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- Alumina (3)
- Alumosol (3)
- Alumo-sol (2)
- Beschichtung (2)
- Inconel-718 (2)
- Oxidationsschutz (2)
- Rheology (2)
- Sol-Gel-Verfahren (2)
- Sol-gel (2)
- 27Al nuclear magnetic resonance (1)
Modified Yoldas sols can be used to prepare crack-free well adherent layers on different substrate materials such as sodalime glass, corundum and metal (IN-718). These layers having thicknesses of 0.6 µm are strain tolerant and withstand thermal cycling between 810 °C and room temperature. Introducing alpha-Al2O3 filler particles into modified Yoldas sols allows the preparation of thicker layers of 1.6 µm, which as well can be thermally cycled without noticeable delaminations. Chemical bonding seems to be the predominant layer bonding mechanism.
Aluminas resulting from sols prepared via a modified Yoldas procedure were studied with differential thermal analysis (DTA), differential thermal gravimetrie (DTG), 27Al nuclear magnetic resonance (27Al MAS NMR) and X-ray diffraction (XRD) concerning their thermal properties, aluminum speciation and phase content.
Hydrolysis of aluminum-sec-butoxide in aluminum nitrate solutions allowed to prepare stable sols with varying NO3-/Al molar ratios, solids contents and pH values. Resulting sols contained different aluminum species including also Al13 polycations. Sol preparation conditions also determined aluminum speciation in solid products obtained after thermal treatments of gels obtained from these sols. Al13 polycations and AlO5 species were found to play an important role for thermally induced transformation from amorphous products via eta-Al2O3 to alpha-Al2O3. Intermediately formed eta-Al2O3 promotes the phase transformation to alpha-Al2O3.
Alumina sols were prepared using a modified
Yoldas procedure, thus resulting in sols with high solids
content, excellent long term stability and small particle size.
PH-value ranged between 3.7 and 0.6. Sols were characterised
using rheological, UV-vis transmission and particle
size measurements. Depending on NO3-/Al molar ratio as
well as solids content, sols vary. Analysis of reduced viscosities
indicate the formation of chainlike polymeric species
above a critical solids fraction.
Influence of polyvinylpyrrolidone (PVP) on alumina sols prepared by a modified Yoldas procedure
(2008)
Al13 polycations containing alumina sols, prepared by a modified Yoldas procedure were mixed with polyvinylpyrrolidone (PVP). Although Al speciation in freshly prepared sols was not affected by PVP addition the decay rate of Al13 polycations was slightly decreased in PVP-containing sols. PVP does not show any influence on particle size and particle growth. The influence of PVP addition on viscosity and flow behavior of modified Yoldas sols depends on their solids content and NO3-/Al molar ratio.
Alumina coatings with differing phase compositions were deposited on Inconel-718 (IN-718) Ni-base
superalloy substrates using solgel processing. Mass gain measurements served for studying the oxidation
behavior of coated metal surfaces if exposed to 800°C and 900°C for up to 4000 h in static air. Itwas found, that
alumina coatings significantly reduce the oxidation related mass gain of IN-718 even after heating to 900°C
for 4000 h. Transmission electron microscopy (TEM) studies revealed diffusion of elements from the metal
substrate (IN-718) into the alumina coatings and to a much lower degree from the coating into the
substrate. These diffusion processes are greatly influenced by the phase composition of the coatings, especially
by the presence of alpha-alumina.
A new method of preparing concentrated alumo-sols, the hydrolysis of Al-tri-sec-butylate in acidic aqueous media at 85°C, was studied in detail by varying the H2O/Al- and NO3 -/Al-ratios in wide ranges. The components of the sols were characterized by 27Al NMR spectroscopy. The pH-value depends on both the chosen H2O/Al- and NO3 -/Al-ratio and on the aging time of the sols and reflects the composition of the sols. Al13 polycations were detected in sols with a pH-value between 3.0 and 3.7. As a new result its presence was shown by NMR below 3.4. The Al13 content of the sols increased with pH and the maximum fraction of Al13 polycations was detected in the sol with the highest pH (3.7). Nearly 65% of the entire aluminium content of this solution is bound in the Al13 polycations. Hence, a new synthetic method for the preparation of Al13 ions containing sols was developed.
Aging studies of the sols showed, that the Al13 polycations were more stable in solutions with higher pH-value. Al13 polycations were detected after an aging time of four months only in sols with a pH-value of 3.7. Tempering the aged sols at 40° to 80°C caused formation of Al13 and also of Al30 polycations.
Alumosols in combination with well dispersed corundum were successfully used to form up to 2.5 ìm thick coatings on the Ni-base alloy SC16. These coatings withstood heat treatments at 700 °C in a water vapour containing atmosphere. The heat treatment caused formation of delta-alumina in the coating and diffusion of chromium and titanium into the coating.
Diffusion of Cr, Fe, and Ti ions from Ni-base alloy inconel-718 into a transition alumina coating
(2012)
Heat treating metals at high temperatures trigger diffusion processes which may lead to the formation of oxide layers. In this work the diffusion of Cr, Fe and Ti into an alumina coating applied to Inconel-718 is being investigated. Mass gain measurements, UV–vis spectroscopy and transmission electron microscopy were applied in order to study the evolution of the diffusion process. It was found that mainly Cr as well as minor amounts of Fe and Ti are being incorporated into the alumina coating upon prolonged heat treatment at 700 °C. It could be shown that alumina coatings being void of Cr have the same oxidation related mass gain as uncoated samples. However, incorporation of Cr into the alumina coating decreased their mass gain below that of uncoated substrates forming a Cr oxide scale only.