Industrial Applications of Pure Rare Earth Metals ... - ACS Publications

9 Industrial Applications of Pure Rare Earth Metals and Related Alloys Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 6, 2016 | http://pubs.acs.org Publication Date: September 3, 1981 | doi: 10.1021/bk-1981-0164.ch009

K. E . DAVIES Rare Earth Products Limited, Widnes, Cheshire, England

In recent years researchers both i n the U.S.A. and Europe have expended c o n s i d e r a b l e time and effort i n seeking t o prepare rare e a r t h metals of very h i g h p u r i t y . For the most part t h e i r work has been s u c c e s s f u l and today it is p o s s i b l e t o o b t a i n s e v e r a l metals having an absolute p u r i t y of 99.99%. Understandably such metals tend t o be available only i n small q u a n t i t i e s and their true cost of p r e p a r a t i o n is o f t e n discounted when set against a particular requirement. By c o n t r a s t , the industrial user must view any raw m a t e r i a l purchase i n terms ofitscost e f f e c t i v e n e s s and in the case of r a r e e a r t h metals t h i s f r e q u e n t l y r e q u i r e s the adoption of low purity specifications. Therefore, i n the context of the industrial a p p l i c a t i o n s to which reference will be made, the term 'pure' will be taken t o i n c l u d e all metals having a purity of not l e s s than 95% - the balance being predominately other r a r e e a r t h s . But before c o n s i d e r i n g the a p p l i c a t i o n s in detail it is perhaps of value t o have some a p p r e c i a t i o n of the s i z e of the market f o r these metals. I f we assume current world production of a l l rare earths t o be of the order of 35,000 tons per year, expressed as r a r e e a r t h oxide, then approximately 1% of t h i s t o t a l , equating t o about 230 tons, represents the current l e v e l of p r o d u c t i o n of pure metals. While i n i t s e l f t h i s f i g u r e may appear i n s i g n i f i c a n t , i t i s necessary t o view i t i n the l i g h t of two other f a c t o r s . F i r s t , expressed i n monetary terms the 1%, equates t o nearer 7% of t o t a l value and secondly, over the past 3 years demand f o r pure metals has been i n c r e a s i n g a t a r a t e approaching 20% per year which, i f s u s t a i n e d , could r a d i c a l l y change the face of the i n d u s t r y w i t h i n a short space of time.(l.) Of course i t can be argued that the growth we are now seeing merely represents the l o g i c a l development of the i n d u s t r y ; once having achieved commercial success i n high volume s e p a r a t i o n of pure oxides by solvent e x t r a c t i o n i t would seem only n a t u r a l that the i n d u s t r y should then t u r n i t s a t t e n t i o n t o l a r g e s c a l e prod u c t i o n of the metals. But i n p r a c t i c e i t has not worked out that 0097-6156/81/0164-0167$05.00/0 © 1981 American Chemical Society Gschneidner; Industrial Applications of Rare Earth Elements ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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way - commercially v i a b l e a p p l i c a t i o n s f o r rare e a r t h metals have been slow to develop and the f a c t that i t has taken almost 20 years to achieve the l i m i t e d success we are now e x p e r i e n c i n g i s a measure of the d i f f i c u l t i e s that have been encountered and f o r the most part overcome. Of those f a c t o r s that have proved a d i s i n c e n t i v e to the wider a p p l i c a t i o n of the metals perhaps none has proved more formidable than c o s t . Unquestionably some r a r e e a r t h metals are expensive when compared w i t h many of the more common metals but then o t h e r s , such as the commercial grades of lanthanum and cerium produced by e l e c t r o l y t i c methods, are r e l a t i v e l y inexpensive. U n f o r t u n a t e l y , f o r most r a r e e a r t h metals one must use a metallothermic r e d u c t i o n process that i s i n h e r e n t l y c o s t l y to operate. However, w h i l e there i s a wide v a r i a t i o n i n the p r i c e of the v a r i o u s metals, from $50 $7,000 per l b . , the o v e r a l l p i c t u r e i s one of r e l a t i v e p r i c e s t a b i l i t y when judged against the movements of many other metals. To some extent t h i s s t a b i l i t y has been born out of the need to encourage p o t e n t i a l users to adopt a p a r t i c u l a r metal i n the face of a competitive product w h i l e i n other cases i t r e f l e c t s more the economy of s c a l e that has been possible once production has passed a given l e v e l . Y t t r i u m metal The f i r s t r a r e e a r t h metal to be produced on a l a r g e s c a l e by metallothermic r e d u c t i o n was y t t r i u m i n the e a r l y 1960 s. At the time researchers at General E l e c t r i c discovered that s t a i n l e s s s t e e l s c o n t a i n i n g both aluminium and y t t r i u m possessed e x c e p t i o n a l r e s i s t a n c e to c o r r o s i o n . However, such s t e e l s as were then produced found only l i m i t e d a p p l i c a t i o n i n the nuclear i n d u s t r y and i t was not u n t i l 5 years ago that t h e i r p o t e n t i a l f o r use i n nonn u c l e a r a p p l i c a t i o n s was f u l l y a p p r e c i a t e d . The p a r t i c u l a r s t e e l that has s i n c e won widest a c c l a i m i s termed ' F e c r a l l o y ' ; as the name suggests i t s c o n s t i t u e n t s are i r o n , chromium, aluminium and y t t r i u m . By comparison w i t h e x i s t i n g s t a i n l e s s s t e e l s F e c r a l l o y possesses e x c e p t i o n a l high temperature c o r r o s i o n r e s i s t a n c e - t h i s f a c t above a l l others has l e d to i t being w i d e l y adopted f o r the f a b r i c a t i o n of furnace h e a t i n g elements and, more i m p o r t a n t l y , i t i s the number one contender to replace ceramic s u b s t r a t e s i n emission c o n t r o l c a t a l y s t s f o r the automobile and motor-cycle industries. The e x c e p t i o n a l p r o p e r t i e s of the a l l o y are due i n no s m a l l way to the y t t r i u m component which together w i t h the aluminium forms a s t a b l e and f i r m l y bound oxide l a y e r that e x h i b i t s e x c e l l ent r e s i s t a n c e to exhaust gas emissions at high temperatures over prolonged periods.(2) At the same time, i t provides an i d e a l surface to r e c e i v e another c o a t i n g of metal or metal oxide which, i n the context of c a t a l y s t a p p l i c a t i o n s , i s most e s s e n t i a l . At the present time most c a t a l y t i c convertors u t i l i s e ceramic s u b s t r a t e s which are prone to damage by both mechanical and thermal shock. !

Gschneidner; Industrial Applications of Rare Earth Elements ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


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Applications

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A l s o i n an i n d u s t r y where weight and space are a t a premium ceramic s u b s t r a t e s occupy a l a r g e r volume than would a u n i t of comparable c a t a l y t i c a c t i v i t y manufactured u s i n g F e c r a l l o y as the s u b s t r a t e . I n the r e l a t i v e l y short time that the a l l o y has been a v a i l a b l e f o r l a r g e s c a l e e v a l u a t i o n i t has already gained acceptance by many p o t e n t i a l users i n the automobile i n d u s t r y and, as a r e s u l t , s p e c i a l t y steelmakers both i n the U.S.A. and B r i t a i n are now able t o o f f e r tonnage q u a n t i t i e s i n v a r i o u s f a b r i c a t e d forms. Y t t r i u m i s a l s o used i n other areas of m e t a l l u r g y notably as a component of c e r t a i n n i c k e l - b a s e and cobalt-base s u p e r a l l o y s of the NiCrAlY and CoCrAlY type.(3) These a l l o y s possess e x c e l l e n t c o r r o s i o n and o x i d a t i o n r e s i s t a n c e , p r o p e r t i e s that have a t t r a c t e d the a t t e n t i o n of the aero-engine i n d u s t r y where they are used as p r o t e c t i v e coatings on t u r b i n e blades. The a l l o y s , when a p p l i e d by vapour d e p o s i t i o n , form an oxide c o a t i n g that e x h i b i t s remarkable adhesion, a property a t t r i b u t e d l a r g e l y t o the y t t r i u m component a c t i n g t o prevent the formation of voids at the o x i d e / s u b s t r a t e interface.(4) Y t t r i u m a l s o f i n d s a p p l i c a t i o n i n t i t a n i u m a l l o y s where a t concentrations of the order of 200 ppm i t improves the d u c t i l i t y and ease of f a b r i c a t i o n of vacuum arc-melted a l l o y s . I t i s a l s o used t o improve the s t r e n g t h of magnesium c a s t i n g s and when used i n combination w i t h z i r c o n i u m , as l i t t l e as 100 ppm y t t r i u m increases the c o n d u c t i v i t y of aluminium t r a n s m i s s i o n l i n e s by as much as 50%. As one might expect, y t t r i u m i s not without i t s competitors; hafnium has been proposed as a replacement f o r i t i n c e r t a i n i r o n based a l l o y s as have other elements, but i n the context of the a p p l i c a t i o n s as d e s c r i b e d y t t r i u m remains the p r e f e r r e d a d d i t i v e . Lanthanum metal A l l r a r e e a r t h metals can be c h a r a c t e r i s e d as being e l e c t r o p o s i t i v e w i t h respect t o most other metals; t h i s f a c t , coupled w i t h t h e i r l a r g e atomic r a d i u s and high r e a c t i v i t y towards nonmetals, p o i n t s the way to t h e i r widespread use as a l l o y i n g c o n s t i t u e n t s . However, i n the manufacture of d u c t i l e i r o n and i n steelmaking i t i s p r a c t i c e t o use mischmetal or mixed r a r e e a r t h s i l i c i d e s as the r a r e e a r t h a d d i t i v e and from p u r e l y cost cons i d e r a t i o n s t h i s s i t u a t i o n i s u n l i k e l y to change s i g n i f i c a n t l y . But as one moves i n t o the f i e l d of s u p e r a l l o y s and other s p e c i a l i s t a l l o y s then t h i s p a r t i c u l a r c o n s t r a i n t tends t o d i m i n i s h and the cost of u s i n g a pure metal can w e l l be j u s t i f i e d . Such i s the case w i t h c e r t a i n high s t r e n g t h n i c k e l a l l o y s which use cerium a t the 100-300 ppm l e v e l t o c o n t r o l sulphur and oxygen. Of f a r g r e a t e r s i g n i f i c a n c e , however, i s the r a p i d l y i n c r e a s i n g use of lanthanum i n high-temperature a l l o y s . Developed o r i g i n a l l y to meet the demanding s p e c i f i c a t i o n s f o r gas t u r b i n e manufacture, these a l l o y s , which i n c l u d e both n i c k e l - b a s e and cobalt-base types, c o n t a i n t y p i c a l l y 200-400 ppm lanthanum.



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Without i t the a l l o y s show s i g n i f i c a n t l y l e s s r e s i s t a n c e to c y c l i c o x i d a t i o n which would suggest t h a t , as w i t h y t t r i u m i n F e c r a l l o y , the lanthanum i s a c t i n g to provide a f i r m l y bound oxide b a r r i e r . A more recent development that i s a t t r a c t i n g c o n s i d e r a b l e a t t e n t i o n i s the use of lanthanum i n high-temperature iron-base a l l o y s . One such a l l o y manufactured i n the U.S.A. (5) combines e x c e l l e n t o x i d a t i o n r e s i s t a n c e to 1100°C w i t h good d u c t i l i t y and ease of f a b r i c a t i o n . By comparison w i t h other a l l o y s possessing s i m i l a r mechanical p r o p e r t i e s , the manufacturers a t t r i b u t e the a l l o y s s u p e r i o r o x i d a t i o n r e s i s t a n c e to what they describe as a small (200 ppm) but e f f e c t i v e a d d i t i o n of lanthanum. P o t e n t i a l l y one of the major a p p l i c a t i o n s f o r lanthanum i s i n the area of hydrogen storage a l l o y s of the LaNi^ type. These and r e l a t e d i n t e r m e t a l l i c compounds possess the a b i l i t y to absorb and desorb l a r g e volumes of hydrogen at moderate temperatures and p r e s s u r e s . They have a t t r a c t e d the a t t e n t i o n of many who regard them not only as an a l t e r n a t i v e to e x i s t i n g high pressure and cryogenic storage systems but more i m p o r t a n t l y as p r o v i d i n g the means of a c h i e v i n g progress i n the f i e l d of energy c o n s e r v a t i o n . Although the subject i s reviewed i n d e t a i l elsewhere i n the Proceedings, i t i s perhaps opportune to consider here the use of pure lanthanum metal as a component of these a l l o y s . The a l l o y L a N i ^ i s g e n e r a l l y considered as the c l a s s i c storage compound i n that i t was the f i r s t r a r e e a r t h i n t e r m e t a l l i c to be c h a r a c t e r i s e d and i t remains one of the most e f f i c i e n t i n terms of both c a p a c i t y and k i n e t i c s but, u n f o r t u n a t e l y , i t s u f f e r s from the disadvantage of being more expensive than the m a j o r i t y of i t s r i v a l s . This f a c t has prompted i t s c r i t i c s to dismiss compounds formulated on the b a s i s of pure lanthanum as ' n o n - s t a r t e r s i n the context of commercial a p p l i c a t i o n s . While i t i s true that they are u n l i k e l y to f i n d favour i n l a r g e s c a l e p r o j e c t s , there i s ample evidence to suggest that they w i l l be adopted f o r use i n s p e c i a l i s e d a p p l i c a t i o n s where performance r a t h e r than cost i s the determining f a c t o r . For example, w i t h i n my own o r g a n i s a t i o n cons i d e r a t i o n i s being given to c o u p l i n g a hydrogen generator d i r e c t to a LiNitj s t o r e ; i n the f i r s t instance t h i s w i l l be employed i n a m i l i t a r y context but already we see s e v e r a l a p p l i c a t i o n s that could b e n e f i t from the a v a i l a b i l i t y of such a system. In the U.S.A. one can now purchase s m a l l hydrogen storage u n i t s and one of the most v e r s a t i l e c u r r e n t l y a v a i l a b l e employs L a N i ^ as the h y d r i d i n g a l l o y . Other examples of systems employing LaNiej are i n the p i p e - l i n e and w h i l e i t would be true to say that i n d u s t r y has so f a r reacted c a u t i o s l y to t h i s new technology, a t t i t u d e s are changing and one may reasonably expect that t h e i r i n t r o d u c t i o n w i l l g a i n momentum i n the p e r i o d ahead. 1

Samarium metal Next l e t us consider samarium, the f i f t h member of the lanthanide s e r i e s * Twelve years ago i t would have been d i f f i c u l t



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to name any a p p l i c a t i o n s i n v o l v i n g i t s use i n amounts greater than a few pounds per year- Today samarium i s the i n d u s t r i e s b r i g h t e s t s t a r accounting f o r n e a r l y two-thirds of a l l metal usage. The reason f o r t h i s t r a n s f o r m a t i o n i s due e n t i r e l y to the d i s c o v e r y and subsequent development of r a r e e a r t h - c o b a l t permanent magnets. In the e a r l y 1970's when samarium-cobalt magnets were f i r s t being proposed as a replacement f o r platinum-cobalt i n microwave tubes, few could have a n t i c i p a t e d the success that was to f o l l o w . Notwithstanding the traumas brought about by a quadrupling of c o b a l t p r i c e s , t h e i r use has grown s t e a d i l y to the p o i n t where they now command a v i t a l l y important p o s i t i o n i n the permanent magnet market. S i n t e r e d magnets of the SmCo5 type, the f i r s t to be manuf a c t u r e d commercially, possess s p e c i f i c magnetic p r o p e r t i e s that place them w e l l above F e r r i t e and A l n i c o magnets i n terms of performance, see Table I . In p a r t i c u l a r t h e i r maximum energy product i s c o n s i d e r a b l y g r e a t e r , due p r i n c i p a l l y to the extremely high c o e r c i v i t y of the a l l o y . This i n t u r n can be a t t r i b u t e d to the f a c t that SmCo5, by v i r t u e of i t s hexagonal s t r u c t u r e , e x h i b i t s a high degree of u n i - a x i a l c r y s t a l l i n e a n i s o t r o p y . TABLE I - COMPARATIVE PROPERTIES FOR VARIOUS TYPES OF MAGNET

Magnet Type

Maximum Energy Product ( )max kJ/m BH

3

Remanence r T

Coercivity B c kA/m

B

H

Anisotropic Ferrite

26

0.37

240

Alnico 5

40

1.20

52

160 56

0.90 0.55

660 400

SmCo Sintered Polymer bonded 5

These p r o p e r t i e s have provided the design engineer w i t h the means of a c h i e v i n g not only m i n i a t t u r i s a t i o n but performance c h a r a c t e r i s t i c s h i t h e r t o u n a t t a i n a b l e . Today such magnets f i n d a p p l i c a t i o n i n a wide range of products i n c l u d i n g e l e c t r o n i c watches, h i - f i equipment, high-power d.c. motors, magnetic bearings and they are even used i n d e n t i s t r y and surgery. The f i r s t generation s i n t e r e d magnets were based on the b i n ary a l l o y SmCo5 and such magnets s t i l l account f o r the bulk of those now manufactured. However, i n order to meet c e r t a i n cost c r i t e r i a i t was soon recognised that i t would be to the b e n e f i t of a l l to provide a l e s s expensive type of magnet based on a modofied composition and t h i s was f i r s t achieved by p a r t i a l



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s u b s t i t u t i o n of the samarium by mischmetal- Today, a l l o y s of t h i s type o f f e r the optimum energy product:cost r a t i o but more i m p o r t a n t l y they provide the magnet i n d u s t r y w i t h the means of expanding production w h i l e conserving s u p p l i e s of samarium oxideThe oxide, the precursor t o the m e t a l / a l l o y , i s c u r r e n t l y a v a i l a b l e t o the extent of about 150-200 tons per year and output may conceivably be doubled by 1985. Even so, when expressed i n terms of samarium metal t h i s q u a n t i t y i s r e l a t i v e l y s m a l l , hence i t i s d e s i r a b l e t o optimise the a v a i l a b l e resources by whatever means are p o s s i b l e . I n Table I I we i l l u s t r a t e one s o l u t i o n t o the problem, namely the production of a l l o y s c o n t a i n i n g much g r e a t e r q u a n t i t i e s of mischmetal. TABLE I I - WORLD PRODUCTION OF SAMARIUM OXIDE EXPRESSED IN TERMS OF DERIVED ALLOYS (metric tonnes per year)

Oxide

Metal

SmCo

150

116

336

500

665

1324

200

155

450

666

889

1769

400

310

900

1333

1778

3537

5

Sm. MM 67

>33

Co

5

Sm. MM Co5 50

-50

The above approach would almost c e r t a i n l y need t o be adopted i f , f o r example, the automobile i n d u s t r y was t o decide t o use R C 0 5 magnets i n the production of t h e i r accessory motors- Such an e x e r c i s e has already been s t u d i e d by at l e a s t two l e a d i n g manuf a c t u r e r s and i f pressures on weight and space become even more acute then the l i k l i h o o d of them u s i n g such magnets must be greater. Samarium i s not the only r a r e e a r t h metal used by the magnet i n d u s t r y . Several other R C 0 5 systems have been s t u d i e d and one i n p a r t i c u l a r , namely P r C o 5 , has a t t r a c t e d much a t t e n t i o n . Unfortuna t e l y , i t s p o t e n t i a l l y very h i g h energy product (>200kJ/m ) i s countered by what appears t o be an inherent i n s t a b i l i t y that so f a r has proved impossible t o overcome. But praseodymium can be used t o some extent as a p a r t i a l replacement f o r samarium t o provide magnets having m a r g i n a l l y b e t t e r performance c h a r a c t e r i s t i c s than SmCo^. Bearing i n mind the v a r i o u s c o n s t r a i n t s already r e f e r r e d t o , i t i s perhaps f o r t u i t o u s that a new generation of r a r e e a r t h c o b a l t magnets should r e c e n t l y have been developed that o f f e r not only enhanced magnetic p r o p e r t i e s , as compared w i t h SmCoij, but a l s o provide f o r savings on raw m a t e r i a l c o s t s . The new magnets are based on what are described as 2:17 a l l o y s , the term r e l a t i n g to the S1112C017 compound from which they are d e r i v e d . 3


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Of those 2:17 a l l o y s c u r r e n t l y i n p r o d u c t i o n the most w i d e l y employed are based on compositions of the type Sm(Co,Cu,Fe,M) where M = Z r , Hf or T i and x i s w i t h i n the range 7.0 - 8.5. Comparative data on t h i s and other types of R C 0 5 magnets are given i n Table I I I . X

RC05

TABLE I I I - COMPARATIVE DATA

AND S m ? ^

7

TYPE MAGNETS


( T y p i c a l Magnetic P r o p e r t i e s )

Content%

SmCo5 S m

S m

fiH

B T

kA/m

BH

R.M.

Cost

Sm

Co

( )max kJ/m

33.5

66.5

160

0.90

660

1.00

3

r

C

(SmCo5=l)

M M

C o

22.8

66.5

110

0.75

520

0.85

P r

C o

17.1

66.5

200

1.00

800

1 +

25.5

50.0

240

1.12

450

0.80

.67 .33 5

.50 .50 5 Sm(Co,Cu,Fe,M)

x

So f a r Japanese i n d u s t r y has been the most a c t i v e i n promoting the use of these newer magnets i n such a p p l i c a t i o n s as stepping motors f o r quartz-analog watches and i n h i g h performance audio pick-ups and loudspeakers. However, there are signs that other c o u n t r i e s are e x o l v i n g t h e i r own 2:17 type magnets which one hopes w i l l soon come i n t o p r o d u c t i o n . Gadolinium and dysprosium metals Of a l l the p r o p e r t i e s of the r a r e earths that c o n t r i b u t e to t h e i r many and v a r i e d a p p l i c a t i o n s one that ranks of s p e c i a l i n t e r e s t i s the extremely h i g h thermal neutron capture c r o s s s e c t i o n a s s o c i a t e d w i t h the elements gadolinium, samarium, europium and dysprosium, see Table IV. TABLE IV - THERMAL NEUTRON ABSORPTION CROSS-SECTION OF NATURAL ELEMENTS (Barns per atom) Gadolinium

40,000

Samarium

5,600

Europium

4,300

Dysprosium

1,100



174

RARE

EARTH

ELEMENTS

As one might expect, the n u c l e a r i n d u s t r y has not been slow to put t h i s p r o p e r t y to good use and today gadolinium, i n the form of i t s oxide, i s an e s s e n t i a l component of c e r t a i n f u e l systems where i t i s employed as a burnable p o i s o n , p r o v i d i n g r a p i d core c o n t r o l under emergency c o n d i t i o n s . By c o n t r a s t , the metals have so f a r found only l i m i t e d a p p l i c a t i o n save f o r one important use i n the f i e l d of nond e s t r u c t i v e t e s t i n g . With the p r o l i f e r a t i o n of r e s e a r c h r e a c t o r s over the past decade, neutron radiography has become a p r a c t i c a l t o o l i n the aerospace, n u c l e a r and e n g i n e e r i n g i n d u s t r i e s , yet without the a v a i l a b i l i t y of gadolinium and dysprosium i n the form of t h i n f o i l s , the technique would be s e v e r e l y r e s t r i c t e d . Neutron radiography shares c e r t a i n f e a t u r e s i n common w i t h X-radiography. In both systems an imaging beam i s passed through the specimen and the attenuated beam i s then detected i n such a way as to produce an image of the i n t e r n a l d e t a i l of the specimen. In neutron radiography the r e c o r d i n g medium i s standard X-ray f i l m but as the f i l m i s r e l a t i v e l y i n s e n s i t i v e to neutrons i t i s necessary to use what are termed converter f o i l s to produce the image. In the most w i d e l y employed method a gadolinium f o i l of t h i c k n e s s 0.025mm i s p l a c e d i n d i r e c t contact w i t h the X-ray f i l m . On exposure to the attenuated thermal neutron beam the gadolinium converter f o i l absorbs neutrons and promptly emits beta r a d i a t i o n thus a c t i v a t i n g the f i l m . When r e q u i r e d to examine i r r a d i a t e d specimens, such as h i g h l y a c t i v e f u e l rods, i t i s neceassary to adopt a somewhat d i f f e r e n t technique. The gadolinium i s r e p l a c e d by a dysprosium f o i l g e n e r a l l y of t h i c k n e s s 0.10mm which i s exposed to the attenuated neutron beam but i n the absence of the X-ray f i l m . The a c t i v a t e d f o i l i s then removed from the beam and i t s beta decay i s used to produce an autoradiograph i n contact w i t h X-ray f i l m . As the gamma cross s e c t i o n s of the f o i l are both small and lead to prompt i n t e r a c t i o n s , t h i s technique i s u n i q u e l y s u i t e d to the examination of h i g h l y r a d i o a c t i v e m a t e r i a l s . Other rare e a r t h metals So f a r i n t h i s review r e f e r e n c e has been made c h i e f l y to the elements that occur i n the f i r s t h a l f of the r a r e e a r t h s e r i e s . Of the remaining elements such as holmium, erbium, thulium y t t e r b i u m and l u t e t i u m i t i s u n f o r t u n a t e l y true that t h e i r r e l a t i v e l y low abundance coupled w i t h h i g h cost has tended to preclude t h e i r use i n a p p l i c a t i o n s o u t s i d e of the l a b o r a t o r y . Even so, demand f o r them i s i n c r e a s i n g year by year and i n some cases q u i t e s p e c t a c t u l a r l y . As an example, both t h u l i u m and erbium are used as t a r g e t m a t e r i a l s i n some sealed-tube neutron generators and erbium i s a l s o used i n other generators designed s p e c i f i c a l l y f o r use i n cancer therapy. The f i n a l a p p l i c a t i o n to which r e f e r e n c e w i l l be made concerns scandium. Although not a lanthanide i t s p o s i t i o n


9.

DAVIES

Applications

of

Pure Metals and

Alloys

175

immediately above y t t r i u m i n Group I I I of the p e r i o d i c t a b l e q u a l i f i e s i t f o r i n c l u s i o n i n the rare e a r t h s e r i e s . Despite i t being a r e l a t i v e l y rare metal, scandium i s now w i d e l y used by the l i g h t i n g i n d u s t r y , a l b e i t i n small amounts, i n the manufacture of what are termed m e t a l h a l i d e ' lamps. For many years i t had been known that c e r t a i n h a l i d e s could be used to change the s p e c t r a l c h a r a c t e r i s t i c s of mercury vapour lamps but i t was not u n t i l q u i t e r e c e n t l y that i t was discovered that mixed h a l i d e systems based on scandium i o d i d e could provide a near p e r f e c t match to n a t u r a l d a y l i g h t . But scandium i o d i d e i s extremely hygroscopic and i t i s d i f f i c u l t to use under normal production c o n d i t i o n s , t h e r e f o r e an a l t e r n a t i v e method had to be found of i n c o r p o r a t i n g i t i n the lamp. The method adopted was to form the i o d i d e i n s i t u by r e a c t i n g a small piece of scandium metal w i t h elemental i o d i n e . Over the past few years the technique has been r e f i n e d c o n s i d e r a b l y and today much of the scandium that i s used i s s u p p l i e d i n the form of small d i s c s of uniform s i z e and weight matched e x a c t l y to the lamp manufacturers requirements. Metal h a l i d e lamps are without question one of the most e f f i c i e n t forms of l i g h t i n g a v a i l a b l e to us today. They d e l i v e r at l e a s t 50% more l i g h t than conventional mercury lamps and by comp a r i s o n w i t h incandescent lamps of s i m i l a r wattage t h e i r l i g h t output i s three times as g r e a t . Although at present they are used mainly f o r outdoor and i n d u s t r i a l l i g h t i n g , low wattage lamps are s h o r t l y to be introduced f o r home use so g i v i n g us a l l the opportunity to e f f e c t some d i r e c t cost savings w h i l s t at the same time c o n t r i b u t i n g to energy c o n s e r v a t i o n . In c o n c l u s i o n , i t i s p a r t i c u l a r l y r e a s s u r i n g to know that research i n v o l v i n g rare e a r t h metals and r e l a t e d a l l o y s i s as a c t i v e today as at any time during the past ten years. A d d i t i o n a l l y , many of the i n d u s t r i a l a p p l i c a t i o n s to which reference has been made are at a r e l a t i v e l y e a r l y stage i n t h e i r development. Taken together these f a c t o r s must p o i n t to an e x c i t i n g f u t u r e f o r r a r e e a r t h metals.


1

Literature Cited

Stellite

1. Moore, C h r i s t i n e M. Rare-earth elements and yttrium. U.S. Dept. of the Interior, Bureau of Mines, 1979. 2. F e c r a l l o y S t e e l s . Trade p u b l i c a t i o n . The Metals and Chemical Technology Centre, A.E.R.E. H a r w e l l , England. 3. Talboom F.P.Jr and G r a f w a l l n e r , J . , U.S. Patent 3,542,530, November 24, 1970. 4. Metal Progress 106, (5) 7,10 (October 1974) 5. Trade p u b l i c a t i o n . Haynes Development Alloy. D i v i s i o n , Cabot C o r p o r a t i o n , Kokomo, Ind.46901, U.S.A. RECEIVED

March 3, 1981.


Industrial Applications of Pure Rare Earth Metals ... - ACS Publications

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