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.     

Mössbauer study of the process of the room-temperature aging of the alloy Cu<Subscript>79</Subscript>Ni<Subscript>14</Subscript>Fe<Subscript>7</Subscript>

Mössbauer spectroscopy was used to investigate the initial stage of the phase separation in the quasi-binary system Cu₇₉Ni₁₄Fe₇ and the subsequent transformation of the alloy structures as a result of prolonged aging at room temperature. For describing the Mössbauer spectra of ferromagnetic particles, which appear upon the spinodal decomposition in a paramagnetic matrix, a model was proposed and approved, which uses particle-size distribution in the approximation of the generalized Lifshitz-Slezov-Wagner (LSW) model and of the linear decrease of the hyperfine field at the ⁵⁷Fe nuclei in the near-surface layers of spherical particles.     

Details on the Formation of Ti<Subscript>2</Subscript>Cu<Subscript>3</Subscript> in the Ag-Cu-Ti System in the Temperature Range 790 to 860 °C

Silver-copper-titanium (Ag-Cu-Ti) ternary alloys are often used as active braze alloys for joining ceramics to metals at temperatures ranging from 780 °C (the melting point of the Ag-Cu eutectic) up to 900 °C. When Ti/Ag-Cu joints are brazed at low temperature (near 800 °C), the intermetallic compound Ti₂Cu₃ (tetragonal, P4/nmm, a = 0.313 nm, c = 1.395 nm) is systematically missing from the interface reaction layer sequence. An experimental investigation based on isothermal diffusion experiments in the Ag-Cu-Ti ternary system has then been undertaken to clarify the issues of thermal stability and formation kinetics of this compound. Evidence has been found for the stability of Ti₂Cu₃ at temperatures ranging from 790 to at least 860 °C. By heat treating Ag-Cu-Ti powder mixtures at 790 °C for increasing times, it has moreover been shown that Ti₂Cu₃ forms at a much slower rate than the two adjacent Ti-Cu compounds: TiCu₄, the first phase to form, and Ti₃Cu₄. This explains why although thermodynamically stable, Ti₂Cu₃ is not obtained when temperature is too low or reaction time too short.     

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 %.     

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.     

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.     

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.     

Pseudo-gap and the nonlinearity of the magnetization of textured polycrystals of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 − <Emphasis Type="Italic">x</Emphasis></Subscript> above the temperature of transition into the superconducting state

Temperature dependences of the third and other higher harmonics of the magnetization of textured polycrystalline samples of YBa₂Cu₃O_{7 ? x} have been studied in the temperature range of 77–120 K. It has been revealed that the nonlinearity of magnetization of YBa₂Cu₃O_{7 ? x} is observed up to temperatures that considerably exceed the temperature of transition into the superconducting state. The observed nonlinearity of magnetization of YBa₂Cu₃O_{7 ? x} is ascribed to the appearance of a pseudogap state at T ～ 102 K in this compound. A method of determining the temperature of the appearance of the pseudogap T * in the high-T _c compounds is suggested, which is based on the measurement of magnetization harmonics.     

Activities in the FeTiO<Subscript>3</Subscript>-NiTiO<Subscript>3</Subscript> Solid Solution from Alloy-Oxide Equilibria at 1273 K

Nine tie-lines between Fe-Ni alloys and FeTiO₃-NiTiO₃ solid solutions were determined at 1273 K. Samples were equilibrated in evacuated quartz ampoules for periods up to 10 days. Compositions of the alloy and oxide phases at equilibrium were determined by energy-dispersive x-ray spectroscopy. X-ray powder diffraction was used to confirm the results. Attainment of equilibrium was verified by the conventional tie-line rotation technique and by thermodynamic analysis of the results. The tie-lines are skewed toward the FeTiO₃ corner. From the tie-line data and activities in the Fe-Ni alloy phase available in the literature, activities of FeTiO₃ and NiTiO₃ in the ilmenite solid solution were derived using the modified Gibbs-Duhem technique of Jacob and Jeffes [K.T. Jacob and J.H.E. Jeffes, An Improved Method for Calculating Activities from Distribution Equilibria, High Temp. High Press., 1972, 4, p 177-182]. The components of the oxide solid solution exhibit moderate positive deviations from Raoult’s law. Within experimental error, excess Gibbs energy of mixing for the FeTiO₃-NiTiO₃ solid solution at 1273 K is a symmetric function of composition and can be represented as:

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.     

Effect of SiC Addition on Oxidation Behavior of ZrB<Subscript>2</Subscript> at 1273 K and 1473 K

The oxidation behavior of ZrB₂–SiC composites with different contents of SiC addition was investigated at 1273 and 1473 K in air for 12 h in this study. The SiC addition contents ranged from 0 to 30 wt%. The results showed that when ZrB₂–SiC composites were oxidized at 1273 K in air, a two-oxide layer-structure forms: a continuous glassy layer and a ZrO₂ layer contained unoxidized SiC. When SiC content is 5 and 10 wt%, the glassy layer is mainly composed by B₂O₃. When SiC content is 20 and 30 wt%, a borosilicate glass could be formed on the top layer, which could improve the oxidation resistance of ZrB₂. When ZrB₂–SiC composites were oxidized at 1473 K in air, the oxide layer was composed of ZrO₂ and SiO₂ and unreacted SiC. Additionally, when SiC addition content was higher than 10 wt%, a continuous borosilicate glass layer could be formed on the top of the oxide layer at 1473 K. With the increase of SiC content in ZrB₂, the oxide layer thickness decreased at both 1273 and 1473 K.     

Physical Characteristics of Ni<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Zr<Subscript>100–<Emphasis Type="Italic">x</Emphasis></Subscript> Alloys Based on Stretching and Heating Processes Using Molecular Dynamics Simulation

The physical characteristics of Ni _x Zr_100–x proportions based on stretching and heating processes were investigated using molecular dynamics (MD) simulation. The physical characteristics included axial stretching, slip vector of dislocation nucleation, radial distribution function, Poisson’s ratio, and internal energy of thermophysical properties of the Ni _x Zr_100–x proportions. The second-moment approximation for tight-binding many-body potential was used for Ni _x Zr_100–x physical characteristics during the MD simulation. Results show that the most Zr atoms of Ni _x Zr_100–x models after axis stretching obviously move and accumulate at the outside of the Ni _x Zr_100–x models due to high-temperature softening behavior and high covalent bond strength. The Poisson’s ratios of the Ni₂₀Zr₈₀, Ni₄₀Zr₆₀, Ni₅₀Zr₅₀ Ni₆₀Zr₄₀, and Ni₈₀Zr₂₀, are 0.401, 0.397, 0.490, 0.437, and 0.385, respectively. The Ni₅₀Zr₅₀ proportion has highest Poisson’s ratio, indicating that the transverse contraction strain to longitudinal extension strain is largest than others proportions. For the thermophysical properties, the atoms of Ni₅₀Zr₅₀ in solids are orientation order.     

Symmetry analysis of magnetic structures with a wave vector k<Subscript>6</Subscript> = μb<Subscript>3</Subscript> possible in the intermetallic compound Ce<Subscript>2</Subscript>Fe<Subscript>17</Subscript>

To estimate the reliability of literature data on the magnetic structure of Ce₂Fe₁₇, we performed a symmetry analysis of the possible magnetic structures characterized by the wave vector k ₆ = μb ₃ = 2π(0, 0, π/c). The previously developed procedure for calculating the basis functions of the irreducible representations of the space group D_3d ⁵ (R[`3]m)(Rbar 3m), which enter into the magnetic representation with the above wave vector, is described briefly and is employed for an analysis. The results of the calculations of these basis functions for Fe atoms located in 6c, 9d, 18f, and 18h positions are given and discussed. A conclusion is made that in this intermetallic compound there can exist magnetic structures of the type of longitudinal or transverse spin waves or of the type of a complicated spiral with the magnetic moment which varies according to a harmonic law as the coordinate z of the site occupied by the Fe atom changes.     

Microstructure and plastic deformation of orthorhombic titanium aluminides Ti<Subscript>2</Subscript>AlNb. IV. Formation of the transformation twins upon the α<Subscript>2</Subscript> → <Emphasis Type="Italic">O</Emphasis> phase transformation

X-ray diffraction transmission electron microscopy, and measurements of microhardness and resistivity were used to study the formation of the orthorhombic O phase upon the α₂ → O phase transformation. It has been found that the orthorhombic O phase is formed by the diffusion mechanism in the form of thin lamellar precipitates (domains). Upon subsequent growth, these domains form packets of twins with a twinning plane {110}. It is shown that the (130) twinning plane is not realized directly, but it appears as a result of joining of twin packets. No formation of an intermediate metastable phase was observed upon this transformation.     

Effects of Na/K ratio on the phase structure and electrical properties of Na<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>K<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>NbO<Subscript>3</Subscript> lead-free piezoelectric ceramics

Lead-free piezoelectric NaxK1-xNbO3(x = 0.3-0.8)(NKN) ceramics were fabricated by normal sintering at 1060°C for 2 h.Microstructures and electrical properties of the ceramics were investigated with a special emphasis on the influence of Na content.The grain size of the produced dense ceramic was decreased by increasing Na content.A discontinuous change in the space distance was found at the composition close to Na0.7K0.3NbO3 ceramic, which indicates the presence of a transitional composition between two dif...     

Specifics of percolation behavior in the polyether–carbon nanotube systems doped with LiClO<Subscript>4</Subscript>

The electrical properties of the polyether/carbon nanotube (CNT) systems doped with LiClO₄ inorganic salt are studied via impedance spectroscopy. The percolation thresholds are evaluated in the context of the standard percolation theory for the nanofilled systems based on polyethylene glycol and polypropylene glycol, being 0.45 and 0.5%, respectively. Two percolation thresholds are observed for the polyether–CNT–LiClO₄ systems. A modified percolation model with consideration of two percolation transitions is proposed.     

Effect of Si and Y<Subscript>2</Subscript>O<Subscript>3</Subscript> Additions on the Oxidation Behavior of Ni–<Emphasis Type="Italic">x</Emphasis>Al (<Emphasis Type="Italic">x</Emphasis> = 5 or 10 wt%) Alloys at 1150 °C

The effect of Si and Y₂O₃ additions on the oxidation behavior of Ni–xAl (x = 5 or 10 wt%) alloys at 1150 °C was studied. The addition of Y₂O₃ accelerates oxidation rate of alloys, especially growth rate of NiO, but improves adherence of the scale to the substrate. The addition of Si facilitates the selective oxidation of Al, suppresses the formation of NiO and therefore reduces the critical Al content to form continuous layer of alumina scale. Higher Al content decreases the oxidation rate of alloys in binary Ni–Al alloys and increases the oxidation rate of alloys in ternary Ni–Al–Si alloys. The effect of third-element Si is more significant and beneficial than that of Al content in ternary Ni–Al–Si alloys.     

Microstructure and plastic deformation of orthorhombic titanium aluminides Ti<Subscript>2</Subscript>AlNb. III. Formation of transformation twins upon the <Emphasis Type="Italic">B</Emphasis>2 → <Emphasis Type="Italic">O</Emphasis> phase transformation

X-ray diffraction and transmission electron microscopy were used to study the O-phase formation in the Ti-25.6% Al-13.9% Nb-0.3% Mo-0.3% Zr alloy upon the B2 → O phase transformation. The formation of pseudotwins is shown to be possible in the B2 phase in the Ti-Al-Nb alloys. It is found that the O phase upon the B2 → O phase transformation does not undergo twinning, and all twins observed in the alloy belong to the intermediate metastable B19 phase. The orthorhombic O phase is formed upon ordering inside the B19-phase twins and save their boundaries.