This research presents a novel and cost-effective electroless nickel plating process on graphite substrates, with a double encapsulation method used as a superior alternative to conventional techniques. A significant advantage of this method is its elimination of the need for precious metal activation, which is typically required to initiate the plating process in traditional approaches. The core methodology involves a two-stage reduction mechanism. Firstly, the nickel ions adsorbed onto the graphite surface undergo an initial reduction to form discrete active centers. Subsequently, a secondary reduction step is employed, where additional nickel ions are reduced preferentially around these established active centers, thereby facilitating the formation of a continuous and relatively dense nickel coating. The successful synthesis of the nickel-coated graphite composite powder is confirmed through comprehensive characterization, including X-ray diffraction (XRD) for phase identification and coating morphology analysis for microstructural evaluation.
To systematically evaluate the functional properties, a series of nickel-coated graphite/paraffin composite samples are fabricated with precisely controlled variations in nickel content. The primary objective of this research is to investigate the influence of nickel loading on the resulting dielectric, magnetic, and ultimately, the microwave absorption properties. This investigation is crucial for determining the optimal compositional ratio and process parameters, thereby aiming to expand and optimize the application of nickel-coated graphite composites in high-performance microwave absorption materials. The electromagnetic parameters, namely the complex permittivity and the complex permeability, which are fundamental to the material's interaction with electromagnetic waves, are measured accurately. The measurements are conducted in the X-band frequency range (8.2-12.4 GHz), which is critical for many radar and telecommunications applications, with a vector network analyzer (Agilent Technologies E8362B) according to the standardized rectangular waveguide method. For the preparation of test specimens, the nickel-coated graphite powders with different Ni contents are used as functional fillers. These are uniformly mixed with paraffin, serving as a non-dispersive binder matrix, at a fixed mass fraction of 30% on a heated plate to ensure homogeneity. The resulting mixtures are then precision-pressed into rectangular samples with standardized dimensions of 22.86 mm in length, 10.16 mm in width, and a thickness of 2.0 mm to fit the waveguide fixtures.
The findings of this research provide detailed insights into the microstructure-property relationships. It is observed that as the nickel content increases, the population of nickel particles on the graphite surface correspondingly rises. These particles begin to interconnect with one another, causing the coating to evolve from a discrete island structure to a more continuous and denser film. However, a critical threshold is identified; when the nickel content is excessively high, it leads to an increase in free nickel, pronounced agglomeration of particles, and a phenomenon prone to explosive nucleation, which is detrimental to coating uniformity. The optimal microwave absorption performance is achieved with a specific Ni content of 45% and a reduced sample thickness of 1.6 mm. Under these optimized conditions, the composite sample exhibits a superior minimum reflection loss of -26.5 dB at a frequency of 10 GHz. Furthermore, it demonstrates an effective absorption bandwidth (defined as the frequency range where reflection loss is below -10 dB) spanning from 9.3 to 10.7 GHz. This research conclusively demonstrates that an excessively dense and continuous coating is, in fact, detrimental to the microwave absorption performance of nickel-coated graphite. The optimal performance is achieved when the flake graphite surface is decorated with an appropriate amount of discrete, irregular nickel particles, which enhances impedance matching and promotes multiple scattering and dissipation of incident microwave energy. The compelling microwave absorption properties and the facile, economical fabrication process of nickel-coated graphite, as established in this research, suggest significant potential for its future commercialization and expansion into practical applications. These include the next generation of radiation-protective fabrics, smart specialty textiles, and other functional composites in the field of industrial textiles.
Key words
nickel-coated graphite /
microwave absorption /
electroless plating /
composites /
reflection loss /
interface
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Funding
Funded Project of the Generic Technology R＆D Platform of Shaanxi Province (2024ZG-GXPT-05)
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