U.S. patent application number 15/777625 was filed with the patent office on 2018-12-27 for piezoelectric transformer.
The applicant listed for this patent is EPCOS AG. Invention is credited to Bernhard Dollgast, Pavol Kudela, Markus Puff, Michael Weilguni.
Application Number | 20180375013 15/777625 |
Document ID | / |
Family ID | 57288458 |
Filed Date | 2018-12-27 |
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United States Patent
Application |
20180375013 |
Kind Code |
A1 |
Weilguni; Michael ; et
al. |
December 27, 2018 |
Piezoelectric Transformer
Abstract
A piezoelectric transformer is disclosed. In an embodiment a
piezoelectric transformer includes an input region and an output
region, wherein the input region is configured to convert an AC
voltage into a mechanical oscillation, wherein the output region is
configured to convert a mechanical oscillation into an electrical
voltage, wherein the piezoelectric transformer includes a longest
edge and a shortest edge, and wherein the longest edge includes a
length that is twenty times a length of the shortest edge or
less.
Inventors: |
Weilguni; Michael;
(Hagenberg, AT) ; Puff; Markus; (Graz, AT)
; Kudela; Pavol; (Deutschlandsberg, AT) ;
Dollgast; Bernhard; (Deutschlandsberg, AT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
EPCOS AG |
Munchen |
|
DE |
|
|
Family ID: |
57288458 |
Appl. No.: |
15/777625 |
Filed: |
November 15, 2016 |
PCT Filed: |
November 15, 2016 |
PCT NO: |
PCT/EP2016/077719 |
371 Date: |
May 18, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 41/107 20130101;
H05H 1/24 20130101; C01B 13/11 20130101; C01B 13/115 20130101; C01B
2201/10 20130101 |
International
Class: |
H01L 41/107 20060101
H01L041/107; H05H 1/24 20060101 H05H001/24; C01B 13/11 20060101
C01B013/11 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 20, 2015 |
DE |
10 2015 120 160.7 |
Claims
1-13. (canceled)
14. A piezoelectric transformer for generating plasma comprising:
an input region and an output region, wherein the input region is
configured to convert an AC voltage into a mechanical oscillation,
wherein the output region is configured to convert a mechanical
oscillation into an electrical voltage, wherein the piezoelectric
transformer comprises a longest edge and a shortest edge, and
wherein the longest edge comprises a length that is twenty times a
length of the shortest edge or less.
15. The piezoelectric transformer according to claim 14, wherein
the longest edge comprises a length of less than 45 mm.
16. The piezoelectric transformer according to claim 14, wherein
the longest edge comprises a length of less than 35 mm.
17. The piezoelectric transformer according to claim 14, wherein a
first external electrode is arranged on a first side surface of the
input region, wherein a second external electrode is arranged on a
second side surface of the input region, the second side surface
arranged opposite the first side surface, wherein the input region
comprises piezoelectric layers and electrodes stacked one above
another in a stacking direction, wherein the electrodes are
contacted either with the first external electrode or with the
second external electrode alternately in the stacking direction,
and wherein the output region comprises a monolithic piezoelectric
layer.
18. The piezoelectric transformer according to claim 14, wherein
the longest edge extends from the input region to the output
region.
19. The piezoelectric transformer according to claim 14, wherein
the shortest edge is perpendicular to the longest edge.
20. The piezoelectric transformer according to claim 14, wherein
the piezoelectric transformer has a resonant frequency of more than
100 kHz.
21. The piezoelectric transformer according to claim 14, wherein
the longest edge comprises a length that is fifteen times a length
of the shortest edge of the piezoelectric transformer or less.
22. The piezoelectric transformer according to claim 14, wherein
the shortest edge comprises a length of between 10 mm and 1 mm.
23. The piezoelectric transformer according to claim 14, wherein
the piezoelectric transformer is configured to avoid plasma
ignitions along the longest edge in the output region based on a
ratio of a length of the longest edge to a length of the shortest
edge.
24. The piezoelectric transformer according to claim 14, wherein
mechanical loadings of a piezoelectric material in the output
region are reduced by a ratio of a length of the longest edge to a
length of the shortest edge.
25. A plasma generator comprising: the piezoelectric transformer
according to claim 14.
26. The plasma generator according to claim 25, wherein the plasma
generator is configured to provide a plasma ignition primarily at
an output-side end side of the piezoelectric transformer.
Description
[0001] This patent application is a national phase filing under
section 371 of PCT/EP2016/077719, filed Nov. 15, 2016, which claims
the priority of German patent application 10 2015 120 160.7, filed
Nov. 20, 2015, each of which is incorporated herein by reference in
its entirety.
TECHNICAL FIELD
[0002] The present invention relates to a piezoelectric
transformer. The piezoelectric transformer can be used for
generating plasma, in particular a non-thermal atmospheric pressure
plasma.
BACKGROUND
[0003] In the case of known parallelepipedal piezoelectric
transformers it has been observed that plasma ignitions occur along
the longitudinal-side edges of the output region. The plasma
ignitions lead to undesired back coupling involving the generation
of mechanical stresses in a piezoelectric material of the output
region, which can lead to cracks in the material. The lifetime of
the piezoelectric transformer can be shortened as a result.
SUMMARY OF THE INVENTION
[0004] Embodiments provide an improved piezoelectric transformer
which can have a longer lifetime, for example.
[0005] Embodiments provide a piezoelectric transformer comprising
an input region and an output region, wherein the input region is
configured to convert an applied AC voltage into a mechanical
oscillation, and wherein the output region is configured to convert
a mechanical oscillation into an electrical voltage. Furthermore,
the piezoelectric transformer comprises a longest edge and a
shortest edge, wherein the longest edge comprises a length that is
a twenty times the length of the shortest edge or less.
[0006] In various embodiments the ratio of the length of the
longest edge to the length of the shortest edge is also referred to
as aspect ratio. It was possible to show that undesired plasma
ignitions along the longitudinal-side edge in the output region can
be avoided in the case of piezoelectric transformers having an
aspect ratio of .ltoreq.20:1. Accordingly, considerable mechanical
loadings of the piezoelectric material in the output region do not
occur in the case of these transformers, with the result that no
cracks arise here and the lifetime is not shortened.
[0007] The piezoelectric transformer can be, for example, a
Rosen-type transformer. The piezoelectric transformer can
furthermore be designed in such a way that the aspect ratio is
.gtoreq.2:1, preferably .gtoreq.5:1.
[0008] In this case, the term edges can denote the lines at which
two side surfaces of the piezoelectric transformer abut one
another.
[0009] The piezoelectric transformer can be substantially
parallelepipedal. In this case, the edges of the transformer can be
rounded. In particular, the edges can be rounded with a very small
radius, which is, for example, .ltoreq.0.5 mm.
[0010] The longest edge can extend in a longitudinal direction from
an input region toward an output region. The shortest edge can be
an edge of an output-side end side. The output-side end side is an
end surface facing away from the input region.
[0011] In various embodiments, the piezoelectric transformer having
an aspect ratio of .ltoreq.20:1 has a longer lifetime since
considerable mechanical loadings of the output region do not occur.
Furthermore, the transformer has further advantages. The preferred
aspect ratio of .ltoreq.20:1 leads to an improved ozone generation
efficiency. In particular, more ozone can be produced with the
input power remaining the same per unit volume of the piezoelectric
transformer given the aspect ratio of .ltoreq.20:1. The improvement
in the ozone generation efficiency is based on an improvement in
the matching of the output impedance of the piezoelectric
transformer to the impedance generated by the plasma, which is
established in the case of the aspect ratio of .ltoreq.20:1.
[0012] The piezoelectric transformer can be a piezoelectric
transformer for generating plasma. In this case, the plasma is
preferably non-thermal atmospheric pressure plasma.
[0013] In various other embodiments, the longest edge comprises a
length of less than 45 mm. Preferably, the longest edge comprises a
length of less than 35 mm. The longest edge can comprise a minimum
length of 10 mm.
[0014] Such a shortened design of the piezoelectric transformer may
have numerous advantages. The polarization voltage required for the
polarization of such a piezoelectric transformer is substantially
dependent on the length of the piezoelectric transformer.
Accordingly, only a low voltage is required for the polarization of
a transformer whose longest edge comprises a length of less than 45
mm, or of less than 35 mm. The method for producing the respective
piezoelectric transformer is simplified as a result.
[0015] In various embodiments, the resonant frequency of the
respective piezoelectric transformer is dependent on the length of
the longest edge. A short length of the longest edge makes it
possible to manufacture piezoelectric transformers having a high
resonant frequency. By way of example, a resonant frequency of more
than 100 kHz can be achieved by means of a longest edge that is
shorter than 35 mm. Such resonant frequencies are advantageous
particularly in the case of plasma generators used in medical
applications.
[0016] In other embodiments, a higher resonant frequency leads to
steeper edges of the output voltage, which in turn have a positive
effect on the ozone generation efficiency.
[0017] A first external electrode can be arranged on a first side
surface of the input region. A second external electrode can be
arranged on a second side surface of the input region, the second
side surface being situated opposite the first side surface. The
input region can comprise piezoelectric layers and electrodes
stacked one above another in a stacking direction. The electrodes
can be contacted either with the first external electrode or with
the second external electrode alternately in the stacking
direction. The output region can comprise a monolithic
piezoelectric layer. The piezoelectric material of the
piezoelectric layer in the output region can be identical to the
piezoelectric material of the piezoelectric layers in the input
region.
[0018] The longest edge can extend from the input region to the
output region. The shortest edge can be perpendicular to the
longest edge.
[0019] The piezoelectric transformer can have a resonant frequency
of more than 100 kHz. These frequencies are advantageous
particularly for applications in the medical sector.
[0020] The longest edge can comprise a length that is a fifteen
times the length of the shortest edge of the piezoelectric
transformer or less. Accordingly, an aspect ratio of .ltoreq.15:1
can be established. Choosing the aspect ratio to be so small makes
it possible to ensure that plasma ignitions do not occur along the
longitudinal-side edges. The ozone generation efficiency can also
be increased further in this way.
[0021] The shortest edge can comprise a length of between 10 mm and
1 mm. Preferably, the shortest edge comprises a length of between 5
mm and 2 mm. The length of the shortest edge is chosen such that an
advantageous aspect ratio of .ltoreq.20:1 is established.
[0022] Plasma ignitions along the longest edge in the output region
can be avoided by means of the ratio of the length of the longest
edge to the length of the shortest edge. Accordingly, an
insulation, for example, a shrinkable sleeve that partly sheathes
the output region, can be dispensed with. The piezoelectric
transformer can thus be free of an insulation in the output region.
The piezoelectric insulator can in particular comprise no
shrinkable sleeve.
[0023] Mechanical loadings of the piezoelectric material in the
output region can be reduced by means of the ratio of the length of
the longest edge to the length of the shortest edge.
[0024] An improved ozone generation efficiency can be achieved by
means of the ratio of the length of the longest edge to the length
of the shortest edge. In particular, this choice of the aspect
ratio can make it possible to reduce the component size and at the
same time to maintain or even improve the ozone generation
efficiency.
[0025] A further aspect of the present invention relates to a
plasma generator comprising the piezoelectric transformer described
above. The plasma generator can be suitable in particular for
generating a non-thermal atmospheric pressure plasma.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] The present invention is described below with reference to
the figures.
[0027] FIG. 1 shows a piezoelectric transformer in perspective
view.
[0028] FIG. 2 shows the results of a measurement in which the ozone
production for the piezoelectric transformer shown in FIG. 1 is
compared with a comparative example.
[0029] FIG. 3 shows the measurement results of a measurement with
consideration of the ozone generation per unit volume.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0030] FIG. 1 shows a piezoelectric transformer 1 in a perspective
view. The piezoelectric transformer 1 can be used in particular in
a device for generating non-thermal atmospheric pressure
plasma.
[0031] A piezoelectric transformer 1 is a design of a resonance
transformer which is based on piezoelectricity and constitutes an
electromechanical system, in contrast to the conventional magnetic
transformers. The piezoelectric transformer 1 is a Rosen-type
transformer, for example.
[0032] The piezoelectric transformer 1 comprises an input region 2
and an output region 3. In the input region 2, the piezoelectric
transformer 1 comprises electrodes 4, to which an AC voltage can be
applied. The electrodes 4 extend in a longitudinal direction L of
the piezoelectric transformer 1. The electrodes 4 are stacked
alternately with a piezoelectric material 5 in a stacking direction
S, which is perpendicular to the longitudinal direction L. The
piezoelectric material 5 here is polarized in the stacking
direction S.
[0033] Electrodes 4 are arranged in the interior of the
piezoelectric transformer 1 and are also referred to as internal
electrodes. The piezoelectric transformer 1 comprises a first side
surface 6 and a second side surface 7 situated opposite the first
side surface 6. A first external electrode 8 is arranged on the
first longitudinal side 6. A second external electrode (not shown)
is arranged on the second side surface 7. The internal electrodes 4
are electrically contacted with either the first external electrode
8 or the second external electrode alternately in the stacking
direction S.
[0034] The input region 2 can be driven with a low AC voltage that
is applied between the electrodes 4. On account of the
piezoelectric effect, the AC voltage applied on the input side is
firstly converted into a mechanical oscillation. In this case, the
frequency of the mechanical oscillation is essentially dependent on
the geometry and the mechanical construction of the piezoelectric
transformer 1.
[0035] The output region 3 comprises piezoelectric material 9 and
is free of internal electrodes. The piezoelectric material 9 in the
output region is polarized in the longitudinal direction L. The
piezoelectric material 9 of the output region 3 can be the same
material as for the piezoelectric material 5 of the input region 2,
wherein the piezoelectric materials 5 and 9 can differ in their
direction of polarization. In the output region, the piezoelectric
material is shaped to form a single monolithic layer, which is
polarized completely in the longitudinal direction L. In this case,
the piezoelectric material in the output region has only a single
direction of polarization.
[0036] If an AC voltage is applied to the electrodes 4 in the input
region 2, then a mechanical wave forms within the piezoelectric
material 5, 9, the wave generating an output voltage as a result of
the piezoelectric effect in the output region 3. The output region
3 comprises an output-side end side 10. The electrical voltage
generated in the output region 3 is thus present between the end
side 10 and the end of the electrodes 4 of the input region 2. In
this case, a high voltage is present at the output-side end side
10. In this case, a high potential difference also arises between
the output-side end side and an environment of the piezoelectric
transformer, the potential difference being sufficient to generate
a strong electric field that ionizes a process gas.
[0037] In this way, the piezoelectric transformer 1 generates high
electric fields that are able to ionize gases or liquids by
electrical excitation. In this case, atoms or molecules of the
respective gas or of the respective liquid are ionized and form a
plasma. An ionization occurs whenever the electric field strength
at the surface of the piezoelectric transformer 1 exceeds the
ignition field strength of the plasma. In this case, the term
ignition field strength of a plasma denotes the field strength
required for ionizing the atoms or molecules.
[0038] Plasma ignitions occur both at the lateral longitudinal
edges 11 and at the edges 12 of the output-side end side 10. The
plasma ignitions along the lateral longitudinal edges 11 lead to
undesired back coupling involving the generation of high mechanical
stresses in the piezoelectric material 9 of the output region 3,
which can lead to cracks during continuous operation of the
piezoelectric transformer 1, as a result of which the lifetime of
the piezoelectric transformer 1 is reduced. In order to avoid such
damage to the piezoelectric transformer 1 on account of the plasma
ignition, the piezoelectric transformer 1 is configured in such a
way that the plasma ignition appears primarily at the output-side
end side 10 and the plasma ignition at the lateral longitudinal
edges 11 is avoided.
[0039] In the case of the piezoelectric transformer 1, a plasma
ignition occurs in particular at the locations at which a maximum
of the electrical potential occurs in the output region 3 of the
piezoelectric transformer 1. A more detailed explanation is given
below of how the piezoelectric transformer 1 is designed to avoid
the plasma ignition at the lateral longitudinal edges 11 if
possible.
[0040] The piezoelectric transformer 1 comprises a parallelepipedal
shape, wherein the piezoelectric transformer 1 comprises a longest
edge 13 and a shortest edge 14. The output-side end side 10
comprises a rectangular shape, wherein the shortest edge of the end
side 10 constitutes the shortest edge 14 of the piezoelectric
transformer 1. The longest edge 13 of the piezoelectric transformer
1 is perpendicular to the output-side end side 10 and extends from
the input region 2 to the output region 3. The longest edge 13 is a
lateral longitudinal edge 11.
[0041] The ratio of the length of the longest edge 13 to the length
of the shortest edge 14 is referred to as aspect ratio. It has been
found that the piezoelectric transformer 1 generates plasma in
particular to an intensified extent at the output-side end side 10
if the piezoelectric transformer 1 is designed in such a way as to
result in an aspect ratio of .ltoreq.20:1.
[0042] The aspect ratio affects the capacitance of the output
region 3 of the piezoelectric transformer 1. The location at which
a maximum of an electrical potential is attained in the output
region 3 shifts in turn depending on the capacitance of the output
region 3. The output capacitance increases as the aspect ratio
decreases. It was possible to show both experimentally and
theoretically that a sufficiently high output capacitance for which
the maximum of the generated output potential lies on the
output-side end side 10 is achieved in the case of an aspect ratio
of .ltoreq.20:1. If the aspect ratio is chosen to be greater than
20:1, the output capacitance decreases and the maximum of the
output potential shifts along the lateral longitudinal edges 11
away from the output-side end side 10 in a direction toward the
input region 2. In this case, undesired plasma ignitions occur
along the edges 11.
[0043] In a first exemplary embodiment, the longest edge 13
comprises a length of 41 mm. The edges 12 of the output-side end
side 10 comprise a length of 6 mm and 3 mm. Accordingly, the 3 mm
long edge of the end side 10 is the shortest edge 14 of the
piezoelectric transformer 1. This results in an aspect ratio of
13.667:1, i.e., an advantageous aspect ratio that is less than or
equal to 20:1.
[0044] In an experimental investigation, ten piezoelectric
transformers 1 in accordance with the first exemplary embodiment
were studied in the course of continuous operation of 1000 hours,
wherein a crack was produced in the output region 3 in none of the
transformers 1 and accordingly no failure of any of the
piezoelectric transformers 1 occurred.
[0045] In accordance with a second exemplary embodiment, the
longest edge 13 can be less than 35 mm. By way of example, the
piezoelectric transformer 1 can comprise a longest edge 13 having a
length of 30 mm. The end side 10 comprises edge lengths of 3 mm and
2.4 mm. The advantageous aspect ratio of .ltoreq.20:1 results in
this case, too. In particular, an aspect ratio of 12.5:1
results.
[0046] The resonant frequency of the piezoelectric transformer 1 is
dependent on the length of the longest edge 13. The second
exemplary embodiment leads to a resonant frequency of more than 100
kHz.
[0047] Piezoelectric transformers 1 having a resonant frequency of
more than 100 kHz are usable in particular for medical
applications. Cutting tissue by means of plasma cutting is an
example of such a medical application. The irritation of nerves
should be avoided in this case. The ionic conductivity of nerves
decreases significantly at frequencies starting from 100 kHz.
Accordingly, plasma generators which operate at frequencies of
above 100 kHz are particularly well suited to the plasma cutting of
tissue. The above-described piezoelectric transformer 1 in
accordance with the second exemplary embodiment can be used in such
a plasma generator.
[0048] FIG. 2 shows the measurement results of a comparative
measurement in which a piezoelectric transformer 1 in accordance
with the first exemplary embodiment was compared with a comparative
transformer comprising a length of 71 mm and an end side having the
cross section of 6 mm.times.3 mm. In both cases, air was used as
process gas and ionized by the respective piezoelectric transformer
1.
[0049] In FIG. 2, the power in W with which the respective
piezoelectric transformer 1 was operated is plotted on the abscissa
axis. The produced amount of ozone in mg/hour is plotted on the
ordinate axis. The measurement results shown in FIG. 2 show that no
significant difference in ozone production can be found. In
particular, with the piezoelectric transformer 1 having the
advantageous aspect ratio of .ltoreq.20:1, a substantially
identical amount of ozone can be produced for the same input power,
wherein the piezoelectric transformer 1 having the aspect ratio of
.ltoreq.20:1 comprises a significantly shorter length than the
comparative transformer.
[0050] FIG. 3 likewise illustrates a comparison between the
piezoelectric transformer in accordance with the first exemplary
embodiment and the comparative transformer, which does not have the
preferred aspect ratio of .ltoreq.20:1. A product of input power
and volume of the respective transformer is considered here. The
product of input power and volume in W.times.cm.sup.3 is plotted on
the abscissa axis. The amount of ozone in mg generated per hour is
once again plotted on the ordinate axis. Air was once again used as
process gas. It is clearly evident that the piezoelectric
transformer 1 having the preferred aspect ratio produces a larger
amount of ozone per unit volume. Accordingly, this piezoelectric
transformer 1 has a higher ozone generation efficiency.
[0051] This experimentally observed improvement in the ozone
generation efficiency in air on account of the advantageous aspect
ratio can also be explained theoretically. The ozone generation
efficiency is dependent on the matching of the output impedance of
the piezoelectric transformer 1 to the impedance generated by the
plasma, and also on the steepness of the edges of the output
voltage. In the case of a sinusoidal voltage, the edge steepness of
the output voltage is substantially dependent on the resonant
frequency. Accordingly, piezoelectric transformers 1 whose longest
side 13 comprises a short length and which accordingly have a high
resonant frequency have steep edges, which in turn have a positive
effect on the ozone generation efficiency.
[0052] It was furthermore possible to show that the advantageous
aspect ratio improves the matching of the impedance of the
piezoelectric transformer 1 to an impedance generated by the
plasma.
[0053] Both the longitudinal edges 11 and the edges 12 of the
output-side end side of the piezoelectric transformer can be
rounded. If the curve radius of the rounded edges is chosen to be
sufficiently small, for example, less than 0.5 mm, the
piezoelectric transformer 1 can be regarded approximately as
parallelepipedal.
[0054] In some exemplary embodiments of the piezoelectric
transformer 1, the latter comprises in the output region 3 an
insulating layer formed, for example, by a shrinkable sleeve. The
insulating layer can leave free the output-side end side 10 and at
least partly cover the remaining side surfaces of the output region
3. The insulating layer is intended to prevent undesired plasma
ignitions along the lateral longitudinal edges 11 and to cover the
latter for this purpose. Since plasma ignitions along the lateral
longitudinal edges 11 are not expected on account of the
advantageous aspect ratio, the piezoelectric transformer 1 can be
free of the insulating layer in the output region 3. In an
alternative exemplary embodiment, such an insulating layer can be
provided in order to further improve the insulation of the output
region 3.
* * * * *