DE102007009907A1 - Piezoelectric sensor, has mono crystalline vibrating body from a piezoelectric material, in which domain-inverted area is formed with inverted crystal axis, where vibrating body is connected with excitation and receiving electrodes - Google Patents

Abstract

The present invention relates to a piezoelectric sensor having a monocrystalline oscillating body of a piezoelectric material in which a domain-inverted region having inverted crystal axes is formed, the vibrating body being connected to excitation and receiving electrodes.

Description

The The following invention relates to a piezoelectric sensor, the in particular for measuring physical properties of fluids suitable is.

Especially oscillating piezoelectric plates can as sensor elements for different measurands be used. Being piezoelectric, they offer the advantage that both the excitation and the readout of the sensor signal with simple electrode structures are realized purely electrically can.

metrics which can be detected with these sensor elements are z. As the viscosity and the density of different fluids (Liquids or gases). A also economically interesting Application example for this is the measurement of the viscosity of engine oils, to timely oil change to signal. Further application examples are different chemical reactions associated with the formation of solid or jelly-like Phases are accompanied and the observation of the homogeneity of Suspensions.

One Another application example for oscillating piezoelectric Platelets is the use as mass sensors, as at Addition of additional mass on its surface a shift of their resonance frequency is measurable, whereby the mass accumulated on its surface becomes determinable. This can z. B. Bioprocesses are observed in which the biomolecules to be detected on the platelet surface remain adherent (eg, antibody-antigen reactions).

A piezoelectric plate sensor which can detect the parameters described above is in patent document WO 2005/043126 A2 described. The proposed there plate structure is in 13 shown. The sensor wafer is composed of several layers: (i) a sensor element 100 , which is usually made of a non-piezoelectric material (eg., Metal plate), (ii) a piezoelectric layer 101 , which is adhered to the sensor element, (iii) an excitation electrode 102 to apply an electrical voltage which is vibrated in conjunction with the piezoelectric layer of the entire sensor, and (iv) a receiving electrode 103 to detect the sensor signal.

This Platelet sensor works such that with the Excitation electrode the plate into a mechanical resonance is stimulated. Is the tile z. B. in one Liquid in the viscosity, the density or both sizes change over time, so will the resonance frequency and the quality of the change according to the mechanical vibration system. Out the magnitude of the changes in the resonant frequency and in goodness can the viscosity and the density of the liquid can be determined.

Of the Sensor according to the described prior art has several disadvantages. So the tile consists of two adhered parts. This requires technology steps, who do not allow a cost-efficient planar technology use. Furthermore, the adhesive layer ages over time. In addition, this adhesive layer absorbs mechanical energy at the resonance movement of the plate. Also is exclusive a bending vibration possible.

Similar Disadvantages also have the piezoceramic layer itself. she is polycrystalline and has a high internal friction, which causes vibration energy is absorbed. These energy losses are particularly important since thereby the effectiveness of the resonance excitation and the size the achievable Q-factors (and thus the quality) reduced become.

The Adhesive layer and the depolarization effects in the piezoceramic also limit the temperature range in which the sensors are used can reach a maximum of 150-180 ° C.

Farther it should be noted that for simultaneous determination the viscosity and the density (which is usually also change at the same time) two measuring parameters necessary are. The prior art sensor is used for this purpose determines the resonant frequency and the quality of the resonance curves. Although the resonance frequencies can also be experimentally good be recorded. However, the quality is difficult to determine. For this reason, the accuracy of the sensor is limited.

It An object of the present invention is a piezoelectric Specify sensor that can be produced inexpensively with a planar technology is.

These The object is achieved by a piezoelectric sensor with a monocrystalline oscillating body from a piezoelectric material in which a domain inverted Area is formed with inverted crystal axes, wherein the Vibrating body connected to excitation and recording electrodes is.

The present piezoelectric sensor can completely dispense with adhesive surfaces or other types of connections between different layers of the vibrating body and he nevertheless allows the measurement of at least two measuring parameters (eg viscosity and density) simultaneously and with great accuracy and over a wide temperature range.

there The vibrating body can be made of a monolithic crystalline Be formed body. The excitation and / or recording electrodes can at least along a part of a longitudinal extension be arranged of the vibrating body. The oscillating body can be formed elongated. The oscillating body can as elongate platelet with substantially rectangular plan or as platelets be formed with a substantially circular outline. Of the Domain-inverted area can be considered a layer in cross-section be formed of the vibrating body. In addition, several domain-inverted regions periodically over the Oscillator length can be arranged distributed, wherein the excitation and recording electrodes interlock but from each other electrically isolated and according to the arrangement of the domain inverted Areas can be arranged.

According to one another preferred embodiment the excitation and / or recording electrodes each as a pair of electrodes be formed on opposite side surfaces of the vibrating body are arranged. Alternatively, the Excitation electrode on a first side surface of the vibrating body be arranged, and the receiving electrode may be on an opposite be arranged on the second side of the vibrating body. Alternatvi For example, the excitation and / or recording electrodes can be common arranged on a side surface of the vibrating body be.

According to one another preferred embodiment more than two electrode pairs may be provided, each one both can be used as excitation as well as recording electrodes.

Of Further, a sensitive for certain substances layer on a portion of the surface of the vibrating body be arranged.

Furthermore the oscillating body can have two end regions, wherein is clamped at least one of these end portions of the vibrating body.

moreover the vibrating body may have an anisotropy, whereby the vibrating body along different crystal orientations has different piezoelectric coefficients.

About that In addition, the portion of the surface of the vibrating body, which comes in contact with a substance to be measured during a measurement, be passivated.

Besides The electrodes can work together on one side of the vibrating body be arranged, the opposite of this side Side can be brought into contact with a substance to be measured, wherein a mechanical barrier against contact with the electrodes Providing side of the vibrating body and to be measured Substance is provided.

According to one Another embodiment, the vibrating body be subdivided into an excitation part and a sensor part, which are separated by a mechanical lock, wherein only the excitation part provided with the electrodes is. In this case, the sensor part can be completely passivated. In addition, the excitation part in a reference chamber and the sensor part be included in a measuring chamber, which are separated from each other.

The The present invention will be described below with reference to several embodiments in conjunction with the accompanying figures closer explained. In these show:

1 a schematic representation of an embodiment of the piezoelectric sensor with one-sided oscillating body attachment ( 1a : Cross-section; 1b : Top view),

2 a schematic representation of another embodiment of the piezoelectric sensor with double-sided oscillating body attachment ( 2a : Cross-section; 2 B : Top view),

3 6 is a diagram illustrating piezo coefficients as a function of a rotation angle in the wafer surface of a 140 ° rotated Y-cut LiNbO ₃ wafer;

4 FIG. 2 a schematic representation of a further exemplary embodiment of the piezoelectric sensor with three electrode pairs (FIG. 3a : Cross-section; 3b : Top view),

5 2 is a schematic representation of a further exemplary embodiment of the piezoelectric sensor with periodically arranged domain-inverted regions (FIG. 5a , Cross-section; 5b , Top view),

6 1 is a schematic representation of a further exemplary embodiment of the piezoelectric sensor with a plurality of periodically arranged electrodes on a vibrating surface ( 6a : Cross-section; 6b : Top view, 6c such a piezoelectric sensor after 6a and 6b , additionally with a passivated surface),

7 a schematic representation of a Another embodiment of the piezoelectric sensor with a plurality of periodically arranged electrodes on a vibrating surface with a separate measuring medium ( 7a : Cross-section; 7b : Top view, 7c such a piezoelectric sensor after 7a and 7b but with a vibrating body with a round rather than a rectangular plan),

8th a schematic representation of another embodiment of the piezoelectric sensor with a plurality of periodically arranged electric on both oscillating body surfaces ( 8a : Cross-section; 8b : Top view),

9 FIG. 2 a schematic representation of a further exemplary embodiment of the piezoelectric sensor with separated excitation and sensor part (FIG. 9a : Cross-section; 9b : Top view),

10 a schematic representation of the vibration modes of a further embodiment of the piezoelectric sensor with separated excitation and sensor part after 9 ( 10a : symmetrical vibration; 10b : asymmetric vibration, 10c : Frequency response),

11 a schematic representation of another embodiment of the piezoelectric sensor with separated excitation and sensor part with passivation.

12 a schematic representation of another embodiment of the piezoelectric sensor with separated excitation and sensor part in a two-chamber arrangement ( 12a : Cross-section; 12b : Top view), and

13 a schematic representation of a known piezoelectric sensor ( 13a : Cross-section; 13b : Top view).

A first embodiment of the present invention is disclosed in 1 shown schematically. It has a monocrystalline body attached to one side 1 (e.g., LiNbC ₃ ) in which a domain-inverted layer is formed by thermal treatment 2 has been generated. The corresponding remaining layer 5 is not domain-inverted. The monocrystalline and monolithic body 1 forms a crystalline sensor element. In the shift 2 the orientation of the crystal axes is inverted by 180 °, which leads to the sign of the piezo coefficients (but also the other material coefficients) is changed. In this way the monocrystalline body becomes 1 transformed into a monolithic piezoelectric bimorph without the use of an adhesive layer.

The body 1 is formed in the present embodiment as a thin plate with a rectangular outline.

Is in the body 1 generates an electric field, for. B. by means of electrode pairs 3 . 4 , which run over the entire platelet length or only over a part thereof and are positioned on the upper and lower platelet sides, deformations with different signs occur in the two layers. These deformations will make the platelet 1 set in motion, as well as z. B. is the case with a piezoceramic bimorph.

Optionally, also a sensitive layer 6 be applied to the platelet surface and / or on the platelet underside to z. B. deleting organic molecules or enrich other substances.

There is also the possibility of the tile 1 on both sides, which increases stability and resonant frequency. Such an embodiment is in 2 shown.

In 1 and 2 becomes the pair of electrodes 3 . 3 as a receiving electrode and the electrode pairs 4 . 4 used as an excitation electrode.

Because the tile 1 is monocrystalline and has an anisotropy, one can (in contrast to the previously used piezoceramics) by the selection of the crystal section stimulate different movements, whereby bending, Torsinns- or a mixing movement consisting of a bending and a Torsionskomponente are possible.

These movement possibilities can be determined by the piezo coefficients of the plate 1 to explain. For example, consider B. a 140 ° rotated Y-cut LiNbO ₃ wafer from which platelets can be sawn with different orientation. The saw edges are oriented at different angles Φ in the wafer surface, thereby changing the relevant piezo coefficients as in 3 shown. In the diagram according to 3 are the piezo coefficient d '' ₂₃ , which describes the bending movement and the piezo coefficient d '' ₂₅ , which describes the torsional motion, shown as a function of the angle of rotation Φ. If the rotation angle Φ = 0 ° selected, the piezo coefficient d '' _{25 is} also equal to zero and the plate will perform a pure bending movement. If, on the other hand, a rotation angle Φ = 54.5 ° is selected, d '' ₂₃ becomes zero and a pure torsional motion is excited in the plate. For all angles between bending and torsional movements of different strengths are excited and it can be shown that for an angle Φ = 29.5 ° both movements can be excited equally strong. The resonance frequencies at The types of movement are different (about a factor of 10) and can be well stimulated and detected separately.

Will the tile 1 , according to the in 1 illustrated embodiment, with the aid of the excitation electrode 4 . 4 resonated (bending or torsion) and then completely or only partially immersed in a liquid, this leads to a change in the resonance frequency. This change is proportional to the depth of immersion and at a constant depth to the viscosity and density of the liquid. From the measured resonance curves, the resonance frequency and the quality (Q-factor) are then determined, from which the viscosity and the density can be calculated. Also, a pure mass occupancy of the platelet surface, z. B. by attachment of biomolecules, leads to a frequency shift, which can then be used for the determination of the additional masses. The crystal cut can be selected so that the wafer can be excited in both bending and torsional resonances at two different frequencies (e.g., 300 Hz for bending and 3000 Hz for torsional resonance). In such a case, both frequencies are shifted under the action of the liquid and from these shifts, the viscosity and the density can be determined with great accuracy. The quality factors of the resonance curves are also available in this case and can be used for increasing the accuracy or for determining other properties of the liquid.

The electrode structure may also have other geometries. In 4 For example, an embodiment is shown with a plate with three pairs of electrodes. These can be used both as excitation and as recording electrodes, they will have different efficiency at different types of movement. So z. For example, if the central electrode is well suited to excite and detect bending movements, it will be weaker on torsion. Predominantly, the middle electrode becomes a receiving electrode 3 , One of the two outer electrodes as at the excitation electrode 4 , and the further outer electrode optionally as an excitation or recording electrode 8th used.

Another embodiment of the present sensor is shown in FIG 5 shown. The domain-inverted areas 10 are not formed in this case as a layer, but distributed periodically over the platelet length. Between domain-inverted regions are non-domain inverted regions 11 arranged. The excitation of a resonance movement (again bending, torsion or mixed resonance possible) takes place with the help of interdigital electrodes 12 . 13 and 14 . 15 that like in 6 shown are positioned. These have arm areas through which these electrodes 12 . 13 respectively. 14 . 15 interlaced, but electrically isolated from each other. The electrodes are applied in pairs to one platelet surface each. A pair applied to the upper surface 12 . 13 serves z. As for the stimulation of platelet movement and applied to the lower surface pair 14 . 15 picks up the sensor signals. The field lines are in 5 represented by the reference F →.

The of the electrode fingers of the electrodes 12 . 13 excited electric fields F → produce surface deformations (eg, expansion or contraction) that propagate along the platelet 1 add up and eventually lead to a movement similar to a monomorph. In order for the surface deformations to cumulate with a periodic change in the direction of the electric field, it is necessary that the orientation of the crystal axes also change with the same periodicity. This is achieved with domain inversion. The excitation and recording electrodes are well spatially separated in this case and will disturb less than in the case of the embodiment according to 1 ,

Another embodiment which is also based on periodic electrodes is shown in FIG 6 shown. In this embodiment, the excitation and pickup electrode pairs 13 . 12 and 14 . 15 arranged on the same platelet side. This results in the possibility of "freeing" one side of the plate from the electrodes and then using this as the "sensor side" for the measuring liquid or mass occupation. This embodiment is especially useful for media (measuring liquids) that are sensitive to metal layers or electric fields.

Also In this embodiment, more than two electrode pairs are positioned on the surface, giving the flexibility of excitation and signal acquisition is increased.

Also a passivation of that surface 20 of the sensor plate, which is in contact with the measuring fluid FI, can be performed here, as in 6c shown.

In order to achieve a complete separation of the measuring medium FI from the electrode side, it is also possible to realize a kind of membrane solution, in which the upper and lower plate sides by a suitable attachment 30 are separated, as in the embodiment according to 7 is shown.

It is also possible to position more than one pair of electrodes on each side of the wafer So shows 8th an embodiment with two pairs per side. This embodiment allows both the excitation strength and the size of the sensor signal to be doubled. In order to achieve this, the two excitation electrodes on both sides of the plate are supplied with control voltage and the sensor signals are also obtained from two receiving electrodes.

An even greater flexibility offer the embodiment 9 in which the excitation part 50 (can also serve as a reference sensor) and the sensor part 60 of the platelet are completely separated from each other. The plate is in this case only partially with electrodes 3 . 4 (flat or even periodic electrodes are possible) coated and in the wall 70 a measuring chamber with measuring fluid FI installed. The electrodes 3 . 4 lie outside the measuring medium FI, so that the measuring part 60 is completely free of electric fields and can be used in liquids that are sensitive to such fields. Is the wall 70 the measuring chamber sufficiently thin, z. B. some 100 microns, so the movement is in the excitation part 60 also in the sensor part 70 transmitted and both parts can be put into resonance.

Mechanically considered, the two parts of the embodiment are after 9 two coupled resonators. The two lowest vibration modes of this system are in 10 shown: (a) symmetric mode with frequency F ₁ and (b) a symmetric mode with frequency F ₂ where F ₁ <F ₂ . The corresponding resonance curve with two maxima is in 10c shown.

Of the Distance between the two frequencies is determined by the strength the coupling of the two resonators, d. H. through the thickness of the chamber wall certainly. It is true that the thicker the wall, the weaker the wall Coupling, whereby the (identical) resonators with (almost) the same Frequencies are vibrating. In contrast, a thin wall requires a strong coupling and the two frequencies drift apart.

amendments in the viscosity or / and density of the measuring liquid lead to changes in the resonance frequencies and can be used as a sensor signal. Because the two Parts of the plate in the same way from the temperature can be influenced to exclude the effect of temperature by using the excitation part as a reference sensor.

It is still possible, the sensor part 60 of the platelet completely passive, ie also free from piezo effect. That's what this part becomes 60 treated with a strong proton exchange in pure benzoic acid, whereby the piezo effect can be reduced to virtually zero. A corresponding embodiment in which the platelet part with passivation also with the reference numeral 80 is marked in 11 shown.

Another embodiment that is in 12 is shown realizes a bicameral system. For this purpose, a reference chamber R is constructed, through the mounting wall 70 is separated from the measuring chamber M. In the reference chamber, a reference liquid, for. B. reference oil, which is only exposed to temperature fluctuations, but otherwise no pollution from the engine and the environment learns to be filled. In this way, not only the temperature influence can be turned off, but also a differential measurement can be realized, in which a calibration of the sensor is not absolutely necessary.

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• WO 2005/043126 A2 [0005]

Claims (19)

Piezoelectric sensor with a monocrystalline Vibrating body made of a piezoelectric material, in a domain-inverted region with inverted crystal axes is formed, wherein the vibrating body with excitation and receiving electrodes is connected. Piezoelectric sensor according to claim 1, wherein the vibrating body of a monolithic crystalline Body is formed. Piezoelectric sensor according to claim 1 or 2, wherein the excitation and / or recording electrodes at least along a part of a longitudinal extension of the vibrating body are arranged. Piezoelectric sensor according to a of claims 1 to 3, wherein the vibrating body extends longitudinally is trained. Piezoelectric sensor according to a of the claims 1 to 4, wherein the vibrating body as elongated plate with a substantially rectangular Plan or as a platelet with essentially round outline is formed. Piezoelectric sensor according to a of claims 1 to 5, wherein the domain inverted Region formed as a layer in the cross section of the vibrating body is. Piezoelectric sensor according to a of claims 1 to 5, wherein a plurality of domain inverted Periodically over the oscillator length are distributed, and wherein the excitation and recording electrodes interlocked but electrically isolated from each other and accordingly the arrangement of the domain inverted areas arranged are. Piezoelectric sensor according to a of the claims 1 to 7, wherein the excitation and / or Receiving electrodes are each formed out as a pair of electrodes, on opposite side surfaces of the vibrating body are arranged. Piezoelectric sensor according to a of the claims 1 to 7, wherein the excitation electrode on a first side surface of the vibrating body is arranged, and wherein the receiving electrode on an opposite second side of the vibrating body is arranged. Piezoelectric sensor according to a of the claims 1 to 7, wherein the excitation and / or Receiving electrodes together on a side surface of the Vibrating body are arranged. Piezoelectric sensor according to a of claims 1 to 10, wherein more than two pairs of electrodes are provided, each as both excitation and as Recording electrodes are available. Piezoelectric sensor according to a of claims 1 to 11, wherein one for certain substances sensitive layer on a subregion of the surface of the Vibrating body is arranged. Piezoelectric sensor according to a of the claims 1 to 12, wherein the vibrating body two Having end portions and wherein at least one of these end portions of the vibrating body is clamped. Piezoelectric sensor according to a of the claims 1 to 13, wherein the vibrating body a Anisothropia, causing the vibrating body along different crystal orientations different piezo coefficients having. Piezoelectric sensor according to a of the claims 1 to 14, wherein the portion of the surface of the vibrating body, with a substance to be measured comes in contact with a measurement, is passivated. Piezoelectric sensor according to a of claims 1 to 15, wherein the electrodes together are arranged on one side of the vibrating body, where at the side opposite this page with a to be measured Substance is contactable, and being a mechanical barrier against the contact of the side of the oscillating body provided with the electrodes and the substance to be measured is provided. Piezoelectric sensor according to a of the claims 1 to 16, wherein the vibrating body in an excitation part and a sensor part is divided, the are separated by a mechanical lock, and wherein only the excitation part provided with the electrodes is. Piezoelectric sensor according to claim 17, wherein the sensor part is completely passivated. Piezoelectric sensor according to claim 17 or 18, wherein the excitation part in a reference chamber and the sensor part are accommodated in a measuring chamber, which from each other are separated.