Wenjea J. Tseng

Water-based sol–gel synthesis of hydroxyapatite: process development

Hydroxyapatite (HA) ceramics were synthesized using a sol-gel route with triethyl phosphite and calcium nitrate as phosphorus and calcium precursors, respectively. Two solvents, water and anhydrous ethanol, were used as diluting media for HA sol preparation. The sols were stable and no gelling occurred in ambient environment for over 5 days. The sols became a white gel only after removal of the solvents at 60 degrees C. X-ray diffraction showed that apatitic structure first appeared at a temperature as low as 350 degrees C. The crystal size and the HA content in both gels increase with increasing calcination temperature. The type of initial diluting media (i.e., water vs. anhydrous ethanol) did not affect the microstructural evolution and crystallinity of the resulting HA ceramic. The ethanol-based sol dip-coated onto a Ti substrate, followed by calcination at 450 degrees C, was found to be porous with pore size ranging from 0.3 to 1 microm. This morphology is beneficial to the circulation of physiological fluid when the coating is used for biomedical applications. The satisfactory adhesion between the coating and substrate suggests its suitability for load-bearing uses.

Rheology and colloidal structure of aqueous TiO2 nanoparticle suspensions

Rheological behavior and suspension structure of anatase titanium dioxide (TiO2) nanoparticles dispersed in pure water have been investigated over a range of volumetric solids concentrations (φ=0.05–0.12) and shear rates (γ=101–103 s−1). The nanoparticle suspensions generally exhibited a pseudoplastic flow behavior, indicating an existence of particle aggregations in the liquid medium. The suspensions became apparently thixotropic as φ was increased above 0.1. Relative viscosity (ηr) of the suspensions followed an exponential form with φ, i.e., ηr=13.47e35.98φ, revealing a pronounced increase in the degree of particle interactions as φ increased. Fractal dimension (Df) was estimated from the suspension yield-stress (τy) and φ dependence, and was determined as Df∼1.46–1.78 for the flocculated nanoparticle suspensions. This suggested that the suspension structure was probably dominated by the diffusion-limited cluster–cluster aggregation, due mostly to the strong attractions involved in the interparticle potentials. Maximum solids loading (φm) of the suspensions was determined as φm=0.146. This relatively low value of φm (compared with the random close packing of monosized particles, φm∼0.64) partially vindicated the existence of a porous, three-dimensional aggregate network of interconnected nanoparticles in the carrier liquid.

Structural evolution of sol–gel-derived hydroxyapatite

Structural evolution upon transformation of sol to gel, and gel to final ceramic during the synthesis of hydroxyapatite is investigated using Fourier transform infrared (FTIR) analysis, X-ray diffraction (XRD), thermal behavior (DTA and TGA), and electron microscopy examination (SEM/TEM). The sol was first thermally aged at 45 C for various time periods up to 120 min. The colloidal sol, which may have an oligomeric structure, was relatively stable against coagulation. Upon drying, the sol particles consolidated into dry gel through van der Waals attraction, and showed X-ray amorphous phosphate structure. The solid gels showed a particulate microstructure, composed of primary particles of about 8-10 nm in diameter. The amorphous gel transformed into crystalline apatite at temperatures > 300 C. The calcined gels showed a nano-scale microstructure, with grains of 20-50 nm in diameter. Through an appropriate heat treatment between 300 and 400d degrees C. the apatite prepared using current process exhibits a nano-scale, low-crystallinity, carbonated apatitic structure, which closely resembles that of human bone apatite.

Aging effect on the phase evolution of water-based sol-gel hydroxyapatite.

In a number of recent reports on the synthesis of sol-gel hydroxyapatite, aging of the precursor solution has been found to be critical in developing an apatitic phase. Critical aging time is required to complete reaction between Ca and P molecular precursors to form a desired intermediate complex that permits a further transformation to apatite phase under appropriate thermal treatment. In this investigation, we employed a water-based sol-gel process recently developed to fabricate hydroxyapatite at relatively low temperatures. The aging effect on apatite formation was systematically studied in terms of aging time and temperature. Experimental results show that the aging time is considerably reduced as aging temperature rises. Long-term thermal aging was unfavorable for apatite formation. The optimal aging parameters for apatite formation were experimentally determined, which was further consolidated into a phase evolution map. Aging kinetic was investigated by monitoring the variation of solution pH, following the determination of an apparent activation energy, which has a value as high as 10.35 kcal/mol, for the chemical reaction occurring upon aging. Optimal solution chemistry was elucidated based on the corresponding phase evolution map.

Effect of polymeric dispersant on rheological behavior of nickel–terpineol suspensions

The rheological behavior of submicrometer nickel powders dispersed in terpineol solvent was investigated in terms of dispersant type, dispersant concentration (0.5–10% of the powder weight) and solids loading (3–10 vol.%) over a shear-rate range 1–1000 s−1. Seven commercially available polymeric dispersants of varying natures were used in the study; among which, the polymeric surfactants featured with nonionic functional groups provided the best dispersion effectiveness. The suspension viscosity reached a minimum as the dispersant concentration exceeding 2 wt.% of the solids to a viscosity level about 60% of the suspension containing no dispersant. The suspensions all appeared pseudoplastic over the shear-rate range investigated, revealing the mixtures were flocculated in structure. The maximum solids concentration (φm) attainable was predicted as φm=0.113±0.014 in the given suspension system. The rheological measurement further revealed that the particulate network formed in the flocculated slurry was most likely of a fractal nature; to which, an attractive van der Waals force dominated the interparticle potentials. An estimated fractal dimension (Df) of 2.13 was determined experimentally, suggesting a reaction-limited cluster–cluster aggregation may be in operative or the aggregates may have undergone substantial flow-induced rearrangement upon shearing.

Sedimentation, rheology and particle-packing structure of aqueous Al2O3 suspensions

Abstract Gravity settling, rheology and particle-packing structure of aqueous alumina suspensions have been investigated using submicrometer α-Al2O3 powders with an average particle size ∼0.2 μm dispersed in pure water. The suspension pH varied from 2 to 11. The interface that separated the supernatant from the sediment appeared to settle linearly with time during the sedimentation experiment, followed then by a gradual reach toward a minimal sediment height; to which, the height was pH and solids loading dependent. The suspensions tended to form a continuous particulate network that defined the gravity settling as φ⩾0.1 at pH 11. This resulted in a flocculated suspension structure which barely settled even after 24 h without disturbance. The flocculated suspensions exhibited correspondingly a shear-thinning flow character over given shear-rate range (γ=1–1000 s−1) examined, in contrast to an apparent transition of flow toward shear thickening from the shear thinning for the better-dispersed suspensions (pH=2) at relatively high shear rates (γ⩾100 s−1). This finding suggested a breakdown of the particulate network into smaller flow units for the flocculated suspensions as shear rate was increased. Films made from the flocculated suspensions showed apparent cracking after drying. Microstructural examinations revealed that a pronounced grain growth occurred in the films made from the pH 2 suspensions after isothermal sintering.

Influence of surfactant on rheological behaviors of injection–molded alumina suspensions

The effect of three different surfactants, namely, stearic acid (SA), oleic acid (OA) and 12-hydroxystearic acid (HSA), on the rheological properties of injection-molded alumina suspensions has been investigated over a temperature range of 120–150°C and a shear-rate range of 1000–15 000 s−1. The suspension mixtures exhibited a pseudoplastic flow behavior and a viscosity (η) order of ηSA<ηOA<ηHSA over the shear-rate range tested. The yield stress of the suspensions, on the other hand, followed a reverse order as that of the viscosity and the suspension flow was highly dependent on the working temperature employed. A particulate network within the mixtures in a form of powder agglomerates was suspected to play a major role in the flow behavior observed, over which, different levels of powder agglomeration were resulted from the varying surfactants used. A model experiment by repeatedly shearing the suspensions under a fixed stress level partially confirmed the hypothesis. The suspension viscosity was reduced in a power-law fashion as the number of shears was increased and eventually a minimum viscosity was reached. This suggests that the particle network restructured itself upon the shear was applied, leading to breakup of the network and improvements in the particle packing.

Rheology of colloidal BaTiO3 suspension with ammonium polyacrylate as a dispersant

Abstract The rheological behavior of barium titanate (BaTiO 3 ) with ammonium polyacrylate (NH 4 PA) as a dispersant has been investigated over a range of volumetric solids concentration ( φ =0.1–0.55) in shear rate 1–1000 s −1 . The addition of NH 4 PA decreased the suspension viscosity pronouncedly. The relative viscosity ( η r ) of the suspension defined as the suspension viscosity over the viscosity of the suspending medium was found increasing substantially when φ exceeds 0.4. A derivative of η r , i.e. 1− η r −1/2 , appeared linearly proportional to φ ; from which, a maximum solids fraction ( φ m ) which presents the theoretical solids concentration at which the suspension viscosity approaches infinity was determined as φ m =0.584 for the given suspension system. The η r – φ relationship determined experimentally was compared with various existing models and the (viscosity) predictive capability of the models also discussed.

Cracking defect and porosity evolution during thermal debinding in ceramic injection moldings

Formation of cracking defect during thermal debinding of a wax-based zirconia–binder system has been investigated at varying binder formulations (volumetric ratio of low-melting vs high-melting binder constitutes ranging from 1.7 to 2.8), heating rates (5–30°C h−1) and the use of wicking powders in ceramic injection moldings. Increase of the low-melting binder ingredients to a critical ratio of 2.8 resulted in an apparent cracking at a relatively low pyrolysis temperature of 250°C as the heating rate was increased to 30°C h−1. The cracking was avoided by use of wicking powders during thermolysis. The most frequent pore diameter of samples debound without the packing powders rapidly increased by nearly 3-fold over a temperature range from 200 to 250°C; contrarily, a gradual increase in the pore size was resulted when moldings were debound under the identical condition but with the packing powders. This finding is correlated with the incidence of cracking and its possible causes are discussed.

The effect of surfactant adsorption on sedimentation behaviors of Al2O3–toluene suspensions

The sedimentation behavior and suspension stability of submicrometer Al2O3 powder–toluene mixtures have been investigated using various organic surfactants to aid powder dispersion in a dilute solids concentration (4.8 vol.%). The surfactants used include a commercially-available, carbonate-based dispersant (hereafter termed the ‘SOL’) and some fatty acids (e.g. stearic acid ‘SA’, oleic acid ‘OA’ and 12-hydroxystearic acid ‘HSA’) in a concentration range 0.3–5 wt.% of the alumina loading. The experiment reveals that the surfactant nature is critical to the suspension stability. The suspensions follow a decreasing order of stability SOL>HSA>SA>OA for the Al2O3–toluene mixtures. This surfactant-dependent behavior is critically related to the adsorption affinity of the surfactant molecules on the powder surface in the carrier medium. The adsorption isotherms exhibit the Langmuir-typed behavior and reach a saturation plateau over a range of 1.3–2.4 mg m−2, revealing a surfactant-specific characteristic.