A radiotracer study of the composition and properties of anodic oxide films on tantalum and niobium

ElcctrochimicaActa. 1965.Vol. 10, pp. 183 to 201.

Pergamon

Press

Ltd. Printed in Northern Ireland

A RADIOTRACER STUDY OF THE COMPOSITION AND PROPERTIES OF ANODIC OXIDE FILMS ON TANTALUM AND NIOBIUM* J. J.

RANDALL,

JR.,

W.

J. BERNARD

and

R.

R.

WILKINSON~

Research Center, Sprague Electric Company, North Adams, Mass., U.S.A. Abstract-The composition of anodic oxide films formed on tantalum in phosphoric and sulphuric acids and on niobium in phosphoric acid has been determined by the use of ss;P- and %-labelled electrolytes. The films consist in all cases of a layer of stoichiometric pentoxide next to the metal and an outer layer characteristic of the anodizing electrolyte. This layered structure can only arise through the movement of both cations and anions during the growth of the fdm. The amount of a particular species that is incorporated into the film from the electrolyte increases with increasing solution concentration and current density and decreases with increasing temperature; variations in the amount of included material with temperature are entirely accounted for by changes in the relative contributions of cation and anion movement during film growth. It is shown that differences between films formed in phosphoric and sulphuric acids are due to the much larger degree of incorporation that occurs with the former. The incorporation of phosphorus into the oxide results in a decrease of dielectric constant and an increase of dielectric strength. R&unGLa composition de films d’oxydes anodiques form& sur le tantale dans les acides phosphorique et,sulfurique et sur le niobium dans l’acide phosphorique a Btt determinke par l’emploi d’&ctrolytes qui contiennent saP et Y& Dans tous ces cas les films sont composCs d’une couche de pentoxyde stoechiom&rique g proximitb du metal et d’une couche extbieure dont la composition dkpend de l’electrolyte anodisant. Cette structure en couches n’est possible que s’il y a mouvement simultane des cations et des anions pendant la croissance du film. La quantite d’une esp&ce provenant de Wectrolyte et incorporke dans le film augmente lorsque la concentration de la solution et l’intens&5 du courant augmenter& et diminue lorsque la tempbature croit ; les variations de la quantite de substance incorpork avec la tempkrature sont expliquees par les changements dans la contribution relative de la migration des cations et des anions & la croissance du film. I1 est montrk que les diffkrences entre les films form& dans les acides phosphorique et sulfurique sont dues au fait qu’une plus grande quantite de substance est incorporke dans les cas du premier Clectrolyte. L’incorporation du phosphore dans l’oxide cause une diminution de la constante di&ctrique et une augmentation de la force dielectrique. Zusammenfassung-Die Zusammensetzung anodischer Oxydfilme, welche im Falle von Tantal in Phosphor- und Schwefelslure und im Falle von Niob in Phosphorslure formiert wurden, ist mit Hilfe von Elektrolyten, die radioaktiven Phosphor, saP, und Schwefel, *5, enthielten, bestimmt worden. In allen Fgllen bestanden die Filme aus einer inneren, an das Metal1 angrenzenden stiichiometrischen Pentoxydschicht und einer lusseren Schicht, deren Zusammensetzung vom Elektrolyten abhing. Diese Schichtstruktur ist nur miiglich, wenn sowohl, Anionen als such Kationen wlhrend der Formierung des Filmes wandern. Der Einbau eines gegebenen Bestandteiles in den Oxydfilm nimmt mit $er Konzentration des Elektrolyten und der Stromdichte zu und mit steigender Temperatur ab. Anderungen in der Anzahl der Einschliisse mit der Temperatur sind durch eine Verschiebung der relativen Beitrlge der Anionen- und Kationenwanderung am Filmwachstum vollstlndig erkllrbar. Es wird gezeigt, dass Unterschiede zwischen Oxydfilmen, die in Phosphor- oder Schwefelslure formiert wurden, mit einer gr&seren Anzahl von Einschliissen im Falle der anodischen Oxydation in Phosphorslure erkllrt werden kiinnen. Der Einschluss von Phosphor in den Oxydfilm verringert die dielektrische Konstante und vergrbssert die dielektrische Festigkeit. INTRODUCTION IN THE extensive

niobium

literature etc little attention

on anodic oxide films formed on aluminum, tantalum, has been directed to the role that the electrolyte plays in

* Manuscript received 26 April 1964. t Present address: Tektronix Inc., Portland 7, Oregon. 183

184

J. J. RANDALL,

JR., W. J. BERNARDand R. R. WILKINSON

determining the properties of the films that are formed. For the most part, it has been tacitly assumed that a more or less stoichiometric oxide results from the electrochemical reaction at the anode. This has been so widely accepted that in a number of papers authors have failed to describe or identify the electrolyte solution employed in their experiments. There are some notable exceptions to thisl-* where it has been shown that the composition of the films-and, in turn, their properties-are affected by the composition of the electrolyte. It is the purpose of the present paper to emphasize this fact and to demonstrate that certain chemical and physical properties of oxide films on tantalum and niobium are modified or controlled by the anodizing electrolyte. The actual presence of material from the electrolyte in anodic films has been established in several cases by radiotracer techniques: Plumb5 has determined that phosphorus (equivalent to 6% POd3-) is an essential part of anodic films grown on aluminum in a phosphate solution. The profile of the distribution of phosphorus was not determined, but a linear distribution was apparently assumed. A logarithmic distribution of sulphur in anodic films formed on tantalum and niobium in O-1 N and lOOo/o HsSO, has been reported by Draper.6 The amount of incorporated sulphur was small in 0.1 N solution (estimated to be about 0.002 per cent by weight of sulphur for a 40-V film) but was much greater in the 100 per cent acid. Bogoyavlenskii’ has shown that 14C and 6oCo can be introduced into oxide films on aluminum by the choice of the proper anodizing electrolyte; the elements were present in an anionic form in both cases. It has also been shown by spectrographic analysis that films formed on aluminum in borate-glycol electrolytes contain a significant amount of boron, and its presence apparently has a measurable effect on the chemical reactivity of the film.3 A more striking example of the effect of modified film composition on film properties is the case in which phosphorus is incorporated into aluminum oxide films formed in phosphate electrolytes. 2,4 Whereas normal amorphous aluminum oxide films are hydrated by boiling water at a very rapid rate, films that contain phosphorus are unattacked even for very long exposures under the same conditions. It has been observed in these laboratories that many of the properties of anodic films formed on tantalum and niobium in phosphoric acid electrolytes are different from those of films obtained in other commonly used anodizing solutions such as sulphuric acid, ammonium chloride, ammonium bromide and sodium sulphate. In view of these observations, a radiotracer investigation was undertaken of anodic film composition as a function of electrolyte concentration and other anodizing conditions. EXPERIMENTAL

TECHNIQUE

Tantalum metal, obtained from the Fansteel Metallurgical Corporation, was in the form of 0.5 mm sheet and had a nominal purity of 99.9 per cent. This sheet was cut into rectangular pieces of 12 cm2 area. As a preliminary cleaning, the pieces were degreased by successive treatments with trichloroethylene and a hot detergent solution. Tantalum wire was then welded to each specimen for the purpose of making electrical contact. The specimens were immersed for a short time in 8 N HNO, to ensure removal of any possible contamination from the copper welding electrodes. Chemical polishing was carried out for 15 s in a solution consisting of 24% HNO,, 18% HF and 58% H,SO, by volume (all acids were commercial concentrated reagents). After

Radiotracer study of anodic oxide films on tantalum and niobium a short immersion in boiling distilled water, each specimen was electropolished mixture of 90 parts concentrated H,SO, and 10 parts concentrated HF at 20 V 100 mA/cm2 for 20 min.* (A lo-min immersion in boiling concentrated HNO, used to remove sulphur, which deposits during electropolishing.) After a final

185

in a and was

boil and rinse in distilled water, the pieces were mounted on a tantalum bar, suspended in a tantalum can and heated in a vacuum furnace (at less than 10e4 torr) for 30 min at 2100°C. We have observed that this final vacuum-heating step markedly improves the behaviour of tantalum during anodizing. After vacuum purification, great care was taken to avoid contamination of samples in any way prior to oxidation. Niobium metal was approximately 99.9 per cent purity and was obtained from the Temescal Metallurgical Corporation. The specimens used were about 2 mm thick and had a total surface area of 13.6 cm 2; these were cleaned in the same manner as the tantalum. Carrier-free 32P tracer was obtained from Iso-Serve Corporation in the form of H,PO, in HCl. (As far as could be determined, the minute concentration of HCl had no noticeable effect on the oxidation rate or on the capacitance of the films.) A given quantity of tracer was added to solutions of different concentrations of H,PO, in sufficient amount to give counting rates of at least several hundred counts per minute on the Geiger-Miiller counter employed; this amounted to 0.2.5-5 mc per 100 ml of solution, depending on the H,PO, concentration. Since the half-life of 32P (a pure beta-emitter) is only 14.3 days, it was necessary to correct the measured counting rate for the isotopic decay. The concentrations of the H,PO, solutions used were 0.001, O-01, 0.1, 1.0 and 14.7 M (the last solution being the commercial concentrated reagent-85% H,PO,). Anodizations and capacitance measurements were carried out in glass cells with Teflon electrode supports and platinized platinum cathodes. Standard samples were prepared by neutralizing aliquots of the active H,PO, solutions with known volumes of NaOH (sufficient to form Na,HPO& and taking small aliquots of the resulting solution; this final aliquot was deposited on an unanodized specimen in very small droplets so that a uniform distribution of material was obtained. Under these conditions the thickness of solid remaining after evaporation is not large enough to require correction for self-absorption of the emitted radiation. The anodization and counting procedure was as follows: a given constant (to better than 1 per cent) current density was passed for a specified length of time; the sample was removed from the cell 2 min after cessation of oxidation, rinsed, treated with boiling water for 30 s, dried with acetone and counted on both sides. The sample was returned to the cell and capacitance was measured at 120 c/s using a standard capacitance bridge. The procedure was repeated as the film was formed to greater thicknesses. After the desired total charge had been passed the distribution of activity was measured by the stepwise dissolution of the film in an HF solution. For tantalum oxide films, a 14 wt-o/0 HF solution was used, while the concentration was decreased to 9 wt-y0 for niobium oxide because of its greater rate of solution. Capacitance during film stripping was measured in 40% H2SO4 for tantalum and in 0.1 y0 H,SO, for niobium; a more dilute solution was used in the latter case because of the greater sensitivity of the anodic films on niobium to strong chemical reagents.

J. J. RANDALL,JR., W. J. BERNARDand R. R. WILKINSON

186

In addition to the phosphate tracer experiments, a brief investigation was made of the incorporation of sulphur from 0.1 M H,S04 solution into anodic films on tantalum. The carrier-free material (in HCI) was also obtained from Iso-Serve. The methods used were essentially the same, except that it was necessary to use a larger quantity of tracer, due to the relatively low energy of the beta particle emitted from YS and the smaller amount of material that is incorporated with the sulphuric acid electrolytes. RESULTS

Comparison of anodization in H,PO, and H,S04 electrolytes Figure 1 shows unitary rates of oxide formation [l/i(dV/dt) expressed as Vs-lmA-km21 for the anodization of tantalum in 0.01 M solutions of H,PO, and H,SO, at constant current density. It is apparent that the rates are greater in the phosphoric

. 001 0 DOI

i “E 0.40

0.28

’

M H,PO, M H,SO,

’ 005 ’ ’ ’ ’ ’0.10 ’

I

Currentdensity,

I

I

mA/cm2

IllIll

0.5

I.0

FIG. 1. Unitary oxide formation rates for anodization of tantalum in 0.01 M H8P04 and 0.01 M H&SO4at 25°C.

acid solution at all current densities by a nearly constant factor. When anodes are oxidized to the same voltage in the two electrolytes at the same current density, measured capacitances are identical, but the interference colour of the specimen anodized in HaPO, indicates (if the same step-gauge is used for both samples) that a thinner film has been formed on this specimen compared to those formed in H,SO,. However, if the experiment is carried out so that equal charges are passed in each case, the results are the reverse of those just described, ie film colours are virtually identical for both specimens, but phosphate films give significantly lower capacitance (by 6-8 per cent). Weight measurements for experiments at I.0 mA/cma indicate quite clearly that the films formed in H,PO, are not pure Ta,O,, for the weight gains are approximately 6 per cent greater than would be expected if the production of oxide were the only

Radiotracer

study of anodic oxide films on tantalum and niobium

187

electrode process. In 0.01 M H,SO,, on the other hand, the measured weight gain corresponds very closely to the theoretical amount.* Anodization

in phosphoric

acid electrolytes

Prior to the radiotracer study of film composition, an investigation was made of the effect of temperature and electrolyte concentration on oxide formation rate and film capacitance for anodization in H,PO,. Table 1 shows that the rate of film formation increases only slightly with increasing acid concentration in the range 0.001 to 1-OM. The formation rate in 14.7 M TABLE 1

HJ’OI concentration M 14.7 EO 0.010 oGO1o 0.001 M + OGOl M HCl 0.1 M HISOl

Fihn formation rate at 1.0 mA/cme Vls 0.60 0.40 0.38 0.37 0.38 0.38 0.34

solution, however, is about 50 per cent greater than in the more dilute solutions. The rate for 0.1 M H,SO, is also included for purposes of comparison. Except for the 14.7 M solution,t all solutions would support film formation to at least 200 V, which was the highest applied voltage in the tracer experiments. The O$lOl M H,PO, + 0.001 M HCI solution is included because the tracer used contained approximately this amount of HCl. It is evident that this concentration of HCl has no effect on the formation rate. Figure 2 shows plots of reciprocal capacitance (l/C) UScharge passed (Q) for films formed at 1 mA/cm2 at 25°C on tantalum in 0.01 M H,PO, and on niobium in O*OOlM H,PO,. Experiments at temperatures other than 25°C showed that the slope of the l/C us Q plot is slightly dependent on the temperature at which the anodization is performed (all capacitances being measured at 25°C) and indicate, since the slope increases with temperature, that the current efficiency of oxide formation is slightly lower at 100 than at 25°C. For a given charge passage, l/C increases by 2 or 3 per cent as the phosphoric acid concentration is increased from 0.001 to 1-O M and is about 50 per cent higher for the 14.7 M solution. Figure 3 gives the applied voltage as a function of charge passed for the same conditions as Fig. 2 and for several different temperatures in the case of tantalum. Figure 4, which shows reciprocal capacitance as a function of applied voltage for films formed on both tantalum and niobium in 0.001 M H,PO,, reveals that reciprocal * This in itself is not conclusive evidence that material from the electrolyte is not included in these films, however. The complete absence of oxygen evolution has not been demonstrated and its occurrence would cause the measured weight increases to be too low. If this were offset by a commensurate increase due to the incorporation of electrolyte, the over-all weight gain would be just unlikely-that the equal to that predicted by the passage of charge. Hence, it is possible--although weight measurements do not give a true measure of the efficiency of oxide formation. t Maximum voltage for this electrolyte is about 95 V; this is achieved in about 150 s at a current density of 1.O mA/cme.

188

J. J. RANDALL, JR., W. J. BERNARDand R. R. WILKINSON

6-

0.2

03 Charge,

0.4 C/Cm’

FIG. 2. Reciprocal capacitance UScharge passed for anodization at 1.0 mA/cme and 25°C of tantalum in 0.01 M HsPOa and niobium in 0901 M H,PO,. capacitance for any particular voltage is about 20 per cent lower for niobium oxide than for tantalum oxide. This is in good agreement with earlier work.9 For the anodization of tantalum at 25°C at a current density of 1-OmA/cm2, for a given charge passage, the product of capacitance and applied voltage was very nearly 240 A n

200

0 l

Tantalum Tantalum Tantalum Tantalum

0°C 25°C 50°C IOO”C

> $- 160 e 2 0 120 al a 2 60

Charge,

C/cm’

FIG. 3. Applied

voltage us charge passed at 1 mA/cm* for anodization of tantalum in O,Ol M H,P04 at various temperatures and of niobium in 0.001 M H,PO, at 25°C.

of H,PO, and in O-1 M H,SO, as well. The significance of this observation will be discussed in a later section. the same for films formed in all concentrations

Tracer experiments

Figure 5 summarizes the data obtained for the stepwise formation of anodic films on tantalum in radioactive solutions of various concentrations of H,PO,. Figure 6

Radiotracer

189

study of anodic oxide films on tantalum and niobium

160 -

Niobium

l

,60_

0 Tontalum

140 -

2

4

Reciprocal FIG.

4.

6

6

I 12

lo

capacitance,

1

cm*/,uF

Applied voltage US reciprocal capacitance for anodization of tantalum niobium in 0.001 M H,PO, at 1.0 mA/cm* at 25°C.

I.6

I.4 -

. 14.7

l

I.0 M H,PO,

. 0.01

“E

H,PO,

A

0 0.1 I.2 -

and

M

M

o 0.001

2

H,PO, M HPO, M

.

/

H,PO,

4

6

Reciprocal

8

copocitonce,

IO

12

I4

cm2+F

FIG. 5. Weight of phosphorus in films formed on tantalum at 1.0 rnA/cm’ in various concentrations of H,PO, at 25°C us reciprocal capacitance.

190

J. J. RANDALL, JR., W. J. BERNARDand R. R. WILKNQN

0

2.0

I 8.0 00 120 4.0 6.0 Reciprocal capacitance, cm*/pF

FIG. 6. Weight of sulphur in tantalum oxide 6hns ILSreciprocal capacitance for anodization in 0.1 M H&SO4at 1.0 mA/cm’ at 25°C.

results for tantalum in O-1 M HaSO4 and Fig. 7 those for niobium in 0401 M HaPOd. If reciprocal capacitance is to be used as a measure of film thickness, then the plots are not directly comparable without a knowledge of the relative dielectric constants of films formed in the different solutions. This will be discussed below. It is clear that the solution composition can markedly influence the composition of the film. shows

0

FIG. 7. Weight of phosphorus in niobium oxide films formed at 1.0 mA/cmS in 0901 M HSP04 at 25T us reciprocal capacitance.

Figure 8 illustrates the dependence, at 25”C, of the amount of phosphorus included in the film on the current density at which the film is formed, and Fig. 9 shows the temperature effect at 1-O mA/cm2; both experiments were performed in 0.01 M H,PO,. The amount of phosphorus is seen to be strongly dependent on the anodizing conditions and increases with either increasing current density or decreasing temperature. Only limited data are presented for the current-density plot because of the

Radiotracer

032 -

study of anodic oxide films on tantalum and niobium

0

I.0 mA/cm'

l

04 mA/cm”

o 0.01

191

mA/cm2

0.26--

- 0.20s 2 % e a B

0.16-

012-

E s? 3

0

i-0

20

Reciprocal

3.0

copocitonce,

4.0

cm’/$F

50

.

FIG. 8. Weight of phosphorus in tantalum oxide fihns formed in 0.01 M H,PO, various current densities at 25°C.

0.9

n

OT

0 259: 08

at

.50-z 0 IOOT

r

0

2

4

Reciprocal

6

8

copocitonce,

'O

_1

cm’@

FIG. 9. Weight of phosphorus in tantalum oxide fihns formed in 0.01 M HIPOl at various temperatures at 1-OmA/cm*.

192

J. J.

JR., W. J. BERNARDand R. R.

RANDALL,

WILKINSON

inordinately long anodizing times required to form films at very low fields; one experiment which was carried out for 13 h (not plotted) showed that the slope of the activity vs l/C plot is constant to very long times. In Figs. 5-9 it is apparent that the curves do not pass through the origin, but indicate that a significant amount of phosphorus is present at zero film thickness. Immersion in the radioactive electrolyte of either a non-anodized specimen or a sample which had been anodized in a non-active solution caused the activity of the 0.9

0.01M H,PO, I.0 l 0 25°C H2S0q l 0°C o lOO*C

0.8

0.1

I.0

mA/cmZ M

mA/cm2

-0.06

"E \o "i

-r305

; 'a - 0.04 2 b -0.03

5 s

-0.02

-

/ 0

L

I 01

I

I

I

0.2

0.3

0.4

.‘.,

0:6 0.5 c,/c

I

I

0.7

I

0.9

09

043

' -0 I.0

Typical plots of weight of phosphorus and sulphur in anodic oxide films on tantalum us reciprocal capacitance during dissolution of the film in HF solution.

FIG. 10.

sample to increase by an amount approximately equivalent to the indicated weight of phosphorus. In view of this it is believed that the amount of material indicated on the figures is adsorbed on to the metal when it is first introduced into the solution and that it remains at the oxide solution interface during the course of the anodization. Anodizations were also performed in 0.01 M solutions of both NaH,PO, and Na,PO,. The amount of phosphorus in the films formed in NaH,PO, was virtually the same as for the 0.01 M acid, but was about 50 per cent less when Na3P0, was used. Film-dissolution

experiments

Figure 10 shows typical plots of the weight of the incorporated species in the film as a function of reciprocal capacitance of the partially dissolved layer. Plots of this type were usually satisfactorily linear, although some slight deviation occasionally occurred as the measured activity approached zero. This was observed as a reduction in the rate at which the activity decreased as the film was dissolved and was accompanied by a mottling of the interference colours, indicating non-uniform dissolution at some sites.

Radiotracer

study of anodic oxide films on tantalum and niobium

193

In all cases the activity of the specimens reached zero before the entire film had been dissolved, indicating that the incorporated species are contained only in the outer portion of the film. When capacitance is used to measure film thickness, then C,,/C at the point where the weight of phosphorus reaches zero is a measure of the fraction of the film which is free of incorporated material. The measured C,,/C values under various film formation conditions are listed in Table 2. In cases where some slight rounding of the plots was observed, an extrapolated value was used. C,,/C tends to decrease with increasing concentration, increasing current density, decreasing temperature and, to a slight extent, with decreasing film thickness. TABLE 2

Charge passed C/cm2

Current density mA/cm*

0.001

0.50 0.50 0.25

1.0 0.01 0.01

25 25 25

0.49 0.59 0.58

0.01

0.50 0.25 0.10 0.50

1.0 1.0 1.0 0.01

25 25 25 25

0.49 0.48 0.47 0.59

0.50 0.50 0.50

1.0 1.0 1.0

0

50 100

0.42 0.50 0.59

0.50 0.50

1.0 1.0

25 25

0.44 0.42

0.10 0.10 0.10 0.50 0.50 0.50 0.50

5.0 1.0 0.01 1.0 1.0 1.0 1.0

25 25 25 25 25 25 25

0.18 0.21 0.39 0.52 0.49 0.49 0.47

HJ’Q concentration M

0.10

1.0 14.1

0.1 M H&S04 0.01 M NaH,PO, 0.01 M Na,PO, Nh-O.001 M HsPO4

Temperature “C

If the presence of phosphorus in the outer layer affects the dielectric constant of that portion of the film, then C,/C is not an accurate measure of the fraction of the film which consists of stoichiometric Ta,O,. Comparison in Table 3 of reciprocal capacitances at the exact point during film dissolution at which the activity of a specimen reaches zero shows that the effect of acid concentration on the thickness of that portion of the film that is phosphorus-free is less pronounced than would be expected from the C,,/C values in Table 2. By the use of these reciprocal capacitances (at zero activity), new estimates were obtained for the fractions of the various films which are free of phosphorus. These values, designated X in the table, are calculated with reference to the O-001 M solution, for which the observed C-,/C is, as a first approximation taken to be equal to X. Using the more accurate estimates (X-values) of the location of the boundary between the layers, the concentration of phosphorus in the portion of the film in which it is contained has been calculated for the passage of O-5 C/cm2 in each of the 6

J. J. RANDALL, JR., W. J. BERNARDand R. R. WILKINSON

194

TABLE3

(l/c)*

&PO, concentration M 14.7 I.0 0.10 0.010 0~0010

GIG

at zero activity, crn”/,uF

X

0.21 0.42 0.44 0.49 0.49

4.2t 5.5 5.8 5.6 5.9

0.35 0.46 0.48 0.47 0.49

0.52

5.9

0.52

0.1 M H$O,

* Anodizations performed at 1.0 mA/cm2 for 500 s, except for 14.7 M. t This value is obtained by extrapolating data for a 100-s anodization, since the 14.7 M solution would not support oxide formation for 500 s at this current density.

H,PO, electrolytes. The results are shown in Fig. 11; the concentration in the film has been expressed as moles of phosphorus per mole of Ta5f, the latter quantity being obtained by multiplying (1 - X) by the total number of Ta5+ ions produced by the charge passed. (It is again assumed that the electrode oxidation process produces only-Ta5+.)

_ 0001

,

, , II,l,l 001

,

, , ,,I111 0.1 H,PO.,

,

( 1 Il,,ll

concenlrotton,

I.0

,

, , ,,I,,1 IO0

M

FIG. 11. Phosphorus

Anodizations

concentration in the outer layer of anodic films on tantalum vs concentration of H,POI. carried out for 500 s at 1.0 mA/cmB (except for the 14.7 M solution, for which data from a 100-s anodization was used).

A number of two-step anodizations were performed in an attempt to determine whether the concentration and distribution of the phosphorus in the outer portion of a film is affected by anodization in a second solution of different H,PO, concentration. Figures 12(a) and (b) show dissolution curves for tantalum oxide films formed in these experiments. The anodizing conditions were chosen so that variations in C,,/C and in the concentration of phosphorus in the film would be as wide as possible:* * Use of the 14.7 M solution would have increased the variation, but in view of the fact that the anodization behaviour of this electrolyte was found to be different from that of the more dilute solutions, its use was purposely avoided.

Radiotracer

study of anodic oxide fihns on tantalum and niobium

FIG. 12(a). Activity us C,/C during film dissolution in HF. first at 0.01 mA/cm* in 0.01 M H,PO,, then at 1.0 mA/cm* in 1-Ohi

l Anodized

H,PO,. 0 Anodized first at 1.0 mA/cm* in 1.0 M H,P04, then at 0.01 mA/cma in 0.01 M H.POd. 0.25 Cmwas passed during each separate anodization.

3ooo5

FIG. 12(b). Activity us C,/C during film dissolution in HF. 0 0.25 C passed at 0.01 mA/cm* in 0.01 M H,PO,. 0 O-25 C passed at 1.0 mA/cmS in I.0 M H,PO,.

195

196

J. J. RANDALL, JR., W. J. BERNARD

and R. R. WILKINSON

TABLE 4

Condition I: 0.25 C/cma at 0.01 mA/cm2-OOl M H9P04 Condition II: 0.25 C/cm2 at 1.0 mA/cm*-1.0 M H,PO, dl Condition Condition Condition Condition

ds

0.59 0.41 0.51 0.48

I alone II alone I followed by II II followed by I

0.74 0.80

a sample was first anodized at a low current density in a solution of low H,PO, concentration and then at a higher current density in a more concentrated solution. The procedure was reversed for the next sample. Table 4 gives the actual anodizing conditions for these experiments. The 4 values in the table represent C,/C at the point during film dissolution at which the activity reaches zero, and d2 represents C,,/C at the point at which a pronounced change in the slope of the curve occurs. Each section of the curves in Fig. 12(a) corresponds to one of the curves in Fig.

0

I 5

I

IO

I

15

I

I

20 Time.

25

I

30

I 35

mn

FIG. 13. C,/C versus time of immersion

in HF solution for a film formed on tantalum in 14.7 M HsPOI at 1.0 mA/cm2 at 25°C.

12(b). Thus, information can be derived from Fig. 12(a) about the nature and position of the various layers formed during the two-step anodizations. The implications of these observations are discussed below. In addition to the markedly higher reciprocal capacitances and voltages (per unit charge) for anodization in 14.7 M H,PO,, the films formed in this solution were also observed to dissolve at a rate that was faster during the first stage of film dissolution

Radiotracer

study of anodic oxide films on tantalum and niobium

197

than that of films formed in dilute solution; during the second stage the rates for films formed in solutions of all concentrations were about the same. Figure 13 illustrates this for films formed in the concentrated solution. The change in slope of the dissolution curve occurs near the location of the boundary between the phosphorus-containing and phosphorus-free layers. Films formed in the more dilute phosphate electrolytes show some unpredictable variation in the rates of dissolution of the outer layers, but the inner portions also dissolve at the same rate as the corresponding portions of films formed in concentrated H,PO,. As expected, films formed in O-1 M H,SO,, which contain a relatively small amount of incorporated sulphur, dissolve at approximately the same rate as the inner portion of those formed in phosphate electrolytes. The results shown in Fig. 13 are at variance with those of Vermilyea,l who found that the outer portion of films formed in 14.7 M H,PO, dissolved more slowly than the inner part. However, the change of slope in Vermilyea’s experiment occurred at approximately the same point in the film as is reported here. DISCUSSION

AND

CONCLUSIONS

Mechanism ofjZm growth It is apparent from the experimental observations that anodic films formed on tantalum and niobium in phosphoric and sulphuric acid electrolytes consist, under a wide variety of conditions, of two distinct layers. The material incorporated from the electrolyte is contained in the outermost layer (that which was in contact with the electrolyte during film formation) and is distributed uniformly within this layer, while the innermost portion of the film consists, as nearly as can be determined, of stoichiometric Ta,O,. Therefore, the linear relationship between the amount of incorporated material and film thickness (Figs. 5-9) does not imply, as has often been assumed, that the entire film contains the material from the electrolyte but, rather, indicates that the mechanism of formation of the film does not change as its thickness increases. Although Vermilyeal has shown by measuring rates of film dissolution that films formed on tantalum in non-aqueous electrolytes consist of two separate layers, the work reported in this paper appears to be the first demonstration of the existence of duplex layers of differing chemical composition for anodic oxide films formed in aqueous solutions. It should be pointed out that the results obtained here in sulphuric acid are in direct contradiction to those of Draper,s who found radioactive sulphur to be distributed throughout the entire film. Considerable controversy has arisen in recent years as to the nature of the mobile ionic species during the electrochemical formation of anodic oxide films. Most of the pertinent literature references have been summarized in a paper by Bernard4 and several more recent investigations have been reported.lO-l2 Observations have generally supported metal-ion motion as the predominant mode, and many investigators have assumed that this mechanism is the most likely because of the small size of the metal ions relative to that of the oxide ion. Notable exceptions are the investigations of Davies et al,11,12in which radioactive rare gas atoms originally injected into tantalum were located, after anodic oxidation, near the middle of the subsequently formed films, implying that metal-ion and oxide-ion motion are equally involved in the oxide growth in this instance. In the case of aluminum, a larger fraction of the film was found to grow by the movement of oxide ions.

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Provided that the radioactive species which is incorporated into the films in the experiments described in this present paper is immobile during and after film formation, its distribution can be used as a means of determining the mechanism of formation of the film. Regardless of the type of ionic motion involved (interstitial or substitutional), this method gives information on the nature of the mobile species, ie metal or oxygen. In the case of 100 per cent cation motion, one would expect the entire film to contain the tracer material, while 100 per cent anion motion would result in zero incorporation. Thus, assuming immobility of the marker, the results obtained here can be explained only on the basis of ionic motion of both species.

Film first

structure after anodization

Film

structure after second anodization

FIG. 14. Spatial relationship of anodic layers resulting from the successive formation of anodic films on tantalum and niobium under two different sets of conditions.

That the incorporated phosphorus is indeed immobile is demonstrated by the results of the two-step anodizations. The structure of these films, as deduced from the film dissolution plots of Fig. 12, is shown in Fig. 14. The entire film is seen to consist of three distinct layers: the layer in contact with the metal substrate is composed of (presumably) stoichiometric oxide, and the outer two layers contain phosphorus in different concentrations. The concentration of phosphorus in the outermost layer (A) is characteristic of that resulting from the use of the second anodizing solution, regardless of the order in which the solutions are used. This outside layer (A) must be formed by tantalum transport through the entire first-formed film; in the process, neither the distribution nor concentration of phosphorus in layer (B) is affected. At the same time, the thickness of the phosphorus-free layer increases during the second anodization by oxygen transport inward, also without disturbing the original phosphorus-containing layer. In addition, it can be seen from Table 4 that d1 for the complex films is simply the average of the dr’s for the duplex films. These observations can only be accounted for by the presence of the phosphorus as an immobile marker. Were the phosphorus mobile to any extent, some mixing of the two outermost layers would be expected, and the final thickness of the stoichiometric oxide adjacent to the metal would not be expected to be simply the sum of the thicknesses from two anodizations on separate specimens. As far as the mechanism of oxide film formation on tantalum and niobium is concerned, these results clearly demonstrate that in aqueous solutions growth occurs with a nearly equal contribution from metal-ion and oxide-ion movement. However, it is probably not valid to generalize from these experiments that the same is true for anodic oxide growth on all metals under all conditions. It is more likely, in view of other papers on this subject, that the type of ion motion that predominates is a function

Radiotracer

study of anodic oxide films on tantalum and niobium

199

of both the metal and the nature of the electrolyte.

Even in the case of tantalum it is aqueous solution, such as 14.7 M H,PO,, the mechanism may be markedly different from that in dilute aqueous solution. In this particular instance cation movement predominates, as shown by the position of the boundary between the phosphorus-free and the phosphorus-containing layers. In view of the fact that weight increases observed for films formed on tantalum in H,PO, are greater than would be expected for an electrode process in which the production of pure Ta,O, is the only electrochemical reaction, it is our belief that either the formation of a tantalum phosphate (in addition to the oxide) or, less likely, physical trapping of HaPOd in the film is the most probable manner of incorporation. The fact that incorporation of material occurs even in Na,PO, solution (pH N 12), although in smaller amount, tends to discount H,PO, trapping as a possibility, for the concentration of undissociated acid in this solution is extremely small. Since the introduction of any species from the electrolyte into the film must occur at the oxide-solution interface during the formation of the film, the increased amount of included phosphorus in films formed in the more concentrated solutions is probably due simply to an increase in the available number of the appropriate species at the interface. The results obtained on neutralization can be interpreted in this manner, since neutralization increases the concentration of available hydroxyl ions relative to phosphate, and hence the latter plays a less significant role in the electrode process. It should be pointed out that the total amount of phosphorus, as opposed to the concentration, can be affected by the position of the boundary between the phosphorusfree and the phosphorus-containing portions of the film. For films formed at the same current density in solutions of various concentrations, except in the 14.7 M solution, the position of the boundary is nearly the same in all cases. However, for films formed in a solution of a given concentration at different current densities it was shown by film dissolution experiments that part of the increase in the total amount of phosphorus with increasing current density is due to a movement of the boundary inward (towards the metal). In addition, the total amount of phosphorus is also increased at the higher current density because of an increase in the phosphorus concentration in the outer layer of the film (shown by the fact that the rate of loss of activity is 20-25 per cent higher for films formed at I.0 mA/cm2 than for those formed at 0.01 mA/cm2). In contrast to the effects of electrolyte concentration and current density, the anodization temperature had no measurable effect on the concentration of phosphorus in the film; the slopes of the dissolution plots (activity, or weight of phosphorus, US l/C) are very nearly identical for films formed at 0, 25, 50 and 100°C (Fig. 10). Hence, the increase of phosphorus with decreasing temperature is entirely due to movement of the boundary inward at the lower temperatures. The effect of temperature is undoubtedly associated with the current-density effect, however, for the field required to pass a particular current density is considerably higher at 0 than at 100°C (by about 30 per cent, from Fig. 3).

apparent that when oxidation is carried out in a very concentrated

Eflect of composition on $lm properties If it is assumed that the anodization process in all solutions is 100 per cent efficient in the production of Ta5+, ie that no oxygen evolution occurs at 1-OmA/cm2 at 25°C

200

J. J. RANDALL,

JR., W. J. BERNARDand R. R. WILKINSON

(none was observed), then the higher reciprocal capacitances observed for films formed in phosphoric acid as compared with those formed in sulphuric acid when the same charge is passed in each case must be attributed either to greater film thicknesses in the case of the phosphate anodizations or to lower dielectric constants. (This follows from C = kc/d where E is the dielectric constant, d the film thickness, and k is a constant.) The difference in reciprocal capacitance, which amounts to 8 per cent in the case of 0.01 M solutions at I.0 mA/cm2, cannot be accounted for by a greater film thickness per coulomb that results from phosphorus incorporation; even if phosphorus were present as H,PO, it would amount to only 1 per cent of the total film weight. Even larger capacitance differences exist between films formed in dilute TABLE 5

Dielectric constant SiOz AL08 T&Q NbZO,

3.8 8.4 28 42

Dielectric strength, V/cm 2.6 9.1 6.7 4.8

x x x x

10’ 108 lOa 106

Reference 14 15 13 16

HaPOd and in the 14.7 M acid. In this case A(l/C)/AQ is greater in the concentrated acid by 50 per cent, yet the amount of incorporated material could amount, at most, to only 5 per cent of the film weight. Hence, it is apparent that incorporation of phosphorus into tantalum oxide significantly lowers the dielectric constant and, furthermore, that differences in film thickness per unit charge passed in the various solutions can account for only a negligible fraction of the observed differences in capacitance. We calculate that in the most extreme case (in the 14.7 M solution) the dielectric constant of the outer portion of the film is reduced from 27.6, the value found for Ta20,,13 to about 21. .Since anodization in all electrolytes to the same voltage yields nearly the same capacitance, an inverse relationship must exist between dielectric constant and dielectric strength; consequently, CV is roughly the same for all solutions. As a result of these compensating effects, any differences in capacitance for films formed to the same voltage in different solutions are small and are observed only when the amounts of incorporated material are widely different. (Films formed to 200 V in I.0 and O-001 M H,PO, actually do show slight differences in capacitance.) The higher dielectric strength of phosphorus-containing films accounts for the faster oxide formation rate observed in phosphoric acid electrolytes, and the lower dielectric constant accounts for the lower capacitance (for the same charge passage). In Table 5 dielectric constants and dielectric strengths are listed for a number of different anodic oxides. It can be seen that the dielectric strength does, in fact, increase with decreasing dielectric constant. Draper r’ has recently reported that the dielectric constants of films formed in 100% H,SO, are the same as for those formed in O-1 N solution, and that the higher A(l/C)/AQ in the 100 per cent acid-by a factor of five-is due to an increased amount of material laid down per coulomb. This conclusion probably results from a misinterpretation of Vermilyea’s report of 210 per cent current efficiency for the formation of films in 86% H,SO, (based on weight gain measurements). From Draper’s

Radiotracer study of anodic oxide films on tantalum and niobium

201

previously published results6 it can be estimated that the sulphur incorporated into films formed inlOO% HaSO, constitutes only about 11 per cent of the total film weight (assuming, as he does, that H,S04 is incorporated). This additional weight would bring about a film thickness increase of only 50 per cent over that calculated for the production of pure TaaOa, and thus cannot entirely explain the five-fold increase in

A(l/'J/AQauthors would like to express their appreciation to Mr Stanley Szpak for his assistance in all aspects of the experimental work and to Dr Thomas Burgess for his help in making the radiotracer measurements. REFERENCES 1. D. A. VERMILYEA, Actu Met. 2,482 (1954). 2. M. S. HUNTER,P. F. TOWNERand D. L. ROBINSON,TechnicalProceedings American Electroplaters’ Society, 46th Annual Conference (1959). 3. W. J. BERNARDand J. J. RANDALL,JR., J. Electrochem. Sot. 108, 822 (1961). 4. W. J. BERNARD,J. Electrochem. Sot. 109,1082 (1962). Acknowledgements-The

5. R. C. PLUMB,J. Electrochem. Sot. 105,498 (1958). 6. P. H. G. DRAPER,Acta Met. 11, 1061 (1963). 7. A. 8. G.

F. B~G~YAVLENSKIIand G. N. DOBROTVORSKII, Zh. Prikl. Z&m. 35, 1557 (1962). W. WENSCH,K. B. BRUCKHART and M. CONNOLY,Met. Progr. 61,81 (1952).

9. L. YOUNG, Cunud. J. Chem. 38,114l (1960). G. AMSELand D. SAMUEL,J. Phys. Chem. Solids 23, 1707 (1962). J. A. DAVIES.J. P. S. PRINGLE.R. L. GRAHAMand F. BROWN,J. Electrochem. Sot. 109,999 (1962). , . , J. A. DA& and B. DOMEIJ,j. Electrochem. Sot. 110, 849 (1963). L. YOUNG, Proc. Roy. Sot. A244,41 (1958). P. F. SCHMIDTand W. MICHEL,J. Electrochem. Sot. 104, 230 (1957). W. J. BERNARDand J. W. COOK,J. Electrochem. Sot. 106,643 (1959). 16. L. YOUNG, Trans. Faraday Sot. 52,502 (1956). 17. P. H. G. DRAPER,Electrochim. Actu 8, 847 (1963).

10. 11. 12. 13. 14. 15.