Hot corrosion behavior of powder metallurgy Rene95 nickel-based superalloy in molten NaCl–Na2SO4 salts

Hot corrosion behavior of powder metallurgy (PM) Rene95 Ni-based superalloy in molten 25%NaCl + 75%Na₂SO₄ salts at 650 °C, 700 °C and 750 °C are investigated by weight loss measurements, X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). Experimental results show that hot corrosion kinetics follow a square power law at 650 °C and linear power laws at 700 °C and 750 °C. The corrosion layers on the surface of PM Rene95 superalloy are detected to be mainly composed of Cr₂O₃, NiO, and Ni₃S₂ at each temperature. Besides, small amounts of NiCr₂O₄ at 700 °C and NaCl at 750 °C are observed respectively. Cross-sectional morphologies and corresponding elemental maps indicate that corrosion layers near scale/alloy interface are composed of oxides at 650 °C while duplex oxides and sulfides at 700 °C and 750 °C. According to these results, a cooperating mechanism of oxidation and sulfuration for hot corrosion of PM Rene95 Ni-based superalloy is confirmed.     

Characteristics of high-k gate oxide prepared by oxidation of multi-layered Hf/Zr/Hf/Zr/Hf metal films

We investigated the physical and electrical properties of Hf-Zr mixed high-k oxide films obtained by the oxidation and annealing of multi-layered metal films (i.e., Hf/Zr/Hf/Zr/Hf, ∼ 5 nm). We demonstrated that the oxidation of multi-layered metal films results in two distinctive amorphous layers: That is, Hf-Zr mixed oxide film was formed on the top of silicate film due to inter-diffusion between Hf and Zr layer. This film shows the improved dielectric constant (k) and the raised crystallization temperature. Compared with HfO₂ and ZrO₂ gate dielectric, the crystallization temperature of Hf-Zr mixed oxides was raised by more than 200 °C. Using AES and XPS, we observed that Zr oxide has more fully oxidized stoichiometry than Hf oxide, irrespective of annealing temperatures. We also found that the thickness of an interfacial layer located between Hf-Zr mixed oxide and Si substrate also increases as annealing temperature increases. Especially, the thin SiO_x interfacial layer starts to form if annealing temperature increases over 700 °C, deteriorating the equivalent oxide thickness.     

Hydrothermal processing of amorphous hydrous zirconia gels in the presence of 1,12-diaminododecane

In the presence of 1,12-diaminododecane (DADD), tetragonal ZrO₂ nanoparticles (ca. 10 nm) with high phase purity (monoclinic volume content: less than 0.1) and large surface area (100 m²/g) and pure monoclinic ZrO₂ nanorods (aspect ratio of ca. 10) have been successfully prepared upon hydrothermal processing at 230 °C for 3 days of zirconyl nitrate-derived hydrous zirconia gels at different pH values. The complementary data from FTIR, Raman, TGA/DTG confirm the presence of hydrocarbon fragments (1-3 wt%) on the surface of DADD-derived zirconia nanocrystals. These organic residues account for their features in the phase composition, particle size and morphology, and surface wettability.     

Vapor-phase synthesis of a solid precursor for α-alumina through a catalytic decomposition of aluminum triisopropoxide

A new solid precursor, hydrous aluminum oxide, for α-alumina nanoparticles was prepared by thermal decomposition of aluminum triisopropoxide (ATI) vapor in a 500 mL batch reactor at 170-250 °C with HCl as catalyst. The conversion of ATI increased with increasing temperature and catalyst content; it was nearly complete at 250 °C with the catalyst at 10 mol% of the ATI. The obtained precursor particles were amorphous, spherical and loosely agglomerated. The primary particle size is in the range 50-150 nm. The ignition loss of the precursor was 24%, considerably lower than 35% of Al(OH)₃, the popular precursor for alumina particles. Upon calcination of the precursor at 1200 °C in the air with a heating rate of 10 °C/min and a holding time of 2 h, the phase was completely transformed into α. The spherical particles composing the precursor turned worm-like by the calcination probably due to sintering between neighboring particles. The surface area equivalent diameter of the resulting α-alumina was 75 nm.     

Atomic layer deposition grown composite dielectric oxides and ZnO for transparent electronic applications

In this paper, we report on transparent transistor obtained using laminar structure of two high-k dielectric oxides (hafnium dioxide, HfO₂ and aluminum oxide, Al₂O₃) and zinc oxide (ZnO) layer grown at low temperature (60 °C-100 °C) using Atomic Layer Deposition (ALD) technology. Our research was focused on the optimization of technological parameters for composite layers Al₂O₃/HfO₂/Al₂O₃ for thin film transistor structures with ZnO as a channel and a gate layer. We elaborate on the ALD growth of these oxides, finding that the 100 nm thick layers of HfO₂ and Al₂O₃ exhibit fine surface flatness and required amorphous microstructure. Growth parameters are optimized for the monolayer growth mode and maximum smoothness required for gating.     

Temperature dependence of the microstructure and resistivity of indium zinc oxide films deposited by direct current magnetron reactive sputtering

Indium zinc oxide (IZO) films were deposited as a function of the deposition temperature using a sintered indium zinc oxide target (In₂O₃:ZnO = 90:10 wt.%) by direct current (DC) magnetron reactive sputtering method. The influence of the substrate temperature on the microstructure, surface roughness and electrical properties was studied. With increasing the temperature up to 200 °C, the characteristic properties of amorphous IZO films were improved and the specific resistivity was about 3.4 × 10^− 4 Ω cm. Change of structural properties according to the deposition temperature was also observed with X-ray diffraction patterns, transmission electron microscopy, X-ray photoelectron spectroscopy, and atomic force microscopy. IZO films deposited above 300 °C showed polycrystalline phases evolved on the amorphous IZO layer. Very flat surface roughness could be obtained at lower than 200 °C of the substrate temperature, while surface roughness of the films was increased due to the formation of grains over 300 °C. Consequently, high quality IZO films could be prepared by DC magnetron sputtering with O₂/Ar of 0.03 and deposition temperature in range of 150-200 °C; a specific resistivity of 3.4 × 10^− 4 Ω cm, and the values of peak to valley roughness and root-mean-square roughness are less than 4 nm and 0.5 nm, respectively.     

Catalytic activity and magnetic properties of barium hexaferrite prepared from barite ore

Barium hexaferrite (BaFe₁₂O₁₉) has traditionally been used in permanent magnets and more recently used for high density magnetic recording. The classical ceramic method for the preparation of barium hexaferrite consists of firing mixture of chemical grade iron oxide and barium carbonate at high temperature. In this paper a mixture of chemical grade hematite, barium oxide and predetermined mixtures of iron oxide ore and barite ore containing variable amounts of coke were used to prepare barium hexaferrite (BaFe₁₂O₁₉) as a permanent magnetic material. The mixtures were mixed in a ball mill and fired for 20 h in a tube furnace at different temperatures (1100, 1150, 1200 and 1250 °C). XRD, magnetic properties, porosity measurements and catalytic activity were used for characterization of the produced ferrite. The results of experiments showed that the optimum conditions for the preparation of barium hexaferrite are found at 1200 °C for the mixture of chemical grade hematite and barium oxide. It was also found that the barium hexaferrite can be prepared from the iron and barite ores at 1200 °C. The addition of coke enhanced the yield of barium hexaferrite and improved its physicochemical properties. Samples prepared from ores with coke% = 0 show the most acidic active sites, they show a higher catalytic activity towards H₂O₂ decomposition. With addition of coke the catalytic activity decreases due to the poisoning effect of carbon on the available active site.     

Auto-correlation function analysis of phase formation in iron ion-implanted amorphous silicon layers

High-resolution transmission electron microscopy (HRTEM) in conjunction with autocorrelation function (ACF) analysis have been applied to investigate the evolution of structural order in iron ion-implanted amorphous silicon layers. β-FeSi₂ nanocrystallites as small as 5 nm in size were detected in 600 °C annealed for 60 min a-Si layers. The embedded nanocrystalline β-FeSi₂ was found to grow in the interlayer with annealing temperature.     

Low-temperature synthesis and microstructure-property study of single-phase yttrium iron garnet (YIG) nanocrystals via a rapid chemical coprecipitation

Single-phase yttrium iron garnet (Y₃Fe₅O₁₂, YIG) nanocrystals have been synthesized via a rapid chemical coprecipitation process with reverse strike operations, followed by calcining the precipitates at the temperature around 750 °C. The formation of YIG nanocrystals from the amorphous precipitates and their microstructural features and magnetic properties were investigated by FT-IR, XRD, TG-DSC, FESEM, TEM and VSM. It has been found that the as-obtained precipitates could be thermally activated to directly form the crystalline phases of garnet structure around 650 °C, including cubic YIG and minor tetragonal YIG but no trace of YFeO₃, which was often involved during the synthesis of YIG or doped-YIG when a chemical coprecipitation method was used. The calcinations could make the tetragonal YIG entirely transform into the cubic phase at 750 °C and allow the crystallites of the latter to grow from ∼22 nm to ∼50 nm in size almost linearly as a function of the temperature ranging from 650 °C to 900 °C. Moreover, the room temperature saturation magnetization of the samples after calcinations at various temperatures showed a nonlinear increase from 0.24 emu g⁻¹ to 24.54 emu g⁻¹, which should be associated with the alignments of atomic magnetic moments in the materials from completely-disordered to partially-ordered firstly and further to completely-ordered and, in the last stage, mainly with the growing YIG nanocrystals.     

Low-temperature mechanochemical–thermal synthesis of α-Al2O3 nanocrystals

The great capability of high-energy ball milled basic polyaluminium chloride gel (PACl) which consisted of the monomeric Al³⁺ ions and polymerized Al³⁺ species such as [Al₁₃O₄(OH)₂₄(H₂O)₁₂]⁷⁺ with a Keggin like structure to enhance the phase transformation into α-Al₂O₃ nanocrystals at low temperature was verified. PACl gel 10 min milled and annealed at 400 °C, partially transformed to nanocrystalline α-Al₂O₃ with the mean XRD crystallite size in the range of 15–17 nm, embedded in an amorphous or transition alumina matrix. Further crystal growth up to ∼50 nm and phase pure α-Al₂O₃ powder was obtained when heat-treated at 1000 °C. In contrast to this, the non-milled PACl gel transforms to the transition θ-Al₂O₃ phase at 1000 °C. The evolution of α-Al₂O₃ nanocrystals was studied by XRD, TEM, selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM) methods.     

Synthesis of FePO4 cathode material for lithium ion batteries by a sonochemical method

Hydrated amorphous FePO₄ was synthesized by a sonochemical reaction method, in which a solution of (NH₄)₂HPO₄ and FeSO₄·7H₂O was irradiated by an ultrasonic wave. From this material, two kinds of cathode materials were easily prepared: (1) an amorphous sample prepared by heating at 350 °C and (2) a crystalline sample prepared by heating at 700 °C. Both samples consisted of homogeneous sub-micron particles. The amorphous sample of FePO₄ exhibited high discharge capacities with more than 100 mAh g⁻¹ in the range of 3.9-2.0 V versus Li/Li⁺ at a current rate of 0.2 C. The sonochemical synthesis proposed herein has the following advantages: no use of oxidation agents for production of trivalent iron ions, reduction in reaction time, control of particle size, and enlargement in surface area for the preparation of the cathode material.     

An anomalous behaviour in the phase stability of the system Fe2O3 and NiO

Iron-nickel mixed oxides containing up to 50 mol% of NiO were prepared by firing the corresponding co-precipitated hydrous oxides; characterization was performed by X-ray diffraction, infrared spectroscopy, magnetic susceptibility, electrical conductivity and thermoelectric power measurements. A non-stoichiometric ferrite phase was formed when a sample containing 20 mol% NiO was sintered at 1050°C. This phase had two- to three-fold higher conductivity than either Fe₂O₃ or the stoichiometric ferrite (NiFe₂O₄). The thermoelectric power of this phase indicated a sharp change of charge carriers from n- to p-type near 350°C. This non-stoichiometric ferrite phase was stable only in a small temperature range and dissociated into -Fe₂O₃ and stoichiometric ferrite above 1200°C. Samples containing 5 and 10 mol% NiO also had small fractions of this non-stoichiometric ferrite phase when sintered at 1050°C.     

Formation of thin β-FeSi2 template layer for the epitaxial growth of thick film on Si (111) substrate

For the epitaxial growth of thick β-FeSi₂ films, we fabricated ultrathin β-FeSi₂ template layers (thinner than 20 nm) on Si (111) substrates with different methods. Surface morphology and crystallinity of the template layers were found to be dependent on the surface conditions of the substrate and the fabrication method. It was revealed that to form a smooth and continuous template, a hydrogen-terminated surface was better than that covered with a several-nanometer oxide layer. Using this surface, continuous (110)/(101)-oriented epitaxial template was obtained by depositing 6-nm iron at 400 °C and subsequent in situ annealing at 600 °C in MBE chamber, namely, a reaction deposition epitaxy (RDE) method. Co-deposition of iron and silicon with atomic ratio of Fe/Si=1/2 allowed the forming of template layers at further low temperature. Co-deposited template layers exhibited better crystallinity and morphology than those prepared by RDE. By using the optimized template layer, we succeeded in growing high-quality thick β-FeSi₂ films on Si (111) substrates with sharp β-FeSi₂/Si interface.     

Preparation of nanosized LaCoO3 perovskite oxide using amorphous heteronuclear complex as a precursor at low temperature

Nanosized LaCoO₃ cobaltite oxide powder with perovskite structure was successfully synthesized at a relatively low calcination temperature using an amorphous heteronuclear complex, LaCo(DTPA)·6H₂O, as a precursor. The precursor decomposed completely into cobaltite oxide above 400 °C according to the DTA and TGA results. XPS revealed that the decomposed species was composed of LaCoO₃ cobaltite oxide after the precursor was calcined at 500 °C for 2 hours. XRD demonstrated that nanosized LaCoO₃ crystalline powder with perovskite structure was formed after the calcination temperature increased to 600 °C. The grain size and the crystal size of LaCoO₃ increased with the calcination temperature from 500 °C to 800 °C, and the heat-treatment time has a less obvious effect on the grain size and the crystal size. It is a useful way to synthesize nanosized perovskite oxides using an amorphous complex as a precursor. This method can be easily quantitatively controlled.     

Temperature-dependent properties of FePO4 cathode materials

The temperature of formation and crystallinity of iron phosphate, FePO₄, is critical in determining its electrochemical behavior in lithium cells. Amorphous FePO₄ formed by heating amorphous iron phosphate dihydrate was found to have good capacity both in chemical and electrochemical lithiation. Initial capacities of up to 140 mAh/g were obtained at a current density of 0.2 mA/cm². Material formed from crystalline iron phosphate dihydrate was found to be much less active, due to both the crystallinity of the product and to the iron being in tetrahedral coordination. The trigonal quartz phase of FePO₄ loses almost all capacity after heating to 700°C, which might be due to glass formation on the surface.     

Deposition and microhardness of SiC from the Si2Cl6-C3H8-H2-Ar system

We obtained SiC coating layers on a graphite substrate using hexachlorodisilane (Si₂Cl₆, boiling point 144° C) as a silicon source and propane as a carbon source. We examined the deposition conditions, contents of carbon, silicon and chlorine in the deposits, and the microhardness. Mirror-like amorphous silicon layers were deposited in the reaction temperature range 500 to 630° C. well-formed silicon carbide layers with good adherency to the substrate were obtained above 850° C. The lowest deposition temperature of SiC was estimated to be 750 to 800° C. The Vickers microhardness of the SiC layer was about 3800 kg mm^–2 at room temperature and 2150 kg mm^–2 at 1000° C.     

Thermal and X-ray studies of the pyrolysis process for bismuth-substituted iron garnet films

The thermal decomposition and crystallization processes of two types of films with various thicknesses prepared by spin-coating aqueous and acetylacetonic solutions made by dissolving metal nitrate hydrates of appropriate ratios for the garnets, are described. It was found that the aqueous films decomposed with endothermic reactions over a broad temperature range from 80–500 °C, while the acetylacetonic films decomposed at two strong exothermic reactions at temperatures of 130 °C and 250 °–400 °C. Both films decomposed to become amorphous oxides, which then began to crystallize at a temperature of about 600 °C. It was also found that when the amorphous oxide films were thinner than 0.3 m, the garnets were formed directly from the amorphous oxides. When the films were thicker than 0.3 m, intermediate orthoferrites were formed which, upon further heating, transformed to the garnets. Differential thermal analysis, thermogravimetry, and X-ray diffraction data in the temperature range 20–750 °C are given and discussed.     

Thermal oxidation of single crystal aluminum nitride — A high resolution transmission electron microscopy study

The impact of the oxidation time on the structures of thermal oxides formed on AlN was determined by high resolution transmission electron microscopy (HRTEM). Oxidation of AlN single crystals was performed for 2 to 6 h at 1000 °C. Oxidation for 2 h produced mostly amorphous oxide layers whereas oxidation for both 4 and 6 h produced partially crystalline oxide layers. The oxide layer thickness varied from 205 to 600 nm for oxidation times of 2 and 6 h respectively. The crystalline oxide was mostly single phase α-Al₂O₃ except at the surface where it was a mixture of γ-Al₂O₃ and α-Al₂O₃. Based on the different structures produced for different oxidation times, we speculate that the oxide formed changes with thickness: first an amorphous oxide, then γ-Al₂O₃, and finally α-Al₂O₃ as the oxide thickness increases. The AlN crystal was nearly defect- and oxygen-free for oxidation at 1000 °C. This could be due to the rapid diffusion of the nitrogen and aluminum interstitials at a high temperature leading to a point-defect equilibrium throughout the nitride. A faceted interface between Al₂O₃ and AlN could be attributed to the surface diffusion to minimize energy.     

An investigation of thermal decomposition of β-FeOOH nanowire arrays assembled in AAO templates

β-FeOOH nanowire arrays were assembled into porous anodized aluminum oxide (AAO) templates by electrochemical deposition in the mixture solution of FeCl₃ and (NH₄)₂C₂O₄. In order to obtain well-crystallized α-Fe₂O₃ and other iron oxides nanowires, β-FeOOH nanowire arrays with amorphous crystal structure were heat-treated at different temperatures from 200 to 600 °C. The decomposition products were characterized by DTA, XRD, FTIR, and Mössbauer spectroscopy. When heat-treated at 200 °C, only 65% of β-FeOOH decomposed, whereas, when the temperature was up to 300 °C, it was completely decomposed and formed poorly crystallized β-Fe₂O₃. This transition temperature is higher than the 200 °C obtained on other β-FeOOH materials. However, when heated above 300 °C, the main products are characterized as poorly crystallized α-Fe₂O₃ nanowires, whereas, well-crystallized α-Fe₂O₃ nanowire arrays can be formed when the temperature was up to 600 °C, and this temperature is also higher compared with those temperatures observed on other β-FeOOH materials. From Mössbauer results, the α-Fe₂O₃ nanowires were composed of fine particles in which 66% of the particles are superparamagnetic.     

Photoluminescence from terbium doped silica-titania prepared by a sol-gel method

Terbium doped (0.5 at.%) TiO₂-SiO₂ (30%/70%) was prepared by a sol-gel method. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to characterize the powder calcined at two different temperatures. At a low temperature of 550 °C an amorphous phase was obtained, but at a higher temperature of 1000 °C, the anatase TiO₂ phase was crystallized in the amorphous SiO₂ phase. Green photoluminescence from ultraviolet excitation was detected after heating to either temperature, but the amorphous sample heated to 550 °C exhibited a higher intensity. X-ray diffraction and photoluminescence excitation data are discussed to explain these observations.