INDIRECT PRESSURE MEASUREMENTS IN DIAMOND CELL UP TO 1 MEGABAR

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INDIRECT PRESSURE MEASUREMENTS IN DIAMOND CELL UP TO 1 MEGABAR

J . Peyronneau, M. Madon, J. Poirier

To cite this version:

J . Peyronneau, M. Madon, J. Poirier. INDIRECT PRESSURE MEASUREMENTS IN DIAMOND CELL UP TO 1 MEGABAR. Journal de Physique Colloques, 1984, 45 (C8), pp.C8-403-C8-404.

�10.1051/jphyscol:1984872�. �jpa-00224374�

JOURNAL DE PHYSIQUE

Colloque C8, supplement au n ° l l , Tome 45, novembre 1984 page C8-403

INDIRECT PRESSURE MEASUREMENTS IN DIAMOND CELL UP TO 1 MEGABAR

J . P e y r o n n e a u , M. Madon and J . F . P o i r i e r

Institut de Physique du Globe, University Paris VI, 4, Place Jussieu, 75230 Paris Cedex OS, France

A b s t r a c t - An i n d i r e c t pressure measurement method f o r the diamond a n v i l c e l l i s d e s c r i b e d . S t r a i n gauges measuring the force applied to the a n v i l s , are c a l i b r a t e d against the actual pressure in the sample, measured by the ruby fluorescence t e c h n i q u e .

Diamond a n v i l c e l l s (DAC) associated w i t h laser heating are of p a r t i c u l a r i n t e r e s t in geophysical s t u d i e s , s i n c e they can reproduce pressure and temperature c o n d i t i o n s of the earth mantle. The c l a s s i c a l determination of the pressure i n a DAC c o n s i s t s i n measuring the wavelength s h i f t of ruby fluorescence l i n e s under pressure. This s h i f t has been c a l i b r a t e d by X-rays methods up t o 1 MBar [ 1 ] , U n f o r t u n a t l e y the ruby fluorescence method may sometimes be inconvenient since the ruby chips may take part in mineral r e a c t i o n s at high temperature, as i t was shown by Madon [ 2 1 : a sample of s y n t h e t i c (Mg0.5Fe0. 5 ) 2s i 04 o l l v 1 n e P°w(ter was compressed with small ruby chips in a diamond c e l l to 240 kbar and laser heated t o 130CPC. Examination of the recovered sample by transmission e l e c t r o n microscopy revealed the presence of = 30 % v o l . of an aluminous ferro-maqnesian garnet produced by the r e a c t i o n between o l i v i n e and ruby. Such r e a c t i o n s are c l e a r l y inconvenient f o r the study of phase t r a n s f o r m a t i o n s . There is t h e r e f o r e a need f o r a method of pressure d e t e r m i a n t i o n t h a t does not i n t e r f e r e w i t h the r e a c t i o n under i n v e s t i g a t i o n . We have developed an i n d i r e c t method f o r measuring p r e s s u r e , usinq s t r a i n gauges.

C a l i b r a t i o n procedure and r e s u l t s

F i g . 1 - S t r a i n gauges Bridge G : gauges, L, l e v e r s , C : c y l i n d e r , S : screw, E : extensometer, R : r e c o r d e r .

The DAC we use i s of the type described by Mao and B e l l [ 3 3 . The a n v i l s are 0.3 carat b r i l l i a n t - c u t diamonds w i t h 16 facets and a f l a t t a b l e 0,5mm in diameter.

Two s t r a i n gauges in s e r i e s , glued on the load t r a n s m i t t i n g members ( f i g . 1) measure t h e i r mean e l a s t i c d e f o r m a t i o n , a l i n e a r f u n c t i o n of the load applied by the screw.

Two other s t r a i n gauges, glued on the body of the c e l l , e q u i l i b r a t e the temperature Résumé - On décrit une méthode de mesure indirecte de la pression en cellule diamant. Des jauges de contrainte mesurent la force appliquée aux enclumes qui a été étalonnée en fonction de la pression dans l'échantillon par la méthode de fluorescence du rubis.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984872

C8-404 JOURNAL DE PHYSIQUE

F i g . 2

-

Force c a l i b r a t i o n F i g . 3

-

Pressure c a l i b r a t i o n i n Ni gasket Open c i r c l e s : 30pm d i m . hole, r u b y f l u o . Closed c i r c l e s : 300pm diam. h o l e , r u b y f l u o . T r i a n g l e s : 300 pm d i m . hole, phase

t r a n s i t i o n s : CuCl 44,2 kbar, AgI 100 kbar, NaCl 300 kbar.

v a r i a t i o n s o f t h e a c t i v e gauges. T h i s h a l f - b r i d g e i s connected t o a P h i l i p s t y p e 9308 extensometer b r i d g e w i t h two i n t e r n a l r e s i s t a n c e s . The system works as a wheastone b r i d g e . The o u t p u t v o l t a g e o f t h e b r i d g e depends on t h e gauges and extensometer used. It has been c a l i b r a t e d a g a i n s t t h e a p p l i e d l o a d u s i n q an I n s t r o n compression machine, d i r e c t l y a p p l y i n g t h e f o r c e t o t h e l e v e r s ( f i g . 2 ) The c e l l i s designed so t h a t t h e a x i a l f o r c e on t h e diamonds i s equal t o 5 t i m e s t h e l o a d a p p l i e d b y t h e screw). The a p p l i e d l o a d was i n t u r n c a l i b r a t e d as a f u n c t i o n o f t h e maximum h y d r o s t a t i c p r e s s u r e between t h e diamonds. Pressure c a l i b r a t i o n s were made u s i n g w e l l known phase t r a n s i t i o n s (CuCl, AgI, NaC1) as w e l l as t h e r u b y f l u o r e s c e n c e technique a t room temperature ( f i g . 3). For t h e r u b y f l u o r e s c e n c e t e c h n i q u e t h e f o l l o w i n g equation, g i v e n b y Mao and B e l l

111,

was used :

where h o = 694 rm i s t h e wave l e n g t h o f t h e f l u o r e s c e n c e l i n e , a t room p r e s s u r e (and A?, i s t h e wave l e n g t h s h i f t . It can be seen f i g . 3 t h a t t h e r e l a t i o n between p r e s s u r e and f o r c e depends on t h e i n i t i a l geometry o f t h e gasket. The method d e s c r i b e d here i s t h e r e f o r e s u c c e s s f u l o n l y i f s p e c i f i c c a l i b r a t i o n c o n d i t i o n s a r e used f o r experiments. Below 250 kbar, w i t h a 300 Fm diameter h o l e ( u s u a l experimental c o n d i t i o n s ) , t h e o b t a i n e d pressure f o r a g i v e n f o r c e i s s t r o n q l y dependant on t h e c o m p r e s s i b i l i t y o f t h e uncompacted powder sample. I n t h i s domain i t

i s d i f f i c u l t o t o b t a i n r e p r o d u c i b l e c a l i b r a t i o n r e s u l t s . It can be noted t h a t a t v e r y h i g h p r e s s u r e t h e two c u r v e s f i g . 3 t e n d towards a common l i m i t and t h e c a l i b r a t i o n becomes almost independant on t h e i n i t i a l experimental c o n d i t i o n s .

The accuracy o f t h e p r e s s u r e measurements b y s t r a i n gauges i s l i m i t e d by t h e accuracy o f t h e r u b y technique. At v e r y h i g h pressure, w i t h a s o l i d medium, t h e r u b y 1 in e s become v e r y broad and an accurate d e t e r m i n a t i o n o f t h e peaks i s n o t p o s s i b l e . The v a l u e s o f p r e s s u r e are o b t a i n e d w i t h an accuracy n o t b e t t e r t h a n 1 0 %.

S t r a i n gauges, c a r e f u l l y used, can t h e r e f o r e q i v e c o n t i n u o u s p r e s s u r e measurements, as accurate as t h e r u b y f l u o r e s c e n c e technique, up t o 1 Mbar, even d u r i n g l a s e r h e a t i n g .

I P G C o n t r i b u t i o n

N o

775.

REFERENCES

(1) MA0 H.K. and P.M. BELL, Carnege I n s t . , Year Book (1976-77), 650 ( 2 ) MADON M., T e r r a Cognita, S I (1981) 89.

(3) MA0 H.K. and P.M. BELL. Carnegie Inst., Year Book (1977-78), 904.