Calorimetric study of ytterbium orthovanadate YbVO4 polycrystalline ceramics

The heat capacity of ytterbium orthovanadate was first measured by adiabatic calorimetry in the temperature range T?=?12.28–344.06?K. No obvious anomalies were observed on the curve obtained. The values of standard thermodynamic functions in the temperature range T?=?0–400 K were calculated. Based on low-temperature calorimetry data obtained, previously published data on the high-temperature heat capacity of ytterbium orthovanadate were corrected. The anomalous contribution to heat capacity for YbVO₄ was compared with the data known for YbPO₄.     

Synthesis and properties of (Hf1-xTax)C solid solution carbides

Thermodynamically stable (Hf_1–xTa_x)C (x?=?0.1–0.3) compositions were selected by First Principle Calculation and synthesized in nanopowders via high-energy ball milling and carbothermal reduction of commercial oxides at 1450?°C. The formation of a solid solution during powder synthesis was investigated. The solid solution carbide powders were sintered at 1900?°C by spark plasma sintering without a sintering aid. As a result, the (Hf_1–xTa_x)C solid solution carbides exhibited high densities, excellent hardness and fracture toughness (ρ: 98.7–100.0%, HVN: 19.69–19.98?GPa, K_IC: 5.09–5.15?MPa?m^1/2) compared with previously reported HfC and HfC–TaC solid solution carbides.     

Methodology for dependence-based integrated constitutive modelling: An illustrative application to SiCp/Al composites

In industrial forming and machining process, the large plastic deformation of material takes place in wide loading ranges of strain-rate and forming temperature. A satisfactory modelling of quasi-static and dynamic material behaviors is of great importance for understanding physical process and processes optimization. A dependence-based integrated methodology, together with an improved weighted multi-objective parameter identification strategy is presented for the development of phenomenological constitutive model and the parameter identification using experimental data from quasi-static and dynamic tests with instantaneous strain rate variations and plastic strain-related temperature changes. The improved multi-objective parameter identification model is reformulated by introducing three weighting factors for valuing different measure errors and fit standard errors in individual objective function corresponding to each test, considering the sampling point number and active material parameter number under different loading conditions, and balancing optimization opportunity of quasi-static and dynamic sub-objective functions. The methodology is verified for feasibility through illustrative constitutive identification for SiC_p/Al composites. This may provide a methodology of constitutive modelling for predicting material behaviors in quasi-static and dynamic modes equally well.     

An investigation on the microstructural and electrical properties of La0.85DxSr0.15–xGa0.8Mg0.2O2.825 (D = Ba and Ca) electrolytes in solid oxide fuel cells

La_0.85D_xSr_0.15–xGa_0.8Mg_0.2O_2.825 (D = Ba and Ca, x?=?0, 0.01, 0.03, 0.05, and 0.07) electrolytes were synthesized using a solid-state reaction method, calcined at 1400?°C for 5?h, and sintered at 1400?°C for 5?h. The microstructures, electrical properties, and cell performances of the electrolytes and fuel cells were analyzed by X-ray diffraction, scanning electron microscopy, impedance analysis, and electrochemical analysis. La_0.85Ba_xSr_0.15–xGa_0.8Mg_0.2O_2.825 (LBSGM) and La_0.85Ca_xSr_0.15–xGa_0.8Mg_0.2O_2.825 (LCSGM) exhibit a dense structure and a cubic perovskite phase. Further, they contain small amounts of a secondary phase. The lattice constants of LBSGM and LCSGM are 0.3913–0.3914?nm and 0.3906–0.3909?nm, respectively. The average grain size of the sample increases with increasing Ba²⁺ or Ca²⁺ content. The conductivity of LCSGM (0.197–0.174?S/cm) is usually higher than that of LBSGM (0.181–0.162?S/cm) at 800?°C. The cells with La_0.85Sr_0.15Ga_0.8Mg_0.2O_2.825 and La_0.85Ca_0.03Sr_0.12Ga_0.8Mg_0.2O_2.825 electrolytes exhibit high open-circuit voltages and maximum power densities of 0.96?V and 542?mW/cm² and 0.94?V and 567?mW/cm², respectively, at 800?°C.     

A modified B2O3 containing Li2O-Al2O3-SiO2 glass with ZrO2 as nucleating agent - Crystallization and microstructure studied by XRD and (S)TEM-EDX

Glass-ceramics based on lithium-alumo-silicate glasses are commercially important for a wide range of applications, due to their special properties, like a vanishing thermal expansion. In order to tailor these properties, the composition of the glass and the temperature/time schedule are crucial factors. For the industrial production of most lithium-alumo-silicate glasses, high melting temperatures are required due to the high viscosities of the respective melt compositions. In this study, a simplified lithium-alumo-silicate glass composition with ZrO₂ as nucleating agent, on the basis of the commercially available Robax^® composition, is studied. Adding boron oxide leads to lower viscosities of the glass melt and notably lower melting temperatures may be supplied. The resulting glass is investigated using X-ray diffraction and transmission electron microscopy. During the crystallization process, phases such as ZrO₂ and β-quartz types are formed. The microstructure of the glass ceramics is notably coarser than that of glass-ceramics which are obtained from lithium-alumo-silicate glasses of standard compositions. EDX-analyses indicate a considerable enrichment of chemical elements in comparatively small areas of the microstructure. Especially boron oxide is found to be enriched in the residual glass of the investigated glass-ceramics.     

High efficiency synthesis of Nd:YAG powder by a spray co-precipitation method for transparent ceramics

A spray co-precipitation method was developed to efficiently synthesize Nd:YAG nano-powders. The effects of spray speeds and solution concentrations on the crystallization processes of calcined precursors have been studied. The results indicated that the pure phase of YAG could be obtained by three different crystallization processes owing to different homogeneity levels of Y and Al mixing. Pure YAG powder was obtained at 850?°C and the phase purity persisted to 1600?°C. Using the obtained powders, transparent ceramics with the in-line transmittance up to 80.2%@400 nm and 83.1%@1064?nm were fabricated by gel-casting method and hot isostatic pressing sintering. Furthermore, the microstructure and laser properties of the transparent ceramics have been measured. The maximum laser output of 7.015?W has been obtained with an oscillation threshold and a slope efficiency of 0.235?W and 59.4%, respectively.     

Effects of deformation rate on properties of Nd,Y-codoped CaF2 transparent ceramics

Different deformation rates of Nd,Y-codoped CaF₂ transparent ceramics were prepared by ceramization of single crystals. The deformation rate effects on the crystallization behaviors, microstructures, mechanical properties, and optical performances were investigated for the first time. The results indicate that the comprehensive performances of Nd,Y-codoped CaF₂ ceramic (△a?=?62%) are the most optimal compared with other ceramics having different deformation rates (△a?=?34%, 40%, 50%, and 75%). In further investigations of the optical properties, the Nd,Y-codoped CaF₂ ceramic (△a?=?62%) sample exhibited a high transparency (T_a?>?91%, 3-mm thick，250?～?1200?nm), low light scattering, superior fracture toughness (K_1c?～?0.71?MPa·m^1/2), strong fluorescence emission, long lifetime (τ?=?348.72?μs), and broad FWHM (29.2?nm), promising a good candidate for high-power laser material.     

Durability of YSZ coated Ti2AlC in 1300°C high velocity burner rig tests

A thermal barrier coating (TBC) system survived 500 hours in aggressive, 1300°C burner rig testing. The yttria-stabilized zirconia (7YSZ) TBC was plasma sprayed on an oxidation-resistant Ti₂AlC-type MAX phase and tested in a jet fuel burner at 100 m/s, using 5 hours cycles. No coating spallation or surface recession was observed; Al₂O₃-scale growth produced a slight 2.4 mg/cm² mass gain. The coating surface exhibited craze-cracked colonies of [111]_flourite textured columns, with no visible moisture attack. The 20 μm alumina scale remained intact under the YSZ face, about twice that producing failure for TBC/superalloy systems. TiO₂ nodules, initially formed on the uncoated backside, were removed, and Al₂O₃ was etched through volatile hydroxides formed in water vapor (~10%). Overall, the test indicated exceptional stability of the YSZ/Al₂O₃/Ti₂AlC system under turbine conditions due in large part to close thermal expansion matching.     

Effect of Yb concentration on the microstructures,spectra, and laser performance of Yb: CaF2 transparent ceramics

Ytterbium-doped calcium fluoride transparent ceramics is considered as a promising laser gain medium. How to prepare low scattering loss ceramics is the main challenge at present. In this study, a simple route of hot-pressing method was introduced to fabricate high optical quality of Yb: CaF₂ laser ceramics with different concentration of Yb ions. The influence of Yb concentration on the particle sizes of the powders, microstructure, fluorescence spectra, output power, and laser emitting spectra of Yb: CaF₂ transparent ceramics was discussed. About 976 nm LD pumped CW laser operation has been demonstrated in 1, 2, 4, and 5 at.% Yb doped CaF₂ ceramics. A maximum average output power of 0.809 W was obtained for the 5 at.% Yb: CaF₂ ceramics with the slope efficiency was 26%.     

Oxidative Stability of Chia (Salvia hispanica L.) and Sesame (Sesamum indicum L.) Oil Blends

Chia and sesame oils are important sources of essential fatty acids; however, their ω-3:ω-6 proportions do not comply with nutritional recommendation. A feasible approach to improve the ratio is to blend different oils, but only after understanding physical and chemical changes of the new matrix. Objective of the investigation was to determine the physico-chemical characteristics and the oxidative stability index (OSI), using the Rancimat method, of chia-sesame oil blends. The four ω-3:ω-6 blends tested (1:4, 1:6, 1:8, and 1:10) were exposed to temperatures of 110, 120, and 130 °C. The OSI values of the mixtures varied between 6.24–8.08, 3.07–4.00, and 1.62–2.01 hours for each temperature, respectively. In addition, their mean activation energy, enthalpy, entropy, and Q₁₀ were 88.4 kJ/mol, 85.2 kJ/mol, −41.1 J/mol K, and 2.0. Finally, a shelf life prediction performed at 25 °C indicated stability times between 80 and 123 days. Therefore, combining chia and sesame oils produced blends with a good balance of essential fatty acids.