Thermal Diffusivity Testing of Thermal Conductive Gel for Photovoltaic Modules

Thermal diffusivity testing of thermal conductive gels for photovoltaic modules measures how efficiently these soft thermal interface materials spread heat away from solar cells, junction boxes, and other heat-sensitive components. Accurate diffusivity data—combined with density and specific heat—allows engineers to derive thermal conductivity, validate supplier datasheets, and feed reliable inputs into module thermal models. This article explains the applicable standards, specimen preparation, measurement methods, and typical applications of thermal diffusivity testing for photovoltaic thermal conductive gels.

Technical Overview

Thermal conductive gels are low-modulus elastomers—most commonly silicone-based—loaded with thermally conductive ceramic fillers such as alumina, aluminum nitride, or boron nitride. In photovoltaic (PV) modules they serve as thermal interface materials (TIMs) placed between cells and backsheets, around junction boxes, beneath bypass diodes, and in frame-level heat-dissipation structures. Their soft, conformable nature eliminates air gaps at interfaces, yet their heat-spreading capability must be quantified before design decisions can be made.

Thermal diffusivity (α) describes the speed at which a temperature disturbance propagates through a material. It is linked to thermal conductivity (k) through the relationship α = k / (ρ × Cp), where ρ is density and Cp is specific heat capacity. For crystalline silicon modules, every degree of cell temperature rise typically reduces power output by roughly 0.3–0.45 percent and accelerates long-term degradation mechanisms such as encapsulant discoloration, solder fatigue, and potential-induced degradation. Reliable diffusivity data therefore supports thermal modeling, material selection, and qualification work performed within the frameworks of IEC 61215 and IEC 61730.

Because gels are soft, compressible, and often translucent, their diffusivity measurement demands dedicated specimen preparation and carefully selected instrument parameters. Accredited third-party laboratories typically perform the measurement according to ISO 22007-4 and ASTM E1461 using the laser flash method, or according to ISO 22007-2 using the transient plane source method, with supporting calorimetry and density measurements to derive thermal conductivity.

Testing Samples

Fully Cured Bulk Gel Discs

The standard specimen for laser flash analysis is a flat disc, typically 10–12.7 mm in diameter and 1–3 mm thick, with parallel faces within ±2 percent of the average thickness. Thickness is measured at a minimum of five positions using a calibrated micrometer. Both faces are coated with a thin graphite layer to ensure uniform absorption of the flash energy and efficient infrared emission from the rear face. A minimum of three specimens per material condition is recommended to demonstrate repeatability.

Cast Two-Component Gel Specimens

Two-component silicone gels are mixed at the supplier-specified ratio, vacuum-degassed to remove entrained air, cast into polytetrafluoroethylene or metal molds, and cured according to a defined schedule (for example, room-temperature cure followed by an elevated-temperature post-cure). Completeness of cure is verified by differential scanning calorimetry, confirming the absence of residual exotherm, or by a hardness plateau check. Incomplete cure, voids, or filler sedimentation are the most common causes of scattered results.

Thin Gel Films on Substrates

For gels applied as printable or dispensable coatings thinner than 1 mm, specimens may be prepared as free-standing films or on a carrier substrate. Such samples are generally better suited to the transient plane source method or modified laser flash configurations with appropriate heat-loss models, since extremely thin, compliant layers are difficult to handle as self-supporting laser flash discs.

All specimen types are conditioned at 23 ± 2 °C and 50 ± 10 percent relative humidity for at least 24 hours in accordance with ISO 291 prior to measurement.

Testing Items

The core scope of a thermal diffusivity test program for photovoltaic thermal gels typically includes the following items:

• Thermal diffusivity in mm²/s at specified temperatures, commonly 25, 50, 75, 100, 125, and 150 °C
• Thermal conductivity in W/(m·K), calculated from measured diffusivity, density, and specific heat capacity
• Specific heat capacity in J/(g·K) determined by differential scanning calorimetry
• Density in g/cm³ determined according to ISO 1183 or ASTM D792
• Temperature-dependent diffusivity curves with regression fitting for simulation input
• Optional comparative thermal impedance measurement according to ASTM D5470 for correlation with interface performance

Testing Methods

Laser Flash Method (LFA)

The laser flash method, standardized in ISO 22007-4, ASTM E1461, and GB/T 22588, is the primary technique for thermal diffusivity of cured gel discs. A short high-energy pulse irradiates the front face of the specimen, and an infrared detector records the temperature rise on the rear face. Diffusivity is calculated from the characteristic half-rise time and specimen thickness, with Cowan or curve-fitting corrections applied for heat losses. The method covers room temperature to 200 °C or higher with typical measurement uncertainty of approximately ±3 percent.

Transient Plane Source Method (TPS)

The transient plane source method described in ISO 22007-2 uses a spiral nickel sensor clamped between two pieces of the gel. The sensor acts simultaneously as heat source and resistance thermometer, and the recorded temperature transient yields both thermal conductivity and thermal diffusivity directly. Because the gel complies around the sensor under low clamping force, contact resistance is minimized, making the technique particularly attractive for soft, highly compliant materials and thin films.

Steady-State Verification

For thermal interface gels, steady-state thermal impedance measurement according to ASTM D5470 provides complementary data on how the material performs under realistic contact pressures. While this method does not produce diffusivity values, correlating bulk diffusivity with interface impedance helps distinguish intrinsic material behavior from contact effects in actual module assemblies.

Supporting Measurements

Specific heat capacity is measured by differential scanning calorimetry using the sapphire method in accordance with ISO 11357-4 or ASTM E1269. Density is determined by the hydrostatic or gas pycnometry approach per ISO 1183 or ASTM D792. Dimensional metrology and balance measurements are traceable to national standards, and every derived thermal conductivity value reports the combined uncertainty of all input quantities.

Applications

Thermal diffusivity test results for photovoltaic thermal gels are used throughout the module value chain:

• Comparison and selection of gel grades for cell encapsulation, backsheet bonding, and heat-spreading layers
• Finite element thermal modeling of modules to reduce nominal module operating temperature in line with IEC 61215-2 test sequences
• Junction box and bypass diode thermal management to prevent overheating under shading and reverse-bias conditions
• Photovoltaic-thermal (PV/T) hybrid collectors and building-integrated photovoltaics where heat extraction is a design requirement
• Incoming quality control and batch-to-batch consistency verification of gel suppliers
• Research and development of highly filled formulations, including retention of thermal properties after damp heat exposure at 85 °C and 85 percent relative humidity
• Adjacent applications such as electric vehicle battery packs, power electronics, and LED assemblies using the same gel chemistries

FAQ

What is the difference between thermal diffusivity and thermal conductivity?

Thermal conductivity measures how much heat a material conducts under a steady temperature gradient, while thermal diffusivity measures how quickly a temperature change propagates through the material. The two are related by α = k / (ρ × Cp). Diffusivity governs transient thermal response—important during irradiance fluctuations—whereas conductivity governs steady-state heat dissipation.

Which method is more suitable for gels: laser flash or transient plane source?

Both are accepted. Laser flash per ISO 22007-4 and ASTM E1461 offers high accuracy and wide temperature coverage for cured disc specimens. Transient plane source per ISO 22007-2 is often preferred for very soft gels and thin films because the material conforms naturally to the sensor. Many programs use both for cross-validation.

Can uncured gel be tested directly?

No. Thermal diffusivity is defined for a stable solid or gel body. Two-component gels must be fully mixed, degassed, cast, and cured before testing, and cure completeness should be verified to avoid drifting results.

What are typical values for photovoltaic thermal gels?

Unfilled silicone gels exhibit thermal conductivity around 0.2 W/(m·K) with diffusivity near 0.1 mm²/s. Highly filled thermal gels used in photovoltaic and power electronics applications commonly reach 1–6 W/(m·K), corresponding to diffusivity of roughly 0.5–2.5 mm²/s depending on filler loading, density, and heat capacity.

At which temperatures should diffusivity be measured?

Since gel properties vary with temperature, measurement at several points from room temperature to 150 °C captures the behavior across the actual operating range of a photovoltaic module, and the resulting curve can be fitted for thermal simulation input.

What accuracy and turnaround can be expected?

Diffusivity by laser flash is typically reported with uncertainty of about ±3 percent, and derived thermal conductivity with combined uncertainty of about ±5–10 percent. Standard programs including specimen casting, conditioning, measurement at multiple temperatures, and data reduction are usually completed within five to ten working days, depending on cure schedule and specimen count.