Design And Mechanics Research Of High Temperature Superconducting Filter

Since the high temperature superconductor?HTS?with critical temperature higher than 77K and the HTS thin film were successfully prepared,the research of HTS microwave circuit based on HTS filter has become mature.Compared with conventional metal microstrip filters,the benefits of HTS filters are mainly reflected in the following aspects.Firstly,the resistivity of HTS films is about 200??,and the low resistivity leads to low loss.Therefore,the band edge steepness can be improved by increasing the order.Secondly,the substrate material of HTS filter has higher dielectric constant,which leads to smaller circuit size.Finally,the fabrication of HTS thin films and the processing technology of microstrip circuits are mature,and people can accept the high performance brought by high cost.As for the design of HTS filters,there are many studies in narrowband and multiband.However,there are still some problems such as parasitic component loss and poor response at high frequencies,which restrict the development of HTS filters.Therefore,the study of the new structure of HTS filters to achieve better filtering performance is conducive to its wider application.In addition,HTS filters will undergo cooling and transmission current changes when they work,and there will be stress inside the filter,which will lead to the change of thermal,electrical and mechanical properties between the superconducting thin film and the substrate,and affect the stability of the filter chip.At present,there are few studies on the mechanics of HTS filters,so the time-dependent distributions of interfacial shear stress and transverse normal stress of thin film-substrate structure during cooling and electrification are studied to provide theoretical basis for the application of HTS filters in complex electromagnetic environments.In this thesis,a narrow-band HTS filter with the center frequency located in 0.8GHz is successfully designed and fabricated based on the material properties of HTS film and the design principle of HTS filter.The filter with the fractional bandwidth of 0.8%is coupled by two compact screw-in stepped impedance resonators.Quarter-wavelength spiral microstrip lines are inserted at the input and output ports,and the transmission zeros are located at the five frequency points of 0.7489GHz,0.8679GHz,1.3592GHz,1.4937GHz and1.5465GHz.The design of the filter gains the out-of-band rejection of about 60dB.Besides,the temperature dependence of microwave nonlinear properties of the HTS filter and the sensitivity related to substrate materials are discussed to explain some parameters selected in the design.The filter is fabricated on a double-sided YBCO thin film grown on the substrate of LaAlO₃.According to the above theoretical analysis,the fabrication of double-sided superconducting thin films and the processing of filter circuit chips are carried out,and the assembled HTS filter is placed at 65K to test its filtering performance.The measured results,which agree well with the simulations,show that the insertion loss and the input return loss are 0.5dB and-15dB at 65K,respectively.In addition,the structure of the resonator is simplified.Combined with the zero-degree feeding port,a dual-band HTS filter is designed.The dual-frequency of 3.5GHz and 5.54GHz are realized,and the circuit structure is simple.Meanwhile,based on the basic model of plane problem of elastic mechanics,a mechanical model of HTS filter thin film-substrate structure is established.The time-dependent distribution of interfacial shear stress and transverse normal stress during cooling and electrification is studied.The peeling behavior of the interface between the thin film and the substrate of the HTS filter is explored,and the conditions for stable operation of the HTS filter in complex electromagnetic environment are obtained.The results show that the interfacial shear stress is concentrated on both sides of the film,which is prone to failure when the shear force is greater than the adhesion force,while the transverse normal stress is concentrated in the center of the film,and the brittle film material is prone to failure when the stress state is tensile stress.Finally,the model is extended to the circular superconducting film-substrate structure,and the interfacial shear stress is calculated under the applied magnetic field.The rationality of the model is verified by comparing with the experimental results or theoretical calculation results.