John Toon of the Georgia Institute of Technology explains how non-destructive investigations could help design nano-scale materials with enhanced properties.
Understanding where and how phase transitions occur is critical to developing new generations of the materials used in high-performance batteries, sensors, energy-harvesting devices, medical diagnostic equipment and other applications. But there has been no reliable way to study and simultaneously map these phenomena at the relevant length scales.
Now, researchers at the Georgia Institute of Technology and Oak Ridge National Laboratory (ORNL) have developed a new non-destructive technique for investigating these material changes by examining the acoustic response at the nanoscale. The technique uses electrically-conductive atomic force microscope (AFM) probes.
The approach has been used in ferroelectric materials, but could also have applications in ferroelastics, solid protonic acids and materials known as relaxors. Though widely used, relaxor-ferroelectrics and PZT are still not well understood. In ferroelectric materials, such as PZT (lead zirconate titanate), phase transitions can occur at the boundaries between one crystal type and another, under external stimuli. Properties can be amplified at the grain boundaries, which are caused by the “confused chemistry” of the materials.
In relaxor-ferroelectrics, for example, it is believed that there are pockets of material in phases that differ from the bulk, a distortion that may help confer the material’s attractive properties. Using their technique, the researchers confirmed that the phase transitions can be extremely localised.
“We have developed a new characterisation technique that allows us to study changes in the crystalline structure and changes in materials behaviour at substantially smaller length scales with a relatively simple approach,” said Nazanin Bassiri-Gharb, an associate professor in Georgia Tech’s Woodruff School of Mechanical Engineering.
The researchers realised they could detect these phase transitions using acoustic techniques in samples at size scales between the bulk and tens of atoms. Using band-excitation piezo-response force microscopy (BE-PFM) techniques developed at ORNL, they analysed the resulting changes in resonant frequencies to detect phase changes in sample sizes relevant to the material applications. To do that, they applied an electric field to the samples using an AFM tip that had been coated with platinum to make it conductive, and through generation and detection of a band of frequencies.
“We’ve had very good techniques for characterising 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,” said Bassiri-Gharb. “To influence the structure of these materials through chemical or other means, we really needed to know where the transition breaks down, and at what length scale that occurs. This technique fills a gap in our knowledge.”
The changes the researchers detect acoustically are due to the elastic properties of the materials, so virtually any material with similar changes in elastic properties could be studied in this way.
“This new method will allow for much greater insight into energy-harvesting and energy transduction materials at the relevant length sales,” noted Rama Vasudeven, a materials scientist at the Center for Nanophase Materials Sciences, a US Department of Energy user facility at ORNL.
Use of the AFM-based technique offers a number of attractive features. According to Bassiri-Gharb, laboratories already using AFM equipment can easily modify it to analyse these materials by adding electronic components and a conductive probe tip.
“It turns out that many energy-related materials have electrical transitions, so we think this is going to be very important for studying functional materials in general,” Bassiri-Gharb added. “The potential for gaining new understanding of these materials and their applications are huge.”