DIRECT  THERMAL  DIFFUSIVITY    MEASUREMENT  ALONG  THE  AXIAL  DIRECTION  OF  CARBON  FIBER  USING  LASER  FLASH:  SPECIMEN  PREPARATION  AND  

RESULTS  Jordan  Burgess1,  Ma0hew  Weisenberger  1,  Sarah  Edrington1,  and  John  Craddock1  

1    University  of  Kentucky  Center  for  Applied  Energy  Research,  Carbon  Materials  Group,  2540  Research  Park  Dr.,  Lexington,  KY  40511    

 Accurate  and  precise  on-­‐axis  thermal  diffusivity  measurement  of  carbon  fiber  is  of  great  interest  for  numerous  applicaTons,  but  can  be  a  parTcularly   difficult   measurement.   In   parTcular,   sample   preparaTon  methodology  can  have  a  profound  influence  on  thermal  diffusivity  results.  Current   on-­‐axis   fiber   thermal   conducTvity   measurements   are   oXen  performed   using   the   ASTM   (E1225)   Guarded   Hot-­‐Plate   method,   which  requires   a   composite   specimen   of   resin   and   fibers   secToned   with   the  fibers  oriented  in  the  desired  direcTon.  While  the  isotropic  resin  thermal  conducTvity   is  well   known,  one  can  back  calculate   the  fiber  conducTvity  suspended  within   the  matrix.     This   requires   some  assumpTons  be  made  about   resin-­‐fiber   thermal   interacTons.   Subsequently   the   thermal  diffusivity  can  be  calculated  with  known  density  and  heat  capacity.  Others  have   invesTgated   single-­‐filament   thermal   tesTng   incorporaTng   the   3-­‐omega  method.   Yet   single   filaments   pose   significant   sample   preparaTon  challenges.   In   this   study,  we  develop  and  describe  a   sample  preparaTon  methodology  using  carefully  collimated  bundles  of  dry  carbon  fibers.    The  difficult-­‐to-­‐measure  axial-­‐direcTon  thermal  diffusiviTes  were  obtained  and  the   results   are   presented.   Our   data,   obtained   using   this   preparaTon  methodology   and   laser   flash   analysis,   show   carbon   fiber   thermal  diffusiviTes  with  very  low  standard  deviaTons.  

This  methodology  poses  a  challenge  to  current  on-­‐axis  diffusivity  measurement  methods.  The  primary  advantages  of  this  method  are:    -­‐Allows  direct,  on-­‐axis  measurement  of  tows  of  fiber  -­‐Well-­‐aligned,  dry  fibers  -­‐Does  not  assume  epoxy-­‐fiber  interacTon      

LFA  TesTng  

The   chart   above     illustrates   thermal   diffusivity  measurements  obtained  by   LFA  at   25   oC   and  under   near   vacuum   pressure.   All   samples   were   run   under   vacuum   to  miTgate   any   thermal  losses.   The   fibers   are   from   a   broad   spectrum   of   high   and   low   thermal   diffusiviTes.   The  magnitude  of  the  values  reported  are  as  expected  for  the  respecTve  fiber  types  and  are  listed  in   order   of   descending   diffusivity;   K1100   (mesophase   pitch),   M55J   (high   modulus),   IM7  (intermediate  modulus),  PAN(CAER)  (in-­‐house  modulus),  and  NARC  Rayon.    

The  graph   to   the   leX  shows  a   shot  from   the   LFA   soXware   (blue)   fit  with  Cape  and  Lehman  model  (red).  From  the  Cape-­‐Lehman  model,   the  LFA   soXware   calculates   the   half-­‐r i se   Tme   and   subsequent ly  calculates   a   diffusivity   value.   The  Tght  fit  of  the  red  curve  is  evidence  that   the   values   reported   are   valid  and   the   sample   preparaTon  methodology   is   viable   for   direct,  on-­‐axis   diffusivity   measurements  on  dry  fiber.