NONDESTRUCTIVE INSPECTION OF NIOBIUM TO IMPROVE SUPERCONDUCTIVITY.

M. C. Oravecz and L. W. Kessler Sonoscan, Inc. , 530 East Green S t r e e t , B e n s e n v i l l e , I L 60106

H. Padamsee Labora to ry of Nuclear S t u d i e s , C o r n e l l U n i v e r s i t y , I thaca, NY 14853

ABSTRACT

T h i s paper demons t r a t e s t h a t t h e in fo rma t ion needed to select t h e h i g h e s t q u a l i t y niobium is o b t a i n a b l e n o n d e s t r u c t i v e l y . Performance improvements of p a r t i - c l e accelerators u s i n g RF s u p e r c o n d u c t i v i t y r e q u i r e s r a p i d n o n d e s t r u c t i v e i n s p e c t i o n o f commerc ia l ly pure niobium sheets i n b u l k f o r a s s u r a n c e of h igh p u r i t y , small g r a i n size, freedom from c r i t i c a l defects and n e a r 100% r e c r y s t a l l i z a t i o n . A t o t a l o f 1.5 s q u a r e meters o f 3.0 and 1.5 mm t h i c k n i o b i u m s h e e t v a s s c a n n e d u i t h a S c a n n i n g Laser A c o u s t i c M i c r o s c o p e (SLAM) u s i n g a c o u s t i c r e s o l u t i o n o f up t o 25-50 microns. We found -- a c o u s t i c a t t e n u a t i o n i n b o t h n i o b i u m p l a t e s and v e l d s d e p e n d s on i n t e r s t i t i a l gas con ten t ; s p a t i a l v a r i a t i o n i n a c o u s t i c transmis- s i o n depends on g r a i n size; u l t r a s o n i c e v i d e n c e of inadequa te r e c r y s t a l l i z a t i o n ; i n t e r n a l d e l a m i n a - t i o n s ; and t h a t i n t e r n a l l o c a l i z e d i n c l u s i o n s and v o i d s are d e t e c t a b l e . A p r i n c i p l e SLAM advan tage is i t s c o m b i n a t i o n o f h i g h s c a n d e n s i t y and h i g h s c a n r a t e (130 l i n e s / m m a n d o n e s q u a r e meter i n 30 minu tes a t 60 MHz) making p r a c t i c a l large q u a n t i t y i n s p e c t i o n (e.g. 100 t o n s p r o p o s e d ) -

INTRODUCTION

S ince E. 0. Lawrence i n v e n t e d t h e c y c l o t r o n i n 1930, new d i s c o v e r i e s i n e l emen ta ry p a r t i c l e phys i c s have been i n t i m a t e l y t i e d t o i n c r e a s e s i n p a r t i c l e e n e r g y . S i g n i f i c a n t i n c r e a s e s i n p a r t i c l e e n e r g y g e n e r a l l y r e s u l t from b r e a k t h r o u g h s i n p a r t i c l e accelerator technology. One t e c h n i c a l development , powerful supe rconduc t ing magnets, h a s s t i m u l a t e d the c o n s t r u c t i o n of v e r y large p ro ton accelerators, such a s t h e p r o p o s e d a n d h i g h l y p u b l i c i z e d S u p e r c o n d u c t i n g S u p e r C o l l i d e r / l / . A n o t h e r t e c h n i c a l development , supe rconduc t ing RF c a v i t i e s (SCRF c a v i t i e s ) , is h o p e d t o p r o v i d e s i m i l a r s t i m u l u s t o the c o n s t r u c t i o n o f v e r y large e l e c t r o n a c c e l e r a t o r s . T h e r e are two a d v a n t a g e s o f SCRF c a v i t i e s compared t o n o r m a l l y c o n d u c t i n g c o p p e r c a v i t i e s c u r r e n t l y used -- much h i g h e r CW RF f i e l d s c a n be o b t a i n e d and e x t r e m e l y low RF l o s s e s are e x h i b i t e d . S t u d i e s have shown t h a t i n p r i n c i p l e t h e a p p l i c a t i o n of SCRF c a v i t i e s t o v e r y large e l e c t r o n a c c e l e r a t o r s is p o s s i b l e /2 ,3/ , and t h e r e s u l t s from SCRF test c a v i t i e s l o o k promising /4 ,5 / .

T h i s work was s u p p o r t e d as P h a s e I of a Small B u s i n e s s I n n o v a t i o n Research P r o j e c t u n d e r U S . Dept. of Energy C o n t r a c t No. DE-AC02-84ER80182.

0090-560718510000-0547 $1.00 0 1985 IEEE

C u r r e n t l y n i o b i u m is t h e p r e f e r r e d s u p e r c o n d u c t o r f o r SCRF c a v i t i e s . U n f o r t u n a t e l y , t h e i m p u r i t i e s and s t r u c t u r a l d e f e c t s found i n commercial niobium l i m i t i t s p e r f o r m a n c e . T h e d e f e c t s and i m p u r i t i e s may b e p r e s e n t i n t h e raw m a t e r i a l a f t e r manufacture , or t h e y may be in t roduced d u r i n g c a v i t y f a b r i c a t i o n p r o c e s s e s such as e l e c t r o n beam we ld ing or t u n g s t e n i n e r t gas w e l d i n g . The r e s u l t o f t h i s is t h a t t h e u l t i m a t e f i e l d s t r e n g t h t h a t c a n be a c h e i v e d is c o n s i d e r a b l y below t h e o r e t i c a l l i m i t s . F o r e x a m p l e , p u r e n i o b i u m s h o u l d h a v e a r e s i d u a l r e s i s t i v i t y r a t i o ( R R R ) of a b o u t 30,000. T h i s " f i g u r e of mer i t " i s t h e r a t i o o f e l e c t r i c a l r e s i s t i v i t y a t room t e m p e r a t u r e t o t h a t a t 4.2 deg ree K. Commercial ly a v a i l a b l e ma te r i a l has a n R R R o f a b o u t 30, or a f a c t o r of 1,000 p o o r e r . A l t h o u g h p u r i f i e d mater ia l i s better t h a n t h i s , m i c r o w a v e c a v i t y p e r f o r m a n c e i s s t i l l a t l e a s t a f a c t o r of 10 b e l o w u h a t c a n be a c h e i v e d . O c c a s i o n a l l y i n smal l s t r u c t u r e s , n e a r theoretical performance h a s been ache ived .

The amount of material r e q u i r e d for proposed accel- erators r anges from 100 t o 2500 s q u a r e meters. This makes t h e i n s p e c t i o n t a s k a c h a l l e n g e . P r e s e n t methods for f i n d i n g d e f e c t s i n v o l v e thermal s e n s o r a r r a y s which are mounted o n t o a s e m i f i n i s h e d d e v i c e w h i l e o p e r a t i n g a t t h e p r o p e r s u p e r c o n d u c t i n g temperature . Because of t h e r e s i s t i v i t y i n c r e a s e i n t h e n e i g h b o r h o o d of a d e f e c t , t h e r e is a l o c a l i z e d h e a t i n g which can be obse rved . However, instrumen- t i n g a s t r u c t u r e w i t h t h e r m a l s e n s o r a r r a y s i s compl i ca t ed , u n r e l i a b l e and e x t r e m e l y time consuming f o r l o c a t i n g a n y t h i n g b u t gross defec ts and t h i s poses a s e r i o u s o b s t a c l e for making f u r t h e r advances i n t h i s f i e l d .

R e c e n t r e s e a r c h h a s p r o v i d e d s i g n i f i c a n t improvements i n c a v i t y pe r fo rmance . P u r e r n i o b i u m is non c o m e r c i a l l y a v a i l a b l e /U, post-product ion p u r i f i c a t i o n t e c h n i q u e s have been deve loped /'I/, and i m p r o v e d i n s p e c t i o n and d i a g n o s t i c t e c h n i q u e s have been d e v e l o p e d /7,8/. F u t u r e i m p r o v e m e n t s may be a c h e i v e d t h r o u g h t h e u s e o f new i n s p e c t i o n t o o l s , s u c h as t h e S c a n n i n g Laser A c o u s t i c M i c r o s c o p e (SLAM). The SLAM may p r o v e u s e f u l s i n c e i t c a n r a p i d l y s c a n large areas, and it can d e t e c t i n t e r i o r and s u r f a c e s t r u c t u r a l d e f e c t s and v a r i a t i o n s i n mater ia l p r o p e r t i e s l a rger t h a n 5 0 m i c r o n s . I n s t u d i e s r e l a t i n g t o ceramic materials /9,10/, v o i d s , p o r o s i t y , i n c l u s i o n s , s u r f a c e d e f e c t s , c r a c k s a n d d e l a m i n a t i o n s have a l l been shown t o p r o j e c t images wi th characterist ic f e a t u r e s , t h e r e b y e n a b l i n g them

1985 ULTRASONICS SYMPOSIUM - 547

t o b e d i s t i n g u i s h e d i n t h e a c o u s t i c images. I n a s t u d y of t i t a n i u m , a c o u s t i c microscopy has been used t o per form m i c r o s t r u c t u r a l c h a r a c t e r i z a t i o n . Diffe- r e n t a l l o y s h a v e been d i s t i n g u i s h e d and gas contami- n a t i o n w i t h i n t h e metal has been obse rved / l l / .

INSTRUMENTATION

A c o u s t i c H i c r o s c o p y is t h e name g i v e n t o h i g h f r equency , 10 MHz t o 3 GHz u l t r a s o n i c v i s u a l i z a t i o n . Acous t i c mic roscopes can produce v e r y h igh r e s o l u - t i o n images. One t y p e of a c o u s t i c m i c r o s c o p e i s known a s a S c a n n i n g A c o u s t i c M i c r o s c o p e ( S A M ) , and i t r e q u i r e s abou t 10 seconds t o produce a n image o f 6 0 0 l i n e s . The o t h e r t y p e o f m i c r o s c o p e e m p l o y s a s c a n n i n g l a se r beam a s a p o i n t u l t r a s o n i c wave de tec tor and is known as t h e Scann ing Laser Acous t i c Microscope (SLAM). U i t h laser beam s c a n n i n g techno- l o g y a c o u s t i c images are produced v e r y r a p i d l y , and i n p a r t i c u l a r a t c o n v e n t i o n a l TV ra tes , v i z . 30 images p e r s e c o n d . C o m p a r e d t o t h e S A M , t h e i m p r o v e m e n t i n s p e e d is 300. The p r i n c i p l e of o p e r a t i o n o f SLAM is d e s c r i b e d i n t h e l i t e r a t u r e /12 , 13, 14/. As a p o i n t o f r e f e r e n c e , a t a 30 MHz o p e r a t i n g f r equency , each SLAM image c o v e r s a sample a r e a o f 1 s q u a r e cm. T h e r e f o r e , w i t h a n area s c a n - n i n g speed o f 30 s q u a r e cm p e r sec, to s c a n a s q u a r e me te r a r e a r e q u i r e s j u s t o v e r 5 m i n u t e s . I n a d d i t i o n t o s p e e d o f s c a n a d v a n t a g e s , t h e SLAM c a n d e t e c t d e f e c t s at t h e s u r f a c e , J u s t b e l o w t h e s u r f a c e , a n d deep w i t h i n t h e b u l k . On t h e o t h e r h a n d , t h e r e l a t i v e a d v a n t a g e s o f S A M a r e h i g h r e s o l u t i o n c a p a b i l i t y i n t h e s u r f a c e and n e a r s u r f a c e r e g i o n .

--

I n pe r fo rming t h e research d e s c r i b e d i n t h i s report, n i o b i u m u a s examined w i t h t h e v a r i o u s modes of o p e r a t i o n o f t h e SLAM u s i n g a w i d e r a n g e of t h e f r e q u e n c i e s e x i s t i n g p r i o r t o t h i s work (10, 24, 30, a n d 100 Hhz) a s w e l l as t h e 60 MHz c a p a b i l i t y d e v e l o p e d as p a r t of t h i s work. The higher f requen- cies are c a p a b l e o f h i g h e r r e s o l u t i o n . However, t h e h i g h e r f r e q u e n c i e s a r e a l s o a s s o c i a t e d w i t h i n c r e a s e d u l t r a s o n i c a t t e n u a t i o n , and v i s u a l i z a t i o n o f g r a i n s t r u c t u r e which produces background t e x t u r e i n t h e image. A l t e r n a t i v e l y , t h e l o w f r e q u e n c i e s h a v e t h e p e n e t r a t i n g power a n d t h e c a p a b i l i t y of c o v e r i n g larger areas of imaging pe r s c a n .

The v a r i o u s modes of t h e SLAM t h a t were employed are as f o l l o w s : The a m p l i t u d e mode produces images on a CRT w h e r e i n t h e g r e y s ca l e , or b r i g h t n e s s o f t h e d i s p l a y , co r re sponds t o t h e u l t r a s o u n d t r a n s m i s s i o n l e v e l t h r o u g h t h e s a m p l e a t t h e p o s i t i o n shown. T h i s mode may be d i s p l a y e d a t a s i n g l e a c o u s t i c f r e q u e n c y a n a l o g o u s t o m o n o c h r o m a t i c o p t i c a l i l l u m i n a t i o n i n an o p t i c a l microscope , or i n t e g r a t e d o v e r a r a n g e of a c o u s t i c f r e q u e n c i e s ( f r e q u e n c y S c a n ) w h i c h i s a n a l o g o u s t o w h i t e l i g h t o p t i c a l i l l u m i n a t i o n . The i n t e r f e r e n c e mode p r o d u c e s a c o u s t i c images which are c h a r a c t e r i z e d by a series o f v e r t i c a l f r i n g e s . T h e f r i n g e p o s i t i o n s a n d c u r v a t u r e , i f any, are i n d i c a t i v e of v a r i a t i o n s i n v e l o c i t y o f sound i n t h e sample .

SAMPLE EXAMINATION PROCEDURE

Data was o b t a i n e d u s i n g 30 n i o b i u m p l a t e s (9.25" x 9.25") from v a r i o u s m a n u f a c t u r e r s u i t h thicknesses of 0.125 a n d 0.062 i n c h a n d w i t h R R R v a l u e s o f

e i t h e r 20-30 , 100-150, or 165. I n a d d i t i o n t h e r e were s e v e r a l s p e c i a l l y p repa red p l a t e s and niobium c o u p o n s w i t h i m p l a n t e d f laws f o r s t u d y . Each n i o b i u m p l a t e was m a n u a l l y s c a n n e d u s i n g t h e SLAM o p e r a t i n g w i t h 30 MHz c o m p r e s s i o n a l w a v e s , 30 MHz s h e a r waves and 100 MHz compress iona l waves. 60 MHz was used t o examine s e l e c t e d samples . C o n s i d e r i n g t h e f i e l d or view size of 30 MHz, for example, is 12 mm by 9 mm, a c a r e f u l manual i n d e x i n g is r e q u i r e d t o examine a l l r o u g h l y 500 f i e l d s on each p l a t e . T h i s problem is e v e n more d i f f i c u l t a t 100 MHz u i t h i ts 3 x 2.3 mm f i e l d of v i ew.

When a flaw was found i t s p o s i t i o n was marked on t h e sample and a c o u s t i c micrographs were t a k e n t o docu- ment t h e flaw characterist ics. T y p i c a l d e f e c t s were e x a m i n e d u n d e r a n SEM w i t h b a c k s c a t t e r e d i m a g i n g and EDAX, m e t a l l o g r a p h y was performed t o d e t e r m i n e g r a i n s i z e a n d m e a s u r e m e n t s were made o f t h e R R R v a l u e . T h i s was i n a d d i t i o n t o o p t i c a l microscopy t h a t was done as needed.

I f s u f f i c i e n t a n a l y s i s c o u l d be made, t h e flaws were c a t e g o r i z e d a s c r i t i c a l or n o t c r i t i c a l w i t h r e s p e c t t o t h e i r a f f e c t o n s u p e r c o n d u c t i v i t y . Some f l a w s r e q u i r e f u r t h e r a n a l y s i s w i t h a l a s e r m i c r o p r o b e mass spectrometer t o f u l l y unde r s t and their n a t u r e . Based on t h e a n a l y s i s and t h e a c o u s t i c images o f t h e f l a w s , a n i d e n t i f y i n g c h a r a c t e r i s t i c ( s ) o f t h e a c o u s t i c image of each f l a w was found.

RESULTS

The basic r e s u l t s of o u r e x a m i n a t i o n of n i o b i u m u s i n g t h e SLAM are summarized below and i l l u s t r a t e d i n t h e accompanying f i g u r e s . A brief d e s c r i p t i o n o f each t y p e of mater ia l v a r i a t i o n or d e f e c t t y p e i s i n c l u d e d w i t h the f i g u r e .

We w i l l b r i e f l y d i s c u s s t h e d e f e c t s i n terms o f t h e i r e f f e c t s on s o u n d t r a n s m i s s i o n -- o v e r a l l c h a n g e s i n t h e a v e r a g e a c o u s t i c t r a n s m i s s i o n , s p a t i a l v a r i a t i o n s i n t r a n s m i s s i o n and r e g i o n s of no t r a n s m i s s i o n .

O v e r a l l change i n a c o u s t i c t r ansmiss ion -

An i m p o r t a n t p a r a m e t e r t o m o n i t o r t o o b t a i n h i g h q u a l i t y niobium is t h e i n t e r s t i t i a l c o n t e n t . T h i s is t y p i c a l l y measured by t h e RRR v a l u e . Unfortuna- t e l y c o r r e l a t i n g t h e a c o u s t i c p r o p e r t i e s o f t h e m a t e r i a l w i t h i t s R R R v a l u e is c o m p l i c a t e d by t h e t e n d e n c y o f l o w e r o x y g e n c o n t e n t s a m p l e s t o a l s o h a v e larger g r a i n s . However, we i d e n t i f i e d a sample w i t h R R R = 2 6 t h a t a l s o h a s a g r a i n s i z e (ASTM 4 - 6.5) c o m p a r a b l e t o t h e R R R - 100-150 m a t e r i a l , i.e. ASTM 3 - 6. T h i s p l a t e was f i r s t i d e n t i f i e d u i t h t h e SLAM by o b s e r v i n g a pronounced s c r a m b l i n g of t h e i n t e r f e r o g r a m f r i n g e s w i t h 30 MHz shear waves.

The a c o u s t i c i m a g e s of t h e two p l a t e s a p p e a r v e r y d i f f e r e n t ( F i g u r e 1 ) . There i s a s i g n i f i c a n t d i f f e r e n c e i n t h e image t e x t u r e ( c o n t r a s t ) a n d i n t h e o v e r a l l l e v e l o f a c o u s t i c t r ansmiss ion . The low R R R mate r i a l i s o n a v e r a g e more a t t e n u a t i n g . I n f a c t 100 MHz u l t r a s o u n d i n c i d e n t on t h e sample a t 10 degrees h a s d i f f i c u l t y p e n e t r a t i n g t h e l o w R R R p l a t e . T h i s e x p l a i n s t h e o v e r a l l l a c k o f d e t a i l i n t h e a c o u s t i c image.

548 - 1985 ULTRASONICS SYMPOSIUM

Similar v a r i a t i o n s can be obse rved i n niobium we lds w i t h d i f f e r e n t l e v e l s o f gas c o n t e n t . F i g u r e 2 shows 30 HHz i n t e r f e r o g r a m s o f e l e c t r o n beam we lds p r o d u c e d u n d e r b o t h good a n d bad vacuum. The contaminated weld shows lower t r a n s m i s s i o n and less c o h e r e n c e i n t h e i n t e r f e r o g r a m f r i n g e s due t o i n c r e a s e d s c a t t e r i n g o f t h e t r a n s m i t t e d p l ane waves.

S p a t i a l T ransmiss ion V a r i a t i o n s -

The t r a n s m i s s i o n o f u l t r a s o u n d through a pol ycrys- t a l l i n e ma te r i a l is a l t e r e d by s c a t t e r i n g o f f t h e i n t e r g r a n u l a r i n t e r f a c e s . Impor t an t parameters i n t h i s s c a t t e r i n g i n c l u d e t h e u l t r a s o n i c a n i s o t r o p y of t h e i n d i v i d u a l g r a i n s , how r a n d o m l y o r i e n t e d t h e g r a i n s are and t h e c o n d i t i o n s a t t h e i n t e r g r a n u l a r boundary (e.g. i n t e r s t i t i a l C o n t a m i n a t i o n ) . (See r e f . I t )

We have found t h a t g r a i n s i z e s t r o n g l y e f f e c t s t h e s p a t i a l f r e q u e n c y of t h e t r a n s m i s s i o n v a r i a t i o n s . T h i s is i l l u s t r a t e d i n computer gene ra t ed p l o t s of t h e n o r m a l i z e d FFT c o e f f i c i e n t s v e r s u s s p a t i a l f r e q u e n c y f o r two s a m p l e s of t h e H e r a e u s n iob ium. The f i r s t p l o t ( F i g u r e 3 ) shows t y p i c a l s p a t i a l v a r i a t i o n s for a s t a n d a r d commercial p l a t e (ASTM 3-6 and R R R = 100-150). The s e c o n d p l o t ( F i g u r e 4 ) shows t y p i c a l v a r i a t i o n s f o r a similar p l a t e af ter i t h a s been a n n e a l e d an a d d i t i o n a l 4 h o u r s t o i n c r e a s e t h e g r a i n size t o abou t 1 mm.

T h e FFT p l o t s show c l e a r l y t h a t t h e t r a n s m i s s i o n v a r i a t i o n s o f t h e smal ler g r a i n e d s a m p l e h a v e a c h a r a c t e r i s t i c a l l y h i g h e r s p a t i a l f r e q u e n c y t h a n those of t h e larger g r a i n e d sample. These s p a t i a l f r equency v a r i a t i o n s a re e a s i l l y c h a r a c t e r i z e d by p e r f o r m i n g t h i s t y p e of h a r m o n i c a n a l y s i s . Also n o t e t h a t t h e t e x t u r e is i s o t r o p i c , t h a t is t h e r e i s no d i r e c t i o n a l i t y t o i t .

We h a v e a l r e a d y d e t e c t e d g r a i n s i z e v a r i a t i o n s w i t h i n two p l a t e s u s i n g t h e i n t e r f e r o g r a m f r i n g e s c r a m b l i n g e f f e c t mentioned above. However, w e feel t h a t t h e 30 HHz s h e a r mode is n o t t h e optimum c h o i c e f o r o v e r a l l i n s p e c t i o n .

S e v e r a l low R R R (20-30) niobium p l a t e s demonstrated an effect w e h a v e named "streaking." F igu re 5 shows two 30 Mhz shear wave i n t e r f e r o g r a m to demonstrate t h e effect, The i n t e r f e r o g r a m of t h e %ormaln p l a t e shows no d i r e c t i o n a l f e a t u r e s o t h e r t h a n the f r i n g e s t h e m s e l v e s , w h e r e a s t h e s t r e a k e d p l a t e has t h i n , d a r k l i n e s c r o s s i n g t h e i n t e r f e r o g r a m f r i n g e s a t a b o u t 45 d e g r e e s . These s t r e a k s a re a l s o S e e n i n ampl i tude micrographs.

Based on a n e x a m i n a t i o n of a n u n a n n e a l e d p l a t e ( F i g u r e 6), we feel t h a t t h e s t r e a k i n g effect may be d u e t o t h e l a c k of 100s r e c r y s t a l l i z a t i o n . The u n a n n e a l e d p l a t e showed a v e r y s i m i l a r p a t t e r n of u n i d i r e c t i o n a l d a r k streaks i n t h e r o l l i n g d i r e c t i o n as was s e e n i n many of t h e niobium p l a t e s . However, t h e effect was many t i m e s s t r o n g e r i n t h e unannealed p l a t e . We feel t h e a n n e a l e d , s t r e a k e d p l a t e s have r e t a i n e d some o f t h e p r e a n n e a l i n g s t r u c t u r e and are c o n c e r n e d t h a t t h i s may be r e l a t e d t o l o c a l i z e d r e g i o n s of higher i n t e r s t i t i a l c o n t e n t .

Regions o f no t r ansmiss ion -

A h i g h l y u s e f u l a s p e c t of u l t r a s o u n d is t h a t t h e t r a n s m i s s i o n o f t h e u l t r a s o u n d can be s i g n i f i c a n t l y a l t e r e d by s t r u c t u r e s w h i c h h a v e a c h a r a c t e r i s t i c l e n g t h much smal le r t h a n t h e w a v e l e n g t h of s o u n d i n c i d e n t on t h a t s t r u c t u r e . An e x t r e m e l y i m p o r t a n t e x a m p l e of t h i s is a sheet o f a i r i n a s o l i d body, i.e. a d e l a m i n a t i o n or voided r e g i o n i n a material. Theoretical c a l c u l a t i o n s p r e d i c t t h a t a 0.1 nm gap i n s t e e l would a l low o n l y abou t 10% t r a n s m i s s i o n of sound . However , t h e p r a c t i c a l l i m i t seems t o be s e v e r a l o r d e r s o f m a g n i t u d e g r e a t e r ( a b o u t 0.1 m i c r o n ) d u e t o f o r e i g n l a y e r s on t h e s u r f a c e , s u r f a c e t e x t u r e , e t c . A p r a c t i c a l e x a m p l e of t h i s is a disbond i n a s tee l can seam weld seen u s i n g t h e SLAM. Examination of a c r o s s - s e c t i o n o f a detected disbond has shown a gap t h i c k n e s s o f abou t 1 micron. T h u s a l l n i o b i u m d e f e c t s a s s o c i a t e d w i t h d e l a m i n a t i o n s and v o i d s can be understood b a s i c a l l y as r e g i o n s of no s i g n a l due t o t h e i n a b i l i t y of t h e u l t r a sound t o p e n e t r a t e t h e a i r f i l l e d l a y e r .

Two c a t e g o r i e s o f d e l a m i n a t i o n were f o u n d i n t h e n i o b i u m p l a t e s . We re fe r t o t h e s e a s e d g e r o l l i n g d e l a m i n a t i o n s and n e a r s u r f a c e r o l l i n g d e l a m i n a - t i o n s . The SLAM r e v e a l e d these defect t y p e s i n t h e new high R R R (100-150 and 165) material. The edge r o l l i n g defect, i n p a r t i c u l a r , was found i n 3 o f 12 100-150 R R R p l a t e s examined . I t i s l i k e l y t h a t p u r e r ma te r i a l w i l l h a v e r o l l i n g r e l a t e d d e f e c t s more o f t e n .

Shown i n F igu re 7 i s a SLAM image of a p o r t i o n o f an i n t e r n a l d e l a m i n a t i o n o c c u r i n g a t t h e e d g e o f t h e niobium p l a t e . The d e l a m i n a t i o n extended a l o n j t h e e d g e of t h e p l a t e f o r a b o u t 6 cm. The w i d t h o f t h e d e l a m i n a t i o n inward from t h e edge v a r i e s a l o n g t h e l e n g t h up t o a maximum of abou t 2 cm. The r e g i o n of t h e p l a t e c o n s i s t i n g of t h e two l a y e r s o f me ta l a p p e a r d a r k i n t h e SLAM images. The d a r k r e g i o n i n d i c a t e s a l a c k of s o u n d t r a n s m i s s i o n d u e t o a voided r e g i o n between t h e two l a y e r s .

The o r i g i n of t h e s e d e l a m i n a t i o n s is a p p a r e n t l y a r e s u l t of t h e r o l l i n g p r o c e s s i t s e l f . D u r i n g t h e r o l l i n g p r o c e s s t h e t o p and bottom s u r f a c e s of t h e p l a t e are pushed backwards due t o f r i c t i o n a l f o r c e s w i t h t h e rolls. A s t h e p l a t e t h i n s , t h e pushed back t o p and bo t tom r e g i o n s a r e s q u e e z e d t oge the r a n d a p p e a r t o be p a r t of t h e s o l i d body of t h e p l a t e . However , t h e r e is o n l y i n t i m a t e c o n t a c t ; t h e s q u e e z e d t o g e t h e r r e g i o n s a re n o t a c t u a l l y bonded t o g e t h e r . The m i l l o p e r a t o r s a r e aware o f t h i s phenomena and c u t t h e end of the r o l l e d p l a t e o f f as a mat ter o f c o u r s e . Our r e s u l t s show t h a t t h e y sometimes unde res t ima te t h e l e n g t h of t h e squeezed together p o r t i o n .

Near s u r f a c e r o l l i n g d e l a m i n a t i o n s have been found r e p e a t e d l y on n i o b i u m p l a t e s w h i c h , u n l i k e o t h e r p l a t e s we r e c i e v e d , were n o t f i n a l annea led and were o n l y l i g h t l y etched. I n a d d i t i o n , t h e s e p l a t e s have a h i g h e r R R R ( 1 6 5 ) t h a n a n y o f t h e o t h e r s d u e t o s p e c i a l p r o c e s s i n g . T h e p l a t e s m e a s u r e 23.5 cm on each s i d e and have a great deal of o p t i c a l l y v i s i b l e s t r e a k i n g on bo th s i d e s .

1985 ULTRASONICS SYMPOSIUM - 549

An example of a n e a r s u r f a c e r o l l i n g d e l a m i n a t i o n is p i c t u r e d o p t i c a l l y a s p a r t of F i g u r e 6. T h i s p a r t i c u l a r f l a w is r a t h e r o b l o n g a n d is a b o u t 2 cm wide. A c o u s t i c a l l y a p o r t i o n of t h e f l a w c a n be e a s i l y s e e n w i t h 30 MHz compressional waves as shown i n F i g u r e 8. The d a r k n e s s i n t h e a c o u s t i c image r e p r e s e n t s no a c o u s t i c t r a n s m i s s i o n t h r o u g h t h e d e l a m i n a t e d region. Th i s flaw was confirmed t o be a d e l a m i n a t i o n when a t h i n l a y e r s e p a r a t e d from t h e rest of t h e material d u r i n g c r o s s - s e c t i o n i n g .

S i m i l a r p l a t e s show areas s u s p e c t e d of b e i n g n e a r s u r f a c e d e l a m i n a t i o n s . I n f a c t some p l a c e s h a v e de lamina ted under f i n g e r p re s su re . The most l i k e l y c a u s e of t h i s d e f e c t t y p e is a c t i o n t h a t o c c u r s d u r i n g r o l l i n g . S ince t h i s material has a h igh RRR v a l u e it a l so has much lower s t r e n g t h than s t a n d a r d commercial niobium. Th i s c a u s e s t h e niobium t o ac t v e r y doughy d u r i n g r o l l i n g w h i c h is t h e opt imum c o n d i t i o n for such d e l a m i n a t i o n s to occur .

CONCLUSIONS

Th i s paper has p re sen ted r e s u l t s which demons t r a t e t h a t t h e SLAM c a n be a u s e f u l s c r e e n i n g t o o l f o r i m p r o v i n g t h e q u a l i t y of n i o b i u m p l a t e u s e d i n s u p e r c o n d u c t i n g RF c a v i t i e s . T h e s e r e s u l t s c o n s i s t e d o f SLAM images a n d d a t a of t h e more c r i t i c a l t y p e s o f mater ia l v a r i a t i o n s a n d d e f e c t s o b s e r v e d d u r i n g e x a m i n a t i o n of a n u m b e r o f commercial niobium p l a t e s . T h i s is o n l y t h e f i r s t i n a series of p a p e r s t h a t w i l l d e s c r i b e t h e e x p e r i m e n t a l t e c h n i q u e s and h a r d w a r e r e q u i r e d for p r a c t i c a l s c r e e n i n g of n i o b i u m f o r u s e i n t h e proposed l a r g e e l e c t r o n accelerators.

REFERENCES

1 ) "To t h e Heart of Matter - The S u p e r c o n d u c t i n g Super C o l l i d e r , " U n i v e r s i t i e s Research A s s o c i a t i o n , Uashington, D.C., March 1985.

2 ) M . T i g n e r and H. Padamsee , " S u p e r c o n d u c t i n g Mic rowave C a v i t i e s i n Accelerators f o r P a r t i c l e P h y s i c s - A Review," CLNS-82/553, December 1982.

3) H. L e n g e l e r , " S u p e r c o n d u c t i n g RF C a v i t i e s f o r Large E l e c t r o n A c c e l e r a t o r s a n d S t o r a g e R ings , " CERN/EF 84-19, 24 November 1984, ( S u b m i t t e d t o Atomkernenergie - Kerntechnik) .

4) P. B e r n a r d , H. L e n g e l e r a n d E. P i c a s s o , "A P o s s i b l e U p g r a d i n g o f LEP E n e r g y w i t h Supe rconduc t ing A c c e l e r a t i n g C a v i t i e s , " LEP Note 524 (CERN/EF/RF 85- 11, 8 Janua ry 1985.

5) Proc. of t h e 2nd Workshop on RF Superconduc t iv i - t y , CERN, Geneva, J u l y 1984, Editor : H. Lengeler .

6) Made by W.C. Heraeus GmbH, Hanau, West Germany.

7) H. Padamsee , "The T e c h n o l o g y of Nb P r o d u c t i o n and P u r i f i c a t i o n , " ref. 5, p. 339.

8 ) G. M u l l e r , " D i a g n o s t i c T e c h n i q u e s and Defect C l a s s i f i c a t i o n , " ref. 5, p. 377.

9) D. E. Yuhas a n d T. E. M c C r a w , " A c o u s t i c Micros- c o p y , S E H , and O p t i c a l Mic roscopy : C o r r e l a t i v e I n v e s t i g a t i o n s i n Ceramics," Scanning E l e c t r o n Microscopy 1979, Vol. 1, SEM, Inc . , pp. 103-110.

Figure 1 - 100 MHz images showing s i g n i f i c a n t d i f - f e r e n c e s i n image t e x t u r e and i n a v e r a g e t r ansmis - s i o n l e v e l a t t r i b u t e d t o t h e lower p l a t e ' s h i g h e r i n t e r s t i t i a l con ten t . The t o p p l a t e has ASTM 3-6 and RRR=100-150; t h e bo t tom p l a t e has ASTM 4-6.5 and RRRz26. F i e l d o f view is 3 mm across.

F i g u r e 2 - 30 MHz i n t e r f e r o g r a m s o f e l e c t r o n beam we lds ( runn ing h o r i z o n t a l l y ) i n niobium. The bottom w e l d (bad vacuum) h a s h i g h e r a t t e n u a t i o n and more s c r a m b l e d f r i n g e s compared t o t h e t o p w e l d (good vacuum). The we lds were ground smooth t o e l i m i n a t e s u r f a c e t e x t u r e . F i e l d of view is 10 mm across.

550 - 1985 ULTRASONICS SYMPOSIUM

10) D. E.Yuhas and L. W. Kessler, "Defec t Charac- 1 2 ) L . W. K e s s l e r a n d D . E. Yuhas , " A c o u s t i c t e r i z a t i o n by Means of t h e Scanning Laser Acoustic Microscopy - 1979," Proc. IEEE, I n v i t e d Manuscript, Microscope (SLAM)," S c a n n i n g E l e c t r o n Microscopy Vol. 67, No. 4, Apri l 1979, pp. 526-536. - 1980, Vol. 1, pp. 385-391.

13) L. W. Kessler and D. E. Yuhas, " P r i n c i p l e s and 1 1 ) D. E. Yuhas and M. G. Oravecz, "Micros t ruc tura l A n a l y t i c a l C a p a b i l i t i e s of t h e Scanning Laser Acous- Charac te r iza t ion of Titanium by A c o u s t i c Microsco- t i c Microscope (SLAM)," SEM/ 1978, Vol. 1, p. 555. py," M a t e r i a l s E v a l u a t i o n , Vol. 41, No. 11, pp. 1304-1309, 1983, ASNT, Inc. 14) L. W. Kessler, "A Review of P r o g r e s s and

A p p l i c a t i o n s i n A c o u s t i c Microscopy," J- A c o u s t i c Socie ty America 55, pp. 909-918, 1974.

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Figures 3 & 4 - Digi t ized SLAM d a t a comparing niobium p l a t e s w i t h d i f f e r e n t g r a i n s i z e . Note t h e h i g h e r s p a t i a l f requencies assoc ia ted w i t h t h e smaller grained m a t e r i a l compared t o t h e l a r g e r grained m a t e r i a l . The FFT's were ca lcu la ted from the h o r i z o n t a l l i n e marked white. F i e l d of view is 4.8 mm h o r i z o n t a l l y .

1985 ULTRASONICS SYMPOSIUM - 551

F i g u r e 5 - 30 HHz s h e a r wave i n t e r f e r o g r a m s of ASTH F i g u r e 6 - O p t i c a l ( t o p ) and 30 HHz s h e a r wave image 20-30 n i o b i u m c o m p a r i n g a " s t r e a k e d " p l a t e ( t o p ) (bottom) of unannea led RRRs165 niobium. The s u r f a c e w i t h a "normal" p l a t e (bottom). The s t r e a k e d p l a t e s t r e a k s , v i s i b l e o p t i c a l l y , are s e e n a c o u s t i c a l l y was r o t a t e d t o show t h e d a r k l i n e s ( s t r e a k s ) cros- t h r o u g h o u t t h e b u l k o f t h e p l a t e . T h i s s t r o n g s i n g t h e f r i n g e s a t 45 degrees . No l i n e s are obse r - s t r e a k i n g p a t t e r n s u g g e s t s t h a t F i g u r e 5's s t r e a k s ved i n normal p l a t e s . F i e l d of v iew is 10 mm across. may be remnants of p r e a n n e a l i n g s t r u c t u r e .

F i g u r e 7 - 30 MHz micrograph of a t y p i c a l p o r t i o n o f F i g u r e 8 - 30 MHz image o f a p o r t i o n o f a n e a r t h e an edge r o l l i n g d e l a m i n a t i o n o b s e r v e d i n s e v e r a l s u r f a c e r o l l i n g d e l a m i n a t i o n . The f law is v i s i b l e RRR=lOO-l50 niobium p l a t e s . F i e l d of v iew is abou t i n t h e bottom c e n t e r of t h e o p t i c a l image i n F i g u r e 12 mm across. 6. The large d a r k area i n d i c a t e s t h e l a c k of acous-

t i c t r a n s m i s s i o n th rough t h e de lamina ted p o r t i o n .

552 - 1985 ULTRASONICS SYMPOSIUM