Time-domain thermoreflectance (TDTR)

Over the past decade, Time-Domain Thermoreflectance (TDTR) has emerged as a versatile and powerful tool for measuring the thermal transport properties of materials. This technique is applicable to a wide range of materials with diverse properties and sample geometries. High-throughput thermal conductivity mapping, facilitated by focused laser spots and a high signal-to-noise ratio, has become a hallmark of TDTR.

The applications of thermal conductivity mapping extend to various fields, including the analysis of metallurgical phase diagrams and the characterization of thermal barriers and coatings for nuclear fuels.

Experimental setup for TDTR thermal diffusivity measurements including pump and probe laser source

TDTR is uniquely advantageous as an optical, non-contact method, making it directly applicable to samples in optical cryostats, high-temperature microscope stages, and environments with extreme conditions such as the high-pressure environment of a diamond anvil cell.

The TDTR approach involves depositing a metal film transducer on the sample, serving as a means for heating via a pump optical pulse and allowing measurement of changes in transducer reflectivity for thermometry.

Data analysis in TDTR is accomplished by employing the analytical solution of the diffusion equation in cylindrical coordinates. Determining thermal transport properties involves adjusting free parameters in a thermal model to obtain the best fit between predicted and measured thermal responses.

The use of a single objective lens and the integration of pump and probe beams into an optical imaging system have streamlined the alignment and focusing of the beams, enhancing efficiency.

In practical applications of TDTR, the sample surface must be sufficiently smooth to avoid any undesired modulation of diffuse scattering that could interfere with the desired signal produced by the thermoreflectance of the metal film transducer.

TDTR is primarily sensitive to thermal conductivity in the through-thickness direction, i.e., heat transport anti-parallel to the surface normal. For thermally anisotropic materials, additional methods, such as a suspended hotwire approach, may be employed to measure in-plane thermal conductivity.

The most common implementation of TDTR uses a Nd:YAG or Ti:sapphire laser oscillator as the light source, producing an optical pulse to locally heat the transducing layer and the underlying sample layer. A continuous-wave (CW) laser in the probe path detects the local temperature rise. The wavelength of the probe laser is selected based on the transducing layer material (e.g., Au, Al, or Pt), ranging from ~473 nm to 532 nm or 785 nm to 808 nm.

The term “thermoreflectance” signifies that the probe measures changes in sample temperature through the dependence of the optical reflectivity (R) of a metal film transducer on temperature (T), where dR/dT represents the coefficient of thermoreflectance.

Raw data of the thermal diffusivity measurement of two SiO2 thin films with varying thickness, measured with the Linseis TF-LFA

Which properties are determined?

TDTR serves as a valuable tool for characterizing the thermal transport properties of materials, particularly thin films, coatings, and the study of thermal interfaces. Its application spans the entire spectrum of thermal conductivity, from high values seen in diamond and metals with high thermal conductivity to ultralow thermal conductivity observed in fullerene derivatives. The versatility of TDTR allows its application to diverse materials, including bulk materials, thin layers, and individual interfaces, making it a comprehensive method for investigating a wide range of thermal properties.

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