Researchers at the University of Hong Kong have developed a 3D-printed microthermoelectric device that enables thermometry in four dimensions (3D space + time) at the microscale, providing a higher spatial resolution of about one micrometer.
Thermocouples have proven to be advantageous because of their simple setup and passive operation, providing minimal disturbance to the sample. However, there have been challenges in miniaturizing these devices to achieve high spatial resolution thermometry.
The introduction of 3D printing in the manufacture of microthermoelectric devices has overcome the limitations faced by traditional thermocouples. Bimetallic 3D printed fabrication in particular can offer a spatial resolution of about one micrometer, allowing exploration of dynamics, such as Joule heating and evaporative cooling, on microscale subjects such as microelectrodes and water menisci.
The printing process
The device consists of independent platinum (Pt) and silver (Ag) microwires that form an electrical junction that acts as an airborne temperature probe. 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 potential to create a wide range of stand-alone on-chip microsensors or microelectronic devices without the design constraints of traditional manufacturing processes.
Pt-Ag microwires were made with printable inks containing Ag or Pt nanoparticles, and dispensed through micropipettes with diameters of ~5 μm.
When 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 trajectory and speed to produce an independent wire, and completion of wire growth was achieved by increasing the speed of movement of the pipette.
The same procedure was then used to fabricate an Ag microwire into a neighboring microelectrode and guide its growth onto 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 microthermometry technique allows researchers to measure the temperature of the environment at designated points without any excitation, they were able to study how heat dissipates in the air under different environmental conditions, such as humidity, which is important for understanding various associated phenomena. to the evaporation and condensation of water in various fields.
The team observed that heat dissipation from the Joule-heated microwire to air becomes faster as relative humidity decreases, leading to lower temperature and slower decay.
This breakthrough has the potential to revolutionize the field of thermodynamics and thermal management in various applications, from scientific research to everyday life. The newly discovered ability to directly measure 4D thermometry at the microscale could pave the way for the development of stand-alone on-chip microsensors or microelectronic devices, removing design constraints imposed by manufacturing processes.
You can read the research article, titled “Additive manufacturing of thermoelectric microdevices for four-dimensional thermometry” in the journal Advanced Materials, at this link.
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