Synthesis and Investigation of Lightweight Sintered Hollow Metallic Spheres

This study investigates the use of Hollow Metallic Spheres (HMS) to enhance the properties of Metal Matrix Composite Syntactic Foam (MMCSF) and Triboelectric Nanogenerators (TENGs), achieving substantial improvements in material consistency, structural integrity, and mechanical resilience. HMS contribute to enhanced porosity and impact energy absorption in MMCSF, while in TENG applications, they serve as effective triboelectric layers due to their lower work function and high dielectric constant, enabling efficient electron transfer and charge storage. The synthesis of HMS involves coating Expanded Polystyrene (EPS) spheres with a metal slurry in a fluidised bed spray system (FBS). Metal powders mixed with a PVA binder are sprayed onto EPS spheres, with Statistical Response Surface Methodology (RSM) optimising parameters like airflow, inlet air temperature, atomising air pressure, and spray rate. FBS ensures uniform coating and prevents agglomeration, achieving a prediction accuracy of R² = 0.9965. SEM analysis confirmed an average shell thickness of approximately 340 µm for Al coated EPS (AlESp) and 110 µm for FeCrNi alloy coated EPS spheres (CS). The EPS core is removed through controlled debinding to produce Green Spheres (GS), followed by high-vacuum sintering. Supersolidus Liquid Phase Sintering (SLPS) at 650°C for Al coated spheres and 1250°C for FeCrNi coated spheres produced optimal density and minimal porosity. Comprehensive chemical, thermal characterisation, and mechanical tests confirmed the structural integrity and purity of HMS, with AlHMS sintered at 650°C yielding the highest collapse strength (2.54 ± 0.15 MPa at a 0.01 s⁻¹ strain rate) and lowest porosity (22 ± 3%). FeCrNi alloy HMS sintered at 1250°C showed reduced surface defects and densities of 0.434 g/cm³ for AlHMS, 0.91 g/cm³ for smaller spheres (SS), and 0.54 g/cm³ for larger spheres (SB), with smaller FeCrNi spheres (2.57 ± 0.48 mm) achieving a collapse strength of 9.1 ± 0.7 MPa at a 0.01 s⁻¹ strain rate. HMSs are chosen for TENG applications due to their robust strength, high elastic limit, conductivity, superior yield strength, and corrosion resistance. Micro-porous structures and consistent chemical composition are crucial for HMS's mechanical robustness and functional performance. Additionally, electronegativity differences and higher work function, compared to EPS spheres, enhance output voltage during HMS based TENG operation. Chapter 6 demonstrates that, for TENG applications, FeCrNi HMS outperformed EPS with a work function of 4.23 ± 0.2 eV versus 4.49 ± 0.2 eV for EPS and a dielectric constant reaching 3.8×10⁷ at 2 MHz, leading to a markedly improved energy output. HMS based TENGs generated an output of 25 V under minimal pressure, significantly outperforming EPS based TENGs. This study highlights the effectiveness of optimised coating and sintering processes in producing HMS with robust physiochemical properties, positioning HMS as a competitive material for MMCSF and TENG applications. These findings open new pathways for lightweight, high-performance solutions in structural and functional roles, marking a significant advancement in HMS synthesis and application.<p></p>