Epitaxial oxidation of zirconium at various temperatures

JOUKNAL

OF THE LESS-COMMON

EPITAXIAL

OXIDATION

XETALS

OF ZIRCONIUM

19

AT VAKIOUS

TEMPERATURES

Transmission electron microscopy on annealed zirconium thin films revealed cpitaxial oxidation to ZrOs at estimated beam temperatures between 100’ and 170°C. Oxidation study using a hot stage at temperatures ranging from 200’ to 700°C resulted in instantaneous oxidation to monoclinic ZrOz. Direct observation of rhombohedrallyshaped dislocations possibly being the nucleation points for epitaxial oxidation on the (0001) plane were detected using a cold stage attachment at liquid-nitrogen temperatures of approximately -175°C. Zirconium hydride was definitely identified to be present in annealed zirconium foils by electron diffraction.

In the past, a number of investigators have studied the oxidation and dislocation aspects in thin foil zirconium. BAILEY’ reported that the formation of Zr02 will depend upon the rate of accumulation of absorbed oxygen, which in turn is directly related to the size of the sample. The same author reported earlier” that the type of dislocations observed is dependent upon the amount of oxygen and nitrogen present in zirconium samples. Recently, DOUGLASS AND V~la LANIJ~YT~ found on stripped oxide fiIms formed by initial oxidation of zirconium metal that uniform oxide growth does not seem to occur. The same authors also reported preferential oxidation which finally ended up in massive formation of oxide during long-time oxidation study between 250~ and 450°C. The problem of studying rate laws with respect to oxidation behavior in zirconium, using electron transmission work, is at present very doubtful due to temperature gradients involved (beam and hot stage) and variation in sample thickness. Rate-law studies using other techniques have been reported by a number of investigators. PORTER reported the oxidation of zirconium below 400°C to follow the cubic rate law. SENSI? reported the parabolic rate of oxidation for the same temperature range. BELLE AND MALI.ETT~ showed the cubic rate law being followed for the 4oo”--8oo’C temperature range. The present study was undertaken to investigate random and preferential oxidation of zirconium as well as direct interaction between the oxidation and dislocation processes. AILIELIWKX~ recently reviewed the relationship between overgrowth ,[. Less-Cornnzon

Metals, 12 (1967)

19-28

20

F. W. VAHLDIEK

and dislocations due to epitaxy, otherwise known as interfacial dislocations. In the present work, the author would like to indicate that oxide epitaxy on the (0001) and {zi?o} planes has been observed; furthermore, that dislocations due to epitaxy on these planes are sometimes observed; last but not least, a hypothesis is presented indicating that epitaxial oxidation on the zirconium planes studied is directly related to interfacial dislocations found on the same planes. Zirconium hydride particles earlier reported to be present in zirconium foil by HOWE et al.* and BAILEY~ were also encountered

in this work.

EXPERIMENTAL

The zirconium foil (99.8% pure and 0.005 cm thick) used in this study was received from Leytess Metal and Chemical Corporation. The total chemical analysis is shown in Table I. The zirconium foil was annealed in a vacuum of 10-6 mm Hg at 750°C for 8 hours. Selected annealed specimens were analyzed and found to have picked up additional 0.7 wt.% of oxygen. Annealed specimens were chemically and/or electro-polished with a mixture of ~HNO~:IHF:~H~O. TABLE CHEMICAL

I ANALYSIS

OF ZIRCONIUM

FOIL

Element

%

Element

%

Si

0.008

w

0.010

Mg

0.002

Ca

o.org

Hf Nb MO Mn

0.015

C 0 N H

0.01g

Sn B Pb Al Fe Ni CU Ti Cr

0.002 0.01

I

0.027 0.006 0.013 0.011 0.020

0.007 0.011

0.001 0.001 0.001

0.010 0.002 0.010

The oxidation experiments were performed inside a JEM-GA electron microscope in a vacuum of 10-5 mm Hg. The specimen region inside the electron microfor beam temperatures with Pt/Pt-ro”/o Rh thermocouples. scope was “calibrated” The wires used for these thermocouples were 0.0025 cm in diameter, with a bead junction of 0.005 cm in diameter. The electron-beam temperature was measured at IOO kV and up to IOO mA. The temperatures measured ranged between 100’ and 170X, depending upon magnification. It should be noted that this is not an absolute measurement of temperature as it happens on a thin film of zirconium foil; however, it gives some idea as to the temperature involved with respect to the beam. For the high-temperature oxidation study, the electron microscope is equipped with a hot stage using a molybdenum-wire resistance heater capable of temperatures up to 12oo’C. The specimen holder containing the zirconium thin film is likewise constructed out of molybdenum. The temperature is measured with a calibrated Pt/Pt-10% Rh thermocouple placed adjacent to the sample and molybdenum holder. Low temperature oxidation studies were carried out in time intervals up to J. Less-Common Metals,

12

(1967)

19-28

EPITAXIAL

OXIDATION

IO min using a coId stage attachment atures of approximately in the temperature maintaining

21

OF ZIRCOMIUM

-175°C.

range

-80”

a given temperature

capable of maintaining

The low-temperature to -175°C.

liquid-nitrogen

experiments

Some difficulties

temper-

were carried out

were encountered

in

for longer periods of time.

I)ISc‘LISSION OF RESULTS

Oxidation at beam temperatuw Several sequences using I- and z-min intervals at beam temperatures ranging from 100’ to 170°C were taken with respect to oxidation of zirconium. Figure I is a typical thin film of zirconium scope, on a near the oxidation

(0001)

(formation

at the beginning

plane, with its electron

of oxidation diffraction

inside the electron microinserted.

Figure

of oxide nuclei) I min after Fig. I was taken.

2

shows

The inserted

Fig. I. Electron micrograph of as-polished zirconium thin film on a near (0001) plant, showing a subboundary and dislocation loops. Electron diffraction of the matrix is inserted.

electron diffraction photograph shows the formation of some of the monoclinic zirconium oxide in addition to zirconium. A relatively heavy oxide layer was formed after subjecting the specimen for IO min to the electron beam (see Fig. 3). The zirconium diffraction pattern can still be identified, even after IO min in the electron microscope. This is explainable by the preferred oxide layer formation, probably due to the variations in thickness and temperature, across the thin film. Figures 4 and 5 show preferred oxidation of zirconium on the (ooox) and (zilo) planes, respectively.

F. W. VAHLDIEK

22

Oxidation at high temperatures The high-temperature oxidation inside the electron microscope mentioned

(zoo”-700°C)

using a hot-stage

that with increasing temperatures

of zirconium

attachment.

the oxidation

was carried out

Generally, it should be process is too rapid to be

studied in detail by the low-speed camera attachment. Attempts were made to use a movie camera at 3 and 6 frames per sec. At present, even with a movie camera attachment it seems quite impossible

to study detailed oxidation

in sita at high temper-

Fig. 2. Initial oxidation of zirconium thin film I min after Fig. I. Monoclinic ZrOz pattern is identifiable in the inserted electron diffraction.

atures. The main experimental difficulty is the problem of attainment of apparent equilibrium temperatures which takes about 15-20 min after a given temperature has been reached. It is obvious that during the total time elapsed of approximately 45 min (heating and temperature equilibrium attainment) the specimens have oxidized almost beyond recognition with respect to the initial thin film. Figures 6 and 7 show zirconium oxidation at 400°C at 5-min intervals. Note the oxide grain nucleation in Fig. 7. An interesting finding was the appearance of more or less spherical oxide nuclei forming the beginnings of an oxide grain structure. Also, due to thermal stresses involved, large crack formation was repeatedly observed. J. Less-Common

Metals,

IZ (1967)

rg-28

Fig. 3, Randomandpreferredoxitlation on the (0001) plane of zirconium after 10 min esposurr to the vlcctron beam. Note the increasing sharpness in inserted diffraction 1)attcrn for monoclinic ZWn.

Fig. 4, Preferred oxide platelets formation on the (0001) plant of zirconium, tron beam approx. 15 min. Fig. 5. Preferred oxidation

and dislocation

netfvorks on a near (ri io) plane.

subjected to the clec-

F. W. VAHLDIEK

24 Oxidation

at low temperatures

The low-temperature experiments, some problems with respect to maintaining

using liquid-nitrogen a given temperature

coolant, indicated for longer periods of

time (about I h or longer). However, the time sequences used for the study were in the order of 30 min or so maximum, which allowed this problem to be overcome. BAILEY~ reported

preferred

oxidation

Fig. 6. Heavy oxidation

of zirconium

and postulated

definite

of zirconium at 4ooT.

Fig. 7, Spherical oxide structure taken 5 min after Fig. 6 at 4ooT

Fig. 8. Preferred oxidation Fig. 9. Epitaxial J. Less-Common

on the (0001) plane at - 160°C.

oxide platelet formation Metals, 12 (1967) Ig-zS

5 min after Fig. 8 at - 160°C.

epitaxial

relation-

ships between the precipitate observed

in the present

study.

and the zirconium The oxidation

matrix.

fairly rapid rate even at temperatures oxide growth was noted with increasing

of -175°C. temperatures

11 typical

sequence

epitaxial

oxidation

behavior

Epitaxy

data indicate

behavior

epitaxial

was again

oxidation

at a

Further increase in the rate of to room temperature and above.

is shown in Figs. 8, q and IO. These

figures represent a sequence taken at - 160°C in 5-min intervals on the (0001) plane of zirconium after apparent equilibrium temperature had been reached. Certain cracks earlier

reported

by B.\ILEY~

visible

between

epitaxial

platelets,

were

sometimes

c~bservetl

i’~g. LO. Heavy platelet formation j min after Fig. 9 at - 16o’C. The inserted electron diffraction pattern identified as monoclinic ZrC), was taken across platelets at the lower left hand corner of the nrca shown in this Fig.

fihns.

Occasionally, rhombohedralIy-shaped dislocations are found in zirconium thin Figure II represents such a dislocation on the (0001) plane. An interesting

analogy seems to be the epitaxg with respect to oxidation of zirconium and epitaxy of dislocations with respect to zirconium. Figure 12 shows the early stage of epitaxial oxidation, again on the (0001) plane of zirconium. Furthermore, carrying these findings a step further, one may say that epitaxy in oxidation and interfacial dislocations present in the (0001) plane of zirconium are likely to be interrelated, meaning the rhol~lb~~hedral dislocations found are the preferred nuclei for the oxidation r~lecl~anism.

Fig. II. Rhombohedral

dislocation

Fig. 12. The beginning of epitaxial fraction of the matrix is inserted. J. Less-Cowman

Metals,

12

on (0001) plane of the zirconium

oxidation

(1967)Ig--28

on the (0001) plane at

jIilm at - 175°C.

-

175°C.

The

electron

dif-

EPIT.KXIAL

0XII)ATION

%irconiuwt hydride In many

27

OF ZIRCONIUM

in zirconimn foil

of the thin

films needle-like

hydride

particles

were found.

These

ranged from 0.2 ,u to I ,u in size. Attempts were made to definitely identify the larger particles by electron diffraction. Figure 13 shows several of these hydride particles. The inserted electron diffraction pattern definitely identifies these particles as tetragonal &-Hz.

The oxidation experiments indicate a random and preferred oxidation mechanism for zirconium. This oxidation behavior can be explained on the basis of differences in sample thickness, temperature gradients, and also by differences in the orientation of the zirconium grains of the specimens. The “room temperature” and lowtemperature experiments indicate a preferred oxidation nucleation on the (0001) and (zIIo) planes of zirconium. Random oxidation was observed at temperatures ranging from 200’ to 700°C; this however does not necessarily preclude preferential oxidation occurring at high temperatures. Monoclinic zirconium oxide was determined by electron diffraction to form instantaneously on the specimens at all temperatures studied. The start of epitaxial oxidation can be interpreted as a result of oxygen ab-

28

F. W. VAHLDIEK

sorption.

The preferential

as “nuclei” tions.

The resulting The

between

with

zirconium

the primary being

be stated

oxide

platelets

oxidation

(000I)Zr growth

oxidation

for the emerging data

is directly phase

related

along

are then preferentially indicate

and zirconium

the oxide

following

to dislocation

the principal formed primary

sites which

crystallographic as monoclinic orientation

act

direc-

ZrOz. relationship

:

II (OOI)ZrOZ oxide

growth

in the < IOO>

occurring

directions.

in the [OOI] direction, The

secondary

orientation

and secondary relationships

oxide may

as:

{zIIo}zr

II (OOI)ZrOZ

(2iIo)zr

II {IOO}ZrOZ

ACKNOWLEDGEMENTS The author with

would

the experiments.

like to thank

Special

thanks

Messrs. D. SWIHART

and R. PENCE for helping

are due to Mr. S. MERSOL in the preparation

of the manuscript

REFERENCES I J. E. BAILEY, The oxidation of thin films of zirconium. InM. TITLBACH (ed.), Electron Microscopy 1964, Vol. A, Non-Biology, Publ. House Czech. Acad. Sci., Prague, 1964, p. 395. 2 J. E. BAILEY, J. Nucl. Mater., 7 (1962) 300. 3 D. L. DOUGLASS AND J. VAN LANDUYT, Acta Met., 13 (1965) 1069. 4 H. A. PORTE, J. G. SCHNITZLEIN, R. C. VOGEL AND D. F. FISCHER, J. Electrochem. SW., 107 (1960)

~36.

5 K. A. SENSE, J. Electrochem. Sot., 109 (1962) 377. 6 J. BELLE AND M. W. MALLETT, J. Electvochem. Sot., IOI (1954) 339. 7 S. AMELINCKX, The Direct Observation of Dislocations, Academic Press, New York, 8 L. M. HOWE, J. L. WHITTON AND J. F. MCGURN, Ada Met., IO (1962) 773.

J. Less-Common Metals, 12 (1967) 19-28

1964. p. 388.