| Solid oxide fuel cells(SOFC)are promising green energy sources due to their high energy conversion efficiency and environmental friendliness,and yttrium oxide stabilized zirconia(YSZ)has become the main electrolyte material for SOFC because of its high oxygen ion conductivity,good chemical stability and stable mechanical properties.However,its high sintering temperature(above 1500℃),above 800℃ to show high oxygen ion conductivity and high operating temperature,limits the further commercial application of YSZ electrolytes.Bi2O3 has a low melting point and high oxygen ion conductivity,making it the most promising additive for reducing the sintering and operating temperatures of YSZ electrolytes.Bi2O3-YSZ composite electrolytes are usually prepared by mechanical mixing and solid phase reaction,which requires high preparation temperature,long reaction time,and uneven mixing leading to incomplete reaction;Bi2O3-YSZ composite powders are prepared by the traditional chemical co-precipitation method,Bi2O3 tends to grow into larger rods(>50 μm),which greatly limits the mixing uniformity of Bi2O3 and YSZ,even worse than mechanical mixing.Therefore,there is an urgent need to develop a new preparation strategy to improve the mixing level between Bi2O3 and YSZ powders,increase the degree of reaction,and thus reduce the preparation temperature of YSZ.In this paper,the controllable preparation of Bi2O3 particle size and Bi2O3-YSZ composite powders was achieved by improved coprecipitation method,and the influence of sintering process on the properties of bulk materials was studied.The stability of YSZ phase structure was improved by introducing Er-doped Bi2O3.The main achievements are as follows:(1)The growth behavior of Bi2O3 was explored by controlling the Bi3+ concentration and precipitant agent,and the Bi2O3-YSZ composite powders with controlled particle size was successfully prepared by further improving the chemical precipitation method.In order to reduce the size of Bi2O3,the water bath temperature of chemical precipitation was increased from room temperature to 90℃ and the number of nucleation was increased to inhibit crystal growth and reduce the size of Bi2O3;at the same time,in the one-step chemical precipitation method,when the mixture of acidic Bi3+solution and YSZ suspension was added into NaOH solution,a violent reaction occurs,which disturbs the growth of Bi2O3 and further reduces the particle size of Bi2O3 powders;however,when the concentration of Bi3+solution was greater than 1.2 mol/L,the particle size of Bi2O3 increases uncontrollably.For these reasons,the onestep precipitation method was further changed by adding half of the NaOH solution to the Bi3+solution,and then adding YSZ powder and the other half of the NaOH solution to reduce the alkaline environment of the reaction solution,thus moderating the intensity of the reaction,and the addition of YSZ provided nucleation sites so that most of the Bi2O3 could grow attached to the YSZ particles,which is two-step chemical precipitation method;through these improvements,even if the doping content of Bi2O3 is as high as 20 mol%,the particle size of Bi2O3 can still be reduced to sub-micron level,and can ensure the mixing uniformity with YSZ powders.(2)Bi2O3-YSZ composite powders with doping content of 1-20 mol%were prepared by two-step precipitation method and sintered in the temperature range of 900-1350℃.The phase composition,microstructure,relative density and bending strength of Bi2O3-YSZ composite electrolytes were studied.The small particle size Bi2O3 and its good mixing homogeneity with YSZ effectively promoted the sintering densification of YSZ:at the doping of 10 mol%Bi2O3 and 5 mol%Bi2O3,the sintering temperature of YSZ was reduced to 900℃ and 1000℃,respectively;the sintering temperatures of YSZ were reduced to 900℃ and 1000℃ at 10 mol%Bi2O3 and 5 mol%Bi2O3 doping,respectively;however,with the increase of Bi2O3 doping content,the volatilization of Bi2O3 was serious,which affected the relative density of YSZ;although dense YSZ bulk could be obtained at 1200℃ and below,Bi2O3 doping entered the YSZ lattice,resulting in the unstable crystal structure of YSZ and the generation of monoclinic phase ZrO2.(3)In order to obtain cubic phase Bi2O3 with better conductivity and phase stability,and to improve the phase stability of YSZ,erbium oxide doped bismuth oxide(ESB)was introduced and face-centered cubic phase Bi2O3 was successfully obtained;it was doped into YSZ to obtain ESB-YSZ composite powders with ESB doping content of 5-20 mol%;sintering in the temperature range of 900-1200℃,it was found that the introduction of Er2O3 made the crystal structure of YSZ more stable,inhibited the formation of monoclinic phase ZrO2,and the YSZ electrolyte with a denseness of 98%could be sintered at 900℃.This study provides a simple and easy method for preparing Bi2O3-YSZ composite powders with controllable particle size and crystal phase,and greatly reduces the sintering temperature of Bi2O3-YSZ composite electrolytes,which lays a foundation for mass production of low-cost YSZ-based SOFC electrolytes. |
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