α-SiAlON Crystals Research Articles

Hot-pressed sintered Ca-α-Sialon ceramics with grains from short prismatic to elongated morphology synthesized via carbothermal reduction and nitridation

Ca-α-Sialon powders was initially synthesized by carbothermal reduction-nitridation (CRN) of SiO2, AlN, CaF2 and carbon black at 1700 °C, subsequently was used as the raw material to prepare Ca-α-Sialon ceramic by hot-pressed (HP) sintering at 1900 °C. The main intermediate nitridated products were Si2N2O and O-Sialon formed at 1400 °C–1600 °C. The resultant Ca-α-Sialon just had hexagonally short prismatic structure at 1700 °C. In order to obtain its elongated crystals, α-Sialon seeding was introduced during CRN process. The results revealed that the seeding facilitated crystal refinement of Ca-α-Sialon and the increase in its aspect ratios from 2 to 5.45. The resultant HP-sintered Ca-α-Sialon ceramic with seeding at 1900 °C had compact structure with fine and elongated grains, forming a highly interlocked microstructure. They had a Vickers indentation hardness of ∼21 GPa independent of the microstructure. The flexural strength and fracture toughness of the Ca-α-Sialon ceramics drastically increased with increasing the amounts of seeds, which was in accordance with that in grains aspect ratio of α-Sialon crystals synthesized by CRN as function of amounts of seeds. The improved flexural strength and fracture toughness should be ascribed to the development of elongated α-Sialon grains, which could enhance crack deflection and grain pullout.

Synthesis of Ca-α-SiAlON phosphors by a mechanochemical activation route

Fine Eu2+-doped Ca-α-SiAlON phosphors have been synthesized by a novel mechanochemical activation route from stoichiometric mixtures of β-Si3N4, AlN, Al2O3, CaO and Eu2O3. Mechanical milling mostly transformed the starting powder mixture into an amorphous phase that contained a large number of well-dispersed nanocrystalline β-Si3N4 grains. Homogeneity of elements at the atomic level was achieved in amorphous phase. The formation mechanism of α-SiAlON grains in unmilled powders was through a solution–diffusion–reprecipitation process, leading to an inhomogeneous distribution of Eu2+ ions in the host phase. Mechanical activation significantly promoted the reaction among the starting powders during post-annealing, and α-SiAlON grains composed of nanocrystals with the intended composition formed directly through rapid precipitation from the homogeneous metastable amorphous phase. This novel thermodynamically unfavorable approach leads to the homogeneous distribution of Eu2+ in α-SiAlON crystals resulting in improved photoluminescence. The mechanochemical activation route is expected to be applicable to the synthesis of a wide range of nanosized (oxo)nitridosilicate phosphors with improved homogeneity and photoluminescence efficiency.

Optimization of α-SiAlON Microstructure by Heat Treatment

Nd-doped α-SiAlON starting composition (Nd0.33Si9.38Al2.62O1.62N14.38) was prepared by gas pressure sintering at 1825°C for 3 hrs. In order to explore the effect of post heat treatment on the developments of elongated α-SiAlON grains, sample was heat treated at 1800°C for 4-12 hrs. It was found that post heat treatments promoted formation of the elongated α-SiAlON grains. The controlling mechanism of grain growth was determined via plotting on a graph the growth in width/length versus time graphics using Image Analysis method. Different growth rates were found between the length and width direction of the α-SiAlON crystals, resulting in anisotropic grain growth in the microstructural development.

Preparation of (Ca+M) (M=Mg, Yb, Sr) Co-Doped α-SiAlON Powders by Combustion Synthesis

In this paper, α-SiAlON powders co-doped with (Ca+M) (M=Mg, Yb, Sr) were prepared by combustion synthesis. The effect of Ca incorporation on the phase composition and grain morphology of reaction products was discussed. The experimental results showed that Ca incorporation promoted the growth of rod-like α-SiAlON crystals in (Ca+Mg) and (Ca+Yb) systems. For (Ca+Sr) system, the addition of Ca assisted Sr into α-SiAlON crystal lattice and increased the relative content of α-SiAlON in reaction product.

Phase transformation and growth of rod-like α-SiAlON particles during combustion synthesis

Yttrium- and ytterbium-stabilized α-SiAlON powders consisting of rod-like microcrystals were prepared by combustion synthesis. A quenching method was used to investigate the phase formation and microstructure evolution during the combustion synthesis of α-SiAlON. Three reaction stages were proposed for the formation of α-SiAlON as follows: nitridation of metal powders, formation of the co-existing liquid phase, and precipitation of α-SiAlON with the dissolution of nitrides. Different nucleation and growth modes of rod-like α-SiAlON crystals were observed: crystallization from liquid phase, or nucleation and growth based on the surface of as-existed large crystals.

Fabrication of (Ca + Yb)- and (Ca + Sr)-stabilized α-SiAlON by combustion synthesis

In this paper, (Ca + Yb)- and (Ca + Sr)-stabilized α-SiAlON powders were fabricated by combustion synthesis. The influence of Ca 2+ incorporation on the phase composition and grain morphology of combustion products was discussed. The experimental results showed that with the incorporation of Ca 2+ well-developed rod-like (Ca + Yb) α-SiAlON crystals could be produced. It was also found that, when only Sr 2+ was used as stabilizing cation, the reaction product was (α + β)-SiAlON composite, in which β-SiAlON was the predominant phase and the relative content of α-SiAlON was low. With the incorporation of Ca 2+, however, both the relative content and the lattice parameters of α-SiAlON were clearly increased. These results indicated that the incorporation of Ca 2+ could assist Sr 2+ into the α-SiAlON lattice structure.

Preparation of Ca α-SiAlON powders with rod-like crystals by combustion synthesis

Ca α-SiAlON powders with rod-like crystals were prepared by combustion synthesis from different starting compositions. The growth mechanism of the rod-like Ca α-SiAlON crystals was investigated and two growth modes were observed. One is liquid phase precipitation, and the other is nucleation and growth on smooth surfaces of the as-developed large crystals. During the development of the rod-like Ca α-SiAlON crystals, anisotropic growth occurred with the [0 0 1] direction being the fast growth direction.

Study on the anisotropic growth of rod-like Yb α-SiAlON crystals prepared by combustion synthesis

In this paper, single-phase Yb α-SiAlON powder with rod-like crystals was fabricated by combustion synthesis. The microstructural development of the rod-like α-SiAlON crystals was studied, and anisotropic grain growth was observed. Furthermore, the growth mechanism of the rod-like α-SiAlON crystals was investigated, and the experimental results indicated that during the anisotropic grain growth, the fast growth plane was the basal plane in the hexagonal crystal lattice.

Effect of Nitrogen Pressure and Additives on Combustion Synthesis of Rod-Like Yttrium α-SiAlON Crystals

Effect of Nitrogen Pressure and Additives on Combustion Synthesis of Rod-Like Yttrium α-SiAlON Crystals

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

Two calcium‐doped α‐SiAlON compositions (Ca0.6Si10.2Al1.8−O0.6N15.4 and Ca1.8Si6.6Al5.4O1.8N14.2) 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.

Toughening Y α-Sialon Ceramics by Using New-Developed Rod-Like Y α-Sialon Crystals as Reinforcing Agents

Toughening Y α-Sialon Ceramics by Using New-Developed Rod-Like Y α-Sialon Crystals as Reinforcing Agents

Microstructure and formation mechanism of combustion-synthesized rodlike Ca α-sialon crystals

The fabrication of rodlike Ca α-sialon crystals through a combustion synthesis process is reported in this paper. The main morphological features in the product included rodlike crystals, two dimensional elongated platelets, and equiaxed particles. By proper adjustment of the combustion parameters, a high productivity of rodlike Ca α-sialon crystals in the final product could be achieved. The combustion reaction mechanism and the relationship between the combustion parameters and the particle morphology were investigated.

Valence evaluation of cerium ions in Ce-doped α-sialon by electron energy loss spectra

For a long time cerium ions, both Ce3+ and Ce4+, have not been considered to be able to enter the interstitial sites in an α-sialon structure until recent successes in the preparation of cerium-doped α-sialon materials using CeO2 as the starting powder. It is then of great importance and necessity to know the chemical valence of these incorporated cerium ions. Meanwhile, the specific structure of the interstices in α-sialon provides quite a different chemical environment upon which the electronic structures of cerium ions could be different from those ever reported in, for instance, metallic compounds and oxides. Electron energy loss spectra (EELS) in the M-edge region of rare-earth elements carries information on the initial state 4f occupancy. We have acquired and examined the M4,5-edge spectra of cerium at different locations in the microstructure. It has been found that all the Ce(IV) ions in the intergranular glass and the majority inside the α-sialon crystals have been reduced to Ce(III) while there exist inside the crystal cerium ions showing 4f0 initial-state peaks on the EELS spectrum. The 4f0 peak locates at about 0.6 eV higher in energy than that for CeO2. An increased and possibly by far the highest weight of f0 state is derived for Ce(IV) ions in α-sialon. The spectral evidences confirm the general suggestion of the unstability of Ce(IV) ions with silicon nitride.