Influence of annealing on magnesioferrite nanoparticles synthesized by a sol-gel/combustion method

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Influence of annealing on magnesioferrite nanoparticles synthesized by a sol-gel/combustion method

Ronaldo Sergio de Biasi^I,*; Lúcia Helena Guimarães Cardoso^I,*; José Brant de Campos^II,*; Dalber Ruben Sanchez^III,*; João Batista Marimon da Cunha^IV,*

^ISeção de Engenharia Mecânica e de Materiais, Instituto Militar de Engenharia, Pr. Gen. Tibúrcio, 80, SE/4, Urca, 22290-270 Rio de Janeiro - RJ, Brazil

^IIDivisão de Processamento e Caracterização de Materiais, Instituto Nacional de Tecnologia, Av. Venezuela, 82, Saúde, 20081-312 Rio de Janeiro - RJ, Brazil

^IIIInstituto de Física, Universidade do Estado do Rio de Janeiro, R. São Francisco Xavier, 524, Maracanã, 20550-013 Rio de Janeiro - RJ, Brazil

^IVInstituto de Física, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves 9500, 91501-970 Porto Alegre - RS, Brazil

ABSTRACT

Nanocrystalline particles of magnesioferrite (MgFe₂O₄) were prepared by a sol-gel/combustion method using iron nitrate, Fe(NO₃)₃.9H₂O, magnesium nitrate, Mg(NO₃)₂.6H₂O, and citric acid, C₆H₈O₇.H₂ O, and annealed for 2 hours at 400, 500 and 600 °C. The average particle size, determined by X ray diffraction, was found to depend on the annealing temperature and varied from 〈D〉 = 8.1 to 〈D〉 = 17.8 nm. By measuring at several temperatures the relative intensity of the Mössbauer spectra due to superparamagnetic particles and to ferrimagnetic particles, we determined the size distribution of the nanoparticles in the samples. It was found to be a log-normal distribution with a most probable diameter that varied from Dm = 6.4 to 17.2 nm and a full width at half-height ΔD in the 5-6 nm range.

Keywords: nanoparticles, mössbauer spectroscopy, magnesioferrite

1. Introduction

Nanosized ferrites display physical and chemical properties which may be quite different from those of their bulk counterparts. Magnesioferrite (MgFe₂O₄) is one of the most interesting, because, due to its small magnetocrystalline anisotropy, superparamagnetic properties are still present at relatively low temperatures and/or high magnetic fields. The purpose of this work was to investigate the influence of annealing on the average particle size and particle size distribution of MgFe₂O₄ nanoparticles prepared using the sol-gel/combustion method¹, a fast and relatively inexpensive technique. The average particle size was determined by X ray diffraction, while the particle size distribution was obtained by measuring at several temperatures the relative intensity of the Mössbauer spectra due to superparamagnetic particles and to ferrimagnetic particles, a method that has already been applied successfully² to another nanosized ferrite, CoFe₂O₄.

2. Experimental Procedure

2.1. Sample preparation

Analytical grade Fe(NO₃)₃.9H₂O, Mg(NO₃)₂.6H₂O, and C₆H₈O₇.H₂ O were dissolved in deionized water to obtain the starting solution. The concentrations of ferric nitrate, magnesium nitrate and citric acid were 0.5, 0.25 and 0.75 M. The solution was stirred for 4 hours at 70 °C, heated to 90 °C and kept at this temperature until the sol turned into a transparent gel. The gel was then heated to 200 °C for 20 minutes so that auto-combustion would take place. Finally, the product was annealed in a furnace for 2 hours at different temperatures.

2.2. Measurements

X ray diffraction patterns were obtained using an XPert Pro Panalitical diffractometer with Cu Kα radiation (λ = 1.5418 Å). The average particle size was calculated from line broadening using the TOPAS application³, academic edition.

Mössbauer spectra were recorded at several temperatures between 25 K and room temperature in a homemade instrument using a source of 57Co(Rh) with an activity of about 50 mCi.

3. Experimental Results and Analysis

Table 1 shows the average particle sizes, as calculated from the X ray spectra shown in Figure 1, for samples as prepared and after annealing at three different temperatures.

The Mössbauer spectra of sample 2 in Table 1 are shown in Figure 2 for several different measurement temperatures. While at room temperature there is a doublet due to superparamagnetic relaxation, at lower temperatures one sees a sextet which is characteristic of bulk magnesioferrite⁴. By taking the ratio of the area under the doublet to the area under the sextet, it is possible to estimate the volume fraction of unblocked particles for each measurement temperature. The result is shown in Figure 3, where the dots are the experimental points and the line is a fit to a cumulative log-normal function

where T is the absolute temperature, erf(T) is the error function, C₁ and C₂ are constants and μ and δ are adjustable parameters. The best fit was obtained with C₁ = 0.64, C₂ = 0.61, μ = 5.70 and δ = 0.64.

The distribution of unblocking temperatures of the system is given by^5,6

where C is a normalization constant.

The temperature dependence of Equation 2 may be converted to a dependence on particle diameter (thus yielding the particle size distribution) using the relation^5,6

where D is the particle diameter, )〈D〉 is the average particle diameter, as estimated from the X ray results, and )〈T_c〉 is the average blocking temperature, given by

The result is shown in Figure 4 (curve 2), which is a log-normal distribution with average diameter )〈D〉 = 13.2 nm, most probable diameter Dm = 11.6 nm and full width at half maximum ΔD = 5.5 nm. Similar results for samples as prepared and annealed at different temperatures are shown in Table 1 and Figure 4.

4. Conclusions

The sol-gel/combustion technique has been used to prepare MgFe₂O₄ nanoparticles, which were annealed at different temperatures. X ray diffraction patterns show broad peaks in the positions corresponding to the crystal structure of magnesioferrite. Mössbauer spectra exhibit superparamagnetic behavior, confirming that the particles are in the nanometric range. Analysis of the broadening of the X ray lines shows that the average particle size increases with increasing annealing temperature. Analysis of the temperature dependence of the Mössbauer spectra of as prepared and annealed samples yields fairly narrow log-normal distributions of particle sizes. The results suggest that the sol-gel/combustion technique may be used to synthesize nanosized MgFe₂O₄ powders whose average size can be controlled by subsequent annealing without appreciably changing the distribution of particle sizes.

Received: September 18, 2008;

Revised: April 2, 2009

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Publication Dates

• Publication in this collection
  24 July 2009
• Date of issue
  June 2009

History

• Accepted
  02 Apr 2009
• Received
  18 Sept 2008