A Comparative Study on Crack-Healing Ability of Al2O3/SiC Structural Ceramic Composites Synthesized by Microwave Sintering and Conventional Electrical Sintering

This study was conducted to assess and compare the crack-healing ability of conventional electrical sintered and microwave sintered Al2O3/x wt. % SiC (x = 5, 10, 15, and 20) structural ceramic composites. The crack-healing ability of both conventional electrical sintered and microwave sintered specimens was studied by introducing a crack of ∼100 µm length by Vickers’s indentation and conducting a heat treatment at 1200°C for dwell time of 1 h in air. The flexural or bending strength of sintered, cracked, and crack-healed specimens was determined by three-point bending test, and the phase variations by X-ray diffraction and SEM micrographs before and after crack-healing of both the sintering methods were studied and compared. The results show that almost all the specimens recovered their strength after crack-healing, but the strength of microwave sintered Al2O3/SiC structural ceramic composites has been shown to be better than that of conventional electrical sintered Al2O3/SiC structural ceramic composites. The microwave sintered crack-healed Al2O3/10 wt. % SiC specimen shows higher flexural strength of 794 MPa, which was 105% when compared with conventional electrical sintered Al2O3/10 wt. % SiC and crack-healed Al2O3/10 wt. % SiC specimen. It was found by X-ray diffractogram that before crack-healing, all the conventional electrical sintered samples have SiO2 phase which reduce the crack-healing ability and microwave sintered samples with 15 and 20 wt. % SiC show lesser SiO2 phase and 5 and 10 wt. % SiC samples have no SiO2 phase before crack-healing. However, after crack-healing treatment, all the samples have distinct SiO2 phase along with Al2O3 and SiC phases. Microwave sintered Al2O3/10 wt. % SiC specimen cracks were fully healed which was evident in SEM micrographs.

Identification of Sand Mineral Content at Beach Tourist Attractions in Sampang Regency through X-Ray Fluorescence and X-Ray Diffraction Testing

Research on the natural mineral content in Sampang Regency has been conducted, namely Camplong Beach, Nepa Beach, and Mandangin Island. The XRF results show that the silica mineral content in Camplong sand at a depth of 0.5 m, 1 m, and 1.5 m are 54.6%, 53.2%, and 57.0%, respectively. Nepa sand shows the highest mineral content is calcium. The calcium content at a depth of 0.5 m, 1 m, and 1.5 m are 51.7%, 58.3%, and 63.0%, respectively. Mandangin Island sand shows the highest calcium content that are 94.69%, 94.65%, and 94.98%, at a depth of 0.5 m, 1 m, and 1.5 m, respectively. XRD test results show that in Camplong sand at a depth of 0.5 m, 1 m, and 1.5 m, SiO2 phase < 94% is formed. In Nepa sand at a depth of 0.5 m, 1 m, and 1.5 m, the SiO2 phase < 80% is formed. For Mandangin Island beach sand with a depth of 0.5 m, 1 m, and 1.5 m, the CaCO3 phase > 90% is formed. This study's results can determine the mineral content of sand, which can be used as a high-tech material to minimize illegal mining.

Crack Healing in Mullite-Based EBC during Thermal Shock Cycle

Crack healing phenomena were observed in mullite and mullite + Yb2SiO5 environmental barrier coating (EBC) materials during thermal shock cycles. Air plasma spray coating was used to deposit the EBC materials onto a Si bondcoat on a SiCf/SiC composite substrate. This study reveals that unidirectional vertical cracks (mud cracks) formed after several thermal shock cycles; however, the cracks were stable for 5000 thermal shock cycles at a maximum temperature of 1350 °C. Moreover, the crack densities decreased with an increasing number of thermal shock cycles. After 3000 thermal shock cycles, cracks were healed via melting of a phase containing SiO2 phase, which partially filled the gaps of the cracks and resulted in the precipitation of crystalline Al2O3 in the mullite. Post-indentation tests after thermal shock cycling indicated that the mullite-based EBC maintained its initial mechanical behavior compared to Y2SiO5. The indentation load–displacement tests revealed that, among the materials investigated in the present study, the mullite + Yb2SiO5 EBC demonstrated the best durability during repetitive thermal shocks.

Sol-Gel and Electrospinning Synthesis of Lithium Niobate-Silica Nanofibers

Lithium niobate-silica fibers were produced by the combination of the sol-gel method and the electrospinning technique. Two sol-gel solutions starting from niobium-lithium ethoxide and tetraethyl orthosilicate were prepared and then mixed with polyvinylpyrrolidone; the solutions were electrospun in a coaxial setup. The obtained lithium niobate-silica polymeric fibers were approximately 760 nm in diameter. Raman spectroscopy confirmed the composite composition by showing signals corresponding to lithium niobate and silica. Scanning electron microscopy showed coaxial fibers with a diameter of around 330 nm arranged as a fibrillar membrane at 800 °C. At 1000 °C the continuous shape of fibers was preserved; the structure is composed of silica and lithium niobate nanoparticles within the fibers. The formation of crystalline lithium niobate and amorphous SiO2 phase was also confirmed by XRD peaks.

Specific Multiphase Assemblages of Carbonatitic and Al-Rich Silicic Diamond-Forming Fluids/Melts: TEM Observation of Microinclusions in Cuboid Diamonds from the Placers of Northeastern Siberian Craton

The microinclusions in cuboid diamonds from Ebelyakh River deposits (northeastern Siberian craton) have been investigated by FIB/TEM techniques. It was found that these microinclusions have multiphase associations, containing silicates, oxides, carbonates, halides, sulfides, graphite, and fluid phases. The bulk chemical composition of the microinclusions indicates two contrasting growth media: Mg-rich carbonatitic and Al-rich silicic. Each media has their own specific set of daughter phases. Carbonatitic microinclusions are characterized by the presence of dolomite, phlogopite, apatite, Mg, Fe-oxide, KCl, rutile, magnetite, Fe-sulfides, and hydrous fluid phases. Silicic microinclusions are composed mainly of free SiO2 phase (quartz), high-Si mica (phengite), Al-silicate (paragonite), F-apatite, Ca-carbonates enriched with Sr and Ba, Fe-sulfides, and hydrous fluid phases. These associations resulted from the cooling of diamond-forming carbonatitic and silicic fluids/melts preserved in microinclusions in cuboid diamonds during their ascent to the surface. The observed compositional variations indicate different origins and evolutions of these fluids/melts.