Alumina fiber-reinforced silica matrix composites with improved mechanical properties prepared by a novel DCC-HVCI method

A novel direct coagulation casting via controlled release of high valence counter ions (DCC-HVCI) method was applied to prepare the alumina fiber-reinforced silica matrix composites with improved mechanical properties. In this method, the silica suspension could be rapidly coagulated via controlled release of calcium ions from calcium iodate and pH shift by hydrolysis of glycerol diacetate (GDA) at an elevated temperature. The influence of tetramethylammonium hydroxide (TMAOH) dispersant amount, volume fraction and calcium iodate concentration on the rheological properties of suspensions was investigated. Additionally, the effect of alumina fiber contents on the mechanical properties of alumina fiber-reinforced silica matrix composites was studied systematically. It was found that the stable suspension of 50 vol% solid loading could be prepared by adding 2.5 wt% TMAOH at room temperature. The addition of 0–15 wt% alumina fibers had no obvious effect on the viscosity of the silica suspension. The controlled coagulation of the suspension could be achieved by adding 6.5 g L⁻¹ calcium iodate and 1.0 wt% GDA after treating at 70 °C for 30 min. Compressive strength of green bodies with homogeneous microstructure was in the range of 2.1–3.1 MPa. Due to the fiber pull-out and fracture behaviors, the mechanical properties of alumina fiber-reinforced composites improved remarkably. The flexural strength of the composite with 10 wt% alumina fibers sintered at 1350 °C was about 7 times of that without fibers. The results indicate that this approach could provide a promising route to prepare complex-shaped fiber-reinforced ceramic matrix composites with uniform microstructure and high mechanical properties.     

In-situ coagulation of yttria-stabilized zirconia suspension via dispersant hydrolysis using sodium tripolyphosphate

A novel in-situ coagulation method without coagulation agent and adjusting pH value for yttria-stabilized zirconia (YSZ) suspension via dispersant hydrolysis is reported. Sodium tripolyphosphate (STPP) is used as dispersant to prepare electrostatic stabilized YSZ suspension. Influences of STPP contents on the dispersion and pH value of YSZ suspension were investigated. It indicated that there was a well-dispersed YSZ suspension with the addition of 0.3 wt% STPP at pH = 10. Influence of coagulation temperature on coagulation process and properties of green body was investigated. The sufficiently high viscosity suspension to coagulate was achieved at 60–80 °C. The coagulation mechanism was different from traditional direct coagulation casting. The suspension was coagulated by directly shifting the isoelectric point to the original state without increasing the ionic strength and adjusting the pH value. It was proposed that the YSZ suspension could be destabilized via decrease of zeta potential by sodium tripolyphosphate hydrolyzing at elevated temperature. Coagulated samples with wet compressive strength of 3.60 MPa could be demolded without deformation by treating 50 vol% YSZ suspension with 0.3 wt% STPP at 60 °C for 30 min. Dense YSZ ceramics with flexural strength of 887 ± 110 MPa and relative density of 98.9% had been prepared by this method sintered at 1450 °C for 3 h.     

Temperature induced gelation of non–aqueous alumina suspension using oleic acid as dispersant

A novel temperature induced gelation method for alumina suspension using oleic acid as dispersant is reported. Non–aqueous suspension with high solid loading and low viscosity is prepared using normal octane as solvent. Influence of oleic acid on the dispersion of suspension was investigated. There was a well disperse alumina suspension with 1.3 wt% oleic acid. Influence of gelation temperature on the coagulation process and properties of green body was investigated. The sufficiently high viscosity to coagulate the suspension was achieved at −20 °C. The gelation temperature was controlled between the melting point of dispersant and solvent. The gelation mechanism is proposed that alumina suspension is destabilized by dispersant separating out from the solvent and removing from the alumina particles surface. The alumina green body with wet compressive strength of 1.07 MPa can be demolded without deformation by treating 53 vol% alumina suspension at −20 °C for 12 h. After being sintered at 1550 °C for 3 h, dense alumina ceramics with relative density of 98.62% and flexural strength of 371±25 MPa have been obtained by this method.     

Direct coagulation casting of yttria-stabilized zirconia using magnesium citrate and glycerol diacetate

Direct coagulation casting via controlled release of high valance counter ions (DCC-HVCI) has been reported in recent years. In this paper, concentrated yttria-stabilized zirconia (YSZ) suspensions were coagulated using DCC-HVCI method with magnesium citrate as coagulating agent assisted by pH shift in the presence of glycerol diacetate. The effect of ammonium polyacrylate (PAA-NH₄) on the dispersibility of YSZ powder was investigated. The influence of concentrations of glycerol diacetate and magnesium citrate on pH and viscosities of YSZ suspensions was studied. The results indicate that concentrated YSZ suspensions can be coagulated by adding 2 vol% glycerol diacetate and magnesium citrate above 0.5 wt% at room temperature for 2–5 h. The compressive strength of coagulated wet samples is above 2.0 MPa. YSZ ceramics sintered at 1450 °C show homogeneous microstructures with relative densities of 98.9–99.2%. Flexural strength of YSZ ceramics is 869±84 MPa.     

Direct Coagulation Casting of Alumina Suspension from Calcium Citrate Assisted by pH Shift

Direct coagulation casting of alumina suspension via controlled release of high valence counter ions (DCC–HVCI) using calcium citrate as coagulating agent was reported. Hydrolysis of glycerol diacetate shifts the pH of suspension to weakly acidic region which helps to decompose calcium citrate and release calcium ions. The effect of concentration of glycerol diacetate and calcium citrate on the pH and viscosity of alumina suspension was investigated at 25°C and 60°C, respectively. The pH of suspensions with glycerol diacetate and calcium citrate decreases to 8.6 and 7.5 treated at 25°C and 60°C, respectively. It is indicated that high viscosity is achieved by adding 2 vol% glycerol diacetate and 0.5 wt% calcium citrate which is enough to coagulate the suspension. Green body with compressive strength of 1.0 MPa is obtained by treating the alumina suspension with 2 vol% glycerol diacetate and 0.5 wt% calcium citrate at 60°C for 1 h. The alumina ceramics sintered at 1550°C have homogeneous microstructure with relative density above 99.0%.     

Reliable high strength alumina fabricated by DCC-HVCI using submicron calcium citrate complex

Ceramics with high strength and reliability are highly demanded in engineering applications. In this paper, a modified direct coagulation casting via high valence counter ions (DCC-HVCI) method for alumina using calcium citrate complex assisted by glycerol diacetate was investigated. Calcium citrate complex suspensions were prepared by mixing tri-ammonium citrate and calcium chloride in water. Effect of reaction time on the chelating properties of the prepared suspensions was investigated. Concentrated alumina suspensions with a solid loading of 50 vol% were prepared by mixing the calcium citrate complex suspensions and alumina powder at pH of 10.5. Then the suspensions were coagulated by adding 3–6 vol% glycerol diacetate at temperatures of 40–70 °C for 2–6 h. The compressive strength of the coagulated wet samples is in the range 1.1–2.4 MPa. Alumina ceramics sintered at 1550 °C shows homogeneous microstructures with flexural strength and Weibull modulus of 455±17 MPa and 30, respectively.     

Rapid and uniform in-situ solidification of alumina suspension via a non-contamination DCC-HVCI method using MgO sintering additive as coagulating agent

A novel rapid, uniform and non-contamination in-situ solidification method for alumina suspension by DCC-HVCI method using MgO sintering additive as coagulating agent was reported. MgO was used to release Mg²⁺ in suspensions via reaction with acetic acid generated from glycerol diacetate (GDA) at elevated temperature as well as to improve density and suppress grain growth of alumina ceramics during sintering. Influence of adding 0.7 wt% MgO with 2.0 vol% GDA in alumina suspension on coagulation process and properties of green bodies and sintered samples were investigated. It was indicated that the controlled coagulation of the suspension could be achieved after treating at 70 °C for 10 min. Homogeneous composition distribution of Mg element in EDS result indicated the uniform solidification of suspensions. Compressive strength of wet-coagulated bodies is 2.09±0.25 MPa. Dense alumina ceramics with relative density of 99.2% and flexural strength of 354±16 MPa sintered at 1650 °C for 4 h present homogeneous microstructure. The result indicated that the novel DCC-HVCI method via a sintering additive reaction with no contamination, short coagulation time and uniform in-situ solidification is a promising colloidal forming method for preparing high-performance ceramic components with complex shape.     

Preparation of silicon carbide ceramics using chemical treated powder by DCC via dispersant reaction and liquid phase sintering

Dense silicon carbide ceramics using chemical treated powder by DCC via dispersant reaction method and liquid phase sintering was reported. Ammonium peroxydisulfate ((NH₄)₂S₂O₈) and ammonium carbonate ((NH₄)₂CO₃) were used as acid and base solutions to treat the silicon carbide powder, respectively. Influence of silicon carbide powder with chemical treatment on the preparation of silicon carbide suspension was studied. It was indicated that 50 vol% and 52 vol% silicon carbide suspensions with viscosities of 0.71 Pa s and 0.80 Pa s could be prepared using acid and base treated powders. Influence of silicon carbide powder with chemical treatment on the coagulation process and properties of green bodies and sintered ceramics were studied. It was indicated that silicon carbide green bodies with compressive strength of 1.13 MPa could be prepared using base treated powder. Dense silicon carbide ceramics with relative density above 99.3% and flexural strength of 697 ± 30 MPa had been prepared by DCC via dispersant reaction and liquid phase sintering using Al₂O₃ and Y₂O₃ as additives at 1950 °C for 2 h.     

Rapid in-situ solidification of SiO2 suspension by direct coagulation casting via controlled release of high valence counter ions from calcium iodate and pH shift

Rapid in-situ solidification of SiO₂ suspension under the joint action of releasing calcium ions and shifting pH has been proposed. When the suspension was heated up to 60 ℃, decomposition of calcium iodate which released calcium ions, as well as hydrolysis of diacetate (GDA) which shifted the pH toward the isoelectric point, both contributed to the solidification of suspension. The controlled coagulation of SiO₂ suspension could be realized via controlled release of high valence counter ions and pH shift at 60 ℃ within 30 min, which could considerably shorten the coagulation time compared with present reported results (1–3 h). Green body prepared by heating the SiO₂ suspension with 6.5 g L⁻¹ calcium iodate and 2.0 vol% GDA at 60 ℃ for 30 min shows uniform microstructure with compressive strength of close to 0.3 MPa. SiO₂ ceramics sintered at 1275 ℃ for 3 h possess homogeneous microstructure with bulk density of 2.06 g cm⁻³ and flexural strength of 40.3 MPa.     

Stabilization of alumina with polyelectrolyte and comb copolymer in solvent mixtures of water and alcohols

Solvent mixtures of water and ethanol and water and isopropanol have been evaluated for processing of concentrated alumina suspensions. The addition of alcohols may increase the long-term stability of suspensions with soluble ceramic species such as magnesia, which is added as a sintering aid. A poly(acrylic acid) and a hydrophilic comb copolymer were used as dispersants for the different solvent mixtures. The aim was to compare the stabilization efficiency at normal processing conditions, pH 9–10, through rheological measurements and to develop a robust system including magnesia with long-term stability. The electrostatic stabilization of the dispersants in the different solvent mixtures was studied by zeta potential measurements. Highly negative zeta potentials were observed for the poly(acrylic acid) at pH 9–10 in the solvent mixtures. A charge contribution was also seen from the adsorbed comb copolymer, however smaller than for the poly(acrylic acid). Low viscosity was obtained for suspensions stabilized with poly(acrylic acid) in solvent mixtures with either 25 vol% ethanol or isopropanol. Higher alcohol to water ratio led to flocculation of the suspension when poly(acrylic acid) was used as dispersant. Alumina suspensions with added magnesia in isopropanol:water 25:75 and poly(acrylic acid) as dispersant showed long-term stability. The viscosity remained almost constant during 4 days of aging. Suspensions stabilized with the comb copolymer dispersant gave stable systems with ethanol and isopropanol concentrations between 25 and 75 vol%. The superior dispersing efficiency of the comb copolymer at alcohol contents above 25 vol% was believed to originate from steric stabilization in combination with low effective particle size, giving low viscosity through lower apparent solid contents of the suspension.     

Fabrication of porous fibrous alumina ceramics by direct coagulation casting combined with 3D printing

A novel forming method for preparing porous alumina ceramics using alumina fibers as raw materials by direct coagulation casting (DCC) combined with 3D printing was proposed. Porous fibrous alumina ceramics were fabricated through temperature induced coagulation of aqueous-based DCC process using sodium tripolyphosphate (STPP) as dispersant and adding K₂SO₄ as removable sintering additives. The sacrificial coated sand molds was fabricated by 3D printing technology, followed by the infiltration of silica sol solution for the subsequent suspension casting. Stable alumina suspension of 40 vol% solid loading was obtained by adding 2.0 wt% STPP and 40 wt% K₂SO₄. The controlled coagulation of the suspension could be realized after heating at 90 °C for about 35 min. The ceramic sample sintered at 1450 °C for 2 h showed the highest compressive strength of 24.33 MPa with porosity of 57.38%. All samples sintered at 1300–1450 °C had uniform pore size distributions with average pore size of 7.2 µm, which indicated the good structure stability when sintered at high temperature.     

Preparation and properties of AlN ceramic suspension for non-aqueous gel casting

A non-aqueous gel casting process based on the mixed solvent (ethanol and polyethylene glycol) and low-toxicity N,N-dimethylacrylamide (DMAA) was developed for an aluminum nitride (AlN) ceramic. In the present work, rheological properties of non-aqueous concentrated AlN suspensions were investigated in the presence of mixed solvent, dispersant, milling time, monomer and solid loading, in order to screen for the most suitable experimental conditions to obtain a good rheological behavior for gel casting. The results showed that the 50 vol% slurry with 0.2 wt% dispersant concentration, 2 h milling time, 6 wt% -monomer content, and a solvent ratio of 3:1, can meet the requirements for the casting process of AlN ceramic slurries. After being dried at 100 °C for 1 h, the optimum bulk density and maximum flexural strength of the AlN green bodies were as high as 1.97 g/cm³ and 18.68 MPa, respectively. SEM photographs revealed that the green body had a relative uniform microstructure when the solid loading was 50 vol%. The shrinkage and deformation of shaped sintered bodies prepared through gel casting were small after sintering. The sintering shrinkage, apparent porosity, bulk density and flexural strength were 14.8%, 0.22%, 3.21 g/cm³ and 310 MPa, respectively.     

Influence of temperature and concentration on tribological properties of silicon nitride in glycerol aqueous solution

Friction and wear behaviors of self-mated Si₃N₄ in glycerol aqueous were investigated by varying the temperature (30 °C, 50 °C, and 70 °C) and concentration (pure water, 5 vol%, 20 vol%, and 50 vol%) of glycerol aqueous solution. Friction tests were conducted on a ball-on-disk apparatus. Normal load and sliding velocity were fixed at 30 N and 0.5 m/s, separately. After each tests, friction coefficients and wear rates were measured to evaluate friction and wear behavior. The results showed that the period of running-in process reduces with the increase of concentration and decrease of temperature. Increase of temperature could intensify wear behavior, and when concentration is larger than 20 vol%, wear rate of glycerol aqueous solution is one order less than that of pure water. Our findings could also guide for the use of glycerol aqueous solution as lubricant at different temperature. At 30 °C, the higher the concentration was, the smaller wear volume and total wear rate were. However, at 50 °C and 70 °C, total wear rates of disk were the largest when concentration is 5 vol%, a concentration of glycerol larger than 20 vol% must be added into water to reduce the wear rate. Wear regimes at different conditions were also given in this paper based on lubrication state number.     

Dispersion of mixtures of submicrometer and nanometre sized titanias to obtain porous bodies

The stability and rheological behaviour of bimodal titania suspensions was studied. Bimodal mixtures were prepared by mixing nanosized TiO₂ powders with an average primary size of ～20–40 nm and surface area of ～50 m² g^?1 and/or a colloidal titania suspension of the same nanopowders dispersed in water with a submicrometer sized titania. The dispersing conditions were studied as a function of pH, type and content of dispersant, and sonication time for a constant solids content of 30 vol% (62 wt%). The mixtures were slip cast and presintered at low temperatures (800–1000 °C) in order to obtain porous materials with anatase as the major phase. The pore size distribution, microstructure and phase composition were characterised using MIP, SEM and XRD techniques, respectively.     

Spark Plasma Sintering of fine alpha-silicon nitride ceramics with LAS for spatial applications

Many space systems such as satellite mirrors and their supporting structures require to be made from very low-thermal expansion materials combining both high hydrostability and relatively high mechanical properties. In this study, we have applied the “composite concept” in order to explore the possibility of fabricating near zero thermal expansion silicon nitride based ceramics. Consequently, a negative thermal expansion material belonged to the lithium aluminosilicate family (LAS powder crystallized under de β-eucryptite structure) was introduced in an alpha-silicon nitride fine powder (5 and 20 vol% of LAS) and the resulting composite system was sintered by Spark Plasma Sintering (SPS) at 1400 and 1500 °C. In the case of 20 vol% LAS compositions, relatively well-densified ceramics (94.4% of the theoretical density) were produced without adding any further sintering additive. The addition of yttria and alumina oxides allowed enhancing the densification level up to 98.2% (20 vol% LAS compositions) or from 62.3% up to 96.7% of the theoretical density in 5 vol% LAS materials. Nevertheless, it was impossible to full consolidate silicon nitride/LAS composite ceramics at temperatures lower than the temperature at which β-eucryptite melts, even by using SPS technology. Moreover, because of the relatively low temperatures involved in SPS, the α to β-Si₃N₄ transformation was avoided, resulting in microstructures composed of fine equiaxed α-Si₃N₄ grains (<200 nm) and of a glassy phase. Even if the effect of having a very large negative thermal expansion material was lost during the sintering step (because of the β-eucryptite melting), ceramics containing only 20 vol% of LAS-based phase exhibited very interesting values as regards of mechanical properties (strength, hardness, toughness, and Young's modulus), thermal conductivity and thermal expansion coefficient. We discuss in this work why we are so interested in developing dense silicon nitride/LAS ceramics sintered without any further additive addition, even though β-eucryptite is melted during the process and the transformation to the β phase is avoided.     

Optimization of pH and dispersant amount of Y-TZP suspension for colloidal stability

In this article, the amount of dispersant agent [i.e., polyethyleneimine (PEI)] and pH was optimized to achieve high colloidal stability in yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) suspension. Rheological properties of aqueous Y-TZP suspension in the presence of different amounts of PEI were evaluated via viscosity tests to identify the optimal amount of dispersant agent. Zeta potential and particle size measurements were employed to determine the suitable pH that can establish good colloidal stability. Mechanical properties (e.g., density and hardness) and morphological properties were also considered in determining the optimal pH. Rheological tests showed that Y-TZP suspension with 0.4 wt% PEI had the lowest viscosity, which is suitable for colloidal processing. pH 2 and 4 were recorded to have the lowest particle size and highest zeta potential, respectively. Characterization tests and morphological analysis showed that pH 2 had the highest density (92.5%), highest hardness (10.36 GPa), and homogeneous microstructure with fine average grain size (486 nm). Thus, the dispersant amount of 0.4 wt% PEI and pH 2 were selected as the optimal parameters for colloidal processing of aqueous Y-TZP suspension. Y-TZP suspension with excellent colloidal stability and reliable final products was produced under these parameters.     

Direct coagulation casting of silicon carbide suspension via polyelectrolyte dispersant crosslink reaction

A direct coagulation casting method for silicon carbide ceramic suspension using dispersant crosslink reaction is reported. Polymer electrolyte (polyethyleneimine, PEI) was used as dispersant to prepare silicon carbide suspension. Common food additives (carboxymethyl cellulose, CMC) were used to coagulate the electrosteric stabilized silicon carbide suspension. There was a well disperse silicon carbide suspension with 0.2 wt% PEI at pH = 5-6. Influence of coagulant on viscosity and zeta potential of the silicon carbide suspension was investigated. It indicates that the high solid loading silicon carbide suspension can be destabilized and coagulated at elevated temperature. It can be attribute to the gradual decrease of electrosteric force due to the crosslink reaction between PEI and CMC. Silicon carbide wet green body with compressive strength of 1.99 MPa could be demolded at 70°C which is higher than that prepared by traditional DCC and dispersant reaction method for nonoxide ceramics. Dense silicon carbide ceramics with relative density above 98.8% and 99.3% had been prepared by liquid phase pressureless and hot pressed sintering, respectively.     

High-frequency welding of polypropylene using dielectric ceramic compounds in composite adhesive layers

In this study, high-frequency welding of polypropylene by melting composite adhesive layers containing dielectric ceramics was investigated. Various dielectric ceramics were mixed in a fixed ratio with polypropylene to make the composite adhesive layers, and the resulting dielectric properties were measured using an impedance analyser. The highest loss factor in the composite adhesive layer was found when 40 vol% silicon carbide (SiC) was used in the mixture. Dynamic viscoelasticity measurements showed that all composite adhesive layers softened at approximately 170 °C and melted (fluidised) at approximately 190 °C. Each composite adhesive layer was inserted between two polypropylene plates, and irradiated at a frequency of 40 MHz. The composite adhesive layers that included 20 vol% anatase-titanium oxide, 20 vol% or 40 vol% zinc oxide and 20 vol% or 40 vol% SiC melted in 40–70 s. The bond strength of the welded material obtained was high, and the adherend failure occurred by a tensile lap shear test. The heating efficiencies of the composite adhesive layers by high-frequency radiation were related to the tanδ/ε′ value of the composites.     

Thermal stability and surface modifications of detonation diamond nanoparticles studied with X-ray photoelectron spectroscopy

Detonation nanodiamond (ND) particles were dispersed on silicon nitride (SiN_x) coated sc-Si substrates by spin-coating technique. Their surface density was in the 10¹⁰–10¹¹ cm^?2 range. Thermal stability and surface modifications of ND particles were studied by combined use of X-ray Photoelectron Spectroscopy (XPS) and Field Emission Gun Scanning Electron Microscopy (FEG SEM). Different oxygen-containing functional groups could be identified by XPS and their evolution versus UHV annealing temperature (400–1085 °C) could be monitored in situ. The increase of annealing temperature led to a decrease of oxygen bound to carbon. In particular, functional groups where carbon was bound to oxygen via one σ bond (C–OH, C–O–C) started decomposing first. At 970 °C carbon–oxygen components decreased further. However, the sp²/sp³ carbon ratio did not increase, thus confirming that the graphitization of ND requires higher temperatures. XPS analyses also revealed that no interaction of ND particles with the silicon nitride substrate occurred at temperatures up to about 1000 °C. However, at 1050 °C silicon nitride coated substrates started showing patch-like damaged areas attributable to interaction of silicon nitride with the underlying substrate. Nevertheless ND particles were preserved in undamaged areas, with surface densities exceeding 10¹⁰ cm^?2. These nanoparticles acted as sp³-carbon seeds in a subsequent 15 min Chemical Vapour Deposition run that allowed growing a 60–80 nm diamond film. Our previous study on Si(100) showed that detonation ND particles reacted with silicon between 800 and 900 °C and, as a consequence, no diamond film could be grown after Chemical Vapour Deposition (CVD). These findings demonstrated that the use of a thin silicon nitride buffer layer is preferable insofar as the growth of thin diamond films on silicon devices via nanoseeding is concerned.     

Aqueous gelcasting of ZrB2–SiC ultra high temperature ceramics

ZrB₂–SiC ultra high temperature ceramics containing B₄C and C as sintering additives were successfully prepared by aqueous gelcasting and pressureless sintering. Polyacrylic acid (PAA) was used as the dispersant throughout this research. The various effects of zeta potential, pH value, dispersant concentration, solid loading and ball-milling time on the rheology and fluidity behavior of ZrB₂–SiC suspension were investigated in detail. A well-dispersed suspension with 50 vol.% solid loading was prepared at pH 10 with 0.4 wt.% PAA after ball milling at 240 rpm for 24 h. Then crack-free green ZrB₂–SiC ceramics were obtained by gelcasting process and then pressureless sintered at 2100 °C to about 98% relative density. The microstructure and mechanical properties were examined, and the flexural strength and fracture toughness were 405 ± 27 MPa and 4.3 ± 0.3 MPa m^1/2, respectively.