Search Results (16)

This paper presents the results of a comprehensive study of the structure, phase composition, thermal corrosion, and tribological properties of multilayer gradient coatings based on YSZ/NiCrAlY obtained using detonation spraying. X-ray phase analysis showed that the coatings consist entirely of metastable tetragonal zirconium dioxide (t’-ZrO₂) phase stabilized by high temperature and rapid cooling during spraying. SEM analysis confirmed the multilayer gradient phase distribution and high density of the structure. Wear resistance, optical profilometry, wear quantification, and coefficient of friction measurements were used to evaluate the operational stability. The results confirm that the structural parameters of the coating, such as porosity and phase gradient, play a key role in improving its resistance to thermal corrosion and CMAS melt, which makes such coatings promising for use in high-temperature applications. It is shown that a dense and thick coating effectively prevents the penetration of aggressive media, providing a high barrier effect and minimal structural damage. Tribological tests in the temperature range from 21 °C to 650 °C revealed that the best characteristics are observed at 550 °C: minimum coefficient of friction (0.63) and high stability in the stage of stable wear. At room temperature and at 650 °C, there is an increase in wear due to the absence or destabilization of the protective layer. Full article

This paper presents the results of a study of layer gradient thermal protection coatings based on NiCrAlY and YSZ obtained by detonation spraying. Modern gas turbines and high-temperature units operate under extreme temperatures and aggressive environments, which requires effective protection of components from wear, corrosion, and thermal shocks. In this study, the use of layer gradient coatings consisting of alternating layers of NiCrAlY and YSZ was investigated with the aim of solving the problem of thermal stress accumulation due to a smooth change in the composition of the layers. Microstructural and phase analysis showed that alternating layers of NiCrAlY and YSZ formed a dense layer gradient structure with clear interphase boundaries and low porosity. Detonation spraying led to a complete transformation of the monoclinic ZrO₂ phase into a tetragonal one, which significantly increased the mechanical strength of the coating and its resistance to thermal shocks. Sample 1D1 demonstrated excellent tribological and corrosion properties in a 3.5% NaCl solution, which can be explained by its higher density and reduced number of pores. Mechanical tests revealed stable values of hardness and wear resistance of the coating, especially for the 1D1 coating. Studies have shown that coatings are resistant to thermal shocks, but thicker layers show a tendency to peel off after thermal cycling. The obtained results indicate high prospects for the use of layer gradient coatings based on NiCrAlY and YSZ for the protection of gas turbine components and other high-temperature installations operating under extreme loads and aggressive environments. Full article

In this study, a NiCr/YSZ coating was applied to a γ-TiAl surface using multi-arc ion plating technology to enhance its high-temperature performance and explore the mechanisms of high-temperature oxidation and thermal corrosion. The thermal corrosion properties of the γ-TiAl matrix and NiCr/YSZ coating were investigated at 850 °C and 950 °C using a constant-temperature corrosion test in a 75% Na₂SO₄ + 25% NaCl mixture. The results indicate that after 100 h, the thermal corrosion weight gain of the coating samples was 70.1 mg/cm² at 850 °C and 118.2 mg/cm² at 950 °C. At these temperatures, sulfide formation on the surface increases, leading to a loose and porous surface. After 100 h of high-temperature corrosion at 850 °C, the primary oxidation product on the surface of the coating was tetragonal-ZrO₂. At 950 °C, Y₂O₃, which mainly acts as a stabilizer in YSZ, reacted with Na₂SO₄, resulting in the continuous consumption of Y₂O₃. This reaction caused a substantial amount of tetragonal-ZrO₂ to transform into monoclinic-ZrO₂, altering the volume of the ceramic layer, which induced internal stress, crack propagation, and minor spallation. A continuous and dense internal thermally grown oxide (TGO) layer effectively impeded the diffusion of molten salt substances and oxygen, thereby significantly improving the thermal corrosion resistance of the thermal barrier coating. Full article

In this work, multilayer gradient coatings obtained by detonation spraying were studied. To obtain a multilayer gradient coating by detonation spraying, two modes with different numbers of shots of NiCrAlY and YSZ were developed. The presented results demonstrate the effectiveness of creating a gradient structure in coatings, ensuring a smooth transition from metal to ceramic materials. Morphological analysis of the coatings confirmed a layered gradient structure, consisting of a lower metallic (NiCrAlY) layer and an upper ceramic (YSZ) layer. The variation in the contents of elements along the thickness of the coatings indicates the formation of a gradient structure. X-ray analysis shows that all peaks in the X-ray diffraction patterns correspond to a single ZrO₂ phase, indicating the formation of a non-transformable tetragonal primary (t′) phase characteristic of the thermal protective coatings. This phase is known for its stability and resistance to phase transformation under changing operating temperature conditions. As the thickness of the coatings increased, an improvement in their mechanical characteristics was found, such as a decrease in the coefficient of friction, an increase in hardness, and an increase in surface roughness. These properties make such coatings more resistant to mechanical wear, especially under sliding conditions, which confirms their prospects for use in a variety of engineering applications, including aerospace and power generation. Full article

The conversion of hydrogen to power via combined external reforming of liquid alcohol and solid oxide fuel cell (SOFC) technology is an effective approach to address future energy challenges. In this study, an La_0.8Ba_0.1Mn_0.8Ni_0.1Cu_0.1O₃ (LBMNCu) perovskite nanofiber with high porosity was synthesized with a modified electrostatic spinning method, which acted as an efficient catalyst for steam reforming of liquid alcohols (methanol and ethanol). After reduction, fine metallic Ni-Cu was uniformly distributed throughout the perovskite nanofiber surface. The obtained composite displayed a methanol conversion above 99.9% at 450 °C and an ethanol conversion above 99% at 600 °C, which was highly superior to the common Ni-Cu/Al₂O₃ catalyst. The catalytic performance of our assembled catalysts also remained stable in methanol and ethanol atmospheres for 50 h and no coking was detected. Furthermore, when the reformed gas was fed into a Y_0.08Zr_0.92O₂ (YSZ)-based SOFC system, the open circuit voltage remained around 1.1 V at 700 °C for 50 h accordingly, without coking, and the voltage remained virtually unchanged at 0.7 V for 50 h at 700 °C and 400 mA cm⁻² during galvanostatic discharge mode, indicating that using LBMNCu nanofiber as a catalyst for hydrogen production and utilization is an efficient strategy. The interaction of the in situ exsolved metallic nanoparticles and nanofibrous perovskite could also be a promising approach for designing a highly active catalyst for H₂ generation. Full article

Solid oxide cells (SOCs) have been considered as a promising energy conversion and storage device. However, state-of-the-art cells’ practical application with conventionally fabricated Ni-(Y₂O₃)_0.08(ZrO₂)_0.92 (YSZ) cermet hydrogen electrode and La_0.8Sr_0.2MnO₃ perovskite oxygen electrode is strongly limited by the unsatisfactory performance. Instead, new advances in cell materials and fabrication techniques that can lead to significant performance enhancements are urgently demanded. Here, we report a high-performance reversible SOC that consisted of a combination of SrSc_0.175Nb_0.025Co_0.8O_3−δ (SSNC) and phase-inversion tape-casted Ni-YSZ, which served as the oxygen and hydrogen electrode, respectively. The hydrogen electrode synthesized from phase-inversion tape-casting showed a high porosity of 60.8%, providing sufficient active sites for hydrogen oxidation in the solid oxide fuel cell (SOFC) mode and H₂O electrolysis in the solid oxide electrolysis cell (SOEC) mode. Accordingly, it was observed that the maximum power density of 2.3 W cm⁻² was attained at 750 °C in SOFC mode and a current density of −1.59 A cm⁻² was obtained at 1.3 V in SOEC mode. Hence, these results reveal that the simultaneous optimization of oxygen and hydrogen electrodes is a pragmatic strategy that improves the performance of SOCs, which may significantly accelerate the commercialization of such an attractive technology. Full article

Thermal barrier coatings (TBCs) prepared using the atmospheric plasma spraying method fail mainly due to coating delamination caused by thermal mismatch in the absence of high temperature assessment. In this study, the thickness optimization of multiple ceramic layers in a TBCs and the influence of the interface structure on the residual stress of the coating were investigated using a finite element simulation method. The results showed that varying the thickness of each layer of a TBCs with multiple ceramic layers affects the distribution and magnitude of the residual stress of the coating. Therefore, a reasonable range of thickness for each layer can be determined. The thickness of the bonding layer should be 110 μm, the thickness of YSZ layer should be about 270 μm, the thickness of tantalate layer should be about 70 μm, and the thickness of Al₂O₃-YAG layer should be about 100 μm. Simultaneously, the results show that a rough interface can be more effective in reducing the relief of stress concentrations compared to a smooth interface, but the stress values increase. Full article

In this study, nickel oxide–Y₂O₃-doped ZrO₂ (NiO-YSZ) composite powder as an anode material was synthesized using a cost-effective combustion method for high-temperature solid oxide fuel cell (SOFC). Further, the effects of sintering temperatures (1200, 1300, and 1400 °C) were studied for its properties in relation to the SOFC performance. The prepared and sintered NiO-YSZ materials were characterized for their surface morphology, composition, structure, and conductivity. The cubic crystalline nature of NiO and YSZ was sufficed by X-ray diffraction, and SEM images revealed an increase in the densification of microstructure by an increase in the sintering temperature. EDX spectrum confirmed the presence of nickel, yttrium, and zirconia without any impurity. Conductivity measurements, under a hydrogen environment, revealed that NiO-YSZ, sintered at 1400 °C, exhibits better conductivity compared to the samples sintered at lower temperatures. Electrochemical performance of button-cells was also evaluated and peak power density of 0.62 Wcm⁻² is observed at 800 °C. The citrate combustion method provided peak performance for cells containing anode sintered at 1200 °C, which was previously reported at higher sintering temperatures. Therefore, the citrate combustion method is found to be a suitable route to synthesize NiO-YSZ at low sintering temperature. Full article

Long term stability is one of the decisive properties of solid oxide fuel cell (SOFC) as well as solid oxide electrolysis cell (SOEC) materials from the commercialization perspective. To improve the understanding about degradation mechanisms solid oxide cells with different electrode compositions should be studied. In this work, Ni-Zr_0.92Y_0.08O_2-δ (Ni-YSZ)| Zr_0.92Y_0.08O_2-δ (YSZ)|Ce_0.9Gd_0.1O_2-δ (GDC)|Pr_0.6Sr_0.4CoO_3-δ (PSC) cells are tested in the SOFC regime for 17,820 h at 650 °C, and in the SOEC regime for 860 h at 800 °C. The SOFC experiment showed a degradation speed of 2.4% per 1000 h at first but decreased to 1.1% per 1000 h later. The electrolysis test was performed for 860 h at 800 °C. The degradation speed was 16.3% per 1000 h. In the end of the stability tests, an electrode activity mapping was carried out using a novel ¹⁸O tracing approach. Average Ni grain sizes were measured and correlated with the results of the oxygen isotope maps. Results indicate that Ni coarsening is dependent on solid oxide cell activity. Strontium, chromium and silicon concentrations were also analyzed using the ToF-SIMS method and compared to the electrode activity map, but significant correlation was not observed. Full article

The degradation process of thermal barrier coatings (TBCs) such as monolayered La₂Zr₂O₇, composite 50% La₂Zr₂O₇ + 50% 8YSZ, and double-ceramic layer (DCL) La₂Zr₂O₇/50% La₂Zr₂O₇ + 50% 8YSZ/8YSZ was investigated. Coatings were deposited using the atmospheric plasma spraying (APS) process (ceramic layer and bond-coat) on the Ni-based superalloy substrate with Ni-22Cr-10Al-1Y bond-coat. The thickness of the ceramic top-coats in all cases were 300 µm. In the case of La₂Zr₂O₇/8YSZ, the internal sublayer was built from 8YSZ powder whereas the outer from La₂Zr₂O₇. Between both sublayers’ “composite” a 50% La₂Zr₂O₇ + 50% 8YSZ zone was present. The “composite” 50% La₂Zr₂O₇ + 50% 8YSZ TBC system was sprayed from two different feedstock powders with equal weight ratios. In the first part of the investigation, the microstructural characterization of the TBCs was presented. The main goals were related to the characterization of the degradation processes in different TBC systems with special emphasis on the phenomenon in the thermally grown oxide (TGO) zone related to oxidation, and the phenomenon related to phase stability in ceramic top-coats as related to temperature influence. The oxidation test was carried out in air at 1100 °C for 500 h. In the second step of the investigation, the numerical simulation of the monolayered TBC 8YSZ and La₂Zr₂O₇ systems was analyzed from the stress distribution point of view. Additionally, the two-layered TBC coating of the DCL type was also analyzed. Full article

Open-celled ceramic composite foams were prepared from NiO and yttria-stabilized zirconia (YSZ) powders by the polymer sponge replication (Schwartzwalder) technique using the respective aqueous dispersions. Mechanically stable NiO–YSZ foams with an average porosity of 93 vol.% were obtained. After chemical reduction of the NiO phase with hydrogen, cellular Ni–YSZ cermet structures were obtained. They are characterized by an electric conductivity up to 19∙10³ S∙m⁻¹ which can be adjusted by both, the Ni volume fraction, and the sintering/reduction procedure. The NiO–YSZ ceramic foams, as well as the cellular Ni–YSZ cermets prepared therefrom, were characterized with respect to their microstructure by scanning electron microscopy, confocal Raman microscopy and X-ray diffraction with Rietveld analysis. In addition, the compressive strength, the electric conductivity and the thermal conductivity were determined. The collected data were then correlated to the sample microstructure and porosity and were also applied for modelling of the mechanical and electric properties of the bulk Ni–YSZ strut material. Full article

Magnesium (Mg) has good hydrogen storage features except for its slow reaction kinetics with hydrogen and high hydride decomposition temperature. Yttria (Y₂O₃)-stabilized zirconia (ZrO₂) (YSZ), nickel (Ni), and graphene were picked as additives to Mg to solve these problems. Samples with a composition of 92.5 wt% Mg + 2.5 wt% YSZ + 2.5 wt% Ni + 2.5 wt% graphene (designated as Mg + YSZ + Ni + graphene) were prepared by grinding in hydrogen atmosphere. The activation of Mg + YSZ + Ni + graphene was finished at the third cycle (n = 3). Mg + YSZ + Ni + graphene had an efficient hydrogen storage capacity (the amount of hydrogen absorbed for 60 min) over 7 wt% (7.11 wt%) at n = 1. Mg + YSZ + Ni + graphene contained Mg₂Ni phase after cycling. The addition of Ni and Ni + YSZ greatly increased the initial hydride formation and decomposition rates, and the amount of hydrogen absorbed and released for 60 min, H_a (60 min) and H_d (60 min), respectively, of a 95 wt% Mg + 5 wt% graphene sample (Mg + graphene). Rapid nucleation of the Mg₂Ni-H solid solution in Ni-containing samples is believed to have led to higher initial decomposition rates than Mg + graphene and Mg. The addition of YSZ also enhanced the initial decomposition rate and H_d (60 min) compared to a sample with a composition of 95 wt% Mg + 2.5 wt% Ni + 2.5 wt% graphene (Mg + Ni + graphene). Full article

The cold gas dynamic spray (CGDS) method has been considered a promising technology to produce a metallic bond coat for thermal barrier coating (TBC) systems. In this study, CoNiCrAlY bond coats produced by CGDS method were coated with yttria-stabilized zirconia (YSZ) by electron beam physical vapor deposition (EB-PVD). TBCs were subjected to 50 wt % V₂O₅ and 50 wt % Na₂SO₄ molten hot corrosion salt combinations at 1000 °C. In the case of YSZ top coat on TBCs, the reaction between Na₂SO₄, V₂O₅, and Y₂O₃ salts generates YVO₄ crystals, and these structures cause the transformation of tetragonal ZrO₂ to monoclinic ZrO₂. This situation occurs under operating conditions that lead to TBC failure. Hot corrosion behavior and the related failure mechanisms of TBC systems were investigated and discussed using scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) analysis, and X-ray diffractometer (XRD). Full article

Tubular oxygen transport membranes (OTMs) that can be directly integrated in high temperature processes have a large potential to reduce CO₂ emissions. However, the challenging processing of these multilayered tubes, combined with strict material stability requirements, has so far hindered such a direct integration. We have investigated if a porous support based on (Y₂O₃)_0.03(ZrO₂)_0.97 (3YSZ) with a dense composite oxygen membrane consisting of (Y₂O₃)_0.01(Sc₂O₃)_0.10(ZrO₂)_0.89 (10Sc1YSZ) as an ionic conductor and LaCr_0.85Cu_0.10Ni_0.05O_3−δ (LCCN) as an electronic conductor could be fabricated as a tubular component, since these materials would provide outstanding chemical and mechanical stability. Tubular components were made by extrusion, dip coating, and co-sintering, and their chemical and mechanical integrity was evaluated. Sufficient gas permeability (≥10⁻¹⁴ m²) and mechanical strength (≥50 MPa) were achieved with extruded 3YSZ porous support tubes. The high co-sintering temperature required to densify the 10ScYSZ/LCCN membrane on the porous support, however, causes challenges related to the evaporation of chromium from the membrane. This chemical degradation caused loss of the LCCN electronic conducting phase and the formation of secondary lanthanum zirconate compounds and fractures. LCCN is therefore not suitable as the electronic conductor in a tubular OTM, unless means to lower the sintering temperature and reduce the chromium evaporation are found that are applicable to the large-scale fabrication of tubular components. Full article

Gas exhaust emissions in vehicles are increasingly restrictive in EU and USA. Diesel engines are particularly affected by limitation in hydrocarbons and NOx concentrations. This work presents a screening of working electrode materials to develop a potentiometric sensor, with the most promising material to detect being C₂H₄ at 550 °C. The device consists of a dense 8YSZ (8 mol% Y₂O₃ stabilized ZrO₂) disk as oxide-ion conducting electrolyte, whereas platinum is screen-printed in the back face as reference electrode. As working electrode, several materials such as Fe_0.7Cr_1.3O₃, ZnCr₂O₄, Fe₂NiO₄, La_0.8Sr_0.2CrO_3−δ (LSC), La_0.8Sr_0.2MnO₃ (LSM), and NiO+5%wt Au were tested to detect C₂H₄. Sensor voltage was measured for several concentrations of C₂H₄ and CO as these are two of the major oxidizable compounds in a diesel exhaust gas. Fe_0.7Cr_1.3O₃ was selected as the most promising material because of its response to C₂H₄ and CO. Not only is the response to the individual analytes important, but the C₂H₄ cross-sensitivity toward CO is also important. Fe_0.7Cr_1.3O₃ showed a good performance to C₂H₄, with low cross-sensitivity to CO. In addition, when 0.16 ppm of phenanthrene is added, the sensor still has a slightly better response to C₂H₄ than to CO. Nevertheless, the sensor exposure to high concentrations (>85 ppm) of polycyclic aromatic hydrocarbons led to signal saturation. On the other hand, the operation in wet conditions induces lower sensor sensitivity to C₂H₄ and higher cross-sensitivity toward CO increase, i.e., the sensor response becomes similar for C₂H₄ and CO. Full article