1,a 1,b 2,c 3,d Silva, C.R - LaMaVlamav.weebly.com/uploads/5/9/0/2/5902800/2014_materials...reported...
Transcript of 1,a 1,b 2,c 3,d Silva, C.R - LaMaVlamav.weebly.com/uploads/5/9/0/2/5902800/2014_materials...reported...
Polycrystalline Tetragonal Zirconia of the form ZrO2: 3 mol% Re2O3 (Re-TZP) for use in oxygen sensors: synthesis, characterization and ionic
conductivity
Muñoz, R.A1,a; Cajas, P.C1,b; Rodríguez, J.E2,c; Rodrigues, A.C3,d; Silva, C.R.M1,e
1Universidade de Brasília- Brasília- DF- Brasil 2-Universidad del Cauca-Popayán-Colombia. .
3 Universidade Federal de São Carlos- São Carlos-Brasil Área Especial, Projeção A,UnB - Setor Leste - Gama CEP: 72444-240
[email protected], [email protected], [email protected], [email protected], [email protected],
Keywords: ionic conductivity, activation energy, oxygen sensors, grain size, tetragonal polycrystalline zirconia.
In this work we propose the synthesis and characterization of tetragonal polycrystalline zirconia for
potential applications in oxygen sensors. The synthesis method used was the Pechini method.
0,280µm ± 0,04 to 0,574 µm ± 0,05 Particles of mean diameter size less than 50 nm were obtained
for this method and pure tetragonal phase was identified according to the diffraction patterns.
Samples were prepared using the cold uniaxial pressing and it were sintered in a resistive furnace in
air at a temperature of 1400°C for two hours. Two different heating schedules were used. Relative
densities greater than 96% of theoretical density were obtained in all cases while grain size was
dependent of the heating schedule used for the sintering of samples. The impedance diagram shows
how changes in the grain size has a direct influence on the electrical behavior of ceramics, showing
an increase in the total ionic conductivity from 1,35E-5 to 2,15E-5 Ω-1cm-1 at 400 °C when the
grain size increases from respectively. Finally, activation energies are presented and compared with
the literature agreeing with the values characteristic of solid electrolytes used in oxygen sensors.
Introduction
Oxygen ion conductors of zirconia based ceramics are a class of materials with technological
applications in several application areas: sensors of chemical species, oxygen pumps, solid oxide
fuel cells among others [1]. For these applications, the zirconia must possess the fluorite type
crystal structure, or close to it. Such oxides with this structure are the classic oxygen ion conductors
[2]. The fluorite structure consists of a cubic lattice of oxygen ions surrounded by cations. The
cations are arranged in a face centered cubic structure with anions occupying tetrahedral positions.
This leads to an open structure with large empty octahedral interstices.
For zirconia containing 3 mol% of ytrium oxide, or yttrium stabilized partially zirconia (Y-
ZTP), the crystalline phase has a distortion of the fluorite structure, mentioned above, with an ionic
conductivity of the same order of magnitude when compared to the fully stabilized zirconia, in the
temperature range 250-600 ° C [3]. For this reason, this material is attractive for its use in electrical
applications, specifically for possible use in oxygen sensors. In this paper, the synthesis and
characterization of polycrystalline tetragonal zirconia as well as the process of conformation of
specimens is described. The rare earth carbonate produced in Brazil by Nuclemon was used as yttria
precursor, from Brazilian monazite. The advantage of this carbonate is its low cost compared to
high purity rare earth. Pechini method allowed synthesis of powder with sub-micrometric particle
size. Samples were cold uniaxially pressed and sintered at resistive furnace. Two sintering
schedules were used, S1 and S2. For the evaluated ceramics it was observed grain size dependence
on the used sintering schedule. Impedance diagrams demonstrated direct influence of electric
behavior with grain sizes.
Materials Science Forum Vols. 798-799 (2014) pp 145-153Online available since 2014/Jun/30 at www.scientific.net© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.798-799.145
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 187.66.190.213, University Federal de São Carlos, São Carlos-SP, Brazil-22/08/14,22:24:38)
Experimental procedure
In this work the following materials were used:
Zirconium tetrabutoxide (TBZ) (Zr[O(CH2)3CH3]4)- Aldrich;
Rare earths carbonate (Re2(CO3)3), Re = Y, Dy, Er, Ho, produced in Brazil by Nuclemon;
Citric acid (H3C6H5O7-H2O), analytical grade;
Nitric acid (HNO3), analytical grade;
Ethylene glycol (C2H6O2), analytical grade;
Isopropyl alcohol, analytical grade;
Distilled water.
The precursors of zirconium oxide (ZrO2) the precursors of rare earths oxides (Re2O3) utilized
in this work were respectively the Zirconium tetrabutoxide from Aldrich and a rare earths carbonate
(Re2(CO3)3). These reagents were used in appropriate proportions to obtain the solid electrolyte of
the form ZrO2:3 mol % Re2O3. The synthesis method used was the polymeric precursor method
named Pechini. Mixtures of nitric acid and ethylene glycol in a ratio ¼:1 were heated up to 70 0C to
ease citric acid dissolution into ethylene glycol. At room temperature yttrium and zirconium
precursors were added under constant mixing and the resulting mixture was heated up to 120 °C for
poly esterification and vaporization, giving rise to a polymeric resin. The obtained resin was heated
up to 250 0C during 18 h causing polymer break down and resin expansion. The residue was
grounded at agate mortar producing a fine and homogeneous black powder as is described in detail
in previous papers [4].
The raw material resulted from this process was characterized by thermogravimetric and
differential thermal analysis (DTA/TGA), X-ray diffractometry (XRD), transmission electron
microscopy (TEM), scanning electron microscopy (SEM) and finally impedance spectroscopy.
These characterization techniques allow to optimize temperatures aiming the production of metal
oxide of interest, particle size and shape control, agglomerates size and shape determination and
mainly its electrical behavior determination.
Results and discussion
One of the main peculiarities of the Pechini method is that the metallic ions of interest are
trapped in an organic network; consequently make it is therefore necessary to understand the
thermal decomposition behavior for this material aiming to define a suitable temperature to obtain
the oxide of interest without interference at particle size increase. The thermal profile DTA/TGA of
the resin from the organic zirconium oxide precursor is showed in Figure 1. The analyses were
performed on samples pre-burned at 250 ° C for 18 hours using platinum crucibles at a heating rate
of 10 ° C / min, in a Shimadzu equipment DTG-60H at the Laboratory of polymers Institute of
Chemistry of the University of Brasilia.
It is observed an endothermic process at 90 ° C related to the waste water vaporization. This
process is followed by a loss in mass of approximately 15%. The first exothermic peak is showed at
360 ° C suggesting the onset of the polymer resin degeneration, with a mass loss of about 28% due
to thermal decomposition of the single connections of the polymer [5].
146 Brazilian Ceramic Conference 57
0 100 200 300 400 500 600 700 800 900
-60
-40
-20
0
20
40
60
80
DTA
TGA
Temperature (°C)
DT
A (
uV
)
20
40
60
80
100
620 °C
432 °C
360 °C
ZrO2:3% Mol Y2O3
TG
A (
%)
90 °C
Exo
Figure 1. Differential thermal analysis (DTA) and thermogravimetric (TGA) of the polymer resin
pre-burned at 250 ° C for 18 hours.
The possibility of the onset of the phase transformation in oxide is discarded due to the
absence of diffraction peaks in the test with thermal treatment at this temperature, as can be seem at
figure 2. A second exothermic peak is showed at 432 °C indicative of thermal decomposition of
stronger connections with the polymer mass loss of approximately 23% and the beginning of the
transformation of phase of oxide of interest, from amorphous solid to crystalline solid. This was
reported by Freitas in 2000 [6] (amorphous oxide zirconia monoclinica+tetragonal) and confirmed
by X-ray diffraction. Finally, the last thermal phenomenon is showed at 620 ° C. It could be related
to a second phase transformation from tetragonal to monoclinic zirconia.
These results show that the compound formation is completed at 620 ° C. X-ray
diffractograms of thermal treated dust samples were obtained in an X-ray diffractometer (XRD)
Shimadzu, model XRD-600 with CuKα radiation (1.5418 Å), and a voltage and operating current of
30 kV and 20 mA, respectively. The angular pitch was 0.05 °, in the range 20<2ϴ<90°. The
software Search-Match was used to identify and to compare the crystal structure obtained in this
work with the data available at International Centre for Diffraction Data (ICDD). Figure 2
illustrates the X-ray diffractogram of the samples pre-burn and thermally treated at 400, 500, 600
and 1100 ° C for two hours. The diffractograms describe the formation of the zirconia tetragonal
phase of the composite. The widths for average height of the different diffraction peaks indicate
nanometric crystallite mid-size. The crystallite size was calculated according to the Scherrer
equation (1) [7]:
(1)
where d is the average crystal size, λ is the wavelength of X-ray, β is the peak width at medium
height of the diffraction peak height (FWHM) (measured in radians), θ is Bragg angle, B is a
numerical constant equal to 0.9. For the diffractogram shown in Figure 2 (b), and treated at 600 ° C
the calculated crystallite size was approximately 24 nm. In Figure 2 (a) can be observed the
development of the crystalline phases present in the samples when the temperature is increased. The
sample pre-burned presented a structure constituted by random atomic arrangements without long-
range order, frequently mentioned as amorphous, consisting mostly of organic material from the
synthesis method. For the thermal treatment at 400 ° C, over two hours, the sample presented an
Materials Science Forum Vols. 798-799 147
overly broad diffraction peak around 30 °, where is located the most intense peak of the tetragonal
and cubic phase. At 500 °C, several diffraction peaks can be seen, possibly related to the phases of
monoclinic, tetragonal and / or cubic zirconia. At 600 ° C, Figure 2 (b), the samples indicates a
substantial increase in their structural order, causing possible correlation with the existing
databases.
A direct comparison with the database indicates the existence of the zirconia tetragonal phase,
JCPDS 50-1089, but the diffraction peaks of monoclinic phase could be superimposed and with
almost imperceptible intensities, causing their identification nearly impossible due to the width of
the peaks occurrence [8], requiring other methods for distinguishing the phases.
(a) (b)
Figure 2. X-ray diffraction for the sample ZrO2 3% Mol Re2O3 thermally treated (a) pre-calcined,
400 °C, 500 °C and (b) 600 °C and 1100 ° C for two hours.
Finally, and considering the last treatment temperature, 1100 ° C, the existence of the zirconia
tetragonal phase can clearly be established. In the Figure 2 (b), it is emphasized the diffraction peak
of low intensity located in 43.12 °, characteristic of crystalline phase. The results of X-ray
diffraction are in complete agreement with the results of thermal analysis, showing that it is possible
to achieve the stabilization of tetragonal zirconia at a low temperature, in this case of less than 650 °
C, with crystallite size of ~ 24 nm.
Figure 3 shows a transmission electron microscopy micrographs (magnification 250K), and a
scanning electron microscopy micrographs of the samples under study. In these images it is possible
to note the high level of agglomeration between the particles of the ceramic powder, as is also
possible to notice submicron particles (particles with sizes less than 50 nm). In order to break the
agglomerates, these were milled in a ball mill. The milling was conducted in isopropyl alcohol
medium using zirconia balls of 2 mm diameter. The milled powder was dried in air at 70 °C for 18
h
20 30 40 50 60 70 80 90
0
20
40
60
80
0
50
100
150
0
20
40
60
8020 30 40 50 60 70 80 90
Inte
nsity (
a.u
)
2 (°)
pre-calcined
400 °C
500 °C
20 30 40 50 60 70 80 90
0
100
200
300
400
500
600
28 30 32
0
100
200
300
400
500
Inte
ns
ida
de
(u
.a)
2 Teta
ZrO2:3% Mol Re2O3
(600 °C )
Inte
nsid
ad
e (
u.a
)
2(°)
20 30 40 50 60 70 80 90
0
500
1000
1500
2000
ZrO2:3% Mol Re2O3 (1100 °C )
Tetragonal Phase (JCPDS 50-1089)
2(°)
42 43 44
0
5
10
15
20
25
30
35
2 Teta
Inte
nsid
ad
e (
u.a
)
Inte
nsity (
a.u
)
43,12°
148 Brazilian Ceramic Conference 57
Finally, the powders were disaggregated in an agate mortar and sieved through a sieve of
mesh 0.106 mm. Immediately after the grinding step, compacts, 10 mm diameter and 5 mm of
thick, were prepared by uniaxially pressing at 187 MPa for 30 s, using a press Marcon MPH-10.The
samples were sintered in a resistive furnace Naberttherm LHT407GN6 at a temperature of 1400 °
C, during two hours in air using two heating schedules, as showed in figure 4.
(a) (b)
Figure 3. Analysis by (a) MET (250K magnification) and SEM (b) of ceramic material obtained
after thermal treatment at 600 ° C, over two hours, suggesting the presence of nanoparticles and
clusters of large size.
Curve S1: The sintering curve S1 (Fig. 4 (a)) has been implemented to achieve dense CPs to
promote grain growth 9. According to Callister [10] the particle growth is a result of the movement
of grain boundaries, which is conducted through two processes: grain boundary diffusion and
migration of grain boundaries. Both processes promote the densification, but the grain boundary
migration that occurs at a higher temperature promotes faster grain growth.
The goal of achieving a high temperature, 1650 ° C in the case, was to enable the faster
migration of grain boundaries. The objective of reducing temperature to 1400 °C is to suppress this
faster migration, but keeping the distribution of the active contour of grain. With this methodology
it was possible to reach the dense test bodies with grain growth at low sintering temperature of 1400
° C.
(a) (b)
Figure 4. Sintering schedules used, (a) S1 and (b) S2, with the purpose of improving microstructural
changes in the specimens and thus increase the electrical properties of the same.
0 100 200 300 400 500 600 700
0
200
400
600
800
1000
1200
1400
1600
1800
1650 °C/5 minutes
Te
mp
era
ture
(°C
)
Time (minutes)
S1
1400 °C/2 hours
0 100 200 300 400 500 600
0
200
400
600
800
1000
1200
1400
1600
Te
mp
era
ture
(°C
)
Time (minutes)
S2
1400 °C/2 hours
Materials Science Forum Vols. 798-799 149
Curve S2: sintering curve S2 (Fig. 4 (b)) is the "traditional" sintering used in previous works
[11]. It has a rapid heating ramp of 10 °C/min until reach a temperature of 1000 °C, maintained for
five minutes. Subsequently, the heating rate was reduced to 3 °C/min to promote the diffusion
mechanism of the grain border until reach 1400 °C, holding this temperature during two hours;
finally a cooling at a rate of 5 °C/min.
Calculation of apparent density of the sintered samples was conducted using Archimedes'
principle, with the immersion of the sample in distilled water. Three measurements were made for
each sample on a precision balance Shimadzu AUY-220. Density of 5.83 g/cm3 were calculated for
the sintered sample with the curve S2 and 5.89 g/cm3 for the sintered sample using the curve S1,
thus resulting in apparent densities of 97.5% and 98.5% of theoretical density, respectively. The
reference value for these assessments was 5.98 g/cm3 and was calculated using a mathematical
model described in the literature [12].
The microstructure of sintered ZrO2:3% Mol Re2O3 ceramics was observed by scanning
electron microscopy (SEM), in a Jeol JSM-7001F microscope (Scanning Electron Microscope), of
the institute of Biology of the University of Brasilia. Measurements of average grain size and
interfacial area per unit of volume were carried out (Figure 5) [13], counting the number of
intersections between the grain boundary and straight lines with known length, which were
designed on the image with the program ImageJ of free access. The number of intersections per
image was more than 400, aiming to achieve better measurement accuracy.
In the case of ceramics sintered with the curve S1, figure 5 (a), the average grain size was
approximately 574 nm and the interfacial area per unit of volume was 3.48 x 10-3
nm2/nm
3. For
ceramics sintered with the curve S2, figure 5 (b), the average grain size was almost 280 nm and the
interfacial area per unit of volume was 7.38 x 10-3
nm2/nm
3.
Qualitative analysis of the difference between these obtained values is related to the sintering
curve, since the two types of samples were processed identically. Other microstructural features are
visible at the micrographs such as low porosity and uniform distribution of grain sizes and shapes
(all with the same size and similar shape). The sintering curve S1 strongly promotes the grain
growth, resulting in the reduction of the density of grain boundaries, fact that could increase its
electrical behavior.
(a) (b)
Figure 5. Analyses by SEM of the surface of the sintered specimens (a), sintered with curve S1,
and, (b) using conventional sintering curve S2.
150 Brazilian Ceramic Conference 57
Finally, the electrical response of sintered solid electrolyte was evaluated by impedance
spectroscopy. That characterization technique allows to establish the dependence of the electrical
behavior with temperature, in different regions of the ceramic, grain and grain boundary. The
samples have apparent density to 96.97% of theoretical. Therefore, the electrical results obtained
are related mainly to the properties and characteristics of the electrolyte [14].
The impedance spectroscopy measurement for samples was carried out on a frequency range
of 1 MHz to 1 Hz, and a voltage of 1000 mV, using a Solartron 1260 equipment. The temperature
range used was between 125 and 400 °C, with measurements at every 25 degrees, giving a total of
twelve (12) measurements for analysis. Paste electrodes of platinum Pt-paste Demetron 308-A were
applied on the parallel faces of the samples and cured at 1100 °C for 20 minutes. Figure 6 shows a
diagram of typical impedance obtained for the samples analyzed at 300 ° C. It is clearly observed
two different semicircles that in the study of ceramic materials, are related to the grain contributions
(high frequencies) and at contributions of the grain boundaries (low frequencies).
From the diameter of these semicircles, the intragranular and intergranular resistivity were
calculated, respectively. It is also evident from Figure 6 (b) that the intragranular resistivity are
slightly affected by the heat treatment sintering, while the intergranular resistivity experiences a
great decline in value.
(a) (b)
Figure 6. Typical impedance diagrams of the samples at a temperature of 300 °C, (a) showing the
decrease in resistivity attributed to grain boundary, (b) expanding the zone of high frequencies.
(Numbers denote the logarithm of frequency).
Thus it is clear that the decrease in the density of grain boundaries significantly affects the
electrical behavior of ceramics under study. With resistivity values obtained was possible to observe
the character thermally activated as a function of test temperature for the grain, grain boundary and
total conductivities, Figure 7(a), (b) and 8(a) respectively. The dependence of the conductivity with
temperature is shown in Figure 7, where one can see a detailed discussion of this increased
conductivity due to the used sintering curve.
0 200 400 600 800 1000 1200 1400 1600 1800
0
200
400
600
800
1000
1200
1400
1600
1800
5
ZrO2: 3% Re2O3 - Sintering curve S1
ZrO2: 3% Re2O3 - Sintering curveS2
- Z
'' (kc
m)
Z' (kcm)
1245
0 50 100 150 200
0
50
100
150
200
- Z
'' (kc
m)
Z' (kcm)
5
4
5
Materials Science Forum Vols. 798-799 151
(a) (b)
Figure 7. Dependence of conductivity with temperature for the grain (a) grain boundary (b)
To complete the electrical analysis, Arrhenius plots (log σ vs. 1000 / T) were constructed,
where is possible to obtain the activation energy for the conduction process, figure 8 (b). It was
observed for both samples a single slope in the temperature range 125-400 ° C and that they do not
deviate the Arrhenius type behavior. From the slope of these lines total activation energies were
obtained, resulting values of 1.00 and 1.01 eV for the samples sintered with the curve S1 and S2
respectively. These values of activation energy are in complete agreement with the values found in
the literature for ion oxygen conductors based on zirconium oxide that are in the range from 1 to 1.2
eV [15].
(a) (b)
Figure 8. Dependence of conductivity with temperature for (c) total conductivity and ( b) Arrhenius
plots for total conductivity.
Conclusions
It was possible to stabilize the tetragonal zirconia polycrystalline form ZrO2 3% Mol Re2O3 at
a temperature lower than 650 ° C with crystallite size of approximately 24 nm. Transmission
electron microscopy revealed that the ceramic powder obtained was composed of nanoparticles with
sizes smaller than the 50 nm. Some particles were agglomerated, requiring the grinding step for
correct specimens conformation.
100 150 200 250 300 350 400
0,0
2,0x10-5
4,0x10-5
6,0x10-5
8,0x10-5
1,0x10-4
1,2x10-4
S1
S2
co
nd
uctivity (
cm
-1)
Temperature (°C)
Conductivity of the grain
100 150 200 250 300 350 400
0,0
5,0x10-6
1,0x10-5
1,5x10-5
2,0x10-5
2,5x10-5
3,0x10-5
Conductivity of the grain boundary
S1
S2
co
nd
uctivity (
cm
-1)
Temperature (°C)
100 150 200 250 300 350 400
0,0
5,0x10-6
1,0x10-5
1,5x10-5
2,0x10-5
2,5x10-5
Total conductivity
S1
S2
co
nd
uctivity (
cm
-1)
Temperature (°C)
1,4 1,6 1,8 2,0 2,2 2,4 2,6
-11
-10
-9
-8
-7
-6
-5
-4
S1
S2
log
(ST
) (
cm
-1)
1000/(T) (K-1)
Total conductivity
152 Brazilian Ceramic Conference 57
Ceramics developed from this raw material had densities higher than 96% of theoretical with
grain size dependent upon the used sintering schedule. The increase in grain size of 280 to 574 nm
caused a decrease in interfacial area per unit volume which is reflected in a decrease in the grain
boundaries densities. This result provides beneficial effects on the electrical behavior of the material
attributed to the resistivity reduction of the grain boundary which causes an increase in the total
conductivity of the ceramic from 1.35 E-5 E-5 to 2.15 Ω-1 cm-1 at 400 ° C.
Bibliography
[1] L. M. Acuña, D. G. Lamas, R. O. Fuentes, I. O. Fábregas, M. C. A. Fantini, A. F. Craievich,
and R. J. Prado, “Local atomic structure in tetragonal pure ZrO 2 nanopowders,” Journal of
Applied Crystallography, vol. 43, no. 2, pp. 227–236, Feb. 2010.
[2] D. G. Lamas, a. M. Rosso, M. S. Anzorena, a. Fernández, M. G. Bellino, M. D. Cabezas, N.
E. Walsöe de Reca, and a. F. Craievich, “Crystal structure of pure ZrO2 nanopowders,”
Scripta Materialia, vol. 55, no. 6, pp. 553–556, Sep. 2006.
[3] Santos, A. P; Domingues, R. Z; Kleitz, M; Eur, J. Ceram. Soc. 18 (1998) 1571.
[4] Muñoz, R.A., Rodriguez, J.E., Silva, C. R. M, 2011. Synthesis and characterization of
nanocrystalline zirconia by pechini method, using as a stabilizing additive a rare earth
elements concentrate for use in oxygen sensors. 21st Brazilian Congress of Mechanical
Engineering. October 24-28, 2011, Natal, RN, Brazil
[5] W. Guo, Z. Lin, X. Wang, G. Song, Microelectronic Eng. 66 (2003) 95.
[6] Freitas, D. (2000), Desenvolvimento de uma Cerâmica à Base de Zircônia Dopada com
Concentrado de Óxidos de Ítrio e Terras Raras para Aplicação em Sensores de Oxigênio.
Tese de Doutorado. Faculdade de Engenharia Química de Lorena – FAENQUIL, Programa
de Pós-Graduação em Ciência e Engenharia de Materiais, Lorena-SP.
[7] J.I. Langford and A.J.C. Wilson, “Scherrer after Sixty Years: A Survey and Some New
Results in the Determination of Crystallite Size,” J. Appl. Cryst. 11 (1978) pp 102-113.
[8] Howard, C.J., HILL, R.J ,KISI, E.H., Neutron Diffraction Studies of Phase Transformations
between Tetragonal and Orthorhombic Zirconia in MagnesiaPartially-Stabilized Zirconia, J.
Am. Ceram. Soc. 73 [10] 2828-33 (1990).
[9] Grzebielucka, E. C. Obtenção e sinterização de nanopartículas de ZrO2-4,5 % Y2O3.
Dissertação de Mestrado, Ponta Grossa, 2009.
[10] Callister, W. D. Ciência e Engenharia de Materiais: Uma Introdução. 5ed. LTC, São Paulo,
2002.
[11] Muñoz, R.A.1; Rodrigues, A.C; Santos, C; Silva, C.R.M. Effect of Rare Earth Addition on
Electrical Properties of Zirconia Based Ceramics. Materials Science Forum Vols. 660-661
(2010) pp 652-657.
[12] Y.-P. Fu, S.-H. Chen, and J.-J. Huang, “Preparation and characterization of Ce0.8M0.2O2−δ
(M=Y, Gd, Sm, Nd, La) solid electrolyte materials for solid oxide fuel cells,” International
Journal of Hydrogen Energy, vol. 35, no. 2, pp. 745–752, Jan. 2010.
[13] Semanate, J. L. N. Obtenção e condutividade elétrica de vitrocerâmica Li AlTi(PO4)com
diferentes microestruturas. Dissertação de Mestrado. São Carlos: UFSCar, 2010.
[14] Muccillo, E. N. S; Buissa Neto, R. C; Tadokoro, S. K; Muccillo, R. Synthesis, sintering and
impedance spectroscopy of calcia-partially stabilized zirconia. Cerâmica 52 (2006) 207-
214.
[15] Mæland, D; Suciu, C; Wærnhus, I; Hoffmann, A. Sintering of 4YSZ (ZrO2 + 4 mol% Y2O3)
nanoceramics for solid oxide fuel cells (SOFCs), their structure and ionic conductivity.
Journal of the European Ceramic Society 29 (2009) 2537–2547
Materials Science Forum Vols. 798-799 153
Brazilian Ceramic Conference 57 10.4028/www.scientific.net/MSF.798-799 ZrO2: 3 mol% Re2O3 10.4028/www.scientific.net/MSF.798-799.145