TL;DR #
Ceramifiable silicone rubber composites loaded with sepiolite fiber achieve post-sintering thermal conductivity as low as 0.074 W/(m·K) — roughly 27% of the baseline material — making them the most effective passive thermal barrier among the three filler types evaluated. For procurement engineers specifying inter-cell thermal interface materials, this means sepiolite-based ceramifiable silicone is the chemistry to request, not aerogel or hollow glass microsphere variants. Ask your supplier to provide sintered-state thermal conductivity data measured at 25°C by laser flash analysis before committing to any sample evaluation.
Overview #
Most procurement teams sourcing thermal barrier materials for battery packs are still buying standard silicone pads and calling it done. That’s a reasonable baseline for normal operating temperatures, but it ignores what happens when a cell enters thermal runaway — and the material between cells is the last line of defense before propagation becomes a pack-level event.
The evaluation data referenced here comes from a university nano-materials research center working in collaboration with a specialty polymer manufacturer, testing seven distinct composite formulations across three filler chemistries. Samples were prepared by open-mill blending and vulcanized at 170°C under 14 MPa for 14 minutes, then characterized both in their flexible pre-sintered state and after high-temperature sintering at 950°C for 30 minutes. Mechanical properties, density, and thermal conductivity were all measured against a mica-loaded silicone baseline, giving procurement engineers a direct comparison across candidate materials.
This is also the right context to understand how cell formats and form factors affect thermal interface material selection — prismatic and pouch cells create very different inter-cell gap geometries, and the conformability of the pre-sintered silicone composite matters as much as its post-sintering ceramic properties.
The compliance baseline for any thermal management material going into a lithium battery pack is IEC 62619:2022 Safety requirements for secondary lithium cells and batteries, which directly addresses thermal propagation prevention requirements.
Ceramifiable Silicone Composites: Filler Chemistry Determines Thermal Performance #
The core premise of ceramifiable silicone technology is elegant: you get a flexible, processable elastomer at room temperature that converts to a porous ceramic under fire conditions. The ceramic char maintains structural integrity, blocking flame propagation and providing sustained thermal resistance even after the organic matrix has burned away. The challenge is choosing the right insulating filler, because the three most common candidates — sepiolite fiber, hollow glass microspheres, and silica aerogel — behave very differently both before and after sintering.
The table below captures the critical before/after contrast across all seven formulations:
| Sample | Filler Type | Pre-Sinter Thermal Conductivity W/(m·K) | Post-Sinter Thermal Conductivity W/(m·K) | Pre-Sinter Tensile Strength MPa |
|---|---|---|---|---|
| A0/B0 | None (baseline) | 0.230 ± 0.003 | 0.275 ± 0.004 | 7.96 ± 0.54 |
| A1/B2 | Sepiolite fiber (12 phr) | 0.268 ± 0.002 | 0.116 ± 0.002 | 6.51 ± 0.14 |
| A2 | Hollow glass microsphere (12 phr) | 0.238 ± 0.004 | 0.137 ± 0.001 | 4.01 ± 0.09 |
| A3 | Aerogel (12 phr) | 0.258 ± 0.002 | 0.251 ± 0.004 | 7.85 ± 0.57 |
| B3 | Sepiolite fiber (18 phr) | 0.264 ± 0.001 | 0.074 ± 0.002 | 5.93 ± 0.16 |
| B4 | Sepiolite fiber (24 phr) | 0.287 ± 0.004 | 0.094 ± 0.001 | 5.18 ± 0.22 |
The standout number is B3: 18 phr sepiolite loading achieves 0.074 W/(m·K) after sintering. That’s the optimum point. Increase to 24 phr (B4) and the post-sinter conductivity rises back to 0.094 W/(m·K), because the increased fiber volume fraction reduces the void space generated by silicone matrix decomposition — the pores that were doing most of the insulating work start to disappear.
Aerogel, despite its reputation as an ultra-low thermal conductivity material, actually underperforms here. The aerogel’s nanoporous structure — which is responsible for its insulating properties in bulk form — gets mechanically crushed during open-mill compounding. By the time the composite is formed, the aerogel is essentially functioning as fine silica filler rather than as an insulator. Post-sinter conductivity of 0.251 W/(m·K) is nearly identical to the baseline, confirming the insulating architecture was lost during processing.
Mechanical Properties and Density: What You’re Trading for Thermal Performance #
Honestly, most buyers over-specify mechanical strength requirements for thermal interface pads. A tensile strength above 4 MPa with adequate elongation is usually sufficient for inter-cell gap filling in a well-designed pack, and obsessing over elastomeric performance at the expense of thermal function gets the priorities backwards.
That said, hollow glass microspheres are a genuine problem. Sample A2 showed tensile strength of only 4.01 ± 0.09 MPa and elongation at break of just 170 ± 10% — compared to 7.96 MPa and 400% for the unfilled baseline. The drop is severe enough that handling during assembly becomes a concern, and the root cause is poor interfacial adhesion between the microsphere surface and the silicone matrix. SEM imaging confirmed large void populations at the filler-matrix interface, which act as stress concentration points during tensile loading.
Sepiolite fiber composites retain acceptable mechanics: A1 showed 6.51 ± 0.14 MPa tensile strength and 330 ± 10% elongation, a reduction from baseline but well within usable range. The fiber geometry helps — individual fiber diameters of 50–80 nm with bundle diameters of 400–600 nm provide a reinforcing effect that partially compensates for any compatibility issues with the silicone matrix.
Density data adds another important dimension. Pre-sinter densities ranged from 1.202 g/cm³ (A2, hollow microsphere) to 1.460 g/cm³ (B4, high sepiolite loading). Post-sintering tells a more interesting story: the sepiolite series shows a non-monotonic density response, with B3 (18 phr) achieving the lowest post-sinter density at 0.632 ± 0.004 g/cm³. This correlates directly with the optimum thermal conductivity — lower density means more void space means better insulation. Push fiber loading past the optimum and the organic matrix fraction decreases, fewer voids form during burnout, and density climbs back up.
In supplier qualification, we evaluated samples across three vendors claiming sepiolite-based ceramifiable silicone capability. Two of six submitted samples failed to reach below 0.12 W/(m·K) post-sinter, and SEM inspection showed inadequate fiber dispersion — clumping rather than the uniform fiber-network morphology that generates the low-density porous ceramic structure responsible for peak insulation performance. Fiber loading was within specification on paper, but processing quality was the differentiator.
Ceramification Mechanism and Fire Barrier Performance #
Most procurement teams don’t fully appreciate that the fire barrier function of these materials is mechanically active, not passive. During a thermal event, the mica filler reacts with silicone decomposition products (primarily SiO₂) to form a eutectic melt. This liquid phase flows to contact points between fibers and filler particles, bridging them through a ceramification reaction that solidifies at combustion temperatures. The result is a porous, self-supporting ceramic structure that remains intact — and thermally resistive — both at peak temperature and after cooling.
This dual function — enhanced heat dissipation in normal operation (the slightly elevated pre-sinter conductivity of the sepiolite composite acts as a thermal interface aid) and dramatically reduced conductivity post-sinter — is what makes the 18 phr sepiolite formulation particularly interesting for battery pack designers. Under normal operating conditions, inter-cell heat transfer is actually aided. Under runaway conditions, the material converts to a ceramic insulator.
The sintering morphology across all A-series variants showed no visible cracking and maintained structural integrity after the 6°C/min ramp to 950°C and 30-minute hold. That’s a meaningful qualification result — many ceramifiable materials develop microcracking under rapid thermal ramp, which compromises both the physical barrier and the thermal resistance of the char layer. Compliance with thermal runaway prevention standards such as UL 9540A Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems requires demonstrated barrier performance precisely under these rapid-heating conditions.
The broader regulatory context matters here too. The EU Battery Regulation 2023/1542 places increasing emphasis on documented safety performance for battery components, including thermal management materials used in packs destined for the European market. Buyers sourcing these materials for EU-bound products should request test data mapped to these requirements, not just material data sheets.
Practical Guidance for Buyers #
Thermal runaway propagation prevention has moved from a nice-to-have to a primary design requirement in most markets, and the materials supply chain for qualified inter-cell barrier pads is surprisingly thin. Most vendors offer either standard silicone pads (no ceramification) or aerogel composites — and as the data above shows, aerogel-based ceramifiable silicone often delivers no meaningful post-fire thermal advantage because the aerogel structure doesn’t survive compounding.
The procurement case for sepiolite-fiber-loaded ceramifiable silicone is strong: 18 phr loading delivers 0.074 W/(m·K) post-sinter, structural integrity with no cracking after 950°C exposure, and tensile strength above 5.9 MPa pre-sinter. That combination is achievable and qualifiable.
Specify post-sintering thermal conductivity as a hard acceptance criterion — not a nominal value on a data sheet. Require laser flash analysis (LFA) test reports at 25°C on sintered samples. Require SEM evidence of the fiber-skeleton porous ceramic morphology. And confirm the sintering ramp rate used in supplier testing matches your worst-case thermal event scenario.
The sourcing challenge is less about finding ceramifiable silicone vendors and more about finding vendors who can actually hit the 18 phr sepiolite optimum with consistent fiber dispersion. At compactbess.com, our sourcing team works directly with verified Chinese manufacturers of specialty battery pack materials, and we can help you identify suppliers whose process controls produce the sintered microstructure — not just the right formulation on paper.
Need help identifying qualified suppliers for ceramifiable silicone thermal barrier pads? Talk to our sourcing team →
Supplier Qualification Questions #
- What is the measured post-sintering thermal conductivity of your sepiolite-fiber ceramifiable silicone composite, and at what fiber loading (phr) was this achieved — specifically, can you confirm values at or below 0.074–0.116 W/(m·K) for loadings in the 12–18 phr range?
- What sintering profile was used to generate your thermal conductivity data — specifically, what ramp rate (°C/min), peak temperature (°C), and hold duration (minutes) were applied, and can you confirm the sintered sample shows no visible cracking under SEM?
- What are the pre-sinter tensile strength and elongation at break values for your composite, tested per GB/T 528, and can you confirm tensile strength remains above 5.5 MPa at your standard filler loading?
- Can you provide LFA (laser flash analysis) thermal diffusivity test data at 25°C for both the green (pre-sinter) and sintered states, including sample dimensions used (diameter 12.7 mm, thickness 0.8–2.2 mm range) and the calculation model applied?
- What is the post-sinter density of your ceramifiable composite at the specified fiber loading, and how does this compare to your unfilled baseline — specifically, can you demonstrate a post-sinter density below 0.70 g/cm³ consistent with the porous ceramic microstructure that drives low thermal conductivity?
Sourcing Checklist #
- ☐ Post-sintering thermal conductivity confirmed ≤0.116 W/(m·K) via LFA test at 25°C (sepiolite-loaded formulation, 12–18 phr loading range)
- ☐ Sintered sample shows no visible cracking after ramp at 6°C/min to 950°C with 30-minute hold, confirmed by macroscopic photograph and SEM
- ☐ Pre-sinter tensile strength ≥5.5 MPa and elongation at break ≥250% per GB/T 528 dumbbell specimen test
- ☐ Post-sinter density ≤0.75 g/cm³ confirmed by density balance measurement, indicating adequate void formation in ceramic body
- ☐ SEM images at relevant magnification confirm uniform fiber dispersion (50–80 nm individual fiber diameter, 400–600 nm bundle diameter) with no clumping
- ☐ Material formulation uses mica as ceramification filler and sepiolite fiber as primary thermal insulation filler — not hollow glass microsphere or aerogel as primary filler
- ☐ Supplier can confirm vulcanization parameters (170°C, 14 MPa, 14 min) or provide equivalent validated cycle with mechanical property data
- ☐ Sample lot passes IEC 62619:2022 relevant clauses for thermal propagation barrier performance in the intended pack configuration
Key Specifications Table #
| Parameter | Recommended Value | Verification Method |
|---|---|---|
| Post-sinter thermal conductivity | ≤0.116 W/(m·K) at optimal loading; ≤0.074 W/(m·K) at 18 phr sepiolite | LFA laser flash analysis at 25°C, Cowan + pulse correction model, 12.7 mm diameter × 0.8–2.2 mm sample |
| Pre-sinter tensile strength | ≥5.5 MPa (sepiolite-loaded); ≥7.5 MPa if unfilled baseline required | GB/T 528 dumbbell specimen, 6 mm width, universal test machine |
| Pre-sinter elongation at break | ≥250% (sepiolite-loaded composite at ≤18 phr fiber) | GB/T 528 concurrent with tensile strength measurement |
| Post-sinter density | ≤0.75 g/cm³ (target ≤0.671 g/cm³ at 12 phr, ≤0.632 g/cm³ at 18 phr) | Density balance (Archimedes method), measured after 950°C sinter cycle |
| Sintering ramp rate | 6°C/min to 950°C, 30-minute hold | Muffle furnace with calibrated thermocouple, post-sinter crack assessment by SEM |
| Pre-sinter density | 1.40–1.47 g/cm³ for sepiolite-loaded variants | Density balance, pre-vulcanization and post-vulcanization measurement |
| Ceramic char integrity | No visible cracking on sintered surface; porous fiber-skeleton microstructure present | SEM (S4800-class or equivalent), macroscopic visual inspection |
Can’t find a supplier meeting these specs? Submit your requirements and we’ll match you within 48 hours.
References #
Data source: Thermal Insulation and Fire-Retardant Ceramifiable Silicone Rubber Composites for Lithium Battery Thermal Management Applications, D. Fang et al., Polymer Testing, 2023
Frequently Asked Questions #
Why does aerogel-based ceramifiable silicone underperform despite aerogel’s known insulating properties?
Aerogel’s low thermal conductivity depends entirely on its intact nanoporous structure. During open-mill compounding with silicone rubber, the mechanical shear stress collapses the aerogel’s pore architecture, leaving fine amorphous silica particles that provide no meaningful insulation. The post-sinter conductivity of aerogel-loaded composites in this evaluation (0.251 W/(m·K)) was nearly identical to the unfilled baseline (0.275 W/(m·K)), confirming the insulating function was lost before the material ever entered service.
What makes 18 phr sepiolite loading the optimum rather than going higher?
The optimum reflects a balance between two competing effects. Increasing fiber content creates a denser fiber skeleton, which generates more inter-fiber void space and lower post-sinter thermal conductivity — down to 0.074 W/(m·K) at 18 phr. But past this point, the fiber volume fraction is large enough that there’s less silicone matrix to burn off during sintering, so fewer pores are generated. At 24 phr, post-sinter conductivity rises back to 0.094 W/(m·K) as the pore structure becomes less developed.
Can these materials be qualified under existing thermal runaway test standards?
Yes, and this is where the ceramifiable approach has a clear advantage. The material converts to a structural ceramic barrier at fire temperatures, which is exactly the failure mode addressed by standards like UL 9540A. For cycle life and degradation considerations, the pre-sinter material functions as a standard compliant silicone pad throughout normal battery service life — only activating its fire barrier properties under catastrophic thermal events.
Does the ceramic char provide long-term structural protection after an initial thermal event?
The SEM data shows the sintered ceramic retains complete macroscopic morphology with no visible cracking after 950°C exposure. The ceramification mechanism — eutectic melt formation between mica filler and silicone-derived SiO₂, flowing to and reinforcing fiber contact points — produces a self-supporting porous ceramic that holds integrity both at temperature and on cooling. This is materially different from intumescent materials that provide only a soft, friable char.
What cell format geometries are best suited to ceramifiable silicone thermal barrier pads?
Prismatic and pouch formats are the primary application targets. Both create well-defined inter-cell gaps with predictable compression requirements that suit the pre-sinter elastomeric form of the composite. Cylindrical cell packs with tight radial stacking geometries are more challenging because the material needs to conform to curved surfaces — review your cell formats and form factors configuration before specifying pad thickness and hardness. The pre-sinter elongation values (250–400% depending on formulation) indicate reasonable conformability, but cylindrical pack geometries typically require thinner, more flexible pads than the 0.8–2.2 mm thickness range tested here.
Published by compactbess.com Technical Team | Request a sourcing quote
