Studies on a new ordered oxide of uranium and yttrium

iJOURNAL OF NUCLEAR

STUDIES

~ATRRIALS

ON A NEW

35 (1970)

126-130.

ORDERED

S. F‘. BARTRAM

0 WORTH-HOLLAND

OXIDE

PUB~ISH~G

OF UR’ANIUM

and E. 8. PITZSIMMONS

AND

YTTRIUM

14 October

Uranium dioxide which is heated in a reducing atmosphere to temperatures above about 1800 “C! becomes hypostoichiometric 132). Upon cooling to room temperature, uranium metal

USA

1969

between 1800 and 2400 “C for time periods from 40 to 165 h to produce various levels of oxygen deficiency resulting from oxygen permeating through the tantalum 2). Tantalumencapsulated UOZ was also included in these tests as a standard of comparisou. All subsequent handling of these pellets was performed in an argon-filled dry box to prevent air oxidation.

precipitates out as a secondary phase in a matrix of stoichiometric UOs. Various metal sesquioxides have been added to UOZ as stabilizers, but the results have been somewhat inconclusive. During studies on the effect of YsOs additions upon UOs, the mode of oxidation of these solid solutions was carefully followed by X-ray and chemica1 analyses. The analytica difficulties which were encountered and the

*

**

CTeneralElectric C~rn~~n~, Nudear Systema Progrcsms, C~~~~~~~, Ohio 45215, Received

CO., AMSTERDAM

Sample preparation : Solid solutions of UOs and 5, 10, and 15 weight percent YsOs were prepared by di~olvi~lg the oxides in 1 : I HNOs,

Analytical methods : After heat treatment, the free uranium contents were determined by the hydridillg technique of Hibbits and Schaefer 3). The amount and distribution of precipitated uranium was also qualitatively determined by metallographic exami~l~tion. Representative pieces of each pellet were weighed in argon and during exposure to ambient air at 21 “C until no further increase in weight could be detected. This oxidation process usually took several days to attain

diluting the solutions and adding NHdOH. The resulting coprecipitate of hydroxides was filtered, dried, oalcined in air at 600 “C, and then reduced in hydrogen at 700 “C. Pellets were prepared by pressing, and then were sintered at 2000 “C for two hours in argon. Metallographic and of these heat treated X-ray examination specimens showed that they were single-phase fluorite-type solid solutions. Chemical analysis indicated that free uranium metal was essentially absent. The pellets were then encapsulated in tantalum and heated in hydrogen at temperatures

equilibrium. These saturation weight gains are listed in table 1 along with the amount of free uranium metal as determined by chemical analysis. The calculated weight gains listed in a footnote to table 1 were obtained by assuming that, after reduction at high temperatures the samples contain tetravalent uranium in an aniondeficient structure and that oxidation at room temperature produces a filled fluorite lattice with an oxygen : metal ratio of 2.0 (U1_sYz02 composition). Note that, the observed weight gains for samples reduced at temperatures up

appearance of a distinct new complex compound are the subject of this short note.

*

This paper originated from work sponsored by the Fuels and Materials Branch, US Atomic

Commission, **

under Contract

Present address:

AT(40-l)-2847.

Fusite

Corp.,

6000 Fernview

Ave., 126

Cjncinnati,

Ohio 45212.

Energy

A NEW ORDERED

Effect

of heat treatment

OXIDE

in hydrogen a on the

OF URANIUM

127

AND YTTRIUM

TABLE 1 oxidation behavior and free uranium content of solid solutions

U&-Y303

Oxidation weight gain b (wt %)

Heat treatment Temperature

(“C) 165 71 17 41 45 92 100 100 90 95 95

1800 1900 2000 2000 2000 e 2000 2000 2000 e 2200 2200 2200 e

1

4.40

0.26

0.63

0.27

0.60

0.25

0.70

4.39

3.13 ~

0.64

0.26 c ~

0.31

4.87

3.90

0.0

0.0

0.03 c

0.34 c

5.29

4.41

0.0

4.57

5.27

0.84 0.84 0.83 3.23 d 0.53 3.38 * 0.35 c 3.87

2.70 3.39

0.12 0.11 0.78 2.22 2.85 3.33 7.32

8.02

8.09

2.63

0.37 0.57 1.11 1.59 2.65 2.65

6.10 6.51

6.24

0.30 1.14 0.92 1.06 2.46

6.15

a Moisture content < 50 ppm. b Calculated weight gains for solid solutions of 5, 10, and 15 percent YzO3 are 0.31, 0.67, and 1.08 percent, respectively. c

After

d

Not

pulverizing.

e

Weight

measured. gain at 700 “C.

to 2000 “C agree quite well with those calculated. The decrease in amount of oxygen absorbed by samples after heat treatment at higher temperatures might be explained by the presence of free uranium within the grains or by the increased grain size (decreased porosity); both of

protective film. All X-ray patterns were taken with filtered Cu-Kb radiation. Some of the reduced specimens were also examined by X-rays after being mounted and polished for metallographic examination. In these instances, the large grain size and preferred

these factors would tend to inhibit the migration of oxygen through the samples. Pulverizing these pellets produced a slight increase in the

orientation made interpretation somewhat more difficult. Oxidized powders were packed into the

amount of oxygen absorbed, but the total amount was still less than that required to fill all of the anion vacancies.

standard Norelco sample holder and powder diffraction patterns were run in the usual way.

X-ray analysis : X-ray samples were prepared by grinding to a fine powder (200 mesh), filling the cavity of a circular brass holder with this powder, and then covering the specimen with 0.006 mm Mylar film held in place by means of a brass ring. This technique provides a sample which is protected from air oxidation and can readily be examined with the X-ray diffractometer. Spontaneous oxidation in ambient air was observed by X-rays after removal of the

The lattice parameters of face-centered cubic solid solutions were determined from the highangle X-ray reflections recorded between 100 and 160” (20). The reported aa values are estimated to be accurate to i 0.001 8. Discu.ssion of results The free uranium analyses in table 1 indicate that YsOs additions do not significantly reduce the amount of free metal which is precipitated from the solid solutions during cooling to room temperature. In fact, there is no difference

128

S. F. BARTRAM

between the free uranium content and the sample containing

AND

E.

of the UOz

uranium

present

after

treatment

as low as 300 “C and after

This suggests a solubility of about 10 mol % YOi.5 in (x-UaOa. The 10 wt o/o YzOa sample oxidizes in a

at

values given in the table are based

on gross sample weight uncorrected

even

heating at 700 “C no fee solid solution is detected.

2200 “C, but it should be pointed out that the analytical

FITZSIMMONS

dominates

5 wt o/o Y203. With

higher yttria contents there is a small decrease in free

S.

completely different way. At 300 “C only t’he free metal is oxidized to OI-UaOa. The major

for yttria

content.

phases are a fee solid solution

Analytical

determination

of these reduced

of the composition

fee solid solutions

expected

orthorhombic

and a new m-

compound.

These

are

is quite

stable to at least 475 “C; at 700 “C both phases

difficult. The usual determination of UOa is complicated by the fact that the Ui_,Y,O2 solid solution dissociates into n-UaOa and a solid solution richer in yttria. From previous

dissociate to produce fee U0.5Y0.502 and 01-Ca08 solid solutions. The oxidation reactions are illustrated by the following diagram: U

work on this system a), under oxidizing conditions the fee limiting composition is UO.~Y~.JO~ which contains pentavalent uranium. In order to understand the oxidation behavior of these solid solutions, X-ray techniques must be used in conjunction with chemical analysis.

400 “C in air

Heat Reduced

treatment,

2

solutions

oxidized

temperature 1 29.7 mol y.

FCC S.S. a0=5.444

in air at ambient

FCC

a0=5.415

b

Major

phase

Trace

fee S.S.

a-Us08

Major

in air at 375 "C (Only

wU3Os

I

detected

ao=5.399 A

amount

fee

in air at 475 “C , No

change

Oxidized

in air at

change

No

fee

orthorhombic

phase

Trace

nr-UsOs

a-UsOs amount fee phase ‘Major S.S. ao=5.390 B Trace

n-UaOs

K-U308

No

change

~Major amounts 9.s. a0=5.35

solution.

amount

S.S. a0=5.390 A

a0=5.400 A

Oxidized

700 “C ~No

Major

d.

Major complex orthorhombic Minor amount fee S.S.

~Trace

A

FCC S.S. II

9s. a0=5.401 Trace

YOi.5

S.S.

a0= 5.436

B

FCC S.S.

FCC S.S. i a0=5.454

(22 “C)

in air at 300 “C

S.S.= solid

at low

21.0 mol 7; Y0i.j

Minor complex

Oxidized

Orthorhombic

Increasing the yttria content to 15 weight percent produces a reduced solid solution which behaves more normally. When oxidized at

~ 11.2 mol y. YOi.5

“C

temperature Oxidized

solid

in Hz at

2000-2400 Oxidized

of UO2-YOi.5

+

solid solution

TABLE analysis

u0.70Y0.2102 /\

a-&OS

The results of X-ray analysis are summarized in table 2 which shows the phases present after the different stages of heat treatment. The sample containing 5 wt o/o YzOa oxidizes in much the same way as UOa. Alpha-UsOa pre-

X-ray

+

of a-UaOa and fee

.L%

change

A NEW

300

ORDERED

IX-Us08 and the phase-limiting

URANIUM

AND

dissociates

previous

is

sample

not

detected

at

any

temperature. These results indicate the problems associated with analysis of UOa-YzOa solid solutions. It cannot be assumed that oxidation at 900 “C will convert all of the UOa to Us08 because the O/U ratio in such solutions is only 2.50 instead of 2.67. Furthermore, subsequent reduction of these solid solutions in hydrogen does not yield an O/U ratio of 2.00, but it may remain as high as 2.25. Therefore, a chemical analysis which involves hydriding of the free uranium metal followed by combustion at 900 “C, assuming only UOZ.S~ formed, will not give correct results. The X-ray diffraction data for the orthorhombic phase were analyzed to determine the composition of this new compound. Since the only systematic absences were the (Ml) reflections when h+ 1 # 2n, the centrosymmetric space group Bmmm was chosen. It was assumed that the atoms occupy a slightly distorted fluorite-type lattice and relative line intensities were calculated with the computer program of Yvon, Jeitschko and Parthe 5) for different distributions of uranium and yttrium in the face-centered sites. The best agreement was obtained with the following arrangement : 2 U atoms in O,O,O; Q,O,$ 2 Y atoms in 011. 110 92J2, 2,2J 0

atoms

in

&

1_114,494,

131. 474749

X-ray

powder

pattern

311. 4,494,

of UYO4

orthorhombic

/ Pbserved

Calculated

phase

I Chl;w$$

mtenslty

101

3.80

3.80

11

14

111

3.15

3.15

125

100

I

2.813

2.816

18

11

200

2.757

2.760

13

11

002

2.626

2.626

20

9

210

2.476

2.478

5

5

121

2.262

2.263

4

7

2’>0 _

1.970

1.971

13

17

022

1.921

1.921

18

16

202

1.903

1.902

20*

15

212

~ 1.800

1.802

8

4

301

1.735

1.736

6

2

131

1.683

1.683

13

14

311

1.660

1.659

9

14

113

1.601

1.600

15

12

222

1.576

1.576

9

8

321

1.477

1.478

5

2

123

1.436

1.436

1

2

040

1.407

1.408

2

2

400

1.379

1.380

1

2

410

1.341

1.340

2

1

232

1.337

1.336

2

1

141

1.321

1.320

1

1

004

1.313

1.313

3

2

014

1.278

1.279

1

1

331

1.275

1.275

3

5

020

i

303

1.270

1.268

2

1

240

1.255

1.254

1

2

133

1.247

1.247

5

5

042

1.241

420

1.239

2

1.239

7

2

313 i 402

1.221

1.221

1

412

1.195

1.194

3

1

024

1.191

1.190

2

2

204

~ 1.187

1.186

2

2

1.237

331 4,414’

These results indicate that the ideal composition of the orthorhombic phase is UYO4, in which the O/U ratio is 2.50. With the limited X-ray data it is not possible to locate the oxygen atoms with any accuracy; they may shift to other positions during oxidation. The X-ray data are presented in table 3. It is difficult to explain the appearance of this new complex compound only in the 10 wt y. YzO3 sample. The ordered 1 : 1 distribution of

3

d (ii)

hkl

129

YTTRIUM

TABLE

solid solution

containing 50 mol o/o YOr.5. The complex orthorhombic phase which was observed in the

8

OF

“C a fee solid solution exists as the major

phase. At 700 “C the solid solution into

OXIDE

1.160

1.158

1

2

1

4

1.156

4

242’

1.133

1.132

3

I 1 4

422

1.122

1.121

1

4

341~’

224

151

*

‘,

1.094

1093

~ 1.080

!

FCC reflection Orthorhombic reflections ao=5.519 163.3

ii3.

1

4

1.093

’

1.080

(4 3

3

superimposed. space

absent

group

when

A, bo=5.632

h+l

-

Bmmm:

(hkl)

# 2n.

& c0=5.252

A. Volume=

S.

130

F.

BARTRAM

AND

E.

S.

FITZSIMMONS

uranium and yttrium

atoms probably

requires

2) E. A. Aitken, H. C. Brassfield and R. E. Fryxell,

carefully

annealing

cooling.

Thermodynamics 2 (IAEA, Vienna, 1966) p. 435 3) J. 0. Hibbits and E. A. Schaefer, Anal. Chem. 38 (1966) 1687 4) S. F. Bartram, E. J. Juenke and E. A. Aitken, J. Am. Ceram. Sot. 47 (1964) 171 5) K. Yvon, W. Jeitschko and E. ParthB, A Fortran IV program for the intensity calculation of powder patterns (1969 Version), Univ. of Pennsylvania, Laboratory for Research on the Structure of Matter, Philadelphia, Pa., 19104

These

controlled conditions

samples. UOa

Further

with

determine

rare

may

earth

the existence

orthorhombic

have

equilibrium

oxides

and varied phase

among

studies

of

are needed

to

of other similar

ordered

compounds.

References 1) J. S. Anderson, H. W. Wormer, G. M. Willis and M. J. Bannister, Nature 185 (1960) 915