全文获取类型
收费全文 | 1870篇 |
免费 | 81篇 |
国内免费 | 1篇 |
专业分类
化学 | 998篇 |
晶体学 | 34篇 |
力学 | 105篇 |
数学 | 442篇 |
物理学 | 373篇 |
出版年
2023年 | 20篇 |
2022年 | 25篇 |
2021年 | 22篇 |
2020年 | 56篇 |
2019年 | 61篇 |
2018年 | 68篇 |
2017年 | 59篇 |
2016年 | 116篇 |
2015年 | 71篇 |
2014年 | 96篇 |
2013年 | 317篇 |
2012年 | 82篇 |
2011年 | 84篇 |
2010年 | 73篇 |
2009年 | 72篇 |
2008年 | 54篇 |
2007年 | 59篇 |
2006年 | 56篇 |
2005年 | 27篇 |
2004年 | 44篇 |
2003年 | 29篇 |
2002年 | 37篇 |
2001年 | 30篇 |
2000年 | 16篇 |
1999年 | 26篇 |
1998年 | 24篇 |
1997年 | 24篇 |
1996年 | 18篇 |
1995年 | 21篇 |
1994年 | 22篇 |
1993年 | 13篇 |
1992年 | 10篇 |
1990年 | 8篇 |
1989年 | 8篇 |
1988年 | 13篇 |
1987年 | 9篇 |
1986年 | 5篇 |
1985年 | 15篇 |
1984年 | 11篇 |
1983年 | 9篇 |
1982年 | 7篇 |
1981年 | 14篇 |
1980年 | 11篇 |
1979年 | 9篇 |
1978年 | 5篇 |
1977年 | 6篇 |
1975年 | 10篇 |
1973年 | 5篇 |
1971年 | 5篇 |
1927年 | 5篇 |
排序方式: 共有1952条查询结果,搜索用时 328 毫秒
991.
Güneş Erdoğan Mohamed Haouari Melda Örmeci Matoglu Okan Örsan Özener 《The Journal of the Operational Research Society》2015,66(10):1742-1754
Airline companies seek to solve the problem of determining an assignment of crews to a pre-determined flight schedule with minimum total cost, called the Crew Pairing Problem (CPP). Most of the existing studies focus on the CPP of North American airlines, which widely differs from that of most European airline companies in terms of the objective function, the flight structure, and the planning horizon. In this study, we develop an optimization-driven heuristic algorithm that can efficiently handle large-scale instances of the CPP that must be solved on a monthly basis. We perform computational experiments using flight schedules of an European airline company to test the performance of the solution method. Our computational results demonstrate that our algorithm is able to provide high-quality solutions to monthly instances with up to 27?000 flight legs. 相似文献
992.
Ahmet Bekir Özkan Güner Burcu Ayhan 《Mathematical Methods in the Applied Sciences》2015,38(17):3807-3817
In this paper, the ‐expansion method is proposed to establish hyperbolic and trigonometric function solutions for fractional differential‐difference equations with the modified Riemann–Liouville derivative. The fractional complex transform is proposed to convert a fractional partial differential‐difference equation into its differential‐difference equation of integer order. We obtain the hyperbolic and periodic function solutions of the nonlinear time‐fractional Toda lattice equations and relativistic Toda lattice system. The proposed method is more effective and powerful for obtaining exact solutions for nonlinear fractional differential–difference equations and systems. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
993.
S. Öz Ü. Ergun M. Yakut I. Svoboda A. Atakol E. Kübra İnal N. Yılmaz O. Atakol 《Russian Journal of Coordination Chemistry》2014,40(8):571-582
1,3-Propanediamine was put to react with 2-hydroxybenzaldehyde and 2-hydroxyacetophenone sequentially in aprotic medium. The crystalline product was examined by high performance liquid chromatography. The composition was 66% asymmetric Schiff base N(2-hydroxybenzylidene)-N′(2-hydroxyacetophenone)-1,3-propanediamine (SALLACH2) and 33% bis-N,N′(2-hydroxyacetophenylidene)-1,3-propanediamine (LACH2). As the crystals were uniform and of appropriate size, the molecular model of the material was revealed by X-ray diffraction. It was seen that two molecules of SALLACH2 and one molecule of LACH2 formed the mixed crystals. The substance was separated to its components and the asymmetric Schiff base was purified with a silica column. The substance was characterized with elemental analysis, FT-IR, MS, 1HNMR, and 13C NMR. In addition, six tri-nuclear complex with the nuclear structure of NiII-NiII-NiII, NiII-CuII-NiII, NiII-MnII-NiII were prepared from this Schiff base and stoichiometry was determined by elemental analysis, FT-IR and thermogravimetry. Finally, the molecular structures of two complexes were brought to light by XRD which highlights the asymmetry of the ligand more clearly. 相似文献
994.
Dursun Ali Köse Banu Öztürk Onur Şahin Orhan Büyükgüngör 《Journal of Thermal Analysis and Calorimetry》2014,115(2):1515-1524
Coumarilate–nicotinamide complexes of CoII and ZnII were synthesized and investigated by elemental analysis, magnetic susceptibility, solid state UV–Vis, direct injection probe mass spectra, FTIR spectra, thermoanalytic TG-DTG/DTA, and crystal X-ray diffraction methods. It was obtained that both complex structures contain 2 mol aqua ligands, 2 mol coumarilate (CCA?) and 2 mol nicotinamide (NA) ligands per formula unit. The CCA? and NA ligands were bonded to metal cations as monodentate through acidic oxygen and nitrogen of pyridine ring, respectively. Thermal decomposition of each complex starts with dehydration and continue removing of 1 mol NA ligand. The thermal dehydration of the complexes takes place in one or two steps. The decomposition mechanism and thermal stability of the investigated complexes are interpreted in terms of their structures. The final decomposition products are found to be metal oxides. 相似文献
995.
Ulviye Acar Çevik Derya Osmaniye Begüm N. Sağlik Serkan Levent Betül K. Çavuşoğlu Abdullah B. Karaduman Ümide D. Özkay Yusuf Özkay Zafer A. Kaplancikli Gülhan Turan 《Journal of heterocyclic chemistry》2020,57(5):2225-2233
Monoamine oxidases (MAO) are enzymes that catalyze the oxidative deamination of monoamines such as dopamine, noradrenaline, adrenaline, and serotonin. Recent studies have shown that numerous benzothiazole derivatives exhibit hMAO inhibitory activity in the micromolar concentration range. In this study, a novel series of benzothiazole-thiadiazole (5a-5l ) was synthesized and characterized their chemical structures by 1H-NMR, 13C-NMR, and Mass spectroscopy. These compounds were evaluated as inhibitors for types A and B MAO enzymes. Compounds 5f and 5l were the most active derivatives in the series with an IC50 values of 0.107 ± 0.003 and 0.128 ± 0.004, respectively. Furthermore, cytotoxicity of compounds 5f and 5l were investigated and found as non-cytotoxic. 相似文献
996.
Hydrophilic, polyacrylate-based, monosized-porous beads with zwitterionic molecular brushes were synthesized as a new stationary medium for hydrophilic interaction chromatography. Monosized-porous poly(glycerol-1,3-diglycerolate diacrylate-co-glycerol dimethacrylate), poly(GDGDA-co-GDMA), beads 5 μm in size were obtained by a staged-shape template polymerization. As an initiator for surface-initiated atom transfer radical polymerization (SI-ATRP), bromine functionality was obtained on the beads by reacting their hydroxyl groups with 3-(aminopropyl)triethoxysilane and α-bromoisobutyryl bromide, respectively. Zwitterionic molecular brushes on the hydrophilic poly(GDGDA-co-GDMA) beads were generated by SI-ATRP of a sulfobetaine monomer, [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl) ammonium hydroxide (MESH). Poly(MESH)-grafted poly(GDGDA-co-GDMA), poly(MESH)g-poly(GDGDAco-GDMA), beads were slurry packed into the microbore columns with 2 mm i.d. and evaluated as stationary medium for the separation of organic acids, nucleosides and peptides using microbore columns in hydrophilic interaction chromatography with the plate numbers up to 30,000 plates m?1. 相似文献
997.
Synthesis of Dihydropyrans and Dihydrofurans via Radical Cyclization of Unsaturated Alcohols and 1,3‐Dicarbonyl Compounds
下载免费PDF全文
![点击此处可从《Helvetica chimica acta》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Hakan Aslan Deniz E. Akpınar Atilla Öktemer Mehtap Yakut Oğuzhan Alagöz 《Helvetica chimica acta》2014,97(5):652-663
The oxidative cyclization reactions of 1,3‐dicarbonyl compounds 1a – 1c and α,β‐unsaturated alcohols 2a – 2f with Mn(OAc)3 were performed, leading to dihydrofurans. Treatment of 1a and 1b with 2‐methylbut‐3‐en‐2‐ol ( 2a ) gave dihydrofurans 3aa and 3ba , and dihydropyrans 4aa and 4ba , as unexpected products. While the reaction of 2‐methylbut‐3‐yn‐2‐ol ( 2b ) with acetylacetone ( 1b ) yielded a bifuran, ethyl acetoacetate ( 1a ) led to a mixture of furan, bifuran, and salicylate derivatives. Besides, surprisingly, styryl‐substituted dihydrofurans were obtained from the reactions of 1,3‐dicarbonyl compounds and (3E)‐2,4‐diphenylbut‐3‐en‐2‐ol. The reaction mechanisms were proposed for the formation of the different products, considering intermediates in these reaction mixtures. 相似文献
998.
O. Şahin E. Ö. Özcan N. Gürbüz İ. Özdemir O. Büyükgüngör 《Journal of Structural Chemistry》2014,55(4):697-702
By treatment of 1,3-bis(3,4-dimethoxybenzyl)-3,4,5,6-tetrahydropyrimidinium chloride (1) with KOBu t and [PtCl2(PEt3)2]2 N-coordinated platinum complex (2) is obtained. The Pt atom is coordinated in square planar arrangements by two chloride ions in a trans-configuration, the N-formyl-N,N′-bisaryltrimethylenediamine nitrogen atom, and the phosphine P atom. An extensive three-dimensional network of three C-H…O hydrogen bonds, two C-H…π and one π…π interactions are responsible for the crystal stabilization. Intermolecular hydrogen bonds and C-H…π interactions produce R 2 2 (6), R 2 2 (22), R 2 2 (24), R 3 3 (23), R 4 4 (26), and R 4 4 (32) rings. 相似文献
999.
A series of novel N-, S-, and O-substituted p-chloranil derivatives were synthesized from the reactions of p-chloranil (1) and related nucleophiles in sodium carbonate (Na2CO3) solution of acetonitrile or in chloroform with Et3N. The structures of novel compounds were characterized by using microanalysis, Fourier transform–infrared, 1H NMR, 13C NMR, and mass spectrometry. 相似文献
1000.
G. Turgut S. Duman F. S. Özçelik E. Sönmez B. Gürbulak 《Journal of Sol-Gel Science and Technology》2014,71(3):589-596
Present study shows the structural, morphological, optical characterization of sol–gel spin coated ZnO:Al film and investigation of device efficiency of Zn/ZnO:Al/p-Si/Al heterojunction diode structure. X-ray diffraction study indicates that film has hexagonal polycrystalline structure with (002) preferential direction. Atomic force microscope and scanning electron microscope images exhibit that surface of ZnO:Al/p-Si consists of homogenously scattered nanoparticles. The surface roughness of ZnO:Al film is found to be 15.24 nm. The band gap value of ZnO:Al film deposited on glass substrate is calculated to be 3.34 eV. The electrical characterization of Zn/ZnO:Al/p-Si/Al heterojunction structure is made by current–voltage (I–V) and capacitance–voltage (C–V) measurements. From these measurements, the heterojunction structure shows a rectifying behavior under a dark condition. The ideality factor and barrier height of Zn/n-ZnO:Al/p-Si/Al structure are calculated as 3.23 and 0.68 eV. The heterojunction structure have diode characteristic with rectification ratio at 64.4 at +2.0 V in the dark. The results suggest that Zn/ZnO:Al/p-Si/Al heterojunction diode can be successfully used in many optoelectronic applications. 相似文献