Calcination Effects on the Properties of Gallium-Doped Zinc Oxide Powders

Highly conductive Gallium-doped zinc oxide (Zn_{1− x} Ga _x O) powder was prepared by calcining precursor that was synthesized from ZnSO₄·7H₂O, GaCl₃, and NH₄HCO₃ by the co-precipitation method. The effects of temperature, heating time, and atmosphere on the calcination process were studied, and we found that these factors are important to control the electrical conductivity, particle size, and agglomeration of Zn_{1− x} Ga _x O powder. By studying these processing conditions, we found that the optimum one is calcining the precursor in H₂ at 540°C for about 1.5 h, and the product has a specific surface area higher than 28 m²/g and volume resistivity lower than 16.8 Ω·cm. The chemistry details of the doping process were also discussed based on the experimental results.     

Low Thermal Conductivity in Garnets

The thermal conductivity of dense, polycrystalline garnet ceramics with compositions of Y₃Al_xFe_(5−x)O₁₂(x = 0.0, 0.7, 1.4, and 5.0) was measured in the temperature range 23° to 1000°C. The high-temperature thermal conductivity of some of these garnets was found to be as low as 2.4 W·m^-1K^-1. The effects of temperature and composition on the observed thermal conductivity are discussed with reference to established theories of thermal conduction. The potential use of yttrium aluminum garnet or YAG (Y₃Al₅O₁₂), in particular, for a specific application of advanced thermal barrier coatings (TBCs) with improved durability is considered.     

Hydrolysis of Pure and Sodium Substituted Calcium Aluminates and Cement Clinker Components Investigated by in Situ Synchrotron X-ray Powder Diffraction

The hydrolysis of pure and sodium-substituted calcium aluminates and cement clinker phases was investigated in situ in the temperature range 25°–170°C, using the angle dispersive powder synchrotron powder X-ray diffraction technique. The final hydrolysis product in all cases was Ca₃Al₂(OH)₁₂. The intermediate phase Ca₄Al₂O₇·19H₂O was formed from the pure calcium aluminates, and the intermediate phases Ca₄Al₂O₇· x H₂O, x = 11, 13, or 19, were formed from the cement clinker phases.     

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.     

Defect Structure and Electrical Properties of La1−ySryFe1−xAlxO3−δ

La_{1− y} Sr _y Fe_{1− x} Al _x O_3−δ perovskites were studied as potential materials for solid-oxide fuel cell (SOFC) cathodes. The phase relations in the LaFeO₃–SrFeO_3−δ–LaAlO₃ system were investigated by X-ray powder diffraction analysis. The defect structure of the La_{1− y} Sr _y Fe_{1− x} Al _x O_3−δ perovskites was investigated by Mössbauer spectroscopy and weight-loss analysis. Relations between the nonstoichiometry and the conductivity of the La_{1− y} Sr _y Fe_{1− x} Al _x O_3−δ perovskites were investigated. The incorporation of aluminum ( x ) into LaFe_{1− x} Al_xO₃ was found to have no influence on the defect structure but to decrease the conductivity. The incorporation of strontium ( y ) into La_{1− y} Sr _y Fe_{1− x} Al _x O_3−δ promotes the formation of anion vacancies and Fe⁴⁺ that lead to higher conductivity.     

Synthesis and Dielectric Properties of Subsituted Lanthanum Aluminate

LaAlO₃ was chemically modified in order to ascertain the effects of substitution of larger cations on the compound's slight rhombohedral distortion from cubic symmetry—a property that often degrades the performance of LaAlO₃ substrates for epitaxial high-temperature superconducting films. La_{1– x} Sr _x Al_{1– x} Ti_xO₃ (x = 0–05, 0.10, 0.15, 0.25),La_{1– x} Sr_xAl_1–xZr_xO₃, La_{1– x} Sr _x Al_1–xMg_xO_3–2xF _2x and La_{1– x} ,Sr_xAl_{1– x} Sc _x O_{3– x} (x = 0.05, 0.10, 0.15) were prepared in polycrystalline form and 1–2 mm single crystals of the Sr,Ti- and Sr,Zr-substituted systems were grown using a PbO–PbF₂–B₂O₃–PbO₂ flux. Shifts in the peak positions of the X-ray powder diffraction patterns confirmed subsitution of the larger cations. The diffraction patterns were also typified by the line-broadening and the decrease in the rhombohedral splitting at all doping levels. The dielectric constant of LaAlO₃ was unchanged for all of the fluoride-containing systems and for the 5% Sr, Ti- and Sr,Zr-systems.     

Lithium Ion-Conducting Glass–Ceramics of Li1.5Al0.5Ge1.5(PO4)3–xLi2O (x=0.0–0.20) with Good Electrical and Electrochemical Properties

NASICON-type structured Li_1.5Al_0.5Ge_1.5(PO₄)₃– x Li₂O Li-ion-conducting glass–ceramics were successfully prepared from as-prepared glasses. The differential scanning calorimetry, X-ray diffraction, nuclear magnetic resonance, and field emission scanning electron microscope results reveal that the excess Li₂O is not only incorporated into the crystal lattice of the NASICON-type structure but also exists as a secondary phase and acts as a nucleating agent to considerably promote the crystallization of the as-prepared glasses during heat treatment, leading to an improvement in the connection between the glass–ceramic grains and hence a dense microstructure with a uniform grain size. These beneficial effects enhance both the bulk and total ionic conductivities at room temperature, which reach 1.18 × 10⁻³ and 7.25 × 10⁻⁴ S/cm, respectively. In addition, the Li_1.5Al_0.5Ge_1.5(PO₄)₃–0.05Li₂O glass–ceramics display favorable electrochemical stability against lithium metal with an electrochemical window of about 6 V. The high ionic conductivity, good electrochemical stability, and wide electrochemical window of LAGP–0.05LO glass–ceramics suggest that they are promising solid-state electrolytes for all solid-state lithium batteries with high power density.     

A Novel Way to Synthesize Yttrium Aluminum Garnet from Metal–Inorganic Precursors

Yttrium aluminum garnet (YAG, Y₃Al₅O₁₂) was synthesized by sol–gel processing from the stoichiometric amounts of aluminum pellets, Y(NO₃)₃·6H₂O, and Al(NO₃)₃·9H₂O or AlCl₃·6H₂O, with suitable kinds of acid (citric acid, acetic acid, etc.) as catalysts. Polycrystalline YAG powder was obtained by drying the YAG precursor followed by calcination at temperatures above 900°C. Thermogravimetry/differential thermal analysis and Fourier transform infrared specotrscopic analyses in air showed an exothermic peak at ∼900°C, attributed to the formation of a polycrystalline YAG phase and weight loss of 60% at 1000°C, caused by the decomposition of hydroxyl and NO₃⁻, etc. X-ray diffraction analysis showed that YAG can be formed at 900°C, and no other intermediate was observed. In particular, the YAG sol can be used for dry-spinning fibers with the aid of some organic polymer.     

Enhanced Thermal Stability of Zinc-Based Titanate (Dielectric) Ceramics Prepared by the Modified Sol–gel Route

A multi-component substitution of Co and Ni was incorporated into ZnTiO₃ to form pure hexagonal Zn_{1− x} (Co_1/2Ni_1/2) _x TiO₃ ( x =0,0.8,0.9,1.0) dielectric ceramic powders by a modified sol–gel route, following heat treatments at 600°C for 3 h and at 800°C for 6 h. Differential scanning calorimetry measurements revealed that the order of increasing thermal stability of solid solution compound Zn_{1− x} (Co_1/2Ni_1/2) _x TiO₃ was ZnTiO₃ (945°C), Zn_0.1Ni_0.9TiO₃ (1346°C), Zn_0.1(Co_1/2Ni_1/2)_0.9TiO₃ (1390°C), and Zn_0.1Co_0.9TiO₃ (>1400°C). Both the dielectric constant and loss tangent reached a maximum at x =0.8 and then decreased with solubility, x , and measurement frequency.     

Synthesis of NiO-Deposited YSZ Composite Powders by Urea Hydrolysis

The urea hydrolysis method was used to prepare NiO-deposited YSZ composite powders. First, micrometer-sized YSZ particles were fabricated, and then the nanosized NiO particles were deposited on the surface of the YSZ particles. The microstructure of composite powders and the sintered bulk were further characterized with the aid of XRD, SEM, and TEM. The results indicated that the mesoporous and microsheet-like Ni(OH)₂· x H₂O ( x =0–1) crystals were deposited on the surface of YSZ particles. As the concentration of Ni²⁺ ion in the stock solution increased, the deposited NiO content and thickness of NiO layer on the YSZ particle surface also increased. In addition, the YSZ particle size showed significant influence on the microstructure and conductivity of Ni/YSZ cermet anode produced by NiO-deposited YSZ composite powders. Such NiO-deposited YSZ composite powders can be easily sintered to form a continuous NiO network.     

Nanosized Zinc-Oxide Particles Derived from Mechanical Activation of Zn5(NO3)2(OH)8·2H2O in Sodium Chloride

Nanosized ZnO particles are successfully synthesized via mechanical activation of a zinc nitrate hydroxide hydrate (Zn₅(NO₃)₂(OH)₈·2H₂O) precursor in NaCl matrix for 15 h. The ZnO particles obtained are in the nanosize range of ∼20 nm, with a well-established hexagonal morphology. They compare favorably with those derived from conventional calcination of the precursor. The decomposition of Zn₅(NO₃)₂(OH)₈·2H₂O precursor and formation of nanocrystalline ZnO cannot be completed by mechanical activation in the absence of NaCl, which acts as both an effective dispersing matrix and drying agent although it remains chemically inert during mechanical activation. The powder derived from calcination at 400°C does not possess powder characteristics comparable to that of the powder derived from the mechanical activation in NaCl, because of the extensive particle coarsening and aggregation at the calcination temperature.     

Subsolidus Equilibria in the System Li2O-Al2O3-Cr2O3

On the join LiCrO₂-LiAlO₂, solid solutions of formulas LiCri- _x Al _x O₂ (0< x <0.6) form. On the spinel join LiAl₅O₈-"LiCr₅O₈," solid solutions of formulas LiAl_{5- x} Al _x O₈ (0< x <3) form and the order-disorder transition, observed in LiAl₅O₈ at 1295°C, occurs at increasingly lower temperatures with increasing Cr content. No evidence was found for the metastable polymorphs of LiAl₅O₈ reported by Datta and Roy. Ternary subsolidus equilibria at 1200° to 1300°C were determined for compositions containing <50% Li₂O.     

Preparation and Properties of Machinable Si2N2O/BN Composites

Si₂N₂O/BN composites were successfully fabricated. With increasing BN content, the elastic modulus and hardness almost linearly decrease while the flexural strength does not exhibit a dramatic degradation. This is attributed to the fact that the homogeneously dispersed nanosized BN particles inhibit the grain growth of Si₂N₂O. The critical thermal-shock resistance temperature of the Si₂N₂O/30 vol% BN composite is enhanced by 400°C than monolithic Si₂N₂O. The introduction of BN significantly improves the dielectric properties and machinability. The Si₂N₂O/BN composites show a combination of high strength, low dielectric constant, good thermal shock resistance, and machinability, indicating that they are promising structural/functional materials.     

Hydration Study of the System Ca3Al2O6–CaSO4· 2H2O–Ca(OH)2–H2O with and without Citric Acid

Hydration occurring in the system Ca₃Al₂O₆–CaSO₄· 2H₂O–Ca(OH)₂–H₂O has been studied at different temperatures and it was found that the reactions are diffusion controlled. The kinetic data obeyed Jander's equation and the rate of reaction increased with increasing temperature. X-ray diffraction studies and calorimetric measurements show that when gypsum is consumed, ettringite is converted into monosulfate. The rate of this conversion also increased with the increasing temperature and decreased in the presence of citric acid. Spectroscopic studies showed that there was some interaction between citric acid and the cement and that the product of hydration is of colloidal nature. Zeta potential measurements show that retardation of Ca₃Al₂O₆ hydration in the presence of gypsum and Ca(OH)₂ is not due to SO²⁻₄ adsorption. Electrical conductivity and thermoelectric potential measurements of solid Ca₃Al₂O₆ show that Ca₃Al₂O₆ is an n -type semiconductor and contains defects. The retardation of Ca₃Al₂O₆ may be due to poisoning of reaction sites by gypsum and Ca(OH)_2.     

Texture and Formation Mechanism of Fibrous Calcium Hydroxyapatite Particles Prepared by Decomposition of Calcium–EDTA Chelates

The texture of fibrous calcium hydroxyapatite (Ca₁₀-(PO₄)₆(OH)₂, CaHAP) particles that were prepared by the decomposition of calcium–ethylenediaminetetraacetic acid (calcium–EDTA) chelates at 100°C under various pH conditions (pH values of 5–10) was investigated by various means. Well-crystallized fibrous CaHAPs were produced at pH .6. The stoichiometry of the CaHAPs with a chemical formula of Ca_{10− x} (HPO₄) _x (PO₄)_{6− x} (OH)_{2− x} (H₂O) _x was improved by increasing the decomposition pH. All the CaHAPs had unit-cell dimensions of a = 0.9436 ± 0.0003 nm and c = 0.6881 ± 0.0006 nm, exhibiting an enlarged a value. The finding of mesoporosity of CaHAPs by nitrogen gas (N₂) adsorption measurement indicated that the CaHAPs were produced by an agglomeration of primary particles. Furthermore, the nonstoichiometric CaHAPs that formed at pH 6 developed ultramicropores, which were accessible to water (H₂O) molecules but not to N₂ molecules, by the elimination of H₂O molecules that were adsorbed in interstices of primary particles in less-orderly crystallized CaHAPs and/or by dehydration of HPO₄²⁻ groups. These findings by gas adsorption techniques could give evidence for the agglomeration mechanism to attain a polycrystalline CaHAP, although they exhibited good crystallinity with large size.     

Fabrication of Translucent Magnesium Aluminum Spinel Ceramics

A precursor for magnesium aluminum spinel powder, composed of crystalline ammonium dawsonite hydrate (NH₄Al(OH)₂CO₃·H₂O) and hydrotalcite (Mg₆Al₂(CO₃)(OH)₁₆·4H₂O) phases, was synthesized via precipitation, using ammonium bicarbonate as the precipitant. The precursor was characterized by differential thermal analysis/thermogravimetry, X-ray diffractometry, and scanning electron microscopy. Reactive spinel powder, which could be densified to translucency under vacuum at 1750°C in 2 h without additives, was obtained by calcining the precursor at 1100°C for 2 h.     

Low-Temperature ac Conductivity of Mixed Mobile Ion Germanate Glasses

The conductivity of alternating current in the x Ag₂O: (1 - x )Rb₂O:4Geo₂ glass series has been measured between 5.0 K and room temperature where x is varied from 0.0 to 1.0. The typical low-temperature conductivity region is observed below 200 K. The temperature and frequency dependence of conductivity in this region can be explained by the thermal excitation of asymmetric double well potential configurations. Further, the classical mixed mobile ion effect is essentially absent, which shows that the effect is associated with the independent movement of the mobile ions.     

Porous Glass-Ceramics with a Skeleton of the Fast-Lithium-Conducting Crystal Li1+xTi2−xAlx(PO4)3

Porous glass-ceramics with a skeleton of the fast-lithium-conducting crystal Li_{1+ x} Ti_{2− x} Al _x (PO₄)₃ (where x = 0.3–0.5) were prepared by crystallization of glasses in the Li₂O─CaO─TiO₂─Al₂O₃–P₂O₅ system and subsequent acid leaching of the resulting dense glass-ceramics composed of the interlocking of Li_{1+ x} Ti_{2− x} Al _x (PO₄)₃ and β-Ca₃(PO₄)₂ phases. The median pore diameter and surface area of the resulting porous Li_{1+ x} Ti_{2− x} Al _x (PO₄)₃ glass-ceramics were approximately 0.2 μm and 50 m²/g, respectively. The electrical conductivity of the porous glass-ceramics after heating in LiNO₃ aqueous solution was 8 × 10⁻⁵ S/cm at 300 K or 2 × 10⁻² S/cm at 600 K.     

Phase Transition of Rare-Earth Aluminates (RE4Al2O9) and Rare-Earth Gallates (RE4Ga2O9)

Lattice parameters of RE₄Al₂O₉ (RE = Y, Sin, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb) prepared at 1600–1800°C and those of RE₄Ga₂O₉ (RE = La, Pr, Nd, Sm, Eu, and Gd) prepared at 1400–1600°C were refined by Rietveld analysis for the X-ray powder diffraction patterns. The parameters increased linearly with the ionic radius of the trivalent rare-earth elements ( r _RE). High-temperature differential calorimetry and dilatometry revealed that both RE₄Al₂O, and RE₄Ga₂O, have reversible phase transitions with volume shrinkages of 0.5–0.7% on heating and thermal hystereses. The transition temperatures (7_tr) decreased from 1300°C (Yb) to 1044°C (Sm) for RE₄A1₂O₉, except for Y₄Al₂O₉ ( T_tr = 1377°C), and from 1417°C (Gd) to 1271°C (La) for RE₄Ga₂O, with increasing ionic radius of the rare-earth elements. These transition temperatures were plotted on a curve against the ionic radius ratio of Al³⁺ or Gd³⁺ and RE³⁺ ( r _A1Ga/r_RE) except for Y₄Al₂O₉.     

Titanium Incorporation in Zn2TiO4 Spinel Ceramics

Compositions in the Zn₂TiO₄+ x TiO₂ system ( x = 0–0.43) were synthesized via the solid-state reaction route, using high-purity (≥99.99%) metal-oxide powders. The incorporation of titanium, in the form of TiO₂, in Zn₂TiO₄ spinel ceramics was investigated by analyzing the crystal structure and measuring the dielectric properties. The results of the crystal structure analyses suggested that TiO₂ levels of x ≤ 0.33 could be incorporated into the Zn₂TiO₄ spinel at temperatures of T > 945°C, whereas the solid solubility of titanium in Zn₂TiO₄ decreased for T < 945°C. When x ≥ 0.28, the Zn₂Ti₃O₈ phase formed in the Zn₂TiO₄ grain interior while cooling after heat treatment. Measurement of the microwave dielectric properties also supported the conclusion that the solubility limit of titanium in Zn₂TiO₄ was close to x = 0.33, as determined through analysis of the crystal structure.