全文获取类型
收费全文 | 360801篇 |
免费 | 3889篇 |
国内免费 | 1305篇 |
专业分类
化学 | 196629篇 |
晶体学 | 5366篇 |
力学 | 15177篇 |
综合类 | 13篇 |
数学 | 39887篇 |
物理学 | 108923篇 |
出版年
2021年 | 2619篇 |
2020年 | 2958篇 |
2019年 | 3285篇 |
2018年 | 4217篇 |
2017年 | 4125篇 |
2016年 | 6340篇 |
2015年 | 4043篇 |
2014年 | 6018篇 |
2013年 | 16108篇 |
2012年 | 12359篇 |
2011年 | 15307篇 |
2010年 | 10367篇 |
2009年 | 10299篇 |
2008年 | 14352篇 |
2007年 | 14422篇 |
2006年 | 13579篇 |
2005年 | 12387篇 |
2004年 | 11132篇 |
2003年 | 9959篇 |
2002年 | 9751篇 |
2001年 | 10880篇 |
2000年 | 8402篇 |
1999年 | 6405篇 |
1998年 | 5211篇 |
1997年 | 5276篇 |
1996年 | 5008篇 |
1995年 | 4294篇 |
1994年 | 4276篇 |
1993年 | 4196篇 |
1992年 | 4562篇 |
1991年 | 4630篇 |
1990年 | 4273篇 |
1989年 | 4255篇 |
1988年 | 4197篇 |
1987年 | 4139篇 |
1986年 | 3950篇 |
1985年 | 5461篇 |
1984年 | 5596篇 |
1983年 | 4477篇 |
1982年 | 4883篇 |
1981年 | 4662篇 |
1980年 | 4470篇 |
1979年 | 4606篇 |
1978年 | 4788篇 |
1977年 | 4776篇 |
1976年 | 4697篇 |
1975年 | 4534篇 |
1974年 | 4424篇 |
1973年 | 4595篇 |
1972年 | 2910篇 |
排序方式: 共有10000条查询结果,搜索用时 699 毫秒
991.
M. C. Fornaciari Iljadica J. C. Furnari I. M. Cohen 《Journal of Radioanalytical and Nuclear Chemistry》2006,270(1):273-275
Summary The evolution of education in Argentina at the university level is described. The detailed search of the educational offer
shows that less than half of the universities (35 out of 92) include chemistry and chemistry related undergraduate programmes
in their curriculum. The revision of the position of radiochemistry in these programmes reveals that only seven courses on
radiochemistry are currently offered. Radiochemistry is included only in few programmes in chemistry and biochemistry. With
respect to the programmes in chemical engineering the situation is worse. This offer is strongly concentrated in Buenos Aires
and its surroundings. 相似文献
992.
W. Gao M. Li R. Klie E.I. Altman 《Journal of Electron Spectroscopy and Related Phenomena》2006,150(2-3):136-149
Oxide catalysts are frequently used to convert toxic species to environmentally benign molecules, and to prevent the formation of toxic species in the first place. In this paper, growth and characterization of model oxide systems employed in both approaches is discussed. An example of the former approach is the selective catalytic reduction (SCR) of NO emitted from power plants by NH3, which employs tungsten and vanadium oxides supported on the anatase polymorph of TiO2. To model SCR catalysts, epitaxial titanium, vanadium and tungsten oxide films were grown using molecular beam epitaxy and magnetron sputtering. Two different anatase orientations were grown on LaAlO3 substrates and their interactions with vanadia were characterized. On LaAlO3 (0 0 1), anatase exposed a (4 × 1) reconstructed (0 0 1) surface. Vanadia lifted the reconstruction and at 1 ML a (1 × 1) surface with mostly V5+ was observed. Continued V2O5 growth led to loss of order, but at high temperatures epitaxial VO2 could be grown; vanadia behaved similarly on anatase films on LaAlO3 (1 1 0). Results suggested that the monolayer is pseudomorphic with O adsorption oxidizing the surface V to 5+, since the anatase structure cannot accommodate more bulk oxygen, only a monolayer can be pseudomorphic and have only V5+. Thus the vanadia monolayer has unique structural and chemical properties that can help explain why vanadia monolayers on TiO2 are much more active than bulk V2O5. For WO3, a series of added row reconstructions were observed as the epitaxial films were reduced. The effect of these structures on surface chemistry was characterized by studying 1-propanol adsorption. The results indicated that the structure of the WO3 surface did not alter its catalytic function but had a strong effect on reaction kinetics. As an example of a system where catalysts prevent the formation of toxic species, the reactivity of oxidized Pd surfaces used in CH4 catalytic combustion were studied. An ordered PdO-like monolayer was found to be less reactive towards CO than adsorbed O on Pd. On the other hand, the PdO layer favored a lower activation energy C3H6 oxidation pathway. The results indicated that Pd oxidation reduces the sticking coefficient of reactive species but once molecules adsorb, the oxide surface can reduce the activation energy for subsequent reaction. 相似文献
993.
994.
E. Baggio Saitovitch F. J. Litterst I. Souza Azevedo R. B. Scorzelli 《Hyperfine Interactions》1989,50(1-4):529-535
The Mössbauer spectra of YBa2(Cu1?x Fe x )3O7 at room temperature show several doublets attributed to Fe in Cu(1) sites with different oxygen configurations. Here we present a systematic study performed at 4.2 K forx=0.005, 0.01, 0.03, 0.05, 0.10, 0.15. To obtain information about the magnetic ordered state two samples, withx=0.005 andx=0.15, have been studied at 4.2 K underB ext=5 T. The Mössbauer spectra indicate that the iron moments are polarized forx=0.005, while in the ordered state (x=0.15) they have an antiferromagnetic or spin-glass-like arrangement with high anisotropy. 相似文献
995.
E. Kyriakopoulos 《General Relativity and Gravitation》1989,21(2):125-136
The class of previously found stationary axisymmetric perfect fluid solutions of Einstein's equations is written inh-orthogonal coordinates,h being a space-like coordinate. Matching of a big number of solutions of the class with each other seems to be possible for a proper choice of some parameters. The exterior solutions of the class are matched explicitly with interior solutions. Also, interior solutions are matched explicitly with each other. 相似文献
996.
The hyperfine fields for the systems140CeLa and140CeLu have been determined by time differential perturbed angular correlations (TDPAC) in the temperature range 4.2 K≤T≤330 K. Magnitude and temperature dependence of the paramagnetic enhancement factor β indicate that cerium in either system
is of intermediate valence. This behaviour is discussed in terms of a theory proposed by Ramakrishnan et al. [1–3]. 相似文献
997.
The three dimensional problem of steady fluid deposition on an inclined rotating disk is solved by similarity transform. For a given spraying rate there may be one, two or no steady state solution. The inclination causes a downward draining flow and a lateral flow. Perturbation solutions compare well with exact similarity solutions when the fluid film is thin. 相似文献
998.
J.T. Elizalde Galindo A.H. Adair C.E. Botez V. Corral Flores D. Bueno Baques L. Fuentes Cobas J.A. Matutes-Aquino 《Applied Physics A: Materials Science & Processing》2007,87(4):743-747
Measurements of ac-susceptibility and dc-magnetization were carried out on samples of Ni1-xZnxFe2O4 nanoparticles (x=0, 0.25, 0.5, 0.75) with average diameters 〈D〉≈7 nm. Values of the superparamagnetic blocking temperature
T̄B were obtained from the characteristic temperature behavior of the imaginary susceptibility χimag. An Arrhenius-type law, which accurately describes the relationship between the observation time τobs and the blocking temperature, was used to determine the effective energy barrier to magnetization reversal Ueff. A Zn-content dependence of the energy barrier is observed, where Ueff changes little for 0≤x≤0.25, it peaks at x=0.5, and decreases back upon further Zn-doping. The large increase of Ueff at x=0.5 is attributed to an enhanced magnetic anisotropy induced by the crossover between two spatial arrangements of spins
in the A and B sub-lattices of the ferrimagnetic inverse spinel.
PACS 75.50.Bb; 75.50.Gg; 75.30.Et 相似文献
999.
In this paper, we study the consistency of a variant of fractionalstep Runge–Kutta methods. These methods are designed tointegrate efficiently semi-linear multidimensional parabolicproblems by means of linearly implicit time integration processes.Such time discretization procedures are also related to a splittingof the space differential operator (or the spatial discretizationof it) as a sum of simpler linear differentialoperators and a nonlinear term. 相似文献
1000.
J Whittaker C Whitehead M Somers 《The Journal of the Operational Research Society》2007,58(7):911-921
A principled technique for monitoring the performance of a consumer credit scorecard through time is derived from Kalman filtering. Standard approaches sporadically compare certain characteristics of the new applicants with those predicted from the scorecard. The new approach systematically updates the scorecard combining new applicant information with the previous best estimate. The dynamically updated scorecard is tracked through time and compared to limits calculated by sequential simulation from the baseline scorecard. The observation equation of the Kalman filter is tailored to take the results of fitting local scorecards by logistic regression to batches of new clients that arrive in the current time interval. The states in the Kalman filter represent the true or underlying score for each attribute in the card: the parameters of the logistic regression. Their progress in time is modelled by a random walk and the filter provides the best estimate of the scores using past and present information. We illustrate the technique using a commercial mortgage portfolio and the results indicate significant emerging deficiencies in the baseline scorecard. 相似文献