Oxidation of several lanthanide elements

JOURNAL OF THE LESS-COMMON METALS

OXIDATION

OF SEVERAL

LANTHANIDE

W. L. PHILLIPS,

139

ELEMENTS

JR.

Engineering Materials Labo~~~o~y, Engineerilzg Research Division, E. I. du Pont de Nemows & Co., Enc., W~~rn~~g~~n,Del. (U.S.A.) (Received

December

rsth, 1963)

SUMMARY The oxidation rates of cerium, praseodymium, neodymium, samarium, gadolinium, terbium, and holmium were measured at elevated temperatures. All elements oxidized according to the linear weight-gain law. All OXides formed were of the MeaOs type. The linear scaling rates and the activation energy were the same for all elements when measured at the same homologous temperature. INTRODUCTION

The lanthanide elements have become generally available only in the last decade and the technology of these metals is still in an early stage of development. LORIERS~ found that lanthanum is initially attacked by dry air until a virtually constant weight increase is reached. PHILLIPS2 has shown that although the linear weight-gain law is obeyed in the temperature range 6oo”-75o’C the weight gain discontinuously jumps to a constant value. This discontinuity is similar to that observed in columbiums. The oxidation of cerium has been studied by LORIERS~ and CU~KOC~~OT~~. They found that cerium oxidizes parabolically between 30% and 125’C but linearly above 125°C. The activation energy for both linear and parabolic oxidation was N rz’kcal. Above 32o’C LORIERS~ found that combustion of cerium occurs in dry air. EXPERIMENTAL

PROCEDURE

The lanthanide elements listed in Table I were purchased from the American Potash and Chemical Companys. The samples were cut, under oil, into rectangular shapes TABLE PHYSICAL

CONSTANTS

OF SEVERAL

Ce Pr Nd Sm Gd Tb Ho $* =

I

LANTHANIDE

ELEMENTS

%303

1.07

Ndz03 Sm&a GdeOs TbaOa Ho&h

1.11 I.2I

PrzO3

AND

OXIDES

I.11 1.20

I.33 I.34

molecular volume of oxide/atomic J. Less-Common

weight of metal. Metals, 7 ($964) 13g-r43

W. L. PHILLIPS,

JR.

OXIDATION

OF LANTHANIDE

r4r

ELEMENTS

Q= 7.4 K cal

12OO~C IOOO~C

I

Fig.

II

RECIPROCAL

ABSOLUTE

constant

oxidized

present

results

demonstrate

that

600 “C

,I

I

1000

1200

TEMPERATURE(K”xl~-G)

us. reciprocal

4 h at ~ooo”C.

DISCUSSION

The

I 600% I

600

1. Scaling-rate

Fig. 2 Samarium

I

600

absolute

x 6.before

tcmpcrature

reproduction

OF RESULTS

the

activation

energy

for the oxidation

of

cerium at high temperatures in moist air is not significantly different from that observed at low temperatures in dry air 4. It appears that the only significant difference is that spontaneous combustion of cerium does not occur in moist air at temperatures above 320°Ci. Zirconia crucibles were used in the present work while alumina crucibles were used previously 1. It was found that alumina dust sprinkled on cerium catalyzed the oxidation and led to spontaneous combustion at 700°C in moist air. The cause for this reaction with alumina is not known. J. Less-Common

Metals,

7 (1964)

139-143

142

W. L. PHILLIPS,JR.

It has been shown that lanthanide series elements with a # value from 1.07-1.34 obey the linear weight-gain law at elevated temperatures. This is at variance with the PILLING-BEDWORTH~ hypothesis. Despite a favorable $ ratio the oxide scales formed at elevated tempertures cracked excessively and exfoliated. One must look for another mechanism to explain the lack of adherance of the oxide scale with resulting poor oxidation resistance of the lanthanide elements. LORIERS~has suggested that the sesquioxide is formed as a continuous oxide layer adhering to the metal and that this oxidizes at a constant rate to form the dioxide in a form porous to molecular oxidation and hence prone to cracking. At high temperatures it may be that the layer of sesquioxide is extremely thin and hence is not detected by X-ray examination. The instability of the lanthanide elements at room temperature prevented the use of normal metallographic practices to determine the presence or absence of this layer.

Fig. 3. Scaling-rateconstantVS.T/T,, whereTmis the melting point.

Although the activation energy for the lanthanide elements investigated is the same, there is considerable variation in the scaling constants for a given temperature. Me203 oxide is formed on all elements during oxidation. Comparison of the data in Tables I and II show that the higher the melting point, the lower the linear scaling rate at a given temperature. This suggests that the scaling rate might be the same in all lanthanide elements at the same homologous temperature. The linear scalingrate constant as a function of T/Tm,where Tm is the melting point in “K is plotted in Fig. 3. The scaling-rate constants, when measured at the same temperature ratio, J. Less-Common Metals, 7 (1964) 139-143

OXIDATION

OF LANTHANIDE

I43

ELEMENTS

are not significantly different. This demonstrates that, for the lanthanide elements considered, the linear scaling rates are the same and the activation energy is identical at the same TIT,+ ACKNOWLEDGEMENTS

The author expresses his thanks to J. preparation of this paper.

DANIELS

for many helpful sessions during the

REFERENCES 1 J. C. LORIERS, Acad. Sci. Paris, zzg (1949) 547; 231 (Igp) 522. 2 W. L. PHILLIPS, Jr., Accepted by J. Electrochem. Sot. 3 T. L. KOLSKI, Trans. Am. Sot. Metals, 55 (1962) 119. 4 D. CUBIOCCIOTI, J. Am. Ceram. Sot., 74 (1952) 1200. 5 Lindsay Rare Earth Chemicals Brochure, American Potash and Chemical Company. 6 N. B. PILLING AND R. E. BEDWORTH, J. Inst. Met&, 29 (1923) 529.

J. Less-Common

Metals, 7 (1964) 139-143