全文获取类型
收费全文 | 318938篇 |
免费 | 3835篇 |
国内免费 | 1121篇 |
专业分类
化学 | 173519篇 |
晶体学 | 4483篇 |
力学 | 13184篇 |
综合类 | 7篇 |
数学 | 37070篇 |
物理学 | 95631篇 |
出版年
2020年 | 2022篇 |
2019年 | 2001篇 |
2018年 | 2008篇 |
2017年 | 1916篇 |
2016年 | 3929篇 |
2015年 | 3319篇 |
2014年 | 4506篇 |
2013年 | 14219篇 |
2012年 | 10913篇 |
2011年 | 13562篇 |
2010年 | 8407篇 |
2009年 | 8380篇 |
2008年 | 12524篇 |
2007年 | 12761篇 |
2006年 | 12405篇 |
2005年 | 11352篇 |
2004年 | 10197篇 |
2003年 | 8996篇 |
2002年 | 8880篇 |
2001年 | 10171篇 |
2000年 | 7791篇 |
1999年 | 6177篇 |
1998年 | 4953篇 |
1997年 | 4806篇 |
1996年 | 4864篇 |
1995年 | 4439篇 |
1994年 | 4181篇 |
1993年 | 4014篇 |
1992年 | 4490篇 |
1991年 | 4356篇 |
1990年 | 4062篇 |
1989年 | 3879篇 |
1988年 | 4167篇 |
1987年 | 3831篇 |
1986年 | 3720篇 |
1985年 | 5415篇 |
1984年 | 5471篇 |
1983年 | 4429篇 |
1982年 | 4833篇 |
1981年 | 4861篇 |
1980年 | 4599篇 |
1979年 | 4728篇 |
1978年 | 4741篇 |
1977年 | 4723篇 |
1976年 | 4654篇 |
1975年 | 4557篇 |
1974年 | 4416篇 |
1973年 | 4568篇 |
1972年 | 2593篇 |
1971年 | 1918篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
972.
A. L'Esprance J. Nicolas D.K. Wilson D.W. Thomson Y. Gabillet G. Daigle 《Applied Acoustics》1993,40(4)
This paper presents a set of acoustical and meteorological data from an outdoor sound propagation experiment. This experiment was done in a farm field near Rock Springs, Pennsylvania, on 7 July 1990. Meteorological and acoustical measurements were recorded simultaneously during six different times in the day. The meteorological measurements permitted determination of the sound speed profiles during each of the measurement sessions, using a method based on surface-layer similarity scaling. The acoustical measurements allowed precise determination of the relative sound pressure levels for a frequency range up to 3150 Hz at six different distances (66, 88, 125, 175, 250 and 350 m). The results show atmospheric conditions have an important effect on sound propagation. At medium and high frequencies, variations of the relative SPL have been measured at distances as short as 62 m. These effects increased with the distances so that variations as great as 30 dB have been measured during that day. Comparisons with the fast field program predictions are also presented, and amply demonstrate the accuracies of this model, especially for the downward refraction cases. 相似文献
973.
974.
975.
A new set of matrix addressing schemes for ferroelectric liquid crystal displays is reported. The schemes use the minimum in the response time-voltage characteristic found in certain mixtures and deliver improved operating speed and contrast ratio compared with previously reported schemes operating in this mode. 相似文献
976.
This paper surveys applications of dynamic programming to decisionproblems faced by active participants in sports. 相似文献
977.
B. Vizoso I. R. Matías M. López-Amo M. A. Muriel J. M. López-Higuera 《Optical and Quantum Electronics》1995,27(10):847-857
The design of a double-parallel amplified recirculating optical delay line utilizing an erbium-doped fibre amplifier for a fibre optical bus is described. This structure is used for amplifying, equalizing and filtering multicarrier signals coming from optoelectronic units attached to fibre couplers. The use of this structure for a hybrid multimode-singlemode fibre bus that includes optoelectronic units for sensors is also demonstrated. 相似文献
978.
E. A. Klop B. J. Lommerts J. Veurink J. Aerts R. R. Van Puijenbroek 《Journal of Polymer Science.Polymer Physics》1995,33(2):315-326
Differential scanning calorimetry and high temperature x-ray diffraction were used to study the perfectly alternating copolymer of ethene and carbon monoxide (polyketone; POKC2). It was found that oriented POK-C2 fibers show a crystalline phase transition at a temperature between 110–125°C with a 10% change in crystalline density. At this temperature, the crystal structure reported recently (POK-α) is transformed to a crystal structure that was reported in the past for room temperature imperfectly alternating polyketone. The latter structure will be designated as POK-β. The influence of chain defects on the crystal structure was studied by synthesizing terpolymers (POK-C2/C3), in which small amounts of propylene-CO units are incorporated into the polymer backbone. The resulting terpolymers differ from the copolymer by the presence of methyl groups randomly distributed along the polyketone backbone chain. Evidence is presented that indicates that the methyl groups are built into the crystal lattice as defects. With more than 5 mole-% propene the terpolymer fibers crystallize exclusively in the β-modification. Below this level the α/β ratio (at room temperature) increases with decreasing amounts of propene. Both as-synthesized and as-spun POK-C2 were found to consist of both POK-α and POK-β; the α/β ratio depends on the method of preparation. Because the drawn POK-C2 fibers studied here consist exclusively of POK-α, the process of spinning and drawing leads to the transformation of unoriented β-rich material into oriented POK-α. © 1995 John Wiley & Sons, Inc. 相似文献
979.
M. J. Gaylard G. C. MacLeod D. P. Smits M. E. West D. J. Walt 《Journal of chemical crystallography》1993,23(9):763-763
Future contributions toJournal of Crystallographic and Spectroscopic Research 相似文献
980.
Summary.
We consider a discontinuous Galerkin finite element method
applied in time to a model Volterra equation of the second kind.
A residual-based computable
Galerkin-error estimate is derived
for .
This estimate does not explicitly
contain the time step and therefore the time step
control must be based
on a heuristic criterion, the estimate can then be used to
demonstrate the integrity, or otherwise, of the finite element
solution. After performing some numerical experiments
we conclude that this approach is at least competetive
with classical discretizations since it is
computationally simple to implement,
but has the added advantage of
reliable error feedback.
Received June 25, 1995 相似文献