J. Cent. South Univ. (2012) 19: 2411-2415
DOI: 10.1007/s11771-012-1289-6
Microstructure characterization of NiFe2O4-NiO solid-solid diffusion couple
SHI Kai-hua(时凯华)1,2, ZHOU Ke-chao(周科朝)1,2, ZHANG Lei(张雷)1,2, LI Zhi-you(李志友)1,2
1. State Key Laboratory of Powder Metallurgy (Central South University), Changsha 410083, China;
2. Powder Metallurgy Research Institute, Central South University, Changsha 410083, China
? Central South University Press and Springer-Verlag Berlin Heidelberg 2012
Abstract: The solid state interdiffusion between NiFe2O4 and NiO in nitrogen atmosphere was studied by means of diffusion couple technique. NiFe2O4/NiO diffusion couple with plane interfaces was made by clamping method and sintering at 1 300 ℃ for 10 h. Scanning electronic microscopy (SEM) and energy-dispersive X-ray spectrometry (EDS) were used to analyze the microstructure and phase composition of the diffusion couples. The results indicate that a porous layer of uniform thickness forms along the NiFe2O4/NiO bonding interface and exhibits a deep penetration in the NiFe2O4 due to the Kirkendall effect. Furthermore, SEM observations reveal that the needle-like nickel ferrite precipitates form in NiO near the interface and the formation mechanism of them are inferred to be diffusion type solid-state phase changes.
Key words: NiFe2O4; microstructure; diffusion; Kirkendall effect
1 Introduction
Nickel ferrite, NiFe2O4, is an important member of the spinel family and one kind of iron-based oxides with technological applications as ferromagnetic material in computer peripherals, telecommunications equipments, permanent magnets, electronic and microwave devices [1-3]. Therefore, the preparation of NiFe2O4 nanoparticles, and the research on magnetic properties are widely reported [4-9]. NiFe2O4 and its derivatives are also believed to be the most promising green anode materials used in aluminum industry for the production of primary aluminum owing to their high melting point, high thermodynamic stability and high corrosion resistance [10-11]. Anecdotal evidence suggests that the NiFe2O4-NiO composite ceramic sintered in nitrogen atmosphere has relatively superior corrosion resistance [12-13]. Consequently, the study on the sintering behavior of NiFe2O4 and NiO in nitrogen becomes very important.
In recent years, sintering microstructure of NiFe2O4-NiO has been investigated intensively. Among these investigations, although some researchers have noted the precipitates appearing in NiO during NiFe2O4-NiO sintering process [14-16], less experimental research is focused on the formation mechanism of them. As we know, the microstructure of materials is generally associated with diffusion at high temperatures. However, few researches on the microstructure changes caused by interdiffusion between NiFe2O4 and NiO have been published, to our knowledge.
In this work, the microstructure characterization in diffusion couples of NiFe2O4 and NiO is presented. This is an attempt to examine the role of elemental transformation behavior in diffusion and microstructure changes in the two phases of NiFe2O4 and NiO.
2 Experimental
2.1 Synthesis of NiFe2O4 powders and preparation of diffusion couples
The powders of NiFe2O4 used in this work were obtained by solid state reaction method, which had been described in detail elsewhere [17]. NiO (Jinchuan, China, 99% purity) and Fe2O3 (Qidong, China, 99.5% purity) powders were used as starting materials and ball-milled in distilled water and ZrO2 milling media for 3 h. The mixture was dried for 10 h at 80-90 ℃ and then calcined at 1 200 ℃ for 6 h in air.
The diffusion couples were prepared by powder metallurgy. The preparation was divided into two steps: first NiO powder was compacted in a steel die at 40 MPa, and then NiFe2O4 powder was put into the mold and, compacted uniaxially at approximately 200 MPa to form wafers of 20 mm in diameter and 10 mm in thickness. The samples then were sintered at 1 300 ℃ for 10 h and then cooled to room temperature in nitrogen atmosphere.
2.2 Composition and microstructure observation
The phase composition of NiFe2O4 powders after synthesis was detected by X-ray diffraction (XRD, RigakuDmax/2550VB+, Japan) at 40 kV and 250 mA using Cu Kα radiation.
The microstructures of NiFe2O4/NiO diffusion couples were observed by scanning electron microscope (SEM, FEI Quanta-200, Netherlands). Simultaneously, the phase composition and Fe, Ni, O contents in diffusion couples were analyzed using energy-dispersive X-ray spectrometry (EDS, FEI Quanta-200, Netherlands).
3 Results and discussion
Figure 1 shows X-ray diffraction pattern of NiFe2O4 powders synthesized at 1 200 ℃ for 6 h. It can be seen that all the diffraction peaks can be perfectly indexed to NiFe2O4 phase and no impurity can be observed, indicating that the starting Fe2O3 powder has completely reacted with the NiO powders.
Fig. 1 XRD pattern of NiFe2O4 powders synthesized at 1 200 ℃ for 6 h
3.1 Formation of porous layer
Figure 2 shows scanning electron micrograph for the cross-section of NiFe2O4-NiO solid-solid diffusion couples sintered in nitrogen at 1 300 ℃ for 10 h. It is obvious that there is a porous layer of uniform thickness along the NiFe2O4/NiO bonding interface.
NiFe2O4 and NiO are both composed of ion crystals. So it is generally accepted that the interdiffusion between them is a kind of substitutional diffusion and the main transmitted ions may be cations rather than anions because the diffusion coefficient of cation is greater [18]. In order to gain insight into the mechanisms of the diffusion between NiFe2O4 and NiO to explain the formation of porous layer, the element contents near interface regions were examined by EDS analysis. Figure 3 illustrates the variation of element content with distance from the interface of NiFe2O4/NiO diffusion couples. It can be seen in Figs. 3(a) and (b) that both the relative contents of Fe in NiO and Ni in NiFe2O4 decrease with increasing the distance from the interface of NiFe2O4/NiO diffusion couples. On the contrary, the variation of oxygen content, as seen in Figs. 3(c) and (d), shows a irregular change with the distance increasing. Hence, it is able to be deduced that Ni2+ (radius of 0.069 nm) in NiO and Fe3+ (radius of 0.064 nm) in NiFe2O4 are the main transmitted ions because O2- (radius of 0.160 nm) is harder to migrate due to its larger radius.
Fig. 2 SEM image of diffusion interface in NiFe2O4-NiO solid-solid diffusion couples
On one hand, the melting point of NiFe2O4 (1 587 ℃) is lower than that of NiO (2 090 ℃). Hence, the diffusion of NiFe2O4, compared with NiO, is dominant. On the other hand, the cations of Fe3+ in NiFe2O4 are inclined to transmit to NiO because of the disparity of Fe3+ concentration between them. As a result, unequal exchange of Ni2+ and Fe3+ leads to large amounts of Fe3+ transmitting from NiFe2O4 to NiO, resulting in the lack of Fe in NiFe2O4, which causes the formation of vacancies in NiFe2O4. When the total number of vacancies exceeds the balanced concentration of space, the widespread macro-pores, called Kirkendall pores, form along the bonding interface in NiFe2O4.
From the above, it is clear that Kirkendall effect is the main factor which leads to the formation of the porous layer.
3.2 Variation of phase composition
Figure 4 shows the scanning electron microscope (SEM) images of different locations from the NiFe2O4/NiO diffusion couples. As shown in Fig. 4(a), a mass of Ni and needle-like nickel ferrite precipitates in NiO are observed near the diffusion interface. However, only a handful of Ni and, nickel ferrite-free precipitates are seen in the central of NiO in Fig. 4(b). Additionally, a similar situation also appears in Figs. 4(c) and (d), where NiO is found more in NiFe2O4 near the interface (Fig. 4(c)) than in the central area (Fig. 4(d)).
Fig. 3 Variation of element contents with distance from interface of NiFe2O4/NiO diffusion couples: (a) Fe in NiO; (b) Ni in NiFe2O4; (c) O in NiO; (d) O in NiFe2O4
Fig. 4 SEM images of different positions in NiFe2O4/NiO diffusion couples: (a) NiO near interface; (b) Center of NiO (c) NiFe2O4 near interface; (d) Center of NiFe2O4
The formation schematic diagram of ferrite nickel precipitates and NiO in NiFe2O4/NiO diffusion couples via cation transmission is shown in Fig. 5. As analyzed above, the transformation between Ni2+ in NiO and Fe3+ in NiFe2O4 is responsible for diffusion of NiFe2O4 and NiO. Consequently, Fe3+ transmits from NiFe2O4 into NiO near the interface and gradually gathers to form supersaturated solid solution, then generates nickel ferrite precipitates by nucleation and growth [15] as the temperature decreases. The needle-like precipitate shown in Fig. 4(a) has certain orientation and its forming process is directly related with ion diffusion. They are analogous to the characteristics of diffusion type solid-state phase changes. So, it can be concluded that the diffusion type solid-state phase change is the main forming mechanism of nickel ferrite precipitates. Analogously, since Ni2+ transmits from NiO to NiFe2O4 constantly and there is the lack of Fe, NiO is formed in NiFe2O4 near the interface.
Fig. 5 Schematic diagram of ferrite nickel precipitates and NiO formation in NiFe2O4/NiO diffusion couples via cation transmission
The formation of Ni in NiO is influenced by sintering atmosphere. As is well known, the oxygen partial pressure of sintering atmosphere is a very important factor in NiFe2O4 [19-20]. If the pressure is low enough, there will be a decomposition reaction of NiO [21]:
NiO=Ni+1/2O2 (1)
The balanced oxygen partial pressure of Eq. (1) at different temperatures are calculated and listed in Table 1.
Consequently, when the NiFe2O4/NiO diffusion couples are sintered in nitrogen atmosphere and the porous layer forms along the NiFe2O4/NiO interface, NiO near the layer will be decomposed into nickel because the effect of sintering inert atmosphere (oxygen partial pressure of about 1.0×10-6 Pa) permeates through the pores layer.
Table 1 Balanced oxygen partial pressure of NiO decomposition reaction at different temperatures
4 Conclusions
1) The diffusion couple technique is introduced to discuss the diffusion behavior of NiFe2O4/NiO couple sintered in nitrogen at 1 300 ℃ for 10 h by employing a combination of scanning electron microscopy and energy-dispersive X-ray spectrometry.
2) A porous layer with uniform thickness forms along the NiFe2O4/NiO bonding interface and exhibits a deep penetration in the NiFe2O4 due to the Kirkendall effect in the diffusion. As a result, NiO near the layer is decomposed into nickel because the effect of sintering inert atmosphere permeates through the pores in porous layer.
3) The transmission of Ni2+ and Fe3+ in NiFe2O4/NiO causes the formation of large amounts of NiO in NiFe2O4 near the interface. Simultaneously, the transmission also leads to nickel ferrite precipitates in NiO because of diffusion type solid-state phase changes in the cooling process.
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(Edited by YANG Bing)
Foundation item: Project(50721003) supported by the National Natural Science Fund for Innovation Group of China; Project(2008AA030501) supported by the National High Technology Research and Development Program of China
Received date: 2011-07-20; Accepted date: 2011-09-28
Corresponding author: ZHOU Ke-chao, Professor, PhD; Tel: +86-731-88836264; E-mail: seasonzg@163.com
