全文获取类型
收费全文 | 5415篇 |
免费 | 391篇 |
国内免费 | 1186篇 |
专业分类
化学 | 5564篇 |
晶体学 | 86篇 |
力学 | 68篇 |
综合类 | 96篇 |
数学 | 32篇 |
物理学 | 1146篇 |
出版年
2024年 | 16篇 |
2023年 | 49篇 |
2022年 | 102篇 |
2021年 | 89篇 |
2020年 | 135篇 |
2019年 | 101篇 |
2018年 | 79篇 |
2017年 | 117篇 |
2016年 | 113篇 |
2015年 | 161篇 |
2014年 | 175篇 |
2013年 | 323篇 |
2012年 | 303篇 |
2011年 | 326篇 |
2010年 | 235篇 |
2009年 | 249篇 |
2008年 | 228篇 |
2007年 | 330篇 |
2006年 | 364篇 |
2005年 | 373篇 |
2004年 | 361篇 |
2003年 | 352篇 |
2002年 | 391篇 |
2001年 | 319篇 |
2000年 | 236篇 |
1999年 | 146篇 |
1998年 | 139篇 |
1997年 | 149篇 |
1996年 | 123篇 |
1995年 | 151篇 |
1994年 | 124篇 |
1993年 | 110篇 |
1992年 | 134篇 |
1991年 | 91篇 |
1990年 | 88篇 |
1989年 | 60篇 |
1988年 | 50篇 |
1987年 | 20篇 |
1986年 | 9篇 |
1985年 | 16篇 |
1984年 | 6篇 |
1983年 | 3篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1980年 | 7篇 |
1979年 | 6篇 |
1978年 | 7篇 |
1975年 | 2篇 |
1973年 | 6篇 |
1972年 | 2篇 |
排序方式: 共有6992条查询结果,搜索用时 15 毫秒
111.
Summary Rare earth elements 1,3,5-benzenetricarboxylates were prepared as solids of the general formula Ln(C9H3O6)·nH2O, where n=6 for La-Dy and n=4 for Ho-Lu,Y. Their solubilities in water at 293 K are of the order 10-4-10-6 mol dm-3. The IR spectra of the complexes indicate that the carboxylate groups are bridging and bidentate chelates. Hydrated 1,3,5-benzenetricarboxylates
lose water molecules during heating in one step (La-Tb), two steps (Y, Ho-Tm) or three steps (Dy, Yb, Lu). The anhydrous complexes
are stable up to 573-742 K and decompose to oxides (Ce-Lu) at higher temperatures. 相似文献
112.
A variety of single component rare earth aryloxides substituted by various alkyl groups [Ln(OAr)3] such as methyl, isopropyl, tert‐butyl have been surveyed in the ring‐opening copolymerization of 2,2‐dimethyltrimethylene carbonate (DTC) and ε‐caprolactone (ε‐CL). It was worthwhile to note that activity of the catalyst varied with both the ligands' structure and the number of alkyl groups on phenyl ring. The stronger ability of electron‐donation of alkyl groups on phenyl ring, and the more numbers of alkyl groups on phenyl ring, the higher catalytic activity. The experimental results show that lanthanum tris(2,4,6‐tri‐tert‐butylphenolate) [La(OTTBP)3] exhibits highest activity in all lanthanum aryloxides. 1H NMR spectral data of copolymer obtained showed that the polymerization mechanism is in agreement with the coordination insertion mechanism. 相似文献
113.
The 4f-4f emissions from lanthanide trication (Ln3+) complexes are widely used in bioimaging probes. The emission intensity from Ln3+ depends on the surroundings, and thus, the design of appropriate photo-antenna ligands is indispensable. In this study, we focus on two probes for detecting hydrogen peroxide, for which emission intensities from Tb3+ are enhanced chemo-selectively by the H2O2-mediated oxidation of ligands. To understand the mechanism, the Gibbs free energy profiles of the ground and excited states related to emission and quenching are computed by combining our approximation—called the energy shift method—and density functional theory. The different emission intensities are mainly attributed to different activation barriers for excitation energy transfer from the ligand-centered triplet (T1) to the Tb3+-centered excited state. Additionally, quenching from T1 to the ground state via intersystem crossing was inhibited by intramolecular hydrogen bonds only in the highly emissive Tb3+ complexes. © 2018 Wiley Periodicals, Inc. 相似文献
114.
利用X射线衍射仪、扫描电镜并配合能谱分析研究了Mg-6Zn-3Y合金4 GPa超高压凝固组织.结果表明:在4 GPa超高压条件下凝固时,实验合金的凝固组织得到显著细化,基体α-Mg相的晶格间距有所减小;Zn在基体α-Mg中溶解度大幅提高,Y不溶于基体α-Mg;实验合金凝固组织中出现呈对称性的四瓣或六瓣花的初晶Y固溶体,该初晶花瓣形貌完整、尺寸较大,显示其充分自由生长;先发生的共晶反应的共晶组织为成“簇”分布的共晶团,共晶相呈点状或棒状;后发生的共晶反应的组织为离异共晶.超高压凝固显著地改变了实验合金的凝固过程. 相似文献
115.
NaYF4:Eu/Sr nanocrystals were synthesized by a hydrothermal method. Tunable photoluminescence of the NaYF4:Eu nanocrystals was successfully achieved by codoping with Sr2+ ions. With increasing Sr2+ concentration, not only the X-ray diffraction peaks of the nanocrystals become broader, but also the positions of them shift toward larger lattice parameters. Eu3+ and Eu2+ have been found to coexist in an NaYF4:Eu/Sr. The Eu3+/Eu2+ emission intensity ratio changed with the Sr2+ concentration and excitation wavelength. More interestingly, the spectral configurations of Eu2+ and Eu3+ also varied with the excitation wavelength, indicating that the nanocrystals have multiple luminescence centers or emitting states. 相似文献
116.
The rate of the Wolff-Kishner reaction of benzophenone hydrazone in butyl carbitol increases as the cation of the alkoxide base is varied in the order K>Na>Li>Mg. The replacement of butyl carbitol by 1-decanol also accelerates the reaction, and an additional increase is caused by the presence of crown ether. On the basis of changes in the activation parameters, it is concluded that the reactivity of the hydrazone anion increases as the contact ion pair becomes more fully solvated and dissociated. 相似文献
117.
本文将奇异值分解法引入因子分析,较好地解决了由于矩阵亏秩引起的困难;并采用正交投影矩阵法实现目标转换,以相似性系数Qta作为新的目标检验判据,使计算结果得到改善.用本法测定并计算了La、Sm、Tm混合物的合成样品,获得满意的结果. 相似文献
118.
本文对Ln(phen)(C6H5COO)3、Ln(phen)2P-(OC6H4CHO)3·3H2O.Ln(phen)2P-(OC6H4COOC2H5)3(Ln=Tb、Eu)固体配合物的制备和光谱性质进行了研究。根据元素分析、红外光谱、摩尔电导测定,X射线粉末衍射、热重差热分析以及紫外可见光谱确定了其组成及配位形式,并对其发光性能进行了研究。 相似文献
119.
The chemical compositions of silicate minerals, sulphide minerals and metal Fe-Ni in theQingzhen meteorite were measured by the microprobe analyses. Based upon these chemicalcompositions, the cosmochemical behaviour and characteristics of the existing forms of themajor elements in the Qinzhen meteorite have been discussed. These characteristics show thatunder the S-rich, O-poor, and strongly reducing conditions, the light metal elements, such asCa, Mg, K, Na may form sulphide, and metal Fe-Ni may contain Si and P. However, we con-sider that the light metal-sulphide can be stable in the lower mantle and there are some Siand P in the Fe-Ni core. Finally, an earth core-mantle model is established, where the Fe-Ni core contains some Si and P; the lower mantle is composed of Mg-rich silicate, SiO_2 andsulphide; the upper mantle, of silicate and oxide. 相似文献
120.
含羧酸共生稀土光敏剂的低密度聚乙烯膜紫外光氧化降解 总被引:3,自引:0,他引:3
含羧酸共生稀土光敏剂的低密度聚乙烯膜紫外光氧化降解林宜超(福建省测试技术研究所福州350003)近年来,许多作者先后研究并开发出含硬脂酸铈(CeSt3)光敏剂的可控光降解低密度聚乙烯(LDPE)[1,2]、高密度聚乙烯[3]、聚丙烯[4]、聚苯乙烯... 相似文献