The Dielectric Piezoelectric and Pyroelectric Properties of VDF-TrFE Copolymers

The e f f e c t s on t h e p i e z o e l e c t r i c P y r o e l e c t r i c 1. I n t r o d u c t i o n copolymer f i l m s w i t h th i cknesses between 40 and 80m. The ' raw ' m a t e r i a l i s d i s s o l v e d i n me thy l -e thy l ke tone (MEK) i n t h e r a t i o 0.15g VDF:TrFE t o 1 m l MEK t o produce s o l u t i o n w i t h s u i t a b l e v i s c o s i t y . The s o l u t i o n i s c a s t on to a q u a r t z p l a t e u s i n g a c o r r u g a t e d spreader t o g i v e n o m i n a l l y 1 O O p t h i c k wet f i l m . T h i c k e r f i l m s a re made by spac ing t h e spreader o f f t h e p l a t e w i t h a number o f m y l a r s t r i p s 1 O O p t h i c k . The f i l m i s d r i e d a t room tempera ture f o r 15 minu tes and then i n an oven a t 6OoC f o r 1 hour . There i s a 10% v a r i a t i o n i n f i l m t h i c k n e s s u s i n g t h i s techn ique. b ) P o l i n g . Aluminium e l e c t r o d e s a r e evapora ted on to The f i l m i s po led b o t h s i d e s o f t h e f i l m and s i l v e r w i r e s a t t a c h e d w i t h conduc t i ve e las tomer . u s i n g conven t iona l s i l i c o n e o i l b a t h techn iques . The e l e c t r i c f i e l d i s a p p l i e d f o r 30 m inu tes a t t he a p p r o p r i a t e p o l i n g tempera tu re and remains a p p l i e d f o r app rox ima te l y 2 hours w h i l e t h e f i l m i s c o o l i n g t o 30 f 5OC i n t h e o i l ba th . 3. D i e l e c t r i c P r o p e r t i e s The d i e l e c t r i c c o n s t a n t and l o s s have been P o l y v i n y l i d e n e f l u o r i d e (PVDF) i s a w e l l known p i e z o e l e c t r i c polymer. i t e x i s t s i n a non -po la r a phase, wh ich must be s t r e t c h e d t o fo rm t h e p o l a r R phase, wh ich can phase and can t h e r e f o r e be rendered p i e z o e l e c t r i c v i n y l i d e n e f l u o r i d e (VDF) and t r i f l u o r o e t h y l e n e (TrFE) c r y s t a l l i s e n a t u r a l l y i n t o t h e p o l a r Q w i t h o u t t he mechanical s t r e t c h i n g s tage r e q u i r e d by PVDF. Hence VDF:TrFE copolymers a r e f a r more v e r s a t i l e than PVDF as they can be i n j e c t i o n o r compression moulded i n t o a lmos t any shape d e s i r e d o r c a s t , sprayed o r spun on to s u b s t r a t e s and p o l e d i n s i t u . I n i t s as-cas t form s t u d i e d as f u n c t i o n s o f tempera ture a t 1500Hz. ___ ~~~ . . then be e l e c t r i c a l l y po led . The copolymers o f 60 T h i s paper d e s c r i b e s t h e e v a l u a t i o n o f copolymers w i t h 56:44 and 70:30 VDF:TrFE r a t i o s 0 20 40 60 80 100 f o r p i e z o e l e c t r i c and p y r o e l e c t r i c appl i c a t i o n s . Temperature 1°C) 1 2 . F i l m P r e p a r a t i o n a l S o l v e n t Cas t ing . A l l t h e r e s u l t s p resen ted use s o l v e n t c a s t F i g u r e 1: D i e l e c t r i c c o n s t a n t and l o s s as a f u n c t i o n o f tempera ture f o r t h e 56:44 VDF:TrFE copolymer. 80 70 =-69 60 Capacitance