Transparent polycrystalline alumina (PCA) is a promising replacement for sapphire. Its optical properties however are highly dependent on the grain size and residual porosity which need to be ...controlled for real inline transmittances (RIT), that are high enough for possible applications.
To achieve high RITs, doping as well as pressure assisted sintering is often used. In this study spark plasma sintering (SPS) and doping are investigated. A systematic experimental design is used to study the influence of Mg, Y and La single or co-doping (75–450
ppm) as well as the SPS sintering pressure and temperature on the RIT and grain size of PCA.
Using optimized sintering parameters, RITs of >50% were attained in the visible wavelength (640
nm) for 0.8
mm thick samples for almost all doping strategies. The best RIT of 57% was for triple-doped samples at a total dopant level of 450
ppm. These results are significantly better than previously published SPS studies and illustrate that SPS sintered alumina can attain high and reproducible optical transmittances under various doping and sintering conditions.
Production of transparent ceramics has become a topic of resurgent interest in recent years, with its promise of near‐net shaping appealing to applications ranging from biomedicine to solar energy. ...However, the mechanisms governing ceramic transparency, translucency, and opaqueness are not entirely understood. Models of both grain boundary and pore scattering have been proposed, but too often without sufficient experimental corroboration. An extensive experimental analysis of transparent alumina samples is presented, establishing a first direct link between the observed transparency, defect size, and porosity. Given the unprecedented experimental detail from the full 3D pore reconstruction from the FIB tomography, how to correctly interpret the experimentally observed transparency is additionally shown. The unprecedented experimental and theoretical agreement for the first time identifies the relative contributions of different scattering mechanisms, thereby paving the way forward for microstructural tuning of transparent polycrystalline alumina.
A full 3D reconstruction of pores in transparent polycrystalline alumina is performed, demonstrating that pores smaller than ≈50 nm have a drastically smaller effect on the degree of transparency than commonly presumed. Furthermore, how to quantitatively incorporate the pore characteristics into the conventional optical transmittance model by deterministically modifying its input parameters is shown.
Use of freeze granulated powders is successfully used as an industrially viable alternative to loose powder sintering for transparent polycrystalline alumina (PCA). Freeze granulation with narrow ...granule size distribution was realized after suspension condition optimization, with very good flowability and regular spherical shapes. The key factors are low viscosity slurries linked to the complex interactions between the organic processing additives and their interaction with dopant ions in solution. Real in-line transmittances (RITs) of 52% were achieved by pulsed electric current sintering (PECS) of dry pressed green bodies from doped granulated powders. This is the first example of a high RIT (>50%) alumina produced from simple dry pressing of a granulated powder. The results indicate that higher granule solid loads and lower organic additive concentrations give better RITs after PECS. Aging of the powder slurry before freeze granulation proves a crucial step for the optical performance of the final transparent PCA.
The effects of influential fatigue testing factors, including loading schemes (e.g., traditional load shedding and staircase load increasing), morphology of crack starters, and precracking approaches ...on the near-threshold fatigue crack growth behaviors for a near lamellar γ-TiAl alloy (Ti-45Al-2Mn-2Nb-1B) were investigated at room temperature and 650 °C. The results showed that the measured fatigue threshold values in lamellar γ-TiAl alloys are very sensitive to the applied testing procedures. For example, the staircase load-increasing method yielded smaller threshold values. When such a load-increasing method was used, the threshold values were measured either from a notch machined by electro-discharge machining or prepared by a compression–compression fatigue loading. Moreover, some differences could be seen with respect to the morphologies of the crack starters. Most of the above influences are associated with the brittle nature of the material and the characteristics of the lamellar microstructures, and closure effects are primarily induced by crack wake roughness or unbroken ligaments.
Strain accumulation ahead of the fatigue crack tip in the martensite lath of a medium-carbon steel was examined using a three-dimensional electron backscatter diffraction (3D-EBSD) technique. The ...objective of this study is to explain the crack propagation mechanism due to the activation of out-of-plane slips with their Burgers vectors having no component of the crack growth direction, which exhibits high resistance to fatigue crack growth. The 3D-EBSD analysis revealed little misorientation in the crystal, concurrent with the fatigue crack propagation in the coarse laths oriented favourably for dislocation glide in their longitudinal directions. This suggests that these laths contributed to strain accommodation. In contrast, strain preferentially accumulated in the coarse laths oriented unfavorably for the longitudinal slip, promoting crack propagation. These indicate that the geometrical anisotropy and distribution of martensite laths dominate the fatigue crack propagation resistance in martensitic carbon steel.
The microhardness and microstructure of linear friction welded Ti–6Al–2Sn–4Zr–6Mo (Ti6246) alloys were studied, in both as-welded and post-weld heat-treated conditions. It has been found that the ...as-welded Ti6246 has a lower microhardness value of about 360HV in the central weld zone than that of the base material of about 420HV. Post-weld heat-treatment of the Ti6246 weld at 600°C for 1h has led to the hardness increase of about 180HV at the central weld zone. Transmission electron microscopy studies show that the microstructure at the central weld zone of the as-welded Ti6246 consists of fine grains with dense acicular orthorhombic α″ martensite. The soft α″ martensite is believed to account for the low hardness measured in the as-welded conditions. Phase transformation from orthorhombic α″ to hexagonal α occurred during the PWHT, resulting in the observed hardness increase.
This work focuses on the development of a facile and scalable wet milling method followed by heat treatment to prepare fluorinated and/or N-doped TiO₂ nanopowders with improved photocatalytic ...properties under visible light. The structural and electronic properties of doped particles were investigated by various techniques. The successful doping of TiO₂ was confirmed by X-ray photoelectron spectroscopy (XPS), and the atoms appeared to be mainly located in interstitial positions for N whereas the fluorination is located at the TiO₂ surface. The formation of intragap states was found to be responsible for the band gap narrowing leading to the faster bacterial inactivation dynamics observed for the fluorinated and N doped TiO₂ particles compared to N-doped TiO₂. This was attributed to a synergistic effect. The results presented in this study confirmed the suitability of the preparation approach for the large-scale production of cost-efficient doped TiO₂ for effective bacterial inactivation.
Molecular dynamics simulations predict differences in calcite growth modification by polyacrylic acid and polyaspartic acid to be a combination of complexing ions in solution, adsorption free ...energies and adsorption conformations.
Molecular dynamics simulations were used to investigate possible explanations for experimentally observed differences in the growth modification of calcite particles by two organic additives, polyacrylic acid (PAA) and polyaspartic acid (p-ASP). The more rigid backbone of p-ASP was found to inhibit the formation of stable complexes with counter-ions in solution, resulting in a higher availability of p-ASP compared to PAA for surface adsorption. Furthermore the presence of nitrogen on the p-ASP backbone yields favorable electrostatic interactions with the surface, resulting in negative adsorption energies, in an upright (brush conformation). This leads to a more rapid binding and longer residence times at calcite surfaces compared to PAA, which adsorbed in a flat (pancake) configuration with positive adsorption energies. The PAA adsorption occurring despite this positive energy difference can be attributed to the disruption of the ordered water layer seen in the simulations and hence a significant entropic contribution to the adsorption free energy. These findings help explain the stronger inhibiting effect on calcite growth observed by p-ASP compared to PAA and can be used as guidelines in the design of additives leading to even more marked growth modifying effects.
Secondary schools are purported to play a significant role in the resilience enhancement of adolescents; however, there are no measures of resilience designed specifically for use in secondary ...schools. To address this shortcoming, a resilience at secondary school (RASS) scale was developed and its psychometric properties explored. Five hundred and sixty-two female secondary school students aged from 12 years to 18 years completed the 20-item RASS, which was adapted from the Resilience at University scale. Confirmatory factor analysis using structural equation modeling revealed a 19-item, seven-factor structure. Convergent validity of all the subscales was tenable. Internal consistency of two subscales was not ideal, indicative of the need for further research. The RASS scale and subscales represent promising empirical measures of resilience at secondary schools in Australia. With further refinement, the RASS offers educators in secondary schools a practical measure founded on the skills and behaviours considered critical for psychological resilience which can be used to inform resilience-building curriculum activities. Author abstract
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