Ceramic bistable deflection element

A bistable ceramic deflection element includes an assembly of two ceramic plates of a ferroelectric ceramic material which can be easily polarized and which has a low coercive field strength, one plate thereof being polarized so that the deflection element is deflected. The other plate is not polarized. The depolarization of the polarized plate can be performed simultaneously with the polarization of the plate non-polarized thus far, because the interchanging of the polarization condition can be simply performed by briefly applying a direct voltage via two extreme electrodes, provided on the plates. The direct voltage causes polarization of the plate not polarized thus far.

BACKGROUND OF THE INVENTION 
The invention relates to a bistable deflection element comprising an 
assembly of two plates which are provided with electrodes on both their 
principal surfaces and which are made of ferroelectric, easily polarizable 
ceramic material having a low coercive field strength. The plates are 
arranged with their principal surfaces one over the other and, are rigidly 
interconnected so that the inner electrodes are also electrically 
accessible. 
The invention also relates to a method of manufacturing such a deflection 
element. 
Unilaterally fixed ceramic deflection elements of polarized, ferroelectric 
ceramic material are normally deflected when a direct voltage is applied 
thereto, the deflection being approximately proportional to the applied 
electric voltage. This means that, when the voltage is interrupted, the 
zero position of the element is substantially reached again. 
In many cases, for example, when ceramic deflection elements are to be used 
for switching purposes, it may be advantageous to maintain the switch 
positions of the deflection element after the interruption of the electric 
voltage; that is to say the deflection is maintained without a further 
electric voltage being required. 
German Patent Application No. P 32 12 576.3 proposes a tristable optical 
switch in the form of a ceramic deflection element which comprises two 
plates of ferroelectric ceramic material which are uniformly depolarized 
in the initial position of the switch, i.e. in the "zero" position of the 
deflection element, while in the switching position each time one plate is 
polarized. This switch is depolarized via a decaying alternating voltage. 
This switch has proven its worth in practice, but has the drawback that the 
generating of the decaying alternating voltage requires a certain 
technical effort. 
SUMMARY OF THE INVENTION 
It is the object of the present invention to provide a ceramic deflection 
element which can occupy two stable positions without the continuous 
presence of an electric voltage being necessary, and which is constructed 
to operate so that no special circuits are required for depolarization. 
To achieve this object, the deflection element in accordance with the 
invention is characterized in that one of the plates is polarized after 
the connection to the other plate so that the assembly is deflected, 
electrical connections being provided on the two extreme electrodes via 
which the assembly of the two plates in series connection can be 
activated, by a direct voltage whose polarity opposes that of the voltage 
used for the polarization of the one plate. 
In a preferred embodiment in accordance with the invention, the plates are 
made of a mixed crystal ceramic material on the basis of lead/rare earth 
metal-zirconate/titanate, notably a composition which within the system 
lead/rare earth metal-zirconate/titanate is in the vicinity of the 
morphotropic phase limit on the rhombohedral side. 
A method of manufacturing such a ceramic deflection element is 
characterized in that it comprises the following steps: 
(a) providing electrodes on principal surfaces of two non-polarized ceramic 
plates of a ferroelectric ceramic material which can be easily polarized 
and which has a low coercive field strength; 
(b) arranging the two plates such that their principal surfaces extend one 
over the other and then rigidly interconnecting the two plates such that 
the electrodes which become situated in the center of the resultant 
assembly remain electrically accessible; 
(c) providing electrical connections on the two extreme electrodes; 
(d) polarizing one of the plates by applying a suitable electric direct 
voltage via the common electrodes which are situated in the center of the 
resultant assembly and via one of the extreme electrodes. 
An advantage obtained by means of the invention is notably that the 
deflection element is capable of occupying two stable positions without 
additional circuits being required for achieving a depolarization of the 
relevant polarized plate of the deflection element. 
The invention is based on the recognition of the fact that the 
depolarization of the polarized plate can be performed simultaneously with 
the polarization of the plate not polarized thus far, because the 
polarization condition of the plates can be simply interchanged by briefly 
applying, via the two extreme electrodes, a direct voltage which results 
in polarization of the non-polarized plate. Because of the series 
connection of the two plates, the shift currents released during this 
polarization procedure ensure exactly that the other plate is depolarized. 
This procedure can be repeated an arbitrary number of times. 
It is a further advantage of the invention that substantially no power is 
required to maintain the switch position, because the voltage required for 
the polarization of the ferroelectric ceramic plates need be applied only 
briefly, i.e. for less than 1 second, so actually a voltage pulse is 
concerned. The substantial length variations occurring during the 
polarization procedure can be used as switching steps for the switching 
over of a switch. Said length variations are always larger than length 
variations of elements in which the polarization condition is not changed. 
After the switching over, the electric voltage may be switched off again, 
after which the deflection element remains in the deflected position.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A deflection element 1 in accordance with the invention consists of two 
plates 3 and 5 which are arranged one over the other. The plates 3 and 5 
are rigidly interconnected, for example, by means of an adhesive, and 
their principal surfaces are provided with electrodes 7, 9, 11 and 13. 
These electrodes 7, 9, 11 and 13 are only shown in FIG. 1 for the sake of 
simplicity; however, they are also assumed to be present in the FIGS. 2 
and 3. The electrodes 7 and 13 constitute the two extreme electrodes of 
the deflection element 1 with its plates 3 and 5. At the clamping end 
(indicated by clamping jaws 15 and 17) the plate 5 is set back with 
respect to the plate 3, so that the electrodes 9 and 11 are free and 
electrically accessible. Via each time two of the connections 19 and 21 or 
23, a voltage source (not shown) can be connected to the electrodes 7, 9 
and 13. In FIGS. 2 and 3, the polarity on the connections as produced by 
the voltage source (not shown) is denoted as "+" and "-". This polarity 
causes deflection of the deflection element. 
The plates 3 and 5 of the deflection element 1 are made of a ferroelectric 
ceramic material. This material can be easily polarized and depolarized 
and is based on lead/rare earth metal-zirconate/titanate, notably a 
composition in accordance with the formula Pb.sub.0.94 La.sub.0.06 
Zr.sub.0.65 Ti.sub.0.35 O.sub.3. The coercive field strength then amounts 
to 7 kV/cm. The thickness of the plates 3 and 5 amounts to approximately 
230 .mu.m; their length amounts to approximately 17 mm and their width to 
approximately 7 mm. The electrodes 7, 9, 11 and 13 are preferably formed 
by vapor-deposited electrode layers. 
FIG. 1 shows the neutral position of the assembly of the two plates 3 and 
5. In this position, the plates 3 and 5 are both non-polarized so that 
they have the same length and the assembly extends rectilinearly. After 
application of a direct voltage pulse to the plate 3 via the connections 
19 and 21, the length of the plate 3 is changed by polarization, and the 
deflection element is deflected; this deflection is also maintained after 
interruption of the voltage (see FIG. 2). In the FIGS. 2 and 3, the 
polarization direction is denoted by an arrow 25, the direction of the 
variation of the length being denoted by the arrows 27. The amplitude of 
the voltage pulse for the polarization should be chosen so that the field 
strength produced thereby exceeds the coercive field strength of the 
material to be polarized. The polarity of the first voltage pulse for the 
polarization is not important, because always a deformation occurs in the 
same direction during the polarization procedure. The deflection element 
thus prepared is now ready for use and occupies one of the two stable 
positions. 
In FIG. 3A this position is represented again. It will be referred to as 
position A. It is characterized in that one of the two plates 3 and 5 
forming the deflection element 1, that is to say the plate 5, is 
non-polarized, while the other plate 3 is electrically polarized. 
The other one of the two stable positions, the position B shown in FIG. 3B, 
is characterized in that the polarization condition of the plates 3 and 5 
has been interchanged, i.e. the initially non-polarized plate 5 is now 
polarized and the initially polarized plate 3 has been depolarized during 
the polarization of the plate 5. This interchanging of the polarization 
condition is simply achieved, without additional structural or 
circuit-technical steps, in that, via the connections 19 and 23, a direct 
voltage is briefly applied to the two extreme electrodes 7 and 13, said 
direct voltage causing polarization of the non-polarized plate. The shift 
currents released during this polarization procedure cause depolarization 
of the polarized plate thanks to the fact that the two plates 3 and 5 are 
connected in series. This procedure can be repeated an arbitrary number of 
times. 
FIG. 4 shows measurement results obtained with a deflection element in 
accordance with the invention. The deflections which can be achieved by 
means of the present deflection element are larger than with a ceramic 
deflection element of the same dimensions in which the polarization 
conditions are not changed. 
FIG. 5 shows, in the form of a model, that the interchanging of the 
polarization condition by a voltage applied to the extreme electrodes only 
enables polarity reversal of the initially polarized plate to exactly the 
depolarized condition. FIG. 5A shows that the non-polarized plate 55 is 
characterized in that the distribution of the polarization directions in 
the ceramic material between all the six spatial directions (I . . . VI) 
is uniform. However, a 100% polarized ceramic material exhibits only a 
single polarization direction (plate 33). During polarization reversal 
(FIG. 5B), each time only the same amount of polarization can be changed 
in both plates by the shift current which is equal in both plates due to 
the series connection of the plates 33 and 55. For the spatial direction 
I, a rotation of the polarization through 180.degree. is possible in both 
plates 33, 55. For the spatial directions II to V, however, only a 
rotation of 90.degree. is possible. The components of the spatial 
directions VI even block one another, so that no rotation is possible. 
Consequently, after the polarization reversal a uniform distribution of 
the polarization directions remains in the upper plate 33 (in FIG. 5B), 
i.e. the material of this plate is depolarized, while the material of the 
plate 55 is complete polarized. 
For the described operation, the material used for the plates 3 and 5 of 
the deflection element 1 should have the following properties: the 
coercive field strength should be as low as possible in order to enable 
the use of as low as possible voltage pulses for the polarization and 
depolarization of the ceramic material. Moreover, the difference between 
the maximum length variation of the ceramic plates due to the applied 
voltage and the remaining length variation after interruption of the 
voltage pulse must be as small as possible. 
The proposed ceramic deflection element can be advantageously used as a 
switch in measuring apparatus for optical data transmission. However, it 
can generally be used as a switching element whenever magnetic fields 
exert a disturbing effect. For example, it can replace relays which must 
be provided with a complex magnetic shield in order to neutralize 
disturbing magnetic influences.