Thermoelectric nanomaterials are engineered to convert temperature differences into electrical energy through the thermoelectric effect at the nanoscale. These materials demonstrate enhanced electrical performance and reduced thermal conductivity due to size-dependent properties.1,2
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Their ability to harness waste heat or thermal energy, even under low-temperature gradients, or to provide efficient cooling makes thermoelectric nanomaterials a valuable option for energy harvesting and temperature management in various applications. Their development contributes to advancements in sustainable energy technologies.1,2
Principle of Thermoelectricity
The thermoelectric effect, or Seebeck effect, refers to the direct conversion of a temperature difference between two electrical conductors or semiconductors into an electrical voltage. When the sides of a thermoelectric material are exposed to different temperatures, a voltage is generated across the material. Conversely, applying a voltage can create a temperature difference, a phenomenon known as the Peltier effect.1,2
At the atomic scale, when a temperature gradient is applied to a thermocouple, holes and electrons at the hotter side move faster and diffuse toward the cooler side, leading to a lower carrier density on the hot side. In p-type materials, holes act as carriers, while in n-type materials, electrons fulfill this role. This results in the generation of an electric field across the thermocouple, described as the Seebeck effect.
The voltage generated, expressed as S × ΔT, depends on the temperature difference (ΔT) and the Seebeck coefficient (S) under thermodynamic equilibrium.1,2
The thermoelectric effect offers several advantages over other energy-harvesting technologies. Thermoelectric conversion operates silently, without mechanical components, and is highly reliable. It is also environmentally friendly, as it generates no heat or chemical/gaseous waste during operation.1,2
This technology is particularly useful in environments like remote outer space, where other energy conversion methods may not be viable. The efficiency of thermoelectric materials is quantified by a dimensionless figure of merit (ZT), defined as ZT= (σ × S2/κ) × T, where T is the absolute temperature, κ is the thermal conductivity, and σ is the electric conductivity.1,2
Simultaneously improving the Seebeck coefficient, electrical conductivity, and thermal conductivity in bulk thermoelectric materials is challenging due to their interdependence. Low-dimensional materials typically have higher ZT compared to their bulk analogs due to quantum confinement effects and lower thermal conductivity.1,2
Nanostructuring is an effective strategy for enhancing thermoelectric performance (ZT). By introducing numerous interfaces, nanostructuring increases phonon scattering, which reduces thermal conductivity without significantly affecting the power factor. Ideally, this approach maintains or improves electrical conductivity, resulting in greater thermoelectric efficiency.1,2
The Thermoelectric Effect – Seebeck & Peltier Effects
Properties of Thermoelectric Nanomaterials
Nanomaterials are a highly desirable class of materials characterized by structures with at least one dimension confined to the nanoscale, specifically between 1 and 1000 nanometers.2
The thermoelectric performance of nanomaterials improves significantly due to the quantum size effect. At the nanoscale, the restriction of carrier motion in specific directions reduces the degrees of freedom for electron and phonon transport, altering the material’s physical and electronic properties.2 This confinement leads to substantial changes in electronic transport behavior.
Nanostructuring also increases the density of states near the Fermi level through quantum confinement, enabling a partial decoupling of electrical and thermal conductivity. This enhances the Seebeck effect, further improving thermoelectric performance.2
For example, silicon-rich silicon-germanium/silicon superlattices with controlled interfaces can achieve a high power factor of approximately 25 μW cm⁻¹ K⁻² and a low thermal conductivity of 2.5 W m⁻¹ K⁻¹ at room temperature in the in-plane direction. In highly doped semiconductors, the mean free path of electrons is significantly shorter than that of phonons.2
By fabricating nanostructures with one or more confined dimensions, multiple surfaces are introduced where phonons can be scattered more selectively and effectively than electrons over a wide range of distances. This selective scattering reduces lattice thermal conductivity significantly while maintaining efficient electronic conduction and high carrier mobility.2
Applications in Semiconductors
Thermoelectric nanomaterials are advancing semiconductor technology with applications in energy harvesting and cooling systems.2,3
Energy Harvesting
Semiconductor nanowires are well-suited for thermoelectric generation applications due to size-dependent effects on transport properties such as the S, σ, and k, resulting in an improvement of the thermoelectric figure of merit.2,3 Specifically, the elongated shape and compatibility of thermoelectric nanowires with microfabrication processes make them ideal for micro thermoelectric generator (TEG) devices used in energy harvesting.2,3
TEGs are primarily used in medical and wearable devices (for example, wristband energy harvesters), electronics for waste heat reutilization, microelectronics and wireless sensor networks, aerospace for energy generation in extreme conditions, and automotive systems to repurpose engine waste heat for powering vehicle devices.2,3
Cooling Systems
Thermoelectric coolers (TECs) utilize thermoelectric materials and nanomaterials for efficient cooling in electronic devices such as miniaturized circuits, central processing units (CPUs), and integrated circuits. These systems provide an efficient alternative to conventional cooling methods, specifically for high-performance electronics, enabling better thermal management and energy efficiency.2,3
Integrated thermoelectric micro-coolers (ITM) are used for microelectronics to cool infrared detectors, enhance the integrated circuit performance, and stabilize the solid-state laser temperature. These devices are based on bismuth telluride alloys or silicon carbide on silicon dioxide substrates and are designed as Peltier modules.2,3
Recent Advances
A paper published in the Journal of Materials Chemistry C reported, for the first time, the thermoelectric properties of cement/single-walled carbon nanotube (SWCNT) nanocomposites over hydration periods of 3, 7, 14, and 28 days. Using cement/SWCNT with the highest realized power factor, a TEG device was fabricated. The TEG achieved a maximum power output of 5.02 μW and a power density of 5.02 mW m−2 under a temperature difference of 50 K.4
Similarly, a recent study in Energy optimized the structural parameters of a thermoelectric module in a concentration photovoltaic–TEG hybrid system. A mathematical model was developed to evaluate the impact of the TEG’s structural parameters on overall system efficiency. The results showed that an optimal structural parameter of 0.36 mm⁻¹ enabled the system to achieve a maximum efficiency of 41.73 % under simulated conditions.5
Conclusion
Thermoelectric nanomaterials offer significant potential for advancing sustainable energy technologies by efficiently converting heat into electricity and facilitating effective temperature management. Their nanoscale properties, such as enhanced thermoelectric performance and reduced thermal conductivity, make them well-suited for energy harvesting and cooling systems applications.
As semiconductor technologies evolve, the development of nanostructured materials will be critical in enhancing the efficiency of thermoelectric devices. Future research should focus on optimizing these materials for broader commercial applications, supporting the transition to more sustainable energy solutions.
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References and Further Reading
- Tzounis, L. (2019). Synthesis and Processing of Thermoelectric Nanomaterials, Nanocomposites, and Devices. Nanomaterials Synthesis. DOI: 10.1016/B978-0-12-815751-0.00009-2, https://www.sciencedirect.com/science/article/abs/pii/B9780128157510000092
- Rashak, M.M., Roy, A., Mallick, TK., Tahir, A. A. (2023). Advancing Thermoelectric Materials: A Comprehensive Review Exploring the Significance of One-Dimensional Nano Structuring. Nanomaterials. DOI: 10.3390/nano13132011, https://www.mdpi.com/2079-4991/13/13/2011
- Galassi, C., Lecis, N. (2023). Thermoelectric Materials and Applications: A Review. Energies. DOI: 10.3390/en16176409, https://www.mdpi.com/1996-1073/16/17/6409
- Vareli, I., et al. (2021). High-performance cement/SWCNT thermoelectric nanocomposites and a structural thermoelectric generator device towards large-scale thermal energy harvesting. Journal of Materials Chemistry C. DOI: 10.1039/D1TC03495B, https://pubs.rsc.org/en/content/articlelanding/2021/tc/d1tc03495b/unauth
- Ge, M., Zhao, Y., Li, Y., He, W., Xie, L., Zhao, Y. (2022). Structural optimization of thermoelectric modules in a concentration photovoltaic–thermoelectric hybrid system. Energy. DOI: 10.1016/j.energy.2022.123202, https://www.sciencedirect.com/science/article/abs/pii/S0360544222001050
