Researchers from the University of Hong Kong have developed a 3D printed micro-thermoelectric device that allows for four-dimensional (3D Space + Time) thermometry at the microscale, providing a higher spatial resolution of approximately one micrometer.
Thermocouples have proven advantageous for their simple configuration and passive operation, providing minimal sample disturbance. However, there have been challenges in miniaturizing these devices to achieve high spatial resolution thermometry.
The introduction of 3D printing in the fabrication of micro-thermoelectric devices has overcome the limitations faced by traditional thermocouples. Fabrication using bi-metal 3D printing in particular can offer a spatial resolution of approximately one micrometer, enabling the exploration of dynamics, such as Joule heating and evaporative cooling, on microscale subjects like microelectrodes and water menisci.
The Printing Process
The device consists of freestanding platinum (Pt) and silver (Ag) microwires forming an electrical junction acting as a temperature probe suspended in air. The temperature at each junction is measured by the thermoelectric voltage generated by the Seebeck effect. The technique enables microscale temperature mapping in three dimensions, offering the possibility of creating a wide range of on-chip, freestanding microsensors or microelectronic devices without the design restrictions of traditional manufacturing processes.
The Pt-Ag microwires were with printable inks containing Ag or Pt nanoparticles, and were dispensed through .micropipettes with diameters of ~5 μm.
As the pipette came into contact with the substrate, a femtoliter ink meniscus was produced, and the nanoparticles rapidly accumulated in the meniscus under solvent evaporation, forming a solidified microstructure on a patterned microelectrode.
The meniscus was then guided with a programmed path and speed to produce a freestanding wire, and termination of wire growth was achieved by increasing the pipette moving speed.
The same procedure was then used to fabricate an Ag microwire on a neighboring microelectrode, and guide its growth towards the top of the Pt wire to create the Pt-Ag thermocouple junction. The Tjunctioned were well-formed, and their cross-sectional area was shown to be as small as 0.38 μm2.
4D Thermocouple Uses
As the 4D micro-thermometry technique allows the researchers to measure the temperature of the environment at designated points without any excitation, they were able to study how heat is dissipated to the air at different ambient conditions such as humidity, which is important for understanding various phenomena associated with evaporation and condensation of water in diverse fields.
The team observed that the heat dissipation from the Joule-heated microwire to the air becomes faster as relative humidity decreases, leading to lower temperature and slower decay.
This advancement has the potential to revolutionize the field of thermodynamics and thermal management in various applications, from scientific research to everyday life. The newfound capability to directly measure 4D thermometry at the microscale could pave the way for the development of on-chip, freestanding microsensors or microelectronic devices, eliminating design restrictions imposed by manufacturing processes.
You can read the research paper, titled “Additive Manufacturing of Thermoelectric Microdevices for Four-Dimensional Thermometry” in the Advanced Materials journal, at this link.
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