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Techniques are currently being developed to allow complex shapes to be created directly from a computer program by the addition of material to, rather than the abstraction of material from, the component. These are mouldless manufacturing operations, variously known as solid free-form fabrication [1,2], rapid prototyping [3] or, more colloquially, "art to part" technology [4, 5]. Various materials processing operations have been adapted for this purpose. In laminated object manufacture (LOM), laser-cut layers of foil are stacked to produce the shape [5]. In stereolithography, an ultraviolet (UV) laser is used to scan and cure a photosensitive monomer [6]. In selective laser sintering, a more powerful laser is used to scan and sinter a deposit of ceramic, metal or polymer powder [1, 7]. Although the last process is attractive for ceramics manufacture, considerable problems of particle packing before sintering and of residual stresses set up during sintefing remain. So far, the main practical use of these techniques is the rapid creation of a prototype from a computer program, but the longer term possibilities for ceramics include the manufacture of high resolution mulfilayer circuits and devices, the fabrication of solid oxide fuel cells of complex design, the preparation of ordered ceramic composites for structural or piezoelectric applications and the manufacture of highly complex small monolithic ceramics. Previous work at Brunel University has considered manufacturing operations for ceramics in which the powder is dispersed in an organic vehicle [8, 9]. These processes do not depend on agglomerated powders and have the capability to achieve the excellent dispersion necessary for high performance ceramics [10]. Similar requirements are applicable in paint and printing operations. Recent work has shown that the random deposition of droplets of a ceramic paint can be used to produce multilayer ceramic composites [11]. Although many printing operations are used in the traditional ceramics industry, only screen printing is widely used for advanced ceramics. Jet printing [12] is a non-contact printing process in which small ( 5 × 10-13m 3) drops of ink are ejected when required from a thermal or piezoelectric activated nozzle of diameter 25-75/xm (drop on demand ink jet printing). In contrast, continuous ink jet printing uses charged drops from a continuously recycled stream, which are diverted to the substrate when required by a voltage applied to deflector plates. Ink jet technology has been used to deposit a binder solution on to layers of roll-compacted ceramic powder to fix particles in place before conventional sintering [13]. This allows the creation of a porous ceramic by computer aided manufacture (CAM). A ceramic ink was prepared by selecting a thermoplastic resin, dispersant and ceramic powder such that the ceramic occupied 60 vol% in the absence of solvent. This volume fraction was chosen to avoid shrinkage cracks [11]. It also confers thermoplastic properties to the dried ink to allow subsequent plastic forming operations if required. This composition was diluted with solvent for printing. The sources of materials and composition of the ink are given in Table I. The polyvinyl butyral

The processing of collagen and chitosan, which exhibit tremendous potential as bioactive materials, has generated a great deal of interest in the last few years (e.g., [1, 2]). In particular, the preparation of collagen [3] and collagen-chitosan films [4] and investigation of their electrical properties has been explored. In a recent letter we were able to produce chitosan films by electrostatic atomization [5] and in this communication we show that ∼30 μm collagen films can also be prepared using the same technique. 35 wt% of water soluble collagen (Type Semed F, supplied by Kensey Nash Corporation, Exton, USA) was dissolved in single distilled water by mixing for 48 h in a beaker using a magnetic stirrer. The density (density bottle method), viscosity (calibrated reverse flow U-tube), surface tension (calibrated Du Novy balance), dc conductivity and relative permittivity (using calibrated cells) of the collagen solution was measured as described previously [6] after calibrating each with single distilled water. Electrostatic atomization was carried out using the set-up shown in Fig. 1 where the stainless steel needle has an inner diameter of 200 μm. The ring-shaped ground electrode is held 8 mm below the exit of the needle. Freshly prepared collagen solution was syringed to the needle at 3 × 10−9 m3 s−1 with the applied voltage set at 8 kV. Under these conditions, with the ammeter in Fig. 1 reading 57 nA, stable cone-jet mode electrostatic atomization was achieved (Fig. 2).

The use of polymeric precursors to produce ceramics is generating considerable interest . Owing to distinct advantages in processability, polymeric precursors have found applications in many areas such as ceramic fibers, foams and coatings. Ceramic coatings, in particular, are of immediate value in industry as a means of providing wear and corrosion resistance for articles used in adverse environments. The coating of engineering parts with ceramics using polymeric precursors is a relatively easy, quick and low-cost process compared with other processing methods such as chemical vapor deposition and plasma-coating.

During the last decade porous ceramic materials have been finding increasing applications due to their favorable properties such as high temperature stability, high permeability, low mass, low specific heat capacity and low thermal conductivity. These characteristics are essential for many technological applications such as catalyst supports, filters for molten metals and hot gases, refractory linings, thermal and fire insulators and porous implants [1, 2]. Ceramic foams can be produced by different methods, principally impregnation of polymer foams with slurries containing appropriate binders and ceramic particles followed by pressureless sintering at elevated temperatures [2–5]. This involves coating an open-cell polymeric sponge with a ceramic slurry several times, pyrolysis of the polymer to form a ceramic skeleton followed by sintering. Ceramic foams produced by this method are generally of low strength as their struts are thin and can contain a hole in the center [2, 6–8]. Recently, a new method to produce silicon carbide (SiC) foams using polymeric precursor solutions was developed by Bao et al. [9] where a polyurethane foam was immersed in a polymeric precursor solution to form a pre-foam which was pyrolyzed in nitrogen. The main advantages of this new approach are the simplicity and ease of control of structure of the final product. This new process was exploited further to prepare silicon carbide-silicon nitride (SiC-Si3N4) composite foams [10]. In this letter we provide microstructural evidence of the improvements in structure of the ceramic foams produced by our method. The polysilane precursor discussed in this study was synthesized by the alkali dechlorination of a combination of chlorinated silane monomers in refluxing toluene/tetrahydrofuran with molten sodium as described previously [11, 12]. The structure of the SiC polysilane precursor synthesized is given below. Ph indicates a phenyl group.

The use of polymeric precursors for the preparation of ceramic materials is generating a great deal of interest, and silicon carbide (SiC) has been produced by the pyrolysis of several polymers. However, it is desirable to develop precursors that can be prepared with a high polymer yield, are melt-processable, and give a high SiC yield on pyrolysis close to the theoretical maximum that can be obtained (with a low amount of impurities).

The dispersion of fine (sub-micrometre size) ceramic powders in polymers which act as a vehicle for their processing has been investigated extensively during the past decade [1-5]. Several novel ceramic fabrication processes, such as injection moulding [6], blow moulding [7], vacuum forming [8] and film blowing [9], are dependent crucially on dispersing homogeneously several chemical types of these fine ceramic powders in a variety of both low and high molecular weight polymers. Dispersion is dependent on several types of ceramic-polymer interactions and these have been reviewed recently by Evans [10]. Good dispersion can be accompanied by a strong attachment of the polymer to the surface of the ceramic powder, and although this could be beneficial in obtaining a homogeneous as-formed microstructure, the subsequent removal of the polymer prior to sintering of the ceramic can be made more difficult [11]. Most ceramic-polymer suspensions used in the forming process mentioned above are prepared by melt processing [12]. Therefore, dispersion of the fine ceramic in the polymer is dependent on shear mixing at a temperature above the softening point of the polymer, but often microstructural investigation of the formulations prepared in this way shows the presence of a variety of defects, including undispersed polymer and relics of agglomerated powder [1]. In addition, demands placed by the necessity to use ultra-fine (nano-sized) powders, which are very agglomerated, to achieve high sintered densities have placed limitations on eren high shear melt processing methods such as twin roll milling and twin screw extrusion [13]. It may be possible to overcome these difficulties if a high volume loading of ceramic powder can be dispersed homogeneously in a suspension of polymer particles in aqueous or nonaqueous medium prior to melt processing. This letter sets the scene for the exploitation of this method in the forming of engineering ceramics. Previously, related studies have been limited to monodisperse model systems, where heterocoagulation in colloidal dispersions containing more than one type of particle has been studied in polymer and paint [14], mineral [15] food and pharmaceutical [16] and composite [17, 18] systems. A major drawback in the assessment of dispersion of fine ceramic powders in polymers is the lack of suitable experimental techniques. A ceramic-polymer suspension can be ashed to remove the polymer and the ceramic powder can be examined microscopically. The polymer can also be dissolved in a suitable solvent and the resulting suspension can be 'studied using light scattering or Coulter counter

Etude de l'utilisation d'agents d'accrochage de silane au cours de moulages par injection de ceramiques. Analyse de l'effet de l'elimination du polymere