Corrosion behavior and strength degradation of Ti3SiC2 exposed to a eutectic K2CO3 and Li2CO3 mixture

The corrosion of polycrystalline Ti₃SiC₂ was studied in the eutectic Li₂CO₃ (68 at.%) and K₂CO₃ (32 at.%) mixture at 650–850 °C. Ti₃SiC₂ exhibited better corrosion resistance at 650 °C. However, the mass loss was fast when temperature was above 700 °C. It was demonstrated that the surface chemical reaction-controlled shrinking core model could be applied to describe the relationship between the degree of the corrosion and reaction time for the corrosion of Ti₃SiC₂ in the 700–850 °C temperature range. The corresponding apparent activation energy was 206 kJ/mol. Corrosion resulted in roughness of specimen surface. The fracture strength of the corroded samples was evaluated by a three-point bending test. The results showed that the degradation of the fracture strength was about 25% of the original values for the corroded specimens up to 10% weight loss. The mechanism of the strength degradation was discussed based on the analysis of the microstructure and composition of the corroded sample.     

Phase and microstructure evolution of Sc2O3–CeO2 –ZrO2ceramics in Na2SO4 + V2O5 molten salts

In this study, the destabilization resistance of Sc₂O₃ and CeO₂ co-stabilized ZrO₂ (SCZ) ceramics was tested in Na₂SO₄ + V₂O₅ molten salts at 750°C–1100 °C. The phase structure and microstructure evolution of the samples during the hot corrosion testing were analyzed with X-ray diffraction (XRD), Raman spectra, scanning electron microscopy (SEM), energy dispersive X-ray spectrum (EDS), and X-ray photoelectron spectroscopy (XPS). Results showed that the destabilization of SCZ ceramics at 750 °C was the result of the chemical reaction with V₂O₅ to produce m-ZrO₂ and CeVO₄, and little ScVO₄ was detected in the Sc₂O₃-rich SCZ ceramics. The primary corrosion products at 900 °C and 1100 °C were CeO₂ and m-ZrO₂ due to the mineralization effect. The Sc₂O₃-rich SCZ ceramics exhibited excellent degradation resistance and phase stability owing to the enhanced bond strength and the decreased size misfit between Zr⁴⁺ and Sc³⁺. The destabilization mechanism of SCZ ceramic under hot corrosion was also discussed.     

Anodic dissolution of U, Zr and U–Zr alloy and convolution voltammetry of Zr|Zr couple in molten LiCl–KCl eutectic

The anodic dissolution of U and Zr metal was studied in LiCl–KCl–UCl₃ and LiCl–KCl–ZrCl₄, respectively, at 773 K by cyclic voltammetry and compared with their respective dissolution behaviour in blank LiCl–KCl eutectic. The anodic dissolution of U–Zr alloy in LiCl–KCl–UCl₃ was also studied at 773 K to compare with the dissolution of U and Zr. The transfer coefficients evaluated by Tafel analysis and the method of Allen–Hickling for U and Zr dissolution were found to be in fair agreement with each other. U dissolution in LiCl–KCl–UCl₃ and Zr dissolution LiCl–KCl–ZrCl₄ were also studied by chronoamperometry and the diffusion coefficient value of U³⁺ was calculated to be in the range of 2.9 × 10⁻⁵ to 3.3 × 10⁻⁵ cm² s⁻¹ which is in agreement with those reported in literature. Convolution voltammetric analysis of Zr⁴⁺/Zr²⁺ redox couple in LiCl–KCl–ZrCl₄ was carried out for the first time to have a comprehensive understanding of the electrode kinetics.     

Ultrafast high-temperature sintering of Li7La3Zr1.75Nb0.25Al0.15O12 (LLZO)

Rapid sintering of Li₇La₃Zr_1.75Nb_0.25Al_0.15O₁₂ (LLZO) is reported. The selection of heating elements, the effect of powder preparation and MgO additions in rapid sintered LLZO are described. Annealing LLZO powder at 750 ºC for 2 h in argon immediately before pressing helped to minimize porosity. A 15–20 s hold at 1372 ºC was sufficient to achieve densities >97%. The total sintering schedule time for rapid sintering represents a 99.7% decrease in sintering time compared to conventional sintering. At 70 °C under a pressure of 4.12 MPa cells had a critical current density of 1020 µA/cm².     

Stability and wettability of ternary carbide Mo2Ga2C in molten metals

Mo₂Ga₂C is a novel member of MAX phases, which is a family of ternary carbide/nitride with layered structure. The stability and wettability of Mo₂Ga₂C in molten metals are crucial for the application of Mo₂Ga₂C in the field of composite materials. In this work, the stability and wettability of Mo₂Ga₂C in three molten metals (Pb, Sn and Zn) were investigated. Mo₂Ga₂C was mixed with metal powders and annealed at different temperatures under vacuum. At 700 °C, Mo₂Ga₂C was not stable in any of the three molten metals. The decomposition of Mo₂Ga₂C was affected by the type of metal matrix as well as the reaction temperature. Mo₂Ga₂C decomposed at 550 °C in Pb, at 450 °C in Sn, or at 650 °C–750 °C in Zn. To characterized the wettability of these metals with Mo₂Ga₂C, the contact angle of molten metals with Mo₂Ga₂C at 650 °C were measured, which are 103° for Pb, 126° for Sn and 0° for Zn, respectively. Mo₂Ga₂C has good wettability with Zn and is not wetted by Pb and Sn.     

Interaction between gadolinium zirconate and LiCl–Li2O melt

The interaction between Gd₂Zr₂O₇ and molten LiCl–Li₂O (2 wt%) was studied for 24–52 h at 650–710 °C in an argon atmosphere. Gd₂Zr₂O₇ is analyzed as a promising structural material for sensors used during pyrochemical reprocessing of spent nuclear fuel and for long-term storage or final disposal of high-level nuclear wastes.The chemical stability of Gd₂Zr₂O₇ relative to the components of the LiCl–Li₂O melt was thermodynamically evaluated. The surface morphology and structure of the samples before and after the experiment were analyzed using an X-ray diffractometer and scanning electron microscopy. The formation of a new Li⁺-doped phase based on Gd₂Zr₂O₇ and the Gd₂O₃ evolution onto the material surface was revealed by the X-ray diffraction analysis (XRD). Changes in the microstructure of the samples confirm the presence of large particles in the surface layer corresponding to the Gd₂O₃ phase, which is in good agreement with the XRD data. A profilometer was used to measure the roughness of the ceramics. Presumably, the thickness of the lithium-doped Gd₂Zr₂O₇ film, which is inhomogeneously distributed over the surface of the samples, was 3 μm. Therefore, it was found that dense Gd₂Zr₂O₇ (F) and Gd₂Zr₂O₇ (P) ceramics can be used in LiCl–Li₂O (2 wt %) as a structural material resistant to the high-temperature chemical attack.     

High Speed Imaging of TATB‐ and HMX‐Based Energetic Material Decomposition in Molten Salts

Immersion of energetic materials into high‐temperature molten‐salt baths, where the energetic materials decompose, is being considered as a method for their safe destruction. In the present research, behaviors of the high explosives LX‐17 (92.5 wt% 1,3,5‐triamino‐2,4,6‐trinitrobenzene (TATB), 7.5 wt% KeI‐F 800 plastic binder) and LX‐04 (85 wt% octahydro‐,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine (HMX), 15 wt% Viton A plastic hbinder) were studied when these materials were immersed into molten salt baths. Pressed cylindrical samples initially 6.35 mm in diameter and length were immersed in molten salt baths, and data were taken photographically. Sample decomposition behaviors were observed for varied salt temperatures in a molten LiCl‐NaCl‐KC1 eutectic and then separately in a molten Li₂CO₃‐Na₂CO₃‐K₂CO₃ eutectic. Bath temperatures ranged from 650 to 750°C. General combustion behaviors such as bubble formation characteristics, gas evolution, and sample lifetimes were observed. Results indicated that sample lifetimes decreased as bath temperatures increased, and that the carbonate eutectic increased initial decomposition rates and decreased sample lifetimes relative to the chloride eutectic.     

Effects of annealing temperature on the phase formation,optical, photoluminescence and magnetic properties of sol-gel YFeO3 films

YFeO₃ (YFO) thin films were deposited onto quartz substrates via sol-gel spin-coating technique and annealed at different temperature ranged between 650 and 900 °C. The impact of annealing temperature on the phase formation, microstructural, optical, photoluminescence (PL) and magnetic properties of the films were systematically investigated. X-ray diffraction analysis revealed an amorphous structure in film annealed at 650 °C and formation of hexagonal-YFO (h-YFO) phase in films annealed at 750–800 °C. The films annealed at 850–900 °C exhibited an orthorhombic-YFO (o-YFO) structure. Atomic force microscopy images of h-YFO films showed homogeneous surface with uniform particles size and shape. The particle size increased and had irregular shape in o-YFO films. The average particle size was 44 and 117 nm, while the root square roughness was 1.38 and 2.55 nm for h- and o-YFO films annealed at 750 and 850 °C, respectively. The optical band gap (E_g) was 2.53 and 2.86 eV for h- and o-YFO films annealed at 750 and 850 °C, respectively. The PL spectra of h-YFO films were red-shifted compared with that of o-YFO films. The PL emission related to near band edge was observed at 459.0 and 441.9 nm for h- and o-YFO films annealed at 750 and 850 °C, respectively. The magnetization was enhanced with the increasing of annealing temperature and has the value of 4.8 and 12.5 emu/cm³ at 5000 Oe for h- and o-YFO films annealed at 750 and 850 °C, respectively.     

High-temperature ultrafast welding creating favorable V2O5 and conductive agents interface for high specific energy thermal batteries

The high interfacial resistance between V₂O₅ cathode materials and conductive agents (molten salt and super carbon) is one of the biggest issues that hinder the development of high specific energy thermal batteries. Designing fast Li⁺ and e^– transport channels in cathode electrodes is considered as an effect method to improve electrochemical performance. Hence, a high-temperature ultrafast welding is proposed to reduce V₂O₅/conductive agents interfacial resistance by reconstructing the transmission channels of Li⁺ and e^– in this paper. The experimental studies reveal the optimum ultrafast welding of 700 °C for 10 s, eliminating gap resistance of cathode electrodes induced by the melt of solid molten salt and rebuilding the more plentiful Li⁺ and e^– transport channels, further reducing the contact resistance and gap resistance. Therefore, the electrodes deliver a high specific capacity of 270.69 mAh g^?1 and a high specific energy of 610.60 Wh kg^?1 at 0.1 A cm^?2 and 500 °C with a cut-off voltage of 1.6 V. The high-temperature ultrafast welding provides guidance to build Li⁺ and e^– transport channels of other cathode materials in thermal batteries.     

Sintering behavior,structure and dielectric properties of CuO-ZnO-MgO-B2O3 glass-ceramics for ULTCC applications

High performance ultra-low temperature co-fired ceramic (ULTCC) materials were prepared from CuO- MgO- ZnO- Al₂O₃- B₂O₃- Li₂O glass-ceramics. The sintering behaviors, crystalline phase evolution, microstructure and dielectric properties, as well as their compatibility with Ag and Al electrodes, were investigated. With the suitable substitution of MgO for ZnO, the dielectric properties of glass-ceramics were improved. It is mainly associated with the fine microstructure, highly crystallinity, and decrease in tetrahedral distortion in the crystal lattice. All the glasses completed the densification at 575–600 °C, and ZnB₄O₇ is the only crystalline phase precipitated from the glasses. Moreover, the glass-ceramic with 1 wt% MgO sintered at 575 °C for 5 h, exhibited low relative permittivity ~ 7.1 and low dielectric loss ~ 6.40 × 10^?4. And the glass-ceramic with 4 wt% MgO sintered at 600 °C for 5 h, also displayed low relative permittivity ~ 7.1 and low dielectric loss ~ 5.77 × 10^?4. Both two glasses have good sintering compatibility with silver and aluminum electrodes, which provided high potential for ULTCC application.     

Comparison of Effects of Calcium and Magnesium Doping on the Structure and Biological Properties of NaTaO3 Film on Tantalum

Tantalum is widely used in hip joint replacement and knee joint repair, but its clinical application is limited due to its poor biological activity and weak ability to promote new bone formation. Ca and Mg ions are thought to be involved in bone metabolism and play an important physiological role in the angiogenesis, growth, and mineralization of bone tissue. In this work, NaTaO₃ films doped with Ca²⁺ and Mg²⁺ were prepared by hydrothermal synthesis and molten salt method. The doping amounts of Ca²⁺ doped at 450, 550, 650 and 750 °C were 0.59, 3.44, 32.75 and 29.88 at%, and that of Mg²⁺ doped at 300, 350, 400, 450, 500, 550 and 650 °C were 0.62, 1.03, 1.54, 20.12, 21.38, 14.37 and 0.74 at%, respectively. Ca²⁺ and Mg²⁺ are evenly incorporated into NaTaO₃ and cause the change of crystal plane spacing without any significant changes of morphologies below 550 and 400 °C respectively. XPS shows that the cations are the A-site substitution of perovskite structure (ABO₃). According to the morphology and composition analysis of Ca-incorporated samples and Mg-incorporated samples, the optimal preparation temperature of them is 550?°C and 400?°C, respectively. The results show that for “550?°C-Ca” and “400?°C-Mg” the hydrophilicity is 13.9° and 96.1°, the roughness is 114.3 and 54.3?nm, the doping ion concentration of Ca and Mg is 3.44 and 1.54 at%, and the 7-day ICP results is 69.8 and 1.4?ppm, respectively. In addition, cell proliferation experiments and cell morphology related to biological activity and osteogenic properties are discussed, and it is found that the performance of “550?°C-Ca” is better than “400?°C-Mg”. Ca²⁺–NaTaO₃ is a promising implantable material that will be extensive used in bone implants, joint replacements and dental implants.

     

Assessment of bismuth oxide-based electrolytes for composite gas separation membranes

Oxide + salt composites can be used in CO₂ and NO_x separation membranes, where high oxide-ion conductivity is crucial to improve performance. Pursuing this goal, the stability of three different bismuth oxide-based electrolytes (Cu + V, Y and Yb-doped) against molten alkali carbonates (Li, Na, K) or nitrates (Na, K) was tested firing them in the 450–550 °C temperature range, and with endurance tests up to 100 h. A well-known ceria-based composite was used as reference (CGO - Ce_0.9Gd_0.1O_1.95). Oxides and composites were studied by X-ray diffraction, scanning electron microscopy and impedance spectroscopy (in air, 140–650 °C temperature range). Bi₂Cu_0.10V_0.90O_5.35 easily reacts with molten salts. Bi_0.75Y_0.25O_1.5 and Bi_0.75Yb_0.25O_1.5 have higher stability against molten carbonates and complete stability against molten nitrates. The Y-doped oxide stability against the molten carbonates was enhanced changing the molten salt composition (Y₂O₃ additions) and using lower firing temperatures. Above all, composites based on Y or Yb-doped Bi₂O₃ with molten alkali nitrates showed impressive 6× or 3× higher electrical conductivity at 290 °C, in air (4.88 × 10⁻² and 2.41 × 10⁻² S cm⁻¹, respectively) than CGO-based composites (7.72 × 10⁻³ S cm⁻¹), qualifying as promising materials for NO_x separation membranes.     

Direct electrochemical reduction of titanium dioxide in molten lithium chloride

The electrochemical reduction of TiO₂ has been carried out in a molten LiCl salt at 650 °C. The direct reduction mechanism of TiO₂ in the molten LiCl was suggested by the CV experiment and the constant voltage electrolysis. TiO₂ was electrochemically converted to the metallic titanium via various reaction intermediates such as LiTiO₂, TiO, and Ti₂O. The interrupted voltage electrolysis was demonstrated to be effective for the direct electrochemical reduction of TiO₂ to achieve a high current efficiency. The surface morphology of the produced metal was investigated by SEM and TEM.     

Influence of precursors of Li2O and MgO on surface and catalytic properties of Li‐promoted MgO in oxidative coupling of methane

The influence of the catalyst precursors (for Li₂O and MgO) used in the preparation of Li‐doped MgO (Li/Mg = 0.1) on its surface properties (viz basicity, CO₂ content and surface area) and activity/selectivity in the oxidative coupling of methane (OCM) process at 650–750 °C (CH₄/O₂ feed ratio = 3.0–8.0 and space velocity = 5140–20550 cm³ g⁻¹ h⁻¹) has been investigated. The surface and catalytic properties are found to be strongly affected by the precursor for Li₂O (viz lithium nitrate, lithium ethanoate and lithium carbonate) and MgO (viz magnesium nitrate, magnesium hydroxide prepared by different methods, magnesium carbonate, magnesium oxide and magnesium ethanoate). Among the Li–MgO (Li/MgO = 0.1) catalysts, the Li–MgO catalyst prepared using lithium carbonate and magnesium hydroxide (prepared by the precipitation from magnesium sulfate by ammonia solution) and lithium ethanoate and magnesium acetate shows high surface area and basicity, respectively. The catalysts prepared using lithium ethanoate and magnesium ethanoate, and lithium nitrate and magnesium nitrate have very high and almost no CO₂ contents, respectively. The catalysts prepared using lithium ethanoate or carbonate as precursor for Li₂O, and magnesium carbonate or ethanoate, as precursor for MgO, showed a good and comparable performance in the OCM process. The performance of the other catalysts was inferior. No direct relationship between the basicity of Li‐doped MgO or surface area and its catalytic activity/selectivity in the OCM process was, however, observed. © 2000 Society of Chemical Industry     

Effects of different sintering additives on the synthesis of γ-Y2Si2O7 powders

Pure γ-Y₂Si₂O₇ powders were synthesized by the solid-liquid reaction method using Y₂O₃ and SiO₂ powders with Li₂O, MgO or Al₂O₃ additives. The effects of the metallic ions Li⁺, Mg²⁺ and Al³⁺ on the synthesis process were systematically investigated by X-ray diffraction and differential scanning calorimetry. The chemical kinetics of the Y₂Si₂O₇ synthetic process was calculated to illuminate the influences of the different metallic ions on the formation of silicate. The results indicate that the additives could effectively reduce the synthesis temperature by 100–300 °C. The apparent activation energy of the synthetic reaction was reduced by 79.75%, 65.16%, or 56.77% when 6 mol.% of Li₂O, MgO, or Al₂O₃ was added, respectively, and the reaction rate was also significantly increased.     

High Reactive MgO‐Y2O3 Nanopowders via Microwave Combustion Method and Sintering Behavior

To decrease the sintering temperature of MgO‐Y₂O₃ composites to avoid undesired grain coarsening, high reactive MgO‐Y₂O₃ nanopowders were synthesized via microwave combustion method. The degree of combustion was enhanced effectively by adding an extra oxidant ammonium nitrate. The as‐synthesized MgO‐Y₂O₃ nanopowders, ~18 nm in size, showed high specific surface area of 64.55 m²/g and low agglomeration. Relative density of 98% was obtained when sintered at a low sintering temperature of 1350°C. The high reactivity can be attributed to the lower activation energy Q (131.13 kJ/mol), compared with samples without extra oxidant (192.97 kJ/mol).     

High velocity oxygen fuel sprayed Cr3C2-NiCr coatings against Na2SO4 hot corrosion at different temperatures

Cr₃C₂-NiCr/NiCrAlY coating was prepared by high velocity oxygen fuel (HVOF) spraying. The microstructure and the Na₂SO₄ hot corrosion behavior at different temperatures of the coating were investigated. The Na₂SO₄ hot corrosion mechanism of Cr₃C₂-NiCr coating was also discussed. The results showed that HVOF Cr₃C₂-NiCr coating was relatively dense and mainly composed of Cr₃C₂, NiCr and a small amount of Cr₇C₃ three phases. The dense Cr₂O₃ layer was formed on the surface of Cr₃C₂-NiCr coating after Na₂SO₄ corrosion at 750 °C to further prevent corrosion. The coating had produced the obvious longitudinal crack with the increase of hot corrosion temperature up to 900 °C. The corrosion mechanism of Cr₃C₂-NiCr coating against the Na₂SO₄ salt at high-temperature was as follows: firstly, the protective oxidizing film was formed at 750 °C, then the protective oxide film dissolved at the interface between the coating and Na₂SO₄ salt with the hot corrosion temperature increasing up to 900 °C, and subsequently the dissolved anions and cations could migrate to the interface between molten salt and air and the loose and unprotected oxides were regenerated, thereby exacerbating the failure of the coating.     

Synthesis of Ga-doped Li7La3Zr2O12 solid electrolyte with high Li+ ion conductivity

Garnet-type Li₇La₃Zr₂O₁₂ (LLZO) Li⁺ ion solid electrolyte is a promising candidate for next generation high-safety solid-state batteries. Ga-doped LLZO exhibits excellent Li⁺ ion conductivity, higher than 1 × 10^?3 S cm^?1. In this research, the doping amount of Ga, the calcination temperature of Ga-LLZO primary powders, the sintering conditions and the evolution of grains are explored to demonstrate the optimum parameters to obtain a highly conductive ceramics reproducibly via conventional solid-state reaction methods under ambient air sintering atmosphere. Cubic LLZO phase is obtained for Li_6.4Ga_0.2La₃Zr₂O₁₂ powder calcined at low temperature 850 °C. In addition, ceramic pellets sintered at 1100 °C for 320 min using this powder have relative densities higher than 94% and conductivities higher than 1.2 × 10^?3 S cm^?1 at 25 °C.     

Crystallization behavior of CMAS and NaVO3+CMAS mixture and its potential effect to thermal barrier coatings corrosion

Calcium-magnesium-alumina-silicate (CMAS) and molten salt corrosion pose great threats to thermal barrier coatings (TBCs), and recently, a coupling effect of CMAS and molten salt has been found to cause even severer corrosion to TBCs. In this study, the crystallization behavior of CMAS and CMAS+NaVO₃ is investigated for potentially clarifying their corrosion mechanisms to TBCs. Results indicated that at 1000 °C and 1100 °C, CMAS was crystallized to form CaMgSi₂O₆, while at 1200 °C, the crystallization products were CaMgSi₂O₆, CaSiO₃ and CaAl₂Si₂O₈. The introduction of NaVO₃ in CMAS reduced the crystallization ability, and as the NaVO₃ content increased, glass crystallization occurred at a lower temperature, with crystallization products mainly consisting of CaAl₂Si₂O₈ and CaMgSi₂O₆. At 1200 °C, CMAS+10 wt% NaVO₃ was in a molten state without any crystallization, which suggested that NaVO₃ addition in CMAS could reduce its melting point, indicating enhanced penetration ability in TBCs and thus increased corrosiveness.