WO1988006927A1

WO1988006927A1 - Ultrasonic instrument

Info

Publication number: WO1988006927A1
Application number: PCT/SE1988/000141
Authority: WO
Inventors: Douglas Mcqueen
Original assignee: Mcqueen Douglas H
Priority date: 1987-03-18
Filing date: 1988-03-18
Publication date: 1988-09-22
Also published as: EP0351416A1; SE8701117L; ATE82530T1; DE3876093D1; SE8701117D0; EP0351416B1; SE456971B

Abstract

Apparatus for exciting a liquid in a vessel by ultrasound for cleaning and/or penetration of an object at least partially immersed in the liquid, and including at least one ultrasonic transducer arranged in contact with the vessel. The apparatus is characterized by the diameter's or diagonal's of the ultrasonic transducer's contact surface against the vessel being less than or equal to the bending wave wavelength in the vessel material and sound wavelength in the liquid.

Description

ULTRASONIC INSTRUMENT

The present invention concerns an apparatus for exciting moti in a liquid contained^' in a vessel by means of ultrasonics f cleaning and/or penetration of an object at least partial submerged in the liquid, arid including t least one ultrasou transducer in physical contact with the vessel.

Background of the invention

Cleaning apparatus using ultrasonics in a conventional manner h an important disadvantage. In spite of the fact that ultrasou should be sound at frequencies above the audible range there a always one or more subharmonics, for example at half t fundamental frequency, which are audiable and strong. The strong tones are firstly an irritant for personnel, and secondl (in many cases) a potential cause of hearing damage. It i necessery to either enclose the ultrasonic apparatus or place i in a room where personnel need not be exposed to the stro tones.

The theory of how these subharmonics are generated is quit inadequate. It is generally accepted that the subharmonics ar generated in connection with cavitation in the bath. Subharmonic are generated when the sound pressure in the liquid exceeds threshold value, between two and four atmospheres (the threshol value for cavitation is about one atmosphere) .

In known ultrasonic apparatus a transducer is used whose contac surface against the bottom of the vessel is as large or large than the active element, which is often a piezoelectric ceramic This means that the whole contact surface moves up and down i time with ultrasound to generate sound in the bath. Such surface, wich is a number of sound wavelengths in size, radiate efficiently into the bath, that is, the radiation coefficient i near unity. This also means that all the energy that enters th bath penetrates exactly that limited area. In a bath with volume of one liter one usually wants about 50 watts of inpu acoustic power. If the contact surface is about 5 x 5 cm one get 2 watts/cm-- input power, which is complemented by power reflecte from the limiting surfaces of the bath, and which furthe increases the intensity by a factor of two or more. In this wa the subharmonic regime is reached, and the apparatus is --clearl audible. The situation is even worse when working with large baths and higher power from a small number of transducers.

Naturally the bath's or vessel's other limiting surfaces als radiate sound to the bath, but that radiation is rather ineffec tive. This is because the vessel is made of thin metal, usuall stainless steel, with a thickness of 0.4 mm, and the ultrasoni vibirations are transmitted in the form of bending waves (fo stainless steel of thickness 0.4 mm and at 40 kHz the wavelengt of the bending waves is 10 mm). These bending waves radiat poorly. The radiation efficiency is about 0.01. Therefore it i very difficult to obtain a high radiation intensity from such surface, whose contribiution to the total radiation is insig nificant. Rather, the radiation is concentrated to the contac surface of the transducer, where the intensity is high. Th result, as is well known, is that the transducer generates stron audible subharmonics. Their production is connected with the wa the ultrasonic transducer (normally containing one or mor piezoelectric ceramics) is made, and its attachment to th vessel. The subharmonics are therefore not produced fay th electrical power supply per se, but rather have an acousti cause.

Today there are two designs on the market, the most common i cylindrically symmetric and is attached to the vessel's under side. A conical front piece is glued to the vessel with the bas of the cone (typical diameter 45 mm) as the contact surface. Th other end of the cone (typical 35 mm) is attached to one or pair of piezoelectric ceramics of diameter 35 mm and thickness mm stacked on each other. On the back side of the piezoelectri ceramics there is a short cylinder (back piece), diameter 35 m and thickness 10 mm. This transducer is held together by a bol which goes through the center of the transducer. Electrode provide the piezoceramics with the voltage. The total thickne is 40 mm, and the transducer has a working frequency of 40 kH corresponding to a vibiration resonance in the transducer. this design the front piece is desiginated positively coned. Th transducer can transmit maximally about 50 watts of ultrasound the bath, according to the manufacturer. The probable reason f this limit is that the ultrasound must be transmitted through limited area (diameter 45 mm) to the bath, whence the intensi is high. When this intensity becomes to high a cavitation pill in the liquid outside the transducer is formed which effective limits power transmission ( see for example Theodor F Hueter Richard H Bolt, Sonics, Wiley, New York, 1955, page 232). If mo ultrasonic power is needed in the bath more transducers must used.

The other type of transducer is disc-shaped and contains piezoelectric ceramic in the form of a plate which is attached the vessel with its plane side as contact surface. Such transducer has a diameter of about 40 mm. Such transducers a very simple, but they are relatively inefficient, and they a therefore used mainly in applications where the desired ultr sonic power is low.

The acoustic bending wave wavelength in these vessel is about mm for frequencies around 40 kHz. The contact surfaces of t above transducers are therefore several bending wave wavelengt in size, and about as large as a wavelength in water at the frequencies. This means that the ultrasound that is generated the transducer is radiated out into the fluid through exact that surface so that the intensity of the ultrasound is appro imately the ultrasound power divided by the contact surface. Th also means that a "shadow angle" is formed, so that only th part of the bath volume that is within a calculable angle fr the normal (usually between 45 and 60 degrees) is irradiat directly with ultrasound. To get a strong ultrasonic field in t fluid the intensity in the fluid outside the transducer must high. It is exactly this high ultrasonic intensity that causes subharmonics and audible sound. High ultrasonic intensities should be avoided, partly because they impose a limit to the power that can be transmitted to the bath, and partly because they cause subharmonics and audible tones. The latter has been studied in detail by Werner Lauterborn et al (see for example Physics Today, January 1986, page S-4, and references therein).

The purpose of the invention and its main characteristics

The purpose of the present invention is to achieve a simple and inexpensive instrument for silent ultrasonic treatment, and which can transmit high ultrasonic powers to fluids. These goals have been realized by making the diameter or diagonal of the ultra- sonic transducer's contact surface against the vessel smaller than or equal in size to a bending wave wavelength in the vessel material and to a sound wave wavelength in the liquid. The transducer can be used at several different frequencies, either one at a time or in combination. The ultrasonic power in a flui in a vessel can be kept high while at the same time the ultra¬ sonic intensity at the limiting surfaces of the vessel remains relatively low. This means that subharmonics are absent and tha ultrasonic irradiation of an object in the fluid comes from man different directions simultaneously.

Description of the drawings

In the following the invention will be described in detail wit reference to an embodiment of the invention shown on the attache drawing.

Fig. 1 shows a section through an instrument for ultrasoni generation according to the invention.

Fig. 2 shows a perspective view of a transducer according to th inventio .

Fig. 3 is a diagram of measurement results from comparison tests. Description of an embodiment

The invention consists of the ultrasonic tranducer's conta surface's being limited to a very small area. By small is mea an area whose diameter is smaller than or of the same order magnitude as the bending wave wavelength is the vessel's wall and whose diameter is simultaneously also smaller than a sou wavelength in the liquid that is in the bath. For water and frequency of 40 kHz the sound wavelength is about 35 mm. Wi these limits one avoids having a radiating surface where t radiation efficiency can be near unity. This means in turn th the sound pressure cannot exceed the threshold value for su harmonic generation, and the bath is silent.

The ultrasonic energy must still be transmitted to the liquid the bath. This takes place through the above mentioned bendi waves in the vessel, which is assumed to be made of a th material, for example 0.4 mm stainless steel. The whole vess vibrates, which means that there is a radiating surface of abo 500 cm-' for one liter of liquid. With a radiation intensity only 0.1 watt/cm² there is still 50 watts of ultrasound in t bath. This radiation intensity is less than -5 % of the intensi that is reached in today's ultrasonic apparatus, not counti reflections, in spite of the fact that the ultrasonic energy the bath is the same, 50 watts/liter. The invention furth allows generation of the desired ultrasonic energy in a ba without concentrating the intensity to the contact surface of t transducer.

Ultrasonic powers of over 100 watts in one liter baths have be achieved without detectable subharmonics. Further, four su transducers have been applied to a larger bath ( 12 liters) a generated over 300 watts of ultrasonic power in that bath witho audible subharmonics. At the same time it has been possible measure the same cleaning efficiency in the one liter ba according to the invention as in conventional ultrasonic baths the same size. Using hydrophones it has been possible to show t presence of cavitation in both cases.

The instrument according to the invention consists of a housing 7 in which there is a vessel 8, for example made of stainless steel.

Attached to the bottom 9 of the vessel is a transducer 1 which will be described below in detail. A generator is designated 10.

The transducer according to the invention is shown one embodiment in figure 2. Such a transducer 1 can for example consist of a front piece 3, one end of which is shaped as a truncated cone 2 with a concave surface .2a. The top surface of the cone or its contact surface 2b against the vessel has an appropriate diameter of 7 mm. The front piece 3 made of aluminium, which for a metal has a relatively low acoustic impedance, and has a total length of 25 mm. The conical part 2 of the front piece has a length of 11 mm. The conical shape should show a continual transitio between the front piece's base diameter (25 mm) and the contac surface 2b (7 mm). The back piece 6 of the transducer is made of steel, which for a metal has a relatively high acoustic im¬ pedance. It is 24.9 mm long. Between the front piece and the bac piece there is a piezoelectric ceramic 5 (thickness about 2 mm) and an electrode 4. For safety reasons there is a thin cerami disc placed between the electrode and the front piece, in orde to insulate it electrically. Electrical current with the desire frequency is coupled to the transducer via the electrode 4 an the back piece 6. It is also possible to have a transducer wit two piezoelectric ceramics stacked on each other and tw additional electrodes.

The whole transducer is glued together with heat cured epox glue, in contrast with most conventional transducers which ar held together by central bolts. Experience shows that the glu works well and simplifies production. The contact surface 2b o the transducer is glued to the steel vessel which can have a wal thickness of 0.4 mm. In this embodiment the three lowes vibration resonances of the transducer are about 45 kHz, abou 100 hKz, and about 170 kHz. One and the same transducer can used for any of these frequencies, and it can deliver at lea 100 watts of acoustic power to a water bath in the vessel 8.

Naturally the transducer can have other dimenesions, depending which frequensies are desired to be generated and what powers a required. The transducer does not need to be glued togeth either, but rather it can be bolted together. Furthur it possible to replace the different materials (aluminum and stee with other materials if such is desirable in a certain appl cation. It is not necessary to use a piezoelectric material, a magnetostrictive materials or other materials can be used. T important thing is that the transducer's contact surface 2b smaller than or of the same order of magnitude as the bendi wave wavelength in the vessel or the sound wavelength in t fluid that is to be excited by the ultra sound.

A 0.6 mm thick ceramic disc is placed between the front piece a the electrode to insulate the front and vessel electrically fr the electrical system. Naturally materials other than cerami can be used, and other thicknesses. It can also be advantageo to use the ceramic disc (or its equivalent) in a mechanic support system for the transducer as a whole. As is know mechanical ^• contacts with the ultrasonic transducer should avoided as they can disturb the resonanse picture and take aw ultrsound in an undesirable manner. A way 'of achieving mechanical contact which does not have these deficiencies is attach the mechanical support at or near a velocity node on t transducer. The ceramic disc is near such a node and c therefore be used as a support point without disturbing t transducer's function. Today's ultrasonic transducers f cleaning purposes can transmit only 50 watts of ultrasound the bath from each transducer. This has to do with the design the transducer. The ultrasonic transducer according to t invention is not affected by the same limit, but rather it h been shown to be able to transmit more than 100 watts ultrasound to a corresponding bath. Further it is possible measure how fast contaminants are removed from an object in ultrasonic bath. This cleaning is quicker in an ultrasoni instrument according to the invention. One of the reasons fo these good results can be development of cavitation (desirable i connection with cleaning) without subharmonics if the soun pressure is kept over the threshold for cavitation (one atmos phere sound pressure or one watt/cm² intensity) and under th threshold for subharmonics (at least two atmospheres soun pressure or at least 4 watt/cm² intensity) . The invention make exactly that possible.

In figure 3 measurement results for the transducer according t the invention are shown. The rate at which a fingerprin disappears from a titanium surface when it is treated wit ultrasound (about 40 watts in 0.9 liters of water at 25°C, o equivalent powers in larger baths) is shown. The^" vertical axi shows the percentage of the fingerprint that has disappeare (measured electrochemically using the Galvani potential, see fo example D H McQeen, "Electrochemical evaluation of ultrasoni cleaning: the Galvani potential", Ultrasonics 24, 49 (1986)), an along the horizontal axis the time is shown. Curve "a" has bee obtained with a conventional apparatus which works at a frequenc of 47 kHz and which generates strong audible tones. The othe curves have been obtained with silent ultrasonic instument according to the invention. The curve marked -"b" is for 45 kHz the curve marked "c" is for 100 kHz; the curve marked "d" is fo 170 kHz." The highest curve, marked "e", is for a combination o 45 kHz ultrasound (half the power) and 170 kHz ultrasound (hal the power), with a total ultrasonic power equal to the ultrasoni power that was used for the three other measurments according t the invention. Similar curves have been obtained for industria grinding paste instead of fingerprints. The process id quicke when higher temperatures and better solvents are used.

The reason that conventional apparatus show poorer cleanin efficiency is that the ultrasound is radiated into the bath fro a limited surface defined by the transducer's contact surface and a shadow angle is formed. The object to be cleaned i irradiated mainly from a single direction. The other curves, fo the instrument according to the invention, show the increasi cleaning efficiency. As the frequency is increased fast cleaning is achieved, in accordance with the theory that w presented by McQueen (Ultrasonics 2A_, 273 (1986)). The highe curve, which is a combination of low and high ultrason frequencies, should give a result corresponding to an average those two frequencies. That is not the case, and instead unexpected higher result is obtained. The same good result obtained with industrial grinding paste on titanium surface. T transducer according to the invention is especially well suit to make use of this positive result, as it can be used at two more frequencies at the same time. Naturally it is appropria that the frequencies be different, for examle one frequen should be at least fifty percent higher than the other. This do not necessarily exclude chosing 20 kHz and 30 kHz, but it clear from figure 3 that it is better touse one frequency betwe about 30 kHz and 50 kHz and the other frequency over about 1 kHz. It should be remembered that higher frequencies can be mo efficient at cleaning, and that higher frequencies can penetra into tight spaces, in precision mechanical devises, for exampl better than lower frequencies can.

According to a modified embodiment the transducer according the invention is equipped with a front piece which has more th one narrowed part and also several contact surfaces. T transducer that has been described above has only one narrow part toward the vessel. The limited, small contact surface can a mechanically weak point, because it is relatively easy to bre the transducer away from the vessel at that point. If one and t same transducer is provided with two or three or more su contact points the whole transducer will be more stable mechan cally. Of course each point must have a contact surface which less than a water wavelength and a bending wave wavelength size.

Of course the invention is not limited to the examples above, b can be varied within the frame of the principles that have be mentioned. Further, the application of the invention is n restricted to cleaning solid objects in a liquid bath, fo instance precision mechanical devices, electrical and electroni components (circuit boards,ceramic substrates, silicon wafers, integrated circuits, etc), optical components (lenses, filters, fiber optics, etc), even if ultrasound has been shown to b excellent for cleaning surface mounted circuits. The inventio can advantageously be used in process industries, for exampl electrochemical processes such as electroplating, biochemica processes such as cell growing, catalytic processes such a sewerage cleaning, separation processes such as ultrafiltratio and chromatography, leaching processes, etc. Further th frequency regime which is intended here is not restricted eithe toward lower or toward higher frequencies. The transducer's powe can be increased to several hundred watts or reduced to a fe watts, for example by changing its diameter.

Claims

1. Apparatus for exciting a liquid in a vessel by ultrasound f cleaning and/or penetration of an object at least partial immersed in the liquid, and including at least one ultrason transducer (1-6) arranged in contact with the vessel, c h a r a^'c t e r i z e d b y , the diameter's or diagonal's of the ultrasonic; transducer contact surface (20) against the vessel being smaller than equal to the bending wave wavelength in the vessel material a the sound wavelength in the liquid.

2. Apparatus according to claim 1, c h a r a c t e r i z e d fa y , the ultrasonic transducer's being arranged to produce a co bination of high and low frequencies, either superomposed fr the same transducer or different frequencies from differe transducers.

3. Apparatus according to claim 1 or 2, c h a r a c t e r i z e d fa y , the end part's of the transducer's (1) front piece (3) connect to the vessel (7) being shaped like a truncated cone (2),pyrami obelise or the like with a concave surface (2a), the top surfa (2b) of the cone being the contact surface against the vessel ( which is connected to the contact surface by a joint, appropri tely a glue joint.

4. Apparatus according to claim 3, c h a r a c t e r i z e d fa y , the front piece's (3) being shaped to include several truncat cones (2) where each top surface is connected to the vessel ( and where each top surface/contact surface fulfills the chara terizating part of the conditions in claim 1.