Improvement of the hydroxyapatite mechanical properties by direct microwave sintering in single mode cavity

The main purpose of this study consists in investigating the direct microwave sintering of hydroxyapatite (HA) in a single mode cavity. Firstly, stoichiometric HA powders were synthesized by a coprecipitation method from diammonium phosphate and calcium nitrate solutions and shaped by slip-casting. Then, using the one-step microwave process, dense pellets with fine microstructures were successfully obtained in short sintering timespan. A parametric study permitted to determine the influence of powder grain size, sintering temperature and dwell time on the sintered samples microstructures. The Young's modulus (E) and hardness (H) were measured by nanoindentation and the values discussed according to the microstructure. Finally, the resulting mechanical properties determined on the microwave sintered samples (E = 148.5 GPa, H = 9.6 GPa, σ_compression = 531.3 MPa and K_IC = 1.12 MPa m^1/2) are significantly higher than those usually reported in the literature, whatever the sintering process, and could allow the use of HA for structural applications.     

Characterization of magnesium-hydroxyapatite functionally graded composites prepared by rapid microwave sintering technique

The use of the microwave technique for the sintering of materials has been noticed due to the reduction in processing time, the achievement of uniform microstructure, and high effectiveness. In this study, magnesium/hydroxyapatite (Mg/HA) functionally graded materials (FGMs) in the form of a 2-layer, 3-layer, and 4-layer samples with different contents of HA (0, 5, 10, and 15 wt%) and also single-layer pure Mg samples were successfully fabricated by powder metallurgy and rapid microwave sintering technique. For this purpose, the prepared powders were first mixed and then poured into a die in a layer-by-layer manner at a predetermined arrangement. The stacked powders were compacted under a pressure of 300 MPa and sintered via microwave technique at 530 °C for 12 min under an argon atmosphere. The samples were investigated by the Archimedes test, optical microscope (OM), scanning electron microscopy (SEM), mechanical and corrosion tests. Achieving high relative densities in a short time for pure Mg (99.5%) and its multi-layer composites suggested the high effectiveness of this technique in processing these materials. In addition to the uniform distribution of HA particles in the Mg matrix, the microstructural studies confirmed the proper inter-layer bonding. The mechanical properties of the samples were evaluated by the microhardness indentation method and the compression test. The results showed that the microhardness and compressive strength of the layered samples is much higher than pure Mg. The highest microhardness (63 ± 5 Hv) and compressive strength (148 ± 10 MPa) were observed in the 3-layer sample. The corrosion behavior of the samples was also examined in simulated body fluid (SBF) at 37 °C through immersion tests which indicated superior corrosion resistance of layered samples compared to pure Mg. The 3-layer sample exhibited the best corrosion behavior.     

Enhanced microwave absorption properties of Zn-substituted SrW-type hexaferrite composites in the Ku-band

Enhanced microwave absorption properties were successfully achievable from SrFe_2-xZn_xFe₁₆O₂₇ (SrFe_2-xZn_xW; x = 0.0, 0.5, 1.0, and 2.0) hexaferrite filler-epoxy resin matrix composites. The composite samples were fabricated with the filler volume fractions (V_f) of 30, 50, 70, and 90%. Compared with fully Zn-substituted SrZn₂W composite (x = 2.0), unsubstituted and partially Zn-substituted SrFe_2-xZn_xW (x = 0.0, 0.5, and 1.0) composites exhibited much higher real and imaginary parts of complex permittivity (ε_r), which is attributable to higher electron hopping between Fe²⁺ and Fe³⁺ ions, and also slightly higher real and imaginary parts of complex permeability (μ_r) due to higher saturation magnetization (M_s). Among all samples, a 2.8 mm-thick SrFe_1·5Zn_0·5W (x = 0.5) composite with the V_f of 90% exhibited the most appropriate for application in the region of 3.4–3.8 GHz, having the minimum reflection loss (RL_min) of ?46 dB at 3.6 GHz with the bandwidth of 0.43 GHz (3.38–3.81 GHz) below ?10 dB, while a 2.15 mm-thick SrFeZnW (x = 1.0) composite with the V_f of 70% showed the most appropriate for application in the region of 5.9–7.1 GHz, possessing the RL_min value of ?23.7 dB at 6.6 GHz with the bandwidth of 1.38 GHz (5.85–7.23 GHz) below ?10 dB. Consequently, partially Zn-substituted SrW-type hexaferrites are very promising microwave absorbers for 5G mobile communications in the Ku band (0.5–18 GHz).     

Flexural behaviors and microstructures of SiC/SiC composites fabricated by microwave sintering assisted with heat molding process

To improve densification degree and reduce process time, microwave sintering and heat molding method were combined to prepared SiC matrix reinforced SiC (SiC/SiC) composite via polymer infiltration and pyrolysis process (PIP). The effects of heat molding pressures on the densification process, flexural behaviors and failure modes of the fabricated SiC/SiC were examined via scanning electron microscopy (SEM), computed tomography (CT) technique and mercury intrusion test. Results indicate that heat molding process promoted the densification degrees of SiC/SiC and adjusted the interphase bonding between SiC matrix and SiC fibers on the basis of rapid microwave heating. Owing to the appropriate interphase bonding, SiC/SiC composites fabricated under the heat molding pressure of 3 MPa had preferable flexural properties and failure mode.     

Mechanical properties of the SiCf/SiC composites reinforced with KD-I and KD-II fibers fabricated assisted by a microwave heating method

SiC_f/SiC composites using KD-I and KD-II SiC fibers braided preforms as the reinforcements were fabricated by applying the polymer impregnation and pyrolysis (PIP) technique with a microwave heating assistance. The microwave heating temperature was 1100 °C, 1200 °C, 1300 °C, and 1400 °C, respectively. Microstructures, flexure properties, and fracture behaviors of the composites were investigated. The KDIISiC_f/SiC composites exhibited higher flexure properties and improved non-brittle fracture characteristics than those of the KD-ISiC_f/SiC composites. The differences in the flexural properties, fracture behaviors and microstructures between the KD-I and KDIISiC_f/SiC composites were discussed based on the tensile properties of the SiC filaments and the interfacial bonding statues in the composites.     

Microwave sintering study of strontium-doped lanthanum manganite in a single-mode microwave with electric and magnetic field at 2.45 GHz

The objective of this work is to study the changes in the physical and mechanical properties of strontium-doped lanthanum manganite (LSM) material and LSM-YSZ (ZrO₂ doped with 8 mol% yttria) composite, obtained by colloidal processing and sintered by 2.45 GHz microwave sintering at 1200 and 1300 °C using two different single-mode cavities. One circular cavity with TE₁₁₁ mode that has maximum in the electric field (E-field) and one rectangular cavity with TE₁₀₂ mode that has maximum in the magnetic field (H-field). As compared to conventional sintering at 1300 and 1400 °C, the microwave-heated samples exhibited a denser structure for shorter sintering times. LSM-based materials showed higher heating behavior in H-field, which translates into higher energy absorption. This fact can be attributed to an electromagnetic pressure induced by the combined effect of current loops subjected to H-field. Therefore, the interaction between the material and the electromagnetic waves depends on the dominant field of them.     

Ultra-low temperature sintering and microwave dielectric properties of a novel temperature stable Na2Mo2O7-Na0.5Bi0.5MoO4 ceramic

The novel ultra-low temperature sintering (1-x)Na₂Mo₂O₇-xNa_0.5Bi_0.5MoO₄ ceramics have been obtained via solid-state reaction method for passive integration use. The Na₂Mo₂O₇ and Na_0.5Bi_0.5MoO₄ crystal phases are found to be compatible with each other from the results of XRD and SEM-EDS. With the x value changing from 0.36 to 1.00, the ε_r increases from 16.0 to 32.0 and the τ_f value varies from ?58 to 47 ppm/°C. At x = 0.75, the 0.25Na₂Mo₂O₇-0.75Na_0.5Bi_0.5MoO₄ ceramic sintered at an ultra-low sintering temperature of 580 °C can be densified (>96%) and possesses good microwave dielectric properties of an ε_r of 24.0, a Q × f value of 13,000 GHz (at 6.2 GHz), and a τ_f value of 3 ppm/°C. The theoretical ε_r and τ_f of the (1-x)Na₂Mo₂O₇-xNa_0.5Bi_0.5MoO₄ composites were calculated using the mixing law and in accordance with the measured values.