α-Al2O3 Films Research Articles

Effect of Surface Morphology on the Response Speed of Moisture Sensors

Relative humidity sensors are used for high-humidity measurement and moisture sensor or dew point sensors are used for low-humidity measurement (<1 ppmv). We fabricated moisture sensors using α-Al2O3 films as porous dielectric materials deposited by anodic spark deposition. In this study, a variety number of small pores has been studied to investigate the response speed of moisture sensors. Three different surface morphologies have been studied using scanning electron microscopy. One sample has the maximum number of small pores, one sample has the medium number of small pores, and the last sample has the minimum number of small pores. Small pores are defined as pore size ranging from 40 nm to 200 nm. We found that the sensor made as porous material with the maximum number of small pores has the fastest response speed, and that from the minimum number of small pores has the slowest response speed either from high humidity to low humidity or from low humidity to high humidity.

Intrinsic exchange bias from interfacial reconstruction in an epitaxial NixCoyFe3−x−yO4(111)/α-Al2O3(0001) thin film family

Intrinsic exchange bias is known as the unidirectional exchange anisotropy that emerges in a nominally single-component ferro-(ferri-)magnetic system. In this work, with magnetic and structural characterizations, we demonstrate that intrinsic exchange bias is a general phenomenon in (Ni, Co, Fe)-based spinel oxide films deposited onα-Al2O3(0001) substrates, due to the emergence of a rock-salt interfacial layer consisting of antiferromagnetic CoO from interfacial reconstruction. We show that in NixCoyFe3-x-yO4(111)/α-Al2O3(0001) films, intrinsic exchange bias and interfacial reconstruction have consistent dependences on Co concentrationy, while the Ni and Fe concentration appears to be less important. This work establishes a family of intrinsic exchange bias materials with great tunability by stoichiometry and highlights the strategy of interface engineering in controlling material functionalities.

Porous Dielectric Structure and Response Speed of Moisture Sensors

We made moisture sensors using α-Al2O3 films as porous dielectric materials deposited through the anodic spark deposition (ASD). In this study, a variety number of small pores has been studied to investigate the response speed of moisture sensors. Three different surface morphologies have been studied using scanning electron microscopy (SEM). Sample DM23 has the maximum number of small pores. Small pores are defined as pore size ranging from 40 nm to several hundred nm. Sample TA40 has the minimum number of small pores. Sample FW06 has the medium number of small pores. We found that the sensor made from dielectric material with the maximum number of small pores has the fastest response speed, and that from the minimum number of small pores has the slowest response speed either from high humidity to low humidity or from low humidity to high humidity.

A novel method for preparing α-Al2O3 (Cr2O3)/Fe–Al composite coating with high hydrogen isotopes permeation resistance

α-Al2O3 as a preferred material for hydrogen isotopes permeation barrier (HIPB) can effectively prevent hydrogen embrittlement in structural steel and has higher thermal stability. However, its application is limited by the issue that it is hard to prepare α-Al2O3 at relatively low temperature and its formation temperature is too high (>1000 °C) to damage the mechanical properties of structural steel. To overcome this issue, this paper presents a novel method (combining Al electrodeposition, aluminizing, Cr deposition and air oxidation) for the preparation of α-Al2O3 (Cr2O3)/Fe–Al composite coating on 316 L stainless steel for the first time. The formation temperature of α-Al2O3 was successfully reduced to 800 °C. The post-oxidation samples were characterized by SEM, XPS, XRD, EDS and TEM. The results showed that the oxide film was composed of highly crystalline α-Al2O3 and Cr2O3 nanoparticles, where the later were diffusely distributed in the internal α-Al2O3 film. And there was an epitaxial growth relationship between α-Al2O3 and Cr2O3. The low-temperature formation of the α-Al2O3 film was due to the fact that the Cr2O3 nanoparticles formed during the oxidation process acted as templates and nucleation sites, which reduced the nucleation barrier of α-Al2O3. Deuterium permeation tests showed that its deuterium permeation reduction factor (DPRF) was 8892 at 500 °C, which met the standard for practical application.

Structural characterization of Zn2SnO4 epitaxial single crystal films prepared on c-cut α-Al2O3 films by magnetron sputtering

Amorphous zinc stannate (Zn2SnO4) films were deposited on α-Al2O3 (0006) substrate by magnetron sputtering. And single crystal Zn2SnO4 films were obtained after thermal-annealing. The structural and optical properties of Zn2SnO4 films at different annealing temperatures were systematically studied. The structural analyses revealed that the sample annealed at 900 °C exhibited the best crystallization quality with an inverse spinel structure. The heteroepitaxy mechanisms were further elucidated by XRD and HETEM. The epitaxial relationship between thin film and substrate can be concluded as Zn2SnO4 (111)//α-Al2O3 (0006) with Zn2SnO4 [0 11‾]//α-Al2O3 <1‾ 100>. The obtained Zn2SnO4 films showed excellent transparency with an average transmittance of 88.8% in the visible region. The optical band gaps of Zn2SnO4 films annealed at temperatures ranging from 600 to 1100 °C was found to be within the range of 3.7–4.2 eV.

A study of the morphological effect of an α-Al2O3 layer on the creep life for nickel-based superalloys using microstructure-based geometrical models

A study on the role of the irregular geometry of the α-Al2O3 film during the creep rupture test was conducted on samples of DD6 Ni-based single-crystal superalloy at 980 °C/250 MPa. This study involved experimental and finite element analyses of creep-oxidation specimens. The experimental results showed that the creep rupture life of the 0.67 mm-thin samples was shorter than that of the 0.81 and 1.12 mm specimens. Microscopic examination of the oxidation-affected zones showed that the morphology of the inner α-Al2O3 layer of a 0.67 mm-thin sample was discontinuous and incomplete, and the sample (1.12 mm thick) included a continuous and complete α-Al2O3 film. Hence, the influence of different morphologies of the α-Al2O3 layer on the stress distribution was studied numerically using the ABAQUS software. The finite element modeling results showed that the maximum stress was concentrated in the blocks of a discontinuous α-Al2O3 layer, which increased the diffusion of ions and metal cations. As a result, excessive growth of the oxidation-affected zone of the thin samples and the formation of AlN particles in high density occurred. It also contributed to an increase in the surface crack growth rate in the thin wall sample, which reaches the critical value of crack length in a shorter time than thick ones. Microstructure-based modeling has provided further insight into the role of the α-Al2O3 layer morphology in the deterioration of the mechanical properties of thin creep specimens.

High-temperature annealing of porous anodic aluminium oxide prepared in selenic acid electrolyte

Selenic acid electrolyte provides porous anodic aluminium oxide (AAO) films with low porosity and high optical transparency. Here, the high-temperature behaviour of AAO obtained in a selenic acid electrolyte is reported for the first time. The crystallization of as-prepared amorphous AAO at about 800 °C is accompanied by the elimination of electrolyte impurities, an increase in pore diameter from 15 to 21 nm, and an increase in specific surface area to 55 m2·g−1 due to the mesopore formation inside the cell walls. The second crystallization stage at about 1150 °C results in the formation of coarse-grained corundum (α-Al2O3) films with a mean grain size of about 11 μm. Each of the grains consists of regularly arranged 27-nm-diameter pores, which are stable up to 1300 °C. AAO physicochemical properties depend on the annealing temperature. In particular, chemical stability is up to three orders of magnitude higher after two-step crystallization. Optical properties and thermal expansion of selenic-acid-made porous AAO films with different phase composition are quantified. The crystallization of as-prepared AAO to low-temperature Al2O3 polymorphs has a minor influence on optical transmittance and effective refractive index of AAO, whereas the coarse-grained structure of corundum causes an order of magnitude decrease in average transmittance in the visible region. The coefficient of thermal expansion increases by 25% to 8.5 × 10−6 K−1 for α-Al2O3 porous films compared to as-prepared AAO.

Effect of substrate morphology on the deposition behavior of α-Al2O3 films by room temperature granule spray in vacuum process

In this study, we investigated the influence of surface morphology of a hard substrate on the deposition behavior of α-Al2O3 films coated by a granule spray in vacuum (GSV) process at room temperature. Three types of α-Al2O3 films were prepared by chemical vapor deposition (CVD) on a WC-Co base material: (006) textured film with a uniform and small mold-type morphology, (110) and (012) textured films with large grains and a relatively flat morphology, and flat (006) textured film after polishing. These CVD α-Al2O3 films were further coated with nano-grained α-Al2O3 films by the GSV process. The deposition behavior, adhesion strength, and residual surface stress were found to be strongly influenced by the surface morphology of the hard substrate. For the (006) textured CVD α-Al2O3 film with steep crests and troughs on the surface, the GSV α-Al2O3 film was deposited uniformly, possibly because the powder can be directly anchored by the molding effect and subsequent hammering effect. Such different deposition characteristics also led to a variation in the surface residual stress.

Enhanced Corrosion Resistance of an Alumina-forming Austenitic Steel Against Molten Al

The development of corrosion resistant steel for molten Al has always attracted the interests from die casting industry for several decades. In this paper, we reported the enhanced corrosion resistance in 750 °C molten Al of an alumina-forming austenitic steel (AFA) via deliberately producing oxide layer after different high-temperature oxidation processes. The results showed that the corrosion resistance against molten Al of the AFA was improved significantly due to the presence of a stable, compact and continuous oxides barrier, in comparison with H13 tool steel as well as unoxidized AFA without Al2O3 film. Apparently, the non-wetting α-Al2O3 film, which was formed after high-temperature oxidation, effectively protected the substrate from inner-diffusion and penetration of the molten Al. These findings provide one of cost-effective ways to improve the corrosion resistance of steels against molten Al.

Effects of corundum-structured seeds on the low-temperature growth and hardness of the alumina-based films

The corundum-structured seeds have been introduced into alumina films using Al + α-Al2O3 composite and Al100−xCrx (x = 10, 20, 30) alloy as targets by reactive magnetron sputtering. The effects of the seeds on the elemental composition, phase composition, structure characteristic, surface morphology and nano-hardness of the alumina-based films have been investigated. The Al target has also been sputtered to prepare the alumina film as reference. The film is composed of single-phase α-Al2O3 by sputtering the Al + α-Al2O3 composite target at 550 °C, while the solid solution α-type (Al0.7Cr0.3)2O3 was detected in the film prepared with the Al70Cr30 target at the same temperature. The nano-hardness of the pure α-Al2O3 film and the α-(Al0.7Cr0.3)2O3 film were measured as ∼23.5 GPa and ∼28.5 GPa, respectively, which are much higher than that of the film deposited from Al target (∼16.3 GPa).

Coarse-grain alpha-alumina films with highly ordered porous structure

Abstract Application of porous anodic alumina as membranes and templates is restricted by its rapid degradation in acidic and alkaline media. Chemical stability of the anodic alumina could be strongly enhanced by annealing leading to crystallization of as-prepared amorphous material into a mixture of low-temperature Al2O3 polymorphic modifications and then to α-Al2O3 phase. Here an approach to choosing an annealing protocol for crystallization of porous anodic alumina into the corundum phase preserving its starting morphology is suggested. Formation of corundum films with hexagonally-packed 27-nm-diameter channels by multi-step annealing of anodic alumina prepared in 0.3 М sulfuric acid at 25 V is demonstrated. According to electron backscatter diffraction, α-Al2O3 films consist of porous single-crystalline grains with an average size of ~ 5 ÷ 10 μm. Crystallization of α-Al2O3 phase from amorphous anodic alumina results in the enhancement of its chemical stability by two orders of magnitude in comparison with porous films consisting of low-temperature polymorphic modifications of Al2O3.

Low-temperature deposition of α-Al2O3 film using Al+α-Al2O3 composite target by radio frequency magnetron sputtering

The realization of low temperature deposition of α-Al2O3 films is important for expanding its application in cutting tools, diffusion barrier and anti-oxidization coating. Here, the alumina films were deposited on Si(100) substrates by radio frequency magnetron sputtering using various targets of Al, α-Al2O3, and Al + 15 wt% α-Al2O3 composite. The elemental composition, phase constitution, microstructure, morphology and mechanical property of the as-deposited films were characterized by various techniques. The results show that the film deposited from the α-Al2O3 target is composed of amorphous and α-Al2O3, while the single phase α-Al2O3 film was successfully deposited by reactively sputtering the composite target at a substrate temperature of 550 °C. The hardness of the α-Al2O3 film deposited from Al+α-Al2O3 composite target was measured as ∼23.8 GPa.

20 Hz synchrotron X-ray diffraction analysis in laser-pulsed WC-Co hard metal reveals oscillatory stresses and reversible composite plastification

The lifetime of WC-Co inserts used in cutting processes, such as milling, is limited by millisecond temperature and mechanical pulses, which occur as a result of interrupted tool-workpiece contact, thermal fatigue and wear. In the current work, synchrotron X-ray diffraction (XRD) was used in conjunction with a pulsed laser heating set-up to characterise the time-dependent development of stresses and microstructure in locally irradiated WC-Co inserts coated by chemical vapour deposition with 6.5 and 3.5 μm thick TiCN and α-Al2O3 films, respectively. Diffraction data from the WC phase were used to evaluate the time and temperature-dependent evolution of in-plane stresses, thermal strains and integral breadths of WC diffraction peaks in experiments with a single and five successive laser shocks applied within 2.2 and 20 s, respectively, using a laser spot diameter of ~5.8 mm and an X-ray beam size of 1 × 1 mm2. The laser heating induces the formation of compressive stresses in the inserts' substrates. Above a temperature of ~750 °C, at the onset of WC-Co composite plastification, compressive stresses relax and then vanish in WC at the maximal applied temperature of ~1300 °C, followed by the build-up of tensile stresses. The applied cyclic heating up and cooling down led to the repetitive formation of compressive and tensile stresses, with temperature dependencies oscillating with the number of applied laser pulses. The observed relatively high tensile stress level of ~1100 MPa in WC was a consequence of the stabilising function of the coating, which hindered the initiation of surface hot cracks and stress relaxation. The stress evolution was coupled with changes in XRD peak broadening, which however strongly depended on the particular hkl reflections and showed oscillatory behaviour within a single temperature cycle. In summary, the unique diffraction set-up revealed stress levels and provides insight into the WC-Co composite plastification mechanism governing the stress build-up and relaxation in locally thermo-shocked WC-Co inserts at millisecond time resolution.

Low-temperature Deposition of α-Al2O3 Films by Reactive Sputtering Al+α-Al2O3 Target

Low-temperature Deposition of α-Al2O3 Films by Reactive Sputtering Al+α-Al2O3 Target

Solid-Phase Epitaxial Growth of an Alumina Layer Having a Stacking-Mismatched Domain Structure of the Intermediate γ-Phase.

Solid-phase epitaxy (SPE), a solid-state phase transition of materials from an amorphous to a crystalline phase, is a convenient crystal growing technique. In particular, SPE can be used to grow α-Al2O3 epitaxially with a novel structure that provides an effective substrate for improved performance of light-emitting diodes (LEDs). However, the inevitable two-step phase transformation through the γ-Al2O3 phase hinders the expected improved crystallinity of α-Al2O3, and thereby further enhancement of LED performance. Herein, we provide a fundamental understanding of the SPE growth mechanism from amorphous to metastable γ-Al2O3 using transmission electron microscopy (TEM) and density functional theory (DFT) calculations. The nanobeam precession electron diffraction technique enabled clear visualization of the double-positioning domain distribution in the SPE γ-Al2O3 film and emphasized the need for careful selection of the viewing directions for any investigation of double-positioning domains. Void and stacking fault defects further investigated by high-resolution scanning TEM (STEM) analyses revealed how double-positioning domains and other SPE growth behaviors directly influence the crystallinity of SPE films. Additionally, DFT calculations revealed the origins of SPE growth behavior. The double-positioning γ-Al2O3 domains randomly nucleate from the α-Al2O3 substrate regardless of the α-Al2O3 termination layer, but the large energy requirement for reversal of the γ-Al2O3 stacking sequence prevents it from switching the domain type during the crystal growth. We expect that this study will be useful to improve the crystallinity of SPE γ- and α-Al2O3 films.

Preparation of Dense Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; Film on 8YSZ Coating by Sol-Gel Method

The 8 wt.% yttria-stabilized zirconia (8YSZ) thermal barrier coatings (TBCs) are widely used to protect hot superalloy components of gas turbine and jet engine. However, Cracks and pores in the 8YSZ coating would accelerate the oxidation of the bondcoat alloy and the corrosion of 8YSZ topcoat by molten salts and finally result in permanent failure of the TBCs. Hence, in this work, a sol-gel spin-coating method was employed to prepare a dense α-Al2O3 film on the surface of air-plasma-sprayed 8YSZ coatings, in order to block the paths of cracks and pores while keeping the stress tolerance of 8YSZ coatings. The influences of the polyvinylpyrrolidone (PVP) and alumina content of the sol on the microstructure of the α-Al2O3 film were investigated. It is found that a sol with PVP addition amount of 50 mg/mL and alumina solid content of 10% let to the achievement of a compact α-Al2O3 film. The positive results in this work would inspire more efforts to prepare dense films on TBCs and therefore improve their performance under service conditions.

Measurements of the band alignment at coherent α-Ga2O3/Al2O3 heterojunctions

The band alignment at a coherent α-Ga2O3/Al2O3 heterointerface was evaluated by analyzing X-ray photoemission spectra for an α-Al2O3 film and an α-Ga2O3/Al2O3 multi-quantum wells grown coherently on sapphire substrates. The measured bandgaps of α-Ga2O3 and α-Al2O3 were 5.7 and 8.7 eV, and the conduction- and valence-band offsets were 2.7 and 0.3 eV, respectively. The smaller valence-band offset is attributed the large contribution of localized O 2p orbitals at the top of the valance band in the oxides. These results complete our understanding of the band alignment of the α-(AlxGa1−x)2O3 system in conjunction with the previously reported band offsets at α-Ga2O3/(AlxGa1−x)2O3 heterojunctions (0.1 ≤ x ≤ 0.8).

Enhancement of methylmercaptan sensing response of WO3 semiconductor gas sensors by gas reactivity and gas diffusivity

WO3-based semiconductor-type gas sensors were fabricated, and their sensing properties to methylmercaptan (CH3SH) were examined in this study. The Ru loading on WO3 was an effective way of an increase in the CH3SH response. In addition, the CH3SH response increased with a decrease in the operating temperature as well as with an increase in the thickness of the Ru-loaded WO3 sensing layer. The increase in the porosity of the Ru-loaded WO3 sensors, which were fabricated by utilizing polymethylmethacrylate microspheres as a template, was also effective in improving the CH3SH response, especially at a low temperature of 150 °C. In addition, the lamination of the Ru-loaded WO3 sensor with an α-Al2O3 film improved the CH3SH response at 200 °C. Moreover, the Ru loading on the WO3 powder increased the catalytic activities of CH3SH oxidation, and CH3SH was partially oxidized to CH3SSCH3 at temperatures less than 330 °C. It was suggested that the increase in the positively charged adsorption of the partially oxidized products onto the bottom part of the sensing layer is effective for enhancing the CH3SH response, especially at lower operating temperatures.

Influence of surface roughness on the oxidation behavior of a Ni-4.0Cr-5.7Al single crystal superalloy

The high-temperature oxidation dynamics and mechanisms of a Ni-based single crystal superalloy with four kinds of surface roughnesses were investigated by virtue of XRD, OM, SEM and EDS at 1000 °C. In the initial oxidation stage, outer (Ni, Co)O was mainly produced on the surfaces of the samples with Ra = 90 nm and 19 nm. Correspondingly, outer Cr2O3 and transient θ-Al2O3 were mainly formed on the surfaces with Ra = 509 nm and 182 nm. After 180 min oxidation, the values of instantaneous parabolic mass gain coefficients (kp) of the samples with all surface roughnesses were gradually consistent with the data of the growth parabolic coefficient of α-Al2O3. The oxidation mechanisms of Ni-based superalloy with different surface roughnesses were discussed by a model. The external diffusion flux of Al (DAl) increases with the increases of surface roughness. Thus, the required Al concentration decreases with the increases of surface roughness when the selective oxidation of Al occurrs to form a protective single α-Al2O3 film.

Effect of oxide films on formation of Fe-rich intermetallic phases in aluminum alloys

The aim of the present work was to study the effect of the α-Al2O3 and MgAl2O4 oxide films, normally found in cast aluminum alloys, on formation of Fe-rich intermetallic phases. In order to investigate the role of these oxide films, experiments were carried out upon conditions in which formed in situ oxide films by natural oxidation were present. Results in this research agreed well with previous reports suggesting that Fe-rich intermetallic phases nucleate upon the oxide films. This is based on some Fe-rich intermetallic phases that are clearly related to oxide films within the microstructure. The intermetallic phases formed on oxide films were identified with the help of techniques such as SEM/EDS and X-ray diffraction.