全文获取类型
收费全文 | 2047篇 |
免费 | 289篇 |
国内免费 | 306篇 |
专业分类
化学 | 1667篇 |
晶体学 | 29篇 |
力学 | 97篇 |
综合类 | 23篇 |
数学 | 284篇 |
物理学 | 542篇 |
出版年
2024年 | 2篇 |
2023年 | 34篇 |
2022年 | 53篇 |
2021年 | 44篇 |
2020年 | 66篇 |
2019年 | 73篇 |
2018年 | 74篇 |
2017年 | 59篇 |
2016年 | 86篇 |
2015年 | 96篇 |
2014年 | 103篇 |
2013年 | 140篇 |
2012年 | 185篇 |
2011年 | 184篇 |
2010年 | 128篇 |
2009年 | 119篇 |
2008年 | 138篇 |
2007年 | 126篇 |
2006年 | 113篇 |
2005年 | 101篇 |
2004年 | 101篇 |
2003年 | 78篇 |
2002年 | 94篇 |
2001年 | 71篇 |
2000年 | 57篇 |
1999年 | 58篇 |
1998年 | 47篇 |
1997年 | 28篇 |
1996年 | 29篇 |
1995年 | 18篇 |
1994年 | 23篇 |
1993年 | 20篇 |
1992年 | 14篇 |
1991年 | 15篇 |
1990年 | 13篇 |
1989年 | 8篇 |
1988年 | 5篇 |
1987年 | 5篇 |
1986年 | 6篇 |
1985年 | 8篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 5篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1977年 | 1篇 |
排序方式: 共有2642条查询结果,搜索用时 140 毫秒
991.
3,3’-二氰基二呋咱基醚(FOF-2)的合成、表征及量子化学研究 总被引:4,自引:0,他引:4
以3-氨基-4-酰胺肟基呋咱(AAOF)为原料, 经两次氧化、分子间醚化得到3,3’-二氰基二呋咱基醚(FOF-2), 收率90.9%, 并通过IR, 1H NMR, 13C NMR, MS, 元素分析, DSC等分析手段对其结构进行了表征. 初步探讨了硝基分子间醚化合成FOF-2的反应机理; 确定了Caro’s acid氧化、分子间醚化的最佳条件. 用B3LYP方法, 在6-31G(d,p)基组水平上对其结构进行了计算, 得到了其稳定的几何构型和键级; 在振动分析的基础上求得体系的振动频率、IR谱及不同温度下的热力学性质, 并得到了温度对热力学性能影响的关系式; 探讨了其热解机理, 计算出醚键断裂时的活化能. 相似文献
992.
Aileen Höhne Lian Yu Linjing Mu Dr. Markus Reiher Prof. Dr. Ulrike Voigtmann Dr. Ulrich Klar Dr. Keith Graham Dr. P. August Schubiger Prof. Dr. Simon M. Ametamey Prof. Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(15):3736-3743
Water stable! Radiochemists have recently discovered silicon chemistry as a tool for the introduction of 18F into biomolecules for positron emission tomography (PET) imaging. 18F‐labeled PET tracers must be stable towards defluorination under physiological conditions. Here, a theoretical model of organofluorosilane hydrolysis is developed that correlates with the experimentally determined hydrolytic half‐lives and allows estimation of the stability of newly designed compounds (see scheme).
993.
聚碳酸1,2-丙二酯/聚琥珀酸丁二酯/邻苯二甲酸二烯丙酯共混体系的研究 总被引:1,自引:0,他引:1
采用熔融共混的方法制备了聚碳酸1,2-丙二酯(PPC)/聚琥珀酸丁二酯(PBS)共混物和PPC/PBS/DAOP(邻苯二甲酸二烯丙酯)增塑共混物,对共混物的相容性、热性能、结晶性和物理机械性能进行了初步研究.研究结果表明PPC/PBS共混物为不相容体系,PPC对PBS的结晶度影响很小;PBS的加入提高了共混物的起始热分解温度(Td-5%),当共混物中PBS含量从10%增加到90%时,共混物的Td-5%可分别增加15℃到59℃.DAOP对PPC/PBS共混物有增塑作用,当PPC/PBS/DAOP的比例从30/70/0变化到30/70/30时,共混物玻璃化转变温度(Tg)下降了36.9℃.与PPC/PBS共混物相比,组成优化的DAOP增塑共混物PPC/PBS/DAOP(PPC/PBS/DAOP=30/70/5)的断裂伸长率和断裂能最大可提高31倍和34倍,分别达到655.1%和3.4 J/mm2,因此引入DAOP尽管使共混材料的热稳定性有所下降,但拓宽了PPC/PBS共混材料的使用温度窗口. 相似文献
994.
Highly efficient Michael addition reactions of malonates to nitroalkenes catalyzed by novel chiral thioureas derived from optically pure BINOL and amino acids are reported. Various trans-nitroalkenes reacted with malonates affording the desired products in up to 95% yield with excellent enantioselectivities (up to 97% ee). 相似文献
995.
Particle swarm optimization (PSO) algorithm has been developing rapidly and many results have been reported. PSO algorithm has shown some important advantages by providing high speed of convergence in specific problems, but it has a tendency to get stuck in a near optimal solution and one may find it difficult to improve solution accuracy by fine tuning. This paper presents a dynamic global and local combined particle swarm optimization (DGLCPSO) algorithm to improve the performance of original PSO, in which all particles dynamically share the best information of the local particle, global particle and group particles. It is tested with a set of eight benchmark functions with different dimensions and compared with original PSO. Experimental results indicate that the DGLCPSO algorithm improves the search performance on the benchmark functions significantly, and shows the effectiveness of the algorithm to solve optimization problems. 相似文献
996.
Lian Chen Feilong Jiang Mingyan Wu Ning Li Wentao Xu Chunfeng Yan Maochun Hong 《无机化学与普通化学杂志》2009,635(2):379-383
Reactions of VO2(acac) (Hacac = acetylacetone) and 2,6‐Di(hydroxymethyl)‐4‐methylphenol (H3L) in different organic solvents give rise to two pseudopolymorphs of a new dinuclear oxovanadium(V) compound, [Et3NH]2[V2O4(HL)2] ( 1 ) and [Et3NH]2[V2O4(HL)2]·H2O ( 2 ). The compounds have been synthesized and characterized by elemental analyses, IR spectra and single‐crystal X‐ray diffraction. X‐ray diffraction analyses reveal that 1 and 2 have different weak interactions and display remarkably distinct hydrogen‐bonded networks. 相似文献
997.
Xiu-Hui Lu Yue-Hua Xu Le-Yi Shi Jun-Feng Han Zhen-Xia Lian 《Journal of organometallic chemistry》2009,694(25):4062-4068
The cycloaddition mechanism of the reaction between singlet dimethyl germylidene and formaldehyde has been investigated with MP2/6-31G* method, including geometry optimization and vibrational analysis for the involved stationary points on the potential energy surface. The energies of the different conformations are calculated with CCSD (T)//MP2/6-31G* method. From the potential energy profile, we predict that the cycloaddition reaction between singlet dimethyl germylidene and formaldehyde has two dominant reaction pathways. First dominant reaction pathway consists of three steps: (1) the two reactants (R1, R2) firstly form an intermediate INT1a through a barrier-free exothermic reaction of 43.0 kJ/mol; (2) INT1a then isomerizes to a four-membered ring compound P1 via a transition state TS1a with an energy barrier of 24.5 kJ/mol; (3) P1 further reacts with formaldehyde(R2) to form a germanic heterocyclic compound INT3, which is also a barrier-free exothermic reaction of 52.7 kJ/mol; Second dominant reaction pathway is as following: (1) the two reactants (R1, R2) firstly form a planar four-membered ring intermediate INT1b through a barrier-free exothermic reaction of 50.8 kJ/mol; (2) INT1b then isomerizes to a twist four-membered ring intermediate INT1.1b via a transition state TS1b with an energy barrier of 4.3 kJ/mol; (3) INT1.1b further reacts with formaldehyde(R2) to form an intermediate INT4, which is also a barrier-free exothermic reaction of 46.9 kJ/mol; (4) INT4 isomerizes to a germanic bis-heterocyclic product P4 via a transition state TS4 with an energy barrier of 54.1 kJ/mol. 相似文献
998.
Chun-Feng Yan Fei-Long Jiang Lian Chen Rui Feng Ming Yang Mao-Chun Hong 《Journal of solid state chemistry》2009,182(11):3162-3170
Assembly of 5-sulfosalicylic acid (H3L) and d10 transition metal ions (CdII, AgI) with the neutral N-donor ligands produces five new complexes: [Cd2(HL)2(4,4′-bipy)3]n·2nH2O (1), {[Cd2(μ2-HCO2)2(4,4′-bipy)2(H2O)4][Cd(HL)2(4,4′-bipy)(H2O)2]}n (2), {[Cd(4,4′-bipy)(H2O)4][HL]·H2O}n (3), [Cd(HL)(dpp)2(H2O)]n·4nH2O (4), {[Ag(4,4′-bipy)][Hhbs]}n (5) (4,4′-bipy=4,4′-bipyridine, dpp=1,3-di(pyridin-4-yl)propane, H2hbs=4-hydroxybenzenesulfonic acid, the decarboxylation product of H3L). Complex 1 adopts a 5-connected 3D bilayer topology. Complex 2 has the herring-bone and ladder chain, which are extended to a 3D network via hydrogen bonding. In 3–4 complexes, 3 is a 3D supermolecular structure formed by polymeric chains and 2D network of HL2−, while 4 gives the double-stranded chains. In 5, ladder arrays are stacked with the 2D networks of Hhbs− anions in an –ABAB– sequence. Complexes 1–4 display green luminescences in solid state at room temperature, while emission spectra of 3 and 4 show obvious blue-shifts at low temperature. 相似文献
999.
Fengtao Fan Keju Sun Zhaochi Feng Dr. Haian Xia Bo Han Yuxiang Lian Pinliang Ying Dr. Can Li Prof. Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(13):3268-3276
The nucleation process of iron‐exchanged zeolite Fe‐ZSM‐5, from the assembly of distorted tetrahedrally coordinated iron species and silicate rings in the precursor to the final Fe‐ZSM‐5 crystals, as well as variations in the coordination environment of iron, were studied by UV resonance Raman spectroscopy and complementary techniques.
1000.
报道了一种抽运-检测型的非线性磁光旋转铷原子磁力仪.其原理是线偏振光通过处于外磁场环境中被极化的原子介质后,由于原子对线偏振光中左、右圆偏成分不同的吸收和色散,导致光的偏振方向会产生与磁场相关的转动.分析了该磁力仪的工作原理,并测试了它对不同磁场大小的响应.测试结果表明,磁力仪测量范围为100—100000 nT,极限灵敏度为0.2 p T/Hz~(1/2),磁场分辨率为0.1 p T.进一步研究了不同磁场下原子系综极化态的横向弛豫时间,讨论了原子磁力仪高磁场采样率的获得方法.本文的原子磁力仪在5000—100000 n T的磁场测量范围内磁场采样率可实现1—1000 Hz范围内可调,能够测量低频的微弱交变磁场.本文的研究内容为大磁场测量范围、高灵敏度、高磁场采样率的原子磁力仪研制提供了重要参考. 相似文献