National Physical laboratory (NPL), New Delhi has successfully developed a technology related to non-toxic, non-radioactive powders that can be coated onto almost any object that continuously glows in the dark.

The material developed is called ‘Long Decay Phosphor’ (LDP). Phosphors are usually made from a suitable host material, to which an activator is added. The best known type is copper-activated zinc sulfide and silver-activated zinc sulfide (zinc sulfide silver).The host materials are typically oxides, sulfides, selenides, halides or silicates of zinc, cadmium, manganese, aluminum, silicon, or various rare earth metals. These activators prolong the emission time (afterglow). Phosphors based on ZnS with radioisotopes were so far in use. However, with the ban imposed on the use of radioisotopes, development of a radioactive-free long persistence phosphor has become very important. Thus Aluminate type Long Decay Phosphor has been developed. Aluminate type LDP has a decay time of 720 minutes while Sulfide type LDP has a decay time of 30 minutes. The decay time depends upon the type of the phosphor, intensity of the source of excitation and ambient light.

The material absorbs the daylight, house-hold tungsten lamp, UV light or artificial lights (except red/yellow lights from neon/sodium lamps) and emits green-yellow light when there is sudden blackouts. Phosphors are thus able to convert absorbed (invisible) energy in the form of moving particles or quanta of radiation into visible light. Luminescence is a two step process and involves excitation of electronic system and emission of photons.

PROCESS

Raw materials, SrCO₃, Al₂O₃, Eu₂O₃, Dy₂O₃, H₃BO₃ and charcoal powder are grounded individually to <50 mm size and then mixed in specified proportions thoroughly. The mixture is then packed in a ceramic container and heated at high temperature in the range of 800-1500^oC in a controlled atmosphere for 1-5 hours. Sintered mass obtained is yellowish green emitting phosphor. It is ground to desired particle size depending on desired application.

Raw materials: SrCO₃, Al₂O₃, Eu₂O₃, Dy₂O₃, H₃BO₃ and charcoal powder.

PLANT & MACHINERY

a) Aluminate type LDP

Furnaces 500-1500 ^oC, Roller mill, ball Mill, DI Water Plant, Jaw Crusher SS, Heat Treatment Trays, Sieves with Shakers, Ovens 0-300°C.

b) Pure ZnS

Reaction vessel, Filtration Vessel, Balance, Glass Trays, Balance, UV Box, Storage Vessel, Air Compressor, DI Water Plant, Ovens 0-300°C, Furnaces 500-900 C, Grinder.

c) Sulfide type Long After glow Luminescent Powder

Phosphor Glass Trays, Ovens 0-300°C, Sieves with Shaker, Furnaces 500-1200 C, Heating Tray Making Set-up, Ball mill.

APPLICATIONS

The LDP powder can be processed into many different media and can be used in a variety of applications. Some of them are as mentioned below:

Switches, clocks, escape route marking , Warning signs on highways, Toys, warning signs in theaters, Flexible and rigid plastics goods, Sports equipment, Enamels and ceramic tiles, Marking of important machinery, Back lighting of LC displays. The coating can be on emergency boards, public guide boards, article goods, toys, household consumer items etc.

COST

Approximate Cost Of Production : 3800-4200 per kg (For a Plant of 500kg/annum)

Approximate Selling Price : 6000-7000 per kg

SPECIFICATIONS

Sl. No.	Property	Sulfide Type Long Decay phosphor	Aluminate Type Long Decay Phospor
	Chemical identity	Non-hazardous ZnS based	Non-hazardous SrAl₂O₄based
	Odour	Nil	Nil
	Average particle size	< 50mm	<150mm
	Specific Gravity	4.0	3.6
	Excitation Wavelength	200-450nm	200-450nm
	Excitation Source	Sunlight Fluorescent tubes	Sunlight Fluorescent tubes
	Excitation Time	1-2 minutes	3-5 minutes
	Glow Colour	Greenish Yellow	Yellowish Green
	Emission Peak	520-540nm	520-540nm
	Afterglow time	15-30 min	10-15 Hours
	Chemical stability	Avoid direct contact with acids	Avoid direct contact with acids and water
	Storage	Cool, dry and clean place	Cool, dry and clean place

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