Sintering Of Oxide Research Articles

Effects of Co content in TiO2–Co composite prepared by alloying–recomposition–oxidation–sintering process on its mechanical properties and microstructure

TiO2–Co composite powders with various Co contents were prepared by the alloying-recomposition-oxidation-sintering process. For comparison, conventionally mixed TiO2–Co composites with the same compositions were sintered at 1000, 1100, 1200, 1300, and 1400℃. Structural characterizations were performed using X-ray diffraction, field-emission scanning electron microscopy, energy-dispersive spectroscopy, and transmission electron microscopy. All of the sintered samples were more densified. A melted matrix was observed at a temperature higher than 1300℃. The flexural strength and the fracture toughness of the TCA sample were higher than those of the conventionally mixed TiO2–Co sample at the same sintering temperature, while the Vickers hardness exhibited the opposite relationship. The flexural strength and the fracture toughness of the TCA sample increased until a Co content of 14 vol%, followed by decrease at 18 vol%, while those of the conventionally mixed TiO2–Co sample increased in the entire Co content range. The highest flexural strength and fracture toughness were observed for T14CA sintered at 1400℃ (161.3 MPa and 6.39 MPa m−1/2, respectively). Consequently, the desirable Co content in the TiO2–Co composite prepared by the alloying–recomposition–oxidation–sintering process was 14 vol%.

The Influence of Cu and Al Additives on Reduction of Iron(III) Oxide: In Situ XRD and XANES Study.

The reduction of Fe-based nanocomposite catalysts doped with Al and Cu has been studied using in situ X-ray diffraction (XRD), in situ X-ray absorption near-edge structure (XANES), and temperature-programmed reduction (TPR) techniques. The catalysts have been synthesized by melting of iron, aluminum, and copper salts. According to XRD, the catalysts consist mainly of Fe2O3 and Al2O3 phases. Alumina is in an amorphous state, whereas iron oxide forms nanoparticles with the protohematite structure. The Al3+ cations are partially dissolved in the Fe2O3 lattice. Due to strong alumina-iron oxide interaction, the specific surface area of the catalysts increases significantly. TPR and XANES data indicate that copper forms highly dispersed surface CuO nanoparticles and partially dissolves in iron oxide. It has been shown that the reduction of iron(III) oxide by CO proceeds via two routes: a direct two-stage reduction of iron(III) oxide to metal (Fe2O3 → Fe3O4 → Fe) or an indirect three-stage reduction with the formation of FeO intermediate phases (Fe2O3 → Fe3O4 → FeO → Fe). The introduction of Al into Fe2O3 leads to a decrease in the rate for all reduction steps. In addition, the introduction of Al stabilizes small Fe3O4 particles and prevents further sintering of the iron oxide. The mechanism of stabilization is associated with the formation of Fe3- xAl xO4 solid solution. The addition of copper to the Fe-Al catalyst leads to the formation of highly dispersed CuO particles on the catalyst surface and a mixed oxide with a spinel-type crystalline structure similar to that of CuFe2O4. The low-temperature reduction of Cu2+ to Cu0 accelerates the Fe2O3 → Fe3O4 and FeO → Fe transformations but does not affect the Fe3O4 → FeO/Fe stages. These changes in the reduction properties significantly affect the catalytic performance of the Fe-based nanocomposite catalysts in the low-temperature oxidation of CO.

High-entropy oxide phases with magnetoplumbite structure

Abstract Sintering of oxides and carbonates at 1400 °C gives crystalline high-entropy single phase products with a BaFe12O19 (magnetoplumbite) structure containing 5 doping elements at high concentration level: Ba(Fe6Ti1·2Co1·2In1.2Ga1.2Cr1.2)O19. The complete list of explored substitutions includes K, Ca, Sr, Pb, La, Bi, Al, Ga, In, Ti, V, Cr, Mn, Co, and Ni. Loading to the batch more than 5 dopants or introduction of NiO, or V2O5 initiates formation of second phases like spinels or vanadates. Bi2O3 and K2O are too volatile at sintering temperature and were evaporated from the samples. Due to large ionic radius, the In3+ cation is likely to be incorporated not only on Fe3+ sites, but also on Ba2+ sites, that follow from resulting crystal composition. The smallest di-valent and tri-valent cations, Sr2+ and Al3+, are found to preferably concentrate together in the same magnetoplumbite phase crystals within one sample. According to elemental analysis of selected hexagonal crystals in the investigated 8 doped samples the most prospective compositions for obtaining high-entropy single phase with magnetoplumbite structure can be formulated as (Ca,Sr,Ba,Pb,La)Fex(Al,Ga,In,Ti,Cr,Mn,Co)12-xO19 where x = 1.5–6.

Mechanism studies of hydrothermal cold sintering of zinc oxide at near room temperature

AbstractZinc oxide densification mechanisms occurring during the cold sintering process (CSP) are examined by investigating specifically the effects of ion concentration in solution, temperature, pressure, and die sealing. The experiments suggest that mass transport through solution is a primary densification mechanism and that either a pre‐loaded solution or grain dissolution can supply migrating ions. Additionally, results indicate cold sintering zinc oxide requires a critical pressure value, above which densification is relatively pressure independent under the majority of process conditions. This critical pressure is related to thermal expansion of the liquid and determines the uniaxial pressure threshold for densification. The data supports a three‐stage interpretation of cold sintering, which includes quick compaction, grain rearrangement, and dissolution‐reprecipitation events. Further, it is observed that under the lowest temperature conditions a net decrease in particle size can occur during the cold sintering process.

Oxidation and ablation protection of plasma sprayed LaB6-MoSi2-ZrB2 coating for carbon/carbon composites

To improve the high-temperature stability of carbon/carbon (C/C) composites, a LaB6-MoSi2-ZrB2 (LMZ) coating was deposited on SiC-encapsulated C/C composites by supersonic atmospheric plasma spraying. The sprayed LMZ coating showed a significant improvement of oxidation and ablation protective abilities relative to MoSi2 and ZrB2 coatings, which could protect C/C composites for more than 80 h at 1773 K in static air and 60 s above 2000 K under oxyacetylene flame. La and Zr acted as the role in accelerating the liquid phase sintering of SiO2-based oxides, forming a compact Si-Zr-La-O compound scale to enhance the high-temperature stability of LMZ coating.

Dielectric relaxation and electrical properties of Bi2.5Nd0.5Nb1.5Fe0.5O9 ceramics

Abstract Aurivillius single-phase Bi2.5Nd0.5Nb1.5Fe0.5O9 (BNNF) ceramics was synthesized via solid state mixed oxide sintering method. Electrical properties of the samples were intensely studied in a wide range of temperature (25 °C–650 °C). Dielectric behaviors were investigated by dielectric and impedance spectroscopies in the range (40 Hz−10 MHz) of frequency. Single dielectric relaxation was observed due to the grains contribution. The phase transition temperature (TC) was noticed at ∼270 °C. Kinetic analysis of dielectric data was used to understand the possible relaxation-conduction behaviors in the ceramics. Above ∼250 °C, relaxation and conduction of the ceramics was assigned to the motion of ionized oxygen vacancies. The Aurivillius ceramics has promising industrial applications in dielectric capacitors and piezoelectric transducers due to low leakage below ∼200 °C. The results could be valuable for designing and/or modifying properties of BNNF related materials.

Highly-porous hierarchical SiC structures obtained by filament printing and partial sintering

Three-dimensional (3D) porous silicon carbide (SiC) structures with total porosity in the range of 64–85% are achieved from nanosized SiC powders by filament printing and partial sintering in a spark plasma sintering (SPS) furnace. The effects of the SPS temperature (1500 and 1700 °C) and the addition of oxide sintering aids (7 wt. % Y2O3+Al2O3) on the porosity of the scaffolds are quantitatively compared. More specifically, hierarchical porosity consisting of meso-pores (< 50 nm) in the struts and open macro-pores (in the range of 500–700 μm) defined by the patterned structure is verified for the 1500 °C SPS treatment. The strength of the structures as a function of the density as well as their cooling profiles, once subjected to rapid heating under a direct flame, are analyzed. By following this easy route, an adaptable family of robust light 3D SiC structures with hierarchical porosity is accomplished.

The role of point defects and defect gradients in flash sintering of perovskite oxides

The present study investigates the impact of point defects and their redistribution on the flash sintering process. Strontium titanate was chosen as a model system for the group of perovskite ceramics. The characteristics of flash sintering of strontium titanate were analyzed with different acceptor dopant concentrations. The onset of flash sintering was found to be dependent on the acceptor dopant concentration, as expected by the increasing conductivity. A gradient in the microstructure was found after flash sintering with larger grain sizes at the negative electrode. TEM-EDS measurements indicated Ti enrichment at the positive electrode for undoped strontium titanate and strong acceptor segregation for doped strontium titanate. In contrast, grain boundaries at the negative electrode were found to be stoichiometric for the undoped case and the acceptor segregation was less obvious for the doped case.Based on these results and the space charge behavior of strontium titanate, we infer that a gradient of the oxygen vacancy concentration is induced by the electric field during flash sintering: at the positive electrode the oxygen vacancy concentration is higher than at the negative electrode. For strontium titanate it is well known that a high oxygen vacancy concentration reduces the space charge and, hence, acceptor segregation, which agrees well with the experimental findings. Overall, the present study highlights the importance of point defect gradients and space charge for flash sintering of strontium titanate, which may be applicable for many other functional ceramics.

On the Phase Associations and Crystal-Chemical Inheritance of Be–Mg–Al Silicates during Subsolidus Crystallization

During the solid-phase synthesis of Be–Mg–Al silicates, i.e., beryllian indialite (BI), the crystal-chemical proximity is due to the transfer of elements of chemical and structural inheritance. BI crystallization from a gel is carried out via the structural adaptation of multicomponent metastable phases transforming into BI, which is implemented in two stages: diffusion and the formation of microcrystallites fed by a melt film. During the sintering of dry oxides, parageneses are transferred from a previous temperature stage to the subsequent one so that several phases of the former stage are retained and the fraction of stable phases increases because of the consumption of intermediate phases. Phase transformations whereby BI is crystalized in the subsolidus region are interpreted from the viewpoint of the evolutionary regularities of mineralogenesis.

Fabrication of Yb-doped Lu2O3-Y2O3-La2O3 solid solutions transparent ceramics by self-propagating high-temperature synthesis and vacuum sintering

A comparative analysis of sintering and grain growth processes of lutetium oxide and lutetium-yttrium-lanthanum oxides solid solutions, as well as optical properties, luminescence and laser generation of (LuxY0.9-xLa0.05Yb0.05)2O3 transparent ceramics are reported. Fabrication of highly dispersed initial powders of these compounds was performed via nitrate-glycine self-propagating high-temperature synthesis (SHS) method. The powders were compacted at 300 MPa and vacuum sintered at temperatures up to 1750 °С. Optical ceramics of (Lu0.65Y0.25La0.05Yb0.05)2O3 elemental composition were shown to have the highest in-line transmittance, which achieved 78% at the wavelength of 800 nm. Generation of laser radiation at a wavelength of 1032 nm with the differential efficiency of 20% was demonstrated in the (Lu0.65Y0.25La0.05Yb0.05)2O3 ceramics.

Investigation of the effects of sintering and indium-doping of zinc oxide using 67Zn magic angle spinning NMR analysis

Indium-doped zinc oxide, a potential alternative material to indium tin oxide, was analyzed in powder form via 67Zn magic angle spinning nuclear magnetic resonance (MAS NMR). The 67Zn MAS NMR results show that the line shapes of zinc oxide were broadened by sintering, which was also observed for indium-doped zinc oxide, in which the broadening also depended on the sintering time. Furthermore, the line shapes of indium-doped zinc oxide were significantly broader than those of the corresponding zinc oxide, and were independent of the degree of indium doping. This indicates that indium atoms are associated into cluster-like structures in this compound.

Green laser sintering of copper oxide (CuO) nano particle (NP) film to form Cu conductive lines

Copper oxide (CuO) nanoparticle (NP) ink is a potential candidate for low-cost alternatives to other metal-based nano-particle inks (e.g., Au, Ag.) in printed electronics. To obtain Cu patterns from CuO NP ink, CuO NP inks should be converted to Cu particles, and be fused to form a connected conductive line. For this purpose, photonic sintering methods have been widely used, which generate the heat required for sintering via the absorption of light. In this study, we used continuous wave (CW) green laser with 532 nm wavelength, since the laser has the advantage of selective sintering by irradiation of light only on the target place. We investigated the optimal sintering parameters, such as laser power and scanning speed, using the green laser, in order to obtain low resistivity. We also investigated the pre-treatment conditions, such as pre-baking, which is required to evaporate solvents in the ink. We found that over-baking of deposited film will adversely affect the sintering, because film can be easily damaged from laser irradiation. As a result of laser sintering, we obtained the resistivity of (9.5 and 71.6) μΩ·cm when the pre-baked thicknesses of CuO films were (546 and 889) nm, respectively. In such cases, the thicknesses were significantly reduced to (141 and 270) nm, respectively.

Effect of heat treatment on the creep behavior of α/β SiAlON sintered with multication oxide sintering additives

AbstractSiAlONs are important materials for high‐temperature applications and creep properties of SiAlONs are largely controlled by the amount and type of sintering additives. It has been established that heat treatment can reduce the amount of amorphous intergranular phase through crystallization. However, there is no study on the creep behavior of heat‐treated SiAlON ceramics containing multication sintering additives. Therefore, the aim of the study was to investigate the effect of heat treatment on the creep properties of multication containing (Y‐Sm‐Ca oxides) α/β‐SiAlON ceramics. The heat treatments of the sintered samples were carried out at 1600°C for 2 hours. The creep tests were carried out in the range 1300‐1400°C under different loads (50‐150 MPa). The existing phases and the microstructures of samples before and after creep were investigated using XRD and SEM techniques. It was found that heat treatment resulted in a better creep performance compared to as‐sintered samples. The activation energy and stress exponent for heat‐treated SiAlONs were also calculated as 708 ± 45 kJ/mol and 1.4, respectively. Compared to the sintered sample values, the results suggested that the acting creep mechanism of grain‐boundary sliding and cavitation was reduced with the heat treatment.

The relationship between phase composition and conditions of sintering seawater derived magnesium oxide

This study has focused on the favourable effect of the TiO2 addition (1, 2 and 5 wt%) on the reduction of B2O3 content during activated sintering of magnesium oxide from seawater at temperatures of 1400, 1500 and 1700°C for the duration of 1, 2 and 4 h. A mathematical model of dependence between the B2O3 mass fraction in the sintered sample, the temperature of isothermal sintering, the isothermal sintering time and the mass fraction of TiO2 added have been proposed. Magnesium oxide was obtained from seawater by substoichiometric precipitation, with the addition of 80% of dolomite lime as the precipitation agent. New phases formed in magnesium oxide samples were examined by the XRD and SEM/EDS analysis. The results indicate that during activated sintering of seawater-derived magnesium oxide with a TiO2 addition, reactions of formation of Ca2B2O5, CaTiO3 and Mg2TiO4 took place simultaneously. The thermodynamics analysis of experimental results, based on the Onsager reciprocity relations (symmetry relations), was applied and phenomenological coefficients were calculated to describe the interference of these three irreversible processes.

Creation of individual few-layer graphene incorporated in an aluminum matrix

3D-networks of few-layer graphene (FLG) platelets at grain boundaries, sandwiched between thin amorphous Al2O3 layers, were fabricated by spark plasma sintering (SPS) of graphene oxide (GO)/Al mixed powders. The GO was prepared by a modified Hummers’ method, and was thermally reduced to FLG simultaneously during SPS densification. Subsequent plastic flow of the Al matrix during the hot extrusion process caused the destruction of this structure, rearranged the FLG platelets individually into the uniaxial direction, and made them incorporate in the Al matrix. Observations by high-resolution transmission electron microscopy proved the existence of a direct-contact interface between the FLG and the Al matrix without any interfacial compounds, and revealed that the Al matrix featured a fairly low dislocation density. Consequently, the mechanical strength of Al matrix was noticeably enhanced by FLG incorporation, agreeing with the potential strengthening effect predicted by the load transfer mechanism.

Evaluation of mixed oxide formation and sintering behavior in thermal barrier coatings on nickel‐based superalloy

AbstractThermal barrier coatings are widely used in aircraft turbines to protect nickel‐based superalloys from the effect of high temperature oxidation and hot corrosion. In this study, both NiCrAlY bond coat and yttria‐stabilized zirconia top coat were deposited using atmospheric plasma spray technique. After coating production, specimens were exposed to oxidation in air atmosphere at 900 °C, 1000 °C and 1100 °C for different periods of time up to 50 h. Microstructural transformations in the ceramic top coat and growth behavior of the thermally grown oxide layer were examined using scanning electron microscopy, porosity calculation, elemental mapping and hardness measurement. Formation of different types of oxides in the thermally grown oxide layer shows that this process strongly depends on deposition technique as well as on oxidation time and temperature. Hardness values of the top coat increased with a decrease in the porosity of the top coat. Uniformity and homogeneity of the thermally grown oxide layer and densification of the top coat were evaluated in terms of the structural durability of thermal barrier coating systems.

Volumetric Shrinkage Characteristics and Kinetics Analysis of Vanadium Titanomagnetite Carbon Composite Hot Briquette during Isothermal Reduction

The volumetric shrinking characteristics and kinetics of vanadium titanomagnetite carbon composite hot briquette (VTM-CCB) during isothermal reduction were investigated in this paper. It was found that the volumetric shrinking occurs as a combination of a loss of carbon and oxygen from VTM-CCB as well as sintering of iron oxide, suppression of growth of iron whiskers, formation of molten phases, and residual solid carbon content. In the temperature range from 900°C to 1100°C, the carbon gasification is viewed as the primary factor for the shrinking behavior. In the temperature range higher than 1100°C, the shrinking of VTM-CCB is mainly caused by the formation of molten slag, liquid iron, and residual solid carbon. It also should be pointed out that, in the higher temperature range, the volumetric shrinkage of VTM-CCB decreased with increasing FC/O ratio, which is mainly due to the fact that a higher FC/O ratio leads to a decreasing of molten slag generation proportion and a suppression of aggregation growth of liquid iron by the increasing of residual solid carbon. Based on the kinetics analysis, The formula for the shrinkage of VTM-CCB during reduction can be obtained, and the value of shrinking activation energy of VTM-CCB decreased obviously from 337.86 kJ/mol to 84.85 kJ/mol with increasing FC/O ratio from 0.8 to 1.4.

Phase stability, microstructure and high-temperature properties of NbSi2- and TaSi2-oxide conducting ceramic composites

NbSi2- and TaSi2-based electroconductive ceramic composites with the addition of 40–70 vol% Al2O3 and ZrO2 particles were fabricated by high-temperature sintering (1400–1600 °C) under argon. Their phase stability, microstructural evolution, oxidation kinetics and electrical properties were studied at high temperatures. The densification of the composites was improved by increasing the oxide phase content and sintering temperature. The interaction of the starting metal disilicides with residual oxygen sources resulted in the formation of the hexagonal-structured 5–3 metal silicide (Nb5Si3 and Ta5Si3) phases. The increasing sintering temperature and volume percentage of the oxide phase reduced the pest oxidation, particularly for the silicide–alumina composites, which exhibited lower oxidation-induced mass changes than their dense monolithic metal silicides. Depending on the silicide–oxide volume percentage, their electrical conductivities ranged from 5.3 to 111.3 S/cm at 900 °C. Their phase stability, reduced oxidation rates and high electrical conductivities at high temperatures show promise for future high-temperature applications in advanced sensing.

Fabrication and properties of Si3N4 bioscaffolds with orderly–interconnected big pore channels and well–distributed small pores

This paper focuses on investigating the technical potential for fabricating porous ceramic bioscaffolds for the repair of osseous defects from trauma or disease by inverse replication of three–dimensional (3–D) printed polymer template. Si3N4 ceramics with pore structure comprising orderly–interconnected big pore channels and well–distributed small pores are successfully fabricated by a technique combining 3–D printing, vacuum suction filtration and oxidation sintering. The Si3N4 ceramics fabricated from the Si3N4 powder with addition of 10 wt% talcum by sintering at 1250 °C for 2 h have little deformation, uniform microstructure, low linear shrinkage of 4.1%, high open porosity of 58.2%, relatively high compression strength of 6.4 MPa, orderly–interconnected big pore channels and well–distributed small pores, which are promising bioscaffold in the field of bone tissue engineering.

Successful Laser Sintering of Aluminum Oxide

Successful Laser Sintering of Aluminum Oxide