收稿日期：19 July 2013
基金：financially supported by the National Natural Science Foundation of China (Nos. 51362011 and 51362012);the Chemistry Discipline Master’s Site Construction Open Foundation of Honghe University of Yunnan Province (No. HXZ1308);
Fabrication of LSGM thin films on porous anode supports by slurry spin coating for IT-SOFC
Hong-Yan Sun Wei Sen Wen-Hui Ma Jie Yu Jian-Jun Yang
College of Science, Honghe University
Yunnan Tin Company Limited
National Engineering Laboratory for Vacuum Metallurgy
Abstract：
La0.9Sr0.1Ga0.8Mg0.2O3-δ(LSGM) and La0.7Sr0.3Cr0.5Mn0.5O3-δ(LSCM) powders were synthesized by glycine-nitrate process, and LSGM electrolyte thin film was successfully fabricated on porous anode substrate of LSCM by slurry spin coating technology. Some technical parameters for the preparation of LSGM thin films were systematically investigated, including ink composition,sintering temperature, and spin coating times. The electrolyte films with the best compactness and somewhat rough are obtained when the operating parameters are fixed as follows: the content of ethyl cellulose as binder is 5 wt%, the content of terpineol as modifier is 5 wt%, the optimum coating cycle number is 9 times, and the best post-deposition sintering temperature is 1,400 °C for 4 h.
Keyword：
Solid oxide fuel cell; Slurry spin coating; LSGM; Thin film;
Author： Hong-Yan Sun e-mail: sunhongyan0404@126.com;
Received： 19 July 2013
1 Introduction
Solid oxide fuel cell (SOFC), an energy conversion device which can convert the chemical energy of the fuel into electrical energy directly, attracts more and more attention attributing to high efficiency and environmental protection[1, 2]. The key current research is the fabrication of SOFC operating at lower temperature (under 800 °C) which is usually called as intermediate temperature solid oxide fuel cell (IT-SOFC). In order to achieve predominant output of cell, reducing the thickness of LSGM electrolyte and transforming the traditional electrolyte-supported type into anode-supported type are the promising technology routes [3, 4]. Doped lanthanum gallate (LSGM) has a superior oxygen ion conductivity of about 0.1 S?cm-1at 750 °C, negligible electronic conduction, and high chemical sta- bility over a broad range of oxygen partial pressures (1 9 10-15–1 9 105Pa) [5]. Therefore, LSGM is a promising material as the electrolyte for the IT-SOFC. Recently, various attempts have been made to fabricate LSGM thin films on porous anode supports for SOFC, including electrochemical vapor deposition (EVD), chem- ical vapor deposition (CVD), physical vapor deposition (PVD), thermal spray coating, sol–gel dip-drawing, tape casting, magnetron sputtering [6, 7], spin coating [8–10], and screen printing [11]. Compared with the other tech- niques, slurry spin coating method is cost-effective, easy to realize, and suitable for mass production.
In this paper, the effects of some technical parameters on LSGM electrolyte film including slurry viscosity deter- mined by the type and additive amount of binder and mod- ifier, heating temperature, and spin coating number were investigated in detail, and the parameters were optimized.
2 Experimental
2.1 Preparation of powders
LSGM and LSCM powders were synthesized by glycine- nitrate process (GNP) [12–14] using nitrates of each element and glycine as the starting material. Nitrates were dissolved in deionized water at a definite stoichiometric ratio, and glycine was then added acting as a complexation agent. The solution was stirred and heated on a hot plate until self-combustion occurred. The as-ignited materials were pulverized in an agate mortar for 30 min and then sintered at proper temperature (LSGM powder was cal- cined at 1,400 °C for 15 h, and LSGM powder was cal- cined at 1,000 °C for 2 h) in a furnace to remove carbonaceous residues and form the desired phase.
2.2 Preparation of porous supports
The green porous LSCM substrates were prepared by con- ventional powder pressing and sintering processes. The as- prepared LSCM powers and proper quantities of starch acting as pore former were mixed and ground in an agate mortar and then pressed into pellets with 20 mm in diameter and 0.8 mm in thickness under the pressure of 8.6 9 104N. After being sintered at 1,350 °C for 5 h, the pellets can be used as substrates for the slurry spin coating process.
2.3 Fabrication of LSGM film by slurry spin coating
The as-prepared LSGM powder was ground with different organic additives (binders and modifiers) in an agate mortar to form a uniform and stable electrolyte slurry with ethanol as medium finely. The organic materials in elec- trolyte slurry used for spin coating are listed in Table 1. LSGM thin films were prepared by spin coating method, which was performed by placing a few drops of LSGM slurry in the middle of the substrate and spinning the sample at 6,000 r?min-1for 40 s using a spin coater (KW- 4A type, Chinese Academy of Science) to form a uniform layer. To avoid cracks and pores in the film induced by the evaporation of the organics during the following co-sin- tering process, each layer of the film was baked at 400 °C for 10 min with a heating rate of 10 °C?min-1before coating the next layer [15, 16]. Furthermore, multiple coating cycles were needed in order to get a film withsuitable thickness and repair the defects in the former layer without cracks and holes [17, 18]. In the study, different coating times (3, 5, 7, and 9 times) were employed to get green LSGM films, which were co-sintered with the anode substrates at different temperatures (1300, 1350, 1400, and 1450 °C) for 4 h with a heating rate of 5 °C?min-1. The surface and cross-sectional microstructures of LSGM electrolyte films were detected with a scanning electron microscope (SEM, Philips XL-30ESEM).
Table 1 Compositions of LSGM electrolyte slurries 下载原图
Table 1 Compositions of LSGM electrolyte slurries
3 Results and discussion
3.1 Characteristics of powders
X-ray diffraction (XRD) analysis was carried out to iden- tify phase formation using Cu Ka radiation at a scanning step of 0.02°. Figure 1 shows the XRD patterns of the as- prepared LSGM and LSCM powders and LSGM–LSCM (50:50, mass ratio) mixture sintered at 1,200 °C for 15 h. As shown in Fig. 1a, LSGM and LSCM powders still remain their own pure perovskite structures. No new impurity phase can be detected for LSGM–LSCM mixture, indicating that they have a good chemical compatibility. Unfortunately, as shown in Fig. 1b, the enlarged XRD patterns of the main peaks of LSGM–LSCM mixture shift to the right slightly compared with those of the LSCM and LSGM. It is because that LSGM and LSCM have different lattices. And they will interact with each other, and the lattice distortion phenomenon will take place possibly when they form composite, which was also previously reported in other literatures [19, 20].
3.2 Effect of slurry composition
Figure 2 shows SEM images of LSGM thin films prepared by slurry spin coating sintered at 1,450 °C for 4 h. It can be readily observed that the LSGM electrolyte film has denser and better adherence with anode substrates when using the terpineol as a modifier (Fig. 2b). This is because glycerol has a larger viscosity, leading to that it is difficult for the electrolyte powders to distribute into glycerol uniformly. So, terpineol is considered to be the preferable modifier for the LSGM electrolyte slurry. When comparing Fig. 2c with d, it can be obviously seen that when using the ethyl cel- lulose as the binder, the LSGM film is much denser than that using polyvinyl-butyral. It also indicates that ethyl cellulose is more suitable than polyvinyl-butyral as the binder for the LSGM electrolyte slurry. As shown in Fig. 2d–f, it is observed that LSGM film becomes denser and better adhered to the anode substrate when the content of ethyl cellulose is 5 wt% (Fig. 2d), which has an important effect on the quality of the films. At thisexperimental condition, the thickness of LSGM film obtained is about 40 lm. On the one hand, while the content of ethyl cellulose is too low, the viscosity of the slurry will be too light to attach to the substrate [21], owing to that the film has an uneven and porous structure (Fig. 2f), and the thickness of LSGM film becomes thinner which is about 30 lm. On the other hand, the viscosity of the slurry will be too high when the content of ethyl cel- lulose is too much. The evaporation of large amount of organics results in large holes left and reducing the com- pactness of the LSGM films (Fig. 2e). The thickness of the film is about 60 lm which is thicker than that of the films prepared with 5 wt% and 3 wt% addition of ethyl cellulose. These results demonstrate that the content of ethyl cellulose has an important effect on the thickness and microstructure of the LSGM films, and the optimum con- tent of ethyl cellulose is 5 wt%, which acts as a binder in electrolyte slurry.
Fig. 1 XRD patterns of LSCM and LSGM powders and LSCM–LSGM mixture sintered at 1,200 °C for 15 h a and zoom in main peaks of XRD patterns b
Fig. 2 SEM images of LSGM films prepared by slurry spin coating with different types of electrolyte slurries: a 8 wt% glycerol as modifier (Sample 1), b 8 wt% terpineol as modifier (Sample 2), c 5 wt% polyvinyl-butyral as binder and 5 wt% terpineol as modifier (Sample 3), d 5 wt% ethyl cellulose as binder and 5 wt% terpineol as modifier (Sample 4), e 10 wt% ethyl cellulose as binder and 5 wt% terpineol as modifier (Sample 5), and f 3 wt% ethyl cellulose as binder and 5 wt% terpineol as modifier (Sample 6)
XRD patterns in Fig. 3 show the phase composition of the LSGM film prepared with 5 wt% ethyl cellulose as binder and 3 wt% terpineol as modifier and post-sintered at 1,400 °C for 4 h. Figure 3 indicates that the film after being sintered forms single phase of LSGM perovskite- type structure and contains only traces of secondary phase peak, which may be attributed to impurities coming from electrolyte slurry grinding process or some elementssegregation. Meanwhile, the EDS spectra analysis of LSGM film prepared at the same condition was carried out as shown in Fig. 4. It is apparent that the film contains only La, Sr, Ga, Mg, and O elements without any other impu- rities, which is consistent with the XRD analysis (Fig. 3). So, it is most likely that for the LSGM component ele- ments, segregation phenomena occur, which results in the trace amount of secondary phases.
Fig. 3 XRD pattern of LSGM film prepared with 5 wt% ethyl cellulose as binder and 3 wt% terpineol as modifier (Sample 6) and post-sintered at 1,400 °C for 4 h
Fig. 4 EDS spectra of LSGM film prepared with 5 wt% ethyl cellulose as binder and 3 wt%terpineol as modifier(Sample 6)and post-sintered at 1,400°C for 4 h
3.3 Effect of sintering temperature
Figure 5a–d shows the surface images of LSGM films sintered at 1300, 1350, 1400, and 1450 °C for 4 h (the electrolyte slurry contains 5 wt% ethyl cellulose and 5 wt% terpineol), respectively. It is obvious that the mor- phologies of the electrolyte vary depending on sintering temperature. As shown in Fig. 5, the grain size in the LSGM film increases with the increase of sintering tem- perature. The LSGM films sintered at 1,300 and 1,350 °C exhibit porous structure, while the one sintered at 1,400 °C is dense, and all pin-holes disappear. The crystal grains continue to grow with the increase of sintering temperature. When the thin film was heat-treated at 1,450 °C, abnormal large grains and an over-sintering phenomenon appear, and the LSGM film is not dense any more. So, the lowest sintering temperature at which dense LSGM film was prepared is 1,400 °C.
3.4 Effect of spin coating times
Typical SEM images of LSGM electrolyte films with different spin coating cycles are shown in Fig.6.A few poresare observed in LSGM films as shown in Fig. 6a–c. It is seen from the surface view that the number of holes decreases with the increase of spin coating times. The LSGM film fabricated by a nine-time coating cycle exhibits the densest and somewhat rough structure, which is bene- ficial to expand the triple phase boundary (TPB) area for SOFC (Fig. 6d). The properties of LSGM films used in SOFC will be studied further in next research work systematically.
Fig. 5 SEM images of surface microstructures of LSGM electrolyte sintered at different temperatures: a 1,300 °C, b 1,350 °C, c 1,400 °C, and d 1,450 °C
Fig. 6 SEM images for surface views of LSGM films with different spin coating times: a 3 times, b 5 times, c 7 times, and d 9 times
4 Conclusion
In this study, LSGM and LSCM powders with pure single perovskite structure were prepared by GNP process. They have good chemical compatibilities at high sintering tem- perature. Several important process parameters for slurry spin coating and their effects on the fabricated LSGM thin films were investigated and optimized: the content of ethyl cellulose as binder is 5 wt%, the content of terpineol as modifier is 5 wt%, the optimum coating number is 9 times, and the best post-deposition sintering temperature is 1,400 °C for 4 h. Slurry spin coating is a promising tech- nique to fabricate LSGM electrolyte thin films on porous LSCM anode supports for SOFC.
