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MICROCOPY RESOLUTION TEST CHART
NATIONAL BUREAU OF STANDARDS - 1963
ORNU P-1899
CONF-651055-1
PREPARATION OF ISOTOPIC METALLIC FOILS AND BULK ISOTOPIC METALS FROM OXIDES
MASTERS

E. H. Kobisk
ORNL - AEC - OFFICIAL
KALTASID FOR ANOUKCEMINI
JAN 2 0 1966

Isotopes Development Centar
Oak Ridge National Laboratory
Oak Ridge, Tennessee
IN MUOLLAR SCIDNCE ABSTRACTS
Inipurities in isotopic materials used to study nuclear reactions and proper-
ties frequently cause inaccuracies in resolution by broadening observed
resonances with overlapping peaks of similar energies from other isotopes
C: the same element or different elements, or by masking weak resonances
or specific nuclei by stronger impurity peaks in the same energy region.
Furthermore, the specific number of atoms in a sample must be known for
accurate cross-section measurements, and impurity atoms of unknown number
and type can adversely affect measurement results. For these reasons, the
Oak Ridge National Laboratory Target Center initiated a program several
years ago to produce high-purity isotopes for use in making targets in thin
film, rolled foil, or bulk forms. Oxygen and nitrogen content have been
reduced by melting metals in vacuum using electron-bombardment heating.
Other techniques being developed to enhance the chemical purity of isotopes
and to form them into targets include zone refining, pyrolytic deposition
of metals, single-crystal preparation, and vacuum reduction of oxides to
metals with simultaneous distillation of the product.
Both resistance heating and electron-bombardment heating have been used to
convert various oxides to their elemental forms. The use of a variety of.
metal reductants has been studied, particularly for the preparation of the
metals of rare-earth isotopes. Isotopes of the rare earths in metallic
form for use in physical research have previously been unobtainable.
Oxide reduction at high temperature was first investigated to provide
metallic magnesium isotopes either in bulk quantity or in thin film form.
Powdered magnesium oxide was mixed with a slight stoichiometric excess
of aluminum powder and heated on a tartalum filament to ~1100°C. Magnesium
was produced by the following reaction:
3MgO + 2A1 → 3Mg 1 + A1203
Use of a simple resistance filament, however, caused excessive loss of
pressure is ~1Q®* torr at >1020°С. Filament geometry itself prevented
good temperature control.
To obtain a high-purity product at a low distillation temperature, several
resistance heating methods were studied; only the most productive will be
noted here. A mixture of powdered magnesium oxide and a 100% stoichiometric
excess of aluminum powder, in the form of a pressed pellet, was placed in a
tantalum crucible (Fig. 1). Pelleting was used to achieve intimate contact
of reactants and to obtain better heat conduction. .
NL-AEC - OFFICIAL
*Research sponsored by the U. S. Atomic Energy Commission under
contract with the Union Carbide Corporation.
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Since the pellet remained intact during the reaction, handling problems
and crucible contamination were reduced to a minimum. A cap with a 40-
mil-dia effusion port was placed on the crucible 80 as to collimate the
vapors. The crucible was heated resistively with low-voltage high-current
ac power; crucible tempera
ible temperature was measured with a chromel-alumel thermo-
couple spot-welded to the crucible wall. Rapid reduction with simultaneous
distillation of the product was found to occur at -2018°C. The metal
distillate could be condensed onto a glass plate substrate and removed by
peeling without using a parting agent or without exposing it to water. At
-780°C with a pressed pellet, a product containing <200 ppm of aluminum
was routinely obtained.
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To obtain bulk material, a water-cooled, copper substrate was placed over
the effusion port ~1/16 in. above the cap. As the metal distilled out of
the crucible, a stalactitic growth formed on the substrate blocking the
e t'fusion port. Continued distillation of metal inside the plugged crucible
formed a crystalline deposit inside the cap as an extension of the stalac-
tite. The polycrystalline nature of the magnesium "flowers" cen be seen
in Fig. 2. Extremely high-purity metal was obtained in this manner because
the deposits were grown in an atmosphere of the metal vapor after initial
exclusion of oxygen, nitrogen, and other residual gases during the growth
of the stalactite from the water-cooled substrate. Although activation
analyses have not been performed on this metal, the purity level is such
that foils of magnesium can be prepared by cold rolling - a feat performed
only on normal magnesium having very high purity.

eur
The condensation zone for the magnesium metal distillate was maintained
at a lower temperature than the reaction zone so as to obtain the flower
configurations. If the temperature was too low, growth did not occur,
e entire cap of the crucible was coated with metal. Single-
crystal growth could be achieved, but the narrow temperature interval
was difficult to maintain because of changes in the rate of heat dissipa-
tion in the crystal as it became larger. A single crystal ~6 mm in dia
of 24 Mg was grown in this manner.
WITAMIN
2
Rare-earth metals of high isotopic purity have been prepared by similar
techniques and rolled into thin foils < mg/cm? in areal density of nego
min. thick. Figure 3 illustrates: flowers, pressed pellets, and a rolled
foil of ytterbium metal produced in this manner. Rare-earth metals suc-
cessfully prepared by reduction-condensation with <300 ppm of reductant
as impurity are Eu, Gd, Ho., Er, Yo, and Lu. The reductant used for each
of the metal oxides is given below:
Metal Oxide
Eu203
Gd203
RELEASED FOR ANNOUNCEMENT
'IN MUCLEAR SCIDICE ABSTRACTS
Ho203
Er203
Reductant
Aluminum powder
Zirconium powder
Tantalum filings
Zirconium powder
Aluminum powder
Zirconium powder
Yb2O3
Lugog
Selection of the reductant became more important when higher distilla-
tion temperatures were required so as to minimize contamination of the
metal with co-distilled reductant.
Calcium metal isotopes have also been prepared as flcwers using aluminum
or lanthanum as reductants. Lengthy distillations at
temperature wore required to keep aluminum contamination to a minimum. A
*2Ca foil of areal: density < mg/cm² was produced by reducing 50 mg of
*CaO with aluminiin, condensing the metal, and cold rolling in an argon
atmosphere. This technique has also been applied to the production of 3 g
of 21+ Am in very ductile and bright metallic fcrm. Similarly, strontium
1sotopes have been prepared as metal and rolled into foils. It should be
noted that both electron-bombardment and resistance heating techniques
have been used to prepare these metals by oxide reduction and vaporization-
condensation of the metal products.
Another technique that has proved most useful in preparing high-purity
metal isotopes is that of pyrolytic decomposition of a volatile compound,
usually a metal iodide of the desired element. For example, zirconium
iodide, formed by direct reaction with elemental iodine at -350°C, can
be volatilized in a chamber containing an electrically heated tungsteni
filament at 1200°C; the purified zirconium metal is deposited through
pyrolytic decomposition on the filament and the free iodine can return
to the impure zirconium metal reservoir to react again. This technique
works only where impurities do not form volatile iodides. With this
technique, classically referred to as the Van Arkls-de Boer process, we
have successfully prepared samples of between 1 and 10 g of zirconium,
hafnium, and titanium isotopes and purified niobium metal. In all cases
the impurity levels have been <200 ppm. Physical evidence of this low
impurity content may be noted by the high ductility of the materials
and our capability of rolling these purified isotopes to thicknesses
of ~0.5 mg/cm.
Rolling techniques employed at the ORNL Target Center do not vary signi-
ficantly from most standard rolling processes. To be rolled, a sample of
the metal isotope is usually melted into a bead which is then pressed and
rolled in a sandwich of Type 304 stainless steel. Two types of rolling
mills have been employed: a 4-high mill having 1.5-in.-dia work rolls
and another having 1.75-in.-dia work rolls. Iwo small 2-in. hand mills
have been motorized and placed in glove boxes under argon atmospheres so
that reactive metals like calcium, strontium, and rare-earth metals can
be rolled to very low thickness. At present our technology has allowed
a minimum thickness of 2250 ug/cm of nickel to be rolled. Almost all
materials can be rolled in the cold form rather than by using hot rolling
techniques.
12/28/65
LEGAL NOTICE
The report we prepared as an account of Government sponsored work, Nelther the United
Statas, nor the Commission, nor Lay person acting on behalf of in Commission.
A. Makes my warranty or representation, exproned or implied, with reopect to the accu-
racy, completament, or w halaose of the Information contained in the report, or that the wo
of way information, apparatus, method, or process declared in this report may not infringo
primtaly owned rights or
B. Ammos hay liabilities with racprot to the use of, or for damago, resulting from the
un of way tnformation, apparatus, method, or procon dinclound la dels report.
As wood in the above, "person acting on ball of the Commuky" includes may one
ployee or contractor of the Counluston, or employee of such contractor, to the extent that
soch employee or contractor of the Commission, or employee of such contractor prepares,
denominatas, or provides access to, wy taformation purnimat to Woonployment or contract
with the Commission, or dis emaployment with such contractor,
ORNI - AEC - OFFICIAL
FIGURE LIST
Fig. 1. Tantalum Metal Crucible Used for Resistive Heating of
Magnesium Oxide Aluminum Metal Pellets. Effusion plug (left) with
0.040-in. port is used to cap the crucible during growth of magnesium
"flowers."
Fig. 2. Magnesium Metal Condensed from tho Vapor Phase During
Distillation from a Magnesium Oxide Aluminum Metal Pellet.
Fig. 3. Ytterbium Metal Condensed from the Vapor Phase During
Distillation from a Ytterbium Oxide Aluminum Metal Pellet. · Metal beads
are shown pressed (lower right hand corner) and finally rolled into
foil 1 mg/cm2 thick.
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