Differential thermal analysis (DTA) of thermal conductive gel is a critical experiment for verifying the thermal reliability, curing behavior, and long-term stability of thermal interface materials used in photovoltaic modules. This article explains the testing principle, sample requirements, measurable parameters, standard methods based on ISO 11357 and ASTM practices, and practical applications in material qualification and failure analysis within accredited third-party laboratories.
Technical Overview
Thermal conductive gels are soft, silicone-based or organic polymer-based composite materials heavily loaded with thermally conductive fillers such as aluminum oxide, zinc oxide, or boron nitride. In photovoltaic modules, they are applied in junction boxes around bypass diodes, behind high-power cells, and inside module-level power electronics to transfer waste heat away from heat-sensitive components. Because these gels must maintain their thermal and mechanical function for more than 25 years under damp heat, thermal cycling, and ultraviolet exposure, their thermal transition behavior must be characterized precisely. Differential thermal analysis (DTA) and its quantitative counterpart, differential scanning calorimetry (DSC), are the core thermal analysis experiments used for this purpose in accredited third-party laboratories.
Role of Thermal Conductive Gel in Photovoltaic Modules
Modern high-power photovoltaic modules concentrate significant thermal loads in the junction box, where bypass diodes can reach junction temperatures above 150 degrees Celsius during shading events. The thermal conductive gel fills the air gaps between the diode, soldered terminals, and plastic housing, replacing poorly conducting air with a material whose thermal conductivity typically ranges from 1 to 6 W/(m·K). If the gel cures incompletely, softens prematurely, or degrades oxidatively, heat accumulates, diode temperatures rise, and the safety margins defined in IEC 61730 and IEC 61215 qualification sequences may be exceeded. Thermal analysis experiments therefore directly support module safety, performance, and service-life assessment.
Principle of Differential Thermal Analysis
In a DTA experiment, a small gel specimen (typically 5 to 15 mg) and an inert reference material (usually an empty aluminum pan or alpha-alumina) are heated or cooled at a controlled linear rate inside a calibrated furnace. Thermocouples monitor the temperature difference between the sample and the reference. When the gel undergoes an endothermic event such as glass transition step, melting, or evaporation of volatile silicone oligomers, the sample lags behind the reference and a negative peak appears. Exothermic events such as additional crosslinking of an under-cured gel or oxidative decomposition produce positive peaks. Modern heat-flux DSC instruments convert this temperature difference into a quantitative heat flow signal, enabling determination of transition temperatures, enthalpies, and specific heat capacity in accordance with ISO 11357 series standards.
Testing Samples
Proper specimen selection is essential because thermal analysis results are highly sensitive to curing state, thermal history, and contamination. The following sample categories are commonly submitted for differential thermal analysis of photovoltaic thermal conductive gels.
Uncured Two-Component Gel (Part A and Part B)
For addition-cure or condensation-cure silicone gel systems, the base component (Part A) and the crosslinker or catalyst component (Part B) are tested individually to establish baseline transition behavior. A freshly mixed, uncured sample is also tested immediately to record the complete curing exotherm, which provides the total heat of reaction used later to calculate degree of cure.
Fully Cured Gel Specimens
Gel samples cured under production conditions are cut or punched from junction boxes or from cast plaques. These specimens are used to measure glass transition temperature, residual cure, melting or crystallization behavior of any crystalline filler or polymer phase, and specific heat capacity. Specimens should be taken from representative locations, since filler settling during storage or curing can cause local variations in thermal behavior.
Aged and Field-Returned Gel Samples
Gel recovered from modules after damp heat exposure (85 degrees Celsius and 85 percent relative humidity), thermal cycling between -40 and +85 degrees Celsius, UV exposure, or actual field service is analyzed to quantify degradation. Shifts in glass transition temperature, disappearance of residual cure peaks, or early onset of oxidative exotherms are indicators of aging-induced structural change.
Testing Items
A comprehensive differential thermal analysis experiment for photovoltaic module thermal conductive gel covers the following measurable parameters:
- Glass transition temperature (Tg) of the cured polymer network
- Curing exotherm peak temperature, onset temperature, and enthalpy of the uncured gel
- Degree of cure and residual cure of gel taken from finished junction boxes
- Melting temperature and melting enthalpy of crystalline phases or low-molecular-weight fractions
- Crystallization temperature and crystallization enthalpy during controlled cooling
- Thermal decomposition onset temperature and degradation enthalpy
- Oxidation induction time (OIT) and oxidation induction temperature (OIT) in oxygen atmosphere
- Specific heat capacity (Cp) as a function of temperature for thermal simulation input
- Volatile content indicated by low-temperature endothermic evaporation events
Glass Transition Temperature
The glass transition is observed as a step-like change in the heat flow baseline, and it defines the lower service temperature boundary at which the gel loses its compliant, stress-relieving character. For silicone gels, Tg values well below -40 degrees Celsius are expected so that the material remains soft during the cold steps of the IEC 61215 thermal cycling test. A significantly elevated or broadened Tg in an aged sample indicates filler-polymer interaction, post-curing, or oxidation.
Curing Behavior and Degree of Cure
By integrating the exothermic curing peak of a mixed, uncured gel and comparing it with the residual exotherm of a production-cured sample, the degree of cure is calculated as the percentage of reacted crosslinking sites. Junction box gels are generally expected to reach a degree of cure above 90 to 95 percent, because residual reactive groups can continue to evolve gas, soften, or degrade during long-term field operation.
Oxidative Stability (OIT)
In the oxidation induction time experiment, the specimen is held isothermally at an elevated temperature, typically between 180 and 240 degrees Celsius, under oxygen after equilibrating under nitrogen. The time elapsed until the abrupt onset of an exothermic oxidation peak is recorded. OIT values are used to compare antioxidant packages, to screen incoming raw materials, and, through Arrhenius extrapolation, to estimate the thermal oxidative lifetime of the gel at realistic photovoltaic operating temperatures.
Testing Methods
The experiment is performed with a calibrated heat-flux or power-compensation DSC/DTA analyzer following internationally recognized standards. Temperature and enthalpy calibration are carried out using certified pure metal standards such as indium, zinc, and tin, while baseline and heat capacity calibration follow the requirements of ISO 11357-1 and ASTM E967 and E968 practices. Chinese national standard GB/T 19466, which is technically aligned with ISO 11357, applies to domestic projects.
Sample Preparation and Instrumentation
- Specimen mass: 5 to 15 mg, accurately weighed to 0.01 mg using an analytical balance
- Pans: crimped aluminum pans with a vented or pierced lid for volatile detection, or hermetic pans for volatile silicone gel systems
- Atmosphere: high-purity nitrogen at 50 mL/min for inert scans; oxygen for OIT determination
- Reference: an empty aluminum pan matched to the sample pan configuration
- Cooling capability: intracooler or liquid nitrogen attachment for sub-ambient scans down to -90 degrees Celsius where Tg verification is required
Test Program and Temperature Profiles
A typical multi-segment program follows a heat-cool-heat protocol. The first heating from -80 or -60 degrees Celsius to 250 degrees Celsius at 10 degrees Celsius per minute records as-cured thermal history, residual cure, and volatiles. The controlled cooling segment captures crystallization events. The second heating segment provides the intrinsic glass transition and melting data after thermal history has been erased. For curing studies of freshly mixed gel, a single dynamic heating run from 25 to 250 degrees Celsius at 5 to 10 degrees Celsius per minute is used to capture the complete exotherm, complemented by isothermal holds at the production cure temperature when cure kinetics are required.
Applicable Standards
- ISO 11357 series (Plastics - Differential scanning calorimetry), including Part 2 for glass transition, Part 3 for melting and crystallization, Part 4 for specific heat capacity, and Part 6 for oxidation induction time
- ASTM E1356 for assignment of glass transition temperature by DSC
- ASTM D3418 for transition temperatures and enthalpies of melting and crystallization
- ASTM D3895 for oxidative induction time of polyolefins by DSC, widely adapted for filled silicone systems
- ASTM E1131 for thermogravimetric compositional analysis when filler content confirmation is required
- GB/T 19466 series as the national equivalent of ISO 11357 for the Chinese market
- IEC 61215 and IEC 61730 as the module-level qualification frameworks that define the aging conditions for comparative gel testing
Data Interpretation and Acceptance Criteria
Interpretation focuses on comparing the measured values against supplier specifications and against the requirements of the module design. Key acceptance considerations include a Tg safely below the minimum operating temperature, a degree of cure above the agreed threshold, no measurable residual exotherm after post-curing, decomposition onset temperature well above the maximum junction temperature reached during shading events, and OIT values consistent across batches. Repeatability of the glass transition midpoint within 2 degrees Celsius across duplicate specimens is normally required for a valid report.
Applications
Differential thermal analysis of thermal conductive gel serves multiple functions across the photovoltaic value chain:
- Incoming material quality control, verifying that delivered gel lots match the qualified reference material in Tg, cure enthalpy, and OIT
- Degree-of-cure verification for junction box potting lines, detecting under-cure caused by incorrect mixing ratios, insufficient cure time, or low ambient temperature
- Material qualification and comparative screening of alternative gel formulations during new module development
- Correlation analysis with IEC 61215 damp heat and thermal cycling results, linking changes in Tg and OIT to measured thermal resistance and diode temperature rise
- Service-life estimation through Arrhenius extrapolation of OIT and decomposition kinetics data
- Failure analysis of discolored, hardened, or delaminated gel recovered from field-returned modules
- Generation of temperature-dependent specific heat capacity inputs for finite-element thermal models of junction boxes and module-level power electronics
- Supporting documentation for safety certification submissions under IEC 61730 and UL 61730
Because the results influence safety-critical design decisions, manufacturers typically commission these experiments in accredited third-party laboratories with ISO/IEC 17025 compliant quality systems, ensuring traceable calibration, validated methods, and internationally accepted test reports.
FAQ
What is the difference between DTA and DSC for gel testing?
Classical DTA records the temperature difference between the sample and a reference, giving qualitative or semi-quantitative transition temperatures. DSC measures heat flow and provides quantitative enthalpies and specific heat capacity. Modern heat-flux DSC instruments are essentially quantitative DTA systems, and most current standards such as ISO 11357 are written in DSC terms, which is why the two techniques are used interchangeably in practice.
How much sample is required for the experiment?
Only 5 to 15 mg of gel is needed for each run. Because thermal analysis is sensitive to local filler distribution, several specimens from different locations of a junction box or plaque should be tested to confirm homogeneity.
What does an exothermic peak during the first heating of a cured gel mean?
It almost always indicates residual curing, meaning additional crosslinking reactions that were not completed during production curing. A significant residual exotherm signals under-cure, which can lead to post-curing shrinkage, volatile evolution, or degraded thermal contact during module service.
Can differential thermal analysis measure thermal conductivity?
No. DTA and DSC measure thermal transitions, enthalpies, and heat capacity. Thermal conductivity and thermal interface resistance must be measured with separate techniques such as the transient plane source or laser flash method, although the DSC-derived specific heat capacity serves as a valuable input for those calculations.
How is OIT used to estimate the service life of a gel?
OIT is measured at several elevated temperatures, and the natural logarithm of OIT is plotted against the inverse absolute temperature. The resulting Arrhenius relationship is extrapolated to the expected photovoltaic operating temperature to compare formulations and antioxidant systems. Absolute life prediction should always be combined with damp heat and thermal cycling data rather than used alone.
Which aging conditions are most relevant for comparative gel testing?
The standard 85 degrees Celsius and 85 percent relative humidity damp heat test and the -40 to +85 degrees Celsius thermal cycling sequence from IEC 61215 are the most widely used conditioning profiles, because they reproduce the dominant field degradation drivers for junction box gels.
How long does a typical experiment take?
A single heat-cool-heat program covering -80 to 250 degrees Celsius at 10 degrees Celsius per minute requires approximately two to three hours including equilibration, while an isothermal OIT run may take one to four hours depending on the stability of the material. Complete report issuance, including duplicate specimens and calibration verification, generally requires three to seven working days in an accredited third-party laboratory.