1. Introduction

Solid oxide fuel cells (SOFCs) are environmentally friendly energy conversion systems to produce electrical energy with minimal environmental impact[1-4]. The efficient operation of SOFC anode is strongly depends on its microstructural parameters such as particle size, composition and spatial distributions of the constituent phases. Most present SOFCs developers use yttria-stabilized zirconia (YSZ) as electrolyte, strontium-doped lanthanum manganite (LSM) as cathode and Ni-YSZ ceramic metallic composite (cermet) as supported anode materials[5-7]. Ni-YSZ cermet is an electronic conductive material formed during the SOFC stack operation by the reduction of NiO-YSZ used at the anode site during cell fabrication process. NiO-YSZ is an insulating material and its complete reduction to Ni-YSZ is necessary to assure good anodic performance. Ni-YSZ cermets have been used as SOFC anodes mainly due to their high electronic conduction, good electrochemical performance in the intermediate temperature range (750–850^oC) and high catalytic activity in reform reactions and the comparatively low cost of nickel[7-8]. During the chemical reaction occurs between NiO and ZrO₂, the ionic radius of Ni⁺² being relatively small compared to the size of ZrO₂ and this gives rise to a solid-solution of nickel oxide and zirconia. The particle size of nickel oxide has a great effect on the stabilization of tetragonal phase of zirconia[9-11]. The formation of the tetragonal solid-solution of NiO₂-ZrO₂ is believed to be due to the smallness of grains, hence, the enhanced surface reactivity and diffusion of the atoms at the surface. In general, the stabilization of tetragonal/cubic phase of zirconia has been found to be dependant on many parameters such as ionic size of dopant, valency, electronegitivity etc.[12].

The presence of the tetragonal phase is due to the effect of either the dopant concentration or the grain size of the dopant. The formation of the tetragonal solid solution of NiO-ZrO₂ is believed to be due to the smallness of the grains, hence, the enhanced surface reactivity and diffusion of the atoms at the surfaces[13-14]. In general, stabilization of the tetragonal/cubic phase of ZrO₂ has been found to be dependent on many parameters such as ionic size of the dopant, valence, and electronegativity[15].

Till today, so many works have been done to obtain the high temperature cubic phase and tetragonal phases in a thermodynamically stable at low temperature using different synthesis techniques like sonochemical, hydrothermal, chemical precipitation and sol-gel etc. In this paper, we are emphasis on the synthesis and stabilization of tetragonal zirconia by varying the Ni-salt concentration. Thermal, structural, microstructure analysis and as well as IR spectroscopy were carried out

2. Experimental Procedures

2.1. Synthesis

Nano sized particles of Ni stabilized zirconia particles was prepared through co-precipitation technique using NH4OH solution. An aqueous solution of 1M (where M is the molarity) concentration of Zr-salt (ZrOCl₂.8H₂O) were prepared from high purity Zr-salt which is highly acidic in nature (having a pH=0.3), while the pH of NH4OH solution was found to be 12.83 which is highly basic in nature. NH₄OH solution was added drop wise to a beaker containing 1 M(1-x) mol% Zr-salt and 1Mx mol% Ni-salt (NiCl₂.6H₂O)[where x = 0, 20 and 40] solution with constant stirring by a magnetic stirrer. A gel was formed and excess addition of NH₄OH solution leads to precipitation was allowed to settle out. The precipitate was washed with hot water for several times to remove chlorine from solution. The precipitate was dried at 80℃ for 24 hours. The obtained dried sample was crushed and grinded to obtain very fine powder. The fine powder was calcined at 700℃, 800℃ and 900℃ for 1 hour. The whole preparation process is schematically illustrated in Fig.1.

Figure 1. Schematic diagram of the preparation of Ni-doped ZrO2 powder

2.2. General characterization

Thermal decomposition of ZrO(OH)₂ gel to an amorphous ZrO₂ powder followed by its reconstructive nucleation of t-ZrO₂ nanoparticles were studied using thermogravimetric and differential scanning calorimetric (TG-DSC) by heating the sample at 100^oC/min in N2 atmosphere in a thermal analyser (Netzsch, STA 449C). Alpha alumina was used as reference material. Phase analysis was studied using the room temperature powder X-ray diffraction (PW 1830 Diffractometer, Phillips, Netherland) with filtered 0.154 nm Cu Kα radiation. Samples are scanned in a continuous mode from 20o-80o with a scanning rate of 0.04^o/sec. Microstructural features were studied using Scanning Electron Microscope (JSM 6480 LV JEOL, Japan).

Size of the particles is usually obtained with the help of scanning electron microscopy (SEM). The size corresponds to the mean value of the crystalline domain size of the particles is determined from the X-ray line broadening using Debye-Scherrer formula with correction factor as given below,

Where β is the angular line width of half maximum intensity, d is is the crystallite size, λ is the X-ray wavelength used, and θ is the Bragg’s angle in degree.

3. Results and Discussion

3.1. Thermal Analysis

Figure 2. DSC-TG of pure ZrO2

The thermal behavior of the nanocrystalline Ni doped ZrO₂ powder is studied up to 1000℃ using DSC/TG analysis. It is clearly shown from figure 2 that the as prepared ZrO₂ powder exhibits an endothermic peak at a temperature of 116℃ due to evolution of water absorbed on the as-prepared powder. With increasing temperature, it shows a very sharp exothermic peak at 446℃ which is related to the fast crystallization into metastable tetragonal Zirconia. To make certain of the structure of the as-synthesized powder, the thermogravimetric analysis (TGA) is conducted. The TGA analysis shows that the weight loss is approximately 31%. The humps in the range of 300-400℃ indicate the decomposition of Zirconium hydroxide ZrO(OH)₂ to ZrO₂. Figure 3 shows the DSC-TG graph of 40 mol% Ni doped ZrO₂ powder; it has two endothermic peaks at temperature 123℃ and 329℃. It may be due to the evaporation of water absorbed by as prepared 40 mol% Ni doped ZrO₂. Further increase in temperature shows a sharp exothermic peak at a temperature 630℃ related to the fast crystallization into metastable tetragonal zirconia. To make certain of the structure of the as-synthesized powder, the thermogravimetric analysis (TGA) is conducted.

Figure 3. DSC-TG of 20 mol%, and 40 mol% Ni doped ZrO2

3.2. X-ray Diffraction

Figure 4. XRD patterns of pure ZrO2 calcined at different temperatures

By co-precipitation technique, 1M ZrO₂, 0.2M NiO-0.8ZrO₂ and 0.4M NiO-0.6ZrO₂ were reacted with NH₄OH by maintaining a final pH to be ~10. Fig.4 shows the XRD patterns of the synthesized ZrO₂ nanopowder calcined at three different temperatures, 700, 800, and 900℃. Fig. 5 shows the XRD patterns of the synyhesized ZrO₂ nanopowder doped with 20 mol% Ni and calcined at three different temperatures, 700, 800, and 900^oC. Fig. 6 shows the XRD patterns of the synyhesized ZrO₂ nanopowder doped with 40 mol% Ni and calcined at three different temperatures, 700, 800, and 900℃.

Figure 5. XRD patterns of 20 mol% Ni doped ZrO2 calcined at different temperatures

Figure 6. XRD patterns of 40 mol% Ni doped ZrO2 calcined at different temperatures

At 700℃ calcination temperature, it is quite clear that the presence of monoclinic phase in pure zirconia powder is dominant. The crystallite NiO peaks were observed as a second phase at 2θ = 36.83^o, 42.88^o, and 62.52^o. From XRD pattern, it is clearly shown that the monoclinic phase in as prepared zirconia powder is dominant. With the addition of 20 mol% of nickel, the monoclinic phase was suppressed and the tetragonal phase was enhanced. Development of NiO along with t-ZrO₂ was observed for 40 mol% Ni doped ZrO₂ powders. The crystallite size of pure zirconia, 20 mol% and 40 mol% nickel doped ZrO₂ was calculated using scherrer’s formula and was found to be 46.3nm, 27.8 nm, and 139.5 nm respectively.

At 800℃ calcination temperature, it is quite clear that in pure zirconia powder the presence of monoclinic phase is dominant. With 20 mol% addition of nickel, the monoclinic phase was suppressed and the tetragonal phase has been enhanced. Further addition of Ni, i.e.40 mol%, leads to development of NiO along with t- ZrO₂. By using scherrer’s formula, the crystallite size of pure zirconia, 20 mol%, and 40 mol% nickel doped ZrO₂ was calculated and found to be 24.4nm, 27.8 nm and 22nm respectively.

While at 900℃ calcination temperature and by analyzing the XRD pattern it was found that in case of pure zirconia powder the major phase is monoclinic with minor amount of tetragonal phase (~99% monoclic and ~1% tetragonal). At 20 mol% Ni addition the monoclinic phase suppressed to 40 vol% and tetragonal phase developed to ~60 vol % with presence of minor amount of NiO phase (~5 vol%). Further addition of nickel concentration leads to a decrease in tetragonal phase and an increase in monoclinic as well increase of NiO phase. This may be explained as follows: the increased concentration of dopant NiO helps nucleation of crystallites and enhances grain growth, the grain size was found to be larger in 40 mol% compared to other composition of powders. Higher grain size is inversely propotional to surface area so reduced reactivity. Crystallite size of pure zirconia, 20 mol% and 40 mol% nickel doped ZrO₂ were calculated using scherrer’s formula and found to be 37.7nm, 37.9 nm and 59.6 nm respectively.

Here we may conclude that at higher concentration of Ni in Ni-ZrO₂ composite which is calcined at high temperature, tetragonal phase of ZrO₂ is completely stabilized and it also shows the effect of NiO prominently. So, this is the perfect condition of the sample which is liable for practical application.

3.3. IR spectroscopy

Infra-Red spectra in the range of 500-4000 cm-1 for the as-prepared ZrO₂ as well as the Ni doped ZrO₂ powders with different concentrations were observed. Figure 7 shows the typical IR spectra of both the as-prepared ZrO₂ and that calcined at 800℃. On the other hand, figure 8 shows the IR spectra of both the as-prepared 20 mol% Ni doped ZrO₂ and that calcined at 800℃.

From the IR spectras, it is clearly shown that the as-prepared powders reveal that the ZrO₂ nano powders have a significant amount of surface-adsorbed H₂O molecules has been assigned at 3401and 3446 cm^-1 for the as prepared and the calcined ZrO₂ respectively. While the as-prepared ZrO₂ as well as Ni doped ZrO₂ that calcined at 800℃ shows Zr-O vibration as observed at 740 and 680 cm^-1.

The size, shape and agglomeration behaviour of Ni doped ZrO₂ was obtained by using scanning electron microscopy (SEM). Fig. 9 (a), (b) and (c) show the microstructural SEM secondary and backscattered images of the as prepared ZrO₂, 20 mol% Ni doped ZrO₂, and 40 mol% Ni doped ZrO₂ powder respectively. The particle size was found to be nearly spherical and agglomerate in nature.

Figure 7. IR spectra of as-prepared ZrO2 and calcined ZrO2 at 800℃

Figure 8. IR spectra of as-prepared 20 mol% Ni doped ZrO2 and calcined 20 mol% Ni doped ZrO2 at 800℃

Figure 9. SEM images of (a) ZrO2, (b) 20 mol % Ni doped ZrO2, and (c) 40 mol % Ni doped ZrO2

4. Conclusions

Co-precipitation route using NH₄OH has been successfully employed to synthesis nonocrystalline Ni-ZrO₂ powder for SOFCs. Characterization by X-ray diffraction and IR spectroscopy confirmed the presence of nanocrystallite phases of NiO and ZrO₂. Stabilization of metastable t- ZrO₂ is observed at 20 mol% of nickel salt at a calcination temperature of 800℃. From XRD results it has been concluded that small crystallites (27.8 nm) stabilizes metastable t-ZrO₂ at intermediate temperature range. Due to the presence of lattice strain above a certain concentration of nickel salt, there is decrease in tetragonal phase.