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 transformer cores, and magnetic memories and for their catalytic activity [1, 2]. They have also been found to be useful anticorrosive pigments for ceramic applications [3]. Cobaltites also show significant catalytic activity [4–6]. These spinels have also been developed as the basis for a number of undergraduate laboratory experiments, such as the synthesis of lithium manganite cathode materials [5], copper ferrite catalysts [7] and magnetite ferrofluids [8]. Spinels are also used as a basis for the conceptual development and application of crystal field theory, particularly, through the calculation of site preference energies in determination of inverse or normal spinel structures [8, 9] which has also been demonstrated through the use of ping-pong ball models to aid the understanding of structure development [9, 10].

Various methods for the synthesis of spinels have been studied, such as co-precipitation of the hydroxides [11] and oxylates [12], complexometric (sol–gel) synthesis through glycolate [13], ethylenediaminetetraacetate (EDTA) [14], fumarate [15, 16] and citrate intermediates [17, 18], organometallic synthesis in a polymer matrices [19], mechanochemical methods using binary oxides [20] and synthesis within silica ampoules [21, 22]. This article investigates a simple co-precipitation method using sulphate precursor solutions precipitated with sodium carbonate/ammonia solution followed by calcination at elevated temperature. The calcination process was characterised using thermogravimetric analysis coupled with mass spectrometry (TG–MS) for evolved gas analysis; characterisation of the calcination products was carried out using X-ray diffraction (XRD).

Experimental

Materials

All chemicals used in the preparation of the spinels were analytical ACS reagents supplied by Sigma-Aldrich; CuSO4·5H2O, CoSO4·7H2O, (NH4)2Fe(SO4)2·6H2O, MgSO4, NiSO4(H2O)6 and ZnSO4(H2O)7, Na2CO3 and saturated ammonia solution.

Procedure

Precursor solutions in the required stoichiometric ratios were prepared by mixing 100 mL of 0.2 M solutions of ammonium iron (II) sulphate (for ferrite synthesis) or cobalt (II) sulphate (for cobaltite synthesis) with 100 mL of 0.1 M solutions of MIISO4, where MII = Cu, Co, Mg, Ni or Zn. Basic carbonates were then precipitated by adding a 1 M sodium carbonate solution/0.59 M ammonia solution, with stirring, in 5 mL aliquots until a pH of 8 was reached (ca. 30–40 mL). The resulting precipitate was then recovered by vacuum filtration and dried in an air oven at 110 °C overnight.

Before calcination, each mixed, dried basic carbonate is ground to a fine powder using a mortar and pestle, and approximately 100 mg is reserved for thermal analysis. The mixed basic carbonates are then calcined at elevated temperature for 1 h: 500 °C for the cobaltites and 800 °C for the ferrites.

Composition and phase analysis

XRD patterns were obtained over a range of 5°–90° 2θ at a step of 0.02° with 2 s accumulation per step, using Siemens D5000 Diffractometer, with a Cu-Kα radiation source. Phase identification was carried out using ICDD (International Committee for Diffraction Data) JCPDS (Joint Committee for Powder Diffraction Standards) database to match the JCPDS Powder Diffraction File (PDF) card data.

The samples were examined by TG–MS using a Setaram Setsys 16/18 Thermobalance coupled with a Balzers ThermoStar mass spectrometer for evolved gas analysis (EGA). Experiments were carried out by placing circa 35 mg of the mixed carbonate sample into a 100 μL platinum crucible and heating at a rate of 10 °C/min from ambient temperature to 1000 °C in an air atmosphere (instrument grade air supplied by BOC Australia) at a flow rate of 20 mL/min.

The peaks in the mass spectra chosen for analysis were at 14, 15, 18, 28, 44, 46 and 64 amu with an aim to identify the presence of water (H2O+), carbon dioxide (C+ and CO2 +), sulphur dioxide (SO2 +) and ammonia (NH+) as decomposition products. As only water and carbon dioxide were detected using EGA, only the data for these mass species are reported.

Results

TG–MS data for the series of ferrites is shown in Fig. 1 and for the cobaltites in Fig. 2. The TG data suggest that the calcination process for the cobaltites is complete by 500 °C; hence, this was the temperature selected for the calcination of the cobaltites. This low temperature is also prescribed by the necessity to calcine at a temperature where the oxidised CoIII state is present as at higher temperatures thermal reduction to the MII state occurs. This is observed for the cobaltites between 800 and 900 °C. The ferrites are observed to undergo mass loss up to temperatures above 600 °C; in each case, calcination appears to be complete by 800 °C; hence, this was the temperature selected for the calcination of the ferrites.

Fig. 1
figure 1

TG–MS data for the thermal decomposition of ferrites in an air atmosphere; a zinc ferrite, b nickel ferrite, c cobalt ferrite and d magnesium ferrite

Full size image
Fig. 2
figure 2

TG–MS data for the thermal decomposition of cobaltites in an air atmosphere; a zinc cobaltite, b nickel cobaltite, c magnesium cobaltite and d copper cobaltite

Full size image

EGA suggests that, for both the ferrites and the cobaltites, the precursor precipitates contain significant proportions of hydroxide and carbonate. Mass losses listed in Table 1 confirm the presence of both carbonate and hydroxide species suggesting that the decomposition of the precipitate precursors is of the form:

$$ \begin{gathered} \left( { 1- x} \right){\text{M}}^{\text{II}} \left( {{\text{CO}}_{ 3} } \right){\cdot}x{\text{M}}^{\text{II}} \left( {\text{OH}} \right)_{ 2}{\cdot}2\left( { 1- y} \right){\text{Fe}}\left( {{\text{CO}}_{ 3} } \right){\cdot}2y{\text{Fe}}\left( {\text{OH}} \right)_{ 2} + {\text{O}}_{ 2} \hfill \\ \to {\text{M}}^{\text{II}} {\text{Fe}}_{ 2} {\text{O}}_{ 4} + \, \left( { 3 { }-x-{ 2}y} \right){\text{CO}}_{ 2} + \, \left( {x + { 2}y} \right){\text{H}}_{ 2} {\text{O}} \hfill \\ \end{gathered} $$
(1)
$$ \begin{gathered} \left( { 1- x} \right){\text{M}}^{\text{II}} \left( {{\text{CO}}_{ 3} } \right) {\cdot}x{\text{M}}^{\text{II}} \left( {\text{OH}} \right)_{ 2}{\cdot}2\left( { 1- y} \right){\text{Co}}\left( {{\text{CO}}_{ 3} } \right){\cdot}2y{\text{Co}}\left( {\text{OH}} \right)_{ 2} + {\text{O}}_{ 2} \hfill \\ \to {\text{M}}^{\text{II}} {\text{Co}}_{ 2} {\text{O}}_{ 4} + \, \left( { 3 { }-x-{ 2}y} \right){\text{CO}}_{ 2} + \, \left( {x + { 2}y} \right){\text{H}}_{ 2} {\text{O}} \hfill \\ \end{gathered} $$
(2)
Table 1 List of mass losses on the formation of the spinel
Full size table

With the exception of the zinc cobaltite, the cobaltites are predominantly carbonate; the ferrites, on the other hand, are predominantly hydroxide. It was not possible to identify which of these species were present in the precursor precipitates from XRD, as the patterns produced only very broad diffuse peaks (data not shown) which did not correlate well with carbonate or hydroxide species of corresponding metal ions present. The X-ray amorphous character of the basic carbonate precipitate suggests that the precipitate is finely divided and, at best, microcrystalline.

The basic carbonate decomposition step is accompanied by the oxidation of FeII to FeIII and CoII to CoIII to produce the spinel structure which is confirmed by the XRD data for ferrites calcined at 800 °C shown in Fig. 3 and the cobaltites calcined at 500 °C in Fig. 4. The XRD patterns for the phases present correlate well with the spinel structures based on JCPDS PDF card data. The ferrites produced products with a high degree of crystallinity with minimal presence of contaminant phases. The cobaltites produced predominantly cobaltite spinels as the main product, but in each case, a trace of the MII oxide is present (ZnO, NiO, MgO and CuO). It is possible that the reaction has not reached completion which may be addressed by either raising the temperature or increasing the calcination time, although the formation of cobalt oxide spinel (CoIICo III2 O4) may also be responsible for the incomplete reaction.

Fig. 3
figure 3

XRD data for the calcined ferrites; (a) zinc ferrite, (b) nickel ferrite, (c) cobalt ferrite and (d) magnesium ferrite

Full size image
Fig. 4
figure 4

XRD data for the calcined cobaltites: (a) zinc cobaltite, (b) nickel cobaltite, (c) magnesium cobaltite and (d) copper cobaltite

Full size image

Conclusions

The article reports a simple synthesis of ferrite and cobaltite spinels through the co-precipitation method. Spinels are the main products of the reaction although, for the cobaltites, the individual oxide phases were also observed. TG–MS allowed the decomposition reaction to be monitored and provided the basis for determining the calcination temperatures of the basic carbonates; 500 °C for the cobaltites and 800 °C for the ferrites. The calcination temperature for the ferrites appears to be optimal. Mixed oxide phases present along with the cobaltite spinels suggest that some further optimisation of the cobaltite calcination temperature may be achieved.