全文获取类型
收费全文 | 632443篇 |
免费 | 6331篇 |
国内免费 | 1615篇 |
专业分类
化学 | 319424篇 |
晶体学 | 9408篇 |
力学 | 31470篇 |
综合类 | 22篇 |
数学 | 86144篇 |
物理学 | 193921篇 |
出版年
2021年 | 5023篇 |
2020年 | 5412篇 |
2019年 | 6135篇 |
2018年 | 13300篇 |
2017年 | 13617篇 |
2016年 | 14392篇 |
2015年 | 7585篇 |
2014年 | 11430篇 |
2013年 | 26140篇 |
2012年 | 22781篇 |
2011年 | 31944篇 |
2010年 | 22205篇 |
2009年 | 22303篇 |
2008年 | 29324篇 |
2007年 | 31211篇 |
2006年 | 20789篇 |
2005年 | 22371篇 |
2004年 | 19179篇 |
2003年 | 17484篇 |
2002年 | 16177篇 |
2001年 | 17046篇 |
2000年 | 13041篇 |
1999年 | 10046篇 |
1998年 | 8323篇 |
1997年 | 8165篇 |
1996年 | 7937篇 |
1995年 | 7104篇 |
1994年 | 7073篇 |
1993年 | 6816篇 |
1992年 | 7620篇 |
1991年 | 7802篇 |
1990年 | 7430篇 |
1989年 | 7312篇 |
1988年 | 7283篇 |
1987年 | 7154篇 |
1986年 | 6761篇 |
1985年 | 8738篇 |
1984年 | 8964篇 |
1983年 | 7212篇 |
1982年 | 7303篇 |
1981年 | 7224篇 |
1980年 | 6699篇 |
1979年 | 7365篇 |
1978年 | 7497篇 |
1977年 | 7474篇 |
1976年 | 7442篇 |
1975年 | 6865篇 |
1974年 | 6772篇 |
1973年 | 6952篇 |
1972年 | 5008篇 |
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
991.
The possibilities of controlling the laser beam properties by a deformable mirror introduced into the laser optical cavity
were studied theoretically and experimentally. The experiments were performed under conditions of an industrial high power
transverse flow cw CO2 laser operating with a stable resonator of a folded configuration. A deformable bimorph mirror of a surface profile controlled
by the voltage applied to the mirror electrodes is implemented to the laser system as a back cavity mirror or as a one of
the inner folding mirrors. The near-and far-field characteristics of the laser beam versus the resonator configuration controlled
by the changes of the focal length of the deformable mirror are discussed in the paper. The analysis reveals that the resonator
with an inner deformable mirror is much more sensitive to the mirror curvature variations than the resonator in which the
deformable mirror is used as a back cavity mirror. The presented results show that dynamic and controllable changes in the
resonator properties result in the controlled modification and optimisation of the laser output power and spatial parameters
of the laser radiation. 相似文献
992.
The theory of dielectric space-charge polarization losses describes well both the model of an inhomogeneous dielectric [1]
and the polarization resulting from mosaic blocks of alkali-halide crystals [2]. The Debye frequency dependences ε*(ω) and
tan δ(ω) with non-Arrhenius relaxation time are calculated in the first approximation of perturbation theory [3, 4] with the
use of a nonlinear system of the Fokker-Planck and Poisson equations for the interlayer polarization with allowance for tunnel
transitions of relaxation oscillators. For the Maxwell mechanism of space-charge relaxation, tan δ(ω) also has the Debye form
[5]. It should be noted that in studies cited above the electric field was considered uniform, and the nonlinearity of the
initial system of equations was not investigated. This paper removes these restrictions and elaborates a theory of relaxation
mechanism.
__________
Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 28–34, November, 2006. 相似文献
993.
A. P. Avdeenko S. A. Konovalova O. N. Ludchenko 《Russian Journal of Organic Chemistry》2006,42(5):683-688
Reactions of halogens with N-arylsulfonyl-1,4-benzoquinone monoimines occur with the formation of a halogenonium ion that either transforms into a carbocation where the first halogen atom adds to the carbon in the ortho-position relative to the carbonyl carbon, or the halogenonium ion adds directly the second halogen atom. 相似文献
994.
In this paper we propose a general integration scheme for a Multi-Criteria Decision Making model of the Multi-Attribute Utility
Theory in Constraint Programming. We introduce the Choquet integral as a general aggregation function for multi-criteria optimization
problems and define the Choquet global constraint that propagates this function during the Branch-and-Bound search. Finally the benefits of the propagation
of the Choquet constraint are evaluated on the examination timetabling problem. 相似文献
995.
S.N. Abdulmadjid M.M. Suliyanti K.H. Kurniawan T.J. Lie M. Pardede R. Hedwig K. Kagawa M.O. Tjia 《Applied physics. B, Lasers and optics》2006,82(1):161-166
We report in this paper the results of an experimental study on hydrogen analysis of solid samples in high pressure helium
ambient gas employing the basic scheme of laser induced breakdown spectroscopy (LIBS). It is shown that the metastable excited
state of helium atom can be utilized to induce delayed excitation of the ablated hydrogen atoms, and thereby avoid the Stark
broadening effect as well as overcoming the undesirable mismatch effect, which are responsible for inefficient excitation
respectively. It is further demonstrated that for samples of high boiling-point materials such as zircaloy, successful hydrogen
analysis can be achieved by a newly introduced double excitation technique employing single laser realized in a modified configuration
of the conventional LIBS method.
PACS 51-52 相似文献
996.
We present a new verified optimization method to find regions for Hénon systems where the conditions of chaotic behaviour
hold. The present paper provides a methodology to verify chaos for certain mappings and regions. We discuss first how to check
the set theoretical conditions of a respective theorem in a reliable way by computer programs. Then we introduce optimization
problems that provide a model to locate chaotic regions. We prove the correctness of the underlying checking algorithms and
the optimization model. We have verified an earlier published chaotic region, and we also give new chaotic places located
by the new technique. 相似文献
997.
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
998.
M. Mayr K. -H. Speidel M. Knopp W. Karle T. Faestermann F. Hagelberg H. -J. Simonis P. N. Tandon J. Gerber 《Zeitschrift für Physik A Hadrons and Nuclei》1987,327(2):157-161
[Theg-factor ratio of the first excited 3? and 5? states in40Ca was measured to beg 3/g 5=1.01(10) employing the implantation perturbed angular correlation technique. The static hyperfine fields (SF) in Fe and Gd hosts were used. In addition the lifetime of the 5? state was measured to be τ=426(7)ps. The values of the SF in Gd and Fe hosts were deduced and compared with systematics in this element region. 相似文献
999.
1000.