M. Nangrejo

Preparation of silicon carbide foams using polymeric precursor solutions

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.

Preparation of silicon carbide–silicon nitride composite foams from pre-ceramic polymers

Abstract A new method of forming silicon carbide–silicon nitride composite foams is presented. These are prepared by immersing a polyurethane foam in a polysilane precursor solution mixed with Si3N4 powder to form a pre-foam followed by heating it in nitrogen at >900°C. X-ray diffraction patterns indicate that a SiC–Si3N4 composite was formed after sintering the ceramic foam at >1500°C. Micrographs show that most of these foams have well-defined open-cell structures and macro-defect free struts. The shrinkage is reduced considerably due to the addition of Si3N4 particles.

Preparation of Polymeric and Ceramic Porous Capsules by a Novel Electrohydrodynamic Process

The preparation of capsules for medical and industrial use can be achieved via several conventional routes, yielding either hard or soft receptacles, depending on the type and the content of the material to be encapsulated. Together with tablets, capsules are amongst the most commonly used means of administering medication and this makes progress in capsule preparation technology a key area of drug delivery research. Here we uncover new technology for the preparation of capsules with porous chambers. The novelty is signified in the use of an electrohydrodynamic process engineering route and its potential is elucidated using a polymeric material; polymethylsilsesquioxane, which can be converted into an identical ceramic form by means of simple pyrolysis. Thus, both polymeric and ceramic capsules have been prepared. The effects of process control parameters such as the applied voltage and flow rate, on the characteristics of the capsules prepared are discussed.

Porosity and Strength of Silicon Carbide Foams Prepared Using Preceramic Polymers

Two types of polymeric precursors for silicon carbide (SiC) were dissolved in dichloromethane. Subsequently, between 10–80 wt% of silicon nitride (Si3N4) and titanium carbide (TiC) powder were added separately into the solutions to make SiC-Si3N4 and SiC-TiC suspensions. Cubes of polyurethane (PU) foams were soaked in precursor solution and suspensions and pyrolyzed in flowing nitrogen to produce SiC, SiC-Si3N4 and SiC-TiC composite foams. Some foams were heated further in nitrogen to 1600°C. Shrinkage observed after pyrolysis and further heating the foams was measured and can be reduced by varying the concentration of polymeric precursor, Si3N4 and TiC content. The foams produced have porosities in the range 85–96%. The average compressive strength of the foams is in the range of 1.1–1.6 MPa.

The structure of ceramic foams produced using polymeric precursors

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.

Silicon carbide–titanium carbide composite foams produced using a polymeric precursor

Abstract A polysilane solution used as a silicon carbide (SiC) precursor was mixed with different amounts of titanium carbide (TiC) powder and open cell polyurethane (PU) foams were dipped in these suspensions. The resulting pre-foams were pyrolyzed at 900°C in nitrogen and then heated further at various temperature between 1100 and 1600°C in the same atmosphere to produce SiC–TiC composite foams. The evolution of the composite foams has been studied using thermogravimetry and X-ray diffraction. The PU foam, pre-foams and the SiC–TiC composite foams were characterized by optical and scanning electron microscopy. These studies show that the pre-foams retained their shape well during pyrolysis and the composite foams produced consist of an open cell structure and hole-free solid struts. Prevention of cracking in the foams were dependent on the TiC content. The shrinkage observed during the conversion of the pre-foams to the composite ceramic foams can be controlled by varying the TiC content and the sintering temperature.

Ceramic encapsulation with polymer via co-axial electrohydrodynamic jetting

Co-flowing media of a polymeric solution (30 wt% polymethylsilsesquioxane in ethanol) and a ceramic suspension (10 wt% alumina in glycerol) were subjected to an electric field. The flow rates of the media (10–30 µL min−1) and the applied voltage (0–11 kV) were varied systematically during the experimentation by making gradual increments to each variable, which enabled the construction of a mode selection map. Under co-flowing conditions, with the flow rate of polymer solution (outer needle) twice that of the ceramic suspension (inner needle), encapsulated droplets of polymer-coated alumina were produced within stable cone-jet mode. These were collected in a thin film of water and the resultant particle size varied between 1 and 38 µm. Encapsulation was confirmed with scanning electron microscopy and element analysis.

Processing of ceramic foams from polymeric precursor-alumina suspensions

A polysilane was used as the precursor for silicon carbide (SiC) and different amounts of it was dissolved in dichloromethane. Subsequently, between 10 to 80 %wt of alumina (Al2O3) powder was added into the solutions to make SiC-Al2O3 suspensions. Cubes of polyurethane (PU) foams with open cells in the size range 500-1200μm were soaked in these suspensions and pyrolysed in flowing nitrogen to produce SiC-Al2O3 composite foams. Some foams were heated further in nitrogen to 1300°C. The foams produced consist of an open cell structure and hole-free solid struts which were also cracks free in the polysilane-Al2O3 80:2Owt% formulation. The retention of shape during processing was excellent. Shrinkage observed after pyrolysis and further heating the foams was measured and can be controlled by varying the Al2O3 content. The foams produced have porosities in the range 87 to 95%. The maximum compressive strength of the pyrolysed foams prepared using the polysilane-Al2O3 80:20 wt% formulation was 2.3MPa.

Coating with amorphous silicon carbide using polymeric precursors

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.