Ary Machado de Azevedo

Natural lignocellulosic fibers are potential reinforcements for lightweight polymer composites, but their geometrical heterogeneity complicates mechanical characterization and material selection. This study investigated the influence of fiber diameter on the tensile behavior of commercial Typha fibers through integrated geometrical, morphological, chemical, thermal, mechanical, and statistical analyses. The fibers exhibited an apparent geometrical density of approximately 0.71 g/cm³, a moisture-related mass loss of 10.62%, and a main thermal degradation onset at 278.61 °C. Tensile properties varied markedly among diameter classes: mean tensile strength decreased from 438.85 MPa for fibers measuring 0.06–0.16 mm to 37.31 MPa for fibers measuring 0.50–0.80 mm, while Young’s modulus decreased from 7.49 to 1.43 GPa. An empirical power-law model described the relationship between class-averaged tensile strength and diameter (R² = 0.983). Weibull moduli ranged from 2.71 to 8.30 within individual diameter classes but decreased to 1.25 when the classes were pooled. These findings indicate that combining dimensionally distinct fiber populations can substantially broaden the apparent strength distribution. Although the influence of cross-sectional geometry and intrinsic structural variability could not be separated, the results demonstrate the importance of diameter-aware characterization for reliable mechanical-property reporting and preliminary reinforcement selection. Further composite-level investigations are required to establish the practical implications of these findings.

Composite materials, particularly those with fiber-reinforced polymer matrices, align well with current demands for combining low density with high mechanical strength. This has motivated extensive research into natural lignocellulosic fiber-reinforced polymeric matrices. In this context, the present study evaluates the toughness of untreated and alkali-treated 30 vol.% sedge fiber (Cyperus malaccensis)-reinforced epoxy matrix composites. This study provides a comprehensive assessment of dynamic and quasi-static mechanical properties. The dynamic toughness performance was evaluated through Izod and Charpy impact, whereas quasi-static toughness was evaluated using tensile, flexural, and compressive tests. To allow for a proper comparison, toughness units were all considered in joules (J) by taking into account the specimen dimensions. The fracture surfaces were examined to evaluate the influence of fiber treatment on failure mechanisms. Scanning electron micrographs depicted a substantial reduction in interfacial delamination of composites reinforced with treated fibers, suggesting improved fiber–matrix adhesion. The results revealed a significant difference attributable to the alkali treatment. Specifically, the treated-fiber composite group showed the highest impact energies, achieving values of 7.84 J for Izod and 15.5 J for Charpy. The alkali-treated fiber composites exhibited quasi-static energy absorption values of 2.78 J, 4.32 J, and 11.06 J under tensile, flexural, and compressive loading, respectively.

This work investigates the development of multifunctional bioceramic composites based on hydroxyapatite reinforced with the lead-free piezoelectric phase sodium potassium niobate, Na_0.5K_0.5NbO₃. Hydroxyapatite powder was synthesized by aqueous precipitation and combined with sodium potassium niobate synthesized by solid-state reaction to produce bulk ceramics containing 0, 10, 20, and 30 wt.% sodium potassium niobate, consolidated by conventional sintering. The materials were characterized in terms of density, linear shrinkage, phase composition, microstructure, elemental composition, flexural strength, and antibacterial response. X-ray diffraction analysis confirmed the coexistence of hydroxyapatite and orthorhombic sodium potassium niobate, with the intensity of sodium potassium niobate-related reflections increasing with its content, together with minor secondary phases. Scanning electron microscopy revealed the progressive incorporation of sodium potassium niobate grains into the hydroxyapatite matrix and showed a strong dependence of porosity on sodium potassium niobate content. Flexural tests showed that intermediate sodium potassium niobate additions improved strength compared with monolithic hydroxyapatite, whereas highly porous compositions exhibited reduced mechanical performance. Agar-diffusion tests against representative Gram-positive and Gram-negative strains showed measurable inhibition zones, although the differences between pure hydroxyapatite and composites containing sodium potassium niobate were limited and species-dependent. Slight increases were observed for P. aeruginosa and S. aureus, whereas no appreciable changes were detected for the other strains. Therefore, these findings should be regarded as preliminary evidence of antibacterial response rather than as confirmation of an intrinsic antibacterial effect of sodium potassium niobate. Overall, the structural and mechanical results indicate that hydroxyapatite/sodium potassium niobate composites are promising lead-free candidates for the development of multifunctional bioceramics, while their antibacterial response requires further validation.

Hydroxyapatite (HAp) is a key bioceramic for biomedical applications, but conventional pressureless sintering (PS) requires high temperatures that can promote phase degradation. Here, we compare PS (1100 °C/180 min) and cold sintering process (CSP) (150 °C/450 MPa/30 min) for pure HAp and an HAp composite containing 4 wt.% niobium–phosphate bioglass (BG), using a 2 M H₃PO₄ transient liquid (10 wt.%). CSP increased relative density from 73.10% to 79.92% for HAp and from 68.43% to 83.54% for HAp/BG, representing up to a 22.1% gain compared with PS. One-way ANOVA confirmed a significant effect of processing route/composition on relative density (F(3,24) = 919.69, p < 0.05), and Tukey HSD indicated that all groups differed statistically. SEM revealed a markedly more consolidated and homogeneous microstructure for CSP, particularly for HAp/BG, consistent with enhanced dissolution–reprecipitation and pore filling. XRD showed that PS at 1100 °C led to partial HAp degradation with β-TCP formation, whereas CSP preserved the HAp phase with broader peaks, smaller crystallite size, and higher specific surface area. These results demonstrate CSP as an efficient low-temperature alternative for densifying HAp-based bioceramics, with BG addition further improving consolidation.

Conductive composites are a flexible class of engineered materials that combine conductive fillers with an insulating matrix—usually made of ceramic, polymeric, or a hybrid material—to customize a system’s electrical performance. By providing tunable electrical properties in addition to benefits like low density, mechanical flexibility, and processability, these materials are intended to fill the gap between conventional insulators and conductors. The increasing need for advanced technologies, such as energy storage devices, sensors, flexible electronics, and biomedical interfaces, has significantly accelerated their development. The electrical characteristics of composite materials, including metallic, ceramic, polymeric, and nanostructured systems, are thoroughly examined in this review. The impact of various reinforcement phases—such as ceramic fillers, carbon-based nanomaterials, and metallic nanoparticles—on the electrical conductivity and dielectric behavior of composites is highlighted. In addition to conduction models like correlated barrier hopping and Debye relaxation, the study investigates mechanisms like percolation thresholds, interfacial polarization, and electron/hole mobility. Because of the creation of conductive pathways and improved charge transport, developments in nanocomposite engineering, especially with regard to graphene derivatives and silver nanoparticles, have shown notable improvements in electrical performance. This work covers the theoretical underpinnings and physical principles of conductivity and permittivity in composites, as well as experimental approaches, characterization methods (such as SEM, AFM, and impedance spectroscopy), and real-world applications in fields like biomedical devices, sensors, energy storage, and electronics. This review provides important insights for researchers who want to create and modify multifunctional composite materials with improved electrical properties by bridging basic theory with technological applications.

This study presents the design, development, and experimental validation of a compact electronic circuit for gamma radiation detection, specifically engineered for integration into UAV platforms. The system employs a 1” NaI (Tl) scintillation detector, along with customized preamplifier and amplifier stages optimized for low power consumption and reduced dimensions. Experimental tests using a sealed Cs-137 source at multiple distances (0, 1, 2, and 3 meters) demonstrated that the circuit maintains spectral integrity up to 1 meter, with performance degrading at greater distances due to air attenuation and reduced interaction statistics. Importantly, no detector saturation was observed even at zero distance. The study confirms the feasibility of the proposed electronics for drone-based radiological monitoring in short-range operations. Further improvements may include adapting portable high-voltage sources or employing alternative sensors, such as photodiode arrays, to enhance system practicality and reduce payload. These findings contribute to the advancement of miniaturized nuclear instrumentation for rapid deployment in radiological safety and emergency response scenarios.