Foamed material, electrical properties

Carbon microspheres yield syntactic foams with resistivities that are astonishingly low for these materials. Novolac syntactic foams with carbon microspheres have resistivities of 0.02-0.5 Ohm m (depending on the filler concentration)77 this is ten orders of magnitude lower than for glass microspheres in the same binder For materials made from carbon microspheres and silicone rubbers, the resistivity depends exponentially on the temperature, viz. 0.08 Ohm m at 20 °C, 0.2 Ohm m at 60 °C, and 200 Ohm m at 95 °C 1). Consequently, carbon microspheres make it possible to produce syntactic foams with electric properties appropriate for semiconductors. 

Electrical Insulation. The substitution of a gas for part of a soHd polymer usuaUy results in large changes in the electrical properties of the resulting material. The dielectric constant, dissipation factor, and dielectric strength are aU generaUy lowered in amounts roughly proportional to the amount of gas in the foam. 

Phenol—formaldehyde resins are used as mol ding compounds (see Phenolic resins). Their thermal and electrical properties allow use in electrical, automotive, and kitchen parts. Other uses for phenol—formaldehyde resins include phenoHc foam insulation, foundry mold binders, decorative and industrial laminates, and binders for insulating materials. 

Cables with multiple-layer sheathing have plastic-insulated cores. Solid PE or sintered PE is used as the plastic. Sintered PE is a foamed polyethylene material that has different electrical properties than solid PE. Under certain circumstances the core region is filled with a petrolatum material to give protection against... 

Also of interest are salts of melamine (see Chapter 24). In the nylons these can be used with bright colours (unlike red phosphorus) and do not adversely affect electrical properties. They do, however, decompose at about 320°C. Similar materials are very important in giving flame-retardant properties to polyurethane foams. 

The volximetric weight, a parameter characteristic for the relative contents of both the solid and gas phase in a material, is the fundamental morphological parameter of foamed polymers and is related to all relevant physical properties of foamed plastics such as strength, thermophysical and electric properties. 

BFRs are compounds widely used to impart fire retardant properties to textiles, plastics and electronic equipment. They are incorporated into many household products including children s pajamas, upholstery fabrics, polyurethane foams (furniture cushion padding), construction materials, electrical equipment, and personal computers. 

Foams were proved to be highly suitable as catalytic carrier when low pressure drop is mandatory. In comparison to monoliths, they allow radial mixing of the fluid combined with enhanced heat transfer properties because of the solid continuous phase of the foam structure. Catalytic foams are successfully used for partial oxidation of hydrocarbons, catalytic combustion, and removal of soot from diesel engines [14]. The integration of foam catalysts in combination with microstructured devices was reported by Yu et al. [15]. The authors used metal foams as catalyst support for a microstructured methanol reformer and studied the influence of the foam material on the catalytic selectivity and activity. Moritz et al. [16] constructed a ceramic MSR with an inserted SiC-foam. The electric conductive material can be used as internal heating elements and allows a very rapid heating up to temperatures of 800-1000°C. In addition, heat conductivity of metal or SiC foams avoids axial and radial temperature profiles facilitating isothermal reactor operation. 

---