High-entropy oxide phases with magnetoplumbite structure

Sintering of oxides and carbonates at 1400 °C gives crystalline high-entropy single phase products with a BaFe₁₂O₁₉ (magnetoplumbite) structure containing 5 doping elements at high concentration level: Ba(Fe₆Ti_1·2Co_1·2In_1.2Ga_1.2Cr_1.2)O₁₉. The complete list of explored substitutions includes K, Ca, Sr, Pb, La, Bi, Al, Ga, In, Ti, V, Cr, Mn, Co, and Ni. Loading to the batch more than 5 dopants or introduction of NiO, or V₂O₅ initiates formation of second phases like spinels or vanadates. Bi₂O₃ and K₂O are too volatile at sintering temperature and were evaporated from the samples.Due to large ionic radius, the In³⁺ cation is likely to be incorporated not only on Fe³⁺ sites, but also on Ba²⁺ sites, that follow from resulting crystal composition. The smallest di-valent and tri-valent cations, Sr²⁺ and Al³⁺, are found to preferably concentrate together in the same magnetoplumbite phase crystals within one sample.According to elemental analysis of selected hexagonal crystals in the investigated 8 doped samples the most prospective compositions for obtaining high-entropy single phase with magnetoplumbite structure can be formulated as (Ca,Sr,Ba,Pb,La)Fe_x(Al,Ga,In,Ti,Cr,Mn,Co)_12-xO₁₉ where x = 1.5–6.     

Crystal structure and luminescence properties of green-emitting Sr1−xAl12O19:xEu2+ phosphors

In this paper, a series of novel luminescent Sr_1−xAl₁₂O₁₉:xEu²⁺ phosphors were synthesized by a high temperature solid-state reaction. The phase structure, photoluminescence (PL) properties, as well as the decay curves were investigated. The quenching concentration of Eu²⁺ in SrAl₁₂O₁₉ was about 0.15 (mol). Upon excitation at 378 nm, the composition-optimized Sr_0.85Al₁₂O₁₉:0.15Eu²⁺ exhibited strong broad-band green emission at 530 nm with the CIE chromaticity (0.2917, 0.5736). The results indicate that Sr_1−xAl₁₂O₁₉:xEu²⁺ phosphors have potential applications as green-emitting phosphors for UV-pumped white-light LEDs.     

Structural and magnetic properties of CaAl4Fe8O19

A new compound, CaAl₄Fe₈O₁₉, was synthesized for the first time and characterized by X-ray diffraction. It was found to have a hexagonal magnetoplumbite structure with lattice parametersa=5·83 Å andc=22·14 Å. The electrical studies showed that the compound was a semiconductor with energy of activationq=0·86 eV. The magnetic susceptibility was studied in the temperature range 300 K to 850 K, in which the compound was paramagnetic with a Curie molar constant of 31·03.     

Thermal-shock resistance of LnMgAl11O19 (Ln = La, Nd, Sm, Gd) with magnetoplumbite structure

Lanthanide hexaaluminates including LaMgAl₁₁O₁₉, NdMgAl₁₁O₁₉, SmMgAl₁₁O₁₉ and GdMgAl₁₁O₁₉ were synthesized via Sol–Gel method. Due to the anisotropic crystal growth, these oxides crystallize in the form of platelets and the platelet thickness increases with the decrease of rare-earth ionic radius. It was observed that the thermal-shock resistances of LaMgAl₁₁O₁₉, NdMgAl₁₁O₁₉ and SmMgAl₁₁O₁₉ oxides were superior to 8YSZ as proved by water quenching tests. In addition, the thinner the platelet, the more interstices are retained in the sintered specimen, and the better thermal-shock resistance the oxide has. Based on SEM images, it can be seen that the SmMgAl₁₁O₁₉ sample exhibits a mixture of the intergranular and transgranular fracture after thermal cycling failure.     

Thermal properties of oxides with magnetoplumbite structure for advanced thermal barrier coatings

Oxides having magnetoplumbite structure are promising candidate materials for applications as high temperature thermal barrier coatings because of their high thermal stability, high thermal expansion, and low thermal conductivity. In this study, powders of LaMgAl₁₁O₁₉, GdMgAl₁₁O₁₉, SmMgAl₁₁O₁₉, and Gd_0.7Yb_0.3MgAl₁₁O₁₉ magnetoplumbite oxides were synthesized by citric acid sol-gel method and hot-pressed into disk specimens. The thermal expansion coefficients (CTE) of these oxide materials were measured from room temperature to 1500 °C. The average CTE value was found to be ∼ 9.6 × 10^− 6/C. Thermal conductivity of these magnetoplumbite-based oxide materials was also evaluated using steady-state laser heat flux test method. The effects of doping on thermal properties were also examined. Thermal conductivity of the doped Gd_0.7Yb_0.3MgAl₁₁O₁₉ composition was found to be lower than that of the undoped GdMgAl₁₁O₁₉. In contrast, thermal expansion coefficient was found to be independent of the oxide composition and appears to be controlled by the magnetoplumbite crystal structure. Preliminary results of thermal conductivity testing at 1600 °C for LaMgAl₁₁O₁₉ and LaMnAl₁₁O₁₉ magnetoplumbite oxide coatings plasma-sprayed on NiCrAlY/Rene N5 superalloy substrates are also presented. The plasma-sprayed coatings did not sinter even at temperatures as high as 1600 °C.     

Synthesis and optical properties of Ni/Co/Cr doped BaMg6Ti6O19

Novel green and blue chromophores based on Ni/Co/Cr doped BaMg₆Ti₆O₁₉ solid solutions are successfully synthesized through a solid-state reaction. The crystal structure of all the samples belongs to the magnetoplumbite structure with the space group of P6₃/mmc. The BaMg_6-x/2Ti_6-x/2Ni_xO₁₉ (0 ≤ x ≤ 0.3) compounds exhibit a light green (x = 0.1) to yellow-green (x = 0.3) color. The oxidation state of Ni is confirmed to be +2 valence and the d-d transition of Ni²⁺ in octahedral sites is responsible for color. BaMg_6-x/2Ti_6-x/2Co_xO₁₉ (0 ≤ x ≤ 0.3) series show a blue color and the intensity of blueness is increasing with the increase of Co content. The blue color is due to d-d transitions within Co²⁺ present in the tetrahedral sites. For BaMg_6-x/2Ti_6-x/2Cr_xO₁₉ (0 ≤ x ≤ 2) phases the color varies from light green (x = 0.2) to green (x = 2). Chromium exists in +3 and + 6 oxidation states and the observed color is due to charge transfer transition between Cr³⁺–Ti⁴⁺ and d-d transitions within octahedral Cr³⁺ sites resulting in strong absorption in the visible region. The synthesized colored oxides are mixed with PMMA to prepare novel green and blue PMMA polymer composites to evaluate their compatibility in plastics.     

Thermal cycling failure of new LaMgAl11O19/YSZ double ceramic top coat thermal barrier coating systems

New LaMgAl₁₁O₁₉ (LaMA)/YSZ double ceramic top coat thermal barrier coatings (TBCs) with the potential application in advanced gas-turbines and diesel engines to realize improved efficiency and durability were prepared by plasma spraying, and their thermal cycling failure were investigated. The microstructure evolutions as well as the crystal chemistry characteristics of LaMA coating which seemed to have strong influences on the thermal cycling failure of LaMA and the new double ceramic top coat TBCs based on LaMA/YSZ system were studied. For double ceramic top coat TBC system, interface modification of LaMA/YSZ by preparing thin composite coatings seemed to be more preferred due to the formations of multiple cracks during thermal cycling making the TBC to be more strain tolerant and as well as resulting in an improved thermal cycling property. The effects of the TGO stresses on the failure behavior of the TBCs were discussed through fluorescence piezo-spectroscopy analysis.     

Enhancement of the densification and thermal properties of Ca2Mg2Al28O46 ceramic by MnO addition

Ca₂Mg₂Al₂₈O₄₆ consists of platelet or platelet-like structural features derived from CaAl₁₂O₁₉. However, its application in dense structural materials is limited due to its poor sinterability and loose structure. In this work, Ca₂Mg₂Al₂₈O₄₆ ceramic was fabricated by solid-state reaction sintering, and the effects of MnO addition on the densification and thermal properties were investigated at 1600-1750 °C. The results showed that the Mn²⁺ solid dissolved into the Ca₂Mg₂Al₂₈O₄₆ lattice by the isovalent substitution of Mg²⁺ at 1650 °C, causing the grain morphology to change from hexagonal to equiaxed and resulting in a distorted magnetoplumbite structure. At 1700 °C, the solid solution reaction accelerated the densification, with the apparent porosity decreasing from 32.5% to 8.4% when 3 wt% MnO is added, promoted grain growth and pore discharge by increasing the number of lattice defects, grain boundary migration rate and in-situ stress. The enhanced thermal expansion coefficient and thermal conduction were mainly due to lattice expansion and grain coarsening. Moreover, the formation of trace amount of the secondary phase Mg_1-xMn_xAl₂O₄ also contributed to the improvement in the thermal properties.