全文获取类型
收费全文 | 2152篇 |
免费 | 128篇 |
国内免费 | 412篇 |
专业分类
化学 | 641篇 |
晶体学 | 22篇 |
力学 | 178篇 |
综合类 | 30篇 |
数学 | 1503篇 |
物理学 | 318篇 |
出版年
2024年 | 5篇 |
2023年 | 32篇 |
2022年 | 44篇 |
2021年 | 60篇 |
2020年 | 55篇 |
2019年 | 64篇 |
2018年 | 63篇 |
2017年 | 59篇 |
2016年 | 73篇 |
2015年 | 50篇 |
2014年 | 86篇 |
2013年 | 192篇 |
2012年 | 88篇 |
2011年 | 92篇 |
2010年 | 91篇 |
2009年 | 139篇 |
2008年 | 139篇 |
2007年 | 134篇 |
2006年 | 122篇 |
2005年 | 120篇 |
2004年 | 112篇 |
2003年 | 123篇 |
2002年 | 88篇 |
2001年 | 91篇 |
2000年 | 88篇 |
1999年 | 70篇 |
1998年 | 72篇 |
1997年 | 53篇 |
1996年 | 58篇 |
1995年 | 27篇 |
1994年 | 24篇 |
1993年 | 34篇 |
1992年 | 28篇 |
1991年 | 26篇 |
1990年 | 15篇 |
1989年 | 8篇 |
1988年 | 14篇 |
1987年 | 5篇 |
1986年 | 3篇 |
1985年 | 7篇 |
1984年 | 6篇 |
1983年 | 2篇 |
1982年 | 7篇 |
1981年 | 7篇 |
1980年 | 6篇 |
1979年 | 1篇 |
1978年 | 4篇 |
1977年 | 3篇 |
1976年 | 2篇 |
排序方式: 共有2692条查询结果,搜索用时 218 毫秒
41.
采用阳离子表面活性剂十六烷基三甲基溴化铵为模板剂,以正硅酸乙酯为硅源,以硫酸铝为铝源,水热合成了以催化油浆窄馏分为添加剂的高比表面积Al-MCM-41介孔分子筛,并利用XRD,HREM,TG-DTA及N2吸附-脱附等测试手段对合成样品进行了表征.合成的Al-MCM-41分子筛的比表面积和孔体积分别达到1295m2/g和1.5cm3/g,其平均孔径为4.3nm.重点研究了Al-MCM-41分子筛结晶度、晶胞参数、比表面积、孔体积及平均孔径等结构性质随催化油浆窄馏分添加量及其组成的变化规律,并从合成机理上进行了解释. 相似文献
42.
DEHYDROGENATION AND AROMATIZATION OF METHANE IN THE ABSENCE OF OXYGEN OVER DOPED Mo/HZSM-5 CATALYSTS
下载免费PDF全文
![点击此处可从《天然气化学杂志》网站下载免费的PDF全文](/ch/ext_images/free.gif)
研究了在Mo/HZSM-5催化剂上添加助剂以及不同的反应预处理温度对甲烷无氧脱氢芳构化反应的影响。实验结果表明,由于第二组分的添加,Mo/HZSM-5催化剂的活性和选择性都得到了较大程度的改善。预处理温度是影响催化剂反应性能的关键因素。Mo-Ru/HZSM-5催化剂经过873K空气预处理后,甲烷在973K的转化率约为10%,催化剂的稳定性也得到较大程度的提高。TPSR实验结果表明,Ru的加入降低了芳烃生成的温度。TPO和DTA实验结果表明,在Mo-Ru/HZSM-5催化剂上可生成较多的碳物种,结合反应结果,可以认为反应过程中生成的碳物种对甲烷的无氧脱氢芳构化反应是起积极作用的 相似文献
43.
Katarzyna Samson Anna Klisiska Irena Gressel Barbara Grzybowska 《Reaction Kinetics and Catalysis Letters》2002,77(2):309-315
VOx/TiO2 and MoOx/TiO2 catalysts with the addition of Re (Re/V or Mo = 0.5) were synthetized and tested in oxidative dehydrogenation of propane and in reduction by propane. XPS measurements showed depletion of the surface in Re. The Re additive does not affect the total conversion of propane, but increases the selectivity to propene. The effect is more pronounced for the MoOx/TiO2 catalyst. The increase in the selectivity to propene is accompanied with the increase in the reducibility of the catalysts. 相似文献
44.
以万古霉素为流动相添加剂的高效液相色谱法手性分析系统的建立 总被引:2,自引:0,他引:2
以丹酰化苯甘氨酸、丹酰化苯丙氨酸、丹酰化丝氨酸、丹酯化丙氨酸为模型化合物,通过对色谱柱类型、缓冲液类型、pH和盐浓度、万古霉素用量、甲醇用量、柱温等色谱参数进行考察,建立了以万古霉素为手性流动相添加剂在普通色谱柱上进行手性分析的色谱系统,并在该系统中成功地拆分了酮洛芬对映体。 相似文献
45.
Pilar Jiménez Maria Victoria Roux JiŘi Kulhánek Otto Exner 《Structural chemistry》1996,7(5-6):375-381
The energy of combustion of 2,5-dimethoxybenzoic acid has been determined using a static bomb calorimeter. The vapor pressures of the compound have been measured over a 18 K temperature interval by the Knudsen effusion technique. Heat capacity measurements betweenT=270 K andT=338 K were carried out by DSC. From these experimental results the standard molar enthalpies of combustion, sublimation, and formation in the crystalline and gaseous state at the temperature 298.15 K have been derived. With this compound, the series of mono- and dimethoxy-benzoic acids have been completed. Theirf H
m
o
values were expressed by an additive relationship, taking into account the number of methoxy groups and the number of all 1,2 interactions: an accuracy of 3.3 kJ·mol–1 was achieved. In an alternative approach the substituent effect of the methoxy groups was evaluated within the framework of isodesmic reactions. The effect of disubstitution was referred to mono derivatives and the excess energy—the so-called buttressing effect—was evaluated (2–24 kJ· mol–1 for individual bis derivatives). These values were explained in terms of the conformation of the methoxy group around the Car-O bond. 相似文献
46.
47.
The influence of spent catalyst from catalytic cracking in fluidized bed on the hydration process of cement and the properties
of cement mortars were studied. The spent catalyst was used as an additive to cement in the mortars (10 and 20% of cement).
The samples of mortars kept in water for28 days, then they were placed in sulfate and chloride media for 2 months (the control
samples were kept in water for 3 months). After this time they were subjected to bending strength and compressive strength
determinations. Thermogravimetric and infrared absorption studies were performed and capillary elevation, capability of binding
heavy metals, and changes in mass and apparent density were determined too. The studies disclosed the pozzolana nature of
spent catalyst and its influence on cement mortars being in contact with corrosive media.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
48.
Stefan Müller-Stach 《K-Theory》1995,9(4):395-406
LetX be a smooth projective variety over an algebraically closed fieldk. We repeat Bloch's construction of aG
m
-biextension (torseur)E over CH
hom
p
(X)×CH
hom
q
(X) forp+q=dim(X)+1. First we show that in characteristic zeroE comes via pullback from the Poincaré biextension over the corresponding product of intermediate Jacobians which has been conjectured by Bloch and Murre. Then the relations betweenE and various equivalence relations for algebraic cycles are studied. In particular we reprove Murre's theorem stating that Griffiths' conjecture holds for codimension 2 cycles, i.e. every 2-codimensional cycle which is algebraically and incidence equivalent to zero has torsion Abel-Jacobi invariant. 相似文献
49.
Three-dimensional systems possessing a homoclinic orbit associated to a saddle focus with eigenvalues ±i, – and giving rise to homoclinic chaos when the Shil'nikov condition < is satisfied are studied. The 2D Poincaré map and its 1D contractions capturing the essential features of the flow are given. At homoclinicity, these 1D maps are found to be piecewise linear. This property allows one to reduce the Frobenius—Perron equation to a master equation whose solution is analytically known. The probabilistic properties such as the time autocorrelation function of the state variablex are explicitly derived. 相似文献
50.
Jacek R. Jachymski 《Proceedings of the American Mathematical Society》1996,124(10):3229-3233
Let be a continuous self-map of the unit interval . Equivalent conditions are given to ensure that has a common fixed point with every continuous map that commutes with on a suitable subset of . This extends a recent result of Gerald Jungck.