Interaction of niobium with active gases at very low pressures

INTERACTION

OF NIOBIUM

WITH

ACTIVE

I. Adsorption

GASES

of Nitrogen

R. A. PASTERNAK

AT

VERY

LOW

PRESSURES*

by Niobium

and R. GIBSON?

The interaction of nitrogen with niobium filaments has been studied over wide ranges of pressure (6 x 10-O to 6 x 10-s torr) and of temperature (26 to 1SOO’C). At room temperature, the sticking probability on the clean surface is about 0.5, and adsorption leads to a saturated surface layer. At high temperatures, the sticking probability is about 0.1, independent of pressure and temperature, and the gas diffuses so fast into the bulk that the surface remains virtually clean for an extensive time interval. The dependence of the observed sorption No significant activation energy is required for adsorption. rates on temperature and pressure is discussed in terms of simultaneous adsorption on the surface and diffusion into the bulk. INTERACTION

DU

NIOBIUM

AVEC

DES

I. Absorption

GAZ

ACTIFS

AUX

TRES

FAIBLES

PRESSIONS

de l’azote par le Niobium

L’interaction de l’azote avec des filaments de niobium a Qte Btudiee dans une large gamme de pressions (6 x 10-O a 6 x 10e6 torr) et de temperatures (26 a 1800°C). A la temperature ambiante, la probabilite de captage sur la surface propre est environ 0,5, et l’adsorption conduit Q une couche superficielle saturee. Aux hautes temperatures, la probabilite de captage est environ O,l, independante de la pression et de la temperature, et le gas diffuse suffisamment vite dans la masse pour que la surface Aucune Bnergie d’activation significative n’est reste virtuellement propre pour une duree prolongbe. requise pour l’adsorption. Les auteurs discutent la dependence des vitesses de sorption observees envers la temperature et la pression sur la base del’adsorption sur la surface et de la diffusion simultanee dans la masse. DIE

WECHSELWIRKUNG

VON

NIOB

AKTIVEN

GASEN

BE1

SEHR

KLEINEN

DRUCKEN

Die Wechselwirkung van Stickstoff mit Niobdrahten wurde in einem grol3en Druck (6 x 10-O bis 6 x lo-” torr-und Temperaturbereich (25 bis 18OO’C) untersucht. Bei Raumtemperatur ist die Haftwahrscheinlichkeit auf der sauberen Oberfliiche etwa 0,5, und die Adsorption ftihrt zu einer gesiittigten Oberfliichenschicht. Bei hohen Temperaturen ist die Haftwahrscheinlichkeit etwa O,l, unabhiingig van Druck und Temperatur, und das Gas diffundiert so sohnell in das Innere, da13 die Oberfliiche liingere Zeit fast sauber bleibt. Fiir die Adsorption ist keine nennenswerte Aktivierungsenergie erforderlich. Die Temperatur- und Druckabhiingigkeit der beobaohteten Sorptionsgeschwindigkeiten wind diskutiert im Hinblick auf gleichzeitige Adsorption an der Oberfliiche und Diffusion in das Innere.

INTRODUCTION

The reaction of nitrogen, with metals is of considerable of corrosion

and embrittlement.

search on these systems

in this communication.

oxygen and hydrogen interest in the context The extensive

denum and tungsten, active

re-

gases;

Niobium, in contrast to molybhas a significant

thus the observed

solubility

effects involve

for both

surface and bulk mechanisms.

has in general been done at

such high pressures, that the observed

were

EXPERIMENTAL

Concept

dominated

by the reaction

compound

surface layers, and diffusion through these

The kinetics of gas sorption at very low pressure have

layers. Studies of the primary process between the gas and the metal atoms themselves which is of special

in general been investigated by the flash filament technique using constant flo~.(l-‘~) Since in that

fundamental interest require special low-pressure techniques and have been carried out mainly with re-

approach the pressure seen by the sample is not constant but depends on the sorption rate, wall and ion

fractory

between

kinetics

the gas and the

metals, primarily tungsten and molybdenum.

An ultrahigh vacuum investigation of niobium with different active gases is in progress. Some of the results for the nitrogen-niobium system are presented Received February 11, 1965; revised March 26, 1965. * This research has been supported in full by the U.S. Atomic Energy Commission, Fuels and Materials Development Branch, Division of Reactor Development, Contract AT(04-3)-115 Project Agreement 36. t Stanford Research Institute, Menlo Park, California. ACTA

METALLURGICA,

VOL.

13, OCTOBER

1965

gauge effects

are variable

and may thus introduce

ill-defined errors; moreover, the effect of the important parameter, pressure, cannot be investigated independently. A constant pressure technique which virtually eliminate these drawbacks (see also della Porta( has been developed based on automatic pressure control.02) Figure 1 is a schematic drawing of the system. Constant pressure is maintained in the sorption cell, V’S,by a Granville-Phillips control unit which compares 1031

ACTA

1032

~~TALL~R~I~A,

“r”

Experimental

t

FIG. 1. Schematic

t ION GAUGE CONTROL

drawing of constant

pressure system.

the pressure, p,, measured by an ion gauge, with a preset value. On imbalance, the drive mechanism for the controlled leak valve is activated, opening or closing it, until the desired pressure in the sorption cell is re-established. When the sample is activated, the only observable effect in the system is a change in pressure pz, and the rate of sorption R is

where F is the conduc~nee between the volumes V, and V,, and pZOis the steady state pressure corresponding to an inactive sample. For gas uptake, p, is larger, and for degassing, p, is smaller than pGO. The sticking probability (or pumping efficiency) S, which will be used here to present most of the kinetic data, is by definition R SE-.-..-= P(Pp, - P1,“) ps . 21. A p, . Y . A where v is the incidence rate of molecules on the unit area at unit pressure; and A is the sample area. The amount of gas, M, taken up or released by the sample in the time interval t, -+ t, is:

iw =

F

13,

1965

of this experimental approach and of sources of error will be given elsewhere. *

RECOUOER

COWlROlLER

VOL.

s12(p,

p,“) at

t1

Thus, measurement of the sorption or degassing rate and of total amount of gas sorbed or released is reduced to measuring the one pressure, p,, as function of time. Because the pressure-measuring device is outside the sorption cell, a mere change in its pumping speed does not affect the gas flow into the cell, since the servomechanism compensates for it. Low pressure flow techniques are in general suitable only for the measurement of high sticking probabilities. In the present study, the lowest value measurable is about S = 3 x 10-4, and the precision is this value or 5%, whichever is larger-A more detailed discussion

unit

The pumping system was an ion getter pump in combination with a titanium sublimation pump; the pumps were separated by a bakable metal valve from the main system. Two similar sorption cells, one in metal, the other in glass were used. The metal cell had a pin hole in a copper foil as conductance, with F = 0.060 litre Nzlsec (calculated from its dimensions); in the glass unit a capillary had the calculated conductance of F = 0.13 litre N,/sec. The niobium sample was spot welded to heavy electrical throughputs. The walls of the sorption cell were cooled with water or/and with air for reducing degassing by radiation heating from the sample.

Ribbons were cut from foils rolled from a zonemelted specimen supplied by the Metallurgy Department at Oak Ridge National Laboratory; ~on~minants, in ppm, were reported as follows: 0, : 9, N, : 5, Ha: 2, C : 38. The rolling operation, carried out under oil, might have increased the carbon, hydrogen, and oxygen content. Etching of the ribbons with a mixture of hydrofhroric and nitric aeids before mounting in the unit, and extended high temperature degassing at about 2OOO’C undoubtedly reduced the contaminant level again. Three ribbons were studied, Nos. 1 and 2 in the metal, No. 3 in the glass unit; they had dimensions of approximately 25 x 0.1 x 0.004 cm. One wire filament, diameter 0.02 and length 25 cm, (obtained from the same source) was also investigated. Contamination was quite high (0,: 1100, N,:3OO, H,:36, C: 210 ppm), but most of it was very likely removed during the high temperature heating in ultrahigh vacuum. The filament diameter varied by as much as loo/& and the sample broke after a limited number of experiments. The crystallographic orientation of samples similar to those used in the sorption studies was determined by X-rays. The filament sample showed preferred orientation along the [l lo] direction as expected for a body centered cubic metal. Two non-annealed ribbon samples were microcrystalline and exhibited on the surface mainly (100) planes. However, another ribbon sample which has been used in sorption studies and had undergone extensive exposure to nitrogen and to oxygen at higb temperature exhibited mainly (110) * Paper in preparation.

PASTERNAK

AND

GIBSON:

planes, and the individual crystallites were of the order The relative intensity data for these three ribbon samples as measured with an X-ray spectrometer are given in Table 1. This rolling and annealing texture is generally found for body centered cubic metals; the high degree of orientation is however rather surprising.

ofone millimeter.

TABLE

1. Relative intensities of X-ray reflections surface planes (ME) on rolled niobium foils

(h/cl)

-

Intensity

Random Orientation (literature) 110 211 200

from

100 32 20

Not Annealed (a) (b) 100 600 3300

100 1000 6000

Annealed 100 3 1

Gas Purified nitrogen from a lecture bottle (total impurity 50 ppm) was dried over liquid nitrogen and stored in a reservoir baked previously with the rest of the system. Temperature

measurements

The samples were heated by direct current supplied by lead batteries, and their resistance R was measured with a Kelvin Bridge. For the sample in the glass unit the brightness temperatures were determined with a micropyrometer; they were converted to true temperatures by correcting for absorption by the glass and for emissivity (Ed = 0.35). (i3) The filament temperature was uniform within +20” at all temperatures, to closer than 1 cm from the leads. The range of measurements was 900 to 2000°C. A parabolic temperature-versus-resistance-curve was drawn through the resistance values at high temperatures and the measured resistance at 25°C; this curve was used for all samples to convert resistance to temperature. Experimental

procedure

The pumps were first baked separately to about 300°C under continuous evacuation by an auxiliary pump; the system itself was then baked in ultrahigh vacuum at 300-350% for 15 hr. At most two bakeout cycles were required to reach a base pressure of 5 x lo-lo torr or lower. The sample required initially extensive and repeated heating to about 2000°C for the removal of surface layers and of gas dissolved in the bulk; the surface was considered to be clean when at room temperature initial sticking probability and saturation coverage for nitrogen approached constant values. Subsequently, the sample required only a one-second flash to

ADSORPTION

OF

N BY

Nb

1033

1OOO’C or higher for reactivation after exposure to nitrogen, and only a small gas burst was observed, since the absorbed gas dissolved in the metal during heating. Whenever a significant amount of nitrogen had been dissolved, the sample was degassed again at 1800-2000°C. For measuring sorption, two techniques were used; they gave approximately the same results: (1) steady state pressure was established first, and the sample activated by flashing (with the controller frozen during the short disturbance); (2) the sample was flashed in ultrahigh vacuum, and after it had cooled to the desired temperature, nitrogen was admitted and constant pressure established. In either method the control pressure was attained within 10 seconds after reactivating the pressure controller. The first method is more convenient, is faster, and is insensitive to wall effects. The second method permits a more reliable study of the early stage of sorption, since the sample cools down before sorption is initiated; about 20 see are required to reach a temperature of 1OO’Cafter high temperature flashing. RESULTS

In Fig. 2 typical sticking probability curves at p = 2 x 1O-6 torr and different tem~ratures are shown for sample No. 1. At room temperature the sticking probability S remains constant to about half coverage, and gas uptake ceases when about 7 x lo14 atoms/cm2 are adsorbed. Virtually identical room temperature curves were obtained with this sample at 6 x 10Ps and 6 x 10es torr, the initial sticking probabilities being 0.56 to 0.58. Constant sticking probability is established immediately, within the resolution of the experimental techniques. For example, at 6 x 1O-s torr the amount adsorbed before S stabilized is estimated at about 1% of the complete layer. The sticking probability drops also at higher temperatures with amount sorbed but approaches a finite quasi-steady state value; gas uptake continues beyond the amount equivalent to a monolayer (Fig. 2). The quasi steady state sticking probability increases and the initial sticking probability decreases with temperature; thus, the distinction between them disappears at high temperature, and S becomes insensitive to amount sorbed and to temperature. Sticking probability curves at 2 j< loss torr are shown in Fig. 3 for temperatures above 170°C; S is plotted here versus time. (At such high pressure, the sticking probability curve at room temperature cannot be observed since a monolayer is formed in a fraction of a second.) S drops again rapidly wit.h time to

ACTA

1034

0.001

I

1

I

METALLURGICA,

I

I

2

0

I

4

I 6

M (atams/cm2)

VOL.

13,

1965

Y 8

IO

12

X lOI

FIG. 2. Sticking probability of nitrogen as function of amount sorbed and of temperature. p = 2 X 1O-8 torr. Sample 1.

I

I

I

I

I

I

I

I

I

--

0.1

T>llOOT

0

60

120 180 240 300 340 420 480 540 600 TIME --SBC

Fra. 3. Sticking probability of nitrogen a8 function of time and temperature. p = 2.4 x 10e6 torr. Sample 1. Amounts sorbed at end of ruu~~ III IV II I C&V0 Atom&me x 1O-16 0.7 1.5 4 11

approach a quasi steady state value which increases with temperature; at the higher temperatures S is, as at low pressure, approximately independent of time (or alternatively, of amount sorbed) and of temperature. Sticking probability curves similar to those illustrated by Figs. 2 and 3 were obtained at pressures between 6 x 1O-e and 6 x 1O-s torr. The results are summarized in Fig. 4. The initial sticking probabilities at lower temperatures, indicated by the dotted lines, are observable only at the three lower pressures; they are independent of pressure, but decrease with temperature. The quasi steady state sticking probabilities are shown by the solid lines; they increase with temperature to reach the same, approximately constant value of 0.07 to 0.09 independent of pressure and temperature. The range of constancy moves, however, to higher temperatures with increasing pressure; this is a consequence of the more general effect that the steady state sticking probabilities at the lower temperatures decreases with pressure. In this lower temperature range the sticking probabilities are not well reproducible and are too small to be measured with precision; thus a quantitative correlation is not possible on the basis of the present data. Finally, at the highest temperatures the sticking probabilities decrease again; the magnitude of this effect depends on how vigorously the sample has been degassed previously. In order to obtain information on the dependence of steady state surface coverage on temperature, some

PASTERNAK

GIBSON:

AND

ADSORPTION

OF

N

Nb

BY

1035

0.1

i

1

0.01

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

(d)

1

P s 2.4 x IO6 torr i

(b)

(4 I_

,

I

I

I

I

I

I

I

1

p - 6 x IO.’ torr

0.1

0.01

1

0

I

I

I

I

I

200

400

6W

800

1000

I ml

I

I

14M)

1600

I MW

0.01

0

200

400

TEMPERATURE-‘C

600

800

1000

room

1600 I800 RO 0151h30

1400

(f)

(c) Fm.

1200

TEMPERATURE-‘C

4. Sticking probability of nitrogen as function of temperature and pressure. Sample 1.

temperature

sticking

probability

curves

were

These curves constitute

direct evidence

that steady

recorded subsequent to exposure of the sample at different temperatures to nitrogen. The same room temperature sticking probability curves as the one in

state surface coverage increases with decreasing temperature, if one assumes that no large changes

Fig. 2 was obtained

The other samples studied exhibited properties very similar to those presented in detail for sample 1. Table 2 gives average initial sticking probabilities and

after the sample was heated for

short periods to any temperature in or above the range of constant sticking probability; this curve is undoubtedly characteristic for a clean surface. However, the initial sticking probability and the total amount sorbed at room temperature are lower when the sample has previously attained steady state at an intermediate temperature, as shown by curves I and II of Fig. 5.

occur during cooling.

amount sorbed at room temperature, and the approximately constant, high temperature sticking probabilities for all samples. The sticking probabilities and the room temperature coverage of ribbon 3 were systematically

lower

compared

to

the

other

two

1036

ACTA

,

IL

METALLURCICA,

I

I

13,

VOL.

,

I

I

I

I

it

II

I1

2

4

6

8

1

1::

I

I

1965

I

0

Ij

0.5 -

Ill

0.001 0

M (atomr/cm*)

10

0

I

2

3

x IOf4

FIQ. 5. Sticking probability of nitrogen at elevated temperature and subsequently at room temperature. Sorption curves (b) we obtained immediately after curves (R). Sample 1. TABLE 2. Average

Sample Ribbon 1 Ribbon 2 Ribbon 3 Filament 4

sticking probabilities

Room ~rnpe~t~ S iJ$ x lOI* atoms/cm$ 0.57 0.5 0.4 0.25

7 : 6

and amounts sorbed High temperature S 0.09 0.09 0.06 0.08

ribbons. It is suspected that this discrepancy is due to experimental errors, since the samples were studied in different units. The filament shows the same sticking probability as the ribbons 1 and 2 at high temperature but a lower one at room temperature. This difference may be significant. DISCUSSION

The surface coverage at room ~mperature was found to be between 4 and 8 x 1O1*atoms/cm2. The spread of the data may be partly due to variation in crys~~ographic orientation and variation in surface roughness which was assumed throughout to be unity. The averclge amount adsorbed is of the same magnitude as found in the tungsten- and the molybdenumnitrogen systems (5 x IOlp atomsfcm2), but it does not seem justified to postulate, because of the spread in the data, a definite atom ratio for the adsorbateadsorbent complex. Similar un~ert~inities are undoubtedly present in most other ultrahigh vacuum studies of adsorption. The room ~mper&ture sticking prob~bi~ty curves for nitrogen on niobium are characterized by a high initial value of S, between 0.25 to 0.6 (see Table 2) and by approximate constancy of S to about half saturation (Fig. 2). The difference in S between filament and

ribbons appear to be significant and may be attributed to the different crystallogr&phic orientation. Constant high sticking probability is attained virtually instantaneously. In an ultrahigh vacuum study of the adsorption of nitrogen and of deuterium by evaporated films, no induction period was observed either.* Sticking probabilities initially increasing with time have, however, been reported for the adsorption of nitrogen on molybdenum. Similar sticking probability curves as reported here have been observed for nitrogen, oxygen, hydrogen, carbon monoxide on tungsten and on molybdenum;f1-10.15) and for oxygen on niobium.* The initial sticking probabilities were between about 0.1 and 0.7. However, an initial probability of less than 0.01 has been found for hydrogen on a clean platinum filament, and no nitrogen was adsorbed at a11.‘L6)Thus a wide spread in sticking probabilities exists; it has been suggested that high values are associated with large heats of adsorption, and low values with low heata. The independence on pressure of the initial sticking probability at all temperatures cIearly shows that the arrival rate of gas molecules on the clean surface is the ran-determining step. The constancy of S at room temperature to relatively high coverages is less understandable and has been the subject’ of speculations and of theoretical treatment,(l*) but no completely satisfactory q~iantitative explanation has been proposed so far. The high value close to unity of the initial sticking * Work to be published.

PASTERNAK

probability

at room temperature

significant

activation

constancy

energy

AND

excludes

for

GIBSON:

ADSORPTION

a priori

a

adsorption.

The is further indica-

of S at high temperatures

with tem-

perature which has also been reported for sorption nitrogen on tungsten(ls6) and on molybdenum(lO) be an entropy The

of

must

bulk diffusion.(le)

approximately

probability

at high

temperature

is

interval between consecutive

d/n,

would

minimize

crystallographic

effect may

effects

process itself.

atures, even only slightly

is approximately

below

for

three

be possibly

due to differences

in

However,

only with

at all temper-

of dnb

and

two

is given

pressures.

log dLa v = 10-a tom

10-e torr -0.6 3.2 4.5

higher than room temper-

The quantity nb decreases with pressure and increases

discussed

here in detail,

of absorption it must

will not be

be considered

for

the presented data.

The observed general dependence of the sticking probabilities (or sorption rates) on pressure, temperature, and the amount sorbed can be explained on the basis of two sequential rate processes, adsorption and diffusion into the bulk. The rate of adsorption depends on pressure and surface coverage; in addition

the rate of diffusion

to surface coverage,

The following

simple estimate

discuss the relative significance

on temper-

permits

of the two steps.

us to It is

that nitrogen dissolves without forming a The nitride may be stable at the lower

nitride phase.

and

however, nucleation

higher

pressures

of

this

study;

is very unlikely at the extremely

concentrations

and low temperatures.

For n,, much larger than unity, the

with temperature.

adsorbed gas moves into the bulk virtually as fast as it sequently

low nitrogen

The logarithm

0.4 4.2 5.5

227 727 1227

is adsorbed;

temperatures

(2)

equal to the average displace-

$2=

the mechanism

assumed

[-35,00O/RT]

temperatures

PC

Although

ature.

on a site is

a

ature, adsorption and absorption occur simultaneously.

depends,

adsorption

orientation.

The discussion so far has been concerned

explaining

energy for

the same for all samples (ignoring the

This temperature

the adsorption

1037

ment (expressed in lattice spacings) from the surface

due to a transition from localized to mobile adsorption, which

Nb

= 108.p-iexp

no =: .-

value for ribbon 3), whereas the room temperature activity of filament and ribbon were found to be different.

BY

The number of jumps nb in the time

in this time interval.

effect.

sticking

N

where AH D = 35,000 cal is the activation

tion that chemisorption of active gases on clean metals does not require significant activation energy. The decrease of the initial sticking probability

OF

a clean surface is maintained,

the sticking probability

and con-

remains at its con-

stant, high value. According to the data (tabulated above) this condition is satisfied at temperatures above

700°C for the pressures

of this study.

The

experimental temperatures

curves (Fig. 4) show that the transition to constant sticking probability increase with pressure as predicted; the experimental temperasomewhat higher than the tures are, however, calculated

ones.

In view of the crudeness of the esti-

mate, the over-all agreement is quite acceptable. If nb is not large, the diffusion process is not sufficiently fast to keep the surface clean, and to distribute the dissolved quasi-steady

gas uniformly

through the bulk.

At the

state the surface is now partly covered,

and as a consequence

the rate of sorption is diminished

relative to that on the clean surface; tion gradient

is established

also, a concentra-

in the bulk.

This con-

It is assumed further that no significant energy barrier exists for the movement of atoms across the surface-

centration gradient, and therefore the surface coverage, changes only slowly with time since very little gas is

Finally, only the case of very low bulk interface. concentration of gas in the bulk is considered so that

dissolved at the low pressures of the experiments. The steady state sorption rate at a given pressure

the degassing reaction can be disregarded. The number of hits per second per pair of surface sites

increases with temperature,

is approximately vn m lo6 p, where p is the pressure in torr, and the number of surface sites per cm2 is

ature is accompanied by higher sorption rates also; however, since the surface coverage is simultaneously

taken as 6 x 1014. For a sticking probability of 0.1 the number of molecules sorbed per second on an unoccupied site is then v, = lo5 . p. The jump fre-

augmented,

quency vb for diffusion in the bulk is approximately

controlled primarily by diffusion, and pressure has barely any effect; at high temperature, adsorption on the clean surface is rate-determining, and the sorption

rb =

V.

exp

[-AHDIRT]=

1013exp

[-35,00O/RT]

(1)

decreases.

since the surface coverage

An increase in pressure at a fixed temper-

the rate increases less than proportional to In the limit, at lower temperature, the pressure. surface is virtually completely covered; the rate is

rate is proportional

to pressure.

1038

The

ACTA

experimental

mechanism.

data

fit well

METALLURGICA,

this

postulated

The curves in Fig. 4 show the over-all

dependence

of the sticking

probabilities

on pressure

VOL.

13, 1965 ACKNOWLEDGMENT

Part of the experimental Mr. Bert Evans.

work was performed

The authors are greatly indebted

by to

and temperature as postulated. The approach to the quasi steady state is shown in Fig. 2 for low pressures, the decrease in sticking probability with amount

Dr. B. Bergsnov-Hansen for stimulating discussion and constructive criticism of the manuscript.

sorbed indicating

1. J. A. BECKER and C. D. HARTMAN, J. Phys. C?wm. 57, 153 (1953). 2. J. A. BECKER, Advances in CataZysis VII, 139 (1955). 3. G. EHRLICH, J. Phys. Chem. 60, 1388 (1956). 4. J. EISINGER, J. Chem. Phy8. 29, 1154 (1958). 5. T. W. HIOKMOTTand G. EERLICH, J. Phya. Chem. Solid8 5, 47 (1948). 6. P. KISLIUK, J. Chem. Phys. 50, 174 (1959). 7. P. L. JONES and B. A. PETHICA, Proc. Roy. Sot. A&%, 454 (1960). 8. T. W. HICKMOTT,J.Chem. Phys. S2, 810 (1960). 9. P. A. REDHEAD, Trans. Faraday Sot. 57, 641 (1961). 10. R. A. PASTERNAKand H. WIESENDANGER, J. Chem. Phy8. 84, 2062 (1961). 11. PAOLA DELLA PORTA and FRANCE RICCA, 1960 Transactions, Seventh National Vacuum Symposium, American Vacuum Society, pp. 352-363. 12. R. Gibson, B. Bergsnov-Hansen, N. ENDOW and R. A. PASTERNAK, 1963 Transactiona, Tenth Nation& Vacuum Sympokum, American Vacuum Society, pp. S&92. 13. J. R. COST and C. A. WERT, T and AM Report No. 205, University of Illinois, 1961. 14. D. LEE, H. TOMASCHKE, and D. ALPERT, Coordinated Science Laboratory Rep. l-104, University of Illinois (1961). 15. TAKEO OQURI, J. Phys. Sot. Japan 19, 77 (1964). 16. H. U. D. WIESENDANGER, J. Catalyti 2, 538-41 (1963). 17. G. EHRLICH, Structure and Proper&s of Thin Filma, John Wiley and Sons, Inc., 1959, p. 423. 18. P. KISLIKJK, J. Phys. Chem. Solids 2, 95 (1957); 5, 78 (1958). 19. R. W. POWERS and W. V. DOYLE, J. Appl. Phys. 80,514 (1959).

that the surface becomes

gradually

covered. A significant surface coverage at steady state, decreasing with temperature, is directly confirmcurves in Fig. 5. Finally,

ed by the sticking probability the sticking

probability

curves

at higher

pressures

(Fig. 3) represent the transient state, during which the surface becomes

partly covered and a concentration gradient is built up. Possibly a supersaturated solution or a subnitride is formed; the stoichiometric nitride Nb,N appears to be excluded,

since the sticking pro-

of nitrogen urably

to be

small as on a saturated

Nb

layer. The quasi steady state sorption here, drop in long-time

approached.

sorption

The observed

probability

at the highest

effect, since the solubility

temperature. absorption

The

and degassing,

of a second communication.

rates, as discussed

runs as saturation

decrease of nitrogen

of

is

of the sticking

temperatures

kinetics

in the nitrogen-niobium

. . . N surface

high

is due to this decreases

with

temperature

and the solution equilibrium

system

will be the subject

REFERENCES