Solid-State Synthesis of LaCoO3 Perovskite Nanocrystals

A solid-state reaction process has been developed to synthesize perovskite-type LaCoO₃ nanocrystals with grain diameters of 15–40 nm. In the first step of the preparation, ∼5 nm composite hydroxide nanoparticles are synthesized by the solid-state reaction of La(NO₃)₃· n H₂O and Co(NO₃)₂·6H₂O with KOH at ambient temperature. A perovskite-type rhombohedral LaCoO₃ phase appears at 550°C, after the hydroxide has been calcined at various temperatures. The phase transformation process is complete at ∼800°C, yielding a single-phase binary oxide. The results indicate that the new process is convenient, inexpensive, and effective for obtaining LaCoO₃ nanocrystals with high yield.     

Low-Temperature Synthesis of Calcium Hexa-Aluminate

Calcium hexa-aluminate (CaO·6Al₂O₃) has been prepared from calcium nitrate and aluminum sulfate solutions in the temperature range of 1000°–1400°C. A 0.3 mol/L solution of aluminum sulfate was prepared, and calcium nitrate was dissolved in it in a ratio that produced 6 mol of Al₂(SO₄)₃·16H₂O for each mole of Ca(NO₃)₂·4H₂O. It was dried over a hot magnetic stirrer at ∼70°C and fired at 1000°–1400°C for 30–360 min. The phases formed were determined by XRD. It was observed that CaO·Al₂O₃ and CaO·2Al₂O₃ were also formed as reaction intermediates in the reaction mix of CaO·6Al₂O₃. The kinetics of the formation of CaO·6Al₂O₃ have been studied using the phase-boundary-controlled equation 1 − (1 − x )^1/3= K log t and the Arrhenius plot. The activation energy for the low-temperature synthesis of CaO·6Al₂O₃ was 40 kJ/mol.     

Fabrication of NiO Nanoparticle-Coated Lead Zirconate Titanate Powders by the Heterogeneous Precipitation Method

NiO nanoparticle-coated lead zirconate titanate (PZT) powders are successfully fabricated by the heterogeneous precipitation method using PZT, Ni(NO₃)₂·6H₂O, and NH₄HCO₃ as the starting materials. The amorphous NiCO₃·2Ni(OH)₂·2H₂O are uniformly coated on the surface of PZT particles. XRD analysis and the selected-area diffraction (SAD) pattern indicate that the amorphous coating layer is crystallized to NiO after being calcined at 400°C for 2 h. TEM images show that the NiO particles of ∼8 nm are spherical and weakly agglomerated. The thickness of the nanocrystalline NiO coating layer on the surface of PZT particle is ∼30 nm.     

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.     

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.     

Synthesis and Properties of Dielectric Ceramics from Metallo-Organic Precursors through the Solid-State Reaction of Mixed Gels

A perovskite structure of 0.4Pb(Mg_1/3Nb_2/3)O₃·0.3Pb(Mg_1/2W_1/2)O₃·0.3PbTiO₃ was prepared from metallo-organic precursors through the solid-state reaction of the mixed gels. Three types of mixed gels were crystallized to obtain PbTiO₃, MgNb₂O₆, and MgWO₄ powders. These powders were calcined at 900°C after mixing with a stoichiometric amount of Pb(CH₃COO)₂·3H₂O. The dielectric constant of the ceramic fired at 900°C was improved by adding an excess of 10 mol% Mg(OC₂H₅)₂, and the ceramic achieved X7T specification of the Electric Industries Association standard. The dielectric loss was reduced by adding an excess of 5 mol% Pb(CH₃COO)₂·3H₂O.     

Low-Temperature Fabrication of Transparent Yttrium Aluminum Garnet (YAG) Ceramics without Additives

A carbonate precursor of yttrium aluminum garnet (YAG) with an approximate composition of NH₄AlY_0.6(CO₃)_1.9(OH)₂·0.9H₂O was synthesized via a coprecipitation method from a mixed solution of ammonium aluminum sulfate and yttrium nitrate, using ammonium hydrogen carbonate as the precipitant. The precursor precipitate was characterized using chemical analysis, differential thermal analysis/thermogravimetry, X-ray diffractometry, and scanning electron microscopy. The sinterability of the YAG powders was evaluated by sintering at a constant rate of heating in air and vacuum sintering. The results showed that the precursor completely transforms to YAG at ∼1000°C via the formation of a yttrium aluminate perovskite (YAP) phase. YAG powders obtained by calcining the precursor at temperatures of ≤1200°C were highly sinterable and could be densified to transparency under vacuum at 1700°C in 1 h without additives.     

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.     

Synthesis of Mesoporous Needle-Shaped Ytterbium Oxide Crystals by Solvothermal Treatment of Ytterbium Chloride

The reaction of rare-earth (RE; Y, Er, and Yb) chloride hydrates in 1,4-butanediol at 300°C for 2 h gave mixtures of RE(OH)₂Cl and RE₂O₃· x H₂O, and the products were composed of irregularly shaped particles. A prolonged reaction (10 h) yielded a mixture of RE(OH)₂Cl and RE₂O₃· x H₂O for Er or Y, but phase-pure RE₂O₃· x H₂O was obtained for Yb. The product for Yb comprised needle-shaped single crystals of Yb₂O₃· x H₂O with a width of 0.2–0.6 μm and a length of 5–15 μm. The Yb₂O₃· x H₂O phase decomposed to Yb₂O₃ at 350°–500°C, preserving the needle-shaped morphology; this was maintained even after calcination at 1100°C. Single crystals of Yb₂O₃ obtained by the calcination of Yb₂O₃· x H₂O at 500°C had very small voids and the voids were enlarged to 35 Å in diameter by calcination at 800°C.     

Nanometer-Sized ZrO2 Particles Prepared by a Sol–Emulsion–Gel Method

ZrO₂ powder was prepared by a sol–emulsion–gel method at temperatures below 140°C from ZrO(NO₃)₂· n H₂O. The asprepared powder was amorphous, but crystallized into the tetragonal structure by 600°C. The metastable tetragonal powder (600°C) was comprised of ultrafine 4- to 6-nm size particles. On heat treatment, the tetragonal form completely transformed into the monoclinic state at 1100°C. Preliminary studies indicate good sinterability with densities greater than 94% at 1100°C and with a grain size of 0.25 μ.     

The System MgO─P2O5─H2O at 25°C

The phase diagram for the ternary system MgO─P₂O₅─H₂O at 25°C has been constructed. The magnesium phosphates represented are Mg(H₂PO₄)₂· n H₂O ( n = 4, 2, 0), MgHPO₄·3H₂O, and Mg₃(PO₄)₂· m H₂O ( m = 8, 22). Because of the large differences in the solubilities of these compounds, the technique which involves plotting the mole fractions of MgO and P₂O₅ as their 10th roots has been employed. With the exception of MgHPO₄·3H₂O, the magnesium phosphates are incongruently soluble. Because incongruency is associated with a peritectic-like reaction, the phase Mg₂(PO₄)₃· 8H₂O persists metastably for an extended period.     

Preparation of Fine, Spherical Yttria-Stabilized Zirconia by the Spray-Pyrolysis Method

Fine yttria-stabilized zirconia powders were prepared by the spray pyrolysis of aqueous solutions of ZrOCl₂·8H₂O and Y(NO₃)₃·5H₂O (3 mol%). An appropriate thermal treatment resulted in slightly porous spherical particles with a narrow size distribution. The sintering ability of these powders was evaluated.     

Mechanochemical Changes of Weinschenkite-Type RPO4·2H2O (R = Dy, Y, Er, or Yb) by Grinding and Thermal Reactions of the Ground Specimens

It is reported that, on mechanochemical treatment, weinschenkite-type RPO₄·2H₂O (R = Dy, Y, or Er) gradually transforms into rhabdophane-type RPO₄· nH₂O (n = 0.5 to 1) and weinschenkite-type YbPO₄·2H₂O into xenotime-type YbPO₄, at room temperature in air. Rhabdophane-type YPO₄·0.8H₂O and ErPO₄·0.9H₂O obtained by grinding weinschenkite-type RPO₄·2H₂O (R=Y or Er) are new. The new rhabdophane-type YPO₄·0.8H₂O and ErPO₄·0.9H₂O gradually transform to xenotime-type YPO₄ and ErPO₄ when heated above 900°C (R = Y) and 700°C (R = Er) in air.     

Effect of Nitrate Salts as Sintering Additives during the Ball-Milling Process of Silicon Nitride Powders

A chemical adsorption method in a Si₃N₄ slurry that contained a nitrate solution was studied during ball milling, with particular interest in increasing the oxide layer in the Si₃N₄ powder and improving the distribution homogeneity of the sintering additives. The nitrate salts Al(NO₃)₃·9H₂O and Y(NO₃)₃·6H₂O were selected as sintering additives. The following characterization techniques were used: oxygen–nitrogen analysis, X-ray photoelectron spectroscopy, high-resolution electron microscopy (coupled with energy-dispersive X-ray spectroscopy), and X-ray imaging (using wavelength-dispersive X-ray spectroscopy). The thickness of the amorphous layer and the oxygen content of the Si₃N₄ powder were greater for samples that were milled with nitrate additives, which were heat-treated at 600°C, than those of powders that were milled with oxide additives. The chemical composition of the oxygen-containing layer—that is, the amorphous layer that formed and/or changed on the Si₃N₄ surface—was similar to Si₂N₂O in heat-treated Si₃N₄ powder with nitrate additives, whereas the composition of heat-treated Si₃N₄ powder with oxide additives was similar to SiO₂. Furthermore, a homogeneous distribution of the additives was achieved via the incorporation of aluminum and yttrium into the amorphous layer on the Si₃N₄ surface. The metal ratio (Y:Al) of the adsorbates was somewhat higher than that of the additives.     

Synthesis of Nanosized Spherical Rhabdophane Particles

Spherical 10 nm rhabdophane (LaPO₄·H₂O) particles were made by controlled precipitation in water using lanthanum citrate chelate and phosphoric acid (La-Cit⁺/H₃PO₄) at a temperature of 30°C. Rod-shaped 10 nm × 100 nm rhabdophane particles were made by direct precipitation from lanthanum nitrate and phosphoric acid (La(NO₃)₃/H₃PO₄). The lanthanum nitrate to phosphoric acid molar ratios (La:P) were fixed at 1:1 and 1:5 for both methods to study their effect on particle size and shape. The particles were characterized with DTA/TGA, XRD, TEM, SEM, Fourier Transform infrared spectroscopy, and ζ potential measurements. The spherical particle surfaces had absorbed citric acid. The point of zero charge of the rod-shaped and spherical particles was pH 5.4 and 4.3, respectively. Formation mechanisms for the spherical particles are discussed.     

Fiber Strength Retention of Lanthanum- and Cerium Monazite-Coated Nextel™ 720

Nextel™ 720 fibers were coated with LaPO₄ and CePO₄ monazite. The coatings were applied using washed and unwashed rhabdophane sols derived from La(NO₃)₃/(NH₄)₂HPO₄ and a washed sol derived from Ce(NO₃)₃/H₃PO₄. The coatings were cured in-line at 900°–1300°C. Multiple coatings were also applied. Fiber strength was retained after coating with washed sols, but not with unwashed sols. These results are consistent with earlier work on LaPO₄ monazite fiber coatings derived from La(NO₃)₃/H₃PO₄.     

Phase Transformation of Hydrous-Zirconia Fine Particles Containing Cerium Hydroxide

Monoclinic hydrous-zirconia fine particles that contained cerium(IV) hydroxide (Ce(OH)₄) were heated from 200°C to 600°C, to investigate the phase transformation to CeO₂-doped tetragonal ZrO₂. Both ZrOCl₂·8H₂O and CeCl₃·7H₂O were dissolved in aqueous solutions and then boiled to prepare the hydrous-zirconia particles. The Ce(OH)₄-containing hydrous-zirconia particles were prepared by adding aqueous ammonia into the boiled solutions. The monoclinic-to-tetragonal ( m right arrow t ) phase transformation of the Ce(OH)₄-containing hydrous zirconias was observed at 300°C using X-ray diffraction (XRD). XRD and Brunauer-Emmett-Teller (BET) specific surface area measurements revealed that the Ce(OH)₄-containing hydrous zirconias had a tendency to transform from the monoclinic phase to the tetragonal phase at lower temperatures as the primary particle size of the hydrous zirconia decreased and the Ce(OH)₄ content increased. These tendencies for the m right arrow t phase transformation agree with the conclusions that have been derived from thermodynamic and kinetic considerations.     

Hot Isostatic Pressing of Reactive SnO2 Powder

Tin(IV) oxide (SnO₂) crystallizes at room temperature by adding hydrazine monohydrate ((NH₂)₂· H₂O) to a hydrochloric acid solution of tin, followed by washing and drying. Well-densified SnO₂ ceramics (99.8% of theoretical) with an average grain size of 0.9 μm have been fabricated by hot isostatic pressing for 2 h at 900°C and 196 MPa. Their Vickers hardness and bending strength are 14.4 GPa and 200 MPa, respectively. They exhibit an electrical conductivity of 2 × 10⁻³−9 × 10⁻³ S·cm⁻¹ at room temperature.     

Characterization and Sintering of Reactive Cerium(IV) Oxide Powders Prepared by the Hydrazine Method

Nanocrystalline cerium(IV) oxide (CeO₂) powders have been prepared by adding hydrazine monohydrate to an aqueous solution of hydrous cerium nitrate (Ce(NO₃)₃·6H₂O), followed by washing and drying. The lattice parameter of the as-prepared powder is a = 0.5415 nm. The powder characteristics and sinterability of reactive CeO₂ have been studied. The surface areas of powders that have been heated at low temperatures are high, and these surface areas do not decrease to 10 m²/g until the temperature is >1200°C. Crystallite size and particle size are strongly dependent on the heating temperature. Optimum sintered densities are obtained by calcining in the temperature range of 700°–800°C. Ceramics with almost-full density can be fabricated at a temperature as low as 1150°C.     

Synthesis of Phase-Pure Pb(ZnxMg1–x)1/3Nb2/3O3 up to x= 0.7 from a Single Mixture via a Soft-Mechanochemical Route

Phase-pure perovskite Pb(Zn _x Mg_{1– x} )_1/3Nb_2/3O₃ solid solution (PZ _x M_{1– x} N) is obtained for x ≦ 0.7 by heating a milled stoichiometric mixture of PbO, Mg(OH)₂, Nb₂O₅, and 2ZnCO₃·3Zn(OH)₂·H₂O at 1100°C for 1 h. Percent perovskite ( f _P) with respect to total crystalline phase decreases with increasing temperature of subsequent heating then increases to 900°C for the mixtures where x ≦ 0.8 and milled for 3 h. For mixtures with x = 0.9 and x = 1, f _P decreases monotonically. Curie temperature increases almost linearly with increasing x up to x = 0.7. The maximum dielectric constant at 1 kHz is 2×10⁴ and 1.7×10⁴ for the mixture with x = 0.4 and x = 0.7, respectively. The stabilization mechanism of strained perovskite is discussed.