J. Cent. South Univ. Technol. (2009) 16: 0703-0707 

DOI: 10.1007/s11771-009-0116-1

Effect of BaO addition on relative density and electric conductivity of xNi/10NiO-NiFe₂O₄ cermets

HE Han-bing(何汉兵), LI Zhi-you(李志友), ZHOU Ke-chao(周科朝), HUANG Bai-yun(黄伯云)

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

Abstract: xNi/10NiO-NiFe₂O₄ (x=5, 10, 17) cermets and those doped with 1% BaO (mass fraction) were prepared by cold isostatic pressing at 200 MPa and sintering in nitrogen atmosphere at 1 473 K. The effects of BaO addition on relative density, microstructure and electric conductivity of cermets were investigated. The results show that relative densities of xNi/10NiO-NiFe₂O₄ cermets (x=5, 10, 17) doped with 1% BaO at 1 473 K in nitrogen atmosphere are increased by 0.49%, 1.45% and 2.99% compared with those of the undoped BaO cermets, respectively. Moreover, the electric conductivities (21.98, 28.37 and 50.10 S/cm) of xNi/10NiO-NiFe₂O₄ cermets (x=5, 10, 17) doped with 1% BaO at 1 233 K are improved compared with those (18.70, 22.79 and 39.58 S/cm) of xNi/10NiO-NiFe₂O₄ cermets (x=5, 10, 17), respectively. This indicates that perhaps the addition of BaO or formation of BaFe₂O₄ and Ba₂Fe₂O₅ has an active effect on electric conductivities of xNi/10NiO-NiFe₂O₄ (x=5, 10, 17) cermets.

Key words: Ni/10NiO-NiFe₂O₄ cermets; BaO doping; inert anode; electric conductivity; densification

1 Introduction

The research on an inert anode material has been a hot issue in pursuit of new technology. Early work on candidate materials for inert anode selection was concentrated on ceramic oxides, alloys and cermets [1]. Recently, cermets have received much more attention due to the combination of advantages of ceramic material and metallic material, such as low corrosion, low oxidation, good electric conductivity and high thermal shock resistance [2-3]. Nickel ferrite is one of the most common compounds in a family of spinel with cubic structure. NiFe₂O₄-based cermet offers a good combination of physical and chemical properties such as high melting point, high resistance to chemical attack, and good thermal stability, and consequently it is the preferred material as inert anode. But it is well known that the poor electric conductivity of xNi/10NiO-NiFe₂O₄ (x=5, 10, 17) cermet restricts its application in the inert anode [4-5]. Sintering additive is an effective method to increase densification and electric conductivity of cermets. GHOSH et al [6] obtained relative density of 99% in magnesium aluminate spinel using ZnO as additive and suggested that the formation of anion vacancy in the presence of ZnO improves density and mechanical properties. SARKAR et al [7] observed that the densified temperature for magnesium aluminate spinel decreased by about 100 K with addition of Cr₂O₃ up to 1% (mass fraction). JIAO et al [8] reported a gradual improvement in sintered density for nickel ferrite products by addition of TiO₂ up to 1% (mass fraction). XI et al [9] pointed out that the addition of MnO₂ increased sintered density of nickel ferrite, refined grain and improved thermal shock resistance for MnO₂- containing samples. So it can be seen that the sintering additive has a great influence on the sintering process and microstructure of composites. XI et al [10] found that when the mass fraction of V₂O₅ was 2%, the electric conductivity of the sample was 7 times as large as that of samples without V₂O_5.  TIAN et al [11] revealed that the addition of SnO₂ decreased the activation energy and improved electric conductivity of samples. LAI et al [12] reported that the maximum electric conductivity of 16.29 S/cm at 1 233 K was obtained for composites doped with 2% CaO (mass fraction), compared with that 1.03 S/cm of the undoped composites. ZHANG et al [13] proved that after adding TiO₂, the electric conductivity of NiFe₂O₄ samples was improved largely.

In order to give some available advice for the choice of anode constituent, the effects of BaO addition on relative densities, microstructure and electric conductivity of xNi/10NiO-NiFe₂O₄ cermets were investigated in this work.

2 Experimental

2.1 Preparation of xNi/10NiO-NiFe₂O₄ composites doped with BaO

xNi/10NiO-NiFe₂O₄ (x=5, 10, 17) cermets and those doped with 1% BaO (mass fraction) were prepared by the conventional ceramic method with reagent grade raw materials of Fe₂O₃, NiO and BaO. The mixture of Fe₂O₃ and NiO in the molar ratio of 1.35 was calcined in a muffle furnace at  for in static air atmosphere to form 10NiO-NiFe₂O₄ ceramic powder. The synthesized powders, Ni and BaO powders, were ground in the media containing dispersant and adhesive. The mass fraction of BaO was 1%. The dried mixtures were compacted at a 200 MPa to get cylindrical blocks (d 20 mm×45 mm). Then, xNi/10NiO-NiFe₂O₄ (x=5, 10, 17) cermets and those doped with 1% BaO were sintered at 1 473 K for 4 h in nitrogen atmosphere of availably controlled oxygen partial pressure [14-15].

2.2 Characterization

Phase compositions were identified by X-ray diffractometry using a Philips PW1390 X-ray diffractometer with Cu K_α radiation. The microstructure was analyzed with a JSM-6360LV scanning electron microscope and an EDX-GENESIS energy dispersive spectrometer. Bulk densities were tested according to the Archimedes’ method. High temperature electric conductivity based on the conventional direct current four-electrode technique was tested using an experimental furnace by treating the powders at a heating rate of 5 K/min in air.

3 Results and discussion

3.1 Effect of BaO addition on relative density and microstructure

The relative densities of xNi/10NiO-NiFe₂O₄ (x=5, 10, 17) cermets and those doped with 1% BaO are shown　in Table 1. Relative densities of xNi/10NiO-NiFe₂O₄ (x=5, 10, 17) cermets doped with 1% BaO, sintered at  1 473 K in nitrogen atmosphere, are increased by 0.49%, 1.45% and 2.99% compared with those of the undoped cermets respectively. Perhaps because BaO doping causes the presence of inherent oxygen vacancy of xNi/10NiO-NiFe₂O₄ cermets, which helps the transport of oxygen ion, resulting in greater extent of atomic diffusion and mass transfer.

Table 1 Relative densities of xNi/10NiO-NiFe₂O₄ samples and those doped with 1% BaO

Fig.1(a)，Fig.2(a) and Fig.3(a) illustrate the SEM images of 5Ni/10NiO-NiFe₂O₄, 10Ni/10NiO-NiFe₂O₄ and 17Ni/10NiO-NiFe₂O₄ cermets, respectively. It can be inferred that with increasing metallic nickel contents, the relative densities of samples increase. However, the relative densities of samples doped with 1% BaO do not change largely according to Table 1, and the doped samples show full densification due to tight combination of particles and removal of pores (Fig.1(b), Fig.2(b) and Fig.3(b)). Based on phase diagram, it can be concluded that BaO can react with NiFe₂O₄ at about 1 423 K to generate BaFe₂O₄ and Ba₂Fe₂O₅ [16].

NiFe₂O₄+xBaO=NiFe_2-2xO_4-3x+xBaFe₂O₄         (1)

NiFe₂O₄+2xBaO=NiFe_2-2xO_4-3x+xBa₂Fe₂O₅       (2)

So the samples doped with BaO achieve densifica- tion by means of the mass transfer of iron vacancy and oxygen vacancy, which is used as transferred carrier to accelerate mass mobility and viscous flow.

Fig.1 SEM images of doped and undoped 5Ni/10NiO-NiFe₂O₄ cermets at 1 473 K: (a) 5Ni/10NiO-NiFe₂O₄ cermets; (b) 5Ni/10NiO- NiFe₂O₄ cermets doped with 1% BaO

Fig.2 SEM images of doped and undoped 10Ni/10NiO-NiFe₂O₄ cermets at 1 473 K: (a) 10Ni/10NiO-NiFe₂O₄ cermets; (b) 10Ni/ 10NiO-NiFe₂O₄ cermets doped with 1% BaO

Fig.3 SEM images of doped and undoped 17Ni/10NiO-NiFe₂O₄ cermets at 1 473 K: (a) 17Ni/10NiO-NiFe₂O₄ cermets; (b) 17Ni/ 10NiO-NiFe₂O₄ cermets doped with 1% BaO

3.2 Effect of BaO addition on electric conductivity

Fig.4 displays the electric conductivity of xNi/ 10NiO-NiFe₂O₄ (x=5, 10, 17) cermets and those doped with 1% BaO. From Fig.4, it is obvious that electric conductivities of samples doped with 1% BaO are apparently larger than those of the undoped samples. This implies that the addition of BaO or formation of BaFe₂O₄ and Ba₂Fe₂O₅ has an active effect on electric conductivity of xNi/10NiO-NiFe₂O₄ (x=5, 10, 17) cermets. The maximum electric conductivity of    50.10 S/cm is obtained by addition of 1% BaO to 17Ni/ 10NiO-NiFe₂O₄ cermets.

The relationship between electric conductivity and temperature for solid material can be described by Arrhenius relationship shown in Eqs.(3)-(5):

σ=σ_cχ                                       (3)

σ=σ₀exp[-E/(2kT)]                            (4)

σ₀=Nq²δ²v₀/(kT)                              (5)

where  σ_c is the electric conductivity of ceramic phase; χ is the valid transmission distance of charge, which is determined by the content of metals and the metal distribution in ceramics; E is the activation energy, which includes formation energy and migration energy of

Fig.4 Electric conductivity—temperature curves of xNi-10NiO- NiFe₂O₄ cermets and those doped with 1% BaO in air

defects; k is the Boltzman constant; σ₀ is an electric conductivity, which is determined by the concentration of carrier (N), charge (q), transition frequency (ν₀), average transition distance (δ) and environmental temperature (T).

The variations of electric conductivity with sintering temperature of samples are shown in Fig.4. There is an obvious increase in electric conductivity with increasing temperature. The electric conductivities of the doped samples sintered at 1 473 K in nitrogen atmos- phere, are larger than those of the undoped BaO samples. The electric conductivities of xNi/10NiO-NiFe₂O₄ (x=5, 10, 17) doped with 1% BaO are 1.83, 4.60 and     30.58 S/cm at 303 K and 21.98, 28.37 and 50.10 S/cm at 1 233 K, respectively. The electric conductivities undoped BaO samples are 1.28, 1.90 and 3.48 S/cm at 303 K and 18.70, 22.79 and 39.58 S/cm at 1 233 K, respectively. These results are explained by the assumption that oxygen atoms run out of crystal and leave the oxygen vacancy. The emergence of oxygen vacancy changes the original phase of NiFe₂O₄ into a new phase NiFe₂O_4-y [17-18], which can be denoted by a sign of  as a kind of oxygen deficiency with two electrons left by an oxygen atom. Just a little energy can stimulate these electrons to conductive band and strengthen the electric property of materials. The ionized process is shown by the following equations:

→                          (6)

→                          (7)

In Eqs.(6)-(7), the first ion energy and the second ionized energy of the oxygen deficiency are  and respectively. The energy level of this oxygen deficiency is below that of conductive band, so the NiFe₂O₄ ceramic with oxygen deficiency is a kind of n-type semiconductor.

According to Fig.4, fitting results of electric conductivity—temperature linear relationship of xNi/ 10NiO-NiFe₂O₄ (x=5, 10, 17) cermets and those doped with 1% BaO, which are sintered at 1 473 K, are listed respectively in Table 2. It is observed that for the undoped cermets σ₀ of samples increases with the increase of content of Ni, but activation energy (E) does not change largely. σ₀ of cermets doped with 1% BaO increases, but activation energy E decreases. This reveals that content of 1% BaO is beneficial to the densification and electric conductivity of xNi/10NiO-NiFe₂O₄ (x=5, 10, 17) cermets.

Table 2 Fitting results of electric conductivity—temperature linear relationship of xNi/10NiO-NiFe₂O₄ (x=5, 10, 17) cermets and those doped with 1% BaO

4 Conclusions

(1) The relative densities of xNi/10NiO-NiFe₂O₄ (x=5, 10, 17) cermets doped with 1% BaO (mass fraction), sintered at 1 473 K in nitrogen atmosphere, are increased by 0.49%, 1.45% and 2.99% compared with those of the undoped cermets, respectively.

(2) The electric conductivity of samples doped with BaO is higher than that of the undoped samples in the range from 303 and 1 233 K. The electric conductivities of samples doped with 1% BaO are 1.83, 4.60 and    30.58 S/cm at 303 K, and 21.98, 28.37 and 50.10 S/cm at 1 233 K, respectively. The electric conductivities of the undoped samples are 1.28, 1.90 and 3.48 S/cm at 303 K, and 18.70, 22.79 and 39.58 S/cm at 1 233 K, respectively.

(3) The formation of BaFe₂O₄ and Ba₂Fe₂O₅ has an active effect on electric conductivity of xNi/10NiO- NiFe₂O₄ (x=5, 10, 17) cermets. The maximum electric conductivity of 50.10 S/cm is obtained for 17Ni/10NiO- NiFe₂O₄ cermets doped with 1% BaO.

(4) With increasing Ni content, σ₀ of undoped samples increases; but activation energy (E) does not change largely. σ₀ of cermets doped with 1% BaO increases, but activation energy (E) decreases. Moreover, 17Ni/10NiO-NiFe₂O₄ cermet has the maximum σ₀, but it has not the minimum activation energy (E) and electric conductivity.

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(Edited by CHEN Wei-ping)

Foundation item: Project(2005CB623703) supported by the National Basic Research Program of China; Project(50721003) supported by the National Natural Science Fund for Innovation Group of China; Project(2008AA030501) supported by the National High-Tech Research and Development Program of China

Received date: 2008-12-25; Accepted date: 2009-03-20

Corresponding author: ZHOU Ke-chao, Professor; Tel: +86-731-88830464; E-mail: zhoukechao@mail.csu.edu.cn