A mirage-effect thermal-wave method for the measurement of thermal diffusivities of solids is described. Data from two different laboratories are provided for various pure elements and compound semiconductor materials. In most cases the agreement with literature values is good.

S t a t e U n i v e r s i t y , U e t r o i t , M I 482U2 Abstract A one-dimensional a n a l y s i s o f t h e inechani sms by which a c o u s t i c waves a r e generated i n t h e thermoacoustic microscope i s presented. Thermal waves and a c o u s t i c waves a r e t r e a t e d as eigen-modes o f t h e system, so t h a t t h e r e i s no a c o u s t i c wave generated by thermal waves i n t h e bulk. It i s then shown t h a t t h e r e a r e f o u r channels by which t h e i n c i d e n t energy can be converted i n t o a c o u s t i c energy which can produce an image. One i s p u r e l y acoustic, and t h e others a l l i n v o l v e mode-conversion o f thermal waves t o a c o u s t i c waves. An important consequence i s t h a t thermoacoustic microscope images can r e f l e c t d i f f e r e n t combinations o f p h y s i c a l parameters than do images made w i t h microscopes based on pure thermal wave s c a t t e r i n g . s c a t t e r i n g mechanisms t o e x h i b i t r e s o l u t i o n which i s much b e t t e r than e i t h e r t h e thermal o r a c o u s t i c wave l e n g t h . We comment on t h e a b i l i t y o f 1. I n t r o d u c t i o n The development o f t h e new f i e l d o f thermal wave imaging has r e s u l t e d i n t h e i n v e n t i o n o f several new types o f Inicroscopes which a r e capable o f imaging near sub-surface f e a t u r e s o f opaque s o l i d s . microscope which uses a modulated p a r t i c l e beam ( e l e c t r o n s , photons o r i o n s ) as a heat source on one sample surface, and a p i e z o e l e c t r i c transducer attached t o t h e sample as an a c o u s t i c de5ector.6 The modulation frequency i s t y p i c a l l y 10 t o 10 Hz so t h e s i g n a l i s i n t h e u l t r a s o n i c range. When t h e e l e c t r o n beatn i n t h i s device i s scanned i n a r a s t e r over t h e sample surface, w i t h t h e l o c k -i n -d e t e c t e d transducer s i g n a l being simultaneously scanned over t h e screen of a video monitor, an image o f surface and near-surface f e a t u r e s appears on t h e screen. Depending on t h e n a t u r e o f t h e sample, these f e a t u r e s could i n c l u d e such t h i n g s as metal1 u r g i c a l g r a i n boundaries, implanted regions i n semiconductors, o r cracks i n the m a t e r i a l . However, i n order t o make f u l l use o f t h e microscope as a q u a n t i t a t i v e NDE t o o l , one must be a b l e t o i n t e r p r e t t h e imayes i n terms o f t h e physical p r o p e r t i e s o f t h e f e a t u r e s being imaged. For example, when one sees a g r a i n boundary whose image i s dark on one s i d e and l i g h t on t h e other, what i s t h e p h y s i c a l p r o p e r t y which i s responsible f o r t h e c o n t r a s t ? Is i t a change i n thermal Oye o f these i s t h e thermoacoustic The a p p l i c a t i o n s t o NUE are obvious. c o n d u c t i v i t y , a change i n e l a s t i c constants, a change i n thermal e x p a n s i v i t y , o r some o t h e r p r o p e r t y which i s bei...

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A contactless method of generating ultrasonic Rayleigh waves at MHz frequencies in metals using the electromagnetic generation technique is described. The method requires the use of photographic printed circuit techniques for the production of the meander coil transducers. Excellent efficiency is achieved and insertion losses are as low as 78 dB in aluminum with an external magnetic field of 5 kG and a frequency of 4.75 MHz.