A colloidal method for manganese oxide addition to alumina powder and investigation of properties

Various methods can be applied to introduce additives to ceramic materials. Of these methods, mechanical mixing may not always be suitable to obtain a uniform distribution of the small quantities of additive within the structure, requiring colloidal methods to be applied for the purpose. The addition of manganese oxide to alumina powder has been studied using a colloidal method. The effect of the manganese addition on alumina microstructure and the later stages of the densification behaviour was investigated, together with the hardness and mechanical strength. No evidence of secondary phase formation was detected between the manganese cation and alumina powder for up to 0.5wt% manganese addition, suggesting that manganese is in solid solution with alumina. The microstructural evidence presented suggests that small quantities of a manganese addition to alumina enhance the densification process through the formation of fast diffusion paths within the crystalline structure, similar to the effect of TiO₂ addition.     

Resonance paramagnetique electronique du manganese divalent dans l'alumine β

Single crystals of β alumina doped with manganese ions have been prepared by high frequency induction melting and slow cooling, followed by Na⁺ - Mn²⁺ exchange in molten MnCl₂. EPR study shows that Mn²⁺ ions occupy two kinds of sites. The first one, with an axial zero field tensor corresponds to the tetrahedral positions in the spinel blocks, close to the sodium planes. This seem to indicate that aluminium vacancies in β alumina are partly located in this site. The other, with an orthorhombic zero field tensor, and high D and E values, belongs to the conduction planes.     

Production and characteristics of glass-ceramics derived from manganese crust tailings

Glass-ceramics have been produced via vitrification from manganese crust tailings with over 23% reduction in tailings volume. The crystalline behaviour of parent glass and glass-ceramics were investigated using DTA, TGA, XRD, and SEM/EDS. XRD analysis revealed that the major crystalline phase was iron manganese oxide. The Vickers microhardness (H _v) was 9.74 MPa, the indentation strength (K _c) was 1.88 Mpa m^1/2, and elastic modulus (E) was 140 MPa. The properties of the glass-ceramic compared well with known research and industrial glass-ceramic materials. Results suggest that manganese crust tailings have potential to be vitrified into useful, marketable glass-ceramic materials.     

Effect of iron particle addition on the pseudocapacitive performance of sol–gel derived manganese oxides film

Manganese oxide electrodes with promising behavior were prepared successfully by sol–gel process. Manganese oxide films were also modified with the addition of submicron-crystalline iron powders. Effects of post heat treatment and iron submicron-powder addition on the material characteristics and electrochemical capacitance of the manganese oxide electrodes were investigated. The experimental results showed that manganese oxide films composed from metal-organic precursors at 250 °C heat treatment, while formation of MnO₂, Mn₂O₃, Mn₃O₄, Fe₂O₃, and Fe₃O₄ phases were observed after heat treatment at a temperature higher than 300 °C. The specific capacitances were 48.9, 140.1, 212.7, and 81.1 F g⁻¹ for manganese oxide films heat treated at 250, 300, 350, and 400 °C, respectively. The specific capacitances were 75.7, 227.3, 247.9, and 152.9 F g⁻¹ for manganese/iron oxide films (Mn:Fe = 100:1) heat treated at 250, 300, 350, and 400 °C, respectively. The manganese/iron oxide films (Mn:Fe = 100:1) treated at 350 °C exhibited the highest specific capacitance 247.9 F g⁻¹ of the electrodes investigated in the present study. After 1000 cyclic voltammetry tests, the specific capacitance decreased by only 10 percent. The surface morphology of this film exhibited powders with linked nano-sized particles. The number of special particles reached a maximum after heat treatment at 350 °C. The experimental results showed that post heat treatment and iron submicron-particle addition may change the surface morphology and structure, increase the specific capacitance, and improve the electrochemical performances of the manganese oxide electrodes.     

A redox-assisted supramolecular assembly of manganese oxide nanotube

In this paper, we report the hydrothermal synthesis of manganese oxide nanotube from an aqueous medium of pH 7, using KMnO₄ and MnCl₂ as inorganic precursors, polyoxyethylene (10) nonyl phenyl ether (TX-10) a surfactant and acetaldehyde an additive. The characterization of X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and N₂ adsorption at 77 K (BET) reveals that the synthesized manganese oxide nanotube has a mesopore size of ca. 3.65 nm and a wall thickness of ca. 12 nm, with the wall being composed of microporous crystals of monoclinic manganite. The X-ray photoelectron spectroscopy (XPS) result demonstrates a decrease of the binding energy of the Mn³⁺ in the manganese oxide nanotube, which may be related to both the nanotubular morphology and the crystalline pore wall. A mechanism of a redox-assisted supramolecular assembly, regulated by acetaldehyde, is postulated.     

Potentiodynamical co-deposited manganese oxide/carbon composite for high capacitance electrochemical capacitors

Manganese oxide/carbon composite materials were prepared by introducing the carbon powders into the potentiodynamical anodic co-deposited manganese oxide in 0.5 mol L^− 1 MnSO₄ and 0.5 mol L^− 1 H₂SO₄ mixed solution at 40 °C. The surface morphology and structure of the composite material were examined by scanning electron microscope and X-ray diffraction. Cyclic voltammetry tests and electrochemical impedance measurements were applied to investigate the performance of the composite electrodes with different ratios of manganese oxide and carbon. These composite materials with rough surface, which consisted of approximately amorphous manganese oxide, were confirmed to possess the ideal capacitive property. The highest specific capacitance of manganese oxide/carbon composite electrode was up to 410 F g^− 1 in 1.0 mol L^− 1 Na₂SO₄ electrolyte at the scan rate 10 mV s^− 1. The synthesized composite materials exhibited ideal capacitive behavior indicating a promising electrode material for electrochemical supercapacitors.     

Adsorptive desulfurization by activated alumina

This study reports usage of commercial grade activated alumina (aluminum oxide) as adsorbent for the removal of sulfur from model oil (dibenthiophene (DBT) dissolved in n-hexane). Bulk density of alumina was found to be 1177.77 kg/m³. The BET surface area of alumina was found to decrease from 143.6 to 66.4 m²/g after the loading of DBT at optimum conditions. The carbon-oxygen functional groups present on the surface of alumina were found to be effective in the adsorption of DBT onto alumina. Optimum adsorbent dose was found to be 20 g/l. The adsorption of DBT on alumina was found to be gradual process, and quasi-equilibrium reached in 24 h. Langmuir isotherm best represented the equilibrium adsorption data. The heat of adsorption and change in entropy for DBT adsorption onto alumina was found to be 19.5 kJ/mol and 139.2 kJ/mol K, respectively.     

Nucleation and growth kinetics of reaction-product formation for copper–titanium and silver–titanium alloys on alumina

A nucleation and growth mechanism is proposed for the formation of the reaction product at the interface between polycrystalline alumina and liquid-metal alloy drops containing titanium. The reaction product had been previously identified to be an oxide of titanium. The growth of reaction product islands was clearly observed at the alumina–metal interface using optical microscopy after dissolving the metal droplets with acid. The fractional coverage was quantified as a function of time and, by assuming Avrami-type reaction kinetics, surface reaction rate constants, k, were calculated for copper–titanium and silver–titanium alloys on alumina. Reaction rate constants between 1.4 × 10^-4 and 18 × 10^-4 s^-2 were obtained for copper–titanium alloys on alumina. The k values for silver–titanium alloys were found to be an order of magnitude lower (2.5 × 10^-6 and 7.2 × 10^-6 s^-2) then the k values obtained for copper–titanium alloys on alumina. This revised version was published online in November 2006 with corrections to the Cover Date.     

High resolution electron energy loss spectroscopy of manganese oxides: Application to Mn3O4 nanoparticles

Manganese oxides particularly Mn₃O₄ Hausmannite are currently used in many industrial applications such as catalysis, magnetism, electrochemistry or air contamination. The downsizing of the particle size of such material permits an improvement of its intrinsic properties and a consequent increase in its performances compared to a classical micron-sized material. Here, we report a novel synthesis of hydrophilic nano-sized Mn₃O₄, a bivalent oxide, for which a precise characterization is necessary and for which the determination of the valency proves to be essential. X-ray diffraction (XRD), Transmission Electron Microscopy (TEM) and particularly High Resolution Electron Energy Loss Spectroscopy (HREELS) allow us to perform these measurements on the nanometer scale. Well crystallized 10–20 nm sized Mn₃O₄ particles with sphere-shaped morphology were thus successfully synthesized. Meticulous EELS investigations allowed the determination of a Mn³⁺/Mn²⁺ ratio of 1.5, i.e. slightly lower than the theoretical value of 2 for the bulk Hausmannite manganese oxide. This result emphasizes the presence of vacancies on the tetrahedral sites in the structure of the as-synthesized nanomaterial.     

Novel strategies for evaluating the degradation of protective coatings on superalloys

Novel approaches involving new specimen preparation methods, ¹⁸O tracer experiments and secondary ion mass spectrometry have been developed to determine the high-temperature oxidation mechanisms and kinetics of gas turbine blades. The industrial gas turbine blades investigated in this work consisted of an oxide–Si-aluminide coating–directionally solidified nickel-based superalloy system. The oxide scales were characterised after various exposure times from 0 to 15 000 h in a gas turbine environment and also after laboratory annealing in oxygen atmospheres (¹⁶O₂ and ¹⁸O₂) at high temperature. The oxide and coating layers were analysed by micro-machining cross-sections on the surface of the blades using a Ga⁺ focused ion beam system (FEI-FIB200) combined with secondary ion mass spectroscopy. The powerful milling technique and the sub-micron resolution of the focused ion beam instrument, widely used within the semiconductor industry, were successfully adapted to our study. The FIB200 experiments provided essential results concerning the microstructural evolution of the oxide during high-temperature exposure yielding the thickness, the porosity, the pathways of diffusion, the chemical composition and the distribution of the different phases present in these layers. Further annealing experiments at high-temperature were also performed in an atmosphere enriched with the stable oxygen isotope ¹⁸O₂ to determine the oxygen diffusion mechanism through the oxide scale. The oxygen diffusion characteristics were investigated by depth profiling and imaging (elemental mapping ¹⁶O⁻ and ¹⁸O⁻) using conventional and high-resolution SIMS (ATOMIKA 6500, FIB200). An Atomic Force Microscope (Quesent Resolver) and an Interferometric Optical Microscope (Zygo) were used to measure the SIMS craters for depth calibration. The combination of these high-resolution methods has provided a basis for a fundamental understanding of the oxidation behaviour of the protective coatings on superalloys.     

The growth and electrical transport properties of self-organized metal/oxide nanostructures formed by anodizing Ta-Al thin-film bilayers

Anodizing of Ta-Al metal bilayers (Al on Ta) sputter-deposited onto SiO₂ substrates was performed in oxalic acid electrolytes at anode potentials of 53 to 21.5 V in order to form nanoporous alumina layers and sequentially oxidize the tantalum underlayers through the alumina pores. The films formed consist of arrays of tantalum oxide nanohillocks percolating through the residual tantalum layer down to the substrate, so that a self-organized network of tantalum nanowires forms between the substrate and the alumina film. The average width (25–<10 nm), length (70–35 nm), and population density (10⁹–10¹¹ cm^-2) of the nanowires are systematically defined by the initial tantalum thickness (8–22 nm) and the anodizing conditions. The mesh-like, nano-sized morphologies of the tantalum underlayers result in a remarkably wide range of potential-dependent, controlled electrical sheet resistances (10²–10⁷ _Ω/sq). The periodical, tunable, metal/insulator film structure, allowing an increased transition to hopping or tunneling conduction at elevated temperature, leads to negative temperature coefficients of resistance, ranging 300 to 5 ppm/K. Oscillations of the potential-dependent dc conductance registered in the films at room temperature are attributed to the quantum-size effects in the metal/oxide nanostructures. The films are of technological importance for fabrication of thin-film, planar, adjustable resistors with significantly improved performances.     

Tetrapropylammonium-manganese oxide/polypyrrole hybrid nanocomposite thin films as novel electrode materials for supercapacitors

Tetrapropylammonium-manganese oxide/polypyrrole (TPA-MO/Ppy) hybrid nanocomposite with molar ratios of TPA-MO/Ppy 4:1, 2:1 and 1:1 were successfully prepared by a combination of in situ polymerization and the sol–gel process. The microstructure of hybrid nanocomposite thin film samples was observed to be significantly affected by synthesis parameters, most notably the molar ratio of reactants and post-synthesis calcination temperature. Samples with higher pyrrole contents appeared to possess higher specific surface areas, which ranged from 132 to 281 m² g⁻¹. SEM micrographs indicated that all nanocomposite thin films were highly fibrous and porous in nature. Optimum doping of manganese oxide with conducting polypyrrole had led to the formation of novel nanocomposite with nanofibrillar structures which consisted of interconnected manganese oxide and polypyrrole nanoclusters. Optimized nanocomposite films showed higher charge capacities which could be attributed to enhanced material utilization as a result of optimized microstuctural parameters in particular, specific surface areas.     

Preparation of alumina films by the sol-gel method

This review describes our study on preparation of alumina films by a sol-gel process and their several applications that have been investigated since 1986. Alumina films were prepared from alkoxide or inorganic salt. Both as-prepared alumina films were transparent in ultraviolet, visible and near infrared regions. The alumina from inorganic salt (inorganic alumina) was structureless even after annealed at 300–700°C in air, while the alumina from alkoxide (alkoxide alumina) was in pseudo-boehmite at an annealing temperature lower than 400°C and was in - or -type at 400–700°C. Both alumina films became opaque after annealed at temperatures above 1000°C. The inorganic alumina film annealed at 800°C showed a gas permeability that was influenced by physico-chemical properties of penetrant and alumina. Composite films of alumina and poly(vinyl alcohol) (PVA) were hydrophilic but insoluble in water, and removal of PVA from the composite films by annealing at 600°C led to formation of transparent alumina films. Such properties enabled us to use a counter diffusion method for fabricating -Fe₂O₃-doped alumina films. Alumina films doped with organic dyes such as laser dyes, hole-burning dyes and non-linear optical dyes, which were fabricated by gelation of dye-added alumina sol, exhibited laser emission, hole-spectra and second- or third-harmonic generation properties, respectively. Hydrogenation of alkene was catalyzed by Ni nanoparticles doped alumina films that were prepared by gelation of Ni²⁺ solution-added alumina sol and annealing the Ni²⁺-doped alumina gel in hydrogen gas. Nonlinear optical properties were observed for alumina films doped with CdS, Au and Ag nanoparticles, which were fabricated by gelation of Cd²⁺, HAuCl₄ and AgNO₃ solution-added alumina sols and annealing the Cd²⁺-doped alumina gel in H₂S gas and the Ag⁺- and Au³⁺-doped alumina gels in H₂ gas. Rare earth metal ion-doped alumina films, which were prepared by gelation of rare earth metal ion solution-added alumina sol and annealed the ion-doped alumina gel, exhibited not only normal luminescence but also up-conversion emission, energy transfer type luminescence and long lasting luminescence.     

Under-surface observation of thin-film alumina on NiAl(100) with scanning tunneling microscopy

Scanning tunneling microscopy (STM) was used to investigate the oxide structures underlying the surface of alumina thin-film grown on NiAl(100). At a bias voltage (on the sample) below 2.0 V, STM topography images of the alumina layer beneath the surface were obtained. A probe with depth of 2-8 Å was readily attained. The under-surface observation shows that the film consists of stacked layers of alumina whereas the layered alumina unnecessarily comprises entire θ-Al₂O₃ unit cells. The lattice orientation of the upper alumina layer differs from that of the lower one by 90° — the newly grown oxide structurally matching the horizontal oxide rather than the lower oxide. The results indicate a growth process competing with the typical mode of epitaxial growth for the growth of alumina film.     

Guided Assembly of Microporous/Mesoporous Manganese Phosphates by Bifunctional Organophosphonic Acid Etching and Templating

Manganese (Mn)‐based compounds are important materials for both energy conversion and energy storage. Unfortunately, it has been a significant challenge to develop highly ordered microporous/mesoporous structures for them to provide more active sites for these applications. In order to do so using the soft‐templating method, three conditions have to be met, namely, a strong interaction between the inorganic precursor and the organic templates; eliminating the formation of bulk manganese phosphate; and the preservation of the manganese phosphate framework without it collapsing upon template removal. Herein, a soft‐templating approach is reported using an organophosphonic acid (n‐hexylphosphonic acid) as both the etching and the templating agent, followed by high‐vacuum‐assisted annealing. This approach simultaneously satisfies the above conditions. Both microporous and mesoporous manganese phosphates with uniform pore sizes and well‐defined pore structures are obtained. The utilization of the organophosphonic acid is shown to be the key in the transformation from bulk manganese oxide into a highly ordered microporous phosphate. A very high surface area of 304.1 m² g^?1 is obtained for the microporous manganese phosphate, which is the highest among the reported values for Mn‐based compounds. The ultrafine micropores and high specific surface area of our manganese phosphate promote electrocatalytic activity for the oxygen evolution reaction.     

Fast fabrication of long-range ordered porous alumina membranes by hard anodization

Nanoporous anodic aluminium oxide has been widely used for the development of various functional nanostructures. So far these self-organized pore structures could only be prepared within narrow processing conditions. Here we report a new oxalic-acid-based anodization process for long-range ordered alumina membranes. This process is a new generation of the so-called "hard anodization" approach that has been widely used in industry for high-speed fabrication of mechanically robust, very thick (>100 microm) and low-porosity alumina films since the 1960s. This hard anodization approach establishes a new self-ordering regime with interpore distances, (D(int))=200-300 nm, which have not been achieved by mild anodization processes so far. It offers substantial advantages over conventional anodization processes in terms of processing time, allowing 2,500-3,500% faster oxide growth with improved ordering of the nanopores. Perfectly ordered alumina membranes with high aspect ratios (>1,000) of uniform nanopores with periodically modulated diameters have been realized.     

One-step through-mask electrodeposition of a porous structure composed of manganese oxide nanosheets with electrocatalytic activity for oxygen reduction

Potentiostatic electrolysis of a mixed aqueous solution of Bu₄NBr and MnSO₄ at +1.0 V (vs. Ag/AgCl) on Pt electrode led to the oxidation of Br⁻ and Mn²⁺ ions. X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and X-ray diffraction (XRD) revealed that this anodic process was followed by the deposition of insulating crystals of bromide salt of Bu₄N⁺ and the subsequent formation of layered manganese oxide in the interstitial spaces of the bromide grains already grown. Dissolution of the bromide crystals in water left a well-dispersed porous texture composed of manganese oxide nanosheets. The resulting MnO_x-modified electrode exhibited a larger catalytic current for the reduction of oxygen in alkaline solution, compared to the bare Pt electrode.     

Analytical study of the evolution of alumina scales formed on an ODS alloy

Abstract

An original deflection technique, in association with TEM observations, allowed study of the transformation of metastable alumina phases to α phase. This transformation is mainly characterised by a 14% volume decrease. Deflection experiments were performed on PM 2000 after pre-oxidation, in order to create a 3 µm alumina scale, and subsequent mechanical oxide removal of one large sample face. This technique allowed evaluation of the kinetics of transformation. The microstructural and chemical evolutions of the oxide, the interface (morphology, segregation) and the alloy (aluminium depletion, oxide dispersion) were characterised using TEM analyses and thermogravimetric measurements were done in the same conditions as the deflection tests to determine the oxidation rate constants     

Biopolymer-manganese oxide nanoflake nanocomposite films fabricated by electrostatic layer-by-layer assembly

Bio-nanocomposite films based on chitosan and manganese oxide nanoflake have been fabricated via the layer-by-layer (LBL) self-assembly technique. UV–vis absorption spectra showed that the subsequent growth of the nanocomposite film was regular and highly reproducible from layer to layer. X-ray photoelectron spectroscopy (XPS) spectra confirmed the incorporation of chitosan and manganese oxide nanoflake into the films. Scanning electron microscopy (SEM) images revealed that the nanocomposite film had a continuous surface and a layered structure. A sensitive hydrogen peroxide (H₂O₂) amperometric sensor was fabricated with the chitosan–manganese oxide nanoflake nanocompoite film. The sensor showed a rapid and linear response to H₂O₂ over the range from 2.5 × 10^? 6 to 1.05 × 10^? 3 M, with a sensitivity of 0.038 A M^? 1 cm^? 2.     

Sintering LiTaO3 and KTaO3 with the aid of manganese oxide

The sintering of LiTaO₃ and KTaO₃ with the aid of manganese oxide was studied at 1080 to 1300° C by X-ray analysis, scanning electron microscopy (SEM) and X-ray microanalysis (XMA). The sintering of pure LiTaO₃ and KTaO₃ proceeds concurrently with grain growth, but only achieves a density of 83% at 1300° C and of 72% at 1280° C, respectively. The addition of 3 wt% of MnO₂ or Mn₃O₄ results in rapid densification of LiTaO₃ to 90 to 92% within 30 min at 1190° C. In the presence of manganese oxide, KTaO₃ densifies to 95% at 1280° C. The action of manganese oxide is attributed to the substitution of Mn³⁺ for Ta⁵⁺ in the LiTaO₃ or KTaO₃ lattice which enhances the diffusion of the rate-determining species, oxygen, in the oxide. In addition, a liquid phase formed at 1250° C in KTaO₃ may significantly contribute to the achievement of 95% densification.