SI20046A - Ultrasonic transducer and procedure for its manufacture - Google Patents

Abstract

The ultrasonic transducer for the transmission and receipt of ultrasound in gases has been made, keeping in mind the reduction of cost, as a group from a homogenous, piezoceramic wafer (1) metallized on both sides, where parallel grooves are cut first, while later the side walls (7) of these channels are metallized and brought in contact with the electrode (31). Onto such a pre-processed wafer (1) an acoustic adaptive layer (5) made from a mixture of epoxi resin and hollow plastic balls is applied. After solidification of the layer (5) the wafer is cut through longitudinal and transverse cuts of a diamond saw into individual transducers. This allows a decrease in production costs by factor 20 or more. The individual ultrasonic transducers are prism shaped and polarised by thickness, which oscillates when the transducer is excited. The adaptive layer (5) is resistive to organic solvents, which can often be traced in natural gases.

Description

THE ULTRASONIC CONVERTER AND THE PROCEDURE FOR ITS MANUFACTURING

The field of technology

The invention belongs to the field of technology of ultrasonic piezoelectric transducers. According to MPK, it belongs to class H 04 R 7/10.

Technical problem

The invention solves the technical problem of designing and manufacturing a small-size piezoelectric ultrasonic transducer, low cost and good acoustic gas tuning at frequencies above 200 kHz.

The state of the art

Ultrasonic gas flow meters use ultrasonic transducers to transmit and receive ultrasonic waves. The difference in flow time in the direction of flow and flow is the basis for determining the velocity or free flow of gas. Household gas meters are large-scale products where low cost in comparison to bellows meters is extremely important for successful sales. Lowering the price of ultrasonic transducers, however, is one way to reduce the price of the meter.

In prior art solutions (DE 2537788, EP 0347096), ultrasonic transducers for gas flowmeters have an active piezoelectric plate, most commonly circular, a passive adaptation layer for better acoustic adaptation to gas, and an enclosure into which the ultrasonic transducer is filled with a pouring mass, which is in each case chosen to suppress the oscillation of the inverter in order to reach a wider frequency band. Such construction requires either the individual production of ultrasonic transducers or the simultaneous production of a large number of transducers in more sophisticated and also more expensive tools.

The price of a piezoelectric ultrasonic transducer consists of the price of the piezoelectric material, which is proportional to the mass and the cost of manufacture, which is independent of the size of the piezoelectric plate. Existing technology therefore makes it impossible to significantly reduce the cost of ultrasonic transducers.

Description of the new solution

The essence of the new solution is the simultaneous production of a large number of transducers with the cutting of a larger piezoelectric plate.

The new solution will be explained in more detail below and with the help of pictures showing:

Figure 1: Ultrasound transducer of the invention.

Figure 2: Piezoceramic tile with metallic channels in (cross section).

Figure 3: Piezoceramic tile with channels filled with electrically conductive resin (cross section).

Figure 4: Piezoceramic tile with an adjustable layer and pointed cuts.

Figure 5: A group of transducers according to the invention (in cross section).

The ultrasonic transducer of the invention is made of piezoelectric ceramics. The piezoelectric plate 11 is polarized in thickness, which means that the dipole electric moment is oriented in the thickness direction of the plate 11 or perpendicular to the transmitter surface 31 of the piezoelectric plate 11 (Figure 1). The upper surface 31 (Figure 1) and the lower surface 32 of the converter have electrodes in the form of a thin layer of charged metal and are electrically coupled to an electronic circuit. In electrical excitation, plate 11 oscillates with the closest or a combination of the nearest more pronounced resonant frequencies. In the case of the converter of the invention, the plate 11 is rotated in the planar mode. This is a sophisticated plane oscillation in which the piezoelectric plate 11 extends and shrinks in both orthogonal transverse directions. Consequently, the plate 11 deforms in the thickness direction with the opposite phase relative to the transverse oscillation. The resulting thickness mode vibration of the transducer is the basis for the generation of sound waves in the gas. The top surface 31 of the transducer is used for ultrasound delivery. Due to the large difference in acoustic impedances, that is, the product of sound velocity and density in gas and ceramics, an efficient impedance adaptation layer 5 is required for efficient energy transfer from the upper surface 31 of the converter to the gas and vice versa. The resonant frequency of the ultrasonic transducer is inversely proportional to the corresponding piezoelectric dimension. tile 11. In the case of planar oscillation, this is the width or length of tile 11, and in the case of thickness oscillation, it is the thickness of the tile Π. In the ultrasonic transducer of the invention, a planar oscillation method is used due to the acceptable dimensions of the plate 11 in the transverse direction (about 5 mm) at an ultrasound frequency of several hundred kHz. In the case of thickness fluctuations with the same frequency, the thickness of the piezoceramic tile 11 should be about 5 mm, which means that the mass of the piezoelectric material is greater with the same surface of the radiation surface. This also means a higher material cost and less homogeneity of ceramics.

However, acoustic adaptation to gas or air is also required for good inverter energy efficiency. The known solutions so far use layer 5 with thickness dm = λ / 4, whose specific acoustic impedance is the product of the density of the layer and the velocity of sound in that layer) is close to the geometric mean of the acoustic impedances of the piezoelectric ceramic and gas. In the case of piezoelectric ceramics and air as a medium, with acoustic impedances of 34 Mrayl or 410 Rayl in monochromatic operation, the impedance of the adaptation layer 5 0.12 MRayl is theoretically required. This can be achieved by reducing the density of layer 5 and reducing the speed of sound in layer 5, while the wave absorption in layer 5 should not be significantly increased. In the present solutions (eg DE 25 37 788) this layer is made as a mixture of hollows

-4 · Glass or silica (S1O2) beads and viscoelastic resins (eg epoxy resin or polystyrene). In the case of polystyrene and glass beads with a density of 0.3 g / cm ³ , a specific acoustic impedance of 0.36 MRayl was achieved, and using quartz beads with a density of 0.18 g / cm ³ about 0.26 Mrayl. Using benzene as the diluent, this impedance was reduced to about 0.21 MRayl, at a layer 5 density of about 0.16 g / cm ³ . Due to the sensitivity of polystyrene to certain organic solvents that may be present in natural gas, this mixture is unsuitable for operation in such a medium. A mixture of hollow glass beads and o

epoxy resins. With this mixture, a density of about 0.5 - 0.6 g / cm and a sound speed of about 2200 - 2400 m / s are available, which means that the specific acoustic impedance is about 1-1.4 Mrayl. Adaptation layer 5 for gases containing organic solvents can be improved in terms of impedance reduction using hollow plastic beads and low-viscosity epoxy resin. Densities of 0.34 g / cm were obtained using the LY5138 impregnation epoxy resin and acrylonitrile hollow beads ³ and an audio speed of 1470 m / s, which means that the impedance is about 0.5 Mrayl, with the hollow balls having a medium diameter

However, at a density of about 0.03 g / cm, in v _e about 60 μπι m. Unfortunately, the sound absorption is about four times higher than that of glass hollow beads. For thicker layers, this is annoying because of the greater attenuation of the useful signal, however, when switching to a higher frequency of operation f and a mixture with a lower speed of sound Cm, thinner adaptation layers dm are required - λ / 4 ⁼ Cm / f

In this case, the attenuation is smaller.

For a square-shaped ultrasonic transducer with a = 4.55 mm side, the operating frequency is f = 350 kHz, which at an attained acoustic velocity of Cm = 1470 m / s in adaptation layer 5 means a dm thickness of approximately 1 mm (Figure 1). This is about a quarter of the thickness of layer 5 as in the case of ultrasonic transducers operating at frequencies of 150 - 200 kHz and using an adaptive layer 5 of epoxy resin and hollow glass or quartz beads. Due to four times the absorption losses in layer 5 containing hollow plastic beads, this mixture is only applicable to ultrasonic transducers operating at frequencies above 300 kHz. In this case, the absorption losses are no greater than for the mixture containing glass hollow beads for transducers operating at frequencies below 200 kHz. In terms of acoustic adaptation to gas, however, the mixture with plastic hollow beads is more favorable because of the lower acoustic impedance. A theoretical estimate for the transition of energy from piezoceramics to gas for the one-dimensional and monochromatic case can be made using transmission line theory

T _e = _ * - Z _k -Z _p _ {ζ, + zjf * cos ² kd + (z "-sin ² k" d where Te is the energy transfer coefficient between piezoelectric ceramics and gas, Zk and Z _{P is the} specific acoustic impedance piezoelectric ceramics or gas, km wave vector in the adaptation layer, dm the thickness of this layer and Zm its specific acoustic impedance. In case dm = λ / 4 is

For acoustic impedance of ceramics Zk = 34 Mrayl and gas Z _P = 300 Rayl, for the adaptation layer 5 with plastic hollow beads with impedance Zm = 0,5 Mravl the energy transfer is -8 dB, for the adaptation layer 5 of hollow glass beads with impedance Zm = 1.4 Mrayl pa -17 dB. This means that for the 350 kHz converter and the plastic beads in the adaptation layer 5, the signal amplitudes are 18 dB higher than for the 170 kHz inverter and the hollow glass beads in the adaptation layer 5.

In the case of transducers with a series resonance frequency fs of approximately 350 kHz and a parallel frequency f _{P of} approximately 390 kHz, the base side a of the converter is 4,55 mm, piezoceramic plate thickness 2 mm, channel depth 0,8 mm, notched channel width 0,4 mm , depth of contact side of transducer side 0.1 mm m adaptive layer thickness 5 dm = 0.95 mm. The maximum transfer function of the inverter pair is about f _s and the bandwidth at - 3 dB is 40 kHz. The converter made in this way has an ignition time τ = 20 ps, which means an increase in the ignition signal from 10% to 90% of the maximum value in 7 periods. No additional mechanical damping of this converter is necessary, so the mounting is elastic and made by pouring into a soft polyurethane seal, which offers sufficient acoustic insulation against mechanical waves that could partially extend through the housing. Due to the small mass of the converter, such a seal also provides sufficient support for positioning.

In the case of the need to obtain a plane wave approximation in a part of the space wider than the side length a of the converter, in this case more than 5 mm, it is possible to use several inverted and equally oriented transducers which are sealed in a common polyurethane seal. Such a structure can be single or multi-row. In this way, the set of ultrasonic transducers 1 fluctuates much like a transducer with a larger transverse dimension. Due to the sophisticated and more uniform surface oscillation amplitudes of the upper radiation plane 51

-6of the individual transducer (Fig. 2), the oscillation of the described composition is also more homogeneous m sophisticated than with one larger transducer of the same lateral dimensions. This then means creating a more homogeneous flat wave.

A further option is the assembly of a group of ultrasonic transducers, the undersides of 32 (electrodes) as shown in Figure 5 are interconnected by a thin layer of uncut ceramic plate 1. Such a connection allows the composition and orientation of the individual ultrasonic transducers to be provided without interfering the coupling of mechanical oscillations between the individual pre-pilot units, which would significantly affect the oscillation of such a composition.

In order to produce the ultrasonic transducers described above with the least cost, a special procedure has been developed, which originates from a two-sided metallized piezoelectric plate 1 into which parallel channels are first cut. The side walls of the ducts are then metallized and brought into electrical contact with the upper electrode 31 by the process of sputtering the solder metal layer 2 or filling the ducts with conductive epoxy resin 6 (Figure 3). In the case of sputtering, we perform two-step work, first spraying a layer of Ni or similar metal to provide adhesion to ceramics and then an Au layer for good soldering. In both cases, the thicknesses of the metal layers are in the submichrometer region. The fillings are then filled with a tear-off cover 4. The filling is done with a movable syringe which has a feed along the channel synchronized with the flow of varnish. The abrasive cover is then cured in an oven at about 100 ° C. This is followed by the application of a mixture of low-viscosity epoxy resin and hollow plastic beads, which is evenly distributed throughout the surface of the ceramic plate with a scraper. After curing the mixture, this layer can be sanded to the required thickness. The plate 1 thus treated must be further cut and transected longitudinally into individual tiles of 11 ultrasonic transducers. The procedure will be described in more detail in the embodiment.

The piezoelectric plate 1 is fixed with thermal adhesive to the pallet, which ensures a repeatable position of the fixture in the devices in all the processing stages. The cutting is done with a saw for cutting ceramic materials, which usually has pneumatic guides with a precision of 0.005 mm and high speeds (15000 rpm). The saw blade is 0.35 mm thick and has diamond dust attached to the cutting edge, so the cutting width is about 0.4 mm. During sawing, the cutting site is cooled to prevent depolarization of the piezoelectric material due to overheating. After cutting the channels, the ceramic plate is well cleaned of refrigerant and chips. The lateral walls of the ducts are then metallized and electrically contacted with the electrode 31 by a sputtering process of the solder metal layer 2 in.

-7 vacuum sputtering devices. Alternatively, the channels can be filled with conductive epoxy resin 6 (Figure 3). In the case of spraying, this process is performed in two steps, first spraying a layer of Ni or similar metal to ensure good adhesion to the ceramic and then an Au layer for good soldering. In both cases, the thicknesses of the layers are in the submichrometer range. The channels are then filled with a retaining cover 4 (protective lacquer) by means of a movable syringe having the displacement along the channel synchronized with the flow of the syringe varnish. Hardening of the tile roof is carried out in an oven at a temperature of about 100 ° C. This is followed by the application of a mixture of low-viscosity epoxy resin and hollow plastic beads, which is evenly distributed over the surface of the ceramic plate with a scraper. The scraper is a part of a dedicated device with guides that allow for a planar parallel of ± 0.01 mm. The treatment of the mixture by means of a scraper is carried out in large increments over several tens of mm thick polyester (or similar) foil, which is coated with a separating agent and in contact with the lower surface of the mixture. The purpose of the foil is to prevent the scraper knife from pulling the mixture behind when handling. This type of application is necessary mainly because the mixture must not have internal pressures that imply the compression of plastic hollow beads. Therefore, the clamping part of the preparation for the application of the admixture is designed so that the excess mixture has a large space for extrusion. After melting the mixture, wipe the adjustment mixture layer to the required size. The plate 1 thus treated is further cut with longitudinal and transverse cuts into individual inverters (Figure 4). Work is done with a saw with which we have already cut the channels. After cutting, the pallet with the cut plate 1 is heated to a temperature of about 120 ° C to allow the adhesive to loosen. The bottom electrodes of the 32 transducers are cleaned of adhesive residues with an organic solvent (eg Solkan). The longitudinal cuts are made through the filled channels so that on the lateral surface 7 of the channel the area of metallization 2 is in contact with the upper electrode 31. In the case of spraying (Figure 2), it is necessary to mechanically remove the tear-off cover 4 and then to the metallized part 7 pages pripajka electrical guide. If the channels are filled with conductive epoxy resin, then contact with such resin. Depending on the transducer technology and contact method, a piezoelectric thickness of 1 plate is recommended to be about 2 mm. Too thin panel 1 causes contact problems, and too thick means a greater material cost.

The usable frequency band for measuring gas flow by passing the ultrasonic signal through the gas reaches up to about 500 kHz. The proposed converter design is a prism in which the transverse dimension of the piezoceramic plate determines the required operating frequency. For the frequency range of 200 to 500 Hz, the transverse dimension of the transducer is in the range of 3-8 mm. It is most advantageous that the basic plane of the prism is a square with side a «λ / 2, because in this way both transverse oscillations are equal

-8and they contribute to the oscillation of the upper, transmitter surface of the converter. With regard to the size of the basic piezoelectric plate 1, it is advantageous to use the largest dimension plate. On the other hand, the larger lateral dimensions of the piezoelectric plates 1 imply the poorer homogeneity of the piezoelectric material at the same technological manufacturing costs, or an increase in these costs while ensuring the same homogeneity. Meaningful maximum dimensions of panels 1 are therefore 50 x 50 mm to 110 x 110 mm, depending on the technological capabilities of the supplier. In our case, the transducers with the side of the square a = 4,55 mm were cut from a panel 1 measuring 60 x 60 mm and 144 converters were made from a single panel 1. This has allowed the production cost to be reduced by a factor of 20, according to current technology. All transducers made from the same panel 1 have comparable acoustic and electrical properties, which increases the reciprocity of the preconception pairs and allows the assembly of transducer groups. Such groups are suitable for forming wave fronts of the desired shape or for simultaneously transmitting and receiving ultrasound.

Claims (6)

Patent claims An ultrasonic transducer for transmitting and receiving ultrasound in gas, consisting of a piezoceramic tile (11) and an acoustic adaptation layer (5), characterized in that the tile (11) is polarized in thickness and prismatic, extends to the lateral surface (7) The solder duct is applied to a soldering metal layer which is in electrical contact with the adjacent electrode (31), and the adjustment layer (5) is made of a mixture of hollow plastic beads and an epoxy resin. An ultrasound transducer according to claim 1, characterized in that the lateral surface (7) is not metallized, but an electrically conductive epoxy resin (6) which is in electrical contact with the electrode (31) is applied to it. A group of ultrasonic transducers according to claim 1 or 2, characterized in that they are interconnected with a portion of the uncut piezoceramic plate (1). 4. A method for manufacturing an ultrasonic transducer, characterized in that it cuts the channels into one side of the metallized piezoceramic plates (1), metallizing the side walls (7) of these channels, contacting the metallized side walls (7) with an electrode (31), filling channels with openings (4), application of the adaptation layer (5), grinding to the prescribed size after hardening and final cutting of the treated plate (1). Method according to claim 4, characterized in that the side walls (7) are not metallized but the channels are filled with an electrically conductive epoxy resin (6) instead of a retaining cover (4) Method according to claim 4 or 5, characterized in that one or more cuts have a depth less than the thickness of the plate (1), thus forming one or more groups of transducers that are mechanically interconnected with a part of the uncut plate (1) ).