Stability of rare‐earth‐doped spherical yttria‐stabilized zirconia synthesized by ultrasonic spray pyrolysis

Phase stability is one of the crucial requirements for any material that can be used at elevated temperature applications such as thermal barrier coating (TBC). The most traditional TBC material, partially stabilized zirconia, limits the operating temperature due to its phase transformation. Conversely, the low thermal conductivity of fully stabilized zirconia (YSZ) may enable effective reduction in the surface temperature on the coated component, while avoiding deleterious phase transitions, although still being subjected to sintering and grain growth. It has been reported that addition of rare‐earths as dopants to YSZ can significantly increase resistance to grain growth at high temperature. Keeping this under consideration, this work investigates the role of rare‐earths (lanthanum and gadolinium) in increasing thermal stability of YSZ microspheres, the building blocks for high‐temperature photonics for reflective TBC. The spheres were produced by ultrasonic spray pyrolysis, and the doping led to significant improvement of stability by significant inhibition of grain growth. While the individual dopants showed significant growth and sintering inhibition up to 900°C, co‐doping with 4% (in mol) of each (Gd and La) led to coarsening resistance up to 1000°C for 3 hours, when particles retained reasonable spherical features with nanometric crystallite sizes.     

Influence of Plasma Spray Parameters on Formation and Morphology of ZrO2–8 wt% Y2O3 Deposits

Spray prints of thermal spray coatings were created on glass slides for air-plasma-sprayed 8-wt%-yttria-partially-stabilized zirconia (YSZ) deposits. The spray parameters such as carrier gas flow rate, standoff distance, and torch power were systematically changed to investigate the influence of these parameters on the YSZ deposit characteristics. The deposit properties such as deposition efficiency (DE), substrate coverage, deposit thickness, and roughness were measured. The deposits sprayed with a 3.5–4.0 L/min carrier gas flow rate at an 80 mm standoff distance exhibited higher values of DE within the range of studied process parameters. The DE increased as much as 25% by varying the carrier gas flow rate from 2.0 to 4.0 L/min. The deposits sprayed at a higher standoff distance and low torch power gave poor deposit characteristics. The deposit characteristics were compared with the in-flight particle parameters and revealed that the deposit characteristics strongly depended on the in-flight particle temperature. Using the in-flight particle properties, the flattening ratio and the splat thickness were calculated. The average size of particles adhering to the substrate was found to drastically change with a change of process conditions, being much less than the average size of the starting powder.     

Influence of Plasma Spray Parameters on In-Flight Characteristics of ZrO2–8 wt% Y2O3 Ceramic Particles

Yttria-partially-stabilized zirconia was atmospherically plasma sprayed by systematically varying the process conditions including carrier gas flow rate, torch power, standoff distance, and Ar/H₂ ratio in the plasma gas mixture. The in-flight particle parameters such as temperature, velocity, number, and size were determined using a commercially available diagnostic system. The particle parameters were controlled by the particle trajectory in the plume and plasma jet characteristics. The average temperature and the velocity of particles, which reached their maximum at an intermediate carrier gas flow rate of 3.5 L/min, varied as much as 6% and 25%, respectively, with a 75% variation in the carrier gas flow rate by going from the lowest to the intermediate rates. The average temperature and the velocity of particles were lower for a lower torch power, a higher Ar/H₂ ratio, and a larger standoff distance. It was necessary to obtain data on particle populations larger than 1000 for statistically reliable and reproducible information from the diagnostic system.     

Induced p-type semiconductivity in yttria-stabilized zirconia

8 mol% yttria-stabilized zirconia (8YSZ) ceramic is an oxide ion conductor at atmospheric pressure but shows the onset of p-type semiconduction, in addition to the preexisting oxide ion conduction, on application of a dc bias in the range 4-66 Vcm⁻¹ and at temperatures in the range 150°C-750°C. The p-type behavior is attributed to the location and hopping of holes on oxygen. This contrasts with the commonly observed introduction of n-type conduction under reducing conditions and high fields. The hole conductivity increases with both dc bias and pO₂. Its occurrence may contribute to the early stages of flash phenomena in 8YSZ ceramics.     

Comparison of temperature dependent deformation mechanisms of 8YSZ thermal barrier coatings prepared by air-plasma-spray and D-gun thermal spray: An in situ study

8 weight percent yttria stabilized zirconia (8YSZ) has gained widespread use in thermal barrier coatings for the hot sections of aero and power generation turbines due to its superb thermal and mechanical properties. In this study, in situ microcompression tests were conducted to evaluate the mechanical performance of 8YSZ coatings with dense vertically cracked (DVC) microstructures produced by detonation gun thermal spray to those deposited by air plasma spray (APS). At room temperature, the APS coatings showed high variability in fracture strength resulting from cracks and pores in the coating. DVC coatings, conversely, exhibited fracture strengths ranging from 3.9 to 6.6 GPa and less variability in fracture strength attributed to the relatively dense and less defective microstructure. At 500 °C, both coatings showed better consistency of fracture strength and enhanced deformability owing to deformable pores, ferroelastic domain switching, and dislocation activities.     

Cation-Cation Interaction in Dehydrated Zeolites X and Y Monitored by Modulus Spectroscopy

We report impedance measurements on zeolites NaX and NaY. By means of a new evaluation technique, i.e., the modulus master plot technique, two distinct relaxation processes within one spectral presentation could be visualized, a local dipolar relaxation and a long-range charge carrier process. The corresponding activation energies reflect the enhanced cation-cation interaction in NaX, which results from the smaller Si/Al ratio causing a larger number of occupied cation sites per unit cell. This demonstrates that from the macroscopic impedance response one can deduce to elementary processes on the microscopic scale, when structural details are taken into account.     

Flash sintering of yttria-stabilized zirconia powders coated with nanoscale films of alumina by atomic layer deposition

The addition of small quantities of aluminum oxide (Al₂O₃) to 8 mol% yttria-stabilized zirconia (8YSZ) benefits conventional sintering by acting as a sintering aid and altering grain growth behavior. However, it is uncertain if these benefits observed during conventional sintering extend to flash sintering. In this work, nanoscale films of Al₂O₃ are deposited on 8YSZ powders by particle atomic layer deposition (ALD). The ALD-coated powders were flash sintered using voltage-to-current control and current rate experiments. The sintering behavior, microstructural evolution, and ionic conductivities were characterized. The addition of Al₂O₃ films changed the conductivity of the starting powder, effectively moving the flash onset temperature. The grain size of the samples flashed with current rate experiments was ~65% smaller than that of conventionally sintered samples. Measurement of grain size and estimates of sample density as a function of temperature during flash sintering showed that small quantities of Al₂O₃ can enhance grain growth and sintering of 8YSZ. This suggests that Al₂O₃ dissolves into the 8YSZ grain boundaries during flash sintering to form complexions that enhance the diffusion of species controlling these processes.     

Microstructure and mechanical properties of yttria stabilized zirconia coatings prepared by plasma spray physical vapor deposition

Plasma spray physical vapor deposition (PS-PVD) was used to deposit yttria stabilized zirconia (YSZ) coatings with different columnar morphologies by varying the spray distance. Although similar quasi-columnar structures were formed at the spray distances of 600 mm and 1400 mm, the formation mechanisms of particles in the coatings were different. Besides, an electron beam physical vapor deposition (EB-PVD) like columnar coating out of pure vapor was deposited at a spray distance of 1000 mm and the columnar consisted of elongated nano-sized secondary columns. The hardness and Young׳s modulus of the coatings were investigated. Compared to the other two quasi-columnar structures, the EB-PVD like columnar coating exhibited higher hardness (~9.0 GPa ) and Young׳s modulus (~110.9 GPa), mainly due to its low porosity and defect.     

热处理工艺对纳米氧化锆粉体微观结构与涂层性能的影响

研究了高温煅烧、等离子炬和等离子流场3种热处理工艺对ZrO2-8%(mol)Y2O3 (8YSZ)球形颗粒及其等离子喷涂涂层微观组织结构的影响. 结果表明,由等离子炬处理后的8YSZ颗粒制备的等离子喷涂涂层的结合强度最高,平均为25 MPa,抗热震性能最好,1200℃恒温5 min,水冷、热循环达41次;而采用等离子流场处理的颗粒所制涂层结合强度最差,平均为11 MPa,热震时涂层易开裂,热循环次数为17次;高温煅烧的颗粒所制涂层性能依赖于煅烧温度和时间,其中1200℃下煅烧2 h的颗粒所制涂层力学性能最优,平均结合强度为21 MPa,热循环次数为38次.     

Preparation of long-lifetime thermal barrier coatings and toughening mechanism by using hierarchy structured zirconia-based microspheres

The thermal cycling lifetime of thermal barrier coatings was doubled when deposited by electro-sprayed (ESP) microspheres instead of by commercial hollow spherical powders. It was believed that partial-molten nodules with featured microstructures inherited from the feedstock microspheres were the main contributor for prolonged thermal cycling durability due to improved fracture toughness and strain tolerance. The maximum lifetime was observed on samples with 20?30 vol.% of partial-molten microspheres. The hierarchy pores may both slow down the crack propagation by triggering multi-deflecting and promote cracking by reducing the tendency of interfacial deflection, the net effect depends on situation. The ESP coatings exhibited bimodal Weibull moduli upon indentation, which was regarded as originated from the hierarchy porous structure. Finally, the criterion was verified by micro-indentation and residual stain-stress evaluation by Raman spectroscopy.     

CMAS corrosion of YSZ thermal barrier coatings obtained by different thermal spray processes

Degradation of yttria-stabilized zirconia (YSZ) layers by molten CaO-MgO-Al₂O₃-SiO₂ (CMAS)-based deposits is an important failure mode of thermal barrier coating (TBC) systems in modern gas turbines. The present work aimed to understand how the chemical purity and microstructure of plasma-sprayed YSZ layers affect their response to CMAS corrosion. To this end, isothermal corrosion tests (1 h at 1250 °C) were performed on four different kinds of YSZ coatings: atmospheric plasma-sprayed (APS) layers obtained from standard- and high-purity feedstock powders, a dense – vertically cracked (DVC) layer, and a suspension plasma sprayed (SPS) one. Characterization of corroded and non-corroded samples by FEG-SEM, EBSD and micro-Raman spectroscopy techniques reveals that, whilst all YSZ samples suffered grain-boundary corrosion by molten CMAS, its extent could vary considerably. High chemical purity limits the extent of grain-boundary dissolution by molten CMAS, whereas high porosity and/or fine crystalline grain structure lead to more severe degradation.     

Microstructure and electrical properties of 3Y-TZP/Al2O3 composite obtained using the citrate gel method

3Y-TZP sinters with 1, 5, 10, and 15 mol% of Al₂O₃ were prepared using two different procedures from a 3-YSZ powder synthesized via the citrate process. In the first procedure, alumina was introduced during synthesis via the citrate method. In the second one, the 3-YSZ powder was impregnated with aluminum nitrate. All samples were sintered at 1773 K. The prepared composites were evaluated in terms of their microstructure, chemical and phase composition, and electrical properties. The total conductivity of the 3Y-TZP/Al₂O₃ composite material, which contained primarily the tetragonal phase, was found to increase with temperature, and to decrease for reduced concentrations of alumina in 3Y-TZP. In the case of the samples which had alumina introduced via impregnation, its higher content was not associated with significantly lower electrical conductivity. These samples generally exhibited higher conductivity than the samples to which alumina had been added via chemical synthesis.     

A highly strain and damage‐tolerant thermal barrier coating fabricated by electro‐sprayed zirconia hollow spheres

An approach to make air plasma sprayed (APS) thermal barrier coatings (TBCs) with the enhanced strain and damage tolerance was reported, using a novel hollow spheres produced by electro‐spraying (ESP) technique. Compared with agglomerated & sintered (A&S) and hollow spherical (HOSP) yttria‐stabilized zirconia (YSZ) powders, the ESP powder showed a unique network microstructure and the TBCs exhibited a 2‐3 times longer thermal cycling lifetime. The splat morphology and the top coats microstructure were investigated. Some semi‐melted ESP particles were observed in the as‐sprayed top coat. The indentation coupled with the Raman mapping technique was employed to evaluate the strain and damage tolerance of the TBCs. The coatings deposited by the ESP powder show a lower in‐plane stiffness determined by three‐point bending tests. It is proposed that the superior performance is attributed to the lower amount of the short microcracks (0.5‐4 μm) with low angle (<45°) and the semi‐melted ESP particles remained in the YSZ top coat.     

Preliminary study on suspension plasma sprayed ZrO2 + 8 wt.% Y2O3 coatings

ZrO₂ + 8 wt.% Y₂O₃ powder of a mean diameter d_VS = 38 μm was milled to obtain fine particles having mean size of d_VS = 1 μm. The fine powder was used to formulate a suspension with water, ethanol and their mixtures. The zeta potential of obtained suspensions was measured and found out to be in the range from −22 to −2 mV depending on suspension formulation. The suspension was injected through a nozzle into plasma jet and sprayed onto stainless steel substrates. The plasma spray experimental parameters included two variables: (i) spray distance varying from 40 to 60 mm and (ii) torch linear speed varying from 300 to 500 mm/s. The microstructure of obtained coatings was characterized with scanning electron microscope (SEM) and X-ray diffraction (XRD). The coatings had porosity in the range from 10% to 17% and the main crystal phase was tetragonal zirconium oxide. The scratch test enabled to find the critical load in the range of 9-11 N. Finally, thermal diffusivity of the samples at room temperature, determined by thermographic method, was in the range from 2.95 × 10⁻⁷ to 3.79 × 10⁻⁷ m²/s what corresponds to thermal conductivities of 0.69 W/(mK) and 0.97 W/(mK) respectively.     

Development of yttria-stabilized zirconia and graphene coatings obtained by suspension plasma spraying: Thermal stability and influence on mechanical properties

This study investigated the feasibility of depositing graphene nanoplatelet (GNP)-reinforced yttria-stabilized zirconia (YSZ) composite coatings. The coatings were deposited from an ethanol-based mixed YSZ and GNP suspension using suspension plasma spraying (SPS). Raman spectroscopy confirmed the presence of GNPs in the YSZ matrix, and scanning electron microscopy (SEM) analysis revealed a desired columnar microstructure with GNPs distributed predominantly in the inter-columnar spacing of the YSZ matrix. The as-deposited YSZ-GNP coatings were subjected to different isothermal treatments—400, 500, and 600 °C for 8 h—to study the thermal stability of the GNPs in the composite coatings. Raman analysis showed the retention of GNPs in specimens exposed to temperatures up to 500 °C, although the defect concentration in the graphitic structure increased with increasing temperature. Only a marginal effect on the mechanical properties (i.e., hardness and fracture toughness) was observed for the isothermally treated coatings.     

Oxygen‐ion conduction in scandia‐stabilized zirconia‐ceria solid electrolyte (xSc2O3–1CeO2–(99−x)ZrO2, 5 ≤ x ≤ 11)

Solid oxide fuel cells (SOFCs) operating at intermediate temperature (500°C‐700°C) provide advantages of better durability, lower cost, and wider target application market. In this work, we have studied Sc₂O₃ (5‐11 mol%) stabilized ZrO₂–CeO₂ as a potential solid electrolyte for application in IT‐SOFCs. Lower Sc₂O₃ doping range than the traditional 11 mol% Sc₂O₃‐stabilized ZrO₂ is an interesting research topic as it could potentially lead to an electrolyte with reduced oxygen vacancy ordering, lower cost, and higher mechanical strength. XRD and Raman spectroscopy was used to study the phase equilibrium in ZrO₂–CeO₂–Sc₂O₃ system and impedance spectroscopy was done to estimate the grain, grain boundary, and total ionic conductivities. Maximum for the grain and grain‐boundary conductivities as well as the tetragonal‐cubic phase boundary was found at 8‐9 Sc₂O₃ mol% in ZrO₂‐1 mol% CeO₂ system. It is suggested that the addition of 1 mol% CeO₂ in the ZrO₂ host lattice has improved the phase stability of high‐conductivity cubic and tetragonal phases at the expense of low‐conductivity t′‐ and β‐phases.     

Granule spray process for fabrication of adherent,low thermal conductivity ceramic coatings

This study proposes an effective strategy to fabricate ceramic thick films with controllable porosity but strong mechanical adhesion. The technique uses pre-heat-treated granules for a dry spray coating process, granule spray in vacuum. For proof-of-concept, Y₂O₃ spherical granules were prepared and subsequently pre-heat-treated at various temperatures to control their strength. During film deposition, hard granules (i.e., pre-heated at 1400 °C) caused damage to the substrate and pre-deposited film, owing to a strong hammering effect, which lead to limited film growth (1–2 μm). Contrarily, soft granules (i.e., pre-heated below 800 °C) produced a powder compact with low adhesion, resulting in delamination of some parts. This is attributed to the reduction in kinetic energy caused by the elastic cushioning effect during deposition. Regarding the granules with appropriate strength (i.e., pre-heated at 1000 °C), relatively porous (relative density: 74%) but mechanically well-adhering (adhesion strength: 41 MPa) thick films (~60 μm) were successfully coated on Al substrates. These films exhibited four times higher adhesion strength than those prepared with Y₂O₃ coatings using atmospheric plasma spraying. The films showed low thermal conductivities (~0.76 W/m·K at R.T.), suggesting potential application of our approach in the field of thermal insulation.     

Cadmium sulfide activated zinc oxide coatings deposited by liquid plasma spray for room temperature nitrogen dioxide detection under visible light illumination

Zinc oxide (ZnO) is well-known to be used as a gas sensing material. However, due to its high operation temperature, the chemical and thermal stability of ZnO based gas sensors are relatively low. In recent years, some researchers adopt light illumination as activated source to replace heating and obtain high gas sensing performance at low temperature. The study that follows is an attempt to use cadmium sulfide (CdS) as sensitizer to activate ZnO at room temperature with assistance of visible-light illumination. CdS–ZnO coatings were deposited by liquid plasma spray with aqueous solution containing zinc acetate and cadmium sulfide as precursors. The crystal structure of as-sprayed CdS–ZnO coatings was characterized by X-ray diffractometer (XRD) and field-emission scanning electron microscopy (FE-SEM). The sensing performance of sensors based on CdS–ZnO coatings were tested with 1 ppm nitrogen dioxide (NO₂) at room temperature illuminated under various visible lights. The results demonstrated that the LPS process was a straightforward method for deposition of high performance CdS–ZnO sensitive layers and the obtained sensors showed high responses to NO₂ at room temperature.     

An efficient Sm and Ge co-doped ceria nanocomposite electrolyte for low temperature solid oxide fuel cells

In this study we present a new nanocomposite electrolyte based on samarium (Sm) and germanium (Ge) co-doped ceria Ce_0.7Sm_0.15Ge_0.15O_2-δ (SGeDC). The nanocomposite electrolyte was prepared using co-precipitation method. The crystal structure and surface morphology were determined using x-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Four probe dc conductivity indicated the value of 0.074 S/cm at 650 °C. The Fuel cell performance was carried out using hydrogen as fuel. The maximum OCV observed was 0.95 V while the peak power density came out to be 600 mW/cm² at 600 °C. It is suggested that adding Ge, the conductivity as well as performance of this new nanocomposite electrolyte is comparatively enhanced and it can find potential applications in low temperature solid oxide Fuel cells (LTSOFCs).