全文获取类型
收费全文 | 1530篇 |
免费 | 122篇 |
国内免费 | 4篇 |
专业分类
化学 | 1289篇 |
晶体学 | 19篇 |
力学 | 20篇 |
数学 | 40篇 |
物理学 | 288篇 |
出版年
2023年 | 15篇 |
2022年 | 23篇 |
2021年 | 41篇 |
2020年 | 37篇 |
2019年 | 62篇 |
2018年 | 54篇 |
2017年 | 29篇 |
2016年 | 48篇 |
2015年 | 57篇 |
2014年 | 63篇 |
2013年 | 105篇 |
2012年 | 152篇 |
2011年 | 149篇 |
2010年 | 67篇 |
2009年 | 49篇 |
2008年 | 92篇 |
2007年 | 82篇 |
2006年 | 77篇 |
2005年 | 65篇 |
2004年 | 57篇 |
2003年 | 54篇 |
2002年 | 38篇 |
2001年 | 13篇 |
2000年 | 23篇 |
1999年 | 12篇 |
1998年 | 8篇 |
1997年 | 12篇 |
1996年 | 6篇 |
1995年 | 7篇 |
1994年 | 8篇 |
1993年 | 5篇 |
1992年 | 6篇 |
1990年 | 8篇 |
1988年 | 7篇 |
1987年 | 6篇 |
1986年 | 5篇 |
1985年 | 12篇 |
1984年 | 5篇 |
1983年 | 7篇 |
1982年 | 7篇 |
1980年 | 4篇 |
1979年 | 12篇 |
1978年 | 9篇 |
1977年 | 8篇 |
1975年 | 8篇 |
1974年 | 4篇 |
1970年 | 4篇 |
1969年 | 4篇 |
1967年 | 6篇 |
1966年 | 5篇 |
排序方式: 共有1656条查询结果,搜索用时 15 毫秒
951.
The loss and phase advance due to diffraction are experimentally observed by measuring the amplitude and phase of radio frequency (rf) tone burst signals in the VHF range, in an ultrasonic transmission line consisting of a buffer rod with an ultrasonic transducer on one end, a couplant of water, and a solid specimen of synthetic silica glass. The measured results agree well with the calculated results from the exact integral expression of diffraction. The diffraction effects on the velocity and attenuation measured in this frequency range and their corrections are investigated to realize more accurate measurements. It is shown that attenuation measurements are influenced by diffraction losses and can be corrected by numerical calculations, and that velocity measurements are affected by the phase advance caused by diffraction. This investigation demonstrates that, in complex-mode velocity measurements, in which the velocity is determined from the measured phase of the signals, the true velocity at each frequency can be obtained by correction using the numerical calculation of diffraction. Based on this result, a new correction method in amplitude-mode velocity measurements is also proposed. In this new method, the velocity is determined from the intervals of interference output obtained by sweeping the ultrasonic frequency for the superposed signals generated by the double-pulse method. Velocity may be measured accurately at frequencies in the Fresnel region, and diffraction correction is essential to obtain highly accurate values with five significant figures or more. 相似文献
952.
Optical Review - Using the finite element method, we numerically investigated the transient behavior of molecular orientations of a liquid crystal (LC) lens with a circular electrode beside a... 相似文献
953.
Experimental and Numerical Analysis of Thermal Deformation of Electronic Packages,QFP and MCM 总被引:1,自引:0,他引:1
0Introduction Anelectronicpackageisgenerallyconstructedwithanactivesiliconchip,mountisland,gold wires,leadframesandsoldersasshowninFig.1(a).Toprotectfromtheenvironment,thesilicon chipisusuallyencapsulatedinresin.SincethesematerialshavedifferentCTE(coefficientofthermal expansion),stressandstrainaregeneratedbyinternalheatinginserviceoperationorthermalloading fromtheenvironment[1].Insomecases,thismaycausefailureofthewireandcrackinganddebonding insidethepackage.Thereliabilityofelectronicpackages… 相似文献
954.
K. Arakawa T. Mada H. Komatsu T. Shimizu M. Satou K. Takehara G. Etoh 《Experimental Mechanics》2007,47(2):277-282
The oblique impact between a golf ball and a polymethyl methacrylate (PMMA) target with smooth transparent surfaces was studied
using a high-speed video camera. Video images of the impact were recorded from the backside of the target and were used to
determine the contact time, contact area, and the displacement and rotation of the ball along the target. The average tangential
and angular velocities were determined as functions of the inbound ball velocity. As the inbound ball velocity increased,
the contact area and average tangential and angular velocities also increased while the contact time decreased. An oiled PMMA
target was used to study the effect of reduced friction between the ball and target. The results were compared with earlier
data for a steel target with relatively rough surfaces. The contact area and time were unaffected by friction, but the average
tangential velocity increased while the average angular velocity decreased as the friction decreased. 相似文献
955.
956.
To develop a system in which a π-conjugation circuit switched by a redox reaction between 3-pyridone and 3-hydroxypyridine, a ring-contracted analog of oxypyriporphyrin, 2-oxypyritriphyrin(1.2.1) was synthesized for the first time. 2-Oxypyritriphyrin(1.2.1) contains a 14π aromatic conjugation circuit with the keto-form of the 3-oxypyridine ring. When 2-oxypyritriphyrin was treated with NaBH4, not only the 3-oxypyridine group, but also the meso-carbons were also reduced to give a colorless porphyrinogen analog. The reduced compound could be oxidized again to provide 2-oxypyritripyhrin in a reversible manner. 相似文献
957.
Preparation of Amylose-Oligo[(R)-3-hydroxybutyrate] Inclusion Complex by Vine-Twining Polymerization
In this study, we attempted to prepare an amylose-oligo[(R)-3-hydroxybutyrate] (ORHB) inclusion complex using a vine-twining polymerization approach. Our previous studies indicated that glucan phosphorylase (GP)-catalyzed enzymatic polymerization in the presence of appropriate hydrophobic guest polymers produces the corresponding amylose–polymer inclusion complexes, a process named vine-twining polymerization. When vine-twining polymerization was conducted in the presence of ORHB under general enzymatic polymerization conditions (45 °C), the enzymatically produced amylose did not undergo complexation with ORHB. However, using a maltotriose primer in the same polymerization system at 70 °C for 48 h to obtain water-soluble amylose, called single amylose, followed by cooling the system over 7 h to 45 °C, successfully induced the formation of the inclusion complex. Furthermore, enzymatic polymerization initiated from a longer primer under the same conditions induced the partial formation of the inclusion complex. The structures of the different products were analyzed by X-ray diffraction, 1H-NMR, and IR measurements. The mechanism of formation of the inclusion complexes discussed in the study is proposed based on the additional experimental results. 相似文献
958.
Liquid crystal cells having polymer layers produced from copolymers of 4,4′-dimethacryloyl-oxy-biphenyl (4,4′-DMABiph) and anthracene-carrying monomers, 2,7-dimethacryloyl-oxy-anthracene (2,7-DMAAnth) and 2-methacryloyl-oxy-anthracene (2-MAAnth), were fabricated, and generation mechanism of residual direct current voltage (VrDC) was clarified. The VrDC was improved with the LC cells having the polymer layers produced from the copolymers of 4,4′-DMABiph and 2,7-DMAAnth due to large rate constant of polymerisation. In contrast, the VrDC was increased with the LC cells having the polymer layers produced from the copolymer of 4,4′-DMABiph and 2-MAAnth. The increase in the VrDC was derived from the increase in the concentration of polymerised- or unpolymerised-terminally radical and ion generated from 2-MAAnth. The results suggest that the monomers should show large rate constant of polymerisation with plural number of polymerised groups for improving the VrDC. 相似文献
959.
Keiki Kishikawa Yuki Furukawa Tomohiro Watanabe Michinari Kohri Tatsuo Taniguchi Shigeo Kohmoto 《Liquid crystals》2017,44(6):956-968
Novel C2-symmetric chiral dopant derivatives, namely, N-substituted (2R, 3R)-2,3-bis(4-(4-octyloxyphenyl)benzoyloxy)succinimides1a-h, were synthesised, and the effects of the N-substituent and imide-carbonyl groups on the helical twisting powers (HTPs) were investigated in two nematic liquid crystalline compounds, 4-n-pentyl-4?-cyanobiphenyl (5CB) and N-(4-ethoxybenzylidene)-4-n-butylaniline (EBBA). As a result, it was clarified that the bulkiness of the N-substituents has a significant correlation with the HTPs, and the imide-carbonyl group interacts strongly with the cyano group of 5CB to give high HTPs in the nematic phases. However, it is assumed that the imide-carbonyl groups of the dopants do not have strong electrostatic attractive interactions with EBBA molecules in the nematic phase to afford the moderate HTPs. 相似文献
960.
Synthesis of 8-aryl-3,5,7,3′,4′-penta-O-methylcyanidins from the corresponding quercetin derivatives by reduction with LiAlH4 is reported. Regioselective iodination at the 8-position of penta-O-methylquercetin followed by a Suzuki-Miyaura reaction gave the 8-arylated quercetin derivatives. By the reduction of 8-arylated quercetins using 4 equiv. of LiAlH4 at room temperature for 30 min, the corresponding anthocyanidins were obtained with a good yield. 相似文献