Study On YSZ And Ni-YSZ Functional Coatings Prepared By EB-PVD Process

Electron beam physical vapor deposition (EB-PVD) technique was employed to prepare the Ni-YSZ anode coatings as well as the YSZ electrolyte coatings for SOFCs in this study. The chemical composition and microstructure of the coatings were analyzed by x-ray fluorescence spectra (XRF), x-ray diffraction (XRD), scanning electron microscope (SEM), electron probe mciroanalysis (EPMA), transmitting electron microscope (TEM), and atom force microscope (AFM). Then the properties of these coatings, such as porosity, gas tightness, electrical conductivity and so on, were measured and characterized. At the same time, unit cells were assembled and output performances were investigated.8YSZ electrolyte coatings were prepared by EB-PVD process on porous NiO-YSZ substrates. Both the as-deposited coating and the coating annealed at 1000°C consisted of a single phase of cubic YSZ. SEM analysis demonstrated that electrolyte coating exhibited a characteristic columnar structure. In gaps between the columns, there were many pores with the magnitude of micrometers. A uniform distribution of zirconium, yttrium and oxygen was realized along the direction across coating thickness according to EPMA. TEM analysis indicated that there were many crystalline grains with shapes of strip and pyramid in coating cross-section. Many white lines, with the width of several nanometers, appeared within these grains and inclined with some angle.Gas-tightness measurement results demonstrated that a gas permeability of 9.78×10-5 cm4·N-1·s-1 was obtained for YSZ electrolyte prepared by EB-PVD, which was lower than that of electrolyte coating prepared by atmosphere plasma spraying by 60%. The electrical conductivity of YSZ electrolyte coating exhibited a typical anisotropy. In temperature range of 500°C~800°C, the electrical conductivity in direction perpendicular to coating surface was obviously higher that in direction parallel to coating surface. According to Arrhenius equation, activation energies for electrical conduction in the directions both perpendicular and parallel to coating surface were 80.884kJ/mol and 110.147kJ/mol respectively.A sol infiltration device at subatmospheric pressure was used to densify the YSZ electrolyte coatings. Then the post-sintering process was determined according to the analysis results of DSC-TG and XRD for sol precursor. After the sol infiltration treatment, the coefficient of gas permeability dropped from 9.78×10-5 cm4·N-1·s-1 for as-deposited coating to 3.18×10-5 cm4·N-1·s-1 and 9.56×10-6 cm4·N-1·s-1 for coating after 4 and 8 times of infiltration treatment respectively. SEM analysis showed that the intergranular pores and gaps on the surface of as-deposited coating had been stuffed by the decomposed products from the sol. The coating surface became smoother and denser after densification treatment. A further SEM analysis of fracture surface of the infiltrated coating demonstrated that the sol had penetrated into the coating with depth of up to 3.5μm. The electrical conductivity in direction perpendicular to coating surface after 4 and 8 times of infiltration treatment increased no more than 10% comparable to that of as-deposited coating. Furthermore, an obvious increment in the electrical conductivities of electrolyte coatings between 4 and 8 times of infiltration treatment did not occurred. The electrical conductivity in direction perpendicular to coating surface after infiltration treatment was still lower than that of bulk 8YSZ by 30%.According to the measurement results of output performances of unit cells at 800°C, an open circuit voltage (OCV) of 0.820V was obtained for the cell using as-deposited electrolyte coating. At the same time, the OCV increased to 1.017V and 1.036V for cells using electrolyte coatings after 4 and 8 times of infiltration treatments respectively. For the cell employing as-deposited coating as electrolyte, a maximum output powder density of 70mW/cm2 was obtained. On the contrary, the maximum output powder densities increased to 140mW/cm2 and 153mW/cm2 for cells using electrolyte coatings after 4 and 8 times of infiltration treatments respectively.At the same time, Ni-YSZ coatings were prepared by EB-PVD through simultaneously evaporating two ingots of metal Ni and YSZ. SEM analysis demonstrated that these coating exhibited a typical columnar growth pattern. Between the loose columns, there were many gaps and pores. TEM analysis showed that many fine grains and nano-sized pores existed in this coating. According to N2 adsorption results, the surface specific area of this coating was up to 3.232m2/g. At the same time, the average pore diameter of 25nm and total pore volume of 0.0202cm3/g were obtained. Due to the strengthening effects of the addition of the second phase particles on coating porosity, the porosity of this coating increased from 12% to 19% when the Ni content in the coating increased from 18% to 44%. On the contrary, when the Ni content further increased to 63%, the coating porosity decreased from 19% to 11%. The electrical conductivity behavior of Ni-YSZ coating showed an S-shaped curve accompanying the increase of Ni content.NiO-YSZ coatings were prepared by EB-PVD through simultaneously evaporating two ingots of NiO and YSZ and then the final Ni-YSZ coatings were obtained after a reduction treatment at 800oC in hydrogen for 2h. SEM analysis showed that the pore diameters were mainly in the range from tens of nanometers to hundreds of nanometers and the characteristics of columnar grains vanished. Comparable to the Ni-YSZ coatings prepared through directly evaporating two ingots of Ni and YSZ, the specific surface area increased from 3.232m2/g to 4.330m2/g for final Ni-YSZ coatings obtained from NiO-YSZ coatings a reduction treatment. At the same time, the total pore volume also increased from 0.02024cm3/g to 0.03460cm3/g. A graded coating, with gradient not only in element distribution but also in porosity distribution along the direction across coating thickness, was realized through adjusting and controlling the electron beam currents. In this graded coating, a high porosity and big pore diameter were achieved in one side closed to substrate, while a low porosity and small pore diameter were obtained in the other side close to coating surface. On the basis of anode preparations mentioned above, a continuous deposition of YSZ electrolyte coating was realized through shutting off electron beam current for evaporating NiO ingot, consequently resulting in a continuous deposition of anode and electrolyte coatings for SOFCs.