全文获取类型
收费全文 | 2445篇 |
免费 | 109篇 |
国内免费 | 4篇 |
专业分类
化学 | 2135篇 |
晶体学 | 5篇 |
力学 | 27篇 |
数学 | 216篇 |
物理学 | 175篇 |
出版年
2024年 | 3篇 |
2023年 | 26篇 |
2022年 | 116篇 |
2021年 | 130篇 |
2020年 | 61篇 |
2019年 | 66篇 |
2018年 | 54篇 |
2017年 | 40篇 |
2016年 | 114篇 |
2015年 | 82篇 |
2014年 | 112篇 |
2013年 | 143篇 |
2012年 | 156篇 |
2011年 | 214篇 |
2010年 | 111篇 |
2009年 | 118篇 |
2008年 | 177篇 |
2007年 | 158篇 |
2006年 | 136篇 |
2005年 | 125篇 |
2004年 | 94篇 |
2003年 | 80篇 |
2002年 | 59篇 |
2001年 | 21篇 |
2000年 | 11篇 |
1999年 | 16篇 |
1998年 | 15篇 |
1997年 | 11篇 |
1996年 | 14篇 |
1995年 | 13篇 |
1994年 | 7篇 |
1993年 | 3篇 |
1992年 | 3篇 |
1991年 | 6篇 |
1990年 | 4篇 |
1988年 | 3篇 |
1986年 | 3篇 |
1985年 | 5篇 |
1984年 | 2篇 |
1983年 | 10篇 |
1982年 | 4篇 |
1981年 | 3篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1972年 | 2篇 |
1933年 | 2篇 |
排序方式: 共有2558条查询结果,搜索用时 15 毫秒
991.
Olga Rac-Rumijowska Marta Fiedot Iwona Karbownik Patrycja Suchorska-Woźniak Helena Teterycz 《Cellulose (London, England)》2017,24(3):1355-1370
We present a method for synthesis of silver nanoparticles in N-methylmorpholine N-oxide (NMMO) and the associated mechanism, as well as their use for in situ volume modification of cellulose fibers. The synthesized particles had diameter of about 4 nm, and their colloid solution was stable for 1 year. The nanoparticles were stabilized using polyethylenimine, which apart from preventing nanoparticle agglomeration, also accelerated Ag+ ion reduction and prevented NMMO degradation. A mechanism for the nanoparticle synthesis is suggested based on the electrochemical potentials of all ions in solution, with perhydroxyl ions resulting from NMMO reducing the silver ions. We also created nanocomposites from fibers and silver nanoparticles, in which the latter showed very good dispersion in the fiber volume. Such spun fibers showed improved mechanical parameters in comparison with unmodified fibers. 相似文献
992.
Marta Worzakowska 《Journal of Thermal Analysis and Calorimetry》2017,127(3):2025-2035
The oxidative thermal stability along with the identification of the volatile decomposition products under heating of terpene acrylate homopolymers by using TG/DSC/FTIR/QMS-coupled method was presented. It was found that the decomposition of poly(geranyl acrylate) and poly(neryl acrylate) had quite different course as compared to the decomposition process of poly(citronellyl acrylate) under oxidative conditions. FTIR and QMS analyses confirmed mainly the formation of terpene hydrocarbons, propane, propene, acetic acid, CO, CO2 and H2O as the volatile decomposition products under heating of the hompolymers. The results obtained indicated the complex decomposition process of terpene acrylate homopolymers including the random ester bond scissors, the random main carbon chain scissors, decarboxylation, dehydration and oxidation processes of formed gaseous decomposition products and a residue which led to the full decomposition of homopolymers at ca. 600 °C under oxidative conditions. 相似文献
993.
Degradation of organic water pollutants through sonophotocatalysis in the presence of TiO(2) 总被引:5,自引:0,他引:5
The degradation of 2-chlorophenol and of the two azo dyes acid orange 8 and acid red 1 in aqueous solution was investigated kinetically under sonolysis at 20 kHz and under photocatalysis in the presence of titanium dioxide particles, as well as under simultaneous sonolysis and photocatalysis, i.e. sonophotocatalysis. The influence on the degradation and mineralisation rates of the initial substrate concentration and of the photocatalyst amount was systematically investigated to ascertain the origin of the synergistic effect observed between the two degradation techniques. The evolution of hydrogen peroxide during kinetic runs was also monitored. Small amounts of Fe(III) were found to affect both the adsorption equilibria on the semiconductor and the degradation paths. Ultrasound may modify the rate of photocatalytic degradation by promoting the deaggregation of the photocatalyst, by inducing the desorption of organic substrates and degradation intermediates from the photocatalyst surface and, mainly, by favouring the scission of the photocatalytically and sonolytically produced H(2)O(2), with a consequent increase of oxidising species in the aqueous phase. 相似文献
994.
Marta Moraschi Giovanni Giulietti Federico Giove Manuela Guardati Girolamo Garreffa Nicola Modugno Claudio Colonnese Bruno Maraviglia 《Magnetic resonance imaging》2010
Parkinson's disease is a neurological disorder associated with the disfunction of dopaminergic pathways of the basal ganglia, mainly resulting in a progressive alteration in the execution of voluntary movements. We present a functional magnetic resonance imaging (fMRI) study on cortical activations during simple motor task performance, in six early–stage hemiparkinsonian patients and seven healthy volunteers. We acquired data in three sessions, during which subjects performed the task with right or left hand, or bimanually. We observed consistent bilateral activations in cingulate cortex and dorsolateral prefrontal cortex of Parkinsonian subjects during the execution of the task with the affected hand. In addition, patients showed both larger and stronger activations in motor cortex of the affected hemisphere with respect to the healthy hemisphere. Compared with the control group, patients showed a hyperactivation of the dorsolateral prefrontal cortex of the affected hemisphere. We concluded that a presymptomatic reorganization of the motor system is likely to occur in Parkinson's disease at earlier stages than previously hypothesized. Moreover, our results support fMRI as a sensitive technique for revealing the initial involvement of motor cortex areas at the debut of this degenerative disorder. 相似文献
995.
Lourdes I. Cabrera Marta Martínez Dolores Reyman Patricia Crespo M. Puerto Morales Pilar Herrasti 《Journal of nanoparticle research》2011,13(12):6931-6939
This work reports the preparation of hybrid nanoparticles with magnetic and fluorescent properties. The material is based
on magnetite nanoparticles (NPs) coated with fluorophore methylene blue (MB). The synthesis of a multifunctional material
with magnetic and fluorescent features is carried out in a single step by electrooxidation. The effect of the presence of
methylene blue in the synthetic medium is discussed. The presence of MB polymer at the NP surface is demonstrated with visible
UV, infrared and Raman spectroscopy. The NPs morphology, structure and size are determined by transmission electron microscopy
(TEM) and X-ray diffraction. The magnetic properties are measured with a vibrating sample magnetometer (VMS). In overall,
the results show that magnetite NPs generated electrochemically in the presence of MB present a core/shell structure, being
the NP at the core surrounded by methylene blue polymer, leading to a nanocomposite or hybrid material. 相似文献
996.
Mikel Sanz Rebeca de Nalda José F. Marco Jesús G. Izquierdo Marta Castillejo 《Applied Surface Science》2009,255(10):5206-5210
Nanostructured deposits of TiO2 were grown on Si (1 0 0) substrates by laser ablating a TiO2 sintered target in vacuum or in oxygen using a Ti:sapphire laser delivering 80 fs pulses. The effect of the laser irradiation wavelength on the obtained nanostructures, was investigated using 800, 400 and 266 nm at different substrate temperatures and pressures of oxygen. The composition of the deposits was characterized using X-ray photoelectron spectroscopy (XPS) and the surface morphology was studied by environmental scanning electron microscopy (ESEM) and atomic force microscopy (AFM). Deposits are absent of microscopic droplets in all conditions explored. The best deposits, constituted by nanoparticles of an average diameter of 30 nm with a narrow size distribution, were obtained at the shorter laser wavelength of 266 nm under vacuum at substrate room temperature. 相似文献
997.
The suppression of high pT hadron production in heavy ion collisions is thought to be due to energy loss by gluon radiation off hard partons in a QCD medium. Existing models of QCD radiative energy loss in a color-charged medium give estimates of the coupling strength of the parton to the medium which differ by a factor of 5. We will present a side-by-side comparison of two different formalisms to calculate the energy loss of light quarks and gluons: the multiple soft scattering approximation (ASW-MS) and the opacity expansion formalism (ASW-SH and WHDG-rad). A common time-temperature profile is used to characterize the medium. The results are compared to the single hadron suppression RAA at RHIC energies. 相似文献
998.
Elena?A.?Golysheva Marta?De?Zotti Claudio?Toniolo Fernando?Formaggio Sergei?A.?DzubaEmail authorView authors OrcID profile 《Applied magnetic resonance》2018,49(12):1369-1383
Data on neutron scattering in biological systems show low-temperature dynamical transition between 170 and 230 K manifesting itself as a drastic increase of the atomic mean-squared displacement, 〈x2〉, detected for hydrogen atoms in the nano- to picosecond time scale. For spin-labeled systems, electron spin echo (ESE) spectroscopy—a pulsed version of electron paramagnetic resonance—is also capable of detection of dynamical transition. A two-pulse ESE decay in frozen matrixes is induced by spin relaxation arising from stochastic molecular librations, and allows to obtain the 〈α2〉τc parameter, where 〈α2〉 is a mean-squared angular amplitude of the motion and τc is the correlation time lying in the sub- and nanosecond time ranges. In this work, the ESE technique was applied to spin-labeled amphiphilic molecules of three different kinds embedded in bilayers of fully saturated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and mono-unsaturated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipids. Two-pulse ESE data revealed the appearance of stochastic librations above 130 K, with the parameter 〈α2〉τc obeying the Arrhenius type of temperature dependence and increasing remarkably above 170–180 K. A comparison with a dry sample suggests that onset of motions is not related with lipid internal motions. Three-pulse ESE experiments (resulting in stimulated echos) in DPPC bilayers showed the appearance of slow molecular rotations above 170–180 K. For D2O-hydrated bilayers, ESE envelope modulation experiments indicate that isotropic water molecular motions in the nearest hydration shell of the bilayer appear with a rate of ~?105 s?1 in the narrow temperature range between 175 and 179 K. The similarity of the experimental data found for three different spin-labeled compounds suggests a cooperative character for the ESE-detected molecular motions. The data were interpreted within a model suggesting that dynamical transition is related with overcoming barriers, of 10–20 kJ/mol height, existing in the system for the molecular reorientations. 相似文献
999.
Marta Tyran-Kamińska 《Journal of Mathematical Analysis and Applications》2009,357(2):385-402
Necessary and sufficient conditions are given for a substochastic semigroup on L1 obtained through the Kato-Voigt perturbation theorem to be either stochastic or strongly stable. We show how such semigroups are related to piecewise deterministic Markov process, provide a probabilistic interpretation of our results, and apply them to fragmentation equations. 相似文献
1000.
We present an algorithm for computing exact shortest paths, and consequently distance functions, from a generalized source (point, segment, polygonal chain or polygonal region) on a possibly non-convex triangulated polyhedral surface. The algorithm is generalized to the case when a set of generalized sites is considered, providing their distance field that implicitly represents the Voronoi diagram of the sites. Next, we present an algorithm to compute a discrete representation of the distance function and the distance field. Then, by using the discrete distance field, we obtain the Voronoi diagram of a set of generalized sites (points, segments, polygonal chains or polygons) and visualize it on the triangulated surface. We also provide algorithms that, by using the discrete distance functions, provide the closest, furthest and k-order Voronoi diagrams and an approximate 1-Center and 1-Median. 相似文献