Sol-gel zinc oxide nanoparticles: advances in synthesis and applications

Zinc oxide nanoparticles (ZnO) exhibit numerous characteristics such as biocompatibility, UV protection, antibacterial activity, high thermal conductivity, binding energy, and high refractive index that make them ideal candidates to be applied in a variety of products like solar cells, rubber, cosmetics, as well as medical and pharmaceutical products. Different strategies for ZnO nanoparticles’ preparation have been applied: sol-gel method, co-precipitation method, etc. The sol-gel method is an economic and efficient chemical technique for nanoparticle (NPs) generation that has the ability to adjust the structural and optical features of the NPs. Nanostructures are generated from an aqueous solution including metallic precursors, chemicals for modifying pH using either a gel or a sol as a yield. Among the various approaches, the sol-gel technique was revealed to be one of the desirable techniques for the synthesis of ZnO nanoparticles. In this review, we explain some novel investigations about the synthesis of zinc oxide nanoparticles via sol-gel technique and applications of sol-gel zinc oxide nanoparticles. Furthermore, we study recent sol-gel ZnO nanoparticles, their significant characteristics, and their applications in biomedical applications, antimicrobial packaging, drug delivery, semiconductors, biosensors, catalysts, photoelectron devices, and textiles.

Hydrogen evolution via noble metals based photocatalysts: A review

In recent decades, the use of photocatalysts in the evolution of hydrogen (H2) has received much attention. However, the use of the well-known titanium oxide and another photocatalyst as a base for noble metals is limited due to their major weakness in electron-hole pair separation. The use of cocatalysts can be a good way to overcome this problem and provide better performance for the evolution of hydrogen. In this review, suitable high-efficiency cocatalysts for solar hydrogen production have been thoroughly reviewed. New strategies and solutions were examined in terms of increasing the recombination of charge carriers, designing reactive sites, and enhancing the wavelengths of light absorption. Several new types of cocatalysts based on semiconductors in noble groups and dual metals have been evaluated. It is expected that these photocatalysts will be able to reduce the activation energy of reaction and charge separation. In this regard, the existing views and challenges in the field of photocatalysts are presented. The characteristics of monoatomic photocatalysts are reviewed in this manuscript and the latest advances in this field are summarized. Further, the future trends and upcoming research are also briefly discussed. Finally, this review presents noble metal-based photocatalysts for providing suitable photocatalysts on a larger scale and improving their applicability.

Microstructure and mechanical properties of Ti3SiC2 MAX phases sintered by hot pressing

The Ti3SiC2 used in this project has been purchased ready-made. This study aimed to investigate the effect of sintering temperature on samples' microstructure and mechanical properties, including three-point flexural strength, Vickers hardness, and fracture toughness. Therefore, Ti3SiC2 samples were sintered under a vacuum atmosphere at a pressure of 35 MPa for 30 minutes at two temperatures of 1500 °C and 1550 °C by hot pressing. The microstructure obtained from the fracture cross-section of the samples shows that by increasing the sintering temperature to 1550 °C, the microstructure of this sample becomes larger than the sintered sample at 1500 °C. Also, increasing the sintering temperature to 1550 °C causes the decomposition of Ti3SiC2 to TiC, which can be seen in the X-ray diffraction pattern (XRD). In addition, the relative density of the sintered sample at 1550 °C is 98.08% which is higher than that of the sintered sample at 1500 °C with the result of 89%. On the other hand, the three-point flexural strength (227.5 MPa), the Vickers hardness (~9 GPa), and the fracture toughness (8.6 MPa.m1/2) of the sintered sample at 1500 °C are higher due to the fine-grained structure.

Role of Ti3AlC2 MAX phase on characteristics of in-situ synthesized TiAl intermetallics. Part II: Phase evolution

In this research, the 2nd part of a series of papers on the processing and characterization of TiAl–Ti3AlC2 composites, the phase evolution during the manufacturing process was investigated by X-ray diffraction (XRD) analysis and Rietveld refinement method. Metallic Ti and Al powders with different amounts of previously-synthesized Ti3AlC2 additives (10, 15, 20, 25 and 30 wt%) were ball-milled and densified by spark plasma sintering (SPS) under 40 MPa for 7 min at 900 °C. Before the sintering process, XRD test verified that the powder mixtures contained metallic Ti and Al as well as Ti3AlC2 and TiC (lateral phase synthesized with Ti3AlC2) phases. In the sintered composites, the in-situ synthesis of TiAl and Ti3Al intermetallics as well as the presence of Ti3AlC2 and the formation and Ti2AlC MAX phases were disclosed. The weight percentage of each phase in the final composition of the samples and the crystallite size of different phases were calculated by the Rietveld refinement method based on the XRD patterns. The size of Ti3AlC2 crystallites in sintered samples was compared with the crystallite size of synthesized Ti3AlC2 powder.

Challenges toward applying UHTC-based composite coating on graphite substrate by spark plasma sintering

In this study, the UHTC-based composite layers where applied on the graphite substrates using SPS method to protect them against ablation. The protective layers had some defects and problems such as crack, fracture, separation, melting, and weak adhesion to the substrate. Several factors such as the thickness of composite layer, the number of protective layers, the SPS conditions (temperature, applied pressure, soaking time and mold), the chemical composition of the layers, the type of the substrate and the mismatch between the thermal expansion coefficients of the substrate and the applied layer(s) affected the quality and connection of the protective layer to the graphite substrate. The amount of additive materials influenced the melting phenomenon in the composite layer; for example, further MoSi2 in the layer led to more melting. The mismatch between the thermal expansion coefficients of the graphite substrate and the composite layer caused stresses during the cooling step, which resulted in cracks in the applied layer. Hence, proximity in the thermal expansion coefficients seems to be necessary for the formation of an acceptable adhesion between the layer and the substrate.

Sintering behavior and microwave dielectric properties of CaTi1-x(Nb1/2Al1/2)xO3

CaTi1-x(Nb1/2Al1/2)xO3 with x=0.1-0.5 ceramics were processed through solid state sintering. X-rays diffraction (XRD) patterns of the compositions showed that the samples have orthorhombic crystal structure with symmetry (Pbnm). The symmetry was further confirmed using Raman spectroscopy. A total of 13 Raman modes were detected, which were in agreement with the XRD results. Microstructure analysis of the samples showed porosity in the samples, presumably due to the substitution of Al, having high melting point. As the concentration of Al and Nb increased, relative permittivity (er), quality factor (Q×fo) and temperature coefficient of resonance frequency decreased. Optimum microwave dielectric properties were achieved for the composition x=0.5 sintered at 1650 °C for 8 h i.e., er ~27.09, Q×fo ~17378 GHz and tf ~ -2.5 ppm/°C.

Thermodynamically study of phase formation of Ni-Ti-Si nanocomposites produced by self-propagating high-temperature synthesis method

Understanding the phase formation mechanisms in self-propagating high-temperature synthesis from the thermodynamical aspect of view is important. In this study, the phase formation of the ternary system of nickel-titanium-silicon was studied by using the HSC software V6.0, and phase formation is predicted by calculating the adiabatic temperature of exothermic reaction between reagents. Then, by using X-ray diffractometer analysis, the results of the simulation were evaluated by experimental achievements. Results showed a good correlation between thermodynamical calculation and prediction with experimental. It could be concluded that the equilibrium mechanism is the dominant mechanism in phase formation in the SHS synthesis method. NiTiSi solid solution phase is obtained from the reaction between Ti5Si3 and Ni2Si and Ni.

Sintered transparent polycrystalline ceramics: the next generation of fillers for clarity enhancement in corundum

A significant proportion of mined natural corundum (ruby and sapphire) contain fractures, which negatively affects a gemstone’s clarity and value. Over the past decades, heat treatment techniques have been developed for either fracture healing, or filling to make such gems marketable. The clarity enhancement processes are mainly based on techniques which are either not durable, as in the case of lead silicate fillers, or do not yield perfect transmittance through a fracture, as in the case of borax based fluxes. Therefore, the gemstone treatment community is actively in pursuit of better techniques for clarity enhancement in corundum. Given that application of pressure is a recent advancement in the heat treatment processes of natural sapphire, it is essential to explore the possibilities regarding different outcomes such treatments can have. In this perspective paper, we have briefly described how application of pressure during heat treatments can lead to in-situ sintering of transparent polycrystalline ceramics within the fractures of corundum, which can result in clarity enhancement. Spinel-structure based fillers can be tailored to mimic corundum in terms of tribological, chemical and optical properties. Therefore, gemstones treated with such fillers will be durable, unlike currently used glass-based filler material. We also provide a possible explanation for ghost-fissures in sapphires heated under pressure, as being a by-product of in-situ sintering process of ceramic fillers that are thermodynamically compatible with Al2O3. The prospect of transparent polycrystalline ceramics in the gem and jewelry industry opens a new field of research in this area, given that ceramic fillers can outperform currently used methods and material for clarity enhancement in gemstones. In essence, we present a novel application for sintered transparent polycrystalline ceramics.

Tribological behavior and mechanical properties of friction stir processed HDPE/Fe-Fe3O4 composites

In the current work, high density polyethylene (HDPE) composites were fabricated via Friction Stir Processing (FSP). A two-phase Fe-Fe3O4 powder was used as the reinforcing agents. The extremely low cost powder was obtained from shot-blasting of as-forged low carbon steel components. X-ray diffraction (XRD) was used to phase analysis and evaluation of the purity of the as-received powder. The size distribution of the powder was determined by Laser Particle Size Analysis (LPSA). Also, Scanning Electron Microscopy (SEM) was employed to investigate the particles morphology. The processing used a cylindrical tool to impose the severe plastic deformation and material stirring in order to improve the mechanical properties and particles distribution. The tribological and mechanical properties of the fabricated samples were examined. According to the results, both the friction coefficient and specific wear rate of FSPed samples reduced remarkably. The hardness and tensile strength of the FSPed composites were higher than the FSPed HDPE samples; however, their elongations were lower.

Role of Ti3AlC2 MAX phase on characteristics of in-situ synthesized TiAl intermetallics. Part I: sintering and densification

Five TiAl–Ti3AlC2 composite samples containing (10, 15, 20, 25 and 30 wt% Ti3AlC2 MAX phase) were prepared by spark plasma sintering technique at 900 °C for 7 min under 40 MPa. For this purpose, metallic titanium and aluminum powders (aiming at the in-situ formation of the TiAl matrix phase) were ball-milled with predetermined contents of Ti3AlC2 MAX phase, which already was synthesized using the same metallic powders as well as graphite flakes. Displacement-time-temperature variations during the heating and sintering steps, displacement rate versus temperature, displacement rate versus time, and densification behavior were studied. Two sharp changes were detected in the diagrams: the first one, ~16 min after the start of the heating process due to the melting of Al, and the second one, after ~35 min because of the sintering progression and the applied final pressure. The highest relative densities were measured for the samples doped with 20 and 25 wt% Ti3AlC2 additives. More Ti3AlC2 addition resulted in decreased relative density because of the agglomeration of MAX phase particles.