Studies on phase formation,microstructure development and elastic properties of reduced NiO–8YSZ anode supported bi-layer half-cell structures of solid oxide fuel cells

Half-cell structures of solid oxide fuel cells (SOFCs) with a thin and dense electrolyte layer of 8YSZ supported by a thick and porous NiO–8YSZ anode precursor structure were reduced in a gas mixture of 5% H₂–95% Ar at 800 °C for selected time periods in order to fabricate cermets with desired microstructure and composition, and to study their effects on the elastic properties at ambient and reactive atmospheres. It appears that 2 h of exposure to the reducing conditions is enough to reduce ～80% of NiO with an enhanced porosity value of 35%. The Ni–8YSZ cermet phase formation in the anode was analyzed with X-ray diffraction (XRD) in correlation with its microstructure. The elastic properties were determined after the reduction, at room and elevated temperatures using the impulse excitation technique. At room temperature the decrease in the Young's modulus was about 44% (after 8 h of reduction) and can be attributed mainly to the changes in the microstructure, particularly the increase in porosity from ～12% to 37%. Young's moduli of the as-received precursor and reduced anodes were evaluated as a function of temperature in air and reducing atmosphere. The results were explained in correlation to the initial porosity, composition and oxidation of Ni at the elevated temperatures.     

Synthesis and characterization of nanocrystalline Ni–YSZ cermet anode for SOFC

Ni–YSZ cermet anode has been synthesized in one step using a simple and cost effective combustion synthesis process. The processed powder of NiO–YSZ is found to be nanocrystalline with crystallite sizes of 29 and 22 nm for NiO and YSZ respectively by X-ray diffraction and transmission electron microscopy analysis. X-ray diffraction analysis also shows that the precursor salts are converted to highly crystalline phases of NiO and YSZ (8 mol% Y₂O₃) without any intermediate calcination step and no undesirable phases are present. Comparison with the X-ray diffraction pattern of a commercial YSZ sample shows that the process is also effective in maintaining a close compositional control. The microstructure of the sintered and reduced sample shows a well defined network of pores which is necessary for the effective functioning of the anode. The electrical conductivity as a function of temperature shows metallic behavior.     

Fabrication of NiO/YSZ anode material for SOFC via mixed NiO precursors

NiO/YSZ anode material with 45 wt.% NiO for SOFC was synthesized by surface-modification of YSZ powder with mixed nickel oxide precursors. YSZ was treated with acidic nickel nitrate and then followed by basic nickel carbonate. Such prepared NiO/YSZ precursor showed nano-size homogeneous mixture upon calcination at 800 °C. The mixture was ground and subjected to calcination at 1200 °C to give rise to agglomerate-free NiO/YSZ composite powder with particle size of ∼ 0.4–0.7 μm. The composite powder was analyzed by XRD, zetapotential measurements, and SEM. The composite powder produced the Ni/YSZ cermet of homogeneous microstructure with a rigid YSZ skeleton, porosity of 33%, and an electrical conductivity of ∼ 430 S/cm at 800–1000 °C, which is much higher than that (∼ 180 S/cm) prepared by a conventional mixed oxide process.     

Electrochemical property of multi-layer anode supported solid oxide fuel cell fabricated through sequential tape-casting and co-firing

In this work, a multi-layer anode supported solid oxide fuel cell (SOFC) is designed and successfully prepared through sequential tape casting and co-firing. The single cell is consisted of NiO-3YSZ (3YSZ: 3 mol.% yttria doped zirconia) anode support, NiO-8YSZ (8YSZ: 8 mol.% yttria stabilized zirconia) anode functional layer, dense 8YSZ electrolyte layer, and porous 3YSZ cathode scaffold layer with infiltrated La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ cathode. The clear interfaces and good contacts between each layer, without element inter-diffusion being observed, suggest that this sequential tape casting and co-firing is a feasible and successful route for anode supported single cell fabrication. This cell exhibits remarkable high open circuit voltage of 1.097 V at 800°C under room temperature humidified hydrogen, with highly dense and gastight electrolyte layer. It provides a power density of 360 mW/cm² under operation voltage of 0.75 V at 800°C and a stable operation of ～110 h at 750°C under current density of ?300 mA/cm². Furthermore, this cell also presents encouraging electrochemical responses under various anode hydrogen partial pressures and maintains high power output at low fuel concentrations.     

Properties of Ni/YSZ porous cermets prepared by electroless coating technique for SOFC anode application

A conceptual nickel–yttria stabilized zirconia (Ni/YSZ) cermet has been prepared by coating YSZ particles with metallic nickel using electroless coating technique. Concentration of nickel was varied between 5 and 60 vol%. Bulk samples were prepared by uniaxial pressing followed by sintering in the temperature range 1,200–1,350 °C with a soaking time of 2–6 h. Thorough investigation on the electrical characteristics, thermal expansion behavior of the samples have been performed as a function of Ni-content in the cermet. Thermal expansion behavior and conductivity measurement results suggest that the samples prepared by this technique are suitable for solid oxide fuel cell (SOFC) anode application at Ni concentration as low as 20 vol%.     

A new method of preparing Ni/YSZ cermet materials

Ceramic–metal composites (cermets) containing yttria-stabilized zirconia(YSZ) and Ni particles as anode materials in solid oxide fuel cells were prepared by a new method. The method encompasses nickel oxalate dihydrate precipitation on the nanometre YSZ powder, and decomposition at 633 K in inert atmosphere. The composite powders containing 30, 40, and 50% Ni manufactured by the oxalate method were compacted into disk pellets, and sintered in Ar containing 10% H₂. The structure of the cermet materials was characterized by means of X-ray diffractometry (XRD), scanning electron microscopy (SEM), and mercury porosimetry. The thermal expansion coefficient (TEC) was determined by the dilatometric method. Electrochemical impedance spectroscopy (EIS) was used to determine electrical conductivity. The oxalate method leads to obtaining the Ni/YSZ anodes with the Ni content reduced to 40 wt% and well obeying based requirements for anode material in SOFCs.     

Young’s modulus evolution at high temperature of SiC refractory castables

The evolution of Young’s modulus versus temperature has been evaluated in SiC-based hydraulically bonded refractories used in waste-to-energy (WTE) plants. Two types of low cement castables (LCC) with 60 and 85 wt% of SiC aggregates have been considered. The study was conducted by the way of a high temperature ultrasonic pulse-echo technique which allowed in situ measurement of Young’s modulus during heat treatment starting from the as-cured state up to 1400 °C in air or in neutral atmosphere (Ar) and during thermal cycles at intermediate temperatures (1000 and 1200 °C). For comparison in order to facilitate interpretation, thermal expansion has also been followed by dilatometry performed in the same conditions. Results are discussed in correlation with phase transformations occurring in the oxide matrix (dehydration at low temperature, crystallization of phases in the CaO–Al₂O₃–SiO₂ system) above 800 °C and damage occurring when cooling. The influence of oxidation of SiC aggregates on elastic properties is also discussed.     

Solubility of NiO in Pechini-derived ZrO<Subscript>2</Subscript> examined with SQUID magnetometry

The solubility of NiO in ZrO₂ was studied by X-ray diffraction, TEM, and SQUID magnetometry. Lattice parameter measurements from a similar, established oxide system, NiO−10YSZ, were first used to show that SQUID magnetometry can effectively measure solubility. ZrO₂ specimens with 0, 0.5, 1, 2, 3, and 5 percent by mol NiO were prepared via the Pechini method. The specimens were calcined in air at 500, 600, and 1000 °C. The paramagnetic response of the specimens measured with SQUID magnetometry revealed that up to 5 percent by mol NiO is soluble in ZrO₂ for specimens calcined at 500 and 600 °C. The relatively large solubility compared with NiO−10YSZ occurs due to the very fine grain size (5–10 nm). The fine grain size is also responsible for stabilizing the tetragonal phase of ZrO₂. At the 1000 °C calcination temperature, the ZrO₂ is entirely monoclinic, exhibits larger grains (>45 nm), and only dissolves about 0.5 percent by mol or less NiO. The correlations between grain size, ZrO₂ polytype, and NiO addition are discussed.     

Novel Co-precipitation method to synthesize NiO–YSZ nanocomposite powder for solid oxide fuel cell

As the Ni composition, particle size and its homogeneous distribution in the SOFC anode of Ni-yttria stabilized zirconia (YSZ) composite are important factors in electrochemical performance, better control of the properties of NiO–YSZ nanocomposite particles as starting powder materials is essential for higher SOFC performance. In this study, their novel co-precipitation method was developed. It aimed the interaction of anionic Zr carbonate complex with cationic Ni and Y ions to achieve its uniform sedimentation. As a result, NiO–YSZ nanocomposite particles with their uniform size were successfully obtained at a facile condition with almost 100% yield. The Ni/YSZ anode was fabricated by screen-printing the paste made by the NiO–YSZ nanocomposite particles and its sintering at 1350 °C and subsequent reduction. The anode had a uniform porous structure consisting of fine grains in the range of 200–400 nm, and exhibited low polarization resistance of 0.62, 0.34 and 0.23 Ω cm² at 700 °C, 750 °C and 800 °C, respectively.     

Weibull modulus of nano-hardness and elastic modulus of hydroxyapatite coating

Here we report the microstructural dependence of nano-hardness (H) and elastic modulus (E) of microplasma sprayed (MIPS) 230 μm thick highly porous, heterogeneous hydroxyapatite (HAP) coating on SS316L. The nano-hardness and Young’s modulus data were measured on polished plan section (PS) of the coating by the nanoindentation technique with a Berkovich indenter. The characteristic values of nano-hardness and Young’s modulus were calculated through the application of Weibull statistics. Both nano-hardness and the Young’s modulus data showed an apparent indentation size effect. In addition, there was an increasing trend of Weibull moduli values for both the nano-hardness and the Young’s modulus data of the MIPS-HAP coating as the indentation load was enhanced from 10 to 1,000 mN. An attempt was made in the present work, to provide a qualitative model that can explain such behavior.     

Young’s modulus and damping in dependence on temperature of Ti–6Al–4V components fabricated by shaped metal deposition

Young’s modulus and damping behavior is investigated by the impulse excitation technique in vacuum up to 1100 °C for Ti–6Al–4V components, fabricated by shaped metal deposition (SMD). This is a novel additive manufacturing technique where near net-shape components are built layer by layer by tungsten inert gas welding. The Young’s modulus decreases linearly from 118 GPa at room temperature to 72 GPa at 900 °C, followed by a stronger decrease up to 1000 °C and during the first heating a plateau thereafter. The damping exhibits an exponential increase with temperature superimposed by two peaks around 700 and 900 °C during the first heating. During cooling and follow-up cycles only the damping peak around 700 °C appears. The change in Young’s modulus and the damping behavior is interpreted by different processes like α/β transformation, O alloying and grain boundary sliding. These results indicate that components fabricated by SMD contain a non-equilibrium α phase which transforms to the β phase at higher temperatures than the equilibrium α phase. Furthermore, the vacuum between 2.4 and 5.3 × 10⁻⁴ mbar proved at high temperatures to be not good enough to rule out the contamination by O, which leads to α casing, stiffening, and hardening.     

Spark plasma sintering synthesis and mechanical spectroscopy of the ω-Al0.7Cu0.2Fe0.1 phase

Starting from a mixture of Al–Cu–Fe quasicrystalline (QC) particles and Al powder, a fully dense and almost Al–Cu–Fe ω single-phase alloy was produced by spark plasma sintering. This technique allows synthesising large samples with sizes suitable for mechanical spectroscopy experiments. Mechanical spectroscopy was selected because it is a relevant tool for detecting the presence of structural defects at both nano and microscopic scales. Young’s moduli were measured in the 15 kHz range as a function of temperature by the resonant frequency method. Young’s moduli behave similarly for typical metals and exhibit values that are comparable to those of the Al–Cu–Fe QC phase. The damping coefficient Q ⁻¹ was determined at various temperatures between room temperature and 840 K over a large frequency range, i.e. between 10⁻³ and 10 Hz. The results suggest that solid friction effects do occur. In addition, a relaxation peak is observed in the intermediate temperature range.     

Low temperature-elastic moduli, Debye temperature and internal dilational and shear frictions of fused quartz

Longitudinal and transverse wave velocities, five kinds of elastic parameters (Young’s, shear and bulk moduli, Lame parameter, Poisson’s ratio), Debye temperature, and dilational and shear internal frictions for fused quartz were simultaneously measured over the temperature range from 73 to 400 K, using the ultrasonic pulse wave with frequency of 7.7 MHz. Large increase in Young’s and bulk moduli and small increase in shear modulus and Lame parameter suggest enhancement of rigidity for KI mode on heating. This would be explained by quasi-crystallization which is associated with a lateral motion of oxygen atoms and the resulting relief of macroscopic strains. The 99 and 137 K peaks and 360 K one in shear friction are probably related to dielectric loss peaks arising from Al3+–Na+ and Al3+–K+ substitutional–interstitial paired defects and to β1/β2-tridymite phase transition, respectively. This revised version was published online in November 2006 with corrections to the Cover Date.     

Influence of granular strontium chloride as additives on some electrical and mechanical properties for pure polyvinyl alcohol

A matrix composed of polyvinyl-alcohol (PVA) mixed with different concentration ratios of the granular strontium chloride (SrCl₂·6H₂O) were prepared by casting technique method at room temperature (about 30°C). The electric and dielectric properties such as a.c. electrical conductivity by a conventional method, using Keithly 616 digital electrometer, dielectric constant, and dielectric loss were measured. Calculated equilibrium properties such as lattice constant, bulk modulus and elastic constants are in good agreement with experimental results. The calculated activation energy values agree well with experiment only when the SrCO₂ molecules are allowed to displace under strain, indicating the importance of inner strain relaxation. From the elastic constants, theoretical values of the Young’s modulus, shear modulus, Poisson’s ratio, of SrCl₂ are obtained. In addition mechanical properties such as Young’s modulus, creep relaxation, and energy stored properties for these samples were also determined at room temperature.     

Microstructural control of Ni-YSZ cermet anode for planer thin-film solid oxide fuel cells

Ni-Y₂O₃-stabilized ZrO₂ (Ni-YSZ) cermet anode was fabricated for solid oxide fuel cells (SOFCs) by conventional ceramic processing using NiO-YSZ composite particles. Microstructures of the anode were carefully characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The Ni-YSZ cermet anode was consisting of fine YSZ connections, as the conducting pass of oxygen ions, on the surface of Ni network, as that of electrons, with continuous pore structure and as that of gaseous species. No amorphous phases were present at the interface between Ni and YSZ, and there was an orientation relationship between Ni and YSZ grains, (111)_Ni//(111)_YSZ. The cermet anode showed a high electrical performance at 800 °C. These results indicated that the electrochemical activity of the Ni-YSZ cermet anode was enhanced with the present microstructure.     

Elastic properties of powders during compaction. Part 2: elastic anisotropy

Isotropy in the elastic properties of powders undergoing uniaxial compaction in a cylindrical die was evaluated from in situ measurements of elastic wave speed. Shear and bulk longitudinal wave speeds were measured in both the axial (pressing) and radial directions. For the five different metal powders studied, wave speeds were generally higher in the axial direction. As such, the powder body was best described as a transversely isotropic material; complete isotropy was approached only when the powder was close to the loose packed state, or completely solid. Transversely isotropic elastic moduli analogous to the common isotropic ‘engineering’ moduli (Young’s modulus, Poisson’s ratio, etc.) were calculated by combining elastic wave speed measurements with the Saint-Venant approximation. Pseudo-isotropic elastic moduli (calculated from axial wave speed measurements and assuming elastic isotropy) were found to be only qualitatively similar to transversely isotropic elastic moduli for the axial plane.     

A review of anode materials development in solid oxide fuel cells

High temperature solid oxide fuel cell (SOFC) has prospect and potential to generate electricity from fossil fuels with high efficiency and very low greenhouse gas emissions as compared to traditional thermal power plants. In the last 10 years, there has been significant progress in the materials development and stack technologies in SOFC. The objective of this paper is to review the development of anode materials in SOFC from the viewpoint of materials microstructure and performance associated with the fabrication and optimization processes. Latest development and achievement in the Ni/Y₂O₃-ZrO₂ (Ni/YSZ) cermet anodes, alternative and conducting oxide anodes and anode-supported substrate materials are presented. Challenges and research trends based on the fundamental understanding of the materials science and engineering for the anode development for the commercially viable SOFC technologies are discussed.     

Physical characterization of Y<Subscript>2</Subscript>O<Subscript>3</Subscript>–CeO<Subscript>2</Subscript>–TiO<Subscript>2</Subscript> (YCT) mixed oxides and Ni/YCT cermets as anodes in solid oxide fuel cells

Mixed oxides in the binary Y₂O₃–CeO₂ (YC) and ternary Y₂O₃–CeO₂–TiO₂ (YCT) systems as well as the corresponding Ni cermets were evaluated in terms of application as anodes in solid oxide fuel cells (SOFCs) between 650 and 900 °C. X-ray diffraction (XRD) analysis of the YCT powders calcined up to 1,400 °C showed the cubic fluorite structure of YC and also the formation of an additional phase with pyrochlore structure. The thermal expansion of the ceramics measured in air and Ar/4% H₂ showed no significant differences in the temperature range of 25–800 °C. The absolute values of the total electrical conductivity of the ceramics measured between 450 and 900 °C in Ar/4% H₂ increased by about 1–2 orders of magnitude compared to those measured in air. Ni/Y_0.20Ce_0.80O_1.9 and Ni/Y_0.20C_0.75Ti_0.05O_1.9 cermets with 40 vol% Ni exhibited improved long-term stability regarding their electrical conductivity after annealing at 1,000 °C. The diffusion coefficient of Ce in the 8YSZ electrolyte was measured by compatibility tests. Electrochemical measurements on single SOFCs showed high polarization resistance at the anode/electrolyte interface.     

Elastic properties of powders during compaction. Part 1: Pseudo-isotropic moduli

The elastic moduli of powdered materials undergoing uniaxial compaction was investigated, paying particular attention to effects of solid phase material properties and initial particle shape. Elastic properties were characterised by the isotropic elastic moduli Poisson’s ratio and Young’s modulus, calculated from elastic wave speeds measured in the axial (pressing direction). To isolate material property effects, three different ductile metal powders (copper, stainless steel, and aluminium) with equivalent particle shape (spheroidal) were tested. Comparison with similar measurements for a brittle spheroidal powder (glass) illustrated that solid phase yield mechanism affects the evolution of pore character, and hence bulk elastic properties of the powder compact. Pore character was also studied separately by comparing copper powders with differing particle shapes (spheroidal, irregular, and dendritic). For all powders, Young’s modulus increased monotonically with compaction (reducing porosity). For the ductile spheroidal powders, differences in evolution of Young’s modulus with compaction were accounted for by solid phase elastic properties. The different morphology copper powders showed an increase in compact compliance as particle (pore) ruggedness increased. Poisson’s ratio followed a concave porosity dependence: decreasing in the initial stages of compaction, then increasing as porosity approached zero. Comparison between powders indicated the initial decrease in Poisson’s ratio was insensitive to solid phase material properties. However, as the compact approached solid phase density, the Poisson’s ratio—porosity locus diverged towards corresponding solid phase values for each particle material, indicating an influence of solid phase elastic properties.