全文获取类型
收费全文 | 420篇 |
免费 | 19篇 |
专业分类
化学 | 269篇 |
晶体学 | 9篇 |
力学 | 7篇 |
数学 | 56篇 |
物理学 | 98篇 |
出版年
2024年 | 1篇 |
2021年 | 5篇 |
2020年 | 5篇 |
2019年 | 2篇 |
2018年 | 10篇 |
2017年 | 7篇 |
2016年 | 19篇 |
2015年 | 22篇 |
2014年 | 18篇 |
2013年 | 27篇 |
2012年 | 25篇 |
2011年 | 31篇 |
2010年 | 20篇 |
2009年 | 10篇 |
2008年 | 31篇 |
2007年 | 27篇 |
2006年 | 20篇 |
2005年 | 23篇 |
2004年 | 22篇 |
2003年 | 13篇 |
2002年 | 8篇 |
2001年 | 5篇 |
2000年 | 7篇 |
1999年 | 6篇 |
1998年 | 3篇 |
1997年 | 2篇 |
1996年 | 5篇 |
1995年 | 2篇 |
1994年 | 2篇 |
1993年 | 4篇 |
1992年 | 2篇 |
1991年 | 5篇 |
1990年 | 1篇 |
1989年 | 3篇 |
1988年 | 4篇 |
1987年 | 4篇 |
1986年 | 6篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1982年 | 8篇 |
1981年 | 7篇 |
1980年 | 1篇 |
1977年 | 2篇 |
1973年 | 1篇 |
1967年 | 1篇 |
1966年 | 1篇 |
1938年 | 1篇 |
1928年 | 2篇 |
排序方式: 共有439条查询结果,搜索用时 31 毫秒
131.
132.
133.
134.
We study the asymptotic zero distribution of type II multiple orthogonal polynomials associated with two Macdonald functions (modified Bessel functions of the second kind). On the basis of the four-term recurrence relation, it is shown that, after proper scaling, the sequence of normalized zero counting measures converges weakly to the first component of a vector of two measures which satisfies a vector equilibrium problem with two external fields. We also give the explicit formula for the equilibrium vector in terms of solutions of an algebraic equation. 相似文献
135.
Fast screening of trace amounts of the perfluorooctane sulfonate anion (PFOS) in water samples was performed following a simple, fast and efficient sample preparation procedure based on vortex-assisted liquid-liquid microextraction (VALLME) prior to liquid chromatography-mass spectrometry. VALLME initially uses vortex agitation, a mild emulsification procedure to disperse microvolumes of octanol, a low density extractant solvent, in the aqueous sample. Microextraction under equilibrium conditions is thus achieved within few minutes. Subsequently, centrifugation separates the two phases and restores the initial microdrop shape of the octanol acceptor phase, which can be collected and used for liquid chromatography-single quadrupole mass spectrometry analysis. Several experimental parameters were controlled and the optimum conditions found were: 50 μL of octanol as the extractant phase; 20 mL aqueous donor samples (pH=2); a 2 min vortex extraction time with the vortex agitator set at a 2500 rpm rotational speed; no ionic strength adjustment. Centrifugation for 2 min at 3500 rpm yielded separation of the two phases throughout this study. Enhanced extraction efficiencies were observed at low pH which was likely due to enhanced electrostatic interaction between the negatively PFOS molecules and the positively charged octanol/water interface. The effect of pH was reduced in the presence of sodium chloride, likely due to electrical double layer compression. The linear response range for PFOS was from 5 to 500 ng L(-1) (coefficient of determination, r(2), 0.997) and the relative standard deviation for aqueous solutions containing 10 and 500 ng L(-1) PFOS were 7.4% and 6.5%, respectively. The limit of detection was 1.6 ng L(-1) with an enrichment factor of approximately 250. Analysis of spiked tap, river and well water samples revealed that matrix did not affect extraction. 相似文献
136.
A study of the radical copolymerization of methyl methacrylate and 1-naphthyl methacrylate in benzene, chlorobenzene, and o-dichlorobenzene was made at 50°C. There is a marked effect of solvent on both r1 and r2 in all these systems, which can be correlated with the variation in the polarity of solvents. The glass transition temperatures of copolymers were discussed taking into consideration the sequence distribution of the copolymers and the homopolymers tg - values. 相似文献
137.
B. B. Romão F. R. X. Batista J. S. Ferreira H. C. B. Costa M. M. Resende V. L. Cardoso 《Applied biochemistry and biotechnology》2014,172(7):3670-3685
Nowadays, hydrogen produced globally has been synthesized from fossil fuel with limited source. Therefore, research has been developed in order to explore biological H2 production by dark fermentation. The purpose of this work was to evaluate the effect of initial pH and ferrous sulfate and ammonium sulfate concentrations on the production of biohydrogen by dark fermentation. The process was carried out in batch mode under anaerobic conditions, in the absence of light, and at standard room temperature and pressure. A microbial consortium provided by the effluent treatment plant of a local dairy company was inoculated into a synthetic medium supplemented with cheese whey permeate (20 g/L of lactose) as a carbon source. The influence of three variables was analyzed by a central composite design 2(3), and the optimum results of hydrogen yield (4.13 mol H2/mol lactose) and productivity (86.31 mmol H2/L/day) were achieved at initial pH 7.0 and FeSO4 and (NH4)2SO4 concentrations of 0.6 and 1.5 g/L, respectively. Under these conditions, the kinetic parameters of fermentation were investigated by analyzing the profile of H2 yield and productivity, metabolite concentrations, pH, and concentration of dissolved iron. In the kinetic analysis, the modified Gompertz equation described adequately the fermentative hydrogen production from cheese whey permeate (R 2?=?0.98). The profile of ethanol and volatile organic acids showed that lactic acid and butyric acid were the main metabolites produced, and the sum of both by-products corresponded to about 58 % of the total metabolites. 相似文献
138.
139.
R. Fermento J. B. González-Díaz A. Cebollada G. Armelles M. Díaz L. Martínez E. Román Y. Huttel C. Ballesteros 《Journal of nanoparticle research》2011,13(6):2653-2659
The optical and magneto-optical properties of hybrid Co–SiOx systems are studied as a function of Co concentration. The structures were prepared by alternate depositions of SiOx thin films and layers of 10-nm-diameter Co nanoparticles produced by an Ion Cluster Source. Both optical and magneto-optical
constants of the system gradually increase with the amount of Co, though maintaining low optical absorption values in the
visible range. The experimental results are well reproduced assuming that the nanoparticles have a cobalt core (7–8 nm in
diameter) surrounded by a cobalt oxide shell (1–2 nm thick). The magneto-optical activity versus optical absorption figure
of merit of this system is compared with other magneto-optical dielectric systems. 相似文献
140.
We investigate the stability of the topological phase of the toric code model in the presence of a uniform magnetic field by means of variational and high-order series expansion approaches. We find that when this perturbation is strong enough, the system undergoes a topological phase transition whose first- or second-order nature depends on the field orientation. When this transition is of second order, it is in the Ising universality class except for a special line on which the critical exponent driving the closure of the gap varies continuously, unveiling a new topological universality class. 相似文献