| Solid oxide fuel cell is an environmentally friendly power generation device that integrates many advantages such as high energy conversion efficiency,low environmental pollution and wide fuel adaptability.It can directly convert chemical energy into electrical energy.With the development of new technologies,the drop in operating temperature,it has broad application potential for solving energy shortages and low carbon emissions,also known as intermediate temperature SOFC（IT-SOFC）.However,at this stage,many key material obstacles limit its application.As an important component of SOFC integrated system,the interconnect has the function of supporting the stack,fuel gas introduction and current transmission.In the process of long-term service,strict requirements are put forward on the thermal expansion coefficient,high temperature oxidation resistance and electrical conductivity of the material.At present,most of the reported interconnect materials can’t meet the needs of practical application,so it is urgent to develop new and stable materials.In this project,ceramic and metal dual phase materials with high conductivity and oxidation resistance are combined to prepare Ti N/Ni Composites.The comprehensive properties of the composites are optimized step by step from four aspects of ceramic phase,introduction of second phase,metal matrix phase and structure optimization.The preparation process,microstructure,oxidation resistance and conductivity of the composites are systematically studied.The goal is to obtain Ti N/Ni composites with appropriate CTE,excellent and stable oxidation resistance and electrical conductivity,accelerating commercial application process of IT-SOFC.The main research contents are as follows:（1）The comprehensive properties of the composites were optimized from the perspective of ceramic phase（Chapter 3）.The effects of particle size,introduction mode and content of ceramic phase on the microstructure and properties of the composites were analyzed.Firstly,Ti N/Ni Composites with different particle sizes were prepared by SPS sintering,and the relative density was above 98%.When the particle size of Ti N is 0.05μm,the sintered Ti N phase presents series growth,and the oxidation resistance is very poor.The CTE of 10μm Ti N composite is the lowest,while is 12.31×10-6℃-1,while the oxidation resistance of 1μm Ti N composite is the best,the oxidation weight gain is 7.23 mg·cm-2,and the electrical conductivity is up to 1.25×104S·cm-1.Secondly,Ti N/Ni composites were prepared by in-situ and semi in-situ reaction sintering of Ti with different particle sizes in N2 atmosphere.Using the phase diagram and△G to analyze the sintering mechanism of in-situ Ti N/Ni.The results illustrate that Ti with particle size of 0.05μm has the best oxidation resistance,the oxidation weight gain（800℃/120h）is 2.42 mg·cm2,and the conductivity is the highest,which can be maintained at 1.33×104S·cm-1after 120h.（2）The comprehensive properties of the composites were optimized by introducing the second phase（Chapter 4）.Y2O3,Cr-Mn and Nb C were introduced to enhancing the oxidation process of the composites.Y2O3 can promote the nucleation and growth of tin phase in Ni matrix,and decrease CTE from 12.4×10-6℃-1 to 11.91×10-6℃-1.When the content is 3 wt.%,the oxidation weight gain（800℃/120h）is 3.87 mg·cm-2,and the conductivity is 1.34×104 S·cm-1.There is a dissolution limit of Cr in Ti N/Ni.High content of Cr（>15 wt.%）can quickly form a dense and thin oxide layer,which makes the composite obtain excellent oxidation resistance.The oxidation weight gain is 0.57 mg·cm-2,and the conductivity is 1.491×104 S·cm-1.But at the same time,a certain amount of Mn should be introduced to solve the problem of chromium poisoning.When the Cr is 15 wt.%and the Mn is 0.6-1.2 wt.%,the composite has the best comprehensive properties.The introduction of Nb C can reduce the vacancies of Ti and O in Ti N/Ni Composites,thus changing the oxidation process.When the content of Nb C is 9 wt.%,the comprehensive performance is the best,CTE is 11.75×10-6℃-1,the oxidation weight gain is 2.18 mg·cm-2,the electrical conductivity is 1.382×104 S·cm-1.（3）The comprehensive properties of the composites are optimized by the metal matrix phase（Chapter 5）.The properties of pure Ni are relatively single.It is necessary to further strengthen the oxidation resistance of the metal matrix in order to achieve a qualitative leap in the overall properties of the composites.According to the previous research results,the Ni Cr Co Ti Mn high entropy alloy with independent intellectual property rights was prepared by mechanical alloying.The melting point was 1148℃and CTE was 11.82×10-6℃-1.The new alloy is completely resistant to oxidation below600°C,the oxidation weight gain at 800°C is 2.12 mg·cm-2,and the oxidation resistance above 1000°C is poor.Ti N/HEA composite was prepared by adding strengthened metal matrix.The effects of content change on microstructure,CTE,oxidation resistance and electrical conductivity of the composite were analyzed.When the content of metal matrix is 75 wt.%,CTE is 11.2×10-6℃-1,oxidation weight gain（800℃/120h）is 3.55 mg·cm-2,and the electrical conductivity is 1.55×104 S·cm-1.（4）The comprehensive properties of composites were optimized from the perspective of structure（Chapter 6）.Based on the research results of the previous three chapters,considering the diversity of material shape and performance requirements in the practical application environment,ABA and ABC configurations were proposed to optimize and upgrade the structure,which can realize the dual characteristics of low thermal expansion and high oxidation resistance of composites.Taking the prepared Ti N/60Ni-Ti N/40Ni-Ti N/60Ni composite with ABA structure as an example,the effects of structure on CTE,mechanical properties,oxidation properties and electrical conductivity of the composite were studied,and the general change rules were obtained,which laid a theoretical foundation for promoting the diversified application of the composite in IT-SOFC interconnects. |
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