ATOM-PROBE STUDY OF ABSORBED HYDROGEN IN NIOBIUM

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ATOM-PROBE STUDY OF ABSORBED HYDROGEN IN NIOBIUM

K. Okuno, K. Oida, H. Yamashita, O. Nishikawa

To cite this version:

K. Okuno, K. Oida, H. Yamashita, O. Nishikawa. ATOM-PROBE STUDY OF ABSORBED HYDROGEN IN NIOBIUM. Journal de Physique Colloques, 1987, 48 (C6), pp.C6-481-C6-486.

�10.1051/jphyscol:1987679�. �jpa-00226887�

ATOM-PROBE STUDY OF ABSORBED HYDROGEN IN NIOBIUM

K. Okuno, K. ~ i d a * , H. ~ a m a s h i t a * a n d 0. ~ i s h i k a w a *

Department of Electrical Engineering, Nagasaki Institute of Applied Science, 536 Abamachi, Nagasaki 851-01, Japan

* ~ e ~ a r t m e n t of Materials Science and Engineering, The Graduate School a t Nagatsuta, Tokyo Institute of Technology, 4259 Nagatsuta,

Midori-ku, Yokohama 227, Japan

Abstract

-

The presence of absorbed hydrogen in Nb metal has been observed visibly by field ion microscope. The behavior and states of hydrogen in Nb metal are of interest. However this information can not be obtained by FIM. Therefore experiments using high mass resolution time-of-flight atom-probe were per- formed to investigate the hydrogen concentration in Nb metal, the characteristics of hydrogen and hydride distribution , and their temperature dependence.

I

-

INTRODUCTION

Interactions of the surface metal atoms and gas atoms are basic problems of metal-gas interfaces, and many papers have been reported about these systems[l]. These interactions are well known to promote field evaporation of the surface metal atoms[2-41. The field induced dipole moment of the absorbed rare gas under a high field was also observed to give the same effect[51. Muller et a1 has observed the reduction effect of a field evaporation voltage on the Nb(Ol1) atomic layer due to Nb atom- hydrogen atom interactions under a hydrogen pressure of 10-~-10-9 torr [61

.

Hydrogen absorption in Nb metal has negative enthalpy energy[7].

Therefore hydrogen is trapped at interstitial tetrahedral lattice sites, and the presence of absorbed hydrogen in a Nb tip has been visibly observed by field ion microscope(FIM)[8]. However the charac- taristics of such hydrogen states and concentrations in Nb metal can not be observed with FIM. Therefore a high mass resolution time-of- flight atom-probe was used to investigate in detail the character- istics of hydrogen in Nb metal.

I1

-

EXPERIMENTAL

Nb tip were analyzed by high mass resolution time-of-flight atom-probe (A-P) [9], at a base pressure of the order of Pa. Nb tips were prepared using a HF(O.1)

+

HN03(0.9) solution and 1 to 3 Vdc. Hydrogen concentration in N ~ ( H / N ~ ) can be controled by tip heating(room temper- ature

-

1100K) and aging time. Atom-probe analysis at room temper- ature(R.T) detected many hydride Nb as N ~ H ~ + . However at the liquid nitrogen temperture(80K) and cryomini temperture(20K), very few hydride ions were detected. To investigate the distribution and states of ~ b 2 + , ~ b ~ + , N ~ H Z + and H+ ion species, the experiment was performed at R.T.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987679

C6-482 JOURNAL DE PHYSIQUE I11

-

A-P ANALYSES OF HYDROGEN ABSORBED Nb TIP

A histogram analysis of A-P data for a non-heated Nb tip after electro- lytic etch is shown in Fig.1. Identified ion species were H,' ~ b 2 +

,

FJb3+ and N ~ H ~ + ions respectively. H2+ ions were not detected, since hydrogen in Nb is in a dissociated state. From the detected rate of H and Nb atoms, hydrogen concentration H/Nb was found to be 0.75 as shown by the linear line in Fig.2, and the distribution was constant over all the depth of the Nb tip. The analyzed hydrogen concentration is in good agreement with that reported as 0.79, at R.T[8]. A depth profile of detected ion species is shown in Fig.3. The result at 80 K also is shown in Fig.4. At 80 K, the detected rate of Nb3+ ions is larger than that at R.T, and no hydride ions, N ~ H ~ + were found by increasing the evaporation field.

Fig.2. Hydrogen concen- tration for the non- heated Nb tip was ob- served H/Nb = 0.75, from the ratio of the number H atoms to that of Nb atoms.

IV

-

HYDROGEN DESORPTION AND ABSORPTION IN Nb

When the Nb tip was heated to various temperature, the absorbed hydro- gen desorbed to the less than 0.2 within 1100 K. An activation energy for the hydrogen desorption in Nb tip from Arrhenius plots was measured as Ea = 0.32 eV. Depth profiles for various ions analyzed for hydro-

gen desorption are shown in Fig. 5. Nb3+

ions were more detected

bb2*

I"'

in greater number than

NM~* from the non-heated Nb

tip. This indicates that the evaporation field of Nb was in- creased by the hydrogen desorption. After the hydrogen was desorbed, the Nb tip was exposed

. I

in hydrog n pressure

l

^{1.3 X} night. The histogram ^{~a over}

20 $0 40

50

60 ⁷⁰

8b

Moss to charge ratio m/n

Fig.1. A histogram ana- lysis of A-P data for a

No heated a Tip non-heated Nb tip.

I b

Nvnber of

a

otans

easily even in a residual gas pressure of 10-8 Pa. However these Nb oxides were stratified in several of the surface layers. A further very interesting result was observed in that hydrogen in Nb02 layer was detected as the complex ions N ~ O ~ H +

,

N ~ o ~ H ~ + and N ~ o ~ H ~ + , because of oxygen trapped with the hydrogen[9]. However in the Nb monoxide layer, no complex ions with hydrogen are detected. The depth profiles of this phenomena are shown in Fig.7, with more detailed results in Fig.8. At the be inning of the analysis, the complex ions N~O~H', and Nb02H23q were detected in urface la ern and

S

below the Nb oxide layer were detected bJb2+, Nbf+ and NbH + ions, corresponding to succesive Nb substrate layer respectively. Therefore the interface of the Nb oxide layer and Nb hydride layer is clearly indicated.

V

-

A REDUCTION EFFECT OF THE EVAPORATION FIELD OF THE Nb ATOM DUE TO ABSORBED HYDROGEN IN Nb

Detected at LN2.T NO heated ~b np

I I The interactions of gas

r..-..-I

-..

7-..-.."

0 50 I-- ^{I .- I} -

Tatd number of ions( Nb2++

90-

Pig.4. A depth profile of ion analysis for the non-heated Nb tip at 80 K.

1

^{.Jp . y 4} dependence of the evapo-

ration voltage of the

6 60-

-

Nb (011) atomic plane

under a hydrogen pressure of 10-4 10-9 torr. The result was an observed reduction ef- Fig.3. A depth profile of ion analysis for the

,

non-heated Nb tip at R.T

0 70 140 210 280

Total number of ions( Nb2*+IUb3*+NbHZ* + H?

1

Detected at R.T ^,.F'

,.f' No heated Fb Tip,

NbHan _,/' -

N~H~:./"

f'

,.-

atoms and metal surface atoms are well known to promote the field evapo- ration of surface metal

[2

-

41. Muller et a1 examined. the pressure

C6-484 JOURNAL DE PHYSIQUE

f e c t o f t h e e v a p o r a t i o n v o l t a g e o f 40 % i n t h a t p r e s s u r e r a n g e . How- e v e r t h e i r e x p e r i m e n t s were done on t h e m e t a l - g a s i n t e r f a c e i n a g a s atmosphere. T h e r e f o r e t h i s e x p e r i m e n t examined t h e r e d u c t i o n e f f e c t of an e v a p o r a t i o n f i e l d of Nb due t o t h e absorbed hydrogen atoms i n Nb m e t a l . The d e t e c t e d r a t e of bTb3+ i o n s (F=4.2

v/A)

t o Nb2+ i o n s

(F=3.5

v/&)

i s s m a l l e r f o r t h e t i p w i t h t h e l a r g e r hydrogen concen- t r a t i o n H/Nb. Hence t h e r e d u c t i o n e f f e c t s o f t h e e v a p o r a t i o n f i e l d of t h e Nb atom due t o a b s o r b e d hydrogen i n Nb m e t a l was e v a l u a t e d from t h e d e t e c t e d r a t e ~ b ~ + / ~ b ~ + . The e x p e r i m e n t a l r e s u l t s f o r t h e Nb t i p w i t h v a r i o u s hydrogen c o n c e n t r a t i o n s i s shown i n F i g . 9 . A s o l i d l i n e i n f i g u r e i n d i c a t e s t h a t ~ b ~ ' / ( ~ b ~ +

+

^{~ b}

+

^~P7blI2+) ⁺ was c a l c u - l a t e d f o r v a r i a t i o n s of t h e hydrogen c o n c e n t r a t i o n o n l y and t h e

FIb3+/Nb2+ had t h e c o n s t a n t v a l u e s 1 . 5 , 1.0 and 0.25 r e s p e c t i v e l y . The d e t e c t e d r a t e of N ~ H ~ + i o n s was o b t a i n e d from mapping d e t e c t e d i o n s . The s l o p e of t h e e x p e r i m e n t a l d a t a ( d o t t e d l i n e ) a r e l a r g e r t h a n t h a t of t h e e a s t i m a t e d s o l i d l i n e s . T h i s i n d i c a t e s t h a t ~ b ~ + / f i l b ~ + is e f f e c t e d by t h e hydrogen c o n c e n t r a t i o n . Hence t h e p r e s e n c e of a r e - d u c t i o n e f f e c t of an e v a p o r a t i o n f i e l d o f N b atoms due t o absorbed hydrogen i n Nb m e t a l was r e c o g n i z e d .

Fad*

NWP,

0 20 40 60 80

Mass to cbarge ratio rn/n

F i g . 5 . A d e p t h p r o f i l e o f i o n a n a l y s i s f o r t h e hydrogen d e s o r b e d Nb t i p (H/Nb = 0.14) a t R.T.

F i g . 6 . A h i s t o g r a m a n a l y s i s o f A-P d a t a f o r a Nb t i p exposed i n a hydrogen p r e s s u r e of 1 . 3 X Pa o v e r n i g h t , a f t e r absorbed hydrogen i n Nb was d e s o r b e d . A-P a n a l y s i s were done a t R.T.

A-P a n a l y s e s f o r a non-heated Nb t i p gave an observed hydrogen concen- t r a t i o n of n e a r l y 0.75. Depth dependency of t h e hydrogen concen- t r a t i o n was n o t observed. The hydrogen i n Nb desorbed t o l e s s t h a n 0.20 w i t h i n 1100 K . An a c t i v a t i o n energy f o r t h e hydrogen d e s o r p t i o n was measured a t 0.32 eV. The Nb s u r f a c e i s e a s i l y formed a Nb o x i d e l a y e r even a r e s i d u a l p r e s s u r e of Pa, and hydrogen i n Nb d i -

o x i d e NbO2 l a y e r was d e t e c t e d a s complex i o n s such a s N ~ o ~ H + , N ~ o ? H ~ ~ +

and N b 0 2 ~ ~ 3 + . However i n t h e Nb monoxide NbO l a y e r , no complex l o n s w i t h hydrogen found. Reduction e f f e c t s of an e v a p o r a t i o n f i e l d of Nb atoms due t o absorbed hydrogen i n Nb t i p s were e v a l u a t e d from t h e d e t e c t e d r a t e of Nb3+/~b2+. The r e s u l t i n d i c a t e d t h a t t h e r a t e of d e t e c t e d Nb3+ions d e c r e a s e s w i t h i n c r e a s i n g hydrogen c o n c e n t r a t i o n . Hence d e c r e a s e s of t h e e v a p o r a t i o n f i e l d of Nb atoms due t o absorbed hydrogen i n a Nb t i p were observed.

50 I , ^I

Detected at R.T Exposed to hydrogen of 13 x Pa fw 10 hours

,100-

E

-

F i g . 7 . A d e p t h p l o f i l e

.- of i o n a n a l y s i s f o r t h e

-

hydro e n exposed ( 1.3

X 10-2 Pa) Nb t i p .

t

⁽ c o r r e s p o n d i n g t o F i g .

I ^{6 ) .}

50-

0 100 200 500 400 500

Totd nunber of ions( N b + ~ t W t N b O i ! + H + H ~ O )

Detected at R.T

. ,

, . . .. . .

..@

,....

,... ^{--- r-}

-

...,

f

, r J W 0 2 --s-'NbO

..I-

0 po ²⁰⁰

Total m b e r of b s (Nb+NbH+NbO+MOi?iNbOi?HntH+M),

Fig.8 More d e t a i l e d d e p t h p l o f i l e s analyzed f o r hydrogen exposed Nb t i p . ( c o r r e s p o n d i n g t o F i g . 6 )

.

C6-486 JOURNAL DE PHYSIQUE

Fig.9. A dotted line shows experimental data

.

Solid lines were calculated for vari- ations of the hydrogen concen rati n only, the NbS+/Nbq+ had the constant values 1.5

,

1.0 and 0.25 respect- ively. The rate of

~ b ~ 2 + i o n s was obtained from mapping detected ions. This indicates that ~ b ~ + / ~ b ~ + is effected by the hydro- gen concentration in Nb.

-

⁸⁰

- s

VII

-

ACKNOWLEDGEMENT

I I I I I

Detected at RT

-

ooExperimenta1 data

-

The author wishes to gratefully acknowledge the technical support of the members of the surface science laboratory, Tokyo Institute of Technology.

VIII

-

REFERENCES

[l] Robert Gomer, Solid State Physics. g ( 1 9 7 5 ) 93. Chemisorption on Metal. ( Academic Press. New York San Francisco London ) . [2] O.Nishikawa, J.Vacuum Sci.Techno1. 2(1971)847.

[3] 0-Nishikawa and E.W.Muller, Surface Sci. u(19681247.

[4] O.Nishikawa, J.Chem.Phys. =(1970)1978.

[5] O.Nishikawa, Surface Sci, 131(1983)239.

[6] E.W.Muller, S.Nakamura, 0 . G h i k a w a and S.B.Mclane, J.Appl.Phys. %(I9651 2496.

[7] G.Alefeld and J.Volk1, Hydrogen in metals _- I, Splinger-Verlag, Berlin (1979).

181 E.Krautz. W.Polanschutz and G.Haim1, Proc.29th Field Emission - -

symposi&, Goteborg 29,151 (1982). .

[9] 0.Nishikawa et al, Rev.Sci.1nstrum. 52(1981)810.

[lOIP.E.Zapp and Birnbaum, Acta Metall. 23(1975)973.