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Sol Gel Approach: Lanthanum Silicates as a Replacement for Yttria Stabilized Zirconia (YSZ) in Solid Oxide Fuel Cell (SOFC) Electrolytes September 1, 2006 Aminah Rumjahn Chemical Engineering and Material Science, UC Davis Principal Investigator: Martha Mecartney Graduate Student: Mai Ng Chemical Engineering and Material Science, UC Irvine Abstract: New material systems must be considered to achieve high ionic conductivity for Solid Oxide Fuel Cells (SOFC) at lower operating temperatures in the range of 600-800°C. Recent studies have shown that oxy-apatites, such as La9.33(SiO4)6O2, exhibit higher oxygen ion conductivity at lower temperatures than the traditional YSZ, which typically operate around 1000°C. Among these apatites, lanthanum silicates exhibit the highest ionic conductivity. Therefore, this study focuses on the development and characterization of lanthanum silicates, specifically via a sol gel route to ensure a homogenously mixed product that has a lower crystallization temperature than that of solid state methods. Hydrated lanthanum nitrate and TEOS (tetraethylorthosilicate) were used as polymer precursors and heat treated to obtain a fine grained powder composed of lanthanum silicate (La9.33(SiO4)6O2). A unique cryomilling process was then used in hopes of decreasing the grain size into the nanometer scale. X-ray diffraction (XRD) was used to study the phases present in our samples and enabled the use of the Scherrer formula to calculate the crystallite size, scanning electron microscopy (SEM) gave us approximate grain sizes and future work with impedance spectroscopy (IS) will determine the material’s ionic conductivity. Introduction: Extensive research is being done on the development of more efficient materials as electrolytes of Solid Oxide Fuel Cells (SOFC) in an effort to decrease the high operating temperatures [1-3]. The electrolyte is one of three main components in a SOFC unit; the other two being an anode and a cathode between which the electrolyte is placed. High ionic conductivity is a vital requirement of the electrolyte due to the fact that a higher ionic conductivity results in a more efficient conversion of chemical energy to electrical energy. Ionic conductivity is governed by grain boundaries and interfacial regions. More grain boundaries results in higher ionic conductivity since ions travel faster in interfacial regions [4,5]. In order to achieve more grain boundaries, the grain size must be significantly decreased; hence the research into nanocrystalline materials for electrolytes. Yttria stabilized zirconia (YSZ) films are most commonly used as SOFC electrolytes because of their high oxygen ion conductivity. However, this high ion conductivity is only exhibited at very high temperatures in the range of 1000°C, which requires expensive materials and results in a decreased lifetime of the system due to material degradation. Thus, new oxygen ion conductors have been investigated, specifically with success in the rare earth oxy-apatites [4,6-9]. These oxy-apatites exhibit high oxygen ion conductivity at lower temperatures (<800°C) than YSZ and have a low activation energy. The general formula for apatite-type oxides is M10(XO4)6O2 where M is a rare earth or alkaline earth metal and X is a p-block element like P, As, Si, Ge [8]. Among these apatites, lanthanum silicates have exhibited the highest ionic conductivity. However, pure apatite-type La10(SiO4)6O2 is difficult to obtain [4,10]. During solid state reactions, the presence of the secondary phase La2SiO5 implies that La10(SiO4)6O2 is not an equilibrium phase and thus not stable. The difficulty in obtaining pure La10(SiO4)6O2 may also be contributed to non-homogenous mixing of the oxide precursors. La9.33(SiO4)6O2, unlike La10(SiO4)6O2, is readily acquired since it is in equilibrium with the secondary phase La2SiO5 according to the La2O3-SiO2 phase diagram [11]. Thus, this study focuses on the fabrication of La9.33(SiO4)6O2. The two main routes of preparing apatite-type lanthanum silicates are by solid state reaction or through sol gel methods. Sol gel methods are chemical processes in which uniform inorganic materials are formed and solid state reactions involve the mechanical mixing of reactants together and placing them through heat treatments to react. The sol gel route requires lower heat treatments and results in a more controlled and homogenous mixture[6]. Additionally, the secondary phase La2SiO5 that is formed during solid state reactions is difficult to remove. It was therefore in our best interest to fabricate La9.33(SiO4)6O2 via a sol gel route. Intensive studies have been made concerning the sol gel route for the synthesis of La9.33(SiO4)6O2.[4-6,8] The procedure from Célérier et al. was followed closely. Lanthanum nitrate and tetraethylorthosilicate (TEOS) were used as precursors along with stoichometric amounts of ethanol and acetic acid. The success of Célérier et al. increasing the density of the final powder to 92% dense by the addition of an attrition milling step furthered prior interest into cryomilling ceramics. Although intensive studies have yet to be conducted on the cryomilling of ceramics, recent success of cryomilling metals into homogenously mixed nanocrystalline particles provided enough encouragement that cryomilling of ceramics might be just as successful [12-16]. Therefore, instead of attritor milling our powders as Célérier et al. had done, we cryomilled them. So as to determine the effects of cryomilling on our oxy-apatite lanthanum silicate powder, we subjected our specimen to numerous tests. X-ray diffraction (XRD) conducted a particle analysis to see whether our specimen had any unpredicted phases or contaminations and was also used to determine the crystallite size of the powder through the Scherrer formula. Although many of the previous studies of cryomilling reported contamination from their cryomilling procedure, no specific precautions were used during our cryomilling run [14,15,17]. Impedance spectroscopy (IS) using AC current will be conducted in the near future to test the conductance of our specimen in order to determine if our procedure was indeed successful in producing a high ionic conducting material. Experimental Methods: La9.33(SiO4)6O2 was prepared via a sol gel route. The procedure consisted of reacting the precursors through heat treatments and then cryomilling the resulting powders. Lanthanum nitrate hexahydrate La(NO3)3 · 6H2O (Fluka, 99+%) and tetraethylorthosilicate TEOS (Gelest, 99+%) were used as the polymer precursors. The La9.33(SiO4)6O2 gels were synthesized in 200 proof ethanol and catalyzed by glacial acetic acid. A mixture of 10mL of ethanol and 10mL of acetic acid was used to dissolve 15g of La(NO3)3 · 6H2O. 5 mL of TEOS was then added into the solution and stirred at room temperature for 1 hour. The resulting clear solution was left overnight to dry at 80°C on a hot plate. The sample was decomposed in air at 600°C for 4 hours with a heating rate of 2°C/min using a Fisher Isotemp programmable ashing furnace (model 495). Calcination was then performed in air at 1000°C for 2 hours with a heating rate of 2°C/min in a Fisher Isotemp programmable muffle furnace. A mortar and pestle was used to grind the resulting sample into the fine powder that made up our standard La9.33(SiO4)6O2 . A second set of powders was synthesized following the same procedure, but with the addition of a unique cryomilling step after the calcination. A modified Union Process 1-S attritor (Szegvari), along with a stainless steel stir bar and YTZ grinding media (Tosoh, 5mm diameter) were used to cryomill our sample for 8 hours. Liquid nitrogen was used to create a slurry with the powder and balls and had to be frequently refilled during the milling to ensure a complete immersion of the milling media. Once the 8 hours was completed, the mill was stopped and left alone until the level of liquid nitrogen had reached approximately 1 cm above the milling media. The resulting slurry was then poured into a sieve (Fritsch Analysette Automatic Microsieve) for two 2 minute intervals to separate the powder from the milling media as the liquid nitrogen evaporated away. The time in between the intervals was used to place any excess powders retrieved from the milling bowl into the sieve. Characterization of our materials was done using XRD and SEM. Powder XRD analyses were done on a Siemens Diffraktometer D5000 XRD machine using a Cu filament. The Zeiss Ultra 55 SEM was then used to observe the powder morphology as well as the grain sizes. Results: The following figures are from powder XRD and SEM analyses conducted on the standard and cryomilled samples of La9.33(SiO4)6O2. XRD for Standard Sol Gel La 9.33(SiO4)6O2 14000 12000 Lin (Counts) 10000 8000 6000 4000 2000 0 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 -2000 2-Theta Scale Figure 1: X-ray diffraction pattern for powders after decomposition at 600°C and calcination at 1000°C (standard sample) XRD for Cryomilled Sol Gel La9.33(SiO4)6O2 7000 6000 Lin (Counts) 5000 4000 3000 2000 1000 0 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 -1000 2-Theta Scale Figure 2: X-ray diffraction pattern for powders after heat treatments and 8 hours of cryomilling Figure 3: Scanning electron micrograph of agglomerated powders after decomposition at 600°C and calcination at 1000°C (standard sample) Figure 4: Scanning electron micrograph of agglomerated powders after heat treatments and 8 hours of cryomilling Figure 5: Scanning electron micrograph of a single particle from the powders after decomposition at 600°C and calcination at 1000°C (standard sample) Figure 6: Scanning electron micrograph of a single particle from the powders after heat treatments and 8 hours of cryomilling Discussion: The fabricated powders were tested via XRD and SEM and produced data with interesting information that enabled the comparison of size and composition between the standard and cryomilled samples. XRD confirmed the composition of the standard and cryomilled powders to be La9.33(SiO4)6O2. However, XRD also verified water contamination of the cryomilled powders. The Scherrer formula determined that the cryomilled powder was indeed smaller than the standard powder by about 33%. Additionally, SEM showed that cryomilling broke the agglomerated powders into smaller agglomerates and decreased the particle size as well. The X-ray diffraction pattern for the standard powders that were decomposed at 600°C and calcined at 1000°C (Fig.1) displayed the same peaks as those in the reference patterns [8]. The major peak occurred at 30.8° and there was no sign of any type of contamination in this sample. The X-ray diffraction pattern for the cryomilled powders (Fig.2) also displayed the same major peaks as those in the reference patterns and has the same major peak at 30.8° as the standard sample. However, there is an extra peak at 21.1° and only part of a missing peak at 31.8°. The extra peak is due to the formation of the secondary phase La2SiO5 which resulted from water contamination during the cryomilling process [8]. The reason behind the missing peak is not completely known, but is most likely attributed to water or some other contamination. Previous studies have indicated the water sensitivity of lanthanum silicates [8]. The observation that water affects the composition of the sample confirms the hygroscopic and water sensitive nature of the material. Unfortunately, our cryomilling process did not have any special methods of preventing water contamination other than trying to scrape off ice from the mill and collecting the powders soon after the end of each run so as to limit the condensation of water into our system. Because the X-ray diffraction patterns confirm a secondary phase from water contamination, better methods must be devised for the cryomilling process, specifically with regards to the collection method. Approximate crystallite sizes were calculated using data from the X-ray diffraction patterns and the Scherrer formula [18]. The Scherrer formula requires the wavelength of the filament used in the XRD machine and the width and angle of a peak from the XRD patterns. It cannot be applied to systems where the crystallite size is less than approximately 200nm. The Scherrer equation is t 0 .9 where t is the crystallite B cos B size, is the wavelength of the filament used in the XRD machine, B is the width of a peak at half of its intensity and B is the angle of the same peak. The Siemens Diffraktometer D5000 XRD machine used in this experiment had a Cu filament with a wavelength of 1.54Å. The resulting crystallite sizes for our standard and cryomilled samples were 21nm and 14nm, respectively, demonstrating that cryomilling does decrease our grain sizes. However, some error must be accounted for in these calculations since the width and angle of the peaks were calculated in a crude manner using only a ruler and personal judgment. Therefore, the calculated crystallite sizes are approximate. Further characterization on the powders was done on the Zeiss Ultra 55 SEM. Comparison between the agglomerated powders of the standard (Fig.3) and the agglomerated powders of the cryomilled (Fig.4) displayed that cryomilling breaks down the agglomerations. The agglomerations of the standard powders were approximately 2-6 μm in diameter while the agglomerations of the cryomilled powders were approximately 1-3 μm in diameter. Furthermore, SEM allowed the determination of particle size. The standard sample (Fig.5) had a particle size of 80-90nm while the cryomilled sample (Fig.6) had a particle size of 20-50nm. These results further support that cryomilling breaks the powders into smaller pieces. Conclusions: La9.33(SiO4)6O2 was successfully fabricated via a sol gel process and cryomilled to determine the effects of cryomilling on the material. X-ray diffraction patterns of the cryomilled powders revealed a secondary phase that formed from water contamination, confirming that lanthanum silicates are sensitive to water. Crystallite sizes were calculated with the Scherrer formula and determined to be approximately 21nm for the standard sample and approximately 14nm for the cryomilled sample, displaying that cryomilling decreased the grain size. SEM showed that cryomilling broke the powders into smaller agglomerates and smaller particles, thus supporting the fact that cryomilling helped decrease the particle and grain size of our material. In order to be considered as an electrolyte material for SOFC, the lanthanum silicate must be determined as having a high oxygen ion conductivity. Thus, sintered pellets of the La9.33(SiO4)6O2 powders must be fabricated in order to determine the material’s ionic conductance through impedance spectroscopy (IS). Further studies into cryomilling must also be done in order to maximize the effects of this special milling process. A better method of extracting our powders from the slurry of liquid nitrogen and milling media must be devised in order to minimize the water contamination. Varying the time of the cryomilling runs must be explored to determine whether a threshold time exists at which the grains do not get any smaller. Density measurements and additional characterization tests must be conducted to give a more complete study of the cryomilling effects. Acknowledgements: This work would not have been made possible without the guidance and support of Professor Martha Mecartney and graduate student Mai Ng. Thank you to the Mecartney and Mumm groups for their spirit, help and support as well as my fellow IMSURE participants for being such an amazing group. Special thanks to Scott Wilhour whom I worked closely with. Thank you UC Irvine and the UROP team for the IMSURE program, to NSF for financial funding and the Zeiss Center of Excellence for microscopy support. References: [1] S. Sarat, N. Sammes, A. Smirnova, J. Power Sources (2006). [2] K. Kawamura, K. Watanabe, T. Hiramatsu, A. Kaimai, Y. Nigara, T. Kawada, J. Mizusaki, Solid State Ionics 144 (2001) 11-18. [3] X. Xu, C. Xia, S. Huang, D. Peng, Ceram. International 31 (2005) 1061-1064. [4] S. Tao and J.T.S. Irvine, Mater. Res. Bull. 36 (2001) 1245-1258. [5] H. Yoshioka, J. Alloys and Compounds 408-412 (2006) 649-652. [6] S. Célérier, C. Laberty-Robert, F. Ansart, C. Calmet, P. Stevens, J. Eur. Ceram. Soc. 25 (2005) 2665-2668. [7] L. León-Reina, J. Manuel Porras-Vásquez, E. Losilla, M.A.G. Aranda, Solid State Ionics (2006). [8] S. 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