全文获取类型
收费全文 | 5334篇 |
免费 | 561篇 |
国内免费 | 153篇 |
专业分类
化学 | 2684篇 |
晶体学 | 416篇 |
力学 | 85篇 |
综合类 | 7篇 |
数学 | 71篇 |
物理学 | 2785篇 |
出版年
2024年 | 5篇 |
2023年 | 38篇 |
2022年 | 42篇 |
2021年 | 70篇 |
2020年 | 154篇 |
2019年 | 123篇 |
2018年 | 155篇 |
2017年 | 161篇 |
2016年 | 304篇 |
2015年 | 174篇 |
2014年 | 303篇 |
2013年 | 795篇 |
2012年 | 244篇 |
2011年 | 311篇 |
2010年 | 230篇 |
2009年 | 318篇 |
2008年 | 308篇 |
2007年 | 370篇 |
2006年 | 288篇 |
2005年 | 252篇 |
2004年 | 194篇 |
2003年 | 175篇 |
2002年 | 133篇 |
2001年 | 124篇 |
2000年 | 100篇 |
1999年 | 116篇 |
1998年 | 98篇 |
1997年 | 44篇 |
1996年 | 64篇 |
1995年 | 41篇 |
1994年 | 39篇 |
1993年 | 31篇 |
1992年 | 28篇 |
1991年 | 17篇 |
1990年 | 31篇 |
1989年 | 13篇 |
1988年 | 25篇 |
1987年 | 13篇 |
1986年 | 16篇 |
1985年 | 12篇 |
1984年 | 20篇 |
1983年 | 7篇 |
1982年 | 13篇 |
1981年 | 5篇 |
1980年 | 8篇 |
1979年 | 10篇 |
1977年 | 4篇 |
1975年 | 6篇 |
1974年 | 3篇 |
1973年 | 7篇 |
排序方式: 共有6048条查询结果,搜索用时 203 毫秒
991.
We have been investigating the high-temperature thermoelectric properties of some novel rare earth borides with a structure containing B12 icosahedra. Doping effects on the TE properties in such systems were investigated for the first time. A series of Nb-doped YB66 and C-doped YB66 single crystals were grown by the floating zone method. The Nb-doped compounds have approximate chemical formulas ranging from YNb0.30B66 to YNb0.33B66 while the C-doped compound has a formula of YB66C0.6. The effect of Nb-doping on the thermoelectric properties was not monotonic and appears to be complex. As a result of Nb-doping, the room temperature resistivity and the characteristic temperature T0 were considerably reduced. At room temperature the power factor of the Nb-doped YB66 sample with 89% site occupancy was three times greater than that of non-doped YB66. However, in the important high-temperature region, the non-doped sample actually exhibited the highest power factor for . Furthermore, owing to a structural feature of YB66, thermal conductivity actually increases with doping of transition metals. Taking into account all the thermoelectric properties, transition metal doping of YB66 is therefore not suitable for our purposes. On the other hand, doping of carbon, which is assumed not to go into the same sites as the transition metals, yielded a lowering of the thermal conductivity. Furthermore, contrary to Nb-doping, carbon doping did not result in a reversal of the relative magnitude of resistivity at extremely high temperatures and therefore, an increase in the figure of merit of factor 2 was realized at 1000 K. 相似文献
992.
993.
The relaxation of director fields in freely suspended smectic films is studied experimentally by means of polarizing microscopy,
and analyzed by solving the torque balance equation under appropriate initial and boundary conditions. We consider in particular
the role of anchoring conditions of the c-director at particles and defects in the film. The structure of regular relaxation
patterns allows to determine the elastic anisotropy of smectic materials. The splay elastic constant can exceed the bend constant
by a factor of two and more. A remarkable consequence of this anisotropy is the stick-slip-like relaxation around a central
defect of topological strength s = + 1. 相似文献
994.
We show that defect-free splay-bend textures are less energetic compared to uniform states in liquid-crystalline phases that
possess both quadrupolar (nematic) and octupolar (tetrahedratic) order. This is because, in such systems, there is a symmetry-allowed
linear gradient term in the energy. Another unusual feature of these splay-bend textures is the fact that they have a non-homogeneous,
space-dependent free-energy density. These results may help clarify some mysterious features noted for the B7 liquid-crystal
phase formed by achiral banana-shaped molecules. 相似文献
995.
Pelzl G Schröder MW Eremin A Diele S Das B Grande S Kresse H Weissflog W 《The European physical journal. E, Soft matter》2006,21(4):293-303
Three homologous achiral five-ring bent-core mesogens are presented where 4-chlororesorcinol is the central core and the aromatic
rings are linked by ester groups. These compounds form smectic phases with a tilted arrangement of the molecules (tilt angle
≈ 45°). On cooling the isotropic liquid this phase adopts a fan-like texture which shows for two homologues at relatively high
electric fields ( 25-35V μm^-1) an antiferroelectric electro-optical response based on the collective rotation of the molecules
around their long axes. At lower temperature the application of a sufficiently high electric field leads to a continuous transition
into a non-birefringent texture which exhibits randomly distributed domains of opposite handedness. These domains can be reversibly
switched into a state of opposite chirality by reversal of the field polarity. This switching is bistable and shows a current
response typical for a ferroelectric ground state. The possible mechanism of the field-induced phase transition, of the ferroelectric
switching and of the field-induced inversion of the chirality is discussed on the base of XRD, 13C- and 1H-NMR investigations, dielectric and electro-optical measurements. 相似文献
996.
It is shown that a smectic A droplet deposited on a solid substrate treated for strong homeotropic anchoring is faceted at
the top in spite of the fact that there are no steps at the free surface, but instead edge dislocations in the bulk. The radius
of the facet and the full profile of the curved part of the droplet are determined as a function of the temperature in the
vicinity of a nematic-smectic A phase transition. It is shown that the observed profiles do not correspond to the actual equilibrium
shape, but to metastable configurations close to their point of marginal stability. In addition, we predict that the profiles
must be different for a given temperature depending on whether the droplet has been heated or cooled down to reach this temperature.
Finally, we discuss the problem of the formation of giant dislocations in big droplets (Grandjean terraces). 相似文献
997.
Sebastião PJ Simeão Carvalho P Chaves MR Nguyen HT Ribeiro AC 《The European physical journal. E, Soft matter》2006,20(1):55-61
We present a polarising optical microscopy study of the low-temperature anticlinic-like tilted mesophase of the liquid-crystal
compound octylphenyl-2-chloro-4-(p-cyano-benzoyloxy) (DB8Cl). This mesophase has been described as a bilayer smectic structure in which the molecules within each layer are organised
in an anticlinic way. The optical textures observed in samples with planar orientation show a double stripe pattern, with
the lines aligned parallel to the rubbing direction, characteristic of a double periodic modulation of the refractive index
of the material. The long-period modulation is temperature dependent and disappears for thin sample cells (< 5μm). The short-period
modulation is nearly independent of the thickness of the cells. The experimental results are analysed in terms of a model
which considers that there is a special distribution of the principal optical axis which may be in or out of the polariser-analyser
plane. The observed periodic variation of the principal optical axis could not be interpreted in terms of the original structure
proposed for this phase. DB8Cl presents a structure formed by dimers that can be viewed as flexible bent-core-like molecules, showing similarities with
phases found in banana-like systems, but exhibiting a much more complex structure. 相似文献
998.
999.
1000.
光子晶体是一种介电常数周期变化、具有光子带隙、对光路可控的新型功能材料。将对外界刺激敏感性材料引入光子晶体空隙就得到可响应光子晶体。电场响应光子晶体是由电活性材料与光子晶体结构相结合所得,可以应用于反射型彩色显示并表现出其他显示技术所没有的独特优点。本文重点介绍了电场响应光子晶体器件的结构和响应机理,并按照引入电活性材料的不同将电场响应光子晶体分为基于液晶、聚电解质水凝胶、金属有机聚合物凝胶、导电聚合物以及核壳式电场响应光子晶体,总结了近几年各类电场响应光子晶体的研究进展,提出了在反射型彩色显示器件方面的应用以及存在的问题和展望。 相似文献