The 'hidden decay' of solid-state sheet-like absorbing materials: Why does the thermal conductivity network fail before the absorbing performance after thermal cycling

In high power density scenarios such as 5G communication base stations and new energy vehicle electronic control systems, thermal absorption plates not only have to bear the function of electromagnetic wave loss, but also need to serve as a high-speed channel for heat transfer. A wave absorbing plate with an initial thermal conductivity of 3.0 W/(m · K) may perform perfectly during installation testing, but after hundreds of thermal cycles, the thermal resistance value significantly increases, the heat dissipation capacity decreases, and the absorption efficiency also decreases.

This failure mode has an easily overlooked feature: decay does not begin with the degradation of electromagnetic parameters of the absorbing filler. The first thing that thermal cycling destroys is the interface bonding between the filling particles and the polymer matrix. Once the interface bonding force is damaged by thermal stress, the thermal conductivity network is cut off, microcracks and interface debonding occur inside the material, and the absorption efficiency will significantly decrease due to structural looseness. In other words, the failure of the thermal conduction path is a precursor signal of the decline in absorption performance.

Mismatch of thermal expansion coefficient: the physical root of decline

The thermal expansion coefficient of polymer matrices (such as silicone and polyurethane) is usually in the range of 100-300 ppm/° C, while the thermal expansion coefficient of common absorbing fillers - carbonyl iron powder, flake FeSiAl, ferrite - is in the range of 5-15 ppm/° C. The difference between the two is more than one order of magnitude.

During the thermal cycling process, the magnitude of matrix expansion and contraction is much greater than that of filler particles. This difference generates periodic shear stress at the particle matrix interface. Each thermal cycle accumulates micro damage at the interface, and after hundreds of cycles, interface debonding and micro cracks expand from local to continuous networks. The thermal conductivity pathway relies on the close contact between particles and the continuous wrapping of the matrix. Once the interface becomes detached, the path of heat transfer is cut off, and the thermal resistance increases accordingly.

For sheet fillers, there is a structural mitigating factor to this problem. The in-plane size of sheet-like particles is much larger than their thickness, and the matrix has a larger "wrapping" area for sheet-like particles during thermal expansion, resulting in more interface contact points per unit volume. Even if some interfaces experience partial debonding, the remaining contact surfaces may still maintain the basic heat conduction path. The contact between spherical particles is point contact, and once debonded, the entire thermal chain breaks; The contact between sheet-like particles is surface contact, which has higher redundancy.

How to improve thermal cycling stability with sheet-like morphology

The tolerance of sheet-like particles to thermal cycling is not only determined by contact geometry, but also by their orientation structure. In the magnetic field orientation process, sheet-like particles are arranged in parallel along the in-plane direction, forming a continuous thermal conduction path along the plane of the sheet. When the matrix expands during thermal cycling, the relative position change between the sheet-like particles is relatively small because they have already formed a tight overlap in the orientation direction. The stress generated by thermal expansion mismatch is more released along the direction of the inter sheet interface, rather than pulling apart the particle spacing perpendicular to the plane of the sheet.

The molecular dynamics simulation results support this judgment: in the oriented sheet-like particle/polymer composite system, the anisotropy of the thermal expansion coefficient decreases, and the thermal expansion coefficient in the thickness direction is closer to the intrinsic value of the filler. This means that in thermal cycling, the size changes of the material are more predictable, and the cumulative strain changes at the interface are smaller.

Low volatility characteristics: overlooked long-term reliability indicators

Another accelerating factor for thermal cycling degradation is the small molecule volatiles in the matrix resin. During the temperature cycling process, incompletely cured resin components and low molecular weight additives may migrate to the interface region, forming a weak boundary layer. These weak boundary layers are more prone to debonding under thermal stress, and volatile substances may accumulate in the sealed cavity and contaminate adjacent precision components.

The requirements for volatile compounds in aerospace grade material standards can be used as a reference. According to ASTM E595 standard, products with total mass loss (TML) controlled below 1% and condensable volatile matter (CVCM) controlled below 0.1% have extremely low long-term volatile matter release levels. For high-power density scenarios that require long-term continuous operation, choosing a low volatility absorbing plate can effectively reduce the risk of thermal cycling degradation caused by interface weakening.

Thermal cycle judgment in selection

For thermal absorbing wave plates, it is not enough to only focus on the thermal resistance and thermal conductivity at room temperature. The following three dimensions directly determine the long-term performance of materials under thermal cycling conditions.

The high-temperature resistant absorbent/polyimide composite material with oriented sheet-like structure exhibits excellent radar absorption performance in the 1-8 GHz frequency band. After high-temperature treatment at 450 ° C/1h, the electromagnetic parameters change little when the doping ratio is 50% -60%. This data indicates that by selecting a high-temperature resistant matrix and controlling the doping ratio of fillers, the thermal cycling stability can be significantly improved while maintaining the absorption performance.