全文获取类型
收费全文 | 21694篇 |
免费 | 533篇 |
国内免费 | 443篇 |
专业分类
化学 | 4045篇 |
晶体学 | 933篇 |
力学 | 395篇 |
综合类 | 46篇 |
数学 | 13402篇 |
物理学 | 3849篇 |
出版年
2023年 | 62篇 |
2022年 | 155篇 |
2021年 | 159篇 |
2020年 | 163篇 |
2019年 | 427篇 |
2018年 | 441篇 |
2017年 | 297篇 |
2016年 | 339篇 |
2015年 | 344篇 |
2014年 | 640篇 |
2013年 | 1471篇 |
2012年 | 650篇 |
2011年 | 1700篇 |
2010年 | 1503篇 |
2009年 | 1435篇 |
2008年 | 1527篇 |
2007年 | 1497篇 |
2006年 | 1113篇 |
2005年 | 954篇 |
2004年 | 821篇 |
2003年 | 640篇 |
2002年 | 662篇 |
2001年 | 456篇 |
2000年 | 437篇 |
1999年 | 427篇 |
1998年 | 408篇 |
1997年 | 305篇 |
1996年 | 356篇 |
1995年 | 355篇 |
1994年 | 344篇 |
1993年 | 261篇 |
1992年 | 256篇 |
1991年 | 181篇 |
1990年 | 191篇 |
1989年 | 180篇 |
1988年 | 117篇 |
1987年 | 129篇 |
1986年 | 95篇 |
1985年 | 161篇 |
1984年 | 120篇 |
1983年 | 74篇 |
1982年 | 96篇 |
1981年 | 126篇 |
1980年 | 109篇 |
1979年 | 61篇 |
1978年 | 111篇 |
1977年 | 92篇 |
1976年 | 91篇 |
1975年 | 39篇 |
1974年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 750 毫秒
991.
The purpose of this work is to study the incorporation of nitrogen into barium titanate at oxygen sites by ammonolysis. The nitrogen content of the resulting barium titanate powders strongly depends on the flow rate of ammonia and the ratio Ba/Ti. The amount of resulting oxygen vacancies is lowered by co-doping with fluoride and tantalum.By avoiding an excess of BaO and exchanging 10 mol% Ti by Ta it is possible to produce powders with nitrogen content of more than 1 wt%. Partial substitution of barium oxide by barium fluoride also increases the nitrogen content. The nitrogen containing powders are colored and show an additional absorption at 500 nm measured in the UV/vis reflectance spectrum. The concentrations of nitrogen and fluorine in the samples were determined using ion-sensitive electrodes. X-ray powder diffraction shows the composition of the powders. The thermal stability was investigated by thermogravimetric analysis, and in order to calculate the specific surface area the three-point BET method was used. 相似文献
992.
Impedance characterization at different temperatures has been used to investigate the conductive behavior of Poly(3-hexylthiophene) (P3HT), prepared in different solvents, as the semiconductor layer in organic multilayer capacitor. It has been found that the P3HT films using chloroform and toluene solvents exhibit an enhancement in conductivity by heating following an Arrhenius law with an activation energy transition from 0.004 to 0.026 eV and from 0.002 to 0.015 eV at ~313 K, respectively, which originates from band tail hopping that occurs around the Fermi edge. The boiling point of the used solvents can affect P3HT crystallization process, which causes the difference in conduction and activation energy. 相似文献
993.
Yankuan Liu Han Wang Yongchun Zhu Xiaoqing Wang Xiaoyan Liu Haibo Li Yitai Qian 《Solid State Communications》2009,149(37-38):1514-1518
The concentration of the Mn(NO3)2 solution has significant influence on the morphologies and the phases of the MnO2 products. A large number of ε- MnO2 nanowires were prepared via a simple pyrolysis under lower reaction concentration. The nanowires have lengths up to tens of micrometers and diameters in the range of 20–100 nm. The β- MnO2 nanobundles and nanoflowers were prepared by increasing the concentration of Mn(NO3)2 solution. The superparamagnetism of ε- MnO2 nanowires and paramagnetism of β- MnO2 nanoflowers indicate their potential applications in magnetic materials. 相似文献
994.
By the aid of temperature- and magnetic-field-dependent Hall effect measurements, we have extracted the multi-carrier transport information in N-doped and N–In codoped p- ZnO thin films grown on Si substrates through mobility spectrum analysis. It is found that owing to the compensation between free electrons and holes, the two-dimensional hole gas from ZnO/Si interface layers becomes determinant and results in the high p-type conductivity and high hole mobility in the ZnO samples. Compared with N-doping, the N–In codoping introduces many In donors and increases acceptor incorporation, as well as enhancing the free hole mobility due to the short-range dipole-like scattering. 相似文献
995.
Hongyan Mao Changzheng Zhang Haiyun Shi 《Journal of Physics and Chemistry of Solids》2011,72(11):1230-1238
Two lanthanide coordination complexes [Nd(NO3)3(CH3OH)2(4,4′-bipy)2] (1) (4,4′-bipy=4,4′-bipyridine) and [4,4′-Hbipy][La(NO3)4(H2O)2(4,4′-bipy)] (2), with a salt of cationic diprotonated 4,4′-bipy, [2(4,4′-H2bipy)][4(NO3)] (3), have been identified and isolated from a methanol solution of Ln(NO3)3·6H2O, 4,4′-bipyridine and pyrazine in 1:2:1 ratio. Their structures have been determined by single-crystal X-ray diffraction analyses, which reveal that 1 has an interesting three-dimensional supramolecular architecture containing 21 double-stranded helical chains through hydrogen bonding and π–π interactions, while 2 and 3 have well defined infinite chiral 3D open networks that undergo self-interpenetration. The electrospray ionization mass spectra (ESI-MS) indicate that the covalent complex has higher stability than the electrostatic bonding one. ESI-MS/MS of these ions reveal that the Ln–O bond forms a stronger coordinated bonding than that of Ln–N system and the nitrate anion remains bound to the lanthanide centers after complete dissociation in methanol solution. 相似文献
996.
David Poger Wilfred F. Van Gunsteren Alan E. Mark 《Journal of computational chemistry》2010,31(6):1117-1125
A new force field for the simulation of dipalmitoylphosphatidylcholine (DPPC) in the liquid‐crystalline, fluid phase at zero surface tension is presented. The structure of the bilayer with the area per lipid (0.629 nm2; experiment 0.629–0.64 nm2), the volume per lipid (1.226 nm3; experiment 1.229–1.232 nm3), and the ordering of the palmitoyl chains (order parameters) are all in very good agreement with experiment. Experimental electron density profiles are well reproduced in particular with regard to the penetration of water into the bilayer. The force field was further validated by simulating the spontaneous assembly of DPPC into a bilayer in water. Notably, the timescale on which membrane sealing was observed using this model appears closer to the timescales for membrane resealing suggested by electroporation experiments than previous simulations using existing models. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010 相似文献
997.
《Mathematische Nachrichten》2017,290(2-3):349-366
In this paper, we give a new characterization for the boundedness of weighted differentiation composition operator from logarithmic Bloch spaces to Bloch‐type spaces and calculate its essential norm in terms of the n‐th power of induced analytic self‐map on the unit disk. From which a sufficient and necessary condition of compactness of this operator follows immediately. 相似文献
998.
999.
Bin Yu 《Journal of Physics and Chemistry of Solids》2010,71(4):468-116
Casein, a natural biopolymer contained in milk, has been successfully intercalated into a Ca-Al-LDH host structure. Synthesis was performed by rehydration of tricalcium aluminate in the presence of casein. The resulting nanohybrids were characterized by powder X-ray diffraction (XRD), elemental analysis, infrared spectroscopy (IR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Further experiments revealed that the single protein strains intercalate instead of the entire casein sub micelles, which are present in milk. Additionally, the pure phospho protein fractions α- and β-casein, which make up ∼80 wt% of total casein were isolated and intercalated into the Ca-Al-LDH host structure, yielding a biopolymer-inorganic hybrid material. 相似文献
1000.
The potential of using encapsulation by MEA, DEA and TEA to control the morphology of ZnO powders was investigated. The crystallite and particle size decreased as a function of aminoalcohol concentration. We found that aminoalcohols can inhibit the crystal growth of ZnO along the c-axis. A steric effect by TEA more strongly influenced the formation of different ZnO shapes than did MEA and DEA. The value of the band gap was dependent on the crystallite size, particle size and particle shape. 相似文献