Chemisorption of Organofunctional Silanes on Silicon Nitride for Improved Aqueous Processing

A new colloidal route leading to the production of ~99% dense 3 mol% yttria stabilized zirconia nanostructured ceramics, whilst retaining a final average grain size of ~75 nm, has been developed. The process was based on the production of stable, homogeneous nanosuspensions with solids contents of up to 28 vol% (70 wt%) but viscosities less than 0.05 Pa s at any shear rate in the range of study were obtained. The suspensions were formed by the concentration and optimization of precursor, dilute (5.0 vol%) commercial nanosuspensions, the approach requiring a change of pH, from the 2.4 of the as-received suspension to 11.5, and the use of an appropriate anionic dispersant. Exposure of the nanosuspensions to ultrasound also helped to reduce the viscosity further, though it only worked when the dispersant was optimized. The nanosuspension was slip cast to form homogenous green bodies with densities of ~55% of theoretical without agglomeration in the nanostructure; these were subsequently densified using a two-step sintering technique.

Section: Resultsmentioning

confidence: 99%

Preparation of High Solids Content Nanozirconia Suspensions

Santacruz¹,

Anapoorani²,

Binner³

2008

“…The mullite to alumina ratio was selected to achieve high packing densities of the powder body and low shrinkage in the following sintering step. 37,38 The network can be packed to a high density via pressure filtration, and the so-formed consolidated body can subsequently be fluidized again via vibration. Tetraethylammonium hydroxide (TEA-OH) was used to maintain the pH above 11, allowing electrostatic repulsive interactions to develop between the oxide particles.…”

Section: (1) Materials and Composite Processingmentioning

confidence: 99%

“…24,28 Lower alumina content would give fewer sintering necks between mullite particles, whereas mixtures with higher alumina content will densify relatively fast above 1200°C. 32,37,38 Using the weight difference method, the volume fraction of solids within the consolidated bodies was determined to 54 Ϯ 1%. A 2 wt% amount (relative to the solids) of poly(ethylene oxide) urethane silane (PEG-silane, Gelest, Inc.) was added to induce a steric dispersing effect.…”

Section: (1) Materials and Composite Processingmentioning

confidence: 99%

Processing and Properties of a Porous Oxide Matrix Composite Reinforced with Continuous Oxide Fibers

Holmquist

Lange

2003

A process to manufacture porous oxide matrix/polycrystalline oxide fiber composites was developed and evaluated. The method uses infiltration of fiber cloths with an aqueous slurry of mullite/alumina powders to make prepregs. By careful manipulation of the interparticle pair potential in the slurry, a consolidated slurry with a high particle density is produced with a sufficiently low viscosity to allow efficient infiltration of the fiber tows. Vibration‐assisted infiltration of stacked, cloth prepregs in combination with a simple vacuum bag technique produced composites with homogeneous microstructures. The method has the additional advantage of allowing complex shapes to be made. Subsequent infiltration of the powder mixture with an alumina precursor was made to strengthen the matrix. The porous matrix, without fibers, possessed good thermal stability and showed linear shrinkage of 0.9% on heat treatment at 1200°C. Mechanical properties were evaluated in flexural testing in a manner that precluded interlaminar shear failure before failure via the tensile stresses. It was shown that the composite produced by this method was comparable to porous oxide matrix composites manufactured by other processes using the same fibers (N610 and N720). The ratio of notch strength to unnotch strength for a crack to width ratio of 0.5 was 0.7–0.9, indicating moderate notch sensitivity. Interlaminar shear strength, which is dominated by matrix strength, changed from 7 to 12 MPa for matrix porosity ranging from 38% to 43%, respectively. The porous microstructure did not change after aging at 1200°C for 100 h. Heat treatment at 1300°C for 100 h reduced the strength for the N610 and N720 composites by 35% and 20%, respectively, and increased their brittle nature.

“…To manipulate the rheological behavior, several investigations were undertaken to alter the depth of the secondary potential well, either by increasing the concentration of counterions added to dispersed slurries, 4,6,11,12 changing the size of the counterions used to collapses the double layer, 9 or using short-range steric barriers of different sizes. [13][14][15][16][17] Colic et al 17 used a different approach to produce short-range repulsive potentials for Si 3 N 4 powders. Different aminosilane molecules were chemadsorbed on Si 3 N 4 powder to develop a short-range repulsive potential that persists during consolidation and leads to plastic bodies.…”

Section: Introductionmentioning

confidence: 99%

Effect of the Interparticle Pair Potential on the Rheological Behavior of Zirconia Powders: I, Electrostatic Double Layer Approach

Klein

Fisher

Franks

et al. 2000

Self Cite

Aqueous slurries containing 20 vol% ZrO 2 powder doped with 3 mol% Y 2 O 3 were prepared by first dispersing the powder at pH 11, then adding 0.1M to 1.0M tetramethylammonium chloride (TMACl), or 1.0M of TPACl, CsCl, and LiCl to produce different, weakly attractive particle networks. The particle pair potentials in the slurries were investigated by viscosity versus shear rate measurements. Slurries exhibited increasing viscosities (at a given shear rate) with increasing salt concentration and decreasing (unhydrated) counterion size. The viscosities for these weakly attractive networks were intermediate to dispersed (pH 11 without added salt) and flocced (isoelectric point, pH 7.5) slurries. Cylindrical bodies were consolidated from these slurries by pressure filtration at different applied pressures. The bodies consolidated from slurries formulated with TMACl had the highest packing densities relative to those consolidated from a flocculated slurry, but the relative densities were much lower than those achieved from bodies consolidated from a dispersed slurry. The plastic or brittle nature of these bodies was determined in uniaxial compression. Powder compacts consolidated from flocced slurries and slurries coagulated with 1M TMACl, CsCl, and LiCl showed plastic behavior for filtration pressures <7.5 MPa. Results for ZrO 2 will be compared with those previously obtained for Al 2 O 3 , which produces plastic, consolidated bodies over a much broader range of slurry conditions.