全文获取类型
收费全文 | 345609篇 |
免费 | 4329篇 |
国内免费 | 1678篇 |
专业分类
化学 | 189673篇 |
晶体学 | 4862篇 |
力学 | 14686篇 |
综合类 | 66篇 |
数学 | 38325篇 |
物理学 | 104004篇 |
出版年
2020年 | 2675篇 |
2019年 | 2875篇 |
2018年 | 3678篇 |
2017年 | 3722篇 |
2016年 | 5672篇 |
2015年 | 3696篇 |
2014年 | 5518篇 |
2013年 | 14216篇 |
2012年 | 11638篇 |
2011年 | 14353篇 |
2010年 | 9993篇 |
2009年 | 9896篇 |
2008年 | 13335篇 |
2007年 | 13475篇 |
2006年 | 12763篇 |
2005年 | 11697篇 |
2004年 | 10358篇 |
2003年 | 9167篇 |
2002年 | 9102篇 |
2001年 | 10060篇 |
2000年 | 7717篇 |
1999年 | 5784篇 |
1998年 | 4894篇 |
1997年 | 4857篇 |
1996年 | 4656篇 |
1995年 | 4213篇 |
1994年 | 4259篇 |
1993年 | 4045篇 |
1992年 | 4469篇 |
1991年 | 4633篇 |
1990年 | 4295篇 |
1989年 | 4185篇 |
1988年 | 4088篇 |
1987年 | 3976篇 |
1986年 | 3918篇 |
1985年 | 5135篇 |
1984年 | 5304篇 |
1983年 | 4416篇 |
1982年 | 4772篇 |
1981年 | 4464篇 |
1980年 | 4209篇 |
1979年 | 4508篇 |
1978年 | 4833篇 |
1977年 | 4759篇 |
1976年 | 4848篇 |
1975年 | 4499篇 |
1974年 | 4638篇 |
1973年 | 4684篇 |
1972年 | 3482篇 |
1971年 | 2859篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
We examine the ferroelectric-relaxor behavior of (Ba0.65Sr0.35)(Zr0.35Ti0.65)O3 (BSZT) ceramics in the temperature range from 80 to 380 K. A broad dielectric maximum, which shifts to higher temperature with increasing frequency, signifies the relaxor-type behavior of these ceramics. The value of the relaxation parameter γ∼2 estimated from the linear fit of the modified Curie-Weiss law, indicates the relaxor nature of the BSZT ceramics. The dielectric relaxation rate follows the Vogel-Fulcher relation with TVF=107 K, Ea=0.121 eV, and ν0=6.83×1014 Hz, further supports such relaxor nature. The slim P-E hysteresis loop and ‘butterfly’ shape dc bias field dependence of permittivity at T>Tm (Tm, the temperature of permittivity maximum) clearly signifies the occurrence of nanopolar clusters, which is the typical characteristic of ferroelectric relaxor. At 300 K and 10 kHz, the dielectric constant and loss tan δ are ∼1100 and 0.0015, respectively. The high tunability (∼25%) and figure of merit (∼130) at room temperature show that the BSZT ceramics could be a promising candidate for tunable capacitor applications. 相似文献
992.
Lancaster G.P.T. Häffner H. Wilson M.A. Becher C. Eschner J. Schmidt-Kaler F. Blatt R. 《Applied physics. B, Lasers and optics》2003,76(8):805-808
We present a scheme for employing a violet extended-cavity diode laser in experiments with single, trapped ions. For this the grating-stabilised laser is spatially and spectrally filtered and referenced to a Fabry–Pérot cavity. We measure an upper limit to the line width by observing a 305-kHz FWHM beat note with the second harmonic of a titanium sapphire laser. The laser is subsequently used to optically cool a single 40Ca+ ion close to the Doppler limit. PACS 03.67.Lx; 32.80.Pj; 42.55.Px 相似文献
993.
D. Chithrani R. L. Williams J. Lefebvre P. J. Poole G. C. Aers 《Physica E: Low-dimensional Systems and Nanostructures》2004,21(2-4):290
We discuss the preparation and spectroscopic characterisation of a single InAs/InP quantum dot suitable for long-distance quantum key distribution applications around λ=1.55 μm. The dot is prepared using a site-selective growth technique which allows a single dot to be deposited in isolation at a controlled spatial location. Micro-photoluminescence measurements as a function of exciton occupation are used to determine the electronic structure of the dot. Biexciton emission, shell filling and many-body re-normalization effects are observed for the first time in single InAs/InP quantum dots. 相似文献
994.
The logarithmic decrement and shear modulus of a bulk amorphous Zr52.5Ti5Cu17.9Ni14.6Al10 alloy were studied with an inverse torsion pendulum in the range from room temperature to the crystallization temperature and in the frequency range 5–40 Hz. The activation energy spectra of reversible and irreversible structural relaxation were estimated. The results obtained are discussed in the context of a two-energy-level model. 相似文献
995.
996.
997.
The general theory on the transformation of elastic waves at a rigid nodal junction of an arbitrary number of thin plates is considered. The notion of transformation marix is introduced, and a way for computing it and numerical examples are given. Properties of the transformation matrix resulted from the reciprocity principle and the energy conservatin law are studied. In particular, a new relation that expresses the energy conservation low for the case where a wave damped in the direction of the joint of plates is a source of a field is obtained. Bibliography: 18 titles. 相似文献
998.
Lasing and discharge in mixtures of SF6 with hydrogen and hydrocarbons are investigated. If the chemical reaction is initiated by a self-sustained discharge (a generator
with both an inductive and capacitive energy storage can be used), the maximum lasing efficiency is attained at high values
of the ratio E/p (where E is the electric field intensity and p is the pressure in the mixture) if shaped electrodes and preionization of the discharge gap are employed. For the first time,
the lasing efficiency obtained in a nonchain HF laser is as high as 10% of the energy deposit and 4.5% of the stored energy
for a specific radiation energy of ≈140 J/(l atm). At high efficiencies, the emission spectrum of the nonchain HF laser significantly
broadens and cascade lasing at P(3-2) → P(2-1) → P(1-0) vibrational transitions for several rotational lines is realized. 相似文献
999.
1000.
Bradley L. Thiel 《Mikrochimica acta》2004,145(1-4):243-247
The role of positive gaseous ions in the formation of secondary electron images in low vacuum scanning electron microscopes is discussed. This paper describes the charging processes and related effects that occur during high vacuum imaging of insulators and then discusses the influence of ions on those processes. The ions are responsible for a number of phenomena, including distortion of the electric field above and below the specimen surface due to space charge, removal of excess negative charge from the specimen, alteration of the specimen surface barrier, and scavenging/filtering of the secondary electron emission. The resulting electron-specimen-ion interactions can give rise to interesting contrast effects that are unique to this class of instruments. 相似文献