New Technique Developed for Studying Thermal Conductivity

By Ralph Dinwiddie, HTML, Oak Ridge National Laboratory

Thermal conductivity is a property of prime importance in many advanced applications. Using traditional techniques, it is difficult if not impossible to directly measure the thermal conductivity of microscopic structures and features such as fibers, fiber coatings, whiskers, grain boundaries, grains, powder particles, functionally gradient coatings and intergranular phases.

Microscopic thermal conductivity measurements of such features will allow better interpretations and modeling of bulk properties. Materials scientists need to be able to directly study microscopic structures, which will lead to a better understanding of processing and design parameters required in the development of new engineered materials.

The Scanning Thermal Conductivity Microscope (STCM), currently under development, can detect differences in thermal conductivity of microscopic structures, such as, fibers, grains, particles, coatings, and intergranular phases.

Effects of Processing Conditions

This information, in conjunction with bulk thermal conductivity measurements, can give a better understanding of the effects of processing conditions on the thermal conductivity of the composites. This in turn will lead to the ability to predict the thermal conductivity of CFCCs as a function of architecture, porosity, fiber-loading and processing conditions.

The STCM employs a modified probe in an Atomic Force Microscope (AFM). The probe is operated at an elevated temperature relative to that of the test specimen. When the probe is brought into contact with the test specimen, the tip cools due to heat conduction from the probe into the specimen. The amount of power required to maintain the probe at a constant temperature is directly related to the thermal conductivity of the test specimen.

Topographic Image

This information in combination with the probe position is used to construct a digital gray-scale or false-color image of the surface with submicron resolution. As with the AFM, topographic information is also obtained while maintaining a constant force between the specimen and probe. The topographic image is acquired simultaneously with the thermal conductivity information. The two resulting images are displayed in the figures.


Topographic (1a) and thermal conductivity (1b) images of 14 m SiO(2) pads on Si substrate. Each image is 50 m by 50 m. In the topographic image, lighter shaded areas-SiO(2)-are higher than the dark areas (Si). In the thermal conductivity image (b) the brighter shades represent areas of higher thermal conductivity (Si).
In the topographic image (Fig.1a) silicon oxide square pads are clearly visible as light colored objects approximately 14 m wide. The trenches between the pads are clearly visible. This area appears much darker, representing a relatively deep trench, at the bottom of which is a silicon wafer.The thermal conductivity image (Fig.1b) may easily be compared with the topographic image. Here the silicon oxide pads appear to be relatively darker than the lower areas between the pads. This lower area, which shows up as black in the topographic image, is bright in the thermal conductivity image. This correctly indicates that the silicon has a higher thermal conductivity than the silicon oxide pads. These silicon features are approximately 4m wide.



Topographic (2a) and thermal conductivity (2b) images of CVI SiC/SiC composite. Each image is 50 m x 50 m.

Fig. 2a shows the topographic image and Fig. 2b the thermal conductivity image of an SiC/SiC composite. In the topographic image, the fibers are slightly lower than the matrix due to specimen preparation. In the thermal conductivity image (2a), the fibers are clearly visible and appear to have a lower thermal conductivity than the matrix. Each image is 50 m by 50 m.

Continuing Work

Work is continuing on improving the resolution and sensitivity of the thermal conductivity probes. The probe is also being calibrated using isotropic high purity standards. Calibrated probes will allow the measurement of absolute thermal conductivity of microscopic structures. Further work is also continuing on a survey of the thermal conductivity of fibers.

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Comments to: mgc@ornl.gov

URL: http://www.hsrd.ornl.gov/cfcc/n4/thermal.html, Revised: July 5, 1995