Study On The Process And Properties Of Perovskite Type Ceramic Coatings Prepared By Sol-Gel Method

The purpose of this thesis is to develop a technique, which is simple, economical and applicable for preparing coatings on any surface (such as large surface, complex shapes) and can be synthesized at low temperature. Two kinds of technology were used, composite sol-gel route and conventional sol-gel method, to prepare perovskite-type ceramic coatings on different substrates. The fabrication process and microstructure were systematically investigated by means of thermogravimetric analysis (TG), differential scanning calorimetry (DSC), X-ray diffractometer (XRD) and scanning electronic microscopy (SEM). The hardness, bonding intensity and electrical properties of coatings prepared from different processes were also investigated.The results indicated that the preparation of La_0.8Sr_0.2MnO₃ coating by composite sol-gel route was feasible. The optimized process was as follows: La_0.8Sr_0.2MnO₃ powders were added to precursor sol with the same composition to form a stable slurry; After being cleaned ultrasonically in acetone and alcohol, alumina substrates were dipped into the composite slurry and pulled out at a linear speed of 1 cnvmin^-1; The coated samples were then dried at 120℃for 5 min, and pre-fired at 600℃for 10 min. This dipped, dried and pre-fired step was repeated several times to achieve the desired thickness. And then, the coatings were calcined at 800℃for 1-4 h to decompose residual organic matter and crystallize the perovskite phase. Finally, the coatings were densified with La_0.8Sr_0.2MnO₃ precursor sol. The densification process was as follows: the deposited coatings were dipped into La_0.8Sr_0.2MnO₃ precursor sol, dried, pre-fired at 600℃for 10 min, and finally calcined at 800℃for 1 h.It was shown by XRD and SEM that a single La_0.8Sr_0.2MnO₃ perovskite phase was obtained on alumina substrate. The microstructure of coatings was significantly modified and cracks were eliminated by adding La_0.8Sr_0.2MnO₃ powders to the precursor sol. The thickness of coatings increased with increasing the amount of La_0.8Sr_0.2MnO₃ powders and the number of coating applications. The results also suggested that a smooth and continuous coating in both surface and cross-section can be obtained when the powder content increased to 0.6 g. Furthermore, the sheet resistance of coatings decreased as the amount of La_0.8Sr_0.2MnO₃ powders and the number of coating applications increased. The sheet resistance of coatings also decreased with the content of Sr-doping, and the minimial value was obtained when x=0.3.In the case of La_1-xCa_xCrO₃ coating, the optimized process was as follows: 20G substrate was dipped into precursor sol for certain minutes, withdrawn at a constant rate of 1 cm·min^-1, dried at 80 °C, pre-fired at 400 °C for 10 min, then this step was repeated several times, and finally calcined at 800 °C for 1 h in air.It was shown by XRD and SEM that coatings on 20G substrates consisted of two parts, i.e. ceramic layer which was composed of single La_0.7Ca_0.3CrO₃ perovskite phase, and oxide layer. The thickness of ceramic and oxide layer was approximately 12.5～25μm and 25～50μm respectively. It was also found that the bond strength of coatings to substrates was influenced by the oxide layer. The microporosity, roughness, inhomogeneity and defect of the coatings increased with the precursor concentration. The porosity increased and microstructure became porous as the pH value of precursor sol increased. Coatings showed denser and more homogeneous microstructure with reduced porosity as the number of coating applications increased, which was the result of the filling of pores. Moreover, the thickness and bond strength of coatings increased with increasing the number of coating application. However, the efficiency will be decreased. A dense and homogeneous coating could be obtained when the precursor concentration was 0.3 mol·L^-1 and the dipped, dried and pre-fired step was repeated six times.