Cenospheres at Ceno Technologies - USA

Cenospheres - EMI Shielding

Cenospheres - With the widespread use of digital electronic devices in a myriad of consumer, industrial and commercial applications, control of electromagnetic interference (EMI) from internal or "stray" signals has become increasingly important.

With the ever increasing speed of circuits and devices, crosstalk becomes a design problem. So does the ever more crowded communications spectrum both in bandwidth and proximity. The reflective solution only redirects the energy, and that's sometimes good enough. However, at higher frequencies it can remain a problem. You can simply eliminate the issue with absorption. Less design time, fewer performance surprises, higher quality at equal or less cost.

EMI Shielding can be incorprated into plastics and materials of consumer electrical and electronic goods
EMI essentially consists of radio frequency (RF) electromagnetic energy interacting with the components of an electronic device. EMI can negatively affect the performance of consumer electronics equipment, such as televisions and stereos. Ever notice your TV screen when someone uses an electric mixer or electric razor nearby? That static is the result of EMI. It can even literally "crash" a computer system. If the electronic equipment design is unable to reject these unwanted signals, then the equipment must be protected (shielded) in order to obtain the desired performance or to prevent catastrophic failure.

This protection from RF energy can be provided by a surface that reflects the unwanted energy, shielding the sensitive internal electronic hardware. In the example above, shielding can be utilized either in the TV to stop outside signals from entering, or in the electric mixer to prevent the signals from leaving the device in the first place. In fact, an entire room can be EMI shielded to electrically isolate the equipment in it from interference coming from other large areas, such as a control room in a factory setting.

The Ideal Solution

If we were to design the ideal particle for RF shielding, we must break the requirements into individual components and choose the optimum solution for each one.

The ideal particle for RF shielding is highly conductive and resistant to oxidation, leading to the use of metals for the majority of shielding applications. Choices include: gold, platinum, palladium, and the less expensive silver. Conductive coatings containing silver particles have been available for a long time and provide excellent shielding performance, but are still expensive and very heavy due to the large amount of silver metal required to produce a complete paint film.

From a shielding system, most of the RF energy is reflected by the surface of the metal particle, leaving the metal core essentially unused. This "skin effect" indicates that a hollow metal particle would provide as much shielding performance as a solid particle, resulting in weight and cost savings.

The ideal shape for this hollow particle is a sphere, which allows the particles to fill the most volume while using the least surface area. The optimum particle size would be about 50 microns, which provides good coverage with thin films and can be sprayed from conventional spray gun orifices. A hollow silver sphere of this type would be very fragile, and would need internal reinforcement. Remember that the inside of the metal sphere does not affect the conductivity of the sphere, so we can choose something strong and inexpensive.

A hollow ceramic by-product of coal-fired power plants, know as cenospheres, provides an excellent substrate for the silver metal shell. The spherical shape allows for isostatic strength of 4,000 psi, and the silver metal strongly adheres to the ceramic, unlike other available substrate. This results in a strong, tough and highly conductive particle with a density around 0,7g/cc, compared to solid silver particles at 10.5g/cc.

These hollow silver coated cenospheres or microspheres would enable significant RF attenuation (reduction of RF interference) in much the same way as traditional solid silver particles, by developing chains of contacting particles into a vast network within the paint film. Solid silver particles rely on high loading and settling to obtain sufficient chaining. Another advantage of using cenospheres as the metal substrate is residual magnetic materials embedded in the ceramic sphere wall during formation. This magnetism helps pull the silver coated spheres together, forming chains without requiring high loadings. This lowers the cost even further, and eliminates spray problems such as sagging associated with solid particles. Cenospheres or microspheres can also be incorporated into plastics, polymers, composites and rubber compounds during the forming or molding stage, thus providing excellent EMI shielding as an integral part of the finished product.

The ideal particle for an EMI shielding system is a thin silver coating over a hollow ceramic sphere near 50 microns in diameter. Magnetic materials embedded in the ceramic wall enhance the chaining ability of the particles, allowing for high conductivity at lower loadings. This ideal particle does exist in the form of SCC, silver coated cenospheres or microspheres manufactured by Ceno Technologies.

Resolving the Issues

cenospheres can be loaded into paints from spray-on EMI shielding The light weight of Silver Coated Cenospheres and hollow glass microspheres is a major advantage in the formulation of paints, since heavy solid particles are difficult to suspend in liquid paints. SCC eliminate the tendency of settling in pump hoses and transfer lines, decreasing paint loss by reducing the need to flush the spray system lines after a short standing time. Use of solid metal particles can also lead to sag when sprayed onto vertical surfaces, which degrades cosmetic appearance and may disrupt shielding performance. SCC eliminate this tendency, increasing product yield.

Silver Coated Cenospheres or microspheres can be incorporated into many different resin systems. Polyurethane and epoxy paints are available as solvent-based systems, and additional low VOC systems are available for special applications. For increased environmental acceptance, SCC can be added to water-based urethane and acrylic dispersions. The final shielding performance of the dry film is the same, regardless of the solvent or resin used.

Any of these types of paints can be spray applied using conventional spray equipment and are compatible with automated spray systems as well. A wide range of formulations are available within each category, and Ceno Technologies has expert chemists able to recommend the best system to meet the needs of the client, regardless of the requirements.

The Perfect Solution...Perfectly Applied

The weight of a gallon of paint incorporating SCC is about 7 lbs., contrasted to a solid metal particle gallon paint with a weight of around 30 lbs. The SCC coating appears as a smooth, light tan film, weighing less than 0.05 lbs/ft2. This film produces a surface conductivity less than 0.1 ohms/sq. with a coating thickness as low as 5 mils. The shielding effectiveness of this type film is 60dB from 100 MHz to 10 GHz and higher. These films can withstand flexing of more than 90 degrees without flaking or loss of conductivity, and forward or reverse impact does not cause chipping. Once applied, the coating can be over coated with traditional color paints, wallpaper, or panelling without affecting the shielding performance. It is hidden from view when used on the interior of electronic cabinetry.

EMI-Fighting Paints...and a lot more

Ceno Technologies can offer a wide range of standard products for EMI shielding including conductive cenospheres for inclusion in paints and coatings, conductive adhesives, gaskets, foams and tapes and impregnated or coated fabrics applications. Ceno Technologies also offers product development and manufacturing assistance in the fabrication of paints, tiles or composites with various binders, resins or other engineered materials at highly competitive rates.

High performance, lightweight, low cost conductive EMI shielding materials are available today.