Sintering behavior and mechanisms of NiO-doped 8 mol% yttria stabilized zirconia

Sintering behavior and densification mechanisms of NiO-doped YSZ were investigated by using a dilatometer, combined with XRD, SEM and HRTEM characterization. The solubility of NiO in YSZ is found to be 0.5-1 mol% at 1500 °C by XRD, and TEM reveals that, beyond solubility limit, the undissolved NiO exists in the form of nano and/or micro-sized crystals depending on the doping amount. The sintering model was used to address the enhanced sintering of YSZ as a result of small additions of NiO. Lattice diffusion is examined to be the rate-determining mechanism for the intermediate-stage sintering of both undoped and NiO-doped YSZ. However, the apparent activation energy for densification of YSZ is reduced by ∼70 kJ/mol upon NiO doping. It is concluded that the dissolved NiO contributes to the lowering of the activation energy and therefore the enhanced lattice diffusivity.     

Novel polymer fibers prepared by electrospinning for use as the pore-former for the anode of solid oxide fuel cell

One-dimensional polyvinyl alcohol (PVA) fibers prepared by electrospinning technique are used as a novel pore-former for the conventional NiO/yttria-stabilized zirconia (YSZ) anodes of solid oxide fuel cell (SOFC). This pore-former forms wire-like pores in the anode substrates, which are beneficial for rapid transport of the fuel and byproduct. The advantage of using this pore-former over the conventional ones (e.g. wheat flour and carbon) is that only a small amount of fibers could generate large amount of continuous pores within the anode for gas transport. In addition the cell with PVA fibers as pore-former for anode exhibits enhanced anode electrocatalytic activity and the cell performance significantly, compared to the cells without pore-former and with wheat flour as pore-former. In this research, an anode-supported SOFC with PVA fibers as pore-former for anode exhibits an open-circuit voltage (OCV) of 1.08 V and maximum power density of 751 mW cm⁻² at 800 °C.     

Synergetic effect of NiO and SiO2 on the sintering and properties of 8 mol% yttria-stabilized zirconia electrolytes

Yttria-doped zirconia electrolytes (e.g., 8 mol% yttria-stabilized ZrO₂, 8YSZ) have been considered to be the most promising candidates for applications in solid oxide fuel cells (SOFC). Due to the ubiquitous presence of SiO₂ impurities and wide use of Ni-containing anodes, it is therefore of great technical importance to understand the synergetic effect of NiO and SiO₂ on densification, grain growth and ionic conductivities (especially the grain boundary (GB) conduction) of zirconia electrolytes. In this study, three groups of 8YSZ ceramics, with Si contents of ∼30, ∼500 and ∼3000 ppm, have been designed. 1 at% NiO was added into these materials by a wet chemical method. The addition of SiO₂ has a negative effect on the sintering and densification, while the introduction of 1 at% NiO reduced the sintering temperature and promotes grain growth of the zirconia ceramics. However, the presence of small amount of NiO prevented full densification of 8YSZ ceramics. NiO also led to a decrease by ∼33% in grain interior (GI) conductivity, with little effect on the GB conduction of high-purity 8YSZ (∼30 ppm SiO₂). However, the coexistence of NiO and SiO₂ is extremely detrimental to total conductivity by significantly reducing the GB conduction. Moreover, it is observed that, unlike the 8YSZ-doped SiO₂ with only, whose GB conduction increases greatly with increasing sintering temperature, the GB conduction of the NiO and SiO₂ codoped samples is less sensitive to sintering temperature.     

High-pressure sintered yttria stabilized zirconia ceramics

Fast densification of 8YSZ ceramics under a high pressure of 4.5 GPa was carried out at different temperatures (800, 1000, 1450 °C), by which a high relative density above 92% could be obtained. FT-Raman spectra indicate that the 8YSZ underwent a phase transition from partially tetragonal to partially cubic phase as temperatures increase from 1000 to 1450 °C when sintering under high pressure. The electrical properties of the samples under different high-pressure sintering conditions were measured by complex impedance method. The total conductivity of 0.92 × 10⁻² S cm⁻¹ at 800 °C has been obtained for 8YSZ under high pressure at 1450 °C, which is about 200 °C lower than that of the samples prepared by conventional pressureless sintering.     

Model for the cold sintering of lead zirconate titanate ceramic composites

A model was developed to describe the cold sintering process (CSP) of lead zirconate titanate (PZT) using moistened lead nitrate as a sintering aid. The densities of PZT powder with different volume fractions of lead nitrate were evaluated after cold sintering at 300°C and 500 MPa for 3 hours. The densities were categorized into three zones. In zone I, the relative density following cold sintering increases from 66% to 80%, as the lead nitrate contents rise from 0 to 14 vol%. In this case, the lead nitrate acts to fill some of the pore volume between PZT grains. Zone II serves as a transition region, where there is both pore filling and dilution of the PZT grains associated with lead nitrate contents from 14 to 34 vol%. In zone III, the relative density drops due to dilution at lead nitrate contents exceeding 34 vol%. To slow the process down so that the kinetics could be studied more readily, samples were cold sintered at room-temperature and 500 MPa. It was found that during the first few seconds of compaction, 85PZT/15Pb(NO₃)₂ rapidly densified from 51% to 61% relative density due to particle re-arrangement. For longer times at pressure, the CSP improved the packing relative to PZT compacted without the lead nitrate, yielding a higher relative density. The late stages of the PZT/Pb(NO₃)₂ CSP could be well described using a viscous sintering model for pressures from 50 MPa to 1000 MPa and temperatures from 25°C to 300°C.     

Improvements to the sintering of yttria-stabilized zirconia by the addition of Ni

We have studied the effect of nickel oxide (NiO) on the sintering of yttria-stabilized zirconia (YSZ) at temperatures from 1100 to 1400 °C. Differences in the densification behaviour were observed between the direct use of NiO powders and Ni metal as precursor. Our results show that with the addition of Ni into YSZ, sintering was completed at 1300 °C instead of 1400 °C, a 100 °C reduction. The addition of Ni also increased the shrinkage rate at 1200 °C from −0.29×10⁻⁶ s⁻¹ to −0.46×10⁻⁶ s⁻¹. Young's modulus of the samples heat treated at 1200 °C measured by microindentation also increased from 26 GPa for YSZ to 65 or 191 GPa for YSZ plus NiO or Ni, respectively. Addition of NiO or Ni also stabilised the cubic phase and promoted grain growth in YSZ during sintering.     

Effects of Al2O3 on the properties and densification of 8YSZ after pulsed current activated sintering

Dense 8YSZ was subjected to pulsed current activated sintering (PCAS) within 1 min of 8YSZ nanopowder preparation using a co-precipitation method. Sintering was accomplished by combining a pulsed current and mechanical pressure. Highly dense 8YSZ with a relative density of up to 99% was produced under simultaneous application of a pressure of 80 MPa and the pulsed current. The effects of the addition of Al₂O₃ on the sintering behavior, mechanical properties, and ionic conductivities of 8YSZ were investigated.     

Performance of mix‐impregnated CeO2‐Ni/YSZ Anodes for Direct Oxidation of Methane in Solid Oxide Fuel Cells

CeO₂‐Ni/YSZ anodes for methane direct oxidation were prepared by the vacuum mix‐impregnation method. By this method, NiO and CeO₂ are obtained from nitrate decomposition and high temperature sintering is avoided, which is different from the preparation of conventional Ni‐yttria‐stabilised zirconia(YSZ) anodes. Impregnating CeO₂ into the anode can improve the cell performance, especially, when CH₄ is used as fuel_. The investigation indicated that CeO₂‐Ni/YSZ anodes calcined at higher temperature exhibited better stability than those calcined at lower temperature. Under the testing temperature of 1,073 K, the anode calcined at 1,073 K exhibited the best performance. The maximum power density of a cell with a 10 wt.‐%CeO₂‐25 wt.‐%Ni anode calcined at 1,073 K reached 480 mW cm^–2 after running on CH₄ for 5 h. At the same time, high discharge current favoured cell operation on CH₄ when using these anodes. No obvious carbon was found on the CeO₂‐Ni anode after testing in CH₄ as revealed from SEM and corresponding linear EDS analysis. In addition, cell performance decreased at the beginning of discharge testing which was attributed to the anode microstructure change observed with SEM.     

Interaction of a ceria‐based anode functional layer with a stabilized zirconia electrolyte: Considerations from a materials perspective

We report on the materials interaction of gadolinium‐doped ceria (GDC) and yttria‐stabilized zirconia (YSZ) in the context of high‐temperature sintering during manufacturing of anode supported solid oxide fuel cells (AS–SOFC). While ceria‐based anodes are expected to show superior electrochemical performance and enhanced sulfur and coking tolerance in comparison to zirconia‐based anodes, we demonstrate that the incorporation of a Ni–GDC anode into an ASC with YSZ electrolyte decreases the performance of the ASC by approximately 50% compared to the standard Ni–YSZ cell. The performance loss is attributed to interdiffusion of ceria and zirconia during cell fabrication, which is investigated using powder mixtures and demonstrated to be more severe in the presence of NiO. We examine the physical properties of a GDC–YSZ mixed phase under reducing conditions in detail regarding ionic and electronic conductivity as well as reducibility, and discuss the expected impact of cation intermixing between anode and electrolyte.     

Microstructural characterization of spray-dried NiO-8YSZ particles as plasma sprayable anode materials for metal-supported solid oxide fuel cell

The plasma spray method is widely used to produce NiO-8YSZ (composed of nickel oxide (NiO) and 8 mol% yttria-stabilized zirconia) anode layers in metal-supported solid oxide fuel cell (SOFC). Flowability control of microsized particles is important for achieving consistent performance of the SOFC anode layer. When microsized particles are fabricated via spray drying and sintering, the most significant factors that influence flowability are their sizes, distribution, and surface conditions. Thus, the aim of this study is to analyze the fabrication conditions for microsized NiO-8YSZ cermet particles made from a nanoscale, sinterable NiO-8YSZ dispersion solution by using an appropriate spray-drying and sintering process. The characteristics of the as-sprayed and sintered NiO-8YSZ composite particles (such as size, distribution, roughness, and nanostructure) were analyzed via field emission scanning electron microscope (FE-SEM), energy dispersive spectroscopy (EDS), particle size distribution (PSD), Brunauer–Emmett–Teller (BET) surface area, and atomic force microscopy (AFM). The as-sprayed microsized NiO-8YSZ particles became smaller and more uniformly distributed as the rotational speed used for spray drying increased. As a result of sintering, the extent of shrinkage of as-sprayed microsized NiO-8YSZ particles generated at high RPMs was lower than that of particles formed at low RPMs. No significant difference was observed in the distribution of the nanosized NiO and 8YSZ particles at different rotational speeds. Furthermore, the highest BET surface areas were observed for particles generated at 8000 RPM before sintering at 13.74 m²/g. After sintering, the highest BET surface area was 0.94 m²/g for particles generated at 16,000 RPM. Differences in nanostructure and surface roughness between as-sprayed and sintered microsized NiO-8YSZ particles were identified via AFM. This study is expected to provide important fundamental information useful for optimizing SOFC efficiency by promoting flowability control during the production of SOFC anodes via plasma spraying.     

Interfacial tension of marine lipids in contact with high pressure carbon dioxide

Interfacial tension (IFT) of fish oil triglycerides (TG) and fatty acid ethyl esters (FAEE) in contact with carbon dioxide (CO₂) was measured according to the pendant drop method at 40, 55 and 70 °C and pressures up to 25 MPa. The IFT of both TG and FAEE decreased substantially with CO₂ pressure. The IFT of FAEE vanished at elevated pressures, whereas that of TG decreased to a fairly constant level of about 2 mN/m. The IFT was correlated using a model taking into account the density, pressure and temperature of CO₂, thereby facilitating the calculation of the ideal pendant drop volume as well as the surface excess concentration of CO₂. In the pressure range studied, the pendant drop volume for FAEE decreased with pressure, whereas for TG it increased at elevated presssures due to the predominant effect of buoyancy. Furthermore, the change in IFT over time was determined at 55 °C for TG in contact with CO₂ at pressures up to 11.4 MPa showing a decrease of IFT over time at low pressures, whereas at higher pressures it remained nearly constant. IFT influences drop formation as well as the disintegration of falling films thereby affecting the performance of separation processes.     

Electrophoretically deposited thin film electrolyte for solid oxide fuel cell

Abstract

Thin films of 8 mol% yttria stabilised zirconia (YSZ) electrolyte have been deposited on non-conducting porous NiO–YSZ anode substrates using electrophoretic deposition (EPD) technique. Deposition of such oxide particulates on non-conducting substrates is made possible by placing a conducting steel plate on the reverse side of the presintered porous substrates. Thickness of the substrates, onto which the deposition has been carried out, varied in the range 0·5–2·0 mm. Dense and uniform YSZ thin films (thickness: 5–20 μm) are obtained after being cofired at 1400°C for 6 h. The thickness of the deposited films is seemed to be increased with increasing porous substrate thickness. Solid oxide fuel cell (SOFC) performance is measured at 800°C using coupon cells with various anode thicknesses. While a peak power density of 1·41 W cm^?2 for the cells with minimum anode thickness of 0·5 mm is achieved, the cell performance decreases with anode thickness.     

Synthesis and thermal behavior of Cu/Y2W3O12 composite

Cu metal matrix composite with Y₂W₃O₁₂ as a thermal expansion compensator was fabricated by high energy ball milling followed by compaction and sintering, and its thermal properties were explored for the potential applications as heat sinks in electronic industries, high precision optics, and space structures. The volume fraction of reinforcement was varied from 40% to 70% in order to tailor the composite for the simultaneous accomplishment of low thermal expansion and high thermal conductivity. The synthesis technique was optimized by varying the parameters like milling time from 1 to 20 h and sintering temperature from 600 to 1000 °C in order to achieve densified composites. The relative density of the composites is found to be around 90% for the 10 h milled powders followed by compaction at a pressure of 700 MPa and sintering at a temperature of 1000 °C. The thermal expansion of the composites exhibits linear behavior in the temperature range 200 to 800 °C and the low coefficient of thermal expansion (CTE) is found to be for Cu–70%Y₂W₃O₁₂ composite whose value, 4.32±0.75×10⁻⁶/°C, matches with that of Si substrate. The thermal conductivities are found to increase with a decrease in the volume fraction of the reinforcement and decrease with an increase in the temperature for all the samples. The experimentally determined CTE and thermal conductivity values are found to be comparable to those predicted by the thermal expansion based Kerner and Turner model and the thermal conductivity based Maxwell model, respectively.     

Effects of the oxygen pressure on the crystalline orientation and strains of YSZ thin films prepared by E-beam PVD

Yttria−stabilized zirconia, YSZ, thin films were prepared by E-beam physical vapor deposition (PVD) at 200 °C under oxygen pressure of 1 × 10⁻³∼1 × 10⁻⁵ Torr. Observations by Field Emission Scanning Electron Microscope (FESEM) proved that different oxygen pressures influenced the thickness of interfacial SiO_x layer formed between the YSZ thin films and Si(100)-substrate. X-ray diffraction (XRD) patterns were used to determine the crystalline structure and calculate the surface grain size of deposited YSZ thin films. XRD patterns also showed that the peaks corresponding to planes (111), (200), (220), and (311) were found and the YSZ thin films revealed the fluorite structure. At lower oxygen pressure (1 × 10⁻⁵∼1 × 10⁻⁴ Torr) YSZ thin films revealed the (111) preferred orientation and at higher oxygen pressure (5 × 10⁻⁴∼1 × 10⁻³ Torr) YSZ thin films revealed the (200) preferred orientation. The effects of oxygen pressure on the lattice constants and the internal strains of YSZ thin films were also investigated.     

Constrained sintering kinetics of 3YSZ films

3YSZ green layers approximately 10 μm thick were screen-printed onto 3YSZ substrates and their constrained sintering kinetics were measured at 1100-1350 °C using an optical dilatometer. The densification rates of the same powder in the form of pellets and free-standing films were also measured. The constrained densification rate was greatly retarded compared with the free densification rate at a given temperature and density. The retardation increased with increasing density and temperature and could not be properly accounted for by existing theories of constrained sintering. As a result the apparent activation energy is much lower for constrained sintering (135 ± 20 kJ mol⁻¹) than for free sintering (660 ± 30 kJ mol⁻¹). It is proposed that this is because the constrained microstructure exhibits larger and more widely separated pores at the higher temperatures.     

Uniaxial compaction behaviour and elasticity of cohesive powders

The compression and compaction behaviour of bentonite, limestone and microcrystalline cellulose (MCC) — three cohesive powders widely used in industry were studied. Uniaxial compression was performed in a cylindrical die, 40 mm in diameter and 70 mm high, for three selected cohesive powder samples. The initial density, instantaneous density and tablet density were determined. The influence of maximum pressure and deformation rate was examined. The secant modulus of elasticity E_sec was calculated as a function of deformation rate v, maximum pressure p and powder sample. After compaction experiments in hydraulic press at three pressures - p = 30, 45 and 60 MPa - and two different deformation rates, the strength of the produced tablets was examined in a material strength testing machine.From uniaxial compression tests performed on the universal testing machine for loading and unloading, the modulus of elasticity E was calculated on the basis of the first linear phase of unloading. The total elastic recovery of tablets was also obtained.     

Microstructure evaluation of sintered combustion-derived fine powder NiO–YSZ

Citrate–nitrate combustion synthesis was used for the preparation of NiO–YSZ. The main advantage of the preparation method used was reflected in the fact that after the synthesis both phases NiO and YSZ were randomly distributed on a nanometre level. The prepared NiO–YSZ powder composites were shaped, sintered and reduced to Ni–YSZ and subsequently submitted to microstructure investigations. Relative sintered densities higher than 90% were obtained at sintering temperatures as low as 1200 °C. A sintering temperature 1200 °C was also recognized as the preparation temperature that provided the smallest Ni grains in the final Ni–YSZ cermet with an average Ni-particle diameter as low as 0.27 μm.     

Effect of microstructure on the electrochemical performance of Ni-ScSZ anodes

The effects of NiO powder morphology and sintering temperature on the microstructure and the electrochemical performance of Nickel-scandia-stabilized zirconia (Ni-ScSZ) cermet anodes for solid oxide fuel cells (SOFCs) were investigated. The particle size and agglomeration of the starting powders were found to affect both the microstructure and electrochemical performance of the Ni-ScSZ cermet anodes. The lowest polarization resistance, 0.690 Ω cm² at 700 °C, was measured for the Ni-ScSZ anode prepared with fine NiO powder (~0.5 µm grain size). This was attributed to the increase in the number of reaction sites afforded by the small grains and well-dispersed Ni and ScSZ phases. The effect of the anode sintering temperature was also found to affect the anode microstructure, adhesion with the electrolyte, and consequently anode polarization resistance. The lowest polarization resistance was observed for the anode sintered at 1400 °C and this was 3–5 times lower than the corresponding values for anodes sintered at lower temperatures.     

Effect of Nickel Oxide/Yttria-Stabilized Zirconia Anode Precursor Sintering Temperature on the Properties of Solid Oxide Fuel Cells

An NiO/yttria-stabilized zirconia (YSZ) layer sintered at temperatures between 1100° and 1500°C onto dense YSZ electrolyte foils forms the precursor structure for a porous Ni/YSZ cermet anode for solid oxide fuel cells. Conflicting requirements for the electrochemical performance and mechanical strength of such cells are investigated. A minimum polarization resistance of 0.09 Ω^.cm²at 1000°C in moist hydrogen is obtained for sintering temperatures of 1300°–1400°C. The mechanical strength of the cells decreases with increased sintering temperature because of the formation of channel cracks in the electrode layers, originating in a thermal expansion coefficient mismatch between the layers.     

High-pressure compaction of municipal solid waste to form densified fuel

A unique piston-in-mold (punch-in-die) process, developed at the Capsule Pipeline Research Center (CPRC), University of Missouri-Columbia, USA, was used to study the compaction of the combustible components found in municipal solid waste (MSW) stream to produce a densified fuel. The compaction was performed under room temperature without binder and at pressures up to 138 MPa. The materials included waste paper, plastics (both film and hard products), textiles, and wood. They were compacted into 49-mm-diameter logs in different mixtures and under various moisture contents and pressures. The mixtures compacted include: (1) paper and plastic film, (2) paper and hard plastics, (3) paper and mixed plastics (both filmed and hard), and (4) paper, plastics, textiles, and wood. The properties of the logs, including density, abrasion resistance, impact resistance, combustion characteristics, and long-term performance were tested. The results show that the waste paper in the mixtures played a role of binder in the formation of the compacted logs. Moisture content and compaction pressure were two key parameters for producing good logs. For all these mixtures, a moisture content of less than 15% is necessary to produce dense (dry density equal to or higher than 0.8 g/cm³) and strong logs. A minimum compaction pressure of 70 MPa is needed for mixtures 1, 2, and 3, and 100 MPa for mixture 4. The logs made from the mixtures of the combustible wastes have a heating value equivalent to that of subbituminous coal, and can be cofired with coal in power plants.