Understanding zT
The thermoelectric figure-of-merit, known as zT, is the fundamental metric that determines material performance. zT encapsulates the interplay between three material properties: electrical conductivity, thermal conductivity, and Seebeck coefficient (which quantifies the voltage generated per degree of temperature difference).
The formula is: zT = (S²σ/κ) × T, where S is Seebeck coefficient, σ is electrical conductivity, κ is thermal conductivity, and T is absolute temperature.
What this reveals is the central design challenge: you want high electrical conductivity (to transport current), high Seebeck coefficient (to generate voltage from heat), and low thermal conductivity (to maintain temperature difference). These properties are often coupled in unfortunate ways. Materials with high electrical conductivity tend to have high thermal conductivity. Boosting Seebeck coefficient often reduces electrical conductivity. Good thermoelectric materials require breaking these coupling constraints.
From BiTe to PbTe to TAGS
Bismuth telluride (BiTe) has been the workhorse material for decades. It achieves zT ~1.0 at moderate temperatures (200-300°C), which translates to roughly 5-7% conversion efficiency. It's mechanically robust, relatively easy to manufacture, and chemically stable.
But at higher temperatures (400°C+), where the highest-value industrial waste heat exists, BiTe's performance degrades. Lead telluride (PbTe), by contrast, has superior theoretical zT (~1.5-2.0 at 440°C), enabling 10-14% conversion efficiency. The challenge is manufacturability and stability. PbTe is brittle, chemically reactive, and historically difficult to work with.
TAGS – a quaternary alloy combining tellurium, antimony, germanium, and silver – represents a middle path. It achieves zT values of 1.2-1.5 across a wide temperature range (300-500°C) with superior mechanical and thermal stability. It's more robust than pure PbTe and more high-performing than BiTe.
MPG's current platform uses both: BiTe for the low-temperature tail (cooling, sub-~250°C power) and PbTe/TAGS for the mid-to-high band (300–1000°C+ power). Not a sequence we left behind – a division of labour by physics.
The Energy-Sorting Barrier – A Different Kind of Breakthrough
The most important architectural innovation in MicroPower's IP isn't a material or a composition. It's the energy-sorting barrier layer – a thin, MBE-grown structure inside the chip that sorts electrons by energy.
To see why this matters, consider what's happening inside a normal thermoelectric. Hot electrons on the hot side want to flow to the cold side, driven by the temperature gradient. But slow, low-energy electrons also flow the other way – the ohmic return current – and the two largely cancel. The useful net current you get out is a small fraction of what the temperature gradient could in principle drive.
The energy-sorting barrier sits inside the chip and changes that calculus. It permits the hot, high-energy electrons through to the cold side while blocking the ohmic backflow. The chip's effective thermopower in the region around the barrier rises substantially, and the same temperature gradient produces a larger open-circuit voltage and short-circuit current than the underlying base material can do on its own.
The physics was originally documented by Hagelstein (MIT) and Kucherov (ENECO) in Applied Physics Letters Vol 81, No 3 (July 2002) and developed in detail in Journal of Applied Physics Vol 97, No 9 (May 2005). Foundational US Patent 6,396,191 covers the device structure. The work was DARPA-funded, and the enhanced-voltage signature was independently replicated in 2001–2002 by NIST, Bechtel-Bettis, and Lockheed Martin KAPL on the original HgCdTe and InSb material systems.
On modern PbTe chips the demonstrated chip-level effect is 1.5–1.8× power density enhancement per barrier, with multiple barriers stackable on a single chip. Importantly, this is an enhancement layer – part of MicroPower's post-funding production roadmap, not a feature of MicroPower's current shipped modules. The 14% module efficiency at 550°C demonstrated by today's standard modules was achieved without it, on the strength of the PbTe/TAGS material system and the high-temperature contact and thermal-interface work, informed by MicroPower's early ARL collaboration and substantially evolved internally since. Reintroducing the barrier layer multiplies power density on top of that 14% baseline.
Recent Lab Advances
The most recent material science advances are pushing zT beyond 2.0 in controlled lab conditions. We've achieved zT ~2.1 in optimization experiments with modified TAGS compositions and novel nanostructuring. These aren't theoretical projections – they're reproducible lab results.
At zT values of 2.0+, theoretical conversion efficiencies approach 16-18%. This would represent a generational improvement over current commercial thermoelectrics. The challenge now is scaling these lab results to production scale while maintaining cost-effectiveness and reliability.
The Path to 14% and Beyond
The 14% conversion efficiency we've achieved is not a ceiling – it's a platform for further improvement. As material science continues advancing, as nanostructuring techniques improve, and as manufacturing processes optimize, efficiency will continue climbing.
For industrial operators and technology investors, the significance is clear: thermoelectric technology is in a period of rapid improvement. The gains of the past decade weren't anomalies; they represent a structural shift in material capabilities. The next decade will likely bring similar improvements, making thermoelectrics increasingly competitive with mechanical and chemical alternatives.