Patent of the month: Optimising thermal conductivity

Materials World magazine
,
1 Dec 2017

Dr Jennifer Unsworth of intellectual property firm Withers & Rogers looks at a newly patented way to manufacture an improved thermal interface material.

Thermal interface materials are designed to be used in the gaps between heat-generating components and heat-dissipating components, in order to enhance thermal coupling. 

Such materials are widely used in the electronics industry, where they can be placed between a microprocessor and a heat sink, for example. They are available in a range of forms – as a paste, a grease or as a sheet of a phase-change material, such as paraffin.

The materials are typically composed of an organic base loaded with a thermally-conductive filler material. To achieve the desired conductivity, very high loading levels of the base material are needed, which can degrade its other properties such as flow, cohesion and adhesion. In order to produce thermal interface materials, it is necessary to balance the desired conductivity with the need for the base material’s other properties. The net result is a material with less than optimal thermal conductivity.

In a bid to solve this problem, global electronics technology company, IBM, has developed a new thermal interface material consisting of graphite nanofibres, along with a new system for producing such materials. Both are the subject of UK patent GB2498464, which was granted on 25 October 2017. 

Graphite nanofibers, varying from five to 100μm in length and diameter, have received considerable attention in the electronics field due to their remarkable thermal conductivity. 

Their thermal conductivity is anisotropic, which means it exhibits properties with different values when measured in different directions. 

The patented technology relates to a method of preparing a thermal interface material using graphite nanofibres. 

This method is being described as graphite nanofibres grown from graphite particles around a magnetic, catalytic seed layer. 

The base material is melted and the nanofibres are randomly dispersed throughout. The interface material is then spread out to form a sheet and a magnetic field is used to align the nanofibres. 

Following that, the sheet is cooled, cut into pieces as required and positioned between two components. 

In order for this alignment to occur, the catalytic seed for producing the nanofibres must be magnetic. The alignment of the nanofibers also uses their properties of anisotropic conduction, which vastly improves the thermal conductivity of the resulting thermal interface material.

The patent shows how the thermal conductivity of a typical paraffinic phase change material can be increased 20 times by displacing about 5% of its weight with randomly dispersed graphite nanofibres. Using the patented technology to align the graphite nanofibres results in a material that is up to 300 times more effective at conducting heat. 

In this case, the application of graphite nanofibres and the use of magnetic fields to produce new thermal interface materials has boosted their thermal conductivity exponentially. Importantly, this has been achieved while retaining the base material’s other properties of flow, cohesion and adhesion.

To view the patent, visit bit.ly/2iNzn4y