New high-performance materials for power devices, sensors and other applications will only be possible when materials scientists have a better understanding of how they can exploit phase transitions. Now, researchers at the Georgia Institute of Technology and Oak Ridge National Laboratory (ORNL) in the USA have developed a new non-destructive technique for investigating phase transitions using the acoustic response at the nanoscale detected with an electrically conductive atomic force microscope (AFM) probe could lead to novel ferroelectric materials. The same technique, described in the journal Advanced Functional Materials [DOI: 10.1002/adfm.201504407], could also be used to investigate ferroelastics, solid protonic acids and relaxors.
"We have developed a new characterization technique that allows us to study and map changes in the crystalline structure and changes in materials behavior at substantially smaller length scales with a relatively simple approach," explains Georgia Tech's Nazanin Bassiri-Gharb. "Knowing where these phase transitions happen and at which length scales can help us design next-generation materials."
External stimuli can trigger a phase transition in various ferroelectric materials, such as lead magnesium niobite-lead titans, commonly known as PMN-PT, and used in high-resolution ultrasound transducers and pyroelectric devices (infrared detectors). The phase transition occurs at the boundary between one crystal type and another and can amplify piezoelectric and dielectric effects. Observing the changes in the bulk material or with electron microscopy is possible but now the US teams have demonstrated how an acoustic technique can home in on the scale that lies between the bulk and the tens of atoms domain.
Their approach more formally known as band-excitation piezoresponse force microscopy (BE-PFM), which was developed at ORNL, allowed the researchers to analyze the changes in resonant frequencies to betray a phase change. "We've had very good techniques for characterizing these phase changes at the large scale, and we've been able to use electron microscopy to figure out almost atomistically where the phase transition occurs, but until this technique was developed, we had nothing in between," adds Bassiri-Gharb. "To influence the structure of these materials through chemical or other means, we really need to know where the transition breaks down, and at what length scale that occurs. This technique fills a gap in our knowledge."
In order to build a complete picture, the team also modeled the relaxor-ferroelectric materials using computational thermodynamic methods and demonstrated that these corroborated the experimental results revealing the existence of a phase transition and the evolution of a complex domain pattern. Specifically for relaxor-ferroelectrics, it had been assumed that localized pockets of material in a different phase than the bulk exist and endow the material with its particular characteristics. Moreover, the greatest response occurs at these locations.
"We would now like to use this method to probe a wide range of phase transition-related phenomena in ferroelectric systems," Bassiri-Gharb told Materials Today. "For example, we would like to know at which length-scale (be these sample or grain size), the phase transition might become suppressed due to boundary conditions. This is, for example, of paramount importance in ferroelectric thin films, where the presence of a phase transition and the accompanying large enhancement in the piezoelectric response is key in processing smaller and smaller piezoelectric micro- and nano-electromechanical systems."
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the bestselling science book "Deceived Wisdom".