Thermal Conductivity Determination of Thermally Conductive Gel for Photovoltaic Modules

Thermal conductivity determination of thermally conductive gel is a critical quality-control and qualification test for photovoltaic (PV) modules, where these compliant silicone-based interface materials transfer heat from cells, bypass diodes, and power electronics to heat-spreading structures. Accurate measurement requires standardized methods such as ASTM D5470, ISO 22007-2, and ISO 22007-4, performed under controlled pressure and temperature conditions. This article explains the testing principle, sample requirements, applicable methods, typical result ranges, and practical considerations for laboratories and PV manufacturers.

Technical Overview

Thermally conductive gel is a low-modulus, silicone- or polyurethane-based thermal interface material (TIM) heavily loaded with ceramic fillers such as alumina, aluminum nitride, or zinc oxide. It is dispensed as a one-part or two-part paste and cures in place into a soft, tacky, non-flowing gel that conforms to irregular surfaces, fills gaps of varying thickness, and maintains intimate thermal contact over the 25-year-plus service life of a photovoltaic module.

Within PV modules, thermal gel is applied between bypass diodes and junction-box housings, between cells and heat-spreading backrails or substrates, and in module-level power electronics. Because crystalline silicon power output degrades at approximately −0.30% to −0.40% per °C of cell temperature rise, effective heat extraction directly preserves energy yield and long-term reliability. Thermal conductivity (λ, expressed in W/(m·K)) is therefore the primary acceptance metric for these materials, and its accurate determination supports supplier qualification, incoming lot inspection, and thermal simulation of module designs.

Testing Samples

Cured Gel Plaques and Sheets

Fully cured, planar plaques or sheets are the preferred specimen form. Typical requirements include a minimum size of 100 mm × 100 mm with thickness between 1 mm and 3 mm, allowing coupons to be cut to the dimensions required by each instrument. Thickness uniformity within ±5% across the measurement area is essential, since thickness error propagates directly into the conductivity result.

Two-Component and Dispensed Gels

Two-part gels are mixed at the manufacturer-specified ratio, vacuum-degassed to remove entrapped air, molded into plaques, and cured according to the recommended schedule (for example, 24 hours at 25 °C or 1 hour at 80 °C). Specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity prior to testing, in accordance with ISO 291 or GB/T 2918. Degree of cure should be verified, typically by differential scanning calorimetry, before measurement.

Finished Module Coupons and Assemblies

For assembly-level verification, coupons containing gel sandwiched between actual substrates (for example, diode-to-housing or cell-to-backsheet interfaces) may be submitted. In this case the measured quantity is the total thermal impedance of the stack, from which the apparent conductivity of the gel layer can be derived when geometry is known.

Recommended submission quantity is 200–500 g of uncured material or at least three cured coupons per test condition. Shelf life and pot life must be respected, and both should be declared on the submission sheet.

Testing Items

• Thermal conductivity (λ) at 25 °C, in W/(m·K)
• Thermal diffusivity (α), in mm²/s
• Specific heat capacity (cp), in J/(g·K)
• Density (ρ), in g/cm³, for cross-validation using λ = α × ρ × cp
• Apparent thermal conductivity as a function of clamping pressure (ASTM D5470)
• Thermal resistance and thermal impedance versus thickness, including contact resistance extraction
• Thermal conductivity temperature sweep, typically from −40 °C to 150 °C
• Supporting characterization where required: Shore 00 hardness (ASTM D2240), volume resistivity, and post-aging conductivity after damp-heat exposure per IEC 61215 (MQT 13)

Testing Methods

Steady-State Heat Flow Method (ASTM D5470)

This is the reference method for thermal interface materials. A cured disc or square coupon, commonly 25.4 mm or 50.8 mm across and 0.5–3 mm thick, is clamped between metered bars under controlled pressure, typically 0.17–3.45 MPa. Heat flux and the temperature drop across the specimen are measured at steady state. By testing at several thicknesses and extrapolating total resistance to zero thickness, contact resistance is separated from bulk resistance, yielding true bulk conductivity. The method is well suited to gels in the 0.1–10 W/(m·K) range with a typical accuracy of ±3% to ±7%, and it aligns with most supplier datasheets.

Transient Plane Source Method (ISO 22007-2)

A hot-disk sensor is sandwiched between two pieces of cured gel, each at least as large as the sensor diameter and thick enough to satisfy the sensor's volumetric requirements. The method covers 0.005–500 W/(m·K), completes in seconds to minutes per point, and is ideal for temperature sweeps. Because gels are very soft, sensor embedding must be checked, and results should be compared against ASTM D5470 during method validation.

Laser Flash Method (ISO 22007-4 / ASTM E1461)

A cured disc, typically 12.7 mm in diameter and 1–3 mm thick, is irradiated on one face and the temperature response of the opposite face is recorded to obtain thermal diffusivity. Specific heat is measured by DSC (ISO 11357-4) and density by ISO 1183-1, and conductivity is calculated as λ = α × ρ × cp. Translucent gels must be surface-coated to ensure proper energy absorption. This method excels at wide temperature-range mapping and post-aging studies.

Guarded Hot Plate and Heat Flow Meter (ASTM C177, ASTM C518, ISO 8302, GB/T 10294/10295)

These steady-state slab methods serve as complementary or verification techniques for thicker cured pads and lower-conductivity gel formulations, and they provide traceable cross-checks for laboratory quality systems.

Regardless of method, testing should be performed by accredited third-party laboratories operating under ISO/IEC 17025, using calibrated reference materials (such as stainless steel, Pyroceram, or PMMA), with a minimum of three replicates and a stated expanded measurement uncertainty, typically within ±5% to ±10% (k = 2).

Applications

• Junction-box thermal management: transferring bypass-diode heat into the potting compound and housing to suppress diode temperature during shading events
• High-power module designs (182 mm and 210 mm cells, half-cut, and shingled layouts): gap filling between cells, backsheets, and aluminum backrails to reduce operating temperature
• Building-integrated and vehicle-integrated photovoltaics, where restricted rear-side ventilation demands higher-performing interface materials
• Module-level power electronics, including microinverters and optimizers, where gels couple heat from power devices to housings
• Reliability qualification: re-measuring conductivity after damp-heat (85 °C / 85% RH) or thermal-cycling exposure to quantify aging drift in support of IEC 61215 and IEC 61730 sequence assessments
• Thermal modeling input: validated λ values for finite-element and computational fluid dynamics simulations of module temperature fields

FAQ

What thermal conductivity values are typical for PV thermal gels?

Commercial photovoltaic thermal gels generally fall between 1 W/(m·K) and 6 W/(m·K), with premium formulations reaching 4–6 W/(m·K). Measured values should be compared against the supplier datasheet under the same method, pressure, and temperature.

Which method best matches supplier datasheet values?

ASTM D5470 at a stated clamping pressure and 25 °C is the most widely cited basis for TIM datasheets and provides the closest apples-to-apples comparison. ISO 22007-2 is excellent for rapid screening and temperature sweeps, while laser flash is preferred for extended temperature mapping.

Can uncured gel be tested directly?

No. Gel must be fully cured and conditioned before measurement, since incomplete cure changes filler network structure and yields falsely low or unstable conductivity. Degree of cure should be verified by DSC before testing.

How much sample is required and what is the typical turnaround?

A submission of 200–500 g of uncured material, or cured plaques of at least 100 mm × 100 mm, is sufficient for full characterization. Routine testing at ambient temperature typically completes within 5–10 working days; temperature sweeps and post-aging studies require additional time for conditioning and exposure.

Why does clamping pressure matter for such soft materials?

Although compliant gels exhibit low contact resistance, apparent conductivity still varies with pressure because compression changes layer thickness and wetting. Reporting the pressure (and thickness) alongside the result is essential for reproducible, comparable data.

What accuracy and replicates should be expected?

Reputable testing provides at least three replicates with an expanded uncertainty typically within ±5% to ±10% (k = 2), supported by calibration against certified reference materials under an ISO/IEC 17025 quality framework.