The transport and deposition reactions involved in the production of zirconium coatings from mixed iodide vapours

Journal of the Less-Common Metals Elsevier Sequoia S.A., Lausanne - Printed in

THE

TRANSPORT

PRODUCTION

AND

DEPOSITION

OF ZIRCONIUM

53

The Netherlands

COATINGS

mfmroxs

INVOLVED

FROM MIXED

IODIDE

rhT THE VAPOURS

F. Ii. SALE Metnllurgy lhpuvtment, (Received

Univcvsity of Sheffield, Shzffiiald, Yorks. {Gt. Britain)

May x3&, rg6g)

SUMMARY

The transport and deposition mechanisms involved in the production of zirconium coatings by a flow iodide process have been considered with reference to the results of studies on the thermodynamic properties of zirconium tetraiodide, triiodide and diiodide. The possible sponge bed reactions are analysed so that the components of the mixed-iodide vapour stream can be identified, and the concomitant disproportionation of zirconium diiodide, and thermal decomposition of tetraiodide, is proposed as the mechanism by which the low-temperature deposition of zirconium takes place.

INTRODUCTION

In recent years, considerable interest has been shown in the vapour-phase production of zirconium coatings by an adaptation of the Van Arkel iodide process of refining zirconiuml. This entails the conversion of a static, sealed-bulb system into a dynamic flow system. The former process requires the continuous fo~ation and decomposition of zirconium tetraiodide within the sealed apparatus, whilst the latter requires the passage of zirconium tetraiodide vapour over a heated target using either a carrier gas or mechanical pumping. The main difference between the two systems lies in the methods of removal of the gaseous product of the decomposition reaction, In the sealed system, the product is removed by reaction with impure sponge zirconium, whilst in the flow system the product is swept away from the reaction site. Such a flow process has been describedz, and zirconium deposition was accomplished at temperatures as low as Q50°-900aC, although the mechanism of the metal transport and deposition was not fully understood. The conclusions of this study were that the lower iodides of zirconium were involved in the transport process, but no definite mechanism could be described because of the lack of published information on these compounds. The discussion of this paper3 showed there was a considerable need for fundamental work on the equilibrium conditions of the zirconium iodides. A study has been made, therefore, of the preparation and disproportionation J. Less-&ommo?z Metals, rg (1969) 53-62

F. R. SALE

54

reactions of zirconium triiodide and diiodide, in order that the mechanism conium deposition in the flow-plating process can be described fully. THERMODYNAMIC

PROPERTIES

OF ZIRCONIUM

The iodides of zirconium 3Zr14(gf + zZr13(sj

Zm

of zir-

IODIDES

possess the following

chemical

interconnections:

=qZrI3w

(11

=Zr12(s)+ZrIs(,)

(2)

2Zr12cS)=Zr14(,)+Zrcs)

(3)

so that the equilibrium ZriS) +2&j

=ZrI4kt

(4)

is very much an oversimplification of the reactions which are likely to exist in a flow-plating process. Consequently, an investigation of each of the above reactions Was carried out to determine the experimental conditions under which each could be expected to occur. The ~formation obtained from these studies can conveniently be divided in two sections; one dealing with the stability of zirconium tetraiodide, the other dealing with the lower iodides of zirconium.

From the pub~shed values of the standard enthalpy and entropy changes at 2g8”K for the formation4 and the vapourisations of solid zirconium tetraiodide, given in Table I, an approximate free-energy/temperature relationship of AGO*= -116,500 1-31.0 T cal can be obtained for reaction (4) using the approximated form of the Gibbs-Kelmholtz equation : AG% = AZP,,, - TAS*z9s The use of the standard values of enthalpy and entropy changes at 298°K in this manner assumes that any variation of CP with temperature will be small, so that negligible changes in the corresponding enthalpy and entropy occur. TABLE

I

STANDARD

ENTHALPIES

Reaction

ENTROPIES

OF FORMATION

AHOass

Zh3 + 21~ ZfIe(st

AND

=

= Z&i,)

AS0298

(kcal)

(call”C)

- 145-7

-73.0

zz&t

29.2

42.0

OF ZIRCONIUM

TETRAIODIDE

AT

2g8”x

Ref.

4 5

Reaction (4), involving molecular iodine vapour, will occur at the zirconium sponge-bed temperatures of approximately 450°C described in the flow-plating process2, but the zirconium deposition reaction, at temperatures of the order of g5o”C, will involve the production of atomic iodine, as gaseous iodine dissociates more readily than any of the other halogens 6. Figure I shows the relationship‘between the percentage of molecular iodine and total pressure at various temperatures, J. Less-Comvto~ Metals, xp (rg6g)

53-62

PRODUCTION OF ZIRCONIUM COATINGS

55

.e60m 80.

E

600°C

‘5

5

a

A! 040

E

800°C

1ooo”c

14

1200°C

8 E 20

P c

1000

100

Pressure(mm

Fig. I. Relationship tures.

IO

1.0

0.1

Hg)

between percentage

molecular iodine and total pressure at various tempera-

calculated from the data of BREWERY. From this it can be seen that all the iodine vapour will be dissociated into atomic iodine at 950°C and a total pressure of I mmHg. The decomposition of zirconium tetraiodide under such conditions must be represented, therefore, by the equation :

and the thermochemical data for AH0298= 72.2 kcal AS0298=48.3 cal/“C, must be included in the estimation of the free-energy/temperature this reaction. This yields : AGOT=188,700-79.3

relationship for

T cal for reaction (5).

This dissociation of the molecular iodine is an important factor in the thermal decomposition of zirconium tetraiodide because it provides a large increase in the number of gaseous molecules produced in the reaction and, consequently, the entropy change has a large positive value. This means that, although the heat of formation of the tetraiodide is a fairly large, negative value, the free energy of decomposition decreases rapidly with increasing temperature. As the equilibrium constant for reaction (5) contains the fourth power of the iodine pressure and only the first power of the tetraiodide pressure, the thermal decomposition will be markedly pressure sensitive. By inserting the thermodynamic data estimated for reaction (4) into the relationship : AG% = - RT In Kp, the equilibrium constant for the reaction can be calculated at the zirconium sponge bed temperature used in the plating process. At 723”K, the value of K, is of the order of 1030 and, consequently, there are no thermodynamic restrictions on the production of zirconium tetraiodide at the sponge bed. As the equilibrium vapour pressure of gaseous zirconium tetraiodide produced according to reaction (4), and the saturated J. Less-Common

Metals,

19 (1969)

53-62

F. R. SALE

56

equilibrium vapour pressure of solid zirconium tetraiodides, at the sponge bed temperatures, are both far greater than the actual pressures in the plating system, it is apparent that the rate of production of zirconium tetraiodide vapour is controlled by the flow rate of iodine in the system, or some other kinetic limitation. If the thermodynamic data for the thermal decomposition of zirconium tetraiodide according to reaction (5) are considered, the equilibrium decomposition temperature can be calculated to be 25oo”K, if all the reactants are in their standard states. (Pure zirconium, activity equals unity, and one atmosphere pressure standard state for all gaseous species.) From the difference in the numbers of gaseous molecules involved in the different sides of the decomposition equilibrium, it can be seen that reduction in pressure of the system would cause a lowering in the decomposition temperature. The absolute effect of the pressure change can be calculated for various partial pressures of the gaseous species. ZrcS) +4I(,)

=Zr14tg)

AGr = AG% + RT In Kp, therefore AGT = AG% + RT In Pzrr4 - qRT In PI for non-standard gaseous states. Figure 2 shows the effect of reductions in the pressure of the gaseous components on the equilibrium decomposition temperature, which is defined by the intercept of the free-energy OS. temperature line and the temperature axis. Even at pressures of 10-3 atm for the gases, the equilibrium decomposition temperature is still approximately 1600“K. Zirconium deposition should not be appreciable, thereTemperature CK) 2000 1500

2500

0 0

00

I

i Fig. 2. Relationship between free energy of formation for various pressures of gaseous species. Fig. 3. Furnace and U-tube arrangement conium tetraiodide and zirconium sheet. J. Less-Common

Metals, Ig (1969) 53-62

of zirconium

used to investigate

tetraiodide

and temperature

the reaction between gaseous zir-

PRODUCTION

OF ZIRCONIUM

COATINGS

57

fore, at target temperatures below IOOOT; a view which is reinforced by experimental work on the normal iodide refining cells?g. The lower iodides of zirconium As outlined in reactions (I), (2) and (3), the three iodides of zirconium possess interesting chemical interconnections by various disproportionation equilibria. A substantial quantity of experimental work has been performed to elucidate the effect of the lower iodides in the iodide refining processlo-12, but no quantitative measurements have been made upon these disproportionation reactions. Some qualitative measurements have been made on the disproportionation of zirconium triiodideia~la, but reported values of the disproportionation temperature differ by 140°C. An initial series of experiments was conducted to investigate the reaction of zirconium (in either sheet or sponge form) with zirconium tetraiodide vapour at various temperatures and pressures. It was anticipated that the results obtained would give an insight into possible reactions in the sponge bed of the plating system, and also provide a method of preparing pure zirconium triiodide. Zirconium tetraiodide, prepared by the reaction of hafnium-free zirconium sponge with iodine vapour in an otherwise evacuated glass reaction tubes, was sublimed from one limb of a U-tube, over zirconium sheet, into a small reservoir (as shown in Fig. 3)) to determine the conditions under which reaction (I) might be expected to occur. ZrcS)+ 3Zr14cg)=@k13cs).

(1)

At temperatures below approximately 3oo”C, and pressures of tetraiodide ranging from 10-4 to I atm, a black-brown surface film of triiodide formed on the zirconium metal. At temperatures above 48o”C, and the entire range of zirconium tetraiodide pressures, the surface film was analysed to be the diiodide. A metallic film of zirconium was also plated on to the walls of the reaction vessel, implying that the diiodide had volatilised and then disproportionated at the hotter capsule walls. At temperatures between 300°C and 48o”C, zirconium triiodide was formed when sufficiently high pressures of tetraiodide were present (of the order of 0.5 to I atm), whilst diiodide layers were produced at lower pressures. This behaviour can be explained by a consideration of reaction (2) : zZr13(s)=ZrIz(s) +ZrI4(,).

In this reaction, zirconium triiodide will be stabilised by high pressures of zirconium tetraiodide, because the reaction is pressure sensitive. AGr = AGOr+ RT In Pzrl, -

for non-standard state ZrI4.

These experiments led to a method of preparing centigram quantities of triiodide by the reaction of liquid tetraiodide, under pressure, with zirconium sheet at 510°C. The diiodide was produced by the disproportionation of the triiodide at 400°C under vacuum. The detailed preparations, and structure characterisations by an X-ray diffraction technique, of zirconium triiodide and diiodide have been described elsewhereid. The disproportionation equilibria of the lower iodides, according to reactions (2) and (3), were studied using a continuous, gravimetric Knudsen-cell technique to J. Less-Common

Metals,

19 (1969)

53-62

58

F. R. SALE

measure the equilibrium vapour pressures of zirconium tetraiodide. The triiodide was shown to disproportionate aiu an intermediate phase over the temperature range ~275~-4oo”C,without any evidence of sublimation: (a) SZrIa(s)= 6ZrIa *Zr&) + Zr&) logpm,H,(Zr14)=

-8700/T+1247,

275”-325”c.

AH=39.9+2

(b) 6ZrIs *ZrIz(s) = 4Zrb(,) + $ZrIa(g)

kcal.

350°-400”c.

For this reaction the total ZrI4 pressure is:

The experimental method and the significance of this intermediate phase have been outlined in an earlier reportls. The &iodide was shown to sublime simultaneously and disproportionate according to reaction (3) at temperatures in the range 450” 520X, but actual equilibrium pressures of tetraiodide were not determined for this reaction because the total weight loss of the Knudsen cell could not be proportioned between the two volatile species. Further experimental work, in which the vapour effusing from the Knudsen cell is analysed, is required to obtain the vapour pressure of the diiodide, and the disproportionation pressure of tetraiodide, in this temperature range. MECHANISM

OF ZIRCONIUM

DEPOSITION

IN THE IODIDE FLOW PROCESS

An attempt can now be made to describe the mechanism of zirconium transport and deposition in the plating process with reference to the results of the studies on the zirconium iodides. It is convenient to consider the schematic representation of the plating process shown in Fig. 4. From this it can be seen that interest is centred on the reactions which occur at two main sites, namely, the zirconium sponge bed and the plating target. Sponge bed reactions The intended role of the sponge bed in the plating process was to produce gaseous zirconium tetraiodide by reaction with iodine vapour. From experiments conducted on the preparation of zirconium tetraiodide by the reaction of iodine vapour and zirconium sponge in otherwise evacuated capsules, there is no doubt that when iodine vapour from the temperature-controlled reservoir contacts the heated zirconium sponge bed, zirconium tetraiodide is formed. This reaction occurs to a greater or lesser extent, depending on the iodine pressure and flow rate and the sponge temperature. It is, however, important to realise that the tetraiodide formed at end A of the sponge bed (see Fig. 4), has to permeate through the remainder of the bed before it leaves this reaction zone and is pumped to the target area. Consequently, there is opportunity for the reduction of some tetraiodide vapour by zirconium, and the product of this reduction reaction will depend upon the temperature of the zirconium sponge and the pressure of tetraiodide vapour. The conditions in the bed at this stage closely approximate to those which existed in the U-tube experiments. These latter experiments showed that, in such dynamic systems, zirconium triiodide is formed on the zirconium surfaces at temperatures in the range 250”-300X, and zirconium diiodide is produced at temperatures above 480°C for all tetraiodide J. Less-Common

&fetals, 19 (1969)

53-62

PRODUCTION

OF ZIRCONIUM

59

COATINGS

pressures examined. Between these two temperature ranges, the product of the reduction depended upon the pressure of tetraiodide vapour. High pressures prevented the disproportionation of zirconium triiodide, whereas lower ones did not. As the temperature increased within the range 3oo”-480’C, the pressure of tetraiodide I, vapourfrom I

resevoir

bed

oHigh frequency

0

a 0

//Oil diffusion Thermocwple pump Fig.

4. Schematic

representation

of the plating

system

used by

OWENS.

required to stabilise the triiodide also increased. The tetraiodide pressure in the plating system is not known, but as the reaction vessel is continuously evacuated by an oil diffusion pump, the pressure is unlikely to exceed a few millimetres of mercury. At the sponge temperature used in the majority of the plating experiments (approx. 45073, the pressure of tetraiodide is not expected to be high enough to stabilise any triiodide. Using the data determined for the disproportionation of zirconium triiodide by the effusion methodIs, the variation of disproportionation temperature with pressure of zirconium tetraiodide can be calculated. The results of such calculations are listed in Table II. A one atmosphere pressure of tetraiodide can be seen to be in equilibrium with solid triiodide at 485°C whereas a tetraiodide pressure of I mm Hg TABLE

II

VARIATION

OF

EQUILIBRIUM

TRIIODIDE

DISPROPORTIONATION

TEMPERATURE

WITH

PRESSURE

OF

TETRAIODIDE

Tetraiodide fwessure (++tmHs) 0.001

0.01 0.1 I.0 IO 100

760

Disproportionation PC)

temp

376 391 405 426 445 465 485

J. Less-Common

Metals, 19 (1969) 53-62

60

F. R. SALE

is in equilibrium at 426°C. Zirconium &iodide will be produced, therefore, in the sponge bed at 450°C by the reaction: Zr(,) +Zr14cg) = 2Zr12(s), which is only a summation of reactions Zr(s) &-I3(s)

+ 3ZrI4k)

and

(2).

= qZrI3(,)

= 2ZrI26)

Addition ZrcS)+ZrI4(,)

(I)

+ 2Zr14cg)

(I) (2)

multiplied by

2.

= 2Zr12cs)

At 45o”C, the results of the Knudsen cell measurements indicate that the &iodide will have a significant vapour pressure, therefore the vapour stream leaving the sponge bed will consist of a mixture of tetraiodide and Go&de. At the lower sponge temperatures used in the plating process, zirconium triiodide will be produced, but as it possesses a negligible vapour pressure, as shown in capsule and effusion experiments, it will not be transported to the target region. Deposition reactions In the iodide refining process, zirconium deposition occurs only by the decomposition of zirconium tetraiodide, because the ambient bulb temperature (zirconium sponge temperature) is usually only 260°C. Consequently, the only lower iodide which is likely to form at this site is the triiodide, and this does not take part in the vapour phase transport process because of its low vapour pressure. The formation of zirconium triiodide has been proposed to account for the removal of tetraiodide in sealed-bulb processes, with the consequent reduction in process yieldlo. From the analysis of the sponge bed reactions in the flow-plating process, however, it is apparent that zirconium deposition can occur by the decomposition of either of the volatile id&s present. In the plating system, the temperature gradient between the sponge bed and the induction-heated specimen is not known, hence it is difficult to describe the reactions which occur in this region. Condensation of the tetraiodicle or diiodide vapour on the walls of the reaction vessel between the sponge bed and the target would be expected if a severe temperature drop was present between these two sites. As no such condensation has been reported, it may be assumed that either there is a gradual increase in temperature over this region, or any temperature drop which occurs is insignificant. A dark metallic deposit, which was analysed as 95 o/ozirconium, has been reported to be formed on the walls of the silica vessel in the vicinity of the specimen16. This area is heated by radiation from the target and conduction from the sponge bed and, consequently, its temperature is unlikely to exceed 750’~-Soo°C at specimen temperatures of g5o”C. Two deposition sites are, therefore, present in the reaction system. From the thermodynamic analysis of the stability of zirconium tetraiodide, and the effect of reduced pressure on the decomposition equilibrium, a large degree of dissociation of the tetraiodide cannot be expected at the lower target temperatures used in the deposition study. This result is confirmed by CAMPBELL et a1.17, who failed to obtain reasonable deposition efficiencies at these lower temperatures using a source of pure zirconium tetraiodide. J.

Less-Common

Metals,

19 (1969) 53-62

A mixed iodide vapour has been shown to be produced in the flow-plating process of OWENS, and so the disproportionation reaction of zirconium diiodide:

2Zr12(g)=Zrts) f

ZrL(d,

must be considered as an alternative deposition process. In both the U-tube experiments, and the effusion measurements, the above reaction proceeded at temperatures in excess of 500°C. A large excess partial pressure of zirconium tetraiodide could cause the disproportionation to occur at slightly higher temperatures, but these values will be much lower than those required for the thermal decomposition of the tetraiodide. The deposition of zirconium on the silica vessel walls at temperatures below that of the target can be explained by this disproportionation reaction, and the target deposition, by the thermal decomposition of tetraiodide plus the disproportionation of any remaining diiodide. CONCL’l:SIOX

A mechanism has been proposed for the low-temperature deposition of zirconium in which the dispropo~ionation reaction of ~irconiunl diiodide occurs concomitant with the thermal decomposition of zirconium tetraiodide. In such deposition systems, the proportion of gaseous diiodide which is transported to the decomposition site in the vapour stream, should be as high as possible to give reasonable low-temperature deposition efficiencies. From the study of the formation of the diiodide, it is apparent that the temperature and physical condition of the zirconium sponge bed are the most important factors in the preparation of the diiodide, assuming sufficient gaseous tetraiodide is available. Zirconium tetraiodide, produced by the reaction of iodine and zirconium in a deep sponge bed, has a greater chance of reduction than in a shallow bed because of the longer contact time as it permeates through the sponge. This, in turn, means that at bed t~l~peratures of approximately 45o”C, the proportion of diiodide in the vapour phase at the target will be increased and, hence, a greater low-temperature deposition efficiency will be obtained. The temperature of the sponge bed is important, because zirconium diiodide is only formed at temperatures in the range 430’~-520%. The upper limit occurs because at this temperature zirconium diiodide disproportionates under the bed conditions. At temperatures below 450”C, zirconium triiodide is formed by the reduction of the tetraiodide, but as this has a negligible vapour pressure it does not participate in the deposition reaction.

The financial support of A.W.R.E., Aldermaston, is acknowledged over the period of time during which the experiments were performed. Thanks are also due to Dr. R. A. J. SHELTON of Manchester University for his interest in the preparation and characterisation of the lower iodides of zirconium. REFERENCES

I .‘,. 1:. VAN !.RKEL, PiaySiCtZ, 3 (1923) 75. L L. W. OWEN AND L. FAIRMAN, Tpuns. Inst. &I&d ~~~z~s~~~~~g, 39 (1962) 98-103. 3 B. S. SPENCER-TEMPTS AND L. W. OWEN,TYHZS. Inst.Mctal FmishiPzg, 39 (1962) 132-134. J. Less-Common

Metals,

19 (1969) 53-62

F. R. SALE

62 4 0. KUBASCHEWSKI, E. LL. EVANS AND C. B. ALCOCK, MetallurgicalThermochemistry, 4th. Edn., 1967, F. R. SALE AND R. F. ROLSTEN, L. BREWER, in

8 9 IO II 12

I3 I4 I5

16

I7

Pergamon,

p. 495. R. A. J. SHELTON, J. Less-Common Metals, 9 (1965) 54-59. Iodide M6tal.s and Metal Iodides, Wiley, New York and London, 1961, p. 17. L. L. QUILL (ed.), Chemistry and Metallurgy of Miscellaneous Materials: Thermodynamics, McGraw-Hill, New York, 1950. 2. M. SHAPIRO, in B. LUSHMAN AND F. KERZE, JR. (eds.),MetallurgyofZirconium,McGraw-Hill, New York, 1955, Chapter 5. V. S. EMELYANOV, P. D. BYSTROV AND A. I. EUSTYUKHIN, J. Nucl. Energy II, 3 (1956) 121-131. J. D. FAST, Z. Anorg. Allgem. Chem., 239 (1938) 145. V. S. EMELYANOV et al., J. Nucl. Energy 11, 4 (1957) 253. J. M. DORING AND K. MOLIERE, Z. Elektrochem., 56 (1952) 493. E. M. LARSEN AND J. J. LEDDY, J. Am. Chem. Sot., 78 (1956) 5983. F. R. SALE AND R. A. J. SHELTON, J. Less-Common Metals, 9 (1965) 60-63. F. R. SALE AND R. A. J. SHELTON, J. Less-Common Metals, 9 (1965) 64-69. R. MORRISON, A.W.R.E., Aldermaston, personal communication. I. E. CAMPBELL et al., U.S.A.E.C. Rept. BMI-887, 1953, Battelle Memorial Institute, Ohio.

J. Less-Common

Metals, 19 (1969) 53-62