Chemically stimulated luminescence in rare earth oxides

Chemically stimulated luminescence in rare earth oxides

Volume 33A, n u m b e r 3 CHEMICALLY PHYSICS STIMULATED LETTERS LUMINESCENCE 19 October 1970 IN RARE EARTH OXIDES* J. R. SWEET, W . B . W H...

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Volume 33A, n u m b e r 3

CHEMICALLY

PHYSICS

STIMULATED

LETTERS

LUMINESCENCE

19 October 1970

IN

RARE

EARTH

OXIDES*

J. R. SWEET, W . B . W H I T E **, H . K . H E N I S C H *** a n d R. ROY ** Materials Research Laboratory, The Pennsylvania State Uni~,ersity University Park, Pennsylvania 16802, USA Received 26 August1970

Strong luminescence from Tb3+-(~ 0.1%) doped Y203 and Lu203 is activated by a hydrogen diffusion flame. 45 s p e c t r a l lines a r e resolved, the s t r o n g e s t of which exhibits maximum intensity at 160°C. Activation appears to be by neutral OH* species in the flame.

C a n d o l u m i n e s c e n c e i s d e f i n e d a s a p r o c e s s of light emission from solids under flame excitat i o n , in e x c e s s of t h e r m a l r a d i a t i o n . T h e p h e n o m e n o n w a s o n c e a s u b j e c t of g r e a t i n t e r e s t in c o n n e c t i o n w i t h the o p e r a t i o n of g a s m a n t l e s a n d received extensive, although mainly qualitative, i n v e s t i g a t i o n in the f i r s t h a l f of t h i s c e n t u r y [1,2]. M o r e r e c e n t w o r k h a s a p p e a r e d p r i m a r i l y in the R u s s i a n l i t e r a t u r e [3] a n d a r e p o r t by M a s o n [4] w h o i n v e s t i g a t e d m o s t of t h e r a r e e a r t h o x i d e s a n d n u m e r o u s c o m m e r c i a l p h o s p h o r s . M o s t of the earlier work was concerned with materials at or near incandescent temperatures, although some low-temperature observations have been reported. The more recent investigations have been concerned with elucidating the excitation m e c h a n i s m of t h i s l u m i n e s c e n c e . O n e v i e w h a s b e e n t h a t r e c o m b i n a t i o n of H a t o m s on the p h o s p h o r s u r f a c e s o m e h o w p r o v i d e d the e n e r g y f o r t h e l u m i n e s c e n c e [5]. T h e s c a t t e r e d s p e c t r a l m e a s urements which have been made generally show a t m o s t a few b r o a d b a n d s w i t h n o f i n e s t r u c t u r e . O u r p u r p o s e in t h i s n o t e i s to c a l l a t t e n t i o n to a p h e n o m e n o n w h i c h i s m o r e w i d e s p r e a d t h a n g e n e r a l l y b e l i e v e d . I t s s i g n i f i c a n c e l i e s in the low d o p i n g l e v e l s i n v o l v e d , t h e r e m a r k a b l e d e t a i l e x h i b i t e d , a n d t h e e f f e c t of t e m p e r a t u r e on intensity as detailed below. M e a s u r e m e n t s w e r e m a d e by p l a y i n g a s i m p l e h y d r o g e n d i f f u s i o n f l a m e on a l a y e r of p h o s p h o r p a i n t e d on a K a n t h a l rod. T h e p h o s p h o r w a s p r e p a r e d a s a s l u r r y in w a t e r , p a i n t e d on t h e r o d * Supported by the Pennsylvania Science and Engine e r i n g Foundation under a g r e e m e n t n u m b e r 9. ** Also affiliated with the D e p a r t m e n t s of Materials Science. and G e o c h e m i s t r y and Mineralogy. *** Also affiliated with the D e p a r t m e n t of Physics.

a n d a l l o w e d to d r y . T h e h y d r o g e n d i f f u s i o n f l a m e i m p i n g e d on t h e p h o s p h o r s u r f a c e i m m e d i a t e l y a d j a c e n t to, but not d i r e c t l y on, a t h e r m o c o u p l e w e l d e d to the r o d . E l e c t r i c a l h e a t i n g a n d w a t e r c o o l i n g of t h e r o d p e r m i t t e d m e a s u r e m e n t of spectra above or below the equilibrium temperat u r e p r o d u c e d by t h e f l a m e a l o n e , w i t h o u t v a r y ing the combustion rates. Spectra were recorded photographically. The oxides La203, Y203, Gd203 and Lu203 a t the u s u a l 99.9% p u r i t y l e v e l s a l l e x h i b i t s o m e luminescence under these conditions. The backg r o u n d l u m i n e s c e n c e of a n o m i n a l l y 9 9 . 9 9 9 9 % p u r e (with r e s p e c t to o t h e r r a r e e a r t h s ) Y 2 0 3 w a s low e n o u g h to p e r m i t i t s u s e a s a h o s t f o r systematic doping experiments. A green phosphor w a s p r e p a r e d b y a d d i n g 0.00143 m o l e f r a c t i o n of T b 2 0 3 to t h i s Y 2 0 3 a n d r e a c t i n g in H 2 a t 1 3 0 0 ° C f o r 12 h o u r s . T h e s p e c t r u m of the f l a m e - e x c i t e d luminescence from this phosphor is very similar to p h o t o l u m i n e s c e n c e s p e c t r a r e p o r t e d [6] f o r 5% t e r b i u m in v a r i o u s h o s t s , e x c e p t t h a t the l i n e s s e e m u n u s u a l l y s h a r p . A s a m p l e of a 99.9% p u r e Lu203 was found that gave a spectrally identical, t h o u g h m o r e i n t e n s e , e m i s s i o n d u e to Tb 3+ a s a n i m p u r i t y at .:: 0.001 m o l e f r a c t i o n . T h i s s p e c t r u m (fig. 1) c o n s i s t s of 45 d i s c r e t e l i n e s b e t w e e n 4 8 0 a n d 690 rim, not a l l of w h i c h a r e d i s c e r n i b l e in t h e f i g u r e . N e i t h e r of t h e s e p h o s p h o r s i s l u m i n e s c e n t u n d e r 2537 o r 3650 A p h o t o e x c i t a t i o n . B o t h the L u 2 0 3 a n d Y 2 0 3 p h o s p h o r s s h o w e d a n o p t i m u m t e m p e r a t u r e f o r l u m i n e s c e n c e of a b o u t 1 6 0 ° C . Fig. 2 s h o w s t h e r e l a t i v e i n t e n s i t y of the 550 n m b a n d of Y 2 0 3 : T b (0.143%) a s a f u n c t i o n of t e m p e r a t u r e . F o r t h i s m e a s u r e m e n t t h e p h o t o g r a p h i c p l a t e w a s r e p l a c e d by a p h o t o m u l t i p l i e r p o s i t i o n e d to r e c e i v e the 550 n m e m i s s i o n , a n d the s l i t o p e n e d to 3 m m to a d m i t a s 195

Volume 33A. number 3

PHYSICS

LETTERS

[9 October 1970

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Fig. 2. Luminescent inte, nsit.v as a lunetion o[ temper:~trite for the 550 nm band of Y203:Tb (0 143 ~:) 480

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Fig. 1. Candoluminescenee s p e c t r u m oI Lu203:Tb (0.U..{) shown as a d e n s i t o m e t e r trace t'rom a photo graphic plate. Note the extremely intense band in the green at 550 nm, w h i e h i s shown in the inset ~t a lo\ter exposure. The a r r o w indicates the D-line of sodium from the flame. m u c h of the g r e e n r e g i o n a s p o s s i b l e . The t e m p e r a t u r e was then a l t e r n a t e l y r a i s e d and l o w e r e d , the p o i n t s s h o w n in fig. 2 b e i n g t a k e n o v e r a t w o h o u r p e r i o d , and o v e r two h e a t i n g - c o o l i n g c y c l e s . A m i c r o w a v e d i s c h a r g e w a s u s e d to i d e n t i f y the f l a m e s p e c i e s r e s p o n s i b l e f o r the c a n d o l u m i n e s c e n c e by p r o d u c i n g a s i m i l a r l u m i n e s c e n c e w i t h o u t a f l a m e . S a m p l e s of L u 2 0 3 w e r e p l a c e d on a c o p p e r p r o b e w i t h i n a s i l i c a tube p a s s i n g through a microwave cavity. Gases or gas mixt u r e s w e r e p a s s e d t h r o u g h the tube at c o n t r o l l e d p r e s s u r e s a n d flow r a t e s , w h i l e the t e m p e r a t u r e of the p r o b e w a s m o n i t o r e d by a t h e r m o c o u p l e . T h e m i c r o w a v e d i s c h a r g e p r o d u c e s f r o m the g a s n e u t r a l a t o m s and r a d i c a l s w h i c h flow d o w n the t u b e . S o m e i o n i c s p e c i e s a r e p r o d u c e d , but t h e i r

196

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550 wavelength,

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d e c a y t i m e s a r e m u c h f a s t e r than t h o s e of n e u t r a l s p e c i e s , and they do not t h e r e f o r e m i g r a t e f a r f r o m the c a v i t y . A g r e e n l u m i n e s c e n c e w a s o b s e r v e d in an H 2 0 a t m o s p h e r e at 5 to 10 t o r t . but not in an H2 a t m o s p h e r e at any p r e s s u r e d o w n to l e s s than 1 t o r r . In f a c t s m a l l a m o u n t s of H 2 a d d e d to the H 2 0 a t m o s p h e r e q u e n c h e d the l u m i n e s c e n c e . The luminescence intensity was visually monitored with i n c r e a s i n g t e m p e r a t u r e and w a s o b s e r v e d tu h a v e the s a m e b e h a v i o r a s c a n d o l u m i n e s c e n c e p r o d u c e d by a h y d r o g e n f l a m e . T h i s e x p e r i m e n t s h o w s that the l u m i n e s c e n c e is f r o m the p h o s p h o r , not f r o m the f l a m e a s s u c h , and t h a t the e x c i t e d s p e c i e s a p p e a r s to b e OH* r a t h e r than H* a s c l a i m e d in m o s t of the l i t e r a t u r e . The m e c h a n i s m of e n e r ~ " t r a n s f e r i s u n d e r i n v e s t i g a t i o n . The e x p e r i m e n t a l r e s u l t s s u g g e s t t h a t m a n y , if not p e r h a p s a l l , of the p h e n o m e n a r e p o r t e d in t h e l i t e r a t u r e a s c a n d o l u m i n e s c e n c e of p u r e m a t e r i a l s owe t h e i r o r i g i n to r a r e e a r t h o r t r a n s i t i o n m e t a l d o p i n g of the h o s t m a t e r i a l s in t r a c e a m o u n t s .

Volume 33A, n u m b e r 3

PHYSICS

LETTERS

19 October 1970

References [1] E. L. Nichols. H. L. Howes and D. T. Wiiber, Cathodoluminescence and the luminescence of incandescent solids (Carnegie Institution of Washington. Publ. No. 384. 1928). [2] L. T. Minchin. T r a n s . Faraday Soc. 35 (1939) 163. [3] V. A. Sokolov, Izv. Akad. Nauk USSR. Ser. Fiz. 21 (1957) 528; 26 (1962) 514.

REMARKS

[4] D. M. Mason. Amer. Chem. Soc. Div. Fuel Chem. P r e p r i n t s . P a r t 2. 11 (2) (1967)540. [5] J. R. A r t h u r and D. T. A. Townend. J. Inst. Fuels 26 (1953) 44. [6] G. B l a s s e a n d A . Bril. Philips Res. Repts. 22 0967) 481.

ON FROHLICH'S MICROSCOPIC DERIVATION ON THE NAVIER-STOKES EQUATIONS

G. J. H Y L A N D a n d G. R O W L A N D S

School of Physics, University of Warwick, Coventry, UK Received 16 September 1970

It is shown that the l i n e a r i z a t i o n procedure used by F r 0 h l i c h in the specification of the force density. in o r d e r to derive the Navier-Stokes equation, is not inconsistent with the retention of the n o n - l i n e a r convective derivative t e r m .

In a m i c r o s c o p i c d e r i v a t i o n of t h e e q u a t i o n s of h y d r o d y n a m i c s , F r i ~ h l i c h [1] s u c c e e d e d i n o b t a i n i n g t h e u s u a l f o r m of t h e N a v i e r - S t o k e s e q u a t i o n s o l e l y b y e x p l o i t i n g t h e s y m m e t r y , u n d e r p a r t i c l e i n t e r c h a n g e , of t h e two p a r t i c l e c o r r e l a t i o n d e n s i t y P(x, y) i n a d e v e l o p m e n t of t h i s q u a n t i t y a b o u t a s t a t e of hydrodynamic equilibrium*. This equilibrium, apart from zero fluid velocity, is characterized by translational and rotational symmetry, whence the macrodensity is spatially independent and P is a funct i o n of Ix-y I. It i s not i m m e d i a t e l y a p p a r e n t f r o m t h i s w o r k t h a t t h e l i n e a r i z a t i o n p r o c e d u r e u s e d to o b t a i n t h e f o r m of t h e v i s c o u s t e r m s c a n b e r e c o n c i l e d w i t h t h e r e t e n t i o n of t h e n o n - l i n e a r c o n v e c t i v e d e r i v a t i v e t e r m . I n v i e w of t h e i m p o r t a n c e of t h i s w o r k we f e e l t h a t i t s h e u r i s t i c a p p e a l w o u l d b e e n h a n c e d b y t h e r e m o v a l of t h i s a p p a r e n t i n c o n s i s t e n c y . T o t h i s e n d we m a k e a s y s t e m a t i c , G a l i l e a n i n v a r i a n t r e d e v e l o p m e n t of P in t e r m s of t h e g r a d i e n t s of t h e m a c r o s c o p i c d e n s i t y crix) , v e l o c i t y v(x) a n d t h e i n t e r n a l k i n e t i c e n e r g y d e n s i t y T°(x) ( e f f e c t i v e l y t h e k i n e t i c p r e s s u r e ) , not a b o u t F r S h l i c h ' s s p a t i a l l y i n d e p e n d e n t e q u i l i b r i u m s t a t e , b u t a b o u t a s t a t e of l o c a l e q u i l i b r i u m , c h a r a c t e r i z e d b y (~(x) a n d T°(x). In t h i s r e d e v e l o p m e n t s e c o n d d e r i v a t i v e s a n d q u a d r a t i c p r o d u c t s of f i r s t d e r i v a t i v e s of (zix), v i x ) a n d T°(x) w i l l b e c o n s i d e r e d to b e of t h e s a m e o r d e r of s m a l l n e s s . In t h i s w a y t h e a p p a r e n t i n c o n s i s t e n c y i s r e m o v e d a n d t h e N a v i e r - S t o k e s e q u a t i o n s o b t a i n e d b u t t h e v i s c o s i t y c o e f f i c i e n t s a r e now f u n c t i o n s of t h e l o c a l v a r i a b l e s , nix) a n d T°(x) a n d not of t h e s p a t i a l a v e r a g e s of t h e s e q u a n t i t i e s a s i n r e f . [1]. In r e f . [1] a n e x a c t m o m e n t u m e q u a t i o n i s o b t a i n e d w h i c h i n v o l v e s the f o r c e d e n s i t y FV, k, w h i c h i t s e l f d e p e n d s on P ( x , y ) . T o e x t r a c t t h e N a v i e r - S t o k e s e q u a t i o n i t i s o n l y n e c e s s a r y to s p e c i f y P n e a r e q u i l i b r i u m i n a f o r m w h i c h s a t i s f i e s v a r i o u s s y m m e t r y a n d i n v a r i a n c e r e q u i r e m e n t s . It f o l l o w s f r o m t h e d e f i n i t i o n of P i x , Y), i n t e r m s of f i e l d o p e r a t o r s , t h a t Pix, y) i s G a l i l e a n i n v a r i a n t a n d t h a t Pix, y) = = P(y, x). In h y d r o d y n a m i c e q u i l i b r i u m F v k--- 0, w h e n c e P i s a f u n c t i o n of l r l ( = / y - x ] ) a n d of t h e macroscopic parameters, a a n d T o, ( p e ( l ~ ] , a, T O ) s a y ) w h i c h i n t h i s c a s e a r e s p a t i a l l y i n d e p e n d e n t . In g e n e r a l n e a r t h i s e q u i l i b r i u m w e w r i t e

P(x, y) = Pe (tr ], ~, ~o) + 6P(x, y) where

~ = ~(~x+~y)l 1 a n d ,~o = T ° (½x + ~y),l t h e f i r s t t e r m r e f e r r i n g to l o c a l e q u i l i b r i u m , e x p l i c i t l y

* The notation and definitions will adhere to those given in ref. [1]. 197