Ca Α-sialon Research Articles

Combustion synthesis and luminescence properties of Eu-doped Ca–α-SiAlON

Investigated were the combustion synthesis of Eu-doped Ca–α-SiAlONs and their luminescence properties. Experimentally established were (a) green compositions ensuring maximal extent of conversion, (b) influence of elemental composition on phase composition and microstructure of synthesized Ca–α-SiAlONs, and (c) influence of elemental composition on the excitation and emission spectra of Ca–α-SiAlON luminophors.

Phase transformation of high aluminium fly ash in carbothermal reduction–nitridation at high temperatures

Sialons (Si5AlON7, Si4Al2O2N6, Si3Al3O3N5 and SiAl4O2N4) were synthesised from high aluminium fly ash by using a carbothermal reduction–nitridation process. The effects of temperature and the addition of carbon black on the phase compositions and morphologies have been investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Thermodynamic analysis has been used to study phase transformations. For the sample with the stoichiometric carbon black content, the product formed at 1500°C was single phase short prismatic Si3Al3O3N5. The products at 1600°C were Ca–α-Sialon accompanied by AlN, whereas at 1700°C, Si4Al2O2N6 and SiAl4O2N4 formed. Increasing the carbon black at 1500°C, Si3Al3O3N5, β-SiC, 15R and FexSi were produced as reaction products.

Photoluminescence properties of Ca–α-SiAlON:Ce3+ phosphors as function of composition and microstructure

Abstract Cerium-doped Ca–α-SiAlON phosphors with Can−1.5xSi12−m−nAlm+nOnN16−n:Cex (m=2n, 0.8≤m≤2.4, and 0.02≤x≤0.40) nominal compositions have been synthesized by gas pressure sintering (GPS). The photoluminescence (PL) properties were studied in relation to phase composition, particle morphology, and microstructure. Continuous addition of Ce rare-earth (RE) could enhance the emission intensity of phosphors, which was partially attributed to the formation of increased amount of planar defects where dense doping of Ce ions is achieved. Such enhancement could hold up to x=0.32, above which the emission intensity fell slightly due to the formation of more secondary phases and a surface glassy phase rather than concentration quenching. Finally, possible ways to improve the luminescence properties of Ce-doped α-SiAlON phosphors were discussed through tailoring the composition and microstructure.

Preparation of Ca-α SiAlON powder by reduction-nitridation in a gas mixture of C<sub>3</sub>H<sub>8</sub> and NH<sub>3</sub>

Ca-α SiAlON powders were synthesized from a SiO2–Al2O3–CaCO3 powder mixture by reduction-nitridation with C3H8–NH3 gas. Ca-α SiAlON with small amounts of AlN and β-SiAlON phases was produced at 1450°C for 120 min. Angular particles and agglomerated and bonded particles with irregular shapes were formed. In the product prepared at 1450°C without any holding time, Si2N2O was detected as an intermediate phase, in addition to the Ca-α SiAlON, β-SiAlON and AlN phases. A large number of agglomerated and bonded particles were observed.

High-Temperature Mechanical Spectroscopy of Nitrogen-Rich Ca-α-SiAlON Ceramics

Nitrogen-rich Ca–α-SiAlON ceramics made with different starting powder compositions have been studied by high-temperature mechanical spectroscopy in parallel with compressive deformation in a spark plasma sintering equipment. The mechanical loss spectra measured upon heating show a relaxation peak at about 1150 K and a high-temperature exponential background at higher temperatures (>1400 K), which are attributed to the α-relaxation in the glassy phase and to grain-boundary sliding, respectively. A theoretical interpretation of the results shows that the peak position is mainly a function of glass viscosity. The amplitude of the peak is not only affected by the glassy phase quantity but also depends on the restoring force due to grain elasticity. Therefore, despite a higher amount of glassy phase specimens containing elongated grains may show a lower peak. The amplitude of the internal friction peak corresponding to α-relaxation can be used to predict the compression creep of silicon-nitride-based ceramics.

Preparation of Ca-α SiAlON hollow spheres by carbothermal reduction–nitridation of CaO–Al 2O 3–SiO 2 glass

Ca-α SiAlON powders were prepared by carbothermal reduction–nitridation of CaO–Al 2O 3–SiO 2 glasses at 1450 °C for 2 h in nitrogen. The content of Ca-α SiAlON phase was 65–81% in the products, and the other phases were AlN and β-SiAlON. The products contained hollow spheres with the size of around 5 μm in diameter. The particle morphology was almost identical to that of Ca-α SiAlON prepared from CaCO 3–Al 2O 3–SiO 2 powder mixtures.

Ca-α SiAlON hollow spheres prepared by carbothermal reduction–nitridation from different SiO 2 powders

The formation process of hollow spheres composed of nanosized Ca-α SiAlON particles was investigated using SiO 2 starting powders with different characteristics in particle size, shape and crystalline state. TEM observations showed Ca-α SiAlON hollow spheres composed of a large number of nanosized particles in the products prepared at 1450 °C for 120 min in nitrogen. In all systems, the Ca-α SiAlON hollow spheres were always produced through an intermediate Si–Al–Ca–O liquid phase in the same mechanism, regardless of the characteristics of SiO 2 starting powders used. Spherical solid particles consisted of amorphous phase containing Si, Al, Ca, O and a small amount of N were generated at the initial stage of carbothermal reduction–nitridation. These spherical solid particles changed into hollow particles with the progression of the reaction from the liquid phase to the crystalline Ca-α SiAlON with increasing temperature.

Effects of starting composition and carbon content on the formation of Ca-.ALPHA. SiAlON powders by carbothermal reduction-nitridation

Ca-α SiAlON powders were prepared at different starting compositions and carbon contents at 1450°C for 2 h in N2 from mixtures of SiO2, Al2O3, CaCO3 and carbon powders by carbothermal reduction–nitridation. Crystalline phases and particle morphology of the products were investigated. With increasing the x value in nominal Ca-α SiAlON compositions CaxSi12−3xAl3xOxN16−x, the number of small hollow spheres decreased and the size of primary grains consisting of the hollow spheres increased. The addition of carbon powder to more than 1.2 times of the necessary stoichiometric value was required to form Ca-α SiAlON hollow spheres composed of ultrafine grains.

High-Temperature Compressive Deformation of SiAlON Polycrystals Prepared without Additives

The high-temperature deformation of dense nano-sized Ca-α SiAlON and submicron- sized β-SiAlON polycrystals were investigated by compression tests. These polycrystals were fabricated from Ca-α SiAlON and β-SiAlON nano powders respectively, and contained almost no intergranular glass phase. Both SiAlONs exhibited large plastic deformations with the stress exponent n ~1 in the higher stress region, and n ~2 in the lower stress region. Shear-thickening creep (n < 1), which has been reported in liquid contained SiAlONs, was not observed.

Formation Process of Calcium-α SiAlON Hollow Balls Composed of Nanosized Particles by Carbothermal Reduction–Nitridation

Carbothermal reduction–nitridation (CRN) of SiO2–Al2O3–CaO powders was performed under various firing conditions to investigate the formation process of Ca-α sialon hollow balls composed of nanosized particles. Scanning electron microscopy and transmission electron microscopy observations of the samples obtained at different firing temperatures confirmed that solid spherical particles were formed at the early stage of the reaction, and nanosized particles were subsequently produced on the surface of these solid balls. From X-ray diffraction and energy-dispersive spectrometry analyses, it was found that the solid balls initially formed at 1450°C were mainly amorphous and contained Si, Al, Ca, O, and a small amount of N. Further nitridation at 1450°C gradually converted the solid balls into Ca-α sialon hollow balls over time. The results revealed that the formation of Ca-α sialon hollow balls depends on the formation of solid balls from the Si–Al–Ca–O liquid phase at the initial stage of the CRN process.

Shape deformation and texture development of consolidated Ca α-SiAlON ceramics prepared by hot-forging

Consolidated Ca α-SiAlON ceramics with gradually varying microstructure and property was prepared by hot-forging. The shape deformation as well as texture development of the sample during forging was studied in detail. It was found that the forging process promoted the growth of elongated α-SiAlON grains and enhanced their preferred orientation with the longitude perpendicular to the pressing force. The strong texture offered the hot-forged sample an increased fracture toughness of 7.9 MPa m 1/2, which was primarily attributed to the pull-out and debonding behaviors of elongated grains.

High-temperature deformation of α-SiAlON nanoceramics without additives

A Ca-α SiAlON nanoceramic was fabricated from Ca0.8Si9.6Al2.4O0.8N15.2 nanopowder without using any sintering additive. The flow stress of the SiAlON nanoceramic, which contained almost no glass phase, was higher than that of liquid phase sintered SiAlONs containing a large amount of glass phase. The deformation was characterized by Newtonian flow in a wide range of stresses, and shear-thickening creep, which has been reported in liquid containing SiAlONs, was not observed.

Effect of Microstructure on Sliding Wear of Ca α-SiAlON Ceramics

The wear behaviour of Ca a-sialon ceramics of two distinct microstructures, fine equiaxed grains (EQ) and large elongated grains (EL), with the same chemical composition was investigated as a function of apparent contact pressure and sliding speed, using ball-on-disc type tribometers at room temperature and at 600°C. For room temperature tests, the EL microstructure exhibited a lower wear rate than EQ in the severe wear regime due to a greater resistance to large crack-induced material removal. As the apparent contact pressure decreased, mild wear appeared for both microstructures. The mechanism that dominated the material removal in EQ was grain pullout. In contrast, the controlling mechanism for EL was transgranular fracture. Therefore, EL had a lower wear rate than EQ in the mild wear regime. For wear tests at 600°C, crack-induced severe wear occurs in both EQ and EL samples for all contact pressures. EL had a slightly lower wear rate than EQ. Wear particles were generated on the wear track, but no tribofilm was observed and no oxidation products were detected. Wear models revealed that the grain aspect ratio plays a more important role than grain diameter in influencing the crack propagation during severe wear and grain pull-out during mild wear.

Fabrication of one-dimensional rod-like α-SiAlON powders in large scales by combustion synthesis

α-SiAlON powders consisting of one-dimensional rod-like microcrystals were prepared by combustion synthesis in large scales. No obvious negative effect on reaction products was noticed by the increase of sample size, and α-SiAlON was synthesized as predominant phase in all the samples, even in CS-CL with the weight of 1000 g. These results showed good repeatability of combustion synthesis and its great potential in industrial production of α-SiAlON powders in large quantities. According to the phase composition of intermediate product, the reaction procedure in combustion synthesis of Ca α-SiAlON was proposed. At the same time, the strong influence of different starting compositions on the grain morphology of Yb α-SiAlON products was explained with the consideration of reaction temperature and grain growth character.

Synthesis of Ca-.ALPHA. SiAlON using Zeolite as a Raw Material

Ca-α SiAlON powders were synthesized by carbothermal reduction nitridation (CRN) of zeolite (Ca-zeolite Y). First, the Ca-zeolite Y was converted into an amorphous phase below 1100°C, and then into Ca-α SiAlON via mullite, Si2N2O and (Ca, Si, Al)xOy liquid at 1450°C. In converting zeolite into Ca-α SiAlON, it was found that Ca-α SiAlON particles were of a similar size to that of the raw material, indicating that it is possible to control the particle size of Ca-α SiAlON using zeolite as a raw material.

Highly efficient white-light-emitting diodes fabricated with short-wavelength yellow oxynitride phosphors

We have already reported orangish yellow Ca–α-SiAlON:Eu2+ phosphors, and applied them to fabricate warm white light-emitting diodes (LEDs). In this letter, we report on greenish yellow Li–α-SiAlON:Eu2+ phosphors, and use them to create daylight when coupled to an InGaN blue LED chip (460nm). The newly discovered Li–α-SiAlON:Eu2+ phosphors emit at shorter wavelengths of 573–577nm under the 460nm excitation, and exhibit a smaller Stokes shift than Ca–α-SiAlON:Eu2+ does. By using this short-wavelength yellow oxynitride phosphor, bright daylight emissions from white LEDs can be generated. Thus, highly efficient white LEDs with tunable white light can be fabricated with α-SiAlON:Eu2+ phosphors, enabling them for a wider range of applications.

Sliding wear behaviour of Ca α-sialon ceramics at 600 °C in air

As an extension of a previous investigation on the wear behaviour of Ca α-sialon ceramics of differing microstructures at room temperature, wear testing was conducted at 600 °C in air to explore the effects of microstructure, contact pressure and sliding speed on the wear behaviour. Under all loading conditions from 1 MPa to 1 GPa, a constant high friction coefficient was observed and a severe wear process was dominant, in which the sliding contact induced cracks were observed in different microstructures. Wear particles were generated along the wear track, but no tribofilm was detected. Increasing the sliding speed from 10 to 23 cm/s was found to significantly increase wear rate. However, variations in microstructure had little impact. That is, large elongated-grained α-sialon exhibited only a slightly lower wear rate than fine equiaxed-grained α-sialon.

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.

Sliding wear of calcium α-sialon ceramics

The sliding wear behaviour of calcium α-sialon ceramics having two distinct microstructures, large elongated-grained and fine equiaxed-grained was revealed by observing the evolution of wear rate, wear track and debris as a function of apparent contact pressure and sliding speed, using a ball-on-disc type wear tester. The thresholds for the transition from mild to severe wear in terms of apparent contact pressure and the controlling material removal mechanisms were identified for both microstructures. The large elongated-grained microstructure had a higher wear transition threshold and exhibited a reduced wear rate in the severe wear regime, compared to the fine equiaxed-grained microstructure. This behaviour is attributed to a greater resistance to crack extension arising from the large elongated-grained microstructure. As the apparent contact pressure decreased, mild wear occurred at pressures below the thresholds for each of the microstructures. The material removal for the large elongated-grained microstructure was controlled by transgranular fracture, whereas grain pull-out prevailed in the fine equiaxed-grained microstructure. Consequently, a greater wear rate results for the fine equiaxed-grained microstructure in mild wear regime. An increase in sliding speed caused a slight increase in wear rate for both microstructures, more evidently in the mild wear regime. Models were developed to clarify the effect of microstructure on severe and mild wear of Ca α-sialon. They revealed that grain aspect ratio plays a more important role than grain diameter in the control of wear behaviour.

Application of Focused Ion Beam Miller in Fracture Characterization

Abstract: Focused ion beam (FIB) miller is a new tool used for the examination of micro-fracture in ceramics. In this work, a FIB miller was employed to investigate fracture events involved in indentation and scratch tests of two differentα-sialon microstructures of the same chemical composition. The subsurface cross-sections were prepared via FIB milling and crack characteristics in both the surface and subsurface were identified. Crack interactions and the effect of microstructure on cracking were revealed. Keywords: Focused ion beam (FIB) miller, microstructure, fracture, indentation Introduction FIB millers are commonly found in the semiconductor industry, where they are used to deposit, etch, mill and image specimens during circuit modification and defect analysis. However, the FIB is becoming increasingly popular in studying fracture behaviour of ceramics, particularly for analysing subsurface damage following surface contact [1, 2]. The FIB uses a liquid metal ion source to emit gallium ions in a high vacuum environment. These particles are accelerated by an energy of between, typically, 5 and 50 keV, forming a fine (~10 nm diameter) energetic beam of gallium ions, which is then focusedonto the specimen surface by electrostatic lenses. When the ions impact and/or implant into the specimen, secondary electrons, secondary ions and atoms are sputtered from the surface. If the beam current is large (~ 10nA), through the insertion of a large beam-limiting aperture, the sputtering rate is very high and sections can be milled rapidly into the surface of the specimen. This is achieved by using the system software to draw a “mill-box” on the area of interest, so that milling occurs locally around specific microstructural features. After milling the beam current can then be reduced to a lower level (~ 10pA), by the insertion of a smaller aperture, and either the positive secondary ions or secondary electrons can be detected and used to form images similar to those acquired by a SEM. By using this method, subsurface features can be rapidly and precisely imaged. However, the FIB can be used in other ways, for example rastering the ion beam over the surface of specimen, at intermediate beam currents (~350 pA) will lead to localised surface etching and the generation of high contrast from multiphase materials. In this way, the relationship between surface features, such as cracks, and microstructure, can be readily assessed [2]. These features of the FIB can be utilized for the characterization of cracks in ceramics. In the past, cracks induced in ceramics by either indentation [3] or scratching [4] were identified typically from direct observation on the sample surface using either optical microscopy or SEM. When asubsurface cracks became critical, either ceramographic polishing [5] or a bonded-interface technique [6] was used for the preparation of subsurface cross-sections. It is noted that both techniques may introduce damage into the original cracks and consequently affect the reliability of identification. Furthermore, the two techniques could not reveal the interaction of microstructure and cracks, which is vital to fracture analysis and design of tougher ceramics. However, using the FIB, the subsurface cross-sections of ceramics can be prepared rapidly with minimal damage. Further, the area of interest can be etched to reveal the effect of microstructure on cracking. In this study,Ca α-sialon ceramic was used as a model material. he FIB was first sed toT u characterize the indentation-induced cracks in the samples, and underscore the effect of