Title: Piezoelectric element and an oscillation transducer with a piezoelectric element
Abstract: There is put forward a piezolectric element for converting pressure signals into electrical signals and vice versa, with a porous homogeneous ceramic body and with at least two electrodes attached to the ceramic body. The porous ceramic body comprises open pores and is preferably hermetically sealed on the whole surface with an elastic coating. Furthermore there is suggested an oscillation transducer with the piezolement which is accommodated in a housing. The piezolectric element is with one end face rigidly connected to the base of the housing. The other end face represents a surface which is sensitive to oscillations, which preferably is not covered by the housing. The volume of the housing is filled with a casting compound, wherein the piezolectric element is mechanically decoupled from the casting compound.
Patent Number: 6,897,601 Issued on 05/24/2005 to Birth, et al.
| Inventors:
|
Birth; Michael (Berlin, DE);
Badaljan; Petros (Dresden, DE);
Lupeiko; Timofej (Rostov am Don, RU);
Poljakova; Svetlana (Rostov am Don, RU);
Brajceva; Elena (Rostov am Don, RU)
|
| Assignee:
|
Holmberg GmbH & Co. Kg (Berlin, DE)
|
| Appl. No.:
|
206600 |
| Filed:
|
July 24, 2002 |
Foreign Application Priority Data
| Jul 27, 2001[DE] | 101 37 425 |
| Jul 27, 2001[DE] | 101 37 424 |
| Current U.S. Class: |
310/334; 310/326; 310/340; 310/358 |
| Intern'l Class: |
H01L 041/08 |
| Field of Search: |
310/328,330-332,340,358,334,338
|
References Cited [Referenced By]
U.S. Patent Documents
| 2814575 | Nov., 1957 | Lange, Jr.
| |
| 3230402 | Jan., 1966 | Nightingale et al.
| |
| 3360665 | Dec., 1967 | Boswell.
| |
| 4707311 | Nov., 1987 | Okazaki.
| |
| 4928264 | May., 1990 | Kahn.
| |
| 4939405 | Jul., 1990 | Okuyama et al.
| |
| 4952836 | Aug., 1990 | Robertson.
| |
| 5148077 | Sep., 1992 | Grawey et al.
| |
| 5305507 | Apr., 1994 | Dvorsky et al.
| |
| 5550790 | Aug., 1996 | Velamoor et al.
| |
| 5894651 | Apr., 1999 | Dvorsky et al.
| |
| 6111339 | Aug., 2000 | Ohya et al.
| |
| Foreign Patent Documents |
| 30 23 155 | Jun., 1980 | DE.
| |
| 40 29 972 | Mar., 1992 | DE.
| |
| 198 14 697 | Oct., 1999 | DE.
| |
| 0 515 521 | Dec., 1992 | EP.
| |
| 060780700 | Jan., 1988 | JP.
| |
| 06269094 | Sep., 1994 | JP.
| |
Other References
IEEE Ultrasonics Symposium 1994 S. 561-564.
IEEE Ultrasonic Symposium an application on Ferroelectric, 1998, ISAF 98, S. 373-376.
|
Primary Examiner: Budd; Mark
Attorney, Agent or Firm: Marshall & Melhorn, LLC
Claims
1. An oscillation transducer comprising a piezoelectric element for converting
pressure signals into electrical signals and vice versa, with a porous ceramic
body and with at least two electrodes attached onto the ceramic body, wherein the
porous ceramic body comprises open pores and at least on the surface which is not
taken up by the electrodes is provided with an elastic coating for increasing the
pressure gradient within the ceramic body;
said piezoelectric element being accommodated in a housing, wherein one end face
of the piezoelectric element is rigidly connected to the housing base and the opposite
end face represents a surface which is sensitive to the oscillations from the outer
space, and the piezoelectric element in the housing is surrounded by a casting
compound and is mechanically decoupled from this.
2. An oscillation transducer according to claim 1, wherein the lateral circumferential
surface of the piezoelectric element comprises a cover which is loosely connected
to this, as a mechanic decoupling, which separates the casting compound from the
piezoelectric element.
3. An oscillation transducer according to claim 1, wherein the electrodes of
the piezoelectric element are connected to an impedance converter circuit, which
is cast into the casting compound.
4. An oscillation transducer according to claim 1, wherein the sensitive end
surface of the piezoelectric element projects beyond the housing.
5. An oscillation transducer according to claim 1, wherein the sensitive end
face is covered over by a metal foil or a wire fabric or a metallized plastic foil
or an electrically conducting elastomer.
6. An oscillation transducer according to claim 1, wherein the housing is placed
onto a frame of oscillation-absorbing material and is connected to this.
7. An oscillation transducer according to claim 6, wherein the frame comprises
a sleeve at least partly surrounding the housing, and a base plate, wherein between
the sleeve and the housing there is provided an intermediate space which is filled
with an oscillation-damping medium.
8. An oscillation transducer according to claim 7, wherein the sleeve is fastened
on a flange of the housing and a base plate is a constituent part of a cover cap
surrounding the housing.
9. An oscillation transducer according to claim 1, wherein the elastic coating
is formed hermetically sealing and covers the whole surface of the ceramic body
with electrodes.
10. An oscillation transducer according to claim 1, wherein the porous ceramic
body is manufactured of a lead titanate zirconate mixture.
11. An oscillation transducer according to claim 1, wherein the porosity is at
least 10%.
12. An oscillation transducer according to claim 11, wherein the porosity lies
between 50 and 70%.
13. An oscillation transducer according to claim 1, wherein the coating consists
essentially of silicone rubber, isoprene rubber or polyurethane or likewise.
14. An oscillation transducer according to claim 1, wherein the thickness of
the coating lies in the region of 0.1 to 1.5 mm.
15. An oscillation transducer according to claim 1, wherein the elasticity of
the coating lies in the range of 10 to 50 Shore A.
16. An oscillation transducer according to claim 1, wherein the porous ceramic
body with open pores is essentially homogeneous.
17. An oscillation transducer according to claim 11, wherein the porosity is
more than 30%.
18. An oscillation transducer according to claim 14, wherein the thickness of
the coating lies in the region of 0.1 to 1.0 mm.
19. An oscillation transducer according to claim 18, wherein the thickness of
the coating lies in the region of 0.1 to 0.5 mm.
20. An oscillation transducer according to claim 15, wherein the elasticity of
the coating lies in the range of 10 to 30 Shore A.
Description
BACKGROUND OF THE INVENTION
The invention relates to a piezoelectric element for converting pressure signals
into electrical signals and vice versa, according to the preamble of the independent
claim as well as to an oscillation transducer with a piezolectric element.
From DE 40 29 972 there is known a piezoelement which is designed as an ultrasound
tranducer and which consists of several layers of a porous piezoceramic arranged
over one another, with electrodes connected therebetween. Each layer of the porous
piezoceramic is manufactured by pulling a plastic foil through a supply vessel
which contains a ceramic slip mixed with a pearl polymeride. The foil stack is
subsequently pressed and fired. Within each layer the porous piezoceramic with
respect to the porosity has a gradient, wherein on the border surfaces there is
present a minimal porosity for improving the contact to the electrodes.
Oscillation transducers are used for the most different of applications,
they may be used in microphones, in particular contact microphones for the purpose
of news transmission, in acceleration recorders, apparatus for auscultative diagnosis,
seismic ground examination or likewise, in safety signal systems and other installations.
From EP 0 515 521 B1 there is known a piezoceramic acceleration recorder which
comprises a box-like two-part housing of glass or ceramic in which there is formed
a hollow space, wherein between the two parts of the housing there is fastened
a piezoceramic plate which engages into the hollow space. The piezoceramic plate
is provided with electrodes in the edge corner region which serves for fastening
between the housing halves, wherein the housing halves have metallisations for
connecting the electrodes with contact connection locations to an electrical circuit.
OBJECT OF THE INVENTION
It is the object of the invention to provide a piezolectric element for converting
pressure signals into electrical signals and vice versa with which the sensitivity
is increased and simultaneously a sufficient mechanical stability is made available,
wherein with regard to the oscillation transducer the increased sensitivity is
designed in particular in one direction.
According to the invention this object is achieved by the feature of the
independent claim and the dependent claims.
By way of the measures specified in the dependent claims advantageous further
developments and improvements are possible.
A SUMMARY OF THE INVENTION
Porous piezoceramic is to be understood as piezoceramic material, e.g. based
on a soldified lead titanium zirconate mixture, having piezoelectric properties
and having pores. Depending on whether the ceramic body encloses closed, i.e. isolated,
or open, i.e. communicating pores, the porous piezoceramic belongs to the piezocomposites
which correspond to a bond 3-0 or 3-3.
It has been shown that with the transition from highly rigid, low-pore ceramics
to highly porous piezoceramics the piezoelectric voltage constant g
H
(air sound sensitivity/thickness piezoceramic) which is a measure of the sensitivity,
is significantly increased, wherein in particular with a porosity of more than
30% by volume there occurs a non-linear increase of the elastic deformability of
the ceramic and a reduction in the Poisson number, i.e. the transversal contraction
coefficient, corresponding to the increase in the ceramic porosity. The first property
ensures a high sensitivity and the second has the effect that with a mechanical
oscillation having a spacial effect on all sides, the size of the piezoelectric
constants g
H, i.e. the sensitivity remains in its full extent in the
polarisation axis, whilst with solid ceramics it reduces on account of the superposition
of the signal components of the main axis, i.e. the polarisation axis with counter-phased
signal components of the secondary axes, thus becomes less sensitive.
This effect is shown most clearly with the porous ceramic with open pores, wherein
however with the increase of the porosity of the ceramic body, its mechanical strength
is reduced, so that highly porous ceramic is not usually used. It is known for
improving the mechanical stability of the highly porous piezoceramic with open
pores to use various polymer composites for filling the pores, although at the
same time the volumetric piezosensitivity is considerably reduced. Typical fillers
are epoxy resin and silicone rubber.
By way of the fact that according to the invention the porous ceramic body comprises
open pores and is provided with an elastic coating at least on the surface which
is not taken up by electrodes, the piezoelectric voltage constant of the piezoelectric
element is considerably increased, wherein the elastic coating improves the mechanical
strength of the ceramic body. At the same time it is particularly advantageous
when the ceramic body on its whole surface is hermetically covered with the coating.
The role of the elastic coating, apart from increasing the mechanical strength
of the ceramic body with a higher porosity by which means its use in a piezoelectric
element for converting pressure signals into electrical signals and vice versa
is possible, lies in the fact that thanks to this coating with the an effect for
example of an acoustic wave there arises a pressure drop between the inner volume
of the piezoelement and the surrounding medium, and a correspondingly increased
deformability of the piezoelement is effected. On account of the low Poisson number
of the highly porous piezoceramic, the increased volumetric deformation is transformed
into a single-axis, mainly longitudinal deformation which as a result of a direct
piezoeffect activates an electrical charging at the electrodes of the piezoelement.
In the end result the volumetric exit signal is transformed by the piezoelement
of porous ceramic into an adequately high electrical signal. With the abscence
of the elastic coating the deformation of the piezoelement is considerably smaller
since there arises no pressure gradient and thus its piezoelectric voltage constant
is likewise lower.
In an advantageous manner the porous ceramic body over the whole sectional area
is essentially homogeneous as to the distribution of the open pores communicating
with each other. In order not to destroy the "breathing" by way of which the pressure
gradient within the ceramic body is increased, due extraneous material, the pores
are filled with air or gas only and not with solid filling material and a sandwich
structure with different material is prevented.
Also the thickness of the elastic coating which has an elasticity in the region
between 10 and 50 Shore A, preferably may lie between 10 and 30 Shore A, is to
be selected depending on the material such that the deformation is not inhibited.
This thickness lies in the range of 0.1 to 1.5 mm, preferably it is about 0.1 to
0.5 mm.
The porosity is advantageously to be selected as high as possible, the upper
limit is limited by the required strength, the desired sensitivity and the method
of manufacture.
By way of the fact that the oscillation transducer according to the invention
comprises a piezoelement with the porous ceramic body with open pores, which with
one end face is essentially rigidly fastened to the base of a housing and the opposite
end face of the piezoelement represents the sensitive surface and the volume of
the housing at least is partly filled with a casting compound, wherein the piezoelement
is mechanically decoupled fom the casting compound, one achieves a high sensitivity
of the oscillation transducer to oscillation signals with a good signal-to-noise
ratio and by way of the mechanical decoupling its amplitude-frequency response
is improved, wherein on account of the rigid connection of the porous ceramic body
to the base of the housing and on account of the surrounding casting compound the
sensitivity is directed.
It is further advantageous that the housing is set on a frame of oscillation-absorbing
material and is connected to this, i.e. is elastically and oscillation-dampingly
mounted, since with this the sensitivity of the transducer to extraneous oscillations
is greatly decreased whilst maintaining the high sensitivity with respect to the
oscillations which impinge on the front side, wherein this effect is improved even
more if the frame consists of a sleeve surrounding the housing and a base plate,
wherein between the sleeve and the housing, and the base plate and the housing
there is provided an intermediate space which is filled with an oscillation-damping
or oscillation-absorbing medium.
Embodiment examples of the invention are shown in the drawings and are
explained in more detail in the subsequent description. There are shown in:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 a section through the piezoelectric element according to one embodiment
example of the present invention;
FIG. 2 a section through an oscillation transducer according to a first embodiment
example, and
FIG. 3 a section through a further embodiment example of the oscillation transducer
according to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
In FIG. 1 a porous, essentially homogeneous ceramic body is indicated at
1,
which comprises open pores, wherein the volume taken up by the pores is not smaller
than 10%, preferably larger than 30%, for example the porosity lies between 50
and 70%. The pores are usually filled with air, but may also contain another gases.
On two opposite surfaces of the ceramic body
1 there are attached two electrodes
2 which are connected to connection wires
3. The ceramic body
1
is provided over its whole periphery, where appropriate disregarding the surface
taken up by the electrodes
2, with a hermetically tight elastic coating
4, for example a polymer such as polyurethane, silicone rubber, isoprene
rubber or likewise. The elastic coating may only be so thick that the deformability
of the ceramic body remains on account of the oscillations, i.e. the deformation
is not prevented, wherein this to the same extent applies to the elasticity of
the coating. The elasticity should for example lie in a region of 10 to 50 Shore
A, preferably a range of 10 to 30 Shore A. The layer thickness of the elastic coating
may lie in a range between 0.1 to 1.5 mm, preferably between 0.1 to 1.0 mm depending
on the used material. Particularly good results are achieved between 0.1 and 0.5 mm.
EXAMPLES
As concrete exemplary embodiments piezoelements were manufactured of piezoceramic
material PTZ-36 with a porosity of 62-63% by volume in the form of disks with a
diameter of 12 mm and a height of 5 mm. After depositing a metal layer onto the
end faces of the disks, soldering-on wire for taking the signals at these layers
and polarisation of the ceramic body for the purpose of comparison, these surfaces
of the elements were sealed with a layer of different elastic polymers such as
polyurethane, silicone rubber and synthetic rubber.
The piezolectric voltage constant g
H were determined according to
the formula
wherein γ is the air sound sensitivity of the ceramic body in mV/Pa
and h the height of the ceramic body, measured in a reflection-poor space at a
frequency of 1000 Hz. The results achieved are specified in Table 1.
| TABLE 1 |
| |
coating material with |
|
| |
the parameter layer thickness |
| Pos. No. |
h = 0.5 mm |
gH [mV · m/N] |
| |
| 1 |
epoxy resin |
28 |
| 2 |
polymethyl methacrylate |
28 |
| 3 |
polyurethane |
252 |
| 4 |
silicone rubber |
280 |
| 5 |
synthetic isoprene rubber |
294 |
| 6 |
without coating |
14 |
From the specified data it results that by way of using elastic coatings one
maintains a high piezoelectric voltage constant of the elements of open porous
ceramic, wherein the sensitivity of the same ceramic body without coating is exceeded
by a factor of 20. It is likewise deduced from Table 1 that an inelastic coating
leads to an increase of the piezolelectric voltage constants only by a factor of 2.
The positive influence of an elastic coating on the piezosensitivity occurs in
a wide frequency bandwidth, wherein in Table 2 there are specified the piezolectric
voltage constants g
H(mV·m/N) in dependence on the frequency, for
an elastic and an inelastic coating.
| TABLE 2 |
| frequency (in Hz) |
10 |
100 |
1000 |
5000 |
10000 |
15000 |
| |
| gH with elastic |
230 |
250 |
280 |
230 |
250 |
265 |
| coating |
| gH with inelastic |
10 |
14 |
28 |
28 |
28 |
28 |
| coating |
In contrast to the nature of the porosity (open or closed pores) the size of
the
pores only slightly influences the piezoesensitivity. Table 3 shows the sensitivity
values of piezoelements with the same open porosity in dependence on the pore size,
with a coating with silicone rubber.
| |
TABLE 3 |
| |
| |
pore size (in μm) |
gH [mV · m/N] |
| |
| |
20-80 |
280 |
| |
50-200 |
308 |
| |
100-500 |
300 |
| |
500-1000 |
280 |
| |
In a further example according to Table 4 the piezosensitivity is represented
in dependence on the coating thickness of the coating material without additional
mass impingement, with a porosity of 56%.
| TABLE 4 |
| layer thickness [mm] |
0.0 |
0.1 |
0.5 |
1.0 |
1.5 |
2.0 |
2.5 |
| airborne sound |
0.02 |
0.7 |
0.7 |
0.7 |
0.45 |
0.33 |
0.21 |
| sensitivity [mV/Pa] |
| solid-borne sound |
1.2 |
25 |
27 |
20 |
15 |
11 |
7 |
| sensitivity [mV/g] |
It is to be recognised that with a layer thickness of 0.1 and 0.5 mm one may
achieve
the largest sensitivities.
With the piezoelectric element one achieves a very much higher piezo-sensitivity
than is achieved in the state of the art, and it has a more simple design. In comparison
to piezolements of solid piezoceramic the piezolement according to the invention
has a piezoelectric voltage constant which is almost two size orders higher and
is characterised by a broad frequency spectrum (10 Hz to 200 Hz) and a uniform
amplitude frequency response with which the fluctuations in the audio frequency
bandwidth do not exceed 6 dB.
In FIG. 2 there is shown an oscillation transducer which uses the above described
piezoelement. The oscillation transducer comprises a piezoelectric element 22
accommodated in a housing 22, wherein the piezolectric element 22
is surrounded by a casting compound 5 filling up the housing 8. The
housing 8 is formed e.g. pot-shaped and is open at one side, in FIG. 2 open
at the top, wherein the piezoelectric element 22 projects somewhat beyond
the level of the casting compound 5 and represents the sensitive sensor
surface of the piezoelement 22. The housing 8, i.e. the piezoelement
22 and the casting compound 5 are covered with a shielding foil 6,
consisting of a metal foil or a fine wire fabric or metallised plastic foil or
electrically conducting polymer. The housing comprises a flange-like edge which
is surrounded by a flanged ring 7, wherein the ring 7 engages over
the shielding foil 6 and the edge of the housing 8.
The piezoelement 22 is provided with the electrodes for transmitting further
the electrical signals. The electrodes 2 are connected to an impedance converter
circuit 20 by way of which the corresponding electrical adaptation is realised.
The upper electrode 2 is connected to the shielding foil 6 via a
connection 16, whilst the lower electrode 2 is connected to the impedance
converter circuit 20 via the lead 10. A connection cable 18
leads to the outside.
The porous ceramic body 1 for a mechanical stabilisation and for increasing
the pressure gradient is provided with an elastic coating 4 which hermetically
surrounds the ceramic body 1 on its whole surface.
On manufacture of the oscillation transducer according to FIG. 2, the porous
ceramic
body 1 provided with an elastic polymer coating 4, with one end face
via a rigid connection 19 is connected e.g. to an adhesive layer on the
base of the housing. The ceramic body 1 provided with the elastic coating
4 is connected to a loose cover 9, i.e. not rigidly connected to
it, which for example is formed as a silicone tubing which is pushed over the ceramic
body 1, and subsequently the housing is completely or partly filled with
a casting compound, with an epoxy resin or with another weakly elastic compound.
With this the cover 9 separates the piezoelement 22 from the casting
compound 5 and forms the mechanical decoupling. The impedance converter
circuit connected to the electrodes is co-cast and is surrounded by the casting
compound 5.
As a result of such an arrangement according to FIG. 2 it is achieved that the
rear side of the ceramic body is maximally "loaded" with the inert mass of the
housing 8 and its front side is maximally "relieved", which increases the
sensitivity of the oscillation transducer on it front side and simultaneously reduces
its sensitivity on the rear side. The use of the protective cover 9 for
the cover surface or the side surface of the piezoelement furthermore permits the
use of a weakly elastic element for casting the housing without compromising the
transducer sensitivity, which makes it possible to shift the self-resonance of
the transducer into the high frequency region and thus to improve its amplitude
frequency response in the low frequency region.
Such a transducer may, as already mentioned, be used as a contact or impact
noise microphone, wherein then only the end face of the piezoelement which projects
beyond the front side of the housing 8 has contact with the surface of a
signal source.
One example of a transducer shown in FIG. 2 has a height of 5.8 mm and a diameter
of 21.5/18.0 mm and a weight of about 7 g, wherein the impact noise transmission
coefficient is 1500 mV/g and the amplitude frequency response in the frequency
range of 50-5000 Hz runs within a 6 dB tolerance and the ratio of the sensitivity
of its front and rear side has a value of around 20 dB.
A further embodiment example is shown in FIG. 3, wherein the transducer is indicated
at 10 and may have the construction according to FIG. 2.
As is to be recognised, the housing 9 over its edge 13 is supported
by a cylindrical sleeve 11 of elastic sound-absorbing material which surrounds
the housing 8 on its circumference. Furthermore there is provided an again
pot-shaped cover cap 12 into which the housing 8 with the cylinder
11 is placed, wherein the cylinder 11 of sound-absorbing material
via connection locations 14 on the one hand is connected to the flange-like
edge 13 and on the other hand to the base of the cover cap 12. At
the same time between the base of the housing 1 and the base of the cartridge
12 there is provided an intermediate space 15 which is filled with
an oscillation absorbing medium. This may be air, a vacuum, fluid or other.
In the present embodiment example a cover cap is used, other designs may also
be provided in order to render the oscillation transducer 10 insensitive
to extraneous oscillations, wherein however the basic concept of the elastic sound-absorbing
suspension of the transducer and of the intermediate space remains in the manner
of a double base.
On account of the arrangement one achieves a great reduction or for certain frequencies
a practically complete suppression of the sensitivity of the transducer to extraneous
oscillation whilst maintaining the high sensitivity to oscillation signals which
impinge it from the front side, with a minimal increase in the transducer thickness.
The sleeve of elastic sound-absorbing material surrounding the housing of the transducer,
which may be formed as a cylinder, is fastened with one side to the front side
of the housing and with the other side to the base of the cover cap pushed onto
the transducer, or also only to one outer disk. At the same time between the housing
of the transducer and the disk or between the housing and the base of the cover
cap there is formed an intermediate space in the manner of a double base which
may either be filled with a gas, a vacuum, a fluid or another oscillation-absorbing medium.
*
