Download Sol Gel Approach: Lanthanum Silicates as a Replacement for Yttria

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. Célérier, C. Laberty, F. Ansart, P. Lenormand, P. Stevens, Ceram. International 32
(2006) 271-276.
[9] H. Yoshioka, Chem. Lett. 33 (2004) 392-393.
[10] S. Nakayama, M. Sakamoto, J. Eur. Ceram. Soc. 18 (1998) 1413-1418.
[11] I.A. Bondar, Izv. Akad. Nauk SSSR, Neorg. Mater. 15 (1979) 1008; Inorg. Mater
(Engl Transl) 15 (1979) 793.
[12] D.B. Witkin, E.J. Lavernia, Prog. In Mater. Sci. 51 (2006) 1-60.
[13] F. Zhou, D. Witkin, S.R. Nutt, E.J. Lavernia, Mater. Sci. Eng. A 375-377 (2004)
917-921.
[14] J.A. Picas, A. Forn, L. Ajdelsztajn, J. Schoenung, Powder Tech. 148 (2004) 20-23.
[15] J. Lee, F. Zhou, K.H. Chung, N.J. Kim, E.J. Lavernia, Metallurgical Mater. Trans. A
32A (2001) 3109-3114.
[16] J. He, E.J. Lavernia, J. Mater. Res., 2001, vol. 16, no. 9, pp. 2724-2732.
[17] R. Rodriguez, R.W. Hayes, P.B. Berbon, E.J. Lavernia, Acta Materialia 51 (2003)
911-929
[18] Y. Jiang, S.V. Bhide, A.V. Virkar, J. Solid State Chem. 157 (2001) 149-159.