Search

Over the past two decades it has been established that bone repair in the presence of implant materials can be optimized by control of their surface chemistry. Certain calcium phosphate materials are similar in composition to bone mineral and have been found to be osteoconductive [1–3]. These ceramic materials have therefore been developed as biomaterials for a range of different applications. However, although they promote direct bone apposition, their mechanical properties are relatively poor compared with other monolithic ceramics. Therefore, classes of applications have been developed to exploit the excellent biological properties of the materials while maintaining mechanically viable implants [4, 5]. There is now increasing evidence that surface topography both on the micro and nano-scale are important in determining the cell response to biomaterials [6– 11]. A number of studies have indicated that cell activity can be up-regulated through optimization of the surface properties of the substrate. Investigations originally concentrated on surface features of several tens of micrometers in scale, but more recently, evidence suggests that surface topography at a much finer scale may influence the cell response. Hydroxyapatite (HA) has been used in a number of different forms and several powder processing routes have been investigated in conjunction with HA [12– 18]. The application of jet-based suspension processing methods, such as ink-jet printing (IJP) [19, 20] and electrostatic atomization printing (EAP) [21, 22] allows solid freeforming of fine structures of advanced materials. In particular, EAP in the cone-jet mode enables the formation of

A high performance 500 nozzle in-line piezoelectric drop-on-demand print-head mounted on a linear table with z-axis control was used to prepare arrays of fine ceramic pillars. Such structures may find applications in miniature heat exchangers, catalyst supports, for cell up-regulation in prosthetics or in polymer-ceramic piezoelectric 1–3 composites. The ink conveyed 14 vol% fine zirconia and deposited a wax-based suspension in an octane-alcohol mixture. The dried ink contained 63 vol% zirconia.

Different dispersants were used to develop ethanol-based silver inks for continuous ink-jet printing. Inks were made using varying amounts of dispersant and silver. Various parameters, such as sedimentation behaviour, sediment volume, half-value time and viscosity, were used to assess the dispersion and stability of these inks over 10 days.

A simple method was developed to produce silicon carbide foams using polysilane polymeric precursors. Polyurethane foams were immersed in polysilane precursor solutions to prepare pre-foams. Subsequently, these were heated in nitrogen at different temperatures in the range of 900°C to 1300°C. The silicon carbide foams produced in this manner showed well-defined open-cell structures and the struts in the foams were free of voids. The shrinkage which accompanies pyrolysis of the pre-foams was reduced with increasing the concentration of the polymeric precursor solutions.

Several polysilanes with different overall functionalities have been synthesized and pyrolyzed to produce porous silicon carbide. The polysilanes and their ceramic products have been characterized using gel permeation chromatography, Fourier transform-infrared spectroscopy, thermogravimetry, X-ray diffractometry and microscopy. Some products were foams while others were micro-porous ceramics. The effect of the final pyrolytic yield on the Type of ceramic produced, its pore structure and shape retention are discussed. Two polysilanes were blended in various ratios to control the pyrolysis process more precisely. This allowed the Type, shape and pore-structure of the silicon carbide produced to be controlled more efficiently. There exists a relationship between the composition and structure of the precursors and their final pyrolytic yield and this determines the Type, shape retainability and pore structure of the ceramics produced. In this work, precursors or their blends which gave a final pyrolytic yield of 50–60 wt % produced the best silicon carbide foams.

In this final part of our investigation, the copolymers prepared (reported in Part III) have been pyrolysed to yield silicon oxynitride. Thermogravimetric studies performed on several copolymers in nitrogen, ammonia/nitrogen mixtures, air and oxygen are discussed in the present paper. One copolymer was used for further investigation and its pyrolysis conditions (variation of pyrolysis atmosphere according to temperature) were optimized to produce the maximum yield of white amorphous residue which was crystallized by heating to 1600°C to produce silicon oxynitride powder. The powder particles were equiaxed in shape and in the size range 50–150 nm but in agglomerates of size 0.3–0.1 μm. Crystallization at higher temperatures resulted in a mixture of a silicon nitride-silicon oxynitride.

A shrinking undegraded core and a porous outer layer result, if the organic vehicle used for shaping ceramic or metal powder mouldings recedes in the interparticle space of the moulded body during pyrolysis. In the present work, a numerical model has been used which simulates the undegraded shrinking core situation and quantifies degradation of the organic vehicle and the diffusion of the resulting products in solution in the organic phase during pyrolysis of a ceramic moulding. This model is extended to include gaseous mass transport in the porous outer layer for a moulding in the shape of an infinite cylinder. The effect of resistance to gaseous mass transport in the porous outer region on defects originating in inner regions was estimated. It is shown that the greatest obstruction to mass transport is diffusion of degradation products in solution in the organic phase. However, the permeability coefficient for gas transport in the outer region begins to affect the critical heating rate required for avoidance of defects only when it is less than 10−15m2.

In the present work, homopolymerization and copolymerization of methylcyclosiloxanes and methylcyclosilazanes are investigated. Homopolymers and copolymers produced are characterized using 1 H NMR and Fourier transform infrared spectroscopy. It is found that the reactivity of methylcyclosiloxanes is much lower than that of methylcyclosilazanes. The structures of the homopolymers and copolymers produced are discussed and it is suggested that the latter consists of a block structure containing siloxanes and silazanes in which the siloxane units are the smaller

Proton nuclear magnetic resonance ( 1 H NMR) spectroscopy, Fourier transform infrared (Ft-IR) spectroscopy and gas chromatography (GC) have been used to study the products of ammonolysis of dichloromethylsilane in diethyl ether at 0 o C. Results indicate that the major products are trimethylcyclotrisilazane and tetramethylcyclotetrasilazane. Hydrolysis of the cyclosilazanes has also been qualitatively investigated. A simple and useful method for the identification of cyclosilazanes has been developed using the 1 H NMR technique

The mechanical properties of alumina test bars fabricated using injection moulding as the shaping operation, are described. The modulus of rupture, hardness, fracture toughness and wear behaviour of the ceramic specimens are reported. Test bars were injection moulded using a polypropylene-based high molecular weight organic vehicle. The processing and microstructure of the test bars were discussed in Part I. The alumina powder was selected in a previous investigation, which considered the compounding, compression moulding, removal of organic vehicle and sintering of several formulations containing different alumina powders.

Previous studies suggest that silicon oxynitride can be synthesized by pyrolysing polymers which are produced by the copolymerization of cyclic polysiloxanes and cyclic polysilazanes. The cyclic polysiloxanes required are [CH3Si(H)O]m with m = 4, 5 and 6, and these can be produced by the hydrolysis of dichloromethylsilane. In the present work, a significantly improved combined yield of these cyclic polysiloxanes is reported. Three main reasons are attributed to this achievement: an improved hydrolysis procedure; optimization of the initial acidity of the reaction medium; and changes in the total reaction time. Thus a mixture containing > 95 mol% cyclic tetramer, pentamer and hexamer has been prepared and this represents a --~ 20 mol% improvement over previously reported results. Investigations of the preparation and characterization of the cyclic polysilazanes, copolymers and silicon oxynitride will be described in Parts II-IV, respectively.

Ceramic suspensions were prepared from six alumina powders using the same high molecular weight organic vehicle. 18 mm diameter cylinders were compression moulded from four of these suspensions which were suitable for ceramic injection moulding. The organic vehicle in each cylinder was removed by thermal degradation according to the same temperature ramp. The defects present in the cylinders after removal of the organic vehicle are discussed in terms of the powder characteristics. Criteria for the selection of ceramic powders that could be used together with a high molecular weight organic vehicle for this shape-forming method, are deduced from the results obtained.

A polypropylene-based binder system was used to injection mould test bars containing 65 vol % aluminium powder. Specimens, 3 and 6 mm thick, made from these bars were used for pyrolytic binder removal experiments in static air and nitrogen. The development of a carefully defined experimental procedure for the determination of the heating rate at which binder removal can be carried out at a given temperature without the creation of macro defects is fully described. The use of isothermal heat treatments during pyrolysis are also considered and results are presented as temperature-heating rate diagrams for each atmosphere and thickness investigated. These diagrams show a lower and an upper boundary. Defect formation occurs if the temperature-heating rate relationship lies between the boundaries. Near optimum binder removal schedules deduced from each diagram have been experimentally verified.

Ceramic injection moulding suspensions consisting of two alumina powders of surface area 9.5 m2 g−1 and 0.3 m2 g−1 were prepared at 60 vol % ceramic loading in the same organic composition. Cylinders of various diameters of each suspension were companion fired on an identical temperature ramp in nitrogen. For the fine powder, bloating occurred for all diameters above 8 mm. The coarse powder cylinders of 25 mm diameter could be prepared free from internal voids. The problems presented by fine powders during binder removal are discussed in terms of adsorbed layers, water emission and catalytic degradation.

The ability to control the sprue solidification time by using modulated pressure was investigated over a wide range of mould temperatures using a silicon nitride suspension. The mould temperature needed for infinite sprue solidification time was reduced from 158 to 75° C by the application of a pressure of 132 MPa modulated at 0.5 Hz. Under similar moulding conditions, macroscopic moulding defects in the hub of a solid rotor were avoided. Such defects were present in all static pressure mouldings. The thermal power input to the sprue resulting from oscillatory flow was estimated.

Ceramic injection-moulding blends containing 56 vol % fine silicon nitride and 65 vol % coarse silicon powder in a polypropylene and wax vehicle were prepared by dispersive mixing with and without 2 p.p.h. silane coupling agent based on the weight of powder. The addition of silane reduced the viscosity and the pseudoplasticity of the suspensions considerably. The mechanical strength of the moulding compositions, which may influence the tendency to crack during solidification in the cavity, was marginally increased. The treated silicon powder was strongly hydrophobic but this was not the case with the silicon nitride powder.[/p]

Five ceramic injection moulding formulations with 65vol% powder and polypropylene as the main organic component were injection -moulded under identical conditions. The extent of mould filling and the production of moulding defects in thick sections were examined in the light of rheological and shrinkage properties of the formulations.