Realizing record-high transverse thermoelectric figure of merit at room temperature in artificially tilted multilayers based on high power factor NiFe alloy

Technical Summary: Record-High Transverse Thermoelectric Figure of Merit in NiFe/BST Artificially Tilted Multilayers

Problem Statement
Conventional longitudinal thermoelectric devices rely on complex $\pi$-type modules composed of multiple p- and n-type semiconductor pairs. This architecture suffers from inherent structural limitations, including mechanical fragility and significant efficiency losses due to cumulative interfacial electrical and thermal resistances at numerous junctions. While transverse thermoelectric conversion offers a monolithic, junction-free alternative by aligning charge and heat currents orthogonally, achieving competitive room-temperature performance without an external magnetic field remains a critical challenge. Existing field-free approaches, such as the anomalous Nernst effect (ANE) in permanent magnets, typically yield low figures of merit ($z_{yx}T \sim 10^{-3}$), while structural symmetry breaking via off-diagonal Seebeck effect (ODSE) in artificially tilted multilayers (ATMLs) has previously reached $z_{yx}T \approx 0.20$ to $0.30$, often relying on calculated rather than directly measured thermal conductivity.

Methodology
The authors engineered an ATML system by alternately and obliquely stacking two constituent materials with contrasting transport properties: a high-power-factor $Ni_{50}Fe_{50}$ (NiFe) alloy and a p-type $Bi_{0.2}Sb_{1.8}Te_3$ (BST) semiconductor.

• Design Strategy: The study leveraged the large negative Seebeck coefficient and high electrical/thermal conductivity of NiFe against the large positive Seebeck coefficient and lower conductivities of BST. Analytical models were used to calculate the optimal geometric parameters (tilt angle $\theta$ and thickness ratio $t$) to maximize the transverse figure of merit ($z_{yx}T$).
• Fabrication: Multilayers were fabricated using Spark Plasma Sintering (SPS) at 500 °C under 30 MPa. The resulting blocks were cut to achieve a tilt angle of $\theta = 20^\circ$ and a NiFe thickness ratio of $t = 0.52$.
• Characterization:
  • Microstructure: Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDS) were used to verify layer integrity and analyze interfacial diffusion.
  • Interfacial Transport: Electrical resistance and temperature distributions were mapped across the stacking direction to quantify interfacial electrical and thermal resistances.
  • Performance Measurement: All critical transport parameters—transverse thermopower ($S_{yx}$), electrical conductivity ($\sigma_{yy}$), and thermal conductivity ($\kappa_{xx}$)—were directly measured at room temperature (300 K) under zero magnetic field. Unlike previous studies that relied on calculated $\kappa_{xx}$, this work employed steady-state methods for direct measurement.

Key Results

• Record Performance: The NiFe/BST-based ATML achieved a transverse thermoelectric figure of merit of $z_{yx}T = 0.36$ at room temperature without an external magnetic field. This value represents the highest reported $z_{yx}T$ for room-temperature ATML systems to date.
• Experimental Validation: The measured $z_{yx}T$ of 0.36 is in excellent agreement with the analytical prediction of 0.36. The direct measurements yielded:
  • Transverse thermopower ($S_{yx}$): $-68.4 \pm 0.2$ $\mu$V K$^{-1}$.
  • Electrical conductivity ($\sigma_{yy}$): $1.02 \times 10^6$ S m$^{-1}$.
  • Thermal conductivity ($\kappa_{xx}$): $4.00$W m$^{-1}$K$^{-1}$.
• Interfacial Quality: Microstructural analysis revealed a stratified interfacial reaction layer (~50 $\mu$m) consisting of FeTe and NiTeSb alloys, which acts as a robust adhesive. Crucially, quantitative analysis showed that interfacial electrical and thermal resistances account for only ~5% and ~7% of the bulk resistances, respectively. This minimization of parasitic resistances ensures that the macroscopic transport contrast required for ODSE is preserved.
• Comparison: The performance surpasses previously reported ATML systems combining BST with SmCo$_5$, Ni, n-type $Bi_2Te_{2.7}Se_{0.3}$, and Co, particularly noting that this result was achieved under zero magnetic field and with fully experimentally determined parameters.

Significance and Claims
The paper claims that these results pave the way for the practical implementation of transverse thermoelectric materials around room temperature. By demonstrating that high-performance transverse conversion can be achieved solely through ODSE in a field-free environment, the study addresses a primary bottleneck in the field. The successful combination of a high-power-factor metallic alloy (NiFe) with a conventional p-type semiconductor (BST), coupled with the rigorous direct measurement of all transport coefficients, validates the viability of ATMLs for efficient thermal energy harvesting and electronic cooling. The authors conclude that while the current performance is record-breaking, future work must systematically characterize the temperature dependence of these coefficients to determine if further improvements are possible for waste heat recovery and solid-state cooling applications.