APPENDIX

APPLICATION OF PHOSPHORS IN OPTICAL DEVICES

A.l. Introduction

In recent years, there has been a growth of

interest on luminescent materials based on the rare earths,

either as host lattice constituents or activators [1]. Rare

earth phosphors have found important commercial applications in

such field as lasers, colour television and lighting. Rare

earth phosphors have also been useful in theoretical studies of

luminescence mechanisms. The red emitting europium-activated

phosphors have been of particular commercial interest since the

introduction of Yttrium Vanadate in 1964 as an efficient

cathode-ray phosphor [2]. From 1964 to the present, europium

activated phosphors have been adopted as the standard red

primary in colour television picture tubes. These phosphors

include (Y Eu)VO , (Y,Eu,Bi)VO 4 4

(Y, Eu) 0 5, 2 2

(Y, Eu) 0, 2 3

(Gd , Eu) 0 , and ( Gd , Y , Eu) 0 . 2 3 2 3

Certain primary colour phosphors

which are suitable for TV screen are briefly discusssed here.

246

6

5

4 -,~ C :J

.ci '-

"' - 3 >-. .~ Vl C QJ +" C -

2

REO PHOSPHOR

5 1 D~~

599 614 633 703 710 Wavelength (n rn)

Fig. lA. Fluorescencl emission spectrum of Y202SIEU + •

247

A.2. Trivalent Europium activated Yttrium Oxysulphide

3+ (Y 0 5 : Eu )

2 2 - Red phosphor

Trivalent europium activated yttrium Oxysulphide

3+ (Y 0 S : Eu ), one of the rare earth oxysulphide group of

2 2

phosphors, is of great interest due to its potent.ial

application in colour television picture tubes as red

component. It has better luminous efficiency and brightness

and high resistance to acid treatment during t.he coat.ing

process when compared wit.h other red phosphors for colour

television pict.ure tubes such as: Zn 3

2+ (PO ) : Mn , (Zn Cd ) S : Ag ,

" 2 3+ 3+

Y (VO ) : Eu and Y 0 : Eu . " 3 2 3

Preparation of Y 0 S : Eu3 + is carried out by 2 2

flux method. This involves mixing of appropriate quantities of

AR grade Yttrium Oxide, Europium Oxide, Sulfur, Sodium

Carbonate and Potassium Phosphate and firing the temperature

o range of 1000-1100 C for 1-3 hours. Flux in this case will be

sodium polysulphide. After the firing, excess flux was removed

by washing with demineralised water and the traces of unreact.ed

Y 0 by dilute HCl. The powder thus obtained was 2 3

converted

into pellets of 10 mm radius and 3 mm thickness. The phosphors

248

...... 1/'1 -C :J

.d l-tU ->-t-t-4 I.f)

z W t-Z t-4

)8

12

10

8

2

BLUE PHOSPHOR 480

450 500 WAVELENGTH(nm)

Fiq. 2A Fluorescence emission spectrum of calcium tungstate.

249

were excited using the nitrogen laser as described previously.

As shown in Fig.1A,- line emission spectrum is obtained in the

orange-red region which corresponds to the

and 50 ~7F transitions. o :I

A.l. Calcium Tungsta~e - Blue phosphor

In calcium tungstate crystal phosphor which has

the structure and composition of the mineral 'scheelite',

luminescence without external activators is due to the complex

tungstate ions W02 - in the lattice [3]. The tungstate ion is 4

coordinated by four oxygen having predominantly a covalent

bond. The calcium is coordinated by six oxygen ions having

predominantly an ionic bond.

A mixture of calc.ium carbonate and tungstic acid

in a suitable mole ratio is ground into a slurry with the

addition of suitable quantity of water. The yellow slurry is

ground thoroughly and dried, in an air oven at about

that the resultant mass is nearly white as possible.

due to the formation of tungsten dihydrate H WHO 242

so

This is

or WO 2H 0 :I 2

which is white in colour. The powdered mass is subsequently

250

-..., c :J .rl ... IQ ->-.-~

Vl z UJ .-Z H

9

a

7

6

5

4

3

2

520 GREEN PHOSPHOR

500 600

WAVELENGTH( nm)

~ig.3A Fluorescence e~1ssion spectrum of zinc silicate.

251

heated in air at a temperature in the vicinity of 800-1000o C

for about 2 hours. The resulting mass is finaly ground and

pelletised. As shown in Fig.2A, a broad band peaking at 480 nm

was observed.

A.~.Zinc Silicate - Green phosphor

Zinc Silicate matrix is known to be rhombohedral

in its structure. The configurational coordinate diagram for

Zn 50 : Mn has been worked out by various workers. The 2 4

emission transition is attributed to a spin reversal of one of

2+ the 3d electrons of the Mn ion. The mode of vibration is a

radial one for the four oxygen ions surrounding. the manganese

ion. 2+ •

Depending on the interaction between the Mn Ion and the

host lattice the luminescence peak centred at 520 nm (Fig.3A),

the mechanism being explained by a configurational coordinate

scheme.

252

REFERENCES

[1] Analysis and Application of Rare Earth Materials;

Odd.B. Michelsen (ed.), Universitets Forlaget, Norway

(1973); P.241, "Production of rare earth red phosphors for

color television and lighting applications",by J.E.Mathers.

[2] A.K.Levine and F.C.Palilla,Appl.Phys.Lett., 5, 118 (1964).

[3] On Luminescent Materials and Devices,Technical Note, CECRr

(1986).

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