NPL is designing a new facility that will provide significantly reduced uncertainties and expanded the scope for thermal conductivity measurements on composites, polymers and ceramics from -100 °C to 250 °C. 

Although NPL is one of the few places in the world that has acknowledged expertise in making thermal conductivity measurements on composites, polymers and ceramics, the majority of customers are not able to provide samples of sufficient dimensions for our existing national standard apparatus. Therefore our existing measurement capability has to be based on a commercial apparatus with an estimated measurement uncertainty of ±7.5% (k=2), which cannot provide the accuracy (±3.0% or better) required for advanced material research. The existing facility mainly serves the aerospace, automotive and energy generation industries, but also provide measurements to most other engineering and process industries.

NPL is currently designing a new facility that will provide significantly reduced uncertainties and expanded the scope of our capability by allowing measurements on thin sections of medium thermal conductivity materials at temperatures from -100 °C to 250 °C (possibly up to 300 °C) in different gas pressure environments and under compressive loads.  The main technical challenges in designing such an apparatus involve measuring the surface temperature on thin section of material under compressive loads, over a wide temperature range and most significantly, in regions of high heat flux.

An overall concept and design for the apparatus has been completed and includes a new style of heater-plate with a construction that allows for a reduced thickness. The heater also includes a specially shaped guard-centre gap, developed with that aid of infinite element thermal modelling, which will reduce uncertainty in the metering area dimension when measuring thin specimens. The design also includes a chamber that will contain the plate-specimen stack in an environment that is controlled at the mean specimen temperature (-100 °C to 250 °C). This will reduce heat loss or gain from both the edges of the plates and specimen, and also limit heat transfer along the power cables and sensor leads. It has also been designed to provide the flexibility to measure the thermal conductivity of materials in different gases and at different pressures.

For more information, please contact Clark Stacey.