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 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.     

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.     

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.     

Plasticization of cellulose diacetate by reaction with maleic anhydride,glycerol, and citrate esters during melt processing

To accomplish the stable internal plasticization of cellulose diacetate (CDA), maleic anhydride (MAH) and glycerol (Gly) were used as reactive plasticizers. The plasticization method used was based on a melt‐processing reaction of CDA with MAH and Gly. MAH and Gly (MG)‐plasticized CDA showed stiff and brittle properties; that is, low elongation at break and high modulus. Thus, citrate esters were used as coplasticizers to improve physical properties. The resulted plasticized materials were optically clear, and showed attractive mechanical properties. The grafting of MG oligoesters to the free hydroxyl groups in CDA and their homo‐oligomerization were accelerated by two‐step kneading process, and verified by FTIR and GPC measurements. Differential scanning calorimeter (DSC) analysis revealed decreases of 80–100°C in the glass transition temperature (T_g) of CDA by these plasticizations. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 243–250, 2001     

Direct coagulation casting of YSZ powder suspensions using MgO as coagulating agent

Direct coagulation casting (DCC) of aqueous 8 wt% yttria stabilized zirconia (YSZ) powder suspensions prepared using ammonium poly(acrylate) dispersant has been studied using MgO as coagulating agent. Small amount (<0.1 wt% based on YSZ) of MgO powder dispersed in the YSZ powder suspension at ∼5 °C set the suspension in to stiff wet-coagulated body when exposed to room temperature (30 °C) due to the reaction between ammonium poly(acrylate) and MgO. MgO concentration equivalent to react with dispersant did not coagulate the YSZ powder suspension though it precipitate the whole ammonium poly(acrylate) dispersant as Mg-poly(acrylate). This is because of the ability of the YSZ powder to disperse in water at alkaline pH (∼9.5) without any dispersant by electrostatic mechanism. The YSZ powder suspensions form stiff coagulated bodies at MgO concentration double or more of the equivalent amount required for reacting with the dispersant. Setting of the YSZ powder suspension is due to the heterocoagulation of the YSZ particles and MgO particles added in excess of the equivalent amount to react with the dispersant, having opposite surface charges. The wet-coagulated body showed relatively high compressive yield strength (155 kPa) and Young’s modulus (3.1 MPa). The green bodies prepared by humidity controlled drying of the wet-coagulated bodies sintered to >98% TD at 1550 °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.     

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.     

Enhanced densification and dielectric properties of CaTiO3-0.3NdAlO3 ceramics fabricated by direct coagulation casting

Severe sintering conditions always cause defects that increase the dielectric loss of microwave ceramics. Direct coagulation casting (DCC) of submicron powder suspension, was applied to accelerate densification and enhance dielectric properties of 0.7CaTiO₃-0.3NdAlO₃ (CTNA) ceramics. The rapid in-situ coagulation of suspension was achieved by tailoring the pH value to the isoelectric point to weaken the repulsion of electric double layer and steric hindrance effect. Compared with the dry-pressed samples, the DCC-fabricated samples are more compacted, which provides a high initial densification value and a shortened diffusion path, and consequently promotes the material transportation and composition homogenization that inhibits the segregation of CaAl₁₂O₁₉. Additionally, the submicron powders accelerate the phase transition and domains growth, leading to a significant reduction of lattice defects in microstructure. Therefore, Q⋅f value of CTNA ceramics reaches more than 40,000 GHz, which is improved by 15 % compared with that of dry-pressed samples sintered under the same conditions.     

MgO calcination for easy direct coagulation casting of aqueous alumina slurries

The reactivity of MgO with ammonium poly(acrylate) and diammonium hydrogen citrate dispersants was decreased by high-temperature calcination which enabled easy preparation of direct coagulation casting slurries without cooling. The decrease in reactivity of MgO with an increase of calcination temperature (30–1200?°C) was due to the decrease of surface area (52.7–0.7?m²/g) as a result of an increase of average particle size (285–2075?nm) as well as a change of particle morphology from flaky to near spherical. The MgO calcined at a temperature of 1000?°C and above provided sufficient time for mixing with aqueous alumina slurries by ball milling at room temperature (~30?°C) without producing an adverse increase in viscosity before casting. The setting time of 55?vol% alumina slurries was in the ranges of 260–1070 and 10–50?min at room temperature and at 70?°C, respectively, at MgO concentrations in the range of 0.1123–1.2?wt%. The faster setting at 70?°C was due to a combination of faster dispersant-MgO reaction, faster hydration of MgO and high valance counter ion effect.     

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.     

Pyrolysis mechanism of magnesium citrate nonahydrate and microstructural evolution during the process

The key characteristics of the porous carbon materials and ceramic composites derived from citrates are directly affected by the pyrolysis mechanism of parent citrates and the microstructural revolution during the process. The pyrolysis mechanism of magnesium citrate nonahydrate (MCN) and the microstructural evolution during its pyrolysis were investigated by analysing the C/MgO nanocomposite powders from MCN pyrolyzed in carbon embedded condition and flowing argon atmosphere. The pyrolysis process of MCN was composed of the following stages: (1) MCN dehydrated to magnesium citrate at about 150 °C; (2) magnesium citrate decomposed into itaconic acid magnesium and MgO at about 300 °C; (3) itaconic acid magnesium decomposed into carbon, MgO and CH₄ at around 500 °C; (4) CH₄ was pyrolyzed and graphene was deposited on MgO. The carbon produced in stage (3) was turbostratic while that derived from the pyrolysis-deposition of CH₄ was few-layered-graphene. The MgO nano grains produced in stage (2) precipitated and agglomerated while those derived from itaconic acid magnesium were much smaller in size. In carbon embedded condition, the few-layered-graphene not only deposited on the MgO aggregates surface but also inserted into the MgO nano grain boundaries, which suppressed the growth and sintering of MgO nano grains.     

Fabrication and electrochemical properties of SOFC single cells using porous yttria-stabilized zirconia ceramic support layer coated with Ni

A porous yttria-stabilized zirconia (YSZ) ceramic supported single cell with a configuration of porous YSZ support layer coated with Ni/Ni–Ce_0.8Sm_0.2O_1.9 (SDC) anode/YSZ/SDC bi-layer electrolyte/La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ cathode was fabricated. The porosity, mechanical strength, and microstructure of porous YSZ ceramics were investigated with respect to the amount of poly(methyl methacrylate) (PMMA) used as a pore former. Porous YSZ ceramics with 56 vol.% PMMA showed a mechanical strength of 24 ± 3 MPa and a porosity of 37 ± 1%. The electrochemical properties of the single cell employing the porous YSZ support layer were measured using hydrogen and methane fuels, respectively. The single cell exhibited maximum power densities of 421 mW/cm² in hydrogen and 399 mW/cm² in methane at 800 °C. Moreover, at a current density of 550 mA/cm², the cell maintained 91% of its initial voltage after operation in methane for 13 h at 700 °C.     

Preparation and properties of fused silica ceramics by Isobam spontaneous coagulation casting

By surface modification with APTMS, spontaneous coagulation casting (SCC) of fused silica based on Isobam was achieved and the possible coagulation and molding mechanism is proposed. Through the interaction between the polar groups on the Isobam molecular chain and the incorporated –NH₂ groups on the surface of the silica particles, Isobam molecular chains were adsorbed on the surface of particles, which initiate the formation of flocs and the solidification of the suspension. The addition of dispersant TMAH results in the hydrolyzation of Isobam, forming more –COO^–, which effectively improves the fluidity and stability of the suspension. Then the zeta potential, rheological properties and coagulation behavior of the suspension were systematically investigated and the fused silica suspension with high solid content (up to 52 vol%), low viscosity and good coagulation properties were prepared at 1.8 wt% TMAH and 0.5 wt% Isobam dosage. After sintering at 1260 °C for 4 hours, the fused silica ceramics (50 vol% solid content) shows a high bending strength of 61.59 MPa, the lowest dielectric loss tanδ of 8.46×10^-4 and the dielectric constant of 3.72. Thus, this work provides a simple and effective method for preparing fused silica and other ceramics with negative surface charge by Isobam SCC.     

A new direct coagulation casting process for alumina slurries prepared using poly(acrylate) dispersant

Direct coagulation casting (DCC) of concentrated aqueous alumina slurries prepared using ammonium poly(acrylate) dispersant has been studied using MgO as coagulating agent. Addition of small amounts of MgO increased the viscosity of the concentrated alumina slurries with time and finally transformed it in to a stiff gel. Sufficient working time for degassing and casting could be achieved by cooling the slurries to a temperature of ∼5 °C after proper homogenization after the addition of MgO. The DCC slip with alumina loading in the range of 50–55 vol% showed relatively low viscosity (0.12–0.36 Pa s at shear rate of 93 s⁻¹) and yield stress (1.96–10.56 Pa) values. The wet coagulated bodies prepared from slurries of alumina loading in the range of 50–55 vol% had enough compressive strength (45–211 kPa) for handling during mould removal and further drying. The coagulated bodies prepared from slurries of alumina loading in the range of 50–55 vol% showed linear shrinkage in the range of 4.8–2.3 during drying and 17.1–16.2 during sintering respectively. Near-net-shape alumina components with density >98% TD could be prepared by the DCC process.     

Compression strength and fracture mechanism of a hierarchical porous yttria-stabilized zirconia microsphere prepared using electro-spraying associated with phase inversion technique

The compression strength and breakage mechanism of a hierarchical porous sphere in hundred-micron size were investigated in the present work. 3 mol% yttria-stabilized zirconia (YSZ) microspheres were prepared by electro-spraying associated with phase inversion (ES-PI) technique. The characteristic compression strengths of the ES-PI microspheres were measured by quasi-static uniaxial compression test, which increased from 19 MPa to 155 MPa as the sintering temperature increased from 1100 °C to 1400 °C. With the similar porosity, the compression strength of the hierarchical structure microsphere was almost three times higher than that of the hollow microsphere. Further, the breakage mechanism of the ES-PI microspheres was proposed by the honeycomb model of cellular materials, which suggested that the breakage of the ES-PI microsphere initiated from the elastic instability of the walls around the finger-like pores. These findings can help the mechanical performance optimization for ceramic microspheres with lightweight structure.     

A simple and effective method for gel casting of zirconia green bodies using phenolic resin as a binder

A simple, effective method for fabricating zirconia green bodies is described that utilizes phenolic resin as a binder in the gel casting process. Both the zeta potential of zirconia particles and the rheological behavior of the slurries were measured. To prepare stable, homogeneous, fluidic zirconia slurry with high solid loading, the zeta potential was adjusted by varying the phenolic resin content of the premixed solution. This represented a departure from normally adjusting the slurry pH by using an acids, alkali or dispersants. To promote gelation of the slurry, a curing agent was added. Gel casting a mixture of a 42 vol.% zirconia slurry containing 13 wt.% phenolic resin yielded an easily sintered, very homogeneous green body with the desired strength. The gelation time of the slurries and the mechanical strength of the green bodies were controlled by adjusting the quantity of the curing agent in the slurries.