YSZ Phase Research Articles

Comparison of X-ray Nanotomography and FIB-SEM in Quantifying the Composite LSM/YSZ SOFC Cathode Microstructure

X-ray nanotomography and focused ion beam scanning electron microscopy (FIB-SEM) provide unique opportunities interrogate the 3D microstructure of solid oxide fuel cell (SOFC) electrodes with unprecedented spatial resolution with elemental specificity. Using x-ray nanotomography with differential absorption imaging across the Mn K-absorption edge (6539 eV), and energy selected backscatter detection for FIB-SEM, these two techniques have been applied to obtain 3D elemental mapping of the LSM, YSZ and pore phases in the cathode microstructure. Measurements taken by non-destructive x-ray imaging and FIB-SEM serial sectioning include three-phase boundary areas (nominal and effective); phase volume fraction; and phase contiguity. The results presented demonstrate the viability of nanotomography as a characterization technique and provide key insights into the SOFC cathode microstructure. The effects of instrument resolution on microstructural characterization are addressed and the roles of FIB-SEM and x-ray nanotomography in future SOFC microstructural characterization efforts are discussed.

Comparison of SOFC cathode microstructure quantified using X-ray nanotomography and focused ion beam–scanning electron microscopy

X-ray nanotomography and focused ion beam scanning electron microscopy (FIB‐SEM) have been applied to investigate the complex 3D microstructure of solid oxide fuel cell (SOFC) electrodes at spatial resolutions of 45 nm and below. The application of near edge differential absorption for x-ray nanotomography and energy selected backscatter detection for FIB–SEM enable elemental mapping within the microstructure. Using these methods, non‐destructive 3D x-ray imaging and FIB–SEM serial sectioning have been applied to compare three‐dimensional elemental mapping of the LSM, YSZ, and pore phases in the SOFC cathode microstructure. The microstructural characterization of an SOFC cathode is reported based on these measurements. The results presented demonstrate the viability of x-ray nanotomography as a quantitative characterization technique and provide key insights into the SOFC cathode microstructure.

Glycine nitrate process에 의한 제조된 SOFC anode용 Ni-YSZ cermet의 물성

Ni-YSZ(Yttria Stabilized Zirconia) composite powders를 glycine nitrate process으로 만들었다. 합성된 분말은 <TEX>$1300{\sim}1400^{\circ}C$</TEX> 4시간 동안 소성하였으며 <TEX>$1000^{\circ}C$</TEX>에서 2시간 동안 질소 및 수소 분위기에서 환원 소성하여 Ni-YSZ cermet을 제조하였다. Ni의 부피비를 변화 시켜 각기 그들의 미세구조, 전기전도도, 열팽창 및 강도 특성을 알아보았다. Ni과 YSZ 상 사이에 상호 연결된 균질하게 분포된 다공성 미세구조를 얻을 수 있었다. 기공률, 전기전도도, 열팽창계수 및 곡강도 모두 Ni의 양에 민감하게 영향 받는 것을 알 수 있으며 40 vol%의 Ni를 함유한 Ni-YSZ cermet가 전극재료로 가장 적당하였다. <TEX>$1350^{\circ}C$</TEX>에서 소성한 40 vol% Ni-YSZ 시편의 경우 30 %의 기공율, 65.5 Mpa의 강도, 917.4 S/cm의 전기 전도도(<TEX>$1000^{\circ}C$</TEX>)및 <TEX>$12.6{\times}10^{-6}^{\circ}C^{-1}$</TEX>의 열팽창계수(<TEX>$1000^{\circ}C$</TEX>)를 가져 YSZ 전해질의 음극재료로 가장 적합하였다. Ni-YSZ (Yttria Stabilized Zirconia) composite powders were fabricated by glycine nitrate process. The prepared powders were sintered at <TEX>$1300{\sim}1400^{\circ}C$</TEX> for 4 h in air and reduced at <TEX>$1000^{\circ}C$</TEX> for 2 h in a nitrogen and hydrogen atmosphere. The microstructure, electrical conductivity, thermal expansion and mechanical properties of the Ni-YSZ cermets have been investigated with respect to the volume contents of Ni. A porous microstructure consisting of homogeneously distributed Ni and YSZ phases together with well-connected grains was observed. It was found that the open porosity, electrical conductivity, thermal expansion and bending strength of the cermets are sensitive to the volume content of Ni. The Ni-YSZ cermet containing 40 vol% Ni was ascertained to be the optimum composition. This composition offers sufficient open porosity of more than 30 %, superior electrical conductivities of 917.4 S/cm at <TEX>$1000^{\circ}C$</TEX> and a moderate average thermal expansion coefficient of <TEX>$12.6{\times}10^{-6}^{\circ}C^{-1}$</TEX> between room temperature and <TEX>$1000^{\circ}C$</TEX>.

Electrochemical and mechanical properties of solid oxide fuel cell Ni/YSZ anode fabricated from NiO/YSZ composite powder

The electrochemical and mechanical properties were investigated on Ni/Yttria-stabilized zirconia (YSZ) solid oxide fuel cells (SOFCs) anode fabricated from a NiO/YSZ composite powder. The composite powder was fabricated by dry milling of NiO and YSZ nanoparticles using attrition type apparatus without media balls for 30 min. The composite powder suppresses the microscopic as well as macroscopic segregation of the component during tape casting and subsequent sintering, resulting fine as well as the highly uniform anode microstructure. The anode supported SOFC made from the composite powder showed the maximum power density of 0.25 and 0.75 W·cm−2 at 700 and 800°C, respectively, which is significantly higher than that fabricated from the conventional powder mixture. Better performance can be attributed to the lower interfacial resistance due to the formation of huge amount of triple phase boundary consisting of Ni, YSZ and pore phases. The higher electrical conductivity would also contributed to the high performance. The equibiaxial flexural strength of NiO/YSZ fabricated from the composite powder was higher than that from the powder mixture.

Glycine nitrate process에 의한 SOFC용 Ni-YSZ cermets 제조

SOFC용 Ni-YSZ(Yttria Stabilized Zirconia) composite powders를 glycine nitrate process를 이용하여 만들었다. <TEX>$ZrO(NO_3)_2{\cdot}2H_2O$</TEX>, <TEX>$Y(NO_3)_3{\cdot}6H_2O$</TEX>, <TEX>$Ni(NO_3)_2{\cdot}6H_2O$</TEX>와 glycine을 출발원료로 하였으며 Ni의 부피비를 변화시켜 각기 그들의 소결 및 환원 특성을 알아보았다. Ni과 YSZ 상들이 상호 연결된 균질하게 분포된 다공성 미세구조를 관찰 할 수 있었으며 Ni의 첨가량에 따라 가공률이 민감하게 변화함을 알 수 있었다. 35 vol% 이상의 Ni를 함유한 Ni-YSZ cermet가 SOFC용 전극재료로 사용되는데 필요한 30% 이상의 공극을 갖는 조성임을 알 수 있었다. Ni-YSZ (Yttria Stabilized Zirconia) composite powders for SOFC were fabricated by glycine nitrate process. <TEX>$ZrO(NO_3)_2{\cdot}2H_2O$</TEX>, <TEX>$Y(NO_3)_3{\cdot}6H_2O$</TEX>, <TEX>$Ni(NO_3)_2{\cdot}6H_2O$</TEX> and glycine were chosen as the starting materials. The structural properties of the sintered Ni-YSZ cermets have been investigated with respect to the volume contents of Ni. A porous microstructure consisting of homogeneously distributed Ni and YSZ phases together with well-connected grains was observed. The sintered Ni-YSZ cermets showed a porous microstructure consists of homogeneously distributed Ni and YSZ phases and the grains were well-connected. It was found that the open porosity is sensitive to the volume content of Ni. The Ni-YSZ cermet containing 35 vol% Ni seems to be suitable for the electrode material of SOFC since it provides sufficient open porosity higher than 30%.

Rapid solidification and fracture behavior of ternary metastable eutectic Al 2O 3/YAG/YSZ in situ composite ceramic

Directionally solidified oxide ceramic eutectic in situ composites are promising candidate materials for high-temperature structural applications in oxidizing environments. The rapid solidification characteristics of ternary metastable eutectic composition of Al 2O 3/YAG/YSZ in situ composite ceramics grown by laser zone-remelting technique are investigated. The fracture toughness and crack propagation behavior of the as-solidified composites are examined by the Vickers indentation technique. The metastable eutectic Al 2O 3/YAG/YSZ ceramic consists of Al 2O 3, YAG and YSZ phases, showing a typical ultra-fine irregular lamellar microstructure morphology with eutectic spacing as fine as 0.5 μm. Moreover, the ternary eutectic microstructures with coupled growth characteristics are also formed at the off-eutectic composition, effectively enlarging the eutectic growth range. The mean fracture toughness of the metastable eutectic Al 2O 3/YAG/YSZ ceramic is 3.9 ± 1.0 MPa m 1/2. The addition of ZrO 2 which weakens the interface bonding between YAG and Al 2O 3 phases, crack deflection, branching and microcrack toughening are the predominant toughening mechanisms.

Microstructure of porous composite electrodes generated by the discrete element method

Abstract Typical microstructures of LSM/YSZ, NiO/YSZ, Ni/YSZ composite electrodes are simulated by the discrete element method. The numerical microstructures are generated by taking into account in a realistic manner the sintering process. This allows complex microstructures such as bilayers or microstructures containing pore formers to be obtained. NiO particles in the NiO/YSZ composite electrodes are reduced to Ni. Reduction is carried out with the discrete element formalism which allows particle rearrangement to be taken into account. We show that the mechanical percolation of the YSZ phase plays an important role during the reduction of NiO. The various numerical microstructures generated by sintering and reduction are analyzed to evaluate important microstructural features such as macroscopic porosity, pore surface area and Triple Phase Boundary length.

Processing and microstructural study of surface laser remelted Al 2O 3–YSZ–YAG eutectic plates

Surface modification of ternary Al 2O 3–Zr 2O 3–Y 2O 3 eutectic oxides was produced by laser melting. Preheating the ceramics and using a protecting atmosphere, we obtained homogeneous eutectic surface layers of 7 mm × 25 mm and 400 μm thick. The remelted layers were dense using processing rates below 500 mm/h but showed bubble porosity at faster processing rates. The phases in the resultant material were cubic zirconia (YSZ), α-Al 2O 3 and Y 3Al 5O 12 (YAG). At 20 mm/h, the remelted layer presented a Chinese-script like microstructure formed by an interpenetrating network of Al 2O 3 and YAG. At travelling speeds greater than 500 mm/h colonies were formed. At the largest processing rates the microstructure becomes fibrous like, the YSZ phase locating at the interface between Al 2O 3 and YAG phases. The chemical composition was independent of position into the layer. The very fine microstructure allows an in-line optical transmittance in the NIR of 60% for a 380 μm thick layer processed at 3500 mm/h.

In-situ neutron diffraction study of phase stress evolutions in a Ni-based porous anode solid oxide fuel cells under uniaxial load

Ni/YSZ porous composite is used widely as anode for solid oxide fuel cells. In this study, neutron diffraction patterns were recorded in-situ while a bulk porous Ni-NiO-YSZ anode was loaded under uniaxial compression. Single peak refinement was used to calculate the lattice strains of each phase in the composite, and the local stress state of each phase was derived from the measured lattice strains and the corresponding diffraction elastic constants. An internal triaxial stress state was observed to develop in the bulk of the specimen under plastic deformation, specifically in the Ni phase. Meanwhile, the NiO and YSZ phases are deforming elastically even in the macroscopically plastic regime of deformation. The von Mises equivalent stress was used to quantify the phase stress evolution. A significant stress concentration induced by the presence of pores becomes manifest in all three phase components. The reduction of stress concentration factor in Ni above the yield point of the composite can be attributed to a gradual change of the grains-pores morphology during the plastic deformation.

Biocompatibility and water durability of alumina-zirconia ceramics blended with microsized HA particles

To improve the bioactivity of alumina containing yttria stabilized zirconia (Al-YSZ), composite ceramics consisting of Al-YSZ and 0–10 wt % of hydroxyapatite (HA) were prepared. The XRD patterns of sintered disks with an HA content above 3 wt % had monoclinic and cubic YSZ phases and a β-tricalcium phosphate phase. Although the Vickers hardness decreased as the HA content increased, all the prepared ceramics had the same or greater hardness (1000–1300 Hv with a load of 0.49 N) compared to typical YSZ ceramics (900–1000 Hv) commonly used in clinics. Furthermore, the bonelike apatite formation ability in simulated body fluid, which was examined as a screening of bioactivity, suggested that all the surfaces of Al-YSZ ceramics blending with a small amount of HA may possess the ability to connect bone. After the accelerated low temperature degradation (LTD) test performed in normal saline at 120°C for five days using composite ceramics with the HA content of 0, 1, 3, and 10 wt %, the tetragonal zirconia phase of all composite ceramics were partially transformed into the monoclinic phase with a transition ratio of ca. 16–18 mol % (for HA = 0 and 10 wt %) and ca. 4–5 mol % (for HA = 1 and 3 wt %). Blending microsized HA powder in Al-YSZ ceramics has both positive and negative effects on the LTD durability of Al-YSZ ceramics. An advantage is that calcium ion substitution in YSZ seems to stabilize the tetragonal (or cubic) zirconia phases, but the increased surface area exposed to the solution is a disadvantage.

J0802-2-2 FIB-SEM再構築構造を用いたSOFC燃料極の三次元数値シミュレーション([J0802-2]SOFC(1))

Different samples of Ni-YSZ anode were reconstructed by using a dual beam focused ion beam scanning electron microscopy (FIB-SEM). In these samples, tortuosity factors were calculated to evaluate area dependency. Tortuosity factors of pore and YSZ phases converged as cross sectional area increased, but that of Ni phase did not. This is because Ni particles were larger than the others. The overpotential calculations showed nearly the same results for different samples. Overpotential calculations were also conducted under different steam (3, 10, 20% H_2O), and temperature conditions (1023K, 1073K, 1123K). The results for different steam conditions showed that diffusion and exchange current density models must be modified. On the other hand, activation energy used in the model reproduced well the temperature dependence of the experimental data.

Preparation and structure development of NiO/YSZ nanocomposite powders by urea hydrolysis

NiO/YSZ composite powders, with various NiO contents, have been prepared by the urea hydrolysis method. The crystallization behavior and microstructure of composite powders has been studied in detail, using differential scanning calorimetry analysis, X-ray diffraction, and transmission electron microscope. The results indicated that the actual NiO content of the NiO/YSZ powders largely deviated from the nominal value, and finally reached a saturated value. The NiO addition would retard the crystallization of NiO/YSZ composite. When the calcination temperature was increased, the NiO crystallites first precipitated at around 500 °C, and then the YSZ phase presented at about 600 °C. The calcined powders consist of NiO/YSZ nanocomposite particles, which are comprised of nano-sized NiO and YSZ crystals. In addition, with the aid of H 2 plasma treatment, it is easier to distinguish the Ni and YSZ phases of Ni/YSZ cermets after sintering and subsequent reduction. This could reveal that such Ni/YSZ cermets exhibited a uniform microstructure that has fine Ni particles homogeneously dispersed within the YSZ matrix. As the NiO content was increased, the size and density number of the Ni phase within an YSZ matrix was increased.

Quantitative Evaluation of Transport Properties of SOFC Porous Anode by Random Walk Process

Microstructural properties of an SOFC anode, such as tortuosity, surface-to-volume ratio and permeability, are quantitatively evaluated using three-dimensional data obtained by a system consisting of a focused ion beam and scanning electron microscope, FIB-SEM. A method employed to evaluate these properties is based on a random walk process in which the diffusion of imaginary particles is considered. In addition, the connectivities of three phases, i.e., the Ni, YSZ and pore phases are also analyzed. The relationships among these properties and their effects on transport phenomena in a porous anode are discussed. It is revealed that the low connectivity and complex microstructural configuration of the porous anode result in large tortuosities in the Ni and YSZ phases, respectively.

Ethanol Fuelled SOFC with ZnO Addition in the Anode

The solid oxide fuel cell (SOFC) has attracted a great deal of attention due to its high efficiency and variety of usable fuels like hydrocarbon or alcohols. In this work, aiming at the use of carbon-containing fuels in SOFC, ZnO was added to the SOFC anode ceramic-metal composite yttria-stabilized zirconia/nickel (YSZ/Ni) in a wide range of relative compositions. Apparent density measurements confirmed that ZnO is a sintering aid for YSZ matrix. X-ray diffraction revealed the presence of YSZ, Ni, ZnO and NiO phases in all samples. The impedance analysis show that the higher is the ZnO concentration, until 50% vol., lower is the electrical resistance of the samples. The single cells preliminary experiments suggest the higher efficiency of ethanol fuelled SOFCs with YSZ/Ni/ZnO anode. Further experiments using doped-ceria instead of YSZ will be accomplished to enhance the SOFC performance at temperatures as low as 500 {degree sign}C.

A stochastic geometry based model for total triple phase boundary length in composite cathodes for solid oxide fuel cells

An analytical equation is derived for total triple phase boundary length per unit volume ( L TPB ) in an isotropic uniform random microstructure of LSM/YSZ composite cathode. The equation is applicable to YSZ and LSM particles of any convex shapes and size distributions. The equation explicitly relates L TPB to the shapes, mean sizes, coefficient of variation (a measure of the spread in a size distribution) and skewness of YSZ and LSM particle populations, and volume fractions of YSZ, LSM, and porosity. The equation is verified using available experimental data, and compared with the results of earlier simulations and models. The parametric analysis reveals that (1) non-equiaxed plate-like, flake-like, and needle-like YSZ and LSM particle shapes can yield substantially higher L TPB ; (2) mono-sized YSZ and LSM powders lead to higher L TPB as compared to the powders having size distributions with large coefficient of variation; (3) L TPB is inversely proportional to the mean sizes of YSZ and LSM particles; (4) high value of L TPB is obtained at the lowest porosity volume fraction that permits sufficient connectivity of the pores for gas permeability; and (5) L TPB is not sensitive to the relative proportion of YSZ and LSM phases in the regime of interest in composite cathode applications.

Three-Dimensional Analysis of Solid Oxide Fuel Cell Ni-YSZ Anode Interconnectivity

A method is described for quantitatively analyzing the level of interconnectivity of solid-oxide fuel cell electrode phases. The method was applied to the three-dimensional microstructure of a Ni-Y2O3-stabilized ZrO2 (Ni-YSZ) anode active layer measured by focused ion beam scanning electron microscopy. Each individual contiguous network of Ni, YSZ, and porosity was identified and labeled according to whether it was contiguous with the rest of the electrode. It was determined that the YSZ phase was 100% connected, whereas at least 86% of the Ni and 96% of the pores were connected. Triple-phase boundary (TPB) segments were identified and evaluated with respect to the contiguity of each of the three phases at their locations. It was found that 11.6% of the TPB length was on one or more isolated phases and hence was not electrochemically active.

Compositional influence of LSM-YSZ composite cathodes on improved performance and durability of solid oxide fuel cells

The use of a dual-composite approach, in which both LSM and YSZ nanoparticles are placed on YSZ core particles, allows for the development of an ideal cathode microstructure with improved phase contiguity and durability for use in solid oxide fuel cells. The volume fraction of the conjugated YSZ phase plays a critical role in the optimization of the cathode microstructure for achieving good durability because it acts as an interconnecting bridge between YSZ core particles. However, the presence of excess conjugated YSZ phase interrupts the formation of sufficient triple phase boundary sites by disturbing the contacts between the LSM and YSZ phases. Impedance spectroscopy analysis and microstructural observations provide a better understanding of the influence of the composition on the electrochemical performance and durability of these dual composite cathodes.

Nanostructured LSM/YSZ composite cathodes for IT-SOFC: A comprehensive microstructural study by electrostatic spray deposition

The deposition of a composite electrode, which is consisting of a mixture of a solid electrolyte (8YSZ) and an electrocatalytic material (LSM) on YSZ substrate was studied by electrostatic spray deposition (ESD), a very successful technique to vary the surface morphology. The influence of two variable parameters such as YSZ content, nozzle to substrate distance, and their combinations were investigated on the films morphology. The surface morphology was also strongly influenced by the deposition temperature. Processes involved in the porous film formation are discussed. Nanostructured films based on pure LSM and YSZ phases with no traces of secondary phases were successfully deposited.

Synthesis of nano-crystalline NiO-YSZ by microwave-assisted combustion synthesis

Nano-crystalline NiO-YSZ composite powders have been successfully prepared by microwave-assisted combustion of a gel derived from an aqueous solution containing ZrO(NO 3) 2·6H 2O ,Y(NO 3) 3·6H 2O, Ni(NO 3) 2·6H 2O and glycine. The as-prepared powders were examined using X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM) techniques. It was found that the process took only a few seconds to obtain NiO-YSZ composite powders. The as synthesized composite powder was a homogeneous mixture of YSZ, NiO and Ni phases with crystallite size of 24.2, 33.6 and 25.3 nm respectively.

A nanocomposite material for highly durable solid oxide fuel cell cathodes

We have synthesized well-engineered nanocomposite particles for the achievement of highly durable solid oxide fuel cell cathodes. The use of a dual-composite approach in which both LSM and YSZ phases are placed on a YSZ grain allows for the development of an ideal cathode microstructure with improved phase contiguity and interfacial coherence. Microstructural observations in conjunction with impedance analysis during accelerated lifetime tests clearly reveal that three-dimensionally interconnected percolative networks of both electronic and ionic conduction give cells based on the dual-composite cathode better long-term stability. A cell with such a well-controlled cathode maintains nearly constant power density over 500 h. In contrast, a cathode prepared by mechanical mixing undergoes significant degradation during the stability test due to thermochemically and electrochemically driven coarsening and shrinkage of the LSM phase.