A potentiostatic transient study of the passivation of carbon steel in 1 M NaOH

The potentiostatic transient behaviour of carbon steel in 1 M NaOH at 20 ± 2°C is described. The data suggest that the initial film on the metal surface forms by a nucleation and lateral growth mechanism. Film thickening and reduction transients were found to obey the empirical relationship i = kt^?n. Both the exponent, n, and the growth constant, k, are potential dependent. However, the observed potential dependencies for these constants suggest that none of the current growth theories are capable of describing quantitatively the formation of oxide—hydroxide films on carbon steel in 1 M NaOH at ambient temperature.     

Studies on viscosity behaviour of alkali halides in aqueous N,N′ dimethyl urea solution

The viscosity behaviour of ternary systems comprising alkali halides (concentration range 0.125–3 M) in aqueous N,N′ dimethyl urea (0.6 M) solution, over the temperature range 25–40°C has been investigated. It has been found that Moulik's equation (ν/ν_$\text{0}$)² = M + KC² holds good for these systems for the concentrated region. From the relative viscosity data, the “effective” rigid molar volume, V_e and apparent B coefficient have been computed employing the Breslau-Miller procedure at temperatures 25, 30, 35 and 40°C. These data have been used to explain the structuring effect in N,N′ dimethyl urea solution.     

Electrical and sensing properties of indium-doped barium cerate

Systematic analysis of electrical characteristics of BaCe_0.75In_0.25O_3-δ (BCI25) sintered sample was performed in a dry and a wet argon atmosphere in the 250 °C–700 °C temperature range. The water vapor sensing properties of BCI25 porous film and its response and recovery times were investigated under different conditions of temperature and water vapor concentration. The 30 μm thick film obtained from the powder calcined at 1050 °C exhibited sensitivity comparable to that of the sintered sample with a significantly shorter response and recovery times. While the sensitivity of the film gradually decreased with a decrease in partial pressure of water vapor (p(H₂O)), a noticeable sensitivity was still observed at p(H₂O) of 200 Pa. Decrease in conductivity depended logarithmically on the partial pressure of water with the slope of 0.52 that is close to the theoretical value. After several cycles, the reusability test proved an almost unchanged ratio between the impedance value in the dry and the wet Ar atmosphere (p(H₂O) = 2.34 kPa), which implied that BCI25, having good stability and sensitivity, is a promising high-temperature humidity sensor.     

Electronic states and cation distributions of MgAl2O4 and Mg0.4Al2.4O4 microwave dielectric ceramics

The electronic state and microwave dielectric properties of MgAl₂O₄ prepared using solid-state (MA-S) and molten salt (MA-M) methods and those of Mg_0.4Al_2.4O₄ (M04A24) were investigated. The λ values, which correspond to the fraction of Al³⁺ cations in tetrahedral sites, for MA-S, MA-M, and M04A24 were 0.23, 0.41, and 0.60, respectively. In molecular orbital calculations, a larger overlap was observed between Al-3s or Al-3p in tetrahedral sites and O-2p orbitals for M04A24, and the bond order for AlO at tetrahedral sites of M04A24 (0.241) was higher than those for MA-S (0.178) and MA-M (0.205). The dielectric constant, ε_r, for M04A24 (7.6) was lower than those for MA-S and MA-M (both 7.9), and the highest quality factor, Q·f, was obtained for M04A24 (235, 800 GHz). It was found that the covalency of the AlO bonds in the MO₄ tetrahedra is closely related to the Q?f values of the present ceramics.     

Influence of cooling rate on the heat capacity and thermal transitions of amorphous polyhexene-1

Heat capacity measurements have been made on an amorphous polyhexene-1 sample between 20 and 300K, with an adiabatic calorimeter giving a low dispersion on the C_p curve (0·2–0·4%). The measured sample was prepared by a new method increasing its thermal diffusivity. The heat capacities were measured for different cooling rates (250 K/h and 0·15 K/h) through the glass transition. This large difference of the cooling rates did not change T_g very much (215·5 to 213·5K), but a peak was added to the usual ΔC_p at T_g. An anomalous behaviour was observed at 80K, which is consistent with an experiment described elsewhere. Around 80K dielectric and mechanical relaxations were found in other experiments. In conclusion, the importance of the glass transition is pointed out, its effect being visible down to the lowest temperatures.     

Causticization of sodium carbonate with rock-salt type magnesium aluminium oxide formed by the thermal decomposition of hydrotalcite-like layered double hydroxide

The causticization behaviour of Na₂CO₃ with rock-salt type oxide, Mg_2(1-x/(2+x) O (M = Al³⁺ and Fe³⁺, x = 0.2 and 0.33), produced by the thermal decomposition of hydrotalcite-like layered double hydroxide was investigated at 30–60°C. The degree of causticization increased with increasing the molar ratio of [rock-salt type oxide]/[Na₂CO₃] and increasing the temperature but decreased with increasing the initial concentration of Na₂CO₃. The causticization behaviour could be adequately described by the equation of anion exchange equilibria between CO₃^2- and OH^? for layered double hydroxide.     

Influence of oxygen ion enrichment on optical,mechanical, and electrical properties of LSCF perovskite nanocomposite

La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) is a mixed ionic electronic conductor with excellent surface catalytic activity for oxygen reduction. This work demonstrated that introduction of pure oxygen ion conductor to LSCF particles can significantly influence in-plane electronic conduction at the surface of LSCF-samarium-doped ceria (SDC) composite. The composite functional layer was prepared by mixing 50?wt% SDC particles with LSCF particles obtained from glycine–nitrate process. Homogeneous LSCF-SDC composite layer deposited by screen printing on an SDC substrate has been studied with and without LSCF current-collecting layer (CCL). The microstructural, optical, Raman, mechanical and electrical properties, and interfacial polarization resistance (R_p) of the prepared powders were evaluated. Results revealed that addition of oxygen ion conductor SDC exerted negligible effect on the phase structure and specific surface area but significantly influenced the band gap, oxygen vacancies, and electrical conductivity of LSCF. SDC addition significantly increased area specific resistance (ASR) of LSCF from 0.138?Ω?cm² to 0.481?Ω?cm² at 800?°C, thereby blocking the conduction path among LSCF particles. R_p value of LSCF-SDC composite can be improved by more than six times by enlarging the in-plane electronic conduction with thin CCL. Electrochemical measurement revealed that LSCF CCL reduced the R_p value, resulting in the lowest ASR of 0.087?Ω?cm² at 800?°C for the LSCF–SDC composite.     

Effect of Li ions on reduction of Fe oxides in aqueous alkaline medium

It has long been known that the performance of the Fe negative electrode in Fe/Ni or Fe/air batteries is improved by the presence of lithium ions in the electrolyte. This work therefore investigated quantitatively the effect of Li⁺ on the reduction of some Fe^II and Fe^III oxides, as possible intermediates of Fe reduction/oxidation cycles, by comparing the extent of the reduction in pure 6.0 M KOH with that in lithiated alkaline media. Fe oxides were studied as pressed powder samples embedded in Ni foam. It was found that, in 6.0 M KOH, only FeO is electrolytically reduced at room temperature, whereas Fe₂O₃ and, to an even greater extent Fe₃O₄, are very recalcitrant to the reduction process. The situation dramatically changes in, for instance, 4.0 M KOH + 2.0 LiOH electrolyte, in which the extent of the reduction, even at room temperature, becomes significant for all compounds. This behaviour is very probably due to the reduction of Li⁺ within the oxide lattice to produce Li_xFe_yO_z intercalation-compound intermediates, which are then reduced to metallic Fe and Li hydroxide.     

Synthesis of Ca–Co hydroxides and their use in facile fabrication of textured CayCoO2 thermoelectric ceramics

A brucite-type hydroxide of Ca_zCo_1?z(OH)₂ with a high Ca content was synthesized and utilized as a single-source precursor for the fabrication of bulk ceramics of a semiconducting layered calcium cobaltite Ca_yCoO₂. Plate-like Ca_zCo_1?z(OH)₂ particles with z up to 0.22 were reproducibly obtained by a reverse co-precipitation method using KOH as an alkaline source. Uniaxial pressing of the plate-like Ca_zCo_1?z(OH)₂ particles (z?=?0.22), which was calcined at 373?K beforehand, at a pressure as high as 500?MPa produced a highly dense green pellet where the plate-like particles were stacked along the pressing direction. The green pellet was then converted by a heat treatment at 923?K into an oxide ceramic of preferentially (010)-oriented Ca_yCoO₂ via a topotactic-like pyrolytic reaction maintaining edge-sharing CoO₆ octahedra. Although the ceramic also contained a minor insulating Co₃O₄ phase (16.7?±?0.7?mol% estimated from thermal analysis), the electrical measurements proved its p-type semiconducting behavior resulting from the Ca_yCoO₂ phase with enhanced Seebeck coefficient exceeding 160 μV/K at room temperature.     

Synthesis of fly ash based geopolymer mortar considering different concentrations and combinations of alkaline activator solution

Geopolymerisation is a process that can transform alumina and silica rich waste materials into valuable binding materials, having excellent mechanical properties. The present experimental study shed a light on the variation in compressive strength of fly ash based geopolymer mortar by varying the molarity of sodium hydroxide as 12 M, 14 M, 16 M and accompanying by sodium silicate (Na₂SIO₃) in 2:1 (Na₂SIO₃/ NaOH) with same molarities. All the geopolymer mixes were oven cured at 80 °C for 24 h and after that kept at room temperature up to the time of testing. The compressive strength was checked subsequently at the ages of 3, 7, 14 and 28 days. The experimental results reveal that the addition of sodium silicate enhances the strength development in geopolymer mortar. The ultimate compressive strength of 40.42 MPa was obtained by incorporating sodium silicate along with 16 M concentrated sodium hydroxide. Furthermore, increasing trend of the compressive strength was found with increasing molar concentration of sodium hydroxide and curing period.     

Phase-formation,microstructure, and piezoelectric/dielectric properties of BiYO3-doped Pb(Zr0.53Ti0.47)O3 for piezoelectric energy harvesting devices

This paper proposes a method for the composition and synthesis of lead zirconate titanate (PZT) piezoelectric ceramic for use in energy harvesting systems. The proposed material consists of (1?x)Pb(Zr_0.53Ti_0.47)O₃–xBiYO₃ [PZT–BY(x)] (x=0, 0.01, 0.02, 0.03, 0.04, and 0.05 mol) ceramics near the morphotropic phase boundary (MPB) region, prepared by a solid-state mixed-oxide method. The optimum sintering temperature was found to be 1160 °C, which produced high relative density for all specimens (96% of the theoretical density). Second phases were found to precipitate in the composition containing x≥0.01 mol of BY. It is shown that the addition of BY inhibits grain growth, and exhibits a denser and finer microstructure than those in the un-doped state. Fracture surface observation revealed predominant intergranular fracture for x=0 and x=0.01, while a mixed mode of transgranular and intergranular fracture appeared for x≥0.02. The optimal doping level was found to be x=0.01, for which a dielectric constant (K₃₃^T) of 750, a Curie temperature (T_C) of 373 °C, a remnant polarization (P_r) of 50 µC/cm², a piezoelectric constant (d₃₃) of 350 pC/N, and an electro-mechanical coupling factor (k_p) of 65% were obtained. In addition, the piezoelectric voltage constant (g₃₃), and transduction coefficient (d₃₃×g₃₃) of PZT–BY(x) ceramics have been calculated. The ceramic PZT–BY(0.01) shows a considerably lower K₃₃^T value, but higher d₃₃ and k_p. Therefore, the maximum transduction coefficient (d₃₃×g₃₃) of 18,549×10^?15 m²/N was obtained for PZT–BY(0.01). The large (d₃₃×g₃₃) indicates that the PZT–BY(0.01) ceramic is a good candidate material for energy harvesting devices.     

Contribution of the edge effect to physical adsorption in micropores of activated carbons

The adsorption process in activated carbon micropores can be viewed as a two-dimensional condensation on micropore walls at a critical condensation pressure, p_c, which evolves into volume filling. p_c on a surface of micropore walls is calculated in respect to a critical condensation pressure on the reference nonporous surface, p_c^ref. Although theoretical treatment predicts that p_c^ref should be a parameter of adsorption isotherms, experimental isotherm equations are expressed in terms of the bulk saturation pressure, p_s. Experimental values of p_s/p_c^ref for a graphitized carbon black, which is often considered as a model of activated carbon surfaces, range from 37 to 4000 for typical adsorbates and are by no means close to unity. It is proposed that this apparent discrepancy between theory and experiments has its origin in the edge effects: carbon blacks are modeled by semi-infinite slabs, whereas the magnitudes of thickness and diameters of micropore walls are of the order of a nanometre. Factors associated with edge effects reduce the adsorption energies on surfaces of finite particles resulting in shifts of condensation pressures to higher values. For particles with diameters about 1.4 and 1.8 nm, adsorption energies decrease to ≈0.72ε₁*^∞ and ≈0.81ε₁*^∞, respectively, where ε₁*^∞ is the energy of adsorption on an infinite plane, and p_c approaches p_s. This phenomenon substantiates the application of p_s in adsorption equations and the success of the Dubinin equations probably owes much to this fact.     

Vacuum infiltration of copper aluminate by liquid aluminium

This paper studies attained microstructures and reactive mechanisms involved in vacuum infiltration of copper aluminate preforms with liquid aluminium. At high temperatures, under vacuum, the inherent alumina film enveloping the metal is overcome, and aluminium is expected to reduce copper aluminate, rendering alumina and copper. Under this approach, copper aluminate toils as a controlled infiltration path for aluminium, resulting in reactive wetting and infiltration of the preforms.Ceramic preforms containing a mixture of Al₂O₃ and CuAl₂O₄ were infiltrated with aluminium under distinct vacuum levels and temperatures, and the resulting reaction and infiltration behaviour is discussed. Copper aluminates stability ranges depend on vacuum level and oxygen partial pressure, which determine both CuAl₂O₄ and CuAlO₂ ability for liquid aluminium infiltration. At 1100 °C and 0.76 atm vacuum level CuAl₂O₄ is stable, indicating pO₂ above 0.11 atm. Reactive infiltration is achieved via reaction between aluminium and CuAl₂O₄; however, fast formation of an alumina film blocking liquid aluminium wicking results in incipient infiltration. At 1000 °C and 3.8 × 10⁻⁷ atm vacuum level, CuAlO₂ decomposes to Cu and Al₂O₃ indicating a pO₂ below 6.0 × 10⁻⁷ atm; infiltration of the ceramic is hindered by the non-wetting behaviour of the resulting metal alloy. At 1000 °C and 1.9 × 10⁻⁶ atm vacuum level CuAlO₂ is stable, indicating pO₂ above 6.0 × 10⁻⁷ atm. Extensive infiltration is achieved via redox reaction between aluminium and CuAlO₂, rendering a microstructure characterised by uniform distribution of alumina particles amid an aluminium matrix.This work evidences that liquid aluminium infiltration upon copper aluminate-rich preforms is a feasible route to produce Al-matrix alumina-reinforced composites. The associated reduction reaction renders alumina, as fine particulate composite reinforcements, and copper, which dissolves in liquid aluminium contributing as a matrix strengthener.     

Absorption of CO2 by alkaline electrolyte and its effect on electrical discharge

Freshly prepared 37% potassium hydroxide was exposed to the atmosphere. Samples were taken periodically to measure amount of absorbed CO₂ and their specific conductance. After exposure for 10 months, the solution was virtually neutralized by absorption of CO₂. Effects of CO₂ content and KOH concentration on the discharge characteristics of the silver-zinc cell were tested. Discharge capacity was decreased when CO₂ content reached beyond 3%, and KOH concentration dropped to below 30%. Polarization experiments on silver oxide and zinc electrodes showed that only the zinc electrode was affected. Observation of the current-time behaviour lead to the equation (i-i ₁)=kt _p ^?1/2 , and X-ray diffraction showed that the passivation layer was ZnO.     

Polarographic behaviour of p-nitrotoluene and p-nitroaniline in water-alcohol mixtures

The polarographic behaviour at the dme of p-nitrotoluene and p-nitroaniline is investigated in water-methanol, water-ethanol and water-isopropanol, in presence of 0.1 M LiCl as indifferent electrolyte. In water-methanol solutions one wave is observed. In essentially aqueous solutions (> 80 wt-% H₂O) and essentially alcoholic solutions (> 80 wt-% alcohol) of water-ethanol and water-iso-propanol, one wave is also obtained. At moderate ethanol and iso-propanol concentrations, (20–80 wt-% alcohol) splitting into two waves takes place. The reduction mechanism of the nitro compounds and the nature of the waves are discussed.     

The evaluation of ΔC°p for acid-base equilibria from pK measurement

The use of the temperature variation of the dissociation constant for evaluating changes in ΔC°_p is reviewed with a view to establishing the degree of reliability of such values.ΔC°_p values obtained from dissociation constant measurements are compared, where possible, with the calorimetrically determined values. Significant differences are apparent, suggesting serious shortcomings in the indirect methods of evaluation used so far.The related question of the appropriate form of the pK(T) equation is also discussed. When the “most appropriate” form of pK(t), viz a five-constants equation is used with the accurate data of Ives for the dissociation of cyano-acetic acid, according to the method of Clarke and Glew, the results still retain large uncertainties on ΔC°_p.Model exact pK data can be calculated from known assumed ΔG°, ΔH°, ΔC°_p, first and second temperature derivatives of ΔC°_p at a chosen reference temperature. By introducing variations of various magnitudes and signs on these model values, very accurate “experimental-like” pK values can be generated. When these accurate pK values are analysed on the computer, using the method of Clarke and Glew, and the results are compared, it becomes apparent that over the normal experimental range (O°?50°C or 60°C) even very small errors (ca 0·00025 pK units) can lead to very large uncertainties on ΔC°_p (especially at the two extremes of the temperature range) exceeding in some cases 20 cal. The identification of the true ΔC°_p(T) becomes impossible.It is concluded that the dissociation constant method is not capable as yet of furnishing reliable ΔC°_p values; the extension of the temperature range to the neighborhood of 100°C and an increase in the degrees of freedom (eg by measurements at 5° intervals) could combine to furnish fairly reliable ΔC°_p values.     

Frequency response analysis of the Ag/Ag+ system: a partially active electrode approach

The dc and ac polarization behaviour of the Ag/Ag⁺ system was studied theoretically and experimentally. The model proposed is based on bulk diffusion towards a partially active electrode surface. The experimental data obtained on stationary and rotated disc electrodes of polycrystalline and monocrystalline silver in 0.1 M AgNo₃ + 1.0 M HClO₄ solution are interpreted in terms of different model parameters characterizing the dc and ac behaviour of the system quantitatively. It is demonstrated that surface relaxation may give rise to an inductive part in the ac-impedance diagram.     

Some aspects of the electrochemical response of iron hydroxide precipitated on different conducting substrates

The potentiodynamic behaviour of different conducting substrates (Pt, Au and vitreous carbon) covered with iron hydroxide formed by colloidal precipitation in 0.01 M NaOH is investigated in the potential range associated with the electrochemical stability of water at 25° C. The electrochemical characteristics are mainly related to the Fe(OH)₂/FeOOH redox couple formed from both thin and thick iron hydroxide layers. The results can be interpreted through the complex reaction pattern already discussed for the electrochemical behaviour of Fe in alkaline solutions.     

Voltammetric investigation of the two step reduction process of hydrogen peroxide in alkaline hydroxide solutions on HMDE

The voltammetric behaviour of hydrogen peroxide has been studied at the hmde in alkaline medium. It has been confirmed that the first of the two reduction peaks of hydrogen peroxide is catalytic in nature; traces of ferric iron act under these conditions as a catalyst. In thoroughly purified potassium hydroxide solutions the height of this peak decreases and it can be also completely eliminated, even in the presence of Fe(III), if some agents capable of forming complexes with Fe(III) are added to a solution. The rate constant of the catalytic reaction proceeding between the hydroxo complex of Fe(II) and hydrogen peroxide in 1 M potassium hydroxide solution was estimated to be 5.6 × 10⁹ mol^?1 1.s^?1.     

Analysis of the milling rate of pharmaceutical powders using the Distinct Element Method (DEM)

The milling behaviour of microcrystalline cellulose (MCC) and α-lactose monohydrate (αLM) in an oscillatory single ball mill has been analysed by using the Distinct Element Method (DEM). The experimental results suggest that the milling behaviour of αLM is more strongly influenced by the milling frequency as compared to MCC. A similar conclusion is also drawn from the DEM results. The milling behaviour of MCC and αLM is described by a first order rate process, and its rate constant, K_p, is found to correlate very well with the milling power, P_n, determined by the DEM simulation, except for the milling behaviour of αLM at 18 Hz. For the latter, there appears to be an incubation time after which the milling rate increases substantially. The results presented here provide a basis for predicting the milling behaviour of a material systematically based on the fundamental material properties and the machine dynamics without the need for extensive experiment and use of large quantities of materials.