January 2015 Archives

#200 Radio-Wave Absorbent Paints

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Electromagnetic shielding is designed to prevent electromagnetic wave leakage to the outside or entry from the outside by blocking its transmission path and reflecting nearly 100% of waves. The most effective material for electromagnetic wave shielding is metal.
On the other hand, radio-wave absorbent materials absorb and eliminate radio waves entering through the materials by converting them into heat energy.
Based on their absorption pattern, radio-wave absorbent materials are classified into the absorption type with larger transmission attenuation and the matching type that utilizes multipath reflection between the metal surface and the absorbent.
Radio-wave absorbent materials have the following functions: [1] A function utilizing ohm loss from the resistive element; [2] A function utilizing dielectric loss from the dielectric substance; [3] A function utilizing magnetic loss from the magnetic body; [4] A combination of any of the above three functions.

[1] is an absorption type whereby metal powders or carbon powders are dispersed in an appropriate medium. This would be suitable for an anechoic chamber. However, it requires the thickness of 1/4 wavelength or more in order to exert sufficient absorption capacity.
The matching type of [3] is designed to attenuate radio waves by placing a magnetic material in the high-field area in front of the metal backing plate used to generate standing waves.
The absorption type is also available with [3].This type is designed to absorb permeable waves by magnetic loss. Carbonyl iron and special ferrite magnetic powders are used as the magnetic substance.

Single-layer radio-wave absorbent paints and double-layer broadband radio-wave absorbent paints are available as the ferrite radio-wave absorbent.Radio waves are absorbed by the single-layer type by coating the metal surface with the resin mixed or dispersed with ferrite powders and carbon black or metal powders.These paints are used for the marine vessels radar.

As shown in [Fig.1], the double-layer radio-wave absorbent paints are comprised of two layers: the metamorphic layer and the absorbing layer that absorbs electromagnetic energy on the metal plate. They are compatible with electromagnetic waves in a broad spectrum.
The film thickness of approximately 1.3 mm can deliver sufficient performance by using the composition of epoxy, ferrite, and brass fiber for the absorbing layer.
A metamorphic layer uses silicate non-organic pigments besides the ferrite in order to improve performance, including the mechanical property and the permeation preventing capacity of the paint film.
Modifying the mixture ratio of these can change the broadband absorption characteristics.
[Fig.1] Double-layer radio-wave absorbent paints

#199 Electrically-Conductive Coating

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This is a type of paint that forms electrically-conductive coating films, which are used in various applications, including electromagnetic shielding, radio-wave shielding, prevention of static charge or surface contamination, and for electronic devices.

The trend of making electrical devices more compact and slim, increasing the density of frequency distribution of electromagnetic energy, and growing use of plastic materials requires more effective shielding technology of electromagnetic waves in a comprehensive manner. Electromagnetic wave shielding (EMI shielding) paints are now attracting attention.

To be able to shield electromagnetic waves effectively, it is necessary to consider various measures, such as chassis shielding and designing, the grounding method, noise filter application, wire separation, and circuit design, in an integrated fashion.
[Table 1] is a brief summary of paints with electrical properties.

[Table 1] Paints with electrical properties
Electrical resistance Ω/□ApplicationFiller
High conductive paste0.05Printed circuitsAg、Au、Pt、Cu
Electrically-conductive coating0.5Electromagnetic wave shieldingNi, Cu, compound materials
102Transparent electrodeITO、SnO2
Sheet heating elementC
105Static protectionSnO2、ZnO、C
108Floor materialC、ZnO

Conductive paints are largely classified into the genuine type (polymeric derivatives utilized) and the dispersion type (conductive filler dispersed).
The applications include EMI and RFI shielding, packaging, electronics, and electrode material fields, etc.
They are also used in the transparent conductive film field.

Conductive packaging materials were developed as the packing materials designed to prevent electrostatic hazards. The conduction technology to coat materials with electrically-conductive paints is now attracting attention.
The factors to consider include the carbon addition rate vs. conductivity, kneading vs. conductivity changes, the required application amount of conductive paints, flexibility of the film, workability, and static charge prevention by pattern coating, etc.
Transparent hard-coating materials designed for use with plastics for static charge prevention have also been developed.

Conductive polymers are frequently used in the aerospace industry.
Polymer insulating materials accumulate secondary electrons by high-energy electrons.
If the electrostatic potential by secondary electrons exceeds the dielectric strength of the polymer, this will cause a fracture that compromises normal functionalities.
That is where conductive polymers are beneficial.

The coating performance of conductive paints is determined by the adhesion strength to plastic cases or the characteristics required by users. General evaluation methods include heat resistance tests, damp heat tests, heat cycle tests, salt spray tests, tape peel tests, staining, metal power isolation, sheet resistance, and shielding effects.

Drawbacks of the conductive paints are [1] conductive filler's unevenness in the coating caused by insufficient mixing of the paints; [2] cracks on the coating and the base material caused by using an incompatible solution for the plastic base material; and [3] whitening occurred under high-temperature/humidity environments.
It is necessary to consider the workability when selecting the solution. In addition, it is important to consider dilution stability.

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