Thermal Conductivity of β-Si3N4: III, Effect of Rare-Earth (RE = La, Nd, Gd, Y, Yb, and Sc) Oxide Additives

β-Si₃N₄ ceramics sintered with a series of rare-earth (RE = La, Nd, Gd, Y, Yb and Sc) oxide additives were fabricated by hot pressing and subsequent annealing. Their microstructures, lattice oxygen contents, and thermal conductivities were evaluated. Mean grain size increased, while lattice oxygen content decreased, and hence, thermal conductivity increased with decreasing ionic radius of the rare-earth element. In all cases, a marked change was observed in the order of ionic radius from La to Nd to Gd, and a little change was observed below them. Rare-earth oxide additives significantly influenced the thermal conductivity of β-Si₃N₄, unlike in the case of AlN.     

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.     

Subsolidus Phase Relationships in Si3N4–AlN–Rare-Earth Oxide Systems

The subsolidus phase relationships in Si₃N₄–AlN–rare-earth oxide (Me₂O₃ where Me=Nd, Sm, Gd, Dy, Er, and Yb) systems were studied. Solid-solution regions with the α-Si₃N₄ structure were delineated along the Si₃N₄–"Me₂O₃:9AIN" joins for all of the rare-earth oxide systems studied. The solubility limits of these solid solutions increased with decreasing size of the rare-earth ions.     

Characterization of Anion Disorder in Zirconate A2B2O7 Compounds by Raman Spectroscopy

Raman spectra were measured at room temperature and at liquid nitrogen temperature on six rare-earth zirconates, RE₂Zr₂O₇ (RE=Nd, Sm, Gd, Dy, Er, and Yb). The spectra of the three compounds with large rare-earth ions exhibited most of the expected Raman bands of the pyrochlore structure but with large, temperature-independent half-widths that could be related to the loss of translational symmetry imposed by incipient randomization of oxygen vacancies over two available anion sites. Compounds with the smaller rare-earth ions have the fluorite structure, in which the disorder arises from the complete random distribution of seven oxygen ions over eight crystallographically equivalent sites. This degree of disorder reduces the Raman spectrum to a broad featureless density-of-states continuum.     

Electronic Structures of Ln3+α-SiAIONs with Correlations to Solubility and Solution Effects

First-principles molecular-orbital calculations using the discrete-variational Xα method have been made on model clusters of α-Si₃N₄ and its solid solutions with lanthanide elements, which occupy interstitial sites in the structure. The formula is Ln_χSi_12–4.5χAl_4.5χO_1.5χN16–1.5χ (Ln = La, Nd, Gd, Dy, Ho, Er, Tm, Yb), i.e., a Ln-α-SiAION solid solution. Covalent bond strength between Si and N, evaluated by overlap population, increases because of the presence of trivalent charges at the interstitial sites. When a Ln³⁺ ion is present, antibondings occur between Ln orbitals and N/Si orbitals, and they depend significantly on the ionic radius of Ln³⁺. The total overlap population for the whole cluster is determined by the balance of Si-N bond reinforcement and Ln-N/Si antibonding. Although no lattice relaxation around the Ln³⁺ ion is included in the present calculation, good correlation between maximum solubility and the total overlap population for the whole cluster is demonstrated for the first time.     

Effect of Cation Substitution and Non-Stoichiometry on the Microwave Dielectric Properties of Sr(B'0.5Ta0.5)O3 [B'=Lanthanides] Perovskites

The Sr(B'_0.5Ta_0.5)O₃ ceramics where B'=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, and Yb have been prepared by the conventional solid-state ceramic route and their microwave dielectric properties have been investigated. The structure and microstructure of the ceramics have been characterized by X-ray diffraction and scanning electron microscope techniques. The relative permittiviy (ɛ_r) varies linearly with B'-site ionic radii, except for La, and the temperature coefficient of resonant frequency (τ_f) varies linearly with the tolerance factor. The Sr(B'_0.5Ta_0.5)O₃ ceramics have ɛ_r in the range 25.9–32, Q _u× f =4500–54 300 GHz, and τ_f=−79 to −42 ppm/°C. A slight deviation from stoichiometry affects the dielectric properties of these double perovskites. Partial substitution of Ba for Sr could tune the dielectric properties. Addition of rutile (TiO₂) lowered the sintering temperature and improved the dielectric properties of Sr(B'_0.5Ta_0.5)O₃ ceramics.     

Phase Relations in the Si3N4-Rich Portion of the Si3N4–AlN–Al2O3–Y2O3

The phase relations in the Si₃N₄-rich portion of the Si₃N₄–AlN–Y₂O₃ rystem were investigated using hot-pressed bodies. The one-phase fields of β3 and α, the twophase fields of β+α, β+M (M=melilite), and α+M, and the three-phase fields of β+α+M were observed in the Si₃N₄-rich portion. The α- and β-sialons are not two different compounds but an allotropic transformation phase of the Si–Al–O–N system, and an α solid solution expands and stabilizes with increasing Y₂O₃ content. Therefore, the formulas of the two sialons should be the same.     

High-Temperature Strength of Liquid-Phase-Sintered SiC with AlN and Re2O3 (RE = Y, Yb)

Using AlN and RE₂O₃ (RE = Y, Yb) as sintering additives, two different SiC ceramics with high strength at 1500°C were fabricated by hot-pressing and subsequent annealing under pressure. The ceramics had a self-reinforced microstructure consisting of elongated α-SiC grains and a grain-boundary glassy phase. High-temperature strength up to 1600°C was measured and compared with that of the SiC ceramics fabricated with AlN and Er₂O₃. SiC ceramics with AlN and Y₂O₃ showed the best strength (∼630 MPa) at 1500°C, while SiC ceramics with AlN and Er₂O₃ the best strength (∼550 MPa) at 1600°C.     

Preparation, Characterization, and Microwave Properties of RETiNbO6 (RE = Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Y, and Yb) Dielectric Ceramics

Microwave ceramic dielectric resonators (DRs) based on RETiNbO₆ (RE = Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Y, and Yb) have been prepared using the conventional solid-state ceramic route. The DR samples are characterized using XRD and SEM methods. The microwave dielectric properties are measured using resonant methods and a net work analyzer. The ceramics based on Ce, Pr, Nd, and Sm have dielectric constants in the range 32–54 and positive coefficient of thermal variation of resonant frequency (τ_f). The ceramics based on Gd, Tb, Dy, Y, and Yb have dielectric constants in the range 19–22 and negative τ_f.     

Stability and Oxidation Properties of RE-α-Sialon Ceramics (RE = Y, Nd, Sm, Yb)

Oxidation studies of hot-pressed RE-α-sialons, RE _x -Si_{12-4.5 x} Al_{4.5 x} O_{1.5 x} N_{16-1.5 x} (with x = 0.40 for RE = Nd, Sm, Yb; and x = 0.48 for RE = Y) were carried out in oxygen in a TG apparatus for ca. 20 h. Very good oxidation resistance was found for the Yb-doped samples, with parabolic rate constants K _p similar/congruent 0.09 10^-6-3 10^-6 mg²cm^-4s^-1 in the temperature range 1250-1350°C. The promising performance of this material was corroborated by long-term oxidation experiments (5 days) in air at 1350°C. Although the oxidation kinetics can be described by simple equations related to the parabolic rate law (e.g., the arctan equation, Δ W / A ₀=α arctan bt + c t ), the oxidation process in these materials is likely to be complex. The significantly lower oxidation resistance of the RE = Nd, Sm doped α-sialons, especially at higher temperatures, is related to the formation of melilite, RE₂Si_{3− y} Al _y O_{3+ y} N_{4− y} ( y ∼ 1), in these systems. The melilite phase is also responsible for the thermal instability of the Nd- and Sm-α-sialons.     

Low Loss Dielectrics in the Ca(B'1/2Nb1/2)O3 [B'=Lanthanides, Y] System

The Ca(B'_1/2Nb_1/2)O₃ [B'=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, Yb, and In] complex perovskites have been prepared by conventional solid-state ceramic route. The structure and microstructure of the ceramics have been characterized by X-ray diffraction and scanning electron microscopy methods. The ceramics have dielectric constant (ɛ_r) in the range 23–32, normalized Q -factor ( Q _u× f ) 11 000–38 000 GHz and temperature coefficient of resonant frequency (τ_f) −43–5.2 ppm/°C. The microwave dielectric properties of Ca(B'_1/2Nb_1/2)O₃ ceramics are found to depend on the ionic radii of B'-site elements and tolerance factor ( t ). The substitution of Ba²⁺ and Sr²⁺ for Ca²⁺ resulted a phase transition in Ca(B'_1/2Nb_1/2)O₃ ceramics. The (Ca_0.05Ba_0.95) (Y_1/2Nb_1/2)O₃ has τ_f close to zero (1.2 ppm/°C) with ɛ_r=35 and Q _u× f =48 500 GHz and is proposed as a useful material for base station applications. Dielectric properties of the Ca(B'_1/2Nb_1/2)O₃ ceramics were tailored by the addition of TiO₂ and CaTiO₃.     

Structural Disorder and Intracrystalline Microtexture of α'H-(Ba0.24Ca0.76)2SiO4

The structural disorder and twin microtexture of (Ba_0.24Ca_0.76)₂SiO₄ crystal were investigated by X-ray powder diffraction, precession method, and optical microscopy. The crystal at 298 K was orthorhombic (space group Pnma , Z =4) with a =0.70490(5) nm, b =0.55399(4) nm, c =0.95532(7) nm, and V =0.37306(4) nm³. The crystal structure was satisfactorily described by a split-atom model, involving the positional disorder of Ba/Ca and O atoms across the (010) mirror plane. The crystal was found to be isostructural with α'_H-Ca₂SiO₄, thus regarded as the stabilized α'_H phase. The crystal grain was composed of orthorhombic domains in three different orientations. The unit cells within each domain were related to one another by the threefold pseudo-symmetry axis lost during the α-to-α'_H transition. Each domain was most probably made up of sub-domains of the two mirror-related structural configurations with P 2₁/ n symmetry.     

Reaction Densification of α'-SiAlON: 1, Wetting Behavior and Acid-Base Reactions

Phase development during reaction hot pressing of α-Si₃N₄, AlN, Al₂O₃, and M₂O_Z powder mixtures (M = Li, Mg, Ca, Y, Nd, Sm, Gd, Dy, Er, and Yb) forming α'-SiAlON has been studied. The wetting behavior of the ternary eutectic melt of M₂O_z-Al₂O₃-SiO₂ was found to control the reaction sequence during hot pressing. Li, Ca, Mg, Nd, Sm, and Gd were found to preferentially wet and react with Si₃N₄ first, whereas Dy, Er, and Yb preferentially wetted and reacted with AlN first. The intermediate phases are Si rich in the former and Al rich in the latter case. Pearson's principle of acid-base theory, which predicts decreasing basicity of oxides of Li, Ca, Mg, Nd, Gd, Sm, Dy, Er, and Yb and decreasing acidity of oxides of Si and Al, is used to understand these reactions.     

The Effect of Dopants on the Dielectric Properties of Ba(B'1/2Ta1/2)O3 (B'=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Yb, and In) Microwave Ceramics

Low-loss dielectric ceramics based on Ba(B'_1/2Ta_1/2)O₃ (B'=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Yb, and In) complex perovskites have been prepared by the solid-state ceramic route. The dielectric properties (ɛ_r, Q _u, and τ_f) of the ceramics have been measured in the frequency range 4–6 GHz by the resonance method. The resonators have a relatively high dielectric constant and high quality factor. Most of the compounds have a low coefficient of temperature variation of the resonant frequencies. The microwave dielectric properties have been improved by the addition of dopants and by solid solution formation. The solid solution Ba[(Y_{1− x} Pr _x )_1/2Ta_1/2]O₃ has x =0.15, with ɛ_r=33.2, Q _u× f =51,500 GHz, and τ_f≈0. The microwave dielectric properties of Ba(B'_1/2Ta_1/2)O₃ ceramics are found to depend on the tolerance factor ( t ), ionic radius, and ionization energy.     

Size and Charge Effects of Dopant M on the Unit-Cell Parameters of Monoclinic Zirconia Solid Solutions Zr0.98M0.02O2 –δ (M = Ce, La, Nd, Sm, Y, Er, Yb, Sc, Mg, Ca)

The crystal structure of monoclinic phase [ P 2¹/ c Z = 4] has been refined by the Rietveld analysis of X-ray powder diffraction data to study the size and charge effects of dopant M ^{n +} on the unit-cell parameters of monoclinic ZrO₂−2 mol% MO _{n /2} solid solutions ( n = 4 for M = Ce; n = 3 for M = La, Nd, Sm, Y, Er, Yb, Sc; and n = 2 for M = Mg and Ca). For trivalent dopant ( n = 3), the unit-cell parameters a _m , b_m , c_m and unit-cell volume increase and β m decreases with an increase of dopant size. Unit-cell volume increases with increasing of dopant charge n .     

Fabrication of Nano-Scaled α-Al2O3 Crystallites Through Heterogeneous Precipitation of Boehmite in a Well-Dispersed θ-Al2O3-Suspension

The possibility of eliminating finger or vermicular growth of α-Al₂O₃ particles obtained by calcination of boehmite was examined. Heterogeneous precipitation of boehmite in a well-dispersed θ-Al₂O₃ suspension was first prepared, in which the mass ratio of boehmite to θ-crystallite was evaluated to form agglomerates of similar sizes that will form α-Al₂O₃ crystallites of <100 nm in diameter. θ- to α-phase transformation of alumina experiences a nucleation and growth mechanism, with the critical size of nucleation being ∼25 nm for θ-Al₂O₃ and the size for accomplishment of transformation followed by finger growth being ∼100 nm. Hence, fabricating agglomerates that would form α-Al₂O₃ crystallites with sizes <100 nm accompanied with appropriate thermal treatments can be a method for obtaining α-Al₂O₃ crystallites free of finger growth. It is found that proper preparation of the agglomerate with appropriate size may initiate a simultaneous and lower temperature θ- to α-Al₂O₃ phase transformation for such powder systems, substantially limiting the mass transfer among the newly formed α-Al₂O₃ particles. Moreover, α-Al₂O₃ crystallites free of finger growth can be obtained.     

High-Temperature Transmission Electron Microscopy and X-Ray Powder Diffraction Studies of Polymorphic Phase Transitions in Ba4Nb2O9

Polymorphic phase transitions in Ba₄Nb₂O₉ were studied by thermal analyses, high-temperature transmission electron microscopy and X-ray powder diffractometry. Two stable polymorphs were isolated, low-temperature α-modification and high-temperature γ-modification, with the endothermic phase transition at 1176°C. The α→γ transformation is accompanied by the formation of a 120° domain structure, which is a consequence of hexagonal→orthorhombic unit cell reconstruction. Reheating the presintered γ-Ba₄Nb₂O₉ results in the formation of a metastable γ'-modification (formerly known as β-polymorph) in the temperature range between 360° and 585°C, before the γ→α transformation at 800°C. Above ∼490°C Ba₄Nb₂O₉ becomes moderately sensitive to a loss of BaO. In air the surface of Ba₄Nb₂O₉ grains decomposes to nanocrystalline Ba₅Nb₄O₁₅ and BaO, which instantly reacts with atmospheric CO₂ to form BaCO₃. Surface reaction delays γ→α transformation up to 866°C in air. In vacuum the loss of BaO is even more enhanced and consequently the formation of minor Ba₃Nb₂O₈ phase is observed above 1150°C.     

Mechanical Property and Oxidation Behavior of Self-Reinforced Si3N4 Doped with Re2O3 (Re=Yb, Lu)

In this work, self-reinforced silicon nitrides with β-Si₃N₄ seeds doped with Re₂O₃ (Re=Yb, Lu) were investigated. Firstly, the two kinds of seeds were obtained by heating α-Si₃N₄ powder with Yb₂O₃ or Lu₂O₃, respectively. Then the self-reinforced silicon nitride ceramics were prepared by HP-sintering of α-Si₃N₄ powder, Re₂O₃ as additive, and the as-prepared seeds. Oxidation test was carried out at 1400°C in air for 100 h with thermogravimetry analysis (TGA) measurement. Mechanical properties, scanning electronic microscopy microstructures, and X-ray diffraction patterns were measured before and after oxidation. The results indicated that the introduction of the seeds doped with Re₂O₃ (Re=Yb, Lu) could obviously increase the toughness and keep the room temperature and high-temperature strength of the ceramics at high values. After oxidation, the crystalline phase in grain boundary changed and the mechanical properties decreased. TGA showed a parabolic weight gain and the oxidation mechanism was discussed.     

Trap Levels in Eu-Doped SrAl2O4 Phosphor Crystals Co-Doped with Rare-Earth Elements

SrAl₂O₄:Eu²⁺ phosphor crystals co-doped with auxiliary activators such as La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, or Y have been grown by the floating zone (FZ) technique. Photoluminescence spectrum (PL), time-resolved PL, and thermally stimulated luminescence (TSL) are evaluated to clarify the long-duration phosphorescence mechanism of SrAl₂O₄, Eu, and Ln phosphors. TSL spectra were measured in the temperature range from RT to 600 K to evaluate the depth and densities of the traps generated by the doping of auxiliary activators that are responsible for the long-duration phosphorescence. The peak wavelength of PL does not vary with auxiliary activator elements, while decay curves vary greatly with the auxiliary activators. The trap depth and the densities of the trapped carriers estimated based on the hole trap model also vary with the auxiliary activator elements. The traps generated at around E =0.5 eV by the auxiliary activators, Nd, Dy, and Tm, with sufficient densities are effective for the long-duration phosphorescence.     

Effects of (B2O3, P2O5) Additives on Microstructural Development and Phase-Transformation Kinetics of Stoichiometric Cordierite Glasses

The microstructural development and phase-transformation kinetics of stoichiometric cordierite glasses containing B₂O₃ and/or P₂O₅ additives were highly affected by the microstructural characteristics of the μ-cordierite and the type of additives. The addition of B₂O₃ tended to cause the formation of μ-spherulitic dendrites with thin dendritic arms, which promoted the formation of α-cordierite, either from crystallization of the residual glass or from transformation of μ-cordierite. P₂O₅ had the opposite effect: Increasing the temperature increased the growth rate of α-cordierite more than that of μ-cordierite.