Phonon-glass electron-crystal thermoelectric clathrates: Experiments and theory

Type-I clathrate compounds have attracted a great deal of interest in connection with the search for efficient thermoelectric materials. These compounds constitute networked cages consisting of nano-scale tetrakaidecahedrons (14 hedrons) and dodecahedrons (12 hedrons), in which the group 1 or 2 elements in the periodic table are encaged as the so-called rattling guest atom. It is remarkable that, though these compounds have crystalline cubic-structure, they exhibit glass-like phonon thermal conductivity over the whole temperature range depending on the states of rattling guest atoms in the tetrakaidecahedron. In addition, these compounds show unusual glass-like specific heats and THz-frequency phonon dynamics, providing a remarkable broad peak almost identical to those observed in topologically disordered amorphous materials or structural glasses, the so-called Boson peak. An efficient thermoelectric effect is realized in compounds showing these glass-like characteristics. This decade, a number of experimental works dealing with type-I clathrate compounds have been published. These are diffraction experiments, thermal and spectroscopic experiments in addition to those based on heat and electronic transport. These form the raw materials for this article based on advances this decade. The subject of this article involves interesting phenomena from the viewpoint of not only physics but also from the view point of the practical problem of elaborating efficient thermoelectric materials. This review presents a survey of a wide range of experimental investigations of type-I clathrate compounds, together with a review of theoretical interpretations of the peculiar thermal and dynamic properties observed in these materials.Comment: 51pages, 43 figure

Interacting Dipoles in Type-I Clathrates: Why Glass-like though Crystal?

Almost identical thermal properties of type-I clathrate compounds to those of glasses follow naturally from the consideration that off-centered guest ions possess electric dipole moments. Local fields from neighbor dipoles create many potential minima in the configuration space. A theoretical analysis based on two-level tunneling states demonstrates that interacting dipoles are a key to quantitatively explain the glass-like behaviors of low-temperature thermal properties of type-I clathrate compounds with off-centered guest ions.From this analysis, we predict the existence of a glass transition

Accounting for Paramagnetic Influence in Wetted Surface Area Studies of Particle Suspensions Using Solvent Relaxometry

Polymer matrix composites (PMC) are structural materials sustained by the synergy between a reinforcing fiber, also known as filler, and a resin matrix. Since performance relies on interfacial interactions between the PMC components, the surface area of particle fillers is a key design consideration. Here, the characterization of Kevlar® pulps and micropulps, derived from milling pulps, was conducted using a multi-technique approach, focusing on the specific surface area. Conventional techniques used to determine surface area require aggressive outgassing steps that can impact the structure of polymeric particles. Hence, a convenient alternative is explored by deploying time-domain nuclear magnetic resonance (TD-NMR) under conditions close to those experienced in-situ. In this thesis, the correlation between NMR relaxation rates and surface area acquired from BET analysis has been established. The specific surface area of Kevlar® pulps was found to increase by as large as a factor of three upon milling pulp materials (8 – 18 m2 g-1) to micropulps (20 – 24 m2 g-1). No changes were found in the chemical structure of Kevlar® following the particle size reduction, however, trace iron (Fe) was identified in the range 11 - 2,633 ppm, which may perturb the NMR signal. To address the influence of Fe, the surface relaxivity of Kevlar® (0.7 ± 0.1 μm s-1) was determined based on a linear relationship between the Fe content and its perturbation of the relaxation time T2. Lastly, the wetted specific surface area was calculated from the NMR data, yielding values within 20% difference of those determined by BET of freeze-dried samples. The obtained trends agree with foreseen effects of polymer drying and particle size reduction. The presented data indicate the value of determining the surface relaxivity of different materials, enabling a deeper understanding of interfacial interactions, as well as the rapid determination of wetted surface area for a variety of polymeric systems

Glasslike vs. crystalline thermal conductivity in carrier-tuned Ba8Ga16X30 clathrates (X = Ge, Sn)

The present controversy over the origin of glasslike thermal conductivity observed in certain crystalline materials is addressed by studies on single-crystal x-ray diffraction, thermal conductivity k(T) and specific heat Cp(T) of carrier-tuned Ba8Ga16X30 (X = Ge, Sn) clathrates. These crystals show radically different low-temperature k(T) behaviors depending on whether their charge carriers are electrons or holes, displaying the usual crystalline peak in the former case and an anomalous glasslike plateau in the latter. In contrast, Cp(T) above 4 K and the general structural properties are essentially insensitive to carrier tuning. We analyze these combined results within the framework of a Tunneling/Resonant/Rayleigh scatterings model, and conclude that the evolution from crystalline to glasslike k(T) is accompanied by an increase both in the effective density of tunnelling states and in the resonant scattering level, while neither one of these contributions can solely account for the observed changes in the full temperature range. This suggests that the most relevant factor which determines crystalline or glasslike behavior is the coupling strength between the guest vibrational modes and the frameworks with different charge carriers.Comment: 8 pages, 4 figures, 4 tables, submitted to Phys. Rev.

Cage-size control of guest vibration and thermal conductivity in Sr8Ga16Si30-xGex

We present a systematic study of thermal conductivity, specific heat, electrical resistivity, thermopower and x-ray diffraction measurements performed on single-crystalline samples of the pseudoquaternary type-I clathrate system Sr8Ga16Si30-xGex, in the full range of 0 < x < 30. All the samples show metallic behavior with n-type majority carriers. However, the thermal conductivity and specific heat strongly depend on x. Upon increasing x from 0 to 30, the lattice parameter increases by 3%, from 10.446 to 10.726 A, and the localized vibrational energies of the Sr guest ions in the tetrakaidekahedron (dodecahedron) cages decrease from 59 (120) K to 35 (90) K. Furthermore, the lattice thermal conductivity at low temperatures is largely suppressed. In fact, a crystalline peak found at 15 K for x = 0 gradually decreases and disappears for x > 20, evolving into the anomalous glass-like behavior observed for x = 30. It is found that the increase of the free space for the Sr guest motion directly correlates with a continuous transition from on-center harmonic vibration to off-center anharmonic vibration, with consequent increase in the coupling strength between the guest's low-energy modes and the cage's acoustic phonon modes.Comment: 7 pages, 7 figures, submitted to PR

Significance of Off-Center Rattling for Emerging Low-lying THz Modes in type-I Clathrates

We show that the distinct differences of low-lying THz-frequency dynamics between type-I clathrates with on-center and off-center guest ions naturally follow from a theoretical model taking into account essential features of the dynamics of rattling guest ions. Our model analysis demonstrates the drastic change from the conventional dynamics shown by on-center systems to the peculiar dynamics of off-center systems in a unified manner. We claim that glass-like plateau thermal conductivities observed for off-center systems stem from the flattening of acoustic phonon dispersion in the regime |k|<|G|/4. The mechanism is applicable to other systems such as glasses or relaxers