全文获取类型
收费全文 | 65232篇 |
免费 | 9497篇 |
国内免费 | 6396篇 |
专业分类
化学 | 16008篇 |
晶体学 | 1166篇 |
力学 | 11769篇 |
综合类 | 1110篇 |
数学 | 23783篇 |
物理学 | 27289篇 |
出版年
2024年 | 117篇 |
2023年 | 638篇 |
2022年 | 1213篇 |
2021年 | 1454篇 |
2020年 | 1772篇 |
2019年 | 1564篇 |
2018年 | 1584篇 |
2017年 | 2138篇 |
2016年 | 2482篇 |
2015年 | 1927篇 |
2014年 | 3284篇 |
2013年 | 4578篇 |
2012年 | 3849篇 |
2011年 | 4494篇 |
2010年 | 3990篇 |
2009年 | 4480篇 |
2008年 | 4467篇 |
2007年 | 4312篇 |
2006年 | 4006篇 |
2005年 | 3558篇 |
2004年 | 3174篇 |
2003年 | 2843篇 |
2002年 | 2580篇 |
2001年 | 2151篇 |
2000年 | 1991篇 |
1999年 | 1817篇 |
1998年 | 1642篇 |
1997年 | 1365篇 |
1996年 | 1193篇 |
1995年 | 941篇 |
1994年 | 838篇 |
1993年 | 683篇 |
1992年 | 680篇 |
1991年 | 511篇 |
1990年 | 445篇 |
1989年 | 382篇 |
1988年 | 303篇 |
1987年 | 243篇 |
1986年 | 176篇 |
1985年 | 209篇 |
1984年 | 184篇 |
1983年 | 98篇 |
1982年 | 153篇 |
1981年 | 107篇 |
1980年 | 81篇 |
1979年 | 113篇 |
1978年 | 78篇 |
1977年 | 72篇 |
1976年 | 41篇 |
1974年 | 31篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Slow pyrolysis of walnut shell which is a cheap and abundantly available solid waste was carried out using thermogravimetric
analysis. The effects of raw material heating rate on the pyrolysis properties and kinetic parameters were investigated. A
two-step consecutive reaction model were used to simulate the pyrolysis process. The kinetic parameters were established by
using the pattern search method. Comparison between experimental data and the model prediction indicated that the two-step
consecutive reaction model can better describe the slow pyrolysis of walnut shell as the formation of an intermediate during
the pyrolysis process was taken into account. 相似文献
992.
993.
994.
在Na_2O-1.6-C_6H_(14)N_2-SiO_2-TiO_2-H_2O体系中首次用快速动态法和静态法合成了TiZSM-48型分子筛,考察了两种合成方法的晶化动力学。利用XRD、IR、XPS、SEM、DRS、TG-DTA等测试手段对TiZSM-48型分子筛进行了表征,结果表明,Ti进入了分子筛骨架,动态法合成比静态法合成快得多,且样品的晶粒较小。热分析表明,两种方法合成的样品热稳定性均较好。 相似文献
995.
目视催化褪色计时法测定超痕量钒的研究 总被引:6,自引:1,他引:6
本文基于在酸性介质中钒(V)催化溴酸钾氧化酸性铬蓝K褪色的新指示反应,建立了一个催化褪色计时法测定超痕量钒的新方法。在体系中引入活化剂抗坏血酸,灵敏度达2×10~(-12)ng/ml,测定范围在0.0l~3.0ng/ml,同时探讨了反应机理。确立了反应速率方程,方法简便经济,已用于天然水中痕量钒的测定。 相似文献
996.
997.
The effects of radiofrequency (RF) (1–4) and magnetic fields (5–9) on the behavior of aqueous solutions and suspensions have been a popular subject in recent years. The mechanism of the magnetic “water memory” effect, though, is still largely unknown (5). In this work, we present evidence that the primary “receptor” of the electromagnetic radiation is a gas/liquid interface. Gas can be either already present in water or produced by the effects of electromagnetic fields. Perturbed gas/liquid interfaces require hours to equilibrate. Certain RF and magnetic signals also produce reactive oxygen and hydrogen species (superoxide, hydrogen peroxide, hydrogen, atomic hydrogen). The perturbed gas/liquid interface modifies the hydrogen bonding networks in water and also the hydration of ions and interfaces. Careful outgassing removes all of the effects of the electromagnetic fields, including the magnetic memory effect. The amplitude of the applied field influences the observed effects. Different amplitudes of RF radiation perturb the interfacial water in different ways and consequently affect the behavior of colloids and ions in specific manners. For instance, the bulk and template precipitation of calcium carbonate, zeta potentials of suspended colloids, rate of dissolution of colloidal silica, and attachment of colloidal silica to metal surfaces are modified in specific ways with the low amplitude or high amplitude RF treatments described in this paper. The solubility/diffusivity of gas species is also modified in a different manner, and it is probably at the core of the specificity of the RF amplitude effects. 相似文献
998.
配合物形成的三维超分子体系—[Ni(C_6H_4O_2N)_2(H_2O)_4]的水热法合成、晶体结构及热分析 总被引:7,自引:0,他引:7
采用水热法用Ni(NO3)26H2O和异烟酸制备出了一种新的由配合物形成的三维超分子体系—[Ni(C6H4O2N)2(H2O)4],并通过X射线衍射对其晶体结构进行了测定。 该晶体属三斜晶系,空间群为Pī, 所得晶胞参数为: a = 6.9228(4), b = 9.6664(19),c = 6.322(1) , a = 96.86(3), b = 113.33(3), g = 110.35(3)°, V = 347.6(1) 3, Z = 1, Mr = 374.98, Dc = 1.791 g/cm3, F(000) = 194, m = 1.443 mm-1。用1362个可观察的 (I > 2s(I))衍射点,修正123个结构参数, 最终偏离因子R = 0.0444,wR = 0.1271。在组成该化合物的基本结构单元[Ni(C6H4O2N)2(H2O)4]中,Ni处于1个稍微拉长的八面体的中心; 各个结构单元之间通过氢键OH…O相互连接,形成了无限伸展的具有层状结构的三维超分子体系。 另外,从差热及热重曲线可以看出,该化合物加热到154 ℃时开始分解, 首先失去4个H2O,再失去2个异烟酸根,最后残余物为NiO。 相似文献
999.
Björn O. Roos Valera Veryazov Per-Olof Widmark 《Theoretical chemistry accounts》2004,111(2-6):345-351
New basis sets of the atomic natural orbital (ANO) type have been developed for the atoms Li–Fr and Be–Ra. The ANOs have been obtained from the average density matrix of the ground states and the lowest excited states of the atom, the positive ion, and the dimer at its equilibirium geometry. Scalar realtivisitc effects are included through the use of a Douglas–Kroll Hamiltonian. Multiconfigurational wave functions have been used with dynamic correlation included using second-order perturbation theory (CASSCF/CASPT2). The basis sets are applied in calculations of the ground-state potentials for the dimers. Computed bond energies are accurate to within 0.05 eV for the alkaline dimers and 0.02 eV for the alkaline-earth dimers (except for Be2).Acknowledgments.ensp;B.O.R. would like to express his gratitude to Prof. Jacopo Tomasi for all the inspiration that his scientific work has given him through the years and continues to do in particular through the work on solvent effects on molecular properties. This work has been supported by a grant from the Swedish Science Research Council, VR.Contribution to the Jacopo Tomasi Honorary Issue 相似文献
1000.