Solidification microstructure of laser remelted Al2O3ZrO2 eutectic

Surface resolidification experiments using a high power CO₂-laser have been performed on an Al₂O₃ZrO₂ containing 36.8 at.% ZrO₂ eutectic alloy at beam velocities between 0.3 and 8 mm·s⁻¹. The local growth rate has been measured by observation of the orientation of the microstructure using scanning electron microscopy. In the whole range of velocities, the structure is essentially a regular lamellar eutectic and the value of the growth productλ²V was found to be ≈ 9.6·10⁻¹⁷ m³·s⁻¹. The measured eutectic spacings were compared with Jackson and Hunt model. Using thermophysical properties from the literature, the measured spacings were more than four times larger than the calculated ones. Assuming all parameters of the growth relationship except the diffusion coefficient to be of the right order of magnitude or to have a negligible influence, agreement is found when using a larger liquid diffusion coefficient,D_L≈5·10⁻¹⁰m²·s⁻¹.     

The passivation of dilute Cu(Hg) in alkaline solutions

The anodic oxidation of dilute Cu(Hg)-pool electrodes (maximum mole fraction of Cu = 0.031) in aqueous solutions of Na₃PO₄ and NaOH pH range 10.5–12.2, yielded a series of potential-time transients that are found to correspond to the formation of Cu₂O, CuO and Cu(OH)₂ at the higher pH value, and Cu₂O and two soluble species, believed to be CuO²- and HCuO₂-, at the lower pH value. No sign of processes involving Hg appeared even at a mole fraction of Cu as low as 10⁻⁴. Theoretical calculation based on a model of competition between species of Cu and Hg for the available OH- predicts that the electrode should behave electrochemically as pure Cu down to a Cu activity of ca. 10⁻¹². For both anodic and cathodic processes i-τ−1/2 plots are found linear, proposing the applicability of the law τ = ai⁻ⁿ, with n = 2.     

Galvanic cells encountered in the corrosion of steel reinforcement —I. Differential pH cells

Corrosion cells resulting from differential pH values have been investigated in the absence and presence of Cl⁻ ions. Measurements of potential, galvanic current for separate and coupled electrodes, as well as experimental determination of Evans diagrams, were carried out. The results of coupling rvealed that the steel at lower pH (7 and 8) suffered from corrosion while that at the. higher pH (12 and 12·5) was completely protected even in presence of high chloride concentrations The increase in the pH range caused a relative enhancement of the anodic process. The presence of 10⁻³ or 10⁻¹M Cl⁻ ions increased the rate of the anodic reaction by 8- or 12-fold. The rate of the cathodic reaction was of the same order of magnitude both in presence or absence of Cl⁻ ions. The % anodic and cathodic control were found to be 6·4 and 93.6, respectively, in the presence of Cl⁻ ions as compared with 34·3 and 65·7 in the absence of Cl⁻ ions.     

Luminance materials containing carbazole and triphenylamine exhibiting high hole-transporting properties

Two novel 2-substitutd-8-hydroxyquinoline derivatives were designed and synthesized. Their luminance properties and carrier transporting abilities were studied when integrated in four different organic light-emitting device structures. The four devices are structured as following—device A: indium tin oxide glass substrate (ITO)/2-TNATA/NPB/(2-[2-(9-ethyl-9H-carbazol-2-yl)-vinyl]-quinolato zinc (1) and 2-[2-(4-diphenylamino-phenyl)-vinyl]-quinolato zinc (2))/Alq₃/LiF/aluminum (Al); device B: ITO/2-TNATA/NPB/1 or 2/LiF/Al; device C: ITO/2-TNATA/1 or 2/Alq₃/LiF/Al; device D: ITO/2-TNATA/1 or 2/Alq₃/LiF/Al. In the device A, the maximum brightness of compound 1 is 5857 cd m⁻² at 11 V with the luminance efficiency 1.84 cd A⁻¹. For 2 it is 6047 cd m⁻² at 10 V with an efficiency of 2.22 cd A⁻¹. In the device B, the maximum brightness of 1 is 745 cd m⁻² at 11 V and efficiency is 0.32 cd A⁻¹. These values are 1748 cd m⁻² at 10 V and 0.42 cd A⁻¹ for 2, respectively. In the device C, the maximum brightness of 1 and 2 are 8729 and 5838 cd m⁻² at 10 V, respectively, with an efficiency of 1.99 and 1.71 cd A⁻¹. In the device D, the maximum brightness and efficiency of 1 are 8512 cd m⁻² at 13 V and 3.10 cd A⁻¹, and they are 9818 cd m⁻² at 13 V and 0.42 cd A⁻¹, for 2. Our results also show that both 1 and 2 are good bipolar and bifunctional molecules with high hole-transporting and luminance properties.     

Influence of the prior oxide film on the growth and structure of oxides formed on Fe---26Cr alloys at 600°C

A Fe---26 Cr alloy has been oxidized at 600°C in 5 × 10⁻³, 5 × 10⁻² and 5 × 10⁻¹ torr oxygen to examine the influence of the prior oxide film on the growth and structure of oxides formed at high temperature. Different prior oxides were produced either by electropolishing or by annealing the electropolished specimen in vacuum at 600°C. Auger electron spectroscopy (AES) showed the Cr content of the prior oxide film to be increased from 50 to 95% during annealing, and electron diffraction indicated a change in oxide structure from amorphous to crystalline. At 5 × 10⁻³ torr, electropolished Fe---26 Cr oxidizes faster than the vacuum annealed specimens because the amorphous prior oxide gives rise to a finer-grained cubic oxide with more grain boundary easy diffusion paths for cation transport. From AES and electron back-scattering Fe⁵⁷ Mössbauer spectroscopy it is concluded that this cubic oxide is a duplex layer of inner γ-Cr₂O₃ and outer Fe₃O₄. The oxidation rate slows markedly when nucleated α-Fe₂O₃ covers the cubic oxide. With increased oxidation time Fe₃O₄ converts to α-Fe₂O₃ and the γ-Cr₂O₂ to α-Cr₂O₃. Annealed Fe---26 Cr oxidizes slower primarily because of a lower cation transport through a coarser-grained cubic oxide rather than because of a higher Cr content in the prior oxide. α-Fe₂O₃ nucleates at an earlier stage in the oxidation and essentially stifles the reaction. The extent of Cr incorporation into any of the Fe oxides produced in 5 × 10⁻³ torr oxygen is small ( 5%). Increasing the oxygen pressure from 5 × 10⁻³ to 5 × 10⁻² and 5 × 10⁻¹ torr has little effect on the mechanism of oxidation of vacuum annealed Fe---26 Cr, except that the overall extent of oxidation is less because of earlier α-Fe₂O₃ formation and, after a few hours of oxidation, up to 20% Cr is incorporated into the α-Fe₂O₃ lattice. On electropolished Fe---26 Cr at 5 × 10⁻² and 5 × 10⁻¹ torr oxygen nodules of α-Cr₂O₃ form and continue to grow both at grain boundaries and within the grains. Possible mechanisms for this nodule formation, which is exclusive to electropolished specimens oxidized at the higher pressures, are considered.     

Thermodynamic studies on LaFeO3(s)

The enthalpy increments and the standard molar Gibbs energies in the formation of LaFeO₃(s) have been measured using a high-temperature Calvet micro-calorimeter and a solid oxide galvanic cell, respectively. The corresponding expression for enthalpy increments is given as:

H°(T)−H°(298.15 K)(J mol⁻¹)(±1.2%)=−36887.27+103.53 T(K)+25.997×10⁻³T²(K)+11.055×10⁵/T(K).

The heat capacity, the first differential of H°(T)−H°(298.15 K) with respect to temperature, is given as:

C_p,m°(T)(JK⁻¹mol⁻¹)=103.53+51.994×10⁻³T(K)−11.055×10⁵/T²(K).

From the measured e.m.f. of the cell, (−)Pt/(LaFeO₃(s)+La₂O₃(s)+Fe(s))//CSZ//(Ni(s)+NiO(s))/Pt(+), and the relevant Δ_fG_m°(T) values from the literature, the Δ_fG_m°(LaFeO₃, s, T) was calculated, and is given as:

Δ_fG_m°(LaFeO₃, s, T)(kJmol⁻¹)(±0.72)=−1319.2+0.2317T(K).

The calculated Δ_fH_m°(LaFeO₃, s, 298.15 K) and S°(298.15 K) values obtained using the second law method are −1334.7 kJ mol⁻¹ and 128.9 J K⁻¹ mol⁻¹, respectively.     

Failure analysis in superplastic materials

Superplastic alloys and metals possess the ability to undergo large uniform strains prior to failure. Isothermal superplasticity of sheet metal is a phenomenon due to both peculiar process condition and material intrinsic characteristics. The material must have a grain size of less than 10 μm, the forming temperature of around half the absolute melting point and a very low strain rate (in the order of 10⁻⁵–10⁻³ s⁻¹).The instability of superplastic flow under uniaxial stress state has been the subject of different studies. In this paper, under biaxial stress conditions, instability analysis of superplastic PbSn60 alloy using the finite element method is investigated. An original model has been implemented successfully in commercial finite element code in order to predict the imminent failure of material during superplastic forming processes.     

Interfacial interaction of solid nickel with liquid Pb-free Sn–Bi–In–Zn–Sb soldering alloys

The dissolution process of nickel in liquid Pb-free 87.5% Sn–7.5% Bi–3% In–1% Zn–1% Sb and 80% Sn–15% Bi–3% In–1% Zn–1% Sb soldering alloys has been investigated by the rotating disc technique at 250–450 °C. The temperature dependence of the nickel solubility in soldering alloys obeys a relation of the Arrhenius type c_s = 4.94 × 10² exp(−39500/RT)% for the former alloy and c_s = 4.19 × 10² exp(−40200/RT)% for the latter, where R is in J mol⁻¹ K⁻¹ (8.314 J mol⁻¹ K⁻¹) and T is in K. Whereas the solubility values differ considerably, the dissolution rate constants are rather close for these alloys and fall in the range (1–9) × 10⁻⁵ m s⁻¹ at disc rotational speeds of 6.45–82.4 rad s⁻¹. Appropriate diffusion coefficients vary from 0.16 × 10⁻⁹ to 2.02 × 10⁻⁹ m² s⁻¹. With both alloys, the Ni₃Sn₄ intermetallic layer is formed at the interface of nickel and the saturated or undersaturated melt at dipping times of 300–2400 s. The other Ni–Sn intermetallic compounds are found to be missing. A simple mathematical equation is proposed to evaluate the Ni₃Sn₄ layer thickness in the case of undersaturated melts. The tensile strength of the nickel-to-alloy joints is 94–102 MPa, with the relative elongation being 2.0–2.5%.     

Electrochemical characterization of the different surface states formed in the corrosion of carbon steel in alkaline sour medium

In this study the different surface states that manifest in the corrosion process of 1018 carbon steel in alkaline sour environment, solution prepared specifically to mimic the sour waters occurring in the catalytic oil refinery plants of the Mexican Oil Company (PEMEX) (0.1 M (NH₄)₂S and 10 ppm NaCN at pH 9.2) were prepared and characterized. The surface states of the carbon steel were formed by treating the surface with cyclic voltammetry at different switching potentials (E_λ+), commencing at the corrosion potential (E_corr=−0.890 V vs sulfate saturated electrode, SSE). The surface states thus obtained were characterized using electrochemical impedance spectroscopy and scanning electron microscopy techniques. It was found that for E_λ+=−0.7 and −0.6 V vs SSE a first product of corrosion formed, characterized by a high passivity. Moreover, it was very compact (with a thickness of 0.047 μm). However, at more anodic potentials (E_λ+>−0.5 V vs SSE) a second corrosion product with non-protective properties (porous with a thickness of 0.4 μm and very active) was observed. The diffusion of atomic hydrogen (H⁰) was identified as the slowest step in the carbon steel corrosion process in the alkaline sour media. The H⁰ diffusion coefficients in the first and second products that formed at the carbon steel–sour medium interface were of the order of 10⁻¹⁵ and 10⁻¹² cm²/s respectively.     

Hydrogen permeation in gamma titanium aluminides

The permeation of hydrogen in gamma titanium aluminides was studied using the Devanathan–Stachurski (DS) cell. Thin disk-shaped samples of gamma titanium aluminide with three different microstructures were appropriately prepared and cathodically charged in an aqueous 0.1 N NaOH solution. The permeation current was monitored as a function of time. Permeation parameters were calculated using the time lag criterion (non-steady state lag). Values of the apparent diffusion coefficient of hydrogen in gamma titanium aluminide varying from 1.87 × 10⁻⁷ cm²/s to 3.75 × 10⁻⁶ cm²/s were obtained. This slight variation is attributed to differences in microstructure.     

Adsorption of platinum(IV) onto D301R resin

Pt(IV) was quantitatively adsorbed by D301R resin in the medium of pH = 3.47. The statically saturated adsorption capacity is 410 mg/g. Pt(IV) adsorbed on D301R resin can be eluted by 1.0-2.0 mol/L NaOH. The rate constant is k₂₉₈ = 5.43 × 10⁻⁵S⁻¹. The adsorption of Pt(IV) on D301R resin obeys the Freundlich isotherm. The adsorption parameters of thermodynamics are as follows: enthalpy change ΔH = 4.37 kJ/mol, Gibbs free energy change ΔG = −5.39 kJ/mol, and entropy change ΔS = 32.76 J/(mol·K). The apparent activation energy is E_a = 22.5 kJ/mol. The coordination molar ratio of the functional group of D301R resin to Pt(IV) is 2:1.     

Spontaneous magnetostriction of R2Fe13.6Si3.4 (R = U, Lu)

The thermal expansion of U₂Fe_13.6Si_3.4 and Lu₂Fe_13.6Si_3.4 has been measured by X-ray powder diffraction. Both compounds exhibit a large spontaneous magnetostriction. In the ground state, the volume effect 11.2 × 10⁻³ in U₂Fe_13.6Si_3.4 consists of almost equal contributions from the Fe–Fe and U–Fe exchange interactions (6 × 10⁻³ and 5 × 10⁻³, respectively). In Lu₂Fe_13.6Si_3.4, the volume effect is 8.9 × 10⁻³.     

Electrochemical investigations of self-doped polyaniline nanofibers as a new electroactive material for high performance redox supercapacitor

Self-doped polyaniline (SDPA) nanofibers were deposited on platinum (Pt) electrode by reverse pulse voltammetric (RPV) method and their electrochemical performance was evaluated in an aqueous redox supercapacitor constituted as a two electrode cell in a weak acidic medium. Scanning electron micrographs clearly revealed the formation of nanofiber structures with diameters in range of 60–90 nm under optimum experimental conditions. Different electrochemical methods including galvanostatic charge–discharge (CD) experiments, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were carried out in order to investigate the applicability of the system as a redox supercapacitor. Based on the charge–discharge results obtained, the SDPA represented high specific capacitance, specific power and specific energy values of 480 F g⁻¹, 436 W kg⁻¹ and 9.40 Wh kg⁻¹, respectively, at a current density of 5 mA cm⁻². The present study introduces new nanofiber materials for electrochemical redox capacitors with advantages including low cost, long cycle-life and stable at low acidic solutions of pH 3.     

Synthesis and crystal structure of Ba3Si4C2

SiC powder prepared by the Na flux method at 1023 K for 24 h and Ba were used as starting materials for synthesis of tribarium tetrasilicide acetylenide, Ba₃Si₄C₂. Single crystals of the compound were obtained by heating the starting materials with Na at 1123 K for 1 h and by cooling to 573 K at a cooling rate of −5.5 K/h. The single crystal X-ray diffraction peaks were indexed with tetragonal cell dimensions of a = 8.7693(4) and c = 12.3885(6) Å, space group I4/mcm (No.140). Ba₃Si₄C₂ has the Ba₃Ge₄C₂ type structure which can be described as a cluster-replacement derivative of perovskite (CaTiO₃), and contains isolated anion groups of slightly compressed [Si₄]⁴⁻ tetrahedra and [C₂]²⁻ dumbbells. The electrical conductivity measured for a not well-sintered polycrystalline sample was 2.6 × 10⁻²–7 × 10⁻³ S cm⁻¹ in the temperature range of 370–600 K and slightly increased with increasing temperature. The Seebeck coefficient showed negative values of around −200 to −300 μV K⁻¹.     

Conductivity of a new pyrophosphate Sn0.9Sc0.1(P2O7)1−δ prepared by an aqueous solution method

New pyrophosphate Sn_0.9Sc_0.1(P₂O₇)_1−δ was prepared by an aqueous solution method. The structure and conductivity of Sn_0.9Sc_0.1(P₂O₇)_1−δ have been investigated. XRD analysis indicates that Sn_0.9Sc_0.1(P₂O₇)_1−δ exhibits a 3 × 3 × 3 super structure. It was found that Sn_0.9Sc_0.1(P₂O₇)_1−δ prepared by an aqueous method is not conductive. The total conductivity of Sn_0.9Sc_0.1(P₂O₇)_1−δ in open air is 2.35 × 10⁻⁶ and 2.82 × 10⁻⁹ S/cm at 900 and 400 °C respectively. In wet air, the total conductivity is about two orders of magnitude higher (8.1 × 10⁻⁷ S/cm at 400 °C) than in open air indicating some proton conduction. SnP₂O₇ and Sn_0.92In_0.08(P₂O₇)_1−δ prepared by an acidic method were reported fairly conductive but prepared by similar solution methods are not conductive. Therefore, the conductivity of SnP₂O₇-based materials might be related to the synthetic history. The possible conduction mechanism of SnP₂O₇-based materials has been discussed in detail.     

Hydro-chemical conversion of galena in FeCl3-KCl solution

The behaviours of complexation and dissolution of PbCl₂ on the surface of galena were investigated to explore the process of hydro-chemical conversion of galena (PbS) in chloride media. By means of solution chemistry calculation, the production and dissolution of the products PbCl₂ were studied. And the passivation of the galena was studied by Tafel curve. The results show that PbCl₄²⁻ is the main form of PbCl₂ presented in the saturated potassium chloride (KCl) solution. The PbCl₂ crystal is easy to precipitate when the total concentration of chloride ion ([Cl⁻]_T) is equal to 0.92 mol/L, and it is inclined to dissolve when [Cl⁻]_T is more than 0.92 mol/L. The chloride complexing reaction rate strongly depends on the Fe³⁺ion concentration when it is less than 6×10⁻⁴mol/L, while passivation occurs on the surface of the electrode when Fe³⁺ concentration is larger than 6×10⁻⁴mol/L. The reaction rate increases obviously when KCl is added, since the activity of Cl⁻ increases; thus accelerates the dissolution of PbCl₂.     

Spin and charge density on iron nuclei in the BCC Fe–Mo alloys studied by Fe Mössbauer spectroscopy

Iron molybdenum alloys were prepared for the molybdenum concentration range 0–50 at.% by the arc melting method. X-ray diffraction patterns show single BCC phase for the Mo concentration up to 18 at.% with the lattice constant increasing upon addition of Mo. Sample with 21 at.% of Mo looks like composed of many BCC phases differing by the lattice constant. Finally, sample with 50 at.% of Mo looks like being composed of two BCC phases with various lattice constants and some amount of the non-stoichiometric λ-phase having symmetry P6₃/mmc. Mössbauer data indicate random solutions up to 12 at.% of Mo with magnetic order at room temperature. Room temperature paramagnetic phase appears for the sample with 18 at.% of Mo and its content increases with the increasing concentration of Mo. Traces of magnetically ordered phase (at room temperature) are seen for the sample with 40 at.% of Mo. Sample with 50 at.% of Mo is paramagnetic at room temperature. Contributions to the hyperfine field and isomer shift on the iron nuclei have been determined as the function of the distance between iron nucleus and Mo impurity up to the third co-ordination shell within the single-phase random solution range. Mo atom as the nearest iron neighbor changes iron hyperfine field by −4.18 T, as the second neighbor makes change by −2.30 T and finally as the third neighbor changes the field by +0.51 T. Corresponding changes in the isomer shift are as follows: −0.033 mm/s, −0.005 mm/s and +0.003 mm/s. The average hyperfine field and the isomer shift decrease linearly versus Mo concentration at rates −0.383 T/at.% and −5.06 × 10⁻⁴ mm/(s at.%), respectively. Hence, addition of Mo increases the electron density on the iron nucleus at the rate +1.7 × 10⁻³ electron a.u.⁻³(at.%)⁻¹.     

Magnetite stability in aqueous solutions as a function of temperature

Magnetite solubility, as a function of temperature and partial hydrogen pressure, with reference to the typical conditions of the operating fluid of a steam generator of a thermal power plant, has been studied by rigorously solving the problem of equilibria and adopting the scheme proposed by Sweeton and Baes [J. Chem. Thermodynamics2, 479 (1970)]. Stoichiometric calculations have proved that magnetite solubility attains its maximum value, which depends on the characteristics of the electrolytic solution, when the temperature is about 100°C, independently of the type of environment. A rigorous pH calculation was carried out using the method of the characteristic function, which can be applied also to complex systems, and assuming that the effect of the ionic strength may be neglected. The main aim of this study, besides helping power plant chemists to select a proper feedwater conditioning, was to calculate the pH, on a molal basis, of a solution through the best-fit of its exact values, as a function of ammonia concentration inside the inverval 1.0 × 10⁻⁸ to 9.0 × 10⁻³ m with a third-degree logarithmic polynomial. The results, which were obtained in the case of a solution containing NH₄OH and H₂CO₃, demonstrate the validity of this technique which allows the pH of a fairly complex system to be computed accurately. It also allows the correct amount of magnetite dissolution products to be evaluated without considering in detail its chemical equilibria when the solution temperature is above 200°C. This remark was derived from the pH calculations of an ammonia containing solution, which showed its independence of partial hydrogen pressure in the high temperature region, at least as far as the interval 0–1 atm was concerned. The determination of the pH, on a molar basis, of a solution at temperatures of 200, 250, 300 and 350°C, contaminated with sea water so that its acid conductivity was 300μΩ⁻¹cm⁻¹, has been performed. These results have shown that the buffering effectiveness of ammonia is negligible when its concentration falls within the interval 1.0 × 10⁻⁶ to 2.0 × 10⁻⁵ M, whereas in the range 6.0 × 10⁻⁵ to 3.0 × 10⁻⁴ M, its effect is quite pronounced.     

Environmental effects in the stress corrosion cracking of turbine disc steels

The effects of environmental parameters, such as the impurity concentration, oxygen content and pH value on the stress corrosion cracking (SCC) of 3.5Ni-Cr-Mo-V and 2Cr-Ni-Mo steels were studied. SCC of these two alloys only occurs in steams containing both oxygen and contaminating ions. A concentration of 40 ppb of Cl⁻ or around 5 ppm of SO₄²⁻ in steam is found to be enough to induce SCC on both alloys when oxygen exists in the steam. Sulfate ions suppress the severity of chloride ions in causing SCC of both materials in a Cl⁻ and SO₄²⁻ containing steam. This effect was proposed to result from the iron sulfate salt deposition. SCC of these steels in steam is accomplished by corrosion of active path, while the reaction rate is determined by the rate of cathodic reaction of the SCC system. Unless the pH value is 3 or less, the change in pH of de-aerated environments induces no SCC on both materials. SCC in the pH 3 environment is a result of the change of the cathodic reaction from water decomposition to proton reduction in this SCC system.     

The kinetics and mechanism of cathodic oxygen reduction on zinc and zinc–aluminium alloy galvanized coatings

A rotating disk electrode technique is used to investigate the kinetics and mechanism of O₂ reduction as it occurs at the surface of various hot-dip Al–Zn alloy coatings (on steel) immersed in weakly alkaline (pH 9.6) aqueous sodium chloride. The zinc component of coatings behaves electrochemically as though it were free zinc and the O₂ reduction pathway is determined by the potential dependent state of zinc. A 2e⁻ reduction to H₂O₂ predominates at potentials near the free corrosion potential, where zinc is (hydr)oxide covered. A 4e⁻ reduction to OH⁻ predominates at potentials where zinc is bare. Tafel slopes (∂E/∂log i) of 0.058 V dec⁻¹ and 0.132 V dec⁻¹ are determined for 2e⁻ and 4e⁻ O₂ reduction on pure zinc, respectively. Aluminium is virtually inert and varying aluminium content between 0.1% and 55% exerts little influence on O₂ reduction kinetics. However, all the Zn–Al alloy surfaces give very much higher O₂ reduction currents at low polarization than does pure zinc and it is proposed that this arises through an electrocatalysis of 2e⁻ O₂ reduction by traces of substrate derived iron.