Dispersant-Binder Interactions in Aqueous Silicon Nitride Suspensions

The interaction of dispersant and binder on the surface of particles was studied to identify the effect of these additives on aqueous ceramic powder processing. Poly(methacrylic acid) (PMAA) and poly(vinyl alcohol) (PVA) were used as the dispersant and binder, respectively. The adsorption isotherms of the organic additives on silicon nitride were determined. The adsorption of PMAA was differentiated from PVA in the mixed additive system via ultraviolet spectroscopy. The electrokinetic behavior of silicon nitride was measured by using an electrokinetic sonic amplitude analyzer. As the PMAA concentration increased, the isoelectric point (pH_iep) of silicon nitride shifted from pH 6.7 ± 0.1 to acidic pH values. The magnitude of the shift depended on the surface coverage of PMAA. PVA did not affect the pH_iep of suspensions but did cause a moderate decrease in the near-surface potential. Finally, the rheological behavior of silicon nitride suspensions was measured to assess the stability of particles against flocculation in aqueous media; this behavior was subsequently correlated with the electrokinetic and adsorption isotherm data.     

Interfacial Characterization of Silicon Nitride Powders

The composition of the surface and the behavior in aqueous suspensions of three silicon nitride powders were investigated using electron spectroscopy for chemical analysis (ESCA), potentiometric titrations, leaching experiments, and electrophoretic mobility. ESCA shows that the as-received powders have a surface-layer composition similar to that identified as an intermediate state between silica and silicon oxynitride. The original differences in pH_iep between the three powders disappears by aging the powders. The common pH_iep of 6.8 ± 0.3 for the three powders is interpreted as the equilibrium pH_iep for silicon nitride in aqueous suspensions.     

Stabilization and Processing of Aqueous BaTiO3 Suspension with Polyacrylic Acid

Polyacrylic acid (PAA) has been chosen to stabilize BaTiO₃ powder in aqueous solution. Zeta potential studies show that the Ba-rich particle surface is positively charged in the pH range from 2 to 11 without PAA. Adsorption of PAA decreases the zeta potential in acidic solution and changes the surface to a negative charge in basic solution. Adsorption of PAA onto BaTiO₃ surfaces is found to be related to the pH-dependent PAA conformation in solution. The amount of PAA adsorbed at pH 1.5 is one order of magnitude more than at pH 10.5, due to a more compact polymer conformation. Colloid stability of aqueous BaTiO₃ suspension is related to the dissociation of PAA as a function of pH. BaTiO₃ suspensions can be stabilized at pH values higher than 7, where more than 99% of carboxylic groups are ionized. At pH 10.5, suspension stability is related to the PAA coverage on BaTiO₃ particle surface. Stabilization can be achieved only when conditions of both PAA ionization and BaTiO₃ surface coverage are satisfied, suggesting an electrosteric stabilization mechanism. Green density of 62% can be achieved by slip casting at a pH above 10, compared to the best density of 58.6% by isostatic pressing at 242 MPa.     

Adsorption of PAA on the α-Al2O3 Surface

The specific system of interest is the polyacrylic acid (PAA) and (0001) α-Al₂O₃ surface, which was modeled and simulated by Cerius² 4.9 software with empirical potentials. The simulation predicted that the adsorbed conformations of PAA with a molecular weight ( M _w) of 5000 were train and tail at pH <4 and >10, respectively. After gradually inserting additional PAA molecular chains, the adsorption reached a saturated amount. Gel permeation chromatography experimental results showed that the adsorption amount at pH 3.6 was three times greater than that at pH 11. Based on the results from simulations and experiments, a successively increasing pH environment was modeled to illustrate the possibility of optimizing electro-steric effects by combining the higher adsorption density at a lower pH and strong steric repulsion of tail-adsorbed configuration at a higher pH.     

Optimization of a Nanoparticle Suspension for Freeze Casting

Poly(acrylic acid) (PAA) dispersant concentration, suspension pH, and Al₂O₃ solids loading effects on PAA adsorption onto Al₂O₃ nanoparticles were studied; the stability and rheology of the Al₂O₃ nanoparticle suspensions were examined. The most desirable suspension conditions were 7.5–9.5 for pH and 2.00–2.25 wt% of Al₂O₃ for the PAA concentration. Electrical double-layer thickness and PAA adsorption layer thickness comparison showed that electrosteric stabilization was dominant. 45.0 vol% Al₂O₃ solids loading can be achieved for freeze casting. The maximum solids loading was predicted to be 50.7 vol%. The freeze-cast sample showed that pre-rest before freezing was critical for achieving desirable microstructures.     

Surface Chemistry, Dispersion Behavior, and Slip Casting of Ti3AlC2 Suspensions

The surface chemistry and dispersion properties of aqueous Ti₃AlC₂ suspension were studied in terms of hydrolysis, adsorption, electrokinetic, and rheological measurements. The Ti₃AlC₂ particle had complex surface hydroxyl groups, such as ≡Ti–OH,=Al–OH, and −OTi–(OH)₂, etc. The surface charging of the Ti₃AlC₂ particle and the ion environment of suspensions were governed by these surface groups, which thus strongly influenced the stability of Ti₃AlC₂ suspensions. PAA dispersant was added into the Ti₃AlC₂ suspension to depress the hydrolysis of the surface groups by the adsorption protection mechanism and to increase the stability of the suspension by the steric effect. Ti₃AlC₂ suspensions with 2.0 dwb% PAA had an excellent stability at pH=∼5 and presented the characteristics of Newtonian fluid. Based on the well-dispersed suspension, dense Ti₃AlC₂ materials were obtained by slip casting and after pressureless sintering. This work provides a feasible forming method for the engineering applications of MAX-phase ceramics, wherein complex shapes, large dimensions, or controlled microstructures are needed.     

Forces Measured between Zirconia Surfaces in Poly(acrylic acid) Solutions

We have studied the forces between a sphere and a plane surface of yttria-partially-stabilized tetragonal-zirconia immersed in aqueous solutions of low-molecular-weight ( M _w= 10 000) poly(acrylic acid) (PAA) using atomic force microscopy. The measurements are performed at high pH where the adsorbed, highly charged anionic polyelectrolyte extends far into the solution, resulting in a combination of polymeric (steric) and electrostatic interactions. Analysis of the experimental data using scaling theory shows that the polymeric contribution dominates and that the electrostatic contribution is small at relatively high ionic strength (0.01 M NaCl). We find that the measured forces are highly dependent on time and interaction history of the absorbed PAA layer; consecutive compression-decompression cycles result in an increase of the surface coverage and the range of the repulsive polymeric interaction. This buildup of PAA at the interface is strongly related to attractive bridging interactions manifested as strong adhesion during decompression at less than full surface coverage. The force results are compared to rheological observations of zirconia suspensions stabilized by the same dispersant; the poor colloidal stability and high viscosity at low surface coverage of PAA are related to the attractive bridging interactions.     

Hydrolysis and Dispersion Properties of Aqueous Y2Si2O7 Suspensions

The dispersion of aqueous γ-Y₂Si₂O₇ suspensions, which contain only one component but have a complex ion environment, was studied by the introduction of two different polymer dispersants, polyethylenimine (PEI) and polyacrylic acid (PAA). The suspension without any dispersant remains stable in the pH range of 9–11.5 because of electrostatic repulsion, while it is flocculated upon stirring due to the readsorption of hydrolyzed ions on the colloid surface. However, suspensions with 1 dwb% PEI exhibit greater stability in the pH range of 4–11.5. The addition of PEI shifts the isoelectric point (IEP) of the suspensions from pH 5.8 to 10.8. Near the IEP (pH_IEP=10.8), the stability of the suspensions with PEI is dominated by the steric effect. When the pH is decreased to acid direction, the stabilization mechanism is changed from steric hindrance to an electrosteric effect little by little. PAA also has the effect of reducing the hydrolysis speed via a "buffer effect" in the basic pH range, but the lack of adsorption between the highly ionized anionic polymer molecules and the negative colloid particle surfaces shows no positive effect on hydrolysis of colloids and on the stabilization of Y₂Si₂O₇ suspensions.     

Development of a Self-Forming Ytterbium Silicate Skin on Silicon Nitride by Controlled Oxidation

A dense and uniform polycrystalline ytterbium silicate skin on silicon nitride ceramics was developed by a controlled oxidation process to improve the hot corrosion resistance of silicon nitride. The process consists of purposely oxidizing the silicon nitride by heating it at high temperatures. It was found that the ytterbium silicate phase was formed as an oxidation product on the surface of the silicon nitride when it was exposed to air at temperatures above 1250°C. The volume fraction of ytterbium silicate compared with that of SiO₂ on the silicon nitride surface increased with increasing oxidation time and temperature. The formation and growth of ytterbium silicate on the surface of silicon nitride is attributed to a nucleation and growth mechanism. Ultimately, a dense and uniform ytterbium silicate skin with 3–4 μm of skin thickness was obtained by oxidation at 1450°C for 24 h. The ytterbium silicate layer, formed by oxidation of the silicon nitride, is associated with the reaction of SiO₂ on the surface of silicon nitride with Yb₂O₃ introduced in the silicon nitride as a sintering additive. Preliminary tests showed that the ytterbium silicate skin appears to protect silicon nitride from hot corrosion. No observable evidence of a reaction between the skin and molten Na₂SO₄ was found when it was exposed to molten Na₂SO₄ at 1000°C for 30 min.     

Effect of Zwitterionic Surfactants on Interparticle Forces, Rheology, and Particle Packing of Silicon Nitride Slurries

Phosphocholine (PC) zwitterionic surfactants, with different hydrocarbon chain lengths (C₆C₆PC to C₉C₉PC), were absorbed on the surface of silicon nitride near the isoelectric point (pH 6). Adsorption of the surfactants changed the lateral and normal surface forces, the rheology, and the consolidation behavior of the particles. The normal force between two silicon nitride surfaces as a function of separation and the lateral (friction) forces were measured using an atomic force microscope (AFM). These measurements indicated that surfactant adsorption reduced the magnitude of the long-range attractive van der Waals force and produced a repulsive short-range force. Although the adsorbed layers provided a barrier to particle contact, they could be ejected with a critical force that increased with the hydrocarbon chain length. The effect of an adsorbed layer on the viscosity and consolidation of slurries was also measured. The viscosity of all slurries decreased with increasing shear rate, indicative of attractive particle networks. The highest viscosity was observed for slurries formulated at the isoelectric point without added surfactant. Much lower viscosities were observed when the surfactant concentration was greater than the critical micelle concentration (cmc). A relative density of 0.46 was obtained via pressure filtration at 4 MPa without a surfactant, and between 0.46 to 0.59 (C₆C₆PC to C₉C₉PC, respectively) for surfactant concentrations greater than the cmc. Comparing force measurements with rheology and packing density provides a basis for discussing the role of interparticle forces in ceramic powder processing via colloidal routes.     

pH-induced conformational changes of loosely grafted molecular brushes containing poly(acrylic acid) side chains

A series of water-soluble loosely grafted poly(acrylic acid) (PAA) brushes with four different grafting densities were synthesized by the “grafting from” approach using atom transfer radical polymerization (ATRP). Gel permeation chromatography (GPC) and ¹H NMR spectroscopy were used to provide evidence for formation of the well-defined backbones and the resulting brush copolymers. Atomic force microscopy was used to study the conformation of adsorbed brushes as a function of pH. The adsorbed molecules undergo a globule-to-extended conformational transition as the solution is changed from acidic to basic. This transition was monitored on a mica surface by imaging individual molecules with atomic force microscopy (AFM). The conformational behavior was compared with 100%-grafted PAA brushes. Unlike the loose brushes, the 100%-grafted molecules remained fully extended in a broad range of pH values (pH = 2-9) due to steric repulsion between the densely grafted side chains which is strongly enhanced upon adsorption to a substrate.     

Dispersing Behavior of Hydroxyapatite Powders Produced by Wet-Chemical Synthesis

Three hydroxyapatite powders with different surface properties were produced by wet-chemical synthesis and characterized. The electrokinetic properties of powders dispersed in water were investigated by electroacoustic spectroscopy measurements. The different surface reactivity (pH_iep and ζ potential versus pH curves) was related to the interplay of dissolution and adsorption of Ca²⁺ ions. With a view toward the preparation of porous bodies by sponge impregnation, the behavior of powder suspensions was studied. Four deflocculants were tested, and the optimum dispersing conditions for each powder were found. Anionic polyelectrolytes resulted in the best effective dispersing agent, with different optimum amounts added to the suspensions.     

Strength of Green Ceramics with Low Binder Content

Acrylic-based polymers are common binders that impart high green strength (>2 MPa) at low concentrations (<5.0 vol%). Strength at low binder concentrations may be determined by chemical bonding at the ceramic–polymer interface. We have studied the binding mechanisms as a function of ceramic surface chemistry using a cross-linkable binder, which is based on a soluble poly(acrylic acid) (PAA, MW = 60 000) and glycerol. The cross-linked PAA binder system has been integrated into a solid freeform fabrication process, which provides a means of fabricating very reproducible green bodies, including SiO₂, TiO₂, Al₂O₃, multicomponent oxides, and non-oxides, with uniform density and composition. The ceramic parts contain only 2.5 vol% binder (solids basis), which increases the strength of the ceramic systems by at least a factor of 8 while the strength of Al₂O₃ components increases by a factor of ∼24 (0.3 to 7.6 MPa). Addition of the binder improves the toughness of the ceramic bodies by an order of magnitude with SiO₂ representing the largest relative increase (2.8 × 10⁻³ to 4.4 × 10⁻² MPa·m^1/2). The mechanical properties are dictated by two binding mechanisms: binder adsorption and mechanical interlocking. High green strengths result from adsorption of the binder onto the ceramic surface whereas toughness is enhanced by poor adhesion of the binder to the ceramic surface.     

Role of Ionic Depletion in Deposition during Electrophoretic Deposition

A model is developed for deposition during the electrophoretic deposition (EPD) process. It suggests that ions that move with the charged particles in suspension are depleted at the depositing electrode, locally changing the pH toward the isoelectric point (pH_iep) to give coagulation. The variation of zeta (zeta) potential is modeled via chemical-equilibrium and surface-adsorption isotherms. The model successfully fits the experimental data for Al₂O₃ particles in ethanol when the Freundlich surface-adsorption isotherm is assumed. Calculations predict the co-ion concentration gradient as a function of location within the suspension, and the deposition time and its role in the coagulatoin process during EPD.     

Preparation of Mesoporous Silicon Nitride via a Nonaqueous Sol–Gel Route

A silicon diimide gel Si(NH) _x (NH₂) _y (NMe₂) _z was prepared by an acid-catalyzed ammonolysis of tris(dimethylamino)silylamine. Pyrolysis of the gel at 1000°C under NH₃ flow led to the formation of an amorphous silicon nitride material without carbon contamination. All of the gel and pyrolyzed products exhibited a mesoporous structure with a high surface area and narrow pore-size distribution. The effective surface area of the pyrolyzed silicon nitride residues decreases with increasing temperature, but the heating rate during pyrolysis has little influence on the surface area and pore-size distribution of the final mesoporous ceramic Si₃N₄ products because of the highly cross-linked structures of the precursor silicon diimide gel.     

Oxidation of a Silicon Nitride-Titanium Nitride Composite: Microstructural Investigations and Phenomenological Modeling

The high-temperature oxidation of a silicon nitride-titanium nitride (Si₃N₄–TiN) composite has been investigated via scanning electron microscopy and energy-dispersive and wavelength-dispersive spectrometry. At 1150°C, the oxidation of both the silicon nitride and titanium nitride phases takes place. Several oxidation processes act simultaneously and/or successively. First, the oxidation of the titanium nitride occurs and leads to the formation of a continuous titanium oxide (TiO₂) crystal layer at the surface. Next, the TiO₂ formation takes place in the sublayer at the same time as the Si₃N₄ oxidation. The oxidation of this last phase leads to the formation of vitreous silica (SiO₂). For long a duration of oxidation (>50 h), a continuous layer of SiO₂ is formed under the outer TiO₂ scale. Large pores grow in this layer and deform the outer oxide layers, whereas the oxidation occurs in the material. Based on these results and bibliographical data, a phenomenological model is proposed to describe the stages of the high-temperature oxidation of Si₃N₄–TiN materials.     

Electrolyte Effects on Nonionic Steric Layers: Bis-hydrophilic PMAA-PEO Diblock Copolymers Adsorbed on Barium Titanate

The steric layers of bis-hydrophilic diblock copolymers formed of poly(methacrylic acid) (PMAA) and poly(ethylene oxide) (PEO) have been investigated through the direct examination of repulsive forces using the atomic force microscopy (AFM) colloidal probe technique. Increasing the KNO₃ and Ba(NO₃)₂ concentrations causes a reduction in the extension of the PEO chains into the solution, but attractive forces are never observed. It is believed that the electrolyte level through the hydration of the polymer affects the conformation of the stabilizing PEO chains. Increasing electrolyte levels disrupt the hydrogen bonding between the polymer and solvent needed to create an extended polymer conformation.     

Elucidation of mechanism of corrosion inhibition by polyacrylic acid and synergistic action with iodide ions by in-situ AFM

Corrosion inhibition mechanism of aluminium in H₂SO₄ by polyacrylic acid (PAA) in the absence and presence of iodide ions was investigated using in-situ atomic force microscopy (AFM). Results indicate that PAA inhibited the corrosion of aluminium by adsorption onto the aluminium surface. Inhibition efficiency of PAA was found to enhance on addition of iodide ion which could be ascribed to synergistic effect. In-situ AFM morphology of the surface film suggests that PAA in the presence of iodide ion was adsorbed on aluminium surface after taking a more ordered arrangement, thus providing a more uniform coverage at potentials below and above potential of zero charge.     

Aqueous Processing of Sintered Reaction-Bonded Silicon Nitride: I, Dispersion Properties of Silicon Powder

An aqueous-based system (Si-Al₂O₃-Y₂O₃-Fe₂O₃) for processing sintered reaction-bonded silicon nitride (SRBSN) was investigated with an emphasis on chemical control of suspension component interactions. Chemical stability and dispersion properties of a commercial silicon powder were characterized using electroacoustic, adsorption isotherm, and rheological measurements. The interactions of silicon with nitriding agent, sintering aids, dispersants, and binder were considered. The effects of pH, electrolyte, aging, particle size, and solids loading were examined. The suspension properties of the silicon powder were influenced by the native oxide film and powder treatment history. The silicon-oxide composite particles exhibit dispersion behavior similar to silica, characterized by a negative surface potential above pH 2. A method to improve the dispersion and homogeneity of suspension components based on the use of quaternary amine dispersants is proposed.     

Electrokinetic Properties of Nanosized SiC Particles in Highly Concentrated Electrolyte Solutions

In this research, the electrokinetic behavior and stability of nanosized SiC particles suspended in various electroplating solutions were studied. Analyses were performed using electrophoretic mobility photometry and streaming current (SC) techniques. The electrolytes included NiCl₂, Ni(SO₃NH₂)₂, and Na₃Co(NO₂)₆, which are currently used in composite plating solutions with concentrations as high as 0.5 M . The results showed that the adsorption of dissolved Ni²⁺ ions onto the surface of the SiC in the pH range 4–8 changed the sign and magnitude of the surface potential. Moreover, trivalent complex species Co(NO₂)₆³⁻ replaced nickel species on the SiC surface and decreased the surface charge of SiC to between pH 3 and pH 5. Even in a highly concentrated electrolyte solution, the SiC particles still maintained a positive charge in a Ni(SO₃NH₂)₂ suspension with nickel coplating on the cathode. The difference between the SC reading and the zeta potential, as well as the surface adsorption of various species onto the SiC, are discussed here.