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Synthesis and characterization of ZrO2 and YSZ thin films
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 14 (2019) 92–95
www.materialstoday.com/proceedings
SLAFES XXIII
Synthesis and characterization of ZrO2 and YSZ thin films Y. Mansilla a,*, M. Arcea,b, C. Gonzalez Olivera,b, H. Troiania,b, A. Serquisa,b a
Centro Atómico Bariloche, Comisión Nacional de Energía Atómica, Av. Bustillo 9500, Bariloche (8400), Argentina b Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina
Abstract Zirconia-based ceramics possess a wide range of relevant applications due to its unique mechanical, electrical and optical properties. On the one hand, ZrO2 has a high mechanical and chemical resistance, which is suitable as protective coating. On the other hand, yttria-stabilized zirconia (YSZ) is one of the main electrolyte materials for solid oxide fuel cells (SOFC) devices due to its high ionic conductivity at high temperatures. In this work, ZrO2 and YSZ thin films synthesized by sol-gel method and deposited by dip coating is studied. ZrO2 films are synthesized as coatings for Zircaloy rod spacer grids used for critical heat flux tests, while YSZ films are synthesized for non-electrolyte supported SOFC. Morphological characterization of these films is performed by SEM, while TEM and XRD provide insight on the crystallographic properties. Electrical properties of these materials are measured by conductivity experiments. Highly dense films with good adherence to substrates and thickness above 200 nm are achieved by this method. The nano grain size allows to obtain high symmetry phase stabilization, which enhance the desired properties in both materials for the proposed applications. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the organizing committee of the XXIII Latin American Symposium on Solid State Physics (SLAFES XXIII), San Carlos de Bariloche, Argentina, 10–13 April 2018. Keywords: Zirconia; Yttria Stabilized Zirconia (YSZ); thin films; sol-gel
1. Introduction Zirconia based materials possess a wide variety of attractive properties, such as unique mechanical, electrical and optical properties, which are responsible for the different applications in which they are used, mainly in the fields of biomedicine and energy [1,2]. As thin films, pure zirconia is used as protective coatings with electrical insulating properties. The properties of these films are highly dependent on the deposition method used for its synthesis, which can be chemical vapor deposition (CVD), sputtering, spray pyrolysis and dip-coating, among others. Major changes in the phase diagrams, thus in the properties of this material, may be introduced when the crystallite size reaches the nanoscale (below ~25nm). In this last case, cubic or tetragonal phases may be retained (instead of the monoclinic 2214-7853© 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the organizing committee of the XXIII Latin American Symposium on Solid State Physics (SLAFES XXIII), San Carlos de Bariloche, Argentina, 10–13 April 2018.
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phase) at lower temperatures than predicted from bulk phase diagram and these phases possess better electrical and mechanical properties [3,4]. In order to achieve this grain size, sol-gel route is one of the most suitable methods. For energy applications, zirconia doped with Y2O3, i.e. yttrium stabilized zirconia (3, 8 or 10% yttria, YSZ) is the state-of-art electrolyte for solid oxide fuel cells (SOFC). Yttria stabilization induces the production of oxygen vacancies while providing chemical stability and good mechanical properties at high temperatures needed in these devices. However, high ionic conductivity is achieved at temperatures above 800 ºC, which means high costs, for example in interconnect materials, higher degradation rates, etc. Thus, different strategies have been proposed to lower the operating temperature of SOFC, being one of them the development of thinner electrolytes [5]. In this work pure ZrO2 and YSZ thin films are synthesized based on sol-gel method as electrical insulators (coatings for Zircaloy rod spacer grids used for critical heat flux tests) and SOFC electrolyte materials, respectively. Morphological, crystallographic and electrical properties are studied on these materials in order to determine its application feasibility. 2. Experimental 2.1. ZrO2 and YSZ synthesis A solution containing yttrium and zirconium precursors was synthesized by sol-gel method using the alkoxide route. The zirconium alkoxide precursor is zirconium (IV) propoxide (Zr(OPr)4), 70 wt% solution in 1-propanol. Triethanolamine is added as a complexing agent to limit the reactivity of zirconium propoxide and to obtain ZrO2 sol. Yttrium (III) nitrate hexahydrate, dried at 280°C [6], is then added to obtain ZrO2-3%Y2O3 sol used to synthesize Yttria Stabilized Zirconia (YSZ). Different substrates were used depending on the applications. ZrO2 thin films were deposited on glass and zircaloy-4 substrates by dip coating with one and two depositions, dried at 25°C for 12 h and heat-treated at 450°C for 6 h. YSZ thin films were deposited on sintered Gadolinium Doped Ceria (GDC) pellets by dip coating, dried at 25°C for 12 h and heat-treated at 550°C during 6 h. 2.2. Characterization ZrO2 and YZS powders heat-treated at different temperature were characterized by X-Ray Diffraction (XRD) with a Panalytical Empyrean diffractometer. Crystallite size was determined by Scherrer formula [7]. Morfology and thin films thickness were studied by Scanning Electron Microscopy (SEM) using a FEI Nova NanoSEM 230 and FIB-SEMZEISS Crossbeam 340. ZrO2 thin films were examined by Transmission Electron Microscopy (TEM) with a Philips CM200 microscope, using samples prepared by scratching the films off the substrates. Electrical conductivity measurements of ZrO2 films were performed using two-probe DC method with an Agilent 34972A Data Acquisition Unit at temperature range between room temperature and 450°C. 3. Results and discussion ZrO2 powders heat-treated at 450°C and 800°C were studied by XRD. XRD patterns are shown in Fig. 1a. The presence of the (1 0 2) diffraction peak is characteristic of the tetragonal phase, this peak enables to differentiate the tetragonal phase from the cubic phase. As can be seen from Fig. 1a. tetragonal phase is obtained in powders treated at 450°C, with crystallite size of 8-9 nm. Powders treated at 800°C have a mixture of tetragonal and monoclinic phases, calculated crystallite size is ~25 nm in the tetragonal phase and ~30 nm in the monoclinic phase. XRD patterns for YSZ (ZrO2-3%Y2O3) powders treated at 500°C and 800°C are displayed in Fig. 1b. At 800°C the tetragonal phase is clearly retained with a crystallite size of 50 nm, however at 500°C the particle size broadening (crystallite size of 7 nm) does not allow correct phase identification by XRD and could either be cubic or tetragonal. Further studies using Raman spectroscopy should be performed to correctly assign crystal structure in this case. According to bulk phase diagram of ZrO2, at temperatures below 1000°C the monoclinic phase is the most stable
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one. However, when crystallite size is below 20 nm, tetragonal phase could either be retained as a metastable phase [8] or be the thermodynamically stable phase [9].
Fig. 1. (a) XRD patterns of ZrO2 powders synthesized by sol-gel method, heat-treated at 450°C and 800°C for 2 h; (b) XRD patterns of YSZ powders synthesized by sol-gel method, heat-treated at 500°C and 800°C for 2 h.
Fig. 2 shows SEM and TEM images of ZrO2 thin films with one deposition on glass. It can be observed that these films are uniform, have very low porosity and present good adherence to substrate. The thickness observed in thin films with one deposition is ~200 nm and ~500 nm with two depositions. Crystallite size between 5 to 10 nm is observed by TEM. Fig. 2c displays the electrons diffraction pattern produced by ZrO2 film, this pattern was indexed using a tetragonal unit cell. The presence of the (1 0 2) diffraction allows to differentiate the tetragonal phase and the cubic phase, which was also observed in XRD pattern.
Fig. 2. ZrO2 film with one deposition and heat-treated 550ºC-6h (a) SEM image; (b) High resolution TEM images of nanocrystallites; (c) electron diffraction pattern.
Electrical conductivity test of a ZrO2 thin film of 500 nm thickness, 6 mm width and 2 mm length between electrodes, deposited on Zircaloy-4, as a function of temperature was performed. The conductivity of tetragonal ZrO2 films is approximately 3.10-5 (Ω.cm)-1 at 300 °C. In previous studies in tetragonal zirconia disks it was shown that the electrical conductivity at 300 °C is 10-5 (Ω.cm)-1 [10]. The differences between reported ZrO2 conductivity and literature data could be arise from the effect that the thin film with nanosized grains has on the conductivity. Using a similar synthesis route for ZrO2 but adding Yttrium nitrate, dense and homogeneous YSZ thin films were obtained on top of a GDC pellet used as substrate. Then, the films were heat-treated at 500°C for 6 h, SEM image is shown in Fig. 3. The observed thickness is 300-400 nm. It is very interesting to notice that the tetragonal phase can be retained with only 3 mol% Y2O3.
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Fig. 3.SEM image of YZS thin film heat-treated at 500°C for 6 h.
4. Conclusions ZrO2 and YSZ thin films were successfully synthesized by sol-gel method and deposited by dip coating in different substrates. ZrO2 films were prepared as coatings for Zircaloy rod spacer grids used for critical heat flux tests, while YSZ films were synthesized for non-electrolyte supported SOFC. Highly dense films with good adherence to substrates and thickness above 200 nm were achieved by this method. In pure ZrO2 films, the tetragonal phase was retained even at room temperature due to nanoscale grain size, while the addition of Y2O3 in YSZ films with as low as 3 mol% of dopant was enough to retain the tetragonal phase. High symmetry phase stabilization enhances the desired properties in both materials for the proposed applications. 5. Acknowledgements This work was supported by CAB - CNEA (Centro Atómico Bariloche – Comisión Nacional de Energía Atómica), CONICET (Consejo Nacional de Investigaciones Científicas y Técnicas), and ANPCyT (Agencia Nacional de Promoción de Ciencia y Tecnología). 6. References [1] Y. W. Chen, J. Moussi, J. L. Drury, J. C. Wataha, Expert Review of Medical Devices 13 (2016) 945-963. [2] Mahato, Neelima; Banerjee, Amitava; Gupta, Alka; Omar, Shobit; Balani, Kantesh, Prog. Mater. Sci. 72 (2015) 141-337. [3] R.C. Garvie, J. Phys. Chem. 69 (1965)1238. [4] E.P. Butler, Mater. Sci. Technol. 1 (1985) 417-432. [5] A. Tarancón, Energies 2 (2009) 1130-1150. [6] P. Melnikov, V. A. Nascimento, L. Z. Z. Consolo, A. F. Silva, J.Therm. Anal. Calorim. 111 (2013), 115-119. [7] P. Scherrer, Gottinger Nachrichten 2 (1918) 98-100. [8] N-L. Wu, T-F. Wu, I. A. Rusakova J. Mater. Res. 16 (2001) 666. [9] D. G. Lamas, A. M. Rosso, M. Suarez Anzorena, A. Fernández, M. G. Bellino, M. D. Cabezas, N. E. Walsöe de Reca, A. F. Craievich, Scr. Mater. 55 (2006) 553–556. [10] T. K. Gupta, R. B. Grekila, E. C. Subbarao, J. Electrochem. Soc. 128 (1981) 929-931.
ScienceDirect Materials Today: Proceedings 14 (2019) 92–95
www.materialstoday.com/proceedings
SLAFES XXIII
Synthesis and characterization of ZrO2 and YSZ thin films Y. Mansilla a,*, M. Arcea,b, C. Gonzalez Olivera,b, H. Troiania,b, A. Serquisa,b a
Centro Atómico Bariloche, Comisión Nacional de Energía Atómica, Av. Bustillo 9500, Bariloche (8400), Argentina b Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina
Abstract Zirconia-based ceramics possess a wide range of relevant applications due to its unique mechanical, electrical and optical properties. On the one hand, ZrO2 has a high mechanical and chemical resistance, which is suitable as protective coating. On the other hand, yttria-stabilized zirconia (YSZ) is one of the main electrolyte materials for solid oxide fuel cells (SOFC) devices due to its high ionic conductivity at high temperatures. In this work, ZrO2 and YSZ thin films synthesized by sol-gel method and deposited by dip coating is studied. ZrO2 films are synthesized as coatings for Zircaloy rod spacer grids used for critical heat flux tests, while YSZ films are synthesized for non-electrolyte supported SOFC. Morphological characterization of these films is performed by SEM, while TEM and XRD provide insight on the crystallographic properties. Electrical properties of these materials are measured by conductivity experiments. Highly dense films with good adherence to substrates and thickness above 200 nm are achieved by this method. The nano grain size allows to obtain high symmetry phase stabilization, which enhance the desired properties in both materials for the proposed applications. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the organizing committee of the XXIII Latin American Symposium on Solid State Physics (SLAFES XXIII), San Carlos de Bariloche, Argentina, 10–13 April 2018. Keywords: Zirconia; Yttria Stabilized Zirconia (YSZ); thin films; sol-gel
1. Introduction Zirconia based materials possess a wide variety of attractive properties, such as unique mechanical, electrical and optical properties, which are responsible for the different applications in which they are used, mainly in the fields of biomedicine and energy [1,2]. As thin films, pure zirconia is used as protective coatings with electrical insulating properties. The properties of these films are highly dependent on the deposition method used for its synthesis, which can be chemical vapor deposition (CVD), sputtering, spray pyrolysis and dip-coating, among others. Major changes in the phase diagrams, thus in the properties of this material, may be introduced when the crystallite size reaches the nanoscale (below ~25nm). In this last case, cubic or tetragonal phases may be retained (instead of the monoclinic 2214-7853© 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the organizing committee of the XXIII Latin American Symposium on Solid State Physics (SLAFES XXIII), San Carlos de Bariloche, Argentina, 10–13 April 2018.
Y. Mansilla et al. / Materials Today: Proceedings 14 (2019) 92–95
93
phase) at lower temperatures than predicted from bulk phase diagram and these phases possess better electrical and mechanical properties [3,4]. In order to achieve this grain size, sol-gel route is one of the most suitable methods. For energy applications, zirconia doped with Y2O3, i.e. yttrium stabilized zirconia (3, 8 or 10% yttria, YSZ) is the state-of-art electrolyte for solid oxide fuel cells (SOFC). Yttria stabilization induces the production of oxygen vacancies while providing chemical stability and good mechanical properties at high temperatures needed in these devices. However, high ionic conductivity is achieved at temperatures above 800 ºC, which means high costs, for example in interconnect materials, higher degradation rates, etc. Thus, different strategies have been proposed to lower the operating temperature of SOFC, being one of them the development of thinner electrolytes [5]. In this work pure ZrO2 and YSZ thin films are synthesized based on sol-gel method as electrical insulators (coatings for Zircaloy rod spacer grids used for critical heat flux tests) and SOFC electrolyte materials, respectively. Morphological, crystallographic and electrical properties are studied on these materials in order to determine its application feasibility. 2. Experimental 2.1. ZrO2 and YSZ synthesis A solution containing yttrium and zirconium precursors was synthesized by sol-gel method using the alkoxide route. The zirconium alkoxide precursor is zirconium (IV) propoxide (Zr(OPr)4), 70 wt% solution in 1-propanol. Triethanolamine is added as a complexing agent to limit the reactivity of zirconium propoxide and to obtain ZrO2 sol. Yttrium (III) nitrate hexahydrate, dried at 280°C [6], is then added to obtain ZrO2-3%Y2O3 sol used to synthesize Yttria Stabilized Zirconia (YSZ). Different substrates were used depending on the applications. ZrO2 thin films were deposited on glass and zircaloy-4 substrates by dip coating with one and two depositions, dried at 25°C for 12 h and heat-treated at 450°C for 6 h. YSZ thin films were deposited on sintered Gadolinium Doped Ceria (GDC) pellets by dip coating, dried at 25°C for 12 h and heat-treated at 550°C during 6 h. 2.2. Characterization ZrO2 and YZS powders heat-treated at different temperature were characterized by X-Ray Diffraction (XRD) with a Panalytical Empyrean diffractometer. Crystallite size was determined by Scherrer formula [7]. Morfology and thin films thickness were studied by Scanning Electron Microscopy (SEM) using a FEI Nova NanoSEM 230 and FIB-SEMZEISS Crossbeam 340. ZrO2 thin films were examined by Transmission Electron Microscopy (TEM) with a Philips CM200 microscope, using samples prepared by scratching the films off the substrates. Electrical conductivity measurements of ZrO2 films were performed using two-probe DC method with an Agilent 34972A Data Acquisition Unit at temperature range between room temperature and 450°C. 3. Results and discussion ZrO2 powders heat-treated at 450°C and 800°C were studied by XRD. XRD patterns are shown in Fig. 1a. The presence of the (1 0 2) diffraction peak is characteristic of the tetragonal phase, this peak enables to differentiate the tetragonal phase from the cubic phase. As can be seen from Fig. 1a. tetragonal phase is obtained in powders treated at 450°C, with crystallite size of 8-9 nm. Powders treated at 800°C have a mixture of tetragonal and monoclinic phases, calculated crystallite size is ~25 nm in the tetragonal phase and ~30 nm in the monoclinic phase. XRD patterns for YSZ (ZrO2-3%Y2O3) powders treated at 500°C and 800°C are displayed in Fig. 1b. At 800°C the tetragonal phase is clearly retained with a crystallite size of 50 nm, however at 500°C the particle size broadening (crystallite size of 7 nm) does not allow correct phase identification by XRD and could either be cubic or tetragonal. Further studies using Raman spectroscopy should be performed to correctly assign crystal structure in this case. According to bulk phase diagram of ZrO2, at temperatures below 1000°C the monoclinic phase is the most stable
94
Y. Mansilla et al. / Materials Today: Proceedings 14 (2019) 92–95
one. However, when crystallite size is below 20 nm, tetragonal phase could either be retained as a metastable phase [8] or be the thermodynamically stable phase [9].
Fig. 1. (a) XRD patterns of ZrO2 powders synthesized by sol-gel method, heat-treated at 450°C and 800°C for 2 h; (b) XRD patterns of YSZ powders synthesized by sol-gel method, heat-treated at 500°C and 800°C for 2 h.
Fig. 2 shows SEM and TEM images of ZrO2 thin films with one deposition on glass. It can be observed that these films are uniform, have very low porosity and present good adherence to substrate. The thickness observed in thin films with one deposition is ~200 nm and ~500 nm with two depositions. Crystallite size between 5 to 10 nm is observed by TEM. Fig. 2c displays the electrons diffraction pattern produced by ZrO2 film, this pattern was indexed using a tetragonal unit cell. The presence of the (1 0 2) diffraction allows to differentiate the tetragonal phase and the cubic phase, which was also observed in XRD pattern.
Fig. 2. ZrO2 film with one deposition and heat-treated 550ºC-6h (a) SEM image; (b) High resolution TEM images of nanocrystallites; (c) electron diffraction pattern.
Electrical conductivity test of a ZrO2 thin film of 500 nm thickness, 6 mm width and 2 mm length between electrodes, deposited on Zircaloy-4, as a function of temperature was performed. The conductivity of tetragonal ZrO2 films is approximately 3.10-5 (Ω.cm)-1 at 300 °C. In previous studies in tetragonal zirconia disks it was shown that the electrical conductivity at 300 °C is 10-5 (Ω.cm)-1 [10]. The differences between reported ZrO2 conductivity and literature data could be arise from the effect that the thin film with nanosized grains has on the conductivity. Using a similar synthesis route for ZrO2 but adding Yttrium nitrate, dense and homogeneous YSZ thin films were obtained on top of a GDC pellet used as substrate. Then, the films were heat-treated at 500°C for 6 h, SEM image is shown in Fig. 3. The observed thickness is 300-400 nm. It is very interesting to notice that the tetragonal phase can be retained with only 3 mol% Y2O3.
Y. Mansilla et al. / Materials Today: Proceedings 14 (2019) 92–95
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Fig. 3.SEM image of YZS thin film heat-treated at 500°C for 6 h.
4. Conclusions ZrO2 and YSZ thin films were successfully synthesized by sol-gel method and deposited by dip coating in different substrates. ZrO2 films were prepared as coatings for Zircaloy rod spacer grids used for critical heat flux tests, while YSZ films were synthesized for non-electrolyte supported SOFC. Highly dense films with good adherence to substrates and thickness above 200 nm were achieved by this method. In pure ZrO2 films, the tetragonal phase was retained even at room temperature due to nanoscale grain size, while the addition of Y2O3 in YSZ films with as low as 3 mol% of dopant was enough to retain the tetragonal phase. High symmetry phase stabilization enhances the desired properties in both materials for the proposed applications. 5. Acknowledgements This work was supported by CAB - CNEA (Centro Atómico Bariloche – Comisión Nacional de Energía Atómica), CONICET (Consejo Nacional de Investigaciones Científicas y Técnicas), and ANPCyT (Agencia Nacional de Promoción de Ciencia y Tecnología). 6. References [1] Y. W. Chen, J. Moussi, J. L. Drury, J. C. Wataha, Expert Review of Medical Devices 13 (2016) 945-963. [2] Mahato, Neelima; Banerjee, Amitava; Gupta, Alka; Omar, Shobit; Balani, Kantesh, Prog. Mater. Sci. 72 (2015) 141-337. [3] R.C. Garvie, J. Phys. Chem. 69 (1965)1238. [4] E.P. Butler, Mater. Sci. Technol. 1 (1985) 417-432. [5] A. Tarancón, Energies 2 (2009) 1130-1150. [6] P. Melnikov, V. A. Nascimento, L. Z. Z. Consolo, A. F. Silva, J.Therm. Anal. Calorim. 111 (2013), 115-119. [7] P. Scherrer, Gottinger Nachrichten 2 (1918) 98-100. [8] N-L. Wu, T-F. Wu, I. A. Rusakova J. Mater. Res. 16 (2001) 666. [9] D. G. Lamas, A. M. Rosso, M. Suarez Anzorena, A. Fernández, M. G. Bellino, M. D. Cabezas, N. E. Walsöe de Reca, A. F. Craievich, Scr. Mater. 55 (2006) 553–556. [10] T. K. Gupta, R. B. Grekila, E. C. Subbarao, J. Electrochem. Soc. 128 (1981) 929-931.