Research Progress on Modified Cellulose Wave-Absorbing Composites

With the increasing reliance on electronic and communication technologies in modern society, the widespread application of electromagnetic waves in various fields such as daily life and healthcare has brought convenience, but has also led to serious electromagnetic pollution issues. This not only affects human health but also threatens the normal operation of electronic devices and even national security. Therefore, developing efficient wave-absorbing materials has become an urgent task. Traditional ceramic-based wave-absorbing materials have outstanding chemical stability and wide frequency response, but they are often costly; iron-based wave-absorbing materials have good magnetic properties but have a high density and limited frequency response range, which restricts their application in lightweight scenarios.

Cellulose, as a natural renewable resource, is widely sourced and cost-effective. It can be extracted from various plants and is a biodegradable material, which gives it significant advantages in terms of environmental impact and cost. The ease of processing cellulose and its high affinity with other atoms or molecules offers the possibility of preparing composite materials with special functions. Particularly, cellulose has a rich porous structure, which not only increases the specific surface area of the material but also provides more interfaces for the absorption of electromagnetic waves. During the carbonization process, the pores of cellulose expand, forming complex structures that allow electromagnetic waves to reflect and scatter multiple times within them, gradually attenuating the energy. Although cellulose is porous and easy to process, it is non-magnetic and has low dielectric loss. Therefore, researchers are combining it with other materials to develop composites with excellent wave-absorbing properties, which is a current research focus.

The author explored the potential of cellulose as a matrix for wave-absorbing materials, focusing on composite modification with magnetic metals, carbon materials, and conductive polymers. Specifically, embedding magnetic metals in the cellulose matrix can enhance magnetic loss and absorption capabilities; combining with carbon materials can construct porous multilayer structures to improve electromagnetic wave reflection and scattering effects; introducing conductive polymers can increase the number of charge carriers, thereby optimizing the material's magnetic and dielectric properties and enhancing wave-absorbing performance. Finally, a detailed analysis and summary of recent research work on modified cellulose wave-absorbing materials is provided, along with an outlook on future research trends.

The wave-absorbing mechanism of absorbing materials mainly relies on their dielectric loss, magnetic loss, multiple scattering, interfacial polarization, and charge carrier migration. These mechanisms work together to convert and dissipate the electromagnetic wave energy that penetrates the material. When absorbing electromagnetic waves, absorbing materials need to consider impedance matching to ensure that electromagnetic waves can enter the material as much as possible without reflection, while also considering attenuation matching for efficient absorption of electromagnetic waves. Ideally, electromagnetic waves enter the material without reflection and fully transmit; practical research focuses on reducing impedance differences to enhance absorption rates. Attenuation matching involves the rapid absorption and energy conversion of electromagnetic waves; thin materials achieve loss through surface wave interference, while multilayer materials increase internal reflections to convert energy. Additionally, the energy loss of electromagnetic waves is positively correlated with the wave-absorbing properties of the material, which depends on the material's dielectric constant and permeability. The larger the imaginary part of the dielectric constant and permeability, the stronger the loss characteristics. Therefore, researchers can use study results to adjust the imaginary part of the material's dielectric constant.ε″Imaginary part of magnetic permeabilityμ"The difference to increase reflection loss. The reflection loss can be calculated according to formula (1-2).

In the formula, RL is the reflection loss, in dB;ZThe impedance of the material;ε_rThe relative permittivity of the material.μ_rThe relative permeability of the material;fThe frequency of electromagnetic waves, GHz;dFor the thickness of the material, mm;cFor the speed of light, 3.0 × 10⁸ m/s。

When the reflection loss (RL) is -10dB, the absorbing material can absorb 90% of the electromagnetic waves. Therefore, the bandwidth where RL is lower than -10dB is generally considered the effective bandwidth of the material. The larger the imaginary part of the complex permeability and complex permittivity, the greater the attenuation coefficient, but the material's impedance will also be affected, resulting in fewer electromagnetic waves entering the material. Therefore, appropriate electromagnetic parameters (permeability and permittivity) are key factors for achieving good wave-absorbing performance in materials.

Magnetic metals have high saturation magnetization and strong magnetic exchange coupling, exhibiting large magnetic permeability at high frequencies, which enables them to effectively function as magnetic absorbers in the gigahertz frequency range. Therefore, modifying cellulose by adding magnetic materials can enhance its microwave absorption performance. When cellulose is composited with metal nanoparticles, these nanoparticles can be uniformly distributed within the porous structure of the cellulose, forming a three-dimensional conductive network. This not only increases the interface for electromagnetic wave interaction with the material but also improves impedance matching levels, effectively mitigating the influence of eddy current effects. To date, researchers have attempted various magnetic metals to modify cellulose.

In addition to magnetic metals, recent research indicates that non-magnetic metal ions also possess the ability to absorb and attenuate electromagnetic waves. Silver (Ag), as a non-magnetic metal, exhibits excellent electrical conductivity, thermal conductivity, and thermal stability. The high conductivity of Ag determines that electromagnetic waves are easily reflected off its surface; therefore, Ag is generally used as a reflective material for electromagnetic interference shielding.

In summary, incorporating metal into a cellulose matrix to form wave-absorbing materials combines the advantages of both: it retains the lightweight strength and environmental characteristics of cellulose while exhibiting the excellent electromagnetic properties of metals, providing efficient wave absorption capability.

Modifying cellulose with carbon materials can yield composites with high reflection loss performance. Carbon-based absorbing materials have become a research hotspot due to their light weight, good flexibility, and high electromagnetic performance. Graphene is a hexagonal two-dimensional single atomic layer carbon material with high conductivity and abundant surface defects and terminal groups. In the field of microwave absorption, graphene is of great interest due to its stable structure, high specific surface area, excellent electrical properties, and unique boundary effects. However, pure graphene has high conductivity, which leads to poor absorption performance due to impedance mismatch. Constructing cellulose/graphene composite absorbing materials using cellulose as a matrix can introduce the effective absorbing medium graphene while fully utilizing the structural characteristics of the cellulose matrix to prevent self-stacking of graphene sheets.

In addition to graphene, carbon nanotubes (CNTs) also have a high electron mobility, which means they can effectively capture and transfer electrons in electromagnetic waves, thereby improving the electrical performance of cellulose.

Using carbon materials to modify cellulose not only retains the lightweight characteristics of cellulose but also improves its dielectric properties through the carbon materials. This composite strategy not only enhances the material's ability to absorb electromagnetic waves but also introduces different types of loss mechanisms, such as dielectric loss, conductive loss, and interfacial polarization loss.

The modification of cellulose using conductive polymers can improve the dielectric loss of cellulose and provide additional polarization centers, thereby achieving a wider absorption bandwidth and higher absorption efficiency. In the porous structure of cellulose, conductive polymers can fill the pores, forming an effective electromagnetic wave absorption layer while providing additional interfaces for the absorption and scattering of electromagnetic waves. The large π bonds or conjugated double bonds in conductive polymers (such as polypyrrole (PPy), polyaniline (PANI), and polythiophene (PTh)) enhance the conductivity and polarization capability of the material by providing delocalized electron orbitals, thereby improving the material's ability to absorb and attenuate electromagnetic waves. Combining cellulose with polyaniline can improve the moldability and brittleness of polyaniline.

In addition to polyaniline, polypyrrole is often used to modify cellulose due to its advantages such as easy synthesis, adjustable conductivity, low density, and stable performance.

Cellulose has the advantages of low cost, environmental friendliness, and lightweight, showing great potential in the field of wave-absorbing materials. In the further promotion of the development of cellulose-based wave-absorbing materials, there are several challenges and corresponding development directions:

(1) Structural design and electromagnetic property optimization. Currently, the application of cellulose in absorptive materials is mainly limited by the design and optimization of its original structure. Future research should focus on exploring how to improve its electromagnetic properties through innovative porous structure design, while achieving wideband absorption goals at thinner material thicknesses.

(2) In-depth study of composite wave-absorbing mechanisms. The combination of cellulose with magnetic or non-magnetic materials is an effective way to improve its wave-absorbing performance. However, the specific wave-absorbing mechanisms of these composites have not yet been fully understood. Future research should focus on systematically exploring the interactions between different materials and their impact on wave-absorbing performance, in order to more accurately design efficient wave-absorbing materials.

(3) Energy Conversion and Thermal Energy Application. The thermal energy released during the absorption of electromagnetic waves by cellulose-based absorbing materials offers a new research direction for energy conversion and thermal energy application. Future research should consider how to utilize the released thermal energy to activate microscopic particles within the material, thereby enhancing the material's absorption of electromagnetic waves and improving the wave-absorbing performance of cellulose-based absorbing materials.

In summary, future research on cellulose-based absorbing materials will focus on achieving lightweight, wideband, and high-efficiency absorption performance. To overcome existing shortcomings, innovations are needed in improving electromagnetic properties, enhancing environmental stability and durability, simplifying processing techniques, and increasing multifunctionality and cost-effectiveness. This will enable cellulose-based absorbing materials to better meet the demands of modern technology and play a more significant role in future electromagnetic wave absorption technologies.