Dense <scp>YSZ</scp> Laminates Obtained by Aqueous Tape Casting and Calendering

Abstract

Dense electrolytes and flawless laminated YSZ electrolytes can be produce combining aqueous tape casting and constrained calendering. During calendering two or more layers are laminated continuously and the edges (entering and final) are not sealed at the same time avoiding air bubbles to trap between the layers. In this study constrained calendering laminates showed adequate adhesion between layers, and almost 20% thickness reduction after sintering as well as high green and sintered densities. Solid oxide fuel cells (SOFC) are high temperature devices aimed to stationary heat and power generation.1-3 Compared to other fuel cells, SOFC present the highest efficiency, being flexible in relation to fuel characteristics and operation temperature range (700–1000 °C). SOFC devices consist of a dense electrolyte and two porous electrodes.4-10 The efficiency of the cells is determined by the performance of the electrolyte, which depends in its turn on the ionic conductivity and density of the material. Yttria-stabilized zirconia (YSZ) is widely used as the electrolyte material due to high chemical stability and mechanical toughness, as well as ionic conductivity above 700 °C.10-14 Alternatively, scandia-stabilized zirconia (ScSZ) electrolytes show high ionic conductivity above 750 °C comparable to that of YSZ at 1000 °C. However, high cost and instability of ScSZ at temperatures below 600 °C may limit the wide of this material. Doped cerium oxides, like gadolinia-doped ceria (GDC) and samaria-doped ceria (SDC), have a high ionic conductivity, however, they tend to be chemically instable in reducing atmospheres.15-17 Different routes have been applied for the fabrication of planar SOFC electrolytes. By tape casting, 10–500 µm thick films may be produced. Typical electrolyte thickness in an electrolyte-supported cell is 150–200 µm.16 In order to produce SOFC structures, usually two or more cast tapes are laminated to produce dense SOFC structures. Interlayer delamination and interconnected porosity are not desirable because they decrease the ionic conductivity. Warm pressing is commonly used to laminate cast tapes.18, 19 An alternative technique was reported20-24 so that 0.25–2.5 mm thick films were prepared by rolling a plastic ceramic mass to produce laminated with two or more layers. The aim of the present work was to fabricate high dense 8YSZ electrolytes by aqueous tape casting followed by warm pressing or calendering. The influence of different processing routes on the density, microstructure, and mechanical properties of the laminates was investigated. SOFC electrolyte green tapes were produced by aqueous tape casting. Prior to casting, the slurry with 55 wt% YSZ powder (8YSZ, 8 mol% Y2O3-stabilized ZrO2, Sigma–Aldrich) was deagglomerated in deionized water with 1 wt% dispersant (Darvan 821A, Vanderbilt) using ball milling for 24 h. After deagglomeration, binder (Mowilith LDM-6138, Clariant), antifoam (Antifoam A, Sigma–Aldrich) and surfactant (coconut diethanolamide, Stepan) were added and the slurry was mixed for further 30 min. The as-fabricated slurry was cast at 25 °C by a tape caster (CC-1200, Mistler) with a Mylar carrier coated with a silicon layer (G10JRM, Mistler). A casting speed of 6 cm min−1 was used. The gap between the blade and the carrier was adjusted to obtain a tape thickness of 90–200 µm. The green tapes were dried at 25 °C for 24 h, cut into 5 × 5 cm2 samples and laminated at 40 °C by warm pressing and calendering. Two sheets were laminated and laid: one in the cast direction and the other perpendicular to the cast direction. Lamination by warm pressing (LA-4,5, Bürkle) was carried out in a steel die. Pressure varying between 16 and 19 MPa was applied for 5 min. The lamination by calendering (CA5, Sumet) was performed at 10–15 MPa with a roll speed of 10 m s−1. Tapes were either passed freely through the calender or placed between two copper sheets with 0.5 mm thickness and passed through calender rolls. Debinding and sintering were carried out in an electrically heated furnace (HT-16/17, Nabertherm) in air. Debinding of the green laminates was performed by heating up to 550 °C with the rate of 0.5 °C min−1 and holding time of 1 h. The laminates were then heated up to 1600 °C with the rate of 5 °C min−1. After holding time of 1 h, the laminates were cooled down with the rate of 5 °C min−1. The microstructure of sintered laminates was analyzed by scanning electron microscopy (SEM, Quanta 200, FEI). The SEM samples were polished up to a 1 μm diamond finish prior to sputtering with gold. Densities of the green and sintered tapes were measured geometrically and by helium pycnometry (AccuPyc 1330, Micromeritics). Open porosity and theoretical density were calculated from the bulk and pycnometric density. A universal testing machine (Instron 5565) was used to measure mechanical properties of sintered samples at room temperature by the ball-three-ball method with a crosshead speed of 0.5 mm min−1. Discs (4.8 mm diameter, 50 samples) were cut by Nd:YAG solid state laser (SMP 065, Rofin). The characteristic strength and the Weibull modulus were determined by linear regression.25 Table 1 summarizes the open porosity and density of the green laminates after debinding. All values for both open porosity and green density are equivalent, considering the dispersion of results. Stable slurries were prepared to produce defect-free and homogeneous green tapes in order to improve the performance of the electrolyte in the cell, increasing the ionic conductivity and the mechanical strength. When two or more sheets are laminated, perfect adhesion is required to form a homogeneous structure. An appropriate lamination process should provide homogeneous junctions between the layers. After debinding and sintering, the interfaces between the tapes should disappear and one single homogeneous piece should be obtained. The choice of the appropriate lamination method is very important, because it affects directly the flatness of the sample and the adhesion between the layers.26 Figure 1 shows SEM images of the polished fracture cross-section and the surface of the sintered electrolytes laminated by three different methods: warm pressing (WP), free calendering (FC), and constrained calendering (CC). In Figure 1a, delamination is observed along the sample. Air bubbles trapped between the layers after lamination by WP can cause delamination during sintering and also affect the quality of the final laminate. In addition, in Figure 1a and b, it is possible to observe a large amount of pores and small defects. In Figure 1c and e, no delamination was noticed. It is possible to distinguish a uniform defect-free structure. During calendering two or more layers are laminated continuously and the edges (entering and final) are not sealed at the same time, like in warm pressing, avoiding air bubbles to trap between the layers. Final pieces laminated by calendering (FC and CC) showed adequate adhesion between layers after sintering. In Figure 1b,d, and f, it may be seen that the electrolyte samples presented high density. Samples laminated by WP (Figure 1b) and FC (Figure 1d) showed small defects and pores. The micrographs of samples sintered at 1600 °C clearly showed a significant grain growth of ≈3 µm whereas the average particle size of the starting power was 0.45 µm. As seen in Figure 2, warpage was observed in the samples laminated by WP (A), FC (B), and CC (C). This phenomenon can be attributed to the residual stresses generated after lamination when the sample was removed from the matrix. Constrained calendering allowed the production of laminates at low pressures and reduced deformation when compared to free calendering and warm pressing. The quality of the final product is determined by quality of the laminate interface and its stability during thermal treatment, which is also determined by the lamination process.26 Laminates with thickness between 150 and 200 µm were prepared by constrained calendering. Thin laminates are desirable because the ionic conductivity of YSZ improves with the reduction of the thickness, which determines the performance of the cell. Figure 3 shows the percentage of thickness reduction of the laminates obtained by CC and after sintering by the three lamination methods. The highest percentage of thickness reduction after sintering, almost 20%, was obtained in the CC laminates. Moreover, a considerable reduction was attained by calendering, around 11%. Both FC and CC samples showed ≈9% reduction after lamination. Laminates produced by WP showed no thickness reduction after lamination and ≈7% reduction after sintering. Figure 4 shows the theoretical density of the green and sintered species laminated by the three methods. The theoretical density of the green bodies laminated by both FC and CC was ≈53. Compared to WP, there was an increase of 3%. In the case of sintered samples, a large difference in theoretical density between the three lamination methods was observed. WP and FC laminated samples presented 93 and 94.3% theoretical density, respectively. A large increment of ≈6% was obtained in the CC laminates showing ≈99% theoretical density. The electrolyte samples were found to be well sintered at 1600 °C corresponding to 93, 94.3, and 99.5% theoretical density. These results are consistent to the microstructural analyses, where WP laminates showed a large amount of pores. True (pycnometric) density and open porosity of the samples laminated by the three methods are summarized in Table 2. In electrolyte-supported SOFCs, the electrolyte which serves as a structural support, should have a high mechanical strength. Fracture strength values of three different laminates measured by ball-on-three-ball test are summarized in Table 3. The highest fracture strength was attained by the CC samples (≈997 MPa). The maximum fracture strength, 1.2 GPa, was obtained by one CC laminated sample. When comparing the fracture strength of the three different laminates, one can be observe that the values of WP and FC laminates are very close. On the other hand, CC and WP samples can achieve higher fracture strengths as well as Weibull modulus. The Weibull modulus indicates how reliable the data are; the highest the modulus, the more reliable the results of the fracture strength. WP and CC laminates presented the Weibull modulus ≈8, as shown in Table 3, while for FC laminates the value was 6. In this regard, there is no statistical difference between the three lamination methods. However, the results are consistent to the microstructural results and porosity. Laminates with more dense structure showed higher fracture strength. High dense 8YZS electrolytes were successfully prepared by aqueous tape casting followed by lamination. Warm-pressed laminates showed delamination due to open pores. Theoretical density of 93 and 94.3% and fracture strength of ≈986 and ≈957 MPa was obtained by warm pressing and free calendering, respectively. Both warm-pressed and free-calendered laminates presented warpage. Samples prepared by constrained calendering showed no delamination or warpage and a theoretical density of 99.5%. In this case, fracture strength of ≈997 MPa was achieved. According to the results obtained in this work, high dense and flat 8YSZ electrolytes may be produced by aqueous tape casting followed by calendering between metal plates. Constrained calendering seems to be an alternative laminating process to overcome the disadvantages of free calendering and warm pressing.