Doped Yttria-stabilized Zirconia Research Articles

Mixed Electronic-Ionic Conductivity and Stability of Spark Plasma Sintered Graphene-Augmented Alumina Nanofibres Doped Yttria Stabilized Zirconia GAlN/YSZ Composites

Graphene-doped ceramic composites with mixed electronic-ionic conductivity are currently attracting attention for their application in electrochemical devices, in particular membranes for solid electrolyte fuel cells with no necessity to use the current collector. In this work, composites of the Y2O3-ZrO2 matrix with graphene-augmented γ-Al2O3 nanofibres (GAlN) were spark plasma sintered. The conductivity and electrical stability in cyclic experiments were tested using electrical impedance spectroscopy. Composites with 0.5 and 1 wt.% GAlN show high ionic conductivity of 10-2-10-3 S/cm at 773 K. Around 3 wt.% GAlN percolation threshold was achieved and a gradual increase of electronic conductivity from ~10-2 to 4 × 10-2 S/cm with an activation energy of 0.2 eV was observed from 298 to 773 K while ionic conductivity was maintained at elevated temperatures. The investigation of the evolution of conductivity was performed at 298-973 K. Besides, the composites with 1-3 wt.% of GAlN addition show a remarkable hardness of 14.9-15.8 GPa due to ZrC formation on the surfaces of the materials.

Stresses within rare-earth doped yttria-stabilized zirconia thermal barrier coatings from in-situ synchrotron X-ray diffraction at high temperatures

There is a growing interest for smart coatings that can be integrated into turbine engines for in-situ temperature measurements or health monitoring. The addition of rare-earth dopants into standard thermal barrier ceramic top coat materials is used to obtain luminescent coatings that enable spectral measurements, for real-time temperature or health monitoring. The thermomechanical performance and durability of such novel coating compositions in extreme environments still remains to be evaluated. Consequently, the ability to manufacture sensor coatings which present suitable thermal properties needs to be demonstrated. For this study, highly luminescent erbium and europium doped yttria-stabilized zirconia and state-of-the-art yttria-stabilized zirconia coatings manufactured by air plasma spray were characterized to determine the effects of the embedded rare-earth dopants on coating internal strain and stress and to quantify and compare their high temperature response using synchrotron X-ray diffraction. In-situ depth-resolved strain measurements were performed at 15 μm intervals along the depth of the coatings to evaluate materials response at key locations, specifically at layer interfaces. In-plane stress was calculated for the coatings and a finite element model was implemented to supplement the results and enable further predictions. The results show that the sensor coatings that were manufactured in this work revealed only minor variations in the strain response of sensor coatings under a typical thermal cycle and especially at temperatures closer to that of gas turbine operating conditions, compared to state-of-the-art coatings. This work demonstrates the viability of manufacturing rare-earth doped yttria-stabilized zirconia coatings that provide beneficial spectroscopic monitoring capabilities while having minimal impact on the thermomechanical response of the thermal barrier coatings.

Conductivity Degradation of Transition Metal (Mn, Fe, Co) Doped Yttria Stabilized Zirconia by Phase Transformation

Conductivity degradation behavior of 1 mol% transition metal (Mn, Fe, Co) doped yttria stabilized zirconia (YSZ) under SOFC operation atmospheres was examined by AC impedance technique under an OCV condition, using a test cell consisting of dry air, Pt / transition metal (Mn, Fe, Co) doped 8YSZ / Pt, 1.2 % H2O–H2, at 1173 K for 300 h. Phase transformation of YSZ from the cubic to the tetragonal phase was analyzed by Raman spectroscopy. The phase transformation occurs significantly in the area corresponding to the transition metal oxides reduction in the YSZ lattice. Cross section analysis of the tested electrolyte revealed that color changed into dark from the anode side surface to inside the electrolyte. Micro-Raman analysis indicated the phase transformation of transition metal (Mn, Fe, Co) doped 8YSZ from the cubic phase to the tetragonal phase. We have experimentally demonstrated that the conductivity degradation of transition metal (Mn, Fe, Co) doped YSZ is closely associated to the phase transformation under SOFC operation atmospheres at 1173 K. All the transition metal (Mn, Fe, Co) doped YSZ showed a lesser conductivity degradation and a lower phase transformation than the Ni doped YSZ. The Fe doped YSZ showed the least Conductivity degradation and lowest phase transformation during the transition metal (Mn, Fe, Co, Ni) doped YSZ under SOFC operation atmospheres at 1173K.

Conductivity Degradation of Transition Metal (Mn, Fe, Co) Doped Yttria Stabilized Zirconia by Phase Transformation

Conductivity degradation behavior of 1 mol% transition metal (Mn, Fe, Co) doped yttria stabilized zirconia (YSZ) under SOFC operation atmospheres was examined by AC impedance technique under an OCV condition, using a test cell consisting of dry air, Pt / transition metal (Mn, Fe, Co) doped 8YSZ / Pt, 1.2 % H2O–H2, at 900°C for 300 h. Phase transformation of YSZ from the cubic to the tetragonal phase was analyzed by Raman spectroscopy. Micro-Raman analysis indicated the phase transformation of transition metal (Mn, Fe, Co) doped 8YSZ from the cubic phase to the tetragonal phase. We have experimentally demonstrated that the conductivity degradation of transition metal (Mn, Fe, Co) doped YSZ is closely associated to the phase transformation under SOFC operation atmospheres at 900°C. All the transition metal (Mn, Fe, Co) doped YSZ showed a lesser conductivity degradation and a lower phase transformation than the Ni doped YSZ. The Fe doped YSZ showed the least conductivity degradation and lowest phase transformation during the transition metal (Mn, Fe, Co, Ni) doped YSZ under SOFC operation atmospheres at 900°C.

Doping-level control of atomic layer deposited YSZ surface layer on platinum catalyst

While local surface composition of fuel cell electrode is critical to performance, precise control of the surface composition at the thickness scale of <10 nm has been challenging. Here we report on the fabrication of ultra-thin (5 nm-thick) yttria-stabilized zirconia (YSZ) surface layers whose compositions are precisely controlled by using atomic layer deposition (ALD), and their applications to cathodic overlayers of low-temperature solid oxide fuel cells. Exchange current density was improved by 80% by optimizing the doping level (16 mol%) of YSZ overlayer. Activation resistance of the cell with 16 mol% doped ALD YSZ overlayer decreased by 27%, and the cell performance improved by 70% compared to the cell without the overlayer. Such improvement was attributed to the ability of the doping-controlled ALD YSZ overlayer to locally improve O2− incorporation, which is the rate-determining step of oxygen reduction reaction, at the cathode surface.

A novel Ni-B/YSZ nanocomposite coating prepared by a simple one-step electrodeposition at different duty cycles

A novel Ni-B nanocomposite coating reinforced by yttria-stabilized zirconia (YSZ) nanoparticles were fabricated by a simple one-step electrodeposition process. The influences of pulse duty cycle on the structure, surface, and properties of the Ni-B/YSZ nanocomposite coating were examined. All coatings exhibit compact, crack-free and nodular-like structures. The Ni-B/YSZ coatings present nanocrystalline structure with 10 nm in size. The preferred orientation was the Ni (111) texture. The topography and roughness were evaluated by AFM. It indicated that duty cycle influences the particle content, nodule size, roughness, microhardness and corrosion behaviors. The roughness (Sa) are 31–51 nm, depending on the duty cycles. The high duty cycle (90%) could enlargen the nodule size. The doped YSZ nanoparticles and the low duty cycle could enhance the hardness, corrosion and wear resistance. A maximum hardness of 823.5 HV was obtained at the duty cycle of 30%. The duty cycle of 30% is the optimal value for the best corrosion and wear resistance, hardness, and desirable electrochemical stability of the nanocomposite coating.

Gradual Conductivity Degradation of Nickel Doped Yttria Stabilized Zirconia by Phase Transformation at Operating Temperature

The conductivity degradation of yttria stabilized zirconia(YSZ) due to the phase transformation from cubic phase to tetragonal phase is one of the issue in solid oxide fuel cells [1,2]. The phase transformation is enhanced by the reduction of nickel oxide which dissolved into YSZ electrolyte when making an anode-support-type-cells. The correlation of conductivity degradation and phase transformation ratio and dependence on oxygen profile in electrolyte for the area of tetragonal formation region about the 1 mol % nickel doped 8mol% yttria stabilized zirconia (Ni-YSZ) at 1173K were reported in our previous studies [2,3,4]. In this study, in order to investigate the factor of temperature for this phase transformation phenomena in nickel doped YSZ electrolyte, conductivity degradation behaviors were measured at various conditions. Even at around 750oC which similar to operating temperature of industrial SOFCs, the tetragonal formation was slightly observed at the area where the reduction of nickel oxide was occurred after over 500h measurement. The time constant for the conductivity degradation due to the tetragonal formation is well fitted on the Arrhenius type plot. This results show the strong temperature dependence for phase transformation. This is an important factor for rapid evaluation of durability of SOFCs.

Local atomic structure evolution in YSZ solid solutions upon Mn doping

Abstract Local atomic structure in Mn doped Yttria Stabilized Zirconia was investigated using X-ray absorption spectroscopy at cation K-edges. Both XANES and EXAFS results show that the most significant changes in the local oxygen environment caused by doping appear in the Mn neighborhood. First shell FEFF modeling shows that both vacancies and distortions occur in Mn local environments. Single scattering fits of the EXAFS spectra of all the cations reveal distortions in the first oxygen coordination shells that are most evident for Mn and less pronounced for Zr and Y cations. Mn EXAFS spectra also confirm the presence of Mn 3 O 4 precipitations at high doping.

Gradual Conductivity Degradation of Nickel Doped Yttria Stabilized Zirconia by Phase Transformation at Operating Temperature

In this study, in order to investigate the factor of temperature for phase transformation phenomena in nickel doped YSZ electrolyte, conductivity degradation behaviors were measured under various conditions. Even around 750oC which is close to operating temperature of industrial SOFCs, the tetragonal formation was slightly observed at the area where the reduction of nickel oxide occurred after over 500h measurement. The time constant for the conductivity degradation due to the tetragonal formation is well fitted to the Arrhenius type plot. Results show the strong temperature dependence for phase transformation. This is an important factor for rapid evaluation of durability of SOFCs.

Phase Transformation and Lattice Parameter Changes of Trivalent Rare Earth Doped YSZ as a Function of Temperature

Yttria-stabilized zirconia (YSZ) co-doped with trivalent oxide Sc2O3 and Yb2O3 is prepared using mechanical alloying and high-temperature sintering. High-temperature XRD analysis was performed to study the phase transformation and lattice parameter changes of various phases in the baseline YSZ and co-doped samples. The results show that the structure for the co-doped samples tends to be more thermally stable at test temperature above critical value. The lattice parameters for all samples increase with temperature at which XRD is carried out, and the lattice parameters for the two trivalent rare earth oxides co-doped YSZ are smaller than that for 7YSZ under the same temperature.

Fast firing of bismuth doped yttria-stabilized zirconia for enhanced densification and ionic conductivity

Fast firing of bismuth doped yttria-stabilized zirconia for enhanced densification and ionic conductivity

Transition metal-doped yttria stabilized zirconia for low temperature processing of planar anode-supported solid oxide fuel cell

An attempt has been made to reduce the processing temperature for fabrication of anode-supported solid oxide fuel cell using transition metal doped 8mol% yttria stabilized zirconia (YSZ) as the material for electrolyte and Ni-YSZ anode support. Through a simple mixing technique, the sintering temperature of a widely used commercial YSZ powder is possible to reduce substantially by adding 2mol% transition metal oxides e.g. cobalt, manganese and iron. More than 96% of theoretical density is achieved at 1175°C. No difference in thermal expansion behaviour is observed for the doped YSZ with respect to pure YSZ (without any additive). Using tape casting and lamination techniques, planar anode-supported half cells (Ni-YSZ/YSZ) were fabricated with such doped YSZ and co-sintered at two different temperatures viz., 1175°C and 1350°C. Finally, single cells (dia. ∼15mm) were fabricated through screen printing of La(Sr)MnO3 (LSM)–YSZ based cathode active layer and LSM current collector layer and tested with hydrogen as fuel and oxygen as oxidant in the temperature range 700–800°C. A reasonably good electrochemical performance (∼1.6A/cm2 at 0.7V and 800°C) is achieved for single cells when half-cell sintering temperature is 1350°C. However, under identical conditions, the current density dropped below 0.2A/cm2 when the corresponding half-cell is sintered at 1175°C. For such a cell, although the open circuit voltage (OCV) is found to be very stable (∼1.1V), the current drawing capability fell abruptly with increasing current load and a corresponding decrease of cell voltage. The cell microstructure revealed that while sintering half-cell at 1175°C, complete densification of the doped YSZ electrolyte film is not achieved. It is anticipated that redesigning of fabrication steps could be required to utilize the benefits of low sintering temperature of cell components.

Microstructure and thermal properties of inflight rare-earth doped thermal barriers prepared by suspension plasma spray

Rare-earth doped yttria-stabilized zirconia (YSZ) coatings with lower thermal conductivity have been fabricated via suspension plasma spray by dissolving rare-earth nitrates into YSZ powder-ethanol suspensions prior to plasma spraying. The effect of dopant concentration and dopant type on properties of the coatings was determined by comparing two coatings containing different concentrations of the same dopant pair (Nd 2 O 3 /Yb 2 O 3 ), and three coatings having similar concentrations of different dopant pairs (Nd 2 O 3 /Yb 2 O 3 , Nd 2 O 3 /Gd 2 O 3 , and Gd 2 O 3 /Yb 2 O 3 ). The porosity content of the coating was found to increase with increased total rare-earth dopant concentration but did not significantly change with dopant pairs. The cross-sectional morphology of every coating displayed a cauliflower-like structure. However, the most heavily doped coating exhibited a larger surface roughness and feathery features in the columnar structures. The thermal conductivity measurement showed that the thermal conductivity decreased with increased Nd 2 O 3 /Yb 2 O 3 concentration. Among coatings containing different dopant pairs, the Gd 2 O 3 /Yb 2 O 3 doped coating exhibited lowest conductivity.

Recent Developments in Surface Engineering of Materials

Engineering approaches to improve surface properties such as surface finish/appearance, hardness/strength, wear resistance, corrosion resistance, environment resistance, and heat resistance have been known since prehistoric times. The manufacturing of several ancient objects can often be traced to processes like surface painting (use of natural pigments), machining (use of sandstone, bone, and metal tools), and heat treatments (use of soybean grains for carbonitriding of steel). While most of the modern surface engineering approaches have evolved from these processes, the past century has also witnessed rapid development of newer processes based on electroplating, thermal spraying, directed beam (ion, laser, and electron) processing, and cold spraying. These processes are now utilized for depositing a range of nanostructured, amorphous, or composite materials for multiple functionalities (hardness/wear resistance, strength/ toughness, corrosion resistance, heat/environment resistance, biocompatibility and cell adhesion, and hydrophobicity). Surface engineering is a very active area, introducing regular developments on various fronts, including fundamental research, technology development, and extension in newer applications. Some of these developments were captured at the symposium Advances in Surface Engineering: Alloyed and Composite Coatings II at the TMS 2013 Annual Meeting (San Antonio, TX; March 3–7, 2013). This symposium was sponsored by the Surface Engineering Committee of the TMS Materials Processing & Manufacturing Division (MPMD) and was partially funded by the U.S. Office of Naval Research (ONR). The symposium featured 59 presentations including 14 invited talks and several student presentations. This was a well-attended, hugely successful symposium, with discussions resulting in an important direction for the future activities of the committee. Most of the recent surface engineering developments discussed in the TMS 2013 symposium are highlighted in the articles published in this JOM issue. These articles represent the current state-ofthe-art and indicate emerging areas in surface engineering. In ‘‘Advances in Laser Surface Engineering: Tackling the Cracking Problem in Laser Deposited Ni-Cr-B-Si-C Alloys,’’ I. Hemmati et al. examine the idea of microstructural refinement for reducing the cracking tendency of Ni-Cr-B-Si-C alloys deposited by laser cladding. This work indicates that effective toughening of these alloys could not be reached solely by refinement of the hard precipitates and that the modification of the eutectic structure or disruption of its continuous network is needed. In ‘‘Nanomechanical Properties and Thermal Conductivity Estimation of Plasma Sprayed Solid Oxide Fuel Cell Components: Ceria Doped Yttria Stabilized Zirconia Electrolyte,’’ N. Mahato et al. demonstrated layered fabrication of solid oxide fuel cell (SOFC) components using an atmospheric plasma spraying method. The nanomechanical behavior of the deposited SOFC composite layers is evaluated. In ‘‘Characterization of Nanostructured and Ultrafine Grain Aluminum-Silicon Claddings using the Nano-Impact Indentation Technique,’’ J. Arreguin-Zavala et al. report that dynamic hardness of the material becomes independent of load using a nano-impact indentation technique. Their analysis shows that better correlation can be established between dry sliding wear and dynamic hardness. In ‘‘Abnormal Nitride Morphologies upon Nitriding Iron-Based Substrates,’’ Sai Ramudi Meka and Eric Jan Mittemeijer provide an overview of different nitride morphologies formed upon Sandip P. Harimkar, Chair of the Surface Engineering Committee of the TMS Materials Processing & Manufacturing Division (MPMD); Srinivasa Rao Bakshi, Vice Chair of the Surface Engineering Committee of TMS; and Arvind Agarwal are the JOM advisors for the Surface Engineering Committee. JOM, Vol. 65, No. 6, 2013

Effective Concentration of Mobile Oxygen-Vacancies in Heavily Doped Cubic Zirconia: Results from Combined Electrochemical Impedance and NMR Spectroscopies

The effective concentration of mobile oxygen vacancies in heavily doped solid electrolytes (SE) used in electrochemical applications such as oxide fuel cells, electrolyzers, and gas sensors has long been of great interest. It is well-known that not all the oxygen vacancies in such heavily doped SEs are sufficiently mobile due to their association with the dopant cations, and thus, the effective number of mobile oxygen vacancies is smaller than the total number created by acceptor doping. However, the effective concentration of oxygen vacancies in a heavily doped SE cannot be directly obtained from electrical conductivity measurements, unless their mobility is known a priori. Here, we report the combined application of dopant–cation magic angle spinning nuclear magnetic resonance (MAS NMR) and electrochemical impedance (EI) spectroscopic techniques to determine the effective concentration and the mobility of the oxygen vacancies in a heavily doped (59 cat % Y) yttria stabilized zirconia (YSZ). The results clearly demonstrate that the effective concentration of oxygen vacancies in this SE is lower than their nominal concentration by more than a factor of 2, even at 700 °C. Furthermore, their mobility in this heavily doped YSZ is lower than that in 17 cat % Y-doped zirconia, one of most widely used SE, by orders of magnitude and is characterized by an activation energy (1.38 eV) that is significantly higher in the former than (1.02 eV) in the latter. These results provide a unique and direct mechanistic understanding of the respective roles of concentration and mobility in controlling ionic transport in SEs.

The Effect of Transition-Metal Doping on Chromium Deposition at Pt/YSZ Cathode Interfaces

Understanding compositional effects, especially at solid oxide fuel cell operating conditions, is important for the selection of materials that minimize the impact of chromium deposition. In this study, the effect of yttria-stabilized zirconia (YSZ) doping on Cr deposition at a Pt electrode/YSZ electrolyte interface was investigated. A comparison between undoped and doped YSZ electrolyte surfaces after cathodic polarization in moist air at in the presence of chromia indicated that the presence of Mn, Co, or Fe in the electrolyte significantly promotes Cr deposition at the electrode/electrolyte interface. Electron microscopy of the Co-doped YSZ electrolyte surface after cathodic polarization for 10–45 h clearly showed that with Co doping, the deposition of Cr occurs preferentially at YSZ grain boundaries. Impedance spectroscopy results showed that chromium deposition led to a decrease in resistance with Mn- and Fe-doped YSZ electrolyte, which was attributed to the increase in the three-phase boundary length. However, this beneficial effect of Cr deposition was not observed with Co doping, which may be related to the preferential deposition of chromia at the YSZ grain boundary, resulting in fewer Cr deposits at the cathode/electrolyte interface.

Influence of elastic properties on superplasticity in doped yttria-stabilized zirconia

We report the first principles calculations of elastic and electronic properties of yttria-stabilized tetragonal zirconium dioxide (YZP) doped with GeO 2 , TiO 2 and SiO 2 . Electronic structure and isotropic elastic properties of YZP do not change upon addition of dopants. Addition of dopants affects the shear C 66 elastic constant that decreases with the increasing dopant concentration. A simple model that connects elastic softening to enhancement of superplasticity in doped fine-grained zirconia ceramics is proposed.

Experimental study of the thermal conductivity of metal oxides co-doped yttria stabilized zirconia

Doping of yttria stabilized zirconia (YSZ) with metal oxides has been reported to have the potential of increasing the thermal insulation capability and phase stability of YSZ at higher application temperatures. In order to select the dopant oxides, it is important to understand the effects of the dopants, in terms of the ionic radius, mass, and valence, on various thermal and mechanical properties of the doped YSZ. In this study, a series of metal oxides, tantalum pentoxide (Ta 2O 5), niobium pentoxide (Nb 2O 5), scandium oxide (Sc 2O 3), ytterbium oxide (Yb 2O 3) and cerium oxide (CeO 2) were incorporated to 7YSZ (7 wt% yttria) by mechanical alloying and sintering; the thermal conductivity changes of the doped 7YSZ were investigated. The oxide dopants that exist as defect clusters within the zirconia lattice structure played an important role in reducing the thermal conductivity of the ternary oxide system. Yb 2O 3 co-doped 7YSZ exhibited the most significant effect on reducing the thermal conductivity while CeO 2 co-doped 7YSZ showed marginal effect. The measured results were also found to be consistent with that obtained from a semi-empirical thermal conductivity model established based on the assumption of defect clusters acting as the phonon scattering centers.

Novel microstructure of EB-PVD double ceramic layered thermal barrier coatings

New ceramic candidate materials for thermal barrier coating (TBCs) application and modifications of yttria stabilized zirconia (YSZ) have been increasingly studied, aiming at improving thermal cycling resistance and thermal insulation of TBCs. A novel Gd 2O 3–Yb 2O3 doped YSZ/YSZ double ceramic layered TBC was prepared by means of electron beam physical vapor deposition (EB-PVD), where the undoped YSZ top layer is featured with clusters of columns with sub-grains growing around columns, while the doped YSZ bottom layer feather-like structure. The TBC showed structure stability after 190 h heat treatment at 1373 K. The double layered TBC exhibited thermal cycling lifetime of more than 360 h, much higher than that of the single YSZ TBC.

Oxygen detection using junctions based on thin films of yttria-stabilized zirconia doped with platinum nanoparticles and pure yttria-stabilized zirconia

Here we develop new types of ceramic junctions composed of two plane-parallel ceramic electrodes: one of them consists of yttria-stabilized zirconia (YSZ), the other one is composed of YSZ doped with platinum nanoparticles. The bottom electrode is either of pure YSZ or of doped YSZ + Pt with the top electrode being of the opposite material. The concentration of platinum has been varied at 2.5, 4.5 and 6.5 mol%. The tests are made in an experimental setup allowing the exchange of gas atmosphere with variable oxygen content at fixed temperature or the gradual change in the temperature up to about 400 °C in a pure nitrogen medium. For YSZ/YSZ + Pt junctions the potential difference is positive, while for YSZ + Pt/YSZ junctions it is negative. At a fixed temperature we investigate the rough response to oxygen of different structures by periodically exchanging the oxygen from 0.9% (technical grade nitrogen) to 21% (air). Good reproducibility of the results is maintained both in the regime of nitrogen flowing and air flowing. The detailed investigation of the kinetics of potential relaxation revealed that it is governed by the diffusion of oxygen in the ceramics. The fastest response is detected with the junctions YSZ + Pt/YSZ with 6.5 mol% of platinum. We also study the potential difference at varying oxygen concentration by tuning the ratio between air and nitrogen. The functional dependence is Nernstian type like in potentiometric gas sensors, which is well-described by the respective equation if the junction is considered as a sort of thermocouple with the electrodes being the contact point. This makes our junctions promising as oxygen sensors at low working temperature and very low oxygen content.