Leveraging Ronchi Rulings as Reconfigurable Microscale Joule Heaters

Microscale heating platforms capable of generating localized temperature rises are relevant for nanomaterials synthesis, processing, and characterization and for thermal metrology applications including the validation of microscale thermometry techniques. While it is typical to design and fabricate custom heater lines for microscale heating, a lower-cost and off-the-shelf alternative would require less time and effort to be spent on device fabrication. Here, commercially available optical calibration samples called Ronchi rulings, consisting of a periodic array of chrome lines on a float glass substrate in a 1:1 line width to line spacing ratio, are demonstrated as reconfigurable Joule heaters. Ronchi rulings allow ultrasonic wire bonds to be made anywhere along the chrome line, forming electrical connections that enable Joule heating of individual lines, which can be monitored using electrical resistance thermometry. Four point-probe electrical resistance measurements across multiple heater lines demonstrate a negative temperature coefficient of resistance (TCR) of -6.93 ×10^-4 ± 8.18 × 10^-5 K^-1, in agreement with limited existing studies on the electrothermal properties of chromium thin films. Joule heating temperature rises exceeding 100 K are recorded and modifying the length of the heater line under a given applied current or changing the input power allows the temperature rise to be easily scaled for different experimental requirements. A large aspect ratio, with chrome line lengths on the order of 10s of millimeters, widths of 100s of microns, and a height of less than 100 nanometers, combined with the 1:1 area ratio between the chrome lines and glass means that randomly dispersed nanoparticles are readily found on both materials. Taking advantage of this fact, ratiometric luminescence thermometry was performed using NaYF₄:Yb³⁺, Er³⁺ upconverting nanoparticles (UCNPs). Temperature measurements from an individual UCNP on glass and a small UCNP cluster on chrome are in good agreement with the temperature profile predicted by a finite element simulation. With the thermal conductivity of glass as the only free parameter, ranging from 0.9 to 1.0 W m^-1 K^-1 based on accepted values, the simulation input power matches the maximum input power used to Joule heat the chrome line for UCNP measurements. Since over 50% of the peak temperature rise on the chrome line is maintained along the neighboring glass region, local heating of nanomaterials is easily accomplished on either the chrome or glass as verified by the UCNP measurements, further showcasing Ronchi rulings as versatile off-the-shelf microscale Joule heaters. In the future, a smaller wire bonding tip and wire could be used on Ronchi rulings with smaller width lines, providing steeper temperature profiles.