全文获取类型
收费全文 | 2571篇 |
免费 | 673篇 |
国内免费 | 179篇 |
专业分类
化学 | 1520篇 |
晶体学 | 185篇 |
力学 | 57篇 |
综合类 | 17篇 |
数学 | 37篇 |
物理学 | 1607篇 |
出版年
2024年 | 9篇 |
2023年 | 22篇 |
2022年 | 47篇 |
2021年 | 64篇 |
2020年 | 67篇 |
2019年 | 61篇 |
2018年 | 66篇 |
2017年 | 105篇 |
2016年 | 144篇 |
2015年 | 83篇 |
2014年 | 117篇 |
2013年 | 448篇 |
2012年 | 190篇 |
2011年 | 136篇 |
2010年 | 144篇 |
2009年 | 143篇 |
2008年 | 149篇 |
2007年 | 119篇 |
2006年 | 171篇 |
2005年 | 127篇 |
2004年 | 106篇 |
2003年 | 104篇 |
2002年 | 135篇 |
2001年 | 73篇 |
2000年 | 85篇 |
1999年 | 82篇 |
1998年 | 63篇 |
1997年 | 48篇 |
1996年 | 43篇 |
1995年 | 45篇 |
1994年 | 32篇 |
1993年 | 30篇 |
1992年 | 23篇 |
1991年 | 11篇 |
1990年 | 11篇 |
1989年 | 11篇 |
1988年 | 7篇 |
1987年 | 8篇 |
1986年 | 8篇 |
1985年 | 15篇 |
1984年 | 15篇 |
1982年 | 14篇 |
1981年 | 11篇 |
1980年 | 4篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1976年 | 6篇 |
1975年 | 2篇 |
1973年 | 4篇 |
1972年 | 2篇 |
排序方式: 共有3423条查询结果,搜索用时 593 毫秒
931.
采用固相反应法制备了不同含氧量的BiFeOδ多晶陶瓷样品,利用HP4294A阻抗分析仪测量了样品的介电特性随频率和氧含量的变化,用正电子湮没寿命谱学的方法研究了样品中因氧含量的变化所引起的结构缺陷. 实验结果表明:引入氧空位和氧填隙离子缺陷都会使介电常数减小,而介电损耗则随氧含量的增加而增加,二者的变化范围均在10%—35%之间;对不同氧含量的BiFeOδ样品,介电常数和介电损耗随测量频率的增加而减小. 氧空位的引入使得局域电子密度变小,正电子平均寿命τm增加. 在氧含量δ=2.99时电子密度最大(ne=3.90×1023/cm3),继续增加氧含量对正电子寿命与局域电子密度的影响不大. BiFeOδ样品的介电常数和介电损耗随氧含量的变化可以在空间电荷限制电导的框架下来理解. 相似文献
932.
Electrical and dielectric properties of conducting polypyrrole–wide band gap silica (PPY–SiO2) nanocomposites have been investigated as a function of temperature and frequency for different concentrations of polypyrrole.
The average grain size of the nanocomposites is in the range of 40–80 nm. Impedance spectra reveal two distorted semicircles
corresponding to grain and grain boundary effects. The magnitude of conductivity and its temperature variation are significantly
different from polypyrrole and silica. A very large dielectric constant of about 4800 at 30 kHz and at room temperature has
been observed for the highest concentration of silica. Inhomogeneous behavior of nanocomposites gives rise to high dielectric
constant. 相似文献
933.
During the period 1949–1961 Szigeti published four seminal papers on the dielectric behaviour of crystals. Szigeti’s theory
is applicable to isotropic and anisotropic, ionic and covalent crystals with different structures. Szigeti’s theory connects
dielectric, spectroscopic and elastic properties. An important outcome of Szigeti’s theory is the concept of the effective
ionic charge (s). It is pointed out that s correlates with a number of physical properties and is a measure of ionicity of the interatomic bond. Since Szigeti’s work,
several theoretical models have been proposed to account for the fact that s < 1. These models provide an insight into the complex polarization mechanisms in solids. This review summarizes Szigeti’s
work and the work that followed; the implications and applications of Szigeti’s theory are discussed. Some new results are
also included.
Dedicated to the memory of late Prof. P S Narayanan. 相似文献
934.
Kazuyuki Sumida Kosuke Hiramatsu Wataru Sakamoto Toshinobu Yogo 《Journal of nanoparticle research》2007,9(2):225-232
Transparent BaTiO3 nanoparticle/polymer hybrid was synthesized by polymerization and hydrolysis of barium titanium alkoxide modified with 2-vinyloxyethoxy
ligand. Barium alkoxide, titanium alkoxide and 2-vinyloxyethanol were reacted affording a BaTiO3 precursor, which was then hydrolyzed and polymerized to form BaTiO3 particle/polymer hybrids below 100°C. BaTiO3 particles increased in crystallinity with increasing water amount for hydrolysis. The absorption edge of the hybrid film
on silica plates shifted to shorter wavelength with decreasing crystallite size. Nano-sized BaTiO3 particle/polymer hybrid polymerized with methyl methacrylate (MMA) was shaped into a transparent and self-standing film with
a refractive index of 1.595 at 589 nm. 相似文献
935.
Soft magnetic properties of Co-based amorphous alloy of the composition Co67Fe4Ni2Si15B12 have been investigated by isothermal heat treatment up to the conventional crystallization temperature. In the as-cast condition the Curie temperature of the sample is 272 °C and saturation magnetization is 74 emu/g. Magnetic properties undergo variation depending on the heat treatment temperature. For the heat treatment temperatures of around 420 and 490 °C, superior soft magnetic properties are obtained. For both the temperatures initial permeability, μ′ reaches value up to ten times the value of permeability in the as-received samples. Annealing effect on giant magneto-impedance has been observed for the current-driving frequencies of 4.5 and 6 MHz. Field dependence of magneto-impedance shows hysteresis at low field, which is related to the changes in the magnetization process of the sample. 相似文献
936.
用溶胶-凝胶方法在Pt/Ti/SiO2/Si衬底上制备了Na+的不同浓度均匀掺杂和成分梯度掺杂(上梯度)钛酸锶钡(Ba0.25Sr0.75TiO3)薄膜.电性能测试表明随着均匀掺杂浓度的增加,薄膜介电常数和损耗都减小,而漏电流先减小(掺杂浓度小于2.5mol%时)后又逐渐增加.场发射扫描电镜分析表明,均匀掺杂浓度增加到2.5mol%后薄膜表面呈疏松多孔状结构,这可能是导致漏电流又逐渐增大的原因.Na+的上梯度掺杂避免了掺杂浓度增加到2.5mol%后薄膜生长过程中出现的孔洞现象,于是薄膜的综合电性能得到了进一步提高.深入、系统地分析了杂质不同分布方式对薄膜结构和性能有不同影响的原因. 相似文献
937.
Temperature dependence of ratio between dielectric anisotropy and order parameter in fluorinated nematic liquid crystals
下载免费PDF全文
![点击此处可从《中国物理 B》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Temperature dependence of ratio between dielectric anisotropy and order parameter of fluorinated nematic liquid crystal is investigated by using a semi-empirical molecular orbital package that can accurately calculate an angle between molecular dipole moment and long axis. We optimize the molecular conformations with three semi-empirical Hamiltonians AM1, PM3 and PM5, and then make a comparison between computational results and experimental measurements. It is shown that the results obtained from AM1 method are in good agreement with the measurements. The present study offers an applicable method to predict the dielectric properties of liquid crystal material. 相似文献
938.
The formulas of the quantum electrodynamics have been applied to calculate the spontaneous emission rate of excited atom in dielectric microcavity.The results exhibit damping oscillating Patterns which depend sensitively on the scaling parameter and geometrical structure.Compared with the case that the emitting atom is immersed in dielectric,the spontaneous emission rate is depressed obviously and the center or the mean value of the oscillations is intimately related to the real refractive index of the local position where the atom is.In order to explain this phenomenon,we utilize the closed-orbit theory to deal with the classical trajectories of the emitted photon.and extract the corresponding frequencies of the oscillations by Fourier transform.It is found that the oscillations can be represented in terms of the closed-orbits of the photon motion constrained in dielectric microcavity,thus providing another perspective on the spontaneous emission of atom sandwiched by dielectric slabs. 相似文献
939.
S. Tarapov T. Bagmut A. Granovsky V. Derkach S. Nedukh A. Plevako S. Roschenko I. Shipkova 《International Journal of Infrared and Millimeter Waves》2004,25(11):1581-1589
The millimeter waveband giant magnetoimpedance (GMI) and magnetoresonance (FMR) properties of thin-layered magnetic nanostructures are analyzed experimentally for wide temperature range. The correlation between FMR and GMI features are under discussion. The manifestation of the anisotropy established by the millimeter waveband FMR experiments and possible reasons of anisotropy appearance are given. 相似文献
940.
Hongwei Chen Chuanren Yang Chunlin Fu Dayu Zhou 《International Journal of Infrared and Millimeter Waves》2005,26(2):307-313
The microwave dielectric properties and microstructures of Ba(Mg1/3Ta2/3)O3 (BMT) ceramics were investigated. In order to reduce the evaporation of MgO at high temperature, burying in MgO powders sintered BMT ceramics. The relationship between the composition and Q value of BMT ceramics was discussed. Dielectric resonators using in millimeter wave band have been designed and fabricated. Their attenuations at central resonation frequency (f) are less than 35dB. Temperature coefficients of resonant frequency are adjustable and less than 2 ppm/°C. 相似文献