Cenospheres are hollow particles in fly ash, a by-product of coal burning, and are widely used as a reinforcement when developing low-density composites called syntactic foams. This study has investigated the physical, chemical, and thermal properties of cenospheres obtained from three different sources, designated as CS1, CS2, and CS3, for the development of syntactic foams. Cenospheres with particle sizes ranging from 40 to 500 μm were studied. Different particle distribution by size was observed, and the most uniform distribution of CS particles was in the case of CS2: above 74% with dimensions from 100 to 150 μm. The CS bulk had a similar density for all samples and amounted to around 0.4 g·cm⁻³, with a particle shell material density of 2.1 g·cm⁻³. Post-heat-treatment samples showed the development of a SiO₂ phase in the cenospheres, which was not present in the as-received product. CS3 had the highest quantity of Si compared to the other two, showing the difference in source quality. Energy-dispersive X-ray spectrometry and a chemical analysis of the CS revealed that the main components of the studied CS were SiO₂ and Al₂O_3. In the case of CS1 and CS2, the sum of these components was on average from 93 to 95%. In the case of CS3, the sum of SiO₂ and Al₂O₃ did not exceed 86%, and Fe₂O₃ and K₂O were present in appreciable quantities in CS3. Cenospheres CS1 and CS2 did not sinter during heat treatment up to 1200 °C, while sample CS3 was already subjected to sintering at 1100 °C because of the presence of a quartz phase, Fe₂O₃ and K₂O. For the application of a metallic layer and subsequent consolidation via spark plasma sintering, CS2 can be deemed the most physically, thermally, and chemically suitable.

Cenospheres are hollow particles in fly-ash, a by-product of coal burning, and are widely used as reinforcement for developing low density composites called syntactic foams. This study investigates the physical, chemical, and thermal properties of cenospheres obtained from 3 different sources, CS1, CS2, and CS3, for the development of syntactic foams. Description of floatation method to separate broken particles is given, and it was seen that up to 11 % of the particles were damaged. Post heat treatment samples show development of SiO2 phase in the cenosphere, which is not present in the as received product. CS3 had the highest quantity of Si element, compared to the other two, showing the difference in the source quality. The particle size distribution for CS2 is very narrow while for the others is much broader. All censopheres have porous walls but the morphology of CS2 is the most uniform and smooth. For the application of metallic layer and subsequent consolidation via spark plasma sintering, CS2 was deemed the most physically, thermally, and chemically suitable.

The single-phase NiFe2O4 and CoFe2O4 ferrites were synthesized by four methods: the high-frequency plasma chemical synthesis (“plasma”), sol-gel self-propagating combustion method (“combust”), and co-precipitation technology, combined with the hydrothermal synthesis (“hydrotherm”) or spray-drying (“spray”). The specific surface area (SSA), crystallite size, and magnetic properties of the synthesized products have been determined. The synthesized ferrites are nanocrystalline single-phase materials with crystallite size of 5-40 nm. The SSA of nanoparticles synthesized in plasma is 28-30 m2/g, the particle size distribution is in the range of 10-100 nm, with some individual particles up to 200 nm. The SSA of the ferrites obtained by the self-combustion and hydrothermal synthesis is 40 ± 3 and 60 ± 5 m2/g, respectively. The SSA of the samples obtained by the spray-drying method is 80-90 m2/g, and the calculated particle size is 13-15 nm. In this process, pellets up to 10 μm are obtained. After synthesis, CoFe2O4 are characterized by the saturation magnetization Ms of 75 emu/g (“plasma”), 53 emu/g (“combust”) and 57 emu/g (“hydrotherm”). The Ms of NiFe2O4 is 44, 29, and 30 emu/g, respectively. The products obtained by the spray-drying method are partially X-ray amorphous and show magnetic properties only after heating above 450°C. These nanopowders were used in sintering studies.

The present study focuses on the synthesis and characterisation of a lightweight ceramic material with electromagnetic interference (EMI) shielding properties, achieved using mullite containing micrometre-sized hollow spheres (cenospheres) and CoFe₂O₄ nanoparticles. This research explores compositions with varying CoFe₂O₄ contents ranging from 0 up to 20 wt.%. Conventional sintering in an air atmosphere is carried out at a temperature between 1100 and 1300 °C. The addition of ferrite nanoparticles was found to enhance the process of sintering cenospheres, resulting in improved material density and mechanical properties. Furthermore, this study reveals a direct correlation between the concentration of ferrite nanoparticles and the electromagnetic properties of the material. By increasing the concentration of ferrite nanoparticles, the electromagnetic shielding effect of the material (saturation magnetisation (M_s) and remanent magnetisation (M_r)) was observed to strengthen. These findings provide valuable insights into designing and developing lightweight ceramic materials with enhanced electromagnetic shielding capabilities. The synthesized ceramic material holds promise for various applications that require effective electromagnetic shielding, such as in the electronics, telecommunications, and aerospace industries.

Solid solutions of Co_1−xZn_xFe₂O₄ and Ni_1−xZn_xFe₂O₄ (0 < x < 1) nanoparticles were synthesized by sol-gel self-propagating combustion method. The obtained single cubic phase product has a specific surface area 25 m²∙g⁻¹ to 33 m²∙g⁻¹ and crystallite size 25 nm to 40 nm. Lattice parameters change linearly from 8.371 A (CoFe₂O₄) and 8.337 A (NiFe₂O₄) to 8.431 A (ZnFe₂O₄). The saturation magnetization (Ms) changes non-linearly from 60.8 emu∙g⁻¹ (CoFe₂O₄), respectively, from 35.6 emu∙g⁻¹ (NiFe₂O₄) to 3.3 emu∙g⁻¹ (ZnFe₂O₄) reaching maximal value 76.1 emu∙g⁻¹ for Co_0.8Zn_0.2Fe₂O₄ and 64.9 emu∙g⁻¹ – for Ni_0.6Zn_0.4Fe₂O₄.

The ferrite materials, especially of nickel and cobalt, are popular due to their unique mechanical and magnetic properties. The single phase NiFe₂O₄ and CoFe₂O₄ ferrites were synthesized by three methods: the high frequency plasma chemical synthesis, the sol-gel self-combustion, and the co-precipitation technology combined with the hydrothermal synthesis method. Magnetic properties, crystallite size, the specific surface area of synthesized products are characterized. Nanopowders synthesized in the high frequency plasma have a specific surface area in the range of 28–30 m²/g (the average particle size 38–40 nm, crystallite size ~40 nm), but a wider particle size distribution is in the range of 10–100 nm with some particles of 200 nm. The ferrite nanopowders obtained by the sol-gel self-combustion and hydrothermal synthesis method have the specific surface area of 40 ± 3 and 60 ± 5 m²/g (average particle size 25–30 nm and 19–26 nm, crystallite size 10–20 nm and 12–20 nm), respectively. Cobalt ferrites after synthesis are characterized by the saturation magnetization M_S of 75 emu/g (for plasma chemical synthesis), 53 emu/g (for the self-combustion method), and 57 emu/g (hydrothermal synthesis). The saturation magnetization for nickel ferrites is 44, 29 and 30 emu/g, respectively. After the thermal treatment up to 900 ºC the particle size of all ferrites increases, but their magnetic properties are approaching those of monolithic ferrites. DOI: http://dx.doi.org/10.5755/j01.ct.67.1.15824