全文获取类型
收费全文 | 3660篇 |
免费 | 630篇 |
国内免费 | 846篇 |
专业分类
化学 | 2361篇 |
晶体学 | 27篇 |
力学 | 256篇 |
综合类 | 33篇 |
数学 | 126篇 |
物理学 | 1597篇 |
无线电 | 736篇 |
出版年
2024年 | 23篇 |
2023年 | 60篇 |
2022年 | 128篇 |
2021年 | 136篇 |
2020年 | 123篇 |
2019年 | 130篇 |
2018年 | 105篇 |
2017年 | 158篇 |
2016年 | 172篇 |
2015年 | 157篇 |
2014年 | 223篇 |
2013年 | 275篇 |
2012年 | 279篇 |
2011年 | 283篇 |
2010年 | 193篇 |
2009年 | 264篇 |
2008年 | 240篇 |
2007年 | 266篇 |
2006年 | 205篇 |
2005年 | 204篇 |
2004年 | 204篇 |
2003年 | 170篇 |
2002年 | 142篇 |
2001年 | 134篇 |
2000年 | 134篇 |
1999年 | 118篇 |
1998年 | 113篇 |
1997年 | 77篇 |
1996年 | 64篇 |
1995年 | 54篇 |
1994年 | 50篇 |
1993年 | 45篇 |
1992年 | 48篇 |
1991年 | 25篇 |
1990年 | 20篇 |
1989年 | 18篇 |
1988年 | 12篇 |
1987年 | 13篇 |
1986年 | 7篇 |
1985年 | 16篇 |
1984年 | 2篇 |
1983年 | 6篇 |
1982年 | 11篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 13篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1973年 | 4篇 |
排序方式: 共有5136条查询结果,搜索用时 0 毫秒
101.
The relationship between temperature and oxygen vacancy concentration is deduced in this paper. Based on the data of thermal weight-loss experiment, the formation enthalpies of congruent and several doped LN crystals have been calculated. It was found that the formation enthalpy of oxygen vacancies can be decreased evidently by doping valence-changeable ions. The experimental results were discussed and a new reduction process of the photorefractive LN crystal at a relatively low temperature was proposed, and the reduced crystals showed a good effect in practical use. 相似文献
102.
合成了一种酰腙类含硫Schiff碱4-(二乙胺基)水杨醛缩2-噻吩甲酰腙一水合物(H2L·H2O,1),采用扩散法制备了它的铜配合物[Cu(phen)L](2,phen=菲咯啉),通过元素分析、红外光谱、紫外光谱和X射线单晶衍射对它们进行了表征。1的晶体属正交晶系,P21212空间群;2的晶体属单斜晶系,P21/c空间群。在2中H2L以烯醇式脱除羟基质子与铜配位,中心离子的配位环境为畸变的四方锥构型。应用Gaussian 09程序,采用密度泛函方法(DFT),分别在B3LYP/6-31G(d)和UB3LYP/6-31G(d)水平对1和2进行了量化计算,对分子的前线分子轨道的能量和组成以及电子光谱进行了分析讨论。测试、分析了化合物的抗氧化活性。 相似文献
103.
Yaosong Huang Yugong Chen Mingfei Zhou 《International journal of quantum chemistry》2020,120(24):e26415
Hexamethyldisiloxane [HMDSO, (CH3)3-SiOSi-(CH3)3] is an important precursor for SiO2 formation during flame-based silica material synthesis. As a result, HMDSO reactions in flame have been widely investigated experimentally, and many results have indicated that HMDSO decomposition reactions occur very early in this process. In this paper, quantum chemical calculations are performed to identify the initial decomposition of HMDSO and its subsequent reactions using the density functional theory at the level of B3LYP/6-311+G (d, p). Four reaction pathways—(a) Si O bond dissociation of HMDSO, (b) Si C bond dissociation of HMDSO, (c) dissociation and recombination of Si O and Si C bonds, and (d) elimination of a methane molecule from HMDSO—have been examined and identified. From the results, it is found that the barrier of 84.38 kcal/mol and Si O bond dissociation energy of 21.55 kcal/mol are required for the initial decomposition reaction of HMDSO in the first pathway, but the highest free energy barrier (100.69 kcal/mol) is found in the third reaction pathway. By comparing the free energy barriers and reaction rate constants, it is concluded that the most possible initial decomposition reaction of HMDSO is to eliminate the CH3 radical by Si C bond dissociation. 相似文献
104.
The chalcogenido indates K6In2Q6 (Q = S, Se, Te) were synthesized from melts of the pure elements at a maximum temperature of 700 °C. All three potassium salts contain dinuclear units [In2Q6]6– of two edge-sharing [InQ4] tetrahedra. The sulfido and the selenido indate are isotypic and crystallize in the K6Mn2O6-type structure [monoclinic, space group P21/c, a = 784.32(9)/809.32(3), b = 1274.58(14)/1322.37(4), c = 836.48(9)/870.53(3) pm, β = 97.900(2)/97.5877(8)°, Z = 2, R1 = 0.0123/0.0109; for Q = S/Se]. The tellurido indate K6In2Te6 crystallizes in a new orthorhombic structure type [space group Pnma, a = 1793.70(12), b = 1491.55(11), c = 837.40(6) pm, Z = 4, R1 = 0.0157]. In this structure, the telluride anions form a hexagonal close packing, in which K+ cations occupy all octahedral voids; the In3+ ions take 1/6 (but always adjacent) tetrahedral voids. This structure-chemical relation to the h.c.p. packing, which is similarly found for most of the sodium dimetallates (e.g. Na6Fe2S6), is substantiated by a full crystallographic group-subgroup tree. The crystal chemistry of the new indates is discussed and compared with that of alkali chalcogenido metallates(III) of Fe, Al and Ga containing [M2Q6]6– dimers, which overall form as many as ten different structure types. DFT band structure calculations of the three title compounds exhibit bandgaps, which continuously decrease from the S to the Te compound and which are also in accordance with the pale yellow (S), bright yellow (Se) and red-brown (Te) color of the compounds. The chemical bonding in the salts and within the metallate anion is discussed on the basis of the partial DOS and a Bader analysis of the calculated electron density. 相似文献
105.
106.
Density functional calculations have been performed to comparatively investigate two possible pathways of Au(I)‐catalyzed Conia‐ene reaction of β‐ketoesters with alkynes. Our studies find that, under the assistance of trifluoromethanesulfonate (TfO), the β‐ketoester is the most likely to undergo Model II to isomerize into its enol form, in which TfO plays a proton transfer role through a 6‐membered ring transition state. The coordination of the Au(I) catalyst to the alkynes triple bond can enhance the eletrophilic capability and reaction activity of the alkynes moiety, which triggers the nucleophilic addition of the enol moiety on the alkynes moiety to give a vinyl‐Au intermediate. This cycloisomerizaion step is exothermal by 21.3 kJ/mol with an energy barrier of 56.0 kJ/mol. In the whole catalytic process, the protonation of vinyl‐Au is almost spontaneous, and the formation of enol is a rate‐limiting step. The generation of enol and the activation of Au(I) catalyst on the alkynes are the key reasons why the Conia‐ene reaction can occur in mild condition. These calculations support that Au(I)‐catalyzed Conia‐ene reactions of β‐ketoesters with alkynes go through the pathway 2 proposed by Toste. 相似文献
107.
《Journal of Coordination Chemistry》2012,65(1-2):91-99
Abstract Fenske-Hall molecular orbital calculations on the complexes CpFe(CO)2(CX)+ (X = O, S, Se, and Te) have been used to quantify the nature of bonding between the CX ligands and the metal atom. In addition, conclusions have been reached about the reactivity of the complexes under both nucleophilic and electrophilic attack. The previously established trend of increasing metal—ligand bond strength as X changes from O to S to Se is demonstrated by our molecular orbital calculations, and found to extend to Te. The mechanism for nucleophilic attack, variously explained in the past by either charge control or orbital control, is quantitatively ascribed to orbital control only. The nature of electrophilic attack on these complexes is also found to begin with orbital control. 相似文献
108.
基于《化学教育》等期刊文献和核心素养的研究成果,分析了关系式在初中化学计算中的地位和作用。通过“关系式在初中化学计算中的应用”课例,提出了高效复习的建议:要宏观认识复习课的目标和功能,帮助学生形成基本的化学观念、提高问题的解决能力、树立自信心。还应精心设计好各个微环节的活动,达到复习效益最大化。 相似文献
109.
水杨醛缩苯胺类席夫碱构象异构体的稳定性及其非线性光学性质的理论研究 总被引:7,自引:0,他引:7
运用量子化学有限场FF/PM3方法,对水杨醛缩苯胺类席夫碱几种异构体稳定构象的能量、结构、非线性光学系数进行了分析,探讨氢键及氢迁移过程对体系稳定性和非线性光学性质的影响.结果表明:水杨醛缩苯胺类席夫碱分子具有较好的二阶非线性光学性质,并可能成为一类良好的光学材料. 相似文献
110.