Growth and optical properties of thulia-doped cubic yttria stabilized zirconia single crystals

Single crystals of yttria stabilized zirconia (YSZ) doped with different thulia (Tm₂O₃) contents (0.2–3.0 mol%) (abbreviated to Tm₂O₃: YSZ) were grown by the optical floating zone method. The crystals were transparent and inclusion free. These samples were then analyzed by X-ray diffraction (XRD), thermogravimetric-differential thermal analysis (TG-DSC), and Raman spectroscopy, and their optical properties were determined with Ultraviolet–visible (UV–Vis) and Photoluminescence spectroscopy (PL). The Tm₂O₃: YSZ single crystals were in the cubic phase, and the lattice parameters first increased and then decreased with increasing Tm₂O₃ content. The absorption spectra showed four peaks at around 356 nm (³H₆→¹D₂), 460.5 nm (³H₆→¹G₄), 678.5 nm (³H₆→³F_2,3) and 784 nm (³H₆→³H₄) in the visible region, and the optical bandgap energy increased with increasing Tm₂O₃ content, as a result of the Moss-Burstein effect. Measurements of PL spectra indicated a strong blue emission peak at 458 nm (¹D₂→³F₄), and three weak emission peaks at 487 nm (¹G₄→³H₆), 497 nm (¹D₂→³H₅), and 656.5 nm (¹G₄→³F₄) when the crystals were excited by light with a wavelength of 356 nm. The intensities of the emission peaks were strongly affected by the Tm₂O₃ content of the YSZ single crystals; the intensity increased with Tm₂O₃ content at low doping levels, reached the maximum at 0.5 mol% Tm₂O₃, then decreased with further increase in Tm₂O₃ content due to the concentration quenching effect. Additionally, the color changed from blue to cyan as the Tm₂O₃ content was increased. Overall, this work demonstrates that the cubic YSZ single crystal is a suitable host material for solid state luminescence.     

Preparation and optical properties of samaria-doped yttria-stabilized zirconia single crystals

Single crystals of yttria-stabilized zirconia (YSZ) doped with various concentrations of Sm₂O₃ were synthesized by the optical floating zone method, and their structure and spectroscopic properties characterized using powder X-ray diffraction (XRD), optical absorption, and luminescence measurements. XRD showed that the cubic phase of ZrO₂ was stabilized by addition of Y₂O₃ and a series of absorption peaks characteristic of Sm³⁺ was detected by UV-visible absorption spectroscopy. Three phenomenological J-O oscillator strength parameters Ω_t (t = 2, 4, 6) were calculated to evaluate the local structure and bonding in the vicinity of the Sm³⁺ for the 0.75 mol% Sm₂O₃: YSZ sample. These were in order of Ω₂ < Ω₄ < Ω₆ with values of Ω₂ = 5.46 × 10⁻²⁰ cm², Ω₄ = 10.70 × 10⁻²⁰ cm², and Ω₆ = 12.67 × 10⁻²⁰ cm², and indicate a relatively high symmetry and low covalent character for the Sm-O bond at the Sm³⁺ sites in the YSZ matrix. Photoluminescence spectra recorded under an excitation of 404 nm showed four emission bands centered at 571 nm, 621 nm, 652 nm, and 716 nm corresponding to the transitions ⁴G_5/2→⁶H_j (j = 5/2, 7/2, 9/2, 11/2). The transition probability, lifetime, and branching ratio obtained from the emission spectrum of the 0.75 mol% Sm₂O₃: YSZ crystal showed that YSZ is a potential host for Sm³⁺ to achieve a reddish-orange laser output, and the Sm³⁺-modified YSZ crystals have a possible use in reddish-orange laser and lighting devices.     

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.     

Influence of flash sintering on the ionic conductivity of 8 mol% yttria stabilized zirconia

The ionic conductivity of flash-sintered, polycrystalline 8 mol% yttria stabilized zirconia (8YSZ) was enhanced compared with that of conventionally-sintered specimens. Flash sintering was carried out at a furnace temperature of 850 °C with an electric field of 100 V cm^–1 to initiate flash. The current density limit was varied between 60 and 100 mA mm^–2. Post-flash impedance measurements over the range 215–900 °C showed that both bulk and grain boundary conductivities had increased with the increased current density limit which was set prior to flash. The conductivity increases post-flash were ionic, not electronic, although electronic conductivity probably occurred, in addition to ionic conductivity, during flash. The conductivity increases were not attributable to sample densification or microstructural changes. The higher ionic conductivities are attributed to a change in YSZ defect structure that led to an increased concentration of mobile charge carriers; possible explanations for this are discussed.     

Vacancy-ordered yttria stabilized zirconia as a low-temperature electronic conductor achieved by laser melting

Laser melting is known to be capable in initiating thorough evolution in microstructure and bringing novel functional performance in metals. But realization of this potential in ceramics only reaches a preliminary stage that needs further investigation. Here we demonstrate zirconia, traditionally an insulative ceramic at low temperature, could be transformed into an electronic conductor with the conductivity on order of 10⁻³ S⋅cm^-1 at room temperature by a simple laser melting process without inducing metallic phases. Transmission electron microscopy and ab-initio simulation show that oversaturated oxygen vacancies, together with their ordered metastable distribution along <001 > , are introduced during this non-equilibrium process, and result in a clear defect level significantly narrowing bandgap to less than 1 eV, leading to the considerable electronic conductivity. These results identify a strategy of utilizing this non-equilibrium method in oxide ceramics to realize some unconventional performances determined by metastable structure thoroughly altered down to atomic level.     

High temperature mechanical properties of zirconia metastable t'-Phase degraded yttria stabilized zirconia

Air plasma sprayed (APS) 8 wt%-yttria stabilized zirconia (8YSZ) with metastable tetragonal prime phase (t′) has been widely applied as thermal barrier coatings (TBCs) for gas turbine blades because of its outstanding mechanical properties at high temperatures. In the present research, a carefully designed process was used to prepare 8YSZ samples with different phase composition (t′, t and c) simulating the phase degradation of the material during operation conditions. High temperature (1000–1200 °C) bending strength, elastic modulus, and thermal expansion coefficient were measured, which exhibit strong dependence on the phase degradation during heat treatment. Effect of the phase composition on high temperature thermo-mechanical properties and the enhancement of the bending strength have been discussed, providing a new perspective for further improvements.     

Effects of manganese oxide addition and reductive atmosphere annealing on the phase stability and microstructure of yttria stabilized zirconia

The effects of Mn₃O₄ addition and reductive atmosphere (N₂:H₂ = 97:3) annealing on the microstructure and phase stability of yttria stabilized zirconia (YSZ) ceramics during sintering at 1500 °C for 3 h in air and subsequent annealing in a reductive atmosphere were investigated. Mn₃O₄ added 6 mol% YSZ (6YSZ) and 10 mol% YSZ (10YSZ) ceramics were prepared via the conventional solid-state reaction processes. The X-ray diffraction results showed that a single cubic phase of ZrO₂ was obtained in 1 mol% Mn₃O₄ added 6YSZ ceramic at a sintering temperature of 1500 °C for 3 h. A trace amount of monoclinic ZrO₂ phases were observed for 1 mol% Mn₃O₄ added 6YSZ ceramics after annealing at 1300 °C for 60 cycles in a reductive atmosphere by transmission electron microscopy. Furthermore, a single cubic ZrO₂ phase existed stably as Mn₃O₄ added 10YSZ ceramics was annealed at 1300 °C for 60 cycles in reductive atmosphere.     

钇稳定纳米氧化锆的制备工艺研究

用溶胶-凝胶法制备了钇稳定纳米氧化锆(YSZ),系统研究了沉淀剂滴加方式、锆离子浓度、溶剂、干燥方式和煅烧温度对产物性能的影响,并采用FT-IR、TG-DSC、XRD、SEM等对前驱体干凝胶粉末和产物进行了表征。结果表明：以水作溶剂,采用正加方式,选择锆离子浓度为0.5 mol/L,真空干燥,550 ℃煅烧2 h,可得到粒径为39 nm左右、组分为单一四方相的YSZ粉体。     

Laser surface modification of porous yttria stabilized zirconia against CMAS degradation

Here, we present a new combined freeze-casting and laser processing method for the design of yttria-stabilized zirconia (YSZ) based thermal-barrier coatings. YSZ ceramics with unidirectionally-aligned pore channels were created using the freeze-casting method. After sintering, top view and cross-sectional scanning electron microscopy (SEM) revealed the structural features of the preform, which exhibits a 74 ± 2% volume fraction of porosity and an average pore channel size of 30 ± 3 μm. The measured thermal conductivity of this porous structure was 0.27 ± 0.02 W/(m K), which is eight times lower than that of reported values for dense YSZ. Though high porosity is beneficial both from a structural and thermal response perspective, the open porosity could potentially be an issue from an application stand-point when evaluating the resistance of materials to calcium–magnesium–aluminum–silicon oxide (CMAS) attack. CMAS attack, which can originate from deposits of molten sand, ash, and dust, is one of the major causes of thermal barrier coating failure. Therefore, the surface of the porous samples was modified using a laser process to create a barrier to CMAS infiltration. SEM micrographs aided in determining the optimum laser parameters required to fully seal the surface using a laser treatment. The performance of the original porous and surface-modified YSZ was compared by conducting CMAS infiltration studies. Laser modification was shown to be a viable technique to significantly reduce CMAS infiltration in porous thermal barrier coatings.     

Thermal conductivity of yttria-stabilized zirconia thin films grown by plasma-enhanced atomic layer deposition

Zirconia doped with yttrium, widely known as yttria-stabilized zirconia (YSZ), has found recent applications in advanced electronic and energy devices, particularly when deposited in thin film form by atomic layer deposition (ALD). Although ample studies reported the thermal conductivity of YSZ films and coatings, these data were typically limited to Y₂O₃ concentrations around 8 mol% and thicknesses greater than 1 μm, which were primarily targeted for thermal barrier coating applications. Here, we present the first experimental report of the thermal conductivity of YSZ thin films (∼50 nm), deposited by plasma-enhanced ALD (PEALD), with variable Y₂O₃ content (0–36.9 mol%). Time-domain thermoreflectance measures the effective thermal conductivity of the film and its interfaces, independently confirmed with frequency-domain thermoreflectance. The effective thermal conductivity decreases from 1.85 to 1.22 W m⁻¹ K⁻¹ with increasing Y₂O₃ doping concentration from 0 to 7.7 mol%, predominantly due to increased phonon scattering by oxygen vacancies, and exhibits relatively weak concentration dependence above 7.7 mol%. The effective thermal conductivities of our PEALD YSZ films are higher by ∼15%–128% than those reported previously for thermal ALD YSZ films with similar composition. We attribute this to the relatively larger grain sizes (∼23–27 nm) of our films.     

Ultrasonic cavitation erosion of as-sprayed and laser-remelted yttria stabilized zirconia coatings

Cavitation erosion resistance of 8 wt.% yttria stabilized zirconia has been investigated in specimens prepared by atmospheric plasma spraying and laser remelting post treatment. The results indicate that as-sprayed coatings involve defects such as primary cavities and initial micro cracks inside a particle and among the interfaces of particles. When the specimens are subjected to cavitation erosion, the micro cracks initiate and coalesce along with chip removals. Laser remelting produces a dense glazed layer with some cracks though the coatings. With the increasing of erosion time, large pieces are delaminated from coating-substrate interface leading to a significant mass loss. However, the resistance of laser remelted coatings to cavitation erosion is significantly improved when they are impregnated with epoxy by vacuum castable mounting. The relationship between cracks formed inside the laser remelted YSZ coatings and their damage mechanism under cavitation is discussed.     

Robocasting of dense zirconia parts using commercial yttria-stabilized zirconia granules and ultrafine particles. Paste preparation,printing, mechanical properties

In this work, a commercial 3 mol% yttrium oxide partially stabilized zirconia spray-dried granules were used as such, and after intensive milling, for the production of dense samples by robocasting technique. The aim was to evaluate the suitability of using spray-dried granules instead of submicron particles in robocasting of ceramic parts. To this aim, robocasting pastes were produced from the as-received granules as well as from nanometric particles obtained from an effective ball milling of the granules. The formulation of the pastes was optimized in order to achieve optimal printability. The drying conditions were optimized as well, and the control of temperature and humidity either in the printing and drying chambers allowed to eliminate any warping issue.After sintering at 1450 °C, the monolithic dense parts were characterized by ultra-fine and highly sintered microstructures, with the highest density achieved by the sample produced from granules. This material not only did not show the typical defects of granules-derived ceramics but was also characterized by fewer and smaller defects than the sample produced by the fine particles. It seemed that the presence of large granules, and thus large intergranular space, favored the debinding process and limited the formation of the typical processing flaws. The ball-on-three balls test was used for the evaluation of the mechanical properties of the dense samples. Biaxial flexural strength values in the range from ～100 to 500 MPa (with an average value of 255 ± 93 MPa) were determined for the samples made from granules while these values ranged from ～100 to 300 MPa (with an average strength of 198 ± 52 MPa) for the samples prepared from the milled particles. In spite of low, these values are in line with literature data, obtained on 3 mol% Y₂O₃–ZrO₂ samples fabricated by the robocasting technique, and suitable to demonstrate the feasibility to use as-received granules as feeding material for robocasting parts.     

Solution combustion synthesis of calcia-magnesia-aluminosilicate powder and its interaction with yttria-stabilized zirconia and co-doped yttria-stabilized zirconia

Thermal Barrier Coatings (TBCs) play a significant role in improving the efficiency of gas turbines by increasing their operating temperatures. The TBCs in advanced turbine engines are prone to silicate particles attack while operating at high temperatures. The silicate particles impinge on the hot TBC surfaces and melt to form calcia-magnesia-aluminosilicate (CMAS) glass deposits leading to coating premature failure. Fine powder of CMAS with the composition matching the desert sand has been synthesized by solution combustion technique. The present study also demonstrates the preparation of flowable yttria-stabilized zirconia (YSZ) and cluster paired YSZ (YSZ-Ln₂O₃, Ln = Dy and Gd) powders by single-step solution combustion technique. The as-synthesized powders have been plasma sprayed and the interaction of the free standing TBCs with CMAS at high-temperatures (1200 °C, 1270 °C and 1340 °C for 24 h) has been investigated. X-ray diffraction analysis of CMAS attacked TBCs revealed a reduction in phase transformation of tetragonal to monoclinic zirconia for YSZ-Ln₂O₃ (m-ZrO₂: 44%) coatings than YSZ (m-ZrO₂: 67%). The field emission scanning electron microscopic images show improved CMAS resistance for YSZ-Ln₂O₃ coatings than YSZ coatings.     

Catalytic composites based on yttria stabilized zirconia for oxidative dehydrogenation of ethane

LiCl/YSZ is found to be a very effective catalyst for the oxidative dehydrogenation of ethane. LiCl supported on YSZ-MgO composite shows increase in catalytic activity and ethylene selectivity. Addition of Mn and Sn as promoters to this system leads to 85% ethane conversion, 77% ethylene selectivity and 65% ethylene yield at 662 °C. Use of Li₂O in the place of LiCl results in lower ethylene yields. Further modification is needed to improve the catalyst stability.     

Correlation between yttria stabilized zirconia particle size and morphological properties of NiO–YSZ films prepared by spray coating process

A systematic approach was taken to investigate the morphology of NiO–yttria stabilized zirconia (YSZ) films deposited by a spray coating process. The final morphological aspects of anode films were influenced by the particle size of YSZ powders and the milling time of the slurries used for film deposition. YSZ powders with average particle size of 17 and 52 nm were obtained from powders calcined at 800 and 1000 °C, respectively. The results obtained by rheological studies pointed out that slurries prepared from YSZ powders calcinated at 1000 °C and milling time of 20 h had more stability. All slurries presented thixotropic and pseudoplastic behaviors.     

Microstructure and thermal properties of nanostructured gadolinia doped yttria-stabilized zirconia thermal barrier coatings produced by air plasma spraying

In this article, the nanostructured 2 mol% Gd₂O₃-4.5 mol% Y₂O₃-ZrO₂(2GdYSZ) coating was developed by the atmospheric plasma spraying technique. And the microstructure and thermal properties of plasma-sprayed 2GdYSZ coating were investigated. The result from the investigation indicates that the as-sprayed coating is characterized by typical microstructure consisting of melted zones, nano-zones, splats, nano-pores, high-volume spheroidal pores and micro-cracks. The 2GdYSZ coating shows a lower resistance to destabilization of the metastable tetragonal (t′) phase compared to the yttria stabilized zirconia(YSZ). The thermal diffusivity and thermal conductivity of the nano-2GdYSZ coating at room temperature are 0.431 mm² s⁻¹ and 1.042 W/m K, respectively. Addition of gadolinia to the nano-YSZ can significantly reduce the thermal conductivity compared to the nano-YSZ and the conventional YSZ. The reduction is mainly attributed to the synergetic effect of gadolinia doping along with nanostructure.     

Mechanical properties of ceria-calcia stabilized zirconia ceramics with alumina additions

Ceria-stabilized zirconia-based composites have been developed aiming to obtain ceramic materials with enhanced hardness, strength, fracture toughness, and resistance to low temperature degradation. These composites are based on ceria-calcia stabilized zirconia (10 mol% CeO₂-1 mol% CaO TZP) and α-alumina (0?15 wt%) as a second phase. Raw materials in the form of powders were dispersed through ball milling, dried by slip casting, and subsequently grounded before being pressed and conventionally sintered at 1450 °C. Compared to the strength and hardness of 10Ce-TZP ceramics (typically 500 MPa and 6 GPa), an increase was observed for all compositions, especially for 10Ce-1CaO-5Al₂O₃ (739 MPa and 10.2 GPa). Single Edge V-Notched Beam fracture toughness values ranged from 5.1 to 6.6 MPa?√m, indicating a loss of transformability for all compositions. As in 10Ce-1CaO-TZP co-doped ceramics, the aging resistance of all alumina-containing composites was also excellent.     

Microstructure and thermo-physical properties of yttria stabilized zirconia coatings with CMAS deposits

Yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) are used to protect hot-components in aero-engines from hot gases. In this paper, the microstructure and thermo-physical and mechanical properties of plasma sprayed YSZ coatings under the condition of calcium-magnesium-alumina-silicate (CMAS) deposits were investigated. Si and Ca in the CMAS rapidly penetrated the coating at 1250 °C and accelerated sintering of the coating. At the interface between the CMAS and YSZ coating, the YSZ coating was partially dissolved in the CMAS, inducing the phase transformation from tetragonal phase to monoclinic phase. Also, the porosity of the coating was reduced from ∼25% to 5%. As a result, the thermal diffusivity at 1200 °C increased from 0.3 mm²/s to 0.7 mm²/s, suggesting a significant degradation in the thermal barrier effect. Also, the coating showed a ∼40% increase in the microhardness. The degradation mechanism of TBC induced by CMAS was discussed.     

Thermophysical behavior of thermal sprayed yttria stabilized zirconia based composite coatings

The effective thermal conductivity of a composite coating depends on intrinsic thermal conductivity of the constituent phases, its characteristics (size, shape) and volume fraction of porosities. The present study concerns studying the effect of CoNiCrAlY and Al₂O₃ content on the coefficient of thermal expansion and thermal conductivity of the YSZ (YSZ-CoNiCrAlY and YSZ-Al₂O₃) based composite coatings developed by thermal spray deposition technique. The coefficient of thermal expansion and thermal conductivity of the composite coatings were measured by push rod dilatometer and laser flash techniques, respectively, from room temperature to 1000 °C. Variation in density, porosity, coefficient of thermal expansion, and thermal conductivity was observed in the composite coatings with the addition of different volume fraction of CoNiCrAlY and Al₂O₃ powders in YSZ-CoNiCrAlY and YSZ-Al₂O₃ composites, respectively. Comparison between the theoretical and experimental thermal conductivities showed a mismatch varying from 4% to 58% for YSZ-CoNiCrAlY composite coatings and from 58% to 80% for YSZ-Al₂O₃ composite coatings. Model based analyses were used to understand the mechanism of thermal conductivity reduction in the composite coatings. It was concluded that the morphology of porosities varied with composition.