Powders are used extensively in a variety of applications, including in powder metallurgy, electronics and electrical infrastructure. The thermal conductivity of the powders used in these applications is important to ensuring that the product performs properly, however the measurements can be tricky to complete accurately. The Hot Disk Transient Plane Source is well respected in the thermal conductivity measurement field for providing rapid and reliable results without the need for a complicated set up. Hot Disk instruments are able to measure thermal conductivity over a range of 0.005 to 1800 W/mK, and at temperatures from -160°C to 1000°C, making them incredibly versatile pieces of equipment. These characteristics are the result of over twenty years of thermal conductivity research and development at Hot Disk, where the creators have continually striven to expand the capabilities of the system. Though powder thermal conductivity can be a complicated measurement, the ability of the Hot Disk TPS to accurately measure them has been demonstrated through thorough research.
Figure 1. The Hot Disk TPS 3500.
A good sample set-up can mean the difference between a poor or an accurate measurement. With this in mind, a specialized powder cell sample holder has been developed and is available for powder testing. This cylinder is composed of four pieces, which when assembled seal a two-sided Hot Disk sensor in the middle of the powder to be tested (Figure 2). The final piece on top applies pressure to the set-up, which ensures excellent thermal contact between the sensor and the powder, and minimizes the chance of air pockets around the sensor. A specialized sample cell was used in both of the investigations described below.
Figure 2. The powder testing cell available from Thermtest enables the two sided Hot Disk sensor to be sandwiched between the powder for testing.
Rare Earth Oxide Powder Thermal Conductivity Measurement
This first example is representative of some of the earliest research performed to quantify the thermal conductivity of powders using the Hot Disk TPS. In 1997, Predeep and Saxena measured the thermal conductivity of three rare earth oxide powders; Gadolinium oxide, Samarium oxide, and Yttrium oxide using the Hot Disk TPS. The objective of the research was to determine if the system could produce results that were in line with theoretical predictions, and to understand some of the factors that affect powder thermal conductivity. The three powders listed above have different particle sizes, therefore Predeep and Saxena (1997) could study the influence of porosity on their results. The experimental results were then compared against a theoretical model developed from previous research.
Predeep and Saxena (1997) determined that porosity had an influence on the thermal conductivity of the powders. As porosity increased, there was more air contained within the powders, which lowered the thermal conductivity. By comparing their measured results to those produced by theoretical modelling, Predeep and Saxena (1997) determined that the Hot Disk TPS could accurately measure the thermal conductivity of powders.
Polyamide 12 Powder Thermal Conductivity Measurement
In a more recent example, Yuan et al. (2013) used the Hot Disk TPS to measure the thermal conductivity of polyamide 12 powder at the University of Texas at Austin. The goal of this research was to gather information that could be used to improve control during the laser sintering (LS) process. The thermal conductivity of both fresh and preheated powder was collected using the Hot Disk TPS 500, and these values were then compared to the final product after LS had taken place. Both density and temperature were investigated as possible influential factors over the thermal conductivity of the samples. The density of the samples was varied during the investigations, and thermal conductivity measurements took place over a 130°C range. Measurements were performed at 10°C increments. Some samples were heated prior to measurements taking place.
Yuan et al. (2013) discovered that the thermal conductivity of polyamide 12 powder increased linearly with density and temperature. Samples that had been preheated had similar thermal conductivities to those that had not. The thermal conductivity of the final laser sintered product was much higher than that of the powders.
The researchers concluded that bonding between particles was the major factor of influence in regards to thermal conductivity. An increase in contact between the powder particles through the formation of necks between them resulted in the slightly higher thermal conductivity of the samples that had been preheated. The much higher thermal conductivity of the final bulk product was due to the superior contact between the particles achieved by the laser sintering process.
These two examples showcase the strength of the Hot Disk TPS system in regards to thermal conductivity measurement of difficult materials. The diversity of the capabilities of the TPS system is a result of dedicated research and development work that spans over 20 years. The ability of the Hot Disk TPS to measure solids, liquids, pastes and powders makes it the ideal thermal conductivity testing equipment.
Note: For comprehensive results and in depth discussions, please follow the links in the reference section to the scientific papers