J.M.F. Ferreira

Processing of porous ceramics by ‘starch consolidation’

Abstract A new consolidation method for forming porous ceramics was developed using starch as both consolidator/binder and pore former. Simple and complex-shaped components of porous alumina were shaped and demoulded in wet state. After drying, burn-out and sintering, materials with ultimate porosities between 23 and 70% were obtained. The overall pore structures were dominated by the large spherically shaped pores (10–80 μm) left by the starch partides. Furthermore, the average size of the small pores connecting the large pores, measured by Hg porosimetry, was controlled by the total solids loading and starch content in the originally prepared slips and varied between 0.5 and 9.5 μm. Chemically modified starch was found to give better dimensional control and regularity regarding average size of the connecting pores than native starch owing to more stable properties during water processing. This new forming technique can be used in the manufacture of various porous ceramics for thermal insulation applications, as filters/membranes and gas burners, or as bioceramics. Major advantages are the simplicity of the process, the possibilities of forming complex shapes using various mould materials and the lowcost processing equipment/materials needed.

An in vitro biological and anti-bacterial study on a sol–gel derived silver-incorporated bioglass system

OBJECTIVES The purpose of this study was to evaluate the in vitro antibacterial and biological activity of silver-incorporated bioactive glass system SiO2-CaO-P2O5-Ag2O (AgBG). The bacteriostatic and bactericidal properties of this new quaternary glass system along with the ternary sol-gel glass system SiO2-CaO-P2O5 (BG) have been studied using Escherichia coli as a test micro-organism. The AGBG system thus appears to be a promising material for dental applications, since similar effects might be produced on a film of bacteria and mucous that grows on the teeth. METHODS The SiO2-CaO-P2O5-Ag2O and SiO2-CaO-P2O5 glass systems were synthesized by the sol-gel technique and characterized for their physicho-chemical properties. The antibacterial activity and biological properties were evaluated by determining the minimum inhibitory concentrations (MICs). Release of Ag+ into the culture medium was measured by inductively coupled plasma (ICP) analysis. RESULTS The in vitro antibacterial action of the SiO2-CaO-P2O5-Ag2O was compared with that of its ternary counterpart glass system. The concentrations of Ag-bioglass, in the range of 0.02-0.20 mg of Ag-bioglass per millilitre of culture medium, were found to inhibit the growth of these bacteria. The Ag-bioglass not only acts bacteriostatically but it also elicited a rapid bactericidal action. A complete bactericidal effect was elicited in the early stages of the incubation at Ag-bioglass concentration of 20 mg/ml and the ternary glass system had no effect on bacterial growth or viability. The antibacterial action of Ag-bioglass was exclusively attributed to the leaching of Ag+ ions from the glass matrix. SIGNIFICANCE One of the major advantages of incorporating silver ions into a gel glass system is that the porous glass matrix can allow for controlled sustained delivery of the antibacterial agent to dental material, used even under anaerobic conditions such as deep in the periodontal pocket. This glass system also provides long-term action required for systems which are constantly at risk of microbial contamination.

Combustion synthesis of ternary carbide Ti3AlC2 in Ti-Al-C system

Abstract In the present work, a method for fabrication of Ti 3 AlC 2 by combustion synthesis was developed. Low cost titanium, aluminum and carbon powders were used as raw materials. The effects of the initial ratio among the above-mentioned materials on the combustion behavior and phase formations in the final products were investigated. The experimental results showed that the combustion reaction usually preferred to form a mixture of titanium carbide and aluminum. However, the desired product of Ti 3 AlC 2 can be obtained through adjusting the initial ratio of the elemental powders or adding TiC as additives. The formation mechanism of ternary phases, Ti 3 AlC 2 and Ti 2 AlC, is that the previously formed crystallites of TiC dissolve into the melting Ti–Al composition, and then, ternary phases precipitate from liquid phase and grow into layered morphology.

Processing of porous cordierite bodies by starch consolidation

Porous cordierite ceramics, formed from a mixture of ball clay, talc, alumina, and silica sand, were developed by a new direct consolidation method based on the properties of starch in water. The gelling capability of starch granules in warm water allows the transformation of a stable suspension into a rigid and stiff body that can be machined in the green state. Starch consolidation can be performed in nonporous molds, enabling materials with controlled porosity to be obtained by varying the starch content in the slip composition. The simplicity of the process as well as the low price of starches makes this new consolidation technique very promising from an industrial point of view. The results showed good agreement with the expected porosities of the cordierite sintered bodies, thus enabling the design of porous textures and tailoring of the material to applications.

Aqueous suspensions for tape-casting based on acrylic binders

Abstract The use of water-based systems represents an interesting alternative to the widespread non-aqueous tape-casting but the low strength of water-based binders generally limits their applicability. A tape-casting slurry is a complex system where each organic component has a substantial effect on the rheological behaviour. In this study the effect of the dispersant and binders in alumina aqueous tape-casting slurries were characterized with electrophoretic mobility and rheological measurements. In aqueous medium, a 4,5-dihydroxy-1,3-benzenedisul-fonic acid, was found to be a very effective dispersant for alumina. The amount of dispersant required to achieve a minimum of viscosity was equal to 0.2 wt%. Two aqueous emulsions of acrylic polymers were used as binders. These binders strongly affect the rheology of the suspensions. The best conditions to obtain a homogeneous stable slurry with a high powder loading suitable for tapecasting were determined in terms of order of component addition, rheological behaviour and ageing of the suspensions. Acrylic binders should act through a cohesive mechanism and lead to green tapes with good mechanical strength.

Porous bioactive calcium carbonate implants processed by starch consolidation

Abstract Macroporous calcium carbonate (CaCO 3 ) materials with porous structures suitable for implantation purposes were prepared in the present work. A new “direct consolidation” technique that uses starch granules as consolidator agent and as pore formers, combined with other larger organic inclusions, enabled us to tailor the porous microstructure for the intended application. Pore sizes as large as several hundreds of micrometers could be generated without matrix cracking, due to the high solid loading of the starting suspensions. The macroporous CaCO 3 bodies fabricated exhibit an accentuated bioactivity even for short soaking time periods. The crystalline calcium phosphate phases precipitate preferentially in the pores, pore boundaries, or in other strained sites at the surface of the samples.

Synthesis and structural characterization of strontium- and magnesium-co-substituted β-tricalcium phosphate

The synthesis of five different Sr(2+)- and Mg(2+)-co-substituted beta-tricalcium phosphate (beta-TCP) has been obtained by heating the calcium-deficient apatites above 800 degrees C. With the investigated concentrations of Sr(2+) and Mg(2+) from the present study, no additional phases other than beta-TCP have been detected. The synthesized powders have been characterized by X-ray diffraction, Fourier transform infrared spectrometry, elemental analysis and Rietveld refinement studies. The co-substitution of Sr(2+) and Mg(2+) in the beta-TCP has resulted in the formation of crystalline beta-TCP at hexagonal setting (space group R3c). The reduction of lattice a- and c-axis parameters with the combined substitution of Sr(2+) and Mg(2+) in the beta-TCP has been found evident from the present results. Sr(2+) has been found occupying the Ca(1,2,3,4) sites and Mg(2+) was found at the sixfold coordinated Ca(5) site of beta-TCP structure.

Fabrication of hydroxyapatite bodies by uniaxial pressing from a precipitated powder.

Hydroxyapatite (OHAp) has been obtained by using a precipitation method. Thermal stability of the precipitated powder has been checked to define an appropriate working range to process ceramic bodies without the appearance of odd phases. Pellets of hydroxyapatite have been manufactured by means of uniaxial pressing and convenient thermal treatments have been investigated to achieve high density OHAp bodies. Finally, factors that could affect the ultimate mechanical properties of the sintered bodies prepared by uniaxial pressing have been discussed.

Influence of the stabilising mechanism and solid loading on slip casting of alumina

Abstract Two commercial surface active agents that act through different stabilisation mechanisms, were used to disperse aqueous alumina suspensions. Electrophoretic measurements were performed to determine the zeta potential curves of the bare and surface charge modified alumina particles and, hence, to determine the stabilising performance of the dispersants. Drying–shrinkage behaviour (Bigot’s curves) and Hg porosimetry were used to characterise the slip cast bodies prepared from the suspensions at different solid loading. The rheological behaviour of the starting alumina suspensions was also determined in order to relate the flow properties with the superficial nature of the suspended particles and the wet body microstructure. Rheological measurements have showed that the slips containing the dispersant that confers the highest zeta potential values to the alumina particles were more viscous than when using the less capable dispersant in terms of surface charge modifier. These findings were explained by a larger interaction size of the particles dispersed through a pure electrostatic mechanism which resulted in a higher effective volume solid fraction of these suspensions. For both dispersants used, the Bigot’s curves indicated that the shrinkage and the slopes of the straight lines corresponding to the first stage of the drying decrease with increasing solids concentration. The variations in the slope derive from a broadening pore size distribution as a result of the increasing difficulties in stabilising the concentrated alumina suspensions, in agreement with the rheological measurements. Good correlations were found between the green microstructure, the rheology of the suspensions and the particle interaction forces.

Suitability evaluation of sol–gel derived Si-substituted hydroxyapatite for dental and maxillofacial applications through in vitro osteoblasts response

UNLABELLED Si-hydroxyapatite (Si-HAP) has been used in orthopedic, dental, and maxillofacial surgery as a bone substitute. OBJECTIVE The aim of this investigation was to study the effect of Si substitution into the hydroxyapatite matrices and evaluate the biocompatibility effects of Si-HAP material in vitro with human osteoblasts. METHODS Silicon-substituted hydroxyapatite (Si-HAP) bioceramic materials were prepared by incorporating small amounts of silicon into the structure of hydroxyapatite [Ca10(PO4)6(OH)2, HAP] through a sol-gel method. A series of silicon substitutions ranging from 0, 1, 3 and 5 mol%, which are comparable to the measured silicon contents in natural bone, were performed. RESULTS Single-phase Si-HAP was obtained upon calcining the as-prepared powders up to 800 degrees C since no secondary phases, such as tricalcium phosphate (TCP), tetracalcium phosphate (TeCP) or calcium oxide (CaO), were identified by X-ray diffraction analysis. The effects of silicon-substituted hydroxyapatite (Si-HAP) materials towards the responses of human osteoblast-like (HOB) cells were investigated and compared with pure hydroxyapatite. SIGNIFICANCE The Si-HAP indicated a significant increase in cell growth density with culture time irrespective of the amount of Si substituted in HAP. A high Si content (5 mol%) appears to promote rapid bone mineralization, since large amount of calcium phosphate minerals started to develop across the ECM by day 31 for a sample containing 5 mol% Si. On the other hand, a high Si content may result in fast dissolution of the material, owing to a decrease of HAP crystallite size, which might not be ideal for cell attachment for prolonged time periods. An optimum level of Si appears to exist at 3 mol%, which balances these effects.