Preparation of Na-beta″-alumina film by tape casting process

This study focused on the preparation of Na-beta″-alumina film by a tape casting process. The effects of solvent, dispersant, binder, plasticizer contents, as well as milling time on the rheological properties of the slurry were investigated. The dispersion of the ceramic powder in the slurry was optimized by ball milling an azeotropic mixture of methyethylketone (MEK) and ethanol (EtOH) as the solvent, 2 wt% triethanolamine as the dispersant, 7 wt% PVB as the binder for 4 h. Green Na-beta″-alumina thick films with thickness of 100–300 μm and homogeneous microstructure were obtained and further sintered at 1600 °C to obtain a relatively dense membrane with a thickness of ca. 100 μm. The conductivity of the sintered ceramic was comparable to that obtained by the conventional isostatic pressing and sintering method.     

Optimization of non-aqueous tape casting of high solid loading slurry for aluminum nitride ceramic substrates

Aluminum nitride (AlN) ceramic substrates have been fabricated using non-aqueous tape casting and pressureless densification under flowing N₂ atmosphere. Considering the economic and environmental impact, a new strategy of solvent and dispersant system was adopted to prepare AlN slurries with high solid loading. According to the viscosity characteristics of AlN slurries, dispersant content was adjusted to be 0.5 wt% of AlN powder in order to optimize the rheological behavior of AlN slurries. The addition contents of binder and plasticizer were both optimized as 5 wt% of AlN powders by combining the viscosity of slurries and tensile strength of green tapes. Green AlN tapes were fabricated with an optimized tape casting process such as dry temperature. The exclusion process of organic additives was investigated by employing thermogravimetric analysis. Flat and dense AlN ceramic substrates with a relative bulk density over 99.75 % were achieved after being sintered under 1800°C for 6 hours, which had a maximum size of 110 × 110 mm. The thermal conductivity of the AlN substrate could reach 145 Wm⁻¹K⁻¹.     

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.     

Enhanced thermal conductivity of low-temperature sintered borosilicate glass–AlN composites with β-Si3N4 whiskers

The effects of β-Si₃N₄ whiskers on the thermal conductivity of low-temperature sintered borosilicate glass–AlN composites were systematically investigated. The thermal conductivity of borosilicate glass–AlN ceramic composite was increased from 11.9 to 18.8 W/m K by incorporating 14 vol% β-Si₃N₄ whiskers, and high flexural strength up to 226 MPa were achieved along with low relative dielectric constant of 6.5 and dielectric loss of 0.16% at 1 MHz. Microstructure characterization and percolation model analysis indicated that thermal percolation network formation in the ceramic composites led to the high thermal conductivity. The crystallization of the borosilicate microcrystal glass also contributed to the enhancement of thermal conductivity. Such ceramic composites with low sintering temperature and high thermal conductivity might be a promising material for electronic packaging applications.     

Aqueous tape casting process for SiC

The condition for the preparation of stable SiC slurries by aqueous tape casting was identified. To acquire stable uniform slurries, the influences of dispersant, solid loading, sintering additives, binder and plasticizer on the rheological properties and viscosity were investigated. The conditions for preparing stable SiC slurries were studied and optimized. After tape casting and drying, the green SiC sheets showed smooth surface and homogeneous microstructure. The SiC ceramic can be densified to 98.89% after hot-pressing at 1850 °C (at 25 MPa in Ar for 30 min). The flexural strength, hardness, and toughness are 779.5 ± 39.2 MPa, 21.51 ± 0.70 GPa and 5.54 ± 0.26 MPa m^1/2, respectively. SEM shows a fine microstructure with few pores in the sintered samples. The fracture surface exhibited predominantly intergranular fracture type.     

Boron carbide green sheet processed by environmental friendly non-aqueous tape casting

An environmental friendly non-aqueous tape casting process for producing boron carbide green sheet has been discussed in the present paper. An ethanol-based formula was developed by using castor oil, single-oleic acid glycerol, polyvinyl butyral and di-n-butyl phthalate as the dispersant, wetting agents, binder and plasticizer, respectively. The influence of binding system on rheological behavior of the slurry was studied. In addition, the effects of drying condition, milling time and binding systems on green sheet were also investigated. The results indicate that the optimal composition of the boron carbide slurry is 47.0 wt.% boron carbide powder with particles size of 3.5 μm, 2.0 wt.% dispersant, 2.0 wt.% wetting agent, 39.0 wt.% EtOH, 5.0 wt.% binder and 5.0 wt.% plasticizer. The green sheet with high-quality can be obtained by the combination of the three-stage milling method and drying process conducted at room temperature without air convection. The density and tensile strength of the boron carbide green sheet are 1.64 ± 0.06 g/cm³ and 2.66 MPa, respectively.     

Aqueous gelcasting of yttrium iron garnet

N,N-Dimethylacrylamide (DMAA)/N,N′-methylenebis (acrylamide) (MBAM) system was used in the aqueous YIG (yttrium iron garnet) slurry. YIG powders were prepared by using Y₂O₃ and Fe₂O₃ as raw materials through the traditional oxide method. In order to reduce the viscosity and improve the stability of slurries, SD-03 (ammonium polyacrylate) was selected as the dispersant. Zeta potential, pH, dispersant dosage, solid loading and milling time have been optimized. The best conditions were pH 9.86, dispersant dosage 0.2 wt.% and milling time 9 h, which helped to prepare the concentrated slurry with low viscosity and good liquidity. The maximum bending strength of the green body could be up to 13 MPa. The shrinkage and deformation of shaped sintered bodies prepared through gelcasting were small after sintering. The sintering shrinkage, apparent porosity and bulk density were 17%, 0.17% and 5.07 g/cm³, respectively. The dielectric constant and dielectric loss were 14.0 and 2.06 × 10^?4, respectively.     

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.     

Role of organic additives on non-aqueous tape casting of SiAlON ceramics

Suspensions consisting of precursor α/β SiAlON forming powders, azeotropic solvent mixture of 60 MEK/40E, dispersant, binder, and plasticizer were optimized for tape casting by rheological measurements and tape properties. Sodium tripolyphosphate (STPP) was introduced as a dispersant for low temperature applications of α/β-SiAlONs. Optimum STPP amount was determined as 0.012 g/m² (of the particle surface) for stable α/β-SiAlON suspensions. Different amounts of binder/plasticizer mixtures were added to the slurries and the effects on rheological and green tape properties were investigated. Green tapes with dibutyl phthalate (DBP), and plasticizer mixture, poly(ethylene glycol) (PEG) and DBP, exhibited centered cracks with high plasticity, on the other hand, polyvinyl butral (PVB) and PEG showed no crack but low plasticity. Therefore, many different parameters were found to be effective on final tape properties. In addition, tapes were prepared with 6 vol% PVB + PEG, sintered at 1800 °C for 2 h and exhibited almost 97%TD in room temperature applications of α/β SiAlONs.     

The preparation of silicon nitride ceramics by gelcasting and pressureless sintering

A new gelling system based on the polymerization of hydantion epoxy resin and 3,3′-Diaminodipropylamine (DPTA) was successfully developed for fabricating silicon nitride (Si₃N₄) ceramics. The effects of pH value, the dispersant content, solid volume fraction and hydantion epoxy resin amount on the rheological properties of the Si₃N₄ slurries were investigated. The relative density of green body obtained from the solid loading of 52 vol% Si₃N₄ slurry reached up to 62.7%. As the concentration of hydantion epoxy resin increased from 5 wt% to 20 wt%, the flexural strength of Si₃N₄ green body enhanced from 5.3 MPa to 31.6 MPa. After pressureless sintering at 1780 °C for 80 min, the sintered samples exhibited the unique interlocking microstructure of elongated β-Si₃N₄ grains, which was beneficial to improve the mechanical properties of Si₃N₄ ceramics. The relative density, flexural strength and fracture toughness of Si₃N₄ ceramics reached 97.8%, 687 MPa and 6.5 MPa m^1/2, respectively.     

Sintering behaviour and properties of magnesium orthosilicate-hydroxyapatite ceramic

The effect of pressureless sintering on the properties of magnesium orthosilicate-hydroxyapatite (MO-HA) ceramic has been studied. The amount of MO composition in the green body was varied from 10 wt% to 50 wt% through mechanical ball milling and was subsequently sintered at varying temperatures in air atmosphere from 1000 °C to 1300 °C for 2 h. The magnesium orthosilicate phase was stable during sintering but the hydroxyapatite phase decomposed to tricalcium phosphate. The MO-HA composites generally exhibited lower mechanical properties across all the investigated composition. Nevertheless, a high fracture toughness of 2.5 MPam^1/2 was recorded for sintered body that contained 20 wt% MO. This finding indicates the potential of this ceramic composite to be used for biomedical applications.     

Preparation of concentrated barium titanate suspensions incorporating nano-sized powders

In order to reduce agglomeration and overcome the low packing density issues of working with nano-sized powders, a colloidal processing route has been chosen in this study. Commercial BaTiO₃ (BT) powders with a particle size in the range of 50 nm have been dispersed in the aqueous media. Rheological properties have been analyzed on suspensions with different solids loading, dispersant concentration, and pH conditions. Optimum dispersing conditions were obtained for suspensions prepared at basic pH (pH 10) with 0.646 wt% ammonium poly (acrylic acid) (NH₄PAA) as a dispersant. Suspensions have been centrifugally cast to obtain the green body, and the sintering conditions have been investigated by examining the phase evolution, microstructures and electrical properties of the sintered samples through XRD, SEM and dielectric measurements, respectively. The results show that for a 45 vol% suspension sintered at 1325 °C, the density of bulk ceramic can reach 5.85 g/cm³, nearly 97.0% of the theoretical density.     

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.     

High-strength AlN ceramics by low-temperature sintering with CaZrO3–Y2O3 co-additives

Aluminum nitride (AlN) ceramics with the concurrent addition of CaZrO₃ and Y₂O₃ were sintered at 1450-1700 °C. The degree of densification, microstructure, flexural strength, and thermal conductivity of the resulting ceramics were evaluated with respect to their composition and sintering temperature. Specimens prepared using both additives could be sintered to almost full density at relatively low temperature (3 h at 1550 °C under nitrogen at ambient pressure); grain growth was suppressed by grain-boundary pinning, and high flexural strength over 630 MPa could be obtained. With two-step sintering process, the morphology of second phase was changed from interconnected structure to isolated structure; this two-step process limited grain growth and increased thermal conductivity. The highest thermal conductivity (156 Wm⁻¹ K⁻¹) was achieved by two-step sintering, and the ceramic showed moderate flexural strength (560 MPa).     

Inhibition of grain growth in liquid-phase sintered SiC ceramics by AlN additive and spark plasma sintering

SiC ceramics were prepared from nanosized β-SiC powder with different compositions of AlN and Y₂O₃ sintering additives by spark plasma sintering (SPS) at 1900 °C for 600 s in N₂. The relative density of the sintered SiC specimens increased with increasing amount of AlN, reaching a relative density higher than 99%, while at the same time grain size decreased significantly. The smallest average grain size of 150 nm was observed for SiC sample sintered with 10 vol% of additives consisting of 90 mol% AlN and 10 mol% Y₂O₃. Fully dense nanostructured SiC ceramics with inhibited grain growth were obtained by the AlN additive and SPS technique. The flexural strength of the SiC body containing 70 mol% AlN and 30 mol% Y₂O₃ additives reached the maximum value of 1000 MPa. The SiC bodies prepared with AlN and Y₂O₃ additives had the fracture toughness of around 2.5 MPam^1/2.     

High-toughness silicate ceramic

A silicate ceramic that is similar to porcelain and exhibits a maximum toughness of 4.6 MPa m^1/2 was obtained by tape casting from kaolin and 3 vol% of alumina fibers. Improved toughness and strength are achieved with the organized micro-composite microstructure that results from preferential orientation during the shaping of kaolinite particles and fibers in-plane of layers. During sintering, typical nucleation and growth processes of mullite produce specific microstructural characteristics, such as bulk zones, oriented fibers and large interfacial zones between the fibers and the bulk. Toughening is attributed to the decreased crack energy in the bulk ceramic, in which a dense and organized network of short mullite occurs, and in interfacial zones containing a superimposed network of large mullite. The silicate ceramic that is reinforced by only 3 vol% of the alumina fibers is strong (95 MPa) and tough (4.6 MPa m^1/2); although these properties are often mutually exclusive.     

Stabilization of highly-loaded boron carbide aqueous suspensions

Injection molding of boron carbide (B₄C) slurries affords the production of complex-shaped personal armor. To injection mold, however, requires preparation of a well dispersed, flowable suspension with >45 vol% B₄C loadings to reduce porosity that must be removed during sintering. In the present study, the preparation of highly-loaded B₄C suspensions is investigated using zeta potential and rheological measurements, varying dispersant type, molecular weight, and amount. Of those dispersants investigated, polyethylenimine (PEI) with a molecular weight of 25,000 g/mol was found to produce suspensions with up to 56 vol% B₄C and the requisite rheological properties suitable for injection molding. A PEI concentration of 1.83 mg/m² was established as the appropriate to produce highly-loaded B₄C suspensions. The effect of a prior B₄C powder treatment (ethanol washed or attrition milled) on rheological properties of the suspensions was also investigated. The PEI was completely burned out in argon, nitrogen, and air at 450 °C.     

Direct coagulation casting of silicon nitride suspension via a dispersant reaction method

A direct coagulation casting method for silicon nitride suspension via dispersant reaction was reported. Tetramethylammonium hydroxide (TMAOH) was used as dispersant to prepare silicon nitride suspension with high solid loading and low viscosity. Influences of TMAOH and pH value on the dispersion of silicon nitride powder were investigated. Glycerol diacetate (GDA) was used to coagulate the silicon nitride suspension. Influences of the concentration of glycerol diacetate on the viscosity and pH value of the suspension were investigated. It was indicated that high viscosity sufficient to coagulate the suspension was achieved by adding 1.0–2.0 vol% glycerol diacetate at 40–70 °C. The coagulation mechanism was proposed that the silicon nitride suspension was destabilized by dispersant reacting with acetic acid which was hydrolyzed from glycerol diacetate at elevated temperature. Coagulated samples could be demolded without deformation by treating 50 vol% silicon nitride suspensions with 0.2 wt% tetramethylammonium hydroxide and 1.0–2.0 vol% glycerol diacetate at different temperatures. Dense silicon nitride ceramics with relative density above 98.8% had been prepared by this method using glycerol diacetate as coagulating agent sintered by different methods.     

Pressureless sintering of ZrO2–ZrSiO4/NiCr functionally graded materials with a shrinkage matching process

Shrinkage matching was applied as the basic concept for the processing of five-layered ZrO₂–ZrSiO₄/NiCr functionally graded materials (FGMs) by pressureless sintering. Shrinkage behavior of the constituent layers of FGM was matched based on the strategy of reducing the mismatches of shrinkage rate between the adjacent layers by a systematic adjustment of the ZrSiO₄ inclusion content and the particle size of ZrO₂ matrix. Before shrinkage matching, sintering defects such as warping towards the ceramic-rich side of FGM and cracking in the 100 vol% ceramic layer were generated, due to the relatively high mismatches of shrinkage rate between the ceramic- and the metal-rich sides and at the interface of 100 and 75 vol% ceramic layers, respectively. After shrinkage matching, FGMs without any sintering defects were obtained, whereas the green bodies warped towards the metal-rich side already before sintering, due to the relatively higher shrinkage of the metal-rich side during isostatic pressing.     

Non-aqueous gelcasting of AlN ceramics using a low-toxicity monomer (DMAA) as gelling agent

In this study, high-performance aluminum nitride (AlN) samples were prepared by gelcasting based on a low-toxicity monomer and organic solvents. We prepared a 50 vol% solid content AlN suspension with a solvent ratio of 3:1, 6 wt% N,N-dimethylacrylamide (DMAA) and 0.15 wt% Polyethyleneimine (PEI); after degassing, low viscosity and high stability were observed. It was proven that the AlN green bodies exhibited a bending strength of 18.68 MPa, which met the requirements for machining. An optimum debinding process was developed and the TG-DSC curves showed that the AlN green bodies were heated at 1 °C/min from 450 to 800 °C. The relative density, apparent porosity, bending strength, and thermal conductivity of the AlN sintered bodies prepared by non-aqueous gelcasting were 98.5%, 0.22%, 310 MPa, and 159 W m^?1 K^?1 respectively, representing optimum conditions. The uniform miscrostructure of the sintered bodies was observed by SEM images.