Powders

The Need for Thermal Conductivity of Powder Materials

Powders are a type of bulk solid comprised of fine particles that are loosely packed and are able to flow freely. They can come in many different forms and sizes, which vastly change their properties and applications. Very coarse powders can often be found in geological applications, such as loosely packed soil or gravel, and their properties are often needed to characterize the quality of a building material, or the environmental conditions of an area. Finer powders are often used for explosive applications, or in additive manufacturing where metal powders help generate new parts. Increasingly, metal powders are being used as an additive to increase the thermal performance of traditionally insulative materials like polymers or novel composites.

The wide use of powders in industry and research makes it important to be able to accurately and rapidly characterize properties such as thermal conductivity, as this is a strong indicator into the thermal performance of the end material.

The MTPS thermal conductivity sensor, equipped with a powder cell. Powder is placed atop the sensor, and is tamped down using the provided weight.
A scientist using the TLS needle probe in a thermal chamber, the probe is being placed into a vial containing the sample powder.
Metal powder, commonly used in metal additive manufacturing. These powders can be quite expensive, meaning that small test volumes are desirable.

Measuring the Effective Thermal Conductivity of Powders

Characterizing a material in powder form presents many different challenges and considerations than the same material as a rigid body. Particularly, powders are much more susceptible to environmental changes due to their small size and lack of strong connection between particles. Therefore, it is often much more useful to measure the effective thermal conductivity of a powder, which considers the conditions of the surroundings as a whole.

C-Therm’s Trident Thermal Conductivity Instrument, configured with both the TLS and MTPS sensors, able to measure the thermal conductivity of powder materials

For example, since powders are loosely packed, there is air present throughout the sample. The amount of air present will change depending on the compressive load placed upon the powder; a higher force will result in a tighter powder-packing, and thus less air. Measuring the effective thermal conductivity takes this into account, which is important as the sample can be arranged to best represent end-use conditions.

C-Therm’s Trident Thermal Conductivity Instrument provides multiple ways to measure the effective thermal conductivity of powders, depending on the type of powder being examined. In particular, the Modified Transient Plane Source (MTPS) sensor and the Transient Line Source (TLS) sensor are both designed for rapid and accurate measurement of the thermal conductivity of powders. In particular, the MTPS requires volumes as small as 1.5 mL making it desirable for both energetic and expensive powders.

National Aeronautics and Space Administration (NASA)

We purchased the C-Therm Thermal Conductivity Analyzer after seeing a demonstration of how fast and easy it is to operate. The instrument provides unequivocal results and provides the flexibility to test powders and liquids. In terms of our satisfaction with the purchase, I’d give it a 10 out of 10 - extremely satisfied.

Dr. Enrique Jackson,
NASA (Sector: Aerospace)
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Centre for Industrial Rheology

Our lab is frequently asked to identify the best practical methods for characterising fundamental physical properties of soft materials, for a myriad of industries including pharma, food, cosmetics, specialty chemicals, and electronics. The MTPS method available on C-Therm’s Trident System is a perfect fit for us, providing a simple, highly accurate, practical, and versatile route for measuring thermal conductivity of semi-solids, powders, and fluids. The MTPS is a welcome addition to our capabilities, helping to complete our service offering, and has been a boon to the growth of our business.

Neil Cunningham,
Founder & CEO
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Case Highlights

Hydrogen Storage Systems Using Metal Hydrides

In recent years, there has been extensive research into the development of a hydrogen-based energy economy in order to eliminate carbon-based energy sources. Consequently, metal hydrides have shown great promise for their ability to reliably release hydrogen simply by being heated. It is crucial to know the thermal properties of metal hydride powders such as NaAlH₄ in order to determine their thermal response, and the rate they expel hydrogen.

C-Therm’s Thermal Conductivity Instrument employing the Modified Transient Plane Source (MTPS) sensor was used to measure the thermal conductivities of all the metal hydrides. Measurements for the hydrides were taken both in powder form, as well as pellet form composed of powder that was pressed under six tons. The results can be seen below.

Graph showing the thermal conductivities of various metal hydrides in both packed and powder form

As expected, the pressed samples exhibited much higher thermal conductivities, indicating that pressed samples may be better suited to hydrogen storage, since they will approach the required heat more rapidly.

This information is originally from the paper Thermal conductivity and specific heat measurements of metal hydrides and can be found here.

High Temperature Simulation Modeling for Metal Additive Manufacturing

Oftentimes in metal additive manufacturing, simulations are required to help part design. When dealing with the laser melting process, it is important to have accurate thermophysical data for the powder so that it will behave as expected during the manufacturing process. Currently, generating thermal conductivity values is quite challenging, and is usual done by approximation. Common solutions include assuming thermal conductivity is independent of temperature, that it is 20-60% of the bulk material, or running an in-depth simulation for thermal conductivity at the process conditions. These solutions range from inaccurate to extremely resource intensive.

One way to get accurate data up is by using the Transient Line Source (TLS) sensor. The TLS-HT300 is able to accurately measure the thermal conductivity of powders up to 300°C.

TLS-HT300 incased in a vial filled with sample Ti-6Al-4V powder used in additive manufacturing

The TLS probe is able to take repeatable measurements up to 300°C, allowing for an improved material model with more accurate thermal properties, without the need for additional computational resources.

This data was collected by the University of Hamburg, and can be seen in a joint webinar here.

Resources

SIMPLIFYING THERMAL CONDUCTIVITY

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