(Nanowerk Spotlight) The field of thermoelectric materials has long sought to harness waste heat and convert it into useful electricity. This technology holds immense potential for improving energy efficiency across numerous industries and applications. However, developing materials that can effectively convert heat to electricity, especially at high temperatures, has proven challenging.
Thermoelectric devices rely on the Seebeck effect, the phenomenon where a temperature difference across a material generates an electrical voltage. The efficiency of this process depends on several material properties, including electrical conductivity, thermal conductivity, and the Seebeck coefficient. Finding materials that optimize these properties in combination has been an ongoing pursuit for researchers.
The search for high-performance thermoelectric materials has focused primarily on semiconductor compounds like bismuth telluride and lead telluride. While these materials perform well at low to moderate temperatures, they lose effectiveness at high temperatures due to melting, oxidation, or decomposition. This limitation has restricted the use of thermoelectric generators in many high-temperature industrial settings where substantial waste heat is produced.
In recent years, attention has turned to more thermally stable ceramic materials as potential high-temperature thermoelectrics. One class of ceramics that has garnered interest is the MAX phases – a family of layered, ternary compounds with the general formula Mn+1AXn, where M is an early transition metal, A is an A-group element, X is carbon or nitrogen, and n is 1, 2, or 3.
MAX phases have been known since the 1960s but saw renewed interest in the 1990s due to their unique combination of metallic and ceramic properties. They exhibit high electrical and thermal conductivity, resist oxidation and thermal shock, and maintain strength at high temperatures. These characteristics made them attractive for various structural and electrical applications.
However, the high thermal conductivity of most MAX phases seemed to preclude their use as thermoelectric materials. Efficient thermoelectric conversion requires low thermal conductivity to maintain a temperature gradient. Consequently, research on MAX phases for thermoelectrics remained limited, with these materials often serving as poor-performing reference points in thermoelectric studies.
A new study published in Physical Chemistry Chemical Physics (“Beyond Metals: Theoretical Discovery of Semiconducting MAX Phases and their Potential Application in Thermoelectrics”) challenges this conventional wisdom about MAX phases. Through extensive computational modeling and analysis, the team has identified several MAX phase compounds that exhibit semiconducting behavior rather than the typical metallic properties. This discovery opens up new possibilities for using MAX phases in thermoelectric applications, particularly at high temperatures where their thermal stability gives them an advantage over traditional semiconductor thermoelectrics.
The image illustrates the structure and composition of semiconducting MAX phases, highlighting their potential for use in high-temperature thermoelectric applications. It features a schematic representation of the layered structure typical of MAX phases, labeled with the elements M (transition metal), A (A-group element), and X (carbon or nitrogen). On the right, specific compounds identified as promising thermoelectric materials – Sc2SC, Y2SC, Y2SeC, Sc3AuC2, and Y3AuC2 – are listed, underscoring their significance in recent research breakthroughs. (Image: Courtesy of the authors)
The international research team employed a series of first-principles calculations to investigate the electronic structures of 861 dynamically stable MAX phases. Among these, they identified five compounds – Sc2SC, Y2SC, Y2SeC, Sc3AuC2, and Y3AuC2 – as narrow bandgap semiconductors with gaps ranging from 0.1 to 0.63 electron volts.
This semiconducting behavior arises from the specific elemental compositions and crystal structures of these compounds. The researchers found that the transition metals (scandium and yttrium) achieve their highest oxidation state by donating electrons, while the carbon, sulfur, selenium, and gold atoms gain electrons. The resulting crystal field produced by carbon and the A-element (S, Se, or Au) on the d orbitals of scandium and yttrium creates the observed band gaps.
To assess the potential of these semiconducting MAX phases for thermoelectric applications, the team conducted detailed analyses of their thermal and electrical transport properties. They calculated lattice thermal conductivities using density functional theory and the Boltzmann transport equation. The results showed remarkably low thermal conductivities ranging from 3 to 15 watts per meter-kelvin at room temperature – values comparable to or lower than many established thermoelectric materials.
The researchers attributed this low thermal conductivity to several factors, including the layered crystal structure of MAX phases and the presence of heavy elements like gold in some compounds. They also noted that the thermal conductivity could be further reduced through nanostructuring, with their calculations predicting that grain sizes of around 50 nanometers could halve the lattice thermal conductivity.
On the electrical side, the team calculated Seebeck coefficients, electrical conductivities, and carrier mobilities for the semiconducting MAX phases. They found Seebeck coefficients exceeding 200 microvolts per kelvin over a temperature range of 300 to 700 kelvin – values comparable to high-performance thermoelectric materials. The predicted carrier mobilities ranged from 50 to 400 square centimeters per volt-second at room temperature, indicating good electrical transport properties.
Combining these thermal and electrical characteristics, the researchers calculated the thermoelectric figure of merit (zT) for the semiconducting MAX phases. This dimensionless parameter quantifies the overall thermoelectric performance of a material, with higher values indicating greater efficiency. At a temperature of 700 kelvin, the team predicted maximum zT values of up to 2.45 for Y3AuC2. This value exceeds the performance of many current high-temperature thermoelectric materials and approaches the threshold (zT > 3) where thermoelectric generation becomes economically competitive with traditional power generation methods for some applications.
The study also revealed interesting anisotropic behavior in these MAX phases, with thermal and electrical properties differing between the in-plane (parallel to the layers) and out-of-plane (perpendicular to the layers) directions. Generally, the in-plane thermoelectric performance was found to be superior due to lower thermal conductivity in this direction.
While these computational results are promising, it’s important to note that experimental verification is still needed. The researchers assessed the thermodynamic stability of their predicted semiconducting MAX phases through ternary phase diagram calculations. They found that most of the compounds lie on or very close to the convex hull of stability, suggesting they should be experimentally synthesizable. However, actual synthesis and characterization of these materials remain to be performed.
If realized experimentally, these semiconducting MAX phases could significantly advance high-temperature thermoelectric technology. Their predicted performance at 700 kelvin, combined with the inherent thermal stability and oxidation resistance of MAX phases, makes them promising candidates for waste heat recovery in industrial processes, automotive exhaust systems, and other high-temperature environments.
Moreover, the discovery of semiconducting behavior in MAX phases broadens the potential applications of this material family beyond their current uses in protective coatings, electrical contacts, and other structural or electrical roles. It also provides new avenues for tuning the properties of MAX phases through compositional variations and doping strategies.
This research highlights the power of computational materials science in identifying new candidates for functional applications. By systematically exploring a large space of possible compositions and analyzing their properties through first-principles calculations, the team uncovered unexpected behavior in a well-studied class of materials. Such computational approaches, combined with experimental validation, can accelerate the discovery and development of new materials for energy applications and other technological needs.
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