Experimental Research On Mechanical-electrical Coupling Indentation Test Of PMN-PT For Cryogenic Temperature

Energy exploration can not live without cryogenic temperature in the field of polar region and outer space,temperature is also a key factor affecting material properties.Relaxor ferroelectrics have become a bridge of mutual conversion between mechanical,electric and heat energy due to their excellent piezoelectric,pyroelectric and ferroelectric properties.Testing the mechanical-electrical properties of relaxor ferroelectric single crystal-PMN-PT at cryogenic temperature has therefore great meaning to guide the production and application of materials.With simple sample-preparation,convenient operation and high resolutions of measurement and orientation,the nanoindentation testing technique has gradually been a common method to evaluate mechanical properties of materials in micro/nano scales.In this paper,combining the current research status at home and abroad of ferroelectric materials and nanoindentation testing technique for low temperature,we decided to analyse the effect of cryogenic temperature on electrical and mechanical performance parameters of PMN-PT by hysteresis loop and indentation test using ferroelectric analyzer and micro/nanoindentation testing device separately.The process of data analysis using electromechanical coupling theory for ferroelectric and Oliver-Pharr method for nanoindentation test was given and the structure and working principle of nanoindentation testing device was briefly introduced.We calibrated the device before test,the indentation curve of calibrated device on standard fused silica was well coincided with that of commercial indentation tester,and the testing result of hardness and elastic modulus was only 2.2% and 1.8% off the standard with high repeatability and accuracy.Then,we proposed and compared two correction methods on improving temperature drift for low temperature,and the correcting effect of indenter cooling with copper wire wound was tested.Next,we gave the transient thermal simulation analysis on extended stage of the tester,getting the required setting temperature at every test temperature with the approximate temperature fitting formula between specimen and cooling side,test efficiency was therefore improved.Finally,for testing safety,we found the breakdown voltage of aviation plug larger than the maximum loading voltage,and testing devices such as ferroelectric analyzer and signal generator were integrated outside vacuum system.The temperature-tunable ferroelectric hysteresis loop test explored the effect of vacuum,temperature and frequency on electrical properties of PMN-PT.It was shown that the spontaneous,remanent polarization and coercive field increased in a vacuum environment.When temperature decreased from 293 K to 100 K,the spontaneous and remanent polarization decreased while the coercive field increased,and the required voltage for a saturated hysteresis loop increased as well.When temperature and test voltage are constant,the spontaneous and remanent polarization decreased while the coercive field increased with the test frequency increased,and the effect of frequency on hysteresis loop decreased with voltage increased while increased with temperature decreased,indicating a sensitive electrical performance parameters of PMN-PT for cryogenic temperature.The electric-field-tunable indentation test at variable temperature explored the effect of temperature and electric-field strength on mechanical properties of PMN-PT,and changes in material properties were analyzed via the Olympus optical microscope.We found that the hardness and elastic modulus of PMN-PT at 293 K and 173 K were 4.67 GPa,330.34 GPa and 5.22 GPa,256.63 GPa,respectively.It was shown that the hardness increased and the elastic modulus decreased with temperature decreased from 293 K to 173 K.At constant temperature,the hardness increased and the elastic modulus decreased with the increase of test voltage which gave a higher degree of polarization on PMN-PT.In the temperature range of 293 K to 173 K,the lower the test temperature,the smaller the rangeability of each parameter at negative voltage,while the rule at positive voltage is on the contrary,meaning the electrostrictive strain and the corresponding electrostrictive coefficient increased with temperature decreased.The effect of vacuum degree on the indentation curve and the mechanical parameters of PMN-PT could be neglected.