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What is SPS?

1.  Introduction

Spark  plasma  sintering  (SPS)  or  pulsed  electric  current  sintering  (PECS)  is  a  sintering  technique  utilizing  uniaxial  force  and  a  pulsed  (on-off)  direct electrical  current  (DC)  under  low  at‐mospheric  pressure  to  perform  high  speed  consolidation  of  the  powder.  This  direct  way  of heating  allows  the application  of  very  high  heating  and  cooling  rates,  enhancing  densifica tion  over  grain  growth  promoting  diffusion  mechanisms  (see  Fig.  1),  allowing maintaining the  intrinsic  properties  of  nanopowders  in  their  fully  dense  products.

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It  is  regarded  as  a  rapid  sintering  method  in  which  the  heating  power  is  not  only  distributed over  the  volume  of  the  powder  compact  homogeneously  in  a  macroscopic  scale,  but  more‐ over  the  heating  power  is  dissipated  exactly  at  the  locations  in  the  microscopic  scale,  where energy  is  required  for  the  sintering  process,  namely  at  the  contact  points  of  the  powder  particles  (see  Fig.  2).  This  fact  results  in  a  favourable  sintering  behaviour  with  less  grain  growth and  suppressed  powder  decomposition.  Depending  on  the  type  of  the  powder,  additional advantageous  effects  at  the  contact  points  are  assumed  by  a  couple  of  authors.

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SPS  systems  offer  many  advantages  over  conventional  systems  using  hot  press  (HP)  sintering,  hot  isostatic  pressing  (HIP)  or  atmospheric  furnaces,  including  ease  of  operation  and  ac‐curate  control  of  sintering  energy  as  well  as  high  sintering  speed,  high  reproducibility,safety  and  reliability.  While  similar  in  some  aspects  to  HP,  the  SPS  process  is  characterized by  the  application  of  the  electric  current  through  a  power  supply,  leading  to  very  rapid  and efficient  heating  (see  Fig.  3).  The  heating  rate  during  the  SPS  process  depends  on  the  geometry  of  the  container/sample  ensemble,  its  thermal  and  electrical  properties,  and  on  the  electric  power  supplier.  Heating  rates  as  high  as  1000  °C/min  can  be  achieved.  As  a consequence,  the  processing  time  typically  takes  some  minutes  depending  on  the  material,dimensions  of  the  piece,  configuration,  and  equipment  capacity.

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On the contrary, in conventional HP techniques, the powder container is typically heated by radiation  from  the  enclosing  furnace  through  external  heating  elements  and  convection  of  inert gases if applicable. Therefore, the sample is heated as a consequence of the heat transfer occurring by conduction from the external surface of the container to the powders. The resulting heating rate is then typically slow and the process can last hours. In addition, a lot of heat is wasted as the whole volume of space is heated and the compact indirectly receives heat from the hot environment. On the other hand, SPS processes are characterized by the efficient use of the heat input, particularly when electrically insulating powders is used and the pulsed electric current is applied.

It  should  be  however  mentioned  that  in  SPS  processes  the  problem  of  adequate  electrical  conductivity of the powders and the achievement of homogenous temperature distribution is particularly  acute.  In  this  way,  the  electric  current  delivered  during  SPS  processes  can  in  general assume different intensity and waveform which depend upon the power supply characteristics.

In  order  to  permit  a  homogeneous  sintering  behaviour,  the  temperature  gradients  inside  the specimen should be minimized. Important parameters that are drastically determining the teperature distribution inside the sample are the sample material's electrical conductivity, the diewall  thickness  and  the  presence  of  graphite  papers  used  to  prevent  direct  contact  between graphite parts and the specimen and used to guarantee electrical contacts between all parts.

The  application  of  external  electric  current  to  assist  sintering  was  initiated  by  Taylor  in  1933, who  incorporated  the  idea  of  resistance  sintering  during  the  hot  pressing  of  cemented  carbides  [Taylor,  1933].  Later,  Cramer  patented  a  resistance  sintering  method  to  consolidate copper,  brass  and  bronze  in  1944  in  a  spot  welding  machine  [Cremer,  1944].  The  concept  of compacting  metallic  materials  to  a  relatively  high  density  (>90%  of  theoretical)  by  an  electric discharge  process  was  originally  proposed  by  Inoue  in  the  1960s  [Inoue,  1965].  Inoue  argued that  a  pulsed  current  was  effective  for  densification  at  the  initial  sintering  stages  for  low melting  point  metals  (e.g.,  bismuth,  cadmium,  lead,  tin)  and  at  the  later  sintering  stage  for high  melting  metals  (e.g.,  chromium,  molybdenum,  tungsten).  In  the  United  States,  Lenel  also  used  a  spot welding  machine  for  the  sintering  of  metals  [Lenel,  1955].  In  addition  to  continuous  pulses,  some  researchers  also  investigated  a  single discharge  method,  i.e.,  the powders  were  densified  by  a  single  discharge  generated  from  a  capacitor  bank.  In  the  late 1970s,  Clyens  et  al. [Clyens  et  al.,  1976],  Raichenko  et  al.  [Raichenko  et  al.,  1973]  and  Geguzin  et  al.  [Geguzin  et  al.,  1975]  studied  the  compaction  of  metal  powders  using  electric  discharge  compaction  (EDC)  or  electric  discharge  sintering  (EDS).  In  all  the  methods  cited, electrically  conductive  powders  are  heated  by  Joule  heating  generated  by  an  electric  current.

In  1990  Sumitomo  Heavy  Industries  Ltd.  (Japan),  developed  the  first  commercially  operated plasma  activated  sintering  (PAS)  and  spark  plasma  sintering  (SPS)  machines  with  punches and  dies  made  from  electrically  conductive  graphite  [Yanagisawa  et  al.,  1994].  One  of  the  salient  features  of  these  machines  was  that,  in  addition  to  electrically  conductive  powders, high  density  was  also  achieved  in  insulating  materials.  In  PAS  process,  a  pulsed  direct  current  is  normally  applied  at  room  temperature  for  a  short  period  of  time  followed  by  a  constant  DC  applied  during  the  remainder  of  the  sintering  process  (see  Fig.  4a).  This  procedure is  often  referred  to  in  the  literature  as  a  "single  pulse  cycle  process”.  In  the  SPS  process,  a pulsed  DC  is  applied  repeatedly  from  the  beginning  to  the  end  of  the  sintering  cycle  (see Fig.  4b).  In  this  case  the  procedure  is  referred  to  as  a  “multiple  pulse  cycle  process”. so  used  a  spot  welding  machine  for  the  sintering  of  metals  [Lenel,  1955].  In  addition  to  continuous  pulses,  some  researchers  also  investigated  a  single  discharge  method,  i.e.,  the powders  were  densified  by  a  single  discharge  generated  from  a  capacitor  bank.  In  the  late 1970s,  Clyens  et  al.  [Clyens  et  al.,  1976],  Raichenko  et  al.  [Raichenko  et  al.,  1973]  and  Geguzin  et  al.  [Geguzin  et  al.,  1975]  studied  the  compaction  of  metal  powders  using  electric  discharge  compaction  (EDC)  or  electric  discharge  sintering  (EDS).  In  all  the  methods  cited, electrically  conductive  powders  are  heated  by  Joule  heating  generated  by  an  electric  current.

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2.  The  basic  SPS  configuration  and  process

The  basic  configuration  of  a  typical  SPS  system  is  shown  in  Figure  5.  The  system  consists  of a  SPS  sintering  machine  with  vertical  single-axis  pressurization  and  built-in  water-cooled special  energizing  mechanism,  a  water-cooled  vacuum  chamber,  atmosphere  controls,  vacuum  exhaust  unit,  special  sintering  DC  pulse  generator  and  a  SPS  controller.  The  powder materials  are  stacked  between  the  die  and  punch  on  the  sintering  stage  in  the  chamber  and held  between  the  electrodes.  Under  pressure  and  pulse  energized,  the  temperature  quickly rises  to  1000~2500  °C  above  the  ambient  temperature,  resulting  in  the  production  of  a  high quality  sintered  compact  in  only  a  few  minutes.

3.  Principles  and  mechanism  of  the  SPS  process

The  SPS  process  is  based  on  the  electrical  spark  discharge  phenomenon:  a  high  energy,  low voltage  spark  pulse  current  momentarily  generates  spark  plasma  at  high  localized  temperatures,  from  several  to  ten  thousand  ℃  between  the  particles  resulting  in  optimum  thermal and  electrolytic  diffusion.  SPS  sintering  temperatures  range  from  low  to  over  2000  ℃  which are  200  to  500  ℃  lower  than  with  conventional  sintering.  Vaporization,  melting  and  sintering  are  completed  in  short  periods  of  approximately  5  to  20  minutes,  including  temperature rise  and  holding  times.  Several  explanations  have  been  proposed  for  the  effect  of  SPS:

3.1.  Plasma  generation

It  was  originally  claimed  by  Inoue  and  the  SPS  process  inventors  that  the  pulses  generated sparks and even plasma discharges between the particle contacts, which were the reason that the processes were named, spark plasma sintering and plasma activated sintering [Inoue, 1965, Yanagisawa et al., 1994]. They claimed that ionization at the particle contact due to spark discharg‐es  developed  “impulsive  pressures”  that  facilitated  diffusion  of  the  atoms  at  contacts.  Groza [Groza et al, 1999] suggested that a pulsed current had a cleaning effect on the particle surfaces based  on  the  observation  of  a  grain  boundary  without  oxidation  formed  between  particles.

Whether plasma is generated or not has not yet been confirmed directly by experiments. Therefore, there is no conclusive evidence for the effect of a plasma generation in SPS. The occurrence of a plasma discharge is still debated, but it seems to be widely accepted that occasional electric discharges may take place on a microscopic level [Groza et al., 2000].

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3.2.  Electroplastic  effect

Metal  powders  have  been  observed  to  exhibit  lower  yield  strength  under  an  electric  fieldRaichenko  et  al.  [Raichenko  et  al.,  1973]  and  Conrad  [Conrad,  2002]  independently  studied electroplastic  phenomena.

3.2.  Electroplastic  effect

Metal  powders  have  been  observed  to  exhibit  lower  yield  strength  under  an  electric  fieldRaichenko  et  al.  [Raichenko  et  al.,  1973]  and  Conrad  [Conrad,  2002]  independently  studied electroplastic  phenomena.

3.3.  Joule  heating

Joule  heating  due  to  the  passage  of  electric  current  through  particles  assists  in  the  welding  of the  particles  under  mechanical  pressure.  The  intense  joule  heating  effect  at  the  particle  conducting  surface  can  often  result  in  reaching  the  boiling  point  and  therefore  leads  to  localized vaporization  or  cleaning  of  powder  surfaces  [Tiwari  et  al.,  2009].  Such  phenomenon  ensures favourable  path  for  current  flow.

3.4.  Pulsed  current

The  ON-OFF  DC  pulse  energizing  method  generates:  (1)  spark  plasma,  (2)  spark  impact pressure,  (3)  Joule  heating,  and  (4)  an  electrical  field  diffusion  effect.

In  the  SPS  process,  the  powder  particle  surfaces  are  more  easily  purified  and  activated  than in  conventional  electrical  sintering  processes  and  material  transfers  at  both  the  micro  and macro  levels  are  promoted,  so  a  high-quality  sintered  compact  is  obtained  at  a  lower  temperature  and  in  a  shorter  time  than  with  conventional  processes.  Figure  6  illustrates  how pulse  current  flows  through  powder  particles  inside  the  SPS  sintering  die.

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The  SPS  process  is  an  electrical  sintering  technique  which  applies  an  ON-OFF  DC  pulse voltage  and  current  from  a  special  pulse  generator  to  a  powder  of  particles  (see  fig.  7), and  in  addition  to  the  factors  promoting  sintering  described  above,  also  effectively  discharges  between  particles  of  powder  occurring  at  the  initial  stage  of  the  pulse  energizing for  sintering.

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High  temperature  sputtering  phenomenon  generated  by  spark  plasma  and  spark  impact pressure  eliminates  adsorptive  gas  and  impurities  existing  on  the  surface  of  the  powder  particles.  The  action  of  the  electrical  field  causes  high-speed  diffusion  due  to  the  high-speed  migration  of  ions.

3.5.  Mechanical  pressure

When  a  spark  discharge  appears  in  a  gap  or  at  the  contact  point  between  the  particles  of  a material,  a  local  high  temperature-state  (discharge  column)  of  several  to  ten  thousands  of degrees  centigrade  is  generated  momentarily.  This  causes  evaporation  and  melting  on  the surface  of  powder  particles  in  the  SPS  process,  and  "necks"  are  formed  around  the  area  of contact  between  particles.  Figure  8  shows  the  basic  mechanism  of  neck  formation  by spark  plasma

Figure  9a  shows  the  behavior  in  the  initial  stage  of  neck  formation  due  to  sparks  in  the  plasma.  The  heat  is  transferred  immediately  from  the  center  of  the  spark  discharge  column  to the  sphere  surface  and  diffused  so  that  intergranular  bonding  portion  is  quickly  cooled.  As seen  in  Figure  9b  which  show  several  necks,  the  pulse  energizing  method  causes  spark  discharges  one  after  another  between  particles.  Even  with  a  single  particle,  the  number  of  positions  where  necks  are  formed  between  adjacent  particles  increases  as  the  discharges  are repeated.  Figure  9c  shows  the  condition  of  an  SPS  sintered  grain  boundary  which  is  plastic deformed  after  the  sintering  has  progressed  further.

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SPS hot [D]

Spark Plasma Sintering

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