全文获取类型
收费全文 | 477篇 |
免费 | 6篇 |
国内免费 | 2篇 |
专业分类
化学 | 254篇 |
晶体学 | 7篇 |
力学 | 8篇 |
数学 | 95篇 |
物理学 | 121篇 |
出版年
2022年 | 4篇 |
2021年 | 5篇 |
2020年 | 12篇 |
2019年 | 6篇 |
2018年 | 4篇 |
2017年 | 4篇 |
2016年 | 10篇 |
2015年 | 12篇 |
2014年 | 7篇 |
2013年 | 21篇 |
2012年 | 34篇 |
2011年 | 36篇 |
2010年 | 19篇 |
2009年 | 18篇 |
2008年 | 24篇 |
2007年 | 45篇 |
2006年 | 31篇 |
2005年 | 16篇 |
2004年 | 16篇 |
2003年 | 16篇 |
2002年 | 21篇 |
2001年 | 18篇 |
2000年 | 12篇 |
1999年 | 6篇 |
1998年 | 5篇 |
1997年 | 6篇 |
1996年 | 6篇 |
1995年 | 7篇 |
1994年 | 6篇 |
1993年 | 9篇 |
1992年 | 3篇 |
1991年 | 2篇 |
1990年 | 6篇 |
1989年 | 5篇 |
1988年 | 6篇 |
1987年 | 3篇 |
1986年 | 6篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1980年 | 4篇 |
1979年 | 1篇 |
1978年 | 3篇 |
1977年 | 1篇 |
1974年 | 1篇 |
1973年 | 3篇 |
1971年 | 1篇 |
排序方式: 共有485条查询结果,搜索用时 15 毫秒
391.
M.F. Gerard N.M.C. Casado M. Perec R. Calvo 《Journal of Physics and Chemistry of Solids》2007,68(8):1533-1539
Magnetic and EPR data have been collected for complex [Cu(L-Arg)2](NO3)2·3H2O (Arg=arginine). Magnetic susceptibility χ in the temperature range 2-160 K, and a magnetization isotherm at T=2.29(1) K with magnetic fields between 0 and 9 T were measured. The observed variation of χT with T indicates predominant antiferromagnetic interactions between Cu(II) ions coupled in 1D chains along the b axis. Fitting a molecular field model to the susceptibility data allows to evaluate g=2.10(1) for the average g-factor and J=−0.42(6) cm−1 for the nearest neighbor exchange coupling (defined as Hex=-∑JijSi·Sj). This coupling is assigned to syn-anti equatorial-apical carboxylate bridges connecting Cu(II) ion neighbors at 5.682 Å, with a total bond length of 6.989 Å and is consistent with the magnetization isotherm results. It is discussed and compared with couplings observed in other compounds with similar exchange bridges. EPR spectra at 9.77 were obtained in powder samples and at 9.77 and at 34.1 GHz in the three orthogonal planes of single crystals. At both microwave frequencies, and for all magnetic field orientations a single signal arising from the collapse due to exchange interaction of resonances corresponding to two rotated Cu(II) sites is observed. From the EPR results the molecular g-tensors corresponding to the two copper sites in the unit cell were evaluated, allowing an estimated lower limit |J |>0.1 cm−1 for the exchange interaction between Cu(II) neighbors, consistent with the magnetic measurements. The observed angular variation of the line width is attributed to dipolar coupling between Cu(II) ions in the lattice. 相似文献
392.
A wide library of 2,3,4,7-tetrahydro[1,4]thiazepines have been prepared by simple heating of thiazolidine and β-enaminonitrile derivatives in acetonitrile. The procedure, whose yields depend on the nature and position of the substituents, gave good results if the substituents were not very bulky but it is less effective when starting from 2-substituted thiazolidines. 相似文献
393.
394.
Summary. A numerical method for solving the thermal subproblem appearing in the modelization of polythermal ice sheets is described.
This thermal problem mainly involves three nonlinearities: a reaction term due to the viscous dissipation, a Signorini boundary
condition associated to the geothermic flux and an enthalpy term issued from the two phase Stefan formulation of the polythermal
regime. The stationary temperature is obtained as the limit of an evolutive problem which is discretized in time with an upwind
characteristics scheme and in space with finite elements. The nonlinearities are solved either by Newton-Raphson method or
by duality techniques applied to maximal monotone operators. The application of the algorithms provides the dimensionless
temperature distribution approximation and allows to identify the cold and temperate ice regions.
Received February 1, 1998 / Published online July 28, 1999 相似文献
395.
Phase-Doppler anemometry (PDA) is a powerful tool for two-phase flow measurements and testing. Particle concentration and
mass flux can also be evaluated using the raw particle data supplied by this technique. The calculation starts from each particle
velocity, diameter, transit time data, and the total measurement time. There are two main evaluation strategies. The first
one uses the probe volume effective cross section, and it is usually simplified assuming that particles follow quasi one-directional
trajectories. In the text, it will be called the cross section method. The second one includes a set of methods which will
be denoted as “Generalized Integral Methods” (GIM). Concentration algorithms such as the transit time method (TTM) and the
integral volume method (IVM) are particular cases of the GIM. In any case, a previous calibration of the measurement volume
geometry is necessary to apply the referred concentration evaluation methods. In this study, concentrations and mass fluxes
both evaluated by the cross-section method and the TTM are compared. Experimental data are obtained from a particle-laden
jet generated by a convergent nozzle. Errors due to trajectory dispersion, burst splitting, and multi-particle signals are
discussed. 相似文献
396.
Synthesis,chemical and microstructural characterization of micro macroporous biphasic calcium phosphate granules
下载免费PDF全文
![点击此处可从《X射线光谱测定》网站下载免费的PDF全文](/ch/ext_images/free.gif)
M. A. Garcés Villalá J. L. Calvo Guirado D. Granados S. P. Limandri V. Galván Josa 《X射线光谱测定》2017,46(4):237-241
A route based on calcium phosphate emulsions with addition of glycolic acid as pore former was developed for synthesizing porous nanocrystalline biphasic calcium phosphate ceramics. The method is low cost and gives a biphasic calcium phosphate composed of 88% β‐tricalcium phosphate and 12% hydroxyapatite. The material obtained is characterized by scanning electron microscopy, X‐ray diffraction and X‐ray fluorescence techniques, and the specific surface area is determined by the Brunauer, Emmett and Teller method. The small crystalline domain sizes obtained (68 and 87 nm for β‐tricalcium phosphate and hydroxyapatite phases, respectively) allow a major contact reaction and stability in the interphase between the implanted material and natural bone, as well as a better promotion effect on the early bone in‐growth. The improvement of the physical, chemical and structural properties by the balanced combination of the ceramic phases, the small crystallite size, the high porosity and high specific surface area obtained is a desirable characteristic in bone tissue engineering and encourages the performance of animal studies in vivo to evaluate their use for applications such as bone replacement in humans. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
397.
J. L. Mateo M. Calvo P. Bosch 《Journal of polymer science. Part A, Polymer chemistry》2001,39(14):2444-2453
The kinetics and mechanism of the photoinitiated polymerization of tetrafunctional and difunctional methacrylic monomers [1,6‐hexanediol dimethacylate (HDDMA) and 2‐ethylhexyl methacrylate (EHMA)] in a polybutadiene matrix (PB) have been studied. The maximum double‐bond conversion, the maximum polymerization rate, the intrinsic reactivity, and the kinetic constants for propagation and termination have been calculated. Unlike the behavior followed by the SBS‐HDDMA and PS‐HDDMA systems, where a reaction‐diffusion mechanism occurs from the start of the polymerization at low monomer concentrations (<30–40%), in the PB‐HDDMA system the reaction diffusion controls the termination process only after approximately 10% conversion is reached, as for the bulk polymerization of polyfunctional methacrylic monomers. Before reaching 10% conversion the behavior observed can be better explained by a combination of segmental diffusion‐controlled (autoaccelerated) and reaction‐diffusion mechanisms. This is probably a consequence of the lower force of attraction between the monomer and the matrix and between the growing macroradical and the matrix than those corresponding to the other systems mentioned. For the PB‐EHMA system, the termination mechanism is principally diffusion‐controlled from the beginning of the polymerization for monomer concentrations below 30–40%, and for higher monomer concentrations, a standard termination mechanism takes place (kt ≈ 106) at low double‐bond conversions, which is diffusion‐controlled for high conversions (>40%). For PB‐HDDMA and PB‐EHMA systems, crosslinked polymerized products are obtained as a result of the participation of the double bonds of the matrix in the polymerization process. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2444–2453, 2001 相似文献
398.
David Prez-Gallego Julian Gonzalez-Ayala Antonio Calvo Hernndez Alejandro Medina 《Entropy (Basel, Switzerland)》2021,23(12)
A model for a pumped thermal energy storage system is presented. It is based on a Brayton cycle working successively as a heat pump and a heat engine. All the main irreversibility sources expected in real plants are considered: external losses arising from the heat transfer between the working fluid and the thermal reservoirs, internal losses coming from pressure decays, and losses in the turbomachinery. Temperatures considered for the numerical analysis are adequate for solid thermal reservoirs, such as a packed bed. Special emphasis is paid to the combination of parameters and variables that lead to physically acceptable configurations. Maximum values of efficiencies, including round-trip efficiency, are obtained and analyzed, and optimal design intervals are provided. Round-trip efficiencies of around 0.4, or even larger, are predicted. The analysis indicates that the physical region, where the coupled system can operate, strongly depends on the irreversibility parameters. In this way, maximum values of power output, efficiency, round-trip efficiency, and pumped heat might lay outside the physical region. In that case, the upper values are considered. The sensitivity analysis of these maxima shows that changes in the expander/turbine and the efficiencies of the compressors affect the most with respect to a selected design point. In the case of the expander, these drops are mostly due to a decrease in the area of the physical operation region. 相似文献
399.
Dr. Moritz Modl Dr. Sebastian Heinl Dr. Gabor Balázs Dr. Fuencisla Delgado Calvo Dr. Maria Caporali Dr. Gabriele Manca Martin Keilwerth Prof. Dr. Karsten Meyer Prof. Dr. Maurizio Peruzzini Prof. Dr. Manfred Scheer 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(25):6300-6305
The reactivity of ruthenium and manganese complexes bearing intact white phosphorus in the coordination sphere was investigated towards the low-valent transition-metal species [Cp′′′Co] (Cp′′′=η5-C5H2-1,2,4-tBu3) and [L0M] (L0=CH[CHN(2,6-Me2C6H3)]2; M=Fe, Co). Remarkably, and irrespective of the metal species, the reaction proceeds by the selective cleavage of two P–P edges and the formation of a square-planar cyclo-P4 ligand. The reaction products [{CpRu(PPh3)2}{CoCp′′′}(μ,η1:4-P4)][CF3SO3] ( 5 ), [{CpBIGMn(CO)2}2{CoCp′′′}(μ,η1:1:4-P4)] ( 6 ) and [{CpBIGMn(CO)2}2{ML0}(μ,η1:1:4-P4)] (CpBIG=C5(C6H4nBu)5; L0=CH[CHN(2,6-Me2C6H3)]2; M=Fe ( 7 a ), Co ( 7 b )), respectively, were fully characterized by single-crystal X-ray diffraction and spectroscopic methods. The electronic structure of the cyclo-P4 ligand in the complexes 5 – 7 is best described as a π-delocalized P42− system, which is further stabilized by two and three metal moieties, respectively. DFT calculations envisaged a potential intermediate in the reaction to form 5 , in which a quasi-butterfly-shaped P4 moiety bridges the two metals and behaves as an η3-coordinated ligand towards the cobalt center. 相似文献
400.