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.     

Aluminum-Containing Nilrogen Melilite Phases

A significant solubility of Al in N-melilite phases (M) has been observed, and this results in the formation of a melilite solid solution (M'_ss) of general formula Ln₂Si_{3 − x} Al _x O_{3 + x} N_{4 − x} (Ln = rare earth). Up to one Si can be replaced by Al without change of structure, and the M'solid solution terminates at Ln₂Si₂AlO₄N₃ in samarium SiAlON systems. M'_ss may appear as an intermediate phase during the sintering of SiAlONs, and its melting temperature is critical to the densification of the materials. For example, samarium M'_ss melts at a temperature lower than neodymium M'_ss, and as a result, samarium oxide shows better densification behavior in the preparation of α-SiAION ceramics than does neodymium oxide. Devitrification of M'_ss from an amorphous grain boundary phase occurs above 1500°C during post heat-treatment. The M'_ss is refractory and may offer better oxidation resistance than N-melilite because of the replacement of Al─O for Si─N in the structure. Therefore M'_ss, is considered to be a most desirable grain boundary phase for α and α–β SiAlON ceramics.     

Solubility Limits of α'-SiAION Solid Solutions in the System Si,Al,Y/N,O

The solubility limit of α'-SiAION solid solutions on the Si₃N₄─YN:3AIN composition join in the system Si₃N₄─YN─AIN has been determined at 1800°C. The end members of these solid solutions are Y_0.43Si_10.7Al_1.3N₁₆ and Y_0.8Si_9.6Al_2.4N₁₆. Unit-cell dimensions of the α'-SiAION solid solutions in the system Si,Al,Y/N,O can be expressed as follows: a _o(Å) = 7.752 + 0.045 m + 0.009 n , c _o(Å) = 5.620 + 0.048 m + 0.009 n , where the α'-SiAION solid solution has the formula Y _x Si_{12-( m+n )}Al _m+n N_{16- n} O _n . The single-phase boundary of the solid solution α'-SiAION on the composition triangle Si₃N₄─YN:3AIN─AIN:Al₂O₃ is delineated. The present paper also reports the phase relationships involving α'-SiAION.     

A New, Low-Temperature Polymorph of Ó-SiAlON

A new phase in the Si-Al-O-N system has been identified, following syntheses based on the nitridation of silicon/clay mixtures at low temperatures (<1350°C). The structure of the new phase was determined using a combination of diffraction and high-resolution imaging techniques, and this new phase possessed the same sheet structure as Ó-SiAlON (Si_{2− x} Al _x O_{1+ x} N_{2− x} ) but with a different stacking arrangement. It is considered to be a low-temperature polymorph of Ó-SiAlON and transforms to conventional Ó-SiAlON at temperatures greater than ∼1350°C.     

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.     

Photoluminescence of Rare-Earth-Doped Ca-α-SiAlON Phosphors: Composition and Concentration Dependence

Rare-earth-doped Ca-α-SiAlON phosphors, with the compositions of (Ca_{1−3/2 x} RE _x ) _{m /2}Si_{12− m − n} Al _m+n O _n N_{16− n} (RE=Ce, Sm, and Dy, 0.5≤ m =2 n ≤3.0), were prepared by sintering at 1700°C for 2 h under 10 atm N₂. The concentration of rare earths varied from 3 to 30 at.% with respect to Ca. The photoluminescence (PL) properties were investigated as functions of the composition of the host matrix (i.e., m ) and the concentration of rare earths (i.e., x ). The results show that the emission properties can be optimized by tailoring m and x . The Ce³⁺ luminescence originating from the 4 f –5 d interconfigurational transitions is greatly affected by the environment surrounding the Ce³⁺ ions, which differs from the Sm³⁺ or Dy³⁺ luminescence arising from the 4 f –4 f intraconfigurational transitions. X-ray diffraction and scanning electron microscopy were used to explain the composition and concentration dependence of PL properties.     

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.     

Melilite Formation in a Samarium-Stabilized α-Sialon Ceramic during Postsintering Heat Treatments

The formation of the melilite solid solution phase (M'), Sm₂Si_3−xAl_xO_3+xN_4−x, in an α-sialon sample of overall composition Sm_0.6Si_9.28Al_2.69O_1.36N_14.76, was studied as a function of time in the temperature interval 1375–1525°C. The alpha-sialon ceramic contained only minor amounts of the 21R sialon polytype and some residual grain-boundary glass before heat treatment. In situ studies by high-temperature X-ray diffraction were combined with postsintering heat treatment followed by quenching. The M'-phase was found to be formed by two different mechanisms: either crystallization of the residual grain-boundary liquid or a direct decomposition of the α-sialon phase. The liquid crystallized during the first 10–15 min of heat treatment, yielding a rapid M'-phase formation, and further formation of M'-phase continued at a much slower rate, related to the decomposition of α-sialon.     

Subsolidus Phase Relationships in the Ga2O3–Al2O3–TiO2 System

Subsolidus phase relationships in the Ga₂O₃–Al₂O₃–TiO₂ system at 1400°C were studied using X-ray diffraction. Phases present in the pseudoternary system include TiO₂ (rutile), Ga_{2−2 x} Al_{2 x} O₃ ( x ≤0.78 β-gallia structure), Al_{2−2 y} Ga_{2 y} O₃ ( y ≤0.12 corundum structure), Ga_{2−2 x} Al_{2 x} TiO₅ (0≤ x ≤1 pseudobrookite structure), and several β-gallia rutile intergrowths that can be expressed as Ga_{4−4 x} Al_{4 x} Ti _{n −4}O_{2 n −2} ( x ≤0.3, 15≤ n ≤33). This study showed no evidence to confirm that aluminum substitution of gallium stabilizes the n =7 β-gallia–rutile intergrowth as has been mentioned in previous work.     

Grain Growth of α-SiAlON in the Calcium-Doped System

Two calcium-doped α-SiAlON compositions (Ca_0.6Si_10.2Al_1.8−O_0.6N_15.4 and Ca_1.8Si_6.6Al_5.4O_1.8N_14.2) were prepared by hot pressing at 1600° and 1500°C, respectively, for complete phase transformation from α-Si₃N₄ to α-SiAlON. Both samples were subsequently fired at different temperatures for different periods of time to study the grain growth of α-SiAlON. Elongated α-SiAlON grains were developed in both samples at high temperatures. The kinetics of grain growth was investigated based on the variations in length and width of the α-SiAlON grains under different sintering conditions. Different growth rates were found between the length and width directions of the α-SiAlON crystals, resulting in anisotropic grain growth in the microstructural development.     

Photoluminescence of Cerium-Doped α-SiAlON Materials

Cerium-doped α-SiAlON (M _x Si_{12−( m + n )}Al _{m + n} O _n N_{16– n} ) materials have been prepared by gas-pressure sintering and post-hot-isostatic-press (HIP) annealing, using four powder mixtures of α-Si₃N₄, AlN, and either (i) CeO₂, (ii) CeO₂+ Y-α-SiAlON seed, (iii) CeO₂+ Y₂O₃, or (iv) CeO₂+ CaO. Cerium-containing CeAl(Si_{6– z} Al _z )(N_{10– z} O _z ) (JEM) phase, rather than Ce-α-SiAlON phase, forms in the sample with only CeO₂, whereas a single-phase α-SiAlON generates in samples with dual doping (CeO₂+ Y₂O₃ and CeO₂+ CaO). On ultraviolet-light excitation, JEM gives one broad emission band with maximum at 465 nm and a shoulder at 498 nm; α-SiAlON shows an intense and broad emission band that peaks at 500 nm. The unusual long-wavelength emissions in JEM and α-SiAlON are due to increases in the nephelauxetic effect and the ligand-field splitting of the 5 d band, because the coordination of Ce³⁺ in JEM and α-SiAlON is nitrogen enriched.     

Reaction Densification of α'-SiAION: II, Densification Behavior

Reaction hot-pressing behavior of α-Si₃N₄, Al₂O₃, A1N, and M₂O_Z powder mixtures (M = Li, Mg, Ca, Y, Nd, Sm, Gd, Dy, Er, and Yb) forming α'-SiAlON has been studied. Five characteristic temperatures are found to control the densification behavior of these materials. The densification proceeded in three major stages. The first two stages were formation of ternary oxide eutectic and wetting of majority nitride powder. The third stage involved dissolution/melting of intermediate phase. Variation from this behavior sometimes occurs due to localization of wetting liquid at A1N, extremely high melting/dissolution temperature of Mg₂Al₄Si_sO₁₈ and Nd and Sm melilite, and secondary precipitation of Dy-α'-SiAlON. The dominant densification mechanism was found to be massive particle rearrangement, irrespective of the wetting and dissolution/melting behavior. The efficiency of this mechanism is mostly affected by the amount of available liquid and less by its viscosity. Fully dense, single-phase ceramics were obtained in all cases except Mg when hot-pressed at a constant heating rate to 1750°C, and considerably lower temperatures for Li, Ca, Gd, Dy, Er, and Yb-SiAlON when held isothermally.     

Near-Net Shape β-Si4Al2O2N6 Parts by Hydrolysis Induced Aqueous Gelcasting Process

In this paper, a new net-shaping process, an hydrolysis-induced aqueous gelcasting (GC) (GCHAS) has been reported for consolidation of β-Si₄Al₂O₂N₆ ceramics from aqueous slurries containing 48–50 vol%α-Si₃N₄, α-Al₂O₃, AlN, and Y₂O₃ powders mixture. Dense ceramics of same composition were also consolidated by aqueous GC and hydrolysis assisted solidification routes. Among three techniques used, the GCHAS process was found to be superior for fabricating defect-free thin wall β-Si₄Al₂O₂N₆ crucibles and tubes. Before use, the as purchased AlN powder was passivated against hydrolysis. The sintered β-Si₄Al₂O₂N₆ ceramics exhibited comparable properties with those reported for similar materials in the literature.     

Duplex α,β-Sialon Ceramics Stabilized by Dysprosium and Samarium

Duplex αβ,-sialon ceramics with a minimum volume fraction of residual intergranular glass have been prepared using Dy or Sm as the α-sialon stabilizing element. These microstructures contained high aspect ratio β-sialon grains homogeneously distributed in an α-sialon matrix. A number of the larger α-sialon grains contained dislocations and showed a core/shell structure. Dy gave an α-sialon which was stable over a wide temperature range (1350–1800°C) for long holding times, while the use of Sm resulted in less stable α-sialon structures at medium temperatures (1450°C) and the formation of melilite, R₂Si_3−xAl_xO_3+xN_4−x, β-sialon, and the 21R sialon polytype during prolonged heating. High α-phase contents gave a very high hardness ( H _V10 is approximately 22 GPa) but a comparatively low indentation fracture toughness (around 4.4 MPam^1/2). Duplex sialons fabricated from powder mixtures corresponding to an α-to-β sialon ratio of around 50:50 resulted in a sialon material with a favorable combination of high hardness (around 22 GPa) and increased toughness (to around 5.5 MPam^1/2).     

Densification Behavior and Properties of Y2O3-Containing α-SiAlON-Based Composites

Different SiAION composites based on α'-SiAION are investigated, with respect to the phase relationships, densification behavior, and mechanical properties. The compositions are located on a phase-diagram line parallel to the Si₃N₄-Y₂O₃ 9AIN compound in the Si₃N₄-SiO₂-AlN-Al₂O₃-Y₂O₃-YN system. Analysis of the reaction sequences shows that the formation of the composites is associated with the transient appearance of Y₄A1₂O₉ (YAM), yttrium-aluminum-garnet (YAG), melilite, and a nitrogen-rich liquid phase. The small shift of compositions on the Si₃N₄-Y₂O₃-9AIN compound phase-diagram line toward the Al₂O₃-rich side offers the advantage of a higher sinterability and the removal of the melilite phase from a wide range of compositions containing α'-SiAlON and polytypes. The α'/β'-SiAlON composites show better mechanical properties in comparison to pure α'-SiAlON and composites of α'-SiAION and polytypes. A post-heat-treatment causes the crystallization of YAG as a grain-boundary phase and leads to excellent strength retention up to temperatures of 1350°C.     

Crystallization of a Rare Earth-Rich Aluminoborosilicate Glass With Varying CaO/Na2O Ratio

The effect of varying R =[CaO]/([CaO]+[Na₂O]) ratio on the crystallization of a rare earth-rich aluminoborosilicate glass (16 wt% RE₂O₃, RE=Nd or La) is investigated. The crystallization of a silicate apatite with Ca_{2+ x} RE_{8− x} (SiO₄)₆O_{2−0.5 x} composition ( x ≈0.4–0.7), is responsible for a drop of the rare earth solubility in the melt. When successive nucleation and growth stages are performed, crystallization processes change across the glass series as a consequence of glass-in-glass phase separation. An exotic phase of composition close to Ca₁₀Nd₇Si_20.75O₆₂ grows at the expense of silicate apatite.     

Sub-Solidus Synthesis and Microwave Dielectric Characterization of Plagioclase Feldspars

Na _x Ca_{1− x} Al_{2− x} Si_{2+ x} O₈ plagioclase solid solutions (0≤ x ≤1) were synthesized under sub-solidus conditions using a solid-state reaction technique. The plagioclase formation and the sintering temperature decreased with an increase in x from the anorthite (CaAl₂Si₂O₈; x =0) to the albite (NaAlSi₃O₈; x =1).
Microwave (MW) dielectric measurements revealed that slow-cooled ( P 1 ) anorthite exhibited higher Q × f values than fast-cooled ( I 1 ) anorthite. Slow cooling also considerably improved the Q × f values of the sodium-rich Na _x Ca_{1− x} Al_{2− x} Si_{2+ x} O₈ solid solutions (0.8≤ x ≤1), where the highest Q × f value of 17 600 GHz was obtained for slow-cooled Na_0.8Ca_0.2Al_1.2Si_2.8O₈. The temperature coefficient of resonant frequency (τ_f) approached zero for 0.8≤ x ≤1.     

Oxidation Behavior of Rare-Earth Disilicate–Silicon Nitride Ceramics

The oxidation behavior and microstructure of the oxidized surfaces of RE₂Si₂O₇–Si₃N₄ ceramics were investigated. The high oxidation resistance of these materials at 1400°C is attributed to the minimization of amorphous phases via devitrification to disilicates that are in equilibrium with SiO₂, the oxidation product of Si₃N₄. Crystals of RE₂Si₂O₇ grew out of the surface silicate in prefered orientations that were dictated by crystal structure. The morphology of the microstructure of the oxidized surfaces was shown to be partially dependent on the concentration of impurities; the presence of Ca was found to coincide with the growth of Gd₂Si₂O₇ and Dy₂Si₂O₇ crystals with high aspect ratios.     

Homogeneity Region and Thermal Stability of Neodymium-Doped α-Sialon Ceramics

Dense sialon ceramics along the tie line between Si₃N₄ and Nd₂O₃·9AlN were prepared by hot-pressing at 1800°C. The materials were subsequently heat-treated in the temperature range 1300–1750°C and cooled either by turning off the furnace (yielding a cooling rate (T_cool) of ∼50°C/min) or quenching (T_cool≥ 400°C/min). It was found necessary to use the quenching technique to reveal the true phase relationships at high temperature, and it was established that single-phase α-sialon forms for 0.30 x 0. 51 in the formula Nd_xSi_{12–4S x} Al_{4.5 x} O_{1. 5 x} , N_{16–1.5 x} . The α-sialon is stable only at temperatures above 1650°C, and it transforms at lower temperatures by two slightly different diffusion-controlled processes. Firstly, an α-sialon phase with lower Nd content is formed together with an Al-containing Nd-melilite phase, and upon prolonged heat treatment thus-formed α-sialon decomposes to the more stable β-sialon and either the melilite phase or a new phase of the composition NdAl(Si_6-zAl_z)N_10-zO_z. Nd-doped α-sialon ceramics containing no crystalline intergranular phase show very high hardness (HV10 = 22. 5 GPa) and a fracture toughness ( K _lc= 4.4 MPa·m^1/2) at room temperature. The presence of the melilite phase, which easily formed when slow cooling rates were applied or by post-heat-treatment, reduced both the fracture toughness and hardness of the materials.     

Phase Relations in the Ternary Systems Nd─B─N, Sm─B─N, and Gd─B─N

Phase relations and phase stabilities have been derived for the ternary systems RE─B─N (RE = Nd, Sm, or Gd) at elevated temperatures (1400°C and above) by means of X-ray powder analysis. Under the experimental conditions selected, various ternary compounds are found to be stable: Nd₃B₂N₄ with the Ce₃B₂N₄ type and (Nd,Sm,Gd)BN₂ with the PrBN₂ type. Phase equilibria at 1400°C and under 10⁵ Pa of argon are mainly characterized by the incompatibility of the RE metals Nd, Sm, and Gd with BN due to the competing equilibria between the RE tetraborides and the RE mononitrides. Each of the ternary compounds, however, is found to be in a two-phase equilibrium with hex -BN. Because of the different thermodynamic stabilities within the various structure series of ternary rare-earth boron nitrides RE₃B₂N₄ and REBN₂, the compound Nd₃B₂N₄ is observed only at temperatures below 1800°C and under 10⁵ Pa of Ar, whereas GdBN₂ is found to be stable only at temperatures above 1400°C under a partial pressure of 10⁵ Pa of N₂.