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.     

Sintering Behavior of Cadmium-Doped Pb(Ni1/3Nb2/3)O3–PbZrO3–PbTiO3 Ceramics

Substituting Cd²⁺ ions into the A and B sites in a Pb(Ni_1/3Nb_2/3)O₃PbZrO₃PbTiO₃ (PNNPZPT) ternary perovskite material made it possible to determine the effects of the Cd²⁺ ion substitution site on sintering behavior. The substitution site of the Cd²⁺ ion was identified by using X-ray photoelectron spectroscopy spectra. Although Cd²⁺ ions were substituted into the A and B sites in PNNPZPT, the Cd²⁺ ions preferred the A site over the B site. When Cd²⁺ ions replaced Pb²⁺ ions, a weight gain was observed during sintering. On the other hand, replacing Ni2 ions with Cd²⁺ ions promoted weight loss. Those weight changes indicated that Cd²⁺ ions change the bonding strength between the B-site cations and the oxygen of the octahedron in a perovskite structure. Density was influenced by the Cd²⁺ ion substitution site, and the A-site-doped compositions had higher densities than the B-site-doped compositions.     

Defect Structure of PLZT Doped with Mn, Fe, and Al

Practically pore-free Mn-, Fe-, and Al-doped PLZT ceramics were prepared using isostatic hot-pressing. The incorporation of the dopants in the perovskite lattice of PLZT (Pb_0.9La_0.1 |Zr_0.5Ti_o.5O₃₊δ) ceramics was analyzed from measurements of the density, lattice constants, and weight loss during sintering. It was deduced that Mn, Fe, and Al are incorporated as trivalent ions at (Zr, Ti)⁴⁺ sites and that charge compensation is effected by elimination of cation vacancies present in the undoped PLZT.     

Effect of Preparation Temperature on the Lattice Parameter of Nickel Aluminate Spinel

Nickel aluminate spinels were prepared by solid-state reaction. Their lattice parameters ( a ₀) changed with preparation temperature, which was explained by cation distribution, vacancies formation, and cation entrance. When the preparation temperature increased from 1100° to 1200°C, a ₀ decreased with temperature as the result of the uptake of Al₂O₃. After 1200°C, a ₀ increased to temperature, which was attributed to some Ni²⁺ exchange with tetrahedral sites and some vacancies occupancy by Ni²⁺ and Al³⁺.     

Mechanism of Solid-state Reaction Between Magnesium Oxide and Aluminum Oxide and Between Magnesium Oxide and Ferric Oxide

Inert marker experiments have shown that the solid-state reactions forming MgAl₂O₄ and MgFe₂O₄ occur by counterdiffusion of the Mg²⁺, Fe³⁺, and A1³⁺ ions through the relatively rigid oxygen lattice of the spinel or ferrite. It is suggested that earlier claims that ZnAl₂O₄ forms by diffusion of Zn²⁺ and O^2- ions were probably due to a splitting of the markers, leaving some marker on the specimen surface. Pores in one component of a reacting couple are shown to serve as inert markers. The formation of those materials, e.g., ferrites and stannates, which dissolve marker materials can now be studied.     

Methane Oxidation on Perovskite-Type Ca(Mn1−xTix)O3−δ

X-ray photoelectron spectroscopy was performed to elucidate the catalytic activity of CH₄ oxidation on perovskite-type Ca(Mn_{1− x} Ti _x )O_3−δ synthesized at 1173 K in a flow of oxygen from a gel with citric acid and ethylene glycol. The Mn ion content decreases and the ratio of the Mn³⁺ ion in the Mn ion increases with increases in x . Ca(Mn_{1− x} Ti _x )O_3−δ has a high catalytic activity of CH₄ oxidation at x =0.4. These results indicate that the catalytic activity strongly depends on the Mn³⁺ ion content of the surface.     

Electrical Properties and Phase Stability of a Zinc Ferrite

The electrical conductivity and theromoelectric power of a zinc ferrite of nominal composition 0.46ZnO·0.54Fe₂O₃ were examined as a function of oxygen partial pressure under equilibrium conditions at elevated temperatures. The isothermal variations of both transport properties were found to be closely correlated with P _O2-induced phase transitions from wustite to ferrite spinel to hematite with successively increasing P _O2's. The region of thermodynamic stability of each phase was determined from either of the two property isotherms. Within the single-phase field of the spinel ferrite, both properties are P _O2 dependent. Applying the small polaron hopping model to these results, the P _O2 and temperature dependencies of the octahedral ferrous and ferric ion concentrations were extracted and interpreted in terms of the buffering capability of the nominal A-site cation and the variability of the partial molar enthalpy of oxygen with the progress of the oxidation reaction of Fe²⁺ to Fe³⁺.     

Effect of MnO2 Addition on the Structure and Electrical Properties of Pb(Zn1/3Nb2/3)0.20(Zr0.50Ti0.50)0.80O3 Ceramics

Pb(Zn_1/3Nb_2/3)_0.20(Zr_0.50Ti_0.50)_0.80O₃ ceramics of pure perovskite structure were prepared by the two-stage method with the addition of 0–3.0 wt% MnO₂ and their piezoelectric properties were investigated systematically. The MnO₂ addition influences in a pronounced way both the crystal structure and the microstructure of the materials. The materials are transformed from the tetragonal to the rhombohedral structure, and the grain size is enhanced when manganese cations are added. The distortion of crystal structure for samples with MnO₂ addition can be explained by the Jahn–Teller effect. The values of electromechanical coupling factor ( k _p) and dielectric loss (tan δ) are optimized for 0.5-wt%-MnO₂-doped samples ( k _p= 0.60, tan δ= 0.2%) and the mechanical quality factor ( Q _m) is maximized for 1.0-wt%-MnO₂-doped samples ( Q _m= 1041), which suggests that oxygen vacancies formed by substituting Mn³⁺ and Mn²⁺ ions for B-site ions (e.g., Ti⁴⁺ and Zr⁴⁺ ions) in the perovskite structure partially inhibited polarization reversal in the ferroelectrics. The ceramics with 0.50–1.0 wt% MnO₂ addition show great promise as practical materials for piezoelectric applications.     

A Study of Conductivity in the Sr(Fe1−xTix)O3−δ System

The conductivity and conductivity-temperature characteristics of the Sr(Fe_1−xTi_xO_3−δ system fired in air are studied by means of Mössbauer spectroscopy at room temperature and at 78 K. On the basis of quantitative analysis of the Fe⁴⁺ content in the solid solution, the concentrations of oxygen vacancies are calculated and the relationship between Fe⁴⁺ content and conductivity is found. It is also found that the turning points of conductivity and material constant B, as well as Fe⁴⁺ and the percentage of oxygen vacancies, occur at x=0.7 on the curve. When x=0.7, Fe³⁺ (II), which has never been reported before, disappears and the lattice parameters no longer vary with x. Besides Fe⁴⁺, Fe³⁺(II) is also a factor affecting the conductivity of this system.     

Exsolution of β-Ga2O3 Crystalline Solutions in the System MgAl2O3-Ga2O3

The subsolidus phase equilibrium diagram for the pseudobinary join MgAl₂O₄-Ga₂O₃ was determined. The shape of the exsolution boundary was obtained by heat-treating samples pre- equilibrated at 1600°C. Crystalline solubility of Ga₂O₃ in MgAl₂O₄ decreased from 73 mole % at 1600°C to 55 mole % at 1200°C. The crystalline solution was formed by the replacement of Mg²⁺ions by Ga³⁺ ions to produce a cation defect spinel. The phase precipitated was the mono-clinic δ-Ga₂O₃ (=δ-Al₂O₃ structure). Changes in the ratios of relative X-ray diffraction intensities indicated that the crystalline solutions also disorder with temperature.     

Thermochemistry of Charge-Coupled Substitutions in Silicate Glasses: The Systems Ml/nn+ AlO2-SiO2 (M = Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Pb)

Enthalpies of solution of framework aluminosilicate glasses x M_{l/ n} ^{n +} AlO₂-(1- x )SiO₂ (M = Cs, Rb, K, Na, Li, Ba, Pb, Sr, Ca, Mg) in molten 2PbO.B₂O₃ near 973 K were measured. The sequence above represents increasing ability of the nonframework cation M to bond to the bridging oxygen. In that sequence, the enthalpy of the substitution Si⁴⁺→+ Al³⁺+ 1/ n M ^{n +} in the glass becomes less exothermic, the enthalpy of vitrification increases, and the tendency toward glass-glass immiscibility becomes more pronounced. These parameters correlate smoothly with the ionic potential, z/r , of the substituting cation.     

Chemical Instability of Metal-Deficient Hexagonal Ferrites with W Structure

The hexagonal ferrite W=BaFe²⁺₂Fe³⁺₁₆O₂₇, which is in equilibrium with oxygen under 0.015 atm at 1350^°C, is oxidized in the range 250^° to 1100^°C in the same atmosphere to BaFe³⁺_16+2x Fe²_2-3x> _x O₂₇ with 2–3 x <0.03. The metal-deficient W structure is chemically unstable and decomposes after longer annealing at the same temperature. This difference in oxidation and decomposition rate is attributed to a difference in the diffusion rate of barium and iron ions.     

Alkali Metal Ions-Assisted Controllable Synthesis of Bismuth Ferrites by a Hydrothermal Method

Various bismuth ferrite compounds have been controllably synthesized by a hydrothermal method assisted by alkali metal ions (K⁺, Na⁺, and Li⁺). The as-prepared powders were characterized by X-ray powder diffraction, energy-dispersive spectroscopy, and transmission electron microscope. It was suggested that different alkali metal ions can result in the formation of rhombohedral BiFeO₃, orthorhombic Bi₂Fe₄O₉, and cubic Bi₁₂(Bi_0.5Fe_0.5)O_19.5, respectively.     

Ethene/Ethane (C2H4/C2H6) Separation through an Inorganic–Organic Hybrid Membrane Containing Silver(I) Ions as Olefin Carriers, Using Poly(N-vinylpyrrolidone) as a Mediation Agent

Novel inorganic–organic membranes that contained Ag⁺ ions as olefin carriers were prepared using sol–gel and dip-coating processes. The permeance of the membranes for nitrogen, helium, ethane (C₂H₆), and ethene (C₂H₄) were evaluated using the single-gas permeation method at temperatures of 298, 373, and 423 K. The results showed that the selectivity of the membranes to C₂H₄ against C₂H₆ increased as the measurement temperature increased, because the decomplexation rate of C₂H₄ molecules from Ag⁺ sites is enhanced by increases in the temperature. Fourier transform infrared spectrophotometry of the hybrid membranes and the performance of the membranes at 373 and 423 K indicated that poly( N -vinylpyrrolidone) (PVP) had a role in increasing the flexibility of the inorganic network and also served as a mediation agent to fix Ag⁺ ions in the polymer segments, because of the coordination interaction between the Ag⁺ ions and the PVP.     

Defect Structure and Electrical Conductivity of ThO2-Y2O3 Solid Solutions

Compositions in the system ThO₂-YO_1.5 were coprecipitated as oxalates and converted to oxides. Disks were pressed and sintered in oxygen at 1400° to 2200°C. Densities of the sintered disks were 96 to 98% of theoretical. Solid solutions with the fluorite-type structure were formed up to 20 to 25 mole % YO_1.5 at 1400°C and up to 45 to 50 mole % YO_1.5 at 2200°C. Density data showed that these solid solutions correspond to Th_{1— x} Y _x O_{2—0.5 x} , having a complete cation sub-lattice filled by Th⁴⁺ and Y³⁺ ions, and vacancies in the anion sublattice. The observed increase in electrical conductivity with increase in YO_1.5content is consistent with charge transport by oxygen ions through a vacancy mechanism. Approximately 7 mole % ThO₂ is soluble in YO_1.5 at 2200°C. Density results indicate an anion interstitial structure for the Y₂O₃ phase. Transference number measurements indicate that the electrical conductivities are only partly due to ions.     

Copper ⇌ Alkali Ion Exchange of Alkali Aluminosilicate Glasses in Copper-Containing Molten Salt: II, Divalent Copper Salts, CuCl2 and CuSO4

The ion-exchange mechanism between copper and alkali ions, when 20R₂O · 10Al₂O₃· 70SiO₂ (R = Li, Na, and K) glasses are immersed in divalent copper-containing molten salts in air and nitrogen at 550°C, has been investigated. In molten CuCl₂, the ion-exchange behavior in both air and nitrogen was very close to that in molten CuCl in air reported previously. This is explained by assuming that CuCl₂ decomposes into CuCl and Cl₂ at 550°C and the Cu⁺ ions thus formed mainly diffuse in glasses to replace alkali ions, where Cl₂ acts as an oxidizing agent just like oxygen. In the case of molten CuSO₄─₂SO₄, a small amount of Cu⁺ which is present in the molten state plays a primary role in the Cu ⇌ R⁺ ion exchange process, although the contribution of direct Cu²⁺⇌ 2R⁺ ion exchange cannot be ignored.     

Chemical Control of the Grain-Boundary Migration of SrTiO3 in the SrTiO3─BaTiO3─CaTiO3 System

The grain-boundary migration induced by chemical instability has been studied in SrTiO₃. Sintered SrTiO₃ has been packed with various BaTiO₃/CaTiO₃ powder mixtures and annealed at 1400°C for various times. At the surface region of most SrTiO₃ specimens, the grain boundary has migrated with the diffusion of Ba and Ca ions. The ratio of Ba and Ca ions present in the migrated region varies with the depth from the surface and the packing-powder composition, and is not equal to the cation ratio of packing powder. When the Ba/Ca ratio of packing powder is 1, cessation of migration has been observed. The composition of the area without migration is estimated to be approximately equal to the composition for the matching of crystal lattices between the original grain (SrTiO₃) and the layer formed by diffusion of Ba and Ca ions. The coherency strain energy induced by diffusion of solute atoms is believed to be the major driving force for the grain-boundary migration of SrTO₃ under the chemical instability. The grain-boundary migration can be accordingly controlled by lattice parameter change and matching.     

X-Ray Study of Phase Transitions in Ferroelectric PbNb2O6 and Related Materials

The phase transitions in PbNb₂O₆ and in compositions of the type Pb_1+x (B_xNb_1-x)O₆, where B = Ti⁴⁺, Zr⁴⁺, or Sn⁴⁺, have been investigated between 25° and 650°C. using X-ray and dilatometric techniques. The modified PbNb₂o₆ compositions possess orthorhombic PbNb₂O₆-type structure, with the additional Pb²⁺ ions occupying vacant lattice sites. The lattice parameters a and c expand and b contracts during heating until, at the ferroelectric Curie temperature, a and b suddenly coincide and c expands slightly. Besides this phase change at the Curie temperature, the nonstoichiometric compounds show an additional phase transition in the range 450° to 300°C. depending on composition. The intermediate phase of Pb_l+x(Ti₂Nb_1-z)₂O₆ appears to possess orthorhombic symmetry.     

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.     

Effect of Reoxidation on the Grain-Boundary Acceptor-State Density of Reduced BaTiO3 Ceramics

To investigate the effect of reoxidation on the grain-boundary acceptor-state density of reduced barium titanate, n -doped BaTiO₃ ceramics are sintered in a reducing atmosphere (2% H₂+ 98% N₂) and then annealed in oxygen. After annealing at 1150°C for different times, the experimental results show a relationship between temperature-averaged acceptor-state density and annealing time as N _s= N _so Bt ^1/n with n between 2 and 3. An inherent acceptorstate density N _so= 4.25 × 10¹² cm⁻² is obtained with an increase rate B = 4.8 × 10¹² cm⁻². min^−1/3, when n reaches 3. The inherent grain-boundary acceptor states in the reduced n -doped BaTiO₃ ceramics are believed not due to adsorbed oxygen ions.