Electrochemical Studies in Glass: I, The System NiO-Na2Si2O5

A solid electrolyte electrochemical cell of the type Pt|Ni:NiO _{a =1}∥ZrO₂+7.5% CaO∥Ni:NiO _{a <1}+glass|Pt was used to measure the activities of NiO in sodium disilicate glass from 750° to 1100°C. The data indicate a solubility varying from 11 mol% (5.0 wt%) at 800° to 20 mol% (9.3 wt%) at 1100°C. From the variation in NiO activity, the activity of sodium disilicate in glass solution was estimated; from these combined data partial molar free energies and entropies of solution of NiO and Na₂Si₂O₅ and free energies and entropies of mixing were calculated. A partial phase diagram for the system NiO-Na₂Si₂O₅ proposed from solubility data indicates a eutectic at ∼12 mol% (5.3 wt%) NiO at 830°C.     

Activity–Composition Relations in the Nickel Oxide–Magnesium Oxide System at 1300 K by the Solid-State Galvanic Cell Technique

The activity of NiO in NiO-MgO rock salt solid solution has been measured at 1300 K by employing a solid-state galvanic cell: Pt,Ni+ NiO||(CaO)ZrO₂||Ni + (Ni_x,Mg_l-x)O, Pt. A high-density tube of Zr0₂-15 mol% CaO has been used as the solid electrolyte for the emf measurements. The activities of the component oxides in the rock salt solid solution exhibit negative deviation from ideality at the temperature of investigation. The solid solution obeys regular solution behavior at 1300 K. The value of the regular solution parameter is found to be -12000 ((l000) J mol^-1. The composition dependence of ΔG^Ex obtained in this study agrees reasonably well with the calorimetric data reported in the literature for NiO-MgO solid solution.     

Activity–Composition Relations in NiAl2O4─MnAl2O4 Solid Solutions and Stabilities of NiAl2O4 and MnAl2O4 at 1300° and 1400°C

Activities of NiO were measured in the oxide and spinel solutions of the system MnO–NiO–Al₂O₃ at 1300° and 1400° C with the aim of deriving information on the thermodynamic properties of the spinel phases. Synthetic samples in selected phase assemblages of the system were equilibrated with metallic nickel and a gas phase of known oxygen partial pressures at a total pressure of 1 atm. The data on NiO activities and directions of conjugation lines between coexisting oxide and spinel phases were used to establish the activity–composition relations in spinel solid solutions at 1300° and 1400°C. The MnAl₂O₄–NiAl₂O₄ solid solutions exhibit considerable negative deviations from ideality at these temperatures. The free energy of formation of MnAl₂O₄ from its oxide components (MnO + Al₂O₃) at 1300° and 1400°C is calculated to be −24.97 and −26.56 kJ. mol⁻¹, respectively. The activities determined in the stoichiometric spinel solid solutions are more negative as compared with those predicted from cation distribution models.     

Li-(Mg,Zn,Ni) Vanadates

Phase relations were determined in the systems of Li-(Mg,Zn,Ni) vanadates in the temperature range between 500° and 800°C. Each of the ternary systems Li₂O-(Mg, Zn, or Ni)O-V₂O₅ is characterized by an orthovanadate: olivine-type LiMgVO₄, phenakite-type LiZnVO₄, and spinel-type LiNiVO₄. Mutual solid solutions form between them. Two solid solutions, a large region of the NiV₂O₆ type and a complete series between MgV₂O₆ and ZnV₂O₆, exist in the system (Mg,Zn,Ni)V₂O₆. The system (Mg,Zn,Ni)₂V₂O₇ has an extensive solid-solution region based on Ni₂V₂O₇, whereas the system (Mg,Zn,Ni)₃V₂O₈ is characterized by a complete series.     

Stability of CaNiSi2O6 ("Niopside") and Activity–Composition Relations of CaMgSi2O6– CaNiSi2O6 Solid Solutions at 1350°C

A complete solid-solution series exists between diopside (CaMgSi₂O₆) and its nickel analogue, "niopside"(CaNiSi₂O₆). Activity–composition relations within this solid solution, and the stability of the end member CaNiSi₂O₆, have been determined by equilibrating CaNiSi₂O₆ with SiO₂, CaSiO₃, and metallic Ni in atmospheres of known oxygen pressures. Within limits of accuracy of the experiments, the solution is ideal at 1350°C. From the experimental data obtained in the present investigation, the standard free energy (Δ G °) of formation of CaNiSi₂O₆ according to the equation CaO + NiO + 2SiO₂= CaNiSi₂O₆ is calculated to be Δ G °=−165862 + 42.40 T J. Experiments in the system CaO–NiO–SiO₂ have shown that the nickel analogue of the phase pseudo-enstatite (MgSiO₃) is unstable with respect to SiO₂ and nickel olivine (Ni₂SiO₄), and the nickel analogues of the phases akermanite (Ca₂MgSi₂O₇) and monticellite (CaMgSiO₄) are unstable relative to the phase assemblage pseudo-wollastonite (CaSiO₃) plus NiO. In the system CaO–MgO–NiO–SiO₂, however, substitution of Ni for Mg in these phases was observed. The percentage substitution of Ni for Mg in the phases is given in parentheses: diopside (100%), olivine (100%), enstatite (18%), akermanite (20%), and monticellite (57%).     

Composition Dependence of 63Ni Diffusion in Single-Crystal NiO-MgO Solid Solutions

Diffusion coefficients were determined for ⁶³Ni diffused into single-crystal NiO-MgO solid solutions for 16.2 h at 1303°C. The solid solutions ranged in composition from 0 to 67.9 at.% NiO and were prepared by prior interdiffusion between NiO-MgO powder compacts and single-crystal MgO at conditions (10 days at 1800°C) selected to provide concentrations which were uniform to at least 5 at.% NiO within the subsequent isotope diffusion zone. The diffusion coefficient of ⁶³Ni increased with increasing Ni content of the solid solution with an exponential dependence given by D *= (1.55±0.12±10⁻¹¹) exp (3.16±0.46 [NiO]). The result confirms data previously obtained from Boltzmann-Matano analysis of interdiffusion specimens, but under conditions where the precise shape of the solute gradient is not essential and where the assumption of rapid local equilibration of defect structure need not be made.     

Fabrication and Characterization of Anode-Supported Tubular Solid-Oxide Fuel Cells by Slip Casting and Dip Coating Techniques

High-performance anode-supported tubular solid-oxide fuel cells (SOFCs) have been successfully developed and fabricated using slip casting, dip coating, and impregnation techniques. The effect of a dispersant and solid loading on the viscosity of the NiO/Y₂O₃–ZrO₂ (NiO/YSZ) slurry is investigated in detail. The viscosity of the slurry was found to be minimum when the dispersant content was 0.6 wt% of NiO/YSZ. The effect of sintering temperature on the shrinkage and porosity of the anode tubes, densification of the electrolyte, and performance of the cell at different solid loadings is also investigated. A Ni/YSZ anode-supported tubular cell fabricated from the NiO/YSZ slurry with 65 wt% solid loading and sintered at 1380°C produced a peak power output of ∼491 and ∼376 mW/cm² at 800°C in wet H₂ and CH₄, respectively. With the impregnation of Ce_0.8Gd_0.2O₂ (GDC) nanoparticles, the peak power density increased to ∼1104 and ∼770 mW/cm² at 800°C in wet H₂ and CH₄, respectively. GDC impregnation considerably enhances the electrochemical performance of the cell and significantly reduces the ohmic and polarization resistances of thin solid electrolyte cells.     

Decomposition of Ni2SiO4 in an Oxygen Potential Gradient

Polycrystalline Ni₂SiO₄ was exposed to a gradient in oxygen potential at 1336°C to cause kinetic decomposition into its component oxides, NiO and SO2. At the higher-oxygen-potential side NiO formed, and at the lower-oxygen-potential side SiO₂ formed. This spatial distribution of SiO₂ and NiO is consistent with diffusion data for silicate olivines which indicate that Ni diffuses much faster than either Si or O.     

Zirconia-Based Metastable Solid Solutions through Self-Propagating High-Temperature Synthesis: Synthesis, Characterization, and Mechanistic Investigations

Cubic Zr_{1− x} Me _x O _y (Me = Fe, Co, Ni, Cu) metastable solid solutions with metal content significantly higher than equilibrium levels have been synthesized by the self-propagating high-temperature synthesis method based on a thermite reaction between metallic zirconium and the transition-metal oxides CoO, Fe₂O₃, CuO, and NiO. Through in situ XRD analysis, it was determined that when heated to 1100°C, the cubic solid solution transformed to the tetragonal phase with the concomitant formation of iron oxide. When cooled to lower temperatures, the tetragonal phase transformed to the monoclinic phase at or below 500°C. Results of auxiliary experiments strongly suggest that the formation of the solid solution takes place behind the combustion front by a reaction between zirconia and the metal.     

Microstructural and Thermoelectric Characteristics of Zinc Oxide-Based Thermoelectric Materials Fabricated Using a Spark Plasma Sintering Process

M-doped zinc oxide (ZnO) (M=Al and/or Ni) thermoelectric materials were fully densified at a temperature lower than 1000°C using a spark plasma sintering technique and their microstructural evolution and thermoelectric characteristics were investigated. The addition of Al₂O₃ reduced the surface evaporation of pure ZnO and suppressed grain growth by the formation of a secondary phase. The addition of NiO promoted the formation of a solid solution with the ZnO crystal structure and caused severe grain growth. The co-addition of Al₂O₃ and NiO produced a homogeneous microstructure with a good grain boundary distribution. The microstructural characteristics induced by the co-addition of Al₂O₃ and NiO have a major role in increasing the electrical conductivity and decreasing the thermal conductivity, resulting from an increase in carrier concentration and the phonon scattering effect, respectively, and therefore improving the thermoelectric properties. The ZnO specimen, which was sintered at 1000°C with the co-addition of Al₂O₃ and NiO, exhibited a ZT value of 0.6 × 10⁻³ K⁻¹, electrical conductivity of 1.7 × 10⁻⁴Ω⁻¹·m⁻¹, the thermal conductivity of 5.16 W·(m·K)⁻¹, and Seebeck coefficient of −425.4 μV/K at 900°C. The ZT value obtained respects the 30% increase compared with the previously reported value, 0.4 × 10⁻³ K⁻¹, in the literature.     

Phase Relations in the System Iron Oxide–NiO–SiO2 under Strongly Reducing Conditions

Liquidus and solidus phase relations have been determined for the system iron oxide–NiO–SiO₂ under strongly reducing conditions obtained by using CO₂–CO gas mixtures in controlled proportions. The phase relations were determined with the well-known quenching method: oxide mixtures were equilibrated in vertical tube resistance furnaces, followed by quenching to room temperature and identification of phases with transmitted- and reflected-light microscopy and X-ray diffraction. Three crystalline phases are present on the liquidus surface: olivine (Fe₂SiO₄–Ni₂SiO₄ solid solutions), oxide ("FeO"–NiO solid solutions), and silica (tridymite or cristobalite, depending on temperature). The "ternary peritectic" point where these three phases coexist with liquid is at 1571°C, with a liquid composition of approximately 19 wt%"FeO", 47 wt% NiO, 34 wt% SiO₂.     

The System MgO–MgAl2O4

In the determination of the liquidus, solidus, and subsolidus of the system MgO-MgAl₂O₄ the limits of the solid solution of A1 ions in periclase and Mg ions in spinel were measured. By using both X-ray diffraction and optical techniques, the maximum periclase solid solution was found at 82 wt% MgO, 18 wt% A12O3 (9.5% A1³⁺) and maximum spinel solid solution at 39% MgO, 61 % A1203 (6% Mg⁺⁺). Periclase and spinel solid solutions existed stably in easily detectable amounts at temperatures above approximately 1500°C.     

Fabrication of NiO Nanoparticle-Coated Lead Zirconate Titanate Powders by the Heterogeneous Precipitation Method

NiO nanoparticle-coated lead zirconate titanate (PZT) powders are successfully fabricated by the heterogeneous precipitation method using PZT, Ni(NO₃)₂·6H₂O, and NH₄HCO₃ as the starting materials. The amorphous NiCO₃·2Ni(OH)₂·2H₂O are uniformly coated on the surface of PZT particles. XRD analysis and the selected-area diffraction (SAD) pattern indicate that the amorphous coating layer is crystallized to NiO after being calcined at 400°C for 2 h. TEM images show that the NiO particles of ∼8 nm are spherical and weakly agglomerated. The thickness of the nanocrystalline NiO coating layer on the surface of PZT particle is ∼30 nm.     

Activity of Nickel(II) Oxide in Silicate Melts

Activity–composition relations of NiO have been determined at 1435°C in melts of the system CaO–NiO–SiO₂ and at 1400°C in melts of the systems CaO–MgO–NiO–SiO₂, CaO–MgO–NiO–Al₂O₃–SiO₂, and CaO–MgO–NiO–K₂O–SiO₂. In the CaO–NiO–SiO₂ and CaO–MgO–NiO–SiO₂ systems the activity coefficient of NiO (γ_NiO) decreases as the polymerization of the melt increases (basicity decrease). γ_NiO stays contant up to several weight percent NiO for melts with similar basicity, an indication of Henry's law behavior. Minima in the NiO activity coefficient are observed in melts of the CaO–MgO–NiO–Al₂O₃–SiO₂ and CaO–MgO–NiO–K₂O–SiO₂ systems at NBO/Si ratios between 1.0 and 1.5; i.e., γ_NiO decreases with decreasing basicity for NBO/Si ratios >1.5 and increases with decreasing basicity for melts with NBO/Si ratios <1.0 (NBO/Si; nonbridging oxygens per silicon). The addition of Al₂O₃ and K₂O to the CaO–MgO–NiO–SiO₂ system results in an increase in the activity coefficient of NiO.     

Investigations of NiO-CuO Solid Solutions Using Transmission Electron Microscopy: II, Evidence for Long-Range Cation Order

Further studies of rock salt solid solutions in the (Ni _x Cu_{1- x} )O system have given direct structural evidence of long-range cation order. Using transmission electron microscopy a structural modulation has been observed at high NiO concentration, x < 0.85, and superlattice reflections and fringes confirmed the formation of an ordered compound Ni₂CuO₃. The cationic arrangement in Ni₂CuO₃ is analogous to the atomic arrangement in the alloy Ni₂V The proposed structure is orthorhombic with a = a _cubic/√2, b = 3a_cubic/√2, and c = a _cubic, and it shows considerable microtwinning. The arrangement of cations is compared to that in Cu₂MgO₃ and these two compounds are found to be closely related. The structural models are discussed in terms of the energetics of the (Ni_x, Cu_{1- x} )O system. The behavior of CuO-MO systems, containing Ni²⁺, is compared and contrasted to that in the Co and Mg systems.     

Superior Perovskite Oxide-Ion Conductor; Strontium- and Magnesium-Doped LaGaO3: III, Performance Tests of Single Ceramic Fuel Cells

The performance of La_0.8Sr_0.2Ga_0.83Mg_0.17O_2.815 (LSGM) as an optimized electrolyte of a solid oxide fuel cell was tested on single cells having a 500-µm-thick electrolyte membrane. The reactivity of NiO and LSGM suggested use of an interlayer to prevent formation of LaNiO₃. The interlayer Sm-CeO₂ was selected and sandwiched between the electrolyte and anode. Comparison of Sm-CeO₂/Sm-CeO₂+ Ni and Sm-CeO₂+ Ni as anodes showed that Sm-CeO₂/Sm-CeO₂+ Ni gave an exchange current density 4 times higher than that of Sm-CeO₂+ Ni. The peak power density of the interlayered cell is 100 mW higher than that of the standard cell without the interlayer. This improvement is due to a significant reduction of the anode overpotential; the overpotential of the cathode La_0.6Sr_0.4CoO_3-delta (LSCo) remained unchanged. Comparison of the peak power density in this study and with that of a previous study, also with a 500-µm-thick electrolyte, indicates a factor of 2 improvement, i.e., from 270 mW/cm² to 550 mW/cm² at 800°C. The excellent cell performance showed that an LSGM-based thick membrane SOFC operating at temperatures 600° < T _op < 800°C is a realistic goal.     

Thermodynamic Properties of the System Indium-Oxygen

The free-energy change for the reaction 2In( l )+3NiO( s ) = In₂O₃( s )+3Ni( s ) was determined from 550° to 800°C from emf measurements on solid-oxide galvanic cells. The results were used to develop an equation for the standard molar free energy of formation of In₂O₃, i.e. ΔG°_ln2O₃= -215,550+72.63 T ±450 cal/ mol. The standard molar enthalpy and entropy of formation of In₂O₃ at 298°K were calculated to be -214,000±1500 cal/mol and -69.03±o0.22 eu, respectively, using the available thermo-chemical data. The absolute entropy of In₂O₃ at 298°K was calculated to be 32.23±0.22 eu. The free-energy results of this study were used in conjunction with literature data to calculate partial pressures of the gaseous species over In₂O₃ for different experimental conditions.     

Precipitation and Solid Solubility in the System NiO-Cr2O3

The precipitation of NiCr₂O₄ and the solid solubility of Cr³⁺ in NiO were investigated. Homogeneous precipitation occurs in this system, and the decomposition of NiO-Cr₂O₃ solid solution involves only one stable equilibrium phase, NiCr₂O₄. The small lattice misfit between precipitate and matrix results in the formation of randomly distributed spherical precipitates about 3 to 10 nm in diameter in the initial stage of growth. Above that size, precipitates form as ellipsoidal plates lying on (100) with all directions of the precipitate and matrix mutually parallel. Strain-energy effects provide a rational explanation for the observed precipitate morphology during nucleation and subsequent growth. The precipitates grow along 100 to a size at which coherency is no longer maintained. The loss of coherency is associated with the development of interfacial dislocations, and the critical size at which coherency is lost is ∼300 nm. Dissolution of coherent precipitates and growth of semicoherent precipitates was directly observed to occur si multaneously when both are present. The NiO-NiCr₂O₄ two-phase boundary (the NiO(Cr)(ss) solvus line) was experimentally determined between 950° and 1150°C. Concentration profiles measured at two-phase interfaces indicate that the growth of the NiCr₂O₄ phase is controlled by volume diffusion. In the present study, interface concentrations obtained from coherent precipitates are indistinguishable from those obtained from semicoherent precipitates. The maximum solubility of Cr in NiO was found to be 0.98°0.10 at 950°, 1.80°0.15 at 1050°, and 3.60°0.31 at 1150°C.     

Thermal Expansion of Homogeneous and Gradient Solid Solutions in the System KZr2(PO4)3─KTi2(PO4)3

Low-thermal-expansion ceramics having arbitrary thermal expansion coefficients were synthesized from homogeneous solid solutions in the system KZr₂(PO₄)₃─KTi₂(PO₄)₃ (KZP–KTP). Dense and strong ceramics were fabricated by sintering at 1100° to 1200°C with 2 wt% MgO. The thermal expansion coefficient increased from 0 to +3 × 10⁻⁶/°C with increasing x in KZr_{2 − x}Ti_x (PO₄)₃ (KZTP). In addition, a functionally gradient material with respect to thermal expansion was prepared by forming a series of KZTP solid solutions in a single ceramic body. By heating a pile of KZP and KTP ceramics in contact with each other, KZP and KTP bonded together to form a KZTP gradient solid solution near the interface.     

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.