Process Analytical Technology

In-Line Thermal Conductivity & Effusivity Sensors

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In-line Monitoring of Filler Distribution in Dispensing Adhesives

With the emergence of Industry 4.0, the need for real-time, automatic quality assurance has grown substantially. One area of growing significance for this movement is the electric vehicle (EV) industry. As EVs become more prominent, the call for appropriate thermal management increase, particularly of the battery pack.

The safety concern, and cost associated with scrapping battery pack demands accurate process monitoring to ensue adequate thermal management. One common method to dissipate heat from the battery has been with the use of two component (2k) adhesives with conductive fillers.

In small volumes, this is a valid solution; however, when scaling to the volume required for the EV industry, problems arise with filler sedimentation.

Sedimentation will cause inconsistencies in the adhesive and thus, poor thermal performance. Mixing the adhesive will introduce air bubbles into the adhesive, which will also ruin the thermal performance. Therefore, it is necessary to monitor the degree of settling to ensure that only adhesives with an appropriate amount of conductive fillers are dispensed onto the the battery pack.

C-Therm’s Process Analytical Technology

C-Therm’s Modified Transient Plane Source (MTPS) sensor has a proven track record for in-line and at-line process monitoring. Using rapid thermal conductivity and effusivity measurements, C-Therm’s Process Analytical Technology is able to detect low filler concentration, air bubbles, and sedimentation of storage tanks using 3 second test times.

Case Highlights

In-Line and At-Line Monitoring of EV Dispensing Adhesives

Thermal Interface Materials (TIMs) have been used in the electronic industry for decades from CPUs to phone batteries. However, as electric vehicles (EVs) become increasingly prominent, the volume of TIMs required for adequate battery thermal management has increased dramatically. With this new volume, new problems are being encountered by both thermal adhesive manufacturers and adhesive dispensing groups. 

In particular, particle sedimentation has proven to be a challenge for these groups. In order to increase the adhesive’s thermal conductivity, conductive fillers are added to the mixture. However, given enough time, these conductive particles will succumb to settling as gravity draws them out of suspension towards the bottom of their storage tank, causing drastically uneven thermal properties. This problem is magnified given the volume requirement of EVs, as well as the safety and liability risk of having poor thermal management.

The Modified Transient Plane Source (MTPS) technique is able to rapidly monitor particle settling for quality control applications. As conductive fillers settle towards the bottom of the storage tank, the thermal conductivity will increase from their presence. Consequently, the thermal conductivity at the top of the adhesive tank will diminish. The MTPS can be retrofitted for in-line process monitoring, using thermal conductivity and effusivity measurements. Data for this application can be seen below: 

As can be seen, the MTPS sensor at the bottom of the adhesive sample shows a significantly higher thermal conductivity over time, increasing as the weight percentage of filler increases. This shows the effects that particle settling has on thermal adhesives, and how the MTPS sensor is able to monitor this for when the adhesive is out of spec.

Metal Injection Molding Quality Assurance

In the Powder Metallurgy (PM) process, it is crucial to have consistent and uniform products. A big part of achieving this goal is ensuring a homogenous powder blend. However, a challenge with this is creating a process that achieves good homogeneity and operates efficiently. Currently, a common approach for determining mixture homogeneity is to compare the final blend properties with the predicted values required. However, this method is unable to determine how long the blend will take to homogenize; therefore, process optimization is often done through trial and error.

Thermal effusivity is a material property that combines thermal conductivity, density, and heat capacity, meaning that it can differentiate between solids, liquids, and powders based on their heat transfer properties. In PM mixtures, the effusivity will change as the blending process reaches steady state, and will exhibit noticeable differences as a response to non-typical events such as particle segregation.

Figure 1: Various blend scenarios as characterized by thermal effusivity measurements. Steady state conditions show stable, homogeneous blend.

The measurement set up four in-line Modified Transient Plane Source (MTPS) sensors embedded in the lid of the blending unit. During the blending process, the powder will be “quasi-stationary” for a time, providing good contact for the sensors. During dynamic blend trials, the stable region of blend homogeneity can be identified based on thermal effusivity as seen in Figure 2.

Figure 2: Thermal effusivity versus blend time. Note the steady state region of thermal effusivity, indicating a completed blending. Continuing blending caused large deviation in RSD, indicating over blending and particle separation.

The use of in-line MTPS sensors was shown to have strong correlations between various phenomena that occur while blending including changes in apparent density, over blending, and mixing time optimization.

This case highlight is take from the paper Monitoring of Powder Homogeneity During Double-Cone Blending, which can be read here.

Pharmaceutical Process Monitoring

Lubricants play an important role in the pharmaceutical industry. In particular, magnesium stearate (MgSt) is a common lubricant, however its properties can vary significantly depending on the way it was produced. These can impact the rate at which active ingredients are released, meaning that determining the optimal mixing time and state are crucial.

Thermal effusivity can be used to quickly and non-destructively monitor blending uniformity for pharmaceutical ingredients. By embedding multiple Modified Transient Plane Source (MTPS) sensors in the lids of the V-type mixer, thermal effusivity can be measured, giving insight to blend quality and lubricant adhesion levels, as seen in Figure 1:

Figure 1: The change in thermal effusivity by blending, reflects the changing adhesion of the added MgSt

These results allow for the determination of the optimal blending time, regardless of the particular physicochemical properties. This study concludes that in-line thermal effusivity testing can effectively determine uniformity without intricate sampling processes, and serve as an efficient monitoring tool for pharmaceutical ingredients.

This case highlight is from a study called Evaluation of physicochemical properties on the blending process of pharmaceutical granules with magnesium stearate by thermal effusivity sensor and can be read here.

  • C-Therm's Process Analytical Technology can be embedded into dispensing or blending units for in-line real time measurements

    C-Therm's Process Analytical Technology can be embedded into dispensing or blending units for in-line real time measurements

  • The traditional MTPS sensor can also be modified for at-line testing; here, sample can be dispensed

    The traditional MTPS sensor can also be modified for at-line testing; here, sample can be dispensed

  • The MTPS sensor face embedded flush with the blender surface ensuring good, representative contact

    The MTPS sensor face embedded flush with the blender surface ensuring good, representative contact

  • Metallurgy powder blender with four in-line sensors applied. This allows for homogeneity monitoring, allowing for optimized blending times.

    Metallurgy powder blender with four in-line sensors applied. This allows for homogeneity monitoring, allowing for optimized blending times.

  • MTPS
  • Modified Transient Plane Source (MTPS)

    Modified Transient Plane Source (MTPS)

    Simple and Precise. The MTPS method employs a single-sided sensor to directly measure thermal conductivity and effusivity of materials. The MTPS method has the highest precision, highest sensitivity, shortest test time, and is the easiest to use among all three techniques.

    Principles of Operation

    Principles of Operation

    Trident’s primary sensor employs the Modified Transient Plane Source (MTPS) technique in characterizing the thermal conductivity and effusivity of materials. It employs a single-sided, interfacial heat reflectance sensor that applies a momentary constant heat source to the sample. Typically, the measurement pulse is between 1 to 3 seconds. Thermal conductivity and effusivity are measured directly, providing a detailed overview of the heat transfer properties of the sample material.

    How It Works

    1. A known current is applied to the sensor's spiral heating element, providing a small amount of heat.
    2. A guard ring surrounds the sensor coil to support a one-dimensional heat transfer into the sample. The applied current results in a rise in temperature at the interface between the sensor and the sample, which induces a change in the voltage drop of the sensor element.
    3. The rate of increase in the sensor voltage is used to determine the thermal properties of the sample. The voltage is factory-calibrated to temperature. The thermal conductivity is inversely proportional to the rate of increase in the temperature at the point of contact between the sensor and the sample. The voltage is used as a proxy for temperature and will rise more steeply when lower thermal conductivity materials (e.g. foam) are tested. Conversely, the voltage slope will be flatter for higher thermal conductivity materials (e.g. metal). With the C-Therm Trident, tabular thermal conductivity results are reported in real-time making thermal conductivity measurement fast and easy. No regression analysis is required.


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