Impact of Encapsulated Phase Change Material Additives for Improved Thermal Performance of Silicone Gel Insulation

Energy density in power electronics is increasing rapidly to enable efficient implementation of all high-performance electric transportation. Increased power to weight and volume ratios are important for maximizing the electric capability of transportation systems. Transitioning from silicon to silicon carbide (SiC) reduces the size of power switching devices and increases their temperature stability up to 175 °C, introducing larger thermal gradients within the packaging architecture [1]. Reliability issues in these devices primarily occur because of thermal stresses induced throughout the electronic packaging but are also connected to degradation of electrical insulations and other packaging materials [2]. Current thermal management solutions are not adequate for the next generation of power electronics and will limit improvements to overall system energy density due to challenges managing large energy dissipations. Efforts to enable reliable use of compact designs focus on reducing thermal resistance throughout the device. Innovative cooling structures have brought thermal management closer to the internal device structure, but such solutions are often infeasible due to manufacturing constraints or newly introduced reliability concerns. To continue effective improvement of electronic packages, material functionality needs to be enhanced.

Passive cooling is a popular method for increasing a system's thermal stability without increasing the primary cooling solution's power requirements. Phase change materials (PCM) constitute one technique where materials absorb large amounts of energy (latent heat) when going through an internal phase transition, commonly solid to liquid. They can be used to mitigate thermal gradients in a system or to prevent thermal overshoot in the device. These phase transitions are isothermal processes and can be effectively implemented to absorb energy during high power cycles before later releasing the energy back to the system during down cycles. Solid to liquid phase change materials are most common due to their high latent heat capacity, but present implementation challenges as the liquid phase must be appropriately contained in additional heat sink structures.

Encapsulated phase change materials (ePCMs) provide one solution for implementing solid to liquid PCMs within a self-contained delivery mechanism. An exterior shell is encased around the PCM core to create ePCMs. This protects the PCM in the core of spherical particles and allows self-contained solid–liquid phase transitions. Encapsulating the PCM with a durable and high temperature stable shell material improves mechanical and thermal stability of the ePCM and increases the operating temperature range [3,4]. The shell material can also improve thermal conductivity and heat transfer into the PCM for improved cyclic stability of phase transitions [5]. ePCM particles are typically 100 s to 1000 s of nanometers in diameter, which allows them to effectively embedded in other materials for the sake of supplying thermal capacitance. ePCMs have been added to liquid forced convection systems to provide mechanically stable forms of passive cooling within the fluid [6]. They have also been effectively embedded in polymers with flexible applications such as wearable devices [7]. In battery applications, the shell can mitigate fire risks in case of thermal runaway [5]. Encapsulation of the PCM allows thermal energy storage to be applied within composites, combining the favorable properties of ePCMs and host material concurrently.

Silicone gels are effective materials to use as electrical insulation within power modules [8]. The crosslinked structure of the silicone gels helps provide dimensional balance while allowing stress relief in the system [9]. They work well as soft encapsulants to protect devices from contamination that may lead to partial discharges or arcing [10]. The gels are thermally stable at temperatures higher than device operating temperatures, often above 200 °C [11]. Though used to protect against contamination, degradation of silicone gels can occur due to moisture ingress and high temperatures over time [12]. They are soft and flexible enough to avoid introducing large amounts of stress into modules when influenced by thermal gradients or cycling [13]. These gels also have self-healing capabilities allowing the gels to recover from some damages induced by mechanical and electrical stresses [14]. The gels are typically formed using a two-part liquid mixture specifically designed for self-setting around the devices [15]. Various process techniques such as vacuum degassing and elevated temperature curing can be used to remove bubble formation when setting, ensuring continuous electrical insulation across the encapsulation [16]. Many studies focused on improving the electrical insulation properties of silicone gel by changing chemical structures and fabrication processes to improve high voltage capability.

The gels must maintain a high dielectric strength throughout operation to prevent unwanted electrical connections. In gel breakdowns, treeing formations or streamers can form due to localized electric fields generated by the module components and ultimately lead to device failure [17]. Transitioning from silicon-based devices to SiC devices will increase the electric stress in modules because the blocking voltage for SiC devices is higher and the volume of SiC devices is smaller than that of silicon [18]. Commonly, breakdown characterization of the material is determined by applying a voltage across two electrodes within the gel [19]. These electrodes can be of various shapes to mimic different electric features which may be producing the electric field [8,20]. Point–point electrodes have been used to represent the impact of electric field concentrations that can occur at sharp corners in modules [21]. External factors or internal changes in the silicone due to impurities, aging-related degradation, or elevated temperatures can alter the breakdown strength of the material and lead to treeing over time [22,23]. Material additives to silicone gels can be beneficial for electrical properties as barium titanate reduces the influence of module switching frequency on the breakdown voltage of the composite [24]. SiC, graphene, and boron nitride, nanofillers have been used to increase the thermal conductivity and stability of silicone gels while also reducing coefficient of thermal expansion (CTE) [25,26]. Other ceramic fillers like silica can be used to increase the electrical breakdown strength of insulation materials though agglomeration becomes a concern with many ceramic particles [27,28]. Moisture ingress due to humidity aging will lead to reductions in dielectric strength, especially if ionic particles such as salt are present [20,29]. Understanding change in bulk dielectric strength based on material addition is key for introducing particles into dielectric composites.

This study delivers ePCMs used as an additive to increase the passive cooling potential of silicone gels. To understand the potential of silicone gels with ePCMs, the thermal, mechanical, and electrical performances are assessed as a function of ePCM loading.

This section details the fabrication of silicone–EPCM composites for achieving enhanced thermal performance within an electrically insulating and mechanically protective material.

A dielectric silicone gel from Silicone Solutions served as the base for the silicone–EPCM composites. This gel has a reported dielectric strength of 20 kV/mm and 3.1 dielectric constant. The volume resistivity is 4E15 and the material has a dissipation factor of 0.01. It is a 2-part liquid (1:1 ratio) that cures to a translucent green solid in 60 min under ambient conditions. It has a specific gravity of 0.9 and a durometer hardness value of 59 shore 00 as rated by the manufacturer.

In this study, paraffin is the core PCM and titania (⁠$TiO2$⁠) is used as the protective shell to form the ePCM. Titania was chosen because of the proven fabrication process of these ePCMs and for the potential ability of titania fillers to increase the breakdown voltage of silicone gel composites [30]. Paraffin is a common PCM used in many thermal management studies. The nanosized particles of this composite were prepared using the sol–gel method. Complete details of the sol–gel synthesis process can be obtained from prior research of the authors [3]. An SEM image of the ePCM particles is provided in Fig. 1.

The size distribution of the ePCM particles was measured using dynamic light scatterings and the resulting plot is shown in Fig. 2. The particles have a mean size around 308 nanometers.

The combination of dielectric silicone and ePCMs is achieved by mixing cleaned and dried ePCMs into already mixed silicone gel. Two-part silicone gel was mixed per manufacturer instructions. Silicone gel and ePCMs were mechanically stirred and the final composite was cured for 3 h, higher than the manufacturer specified 1-h cure time, before testing to ensure complete solidification across ePCM concentrations. Final samples consisted of 2.5%, 5%, 10%, 20%, 30%, and 40% ePCM by weight.

This study aims to maintain favorable mechanical and electrical properties while enhancing the thermal capabilities of the new ePCM gels. Combining benefits of both materials will improve the system's thermal regulation, leading to enhanced system reliability. This section details material characterizations demonstrating the influence of ePCM concentration on thermal, electrical, and mechanical properties of the dielectric composite.

The impact of ePCM loading on the thermal properties was observed using differential scanning calorimetry (DSC, DSC25—TA Instruments). From these curves, latent heat capacity, and specific heat capacity can be calculated. All samples were hermetically sealed within aluminum pans, preventing material loss during testing.

For determining heat flow curves, samples were temperature ramped from 0 °C to 150 °C at a rate of 3 °C per minute. The samples were equilibrated at 0 °C prior to starting the temperature ramp and again at 150 °C, between temperature increase and temperature reduction data collection, to ensure uniform starting points. Latent heat capacity is found by integrating over the heat flow curves from 25 °C to 65 °C encompassing the typical melting range of the paraffin ePCMs.

As shown in the heat flow curves from Figs. 3 and 4, the ePCMs provide a two stage phase transition between 30 °C and 60 °C. The magnitude of these peaks corresponds to the amount of energy stored by the composites due to their phase transitions. The silicone gel does not provide any additional thermal capability in this temperature range, so all favorable thermal changes are due directly to the ePCM loading percentage in each composite.

To better understand the distribution of properties across the bulk gel composites, solid–liquid and liquid–solid temperatures are provided in Fig. 5 for three randomly selected segments for each of the silicone control, 2.5%, 5%, 10%, 20%, 30%, and 40% ePCM loaded samples. The peak temperatures correspond to the maximum heat flow experienced in each sample, denoting the apex of phase transitions. The peak heat flow temperature for the control ePCMs is approximately 55 °C. Peak solid–liquid temperatures, shown in part A of Fig. 5, decrease as a function of increased ePCM inclusion. The change is more pronounced in low percentages of added ePCMs and levels off as ePCM concentration approaches 100%. At lower loading fractions, peak heat flow values are reduced, and the transition range decreases because not enough material is going through phase transition to form the complete peaks. The liquid–solid temperatures are lower than the solid–liquid temperatures, indicating the presence of slight undercooling in the PCM. Liquid–solid transition temperatures are shown in part B of Fig. 5. These temperatures remain consistent at around 51 °C for 10% and higher ePCM samples. The 2.5% and 5% samples have significant drops in the peak liquid–solid temperatures due to the smaller phase transition becoming more prominent than the main transition as observed in high ePCM samples. The leveling out of the heat flow peaks is shown in Fig. 4.

The latent heat capacity shows how much thermal energy is absorbed through the phase transition and is a good metric for the thermal capacity of the composite and its potential as a passive cooling component. This property can be determined as the area under the curve across the phase transition regions, between 25 °C and 65 °C, from Figs. 3 and 4. The latent heat capacity for the solid–liquid curves is given in Fig. 6. Across each loading fraction, 3 samples were tested (5 for the ePCM control) to demonstrate the variation of thermal properties within each of conditions. The latent heat value should have a strong correlation to the weight % of ePCMs added into the silicone gel. Since paraffin is the only contributing material to the phase transition in these composites, the mass of paraffin (core PCM of the ePCM particles) will correspond to the thermal energy storage capacity. The latent heat capacity of paraffin was found to be 193.5 J/g for this study as a comparison point for the ePCM particles that have an average latent heat capacity of 132.9 J/g for reference. As expected, the silicone control sample does not display thermal energy storage related to a phase transition in the temperature range observed. The ePCM loaded samples of 2.5%, 5%, 10%, 20%, 30%, and 40% ePCM samples have latent heat capacities corresponding to their respective percent of ePCM loaded as compared to the control ePCM sample within an acceptable margin of variation. The small differences can be attributed to variance in the size of the ePCM particles and their dispersion within the gel. The size distribution is provided in Fig. 2. ePCM control has the widest variation among the samples. This can be attributed to the fabrication process of the ePCMs that create a range of particle. Smaller particles will have a larger shell to PCM ratio and lower latent heat capacity on weight % basis. ePCM loading fraction can be used as a solid indicator of the energy storage ability of each composite.

Thermally, particle collections can be beneficial as they demonstrate a high latent heat potential and will be very useful as passive cooling mitigation. Small PCM dense pockets spread throughout the sample combined with a more even distribution of the nonagglomerated particles will provide adequate thermal benefit for hotspot mitigation and passive cooling close to the energy generation points in modules. Further process refinements will lead to more uniform particle creation and dispersion, allowing for improved material consistency.

Specific heat capacity specifies the necessary energy input to raise the temperature of a material. For this study, specific heat capacity is found using a modulated temperature ramp of sinusoidal nature. Samples are equilibrated at 0 °C and ramped using modulation, with a 1 °C temperature amplitude and 120 s modulation period, to 150 °C at a 2 °C per minute ramp rate. Specific heat plots are shown in Fig. 7 as a function of temperature. They are determined by dividing heat flow into the sample by the heating rate and then normalizing by sample mass.

Comparison values are taken at 30 °C, 75 °C, 125 °C to show variations across the applicable operating temperature range. These values are reported in Fig. 8. The specific heat capacity values tend to increase linearly with increasing PCM concentration. This trend holds across the temperature range as shown. Variability of measurements from each sample fit within the trend, though the ePCM control for 75 °C is lower than anticipated for the rising trend.

In relation to the overall energy storage of the encapsulant material, ePCM addition provides two benefits over the control silicone. The first benefit is the ability to absorb more energy per unit mass and the second is to absorb more energy without increasing the material's temperature. The latent heat capacity of the ePCM for the 20% loaded sample of 17.707 J/g equates to a temperature change of an extra 11.5 °C in the control silicone. Using the specific heat capacity of silicone gel, 1.545 J/g-° C, the 20% EPCM sample would be 11.5 °C cooler than the control silicone if enough energy is generated to utilize the PCM melt. 11.5 °C is a large difference in temperature that can be achieved in the encapsulant leading to smaller thermal stresses and increased reliability of electrical system with respect to thermal cycling with only a 20% loading of ePCM.

The mechanical properties of encapsulants are also of considerable importance to the protection and reliability of a module. Silicone gels are soft and flexible to prevent introducing additional stresses into the module via thermal cycling. Though the additional thermal benefits of the gels studied here will reduce the magnitude of thermal gradients experienced, it is still crucial that introduction of ePCMs does not drastically change the mechanical properties in a stress increasing manner.

Coefficient of thermal expansion, shown in Fig. 9, has been found using a thermomechanical analyzer (TMA, TA Instruments Q400). Samples were cut into 3.00–3.15 mm tall squares and loaded on the TMA stage to record the linear change in dimension as samples underwent a temperature ramp of 5 °C per minute from 40 °C to 150 °C. The resulting CTE values were determined by dividing the linear dimension change by the coinciding temperature change and the initial sample length.

Coefficient of thermal expansion increases as a function of higher ePCM concentrations in the gel. The CTE increased by approximately 55% from the control silicone sample to the 5% and 10% ePCM weighted samples. The samples with 20–40% ePCM loading saw between 90% and 100% increase in the CTE value. CTE has an important effect on the stresses induced within the module. Increased CTE values will lead to larger expansion of the gel during use and will increase problematic stress generation.

Another component in thermal stress is the elastic modulus of the material. Higher stiffness values in encapsulants lead to larger stress gradients applied across the packaging structure because stiff encapsulant materials do not provide compliance in expansion or contraction cycles of the module. Low stiffness encapsulants induce less stress on the packaging when undergoing thermal expansion or contraction. Ideal encapsulant material will have a low elastic modulus to prevent introducing damaging thermal stress. Fillers often cause an increase in elastic modulus due to additional particle to network and particle to particle interactions [31]. The elastic modulus is experimentally found in compression for the gel samples using a Q400 thermomechanical analyzer, TMA, from TA Instruments. Samples are subjected to linear force ramp of 0.050 N per minute from 0.020 to 1.000 N. The tests are run at 25, 45, 65, and 100 °C to observe stiffness changes with respect to temperature. To maintain consistency, each sample is allowed to equilibrate at temperature for 10 min before applying the force ramp. For each temperature and ePCM loading condition, three square samples with heights between 3.00 and 3.15 mm were tested.

The elastic modulus results are provided in Fig. 10 as a function of the ePCM loading fraction and separated by testing temperature. All the data, excluding the 20% ePCM loading and 25 °C condition, fall within a 4–10 MPa range for the bulk elastic modulus as measured in compression. The data range holds across the various temperatures showing the composites are not significantly impacted by the bulk composite's temperature. This is important to note that the 45 °C and 65 °C data sets represent elastic moduli in solid PCM phase and liquid PCM phase, respectively. The minimal changes between these two temperature dependent sets indicate the PCM phase does not correlate to the elastic modulus and mechanical behavior of the encapsulant material. This is beneficial because it removes thermomechanical concerns across temperatures and simplifies the process of implementation within modules, where temperature swings and gradients will exist. The 100 °C dataset correlates well to the 45 °C and 65 °C data sets matching both the trends across ePCM loading fraction and the variance in each condition's three samples to demonstrate acceptability at higher temperature applications. The consistency across temperatures is likely due to gained material stability from the titania shell, preventing the paraffin core from causing significant mechanical changes across different phases. The dataset for 25 °C differs at the 20% loading condition where it experiences an increase of the elastic modulus to approximately 15 MPa for some tests.

Encapsulated phase change material loading fractions in many of the testing conditions lead to a decrease in elastic modulus, theoretically improving the thermomechanical response of the composites. The elastic modulus in the 20% ePCM samples is on par with the modulus of the silicone control samples for the elevated temperatures. In the 2.5%, 5%, 10%, 30%, and 40% ePCM samples the elastic modulus is lower than that of the silicone control. The results from this dataset indicate the ePCMs do not have a negative performance impact on a key factor in thermal stress creation, meaning ePCMs do not significantly change composite stiffness.

To provide adequate voltage isolation as an encapsulant the new composites must have sufficient electrical insulation to prevent electrical connection between various components through the gels. The dielectric strength of material measures its ability to resist electrical breakdown within strong electric fields generated by large voltage biases within the module. Breakdown voltage tests are used to characterize this ability. Figure 11 shows the testing setup for needle–needle breakdown voltage tests adapted from fluid breakdown studies by Iradukunda et al. [21].

To characterize the breakdown voltage, point–point electrodes were used to apply the voltage and create the electric field within the material. These needles were inserted into the gel from the exterior of the gray sample enclosure in Fig. 11 using micrometer dials to set a 1 mm gap between the needle tips in the center of the gel composites. Voltage was applied across the needles and steadily increased at 100 V increments from 0 V to breakdown of the material or the 28 kV limit for this testing apparatus was reached. The current flowing between the two needles was monitored and breakdown of the materials was assessed when the current exceeded 1 mA across the material separating the electrodes. Moisture ingress and long-term sample storage are expected to impact the dielectric strength of the composite. This may be more profound in high ePCM samples if the ePCMs are more susceptible to moisture ingress. The samples for this study are prepared directly prior to testing to avoid this possibility and further testing in the future will be needed to address concerns with material degradations.

The results from the breakdown testing are given in Fig. 12 where 5 tests were run for each sample. Each recorded point was tested at a different location in the sample. It is expected that the control silicone would have the highest breakdown voltage because it does not have any additional particles that may provide electrical pathways to lower the dielectric strength. Dielectric breakdown strength decreased as a function of increasing ePCM fraction, though there is a large spread across each sample. Particle placement in relation to the needle electrodes dictates the likelihood of significant reductions in breakdown strength as the particles provide electrical pathways for breakdown phenomena. Highly concentrated samples have a higher likelihood of containing enough particles in between the needles to form pathways for early electrical breakdown of the composite, but all samples retain a level of insulation strength that can be used for various module designs.

Needle–needle breakdown testing is often considered to produce higher electric stress in the surrounding material, mimicking the high field stresses apparent in modules around sharp features [32]. Breakdown voltage is highly dependent on electrode configuration and electrode spacing [33]. The alignment of the electric field lines will dictate the voltage level required to breakdown a material across different configurations. The gap spacing between electrodes will determine where the electric field lines intercept the high voltage gap [33]. If the lines are aligned close to the electrode, lower voltage will be needed to initiate breakdown [33]. To observe dielectric breakdown under different geometric conditions needle-plane breakdown testing is also conducted under room temperature. This testing configuration provides a larger area spread across one side of the apparatus for breakdown events to find a pathway for formation though reducing the strength of the generated electric field surrounding the electrodes.

The experimental setup for the needle-plane breakdown tests is shown in Fig. 13. A needle electrode and plane electrode are positioned opposite each other. The needle electrode is connected to a fixture with displacement control in the X, Y, and Z directions using micrometer actuators. A lead is soldered to the copper plane. The sample is poured over the plane and cured to create a complete insulation layer. The needle is inserted into the sample material at a height of 0.10 and 0.25 mm in a new position within the sample for each measurement. Voltage bias is applied to the needle and the plane starting at 0 V and ramping until failure or 28 kV at 100 V increments. Each condition was tested 5 times to capture sample variance.

The results of the needle-plane breakdown testing are shown in Fig. 14. Higher ePCM concentrated samples show a reduction in breakdown voltage for both distances as expected. A larger drop-off in dielectric strength occurs after 5% that starts to level off after 20% for the higher concentrations. The initial drop likely occurs as the sample reaches a particle spacing threshold that allows for more favorable breakdown paths across particles to form. After 20%, enough of these particle pathways have formed. Adding more of these pathways through even higher concentrations of ePCMs provides diminishing impact of the reduction of dielectric strength. These data demonstrate the ability to retain dielectric capabilities at high particle concentrations for the plane-needle electrode configuration.

Silicone gels imbedded with ePCM particles demonstrate promise as a dielectric composite for combining electrical insulation and passive cooling capability. Thermal energy storage can be achieved as a function of ePCM weight percentage. Ideally, ePCM concentration is maximized to receive the most thermal storage. The limiting factor in ePCM addition is the dielectric strength. Adding ePCMs lowers the dielectric strength of the material but more work is needed to determine direct impact of ePCMs on electrical breakdown. This study demonstrates the 20% ePCM samples to be the best balance of added energy storage and electrical insulation. It adds ∼20 J/g of latent heat capacity while maintaining over 17 kV/mm of dielectric strength. The mechanical properties remain within a favorable range for the material's application. Further testing will outline the capabilities of these new composites and work toward determining optimal composite material combinations.

• The National Science Foundation Engineering Research Center for Power Optimization of Electro-Thermal Systems under Cooperative Agreement (No. EEC-1449548; Funder ID: 10.13039/100000001).

The datasets generated and supporting the findings of this article are obtainable from the corresponding author upon reasonable request.

BDV =

breakdown voltage

CTE =

coefficient of thermal expansion

DLS =

dynamic light scattering

DSC =

differential scanning calorimetry

PCM =

phase change material

ePCM =

encapsulated phase change material

SEM =

scanning electron microscopy

SiC =

silicon carbide

$TiO2$ =

titania (shell material)

TMA =

thermomechanical analysis

TMA =

thermomechanical analyzer