Solvothermal Synthesis of Si3N4 Nanomaterials at a Low Temperature

Silicon nitride nanowires or nanorods have been synthesized from SiCl₄, NaN₃, and metallic Mg at temperatures ranging from 200° to 300°C. X-ray powder diffraction patterns indicated that the as-obtained products were mainly β-Si₃N₄. Scanning electron microscope and high-resolution transmission electronic microscopy showed that the samples mostly consisted of Si₃N₄ nanowires or nanorods. As metallic iron powder was used, α-Si₃N₄ was mainly formed at 250°C.     

Synthesis of the Orthorhombic Phase of 2Y˙ZrO2

A mixture of tetragonal and monoclinic 2Y˙ZrO₂ (2 mol% Y₂O₃–ZrO₂) powder was treated from 400° to 800°C and from 4 to 7 GPa for 30 min. The products were identified by powder XRD, Raman spectroscopy, and TEM. Results indicated that an orthorhombic phase was synthesized at T=400° to 600°C and P>4 GPa. The lattice parameters were obtained as a=0.505, b=0.525, and c=0.509 nm; the density was 6.17 Mg/m³. The orthorhombic phase always coexisted with the tetragonal phase in the products. The amounts of the tetragonal phase before and after treatment remained largely unchanged, whereas the amount of new orthorhombic phase was nearly the same as the decreased amount of the monoclinic phase. It was assumed, therefore, that only the monoclinic phase transformed into the orthorhombic phase.     

Synthesis and Characterization of Nanocrystalline Lanthanide Oxysulfide via a La(OH)3 Gel Solvothermal Route

A La(OH)₃ gel solvothermal process has been developed for the preparation of nanocrystalline lanthanide oxysulfide (La₂O₂S) in polar solvents at 300°C through a reaction between a La(OH)₃ gel and K₂S. X-ray powder diffraction (XRD) indicated that the product was hexagonal La₂O₂S with cell parameters a = 4.046 Å and c = 6.951 Å. Transmission electronic microscopy (TEM) showed that different morphology nanocrystallites were formed, including particles with diameters of about 10 nm and nanorods about 10 nm in diameter and 300 nm in length, depending on the solvent.     

Barium Holmium Zirconate, A New Perovskite Oxide: II, Synthesis as Nanoparticles through a Modified Combustion Process

Nanoparticles of barium holmium zirconate, a new complex perovskite ceramic oxide, has been synthesized using a modified self-propagating combustion process. The solid combustion products obtained were characterized by X-ray diffraction (XRD), electron diffraction, differential thermal analysis, thermogravimetric analysis, infrared spectroscopy, particle size analysis, surface area determination, and high-resolution transmission electron microscopy. The XRD and electron diffraction studies have shown that the as-prepared powder is phase pure Ba₂HoZrO_5.5 and has a complex cubic perovskite (A₂BB'O₆) structure with a lattice constant a = 8.428 Å. The transmission electron microscopic investigation has shown that the particle size of the as-prepared powder was in the range 4–16 nm with a mean grain size of 8.2 nm. The nanoparticles of Ba₂HoZrO_5.5 obtained by the present method could be sintered to 98% theoretical density at 1500°C.     

Nitridation of Silica to an α-Silicon Nitride Nanorod Using NaNH2 in the Autoclave at 700°C

α-silicon nitride nanorods have been synthesized through solid-state reduction–nitridation of silica using NaNH₂ as both a reductant and a nitriding reagent. X-ray powder diffraction patterns show that the products have a hexagonal phase with lattice parameters a =7.767 Å and c =5.630 Å. Transmission electron microscopy reveals that the as-synthesized products are pure nanorods with an average size about 30 nm in diameter and 400 nm in length. X-ray photoelectron spectra indicate that the molar ratio of Si/N is 2.988:4. Fourier-transform infrared spectrum yields a strong Si–N absorption at 926 cm⁻¹ that may be a red shift due to size effect.     

Cell Constants for Dicalcium Silicate-Alkali Fluoride Complexes

X-ray diffraction powder patterns were used to determine the cell constants for the complexes 2CaO·SiO_2·LiF and 2CaO·SiO₂·KF by Lipson's method of indexing, assuming orthorhombic structures. The resulting cell constants are α₀= 18.2, b₀=20.6, and c₀=7.2 Å for the Li complex and α₀=18.7, b₀=20.5, and c₀6.9 Å for the K complex.     

New High-Permittivity AgNb1-xTaxO3 Microwave Ceramics: Part I, Crystal Structures and Phase-Decomposition Relations

The crystal structure of monophasic AgNb_1/2Ta_1/2O₃ was investigated via Rietveld refinement using powder X-ray diffractometry (XRD) data. The study revealed a monoclinic unit cell of the P 2/ m space group with the following unit-cell parameters: a = 3.9286(3) Å, b = 3.9259(2) Å, c = 3.9302(3) Å, and β= 90.49(1)°. High-temperature XRD studies of the decomposition of AgNb_1/2Ta_1/2O₃ showed that the kinetics of the decomposition are influenced by the volatilization of silver. Suppression of silver volatilization by enclosing the sample in a corundum tube and applying an oxygen atmosphere yielded sintered, almost-monophasic AgNb_1/2Ta_1/2O₃ ceramics with >97% of the theoretical density. The Ag₈(Nb,Ta)₂₆O₆₉ phase, which regularly appeared as a result of partial decomposition of Ag(Nb_{1− x} Ta _x )O₃, was studied by energy-dispersive spectroscopy analysis and XRD. The Ag₈(Nb,Ta)₂₆O₆₉ phase exhibited a bronzelike orthorhombic structure with the following unit-cell parameters: a = 37.116(5) Å, b = 12.432(3) Å, and c = 7.826(2) Å. Indexed powder diffraction data for Ag₈(Nb,Ta)₂₆O₆₉ have been reported.     

High-Temperature Structural Phase Transition in Ca0.7Ti0.7La0.3Al0.3O3: Investigation by Synchrotron X-Ray Diffraction

Mixed-oxide prepared Ca_0.7Ti_0.7La_0.3Al_0.3O₃ (CTLA) ceramics (≈96% dense), grain size 6–7 μm, with dielectric properties (at 4 GHz) of ɛ_r≈46, Q × f ≈38 000 GHz, and τ_f+13 ppm/°C, were studied at 25°–1300°C using synchrotron X-ray powder diffraction. At room temperature, CTLA exhibits a distorted orthorhombic structure, with two tilt systems: a =5.40383 (4) Å, b =5.41106 (6) Å, and c =7.64114 (7) Å with space group Pbnm . At 1050°±25°C, there is a transition from orthorhombic ( Pbnm ) to tetragonal ( I 4/ mcm ), with a simpler tilt arrangement. The lattice parameters at 1100°C were: a =5.44285 (4) Å and c =7.68913 (8) Å.     

X-ray Powder Study of 2BaO·CuO

A compound of composition 2BaO·CuO was synthesized during the phase equilibrium study of the BaO-Y₂O₃-CuO_x system. Phase characterization has been carried out by using X-ray powder diffraction. The crystal symmetry was found to be the same as that of Ca₂CuO₃ and Sr₂CuO₃. It is orthorhombic with space group Immm and lattice parameters a=12.9655(14) Å (1.29655 nm), b=4.1007(3) Å (0.41007 nm), c=3.9069(5) Å (0.39069 nm), and V=207.72(3) ų (0.20772 nm³). The experimental pattern shows good agreement, in general, with intensity values calculated by assuming Ba₂CuO₃ to have a structure similar to that of Sr₂CuO₃ and Ca₂CuO₃. Intensity discrepancy for the h00 reflections might be due to preferred orientation.     

Kinetics of Sicl4 Oxidation

The oxidation rate of SiCl₄ was studied at 1100° to 1300°C. The reaction was first order in SiCl₄ and zero order in O₂ up to a 20-fold excess of O₂. At higher O₂ concentrations, the reaction becomes first order in oxygen. The activation energy for the reaction was 96 ± 5 kcal/mol which is approximately the CI₃SiCI bond energy. The reaction SiCl₄→ SiCl₃+ Cl is proposed as the rate-determining step.     

Reaction Synthesis of Magnesium Silicon Nitride Powder

The synthesis of magnesium silicon nitride (MgSiN₂) by direct nitridation of a Si/Mg₂Si/Mg/Si₃N₄ powder mixture is described. A nucleation period at 550°C and stepwise heat-treatment schedule up to 1350°C was adopted for the synthesis of MgSiN₂ powder, based on TG-DTA measurements. The influence of the ratio of constituents on the final phase composition also has been studied. The content of magnesium and silicon in the starting powder should fulfill the conditions Mg₂Si/Mg ≥ 3 and Si₃N₄/Si_tot≥ 0.5 to obtain single-phase MgSiN₂. The silicon particle size of <0.5 μm is preferable to decrease the time of nitridation. The oxygen content of as-synthesized powders is in the range 0.9–1.2 wt%. However, the oxygen content of MgSiN₂ powder decreases further by the addition of 2 wt% CaF₂ or 0.75 wt% carbon and reaching the lowest value of 0.45 wt% oxygen after carbothermal reduction in an alumina-tube furnace.     

Phase Diagram of the System KF–AlF3

The system KF–AlF₃ was reinvestigated precisely by differential thermal analysis, X-ray diffractometry, and visual observation. All of the samples for the present investigation were prepared by solution synthesis. The results verified the existence of 2KF·AlF₃ (K₂AlF₅) and KF·4AlF₃ in the phase diagram; both compounds were orthorhombic. The cell parameters of the compounds were, respectively, a = 10.87 ± 0.03 Å, b = 10.36 ± 0.01 Å, c = 7.83 ± 0.03 Å, and a = 7.89 ± 0.01 Å, b = 7.57 ± 0.01 Å, c = 6.94 ± 0.01 Å. KAlF₄ was confirmed to melt congruently at 575°± 2°C by careful examination.     

Low-Temperature Synthesis of Nanocrystalline α-Si3N4 Powders by the Reaction of Mg2Si with NH4Cl

Nanocrystalline α-Si₃N₄ powders have been prepared with a yield of 93% by the reaction of Mg₂Si with NH₄Cl in the temperature range of 450° to 600°C in an autoclave. X-ray diffraction patterns of the products can be indexed as the α-Si₃N₄ with the lattice constants a = 7.770 and c = 5.627 Å. X-ray photoelectron spectroscopy analysis indicates that the composition of the α-Si₃N₄ samples has a Si:N ratio of 0.756. Transmission electron microscopy images show that the α-Si₃N₄ crystallites prepared at 450°, 500°, and 550°C are particles of about 20, 40, and 70 nm in average, respectively.     

Phase Equilibria in the System CaF2-AlF3

The phase diagram of the system CaF₂-AlF₃ was established from microscopic, powder X-ray diffraction, quench, and DTA data obtained from samples encapsulated in sealed Pt tubes and either reacted in the solid state or melted. Two compounds, CaAlF₅ and Ca₂AlF₇, melted incongruently at 873^°° 3^° and 845^°°3^°C, respectively. Previously unreported Ca₂AlF₇ was successfully indexed as orthorhombic with α₀= 18.22 Å, b ₀=9.06 Å, and c ₀= 7.11 Å. The only eutectic in the system exists at 836^°° 3^°C and 37.5 mol% AlF₃.     

Crystal Structure and Dielectric Properties of LaYbO3

The crystal structure and dielectric properties of LaYbO₃ ceramics prepared by the mixed-oxide route have been investigated. Rietveld refinements performed on X-ray and neutron diffraction data show the room-temperature structure to be best described by the orthorhombic Pnma space group [ a =6.02628(9) Å, b =8.39857(11) Å, and c =5.82717(7) Å; Z =4, and theoretical density, D _x =8.1 g/cm³] in agreement with electron diffraction experiments. LaYbO₃ ceramics fired at 1600°C for 4 h attain ∼97% of D _x and their microstructures consist of randomly distributed equiaxed grains with an average size of ∼8 μm. Conventional transmission electron microscopy shows densification to occur in the absence of a liquid phase and reveals domain-free grains. The relative permittivity, ɛ_r, of LaYbO₃ ceramics at radio frequencies is ∼26 in the range ∼10–300 K; however, a small dielectric anomaly is detected at ∼15 K. At room temperature and microwave frequencies, LaYbO₃ ceramics exhibit ɛ_r∼26, Q × f _r∼20 613 GHz (at 7 GHz), and τ_f∼−22 ppm/K. Q × f _r show complex subambient behavior, decreasing from a plateau value of ∼20 000 GHz between ∼300 and 200 K to a second plateau value of ∼6000 GHz at ∼90 K before decreasing to <1000 GHz at ∼10 K. The large decrease in Q × f _r at low temperature may be related to the onset of antiferromagnetism at ∼2.7 K. ¹     

Preparation and Characterization of High-Purity HfTiO4

The double decomposition of hafnium and titanium tetrakis tertiary amyloxides was used to produce high-purity stoichiometric submicron HfTiO₄ powders. The particle size range was 50 to 400 Å. X-ray diffraction of a 1:1 mixture, as-prepared and heated to ∼750°C, indicated that the material is 95% orthorhombic HfTiO₄. The TGA, BET, DTA, and high-temperature X-ray analysis of the as-prepared powders are also reported; the ir absorption and reflection frequencies of HfTiO₄ were determined. The homogeneity and stoichiometry of the powder were demonstrated by electron microscopy and wet chemical analysis. Mixed oxides prepared in this manner were sintered in an ambient atmosphere at 1650° to 1700°C for 4 h or Ionger into a body of nearly theoretical density and uniform microstructure.     

Atomic Resolution Transmission Electron Microscopy of the Microstructure of Ordered Ba(Cd1/3Ta2/3)O3 Perovskite Ceramics

Microstructures of ordered Ba(Cd_1/3Ta_2/3)O₃ perovskite dielectric ceramics with and without a boron additive have been observed by atomic resolution transmission electron microscopy (TEM). The selected area electron diffraction and lattice image show a well-ordered structure with hexagonal symmetry (lattice constants of a ∼5.8 Å and c ∼7.1 Å) in the ordered Ba(Cd_1/3Ta_2/3)O₃ with a boron additive, which is similar to those in ordered Ba(Zn_1/3Ta_2/3)O₃ and Ba(Mg_1/3Ta_2/3)O₃ ceramics. Ordered domains with a twin crystallographic relationship and high-density domain interfaces induced by ordering were observed in the ordered Ba(Cd_1/3Ta_2/3)O₃ without a boron additive sintered at a relatively high temperature. Atomic resolution TEM further revealed the conservative twin boundaries along (001) and (110) planes and non-conservative antiphase boundaries with a projected displacement vector of the type [001] in the ordered Ba(Cd_1/3Ta_2/3)O₃ without a boron additive. Finally, the energetics of different domain interfaces are discussed with the interfacial structures in ordered Ba(Cd_1/3Ta_2/3)O₃ ceramics revealed by an electron microscope.     

Zinc Vanadates in Vanadium Oxide-Doped Zinc Oxide Varistors

Convergent-beam electron diffraction has been used to determine the space groups of β- and γ-Zn₃(VO₄)₂ particles in vanadium oxide-doped zinc oxide varistors. The crystal structure of β-Zn₃(VO₄)₂ has been determined to be monoclinic with space group P 2₁ and lattice parameters of a = 9.80 Å, b = 8.34 Å, c = 10.27 Å, and β= 115.8°, whereas that of γ-Zn₃(VO₄)₂ is monoclinic with space group Cm and a = 10.40 Å, b = 8.59 Å, c = 9.44 Å, and β= 98.8°. Energy-dispersive X-ray microanalysis of these two phases shows significant deviations from their expected stoichiometry. It is apparent that the β-phase is, in fact, the metastable Zn₄V₂O₉ phase, whereas the γ-phase either is a new oxide that consists of zinc, vanadium, and manganese or, more likely, is a zinc vanadate phase with a Zn:V atomic ratio of 1:1 that has the ability to go into solid solution with manganese.     

Preparation of Ultrafine Silicon Nitride, and Silicon Nitride and Silicon Carbide Mixed Powders in a Hybrid Plasma

Ultrafine Si₃N₄ and Si₃N₄+ SiC mixed powders were synthesized through thermal plasma chemical vapor deposition (CVD) using a hybrid plasma which was characterized by the superposition of a radio-frequency plasma and an arc jet. The reactant, SiCl₄, was injected into an arc jet and completely decomposed in a hybrid plasma, and the second reactant, CH₄ and/or NH₃, was injected into the tail flame through multistage ring slits. In the case of ultrafine Si₃N₄ powder synthesis, reaction effieciency increased significantly by multistage injection compared to single-stage injection. The most striking result is that amorphous Si₃N₄ with a nitrogen content of about 37 wt% and a particle size of 10 to 30 nm could be prepared successfully even at the theoretical NH₃/SiCl₄ molar ratio of ∼ 1.33, although the crystallinity depended on the NH₃/SiCl₄ molar ratio and the injection method. For the preparation of Si₃N₄+ SiC mixed powders, the N/C composition ratio and particle size could be controlled not only by regulating the flow rate of the NH₃ and CH₄ reactant gases and the H₂ quenching gas, but also by adjusting the reaction space. The results of this study provide sufficient evidence to suggest that multistage injection is very effective for regulating the condensation process of fine particles in a plasma tail flame.     

Microwave Dielectric Properties of Low Firing Temperature Bi2W2O9 Ceramics

Phase stability, sinterability, and microwave dielectric properties of Bi₂W₂O₉ ceramics and their cofireability with Ag, Cu, and Au electrodes have been investigated. Single-phase Bi₂W₂O₉ powder was synthesized by solid-state reaction in air at 800°C for 3 days. X-ray powder diffraction data show Bi₂W₂O₉ to have an orthorhombic crystal structure described by the noncentrosymmetric space group Pna 2₁, with lattice parameters a =5.4401(8), b =5.4191(8), c =23.713(4) Å. Ceramics fired at temperatures up to 865°C remain single-phase but above this temperature ferroelectric Bi₂WO₆ appears as a secondary phase. The measured relative permittivity of Bi₂W₂O₉ ceramics increases continuously from 28.6 to 40.7 for compacts fired between 860° and 885°C. The bulk relative permittivity of Bi₂W₂O₉ corrected for porosity was calculated as 41.3. Bi₂W₂O₉ ceramics fired up to 875°C exhibit moderate quality factors, Q × f _r, ∼7500–7700 GHz and negative temperature coefficient of resonant frequency, ∼−54 to −63 ppm/°C. Chemical compatibility experiments show Bi₂W₂O₉ ceramics to react with both Ag and Cu electrodes, but to form good contacts with Au electrodes.