Preparation Of DC SQUID Gradiometer Based On Y-Ba-Cu-O High Temperature Superconductin

With the development of The Times and the progress of science and technology,the performance and application requirements of magnetic sensors in modern industry,electronic communication and biomedical fields are increasing day by day.Superconducting quantum interference device(SQUID)based on Josephson effect has extremely high sensitivity,can effectively detect magnetic field below 1 n T,and has a wide range of applicability,can measure any physical quantity that can be converted into magnetic flux,such as magnetic field,magnetic field gradient,susceptibility,current,voltage and mechanical displacement,etc.With the discovery of YBCO,HTS SQUID based on YBCO is widely used in biomagnetic measurement,geophysical survey and material non-destructive evaluation.However,for many practical applications,it is not possible to place the signal source very close to SQUID,and it is necessary to detect extremely weak signals when the background noise is several orders of magnitude higher than the actual signal.In this case,because gradiometers measure the gradient of magnetic field changes and are not affected by magnetic fields that can be treated as uniform variations,such as distant noise signals and ambient noise,they are effectively shielded.Hence,SQUID gradiometer is more desirable and economical to detect distant signal sources.This paper is based on high-temperature superconducting materials YBCO,explore the preparation of superconducting film,and the prepared film used in SQUID gradiometer.The main results are as follows:(1)The epitaxial conditions of YBCO/STO superconducting thin films using PLD were explored.Before the experiment,STO substrate was annealed at 1000℃for 1 h to make the substrate surface more smooth,and then the film was grown by PLD system.The specific experimental conditions were as follows:The oxygen concentration in the vacuum chamber is 20 Pa,the substrate temperature is 850℃,the energy density of pulsed laser is 3 J/cm~2,the pulse laser frequency is 1 Hz,and the target distance is 6 cm.Finally,after the deposition,5×10~4Pa oxygen is passed into the vacuum chamber and kept at 500℃for 1.5 h,so that the YBCO film could fully absorb oxygen and transform into ortho-phase superconducting film.(2)The structure and superconductivity properties of YBCO/STO superconducting thin films are characterized.The surface roughness of the films is less than 10 nm,with high crystallinity,good c-axis orientation and no other heterophase.The critical temperature of the film can reach above 91 K,and the critical current at 77 K ranges from tens of milliamps to thirty milliamps.(3)A direct coupled first-order planar gradiometer is designed.After obtaining YBCO/STO superconducting thin film,all the processes from film to device,such as magnetron sputtering,photography and etching,etc.are investigated.Finally,A SQUID gradiometer is prepared.Its critical temperature can still reach about 90 K,and its critical current density is about 4.2×10~6A/cm~2,which meets the requirements of superconductivity.