Corundum Substrates Research Articles

Effect of MgO on wetting and corrosion behaviour of corundum substrate by CaO–SiO2–MgO (–Al2O3) slags

ABSTRACT Effect of MgO content on wetting and corrosion behaviour of corundum substrate by CaO–SiO2–MgO (−15%Al2O3) molten slags was investigated at 1450°C via joint application of a sessile drop technique, SEM–EDS detection and FactSage thermodynamic software. All slags exhibit good wettability on the substrate, and the contact angle values are small. Increasing MgO content from 7% to 9% in CaO-SiO2-MgO slags can increase the contact angle between the droplet and the substrate, while the substrate dissolution thickness and the slag penetration depth tend to decrease. Increasing MgO content from 8% to 15% in CaO-SiO2-MgO-15% Al2O3 slags can also increase the contact angle, but significantly decrease the slag penetration depth. The indirect dissolution thickness increases instead due to the formation of a large amount of MgAl2O4 by the interface reaction. The wettability can give an indication of the slag penetration depth in the substrate but not the dissolution thickness of the substrate.

Film casting technology for aluminium oxide ceramic substrates

The paper presents the technology of aluminum oxide ceramic substrates formation by film casting using "KEKO" line (Slovenia). Components of binder (dispersion phase) including polymeric bonding agent, plasticizer, dispersant, solvent are presented. The following binder components were used: polymeric binder - polyvinyl butyral, plasticizer - dibutyl phthalate, dispersant - fish oil and solvent - mixture of toluene and ethanol. The preparation regimes for the slurry, casting and lamination of the ceramic strip are given. According to the thermal analysis (DSC and TG) of raw corundum substrate the main mass of the removed binder occurs in the temperature range 240-450oC, the observed exo-effects in this temperature range are caused by successive processes of decomposition and removal of binder components. Based on the results of thermal analysis the regime of organic component removal from the "raw blanks-substrates" which provides for heating to a temperature of 500oC with a given rate of heating and cooling is determined. The technology of film-casting provides obtaining raw billet-substrate with a given density and the necessary technological strength. We experimentally set the firing mode of corundum substrates in vacuum (0.1-1.0 Pa) at a speed of temperature 100оС/h to 1450оС with an exposure time of 2 hours, then at a speed of 60оС/h to a maximum temperature of 1620-1660оС with an exposure time of 4 hours. The substrates were cooled at a rate of 100oC/h. The structure of the annealed substrates is represented by corundum crystals with sizes predominantly not more than 10 microns, intergrain porosity not more than 0.5%. Sintered products have high density and minimum number of defects.

Гетерометаллические (никель, титан)-содержащие диметилглиоксиматы как прекурсоры для нанесения покрытий титаната никеля

Heterometallic (nickel, titanium) dimethylglyoximate (II) with a nickel:titanium ratio of 1, was synthesized. based on nickel dimethylglyoximate (I) and titanium tetrachloride TiCl4 in chloroform The obtained complex II was characterized by XRD, IR and EPR spectroscopy, and elemental analysis. It was shown that the synthesized complex II is a solvate. On the basis of the analysis results, an assumption was made about the possible structure of complex II. It was found that the thermal decomposition of complex II starts at 44 °C. A constant mass of the decomposition product is observed in the temperature range of 590 – 850 °C. According to the XRD analysis of the thermolysis product, the main crystalline phase is nickel titanate NiTiO3, it is also present up to 5 wt. % oxides of nickel NiO and titanium TiO2, as well as carbon. Complex II was used to obtain a nickel titanate coating of a corundum substrate. A solution of complex II dissolved in dimethyl sulfoxide was applied to a preliminarily prepared surface of alumina (corundum) by immersing the substrate in the solution, after which firing was carried out at 850 °C, as a result of which the organic part (dimethylglyoxime) (III) pyrolyzed and a coating was formed. According to XRD data, the resulting coating is characterized by the perovskite structure and refers to nickel titanate. An insignificant impurity of the rutile phase was found, the content of which does not exceed 5 wt. %.

Effect of Silicate Additive on Structural and Electrical Properties of Germanium Nanowires Formed by Electrochemical Reduction from Aqueous Solutions.

Layers of germanium (Ge) nanowires (NWs) on titanium foils were grown by metal-assisted electrochemical reduction of germanium oxide in aqueous electrolytes based on germanium oxide without and with addition of sodium silicate. Structural properties and composition of Ge NWs were studied by means of the scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and Raman spectroscopy. When sodium silicate was added to the electrolyte, Ge NWs consisted of 1–2 at.% of silicon (Si) and exhibited smaller mean diameter and improved crystallinity. Additionally, samples of Ge NW films were prepared by ultrasonic removal of Ge NWs from titanium foils followed with redeposition on corundum substrates with platinum electrodes. The electrical conductivity of Ge NW films was studied at different temperatures from 25 to 300 °C and an effect of the silicon impurity on the thermally activated electrical conductivity was revealed. Furthermore, the electrical conductivity of Ge NW films on corundum substrates exhibited a strong sensor response on the presence of saturated vapors of different liquids (water, acetone, ethanol, and isopropanol) in air and the response was dependent on the presence of Si impurities in the nanowires. The results obtained indicate the possibility of controlling the structure and electrical properties of Ge NWs by introducing silicate additives during their formation, which is of interest for applications in printed electronics and molecular sensorics.

Direct Epitaxial Growth of Polar Hf0.5Zr0.5O2 Films on Corundum.

Single-phase epitaxial Hf0.5Zr0.5O2 films with non-centrosymmetric orthorhombic structure have been grown directly on electrode-free corundum (α-Al2O3) substrates by pulsed laser deposition. A combination of high-resolution X-ray diffraction and X-ray absorption spectroscopy confirms the epitaxial growth of high-quality films belonging to the Pca21 space group, with [111] out-of-plane orientation. The surface of a 7-nm-thick sample exhibits an atomic step-terrace structure with a corrugation of the order of one atomic layer, as proved by atomic force microscopy. Scanning transmission electron microscopy reveals that it consists of grains with around 10 nm lateral size. The polar nature of this film has been corroborated by pyroelectric measurements. These results shed light on the mechanisms of the epitaxial stabilization of the ferroelectric phase of hafnia.

The Effect of Trace Oxygen Addition on the Interface Behavior of Low-Alloy Steel.

This work aims to assess the effect of an oxygen content graded in minimal quantities, on the order of hundreds of ppms, on the determination of surface tension of low-alloy FeCOCr and FeCONi steels in contact with a corundum substrate. Oxygen, as a surface-active element, was segregated at the surface where it interacted with the major components of the alloys, leading to a reduction in surface tension. The sessile drop method was used for wetting tests in the temperature range from steel liquidus temperatures to 1600 °C under nonoxidizing conditions. The effect of oxygen on surface tension and wetting angles was verified by statistical analysis using the Kruskal–Wallis test, which supported the results stating that the values of these quantities decreased with increasing oxygen content. Furthermore, liquidus temperatures, which are of practical importance, were determined by the optical and DTA methods and then compared with theoretically calculated temperature values. It turned out that the increased chromium content causes difficulties in determining surface tension up to 1550 °C due to the formation of a thin Cr2O3 layer. In addition, SEM and XRD analyses accompanied by calculations in the FactSage oxide database were performed to better understand the wetting mechanism.

Experimental phase equilibria study and thermodynamic modelling of the PbO-“FeO”-SiO2-ZnO, PbO-“FeO”-SiO2-Al2O3 and PbO-“FeO”-SiO2-MgO systems in equilibrium with metallic Pb and Fe

Phase equilibria of the PbO-“FeO”-SiO2-ZnO, PbO-“FeO”-SiO2-Al2O3 and PbO-“FeO”-SiO2-MgO slags with liquid Pb metal, solid or liquid Fe metal and solid oxides (cristobalite and tridymite SiO2, willemite (Zn,Fe)2SiO4, wustite (Fe,Al)O1+x, spinel (Fe,Al)3O4, olivine Fe2SiO4, corundum (Al,Fe)2O3, mullite Al6Si2O13 and pyroxene (Mg,Fe)SiO3) were investigated at 1125–1670 °C. These conditions correspond to the minimum solubility of PbO in slag in presence of Pb and Fe metals at reducing conditions and represent the limit of lead smelting and slag cleaning process. High-temperature equilibration on silica, corundum or iron foil substrates, followed by quenching and direct measurement of Pb, Fe, Si, Zn, Al and Mg concentrations in the liquid and solid phases with the electron probe X-ray microanalysis (EPMA) was used. Present data can be used to improve the thermodynamic models for all phases in this system.

Effect of applied voltage on wetting and corrosion of corundum refractory by CaO–SiO2–MgO molten slag

The wetting and corrosion behavior of the corundum substrate anode by CaO–SiO2–MgO molten slag was investigated via the joint application of the sessile drop method with applied voltage and SEM-EDS technique. The slag drop exhibited a good wettability on the corundum substrate. The apparent contact angle at zero voltage slightly exceeded that at a 1 V applied voltage but was lower than those at 1.5 V and 2 V ones. Low applied voltage of 1 V had little effect on the corundum substrate's direct dissolution corrosion processes; high ones of not less than 1.5 V caused the electrode reaction to occur. The stirring effect of O2 bubbles from the anode reaction aggravated the substrate's direct dissolution and physical stripping. It was found that the applied voltage could inhibit the slag penetration, and the apparent contact angle had no obvious relation with the direct dissolution thickness and penetration depth. A thin but almost continuous MgO⋅Al2O3 (MA) layer could form at the slag/substrate interface at the applied voltage of 1.5 V. These results indicate the positive effect of applied voltage on the distribution of interfacial products and the molten slag penetration in reducing the corrosion of corundum anode under certain conditions. However, when the applied voltage was too high, the vigorous electrode reaction could aggravate the direct dissolution and physical stripping of the corundum anode, and damage the continuation of the formed interface product layer with a high melting point.

Interfacial phenomena between alumina substrate and nickel containing low-alloy steel

A comprehensive description of the phenomena at the low-alloy Fe-C-O-Ni steel/corundum interface at elevated temperatures up to 1600 °C was performed using the sessile drop method, Differential Thermal Analysis (DTA), 3D Differential Scanning Calorimetry (3D DSC) as well as several supporting methods, namely Energy Dispersive X-Ray (EDX), X-Ray Diffraction (XRD) analyses and Scanning Electron Microscopy (SEM). Steel samples with different carbon (0.04–1.56 wt%) and nickel (1.08–4.48 wt%) contents were selected to assess the effect of temperature and chemical composition on the related quantities, namely density, surface and interfacial tension, wetting angles, and the work of adhesion. While increased nickel content caused a decrease in wetting angle, and surface and interfacial tension values, the increased carbon content had the opposite effect. Two distinctly different modes of high-temperature interaction at the phase interface were also described. Little interaction was observed between the steel with lower carbon content (<0.1 wt%) and the corundum substrate. On the contrary, a more substantial interaction occurred in steel with higher carbon content and thus lower liquidus temperature. There was significant disruption of the corundum substrate at the surface and in depth up to 140 µm. The original corundum structure was replaced by new structural phases, particularly hibonite, grossite and gehlenite. In addition, when a higher chromium content was present in the steel, chromium-doped corundum was formed.

The slagging propensity of Zn-rich scrap tyre ash in reducing environments consisting of CO and CO2 and its promotion of the slagging of Si-rich coal ash

This study includes a comprehensive investigation on the ash slagging characteristics of zinc (Zn)-rich tyre ash and its interaction with silica (Si)-rich coal ash blended at different ratios in a 5% CO in CO2 reducing environment. Such an environment is expected to occur near the burner in a combustor, as well as serves as a fundamental study on the slagging propensity upon the effect of CO, which is a principal reducing agent in a gasifier. An approach consisting of experimental studies using modified inclined plane (M-IP) methodology, advanced analytical techniques including the scanning electron microscopy (SEM) and x-ray photoelectron (XPS) spectroscopy, and thermodynamic equilibrium calculations were used to elaborate the slagging propensities of the single and blended ashes. Furthermore, the fate of Zn within tyre ash and its potential implications on the slagging properties were evaluated. Tyre ash easily melts to liquid slag upon increasing temperature compared to the silica-rich coal ash. Zinc does not evaporate but rather remains within tyre slag mostly as a hemimorphite (Zn4(Si2O7)(OH))-like coordination structure attributing to the characteristic blue colour of tyre slag. The low ionic potential value of 2.7 for Zn2+ is characteristic of basic ions that have a greater tendency to depolymerise the Si-rich ash slag matrix and acts as a fluxing agent in ash slag. Moreover, it was proven that Zn2+ plays a synergistic role with Ca2+ in enhancing the flowability of the Si-rich slag remarkably. Furthermore, for the pure tyre slag, Zn was found to preferentially deposit at the corundum (Al2O3) substrate interface and even penetrate/react with the porous corundum, causing a strong corrosion on the substrate. However, blending of tyre ash with coal ash alleviates this problem by a preferred scavenging of Zn into the entire Si matrix, which in turn protects the corundum plate.

Formation of the Phase Composition, Porous Structure, and Morphology of Membrane Catalysts Based on Tungsten Carbides

Membrane catalysts are prepared by the chemical vapor-phase deposition of tungsten oxides on a corundum porous microfiltration membrane. The massive layer of tungsten oxides formed on the substrate is converted to carbides by temperature-programmed carbidization with С3Н8/Н2 and СН4/Н2 mixtures. Changes in the porous structure and phase composition of the catalytic layer at stages of vapor-phase deposition and carbidization of oxides deposited on the corundum substrate are considered. Dependence of the depth of distribution of tungsten compounds on the deposition temperature is studied by electron scanning microscopy and X-ray spectroscopy. A relationship is established between the synthesis conditions of tungsten carbide-based membrane catalysts and their phase composition, porous structure characteristics, and morphology.

Synthesis and Sensor Characteristics of Nanoscale Thin Films in the In2O3–SnO2 and Y2O3–TbOx(CeOx) Systems

(In2O3)0.95(SnO2)0.05, (TbOх)0.50(Y2O3)0.50, and (CeO2)0.80(Y2O3)0.20 single-phase cubic solid solution xerogels and nanocrystalline powders have been synthesized via coprecipitation of hydroxides. Using these powders as precursors, nanoscale film electroconductive materials on a corundum substrate have been produced by screen printing. The highest speed of sensor response to ozone is attributed to In0.95Sn0.05O1.5 films, reaching 8–10 s at a temperature of 473 K. (TbOх)0.50(Y2O3)0.50 thin film is found to be sensitive to changes in the concentrations of (CO + CO2) (the sensitivity coefficient S = 25.11) and ozone (S = 3.16–7.94). (CeO2)0.80(Y2O3)0.20 film exhibits a sensor response to (CO + CO2) (S = 3.10). This study opens up the prospects of using the obtained nanopowders and relevant thin films as materials for resistive gas sensors operating at oxygen pressures of 103–104 Pa.

Etching by a beam of fast argon atoms of gas dynamic grooves on the surface of ceramic substrates

A method of manufacturing the grooves of the end seals from high-strength dielectric materials is proposed. Uneven depth (no more than 10%) imposes a limit on the processing time. Grooves with a depth of ∼5 μm in corundum substrates can be made using a source of fast argon atoms in ∼1 hour with their energy of 3 keV and equivalent beam current of 1 A.

Growth of ultrathin Ru oxide films on perovskite and corundum substrates

The ability to control the dispersion of RuO2 monolayers on a range of perovskite- and corundum-structured oxide substrates was investigated using complementary experiment and theory. Controlled deposition under UHV conditions and subsequent in situ characterization by x-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS), reflection high energy electron diffraction (RHEED) and low energy electron diffraction (LEED) was used to follow the chemical state, dispersion and structure of the RuO2 layers as a function of growth and post-growth processing conditions. The experimental results reveal that monolayer RuO2 on ferroelectric PbxZr1-xTiO3(001) and non-polar SrTiO3(001) α-Al2O3(11¯02) tends to dewet the surface to form three-dimensional clusters, though some kinetic stability of dispersed RuO2 monolayers was achieved on the SrTiO3 surface. Although an RuO2 termination is not favored for SrRuO3(001), deposition onto SrRuO3(001) yields a stable RuO2 surface which ISS suggests is capped by chemisorbed oxygen. The complementary density functional theory modeling and ab initio thermodynamics indicate that Ru oxide monolayers on the perovskite substrates tend to be unstable under the low oxygen pressure experimental conditions; however, either lower temperatures or higher oxygen pressures can lead to a kinetic and even thermodynamic stability. The theoretical findings also provide guidance on how the instability of dispersed RuO2 monolayers can be overcome by inserting buffer layers and increasing the partial pressure of molecules that selectively, strongly adsorb at coordinatively unsaturated Ru sites on the oxide surface.

Interface reactions between rutile coatings and molten aluminium or AlSi7Mg0.6 alloy

Rutile coatings deposited on corundum substrates are considered as promising functional elements improving the efficiency of the filtration of oxide inclusions out of aluminium melts. This contribution describes the reactions between rutile and two kinds of the aluminium melts and discusses the consequences of these reactions for the filtration process. It was found that the contact of rutile coatings with molten aluminium leads to the formation of a corundum layer at the solid/liquid interface. The exposure of the rutile coatings to molten AlSi7Mg0.6 alloy produces an interface layer of MgTiO3. The interface layers possess defined orientation relationship to rutile which is characteristic for locally heteroepitaxial growth. The density functional theory calculations revealed that the TiO2/α-Al2O3 and TiO2/MgTiO3 interfaces with the orientation relationships observed experimentally have low interface energies. The mechanisms of the interface layer formation and the impact of these layers on the degradation of the rutile coatings are discussed.

Post-deposition annealing of thin RF magnetron sputter-deposited VO2 films above the melting point

Vanadium dioxide (VO2) is a promising material for electronics and optics due to abrupt change in conductivity of VO2 that occurs at ∼68 °C, which is the metal-insulator transition temperature. The abruptness and magnitude of this transition is governed by the crystal structure of VO2. The growth of thin VO2 films therefore requires substrate heating during deposition and, as well as the occasional need for additional post-deposition annealing. The processes that occur during annealing depend upon the deposition technique. However, the published reports related to these studies are rarely systematic and lack consistency. The annealing of thin nanocrystalline VO2+x films at temperatures above the melting point is reported here. Thin films were deposited onto a polycrystalline corundum substrate by means of radio-frequency reactive magnetron sputtering, with the thin film composition fine-tuned by applying negative substrate bias during deposition and proceeding with ex-situ control of the vanadium oxidation state. Post-deposition annealing causes recrystallization, which is in accordance with the VO phase diagram. No epitaxial or other substrate-induced processes were detected. However, the sharpest transition was attributed to the film possessing the most uniform grain size, as opposed to the film with the largest VO2 crystallites. The temperature dependency of the microwave radiation transmittance was investigated over the 8–18 GHz range, with the samples measured in free-space conditions. This was intended to reveal any frequency-dependent features, which can possibly appear as benefits or obstacles during the development of designs in the microwave range that are based on the VO2 films being studied.

Influence of a high-power pulsed ion beam on the mechanical properties of corundum ceramics

The mechanical properties of near-surface layers of corundum ceramics treated by high-power pulsed ion beam of carbon are investigated. The samples for investigation were prepared from corundum substrate, which is usually used in microelectronic. The ion treatment was carried out at the TEMP-4M facility under the following conditions: an accelerating voltage of 160-200 keV, the current density in the pulse varied within 15-85 A/cm2. It was found that ion irradiation changes the structure and properties of near-surface layers of corundum ceramics. At the same time, melting and erosion of the surface layer takes place. These processes are accompanied by the formation of a network of microcracks. Microcracks are propagated only by the depth of melting layer. The mechanical properties were measured using a NanoTest600 nanohardness testing instrument. It was found that the nanohardness depends of the treatment modes. At a current density of 15A/cm2, with an increase treatment dose, the nanohardness of the irradiated surface layer increases in comparison with the initial value before irradiation. At higher current densities, the nanohardness of irradiated ceramics decreases relatively to the initial value before irradiation. The dependences of nanohardness off the irradiation dose in this case have the view of a curves with a minimum at irradiation doses of 2.5·1014 and 1.3·1014 cm-2, for current densities of 50 and 85 A/cm2, respectively.

Xinjiang lignite ash slagging and flowability under the weak reducing environment at 1300 °C – A new method to quantify slag flow velocity and its correlation with slag properties

In this study, a novel measurement method for the direct visualisation and quantification of lignite slag flowability has been established. The ash slagging was tested under a weak reducing environment (1% CO2 in nitrogen) at temperature range from 1000 to 1300°C to mimic a cyclone combustion furnace for the Chinese Xinjiang lignite. Five different ash samples with a broad variation on the basicity, i.e. base/acid ratio from 1.98 to 0.27, and their blends with up to 40wt% clay were loaded on a 25°–inclined corundum substrate and exposed to the above-mentioned reducing gases in a pre-heated horizontal furnace. The thermodynamic equilibrium software, FactSage 6.4 and typical viscosity equations were also employed to quantify the liquid fraction in each slag and its viscosity, respectively. Efforts were further made to correlate the calculated liquid fraction and viscosity of a slag and its flow velocity. As has been found, the neutral ash with the lowest ash fusion temperature is the only original ash sample that can melt and flow at 1300°C without the use of clay additive, although its flow velocity is quite marginal. The addition of clay into basic ashes, at 8–10wt% is able to improve the slag flow velocity by 6–7 times at 1300°C. Both ash basicity and liquid fraction within an ash are critical in determining the slag flow velocity. However, neither has proven to be a sole and sufficient factor affecting the slag flowability. The slag viscosity is believed to be a key factor counter balancing the liquid fraction within a slag. This hypothesis was proven by the addition of 5–10wt% MgO into a basic ash that decreased both the liquidus fraction and slag viscosity. Since the decrease on the slag viscosity (based on the calculation) was more remarkable than the liquidus fraction, the slag flow velocity was improved consequently. Apart from providing a new method to quantify slag flowability, this study also paved a direction for the future study focusing on lignite ash slag viscosity and modelling approach to correlate lignite slag properties and its flow velocity. A universal method has yet to be established for either of them.

Thermocapillary migration mechanism of molten silicon droplets on horizontal solid surfaces

Effective lubrication under extreme conditions such as high temperature is of considerable importance to ensure the reliability of a mechanical system. New lubricants that can endure high temperatures should be studied and employed as alternatives to traditional oil-based lubricant. In this paper, a thermocapillary model of a silicone-oil droplet is developed by solving the Navier–Stokes and energy equations to obtain the flow, pressure, and temperature fields. This is accomplished using a conservative microfluidic two-phase flow level set method designed to track the interface between two immiscible fluids. The numerical simulation accuracy is examined by comparing the numerical results with experimental results obtained for a silicone-oil droplet. Hence, the movement and deformation of molten silicon droplets on graphite and corundum are numerically simulated. The results show that a temperature gradient causes a tension gradient on the droplet surface, which in turn creates a thermocapillary vortex. As the vortex develops, the droplet migrates to the low-temperature zone. In the initial stage, the molten silicon droplet on the corundum substrate forms two opposite vortex cells, whereas two pairs of opposite vortices are formed in the silicone fluid on the graphite substrate. Multiple vortex cells gradually develop into a single vortex cell, and the migration velocity tends to be stable. The greater the basal temperature gradient, the stronger the internal thermocapillary convection of the molten silicon droplet has, which yields higher speeds.

Morphology of polycrystalline cassiterite films

Polycrystalline cassiterite films have been grown by the hydropyrolytic method from a 10(H2O) + 5(SnCl2 · 2H2O) solution (in weight fractions) on corundum substrates. The crystallization regularities are considered and a comparative analysis of the properties of natural and artificial cassiterite crystals is performed. The surface morphology is investigated and the size of crystalline grains is determined by scanning electron microscopy. X-ray microprobe analysis showed that all films contain tin and oxygen atoms in a ratio corresponding (within the experimental error) to the chemical formula of tin dioxide. It is established that the surface morphology of cassiterite films is characterized by both single crystallites and aggregates of two or more crystals typical of twins. It is suggest that doping can efficiently be used to control the concentration of twins and the stability of their formation.