Search Results

You are looking at 1 - 10 of 157 items for :

  • Chemistry and Chemical Engineering x
  • All content x
Clear All

Abstract  

With the aim of obtaining materials with applications in pigments, CoxZn7-xSb2O12 spinels were synthesized using the Pechini method. This method consists in the formation of a polymeric net, where the metallic cations are homogeneously distributed. In this work, two types of alcohol (ethyl glycol and ethylene glycol) were used for the synthesis of a zinc antimoniate spinel, CoxZn7-xSb2O12 (x=0-7). The materials were characterized by termogravimetry (TG) and differential thermal analysis (DTA). TG results indicated a decrease in total mass loss when cobalt was added to the solution substituting zinc, for samples prepared using the two different alcohols. Decomposition temperatures, obtained by TG and DTA, presented a decreasing behavior as cobalt was added to the material. In relation to the alcohols, all results indicated a better polymerization of the resin when ethylene glycol was used, being the most indicated one for cation immobilization. X-ray diffraction did not show differences between the two alcohols - both presented the spinel phase (Co, Zn)2.33Sb0.67O4. Samples with higher quantity of cobalt also presented ilmenite phase (Co, Zn)Sb2O6.

Restricted access

Abstract  

Soon after the discovery of the Mössbauer effect, studies were performed on spinels containing various transition metal ions (mostly Fe2+). This method proved very useful for investigating the local symmetry at transition metal ions. In spite of the numerous results, the correct interpretation of the complex quadrupole split spectra is still not given for numerous spinel structures. Since spectra of different shapes were measured for FeAl2O4 and FeCr2O4 by different authors, we performed new measurements on these spinels. The results on FeAl2O4 showed that the statistical distribution of another kind of ions in the positions A may influence the electric field gradient at the Fe2+ ions in the tetrahedral interstices. In FeCr2O4 and in the mixed Fe0.5Mg0.5Cr2O4, the electric field gradient exists at the Fe2+ ions at room temperature indicating that the degeneracy of the orbital doublet of the Fe2+ is removed.

Restricted access

Introduction Spinels are an important class of material due to their catalytic and magnetic properties. Ferrites are manufactured for their magnetic properties, in particular, for their high frequency response in high frequency

Restricted access

Introduction Magnetic nanoparticles of spinel ferrites are of great importance not only for its application potential but also for fundamental research [ 1 – 4 ]. Understanding the structure and size dependent properties is the

Restricted access

hydrocarbons [ 17 – 22 ] and unsaturated aldehydes [ 12 , 23 – 25 ], water–gas shift reaction [ 26 ], and many other processes [ 27 , 28 ] where the optimal carriers for platinum are a major issue. Magnesium aluminate spinel (MgAl 2 O 4 ) is a favorable

Restricted access

Introduction In the recent years, research has been carried out on mixed oxides of transition metals, of the type A II B 2 III O 4 with spinel structure, which are active and thermally stable. In addition to this, there is

Restricted access

crystallizes in cubic system, has a spinel structure, and has a low Curie temperature. The ternary metal oxide spinel has potential catalytic and chemical sensing applications [ 3 – 6 ]. Unlike ferrites, the chromite spinels have no B-site Fe 3+ . Still, these

Restricted access

Abstract  

The novel two-step synthesis method decreasing the calcining temperature necessary for formation of spinel lattice and as well for reaching of bright and clear hue of the pigments prepared were investigated. This work aims to utilization of DTA for synthesis monitoring. Influence of raw materials and temperature of calcination on color properties were observed. The characterization of the samples was performed by X-ray diffraction and colorimetry in the CIE L*a*b* system. It was proved that the black spinel pigment Cu2.3Mn2.8Cr4.9Ox can be prepared at finally temperature 700C as one-phase system with high quality black hue.

Restricted access

garnets and spinels. Many rare earth aluminate garnets are used as laser host when doped with Nd(III) or Yb(III) and also as scintillators and phosphors. Upon doping, interesting properties have been observed. Eu 2+ , R 3+ -doping for example, brings

Restricted access

Kinetics of low temperature hydrogen reduction of the metastable spinels

Magnetite and solid solutions with Mn, Co, Ni and Cu

Journal of Thermal Analysis and Calorimetry
Authors: D. Drakshayani and R. Mallya

Abstract  

The kinetics of reduction at relatively low temperatures with hydrogen of pure and doped metastable non-stoichiometric magnetite with 1 at% Mn, Co, Ni and Cu and also with 5 at % Ni and Cu have been investigated by using isothermal thermogravimetry in the temperature range 300–400°C. With increase in the concentration of the dopant (5 at% Ni and Cu), the reactivity increases. The activation energies for pure magnetite varies from 7 to 9 kcal/mole with the preparation temperature of precursorf Fe2O3 (250–400°C), being the lowest for those prepared at the lowest temperatures. The corresponding activation energies for the reduction of doped samples (Fe, M)3−zO4, it depends, apart from their porosity and surface areas, on the nature of the solute atom, amount of disorder, whether it occupies the tetrahedral (A) or octahedral (B) sites in the non-stoichiometric spinel and possibly on hydrogen ‘Spill over’ effects.

Restricted access