Electrical Properties of Cerium-Doped BaTiO3

The high-temperature equilibrium electrical conductivity of Ce-doped BaTiO₃ was studied in terms of oxygen partial pressure, P (O₂), and composition. In (Ba_1−xCe _x )TiO₃, the conductivity follows the −1/4 power dependence of P (O₂) at high oxygen activities, which supports the view that metal vacancies are created for the compensation of Ce donors, and is nearly independent of P (O₂) where electron compensation prevails at low P (O₂). When Ce is substituted for the normal Ti sites, there is no significant change in conductivity behavior compared with undoped BaTiO₃, indicating the substitution of Ce as Ce⁴⁺ for Ti⁴⁺ in Ba(Ti_1−yCe _y )O₃. The Curie temperature ( T _c) was systematically lowered when Ce³⁺ was incorporated into Ba²⁺ sites, whereas the substitution of Ce⁴⁺ for Ti⁴⁺ sites resulted in no change in this parameter.     

Reaction Sequence and Microstructrual Development of CeO2-Doped Reaction-Bonded Mullite

A promising technique for the fabrication of mullite ceramics and mullite-matrix composites with low dimensional changes ("near-net-shape processing") is reaction bonding using Si metal and α-Al₂O₃ as starting materials, because sintering-induced shrinkage is compensated by Si-oxidation-induced volume expansion. A mullite reaction bonding (RBM) route which proceeds at much lower temperatures (lessthan equal to1350°C) than in conventional RBM systems (greaterthan equal to1500°C) is based on Ce doping which provides accelerated Si oxidation and mullite formation due to the formation of transient, low-viscosity Ce-Al-Si-O liquids. The present study shows that the required Ce-Al-Si-O liquids form in a reducing environment with Ce occurring as Ce³⁺. In an oxidizing environment, Ce is present as Ce⁴⁺, giving rise to precipitation of crystalline CeO₂. Ce³⁺ left and right arrow Ce⁴⁺ redox reactions in the temperature range under consideration appear to be controlled by the presence of nonoxidized Si in the samples. According to the present investigation the amount of CeO₂ added to the starting powders must be tailored carefully: Exaggerated CeO₂ content produces large amounts of low-viscosity Ce-Al-Si-O liquids which may have the disadvantage of excessive sealing of the open porosity. This slows the oxygen diffusion velocity into the specimen considerably, with the consequence that nonoxidized Si and a residual Ce-Al-Si-O glass coexist in the ceramics after processing. A solution to this problem is to simultaneously enhance mullite crystal growth through seeding which works against excessive liquid-phase-induced shrinkage of the samples. This in turn enables complete oxidation and recrystallization of all liquid phases.     

Phase Separation in 12 mol% Ceria-Doped Zirconia Induced by Heat Treatment in H2 and Ar

The phase separation in 12 mol% CeO₂─ZrO₂ ceramic heattreated in a mixture of H₂ and Ar was investigated by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy, and Raman scattering. After heat treatment at temperatures above 1200°C, the tetragonal solid-solution phase separated into Zr₂Ce₂O₇ and the monoclinic phase. Raman scattering measurements also provided supplementary evidence for the phase separation. XPS showed that the valence change from Ce⁴⁺ to Ce³⁺ predominantly occurred, whereas the reduction from Zr⁴⁺ to Zr³⁺ took place above 1200°C. It is concluded, that in the highly reduced sample, where the valence changes from Ce⁴⁺ (Zr⁴⁺) to Ce³⁺ (Zr³⁺), the phase separation is noticeably promoted. Below 1000°C the phase separation was suppressed because of no appreciable valence change to trigger the phase separation, and the single tetragonal phase was retained.     

Synchrotron Radiation Ti-k XANES Study of TiO2─Y2O3-Stabilized Tetragonal Zirconia Polycrystals

Valence state and site symmetry of Ti ions in TiO₂–Y₂O₃–ZrO₂ powders with 2 mol% Y₂O₃ and 5, 10, 15, and 20 mol% TiO₂, respectively, are studied by X-ray absorption near-edge spectroscopy (XANES). Tetravalent Zr⁴⁺ ions are replaced predominantly by Ti⁴⁺ ions. Within the solubility region of Ti ions, a subsequent displacement of Ti ions from the center of symmetry is observed with increasing TiO₂ content in TiO₂–Y₂O₃-stabilized tetragonal ZrO₂ polycrystals (Ti-Y-TZP) under investigation. This behavior cannot be interpreted with a random substitution of Ti⁴⁺ ions on Zr⁴⁺ lattice sites. On the contrary, this correlation between the TiO₂ content in Ti-Y-TZP and the shift of Ti ions indicates an increasing interaction between the Ti ions with growing TiO₂ content, caused by a subsequent clustering of Ti ions.     

Grain Boundary Mobility in Y2O3: Defect Mechanism and Dopant Effects

The effects of the dopants, Mg²⁺, Sr²⁺, Sc³⁺, Yb³⁺, Gd³⁺, La³⁺, Ti⁴⁺, Zr⁴⁺, Ce⁴⁺, and Nb⁵⁺, on the grain boundary mobility of dense Y₂O₃ have been investigated from 1500° to 1650°C. Parabolic grain growth has been observed in all cases over a grain size from 0.31 to 12.5 μm. Together with atmospheric effects, the results suggest that interstitial transport is the rate-limiting step for diffusive processes in Y₂O₃, which is also the case in CeO₂. The effect of solute drag cannot be ascertained but the anomalous effect of undersized dopants (Ti and Nb) on diffusion enhancement, previously reported in CeO₂, is again confirmed. Indications of very large binding energies between aliovalent dopants and oxygen defects are also observed. Overall, the most effective grain growth inhibitor is Zr⁴⁺, while the most potent grain growth promoter is Sr²⁺, both at 1.0% concentration.     

DTA and X-Ray Analysis Study of Nucleation and Crystallization of MgO-Al2O3-SiO2 Glasses Containing ZrO2, TiO2, and CeO2

Crystallization sequences of glasses with compositions in the tridymite primary phase field of the MgO-Al₂O₃-SiO₂ system were studied by DTA, X-ray diffraction, and other techniques. Crystallization was catalyzed by the addition of 7 wt% of either ZrO₂ or TiO₂. Up to 10 wt% CeO₂ was also added to some glasses. Metastable solid solutions with the high-quartz structure exhibiting varying lattice parameters commonly occurred at low temperatures, transforming into a high cordierite at higher temperatures. Depending on the composition and heat treatment, other phases also appeared, e.g. Ce₂Ti₂O₄ (Si₂O₇). The rate of crystallization was markedly dependent on the catalyst. Colloidal precipitation of the catalyst accompanied by bulk crystallization of the glass was observed with ZrO₂, but no crystalline TiO₂ was detected. In the presence of CeO₂, TiO₂ was a more effective catalyst than ZrO₂. Although CeO₂ lowered the melting temperatures of the glass-ceramics, it increased the stability of the glasses and inhibited volume nucleation, causing coarse structures to form on crystallization.     

Preparation and Physical Properties of Radiofrequency-Sputtered Amorphous Films in the System AlPO4–TiO2

Amorphous films in the system AlPO₄–TiO₂ were prepared by an rf-sputtering method, and their physical properties, such as density, refractive index, and thermal expansion coefficient, and the infrared absorption spectra were measured. The thermal expansion coefficient increased linearly with increasing TiO₂ content. The results of the molar refractivity and the infrared absorption spectra indicated that the coordination number of titanium ions in these films is higher than that in SiO₂–TiO₂ glasses with a negative thermal expansion, in which Ti⁴⁺ ions are tetrahedrally coordinated. In order to confirm the coordination state of the titanium ions in these amorphous films, titanium K -band emission spectra were obtained by X-ray emission spectroscopy, revealing sixfold coordination. The higher coordination state of Ti⁴⁺ was considered to account for these amorphous films not exhibiting negative thermal expansion, as in the SiO₂–TiO₂ system.     

Synthesis and Characterization of (Ce0.67Tb0.33)MnxMg1−xAl11O19 Phosphors Derived by Sol–Gel Processing

A (Ce_0.67Tb_0.33)Mn _x Mg_{1− x} Al₁₁O₁₉ phosphor powder was synthesized, using a simple sol–gel process, by mixing citric acid with CeO₂, Tb₄O₇, Al(NO₃)₃·9H₂O, Mg(OH)₂·4MgCO₃·6H₂O, and Mn(CH₃COO)₂. The phosphor crystallized completely at 1200°C, and the phosphor particle size was between 1 and 5 μm. The excitation spectrum was characteristic of Ce³⁺, while the emission spectrum was composed of lines from Tb³⁺ and Mn²⁺. The Mn²⁺ gave a green fluorescence band, and concentration quenching occurred when x > 0.10. The luminescent properties of the phosphor were explained by a configurational coordinate model.     

Control of Grain-Boundary Pinning in Al2O3/ZrO2 Composites with Ce3+/Ce4+ Doping

The control of the microstructure of Ce-doped Al₂O₃/ZrO₂ componsites by the valence change of cerium ion has been demonstrated. Two distinctively different types of microstructure, large Al₂O₃ grains with intragranular ZrO₂ particles and small Al₂O₃ grains with intergranular ZrO₂ particles, can be obtained under identical presintering processing conditions. At doping levels greater than ∼ 3 mol% with respect to ZrO₂, Ce³⁺ raises the alumina grain-boundary to zirconia particle mobility ratio. This causes the breakaway of grain boundary from particles and the first type of microstructure. On the other hand, Ce⁴⁺ causes no breakaway and produces a normal intergranular ZrO₂ distribution. The dramatic effect of Ce³⁺ on the relative mobility ratio is found to be associated with fluxing of the glassy boundary phase and is likewise observed for other large trivalent cation dopants. The ZrO₂ second phase acts as a scavenger for these trivalent cations, provided their solubility limit in ZrO₂ is not exceeded.     

Homogeneous Precipitation of TiO2 Ultrafine Powders from Aqueous TiOCl2 Solution

Crystalline TiO₂ powders were prepared by the homogeneous precipitation method simply by heating and stirring an aqueous TiOCl₂ solution with a Ti⁴⁺ concentration of 0.5 M at room temperature to 100°C under a pressure of 1 atm. TiO₂ precipitates with pure rutile phase having spherical shapes 200-400 nm in diameter formed between room temperature and 65°C, whereas TiO₂ precipitates with anatase phase started to form at temperatures >65°C. Precipitates with pure anatase phase having irregular shapes 2-5 µm in size formed at 100°C. Possibly because of the crystallization of an unstable intermediate product, TiO(OH)₂, to TiO₂ x H₂O during precipitation, crystalline and ultrafine TiO₂ precipitates were formed in aqueous TiOCl₂ solution without hydrolyzing directly to Ti(OH)₄. Also, formation of a stable TiO₂ rutile phase between room temperature and 65°C was likely to occur slowly under these conditions, although TiO₂ with rutile phase formed thermodynamically at higher temperatures.     

Enhanced Visible Light-Driven Photocatalytic Performance of La-Doped TiO2−xFx

La-doped TiO_{2− x} F _x (La–TiO_{2− x} F _x ) powders were prepared by the sol–gel method. X-ray diffraction results showed that La efficiently inhibited grain growth. X-ray photoelectron spectroscopy spectra revealed that La₂O₃ and O–Ti–F bonds have formed, the La₂O₃ maintained the high surface area of TiO_{2− x} F _x after calcination at a temperature above 500°C, while the O–Ti–F bonds increased the oxidation potential of the photogenerated hole in the valence band. The UV-vis spectroscopy of the La–TiO_{2− x} F _x showed that the presence of intraband gap states was likely responsible for its absorption of visible light. When the molar ratios of La and F to Ti were 1.5:100 and 5:100, respectively, and calcined at 500°C, the photocatalytic degradation rate of methylene blue over La–TiO_{2− x} F _x was about 1.5 times higher than that of F-doping TiO₂.     

Solubility of TiO2 in BaTiO3

Scanning electron microscopy and electron probe micro-analysis were used to investigate the microstructure of both slow-cooled and quenched polycrystalline BaTiO₃ specimens with a small excess of TiO₂ (Ba/Ti=0.995 to 0.999) or of BaO (Ba/Ti=1.002 and 1.005). The electron micrographs of polished and etched TiO₂-excess BaTiOs samples, and of fracture surfaces of quenched samples, showed a second phase in the grain boundaries and triple-point regions, whereas no second phase was observed in samples having Ba/Ti=1.000. Microprobe analysis of the second phase gave compositions near that of the reported adjacent phase of higher TiO₂ content, Ba₆Ti₁₇O₄₀. The results indicate that the solubility of TiO₂ in BaTiO₃ is <0.1 mol%.     

Effect of Nd2O3 Concentration on the Defect Structure of CeO2–Nd2O3 Solid Solution

An extensive X-ray study of CeO₂–Nd₂O₃ solid solutions was performed, and the densities of solid solutions containing various concentrations of NdO_1.5 were measured using several techniques. Solid solutions containing 0–80 mol% NdO_1.5 were synthesized by coprecipitation from Ce(NO₃)₃ and Nd(NO₃)₃ aqueous solutions, and the coprecipitated samples were sintered at 1400°C. A fluorite structure was observed for CeO₂–NdO_1.5 solid solutions with 0–40 mol% NdO_1.5, which changed to a rare earth C-type structure at 45–75 mol% NdO_1.5. The change in the lattice parameters of CeO₂–NdO_1.5 solid solutions, when plotted with respect to the NdO_1.5 concentration, showed that the lattice parameters followed Vegard's law in both the fluorite and rare earth C-type regions. The maximum solubility limit for NdO_1.5 in CeO₂ solid solution was approximately 75 mol%. The relationship between the density and the Nd concentration indicated that the defect structure followed the anion vacancy model over the entire range (0–70 mol% NdO_1.5) of solid solution.     

Preparation of TiO2 Nanoparticles on Different SiO2 Supports by the Adsorption Phase Technique

The influence of supports on the preparation of TiO₂ nanoparticles by the adsorption phase technique is studied in detailed. Series temperature experiments of two types of supports (named as SiO₂ A and B) were used. Energy-dispersive analysis by X-ray indicates that the concentration of TiO₂ on both supports decreases with temperature increasing. TiO₂ quantity on SiO₂ A decreases sharply between 40° and 60°C, whereas the temperature range for SiO₂ B is between 30° and 50°C. X-ray diffraction (XRD) shows that grain size of TiO₂ particles on two SiO₂ surfaces is all below 7 nm. It is also shown by XRD that particles on SiO₂ A decrease sharply as in the quantity curve of TiO₂, but particles on SiO₂ B all change gradually and TiO₂ particles on SiO₂ B are more uniform in transmission electron spectroscopy. The similarly of both supports is considered to be the reason for the similar changes in Ti concentration, and the different characteristics of the internal/external surface lead to variant quantity and grain size, as well as characteristics of TiO₂.     

Influence of Crystal Structure on the Fatigue Properties of Pb1−xLax(Zry', Tiz)O3 Thin Films Prepared by Pulsed-Laser Deposition Technique

Lead lanthanum zirconate titanate (Pb_{1− x} La _x (Zr _y ,Ti _z )O₃, PLZT) films containing [00 l ] preferentially oriented grains were produced successfully on YBa₂Cu₃O_{7− x} -coated (YBCOcoated) SrTiO₃ (STO) or YBCO/CeO₂-coated silicon substrates; films containing randomly oriented grains were created on platinum-coated silicon substrates. The latter possessed significantly inferior ferroelectric properties, a fact ascribed to the presence of a paraelectric phase (TiO₂) at the PLZT/platinum interface. On the other hand, the PLZT/YBCO/STO films exhibited better electrical properties than did the PLZT/YBCO/CeO₂/Si films, and this phenomenon was attributed to better alignment of the grains in normal and in-plane orientations. In terms of fatigue properties, the [00l] textured films that were deposited on YBCO/CeO₂/Si substrates possessed substantially superior polarization-switching-cycle endurance versus the randomly oriented films grown on Pt(Ti)/Si substrates. Moreover, the tetragonal films behaved much more satisfactorily than did the rhombohedral PLZT films. The ferroelectric parameters of tetragonal PLZT films showed no significant degradation up to 10⁹ polarization switching cycles, whereas the remnant polarization and coercive force of the rhombohedral PLZT films had already degraded to 80% of their initial values after 10⁸ cycles.     

Preparation of Composite Particles by Pulsed Nd-YAG Laser Decomposition of [(CH3)2N]4Ti to TiN-Coat TiO2, Al2O3, or Si3N4 Powders

Irradiation of Ti[N(CH₃)₂]₄ by the 1.064-μm line of a pulsed Nd: YAG laser in the presence of TiO₂, Al₂O₃, or Si₃N₄ particles has been found to form amorphous deposits on the oxide particles. The resulting materials can be processed into TiN/TiO₂, TiN/Al₂O₃, or TiN/Si₃N₄ composites with the TiN component on the surface of the particles. The powders have been characterized by Raman spectroscopy and X-ray powder diffraction studies. The surface analysis of the composites by X-ray photoelectron spectroscopy and high-resolution electron microscopy is presented.     

Preliminary Observations on Phase Relations in the "V2O3–FeO" and V2O3–TiO2 Systems from 1400°C to 1600°C in Reducing Atmospheres

Phase relations within the "V₂O₃–FeO" and V₂O₃–TiO₂ oxide systems were determined using the quench technique. Experimental conditions were as follows: partial oxygen pressures of 3.02 × 10⁻¹⁰, 2.99 × 10⁻⁹, and 2.31 × 10⁻⁸ atm at 1400°, 1500°, and 1600°C, respectively. Analysis techniques that were used to determine the phase relations within the reacted samples included X-ray diffractometry, electron probe microanalysis (energy-dispersive spectroscopy and wavelength-dispersive spectroscopy), and optical microscopy. The solid-solution phases M₂O₃, M₃O₅, and higher Magneli phases (M _n O_{2 n −1}, where M = V, Ti) were identified in the V₂O₃–TiO₂ system. In the "V₂O₃–FeO" system, the solid-solution phases M₂O₃ and M₃O₄ (where M = V, Ti), as well as liquid, were identified.     

Internal Oxidation of Ce3+-BaTiO3 Solid Solutions

The reoxidation process in highly Ce³⁺-doped BaTiO₃ ceramics was studied using TEM. Samples of two different types of solid solutions, Ba_1−XCe³⁺ _X Ti_1−X/4( V _Ti) X/4 O₃ and Ba_1−XCe³⁺ _X Ti⁴⁺_{1− X} Ti³⁺ _X O₃, were prepared by sintering oxide mixtures in air and in a reducing atmosphere, respectively. The solid solutions were reoxidized by annealing in air at high temperatures (1000°—1100°C). As a result of internal oxidation of Ce³⁺ and Ti³⁺, fluorite CeO₂ and monoclinic Ba₆Ti₁₇O₄₀ phases were precipitated in the perovskite matrix. In Ba_1−XCe³⁺ _X Ti_1−X/4( V _Ti)X/4O3 solid solution precipitates nucleate heterogeneously at grain boundaries and at extended defects inside the grains, whereas in Ba_1−XCe³⁺_XTi⁴⁺_1−XTi³⁺_XO₃ solid solution precipitates are nucleated mainly homogeneously inside reoxidized perovskite grains. The form of the precipitates and their orientational relationship with the matrix, as well as the mechanism of internal oxidation, are discussed.     

High-Brightness LaPO4:Ce3+, Tb3+ Nanophosphors: Reductive Hydrothermal Synthesis and Photoluminescent Properties

This paper definitely reveals that LaPO₄:Ce³⁺, Tb³⁺ (LAP) nanophosphors prepared by normal hydrothermal method suffer significant loss of luminescence due to the oxidation of Ce³⁺–Ce⁴⁺ at hydrothermal stage. To effectively protect Ce³⁺ from oxidation, a reductive hydrothermal process using hydrazine hydrate as a protecting agent is proposed to synthesize LAP nanophosphors with different Ce³⁺ and Tb³⁺ concentrations, which exhibited much stronger green photoluminescence (PL) and longer lifetime than the products prepared by normal hydrothermal method. Furthermore, the high-brightness LAP nanophosphors exhibited high-quenching concentration of Tb³⁺; the La_0.4Ce_0.4Tb_0.2PO₄ nanophosphor showed almost the same PL intensity as that of the commercially used La_0.7Ce_0.2Tb_0.1PO₄ bulk powder.     

The System HfO2-TiO2

The system HfO₂-TiO₂ was studied in the 0 to 50 mol% TiO₂ region using X-ray diffraction and thermal analysis. The monoclinic ( M ) ⇌ tetragonal ( T ) phase transition of HfO₂ was found at 1750°± 20°C. The definite compound HfTiO₄ melts incongruently at 1980°± 10°C, 53 mol% TiO₂. A metatectic at 2300°± 20°C, 35 mol% TiO₂ was observed. The eutectoid decomposition of HfO_2,ss) ( T ) → HfO_2,ss ( M ) + HfTiO_34,ssss occurred at 1570°± 20°C and 22.5 mol% TiO₂. The maximum solubility of TiO₂ in HfO_2,ss,( M ) is 10 mol% at 1570°± 20°C and in HfO_2,ss ( T ) is 30 mol% at 1980°± 10°C. On the HfO₂-rich side and in the 10 to 30 mol% TiO₂ range a second monoclinic phase M of HfO₂( M ) type was observed for samples cooled after a melting or an annealing above 1600°C. The phase relations of the complete phase diagram are given, using the data of Schevchenko et al. for the 50% to 100% TiO₂ region, which are based on thermal analysis techniques.