Porous Metals Made by Dealloying for Supercapacitor Applications Tom Swift, Ian McCue, Jonah Erlebacher

Department of Materials Science and Engineering Johns Hopkins University Baltimore, MD 21218, USA

INTRODUCTION  I.  Abstract  Tantalum   and   niobium   are   used   to   make   capacitors   for   the  computer   industry   due   to   their   high   dielectric   constants   and  ability   to   support   high   current   densi:es.   Typical   industry          Ta  /Nb  capacitors  are  made  by  sintering  powders  into  porous  pellets   with   surface   area   of   roughly   1  m2/g.   The   aim   of   this  project   is  to  design  a  capacitor  fabrica:on  process  that  more  efficiently   uses   Ta   and   Nb   by   using   the   new   liquid   metal  dealloying  (LMD)  technique.  

Capacitors   are   used   to   store   electrical   charge,   where   two  conduc:ve  plates  are  separated  by  a  dielectric  material.                  The  characteris:c  capacitance  equa:on  is  given  by:          where   C   is   capacitance,   εr   is   the   dielectric   constant   of   the  oxide   Ta2O5   (27)   or   Nb2O5   (40),   ε0   is   the   permiRvity   of   free  space,   A   is   the   area   of   the   plates,   and   d   is   the   distance  between  the  plates.  

II.  Background  

!!C=

εrε0Ad

Anode  (MnO2)  Dielectric  (Ta2O5    or  N  b2O5)  Cathode  (Ta  or  Nb)  Copper  

Fig  1.  Schema3c  of  a  parallel  plate  capacitor.      

III.  Methods  

Fig  2.  Schema3c  of  the  fabrica3on  process  for  porous  Ta/Nb  supercapacitors.  

DESIGN  

Fully  dealloyed  samples  oUen  resulted  in  flimsy  foils  that  turned  to   powder   upon   anodizing.   The   oxide   made   in   this   process  creates  addi:onal   strains   in   the  material  by  changing   its   crystal  structure.   In   this   process,   individual   grains   pop   out   of   samples  due  to  Ta/Nb  segrega:on  (Fig.  3A).  The  sample  then  becomes  a  powder   of   individually   dealloyed   grains   (Fig.   3B).   This   can   be  explained   by   the   mechanism   of   LMD,   where   dealloying    propagates  faster  at  grain  boundaries  (Fig.  3C).  To  overcome  this  challenge,   samples   were   only   par:ally   dealloyed   in   order   to  leave   a   mechanically   stable   core   for   the   grains   to   remain  aXached  to  (Fig.  3D).    

Fig   3.   (A)   SEM   image   where   a   grain   has   popped   out   of   a   sample.              (B)  Op3cal  image  of  individually  dealloyed  grains.  (C)  Cross-­‐sec3onal  SEM   image  of  a  par3ally  dealloyed  sample.   Individual  grains  can  be  seen   due   to   grain   propaga3on   of   dealloying.   (D)   Schema3c   of   a  par3ally  dealloyed  sample.    

IV.  Par:al  Dealloying  

RESULTS  

VIII.  Future  Work    Ø  Solving  grain  boundary  detachment  during  LMD:  

Ø  using  variant  chemical  composi:ons    TiAX1-­‐A  (X  =  Ta,  Nb)  

Ø  using  addi:onal  metallic  addi:ves  TiAXBY1-­‐A-­‐B  (X  =  Ta,  Nb;  Y  =  V,  Mo,  Co,  W)  

Ø  Conduct  addi:onal  work    on  the  reduc:on  of  leakage  current  using  MnO2  

IV.  Acknowledgements  Thank  you  to  Orla  Wilson  for  research  guidance,  and  the  Erlebacher  Lab  for  collabora:on  and  advice.  

(1)   Gill,   J.   Basic   tantalum   capacitor   technology.   AVX   Technical   Publica:on,   AVX  Corpora:on.   (2)   Chen,   Q.;   Sieradzki,   K.   Spontaneous   evolu:on   of   bicon:nuous  nanostructures   in   dealloyed   Li-­‐based   systems.   Nature   Materials   2013,   12,  1102-­‐1106.   (3)   Erlebacher,   J.   An   atomis:c   descrip:on   of   dealloying   -­‐   Porosity  evolu:on,   the   cri:cal   poten:al,   and   rate-­‐limi:ng   behavior.   J.   Electrochem.  Soc.  2004,  151,  C614-­‐C626.    

X.  References  

Ti0.65X0.35  (X  =  Ta,  Nb)  is  immersed  in  molten  Cu  with  3me  and  temp  as  

ligament  size  determining  parameters    

Resultant  CuTi  is  dissolved  away  using  5M  HNO3  to  yield  porous  Ta/Nb  

The  surface  of  Ta/Nb  is  anodized  (oxidized)  by  applying  a  voltage  to  the  sample  in  0.1M  H3PO4  

MnO2  is  grown  in  the  pores  through  thermal  decomposi:on  of  0.1M  

Mn(NO3)2  

1.  

2.  

3.  

4.  

CONCLUSION  VII.  Conclusion  We  have   supported   the  ability   and   scalability  of   crea:ng  porous   Ta   and   Nb   structures   with   the   LMD   technique.  Difficul:es   in   maintaining   the   mechanical   integrity   of  samples   was   overcome   through   par:al   dealloying,   and      both   material   systems   exhibited   ample   capaci:ve  behavior   rela:ve   to   current   devices.   Par:cularly,  capacitance   is   shown  to   increase  as   the   ligament   feature  size  is  decreased,  and  an  op:mal  ligament  size  of  around  130  nm  is  found  for  devices  with  20  nm  of  oxide.  

V.  Characteriza:on  Samples  were   imaged   by   SEM   to   evaluate   their   porosity  and  determine  ligament  size  (Fig.  4).    

VI.  Capacitance  Capacitance   measurements   were   taken   using   an   LCR  meter  at  100  Hz,  0.25  V,  and  in  0.1M  H2SO4.  Ligament  size  was   ploXed   versus   capacitance,   normalized   by   material  dielectric  constant   (Fig.  5),   to  accommodate  both  Ta  and  Nb   samples.   An   inverse   rela:on   can   be   seen,   showing  increased   capacitance   as   ligament   size   decreases.   The  highest  capacitance  is  seen  at  a  ligament  size  of  130  nm.  

A   dielectric   thickness   of   20   nm  was  grown  on  all  samples,  leading  to   the   op:miza:on   of   surface  area   when   ligament   size   is   near  130  nm.        

Fig 7. Schematic displaying how oxide growth can decrease surface area and capacitance when ligament size is small.

Fig 4. SEM images of (A) 30 nm Ta, (B) 130 nm Nb, (C) 760 nm Nb, and (D) 3900 nm Nb ligament sizes.

Fig 5. Capacitance, normalized by mass and dielectric constant (µF/gεr), versus the ligament size (µm).

Ligament  Size  (μm)  

Normalized

 Cap

acita

nce  (μF/gε

r)  

Niobium  Tantalum  

Fig 6. Area per mass (m2/g) as a function of ligament size (µm). Ligament  Size  (μm)  

Area  Per  M

ass  (m

2 /g)  

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