The thermal electromagnetic coupling dilemma of flexible microwave absorbing sheets: when the absorbed energy becomes heat

The core function of flexible microwave absorbing sheets is to irreversibly convert electromagnetic energy into thermal energy. This description usually appears at the beginning of a product introduction, but few people ask a follow-up question when selecting: where does the converted heat go.

In low-power scenarios, this issue is irrelevant. Absorbing a few dBm of noise power, the temperature rise of the material is negligible, and the electromagnetic parameters will not change as a result. But when the absorbing plate faces high-power microwave devices, high-power antenna arrays, or continuously operating RF power amplifiers, the situation is completely different. The absorbed electromagnetic energy accumulates inside the material in the form of Joule heating, and the temperature of the material continues to rise. The change in temperature directly changes the material's complex magnetic permeability and complex dielectric constant. The impedance matching window shifts accordingly, resulting in a decrease in absorption efficiency. Unabsorbed electromagnetic waves are reflected back into the system, and the reflected waves may form secondary resonance in the cavity, further exacerbating local power density.

This is a typical positive feedback loop. However, due to the fact that the substrate of flexible wave absorbing plates is usually a polymer, their thermal conductivity is much lower than that of metals or ceramics, and the diffusion rate of heat inside the material is slow, making it easier for local hotspots to form. The failure of absorbing materials under high-power microwave irradiation is essentially not the failure of electromagnetic properties, but the failure of thermal management capabilities.

Why is the thermal problem of flexible absorbing plates more difficult than that of rigid absorbing bodies

Rigid ceramic or metal based absorbers can enhance heat dissipation through structural design, such as heat dissipation fins, ventilation ducts, and metal inserts. Flexible waveplates do not have this option. Its flexibility comes from the balance between the polymer matrix and low filler filling, both of which happen to be unfavorable for heat conduction.

The fundamental contradiction is that improving thermal conductivity requires increasing the content of high thermal conductivity fillers, but high thermal conductivity fillers (such as boron nitride and aluminum oxide) themselves do not contribute to magnetic or dielectric losses. Increasing their content will dilute the concentration of magnetic fillers and reduce the absorption efficiency per unit thickness. If the thickness is increased to compensate for the absorption performance, the thermal resistance will also increase, making it more difficult to dissipate heat.

Porous structure: providing a pathway for heat to travel

Porous structure is the most common thermal management method in flexible absorbing plates, but its mechanism of action is often simplified as "increasing the heat dissipation area". A more accurate understanding is that the porous structure changes the mode of heat transfer within the material.

The study of MWCNT-OH/PDMS porous elastic absorbing materials provides a valuable reference case. This material achieves a minimum reflection loss of -37.84 dB at 12.66 GHz, an effective absorption bandwidth of 5.53 GHz, a tensile strength of 0.48 MPa, and a fracture elongation of 170%. Its three-dimensional porous structure promotes heat dissipation, allowing the composite material to maintain structural and functional integrity even at 250 ° C. The thermal management advantage of porous structures lies in the fact that although the air in the pores has poor thermal conductivity, it allows the material to undergo thermal expansion deformation when heated without significant internal stress accumulation, thereby avoiding structural cracking and conductive network fracture caused by thermal stress.

For wearable devices and flexible electronics, a temperature tolerance of 250 ° C has covered the vast majority of application scenarios. But if the target scenario involves higher power density or longer continuous working time, the porous structure itself is not enough to solve the problem, and an active thermal conduction path is also needed.

Combination strategy of thermal conduction pathway and phase change thermal storage

When the absorbed power density reaches a certain level, passive heat dissipation alone cannot control the material temperature within a stable range of electromagnetic parameters. At this point, two paths need to be coordinated: one is to establish a thermal conduction path from the inside of the material to the outside, and the other is to set up a "thermal buffering" mechanism inside the material to stabilize the temperature peak.

The construction of a thermal conductivity pathway can be achieved by introducing directionally arranged thermal conductive fillers into the absorbing layer. Under the action of shear or electric fields, sheet-like boron nitride or graphene can form oriented arrangements in the polymer matrix, constructing continuous thermal conductivity paths in the thickness direction and maintaining low thermal conductivity in the in-plane direction to avoid heat diffusion to sensitive electronic components. The key to this anisotropic thermal conductivity design is that heat generated from the absorbing layer is preferentially conducted along the thickness direction to the heat dissipation substrate or metal shell on the back, rather than accumulating within the surface.

Phase change thermal storage provides another approach. Dispersing microencapsulated phase change material (PCM) in the absorbing layer, when the material temperature rises to the phase transition point, PCM absorbs latent heat without significant heating. The study of bio based multifunctional composite phase change films shows that the material simultaneously achieves a thermal conductivity of 1.13 W/(m · K), microwave absorption performance of -41.36 dB, and a phase change enthalpy of 104 J/g. A phase transition enthalpy of 104 J/g means that within the phase transition temperature range, each gram of material can absorb 104 joules of heat without generating a temperature rise. For intermittent high-power pulse scenarios such as radar pulse transmission or communication base station uplink bursts, phase change thermal storage can absorb heat during the pulse period and slowly release it during the pulse interval, keeping the material temperature within a stable range of electromagnetic parameters.

The following table compares the applicable characteristics of three thermal management strategies in flexible absorbing plates:

Power density judgment in selection

For flexible wave absorbing plates that need to be used under non low power conditions, power processing capability should be considered as an independent selection dimension, rather than just focusing on reflection loss and frequency band coverage.