Influence of calcination temperature on structural and magnetic properties of nanocomposites formed by Co-ferrite dispersed in sol-gel silica matrix using tetrakis(2-hydroxyethyl) orthosilicate as precursor

Effects of calcination temperatures varying from 400 to 1000°C on structural and magnetic properties of nanocomposites formed by Co-ferrite dispersed in the sol-gel silica matrix using tetrakis(2-hydroxyethyl) orthosilicate (THEOS) as water-soluble silica precursor have been investigated. Studies carried out using XRD, FT-IR, TEM, STA (TG-DTG-DTA) and VSM techniques. Results indicated that magnetic properties of samples such as superparamagnetism and ferromagnetism showed great dependence on the variation of the crystallinity and particle size caused by the calcination temperature. The crystallization, saturation magnetization Ms and remenant magnetization Mr increased as the calcination temperature increased. But the variation of coercivity Hc was not in accordance with that of Ms and Mr, indicating that Hc is not determined only by the crystallinity and size of CoFe2O4 nanoparticles. TEM images showed spherical nanoparticles dispersed in the silica network with sizes of 10-30 nm. Results showed that the well-established silica network provided nucleation locations for CoFe2O4 nanoparticles to confinement the coarsening and aggregation of nanoparticles. THEOS as silica matrix network provides an ideal nucleation environment to disperse CoFe2O4 nanoparticles and thus to confine them to aggregate and coarsen. By using THEOS as water-soluble silica precursor over the currently used TEOS and TMOS, the organic solvents are not needed owing to the complete solubility of THEOS in water. Synthesized nanocomposites with adjustable particle sizes and controllable magnetic properties make the applicability of Co-ferrite even more versatile

Is a Molecular Adiabatic Approximation Appropriate to Positronic Atoms and Molecules?

The adiabatic approximation to positronic atoms and molecules was considered as an option to the computationally unfeasible methods that treat all particles in a common footing, in two different approaches communicated in the 37th PSPA. Here we present further assessment and comparison of the two approaches as a way of evaluating the potential of adiabatic or, as we found preferable, molecular approaches

Is a Molecular Adiabatic Approximation Appropriate to Positronic Atoms and Molecules?

The adiabatic approximation to positronic atoms and molecules was considered as an option to the computationally unfeasible methods that treat all particles in a common footing, in two different approaches communicated in the 37th PSPA. Here we present further assessment and comparison of the two approaches as a way of evaluating the potential of adiabatic or, as we found preferable, molecular approaches

Molecular Structure of Water-Like Positronium Complexes

We resort to a previously introduced molecular model for positron complexes to study the molecular structures of Ps$\text{}_{2}$O and PsOH and to explore their analogies and differences from the molecular structure of water

A Theoretical Study of Rate Coefficients for the O + NO Vibrational Relaxation

Quasi-classical trajectories have been integrated to study the vibrational relaxation of the O + NO(v) process as a function of the initial vibrational quantum number for T = 298 K, 1500 K, and 3000 K. Two reliable potential energy surfaces have been employed for the A‘ and A‘ ‘ doublet states of NO2. The calculated vibrational relaxation rate constants show a nearly v-independent behavior at room temperature and a moderate increase with v for higher temperatures. Although deviating significantly from the recommended values, good agreement with recent experimental results has been obtained. The importance of multi-quantum transitions is also analyzed

Resonating Valence Bond calculations on small anionic lithium clusters

We recast the Resonating Valence Bond theory, first introduced by Linus Pauling, in a nonorthogonal ab initio Valence Bond formalism and apply the method to study some properties of the anionic clusters ${\rm Li}_{n}^ -$ $(2\le n\le 5)$. We show how to choose appropriate structures and orbitals, and also how to use the so-called metallic orbitals. The problem of interpreting the role of a specific Valence Bond structure looking up its weight in the general wave function is elucidated. Information about the excited states of the systems is obtained. The theory can make good qualitative predictions on the electronic behaviour of the clusters by using a wave function that is a linear combination of a small set of structures. Pauling's theory is shown to be quite appropriate for describing anionic systems, specially the small ones, where the loosely bounded electron largely influences the properties of the systems. We verify the preference of some clusters for linear geometries