Robert W. McKinney

Potential for high thermoelectric performance in n-type Zintl compounds: a case study of Ba doped KAlSb4

High-throughput calculations (first-principles density functional theory and semi-empirical transport models) have the potential to guide the discovery of new thermoelectric materials. Herein we have computationally assessed the potential for thermoelectric performance of 145 complex Zintl pnictides. Of the 145 Zintl compounds assessed, 17% show promising n-type transport properties, compared with only 6% showing promising p-type transport. We predict that n-type Zintl compounds should exhibit high mobility μn while maintaining the low thermal conductivity κL typical of Zintl phases. Thus, not only do candidate n-type Zintls outnumber their p-type counterparts, but they may also exhibit improved thermoelectric performance. From the computational search, we have selected n-type KAlSb4 as a promising thermoelectric material. Synthesis and characterization of polycrystalline KAlSb4 reveals non-degenerate n-type transport. With Ba substitution, the carrier concentration is tuned between 1018 and 1019 e− cm−3 with a maximum Ba solubility of 0.7% on the K site. High temperature transport measurements confirm a high μn (50 cm2 V−1 s−1) coupled with a near minimum κL (0.5 W m−1 K−1) at 370 °C. Together, these properties yield a zT of 0.7 at 370 °C for the composition K0.99Ba0.01AlSb4. Based on the theoretical predictions and subsequent experimental validation, we find significant motivation for the exploration of n-type thermoelectric performance in other Zintl pnictides.

Thermoelectric properties of bromine filled CoSb3 skutterudite

Historically, the improved thermoelectric performance of skutterudite compounds has largely been driven by the incorporation of electropositive donors on interstitial sites. These “rattlers” serve to optimize both electronic and thermal properties by tuning the carrier concentration and scattering phonons. In this work, we show that interstitial bromine can be incorporated into CoSb3 and assess the impact on electronic and thermal transport. In contrast to prior high pressure syntheses with iodine, interstitial bromine incorporation is achieved at ambient pressure. Transport properties are stable up to at least 375 °C. Bromine serves as an electronegative acceptor and can induce degenerate (>5 × 1019 cm−3) hole densities. In contrast to other p-type skutterudite compositions, bromine preserves the intrinsically high hole mobility of CoSb3 while significantly reducing the lattice thermal conductivity. The development of a stable p-type dopant for the interstitial filler site enables the development of skutterudites with both donor and acceptor interstitials to maximize phonon scattering while maintaining the high mobility of CoSb3.

Developing an optical chopper-modulated capacitive probe for measuring surface charge

Gravitational-wave observatories such as Laser Interferometer Gravitational-Wave Observatory (LIGO) use suspended optics in a Michelson interferometer configuration to measure strains in space between 10 Hz and 3 kHz. One potential noise source in this frequency range is the buildup and motion of surface charge on the optics, which can generate fluctuating electric fields, interfere with position control, and reduce reflectance by attracting dust to the optical surface. We have developed a capacitive probe to measure the magnitude and relaxation time of surface charge deposited on smaller test optics in high vacuum ( approximately 10(-5) Torr). Our device modulates capacitance with a tuning-fork optical chopper between probe and sample, chosen for vacuum compatibility and minimal cost. We have found that the probe has a resolution of (3.5+/-0.5)x10(5) e(-)cm(2) in air, on the order of charging levels that could contribute noise to Advanced LIGO, and sufficient for measuring relaxation times on test optics.

Focused terahertz waves generated by a phase velocity gradient in a parallel-plate waveguide

We demonstrate the focusing of a free-space THz beam emerging from a leaky parallel-plate waveguide (PPWG). Focusing is accomplished by grading the launch angle of the leaky wave using a PPWG with gradient plate separation. Inside the PPWG, the phase velocity of the guided TE1 mode exceeds the vacuum light speed, allowing the wave to leak into free space from a slit cut along the top plate. Since the leaky wave angle changes as the plate separation decreases, the beam divergence can be controlled by grading the plate separation along the propagation axis. We experimentally demonstrate focusing of the leaky wave at a selected location at frequencies of 100 GHz and 170 GHz, and compare our measurements with numerical simulations. The proposed concept can be valuable for implementing a flat and wide-aperture beam-former for THz communications systems.

Search for new thermoelectric materials with low Lorenz number

To date, efforts to reduce thermal conductivity in thermoelectric materials have largely focused on reducing the vibrational component. However, in an optimized thermoelectric material, the electronic component can often contribute an equivalent amount to the total thermal conduction. In principle, the Lorenz number of a bulk semiconductor can be reduced to a small fraction of the Sommerfeld value through the use of a band-pass energy filter in the transport distribution function. One strategy to achieve this band-pass filter is through multiple bands that are offset in energy. Despite a reduction in power factor, we show that a lower Lorenz number can lead to zT enhancement of nearly 40% over the single parabolic band value for materials with electronic properties similar to PbTe. One signature for this behavior is a density of states that rapidly increases deeper into the band. Guided by this insight, we conducted a high-throughput computational search for materials with exceptionally low Lorenz number and high thermoelectric quality factor. From this search, we identify materials that are predicted to have both high quality factor and low Lorenz number. Intriguingly, we find that the vast majority of known thermoelectric materials exhibit these traits; further, new materials have emerged that warrant further investigation.

Ionic vs. van der Waals layered materials: identification and comparison of elastic anisotropy

In this work, we expand the set of known layered compounds to include ionic layered materials, which are well known for superconducting, thermoelectric, and battery applications. Focusing on known ternary compounds from the ICSD, we screen for ionic layered structures by expanding upon our previously developed algorithm for identification of van der Waals (vdW) layered structures, thus identifying over 1500 ionic layered compounds. Since vdW layered structures can be chemically mutated to form ionic layered structures, we have developed a methodology to structurally link binary vdW to ternary ionic layered materials. We perform an in-depth analysis of similarities and differences between these two classes of layered systems and assess the interplay between layer geometry and bond strength with a study of the elastic anisotropy. We observe a rich variety of anisotropic behavior in which the layering direction alone is not a simple predictor of elastic anisotropy. Our results enable discovery of new layered materials through intercalation or de-intercalation of vdW or ionic layered systems, respectively, as well as lay the groundwork for studies of anisotropic transport phenomena such as sound propagation or lattice thermal conductivity.

Waveguide Devices for Terahertz Signal Processing

We introduce two waveguide based devices for signal processing in future terahertz wireless communications systems: a leaky-wave antenna for frequency multiplexing and a Tjunction waveguide for broadband power splitting.