Accounting for the length-scale dependence of thermal diffusivity in 3C-SiC measured with transient thermal gratings

Abstract

Pump-probe optical methods like transient grating spectroscopy (TGS) enable rapid, nondestructive thermoelastic property measurements. But, in phonon-dominated ceramics, they can underpredict bulk thermal diffusivity when long mean free path (MFP) phonons do not equilibrate over experimental length scales. We combine in situ TGS with Si4+ ion irradiation of CVD 3C-SiC (300 and 550C, 0.5-1 dpa) and density functional theory informed Boltzmann transport equation solutions to understand the origins of this offset. We show how the discrepancy between laser flash analysis (LFA) and TGS-measured thermal diffusivity varies with grain-boundary density, temperature, and defect concentration. We introduce a dimensionless suppression factor that accounts for this discrepancy and demonstrate its utility by using it to show an agreement between the thermal defect resistance of neutron irradiated 3C-SiC (measured using LFA) and ion irradiated 3C-SiC (measured using TGS). This theory-informed experimental framework enables quantitative, in situ tracking of ion irradiation damage induced thermal transport degradation in ceramics.

Figures (9)

• Figure 1: Normalized TGS signals for CVD 3C-SiC coated with approximately 36 nm of Au (sputter coating) and 20 nm of W (PVD). These signals were acquired at room temperature and in vacuum. Note that the temporal shift is only for visual purposes. Inset showing TGS signal trace for single crystal tungsten with 100 surface orientation.
• Figure 2: (a) Thermal diffusivity vs temperature, normalized by room temperature thermal diffusivity, measured using TGS (Au- and W-coated SiC) and LFA (SiC). Inset showing absolute values of thermal diffusivity as a function of temperature. (b) Elastic modulus vs temperature, normalized by room temperature elastic modulus, measured using TGS (W-coated SiC) compared to theoretical values (SiC) snead_handbook_2007. Inset showing absolute values of SAW speed as a function of temperature. Shaded region in inset shows $\pm \: 5\%$ bounds on the theoretical value
• Figure 3: Normalized thermal diffusivity of CVD 3C-SiC as a function of dose at 300, 525 and 550 ° C as measured using TGS.
• Figure 4: (a) EBSD map showing surface orientation of grains (b) Bar plot showing normalized areal coverage as a function of grain size. (c) SEM micrograph showing the tungsten delamination region (darker region inside white-dashed ellipse), which aligns with the TGS spot on S3. Green marking shows approximate region where TEM lamella was milled (d) TEM micrograph showing cross section of boundary between the region where the tungsten coating had delaminated and where it was intact. Blue and red lines mark approximate locations of EDS line-scans, with the inset showing tungsten signal intensity.
• Figure 5: (a) DFT+BTE predictions for $K_{eff}(\Lambda)$ for pristine single crystal and polycrystalline 3C-SiC calculated at 27 and 527° C. (b) Spectral contribution to cumulative thermal conductivity (normalized) for single and polycrystalline 3C-SiC. (c) Zoomed in view for $K_{eff}(\Lambda)$ bounded between $\Lambda \: = \: 6 \: \mathrm{\upmu m}$ and $\Lambda \: = \: 9.5 \: \mathrm{\upmu m}$ at 27° C. Also shown are experimental results for $K_{eff}(\Lambda)$ of CVD 3C-SiC taken at nominal $\Lambda \: = \: 6.4$, $7.6$ and $8.8 \: \mathrm{\upmu m}$. The experimental data was acquired at room temperature (between 20 and 22° C).