Control of the γ-alumina to α-alumina phase transformation for an optimized alumina densification

Lamouri, S.; Hamidouche, M.; Bouaouadja, Noureddine; Belhouchet, H.; Garnier, Vincent; Fantozzi, Gilbert; Trelkat, Jean François

doi:10.1016/j.bsecv.2016.10.001

Cited by 187 publications

(104 citation statements)

References 37 publications

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“…It is known that the specific temperatures for alumina transitions depend on the dopant concentration and alumina mean particle size [59]. In particular, the additives that reduce alumina particle growth during sintering and reduce nucleation sites α-Al 2 O 3 (for example, ZrO 2 ) [59] strongly influence phase transition temperatures.…”

Section: Discussionmentioning

confidence: 99%

“…In particular, the additives that reduce alumina particle growth during sintering and reduce nucleation sites α-Al 2 O 3 (for example, ZrO 2 ) [59] strongly influence phase transition temperatures. The second step of alumina nanoparticle transformation (θ-Al 2 O 3 →α-Al 2 O 3 ) in nanocomposites is critical to understanding the sintering.…”

Section: Discussionmentioning

confidence: 99%

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Mechanism of Reduced Sintering Temperature of Al2O3–ZrO2 Nanocomposites Obtained by Microwave Hydrothermal Synthesis

et al. 2018

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A novel method to obtain Al2O3–ZrO2 nanocomposites is presented. It consists of the co-precipitation step of boehmite (AlO(OH)) and ZrO2, followed by microwave hydrothermal treatment at 270 °C and 60 MPa, and by calcination at 600 °C. Using this method, we obtained two nanocomposites: Al2O3–20 wt % ZrO2 and Al2O3–40 wt % ZrO2. Nanocomposites were characterized by Fourier transformed infrared spectroscopy, Raman spectroscopy, X-ray diffraction, and transmission electron microscopy. Sintering behavior and thermal expansion coefficients were investigated during dilatometric tests. The sintering temperatures of the nanocomposites were 1209 °C and 1231 °C, respectively—approximately 100 °C lower than reported for such composites. We attribute the decrease of the sintering temperature to the specific nanostructure obtained using microwave hydrothermal treatment instead of conventional calcination. Microwave hydrothermal treatment resulted in a fine distribution of intermixed highly crystalline nanoparticles of boehmite and zirconia. Such intermixing prevented particle growth, which is a factor reducing sintering temperature. Further, due to reduced grain growth, stability of the θ-Al2O3 phase was extended up to 1200 °C, which enhances the sintering process as well. For the Al2O3–20 wt % ZrO2 composition, we observed stability of the zirconia tetragonal phase up to 1400 °C. We associate this stability with the mutual separation of zirconia nanoparticles in the alumina matrix.

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Mechanism of Reduced Sintering Temperature of Al2O3–ZrO2 Nanocomposites Obtained by Microwave Hydrothermal Synthesis

et al. 2018

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“…The transformation is accompanied by a progressive increase in crystal density, from 3.67 to 3.987 g·cm −3 . Therefore, if densification is not involved, the γ-α transformation yields porosity [70][71][72][73].…”

Section: Amassed Oxide and Energy Consumptionmentioning

confidence: 99%

Review of the Soft Sparking Issues in Plasma Electrolytic Oxidation

Tsai

Chou

2018

Metals

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Abstract:A dense inner layer is highly valued among the surface coatings created through plasma electrolytic oxidation (PEO) treatment, because the PEO coating has been troubled by inherent porosity since its conception. To produce the favored structure, a proven technique is to prompt a soft sparking transition, which involves a sudden decrease in light and acoustic emissions, and a drop in anodic voltage under controlled current mode. Typically these phenomena occur in an electrolyte of sodium silicate and potassium hydroxide, when an Al-based sample is oxidized with an AC or DC (alternating or direct current) pulse current preset with the cathodic current exceeding the anodic counterpart. The dense inner layer feature is pronounced if a sufficient amount of oxide has been amassed on the surface before the transition begins. Tremendous efforts have been devoted to understand soft sparking at the metal-oxide-electrolyte interface. Studies on aluminum alloys reveal that the dense inner layer requires plasma softening to avoid discharge damages while maintaining a sufficient growth rate, a porous top layer to retain heat for sintering the amassed oxide, and proper timing to initiate the transition and end the surface processing after transition. Despite our understanding, efforts to replicate this structural feature in Mg-and Ti-based alloys have not been very successful. The soft sparking phenomena can be reproduced, but the acquired structures are inferior to those on aluminum alloys. An analogous quality of the dense inner layer is only achieved on Mg-and Ti-based alloys with aluminate anion in the electrolytic solution and a suitable cathodic current. These facts point out that the current soft sparking knowledge on Mg-and Ti-based alloys is insufficient. The superior inner layer on the two alloys still relies on rectification and densification of aluminum oxide.

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“…Transitions from RT up to approximately 450°C are associated with the H 2 O and CO 2 release (Fig. 10) [1,41,42], while the following endothermic transition is due to v-AlO(OH) transformation and water release [33]. The forth, weak endothermic event above 1000°C is related to particle growth and/or further (t ?…”

Section: Nanopowders Synthesis and Characterisation Methodsmentioning

confidence: 99%

“…Results and discussion boehmite phase transforms into Al 2 O 3 after annealing in at least 800°C, which was shown in [29] and [33]. Figure 2 shows SEM images for selected compositions in order to demonstrate the change of powders' morphology.…”

Section: Nanopowders Synthesis and Characterisation Methodsmentioning

confidence: 99%

Thermal and physical properties of ZrO2–AlO(OH) nanopowders synthesised by microwave hydrothermal method

Koltsov

Prześniak-Welenc

Wojnarowicz

et al. 2017

J Therm Anal Calorim

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Industrially relevant nanopowder was synthesised by microwave hydrothermal synthesis to obtain wellcontrolled composition (ZrO 2 -AlO(OH) system) which was found to determine a number of physical and thermal characteristics. This study reports variation of particle size, density, specific surface area (SSA BET ), as well as thermal behaviour of nanopowder mixtures of ZrO 2 -AlO(OH) in the whole range of compositions. It was found that the onset temperature (T on ) of physically and chemically bounded water desorption depends on the Al 3? /or AlO(OH) content. The lower content of Al 3? in the ZrO 2 -AlO(OH) system, the higher T on of physically bound water desorption. There are three distinct temperature regions for water decomposition for nanomaterials investigated in air (at approximately 50, 250 and 450°C). These temperature ranges depend on particle size and chemical composition of ZrO 2 -AlO(OH) nanopowders. Materials were divided into three groups characterised by different properties: (1) ZrO 2 with 2-12% of Al 3? , where particle sizes are from 4 to 8 nm, (2) ZrO 2 with 30-67% of AlO(OH), where particle sizes are from 10 to 13 nm, and (3) ZrO 2 with 80-99% of AlO(OH), where particle sizes are from 13 to 23 nm. AlO(OH) content determines thermal and physico-chemical properties of synthesised ZrO 2 -AlO(OH) nanopowders.

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Control of the γ-alumina to α-alumina phase transformation for an optimized alumina densification

Cited by 187 publications

References 37 publications

Mechanism of Reduced Sintering Temperature of Al2O3–ZrO2 Nanocomposites Obtained by Microwave Hydrothermal Synthesis

Mechanism of Reduced Sintering Temperature of Al2O3–ZrO2 Nanocomposites Obtained by Microwave Hydrothermal Synthesis

Review of the Soft Sparking Issues in Plasma Electrolytic Oxidation

Thermal and physical properties of ZrO2–AlO(OH) nanopowders synthesised by microwave hydrothermal method

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