Preparation And Properties Of Mn-Co-Cu Spinel Protective Coatings For SOFC Interconnect

Mn-Co Spinel is considered as the most promising coating materials for the interconnect protection of solid oxide fuel cell.However,in the actual working environment,the coating still has problems such as low conductivity and Cr poisoning the cathode,which leads to the degradation of the stack performance.On the one hand,Cu doping can promote the electronic transition between the mixed valence elements(Co²⁺/Co³⁺and Mn³⁺/Mn⁴⁺)and improve the conductivity of the coating.On the other hand,the thermal expansion coefficient of the coating is increased to improve the thermal matching between the coating and the substrate.In this thesis,two process routes are used to prepare protective coatings.One is to prepare Cu-doped Mn_1.5Co_1.5O₄ spinel powder(Cu_?Mn_1.5-xCo_1.5-yO₄,Mn-Co-Cu)by solid-state reaction method,and the powder is characterized and tested to determine the optimal doping ratio and solid state reaction conditions.Mn-Co-Cu spinel coating is prepared on the substrate surface by atmospheric plasma spraying technology,and the conductivity and Cr resistance of the coating are investigated.The second is to transform Mn-Co-Cu metal coating into spinel coating.Explore the spraying process of metal coating,and the process and mechanism of metal coating into spinel coating.The specific research results are as follows:(1)The optimal Cu doping amount of Cu_?Mn_1.5-xCo_1.5-yO₄(?=0.1,0.2,0.3,0.5,x+y=?)powder was?=0.3.Under this condition,the content of high-conductivity spinel phase was the highest,and the spinel phase was mainly a mixed spinel phase of Mn Co₂O₄/Cu Mn₂O₄.At1000°C/12 h,the content of spinel phase was the highest and the doping effect of Cu element was the best.(2)Spinel powder will be deoxidized and decomposed into oxides with lower conductivity when heated by high temperature plasma jet,but the coating will undergo oxygen absorption reaction when heated in an oxidizing atmosphere,and finally transformed into spinel structure.The Cu_0.3Mn_1.5-xCo_1.5-yO₄ coating was oxidized at 800°C for 97 h,and the electrical conductivity could reach 51 S/cm.Compared with the Mn_1.5Co_1.5O₄ coating conductivity(23 S/cm),Cu doping further improved the coating conductivity.The doping of Cu inhibited the rapid growth of Cr₂O₃ film at the interface between the coating and substrate,and had good Cr resistance.(3)During the high temperature oxidation process of Mn-Co-Cu metal coating,Cu element preferentially diffused to the coating surface to form Cu O.With the increase of oxidation time,Cu diffused gradually to the inside of the coating,while Mn and Co diffused to the outside of the coating,and finally transformed into Mn-Co-Cu spinel phase.No Cr element was found in the coating.The Cr₂O₃ and Mn Cr₂O₄ oxide layers with low electrical conductivity were formed in the middle of the substrate and spinel coating.(4)The spraying power had a great influence on the phase composition,microstructure and electrical conductivity of the metal-transformed Mn-Co-Cu spinel coating.The spinel coating prepared with 51 KW spraying power contained the most high-conductivity Mn Co₂O₄and Cu_xMn_3-xO₄(x=1,1.4,1.5)phases,and the coating was dense,which was the best spraying parameters for preparing high performance spinel coating.