全文获取类型
收费全文 | 34451篇 |
免费 | 945篇 |
国内免费 | 246篇 |
专业分类
化学 | 22918篇 |
晶体学 | 170篇 |
力学 | 797篇 |
综合类 | 1篇 |
数学 | 6121篇 |
物理学 | 5635篇 |
出版年
2021年 | 329篇 |
2020年 | 408篇 |
2019年 | 374篇 |
2018年 | 328篇 |
2017年 | 332篇 |
2016年 | 723篇 |
2015年 | 724篇 |
2014年 | 766篇 |
2013年 | 1658篇 |
2012年 | 1648篇 |
2011年 | 1941篇 |
2010年 | 1091篇 |
2009年 | 964篇 |
2008年 | 1672篇 |
2007年 | 1678篇 |
2006年 | 1760篇 |
2005年 | 1594篇 |
2004年 | 1487篇 |
2003年 | 1257篇 |
2002年 | 1203篇 |
2001年 | 599篇 |
2000年 | 545篇 |
1999年 | 490篇 |
1998年 | 473篇 |
1997年 | 530篇 |
1996年 | 507篇 |
1995年 | 461篇 |
1994年 | 473篇 |
1993年 | 443篇 |
1992年 | 407篇 |
1991年 | 402篇 |
1990年 | 338篇 |
1989年 | 348篇 |
1988年 | 332篇 |
1987年 | 279篇 |
1986年 | 301篇 |
1985年 | 462篇 |
1984年 | 460篇 |
1983年 | 383篇 |
1982年 | 433篇 |
1981年 | 405篇 |
1980年 | 387篇 |
1979年 | 347篇 |
1978年 | 374篇 |
1977年 | 352篇 |
1976年 | 287篇 |
1975年 | 263篇 |
1974年 | 242篇 |
1973年 | 231篇 |
1967年 | 145篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
91.
We give a new and comparably short proof of Gittins’ index theorem for dynamic allocation problems of the multi-armed bandit type in continuous time under minimal assumptions. This proof gives a complete characterization of optimal allocation strategies as those policies which follow the current leader among the Gittins indices while ensuring that a Gittins index is at an all-time low whenever the associated project is not worked on exclusively. The main tool is a representation property of Gittins index processes which allows us to show that these processes can be chosen to be pathwise lower semi-continuous from the right and quasi-lower semi-continuous from the left. Both regularity properties turn out to be crucial for our characterization and the construction of optimal allocation policies. 相似文献
92.
This study analyzes the economics of transshipping biomass from truck to train in a North American setting. Transshipment
will only be economic when the cost per unit distance of a second transportation mode is less than the original mode. There
is an optimum number of transshipment terminals which is related to biomass yield. Transshipment incurs incremental fixed
costs, and hence there is a minimum shipping distance for rail transport above which lower costs/km offset the incremental
fixed costs. For transport by dedicated unit train with an optimum number of terminals, the minimum economic rail shipping
distance for straw is 170 km, and for boreal forest harvest residue wood chips is 145 km. The minimum economic shipping distance
for straw exceeds the biomass draw distance for economically sized centrally located power plants, and hence the prospects
for rail transport are limited to cases in which traffic congestion from truck transport would otherwise preclude project
development. Ideally, wood chip transport costs would be lowered by rail transshipment for an economically sized centrally
located power plant, but in a specific case in Alberta, Canada, the layout of existing rail lines precludes a centrally located
plant supplied by rail, whereas a more versatile road system enables it by truck. Hence for wood chips as well as straw the
economic incentive for rail transport to centrally located processing plants is limited. Rail transshipment may still be preferred
in cases in which road congestion precludes truck delivery, for example as result of community objections. 相似文献
93.
94.
95.
J. C. Raines J. P. Schmidt J. P. Burelbach H. K. Fauske 《Journal of Thermal Analysis and Calorimetry》2006,85(1):53-55
Hydrogen
peroxide is a very versatile oxidizing agent, and it is also environmentally
compatible considering that the products of its exothermic decomposition are
oxygen and water.When kept in a clean temperature-controlled environment,
the self-reaction (decomposition) rate is extremely low. However, it is well
known that even a small amount of contamination will dramatically increase
the reaction rate. This paper describes the use of the fast thermal activity
interpreter (FTAI) instrument to examine the chemical reactivity of commercially
available 50% hydrogen peroxide at two different temperatures (30 and 40°C)
both with and without contamination. The results show that at 30°C a small
amount of rust (330 ppm) increases the reaction rate of 50% hydrogen peroxide
by a factor of 50. When the temperature is increased to 40°C, the reaction
rate is further increased by almost a factor of four. The implication for
reactivity management is that at this contamination level most practical vessel
sizes would require emergency venting capability.
An evaluation
was then performed to determine the emergency venting requirement for the
safe transportation or storage of the contaminated hydrogen peroxide. It was
determined that for quantities of the material less than 5 gallons, conventional
breather vents would be sufficient to accommodate the gas evolved. However,
for larger quantities, a safety relief device would be needed. For example,
for a 400-gallon tote bin at 40°C the required minimum vent area is estimated
to be 4.3 in2, corresponding to a minimum vent
diameter of 2.3 inches. 相似文献
96.
Peter Šimon 《Journal of Thermal Analysis and Calorimetry》2006,83(3):521-522
This
issue of Journal of Thermal Analysis and Calorimetry contains selected papers
based on presentations in the field of thermal characteristics of advanced
polymer systems presented at the 8th International
Symposium on Polymers for Advanced Technologies (PAT 2005) held in Budapest,
September 2005.
Guest Editors 相似文献
97.
F. Schmidt und Hänsch 《Fresenius' Journal of Analytical Chemistry》1884,23(1):530-531
Ohne Zusammenfassung 相似文献
98.
Erhard Schmidt 《Mathematische Zeitschrift》1940,46(1):204-230
Ohne ZusammenfassungHerrn Oskar Perron zum 60. Geburtstag am 7. Mai 1940 gewidmet 相似文献
99.
In this paper, the possibility of using the gradients of the temperature and of the internal parameters as additional state variables in Continuum Thermodynamics is considered. The expressions of the two principles are discussed and Clausius–Duhem inequality is derived under an appropriate statement of the internal entropy production. Several formulations are proposed and compared to existing results in the literature. To cite this article: P. Ireman, Q.S. Nguyen, C. R. Mecanique 332 (2004). 相似文献
100.