ATTENUATION OF A STATIC POLARIZATION ECHO BY 4He FILM

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ATTENUATION OF A STATIC POLARIZATION ECHO BY 4He FILM

M. Poirier, J. Cheeke, P. Morisseau, A. Hikata

To cite this version:

M. Poirier, J. Cheeke, P. Morisseau, A. Hikata. ATTENUATION OF A STATIC POLARIZA- TION ECHO BY 4He FILM. Journal de Physique Colloques, 1981, 42 (C6), pp.C6-876-C6-878.

�10.1051/jphyscol:19816260�. �jpa-00221349�

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JOURNAL DE PHYSIQUE

CoZZoque C6, supptlment au no 12, Tome 4 2 , dbcembre 1981 page C6-876

ATTENUATION OF A STATIC POLARIZATION ECHO BY 4 ~ e FILM

*+

* *

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M. Poirier

,

J.D.N. Cheeke, P. Morisseau and A. Hikata

*~lpartement de Physique, Universitl de Sherbrooke, Sherbrooke, Prov. Qulbec, Canada, J I K 2R1

+*

Brown's University, Providence, R. I., U . S.A.

Abstract. - We have measured by the phonon echo technique the attenuation of an adsorbed 4 ~ e film on silica powder against pressure at 35 MHz and low temperatures. The attenuation curves present the typical shape of a multilayer adsorption isotherm of type I1 according to the BET classification. Because of relatively high RF power no onset of superfluidity has been observed in these films. The attenuation decreases with temperature following the normal viscosity and thermal conductivity contributions to the ultrasonic absorption by '~e.

1. Experiment.

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SiOp powder

(T -

70 pm) is introduced in a copper capacitive cell to which we apply a sequence of three RF pulses (f

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35 MHz) supplied by a Matec 6600 pulse modulator and receiver. The first two pulses (15 usec separation time and 2 psec width) produce the dynamic polarization echo e2 and the third pulse, the static polarization (or memory) echo e3 when it is applied a long time after the first two I . Because of stability the memory echo has been chosen for monitoring the attenuation produced by an adsorbed 4 ~ e film on the individual particles. The film is built up when 'He gas is admitted in the capacitive cell. The attenuation is measured by a Matec attenuation recorder as a function of the cell gas pressure for different temperatures.

The adsorption isotherm measurement is done with 13 g of Si02 powder in a Copper cell of 11 cm3 volume. By admitting precise amounts of 4 ~ e gas, the volume adsorbed by the powder is obtained by measuring the pressure variation in the cell.

The adsorption isotherm is obtained by plotting the total volume adsorbed as a function of the cell equilibrium pressure which takes a few minutes (3 - 5) to esta- blish for both measurements.

2. Results.

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The attenuation curves obtained for different temperatures are pre- sented in Figure 1. They are the result of an adsorbed 4 ~ e film on the individual particles and not an effect of gas loading losses which would give a linear variation with pressure 2 . In Figure 1, there is first a rapid increase of attenuation for p/p, < 0.1. then a flattening up to 0,7 and a rapid increase when saturation pressure is approached.

An example of an adsorption isotherm of 4 ~ e on the same powder is shown in

+ Permanent address : Conseil National de Recherche du Canada, IGM, 750 rue Bel-Air, Montreal, Prov. QuCbec, Canada, H4C 2K3

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19816260

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F i g u r e 2 . Because of s m a l l s p e c i f i c s u r f a c e of a d s o r b a n t , r e s i d u a l g a s e f f e c t s con- t r i b u t e t o t h e a d s o r b e d volume e s p e c i a l l y a t low p r e s s u r e s and h i g h t e m p e r a t u r e s .

F i g . 1: A t t e n u a t i o n v a r i a t i o n of memory F i g . 2: A d s o r p t i o n i s o t h e r m of 4 ~ e : echo v e r s u s r e l a t i v e c e l l p r e s s u r e of 'He. a d s o r b e d volume a t STP p e r g of a d s o r b e n t .

3. D i s c u s s i o n .

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By comparing a t t e n u a t i o n and a d s o r p t i o n i s o t h e r m r e s u l t s we con- c l u d e t h a t we a r e r e a l l y measuring t h e a t t e n u a t i o n produced by t h e a d s o r b e d 4 ~ e f i l m on t h e i n d i v i d u a l p a r t i c l e s . According t o t h e c l a s s i f i c a t i o n of a d s o r p t i o n i s o t h e r m s of

BET^

o u r r e s u l t s show e v i d e n c e of m u l t i l a y e r a d s o r p t i o n . T h e r e i s f i r s t a t p / p o < 0 , l c o m p l e t i o n of t h e f i r s t ( o r two) monolayer compressed i n t h e van d e r Waals f i e l d of t h e s u b s t r a t e which g i v e s a r a p i d i n c r e a s e of a t t e n u a t i o n and a l s o volume a d s o r b e d . Up t o p/po

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^0.7 t h e c o m p l e t i o n of t h e f i r s t normal monolayer p r o c e e d s showing a f l a t t e n i n g and f o r p/po > 0,7 m u l t i l a y e r f o r m a t i o n i s g o i n g on w i t h a f a s t i n c r e a s e i n a t t e n u a t i o n and volume a d s o r b e d . A t s a t u r a t i o n p r e s s u r e , l i q u e f a c t i o n o c c u r s : a t t e n u a t i o n i s maximum and volume a d s o r b e d g o e s t o

m. According t o t h e s l a b t h e o r y of F H H ~ t h e volume a d s o r b e d s h o u l d b e a l i n e a r f u n c t i o n ( s l o p e

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113) of I n ( p o / p ) on a log-log s c a l e when m u l t i l a y e r f o r m a t i o n p r o c e e d s . One may y e r i f y t h a t i t i s i n d e e d t h e c a s e by l o o k i n g a t F i g u r e s 3 and 4 where s u c h p l o t s have been done f o r b o t h s e t s o f r e s u l t s . The d a t a a l i g n f a i r l y w e l l a l o n g t h e l i n e

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113 w i t h a s l o w e r approach f o r a d s o r b e d volume b e c a u s e o f gas e f f e c t s a t low p r e s s u r e s .

I n F i g u r e 1, t h e a t t e n u a t i o n d e c r e a s e s w i t h t e m p e r a t u r e which i s t h e u s u a l v a r i a t i o n a s s o c i a t e d w i t h t h e normal v i s c o s i t y and t h e r m a l c o n d u c t i v i t y c o n t r i b u - t i o n s t o t h e u l t r a s o n i c a b s o r p t i o n 5 . No o n s e t of s u p e r f l u i d i t y i s o b s e r v e d i n t h e s e f i l m s b e c a u s e t o o much RF power i s needed i n phonon echo e x p e r i m e n t . I n f i - g u r e 2 a d s o r b e d volume i s n o t o n l y a f i l m e f f e c t b u t t h e r e i s a c o n t r i b u t i o n from r e s i d u a l g a s which w i l l b e d i s c u s s e d i n a l a t e r p a p e r .

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JOURNAL DE PHYSIQUE

F i g . 3 : A t t e n u a t i o n v a r i a t i o n v e r s u s F i g . 4: Volume a d s o r b e d p e r g a t STP I n (p,/p) o n a l o g - l o g s c a l e . v e r s u s I n (p,/p) on a l o g - l o g s c a l e .

I n c o n c l u s i o n , we h a v e shown t h a t a n a d s o r p t i o n i s o t h e r m model p r o c e e d i n g from a m u l t i l a y e r f o r m a t i o n e x p l a i n s o u r phonon e c h o a t t e n u a t i o n r e s u l t s . I t shows t h a t s u c h a t e c h n i q u e may be u s e d t o m o n i t o r t h e a d s o r p t i o n i s o t h e r m o f 4 ~ e a t low t e m p e r a t u r e s w i t h a n i n t e r e s t i n g s e n s i t i v i t y t o d e t e c t m o n o l a y e r s .

T h i s work was s u p p o r t e d b y t h e N a t u r a l S c i e n c e a n d E n g i n e e r i n g R e s e a r c h C o u n c i l o f Canada a n d t h e G.R.S.D. (Groupe d e R e c h e r c h e s s u r l e s s e m i c o n d u c t e u r s e t l e s D i g l e c t r i q u e s ) .

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