Susana M. Olhero

Synthesis, mechanical and biological characterization of ionic doped carbonated hydroxyapatite/β-tricalcium phosphate mixtures

The influence of ionic substituents in calcium phosphates intended for bone and tooth replacement biomedical applications is an important research topic, owing to the essential roles played by trace elements in biological processes. The present study investigates the mechanical and biological evaluation of ionic doped hydroxyapatite/β-tricalcium phosphate mixtures which have been prepared by a simple aqueous precipitation method. Heat treating the resultant calcium phosphates in a carbonated atmosphere led to the formation of ionic doped carbonated hydroxyapatite/β-tricalcium phosphate mixtures containing the essential ions of biological apatite. The structural analysis determined by Rietveld refinement confirmed the presence of hydroxyapatite as the main phase, together with a considerable amount of β-tricalcium phosphate. Such phase assemblage is essentially due to the influence of substituted ions during synthesis. The results from mechanical tests proved that carbonate substitutions are detrimental for the mechanical properties of apatite-based ceramics. In vitro proliferation assays of osteoblastic-like cells (MC3T3-E1 cell line) to powders revealed that carbonate incorporation can either delay or accelerate MC3T3 proliferation, although reaching the same proliferation levels as control cells after 2 weeks in culture. Further, the powders enable pre-osteoblastic differentiation in a similar manner to control cells, as indirectly measured by ALP activity and Type-I collagen medium secretion.

Synergy of polysaccharide mixtures in gelcasting of alumina

Abstract The synergetic effect of mixtures of agar and galactomannan on their gelling ability was exploited to consolidate concentrated aqueous alumina suspensions by gelcasting. Aqueous polysaccharide solutions were prepared at 60°C and added to stabilised and highly concentrated alumina suspensions maintained at the same temperature. These ceramic/polymer slurries were then poured into non-porous moulds, and direct consolidation was accomplished by cooling. Rheological characterisation was used to study the influence of alumina particles on gel network and to determine the optimal processing conditions for gelcasting. Relatively low amounts of polymers (≈ 1 wt%) were sufficient for consolidation, thereby avoiding the binder burnout step. High green density was achieved by this method due to the bimodal character of the starting alumina powder. However, the presence of relatively coarse particles in the starting powder exerted a deleterious effect on densification and requires the use of an external driving force to full densify the samples.

Mullite-Alumina Refractory Ceramics Obtained from Mixtures of Natural Common Materials and Recycled Al-Rich Anodizing Sludge

This work describes attempts to reuse Al-rich sludge produced from industrial anodizing and surface treatment processes in the fabrication of mullite-based refractory materials. The complete characterization of the residue is reported, including physical and compositional parameters. Mixtures with common natural silica-containing materials, like ball clay, kaolin, and/or diatomite were prepared in order to achieve interesting final fired compositions. Rheological characterization of the suspensions, their slip casting performance, the thermal behavior of the consolidated bodies, and crystalline phase evolution are also detailed. From the knowledge acquired, pretreatment operations have been suggested to facilitate the recycling of the Al-rich sludge and to improve the characteristics of mullite-based materials.

Surface passivation of MgAl2O4 spinel powder by chemisorbing H3PO4 for easy aqueous processing.

A stoichiometric MgAl 2O 4 spinel (MAS) powder was synthesized by heat treating at 1400 degrees C for 2 h a compacted mixture of alpha-Al 2O 3 and calcined caustic MgO, followed by crushing and milling. The surface of this powder was then passivated against hydrolysis with H 3PO 4 and Al(H 2PO 4) 3 in an ethanol solution. The as-passivated powder could then be dispersed in water using tetramethylammonium hydroxide (TMAH) and an ammonium salt of poly(acrylic acid) (Duramax D-3005) as dispersing agents and gelcast to form green consolidates with relatively high strength (>15 MPa). The good dispersing behavior of the passivated powder in water was confirmed by the low viscosity of its suspension containing 41-45 vol % solids, demonstrating the viability of replacing organic solvents by water in colloidal processing of MAS-based ceramics. The Fourier transform infrared (FT-IR), X-ray diffraction (XRD), and energy dispersive X-ray (EDAX) studies revealed that only negligible amounts of phosphate ions at the surface are required to effectively protect the powder from reacting with water.

Surface Passivation of MgAl 2 O 4 Spinel Powder by Chemisorbing H 3 PO 4 for Easy Aqueous Processing

A stoichiometric MgAl 2O 4 spinel (MAS) powder was synthesized by heat treating at 1400 degrees C for 2 h a compacted mixture of alpha-Al 2O 3 and calcined caustic MgO, followed by crushing and milling. The surface of this powder was then passivated against hydrolysis with H 3PO 4 and Al(H 2PO 4) 3 in an ethanol solution. The as-passivated powder could then be dispersed in water using tetramethylammonium hydroxide (TMAH) and an ammonium salt of poly(acrylic acid) (Duramax D-3005) as dispersing agents and gelcast to form green consolidates with relatively high strength (>15 MPa). The good dispersing behavior of the passivated powder in water was confirmed by the low viscosity of its suspension containing 41-45 vol % solids, demonstrating the viability of replacing organic solvents by water in colloidal processing of MAS-based ceramics. The Fourier transform infrared (FT-IR), X-ray diffraction (XRD), and energy dispersive X-ray (EDAX) studies revealed that only negligible amounts of phosphate ions at the surface are required to effectively protect the powder from reacting with water.

Particle segregation phenomena occurring during the slip casting process

Abstract Homogenous microstructures are important to improve the reliability of the final ceramic products. This target is usually hindered by particle segregation phenomena that occur during colloidal processing when the suspended powders present significant differences in average particle size. In the present work binary suspensions have been prepared by mixing fumed silica ( D 50 =0.07 μm), with coarser silica powders (P600, D 50 =2.2 μm and P10, D 50 =19 μm) in different proportions. The effects of milling time, average particle size, proportion of the components in the mixtures and total solids volume fraction on the extent of particle segregation that occurs during unidirectional slip casting were evaluated. Particle size distribution analysis and scanning electron microscopy revealed that successive layers of fine/coarse/fine particles have been successively deposited during casting. The extension of segregation was dependent upon particles’ size ratio, total solids volume fraction and milling time. It could be concluded that the clogging effect of the cake by the fine particles was the main mechanism responsible for particle segregation.

Al-rich sludge treatments towards recycling

A treatment method towards recycling of Al-rich sludges to produce high-alumina refractory ceramics is proposed. These sludges are collected in wastewater treatment units of industrial plants dealing with anodising or surface coating processes. This industrial residue usually contains significant amounts of soluble salts such as Na, K, Mg, Ca and Al sulphates that make direct recycling problematic. The removal of these salts from the as-received sludges by washing is ineffective due to its gel-like consistency. The treatment consists of calcining and subsequent washing operations, followed by removal of sulphates by precipitation as BaSO4. The treated sludges could then be well dispersed and processed by slip casting. # 2002 Elsevier Science Ltd. All rights reserved.

Injectability of calcium phosphate pastes: Effects of particle size and state of aggregation of β-tricalcium phosphate powders.

The present study discloses a systematic study about the influence of some relevant experimental variables on injectability of calcium phosphate cements. Non-reactive and reactive pastes were prepared, based on tricalcium phosphate doped with 5 mol% (Sr-TCP) that was synthesised by co-precipitation. The varied experimental parameters included: (i) the heat treatment temperature within the range of 800-1100°C; (ii) different milling extents of calcined powders; (iii) the liquid-to-powder ratio (LPR); (iv) the use of powder blends with different particle sizes (PS) and particle size distributions (PSD); (v) the partial replacement of fine powders by large spherical dense granules prepared via freeze granulation method to simulate coarse individual particles. The aim was contributing to better understanding of the effects of PS, PSD, morphology and state of aggregation of the starting powders on injectability of pastes produced thereof. Powders heat treated at 800 and 1000°C with different morphologies but with similar apparent PSD curves obtained by milling/blending originated completely injectable reactive cement pastes at low LPR. This contrasted with non-reactive systems prepared thereof under the same conditions. Hypotheses were put forward to explain why the injectability results collected upon extruding non-reactive pastes cannot be directly transposed to reactive systems. The results obtained underline the interdependent roles of the different powder features and ionic strength in the liquid media on determining the flow and injectability behaviours.

Chemisorption of phosphoric acid and surface characterization of As passivated AlN powder against hydrolysis.

By simply refluxing a commercial AlN powder in a mixture solution of ethanol, H(3)PO(4), and Al(H(2)PO(4))(3) for 24 h at 80 degrees C, the powder was successfully passivated against hydrolysis. The phosphate layer formed on the surface of AlN powder was found to be quite stable toward protecting the powder from hydrolysis. The efficacy of the coating was established by suspending the treated and the untreated powders in water for 72 h and subsequently characterizing them by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and Raman analysis. The good dispersing behavior of the treated AlN powder in water was confirmed by the low viscosity of an AlN suspension containing 50 vol % solids demonstrating the viability of replacing organic solvents by water in colloidal processing of AlN-based ceramics.

Phosphoric acid treated AlN powder for aqueous processing of net-shape dense AlN and β-SiAlON parts

Abstract Due to fast hydrolysis of AlN when in contact with water, AlN based ceramics are processed in organic solvents, which are volatile, expensive, harmful to the health and environment. The present work intends to protect AlN powder against hydrolysis in order to enable aqueous processing of AlN based ceramics. A commercial AlN powder was treated in an ethanol solution of H3PO4 and Al(H2PO4)3 kept at 80°C for 24 h to protect it from hydrolysis. The dispersing behaviour of the treated AlN powder in water and the ability to consolidate defect free parts of AlN and β-Si4Al2O2N6 ceramics from aqueous suspensions by slip casting and gelcasting techniques were studied. The consolidated parts were then sintered in a graphite furnace and the sintered bodies exhibited properties [bulk density (BD), apparent porosity (AP), crystalline phase composition and microstructural features] similar to those reported in literature for the same materials processed from organic media.