全文获取类型
收费全文 | 9131篇 |
免费 | 1310篇 |
国内免费 | 634篇 |
专业分类
化学 | 5229篇 |
晶体学 | 53篇 |
力学 | 1038篇 |
综合类 | 159篇 |
数学 | 1953篇 |
物理学 | 2643篇 |
出版年
2024年 | 17篇 |
2023年 | 76篇 |
2022年 | 306篇 |
2021年 | 378篇 |
2020年 | 370篇 |
2019年 | 281篇 |
2018年 | 305篇 |
2017年 | 287篇 |
2016年 | 427篇 |
2015年 | 388篇 |
2014年 | 476篇 |
2013年 | 870篇 |
2012年 | 558篇 |
2011年 | 544篇 |
2010年 | 424篇 |
2009年 | 559篇 |
2008年 | 518篇 |
2007年 | 530篇 |
2006年 | 488篇 |
2005年 | 491篇 |
2004年 | 421篇 |
2003年 | 395篇 |
2002年 | 319篇 |
2001年 | 212篇 |
2000年 | 207篇 |
1999年 | 149篇 |
1998年 | 156篇 |
1997年 | 161篇 |
1996年 | 115篇 |
1995年 | 106篇 |
1994年 | 92篇 |
1993年 | 67篇 |
1992年 | 76篇 |
1991年 | 57篇 |
1990年 | 45篇 |
1989年 | 24篇 |
1988年 | 37篇 |
1987年 | 20篇 |
1986年 | 22篇 |
1985年 | 19篇 |
1984年 | 12篇 |
1982年 | 24篇 |
1981年 | 7篇 |
1980年 | 6篇 |
1979年 | 11篇 |
1978年 | 3篇 |
1977年 | 5篇 |
1973年 | 2篇 |
1971年 | 2篇 |
1959年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
晶化温度对中孔分子筛(M41S)结构转变的影响 总被引:11,自引:6,他引:11
在低表面活性剂浓度及低表面活性剂与氧化硅比的条件下,研究了十六烷基三甲基溴化铵-硅酸钠体系中晶化温度对中孔分子筛M41S材料结构转变的影响。结果有,晶化温度提高至135℃改变了原六方晶相MCM-41的结构,晶化温度提高到150℃后,中孔结构六方MCM-41晶格转变为变不稳定的层状相, 相似文献
102.
103.
The natural products cis-p-menthane-1,7-diol (cis-IV), cis-p-menth-8-ene-1,7-diol (cis-I) and cis-p-menthane-1,7,8-triol (cis-II) are obtained starting from the corresponding cis-cyanohydrins, cis-2 and cis-7, respectively, by chemical transformation of the cyano into the hydroxymethyl group. The key step of the synthesis is the very high cis-selectivity (> or = 96 %) of the MeHNL-catalyzed HCN addition to 4-alkylcyclohexanones. From 4-isopropylcyclohexanone (1) the cyanohydrin cis-2 and from 4-(1-methylvinyl)cyclohexanone (6) the cyanohydrin cis-7 result almost quantitatively. Regioselective hydroxylation of cis-I affords the triol cis-II. X-ray crystal structure determinations of the final products confirm their cis-configuration. 相似文献
104.
105.
Atomic populations and localization [lambda(A)] and delocalization [delta(A,B)] indices (LIs and DIs) are calculated for a large set of molecules at the Hartree-Fock (HF), MP2, MP4(SDQ), CISD, and QCISD levels with the 6-311++G(2d,2p) basis set. The HF method and the conventional correlation methods [MP2, MP4(SDQ), CISD, and QCISD] yield distinct sets of LIs and DIs. Yet, within the four conventional correlation methods the differences in atomic populations and LIs and DIs are small. Relative to HF, the conventional correlation methods [MP2, MP4(SDQ), CISD, QCISD] yield virtually the same LIs and DIs for molecules with large charge separations while LIs and DIs that differ significantly from the HF values--the LIs are increased and DIs decreased--are obtained for bonds with no or small charge separations. Such is the case in the archetypal homopolar molecules HC(triple bond)CH, H2C=CH2, CH3-CH3, and "protonated cyclopropane" C(3)H(7) (+), in which case the bonding may be atypical. Relative to HF, the typical effect of the conventional correlation methods is to decrease the DI between atoms. 相似文献
106.
Simultaneous isolation and determination of esculin and rutin in natural materials using SPE and HPLC 总被引:1,自引:0,他引:1
Summary Esculin (ESC) and rutin (RUT) have been simultaneously isolated from pharmaceutical natural materials by solid phase extraction (SPE). Determination of both substances was performed by reversed phase high performance liquid chromatography (RPHPLC) with UV detection. Optimization of the separation conditions showed that simultaneous isolation and determination of rutin and esculin from pharmaceutical material was possible. The recovery obtained was not lower than 95±2%. 相似文献
107.
Cu-12.7(wt)%Al合金从900℃高温淬至365℃~500℃温度范围等温不同时间(30秒至15分钟)的试样中形成了大量树枝状的γ_2相沉淀粒子。应用配有X线能谱仪(XEDS)和电子能量损失谱仪(EELS)的分析电子显微镜(JEOL JEM-2000FX型)对上还试样研究,结果表明:这些γ_2相粒子具有富铝的“核心”,长大到粒子线度大于约800nm后以不稳定方式生长成“花朵”状。生长过程中伴随着溶质元素Al由基体向γ_2相内的富集。而且在γ_2相粒子内部呈现出Spinodal分解组织的形貌特征,表明γ_2粒子内形成了化学成分调幅。EELS分析表明试样含氧。 相似文献
108.
Bernards T.N.M. Huls B.G. van Bommel M.J. 《Journal of Sol-Gel Science and Technology》1997,10(2):193-202
This article describes the SmartKom Companion, one of three demonstrators developed within the SmartKom project whose goal
has been to provide an intuitive and intelligent interface for non-expert users to everyday computer-based applications in
different scenarios of use. Conceived for the SmartKom Mobile scenario, the Companion allows intuitive multimodal interaction
for drivers and pedestrians. The development of the SmartKom prototypes was constantly accompanied by Wizard-of-Oz experiments
as well as system evaluations at different stages of the project. Some usability evaluation results relative to the SmartKom
Companion will also be presented. 相似文献
109.
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
110.
Wei ZHANG Ni SONG Zhong Zhen LI Ying Xia LI 《中国化学快报》2006,17(3):285-288
Obyanamide 1 is a cyclic depsipeptide that was recently isolated from the marine cyanobacterium Lyngbya confervoides by Moore and co-workers1. This compound displayed a potent inhibitory effect on both KB and LoVo cells in vitro. Several structural analog… 相似文献