Optical Basicity of Titanium(IV) Oxide

The relationship between phosphate and vanadate capacites and optical basicity, Λ, are used in order to settle the optical basicity of TiO₂. Phosphate and vanadate capacity data indicate that the optical basicity of TiO₂ is 0.55. The validity of this value is supported by the good agreement with the value derived from Pauling electronegativity.     

Apatite Formation on Calcium Phosphate Invert Glasses in Simulated Body Fluid

Calcium phosphate invert glasses, which contain P₂O₇²⁻ and PO₄³⁻ ions, have been prepared via the addition of a small amount of TiO₂. The formation of bonelike calcium phosphate apatite on the surface of the phosphate invert glasses was examined in simulated body fluid (SBF) at a temperature of 37°C. Soaking for 20 d resulted in the deposition of leaflike apatite particles on 6CaO·3P₂O₅·TiO₂ invert glass (based on molar ratio). The glass had much-greater chemical durability against SBF, in comparison with a metaphosphate glass; P ions were not dissolved excessively from the 6CaO·3P₂O₅·TiO₂ glass, so the apatite formation was not suppressed.     

Microwave Dielectric Loss of Titanium Oxide

The dielectric loss (tan δ) of titanium dioxide (TiO₂) disks has been measured at a frequency of 3 GHz. High-purity TiO₂ sintered to almost-full density exhibits a very high tan δ, which is interpreted to be due to oxygen deficiency. To counter this, doping with stable divalent and trivalent cations, such as Mg and Al, leads to a low tan δ, probably by preventing Ti⁴⁺ reduction. The tan δ of polycrystalline TiO₂ doped with divalent and trivalent ions with ionic radii in the range of 0.5–0.95 Å at 3 GHz can be very low: 6 × 10⁻⁵ ( Q ∼ 17 000) at a temperature of 300 K. The tan δ of undoped pure TiO₂ disks increases when the disks are cooled from 300 K to ∼100 K. At temperatures <100 K, the tan δ decreases rapidly, which is interpreted as carrier freeze-out. The tan δ for all the high- Q doped TiO₂ polycrystalline samples smoothly decrease to ∼5 × 10⁻⁶ ( Q ∼ 200 000) at 15 K, comparable to that of single crystals.     

Structure and Dielectric Properties of Pb(Sc2/3W1/3)O3–Pb(Zr/Ti)O3 Relaxors

The structure and dielectric properties of (1− x )Pb(Sc_2/3W_1/3)O₃–( x )Pb(Zr/Ti)O₃ ceramics have been investigated over a full substitution range. All compositions with x < 0.5 adopt a cubic perovskite structure; however, for x ≤ 0.25 a doubled cell results from a 1:1 ordered distribution of the B-site cations. The structural order in Pb(Sc_2/3W_1/3)O₃ (PSW) can be described by a random-site model with one cation site occupied by Sc³⁺ and the other by a random distribution of (Sc_1/3³⁺W_2/3⁶⁺). The ordering is destabilized in solid solutions of PSW with PbZrO₃ (PSW–PZ), but stabilized by PbTiO₃ in the (1− x )PSW–( x )PT system. The changes in order are accompanied by alterations in the dielectric response of the two systems. For PSW–PZ the temperature of the permittivity maximum ( T _ɛ,max) increases linearly with x ; however, for PSW–PT T _ɛ,max decreases in the ordered region (up to x = 0.25) and then increases rapidly as the order is lost. Similar effects were produced by modifying the degree of order of (0.75)PSW–(0.25)PT; when the order parameter was reduced from ∼1.0 to ∼0.65, T _ɛ,max increased by more than 60°C.     

Reaction Between Titanium and Zirconia Powders During Sintering at 1500°C

Zirconia–titanium (ZrO₂–Ti) composites have been considered potential thermal barrier graded materials for applications in the aerospace industry. Powder mixtures of Ti and 3 mol% Y₂O₃ partially stabilized ZrO₂ in various ratios were sintered at 1500°C for 1 h in argon. The microstructures of the as-sintered composites were characterized by X-ray diffraction and transmission electron microscopy/energy-dispersive spectroscopy. Ti reacted with and was mutually soluble in ZrO₂, resulting in the formation of α-Ti(O, Zr), Ti₂ZrO, and/or TiO. These oxygen-containing phases extracted oxygen ions from ZrO₂, whereby oxygen-deficient ZrO₂ was generated. For relatively small Ti/ZrO₂ ratios, specimens with ≤30 mol% Ti, TiO were formed as oxygen could be sufficiently supplied by excess ZrO₂. For the specimens with ≥50 mol% Ti, lamellar Ti₂ZrO was precipitated in α-Ti(Zr, O), with no TiO being found. Both m -ZrO_{2− x} and t -ZrO_{2− x} were found in specimens with ≤50 mol% Ti; however, only c -ZrO_{2− x} was formed in the specimen with 70 mol% Ti. As ZrO₂ was gradually dissolved into Ti, yttria was retained in ZrO₂ because of the very limited solubility of yttria in α-Ti(O, Zr) or TiO. The concentration of retained yttria and the degree of oxygen deficiency in ZrO₂ increased with the Ti content. The complete dissolution of ZrO₂ into Ti was followed by the precipitation of Y₂Ti₂O₇ in the specimen with 90 mol% Ti.     

Origin of Microwave Dielectric Loss in ZnNb2O6-TiO2

(1− x )ZnNb₂O₆· x TiO₂ ceramics were prepared using both anatase and rutile forms of TiO₂. At a composition of x = 0.58, a mixture region of ixiolite (ZnTiNb₂O₈) and rutile was observed and the temperature coefficient of resonant frequency (τ_f) was ∼0 ppm/°C. We found that although ɛ_r and τ_f were comparable, the quality factor ( Q × f , Q ≈ 1/ tan δ, f = resonant frequency) of 0.42ZnNb₂O₆·0.58TiO₂ prepared from anatase and rutile was 6000 and 29 000, respectively. The origin of the difference in Q × f of both samples was investigated by measuring electrical conductivity and by analysis of the anatase–rutile phase transition. The anatase-derived sample had higher conductivity, which was related to the reduction of Ti⁴⁺. It is suggested that the increase of dielectric loss originates from an increase in Ti³⁺ and oxygen vacancies due to an anatase–rutile phase transition.     

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.     

Zirconia-Related Phases in the Zirconia/Titanium Diffusion Couple After Annealing at 1100°–1550°C

A diffusion couple of 3 mol% Y₂O₃–ZrO₂ and titanium was isothermally annealed in argon at temperatures between 1100° and 1550°C. The phases and microstructure in the ceramic side were investigated using scanning electron microscopy and transmission electron microscopy, both attached to an energy-dispersive spectrometer. After annealing at 1100°C/6 h, zirconia grains did not grow conspicuously and evolved only traces of oxygen, resulting in t -ZrO_{2− x} but not α-Zr. At temperatures above 1300°C, a significant amount of oxygen evolved from zirconia, reducing the O/Zr ratio, such that α-Zr was excluded from t -ZrO_{2− x} during cooling, yielding a higher O/Zr ratio (≈2). When held at 1550°C/6 h, zirconia grains grew rapidly. The α-Zr was segregated on grain boundaries during cooling by the exsolution of zirconium from ZrO_{2− x} , while twinned t '-ZrO_{2− x} or lenticular t -ZrO_{2− x} , which was embedded in ordered c- ZrO_{2− x} , was found. The ordered c -ZrO_{2− x} was identified by the     {113} superlattice reflections of its electron diffraction patterns.     

Immiscibility Area in the System TiO2–ZrO2–SiO2

A furnace for use in conjunction with the X-ray spectrometer was developed which was capable of heating small powdered specimens in air to temperatures as high as 1850°C. This furnace was also used for the heating and quenching of specimens in air from temperatures as high as 1850°C. An area of two liquids coexisting between 20 and 93 weight % TiO₂ above 1765°± 10°C. was found to exist in the system TiO₂–SiO₂, which is in substantial agreement with the previous work of other investigators. The area of immiscibility in the system TiO₂–SiO₂ was found to extend well into the system TiO₂–ZrO₂–SiO₂. The two liquids were found to coexist over a major portion of the TiO₂ (rutile) primary-phase area with TiO₂ (rutile) being the primary crystal beneath both liquids. The temperature of two-liquid formation in the ternary was found to fall about 80°C. with the first additions of ZrO₂ up to 3%. With larger amounts of ZrO₂ the change in the temperature of the boundary of the two-liquid area was so slight as to be within the limits of error of the temperature measurement. Primary-phase fields for TiO₂ (rutile), tetragonal ZrO₂, and ZrTiO₄ were found to exist in the system TiO₂–ZrO₂–SiO₂. SiO₂ as high cristobalite is known to exist in the system TiO₂–ZrO₂–SiO₂.     

Synthesis and Characterization of (Zn,Cd)TiO3 by a Modified Sol–Gel Method

Ilmenite-type (Zn_{1− x} Cd _x )TiO₃ (0≤ x ≤0.15 and 0.8≤ x ≤1.0) was synthesized by a modified sol–gel route including the Pechini process via two-step heat treatments. The thermal stability of (Zn_{1− x} Cd _x )TiO₃ depended on the amount of cadmium content. The as-synthesized (Zn_{1− x} Cd _x )TiO₃ (0≤ x ≤0.15 and 0.8≤ x ≤1.0) showed higher thermal stability than that of ZnTiO₃. The variation of the dielectric constant of all synthesized (Zn_{1− x} Cd _x )TiO₃ samples for all measurement frequencies showed a similar tendency. The dielectric constant of each (Zn_{1− x} Cd _x )TiO₃ sample decreased first with increasing frequencies and then increased slightly when the frequency was up to 10⁷ Hz. Moreover, the dielectric loss tangent of all synthesized (Zn_{1− x} Cd _x )TiO₃ samples for all measurement frequencies also changed in similar patterns. The dielectric loss tangent decreased with increasing measurement frequencies. The microwave dielectric properties of (Zn_{1− x} Cd _x )TiO₃ were changed with the cadmium doping content in the range of microwave frequency.     

Thermodynamic Assessment of the Lithium-Borate System

The Li₂O–B₂O₃ quasi-binary system is assessed. A two-sublattice ionic solution model, (Li¹⁺) _P (O²⁻, BO₃³⁻, B₄O₇²⁻, B₃O_4.5) _Q , is adopted to describe the liquid phase. All solid phases are treated as stoichiometric compounds. A set of parameters consistent with most of the available experimental data on phase diagram and thermodynamic properties is obtained by using the CALPHAD technique.     

Effects of Annealing Temperature on Microstructural Development at the Interface Between Zirconia and Titanium

The interfacial reaction layers in the Ti/ZrO₂ diffusion couples, isothermally annealed in argon at temperatures ranging from 1100° to 1550°C for 6 h, were characterized using scanning electron microscopy and transmission electron microscopy, both attached with an energy-dispersive spectrometer. Very limited reaction occurred between Ti and ZrO₂ at 1100°C. A β'-Ti(Zr, O) layer and a two-phase α-Ti(O)+β'-Ti(Zr, O) layer were found in the titanium side after annealing at T ≥1300°C and T ≥1400°C, respectively. A three-phase layer, consisting of Ti₂ZrO+α-Ti(O, Zr)+β'-Ti (O, Zr), was formed after annealing at 1550°C. In the zirconia side near the original interface, β'-Ti coexisted with fine spherical c- ZrO_{2− x} , which dissolved a significant amount of Y₂O₃ in solid solution at T ≥1300°C. Further into the ceramic side, the α-Zr was formed due to the exsolution of Zr out of the metastable ZrO_{2− x} after annealing at T ≥1300°C: the α-Zr was very fine and dense at 1300°C, continuously distributed along grain boundaries at 1400°C, and became coarsened at 1550°C. Zirconia grains grew significantly at T ≥1400°C, with the lenticular t -ZrO_{2− x} being precipitated in c -ZrO_{2− x} . Finally, the microstructural development and diffusion paths in the Ti/ZrO₂ diffusion couples annealed at various temperatures were also described with the aid of the Ti–Zr–O ternary phase diagram.     

Effect of Lead Oxide on Niobium-Doped Titania Varistors

The effect of PbO on Nb-doped TiO₂ varistors was investigated. It was found that the PbO dopant had significant effect on the varistor properties of Nb-doped TiO₂ ceramics. There existed an optimal range of PbO starting from 0.25 to 1 mol% for the nonlinear I–V characteristics of the 0.25 at.% Nb-doped TiO₂ ceramics. Within this PbO range, an effective boundary energy barrier of about 0.70 eV was created which yielded nonlinear I–V characteristics with α= 7.6. In contrast, α values of the samples containing PbO dopant outside this optimal range are only about 2 to 3. The effect of PbO on TiO₂ varistors was analyzed by impedance spectroscopy, C–V and dielectric measurements, and X-ray diffraction, as well as scanning electron microscopy. The results are discussed in the text.     

Thermal Expansion, Compressibility, and Polymorphism in Hafnium and Zirconium Titanates

Using X-ray diffraction techniques, thermal expansion and compressibility were measured on the orthorhombic compounds HfTiO₄, Hf_1.26Ti_0.74O₄, and ZrTiO₄ (both quenched and cooled slowly from 1300°C). The thermal expansion of HfTiO₄ is highly anisotropic; the thermal expansion coefficients along the crystallographic axes are α _a =+(8.7±0.5)×10⁻⁶°C⁻¹, α _b =−(5.2±0.5)×10⁻⁶°C⁻¹, and α _c =+ (5.3±0.5)×10⁻⁶°C⁻¹. The thermal expansion of Hf_1.26Ti_0.74O₄ was similar to that of HfTiO₄ but that of ZrTiO₄ was markedly less anisotropic. The compressibilities of HfTiO₄ and ZrTiO₄ also differed markedly. All compounds investigated, however, behaved similarly in exhibiting a polymorphic transition to a high-pressure phase having the monoclinic baddeleyite (ZrO₂) structure. The polymorphism can be explained qualitatively on the basis of crystal structure.     

Solid-State Diffusive Amorphization in TiO2/ZrO2 Bilayers

Thin films of crystalline TiO₂ were deposited on self-assembled organic monolayers from aqueous TiCl₄ solutions at 80°C; partially crystalline ZrO₂ films were deposited on top of the TiO₂ layers from Zr(SO₄)₂ solutions at 70°C. In the absence of a ZrO₂ film, the TiO₂ films had the anatase structure and underwent grain coarsening on annealing at temperatures up to 800°C; in the absence of a TiO₂ film, the ZrO₂ films crystallized to the tetragonal polymorph at 500°C. However, the TiO₂ and ZrO₂ bilayers underwent solid-state diffusive amorphization at 500°C, and ZrTiO₄ crystallization could be observed only at temperatures of 550°C or higher. This result implies that metastable amorphous ZrTiO₄ is energetically favorable compared to two-phase mixtures of crystalline TiO₂ and ZrO₂, but that crystallization of ZrTiO₄ involves a high activation barrier.     

Fabrication of Hydroxyapatite–Zirconia Composites for Orthopedic Applications

Dense hydroxyapatite–zirconia (HAp–ZrO₂) composites are expected to have desired mechanical and biological properties for orthopedic applications. However, due to some processing problems, to date, this material can only be prepared by special techniques. In this paper, we report for the first time a facile route to prepare HAp–ZrO₂ composites. Initially, HAp and ZrO₂ powders were dispersed in aqueous media with polyacrylic acid and glutamic acid as the dispersants. The slurries exhibited a well-stabilized state at a high solid content (>50 vol%) and therefore green samples with high density (>60%) can be obtained after slip casting. These HAp–ZrO₂ green samples can be easily densified by pressureless sintering at 1450°C with 2 h holding. After sintering, only hydroxyoxyapatite Ca₁₀(PO₄)₆O _x (OH)_{2(1− x )} (HOA), ZrO₂, and trace amounts of α-tricalcium phosphate phases were detected. No obvious reactions between HAp and ZrO₂ phase were observed. The HAp–ZrO₂ samples showed excellent mechanical and biological properties. For 40 vol% HAp–60 vol% ZrO₂ samples sintered at 1450°C, the flexural strength and toughness were 220 MPa and 4.37 MPa·m^1/2, respectively. In addition, we observed the attachment, spreading, and proliferation of mesenchymal stem cells on the HAp–ZrO₂ samples' surface. The results showed that the proposed colloidal processing and pressureless sintering process is feasible for preparing HAp–ZrO₂ composites with high mechanical properties and promising bioactivity for orthopedic applications.     

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.     

Processing and Mechanical Behavior of CrN/ZrO2(2Y) Composites

CrN powder consisting of granular particles of ∼3 μm has been prepared by self-propagating high-temperature synthesis under a nitrogen pressure of 12 MPa using Cr metal. Dense pure CrN ceramics and CrN/ZrO₂(2Y) composites in the CrN-rich region have been fabricated by hot isostatic pressing for 2 h at 1300°C and 196 MPa. The former ceramics have a fracture toughness ( K _IC) of 3.3 MPa ·m^1/2 and a bending strength (σ_b) of 400 MPa. In the latter materials almost all of the ZrO₂(2Y) grains (0.36–0.41 μm) are located in the grain boundaries of CrN (∼4.6 μm). The values of K _IC (6.1 MPa · m^1/2) and σ_b (1070 MPa) are obtained in the composites containing 50 vol% ZrO₂(2Y).