April AMP_Digital

R esearch focusing on thermo- electric materials is gaining in im- portance with the current interest in clean energy generation and waste heat recovery. Advanced thermoelec- tric materials require balancing op- posed physical properties. For example, maximizing the power factor, ZT (the thermoelectric figure of merit), requires high electrical conductivity and gener- ated voltage (Seebeck coefficient) and low thermal conductivity. To date, the strategies used to achieve this balance were primarily aimed at enhancing phonon scattering to hinder thermal conductivity in ways that do not un- duly scatter charge carriers (electrons and holes). Among the vast number of po- tential thermoelectric materials, half- Heusler alloys are promising due to their large power factor, ZT, mechani- cal durability, and the great abundance of their constituent elements. These alloys are intermetallic compounds with the chemical formula XYZ, where X and Y are transition metals and Z is either a semiconductor or nonmag- netic metal. The half-Heusler crystal structure can be described as a mix of rock-salt and zinc-blend crystal struc- tures. Due to their unique structure, phonon scattering can be tuned by in- troducing isovalent (same valence) or aliovalent (different valence) substitu- tions. Further, controlled phase sepa- ration can lead to additional scattering. Recently, QuesTek Innovations LLC, Evanston, Illinois, and Intermolecular, San Jose, California, joined forces to further push the boundaries of ther- moelectric materials. To do so, they used a combination of state-of-the-art computational modeling and high throughput experimentation (HTE) to strategically screen the extensive mate- rial space and accelerate learning. EXPERIMENTAL APPROACH While the chemical and structur- al diversity of thermoelectric materials is vast, computational modeling ad- vances have significantly improved the ability to predict electron and phonon scattering and transport. Sophisticated modeling also enables predicting the structure and dopability of ma- terials. In addition, HTE combined with computational simulation en- ables validation and refinement of electron and phonon transport models. A physical vapor deposition HTE platform was used in this study (Fig. 1). Simultaneous depo- sition from up to four magnetron sputtering targets is possible with this setup, enabling fine composi- tional control in multicomponent systems. In addition, several other process parameters (e.g., pressure, substrate temperature, power, and target-substrate distance) can be optimized to obtain optimum de- position and crystal growth. An HTE approach was also used for physical characterization of the samples including compositional, structural, electrical, and thermo- electric analysis (Fig. 2). The (Ni, Co)TiSb alloy is one of the most promising half-Heusler alloys for application as a thermoelectric mate- rial, but there is insufficient thermody- namic and kinetic data to design high- performance thermoelectrics. QuesTek and Intermolecular combined HTE with integrated computational materials engineering (ICME) design to accelerate the discovery and design of thermoelec- tric materials. To validate HTE methodology, thin films of the four-element half-Heusler (Ni, Co)TiSb systemwere synthesized by COMBINATORIAL SCREENING OF THERMOELECTRIC MATERIALS Advanced thermoelectric materials open new horizons for use in solid-state power generation and refrigeration applications. Fig. 1 — High throughput experimentation (HTE) PVD chamber with four gun-target configuration enables many depositions/experiments per 300-mmwafer.