Aluminate

About: Aluminate is a research topic. Over the lifetime, 7037 publications have been published within this topic receiving 89839 citations.

Characterisation of oxide films produced by plasma electrolytic oxidation of a Ti–6Al–4V alloy

Abstract: The paper discusses processing and property aspects of oxide films formed on a Ti–6Al–4V alloy by AC plasma electrolytic oxidation (PEO) in aqueous solutions containing aluminate, phosphate, silicate and sulfate anions and some of their combinations. Structure, composition, mechanical tribological and corrosion resistant characteristics of the films formed are studied by SEM, XRD and microhardness analyses, and by scratch, impact, pin-on-disc friction and potentiodynamic corrosion testing. It is found that the films produced from the aluminate–phosphate electrolyte are dense and uniform and are composed mainly of Al 2 TiO 5 and TiO 2 phases of the rutile form. The films possess a beneficial combination of 50–60 μm thickness, 575 kg/mm 2 hardness and high adhesion and provide a low wear rate (3.4×10 −8 mm 3 /Nm) but a relatively high friction coefficient of μ=0.6–0.7 against steel, caused by material transfer from the counterface. A minimum friction coefficient of μ=0.18 is recorded during the testing of softer rutile–anatase films, 7 μm thick, produced from a phosphate electrolyte. Both of these types of film show good corrosion resistance in NaCl and physiological solutions, where the corrosion current is approximately 1.5 orders of magnitude lower than that of the uncoated substrate. SiO 2 /TiO 2 -based films with 70–90 μm thickness and high bulk porosity produced from silicate and silicate–aluminate electrolytes demonstrate better corrosion behaviour in H 2 SO 4 solution, due to the greater chemical stability of the film phase components in this environment.

The behaviour of sulphur in silicate and aluminate melts

Abstract: As part of a general programme of research on the nature of inorganic melts, a study has been made of the manner in which sulphur is held in liquid silicates and aluminates. Mixtures of CaO-SiO 2 , MgO-SiO 2 , FeO-SiO 2 , CaO-Al 2 O 3 and CaO-SiO 2 -Al 2 O 3 have been brought into equilibrium, at temperatures between 1350 and 1650°C, with gas phases made by mixing H 2 , CO 2 and SO 2 at room temperature. It has been shown that, when p O 2 is less than about 10 –5 atm, a sulphur atom can only enter the melt by displacing a suitable oxygen atom. The sulphur is then held in the slag entirely as sulphide, its concentration being controlled by the general equilibrium, ½S 2 + (O)melt = ½O 2 + (S)melt. When p O 2 is greater than about 10 -3 atm, the sulphur is held as sulphate and the corresponding equilibrium is ½S 2 + 3/2O 2 + (O)melt = (SO 4 )melt∙ In the binary silicate melts there must be three kinds of oxygen atoms present: those bonded to two silicon atoms, those bonded to one silicon and those unattached to silicon. Their proportions are presumably connected by an equilibrium of the type O 2– + ⋮ Si-O-Si ⋮ = 2(⋮ Si-O¯). From the manner in which both the sulphide and sulphate equilibria depend upon the proportions of metal oxide present, it appears that only the oxygen atoms which are unattached to silicon are of importance so far as these equilibria are concerned. The potential capacity of a melt to hold sulphur as sulphide is expressed as its sulphide capacity C s = (wt. % S) ( p O 2 ) ½ /( p s 2 ) ½ . For binary silicate and aluminate melts there is a close parallelism between the value of this capacity and the activity of the basic metal oxide. The activity coefficients for CaS have been found to be the same as for CaSO 4 and approximately constant in CaO-SiO 2 mixtures. The sulphide equilibrium results have been used to improve the free energy of formation curve for the formation of CaO-SiO 2 melts, and to construct a similar curve for CaO-Al 2 O 3 melts. The temperature coefficient of the sulphide capacity for CaO-SiO 2 melts has been combined with other data and used to extend the heat of formation curve for CaO-SiO 2 melts. The sulphur capacities, and presumably the lime activities, of the ternary melts CaO-SiO 2 -Al 2 O 3 , have been shown to be a maximum at compositions for which the molar proportions of SiO 2 and Al 2 O 3 are equal; this behaviour would be expected because the energy of interaction of SiO 2 and Al 2 O 3 is much greater than that of either CaO and SiO 2 or CaO and Al 2 O 3 . It follows the pattern established for FeO activity coefficients in CaO-SiO 2 and CaO-P 2 O 5 melts, these rising to a maximum at CaO : SiO 2 and CaO; P 2 O 5 molar ratios of 2:1 and 3:1 respectively.

A review on magnesium aluminate (MgAl2O4) spinel: synthesis, processing and applications

Abstract: Magnesium aluminate (MgAl2O4) spinel (MAS) is a synthetic material with cubic crystal structure and excellent chemical, thermal, dielectric, mechanical and optical properties. These properties made MAS an indispensable material for optically transparent windows, domes and armours, and for certain refractory applications. High processing cost of dense MAS ceramics is in the main responsible for its limited usage in certain important applications despite its excellent performance in them. The volume expansion (∼8%) associated with MAS phase formation from alumina and magnesia does not allow obtaining dense MAS bodies in a single-stage reaction sintering process. Therefore, dense MAS bodies are made by following a double stage firing process, which is expensive. The existing literature suggests that the processing cost of dense MAS ceramics could be reduced to a great extent by decisive selection of starting raw materials, powder processing and densification conditions, and by understanding the under...

Selective formation of Na-X zeolite from coal fly ash by fusion with sodium hydroxide prior to hydrothermal reaction

Abstract: Hydrothermal treatment of fly ash with alkali gives various types of zeolites such as Na-Pl, Na-A and hydroxysodalite, where the zeolite zone was formed like an egg white, covering the central core of fly ash particles, as evinced in the previous paper. By fusion with sodium hydroxide, most of the fly ash particles were converted into sodium salts such as silicate and aluminate, from which hydrothermal reaction without stirring favourably resulted in the formation of Na-X zeolite. Crystallinity of Na-X zeolite as high as 62% was attained at the optimum condition of NaOH/fly ash = 1.2 and a fusion temperature of 823 K. Fly ash contains 14 wt% mullite (3Al2O3·2SiO2), which was revealed to be a less-active crystalline component for zeolite formation. Aluminium-enriched fly ash gave Na-A in place of Na-X zeolite. Scanning electron microscope images of cubic and octahedral crystals characteristic of Na-A and Na-X zeolite, respectively, obtained from fly ash, are given.