A complete understanding of thermal transport within composite materials post fabrication is difficult to ascertain due to limitations of thermal characterization techniques. Bulk values may be measured, but do not capture the full information about the material’s microscale thermal character. The constituents may be individually characterized before fabrication, but this will not capture any changes to the thermal properties that result during processing. If *in situ* measurements of microscale properties of components within the fully formed composites could be accessed, they could be used as inputs to mesoscale models to inform intelligent design of composite material systems and the fabrication processes used to create them.

In this work, a finite element analysis (FEA) model is developed and implemented to analyze data obtained from an imaging pump-probe technique to extract *in situ* thermal properties of multi-material systems. Thermal property maps are generated, and the spatial limitations of the technique are investigated. The technique examined in this work is known as Frequency Domain Thermoreflectance (FDTR), which is a pump-probe technique traditionally used to measure thermal and dimensional properties in radially symmetric samples. In the most common implementation, a metal transducer layer is patterned onto a sample for converting incident light from the pump laser into heat, which diffuses into the sample. The magnitude and phase of the reflected probe laser are used to monitor changes in reflectivity of the transducer layer, which is proportional to temperature rise at the transducer layer surface. Thermal properties of the sample are fit using an analytical forward model as an input to an inverse solver. Typically, this approach is used to fit for bulk properties of materials, or thin films grown in the same orientation as the transducer layer.

The FEA model presented here extends the data prediction and analysis capability for FDTR from radially symmetric samples to 2 dimensional samples with varying character in one lateral (in-plane) direction and the through-plane (cross-plane) direction. Simulation results are compared to the Rayleigh criterion and a quantitative determination of the lateral resolution of reconstructive images obtained from FDTR is achieved. To validate the model’s predictions, 5 samples were prepared varying critical dimensions of 2 dimensional features. Pitch and depth were varied from 1/10 of probe beam diameter to 10 times probe beam diameter. These samples are measured by a 1-dimensional scan in the lateral direction, and inverse analysis is used to extract the thermal conductivity of critical features. The length scale at which the critical features can no longer be individually distinguished is identified, and implications for future work in imaging are discussed.


Resolution of 2-Dimensional Frequency Domain Thermal Mapping

Division / Sponsors: 
Heat Transfer Division (HTD), Fluids Engineering Division (FED)

Heat Transfer Summer Conference
Paper Proceedings: https://asmedigitalcollection.asme.org/HT/HT2020/volume/83709

18th International Conference on Nanochannels, Microchannels, and Minichannels
Paper Proceedings: https://asmedigitalcollection.asme.org/ICNMM/ICNMM2020/volume/83693

Fluids Engineering Division Summer Meeting
Paper Proceedings: https://asmedigitalcollection.asme.org/FEDSM/FEDSM2020/volume/83716

HT-ICNMM2020 - Virtual Conference, SHTC - Virtual Conference, FEDSM2020 - Virtual Conference

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