全文获取类型
收费全文 | 17095篇 |
免费 | 496篇 |
国内免费 | 857篇 |
专业分类
化学 | 7015篇 |
晶体学 | 81篇 |
力学 | 233篇 |
综合类 | 10篇 |
数学 | 1709篇 |
物理学 | 9400篇 |
出版年
2023年 | 137篇 |
2022年 | 144篇 |
2021年 | 152篇 |
2020年 | 227篇 |
2019年 | 275篇 |
2018年 | 278篇 |
2017年 | 251篇 |
2016年 | 355篇 |
2015年 | 345篇 |
2014年 | 487篇 |
2013年 | 1056篇 |
2012年 | 1097篇 |
2011年 | 1280篇 |
2010年 | 921篇 |
2009年 | 1136篇 |
2008年 | 972篇 |
2007年 | 1104篇 |
2006年 | 796篇 |
2005年 | 638篇 |
2004年 | 586篇 |
2003年 | 518篇 |
2002年 | 479篇 |
2001年 | 293篇 |
2000年 | 298篇 |
1999年 | 283篇 |
1998年 | 287篇 |
1997年 | 176篇 |
1996年 | 189篇 |
1995年 | 204篇 |
1994年 | 156篇 |
1993年 | 144篇 |
1992年 | 140篇 |
1991年 | 112篇 |
1990年 | 99篇 |
1989年 | 96篇 |
1988年 | 75篇 |
1985年 | 179篇 |
1984年 | 190篇 |
1983年 | 135篇 |
1982年 | 197篇 |
1981年 | 177篇 |
1980年 | 135篇 |
1979年 | 152篇 |
1978年 | 156篇 |
1977年 | 204篇 |
1976年 | 198篇 |
1975年 | 191篇 |
1974年 | 228篇 |
1973年 | 219篇 |
1972年 | 114篇 |
排序方式: 共有10000条查询结果,搜索用时 41 毫秒
61.
Infrared and nuclear magnetic resonance (NMR) spectroscopies have been applied for quantitative comparison on unsaturated bonds in tetrafluoroethylene (TFE)-propylene (P)-vinylidene fluoride (VdF) terpolymers. These spectroscopies have led to consistent results while they are based on completely different principles. It is shown that the absolute concentration can be evaluated from NMR measurements. The behavior of unsaturation has been discussed in terms of after-treatment such as wet and dry treatments. The result obtained here can give the important information to improve the performance of fluoroelastomer based on TFE-P-VdF terpolymers. 相似文献
62.
63.
E. García-Matres N. Stüßer M. Hofmann M. Reehuis 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,32(1):35-42
The magnetic structures of Mn1-xFexWO4 with x
= 0.0, 0.16, 0.21, 0.225, 0.232, 0.24, 0.27, 0.29, and 1.0 were refined from neutron powder diffraction data. The magnetic phase
diagram could be completed in the coexistence range of different magnetic structures up to x
= 0.29. For the magnetic state at 1.5 K a commensurate antiferromagnetic structure with a propagation vector
= (±1/4, 1/2, 1/2) was found for x
⩽ 0.22 while the magnetic spins order with
= (1/2, 0, 0) for x
≥ 0.22. In the latter phase, additionally, weak magnetic reflections indexed to an incommensurate ordering with
= (- 0.214, 1/2, 0.457) occur in the diffraction pattern up to x
= 0.29 indicating the occurence of a reentrant phase. For 0.12 ⩽
x
⩽ 0.29 the low temperature phases are separated from a magnetic high temperature phase showing only magnetic reflections indexed
to a spin arrangement with
= (1/2, 0, 0). The magnetic phase diagram is discussed qualitatively considering random superexchange between the statistically distributed
Mn2+- and Fe2+-ions in the coexistence range 0.12 ⩽
x
⩽ 0.29 of different magnetic structures related to those of pure MnWO4 and FeWO4.
Received 9 October 2002 Published online 14 March 2003 相似文献
64.
As predicted by Haldane, spin, S=1 one-dimensional (1D) Heisenberg antiferromagnet (HAF) has an energy gap between the singlet ground state and first excited triplet. On application of magnetic field, the triplet state Zeeman splits and the energy of one of the triplet state becomes zero at a critical field, Hc. Above Hc the system recovers magnetism. Then, we expect that a quasi-1D HAF will show a magnetic long-range ordering (LRO) at low temperatures due to the inter-chain coupling. This field-induced LRO has not been observed before due to complication of the crystal structure in the materials studied so far and/or technical difficulty.From a heat capacity measurement on a single crystal of an S=1 quasi-Q1D HAF, Ni(C5H14N2)2N3(PF6), we found an anomaly at a temperature in finite fields indicating a field-induced phase transition. A magnetic LRO is confirmed by a neutron diffraction measurement on the same sample. The temperature versus magnetic field phase diagram of this compound is constructed and discussed. 相似文献
65.
A. M. Yakunin A. Yu. Silov P. M. Koenraad W. Van Roy J. De Boeck J. H. Wolter 《Superlattices and Microstructures》2003,34(3-6):539
An individual Mn acceptor in GaAs is mapped by cross-sectional scanning tunneling microscopy (X-STM) at room temperature and a strongly anisotropic shape of the acceptor state is observed. An acceptor state manifests itself as a cross-like feature which we attribute to a valence hole weakly bound to the Mn ion forming the (Mn2+3d5+hole) complex. We propose that the observed anisotropy of the Mn acceptor wavefunction is due to the d-wave present in the acceptor ground state. 相似文献
66.
E. A. El-Amri M. I. Al-Jarallah F. Abu-Jarad Fazal-ur-Rehman 《Radiation measurements》2003,36(1-6):453-456
The uniformity in radon exhalation rates for 46 tiles of granite, marble and ceramic used as construction materials were determined using “Can Technique” employing CR-39 nuclear track detectors (NTDs). On each tile, two sealed cans, each enclosing one NTD fixed at the center of the tile surface area covered by the can, were mounted at two different locations of each individual tiles. The track production rates on the NTDs representing radon exhalation rates were measured. The radon exhalation rates from the surface of individual tiles showed uniform exhalations within the calculated uncertainties of the measured values. This makes Can Technique an alternative simple method to measure radon exhalation rates. Calibration required to convert track production rates into radon exhalation rates for the used can and NTD was done using an active technique. The correlation between the measurements by the two techniques shows a good linear correlation coefficient (0.83). 相似文献
67.
通过解析推导和数值计算的方法,得到了平衡态指向矢满足的微分方程和边界条件.研究了表面弹性能K13项对磁场作用下的弱锚定向列液晶盒Fréedericksz转变性质的影响.结果表明,表面弹性能K13项的存在对液晶系统的自由能有很大的影响,从而改变转变的性质,诱导液晶盒在阈值点发生一级Fréedericksz转变.给出了发生一级转变的物理条件,它除了与液晶的结构和材料有关外,还依赖于液晶表面弹性能K13项,同时给出了由此判断K13项是否存在的检验方法.
关键词:
表面弹性能K13项
弱锚定
Fréedericksz转变 相似文献
68.
偏滤器靶板磁预鞘层边界的离子流 总被引:1,自引:1,他引:0
利用流体力学模型研究了偏滤器等离子体条件下磁预鞘层边界离子流和预鞘层宽度之间的关系,并分析了粒子碰撞对预鞘层边界离子流的影响。由于粒子碰撞对离子的驱动效益,预鞘层边界离子流速度随着预鞘层宽度的增加而减少;只有当预鞘层宽度趋于零时,边界离子流沿磁力线的速度才趋近于离子声速。在固定预鞘层宽度的条件下,计算了预鞘层边界离子流速度与碰撞频率之间的函数关系。 相似文献
69.
《Magnetic resonance in chemistry : MRC》2003,41(9):731-734
An NMR study of one new and several known abietane diterpenes isolated from the roots of Aegiphila lhotzkyana is described. In addition to 1D NMR, several 2D shift‐correlated NMR pulse sequences (1H–1H‐COSY, NOESY, HMQC and HMBC) were used to establish all the structures, and unambiguously perform the 1H and 13C chemical shift assignments of the new natural diterpene and three derivatives, the NMR data for which have not been reported previously. Revision of current data assignment for teuvincenone H is also suggested. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
70.
K. L. Janssens B. Partoens F. M. Peeters 《Physica E: Low-dimensional Systems and Nanostructures》2004,21(2-4):349
We investigated theoretically the influence of strain on the exciton in both single and three vertically coupled self-assembled quantum dot systems in the presence of a perpendicular magnetic field. For the single disk, we find that the heavy-hole exciton is the ground state, while for the system of three stacked disks, the light hole state was found to be lower in energy. Results for the diamagnetic shift were compared with experimental results. 相似文献