Processing of YSZ thin films on dense and porous substrates

Surface & Coatings Technology 200 (2005) 1242 – 1247 www.elsevier.com/locate/surfcoat

Processing of YSZ thin films on dense and porous substrates Y. Pan a, J.H. Zhu a,*, M.Z. Hu b, E.A. Payzant c a

Department of Mechanical Engineering, Box 5014, Tennessee Technological University, Cookeville, TN 38505, United States b Nuclear Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6181, United States c Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6064, United States Available online 29 August 2005

Abstract Yttria-stabilized zirconia (YSZ) thin films with a Zr/Y molar ratio of 0.84:0.16 were synthesized on both dense Si substrates and porous Ni – YSZ anodes using the spin coating technique. Two polymeric precursors were used to process the YSZ films on the Si substrates. The first one utilized a commercial precursor, with butanol added as the diluting solvent. By controlling the content of butanol in the solution, dense and crack-free YSZ films with thickness of 500 nm were obtained after 8 coating runs with a final anneal of 700 -C for 4 h. X-ray diffraction was used to monitor the crystallization process in the films during annealing, which indicated that the YSZ films started to crystallize at 300 – 400 -C and became fully crystalline with a cubic structure at temperature 600 -C. In the second case, a new YSZ polymeric precursor was prepared and used for coating. It was found that the viscosity of the precursor solution is critical in controlling the film quality. Relatively thick, dense YSZ films were also synthesized on porous Ni – YSZ anode substrates using a combined colloidal – polymer method. D 2005 Elsevier B.V. All rights reserved. Keywords: YSZ film; SOFC; Sol – gel technique; Spin coating; Porous substrate

1. Introduction A solid oxide fuel cell (SOFC) is an electrochemical power generation device that converts chemical energy directly into electricity with high efficiency and in an environmentally friendly way. In conventional electrolytesupported SOFCs, yttria-stabilized zirconia (YSZ) thick plates (usually 0.1 – 0.5 mm) are used as the electrolyte material. High operating temperatures of 800– 1000 -C are necessary for the efficient operation of thick YSZ electrolyte-supported SOFCs because of the low oxygen ionic conductivity of YSZ at lower temperatures. However, high temperature fabrication processes and long-term operation can cause reactions and interdiffusion between the cell components resulting in cell performance losses and reduced efficiency. Also, the requirements for interconnect materials of SOFCs are very stringent because of the high operating temperature. Current research efforts have

* Corresponding author. Tel.: +1 931 372 3186. E-mail address: [email protected] (J.H. Zhu). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.07.083

focused on the development of SOFCs operating at intermediate temperatures in the range of 600 –800 -C, as the reduction in SOFC operation temperature will broaden the materials choices for SOFC components, especially the interconnect and sealing materials. For example, metallic interconnects based on Fe, Ni, or Cr might be chosen to replace costly, brittle ceramic interconnects (e.g. doped LaCrO3) at such reduced operation temperatures, leading to enhanced manufacturability and long-term durability of SOFC power generation systems. However, oxygen ionic conductivity of the electrolyte and catalytic kinetics of the electrodes may drop significantly with the reduction in SOFC operating temperature. Ohmic losses across the electrolyte can be minimized in two ways: (1) the use of high ionic conductivity materials such as Gd doped ceria (GDC) or strontium and magnesium doped lanthanum gallate (LSGM) as electrolyte and (2) the use of a YSZ thinfilm electrolyte, typically a few microns thick, supported on the anode or cathode substrate. Several techniques have been used to fabricate YSZ thin films, including sputtering [1,2], plasma spraying [3],

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electron-beam physical vapor deposition (EB-PVD) and chemical vapor deposition (CVD) [4]. However, these methods are relatively costly and are not realistic for the synthesis of commercial YSZ thin-film electrolytes. Sol– gel based method such as spin coating or dip coating can also be used for the fabrication of YSZ thin films. Compared to PVD or CVD methods, the sol – gel coating technique is simple, cost-effective, and can be used to synthesize homogeneous thin films of multi-component oxides [5,6]. It has been reported that thin, dense, nanocrystalline YSZ or CeO2 electrolytes can be fabricated using a polymeric precursor spin coating technique [7,8]. The thickness of these films is however usually less than 1 Am; the stability and potential gas crossover through such thin films during extended operation is a concern. Relative thick and pore-free films of 1– 10 Am are desirable for the SOFC electrolyte applications. Furthermore, the polymer precursor spin coating technique is limited in application to dense substrates only. By combined spinning or dip coating with a colloidal suspension and a polymeric precursor, these technical difficulties can be addressed and it is possible to prepare coatings on both porous and dense substrates to yield >1 Am thick, dense films of YSZ after annealing at temperatures < 1000 -C [9,10]. A colloidal suspension containing fine YSZ particles is expected to seal the large pores on the porous substrate surface and to build up the coating thickness, while the polymeric precursor is expected to impregnate the colloidal coating and therefore increase coating density after sintering. In this paper, we successfully deposited a layer of dense and crack-free nanocrystalline YSZ thin films on dense Si substrates by spin coating process using two polymeric precursors (one from a commercial vendor and the other synthesized in our laboratory). Viscosity of these two YSZ polymeric precursors was monitored to produce good coating either by dilution with certain solvent or by controlled polymerization. The quality of the YSZ film prepared with the two different polymeric precursors was compared. A combined colloidal –polymeric method was used to deposit relatively thick coatings on porous Ni –YSZ substrates.

speed of 700 rpm for 6 s and the second one, 2500 – 4000 rpm for 25– 30 s. The deposited film was then dried at 80 -C for 1– 3 min followed by 300 -C for 1 – 3 min. This process was repeated a number of times to increase the coating thickness to typically 0.5 Am. The film was finally annealed in a furnace at 700 -C for 4 h with a heating and cooling rate of 2 -C/min. In addition to the commercial precursor, a second YSZ polymeric precursor was synthesized in our lab, similar to that described in the patent by Anderson et al. [11]. The starting solution, with a nominal molar composition Zr/ Y= 0.84:0.16, was prepared from zirconyl chloride hydrate (ZrOCl2I8H2O) and yttrium nitrate (Y(NO3)3I6H2O) which were pre-standardized thermogravimetrically to confirm the actual cation composition. These chemicals were mixed with H2O, ethylene glycol and glycine to obtain a clear, precipitate-free solution. The solution was then heated on a hot plate to 80 -C to expel the excess water until it turned into a viscous liquid with viscosity of 90– 200 cP. The deposition, drying, and annealing of YSZ thin films on Si substrates were very similar to that with the commercial YSZ precursor.

2. Experimental

2.3. Characterization of YSZ thin films

2.1. Synthesis of YSZ thin films on dense Si substrates

The surface morphology, thickness, and cross section of the as-deposited films after annealing were investigated using scanning electron microscopy (SEM) (Hitachi S4700). The phase evolution in the film after annealing at temperature between 300 and 1000 -C was studied by X-ray diffraction (XRD). The samples were scanned over the 2h range from 20- to 60- with a scan rate of 1- per minute using a Scintag XD2000 diffractometer with CuKa radiation. The crystallite size was estimated using the

YSZ 0007 polymer (Chemat Company, CA), with a molar composition of Zr/Y= 0.84:0.16, was diluted with butanol to an appropriate concentration. The YSZ polymeric precursor was deposited on silicon wafers (dimension 10  10  0.5 mm, b001 orientation, Silicon Sense, Inc.) using a two-stage spin coater (KW-4, Chemat Technology, CA). The first coating stage had a rotation

2.2. Synthesis of YSZ thin films on Ni – YSZ porous substrates YSZ thin films were also deposited on porous Ni – YSZ substrates, which are the most commonly used SOFC anode. To produce a relatively thick YSZ film (> 1 Am), a combined method of colloidal suspension (NexTech Materials, Ohio) and polymeric precursor was used in the deposition process. The colloidal solution contained nanocrystalline YSZ particles with particle size of 40 – 50 nm. Both commercial (YSZ 0007) and synthesized polymeric precursors were used along with the colloidal suspension to produce the thick YSZ coating and it was found that the two precursors yielded essentially the same results. Suspension solution and polymeric precursor were alternatively deposited on the porous substrate and dried at 80 -C and 300 -C for 3 min at each temperature. In this study, a low viscosity polymeric precursor (¨ 90 cP) was used because it was easy to impregnate the colloidal coating, and yielded dense films after sintering at 900 -C for 4 h.

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while when the viscosity was too high, many cracks existed in the thin film. Therefore, it is important to control the viscosity of the precursor to yield continuous, crack-free films. Fig. 1 shows the viscosity of the synthesized precursor with different heating times at 80 -C. The viscosity of the precursor was very low within the first 20 h, and increased slowly afterwards, and eventually rose sharply after the heating time reached 50 h. For the commercial precursor, its viscosity could be adjusted by diluting it with butanol. When the viscosity of the polymeric precursor was controlled between 90 cP and 200 cP, dense, continuous thin films were obtained by the spin coating technique.

350

Viscosity, cP

300 250 200 150 100 50 0

0

10

20

30

40

50

60

70

Time, h Fig. 1. Viscosity of the synthesized polymeric precursor heated at 80 -C for various times.

3.1.2. Phase evolution in the films The structural evolution of the deposited films during annealing was determined by XRD. Fig. 2 shows the XRD patterns of the YSZ films after 6 coating runs with the commercial polymeric precursor diluted with butanol (the ratio of butanol to the precursor is 4.5:1, identified as 4.5:1 commercial precursor hereafter) following 4 h annealing at temperatures between 400 -C and 1000 -C. The results indicated that the YSZ film started to crystallize into a cubic fluorite structure after the polymeric precursor molecules were burned off at temperatures as low as 400 -C. No peaks from tetragonal or monoclinic phases of zirconia were observed. As the annealing temperature increased, the peaks of cubic YSZ became stronger and sharper, implying that the YSZ crystallites in the film grew in size with increasing annealing temperature. The average YSZ crystallite sizes at

Scherrer method, i.e., from the breadth of the diffraction peaks of YSZ films on Si substrates.

3. Results and discussion 3.1. YSZ thin films on dense Si substrates 3.1.1. Effect of precursor viscosity on film quality In general, uniform thin films could not be achieved using polymeric precursors with a very low viscosity (< 50 cP) or very high viscosity (> 500 cP) on Si substrates. When the viscosity was too low, a discontinuous film was formed, 1000

800 o

Relative Intensity

1000 Cx4h

600

o

800 Cx4h

400 o

600 Cx4h

200 o

400 Cx4h

0 25

30

35

40

45

50

55

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2 Theta Fig. 2. XRD patterns of YSZ films on Si substrates with 6 coatings runs using the 4.5:1 commercial polymer annealed at different temperatures.

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250

Relative Intensity

200

150

Commercial precursor

100

50

Synthesized precursor

0 25

30

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2 Theta Fig. 3. XRD patterns of YSZ films (2 coating runs) on Si substrates using the 4.5:1 commercial precursor and the synthesized precursor annealed at 600 -C for 4 h.

different annealing temperatures were calculated from the full width of half maximum intensity of the XRD peaks. The crystallite size was 17 nm when the YSZ thin film was annealed at 700 -C for 4 h, consistent with Chen et al. [7]. After annealing at 1000 -C for 4 h, the crystallite size increased to ¨ 20 nm. The results of YSZ thin films grown from the synthesized polymeric precursor were very similar to those from the commercial precursor. Fig. 3 shows the XRD patterns of YSZ films on Si substrates after 2 coating runs using the 4.5:1 commercial precursor and the synthesized precursor with a final anneal treatment at 600 -C for 4 h. The YSZ film after 2 coating runs using the synthesized precursor had the same cubic structure as that using the commercial precursor. After the same heat treatment, the

peak intensities were almost identical for the two different precursors.

Fig. 4. SEM cross-sectional micrograph of the YSZ film (8 coating runs) on Si substrate using the 4.5:1 commercial polymer annealed at 700 -C for 4 h.

Fig. 5. SEM cross-sectional micrograph of the YSZ film (2 coating runs) on Si substrate using the synthesized precursor annealed at 600 -C for 2 h.

3.1.3. Cross-sectional view of the films Fig. 4 shows the cross-sectional SEM micrograph of the YSZ film on the Si substrate after 8 coating runs using 4.5:1 commercial polymer with a final anneal of 700 -C for 4 h in air. The film was dense and free from microcracks and pores. The grains in the film are small and are unable to be resolved with SEM even at a magnification of 30,000. As can be seen from the micrograph, a very dense and uniform coating was formed on the Si substrate, with a thickness of about 500 nm. Fig. 5 shows the SEM image of the crosssection of the YSZ film after 2 coating runs using the

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thin film from the combined colloidal – polymer method on porous Ni –YSZ substrate. As can be seen from the figures, a layer of dense, crack-free, and continuous YSZ film was successfully deposited on porous Ni– YSZ substrates. No pinholes in the film were observed. Large pores in the Ni– YSZ substrates were covered with the coating. A thickness of 2 Am could be readily obtained after 4 spin coatings with a final anneal treatment at 900 -C for 4 h. Since the anode substrate was not smooth, the coating surface was relatively rough. It is expected that the coating roughness can be reduced if the anode substrate quality is improved.

4. Conclusions

Fig. 6. SEM images of the surface morphology (a) and cross section (b) of YSZ film (4 coating runs) on porous Ni – YSZ using the sequential colloidal suspension and the synthesized polymeric precursor method after a final anneal at 900 -C for 4 h.

By controlling the viscosity of the polymeric precursors, nanocrystalline YSZ thin films with cubic structure were synthesized on dense Si substrates via two different polymeric precursors by spin coating technique. Dense, uniform, and crack-free YSZ films with a thickness of about 0.5 Am were successfully deposited after 8 coating runs on dense Si substrates after a final annealing at 700 -C for 4 h. YSZ films with thickness more than 1 Am were also obtained through a combined colloidal – polymer method. A layer of dense, crack-free, and continuous YSZ film with a thickness of 2 Am was deposited on porous Ni – YSZ substrates after 4 spin coating runs with a final anneal treatment at 900 -C for 4 h. This combined colloidal – polymer precursor approach is promising for fabricating dense, crack-free YSZ thick film (1– 10 Am) with relatively low sintering temperature for SOFC electrolyte applications.

Acknowledgement synthesized precursor on the Si substrate annealed at 600 -C for 2 h. The thickness of YSZ thin film after 2 coating runs was about 100 nm. The thickness of the YSZ thin film could be increased by simply increasing the number of coatings runs. 3.2. YSZ films on porous Ni– YSZ substrates The polymeric precursors were initially used to synthesize YSZ films on porous Ni – YSZ anode substrates. However, no satisfactory films were obtained on porous substrates because the polymeric precursors were sucked into the porous Ni – YSZ substrate and no continuous film could be formed on the surface. Spin coating with YSZ colloidal suspension solution alone yielded a thin film on the porous substrate; however, numerous cracks were observed in the thin film, and it was therefore not suitable for SOFC electrolyte applications. To achieve relatively thick and at the same time, dense and crack-free films, a combined method of sequential spin coating with YSZ colloidal solution and polymeric precursor was employed. Fig. 6 shows the surface (a) and cross-section (b) of the YSZ

This research project was financially supported by the National Science Foundation (# DMR-0238113) and the Division of Materials Science, Office of Basic Energy Sciences (#KC0203010), U.S. Department of Energy. Part of this work was also supported by the Center for Manufacturing Research, Tennessee Technological University (TTU) and by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of FreedomCAR and Vehicle Technologies, as part of the High Temperature Materials Laboratory User Program, Oak Ridge National Laboratory, managed by UT-Battelle, LLC for the U.S. Department of Energy under contract # DE-AC0500OR22725.

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