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SCALING LAW VARIATION OF THE MECHANICAL PROPERTIES OF SILICA AEROGELS

T. WOIGNIER, J. PHALIPPOU
1989 Le Journal de Physique Colloques  
The mechanical behaviour o f s i l i c a aerogels i s studied by t h e three points f l e x u r a l technique. The Young's modulus (E) , t h e f r a c t u r e strength (s) and the toughness KI C are i n v e s t i g a t e d as a f u n c t i o n o f t h e d i f f e r e n t preparation parameters. It i s demonstrated t h a t t h e e v o l u t i o n o f mechanical c h a r a c t e r i s t i c s as a f u n c t i o n o f the apparent d e n s i t y f o l l o w s s c a l i n g laws. The evaluated
more » ... he evaluated exponents are 3.7, 2.6 and 1.6 f o r E, S and KIC respectively. The experimental data are discussed i n terms o f the p e r c o l a t i o n theory. 1 -INTRODUCTION The u t i l i z a t i o n o f glasses and ceramics i n s t r u c t u r a l a p p l i c a t i o n s i s o f t e n hampered by t h e inherent b r i t t l e nature o f these m a t e r i a l s which r e s u l t s i n a substantial v a r i a b i l i t y i n t h e mechanical p r o p e r t i e s . That i s p a r t i c u l a r l y t h r u e f o r aerogels, which are porous ceramics where t h e s o l i d p a r t represents between 1 t o 30% o f the whole volume. Due t o t h e i r uncommon physical p r o p e r t i e s (very 1 ow thermal c o n d u c t i v i t y , high s p e c i f i c surface area, low r e f r a c t i v e index) s i l i c a aerogels have a l a r g e p o t e n t i a l as transparent thermal i n s u l a t o r , Cerenkow r a d i a t o r and c a t a l y z i n g support. Recently, s i l i c a aerogels have been used as precursors f o r t h e synthesis o f pure s i l i c a glass /I/. However, l i t t l e i s known regarding the mechanical p r o p e r t i e s o f such m a t e r i a l s . Moreover a new i n t e r e s t i s a c t u a l l y devoted t o aerogels because o f t h e i r f r a c t a l nature. They o f f e r thus, t h e p o s s i b i l i t y t o experimentally study t h e mechanical properties o f r e a l f r a c t a l objects. Preliminary studies on aerogels reported the measurements o f t h e e l a s t i c constants by B r i l l o u i n s c a t t e r i n g / 2 / and u l t r a s o n i c measurements /3,4/. I n t h i s work we present a more d e t a i l e d study o f t h e mechanical p r o p e r t i e s measured by t h e t h r e e -p o i n t s f l e x u r a l technique. This technique gives informations on t h e e l a s t i c behaviour (Young's modulus) but also on t h e f r a c t u r e features such as strength and toughness. This s t a t i c manner t o determine t h e Young's modulus gives complementary information t o the high frequency measurements. The experiments have been performed on d i f f e r e n t sets o f samples i n order t o i n v e s t i g a t e t h e i n f l u e n c e o f t h e preparation c o n d i t i o n s (pH, d i l u t i o n ) on t h e mechanical properties. Recently, i t has been suggested t h a t t h e r e i s an analogy between g e l a t i o n and p e r c o l a t i o n which i s a c r i t i c a l process f o r c o n n e c t i v i t y . This analogy was supported by experimental studies g i v i n g r e s u l t s i n agreement w i t h model p r e d i c t i o n s . I n t h i s work we w i l l discuss our experimental r e s u l t s i n terms o f t h i s analogy. 2 -EXPERIMENTAL The g e l s are elaborated by h y d r o l y s i s and polycondensation r e a c t i o n s o f tetramethoxysilane (T.M.0.S). The T.M.O.S. was dissolved i n vari,ous amounts o f methanol thereby a d j u s t i n g t h e f i n a l b u l k density. The s o l u t i o n s are hydrolyzed under n e u t r a l , basic (5.10-2 N NHaOH) o r a c i d i c (10-4 N HN03) conditions. The number o f water molecules i s f o u r times t h a t o f TMOS. The g e l s are allowed t o cure a t 55°C f o r two weeks. The alcogels are transformed i n t o aerogels by h y p e r c r i t i c a l evacuation o f t h e methansl. The apparent d e n s i t y i s determined by simply weighing t h e samples o f w e l l defined Article published online by EDP Sciences and available at http://dx.
doi:10.1051/jphyscol:1989429 fatcat:5rzdxmu6qvhkdp75blpyflbcla