Research On Al₂O₃-based Ceramic Insulators Used For The Extremely Cold Areas

With the rapid development of national electric power industry,the scale of the power grid is expanding,and it gradually extends to the extreme climatic areas such as high altitude and extreme cold areas,so the safe and stable operation of the power system is essential.The insulator material is an important component of the transmission line,whose performance will affect the safe and stable operation of the power system directly.Based on the requirements that the insulators used for extremely cold areas must have high strength and excellent freeze-thaw resistance,Al₂O₃-based ceramic insulators for extremely cold areas were prepared with Shandong Aluminum Company?-Al₂O₃,Shandong Jinao Technology Company ZrO₂as the main raw materials via atmospheric pressure sintering technique.Relationships between the composition,preparation process,microstructure and performance of the composites were researched by using modern testing techniques such as XRD,SEM and EPMA.Effects and mechanisms of CeO₂,Sm₂O₃ and their composite additives on the microstructure and performance of the material were investigated.To further improve the freeze-thaw resistance of Al₂O₃-based ceramics,the composite sol and fritted glaze were fabricated as freeze-thaw-resistant surface modified coatings.The ways and mechanisms of improving the freeze-thaw resistance of Al₂O₃-based ceramics were discussed.The main contents and results are as follows:?1?Al₂O₃/ZrO₂ composite ceramic insulator materials were fabricated using?-Al₂O₃,ZrO₂,MgO,TiO₂ as raw materials.The formulation and preparation process of the sample with the best freeze-thaw resistance were determined.Results show that MgO and TiO₂ are effective in promoting the sintering of Al₂O₃ ceramics and reducing the sintering temperature by at least 140°C.ZrO₂ can inhibit the growth of corundum grains and make the grains refine.At the same sintering temperature,the increase in the content of ZrO₂ can improve the strength and density of Al₂O₃ceramics.Sample A5?80 wt%?-Al₂O₃,20 wt%ZrO₂,plus 1 wt%MgO and 1 wt%TiO₂?sintered at 1580°C has the best freeze-thaw resistance.It does not crack after30 freeze-thaw cycles?-50⁴0°C?,but the bending strength decreases from 270.48MPa to 233.10 MPa with a loss rate of 13.82%.The crystal phases of the samples before and after freeze-thaw cycles are all?-Al₂O₃,t-ZrO₂,m-ZrO₂.The low temperature freeze-thaw cycles promote the conversion of part of t-ZrO₂ to m-ZrO₂,and also increase the number and size of pores in the sample.The volume expansion produced by t-ZrO₂ phase transition and the freezing of adsorbed and percolated water is the main reason for the increase of the number and size of the pores,and the loss of bending strength.?2?To improve the freeze-thaw resistance of Al₂O₃/ZrO₂ composite ceramic insulator materials,series B was designed by adding CeO₂,Sm₂O₃ and their compound additives into formula A5,respectively.Effects and mechanisms of the three additives on the microstructure and performance of the material were investigated.Results show that the addition of CeO₂,Sm₂O₃ and the combination of the two can promote the sintering,refine the corundum grains and improve the bending strength,respectively.The best CeO₂ content is 5 wt%,and the bending strength of the sample sintered at 1580 ^o C is 318.13 MPa.The best Sm₂O₃ content is 1wt%,and the bending strength of the sample sintered at 1540^oC is 322.71 MPa.The bending strength of sample B7?plus 5 wt%CeO₂ and 1 wt%Sm₂O₃?sintered at 1560^oC is 329.24 MPa.The addition of CeO₂,Sm₂O₃ and the combination of the two can improve the freeze-thaw resistance of Al₂O₃/ZrO₂ composite ceramic insulator materials,respectively.Sample B7 with the addition of compound additives sintered at 1560 ^oC has the best freeze-thaw resistance and its strength loss rate is 8.50%after30 freeze-thaw cycles.The presence of CeO₂ increases the room temperature stability of t-ZrO₂ and weakens the transition trend of t-ZrO₂ to m-ZrO₂ after freeze-thaw.Therefore,the strength loss after 30 freeze-thaw cycles is attributed to the volume expansion produced by the freezing of adsorbed and percolated water,which destroys the structure of pore wall and increases the number and size of pores.?3?To further improve the freeze-thaw resistance of Al₂O₃-based ceramic insulator materials,the freeze-thaw-resistant surface modified coatings were designed and prepared,the way to block the pores on the surface of ceramic materials was discussed,the coating sealing and bonding with Al₂O₃-based ceramics were studied,the mechanism of the coating to improve the freeze-thaw resistance of Al₂O₃-based ceramics was revealed.The ZrO₂-SiO₂-Al₂O₃ ternary composite sol was prepared by sol-gel method using Al?NO₃?₃·9H₂O,ZrOCl₂·8H₂O,TEOS as the main raw materials,and then coated on the surface of Al₂O₃-based ceramics for sealing treatment.The effects of preparation technology and coating method on the surface properties of Al₂O₃-based ceramics were studied.Results show that the main factors that affect the stability of the composite sol are the reaction temperature,pH,reaction time and viscosity.The temperatures Al?OH?₃ suspension stirred and ternary composite sol mixed should be at 50^oC and 85^oC,respectively.The pH of the solution during the preparation of the SiO₂ sol should be controlled to 2,and the pH of the ternary composite sol should be maintained between 3 and 4.The stirring time should be controlled at 2h after TEOS and ZrOCl₂ being added to Al?OH?₃ suspension.The viscosity of the ternary composite sol should be controlled between 3 and 5 mPa·S,which is suitable for coating.The optimum heat treatment temperature of the composite film is 1150^oC,and the phase composition are t-ZrO₂,?-SiO₂ and?-Al₂O₃.Coating 3 times is more appropriate and the surface of Al₂O₃-based ceramics is completely covered by the film,whose thickness is about 4?m.There is a good sealing effect but a poor adhesion between the film and matrix,which is easy to produce cracks or cracking.?4?The fritted glaze was prepared with frit 707 and Suzhou clay as the main raw materials and then coated on the surface of Al₂O₃-based ceramics by dipping-glaze method.Results show that after 1100 ^oC heat treatment,the Vickers hardness of the glaze is 761.0 HV,and the crystal phases of the glaze is?-SiO₂,ZnO,CaAl₄O₇ and CaSiO₃.The surface of the glaze is smooth without cracks.The thickness of the glaze layer is about 500?m,which is close to Al₂O₃-based ceramics.Compared with the composite film,the sealing effect of the glaze layer is more outstanding and combines with the matrix closely,which is the better sealing coating.After 30 freeze-thaw cycles,the surface of the glaze keeps smooth without cracks,and the adhesion between glaze layer and matrix is great,which indicates that the coated glaze layer can protect Al₂O₃-based ceramics from being frozen.The mechanism,on one hand,is that the glaze layer has a dense structure and the glass phase fills with the pores of matrix surface,which can effectively hinder water on the surface intrude inside the matrix,greatly reducing the demage to the material due to the volume expansion produced by the freezing of water.Thereby,freeze-thaw resistance of the material is improved.On the other hand,the glaze has high hardness and is well integrated with the matrix material,it will increase mechanical strength of the matrix material,which is conducive to the improvement of freeze-thaw resistance of the material.The Al₂O₃-based ceramics coated with the glaze layer are expected to be used as insulator materials for extremely cold areas.