全文获取类型
收费全文 | 21149篇 |
免费 | 2383篇 |
国内免费 | 3662篇 |
专业分类
化学 | 15348篇 |
晶体学 | 155篇 |
力学 | 764篇 |
综合类 | 139篇 |
数学 | 1037篇 |
物理学 | 2968篇 |
综合类 | 6783篇 |
出版年
2024年 | 126篇 |
2023年 | 336篇 |
2022年 | 791篇 |
2021年 | 947篇 |
2020年 | 1323篇 |
2019年 | 937篇 |
2018年 | 735篇 |
2017年 | 742篇 |
2016年 | 917篇 |
2015年 | 905篇 |
2014年 | 1175篇 |
2013年 | 1580篇 |
2012年 | 1355篇 |
2011年 | 1211篇 |
2010年 | 901篇 |
2009年 | 1067篇 |
2008年 | 1039篇 |
2007年 | 1198篇 |
2006年 | 1161篇 |
2005年 | 1034篇 |
2004年 | 995篇 |
2003年 | 935篇 |
2002年 | 911篇 |
2001年 | 618篇 |
2000年 | 585篇 |
1999年 | 437篇 |
1998年 | 406篇 |
1997年 | 363篇 |
1996年 | 350篇 |
1995年 | 348篇 |
1994年 | 307篇 |
1993年 | 261篇 |
1992年 | 242篇 |
1991年 | 154篇 |
1990年 | 139篇 |
1989年 | 124篇 |
1988年 | 132篇 |
1987年 | 106篇 |
1986年 | 62篇 |
1985年 | 58篇 |
1984年 | 34篇 |
1983年 | 16篇 |
1982年 | 33篇 |
1981年 | 22篇 |
1980年 | 21篇 |
1979年 | 17篇 |
1978年 | 6篇 |
1977年 | 5篇 |
1976年 | 8篇 |
1973年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
901.
《Comptes Rendus Mecanique》2014,342(12):671-691
The paper extends the use of the PGD method to viscoelastic evolution problems described by a large number of internal variables and with a large spectrum of relaxation times. The internal variables evolution is described by a set of linear differential equations that involve many time scales. The feasibility and the robustness of the method are discussed in the case of a polymer in a non-equilibrium state under creep and cyclic loading. The relationships between different time scales (loading and internal variables) are also discussed. 相似文献
902.
字幕翻译应通过对源语信息进行浓缩、简化、省略等减译处理,给观众提供通俗易懂、简洁流畅且符合人物个性的字幕语言,让观众迅速准确地明白剧情。翻译的难度在于文化信息的转换,翻译时应尽力保留源语独特的表达方法,诸如词语、比喻、典故、人名等应直译,以体现源语及其文化的瑰丽;不得已时,再采用意译,以达到与源语功能的对等。意译时,可采用适度的娱乐化改写或本土化策略。 相似文献
903.
Coordination compounds of Fe(III), Zn(II), Ni(II), Co(II), Cu(II), Cd(II) and Mn(II) ions were synthesized from the ligand [4,4′‐((((ethane‐1,2‐diylbis(oxy))bis(2,1‐phenylene))bis(methanylylidene))bis(azanylylidene))diphenol]ethane (H2L) derived from the condensation of bisaldehyde and 4‐aminophenol. Microanalysis, magnetic susceptibility, infrared, 1H NMR and mass spectroscopies, molar conductance, X ray powder diffraction and thermal analysis were used to confirm the structure of the synthesized chelates. According to the data obtained, the composition of the 1:1 metal ion–bis‐Schiff base ligand was found to be [M(H2L)(H2O)2]Cln (M = Zn(II), Ni(II), Co(II), Cu(II), Cd(II) and Mn(II), n = 2; Fe(III), n = 3). Magnetic susceptibility measurements and reflectance spectra suggested an octahedral geometry for the complexes. Central metals ions and bis‐Schiff base coordinated together via O2 and N2 donor sites which as evident from infrared spectra. The Gaussian09 program was applied to optimize the structural formula for the investigated Schiff base ligand. The energy gaps and other important theoretical parameters were calculated applying the DFT/B3LYP method. Molecular docking using AutoDock tools was utilized to explain the experimental behaviour of the Schiff base ligand towards proteins of Bacillus subtilis (5 h67), Escherichia coli (3 t88), Proteus vulgaris (5i39) and Staphylococcus aureus (3ty7) microorganisms through theoretical calculations. The docked protein receptors were investigated and the energies of hydrogen bonding were calculated. These complexes were then subjected to in vitro antibacterial studies against several organisms, both Gram negative (P. vulgaris and E. coli) and Gram positive (S. pyogones and B. subtilis). The ligand and metal complexes exhibited good microbial activity against the Gram‐positive and Gram‐negative bacteria. 相似文献
904.
《应用有机金属化学》2017,31(11)
Agx Pt100−x (x = 0, 25, 50, 75 and 100) nanoparticles were grown on the surface of magnetic graphene oxide nanosheets (Fe3O4@GO) for the first time. The as‐prepared nanocomposites were characterized using various techniques such as Fourier transform infrared spectroscopy, powder X‐ray diffraction, field emission scanning electron microscopy, energy‐dispersive X‐ray spectroscopy, transmission electron microscopy, Brunauer–Emmett–Teller surface area analysis, vibrating sample magnetometry and thermogravimetric analysis. The Fe3O4@GO‐Agx Pt100−x catalysts were applied in the reduction of 4‐nitrophenol (4‐NP) to 4‐aminophenol using sodium borohydride (NaBH4). The synthesized nanocomposites exhibited excellent catalytic performance in the reduction of 4‐NP with high recyclability for five consecutive runs. The Fe3O4@GO‐Ag75Pt25 nanocomposite exhibited the best catalytic activity with a rate constant as high as 140.6 × 10−3 s−1. The obtained kinetic data were modelled with the Langmuir–Hinshelwood equation. The energy of activation and thermodynamic parameters including enthalpy, entropy of activation and activation Gibbs free energy were calculated. 相似文献
905.
《Electroanalysis》2017,29(3):898-906
Platinum nanoparticles (NPs) modified with undecafluorohexylamine (UFHA) and octylamine were synthesized as a novel model cathode catalyst for fuel cells. The modified Pt NPs were well characterized by FTIR, X‐ray photoelectron spectroscopy, thermogravimetric analysis, and transmission electron microscopy. These NPs supported on carbon black were applied as electrocatalysts for the oxygen reduction reaction. The UFHA‐modified Pt NP catalyst showed high electrocatalytic activity and durability compared to a commercial catalyst. Besides suppression of undesired oxide formation on the Pt surface, the affinity between the perfluorinated alkyl chains of UFHA and Nafion® improved the catalyst activity by creating a desirable proton conduction path. Additionally, UFHA modification improved durability by suppressing Pt dissolution and carbon corrosion because of restricted water accessibility. The β ‐oxide formation, which is responsible for Pt dissolution, was significantly attenuated by surface modification. 相似文献
906.
《应用有机金属化学》2017,31(12)
A novel tridentate chelating ligand, Ethyl 2‐(2‐(2‐chlorobenzoyl)hydrazono)propanoate and its late transition metal complexes were synthesized, characterized and evaluated for anticancer behavior. The structures were elucidated with the help of elemental analyses, spectral (vibrational, electronic, NMR and mass) and thermo‐gravimetric techniques. Single crystal X‐ray crystallographic studies of the ligand suggest an orthorhombic lattice structure with Pna21 space group. The interaction of ligand and complexes with DNA (CT‐DNA) has been extensively studied using absorption, emission, viscosity and thermal denaturation studies with E. coli DNA. The DNA cleavage ability of ligand and metal complexes was tested using plasmid pBR322 DNA by gel electrophoresis method. The ligand and its copper complex have been evaluated for their in vitro anticancer activity against human cancer cells of different origin such as KB (Oral), A431 (Skin), Mia‐Pa‐Ca (Pancreases), K‐549 (Lung), K‐562 (Leukemia), MCF‐7 (Breast) and VERO by MTT assay and the apoptosis assay was carried out with acridine orange/ethidium bromide (AO/EB) staining method. The studies suggest that ligand and copper complex exhibit significant cytotoxic activity on KB, MCF‐7, A‐431, Mia‐Pa‐Ca‐2 an d A‐549 cell lines compared to K‐562 and VERO cell lines. 相似文献
907.
AFe3O(PO4)3 (A = Ca, Sr and Pb) powder compounds were studied by means of X-ray diffraction (XRD), electron-probe microanalysis (EPMA) coupled with wavelength dispersion spectroscopy (WDS), Raman and diffuse reflectance spectroscopies, specific heat and magnetic properties measurements. Magnetization, magnetic susceptibility and specific heat measurements carried out on AFe3O(PO4)3 (A = Sr, Ca and Pb) powders firmly establish a series of three ferromagnetic (FM)-like second order phase transitions spanned over the 32–8 K temperature range. Room temperature Mössbauer spectroscopy and associated DFT calculations confirm the existence of three crystallographically non equivalent Fe3+ sites in the three compounds. Mössbauer spectra recorded as a function of temperature in the PbFe3O(PO4)3 compound also establishes the occurrence of two purely magnetic and reversible phase transitions at 32 and 10 K. Diffuse reflectance measurements reveal two broad absorption bands at 1047 and 837 nm, in both PbFe3O(PO4)3 and SrFe3O(PO4)3 powders, with peak cross sections ∼10−20 cm2 typical of spin-forbidden and forced electric dipole intraconfigurational transitions. 相似文献
908.
909.
910.
Cross‐Coupling of Organolithium with Ethers or Aryl Ammonium Salts by C−O or C−N Bond Cleavage
下载免费PDF全文
![点击此处可从《Chemistry (Weinheim an der Bergstrasse, Germany)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Ze‐Kun Yang Dong‐Yu Wang Hiroki Minami Hiroyuki Ogawa Takashi Ozaki Dr. Tatsuo Saito Dr. Kazunori Miyamoto Dr. Chao Wang Prof. Dr. Masanobu Uchiyama 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(44):15693-15699
Various aryl‐, alkenyl‐, and/or alkyllithium species reacted smoothly with aryl and/or benzyl ethers with cleavage of the inert C?O bond to afford cross‐coupled products, catalyzed by commercially available [Ni(cod)2] (cod=1,5‐cyclooctadiene) catalysts with N‐heterocyclic carbene (NHC) ligands. Furthermore, the coupling reaction between the aryllithium compounds and aryl ammonium salts proceeded under mild conditions with C?N bond cleavage in the presence of a [Pd(PPh3)2Cl2] catalyst. These methods enable selective sequential functionalizations of arenes having both C?N and C?O bonds in one pot. 相似文献