Ji Cheng Zhao

Invited Article: Micron resolution spatially resolved measurement of heat capacity using dual-frequency time-domain thermoreflectance

A pump-probe photothermal technique - dual-frequency time-domain thermoreflectance - was developed for measuring heat capacity with a spatial resolution on the order of 10 μm. The method was validated by measuring several common materials with known heat capacity. Rapid measurement of composition-phase-property relationships was demonstrated on Ti-TiSi2 and Ni-Zr diffusion couples; experimental values of heat capacity of the intermetallic compounds in these diffusion couples were compared with literature values and CALPHAD (CALculation of PHAse Diagram) calculations. The combination of this method and diffusion multiples provides an efficient way to generate thermodynamic data for CALPHAD modeling and database construction. The limitation of this method in measuring low thermal diffusivity materials is also discussed.

High-throughput diffusion multiples

A diffusion multiple is an assembly of three or more different metal blocks, in intimate interfacial contact, subjected to high temperature to allow thermal interdiffusion to create solid-solution compositions and intermetallic compounds. Using microscale probes, composition-structure-phase-property relationships can be established with an efficiency orders of magnitude higher than conventional one-composition-at-a-time practice. For structural materials, such relationships include phase diagrams, diffusion coefficients, precipitation kinetics, solution strengthening effects, and precipitation strengthening effects. Many microscale probes can also be used to study several materials phenomena. For instance, microscale thermal conductivity measurements can be used to study order-disordering transformation, site preference in intermetallic compounds, solid-solution effect on conductivity, and compositional point defect propensity. This article will use a few examples to illustrate the capabilities and developmental needs of this approach.

Micron-scale measurements of the coefficient of thermal expansion by time-domain probe beam deflection

We describe a pump-probe optical technique, time-domain probe-beam deflection, that enables measurements of the coefficient of thermal expansion (CTE) with micron-scale spatial resolution. Our quantitative model for the beam deflection includes contributions from thermal expansion of thin films deposited on a substrate, thermal expansion of the substrate, elastic deformation of the substrate, heating of air above the surface of the sample, and the lateral temperature gradient at the surface of the sample. The usefulness of this technique for high-throughput CTE measurement is demonstrated by two experiments: we are able to find the Invar alloy composition directly from a simple Fe–Ni diffusion couple and we find abnormal CTE increase in an ≈100μm region of dentin adjacent to the dentin-enamel junction of a human tooth. Such a CTE abnormality would be difficult to find using other techniques.

Generation and detection of gigahertz surface acoustic waves using an elastomeric phase-shift mask

We describe a convenient approach for measuring the velocity vSAW of surface acoustic waves (SAWs) of the near-surface layer of a material through optical pump-probe measurements. The method has a lateral spatial resolution of <10u2009μm and is sensitive to the elastic constants of the material within ≈300u2009nm of the surface. SAWs with a wavelength of 700u2009nm and 500u2009nm are generated and detected using an elastomeric polydimethylsiloxane phase-shift mask which is fabricated using a commercially available Si grating as a mold. Time-domain electromagnetics calculations show, in agreement with experiment, that the efficiency of the phase-shift mask for generating and detecting SAWs decreases rapidly as the periodicity of the mask decreases below the optical wavelength. We validate the experimental approach using bulk and thin film samples with known elastic constants.

Dynamic surface acoustic response to a thermal expansion source on an anisotropic half space

The surface displacement response to a distributed thermal expansion source is solved using the reciprocity principle. By convolving the strain Greens function with the thermal stress field created by an ultrafast laser illumination, the complete surface displacement on an anisotropic half space induced by laser absorption is calculated in the time domain. This solution applies to the near field surface displacement due to pulse laser absorption. The solution is validated by performing ultrafast laser pump-probe measurements and showing very good agreement between the measured time-dependent probe beam deflection and the computed surface displacement.

Spatially Resolved Measurements of Thermal Stresses by Picosecond Time-Domain Probe Beam Deflection

Measurements of thermally induced strains with micron-scale lateral spatial resolution, picosecond time resolution, and sub-picometer vertical sensitivity are achieved using a newly developed experimental method, time-domain probe beam defection (TD-PBD). TD-PBD is a pump-probe optical technique that combines an ultrafast laser oscillator as the light source, high frequency (10 MHz) modulation of the pump beam, and a wide range of time delays (0–4 ns) between the pump and probe. Deflections of the probe beam are measured by a position sensitive detector and an rf lockin amplifier. The beam-deflection data are analyzed using a detailed model of heat transport and thermally generated stresses and strains. Comparisons between the model and the data enable quantitative measurements of the coefficient of thermal expansion with a spatial resolution of 4 µm.