Characteristics of La2O3 thin films deposited using metal organic chemical vapor deposition with different oxidant gas

La₂O₃ films were deposited using O₃ and the structural and electrical properties were investigated and compared with those of La₂O₃ films deposited using O₂. The deposition temperature of the La₂O₃ films using O₃ was slightly reduced compared to that of the La₂O₃ films generated using O₂. After a post-annealing process at 600 and 900 °C, the crystallinity of the La₂O₃ films using O₃ were smaller than that using O₂. The leakage current density increased after annealing at 600 °C due to densification and then decreased after annealing at 900 °C due to interfacial layer growth. The effective dielectric constant of the La₂O₃ films deposited using O₃ decreased at 900 °C due to interfacial layer growth. The La₂O₃ films deposited using O₃ showed better structural and electrical properties in this study.     

Electrochemical and time-of-flight secondary ion mass spectrometry analysis of ultra-thin metal oxide (Al2O3 and Ta2O5) coatings deposited by atomic layer deposition on stainless steel

Ultra-thin (5–50 nm) layers of aluminium and tantalum oxides deposited by atomic layer deposition (ALD) on a stainless steel substrate (316L) for corrosion protection have been investigated by electrochemical methods (linear scan voltammetry, LSV, and electrochemical impedance spectroscopy, EIS) and time-of-flight secondary ion mass spectrometry, ToF-SIMS. The effects of the deposition temperature (250 °C and 160 °C) and coating thickness were addressed. ToF-SIMS elemental depth profiling shows a marked effect of the organic and water precursors used for deposition and of the substrate surface contamination on the level of C and OH trace contamination in the coating, and a beneficial effect of increasing the deposition temperature. The polarization data show a decrease of the current density by up to four orders of magnitude with increasing coating thickness from 5 to 50 nm. The 50 nm films block the pitting corrosion in 0.8 M NaCl. The uncoated surface fraction (quantified from the current density and allowing a ranking of the efficiency of the coating, also confirmed by the capacitance and resistance values extracted from the EIS data) was 0.03% with a 50 nm thick Al₂O₃ film deposited at 250 °C. The correlation between the porosity values of the coatings and the level of C and OH traces observed by ToF-SIMS points to a marked effect of the coating contaminants on the sealing performance of the coatings and on the corrosion resistance of the coated systems.     

Atomic layer deposition of Al2O3 thin films on diamond

Atomic layer deposition (ALD) of aluminum oxide thin films on diamond was demonstrated for the first time, and the film properties as a gate insulator for diamond field effect transistor (FET) were examined. The interface between the aluminum oxide and the diamond was abrupt, and the ratio of aluminum to oxygen in the film was confirmed to be stoichiometric by Rutherford back scattering. Even a bumpy surface of polycrystalline diamond film was conformally covered by the Al₂O₃ films. To evaluate the feasibility of the film for FET gate insulator, the electrical characteristics of the Al₂O₃ films deposited by ALD on diamond were measured using metal–insulator–semiconductor structure. It was found that the Al₂O₃ films deposited by ALD were better than those deposited by conventional methods, which indicates that the ALD-Al₂O₃ films are feasible for gate insulators of diamond FETs.     

Failure mechanism of thin Al2O3 coatings grown by atomic layer deposition for corrosion protection of carbon steel

Combined analysis by electrochemical impedance spectroscopy (EIS), time-of-flight secondary ion mass spectrometry (ToF-SIMS) and field emission scanning electron microscopy (FESEM) of the corrosion protection provided to carbon steel by thin (50 nm) Al₂O₃ coatings grown by atomic layer deposition (ALD) and its failure mechanism is reported. In spite of excellent sealing properties, the results show an average dissolution rate of the alumina coating of ∼7 nm h⁻¹ in neutral 0.2 M NaCl and increasing porosity of the remaining layers with increasing immersion time. Alumina dissolution is triggered by the penetration of the solution via cracks/pinholes through the coating to the substrate surface where oxygen reduction takes place, raising the pH. At defective substrate surface sites of high aspect ratio and concentrated residual mechanical stress (along scratches) presumably exposing a higher steel surface fraction, localized dissolution of the coating is promoted by a more facile access of the solution to the substrate surface enhancing oxygen reduction. De-adhesion of the coating is also promoted in these sites by the ingress of the anodic dissolution trenching the steel surface. Localized corrosion of the alloy (i.e. pitting) is triggered prior to complete dissolution of the alumina film on the elsewhere still coated surface matrix.     

Comparative bioelectrochemical study of two types of myoglobin layer-by-layer films with alumina: Vapor-surface sol-gel deposited Al2O3 films versus Al2O3 nanoparticle films

Two types of layer-by-layer films of myoglobin (Mb) and Al₂O₃ were assembled on different surfaces and compared in Mb electrochemistry and bioelectrocatalysis. One type, designated as {SG-Al₂O₃/Mb}_n, was assembled by alternate deposition of Al₂O₃ by vapor-surface sol-gel method from liquid aluminum butoxide and Mb by adsorption from its solution. Another type, designated as {NP-Al₂O₃/Mb}_n, was constructed by alternate adsorption of Al₂O₃ nanoparticles and Mb from their dispersion or solution in the conventional layer-by-layer way. Quartz crystal microbalance (QCM), UV-vis spectroscopy, and cyclic voltammetry (CV) were used to monitor the growth of the two types of {Al₂O₃/Mb}_n films. UV-vis and IR spectroscopy demonstrated that Mb in both types of {Al₂O₃/Mb}_n films retained its near native structure. While both Mb films assembled on pyrolytic graphite (PG) electrodes exhibited a pair of well-defined, nearly reversible CV reduction-oxidation peaks for Mb heme Fe(III)/Fe(II) redox couple and good electrocatalytic reactivity toward reduction of oxygen and hydrogen peroxide, the {SG-Al₂O₃/Mb}_n films demonstrated distinct advantages over the {NP-Al₂O₃/Mb}_n films in larger maximum surface concentration of electroactive Mb and better biocatalytic performances. This may be mainly attributed to the higher porosity of {SG-Al₂O₃/Mb}_n films than that of {NP-Al₂O₃/Mb}_n films, which may be beneficial to counterion transport in the charge-hopping mechanism and the diffusion of catalytic substrates through the films.     

MOCVD deposition of CoAl2O4 films

Cobalt aluminate (CoAl₂O₄) thin films were grown in a low-pressure hot wall metal organic chemical vapour deposition (MOCVD) reactor on Si(1 0 0) and quartz substrates with a total pressure of 2 Torr using bis(η⁵-cyclopentadienyl)Co(II) [Co(η⁵-C₅H₅)₂] and aluminium dimethylisopropoxide [AlMe₂(OⁱPr)] as precursors at 500 and 900 °C. Films showed a dark-brown and dark-green colouration, respectively, and after an overnight heat treatment in air at 1200 °C, they turned blue. Film microstructure, composition and morphology were investigated in detail by X-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS), scanning electron microscopy (SEM) and secondary ion mass spectrometry (SIMS) analyses. Films were polycrystalline and the UV-vis spectra showed three electronic transitions allowed by the spin (540-630 nm range) characteristic of Co(II) ions with 3d⁷ configuration in tetrahedral coordination. SEM micrographs of the heat-treated CoAl₂O₄ samples revealed the presence of agglomerated crystallites with a highly porous structure.     

Properties of nano-crystalline LiMn2O4 thin films deposited by pulsed laser deposition

Properties of LiMn₂O₄ thin films deposited on polished stainless steel substrates at 400 °C and 200 mTorr of oxygen by pulsed laser deposition have been characterized by electrochemical measurements and physical analyses. The film was mainly composed of nano-crystals less than 100 nm. A maximum specific capacity of 141.9 mAh/g cycled between 3.0 and 4.5 V with a current density of 20 μAh/cm² has been achieved. The film exhibited an excellent cycling stability up to 500 cycles. The low charge-transfer resistance at high potentials as revealed by AC impedance resulted in high charge/discharge potential and more capacity. The effect of overdischarge was limited and Jahn-Teller effect was overcome to a significant extent in this nano-crystalline film. Ex situ XRD, Raman and XPS provided supporting evidence in the changes in structure, reactivity and cycling stability of nano-crystalline LiMn₂O₄ film cathodes under different charge/discharge states and cycling tests. SEM images also revealed the stability of the surface topography after a long-term cycling test.     

Self-healing of Al2O3 containing Ti microparticles

This work explores the possibility of using embedded micron-sized Ti particles to heal surface cracks in alumina and to unravel the evolution of the crack filling process in case of pure solid-state oxidation reactions. The oxidation kinetics of the Ti particles is studied and the results are applied in a simple model for crack-gap filling. An activation energy of 136?kJ/mol is determined for the oxidation of the Ti particles having an average particle size of 10?µm. The almost fully dense alumina composite containing 10?vol% Ti has an indentation fracture resistance of 4.5?±?0.5?MPa?m^1/2. Crack healing in air is studied at 700, 800 and 900?°C for 0.5, 1, and 4?h and the strength recovered is measured by 4-point bending. The optimum healing condition for full strength recovery is 800?°C for 1?h or 900?°C for 15?min. Crack filling is observed to proceed in three steps i.e., local bonding at the site of an intersected Ti particle, lateral spreading of the oxide and global filling of the crack. It is discovered that, although significant strength recovery can be attained by local bonding of the intersected particles, full crack filling is required to prevent crack initiation from the damaged region upon reloading. The experimental results observed are in good agreement with the predictions of a simple discrete crack filling/healing model.     

Effect of ZrO2 on phase transformation of Al2O3

This study investigates the effect of ZrO₂ on phase transformation of alumina. Alumina and alumina–zirconia composite powders were produced by the precipitation method from aluminum sulfate and zirconium sulfate precursors. Precipitates obtained at 15 °C were dried at 80 °C for 72 h, and then calcinated at four different temperatures; 1000 °C, 1100 °C, 1200 °C and 1300 °C for 1 h. Powders calcinated at 1300 °C were pressed uniaxially and sintered at 1600 °C for 1 h. XRD and DSC analyses showed that the presence of zirconia retarded the transformation to α-alumina. SEM studies on the powders calcinated at 1300 °C revealed that both alumina and alumina–zirconia particles were 100–300 nm in size and of near spherical shape. Zirconia additions inhibited abnormal grain growth of alumina and provided homogeneous, equaxied grain structure.     

Selective catalytic reduction of NO by hydrocarbons on Ga2O3/Al2O3 catalysts

Catalytic properties of supported gallium oxides have been examined for the selective reduction of NO by CH₄ in excess oxygen. The activity was greatly affected by the support; Ga₂O₃/Al₂O₃ (Al₂O₃ supported Ga₂O₃) and Ga₂O₃–Al₂O₃ mixed oxide exhibited high activity and selectivity as comparable to Ga-ZSM-5, while unsupported Ga₂O₃ and the other supported Ga₂O₃ were ineffective. For Ga₂O₃/Al₂O₃, the activity changed with Ga₂O₃ content, and was highest at about 30 wt% Ga₂O₃, which corresponds to a theoretical monolayer coverage. Gallium oxide highly dispersed on Al₂O₃ is considered to be responsible for the high activity and selectivity. The reaction characteristics of Ga₂O₃/Al₂O₃ were studied and compared with Ga-ZSM-5 and Co-ZSM-5. Ga₂O₃/Al₂O₃ exhibited the highest activity and selectivity at high temperature. In addition, Ga₂O₃/Al₂O₃ showed higher tolerance against water than Ga-ZSM-5. C₃H₈ and C₃H₆ were also evaluated as reducing agents, and Ga₂O₃/Al₂O₃ showed higher activity than Ga-ZSM-5 above 723 K achieving almost complete reduction of NO to N₂.     

Effect of powder particle size on vibration damping behaviour of plasma sprayed alumina (Al2O3) coating on AISI 304 stainless steel substrate

The damping capacity of plasma sprayed alumina (Al₂O₃) coatings on AISI 304 stainless steel was investigated in this study as a function of particle size of the starting alumina powder. The coatings were prepared from different sizes alumina powder using commercial air plasma spraying (APS) technique. The damping properties of coated samples were characterized by damping capacity (Q^?1) measured experimentally using dynamic mechanical analyzer (DMA). The surface morphology of the coatings was studied using scanning electron microscope (SEM). The results revealed that the coating was porous and was able to improve the damping capacity of bare substrate. It was also observed that the powder particle size had a significant effect on the damping characteristics of the coatings. The damping values were found to be increased with the increase in particle size in the measured strain range. This behaviour was correlated with the microstructure investigated by SEM.     

Study of color change and microstructure development of Al2O3–Cr2O3/Cr3C2 nanocomposites prepared by spark plasma sintering

The densification behaviors of Al₂O₃–Cr₂O₃/Cr₃C₂ nanocomposites prepared by a Spark Plasma Sintering (SPS) were investigated in this work. The initial powders used for sintering were Al₂O₃–Cr₂O₃, which were prepared by metal organic chemical vapor deposition (MOCVD) in a spout bed. Different colors of the compacts such as green, purple and black were observed after densification process at different SPS temperatures from 1200 °C to 1350 °C. These changes of color were relevant to the existence of secondary phase of green Cr₂O₃, pink solid solution of Cr₂O₃–Al₂O₃ and black Cr₃C₂, which were formed under the different SPS temperature. The secondary phase of Cr₂O₃ retarded the processing of densification for spark plasma sintering at 1200 °C. The Cr₂O₃ reacted with Al₂O₃ to form solid solution of Cr₂O₃–Al₂O₃ and with carbon to form Cr₃C₂ as sintering temperature increased to 1350 °C. The characteristics of high heating rate, shorter sintering time for SPS and the formation of secondary phase of Cr₃C₂ effectively reduced the substrate's grain growth, making Al₂O₃–Cr₂O₃/Cr₃C₂ nanocomposites with small grain size.     

Fabrication of Fe-, Ni- and FeNi-coated Al2O3 core-shell microspheres by heterogeneous precipitation

The precursors with NiCO₃·2Ni(OH)₂·2H₂O-, Fe₂O₃·nH₂O-, or both of NiCO₃·2Ni(OH)₂·2H₂O- and Fe₂O₃·nH₂O-coated alumina microspheres were prepared, respectively, by the aqueous heterogeneous precipitation using metal salts, ammonium hydro-carbonate and -Al₂O₃ micropowders as the starting materials. Subsequently, magnetic metallic Ni-, -Fe- and γ-FeNi-coated alumina core-shell structural microspheres were successfully obtained by thermal reduction of the as-prepared precursors at 700 °C for 2 h, respectively. Optimized precipitation processing parameters of the concentration of alumina micropowders (15 g/L), the rate of adding reactants (5 mL/min) and pH value were determined by a trial and error method. Powders of the precursors and the resultant metal (Ni, -Fe, γ-FeNi alloy)-coated alumina micropowders were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The experimental results show that it is possible to adjust metal coating thicknesses and fabricate multilayer structured metal/ceramics core-shell microspherical powder materials and these materials may be applied for high performance of functional materials and devices.     

Characterization and tribological investigation of sol-gel Al2O3 and doped Al2O3 films

Thin films of Al₂O₃ and doped Al₂O₃ were prepared on a glass substrate by dip coating process from specially formulated ethanol sols. The morphologies of the unworn and worn surfaces of the films were observed with atomic force microscope (AFM) and scanning electron microscope (SEM). The chemical compositions of the obtained films were characterized by means of X-ray photoelectron spectroscopy (XPS). The tribological properties of obtained thin films sliding against Si₃N₄ ball were evaluated and compared with glass slide on a one-way reciprocating friction tester. XPS results confirm that the target films were obtained successfully. The doped elements distribute in the film evenly and exist in different kinds of forms, such as oxide and silicate. AFM results show that the addition of the doped elements changes the structure of the Al₂O₃ films, i.e., a rougher and smoother surface is obtained. The wear mechanisms of the films are discussed based on SEM observation of the worn surface morphologies. As the results, the doped films exhibit better tribological properties due to the improved toughness. Sever brittle fracture is avoided in the doped films. The wear of glass is characteristic of brittle fracture and severe abrasion. The wear of Al₂O₃ is characteristic of brittle fracture and delamination. And the wear of doped Al₂O₃ is characteristic of micro-fracture, deformation and slight abrasive wear. The introduction of ZnO is recommended to improve the tribological property of Al₂O₃ film.     

Sulphur trap materials based on mesoporous Al2O3

Different sulphur trap materials based on mesoporous Al₂O₃ supports modified with the storage component Ca or Ba and the oxidation components Pt or Cu or Mn were prepared and the SO₂ uptake behaviour examined in the temperature range 50–600 °C. A comparison between the different oxidation components shows that the presence of Pt is not a necessary precondition for attaining a good storage behaviour under these test conditions. Mn is the most suitable oxidation component.

Tests performed over four temperature cycles show that the removal efficiency for SO₂ gradually decreases over the course of temperature cycles. Moreover, a progressive diminution of the SO₂ uptake is observed especially in the lower temperature range during the temperature cycles. A further modification with Na prevents this drawback. Especially a distinct improvement is observed with the Ca-containing material. It stored 22 wt.% sulphate during the four temperature cycles. This material is regarded as being most suitable for application as sulphur trap material.     

Microhardness and wear resistance of Al2O3-TiB2-TiC ceramic coatings on carbon steel fabricated by laser cladding

Al₂O₃-TiB₂-TiC ceramic coatings with high microhardness and wear resistance were fabricated on the surfaces of carbon steel substrates by laser cladding using different coating formulations. The microstructures of these ceramic coatings with the different coating formulations were investigated using X-ray diffraction, scanning electron microscopy, and energy dispersive spectrometer. The wear resistance and wear mechanism were analyzed using Vickers microhardness and sliding wear tests. The results showed that when the amount of independent Al₂O₃ was increased to 30%, the ceramic coatings had a favorable surface formation quality and strong metallurgical bond with the steel matrix. The cladding layer was uniformly and densely organized. The black massive Al₂O₃, white granular TiB₂, and TiC distributed on the Fe substrate significantly increased the microhardness and wear resistance. The laser cladding ceramic coating had many hard strengthening phases, and thus resisted the extrusion of rigid particles in frictional contact parts. Therefore, the wear process ended with a “cutting-off” loss mechanism.     

Sol-gel derived 6CaO·6SrO·7Al2O3 thin films using metal alkoxides

The novel 6CaO·6SrO·7Al₂O₃ (C6S6A7) thin films were deposited onto soda lime glass substrate using calcium, strontium, aluminium isopropoxide and 2-methoxy ethanol as starting materials via sol-gel dip coating technique. The electrical and optical properties of C6S6A7 films were investigated for films sample annealed at 450 °C for 2 h in air and hydrogen (H₂) atmosphere, respectively. X-ray diffraction pattern and Fourier transformed infrared spectroscopy analysis confirms cubic structure to the C6S6A7 material. The optical transmission spectra of C6S6A7 films showed the high transparency in wide visible range of ∼ 88 and 80% for air and H₂ annealed samples, respectively. The C6S6A7 films sheet resistance of 528 and 0.65 k Ohm/square has been observed for films annealed in air and H₂ atmosphere at 450 °C, respectively.     

Preparation and characterization of single crystalline In2O3 films deposited on MgO (110) substrates by MOCVD

Epitaxial indium oxide (In₂O₃) films have been prepared on MgO (110) substrates by metal-organic chemical vapor deposition (MOCVD). The deposition temperature varies from 500 °C to 700 °C. The films deposited at each temperature display a cube-on-cube orientation relation with respect to the substrate. The In₂O₃ film deposited at 600 °C exhibits the best crystalline quality. A clear epitaxial relationship of In₂O₃ (110)|MgO (110) with In₂O₃ [001]|MgO [001] has been observed from the interface area between the film and the substrate. The average transmittance of the prepared films in the visible range is over 95%. The band gap of the obtained In₂O₃ films is about 3.55–3.70 eV.     

Protective abilities of nanocomposite coatings containing Al2O3 nano-particles loaded by CeCl3

The protective ability of hybrid nano-composite oxysilane coatings, deposited via sol–gel method on AA2024-T3 – aluminium alloy, were studied by linear voltammetry (LVA) and electrochemical impedance spectroscopy (EIS) methods in 0.05 M solution of NaCl. Cerium chloride (CeCl₃) was incorporated as an inhibitor into a sol–gel hybrid matrix in two different routes: directly and via filled porous Al₂O₃ nano-particle aggregates with diameters up to 500 nm. The influences of the inhibitor concentration, as well as the influence of nano-particles on the barrier properties and the susceptibility against corrosion, were evaluated and EIS spectra were fitted by appropriated equivalent circuits. The values for C_coat, R_coat, C_oxy and R_oxy were achieved and their evolution over time was investigated. The investigated coatings possess highly expressed barrier properties (10⁶ to 10⁷ Ω cm²). Despite of the chloride ions inside of the matrix, some samples illustrated a significant durability of over 4000 h during exposure to the corrosion medium before first signs of corrosion appeared. The electrochemical results were compared with the neutral salt spray test. Thus, it was proved that the potential of these coatings is to be used as anticorrosive protective materials and are candidate to replace Cr(VI)-based anti-corrosion coatings.     

Hydrophobicity and transparency of Al2O3-based poly-tetra-fluoro-ethylene composite thin films using aerosol deposition

Alumina and polytetrafluoroethylene (Al₂O₃-PTFE) composite films were fabricated by a simple aerosol deposition (AD) process, to confirm its applicability for various display screens requiring water resistant, anti-smudge and easy-to-clean properties. The surface morphologies, hydrophobic properties, and transparencies of the composite films with different PTFE contents, varying from 0.01 to 1?wt% were investigated. As a result, the composite films with over 0.3?wt% PTFE showed a sudden rise in surface roughness and low transmittance, despite having the highest contact angle of 128° at a PTFE content of 0.3?wt%. From the energy dispersive spectrometer analysis, the crash-cushioning effect of PTFE and agglomerated PTFE particles were determined to be major causes of surface roughness and opacity. In contrast, the transmittance showed a tendency to be enhanced, with an increasing PTFE content in the range of 0.01, 0.05, and 0.1?wt% PTFE, respectively. Especially, the film with 0.1?wt% of PTFE had contact angle of 111° and exhibited a high transmittance of over 75%, which was inferred to be an appropriate amount of PTFE, with a high elongation filling up the surface and the internal defects, leading to an enhancement of transparency. Consequently, these results implied that the AD-prepared Al₂O₃-PTFE composite coatings are promising candidates for various display applications.