Growth and formation mechanism of sea urchin-like ZnO nanostructures on Si

Sea urchin-like nanostructures of ZnO consisting of ZnO nanowires with blunt faceted ends were grown on Si (100) substrates by oxidation of metallic Zn at 600 °C. ZnO nanowires having a diameter of 30–60 nm and length of 2–4 Μm were in similar shape with uniform diameter along its entire length with well faceted blunt ends. X-ray diffraction and transmission electron microscope analysis showed that the as-grown nanostructures were highly crystalline with wurtzite hexagonal structure having lattice constants of a=b=3.25 å and c=5.21 å. Room temperature photoluminescence (PL) measurements showed a weak near band-edge emission at 380 nm, but a strong green emission at 500–530 nm. A model for vapor-solid (VS) growth mechanism of ZnO nanowires was presented, in which nucleation of ZnO is crucial for the growth of the nanostructures.     

Response of structure and mechanical properties of high entropy pyrochlore to heavy ion irradiation

Material with superior damage tolerance, chemical durability, and structure stability is of increasing interest in high-level radioactive waste management and structural components for advanced nuclear systems. In this paper, high-entropy (La_0.2Ce_0.2Nd_0.2Sm_0.2Gd_0.2)₂Zr₂O₇ with pyrochlore-type structure was synthesized through conventional solid-state method. The as-synthesized high-entropy oxide maintained crystalline after being irradiated by using Au³⁺ with 9.0 MeV energy at the fluence of 4.5 × 10¹⁵ ions·cm^-2, indicating its high tolerance to heavy-ion irradiation. The irradiation-induced order-disorder transition from pyrochlore structure to defective fluorite structure occurred in high-entropy (La_0.2Ce_0.2Nd_0.2Sm_0.2Gd_0.2)₂Zr₂O₇. After irradiation, no irradiation-induced segregation was observed at grain boundary. Moreover, the mechanical properties of high-entropy pyrochlore were improved. The heavy-ion irradiation resistance mechanisms of high-entropy pyrochlore were discussed in detail. Our work identified high-entropy (La_0.2Ce_0.2Nd_0.2Sm_0.2Gd_0.2)₂Zr₂O₇ can be a promising candidate for immobilization of high-level radioactive waste as well as advanced nuclear reactor system from the perspective of irradiation resistance.     

Bi2(Sn0.95Cr0.05)2O7: Structure,IR spectra,and dielectric properties

Infrared absorption spectra of the bismuth pyrostannate Bi₂(Sn_0.95Cr_0.05)₂O₇ were investigated in the frequency range 350−1100 cm⁻¹ at temperatures of 110−525 K. Four frequency regions with split absorption lines are distinguished. Softening of frequencies at the structural transitions was observed. The maxima of permittivity measured in the frequency range 1−200 kHz at temperatures 100−400 K were determined. It was found that the magnetic susceptibility changes its sign in the low-temperature region. The correlation between anomalies in the magnetic susceptibility, permittivity, and absorption line intensity was established. Softening of frequencies is explained by the variation in the coefficient of thermal expansion of the lattice. The temperature behavior of permittivity is described using the Debye model.     

Thermal insulation and anti-vibration properties of MoSi2-based coating on mullite fiber insulation tiles

Thermal protection materials with excellent thermal insulation properties and high reliability are crucial for aerospace vehicle. Mullite fiber insulation tiles coated with MoSi₂-borosilicate glass (MFIT@MoSi₂) were prepared by a simple slurry method. Results shown that the surface temperature rapidly reached 1043.1 °C under the heat flux of 450 kW/m², while the cold-surface remained at room temperature. The adjusting effects of MoSi₂-based coating on the thermal response and transfer properties of MFIT were investigated systematically. Compared with MFIT, the surface and internal temperatures of MFIT@MoSi₂ were obviously suppressed, due to the existence of MoSi₂-based coating with high emissivity, which effectively enhanced the thermal radiation of the surfaces. Finally, the structural reliability under coupled environment of heating (q₀ = 450 kW/m²) and random vibration with high magnitude (20–2000 Hz, Grms = 20 g) was also investigated. Destructive cracking and peeling of the coating were not observed, due to the excellent bonding strength between the coating and the substrate. The results provided an important basis for the potential application of insulation tiles with mullite fibers in aerospace vehicles and the design of lightweight thermal protection systems.     

Thermal shock effects on the impact resistance,tolerance and flexural strength of alumina based oxide/oxide ceramic matrix composites

In this study the effects of thermal shock on the impact damage resistance, damage tolerance and flexural strength of Nextel 610/alumina silicate ceramic matrix composites were experimentally evaluated. Composite laminates with balanced and symmetric layup were gradually heated to 1200°C in an air-based furnace and held for at least 30 min before being removed and immersed in water at room temperature. The laminates were then subjected to low velocity impacts via a hemispherical steel impactor. The resultant damage was characterized non-destructively, following which the laminates were subjected to compression tests. Three-point bend tests were also performed to evaluate the effect of thermal shock on the flexural strength and related failure modes of the laminates. Thermally shocked laminates showed smaller internal damage and larger external damage areas in comparison to their pristine counterparts. For the impact energy and resultant damage size considered, the residual compressive strengths for the thermally shocked and pristine laminates were similar.     

Structural characteristics and high-temperature oxidation behaviour of nano-HfO2-doped thermal barrier coatings

The uneven growth of thermally grown oxides (TGOs) in thermal barrier coating systems is an important cause of cracking failure at the coating interface in high-temperature environments. The doping of rare earth elements in the bonding layer can effectively inhibit the formation of spinel oxides in the TGO and improve the high-temperature oxidation resistance of the coating. However, a single rare earth element has a limited effect on inhibiting TGO failure. In this study, a NiCoCrAlYHf coating was prepared using a supersonic flame spraying (HVOF) technique. The effects of HfO₂ doping on the high-temperature oxidation behaviour of the coatings and diffusion behaviour of metallic elements in the coatings were investigated at 1100 °C. The results showed that the nano-sized HfO₂ filled the pores between the powder particles and improved the hardness of the coating. During the high-temperature oxidation process, the oxides formed by Hf and Y had a large size and low solubility, which effectively blocked the diffusion of Al. This slowed the generation of spinel oxides, effectively inhibited the growth of the TGO, it inhibits the initiation and propagation of cracks within the coating, reduces damage to the coating from tensile and compressive stresses at the interface, and improved the high-temperature oxidation resistance of the coating.     

Electrical polarization and ionic conduction properties of β-tricalcium phosphate bioceramics with controlled vacancies by sodium ion substitution

With the aim to understand electric polarization mechanisms of β-tricalcium phosphate as an advanced biomaterial, Na ion-substituted β-Ca₃(PO₄)₂ (Na-β-TCPs) ceramics with controlled lattice vacancies were synthesized and structural refinement was performed by the Rietveld method. The Rietveld analysis revealed that Ca and vacancies at Ca(4) sites in the β-TCP structure decreased with an increase in Na substitution. Electrical measurements by the complex impedance method revealed that the conductivity and the activation energy calculated from Cole-Cole plots rapidly decreased to a constant value with an increase in Na substitution and decrease in vacancies. The thermally stimulated depolarization current (TSDC) curve of the electrically polarized Na-β-TCP showed one large peak at 530–610 °C. However, the accumulated charge decreased with an increase in Na ions and decrease in vacancies up to 2.37 mol%, after which it became constant. These results are consistent with the presumed formation of a dipole moment between aligned Ca²⁺ ions and their vacancies along the direction of the external polarization field applied at high temperature. We conclude that the large amount of stored charge in β-TCP caused by electrical polarization is due to the low site occupancy of calcium ions and vacancies at Ca(4) sites in the β-TCP structure, which is not the case for hydroxyapatite (HAp), as previously reported.     

Tuning of emission by effect of local symmetry in BaLaLiWO6:Dy3+/Eu3+ for WLEDs

The luminescence center energy transfer, crystal field strength, and covalency are limited by the crystal structure of the host and subsequently affect the luminescence efficiency, color, and intensity. Here, we report an excellent red phosphor BaLaLiWO₆:0.40Eu³⁺ and the dependence between symmetry and luminous performance. A model for changing symmetry is drawn by analyzing the Coulomb potential and structure for the application of a double-perovskite phosphor BLLWO: Dy³⁺, Eu³⁺ in white light LEDs. The addition of Dy³⁺/Eu³⁺ makes the W-O bond formed by the B-site and oxygen ion longer and the Li-O bond shorter, and the difference between the eight octahedral around the A-site is reduced, increasing the symmetry of the A-site. Local symmetry was successfully modulated by changing the Eu³⁺ concentration to control the Y/B ratio of Dy³⁺ and the R/O ratio of Eu³⁺ and smoothly achieved (0.382, 0.373) warm white light color coordinate. The phosphor has excellent thermal stability and still has 92.3% intensity at 475 K. The above results show that the wavelength composition of the luminescence is tunable by changing the symmetry of the environment in which the doped ions are located. It applies to single hosts for the regulation of white light emission.     

Probing the effect of R-cation radii on structural,vibrational, optical,and dielectric properties of rare earth (R=La,Pr, Nd) aluminates

The present study correlates the effect of R-cation radii on structural, vibrational, optical, and dielectric properties of rhombohedral rare earth aluminates RAlO₃ (R = La, Pr, Nd). The polycrystalline samples of RAlO₃ have been synthesized using sol-gel synthesis technique. Pure rhombohedral phase of RAlO₃ samples has been confirmed with X-ray diffraction. Systematic decrements in the lattice parameter, bond length, and bond angle have been observed, giving rise to structural distortion due to decrease in ionic radii of R-cation. The phononic properties of RAlO₃ have been investigated through Raman spectroscopy, where the degree of distortion of AlO₆ octahedra can be analyzed with the peak position of E_g and A_1g modes. An increase in the energy bandgap with decreasing R-cation radii shows an interconnection with the decrease in Al–O bond length. Interestingly, the decreasing dielectric constant with decreasing ionic radii of R-cation has been correlated with the difference in electronegativity of cation(R³⁺)-anion(O^2?) pair. Also, a positive linear relationship between dielectric constant and energy bandgap has been investigated using Penn model.