Written by Sarah Ackermann, Laboratory Services Manager (BSc, MSc)

Exploring metal foils, anisotropic polymers, and thin films

Introduction

In electronics, construction, and engineering applications, a good understanding of a material’s heat-transfer properties is key to good design. Thermal conductivity is the property which defines how much heat is lost to the ambient environment, or how much heat can be pulled away from a heat source. On the other hand, thermal diffusivity defines the transient heat-spreading ability of a material. As new materials are developed, thermal conductivity testing is needed to provide the data to support their use in real-world applications.

Thermal conductivity requirements – or what’s considered a “good” thermal conductivity – varies by material application: A heat sink in an electric vehicle needs a high thermal conductivity and diffusivity to respond to changes in heat generation rates within the electronics. By contrast, concrete needs a moderate thermal conductivity. Too high and it will vent too much heat to the outside environment, but too low and it will suffer thermal strain as it heats and cools throughout a day or year. At the lower extreme, insulation materials in many applications need low thermal conductivity to prevent excessive heat loss to the outside environment. 

Thermal conductivity is defined as the rate at which heat transfers through a material with a given temperature gradient. Thermal conductivity is often represented by k or λ, and has units of W/mK. Thermal diffusivity, on the other hand, is typically represented by α and has units of area/time – often expressed in mm²/s in engineering applications or m²/s in physics applications.

Figure 1: Thermal Conductivity is the property which describes the rate of heat flow (in W) through unit length of a material given a unit temperature difference from one side to the other.

Transient Plane Source

The Transient Plane Source (TPS) method is a powerful method for measuring the thermal properties of a sample. Several different mathematical algorithms exist to  accommodate different   sample types for TPS testing, during which the sensor is sandwiched between two halves of the same specimen (Figure 2). 

Figure 2: Setting up for a TPS test.

Thermal Conductivity Testing of Sheet Metal and Metal Foils

Metal foils are used in a variety of applications – including cooking, electronics, electrical and thermal insulation, and packaging, among others.. The challenge with foils and other very thin materials is they will often deform under their own weight – making them inappropriate to methods that suspend the sample in a testing setup from its edges, such as flash diffusivity. Their thin size also makes them inappropriate to transient methods which don’t account for barriers in the through-thickness direction. Thermal conductivity is often a key performance attribute for these materials – and the thermal conductivity is highly sensitive to any flaws in the manufacturing process which could introduce compositional impurities in the product. For this reason, characterizing the thermal conductivity is important both in the design and quality control phases of metal film applications.

Figure 3: Lower half of setup for Aluminum Foil Testing.

For this kind of sample, the TPS method can be employed with the slab testing utility on the Trident Thermal Conductivity Analyzer. Results from this testing is in Figure 4.

Figure 4: Thermal Conductivity Test Results for Aluminum Foils

For more information on the use of the slab testing utility application and theory, check out our method selection guide. 

Thermal Conductivity Testing of Anisotropic Polymers and Polymer Composites

Measuring an anisotropic sample on the bulk TPS utility (keep this term consistent) does not reveal the true thermal conductivity of a sample. Instead, the thermal conductivity will be a geometric average of the component thermal conductivities. However, if the sample is orthotropic (as is the case with wood and many extruded composites) and the volumetric heat capacity is known, the through-plane and in-plane components of thermal conductivity can be separated. This makes the anisotropic utility on C-Therm’s Trident Thermal Conductivity Analyzer an attractive option for convenient testing of anisotropic polymers.

Figure 5: Testing a cylindrical polymer sample.

To demonstrate performance, a sample of highly anisotropic glass-filled polymer composite (Garolite, axis: a = b ≠ c) is measured on the anisotropic TPS utility and compared to a mildly anisotropic polymethylmethacrylate (PMMA) material, Lexan. Figure 6 shows a plot of the resulting thermal conductivity measurements. 

Figure 6: Test Results for Lexan and Garolite G-10.

This thermal conductivity data is plotted above (Figure 3). The axial thermal conductivity of Lexan was found to be 0.174 W/mK (0.167 W/mK literature value¹ (+4%)), and the radial thermal conductivity was found to be 0.265 W/mK (0.25 W/mK literature value¹ (-6%)). In the case of Garolite, the axial thermal conductivity was found to be 0.239 W/mK (0.225 literature value (+6%)), and the radial thermal conductivity was found to be 1.45 (1.55 literature value (-6%)).  TPS shows good performance with both mildly and strongly anisotropic polymers. 

For more information on anisotropic testing applications, you can watch this webinar on anisotropic polymers. 

Thermal Conductivity Testing of Thin Films

In fields ranging from packaging, labels, construction, and electronics, thin films see ever greater application in modern technology. In many of these applications, a thorough understanding of the thermal conductivity is needed. In packaging, thermal conductivity of the outer material can dictate the temperature of the product being shipped. This is particularly important in cold chain applications. In electronics, a flexible circuit’s thermal conductivity will dictate how much heat generation can be tolerated in operation. In construction, thermal conductivity of a sealant film can contribute significantly to the thermal performance of windows and door frames. For these reasons, it’s critical to understand the thermal conductivity of a novel film material – which can often vary significantly from the bulk thermal behavior, owing to greater organization of thin film’s molecules.

Understanding the thermal conductivity of a thin film can be a challenge. These samples are inappropriate to free-standing methods like flash diffusivity as they cannot maintain their shape and often are IR transparent.  Furthermore, they are typically  inappropriate for other transient and steady state methods owing to their thin shape. However, with Trident’s Transient Plane Source method using the Thin Films utility, the testing is fast and straightforward.

Figure 7: Performance on Thin Film Materials.

The test results for these materials can be seen in Figure 7. All samples show good agreement with the literature values. For more information on testing thin samples, check out this application highlight. 

Contract Testing Services

Thermal Analysis Labs offers thermal conductivity and thermal effusivity testing with the TPS sensor, in addition to a variety of other thermal tests. Contact us here if you want more information about our testing services or to to get a quote for testing.