LI Huitong ¹, DU Kai ¹, LIN Yijun ¹, HUANG Yujia ^{1, 2}, XING Yan ³, SUN Shikuan ¹

(1. School of Materials and Energy, Foshan University, Foshan 528000, Guangdong, China; 2. Guangdong Key Laboratory for Hydrogen Energy Technologies, Foshan University, Foshan 528000, Guangdong, China; 3. College of Science, Nanjing University of Posts and Telecommunications, Nanjing 210023, Jiangsu, China)

Extended abstract:

[Background and purposes] With the rapid development of modern radar technology and 5G/6G high-frequency communication, the demand for electromagnetic pollution and stealth protection requires high-performance microwave- absorbing materials. Introducing low-dimensional conductive materials, such as graphene and carbon nanotubes, into semiconductor materials like TiO₂, has become a key research direction for adjusting the dielectric properties of microwave-absorbing materials. However, novel carbon materials are expensive and fail to scale up in production. Traditional graphite materials are readily available, but with issues of aggregation and weak interfacial coupling. Therefore, expanded graphite (EG) was selected, which has facile production techniques and an interlayer space suitable for composites, as an alternative to graphene for constructing EG/TiO₂ binary nanocomposites. Uniformly distributed nanocomposites were prepared using intercalated expanded graphite and chemical coprecipitation methods. Phase composition, chemical components and microstructure of the nanocomposites were systematically characterized. The effects of filler ratio and calcination temperature on electromagnetic parameters were studied to explore the regulation mechanisms of wave-absorbing performance of the nanocomposites.

[Methods] Under vacuum, a precursor solution consisting of tetrabutyl orthotitanate (TBOT) and silane coupling agent KH570 was infiltrated into the EG interlayers. Then, ultrasonic and stirring treatments were carried out simultaneously to ensure uniform mixing of EG and precursor. After hydrolysis by adding sufficient deionized water and continuously stirring for more than 120 min, the hydrolyzed precipitate was collected through filtration. After washing and vacuum drying, the powder was calcined at 450 ℃, 750 ℃ and 1050 ℃, in N₂ atmosphere. Finally, EG/TiO₂ nanocomposites with a mass fraction of 2.5% were obtained. Phase composition of the composite material was characterized using X-ray diffraction (XRD, TD-35000), while microstructure was analyzed using scanning electron microscopy (SEM, TESCAN MIRA LMS). Structural state of the expanded graphite was characterized using a laser confocal Raman spectrometer (Horiba LabRam HR Evolution). Surface elemental chemical state was determined using X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific ESCALAB Xi). Electromagnetic parameters of the samples were measured using an Agilent E5071C vector network analyzer using the coaxial method, over frequency range of 1–18 GHz.

[Results] The TiO₂ nanoparticles (~100 nm) are uniformly distributed on the EG layers. As the calcination temperature increases, the powder begins to crystallize and break the original agglomeration, resulting in a decrease in the particle size of TiO₂ nanoparticles. However, at high temperatures, grain growth occurs, while the phase transition of TiO₂ from anatase to rutile happens. The nanocomposites have a high intensity ratio of the D peak (~1350 cm⁻¹) to the G peak (~1580 cm⁻¹), indicating that abundant defects on EG sheets, which might be oxygen-containing functional group (-OH, -COOH, etc.), as proven by XPS results. As the calcination temperature increases, characteristic peaks of C-O-Ti appear, indicating the presence of interfacial chemical bonding between EG and TiO₂. As the content of the nanocomposites calcined at 1050 ℃ increases to 70%, the dielectric constants increase to ε'≈24.6 and ε''≈7.9. To avoid the interface impedance mismatch caused by high dielectric constants, the absorption performance of the composites with a content of 65% is tested. The minimum reflection loss (RL_min) achieves −43.50 dB, while the optimal bandwidth is 3.37 GHz. As the calcination temperature decreases, the sample calcined at 750 ℃ shows excellent interface impedance matching, owing to the decreased crystallinity and the TiO₂ phase with lower dielectric constants. It shows excellent microwave adsorption properties with an RLmin value of −63.50 dB at 1.94 mm and an effective absorption bandwidth (EAB) of 3.13 GHz (17.43–14.30 GHz, 1.3 mm). In the EG/TiO₂ nanocomposites, EG provides a three-dimensional conductive framework, while TiO₂, as a wide bandgap semiconductor, could dissipate electromagnetic wave energy through polarization relaxation. Meanwhile, the tight combination of defect-rich EG and TiO₂ brings significant interface polarization and dipole polarization, exhibiting excellent impedance matching characteristics and absorption performance. Although the proportion of the nanocomposite materials used here was relatively high (65%), the amount of expanded graphite was relatively low (only 2.5%), achieving higher RLmin values and wider EAB values with less addition and thinner thickness, comparable to similar materials.

[Conclusions] The low-cost and high-efficiency EG acting as the conductive skeleton is combined with uniformly distributedTiO₂ nanoparticles by a chemical co-precipitation method. The effects of filling ratio and calcination temperature on electromagnetic parameters and microwave absorption properties of the nanocomposites are explored. As the filling ratio increases, the dielectric constant increases until an interface impedance mismatch occurs, while the optimal filling ratio is 65%. As the calcination temperature increases, the phases of TiO₂ change from anatase to rutile, resulting in an increase in dielectric constant and a decrease in the degree of impedance matching. Therefore, the optimal calcination temperature is 750 ℃. The EG/TiO₂ nanocomposites exhibit multiple dielectric synergistic loss mechanisms through conductive loss provided by the EG conductive network and defects, and polarization loss caused by the EG/TiO₂ interface dipole. Meanwhile, the interface impedance matching can be effectively regulated. Therefore, the nanocomposites calcined at 750 ℃ with a filling ratio of 65% exhibit excellent absorption performance. The RLmin value reaches −63.50 dB at a thickness of 1.94 mm, while the EAB is 3.13 GHz at a thickness of 1.3 mm. This is a facile process for preparing cost-effective and high-performance microwave-absorbing materials.

Key words: expanded graphite; titanium dioxide; dielectric loss; microwave-absorbing performance