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Deposition of Sm2O3-doped CeO2 layers using the MOCVD method
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CERAMICS INTERNATIONAL
Ceramics International 42 (2016) 1446–1452 www.elsevier.com/locate/ceramint
Deposition of Sm2O3-doped CeO2 layers using the MOCVD method Agata Sawkan, Andrzej Kwatera AGH University of Science and Technology, Faculty of Materials Science and Ceramics, Al. Mickiewicza 30, 30-059 Krakow, Poland Received 13 May 2015; received in revised form 9 September 2015; accepted 16 September 2015 Available online 26 September 2015
Abstract Sm2O3-doped CeO2 layers were synthesized on the inner surfaces of tube shaped quartz glass substrates using the MOCVD method. Ce(tmhd)4 and Sm(tmhd)3 were used as precursors. Argon and air were the carrier gases. Air was also a source of oxygen, necessary for the elimination of carbon (a solid by-product of the reactant pyrolysis). The molar content of Sm(tmhd)3 in the reactant mixture was: 0.1; 0.2; 0.3. The temperature of the synthesis was maintained at 600–800 1C. Such a low synthesis temperature made possible the deposition of amorphous and nanocrystalline layers. The higher the synthesis temperature, the greater the content of the crystalline phase and the larger the crystallites. The obtained layers were analysed by the scanning electron microscopy combined with the EDS and X-ray analyses. This work is a part of research on amorphous and nanocrystalline composite electrolyte CeO2 þSm2O3/ZrO2 þ Y2O3 for solid oxide fuel cells (SOFC). & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: MOCVD method; Nanocrystalline SOFC electrolyte
1. Introduction CeO2-based materials could be used as an electrolyte in oxide fuel cells (SOFC-Solid Oxide Fuel Cells) due to their high ionic conductivity at relatively low temperatures of 500–600 1C. However, they are not resistant to the reducing atmosphere occurring on the anode side [1], and, therefore, despite the many advantageous features have not found yet any practical application. As a result of a contact of CeO2 with the fuel on the anode side, a reduction of Ce4 þ –Ce3 þ ions occurs, and the material also exhibits electron conductivity, which diminishes the cell efficiency. Microcrystalline yttria-stabilized zirconia (YSZ) used currently as electrolyte in SOFC exhibits similar ionic conduction at the operation temperature of about 1000 1C. This electrolyte is obtained by sintering powders at a temperature of 1450 1C [2]. Such high temperatures-both of the preparation and the operation of the electrolyte-require the use of expensive materials for other n
Corresponding author. E-mail addresses: [email protected] (A. Sawka), [email protected] (A. Kwatera). http://dx.doi.org/10.1016/j.ceramint.2015.09.089 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
components of the cell. Consequently, cost of cell manufacturing and energy is high. It is believed that this material could be replaced by CeO2 doped with e.g. Sm2O3, obtained at low temperatures of 600–800 1C, if the new material does not undergo a reduction on the anode side. The use of the new electrolyte should lead to a significant reduction of manufacturing costs for SOFC cells due to the possibility of using cheaper materials to build other parts of the cell. It should be stressed at this point that such electrolyte has been so far obtained by sintering micro- and nanopowders at 1450 1C [2] to produce a non-porous material and, therefore, it has a microcrystalline structure. It is expected that low temperature of the electrolyte manufacturing and its operation will counter the adverse chemical reactions at the electrolyte–electrode contacts, which in turn should increase the stability of the cell and also reduce the cost of electricity production. This study aimed at implementing the above concept by applying a composite electrolyte layer consisting of samariadoped ceria (SDC) on the cathode side with the oxidizing atmosphere and a layer of YSZ on the anode side with the reducing atmosphere (ZrO2 is resistant to both the reducing and oxidizing atmospheres). Synthesis of the electrolyte was
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carried out at temperatures of 600–800 1C with the use of a MOCVD method (Metal Organic Chemical Vapor Deposition). It is commonly agreed that the method is suitable for synthesis of non-porous, uniform in thickness layers at low temperatures on large substrates including those of complex shapes, especially for materials of high melting point and complex composition. Owing to low synthesis temperatures it is expected that the materials will have nanocrystalline microstructure with presence of an amorphous phase. It is also expected that these electrolytes will be characterized by a high ionic conductivity at low temperatures [3,4]. It should be noted that this study was inspired by the work of Eguchi et al. [1,5] who investigated single or double layer electrolytes consisting of disks of yttria-stabilised zirconia (YSZ), or samaria-doped ceria (SDC) and YSZ, respectively, prepared by sintering of powders at temperature of 1450 1C. The current-voltage characteristics of the YSZ electrolyte indicated that at temperature of 800 1C the voltage was 0.1 V at a current density of 0.4 A/cm2 whilst at 1000 1C the voltage increased to 0.4 V at the same current density. In the case of the SDC-YSZ electrolyte at the same current density, the same voltage of 0.65 V was observed at temperatures of 800 and 1000 1C. At such high temperature, a sintering reaction might have proceed between the components during the prolonged annealing which degraded the electrical properties of the double layer composite [5]. Therefore, synthesis of such composite at low temperatures was attempted so that non-porous, nanocrystalline or amorphous material is obtained. There are three main groups of the layer deposition methods: CVD (Chemical Vapour Deposition), PVD (Physical Vapour Deposition) and sol–gel. CVD seems to be the most useful method for this purpose, especially MOCVD due to the fact that metalorganic compounds used in the process are more reactive than their inorganic equivalent and therefore the layers are obtained at lower temperatures than in the traditional CVD process. The amount of deposited product on the substrate is relatively small in the CVD method, particularly in the process controlled by mass diffusion to the substrate, and the diffusion process leads to the growth of non-porous layers owing to the transport of mass in the form of single atoms and small clusters. At low temperatures, the deposited layer may be amorphous or nanocrystalline. The method is suitable for the synthesis of layers made of materials with small diffusion coefficients (materials with a large share of covalent binding). The method allows dense and uniform in thickness layers to be obtained on flat or curved surfaces (for example the inner surfaces of tubes) and on the substrates with large dimensions. It should be noted that the deposited layers may be porous, when a homogeneous nucleation process occurs during their growth. In the case of PVD methods, significantly larger clusters than in the CVD process are transported to the substrate and then the deposited layer where they should be distributed in the form of single atoms by surface diffusion processes in order to obtain a dense layer before the deposition of a next cluster. This is possible when diffusion coefficients of the deposited material are high which is the case for ceramic materials with high share of ionic binding. It is not possible to obtain non-porous layers of materials of a high share of covalent bonding.
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In the sol–gel method, its final step consists in sintering of layers to obtain non-porous and smooth layers. This method is suitable for materials characterized by high or small diffusion coefficients, but then with the use of liquid phase. Owing to the aspects discussed above, the PVD and sol–gel methods did not seem suitable for the synthesis of non-porous electrolytes from the materials in question at low temperatures. The preparation of thin MOCVD (CVD) layers could be considerably facilitated if the process is described by a mathematical equation correlating the process parameters with the properties of a product. The authors have studied modelling of the CVD (MOCVD) deposition of thin layers for long time (eg. [6–16]) and they seek in this study to apply the knowledge gained to the practical syntheses. In this study, the synthesis of the layers was carried below the critical temperature T* i.e. in the temperature range in which the synthesis is controlled by rate of the reaction on the substrate; above this temperature, the CVD (MOCVD) process is controlled by the reactant diffusion from the gas phase to the substrate [7] using Ce(tmhd)4 and Sm(tmhd)3 as precursors. It should be noticed that Jiang et al. [17] deposited SDC layers on small size substrates (NiO þ YSZ and α-Al2O3 disks, ф13 mm) using AAMOCVD method. However, the layers obtained at temperatures of 400–500 1C were amorphous and porous. Crystalline layers were obtained above 530 1C. The layers synthesized at 600–650 1C were characterized by presence of elongated shape crystallites and on their surface fine grains of powders were visible. These powders were formed in the gas phase as a result of homogeneous nucleation process. The deposited layers were rough and had poor adhesion to the substrate. 2. Experimental details A mentioned the SDC layers were synthesized on the inner surfaces of a quartz glass tube (ф 15 mm, L¼ 25 mm) by the MOCVD method using Ce(tmhd)4 (ABCR, Germany) and Sm (tmhd)3 (Sigma Aldrich) as precursors. Air and argon were the carrier gases. The air was also source of oxygen for the oxidation of carbon resulting from the pyrolysis of these reactants. The use of quartz glass instead of not transparent SOFC cathode was due to the following reasons: it does not react with Al2O3 below a temperature of 1100 1C, it is well transparent, and therefore allows for a very quick visual assessment of the obtained layer thickness [18] and thickness distribution. It is also very easy to determine whether during a given synthesis unfavorable homogeneous nucleation process occurred. In the process porous powders formed in the gas phase, which can be deposited on a synthesized layer causing their porosity, thereby lowering the transparency, and its mechanical strength and adhesion. The MOCVD equipment is shown schematically in Ref. [9]. It consists of the following principal elements: supply of gases (cylinders with Ar and air), valves, a tube reactor of 30 mm diameter and 700 mm length with the substrate-placed inside the induction coil of a high frequency generator, a vacuum pump, flowmeters and an evaporator with a gradual temperature
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distribution allowing a simultaneous evaporation of two reactants of different evaporation temperatures: Ce(tmhd)4 and Sm(tmhd)3. All the elements were connected with plastic tubing, rendering the equipment gas tight. The molar content of Sm(tmhd)3 in the gas mixture with Ce(tmhd)4 was: 0.1; 0.2; 0.3. Ar (flow of 0.09 mol/h) and air (flow of 0.4–8 mol/h) were heated to attain the evaporation temperatures of the reactants, lower and higher, respectively. Ar was introduced into the evaporator and air into the gas mixture at the outlet from the evaporator. Substrate temperature was about 600–800 1C and synthesis time about 0.5 h. Static pressure and velocity of gases 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 dense and transparent layers (without homogeneous nucleation), uniform in thickness, to be obtained, which indicated that the synthesis was carried out under a laminar gas flow [8,10]. The Grx/Rex2 criterion [6] is given by: Grx dgΔTβρ1 T 1 ¼ 2 T avðxÞ p1 px þ 12 ρ1 U 21 Re2x where: x–distance from the gas inflow point; values of gas parameters at this distance are subscripted x, 1–gas parameter values at x¼ 0 or above the diffusion layer, d–reactor diameter, g–acceleration due to gravity, ΔT–difference between the substrate temperature TS and that of gas in the bulk, T1 β-volume thermal expansion coefficient of the gas, p–total static gas pressure in the reactor, p1–px ¼ Δp–the 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 [19] or evaluated from: T avðxÞ ¼ ½T avðwÞ SwðxÞ þ T 1 ðSR SwðxÞ Þ=SR
Tav(w)–average gas temperature in the boundary thermal layer evaluated from: T avðwÞ ¼ 0; 68T S þ 0; 32T 1 Sw(x)–magnitude of the cross-section of the reactor occupied by the boundary thermal layer; the thickness of which was determined from [10]: 0; 977 δx ffiffiffiffiffi δTðxÞ ¼ p 3 Pr where: δx–thickness of the boundary layer which can be assessed using the empirical relations given in [19] or from the measurement of the gas velocity distribution in x of a given reactor [20],
Pr–Prandl’s number of the reaction mixture SR–cross-sectional area of the reactor Evaporation temperatures of the reactants were established from their weights, assuming the same time of complete evaporation for the two reactants. Therefore, when the mass of a reactant increased or decreased, it was necessary to increase or decrease the evaporation temperature, respectively. Such procedures allow a constant molar ratio of the reactants in the gas phase to be obtained in spite of decreasing weight of the reactants during the evaporation. Therefore, it can be expected that if the synthesis of the layers is conducted in the mass diffusion controlled process [8,9,11], one gets a constant molar ratio of products (CeO2 and Sm2O3) in the layers synthesized at different temperatures. In a such process, pressure of the i-th component pi(s) on the substrate is close to zero, which may be obtained also at low temperatures for a sufficiently high dilution of the reactants in the carrier gas. Therefore, there should be no competition for adsorption of various reactants on the substrate in various synthesis temperatures. Consequently, the molar ratio of products should be constant. The resulting layers were investigated using a scanning electron microscope SEM NANO NOVA 200 from FEI EUROPE COMPANY with an EDS microanalyser from the EDAX EDS company, and an X-ray powder diffractometer X'Pert Pro from Philips. 3. Results and discussion The samaria-doped ceria layers obtained in the temperature range of 600–800 1C on substrates made of quartz glass for small molar contents of Sm(tmhd)3 in the reactant mixture (0.1; 0.2; 0.3) were visually transparent, which indicates that the homogeneous nucleation process did not occur during the layer growth [8]. Studies performed with the use of the scanning electron microscopy showed that the layers were also smooth and nonporous. Results of investigations for selected samples only are presented below. Fig. 1 shows the surface of the layer synthesized at 600 1C (the molar content of Sm(tmhd)3 was 0.1). A quantitative composition obtained with the use of the EDS is presented in Table 1. It can be concluded from Fig. 1 that there are numerous bright fields – probably a nanocrystalline phase-on the layer surface. The maximum grain size is below 50 nm. The darker fields correspond probably to an amorphous phase. Surface of the layer synthesized at 800 1C with the molar content of Sm(tmhd)3 of 0.1 is shown in Fig. 2. This layer is also smooth and non-porous. The data shown in Table 2 indicate that both cerium and samarium are present in the layer. In the case of the layer synthesized at the higher temperature (Fig. 2), the fraction of bright fields is significantly larger than in the case of the sample presented in Fig. 1. It seems that the crystallization of sample (Fig. 2) is more advanced than of sample (Fig. 1). Grain sizes are of the order of nanometers (max. about 100 nm). The results shown in Tables 1 and 2
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Fig. 1. Surface of the SDC layer (the molar content of Sm(tmhd)3–0.1) synthesized at 600 1C.
Table 1 Quantitative EDS analysis for the sample obtained at 600 1C with the molar content of Sm(tmhd)3-0.1 (Fig. 1). Element
Wt%
At%
OK SiK CeL SmL
49.82 43.38 5.71 1.09
66.16 32.82 0.87 0.15
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indicate that, for the molar content of Sm(tmhd)3 in the reactant mixture of 0.1, lowering the synthesis temperature from 800 to 600 1C causes a reduction of the atomic content of Sm in the layer from 0.11 to 0.07 (the molar content of Sm2O3 respectively from 0.055 to 0.035). The data presented in Tables 1 and 2 also indicate that the atomic content of Ce in the layer synthesized at the higher temperature is lower than in the layer synthesized at the lower temperature for the same molar composition of the reactant mixture. It seems that the layer growth probably occurred in the regime controlled by the rate of surface reaction. In this process, a significant differentiation of adsorption of reactants necessary for the synthesis of the layer is possible both as a function of temperature and concentration of the reactants [8]. The layer shown in Fig. 3 was obtained at 600 1C. The molar content of Sm(tmhd)3 was 0.2. Nanoparticles were larger than in the case of the layer synthesized at the same temperature but with the molar content of Sm(tmhd)3 of 0.1 (Fig. 1). Fig. 4 shows a sample synthesized at 800 1C with the molar content of Sm(tmhd)3 of 0.2 and Table 3 a point EDS analysis. Comparing Figs. 2 and 4, it can be also concluded that the increased molar content of Sm(tmhd)3 in the reactant mixture influenced the content of the crystalline phase and the grain size. In the case of the sample presented in Fig. 4, the grain size is larger than in the case of the sample shown in Fig. 2. The layers obtained both at 600 and 800 1C with the same molar content of Sm(tmhd)3 i.e. 0.2 (Figs. 3 and 4) are dense (non-porous) and smooth. As expected the content of samarium in the layers increased with an increase of the Sm(tmhd)3 M content in the reactant mixture from 0.1 to 0.2 (Table 3). The results of microscopic studies of the SDC layers obtained at 600 1C with the molar content of Sm(tmhd)3 of about 0.3 are presented in Fig. 5(a). Fig. 5(b) shows a linear EDS analysis of a fracture in the layer. The deposited layer is smooth, non-porous, uniform in thickness and contains Ce and Sm. An example of a microstructure of the layer synthesized at 800 1C with the same molar content of Sm(tmhd)3 is illustrated in Fig. 6.
Fig. 2. Surface of the SDC layer (the molar content of Sm(tmhd)3–0.1) synthesized at 800 1C.
Table 2 Quantitative EDS analysis for the sample obtained at 800 1C with the molar content of Sm(tmhd)3-0.1 (Fig. 2). Element
Wt%
At%
OK SiK CeL SmL
47.99 43.83 7.18 1.00
64.95 33.80 1.11 0.14
Fig. 3. Surface of the SDC layer (the molar content of Sm(tmhd)3–0.2) synthesized at 600 1C.
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Quantitative EDS analysis for this sample is presented in Table 4. This layer is also smooth and non-porous. In this case, the maximum crystallite size reaches about 300 nm. The content of Sm in the layer (Table 4) is significantly higher than previously (Tables 1–3). Tables 1–4 also show that increasing the molar content of Sm(tmhd)3 in the reactant mixture from 0.1 to
0.3 produced an increase of atomic content of Sm in the layer from 0.11 to 0.21 at the synthesis temperature of 800 1C. The data shown in Tables 1 and 2 demonstrate that the atomic ratios Ce:Sm were different for layers deposited at 600 and 800 1C in spite of a constant molar ratio of reactants maintained in the gas phase. This indicates that the synthesis process was controlled by the reaction rate on the substrate. As the reaction rate in this process is low, a high concentration of reactants adsorbed on the substrate and a competition between the adsorption of Ce(tmhd)4 and Sm(tmhd)3 may occur. This competition is largely dependent on the substrate temperature and the reactant concentration in the gas phase. It should be noted that the critical temperature T* of transition from the process controlled by the reaction rate on the substrate to the process controlled by the reactant diffusion from the gas phase to the substrate can be significantly reduced (even by 200–300 1C) by a high dilution of the reactants in the carrier gas, and/or by reducing the diffusion coefficients of the reactants. The latter effect can be obtained by introducing a
Fig. 4. Surface of the SDC layer (the molar content of Sm(tmhd)3–0.2) synthesized at 800 1C.
Table 3 Quantitative EDS analysis for the sample obtained at 800 1C with the molar content of Sm(tmhd)3 of 0.2 (Fig. 4). Element
Wt%
At%
OK SiK CeL SmL
37.53 53.16 7.63 1.67
54.50 43.98 1.27 0.26
Fig. 6. Surface of the SDC layer (the molar content of Sm(tmhd)3–0.3) synthesized at 800 1C.
Fig. 5. Fracture of the SDC layer synthesized at 600 1C (the molar content of Sm(tmhd)3–0.3) (a), a linear EDS analysis (b).
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large quantity of gas of a density substantially greater than that of air (for example argon) into the carrier gas, in addition to the air necessary for the combustion of carbon produced by pyrolysis of the metalorganic compounds [7,9]. Fig. 7 shows results of the X-ray analysis of the layers deposited at 600 1C (molar content of Sm(tmhd)3–0.2). It can be concluded that there is a crystalline phase in these layers. In summary it should be noted, that non-porous deposited layers of varying grain size were obtained. The lower the synthesis temperature, the smaller the grains (at a temperature of about 800 1C the grains grew even to above 100 nm). At the current stage of research, it is difficult to point to the optimum size of grains for the SOFC electrolyte. Earlier research shows that the layer synthesis at low temperatures (for example the synthesis of amorphous layers) and then their annealing at a strictly defined temperature for a strictly limited period of time is the most suitable way of controlling the microstructure and grain size in the MOCVD method [15,16]. Such investigations are planned to elucidate the problem. It should be also noted that Laukaitis et al. [21] obtained layers with a high growth rate using an e-beam technique (EB PVD). The deposited layers were characterized by small (nano) grains uniform in Table 4 Quantitative EDS analysis for the sample obtained at 800 1C with the molar content of Sm(tmhd)3-0.3 (Fig.6). Element
Wt%
At%
OK SiK CeL SmL
42.49 44.26 10.25 2.99
61.41 36.44 1.69 0.46
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size. Annealing of the layers at temperatures of about 800 1C did not cause any observable grain growth (the authors did not provide, however, the annealing time). From this point of view, this method seems to be very interesting. However, it should be noted that layers deposited by this method had high porosity. The porosity depended on the substrate temperature and electron gun power. Maximum porosity amounted to even approximately 40%, when the substrate temperature was about 100 1C and the electron gun power 3 kW. At the same value of the gun power, a lower porosity of about 7%) was obtained when the substrate temperature was about 500 1C. The substrate temperature increase to about 600 1C caused a porosity increase to 8%. Authors did not give substrate dimensions. Such information is important, because the layer thickness and grain size may be uniform if the substrates have small dimensions (for example: a diameter of 10 mm and the distance from a source of vapours of 250 mm). If the substrates are larger and there is a point source of vapours an effect of glancing angle orientation should occur [22]. It should be also mentioned that ceramic porous materials may be characterized by significantly reduced mechanical strength due to the stress concentration at pores (the Cook – Gordon mechanism). Such material may easily undergo cracking when subjected to thermal stresses occurring in an electrolyte as a consequence of its lasting contact with electrodes characterized by different coefficients of thermal expansion. A very fast transport of the molecular oxygen (O2) from the cathode to the anode takes place through these cracks and numerous pores, which significantly reduces the cell efficiency (O2 ions should be transported). It may also lead to the cell destruction. Because of these aspects, at this stage, the method is not useful for obtaining of the SOFC electrolyte. 4. Conclusions The study has refined understanding of the synthesis of samaria-doped ceria layers in several aspects:
Fig. 7. X-ray diffraction pattern of the SDC layers synthesized at 600 1C (the molar content of Sm(tmhd)3–0.2).
1. Samaria-doped ceria layers may be obtained at temperatures of 600–800 1C using the MOCVD method. 2. The deposited layers are smooth, non-porous and uniform in thickness. 3. The layers synthesized in the above temperature range are nanocrystalline-amorphous. 4. The higher the synthesis temperature, the greater the content of the crystalline phase. 5. Different atomic ratios of Ce to Sm in the layers synthesized at temperatures of 600 and 800 1C indicate that the layer synthesis, at least at the lower temperature, must have occurred in the process controlled by the reaction rate. 6. Increasing the molar content of Sm(tmhd)3 in the reactant mixture leads also to an increase in the molar content of Sm2O3 in the layer. 7. When the synthesis temperature was about 800 1C, the maximum crystallite size changed from about 100 nm-for the molar content of Sm(tmhd)3 of 0.1-to about 300 nm-for the molar content of Sm(tmhd)3–0.3.
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8. The results obtained open perspective for further detailed research on the layer synthesis in the mass diffusion controlled process. 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] K. Eguchi, Ceramic materials containing rare earth oxides for solid oxide fuel cell, J. Alloy. Compd. 250 (1997) 486–491. [2] H. Yahiro, Y. Eguch, K. Eguchi, H. Arai, Oxygen ion conductivity of the ceria-samarium oxide system with fluorite structure, J. Appl. Electrochem. 18 (1988) 527–531. [3] W.S. Hsieh, P. Lin, S.F. Wang, Characteristics of electrolyte supported micro-tubular solid oxide fuel cells with GDC-ScSZ bilayer electrolyte, Int. J. Hydrog. Energy (2014) 3917267–3917274. [4] H. Hidalgo, E. Reguzina, E. Millon, A.-L. Thomann, J. Mathias, C. Boulmer-Leborgme, T. Sauvage, P. Brault, Ytttria-stabilized zirconia thin films deposited by pulsed-laser deposition and magneto sputtering, Surf. Coat. Technol. 205 (2011) 4495–4499. [5] K. Eguchi, N. Akosaka, H. Mitsuyasu, Y. Nanoka, Process of solid state reactions between doped ceria and zirconia, Solid State Ion. 135 (2000) 589–594. [6] A. Kwatera, C.V.D. Thin, layers of carbon doped silicon nitride on quartz glass, Ceram. Int. 15 (1989) 65–72. [7] A. Kwatera, Models of the processes at the substrate surface in the CVD method, Ceram. Int. 17 (1991) 11–23. [8] A. Kwatera, Uniform thin chemically vapour deposited layers of high density on the inner surfaces of tube-shaped substrates, Thin Solid Films 204 (1991) 313–339. [9] A. Kwatera, Carbon doped α-Al2O3 films synthesized on cemented carbide tools by the metal-organic LPCVD technique, Thin Solid Films 200 (1991) 19–32.
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CERAMICS INTERNATIONAL
Ceramics International 42 (2016) 1446–1452 www.elsevier.com/locate/ceramint
Deposition of Sm2O3-doped CeO2 layers using the MOCVD method Agata Sawkan, Andrzej Kwatera AGH University of Science and Technology, Faculty of Materials Science and Ceramics, Al. Mickiewicza 30, 30-059 Krakow, Poland Received 13 May 2015; received in revised form 9 September 2015; accepted 16 September 2015 Available online 26 September 2015
Abstract Sm2O3-doped CeO2 layers were synthesized on the inner surfaces of tube shaped quartz glass substrates using the MOCVD method. Ce(tmhd)4 and Sm(tmhd)3 were used as precursors. Argon and air were the carrier gases. Air was also a source of oxygen, necessary for the elimination of carbon (a solid by-product of the reactant pyrolysis). The molar content of Sm(tmhd)3 in the reactant mixture was: 0.1; 0.2; 0.3. The temperature of the synthesis was maintained at 600–800 1C. Such a low synthesis temperature made possible the deposition of amorphous and nanocrystalline layers. The higher the synthesis temperature, the greater the content of the crystalline phase and the larger the crystallites. The obtained layers were analysed by the scanning electron microscopy combined with the EDS and X-ray analyses. This work is a part of research on amorphous and nanocrystalline composite electrolyte CeO2 þSm2O3/ZrO2 þ Y2O3 for solid oxide fuel cells (SOFC). & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: MOCVD method; Nanocrystalline SOFC electrolyte
1. Introduction CeO2-based materials could be used as an electrolyte in oxide fuel cells (SOFC-Solid Oxide Fuel Cells) due to their high ionic conductivity at relatively low temperatures of 500–600 1C. However, they are not resistant to the reducing atmosphere occurring on the anode side [1], and, therefore, despite the many advantageous features have not found yet any practical application. As a result of a contact of CeO2 with the fuel on the anode side, a reduction of Ce4 þ –Ce3 þ ions occurs, and the material also exhibits electron conductivity, which diminishes the cell efficiency. Microcrystalline yttria-stabilized zirconia (YSZ) used currently as electrolyte in SOFC exhibits similar ionic conduction at the operation temperature of about 1000 1C. This electrolyte is obtained by sintering powders at a temperature of 1450 1C [2]. Such high temperatures-both of the preparation and the operation of the electrolyte-require the use of expensive materials for other n
Corresponding author. E-mail addresses: [email protected] (A. Sawka), [email protected] (A. Kwatera). http://dx.doi.org/10.1016/j.ceramint.2015.09.089 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
components of the cell. Consequently, cost of cell manufacturing and energy is high. It is believed that this material could be replaced by CeO2 doped with e.g. Sm2O3, obtained at low temperatures of 600–800 1C, if the new material does not undergo a reduction on the anode side. The use of the new electrolyte should lead to a significant reduction of manufacturing costs for SOFC cells due to the possibility of using cheaper materials to build other parts of the cell. It should be stressed at this point that such electrolyte has been so far obtained by sintering micro- and nanopowders at 1450 1C [2] to produce a non-porous material and, therefore, it has a microcrystalline structure. It is expected that low temperature of the electrolyte manufacturing and its operation will counter the adverse chemical reactions at the electrolyte–electrode contacts, which in turn should increase the stability of the cell and also reduce the cost of electricity production. This study aimed at implementing the above concept by applying a composite electrolyte layer consisting of samariadoped ceria (SDC) on the cathode side with the oxidizing atmosphere and a layer of YSZ on the anode side with the reducing atmosphere (ZrO2 is resistant to both the reducing and oxidizing atmospheres). Synthesis of the electrolyte was
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carried out at temperatures of 600–800 1C with the use of a MOCVD method (Metal Organic Chemical Vapor Deposition). It is commonly agreed that the method is suitable for synthesis of non-porous, uniform in thickness layers at low temperatures on large substrates including those of complex shapes, especially for materials of high melting point and complex composition. Owing to low synthesis temperatures it is expected that the materials will have nanocrystalline microstructure with presence of an amorphous phase. It is also expected that these electrolytes will be characterized by a high ionic conductivity at low temperatures [3,4]. It should be noted that this study was inspired by the work of Eguchi et al. [1,5] who investigated single or double layer electrolytes consisting of disks of yttria-stabilised zirconia (YSZ), or samaria-doped ceria (SDC) and YSZ, respectively, prepared by sintering of powders at temperature of 1450 1C. The current-voltage characteristics of the YSZ electrolyte indicated that at temperature of 800 1C the voltage was 0.1 V at a current density of 0.4 A/cm2 whilst at 1000 1C the voltage increased to 0.4 V at the same current density. In the case of the SDC-YSZ electrolyte at the same current density, the same voltage of 0.65 V was observed at temperatures of 800 and 1000 1C. At such high temperature, a sintering reaction might have proceed between the components during the prolonged annealing which degraded the electrical properties of the double layer composite [5]. Therefore, synthesis of such composite at low temperatures was attempted so that non-porous, nanocrystalline or amorphous material is obtained. There are three main groups of the layer deposition methods: CVD (Chemical Vapour Deposition), PVD (Physical Vapour Deposition) and sol–gel. CVD seems to be the most useful method for this purpose, especially MOCVD due to the fact that metalorganic compounds used in the process are more reactive than their inorganic equivalent and therefore the layers are obtained at lower temperatures than in the traditional CVD process. The amount of deposited product on the substrate is relatively small in the CVD method, particularly in the process controlled by mass diffusion to the substrate, and the diffusion process leads to the growth of non-porous layers owing to the transport of mass in the form of single atoms and small clusters. At low temperatures, the deposited layer may be amorphous or nanocrystalline. The method is suitable for the synthesis of layers made of materials with small diffusion coefficients (materials with a large share of covalent binding). The method allows dense and uniform in thickness layers to be obtained on flat or curved surfaces (for example the inner surfaces of tubes) and on the substrates with large dimensions. It should be noted that the deposited layers may be porous, when a homogeneous nucleation process occurs during their growth. In the case of PVD methods, significantly larger clusters than in the CVD process are transported to the substrate and then the deposited layer where they should be distributed in the form of single atoms by surface diffusion processes in order to obtain a dense layer before the deposition of a next cluster. This is possible when diffusion coefficients of the deposited material are high which is the case for ceramic materials with high share of ionic binding. It is not possible to obtain non-porous layers of materials of a high share of covalent bonding.
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In the sol–gel method, its final step consists in sintering of layers to obtain non-porous and smooth layers. This method is suitable for materials characterized by high or small diffusion coefficients, but then with the use of liquid phase. Owing to the aspects discussed above, the PVD and sol–gel methods did not seem suitable for the synthesis of non-porous electrolytes from the materials in question at low temperatures. The preparation of thin MOCVD (CVD) layers could be considerably facilitated if the process is described by a mathematical equation correlating the process parameters with the properties of a product. The authors have studied modelling of the CVD (MOCVD) deposition of thin layers for long time (eg. [6–16]) and they seek in this study to apply the knowledge gained to the practical syntheses. In this study, the synthesis of the layers was carried below the critical temperature T* i.e. in the temperature range in which the synthesis is controlled by rate of the reaction on the substrate; above this temperature, the CVD (MOCVD) process is controlled by the reactant diffusion from the gas phase to the substrate [7] using Ce(tmhd)4 and Sm(tmhd)3 as precursors. It should be noticed that Jiang et al. [17] deposited SDC layers on small size substrates (NiO þ YSZ and α-Al2O3 disks, ф13 mm) using AAMOCVD method. However, the layers obtained at temperatures of 400–500 1C were amorphous and porous. Crystalline layers were obtained above 530 1C. The layers synthesized at 600–650 1C were characterized by presence of elongated shape crystallites and on their surface fine grains of powders were visible. These powders were formed in the gas phase as a result of homogeneous nucleation process. The deposited layers were rough and had poor adhesion to the substrate. 2. Experimental details A mentioned the SDC layers were synthesized on the inner surfaces of a quartz glass tube (ф 15 mm, L¼ 25 mm) by the MOCVD method using Ce(tmhd)4 (ABCR, Germany) and Sm (tmhd)3 (Sigma Aldrich) as precursors. Air and argon were the carrier gases. The air was also source of oxygen for the oxidation of carbon resulting from the pyrolysis of these reactants. The use of quartz glass instead of not transparent SOFC cathode was due to the following reasons: it does not react with Al2O3 below a temperature of 1100 1C, it is well transparent, and therefore allows for a very quick visual assessment of the obtained layer thickness [18] and thickness distribution. It is also very easy to determine whether during a given synthesis unfavorable homogeneous nucleation process occurred. In the process porous powders formed in the gas phase, which can be deposited on a synthesized layer causing their porosity, thereby lowering the transparency, and its mechanical strength and adhesion. The MOCVD equipment is shown schematically in Ref. [9]. It consists of the following principal elements: supply of gases (cylinders with Ar and air), valves, a tube reactor of 30 mm diameter and 700 mm length with the substrate-placed inside the induction coil of a high frequency generator, a vacuum pump, flowmeters and an evaporator with a gradual temperature
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distribution allowing a simultaneous evaporation of two reactants of different evaporation temperatures: Ce(tmhd)4 and Sm(tmhd)3. All the elements were connected with plastic tubing, rendering the equipment gas tight. The molar content of Sm(tmhd)3 in the gas mixture with Ce(tmhd)4 was: 0.1; 0.2; 0.3. Ar (flow of 0.09 mol/h) and air (flow of 0.4–8 mol/h) were heated to attain the evaporation temperatures of the reactants, lower and higher, respectively. Ar was introduced into the evaporator and air into the gas mixture at the outlet from the evaporator. Substrate temperature was about 600–800 1C and synthesis time about 0.5 h. Static pressure and velocity of gases 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 dense and transparent layers (without homogeneous nucleation), uniform in thickness, to be obtained, which indicated that the synthesis was carried out under a laminar gas flow [8,10]. The Grx/Rex2 criterion [6] is given by: Grx dgΔTβρ1 T 1 ¼ 2 T avðxÞ p1 px þ 12 ρ1 U 21 Re2x where: x–distance from the gas inflow point; values of gas parameters at this distance are subscripted x, 1–gas parameter values at x¼ 0 or above the diffusion layer, d–reactor diameter, g–acceleration due to gravity, ΔT–difference between the substrate temperature TS and that of gas in the bulk, T1 β-volume thermal expansion coefficient of the gas, p–total static gas pressure in the reactor, p1–px ¼ Δp–the 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 [19] or evaluated from: T avðxÞ ¼ ½T avðwÞ SwðxÞ þ T 1 ðSR SwðxÞ Þ=SR
Tav(w)–average gas temperature in the boundary thermal layer evaluated from: T avðwÞ ¼ 0; 68T S þ 0; 32T 1 Sw(x)–magnitude of the cross-section of the reactor occupied by the boundary thermal layer; the thickness of which was determined from [10]: 0; 977 δx ffiffiffiffiffi δTðxÞ ¼ p 3 Pr where: δx–thickness of the boundary layer which can be assessed using the empirical relations given in [19] or from the measurement of the gas velocity distribution in x of a given reactor [20],
Pr–Prandl’s number of the reaction mixture SR–cross-sectional area of the reactor Evaporation temperatures of the reactants were established from their weights, assuming the same time of complete evaporation for the two reactants. Therefore, when the mass of a reactant increased or decreased, it was necessary to increase or decrease the evaporation temperature, respectively. Such procedures allow a constant molar ratio of the reactants in the gas phase to be obtained in spite of decreasing weight of the reactants during the evaporation. Therefore, it can be expected that if the synthesis of the layers is conducted in the mass diffusion controlled process [8,9,11], one gets a constant molar ratio of products (CeO2 and Sm2O3) in the layers synthesized at different temperatures. In a such process, pressure of the i-th component pi(s) on the substrate is close to zero, which may be obtained also at low temperatures for a sufficiently high dilution of the reactants in the carrier gas. Therefore, there should be no competition for adsorption of various reactants on the substrate in various synthesis temperatures. Consequently, the molar ratio of products should be constant. The resulting layers were investigated using a scanning electron microscope SEM NANO NOVA 200 from FEI EUROPE COMPANY with an EDS microanalyser from the EDAX EDS company, and an X-ray powder diffractometer X'Pert Pro from Philips. 3. Results and discussion The samaria-doped ceria layers obtained in the temperature range of 600–800 1C on substrates made of quartz glass for small molar contents of Sm(tmhd)3 in the reactant mixture (0.1; 0.2; 0.3) were visually transparent, which indicates that the homogeneous nucleation process did not occur during the layer growth [8]. Studies performed with the use of the scanning electron microscopy showed that the layers were also smooth and nonporous. Results of investigations for selected samples only are presented below. Fig. 1 shows the surface of the layer synthesized at 600 1C (the molar content of Sm(tmhd)3 was 0.1). A quantitative composition obtained with the use of the EDS is presented in Table 1. It can be concluded from Fig. 1 that there are numerous bright fields – probably a nanocrystalline phase-on the layer surface. The maximum grain size is below 50 nm. The darker fields correspond probably to an amorphous phase. Surface of the layer synthesized at 800 1C with the molar content of Sm(tmhd)3 of 0.1 is shown in Fig. 2. This layer is also smooth and non-porous. The data shown in Table 2 indicate that both cerium and samarium are present in the layer. In the case of the layer synthesized at the higher temperature (Fig. 2), the fraction of bright fields is significantly larger than in the case of the sample presented in Fig. 1. It seems that the crystallization of sample (Fig. 2) is more advanced than of sample (Fig. 1). Grain sizes are of the order of nanometers (max. about 100 nm). The results shown in Tables 1 and 2
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Fig. 1. Surface of the SDC layer (the molar content of Sm(tmhd)3–0.1) synthesized at 600 1C.
Table 1 Quantitative EDS analysis for the sample obtained at 600 1C with the molar content of Sm(tmhd)3-0.1 (Fig. 1). Element
Wt%
At%
OK SiK CeL SmL
49.82 43.38 5.71 1.09
66.16 32.82 0.87 0.15
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indicate that, for the molar content of Sm(tmhd)3 in the reactant mixture of 0.1, lowering the synthesis temperature from 800 to 600 1C causes a reduction of the atomic content of Sm in the layer from 0.11 to 0.07 (the molar content of Sm2O3 respectively from 0.055 to 0.035). The data presented in Tables 1 and 2 also indicate that the atomic content of Ce in the layer synthesized at the higher temperature is lower than in the layer synthesized at the lower temperature for the same molar composition of the reactant mixture. It seems that the layer growth probably occurred in the regime controlled by the rate of surface reaction. In this process, a significant differentiation of adsorption of reactants necessary for the synthesis of the layer is possible both as a function of temperature and concentration of the reactants [8]. The layer shown in Fig. 3 was obtained at 600 1C. The molar content of Sm(tmhd)3 was 0.2. Nanoparticles were larger than in the case of the layer synthesized at the same temperature but with the molar content of Sm(tmhd)3 of 0.1 (Fig. 1). Fig. 4 shows a sample synthesized at 800 1C with the molar content of Sm(tmhd)3 of 0.2 and Table 3 a point EDS analysis. Comparing Figs. 2 and 4, it can be also concluded that the increased molar content of Sm(tmhd)3 in the reactant mixture influenced the content of the crystalline phase and the grain size. In the case of the sample presented in Fig. 4, the grain size is larger than in the case of the sample shown in Fig. 2. The layers obtained both at 600 and 800 1C with the same molar content of Sm(tmhd)3 i.e. 0.2 (Figs. 3 and 4) are dense (non-porous) and smooth. As expected the content of samarium in the layers increased with an increase of the Sm(tmhd)3 M content in the reactant mixture from 0.1 to 0.2 (Table 3). The results of microscopic studies of the SDC layers obtained at 600 1C with the molar content of Sm(tmhd)3 of about 0.3 are presented in Fig. 5(a). Fig. 5(b) shows a linear EDS analysis of a fracture in the layer. The deposited layer is smooth, non-porous, uniform in thickness and contains Ce and Sm. An example of a microstructure of the layer synthesized at 800 1C with the same molar content of Sm(tmhd)3 is illustrated in Fig. 6.
Fig. 2. Surface of the SDC layer (the molar content of Sm(tmhd)3–0.1) synthesized at 800 1C.
Table 2 Quantitative EDS analysis for the sample obtained at 800 1C with the molar content of Sm(tmhd)3-0.1 (Fig. 2). Element
Wt%
At%
OK SiK CeL SmL
47.99 43.83 7.18 1.00
64.95 33.80 1.11 0.14
Fig. 3. Surface of the SDC layer (the molar content of Sm(tmhd)3–0.2) synthesized at 600 1C.
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Quantitative EDS analysis for this sample is presented in Table 4. This layer is also smooth and non-porous. In this case, the maximum crystallite size reaches about 300 nm. The content of Sm in the layer (Table 4) is significantly higher than previously (Tables 1–3). Tables 1–4 also show that increasing the molar content of Sm(tmhd)3 in the reactant mixture from 0.1 to
0.3 produced an increase of atomic content of Sm in the layer from 0.11 to 0.21 at the synthesis temperature of 800 1C. The data shown in Tables 1 and 2 demonstrate that the atomic ratios Ce:Sm were different for layers deposited at 600 and 800 1C in spite of a constant molar ratio of reactants maintained in the gas phase. This indicates that the synthesis process was controlled by the reaction rate on the substrate. As the reaction rate in this process is low, a high concentration of reactants adsorbed on the substrate and a competition between the adsorption of Ce(tmhd)4 and Sm(tmhd)3 may occur. This competition is largely dependent on the substrate temperature and the reactant concentration in the gas phase. It should be noted that the critical temperature T* of transition from the process controlled by the reaction rate on the substrate to the process controlled by the reactant diffusion from the gas phase to the substrate can be significantly reduced (even by 200–300 1C) by a high dilution of the reactants in the carrier gas, and/or by reducing the diffusion coefficients of the reactants. The latter effect can be obtained by introducing a
Fig. 4. Surface of the SDC layer (the molar content of Sm(tmhd)3–0.2) synthesized at 800 1C.
Table 3 Quantitative EDS analysis for the sample obtained at 800 1C with the molar content of Sm(tmhd)3 of 0.2 (Fig. 4). Element
Wt%
At%
OK SiK CeL SmL
37.53 53.16 7.63 1.67
54.50 43.98 1.27 0.26
Fig. 6. Surface of the SDC layer (the molar content of Sm(tmhd)3–0.3) synthesized at 800 1C.
Fig. 5. Fracture of the SDC layer synthesized at 600 1C (the molar content of Sm(tmhd)3–0.3) (a), a linear EDS analysis (b).
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large quantity of gas of a density substantially greater than that of air (for example argon) into the carrier gas, in addition to the air necessary for the combustion of carbon produced by pyrolysis of the metalorganic compounds [7,9]. Fig. 7 shows results of the X-ray analysis of the layers deposited at 600 1C (molar content of Sm(tmhd)3–0.2). It can be concluded that there is a crystalline phase in these layers. In summary it should be noted, that non-porous deposited layers of varying grain size were obtained. The lower the synthesis temperature, the smaller the grains (at a temperature of about 800 1C the grains grew even to above 100 nm). At the current stage of research, it is difficult to point to the optimum size of grains for the SOFC electrolyte. Earlier research shows that the layer synthesis at low temperatures (for example the synthesis of amorphous layers) and then their annealing at a strictly defined temperature for a strictly limited period of time is the most suitable way of controlling the microstructure and grain size in the MOCVD method [15,16]. Such investigations are planned to elucidate the problem. It should be also noted that Laukaitis et al. [21] obtained layers with a high growth rate using an e-beam technique (EB PVD). The deposited layers were characterized by small (nano) grains uniform in Table 4 Quantitative EDS analysis for the sample obtained at 800 1C with the molar content of Sm(tmhd)3-0.3 (Fig.6). Element
Wt%
At%
OK SiK CeL SmL
42.49 44.26 10.25 2.99
61.41 36.44 1.69 0.46
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size. Annealing of the layers at temperatures of about 800 1C did not cause any observable grain growth (the authors did not provide, however, the annealing time). From this point of view, this method seems to be very interesting. However, it should be noted that layers deposited by this method had high porosity. The porosity depended on the substrate temperature and electron gun power. Maximum porosity amounted to even approximately 40%, when the substrate temperature was about 100 1C and the electron gun power 3 kW. At the same value of the gun power, a lower porosity of about 7%) was obtained when the substrate temperature was about 500 1C. The substrate temperature increase to about 600 1C caused a porosity increase to 8%. Authors did not give substrate dimensions. Such information is important, because the layer thickness and grain size may be uniform if the substrates have small dimensions (for example: a diameter of 10 mm and the distance from a source of vapours of 250 mm). If the substrates are larger and there is a point source of vapours an effect of glancing angle orientation should occur [22]. It should be also mentioned that ceramic porous materials may be characterized by significantly reduced mechanical strength due to the stress concentration at pores (the Cook – Gordon mechanism). Such material may easily undergo cracking when subjected to thermal stresses occurring in an electrolyte as a consequence of its lasting contact with electrodes characterized by different coefficients of thermal expansion. A very fast transport of the molecular oxygen (O2) from the cathode to the anode takes place through these cracks and numerous pores, which significantly reduces the cell efficiency (O2 ions should be transported). It may also lead to the cell destruction. Because of these aspects, at this stage, the method is not useful for obtaining of the SOFC electrolyte. 4. Conclusions The study has refined understanding of the synthesis of samaria-doped ceria layers in several aspects:
Fig. 7. X-ray diffraction pattern of the SDC layers synthesized at 600 1C (the molar content of Sm(tmhd)3–0.2).
1. Samaria-doped ceria layers may be obtained at temperatures of 600–800 1C using the MOCVD method. 2. The deposited layers are smooth, non-porous and uniform in thickness. 3. The layers synthesized in the above temperature range are nanocrystalline-amorphous. 4. The higher the synthesis temperature, the greater the content of the crystalline phase. 5. Different atomic ratios of Ce to Sm in the layers synthesized at temperatures of 600 and 800 1C indicate that the layer synthesis, at least at the lower temperature, must have occurred in the process controlled by the reaction rate. 6. Increasing the molar content of Sm(tmhd)3 in the reactant mixture leads also to an increase in the molar content of Sm2O3 in the layer. 7. When the synthesis temperature was about 800 1C, the maximum crystallite size changed from about 100 nm-for the molar content of Sm(tmhd)3 of 0.1-to about 300 nm-for the molar content of Sm(tmhd)3–0.3.
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8. The results obtained open perspective for further detailed research on the layer synthesis in the mass diffusion controlled process. 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] K. Eguchi, Ceramic materials containing rare earth oxides for solid oxide fuel cell, J. Alloy. Compd. 250 (1997) 486–491. [2] H. Yahiro, Y. Eguch, K. Eguchi, H. Arai, Oxygen ion conductivity of the ceria-samarium oxide system with fluorite structure, J. Appl. Electrochem. 18 (1988) 527–531. [3] W.S. Hsieh, P. Lin, S.F. Wang, Characteristics of electrolyte supported micro-tubular solid oxide fuel cells with GDC-ScSZ bilayer electrolyte, Int. J. Hydrog. Energy (2014) 3917267–3917274. [4] H. Hidalgo, E. Reguzina, E. Millon, A.-L. Thomann, J. Mathias, C. Boulmer-Leborgme, T. Sauvage, P. Brault, Ytttria-stabilized zirconia thin films deposited by pulsed-laser deposition and magneto sputtering, Surf. Coat. Technol. 205 (2011) 4495–4499. [5] K. Eguchi, N. Akosaka, H. Mitsuyasu, Y. Nanoka, Process of solid state reactions between doped ceria and zirconia, Solid State Ion. 135 (2000) 589–594. [6] A. Kwatera, C.V.D. Thin, layers of carbon doped silicon nitride on quartz glass, Ceram. Int. 15 (1989) 65–72. [7] A. Kwatera, Models of the processes at the substrate surface in the CVD method, Ceram. Int. 17 (1991) 11–23. [8] A. Kwatera, Uniform thin chemically vapour deposited layers of high density on the inner surfaces of tube-shaped substrates, Thin Solid Films 204 (1991) 313–339. [9] A. Kwatera, Carbon doped α-Al2O3 films synthesized on cemented carbide tools by the metal-organic LPCVD technique, Thin Solid Films 200 (1991) 19–32.
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