Alexis Abramson - Department of Mechanical and Aerospace Engineering
Novel Characterization Methods
Three Omega for Soft Films: The three omega method has proven to provide accurate and reliable measurements of thermal conductivity of thin films and other materials. However, if the films are soft and conductive, conventional methodologies to prepare samples for the measurement technique are challenging and often unachievable. Various novel modifications to the approach have been taken in the nanoEngineering laboratory to demonstrate that the differential three omega method may be employed to measure the thermal conductivities of soft conducting films of polyaniline. These include utilizing a shadow mask for metal deposition and developing a recipe for low temperature deposition of an insulating layer on a conductive soft film with a relatively low glass transition temperature. Our results demonstrate that a polyaniline film exhibits an increase in thermal conductivity with temperature, which is largely due to increasing heat capacity.
Nanoscale Thermal Characterization: The Thermal Flash Method: Micro- and nanofibers are one-dimensional structures with diameters ranging from several nanometers to hundreds of micons. These materials offer promise in a wide range of applications, and their accurate thermal characterization is critical for this purpose. The development of rapid, simple, accurate and reliable methods to measure their thermal properties would facilitate their use and expand fundamental understanding of nanoscale thermal transport. With the limited number of steady-state methods developed to-date, thermal contact resistance is often a concern, casting doubt on results presented. Other transient methods commonly have other complexities associated with them. Nonetheless, the nanoEngineering laboratory has developed a transient thermal measurement technique, the “thermal flash method,” which is administered within a scanning electron microscope (SEM) to measure the thermal properties of one-dimensional nanostructures. For this relatively simple technique, thermal contact resistance is inconsequential. This technique builds on our laboratories previous work to measure the mechanical properties of micro/nanowires (refer to APL paper here) and expands on the laser flash method in which a sudden application of radiant heat is applied to a sample after which a transient change in temperature is detected. A wire-wrapped micromanipulator inside the SEM supplies instantaneous heating to one end of the micro/nanostructure; the temperature response is then detected at the other end using a microfabricated sensor. The transient electrical resistance of the sensor allows calculation of thermal diffusivity.
An SEM image of the thermal flash measurement of a microfiber
Nanofluid Thermal Characterization: The Comparative Radial Heat Flow Meter Research and development of various technologies in fields such as food engineering, bioengineering, heat exchanger design, nanofluids and agriculture/soil analysis often require highly accurate thermal conductivity data for liquids. A variety of methods have been employed for the thermal conductivity measurement of these materials, and transient methods are commonly preferred since quick results may be obtained and convection effects are generally negligible in the short time interval of the experiment. Nonetheless, transient thermal conductivity measurement apparatus and relevant analysis techniques can have various assumptions associated with them, leading to errors. The aim of this work was to demonstrate a novel steady state technique for measurement of thermal conductivity of liquids, and in particular, nanofluids, that combines some of the benefits of various previously demonstrated methods. The experimental setup is a coaxial type arrangement to impose heat flow along the radial direction away from the central axis, and the analysis method involves a comparative technique. Radial heat flow ensures that losses are minimized, and heat guards commonly found in other techniques are not required. Moreover, the comparative approach eliminates the necessity of exact knowledge of the heat flux from the source. These factors help reduce uncertainty, and a simple steady state apparatus may be constructed for this purpose. This “Comparative Radial Heat Flow Meter” was used to measure a wide variety of known fluids as well as novel nanofluids.
| Experimental setup: | (a) a schematic cut section illustrating various parts |
| (b) a schematic of a top-down midsection view of the cylindrical setup. |
In-plane Thermal Conductivity: The Dual-mode Heat Flow Meter Efficient and accurate thermal characterization is critical to the continued development of materials with engineered thermal transport properties for applications such as thermal management, thermoelectrics, thermal insulation and more. Thermal characterization techniques should be simple and reliable despite being constrained by the unpredictable nature of the materials being tested. For instance, the measurements should not be affected by the fragility of the samples, their unexpectedly extreme thermal properties (either very low or very high), their constrained geometries and most importantly, their limited availability. Conventional steady-state thermal conductivity measurement techniques like the heat flow meter are simple and easy to use but have been limited to certain ranges of the spectrum of thermal conductivity, often require specific geometries for testing, and even minor heat losses can significantly influence accuracy. In contrast, transient techniques, such as laser-flash or transient hot-strip, are known to be reliable over the entire range but conductivity measurements typically involve relatively complicated data analysis and are contingent upon the accurate determination of specific heat capacity and density. Additionally, most conventional techniques only provide an effective thermal conductivity when used for characterizing materials with anisotropic thermal properties. Thus, a simple and versatile characterization technique that is capable of measuring the thermal conductivity in a particular direction without requiring any knowledge or assumption about the properties in the other directions is desirable. The nanoEngineering laboratory has developed a steady-state thermal characterization technique which can be used for characterization of insulators as well as conductors and is particularly suitable for thin, anisotropic materials. In essence, the method was developed from the standard ASTM C 518 based heat flow meter technique, which compares the steady-state temperature gradient in the sample to that of a reference material to determine the thermal conductivity of the unknown sample. The conventional heat flow meter technique assumes one-dimensional conduction to be the only mode of heat transfer and works very well for determining through-thickness thermal conductivity of thin thermal insulators as long as the requirement that one-dimensional heat flow with negligible losses is strictly maintained. Alternatively for highly conductive materials, establishing a measurable temperature drop is critical, and therefore longer specimens may be required for the measurement; however, due to the high surface area, the effects of convection and radiation may no longer be negligible, thereby casting doubt on the reliability and accuracy of the technique. To overcome these obstacles, the “Dual-mode Heat Flow Meter,” which considers both conduction and radiation modes of heat transfer along a specimen of relatively long length was conceived to broaden the applicability of the heat flow meter technique while maintaining its inherent simplicity. This technique accurately and reliably measures thermal conductivity along the direction of one-dimensional heat flow. Convection heat loss can be minimized by conducting the experiments under conditions of very high vacuum (pressures less than 10-5 Torr).
