Fragmentation and film growth in supersonic nanoaggregate aerosol deposition

Aerosol deposition with gas phase-synthesized chain-like nanoaggregates can yield dense coatings from the impaction of particles on a substrate; however, dense coating formation is not well understood. Here, we study coating consolidation at the single nanoaggregate level. Flame spray pyrolysis-made tin oxide nanoaggregates are mobility (size) filtered, accelerated through a de Laval nozzle, and impacted on alumina substrates. TEM images obtained from low velocity collection and supersonic deposition are compared via quantitative image analysis, which reveals that upon supersonic impact nanoaggregates fragment into smaller aggregates. This suggests that fragmentation is a key step in producing coatings denser than the depositing nanoaggregates themselves. We supplement experiments with detailed particle trajectory calculations, which show that the impact energies per atom during nanoaggregate deposition are below 0.2 eV/molecule. These results suggest that fragmentation can only occur at locations where nanoaggregates bonded by van der Waals and capillary interactions.     

Immobilization of simulated waste into pure Gd2Zr2O7 pyrochlore without space occupancy design

In this study, different molar ratios of Nd:Ce were directly mixed with prepared pure Gd₂Zr₂O₇ powders without space occupancy design. Samples were obtained by performing sparking plasma sintering (SPS) at 1750°C for 5 minutes. X-ray diffraction (XRD) results show that maximum solid solubility of simulated radionuclides can reach 50 mol%. In addition, all samples with the maximum solid solubility have high compactness, and all elements are evenly distributed on the surface of the samples. The samples show a better crystallization effect as the molar ratio of Nd:Ce increases. The maximum solid solubility increases from 42 mol% to 50 mol% when the amount of Nd₂O₃ reaches 66 mol%.     

New insight into the formation and oxygen barrier mechanism of carbonaceous oxide interlayer in a multicomponent carbide

Early transition metal carbides are considered to be superior candidate materials for oxidizing environments at temperatures exceeding 2000°C. Generally, the remarkable oxidation resistance is largely attributed to a carbonaceous oxide interlayer (eg, Hf–O–C, Zr–O–C, and Ta–O–C), located at the interface between the external oxide layer and internal carbide (eg, HfC, ZrC, and TaC), acting as the primary oxygen barrier. However, the oxygen barrier mechanism of the carbonaceous oxide interlayer remains unclear. Herein, through studying the oxidation behavior of a novel multicomponent carbide Hf_0.5Zr_0.3Ti_0.2C in oxidizing environments up to 2500°C, the oxygen barrier mechanism of the carbonaceous oxide was recently revealed. We found that the oxygen barrier resulted from the slow oxygen diffusion through the inner grains of Hf-Zr–Ti–O due to the presence of carbon formed at the grain boundaries because of the existence of compact external oxide layer, beneath which the Hf–Zr–Ti–O–C interlayer possesses much lower oxygen activity and temperature that allow carbon to exist stably. This as-formed carbon strongly retarded the fast diffusion of oxygen along the grain boundaries of oxides. Additionally, desirable synergisms of the designed multicomponent system, particularly, the outward short-circuit diffusion of Ti, lead to the self-healing of the external oxide layer, evidently enhancing integral protection performance against oxidizing environments.     

Reinvestigation of the photostrictive effect in lanthanum-modified lead zirconate titanate ferroelectrics

Photostriction of lanthanum-modified lead zirconate titanate (PLZT) was commonly attributed to the combination of anomalous photovoltaic effect and inverse piezoelectric response. Herein, distinct photostrictions are detected in both poled and unpoled PLZT ceramics under 405 and 520 nm laser illuminations. The maximum photostriction around 0.09% is obtained in unpoled PLZT under 405 nm illumination, which is nine times of previously reported value that deduced from poled PLZT. The photoelectric and photovoltaic characterizations of poled/unpoled PLZT further evidence that the detected photostriction shows no direct association with the photovoltage-induced inverse piezoresponse. The light-induced microstructure changes observed by in situ piezoelectric force microscopy are revealed by domain evolutions along the boundaries. The possible photostriction mechanism of PLZT is attributed to the strong photo-induced electron-lattice coupling, which is suggested by the light-induced changes of the Zr/Ti-O-Zr/Ti bonds reflected by the power-dependent Raman spectra. This study extends the photostriction of PLZT to visible light range and will also stimulate reappraisal of the underlying mechanism of photostrictive effect involving ferroelectrics.     

Sintering behavior of a nanostructured thermal barrier coating deposited using electro-sprayed particles

During high temperature service, a series of microstructure and phase evolutions occur in thermal barrier coatings (TBCs), which result in degradation of thermal insulation and durability. In this study, the sintering behavior of an air plasma sprayed 8 wt% YSZ coating deposited using electro-sprayed nanostructured particles (ESP) as feedstock powder was investigated and compared with conventional YSZ coating deposited using hollow spherical powders (HOSP). Due to the distinct asymmetric porous structure formed by nanosized YSZ particles, the ESP powder was partially melted in the plasma jet during the deposition, which resulted in the formation of a nanostructured coating that consisted of porous nanozones and dense zones. The ESP coating not only shows a significantly lower initial thermal conductivity of 0.70 W/mK, but also exhibits a stronger sintering resistance in terms of phase stability and thermal insulation compared to the conventional coating. When subjected to prolonged sintering at 1400°C for 128 hours, the thermal conductivity of the ESP coating would gradually increase to about half that of the HOSP coating at 1.29 W/mK. These differences are ascribed to the interaction among different sintering behavior between nanozones and dense zones.     

Ferroelastic domain switching and R-curve behavior in lead zirconate titanate (Zr/Ti = 52/48)-based ferroelectric ceramics

In this work, ferroelastic domain switching and R-curve behavior in lead zirconate titanate (Nb/Ce co-doped Pb(Zr_0.52Ti_0.48)O₃, ab. PZT-NC)-based ferroelectric ceramics were investigated, using the indentation-strength-in-bending (ISB) method. Firstly, Vickers indentation test examined the notable fracture anisotropy of PZT-NC ceramics between the poling direction and its perpendicular direction, and the crack open displacement (COD) profiles in the two directions were also theoretically calculated from the indentation fracture mechanics. And then two kinds of ferroelastic domain switching modes (in-plane and out-of-plane) were used for explaining such anisotropic propagation behavior of indentation cracks. The subsequent three-point bending test illustrated the dependence of fracture strength on indentation load and the rising crack growth resistance curves (R-curves) in two directions. The resulted R-curves were fitted by the Hill's type Growth Function successfully, giving the reasonable values of crack extension exponential (n), plateau fracture toughness (K_max), and initial fracture toughness (K_ini). The in-plane ferroelastic domain switching was identified as a more significant toughening mechanism for PZT-NC ceramics than the out-of-plane switching due to more switchable domains.     

Understanding the correlation between calcination temperature and performance in low-temperature methanation over Ni-Zr/Al2O3 catalysts

Low-temperature methanation of CO in the continuous stirred tank reactor (CSTR) over Zr doped Ni/Al₂O₃ catalyst calcined at different temperatures (673, 723, and 823 K) was investigated. XRD, TPR, XPS, ICP, SEM, and S-TPR techniques were employed to characterize the fresh and spent catalysts. Based on the characterization results, it was found that low-temperature (673 K) calcination could effectively prohibit the formation of NiAl₂O₄ spinel, thereby resulting in more reducible NiO particles, which were the essential precursor of methanation active sites over the catalyst surface. Thus, the highest CO conversion of 93.6% was achieved over the 25N3ZA-673 catalyst. In addition, the deactivation rate of 25N3ZA-673 was relatively slow in comparison to 25N3ZA-823 due to the presence of more reducible NiO. The formed nickel carbonyl species (Ni[CO]_x), which quickly decomposed at a higher reaction temperature, was closely related to the catalyst deactivation. Therefore, 25N3ZA-673 possessed much better stability at 593 K than that at 553 K.     

Effects of laser polishing on surface quality and mechanical properties of PLA parts built by fused deposition modeling

Fused deposition modeling (FDM) produces parts through layer by layer on the top of each other, making it almost impossible to obtain smooth printed parts. Hence, there is a huge demand for the postprocessing of the FDM-printed parts. Laser polishing is a novel technique that can be used to polish products to obtain a smoother surface. The aim of this work was to explore the feasibility of surface-finishing FDM-printed polylactic acid (PLA) parts by laser polishing. The surface roughness, surface morphology, dynamic mechanical analysis (DMA), and tensile properties were investigated. The results indicated that the lower laser power and the bigger laser beam diameter within a certain range could facilitate the formation of smoother surface. With optimized parameters, the surface roughness was reduced by 90.4%. DMA showed that the storage modulus (E’) and glass transition temperature of PLA specimens were significantly improved due to the decrease of molecular mobility of denser structures. Moreover, the tensile strength and Young's modulus of the PLA specimen were also significantly increased after laser polishing. The fracture morphologies were observed, and the possible strengthening mechanism was also discussed. These results indicated that laser polishing could be an efficient method for surface polishing of FDM parts. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48288.     

Effects of modified silica on the co-vulcanization kinetics and mechanical performances of natural rubber/styrene–butadiene rubber blends

Rubber blends are widely used for combining the advantages of each rubber component. However, to date, how to determine and distinguish the vulcanization kinetics for each single rubber phase in rubber blends during the co-vulcanization process is still a challenge. Herein, high-resolution pyrolysis gas chromatography–mass spectrometry (HR PyGC-MS) was employed for the first time to investigate the vulcanization kinetics of natural rubber (NR) and styrene–butadiene rubber (SBR) in NR/SBR blends filled with modified silica (SiO₂). The reaction rates of crosslinking of each rubber phase in NR/SBR were calculated, which showed that the crosslinking rates of NR were much lower than those of SBR phase in the unfilled blends and blends filled with unmodified and silane modified silica. Interestingly, the vulcanization rates of NR and SBR phase were approximately same in the vulcanization accelerator modified silica filled blends, showing better co-vulcanization. In addition, the vulcanization accelerator modified silica was uniformly dispersed and endowed rubber blends with higher mechanical strength compared to the untreated silica. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48838.