Sustainable and Environmentally Friendly Microwave Synthesis of Nano-Hydroxyapatite from Decarbonized Eggshells.

The sustainable microwave (MW) synthesis of hydroxyapatite (HAp) from decarbonized eggshells was investigated. Decarbonization of eggshells, as a natural source of calcium carbonate (CaCO3), was carried out in the current study at ambient conditions to reduce the footprint of CO2 emissions on our environment where either calcination or acidic direct treatments of eggshells produce CO2 emissions, which is a major cause for global warming. Eggshell decarbonization was carried out via the chemical reaction with sodium hydroxide (NaOH) alkaline solution in order to convert eggshell waste into calcium hydroxide (Ca(OH)2) and simultaneously store CO2 as a sodium carbonate (Na2CO3) by-product which is an essential material in many industrial sectors. The produced Ca(OH)2 was mixed with ammonium dihydrogen phosphate (NH4H2PO4) reagent at pH~11 before being subjected to MW irradiation at 2.45 GHz frequency for 5 min using 800 Watts to prepare HAp. The prepared Nano-HAp was characterized using X-ray diffraction (XRD) where the crystal size was ~28 nm using the Scherrer equation. The elongated rod-like nano-HAp crystals were characterized using scanning electron microscopy (SEM) equipped with dispersive energy X-ray spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR), and transmission electron microscopy (TEM). MW synthesis of decarbonized eggshells is considered as a sustainable and environmentally friendly route to produce promising bioceramics such as nano-HAp. Concurrently, decarbonization of eggshells offers the ability to store CO2 as a high value-added Na2CO3 material.

Characterization of the Native Oxide Shell of Copper Metal Powder Spherical Particles.

The native oxide layer that forms on copper (Cu) metal spherical particle surfaces under ambient handling conditions has been shown to have a significant effect on sintering behavior during microwave heating in a previous study, where an abnormal expansion was observed and characterized during sintering of Cu compacts using reducing gases. Because microwave (MW) heating is selective and depends greatly on the dielectric properties of the materials, this thin oxide layer will absorb MW energy easily and can consequently be heated drastically starting from room temperature until the reduction process occurs. In the current study, this oxide ceramic layer was qualitatively and quantitatively characterized using the carrier gas hot extraction (CGHE) method, Auger electron spectroscopy (AES), and a dual-beam focused ion beam (FIB)/scanning electron microscope (SEM) system that combines both FIB and SEM in one single instrument. Two different commercial gas-atomized spherical Cu metal powders with different particle sizes were investigated, where the average oxygen content of the powders was found to be around 0.575 wt% using the CGHE technique. Furthermore, AES spectra along with depth profile measurements were used to qualitatively characterize this oxide layer, with only a rough quantitative thickness approximation due to method limitations and the electron beam reduction effect. For the dual-beam FIB-SEM system, a platinum (Pt) coating was first deposited on the Cu particle surfaces prior to any characterization in order to protect and to preserve the oxide layer from any possible beam-induced reduction. Subsequently, the Pt-coated Cu particles were then cross-sectioned in the middle in situ using an FIB beam, where SEM micrographs of the resulted fresh sections were characterized at a 36° angle stage tilt with four different detector modes. Quantitative thickness characterization of this native oxide layer was successfully achieved using the adapted dual-beam FIB-SEM setup with more accuracy. Overall, the native Cu oxide layer was found to be inhomogeneous over the particles, and its thickness was strongly dependent on particle size. The thickness ranged from around 22-67 nm for Cu powder with a 10 µm average particle size (APS) and around 850-1050 nm for one with less than 149 µm.

Microwave Crystallization of Lithium Aluminum Germanium Phosphate Solid-State Electrolyte.

Lithium aluminum germanium phosphate (LAGP) glass-ceramics are considered as promising solid-state electrolytes for Li-ion batteries. LAGP glass was prepared via the regular conventional melt-quenching method. Thermal, chemical analyses and X-ray diffraction (XRD) were performed to characterize the prepared glass. The crystallization of the prepared LAGP glass was done using conventional heating and high frequency microwave (MW) processing. Thirty GHz microwave (MW) processing setup were used to convert the prepared LAGP glass into glass-ceramics and compared with the conventionally crystallized LAGP glass-ceramics that were heat-treated in an electric conventional furnace. The ionic conductivities of the LAGP samples obtained from the two different routes were measured using impedance spectroscopy. These samples were also characterized using XRD and scanning electron microscopy (SEM). Microwave processing was successfully used to crystallize LAGP glass into glass-ceramic without the aid of susceptors. The MW treated sample showed higher total, grains and grain boundary ionic conductivities values, lower activation energy and relatively larger-grained microstructure with less porosity compared to the corresponding conventionally treated sample at the same optimized heat-treatment conditions. The enhanced total, grains and grain boundary ionic conductivities values along with the reduced activation energy that were observed in the MW treated sample was considered as an experimental evidence for the existence of the microwave effect in LAGP crystallization process. MW processing is a promising candidate technology for the production of solid-state electrolytes for Li-ion battery.