|Fitria Rahmawati||Research Group of Solid State Chemistry & Catalysis, Chemistry Department, Sebelas Maret University, Jl. Ir. Sutami 36A Kentingan, Surakarta 57126, Indonesia|
|Ita Permadani||Research Group of Solid State Chemistry & Catalysis, Chemistry Department, Sebelas Maret University, Jl. Ir. Sutami 36A Kentingan, Surakarta 57126, Indonesia|
|Dani. G. Syarif||Center of Science and Applied Nuclear Technology (PSTNT) BATAN, Jl. Taman Sari 73, Bandung, Indonesia|
|Syoni Soepriyanto||Department of Metallurgy, Faculty of Mining and Petrochemical, Institut Teknologi Bandung, Jl. Ganesha 10, Bandung, Indonesia|
Doping yttrium ions, Y3+ into ZrO2 produced Yttria-Stabilized Zirconia, YSZ. Various amount of yttrium ions could provide different ionic conductivity. This research investigated electrical conductivity of various YSZ composition, i.e., 4.5; 8.0 and 10% mol yttrium in ZrO2. The ZrO2 powder used was synthesized from zircon sand, a side product of tin mining plant, Bangka Island, Indonesia. Structural investigation on the prepared YSZ found that yttrium ion doping has changed the crystal structure of ZrO2 from monoclinic to cubic, even though the monoclinic and tetragonal are also still exist. The Y3+ doping changed the cell parameter of ZrO2 crystal. It indicates that the Y3+ entered into the ZrO2 structure and produced vacancy sites. The highest ionic conductivity is provided by 8% mol Yttrium doping or 8YSZ, i.e., 2.74×10-4 S.cm-1 at 700oC with an activation energy of 0.741 eV.
Electrical conductivity; Various composition; YSZ; Zirconia; Zircon sand
Indonesia has tin ore abundant spread over Pulau Karimun, Singkep, in some of the Sumatera lands, and also in Bangka Island, Riau islands, until to the West Kalimantan (Poernomo, 2012). Among of those, Bangka island has the largest Tin abundant with a side product is zircon sand (ZrSiO4). Zircon, ZrSiO4, is a stable molecule due to the strong bond between zirconia and silica. Some methods to extract zirconium from zircon are caustic fusion or with soda ash (El-Barawy et al., 2000), heat plasma dissociation (Ananthapadmanabhan et al., 1993), thermal decomposition (Pavlik et al., 2001), caustic fusion (Rahmawati et al., 2016) and mechanochemical processing (Puclin et al., 1995).
Zirconia, ZrO2, is an important ceramics material due to its strengths and the high melting point temperature (2700oC). In addition, zirconia also has oxygen ions conductivity, low thermal conductivity, high flexibility, and high corrosion resistance (Rivai & Takahashi, 2010). Zirconia is used as a material for the oxygen sensor, fuel cell material, thermal barrier coating, and some high-temperature applications. Tetragonal zirconia has high mechanical strength; meanwhile, cubic zirconia has high ionic conductivity (Shackelford & Doremus, 2008). The addition of some metal oxides such as MnO, NiO, Cr2O3, Fe2O3, Y2O3, and Ce2O3 could stabilize tetragonal
and cubic phase of zirconia at room temperature (Munggaran et al., 2014). Our previous research on preparing ZrO2 from Indonesian zircon sand found a Zr content of 72.1% and the presence of some impurities such as sodium oxide, Na2O at 14.71% and also silica, SiO2 at 3.03%. The sodium oxide is known as the remains of sodium hydroxide reactant to mix with zircon, ZrSiO4, sand. Meanwhile, the silica content refers to the silica network in zircon sand as the raw material (Rahmawati et al., 2016).
A high ionic conductivity of the material is required to develop a solid oxide fuel cell, SOFC. Cubic zirconia is known to has highest ionic conductivity. However, the structure is not stable at room temperature. Doping of yttrium ions into zirconia is known to stabilize the cubic structure at room temperature and increase the ionic conductivity due to the formation of oxygen vacancies when Zr4+ is replaced by Y3+ (Shakthinathan et al., 2012). Low yttrium concentration might produce tetragonal zirconia that has good mechanical strength. Meanwhile, a high concentration of yttrium dopant might produce cubic structure zirconia that has high ionic conductivity (Shackelford & Doremus, 2008). The ionic conductivity depends on the size and concentration of the dopant, in which yttria is the most common dopant for stabilizing the cubic phase of zirconia. When the Y2O3 content increases, the crystal structure of ZrO2 transforms from monoclinic to tetragonal and further transforms to the cubic phase when the yttria addition up to 8 mol% (Sharma et al., 2016). When the ZrO2 is prepared from ZrSiO4 sand as a side product of tin mining plant, it is known that the addition of Y2O3 also transforms the structure to cubic, as confirmed by peaks at 2q of 30.8o, 34.92o, 50.15o, and 59.52o which are in agreement with the YSZ standard diffraction ICSD #75316. However, the monoclinic ZrO2 still exists as small peaks at 2q of 27.91o and 31.09o, and tetragonal phase also present as small peaks at 2q of 35.24o and 60.26o (Rahmawati et al., 2016).
The result is different when the commercial ZrO2 is used, in which doping of > 7 mol% produced the single cubic phase, while a 2–6 mol% Y2O3 content gives a partially stabilized (Sharma et al., 2016). Other researcher found 8–10 mol% Y2O3 that result in a fully cubic phase (Ochrombel et al., 2010). Meanwhile, for YSZ nanowire, the 4–10 mol% Y2O3 doping level produces a dominated cubic phase (Liu et al., 2017). Crystal structure may affect the ionic conductivity. Meanwhile, because of the different purity grade, the similar amount of yttrium doping to ZrO2 that was prepared from Indonesian zircon sand and to the commercial ZrO2 may produce a different crystal structure. Therefore, this research investigates their electrical properties in accordance with the Y2O3 dopant amount. For the commercial grade zirconia, it is found that the highest conductivity level of (ZrO2)1-x(Y2O3)x electrolyte is obtained when x is 0.08 (Kharton et al., 1999). This research is going to understand if the 71.53±0.76% purity of ZrO2 and the presence of some impurities will affect the electrical conductivity as well.
Yttrium stabilized-zirconia, YSZ, was prepared at the various composition of yttrium content, i.e., 4.5%, 8%, and 10% mol. The yttrium dioxide, Y2O3, (Aldrich) was used as yttrium ions precursor. Meanwhile, ZrO2 was synthesized from Indonesian local zircon sand, procured as a side product of Tin Mining Plant in Bangka Island, Indonesia, and it was pre-concentrated in Laboratory of Metallurgy, Faculty of Mining and Oil Engineering, Institut Teknologi Bandung. XRF analysis on the zircon sand concentrate found that the concentrate consists of some elements, i.e., Zr, Si, Na, S, Al at 64.10%, 14.67%, 11.61%, 1.85%, and 1.33%, respectively, and some other small content elements at 6.44%.
Zirconia, ZrO2 was synthesized by caustic fusion method (Soepriyanto et al., 2005; Rahmawati et al., 2012) in which the zircon sand concentrate, ZrSiO4, was crushed with NaOH at a ratio of ZrSiO4: NaOH 1:4. The mixture was then heated at 800oC to produce a greyish white powder. The powder then was leached with distilled water at a volume ratio of 1:10 of powder to water, and followed by filtration. Next step was to leach residue of filtration with 3.5 M of HCl at a volume ratio of 1:10, means that 1 gram of residue was dissolved in 10 mL HCl, at 80oC, and under stirred condition. The result was a cloudy yellow solution that produced a clear yellow solution after filtration. The yellow solution is ZrOCl2.6H2O (ZOC). Zirconia, ZrO2, was precipitated from ZOC by slowly added 3 M of NH4OH. A white precipitate was produced. After decantation and heating at 800oC for 5 hours, a white powder of ZrO2 was founded. Our previous research on the caustic fusion of this Bangka Island zircon sand produced a zirconia powder at 71.53 ± 0.76% of purity (Rahmawati et al., 2014).
Yttrium ions doping was conducted with Y2O3 as yttrium source. A mixture of Y2O3 and ZrO2 was crushed for 2 hours in a pestle at a stoichiometric ratio to produce a 4.5%, 8% and 10% mol of yttrium. Calcination was at 1000oC for 2 hours, then followed by sintering of its green pellets at 1500oC for 5 hours. X-ray diffraction analysis equipped with Le Bail refinement was used to identify the peaks in comparing with YSZ standard diffraction, and also to analyze its crystallinity, crystal structure, and its cell parameters. The prepared materials also were analyzed by SEM to understand their surface morphology. The MeasureIT software (free edition) was used to analyze the average particle size from SEM images. Meanwhile, the electrical conductivity was analyzed with LCR meter (GW Instek) to study their impedance plot and their electrical conductivity. The impedance was measured at 20–5 MHz at 300–700oC. The impedance curve was fitted with Origin 6.0 program (free edition), and the conductivity value was calculated by Equation 1.
where k is the specific conductivity (Ohm-1.cm) (S.cm), R is the resistance that was determined from impedance measurement equipped with the fitting process (Ohm), A is the area of the active electrode (cm2), and l is the thickness of material (cm).
The material characterization has been conducted on the prepared YSZ at 4.5%, 8%, and 10% of mol yttrium ions. The analysis by X-ray diffraction equipped with le bail refinement and the results have been published in our previous paper (Rahmawati et al., 2016). The yttria-stabilized zirconia at 4.5%, 8%, and 10% produced from zircon sand were crystallized in the same structure, i.e., cubic, tetragonal, and monoclinic with the space group of F M 3 M, P 42/ N M C z, and P1 21/C 1, respectively. The specific peaks of ZrO2 are identified at 2?~30.30°; 34.70°; 35.24°; 50.34° and 60.26°. Meanwhile, a peak at 2?~28.36° confirms the presence of monoclinic phase ZrO2 based on ICSD#157403 (Rahmawati et al., 2016).
Impedance measurement at 20–5 MHz for 4.5 YSZ produce impedance curves as plotted in Figure 1. The Figure 1 shows that the impedance curve of 4.5 YSZ is in a semicircle at 400, 500, 600, and 700oC. The impedance values decrease as the temperature increase. A similar trend also can be seen in Figure 2 for 8 YSZ, in which the impedance value decreases as the temperature increases. However, at 400oC, the 8 YSZ shows a two semicircle curve of impedance (Figure 2). Those two semicircle indicates that the ionic conductivity of 8 YSZ consists of grain conduction and grain boundary conduction (Li et al., 2003). The impedance value of 8 YSZ is lower at various temperature, in which at 600oC the impedance of 8 YSZ is around 600 Ohm, which is 5 times lower of 4.5 YSZ impedance value. Meanwhile, the 10 YSZ shows a similar trend of impedance curve with 8 YSZ, in which at 400oC the impedance curve consists of two semicircles indicating grain and grain boundary conductivity. However, the impedance value of 10 YSZ at various temperature are higher than the impedance of 8 YSZ. As it can be seen in Figure 3 that the impedance of 10 YSZ at 600oC is around 3500 Ohm, meanwhile the impedance of 8 YSZ is around 600 Ohm.