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Deposition and characterization of ceria layers using the MOCVD method
Accepted Manuscript Deposition and characterization of ceria layers using the MOCVD method Agata Sawka, Andrzej Kwatera, Paweł Andreasik PII: DOI: Reference:
S0167-577X(17)30894-7 http://dx.doi.org/10.1016/j.matlet.2017.06.012 MLBLUE 22724
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
9 December 2016 23 May 2017 3 June 2017
Please cite this article as: A. Sawka, A. Kwatera, P. Andreasik, Deposition and characterization of ceria layers using the MOCVD method, Materials Letters (2017), doi: http://dx.doi.org/10.1016/j.matlet.2017.06.012
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Deposition and characterization of ceria layers using the MOCVD method Agata Sawka1, Andrzej Kwatera, Paweł Andreasik AGH University of Science and Technology, Faculty of Materials Science and Ceramics, Mickiewicza Ave. 30, 30-059 Krakow, Poland
Abstract Cerium oxide layers on quartz glass substrates were synthesized by means of the MOCVD method using cerium acetyloacetonate or cerium tetramethylheptanedionate in the temperature range 300 to 900 0C. Argon and air were used as carrier gases, while air also provided the oxidizing atmosphere for carbon removal. Temperature of gases was maintained close to evaporator temperature. Parameters of the synthesis process were fixed to assure the value of extended criterion Grx/Rex2<0.01. The microstructure of these layers for selected samples was examined by scanning electron microscopy. X-ray diffraction analysis was also performed. When the layer synthesis process is realized in air, then CeO2 layers were obtained. In the presence of Ar, CeO2-x layers were deposited and they contained carbon (in the form of clusters), which inhibits grain growth in these layers. The deposited layers were nanocrystalline. Keywords: MOCVD method, nanocrystalline SOFC electrolyte
1. Introduction Cerium oxide doped with rare earth metal oxides (eg. Sm2O3) exhibits high ionic conductivity. Therefore, it is an attractive material for manufacturing SOFC (Solid Oxide Fuel Cell) electrolyte materials [1]. The ionic conductivity of SDC (samaria doped ceria) is significantly higher than the currently used YSZ (yttria stabilized 1
Corresponding author at AGH University of Science and Technology, Faculty of Materials Science and Ceramics, 30 Mickiewicza Av., 30-059 Krakow, Poland; tel. +48 12 617 39 03 E-mail address: [email protected]
zirconia) electrolyte. Its disadvantage is, however, a lack of resistance against a reducing atmosphere, which is present at the anode side (in such an atmosphere it becomes an n-type semiconductor). It is resistant to oxidizing atmosphere at the cathode side. Therefore, it seems that it can be used in a thin-film two-layer electrolyte consisting of SDC and YSZ, wherein the SDC layer would be placed on the cathode side, and the YSZ layer on the anode side [2]. Elaboration of a method for preparing this composite electrolyte requires initial recognition of synthesis conditions of oxides This paper presents the results of studies on the synthesis of cerium oxide layers by MOCVD method using Ce(acac)3 and Ce(tmhd)4 as precursors. It should be noted that the MOCVD method is a modification of the CVD method. Metalorganic reactants used in this method are more reactive than inorganic reactants in a traditional CVD process. The use of more reactive metalorganic reactants makes it possible to obtain layers at significantly lower temperatures than in the CVD process. Hence non-porous, amorphous and nanocrystalline layers can be obtained in the MOCVD process, which is impossible in conventional CVD process. The higher the synthesis temperature, the larger the amount of grains (for example micrograins). According to authors, electrolytes should be nanocrystalline, because ionic conductivity of O2- ions in an electrolyte takes place due to diffusion of these ions from the cathode to the anode. O2- ion diffusion is facilitated by the presence of an amorphous phase at the grain boundary. The smaller the grains the higher the amorphous phase content and the higher the ionic conductivity. Therefore, it is possible to decrease the temperature of electrolyte operation by reducing the grain size. It should also be noted that this is currently the only method that allows for obtaining non-porous, amorphous or nanocrystalline bulk samples (for example
electrolytes). The obtained research results will be useful for manufacturing electrolytes capable of operating at low temperatures, which enables the use of cheaper materials for other SOFC elements and reduces the costs of their production.
2. Experimental Cerium oxide layers were synthesized on the inner surfaces of quartz glass tubes (фinner=14 mm, L=25 mm) using the MOCVD method. The details of the experimental set-up used for the synthesis are described elsewhere [3]. The use of quartz glass as a substrate instead of opaque SOFC cathode is an advantageous feature due to its transparency, which makes it easy to observe the thickness distribution of the layers synthesized in different conditions on the basis of interference colors [4] and the possible occurrence of homogeneous nucleation. This is a disadvantageous process. It causes turbidity of layers, which can be easy observed. In the first part of the work cerium acetyloacetonate (Sigma Aldrich) was used as a precursor and then Ce(tmhd)4 (ABCR GmbH&CoKG) for further study. Cerium oxide layers were synthesized in argon and air. Gas pressure in the CVD reactor changed from 10 to 13x10 4 Pa. Argon flow rate was equal to about 5.56x10 -7 Nm3/s and air to 2.78 x10-6 – 2.28x10-5 Nm3/s. Synthesis temperature changed in the range of 300900oC, and evaporation temperature of reactants changed from 60 to 250 oC. Other synthesis variables were selected in such a way that the value of the Grx/Rex2 expression at both the reactor inlet and outlet did not exceed 0.01. This allowed for dense and transparent layers (without homogeneous nucleation) with uniform thickness to be obtained, which indicated that the synthesis process was carried out under a laminar gas flow.
The Grx/Rex2 criterion [5,6] is given by the following equation:
dg∆Tβρ ∞ T∞ Gr x = 2 2] Re x 2Tav ( x ) [( p ∞ − p x ) + 1 ρ ∞ U ∞ 2
where: x – distance from the gas inflow point; values of gas parameters at this distance are subscripted x, ∞ - gas parameter values at x=0 or above the diffusion layer, d – reactor diameter, g – acceleration due to gravity, ∆T – difference between the substrate temperature TS and that of the gas in the bulk, T∞ β - volume thermal expansion coefficient of the gas, p – total static gas pressure in the reactor, p∞-px=∆p – gradient of the static gas pressure along the distance x ρ – gas density, U – average velocity of gas molecules, 1/2ρU2 – the total dynamic gas pressure Tav(x) – average temperature in the gas stream passing the cross section of the reactor S at the distance x from the inflow point, which can be measured [7] or evaluated from:
[
(
)]
T av ( x ) = T av(W ) S w ( x ) + T ∞ S R − S w( x ) / S R
Tav(w) - average gas temperature in the boundary thermal layer evaluated from: T av ( w ) = 0.68T S + 0.32T ∞
Sw(x) – magnitude of the cross-section of the reactor occupied by the boundary thermal layer; the thickness of which was determined from [5]: δ T ( x) =
0.977 δ x 3 Pr
where: δx – thickness of the boundary layer which can be assessed using the empirical relations given in [7] or from the measurement of the gas velocity distribution in x of a given reactor [8], Pr – Prandl’s number of the reaction mixture, SR – cross-sectional area of the reactor
The obtained layers were investigated using a scanning electron microscope JEOL JSM 5400 equipped with an EDS microanalyser (EDS, LINK ISIS 300 from Oxford Instrument), Philips CM 20 transmission microscope with EDS, and an X-ray powder diffractometer X'Pert Pro from Philips.
3. Results and discussion Cerium oxide layers synthesized in air using both cerium acetyloacetonate (Ce(acac)3) and cerium tetramethylheptanedionate (Ce(tmhd)4) were determined as transparent, uniform in thickness and exhibited good adhesion to the substrate. As the sample was broken in half, the layer split together with the substrate. Fig. 1 illustrates the surface of the layer obtained using (Ce(acac)3).
Fig. 1. SEM image of cerium oxide layer surface synthesized at 800oC in air using Ce(acac)3. Synthesis time: 30 min.
Oval shaped crystallites are visible on the surface of the obtained layer. Their maximum size is about 1000 nm. For comparison, the surface of the layer synthesized using Ce(tmhd)4 is presented in Fig. 2. The crystallites, also round in shape, are at least three times smaller than those shown in Fig. 1.
Fig. 2. SEM image of the surface of cerium oxide layer synthesized at 800oC in air using aCe(tmhd)4 precursor. Synthesis time: 30 min.
Fig. 3 illustrates the fracture cross-section of the layer and the substrate along with EDS linear analysis. This layer was deposited using Ce(tmhd)4 in argon at 800oC for 30 min. The results shown in Fig. 3 indicate that the layer contains cerium and oxygen. Due to the dark color of this sample, it is likely that the layer contains carbon as a result of Ce(tmhd)4 pyrolysis. It was observed that the layers synthesized in Ar were thinner, their color was darker, and in case of thicker layers even black. This indicates that carbon is not present in these layers as single atoms but in the form of clusters, in which π and σ bonds exist between carbon atoms (as in the case of graphite). The π bonds enable the absorption of visible light. It seems that due to the nonstoichiometry of cerium oxide, the presence of carbon in the layers may cause the formation of CeO2-x.
Therefore, it is expected that the size of CeO2-x grains should be larger than those of CeO2.
Fig. 3. SEM microphotograph of the fracture cross-section of cerium oxide layer synthesized from Ce(tmhd)4 at 800oC in argon and linear analysis of its chemical composition (EDS). Synthesis time: 30 min.
Results of X-ray diffraction studies are presented in Fig. 4. This analysis proves that the cerium oxide layer synthesized in air from Ce(tmhd)4 at temperatures above 400oC is crystalline. From Fig. 4. it can be seen that the XRD curve of the CeO2 layer synthesized at 400 oC in air is similar to that for CeO2-x obtained at 600 oC in argon. While the XRD curve pertaining to the CeO2 layer deposited at 500oC indicates that layer is more crystallized than the CeO2 layer obtained at 400 oC in air and CeO2-x synthesized at 600 oC. From the above it follows that the type of carrier gas used, as well as synthesis temperature, have an impact on the crystallization process.
Fig. 4. XRD patterns obtained from CeO2 layers synthesized using Ce(tmhd)4 in air and argon.
This process is easier to perform in air than in argon, which confirms the earlier discussion on this problem.
4. Conclusions The investigations carried out in this work indicate that the deposition of dense, smooth, transparent (without pores) and well adherent to the substrate cerium oxide layers on quartz glass tubes using the MOCVD method is possible when both Ce(acac)3 and Ce(tmhd)4 are used as reactants. It is expected that the layer synthesized in air exhibits the chemical formulae CeO2, whereas the layer deposited in argon can be written as CeO2-x. This may be a result of a carbon formation within the layer during the pyrolysis of metalorganic compounds, i.e. Ce(acac)3 and Ce(tmhd)4 in Ar. The reaction only occurs at the interface between CeO2 and carbon and it causes the reduction of CeO2 to CeO2-x at this interface. The higher the carbon content in the layer, the higher the CeO2-x formation. The presence of carbon in the layer also hinders the diffusion process, which is necessary for the occurrence of recrystallization in cerium oxides between CeO2 and CeO2-x grains. Cerium oxide layers exhibit a fine-crystalline
microstructure when Ce(tmhd)4 is used as reactant. Using the same amount of Ce(acac)3 in the synthesis process performed at the same temperature leads to the formation of large crystallites. Ce(tmhd)4 is also characterized by a higher vapour pressure leading to a higher efficiency of the MOCVD process. Moreover, it exhibits a higher stability during its longer storage. It seems that the presence of carbon may affect grain size in the CeO2-x layers.
The results presented in this paper have been obtained within project no 3 T08D 046 28 supported by the Polish Scientific Research Committee References [1] N. Maffei, A. K. Kuriakose, Solid oxide fuel cells of ceria-doped with gadolinium and praseodymium, Solid State Ionics 107 (1998) 67-71 [2] A. Sawka, A. Kwatera, Deposition of Sm2O3 –doped CeO2 layers using the MOCVD method, Ceramics Int. 44 (2016) 1446-1452 [3] A. Kwatera, Uniform Thin CVD Layers of High Density on the Inner Surface of Tube Shaped Substrates, Thin Solid Films 204 (1991) 313-339 [4] C.E. Morosanu, The preparation, characterization and applications of silicon nitride thin films, Thin Solid Films 65 (1980) 171-208 [5] A. Kwatera, Modelling of chemical vapour deposition (CVD) in the regime controlled by the diffusion of reactants to the substrate. Scientific Bulletins of Stanislaw Staszic Academy of Mining and Metallurgy, Ceramics. Bulletin 62, Krakow 1991 (habilitation thesis, in Polish) [6] A. Kwatera, Thin CVD layers of carbon doped silicon nitride on quartz glass, Ceramics Int. 15 (1989) 65-72 [7] J. Van de Ven, G.M.J. Rutten Raaijmakers, J. Giling, Gas phase depletion and flow dynamics in horizontal MOCVD reactor. J. of Cryst. Growth 76 (1986) 352-372 [8] J.M. Coulson, J.F. Richardson, Chemical engineering.Vol. 1 Pergamon Press, London 1957
Deposition and characterization of ceria layers using the MOCVD method • Recognition of the synthesis conditions of cerium oxide layers by MOCVD method. • Synthesis of ceria from Ce(acac)3 or Ce(tmhd)4 in Ar or air. • Influence of the synthesis conditions on their structure and composition. • The relationship between chemical composition of reactants and obtained layers.
S0167-577X(17)30894-7 http://dx.doi.org/10.1016/j.matlet.2017.06.012 MLBLUE 22724
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
9 December 2016 23 May 2017 3 June 2017
Please cite this article as: A. Sawka, A. Kwatera, P. Andreasik, Deposition and characterization of ceria layers using the MOCVD method, Materials Letters (2017), doi: http://dx.doi.org/10.1016/j.matlet.2017.06.012
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Deposition and characterization of ceria layers using the MOCVD method Agata Sawka1, Andrzej Kwatera, Paweł Andreasik AGH University of Science and Technology, Faculty of Materials Science and Ceramics, Mickiewicza Ave. 30, 30-059 Krakow, Poland
Abstract Cerium oxide layers on quartz glass substrates were synthesized by means of the MOCVD method using cerium acetyloacetonate or cerium tetramethylheptanedionate in the temperature range 300 to 900 0C. Argon and air were used as carrier gases, while air also provided the oxidizing atmosphere for carbon removal. Temperature of gases was maintained close to evaporator temperature. Parameters of the synthesis process were fixed to assure the value of extended criterion Grx/Rex2<0.01. The microstructure of these layers for selected samples was examined by scanning electron microscopy. X-ray diffraction analysis was also performed. When the layer synthesis process is realized in air, then CeO2 layers were obtained. In the presence of Ar, CeO2-x layers were deposited and they contained carbon (in the form of clusters), which inhibits grain growth in these layers. The deposited layers were nanocrystalline. Keywords: MOCVD method, nanocrystalline SOFC electrolyte
1. Introduction Cerium oxide doped with rare earth metal oxides (eg. Sm2O3) exhibits high ionic conductivity. Therefore, it is an attractive material for manufacturing SOFC (Solid Oxide Fuel Cell) electrolyte materials [1]. The ionic conductivity of SDC (samaria doped ceria) is significantly higher than the currently used YSZ (yttria stabilized 1
Corresponding author at AGH University of Science and Technology, Faculty of Materials Science and Ceramics, 30 Mickiewicza Av., 30-059 Krakow, Poland; tel. +48 12 617 39 03 E-mail address: [email protected]
zirconia) electrolyte. Its disadvantage is, however, a lack of resistance against a reducing atmosphere, which is present at the anode side (in such an atmosphere it becomes an n-type semiconductor). It is resistant to oxidizing atmosphere at the cathode side. Therefore, it seems that it can be used in a thin-film two-layer electrolyte consisting of SDC and YSZ, wherein the SDC layer would be placed on the cathode side, and the YSZ layer on the anode side [2]. Elaboration of a method for preparing this composite electrolyte requires initial recognition of synthesis conditions of oxides This paper presents the results of studies on the synthesis of cerium oxide layers by MOCVD method using Ce(acac)3 and Ce(tmhd)4 as precursors. It should be noted that the MOCVD method is a modification of the CVD method. Metalorganic reactants used in this method are more reactive than inorganic reactants in a traditional CVD process. The use of more reactive metalorganic reactants makes it possible to obtain layers at significantly lower temperatures than in the CVD process. Hence non-porous, amorphous and nanocrystalline layers can be obtained in the MOCVD process, which is impossible in conventional CVD process. The higher the synthesis temperature, the larger the amount of grains (for example micrograins). According to authors, electrolytes should be nanocrystalline, because ionic conductivity of O2- ions in an electrolyte takes place due to diffusion of these ions from the cathode to the anode. O2- ion diffusion is facilitated by the presence of an amorphous phase at the grain boundary. The smaller the grains the higher the amorphous phase content and the higher the ionic conductivity. Therefore, it is possible to decrease the temperature of electrolyte operation by reducing the grain size. It should also be noted that this is currently the only method that allows for obtaining non-porous, amorphous or nanocrystalline bulk samples (for example
electrolytes). The obtained research results will be useful for manufacturing electrolytes capable of operating at low temperatures, which enables the use of cheaper materials for other SOFC elements and reduces the costs of their production.
2. Experimental Cerium oxide layers were synthesized on the inner surfaces of quartz glass tubes (фinner=14 mm, L=25 mm) using the MOCVD method. The details of the experimental set-up used for the synthesis are described elsewhere [3]. The use of quartz glass as a substrate instead of opaque SOFC cathode is an advantageous feature due to its transparency, which makes it easy to observe the thickness distribution of the layers synthesized in different conditions on the basis of interference colors [4] and the possible occurrence of homogeneous nucleation. This is a disadvantageous process. It causes turbidity of layers, which can be easy observed. In the first part of the work cerium acetyloacetonate (Sigma Aldrich) was used as a precursor and then Ce(tmhd)4 (ABCR GmbH&CoKG) for further study. Cerium oxide layers were synthesized in argon and air. Gas pressure in the CVD reactor changed from 10 to 13x10 4 Pa. Argon flow rate was equal to about 5.56x10 -7 Nm3/s and air to 2.78 x10-6 – 2.28x10-5 Nm3/s. Synthesis temperature changed in the range of 300900oC, and evaporation temperature of reactants changed from 60 to 250 oC. Other synthesis variables were selected in such a way that the value of the Grx/Rex2 expression at both the reactor inlet and outlet did not exceed 0.01. This allowed for dense and transparent layers (without homogeneous nucleation) with uniform thickness to be obtained, which indicated that the synthesis process was carried out under a laminar gas flow.
The Grx/Rex2 criterion [5,6] is given by the following equation:
dg∆Tβρ ∞ T∞ Gr x = 2 2] Re x 2Tav ( x ) [( p ∞ − p x ) + 1 ρ ∞ U ∞ 2
where: x – distance from the gas inflow point; values of gas parameters at this distance are subscripted x, ∞ - gas parameter values at x=0 or above the diffusion layer, d – reactor diameter, g – acceleration due to gravity, ∆T – difference between the substrate temperature TS and that of the gas in the bulk, T∞ β - volume thermal expansion coefficient of the gas, p – total static gas pressure in the reactor, p∞-px=∆p – gradient of the static gas pressure along the distance x ρ – gas density, U – average velocity of gas molecules, 1/2ρU2 – the total dynamic gas pressure Tav(x) – average temperature in the gas stream passing the cross section of the reactor S at the distance x from the inflow point, which can be measured [7] or evaluated from:
[
(
)]
T av ( x ) = T av(W ) S w ( x ) + T ∞ S R − S w( x ) / S R
Tav(w) - average gas temperature in the boundary thermal layer evaluated from: T av ( w ) = 0.68T S + 0.32T ∞
Sw(x) – magnitude of the cross-section of the reactor occupied by the boundary thermal layer; the thickness of which was determined from [5]: δ T ( x) =
0.977 δ x 3 Pr
where: δx – thickness of the boundary layer which can be assessed using the empirical relations given in [7] or from the measurement of the gas velocity distribution in x of a given reactor [8], Pr – Prandl’s number of the reaction mixture, SR – cross-sectional area of the reactor
The obtained layers were investigated using a scanning electron microscope JEOL JSM 5400 equipped with an EDS microanalyser (EDS, LINK ISIS 300 from Oxford Instrument), Philips CM 20 transmission microscope with EDS, and an X-ray powder diffractometer X'Pert Pro from Philips.
3. Results and discussion Cerium oxide layers synthesized in air using both cerium acetyloacetonate (Ce(acac)3) and cerium tetramethylheptanedionate (Ce(tmhd)4) were determined as transparent, uniform in thickness and exhibited good adhesion to the substrate. As the sample was broken in half, the layer split together with the substrate. Fig. 1 illustrates the surface of the layer obtained using (Ce(acac)3).
Fig. 1. SEM image of cerium oxide layer surface synthesized at 800oC in air using Ce(acac)3. Synthesis time: 30 min.
Oval shaped crystallites are visible on the surface of the obtained layer. Their maximum size is about 1000 nm. For comparison, the surface of the layer synthesized using Ce(tmhd)4 is presented in Fig. 2. The crystallites, also round in shape, are at least three times smaller than those shown in Fig. 1.
Fig. 2. SEM image of the surface of cerium oxide layer synthesized at 800oC in air using aCe(tmhd)4 precursor. Synthesis time: 30 min.
Fig. 3 illustrates the fracture cross-section of the layer and the substrate along with EDS linear analysis. This layer was deposited using Ce(tmhd)4 in argon at 800oC for 30 min. The results shown in Fig. 3 indicate that the layer contains cerium and oxygen. Due to the dark color of this sample, it is likely that the layer contains carbon as a result of Ce(tmhd)4 pyrolysis. It was observed that the layers synthesized in Ar were thinner, their color was darker, and in case of thicker layers even black. This indicates that carbon is not present in these layers as single atoms but in the form of clusters, in which π and σ bonds exist between carbon atoms (as in the case of graphite). The π bonds enable the absorption of visible light. It seems that due to the nonstoichiometry of cerium oxide, the presence of carbon in the layers may cause the formation of CeO2-x.
Therefore, it is expected that the size of CeO2-x grains should be larger than those of CeO2.
Fig. 3. SEM microphotograph of the fracture cross-section of cerium oxide layer synthesized from Ce(tmhd)4 at 800oC in argon and linear analysis of its chemical composition (EDS). Synthesis time: 30 min.
Results of X-ray diffraction studies are presented in Fig. 4. This analysis proves that the cerium oxide layer synthesized in air from Ce(tmhd)4 at temperatures above 400oC is crystalline. From Fig. 4. it can be seen that the XRD curve of the CeO2 layer synthesized at 400 oC in air is similar to that for CeO2-x obtained at 600 oC in argon. While the XRD curve pertaining to the CeO2 layer deposited at 500oC indicates that layer is more crystallized than the CeO2 layer obtained at 400 oC in air and CeO2-x synthesized at 600 oC. From the above it follows that the type of carrier gas used, as well as synthesis temperature, have an impact on the crystallization process.
Fig. 4. XRD patterns obtained from CeO2 layers synthesized using Ce(tmhd)4 in air and argon.
This process is easier to perform in air than in argon, which confirms the earlier discussion on this problem.
4. Conclusions The investigations carried out in this work indicate that the deposition of dense, smooth, transparent (without pores) and well adherent to the substrate cerium oxide layers on quartz glass tubes using the MOCVD method is possible when both Ce(acac)3 and Ce(tmhd)4 are used as reactants. It is expected that the layer synthesized in air exhibits the chemical formulae CeO2, whereas the layer deposited in argon can be written as CeO2-x. This may be a result of a carbon formation within the layer during the pyrolysis of metalorganic compounds, i.e. Ce(acac)3 and Ce(tmhd)4 in Ar. The reaction only occurs at the interface between CeO2 and carbon and it causes the reduction of CeO2 to CeO2-x at this interface. The higher the carbon content in the layer, the higher the CeO2-x formation. The presence of carbon in the layer also hinders the diffusion process, which is necessary for the occurrence of recrystallization in cerium oxides between CeO2 and CeO2-x grains. Cerium oxide layers exhibit a fine-crystalline
microstructure when Ce(tmhd)4 is used as reactant. Using the same amount of Ce(acac)3 in the synthesis process performed at the same temperature leads to the formation of large crystallites. Ce(tmhd)4 is also characterized by a higher vapour pressure leading to a higher efficiency of the MOCVD process. Moreover, it exhibits a higher stability during its longer storage. It seems that the presence of carbon may affect grain size in the CeO2-x layers.
The results presented in this paper have been obtained within project no 3 T08D 046 28 supported by the Polish Scientific Research Committee References [1] N. Maffei, A. K. Kuriakose, Solid oxide fuel cells of ceria-doped with gadolinium and praseodymium, Solid State Ionics 107 (1998) 67-71 [2] A. Sawka, A. Kwatera, Deposition of Sm2O3 –doped CeO2 layers using the MOCVD method, Ceramics Int. 44 (2016) 1446-1452 [3] A. Kwatera, Uniform Thin CVD Layers of High Density on the Inner Surface of Tube Shaped Substrates, Thin Solid Films 204 (1991) 313-339 [4] C.E. Morosanu, The preparation, characterization and applications of silicon nitride thin films, Thin Solid Films 65 (1980) 171-208 [5] A. Kwatera, Modelling of chemical vapour deposition (CVD) in the regime controlled by the diffusion of reactants to the substrate. Scientific Bulletins of Stanislaw Staszic Academy of Mining and Metallurgy, Ceramics. Bulletin 62, Krakow 1991 (habilitation thesis, in Polish) [6] A. Kwatera, Thin CVD layers of carbon doped silicon nitride on quartz glass, Ceramics Int. 15 (1989) 65-72 [7] J. Van de Ven, G.M.J. Rutten Raaijmakers, J. Giling, Gas phase depletion and flow dynamics in horizontal MOCVD reactor. J. of Cryst. Growth 76 (1986) 352-372 [8] J.M. Coulson, J.F. Richardson, Chemical engineering.Vol. 1 Pergamon Press, London 1957
Deposition and characterization of ceria layers using the MOCVD method • Recognition of the synthesis conditions of cerium oxide layers by MOCVD method. • Synthesis of ceria from Ce(acac)3 or Ce(tmhd)4 in Ar or air. • Influence of the synthesis conditions on their structure and composition. • The relationship between chemical composition of reactants and obtained layers.