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Solid solutions of ZrO2 and HfO2 are potential electrolyte materials for intermediate-temperature SOFC because both are oxygen-ion conductors. The main challenge for these compounds is to reduce the relatively high value of the activation energies vacancies diffusion, which is influenced by several factors. In this work the thermal evolution of CaO-HfO(2) materials have been investigated. (CaO) y-Hf(1-y)O(2-y) (y = 0.06, 0.14 y 0.2) coatings and powders were synthesized by chemical solution deposition (CSD). Films were deposited onto alumina substrates by Dip Coating technique, the burning of organic waste was carried out at 500 degrees C under normal atmosphere and then the films were thermally treated at intervals of temperature rising to a maximum temperature of 1250 degrees C. By means Glazing Incidence X-ray Diffraction (phi-2 theta configuration) the phases were studied in the annealed films. On the other hand, the thermal evolution and crystallization process of powders were analyzed in-situ by HT-XRD. The phenomena crystallization occurred in films and powders were analyzed. The activation energies of diffusion of oxygen vacancies of HfO2-14 mole% CaO and HfO2-20 mole% CaO films were measured from the thermal evolution of the relaxation constant measured by Perturbed Angular Correlation Technique.
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2009 J. Phys.: Conf. Ser. 167 012052
(http://iopscience.iop.org/1742-6596/167/1/012052)
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Thermal evolution of CaO-doped HfO2 films and powders
S A Barolin1, M C Caracoche2, J A Martínez2, P C Rivas3, M A Taylor2, A F
Pasquevich2 and O A de Sanctis1,4
1Lab. Materiales Cerámicos, FCEIyA, Universidad Nacional de Rosario, IFIR-
CONICET
2Departamento de Física, FCE, Universidad Nacional de La Plata, Argentina – IFLP-
CONICET
3Facultad de Ciencias Agronómicas y Forestales, Universidad Nacional de La Plata,
Argentina - IFLP
oski@fceia.unr.edu.ar
Abstract. Solid solutions of ZrO2 and HfO2 are potential electrolyte materials for intermedi-
ate-temperature SOFC because both are oxygen-ion conductors. The main challenge for these
compounds is to reduce the relatively high value of the activation energies vacancies diffusion,
which is influenced by several factors. In this work the thermal evolution of CaO-HfO2 materi-
als have been investigated. (CaO)y-Hf(1-y)O(2-y) (y = 0.06, 0.14 y 0.2) coatings and powders
were synthesized by chemical solution deposition (CSD). Films were deposited onto alumina
substrates by Dip Coating technique, the burning of organic waste was carried out at 500 ºC
under normal atmosphere and then the films were thermally treated at intervals of temperature
rising to a maximum temperature of 1250 ºC. By means Glazing Incidence X-ray Diffraction
(ϕ-2θ configuration) the phases were studied in the annealed films. On the other hand, the
thermal evolution and crystallization process of powders were analyzed in-situ by HT-XRD.
The phenomena crystallization occurred in films and powders were analyzed. The activation
energies of diffusion of oxygen vacancies of HfO2-14 mole% CaO and HfO2-20 mole% CaO
films were measured from the thermal evolution of the relaxation constant measured by Per-
turbed Angular Correlation Technique.
1. Introduction
The solid solutions based HfO2 have taken very technological importance in recent years, not only for
their applications as solid electrolyte in fuel cells and gas sensors, but also for its use as picture-
Detectors high-energy particles (scintillation) [1]. Furthermore, due to the miniaturization of devices,
the demand for functional materials in restricted dimensions, such as thin films, is imperative.
The required properties of solid solutions based on HfO2 for any of their applications, are
dependent on the structures and the stability of cubic phases metaestables or tetragonal.
The objective of this work is to study the evolution of phases in films and powders in the system
CaO-HfO2 prepared from the precursor solution.
4 Author to whom correspondence should be addressed
XIX Latin American Symposium on Solid State Physics (SLAFES XIX) IOP Publishing
Journal of Physics: Conference Series 167 (2009) 012052 doi:10.1088/1742-6596/167/1/012052
c
2009 IOP Publishing Ltd
1
2. Experimental procedure
2.1. Samples preparation
CaO-HfO2 precursor solutions were prepared by using Calcium acetate and Hafnium buthoxide
(Hf(OC4H9)4), as source materials, with buthanol as solvent. Acetoin (3-hydroxy-2-butanone,
CH3COCH(OH)CH3) was used as chelating agent. Commercial starting materials were used in their
as-received state from Aldrich. Acetoin was added to Hafnium buthoxide - buthanol solution (the mo-
lar ratio of the chelating agent to the alkoxide was R = 4). The compositions were Hf1-x Cax O2-x with
x = 0,06; 0,14; 0,20. The solution was suitable for the deposition of uniform and crack-free layers by
Dipping-coating, with a thickness of ~150 nm per layer deposited onto alumina substrates. After depo-
sition, each layer was dried at 200 ºC for 3 min and pyrolyzed at 500 ºC for 6 min. Additional layers
were dip-coated to build up the desired thickness. Several samples of multilayer coating was heat-
treated by quickly placing it into a preheated furnace at temperatures ranged between 500 and 1250 ºC
for 60 minutes. To derive gel powders the parent solutions were kept overnight in an oven maintained
at 60C under 100 Pa of pressure. After solvent evaporation the residue was heating at 200 ºC. The
dried gel was crushed in a agate mortar to get fine amorphous powders.
2.2. X-ray diffraction
The structure of the films was measured using an X-ray diffractometer with Cu Kα radiation (Philips
X-Pert Pro) and a graphite monochromator (the step size being of 2θ = 0.02º and a 1 second time per
step). For annealed films, a grazing incidence configuration (GIXRD) was used for measuring at room
temperature. High temperature in-situ X-ray diffraction (HT-XRD) patterns of powders were obtained
in air atmosphere making use of an Anton-Paar HTK10 high-temperature chamber. The spectra were
taken at nominal temperatures (temperature of heating filament) equal to the temperatures used in an-
nealing treatment of films. In order to overcome partly uncertainty in the determination of tempera-
tures, a second S-thermocouple was placed onto the sample to measure directly the temperature of the
powders (TR). Diffraction spectra were exploited to calculate average crystallite sizes of the solid so-
lution from the magnitude of the half-width of the main line for c-HfO2 (2θ = 30°), instrumental broa-
dening of 0.130°), using the Scherrer equation.
2.3. Thermal analyses
By means of a DTG-60H Shimadzu analyzer, simultaneous Differential Thermal Analisys (DTA) and
Thermogravimetric Analysis (TGA) measurements were carried out on the as-obtained powder gels
from room temperature to 800C in normal atmosphere. Alumina crucibles and α-alumina reference
were used.
2.4. Oxygen vacancies movement
The activation energy of oxygen vacancies diffusion was measured by PAC experiment. The PAC
method is a nuclear technique by which the hyperfine interaction between a radioactive nucleus acting
as a probe and the internal fields of the solid in the nearest neighbourhood can be measured [2]. PAC
experiment, gives the desired information about the electrical field gradient (EFG) produced by the
surrounding ions at the probe nucleus via the determination of the four quantities, i.e., relative fraction,
quadrupole frequency, asymmetry parameter, and frequency spread, associated with each unequivalent
Hafnium lattice site. In fact, atomic or defect movements cause a further perturbation of the angular
correlation which may be reflected in the PAC spin rotation curves by a damping in the amplitude
obeying to a factor of the form e-λt where λ is called the relaxation constant. In stabilized hafnias or
zirconias, this situation appears with the onset of the thermally activated vacancy-hopping process.
The jump frequency, v obeys an Arrhenius law and the correlation time τ = 1/ ν can then be associ-
ated with the activation energy of the jump through the equation: τ = τ eEac/kT (1). It has already been
reported that an exponential decrease of the relaxation constant with the reciprocal absolute tempera-
ture implies a slow diffusion movement for which the correlation time between two successive atomic
XIX Latin American Symposium on Solid State Physics (SLAFES XIX) IOP Publishing
Journal of Physics: Conference Series 167 (2009) 012052 doi:10.1088/1742-6596/167/1/012052
2
configurations is equal to λ-1. By contrast, an exponential increase of λ with the reciprocal absolute
temperature, generally found at higher temperatures, is typical of a fast relaxation regime, for which
the correlation time is proportional to λ. Hence, considering equation (1), the activation energies for
movement of the vacancies can be drawn from the slopes of the λ (T–1) function [3].
3. Results and discussions.
3.1. Thermal evolution of as-prepared materials.
Figure 1 show DTA – TGA curves for CaO 20 mol% – HfO2 powder (no substantial differences for
other compositions). The TGA curve shows a weight loss assigned to the decomposition and burning
of the residual organic groups and no weight loss at temperatures > 520°C is observed, so the thermal
decomposition of the gel is completed at that temperature. The DTA curves display intense exothermic
peaks, one at centered at 350 ºC and another around 490 ºC (488, 492 and 498 ºC for 6, 14 and 20
mole% of CaO, respectively) that are attributed to burning of the residual organic groups and to re-
moval of hydroxyls, with a simultaneous onset of the crystallization of the cubic/tetragonal phase, re-
spectively.
Figure 1. Thermogravimetric and Dif-
ferential thermal analysis of powder
CaO 20 mol% – HfO2
3.2. Crystalline phases and metastable phase stabilization.
Figure 2 shows in-situ HT-XRD spectra of the CaO– HfO2 powders that were taken at three tempera-
tures for each composition: i) at 500 ºC as nominal temperature, ii) at the highest temperature that the
spectrum only exhibited diffraction peaks corresponding to cubic hafnia and iii) at the temperature
that the monoclinic phase appears. The amorphous phase unambiguously transforms to cubic phase in
the 14 and 20 mole% of CaO doped- HfO2 powders. On the other hand, the metastable crystalline
phase growth in the 6 mole% of CaO doped- HfO2 powder could not be clearly identified as tetrago-
nal phase or cubic phase due to its early destabilization by irruption of monoclinic phase at low tem-
perature. The X-ray diffraction result follows the same trend that showed the DTA analysis, indicating
that for increasing CaO content the crystallization temperature increases.
Figure 2. HT-XRD pattern of CaO - HfO2 powders
2θ [º]
630ºC
510ºC
400ºC
Intensity [a u]
6% CaO
Intensity [a u]
780ºC
700ºC
400ºC
2
θ
[º]
14% CaO
860ºC
780ºC
400ºC
Intensity [a u]
2θ [º]
20% CaO
ΔV [μV]
T [ºC]
Δm/m
XIX Latin American Symposium on Solid State Physics (SLAFES XIX) IOP Publishing
Journal of Physics: Conference Series 167 (2009) 012052 doi:10.1088/1742-6596/167/1/012052
3
Figure 3 shows ex-situ GI-XRD spectra of the CaO–HfO2 films that were taken at room tempera-
ture on samples of annealed films at: i) 500 ºC, ii) the highest temperature that the spectrum only ex-
hibited diffraction peaks corresponding to cubic hafnia and iii) temperature that the monoclinic phase
appears. The appearance of the monoclinic phase in the films occurred at higher temperatures than the
ones in the powders; at 800 ºC, 1000 Cº and 1100 ºC for the 6, 14 and 20 mole% of CaO doped- HfO2
film, respectively.
Table 1 shows the thermal evolution of fractions of cubic phase and monoclinic phase in annealed
films and thermal treated powders of composition xCaO-(1-x)HfO2 with x = 0.06, 0.14 and 0.2.
Table 1. Thermal evolution phase fractions for CaO – HfO2 materials
Comp. Hf
0,94Ca0,06O1,94 Hf 0,86Ca0,14O1,86 Hf 0,20Ca0,20O1,80
Temp. [ºC] Film Powder Film Powder Film Powder
TN TR c m c m c m c m c m c m
500 400 100 0 100 0 100 0 100 0 ¿? ¿? ¿? ¿?
650 510 94 06 100 0 100 0 100 0 100 0 100 0
800 630 79 21 72 28 100 0 100 0 100 0 100 0
900 700 64 36 40 60 100 0 100 0 100 0 100 0
1000 780 39 61 22 78 > 99 < 01 95 05 100 0 100 0
1100 860 0 100 17 83 36 64 71 29 79 21 98 02
1250 1130 0 100 21 79 0 100 61 39 75 25 87 13
The metastable tetragonal/cubic phases become unstable at increasing temperature of thermal treat-
ment if the metastable–monoclinic transformation is not prevented. The Calcium cations are able to be
substituted for Hf+4 ions in the cation lattice and consequently create vacancies in the oxygen sublat-
tice in order to satisfy electroneutrality requirements. The oxygen vacancies increase the cubic phase
stability due to the transformation of the Hf+4 surroundings from 8-folded oxygen coordination to 7-
folded oxygen coordination. The question is to know the minimum dopant concentration for full cubic
stabilization. Table 1 shows that the stability of the cubic phase increases for increasing CaO content
for both films and powders. However, in the powders (TR) the irruption of monoclinic phase occurred
at lower temperatures than in the films (TN). For thermal treatment at temperatures of 1000 ºC, the
cubic maintains as the unique phase in the 14 and 20 mole% of CaO doped-films.
This different behavior between films and powders has been already observed in the system Y2O3
doped-ZrO2 [4]. The highly defective nature of the structure in the film coating, produced by presence
of residual lattice strains and asymmetric grains, were been aiding the cubic phase stabilization.
Intensity [a u]
800ºC
500ºC
650ºC
Intensity [a u]
2
θ
[º]
(111) m (111) m
6% CaO
500ºC
1100ºC
1000ºC
Intensity [a u]
2
θ
[º]
(111) c
14% CaO
1100ºC
1000ºC
500ºC
2θ [º]
20% CaO
Figure 3. GI-XRD pattern of annealed for CaO - HfO2 films
XIX Latin American Symposium on Solid State Physics (SLAFES XIX) IOP Publishing
Journal of Physics: Conference Series 167 (2009) 012052 doi:10.1088/1742-6596/167/1/012052
4
The thermal evolution of the grain size vs. 1/kT(K) of the cubic phase in the HfO2 - 20 mole% of
CaO film and powder, make possible to infer that the mechanisms operated during the growth were
dissimilar in each one of such materials (Figure 4). The change of slope at around 700 ºC would indi-
cate the starting of a recrystallization process during the thermal evolution of the powder, which would
be indicating that in the powder the nucleation developed mainly by homogenous nucleation. Contrar-
ily, the constant slope in the all temperature range and the low value of activation energy of the grow-
ing grain, it would evidence a heterogeneous nucleation in the film.
3.3. Oxygen vacancies diffusion.
Concerning the movement of oxygen vacancies, the λ(T-1) function is plotted in figure 5. For both
CaO 14 mole% doped- HfO2 and 20 mole% CaO 20 mole% doped- HfO2 films a slow-fast two regime
behaviour can be observed. The movements at temperatures lower than 800°C correspond, according
to the dynamic model assumed, to a slow diffusion (τ λ-1) that is related to phenomena that are not
linked to the oxygen vacancies diffusion. For temperatures higher than 800°C, in turn, the fast relaxa-
tion dynamical effect due to oxygen vacancies movement can be considered Arrhenius-like and an
activation energies of 0.54 ± 0.04 eV and 0.70 ± 0.05 eV could be drawn from the λ(T-1) slopes corre-
sponding to HfO2-14 mole% CaO film and HfO2-20 mole% CaO film, respectively. Unexpectedly,
the film that exhibits higher activation energy is the one where the CaO doping is higher, in which
consequently the concentration of oxygen vacancies is higher also. This fact suggests that high levels
of CaO content in the structure of the CaO doped- HfO2 films performs a situation that impedes partly
the oxygen movement. High CaO content would produce an increase of the
[
]
''
CaVO
•• association or
the growth of order nanophases formed by defective-oxygen compounds that were unappreciable in
XRD [5].
Figura 5. Activation energies deduced from the
relaxation constants λ experienced by probes in
the cubic solid solution.
Figure 4. Thermal evolution of the size grain as function of 1/kT(K) of HfO2 - 20 mole% of CaO
1/kBT [eV] 1/kBT [eV]
ln s
ln s
T [ºC] T [ºC]
XIX Latin American Symposium on Solid State Physics (SLAFES XIX) IOP Publishing
Journal of Physics: Conference Series 167 (2009) 012052 doi:10.1088/1742-6596/167/1/012052
5
4. Conclusions
Powders and coatings of metastable cubic hafnia have been successfully prepared by chemical solu-
tion deposition technique. The burning of residual organic groups, hydroxyls removal and onset of
crystallization performed at temperatures lower than 500 ºC.
The maximum temperature up to the stabilization is retained increases when CaO content increases
in both powder and film. However, the irruption of monoclinic phase occurred at higher temperatures
in the films. Films with 14 mole% doped- HfO2 and 20 mole% CaO 20 mole% retained the cubic
phase after annealing treatment at 1000 ºC for 1 hour.
The determination of activation energies for oxygen vacancies movements allows to find that very
high CaO content promotes the phenomena that impede partly the oxygen movement in the CaO –
HfO2 films.
References
[1] M Kirm, J Aarik, M Jürgens and I Sildos 2005 Nuclear Instruments and Methods in Physics
Research A 537 251-255
[2] P C Rivas, M C Caracoche, J A Martínez, A M Rodríguez, R Caruso, N Pellegri and O de
Sanctis 1997 J. Mater. Res. 12 493
[3] J A Gardner, H Jaeger, H T Su, W H Warner and J C Haygarth 1988 Physica B150, 223
[4] R Caruso, E Benavidez, O de Sanctis, M C Caracoche, P C Rivas, M Cervera, A Caneiro and A
Serquis 1997 Phase Structure and Thermal Evolution in Coating Films and Powders
Obtained by Sol-gel Process. Part. II: ZrO2-2.5 % mol Y2O3 J. Mater. Res. 12 [10] 2594-
601
[5] M C Caracoche, J A Martínez, P C Rivas, M A Taylor, A F Pasquevich, S Barolin, O A de
Sanctis 2008 Nanostructures in calcia stabilized hafnia thin films observed by PAC as a
function of temperature Hyperfine Interaction 0304-3843
XIX Latin American Symposium on Solid State Physics (SLAFES XIX) IOP Publishing
Journal of Physics: Conference Series 167 (2009) 012052 doi:10.1088/1742-6596/167/1/012052
6
... In effect, divalent and trivalent dopants introduce oxygen vacancies into the structure of HfO 2 which in turn, stabilize a high temperature phase of HfO 2 such as the cubic phase [5][6][7]. Some of the well-known dopants to stabilize the cubic phase of HfO 2 include CaO, CeO 2 , MgO and Y 2 O 3 [8]. In fact, the latter three crystallize into the fluorite cubic structure and when used as dopants, compel HfO 2 to stabilize also into its inherent fluorite cubic structure. ...
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Relaxation of **1**8**1Ta nuclei due to diffusion of oxygen vacancies in cubic zirconia/yttria has been measured by perturbed angular correlation spectroscopy. The activation energy of the vacancy jump rate is found in a temperature range above 750 degree C and is in reasonable agreement with the activation energy of ionic conduction. Preliminary results of a **1**1**1In/**1**1**1Cd perturbed angular correlation investigation of high-temperature monoclinic and tetragonal zirconia are also reported.
  • M P C Rivas
  • J A C Caracoche
  • A Martínez
  • R Rodríguez
  • N Caruso
  • O Pellegri
  • De Sanctis
P C Rivas, M C Caracoche, J A Martínez, A M Rodríguez, R Caruso, N Pellegri and O de Sanctis 1997 J. Mater. Res. 12 493
  • M Kirm
  • M Aarik
  • I Jürgens
  • Sildos
M Kirm, J Aarik, M Jürgens and I Sildos 2005 Nuclear Instruments and Methods in Physics Research A 537 251-255
  • J A Gardner
  • H Jaeger
  • H T Su
  • W H Warner
  • J C Haygarth
J A Gardner, H Jaeger, H T Su, W H Warner and J C Haygarth 1988 Physica B150, 223