What is ultrasonic?
Cleaning an element means removing all contaminants from its surface. This operation is usually obtained by using a combination of a chemical action, with detergents, and a mechanical action (such as rubbing).
Adding high intensity acoustical waves to a liquid medium will increase the mechanical action in the medium. An acoustoelectronic transducer, similar to a loudspeaker, is capable of transforming electrical energy into sound energy in elastic mediums such as air. Similar acoustoelectronic (ultrasonic) transducers are used to generate high energy sounds in liquids with frequencies above the upper range of human hearing.
In order to understand how an acoustical ultrasonic wave can create a cleaning action in a tank, we can use the example of the drawing below that shows a container with metallic walls, containing liquid, and an electro-mechanical transducer in the bottom that is capable of emitting a specific acoustical frequency.
The acoustical wave propagates into the liquid, at the speed of sound, which is approximately 1500 metres per second in water.
This phenomenon makes the molecules in the liquid vibrate thus creating pressure variations all around them, that in turn make adjacent molecules in the liquid move and so on.
If we consider what is happening at one specific point in the tank, we can see that the molecules vibrate with a frequency that is the same as the one introduced by the transducer, and in the same way that the instantaneous value of the pressure oscillates at the same frequency.
The average pressure value will be the same as the value when there is no acoustical wave, i.e. the value of atmospheric pressure added to that of the volume of water while the instantaneous pressure value oscillates between a minimum value and a maximum value.
We recall that the physical state of a liquid or vapour depends on temperature and pressure. For example, water boils at 100 °C when is there is 1 Bar of atmospheric pressure, but turns into steam at a lower temperature if the pressure is sufficiently low.
If the intensity of the acoustical wave is sufficiently large, when the pressure reaches the critical value, a vapour bubble is created, and continues to increase in size whilst storing up potential energy (how much depends on the length of time Tb).
At the end of this infinitesimal moment, when the pressure increases again, the vapour state is no longer possible and the vapour bubble explodes on a very small point that is virtually invisible and thus releases the stored energy.
Even though it is very small, the implosion of a single bubble creates a very high upsurge of energy, due to the fact that it bursts in a split second.
As a metaphor, we can use the example of a hammer that is capable of storing up energy and transmitting it in a split second in order to produce enormously expanded pressure values.
What frequency should we choose for the acoustical wave?
Based on what we saw above, we can conclude that the intensity and the amplitude of the energy upsurge created by each vapour bubble when it explodes, due to a phenomenon called cavitation, is calculated according to the intensity of the acoustical wave that is applied and the time Tb that the cavitation bubble grows. Tb dimishes when the frequency increases and viceversa.
In order to make use of the majority of the energy created by each bubble, we will thus use the lowest possible frequency: the lower limit consists of the highest frequency audible to man, which is around 16 kHz.
For some applications such as mould cleaning, the best operating frequency is around 19 kHz.
Piezoelectric Acoustoelectronic Transducers
In order to make a transducer, a tool that can transform electrical energy into mechanical energy and in consequence also into acoustic energy, we will discuss here one particular type of technology.
This technology uses the characteristics of certain ceramic materials that modify their internal elastic tension and thus their shape, when an electrical field is applied to them.
The elements used to create ultrasonic acoustic waves normally have the shape of a ring or more precisely a disk some millimetres thick and a few centimetres in diameter. There is a hole in the middle that means that a fixing bolt can be put through it without touching the disk itself.
Each piezoelectric transducer is made up of several parts as shown on the diagram below.
Each transducer is made up by joining two piezoelectric ceramics, one under the other, between the two aluminium and steel parts and fixed with a bolt. The assembly is designed to make a metallic structure with its own oscillating frequency that is similar to that of the required acoustic wave.
In this way we obtain a resonant system that is capable of amplifying the range of movement of the piezolectrical ceramic surfaces, when an alternate magnetic field is applied to them, normally with several hundred volts of tension and whose frequency coincides exactly with the mechanical resonance.
Each transducer is designed to be capable of creating 50 W of ultrasonic power. In order to make a diaphragm that radiates 600 W of power as the drawing shown above, it is necessary to apply 12 elements, wired up in parallel.
Why do we need two piezoelectric ceramics?
Because the bolt, that is necessary in order to hold the two parts firmly together, is made of metal and would create a short-circuit between the two surfaces of the ceramics.
If we use two ceramics, taking care to put two equal polarities opposite each other, we solve the problem.
Why is the steel on the far side away from the tank wall and the aluminium on the side nearest the tank wall?
Because we want to obtain optimum transfer from the ceramics to the tank and, conversely, to reduce the transfer to the air on the other side to a minimum.
By using aluminium, we optimise the adaptation of the acoustic impedance between the liquid and the ceramics.
On the contrary, with steel, we ensure a maximum reflection in relation to the air.
How do we obtain the polarisation of piezoelectric ceramics?
Manufacturing of ceramics involves formulation, correct proportioning and mixture of the different components.
We start off with a special type of sand that is compressed into specific moulds that reproduce the final shape. They are placed in an oven at high temperature for a long period in order to crystallize them.
The normal ceramic appearance is thus obtained. Next we carry out a series of mechanical operations such as correcting and metal-plating the flat surfaces.
The last phase consists of the polarisation process: applying a constant electrical charge of several hundred volts to the electrodes (flat metallic surfaces) of the ceramic whilst it is immersed into a diathermal oil bath at a temperature of several hundred degrees centigrade.
Finally, we leave the ceramics to cool whilst still supplied with electricity.
At this stage, the ceramic is polarised.