Nano-silver sintering

Hayley Everett

Researchers have developed a novel high-efficiency thermoelectric device design that uses silver nanoparticles as a welding filler.

Multiple high-performance thermoelectric materials have been discovered over the past two decades. However, their promise has remained largely unfilled due to a lack of efficient devices capable of converting the energy these materials produce into emission-free power.

Now, an international team of scientists from the University of Houston and the Harbin Institute of Technology have revealed a novel approach to constructing such thermoelectric modules that is expected to accelerate the development of advanced thermoelectric modules for power generation and other applications. The key ingredient: Silver nanoparticles.

Thermoelectric devices so far

Thermoelectric devices can convert heat directly into electricity due to the Seebeck effect, a phenomenon in which a temperature difference between two dissimilar electrical conductors or semiconductors produces a voltage difference between the two substances. Since they are compact and highly reliable, and operate without vibration and noise, these devices have key applications in aerospace and in other fields experiencing extreme conditions.

In recent years, the performance of different families of thermoelectric materials has been significantly improved. However, the development of thermoelectric devices with both high performance and high reliability remains difficult. The key to developing thermoelectric devices lies in the design of two connection interfaces: the semiconductor-metal interface (the connection interface between the thermoelectric material and the contact layer) and the metal-metal interface (the bonding interface between the electrode and the contact layer). To ensure that the thermoelectric material performance does not decay, it is very important to realise connections that minimise interface resistance and thermal resistance while maintaining high connection strength.

However, as the welding temperature increases, problems such as thermoelectric material performance degradation, diffusion of elements between interfaces, and welding residual thermal stress become increasingly serious. Therefore, determining how to achieve both low-temperature connections and high-temperature service in thermoelectric devices to avoid the above problems is of great significance.

A new approach with silver nanoparticles

Addressing this challenge to the development of thermoelectric devices, Professor Qian Zhang from the Harbin Institute of Technology, Professor Zhifeng Ren from the University of Houston, and their research groups proposed a novel connection design in which silver nanoparticles (Ag NPs) are used as a welding filler rather than using a traditional brazing alloy.

Due to the nanoscale size of the Ag NPs, their melting point is reduced while their specific surface energy is increased. Therefore, different metals can be bonded via sintering the Ag NPs at very low temperatures. By exploring the high-temperature mechanical properties of the Ag NP connection interface, the researchers proved that low-temperature-sintered Ag NP solder joints have excellent high-temperature (773K) thermal stability. Due to the low sintering temperature (≤ 573 K) and high remelting temperature (> 1200 K) of the Ag NP joint, this connection scheme can theoretically be applied to most thermoelectric device designs over a wide temperature range.

For medium-temperature PbTe-based thermoelectric devices, the researchers designed and developed two connection layer structures, Fe-Sb/SnTe/p-PbTe and Fe-Sb/n-PbTe, both of which achieved low interface contact resistance and extremely low thermal expansion coefficient difference, thus guaranteeing the devices’ high performance and reliability.

This low-temperature Ag Np sintering process, in combination with structure optimisation via finite element simulation, has reportedly resulted in the best single-stage PbTe-based thermoelectric device reported thus far, achieving an energy conversion efficiency as high as ~11% at a temperature difference of 550K, as well as good thermal cycling reliability.

Professor Ren explains: “This method significantly reduces the welding temperature of medium- and high-temperature thermoelectric devices, ensures the excellent performance of thermoelectric materials, has wide applicability, greatly simplifies the connection process for traditional thermoelectric devices, and shortens the development cycle for new thermoelectric devices.”

Demonstrating applicability

To prove the application potential of this method to produce different thermoelectric devices, the researchers additionally designed and fabricated room-temperature Bi2Te3-based thermoelectric devices and high-temperature half-Heusler thermoelectric devices; both of which achieved high energy conversion efficiency in their corresponding temperature ranges.

“This design is based on nano-silver sintering technology, and it is very important to know more about this technology in high temperature environments,” Professor Zhang concludes. “We expect to promote the application of thermoelectric devices throughout this research.”

Read the full research paper ‘Low-temperature sintering of Ag nanoparticles for high-performance thermoelectric module design’ here

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