Surprising size-confinement effect of magnetocaloric materials
(Nanowerk Spotlight) Certain magnetic materials change their temperature when an external magnetic field is applied to them. This magnetocaloric effect (MCE) could become the heart of future cooling technologies.
At ambient temperature, the prime magnetocaloric standard is still made of gadolinium (Gd). Exposing this rare earth element to a magnetic field of one Tesla results in a temperature change of almost 3 °C.
The Gd5(Si,Ge)4 materials achieved worldwide prominence in the materials science community due to their giant magnetocaloric effect, discovered in 1997 by Pecharsky and Gschneidner. This discovery boosted the research into magnetic refrigeration technology in general and magnetocaloric materials in particular.
The most intriguing result presented in this paper is that size-confinement – going from macroscopic bulk samples towards nanoparticles – leads to drastic changes in the atomic structure of the material: The change from positive thermal expansion at the macroscale towards a negative thermal expansion at the nanoscale.
Temperature dependence of the Gd5Si1.3Ge2.7 nanogranules (blue open circles) and bulk relative unit-cell volume (red open diamonds). The volume values were normalized to V (T = 340K, T = 330K, for nano- and bulk, respectively) and the linear fits in the 90–150, 175–210, and 255–340 K temperature ranges are presented. (Image: João H. Belo, IFIMUP)
Whereas a majority of materials exhibits a positive thermal expansion (PTE), i.e. they expand when heated and contract when cooled down, researchers also have discovered materials with the opposite behavior: negative thermal expansion (NTE).
NTE materials are of increasing technological interest mostly because of the idea of mixing them with a symmetric PTE material in order to create a composite material which has near-zero thermal expansion. The nanoparticle shape is particularly suitable for the design of such composites.
Materials with near-zero thermal expansion could be used to create high-temperature stable thermal contacts; this would be particularly useful in microelectronic and semiconductor devices that undergo thermal cycling operations. Other application areas include precision instruments where any volumetric change induced by temperature can lead to their malfunctioning and erroneous read-outs; fiber-optic and electro-optical sensors; and substrate materials for mirrors in various telescope and satellite applications.
According to Belo, another very interesting point is that nanoscale Gd5(Si,Ge)4 appear to be the first magnetic nanoparticles exhibiting a negative thermal expansion above their transition temperature.
"Our results, in particular the cross-over from positive at the macroscale to negative at the nanoscale, underline the need for understanding what is happening at the nanoscale for such a dramatic change to occur," Belo points out. "Unfortunately, there is scarce information available on the size-reduction effects of these materials and even scarcer concerning the temperature dependence of their atomic structure at nanoscale."
Belo and his co-authors, as well as other scientists, have already suggested that there might be a more general and new mechanism underlying the structural behavior of magnetic nanoparticles promoting their negative thermal expansion. This mechanism is most probably associated with surface effects, since nanoparticles have a much higher surface/volume ratio than macroscopic materials.
The team's results, as presented in their paper, are new on two fronts. Firstly, this appears to be the first time someone has measured the thermal expansion of Gd5(Si,Ge)4 nanoparticles; and secondly, no negative thermal expansion behavior has ever been found in Gd5(Si,Ge)4 compounds before.
"Our overall explanation for this behavior rises from the similarity of producing nanoparticles and applying a pressure to a material: there is an intrinsic surface pressure (very much like in the formation of a drop of water) on every nanoparticle which is inversely proportional to the diameter of the nanoparticle," explains Belo. "We therefore suggest that the negative thermal expansion could also be induced by the application of pressure at macroscopic samples."
"We believe that our results will trigger a renewed interest in the study of the thermal expansion of magnetic nanoparticles in general and nanoparticles of the R5(Si,Ge)4 composition (where R stands for rare Earth element) in particular," Belo adds. "More studies on the origin of this effect should be performed by applying other techniques, such as Raman spectroscopy, which could help unveil what is changing on the lattice vibrations to change from positive to negative thermal expansion."
The next stages of the team's investigation are already underway. They are planning to deepen the understanding of the lattice vibrations of the Gd5(Si,Ge)4 nanoparticles and learn what are the most significant changes in comparison with the bulk counterpart.
Simultaneously, they are optimizing the procedure for synthesizing controlled-size Gd5(Si,Ge)4 nanoparticles with different compositions of Si/Ge ratio.
In addition, they are also planning high-pressure Synchrotron X-ray diffraction experiments to evaluate the similarities between the size-confinement effect and the pressure effect on the atomic structure of these materials.
According to Belo, future work in this field will cover several general aspects: Control and optimization of negative thermal expansion materials; Development of near-zero thermal expansion composites for various applications; Deepening the understanding of the main origins of the negative thermal expansion behavior; and boosting the research effort on the size-confinement effects of R5(Si,Ge)4 compounds.