Constrained sintering of 8 mol% Y2O3 stabilised zirconia films

Constrained sintering kinetics of 8 mol% Y₂O₃/92 mol% ZrO₂ (8YSZ) films approximately 10–15 μm thick screen-printed on dense YSZ substrates, and the resulting stress induced in the films, were measured in the temperature range 1100–1350 °C. The results are compared with those reported earlier for 3YSZ films.Both materials behave similarly, although there are differences in detail. The constrained densification rate was greatly retarded compared with the unconstrained densification rate due to the effect of the constraint on the developing anisotropic microstructure (3YSZ) and, in the case of 8YSZ, considerable grain growth. The stress generated during constrained sintering was typically a few MPa. The apparent activation energies for free sintering, constrained sintering, creep and grain growth are found to cover a wide range (135–670 kJ mol^?1) despite all probably being mainly controlled by grain boundary cation diffusion.     

Effect of Fe2O3 doping on sintering of yttria-stabilized zirconia

This paper reports the effect of Fe₂O₃ doping on the densification and grain growth in yttria-stabilized zirconia (YSZ) during sintering at 1150 °C for 2 h. Fe₂O₃ doped 3 mol% YSZ (3YSZ) and 8 mol% YSZ (8YSZ) coatings were produced using electrophoretic deposition (EPD). For 0.5 mol% Fe₂O₃ doping, both 3YSZ and 8YSZ coatings during sintering at 1150 °C has similar densification. However, a significant grain growth occurred in 8YSZ during sintering, whereas grain size remains almost constant in 3YSZ. XRD results suggest that Fe₂O₃ addition substitutionally and interstitially dissolved into the lattice of 3YSZ and 8YSZ. In addition, colour of 3YSZ and 8YSZ changes differently with doping of Fe₂O₃. A Fe³⁺ ion interstitial diffusion mechanism is proposed to explain the densification and grain growth behaviour in the Fe₂O₃ doped 3YSZ and 8YSZ. A retard grain growth observed in the Fe₂O₃ doped 3YSZ is attributed to Fe³⁺ segregation at grain boundary.     

Low thermal conductivity of atomic layer deposition yttria-stabilized zirconia (YSZ) thin films for thermal insulation applications

Thermal insulation applications have long required materials with low thermal conductivity, and one example is yttria (Y₂O₃)-stabilized zirconia (ZrO₂) (YSZ) as thermal barrier coatings used in gas turbine engines. Although porosity has been a route to the low thermal conductivity of YSZ coatings, nonporous and conformal coating of YSZ thin films with low thermal conductivity may find a great impact on various thermal insulation applications in nanostructured materials and nanoscale devices. Here, we report on measurements of the thermal conductivity of atomic layer deposition-grown, nonporous YSZ thin films of thickness down to 35 nm using time-domain thermoreflectance. We find that the measured thermal conductivities are 1.35–1.5 W m⁻¹ K⁻¹ and do not strongly vary with film thickness. Without any reduction in thermal conductivity associated with porosity, the conductivities we report approach the minimum, amorphous limit, 1.25 W m⁻¹ K⁻¹, predicted by the minimum thermal conductivity model.     

Effect of cation doping on lattice and grain boundary diffusion in superplastic yttria-stabilized tetragonal zirconia

Lattice diffusion coefficients D_l and grain boundary diffusion D_gb coefficients of hafnium were studied for 0.5 and 1 mol% cation-doped yttria-stabilized tetragonal zirconia at the temperature range from 1283 to 1510 °C. The diffusion profiles were determined by two experimental techniques: secondary ion mass spectroscopy and electron microprobe analysis. Additionally the first principle calculations of the electronic states of Zr⁴⁺, dopant cations and O^2? anions and elastic properties in 3Y-TZP were performed. Superplastic strain rate versus stress and inverse temperature was also measured. For 1 mol% doped samples the significant increase of the grain boundary diffusion and superplastic strain rate was observed. Correlations between the calculated ionic net charges and D_gb indicate that enhancement of D_gb was caused by the reduction of ionic bonding strength between metal cation and oxygen anion in zirconia. The new constitutive equation for superplastic flow of yttria-stabilized tetragonal zirconia ceramics was obtained.     

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.     

Fabrication and evaluation of NiO/Y2O3-stabilized-ZrO2 hollow fibers for anode-supported micro-tubular solid oxide fuel cells

In this work NiO/3 mol% Y₂O₃–ZrO₂ (3YSZ) and NiO/8 mol% Y₂O₃–ZrO₂ (8YSZ) hollow fibers were prepared by phase-inversion. The effect of different kinds of YSZ (3YSZ and 8YSZ) on the porosity, electrical conductivity, shrinkage and flexural strength of the hollow fibers were systematically evaluated. When compared with Ni–8YSZ the porosity and shrinkage of Ni–3YSZ hollow fibers increases while the electrical conductivity decreases, while at the same time also exhibiting enhanced flexural strength. Single cells with Ni–3YSZ and Ni–8YSZ hollow fibers as the supported anode were successfully fabricated showing maximum power densities of 0.53 and 0.67 W cm⁻² at 800 °C, respectively. Furthermore, in order to improve the cell performance, a Ni–8YSZ anode functional layer was added between the electrolyte and Ni–YSZ hollow fiber. Here enhanced peak power densities of 0.79 and 0.73 W cm⁻² were achieved at 800 °C for single cells with Ni–3YSZ and Ni–8YSZ hollow fibers, respectively.     

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.     

Fabrication and properties of reduced graphene oxide reinforced yttria-stabilized zirconia composite ceramics

Fully dense yttria-stabilized zirconia (YSZ) ceramics reinforced with reduced graphene oxide (RGO) were fabricated by spark plasma sintering (SPS), and their electrical, thermal, and mechanical properties were investigated. Graphene oxide (GO) was exfoliated by a short sonification in dimethylformamide (DMF)/water solution and uniformly mixed with ZrO₂ powders. The microstructure of the composites showed that undamaged RGO sheets were homogeneously distributed throughout matrix grains. The electrical conductivity of YSZ composites drastically increased with the addition of RGO, and it reached 1.2 × 10⁴ S/m at 4.1 vol.%. However, the thermal diffusivity increased only 12% with RGO addition. The hardness decreased slightly with RGO addition, whereas the fracture toughness significantly increased from 4.4 to 5.9 MPa^1/2. The RGO pull-out and crack bridging contributed to the improved fracture toughness.     

Study of thin films of yttria-stabilized zirconia (YSZ) for the oxidation of some volatile organic compounds

Thin films of YSZ and 1%Pt/YSZ were deposited onto stainless steel tubes by an electrophoretic deposition technique. O₂-TPD from r.t. to 600 °C using induction heating was used to characterize the two films considering (i) the amount of oxygen desorbed (5.1 and 1.4 × 10^− 2 μmol O₂·g^− 1 for 1%Pt/YSZ and YSZ respectively) and (ii) the apparent activation energy of desorption E_app. Finally, complete oxidation of some VOCs (isopropanol and toluene) in air was studied on both films. From r.t. to 400 °C, oxidation of isopropanol can be achieved with either YSZ or 1%Pt/YSZ but only this last catalyst can achieve the complete oxidation of toluene.     

Molten salt attack on t′ yttria-stabilised zirconia by dissolution and precipitation

Degradation due to molten salt attack is one of the failure mechanisms of thermal barrier coatings. Thermochemical attack of the salt mixture Na₂SO₄–30 mol% NaVO₃ on ZrO₂–8 mol% YO_1.5 (8YSZ) at 950 °C was studied by two types of experiments. Sintered compacts were exposed to 25 mg cm^?2 salt dosage for up to 96 h. In the other set of experiments, 10–35 wt.% 8YSZ powder was mixed with the salts to study the dissolution of 8YSZ in the molten salt. The role of volatile losses was also examined. The results show that more than 25 wt.% 8YSZ dissolves in the sulphate-vanadate melt at 950 °C, followed by slow reactions to form YVO₄ and NaYV₂O₇ at 950 °C. The unreacted Y₂O₃ and monoclinic ZrO₂ precipitate out separately during rapid cooling (～300 °C/min). Slow cooling at ～3 °C/min leads to the formation of ZrOS apart from ZrO₂ and Y₂O₃.     

Fabrication and electrochemical properties of SOFC single cells using porous yttria-stabilized zirconia ceramic support layer coated with Ni

A porous yttria-stabilized zirconia (YSZ) ceramic supported single cell with a configuration of porous YSZ support layer coated with Ni/Ni–Ce_0.8Sm_0.2O_1.9 (SDC) anode/YSZ/SDC bi-layer electrolyte/La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ cathode was fabricated. The porosity, mechanical strength, and microstructure of porous YSZ ceramics were investigated with respect to the amount of poly(methyl methacrylate) (PMMA) used as a pore former. Porous YSZ ceramics with 56 vol.% PMMA showed a mechanical strength of 24 ± 3 MPa and a porosity of 37 ± 1%. The electrochemical properties of the single cell employing the porous YSZ support layer were measured using hydrogen and methane fuels, respectively. The single cell exhibited maximum power densities of 421 mW/cm² in hydrogen and 399 mW/cm² in methane at 800 °C. Moreover, at a current density of 550 mA/cm², the cell maintained 91% of its initial voltage after operation in methane for 13 h at 700 °C.     

Direct coagulation casting of yttria-stabilized zirconia using magnesium citrate and glycerol diacetate

Direct coagulation casting via controlled release of high valance counter ions (DCC-HVCI) has been reported in recent years. In this paper, concentrated yttria-stabilized zirconia (YSZ) suspensions were coagulated using DCC-HVCI method with magnesium citrate as coagulating agent assisted by pH shift in the presence of glycerol diacetate. The effect of ammonium polyacrylate (PAA-NH₄) on the dispersibility of YSZ powder was investigated. The influence of concentrations of glycerol diacetate and magnesium citrate on pH and viscosities of YSZ suspensions was studied. The results indicate that concentrated YSZ suspensions can be coagulated by adding 2 vol% glycerol diacetate and magnesium citrate above 0.5 wt% at room temperature for 2–5 h. The compressive strength of coagulated wet samples is above 2.0 MPa. YSZ ceramics sintered at 1450 °C show homogeneous microstructures with relative densities of 98.9–99.2%. Flexural strength of YSZ ceramics is 869±84 MPa.     

Investigation of reactions between vanadium oxide and plasma-sprayed yttria-stabilized zirconia coatings

The phase evolution occurring during the reaction between corrosive V₂O₅ (T_m = 690 °C) and a plasma-sprayed 7 wt.% Y₂O₃–ZrO₂ (YSZ) coating from 700 to 900 °C has been investigated in situ by X-ray diffraction. The temperature and time of interaction between the V₂O₅ and YSZ coating determines the phases observed. Between 700 and 750 °C, reaction products of ZrV₂O₇ and YVO₄ were observed within minutes of reaching the test temperature. m-ZrO₂ was observed after 220 and 60 min at 700 and 750 °C, respectively. The simultaneous formation of both ZrV₂O₇ and YVO₄ at the beginning of the reaction along with the delay of the m-ZrO₂ formation suggests similar reactivity between both Zr and Y with V₂O₅. The weight percent of the ZrV₂O₇ phase began to diminish after 150 and 60 min at 700 and 750 °C, respectively. For reaction temperatures of 800 and 900 °C, there is a rapid decrease in the amount of t′-ZrO₂ and a rapid increase in the amount of m-ZrO₂ with reaction time. YVO₄ was also observed at these reaction temperatures. SEM and TEM microstructural observations confirmed the phases detected from the in situ XRD experiments. Reactions between YSZ and V₂O₅ suggest that the formation of a liquid phase due to the high solubility of both zirconia and yttria in vanadia is the dominate mechanism that damages the coating. The thermal conductivity of a plasma-sprayed YSZ coating reacted with up to 1 wt.% V₂O₅ did not significantly change due to the small volume affected.     

Kinetic model of CaO/fly ash sorbent for flue gas desulphurization at moderate temperatures

Characteristics of the sulphation reaction between SO₂ and CaO/fly ash sorbent were analyzed based on TGA results to develop a kinetic model for a dry moderate temperature (400–800 °C) FGD process. It was found that SO₂ diffusion within sorbent particles involved three sub-processes: inter-particle diffusion, inter-grain diffusion and diffusion through product layers and the diffusion dominated the whole sulphation reaction process. The activation energy for product layer diffusion E_diff of 49.3 kJ mol⁻¹ being greater than the chemical reaction activation energy E_a of 13.9 kJ mol⁻¹ verified the importance of the diffusion. Predictions using the kinetic model in which k₀ varies with temperature agree well with the experimental data.     

Raman study of yttria-stabilized zirconia interfacial coatings on Nicalon™ fiber

The undoped, 3Y- and 9Y-stabilized ZrO₂ interfacial coatings on SiC-based fiber type Nicalon™ were fabricated by sol–gel approach and studied using Raman spectroscopy. Raman spectroscopy proved to be a very successful method for revealing beyond question the monoclinic, tetragonal and cubic modification in the as-prepared and exposed to air ZrO₂-coated Nicalon™ fibers. The quantitative phase analysis in the tetragonal or tetragonal/monoclinic two-phase interfacial zirconia coatings was done using an accurate calibration curve directly determined from the Raman spectra of standard mixtures with known monoclinic and tetragonal phase ratios. It was found that the undoped ZrO₂ coating on Nicalon™ fiber was composed of monoclinic together with tetragonal modification in approximately equal fractions whereas after exposition to air the t → m phase transformation occurred in full extent. The 3YSZ coating also underwent the t → m transformation, with the extent of this transformation being different for various areas of the same filament and for various filaments.A monitoring of the t → m phase transformation within ZrO₂ coating on Nicalon™ fiber using micro-Raman spectroscopy makes it possible quantitatively to evaluate an ability of ZrO₂ as oxidation resistance and readily deformable weak interfacial coating for CMC's.     

Master sintering curves of a nanoscale 3Y-TZP powder compacts

The sintering behavior of commercially available granulated ZrO₂–3 mol% Y₂O₃ (3Y-TZP) powder compacts with an aggregate size of 75 nm was studied. The shrinkage response of the powder compacts during non-isothermal sintering was measured in a sensitive dilatometer at different heating rates. Densification and grain growth were also studied after isothermal firing in air according to different sintering cycles. The sintering and grain growth activation energy was estimated to be Q_S = 485 ± 12 kJ mol^?1 and Q_G = 546 ± 23 kJ mol^?1, respectively. Using the estimated Q-values, the master curves for sintering and grain growth were established and used for prediction of the densification and microstructural development under different thermal histories. A good agreement between the model predictions and experimental result was obtained.     

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.     

Phase-transformation and grain-growth kinetics in yttria-stabilized tetragonal zirconia polycrystal doped with a small amount of alumina

Microstructure development during sintering in 3 mol% Y₂O₃-stabilized tetragonal zirconia polycrystal doped with a small amount of Al₂O₃ was investigated in the isothermal sintering conditions of 1300–1500 °C. At the low sintering temperature at 1300 °C, although the density was relatively high, the grain-growth rate was much slow. In the specimen sintered at 1300 °C for 50 h, Y³⁺ and Al³⁺ ions segregated along grain boundaries within the widths of about 10 and 6 nm, respectively. In grain interiors, the cubic-phase regions were formed by not only a grain-boundary segregation-induced phase-transformation mechanism but also by spinodal decomposition. The grain-growth behavior was kinetically analyzed using the grain-size data in 1300–1500 °C, which indicated that the grain-growth rate was enhanced by Al₂O₃-doping. These phase-transformation and grain-growth behaviors are reasonably explained by the diffusion-enhanced effect of Al₂O₃-doping.     

Isothermal sintering kinetic of 3YTZ and 8YSZ: Cation diffusion

We have studied the sintering kinetic of 3 and 8 mol% of yttria stabilized zirconia under isothermal conditions. Sintering was performed in the temperature range between 1200 and 1450 °C. The sintering kinetic process was followed by measuring density as a function of sintering time. A model was applied to the first stage of densification. Sintering obeys to the grain boundary diffusion mechanism for both materials. It was possible to calculate the activation energy as well as the diffusion coefficients. 887 and 731 kJ mol^?1 were the activation energies for the initial stage of sintering for 3YTZ and 8YSZ respectively.Finally the diffusion activation energy was estimated for both materials. The diffusion coefficients were also estimated at 1400 °C in 4.05×10^?14 and 6.00×10^?11 cm² s^?1 for tetragonal and cubic zirconia respectively. The obtained results support the observations of a faster densification for 8YSZ.     

Thin film yttria-stabilized zirconia electrolyte for intermediate-temperature solid oxide fuel cells (IT-SOFCs) by chemical solution deposition

A 500 nm thick thin film YSZ (yttria-stabilized zirconia) electrolyte was successfully fabricated on a conventionally processed anode substrate by spin coating of chemical solution containing slow-sintering YSZ nanoparticles with the particle size of 20 nm and subsequent sintering at 1100 °C. Incorporation of YSZ nanoparticles was effective for suppressing the differential densification of ultrafine precursor powder by mitigating the prevailing bi-axial constraining stress of the rigid substrate with numerous local multi-axial stress fields around them. In particular, adding 5 vol% YSZ nanoparticles resulted in a dense and uniform thin film electrolyte with narrow grain size distribution, and fine residual pores in isolated state. The thin film YSZ electrolyte placed on a rigid anode substrate with the GDC (gadolinia-doped ceria) and LSC (La_0.6Sr_0.4CoO_3?δ) layers deposited by PLD (pulsed laser deposition) processes revealed that it had fairly good gas tightness relevant to a SOFC (solid oxide fuel cell) electrolyte and maintained its structural integrity during fabrication and operation processes. In fact, the open circuit voltage was 1.07 V and maximum power density was 425 mW/cm² at 600 °C, which demonstrates that the chemical solution route can be a viable means for reducing electrolyte thickness for low- to intermediate-temperature SOFCs.