Industrial Applications of Rare Earth Elements - American Chemical

11 Lamp Phosphors W. A. THORNTON Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 9, 2015 | http://pubs.acs.org Publication Date: September 3, 1981 | doi: 10.1021/bk-1981-0164.ch011

Westinghouse Electric Corporation, Bloomfield, N.J. 07003

This Symposium gives a fine overview of the contributions of the rare earth elements to human need and a c t i v i t y . Their applicability to iron and s t e e l , to other a l l o y s , glasses, abrasives, refineries, electronic parts is most impressive in its scope. But I note, with wry s a t i s f a c t i o n , that the lampmaker's use of rare earths is v i s u a l l y the most spectacular of all -- that of generating brilliant colored l i g h t s to see by. The rare earth ion being a sheltered place, inside, safe from disrupting influences of its environment, it can take a bit of absorbed energy, shape it, and spit it out in one of the purest forms of v i s i b l e l i g h t we know. These pure, brilliant, colored l i g h t s (termed spectral colors or spectral lights)show enormous promise for general illumination. What I would l i k e to do is to extend the description of phosphors, of the previous paper, to modern l i g h t i n g . Not long after the turn of the century, one of the magnificent red-emitting luminescent materials, activated by the rare earth europium, was discovered(1). I t s b r i l l i a n t red-orange l i g h t has a s p e c t r a l power d i s t r i b u t i o n as i n F i g u r e 1A. From that day to t h i s , no more e f f i c i e n t r a r e e a r t h m a t e r i a l f o r generating commercial l a m p l i g h t has ever been produced, and europium has r e c e n t l y begun to p l a y a major r o l e i n l i g h t i n g human a c t i v i t i e s a l l over the world. Generating b r i l l i a n t c o l o r e d l i g h t s i s e s s e n t i a l t o the lampmaker, because the white l i g h t he s e l l s i s composed o f a mixture of b r i l l i a n t c o l o r e d l i g h t s , and because he i s beginning to understand which c o l o r e d l i g h t s are most important to human v i s i o n . Some hundred and f i f t y s p e c t r a l l i g h t s are d i s t i n g u i s h a b l e by the normal human observer. They a r e , o f course, f a r from e q u a l l y e f f e c t i v e i n a i d i n g the human v i s u a l system to f u n c t i o n w i t h maximum e f f i c i e n c y . The v i s u a l system has three independent i n p u t s , a l l o w i n g i t to s o r t incoming l i g h t s i n three dimensions. There must be three independent s p e c t r a l responses, a s s o c i a t e d w i t h these i n p u t s , each s p e c t r a l response sampling a d i f f e r e n t r e g i o n o f the v i s i b l e spectrum, although there may be a great d e a l o f overlap between p a i r s o f responses. Each s p e c t r a l response may be c h a r a c t e r i z e d by 0097-6156/81/0164-0195$05.00/0 © 1981 American Chemical Society In Industrial Applications of Rare Earth Elements; Gschneidner, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 9, 2015 | http://pubs.acs.org Publication Date: September 3, 1981 | doi: 10.1021/bk-1981-0164.ch011

300

Figure 1.

400

500

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ELEMENTS

X)0 NM

8

The brilliant orange-red emission of Eu * (A), contrasted to the bluishwhite spectral power distribution of average daylight (B)

80

60

40 CRI

20

0

-20 400

500 600 WAVELENGTH - NM

Figure 2. The color-rendering index (CRI), of similarity to daylight-rendering, dependence upon choice of triad of spectral lights to form white light of daylightcolor. Wavelengths of two components are fixed at their peaks, and the wavelength of the third component is varied. Optimum combination appears in Figure 3.

Figure 3. The three pure spectral colors, the "prime-colors," uniquely related to normal human vision. Combined, as shown here, they form a white-light mixture the color of sunlight.

_j J I • LA 400 500

iL_l 600 NM

In Industrial Applications of Rare Earth Elements; Gschneidner, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


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a s i n g l e wavelengthC2,3)marking i t s peak, o r mean, s e n s i t i v i t y ; t h e r e s u l t i n g three wavelengths expectably p l a y a unique r o l e i n human v i s i o n , and the s p e c t r a l l i g h t s corresponding t o them may do the same i n i l l u m i n a t i o n . We name these wavelengths the " p r i m e - c o l o r s " o f human v i s i o n (A), and ask where i n the v i s i b l e spectrum they l i e . I f a r t i f i c i a l white l i g h t o f d a y l i g h t c o l o r i s composed o f t r i a d s o f s p e c t r a l l i g h t s , one soon f i n d s that the c o l o r - r e n d e r i n g o f such mixtures v a r i e s from very poor t o very good. E v e n t u a l l y i t i s found how c r i t i c a l the choice i s , that three s p e c i f i c wavelengths are necessary f o r greatest s i m i l a r i t y t o r e a l - d a y l i g h t - r e n d e r i n g , that the c o l o r - r e n d e r i n g o f t h i s unique t r i a d i s very good indeed ( b e t t e r than that o f most commercial lamps marketed), and that d e v i a t i o n i n wavelength from any one o f the three optimum s p e c t r a l l i g h t s r e s u l t s i n r a p i d d e t e r i o r a t i o n o f c o l o r - r e n d e r i n g o f the w h i t e l i g h t mixture(2_ 5) . The f i n a l i t e r a t i o n i s shown i n F i g u r e 2,where two o f the wavelengths are s e t a t optimum v a l u e s and the t h i r d wavelength i s v a r i e d . For each t r i a d , the c o l o r - r e n d e r i n g index (CRI, an index o f s i m i l a r i t y t o d a y l i g h t - r e n d e r i n g ) i s computed(60. I see no e x p l a n a t i o n o f F i g u r e 2 other than that the unique wavelengths mark the mean s e n s i t i v i t i e s o f the three v i s u a l r e s ponses, and can be thought o f as the "sampling p o i n t s " o f the v i s u a l system. I n any case, the w h i t e - l i g h t mixture o f F i g u r e 3, which i s about the c o l o r o f s u n l i g h t , renders the c o l o r o f complexions, foods, c l o t h i n g , f u r n i s h i n g s , p l a n t s , animals, and m i n e r a l s a s t o n i s h i n g l y w e l l , i . e . p l e a s a n t l y and expectably. The widths o f the components can be narrowed without l i m i t , u n t i l almost a l l o f the v i s i b l e spectrum i s empty. Yet moving one o f the components f i f teen nanometers can be d i s a s t r o u s . The c o l o r o f such p r i m e - c o l o r l i g h t may be v a r i e d by a l t e r i n g the r a t i o o f power i n the three components, t a k i n g care not t o a l t e r the mean wavelength o f any component. In a d d i t i o n t o s i m i l a r i t y t o d a y l i g h t - r e n d e r i n g , p r i m e - c o l o r white l i g h t y i e l d s an i l l u m i n a t e d scene which i s p e c u l i a r l y a t t r a c t i v e (_7) shows p e c u l i a r " v i s u a l c l a r i t y " ( 8 ^ , 1 0 ) ,and h i g h p e r c e i v d b r i g h t n e s s per f o o t c a n d l e ( l l ) . Other p s y c h o p h y s i c a l evidence(2,J3, 12,13)suggests t h a t the s p e c t r a l response o f the human v i s u a l s y s tem i s approximated by the dashed envelope o f F i g u r e 4; the three peaked responses are independent although o v e r l a p p i n g . The s o l i d curve i s the t r a d i t i o n a l luminous e f f i c i e n c y f u n c t i o n , a l l o w i n g only one-dimensional v i s i o n , o p e r a t i v e only under uncommon v i s u a l c o n d i t i o n s , y e t wrongly p r e s i d i n g over lamp development f o r more than s i x t y y e a r s . A modern o b j e c t i v e i s t o feed l i g h t i n g power i n t o the v i s u a l system a t i t s peaks o f response, as does the white l i g h t o f F i g u r e 3, f o r example. T h i s a r e v o l u t i o n a r y turnabout from t r a d i t i o n a l views o f how commercial l a m p l i g h t should be designed; the d a y l i g h t i n which we presumably evolved i s a continuum(Figure IB) as are f i r e l i g h t and l i g h t from the o i l lamp and the incandescent lamp. The primary use o f l a m p l i g h t i s t o l i g h t human a c t i v i t i e s , and i t i s h i g h time the l a m p l i g h t i s designed f o r the human v i s u a l system. 9

9


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Generating b r i l l i a n t c o l o r e d l i g h t s l i k e the components of F i g u r e 3 i s something r a r e earths can o f t e n do as no other m a t e r i a l s can. I d e a l l y , the components of l a m p l i g h t should comprise very narrow bands centered near 450nm, 535nm, and 615 nm. V a r i a t i o n of mean wavelength of one of the components by +5nm has l i t t l e d e l e t e r i o u s e f f e c t , but more than t h a t r e s u l t s i n r a t h e r r a p i d degrada t i o n of the v i s u a l e f f i c i e n c y of the l a m p l i g h t . P a r t i c u l a r l y to be s c r u p u l o u s l y avoided, i n the i d e a l case, are the " a n t i p r i m e " c o l o r s , v i o l e t , blue-green, y e l l o w and deep-red. Europium 3+ i s made to order f o r the red-orange p r i m e - c o l o r . The c r y s t a l p l a y i n g host to the europium i m p u r i t y must, however, s t r o n g l y f a v o r the ^ D Q — F e l e c t r i c d i p o l e t r a n s i t i o n , which y i e l d s s t r o n g emission i n the wavelength range 612-618nm. Some c r y s t a l s h o s t i n g Eu 3+ a l l o w magnetic d i p o l e t r a n s i t i o n s and thus strong yellow-orange emission near 590nm, and are u n s u i t a b l e ( 1 4 , 1 5 ) . The u s e f u l e l e c t r i c d i p o l e t r a n s i t i o n s are favored by oxygen-domi n a t e d l a t t i c e s ( s m a l l i o n , l a r g e charge). Perhaps the best luminescent m a t e r i a l of t h i s type, at l e a s t f o r use i n f l u o r e s c e n t lamps, i s Y2O3:Eu^ (Figure I A ) . A c h i e v i n g a p r i m e - c o l o r s p e c t r a l power d i s t r i b u t i o n l i k e t h a t of F i g u r e 3 r e q u i r e s good c o n t r o l of the e m i t t i n g species i n the commercial lamp. At p r e s e n t , the f l u o r e s c e n t lamp o f f e r s the best c o n t r o l . The v i s i b l e c o n t r i b u t i o n of i t s arc i s so f a r i n t r a c t a b l e but amounts to only a few percent of v i s i b l e output. The r e s t d e r i v e s of course from luminescent m a t e r i a l s which convert 254nm r a d i a t i o n from the mercury a r c , to u s e f u l l i g h t . The problem i s t h e r e f o r e to provide e f f i c i e n t luminescent m a t e r i a l s which emit the p r i m e - c o l o r s , p r e f e r a b l y one phosphor per p r i m e - c o l o r f o r ease i n c o l o r adjustment of the r e s u l t i n g l a m p l i g h t . Quantum e f f i c i e n c y ( v i s i b l e photons emitted per 254nm photon absorbed)must be 0.8-0.9 i n order to stay i n the f i e r c e l y c o m p e t i t i v e f l u o r e s c e n t lamp market. So Y 2 0 3 i E u ^ i s , except f o r i t s h i g h c o s t , not f a r from i d e a l as a photoluminescent orange-red prime-color generator. What about the b l u e - v i o l e t , and the green, prime-colors? The w r i t e r knows of no r a r e e a r t h i o n , i n any host c r y s t a l , which i s s t r o n g l y photoluminescent and emits a narrow band i n the b l u e - v i o l e t centered at 45tH5nm. Thulium 3+, i n YVO^ f o r example, emits near 475nm, which i s too g r e e n i s h . Praseodymium, even given an a p p r o p r i a t e host c r y s t a l , would probably be too green i n i t s emission a l s o . However, d i v a l e n t r a r e e a r t h ions can emit s t r o n g bands of r a t h e r pure v i s i b l e l i g h t , although i n t e r a c t i o n w i t h the c r y s t a l environment broadens the emission band f a r more than i s c h a r a c t e r i s t i c of t r i v a l e n t r a r e e a r t h emission. There are a number of host c r y s t a l s , perhaps the best of which i s s t r o n t i u m c h l o r a p a t i t e ( 1 6 ) , f o r d i v a l e n t europium. I t s emission spectrum appears i n F i g u r e 5A. Quantum e f f i c i e n c y i s c l o s e to 0.9, and f o r t u n a t e l y the emission band, w h i l e not i d e a l , feeds i n t o the human system w i t h great v i s u a l e f f i c i e n c y . I t i s the best we have, at any r a t e . I f a r a r e 7


ELEMENTS

2

+

+


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Phosphors

/ /

/

/

/ / / /


y 400

500 600 WAVELENGTH - N M

700

Figure 4. The three-peaked spectral response of the human visual system, the peak responses being marked by the prime-colors: ( ) the luminous efficiecy curve upon which much of modern lighting is wrongly based.

Figure 5. The brilliant blue-violet emission of Eu * (A), and the green emission of zinc silicate activated by Mn * (B). 2

2

Figure 6. The spectral power distribution of a fluorescent lamp containing two rare earth phosphors, those of Figures 1 (Curve A) and 5 (Curve A), and greenemitting zinc silicate:Mn. A closer approximation to Figure 3 is desirable.



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earth-host c r y s t a l combination could be found, which concentrates i t s emission more c l o s e l y around 450nm, i t would be welcome indeed. The s i t u a t i o n i n regard to the green prime-color i s not as s a t i s f a c t o r y . T r i v a l e n t terbium, holmium and erbium have t r a n s i t i o n s i n the d e s i r a b l e wavelength r e g i o n s , but the l a s t two have not y e t been developed i n e f f i c i e n t photoluminescent m a t e r i a l s , and terbium has s p e c i a l problems. We need a narrow emission near 530nm o r 540nm, uncontaminated with s a t e l l i t e emission i n unwanted regions, p a r t i c u l a r l y the yellow. Terbium ions i n c e r t a i n c r y s t a l s make very f i n e phosphors(17), p o s s i b l y unmatched i n e f f i c i e n c y and ruggedness. (The mercury a r c i s a small i n f e r n o , and these i n o r ganic m a t e r i a l s have to be robust.) But terbium always, or at l e a s t so f a r , b r i n g s with i t sidebands i n the yellow and b l u e green. These r a p i d l y degrade the c l a r i t y and c o l o r a t i o n o f a scene i f they are present i n the white l i g h t i l l u m i n a t i n g the scene. So we have had to s t i c k with an o l d and trustworthy phosphor, z i n c s i l i c a t e : M n ^ , to provide our green emission(Figure 5B). I t i s not, however, q u i t e narrow enough, nor rugged enough. I would l i k e to take t h i s opportunity to ask, very s e r i o u s l y , f o r any help readers can give toward i d e n t i f y i n g an e f f i c i e n t pure-greene m i t t i n g luminescent m a t e r i a l . A new phosphor of that s o r t could have a profound e f f e c t on the q u a l i t y o f l i g h t i n g around the world. Hopefully some one o f the r a r e earths could help to b r i n g t h i s about. +

The s p e c t r a l power d i s t r i b u t i o n o f Figure 6 i s as c l o s e as we can p r e s e n t l y get to the i d e a l prime-color mixture. In summary, lamplight i l l u m i n a t e s human a c t i v i t i e s , so lamp phosphors should feed t h e i r l i g h t i n t o the human v i s u a l system with h i g h v i s u a l e f f i c i e n c y . R a r e - e a r t h - a c t i v a t e d phosphors tend to produce narrow, s t r o n g l y s a t u r a t e d , b r i l l i a n t l y c o l o r e d l i g h t s . I t begins to appear that r a r e earth emission i s not only u s e f u l , but made-to-order, f o r the requirements o f the human v i s u a l s y s tem f o r optimum seeing. The v i s u a l system has three w e l l - d e f i n e d peaks o f response, placed at three wavelengths unique to human v i s i o n . When white lamplight i s composed as n e a r l y as p o s s i b l e o f these three pure s p e c t r a l c o l o r s , and the remainder of the v i s i b l e spectrum i s l e f t as n e a r l y empty as p o s s i b l e , at l e a s t four strong p o s i t i v e v i s u a l e f f e c t s r e s u l t : The perceived b r i g h t n e s s per watt of lamplight exceeds that o f normal i l l u m i n a n t s by tens of percent. The v i s i b i l i t y o f a scene per watt of lamplight increases by l a r ger f a c t o r s s t i l l . C l a r i t y , i n the sense o f sharpness of d e t a i l i n a scene, i s enhanced. A t t r a c t i v e n e s s o f c o l o r a t i o n , measured i n terms o f what Judd c a l l e d " p r e f e r r e d c o l o r a t i o n " , exceeds that of d a y l i g h t , which i t s e l f excels normal i l l u m i n a n t s . The three spectr a l c o l o r s so important to human v i s i o n are: b l u e - v i o l e t near 450nm, green near 535nm, and orange-red near 615nm. The f i r s t and l a s t o f these c o l o r e d l i g h t s are f a i r l y s a t i s f a c t o r i l y s u p p l i e d by lamp phosphors a c t i v a t e d by europium 2+ and europium 3+, r e s p e c t i v e l y . The need f o r a b e t t e r pure green emission i s acute.



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Literature Cited 1. Urbain,G.,Ann.Chim.Phys.,1909,18,294. 2. Thornton,W.A.,J.Opt.Soc.Amer.,1971,61,1155. 3. Thornton,W.A.,J.Opt.Soc.Amer.,1972,62,457. 4. Thornton,W.A.,Westinghouse Engineer,1972,32,170. 5. Thornton,W.A.,J.I11.Eng.Soc.,1979,8,78. 6. CIE Pub.#13,E-l.3.2,1965,1st Ed. 7. Thornton,W.A.,J.I11.Eng.Soc.,1974,4,48. 8. Aston,S.N.,Bellchambers,H.E.,Lighting Res.Tech.,1969,1,259. 9. Bellchambers,H.E.,Godby,A.C.,Lighting Res.Tech.,1972,4.,104. 10. Thornton,W.A.,Chen,E.,J.I11.Eng. Soc.,1978,7,85. 11. Thornton,W.A.,Chen,E.,Morton,E.W.,Rachko,D.,J.I11.Eng.Soc., (in press). 12. Thornton,W.A.,J.Color Appearance,1973,II,23. 13. Thornton,W.A.,Lighting Pes.Appl.,1975,5,35. 14. Palilla,F.C.,Electrochem.Tech.,1968,6,39. 15. Blasse,G.,Bril,A.,Philips Tech.Rev.,1970,31,304. 16. Wachtel,A.Netherlands Patent 6906724,1969. 17. McAllister,W.A.,J.Electrochem.Soc.,1966,113,226. RECEIVED

January 8,

1981.


Industrial Applications of Rare Earth Elements - American Chemical

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