ULTRASONIC STUDY OF PHASE TRANSITIONS BY ACOUSTIC MICROSCOPY

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ULTRASONIC STUDY OF PHASE TRANSITIONS

BY ACOUSTIC MICROSCOPY

J. Fossum, J. Cheeke

To cite this version:

J. Fossum, J. Cheeke.

ULTRASONIC STUDY OF PHASE TRANSITIONS BY

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JOURNAL DE PHYSIQUE

C o l l o q u e C10, s u p p l e m e n t a u n 0 1 2 , Tome 46, d k c e m b r e 1 9 8 5 p a g e C10-755

ULTRASONIC STUDY OF PHASE TRANSITIONS BY ACOUSTIC MICROSCOPY

J . O . FOSSUM' AND J.D.N. CHEEKE

Dbpartement de physique, Universite de Sherbrooke,

Sherbrooke, Quebec

J 1 K

2R1, Canada

Rbsumd

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Dans l a technique d i t e s i g n a t u r e s a c o u s t i q u e s on f a i t un ddplacement de l a l e n t i l l e d ' u n microscope a c o u s t i q u e v e r s l a s u r f a c e de 1 ' 6 c h a n t i l l o n b t u d i l . I1 en r 6 s u l t e un phbnomPne d ' i n t e r f b r e n c e e n t r e l a p a r t i e du f r o n t d'onde

P

i n c i d e n c e q u a s i normale e t l ' o n d e de s u r f a c e engendrge dans l'dchan- t i l l o n e t rdbmise dans l e l i q u i d e . On observe une s E r i e de f r a n g e s d ' i n t e r - fErence V(z) en f o n c t i o n du dsplacement z

B

p a r t i r de l a q u e l l e on p e u t d b d u i r e l a v i t e s s e d'ondes d e Rayleigh dans l ' b c h a n t i l l o n . Nous avons f a i t une 6tude p r h l i m i n a i r e de l a t r a n s i t i o n de phase de SrTi03 ii 105 K u t i l i s a n t c e t t e technique a f i n d 1 6 v a l u e r l e p o t e n t i e l de l a microscopie a c o u s t i q u e pour E t u d i e r l e s t r a n s i t i o n s de phase.

A b s t r a c t

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The t e c h n i q u e of Acoustic m a t e r i a l s s i g n a t u r e s i n v o l v e s d i s p l a c e - ment of t h e l e n s of an a c o u s t i c microscope towards t h e specimen s u r f a c e under examination. The r e s u l t i n g i n t e r f e r e n c e of t h e d i r e c t l y r e f l e c t e d wave and t h e leaky wave re-emittedfrom t h e s u r f a c e g i v e s r i s e t o a s e r i e s of i n t e r - f e r e n c e f r i n g e s from which t h e average Rayleigh wave v e l o c i t y can b e deduced. The t e c h n i q u e h a s been used f o r t h e s t u d y of t h e phase t r a n s i t i o n i n s t r o n - tium T i t a n a t e a t 105 K i n o r d e r t o e v a l u a t e i t s u s e f u l n e s s f o r t h e s t u d y of phase t r a n s i t i o n s by u l t r a s o n i c t e c h n i q u e s .

The a c o u s t i c m a t e r i a l s s i g n a t u r e (AMS) i n a c o u s t i c microscopy i s t h e p e r i o d i c s i g n a l V(z) o b t a i n e d i n r e f l e c t i o n a s t h e a c o u s t i c l e n s i s d i s p l a c e d towards t h e sample s u r f a c e . This p e r i o d i c i t y i s an i n t e r f e r e n c e e f f e c t between t h e d i r e c t l y r e f l e c t e d beam and t h e l e a k y wave re-emitted by Rayleigh waves o r pseudo s u r f a c e waves on t h e specimen s u r f a c e . The average s u r f a c e wave v e l o c i t y i s g i v e n by t h e

d i s t a n c e between minima w h i l e t h e a t t e n u a t i o n can i n p r i n c i p l e b e o b t a i n e d by t h e i r depth. These b a s i c i d e a s have been confirmed by experiment and t h e AMS technique has been used f o r a b s o l u t e Rayleigh wave v e l o c i t y and,Rayleigh wave d i s p e r s i o n on l a y e r e d m a t e r i a l s (1) and measurements of t h e a n i s o t r o p y of Rayleigh wave v e l o c i t y by t h e use of c y l i n d r i c a l l e n s e s ( 2 ) .

The p r e s e n t paper p r e s e n t s p r e l i m i n a r y r e s u l t s on a novel a p p l i c a t i o n of t h e AMS technique which i n v o l v e s t h e measurement of t h e Rayleigh wave v e l o c i t y c l o s e t o a

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' P r e s e n t a d d r e s s

:

D e p a r t m e n t

of

C h e m i s t r y a n d C e n t r e

f o r M a t e r i a l s

Science

and

E n g i n e e r i n g ,

MIT,

C a m b r i d g e ,

Ma.

20139,

u.S.A.

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JOURNAL

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Fig. 1 Sample holder used for low temperature AMS measurements. The lens assembly was fixed to a bellows (not shown) to allow precision vertical displacements.

LIQUID

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_-

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phase transition. The 105

K

structural phase transition was chosen, principally because it is well known by other investigations and the

T

fell conveniently in the temperature range of our apparatus. The experimental set up is shown in fig. 1; the usual acoustic microscope configuration is used with liquid propane as the coupling medium. Full details are given elsewhere of sound absorption in propane obtained by this technique (3) as well as of the experimental apparatus used here

(4).

The useful temperature range was 85

-

220

K

principally determined by the sound attenuation in the propane. The measurements presented here were made with a 3 mm diameter 5 mm long cylindrical quartz lens. The reflected signal V(z) was boxcar integrated, recorded by a Tektronix 7 D20 digitiser and analyzed by a Fast Fourier Transform (FFT) in an

HP

9816.

Typical results are shown for the AMS in fig. 2 at T % 108

K

with the corresponding FFT in fig, 3

.

Only one peak is observed in the FFT at all temperatures indica- ting the AMS is due to a single leaky Rayleigh mode. The R'ayleigh wave velocity along the [loo] direction in the (001) surface of SrTi03 obtained by use of a

--

-. -. - - Fig. 2 AMS results for SrTi03 along

' : j [loo] in the (001) surface at

T = 108

K

and f = 150 PlHz. One point on the graph corres-

1

ponds to 0,72 microns. The

! high frequency modulation is

due to a leakage signal from the RF source which becomes

0 I d 2 3

!

P O I Y 1 5

increasingly important near

H O I ~ P L I 2 C D !?RCHl'UDC I- 3 l.aBC-BBI

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F i g . 3 FFT spectrum f o r t h e d a t a of f i g . 2. One p o i n t corresponds t o 1.22 x 10-4 Hz.

c y l i n d r i c a l l e n s i s shown i n f i g . 4. No AMS s i g n a l was o b t a i n e d below 105 K due t o t h e h i g h a t t e n u a t i o n i n t h e SrTi03

.

A t h i g h e r t e m p e r a t u r e s t h e r e s u l t s of f i g . 4 a r e w i t h i n Q f 1% of t h e f u l l c u r v e corresponding t o c o n v e n t i o n a l p u l s e

echo o v e r l a p measurements from r e f . ( 5 ) . We d i d not have s u f f i c i e n t a c c u r a c y t o d e t e c t a n i s o t r o p i c e f f e c t s .

These r e s u l t s show t h a t V ( z ) curves can b e used t o d e t e c t Rayleigh wave anomalies of t h e o r d e r of 5% n e a r t o phase t r a n s i t i o n s . The major l i m i t t o t h e accuracy i n t h e p r e s e n t work i s due t o t h e f a c t t h a t we were o b l i g e d t o measure t h e a b s o l u t e v e l o c i t y , b u t t h i s could i n p r i n c i p l e b e avoided by t i m e r e s o l v i n g of t h e d i r e c t l y r e f l e c t e d echo and t h e l e a k y wave ( 6 ) , ( 7 ) . N e v e r t h e l e s s t h e p o t e n t i a l of t h e t e c h n i q u e i s c l e a r l y demonstrated and i t comports two d i s t i n c t advantages compared t o more c o n v e n t i o n a l m e t h o d s . F i r s t l y t h e method i s non c o n t a c t i n g which a v o i d s

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problems due t o bonds, s t r a i n s , e t c . Secondly o n e c a n s t u d y v e r y s m a l l samples which a r e e x t r e m e l y d i f f i c u l t t o measure by c o n v e n t i o n a l t e c h n i q u e s . Moroever samples of t h i s t y p e , s u c h a s q u a s i 1 and 2 d i m e n s i o n a l s y s t e m s a r e known t o d i s p l a y a r i c h v a r i e t y of phase t r a n s i t i o n s .

Acknowledgements: T h i s work was s u p p o r t e d by t h e N a t u r a l S c i e n c e s and E n g i n e e r i n g Research C o u n c i l o f Canada. The a u t h o r s g r a t e f u l l y acknowledge c o n t r i b u t i o n s from Mario Castonguay f o r t h e a c o u s t i c l e n s e s , AndrL Beaus6jour f o r t e c h n i c a l a s s i s t a n c e

and Luc Germain f o r w r i t i n g some of t h e computer programs u s e d i n t h e d a t a a n a l y s i s .

References

(1) R.G. Wilson and R.D. Weglein, J. App. phys.,

55,

3261, 1984

(2) J. K u s h i b i k i , A. Ohkubo and N. Chubachi, E l e c t r o n i c s L e t t e r s ,

17,

534, 1981 (3) J . O . Fossum and J.D.N. Cheeke, t o b e p u b l i s h e d

(4) J . O . Fossum and J . D . N . Cheeke, t o h e p u b l i s h e d

(5) L. B j e r k a n and K. Fossheim, S o l i d S t a t e Comun.,

2,

1147, 1977

(6) K. L i a n g , S.D. B e n n e t t , B.T. K h u r i

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Yakub and G.S. Kino, App. Phys. L e t t . 41, 1124, 1982