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.     

Effect of Dopants on Zirconia Stabilization—An X-ray Absorption Study: II, Tetravalent Dopants

X-ray absorption spectra at Zr- K , Ce-L₁₁₁ and Ge- K edges in ZrO₂–CeO₂ and ZrO₂–GeO₂ solid solutions have been measured at 10 K. Both Ce and Ge substitute for Zr in the cation network, but CeO₈ and GeO₄ polyhedra are maintained around the dopants. The oversized CeO₈ is compressed to a Ce–O distance of 2.30 Å, which is smaller than the 2.35 Å seen in fluorite-type CeO₂. Meanwhile, the distortion of the Zr-cation network is increased and the tetragonality is reduced. The undersized Ge has a Ge–O distance of 1.80 Å, which is much shorter than the Zr–O, distance of 2.10 Å. A strong tendency for Ge–Zr ordering in tetragonal solid solutions into locally scheelite-like clusters, even within the solubility limit, is observed. This results in an increase in the tetragonality and a decrease in the distortion of the cation network. Despite the opposite effects on tetragonality, both Ce and Ge doping can stabilize tetragonal zirconia albeit via different mechanisms. The structural origin of the cubic phase is also elucidated.     

X-ray Absorption Studies of Ceria with Trivalent Dopants

The local structural environments of Sc³⁺ and Gd³⁺ in ceria were studied for the first time by using EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge structure) spectroscopy. Our results indicate that Sc³⁺ in 10-mol%-Sc₂O₃-doped ceria has a 7-fold-coordinated Sc-ordered structure with two different Sc─O distances (2.13 and 2.26 Å). An octahedrally coordinated Sc─O cluster is present in 5 mol% Sc₂O₃─5 mol% Gd₂O₃─CeO₂, because of the strong scavenging effect of Sc³⁺ ions on oxygen vacancies created by Gd³⁺. The Gd³⁺ in this sample is largely unassociated. Sc₂O₃ (5 mol%) and Gd₂O₃ (5 or 10 mol%) form random solid solutions with CeO₂. These results are consistent with the current concept of defect structures in this structural family.     

Effect of Tetravalent Dopants on Raman Spectra of Tetragonal Zirconia

Tetragonal zirconia solid solutions were prepared by the additions of ceria, titania, germania, and cassiterite into 2-mol%-Y₂O₃-stabilized t -ZrO₂ (2Y-TZP), and the effect of the dopant size on the changes in the Raman spectra of t -ZrO₂ was investigated. The frequencies of the Raman modes that were associated with Zr-O bond stretching decreased as doping with the oversized cation (Ce⁴⁺) increased and increased linearly as doping with the undersized dopants (Ti⁴⁺, Ge⁴⁺, Sn⁴⁺) increased within the solubility limits of the dopants in 2Y-TZP. The frequency of the 640 cm⁻¹ Raman band decreased as the square root of the unit-cell volume increased, suggesting that the ionic size of dopants governs the Raman mode that is related to the short ZrO bond between two ZrO bonds in t -ZrO₂ solid solutions. Although there was no consistent relationship between the change in the tetragonality of t -ZrO₂ solid solutions and the tetravalent cation size, the intensity ratio of the Raman modes at 609 and 640 cm⁻¹ ( I ₆₀₉/ I ₆₄₀) decreased as the ionic size decreased.     

Effects of Lanthanide Dopants on Oxygen Diffusion in Yttria-Stabilized Zirconia

The effects of lanthanide co-dopants on oxygen diffusion in yttria-stabilized zirconia (YSZ) are studied using a combined first principles density functional theory (DFT)/kinetic Monte Carlo (kMC) modeling approach. DFT methods are used to calculate barrier energies for oxygen migration in different local cation environments, which are then input into kMC simulations to obtain long-time oxygen diffusivities and activation energies. Simulation results show a substantial increase in the maximum value of the oxygen diffusivity upon co-doping and in the dopant content at which this value is obtained for Lu-co-doped YSZ; while relatively little change is seen for Gd-co-doped YSZ. Examination of the DFT barrier energies reveals a linear scaling of barrier heights with the size of cations at the diffusion transition state. Using this strong correlation, oxygen diffusivity is examined in YSZ co-doped with several lanthanide elements. The oxygen diffusivity decreases with dopant atomic number (and decreasing dopant ion size) for co-dopants smaller than Y, and changes relatively little when Y is replaced by co-dopants larger than it. These results are broadly consistent with experiment, and are explained in terms of cation-dopant and vacancy concentration-dependent correlation effects, with the aid of a simple analytical model.     

Photo-Thermo-Refractive Glasses: Effects of Dopants on Their Ultraviolet Absorption Spectra

Data accumulated on the UV spectra of photo-thermo-refractive glasses known to be a new class of photo-sensitive materials for nanophotonics is discussed. The paper is aimed to providing specialists in the applied photonics with a complete self-consistent block of the latest information on the UV spectral manifestations of various oxidation states of dopants and on the redox balance between these states. For all oscillators related to the oxidation states of dopants such as cerium, silver, and antimony, the spectroscopic parameters are computed and analyzed. Intercomparison of the data is conducted in terms of the redox reactions between particular oxidation states of dopants. The effects of bromine in the matrix glass composition on the parameter magnitudes for the Ag⁺ and Ce⁴⁺ states are demonstrated and the origin of the effects is discussed. The roles of atomary silver and antimony in the Sb³⁺ state as strong reducers with respect to cerium in the Ce⁴⁺ state are analyzed. Estimates for the molar fractions of Ce³⁺ and Ce⁴⁺ states in the glasses with various dopant combinations are discussed. A possibility to reconstruct the absorption spectrum of commercial photo-thermo-refractive glass using data obtained on the parameter magnitudes for the oxidation states of dopants is shown.     

Crystallization and Phase Transformation Process in Zirconia: An in situ High-Temperature X-ray Diffraction Study

Amorphous zirconia precursors were made by the precipitation of a zirconium tetrachloride solution with either slow (8 h) or rapid additions of ammonium hydroxide at a pH of 10.5. Following calcination at 500°C for 4 h, the rapidly precipitated precursor exhibited predominantly monoclinic ZrO₂ phase, while the slowly precipitated precursor produced the tetragonal ZrO₂ phase. The crystallization and phase transformations were followed by in situ high-temperature X-ray diffraction (HTXRD) for both specimens in helium and in air. Each amorphous precursor first crystallizes as the tetragonal phase at about 450°C. A tetragonal-to-monoclinic phase transformation of the rapidly precipitated material was observed on cooling at about 275°C. Surface impregnation of sulfate ions following precipitation inhibited the tetragonal-to-monoclinic transformation for the rapidly precipitated ZrO₂ sample. The crystallite size for the t -ZrO₂ of all samples, irrespective of whether they transform to monoclinic, was approximately 11 nm, indicating that the t → m transformation in these materials is not controlled by differences in crystallite size. It is therefore suggested that anionic vacancies control the tetragonal-to-monoclinic phase transformation on cooling, and that oxygen adsorption triggers this phase transformation.     

Mass Transport in the Presence of Internal Defect Reactions—Concept of Conservative Ensembles: III, Trapping Effect of Dopants on Chemical Diffusion

Trapping effects of dopant ions on the chemical diffusion due to valence changes of the impurity ions (association with electronic charge carriers) and/or due to association with ionic defects are considered. It is shown that the chemical or ambipolar diffusion coefficient may be substantially changed and may even be lower than both the diffusivity of the ionic and the electronic defects under consideration. Data reinterpretation is exemplified for SrTiO₃ and ZrO₂ (Y₂O₃).     

High-Temperature Optical Absorption Measurement of Single-Crystal, Yttria-Stabilized Zirconia

High-temperature optical absorption measurements were performed on single-crystal yttria-stabilized zirconia. The optical band gap (E_g) can be represented in the temperature range 15° to 1010°C as E_g(T)=3.40-5.16x10⁻⁴T(K)(eV).     

Some Observations on the Role of Dopants in Phase Transitions in Zirconia from Atomistic Simulations

Attempts to calculate the free energy of the polymorphs of zirconia are reported. The results suggest a theoretical basis for the phase transitions in terms of the removal of lattice instabilities (evidenced by imaginary phonon frequencies). Dopant stabilization of the higher symmetry structures is seen to arise because of changes to the phonon frequencies due to the different mass and effective charge of the dopant.     

Hydrogen Dissolution in and Release from Nonmetals: III, Tetragonal Zirconia

The solubility and release kinetics of hydrogen in single-crystal zirconia were investigated by the infusion–extraction method. Equilibration in hydrogen at pressures between 1.4 and 11 atm (0.14 and 1.1 MPa) and at temperatures from 1300° to 1600°C produced H/Zr ratios varying from 4 × 10⁻⁵ to 1.3 × 10⁻⁴. Dissolution was exothermic with an enthalpy of solution of 6.6 kcal/mol (28 kJ/mol). The solubility increased with the extent of substoichiometry of the oxide caused by exposure to hydrogen. The solubility behavior and the kinetics of release indicated that hydrogen in the solid forms defect clusters consisting of a hydrogen atom, two trivalent cations, and a doubly charged oxygen vacancy. The trivalent ions were either impurities or reduced zirconium ions. Rate constants for first-order detrapping were determined. Diffusion was shown not to be a rate-limiting process in hydrogen release.     

Effect of Rare-Earth Dopants on Mechanical Properties of Alumina

We report here about the effect of rare-earth dopants on the improvement of room-temperature mechanical properties of alumina. Rare-earth ions (RE = Yb³⁺, Er³⁺, and La³⁺) of different ionic radii in a minimum concentration of 1000 ppm were added as dopants individually to high-purity alumina and densified by pressureless sintering. High strength of about 700 MPa was attained for Yb-doped alumina sintered at 1400°C. And, high toughness of about 7.0 MPa·m^1/2 was attained for Er- and La-doped alumina samples.     

Fundamental Optical Absorption Edge of Sputter-Deposited Zirconia and Yttria

Zirconia and yttria films were sputter deposited onto unheated fused silica substrates using a metal target and rare gas-oxygen discharges. Double-beam spectrophotometry was used to measure the transmission and reflection as a function of incident photon energy, E , from which the absorption coefficient, α( E ), was calculated. An indirect interband transition at E _i= 4.70 eV and two direct interband transitions at E _g1= 5.17 eV and E _g2= 5.93 eV occur in monoclinic zirconia. Two direct interband transitions at E _g1= 5.07 eV and E _g2= 5.73 eV occur in cubic yttria. The absorption edge structure is modified when unusual phases, such as tetragonal zirconia, and zirconia and yttria with no longrange crystallographic order, are present.     

壬基酚聚氧乙烯醚硫酸掺杂聚苯胺的电导率研究

以壬基酚聚氧乙烯醚(10)硫酸(NPES)为聚苯胺掺杂剂,研究了掺杂剂及其掺杂反应时间、温度等条件对聚苯胺导电性的影响.结果表明:长链NPES的掺杂能够提高聚苯胺导电性,且随着掺杂反应时间的增加,电导率在20h后即达到最大值0.6S/cm;随着掺杂温度的降低,在冰浴条件下,NPES掺杂聚苯胺的电导率较常温时提高两个数量...     

纳米氧化锆添加量对氧化锆制品性能的影响

在电熔单斜氧化锆原料中添加不同数量的CaO稳定剂,制备部分稳定氧化锆,研究CaO加入量和稳定相数量的关系.在制备的CaO部分稳定氧化锆中添加纳米氧化锆粉体,经过造粒、200 MPa压力成型、干燥、1650 ℃×2 h烧成制得试样.测试试样的物理性能、分析矿物相组成、观察显微结构,研究纳米氧化锆粉体添加量对试样性能的影响.研究结果表明:2Ca-PSZ、3Ca-PSZ、4Ca-PSZ试样中,4Ca-PSZ试样稳定化程度最高;3Ca-PSZ试样显气孔率小,体积密度较大,耐压强度高.在3Ca-PSZ试样中加入纳米氧化锆粉体,随着加入量的增加,试样的显气孔率下降、烧成收缩率增加、耐压强度提高.其中纳米氧化锆粉体添加比例为8wt%时,试样气孔率为9.4%,体积密度为5.08 g/cm3,抗压强度达到381 MPa.与3Ca-PSZ试样相比,气孔率下降40%,体积密度提高5%,耐压强度提高70%.     

氧化锆纤维布强度的研究

氧化锫纤维布的强度受到多种因素的影响,当氧化锆纤维布的晶相为四方晶相时,其力学性能最佳;优化布的织物结构可以大幅度提高其强度;PSU和S-PSU对氧化锆纤维布起到增强的作用,PSU也导致其吸碱率和吸碱速率下降,而采用S-PSU代替PSU则对吸碱率和吸碱速率下降会有所改善;通过细化氧化锆晶粒尺寸、提高晶体的完整性、保留气孔结构等方法可以获得高强度的氧化锆纤维布.