Study On Thermal Degradation Mechanism And Structure Optimization Of One-dimensional Photonic Crystal Spectral Selective Emitters

Thermophotovoltaic system is a research hotspot in the thermoelectric conversion field in recent years,and the emitter is one of the key energy conversion components.Onedimensional photonic crystal(1D PhC)selective emitters have good spectral performance and simple preparation method among all emitters,which have great development potential.However,the thermal degradation in the 1D PhC emitters at high temperatures restricts their practical application.And thus,the thermal degradation mechanism and structure optimization of the 1D PhC emitters were analyzed based on the thermal stability experiments.Firstly,the theoretical model of the 1D PhC emitter was built based on the transfer matrix method.Mo and HfO2 were selected as the materials of the emitters,and the structural parameters were designed.Then,the corresponding emitters were prepared.The theoretical calculation results showed that the trilayer structure of HfO2 layer(90 nm)/Mo layer(15 nm)/HfO2 layer(90 nm)has good spectral selectivity.The structure of the prepared emitter meets the design requirements and the boundary of the layered structure is distinguished,which can facilitate the observation of the experimental phenomenon.Subsequently,the heat treatment experiments were carried out on the prepared emitter at 3×10-4 Pa vacuum condition with the temperature of 1000～1500 K.The thermal degradation was characterized by the changes in the spectral characteristics,structural morphology and elemental composition,and the thermal degradation mechanism was studied based on the characterization results.The experimental results showed that the emitter can maintain thermal stability below 1300 K for 1 hour.After heating at 1400 K for 1 hour,the thermal degradation occurred,and the spectral emittance shift to the nonconvertible wave band.When the temperature rose to 1500 K,the thermal degradation intensified,and the spectral emittance decreased sharply in the convertible waveband.The characterization results showed that the Mo oxides and holes appeared in the emitter after thermal degradation,and the grain growth and structure collapse can also be observed.It can be seen from the above phenomenon that the Mo oxidation,grain growth and thermal expansion caused the thermal degradation,among them,the Mo oxidation is the main factor.The Mo oxidation mechanism is complex,MoO2 and volatile MoO3 were mainly generated during the heating.The residual oxygen in the environment contacts the Mo layer through the micro-cracks in the structure,which is the internal cause of the oxidation process.In addition,the structure of the 1D PhC emitter was optimized.The Mo layer was replaced by the HfO2 layer embedded with VO2 nanoparticles,and an emitter structural parameters optimization design process,which is evaluated by the figure of merit,spectral efficiency and spectral cutoff efficiency,was proposed.The results showed that the thickness of the emitter with good spectral performance varies from 105 to 150 nm.When the volume fraction of embedded nanoparticles is 10%,the HfO2 layer(5 nm)/embedded layer(130 nm)bilayer structure and HfO2 layer(5 nm)/embedded layer(100 nm)bilayer structure has the highest energy output density and energy conversion efficiency,respectively.It is found that the spectral selectivity of the emitter is affected by the interference between layers and the scattering characteristics of nanoparticles.And the layer thickness has the greatest influence on the spectral performance,when the thickness of the HfO2 layer increases,it is difficult for the emitter to suppress the spectral emittance in the non-convertible waveband.The proposed optimization model and design process are expected to improve the thermal stability of the 1D PhC emitter and provide a new way of structural parameter design.