CA1090556A - Ferroelectric ceramic materials - Google Patents
Ferroelectric ceramic materialsInfo
- Publication number
- CA1090556A CA1090556A CA253,956A CA253956A CA1090556A CA 1090556 A CA1090556 A CA 1090556A CA 253956 A CA253956 A CA 253956A CA 1090556 A CA1090556 A CA 1090556A
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- Canada
- Prior art keywords
- compositions
- value
- temperature
- birefringence
- electro
- Prior art date
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Classifications
-
- C—CHEMISTRY; METALLURGY
- C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
- C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
- C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
- C04B35/01—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics
- C04B35/48—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics based on zirconium or hafnium oxides, zirconates, zircon or hafnates
- C04B35/49—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics based on zirconium or hafnium oxides, zirconates, zircon or hafnates containing also titanium oxides or titanates
- C04B35/491—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics based on zirconium or hafnium oxides, zirconates, zircon or hafnates containing also titanium oxides or titanates based on lead zirconates and lead titanates, e.g. PZT
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- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/0009—Materials therefor
- G02F1/0018—Electro-optical materials
- G02F1/0027—Ferro-electric materials
Abstract
ABSTRACT
An optical material having an electrically variable birefringence value consist of a ferroelectric ceramic of the composition Pb1-xLax(ZryTiz)1-x/4O3 where x is between 7.5 and 8 atomic percent and where the ratio y/z is between 70/30 and 74/26.
An optical material having an electrically variable birefringence value consist of a ferroelectric ceramic of the composition Pb1-xLax(ZryTiz)1-x/4O3 where x is between 7.5 and 8 atomic percent and where the ratio y/z is between 70/30 and 74/26.
Description
5 ~ PHB ,~2503 ' 25.5.76 ~Ferroelectric ceramic materialsn.
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This invention relates to electro-optic ferro ' electric ceramic materials of the lead lanthanum zir-- conate titanate (PLZT) system and specifically ~o a group Or such materials disclosed in U.S. Patent 3,666,666. This patent discloses optical materials having electrically variable birefringence comprising a ferroelectric ceramic of Pb1 xLax(ZryTiz)l x/43 : with x between 5 and 25 atomic percent and with the . ratio ~ z being from 5/95 to 95/5.
, 10 - According to the invention, there is provided , an optical material having an electrically variable birefringence value, comprising a f'erroelectric ceramic of the composition Pb1 xLax(zryTi~)1 ' xJ43~ where x is betweep 7~ and 8 atomic percent and where the , : , 15 ratio ~ z ~s between 70/30 and 74/26.
. ' The present invention discloses a small range ' of compositions which lie within the wide range of the :
aforementioned patent. These selected compositions have . maximum values-of birefringence ~ n or remanent bire-~ 20 fringence a nrem for transition temperature Tt lying 3 between 10C and 600C. ~lese selected compositions therefore have optimum characteristics for incorpo-ration in devices making use of the transverse electro-optic effect and' devices which may be electrically or ~ ' .
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P~B 32503 l~S~ 25.5.76 thermally addressed. In addition, the selected compo-sitions have other useful features such as low strain and sharp phase transition. These essentially intrinsic compositionaI effects are present, irrespective of the 5 - method of preparation of the ceramic.
The transition temperature of the material may . ~o C
C lie between 10^C and 600C or, but for some applications preferably lies between 25C and 600C..A material with a ratio ~ z of 72/28 in particular has good properties.
.
10 . A material having a rhombohedral crystal struc$ure was . .. sometimes ~ound to give preferable results.
The invention also comprises an electro-optic device, such as a display, filter, imaging system or memory device when including a body of the optical material OI^ the invention.
By way of example, certain embodiments of the invention will be further described with reference to the accompanying drawings, in which:
Figure 1 is a graph showing the vàriation of birefringenQe value ~ n with temperature ror a typical material, Figures 2a and 2b show variation of bire-fringence value ~ n with electric field-E, for tem-peratures below and above the transition temperature, respectively, Figure 3 shvws an arrangement of apparatus for dernonstrating a light gate effect, .
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Figure 4 shows a platelet of PLZT material carrying an electrode structure, . Figure 5 is a graph showing the compositional dependence of birefrin~ence value ~ n for two series 5 . of compositions, Figure 6 is a graph showing the compositional dependence of remanent birefringence value ~ nrem for one series of compositions, . Figure 7 is a graph showing the compositional variation of the width of the phase transition, and, . . Fi-gure 8 is:a partial phase diagram of the lead lanthanum zirconate titanate solid solution sys-tem.
The phenomenon of the transverse electro-optic effect is observed when an electric field i9 applied to . electrodes carried on a slice of an electro-optic cera-mic material which i8 arranged perpendicular to a prob-ing light beam. The temperature dependence of the bire-. . fringence value ~ N of a typical material is shown in Figure 1. In this graph, the birefringence value ~ n is sho~n on the vertical axis whilst temperature T is shown horizontally.
With the ceramic slice in an initially ther-mally depoled state, A, the material shows no biro-fringence until a poling field (of, for example.
20kV/cm) has been applied. During this application, the birefringence reaches the value B. When the field ; - .
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25.5.76 . 109~556 .
is removed the value of the birefringence falls to C
which i9 the remanent birefringence ~ nrem. On heat-ing, the remanent birefringence falls to D having a value near to zero after the temperature has exceeded the transition temperature Tt of the material. The re-manent birefringence then remains at zero when the tem-perature of the material is reduced again to A. However, if there is a sufficient bias ~ield, applied there is no discontinuity at the transition temperature Tt and the path is retraced on cooling.
- The variation of *he birefringence value with . : , field strength is shown in Figures 2a and 2b where the ' birefringence value a n is shown on the vertical axis and field strength E is shown horizontally. The curve in Figure 2a was taken at the temperature A which was below the transition temperature Tt. In this case the effect is not reversible although zero birefringence can be regained by heating the slice to a temperature above the transition temperature Tt, electrically shorting across the électrodes on the slice and cooling the slice.
The curve in Figure 2b was taken at the tem-perature D which was above the transition temperature Tt. In this instance, the effect observed is reversible.
The cycle D-~ F--~D in Figure 1 (which is also shown in Figure 2b) can be used for constructing a fa-! mily of electro-optic devices. As this cycle does not , ¦ ~ P~B 32503 1(~5~0556 25 . 5 . 76 . ~ .
require changes of temperature to effect switching, the devices of this kind are intended to be electri-cally addressed when in operation.
. One member of this family of devices i9 a : 5 light gate apparatus as depicted in Figure 3. The ap-- paratus comprises a PLZT platelet 1 which carries two semicircular electrodes 2 of gold. A light beam 3, from a source 4~ can be passed through a polarizing filter 5, the platelet 1 and then an analysing filter .
6. The intensity of the beam transmitted by the analys-ing filter 6 can be measured by means of a detector 7 . . when different electric poling fields E are applied across the electrodes 2. The polarizing filter 5 i8 orientated so that its plane of polarization P is tilted 45 to the left of vertical, the space-between the electrodes 2 is orientated vertically and the plane , A of the analysing filter 6 is orientated 45 to the right of vertical. The crossed planes of the filters thus ensure that no li.ght is transmitted when the plate-let is in a non-birefringent mode.
It is found that no light is transmitted when the platelet 1 is in state D, whilst a substanti~l pro-portion of the light is transmitted when'the platelet is in state F. Light transmission is a maximum.when the birefringence value ~ nE, at a given field strength E, is sunh that ' , ' ''.
~ 5 S~ PHB. 32,503.
where dEFF is the effective thickness of the platelet and ~ is the wavelength of the light used.
The letter E in Figure 3 and the adjacent arrow indicate the direction of the electric field be-tween the electrodes 2 which produces poling of thematerial of the platelet 1.
In an alternative construction,,the simple electrode arrangement of Figure 3 can be replaced by that of Figure 4 which depicts part of an interdigital electrode structure. Figure 4 shows a section of the PLZT plateIet 1 which now supports two electrode areas 8 arranged such that a substantial proportion of the platelet surface can be placed under tension by the electric field when this is applied. A light gate of this kind can be used for applications such as variable density transmission filters as described for instance in the U.S. Patents 3.512.864, 3.499.704, 3.531.182 and 3.702.724 and British Patent 1.368.378. A set of such light gates, with or without interdigital electrodes, can be arranged to form patterns suitable for numeric and alphanumeric display devices.
A second possible family of devices uses the cycle A--~C--~D--~A in Figure 1. These are generaily known as thermally addressed devices and use electro-optic material having a transition temperature Ttslightly above room temperature, for example, of 50C.
I ~HB ~2503 l~S~ 25.5.76 .
In a light gate structure, no light is transmitted at the temperature A in the thermally depoled stat~. Light is transmitted after electrically poling the material to condition C (compare also with Figure 2a). The in-tensity of light transmitted is a maximum when To reduce the intensity of transmission to zero, the ceramic platelet is heated above the tran-sition temperature Tt with the electrodes place in a ~hort circuit ~ondition and then it is cooled.
¦ Different device configurations w~ch may be thermally addressed include numeric and alphanumeric displays, light gates and memories. Some examples are disclosed British Patent Number 1435914.
~ 15 In order to evaluate the compositions of the j present invention, two series of test samples were pre-¦ - pared with varying proportions of lanthanum, zirconium - and titanium.
All of the test compositions were prepared in the same way and this was done from powdered raw materials PbO, TiO2,ZrO2 and La203 of analytical grade purity. The selected ingredients were weighed, dry-- milled for three hours in PVC-bottles with zirconia balls and then heatod for ten hours at 800C in cover-ed alumina crucibles. After cooling, the crucible con-tents were dry-milled for a further hour in PVC-bottles with zirconia balls. The resulting powders were then ~ . .
~ - 8 ~556 PHB. 32,503.
pressed hydrostatically (4 k bar) without the addition of any binder into prismatic specimens having the dimensions 7 mm x 7 mm x 20 mm.
These specimens were placed in covered alumina crucibles which were coated with platinum foils and then sintered in an oxygen atmosphere for six hours at 1290C.
To help control any loss of PbO by vaporisation, the firing took place in a fixed PbO atmosphere originating from a body of PbZrO3. After this sintering operation, the density of the resulting specimens was found to be about 99% of the theoretical density.
To remove as far as possible the last traces of residual porosity, a method involving an isostatic hotpressing operation was used. The method was similar to that disclosed in United States Patent Number 3853973 and this was the preferred method since it avoided the need to insert each sintered specimen in a closed con-tainer for the treatment. The working gas was argon and the hotpressing parameters for all samples was 200 bar during two hours at 1220C. Under these conditions the residual porosity was able to be eliminated so that full density was achieved.
After these treatments the material of the prismatic specimens was in a suitable condition to be cut up into convenient areas for the construction of electro-optical devices for testing purposes.
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¦ Each prismatic specimen was cut with a diamond saw and the resulting slices were polished to a suit-able final thickness. Each slice was then coated with an evaporated gold layer on a major face to form elec-- trodes.
One series of compositions was prepared ini-tially where the lanthanum concentration was adjusted so that the transition temperature Tt fell close to the ambient temperature. Thus Tt generally fell between 15C
and 35C. The results~of measurements made on platelets formed from these compositions are given in the follow-ing Table I where the heading composition gives the pro-portions x, y and z of lanthanum/zirconium/titanium respectively, Tt gives the transition temperature in C, and a n gives the birefringence value measured with a poling field of 20 kV/cm and at the transition ¦ temperature.
1.
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t - PHB 32503 25.5.76 !
. . . ~_ ~ TABLE I
. Composltion Tt(C) ~ n(20kV/cm~Tt) .
. 10/60/40 -7 7.4 x 10 3 . 9.4/62/38 16 9.8 " ' .
. 8.75/64J36 30 10.8 "
8.35/66/34 36 11.6 ~
: 8.13/68/32 32 11.3 "
. .7.95/70/30 35 13.8 "
8/72/28 25 14.5 "
. . 7.6/74/26 35 13.2 ~l .
A second series of compositions was then pre-pared by keepi~g the proportion of lanthanum constant at 7~ percent and varying the zirconium/titanium ratio.
The same measurements were made on.the compositions prepared as before. In addition, further meaSuremQnts were made of the blrefringence value at a temperature of (Tt ~ 400C) and a To 5, a quantity related to the width of the phase ~ransition The results are shown in the following Tabl- II. ~ -.
- 11 _ :~ .
~ PHB 32503 ~ 556 25.5.76 TABLE II
sition Tt(C) ~ n(20kV/Cm.Tt) ~ nrem(Tt-4C) 5~ C) .
._ 7~/56/44 158 7.3 x 10 31.1 x 10 3 1.4 7~/58/42 147 7-5 " 1.3 " 1.3 7~/50/40 138 8.3 ~' 2.4 " 1.2 7~/62/38 114 10.2 " 3.4 ll 1.1 7~/64/36 92 11.2 " 12.0 " 4.5 7~/66/34 73 11.8 " 12.1 " 11.0 7~/68/32 67 13.1 " 12.2 " 5.
7~/72/28 60 15.5 " 12.7 " 1.5 7 V 7G/Z4 25 12.6 " 8.0 " 9.0 . , '.
In Table III a number of compositions is given lying in the neighbourhood of the composition with op-timum properties.
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TABLE III
iti~Tt(~C)~A n(20kV/cm,5tl~ 5~ Cl~ n(20kV/cm,20C) 8.1/71/29 20 13.0 x 10-3 4.8 13 x 10-3 7.8/74/26 19 13.5 x 10-7.4 13.5 x 10-8.8/70/30 -12 .. 5.5 7.5 x 10-3 8.6/72/28 -14 .. 7.7 7.0 x 10-3 8.4/74/26 -14 .. 17.3 8.0 x 10-3 8.2/76/24 -33 .. 17.0 5.7 x 10-3 8.4/70/30 8 .. 4.2 12.7 x 10-3 8.2/72/28 29 .. 6.0 12.7 x 10-3 8.0/74/26 3 .. 10.3 ~ 12.5 x 10-3 7.8/76/24 -8 .. 12.2 12.5 x 10-3 7.4/74/26 45 13.4 x 10-3 5.8 15.4 x 10-3 7.2/76/24 43 13.7 x 10-3 7.0 15.2 x 10-3 7.4/70/30 71 ~ 10.5 x 10-3 4.1 ~ 14.1 x 10-3 7.2/72/28 73 ; 12.4 x 10-3 3.7 16.4 x 10-3 7,~/7~80 10.2 x 10-3 3 ~ 14.5 x 10-3 From the results given in these tables, it was possible to draw the graph of Figure 5 which depicts the compositional dependence of the birefringence value ~ n for both series of compositions. The graph shows on the vertical axis the values for birefringence ( ~ n x 103) whilst the horizontal axis shows the value of ~.
a~
1~5~ PHB 32503 25.5.76 The points on the graph indicated by a circle are deriv-ed from Table I (15 ~ Tt < 35 series) whilst the points indicated by a cross are derived from Table II
(7~% lantanum series). For comparison, a scale giving the transition temperatures TtC for the 7~% lanthanum . series is marked horizontally at the top of the Figure.
It will be seen from Figure 5 that a peak is . evident for both series of compositions in the regiDn 7~-8/ ~ 100-~ where 70 ~ ~ < 74. Since the transition temperature ls not too far from:ambient:-for these com-p~sitions, they~are likely to be particularly useful for . electrically addressed light gate devices.
The optical materials disclosed in the present speoification generally have a rhombohedral crystal structure. However~ they do not necessarily always haye this structure and they might for example be orthorhom-: bic. In Flgures -5 and 6 the presence.of the crystal struc-. tures tetragonal, orthorhombic and rhombohedral are in-dicated respectively by the letters T, 0 and R. m e material having a rhombohedral crystal structure was sometimes found to be preferabl,e.
Figure 6 shows the.compositional dependence . of the remanent.birefringence value ~ nrem as measured at a temperature of 400C below the transition temperature Tt. The points marked on the graph are for the Table II
(7~% lanthanum series) compositions. The scales show as before values for birefringence ( a nrem x 103) and tho ~ , .
. _ 14 .
~, , .
~ 556 PHB. 32,503.
value of y. For comparison, a temperature scale of (Tt ~ 40C) is marked horizontally at the top of the Figure.
The curve shown in this graph is also seen to reach a peak where 70 ~ _ ~ 73. This property, together with the fact that the transition temperature Tt is some 40C above the ambient temperature (say 20C) makes these compositions parti-cularly suitable for use in thermally addressed light gate devices.
An additional factor of importance to these devices, particularly those of the thermally addressed type is the width of the phase transition at the transition temperature.
A sharp phase transition gives good discrimination between the ON and OFF states in a device. Such a transition also allows cross bar addressing of thermally addressed devices as dis-closed in the afore-mentioned British Patent 1.435.914 which was published on May 19, 1976. Figure 7 shows the composi-tional variation of the width of the phase transition for the Table II (7-1/2% lanthanum series) compositions. In Table II, the value ~ To 5 (C) is given as it is related to the width of the phase transition. This value is the temperature in-terval within which one half of the polarisation is lost. It can be seen that this quantity, which is desired to be low, has a minimum value at the composition 7-1/2/72/28 which is equal or close to the optimum electro-optic effect.
Figure 8 is a partial phase diagram of the lead lanthanum zirconate titanate solid solution system -~IB 32503 0~0 S ~ 25.5,76 ' and the rectangular area ABCD is a range of compositions referred to in US Patent 3.666.666. The present invention discloses the special characteristics of a-selected range of compositions which fall within the small area X in the phase diagram.
As regards mechanical properties pf the CerQ-mic compositions of the invention, the variation of the spontaneous strain as a function of the composition has been measured for the Table II (7~% lanthanum series) compositions. For electro-optic devices, strain needs to be minimized and such a minimum was found to occur at a value close to y = 72 in the rhombohedral phase . ., .
for this series. The fact that minimum strain corresponds to a high birefringence value i8 a very useful feature for devices.
The invention can thus be seen to have provid-ed a range of electro-optic oeramic materials having a maximum birefringence value combined with low strain and .
narrow transi$ion width. ~y the appropriate combination of constituents, the transi*ion temperatures can be tailored to lie within a small range of between 25C
and 600C. Such properties make the materials of the in-vention useful in electro-optic devices since they enable devices of high efficiency to be constructed. ~
The materials are particularly suitable for use in dis-i play devices having a low operating voltage.
The foregoing description o~ embodiments of '~' ..', , ' ' , ,~ .
10g0556 25.5,76 .
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the invention has been given by way of example only and a number of modifications may be made without departing ~ from the scope of the invention as defined by the append-ed claims. For example, instead of using the method of .
. 5 . preparation of the compositions that has been specifi-cally described, an alternative method could be used as has been described in the literature or in the afore-mentioned published patents.
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This invention relates to electro-optic ferro ' electric ceramic materials of the lead lanthanum zir-- conate titanate (PLZT) system and specifically ~o a group Or such materials disclosed in U.S. Patent 3,666,666. This patent discloses optical materials having electrically variable birefringence comprising a ferroelectric ceramic of Pb1 xLax(ZryTiz)l x/43 : with x between 5 and 25 atomic percent and with the . ratio ~ z being from 5/95 to 95/5.
, 10 - According to the invention, there is provided , an optical material having an electrically variable birefringence value, comprising a f'erroelectric ceramic of the composition Pb1 xLax(zryTi~)1 ' xJ43~ where x is betweep 7~ and 8 atomic percent and where the , : , 15 ratio ~ z ~s between 70/30 and 74/26.
. ' The present invention discloses a small range ' of compositions which lie within the wide range of the :
aforementioned patent. These selected compositions have . maximum values-of birefringence ~ n or remanent bire-~ 20 fringence a nrem for transition temperature Tt lying 3 between 10C and 600C. ~lese selected compositions therefore have optimum characteristics for incorpo-ration in devices making use of the transverse electro-optic effect and' devices which may be electrically or ~ ' .
_ 2 ~ .
P~B 32503 l~S~ 25.5.76 thermally addressed. In addition, the selected compo-sitions have other useful features such as low strain and sharp phase transition. These essentially intrinsic compositionaI effects are present, irrespective of the 5 - method of preparation of the ceramic.
The transition temperature of the material may . ~o C
C lie between 10^C and 600C or, but for some applications preferably lies between 25C and 600C..A material with a ratio ~ z of 72/28 in particular has good properties.
.
10 . A material having a rhombohedral crystal struc$ure was . .. sometimes ~ound to give preferable results.
The invention also comprises an electro-optic device, such as a display, filter, imaging system or memory device when including a body of the optical material OI^ the invention.
By way of example, certain embodiments of the invention will be further described with reference to the accompanying drawings, in which:
Figure 1 is a graph showing the vàriation of birefringenQe value ~ n with temperature ror a typical material, Figures 2a and 2b show variation of bire-fringence value ~ n with electric field-E, for tem-peratures below and above the transition temperature, respectively, Figure 3 shvws an arrangement of apparatus for dernonstrating a light gate effect, .
J , ., ~ ~ 3 ~l~ 32503 25.5.76 ~ 556 .
Figure 4 shows a platelet of PLZT material carrying an electrode structure, . Figure 5 is a graph showing the compositional dependence of birefrin~ence value ~ n for two series 5 . of compositions, Figure 6 is a graph showing the compositional dependence of remanent birefringence value ~ nrem for one series of compositions, . Figure 7 is a graph showing the compositional variation of the width of the phase transition, and, . . Fi-gure 8 is:a partial phase diagram of the lead lanthanum zirconate titanate solid solution sys-tem.
The phenomenon of the transverse electro-optic effect is observed when an electric field i9 applied to . electrodes carried on a slice of an electro-optic cera-mic material which i8 arranged perpendicular to a prob-ing light beam. The temperature dependence of the bire-. . fringence value ~ N of a typical material is shown in Figure 1. In this graph, the birefringence value ~ n is sho~n on the vertical axis whilst temperature T is shown horizontally.
With the ceramic slice in an initially ther-mally depoled state, A, the material shows no biro-fringence until a poling field (of, for example.
20kV/cm) has been applied. During this application, the birefringence reaches the value B. When the field ; - .
' .
.
' . ``~
25.5.76 . 109~556 .
is removed the value of the birefringence falls to C
which i9 the remanent birefringence ~ nrem. On heat-ing, the remanent birefringence falls to D having a value near to zero after the temperature has exceeded the transition temperature Tt of the material. The re-manent birefringence then remains at zero when the tem-perature of the material is reduced again to A. However, if there is a sufficient bias ~ield, applied there is no discontinuity at the transition temperature Tt and the path is retraced on cooling.
- The variation of *he birefringence value with . : , field strength is shown in Figures 2a and 2b where the ' birefringence value a n is shown on the vertical axis and field strength E is shown horizontally. The curve in Figure 2a was taken at the temperature A which was below the transition temperature Tt. In this case the effect is not reversible although zero birefringence can be regained by heating the slice to a temperature above the transition temperature Tt, electrically shorting across the électrodes on the slice and cooling the slice.
The curve in Figure 2b was taken at the tem-perature D which was above the transition temperature Tt. In this instance, the effect observed is reversible.
The cycle D-~ F--~D in Figure 1 (which is also shown in Figure 2b) can be used for constructing a fa-! mily of electro-optic devices. As this cycle does not , ¦ ~ P~B 32503 1(~5~0556 25 . 5 . 76 . ~ .
require changes of temperature to effect switching, the devices of this kind are intended to be electri-cally addressed when in operation.
. One member of this family of devices i9 a : 5 light gate apparatus as depicted in Figure 3. The ap-- paratus comprises a PLZT platelet 1 which carries two semicircular electrodes 2 of gold. A light beam 3, from a source 4~ can be passed through a polarizing filter 5, the platelet 1 and then an analysing filter .
6. The intensity of the beam transmitted by the analys-ing filter 6 can be measured by means of a detector 7 . . when different electric poling fields E are applied across the electrodes 2. The polarizing filter 5 i8 orientated so that its plane of polarization P is tilted 45 to the left of vertical, the space-between the electrodes 2 is orientated vertically and the plane , A of the analysing filter 6 is orientated 45 to the right of vertical. The crossed planes of the filters thus ensure that no li.ght is transmitted when the plate-let is in a non-birefringent mode.
It is found that no light is transmitted when the platelet 1 is in state D, whilst a substanti~l pro-portion of the light is transmitted when'the platelet is in state F. Light transmission is a maximum.when the birefringence value ~ nE, at a given field strength E, is sunh that ' , ' ''.
~ 5 S~ PHB. 32,503.
where dEFF is the effective thickness of the platelet and ~ is the wavelength of the light used.
The letter E in Figure 3 and the adjacent arrow indicate the direction of the electric field be-tween the electrodes 2 which produces poling of thematerial of the platelet 1.
In an alternative construction,,the simple electrode arrangement of Figure 3 can be replaced by that of Figure 4 which depicts part of an interdigital electrode structure. Figure 4 shows a section of the PLZT plateIet 1 which now supports two electrode areas 8 arranged such that a substantial proportion of the platelet surface can be placed under tension by the electric field when this is applied. A light gate of this kind can be used for applications such as variable density transmission filters as described for instance in the U.S. Patents 3.512.864, 3.499.704, 3.531.182 and 3.702.724 and British Patent 1.368.378. A set of such light gates, with or without interdigital electrodes, can be arranged to form patterns suitable for numeric and alphanumeric display devices.
A second possible family of devices uses the cycle A--~C--~D--~A in Figure 1. These are generaily known as thermally addressed devices and use electro-optic material having a transition temperature Ttslightly above room temperature, for example, of 50C.
I ~HB ~2503 l~S~ 25.5.76 .
In a light gate structure, no light is transmitted at the temperature A in the thermally depoled stat~. Light is transmitted after electrically poling the material to condition C (compare also with Figure 2a). The in-tensity of light transmitted is a maximum when To reduce the intensity of transmission to zero, the ceramic platelet is heated above the tran-sition temperature Tt with the electrodes place in a ~hort circuit ~ondition and then it is cooled.
¦ Different device configurations w~ch may be thermally addressed include numeric and alphanumeric displays, light gates and memories. Some examples are disclosed British Patent Number 1435914.
~ 15 In order to evaluate the compositions of the j present invention, two series of test samples were pre-¦ - pared with varying proportions of lanthanum, zirconium - and titanium.
All of the test compositions were prepared in the same way and this was done from powdered raw materials PbO, TiO2,ZrO2 and La203 of analytical grade purity. The selected ingredients were weighed, dry-- milled for three hours in PVC-bottles with zirconia balls and then heatod for ten hours at 800C in cover-ed alumina crucibles. After cooling, the crucible con-tents were dry-milled for a further hour in PVC-bottles with zirconia balls. The resulting powders were then ~ . .
~ - 8 ~556 PHB. 32,503.
pressed hydrostatically (4 k bar) without the addition of any binder into prismatic specimens having the dimensions 7 mm x 7 mm x 20 mm.
These specimens were placed in covered alumina crucibles which were coated with platinum foils and then sintered in an oxygen atmosphere for six hours at 1290C.
To help control any loss of PbO by vaporisation, the firing took place in a fixed PbO atmosphere originating from a body of PbZrO3. After this sintering operation, the density of the resulting specimens was found to be about 99% of the theoretical density.
To remove as far as possible the last traces of residual porosity, a method involving an isostatic hotpressing operation was used. The method was similar to that disclosed in United States Patent Number 3853973 and this was the preferred method since it avoided the need to insert each sintered specimen in a closed con-tainer for the treatment. The working gas was argon and the hotpressing parameters for all samples was 200 bar during two hours at 1220C. Under these conditions the residual porosity was able to be eliminated so that full density was achieved.
After these treatments the material of the prismatic specimens was in a suitable condition to be cut up into convenient areas for the construction of electro-optical devices for testing purposes.
~,~
:1 j PHB 32503 ~ S ~ 25.5.76 1 .
¦ Each prismatic specimen was cut with a diamond saw and the resulting slices were polished to a suit-able final thickness. Each slice was then coated with an evaporated gold layer on a major face to form elec-- trodes.
One series of compositions was prepared ini-tially where the lanthanum concentration was adjusted so that the transition temperature Tt fell close to the ambient temperature. Thus Tt generally fell between 15C
and 35C. The results~of measurements made on platelets formed from these compositions are given in the follow-ing Table I where the heading composition gives the pro-portions x, y and z of lanthanum/zirconium/titanium respectively, Tt gives the transition temperature in C, and a n gives the birefringence value measured with a poling field of 20 kV/cm and at the transition ¦ temperature.
1.
.
.~ .
- . -.
!
t - PHB 32503 25.5.76 !
. . . ~_ ~ TABLE I
. Composltion Tt(C) ~ n(20kV/cm~Tt) .
. 10/60/40 -7 7.4 x 10 3 . 9.4/62/38 16 9.8 " ' .
. 8.75/64J36 30 10.8 "
8.35/66/34 36 11.6 ~
: 8.13/68/32 32 11.3 "
. .7.95/70/30 35 13.8 "
8/72/28 25 14.5 "
. . 7.6/74/26 35 13.2 ~l .
A second series of compositions was then pre-pared by keepi~g the proportion of lanthanum constant at 7~ percent and varying the zirconium/titanium ratio.
The same measurements were made on.the compositions prepared as before. In addition, further meaSuremQnts were made of the blrefringence value at a temperature of (Tt ~ 400C) and a To 5, a quantity related to the width of the phase ~ransition The results are shown in the following Tabl- II. ~ -.
- 11 _ :~ .
~ PHB 32503 ~ 556 25.5.76 TABLE II
sition Tt(C) ~ n(20kV/Cm.Tt) ~ nrem(Tt-4C) 5~ C) .
._ 7~/56/44 158 7.3 x 10 31.1 x 10 3 1.4 7~/58/42 147 7-5 " 1.3 " 1.3 7~/50/40 138 8.3 ~' 2.4 " 1.2 7~/62/38 114 10.2 " 3.4 ll 1.1 7~/64/36 92 11.2 " 12.0 " 4.5 7~/66/34 73 11.8 " 12.1 " 11.0 7~/68/32 67 13.1 " 12.2 " 5.
7~/72/28 60 15.5 " 12.7 " 1.5 7 V 7G/Z4 25 12.6 " 8.0 " 9.0 . , '.
In Table III a number of compositions is given lying in the neighbourhood of the composition with op-timum properties.
.. ' ,' , ~ - 12 ,~ .
lO~(~5S6 PHB. 32,503.
TABLE III
iti~Tt(~C)~A n(20kV/cm,5tl~ 5~ Cl~ n(20kV/cm,20C) 8.1/71/29 20 13.0 x 10-3 4.8 13 x 10-3 7.8/74/26 19 13.5 x 10-7.4 13.5 x 10-8.8/70/30 -12 .. 5.5 7.5 x 10-3 8.6/72/28 -14 .. 7.7 7.0 x 10-3 8.4/74/26 -14 .. 17.3 8.0 x 10-3 8.2/76/24 -33 .. 17.0 5.7 x 10-3 8.4/70/30 8 .. 4.2 12.7 x 10-3 8.2/72/28 29 .. 6.0 12.7 x 10-3 8.0/74/26 3 .. 10.3 ~ 12.5 x 10-3 7.8/76/24 -8 .. 12.2 12.5 x 10-3 7.4/74/26 45 13.4 x 10-3 5.8 15.4 x 10-3 7.2/76/24 43 13.7 x 10-3 7.0 15.2 x 10-3 7.4/70/30 71 ~ 10.5 x 10-3 4.1 ~ 14.1 x 10-3 7.2/72/28 73 ; 12.4 x 10-3 3.7 16.4 x 10-3 7,~/7~80 10.2 x 10-3 3 ~ 14.5 x 10-3 From the results given in these tables, it was possible to draw the graph of Figure 5 which depicts the compositional dependence of the birefringence value ~ n for both series of compositions. The graph shows on the vertical axis the values for birefringence ( ~ n x 103) whilst the horizontal axis shows the value of ~.
a~
1~5~ PHB 32503 25.5.76 The points on the graph indicated by a circle are deriv-ed from Table I (15 ~ Tt < 35 series) whilst the points indicated by a cross are derived from Table II
(7~% lantanum series). For comparison, a scale giving the transition temperatures TtC for the 7~% lanthanum . series is marked horizontally at the top of the Figure.
It will be seen from Figure 5 that a peak is . evident for both series of compositions in the regiDn 7~-8/ ~ 100-~ where 70 ~ ~ < 74. Since the transition temperature ls not too far from:ambient:-for these com-p~sitions, they~are likely to be particularly useful for . electrically addressed light gate devices.
The optical materials disclosed in the present speoification generally have a rhombohedral crystal structure. However~ they do not necessarily always haye this structure and they might for example be orthorhom-: bic. In Flgures -5 and 6 the presence.of the crystal struc-. tures tetragonal, orthorhombic and rhombohedral are in-dicated respectively by the letters T, 0 and R. m e material having a rhombohedral crystal structure was sometimes found to be preferabl,e.
Figure 6 shows the.compositional dependence . of the remanent.birefringence value ~ nrem as measured at a temperature of 400C below the transition temperature Tt. The points marked on the graph are for the Table II
(7~% lanthanum series) compositions. The scales show as before values for birefringence ( a nrem x 103) and tho ~ , .
. _ 14 .
~, , .
~ 556 PHB. 32,503.
value of y. For comparison, a temperature scale of (Tt ~ 40C) is marked horizontally at the top of the Figure.
The curve shown in this graph is also seen to reach a peak where 70 ~ _ ~ 73. This property, together with the fact that the transition temperature Tt is some 40C above the ambient temperature (say 20C) makes these compositions parti-cularly suitable for use in thermally addressed light gate devices.
An additional factor of importance to these devices, particularly those of the thermally addressed type is the width of the phase transition at the transition temperature.
A sharp phase transition gives good discrimination between the ON and OFF states in a device. Such a transition also allows cross bar addressing of thermally addressed devices as dis-closed in the afore-mentioned British Patent 1.435.914 which was published on May 19, 1976. Figure 7 shows the composi-tional variation of the width of the phase transition for the Table II (7-1/2% lanthanum series) compositions. In Table II, the value ~ To 5 (C) is given as it is related to the width of the phase transition. This value is the temperature in-terval within which one half of the polarisation is lost. It can be seen that this quantity, which is desired to be low, has a minimum value at the composition 7-1/2/72/28 which is equal or close to the optimum electro-optic effect.
Figure 8 is a partial phase diagram of the lead lanthanum zirconate titanate solid solution system -~IB 32503 0~0 S ~ 25.5,76 ' and the rectangular area ABCD is a range of compositions referred to in US Patent 3.666.666. The present invention discloses the special characteristics of a-selected range of compositions which fall within the small area X in the phase diagram.
As regards mechanical properties pf the CerQ-mic compositions of the invention, the variation of the spontaneous strain as a function of the composition has been measured for the Table II (7~% lanthanum series) compositions. For electro-optic devices, strain needs to be minimized and such a minimum was found to occur at a value close to y = 72 in the rhombohedral phase . ., .
for this series. The fact that minimum strain corresponds to a high birefringence value i8 a very useful feature for devices.
The invention can thus be seen to have provid-ed a range of electro-optic oeramic materials having a maximum birefringence value combined with low strain and .
narrow transi$ion width. ~y the appropriate combination of constituents, the transi*ion temperatures can be tailored to lie within a small range of between 25C
and 600C. Such properties make the materials of the in-vention useful in electro-optic devices since they enable devices of high efficiency to be constructed. ~
The materials are particularly suitable for use in dis-i play devices having a low operating voltage.
The foregoing description o~ embodiments of '~' ..', , ' ' , ,~ .
10g0556 25.5,76 .
. ~ .
the invention has been given by way of example only and a number of modifications may be made without departing ~ from the scope of the invention as defined by the append-ed claims. For example, instead of using the method of .
. 5 . preparation of the compositions that has been specifi-cally described, an alternative method could be used as has been described in the literature or in the afore-mentioned published patents.
, ' ' ' ' ' .
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J : -, - 17 -,
Claims (5)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. An optical material having an electrically variable birefringence value, comprising a ferroelec-tric ceramic of the composition Pb1 - xLax(ZryTiz)1-x/4O3' where x is between 7.5 and 8 atomic percent and where the ratio y/z is between 70/30 and 74/26, the optical material having a transition temperature lying between 10°C and 60°C.
2. A material as claimed in Claim 1, in which the transition temperature lies between 25°C and 60°C.
3. A material as claimed in Claim 1, in which the ratio y/z is 72/28.
4. An electro-optical device including a body of an optical material as claimed in Claim 1.
5. An electro-optical device as claimed in Claim 4 in which the device is a display, filter, imaging system or a memory device.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB24623-75 | 1975-06-09 | ||
| GB2462375A GB1516408A (en) | 1975-06-09 | 1975-06-09 | Ferroelectric ceramic materials |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1090556A true CA1090556A (en) | 1980-12-02 |
Family
ID=10214618
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA253,956A Expired CA1090556A (en) | 1975-06-09 | 1976-06-03 | Ferroelectric ceramic materials |
Country Status (5)
| Country | Link |
|---|---|
| JP (1) | JPS527258A (en) |
| CA (1) | CA1090556A (en) |
| DE (1) | DE2625535A1 (en) |
| FR (1) | FR2332964A1 (en) |
| GB (1) | GB1516408A (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS56131979A (en) * | 1980-03-19 | 1981-10-15 | Hitachi Ltd | Piezoelectric material for transparent vibrator and transparent vibrator |
| JP2532381B2 (en) * | 1986-03-04 | 1996-09-11 | 松下電器産業株式会社 | Ferroelectric thin film element and manufacturing method thereof |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3666666A (en) * | 1969-12-17 | 1972-05-30 | Atomic Energy Commission | Ferroelectric ceramic materials |
| JPS522083B2 (en) * | 1971-10-01 | 1977-01-19 |
-
1975
- 1975-06-09 GB GB2462375A patent/GB1516408A/en not_active Expired
-
1976
- 1976-06-03 CA CA253,956A patent/CA1090556A/en not_active Expired
- 1976-06-05 DE DE19762625535 patent/DE2625535A1/en active Pending
- 1976-06-09 FR FR7617354A patent/FR2332964A1/en not_active Withdrawn
- 1976-06-09 JP JP51066645A patent/JPS527258A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| FR2332964A1 (en) | 1977-06-24 |
| JPS527258A (en) | 1977-01-20 |
| DE2625535A1 (en) | 1976-12-23 |
| GB1516408A (en) | 1978-07-05 |
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