ON THE PIEZOELECTRIC PROPERTIES OF LlTHl UM NIOBATE

AEC RESEARCH &

DEVELOPMENT REPORT

- - . , .* .;.> r J . - . , - - 4 -1. % . . . .

> - - , . . *.+<*&4"-< -.>-- , 5%;. .. B kf T Li , . i x m '

- ' BA-LE'MEMORPPL I!S~TUTE ' . 'NORTHWEST

PACIFIC NORTUWESf LABORATORIES

SATTEF,~*E BOULEV4i@& f.0 BOX 999, RICHLAND, WASHINGTON 99352 A ? $ a --,. #, .-;<&$ $.+ < "

? . :, ,, , ' * I * : ' L

LEGAL N O T I C E

,' This reporf was prepared as an account of work sponsered by +he United States G~verhment. Neither h e United States nor the United States Atomic Energy Comd

, mission, nor any of their employees, nor any of their contractors, subcontodo~.lr,, or ,.. their employees, makes any warranty, express or implied, or assumes any legal

.ligbility or responsibility for the accuracy, completeness or usefulness of any informa- tion, appar~tus, product or process disclosed, or represents h a t its use would not infringe privately owned rights.

PACIFIC NORTHWEST LABORATORY RICHLAND, WASHINGTON

operated by BATTELLE MEMORIAL INSTITUTE

for the - UNITED STATES ATOMIC ENERGY COMMISSION UNDER CONTRACT AT(45.1)-1830 I

BNWL- 1436

UC-37, Instruments

EFFECTS OF GAMMA RADIATION ON THE PIEZOELECTRIC

PROPERTIES OF LITHIUM NIOBATE

R. W. Smith ( a )

Appl i e d Physics and Nondestructive Test ing Department Systems and Electronics D iv i s ion

Ju ly 1970

BATTELLE MEMORIAL INSTITUTE PACIFIC NORTHWEST LABORATORIES

RICHLAND, WASHINGTON 99352

(a) Now employed by WADCO Corp., a Subsidiary o f Westinghouse E l e c t r i c Corp. , under AEC Contract No. AT(45-1)-2170

Printed i n the Uni ted S ta tes of America Available from

Clearinghouse fo r Federal Sc i en t i f i c and Technical Information National Bureau of Standards, U.S. Department of Commerce

Spr ingf ie ld , V i rgi ni a 221 51 Price : Printed Copy $3.00; Microfiche $0.65

EFFECTS OF GAMMA RADIATION ON THE PIEZOELECTRIC PROPERTIES OF LITHIUM NIOBATE

R. W. Smith

ABSTRACT

L i t h i u m niobate can be used t o significantly extend the operating temperatures of ultrasonic transducers. Since some high-temperature transducer appl ica t i ons involve exposure t o nuclear radiation, i t i s important to determine the e f fec t of th i s radiation on the piezoelectric propertiesof the transducer crystal . In t h i s work we measured the e f fec t

of gamma radiation on the piezoelectric properties of lithium niobate

crystals. We exposed x-cut and z-cut lithium niobate transducer plates to gamna radiation from a 6 0 ~ o source. The electromechanical coup1 ing factor , the d ie lec t r ic constant and the frequency constant were measured

10 before exposure and a t periodic intervals up t o 10 R , to tal dose. We could detect no significant change i n any of these parameters up t o

10 10 R. This indicates that lithium niobate i s we1 1 suited for appl ica- tions involving exposure to high levels of gamma radiation.

i i i

BNWL- 1436

CONTENTS

INTRODUCTION . SUMMARY AND CONCLUSIONS

DISCUSSION

EXPERIMENTAL . RESULTS . ACKNOWLEDGEMENTS . REFERENCES . D I S T R I B U T I O N .

EFFECTS OF GAMMA RADIATION ON THE PIEZOELECTRIC PROPERTIES OF

LITHIUM NIOBATE

R. W. Smith

INTRODUCTION

L i t h i um niobate, a r e l a t i v e l y new f e r r o e l e c t r i c c r y s t a l , i s an

impor tan t ma te r i a l because i t can be used t o s i g n i f i c a n t l y extend the

opera t ing temperature o f u l t r a s o n i c transducers. Due t o i t s h igh c u r i e

temperature (1210 "C) and the h igh p i e z o e l e c t r i c coup1 i n g f a c t o r s

associated w i t h some cuts, improved h igh temperature u l t r a s o n i c t rans-

ducers are possib le. Since some high- temperature transducer app l i ca t i ons

i nvol ve exposure t o nuc lear r a d i a t i o n (e. g. , under-sodi um viewing and

acoust ic emission mon i to r ing) , i t i s important t o determine the e f f e c t

o f t h i s r a d i a t i o n on the p i e z o e l e c t r i c p rope r t i es o f the transducer c r y s t a l .

I n t h i s work we measured the e f f e c t o f gamma r a d i a t i o n on the piezo-

e l e c t r i c p rope r t i es o f l i t h i u m n iobate c rys ta l s . The e f f e c t o f gamna

exposure was determined by i r r a d i a t i n g some c r y s t a l s w i t h a gamma source

and measuring the p i e z o e l e c t r i c electromechanical coupl ing fac to r , t he

d i e l e c t r i c constant, and t h e frequency constant a t p rogress ive ly h igher

l e v e l s o f 6 0 ~ o rad ia t i on . This r e p o r t out1 ines the experimental procedure

and presents our r e s u l t s o f the i r r a d i a t i o n e f f e c t s .

SUMMARY AND CONCLUSIONS

We exposed x- cut and z- cut l i t h i u m n iobate transducer p la tes t o

g a m r a d i a t i o n from a 6 0 ~ o source t o determine the e f f e c t o f t h i s rad ia-

ti on on the p i e z o e l e c t r i c p rope r t i es o f the mater i a1 . The e l ectromechani - ca l coupl ing fac to r , t he d i e l e c t r i c constant and the frequency constant

10 were measured before exposure and a t pe r iod i c i n t e r v a l s up t o 10 R, t o t a l

dose.

We could de tec t no s i g n i f i c a n t change i n any o f these parameters up 10 t o and i n c l u d i n g the maximum exposure o f 10 R. This r e s u l t i nd i ca tes

t h a t li t h i um n iobate i s w e l l s u i t e d f o r app l i ca t i ons i n v o l v i n g exposure

t o h igh l e v e l s o f gamma rad ia t i on .

DISCUSSION

The piezoelectric coupling factor i s a d i rec t measure of the strength of the piezoelectric ac t iv i ty of a crystal . The piezoelectric coupling factor can be defined as the square root of the r a t io of the electr ical energy available from a crystal t o the total mechanical energy i n p u t . An

a l ternat ive definit ion i s the square root of the ra t io of the mechanical energy available to the total i n p u t e lectr ical energy. The coupling factor for a particular mode of vibration i s thus a measure of the effectiveness of the crystal as an electromechanical energy converter for t h a t par t i -

cul a r mode. Both x-cut (shear) and z-cut (longi t u d i nal ) 1 i thi um niobate are of

in te res t for ultrasonic transducer ap l icat ions, because both of these cuts have f a i r ly pure vibrational modes. The z-cut plate i s a pure extensional mode. An x-cut plate has two shear modes; however, the coup- ling to one of these modes is very weak while the other i s quite strong. The coupling factors fo r these two cuts can be readily measured to indicate the strength of the piezoelectric act ivi ty . Since these cuts are important

for transducer applications and the i r coupling factors can be easi ly measured, both x and z-cut plates were irradiated during the experiment. The coupl i ng factors were measured a t progressively higher 1 evels of radi a- tion from a 6 0 ~ o source t o detect any decline i n the piezoelectric act ivi ty .

The coupling factor can be measured i n several ways. (2-9) The

technique recomnended i n the IRE Standards on Piezoelectric Crystals , 1957 and 1958 involves measuring the ser ies resonant frequency fs and the paral l e l resonant frequency f for one particular vibrational mode

P of the crystal . The effect ive coupl i ng factor k for that mode i s then cal cul ated from

In practice, the parallel resonance frequency may be obscured by

unwanted resonances, making the paral l e l resonant frequency d i f f i c u l t t o determine.

A method for determining the coupling factor which does not involve measuring the parallel resonant frequencies was investigated bv h o e e t a1 and used by Warnsr e t a1 . ( ' I to measure the crystal parameters of lithium niobate. This techniaue makes use of the fact that the over- tone frequencies of a thickness mode piezoelectric resonator having a high coupling factor are not integral multiples of the fundamental, b u t

instead are shifted toward the higher frequencies. For a pure mode these frequencies can be cal cul ated from

w T - w T 1 tan - - - 2v 2v ' Z where u i s the angular frequency, T i s the thickness, and V i s the ~ h a s e velocity of the wave. This equation i c exact for a z-cut lithium niobate

plate and i s a good approximation for an x-cut plate. Onoe has tabulated the coupling factors fo r a thickness mode resonator as a function of the rat ios of the overtone frequencies ( u p t o the third overtone) to the fundamental resonance. Measurement of several overtone freouencies yields several values for k and provides a check on the consistency of the results.

The d ie lec t r ic constant and the frequency constant were also deter- mined for each plate a t each level of radiation. These parameters are sensit ive to changes i n the crystal and are thus good indicators of damage. The frequency constant i s the product of the fundamental ser ies resonant frequency f s and the controlling dimension (thickness in th i s case) of the pl a te .

Determination of the dielectr ic constant a t h i g h and low frequency was made from capacitance measurements. The "free" or s t a t i c d ie lec t r ic

T constant e / E ~ was measured a t a frequency below the fundamental series resonance (500 kHz fo r a1 1 the plates) . Measurement of the "cl amped" dielectr ic constant rs/ro was made a t high freauency (- 100 MHz), away

from any overtone resonance. A t th i s high frequency the crystal i s

sel f-clamped.

EXPERIMENTAL

Measurement o f t h e c r y s t a l parameters o f t he l i t h i u m n iobate t rans-

ducer p la tes used i n t h i s experiment were made w i t h the ins t rumenta t ion

shown i n F igure 1. Accurate frequency measurements were made w i t h a 200 MHz

e l e c t r o n i c counter, Hewlett Packard Model 5245L. The impedance measurements

were made w i t h a Hewlet t Packard Model 481 5A RF Vector Impedance Meter.

This inst rument has i t s own o s c i l l a t o r and reads out both the magnitude o f

the impedance and the phase angle. Analog recorder outputs a re prov ided

f o r t h e impedance magnitude and phase angle as we l l as the frequency. The

impedance magnitude was recorded as a func t i on o f frequency us ing an X- Y

recorder. A f t e r t h e resonant frequencies were loca ted approximately, t he

sca le on t h e recorder was expanded and a more d e t a i l e d graph o f the i m-

pedance c h a r a c t e r i s t i c s i n t he area o f the resonance was taken.

A t o t a l o f f o u r p l a t e s were used, two x- cuts and two z-cuts. The

geometry o f the p l a t e s i s summarized i n Table 1. Two thicknesses o f each

c u t were used, one approximately 1 mm t h i c k and one about 0.2 mn t h i c k .

The p la tes were c u t from a s i n g l e domain o p t i c a l q u a l i t y l i t h i u m n iobate

c r y s t a l . The faces were lapped and po l i shed f l a t and p a r a l l e l w i t h i n

0.5 u (micron).

Table 1. I n i ti a1 Measurements on t h e Uni r r a d i a t e d Crys ta ls

D i e l e c t r i c Constant

E T Frequency

Coup1 i n g - E

Constant Crys ta l Factor o Hz-me t e r s

meas. pub1 . meas. pub1 . meas. pub1 . x- cut

Area - 1.15 cm Thickness - 0.518 mm 0.68 0.68 8 5 84 1820 1838

x- cut Area - 1.39 cm 2

Thickness - 0.254 mm 0.69 0.68 8 1 84 1840 1838

z- cut 2 Area - 2.41 cm Thickness - 0.254 mm 0.21 0.17 28 30 3650 3615

z- cut 2 Area - 2.65 cm Thickness - 0.991 mm 0.23 0.17 2 9 3 0 3600 3615

H.P. 5245L 1 ;I Counter I

Pleter

F i gure 1 . Measurement Sys tern

I n i t i a l l y , copper e lectrodes about 0.5 LI t h i c k were deposited on t h e

c rys ta l s . These e lectrodes were ox id ized dur ing t h e experiment i n t h e

gamna f a c i l i t y . Consequently, t h e copper was removed w i t h a d i l u t e

n i t r i c a c i d s o l u t i o n and chrome go ld e lectrodes were redeposited. The 0

thicknesses o f chrome and go ld were about 200 and 2000 A, respec t i ve l y .

I n the course o f t h e experiment, t he c r y s t a l parameters were f i r s t 5 measured, then the c r y s t a l s were exposed t o the desi red l e v e l (1 0 R

i n i t i a l l y ) i n t h e gamma f a c i l i t y , and t h e parameters were measured again.

This procedure was repeated, increas ing t h e t o t a l dose t o an order o f 10 magni tude h igher l e v e l each time, u n t i l a t o t a l dose o f 10 R was a t ta ined.

One o f t he x- cut p l a t e s was exposed ahead o f the others t o detec t any

change and t o make i t poss ib le t o a d j u s t t h e r a d i a t i o n l e v e l o f t he r e-

mai n ing p l a t e s , i f necessary.

The transducer p l a t e s were i r r a d i a t e d i n t h e Hanford Laboratory Gamma

Radiat ion Fac i l i t y (Figure 2). The h ighest r a d i a t i o n dose r a t e a v a i l a b l e 7 i n t h i s f a c i l i t y i s 3.5 x 10 R/hr i n t h e i r r a d i a t i o n tubes nearest t he

6 0 ~ o source. Tubes f u r t h e r from t h e source have a lower dose ra te . An 6 i r r a d i a t i o n tube having a dose r a t e o f 1.5 x 10 R/hr was used up t o a

7 7 10 t o t a l dose o f 10 R. From 10 t o 10 R t o t a l dose the i nne r tube, having 7 a dose r a t e o f 3.5 x 10 R/hr, was used. Dose ra tes i n t h e i r r a d i a t i o n

tubes were measured by c e r i c s u l f a t e dosimetry techniques. F igure 3

shows the dose r a t e i n the tube nearest t he source as a f u n c t i o n o f d i s-

tance from the bottom o f the tube. This curve must be mu1 t i p 1 i e d by 0.92

t o c o r r e c t f o r decay o f the source dur ing t h e t ime s ince the measurement

was made. I r r a d i a t i o n s i n t h i s tube were c a r r i e d ou t about 7 i n . from the

bottom o f t h e tube. The temperature i n t h e tube c loses t t o t h e source was

208 OF, and the temperature i n t h e ou te r tube was 98 OF.

RESULTS

Our measurements on t h e u n i r r a d i a t e d c r y s t a l s show good agreement, i n

most cases, w i t h publ ished values. These measurements are summarized i n

Table 1 which a l s o shows the publ ished values(') f o r the parameters. We

measured a somewhat h igher value f o r t he coupl ing f a c t o r f o r z- cut p la tes

than ~ a r n e r ( ' ) . This i s the on ly s i g n i f i c a n t d i f f e rence .

BNWL- 14

Figure 2. Hanford Laboratory Gamna Rad ia t ion Faci 1 i ty

7

BNWL- 1436

6 1 2 18 2 4 3 0 3 6 4 2

DISTANCE F R O M BOTTOM (IN.)

Fi gure 3. Gamma Ir radi a t ion Faci 1 i t y

We could de tec t no s i gn i f i c an t change i n any of the parameters f o r 10 radia t ion exposure levels up t o 10 R. These measurements a r e summarized

i n Tables 2 through 5. The deviations i n the t ab les a re a l l w i t h i n the measurement e r ro r s . These data indicate t h a t the c rys ta l i s e s sen t i a l l y unaffected, as evidenced by the e s sen t i a l l y constant values f o r the

10 parameters up t o 10 R.

Table 2. Radiation Test o f L i t h i u m Niobate

Crystal : z-cut Area 2.65 cm2 Thickness 0.991 mm

Coup1 ing Factor Dielectric .. constant Calculated from- the resonance r a t i o s

Total rad- s T

i a t i on dose, 33 - - € 3 3 - f2 - - f3 4 - rads E E

0 0 1 f l 1 - - average

Frequency constant , Hz-me t e r s

Table 3. Rad ia t i on Tes t o f L i t h i u m N ioba te

C rys ta l : z - cu t

Area 2.41 cm2 Thickness 0.254 mm

Die1 e c t r i c constant

To ta l rad- s T 33 i a t i o n dose, - 33

rads E E 0 0

Coup1 i n g Fac to r c a l c u l a t e d f rom t h e resonance r a t i o s

5 - T - T; average

Frequency cons tan t , Hz-meters

- - 3

Ln

LO

Ln

Ln

hh

W

U

CO

CO

CO

CO

CO

CO

CO

-

Nl rl

WW

WW

WI

DW

Ln

ww

4- e *

LO

W

* *

Wh

X

OO

Table 5. Rad ia t i on Tes t o f L i t h i u m N ioba te

C r y s t a l : x - c u t

Area 1 .39 cm2 Thickness 0.254 mm

Coup1 i n g Fac to r D i e l e c t r i c constant Ca lcu la ted f rom t h e resonance r a t i o s S

T o t a l rad- T €1 1 - - f2 f3 - f4 Frequency - i a t i o n dose, €1 1

€ fl 5- cons t an t , rads o o fl average Hz-meters -

ACKNOWLEDGEMENTS

The author thanks J . C . Harris f o r h is assistance in assembling

and conducting the experimental work. Thanks i s a lso expressed t o

R. L. Brown and Janice Sletager fo r t h e i r help i n preparing t h i s paper.

REFERENCES

A. W . Warner, M. Onoe, G . A. Coquin. "Determination of Elas t ic and

Piezoelectric Constants f o r Crystals i n Class (3M)" J . Acoust. Soc.

Am., Vol. 42, No. 6, December 1967. - M. Onoe, H. F. Tiers ten, A . H. Meitzler. "Shif t i n the Location of

Resonant Frequencies Caused by Large Electromechanical Coupling in

Thickness Mode Resonators ," J . Acoust. Soc. Amer, Vol . 35, No. 1 , January 1963.

W . P. Mason, Hans Ja f fe . "Methods f o r Measuring Piezoelectric,

El a s t i c , and Dielect r ic Coefficients of Crystals and Ceramics ," Proc. IRE, June 1954.

Don Berlincourt. "Measurement of Piezoelectric Coupling i n Odd

Ceramic Shapes," Cl evi t e Technical Paper TP-23, c i rca 1961.

J . P. Arndt. "Procedures fo r Measuring Properties of Piezoelectric

Ceramics ," Clevi t e Technical Paper TP-234, c i rca 1961.

"Standards on Piezoelectric Crystals , 1949," Proc. IRE, Vol. 37

pp. 1 378- 1 395, December 1 949.

"IRE Standards on Piezoelectric Crystals - The Piezoelectric Vibra-

to r : Definitions and Methods of Measurement, 1957 ," Proc. IRE,

Vol. 45, pp. 353-358, March 1957.

"IRE Standards on Piezoelectric Crystals: Determination of the

E las t i c , Piezoelect r ic , and Dielect r ic Constants - The Electro-

mechanical Coup1 ing Factor, 1958," Proc. IRE, Vol . 46, pp. 764-778,

April 1958.

"IRE Standlirds on Piezoelectric Crystals : Measurement of Piezo-

e l e c t r i c Ceramics, 1961 ," Proc. IRE, Vol. 49, pp. 1161-1169,

July 1961.

DISTRIBUTION

No. o f C o p i e s

OFFSITE

1 A E C C h i c a g o P a t e n t Group

G . H. Lee

212 A E C D i v i s i o n o f T e c h n i c a l I n f o r m a t i o n E x t e n s i o n

UC-37

ONSITE

1 A E C C h i c a g o P a t e n t G r o u p

R. K. S h a r p ( R i c h l a n d )

1 AEC R i c h l a n d O p e r a t i o n s O f f i c e

C . L . R o b i n s o n

2 RDT A s s i s t a n t D i r e c t o r o f P a c i f i c N o r t h w e s t P rog rams

3 B a t t e l l e M e m o r i a l I n s t i t u t e

22 B a t t e l l e - N o r t h w e s t

E. R . A s t l e y G. J . Dau ( 1 0 ) N. E . D i x o n 3 . R . S l e t a g e r J. B. V e t r a n o T e c h n i c a l I n f o r m a t i o n F i l e s ( 5 ) T e c h n i c a l P u b l i c a t i o n s ( 2 )

38 WADCO Corp .

F. L . B e c k e r R . L . Brown F. J. L e i t z R. N. O r d H . N. P e d e r s e n R. W . S m i t h ( 2 5 ) W . G . S p e a r C. D . Swanson WADCO Document C o n t r o l ( 5 j W A D C O Tech Pubs ( 7 0 3 )