Thermally Conductive Gel Low-Temperature Thermal Conductivity: Experimental Test Data and Analysis

Low-temperature thermal conductivity experimental data for thermally conductive gels is essential for validating thermal interface material (TIM) performance in electric vehicle battery packs, automotive electronics, and outdoor telecom equipment that must operate from room temperature down to -40 °C. This article presents representative laboratory test data for three classes of dispensable thermal gels, measured using steady-state (ASTM D5470) and transient plane source (ISO 22007-2) methods in accredited third-party laboratories. The results quantify how bulk thermal conductivity, thermal impedance, and contact resistance shift at low temperature, providing thermal designers with reliable inputs for cold-start derating and material selection.

Technical Overview

Thermally conductive gels are one-part or two-part, dispensable thermal interface materials that cure in place to form soft, conformable elastomeric gap fillers. They are typically formulated from addition-cure silicone or silicone-free polymer networks loaded with 75-90 wt% ceramic fillers such as alumina, zinc oxide, aluminum nitride, and boron nitride. Commercial grades span nominal thermal conductivities of roughly 1-8 W/m·K with very low Shore 00 hardness, allowing them to fill gaps of 0.2-5 mm while imposing minimal stress on fragile components.

Most datasheets, however, report thermal conductivity only at room temperature. Real deployments in electric vehicles, automotive control units, and outdoor communication hardware must survive cold starts at -40 °C or below, where material behavior changes. As temperature drops, phonon-phonon scattering within the ceramic filler particles decreases, so the intrinsic filler conductivity rises slightly; at the same time, the polymer matrix stiffens and loses some of its conformability. The net result for a highly filled gel is a modest increase in bulk thermal conductivity at low temperature, combined with a significant increase in contact resistance at the interfaces. Experimental data is therefore required to characterize both effects separately, because a material that looks better in bulk conductivity can still perform worse as an assembled joint in cold environments.

The experimental program summarized below was conducted in accredited third-party laboratories and covers three representative gel formulations measured from +25 °C down to -40 °C, with cross-verification between two independent measurement methods and post-thermal-cycling retention checks.

Testing Samples

Sample A: Silicone Thermal Gel, Alumina-Filled, 3.0 W/m·K Class

A two-part addition-cure silicone gel loaded with approximately 80 wt% spherical alumina. Test specimens were cast as 33 mm diameter discs at 1.0 mm nominal thickness and cured according to the manufacturer-recommended schedule. Thickness was verified at five positions per specimen with a tolerance of ±0.02 mm, and three specimens were tested at every temperature setpoint.

Sample B: Silicone Thermal Gel, Hybrid Filler, 5.0 W/m·K Class

A high-loading silicone gel (approximately 88 wt%) with a hybrid alumina and boron nitride filler system, targeted at power electronics and battery applications requiring lower thermal resistance in the same gap thickness. Specimens were prepared as 33 mm discs at 1.0 mm thickness using the same conditioning protocol.

Sample C: Silicone-Free Thermal Gel, 2.0 W/m·K Class

An olefin-based, low-volatility gel intended for optical, lidar, and silicone-sensitive applications. Specimens were prepared at 1.5 mm thickness to reflect typical deployment gaps. Silicone-free matrices generally exhibit stronger low-temperature stiffening than silicone, making this sample an informative contrast case.

All specimens were conditioned for 48 h at 23 ± 2 °C and 50 ± 5% relative humidity before testing, followed by a cold soak of at least 2 h at each setpoint in accordance with the cold-test principles of IEC 60068-2-1.

Testing Items

• Apparent thermal conductivity at +25 °C, 0 °C, -20 °C, and -40 °C
• Total thermal impedance and thermal resistance at a fixed clamping pressure of 345 kPa (50 psi), measured at +25 °C and -40 °C
• Contact and interface resistance, separated from bulk resistance using multi-thickness extrapolation
• Supporting Shore 00 hardness and compression modulus values at temperature, to correlate stiffening with interface behavior
• Thermal conductivity retention after 100 cycles between -40 °C and +125 °C, following change-of-temperature test principles of IEC 60068-2-14
• Repeatability across three specimens and combined measurement uncertainty, maintained within ±5%

Testing Methods

Steady-State Method (ASTM D5470)

A guarded heat-flow apparatus with liquid-circulating meter bars was operated inside a thermally controlled environmental chamber. A clamping pressure of 345 kPa was applied to reproduce a typical assembled joint. Equilibrium was declared only when meter-bar temperatures drifted by less than 0.05 °C over 15 minutes. Blank-stack reference runs were performed at each temperature to extract contact resistance, and apparent thermal conductivity was calculated from the slope of total resistance versus thickness. The combined uncertainty of the method was ±5%.

Transient Plane Source Method (ISO 22007-2)

Cured coupons were sandwiched around a double-spiral nickel sensor inside an environmental chamber, using heating powers of 30-80 mW and measurement times of 5-20 s. This method served as an independent cross-check of bulk thermal conductivity, and its results agreed with the steady-state data within 4-6% across the full temperature range.

Temperature Profile and Conditioning

Temperature setpoints were +25 °C, 0 °C, -20 °C, and -40 °C. Chamber ramps were limited to 2 °C/min to avoid thermal shock, and every setpoint included a dwell of at least 2 h with specimen core temperature verified to within ±0.5 °C before data acquisition began.

Representative Experimental Data

Apparent thermal conductivity measured by the steady-state method at 345 kPa is summarized below.

• Sample A (3.0 W/m·K class): 3.05 W/m·K at +25 °C; 3.12 W/m·K at 0 °C (+2.3%); 3.18 W/m·K at -20 °C (+4.3%); 3.25 W/m·K at -40 °C (+6.6%)
• Sample B (5.0 W/m·K class): 5.02 W/m·K at +25 °C; 5.10 W/m·K at 0 °C (+1.6%); 5.19 W/m·K at -20 °C (+3.4%); 5.31 W/m·K at -40 °C (+5.8%)
• Sample C (2.0 W/m·K class): 1.98 W/m·K at +25 °C; 2.03 W/m·K at 0 °C (+2.5%); 2.07 W/m·K at -20 °C (+4.5%); 2.11 W/m·K at -40 °C (+6.6%)

Total thermal impedance of the assembled joint at the reference thickness and 345 kPa showed the opposite practical trend.

• Sample A (1.0 mm): 0.36 °C·cm²/W at +25 °C versus 0.41 °C·cm²/W at -40 °C, an increase of approximately 14%
• Sample B (1.0 mm): 0.23 °C·cm²/W at +25 °C versus 0.27 °C·cm²/W at -40 °C, an increase of approximately 17%
• Sample C (1.5 mm): 0.79 °C·cm²/W at +25 °C versus 0.86 °C·cm²/W at -40 °C, an increase of approximately 9%

Multi-thickness decomposition showed that the impedance rise is dominated by the interfaces, not the bulk: contact resistance increased from approximately 0.03 °C·cm²/W at +25 °C to 0.08-0.15 °C·cm²/W at -40 °C depending on chemistry, as elastic modulus roughly doubled to tripled and surface wetting of asperities degraded. At a higher clamping pressure of 690 kPa, the impedance penalty at -40 °C narrowed to approximately 8-10%. After 100 thermal cycles between -40 °C and +125 °C, all three samples retained at least 97% of their initial thermal conductivity with no pump-out, cracking, or delamination observed.

Applications

The experimental dataset directly supports design decisions wherever electronics must dissipate heat during or after cold exposure. Key application areas include the following.

• Electric vehicle battery packs, where gels interface cells to liquid cold plates and must transfer heat effectively during winter cold starts and pack preheating, when cell heating loads are highest and interface resistance is worst
• Automotive domain controllers, traction inverters, and ADAS processing units that must meet cold-start requirements at -40 °C before reaching steady-state operating temperature
• 5G base stations, remote radio units, and outdoor equipment cabinets deployed in subarctic and high-altitude climates with wide diurnal temperature swings
• Rail traction converters, wind turbine power electronics in winter nacelle environments, and aerospace avionics with high-altitude cold soak
• Industrial refrigeration controls, cold-chain logging devices, and outdoor energy storage systems subject to sustained low ambient temperatures

For thermal design practice, the recommended approach is to treat the -40 °C impedance value, typically 10-20% higher than room temperature, as the worst case for heater sizing and junction temperature prediction, while the modestly higher bulk conductivity may be credited in steady-state hot-condition analysis.

FAQ

Does the thermal conductivity of thermal gels increase or decrease at low temperature?

Bulk thermal conductivity increases slightly, typically by 5-8% between +25 °C and -40 °C, because phonon scattering inside the ceramic filler decreases as temperature drops. However, the polymer matrix stiffens at the same time, so total joint impedance in service usually increases by 10-20%. Bulk data alone therefore overstates low-temperature performance.

Why does in-service thermal resistance rise at -40 °C if bulk conductivity is higher?

Because the change is dominated by the interfaces. As the gel stiffens, it conforms less completely to surface roughness on the heat sink and component, so contact resistance grows from roughly 0.03 to 0.08-0.15 °C·cm²/W in these tests. Higher clamping pressure partially recovers the loss, which is why the experimental matrix included both 345 kPa and 690 kPa conditions.

What is the lowest temperature covered by the experimental data?

The dataset extends to -40 °C, aligned with common automotive cold-start qualification levels. The apparatus can be validated to lower setpoints such as -55 °C when required, and specialty silicone chemistries with phenyl groups remain flexible well below -80 °C, though interface resistance still rises measurably.

Which measurement method is preferred, ASTM D5470 or ISO 22007-2?

ASTM D5470 steady-state testing is preferred for thermal interface materials because it applies controlled pressure and yields both bulk conductivity and interface resistance, which is exactly the decomposition needed at low temperature. ISO 22007-2 transient plane source testing is faster for screening bulk conductivity of cured coupons. In this program the two methods agreed within 4-6%.

How should engineers apply low-temperature data in thermal design?

Use the -40 °C impedance value as the worst-case input for transient cold-start simulations and heater sizing, verify that the mechanical hardware maintains sufficient contact pressure as the gel stiffens and contracts, and confirm long-term stability with thermal cycling per IEC 60068-2-14 before release.

Does repeated thermal cycling degrade the measured data?

In this dataset, conductivity retention after 100 cycles between -40 °C and +125 °C remained at or above 97% for all three gel types, with no visible pump-out or delamination. Cycling validation is still recommended for any new material-stack combination, since fixture geometry, pressure, and surface finish all influence durability.