Written by John Clifford, Technical Specialist
When measuring thermal conductivity, powders and liquids can often pose problems. For liquids, the possibility of convection inhibits more conventional ways for measuring thermal conductivity. Meanwhile, the properties of powders vary depending on the compressive load, as that will change the amount interstitial air in the sample. One way that C-Therm has been able to overcome these issues is by requiring very small volumes for test samples.
The Modified Transient Plane Source sensor for measuring thermal conductivity. Equipped with the liquid cell attachment, a researcher is testing a small volume of liquid.
The Modified Transient Plane Source (MTPS) is a single sided sensor, able to measure volumes as small as 1.5 mL. This give many advantages for thermal conductivity measurement; from convenience, to accuracy, to safety, the small volume of the MTPS simplifies the testing process for its users.
For one, the 1.5 mL requirement makes sample testing much more convenient for researchers. Being able to use a small volume of liquid or powder allows many tests to be prepared quickly, using small amounts of material. This is particularly important for metal powders used in additive manufacturing.
Metal powder seen here is often used in metal additive manufacturing. It is important to measure the thermal conductivity of these powders as an indicator of part performance, however this powder can be quite expensive.
Metal powders like Ti-6Al-4V can cost over $400 per kilogram, making it important to use as little as possible for property testing. This optimizes in-line measurements for the purpose of quality testing. This is because a drastic change in thermal conductivity or effusivity from one batch to another signifies a change in quality. With the ability to collect accurate data from small volumes, the MTPS sensor can be implemented as an efficient quality assurance test.
Additionally, the small volume requirement helps ensure the accuracy of the test. This may seem counterintuitive, however having small amounts of sample reduces the inherent difficulties of testing these types of materials. For liquids, the presence of convection will introduce errors when measuring conductivity. However, necessitating a small volume allows for a decreased interface between the liquid and the ambient air, thus reducing the possibility of convection. This, coupled with the rapid heat pulse of the sensor, negates any errors due to convection.
Likewise, requiring a small amount of powder helps control the amount of interstitial air present. The powder sample can quickly and easily be tamped down with a small weight to produce consistent measurements over many trials, as seen below.
This allows for more accurate measurement by replicating the compressive conditions the powder would experience during use.
The MTPS sensor being used to measure the thermal conductivity of ammonium perchlorate, an energetic and explosive powder.
Safety is another area where using a minimal amount of material can be beneficial. Many materials that need to be characterized can exhibit dangerous properties such as a low flash point, volatility, or acidic behavior. Fortunately, one of the most effective risk mitigation strategies is reducing the amount of dangerous material needed. These behaviors are more common in liquids, especially in the automotive and fossil fuel industry, however powders can also pose some risks. Due to their low weight, powders can go airborne which creates a risk of inhalation. Similarly, certain powders can be sensitive to increases in temperature and could pose an explosion risk due to the high surface area of the material.
By requiring a small volume of sample to yield accurate results, the MTPS sensor provides flexibility, convenience, and added safety to its operators. To learn more about powder or liquid testing, see some of the resources below.
This blog post is a part of our Powders and Heat Transfer Fluids applications.
Resources:
– Thermal Conductivity Testing of Minimal Volumes of Energetic Powders
– Why is the thermal conductivity so low? The effect of interstitial air on the effective k of powders
– Avoid Convection Errors in Measuring the Thermal Conductivity of Liquids
About the Author
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John Clifford Technical Specialist
John Clifford is a marketing intern at C-Therm. He is currently in his third year of his Chemical Engineering degree at the University of New Brunswick in Fredericton. |
