全文获取类型
收费全文 | 29810篇 |
免费 | 827篇 |
国内免费 | 236篇 |
专业分类
化学 | 20150篇 |
晶体学 | 138篇 |
力学 | 685篇 |
综合类 | 1篇 |
数学 | 5558篇 |
物理学 | 4341篇 |
出版年
2022年 | 187篇 |
2021年 | 284篇 |
2020年 | 358篇 |
2019年 | 323篇 |
2018年 | 282篇 |
2017年 | 304篇 |
2016年 | 619篇 |
2015年 | 632篇 |
2014年 | 684篇 |
2013年 | 1455篇 |
2012年 | 1463篇 |
2011年 | 1773篇 |
2010年 | 996篇 |
2009年 | 848篇 |
2008年 | 1536篇 |
2007年 | 1524篇 |
2006年 | 1615篇 |
2005年 | 1451篇 |
2004年 | 1366篇 |
2003年 | 1149篇 |
2002年 | 1101篇 |
2001年 | 496篇 |
2000年 | 445篇 |
1999年 | 427篇 |
1998年 | 408篇 |
1997年 | 461篇 |
1996年 | 422篇 |
1995年 | 390篇 |
1994年 | 419篇 |
1993年 | 369篇 |
1992年 | 348篇 |
1991年 | 336篇 |
1990年 | 301篇 |
1989年 | 296篇 |
1988年 | 283篇 |
1987年 | 235篇 |
1986年 | 269篇 |
1985年 | 388篇 |
1984年 | 387篇 |
1983年 | 335篇 |
1982年 | 375篇 |
1981年 | 333篇 |
1980年 | 369篇 |
1979年 | 286篇 |
1978年 | 317篇 |
1977年 | 296篇 |
1976年 | 238篇 |
1975年 | 206篇 |
1974年 | 214篇 |
1973年 | 180篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
72.
Properties of nonparametric estimators of autocovariance for stationary random fields 总被引:1,自引:0,他引:1
Summary We introduce nonparametric estimators of the autocovariance of a stationary random field. One of our estimators has the property that it is itself an autocovatiance. This feature enables the estimator to be used as the basis of simulation studies such as those which are necessary when constructing bootstrap confidence intervals for unknown parameters. Unlike estimators proposed recently by other authors, our own do not require assumptions such as isotropy or monotonicity. Indeed, like nonparametric function estimators considered more widely in the context of curve estimation, our approach demands only smoothness and tail conditions on the underlying curve or surface (here, the autocovariance), and moment and mixing conditions on the random field. We show that by imposing the condition that the estimator be a covariance function we actually reduce the numerical value of integrated squared error. 相似文献
73.
Peter Karduck Norbert Ammann Hans Günter Esser Jörg Winter 《Fresenius' Journal of Analytical Chemistry》1991,341(5-6):315-319
Summary The first wall of the fusion device TEXTOR at the Forschungszentrum Jülich has been coated in situ with an amorphous hydrogen rich carbon/boron film (a-C/B:H) which reduces plasma impurities caused by the plasma surface interaction. The results of the coating process of the 35 m2 large inner wall surface have been controlled by a recently developed modification of the quantitative electron probe microanalysis, which has been applied to 12 samples from specified positions inside the tokamak. The quantification itself is based on a Monte Carlo simulation of electron trajectories providing very accurate results for X-ray intensities emitted by elements present in the electron bombarded sample. The Monte Carlo results are used in the present work to calibrate the measured X-ray intensities emitted by boron and carbon from the a-C/B:H layers deposited on pure silicon substrates. As a result the total deposited mass of the layer per area unit as well as the composition of the layers (except hydrogen) could be determined very accurately. The relative errors were less than 7%. The limit of detectability were found to be in the range of one monolayer for boron as well as for carbon. 相似文献
74.
75.
76.
Ohne ZusammenfassungDiese Arbeit wurde während eines Aufenthaltes von Schmidt in der Bundesrepublik Deutschland als Preisträger der Alexander von Humboldt-Stiftung geschrieben. 相似文献
77.
Butyltins were analyzed in waters from California river and lake marinas and were detected at the part per trillion (ppt) level in most locations. The tributyltin:dibutyltin (TBT/DBT) ratios in the fresh waters were similar to those in their saline counterparts indicating like rates of degradation in the two salinity regimes. 相似文献
78.
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
79.
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
80.