Metal working – Piezoelectric device making
Reexamination Certificate
1998-10-23
2001-02-06
Hall, Carl E. (Department: 3729)
Metal working
Piezoelectric device making
C310S318000, C310S357000
Reexamination Certificate
active
06182340
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention generally relates to flextensional piezoelectric transformers and actuators, and more particularly, to a method of manufacturing a flextensional multi-layer piezoelectric transformer/actuator by cofiring the ceramic composition with the electrode forming metallization.
2. Description of the Prior Art
Wound-type electromagnetic transformers have been used for generating high voltage in internal power circuits of devices such as televisions or in charging devices of copier machines which require high voltage. Such electromagnetic transformers take the form of a conductor wound onto a core made of a magnetic substance. Because a large number of turns of the conductor are required to realize a high transformation ratio, electromagnetic transformers that are effective, yet at the same time compact and slim in shape are extremely difficult to produce.
To remedy this problem, piezoelectric transformers utilizing the piezoelectric effect have been provided in the prior art. In contrast to the general electromagnetic transformer, the piezoelectric ceramic transformer has a number of advantages. The size of a piezoelectric transformer can be made smaller than electromagnetic transformers of comparable transformation ratio. Piezoelectric transformers can be made nonflammable, and they produce no electromagnetically induced noise.
Materials exhibiting piezoelectric and electrostrictive properties develop a polarized electric field when placed under stress or strain. Conversely, they undergo dimensional changes in an applied electric field. The dimensional change (i.e., expansion or contraction) of a piezoelectric or electrostrictive material is a function of the applied electric field.
The ceramic bodies employed in prior piezoelectric transformers take various forms and configurations, including rings, flat slabs and the like. A typical example of a prior piezoelectric transformer is illustrated in FIG.
1
. This type of piezoelectric transformer is commonly referred to as a “Rosen-type” piezoelectric transformer. The basic Rosen-type piezoelectric transformer was disclosed in U.S. Pat. No. 2,830,274 to Rosen, and numerous variations of this basic apparatus are well known in the prior art. The typical Rosen-type piezoelectric transformer comprises a flat ceramic slab
10
which is appreciably longer than it is wide and substantially wider than thick. As shown in
FIG. 1
, a piezoelectric body
10
is employed having some portions polarized differently from others. In the case of
FIG. 1
, the piezoelectric body
10
is in the form of a flat slab which is considerably wider than it is thick, and having greater length than width. A substantial portion of the slab
10
, the portion
12
to the right of the center of the slab is polarized longitudinally, whereas the remainder of the slab is polarized transversely to the plane of the face of the slab. In this case the remainder of the slab is actually divided into two portions, one portion
14
being polarized transversely in one direction, and the remainder of the left half of the slab, the portion
16
also being polarized transversely but in the direction opposite to the direction of polarization in the portion
14
.
In order that electrical voltages may be related to mechanical stress in the slab
10
, electrodes are provided. If desired, there may be a common electrode
18
, shown as grounded. For the primary connection and for relating voltage at opposite faces of the transversely polarized portion
14
of the slab
10
, there is an electrode
20
opposite the common electrode
18
. For relating voltages to stress generated in the longitudinal direction of the slab
10
, there is a secondary or high-voltage electrode
22
cooperating with the common electrode
18
. The electrode
22
is shown as connected to a terminal
24
of an output load
26
grounded at its opposite end.
In the arrangement illustrated in
FIG. 1
, a voltage applied between the electrodes
18
and
20
is stepped up to a high voltage between the electrodes
18
and
22
for supplying the load
26
at a much higher voltage than that applied between the electrodes
18
and
20
.
A problem with prior piezoelectric transformers is that they are difficult to manufacture because individual ceramic elements must be “poled” at least twice each, and the direction of the poles must be different from each other.
Another problem with prior piezoelectric transformers is that they are difficult to manufacture because it is necessary to apply electrodes not only to the major faces of the ceramic element, but also to at least one of the minor faces of the ceramic element.
Another problem with prior piezoelectric transformers is that they are difficult to manufacture because, in order to electrically connect the transformer to an electric circuit, it is necessary to attach (i.e. by soldering or otherwise) electrical conductors (e.g. wires) to electrodes on the major faces of the ceramic element as well as on at least one minor face of the ceramic element.
Another problem with prior piezoelectric transformers is that the voltage output of the device is limited by the ability of the ceramic element to undergo deformation without cracking or structurally failing. It is therefore desirable to provide a piezoelectric transformer which is adapted to deform under high voltage conditions without damaging the ceramic element of the device.
Piezoelectric and electrostrictive devices (generally called “electroactive” devices herein) are also commonly used as drivers, or “actuators,” due to their propensity to deform under applied electric fields. When used as an actuator, it is frequently desirable that the electroactive device be constructed so as to generate relatively large deformations and/or forces from the electrical input. Prior electroactive devices include flextensional transducers which are composite structures composed of a piezoelectric ceramic element and a metallic shell, stressed plastic, fiberglass, or similar structures. The actuator movement of conventional flextensional devices commonly occurs as a result of expansion in the piezoelectric material which mechanically couples to an amplified contraction of the device in the transverse direction. By coupling two or more electroactive devices, the deformation of one “actuator” can cause the deformation of the adjacent coupled actuator.
Another type of transformer, which is disclosed in co-pending patent application Ser. No. 08/864,029 takes advantage of both the electrical properties of electroactive devices as well as the mechanical “actuator” properties. As disclosed in my copending patent application, a piezoelectric transformer can be made by mechanically bonding electroactive devices to each other such that an input voltage is transformed into mechanical movement, which is translated through the mechanical bond to the adjacent electroactive device, which generates an output voltage.
One embodiment of the type of piezoelectric transformer disclosed in my co-pending patent application is illustrated in FIG.
2
. This transformer
1
is manufactured by stacking two ceramic wafers
30
and
48
between three preferably metallic layers
36
,
42
and
54
, bonding them together with four adhesive layers
34
,
40
,
44
and
52
, and simultaneously heating the stack to a temperature above the melting point of the adhesive materials, such as LaRC-SI™ developed by NASA Langley Research Center. The adhesive used is a very strong adhesive and has a coefficient of thermal contraction which is greater than that of most ceramics (and, in particular, is greater than that of the materials of the two ceramic wafers
30
and
48
). The adhesive is used to apply a bond between the respective metallic layers
36
,
42
and
54
and the ceramic wafers
30
and
46
and the bond is sufficient to transfer longitudinal stresses between adjacent layers of the transformer
1
.
After the entire stack of laminate layers have been heated to a temperature above the melting point
Bolduc David J.
Clark Stephen E.
Face International Corp.
Hall Carl E.
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