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At the outbreak of the first world war, ultrasonic have been used to localize submarines by catching ultrasonic waves fly-back. Between 1914 and 1918 the ultrasonic "sonar" has been used to localize enemy submarines.
Constantin Chilowsky, a Russian emigrated to Switzerland, and Paul Langévin, a French distinguished physicist, designed and built an efficient eco-resonant apparatus, and called it hydrophone.
In the first thirties, many French transatlantics have been equipped with eco-resonant detection systems to prevent underwater collisions. However true ultrasonic utilization in industrial applications have been developed at the beginning of second world war with production of ultrasonic analysers to reveal imperfections in metals, have been used particularly to check boat hull and tank shell.
Quartz, during the last world war, for its piezoelectric characteristics, has been declared as strategic material being used for technical applications related to military purposes and in radio transmissions. At the end of the war, the fast electronic development, allowed the design of many apparatus (based on ultrasonic properties) more and more sophisticated to be used for military, medical, industrial and domestic purposes. Further researches brought to the discovery of piezoceramic materials which allowed construction of ultrasonic generators robust, powerful and versatile. Ultrasonic waves, characterized by a short wavelength and high frequency, spread by straight beams with characteristics and conditions similarly to light waves and carry a certain quantity of energy.
Ultrasounds are elastic waves transmitted and propagate through materials, each propagating material have different absorbance index. In-fact, we will have an optimum transmittance through solid and liquid means but a bad propagation through air and gases especially at higher frequencies.
The ultrasounds waves characteristic of carrying high energy through liquids, has brought to application in industrial washing. The elements that allow ultrasonic formation are called transducers that can be piezoelectric magnetostrictor. Their job is to transform electrical energy in mechanical energy (and vice versa). The magnetostrictor transducer is based on the capability to generate vibration in a nickel leaf exposed to a strong magnetic field. They are shaped and are positioned in a coil, attached to a metallic support so that there is a single vibrating pack. This allows a distribution of the mechanical wave (generating ultrasounds), in a uniform way on all the surface of the transducer. High power magnetostrictor transducers, usually, do not exceed 22 Khz. Working frequencies around 20-22 Khz have a very intense cavitationing effect for which are non suitable for some light applications for example for electronic circuits and for soft metals as aluminium and polished brass (the surface of which become spotted), furthermore they irritate operators by reflected waves produced during extraction of objects from bath or shaking materials to be washed in ultrasonic bath: this is due to the fact that waves created by movement of object produced have an audible frequency by the human hearing. Voltage applied to the piezoelectric transducer creates an ultrasonic vibration the frequency if which is typical of that particular type piezoelectric component. In fact, it is mainly formed by a layer of quartz crystal or by a more modern disc of ceramics obtained by Titanate and Lead Zirconate hold in two metal blocks. The AC voltage applied to electrodes generate an alternated buckling creating a vibration of the crystalline synthetic plate. The piezoelectric disc increases or decreases its thickness in relation to the to the specific frequency that have been chosen to generate ultrasounds with frequencies over 20 KHz (one KHz corresponds to 1000 oscillations per second).
Metal blocks are generally stuck in steel flanges (for assembling appropriate resins are used) that are fixed to the walls or on the bottom of washing bath transmitting mechanical energy generated o simply dipping the ultrasonic transducer into the aqueous detergent solution of the bath. Usually, in industrial washing field, piezoelectric transducers are preferably used because are designed for much higher frequencies (up to 42 KHz which means soft powerful ultrasounds) if compared with transducers of magnetostrictive type which have a maximum is around 22 KHz). An ultrasonic cleaner is composed of an electronic generator emitting a frequency between 20 and 40 KHz, the signal is sent to one or more piezoelectric transducers where the electric pulse I changed in a mechanical vibration of the same frequency.
These mechanical vibrations emitted by the ultrasonic transducer are transferred to the washing bath creating ultrasonic pressures and depressions waves, generating micro bubbles of the liquid. During compression phase, micro bubbles get to minimum diameter allowed in size, then they implodes generating a mechanical effect in the detergent liquid. This quick implosion phenomena is called "ultrasonic cavitation". The shock waves during cavitation reach every zone of the liquid and the surface of objects immersed into the washing bath.
It is very important that the detergent liquid be capable of transmitting ultrasonic wave without excessive absorbtion, in this sway ultrasonic waves are spread in the detersive solution till the surface of the object to be washed. The obtained fine cleaning of the surface, (effective even in micro cavity or porosity of surface to be treated) is the caused by ultrasonic waves; this operation is achieved with better results only using a specific detergent for ultrasonic that would improves the cavitation process and develops maximum chemical reaction on dirt (each contaminant needs a different ultrasonic detergent) to be broken and have an absolute no chemical activity on the object. When ultrasonic beam hit a solid obstacle, it is reflected, deviated or diffracted behaving all common laws of waves propagation phenomena; it is absorbed when hitting a soft or porous object, this suggests us that frames and objects will be well washed when they will not have cavities containing air or plastic coatings adsorbing or attenuating ultrasounds.
Propagation of the ultrasonic wave generated and its consequent penetration into washing bath is depending by the associated energy and this is a function of the transmitting frequency of the piezoelectric material used. When a certain waves of a frequency generated by a source, hit a mobile surface, it is reflected with a frequency which is different by the original one. This variation may be over or under the value depending if the object is moving toward or away from the source.
The sound waves are increased in frequency if the obstacle is moving toward and decreased if it is moving away. The reflected ultrasonic waves have a slightly different frequency of incident waves and are out of phase from them, also because the path, from transducer This phenomenon is very useful for ultrasonic washing, obviously it will be useful that the system would be equipped with a way to keep objects moving to get the best results in ultrasonic washing. Often, modern systems have ultrasonic waves oscillating around the base frequency (20 or 40 KHz) to get rid of effects of stationary waves on steady objects to be washed in the liquid that may create zones with high energy concentrations called "waves knots" and others effect of minor concentration. This modulated technique maximises and makes uniform ultrasounds into washing liquid improving remarkably cleaning without the need to mechanically move the objects submitted to ultrasounds.
The immersion washing with ultrasounds, the two cleaning effects are added: chemical degreasing action made by the reagent and the mechanical action of ultrasound waves. This treatment has been found very effective in situations where pollutants are very strong such as residues from polishing, lapping, ecc.
To improve cleaning action the objects are placed in baskets which are rotated in the washing baths so that each object is exposed to action of ultrasounds. The frequency of ultrasonic generator is very important because it establishes the size of bubbles within the detergent liquid which is exposed to ultrasonic action. The higher is the frequency (40Khz) the less is the bubble size and higher is the number of bubbles generated, on the opposite, the less is the frequency (20 KHz) the higher will be the size of bubbles and the less will be the quantity.
On the other side higher frequencies allow a much higher number of bubbles generated in time unit., facilitating a better distribution of cavitation per area unit and "softer" washing effect of ultrasounds; the number of bubbles generated at 40 KHz is actually doubled with respect to the ones created by generators at 20 KHz, then, a system at 40KHz allow to reach very small points per surface units.
For a practical example, we can compare the high frequency fine cavitation to a suit brush very fine while the low frequency to a very rough brush for laundry with les contact points but stronger. Waves generated by ultrasounds are capable to reach each single point of washing bath colliding with objects submerged to be washed with ultrasounds; all residues of oil, grease, polishing pastes, colouring pigments, graphite, micro-dusts, and handling traces are totally removed and emulsionated by the detergent. Ultrasounds equipment, utilizing aqueous detergents instead of solvents for cleaning immersed objects, are more convenient because aqueous detergents remove also micro-powder and inorganic substances.
It is recommended for precise cleaning in various type of objects, and it is usable even in presence of crucial cavities as long and narrow channels, threaded holes, capillary tubes (this cavities are very difficult to reach). The objects are positioned in small baskets or on a frame and then immersed into the various stages of treatment. Ultrasounds con be used to clean any object with aqueous detergents instead of solvents; from large mechanical pieces like moulds for rubber, to very fragile parts like frame glasses, even organic or mineral lenses can be washed with aqueous detergents. Glass or plastic lenses (CR 39) are cleaned from cobbler's wax or cerium oxide with aqueous detergents having a single detergent.
Ultrasounds are suitable also to remove fluxsant from delicate electronic printed circuits. Then it is essential to know how to choose correct detergent that would be inert on the object and develop, at the same working temperature the best cavitation in ultrasonic washing baths. Optimum temperature is within between 50 and 70°C. Energy required to form a small cavitation bubble is proportional to the vapour pressure of the liquid and it is then affected by temperature e by the surface strength (this is lowered by suitable detergent which is used for ultrasonic washing).
Temperature of aqueous solution in an ultrasonic washing bath is very important; it is useful to say once more, that cavitation intensity varies with variation of temperature and with employed detergent. Cavitation intensity increases when the temperature increases, till roughly 70°C, then it decreases and to stop completely at the boiling point of liquid, because with boiling liquid the water vapours generated by cavitation, cannot be condensed by the washing solution and the micro-bubbles cannot implode, vice versa with a with solution too cold vapour creating micro-bubbles needs too much energy so that bubbles will have a lower quantity with limited mechanical output. Ultrasounds cannot be replaced if used with a suitable aqueous liquid detergent or a detersive to remove oils, greases, polishing pastes, lapping residues, filings, small shavings, dust, digital prints and other polluters during production and maintenance of objects, even if they show complex surfaces with small holes. Besides the aqueous detergent must exalt cavitation and not to depress it, it has to be totally soluble in water and possibly be formulated with surfactant substances with very low surface strength \and a chemical stability at working temperature.
Another parameter to be considered is the nature of material of the object to be washed; this is conditioning the chose of detergent to be employed. Detergent must have an appropriate pH to avoid attack to washing container and to the surface of objects (attack can be exalted by temperature and by ultrasounds).
When detergent solution has been freshly prepared, turning on the system, ultrasounds emitted provoke, at beginning, degassing of detergent solution, that is separation and elimination of gases in the solution separating them from liquid forming bubbles that comes up to the surface. Presence of gases into water (CO2, O, N), reaches consistent values and, initially make elastic aqueous detergent, attenuating mechanical energy of ultrasounds. When gas separation is ended, ultrasonic transducer, are in the best conditions, capable to totally transmit energy on the objects to be washed with ultrasounds.
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