Oxidation Resistance of AlPO4 Bonded Ceramic Coating Formed on Titanium Alloy for High‐Temperature Applications

AlPO₄ based coatings were prepared on Ti‐6Al‐2Zr‐1Mo‐1V titanium alloy using aluminum phosphate as a binder and Al₂O₃/Cr₂O₃ based mixing particles as the fillers. The microstructure, phase and chemical composition of the coatings were analyzed by SEM, XRD and EDS techniques. The high temperature infrared emissivity values of coated and uncoated titanium samples were tested. The results show that the coating had a higher infrared emissivity value (>0.8) than titanium substrate (0.15–0.3) in the wide wavelength range of 5–20 mm, which is attributed to the uniform dispersion of high emissivity Al₂O₃ and Cr₂O₃ particles in the AlPO₄ binder matrix. The coated titanium samples exhibited excellent oxidation resistance performance with significantly decreased oxidation rates at 600 and 800°C. The mass gain of the coated sample kept at a low and stable constant of 0.15 mg/cm², significantly lower than that of titanium substrate (0.54 mg/cm²) when oxidized at 600°C up to 100 h.     

Synthesis of La0.9Sr0.1Al0.85Mg0.1Co0.05O2.875 using a polymeric method

Nanocrystalline La_0.9Sr_0.1Al_0.85Mg_0.1Co_0.05O_2.875 (LSAMC) powders were synthesized via a polymeric method using poly(vinyl alcohol) (PVA). The effect of PVA content on the synthesized powders was studied. When the ratio of positively charged valences (Mⁿ⁺) to hydroxyl groups (OH) is 1.5:1, crystalline LaAlO₃ could be obtained at such a low calcination temperature as 700 °C. While at 900 °C the ratio is of less importance, since pure LaAlO₃ perovskite could be formed for all powders after calcination at 900 °C. Thermal analysis (TG/DTA) was utilized to characterize the thermal decomposition behaviour of precursor powders. The chemical structure of the calcined powder was studied by Fourier transform infrared (FTIR) spectroscopy. The powder morphology and microstructure were examined by SEM. Dense pellets with well-developed submicron microstructures could be formed after sintering at 1450 °C for 5 h. Compared with the solid-state reaction method, the sintering temperature is substantially lower for powder prepared by the PVA method. This is due to the ultrafine and highly reactive powder produced.     

High Li-ion conductivity of Al-doped Li7La3Zr2O12 synthesized by solid-state reaction

Li₇La₃Zr₂O₁₂ (LLZO) has cubic garnet type structure and is a promising solid electrolyte for next-generation Li-ion batteries. In this work, Al-doped LLZO was prepared via conventional solid-state reaction. The effects of sintering temperature and Al doping content on the structure and Li-ion conductivity of LLZO were investigated. The phase composition of the products was confirmed to be cubic LLZO via XRD. The morphology and chemical composition of calcined powders were investigated with SEM, EDS, and TEM. The Li-ion conductivity was measured by AC impedance. The results indicated the optimum sintering temperature range is 800–950 °C, the appropriate molar ratio of LiOH·H₂O, La(OH)₃, ZrO₂ and Al₂O₃ is 7.7:3:2:(0.2–0.4), and the Li-ion conductivity of LLZO sintered at 900 °C with 0.3 mol of Al-doped was 2.11×10⁻⁴ S cm⁻¹ at 25 °C.     

Fabrication and performances of high lithium-ion conducting solid electrolytes based on NASICON Li1.3Al0.3Ti1.7-xZrx(PO4)3 (0≤ x≤ 0.2)

Solid electrolytes are the key component in designing all-solid-state batteries. The Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) structure and its derivatives obtained by doping various elements at Ti and Al site acts as good solid electrolytes. However, there is still scope for enhancing the ionic conductivity using simple precursors and preparation methods. In this study, the Li superionic conductors Li_1.3Al_0.3Ti_1.7-xZr_x(PO₄)₃ (LATZP) with 0 ≤ x ≤ 0.2 have been successfully prepared by the solid-state reaction route. The structural, morphological, and ionic transport properties were analyzed using several experimental techniques including powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and impedance spectroscopy (IS). The presence of two relaxation processes corresponding to grain and grain boundary was studied using various formalisms. We have observed that grain effects dominate at lower temperatures (<100 °C) while the grain boundary at higher temperatures (> 200 °C) on ionic conductivity. The relaxation mechanisms of grain and grain boundaries were investigated by the Summerfield scaling of AC conductivity. The highest total ionic conductivity of 2.48 × 10^-4 S/cm at 150 °C and 5.50 × 10^-3 S/cm at 250 °C was obtained for x = 0.1 in Li_1.3Al_0.3Ti_1.6Zr_0.1(PO₄)₃ sintered at 950 °C/6 h in the air. The ionic conductivity value was found to be higher than the ionic conductivity reported for LATP prepared via solid-state reaction mechanism using the same precursors and conditions.     

Thermoelectric properties of Pb0.833Na0.017(Zn0.85Al0.15)0.15Te-Te composite

Nanoparticles of Zn_0.85Al_0.15Te and Pb_0.98Na_0.02Te were used as the starting materials to prepare p-type Pb_0.833Na_0.017(Zn_0.85Al_0.15)_0.15Te-Te composite. The resulting powder was densified, sintered at 380 °C for 24 h in an evacuated and encapsulated ampoule and its thermoelectric transport property was characterized between 300 K and 600 K. At 300 K, the electrical resistivity of Pb_0.833Na_0.017(Zn_0.85Al_0.15)_0.15Te-Te composite is 4.2 mΩ-cm; exhibits nonmetal-like behavior from 300 K to 375 K and degenerate behavior beyond 375 K. The temperature dependence of the electrical conductivity shows deviation from the normal power law (1/T^δ, δ ≈ 1.84–2.27 for lead chalcogenides), suggesting a sharp drop in mobility in 425 K–600 K which is ascribed to defects, grain boundaries, and potential energy fluctuation due to atomic disorders. The maximum thermopower of Pb_0.833Na_0.017(Zn_0.85Al_0.15)_0.15Te-Te is 400 μVK^-1 at 600 K. Assuming acoustic phonon scattering is the dominant mechanism, we calculate the reduced Fermi energy and Lorenz numbers and compare them with other materials. As-calculated Lorenz numbers is used to estimate the lattice thermal conductivity, which is 11% lower than the total thermal conductivity at 300 K. The lattice thermal conductivity varies as κ_L ~ T^-0.46 proving the presence of grain boundary scattering, dislocations, and alloy scattering. The maximum power factor (P.F.) of 17.7 μWcm^-1K^-2 is observed at 400 K. Finally, the Pb_0.833Na_0.017(Zn_0.85Al_0.15)_0.15Te-Te composite exhibits a dimensionless figure-of-merit (zT) of 1.08 at 600 K, demonstrating the material from the current study can compete with many high performing PbTe-based materials.     

Efficient fabrication of spinel copper ferrite with enhanced high infrared radiation properties

Rational design and exploration of high infrared radiation materials with remarkable emissivity at high temperatures are always challengeable. In the work, the spinel copper ferrite products with exceptional infrared radiation performance in the wavenumber range of 3–5 μm are massively fabricated through a simple two-step strategy including hydrothermal treatment and low temperature calcination process. Detailed physicochemical characterizations demonstrate that specific structures, compositions, optical behaviors and infrared radiant properties of resultant CuFe₂O₄ samples are enormously dependent upon the involved hydrothermal temperatures/time and annealing temperatures. The synthetic parameters were optimized as hydrothermal process at 150 °C for 16 h and subsequent calcination at 800 °C. The desirable crystallinity, hetero-composition and lower band gap energy synergistically endow the optimal CuFe₂O₄ sample with super high infrared radiation emissivity of ~0.913 evaluated at the testing temperature of 800 °C. Our contribution here will provide significant guidance for scalably low-temperature synthesis of high infrared radiation materials with superb emissivity at high temperatures.     

Synthesis and characterization of La0.85Sr0.15Ga0.85Mg0.15O3−δ electrolyte by steric entrapment synthesis method

Strontium and magnesium doped lanthanum gallate La_0.85Sr_0.15Ga_0.85Mg_0.15O_3−δ (LSGM) oxygen ionic conducting ceramics were prepared by a steric entrapment synthesis (SES) method, which is a polymeric precursor synthesis method by using polyvinyl alcohol in aqueous solution. The perovskite LSGM phase formed essentially at a calcination temperature of 900 °C. Pure and single perovskite LSGM phase with high relative density of 97.1% was obtained after sintering at 1450 °C, while the relative density of the LSGM sample sintered at the same temperature by solid state reaction (SSR) method was 80.6% in present experiment. Comparing with SSR synthesis method, the sintering temperature by SES can be reduced at least 100 °C. Impedance spectra revealed that the grain-boundary resistivity of LSGM synthesized by SES was smaller than that by SSR method, and the conductivities of the samples by SES were higher than those by SSR method at all measuring temperatures.     

Infrared radiation and thermal cyclic performance of a high-entropy rare-earth hexaaluminate coating prepared by atmospheric plasma spraying

In this study, a high-entropy RMgAl₁₁O₁₉ (HE-RMA, R = La, Pr, Nd, Sm, Gd) and LaMgAl₁₁O₁₉ (LMA) coatings were fabricated by atmospheric plasma spraying. The phase composition, microstructure, thermal stability, infrared emissivity performance and shock resistance were comparatively characterized. The results showed that doping multiple rare-earth cations could be conductive to enhance the infrared emissivity. The as-sprayed HE-RMA coating exhibited the highest infrared emissivity, which reached up to 0.971 at 1000 °C. The reason for the improvement of the infrared emissivity was attributed to introduced impurity energy level resulting from doping cations, which could reduce the forbidden bandwidth and increase probability of electronic transition. Meanwhile, HE-RMA coating exhibited better shock resistance at 1100 °C due to superior fracture toughness (1.84 ± 0.41 MPa·m^1/2) during thermal cycling test at 1100 °C. In addition, HE-RMA coating still exhibited high infrared emissivity (0.932 at 1000 °C) at 1100 °C annealing for 100 h with only a slight reduction.     

Efficient strategy to boost the electrochemical performance of yttrium stabilized zirconia electrolyte solid oxide fuel cell for low-temperature applications

Yttrium stabilized zirconia (YSZ) used as the state-of-the-art electrolyte for solid oxide fuel cells (SOFCs) requires high temperature (over 800 °C) to realize sufficient oxygen ion conductivity. Thus, the high operational temperature is the main restriction for the commercial process of YSZ-based SOFCs. To obtain decent ionic conductivity at intermediate-low temperatures, Sr-free cathode LaNiO₃ is introduced into YSZ to construct a novel LaNiO₃-YSZ composite electrolyte, which is sandwiched by two Ni_0.8Co_0.15Al_0.05LiO_2-δ (NCAL) electrodes to assemble systematical fuel cells. This device presents an excellent peak output of 1045 mW cm^-2 at 600 °C and even 399 mW cm^-2 at 450 °C. A series of characterizations indicates that the oxygen ion conductivity of the LaNiO₃-YSZ composite is significantly promoted in comparison with that of pure YSZ, and the LaNiO₃ component has certain proton conductivity after hydrogenation. Both of the two factors contributes to the superior performance of such devices at intermediate-low temperatures. Furthermore, the sharp decrease in electronic conductivity for LaNiO₃ in hydrogen atmosphere combined with Schottky junction at the anode-electrolyte interface eliminates the short-circuiting problem. Our work demonstrates that incorporating Sr-free cathode LaNiO₃ into the YSZ electrolyte is an efficient strategy to boost the performance and reduce the operational temperature of YSZ-based SOFCs.     

Complex impedance spectroscopy study of a liquid-phase-sintered α-SiC ceramic

The electrical response of a liquid-phase-sintered (LPS) α-SiC with 10 wt.% Y₃Al₅O₁₂ (YAG) additives was studied from near-ambient temperature up to 800 °C by complex impedance spectroscopy. The electrical conductivity of this LPS SiC ceramic was found to increase with increasing temperature, which was attributed to the semiconductor nature of the SiC grains. It was concluded that the contribution of the SiC grains to the electrical conductivity of the LPS SiC ceramic at moderate temperatures (<450 °C) is a somewhat greater than that of the YAG phase. In contrast, at higher temperatures the SiC grains control the electrical conductivity of the LPS SiC ceramic. It was also found that there are two activation energies for the electrical conduction process of the α-SiC grains. These are 0.19 eV at temperatures lower than ∼400 °C and 2.96 eV at temperatures higher than ∼500 °C. The existence of two temperature-dependence conduction regimes reflects the core–shell substructure that develops within the SiC grains during the liquid-phase sintering, where the core is pure SiC (intrinsic semiconductor) and the shell is mainly Al-doped SiC (extrinsic semiconductor).     

Phase transformation and grain-boundary segregation in Al-Doped Li7La3Zr2O12 ceramics

Cubic phase garnet-type Li₇La₃Zr₂O₁₂ (LLZO) is a promising solid electrolyte for highly safe Li-ion batteries. Al-doped LLZO (Al-LLZO) has been widely studied due to the low cost of Al₂O₃. The reported ionic conductivities were variable due to the complicated Al³⁺-Li⁺ substitution and Li_xAlO_y segregation in Al-LLZO ceramics. This work prepared Li_7?3xAl_xLa₃Zr₂O₁₂ (x = 0.00~0.40) ceramics via a conventional solid-state reaction method. The AC impedance and corresponding distribution of relaxation times (DRT) were analyzed combined with phase transformation, cross-sectional microstructure evolution, and grain boundary element mapping results for these Al-LLZO ceramics to understand the various ionic transportation levels in LLZO with different Al-doping amounts. The low conductivity in low Al-doped (0.12~0.28) LLZO originates from the slow Li⁺ ion migration (1.4~0.25 μs) in the cubic-tetragonal mixed phase. On the other hand, LiAlO₂ and LaAlO₃ segregation occur at the grain boundaries of high Al-doped (0.40) LLZO, resulting in a gradual Li⁺ ion jump (6.5 μs) over grain boundaries and low ionic conductivity. The Li_6.04Al_0.32La₃Zr₂O₁₂ ceramic delivers the optimum Li⁺ ion conductivity of 1.7 × 10^?4 S cm^?1 at 25 °C.     

Formation of porous aggregations composed of Al2O3 platelets using potassium sulfate flux

Porous aggregations, with about 10 μm diameter, composed of Al₂O₃ platelet crystals were formed by heating a powder mixture consisting of Al₂(SO₄)₃+2K₂SO₄ (mol ratio) in an alumina crucible at temperatures 1000–1300°C for 3 h and removing the flux component with hot hydrochloric acid after heating. The specific surface area of the aggregations obtained by heating at 1000°C for 3 h was maximum and its value was 5·2 m² g⁻¹. Since the size of Al₂O₃ platelets increased and the number of Al₂O₃ platelets decreased, the specific surface area decreased to 0·7 m² g⁻¹ at 1100°C. When heated at 1300°C, the size of the Al₂O₃ platelets increased with increasing amount of K₂SO₄ in the starting powder mixture. ©     

Direct synthesis of PMN samples by spray-drying

The processing and characterisation of Pb(Mg_1/3Nb_2/3)O₃ (PMN) materials, obtained either by spray-drying the solution of the precursors or by the conventional “columbite” method, were investigated and the morphological and micro-structural characteristics were compared. The acid solution of ammonium-peroxo-niobium complex, magnesium and lead nitrates was spray-dried and the precursor powder obtained was calcined at different temperatures ranging from 350 to 900 °C. The morphologies and the XRD patterns of the powders were compared. The calcined powders exhibited a pyrochlore phase above 400 °C converting into an almost pure perovskite phase at 800 °C. The powder calcined at 350, 500 and 800 °C were sintered at different temperatures, ranging from 950 to 1150 °C, always resulting in a pure perovskite PMN material. The XRD patterns of as-fired surfaces of samples sintered at 950 and 1050 °C showed an unwanted PbO phase together with the main PMN, nevertheless this secondary phase is not present in the ground surfaces. The high reactivity of sprayed powder is reflected in the formation and densification of pure perovskite PMN material with a faster process as regards the conventional one; in particular samples of about 96% theoretical density were obtained starting from the amorphous powder calcined at low temperature (350 °C) through a reaction sintering process. Furthermore, due to the better flowability of the spray-dried powder, the cold consolidation process is highly improved and no binder addition to powder is necessary.     

High-entropy alloy: An alternative approach for high temperature microwave-infrared compatible stealth

To achieve microwave-infrared compatible stealth in high temperature conditions, high-entropy alloys (HEAs) thin films were deposited on Al₂O₃ matrix by magnetron sputtering technology. Films were annealed to investigate thermal stability at 500 °C, 600 °C and 700 °C, respectively. Results from X-ray diffract meter (XRD), atomic force microscope (AFM), scanning electron microscope (SEM), and Fourier transform infrared spectrometer (FTIR) suggested that high-entropy alloy (HEA) film was seriously oxidized when the annealed temperature reached 700 °C for 6 h, causing a significant decrease of infrared reflectivity. Conversely, HEA films showed low infrared emissivity of 0.09 at 600 °C. Additionally, the films possessed excellent thermal stability at 500 °C for 20 h with low infrared emissivity of 0.11. Finally, a simple metamaterial design utilizing HEA films was proposed for infrared-microwave compatible stealth. With the ability of incorporating excellent thermal stability and durable high temperature stealth performance, the study shows great potential of introducing HEAs in the field of high temperature compatible stealth.     

Sintering mechanisms of Yttria with different additives

The sintering behaviour of conventional yttria powder was investigated, with emphasis on the effect of sintering additives such as B₂O₃, YF₃, Al₂O₃, ZrO₂, and TiO₂, etc. at sintering temperatures from 1000 °C to 1600 °C. Powder shrinkage behaviour was analysed using a dilatometer. The powder sintering mechanisms were identified at different temperatures using powder isothermal shrinkage curves. This analysis showed that the sintering additives B₂O₃ and YF₃ could improve yttria sintering by changing the diffusion/sintering mechanisms at certain temperatures, while sintering additives TiO₂, Al₂O₃ and ZrO₂ appeared to retard the powder densification at temperatures around 1000 °C and are more suitable when used at temperatures in excess of 1300 °C. The powder with La₂O₃ added had the slowest densification rate throughout the test temperatures in this experiment and was also found to be more suitable when used at temperatures higher than 1550 °C.     

Electrical conductivity of Al-doped Li2ZrO3 ceramics for Li-ion conductor electrolytes

All-solid-state Li-ion batteries (LIBs) have recently attracted widespread attention for their high energy density and safety. Some research have conducted on the Li₂ZrO₃-based Li-ion conductor electrolytes, while there is little work on the conductivity below 100 °C, although it is very important for LIBs work around room temperature. Here, monoclinic Li₂ZrO₃-based ceramics are prepared via a wet chemistry method, and the conductivities of Li₂ZrO₃ ceramics are tuned by defect engineering of Al³⁺ ions introduction. The conductivity of Al-doped Li₂ZrO₃ reaches up to 3.06 × 10^-4 S cm^-1 at 25 °C, the related activation energy of conduction is less than 0.1 eV. Simulation calculation using bond valence site energy reveals that there is a two-dimensional Li-ion migration network in the crystal structure of Li₂ZrO₃.     

Microwave dielectric properties of Ca1.15RE0.85Al0.85Ti0.15O4 (RE=Nd,La, Y) ceramics prepared by the reaction sintering method

Ca_1.15RE_0.85Al_0.85Ti_0.15O₄ (RE = Nd, La, Y) ceramics were prepared by a reaction sintering method. The sintering behavior, phase composition, microstructure and microwave dielectric performances of ceramics were investigated. X-ray diffraction patterns illustrated that both the Ca_1.15Nd_0.85Al_0.85Ti_0.15O₄(CNAT) and Ca_1.15Y_0.85Al_0.85Ti_0.15O₄(CYAT) ceramics are single-phase structures, and the Ca_1.15La_0.85Al_0.85Ti_0.15O₄(CLAT) ceramic contain LaAlO₃ and CaO phases. The apparent morphology and elemental distribution of the ceramic samples were analyzed by using scanning electron microscope and energy dispersive spectrometer. When the sintering temperature is 1500 °C, the CNAT and CYAT ceramics have the best microwave dielectric properties with ε_r = 19.2, Q × f = 74924 GHz, τ_f = ?1.21 ppm/°C and ε_r = 17.5, Q × f = 27440 GHz, τ_f = ?5.79 ppm/°C, respectively. And the best microwave dielectric properties of ε_r = 17.5, Q × f = 22568 GHz, τ_f = ?14.69 ppm/°C were obtained for the CLAT ceramic sintered at 1525 °C. The reaction sintering method provides a low-cost, economical and straightforward method for the preparation of the Ca_1.15RE_0.85Al_0.85Ti_0.15O₄ (RE = Nd, La, Y) ceramics, which has promising potential for application.     

Fabrication and dielectric properties of barium titanate-based glass ceramics for tunable microwave LTCC application

Low-fired ferroelectric glass ceramics were fabricated from glass powders with a basic composition of 0.65BaTiO₃·0.27SiO₂·0.08Al₂O₃. The combined addition of SnO₂ (or ZrO₂) and SrCO₃ was conducted to modify the dielectric properties of the glass ceramics. The Sr-component could be incorporated preferentially in the perovskite structure after heating at 1000 °C. The bulk and thick film samples obtained by sintering glass powder with a starting composition of 0.65(Ba_0.7Sr_0.3)(Ti_0.85Sn_0.15)O₃·0.27SiO₂·0.08Al₂O₃ at 1000 °C for 24 h showed a broadened ɛ_r–T relation with T_c ≈ 10 °C and ɛ_r(max) ≈ 280 and microwave tunability of 32% at 3 GHz, respectively.     

Infrared radiative performance and anti-ablation behaviour of Sm2O3 modified ZrB2/SiC coatings

To improve the emissivity of ZrB₂/SiC coatings for serving in more serious environment, ZrB₂/SiC coatings with varying contents of high emissivity Sm₂O₃ were fabricated using atmospheric plasma spraying. The microstructure, infrared radiative performance and anti-ablation behaviour of the modified coatings were investigated. The results showed that as the content of Sm₂O₃ increased, the density of the coatings increased because of the low melting point of Sm₂O₃. When the content of Sm₂O₃ was 10 vol%, the coating had the highest emissivity in the 2.5–5 μm band at 1000 °C, up to 0.85, because of the oxygen vacancies promoting additional electronic transitions. Due to the high emissivity, the surface temperature of the coating modified with 10 vol% Sm₂O₃ decreased by 300 °C, which led to little volatilisation of the sealing phase. Further, the mass ablation ratio of the above coating was 3.19 × 10^?4 g/s, decreasing 31% compared to that of a ZrB₂/SiC coating. The formed dense surface structure of the coatings showed considerable oxygen obstructive effects. These findings indicate that the modified coatings show considerable anti-ablation performance, which provides effective anti-ablation protection for the C/C composite substrate.