Deposition Of Alumina Research Articles

Impact of Deposits and Their Morphology on the Active Corrosion of Iron in Chlorine- and Sulfur-Containing Atmospheres in the Temperature Range of 350–500 °C

Iron-based alloys have shown high corrosion rates under ash deposits typical for waste-to-energy plants. The ashes on superheater tubes in waste incineration are multicomponent systems including alkali and alkali–earth chlorides and sulfates. Under and within such salts, the corrosive effect on the alloy is induced by a complicated interplay of such ash products. On the one hand, in chlorine-containing atmospheres iron-based alloys are believed to be attacked by the so-called active corrosion, including the formation of volatile corrosion products and their transformation into stable iron oxides. At the same time, they form complex scales, involving among other compounds iron sulfides, chlorides, and oxides. Thus, in order to directly investigate the influence of a deposit on the corrosion in waste-to-energy plants and to reproduce the scales observed on field tested superheaters, this work compares the scale formation and metal wastage under different chemically inert alumina deposits with different grain sizes to a synthetic salt as well as to an actual deposit taken from a superheater tube in a plant.

EB-PVD alumina (Al2O3) as a top coat on 7YSZ TBCs against CMAS/VA infiltration: Deposition and reaction mechanisms

Al2O3 was deposited as a top coat on a standard 7YSZ layer (or layers) by means of EB-PVD technique and the corresponding morphology of the Al2O3/7YSZ coatings was studied in detail. This multi-layer TBC system was tested against calcium-magnesium-aluminium-silicate (CMAS) recession by performing infiltration experiments for different time intervals from 5 min to 50 h at 1250 °C using two types of synthetic CMAS compositions and Eyjafjallajökull volcanic ash (VA) from Iceland. The results show that the studied EB-PVD Al2O3/7YSZ coatings react quickly with CMAS or VA melt and form crystalline spinel (MgAl2-xFexO4) and anorthite (CaAl2Si2O4) phases. The presence of Fe-oxide in the CMAS has been found to be key element in influencing the spinel formation which was proved to be more efficient against CMAS sealing in comparison to the Fe-free CMAS compositions. Even though a rapid crystallization was assured, shrinkage cracks in the EB-PVD alumina layer produced during the crystallization heat treatment have proven to be detrimental for the CMAS/VA infiltration resistance. To overcome these microstructural drawbacks, an additional alumina deposition method, namely reaction-bonded alumina oxide (RBAO), was applied on top of EB-PVD Al2O3. RBAO acts as a sacrificial layer forming stable reaction products inhibiting further infiltration.

Low-temperature deposition of nanocrystalline Al2O3 films by ion source-assisted magnetron sputtering

In this paper, bipolar pulse reactive magnetron sputtering with ion source assisted deposition method has been utilized for the deposition of nanocrystalline alumina (Al2O3) films at the temperature of 300 °C onto silicon (111) wafers, and cemented carbide substrates. The influence of ion source power, i.e. 0, 1.0, 1.5 and 2.0 kW on the structure, morphology, compressive stress and mechanical properties of the Al2O3 films were investigated. In absence of ion source power assistance, an amorphous Al2O3 film was dominant at deposition temperature of 300 °C. With ion source assisted deposition, the crystalline γ-Al2O3 films were obtained at the same conditions, suggesting an importance of ion source power in crystallinity development of metal oxide films obtained from magnetron sputtering deposition method. Images of surface morphology clearly demonstrated the difference in granular sizes of film surfaces prepared with and without ion source powers. With increasing ion source power from 1.0 to 2.0 kW, the micro-hardness and compressive stress of the films were increased from 7 GPa to 13 GPa and 0.3 GPa to 1.1 GPa, respectively. Results revealed that the reactive magnetron sputtering with the ion source assisted deposition was a simple and effective way to prepare nanocrystalline γ-Al2O3 films at low temperature.

Influence of Ladle Addition Practice on the Castability of Typical Low Carbon Low Silicon Steel

Continuous casting of low carbon low silicon steel processed though Basic Oxygen Furnace (BOF) followed by Online Purging station (OLP) and Ladle Furnace (LF) inherently suffers from the problem of clogging of submerged entry nozzles (SEN) during casting due to gradual alumina deposition at the inner surface of the refractory wall. This restricts the incoming flow of liquid steel from tundish to mould through SEN and thus limits the productivity of the caster. On the other hand, excess alumina deposition and their periodic dislodgement from the nozzle refractory surface gives rise to undue stopper rod movement and melt level fluctuations in the mould. All these abnormalities lead to decreased production of good quality slabs. In the present work, influence of ladle addition practice on the castability of typical low carbon low silicon steel has been investigated for a conventional thick slab caster. For identification of inclusions characteristics at different stages of processing, liquid steel samples were collected using the lollipop and a specially designed large sampler during casting. Also, the clog deposits in the used SEN and the natures of inclusions in the clog deposits as well as in the liquid steel were identified by SEM–EDS analysis. Subsequently, to measure the process the different plant trials were carried out for several casting sequences using different deoxidation practices. The upstream processing parameter and corresponding casting data with nozzle clogging index (NCI) for a complete casting sequence were examined for evaluating their casting performances. Addition practice of deoxidant and lime during the ladle treatment has been found to have an important effect of the liquid steel casting. It has been clearly demonstrated the faulty addition practice can lead to severe nozzle clogging during casting. Though a series of plant trials the best addition practice has been determined and implemented in the regular production in the plant for ensuring the smooth production of the given grade of steel. For the quantification of steel cleanliness, total oxygen content (T [O]) of liquid steel, was measured from the samples collected after each ladle treatment stage. T [O] of liquid steel was found in the range of 60−120 ppm at OLP and 30−55 ppm at tundish of low carbon low silicon grade of steel.

Catalytic Activity of a Bifunctional Catalyst for Hydrotreatment of Jatropha curcas L. Seed Oil

The hydrotreating process of vegetable oils (HPVO) involves the transformation of vegetable oil triglycerides into straight chain alkanes, which are carried out by deoxygenation reactions, generating multiple hydrocarbon compounds, cuts similar to heavy vacuum oil. The HPVO is applied to Jatropha curcas oil on USY zeolite supported with gamma alumina and platinum deposition on the catalytic as hydrogenation component. The acid of additional activity of the supports allows the development of catalytic routes that the intervention of catalytic centers of different nature reaches the desired product. The products of the hydrotreating reaction with Jatropha curcas seed oil triglycerides were identified by Fourier transform infrared spectroscopy and by mass spectroscopy to identify and analyze the generated intermediate and final hydrocarbon compounds.

First principles mechanistic study of self-limiting oxidative adsorption of remote oxygen plasma during the atomic layer deposition of alumina

Plasma-enhanced atomic layer deposition (ALD) of metal oxides is rapidly gaining interest, especially in the electronics industry, because of its numerous advantages over the thermal process. However, the underlying reaction mechanism is not sufficiently understood, particularly regarding saturation of the reaction and densification of the film. In this work, we employ first principles density functional theory (DFT) to determine the predominant reaction pathways, surface intermediates and by-products formed when constituents of O2-plasma or O3 adsorb onto a methylated surface typical of TMA-based alumina ALD. The main outcomes are that a wide variety of barrierless and highly exothermic reactions can take place. This leads to the spontaneous production of various by-products with low desorption energies and also of surface intermediates from the incomplete combustion of -CH3 ligands. Surface hydroxyl groups are the most frequently observed intermediates and are formed as a consequence of the conservation of atoms and charge when methyl ligands are initially oxidized (rather than from subsequent re-adsorption of molecular water). Anionic intermediates such as formates are also commonly observed at the surface in the simulations. Formaldehyde, CH2O, is the most frequently observed gaseous by-product. Desorption of this by-product leads to saturation of the redox reaction at the level of two singlet oxygen atoms per CH3 group, where the oxidation state of C is zero, rather than further reaction with oxygen to higher oxidation states. We conclude that the self-limiting chemistry that defines ALD comes about in this case through the desorption by-products with partially-oxidised carbon. The simulations also show that densification occurs when ligands are removed or oxidised to intermediates, indicating that there may be an inverse relationship between Al/O coordination numbers in the final film and the concentration of chemically-bound ligands or intermediate fragments covering the surface during each ALD pulse. Therefore reactions that generate a bare surface Al will produce denser films in metal oxide ALD.

Parametric Study of the Effect of the Suspension Composition on the Electrophoretic Deposition of Alumina

Electrophoretic deposition (EDP) is gaining increasing attention both in science and industry because this method has allowed the formation of thin films or multilayer films of controlled thickness and morphology. The method enables the formation of films on substrates of complex geometry that suits for various applications.
 This work is a study the effect of polyethylene glycol as a binder-suspension agent, the amount the solid loading (alumina particles), and the effect of the toluene as a dielectric liquid, on the pH of suspension, the final thickness and green density of the deposed parts.
 It has been shown that a certain amount of polyethylene glycol when added to ethanol or ethanol-30%toluene has given good results for both the green density and the thickness.

Improving the high temperature oxidation resistance of pure titanium by shot-peening treatments

Shot-peening (SP) treatments have shown their capacity to improve the oxidation resistance of titanium and zirconium thanks to the large compressive stresses and the surface hardening induced by this mechanical process. However, shot-peening treatments can produce a surface chemical deposit, which can modify the high temperature oxidation resistance.Here, we study pure titanium samples shot-peened with different type of balls: tungsten carbide, alumina or glass. The oxidation behavior was studied at 700°C in dry air by thermo gravimetric analysis for short isotherm oxidation periods up to 100h. Also, long oxidation tests (3000h) at 700°C were performed with an intermittent monitoring of the samples mass. The oxidized samples were characterized by XRD, SEM/EDS, XPS and nuclear reaction analysis. After 100h of oxidation the alumina ball shot-peening treatment produced the most resistant samples. The smallest α-case area was found for the samples that received the most energetic mechanical treatment. The formation of a continuous nitride layer observed underneath the oxide layer can explain the oxidation resistance improvement. Shot-peening treatments with alumina balls produce an alumina deposit on the samples surface that participates to reduce the oxidation. The chemical and structural surface modifications brought by the mechanical treatment participate to the oxidation resistance improvement. For long oxidation periods (3000h), the shot-peening with WC balls followed by a stripping with glass balls was the only treatment that showed an oxidation resistance improvement.

Comparative study of thermal and plasma enhanced atomic layer deposition of aluminum oxide on graphene

Atomic layer deposition of alumina on graphene was studied in thermal and plasma enhanced process. Deposition was controlled by in situ ellipsometry every half cycle, which allows measuring of Al2O3 thickness and graphene equivalent thickness during the process. Properties of graphene were measured by Raman spectroscopy prior and after deposition of dielectric layer. It was shown that plasma enhanced deposition leads to decrease of effective graphene thickness, while thermal deposition does not affect graphene layer. No substantial nucleation lag was observed in both types of deposition.

Characteristics and formation mechanism of compact/porous structures in char layers of EPDM insulation materials

A char layer is the first barrier against the ablation of insulation materials because it is the first to be subjected to thermal-chemical ablation and particle and gas-mechanical erosion. To study the ablation mechanism of the char layer in depth and to develop accurate models of the ablation phenomenon, the structural characteristics of the char layer must be understood. In this work, the structural characteristics of the char layer were studied using an ablation motor and several other analytical techniques. It was found that the char layer usually has a compact/porous structure with a compact surface. But in alumina deposition mode, the middle portion remains relatively compact. A mechanism whereby the compact layer is formed by re-deposition of the pyrolysis gas in the char layer skeleton, which is initially relatively porous, is proposed to explain the formation of the compact/porous structure. It was found that a liquid silica film contributes to the formation of the compact layer; this explains the formation of the porous/compact/porous structure. The gas-pyrolysis-based deposition which forms the surface compact layer can be regarded as being a self-reinforcing mechanism and can be exploited to improve the ablation resistance of insulation materials.

Hydrophobization of metallic surfaces by means of Al2O3-HDTMS coatings

Abstract In this work, the structure and wetting behavior of stainless steel coated with sol-gel derived alumina and hexadecyltrimethoxysilane (HDTMS) deposits were studied. As a typical result, an artificial lotus-leaf like structure with a dual nano/micro-scaled roughness was developed at specific deposition and sintering conditions. Water contact angle measurements indicated that the surface functionalization by HDTMS enhances hydrophobicity, where this effect is highlighted when an alumina interlayer is used. In this regard, the most hydrophobicity level was obtained for the surface coated with the lotus-featured alumina and HDTMS coatings. In conclusion, the relatively inexpensive and facile method used in this work donates outstanding capabilities to metallic surfaces and provides a new driving force for their further developments.

Continuous-Flow Electrophoresis of DNA and Proteins in a Two-Dimensional Capillary-Well Sieve.

Continuous-flow electrophoresis of macromolecules is demonstrated using an integrated capillary-well sieve arranged into a two-dimensional anisotropic array on silicon. The periodic array features thousands of entropic barriers, each resulting from an abrupt interface between a 2 μm deep well (channel) and a 70 nm capillary. These entropic barriers owing to two-dimensional confinement within the capillaries are vastly steep in relation to those arising from slits featuring one-dimensional confinement. Thus, the sieving mechanisms can sustain relatively large electric field strengths over a relatively small array area. The sieve rapidly sorts anionic macromolecules, including DNA chains and proteins in native or denatured states, into distinct trajectories according to size or charge under electric field vectors orthogonally applied. The baseline separation is achieved in less than 1 min within a horizontal migration length of ∼1.5 mm. The capillaries are self-enclosed conduits in cylindrical profile featuring a uniform diameter and realized through an approach that avoids advanced patterning techniques. The approach exploits a thermal reflow of a layer of doped glass for shape transformation into cylindrical capillaries and for controllably shrinking the capillary diameter. Lastly, atomic layer deposition of alumina is introduced for the first time to fine-tune the capillary diameter as well as to neutralize the surface charge, thereby suppressing undesired electroosmotic flows.

Catalyst stabilization by stoichiometrically limited layer-by-layer overcoating in liquid media

The use of metal oxide overcoats over supported nanoparticle catalysts has recently led to impressive improvements in catalyst stability and selectivity. The deposition of alumina is especially important for renewable catalysis due to its robustness in liquid-phase conditions. However, there are limited reports of work on alumina deposition and stabilization that goes beyond atomic layer deposition (ALD). Here, we present a layer-by-layer deposition technique for the controlled formation of conformal alumina overcoats in the liquid phase. This technique is easy to perform in common wet chemistry conditions. Alternated exposure of the substrate to stoichiometric amounts of aluminum alkoxide and water in liquid-phase conditions leads to the formation of a porous overcoat that was easily tunable by varying synthesis parameters. The deposition of 60 Al2O3 layers onto Al2O3-supported copper nanoparticles suppressed irreversible deactivation during the liquid-phase hydrogenation of furfural – a key biomass-derived platform molecule. The porous overcoat leads to highly accessible metal sites, which significantly reduces the partial site blocking observed in equivalent overcoats formed by ALD. We suggest that the ease of scalability and the high degree of control over the overcoat’s properties during liquid-phase synthesis could facilitate the development of new catalyst overcoating applications.

Investigation of inherent differences between oxide supports in heterogeneous catalysis in the absence of structural variations

Oxide supports in heterogeneous catalysts can profoundly influence the catalytic properties of the metal or alloy active phase. Supports prepared by conventional methods can have not only different chemical properties but also different structural properties depending on the oxide. These structural differences can in turn affect the dispersion, distribution, and accessibility of the active phase. In this study, we apply an atomically-controlled synthesis method to isolate the effects arising from the different chemical properties of three oxide supports – titania, alumina and silica – used in Rh catalysts for syngas conversion reactions. We perform atomic layer deposition of titania and alumina to chemically modify the surface of silica gel, without changing its structural characteristics such as surface area and porosity. An inverse catalyst structure is also fabricated by depositing titania and alumina as overlayers onto silica-supported Rh. Titania is found to increase syngas conversion activity and higher hydrocarbon selectivity as both a support layer and an overlayer. An alumina support layer increases Rh nanoparticle dispersion and activity whereas an alumina overlayer decreases the activity of Rh. No significant increase in higher oxygenate selectivity was observed with either titania or alumina.

Grass-like Alumina with Low Refractive Index for Scalable, Broadband, Omnidirectional Antireflection Coatings on Glass Using Atomic Layer Deposition

We present a new type of nanoporous antireflection (AR) coating based on grass-like alumina with a graded refractive index profile. The grass-like alumina AR coating is fabricated using atomic layer deposition (ALD) of alumina and immersion in heated deionized water. Optical transmittance of 99.5% at 500 nm was achieved with average transmittance of 99.0% in the range of 350-800 nm at normal incidence for double-sided coated glass. Angular spectral transmittance (0-80°) of the double-sided AR coated glass was also measured in the range of 350-800 nm and found to have mean spectral transmittance of 94.0% at 60°, 85.0% at 70°, and 53.1% at 80° angles of incidence, respectively. The grass-like alumina AR coating is suitable for mass production with the presented technique: even hundreds of optical components can be coated in parallel. Furthermore, as an ALD-based technique, the coating can be deposited conformally on surfaces with extreme topography, unlike many spin-coating, physical vapor deposition or glancing angle deposition-based coatings used today.

Magnetic alloy nanowire arrays with different lengths: Insights into the crossover angle of magnetization reversal process

Nanoscale magnetic alloy wires are being actively investigated, providing fundamental insights into tuning properties in magnetic data storage and processing technologies. However, previous studies give trivial information about the crossover angle of magnetization reversal process in alloy nanowires (NWs). Here, magnetic alloy NW arrays with different compositions, composed of Fe, Co and Ni have been electrochemically deposited into hard-anodic aluminum oxide templates with a pore diameter of approximately 150nm. Under optimized conditions of alumina barrier layer and deposition bath concentrations, the resulting alloy NWs with aspect ratio and saturation magnetization (Ms) up to 550 and 1900emucm−3, respectively, are systematically investigated in terms of composition, crystalline structure and magnetic properties. Using angular dependence of coercivity extracted from hysteresis loops, the reversal processes are evaluated, indicating non-monotonic behavior. The crossover angle (θc) is found to depend on NW length and Ms. At a constant Ms, increasing NW length decreases θc, thereby decreasing the involvement of vortex mode during the magnetization reversal process. On the other hand, decreasing Ms decreases θc in large aspect ratio (>300) alloy NWs. Phenomenologically, it is newly found that increasing Ni content in the composition decreases θc. The angular first-order reversal curve (AFORC) measurements including the irreversibility of magnetization are also investigated to gain a more detailed insight into θc.

Significant performance enhancement of InGaN/GaN nanorod LEDs with multi-layer graphene transparent electrodes by alumina surface passivation

Nanotextured surfaces provide an ideal platform for efficiently capturing and emitting light. However, the increased surface area in combination with surface defects induced by nanostructuring e.g. using reactive ion etching (RIE) negatively affects the device’s active region and, thus, drastically decreases device performance. In this work, the influence of structural defects and surface states on the optical and electrical performance of InGaN/GaN nanorod (NR) light emitting diodes (LEDs) fabricated by top-down RIE of c-plane GaN with InGaN quantum wells was investigated. After proper surface treatment a significantly improved device performance could be shown. Therefore, wet chemical removal of damaged material in KOH solution followed by atomic layer deposition of only 10 alumina as wide bandgap oxide for passivation were successfully applied. Raman spectroscopy revealed that the initially compressively strained InGaN/GaN LED layer stack turned into a virtually completely relaxed GaN and partially relaxed InGaN combination after RIE etching of NRs. Time-correlated single photon counting provides evidence that both treatments—chemical etching and alumina deposition—reduce the number of pathways for non-radiative recombination. Steady-state photoluminescence revealed that the luminescent performance of the NR LEDs is increased by about 50% after KOH and 80% after additional alumina passivation. Finally, complete NR LED devices with a suspended graphene contact were fabricated, for which the effectiveness of the alumina passivation was successfully demonstrated by electroluminescence measurements.

Band offsets and trap-related electron transitions at interfaces of (100)InAs with atomic-layer deposited Al2O3

Spectral analysis of optically excited currents in single-crystal (100)InAs/amorphous (a-)Al2O3/metal structures allows one to separate contributions stemming from the internal photoemission (IPE) of electrons into alumina and from the trapping-related displacement currents. IPE spectra suggest that the out-diffusion of In and, possibly, its incorporation in a-Al2O3 lead to the development of ≈0.4 eV wide conduction band (CB) tail states. The top of the InAs valence band is found at 3.45 ± 0.10 eV below the alumina CB bottom, i.e., at the same energy as at the GaAs/a-Al2O3 interface. This corresponds to the CB and the valence band offsets at the InAs/a-Al2O3 interface of 3.1 ± 0.1 eV and 2.5 ± 0.1 eV, respectively. However, atomic-layer deposition of alumina on InAs results in additional low-energy electron transitions with spectral thresholds in the range of 2.0–2.2 eV, which is close to the bandgap of AlAs. The latter suggests the interaction of As with Al, leading to an interlayer containing Al-As bonds providing a lower barrier for electron injection.

Passivation of all-angle black surfaces for silicon solar cells

Optical losses at the front surface of a silicon solar cell have a significant impact on efficiency, and as such, efforts to reduce reflection are necessary. In this work, a method to fabricate and passivate nanowire-pyramid hybrid structures formed on a silicon surface via wet chemical processing is presented. These high surface area structures can be utilised on the front surface of back contact silicon solar cells to maximise light absorption therein. Hemispherical reflectivity under varying incident angles is measured to study the optical enhancement conferred by these structures. The significant reduction in reflectivity (<2%) under low incident angles is maintained at high angles by the hybrid textured surface compared to surfaces textured with nanowires or pyramids alone. Finite Difference Time Domain simulations of these dual micro-nanoscale surfaces under varying angles support the experimental results. In order to translate the optical benefit of these high surface area structures into improvements in device efficiency, they must also be well passivated. To this end, atomic layer deposition of alumina is used to reduce surface recombination velocities of these ultra-black silicon surfaces to below 30cm/s. A decomposition of the passivation components is performed using capacitance-voltage and Kelvin Probe measurements. Finally, device simulations show power conversion efficiencies exceeding 21% are possible when using these ultra-black Si surfaces for the front surface of back contact silicon solar cells.

Alumina and titania films deposition by APS/ASPPS dual mode thermal spray equipment using Ar added N2 working gas

In order to develop a low cost oxide film deposition process with short duration time, a 1 kW class Atmospheric thermal plasma spray (APS)/Atmospheric solution precursor plasma spray (ASPPS) dual mode thermal spray equipment was manufactured and film depositions of alumina (Al2O3) and titania (TiO2) by APS and titania film deposition by ASPPS were carried out. Consequently, though intensive fluctuation with intensive abrasion of electrodes occurred during plasma jet generation in case of N2 working gas, the plasma jet was stabilized and the abrasion was dramatically diminished by slight addition of Ar to N2 working gas. Since the suction type feedstock feeder could be confirmed to be available not only for powder feedstock but also solution precursor feed stock. In the case of APS, lamellar structure alumina and titania films could be deposited. However, in the case of titania films deposited by APS, a phase transformation from anatase to rutile occurred partially during film deposition. Also in the case of ASPPS, titania films including rutile and anatase were deposited. From these results, the developed equipment was proved to be available as an APS/ASPPS dual mode thermal spray equipment and this technique was found to have high potential for a low-cost oxide film deposition process.