- Email: [email protected]
Structural behavior of metal tritide films
Journal
of the Less-Common
Elsevier Sequoia
STRUCTURAL
L. C. BEAVIS Strrdio
- Printed
BEHAVIOR
in The Netherlands
OF METAL TRITIDE
FILMS*
and C. J. MIGLIONICO
Lcthorrrtoric5. .4 Ihuquoquc~.
(Received
201
Metals
S.A., Lausanne
December
N.,!d. X7 I I5 (U.S./1.)
14th. 1971)
SUMMARY
Metal tritide films of five different elemental tritides have been observed over a five- to six-year period using interferometry and scanning electron microscopy. Initially the films exhibit a steady dilation with increasing age but no other effects. Later in life (after a few years) the films show considerable spalling which is ascribed to the formation and rupture of bubbles. Tritides of any given element behave in much the same manner, i.e., they show total expansion and bubble formation and rupture at the same age. Tritides of the various elements behave qualitatively in the same way, i.e., they exhibit bubble formation and expansion, but they differ noticeably in time with respect to apparent spalling, bubble density, bubble size, and total expansion. A total film expansion of 10-20 % is observed, bubbles vary from less than 1 to about 50 pm in linear dimension, and apparent spalling starts from two to five years after the tritides are formed.
INTRODUCTION
The energetic reaction between hydrogen isotopes is used as a means to produce neutrons. It is convenient to store the hydrogen isotope as a transition metal hydride. In this way a high density of isotope can be achieved without the inconvenience of a cryogenic system. One of the isotopes commonly used is tritium, which is known to decay with a 12.3-year half4ife to helium-3 and an electron. It is generally believed that the helium-3 generated in the disintegration of tritium is not bound chemically to either the metal or the metal tritide. However, observations in 1960 by Rodin and Surenyants’ on titanium tritide films, of the total quantity of helium-3 released over the first few years after the tritide is formed, indicate that less than I”/;, of the helium produced is emitted from titanium tritide. Any helium-3 produced which is not released must be trapped within the tritide, either as atoms which are atomically dispersed or as bubbles. One wonders about the structural integrity of the tritide as the helium-3 concentration increases. Most often the tritium is stored in a thin foil or film for use as a neutron-producing target. If the foil or film fails structurally by peeling, spalling, or *
This work was supported
J. Less-Common
Metals,
27 (1972)
by the U.S. Atomic
Energy
Commission.
L. C. BEAVIS.
202
C. J. MIGLIONICO
fragmentation, it will no longer intercept the beam and thus will not produce neutrons, although considerable tritium may remain occluded in the material. Information about structural changes due to helium-3 buildup in tritides is sparse ‘p3, therefore we undertook the study of some transition-metal tritide films to determine the degree of swelling and any other structural changes which might take place with time. The purpose of this paper is to discuss some observations made on these samples of aged tritides over the past several years. SAMPLE
PREPARATION
Film samples were prepared on polished sapphire substrates to enhance accurate measurements of length, width, and thickness. The substrates were washed ultrasonically in ethyl alcohol, fired in air for 1 h at lOOo”C, and then vacuum-fired for 3 h at 800” C to a final pressure of 1O-7 Torr. To prevent interaction between the active metal films and the aluminum oxide substrates and to improve light reflection for interferometry, a layer of chromium 10,000-12,000 A thick, 1.5 mm wide, and 9.5 mm long was deposited on each of the substrates (Fig. 1). During chromium deposition by thermal evaporation, the substrates were held at about 450” C. After deposition the samples were measured for width, length, and thickness before being stored in argon until the tritium-occluding metal was deposited upon them. The occluding metals selected for study were erbium, holmium, scandium, titanium, and yttrium. Each of these’metals was deposited on four substrates. The evaporation was made through a molybdenum mask which gave films of stepped structure (Fig. 1) about 6.4 mm long centered on the chromium film. The evaporant was evaporated from a resistively-heated conical tungsten basket in about 100 s. The substrate temperature was neither monitored nor controlled but was about 25” C at
Fig. 1. Diagram of film on substrate. J. Less-Common Metals, 27 (1972)
STRUCTURAL
BEHAVIOR
OF METAL
TRITIDE
FILMS
203
the beginning of the evaporation. The substrates and the evaporant charge were outgassed for 20 min before deposition to a final pressure, while hot, of less than lo-’ Torr. The length and width were measured with a traveling microscope with 1 pm resolution. Thickness was measured with a Watson interferometer using the 5460-A light of mercury. The thickness of the deposited occluder ranged from 2 to 3 pm. Immediately after deposition and measurement, the samples were placed in the tritiding system and exposed to 5 Torr pressure of high-purity tritium at about 450°C for 10 min. The tritium was supplied by heating uranium tritide. While at 4.50”C, the tritium gas was evacuated from the chamber containing those occluders which form tritritides (erbium, yttrium, and holmium). The scandium and titanium samples were allowed to cool in the tritium gas to about 100°C before the gas was evacuated. The samples were measured for length, width, thickness, and radioactivity and then stored in argon-filled bottles to await future measurements. The radioactivity measurement through early calibration was found to give total tritium content within 5 ?<. All of the tritides had atomic ratios of tritium-to-metal of about 1.8. After each subsequent measurement the specimens were re-stored in argon. OBSERVATIONS
The measurement of film width was accurate to about 0.1 “/a, i.e., 2 pm in 1.5 mm. Because of the irregularity of the film-end edges, and the difficulty of measuring to the same location on the end at each measurement, film length was not measured as accurately as width. The length measurement was accurate to 0.8 Y/i,i.e.. 50 pm in 6.4 mm. The thickness measurement was accurate to about 0.53; or 100 A, i.e., l/20 of a fringe separation (2730 A) in 2.S3.0 pm. The length and width of the samples did not change with tritiding, but the increase in thickness raised the bulk volume to account for about 804;1 of that predicted from bulk density changes due to tritiding4. For example, the thickness of the titanium tritide increased 12 7,. This is SO’?;,of the 15 o. increase in volume measured on bulk samples during hydriding. This would seem to indicate that, as deposited, the films were in tension (a common result for evaporated transition metal films) and/or after tritiding they were under a rather high compressive stress. The compressive stress in the films undoubtedly increases as the tritium decays to helium-3. None of the films showed structural failure from the stress and strain caused by tritiding, but they became roughened. This roughening is characteristic of all the films studied. From other studies on hydrides we have prepared, it appears that the linear dimensions of the grains of these tritides should be about 0. I pm. After tritiding, the samples were measured at approximately 6-month intervals for the first 2 years. The length and width measurements did not change during this time and were discontinued in order to minimize exposure to air. After 2 years, the measurement of thickness was extended to 12-month intervals. The samples were never heated above room temperature (z 22” C) after formation. Erhium
tritide
The erbium tritide samples showed a steady increase in film thickness with time. Control sample films of r rbium deutride were produced on identical substrates, but stored in air. They are 1 year older than the erbium tritide samples. These showed J. Less-Common
Metals. 21 (1972)
204
L. C. REAVIS,
C.
Fig. 2. (a) Interferometer photograph ferometer photographs of an erbium
T =9
C. J. MIGLIONICO
mo.
of an erbium film. One fringe displacement Wide filmat variousages.
=2730
A; (b)-(f) Inter-
no increase in thickness with time. Figure 2 shows the interferometric data on aging of a typical erbium tritide sample. Figure 2(a) shows the film before tritiding. Figure 2(b) was taken immediately after tritiding. It is possible to see the fringe shift indicative of the tritiding strain. One can note between the fringes in the light areas a mottled surface due to the strain of tritiding. Figure 2(c) indicates no change in film thickness nor other change from Fig. 2(b). This is typical of a few of the films at an early age. In Fig. 2(d), the film has expanded as seen by the interference fringe shift. No other changes are apparent. At 47 months (Fig. 2(e)) additional expansion is noted, and also considerable surface disruption of the film can be seen in the regions between the interference of fringes. Finally, at age 60 months, further expansion and disruption of the surface can be noted. The magnification in the plane of the original photographs is about x 60. Erbium tritide expanded least of any of the materials investigated. The values plotted in Fig. 3 are the average of the 8 sample points for the total expansion. Figure 4 consists of a number of scanning electron micrographs @EM’s) of sample surfaces. The upper left frame (Fig. 4(a)) is of an erbium deuteride control sample. The lines in the film are due to scratches left in the alumina substrate during its final polishing. The bright spots are due to dust particles. Erbium tritide appears in the remaining frames at increasing magnification. At the time the SEM’s were taken, the erbium deuteride sample was 80 months old and the erbium tritide films were 67 months old. Damage is apparent in all of the erbium tritide pictures. Craters, cracks, and upraised areas in the film appear to be randomly distributed in Fig. 4(b). The damaged regions seem to vary in size from 1 to 50 pm in diameter; in some cases the damage J. Less-Common
Metals, 27 (1972)
STRUCTURAL
BEHAVIOR
OF METAL
TRITIDE
205
FIL.MS
-2
Caysx 10
Fig. 3. Total expansion from the film.
L‘S,time in days and percent helium in the metal tritide film assuming
no helium loss
50~ Crater
a.
u
b.
C.
U 1ol.l
d.
1-J 111
Fig. 4.(a) Scanning electron micrograph of erbium deuteride. graphs of erbium tritide (x 100. x 1000. x 10.000). J. Less-Common
Metals, 27 (1972)
( r 3000):(b)--(d) Scanning
electron
micro-
206
L. C. BEAVIS. C. J. MIGLIONICO
extends through the film thickness, and in other cases a stepped structure is observed in the bottom of the crater (see Fig. 4(c)). Figure 4(d)) is a picture of a small raised area, i.e., a bubble, with a cracked surface. No catastrophic effect, i.e., total film spalling, was observed in the erbium tritide films to 78 months of age. Holmium tritide
The holmium tritide lilms after approximately 1.5 years became almost impossible to photograph. The film surface was very rough and nonreflecting. The interferometry photographs taken at 2 years showed surface damage of a magnitude comparable to that of erbium tritide at 4 years. The expansion rate of these films up to 1 year was at approximately the same rate as the erbium tritide films; after this age, the holmium tritide expanded at an increased rate. After 3 years, holmium tritide was expanding at about twice the rate of expansion for erbium tritide (Fig. 3). At this time the films began to peel and had become extremely cratered. After 4 years, these films had completely peeled from the chromium substrate. No SEM’s of these films were obtained. Scandium tritide
Scandium tritide exhibited an initial expansion rate about 3 times as high as that for erbium tritide, the slowest expanding films. This rate decreased over the years until, after 5 years, the average rate was about 70 ‘Agreater than the erbium tritide rate
Stepped Craters
d. Fig. 5.(a)-(d)
Scanning
J. Less-Common
electron
micrographs
Metals, 27 (1972)
of scandium
tritide ( x 100, x 1000, x 3000, x 3000).
STRUCTURAL
BEHAVIOR
OF METAL
TRITIDE
207
FILMS
of expansion. The total apparent expansion of these films was the greatest of any of the tritides investigated, about 18% after 5 years (Fig. 3). A similar high initial expansion rate followed by a lower expansion rate is reported by Jones3 for bulk yttrium tritide. These films show a roughening during tritiding similar to that seen for erbium tritide. Interferometry pictures of the scandium tritide films taken after 3 years show qualitatively the same change in surface roughness as seen on erbium tritide films at 4 years. The SEM’s taken at 69 months indicate (Fig. 5) that the number of gross defects in the film is greater than on erbium tritide films of the same age. Again, stepped structure in crater bottoms is apparent. It should be noted that the first tritide frame of each set of SEM photographs is the same magnification, i.e., x 100. The size of the largest craters appears to be somewhat larger than in erbium tritide films (Fig. 5(a)). It should also be noted that many of the craters terminate at a roughly straight edge (Fig. 5(b) and 5(c)). I n many cases, in these as well as other films, this edge lies on a scratch in the alumina substrate. Figure 5(d) is a view of the bottom of a crater. This type of structure is characteristic of brittle fracture. These films have, since the SEM pictures were taken, completely peeled (spalled) from the chromium substrate. Titunium
Wide
Between 5 and 12 months of age in 7 of 8 cases, the titanium tritide films peeled at their ends (Fig. 6(a) and 6(b)). The peeled area can be seen as a light area (chromium
Region
d.
T - 31 mo.
Fig. 6.(a)-(f) displacement
Interferometer = 2730 p\.
J. Less-Common
Met&,
b.
T
=5 mo.
e.
T
=43 mo.
photographs
27 (1972),
of
of a titanium
C.
f. tritidc
T
= 12 mo.
T = 56 mo. Cilm at variour
increasing
ages. One fringe
208
L. C. BEAVIS. C. J. MIGLIONICO
substrate) to the right of the original film edge. The remainder of the film did not peel and appeared normal at the time the ends had peeled. A change in the expansion rate for titanium tritide was noted at this time. Figure 6(c), (d), and (e) shows the optical appearance between fringes of the film to be rather unchanging through 43 months, except for the peeled ends. The continual expansion can be noted by the fringe shifts through this period. At 56 months (Fig. 6(f)), the titanium tritide films show a striking change in surface texture. Considerable cratering of the surface is noted as splotches between the interference fringes. Not all films showed this effect at this time. This texture change occurred at about 12 months greater age in these films than it did in the erbium tritide films. In addition, some further peeling was noted at this time on some of the film edges.
Fig. 7.(a) Scanning electron micrograph of a titanium deuteride film (x 3000); (b)-(f) Scanning electron micrographs of a titanium tritide film ( x 100, x 1000, x 3000, x 3000, x IO,OOO).
SEM pictures were taken after 64 months (Fig. 7). Figure 7(a) shows a titanium deuteride film prepared by Blewe? ; the bright spot is due to a dust particle. This film appears much like the erbium deuteride film of Fig. 4(a). The greatest density of craters on any tritides is seen in Fig. 7(b). The sharp demarcation across the frame is at the titanium tritide film step to the chromium substrate. The debris is probably ejected from the craters. In Fig. 7(c) are seen craters, debris from craters, and a chip of film nearly torn away. Figure 7(d) shows another cracked piece of film still in its crater. Figure 7(e) shows some of the nodular structure seen on all of the titanium tritide films. A similar structure is also reported by Blewer to be present on some hydride films5. Again, the structure in the bottom of the craters appears to be a characteristic of brittle fracture. Figure 7(f) depicts a typical hole in the film with what appears to be an uplifted rim. All of the titanium tritide samples had craters in J. Less-Common Metals, 27 (1972)
STRUCTURAL
BEHAVIOR
OF METAL
TRITIDE
FILMS
209
them at this time (age 64 months). Some of the films did not have these craters at age 56 months. The titanium tritide films after 74 months had not completely peeled from their substrates. tritide Yttrium tritide films early in their life behaved much the same as erbium tritide films and showed the same total expansion (Fig. 2). After 4 years, the film character changed as did that of the erbium tritide films, i.e., craters are apparent in the interferometry photographs. In addition, the yttrium tritide films began to peel. Between 4 and 5 years, the films completely peeled from their substrates. Yttrium
DISCUSSION
OF
RESULTS
It appears that early in the life of tritide films most of the helium-3 produced as a result of tritium decay remains in the film. If the helium-3 escaped at an appreciable rate it would be difficult to explain the continued expansion of the films observed in these studies and the results reported in refs. 1,2, and 3. We note furtherh.7 that the lattice parameter of erbium and scandium tritide films changes little or not at all with age. Bulk samples of erbium and scandium tritide6.’ and yttrium tritide3 show an initial (age 1 year) expansion rate similar to the rate measured on erbium, holmium, and yttrium films over the same period ; however, measurement of the microscopic film thickness of these tritides films, as well as those of the other tritides investigated through at least the first 3 years, indicates continued expansion. These apparently conflicting data can be explained if it is assumed that the helium-3 accumulates in bubbles which form as a result of diffusion. Bubbles are defined as an accumulation of enough helium-3 atoms in one location so that these particles act collectively and behave as a gas in the thermodynamic sense (perhaps lo6 atoms). Nothing concerning the physical configuration of the collection is to be inferred from this definition. In fact, most of the bubbles appear elongated in the plane parallel to the film width and length. The reason for this asymmetry is not known, but it must be related to stress distribution in the film. As the helium-3 accumulates at a specific location (bubble) the pressure from the gas causes the tritide to deform by expansion. This expansion has little effect on the crystallographic lattice parameter of the material although it is readily apparent as a bulk dilation (film thickness increase). The deformation after exceeding the yield limit of the tritide (plastic deformation) does not return to its original dimensions even if the helium escapes. As the ultimate strength of the tritide is exceeded, bubbles rupture and cause the film to spall. The apparent bulk dilation is reduced only by the amount of spallation during this process. Continued expansion of the remaining aged tritide is observed as new bubbles develop and expand. This effect appears as little or no change in total expansion in Fig. 3. Another, and less plausible, explanation is that the film is separating slowly and steadily from the substrate. One would expect very early (2 1 year) film peeling if separation were the explanation of the observations. If all of the observed expansion occurred in 100 or less atomic layers the tensile elongation limit for the tritide would be exceeded in 1 year. This explanation also does not account for the observed partially spalled films. J. Less-Common
Metals, 21 (1972)
210
L. C. BEAVIS,
C. J. MIGLIONICO
It is apparent that, qualitatively, all of the tritide films behave the same. That is, they all show signs of bubble formation and subsequent structural failure of individual and collections of bubbles (blisters). The life time to bubble and blister failure and gross film failure is a variable which appears to be related to the metal tritide under consideration. For holmium and yttrium tritides a high expansion rate and large change in surface roughness (evident from the optical quality of the films) was apparent immediately before complete spalling. All of the samples of titanium, erbium, and scandium tritides investigated by SEM showed the same optical behavior as holmium and yttrium tritides, but they did not show the large apparent expansion rate increases seen in 4 years or less by the samples of these two tritides. The increase in expansion rate immediately before complete spallation may indicate gross separation of film and substrate which would more than compensate for the reduced expansion due to blister spall. Holmium and yttrium tritides display this effect early. Scandium tritide shows this effect after 5 years, which is also followed by complete spalling. The erbium and titanium tritides never showed this feature. It should be restated that all of the films had approximately the same atomic ratio of tritium-to-metal when they were formed (1.7-1.8). Figure 3 plots on the upper abscissa the helium-3 concentration assuming that the helium concentration in the film is governed by the decay rate of tritium. This assumption is probably valid only until appreciable cratering occurs. At the time a blister or bubble ruptures, the helium contained therein undoubtedly escapes. Nucleation of bubbles must occur early in the life of the tritide to account for the lack of gross lattice distortion. Times of the order of 1 year or less to bubble formation appear reasonable (Jones3 and Lundin’~‘). The helium concentration at age 1 year is about 3.5 at.%, i.e., MT,,,_XHe, where x=0.1 or, for example, 4x 10zl atoms/cm3 in the titanium films. Eventually the helium pressure in a bubble apparently exceeds the ultimate tensile limit of the material and ruptures. The helium-3 concentration at this stage, approximately 4 years,if one assumes no loss of the helium-3 formed’, is about 13.0%, i.e., x=0.36 or 1.4 x 10” atoms/cm3 in titanium tritide. This rupturing of bubbles is quite apparent in the SEM photographs and the interferometry pictures. The effect of the tendency of the metal to reform to its natural hexagonal crystallographic structure as the helium is formed is unknown and may be of crucial importance to bubble formation and spalling. Film thickness is probably not an important consideration since individual samples of one type of tritide vary by as much as 50’% in thickness, but all tritides of a given metal seem to fail at about the same time but randomly with thickness. Deposition parameters, e.g., rate and substrate temperature, may influence subsequent time to bubble formation and tritide failure. It also seems likely that local high stress concentrations, such as one would expect near replication of substrate scratch marks, cause film failure (Figs. 5 and 7). CONCLUSIONS
(1) The expansion rate of metal tritide films is about 5 to 10 x 10e3 %/day for all of the tritides studied over the first 5 years, except for scandium tritide which starts to expand at a much higher rate. Eventually (at 4-5 years) the scandium tritide average .I. Less-Common
Metals, 27 (1972)
STRUCTURAL
BEHAVIOR
OF METAL
TRITIDE
FILMS
211
rate is about 10 x 10e3 ‘!/day. These rates are much the same as those reported for newly formed bulk tritides (to 1 year)3.6.‘. (2) It appears that all of the metal ditritide films studied fail structurally to a significant extent in 5 years or less through a mechanism related to helium-3 accumulation, as bubbles, in the tritide. (3) Qualitatively all of the metal tritides behave the same. However, quantitative differences are noted such as film expansion rate, total expansion to spalling. time to film spalling (bubble Failure), density of craters, and crater size. (4) In all films investigated, failure seems to be by a brittle fracture mechanism. The fracture occurs at random depths in the film, as one might expect if the helium-3 is initially randomly distributed and its subsequent aggregation is also random. (5) It appears that in some cases bubbles may coalesce to form a blister which eventually fails. This is evident from stepped crater bottoms. (6) The length of time to noticeable tritide film peeling increases in the following order: titanium, holmium, yttrium, erbium, and scandium. The maximum time to which craters appeared in the tritides is holmium (23 months), helium concentration = 6.7 at.‘/, ; scandium (35 months) helium concentration = 9.5 at.‘! ; yttrium (46 months), helium concentration= 12.5 at.‘/“; erbium (47 months), helium concentration = 12.5 at.‘/& and titanium (56 months), helium concentration = 15 at.:,;. (7) The quantitative effects of deposition parameters, film structure, and stress concentration on bubble formation and the related film integrity are unknown but are qualitatively apparent through enhanced failure of aged tritides at substrate scratches. The influence of bubble size and failure on total film spalling is unknown in tritide films, but is considered by Blewe? for helium-implanted deuteride films. ACKNOWLEDGEMENT
The authors wish to acknowledge the assistance of G. L. Knauss who tritided the films and K. D. Mills who took some of the interferometry photographs. REFERENCES I A. M. Rodin and V. V. Surenyants, Coefficient of diffusionol’helium in titanium. Fi:. \Ic,tol. \I~,r~rllo~~e~/.. 10 (1960) 216-222. 2 V. I? Vertcbnyi. M. F. Vlasov, A. L. Kirilyuk, R. A. Zatserkovshii. M. V. Posechnik and V. A. Stepaneko. The behavior of He’ in ZrT, and zirconium, 41. Enrtq. (USSR). 22 (1967) 235-237. 3 P. M. S. Jones. W. Edmondson and N. S. McKenna. The stability 01”metal tritides~ yttrium tritide. J. N~,cl. \Irlter.. 23 (1967) 309-312. 4 W. J. Mueller, J. P. Blackledge and G. C. Libowitz, Metal Hydrides, Academic Press, New York, 1968. 5 R. S. Blewer, private communication, March, 1971. 6 C. E. Lundin, private communication, Denver Res. Inst., February, 1969. 7 C. E. Lundin, private communication, Denver Res. Inst., February, 1970.
J. Less-Common
Metals, 27 (1972)
of the Less-Common
Elsevier Sequoia
STRUCTURAL
L. C. BEAVIS Strrdio
- Printed
BEHAVIOR
in The Netherlands
OF METAL TRITIDE
FILMS*
and C. J. MIGLIONICO
Lcthorrrtoric5. .4 Ihuquoquc~.
(Received
201
Metals
S.A., Lausanne
December
N.,!d. X7 I I5 (U.S./1.)
14th. 1971)
SUMMARY
Metal tritide films of five different elemental tritides have been observed over a five- to six-year period using interferometry and scanning electron microscopy. Initially the films exhibit a steady dilation with increasing age but no other effects. Later in life (after a few years) the films show considerable spalling which is ascribed to the formation and rupture of bubbles. Tritides of any given element behave in much the same manner, i.e., they show total expansion and bubble formation and rupture at the same age. Tritides of the various elements behave qualitatively in the same way, i.e., they exhibit bubble formation and expansion, but they differ noticeably in time with respect to apparent spalling, bubble density, bubble size, and total expansion. A total film expansion of 10-20 % is observed, bubbles vary from less than 1 to about 50 pm in linear dimension, and apparent spalling starts from two to five years after the tritides are formed.
INTRODUCTION
The energetic reaction between hydrogen isotopes is used as a means to produce neutrons. It is convenient to store the hydrogen isotope as a transition metal hydride. In this way a high density of isotope can be achieved without the inconvenience of a cryogenic system. One of the isotopes commonly used is tritium, which is known to decay with a 12.3-year half4ife to helium-3 and an electron. It is generally believed that the helium-3 generated in the disintegration of tritium is not bound chemically to either the metal or the metal tritide. However, observations in 1960 by Rodin and Surenyants’ on titanium tritide films, of the total quantity of helium-3 released over the first few years after the tritide is formed, indicate that less than I”/;, of the helium produced is emitted from titanium tritide. Any helium-3 produced which is not released must be trapped within the tritide, either as atoms which are atomically dispersed or as bubbles. One wonders about the structural integrity of the tritide as the helium-3 concentration increases. Most often the tritium is stored in a thin foil or film for use as a neutron-producing target. If the foil or film fails structurally by peeling, spalling, or *
This work was supported
J. Less-Common
Metals,
27 (1972)
by the U.S. Atomic
Energy
Commission.
L. C. BEAVIS.
202
C. J. MIGLIONICO
fragmentation, it will no longer intercept the beam and thus will not produce neutrons, although considerable tritium may remain occluded in the material. Information about structural changes due to helium-3 buildup in tritides is sparse ‘p3, therefore we undertook the study of some transition-metal tritide films to determine the degree of swelling and any other structural changes which might take place with time. The purpose of this paper is to discuss some observations made on these samples of aged tritides over the past several years. SAMPLE
PREPARATION
Film samples were prepared on polished sapphire substrates to enhance accurate measurements of length, width, and thickness. The substrates were washed ultrasonically in ethyl alcohol, fired in air for 1 h at lOOo”C, and then vacuum-fired for 3 h at 800” C to a final pressure of 1O-7 Torr. To prevent interaction between the active metal films and the aluminum oxide substrates and to improve light reflection for interferometry, a layer of chromium 10,000-12,000 A thick, 1.5 mm wide, and 9.5 mm long was deposited on each of the substrates (Fig. 1). During chromium deposition by thermal evaporation, the substrates were held at about 450” C. After deposition the samples were measured for width, length, and thickness before being stored in argon until the tritium-occluding metal was deposited upon them. The occluding metals selected for study were erbium, holmium, scandium, titanium, and yttrium. Each of these’metals was deposited on four substrates. The evaporation was made through a molybdenum mask which gave films of stepped structure (Fig. 1) about 6.4 mm long centered on the chromium film. The evaporant was evaporated from a resistively-heated conical tungsten basket in about 100 s. The substrate temperature was neither monitored nor controlled but was about 25” C at
Fig. 1. Diagram of film on substrate. J. Less-Common Metals, 27 (1972)
STRUCTURAL
BEHAVIOR
OF METAL
TRITIDE
FILMS
203
the beginning of the evaporation. The substrates and the evaporant charge were outgassed for 20 min before deposition to a final pressure, while hot, of less than lo-’ Torr. The length and width were measured with a traveling microscope with 1 pm resolution. Thickness was measured with a Watson interferometer using the 5460-A light of mercury. The thickness of the deposited occluder ranged from 2 to 3 pm. Immediately after deposition and measurement, the samples were placed in the tritiding system and exposed to 5 Torr pressure of high-purity tritium at about 450°C for 10 min. The tritium was supplied by heating uranium tritide. While at 4.50”C, the tritium gas was evacuated from the chamber containing those occluders which form tritritides (erbium, yttrium, and holmium). The scandium and titanium samples were allowed to cool in the tritium gas to about 100°C before the gas was evacuated. The samples were measured for length, width, thickness, and radioactivity and then stored in argon-filled bottles to await future measurements. The radioactivity measurement through early calibration was found to give total tritium content within 5 ?<. All of the tritides had atomic ratios of tritium-to-metal of about 1.8. After each subsequent measurement the specimens were re-stored in argon. OBSERVATIONS
The measurement of film width was accurate to about 0.1 “/a, i.e., 2 pm in 1.5 mm. Because of the irregularity of the film-end edges, and the difficulty of measuring to the same location on the end at each measurement, film length was not measured as accurately as width. The length measurement was accurate to 0.8 Y/i,i.e.. 50 pm in 6.4 mm. The thickness measurement was accurate to about 0.53; or 100 A, i.e., l/20 of a fringe separation (2730 A) in 2.S3.0 pm. The length and width of the samples did not change with tritiding, but the increase in thickness raised the bulk volume to account for about 804;1 of that predicted from bulk density changes due to tritiding4. For example, the thickness of the titanium tritide increased 12 7,. This is SO’?;,of the 15 o. increase in volume measured on bulk samples during hydriding. This would seem to indicate that, as deposited, the films were in tension (a common result for evaporated transition metal films) and/or after tritiding they were under a rather high compressive stress. The compressive stress in the films undoubtedly increases as the tritium decays to helium-3. None of the films showed structural failure from the stress and strain caused by tritiding, but they became roughened. This roughening is characteristic of all the films studied. From other studies on hydrides we have prepared, it appears that the linear dimensions of the grains of these tritides should be about 0. I pm. After tritiding, the samples were measured at approximately 6-month intervals for the first 2 years. The length and width measurements did not change during this time and were discontinued in order to minimize exposure to air. After 2 years, the measurement of thickness was extended to 12-month intervals. The samples were never heated above room temperature (z 22” C) after formation. Erhium
tritide
The erbium tritide samples showed a steady increase in film thickness with time. Control sample films of r rbium deutride were produced on identical substrates, but stored in air. They are 1 year older than the erbium tritide samples. These showed J. Less-Common
Metals. 21 (1972)
204
L. C. REAVIS,
C.
Fig. 2. (a) Interferometer photograph ferometer photographs of an erbium
T =9
C. J. MIGLIONICO
mo.
of an erbium film. One fringe displacement Wide filmat variousages.
=2730
A; (b)-(f) Inter-
no increase in thickness with time. Figure 2 shows the interferometric data on aging of a typical erbium tritide sample. Figure 2(a) shows the film before tritiding. Figure 2(b) was taken immediately after tritiding. It is possible to see the fringe shift indicative of the tritiding strain. One can note between the fringes in the light areas a mottled surface due to the strain of tritiding. Figure 2(c) indicates no change in film thickness nor other change from Fig. 2(b). This is typical of a few of the films at an early age. In Fig. 2(d), the film has expanded as seen by the interference fringe shift. No other changes are apparent. At 47 months (Fig. 2(e)) additional expansion is noted, and also considerable surface disruption of the film can be seen in the regions between the interference of fringes. Finally, at age 60 months, further expansion and disruption of the surface can be noted. The magnification in the plane of the original photographs is about x 60. Erbium tritide expanded least of any of the materials investigated. The values plotted in Fig. 3 are the average of the 8 sample points for the total expansion. Figure 4 consists of a number of scanning electron micrographs @EM’s) of sample surfaces. The upper left frame (Fig. 4(a)) is of an erbium deuteride control sample. The lines in the film are due to scratches left in the alumina substrate during its final polishing. The bright spots are due to dust particles. Erbium tritide appears in the remaining frames at increasing magnification. At the time the SEM’s were taken, the erbium deuteride sample was 80 months old and the erbium tritide films were 67 months old. Damage is apparent in all of the erbium tritide pictures. Craters, cracks, and upraised areas in the film appear to be randomly distributed in Fig. 4(b). The damaged regions seem to vary in size from 1 to 50 pm in diameter; in some cases the damage J. Less-Common
Metals, 27 (1972)
STRUCTURAL
BEHAVIOR
OF METAL
TRITIDE
205
FIL.MS
-2
Caysx 10
Fig. 3. Total expansion from the film.
L‘S,time in days and percent helium in the metal tritide film assuming
no helium loss
50~ Crater
a.
u
b.
C.
U 1ol.l
d.
1-J 111
Fig. 4.(a) Scanning electron micrograph of erbium deuteride. graphs of erbium tritide (x 100. x 1000. x 10.000). J. Less-Common
Metals, 27 (1972)
( r 3000):(b)--(d) Scanning
electron
micro-
206
L. C. BEAVIS. C. J. MIGLIONICO
extends through the film thickness, and in other cases a stepped structure is observed in the bottom of the crater (see Fig. 4(c)). Figure 4(d)) is a picture of a small raised area, i.e., a bubble, with a cracked surface. No catastrophic effect, i.e., total film spalling, was observed in the erbium tritide films to 78 months of age. Holmium tritide
The holmium tritide lilms after approximately 1.5 years became almost impossible to photograph. The film surface was very rough and nonreflecting. The interferometry photographs taken at 2 years showed surface damage of a magnitude comparable to that of erbium tritide at 4 years. The expansion rate of these films up to 1 year was at approximately the same rate as the erbium tritide films; after this age, the holmium tritide expanded at an increased rate. After 3 years, holmium tritide was expanding at about twice the rate of expansion for erbium tritide (Fig. 3). At this time the films began to peel and had become extremely cratered. After 4 years, these films had completely peeled from the chromium substrate. No SEM’s of these films were obtained. Scandium tritide
Scandium tritide exhibited an initial expansion rate about 3 times as high as that for erbium tritide, the slowest expanding films. This rate decreased over the years until, after 5 years, the average rate was about 70 ‘Agreater than the erbium tritide rate
Stepped Craters
d. Fig. 5.(a)-(d)
Scanning
J. Less-Common
electron
micrographs
Metals, 27 (1972)
of scandium
tritide ( x 100, x 1000, x 3000, x 3000).
STRUCTURAL
BEHAVIOR
OF METAL
TRITIDE
207
FILMS
of expansion. The total apparent expansion of these films was the greatest of any of the tritides investigated, about 18% after 5 years (Fig. 3). A similar high initial expansion rate followed by a lower expansion rate is reported by Jones3 for bulk yttrium tritide. These films show a roughening during tritiding similar to that seen for erbium tritide. Interferometry pictures of the scandium tritide films taken after 3 years show qualitatively the same change in surface roughness as seen on erbium tritide films at 4 years. The SEM’s taken at 69 months indicate (Fig. 5) that the number of gross defects in the film is greater than on erbium tritide films of the same age. Again, stepped structure in crater bottoms is apparent. It should be noted that the first tritide frame of each set of SEM photographs is the same magnification, i.e., x 100. The size of the largest craters appears to be somewhat larger than in erbium tritide films (Fig. 5(a)). It should also be noted that many of the craters terminate at a roughly straight edge (Fig. 5(b) and 5(c)). I n many cases, in these as well as other films, this edge lies on a scratch in the alumina substrate. Figure 5(d) is a view of the bottom of a crater. This type of structure is characteristic of brittle fracture. These films have, since the SEM pictures were taken, completely peeled (spalled) from the chromium substrate. Titunium
Wide
Between 5 and 12 months of age in 7 of 8 cases, the titanium tritide films peeled at their ends (Fig. 6(a) and 6(b)). The peeled area can be seen as a light area (chromium
Region
d.
T - 31 mo.
Fig. 6.(a)-(f) displacement
Interferometer = 2730 p\.
J. Less-Common
Met&,
b.
T
=5 mo.
e.
T
=43 mo.
photographs
27 (1972),
of
of a titanium
C.
f. tritidc
T
= 12 mo.
T = 56 mo. Cilm at variour
increasing
ages. One fringe
208
L. C. BEAVIS. C. J. MIGLIONICO
substrate) to the right of the original film edge. The remainder of the film did not peel and appeared normal at the time the ends had peeled. A change in the expansion rate for titanium tritide was noted at this time. Figure 6(c), (d), and (e) shows the optical appearance between fringes of the film to be rather unchanging through 43 months, except for the peeled ends. The continual expansion can be noted by the fringe shifts through this period. At 56 months (Fig. 6(f)), the titanium tritide films show a striking change in surface texture. Considerable cratering of the surface is noted as splotches between the interference fringes. Not all films showed this effect at this time. This texture change occurred at about 12 months greater age in these films than it did in the erbium tritide films. In addition, some further peeling was noted at this time on some of the film edges.
Fig. 7.(a) Scanning electron micrograph of a titanium deuteride film (x 3000); (b)-(f) Scanning electron micrographs of a titanium tritide film ( x 100, x 1000, x 3000, x 3000, x IO,OOO).
SEM pictures were taken after 64 months (Fig. 7). Figure 7(a) shows a titanium deuteride film prepared by Blewe? ; the bright spot is due to a dust particle. This film appears much like the erbium deuteride film of Fig. 4(a). The greatest density of craters on any tritides is seen in Fig. 7(b). The sharp demarcation across the frame is at the titanium tritide film step to the chromium substrate. The debris is probably ejected from the craters. In Fig. 7(c) are seen craters, debris from craters, and a chip of film nearly torn away. Figure 7(d) shows another cracked piece of film still in its crater. Figure 7(e) shows some of the nodular structure seen on all of the titanium tritide films. A similar structure is also reported by Blewer to be present on some hydride films5. Again, the structure in the bottom of the craters appears to be a characteristic of brittle fracture. Figure 7(f) depicts a typical hole in the film with what appears to be an uplifted rim. All of the titanium tritide samples had craters in J. Less-Common Metals, 27 (1972)
STRUCTURAL
BEHAVIOR
OF METAL
TRITIDE
FILMS
209
them at this time (age 64 months). Some of the films did not have these craters at age 56 months. The titanium tritide films after 74 months had not completely peeled from their substrates. tritide Yttrium tritide films early in their life behaved much the same as erbium tritide films and showed the same total expansion (Fig. 2). After 4 years, the film character changed as did that of the erbium tritide films, i.e., craters are apparent in the interferometry photographs. In addition, the yttrium tritide films began to peel. Between 4 and 5 years, the films completely peeled from their substrates. Yttrium
DISCUSSION
OF
RESULTS
It appears that early in the life of tritide films most of the helium-3 produced as a result of tritium decay remains in the film. If the helium-3 escaped at an appreciable rate it would be difficult to explain the continued expansion of the films observed in these studies and the results reported in refs. 1,2, and 3. We note furtherh.7 that the lattice parameter of erbium and scandium tritide films changes little or not at all with age. Bulk samples of erbium and scandium tritide6.’ and yttrium tritide3 show an initial (age 1 year) expansion rate similar to the rate measured on erbium, holmium, and yttrium films over the same period ; however, measurement of the microscopic film thickness of these tritides films, as well as those of the other tritides investigated through at least the first 3 years, indicates continued expansion. These apparently conflicting data can be explained if it is assumed that the helium-3 accumulates in bubbles which form as a result of diffusion. Bubbles are defined as an accumulation of enough helium-3 atoms in one location so that these particles act collectively and behave as a gas in the thermodynamic sense (perhaps lo6 atoms). Nothing concerning the physical configuration of the collection is to be inferred from this definition. In fact, most of the bubbles appear elongated in the plane parallel to the film width and length. The reason for this asymmetry is not known, but it must be related to stress distribution in the film. As the helium-3 accumulates at a specific location (bubble) the pressure from the gas causes the tritide to deform by expansion. This expansion has little effect on the crystallographic lattice parameter of the material although it is readily apparent as a bulk dilation (film thickness increase). The deformation after exceeding the yield limit of the tritide (plastic deformation) does not return to its original dimensions even if the helium escapes. As the ultimate strength of the tritide is exceeded, bubbles rupture and cause the film to spall. The apparent bulk dilation is reduced only by the amount of spallation during this process. Continued expansion of the remaining aged tritide is observed as new bubbles develop and expand. This effect appears as little or no change in total expansion in Fig. 3. Another, and less plausible, explanation is that the film is separating slowly and steadily from the substrate. One would expect very early (2 1 year) film peeling if separation were the explanation of the observations. If all of the observed expansion occurred in 100 or less atomic layers the tensile elongation limit for the tritide would be exceeded in 1 year. This explanation also does not account for the observed partially spalled films. J. Less-Common
Metals, 21 (1972)
210
L. C. BEAVIS,
C. J. MIGLIONICO
It is apparent that, qualitatively, all of the tritide films behave the same. That is, they all show signs of bubble formation and subsequent structural failure of individual and collections of bubbles (blisters). The life time to bubble and blister failure and gross film failure is a variable which appears to be related to the metal tritide under consideration. For holmium and yttrium tritides a high expansion rate and large change in surface roughness (evident from the optical quality of the films) was apparent immediately before complete spalling. All of the samples of titanium, erbium, and scandium tritides investigated by SEM showed the same optical behavior as holmium and yttrium tritides, but they did not show the large apparent expansion rate increases seen in 4 years or less by the samples of these two tritides. The increase in expansion rate immediately before complete spallation may indicate gross separation of film and substrate which would more than compensate for the reduced expansion due to blister spall. Holmium and yttrium tritides display this effect early. Scandium tritide shows this effect after 5 years, which is also followed by complete spalling. The erbium and titanium tritides never showed this feature. It should be restated that all of the films had approximately the same atomic ratio of tritium-to-metal when they were formed (1.7-1.8). Figure 3 plots on the upper abscissa the helium-3 concentration assuming that the helium concentration in the film is governed by the decay rate of tritium. This assumption is probably valid only until appreciable cratering occurs. At the time a blister or bubble ruptures, the helium contained therein undoubtedly escapes. Nucleation of bubbles must occur early in the life of the tritide to account for the lack of gross lattice distortion. Times of the order of 1 year or less to bubble formation appear reasonable (Jones3 and Lundin’~‘). The helium concentration at age 1 year is about 3.5 at.%, i.e., MT,,,_XHe, where x=0.1 or, for example, 4x 10zl atoms/cm3 in the titanium films. Eventually the helium pressure in a bubble apparently exceeds the ultimate tensile limit of the material and ruptures. The helium-3 concentration at this stage, approximately 4 years,if one assumes no loss of the helium-3 formed’, is about 13.0%, i.e., x=0.36 or 1.4 x 10” atoms/cm3 in titanium tritide. This rupturing of bubbles is quite apparent in the SEM photographs and the interferometry pictures. The effect of the tendency of the metal to reform to its natural hexagonal crystallographic structure as the helium is formed is unknown and may be of crucial importance to bubble formation and spalling. Film thickness is probably not an important consideration since individual samples of one type of tritide vary by as much as 50’% in thickness, but all tritides of a given metal seem to fail at about the same time but randomly with thickness. Deposition parameters, e.g., rate and substrate temperature, may influence subsequent time to bubble formation and tritide failure. It also seems likely that local high stress concentrations, such as one would expect near replication of substrate scratch marks, cause film failure (Figs. 5 and 7). CONCLUSIONS
(1) The expansion rate of metal tritide films is about 5 to 10 x 10e3 %/day for all of the tritides studied over the first 5 years, except for scandium tritide which starts to expand at a much higher rate. Eventually (at 4-5 years) the scandium tritide average .I. Less-Common
Metals, 27 (1972)
STRUCTURAL
BEHAVIOR
OF METAL
TRITIDE
FILMS
211
rate is about 10 x 10e3 ‘!/day. These rates are much the same as those reported for newly formed bulk tritides (to 1 year)3.6.‘. (2) It appears that all of the metal ditritide films studied fail structurally to a significant extent in 5 years or less through a mechanism related to helium-3 accumulation, as bubbles, in the tritide. (3) Qualitatively all of the metal tritides behave the same. However, quantitative differences are noted such as film expansion rate, total expansion to spalling. time to film spalling (bubble Failure), density of craters, and crater size. (4) In all films investigated, failure seems to be by a brittle fracture mechanism. The fracture occurs at random depths in the film, as one might expect if the helium-3 is initially randomly distributed and its subsequent aggregation is also random. (5) It appears that in some cases bubbles may coalesce to form a blister which eventually fails. This is evident from stepped crater bottoms. (6) The length of time to noticeable tritide film peeling increases in the following order: titanium, holmium, yttrium, erbium, and scandium. The maximum time to which craters appeared in the tritides is holmium (23 months), helium concentration = 6.7 at.‘/, ; scandium (35 months) helium concentration = 9.5 at.‘! ; yttrium (46 months), helium concentration= 12.5 at.‘/“; erbium (47 months), helium concentration = 12.5 at.‘/& and titanium (56 months), helium concentration = 15 at.:,;. (7) The quantitative effects of deposition parameters, film structure, and stress concentration on bubble formation and the related film integrity are unknown but are qualitatively apparent through enhanced failure of aged tritides at substrate scratches. The influence of bubble size and failure on total film spalling is unknown in tritide films, but is considered by Blewe? for helium-implanted deuteride films. ACKNOWLEDGEMENT
The authors wish to acknowledge the assistance of G. L. Knauss who tritided the films and K. D. Mills who took some of the interferometry photographs. REFERENCES I A. M. Rodin and V. V. Surenyants, Coefficient of diffusionol’helium in titanium. Fi:. \Ic,tol. \I~,r~rllo~~e~/.. 10 (1960) 216-222. 2 V. I? Vertcbnyi. M. F. Vlasov, A. L. Kirilyuk, R. A. Zatserkovshii. M. V. Posechnik and V. A. Stepaneko. The behavior of He’ in ZrT, and zirconium, 41. Enrtq. (USSR). 22 (1967) 235-237. 3 P. M. S. Jones. W. Edmondson and N. S. McKenna. The stability 01”metal tritides~ yttrium tritide. J. N~,cl. \Irlter.. 23 (1967) 309-312. 4 W. J. Mueller, J. P. Blackledge and G. C. Libowitz, Metal Hydrides, Academic Press, New York, 1968. 5 R. S. Blewer, private communication, March, 1971. 6 C. E. Lundin, private communication, Denver Res. Inst., February, 1969. 7 C. E. Lundin, private communication, Denver Res. Inst., February, 1970.
J. Less-Common
Metals, 27 (1972)