Sputtering experiments were carried out in steady state plasma of a mirror Penning discharge with cathodes made from hydrogen saturated/non-saturated palladium. For the both cases no difference in erosion behavior was found under hydrogen, helium or nitrogen plasma impact, in contrast to observed earlier erosion behavior of deuterium saturated/non-saturated lithium. Possible reasons are suggested and discussed to explain such erosion behavior

Baron, D.I.

Glazunov, G.P.

Hassanein, A.

Konotopskiy, A.L.

Volkov, E.D.

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

PALLADIUM EROSION BEHAVIOR UNDER LOW/HIGH LOCAL HYDROGEN RECYCLING IN STEADY STATE PLASMAS G. P. Glazunov, E. D. Volkov, D. I. Baron, A. L. Konotopskiy, A. Hassanein*Institute of Plasma Physics, NSC KIPT, 61108 Kharkov, Ukraine;* Argonne National Laboratory, 60439 Argonne, Illinois, USASputtering experiments were carried out in steady state plasma of a mirror Penning discharge with cathodes made from hydrogen saturated/non-saturated palladium. For the both cases no difference in erosion behavior was found under  hydrogen,  helium or  nitrogen  plasma  impact,  in  contrast  to  observed  earlier  erosion  behavior  of  deuterium saturated/non-saturated lithium. Possible reasons are suggested and discussed to explain such erosion behavior. PACS: 52.40.Hf1. INTRODUCTIONOne  important  problem  associated  with  the interaction of hydrogen isotopes with structural materials in  fusion  devices  is  the  change  in  hydrogen  recycling coefficient  during  operation.  It  should  be  noted  that  in general  case,  studies  of  recycling process  are  important not  only  for  hydrogen  isotopes,  but  also  for  helium, oxygen, nitrogen, etc. This is of a great interest not only for  fusion  devices  but  also  for  numerous  machines  for plasma  applications.  In  the  latter,  by  the  controlling  of mentioned processes, one can provide, in particular, new conditions  for  material  modification.  Different  material states  (hydrogen  saturated/non-saturated  states)  vary plasma  properties  and  can  also  lead  to  change,  under plasma  irradiation,  of  material  performances  such  as erosion  rate,  reflection  coefficient,  electron  emission coefficient, etc. It was recently shown [1-3] that erosion rate  of  some  materials  (lithium,  carbon,  intermetallide,) decreases  after  hydrogen  saturation  up  to  high concentration. In particular, erosion of deuterium saturated solid lithium under He+ ion bombardment was observed in ≈ 3 times lower than that for non-saturated samples [1]. The physical mechanism of this phenomenon is not clear enough. It is incomprehensible also, if this effect will be working  for  other  materials.  Palladium  is  a  unique material  that  can  provide  high  hydrogen  isotope concentration and high hydrogen flows, because hydrogen in Pd is in an active state and has high mobility. In this work, Pd was used as a convenient material to study the influence  of  both  high  and  low  hydrogen  recycling regimes  on  material  sputtering  behavior  under  plasma impact. Such studies can be useful to understand erosion behavior  of  materials  under  plasma  impact  in  high hydrogen recycling regime and for some applications, e.g., in connection with possible  creation of erosion resistant plasma  facing  components  on  the  base  of  bimetallic membranes containing palladium substrate  for  an active recycling control, as it was recently suggested [4, 5].2. EXPERIMENTAL The experimental setup used for plasma impact studies was DSM-1 device [6] with some modifications. The ground pressure in vacuum chamber before plasma experiments  was  about  6.65⋅10-4 Pa  and  determined  by such residual  gases as H2O, CO2,  N2,  and CO. Working gas (hydrogen,  helium, or  nitrogen) pressure in  vacuum chamber was about 0.266 Pa. A mirror Penning discharge was ignited at magnetic field B≈0.05 T that was produced by two pairs of coils. Cathodes potential values U were in the  range  U =  1-2  kV.  The  ion  energy  E in  hydrogen discharges (0.8 keV-1.6 keV) had taken at the peak of the energy  distribution,  which  equals  E  =  0.8eU,  as  it  was shown earlier [6]. For comparison with literature data in the  case  of  helium  and  nitrogen  plasmas  it  were  used energy values estimated similar to hydrogen plasma. As the weak energy dependence of erosion rate was observed in  the  above  mentioned  energy  range  for  all  kinds  of working gases, so the error is believed not so large. Ion currents and doses were estimated in assumption that ion current  to  the  cathode  is  0.6  times  the  total  measured current in hydrogen plasma discharge, and 0.8 times the total current in helium and nitrogen plasma discharge [7]. Ion current density values were in the range 0.5 mA/cm2 – 5mA/cm2 for  different  discharge  voltages  and  different working gases. Estimated irradiation doses were 1018-1019 ions/cm2 and  in  this  range  it  was  not  found  the  dose dependence of erosion coefficient. The samples for studies were  mechanically  cleaned  and  annealed  in  vacuum at temperature  600°C  Pd  (99.98  purity)  foils  without  and with  hydrogen  saturation  of  concentration  up  to  H/Pd≈0.65. It should be noted, that in the latter case, pressure during discharge increased up to 2.66 Pa due to hydrogen release from Pd targets (or from one target) and the high hydrogen-recycling regime was realized in this case for all kinds of working gas. Note, hereinafter we have in view so-called  local  hydrogen  recycling  process  and  local recycling  coefficient  determined  by  hydrogen sorption/release from targets only. After a few minutes of the  operation  at  that  regime,  the  working  pressure decreased and reverted to the initial values.  The similar experiments  in  diffusion regime,  when back side of the target  was in situ exposed to hydrogen at  pressure of  2 atm., had been also carried out. In this case the working pressure  during  discharge  increased  to  0.29  Pa  due  to hydrogen  diffusion  through  palladium  target,  and  this pressure  was  constant  during whole plasma experiment. The  possibility  existed to  measure  by thermocouple  the temperature  of  the  sample.  The  thermocouple  was mechanically  attached  in  the center  of  the  sample  back side. It was not found the difference between erosion rate under water cooled (50-100°C sample temperatures) and non-cooled cathode regimes. So, all presented results had been  measured  in  non-cooled  regime.  In  this  case,  the usual temperatures of the sample back side were 100-300°80                              Problems of Atomic Science and Technology. 2002. № 5. Series: Plasma Physics (8). P. 80-82C. With ± 10% temperature measurement accuracy it was not  observed  difference  in  the  temperatures  and  in  the erosion  coefficients  for  Pd-samples  made  from  thin  or thick foils (foils were used of 0.2 mm, 0.3 mm and 1 mm thickness).  It  gives  the  reason  to  suppose  that  the  real surface temperature of a sample was near to the measured one, and that sublimation did not influence on an erosion rate.  Erosion  coefficient  values  were  measured  by  the weight loss method [6] and calculated by the equation: S = (P1-P2)/6.25⋅1018⋅I⋅t⋅m,  where  P1 (grams)  is  the  sample mass before exposure to plasma; P2 (grams) is the sample mass  after  exposure  to  plasma;  I  (Amperes)  is  the  ion current;  t  (seconds)  is  exposure  time;  m (grams)  is  the mass  of  Pd  atom.  For  hydrogen  saturated  samples,  P1 means  the  mass  before  hydrogen  saturation.  Then  the sample exposed in hydrogen at pressure of 2 atm. during 72 hours and hydrogen concentration was determined by the  weighting  on  the  VLR-20  analytic  balance  with sensitivity of 10-5 g. After that, the sample was irradiated by plasma in erosion experiments. During plasma erosionFig.1. Energy dependence of palladium erosion coefficient  under nitrogen, helium and hydrogen plasmas impact for hydrogen non-saturated samples (solid lines, Pd), for hydrogen saturated samples (dotted lines, PdHx); for diffusion regime: ○ - hydrogen plasma, ● - helium plasma, ж – nitrogen plasma.experiments not all hydrogen released from the sample, so the irradiated sample was placed in the special  vacuum chamber and was baked at the temperature of 600°C in vacuum 1.33⋅10-4 Pa during one hour to remove dissolved hydrogen. The weight of the sample after this baking is P2.  The  all  presented  erosion coefficient  values  are  the average  of  not  less  than  three  different  measurements including  measurements  with  samples  of  different thickness,  and  they  are  believed  to  be  accurate  with  ≈ 30%.  The  main  error  ≈ 25%  was  determined  by  ion current  measurements  and  caused  by  some  current instability on the initial stage of discharges. 3. RESULTS AND DISCUSSIONThe  measured  values  of  Pd  erosion  coefficient under  hydrogen,  helium or  nitrogen  plasma  impact  are presented in Fig. 1. One can see that erosion coefficient weakly depends on ion energy for all  kinds of working gas.  This  is  in  agreement with numerous literature  data [8], including Cu, Ni and stainless steel erosion behavior studies in the similar Penning discharges [6]. But practical independence of Pd erosion on hydrogen saturation up to concentration  about  H/Pd  =  0.65  (in  high  hydrogen recycling regime) is unexpected result. The point is that in recent works [1-3] the decrease of some material erosion was  found  after  hydrogen/deuterium  saturation  to  high concentration.  The  physical  mechanism  of  that phenomenon is not clear enough. For example, in the work [1] erosion of deuterium saturated solid lithium (D/Li = 1) under He+ ion bombardment was observed in  ≈ 3 times lower  than  that  for  non-saturated  samples.  It  had  been explained  as  preferential  sputtering  for  the  lightest component (deuterium). So, the erosion rate decrease was expected for hydrogen saturated PdHx samples. In reality, the erosion coefficients for the non-saturated Pd samples (solid  lines  in  Fig.1)  are  similar  to  those  for  hydrogen saturated  PdHx samples  (dotted  lines  in  Fig.1),  when  a hydrogen  flow  of  about  665  Pa⋅l/s  from  Pd  targets  is puffed  due  to  the  high  hydrogen  concentration  in palladium (at  0.65 H/Pd concentration  mainly  hydrogen rich β-phase exists in Pd [9]). Such possible reason can be suggested  to  explain  the  obtained  result.  Hydrogen  has very low binding energy in Pd lattice (≈ 0.15 eV [10]) and high mobility at moderate and even at low temperatures [9]. Therefore, when hydrogen in metal lattice is strongly bound  up  with  host  atoms,  it  is  more  probably,  that bombarding  ion  gives  its  energy  to  large  complex hydrogen-host atom. In this situation hydrogen atoms have good  chance  to  be  sputtered  (selective  or  preferential hydrogen sputtering can take place). In Pd case, hydrogen sub-system slightly bound up with host atoms, therefore it is more probably that bombarding ion gives its energy to large Pd atom than to small hydrogen atom (proton). In this situation Pd atoms have good chance to be sputtered and  dissolved  hydrogen  has  not  an  influence  on  the erosion  behavior.  In  such  conditions  hydrogen  receives thermal energy from Pd-lattice, and high hydrogen release during  plasma  impact  could  be  caused  by  thermal diffusion and desorption processes, but not sputtering or radiation enhanced diffusion and desorption. In order to get direct evidence in favor of such mechanism, additional experiments  are  needed,  in  particular,  using  various coatings on plasma facing surface of palladium, including metals with low Z/high Z and high/low hydrogen couple energies, etc.As seen  in  Fig.1  and the  comparison  with  the literature  data  [8]  shows that  erosion  coefficient  of  Pd under  hydrogen  plasma  impact  is  extremely  high.  The control  experiments  with  Cu  and  Kh18N10T  stainless steel  samples  under  hydrogen  plasma  impact  had  also shown enhanced  erosion  rates,  which  are  in  agreement with data in Ref. [6]. So it was difficult to expect for Pd another  result.  But,  as  for  helium and  for  nitrogen  the sufficient agreement was observed, it was supposed that explanation of the erosion increase used in Ref. [6] (self-sputtering) is not right in our case. If it were so one would have expected that erosion rate under nitrogen and helium plasmas would be essential higher. No traces of arcs were observed  on  sample  surface.  So,  it  is  remained  the influence of heavy impurity ions, such as N+, C+, O+. In the nitrogen plasma case the impurities (mainly C+ and O+ ions) have the nearest masses to nitrogen ion mass. So the influence  of  such  impurities  on  erosion behavior  under nitrogen plasma impact will be negligible. There are no literature  data  on  Pd  erosion  under  nitrogen  ion bombardment.  But  obtained  results  are  in  a  sufficient agreement  with  data  [8]  on  Pd  erosion  under  Ne+ ion 810.11100.7 0.9 1.1 1.3 1.5 1.7Ion  energy  E [keV]Erosion coefficient S [atom/ion] N+He+H+PdPdHxbombardment (≈ 1.5 at./ion at 1.5 keV ion energy). So the data on Pd erosion under nitrogen plasma impact one can consider as the hard data enough. We used this fact for calculation of the empirical coefficient “a” in the semi-empirical equation for mass dependence of erosion rate [8]:  (Z1Z2)⋅m1/(Z12/3+Z22/3)0..5⋅(m1+m2),  where  a  =  0.31  is constant,  Z1 and Z2 are atomic numbers of bombarding ion and Pd-target, respectively, m1 and m2 are ion and Pd atom  masses.  In  Fig.2  are  presented  the  Pd  erosion coefficients calculated by that equation for various atomic numbers Z of bombarding ions. The experimental results obtained  under  nitrogen,  helium  and  hydrogen  plasma impact and the data of ref. [8] obtained under H+, He+, and Ne+ ions  bombardment  are  shown  in  Fig.2,  too.  For nitrogen  and  helium  plasmas  the  sufficient  agreement with  the  literature  data  is  observed.  But,  measured  in hydrogen plasma, erosion coefficient is in 40 times higher than that for sputtering coefficient under hydrogen ions bombardment,  i.  e.  overpowering  influence  of  the impurities is observed. So the questions arises on the Fig.2. Experimental and calculated data on Pd erosion coefficientpossible impurity content and on the possible source of high impurity flow. As the ground pressure was 6.65⋅10-4 Pa and working gas pressure in plasma experiments was 0.266 Pa, the impurity content on the initial moment of the discharge was estimated as 0.25%. It is small value to be the reason of the observed erosion coefficient increase under  hydrogen  plasma  impact.  Really,  for  rough estimation of  heavy impurity  content  one can use such equation: S(H) = S(H+)⋅IH/I + S(N+)⋅IN/I, where S(H) is the measured total erosion coefficient under hydrogen plasma impact;  S(H+)  and  S(N+)  are  erosion  coefficients  under hydrogen and nitrogen ion bombardment, respectively; I is  the total  ion current;  IH  and IN are hydrogen ion and nitrogen ion current, respectively. To simplify a problem, only one heavy impurity (N+) was taken into account (it is not large error as ions C+ and O+ have masses near to N+ mass). If to put in equation measured values S(H) = 0.4 and S(N+) = 1.5, and taking into account that S(H+) = 0.01 [8]  << S(N+) it is obtained IN/I  ≈ S(H)/S(N+)  ≈ 0.25, i.e. 25% impurity  content.  The  gas  desorption  from device walls cannot be the reason of such impurity flow as the pressure  in  vacuum  chamber  did  not  change  during plasma experiment (in case of non-saturated Pd target). The possible reason could be impurity flow from targets. Really, if to suppose only one impurity monolayer on the sample surface before discharge (the experimental device is unbaked), we obtain 3⋅1015 impurity molecules could be desorpted from two samples. According to estimation, the number of working gas molecules in the plasma volume (about 60 cm3) is  ≈ 1015 molecules. So, it could be more than  75  %  impurity  content  on  the  initial  stage  of discharges. A life time of desorpted gases in the discharge is  unknown,  but  if  the  current  instability  on  the  initial stage  of  the  discharge  (about  a  few  minutes)  is  also caused by impurities, one can suppose the same time for impurity confinement in the discharge.  On the next stage of  the  discharge  the  influence  of  the  diffused  and desorpted from Pd impurities could be observed (mainly C).  As  shown  earlier  [11],  hydrogen  irradiation  is  the more  effective  method  for  Pd  surface  cleaning (decarburization) in comparison with helium or argon ion bombardment  and,  consequently,  one  can  expect  more high impurity flow from Pd-target under hydrogen plasma impact. For  the  diffusion  regime,  the  downtrend  of erosion  coefficient  values  was  observed  (Fig.1).  In  the case  of  nitrogen  plasma  this  effect  is  negligible,  for helium  plasma  it  is  visible  (≈25%),  and  for  hydrogen plasma the erosion values in diffusion regime (high local hydrogen recycling coefficient)  in 1.5 times lower than that  for  hydrogen  saturated  /non-saturated  samples.  It could  be  explained  if  to  suggest  the  different  impurity contents on the sample surface in different regimes. SUMMARYThe  main  intent  of  this  work  was  to  look  an influence  of  hydrogen  saturation  of  palladium  on  its erosion  behavior.  The  measured  values  of  Pd  erosion coefficient  are  1.5  ± 0.5  at./ion  under  nitrogen  plasma impact and 0.3 ± 0.09 at./ion for helium plasma. For both cases  the  erosion  rate  independence  was  observed  on hydrogen  saturation  of  the  samples  up  to  concentration H/Pd = 0.65,  in contrast  to observed earlier  [1]  erosion behavior of lithium. It could be caused by difference in hydrogen binding energy values for both materials. In the case  of  hydrogen  plasma  impact,  the  measured  erosion values  are  extremely  high  (0.4  ± 0.12)  due  to overpowering impurity influence, especially, on the initial stage of discharge, and additional experiments are needed to confirm the independence of Pd erosion behavior under hydrogen plasma impact on the hydrogen saturation (on the low/high recycling regimes). REFERENCES[1] J.P.Allain, D.N. Ruzic. Nucl. Fusion, v. 42, p.202-210 (2002).[2] E. Salonen, et al. Physical Review B, v. 60, No 20, p. 4005-4008 (1999).[3]  I.  Bitnaya,  et  al.  Journal  Technical  Physics,  Special  Suppl., Warsaw, v. 40, N1, p.485-488 (1999).[4]  G.P.Glazunov,  E.D.Volkov,  A.Hassanein.   In:  Hydrogen and Helium Recycling at Plasma Facing Materials. (Ed. A.Hassanein) NATO Science Series II: Mathematics, Physics and Chemistry, v.54, p.163-176 (2002).[5] G.P.Glazunov, E.D.Volkov, A.Hassanein. Problems of Atomic Scince and Techn. Series “Plasma Physics”,  No 3 (5), p. 102-104 (2000).[6] P.Ya. Burchenko, et al. J. Tech. Phys., v.55, No 11, p.2097-2288 (1985).[7] J.Backus J. Appl. Phys., v.30, No 12, p. 1866-1869 (1959).[8] Y. Yamamura and H. Tawara. NIFS-DATA-23, Nagoya, Japan, 114p. (1995).[9] . G.Alefeld, J.Volkl (Eds.), Hydrogen in Metals, v. 1, 2, Springer-Verlag, Berlin-Heidelberg-New York (1981). 820.0010.010.11101000 5 10 15 20 25 30Atomic number  ZErosion coefficient S [atom Pd/ion]ExperimentRef. [8] Caculated[10]  P.V.  Geld,  R.A.  Ryabov.  Hydrogen  in  metals  and  alloys. Moscow, Metallurgiya publ. (In Russian). (1974).[11] G.P.Glazunov. Int. J. Hydrogen Energy, v. 22, No. 2/3, p. 263-268 (1997).83

Palladium erosion behavior under low/high local hydrogen recycling in steady state plasmas

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Palladium erosion behavior under low/high local hydrogen recycling in steady state plasmas

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