEM、IR等技术对其进行了表征,发现研磨时间对粉体性能有显著影响。1 实验部分1 1 粉体的制备按摩尔比(2∶3)准确称取一定量的分析纯KMnO4和MnCl2·4H2O,分别置于玛瑙研钵中充分研细,再混合研磨,固相反应立即发生,体系颜色逐渐加深,并有刺激性气体产生,充分研磨后70℃恒温12h,固相产物依次经水洗至中性、醇洗、抽滤,真空干燥得黑色粉… 相似文献
273.
YAN Xiaomin ZHAO Lin SHAO Jianhui TAN Xin SONG Zhengxiao School of Chemical Engineering Technology Tianjin University Tianjin China Slichem Liquid Material Co. Ltd. Shijiazhuang China 《Chinese Journal of Reactive Polymers》2004,(Z1)
1. INTRODUCTION Methionine, namely 2-amido-4-thiomethyl butyric acid with a structure of CH3SCH2CH2CHCOOH, is one of the essential amino acids and has two natural enantionmers, D and L-methionine. The mixture of L- and D-isomers can be used as analeptics or nutritive additives to maintain the equilibrium of amino acids of feed [1,2]. L-methionine can release active methyl and accelerate the synthesis of choline, which further speeds up the conversion of the lipid accumulated in liv… 相似文献
274.
Introduction In the previous studies on the oxidation reaction,peroxydisulfate was widely used as an oxidizingagent.1-5 One of the advantages of this oxidant lies in itsstability in a wide range of pH values. The reaction be-tween Fe(CN)5L3- (L=N-aromatic heterocyclic li-gands) and S2O8 2- has been proved to proceed throughan outer-sphere electron transfer mechanism.5 For a re-action [(Eq. (1)] to be under an outer-sphere mechanismthe steps involved are the formation of a reactant … 相似文献
275.
The polyacrylic acid functionalized polystyrene nanospheres were synthesized and self-assembled into irregular, densely packed monolayers in non-aqueous media. The polymer nanoparticles were chemically adhered to substrates. The morphologies of the resulting films were investigated. The impact of the volume fraction of alcohol in the mixed solvents on the particle adsorption and fabrication of nanosphere assembled films was examined. 相似文献
276.
JiangMinCHEN LuLingWU XianHUANG 《中国化学快报》2004,15(2):143-144
2,5-Disubstituted oxazoles were prepared conveniently by treatment of aromatic α-methyl ketones and nitriles with poly[styrene(iodosodiacetate)] in one-pot process. 相似文献
277.
Direct Observation of Non-covalent Complexes for Phosphorylated Flavonoid-protein Interaction by ESI
XiaoLanCHEN TingZHANG HongXiaLIU LingBoQU YouZhuYU YuFenZHAO 《中国化学快报》2004,15(3):343-346
Diethyl flavon-7-yl phosphate was synthesized by modified Atheron-Todd reaction. The result of ESI shows that the phosphated flavonoids possess stronger binding affinities toward proteins such as myoglobin, insulin and lysozyme and are easier to form the non-covalent complexes with them. 相似文献
278.
The First Intramolecular Charge Transfer Transition Based on 2-Ureido-4[1H]-pyrimidinone Binding Module 总被引:1,自引:0,他引:1
The first intramolecular charge transfer transition based on 2-ureido-4[1H]-pyrimidinone binding module was reported. 相似文献
279.
Solid Phase Synthesis of 2-Substituted 1,3-Oxazin-6-ones Using Resin-bound Cyclic Malonic Acid Ester
A facile solid phase synthesis of 2-substituted 1,3-oxazin-6-ones using polymer-supported Meldrum‘s acid has been reported. Reaction of the resin-bound cyclic malonic acid ester with triethyl orthoformate and subsequent double substitution with amide, afforded the corresponding polymer-supported acylaminomethylene cyclic malonic acid ester, which upon thermal treatment led to 1, 3-oxazin-6-ones in good yields and with high purity. 相似文献
280.
Kinetic parameters of amino acid cations in an MK-40 ion-exchange membrane are calculated from the conductivity data. A theoretical quantum-chemical analysis of experimental activation energies for conduction suggests a mechanism of elementary act of transport of amino acid cations in the membrane. 相似文献