Device for Producing a Non-Thermal Atmospheric Pressure Plasma and Method for Operating a Piezoelectric Transformer

Abstract

A device for producing a non-thermal atmospheric pressure plasma and a method for operating a piezoelectric transformer are disclosed. In an embodiment a device includes a piezoelectric transformer, a driver circuit configured to apply an input signal to the piezoelectric transformer and a field probe configured to measure a field strength of an electric field produced by the piezoelectric transformer at a measurement point, wherein the driver circuit is configured to adapt the input signal while taking into account measurement results of the field probe, and wherein the device is configured to produce a non-thermal atmospheric pressure plasma.

Claims

1-36. (canceled)

37. A device comprising: a piezoelectric transformer; a driver circuit configured to apply an input signal to the piezoelectric transformer; and a field probe configured to measure a field strength of an electric field produced by the piezoelectric transformer at a measurement point, wherein the driver circuit is configured to adapt the input signal while taking into account measurement results of the field probe, and wherein the device is configured to produce a non-thermal atmospheric pressure plasma.

38. The device according to claim 37, wherein the field probe is configured to measure a field strength of the electric field at least two measurement points.

39. The device according to claim 37, wherein the driver circuit is configured to adapt a frequency of the input signal while taking into account the measurement results of the field probe.

40. The device according to claim 37, wherein the driver circuit is configured to adapt a voltage of the input signal, a current of the input signal, a phase of the input signal and/or a signal waveform of the input signal while taking into account the measurement results of the field probe.

41. The device according to claim 37, wherein the driver circuit is configured to adapt the input signal while taking into account the measurement results of the field probe, in such a way that a frequency of the input signal corresponds to that of a series resonant frequency of the piezoelectric transformer or in such a way that the frequency of the input signal corresponds to that of a parallel resonant frequency of the piezoelectric transformer.

42. The device according to claim 37, wherein the driver circuit is configured to adapt the input signal while taking into account the measurement results of the field probe, in such a way that a maximum in the field strength of the electric field is formed at an output end side of the piezoelectric transformer or in a half-space, facing away from the piezoelectric transformer, in front of the output end side of the piezoelectric transformer.

43. The device according to claim 37, wherein the field probe comprises a substrate comprising an insulating material, and wherein the field probe comprises metallizations separated from one another, which form at least two measurement points.

44. The device according to claim 43, wherein the field probe comprises a metal face, and wherein the device is configured to determine the field strength at each measurement point by a measurement of a voltage between the metal face and the respective measurement point.

45. The device according to claim 43, wherein at least one passive component is integrated into the substrate, and/or wherein at least one passive component and/or one active component are arranged on a surface of the substrate.

46. The device according to claim 45, wherein the driver circuit is formed fully or partially by one or more passive component integrated into the substrate or arranged on the surface of the substrate, and/or by one or more active components arranged on the surface of the substrate.

47. The device according to claim 37, wherein the field probe comprises at least three metallizations separated from one another, each of which forms a measurement point, and wherein the at least three metallizations are arranged in a row.

48. The device according to claim 47, wherein the field probe is arranged in such a way that the row is arranged parallel to the piezoelectric transformer.

49. The device according to claim 37, furthermore comprising evaluation electronics configured to further process measurement values recorded by the field probe, wherein the evaluation electronics are connected to the driver circuit.

50. The device according to claim 49, wherein the driver circuit is configured to transmit the input signal applied to the piezoelectric transformer to the evaluation electronics, wherein the evaluation electronics are configured to determine an output signal determined by the piezoelectric transformer from the measurement values recorded by the field probe, and wherein the evaluation electronics are configured to compare the input signal with the output signal.

51. The device according to claim 50, wherein, during comparison of the input signal with the output signal, a voltage of the signals and/or a current of the signals and/or an impedance and/or a frequency of the signals and/or a phase of the signals is considered.

52. The device according to claim 50, wherein, during comparison of the input signal with the output signal, a parallel resonant frequency and/or a series resonant frequency of the piezoelectric transformer are determined.

53. The device according to claim 49, wherein the evaluation electronics are formed fully or partially by one or more passive components integrated into a substrate or arranged on the surface of the substrate, and/or by one or more active components arranged on the surface of the substrate.

54. The device according to claim 49, wherein the evaluation electronics are configured to detect a load arranged in front of a output end side of the piezoelectric transformer with aid of a change in the field strength, and wherein the driver circuit is configured that a power sufficient for an ignition of the non-thermal atmospheric pressure plasma is applied to an input region of the piezoelectric transformer only when a load is detected in front of the output end side.

55. The device according to claim 49, wherein the evaluation electronics are embodied fully or partially by software.

56. The device according to claim 37, wherein the driver circuit is embodied fully or partially by software.

57. The device according to claim 37, further comprising a second field probe configured to measure a field strength of an electric field at least two measurement points.

58. The device according to claim 37, wherein the piezoelectric transformer is configured to ignite a plasma at its output end side.

59. The device according to claim 37, wherein the field probe is configured to measure at least one of the following: a pressure, a temperature, a humidity, a conductivity, an ozone concentration, a gas composition, an ion concentration and/or a particle concentration.

60. The device according to claim 37, wherein the driver circuit is configured to take into account at least one of the following during adaptation of the input signal: a pressure, a temperature, a humidity, a conductivity, an ozone concentration, a gas composition, an ion concentration and/or a particle concentration.

61. The device according to claim 37, wherein the driver circuit is configured to carry out a correction of the input signal at established time intervals during ongoing operation of the device while taking into account the measurement results of the field probe.

62. The device according to claim 37, wherein the adaptation of the input signal is carried out continuously and/or in real time.

63. The device according to claim 37, wherein the device is configured to produce excited molecules, ions or radicals, in particular to produce OH radicals and/or nitrogen oxide and/or ozone.

64. A method for operating a piezoelectric transformer, the method comprising: applying an input signal to the piezoelectric transformer; measuring a field strength of an electric field produced by the piezoelectric transformer by a field probe at a measurement point; and adopting the input signal applied to the piezoelectric transformer while taking into account measurement values.

65. The method according to claim 64, wherein measuring the field strength of the electric field by the piezoelectric transformer by the field probe comprising measuring the field strength at least two measurement points.

66. The method according to claim 64, further comprising: stepwise varying a frequency of the input signal; determining a frequency at a maximum field strength at an output end side of the transformer or in a half-space, facing away from the piezoelectric transformer, in front of the output end side of the piezoelectric transformer; and subsequently adjusting the frequency of the input signal to the determined frequency.

67. The method according to claim 64, further comprising: transmitting the input signal to evaluation electronics; determining an output signal of the piezoelectric transformer by the evaluation electronics from measurement values recorded by the field probe; and comparing the input signal with the output signal by the evaluation electronics.

68. The method according to claim 67, wherein, while comparing the input signal with the output signal, determining a parallel resonant frequency of the piezoelectric transformer and adjusting the frequency of the input signal to the parallel resonant frequency.

69. The method according to claim 64, further comprising using the piezoelectric transformer to produce a non-thermal atmospheric pressure plasma.

70. The method according to claim 69, wherein the plasma is used to produce excited molecules, ions or radicals.

71. The method according to claim 64, further comprising performing a calibration of the piezoelectric transformer by comparing the measurement values determined by the field probe with a reference signal.

72. The method according to claim 64, further comprising determining ageing and/or a lifetime of the piezoelectric transformer by comparing the measurement values determined by the field probe with a reference signal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Preferred exemplary embodiments of the invention will be explained in more detail below with the aid of the figures.

[0045] FIG. 1 shows a piezoelectric transformer in a perspective view;

[0046] FIG. 2 shows a device for producing a non-thermal atmospheric pressure plasma;

[0047] FIGS. 3, 4 and 5 respectively show a field probe according to a first exemplary embodiment;

[0048] FIGS. 6, 7 and 8 show a field probe according to a second exemplary embodiment;

[0049] FIG. 9 shows a first signal, which may be applied as an input signal to the piezoelectric transformer;

[0050] FIG. 10 shows measurement values determined by the field probe;

[0051] FIG. 11 shows a second input signal which may be applied to the piezoelectric transformer; and

[0052] FIG. 12 respectively shows, for various signals applied to the piezoelectric transformer, the measurement values which are determined by the individual measurement points of the field probe for the field strength.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0053] FIG. 1 shows a piezoelectric transformer 1 in a perspective view. The piezoelectric transformer 1 may, in particular, be used in a plasma generator for producing non-thermal atmospheric pressure plasma.

[0054] A piezoelectric transformer 1 is a version of a resonant transformer which is based on piezoelectricity and, in contrast to conventional magnetic transformers, represents an electromagnetic system. The piezoelectric transformer 1 is, for example, a transformer of the Rosen type.

[0055] The piezoelectric transformer 1 comprises an input region 2 and an output region 3, wherein the output region 3 follows on from the input region 2 in a longitudinal direction z. In the input region 2, the piezoelectric transformer 1 comprises electrodes 4 to which an alternating voltage may be applied. The electrodes 4 extend in the longitudinal direction z of the piezoelectric transformer 1. The electrodes 4 are stacked alternately with a piezoelectric material 5 in a stack direction x, which is perpendicular to the longitudinal direction z. The piezoelectric material 5 is in this case polarized in the stack direction x.

[0056] The electrodes 4 are arranged inside the piezoelectric transformer 1, and are also referred to as internal electrodes. The piezoelectric transformer 1 comprises a first side face 6 and a second side face 7, which lies opposite the first side face 6. A first external electrode 8 is arranged on the first side face 6. A second external electrode (not shown) is arranged on the second side face 7. The internally lying electrodes 4 are electrically contacted alternately in the stack direction x with either the first external electrode 8 or the second external electrode.

[0057] The input region 2 may be driven with a low alternating voltage which is applied between the electrodes 4. Because of the piezoelectric effect, the alternating voltage applied on the input side is initially converted into a mechanical oscillation. The frequency of the mechanical oscillation is in this case substantially dependent on the geometry and the mechanical structure of the piezoelectric transformer 1.

[0058] The output region 3 comprises piezoelectric material 9 and is free of internally lying electrodes. The piezoelectric material 9 in the output region 3 is polarized in the longitudinal direction z. The piezoelectric material 9 of the output region 3 may be the same material as the piezoelectric material 5 of the input region 2, in which case the piezoelectric materials 5 and 9 may differ in their polarization direction. In the output region 3, the piezoelectric material 9 is formed as a single monolithic layer which is fully polarized in the longitudinal direction z. In this case, the piezoelectric material 9 in the output region 3 has only a single polarization direction.

[0059] If an alternating voltage is applied to the electrodes 4 in the input region 2, a mechanical wave is formed inside the piezoelectric material 5, 9 which produces an output voltage in the output region 3 by the piezoelectric effect. The output region 3 comprises an output end side 10. In the output region 3, an electrical voltage is therefore produced between the end side 10 and the end of the electrodes 4 of the input region 2. A high voltage is then produced at the output end side 10. A high potential difference is in this case also formed between the output end side and an environment of the piezoelectric transformer, this potential difference being sufficient to produce a strong electric field that ionizes a process gas.

[0060] In this way, the piezoelectric transformer 1 produces high electric fields that are capable of ionizing 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. Ionization takes place whenever the electric field strength at the surface of the piezoelectric transformer 1 exceeds the ignition field strength of the plasma. The ignition field strength of a plasma in this case refers to the field strength which is required for ionization of the atoms or molecules.

[0061] FIG. 2 shows a device for producing a non-thermal atmospheric pressure plasma, which comprises the piezoelectric transformer 1 described in connection with FIG. 1. The device furthermore comprises a driver circuit 11 which is designed to apply an input signal to the piezoelectric transformer 1, a field probe 12 which measures a field produced by the piezoelectric transformer 1, and evaluation electronics 13 which are connected to the field probe 12 and to the driver circuit 11.

[0062] It is an aim to make it possible to adapt the input signal constantly so that the piezoelectric transformer 1 is operated at its present series resonant frequency or at its present parallel resonant frequency. The series resonant frequency and the parallel resonant frequency of the transformer 1 are not constant but depend on various parameters. If a plasma is ignited at an output end side 10 of the transformer 1, it interacts as a load with the transformer 1 and then influences the impedance of the transformer 1. In this way, a shift of the series resonance and of the parallel resonant frequency takes place relative to operation of the transformer 1 under no load, during which the applied voltage is not sufficient to ignite a plasma. The working environments, for example the process gas used or the temperature of the transformer 1, also influence the series resonant frequency and the parallel resonant frequency.

[0063] The driver circuit 11 applies the input signal to the piezoelectric transformer 1. The input signal is preferably sinusoidal. However, input signals having a different waveform, for example rectangular or triangular, are also possible. The input signal that is produced by the driver circuit 11 is adapted continuously according to a feedback signal transmitted by the evaluation electronics 13 to the driver circuit 11.

[0064] The field probe 12 is configured to measure a field strength of the electric field produced by the piezoelectric transformer 1. The field probe 12 is arranged in the immediate vicinity of the output region 3 of the piezoelectric transformer 1. In particular, the field probe 12 is configured to determine the local field strength of the electric field produced by the piezoelectric transformer 1 at a plurality of measurement points. Accordingly, a spatial profile of the field strength of the electric field may be determined by the field probe 12. This may make it possible for the field probe 12 to determine the position at which a maximum field strength lies.

[0065] The configuration of the electric field and the position of the maximum field strength depend on the input signal applied to the transformer 1 and on the operating conditions of the piezoelectric transformer 1. In respect of the input signal, the following parameters are in this case crucial for the formation of the field: an input voltage, a current of an input current, a phase relationship between the input voltage and the input current, and a frequency of the input signal. In respect of the operating conditions, the electric field may be influenced by a process medium, an ambient temperature and/or an air humidity. Furthermore, a load arranged in the vicinity of the transformer 1 also influences the potential distribution of the electric field and therefore the position of the maximum field strength.

[0066] The evaluation electronics 13 are configured to evaluate the measurement values that have been determined by the field probe 12 and are furthermore configured to control the driver circuit 11. The evaluation electronics 13 comprise a signal conditioner 14 and a signal processing unit 15.

[0067] The field probe 12 transmits a measurement signal to the evaluation electronics 13. In the evaluation electronics 13, the measurement signal is initially processed by the signal conditioner 14. In particular, the signal conditioner 14 may filter the measurement signal, convert it into a digital signal, amplify it and/or equalize it. The signal processed further by the signal conditioner 14 is then forwarded to the signal processing unit 15. The signal processing unit 15 processes the signal and determines a feedback signal, which is transmitted to the driver circuit 11.

[0068] The signal processing unit 14 may in this case compare the input signal which is applied to the piezoelectric transformer 1 by the driver circuit 11 and an output signal of the transformer 1 with one another. To this end, the input signal applied to the piezoelectric transformer 1 by the driver circuit 11 may also be transmitted to the signal processing unit 14. The output signal of the transformer 1 is determined by the signal processing unit 14 with the aid of the measurement values recorded by the field probe 12. As an alternative, the output signal may also be compared with a reference signal in the signal processing unit 14. This may be used for calibrating the device and/or determining the degree of ageing of the piezoelectric transformer 1 and/or for lifetime monitoring of the piezoelectric transformer 1.

[0069] During the comparison of the input signal with the output signal, voltages of the two signals may be compared with one another. As an alternative or in addition, currents of the two signals may be compared with one another during the comparison of the input signal with the output signal. As an alternative or in addition, phase relationships between the voltages and the currents of the two signals may be compared with one another during the comparison of the input signal with the output signal. As an alternative or in addition, frequencies of the two signals may be compared with one another during the comparison of the input signal with the output signal. As an alternative or in addition, an impedance of the transformer may be taken into account during the comparison of the input signal with the output signal. By considering one or more of the parameters mentioned here during the comparison of the signals, the evaluation electronics 13 may determine the present parallel resonant frequency and/or the present series resonant frequency of the transformer 1. The evaluation electronics 13 then transmit a feedback signal to the driver circuit 11, which causes the driver circuit 11 to adapt the frequency of the input signal and adjust it to the present parallel resonant frequency or the present series resonant frequency of the transformer 1. This process may be repeated, at regular time intervals or continuously in real time, during ongoing operation of the device.

[0070] FIGS. 3, 4 and 5 respectively show a field probe 12 according to a first exemplary embodiment. FIG. 3 shows a lower side of the field probe 12, which faces away from the piezoelectric transformer 1. FIG. 4 shows an upper side of the field probe 12, which faces toward the piezoelectric transformer 1. FIG. 5 shows a cross section through the field probe 12.

[0071] The field probe 12 comprises a substrate 16 made of an insulating material. The insulating material may be a polymer, a ceramic material, FR4, glass or Al.sub.2O.sub.3, for example. According to the first exemplary embodiment, the substrate 16 is single-layered.

[0072] Metallizations 17 are formed on a lower side of the substrate 16, which faces away from the piezoelectric transformer 1. According to the first exemplary embodiment, the metallizations 17 are circular. Differently shaped metallizations 17, for example rectangular metallizations, are also conceivable. Each of the metallizations 17 forms a measurement point, at which the field probe 12 determines a local field strength of the electric field that is produced by the piezoelectric transformer 1. The measurement points are arranged in a row.

[0073] The field probe 12 is arranged at a distance d from the piezoelectric transformer 1, which is large enough to avoid plasma ignitions of the transformer 1 against the field probe 12. The distance d may furthermore be selected so that sparks of a plasma jet produced by the transformer 1 do not reach the field probe 12.

[0074] The field probe 12 is arranged parallel to the output region 3 of the piezoelectric transformer 1 and protrudes beyond the output region 3. The field probe 12 may accordingly measure the field strength in the output region 3 and in a half-space, facing away from the transformer 1, in front of the output end side 10 of the piezoelectric transformer 1.

[0075] The field probe 12 furthermore comprises a metal face 18, to which a reference potential is applied. The metal face 18 forms a frame and is arranged at the edges of the lower side of the field probe 12. The row formed by the measurement points is framed by the metal face 18. In order to determine a local field strength, the voltage between the respective measurement point and the metal face 18 is respectively determined. To this end, the metal face 18 is to be connected to the reference potential. Differently-shaped metal faces 18 are also conceivable. For example, instead of forming a circumferential frame, the metal face 18 could also be U- or -shaped.

[0076] FIGS. 6, 7 and 8 show a field probe 12 according to a second exemplary embodiment. FIG. 6 shows an upper side of the field probe 12, which faces toward the piezoelectric transformer 1. FIG. 7 shows a lower side of the field probe 12, which faces away from the piezoelectric transformer 1. FIG. 8 shows a cross section through the field probe 12.

[0077] According to the second exemplary embodiment, the substrate 16 of the field probe 12 is multilayered. The metallizations 17 which form the measurement points are arranged in an inner layer of the substrate 16. The metallizations 17 furthermore comprise through-contacts 17a, which are fed through to the lower side of the field probe 12. The frame which is formed by the metal face 18 is also formed in the inner layer of the multilayer substrate 16 and comprises a through-contact 18a. A potential of the metal face 18 and of the metallizations 17 may be tapped at the through-contacts 17a, 18a.

[0078] Passive components 19 are furthermore integrated into the multilayer substrate 16 of the field probe 12. A further passive component 20 is arranged on the lower side of the field probe 12. Furthermore, an active component 21 is also arranged on the lower side of the field probe 12. The active and passive components 19, 20, 21 are components of the driver circuit 11 and/or of the evaluation electronics 13. In particular, the evaluation electronics 13 and/or the driver circuit 11 may respectively be formed fully or partially on the field probe 12.

[0079] FIG. 9 shows a first signal which may be applied as an input signal to the piezoelectric transformer 1 by the driver circuit 11. The signal is substantially sinusoidal.

[0080] FIG. 10 shows the measurement values determined by the field probe 12 for the field strength at various positions in the vicinity of the piezoelectric transformer 1. The field probe 12 is arranged in such a way that the measurement points are arranged parallel to the output region 3 and in a region in front of the output end side 10.

[0081] In FIG. 10, the field strength determined by the field probe 12 at the individual measurement points is respectively plotted in volts on the ordinate axis. The position of the measurement points is plotted on the abscissa axis, wherein the position o corresponds to the output end side 10. The output region 3 of the piezoelectric transformer 1 in this case extends in the positive direction. Accordingly, for example, the position 3 mm is arranged in the output region 3 of the transformer 1 and lies at a distance of 3 mm from the output end side 10. The position 3 mm lies in front of the output end side 10 at a distance of 3 mm therefrom.

[0082] FIG. 10 shows the profile of the field strength for two different input signals, each of which has the substantially sinusoidal waveform shown in FIG. 9. The power of the first input signal is 10 W. The power of the second input signal is 5 W. It may be seen in FIG. 10 that, for the first input signal, a maximum of the electric field strength is formed at a distance of about 4 mm in front of the output end side 10. For the second input signal with a power of 5 W, on the other hand, a plateau of a maximum field strength, which extends from an edge region of the input region to a distance of about 6 mm in front of the output end side 10, is formed in the distribution of the electric field strength. This measurement therefore shows that the power of the input signal is crucial for the spatial arrangement of the maximum of the field strength.

[0083] FIG. 11 shows a second input signal which may be applied to the piezoelectric transformer 1 by the driver circuit 11. The second input signal has a periodic profile, but it does not form a regular sine curve.

[0084] FIG. 12 respectively shows, for various signals applied to the piezoelectric transformer 1, the measurement values which are determined by the individual measurement points of the field probe for the field strength. All the applied signals in this case have an input power of 5 W. Curves F1 and F2 correspond to an applied first signal, the profile of which is shown in FIG. 9, and curves L1, L2 and L3 correspond to an applied second signal, the profile of which is shown in FIG. 11.

[0085] FIG. 12 shows that, in the case of the second input signal, a maximum in the field strength which lies in the output region 3 of the piezoelectric transformer 1 at a distance of a few millimeters in front of the output end side is formed. Such a maximum is unfavorable since it may lead to plasma ignitions, by which the piezoelectric transformer 1 may be damaged, along the edges of the output region 3.

[0086] Operation of the transformer 1 with the second input signal is possible, but in this case the input signal should be adapted in such a way that the maximum of the field strength is shifted toward the output end side 10. To this end, for example, the frequency of the input signal may be adapted. A load arranged in front of the output end side 10 also leads to a variation of the field.

[0087] The field probe 12 therefore makes it possible to record the spatial profile of the electric field produced by the piezoelectric transformer 1, since the field probe 12 records the field strength at a plurality of measurement points simultaneously. In this way, as shown in connection with FIG. 12, the influence of different signal waveforms of the input signal on the potential profile of the field produced may be measured. With such a field probe 12, it is furthermore possible to measure the influence of different working conditions on the potential profile of the field produced, in which case the working conditions may for example differ in terms of temperature, air humidity or the process medium used. With such a field probe 12, it is furthermore possible to demonstrate the effect of ageing of the transformer 1, in which case variations of the electric field over the lifetime of the transformer 1 may be measured.

[0088] Evaluation of the measurement values recorded by the field probe 12 makes it possible to obtain quantitative and qualitative information about a load arranged in front of the transformer 1. In particular, the evaluation electronics 13 may be configured to determine whether a load is arranged in front of the piezoelectric transformer 1 with the aid of the measurement values recorded by the field probe 12 and the comparison of the input signal with the output signal. If no load is detected, the evaluation electronics 13 may send a feedback signal to the driver circuit 11, according to which signal the power of the input signal is reduced to such an extent that plasma ignitions do not take place. If a load is detected, the evaluation electronics 13 may send a feedback signal to the driver circuit 11, according a power of the input signal which is sufficient for the ignition of plasma is adjusted. Accordingly, the plasma may be ignited only when a load arranged in front of the transformer 1 is detected.

[0089] The field probe 12 may be integrated into a holder in which the piezoelectric transformer 1 is mechanically fastened and via which the piezoelectric transformer 1 is