Effect of Dopants on Zirconia Stabilization—An X-ray Absorption Study: I, Trivalent Dopants

Local atomic structures of Zr and dopant cations in zirconia solid solutions with Fe₂O₃, Ga₂O₃, Y₂O₃, and Gd₂O₃ have been determined. The Zr ions in both partially stabilized tetragonal and fully stabilized cubic zirconia have their own characteristic structures which are dopant-independent. The dopant cations substitute for Zr ions despite severe local distortions necessitated by the large difference in dopant–O distance ana Zr─O distance. Dopant ionic size determines the preferred locations of oxygen vacancies. Vacancies introduced by oversized dopants (Y and Gd) are located as nearest neighbors to Zr atoms, leaving 8-fold oxygen coordination to dopant cations. Undersized dopants (Fe and Ga) compete with Zr ions for the oxygen vacancies in zirconia, resulting in 6-fold oxygen coordination and a large disturbance to the surrounding next nearest neighbors. Since oxygen vacancies associated with Zr can provide stability for tetragonal and cubic zirconia, these results suggest an explanation for the observation that oversized trivalent dopants are more effective than undersized trivalent dopants in stabilizing cubic and tetragonal phases.     

Reaction Kinetics and Mechanisms between La0.65Sr0.3MnO3 and 8 mol% Yttria-Stabilized Zirconia

The reaction kinetics and mechanisms between 8 mol% yttria-stabilized zirconia (YSZ) and 30 mol% Sr-doped lanthanum manganite (La_0.65Sr_0.30MnO₃, LSM) with A-site deficiency for the application of planar solid oxide fuel cells (SOFCs) were investigated. The LSM/YSZ green tapes were cofired from 1200° to 1400°C for 1 to 48 h and then annealed at 1000°C for up to 1000 h. The results showed that the diffusion of manganese cations first caused the amorphization of YSZ, and then the formation of small La₂Zr₂O₇ (LZ) or SrZrO₃ (SZ) crystals if treated for a longer time at 1400°C. The ambipolar diffusion of the Mn–O pair, transported through the migration of oxygen vacancy, plays an important role in the formation of secondary phases. The diffusion of LSM to YSZ and substitution of Mn for Zr both result in the enhanced concentration of oxygen vacancy, leading to the formation of a void-free zone (VFZ). No additional reaction products in annealed LSM/YSZ specimens, treated at 1000°C for 1000 h, were detected. The interfacial reactions, detailed reaction kinetics, and mechanisms are reported.     

Dependence of the Microstructure and the Electrical Properties of Lanthanum-Substituted (Na1/2Bi1/2)TiO3 on Cation Vacancies

The sintering and electrical characteristics of La-modified Na_1/2Bi_1/2TiO₃ (NBT) was investigated from a defect structure viewpoint. To reveal the role of cation vacancies, two series of ceramics, with different cation vacancies, were processed to compensate the excess positive charge of lanthanum ions. In a region of complete solid solution, the grain size of NBLT-B {[(Na_0.5Bi_0.5)_{1− x} La _x ]Ti_{1−0.25 x} O₃} was smaller than that of NBLT-A {[(Na_0.5Bi_0.5)_{1−1.5 x} La _x ]TiO₃} and densification was enhanced more effectively in NBLT-B. With the aid of thermoelectric power, electric conductivity, and electrotransport measurements, it was found that different sintering behaviors between NBLT-A and NBLT-B specimens were related to the change in the type of cation vacancies present and that lanthanum ion–cation vacancy pairs played an important role in reducing the grain growth and enhancing the densification process.     

Grain-Boundary Reactions in Magnesia-Chrome Refractories: Application of the Electron Probe, II

When a magnesia-chrome refractory is heated in air a reaction layer develops around the chromite grains. This layer is magnesioferrite at 800°C; above 800° it comprises a solid solution of spinel of the type Mg(Al,Cr,Fe³⁺)₂O₄ and a magnesiowustite solid solution. As the temperature increases, the composition of the spinel in the reaction layer changes toward enrichment in chromium and aluminum and impoverishment in iron. A direct-bond chromite-periclase, well defined at about 1750°C, is formed essentially by diffusion. In slowly cooled specimens the average composition of the spinel in the reaction layer in the direct bond approximates to Mg(Al_0.05Cr_0.40-Fe_0.10)₂O₄. The order of diffusion of the individual ions from chromite to periclase is: Fe ≫ Cr > Al. This order can be explained by considering the charge and size of ions involved and the energy required to create cation vacancies. In specimens quenched from high temperatures the concentration of the magnesium-rich phases in the silicate pockets increases in the direction of the periclase grains whereas the calcium-rich phases are concentrated at the center of the pockets.     

Lattice Constants and Compositions of Manganese-Bearing Ferrites

When manganese-bearing ferrites (Mn _x Fe_{3- x} O_4-delta, with x = 0.17-1.09) were prepared in the present study via a solid-state reaction, manganese ions were incorporated into the lattice sites of the ferrites in the divalent state. The lattice constant ( a ₀) of those ferrites increased linearly as the manganese composition x increased. A ferrite with x = 1.28 contained manganese ions with a valence state higher than 3+; however, no further increase in a ₀ occurred at valences higher than 2+.     

Fabrication of Lanthanum Manganese Oxide Thin Films on Yttria-Stabilized Zirconia Substrates by a Chemically Modified Alkoxide Method

Perovskite-type thin films of lanthanum manganese oxide (LaMnO₃) were prepared on yttria (8%) stabilized zirconia substrate by the sol–gel process from an alkoxide solution of lanthanum isopropoxide (La(O- i -C₃H₇)₃) and manganese isopropoxide (Mn(O- i -C₃H₇)₂). The alkoxide solution was chelated with 2-ethyacetoacetate, and further modified with polyethylene glycol (PEG). The obtained LaMnO₃ thin film was transparent and macroscopically crackless. X-ray diffraction, differential thermal analysis–thermogravimetry analysis, and scanning electron microscope observations indicated that single-phase LaMnO₃ thin films with a grain size of 80 to 100 nm are formed when a spin-coated LaMnO₃ gelled film is heated at 600°C for 1 h. The porous and homogeneous grain structure with a grain size of <100 nm can be obtained when the LaMnO₃ gelled film is heated at 600° and 800°C. It was considered that PEG might accelerate the crystallization of the perovskite phase, which indicates that PEG assists the formation of the La-O-Mn frame network during partial hydrolysis and condensation reactions in sol–gel processes.     

The Ternary System MgO-MnO-SiO2

A ternary phase equilibrium diagram is presented for the system Mg0-Mn0-Si0₂. Two peritectic points exist at 1536° and 1538°C. at liquid compositions of (a) 32 MgO, 5 MnO, 63 SiO₂ and (b) 36 MgO, 5.5 MnO, 58.5 SO,. Each of these liquids is in equilibrium with a hodonite (Mn-SiO,) and a high enstatite (MgSiO,) solid solution, the third crystalline phase being cristobalite (SiO₂) with (a) and olivine ((Mg,Mn]₂,Si0₄) with (b). Metasilicate solid solutions form a congruently melting rhodonite series in the central part of the MgSi0₃-MnSi0₃ join, but melt incongruently to olivine plus liquid at the MgSiO₃ end and to tridymite plus liquid at the MnSi0₃ end. X-ray diffraction data are given for two new high enstatite solid solution structures. Rhodonite solid solutions form a continuous structural series extending from MnSi0, to 94.5 weight % MgSi03. Continuous series of solid solutions exist between Mg,Si0₄ and Mn₂SiO₄ and between MgO and MnO. Compositional data are given for coexisting condensed phases, and courses of crystallization are described for certain ternary mixtures. A plot is presented of variation of refractive index of glasses which form an almost colorless series in the region of the rhodonite and high enstatite fields. The quenching technique was used in the investigation, with oxygen partial pressure controlled to maintain manganese as Mn²⁺.     

Transmission Electron Microscope Investigation of the Interface between Titanium and Zirconia

The interfaces between 3-mol%-yttria-partially-stabilized zirconia and commercially pure titanium after reaction at 1750°C were analyzed with a scanning electron microscope and an analytical transmission microscope. Zirconia was reduced to oxygen-deficient zirconia (ZrO_{2- x} ) with an O/Zr ratio as low as 1.53, causing the evolution of oxygen. Part of the oxygen could accumulate at grain boundaries, the remainder being dissolved in titanium as alpha-Ti(O). An ordered titanium suboxide (Ti₃O) could be formed from a solid solution of alpha-Ti(O) during cooling. A fine crystalline ZrO_{2- x} phase (O/Zr similar/congruent 2) was also found along with alpha-Zr near the interface on the zirconia side. The alpha-Zr was twinned with one of the twin planes being indexed as {1012}. The yttria stabilizer was excluded from zirconia as the reaction was progressing, existing as oxygen-deficient yttria. Extensive dissolution of zirconia in titanium gave rise to the formation of alpha-Ti(Zr,O) solid solution. On cooling, lamellae of Ti₂ZrO precipitated from alpha-Ti(Zr,O) with an orientation relationship of {110}_Ti2ZrO//{100}_alpha-Ti and <111>_Ti2ZrO//<011>_alpha-Ti.     

Effect of Dopants on Zirconia Stabilization—An X-ray Absorption Study: III, Charge-Compensating Dopants

X-ray absorption spectra at the Zr-, Y-, and Nb- K edges of ZrO₂-YNBO₄ solid solutions have been measured at 10 K to determine the local atomic structures. Both Y and Nb cations substitute for Zr in the cation network but maintain rather different local oxygen coordination from Zr. In tetragonal zirconia solid solutions, Y adopts a YO₈ structure with a bond length of 2.32 Å. This is the same as the structure found in ZrO₂–Y₂O₃, solid solutions, confirming our previous conclusion that Y is not associated with oxygen vacancies. Nb has an NbO₄ structure with a bond length of 1.90 Å. This is shorter than the Zr–O, distance of 2.10 Å. The strong Nb–O coordination increases the bonding disparity between Zr–O layers, thus increasing tetragonality. This is similar to the trend previously established for ZrO₂-GeO₂ solid solutions. Severe distortion of neighboring cations around the undersized Nb, similar to that previously found for undersized Fe³⁺ and Ga³⁺, is also observed. At higher temperatures, local Y–Nb cation ordering occurs at a concentration below the solubility limit, similar to the Zr–Ge ordering reported previously. This cation ordering mechanism allows the charge-compensating Y-Nb pair to stabilize the tetragonal structure but increase tetragonality.     

Influence of TiO2 Solid Solution on the Thermal Property and Ionic Conductivity Of Partially Stabilized Zirconia

TiO₂(0–20 mol%)-3 mol% yttria-stabilized zirconia (3YSZ) ceramics were prepared by a solid-state reaction. With increasing TiO₂ content in 3YSZ, the structure of the main phase changed from a monoclinic, tetragonal, and cubic mixture to a tetragonal single phase. Increasing TiO₂ content in 3YSZ caused an increase in the average grain size of these ceramics. The thermal conductivity decreased from 4.1 to 2.1 at room temperature with an increase in the TiO₂ content. The specific heat of non-TiO₂-doped 3YSZ was slightly larger than all the doped TiO₂–3YSZ at room temperature. When the TiO₂ content was >8 mol% in 3YSZ, no abrupt expansion, shrinkage, or cracks were observed on heating and cooling these samples; thus, the thermal stability of 3YSZ was improved by TiO₂ solid solution. The ionic conductivity of the samples decreased with increasing TiO₂ solid solution.     

Application of an Ion-Packing Model Based on Defect Clusters to Zirconia Solid Solutions: I, Modeling and Local Structure of Solid Solutions

Oxygen vacancies can be introduced into zirconia solid solution ZrO₂–MO _u ( u = 1 and 1.5) to maintain electroneutrality. Recently, the local structures around Zr⁴⁺ and M^{2 u +} ions in ZrO₂–MO _u solid solutions have been studied through EXAFS spectroscopy, diffuse scattering analysis, and single-crystal structure analysis. The present study constructs an ion-packing model for zirconia solid solutions based on some defect cluster models. The decrease of cell volume with the occurrence of vacancies is assumed to be expressed by decreasing the coordination number (CN) of cations around the vacancy. The distribution of CNs in a solid solution was calculated from a certain defect cluster model. The average interatomic distances, the average CN, and the short-range order parameters were calculated using this distribution of CNs. The local structures calculated from the model were compared with experimental data in the systems ZrO₂–MO_1.5 (M = Y, Gd, Yb, and Ca). In the ZrO₂–YO_1.5 system, the r (s–O) interatomic distance, where s represents Zr⁴⁺ or Y³⁺ and O represents O²⁻, decreased with Y content and therefore vacancy content. The probability of finding Y³⁺ around a vacancy increases with increasing yttria content from a comparison of the calculated results with the ones from recent EXAFS studies. The present model can qualitatively explain compositional and size dependences of the dopant on various local structures.     

Effect of the Presence of Ammonium Peroxodisulfate on the Direct Precipitation of Ceria and Ceria–Zirconia Solid Solutions from Acidic Aqueous Solutions

Direct precipitation of nanometer-sized particles of ceria–zirconia (CeO₂–ZrO₂) solid solutions with cubic and tetragonal structures was successfully attained from acidic aqueous solutions of cerium(III) nitrate (Ce(NO₃)₃) and zirconium oxychloride (ZrOCl₂) through the addition of ammonium peroxodisulfate ((NH₄)₂S₂O₈), because of promotion of the hydrolysis via the oxidation of Ce³⁺ ions, together with the simultaneous hydrolysis of ZrOCl₂ under hydrothermal conditions. Ultrafine CeO₂ particles also could be formed from relatively concentrated aqueous solutions of the same trivalent cerium salt in the presence of (NH₄)₂S₂O₈ via hydrolysis. The crystallite size and lattice strain of as-precipitated solid solutions varied, depending on the composition within the CeO₂–ZrO₂ system. Creation of a solid solution of ZrO₂ into a fluorite-type CeO₂ lattice clearly introduced lattice strain, as a consequence of the decreasing crystallite size. Both the direct precipitation process and the effectiveness of the presence of (NH₄)₂S₂O₈ for the synthesis of CeO₂–ZrO₂ solid solutions were discussed.     

Influence of Non-Stoichiometry on the Structure and Properties of Ba(Zn1/3Nb2/3)O3 Microwave Dielectrics: I. Substitution of Ba3W2O9

A narrow region of Zn-vacancy-containing cubic perovskites was formed in the (1− x )Ba₃(ZnNb₂)O₉−( x )Ba₃W₂O₉ system up to 2 mol% substitution ( x =0.02). The introduction of cation vacancies enhanced the stability of the 1:2 B-site ordered form of the structure, Ba(Zn_{1− x} □ _x )_1/3(Nb_{1− x} W _x )_2/3O₃, which underwent an order–disorder transition at 1410°C, ∼35° higher than pure Ba(Zn_1/3Nb_2/3)O₃. The Zn vacancies also accelerated the kinetics of the ordering reaction, and samples with x =0.006 comprised large ordered domains with a high lattice distortion ( c/a =1.226) after a 12 h anneal at 1300°C. The tungstate-containing solid solutions can be sintered to a high density at 1390°C, and the resultant ordered ceramics exhibit some of the highest microwave dielectric Q factors ( Q × f =1 18 000 at 8 GHz) reported for a niobate-based perovskite.     

Mechanism of Reaction between Lanthanum Manganite and Yttria-Stabilized Zirconia

The reaction of La_{1- x} Ca _x MnO₃ ( x = 0, 0.1, 0.2) with ZrO₂-8 mol% Y₂O₃ (YSZ) has been investigated at temperatures ranging from 1300° to 1425°C in air. Substitution of Ca for La in LaMnO₃ depresses the reactivity with YSZ. A layer of La₂Zr₂O₇ is formed at the La_{1- x} Ca _x MnO₃/YSZ interface after an induction period, and its formation is accelerated when the La_{1- x} Ca _x MnO₃ phase is porous. The reaction proceeds by unidirectional diffusion of La, Mn, and/or Ca ions, mainly Mn ions, into YSZ. The diffusion coefficients of La and Mn ions in YSZ, which are estimated using a LaMnO₃/single-crystal YSZ couple, are much lower than that of oxygen ion. From the experimental data, a reaction mechanism is proposed.     

Stabilization of Cubic Zirconia with Manganese Oxide

A directionally solidified eutectic (DSE) of MnO-ZrO₂ has been investigated using a variety of electron optical techniques. It is found that considerable MnO goes into ZrO₂ to form a substitutional solid solution. About 14 wt% of MnO is soluble in ZrO₂ close to the eutectic temperature. The solubility of ZrO₂ in MnO, however, is quite low, less than 0.50 wt%. Electron diffraction experiments indicate that ZrO₂ (MnO) has the cubic fluorite structure. Diffuse scattering, similar to other cubic zirconias (e.g., CaO, MgO stabilized zirconia), is also observed in manganese-stabilized zirconia. Diffuse scattering indicates the presence of oxygen vacancies and thus confirms the defect nature of the fluorite structure. Electron energy loss spectrometry (EELS) fine structure analysis of the Mn L₂₃ edge provided clear evidence that Mn is present as Mn²⁺ in Mn-stabilized cubic ZrO₂.     

La(Sr)FeO3 SOFC Cathodes with Marginal Copper Doping

(La_0.8Sr_0.2)_0.98Fe_0.98Cu_0.02O_3−δ can be sintered directly onto YSZ (without the need for a protective ceria interlayer). Though subject to an extended "burn-in" period (∼200 h), anode-supported YSZ cells using the Cu-doped LSF achieve power densities ranging from 1.3 to 1.7 W/cm² at 750°C and 0.7 V. These cells have also demonstrated 500 h of stable performance. The results are somewhat surprising given that XRD indicates an interaction between (La_0.8Sr_0.2)_0.98Fe_0.98-Cu_0.02O_3−δ and YSZ resulting in the formation of strontium zirconate and/or monoclinic zirconia. The amount and type of reaction product was found to be dependent on cathode and electrolyte powder precalcination temperatures.     

Synthesis and Characterization of NiO–YSZ Anode Materials: Precipitation, Calcination, and the Effects on Sintering

Nickel oxide yttria-stabilized zirconia (NiO–YSZ) anode materials were synthesized via hydrolysis of the corresponding chloride solutions with NH₃, NH₃+NaOH, and NaOH as precipitation agents. Powder properties such as crystallite size, morphology, and sintering behavior of the final NiO–YSZ materials were also studied. The mechanism of the formation of NiO–YSZ was established for the different co-precipitation techniques by the direct observation of Ni(NH₃) _n ⁺² complexes, Ni(OH)₂ and NiO at different stages of the synthesis process. A direct relationship between the precipitation agent, the order of calcination from dry sample to final product, the final composition, the crystallite sizes and particle sizes of NiO, and the sinterability of the final products was established. A comparison of the powder and individual component properties indicate that the choice of precipitation agent greatly influences the final characteristics. Ni/YSZ materials prepared by NH₃+NaOH precipitation offer higher Ni dispersion and nanocrystallinity of both the Ni and YSZ phases. The conductivity of both prepared materials compares well with mixed-oxide materials of higher Ni content.     

Dysprosium Oxide Ceramics

Ceramic compositions within the systems Dy₂O₃-A1₂0₃, Dy₂O₃-CeO₂, Dy₂0₃-Nb₂0₅, Dy₂O₃-UO₂, and Dy₂0₃ -Zr0₂ are described. Compound formation was observed in the dysprosia-alumina and dysprosianiobia systems whereas solid solutions were observed in the other three systems investigated. The properties that are reported include lattice parameters, density, shrinkage, thermal expansion, water-corrosion resistance, and compound melting points. Dysprosia was found to stabilize zirconia in amounts as low as 10 volume %.     

Oxygen Nonstoichiometry and Defects in Mn-Doped Gd2Ti2O7+x

The oxygen nonstoichiometry in Mn-doped Gd₂Ti₂O₇, Gd₂(Ti_0.975Mn_0.025)₂O_7+x, was measured electrochemically, as a function of temperature and oxygen partial pressure, with the aid of an oxygen titration cell. The analysis of the data shows that the defect equilibrium can be described by considering the dominant point defects to be neutral oxygen interstitials, doubly charged oxygen vacancies, and trivalent and quadrivalent Mn ions substituted in the Ti sites. The enthalpies for the formation of neutral oxygen interstitials and trivalent Mn are determined.     

Phase Relations in the Garnet Region of the System Y2O3–Fe2O3–FeO–Al2O3

Liquidus equilibrium relations for the air isobaric section of the system Y₂O₃–Fe₂O₃–FeO–Al₂O₃ are presented. A Complete solid-solution series is found between yttrium iron garnet and yttrium aluminum garnet as well as extensive solid solutions in the spinel, hematite, orthoferrite, and corundum phases. Minimum melting temperatures are raised progressively with the addition of alumina from 1469°C in the system Y–Fe–O to a quaternary isobaric peritectic at 1547°C and composition Y _0.22 Fe _1.08 Al _0.70 O _2.83* Liquidus temperatures increase rapidly with alumina substitutions beyond this point. The thermal stability of the garnet phase is increased with alumina substitution to the extent that above composition Y _0.75 Fe _0.65 Al _0.60 O ₃ garnet melts directly to oxide liquid without the intrusion of the orthoferrite phase. Garnet solid solutions between Y _0.75 Fe _1.25 O ₃ and Y _0.75 Fe _0.32- Al _0.93 O ₃ can be crystallized from oxide liquids at minimum temperatures ranging from 1469° to 1547°C, respectively. During equilibrium crystallization of the garnet phase, large changes in composition occur through reaction with the liquid. Unless care is taken to minimize temperature fluctuations and unless growth proceeds very slowly, the crystals may show extensive compositional variation from core to exterior.