MSE Professor Featured on the Cover of Review of Scientific Instruments

J.-C. Zhao, Materials Science and Engineering professor, David Cahill, professor at the University of Illinois, and MSE graduate student Changdong Wei published an article that was featured on the front page of  Review of Scientific Instruments (RSI), the most cited multidisciplinary journal in Instruments and Instrumentation covering physical, chemical and life sciences. RSI had 23,613 citations in 2012 alone.

The invited article, Micron resolution spatially resolved measurement of heat capacity using dual-frequency time-domain thermoreflectance, appeared in the August 2013 edition.

Article Summary
Specific heat capacity (C_p) is an important thermodynamic quantity from which the entropy, enthalpy, and Gibbs free energy as a function of temperature can be established. C_p can also be used as a tool to study phase transitions. Recently, J.-C. Zhao and his student Changdong Wei, collaborating with David Cahill of the University of Illinois, invented an ultrafast laser based photo-thermal technique, which can measure the heat capacity with a spatial resolution of about 10 microns. This localized high-spatial-resolution measurement technique is ideal for the demanding high-throughput measurements of desired material properties.

The diffusion-multiple approach is to create composition libraries of solid solutions and intermetallic phases by long-term annealing of junctions of three or more phases/alloys together. It has been extensively used in the study of phase diagrams, materials kinetics, and composition-structure-property relationships of bulk alloys. Localized micro-scale materials properties microscopy tools facilitated those studies. Measurement tools for properties such as elastic modulus, hardness, thermal conductivity, heat capacity, dielectric properties, optical properties, and crystal structures are relatively well developed; while for electrical conductivity, magnetic properties, and compressive yield strength need further improvement or more benchmark studies. All these micro-scale probes are very useful and will have great impact on materials research.

In this C_p measurement paper, the ultrafast laser based experimental set-up is systematically described. The output of the Tsunami laser was split into pump and probe beams. The pump beam traveled through an electro-optical modulator (EOM) while a translational delay stage was focused on the sample surface via a long-working distance objective lens. The probe beam, after it traveled a similar length of optical path, was focused to the same location on the sample surface. The arrival time difference could be adjusted by the delay stage. The reflected probe beam was collected by a Si-photodiode and the signal was sent into computer for analysis. The research team carefully compared the measurement values of several well studied standard materials with literature values. The overall accuracy of the measurement is established as ± 8%, and the spatial resolution is tested as 8-10 microns. We also applied this method to map the heat capacity of two diffusion couples--Ni-Zr and Ti-TiSi₂. Figure 2 is the example of Ni-TiSi₂ case. The two raw maps revealed the exactly same microstructure as in the SEM image. We compared the measured heat capacity values for each intermetallic compounds with literature and CALPHAD calculations, and a very good agreement has been achieved.