Thin Films Of Amorphous Alumina Research Articles

Predicting Fracture Propensity in Amorphous Alumina from Its Static Structure Using Machine Learning.

Thin films of amorphous alumina (a-Al2O3) have recently been found to deform permanently up to 100% elongation without fracture at room temperature. If the underlying ductile deformation mechanism can be understood at the nanoscale and exploited in bulk samples, it could help to facilitate the design of damage-tolerant glassy materials, the holy grail within glass science. Here, based on atomistic simulations and classification-based machine learning, we reveal that the propensity of a-Al2O3 to exhibit nanoscale ductility is encoded in its static (nonstrained) structure. By considering the fracture response of a series of a-Al2O3 systems quenched under varying pressure, we demonstrate that the degree of nanoductility is correlated with the number of bond switching events, specifically the fraction of 5- and 6-fold coordinated Al atoms, which are able to decrease their coordination numbers under stress. In turn, we find that the tendency for bond switching can be predicted based on a nonintuitive structural descriptor calculated based on the static structure, namely, the recently developed "softness" metric as determined from machine learning. Importantly, the softness metric is here trained from the spontaneous dynamics of the system (i.e., under zero strain) but, interestingly, is able to readily predict the fracture behavior of the glass (i.e., under strain). That is, lower softness facilitates Al bond switching and the local accumulation of high-softness regions leads to rapid crack propagation. These results are helpful for designing glass formulations with improved resistance to fracture.

Optical function of atomic layer deposited alumina (0.5–41.0 nm) from 191 to 1688 nm by spectroscopic ellipsometry with brief literature review

Atomic layer deposition (ALD) is widely used in the semiconductor industry to provide atomic level control over film thicknesses and layer conformality. Here, the authors report the thermal (332 °C) ALD of thin amorphous alumina films (0.5–41.0 nm) deposited using water and trimethylaluminum precursors. Alumina deposition was optimized by varying the dose and purge times for both precursors with an eye toward obtaining uniform film thicknesses and constant growth per cycle. Films were characterized by x-ray photoelectron spectroscopy and spectroscopic ellipsometry (from 191 to 1688 nm). The subsequent workup of the SE data was based on a multisample analysis. It considered Cauchy and Sellmeier functions and accounted for surface roughness. This modeling yielded an optical function for ALD alumina over the measured wavelength range. The authors recommend the optical function determined from the Sellmeier model.

Pulsed laser deposited alumina thin films

Thin films of amorphous alumina were fabricated using pulsed laser deposition. Topography, structural, and optical properties of alumina films were investigated depending on process parameters, specifically deposition time under vacuum and partial pressure of argon. Deposited films present good uniformity with RMS roughness ranging from 0.35 to 2.50nm. Alumina films with thickness lower than 40nm deposited under vacuum present a non-negligible void content that induces a decrease of the effective refractive index of the layers. Furthermore, introduction of argon gas (at 5×10−4 and 5×10−2mbar) during the deposition process induces grainy structure of the thin films documented by an increase of RMS roughness from 0.35 to 1.5nm. A decrease of the alumina layers' refractive index is observed in the 300–7500nm spectral range when increasing Ar pressure.

Structural, optical and mechanical properties of amorphous and crystalline alumina thin films

Thin films of amorphous alumina of thickness 350nm were deposited on fused silica substrates by electron beam evaporation. Amorphous films were annealed at several temperatures in the range: 400–1130°C and changes in film crystallinity, short-range structure, optical and mechanical properties were studied. X-ray diffraction studies found that crystallization starts at 800°C and produces γ and δ-alumina, the latter phase grows with heat treatment and the sample was mostly δ and θ-alumina after annealing at 1130°C. The as-deposited amorphous alumina films have low hardness of 5 to 8GPa, which increases to 11 to 12GPa in crystalline sample. 27Al Magic Angle Spinning Nuclear Magnetic Resonance was used to study the short-range order of amorphous and crystalline alumina films and it was found that amorphous alumina film contains AlO5 and AlO4 structural units in the ratio of 1:2. The concentration of AlO5 was significantly suppressed in crystalline film, which contains 48% of Al3+ ions in AlO6, 7% in AlO5 and 45% in AlO4 units.

Influence of titanium and vanadium on the hydrogen transport through amorphous alumina films

The influence of titanium and vanadium on the hydrogen transport rate through thin amorphous alumina films is addressed. Only small changes in the transport rate are observed when the Al 2O 3 are covered with titanium or vanadium. This is in stark contrast to results with a Pd overlayer, which enhances the transport by an order of magnitude. Similarly, when titanium is embedded into the alumina the transport rate is faster than for the covered case but still slower than the undoped reference. Embedding vanadium in the alumina does not yield an increase in uptake rate compared to the vanadium covered oxide layers. These results add to the understanding of the hydrogen uptake of oxidized metals, especially the alanates, where the addition of titanium has been found to significantly enhance the rate of hydrogen uptake. The current findings eliminate two possible routes for the catalysis of alanates by Ti, namely dissociation and effective diffusion short-cuts formed by Ti. Finally, no photocatalytic enhancement was noticed on the titanium covered samples.

Properties of alumina films prepared by low-pressure metal-organic chemical vapour deposition

Thin amorphous alumina films were deposited on AISI 304 by thermal decomposition at low pressure (0.17 kPa (1.25 Torr)) of aluminium-tri- sec-butoxide (ATSB). The effect of the deposition temperature (within the range 200–380 °C) has been studied in relation to film properties including the protection of the underlying substrate against high-temperature corrosion, the chemical composition of the film, the microstructure and its refractive index. The activation energy for the heterogeneous reaction was 65 ± 2 kJ mol −1. Corrosion experiments, performed at 450 °C in a hydrogen sulphide gas, showed that the alumina coatings giving the best protection were obtained at a deposition temperature of 280 °C. The alumina films showed an amorphous structure, in agreement with the index of refraction. Transmission electron microscopy analysis revealed extremely fine grains of γ- Al 2O 3 AlO(OH) . Only OH groups were found as an “impurity” in the oxide film. Carbon was not detected.

The effect of thermal annealing on the properties of thin alumina films prepared by low pressure metal-organic chemical vapour deposition

Thin amorphous alumina films were prepared on stainless steel, type AISI 304, by low pressure metal-organic chemical vapour deposition. The effect of thermal annealing in nitrogen (for 2, 4 and 17 h at 600, 700 and 800 °C) on the film properties, including the protection of the underlying substrate against high temperature corrosion and the chemical composition of the film, were investigated. Corrosion experiments performed at 450 °C in a hydrogen sulphide-containing gas, showed that the thermal annealing process had a detrimental effect on the protective properties of the alumina films. From FTIR and Auger measurements, it was found that the amorphous alumina, containing boehmite, converted to γ-alumina during the annealing process.

Thermal conductivity of thin amorphous alumina films

Alumina films play an important role in microelectronics, e.g. as dielectric insulating layers. The aim of this paper is to contribute to the determination of the heat transportation properties of thin Al 2O 3 films, which have been generated by anodic oxidation. Free-standing films were prepared with thickness 140 nm as suitable substrate materials for depositing thin metallic or semiconducting layers. Analytical investigations (transmission electron spectroscopy, energy dispersive X-ray spectroscopy, selected area diffraction) show that the films have an amorphous structure and a homogeneous composition. In the temperature range 90–350 K the thermal conductivity and the emissivity of the films were determined simultaneously using a steady state method. It was observed that the thermal conductivity increases with rising temperature which is typical behaviour for dielectric amorphous solids in this temperature range. The magnitude and the temperature dependence can be consistently interpreted in terms of the theoretical Al 2O 3 crystal minimum thermal conductivity concept.

Ion Induced Crystallization and Growth of Nanoscale Grains in Ceramics

ABSTRACTSelf supporting thin films of amorphous alumina and zirconia were irradiated with light and heavy ions at various temperatures(25–430°C). Irradiation was found to result in the formation of 10–30 nm large grains well below conventional crystallization temperatures. These grains were quite stable against subsequent thermal growth. Crystallization, grain size, and growth depended on ion species as well as on ‘stabilizing’ additives (yttria or Pt).

The incorporation of a siliceous impurity during the anodic oxidation of aluminum in a sodium tartrate electrolyte

We have observed the occurrence of a high concentration of silicon as an impurity in thin films of amorphous alumina formed by the anodic oxidation of pure aluminum in a neutral sodium tartrate electrolyte. X-ray absorption analysis of the film by a total secondary electron emission method shows that the amount of silicon is proportional to the film thickness and is, therefore, thought to be incorporated in the oxide rather than being a deposit at the metal/oxide interface, or on the surface. An examination of the films by scanning electron microscopy and EDAX microanalysis shows that the impurity takes the form of discrete particles. It is suggested that these siliceous particles, probably colloidal silica, are deposited from the electrolyte during the growth of the oxide layer and are formed as a result of the action of the sodium tartrate solution on the Pyrex glass container. This phenomenon might be a more general occurrence in surface treatments.

The hydrogenolysis of methylcyclopentane on platinum model catalysts: Particle size effect due to a reaction occurring at the phase boundary metal-support

The hydrogenolysis of methylcyclopentane at 520 K on platinum model catalysts was studied and the mechanism of this reaction investigated. Catalysts of low metal dispersion (<0.15) were prepared by high-vacuum evaporation of platinum onto thin films of amorphous alumina. After annealing at 770 K in air and hydrogen treatment at 520 K the mean size of the Pt particles was about 10 nm as determined by electron microscopy. On these catalysts the reaction products are mainly 2-methylpentane and 3-methylpentane and only 9–14% n-hexane is formed, as is to be expected for large Pt particles. On top of these catalysts a thin layer of Al 2O 3 (0.3 nm mean thickness) was then deposited in order to reduce the surface area of platinum in relation to the phase boundary platinum/alumina. Thereafter a decrease of the catalytic activity, as well as a shift in the product distribution, was observed. The n-hexane content was significantly enhanced (up to 22%) and the ratio of the three products was comparable to that usually obtained with Pt catalysts of higher dispersion. This result supports a reaction model which consists of two parallel reactions (i) occurring on the platinum surface and producing mainly 2-methylpentane and 3-methylpentane and (ii) occurring on the phase boundary platinum/support and producing additional n-hexane.

Electron microscopy of [formula omitted] model catalysts : II. Sintering in atmospheres of H 2, O 2 and Ar

Thin films of amorphous alumina were prepared by anodic oxidation of Al foil or by reactive evaporation of Al in oxygen. Onto these supports ultrathin films of Pt (0.05 – 0.5 nm thick) were deposited under high vacuum. The resulting catalysts were characterized in the electron microscope and their particle density, their particle size distribution and their dispersion were studied after various annealing treatments in Ar, H 2, and O 2 at 510 and 720 K. It was shown that a treatment in hydrogen has the strongest effect on sintering and that stable particles can be produced by iterative treatments in H 2 and O 2 at 510 and 720 K, respectively. These stabilized platinum films are well suited as model catalysts for the study of particle size effects.

Inelastic Electron Tunneling Spectroscopy of Phenol, Catechol, Resorcinol and Hydroquinone Chemisorbed on Alumina

Inelastic electron tunneling spectroscopy has been used to probe the irreversible chemisorption at 295 K of phenol, catechol, resorcinol and hydroquinone on thin amorphous alumina films in Al/Al2O3/Pb sandwich structure tunneling junctions. This spectroscopy yields a representation of the vibronic excitations of the chemical bonds in the adspecies. Phenol chemisorbs predominantly as C6H5O- on Al+3 sites with a very small amount of associative chemisorption at high coverages. Catechol and resorcinol both adsorb predominently as the di-ion C6H4O2-2, whereas hydroquinone chemisorbs both as the di-ion and as the single ion 1, 4-C6H4(OH)O- at high surface coverages. Hydrogen bonding is observed among the adsorbates and between the adsorbates and the OH groups on the partially hydroxylated Al2O3 surface. The experimental results are explicable in terms of the geometry and chemical properties of the adsorbates and the oxide surface.