Thermal Analysis Labs

Thermal Conductivity

Performance Characterization of Thermal Interface Materials: Rapid Thermal Conductivity Testing

Thermal interface materials (TIMs) are widely used to reduce thermal resistance at the interface between two materials (often called the thermal contact resistance). There are many different types of TIMs, ranging from thermal greases to thermal glues, thermal gap pads, and thermal adhesive pads. Of these, thermal greases (Figure 1) are popular because they tend to be easy-to-use, non-toxic, and non-permanent solutions to the thermal contact resistance problem.

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Figure 1. A thermal grease compound being tested in the Small Volume Test Kit (SVTK)

Thermal greases are difficult for many traditional methods of thermal conductivity testing to analyze: They are highly viscous, resulting in a very slow pour rate. As a result, air can become entrapped within the material and artificially deflate its thermal conductivity.

The unique planar, one-sided and one-directional thermal conductivity measurement provided by the C-Therm TCi Thermal Conductivity Analyzer provides a useful tool for the performance verification of thermal grease compounds, as the grease may be applied directly to the sensor surface, minimizing the formation of bubbles and enabling a uniform sample distribution over the sensor. Additionally, the small sensor surface area, short test time, and the optional small volume test kit result in a very small sample volume (< 2 mL) required to obtain good data, reducing the wastage in quality-control applications and the synthetic demand in research applications.
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Figure 2. Thermal Conductivity Testing of a Thermal Grease

The C-Therm TCi Thermal Conductivity Analyzer was used to test the thermal conductivity of a thermal grease compound. The observed value was referenced to the manufacturer’s specification for the grease’s thermal conductivity performance. The grease was found to test within 2% of the manufacturer’s specification on all tests. The mean of three tests of five measurements each was 0.727 W/mK. The relative precision for each of the three tests was 0.4%, 0.3%, and 0.2%, respectively, and the relative standard deviation across the three tests was 0.5%.

Characterizing the Thermal Conductivity Performance of Aerogel Samples

Aerogels are a relatively new class of ultralight, porous materials, typically derived from a gel. In an aerogel, the liquid component of the gel has been replaced by a gas (typically air). Owing to the very light nature of most aerogels, most aerogel samples have a transluscent, blueish appearance. Porosity of aerogels is generally in excess of 98% (meaning that, per unit volume, >98% of an aerogel’s volume is pore volume). Aerogels can be made of a variety of chemical compounds.

Aerogel Figure 1

Image Source: NASA/JPL-Caltech

Aerogels (above) are known for their extremely low thermal conductivity, which is often lower than that of air. In this respect, the thermal conductivity of an aerogel material is typically identified as a critical performance specification. This low thermal conductivity makes aerogel materials exciting in the field of insulation research, where engineers are continually looking to improve energy efficiency without adding excessive weight.

Three commercially-available aerogel samples were recently provided by a client seeking performance data on them in comparison to the specification sheet. The samples were analyzed with the C-Therm TCi Thermal Conductivity Analyzer using the Modified Transient Plane Source (MTPS) technique. The results are displayed below:

It can be seen that the thermal conductivity performance measured is in good agreement with the specified thermal conductivity of these commercially-available aerogel samples. Agreement with the specified value in all three cases was better than 4%.

Thermal Conductivity Enhancement of Biodegradable Thermoplastic Blends

Increasingly, work is being devoted to replacing heavy and expensive metallic heat sinks with lower cost, lighter organic-based heat sinks. One way in which to do this is to mix a polymer material with some thermally conductive additive to improve its thermal conductivity. Such a material is referred to as a thermally-conductive polymer composite (TCPC) material. The field of TCPC research is an emerging and highly competitive one – however, historically, such materials have historically possessed very high percolation thresholds – the percolation threshold being the point at which an additive in such a mixture begins to have an appreciable and scalable effect upon the thermal conductivity of the mixture, before which there is little, if any appreciable improvement in thermal conductivity upon successive additions of additive (Figure 1).

TCPC Figure 1

Figure 1. Left: Below the percolation threshold, no pathway exists for effective heat conduction. Right: Above the percolation threshold, a pathway for conduction exists

As a result of the high percolation threshold, very high loadings of additives (30 wt% or more) have been needed to achieve the desired thermal conductivity. The issue with such a high loading is that it sacrifices the mechanical properties, and sometimes the electrical properties, of the composite for the thermal conductivity, and it makes the composite denser, resulting in a heavier consumer good. Research in recent years has therefore focused on ways of reducing the required additive loading.

Scientists from the Chinese Academy of Forestry and the Chinese Academy of Sciences recently published work in the Journal of Composites Science and Technology detailing thermal conductivity improvement of a polylactic acid (PLA)/polycaprolactone (PCL) blend by addition of graphene, a type of elemental carbon one layer thick with an extremely high thermal conductivity. This work is unique, as the additive doping requirement was substantially reduced by trapping the graphene material at the interface of the polymer blend.

TCPC Figure 2

Figure 2. Thermal conductivity of PLA/GE, PCL/GE, and PCL/PLA/GE composite materials as a function of GE loading.

A key finding of their work is summarized in Figure 1. With a C-Therm TCi Thermal Conductivity Analyzer, Huang et al were able to study the effect of graphite content on the various polymer blends. The graphene loading improves the thermal conductivity of the PLA and PCL, but only marginally. However, once it becomes trapped at the interface of the polymer blend, the thermal conductivity of the material substantially improves, resulting in a four-fold improvement of the thermal conductivity of the material with merely a 0.5 volume % doping of graphene into the material. Remarkably, the percolation threshold of the PCL/PLA/GE blend (circled) was 0.11 vol%, representing the lowest percolation threshold for TCPC yet discovered.