Impact of supporting electrolytes on the stability of TiO<Subscript>2</Subscript>–Ti counter electrode during H<Subscript>2</Subscript>O<Subscript>2</Subscript> electrogeneration

This work focuses on the role of common supporting electrolytes (SEs) in the electro-chemical inertness of Ti-based materials employed for the anodic (direct) oxidation coupled with H₂O₂ electro-generation at the graphite cathode for the concurrent decomposition of organic contaminants. SEs are added to boost up the ionic conductivity of solution but a question always remains on the effect of SEs on the stability of anode materials. The use of ClO ₄ ^? is encouraged in the electro-Fenton process as it does not form complexes with Fe²⁺/Fe³⁺; however, it is found that ClO ₄ ^? corroded the TiO₂ coated Ti (TiO₂–Ti) anode very fast (>60 min) and, Ti⁴⁺ ions formed a yellow color complex (λ_max = 380 nm) with H₂O₂. The influence of Cl^–, NO ₃ ^? and SO ₄ ^2? was insignificant on the stability of TiO₂–Ti. The cell current efficiency of H₂O₂ formation dropped sharply with in the case of TiO₂–Ti anode. The TiO₂–Ti corrosion also reduced the mass transfer co-efficient of DO transport from bulk to the cathode surface because of Ti⁴⁺ adsorption on graphite.     

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.     

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 %.     

The effect of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles on tribological and corrosion behavior of electroless Ni–B–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composite coating

In this study, the Ni–B–Al₂O₃ composite was successfully coated on the surface of Ck45 steel by elecroless method. X-Ray diffraction analysis (XRD) and scanning electron microscopy (SEM) were utilized in order to investigate and identify the coating properties. Wear behavior of the coating was studied by the pinon- disk test. Corrosion behavior of the Ni–B and Ni–B–Al₂O₃ coatings was investigated by using Tafel polarization diagrams in the 3.5% NaCl solution at room temperature. The obtained data demonstrate that the addition of Al₂O₃ nanoparticles to the coating has resulted in improving the tribological behavior of the coating due to the presence of the composite nanoparticles. Also, the results of electrochemical testing show that corrosion resistance of the electroless Ni–B coating with Al₂O₃ nanoparticles has dramatically increased.     

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.     

Effect of doping and substitution on the low-temperature decomposition of oxygen-nonstoichiometric Ba<Subscript>2</Subscript>YCu<Subscript>3</Subscript>O<Subscript>7 − δ</Subscript>

Effects of doping (with Ce and Pr) and substitution of Sr, Nd, Eu, and Ni for Ba and Cu on the lowtemperature (T = 200–300°C) decomposition of oxygen-nonstoichiometric Ba₂YCu₃O_{7 ? δ} have been studied. Both the doping and partial substitution for any of the principal components was found to increase the stability of the 123 compounds with respect to the decomposition into oxygen-depleted and oxygen-rich phases. Both doping and substitution to a level of ～2% lead to a narrowing of the immisibility dome and a decrease in the critical temperature. In the first place, the decomposition is suppressed in the bulk of grains. To increase the stability of near-boundary regions of grains, a high degree (～20 at %) of substitution is necessary.     

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.     

Bridge polysilsesquioxane xerogels with a bifunctional surface layer of the ≡Si(CH<Subscript>2</Subscript>)<Subscript>3</Subscript>NH<Subscript>2</Subscript>/≡Si(CH<Subscript>2</Subscript>)<Subscript>3</Subscript>SH composition

Xerogels with a bifunctional surface layer of the ≡Si(CH₂)₃NH₂/≡Si(CH₂)₃SH composition are synthesized by hydrolytic co-polycondensation of bis(triethoxy)silane (C₂H₅O)₃Si(CH₂)₂Si(OC₂H₅)₃ and two trifunctional silanes, namely, 3-aminopropyltriethoxysilane and 3-mercaptopropyltrimethoxysilane. Using IR, ¹H MAS NMR, and ¹³C CP/MAS NMR spectroscopic techniques, it is shown that in addition to complexing groups, the surface layer also contains water, silanol groups that are involved in the hydrogen bond formation and also residual ethoxysilyl groups. According to ²⁹SiCP/MAS NMR spectroscopic data, the degree of polycondensation of synthesized xerogels exceeds 80%. It is found that the use of 1,2-bis(triethoxysilyl)ethane as the structuring agent in place of tetraethoxysilane allows one to synthesize bifunctional xerogels with the highly developed biporous structure (S _sp = 607–680 m²/g, V _c = 1.38–1.47 cm³/g, d = 2.9–3.1 and 18.3 nm). Changing the ratio structuring-silane/functionalizing-silane-mixture from 2: 1 to 4: 1 in the reaction system has virtually no effect on the porous structure parameters of final xerogels.     

Accelerated Hot Corrosion Studies of D-Gun-Sprayed Cr<Subscript>2</Subscript>O<Subscript>3</Subscript>–50% Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Coating on Boiler Steel and Fe-Based Superalloy

In the current investigation, Cr₂O₃–50% Al₂O₃ coating was deposited on ASTM-SA213-T-22 boiler steel and Fe-based superalloy Superfer 800H by D-gun spray process. The high-temperature corrosion performance of the coated as well as bare alloys was evaluated in Na₂SO₄–60%V₂O₅ molten salt, an aggressive environment at 900 °C under cyclic conditions. The kinetics of the corrosion were analyzed by the change in weight measurements which were taken after each cycle (i.e., 1-h heating in a tube furnace followed by 20-min cooling in ambient air) for a total period of 50 cycles. The X-ray diffraction and scanning electron microscopy/energy-dispersive X-ray analysis techniques were used for the analysis of corrosion products. During investigations, it was found that both the selected bare alloys have suffered intensive spallation in the form of removal of their oxide scales, which may be attributed to the formation of non-protective Fe₂O₃-dominated oxide scales, whereas the coated alloys have shown lesser weight gains along with better adhesiveness of the oxide scales with the substrate till the end of the experiment. The oxides of chromium and aluminum were the main phases revealed in the oxide scales of the coated specimens, which are reported to be protective against the hot corrosion.     

Investigation of hybrid PEO coatings on AZ31B magnesium alloy in alkaline K<Subscript>2</Subscript>ZrF<Subscript>6</Subscript>–Na<Subscript>2</Subscript>SiO<Subscript>3</Subscript> electrolyte solution

Detail study of the PEO coatings produced on AZ31B magnesium alloy in Na₂SiO₃–K₂ZrF₆-based electrolyte solution was carried out in this work. ZrO₂, MgF₂, MgO and Mg₂SiO₄ were observed as the major phases in the coatings. Electrolytic decomposition and then the severe thermochemical environment provoked various Zr–F–Mg based complexes in the coatings. From the surface analysis, various structures e.g, nano-grains, pores and volcano shape sintered material were evidenced. Using Vicker hardness test, maximum hardness was recorded as ~1280 HV. Potentiodynamic polarization technique in 3.5 wt % NaCl solution was used to predict the corrosion performances of the specimens. Among the coated samples, highest corrosion resistance was recorded to be ~223.5 × 10³ kΩ/cm² for 15 min coatings.     

Effect of Sealing Treatment on Corrosion Resistance of Plasma-Sprayed NiCrAl/Cr<Subscript>2</Subscript>O<Subscript>3</Subscript>-8 wt.%TiO<Subscript>2</Subscript> Coating

Plasma-sprayed ceramic coatings inherently contain pores and micro-cracks which is deleterious when performed in aggressive environment. Various methods were applied to the as-sprayed coatings in order to improve the corrosion resistance. In the investigation of this study, plasma-sprayed NiCrAl/Cr₂O₃-8 wt.%TiO₂ coatings were sealed by epoxy resin and silicone resin, respectively. Coatings were characterized by scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS), optical microscopy (OM) and x-ray diffraction (XRD). The possible corrosion mechanism was discussed. The results of salt spray test and electrochemical measurements indicated that after the sealing treatment, the porosity of coatings decreased obviously and a compact layer was formed to protect the coating from corrosion. The silicone resin proved to be more effective than epoxy resin in enhancing the corrosion resistance of the coatings used in this research.     

The Effect of Synthesis Conditions on the Phase Composition and Structure of EuBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6 + δ</Subscript> Samples

The composition and the structure of ceramic EuBa₂Cu₃O_{6 + δ} (Eu-123) oxide samples annealed in steps with varying processing conditions (in air or oxygen and argon atmosphere at a temperature of 940–960°С for 1–70 h with or without homogenization) were studied by the X-ray phase and chemical analysis, electron diffraction pattern analysis, elemental analysis, and high-resolution transmission electron microscopy. Regardless of the processing conditions, Eu-123 nanostructured oxide with a tetragonal or orthorhombic structure and domains 1–20 nm in size was obtained as a result of annealing. Nanostructuring of the samples, which was revealed by high-resolution electron microscopy, is attributed to their chemical nature: the presence of identical structural elements in members of the homologous Eu _n Ba _m Cu_{m + n}O _y series of oxides allows them to intergrow coherently and create an illusion of a single crystal. Just like any other member of the Eu _n Ba _m Cu_{m + n}O _y series, oxide Eu-123 is disproportionate depending on the annealing conditions to form other members of this series located on either side of the dominant oxide. Temperature T_c of the superconducting transition of each member of the series depends on the average oxidation state of copper \(\overline {Cu} \). At \(\overline {Cu} \) < 2, all members of the series have a tetragonal structure and do not exhibit superconducting properties. At \(\overline {Cu} \) = 2.28, five members of the Eu _n Ba _m Cu_{m + n}O _y series with matrices (Ba : Cu) 5 : 8, 3 : 5, 2 : 3, 5 : 7, and 3 : 4 exhibit superconducting properties with T_c = 82–90 K.     

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.     

Effect of the atmosphere of low-temperature annealing on the structure and electrophysical properties of nonstoichiometric YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 − δ</Subscript> ceramics

The fine structure and electrophysical properties of nonstoichiometric YBa₂Cu₃O_{7 − δ} ceramics and the effect of low-temperature annealing (t ⩾ 200°C) in various atmospheres on these parameters have been studied. It has been shown that, during annealing in a vacuum, the decomposition is quite sluggish; structures typical of initial stages of decomposition are observed. The decomposition in an inert-gas atmosphere occurs more actively, and structures typical of stages of deep decomposition are realized. It has been found that, during low-temperature annealing, the structure and properties are affected by two factors; these are the decomposition into phases differing in the oxygen content, and water absorption, leading to the transformation with the formation of a pseudo-cubic lattice. The annealing atmosphere substantially affects the kinetics of both processes.     

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.     

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.     

Effect of NiCr Clad BaF<Subscript>2</Subscript>·CaF<Subscript>2</Subscript> Addition on Wear Performance of Plasma Sprayed Chromium Carbide-Nichrome Coating

NiCr clad BaF₂·CaF₂ fluoride eutectic powders were added into chromium carbide-nichrome feedstock to improve the tribological properties of NiCr-Cr₃C₂ coating, and the structures, mechanical, and ball-on-disk sliding wear performance of the coating were characterized. The results show that NiCr cladding can effectively decrease the density and thermophysical difference between the feedstock components, while alleviate the decarburization and oxidization of the constituent phases, and form the coating with a uniform and dense microstructure. However, the addition of BaF₂·CaF₂ has a negative effect on mechanical properties of the coating. When the temperature reaches 500 °C, the BaF₂·CaF₂ eutectic is soften by the heat and smeared by the counterpart, thus the low shear stress lubricating film forms between the contact surface, that improves the tribological properties dramatically. At this temperature, the dominant wear mechanisms also change from splats spallation and abrasive wear at room temperature to plastic deformation and plawing by the counterpart. Within the temperature range from 600 to 800 °C, the friction coefficient, the wear rates of NiCr/Cr₃C₂-10% BaF₂·CaF₂ coating and its coupled Si₃N₄ ball are 20%, 40%, and 75% lower than those of the NiCr/Cr₃C₂ coating, respectively. The NiCr/Cr₃C₂-BaF₂·CaF₂ coating shows superior wear performance to the NiCr/Cr₃C₂ coating without lubricant additive.     

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.     

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.