Scientists have demonstrated that sparse concentrations of nanoparticles can impact the flow of heat energy through solid materials. While this early work was based around a simple ice model, it paves the way for research towards more advanced, complex and efficient thermal insulation materials. The results, based on research at the U.S. Department of Energy’s Advanced Photon Source (APS), were published in the journal Nanomaterials.
Crystals are ordered, repeating arrangements of atoms. At temperatures above absolute zero crystal structures oscillate. As crystals are interconnected by tight microscopic bonds these oscillations propagate through the material. When one atom is displaced from its equilibrium in the lattice it transmits its movement to its neighbour and this distortion travels through the material. These waves of vibrational energy created by oscillating atoms are known as phonons.
Phonons transmit sound and heat through materials. By controlling the phononic properties of materials this transmission of energy can be controlled, opening opportunities in advanced sound and heat insulation. Phononics is a relatively new research field that explores these mechanical vibrations and how to control them.
The propagation of phonons through materials can be controlled with phononic crystals. These are artificial materials made of periodic arrangement of scatterers embedded in a matrix. They are designed to interfere with sound waves in a given frequency band, causing them to slow down, deviate, or become trapped.
There has been success developing phononic crystals that work on kilohertz to megahertz frequencies, covering audible sound and ultrasound. However, as frequencies move higher and higher wavelengths get shorter and shorter. This means that the scatterers in phononic crystals and the spaces between them must become smaller and smaller – eventually entering nanometre sizes – to impact energy propagation.
Heat is transmitted by acoustic phonons with terahertz frequencies. Because of the challenges in developing phonic crystals with the nanoscale properties required to impact these very high frequencies there has been little progress on phonic crystals in the terahertz domain, to control heat transfer.
Sound propagation can also be impacted in a similar way by adding disorder to a system. A random arrangement of particles in a material can interfere with and scatter propagating waves. This could be a simpler way to impact the propagation of terahertz phonons.
To investigate this idea, an international team of researchers have carried out a series of experiments at the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science user facility at DOE’s Argonne National Laboratory, exploring how random arrangements of nanoparticles suspended in a liquid impact the propagation of terahertz phonons. They have found that nanoparticles, even in small concentrations, enhance acoustic damping.
But what works in a liquid does not necessarily work in a solid. Phonon excitations have a much longer lifetime in solids than in liquids. This makes it harder to affect them, so it was not clear if nanoparticles in a solid would also dampen terahertz phonon propagation.
In their latest work, the researchers returned to the APS to explore this. The work was carried out at beamline 30-ID.
The team suspended spherical gold nanoparticles with a diameter of 15 nanometres in water, which they then froze. To probe the phonon spectrum of the ice, they used high-resolution inelastic X-ray scattering. This is an advanced X-ray vibrational spectroscopy technique that can be used to determine the transfer of momentum and energy in materials. Measurements of ice containing gold nanoparticles were compared with those for pure ice.
The team found that the gold nanoparticles did impact the movement of energy through the ice. Even low nanoparticle concentrations of about one percent in volume were found to be sufficient to affect the propagation of acoustic phonons in ice.
While ice is a simple model system, this work shows that random arrangements of nanoparticles can control the movement of terahertz acoustic phonons through materials. This can be built on to understand the phenomenon in more complex systems, potentially paving the way for the development advanced thermal insulation materials.
The natural next step for this work is to replace the nanoparticles with more complex nano-objects with engineered architectures, along the lines of phononic crystals. Researchers could then explore whether – and how – different designs and structures impact terahertz sound propagation.
- Michael Allen
See: A. De Francesco1,2,, L. Scaccia3, F. Formisano1,2, E. Guarini4, U. Bafile5, D. Nykypanchuk6, A. Alatas7, M. Li8, S.T. Lynch9, A. Cunsolo9, “The Effect of Embedded Nanoparticles on the Phonon Spectrum of Ice: An Inelastic X-ray Scattering Study,” Nanomaterials 2023, 13(5), 918 (February 2023)
Author affiliations: 1CNR-IOM & INSIDE@ILL c/o Operative Group in Grenoble; 2Institut Laue-Langevin; 3Universita’ di Macerata; 4Universita’ di Firenze; 5Istituto di Fisica Applicata “Nello Carrara”; 6Brookhaven National Laboratory; 7Argonne National Laboratory; 8Massachusetts Institute of Technology; 9University of Wisconsin-Madison.
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