Study On Delamination Strength Of Second Generation HTS Tape

The second-generation high temperature superconducting(HTS)tape has high commercial value and application prospects in many electrical scenarios for its high current carrying capacity,high transition temperature and high irreversible field in strong magnetic field.Commercial developing of second-generation high temperature superconducting tape mainly focuses on superconducting current limiters,superconducting motors and energy storage devices,etc.,and their commercial value and application prospects are also promising.With the wide application of superconducting material,study on the mechanics of high-temperature superconductors is becoming the limitation on promotion of superconducting technology.The Lorentz force on superconducting tape under high field condition and the thermal stress on superconducting coil impregnated with epoxy resin will both lead to delamination on superconducting tape.Comparing to the other mechanical performance indexes,delamination strength is far more important in engineering practicing.However,the testing process of delamination strength is cumbersome,the testing data is discrete,and there is no uniform industry standard for the testing process.These become a major problem for testing and improving mechanical properties of superconducting tape.At the same time,due to the gradual expansion of the application environment of the HTS tape,delamination of the strip occur from time to time,and there is an urgent need for a method to improve the delamination strength that can be applied in production.To further analyze and solve delamination problem of superconducting tape,this paper improves the testing method of delamination strength,proposes the tape microscopic defect propagation model and factors affecting delamination strength.Based on these,an improving method to Mechanical strength(including delamination strength)of superconducting tape is proposed.In experiments,the model conjecture of delamination mechanism is verified.In this paper,the existing testing method to delamination strength was improved,and a delamination strength testing platform was built.For the improved testing method,the operations were simple and the repeatability is strong.The method is suitable for testing delamination strength of variable types of tapes(such as silver tape)within varying temperature range of 77 K ~ 298 K.The method has been selected as a one of the main options of national testing standard for delamination strength of 2nd generation HTS tape.Also,model conjectures of tape delamination mechanism basing on mechanical analysis and micro model are proposed.The conjecture basing on mechanical analysis explains the delamination process from the perspective of mechanics by analyzes the mechanical properties of each layer of tape.The conjecture basing on micro model explains the delamination process based on two important hypotheses:(1)fracture and delamination only happens on superconducting layer(inside,interface);(2)delamination interface expands along layer gap vertical to c axis of superconducting layer.This conjecture converses and promotes the microdefect expansion model to explain mechanisms of two types of mechanical damage(stretching and bending)of tape,and attempts to include mechanical damages into one model framework.With these two conjectures,five important factors affecting mechanical strength of tape are derived,which are tape material intrinsic parameter(temperature),tape structure(copper plating process),total-amount of superconducting layer defect(thickness of superconducting layer),size of superconducting layer defect and defect extension action.Among these,the first three factors reflect intrinsic characteristics,and the other two could be applied in producing practice with targeted technological innovation.With the idea of reducing size of defect inside of superconducting layer,in this paper traditional mechanical slitting process is replaced by picosecond ultraviolet(UV)laser slitting process.The micro-cracks(major defect)on slitting edge in mechanical splitting is avoided,and the electrical and mechanical properties of slit tape are improved.The strip prepared by the laser slitting process has an increase in delamination strength of more than 10% compared with the traditional mechanical slitting,and the minimum turning diameter of the superconducting surface is increased to 7mm.Meanwhile,the picosecond UV laser slitting process makes narrow incision and small heat-affected zone,it can reduce damage to tape in slitting and has tremendous application prospects.With the idea of stopping the expanding of micro-cracks,this paper proposes a method that introduces columnar superconducting crystal with abnormal texture in superconducting layer by using ultraviolet laser to etch buffer layer.The superconducting crystal columns with abnormal texture can stop the growth of micro-cracks in the superconducting layer,and the overall mechanical properties of tape will be improved.The HTS tapes with a laser-etched buffer layer has an increase in delamination strength of more than 20% compared with the control group,and the highest delamination strength test value at room temperature reaches 80.49 MPa,which has a broader application prospect.