MgFe2O4 pigment obtained at low temperature

doi:10.1016/j.materresbull.2005.07.019

Materials Research Bulletin

Volume 41, Issue 1, 5 January 2006, Pages 183-190

https://doi.org/10.1016/j.materresbull.2005.07.019 Get rights and content

Abstract

This work had the objective of studying the pigment MgFe₂O₄ obtained by the polymeric precursor method, which is based on the formation of a polymeric resin, followed by a heat treatment in order to obtain the final oxide. The powders were calcined between 500 and 1100 °C, and characterized by thermal analysis, X-ray diffraction, nitrogen adsorption, scanning electronic microscopy and diffuse reflectance. The pigment is single phase, with a cubic system and space group Fd3m (2 2 7). A low temperature synthesis was possible, with a crystalline material after heat treatment at 800 °C. The colors obtained varied, according to the crystallization of the MgFe₂O₄.

Introduction

The magnesium ferrite (MgFe₂O₄) has cubic structure of normal spinel-type. A general formula for two to three oxide spinels is AB₂O₄, where A and B denote di and trivalent cations, respectively (Mg²⁺ and Fe³⁺). The unit cell containing eight formula units has the cubic space group Fd3m (2 2 7), where the O²⁻ ions occupy the 32e positions and cations occupy the 8a (43 m) and 16d (3 m) positions. The FCC packing of O²⁻ ions creates 64 tetrahedral and 32 octahedral interstices/unit cell [1]. For stoichiometric two to three spinels, one-eighth of the tetrahedral (43 m) interstices and one-half of the octahedral (3 m) interstices are occupied. This way, the structure may be described as a face-centered complex of A and B cubes, where the A cubes comprise an X₂O₄ group (X cations in 4-coordination) and the B cubes an Y₄O₄ group (Y cations in 6-coordination). When the oxygen parameter, u, is equal to 1/4 (unit cell origin at a center of symmetry (3 m)), the O²⁻ ions form a cubic close packing; for u > 1/4, O²⁻ ions move apart in the [1 1 1] direction from the nearest (43 m) position and the tetrahedral/octahedral radius increases. In that way, 56 tetrahedral sites and 16 octahedral sites stay empty in the interstitial space of the structure that, conceptually, could contain a cation [2].

Different studies show that certain compounds with spinel structure present possible applications of great technological interest, such as pigments (CoAl₂O₄) [3], refractories (MgAl₂O₄) [4], catalysts (ZnCo₂O₄) [5], electrodes (LiMn₂O₄) [6], superconductors (LiTi₂O₄) [7], varistors (Zn₇Sb₂O₁₂) [8], magnetics displays (ferrites) [9], [10], among others. In relation to pigments, MgFe₂O₄ is commercially used as oil paint [11] and ZnFe₂O₄ was already evaluated, with brown color and good properties [12].

Many works present the synthesis of spinel using the conventional ceramic powder preparation process, which involves a solid state reaction. This method has some disadvantages for advanced applications, such as formation of strongly bonded agglomerates, poor sintering behavior, non-homogeneities, such as undesirable phases, abnormal grain growth, poor reproducibility and imprecise control of cation stoichiometry and ratios [13].

In order to improve the properties of ceramic powders, chemical methods as the sol–gel, coprecipitation and polymeric precursors have been investigated in the last years. These methods lead to a more precise stoichiometry and good sinterability, besides particle size and morphology control [14].

The polymeric precursor method was developed by Pechini [15], being a soft chemical method used to synthesize polycationic powders. The process is based on the metallic citrate polymerization using ethylene glycol. A hydrocarboxylic acid, such as citric acid, is used to chelate cations in an aqueous solution. The addition of a glycol, such as ethylene glycol, leads to the formation of an organic ester. Polymerization, promoted by heating, results in a homogeneous resin in which metal ions are uniformly distributed throughout the organic matrix. With additional heating, a polymeric resin is formed, keeping a homogeneous distribution of cations in molecular scale. When the resin is calcined at relatively low temperatures (300 °C), polymer breaking occurs and the powder precursor is obtained. Further calcination leads to mixed-cation oxides with fine structure and controlled stoichiometry [14], [16]. Different pigments with spinel structure were synthesized by our group using the polymeric precursor method [17], [18], with good results.

This work aims to study the MgFe₂O₄ synthesized by polymeric precursor method. The physicochemical behavior of this material and its application as inorganic ceramic pigment was evaluated.

Section snippets

Experimental

Initially, the citric acid (CA) was added to water under constant agitation, with temperature between 60 and 70 °C. Then, the former of the inorganic matrix was added, iron III nitrate (Vetec) and after its dissolution, the modifier of the inorganic matrix was added, magnesium carbonate (Aldrich). A 3:1 citric acid:metal molar ratio was used. Finally, ethylene glycol (EG) (Synth) was added to the solution, at a mass proportion of 60% CA to 40% EG [15], [16]. The temperature was raised to 90–110

Results and discussion

Fig. 1 illustrates the thermogravimetric curve of the MgFe₂O₄ powder precursor. It indicated two decomposition steps. The first one was attributed to the loss of water and some gases adsorbed on the surface. The second step was ascribed to the decomposition of the organic matter. The DTA curve shows a highly exothermic peak, associated to considerable mass loss. This peak is related to the combustion of the organic material.

Fig. 2 shows XRD patterns of samples after heat treatment between 500

Conclusions

The synthesis of the ceramic pigment with an MgFe₂O₄ spinel structure was obtained by the polymeric precursor method at low temperatures. The pigment presented thermal stability, being single phase. A light reddish-yellow hue, characterized by a reflectance in the 600–650 nm range was obtained. Colors varied with heat treatment temperature, due to ordering of the crystalline structure.

Acknowledgment

The authors gratefully acknowledge the financial support of the Brazilian research funding institution CNPq, FAPESP/CEPID.

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MgFe2O4 pigment obtained at low temperature

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgment

J. Alloys Compd.

J. Mol. Catal. A Chem.

Sol. Energy Mater. Sol. Cells

J. Magn. Magn. Mater.

J. Alloys Compd.

Mater. Lett.

J. Cryst. Growth

J. Nucl. Mater.

Mater. Lett.

J. Am. Ceram. Soc.

J. Chem. Phys.

J. Am. Ceram. Soc.

MgFe₂O₄ pigment obtained at low temperature