Oxidation behavior and electrical properties of metal interconnects with Ce-doped Ni–Mn spinel coatings

To improve the high-temperature oxidation resistance and electrical conductivity of ferritic stainless steels, protective Ce-doped NiMn₂O₄ spinel coatings were fabricated on the surface of SUS430 steel by electrophoretic deposition (EPD). The phase structure and microstructure of Ce-doped NiMn₂O₄ in both powder and coating forms were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The high-temperature oxidation of the NiMn₂O₄ spinel coating before and after Ce doping in the air at 800 °C for 168 h was studied by weight gain experiments. The area-specific resistance (ASR) of coatings was measured by a standard four-probe method. It was found that the Ce-doped NiMn₂O₄ spinel powder displayed a stable structure, high crystallinity, fine grain size, and decreased agglomeration when the Ce content was fixed at 0.05 mol?L^?1. The oxidation kinetics of NiMn₂O₄-coated SUS430 steel before and after Ce doping obeyed a parabolic law with parabolic rate constants of 4.58 × 10^?15 g² cm^?4 s^?1 and 1.83 × 10^?15 g² cm^?4 s^?1, respectively. When oxidized at 800 °C for 50 h, the ASR value of the coated samples before and after Ce doping stabilized at about 15.2 mΩ?cm² and 14.5 mΩ?cm², respectively. This work demonstrated that the Ce-doped NiMn₂O₄ spinel coating improved the high-temperature oxidation resistance and the electrical conductivity of metal interconnects.     

Effect of preparation temperature on the formation of Sr2CeO4 phosphor particles in the spray pyrolysis

Sr₂CeO₄ phosphor particles were prepared by spray pyrolysis at various preparation temperatures. The effect of preparation temperatures on the morphology, crystal structure and photoluminescence characteristics of the post-treated Sr₂CeO₄ phosphor particles was studied. Phase pure Sr₂CeO₄ phosphor particles were not produced by spray pyrolysis without post-treatment. The optimum post-treatment temperature to produce the Sr₂CeO₄ phosphor particles with high photoluminescence intensity was 1,000 °C in spray pyrolysis. The spherical morphology of the as-prepared particles obtained at high preparation temperatures above 1,400 °C had maintained after post-treatment at 1,000 ‡C. The relative photoluminescence intensities of the Sr₂CeO₄ phosphor particles varied with the preparation temperatures in the spray pyrolysis. The as-prepared particles obtained by spray pyrolysis at preparation temperatures below 1,400 °C converted into phase pure Sr₂CeO₄ phosphor particles after post-treatment at 1,000 ‡C. The optimum preparation temperature of the as-prepared particles was 1,400 °C to produce the Sr₂CeO₄ phosphor particles with spherical shape and high photoluminescence intensity in the spray pyrolysis.     

Interaction of air plasma-sprayed spinel and yttria coatings with ultra-high temperature ceramic-metal hot-melt

Air Plasma Sprayed (APS) ceramic coatings are conventionally used for protecting metallic parts of hot section components in aero- and land-based gas turbines. Owing to their capability to withstand very high temperatures high specific heat capacity, and thermal shock resistance, such APS coatings are also investigated for the potential to protect the core catcher in nuclear reactors during the high-temperature core-meltdown accidents resulting in corium hot-melt. In this study, candidate plasma-sprayed spinel and yttria coatings on SS 316LN substrates were subjected to an ultra-high temperature (UHT) ceramic-metal hot-melt at ∼2500 °C. Uncoated steel substrates showed melting, whereas coated substrates were found un-attacked at the expense of degradation of the coatings such as surface melting, topcoat sintering, columnar grain growth and thermal shock cracks. The evolution of interfacial fusion was observed in the case of yttria coatings due to the formation of primary YAG and Al₂O₃-YAG eutectic at the interface. Spinel coatings with a comparatively lower thermal conductivity could generate a higher ΔT across the topcoat thickness with limited grain growth in the substrate. Evidence of local melting at the interface and evolution of temperature gradient between the hot-melt and substrate are comprehensively illustrated. Out of the two coatings tested, spinel was found to be more protective to the steel substrate.     

Phase and microstructure evolution in plasma sprayed Yb2Si2O7 coatings

Yb₂Si₂O₇ coatings were deposited on Si/SiC substrates by atmospheric plasma spray (APS). The different power and plasma chemistries used in this work produced mainly amorphous crack-free coatings with compositions shifted to lower SiO₂ content with higher power and H₂ flow. Differences in microstructure and thermomechanical properties (crystallization behavior, thermal expansion coefficient and thermal conductivity) of as-deposited and thermally treated coatings were directly related to the evolution from amorphous to crystalline phases. A Yb₂SiO₅ metastable phase was identified after thermal treatments at temperatures ～ 1000 °C that transformed to its stable isomorph at 1220 °C. This transformation, followed by the growth of the crystal cell volume, promoted the coating expansion and the “healing” of microcracks present in the amorphous as-sprayed coating.     

Thermoelectric properties of non-stoichiometric CaMnO3-δ composites formed by redox-activated exsolution

A novel synthesis route for preparing well-defined composites based on CMO (CaMnO₃) has been established taking advantage of the unique phase relations in the system Ca-Mn-O at reducing- and oxidizing atmosphere, respectively. Samples corresponding to stoichiometric CMO and composites with 5 and 10 vol % of Ruddlesden-Popper (Ca₄Mn₃O₁₀)- and spinel (CaMn₂O₄)-phases, respectively, were prepared with final densities >91 %. The presence of secondary phases significantly enhanced the electrical conductivity compared to stoichiometric CMO. The highest electrical conductivity was observed for CMO with 10 vol % spinel varying between 55 and 75 S/cm at temperatures between 100 and 900 °C. This composition also exhibited the highest figure-of-merit (zT) in this study, reaching 0.083 at 800 °C.     

Spray pyrolysis synthesis of mesoporous TiO<Subscript>2</Subscript> microspheres and their post modification for improved photocatalytic activity

Mesoporous TiO₂ microspheres were prepared by spray pyrolysis for photocatalysis. Post modification of TiO₂ by heat treatment was performed to optimize its photocatalytic performance. First, spherical TiO₂ particles with mesoporous structure were synthesized at pyrolysis temperatures of 500, 600, and 700 °C. After characterization by XRD, SEM, and N₂ adsorption, a sample prepared at 500 °C was found to possess desirable properties for photocatalytic performance through post-modification. In methylene blue degradation, mesoporous TiO₂ microspheres synthesized at 500 °C outperformed other microspheres. Furthermore, samples obtained by spray pyrolysis at 500 °C were calcined at various temperatures as a post-modification process. The sample calcined at 350 °C showed improved photocatalytic activity due to optimal anatase crystallinity and surface area.     

Polymer derived silicon oxycarbide ceramic monoliths: Microstructure development and associated materials properties

Polymer derived SiOC and SiCN ceramics (PDCs) are interesting candidates for additive manufacturing techniques to develop micro sized ceramics with the highest precision. PDCs are obtained by the pyrolysis of crosslinked polymer precursors at elevated temperatures. Within this work, we are investigating PDC SiOC ceramic monoliths synthesized from liquid polysiloxane precursor crosslinked with divinylbenzene for fabrication of conductive electromechanical devices. Microstructure of the final ceramics was found to be greatly influenced by the pyrolysis temperature. Crystallization in SiOC ceramics starts above 1200?°C due to the onset of carbothermal reduction leading to the formation of SiC and SiO₂ rich phases. Microstructural characterisation using ex-situ X-ray diffraction, FTIR, Raman spectra and microscopy imaging confirms the formation of nano crystalline SiC ceramics at 1400?°C. The electrical and mechanical properties of the ceramics are found to be significantly influenced by the phase separation with samples becoming more electrically conducting but with reduced strength at 1400?°C. A maximum electrical conductivity of 10¹ S?cm^?1 is observed for the 1400?°C samples due to enhancement in the ordering of the free carbon network. Mechanical testing using the ball on 3 balls (B3B) method revealed a characteristic flexural strength of 922?MPa for 1000?°C amorphous samples and at a higher pyrolysis temperature, materials become weaker with reduced strength.     

Passive filler loaded polysilazane-derived glass/ceramic coating system applied to AISI 441 stainless steel,part 2: Oxidation behavior in synthetic air

This article features the oxidation behavior of ferritic stainless steel grade AISI 441 coated with protective polymer-derived ceramics (PDC). Two PDC compositions are studied with respect to their oxidation resistance in a flow-through atmosphere of synthetic air at temperatures of up to 1000°C. The coatings contain a combination of six passive fillers: Y-containing ZrO₂, glass microspheres, alumina-yttria-zirconia (AYZ) powder, and three commercial glasses. They are pyrolyzed in air for 1 hour at 800°C with heating and cooling rates of 3 K/min. Detailed microstructural examination of the oxide products formed at the surface of samples after exposure to air at 900°C, 950°C, and 1000°C for 1-48 hours is analyzed. Both uncoated steel and steel coated with two of the protective systems described in part 1 of this article are investigated. Fe, Cr₂O₃, TiO₂, and a spinel of the composition (Mn,Cr)₃O₄ are identified at the oxidized surface of the steel substrate using X-ray diffraction. A significant weight gain of the unprotected steel is measured after all experiments, while oxidation tests of the coated steel show a negligible weight gain after 900°C and 950°C. During the early stages of coating oxidation, the monoclinic-to-tetragonal ratio in the zirconia filler is shifted toward the monoclinic modification. Longer exposures and higher temperatures lead to the formation of yttrium aluminum garnet (YAG) due to glass microsphere crystallization and solid state reactions in the AYZ powder. The crystallization of the three commercial glasses functioning as sealants leads to the formation of Ba(AlSiO₄)₂ also known as hexacelsian, which subsequently transforms to celsian. YZr₈O₁₄ is also formed. The protective effect of the PDC coatings applied to the stainless steel is demonstrated up to 950°C.     

Electrical conductivity measurements as a microprobe for structure transitions in polysiloxane derived Si–O–C ceramics

Polysiloxanes [RSiO_1.5]_n with R=CH₃ (PMS) and C₆H₅ (PPS), respectively, were transformed to Si–O–C ceramics of variable composition and structure upon pyrolysis in inert atmosphere at 800–1500°C. The electrical conductivities of the Si–O–C ceramics in air were measured at room temperature by using a shielded two point configuration. In situ measurements of the dc-conductivity during the pyrolytic conversion from the polymer to the ceramic phase were carried out up to 1500°C with four point contacted carbon electrodes in inert atmosphere. During polymer-ceramic conversion excess carbon precipitates above 400°C (PPS)–700°C (PMS). At temperatures above 800°C (PPS) and 1400°C (PMS) coagulation and growth of the carbon clusters results in a percolation network formation. While below the percolation threshold electrical conductivity can be described according to Motts mechanism by variable-range-hopping of localized charge carriers, regular electron band conduction due to the instrinsic conductivity of turbostratic carbon (8×10⁻⁴ (Ωcm)⁻¹) predominates above. Thus, the in situ measurement of non-linear electrical property changes can be used as a microprobe of high sensivity to detect microstructural transformations during the pyrolysis of preceramic polymers.     

Effect of CeO2 addition on the sintering behavior of pre-synthesized magnesium aluminate spinel ceramic powders

In this work, the effects of 1?wt%, 2?wt%, and 3?wt% CeO₂ as an additive on the sintering behavior of alumina-rich spinel and magnesia-rich spinel powders subjected to sintering at temperatures of 1600?°C, 1650?°C, 1700?°C, and 1750?°C were investigated. The sintering behavior of the ceramics was investigated according to dilatometry measurements, linear shrinkage, bulk density, phase analysis, and microstructure. It was demonstrated that CeO₂ hindered the sintering process in alumina-rich spinel by reacting with Al₂O₃ exsolved from the spinel to form platelet-shaped particles of CeAl₁₁O₁₈ interspersed between the spinel grains. Meanwhile, the presence of CeO₂ promotes the sintering process in magnesia-rich spinel by being distributed in an isolated form among the spinel grains.     

Yttrium doping of Ba0.5Sr0.5Co0.8Fe0.2O3-δ part II: Influence on oxygen transport and phase stability

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) in its cubic perovskite phase has attracted much interest for potential use as oxygen transport membrane (OTM) due to its very high oxygen permeability at high temperatures. However, performance degradation due to a sluggish phase decomposition occurs when BSCF is operated below 840?°C. Partial B-site substitution of the transition metal cations in BSCF by larger and redox-stable cations has emerged as a potential strategy to improve the structural stability of cubic BSCF. In this study, the influence of yttrium doping (0…10?mol-%) on oxygen transport properties and stability of the cubic BSCF phase is assessed by in situ electrical conductivity relaxation (ECR) and electrical conductivity measurements during long-term thermal annealing both at 700?°C and 800?°C. Detailed phase analysis is performed by scanning electron microscopy (SEM) after long-term annealing of the samples in air at different temperatures.     

Thermophysical property and electrical conductivity of titanium isopropoxide – polysiloxane derived ceramics

In this study, high temperature resistant Si-O-C-Ti has been successfully prepared based on the pyrolysis of polysiloxane (PSO) and titanium (IV) isopropoxide (TTIP) at 1200–1400 °C. PSO can homogeneously mix with TTIP to enhance its conversion to TiC. The carbothermal reactions between TiO₂ (product of thermal decomposition of TTIP) and carbon result in the formation of TiC. All the Si-O-C-Ti composites pyrolyzed at 1200–1300 °C are stable up to 1000 °C in an oxidizing air atmosphere. TiC leads to high electrical conductivity at elevated temperatures; the maximum conductivity is 1176.55 S/m at 950 °C, which is the first reported value of >1000 S/m conductivity for Si-O-C-Ti ceramics. However, too high a pyrolysis temperature, such as 1400 °C, can potentially ‘destabilize’ the Si-O-C-Ti system by consuming the free carbon and result in lower conductivities.     

Enhancement of reaction-sintering of alumina-excess magnesium aluminate spinel in presence of titania

Alumina-excess magnesium aluminate spinel finds use in different high temperature applications including steel ladles. Alumina-excess spinel was prepared by solid oxide reaction using magnesia (MgO=10?wt%) and calcined alumina (Al₂O₃ = 90?wt%), in the sintering temperature range of 1500–1700?°C. The role of titania on the densification, spinelisation, evolution of microstructure and phase assemblage was investigated in this MgO-Al₂O₃ system. Titania addition increased the rate of densification 20x compared to undoped composition at 1500?°C under dynamic heating condition. However, under static firing, the beneficial effect of titania on densification could only be discerned at lower temperatures. The microstructure of titania doped sintered alumina-excess spinel compacts contain magnesium aluminium titanate phase in the grain boundary of corundum and spinel grains. The beneficial effect of titania on densification is attributed to magnesium aluminium titanate phase (Mg_xAl_2(1-x)Ti_(1+x)O₅) development and also by incorporation of Ti⁴⁺ into the spinel structure.     

Alternating electrical conductivity of polyphenylenes and their pyrolysis residues after doping

o,m,p-Polyphenylenes were prepared by oxidation-cationic polymerization of biphenyl and then pyrolyzed up to 800°C. The o,m,p-polyphenylenes as well as their pyrolysis residues were doped with anhydrous FeCl₃ from their solution in acetonitrile. The real part of the alternating electrical conductivity σ′ of the undoped and doped materials was determined at room temperature. Decreasing σ′-values of pyrolyzed residues at 600°C without doping were observed because of the large weight losses during the pyrolysis leading to chain scission reactions within the polymer. At higher pyrolysis temperatures (i.e. 700°C, 800°C) the σ′-values are increased due to the formation of closed aromatic systems in the polymer. The doped materials have higher σ′-values in relation to the corresponding specimens without doping, with the exception of the pyrolysis residues at 800°C. The dependence of doping effect and time was also determined. The transition of the organic polymer to the pyropolymers can be followed and characterized by the measurement of alternating electrical conductivity.     

Development of alumina-based hybrid composite coatings for high temperature erosive and corrosive environments

Low alloy steels are being used for structural applications exposed to extreme environmental conditions such as, high temperature, corrosive and erosive environment in petrochemical industry, turbomachinery, chemical reactors, and nuclear industry. These extreme environment conditions severely affect the performance of critical components. In this context, it is attempted to develop the alumina (Al₂O₃) based hybrid composite coatings on ASME SA387-22-2 steel to mitigate the effect of high temperature erosion and corrosion. High-velocity oxygen-fuel (HVOF) thermal spray technique is used to develop the coatings. The physical characteristics of the coatings are recorded in terms of density measurement, thermogravimetric analysis, and surface wettability analysis. Hardness and elastic modulus of the coatings are estimated at room temperature using nanoindentation and microhardness at high temperature (400 °C) using Vickers hardness tests. Metallurgical characterization and eroded surface analysis are done using Scanning electron microscopy (SEM), Energy dispersive X-ray spectroscopy (EDS), Raman spectroscopy and X-ray diffraction (XRD). High temperature (400 °C) erosion performance of the coatings is recorded using solid particle Air-jet erosion tester. The Corrosion performance of the coatings are analyzed using the potentiodynamic polarization technique and electrical impedance spectroscopy (EIS). Significant improvement in erosion performance (Erosion reduction: ≈ 96% at 30° and ≈63% at 90°) and corrosion resistance (≈92% reduction in corrosion rate) is recorded for the coating reinforced with 0.8 wt% ceria (CeO₂) and 3 wt% hexagonal boron nitride hBN as compared to the bare steel. The improved erosive and corrosive characteristics of the coatings are attributed to the refined microstructural aspects, in-situ generation of hard second phases, and induction of hydrophobicity in the composite structure.     

La0.8Sr0.2Ga0.8Mg0.2O3 electrolytes prepared by vacuum cold spray under heated gas for improved performance of SOFCs

High impact velocity of particles has found its common way into the vacuum cold spray (VCS), but heating gas may further intensify this function, resulting in significantly higher impact velocity. That's the original design idea to realize denser ceramic deposition at low temperature in this paper. In this study, a ~ 10?µm thick La_0.8Sr_0.2Ga_0.8Mg_0.2O₃ (LSGM) electrolyte layer for SOFCs is prepared by VCS under heated gas. The effects of gas temperature on the deposition behavior, mechanical and electrical properties of the coatings are investigated. Results show improvements in coating density, hardness and ionic conductivity at elevated temperature. Additionally, the output performance of cell with LSGM electrolytes deposited at gas temperature of 400?°C achieved an open circuit voltage of ~ 1.0?V and the maximum power density of 855?mW/cm² at 750?°C. Overall, these findings testify of the promising aspects of VCS method for preparing solid electrolyte films for IT-SOFCs.     

Conducting fluorine doped tin dioxide (FTO) coatings by ultrasonic spray pyrolysis for heating applications

Fluorine doped tin dioxide (FTO) coatings were deposited on AISI 304 stainless steel (SS) substrates using ultrasonic spray pyrolysis (USP) technique. Four different candidate insulating intermediate layers of TiO₂, MgO, Al₂O₃ and SiO₂ were selected and coated on AISI 304 SS using USP and investigated in order to overcome dielectric breakdown of FTO layer due to conductive nature of stainless steel substrate under the applied voltage. After the optimization of the process parameters for the depositions, only TiO₂ layer was successfully deposited while other oxides did not form continuous films on AISI-304 SS surface despite the efforts. SEM analysis, supported by back-scatter imaging, revealed thickness values of the layers for FTO and TiO₂, individually. Heating performances of the samples with varying sheet resistance values were examined under different applied voltages. Heating experiments showed that 300 °C can be reached with 10 V of applied voltage without occurrence of an electrical breakdown of FTO coating having 160 nm thickness with TiO₂ intermediate layer of 660 nm. 28.6% efficiency for the conversion of electrical energy into heat was calculated for FTO coating while bare stainless steel showed only 7.3% efficiency. Later, the heating experiments were repetitively conducted and the samples were tested under high relative humid environment to determine their resistance against moist conditions.     

Electrical Conductivity of SiOCN Ceramics by the Powder‐Solution‐Composite Technique

SiOCN ceramics have been prepared by the polymer pyrolysis method. The preceramic polymers were synthesized from a polysiloxane cross‐linked with two different N‐containing compounds: a silazane or a ternary amine. The corresponding SiOCN ceramics were obtained by pyrolysis in nitrogen atmosphere at five different temperatures from 1000°C to 1400°C. The electrical conductivity of the powdered SiOCN ceramic samples was determined by the powder‐solution‐composite technique. The results show an increase in room temperature AC conductivity of three orders of magnitude, from ≈10^?5 (S/cm) to ≈10^?2 (S/cm), with increasing pyrolysis temperature from 1000°C to 1400°C. Furthermore, the electrical conductivity of the amine‐derived SiOCN is three to five times higher than that of the silazane‐derived ceramic at each pyrolysis temperature. The combined structural study by Raman spectroscopy and chemical analysis suggests that the increase of electrical conductivity with the pyrolysis temperature is due to the sp³‐to‐sp² transition of the amorphous carbon phase. The higher conductivity of the amine‐derived SiOCN is also discussed considering features like the volume% of the free‐carbon phase and its possible N‐doping.     

Synthesis,spray granulation and plasma spray coating of lanthanum phosphate nanorods for thermal insulation coatings

Nanorods of lanthanum phosphate obtained by a wet chemical precipitation route were granulated to obtain sizes in the range of 10–15 µm by spray drying from aqueous slurry of 35 wt% solid loading and 2 wt% of PVA binder. The powders thus obtained displayed enhanced flowability and were plasma sprayed on to stainless steel substrates resulting in the formation of adherent coatings of 150–180 µm thickness. These coatings were characterized using electron microscopy, X-ray diffraction analysis and Raman spectroscopy. X-ray analysis indicated phase instability of LaPO₄ during plasma spraying resulting in the formation of oxy and polyphosphates of lanthanum (La₂P₄O₁₃ and La₃PO₇). However, post deposition heat treatment of coated samples at 1100 °C for 2 h resulted in the reversible formation of stoichiometric lanthanum orthophosphate (LaPO₄). Raman spectral analysis was used to confirm the phase structure of the coatings deposited at various plasma input powers. The coatings obtained were found to effectively lower the thermal conductivity of the substrates from ~24 W/mK to less than 19 W/mK (~10%) even at 200 °C.     

Effects of Annealing Temperature on the Structure and Capacitive Performance of Nanoscale Ti/IrO2–ZrO2 Electrodes

Electrodes consisting of coating of the Iridium oxide–Zirconium oxide (70%IrO₂–30%ZrO₂) binary oxide were formed on Ti substrates by thermal decomposition and annealing at 340°C–450°C. The effects of the annealing temperature on the structure, surface morphology, surface composition, and capacitive performance of the coatings were investigated using X‐ray diffraction analysis (XRD), transmission electron microscopy (TEM), scanning electron microscopy, X‐ray photoelectron spectroscopy, cyclic voltammetry, and electrochemical impedance spectroscopy (EIS). The XRD and TEM analyses showed that 360°C is greater than but very close to the crystallization temperature of the 70%IrO₂–30%ZrO₂ oxide coating. The 70%IrO₂–30%ZrO₂ oxide coatings annealed at this temperature consisted of an amorphous matrix containing a few IrO₂ nanocrystalline particles (diameter of 1–2 nm). The degree of crystallinity of the coatings was approximately 13.2%. EIS analysis showed that the electrode annealed at 360°C exhibited the highest specific capacitance, which was much higher than that of the electrode annealed at 340°C (which had a purely amorphous structure) as well as those of the electrodes annealed at 380°C and 400°C (which had higher degrees of crystallinity). On the basis of the obtained results, the following conclusion can be drawn: oxide coatings prepared at temperatures slightly higher than the crystallization temperature of the oxide and containing conductive nanocrystalline particles exhibit the best capacitive performance. We suggest that this phenomenon can be explained by the fact that the electronic conductivity of the coating is greatly improved by the presence of the homogeneously distributed conductive nanocrystalline particles in the amorphous matrix. Furthermore, the protonic conductivity and loose atomic configuration of the amorphous structure of the electrode are not adversely affected by the annealing treatment.