γ-Alumina: a single-crystal X-ray diffraction study

Smrčok, Ľubomír; Langer, Vratislav; Křesťan, Jan

doi:10.1107/s0108270106026850

Cited by 139 publications

(120 citation statements)

References 13 publications

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“…This ceramic material is a commonly used catalyst support [68], whose bulk and surface structures are not well understood [69], as indicated by the brief sampling of the literature which follows. There is one report of a single crystal structure [70]. The bulk structure type is usually described as a spinel defect, but the location of its cation vacancies is now known to be considerably more complicated [71] and the real-world material may include hydrogen [72] or foreign ions [73].…”

Section: Catalyst Modelsmentioning

confidence: 99%

The Decomposition of Hydrazine in the Gas Phase and over an Iridium Catalyst

Schmidt

Gordon

2013

Zeitschrift Für Physikalische Chemie

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Hydrazine / Gas Phase / Ir Catalyst / Quantum ChemistryHydrazine is an important rocket fuel, used as both a monopropellant and a bipropellant. This paper presents theoretical results to complement the extensive experimental studies of the gas phase and Ir catalyzed decompositions involved in the monopropellant applications of hydrazine. Gas phase electronic structure theory calculations that include electron correlation predict that numerous molecular and free radical reactions occur within the same energy range as the basic free radical pathways: NN bond breaking around 65 kcal/mol and NH bond breaking around 81 kcal/mol. The data suggest that a revision to existing kinetics modeling is desirable, based on the energetics and the new elementary steps reported herein. A supported Ir 6 octahedron model for the Shell 405 Iridium catalyst used in thrusters was developed. Self-Consistent Field and electron correlation calculations (with core potentials and associated basis sets) find a rich chemistry for hydrazine on this catalyst model. The model catalyst provides dramatically lower NN and NH bond cleavage energies and an even smaller barrier to breaking the NH bond by NH 2 abstractions. Thus, the low temperature decomposition over the catalyst is interpreted in terms of consecutive NH 2 abstractions to produce ammonia and nitrogen. The higher temperature channel, which has hydrogen and nitrogen products, may be due to a mixture of two mechanisms. These two mechanisms are successive NH cleavages with surface H + H recombinations, and the same type of assisted H 2 eliminations found to occur in the gas phase part of this study.

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Section: Catalyst Modelsmentioning

confidence: 99%

The Decomposition of Hydrazine in the Gas Phase and over an Iridium Catalyst

Schmidt

Gordon

2013

Zeitschrift Für Physikalische Chemie

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“…Some of them with cubic symmetry are preferable since specif- ic relations between the interplanar spacing have been observed. Namely, -alumina [9], -alumina [9,10] [9,12] both belonging to the space group 3 Fd m . Finally, the reflexes of copper-nickel aluminates spinel [13] Cu x Ni 1-x Al 2 O 4 (space group 3 Fd m ) also good fit the spots on the rings.…”

Section: Chemical Composition and Structure Of The Binder (Ceramic) Fmentioning

confidence: 99%

Microstructure Investigation of the Spark Plasma Sintered Cu—Al—Ni Shape Memory Material

Monastyrsky¹,

Kоtkо²,

Gilchuk³

et al. 2016

Metallofiz. Noveishie Tekhnol.

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The microstructure of Cu-13.0Al-3.9Ni-0.4Ti-0.2Cr wt.% compacts sintered by spark plasma method from powders prepared by spark-erosion method in liquid argon from master alloy is investigated. Powder is annealed in H 2 atmosphere before the spark plasma sintering. SEM and TEM investigations reveal that sintered samples have a composite structure, which consists of the micron and submicron spherical metallic particles embedded into the binder matrix. This matrix seems to be the product of copper oxide reduction according to the scheme CuO  Cu 4 O 3  Cu 2 O  Cu 8 O and aluminium hydroxides' conversion according to the flowchart aluminium hydroxide  transition alumina  -Al 2 O 3 , during the annealing and/or sintering. TEM study reveals that main phase in metallic particles is self-accommodated 18R martensite. The regular basal plane stacking faults and/or (001) 18R microtwins are dominant defects in 18R martensite. 18R martensite of single orientation occupies entire volume of spherical nanoparticles, being the basal plane stacking faults and/or (001) 18R microtwins, which have different thicknesses.Досліджено мікроструктуру компактів Cu-13,0Al-3,9Ni-0,4Ti-0,2Cr ваг.% спечених плазмово-іскровим методом із електроерозійних порош-ків, виготовлених із мастер-стопу в рідкому арґоні. Перед спіканням по- Встановлено, що основною фазою в металевих частинках є самоузгодже-ний багатоваріянтний 18R-мартенсит, домінантні дефекти якого є реґу-лярними дефектами пакування в базовій площині та/або (001) 18R -мікродвійники. Весь об'єм сферичних наночастинок займає одноваріянт-ний 18R-мартенсит; при цьому дефекти пакування в базовій площині та/або (001) 18R -мікродвійники мають різні товщини.Исследована микроструктура компактов Cu-13,0Al-3,9Ni-0,4Ti-0,2Cr масс.%, спечённых плазменно-искровым методом из электроэрозионных порошков, изготовленных из мастер-сплава в жидком аргоне. Перед спе-канием порошок отжигался в атмосфере водорода. Спечённые образцы имеют композитную структуру, которая состоит из микронных и субмик-ронных сферических металлических частиц, встроенных в связующую матрицу. Последняя является результатом восстановления при отжиге и/или спекании оксида меди согласно схеме CuO  Cu 4 O 3  Cu 2 O  Cu 8 O и гидроксидов алюминия согласно схеме гидроксид алюминия  пере-ходный оксид алюминия  -Al 2 O 3 . Установлено, что основной фазой в металлических частицах является самосогласованный многовариантный 18R-мартенсит, доминантными дефектами которого являются регуляр-ные дефекты упаковки в базовой плоскости и/или (001) 18R -микро-двойники. Весь объём сферических наночастиц занимает одновариант-ный 18R-мартенсит; при этом дефекты упаковки в базовой плоскости и/или (001) 18R -микродвойники имеют разные толщины.

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“…Thus, γ-alumina is sometimes referred to as a defect-spinel. One of the major sources of disagreement is how the aluminium cations are distributed over the octahedral and tetrahedral interstices [25,38,39,44].…”

Section: The Various Forms Of Aluminium Oxidementioning

confidence: 99%

“…It is arguable whether such an amorphous band appears on the diffraction patterns presented here, however the pattern (and hence the electron signal) is dominated by the crystalline phase, making it problematic for such a conclusion to be drawn with a reasonable degree of confidence. 38 The 440 and 311 reflections (along with the 400 reflection) are the three most intense reflections for γ-Al 2 O 3 , and so are expected to give the clearest dark field images. The occurrence of order domains in transition aluminas has been reported in the literature [19].…”

Section: Oxide Film Microstructurementioning

confidence: 99%

“…Despite the significant amount of literature on γ-alumina [25,[34][35][36][37][38][39], there is still a significant uncertainty surrounding its structure [39][40][41][42][43][44], as its inherently disordered structure has been problematic to consistently describe. This is at least partially due to the same issue that affects all the transition aluminas, namely that the crystal structure is dependent on the starting material and its thermal history.…”

Section: The Various Forms Of Aluminium Oxidementioning

confidence: 99%

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A microstructural and kinetic study of molten aluminium oxidation in relation to dross formation

Bonner¹

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Oxidation and dross formation on molten aluminium is estimated to directly cause losses of tens of millions of dollars per annum to the Australian smelting industry. Although there has been significant study of molten aluminium oxidation over the past few decades, the formation mechanisms are still not fully understood. There is also lack of reliable and useful kinetic data that can be applied to computational simulations of industrial melt handling processes involving the oxidation of either quiescent pools or turbulent transfers of molten aluminium. There is additional uncertainty in the literature relating to the physical processes underlying molten aluminium oxidation, and the microstructural evolution of the growing oxide film. Past oxidation studies have been confounded by the detrimental effect of the pre-existing oxide ubiquitously present on aluminium specimens. Recent studies conducted at The University of Queensland have indicated a way forward in this respect, with the development of an experimental technique that enables the melt surface to be skimmed free of pre-existing oxide prior to each oxidation experiment. This project is based on previous outcomes and expands on them to create new insights into the oxidative behaviour of molten aluminium in response to various key parameters, such as melt temperature and isothermal holding time. It also aims to resolve some ambiguities in the literature.Melts of commercial purity aluminium were isothermally held at various temperatures ranging from 750°C to 900°C, and exposed to ambient air for times ranging up to ~20 h. The oxide grown on each melt was removed and treated to separate the oxide itself from the underlying aluminium metal, using a molten salt flux. The mass of oxide was then determined as a function of melt temperature and exposure time. These gravimetric experiments were complemented by morphological and microstructural characterization of grown oxide films, using scanning electron microscopy, transmission electron microscopy, and electron diffraction. These two approaches were combined to develop a simple kinetic model based on the parabolic rate law.The amount of oxide formed was found to be significantly lower than what has previously been reported, and was attributed to the removal of the pre-existing oxide. The amount of oxide that formed increased with increasing temperature, and the oxidation rate continuously decreased with increasing exposure time. These observations are consistent with diffusion-limited growth.Microstructural characterisation showed two stages of oxidation. In the early stage, nano-sized, planar, polyhedral crystallites of γ-alumina nucleate at the metal/oxide interface. In the later stage, flake-like γ-alumina crystallites were observed to grow outwards from the oxide/air interface. The time at which the second stage of growth became active decreased with increasing temperature, and the flake-like crystallites continued to increase in size with increasing exposure time.iii Crystallographic texture was obser...

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γ-Alumina: a single-crystal X-ray diffraction study

Cited by 139 publications

References 13 publications

The Decomposition of Hydrazine in the Gas Phase and over an Iridium Catalyst

The Decomposition of Hydrazine in the Gas Phase and over an Iridium Catalyst

Microstructure Investigation of the Spark Plasma Sintered Cu—Al—Ni Shape Memory Material

A microstructural and kinetic study of molten aluminium oxidation in relation to dross formation

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