Thermal interface materials (TIMs) are incredibly important to the proper function and longevity of modern electronic devices. As our electronics get smaller and more powerful, they produce more heat in a smaller area, which must be quickly removed from the components of the device to maintain a safe operating temperature. TIMs facilitate the movement of heat between the heat producing components and heat sinks by reducing the thermal resistance between them (Figure 1). The thermal conductivity of the TIM is crucial to device performance, and even a slight increase in this value can make a significant difference.
Figure 1. Illustration showing the position of a thermal interface material (TIM) between a heat producing chip and a heat sink1.
Most TIMs use a base material to which ‘filler’ particles with high thermal conductivities are added. The downside to this method is that when high amounts of filler particles are added, the thermal resistance and viscosity increases, which limits the benefit the additions have on the thermal conductivity. Therefore a filler that can have a large impact in small amounts is desirable. Goyal and Balandin (2012) from the University of California set out to develop such a filler, and tested the thermal conductivities of a hybrid graphene-metal filler using the Hot Disk TPS 2500 S thermal conductivity instrument (Figure 2).
Figure 2. The Hot Disk TPS 2500 S thermal conductivity measurement system.
Goyal and Balandin (2012) decided to use graphene as it demonstrates excellent thermal conductivity, and graphene flakes (few-layer graphene/FLG) also possess a high thermal conductivity which is unaffected by incorporation into a matrix. Composite disks made of homogenous hybrid graphene-metal-epoxy were created. Samples with various amounts of graphene were produced, and reference disks containing metal epoxy with carbon black were used as a comparison.
The Hot Disk TPS 2500 is a versatile thermal conductivity measurement instrument that takes absolute measurements of thermal conductivity between 0.005 and 1000 W/mK. It can perform tests on solids, liquids, powders, and pastes and can make isotropic and anisotropic measurements in a single test; producing rapid results with a reproducibility better than 2%. Goyal and Balandin (2012) used a two sided Hot Disk sensor and pressed it between two disks of identical composite mixture to test their thermal conductivity (Figure 3). Measurements were performed across a temperature range to determine if it was a factor that influenced the thermal conductivity. A single sided sensor, the TPS-S, is also available for use with the TPS 2500 S, which is ideal for clients who have small amounts of a sample or have a sample that is expensive or difficult to cut.
Figure 3. On the left is an illustration depicting the position of the TPS sensor in relation to the sample pieces in the set up used by Goyal and Balandin. On the right is the TPS-S, the single sided sensor that can perform the same measurement using one sample piece.
Data from the Hot Disk TPS 2500S on the reference disks containing carbon black indicated there was no significant increase in thermal conductivity as a result of its addition. Conversely, the addition of the graphene-FLG filler had a huge impact on the thermal conductivity of the composite, a volume fraction of just 5% graphene caused the thermal conductivity value of the composite to jump ~500%. Data collected from thermal conductivity tests taken along a temperature range indicated that thermal conductivity increased slightly along with temperature. Researchers concluded that the variation in size of the graphene flakes in the solution had aided in increasing the thermal conductivity, as it helped form a strong percolation network.
Goyal and Balandin concluded from their work with the Hot Disk TPS 2500 that the hybrid composite that they created had excellent potential as a thermal interface material, as it required just a small amount of filler (5% volume fraction) to increase its thermal conductivity by 500%, without affecting the electrical resistivity. This is an exciting step forward in the TIM world, and will contribute to the ability of companies to produce increasingly efficient and high quality electronics in the future.
Note: For comprehensive results and an in depth discussion, please follow the link to the scientific article in the reference section.