全文获取类型
收费全文 | 564篇 |
免费 | 15篇 |
国内免费 | 1篇 |
专业分类
化学 | 484篇 |
晶体学 | 6篇 |
力学 | 1篇 |
数学 | 42篇 |
物理学 | 47篇 |
出版年
2023年 | 6篇 |
2022年 | 38篇 |
2021年 | 40篇 |
2020年 | 17篇 |
2019年 | 8篇 |
2018年 | 11篇 |
2017年 | 8篇 |
2016年 | 27篇 |
2015年 | 16篇 |
2014年 | 33篇 |
2013年 | 45篇 |
2012年 | 35篇 |
2011年 | 45篇 |
2010年 | 22篇 |
2009年 | 22篇 |
2008年 | 31篇 |
2007年 | 31篇 |
2006年 | 28篇 |
2005年 | 16篇 |
2004年 | 21篇 |
2003年 | 19篇 |
2002年 | 15篇 |
2001年 | 2篇 |
2000年 | 3篇 |
1999年 | 5篇 |
1998年 | 8篇 |
1997年 | 6篇 |
1996年 | 2篇 |
1995年 | 7篇 |
1994年 | 1篇 |
1992年 | 2篇 |
1985年 | 1篇 |
1983年 | 1篇 |
1981年 | 4篇 |
1980年 | 2篇 |
1937年 | 2篇 |
排序方式: 共有580条查询结果,搜索用时 0 毫秒
211.
S. L. Druţă-Romaniuc 《Acta Mathematica Hungarica》2013,139(3):228-244
We characterize the almost product and locally product structures of general natural lift type on the cotangent bundle of a Riemannian manifold. We find the conditions under which the cotangent bundle endowed with the constructed almost product (locally product) structure and with a pseudo-Riemannian metric obtained as a general natural lift of the metric from the base manifold, is a Riemannian almost product (locally product) or an (almost) para-Hermitian manifold. Finally, by studying the closedness of the 2-form associated to the obtained (almost) para-Hermitian structure, we characterize the general natural (almost) para-Kählerian structures on the cotangent bundle. 相似文献
212.
Polybenzene unit BT_48 can dimerize either by identification of 8-membered rings to provide a diamond-like fcc net or by identifying the opening 12-membered rings to form intercalated dendrimer-dimers which can further grow and rather quickly superimpose over the diamond-like network. The third hypothetical moiety we consider here is an eclipsed isomer, that can form multitori as negatively curved structures of various complexity. Multitori can evolve spherically or show a linear periodicity, as in rods. The polybenzene (??armchair??) multitori BTA are compared to the (??zig-zag??) BTZ ones, proposed earlier by us. A graph-theoretical study related the structure of multitori to the genus of their embedding surface and established the lower and upper bound values of genus. The total energy per carbon atom, HOMO-LUMO gap and strain energy of BTA and BTZ multitori have been computed and the results obtained point to BTZ multitori to be at least as stable as C60 what suggests BTZ multitori can be eventually synthesized in laboratory. 相似文献
213.
In this paper we solve numerically a degenerate parabolic equation with dynamical boundary condition for pricing zero coupon bond and compare numerical solution with asymptotic analytical solution. First,
we discuss an approximate analytical solution of the model and its order of accuracy. Then, starting from the divergent form
of the equation we implement the finite-volume method of Song Wang (IMA J Numer Anal 24:699–720, 2004) to discretize the differential problem. We show that the system matrix of the discretization scheme is a M-matrix, so that the discretization is monotone. This provides the non-negativity of the price with respect to time if the initial distribution is nonnegative. Numerical
experiments demonstrate second order of convergence for difference scheme when the node is not too close to the point of degeneration. 相似文献
214.
Krzysztof Kozak Beata Koz?owskaTadeusz A. Przylibski Jadwiga MazurAnna Adamczyk-Lorenc Kalina Mamont-Cie?laOlga Stawarz Jerzy DordaBarbara K?os Miros?aw JanikEl?bieta Kochowska 《Radiation measurements》2012,47(1):89-95
The paper presents the results of intercalibration measurements of 222Rn concentration in water samples. The aim of the experiments was to evaluate the different measurement methods of radon determination in water samples performed by eleven laboratories in Poland and to test these methods in field work. In the years 2001-2003 three national intercomparison experiments were organized. The results allowed the participating laboratories to improve their measurement methodologies and to achieve better, i.e. more convergent results of determining radon concentration in water samples. The results show that the use of liquid-scintillation technique seems to be the best measurement method. It should be emphasized that comparative experiments are essential for all laboratories and they should be carried out as often as possible, based both on prepared samples with known concentration values of 222Rn and natural water samples, the most commonly used in laboratory practice. 相似文献
215.
Determination of chemical composition of siderite (Fe, Me)CO3 (where Me = Mg, Ca, Mn) present in siderite concretion is developed. An accurate and precise determination of Mg, Ca, Mn and Fe in siderite required complete separation of this mineral from other materials, e.g. calcite, quartz. For this purpose, selective dissolution in acetic acid (HAc) was applied. HAc concentration from 0.1 to 1 mol L−1 and extraction time from 0.5 to 8 h were investigated. In each step of investigation of selective dissolution, the X-ray diffraction measurements (XRD) of the residues was performed and also calcium (complexometric titration) and iron (XRF) in solution were determined. HAc of concentration 0.25 mol L−1 and extraction time of 2 h was adopted for siderite separation because in these conditions the siderite was not dissolved and, simultaneously, calcite was completely dissolved. In the next step, the nondissolved sample was digested in hydrochloric acid. The solution of the separated siderite was pipetted onto membrane filter and Mg, Ca, Mn and Fe were determined by wavelength-dispersive X-ray fluorescence (WDXRF) spectrometry. The calibration was performed using 11 certified reference materials of iron ores. Matrix effects were corrected using empirical coefficient model for intermediate-thickness samples. 相似文献
216.
Marek Jasiorski Anna Leszkiewicz Stefan Brzeziński Gabriela Bugla-Płoskońska Grażyna Malinowska Beata Borak Iwona Karbownik Agnieszka Baszczuk Wiesław Stręk Włodzimierz Doroszkiewicz 《Journal of Sol-Gel Science and Technology》2009,51(3):330-334
The aim of this study was to investigate antimicrobial activity of textiles doped with silver in different forms. Three types
of textiles were prepared and examined: textiles doped with commercially available Ag nanoparticles, textiles doped with commercial
colloidal silver and textiles doped with silver silica SiO2/Ag spheres. The specimens of silica submicron spheres were synthesized by the sol–gel method as a matrix for biological active
silver. The results of microbiological tests revealed that among three kinds of Ag doped textiles only these doped with SiO2/Ag spheres are bacteriostatically active. During the experiments minimal inhibiting bacteria growth concentration of active
SiO2/Ag spheres added to textiles was determined. 相似文献
217.
Aldona Kostuch Iwona A. Rutkowska Beata Dembinska Anna Wadas Enrico Negro Keti Vezz Vito Di Noto Pawel J. Kulesza 《Molecules (Basel, Switzerland)》2021,26(17)
Platinum is a main catalyst for the electroreduction of oxygen, a reaction of primary importance to the technology of low-temperature fuel cells. Due to the high cost of platinum, there is a need to significantly lower its loadings at interfaces. However, then O2-reduction often proceeds at a less positive potential, and produces higher amounts of undesirable H2O2-intermediate. Hybrid supports, which utilize metal oxides (e.g., CeO2, WO3, Ta2O5, Nb2O5, and ZrO2), stabilize Pt and carbon nanostructures and diminish their corrosion while exhibiting high activity toward the four-electron (most efficient) reduction in oxygen. Porosity of carbon supports facilitates dispersion and stability of Pt nanoparticles. Alternatively, the Pt-based bi- and multi-metallic catalysts, including PtM alloys or M-core/Pt-shell nanostructures, where M stands for certain transition metals (e.g., Au, Co, Cu, Ni, and Fe), can be considered. The catalytic efficiency depends on geometric (decrease in Pt–Pt bond distances) and electronic (increase in d-electron vacancy in Pt) factors, in addition to possible metal–support interactions and interfacial structural changes affecting adsorption and activation of O2-molecules. Despite the stabilization of carbons, doping with heteroatoms, such as sulfur, nitrogen, phosphorus, and boron results in the formation of catalytically active centers. Thus, the useful catalysts are likely to be multi-component and multi-functional. 相似文献
218.
The study on the relationship between the structure and spectroscopic properties of styrylquinolinium dyes were carried out
by measuring the electronic visible absorption, steady-state and time-resolved fluorescence spectra of quinoline based hemicyanine
dyes. The influence of the solvent on absorption and emission spectra and the solvatochromic properties, observed for both
ground and first excited states, for all the dyes were applied for the evaluation of their excited state dipole moments. The
ground state dipole moments of dyes under the study were established by applying ab initio calculations. The measured, using solvatochromic methods, excited state dipole moments of tested hemicyanines are in the
range from 5.38 to 18.90 D and the change in the dipole moments caused by excitation were found to differ from 1.88 to 6.64
D. It was observed that for all tested dyes the dipole moments of the excited states were higher than those of a ground states.
The fluorescence lifetime measurements with picosecond resolution was performed for entire series of hemicyanine dyes possessing
different dialkylamino groups attached to the phenyl ring. The average lifetimes of the dye fluorescence, determined from
the measured data by multi-order exponential decay curve fitting, were in the range from about 120 to 1200 ps at the fluorescence
peak wavelength. The fluorescence lifetime measurements were performed for dyes in ethyl acetate solutions. The time-resolved
fluorescence spectra measurements allowed to propose the mechanism of the dyes excited states deactivation. 相似文献
219.
In this work, separation of multicomponent mixtures containing components with the same and different electrophoretic mobility by using orthogonal pressurised planar electrochromatography is studied theoretically. Additionally, a simple way for determination of a maximum amount of mixture causing volume overload of flat/planar columns used in this technique is presented. In the next stage, effects of change in different parameters on process performance by simulation case studies are investigated. A comparative study of separation productivity of orthogonal pressurised planar electrochromatography with the continuous and periodic modes of mixture delivery and column chromatography is carried out. 相似文献
220.
The infrared image of a microbolometer camera can show significant blurring effects if the object is moving. The blurring mechanism of a microbolometer is different to that encountered with classical CCD and CMOS cameras. The electrical signal in the pixel of a microbolometer detector decays exponentially with a time constant of 10–15 ms; therefore, the moving object is mapped to more pixels, resulting in a blurred image. Because of this blurring effect, the recorded infrared radiation of the object is dispersed and the contrast of the object corrupted. In this paper it is shown how the image can be restored and the blurring eliminated. The point spread function of the microbolometer camera is determined and the impact of the blurring from objects of different sizes is investigated. In order to suppress the noise in the restoration, a Wiener filter is used and it is demonstrated how objects of different sizes can be restored. Examples are presented for objects moving linearly and/or rotating; furthermore, an application is described where this technique has been used for automated thermographical testing. 相似文献