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The upper limit of the performance of solid-state sheet-like absorbing materials is largely determined by the preparation process.
The geometric anisotropy of sheet-like magnetic particles is the physical basis for breaking through the Snoek limit, but this advantage can only be fully utilized when the particles are oriented parallel to the substrate surface. If the sheet-like particles are randomly dispersed in the polymer matrix, and the sheet-like planes of some particles are perpendicular to the substrate surface, the in-plane demagnetization factor increases, the magnetic permeability contribution significantly decreases, and the continuity of the in-plane conductive network is also disrupted. Randomly dispersed sheet-like absorbing materials often have properties between spherical particles and ideally oriented sheet-like materials, and the process investment has not been converted into performance returns.
How to make the sheet-like particles "obediently" arrange in the same direction is the core engineering problem in the preparation of sheet-like absorbing materials. Magnetic field orientation is currently a mature solution in industry, but it has an inherent limitation: it can only control the direction of the magnetic moment of magnetic particles and is powerless to control the orientation of non-magnetic or weakly magnetic sheet fillers. Moreover, magnetic field orientation usually requires a high magnetic field strength, and for thicker sheets, the attenuation of the magnetic field inside the material will weaken the orientation effect. Directional freeze-drying technology provides another orientation control pathway. Its approach is not to directly apply orientation force to particles, but to indirectly guide particle arrangement by controlling the growth direction of ice crystals in the matrix material.
Directional freezing: ice crystals as a "template"
The core of directional freeze-drying is to utilize the anisotropy during the growth process of ice crystals. When a suspension containing sheet-like particles is directionally frozen from the bottom, ice crystals grow into columnar or layered structures along the temperature gradient direction. Flake particles are squeezed and pushed at the forefront of ice crystal growth, gradually forming an arrangement parallel to the direction of ice crystal growth in the gaps between ice crystals.
The physical mechanism of this process is that during the growth of ice crystals, there is a exclusion zone at the solid-liquid interface, where suspended particles are pushed by the interface rather than engulfed. When multiple ice crystals grow in parallel, the particles are confined in the narrow slits between the ice crystals, naturally forming a parallel arrangement along the direction of ice crystal growth. After freezing, the ice crystals are removed during the vacuum sublimation process, leaving behind ordered pore structures complementary to the morphology of the ice crystals, while the sheet-like particles retain their orientation and arrangement.
The key process parameter of this method is the freezing rate. The cooling rate is too fast, and the ice crystals do not have time to grow directionally, resulting in randomly distributed small ice crystals with low particle orientation; The cooling rate is too slow, the ice crystals coarsen, the pore size is too large, and the mechanical strength of the material decreases. Within an appropriate cooling rate window, ice crystals form uniform columnar structures, and the orientation of sheet-like particles can reach a high level.
Electromagnetic benefits of ordered pore structure
The parallel pore structure formed by directional freeze-drying enhances the electromagnetic wave absorption performance from two aspects.
The first layer is the improvement of impedance matching.The parallel arrangement of pores forms a progressive impedance transition in the direction of electromagnetic wave incidence. When electromagnetic waves enter materials from free space, they first encounter regions with higher porosity, lower equivalent dielectric constant, and smaller impedance difference with air. As electromagnetic waves penetrate deeper into the material, the porosity gradually decreases, the material density increases, and the impedance transitions smoothly. This gradient impedance structure reduces surface reflection, allowing more electromagnetic wave energy to enter the material and participate in losses.
The second layer is the enhancement of multiple reflection losses.The walls of parallel pores are equivalent to a series of parallel reflective interfaces. Electromagnetic waves entering the interior of the material undergo multiple reflections between the pore walls, with each reflection accompanied by partial energy absorption. Compared with random pore structures, ordered pores have more regular multiple reflection paths, longer residence time of electromagnetic waves inside the material, and a higher probability of absorption.
Taking the anisotropic aerogel prepared by directional freeze drying as an example, this kind of material forms an ordered pore connection network through the directional ice template, and two-dimensional flaky units are arranged in parallel in the pore wall to build a continuous conductive and thermal conduction path. This ordered structure enables the material to achieve effective electromagnetic wave absorption at extremely low filler loads. Similar methods have also been used to prepare BC/MXene aerogels. An anisotropic structure was constructed by directional freeze drying, which achieved a balance between lightweight and absorption strength.
Additional improvement in mechanical properties
The improvement of mechanical properties by the ordered structure brought about by directional freeze-drying is also worthy of attention. The pore distribution in random pore structures is uneven, and stress concentration is prone to occur in areas with dense pores. Materials tend to crack from the weakest areas during compression or bending. The parallel arrangement of pore structures provides a more uniform stress distribution in the direction of force, and the load transmission efficiency of pore walls in the compression direction is higher.
For solid sheet-like absorbing materials that need to withstand mechanical stress during assembly or use, an ordered pore structure means that higher compressive strength and better dimensional stability can be achieved at the same porosity. If the wave absorbing plate needs to be used as a structural component for load-bearing, the compressive strength and flexural strength of ordered pores are usually better than those of random pore structures.
Selection and judgment of process route
The engineering selection and preparation process evaluation of solid-state sheet-like absorbing materials can be carried out from the following dimensions.
Matching orientation degree with filler type
Directional freeze-drying is suitable for orientation control of sheet-like or two-dimensional sheet-like fillers, including sheet-like FeSiAl, sheet-like carbonyl iron, MXene nanosheets, graphene, etc. For spherical or nearly spherical fillers, the orientation effect of ice crystal template method is limited, and magnetic field orientation or electric field orientation may be a more suitable choice. When selecting, the compatibility between the filler type and the process route should be confirmed first.
Matching pore structure with usage environment
The ordered pore structure formed by directional freeze-drying not only provides electromagnetic performance advantages, but also reduces the density of the material. For scenarios that require high thermal conductivity pathways, the air in the pores is a poor conductor of heat. Although ordered pores are more conducive to forming a continuous thermal conductivity pathway than random pores, the overall thermal conductivity is still lower than that of high-density solid materials. If the application scenario requires both high thermal conductivity and electromagnetic absorption, it is necessary to evaluate the influence of pore structure on thermal conductivity performance, or fill some pores by impregnating high thermal conductivity resin in the post-processing stage.
Feasibility of large-scale preparation
Directional freeze-drying has become relatively mature at the laboratory level, but industrial scale production requires consideration of uniform control of freezing rate, batch consistency, and drying energy consumption. For large-sized sheets, the difficulty of controlling temperature gradient increases with size, and the advancing speed of the freezing front may vary at different positions of the sheet, resulting in spatial non-uniformity of orientation degree. When selecting, it is necessary to confirm the dimensional capability and batch consistency data of the directional freezing process with the supplier.
Advanced Institute (Shenzhen) Technology Co., LtdWe have engineering capabilities in the morphology control, surface coating, and orientation process of sheet-like magnetic particles. The solid-state sheet-like products in our platinum brand absorbing material series cover the 1-18 GHz frequency band and support customized design of sheet-like particle orientation angle and filling ratio.
The matrix system can be made of high-temperature resistant polyimide or epoxy resin, and the temperature resistance range can be adjusted within the range of -40 ° C to 200 ° C according to the matrix selection. The product can provide sheet, roll, and precision die-cutting forms, and supports thermal conductive adhesive options to establish a continuous thermal path from the absorbing layer to the heat dissipation substrate. For more technical information, please visithttp://www.avanzado.cn/.