This work investigates the effects of doping on both the thermodynamics and kinetics of sintering in aluminum‐doped yttrium oxide nanoparticles (Al‐doped Y2O3), with the objective of delineating their interdependent effects at different stages of the process. Direct measurements of surface and grain boundary energies using differential scanning calorimetry showed that Al‐doping decreases both interfacial energies, leading to an increase in dihedral angle (from 152.7 ± 5.6° to 165.8 ± 5.5°) and, therefore, sintering stress. Densification and grain growth analyses showed that despite this increase in sintering stress, the onset of sintering is delayed for the Al‐doped samples, demonstrating that a large dihedral angle is a necessary but not sufficient condition for densification. The measurements of activation energies for densification and grain growth point out that Al suppresses grain boundary mobility by increasing the activation energy from 400 to 448 kJ/mol, hindering densification at the intermediate stages of sintering. At temperatures above 1150℃, grain growth is activated in the Al‐doped samples, which rapidly releases the accumulated sintering stress and exhibits a higher densification rate than in undoped Y2O3. This study demonstrates a complex interconnectivity between the thermodynamics and kinetics at different temperature ranges of sintering and reinforces the need for a comprehensive description for proper design of sintering aids.
This work describes the role of manganese (Mn) as a sintering aid for magnesium aluminate (MgAl2O4) nanoparticles. Mn‐doped MgAl2O4nanoparticles, synthesized by coprecipitation method, showed increased surface area when contrasted to undoped MgAl2O4. Fast firing of compacted‐doped nanoparticles achieved high degree of densification at temperatures as low as 1100°C with very moderate grain growth, resulting in average sizes at the nanoscale (~60 nm). Differential scanning calorimetry was used to quantify the exothermic heat effects of sintering, which combined with quantitative microstructural evolution analysis enabled calculation of both surface and grain boundary energies. The results revealed that Mn effectively reduces the surface and grain boundary energies which led to dihedral angle broadening and consequently increased sintering stress. Experimental data also revealed a concomitant decrease in the activation energy of sintering with Mn doping which dropped from 644 kJ/mol for undoped MgAl2O4to 285 kJ/mol, informing Mn acts as a sintering aid in a thermokinetic manner.
more » « less- NSF-PAR ID:
- 10456452
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Journal of the American Ceramic Society
- Volume:
- 103
- Issue:
- 8
- ISSN:
- 0002-7820
- Page Range / eLocation ID:
- p. 4167-4177
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract -
more » « less
Abstract Surface energy (
γ S) and grain boundary energy (γ GB) of yttrium oxide (Y2O3) were determined by analyzing the heat of sintering (ΔH sintering) using differential scanning calorimetry (DSC). The data allowed quantification of sintering driving forces, which when combined with a thorough kinetic analysis of the process, provide better understanding of Y2O3densification as well as insights into effective strategies to improve its sinterability. The quantitative thermodynamic study revealed moderate thermodynamic driving force for densification in Y2O3(as compared to other oxides) represented by a dihedral angle of 152.7° calculated from its surface and grain boundary energies. The activation energy was determined as 307 ± 61 kJ/mol, consistent with activation energies previously reported for processes relevant to sintering of Y2O3,such as Y3+diffusion and grain boundary mobility. Finally, we propose that a refined deconvolution study on the DSC curve for Y2O3sintering, combined with the associated material's microstructure evolution, may help identify shifts in sintering mechanisms, and therefore, specific activation energies at increasing temperatures. -
more » « less
Abstract The addition of small quantities of aluminum oxide (Al2O3) to 8 mol% yttria‐stabilized zirconia (8YSZ) benefits conventional sintering by acting as a sintering aid and altering grain growth behavior. However, it is uncertain if these benefits observed during conventional sintering extend to flash sintering. In this work, nanoscale films of Al2O3are deposited on 8YSZ powders by particle atomic layer deposition (ALD). The ALD‐coated powders were flash sintered using voltage‐to‐current control and current rate experiments. The sintering behavior, microstructural evolution, and ionic conductivities were characterized. The addition of Al2O3films changed the conductivity of the starting powder, effectively moving the flash onset temperature. The grain size of the samples flashed with current rate experiments was ~65% smaller than that of conventionally sintered samples. Measurement of grain size and estimates of sample density as a function of temperature during flash sintering showed that small quantities of Al2O3can enhance grain growth and sintering of 8YSZ. This suggests that Al2O3dissolves into the 8YSZ grain boundaries during flash sintering to form complexions that enhance the diffusion of species controlling these processes.
-
more » « less
Abstract A magnetic vanadium oxide nanoparticles supported on spinel copper ferrite (CuFe2O4–VOx) are prepared, characterized, and examined for the peroxymonosulfate (PMS) activation to degrade Rhodamine B (RhB) in water solution. Interestingly, the results show that despite the inability of mixture of copper ferrite and vanadium oxides nanoparticles to the effective RhB decomposition, the prepared catalyst exhibits an excellent catalytic ability toward RhB oxidation. The influence of vital parameters, such as temperature, PMS concentration, catalyst loading, and initial pH are discussed comprehensively. The kinetic studies demonstrate that the pseudo‐first‐order model is well fitted for RhB degradation in CuFe2O4–VOx/PMS system and the activation energy is estimated at 19.60 kJ mol−1. Furthermore, it is found out that the concentration of leached metal ions in solution is negligible and PMS activation is done mainly on the surface of the catalyst. A probable mechanism of PMS activation over RhB degradation is proposed based on the results of free radical quenching studies and X‐ray photoelectron spectroscopy (XPS) analysis. Radical quenching experiments using various scavengers suggest SO4•−as a main reactive species in the degradation.
-
more » « less
Abstract Unveiling the underlying mechanisms of properties of functional materials, including the luminescence differences among similar pyrochlores A2B2O7, opens new gateways to select proper hosts for various optoelectronic applications by scientists and engineers. For example, although La2Zr2O7(LZO) and La2Hf2O7(LHO) pyrochlores have similar chemical compositional and crystallographic structural features, they demonstrate different luminescence properties both before and after doped with Eu3+ions. Based on our earlier work, LHO‐based nanophosphors display higher photo‐ and radioluminescence intensity, higher quantum efficiency, and longer excited state lifetime compared to LZO‐based nanophosphors. Moreover, under electronic O2−→Zr4+/Hf4+transition excitation at 306 nm, undoped LHO nanoparticles (NPs) have only violet blue emission, whereas LZO NPs show violet blue and red emissions. In this study, we have combined experimental and density functional theory (DFT) based theoretical calculation to explain the observed results. First, we calculated the density of state (DOS) based on DFT and studied the energetics of ionized oxygen vacancies in the band gaps of LZO and LHO theoretically, which explain their underlying luminescence difference. For Eu3+‐doped NPs, we performed emission intensity and lifetime calculations and found that the LHOE NPs have higher host to dopant energy transfer efficiency than the LZOE NPs (59.3% vs 24.6%), which accounts for the optical performance superiority of the former over the latter. Moreover, by corroborating our experimental data with the DFT calculations, we suggest that the Eu3+doping states in LHO present at exact energy position (both in majority and minority spin components) where oxygen defect states are located unlike those in LZO. Lastly, both the NPs show negligible photobleaching highlighting their potential for bioimaging applications. This current report provides a deeper understanding of the advantages of LHO over LZO as an advanced host for phosphors, scintillators, and fluoroimmunoassays.
