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Solid state lighting involving light emitting diodes (LEDs) continues to play a dramatic role in reduction of electrical energy usage in the USA. Use of LEDs in residential lighting, for example, results in a reduction of electrical power consumption by about 80-88% at equivalent light output.
Several of these light sources consist of LEDs placed on suitable substrates, housed in glass envelopes and driven by sophisticated electronics. The glass envelopes often employ specialized coatings on the surface to achieve one or more objectives: glare reduction, optimal angular distribution of the light, spectral tuning, lumen maintenance etc. The coatings are composed of one or more inorganic materials like oxides, silicates, aluminates and phosphates. The 50% particle size of the powder ranges from 20 μm to 40 μm.
In some applications, the powder layer is deposited via an electrostatic coating process. In this process, the glass envelope is rotated around a vertical axis with a flame impinging on the external surface of the glass. Powder is supplied via a gravimetric feeder into a venturi powered by a carrier gas with controlled psychrometric properties. The gas carries the powder particles into the angled nozzles of a coating gun which has a central high voltage probe running at around 60-75kV that creates a corona discharge to charge the particles. The flame serves to both heat the glass envelope to a temperature for optimal electrical conductivity and to create a ground potential for the envelope. The former is required for creating a mirror charge on the inside glass surface for initial adherence of the charged particles. The latter helps to create the electric field for migration of the particles from the probe to the inside glass surface. A subsequent heat treatment enhances the adherence of the powder layer to the glass.
"Coatings play an important role in lighting and will continue to do so because of the value they add"
The charge to mass ratio (q/m) of the particles is a key parameter that determines the quality of the electrostatic coating. A low value of q/m means poor particle charging and inadequate adhesion of the powder layer. The ratio is directly proportional to the electric field strength and inversely proportional to the particle size and density. The ratio is also influenced by the dielectric constant which varies from 4 to 6 for the materials involved.Typical q/m for the coating process varies from 1-3 μC/g.
The adjoining chart shows the angular light distribution that a LED lamp must comply with in order to be classified as an Omni-directional lamp per the US Energy Star voluntary Program. Not only must a certain amount of light be emitted at low angles near the base but the overall light distribution from 0 to 135˚ needs to have a minimum threshold of uniformity. Electrostatic powder coating of the inside of the glass bulb, via the process discussed here, helps to achieve this Omni-directionality. The coating powder is silica and/or alumina.
In some applications, the light emitted by the LED lamp is tailored to be richer or deficient in certain regions of the visible spectrum. This helps to render objects of color more vividly.
This kind of spectral tuning of the light emitted by LED lamps can also be achieved by using powder coatings. In this case, a coating is applied on the inside of the glass bulb that absorbs yellow light in the 570-590 nm wavelength region. The resulting light is crisp white that renders objects more vividly. The coating material used for this application is a combination of certain silicates, phosphates, and aluminates with optimized spectral characteristics.
In yet otherlighting applications, protective coatings are applied on photoluminescent powderscalled phosphors. Phosphors are materials that convert incident light of one wavelength to an emitted light of a longer wavelength. They are generally inorganic compounds doped with rare earth ions like Ce, Tb, Eu etc.
Protective coatings may be needed to alter the surface chemistry of the phosphor particles. The coating could improve colloidal stability in aqueous media, increase shelf life by reducing solubility and improve performance by lowering affinity for adsorption of certain species. The phosphor particles are usually very fine sized, less than 20 μm in size, and cohesive. Van der Waals type attractive forces become significant compared to gravity forces, which makes the handling of these materials very challenging.
An organo-metallic precursor is adsorbed on the surface of the phosphor particles in a low temperature multi-stage fluidized bed. These precursors are usually pyrophoric,so the process must be designed with layers of redundancy for safety. The adsorbed precursor is subsequently oxidized to the protective metal oxide coating in a second fluidized bed that operates at a temperature higher than the first bed.
Electrostatic and CVD coating technologies have been discussed in this article. In some other lighting applications, water base coatings and RF sputtered coatings are also used. Coatings play an important role in lighting and will continue to do so because of the value they add.