全文获取类型
收费全文 | 7751篇 |
免费 | 1183篇 |
国内免费 | 1001篇 |
专业分类
化学 | 5626篇 |
晶体学 | 50篇 |
力学 | 501篇 |
综合类 | 63篇 |
数学 | 845篇 |
物理学 | 2850篇 |
出版年
2024年 | 20篇 |
2023年 | 143篇 |
2022年 | 253篇 |
2021年 | 297篇 |
2020年 | 313篇 |
2019年 | 299篇 |
2018年 | 223篇 |
2017年 | 203篇 |
2016年 | 322篇 |
2015年 | 373篇 |
2014年 | 384篇 |
2013年 | 506篇 |
2012年 | 613篇 |
2011年 | 707篇 |
2010年 | 448篇 |
2009年 | 436篇 |
2008年 | 498篇 |
2007年 | 438篇 |
2006年 | 427篇 |
2005年 | 412篇 |
2004年 | 289篇 |
2003年 | 256篇 |
2002年 | 256篇 |
2001年 | 222篇 |
2000年 | 171篇 |
1999年 | 161篇 |
1998年 | 152篇 |
1997年 | 132篇 |
1996年 | 143篇 |
1995年 | 153篇 |
1994年 | 118篇 |
1993年 | 106篇 |
1992年 | 75篇 |
1991年 | 100篇 |
1990年 | 60篇 |
1989年 | 60篇 |
1988年 | 48篇 |
1987年 | 31篇 |
1986年 | 22篇 |
1985年 | 21篇 |
1984年 | 11篇 |
1983年 | 11篇 |
1982年 | 11篇 |
1981年 | 5篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1976年 | 1篇 |
1966年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有9935条查询结果,搜索用时 12 毫秒
991.
The optical characteristics of transparent lead magnesium niobate titanate (PMNT) electro-optic ceramic, including the electro-optic phase modulation, electric hysteresis property and thermo-optic coefficient, are investigated in detail. Based on this novel ceramic, a polarization independent electro-optic switch by using fiber Sagnac interferometer (FSI) structure is realized. An initial π-shift is introduced into the Sagnac loop to eliminate the effect of the polarization orientation of the incident light on the switch performance. Then an electrically controllable PMNT phase retarder is used to switch the optical signal between the reflection and transmission ports. Some theoretical analyses are given and the switch performances are also discussed, including the thermal characteristic and different switching frequency response. 相似文献
992.
采用傅里叶模方法,分析了单点金刚石铣削后KDP晶体表面小尺度波纹的周期和幅值对单层增透膜折射率、厚度以及透射率的影响。研究表明:膜层最佳折射率在1.22左右,在此折射率条件下,保证透射率大于99%的单层增透膜的理想厚度范围应为180~220 nm,并且折射率和膜厚值的选取基本不受晶体表面小尺度波纹周期和幅值的影响。若只考虑SPDT法加工后KDP晶体表面小尺度波纹周期和幅值的实际范围,透射率基本不受波纹周期的影响,但却会随波纹幅值的增大而加速下降。理想镀膜条件下透射率最大值大于99%,并且通常在99.67%~99.94%之间。 相似文献
993.
Amorphous Ni-P nanotubes are fabricated through electroless chemical deposition inside an anodic aluminum oxide template. The hysteresis loops of Ni-P nanotube arrays are each found to exhibit an unusual isotropic behaviour, which is believed to be due to the competition results between the shape anisotropy and the magnetostatic interaction among nanotubes. The dynamic dependence of permittivity on the frequency spectrum is fitted to the Lorentzian-type dispersion law. The permeability dispersion behaviours have been fitted based on the Kittel equation. Electromagnetic wave absorption properties of Ni-P nanotubes/paraffin composites with different values of thickness (t) are clearly shown by a three-dimensional graph. Furthermore, the bandwidths of composites with different "t" values can be well presented by a two-dimensional contour graph, which is a novel presentation form. The results show that the composites each have a good microwave absorption performance with t larger than 5.5 mm and with the frequency around 8 gigahertz. 相似文献
994.
F. Fan Y.M. Lu L.L. Ying Z.Y. Liu C.B. Cai R. Hühne B. Holzapfel 《Physica C: Superconductivity and its Applications》2011,471(15-16):471-475
Biaxially textured Ce2Y2O7 (CYO) films were deposited on Ni–5at.%W (Ni–5W) tapes by a DC reactive sputtering technique in a reel-to-reel system. Subsequent YBa2Cu3O7?δ (YBCO) films were prepared using pulsed laser deposition leading to a simplified coated conductor architecture of YBCO/CYO/Ni–5W. X-ray diffraction measurements revealed an epitaxial growth of the CYO buffer layer with a texture spread down to 2.2° and 4.7° for the out-of-plane and in-plane alignment, respectively. Microstructural investigations showed a dense, smooth and crack-free surface morphology for CYO film up to a thickness of 350 nm, implying an effective suppression of cracks due to the incorporation of Y in CeO2. The superconducting transition temperature Tc of about 90 K with a narrow transition of 0.8 K and the inductively measured critical current density Jc of about 0.7 MA/cm2 indicate the potential of the single CYO buffer layer. 相似文献
995.
Based on the generalized Raleigh-Sommerfeld diffraction integral, an analytical nonparaxial propagation formula of a partially
coherent dark hollow beam (DHB) in free space is derived. The paraxial approximation is dealt with as a special case of our
general result. By using the derived formula, the nonparaxial propagation properties of DHBs are studied and are compared
with the paraxial results. It is shown that the nonparaxiality properties of a partially coherent DHB are closely related
to its initial beam parameters (i.e., beam waist size, wavelength, coherence length, dark size) and the propagation distance. 相似文献
996.
Yong?WanEmail author Zhongyu?Cai Qin?Li X.?S.?Zhao 《Applied Physics A: Materials Science & Processing》2011,102(2):373-377
Silicon-based photonic crystal is a promising material for terahertz (THz) waveguide due to its high refractive index contrast.
In this work, we introduce eye-shaped pillars as the feature building blocks for two-dimensional (2D) photonic crystals. The
simulation study shows that larger TE mode band gaps (PBGs) can be created by the arrangement of dielectric eye-shaped pillars
in air. The reflective spectra demonstrate that there are complex PBGs, where the peak position and intensity can be changed
by varying the parameter e. Moreover, the peak of reflective spectra exhibits an obvious blue shift with the increase of incidence angle of light. When
the vacant space in the structure is filled by polystyrene (PS) microspheres of 2 μm in diameter, the peak intensity of reflective
spectra reduces significantly compared with that without PS microspheres, which suggests that this design can act as a sensor
in the fields of biology, agriculture or medicine. 相似文献
997.
基于抽取和连续投影算法的可见近红外光谱变量筛选 总被引:1,自引:0,他引:1
大多数短波CCD硅检测器为2 048或3 648像元,相邻波长间隔小,预处理算法对其适用性差.本文在600.09~980.47 nm光谱范围内,采用等间隔抽取方法重构光谱矩阵.经不同光谱预处理后,分别采用遗传算法(GA)和连续投影算法(SPA),筛选偏最小二乘法(PLS)建模变量.采用留一法交叉验证评价模型的预测能力,... 相似文献
998.
采用了熔盐法新工艺制备了纯相与掺钒的MnNb2O6粉晶,利用X射线衍射仪(XRD),扫描电子显微镜(SEM),能谱分析(EDX),透射电子显微镜(TEM)、高分辨透射电镜(HRTEM)和电子衍射(SAED)分析了其物相、形貌及微结构.结果表明合成产物为正交晶系钶铁矿型MnNb2O6;在不同的熔盐中合成出了棒状、片状与长方体形貌的纯相产物.讨论了温度与掺杂对结构与形貌的影响,HRTEM与SAED分析表明了产物的各向异性生
关键词:
熔盐法
结构与形貌
掺杂
反铁磁性 相似文献
999.
The structural properties, elastic properties and electronic structures of hexagonal Al3RE intermetallic compounds are calculated by using first-principles calculations based on density functional theory. Since there exists strong on-site Coulomb repulsion between the highly localized 4f electrons of RE atoms, we present a combination of the GGA and the LSDA+U approaches in order to obtain the appropriate results. The GGA calculated lattice constants for the hexagonal Al3RE intermetallic compounds are in good agreement with available experimental values. The results of cohesive energy indicate that these compounds can be stable under absolute zero Kelvin and the stability of Al3Gd is the strongest in all of the hexagonal Al3RE compounds. The densities of states for GGA and LSDA+U approaches are also obtained for the Al3RE intermetallic compounds. The mechanical properties are calculated from the GGA method in this paper. According to the computed single crystal elastic constants, Al3La, Al3Sm and Al3Gd are mechanically unstable, while Al3Ce, Al3Pr and Al3Nd are stable. The polycrystalline elastic modulus and Poisson’s ratio have been deduced by using Voigt-Reuss-Hill (VRH) approximations, and the calculated ratio of bulk modulus to shear modulus indicates that Al3La compound is ductile material, but Al3Ce, Al3Pr, Al3Nd, Al3Sm and Al3Gd are brittle materials. 相似文献
1000.
A simple sensor system for high spatial resolution distributed strain field measurement is proposed and demonstrated experimentally. The fiber loop ringdown technique combined with a linear chirped fiber Bragg grating is used to realize the high spatial resolution. A proof-of-concept distributed strain sensor with 2?mm spatial resolution is realized. The sensor network is also explored and researched experimentally. The proposed technique suggests a broad range of applications for real-time distributed physical parameter sensing, such as strain or temperature. 相似文献