High temperature corrosion of Al<Subscript>3</Subscript>Ti−Cr intermetallics in an Ar−1 %SO<Subscript>2</Subscript> gas atmosphere

Ll₂-type Al₃Ti−Cr alloys of compositions 67Al−25Ti−8Cr, 66Al−24Ti−10Cr, and 59Al−26Ti−15Cr (in atomic percent) were corrosion tested between 800°C and 1100°C in an Ar−1% SO₂ gas atmosphere for up to 150h. Corrosion proceeded mainly via oxidation reactions. The scale consisted primarily of a thin Al₂O₃ barrier layer. The alloys tested had much better corrosion resistance than TiAl-base alloys or Hastelloy X. Virtually no sulfides were detected in the scale, because of the strong oxidizing tendency of the Al₃Ti−Cr alloys having high Al concentrations.     

Synthesis and characterization of Ba<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>Co<Subscript>0.8</Subscript>Fe<Subscript>0.2</Subscript>O<Subscript>3−σ</Subscript>

Perovskite-type Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCFO) powders were synthesized using two methods, solid-state reaction (SSR) method and citrate-EDTA complexing method (CC-EDTA). Then the powders were pressed to green disks of 19 mm in diameter and sintered at 1140°C for 5 h. The shrinkage rate and relative density of the membranes prepared from the perovskite-type powders were determined and calculated, and the powders and derived membranes were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that the shrinkage rates of the two kinds of disks are nearly the same (about 10%). The disks prepared by the SSR method had a bigger grain size and lower relative density than those prepared by the CC-EDTA method. The conductivity of the membranes prepared by the SSR method was about 38 S/cm, higher than that of the membranes prepared by the CC-EDTA method, which was about 30 S/cm, at the same temperature of 600°C.     

Spark Plasma Sintering of α/β Si<Subscript>3</Subscript>N<Subscript>4</Subscript> Ceramics with MgO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and MgO-Y<Subscript>2</Subscript>O<Subscript>3</Subscript> as Sintering Additives

In the present work, the α/β Si₃N₄ ceramics were fabricated by spark plasma sintering (SPS) at 1400-1500 °C for 6 min with 3wt.%MgO + 5wt.%Al₂O₃ and 3wt.%MgO + 5wt.%Y₂O₃ as sintering additives. The results showed that the phase composition, microstructure and mechanical properties of α/β Si₃N₄ ceramics were highly dependent on the type of sintering additive. The incomplete phase transformation from α to β occurred in the presence of an oxynitride (Mg-Al(Y)-Si-O-N) liquid phase. Compared with MgO-Al₂O₃, MgO-Y₂O₃ can significantly improve the β conversion rate of as-sintered α/β Si₃N₄ ceramics. And the as-sintered ceramics using MgO + Al₂O₃ as sintering additives had higher mechanical properties.     

Air Oxidation of a Ni<Subscript>53</Subscript>Nb<Subscript>20</Subscript>Ti<Subscript>10</Subscript>Zr<Subscript>8</Subscript>Co<Subscript>6</Subscript>Cu<Subscript>3</Subscript> Glassy Alloy at 400–550 °C

Air-oxidation behavior of a Ni₅₃Nb₂₀Ti₁₀Zr₈Co₆Cu₃ amorphous ribbon was studied at 400–550 °C. The oxidation kinetics of the amorphous alloy followed a two-stage parabolic rate law with its oxidation rates steadily increasing with temperature. The steady-state oxidation rate constants of the alloy were faster than those of pure Ni. Triplex scales formed on the glassy alloy, containing an outer layer of NiO. The scales formed in the intermediate layer consisted of Nb₂O₅, NiO, and uncorroded α-Ni, while an additional Nb₂Zr₆O₁₇ phase was also detected in the inner layer. The formation of multilayered scales is responsible for the faster oxidation for the Ni6-AR.     

Photocatalytic degradation of organic dyes with H<Subscript>3</Subscript>PW<Subscript>12</Subscript>O<Subscript>40</Subscript>/TiO<Subscript>2</Subscript>–SiO<Subscript>2</Subscript>

H₃PW₁₂O₄₀/TiO₂–SiO₂ was synthesized by impregnation method which significantly improved the catalytic activity under simulated natural light. The properties of the samples were characterized by Fourier transform infrared spectra (FTIR), X-ray powder diffraction pattern (XRD), Scanning electron micrographs (SEM), and Zeta potential. Degradation of methyl violet was used as a probe reaction to explore the influencing factors on the photodegradation reaction. The results show that the optimal conditions are as follows: initial concentration of methyl violet of 10 mg·L^?1, pH of 3.0, catalyst dosage of 2.9 g·L^?1, and light irradiation time of 2.5 h. Under these conditions, the degradation rate of methyl violet is 95.4 %. The reaction on photodegradation for methyl violet can be expressed as the first-order kinetic model, and the possible mechanism for the photocatalysis under simulated natural light is suggested. After used continuously for five times, the catalyst keeps the inherent photocatalytic activity for degradation of dyes. The photodegradation of methyl orange, methyl red, naphthol green B, and methylene blue was also tested, and the degradation rate of dyes can reach 81 %–100 %.     

Hydrothermal conversion of Ti-containing minerals in system of Na<Subscript>2</Subscript>O–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript>–CaO–TiO<Subscript>2</Subscript>–H<Subscript>2</Subscript>O

Titanium has a great effect on the digestion of bauxite in the Bayer process because it reacts readily at high temperatures in alkaline sodium aluminate solution. Under this consideration, the hydrothermal conversion of Ti-containing minerals in the system of Na₂O–Al₂O₃–SiO₂–CaO–TiO₂–H₂O with increased temperatures was studied based on the thermodynamic analysis and systematic experiments. The results show that anatase converts to Al₄Ti₂SiO₁₂ at low temperatures (60–120 °C), which is similar to anatase in crystal structure. As the temperature continues to rise, Al₄Ti₂SiO₁₂ decomposes gradually and converts to Ca₃TiSi₂(Al₂Si_0.5Ti_0.5)O₁₄ at 200 °C. When the temperature reaches 260 °C, CaTiO₃ forms as the most stable titanate species for its hexagonal closest packing with O^2? and Ca²⁺. The findings enhance the understanding of titanate scaling in the Bayer process and clarify the mechanism of how additive lime improves the digestion of diaspore.     

Investigations on Li<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>12</Subscript>/Ti<Subscript>3</Subscript>O<Subscript>5</Subscript> Composite as an Anode Material for Lithium–Ion Batteries

The Ti₃O₅ powder with uniform morphology has been successfully obtained and used to synthesize Li₄Ti₅O₁₂/Ti₃O₅ composite material by ball milling for modifying Li₄Ti₅O₁₂-based, lithium–ion battery anodes. Moreover, according to the relative performance investigations, the synthesized Li₄Ti₅O₁₂/Ti₃O₅ composite shows better electrochemical properties than that of the Li₄Ti₅O₁₂. At a high rate (10 C), the capacity of the Li₄Ti₅O₁₂/Ti₃O₅ composite electrode is 139.8 mAhg^?1, whereas the value of Li₄Ti₅O₁₂ is 121.6 mAhg^?1, showing a capacity enhanced about 14.97%. After 100 cycles at 0.2 C, the discharge capacity of Li₄Ti₅O₁₂/Ti₃O₅ remains at 160 mAhg^?1 with a capacity loss of 2.6%. The results indicate that the Li₄Ti₅O₁₂/Ti₃O₅ composite electrode can be used as anode material with a relatively higher rate capability and excellent cycle performance in lithium–ion batteries.     

Electrochemical and interfacial properties of (PEO)<Subscript>10</Subscript>LiCF<Subscript>3</Subscript>SO<Subscript>3</Subscript>−Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanocomposite polymer electrolytes using ball milling

Electrochemical and interfacial properties of (PEO)₁₀LiCF₃SO₃−Al₂O₃ composite polymer electrolytes (CPEs) prepared by either ball milling or stirring are reported. Ball milling was introduced into a slurry preparative technique utilizing PEO, lithium salt and Al₂O₃ powder ranging from 5 to 15 wt.%. The ionic conductivity was increased by ball milling over a range of temperatures. In particular, a significant increase at low temperature below the melting point of crystalline PEO was observed. Interfacial stability between lithium electrode and CPE was significantly improved by the addition of alumina as well as by ball milling. The electrochemical stability window produced by (PEO)₁₀LiCF₃SO₃−Al₂O₃ ball milling was higher than that of stirring, which was about 4.4 V. Charge/discharge performance of Li/CPE/S cells with (PEO)₁₀LiCF₃SO₃−Al₂O₃-12 hr ball milling was superior to that of a pristine polymer electrolyte due to the low interface resistance and high ionic conductivity.     

Densification Kinetics of CeO<Subscript>2</Subscript> Reinforced 8 Mol.% Y<Subscript>2</Subscript>O<Subscript>3</Subscript> Stabilized ZrO<Subscript>2</Subscript> Ceramics

The grain growth kinetics of 8YSZ ceramics processed using spark plasma sintering (SPS) has been investigated in the temperature ranging from 1100°C to 1500°C. The activation energy during SPS densification was obtained as 332 kJ/mol with grain boundary diffusion as a dominant mechanism. Further, the effect of CeO₂ on the densification kinetics of 8YSZ ceramic processed via SPS and conventional sintering (CS) has been delineated. The lower grain boundary mobility of CS-processed composites (an order of magnitude lower than SPS) is attributed to the solute drag and lattice distortion mechanism. However, no significant change in the grain boundary mobility was observed with CeO₂ addition (~?14.7–43.9?×?10^?18 m³/N/s for CS and 107.2–116.7?×?10^?18 m³/N/s for SPS) revealing that the defect concentration is nearly constant in 8YSZ. The study highlights the effect of sintering techniques (SPS and CS) and reinforcement (CeO₂) on engineering the desired microstructure of 8YSZ ceramic.     

Synthesis of LiFePO<Subscript>4</Subscript> using FeSO<Subscript>4</Subscript>·7H<Subscript>2</Subscript>O byproduct from TiO<Subscript>2</Subscript> production as raw material

As the byproduct of TiO₂ industrial production, impure FeSO₄·7H₂O was used for the synthesis of LiFePO₄. With the purified solution of FeSO₄·7H₂O, FePO₄·xH₂O was prepared by a normal titration method and a controlled crystallization method, respectively. Then LiFePO₄ materials were synthesized by calcining the mixture of FePO₄·xH₂O, Li₂CO₃, and glucose at 700°C for 10 h in flowing Ar. The results indicate that the elimination of FeSO₄·7H₂O impurities reached over 95%, and using FePO₄·xH₂O prepared by the controlled crystallization method, the obtained LiFePO₄ material has fine and sphere-like particles. The material delivers a higher initial discharge specific capacity of 149 mAh·g⁻¹ at a current density of 0.1C rate (1C = 170 mA·g⁻¹); the discharge specific capacity also maintains above 120 mAh·g⁻¹ after 100 cycles even at 2C rate. Thus, the employed processing is promising for easy control, low cost of raw material, and high electrochemical performance of the prepared material.     

Leakage currents in Ti−O/Ta<Subscript>2</Subscript>O<Subscript>5</Subscript> thin film deposited on Ta/Ti/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

The electrical characteristics of Ta/Ta₂O₅ films and Ta/Ti−O/Ta₂O₅ films deposited by RF reactive sputtering on Ta/Ti/Al₂O₃ substrates were investigated. Ta was used for the bottom and upper electrodes in order to simplify the fabrication process. Dielectric materials were annealed at 700°C for 60 sec under vacuum. XRD analysis showed that Ta was crystalline and Ta₂O₅ was amorphous in an as-deposited state, but amorphous Ta₂O₅ was transformed to a crystalline state by rapid thermal heat treatment. We compared lnJ-E², C−V, and C−F of both as-deposited and annealed dielectric thin films deposited on the Ta bottom electrode. From these results, we concluded that introducing a Ti−O buffer layer could reduce the leakage current. The conduction mechanisms of Ti−O/Ta₂O₅ could be interpreted appropriately by hopping conduction and space-charge-limited current.     

Formation of Al/(Al<Subscript>13</Subscript>Fe<Subscript>4</Subscript> + Al<Subscript>2</Subscript>O<Subscript>3</Subscript>) Nano-composites via Mechanical Alloying and Friction Stir Processing

Combination of mechanical alloying and friction stir processing was used for the fabrication of Al/(Al₁₃Fe₄ + Al₂O₃) nano-composites. Pre-milled hematite + Al powder mixture was introduced into the stir zone generated on 1050 aluminum alloy sheet by friction stir processing. Uniform and active milled powder mixture reacted with plasticized aluminum to produced Al₁₃Fe₄ + Al₂O₃ particles. Al₁₃Fe₄ intermetallic showed elliptical shape with a typical size of ~ 100 nm, while nano-sized Al₂O₃ exhibited irregular floc-shaped particles that formed clusters with the remnant of iron oxide particles in the fine recrystallized aluminum matrix. As the milling time (1-3 h) of the introduced powder mixture increased, the volume fraction of Al₁₃Fe₄ + Al₂O₃ particles increased in the fabricated composite. The hardness and ultimate tensile strength of the fabricated nano-composites varied from 54.5 to 75 HV and 139 to 159 MPa, respectively; these are much higher than those of the friction stir processed base alloy (33 HV and 97 UTS). The highest hardness and strength were achieved for the nano-composite fabricated using the 3-h milled powder mixture; hard nano-sized reaction products and fine recrystallized grains of Al matrix had major and minor roles on enhancing these properties, respectively.     

Tribological Behavior of Plasma-Sprayed Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-20 wt.%TiO<Subscript>2</Subscript> Coating

Al₂O₃-20 wt.% TiO₂ ceramic coatings were deposited on the surface of Grade D steel by plasma spraying of commercially available powders. The phases and the microstructures of the coatings were investigated by x-ray diffraction and scanning electron microscopy, respectively. The Al₂O₃-20 wt.% TiO₂ composite coating exhibited a typical inter-lamellar structure consisting of the γ-Al₂O₃ and the Al₂TiO₅ phases. The dry sliding wear behavior of the coating was examined at 20 °C using a ball-on-disk wear tester. The plasma-sprayed coating showed a low wear rate (~4.5 × 10^?6 mm³ N^?1 m^?1), which was <2% of that of the matrix (~283.3 × 10^?6 mm³ N^?1 m^?1), under a load of 15 N. In addition, the tribological behavior of the plasma-sprayed coating was analyzed by examining the microstructure after the wear tests. It was found that delamination of the Al₂TiO₅ phase was the main cause of the wear during the sliding wear tests. A suitable model was used to simulate the wear mechanism of the coating.     

Resynthesis of LiCo<Subscript>1−x</Subscript>Mn<Subscript>x</Subscript>O<Subscript>2</Subscript> as a cathode material for lithium secondary batteries

A recycling process involving chemical, mechanical, and electrochemical steps has been applied to recover cobalt from spent lithium ion batteries and resynthesize cathode active materials. LiCo_1−xMn_xO₂ powders using Co salt including Mn from the leach liquor were resynthesized by solid-state reaction as cathode active materials. When the powder mixture with added Li salt was calcined at 950 °C for 8 hours, well crystallined LiCo_1−xMn_xO₂ was successfully obtained. The LiCo_1−xMn_xO₂ powders with a ratio of Co:Mn=10:1 has a discharge capacity of 156.3 mAh/g at a rate of 20 mA/g with no perceptible capacity loss, in sharp contrast to the pure LiCoO₂ as active materials. The resynthesized LiCo_1−xMn_xO₂ was proven to have good characteristics as cathode active materials in charge/discharge capacity and cyclic performance.     

Hot Corrosion Behavior of YSZ and CaZrO<Subscript>3</Subscript>/YSZ Composite Thermal Barrier Coatings in Contact with 50V<Subscript>2</Subscript>O<Subscript>5</Subscript> + 50Na<Subscript>2</Subscript>SO<Subscript>4</Subscript> Salts

A novel thermal barrier coating system was formulated to resist hot corrosion environments. In this coating system, 5% CaZrO₃ was added to conventional yttria-stabilized zirconia (YSZ). The above composite coating system was compared with the standard YSZ system in the presence of a mixture of 50% Na₂SO₄ and 50% V₂O₅ at 950 °C. The results demonstrated that the lifetime of the CaZrO₃-added composite system in this highly hostile environment was longer compared with the standard YSZ system. This is due to the fact that the preferential reaction of NaVO₃ shifted from yttria to calcia, forming CaV₂O₄ instead of YVO₄. The preferential reactions are discussed in terms of free energy changes and acid–base theory of molten salts with ceramic oxides. Furthermore, calculations of lattice distortion also proved that the CaZrO₃-added composite system demonstrated less distortion, thus increasing the overall lifetime of the coating system.     

Effect of the decomposition of the supersaturated β solid solution in melt-quenched Ni<Subscript>65</Subscript>Al<Subscript>35</Subscript> and Ni<Subscript>56</Subscript>Al<Subscript>34</Subscript>Co<Subscript>10</Subscript> alloys on the reversibility of the martensitic transformation

Melt-quenched Ni₆₅Al₃₅ and Ni₅₆Co₁₀Al₃₄ (at %) alloys are studied by electrical resistance measurement and electron microscopy. The effects of the isothermal holding time in the supersaturated β solid solution field and the heating rate during thermal cycling on the restoration of the reversibility of the martensitic transformation are investigated. After short-term aging in the B2 austenite field followed by long-term aging in the L1₀ martensite field, the melt-quenched Ni₆₅Al₃₅ and Ni₅₆Co₁₀Al₃₄ alloys retain their high thermal stability of the reversibility of the martensitic transformation.     

Embrittlement and conditions of the optimization of magnetic properties in the amorphous alloy Co<Subscript>69</Subscript>Fe<Subscript>3.7</Subscript>Cr<Subscript>3.8</Subscript>Si<Subscript>12.5</Subscript>B<Subscript>11</Subscript> in the absence of a viscous–brittle transition

The influence of the holding time upon annealing on the temperature of the viscous–brittle transition (temperature of embrittlement) T_f in a cobalt-based amorphous alloy of the composition Co₆₉Fe_3.7Cr_3.8Si_12.5B₁₁ with a very low saturation magnetostriction λ_s (<10^–7) has been studied. It has been established that the dependence of the embrittlement temperature T_f on the of time of holding t_a can be described by an Arrhenius equation and that the embrittlement at the annealing temperatures above and below 300°C is described by different kinetic parameters. In the alloy under study, irrespective of the holding time, embrittlement occurs in a very narrow range of annealing temperatures, which does not exceed 5 K. Based on the experimental data on the evolution of the hysteresis magnetic properties upon the isochronous annealings and upon the isothermal holding, the regime of heat treatment that ensures a very high (about 50000) magnitude of the permeability µ₅ (H = 5 mOe, f = 1 kHz) without the transition of the alloy into a brittle state has been determined.     

Performance of Sm<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>CoO<Subscript>3−<Emphasis Type="Italic">δ</Emphasis></Subscript> membrane under CO<Subscript>2</Subscript>-containing atmosphere

The permeability and stability of Sm_0.7Sr_0.3CoO_3?δ (SSCO) regarding the special requirements for carbon capture and storage (CCS) application were investigated. Pure CO₂ was used as the sweep gas at 900 °C, leading to that the oxygen permeation flux decreases by about 34 %. Several cycles of changing the sweep gas between helium and CO₂ indicate the good reversibility of this degradation. Both carbonate formation and adsorption of CO₂ on the membrane surface are responsible for the degradation of the membrane performance. The better CO₂ resistance results from the substitution of Sm for Sr due to the higher acidity of Sm₂O₃ (1.278) than that of SrO (0.978) and a discontinuous layer of carbonate.     

Production and Characterization of Metastable ZrO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>Coatings Obtained by APS + Quench

Producing nanostructured materials through metastable phases is an interesting novel route in the field of ceramic materials. Due to their small grain size and uniform structure, these nanostructured bulk materials exhibit very interesting properties. Metastable coatings can be produced starting from microstructured powders through atmospheric plasma spray technique, followed by a quenching route. The initial powders are melted during the spraying and deposited over a substrate that is quenched with liquid nitrogen feeders, producing metastable coatings. The thermal-sprayed coatings have been characterized using x-ray diffraction, scanning electron microscopy, field emission scanning electron microscopy, and energy dispersive spectroscopy. The properties of such coatings have been also studied obtaining promising results.     

Phase and Microstructure Evolution and Toughening Mechanism of a Hierarchical Architectured Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–Y<Subscript>2</Subscript>O<Subscript>3</Subscript> Coating under High Temperature

In recent years, numerous techniques have been developed to mimic nacre-like hierarchical architectures in order to improve the damage tolerance of materials. We present herein a simple strategy to fabricate such a hierarchical architectured Al₂O₃–Y₂O₃ composite coating via atmospheric plasma spraying. The evolution of the phase and microstructure of the Al₂O₃–Y₂O₃ composite coating were characterized under conditions of high-temperature exposure in air at 800-1350 °C. The hardness and porosity of several typical coatings were determined. In situ formation of dense hierarchical architectured Al₂O₃–YAG composite coating with improved hardness was achieved after heat treatment at 1350 °C. Compared with Al₂O₃ coating, elevated toughness was found for the hierarchical architectured Al₂O₃–YAG composite coating, which can be ascribed to the distribution of YAG phase that contributed to crack termination and deflection, and microbridging. After thermal aging treatment at 1350 °C, the hierarchical architectured Al₂O₃–YAG composite coating was quite stable after 100 h of thermal exposure. Furthermore, the Al₂O₃–Y₂O₃ composite coating exhibited superior sintering resistance compared with the Al₂O₃ coating.