Effect of Nb2O5 and MgO/Nb2O5 doping on densification,microstructure and wear resistance of alumina

In this paper, the influence of Nb₂O₅ single-doping and MgO/Nb₂O₅ co-doping on the densification, microstructure and wear resistance of Al₂O₃ has been investigated. The results show that Nb₂O₅ single-doping can increase the density of alumina effectively, but excessive Nb₂O₅ will lead to abnormal grain growth. Comparing with Nb₂O₅ single-doping, MgO/Nb₂O₅ co-doping can further increase the density and suppress the abnormal grain growth of the alumina. Meanwhile, the co-doped samples also show much lower wear rates. Typically, when 500 ppm MgO/1000 ppm Nb₂O₅ co-doped, the alumina ceramics has a minimum wear rate of 0.01‰, which is about 1/5 of the alumina ceramics with 1000 ppm Nb₂O₅ single-doped. The inhibiting effect of co-doping on abnormal grain growth and the reasons for the decrease of wear rate are also discussed.     

The Degradation of Polyglycolide in Water and Deuterium Oxide. Part II: Nuclear Reaction Analysis and Magnetic Resonance Imaging of Water Distribution

Magnetic resonance imaging (MRI) and scanning microbeam nuclear reaction analysis (NRA) were used to monitor changes of water ingress into polyglycolide (PGA) disks with degradation time. MRI detects H₂O, whereas NRA is sensitive to D₂O. The acid-catalysed hydrolysis of the ester is significantly slower in D₂O than H₂O because of the kinetic isotope effect. This behaviour was investigated in Part I. In this paper, NRA was used to investigate PGA hydration in buffers made from D₂O, and NRA and MRI experiments were performed on samples degraded buffers made from a 50% mixture of D₂O and H₂O (D₂O/H₂O 50:50) to allow a comparison between the two techniques. The NRA and MRI results provide direct evidence in support of the four-stage reaction-erosion model reported in previous literature, and show that this model applies to polymer degradation in heavy water and in a buffer made from D₂O/H₂O 50:50. It is believed that this is the first time that NRA and MRI have been compared for the same hydrating system.     

The Simplified Equation of Separation for the Enrichment of Heavy Water in a Batch-Type Thermal Diffusion Column

Abstract

A simple but precise equation of separation for the enrichment of heavy water in a batch-type thermal diffusion column has been derived with the consideration of a pseudobinary mixture. The experiment has also been conducted for various initial concentrations of D₂O and the results are in agreement with the prediction of the theory.     

The role of residual stresses in layered composites of Y–ZrO2 and Al2O3

Laminar composites, containing layers of Y–TZP and either Al₂O₃ or a mixture of Al₂O₃ and Y–ZrO₂ have been fabricated using a sequential centrifuging technique of water solutions containing suspended particles. Controlled crack growth experiments with notched beams of composites were done and showed the significant effect of barrier layer thickness and composition on crack propagation path during fracture. Distinct crack deflection in alumina layers was observed. The increase of crack deflection angle with the alumina layer thickness was also found. In the case of the barrier layer made of a mixture, crack deflection did not occur independently on layer thickness. The observed changes have been correlated with the radial distribution of residual stresses in barrier layers created during cooling of sintered composites from fabrication temperature. The stresses found were the result of the difference in the thermal expansion and sintering shrinkage of alumina and zirconia and the crystallographically anisotropic thermal expansion of the alumina. The residual stress distribution has been measured by piezo-spectroscopy based on the optical fluorescence of Cr⁺³ dopants in alumina.     

Preparation of ultrafine Al2O3 using plasma jet and its application for synthesis of dimethyl carbonate

A direct preparation of Al₂O₃ from aluminum and oxygen under plasma jet conditions has been performed in this work. The XRD characterization of such plasma-prepared alumina shows that a production of i -Al₂O₃ has been achieved. The size of the alumina particles ranged from several tens to 200 nm, based on SEM analysis. The effectiveness of plasma-prepared alumina as a catalyst support has been demonstrated by the catalytic synthesis of dimethyl carbonate over Pd/Al₂O₃.     

Effect of SO2 on NOx reduction by ethanol over Ag/Al2O3 catalyst

A new Ag/Al₂O₃ catalyst for removing NO_x in diesel engine exhaust gas was developed. The influence of SO₂ on the reduction of lean NO_x by ethanol over the Ag/Al₂O₃ catalyst was evaluated in simulated diesel exhaust and characterized using TPD, XRD, XPS, SEM and BET measurements. The Ag/Al₂O₃ catalyst was highly active for the reduction of NO_x with ethanol in the presence of SO₂ although the reduction of NO_x is suppressed at lower temperatures. The activity for NO_x reduction is high even on the Ag/Al₂O₃ catalyst exposed to a SO₂ (200 ppm)/O₂ (10%)/H₂O (10%) flow for 20 h at 723 K and comparable to that on the fresh Ag/Al₂O₃ catalyst. No crystallized Ag metal and Ag compounds were formed by the SO₂/O₂/H₂O exposure. On the other hand, crystallized Ag₂SO₄ was easily formed when the Ag/Al₂O₃ catalyst was exposed to a SO₂ (200 ppm)/O₂ (10%)/NO (800 ppm)/H₂O (10%) flow for 10 h at 723 K. XRD, SEM and XPS studies showed that the formation of crystallized Ag₂SO₄ results in growing of Ag particles in larger size and lowering the surface content of Ag particles. In addition, the specific surface area of the Ag/Al₂O₃ catalyst decreases from 221 to 193 m²/g. Although the dispersion of Ag particles was decreased by the formation of Ag₂SO₄, the activity for the reduction of lean NO_x was, remarkably, not affected. This suggests that the Ag–alumina sites created by the Ag₂SO₄ formation are still active for the lean catalytic reduction of NO_x. This revised version was published online in July 2006 with corrections to the Cover Date.     

Water vapour permeability,diffusion and solubility in latex films

Water vapour diffusion D, solubility S and permeability P coefficients have been determined for films obtained from carboxylated styrene-butadiene (SB) copolymer latexes. The experimental method is water vapour sorption performed in the range 30–60d?C. Using the small angle neutron scattering (SANS) method and a selective labelling with D₂O vapour, it has been shown that water molecules mainly diffuse in the films through the particle-particle interfaces, which consist of a polar carboxyl-rich copolymer. It has been shown that the degree of cross-linking of the particles does not significantly affect the values of D and S. Moreover, the effect of neutralization conditions, regarding both the pH value of the initial latex and the nature of the neutralizing agent, has also been investigated. It has been found that D does not depend on these parameters, whereas S appears to be very sensitive to them. The results have been interpreted on the basis of the structural modifications of the films induced by neutralization. Finally, the hydrophilicity (or hydrophobicity) of the neutralizing agent has been identified as one of the key features for controlling the affinity of the latex film for water vapour and hence its permeability properties.     

The Influence of O2, Hydrocarbons, CO, H2, NO x , SO2, and Water Vapor Molecules on Soot Combustion over LaCoO3 Perovskite

The effect of various gases (O₂, hydrocarbons, CO, H₂, NO _x , SO₂, and H₂O vapor) presenting in the diesel exhaust on soot combustion using LaCoO₃ as a catalytic material was investigated in this paper. A significant promotion of the combustion rate was found following a trend of 10% H₂O addition > 3,000 ppm NO _x  > 1% H₂ or 3,000 ppm C₃H₆ addition, while the improvement in soot oxidation due to the introduction of 3,000 ppm CO or 3,000 ppm CH₄ into the reactant gas is relatively less. The wet pretreatment of LaCoO₃ with 10% steam before soot oxidation hardly affects the combustion behavior. Interestingly, 10% water vapor in the reaction feed produced a significant promoting effect on combustion. In contrast, 30 ppm SO₂ treating led to an obvious deactivation likely owing to the coverage of active sites by sulfate compounds.     

Use of Alumina as an lon Exchanger in the Separation of Carrier-Free RaE from RaD

Abstract

A very simple procedure for the radiochemical separation of carrier-free RaE (^210B) from RaD (^210Pb) has been developed. RaE as sulfate was adsorbed in powdered alumina column, which was later quantitatively washed out with hot cone HNO₃. The radiochemical purity of RaE had been proved by studying the β-decay half-life of the isotope.     

Synthesis and Characterization of Iron Particles Hosted in Porous Alumina

Fe/Al₂O₃ composite consisting of iron particles dispersed in a porous alumina host has been prepared by wet impregnation and subsequent heat treatment in hydrogen. Thermogravimetric and differential thermal analysis was used to study the thermal behavior of Fe(NO₃)₃ loaded in porous alumina. H₂-temperature programmed reduction was adopted to analyze the reduction behavior of α-Fe₂O₃ loaded into porous alumina. The morphology and particle size of the magnetic particles were evaluated by scanning electron microscope, while the phase identification and structural analysis of the samples were examined by X-ray diffraction technique. The magnetic properties of the nanocomposite were investigated by means of vibrating sample magnetometer and ⁵⁷Fe Mössbauer spectrometry. The Mössbauer spectra indicated that all of the α-Fe₂O₃ changed into α-Fe during the reduction process; and, microscopic observations revealed that iron particles with an average diameter of ~200 nm were dispersed homogeneously on the pore walls of the porous alumina.     

The influence of additives on microstrucutre of sub-micron alumina ceramics prepared by two-stage sintering

For various systems two-stage sintering has been reported as a successful way of suppressing the grain growth in the final stage of densification of polycrystalline ceramics. Our previous results on two-stage sintering of high purity submicrometre polycrystalline alumina indicate limited efficiency of the process with respect to suppression of grain growth. The present work deals with the influence of deliberate additions of various metal oxides (500 ppm of MgO, Y₂O₃ or ZrO₂) whose grain growth retarding effect in conventional sintering has been well documented, on two-stage sintering of submicrometre alumina ceramics. The addition of MgO was observed to enhance densification. Addition of yttria and zirconia impaired densification, but addition of all three dopants resulted in suppression of the grain growth and microstructure refinement in comparison to undoped alumina.     

A Study of the Interaction of NO2 with Carbon Particles

The interaction NO₂ in air (0.5–35 ppm) with carbon particles led to three products: NO gas, and NO₂ ^? and NO₃ ^?, removed from the particles by water extraction. At 4 ppm or below, in dry or humid air, the product distribution, in relative molar amounts, was NO₃ ^? = 2NO₂ ^? = 2NO. At 20 ppm and above, the relative amounts of products depended on the presence of water vapor: in dry air NO = 3NO₃ ^? = 6NO₂ ^?; in humid air NO = NO₂ ^? = 2NO₃ ^?. For carbon slurries in water, [NO₂ ^?] = 6[NO₃ ^?] at an input concentration of NO₂ of 4 ppm. In comparison to carbon, alumina particles and glass beads removed NO₂ ineffectively. These results indicate that NO₂ oxidized the carbon particles while it was reduced to NO. NO₂ adsorbed at oxidized sites on the particles formed a surface species that was analyzed as nitrate. At high enough concentration of NO₂ (20 ppm and above), the interaction of NO and water vapor with the surface nitrate produced NO₂ ^?. In slurries NO, generated from interaction of NO₂ with carbon, reacted with surface nitrate or nitric acid in solution to form the relatively large quantities of nitrite. This work suggests that NO_x reactions with carbon in droplets or on wet surfaces could be important sources for the production of nitrous acid in the environment.     

Explosive compaction of Al2O3 nanopowders

Dense bulk alumina (Al₂O₃) has been prepared by explosive compaction and its microstructure has been investigated by optical microscopy, Raman spectroscopy, scanning electron microscopy and transmission electron microscopy. The average microhardness of the alumina compact is about 171 HV0.025. Ultra-thin alumina films with glassy (translucent) appearance formed during the explosive compaction process. Nanograin Al₂O₃ particles are covered by amorphous Al₂O₃ which help to achieve a dense microstructure by promoting interparticles bonding. Phase transformation from γ-Al₂O₃ to α-Al₂O₃ during the explosive compaction process has been confirmed. Formation of the alumina compact is due to the large cooling rate and the high pressure during the explosive compaction.     

Permalloy/alumina soft magnetic composite compacts obtained by reaction of Al-permalloy with Fe2O3 nanoparticles upon spark plasma sintering

Composite sintered soft magnetic materials of permalloy/alumina type have been obtained by reactive spark plasma sintering. The composite compacts have been obtained by sintering of Ni71.25Fe23.75Al5 alloy with 3 and 5% (wt.) Fe₂O₃ nanoparticles. The Ni based alloy with large particles (up to hundreds of μm) have been covered by a thin layer of iron ferric oxide nanoparticles (20–40 nm). The as obtained composite particles have been subjected to sintering process using a homemade installation at 900 °C for 10 min. Upon sintering process several reactions between Ni-based alloy and iron oxide are induced, the main phase resulting from reaction is alumina-Al₂O₃ as it results by X-ray diffraction investigations. According to the scanning electron microscopy and energy dispersive X-ray spectroscopy investigations, alumina forms a matrix embedding the Ni-based particles. The alumina matrix is continuous, but the layer has large variation in width, and offers a high electrical resistivity. A mechanism of formation is proposed for the alumina matrix composite compacts when using Al-permalloy powder and iron oxide. The compacts have been tested in DC and AC for magnetic characteristics.     

Evaluation of alumina reinforced oil fly ash composites prepared by spark plasma sintering

The thermomechanical behavior of micro/nano-alumina (Al₂O₃) ceramics reinforced with 1-5 wt.% of acid-treated oil fly ash (OFA) was investigated. Composites were sintered using spark plasma sintering (SPS) technique at a temperature of 1400°C by applying a constant uniaxial pressure of 50 MPa. It was evaluated that the fracture toughness of micro- and nanosized composites improved in contrast with the monolithic alumina. Highest fracture toughness value of 4.85 MPam^1/2 was measured for the nanosized composite reinforced with 5 wt.% OFA. The thermal conductivity of the composites (nano-/microsized) decreased with the increase in temperature. However, the addition of OFA (1-5 wt.%) in nanosized alumina enhanced the thermal conductivity at an evaluated temperature. Furthermore, a minimum thermal expansion value of 6.17 ppm*K⁻¹ was measured for nanosized Al₂O₃/5 wt.% OFA composite. Microstructural characterization of Al₂O₃-OFA composites was done by x-ray diffraction and Raman spectroscopy. Oil fly ash particles were seen to be well dispersed within the alumina matrix. Moreover, the comparative analysis of the nano-/microsized Al₂O₃/OFA composites shows that the mechanical and thermal properties were improved in nanosized alumina composites.     

Bis(Diphenylcarbamoylmethylphosphine Oxide) Ligand Containing 4, 7, 10 – Trioxatridecane Spacer: Novel Effective Extractant for Actinides and Lanthanides

A novel polydentate neutral organophosphorus ligand 1 containing two Ph₂P(O)CH₂C(O)NH- bidentate moieties connected by a 4, 7, 10 - trioxatridecane spacer through amide nitrogen atoms was synthesized and studied as an extractant for U(VI), Th(IV), and lanthanides(III) ions from HNO₃ solutions. The influence of aqueous and organic phases on the extraction efficiency was elucidated and stoichiometry of the complexes extracted was determined. Bis-CMPO ligand 1 was found to possess a higher extraction efficiency towards U(VI), Th(IV), and lanthanides(III) ions than its mono analog diphenylphosphorylacetic acid N-nonylamide 5 and bis-CMPO ligands containing a di- and a triethylene glycol spacers. The values of D_U, D_Th, and D_Ln for compound 1 are more than two orders of magnitude higher than those for its mono-CMPO analog 5.     

Glycine-nitrate combustion synthesis of finely dispersed alumina

The possibility of synthesizing different crystalline modifications of alumina Al₂O₃ by combustion reactions of a mixture of aluminum nitrate with glycine has been analyzed. Aggregated powders of γ-Al₂O₃ that do not transform into the corundum structure after annealing at a temperature of 1000°C have been prepared. The influence of the synthesis conditions on the specific surface area, bulk density, porosity, and morphology of alumina particles has been investigated.     

Flame-made Cu/ZnO/Al2O3 catalyst for dimethyl ether production

For the single step synthesis of dimethyl ether (DME) from synthesis gas a Cu/ZnO/Al₂O₃-catalyst has been prepared using flame-spray pyrolysis. The resulting powder was co-mixed with γ-alumina to give an admixed system for DME production. The flame-made catalyst was analyzed using the BET method, in situ XRD, N₂O decomposition, TPR and XPS. These studies unraveled that the catalyst exhibited a high Cu surface area including good contact with zinc oxide and alumina as well as small Cu particles resulting in high catalytic activity and product selectivity, also in comparison to a commercially available catalyst.     

Fenton‐ and Fenton‐Like AOPs for Wastewater Treatment: From Laboratory‐To‐Plant‐Scale Application

Abstract

Fenton‐and Fenton‐like AOPs systems have been utilized for the oxidative degradation of some chlorinated pollutants such as chloral hydrate or 1,1,1‐trichloroethane, and for the treatment of real industrial wastewaters. Both ferrous sulfate (FeSO₄ · 7 H₂O) and Mohr's salt (NH₄)₂Fe(SO₄)₂. 6 H₂O have been used as Fe²⁺ ion sources. With Mohr's salt (MS) the Fenton‐and Fenton‐like reaction has been successfully carried out under acidic (pH 3) and neutral (pH 7) reaction conditions. The new Fenton‐like system utilizes zero‐valent iron (Fe^o) instead of ferrous sulfate has been applied for the 1,1,1‐trichloroethane and chloral hydrate degradation. Similarly, the application of catechol‐ and hydroquinone‐driven Fenton reaction for the degradation of chloral hydrate under acidic and neutral pH is a new Fenton‐like AOPs approach. The photo‐Fenton‐like reactions such as Fe³⁺/hν, Fe²⁺/H₂O₂/hν, and ferrioxalate system have been also studied for the degradation of chloral hydrate. As an irradiation source a daily light or sun light have been used. In comparison with photoreactor experiments the best system was observed to be Fe³⁺/hν. In some experiments the influence of standing time prolongation after Fenton reaction on the final degradation efficiency due to hydrolysis of intermediates such as phosgene (CCl₂?O) has also been studied. The Fenton reaction was successfully utilized for the treatment of real industrial wastewaters, in two cases even in plant‐scale applications.     

The Preparation of Highly Dispersed Au/Al2O3 by Aqueous Impregnation

In this work, we report an impregnation method for preparing Au supported on alumina from HAuCl₄. In the literature, impregnation under acidic conditions has been found to lead to poor dispersions of Au and the resulting catalysts are not as active as those prepared by deposition-precipitation. To overcome these problems, we have developed a two-step procedure: in the first step, the acidified Au solution is contacted with alumina to adsorb the Au chloride on the alumina. After washing off the excess Au precursor, we treat the solid with a strong base to convert the chloride to an absorbed hydroxide. Drying and calcination at 400 °C yields a catalyst with Au particles having a number average diameter of 2.4 nm. The reactivity for CO oxidation at room temperature is comparable to catalysts prepared by deposition-precipitation. These catalysts are stable to hydrothermal sintering, with average particle size around 4 nm after sintering in 10 mol% H₂O at 600 °C for 100 h. This work shows that stable Au/Al₂O₃ catalysts having a high reactivity for CO oxidation can be prepared by impregnation under acidic conditions.