全文获取类型
收费全文 | 575篇 |
免费 | 10篇 |
专业分类
化学 | 306篇 |
晶体学 | 12篇 |
力学 | 12篇 |
数学 | 86篇 |
物理学 | 169篇 |
出版年
2017年 | 5篇 |
2016年 | 9篇 |
2015年 | 8篇 |
2014年 | 10篇 |
2013年 | 15篇 |
2012年 | 18篇 |
2011年 | 19篇 |
2010年 | 12篇 |
2009年 | 21篇 |
2008年 | 15篇 |
2007年 | 12篇 |
2006年 | 13篇 |
2005年 | 14篇 |
2004年 | 6篇 |
2003年 | 14篇 |
2002年 | 12篇 |
2001年 | 18篇 |
2000年 | 18篇 |
1999年 | 16篇 |
1998年 | 8篇 |
1997年 | 13篇 |
1996年 | 5篇 |
1995年 | 5篇 |
1994年 | 7篇 |
1993年 | 12篇 |
1992年 | 9篇 |
1990年 | 8篇 |
1989年 | 8篇 |
1988年 | 9篇 |
1987年 | 5篇 |
1986年 | 9篇 |
1985年 | 6篇 |
1984年 | 9篇 |
1983年 | 9篇 |
1982年 | 12篇 |
1981年 | 8篇 |
1980年 | 5篇 |
1978年 | 8篇 |
1968年 | 5篇 |
1966年 | 5篇 |
1933年 | 5篇 |
1932年 | 6篇 |
1930年 | 7篇 |
1929年 | 6篇 |
1927年 | 8篇 |
1926年 | 4篇 |
1925年 | 7篇 |
1924年 | 10篇 |
1923年 | 8篇 |
1913年 | 4篇 |
排序方式: 共有585条查询结果,搜索用时 31 毫秒
91.
Wassim M. Haddad Emmanuel G. Collins Jr. Raymond Moser 《Mathematical and Computer Modelling of Dynamical Systems: Methods, Tools and Applications in Engineering and Related Sciences》2013,19(4):299-318
Models of systems are always inexact. Hence, to better predict the performance of a system it is necessary to take into account uncertainty in a nominal model of a system. The structured singular value was developed to nonconservatively analyze robust stability and performance for systems with multiple-block uncertainty. In practice, optimization techniques are used to compute an upper bound on the structured singular value. For dynamic uncertainty with bounded magnitude and arbitrary phase (i.e., "complex uncertainty"), the standard approach to computing an upper bound involves finding diagonal scaling matrices D(jω) that minimize σmax (D(jω)G(jω)D-1(jω)) over a (theoretically) infinite number of frequencies. The order of the corresponding stable, minimum phase, rational function D(s) (if it exists) is hence arbitrary, which can lead to very high order controllers when D(s) is used for controller synthesis. This paper develops a fixed-structure approach to computing an upper bound for the complex structured singular value. In particular, by relying on results from mixed-norm H2/H∞ analysisD(s) is a priori constrained to be a rational matrix function of a chosen order and a new approach to computing an upper bound on the structured singular value is developed. The results are illustrated using two examples which clearly demonstrate the suboptimality of standard curve fitting. The proposed approach can be extended to mixed uncertainty and structured singular value controller synthesis without D — K type iteration. 相似文献
92.
Abstract Optical infrared (IR) fibers with core-clad structure are of great importance because they have better qualities than unclad fibers for most IR fiber applications, especially in CO2 laser power delivery and radiometry. We have fabricated core-clad polycrystalline silver halide optical fibers with different compositions and core diameters, and although their loss is still higher than that of unclad fibers, they already have many advantages and new capabilities. The behavior of the scattering loss along these fibers and other optical properties was measured and compared with that of unclad silver halide fibers. We show that the higher loss of clad fibers results mainly from excessive scattering. The improvement in the process of fabricating clad fibers enabled the production of new elements such as single-mode fibers (SMFs) and fiber bundles for thermal imaging. 相似文献
93.
94.
We demonstrate 60-fs pulses with an average output power of 84 mW from a diode-pumped Nd:glass laser mode locked by a low-finesse antiresonant Fabry-Perot saturable absorber (A-FPSA). The mode-locked spectrum spreads over most of the available Nd:glass fluorescence bandwidth. At increased pulse energy fluence or decreased negative group-velocity dispersion, multiple pulsing was observed. We experimentally characterize this behavior, which can be explained by the saturation behavior of the A-FPSA and the limited available gain bandwidth. These considerations are significant for the design of saturable absorbers to achieve stable passive mode locking. 相似文献
95.
We have demonstrated 175-fs pulses with 1 W and 300-fs pulses with 1.2 W of average output power at a pulse repetition rate of 117 MHz from a Nd:phosphate (Schott LG 760) glass laser pumped by a 1-cm-wide, 20-W diode laser bar. Stable soliton mode locking was achieved by use of an intracavity semiconductor saturable absorber mirror. We obtained more than 2 W of average power without mode locking. Using cylindrical cavity mirrors, we adapted the laser mode inside the Nd:glass to the highly elliptical pump beam in both dimensions (tangential and sagittal axes) while maintaining a nearly ideal circular TEM(00) output beam with M(2) approximately 1.2 . Overpumping the laser mode in the tangential plane and efficient unidirectional heat removal in the sagittal plane using a 0.8-mm thin Nd:glass also contributed to the good output-beam quality. 相似文献
96.
A. F. Joseph F. P. Treadwell W. T. Hall F. Sutton L. L. de Koninck J. M. Kolthoff F. W. Küster Fr. L. Hahn H. Windisch W. Böttger C. Russo A. Monnier O. Hackl W. M. Thornton jr. J. E. Chapman L. Brandt E. Knecht Eva Hibbert O. Tomiček E. Polidori B. Diethelm F. Foerster W. M. Thornton L. Moser W. A. Hicks K. Schröder 《Analytical and bioanalytical chemistry》1926,68(9-10):369-384
97.
98.
Top-emitting Alas/AlGaAs vertical cavity surface emitting lasers emitting at 765 nm with minimum threshold currents of 0.6 mA and threshold voltages of 1.9 V have been grown by MOVPE. In order to keep the growth time low, we investigated the possibility to grow these structures at growth rates of 5 μm/h. Special attention was paid to the homogeneity that can be achieved over a 2″ wafer under these growth conditions. Spatially resolved reflectivity measurements on GaAs/AlAs distributed Bragg reflectors showed, that the growth rate varies less than 0.3% in the center of the wafer and decreases by 1% at the wafer edge. 相似文献
99.
Shasta?Lee?Moser Valerio?Bertolasi Keith?VaughanEmail author 《Journal of chemical crystallography》2005,35(4):307-312
Methyl 4-((E)-2-{3-[(3-{(E)-2-[4-(methoxycarbonyl)phenyl]-1-diazenyl}-5, 5-dimethyl hexahydro-1-pyrimidinyl)methyl]-5, 5- dimethylhexahydro-1- pyrimidinyl}-1- diazenyl) benzoate (1) has been synthesized by reaction of a mixture of formaldehyde and 2,2-dimethyl-1,3-propanediamine with p-methoxycarbonylbenzenediazonium chloride. The crystal structure of 1 has been determined by single crystal X-ray diffraction analysis. The crystals of 1 displayed problems of disorder; the asymmetric unit is built up by two independent molecules, which display disorder within a methoxycarbonyl group. The two independent molecules in the asymmetric unit are very similar, differing only slightly about the mutual orientation of the aryltriazenyl groups. With respect to the N–C–N bridge between the hexahydropyrimidinyl rings, the two equivalent fragments containing the triazene moieties are in a cis orientation. The N–N single bonds, in the range 1.319(4)–1.348(4) Å, and the N=N double bonds, in the range 1.255(5)–1.275(4) Å, indicate significant conjugations within the triazene moieties. All the hexahydropyrimidine six-membered rings adopt a chair conformation as shown by their puckering parameters. The crystal packing is determined only by simple van der Waals interactions. The crystal structure of 1 is compared with the previously reported structure of the unbranched hexahydropyrimidine analogue 2. Crystal data: 1 C29H40N8O4, triclinic, space group P-1, a = 13.9652(2), b = 14.2908(3), c = 16.6129(4) Å, = 97.279(1), = 90.872(1), = 107.385(1), V = 3133.6(1) Å3, for Z = 4. 相似文献
100.
M. Sbetti E. Moser M. Montagna M. Ferrari S. Chaussedent M. Bettinelli 《Journal of Non》1997,220(2-3):217-221
In this work we report on the temperature depedence of the homogeneous line width (Γh) in 4ZnO · 3B2O3:Eu3+ glass in the region between 30 K and room temperature. The time resolved resonant fluorescence line narrowing technique was applied to the 7F0 ↔ 5D0 transition of Eu3+. We obtain that Γh follows the commonly found behaviour T2 in the whole range of temperature (30–300 K). The Γh is comparable to that measured in borate glasses with a small amount of network modifier. This result, together with a linear increase of Γh with the excitation energy within the inhomogeneous profile, is indicative of a specific local environment around the Eu3+ ion in the zinc borate glass. 相似文献