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Research on Acoustic Emissions from Organic Martensite Analogues

An extremely fast change in the crystal structure of organic crystals results in a few of these crystals jumping around when heated.

© Wiley-VCH

Recently, in the journal Angewandte Chemie, Scientists demonstrated that the crystals give out acoustic signals during this process, which indeed could be useful for examining the characteristics of this phenomenon. The demonstration proved that this process is analogous to martensitic transitions observed in some alloys and steel.

Martensite refers to a form of steel developed by quenching austenite, and gives its name to a specific kind of phase transition. The atoms are prevented from adopting their preferred structure at the lower temperature because of the rapid cooling of the austenite. They instead move in harmony from the martensite lattice. In jumping crystals, an increasing number of atoms also modify their lattice positions in concert. The fact that the crystals frequently explode and the high speed of this phenomenon have earlier made it impossible to establish this theory, comprehend the details and make use of this so called thermosalient effect. The potential of the hopping crystals to convert heat into movement or work in an extremely rapid manner is considered to be potentially useful for the development of microscale robotic arms or artificial muscles.

The team from New York University Abu Dhabi, the German Electron Synchrotron (DESY) in Hamburg, and the Max Planck Institute for Solid State Research in Stuttgart started the research from the assumption that the sudden discharge of the accumulated elastic tension in jumping crystals leads to relatively powerful acoustic waves, similar to seismic waves from an earthquake. Headed by Panče Naumov, the Researchers decided to study crystals of the vegetable amino acid L-pyroglutamic acid (L-PGA). These jumping crystals are capable of changing their crystal structure when heated to between 65 and 67 °C; they then return to their starting structure after being cooled between 55.6 and 53.8 °C, as established by X-ray crystallography with synchrotron radiation.

As postulated, the crystals give out clear acoustic signals during the transition. It is possible to register these signals with a piezoelectric sensor. The amplitude, frequency, number and form of the signals provided the Researchers with information about the mechanism and the dynamics of the effect. The energy and intensity of the initial acoustic wave were majorly higher and the rise time shorter than for succeeding waves. The more efficient propagation of the elastic wave via the defect-free medium at the beginning of the phase transition is considered to be the reason for this. The number of microfissures increases, as the transition progresses, thus decreasing the elastic stress.

The phase boundary between the diverse crystal structures progresses at 2.8 m/s in L-PGA, which is several thousand times quicker than other phase transitions. However, it has been observed that the crystal structures are more similar to each other than expected. The transition deals with a contraction in the third and expansions in two dimensions, all in the range of just 0.5-1.7%.

Our study shows that the jumping crystals are a class of materials analogous to inorganic martensite, and this could be of a tremendous significance for applications such as all-organic electronics. Acoustic emission techniques finally deliver direct insight into these rapid transitions. Our results indicate that organic matter which is normally perceived as soft and brittle, and much harder materials, such as metals and metal alloys are, at least at the molecular level, not that different. The research into the organic solid state could allow us to gain a better understanding of the related macroscopic effects.

Panče Naumov, Head of the research team


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