Thermal conductivity or local temperature can be measured using scanning thermal microscopy (SThM) with nanometer resolution, for example in polymer science and semiconductor devices.
Here, a compact temperature sensor, such as a thermocouple, is positioned at the top of an AFM cantilever tip, which is raster scanned across the surface to capture sample topography as well as temperature-related material properties.
Thermally Conductive Compounds
In most applications, polymers provide several benefits over metallic materials due to high corrosion resistance, lower weight and easier processability, enabling more flexibility in terms of design. Conversely, polymers are known to be poor thermal conductors.Carbon fibers and other additives are extensively used to provide thermal conductivity to polymers.
Heat transport within these compounds can be directional, based on the manufacturing process and additive, for example, in plane instead of through plane, which can help to optimize the thermal management in various applications and environments. Compounds that are thermally conductive are often used in aerospace and automotive cooling systems, temperature sensors, LED luminaires, and even consumer electronics.
In this analysis, the Nanosurf Flex-Axiom system equipped with an AppNano VertiSense Thermal Microscopy Module was used to track the thermal conductivity of carbon fibers integrated in a polymer resin. Polymer-embedded carbon fibers are shown in Figure 1 (top).
Figure 1. AFM imaging and thermal analysis of polymer-embedded carbon fibers. AFM topography (top), tip temperature (uncalibrated Imaging amplifier output, middle) and tip temperature mapping to the 3D sample topography representation (bottom).
In the left column (Figure 1), images exhibit a 40 µm sample area, whereas the images in the right column were captured within the area denoted by the red square shown on the topography image in the left column. In Figure 1 (top), fibers were cut almost perpendicular to the axis of the fiber and polished.
In the thermal conductivity mapping mode (Figure 1, middle), the laser heats up the cantilever tip and its temperature mirrors the thermal conductivity of the underlying substrate - if the material’s thermal conductivity is higher, the cantilever tip temperature will be lower due to heat dissipation from the tip into the sample.
In this case, a higher output value of the imaging amplifier reports a higher tip temperature. Figure 1 (bottom) shows thermal conductivity mapping to the 3D topography representation.
To make concurrent AFM imaging and thermal conductivity measurements, an AppNano VTP500 cantilever was employed in static mode.
After polishing the polymer matrix is slightly protruded by the carbon fibers. After thermal conductivity mapping (Figure 1, middle), it is seen that the carbon fibers display more thermal conductivity (lower tip temperature) compared to the surrounding polymer matrix.
A reduced thermal conductivity is seen in certain areas on the carbon fibers, pointing towards polymeric debris on the apex of the fiber surface. Such debris could be the result of residual material from the polishing or cutting process.
When the thermal conductivity map is overlaid on a 3D representation of the sample topography, clear correlation can be made between sample topography and thermal conductivity properties.
For thermal conductivity mapping the Nanosurf Flex-Axiom system was extended by the AppNano VertiSense system, which features unique cantilevers and an imaging amplifier:
- The imaging amplifier (Figure 2) can be used both with the Nanosurf EasyScan2 controller (signal module needed) and the Nanosurf C3000 controller (signal I/O option needed0. It is controlled through a smartphone or tablet application
- The thermocouple sensor is placed at the top of the cantilever tip (Figure 3), enabling precise measurements of temperature. The material around the tip sensor is thermally insulating to avoid heat loss from the tip into the cantilever material
- By using the configurable Nanosurf controller user input, the output of the imaging amplifier can be directly transformed into tip temperature and can be captured together with sample topography
- Two different modes can be used to operate the system, but in this case the thermal conductivity mapping mode (CMM) was employed
Figure 2. The VertiSense imaging amplifier connects to the Nanosurf AFM controller and the VertiSense cantilever
Figure 3. Scanning electron micrograph of a VertiSense AFM cantilever
In the CMM, the laser is directed just above the cantilever tip (Figure 4, dashed line) and this laser heats the tip of the cantilever containing the thermocouple at its apex. Passing over the sample’s thermally conductive area, heat from the cantilever tip is dissipated more efficiently into the sample, reducing the tip temperature.
On areas that are thermally less conductive, the heat dissipation is comparatively less efficient and the temperature of the tip is reduced to some extent.
Figure 4. Different operating modes of the VertiSense system
In the temperature mapping mode (TMM), the laser is directed behind the tip of the cantilever (Figure 4, solid line) and does not heat up the cantilever tip. The thermocouple at the tip can determine the temperature of the sample.
At a qualitative level, the VertiSense imaging amplifier is capable of reporting the temperature as an output voltage, where a higher voltage matches to a higher tip temperature. With the help of a unique temperature-controlled reference sample, it is possible to calibrate the thermocouple to enable absolute temperature measurements.
This information has been sourced, reviewed and adapted from materials provided by Nanosurf AG.
For more information on this source, please visit Nanosurf AG.