Microstructural evolution of NiFe2O4-10NiO powder prepared by

high temperature solid state reaction

ZHANG Lei(张 雷), ZHOU Ke-chao(周科朝), LI Zhi-you(李志友), YANG Wen-jie(杨文杰)

State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China

Received 29 November 2005; accepted 3 April 2006

Abstract: The NiFe₂O₄-10NiO powder for inert anode of aluminium electrolysis was prepared by high temperature solid state reaction. The microstructural evolution from the raw materials NiO and Fe₂O₃ to the NiFe₂O₄-10NiO powder was studied by SEM. The results show that the domain structure making up of the agglomerate particles of Fe₂O₃ remains after high temperature solid state reaction, and the diffusion of Ni²⁺ into Fe₂O₃ structure is the control step of the reaction process. A microstructure with compact structure and fine grain inside the particle results from the sintering of NiFe₂O₄-10NiO powder.

Key words: high temperature solid state reaction; NiFe₂O₄-NiO; microstructural evolution; inert anode

1 Introduction

The development of inert anode materials for aluminium production has gained considerable attention in recent years[1,2]. The significance of such anodes is the ability to produce environment-friendly O₂ gas during electrolysis instead of greenhouse gases, and dramatically reduce the cost for the production of Al. Several materials have been investigated for developing inert anodes including oxides[3], such as CeO₂, Cr₂O₃- NiO-CuO, NiFe₂O₄, CoFe₂O₄, SnO₂ and ZnFe₂O₄; cermets[4,5] such as NiFe₂O₄+NiO+Cu, NiFe₂O₄+NiO+ Cu+Ag; and metals such as Al-Cu, Ni-Al-Fe-Cu-X, Ni-Fe. The cermets NiFe₂O₄+NiO+M, where M is metals (such as Ni, Cu, Fe) and their alloys, are the most promising candidate materials of inert anode[6]. The composite oxide NiFe₂O₄-NiO has high thermodynamic stability and favorable electro-catalytic activity for oxygen evolution and high corrosion stability in molten cryolite electrolyte. A number of methods can be used to produce this composite powder, such as coprecipitaion of hydroxides, carbonates and oxalates, freeze-drying of mixed carboxylates, and high temperature solid state reaction[7,8]. For processing a great amount of raw NiFe₂O₄-NiO powder for developing inert anode in commercial size, the solid state reaction, a conventional process in advanced ceramic industry, was used in this work. The solid state reaction can achieve high purity, high surface area, and good sintering activity powder with high efficiency[9, 10]. In this work, the synthesis of NiFe₂O₄-10NiO by high temperature solid state reaction process with NiO and Fe₂O₃ powder as raw materials was studied. The crystalline phases of the prepared powders were identified by powder X-ray diffractometry. The evolution of microstructure from the raw materials NiO and Fe₂O₃ to the products, NiFe₂O₄- 10NiO was studied by SEM.

2 Experimental

The initial materials used were: high purity (99.9%, mass fraction) NiO with mean particle size of 40 μm produced by Jinchuan Group Ltd; high purity (99.2%) Fe₂O₃ with mean particle size of 30 μm produced by Hecheng Ferrite Ltd. For the synthesis of NiFe₂O₄- 10NiO, the as-received raw materials were mixed and ground for 2 h in de-ionized water by ball milling, with the mass ratio of NiO to Fe₂O₃ being 61.8?38.6, and the mass ratio of ball to material 5?1. Then the slurry was removed and dried at 120 ℃ for 48 h in air.

20 kg of mixed oxides per batch were heated in a furnace for high temperature solid state reaction in air. According to the research work of ZHANG et al [11] and QIN et al[12], the reaction temperature was risen to 1 250 ℃ at a heating rate of 100 ℃/h, and held for 2 h, then was slowly cooled down to ambient temperature. After solid state reaction, the NiFe₂O₄- 10NiO powder was ball milled for 6 h again with the ball-to-powder mass ratio of 5?1 and water-to-material ratio 2?3.

The particle size distribution of the as-received raw materials and the ball milled NiFe₂O₄-10NiO powder was examined by MICRO-PLUS laser size analyzer. X-ray diffraction analysis was performed on Rigaku 3014 diffractometer with a K_α radiation of Fe anode to determine the phase present in the final samples. The evolution of microstructure from the as-received raw materials NiO and Fe₂O₃ to the NiFe₂O₄-10NiO powder was observed by a JEOL JSM-6400F scanning electron microscope(SEM).

3 Results and discussion

3.1 XRD analysis

The X-ray powder diffraction pattern of NiFe₂O₄- 10NiO synthesized by high temperature solid state reaction is shown in Fig.1. All the diffraction peaks of well crystallized NiO and NiFe₂O₄ are detected in the XRD patterns, which are indentifical with the results in Ref.[13]. It reveals that the sample has typical cubic spinel structure without impurity phases.

Fig.1 XRD pattern of NiFe₂O₄-10NiO powder calcined at 1 250 ℃ for 2 h in air

3.2 Evolution of microstructure

The as-received Fe₂O₃ powder is produced by the oxidation of iron scale, and granulated by spray dry process. The SEM photographs of the as-received Fe₂O₃ powder are shown in Fig.2. It can be seen that the Fe₂O₃ powder is made up of agglomerates, and the agglomerates are composed of small and high density packed subunit which can be called “domains”. The mean particle size of the as-received Fe₂O₃ is 30 μm, and the size of domains is 3-6 μm. Fig.3 shows the SEM photographs of the as-received NiO powder. The NiO powder has crumble and lumpy structure with high surface area (Fig.3(b)). After ball milling, both NiO powder and Fe₂O₃ agglomerates are broken up into fine particle size, and the mean size of the composite oxide powder is 4.36 μm. Figs.4(a) and (b) show the SEM images of the milled NiFe₂O₄-10NiO powder calcinated at 1 250 ℃ for 2 h in air. The NiFe₂O₄-10NiO powder is of fine and polyhedral structure with a mean particle size of 3.55 μm. Fig.4(c) shows the interior conformation of NiFe₂O₄-10NiO particles, where the “domains” structure can be seen once again, and its size is obviously smaller than that of Fe₂O₃.

Fig.2 SEM images of as-received Fe₂O₃ powder

Fig.3 SEM images of as-received NiO powder

In Figs.4 (a) and (b), the polyhedral particles that look like the subunit particles of Fe₂O₃ agglomerates are observed, and the size of the particles is 1-3 μm, similar to the domains’ size of the as-received Fe_­O₃ powder. It can be deduced that the domain structure of Fe₂O₃ remains after high temperature solid state reaction. For the as-received NiO powder, the aggregative structure is constructed by subunit particles which has smaller particle size than the domains of Fe₂O₃. After ball milling and calcining, the structure of NiO particle is destroyed, but excessive NiO powder remains in the compound oxide powder.

Fig.4 SEM images of NiFe₂O₄-10NiO powder calcinated at   1 250 ℃ for 2 h

The special microstructure which has high density and fine grain inside the particle shown in Fig.4(c) is the result of sintering of the composite powder at high temperature.

3.3 Reaction mechanism

Fig.5 shows an illustrative and classical example for elucidating material transport mechanisms in solid-state reactions using the diffusion couple arrangements. The diffusion couple experiments are helpful for illustrating the complexity of reaction mechanisms in ionic compounds, especially ferrites[14].

Fig.5 Schematic diagram of NiFe₂O₄ formation reaction

At high temperature, the products phase nucleates and grows, then separates from the reactants. For relatively simple reaction mechanisms, it is possible to deduce the nature of the diffusing species from the relative growth of the new phase.

The overall reaction of the formation of spinel NiFe₂O₄ is

NiO+Fe₂O₃→NiFe₂O₄                         (1)

The reaction may occur in many mechanisms. The relative amount of the spinel phase formed at the original boundary is indicative of the diffusion mechanism of the reaction, the amount of spinel products formed is in a ratio of 1?3[15]. Due to the larger ionic radius, O^2- ions are expected to have a considerably lower mobility than most cations. When cations are only the transporting species, the mechanism is known as the Wagner mechanism[10]. The formation of the spinel phase can be easily explained by the diffusion of Ni²⁺ and counter- diffusion of Fe³⁺. To maintain electroneutrality, the mechanism involves the counter-diffusion of three Ni²⁺ ions for every two Fe³⁺ ions. While three Ni²⁺ ions produce three NiFe₂O₄, the diffusion of two Fe³⁺ ions results in only one NiFe₂O₄. The results of the evolution of microstructure show that the domain structure which makes up of the agglomerative Fe₂O₃ particle remains after high temperature solid state reaction, indicating that the diffusion of Ni²⁺ to Fe₂O₃ structure controls the reaction process.

The existence of an extensive range of solid solubility can lead to a difference in the rate of consumption of reactants. Fe₂O₃ has a high solubility in the spinel phase through partial reduction of Fe³⁺ to Fe²⁺ to produce magnetite:

→↑            (2)

Reaction (1) begins at 700 ℃, and gains a high reaction rate at 1 000-1 100 ℃[16]. While reaction(2) occurs at about 1 000 ℃ when heated in air[16]. Before the solid state reaction (1) is fully in action, the particles of Fe₂O₃ has been consumed by reaction (2), and the products Fe₃O₄ become the reactant of the solid state reaction (1). Then the formation reaction of nickel ferrite spinel is reaction (3) at higher temperature (＞1 000℃). At high temperature, with the reoxidation of Fe²⁺ back to Fe³⁺, the diffusion of Ni²⁺ to the Fe₃O₄ particle becomes the control step of the reaction, and the NiFe₂O₄ particles are sintered. So the particle size of the composite oxide is smaller than the size of domains in Fe₂O₃.

2Fe₃O₄+3NiO+1/2O₂→3NiFe₂O₄                (3)

4 Conclusions

The NiFe₂O₄-10NiO powder with classic spinel phase, fine particle size, polyhedral structure and high surface area is synthesized by high temperature solid state reaction. The evolution of microstructure from raw materials NiO and Fe₂O₃ to the reaction resultant NiFe₂O₄-10NiO shows that the domain structure which makes up of the agglomerative Fe₂O₃ particle remains after heating process. The analysis results of micro- structural evolution show that the main reaction mechanism is the diffusion of Ni²⁺ to the Fe₃O₄ particles. High temperature has a sintering effect on the composite powder, and the microstructure with high density and fine grain inside the particle is gained.

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 Foundation item: Project(2005CB623703) supported by the National Basic Research Program of China

Corresponding author: ZHANG Lei; Tel: +86-731-8836264; E-mail: fgmzhang@163.com

(Edited by YUAN Sai-qian)