全文获取类型
收费全文 | 342390篇 |
免费 | 4134篇 |
国内免费 | 534篇 |
专业分类
化学 | 171538篇 |
晶体学 | 5203篇 |
力学 | 16176篇 |
综合类 | 11篇 |
数学 | 50191篇 |
物理学 | 103939篇 |
出版年
2021年 | 1899篇 |
2020年 | 2355篇 |
2019年 | 2518篇 |
2018年 | 11617篇 |
2017年 | 12349篇 |
2016年 | 8835篇 |
2015年 | 4367篇 |
2014年 | 5013篇 |
2013年 | 11983篇 |
2012年 | 13925篇 |
2011年 | 23376篇 |
2010年 | 14548篇 |
2009年 | 14264篇 |
2008年 | 20151篇 |
2007年 | 23592篇 |
2006年 | 9374篇 |
2005年 | 15151篇 |
2004年 | 10935篇 |
2003年 | 10050篇 |
2002年 | 7683篇 |
2001年 | 7108篇 |
2000年 | 5613篇 |
1999年 | 4060篇 |
1998年 | 3169篇 |
1997年 | 3060篇 |
1996年 | 3189篇 |
1995年 | 2805篇 |
1994年 | 2737篇 |
1993年 | 2571篇 |
1992年 | 3243篇 |
1991年 | 3093篇 |
1990年 | 3017篇 |
1989年 | 3045篇 |
1988年 | 3026篇 |
1987年 | 3025篇 |
1986年 | 2854篇 |
1985年 | 3737篇 |
1984年 | 3731篇 |
1983年 | 2910篇 |
1982年 | 3093篇 |
1981年 | 3145篇 |
1980年 | 2883篇 |
1979年 | 3229篇 |
1978年 | 3208篇 |
1977年 | 3201篇 |
1976年 | 3120篇 |
1975年 | 2819篇 |
1974年 | 2796篇 |
1973年 | 2772篇 |
1968年 | 1859篇 |
排序方式: 共有10000条查询结果,搜索用时 234 毫秒
911.
Pál Sohár Gábor Bernáth Géza Stájer Angela E. Szabo 《Magnetic resonance in chemistry : MRC》1989,27(9):872-876
Pentacyclic isoxazolines were obtained by the cycloaddition of benzonitrile oxide to norbornene-azetidinone-fused 3,1-oxazines. The constitutions of two of the isomers obtained, and the configurations and conformations of all products, were determined by means of 1H and 13C NMR spectroscopy and DNOE experiments. 相似文献
912.
Summary In the present paper we study truncated projections for the fanbeam geometry in computerized tomography. First we derive consistency conditions for the divergent beam transform. Then we study a singular value decomposition for the case where only the interior rays in the fan are provided, as for example in region-of-interest tomography. We show that the high angular frequency components of the searched-for densities are well determined and we present reconstructions from real data where the missing information is approximated based on the singular value decomposition.The work of the authors was supported by the Deutsche Forschungsgemeinschaft under grant Lo 310/2-4 相似文献
913.
Roger M. Hill 《The Journal of the Operational Research Society》1989,40(11):983-992
The context is that of a retail chain where stock is purchased into a central warehouse from which branch stocks are replenished. This paper considers possible methods of allocating limited warehouse stock to branches, and uses simulation to quantify the resulting benefits. 相似文献
914.
Summary We consider a class of infinite delay equations of neutral type which includes Volterrra type integral und integrodifferential equations. Using abstract approximation results (cf. Trotter-Kato-type) for strongly continuous semigroups we develop an approximation scheme which is based on approximation of the system state by Laguerre (and Legendre) polynomials. Numerical examples demonstrate the feasibility of the scheme and show infinite order convergence for smooth data.Supported in part by the Air Force Office of Scientific Research under Contracts AFORS-84-0398 and AFORS-85-0303 and the National Aeronautics and Space Administration under NASA Grant NAG-1-517 and by NSF under Grant UINT-8521208Supported in part by the Air Force Office of Scientific Research under Contract AFORS-84-0398 and in part by the Fonds zur Förderung der wissenschaftlichen Forschung, Austria, under project No. S3206 相似文献
915.
916.
917.
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
918.
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
919.
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
920.
S. Yu. Kudryashov Yu. I. Arutyunov L. A. Onuchak 《Russian Journal of Physical Chemistry A, Focus on Chemistry》2007,81(1):102-107
New methods for calculating the mean volumetric flow rate of the carrier gas and the retention time of the unretained substance in the column under conditions of gas chromatography were proposed. The methods are based on preliminary isothermal calibrations of the flow rate and holdup time for a packed column. A theoretical substantiation of the methods was given. Procedures of plotting calibration dependences for determining the indicated quantities at a desired temperature were described. The calculation results were compared to experimental data obtained by traditional methods. It was demonstrated that the use of calculation methods substantially simplifies the determination of the specific retention volume over a wide temperature range. 相似文献