Composition-dependent properties of Ca-α-SiAlON:Eu2+ phosphors prepared by combustion synthesis

Herein, we put forward a simple combustion synthesis strategy for the highly efficient preparation of Eu-doped Ca-α-SiAlON yellow phosphors with different composition of Ca_(m/2−x)Eu_xSi_12−m-nAl_m+nO_nN_16−n. The as-synthesized phosphors were endowed with outstanding photoluminescence behavior of yellow emission peaking at ~580 nm under excitation of near-ultraviolet (UV) or blue light. In the designed experiments, products with different compositions, which were determined by the varying m and n values, were obtained by adjusting the proportion of starting reactants. Further, the dependence of composition on the overall properties of products was systematically studied. It was found that the m and n values could have distinct impact on the phase composition, microstructure and photoluminescence properties of as-synthesized phosphors. The most prominent enhancement of spectral intensity was achieved in the sample with composition of m=1.5 and n=0.8 in this research system. The resultant Ca-α-SiAlON:Eu²⁺ phosphors were simultaneously featured with high purity, favorable uniformity and equiaxial morphology. A continuous red shift phenomenon in emission wavelength with the increase of m value and an inverse blue shift with the increase of n value were both observed and rationally uncovered.     

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.     

Oxidation behavior of β-SiAlON powders fabricated by combustion synthesis

The present study presents the effect of addition of diluents on the crystal morphologies and the impact on the oxidation resistance of β-SiAlON powders obtained from combustion synthesis method. Pure β-SiAlON were synthesized with diluents. It was observed by SEM that the diluents resulted in the transformation from large hexagonal column-shaped crystals to fine isotropic grains. Oxidation experiments were conducted by thermogravimetric analysis (TGA) in the temperature range from 1200 °C to 1400 °C. The TGA results indicated that oxidation was controlled by mixed chemical reaction and diffusion process. The chemical reaction step was found to be rate-controlling at low oxidation temperatures (1200 °C and 1300 °C), while at high temperature (1400 °C), diffusion was found to control the reaction rate. Oxidation products were investigated by X-ray Diffraction (XRD) analysis and found to be made up of SiO₂, Al₂O₃. This research aims to provide guidance for the fabrication of β-SiAlON by combustion synthesis, thus facilitating its further application in high temperature industry.     

Optical and adhesive properties of composite silica-impregnated Ca-α-SiAlON:Eu2+ phosphor films prepared on silica glass substrates

The compaction of a Eu-doped Ca-α-SiAlON phosphor powder was performed by electrophoretic deposition (EPD). The effect on the adhesion and optical properties of the silica precursor as both a binder of the powder and a filler of the air voids were evaluated. The adhesion of the silica impregnated composite film to the silica glass substrate was characterized by the tape test. The improvement of the external quantum efficiency was confirmed from the PL spectra measurement after the silica impregnation. The temperature dependence of the external quantum efficiency was also investigated in order to discuss the advantage of using the silica precursor as a binder for high-brightness LED applications.     

Aqueous processing of carbothermally prepared Ca-α-SiAlON and β-SiAlON Powders: powder and suspension characterisation

Properties of carbothermally prepared Ca-α-SiAlON and β-SiAlON powders and aqueous suspensions thereof were determined. The isoelectric points of Ca-α-SiAlON and β-SiAlON were 3·4 and 4·6. After addition of deflocculant, Dolapix CE64, the behaviour of both suspensions is nearly identical. The isoelectric points become 5·5 and 5·3, respectively. Despite differences in bulk composition, grain size distribution, grain size and shape, both SiAlON suspensions show a similar dependence of a zeta potential on pH. Optimum slip casting properties, i.e. lowest viscosity values (below 10 mPa s), the highest absolute zeta potential values, the smallest floc size and sediment volume were found between pH 10–11 for both powders. The potentials of the different suspension characterisation techniques were compared and zeta potential and viscosity measurements were found the most convenient.     

Effects of excess NaClO4 on phases,size and magnetic properties of Ni–Zn ferrite powders prepared by combustion synthesis

Ni_0.4Zn_0.6Fe₂O₄ powders were prepared by combustion synthesis with different amount of NaClO₄. Phases, particle size and magnetic properties of the powders and annealed powders were systematically investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive spectrometer (EDS) and vibrating sample magnetism (VSM). The excess content of NaClO₄ offered significant advantages with respect to the size, morphology and magnetic properties of the powders. After annealing, sub-micro ferrite spherical powders with spinel phase in a range of 500–800 nm can be obtained. With the increase of the NaClO₄ content, the saturation magnetization of the powders shows a maximum value at 68.8 emu/g when w=0.4, whereas the coercivity stayed nearly constant. The maximum saturation of annealed powders by combustion synthesis is much higher than the range reported in the literature.     

Synthesis of single phase Ca-α-SiAlON using Y-type zeolite

Single phase Ca-α-SiAlON was synthesized for the first time by the carbothermal reduction-nitridation of a physical mixture of Y-type zeolite and CaO. Under well-optimized conditions, which included both a Si/Al ratio of 2.8 for the starting Y-type zeolite and a Ca/Al ratio of 0.63–0.75, highly crystalline, pure Ca-α-SiAlON was obtained. The formation of pure Ca-α-SiAlON was also confirmed by ²⁷Al and ²⁹Si MAS NMR measurements.     

Effect of functionalised core–shell microgels prepared by emulsion polymerisation on acrylic coatings properties

The influence of hydroxyl-functionalised acrylic core–shell microgels incorporated into a solvent-borne acrylic binder system on the properties of coatings is described in this paper. Our approach has been to show the usefulness of prepared functionalised microgels as coating modifiers. This subject was shown to be connected with the selection of an appropriate solvent with good affinity for microgels and the film-forming polymer. Therefore the swelling behaviour of microgels in selected solvents as a function of microgel composition is discussed as well. The structured microgels were prepared by first making an aqueous emulsion via the semi-batch emulsion copolymerisation, then dehydrating the system by air drying followed by grinding in a mill. The resulting agglomerates of spherical microgel particles were dispersed in convenient organic solvent media and after that added to the thermosetting solvent-borne acrylic binder system. It was shown that the application of core–shell microgels did not affect the surface appearance and transparency of coatings. Moreover, the presence of microgel network precursors improved corrosion resistance of coatings.     

Effect of annealing temperature on structural and optoelectronic properties of γ-CuI thin films prepared by the thermal evaporation method

High quality transparent conducting CuI thin films were deposited at room temperature via thermal evaporation technique followed by post deposition annealing at different temperatures. The samples were characterised by X-ray diffraction (XRD), UV–Vis spectrophotometry, Scanning electron microscopy and I-V measurements. The structural, morphological and optical properties were studied as a function of the annealing temperature from room temperature (RT) to 200 °C. XRD results revealed that the films were polycrystalline with zinc blende structure of cubic phase. Increasing the annealing temperature increased the crystallite size from 33 to 49 nm whilst the dislocation density and lattice strain shifted to lower values. High transmittance of about 70–80% was exhibited by all films in the entire visible spectral range. The as deposited film possesed the lowest resistivity of 3.0×10⁻³ Ω cm.     

Coarse-grained β-Si3N4 powders prepared by combustion synthesis

Coarse-grained β-Si₃ N₄ powders were prepared by combustion synthesis under N₂ pressure of 6 MPa, with a low diluent content of not more than 10 wt.% and high reaction temperature of >1900°C. β-Si₃ N₄ was obtained as the major phase in the products, except for a small amount of residual Si. The addition of carbon black was effective to reduce the residual Si, but resulted in the formation of β-SiC when too much carbon black was used. The coarse-grained β-Si₃ N₄ powders consisted of β-Si₃ N₄ crystals with an average thickness of more than 10 µm, and some crystals were thicker than 20 µm. The growth mechanism of the coarse β-Si₃ N₄ crystals was discussed, associated with the particular reaction conditions in combustion synthesis.     

Effect of nano- and micro-sized Si3N4 powder on phase formation,microstructure and properties of β′-SiAlON prepared by spark plasma sintering

The phase formation behavior of β′-SiAlON with the general formula Si_6-zAl_zO_zN_8-z was studied comprehensively for z values from 1 to 3 using spark plasma sintering (SPS) as the consolidation technique at synthesis temperatures from 1400 to 1700 °C. The samples were prepared close to the β′-SiAlON composition line: Si₃N₄ ? 4/3(AlN·Al₂O₃) in the phase diagram using (A) nano-sized amorphous Si₃N₄ and (B) micro-sized β-Si₃N₄ precursors. Field-emission scanning electron microscopy (FESEM) was used for microstructural analysis.Most compositions reached almost full density at all SPS temperatures. Compared with the micro-sized β-Si₃N₄ precursor, the nano-sized amorphous Si₃N₄ precursor accelerated the reaction kinetics, promoting the formation of dense β′-SiAlON + O′-SiAlON composites after SPS at synthesis temperatures of 1400–1500 °C. This resulted in very high values of Vickers hardness (Hv₁₀) = 18.2–19.2 GPa for the z = 1 composition related to the hardness of the O′-SiAlON component phase.In general, for samples synthesized from nano-sized amorphous Si₃N₄, which were almost fully dense, containing >95% β′-SiAlON, the hardness values were 13.4–13.8 GPa with a fracture toughness of 3.5–4.6 MPa m^1/2. For equivalent samples synthesized from micro-sized β-Si₃N₄, hardness was in the range 13.9–14.4 GPa with a fracture toughness of 4.3–4.5 MPa.m^1/2. These values are comparable with fully dense β′-SiAlONs, usually containing intergranular glass phase which has been sintered by HIP and other processes at much higher temperatures for longer times.     

Carbon–silica composites prepared by the precipitation method. Effect of the synthesis parameters on textural characteristics and toluene dynamic adsorption

Three synthesis routes are presented here that leads to carbon?Csilica composites. These were characterized by nitrogen physisorption, by thermogravimetric analysis and by dynamic toluene adsorption test similar to Ashrae standard I45.1. The carbon?Csilica composites possess high microporosity and mesoporosity as well as large surface areas. Furthermore, the control of the microporosity as well as pore size distribution is possible because they depend on the amount of carbon used and of the synthesis route. Following routes I and III a wide micro-mesoporous pore size (1?C32?nm) was obtained where as by route II narrow micro-mesoporous pore size (1?C4?nm) was observed. In addition, pore diameters center in the range of 1.13?C1.17?nm was observed when carbon content was 32 or 45?wt%. The dynamic adsorption of toluene was evaluated for carbon?Csilica composites obtained by three preparation routes at two different carbon contents, 32 and 45?wt% The results showed that a composite with 45?wt% carbon content and obtained via preparation route I gave the highest toluene adsorption capacity (27.6?wt% relative to carbon content). The large uptake capacity of this composite was attributed to the presence of high microporosity volume and a wide (1?C32?nm) bimodal pore system consisting of extensive mesopore channels (2?C32?nm) as well as large surface area. These capacity values of carbon?Csilica composites are by weight relative to carbon content and are competitive to, results obtained for commercial coconut activated carbon (31.1?wt%) and significantly better than a commercial alumina-carbon composite (9.5?wt%) at 0% efficiency.     

Effect of Si content on the microstructure and properties of Ti–Si–C composite coatings prepared by reactive plasma spraying

In this study, Ti–Si–C composite coatings were synthesized via plasma spraying of agglomerated powders prepared by a spray drying/precursor pyrolysis technology using Ti, Si, and sucrose powders. The influence of Si content, ranging from 0 wt% to 24 wt%, on the microstructure, mechanical properties, and oxidation resistance of the composite coatings was investigated. Results show that the phase composition of the Ti–Si–C composite coatings changes with the increasing Si content. The coatings without Si addition consist of TiC and Ti₃O; the coatings with 6–18 wt% Si are composed of TiC, Ti₅Si₃, and Ti₃O; the coatings with Si content of 24 wt% form only TiC and Ti₅Si₃ phases. As the Si content increases, the hardness of the Ti–Si–C composite coatings increases first and then decreases, depending on the intrinsic hardness of the ceramic phases, the brittleness of Ti₅Si₃, and the defects such as pores and cracks. The Ti–Si–C composite coatings have high wear resistance due to the in-situ synthesized high-hardness TiC and Ti₅Si₃. Owing to the high brittleness of Ti₅Si₃, the increasing Si content leads to higher wear volume loss at room temperature, which can be partially improved in high-temperature wear tests. The oxidation resistance of Ti–Si–C composite coatings increases with the increase of Si content, and the higher the oxidation temperature, the more obvious the influence of the Si addition on oxidation resistance.     

Effect of a mass force on the combustion of the Ni−Al system

The effect ofg-load (in the range of 1 to 1000g) on the combustion of the Ni−Al system was studied experimentally. It is shown that a mass force has a significant effect on the burning rate of the mixture, compaction, and the chemical and phase compositions of the combustion products. A mechanism for the effect ofg-load on the burning rate and compaction of the system is proposed. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 1, pp. 34–38, January–February, 1998.     

Effect of sol–gel combustion temperature on the luminescent properties of trivalent Dy doped SrAl2O4

Trivalent dysprosium doped strontium aluminates (SrA1₂O₄:Dy³⁺) were synthesized by firing the sol–gel at 600, 700 and 800 °C. The morphology, crystal structure, photoluminescence and long afterglow of the synthesized SrAl₂O₄:Dy³⁺ phosphors were characterized with scanning electron microscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy and photoluminescence spectroscopy, respectively. It is found that SrA1₂O₄:Dy³⁺ phosphors exhibit broadband afterglows with its peak at about 510 nm. As the sol–gel synthesis temperature increases from 600 to 800 °C, the green afterglow of the SrA1₂O₄:Dy³⁺ phosphors becomes weaker in intensity and shorter in lifetime. The results are discussed in terms of thermally generated point defects in the host material.     

The effect of CaO on the physicochemical and biological properties of β-SiAlON ceramics

To investigate the effects of CaO on the physicochemical and biological properties of β-SiAlON, β-SiAlON ceramics containing CaO were prepared using the direct nitriding method. The results of X-ray powder diffraction and scanning electron microscopy demonstrated that β-SiAlON remained in the material phase because the addition of Ca did not result in the formation of Ca-α-SiAlON. However, CaO promoted the sintering of β-SiAlON grains and significantly decreased porosity and increased bulk density and compressive strength. According to a chemical stability study, when β-SiAlON ceramics powder was soaked in deionized water and a cell culture medium, it was noted to have negative electricity. A reaction occurred with the H⁺ and OH⁻ ions in the deionized water, leading to the formation of surface structures, such as Si-OH, Al-OH, and N-H. Moreover, the addition of CaO caused a different chemical reaction among ions or ion groups in the culture medium, and new chemical groups formed on the material surface that interacted with the culture medium, which resulted in an alteration of the zeta potential and surface chemical properties of β-SiAlON. MC3T3-E1 cells cultured on the surface of ceramics proved that the cells cannot adhere well to the surface of β-SiAlON ceramics with CaO, although they could proliferate well around those with only β-SiAlON. Therefore, the change in the surface chemical properties provides good anticell adhesion ability, which makes β-SiAlON with CaO a biocompatible material that can be used to prevent contamination caused by cell adhesion, with possible applications in biosensors or biomedical equipment that must be used in sterile environments.     

The effects of NbC nanoparticles synthesized by combustion method on the mechanical properties of Cu–NbC nanocomposite

In the first stage of this work, the master nanocomposite of niobium carbide (NbC)–Cu (ceramic-based nanocomposite) was synthesized by a mechanically induced magnesiothermic combustion in the Nb₂O₅/CuO/Mg/C system. Ignition time in this system was recorded to be ∼28 min of milling. In the second stage, appropriate amounts of NbC–Cu nanocomposite powder were mixed with pure copper powder to prepare Cu-based nanocomposite with 0, 5, 10, 15, and 20 volume fraction of NbC. The final metal matrix nanocomposite powder was sintered by spark plasma sintering method. The density of nanocomposite specimens decreased with increasing the percentage of NbC nanoparticles, while the microhardness of specimens increased with increasing nano-NbC content. Regarding the tensile test, the sample Cu–10 vol.% NbC nanocomposite with a strength of 372 MPa (∼63% higher than that of nonreinforced copper) was the best composition, and the nanocomposite strength decreased at higher NbC concentrations, mostly due to the agglomeration and nonhomogeneous distribution of reinforcing nanoparticles.     

Effect of particle size and shape of diluent on the reaction process of combustion synthesis of α-Si3N4 powder

α-Si₃N₄ powder was prepared by combustion synthesis using different particle sizes and shapes of Si₃N₄ diluent. The effects of different diluents on combustion temperature, phase composition, and microstructure of the product were investigated. The role of diluents in combustion synthesis is discussed. When no ammonium salt was added, because of the higher reaction temperature, the phase transformation of the fine particle diluent with the best barrier effect was also enhanced, and the α content of the product was the lowest. When the ammonium salt is added, the liquid phase Si content decreases at high temperatures, the lower reaction temperature and the Si₃N₄ generated before Si melting make the barrier effect of the diluent fully play. Finally, Si₃N₄ powder with 86% α content was synthesized by combustion with 2 μm Si₃N₄ diluent.     

Microwave sintering of mullite–cordierite precursors prepared from solution combustion synthesis

Mullite–cordierite composite was synthesized using the solution combustion synthesis method and glycine as a fuel and aluminum nitrate, magnesium nitrate and colloidal silica as the reagents. The effect of fuel to oxidizer ratio on the combustion behavior, as well as chemical composition and morphology of the formed powders was investigated. All synthesized powders were amorphous with submicron particle size. It was found that the change of fuel to oxidizer ratio had no effect on synthesis of this composite without heat treatment. The smallest particle size of composite powder was obtained as 302 nm for ratio less than 1 (rich of fuel). Mullite, cordierite and spinel were detected after microwave heating at 1200 and 1400 °C. Mullite and cordierite were detected as the only crystalline phases when the stoichiometric ratio of fuel to oxidizer was chosen and this composite obtained the highest density of 2.61 g/cm³.     

Effect of sintering conditions and heat treatment on the properties,microstructure and machining performance of α-β-SiAlON ceramics

The properties of SiAlON ceramics are strongly affected by the composition and structure of the intergranular phase, which are controlled by dopants, sintering conditions and starting silicon nitride (Si₃N₄) powder characteristics. In this study, 25α:75β SiAlON compositions were designed with different molar ratios of Y:Sm:Ca (9:0.5:0.5 and 3:6:1). The effects of cation ratios, different cooling profiles (50 °C/min and 5 °C/min) and further heat treatment under different conditions (at 1600 °C for 2, 4 and 6 h) on the final phase composition, the type of the intergranular phase (amorphous or crystalline) formation, the resulting microstructures and the machining performance were studied. It is found that slow cooling and heat treatment have a great influence on crystallisation behaviour and in turn the crystallisation enhance the machining performance of SiAlON materials in cutting tool applications.