PLZT Electrooptic Ceramics and Devices - ACS Symposium Series

16 PLZT Electrooptic Ceramics and Devices GENE H . HAERTLING

Downloaded by UNIV OF ROCHESTER on November 6, 2014 | http://pubs.acs.org Publication Date: September 3, 1981 | doi: 10.1021/bk-1981-0164.ch016

Ceramic Products, Motorola Incorporated, 3434 Vassar N E , Albuquerque, N M 87107

Approximately ten years ago, i t was first reported by Haertling and Land (1) that optical transparency was achieved in a ferroelectric ceramic material. This material was, in reality, not just one composition but consisted of a series of compositions in the lanthanum modified lead zirconate-lead titanate (PLZT) solid solution region. The multiplicity of compositions, each with different mechanical, electrical and electrooptic properties; has led to a decade of study in defining the chemical and structural nature of these materials; in understanding the phenomena underlying their optical and electrooptic properties and in evaluating the practicality of the large number of possible applications (2-12). The purpose of this paper is to review the status of the PLZT materials, dealing particularly with specific compositions, processing and fabrication; and to demonstrate the application of these materials to practical devices. To date, these devices are largely confined to applications involving shutters and modulators, but PLZT ceramics also offer a promising solid state answer to display applications of the future. S p e c i f i c examples of m i l i t a r y and i n d u s t r i a l devices c i t e d i n t h i s paper i n c l u d e (1) the A i r Force sponsored Thermal/Flash P r o t e c t i v e Device, (2) B e l l and Howell's Data Recorder, (3) a stereo-viewing system manufactured by Megatek Corporation and (4) eye s a f e t y viewing d e v i c e s (welding helmet, i n s p e c t i o n goggles) by M o t o r o l a . Materials PLZT Compositional System. The s o l i d s o l u t i o n r e g i o n which forms the b a s i s o f the PLZT m a t e r i a l s i s a s e r i e s of compositions r e s u l t i n g from the complete m i s c i b i l i t y of l e a d z i r c o n a t e and lead t i t a n a t e (commonly designated a t PZT) i n each o t h e r . Modif i c a t i o n s to the PZT system by the a d d i t i o n o f lanthanum oxide has a marked b e n e f i c i a l e f f e c t upon s e v e r a l o f the b a s i c p r o p e r t i e s o f the m a t e r i a l such as decreased c o e r c i v e f i e l d and i n creased d i e l e c t r i c constant, electromechanical c o u p l i n g c o e f 0097-6156/81/0164-0265$05.00/0 © 1981 American Chemical Society In Industrial Applications of Rare Earth Elements; Gschneidner, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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f i c i e n t , mechanical compliance and o p t i c a l transparency. The l a t t e r of these p r o p e r t i e s , o p t i c a l transparency, was only d i s covered i n r e c e n t years but came about as the r e s u l t o f an i n depth study of v a r i o u s a d d i t i v e s to the PLZT system. Results from t h i s work i n d i c a t e t h a t L a ^ , as a chemical m o d i f i e r , i s unique among the o f f - v a l e n t (chemical valency of the m o d i f i e r i s d i f f e r e n t or " o f f - v a l e n t " from t h a t of the i o n i t r e p l a c e s i n the +3 +2 l a t t i c e ; e.g., La r e p l a c i n g Pb ) a d d i t i v e s i n producing t r a n s parency. The reason f o r t h i s behavior i s s t i l l not f u l l y understood; however, i t i s known t h a t lanthanum i s , to a l a r g e extent, e f f e c t i v e because o f i t s h i g h s o l u b i l i t y i n the PZT oxygen o c t a h e d r a l s t r u c t u r e , thus producing an extensive s e r i e s of s i n g l e phase s o l i d s o l u t i o n compositions. The mechanism i s b e l i e v e d t o be one o f lowering the d i s t o r t i o n o f the u n i t c e l l , thereby r e ducing the o p t i c a l a n i s o t r o p y of the c r y s t a l l i n e l a t t i c e and a t the same time promoting uniform g r a i n growth and d e n s i f i c a t i o n of a s i n g l e phase, p o r e - f r e e s t r u c t u r e . A g e n e r a l i z e d formula f o r a l l compositions i n the PLZT system i s : l - x x < y l-y>l-x°3 4 where lanthanum ions r e p l a c e lead ions i n the A s i t e o f the p e r o v s k i t e ABO3 i o n i c s t r u c t u r e shown i n F i g u r e 1. Since L a ^ (added as La203) s u b s t i t u t e s f o r Pb , electrical neutrality i s maintained by the c r e a t i o n o f l a t t i c e s i t e v a c a n c i e s . The l o c a t i o n o f these vacancies i n e i t h e r the A(+2) s i t e s or B(+4) s i t e s o f the u n i t c e l l has not y e t been completely r e s o l v e d d e s p i t e numerous s t u d i e s on the subject; however, i t i s most probable t h a t both A and B s i t e vacancies e x i s t as p o i n t e d out by H a r d t l and Hennings (13). I f both A and B s i t e vacancies are present i n the l a t t i c e , i t i s £ expected t h a t the above f o r mulation would p r o v i d e excess Pb ions which are e x p e l l e d from the l a t t i c e (as PbO vapor) d u r i n g the d e n s i f i c a t i o n process a t elevated temperatures. T h i s behavior does, indeed, occur; and i n f a c t , i t has been r e p o r t e d by Snow (14) t h a t t h i s excess PbO cont r i b u t e s to a c h i e v i n g f u l l d e n s i t y by forming a l i q u i d phase a t the g r a i n boundaries and by i n h i b i t i n g g r a i n growth during the i n i t i a l stages of d e n s i f i c a t i o n . Both o f these e f f e c t s are benef i c i a l to the attainment of t h e o r e t i c a l l y dense m a t e r i a l by e l i minating r e s i d u a l p o r o s i t y before i t becomes entrapped w i t h i n the grains. The PLZT phase diagram i s given i n F i g u r e 2. As can be seen, the o v e r a l l e f f e c t o f adding lanthanum to the PZT system i s one o f decreasing the s t a b i l i t y of the f e r r o e l e c t r i c (FE) phases (a f e r r o e l e c t r i c m a t e r i a l possesses spontaneous i n t e r n a l p o l a r i z a t i o n , P, which can be switched by an e l e c t r i c f i e l d , E, as i l l u s t r a t e d i n the P vs. E h y s t e r e s i s loops i n F i g u r e 2) i n favor of the nonf e r r o e l e c t r i c c u b i c and a n t i f e r r o e l e c t r i c (AFE) phases. At a 65/35 r a t i o of PbZr03 to PbTi03, a c o n c e n t r a t i o n o f 9.5% l a n t h a num i s s u f f i c i e n t to reduce the rhombohedral-cubic phase t r a n s i -


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


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Figure 1. Configuration of the ABO unit cell shown with sites occupied by Pb, La, Zr, Ti and O atoms as in the paraelectric cubic phase of PLZT s

Figure 2. Room temperature phase diagram of the PLZT system illustrating phases present and typical hysteresis loops associated with each phase: compositions 1, 2 and 3 are 9565, 7065 and 12040, respectively


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t i o n (Curie temperature) to below room temperature. Thus, a m a t e r i a l o f t h i s composition (designated as 9.5/65/35 or simply, 9565) i s n o n - f e r r o e l e c t r i c and cubic i n i t s v i r g i n c o n d i t i o n . I t i s i d e n t i f i e d as composition 1 on the phase diagram and w i l l be d i s c u s s e d l a t e r i n r e l a t i o n t o i t s o p t i c a l and e l e c t r o o p t i c properties. I t should be noted t h a t t h i s composition i s a l s o l o cated i n the c r o s s hatched p o r t i o n o f the diagram which i n d i c a t e s a r e g i o n o f metastable f e r r o e l e c t r i c phases t h a t can be e l e c t r i c a l l y induced with a s u f f i c i e n t l y high f i e l d . The phase diagram i s o n l y given to 15 atom percent La s i n c e a l l o f the compositions o f i n t e r e s t l i e w i t h i n t h i s range. A l though not shown, compositions with La c o n c e n t r a t i o n s higher than approximately 30% possess mixed phases and are o p t i c a l l y opaque. Processing and F a b r i c a t i o n . Ceramics are t r a d i t i o n a l l y prepared from powders formulated from the i n d i v i d u a l oxides; however, e a r l y attempts to produce the PLZT powders by t h i s method proved to be inadequate from the standpoint o f chemical and o p t i c a l u n i f o r m i t y . As a r e s u l t , a chemical c o - p r e c i p i t a t i o n method designed s p e c i f i c a l l y f o r the PLZT m a t e r i a l s which u t i l i z e d l i q u i d p r e c u r s o r m a t e r i a l s was developed and s u c c e s s f u l l y implemented as a p r o d u c t i o n process (15). F i g u r e 3 shows i n p i c t o r i a l form the v a r i o u s steps i n v o l v e d i n the powder p r o c e s s i n g and f a b r i c a t i o n o f the PLZT m a t e r i a l s . The high p u r i t y , l i q u i d organometallies, t e t r a b u t y l z i r c o n a t e and t e t r a b u t y l t i t a n a t e , are f i r s t i n t i m a t e l y mixed together i n a high speed blender along with the a p p r o p r i a t e amount of l e a d oxide powder and then p r e c i p i t a t e d by adding the lanthanum a c e t a t e s o l u t i o n while b l e n d i n g . As the lanthanum acetate i s introduced, the zirconium and t i t a n i u m butoxides are hydrolyzed by the water from the lanthanum a c e t a t e s o l u t i o n producing a p r e c i p i t a t e of mixed hydroxides. At the same time, lead oxide and lanthanum acetate r e a c t with the f r e s h l y hydrolyzed p r e c i p i t a t e to produce a f i n a l product c o n s i s t i n g of mixed oxides and hydroxides i n a t h i n s l u r r y form. The s l u r r y i s d r i e d , r e s u l t i n g i n the white p r e c i p i t a t e d powder shown i n F i g u r e 3. T h i s powder i s then c a l c i n e d or c h e m i c a l l y r e a c t e d a t an e l e v a t e d temperature (500°C f o r 16 hours) i n order to produce the d e s i r e d PLZT c r y s t a l l i n e phase. A f t e r c a l c i n i n g , the powder i s wet m i l l e d f o r s e v e r a l hours i n order to promote a d d i t i o n a l chemical homogeneity, d r i e d and prepressed i n t o a s l u g of proper s i z e and shape f o r hot pressing. A t y p i c a l hot p r e s s i n g setup i s given i n F i g u r e 4. Exp e r i e n c e has shown t h a t a simple u n i a x i a l , single-ended hot p r e s s i n g arrangement i s both r e l i a b l e and economical i n producing c o n s i s t e n t , o p t i c a l q u a l i t y m a t e r i a l . The prepressed s l u g i s p l a c e d i n t o a s i l i c o n c a r b i d e mold r e s t i n g on an alumina p l a t e and surrounded completely with a r e f r a c t o r y g r a i n such as magn e s i a or z i r c o n i a i n order to prevent r e a c t i o n with the mold a t high temperature. An alumina p l a t e and push rod are l o c a t e d



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Various stages in the processing of PLZT ceramics

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A typical setup for hot pressing PLZT ceramics


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on top of the s l u g and a modest amount of pressure i s a p p l i e d to the s l u g f o r alignment purposes. Heat-up of the furnace i s s t a r t e d , while a t the same time a vacuum i s drawn on the s l u g v i a a water-cooled vacuum chamber surrounding the furnace. Oxygen i s b a c k - f i l l e d i n t o the chamber a t 700°C, f u l l pressure i s a p p l i e d and the furnace temperature i s r a i s e d to i t s f i n a l value. Typic a l hot p r e s s i n g c o n d i t i o n s are 1250°C f o r 18 hours a t 2000 p s i . A f t e r hot p r e s s i n g , the s l u g i s extracted from the mold, i t s surfaces are cleaned, and i t i s then p o l i s h e d f o r o p t i c a l e v a l u a t i o n . T h i s method of vacuum/oxygen hot p r e s s i n g was s u c c e s s f u l l y used by Dungan and Snow (16) f o r f a b r i c a t i n g o p t i c a l q u a l i t y PLZT slugs up to f i v e inches i n diameter. An a l t e r n a t e method of hot p r e s s i n g i n flowing oxygen r a t h e r than vacuum/oxygen i s a l s o known to produce o p t i c a l q u a l i t y m a t e r i a l , but i t i s g e n e r a l l y l i m i t e d to s l u g s i z e s l e s s than two inches i n diameter. T y p i c a l examples of hot pressed ceramics are given i n F i g u r e 5. M i c r o s t r u c t u r e . Ceramic compositions i n the PLZT system c h a r a c t e r i s t i c a l l y e x h i b i t a h i g h l y uniform m i c r o s t r u c t u r e cons i s t i n g o f randomly o r i e n t e d , equiaxed g r a i n s ( c r y s t a l l i t e s ) i n t i m a t e l y bonded together. An example of such a m i c r o s t r u c t u r e i s shown i n F i g u r e 6 f o r PLZT 9565 thermally etched a t 1100°C. The average g r a i n s i z e of a given m a t e r i a l may vary from about two microns to 15 microns depending on the temperature and time of hot p r e s s i n g , with a t y p i c a l s i z e being approximately e i g h t microns average diameter. A uniform g r a i n s i z e i s a h i g h l y des i r a b l e f e a t u r e from the standpoint of performance. Another d i s t i n c t i v e c h a r a c t e r i s t i c of the PLZT m a t e r i a l s i s t h e i r f u l l y dense, p o r e - f r e e m i c r o s t r u c t u r e which i s devoid of any second phases. T h i s i s r e f l e c t e d i n measured bulk d e n s i t i e s which r o u t i n e l y exceed 99.9% of t h e o r e t i c a l d e n s i t y . The e x i s t ence o f pores or second phases i n the volume of the g r a i n s or i n the g r a i n boundaries i s undesirable s i n c e both a c t to i n c r e a s e l i g h t s c a t t e r i n g and reduce o p t i c a l transparency. O p t i c a l P r o p e r t i e s . The a d d i t i o n o f lanthanum oxide to PZT has a r a t h e r remarkable e f f e c t on the o p t i c a l transparency, e s p e c i a l l y when the amount of lanthanum exceeds seven atom p e r cent. Thin p o l i s h e d p l a t e s c h a r a c t e r i s t i c a l l y transmit about 67% o f the i n c i d e n t l i g h t . When broadband a n t i r e f l e c t i o n c o a t ings are a p p l i e d to the major s u r f a c e s , t h i s transmission i s i n creased to greater than 98%. Surface r e f l e c t i o n l o s s e s are a f u n c t i o n o f the index of r e f r a c t i o n (n = 2.5) of the PLZT. O p t i c a l absorption i n these m a t e r i a l s i s wavelength dependent, becoming extremely high i n the v i o l e t (short wavelength) end of the spectrum near 0.37 microns. In the i n f r a r e d p o r t i o n of the spectrum, transmittance remains high out to approximately 6.5 microns and then g r a d u a l l y decreases i n a r e g u l a r manner unt i l 12 microns, where the m a t e r i a l i s f u l l absorbing. The o p t i c a l t r a n s m i s s i o n c h a r a c t e r i s t i c s of three PLZT comp-



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Examples of the optical transparency of quadratic PLZT ceramics

A typical microstructure of PLZT, composition 9565, illustrating the fully dense structure and uniform grain size


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o s i t i o n s are given i n F i g u r e 7. These compositions (see F i g u r e 2) were s e l e c t e d because they represent m a t e r i a l s of d i s t i n c t l y d i f f e r e n t e l e c t r o o p t i c behavior. Composition 9565 i s s u b s t a n t i a l l y more transparent than e i t h e r of compositions 7065 or 12040. This i s most n o t i c e a b l e i n the blue end of the spectrum where absorpt i o n and l i g h t s c a t t e r i n g predominate. Both compositions 7065 and 12040 are f e r r o e l e c t r i c and hence possess domain w a l l s which produce index of r e f r a c t i o n d i s c o n t i n u i t i e s and l i g h t s c a t t e r i n g from w i t h i n the m a t e r i a l . The t e t r a g o n a l phase composition 12040 i s more transparent than the rhombohedral 7065 composition. A l l samples were measured i n the v i r g i n s t a t e . Electrooptic Properties, The e l e c t r o o p t i c p r o p e r t i e s of the PLZT m a t e r i a l s are i n t i m a t e l y r e l a t e d to t h e i r f e r r o e l e c t r i c properties. Consequently, v a r y i n g the f e r r o e l e c t r i c p o l a r i z a t i o n with an e l e c t r i c f i e l d such as i n a h y s t e r e s i s loop, produces a change i n the o p t i c a l p r o p e r t i e s of the ceramic. In a d d i t i o n , the magnitude of the observed e l e c t r o o p t i c e f f e c t i s dependent on both the strength and d i r e c t i o n of the e l e c t r i c f i e l d . PLZT ceramics d i s p l a y o p t i c a l l y u n i a x i a l p r o p e r t i e s on a microscopic s c a l e , and a l s o on a macroscopic s c a l e when p o l a r i z e d with an e l e c t r i c f i e l d . In u n i a x i a l c r y s t a l s there i s one unique symmetry a x i s , the o p t i c a x i s ( c o - l i n e a r with the f e r r o e l e c t r i c p o l a r i z a t i o n v e c t o r i n ceramic PLZT), which possesses d i f f e r e n t o p t i c a l p r o p e r t i e s than the other two orthogonal axes. That i s , l i g h t t r a v e l i n g i n a d i r e c t i o n along the o p t i c a x i s and v i b r a t i n g i n a d i r e c t i o n p e r p e n d i c u l a r to i t encounters a d i f f e r e n t index of r e f r a c t i o n (r^) than l i g h t t r a v e l i n g i n a d i r e c t i o n 90° to the o p t i c a x i s and v i b r a t i n g p a r a l l e l to i t ( n ) . The absolute d i f ference between the two i n d i c e s i s defined as the b i r e f r i n g e n c e ; i . e . , n - n = An. In ceramic m a t e r i a l s where a s t a t i s t i c a l array of randomly o r i e n t e d c r y s t a l l i t e s e x i s t , the macroscopic or e f f e c t i v e b i r e f r i n g e n c e i s designated by Ah. On a macroscopic s c a l e , "AH i s equal to zero before e l e c t r i c a l p o l i n g and has some f i n i t e value a f t e r p o l i n g , depending on the composition and the degree of p o l a r i z a t i o n . The An value i s a meaningful q u a n t i t y i n t h a t i t i s r e l a t e d to the o p t i c a l phase r e t a r d a t i o n i n the material. L i n e a r l y p o l a r i z e d l i g h t , on entering the e l e c t r i c a l l y energ i z e d ceramic, i s r e s o l v e d i n t o two p e r p e n d i c u l a r components whose v i b r a t i o n d i r e c t i o n s are defined by the c r y s t a l l o g r a p h i c axes o f the c r y s t a l l i t e s a c t i n g as one o p t i c a l e n t i t y . Because of the d i f f e r e n t r e f r a c t i v e i n d i c e s , n and nQ, the propagation v e l o c i t y of the two components w i l l be d i f f e r e n t w i t h i n the m a t e r i a l and w i l l r e s u l t i n a phase s h i f t c a l l e d r e t a r d a t i o n . The t o t a l r e t a r d a t i o n r i s a f u n c t i o n of both An and the o p t i c a l path length t (generally, t i s the p l a t e thickness) according to the r e l a t i o n ship of T = STTE". When s u f f i c i e n t v o l t a g e i s a p p l i e d to the c e r amic, a halfwave r e t a r d a t i o n i s achieved f o r one component r e l a t i v e to the other. The net r e s u l t i s one of r o t a t i n g the v i b r a e

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t i o n d i r e c t i o n of the l i n e a r l y p o l a r i z e d l i g h t by 90°, thus allowi n g i t to be transmitted by the second (crossed) p o l a r i z e r i n the ON c o n d i t i o n . Switching o f the e l e c t r o o p t i c ceramic from a s t a t e o f zero r e t a r d a t i o n to halfwave r e t a r d a t i o n w i l l create an ON/OFF l i g h t shutter. S e l e c t i v e c o l o r f i l t e r i n g of white l i g h t may be achieved by extending the r e t a r d a t i o n beyond h a l f wavelength. Three common types of e l e c t r o o p t i c e f f e c t s are i l l u s t r a t e d i n F i g u r e 8; i . e . , q u a d r a t i c and l i n e a r b i r e f r i n g e n c e and memory s c a t t e r i n g . A l s o i n c l u d e d i n the f i g u r e i s a t y p i c a l setup r e q u i r e d f o r generating each e f f e c t along with the observed behavi o r shown i n terms of l i g h t i n t e n s i t y output (I) as a f u n c t i o n of e l e c t r i c f i e l d (E). The f i r s t and most widely a p p l i e d o f a l l of the e l e c t r o o p t i c responses i s the quadratic (Kerr) e f f e c t . I t i s generally d i s played by those m a t e r i a l s which are e s s e n t i a l l y cubic phase (comp o s i t i o n 1) but are l o c a t e d c l o s e to the f e r r o e l e c t r i c rhombohed r a l or t e t r a g o n a l phases. The d e s i g n a t i o n f o r t h i s e f f e c t i s d e r i v e d from the quadratic dependence o f An on e l e c t r i c f i e l d ; i . e . , An = k E . These m a t e r i a l s , by v i r t u e o f t h e i r n a t u r a l cubic symmetry, do not possess permanent p o l a r i z a t i o n and are not o p t i c a l l y b i r e f r i n g e n t i n t h e i r quiescent s t a t e . As such, they c o n t r i b u t e no o p t i c a l r e t a r d a t i o n to an incoming p o l a r i z e d l i g h t beam; however, when an e l e c t r i c f i e l d i s a p p l i e d t o the m a t e r i a l , an e l e c t r i c p o l a r i z a t i o n (and consequently, b i r e f r i n g e n c e ) i s i n duced i n the m a t e r i a l and r e t a r d a t i o n i s observed between crossed p o l a r i z e r s ( c a l l e d an ON s t a t e ) . On removing the e l e c t r i c f i e l d , the m a t e r i a l r e l a x e s again to i t s cubic s t a t e and i s i n the OFF c o n d i t i o n . Relaxation times to the OFF c o n d i t i o n vary w i t h comp o s i t i o n but g e n e r a l l y range from one to 100 microseconds. Turn ON times are o f the same magnitude and ON-OFF r a t i o s as h i g h as 5000 to one have been measured. A p p l i c a t i o n s f o r the q u a d r a t i c e f f e c t i n c l u d e s h u t t e r s , o p t i c a l gates, d i s p l a y s , s p e c t r a l f i l t e r s , l i g h t modulators and v a r i a b l e d e n s i t y windows. A second type of behavior e x i s t i n g i n the PLZT's i s the l i n ear (Pockels) e f f e c t which i s g e n e r a l l y found i n high c o e r c i v e f i e l d , t e t r a g o n a l m a t e r i a l s (composition 3). This e f f e c t i s so named because o f the l i n e a r r e l a t i o n s h i p between An and e l e c t r i c field. The t r u l y l i n e a r , n o n h y s t e r e t i c character of t h i s e f f e c t has been found to be i n t r i n s i c to the m a t e r i a l and not due to domain r e o r i e n t a t i o n processes which occur i n the q u a d r a t i c and memory m a t e r i a l s . The l i n e a r m a t e r i a l s possess permanent remanent p o l a r i z a t i o n ; however, i n t h i s case the m a t e r i a l i s switched to i t s s a t u r a t i o n remanence, and i t remains i n t h a t s t a t e . O p t i c a l information i s e x t r a c t e d from the ceramic by the a c t i o n of an e l e c t r i c f i e l d which causes l i n e a r changes i n the b i r e f r i n g e n c e , but i n no case i s there p o l a r i z a t i o n r e v e r s a l i n the m a t e r i a l . The experimental setup f o r observing t h i s e f f e c t , as seen i n F i g u r e 8, i s i d e n t i c a l to t h a t f o r the q u a d r a t i c response, except t h a t the PLZT p l a t e i s prepoled t o s a t u r a t i o n remanence before u s i n g . A p p l i c a t i o n s i n c l u d e modulators and s p e c t r a l f i l t e r s ; however, no devices have y e t emerged u t i l i z i n g t h i s e f f e c t . 2


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Figure 8. Operational configurations and typical light output responses of (A) quadratic (B) linear, and (C) memory PLZT materials; the heavy accented portions of the response curves indicate the usable range.



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A t h i r d type o f e l e c t r o o p t i c behavior which i s employed a l most e x c l u s i v e l y f o r d i s p l a y s i s t h a t of e l e c t r i c a l l y c o n t r o l l e d l i g h t s c a t t e r i n g i n a memory m a t e r i a l . T h i s e f f e c t , as observed i n the Cerampic (ceramic p i c t u r e ) device, was f i r s t reported i n 1972 by Smith and Land (17). The experimental arrangement i n v o l v e d i n observing t h i s e f f e c t i s given i n F i g u r e 8. No p o l a r i z e r s are employed s i n c e i t i s predominantly due to l i g h t s c a t t e r i n g from domains (areas o f l i k e p o l a r i z a t i o n ) w i t h i n the material. The o r i e n t a t i o n o f these domains are e l e c t r i c a l l y a l t e r able; and because l i g h t i s p r e f e r e n t i a l l y s c a t t e r e d along the p o l a r d i r e c t i o n o f the domains, the l i g h t transmitted by the PLZT p l a t e i s a l s o e l e c t r i c a l l y c o n t r o l l a b l e . In a d d i t i o n , l o c a l areas can be p o l a r i z e d to d i f f e r e n t l e v e l s l e a d i n g to an a b i l i t y f o r s t o r i n g images with a gray s c a l e c a p a b i l i t y and a r e s o l u t i o n o f a t l e a s t 30 l i n e p a i r s per m i l l i m e t e r . Once a given l o c a l area i s switched to a s p e c i f i c p o l a r i z a t i o n s t a t e , i t i s permanently locked i n u n t i l i t i s e l e c t r i c a l l y switched to a new s t a t e or the m a t e r i a l i s heated above i t s Curie p o i n t (thermally depoled) which erases a l l o f the p o l a r i z a t i o n s t a t e s . The means by which l o c a l areas are switched independently of each other i s provided by the photoconductor l a y e r sandwiched between one of the transparent ITO (indium-tin oxide) e l e c t r o d e s and the PLZT. When l i g h t impinges on the photoconductor l a y e r , i t reduces i t s r e s i s t i v i t y by s e v e r a l orders o f magnitude, e l e c t r o n s from the voltage source are t r a n s f e r r e d from the ITO e l e c t r o d e to the PLZT and the l o c a l p o l a r i z a t i o n i s switched to a new s t a t e . Erasure o f the t o t a l image i s performed by f l o o d i n g the p l a t e with l i g h t while the voltage i s a p p l i e d i n the p o s i t i v e s a t u r a t i o n d i r e c t i o n . The maximum cont r a s t r a t i o may be as h i g h as 100 to 1. In a d d i t i o n to the above three e f f e c t s , there are two others; i . e . , memory b i r e f r i n g e n c e and d e p o l a r i z a t i o n s c a t t e r i n g , which e x i s t i n the PLZT m a t e r i a l s and have been proposed f o r device a p p l i c a t i o n s . These are described i n reference 5. Applications Modes o f Operation. F i g u r e 8 a l s o i l l u s t r a t e s the two b a s i c modes o f o p e r a t i o n used i n e l e c t r b o p t i c devices; i . e . , the t r a n s verse and l o n g i t u d i n a l modes. In the transverse mode, the e l e c t r i c f i e l d i s a p p l i e d i n a d i r e c t i o n normal to the l i g h t propagat i o n d i r e c t i o n while i n the l o n g i t u d i n a l mode, the f i e l d i s ap p l i e d along the l i g h t propagation d i r e c t i o n . In general, the transverse mode of operation i s most e f f e c t i v e f o r v a r i a b l e b i r e f r i n g e n c e devices, and the l o n g i t u d i n a l mode i s b e t t e r s u i t e d f o r variable l i g h t scattering devices. A l s o , c o l o r e f f e c t s can be produced with v a r i a b l e b i r e f r i n g e n c e whereas they cannot with s c a t t e r i n g . V a r i a b l e b i r e f r i n g e n t devices always r e q u i r e the use o f p o l a r i z e d l i g h t ; however, s c a t t e r i n g devices may or may not necessitate polarized l i g h t . I t should be recognized t h a t i n



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order t o produce p o l a r i z e d l i g h t from an incandescent white l i g h t source there i s a s u b s t a n t i a l l o s s i n l i g h t i n t e n s i t y . In the case o f an i d e a l , l i n e a r p o l a r i z e r , t h i s l o s s amounts t o 50% o f the i n c i d e n t l i g h t ; but t h i s l o s s i n c r e a s e s t o approximately 70% with the use o f p l a s t i c sheet p o l a r i z e r s such as P o l a r o i d ' s HN32 material. In t r a n s v e r s e mode devices such as s h u t t e r s o r v a r i a b l e dens i t y f i l t e r s , the e l e c t r i c f i e l d i s g e n e r a l l y a p p l i e d by means o f s u i t a b l e e l e c t r o d e p a t t e r n on one o r both major s u r f a c e s o f a p o l i s h e d p l a t e o f m a t e r i a l . Since viewing i s accomplished through the gap between the p o s i t i v e and negative e l e c t r o d e s , i t f o l l o w s t h a t the a c t i v a t i n g v o l t a g e can be reduced, f o r a given o v e r a l l viewing area, by reducing the gap width and i n c r e a s i n g the t o t a l number o f gaps. T h i s r e s u l t s i n a number o f narrow, i n t e r d i g i t a l e l e c t r o d e s on a given p l a t e . By p l a c i n g the d e v i c e out o f the f o c a l plane o f the o p t i c a l system,the f i n e e l e c t r o d e s ( ~ 0.04mm wide) a r e v i r t u a l l y i n v i s i b l e , and image q u a l i t y through the dev i c e i s e x c e l l e n t . In c o n t r a s t to the l o n g i t u d i n a l mode, the t r a n s v e r s e mode produces l a r g e r e l e c t r o o p t i c e f f e c t s ; and i n the a c t i v a t e d or ON s t a t e , the m a t e r i a l i s o p t i c a l l y c l e a r with essent i a l l y no s c a t t e r i n g . Devices u t i l i z i n g t h i s mode may o r may not e x h i b i t memory, depending on the composition. In the l o n g i t u d i n a l mode, v o l t a g e i s a p p l i e d through the t h i c k n e s s o f the p l a t e n e c e s s i t a t i n g the use o f t r a n s p a r e n t e l e c trodes such as t i n oxide o r ITO. Since t h i s mode g e n e r a l l y a l i g n s the macroscopic o p t i c a x i s o f the m a t e r i a l p a r a l l e l t o the d i r e c t i o n o f viewing, o p t i c a l b i r e f r i n g e n t e f f e c t s are minimal. In t h i s mode, the s t r e n g t h o f the e l e c t r i c a l s w i t c h i n g f i e l d i s dependent on the t h i c k n e s s o f the p l a t e and the s p e c i f i c composition s e l e c t e d , but i s independent o f the area. Thermal/Flash P r o t e c t i v e Device, In 1975 Sandia L a b o r a t o r i e s of Albuquerque, New Mexico, began the design and development o f PLZT goggles f o r the U.S. A i r Force t o p r o v i d e p r o t e c t i o n f o r a i r c r a f t personnel from f l a s h b l i n d n e s s caused by a n u c l e a r e x p l o s i o n (18). A t t h a t time, the technology f o r producing such a d e v i c e was i n i t s i n f a n c y and many o f the techniques r e q u i r e d f o r i t s development and manufacture were non-existent. In the next three years, s e v e r a l new t e c h n o l o g i e s such as PLZT p o l i s h i n g and e l e c t r o d i n g , high performance p o l a r i z e r s , l e n s bonding and the f a b r i c a t i o n o f s p e c i a l i z e d e l e c t r o n i c s were a l l developed and p u t i n t o p r a c t i c e . The f i n a l product, o f f i c i a l l y designated as the EEU-2/P F l a s h b l i n d n e s s F l y e r s Goggles, i s shown i n F i g u r e 9. I t possesses s e v e r a l advantages over i t s predecessor, a l i q u i d photochromic system, among which are i n c l u d e d (1) s m a l l e r s i z e , (2) l e s s weight, (3) s o l i d s t a t e , (4) f a s t e r response and (5) higher p o r t a bility. I t has been i n p r o d u c t i o n f o r the l a s t two years and i s the f i r s t PLZT d e v i c e to reach t h i s stage. The f l a s h b l i n d n e s s goggle i s b a s i c a l l y a transverse-mode shut t e r o f the c o n f i g u r a t i o n shown i n F i g u r e 3(A). The s h u t t e r i s


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Sandia Laboratories

Figure 9.

PLZT Thermal/Flash Protective Goggle developed by Sandia Laboratories for the U.S. Air Force


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operated i n the f u l l y open or energized s t a t e u n t i l a l i g h t hazard i s detected by means of sensors mounted behind the viewing l e n s . When t h i s occurs, the PLZT e n e r g i z i n g voltage i s r a p i d l y discharged causing the goggles to r e v e r t to the c l o s e d s t a t e . When the t h r e a t i s removed, the PLZT i s re-energized and the open s t a t e i s r e s t o r e d . The open and c l o s e d s t a t e s of the device have t y p i c a l l y 20% and 0.006% transmission r e s p e c t i v e l y . Closure time i s l e s s than 150 microseconds. Data Recorder. The second PLZT device to reach the product i o n s t a t e (1979) i s a data d i s p l a y recorder manufactured by B e l l and Howell of Pasadena, C a l i f o r n i a . T h i s device i s shown i n F i g ure 10. The CEC HR-2000 Datagraph works on a p r i n c i p l e not p r e v i o u s l y used i n analog data r e c o r d i n g ; i . e . , a d i g i t a l l y cont r o l l e d e l e c t r o o p t i c s h u t t e r using p o l a r i z e d l i g h t and a PLZT c e r amic p l a t e as the e l e c t r o o p t i c m a t e r i a l (19). By s e l e c t i v e l y passing or b l o c k i n g l i g h t through a l i n e a r array o f hundreds o f t i n y l i g h t gates or s h u t t e r s , each of which i s c o n t r o l l e d by d r i ver e l e c t r o n i c s , the input data s i g n a l s are a c c u r a t e l y reproduced. L i g h t which passes through the l i g h t gates impinges upon d i r e c t p r i n t r e c o r d i n g paper to r e c o r d data waveforms w i t h high f i d e l i t y and accuracy. The o p e r a t i o n a l setup o f t h i s transverse-mode dev i c e i s the same as t h a t d e s c r i b e d i n F i g u r e 8(A). At the h e a r t o f the r e c o r d e r i s the a r r a y o f l i g h t gates composed of a number of PLZT p l a t e s c o n t a i n i n g vacuum deposited e l e c trodes spaced 0.0125 inches apart, thus p r o v i d i n g h i g h r e s o l u t i o n . By using t h i s type o f f i x e d , d i g i t a l l y c o n t r o l l e d s o l i d s t a t e array, the data recorder has e l i m i n a t e d such problems as l i n e a r i t y , beam d e f l e c t i o n , t a n g e n t i a l e r r o r , overshoot and i n e r t i a which l i m i t present galvanometer and CRT r e c o r d i n g d e v i c e s . The instrument has a frequency response from dc to 5 kHz s i n e wave or 10 kHz square wave and a r e c o r d i n g speed of 0.01 to 129 inches of paper per second. Stereo-Viewing Device. T h i s d e v i c e , now being s o l d under the name o f Megavision, has r e c e n t l y been developed by Megatek Corpora t i o n o f San Diego, C a l i f o r n i a . I t makes p o s s i b l e t r u e s t e r e o s c o p i c three-dimensional viewing of images on both v e c t o r r e f r e s h and r a s t e r scan computer graphic d i s p l a y s . The device i s shown i n F i g u r e 11. I t c o n s i s t s of a p a i r of l i g h t w e i g h t (1.5 oz.) viewing g l a s s e s , each l e n s of which i s s e p a r a t e l y e l e c t r o n i c a l l y cont r o l l e d through a small c a b l e to a belt-mounted backup u n i t . Each l e n s i s e s s e n t i a l l y an independently c o n t r o l l e d transverse-mode s h u t t e r of the type d e s c r i b e d i n F i g u r e 8(A). The s h u t t e r s are synchronized to an a l t e r n a t i n g p a i r of d i s p l a y e d images so t h a t only the l e f t eye sees the l e f t - e y e view and the r i g h t eye the r i g h t - e y e view. The views are a l t e r n a t e d a t a r a t e more than 30 Hz f o r each l e n s , a l l o w i n g the observer to p e r c e i v e a s i n g l e , s t e r e o s c o p i c image wrth a l i f e - l i k e sensation of o b j e c t depth. Some a p p l i c a t i o n s of t h i s device are f l i g h t simulators and t r a i n -



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Figure 10.

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Datagraph display recorder developed by Bell and Howell

Megatek Corporation

Figure 11.

Stereo-viewing system developed by Megatek Corporation


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ers, a i r t r a f f i c c o n t r o l , medical imaging, s c i e n t i f i c radar and sonar d i s p l a y s and contour mapping.

ELEMENTS

modeling,

Eye Safety Devices. Personnel eye s a f e t y devices such as the e l e c t r o n i c welding helmet, i n s p e c t i o n goggles and s a f e t y flip-down g l a s s e s mounted on a hard hat are some devices t h a t are i n the l a t t e r stages o f development a t the Ceramic Products department of Motorola, Inc. i n Albuquerque, New Mexico. A l l o f these d e v i c e s operate i n a manner very s i m i l a r to t h a t o f the f l a s h b l i n d n e s s goggles developed by Sandia L a b o r a t o r i e s . They are transverse-mode s h u t t e r d e v i c e s assembled i n a c o n f i g u r a t i o n as d e s c r i b e d i n F i g ure 8(A). An example o f the e l e c t r o n i c welding helmet i s shown i n F i g u r e 12. The l i g h t sensors and power supply are mounted e x t e r n a l l y to the PLZT s h u t t e r s which a c t as the v a r i a b l e d e n s i t y f i l t e r p l a t e . When an arc i s s t r u c k or some other s i m i l a r l y i n t e n s e l i g h t source i s a c t i v a t e d , the sensors d e t e c t t h i s change i n l i g h t l e v e l and remove the v o l t a g e from the s h u t t e r s , causing them to i n s t a n taneously darken to a shade p r e v i o u s l y s e t by the o p e r a t o r . When the a r c i s i n t e r r u p t e d , the s h u t t e r s q u i c k l y and a u t o m a t i c a l l y r e a c t i v a t e to t h e i r f u l l ON c o n d i t i o n . Since t h i s automatic a c t i o n e l i m i n a t e s the n e c e s s i t y o f r a i s i n g and lowering the helmet, the mask can be worn i n the down p o s i t i o n a t a l l times, thus i n c r e a s ing p r o d u c t i v i t y and p r e v e n t i n g a c c i d e n t a l eye burns from neighb o r i n g welding o p e r a t i o n s . A wide range o f f i l t e r p l a t e s from shade 4 (5.2% transmittance) to shade 14 (0.0004%) are a v a i l a b l e . Image Storage Devices. Research and development a c t i v i t i e s are c o n t i n u i n g a t Sandia L a b o r a t o r i e s i n Albuquerque, New Mexico, on image storage devices u t i l i z i n g PLZT ceramics. The Cerampic d e v i c e has r e c e i v e d extensive study f o r the p a s t s e v e r a l years and shows promise f o r image storage a p p l i c a t i o n s o f the f u t u r e . I t i s a l o n g i t u d i n a l s c a t t e r i n g mode d e v i c e as d e s c r i b e d i n F i g ure 8(C). E a r l y designs u t i l i z e d a photoconductor l a y e r which provided the s p a t i a l v a r i a t i o n s o f s w i t c h i n g v o l t a g e when exposed to s p a t i a l v a r i a t i o n s o f l i g h t i n t e n s i t y ( u s u a l l y through a cont a c t negative) needed to produce the image i n the ceramic. This photoconductor was subsequently e l i m i n a t e d by exposing the image with near UV l i g h t c o n t a i n i n g band gap (3.35 eV) or higher energy photons which produce a space charge f i e l d , thus a i d i n g the domain switching process. S i g n i f i c a n t improvements i n the s e n s i t i v i t y o f the exposure and r e c o r d i n g process were r e p o r t e d by Land and Peercy (20) through the use o f i o n i m p l a n t a t i o n (hydrogen and helium) i n the s u r f a c e o f the PLZT. Reductions i n exposure energy by as much as 10,000 times have more r e c e n t l y been achieved through the c o - i m p l a n t a t i o n o f argon and neon. Present exposure energy values o f about 10 /xJ/cm compare f a v o r a b l y with 100 /xJ/cm r e q u i r e d f o r f i n e - g r a i n e d holographic f i l m . An example o f t y p i c a l image q u a l i t y i s shown i n F i g u r e 13. The image i n the ceramic (A) was obtained by c o n t a c t exposure o f a negative produced from the o r i g i n a l photograph (B). 2


2


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Devices

Motorola Incorporated

Figure 12.

Electronic welding helmet developed by Motorola Incorporated

Sandia Laboratories

Figure 13. Example of image storage quality in memory PLZT 7065: (left) stored image and (right) original positive; ceramic device under development at Sandia Laboratories


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Summary The development o f o p t i c a l transparency i n f e r r o e l e c t r i c PLZT (lanthanum modified l e a d z i r c o n a t e t i t a n a t e ) ceramics a decade ago has s t i m u l a t e d a c o n s i d e r a b l e amount o f i n t e r e s t i n the nature o f these m a t e r i a l s / t h e i r e l e c t r o o p t i c behavior and t h e i r a p p l i c a t i o n to e l e c t r o o p t i c d e v i c e s . Although some measure o f o p t i c a l t r a n s parency has now been achieved i n other s i m i l a r f e r r o e l e c t r i c mat e r i a l s , r a r e - e a r t h lanthanum oxide i s unique i n i t s a b i l i t y t o produce the h i g h e s t q u a l i t y m a t e r i a l ; and thus, i t remains the standard o f the i n d u s t r y . The ceramics a r e c h a r a c t e r i z e d by good e l e c t r i c a l and o p t i c a l p r o p e r t i e s , uniform g r a i n s i z e and micros t r u c t u r e , h i g h e l e c t r o o p t i c c o e f f i c i e n t s and e x c e l l e n t moisture r e s i s t a n c e . T h e i r unusual combination o f p r o p e r t i e s have made them u s e f u l m a t e r i a l s f o r such s p e c i f i c a p p l i c a t i o n s as nuclear f l a s h b l i n d n e s s goggles, a data d i s p l a y r e c o r d e r , a stereoviewing system, an e l e c t r o n i c welding helmet and an image storage d i s p l a y device. An estimate o f the annual amount o f lanthanum oxide p r e s e n t l y being used i n a l l PLZT a p p l i c a t i o n s i s approximately 300Kg. T h i s f i g u r e i s c o n s e r v a t i v e l y p r o j e c t e d t o i n c r e a s e twenty-fold i n the next f i v e years as p r o d u c t i o n volumes i n c r e a s e and new a p p l i c a t i o n s f o r these m a t e r i a l s are r e a l i z e d .

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

Haertling, G.H.; Land, C.E. J. Am. Ceram. Soc., 1971, 54, 1-11. Okazaki, K.; Nagata, K. J. Am. Ceram. Soc., 1973, 56, 82-86. Meitzler, A.H.; O'Bryan, H.M. Jr. Proc. IEEE, 1973, 61, 959-966. Keve, E.T.; Annis, A.D. Ferroelectrics, 1973, 5, 77-89 Land, C.E.; Thacher, P.D.; Haertling, G.H., "Applied Solid State Science"; Academic Press, New York, 1974; p. 137-233. Micheron, F.; Rouchon, J.M.; Vergnolle, M. Appl. Phys. Lett., 1974, 24, 605-607. Drake, M.D. Applied Optics, 1974, 13, 347-352. Maldonado, J.R.; Fraser, D.B.; Meitzler, A.H., "Advances in Image Pickup and Displays"; Academic Press, New York, 1975, p. 65-168. Cutchen, J.T.; Harris, J.; Laguna, G. Applied Optics, 1975, 14, 1866-1873. Roese, J.; Khalafalla, A. Ferroelectrics, 1976, 10, 47-51. Land, C.E. Optical Engineering, 1978, 17, 317-326. Samek, N.; Raymond, W. Proc. of the 25th Intl. Instr. Symp., 1979, 16, 485-500. Hardtl, K.H.; Hennings, D. J. Am. Ceram. Soc., 1972, 55, 230231. Snow, G.S. J. Am. Ceram. Soc., 1973, 56, 91-96. Haertling, G.H.; Land, C.E. Ferroelectrics, 1972, 3, 269-280. Dungan, R.; Snow, G. Bull. Am. Ceram. Soc., 1977, 56, 781-782. Smith, W.D.; Land, C.E. Appl. Phys. Lett., 1972, 20, 169-171.


16.

HAERTLING

Electrooptic


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18. Cutchen, J.T. Ferroelectrics, 1980, 27, 173-178. 19. Howes, P.A. Proc. of the 25th Intl. Instr. Symp., 1979, 16, 199-210. 20. Land, C.; Peercy, P. Appl. Phys. Lett., 1980, 37, 39-41. December 19, 1980.


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PLZT Electrooptic Ceramics and Devices - ACS Symposium Series

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