Homogeneous Distribution of Sintering Additives in Liquid-Phase Sintered Silicon Carbide

The addition of sintering additives to silicon carbide particles by electrostatic adsorption of colloidal A1₂O₃ and Y₂O₃ sols has been studied as a way to achieve an optimum homogeneity in the microstructure. The adsorption behavior of the sol particles was examined by electrophoretic measurements and X-ray fluorescence analysis. Both A1₂O₃ and Y₂O₃ sols could simultaneously be adsorbed on the SiC particle surfaces. Viscosity measurements showed that the colloidal sol particles had a stabilizing effect on the slip, and hence slips with relatively high solid loadings could be prepared without adding extra dispersing agent. Liquid-phase-sintered silicon carbide materials (LPS-SiC) with 2 wt% A1₂O₃ and 1 wt% Y₂O₃ were prepared by freeze granulation/ pressing and sintering at 1880^deg;C for 4 h. The homogeneity of the green compacts was quantified using a spot analysis technique in an electron probe microanalyzer. It was clearly shown that the addition of sols gave a more homogeneous microstructure than the reference sample with Y₂O₃ and A1₂O₃ added as powders. The addition of sintering additives as sols also enhanced the sintering behavior.     

Rapid Hardening of Cement by the Addition of a Mechanically Activated Al(OH)3–Ca(OH)2 Mixture

Rapid hardening of cement was achieved in the present study by adding a mechanically activated Al(OH)₃–Ca(OH)₂ mixture to the starting cement paste. Among the dominant parameters for hardening were the mechanical treatment time for the Al(OH)₃ powder and the Al(OH)₃/Ca(OH)₂ ratio. The hardening mechanisms are discussed here in terms of the ionic concentration of the solution and the hydration products created when the Al(OH)₃–Ca(OH)₂ mixture was added to water. Mechanical activation of the Al(OH)₃ powder accelerated dissolution into an aqueous alkaline solution and induced the formation of calcium aluminate hydration products. Those hydration products increased the compressive strength of the cement paste at a very early stage of hardening.     

Pressureless Sintering of Dense Si3N4 and Si3N4/SiC Composites with Nitrate Additives

The effect of aluminum and yttrium nitrate additives on the densification of monolithic Si₃N₄ and a Si₃N₄/SiC composite by pressureless sintering was compared with that of oxide additives. The surfaces of Si₃N₄ particles milled with aluminum and yttrium nitrates, which were added as methanol solutions, were coated with a different layer containing Al and Y from that of Si₃N₄ particles milled with oxide additives. Monolithic Si₃N₄ could be sintered to 94% of theoretical density (TD) at 1500°C with nitrate additives. The sintering temperature was about 100°C lower than the case with oxide additives. After pressureless sintering at 1750°C for 2 h in N₂, the bulk density of a Si₃N₄/20 wt% SiC composite reached 95% TD with nitrate additives.     

Low-Temperature, Single Step, Reactive Sintering of Lead Magnesium Niobate Using Mg(OH)2-Coated Nb2O5 Powders

A low-temperature, single step, reactive sintering method for Pb(Mg_1/3Nb_2/3)O₃ (PMN) and PMN–PbTiO₃ (PMN–PT) processing was developed based on the coating of Mg(OH)₂ on Nb₂O₅. This method simplified the processing of PMN and PMN–PT to a single step of heat-treatment and decreased the sintering temperature to 1000°C. It was found that the pyrochlore phase formation reaction at 500°C reduced the particle size to 130 nm. The overlap of the pyrochlor-perovskite phase transformation between 700° and 900°C and the densification process between 800° and 1000°C improved the sintering process. These two factors were the major reasons of the low temperature sintering.     

Synthesis of High Density and Transparent Forsterite Ceramics Using Nano-Sized Precursors and Their Dielectric Properties

Forsterite (Mg₂SiO₄) ceramics were prepared using Mg(OH)₂ and SiO₂ as precursors, and the effect of powder characteristics of Mg(OH)₂ on calcination and sintering was investigated. The use of highly dispersed Mg(OH)₂ powder (HD powder) resulted in a lower calcination temperature. Forsterite powder of high homogeneity and small particle size prepared from the HD powder enabled synthesis of high-density forsterite ceramics by ordinary sintering without applying external pressure. Moreover, transparent forsterite ceramics were successfully synthesized through addition of excess Mg to the precursors to compensate for Mg evaporated during the sintering process. Subsequent dielectric measurements revealed that the transparent forsterite ceramics had a very low dielectric loss (tan δ<10⁻⁴).     

Vapor-Phase Hydration of Submicrometer CaO Particles

Reaction of water vapor at 25°CC with CaO powder of either high or low surface area yields Ca(OH)₂ with broad XRD peaks. Relative rates of reaction of these hydroxides with CO₂ at 250°CC depend mainly on the sample surface areas per unit weight. SEM observations show that the exterior shapes of porous 1 to 30 μ m particles of CaO formed by decomposition in vacuum of CaCO₃(sr-CaO) or Ca(OH)₂ (h-CaO) are only slightly changed when they are converted to Ca(OH)₂, except for surface roughening, which increases with time of exposure to water vapor. The N₂ adsorption-desorption isotherms of h-CaO and sr-CaO, and of their hydration products, are used to calculate the average particle expansion, the probable pore shapes, and the changes in pore-size distributions that accompany the reaction. These data show that the reaction causes expansion perpendicular to ulterior surfaces of the porous powder particles. A possible mechanism is suggested. Both h-CaO and sr-CaO dissolve in liquid water to yield Ca(OH)₂ by subsequent precipitation.     

Effect of Nitrate Salts as Sintering Additives during the Ball-Milling Process of Silicon Nitride Powders

A chemical adsorption method in a Si₃N₄ slurry that contained a nitrate solution was studied during ball milling, with particular interest in increasing the oxide layer in the Si₃N₄ powder and improving the distribution homogeneity of the sintering additives. The nitrate salts Al(NO₃)₃·9H₂O and Y(NO₃)₃·6H₂O were selected as sintering additives. The following characterization techniques were used: oxygen–nitrogen analysis, X-ray photoelectron spectroscopy, high-resolution electron microscopy (coupled with energy-dispersive X-ray spectroscopy), and X-ray imaging (using wavelength-dispersive X-ray spectroscopy). The thickness of the amorphous layer and the oxygen content of the Si₃N₄ powder were greater for samples that were milled with nitrate additives, which were heat-treated at 600°C, than those of powders that were milled with oxide additives. The chemical composition of the oxygen-containing layer—that is, the amorphous layer that formed and/or changed on the Si₃N₄ surface—was similar to Si₂N₂O in heat-treated Si₃N₄ powder with nitrate additives, whereas the composition of heat-treated Si₃N₄ powder with oxide additives was similar to SiO₂. Furthermore, a homogeneous distribution of the additives was achieved via the incorporation of aluminum and yttrium into the amorphous layer on the Si₃N₄ surface. The metal ratio (Y:Al) of the adsorbates was somewhat higher than that of the additives.     

Dispersion of SiC in Aqueous Media with Al2O3 and Y2O3 as Sintering Additives

It has been well accepted that polyethylene imine (PEI) is an effective dispersant for silicon carbide (SiC) in aqueous media. However, after the addition of sintering additives (Al₂O₃ and Y₂O₃), this dispersing effect is reduced significantly. In this work, a second dispersant, citric acid, was used to resolve this problem. It was found that citric acid could decrease the slurry viscosity (without sintering additives) and enhance the PEI adsorption on SiC particle surface. The optimal amount of citric acid required to achieve a minimum viscosity for 55 vol% SiC suspensions was equal to ∼0.87 wt% (at pH ∼6.8). With the aid of citric acid, well-stabilized SiC suspensions (containing sintering additives) were realized, which exhibited slight shear thinning rheologies. After tape casting and SPS sintering, dense SiC samples were obtained with a homogeneous fine-crystalline microstructure. Results showed that citric acid was an effective dispersant for improving the dispersion of SiC particles containing sintering additives.     

Low-Temperature Fabrication of Transparent Yttrium Aluminum Garnet (YAG) Ceramics without Additives

A carbonate precursor of yttrium aluminum garnet (YAG) with an approximate composition of NH₄AlY_0.6(CO₃)_1.9(OH)₂·0.9H₂O was synthesized via a coprecipitation method from a mixed solution of ammonium aluminum sulfate and yttrium nitrate, using ammonium hydrogen carbonate as the precipitant. The precursor precipitate was characterized using chemical analysis, differential thermal analysis/thermogravimetry, X-ray diffractometry, and scanning electron microscopy. The sinterability of the YAG powders was evaluated by sintering at a constant rate of heating in air and vacuum sintering. The results showed that the precursor completely transforms to YAG at ∼1000°C via the formation of a yttrium aluminate perovskite (YAP) phase. YAG powders obtained by calcining the precursor at temperatures of ≤1200°C were highly sinterable and could be densified to transparency under vacuum at 1700°C in 1 h without additives.     

Double Precursor Solution Coating Approach for Low-Temperature Sintering of [Pb(Mg1/3Nb2/3)O3]0.63[PbTiO3]0.37 Solids

Lead-based piezoelectric ceramics typically require sintering temperatures higher than 1000°C at which significant lead loss can occur. Here, we report a double precursor solution coating (PSC) method for fabricating low-temperature sinterable polycrystalline [Pb(Mg_1/3Nb_2/3)O₃]_0.63-[PbTiO₃]_0.37 (PMN–PT) ceramics. In this method, submicrometer crystalline PMN powder was first obtained by dispersing Mg(OH)₂-coated Nb₂O₅ particles in a lead acetate/ethylene glycol solution (first PSC), followed by calcination at 800°C. The crystalline PMN powder was subsequently suspended in a PT precursor solution containing lead acetate and titanium isopropoxide in ethylene glycol to form the PMN–PT precursor powder (second PSC) that could be sintered at a temperature as low as 900°C. The resultant d ₃₃ for samples sintered at 900°, 1000°, and 1100°C for 2 h were 600, 620, and 700 pm/V, respectively, comparable with the known value. We attributed the low sintering temperature to the reactive sintering nature of the present PMN–PT precursor powder. The reaction between the nanosize PT and the submicrometer-size PMN occurred roughly in the same temperature range as the densification, 850°–900°C, thereby significantly accelerating the sintering process. The present PSC technique is very general and should be readily applicable to other multicomponent systems.     

Low-Temperature Synthesis of Aluminum Silicon Carbide Using Ultrafine Aluminum Carbide and Silicon Carbide Powders

The conditions for preparing α-aluminum silicon carbide (α-Al₄SiC₄) were examined by heating stoichiometric mixtures of ultrafine A1₄C₃ and SiC powders with sizes of <0.1 μm at and below 1600°C. The starting A1₄C₃ powder was obtained by the pyrolysis of trimiethylaluminum; the starting SiC powders were obtained by the pyrolyses of triethylsilane (3ES), tetraethylsilane (4ES), and hexamethyldisilane (6MDS). The reactivity of SiC with Al₄C₃ to form α-Al₄SiC₄ varies according to the kind of starting alkylsilane: 3ES > 4ES > 6MDS. The reaction of 3ES-derived SiC with A1₄C₃ produced α-Al₄SiC₄ at temperatures as low as 1400°C for 240 min, regardless of the presence of A1₄C₃ (trace). Only α-Al₄SiC₄ was formed at and above 1500°C for 60 min; the crystal growth was appreciable.     

The Formation of Magnesium Oxychloride Phases in the Systems MgO-MgCl2-H2O and NaOH-MgCl2-H2O

The reactions leading to the formation of crystalline Mg₃(OH)₅Cl·4H₂O (phase 5), Mg₂(OH)₃,Cl·4H₂O (phase 3), and Mg(OH)₂ are compared for the systems MgO-MgCl₂-H₂O and NaOH-MgCl₂-H₂O. The crystalline phases were determined by X-ray diffraction analysis. The concentration of the total magnesium and chloride in the solution and the pH of the solution determine the reaction product(s) in both systems. The influence of MgO reactivity and the molar ratio of reactants on the formation and stability of reaction products is discussed and the mechanism of the formation of phases 3 and 5 is explained. In the system MgO-MgCl₂-H₂O, MgO serves only to increase the concentration of total magnesium and the pH of the MgCl₂ solution.     

Intermediate Phases in Ti3SiC2 Synthesis from Ti/SiC/C Mixtures Studied by Time-Resolved Neutron Diffraction

The reactive sintering of 3Ti/SiC/C to form the layered ternary carbide Ti₃SiC₂ was studied in situ by time-resolved neutron powder diffraction. A number of intermediate processes occur during the synthesis beginning with the α-β transition in Ti. Concurrent with the α-β transition, two intermediate phases, TiC _x and Ti₅Si₃C _x ( x ≤ 1), form. These phases account for almost the entire sample in the range 1500–1600°C beyond which they react with each other and a small amount of free C to form the product phase Ti₃SiC₂.     

Kinetics of Barium Titanate Synthesis

Reaction curves were obtained at various temperatures and concentrations for the formation of BaTiO₃ from particulate titania in Ba(OH)₂ solution. Kinetic analyses were performed by constructing mathematical models which took into account the particle size distribution of the reactant titania for both the topochemically-rate-controlled and the diffusion-rate-controlled reactions. At [Ba(OH)₂] > ca. 0.1 M the rate-controlling step is the Ba reaction with TiO₂ at the interface. The measured activation energy is 105.5 kJ/mol. The rates are independent of Ba(OH)₂ concentration, indicating that the TiO₂ interface is saturated. At [Ba(OH)₂] < ca. 0.1 M the rate-determining step shifts to diffusion through the product BaTiO₃ layer, the rates are concentration dependent, and the BaTiO₃ particle sizes are inversely proportional to the Ba(OH)₂ concentrations used.     

Oxidation Behavior of Liquid-Phase Sintered Silicon Carbide with Aluminum Nitride and Rare-Earth Oxides (Re2O3, where Re = Y, Er, Yb)

Silicon carbide (SiC) ceramics have been fabricated by hot-pressing and subsequent annealing under pressure with aluminum nitride (AlN) and rare-earth oxides (Y₂O₃, Er₂O₃, and Yb₂O₃) as sintering additives. The oxidation behavior of the SiC ceramics in air was characterized and compared with that of the SiC ceramics with yttrium–aluminum–garnet (YAG) and Al₂O₃–Y₂O₃–CaO (AYC). All SiC ceramics investigated herein showed a parabolic weight gain with oxidation time at 1400°C. The SiC ceramics sintered with AlN and rare-earth oxides showed superior oxidation resistance to those with YAG and Al₂O₃–Y₂O₃–CaO. SiC ceramics with AlN and Yb₂O₃ showed the best oxidation resistance of 0.4748 mg/cm² after oxidation at 1400°C for 192 h. The minimization of aluminum in the sintering additives was postulated as the prime factor contributing to the superior oxidation resistance of the resulting ceramics. A small cationic radius of rare-earth oxides, dissolution of nitrogen to the intergranular glassy film, and formation of disilicate crystalline phase as an oxidation product could also contribute to the superior oxidation resistance.     

Interdiffusion in SiC–AlN and AlN–Al2OC Systems

AlN, Al₂OC, and the 2 H form of SiC are isostructural. Both SiC–AlN and AlN–Al₂OC form homogeneous solid solutions above 2000° and 1950°C, respectively. The kinetics of phase separation in the two systems, however, are quite different. Interdiffusion in both SiC–AlN and AlN-Al₂OC systems was examined in the solid-solution regime in an attempt to elucidate differences in the kinetics of phase separation that occur in the two systems when annealed at lower temperatures. Diffusion couples of (SiC)_0.3(AlN)_0.7/(SiC)_0.7(AlN)_0.3 and (AlN)_0.7(Al₂OC)_0.3/(AlN)_0.3(Al₂OC)_0.7 were fabricated by hot pressing and were annealed at high temperatures by encapsulating them in sealed SiC crucibles to suppress loss due to evaporation. Interdiffusion coefficients in (SiC)_0.3-(AlN)_0.7/(SiC)_0.7(AlN)_0.3 diffusion couples were measured at 2373, 2473, and 2573 K, and the corresponding activation energy was determined to be 632 kJ/mol. (AlN)_0.7(Al₂OC)_0.3/ (AlN)_0.3(Al₂OC)_0.7 samples were annealed at 2273 K. The interdiffusion coefficient measured in the AlN–Al₂OC system was much larger than that in the SiC–AlN system.     

Mass Spectrometric Identification of Si–O–H(g) Species from the Reaction of Silica with Water Vapor at Atmospheric Pressure

A high-pressure sampling mass spectrometer was used to detect the volatile species formed from SiO₂ at temperatures between 1200° and 1400°C in a flowing water vapor/oxygen gas mixture at 1 bar total pressure. The primary vapor species identified was Si(OH)₄. The fragment ion Si(OH)₃⁺was observed in quantities 3 to 5 times larger than the parent ion Si(OH)₄⁺. The Si(OH)₃⁺ intensity was found to have a small temperature dependence and to increase with the water vapor partial pressure as expected. In addition, SiO(OH)⁺, believed to be a fragment of SiO(OH)₂, was observed. These mass spectral results were compared to the behavior of silicon halides.     

Novel Fabrication of Nickel Hydrosilicate Hollow Spheres

Hollow Ni₃Si₂O₅(OH)₄ nanospheres were synthesized via a facile deposition process at room temperature. The diameters of the products are in the range of 300–320 nm, and the average wall thickness is about 10 nm. Furthermore, the synthesis process of Ni₃Si₂O₅(OH)₄ hollow spheres was briefly described and this solution-based approach could be extendable to the preparation of other spherical materials with hollow interiors.     

Spark Plasma Sintering Behavior of Nano-Sized (Ba, Sr)TiO3 Powders: Determination of Sintering Parameters Yielding Nanostructured Ceramics

Nano-powders of BaTiO₃, SrTiO₃, Ba_0.6Sr_0.4TiO₃ (BST64), and a mixture of the composition (BaTiO₃)_0.6(SrTiO₃)₀₄ with particle sizes in the range of 60–80 nm were consolidated by spark plasma sintering (SPS). An experimental procedure is outlined that allows the determination of a "kinetic window," defined as the temperature interval within which the densification process can be kinetically separated from the grain growth one, enabling preparation of dense nanostructured ceramics. The width of this window varied from almost zero for BST64 to 125°C for the (BaTiO₃)_0.6(SrTiO₃)_0.4 mixture. During the densification (sintering) of the (BaTiO₃)_0.6(SrTiO₃)₀₄ mixture, BST64 is formed. The main part of this reaction occurs in a fully densified body through which suggesting that the constitutional phase(s) have a self-pinning effect on the grain growth.     

Synthesis and Characterization of MgAl2O4 Spinel Precursor from a Heterogeneous Alkoxide Solution Containing Fine MgO Powder

A precursor was synthesized from a heterogeneous alkoxide solution that contained fine MgO powder, which allowed the preparation of MgAl₂O₄ spinel powder with high sinterability characteristics. The precursor consisted of a mixture of boehmite (AlO(OH)) and a mixed hydroxide (Mg₄Al₂(OH)₁₄· 3H₂O). The spinel phase formed through two steps: (i) decomposition of the mixed hydroxide at low temperature and (ii) solid-state reaction between MgO and γ-Al₂O₃ at higher temperatures. Dense polycrystalline spinel could be obtained from the calcined powders at sintering temperatures as low as 1400°C.