Bu et al. (Nature Communications, 2022) demonstrated a tellurium-free thermoelectric module pairing inexpensive p-type CdSb with n-type Mg3(Bi,Sb)2 that reached a then-record efficiency above 8% at a 550 K hot-side temperature, but its demonstrated envelope remained bounded by four coupled limitations: the empirically determined ~550 K safe-temperature ceiling of the CdSb legs, radiative heat losses from the unfilled hot side that were included in the paper's efficiency estimates but whose correction at large temperature differences lacks a standard uncertainty-quantified procedure, contact-resistance losses and their leg-length scaling in short-legged modules, and durability demonstrated only over 200 hours of continuous operation plus 80-day aging observations. The open problem is to extend the validated operating envelope of this module technology: establish whether p-type CdSb legs can be stabilized and operated with an efficiency at least matching the source module's >8% under comparable cold-side conditions at source temperatures above ~550-600 K (or determine the fundamental limit), establish uncertainty-quantified correction of parasitic radiation losses in high-temperature-difference module metrology, minimize contact-induced losses in scaled modules, and demonstrate service-relevant (multi-month to multi-year) reliability under realistic conditions.

Background

A thermoelectric generator converts heat directly into electricity via the Seebeck effect: a temperature difference across semiconductor legs drives a voltage. The conversion potential of a leg material is gauged by the dimensionless figure of merit zT=S2T/(ρκ)zT = S^2T/(\rho\kappa), where SS is the Seebeck coefficient, TT the absolute temperature, ρ\rho the electrical resistivity, and κ\kappa the thermal conductivity. A practical device is a module of n-type and p-type legs connected electrically in series and thermally in parallel; its efficiency depends on the average zTzT of both leg materials over the operating range and on parasitic electrical and thermal losses at the joints. Low-grade waste heat (source temperatures below roughly 600 K) constitutes the largest share of industrial waste heat, and the commercial state of the art uses Bi2Te3-based alloys, which lose output above about 450-500 K (narrow bandgap, strong bipolar conduction) and depend on scarce, expensive tellurium. Cheap tellurium-free alternatives emerged in the 2019-2021 period: n-type Mg3(Sb,Bi)2 Zintl-phase alloys (Zintl phases are valence-precise intermetallic compounds; here a Mg-Sb-Bi semiconductor) with zTzT near or above 1 at low temperature and single-leg efficiencies around 10% at 700 K, and p-type candidates MgAgSb (device-grade zTzT, but Ag cost and complicating phase transitions) and CdSb (average zT>1zT > 1 below 600 K, inexpensive elements). Bu et al. (Nature Communications 13, 2022) assembled the first CdSb-based module, pairing p-type Cd0.99Ag0.01Sb with n-type Mg3.1Y0.01SbBi using one-step hot-pressed contacts of Ni/Sb/CdSb and Ni/Fe/Mg3SbBi, where the Sb and Fe interlayers act as diffusion barriers preventing chemical interdiffusion between leg and Ni electrode. The module reached a then-record experimental efficiency above 8% at hot-side 550 K and cold-side 280 K (a 270 K temperature difference), with interfacial contact resistivity (area-normalized electrical resistance of the joint, in μΩ⋅\mu\Omega\cdotcm2^2) below 60 μΩ\mu\Omegacm2^2, and a raw-material-cost-to-power ratio about 1/15 that of Bi2Te3 modules. The same paper's demonstrations were bounded as follows: (i) the safe operating temperature of CdSb legs was determined experimentally to be about 550 K or below (CdSb is metastable against decomposition toward Cd13Sb10 plus Sb; quenched ingots show this decomposition and require a 673 K anneal to rehomogenize), so module performance and materials stability at higher source temperatures were unestablished; (ii) at large temperature differences, the measured heat flow falls below model predictions, an effect ascribed to vertical radiative heat transfer from the exposed (unfilled) hot side to the cold-side substrate; both horizontal and vertical radiation are accounted for in the paper's efficiency model so that the measured maximum efficiency agrees with that model, but the uncertainty of the radiative contribution is not quantified and no standard uncertainty-quantified correction procedure is established for such unfilled layouts, and this parasitic channel scales steeply with temperature; (iii) although the contact resistivity is small, contact resistance contributes non-negligibly to internal resistance for short legs (its relative contribution decreases as leg length grows), leaving a power-density-versus-contact-loss trade-off in scaled modules; and (iv) demonstrated service life was limited to 200 h of continuous operation at 555 K (efficiency drift under 3%, within measurement uncertainty), 15 thermal cycles between 350 and 550 K, and static aging observations over 14 and 80 days, during which the CdSb-leg contact resistance roughly doubled from 1.4 to 2.7 mΩ\Omega. Since 2022, contact-layer and measurement-methodology work has substantially advanced and the tellurium-free module efficiency record has been raised to 12% at 583 K with Ag-containing MgAgSb p-legs, but no CdSb device operates above its ~550 K ceiling and no long-duration field data exist for any tellurium-free antimonide module.

Problem Statement

Extend the validated operating envelope and reliability of the tellurium-free CdSb/Mg3(Bi,Sb)2 module technology for low-grade waste-heat recovery, addressing the four coupled limitations of the source work's demonstrated envelope under the single objective of deploying cheap antimonide modules at higher hot-side temperatures with trustworthy, service-relevant performance: (1) establish whether p-type CdSb legs can operate with a conversion efficiency of at least 8% at a hot-side temperature of at least 600 K with the cold side held at 280 K, given that the material is metastable against decomposition toward Cd13Sb10 plus Sb; (2) quantify and correct the parasitic vertical heat radiation from the unfilled hot side to the cold-side substrate, for which no standard correction procedure existed at the time of the source work, by reporting a radiation-correction procedure that states the efficiency of the module of sub-objective (1) measured at a temperature difference of at least 300 K with a propagated uncertainty no larger than 10% of that efficiency; (3) reduce the interfacial contact resistivity to at or below 30 μΩ\mu\Omegacm2^2 (at most half the source work's reported upper bound of 60 μΩ\mu\Omegacm2^2) and determine the leg-length scaling of contact-induced losses by reporting how the contact share of the module internal resistance depends on leg length in scaled, short-legged modules; and (4) demonstrate module durability and reliability over at least 12 months of operation, including sustained thermal cycling between 300 K and the operating hot side and realistic field conditions with a variable source temperature and load, with efficiency drift no larger than the source work's demonstrated 3% and interfacial contact resistance no more than doubled over that period, beyond the 200-hour continuous and 80-day aging demonstrations. All four sub-objectives are jointly required, and addressing any proper subset of them does not by itself resolve the task: for sub-objectives (2)-(4), resolution requires advancing the corresponding limitation beyond the source work's demonstrated envelope; for sub-objective (1), resolution requires either a demonstration meeting the operating point stated in (1) or a demonstrated fundamental limit that precludes operation at or above 600 K, where such a fundamental limit means an intrinsic thermodynamic or phase-stability demonstration that no composition, defect, or phase engineering of CdSb can yield stable operation at or above 600 K, as distinct from the source work's empirical ~550 K safe-temperature ceiling.

References

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