Into the world of Piezoelectric Ceramics

 Good morning! If your world’s anything like mine, ceramics just woke you from a peaceful sleep. Chances are, your clock or clock radio has an alarm buzzer made from an advanced piezoelectric ceramic. This unusual ceramic vibrates with a loud noise when electricity is applied. If your clock has a quartz mechanism, as most clocks and watches now do, a tiny slice of vibrating piezoelectric quartz ceramic is the timekeeper. However, in this case, the vibrations are so rapid and small that you can’t hear them. Most people have never heard of piezoelectric ceramics, but piezoelectrics are the secret behind a wide variety of products ranging from underwater sonar (submarine searchers) to medical ultrasonic scans to “smart” skis.

What are Piezoelectric Ceramics?  

 

Piezoelectric ceramics are known for what are called the piezoelectric and reverse piezoelectric effects. The piezoelectric effect causes a crystal to produce an electrical potential when it is subjected to mechanical vibration. In contrast, the reverse piezoelectric effect causes the crystal to produce vibration when it is placed in an electric field. Of piezoelectric materials, Rochelle salt and quartz have long been known as single-crystal piezoelectric substances. However, these substances have had a relatively limited application range chiefly because of the poor crystal stability of Rochelle salt and the limited degree of freedom in the characteristics of quartz. Later, barium titanate(BaTiO3), a piezoelectric ceramic, was introduced for applications in ultrasonic transducers, mainly for fish finders. More recently, a lead titanate, lead zirconate system(PbTiO3⋅PbZrO3) appeared, which has electromechanical transformation efficiency and stability(including temperature characteristics)far superior to existing substances. It has dramatically broadened the application range of piezoelectric ceramics. When compared which other piezoelectric substances, both BaTiO3 and PbTiO3⋅PbZrO3 have the following advantages:

        1.High electromechanical transformation efficiency. 

        2.High machinability. 

      3.A broad range of characteristics can be achieved with different material compositions (high degree of freedom in characteristics design). 

       4.High stability. 

       5.Suitable for mass production, and economical.

 Properties of Piezoelectric Ceramics

Piezoelectric ceramics are a type of multi-crystal dielectric with a high dielectric constant and are formed by two processes : first, high temperature firing. After firing, they have the characteristic crystal structure shown in Fig. (a) but do not yet exhibit the piezoelectric property because the electrical dipoles within the crystals are oriented at random and the overall moment of the dipoles is canceled out. To make ceramics piezoelectric they must be polarized. A DC electric field of several kV/mm is applied to the piece of ceramic to align the internal electrical dipoles in a single orientation (see Fig. (b)). Due to the strong dielectric property of the ceramic, the dipole moment remains unchanged after the electric field is removed, and the ceramic thus exhibits a strong piezoelectric property (see Fig. (c)). When an AC signal is applied to a piezoelectric ceramic (piezoelectric transducer) in a frequency matching the specific elastic frequency of the ceramics (which depends on the shape of the material), the ceramic exhibits resonance. Since the ceramic has a very high electromechanical transforming efficiency at the point of resonance, many applications use this resonance point. Also piezoelectric ceramics when molded in certain shapes have more than one point of resonance depending on vibration mode. In such a case, the vibration mode most suited for the application is selected.

 

       Quartz Watches

     Perhaps the most common piezoelectric application is in the quartz watch. A quartz watch contains a small slice of quartz crystal, cut in just the right way to make it piezoelectric. Connected to the battery in the watch, the slice of quartz vibrates at a constant frequency (about 80,000 vibrations per second). This information feeds into a silicon chip microprocessor that then controls a digital readout. The quartz watch accurately measures time and is much more dependable than old-fashioned mechanical watches with their springs, levers, gears, and bearings.

        Noisemakers and Beautiful Music

 If you hum you can feel the vibrations on your Adam’s apple. Sounds you hear are simply vibrations. Vibrations with the peaks of the waves close together (short wavelength, high frequency) are high pitched, like the sounds made by an opera  singer reaching high C or the notes that result from the keys on the far right of a piano keyboard. Vibrations with the peaks of the waves farther apart are lower pitched, like those made by the cello in the orchestra rather than the violin. How do we get different pitches of sound? In stringed musical instruments, sound is controlled by the diameter and length of the string. Small-diameter, short, tightly strung strings produce high notes. Strings that are bigger in diameter, longer, and strung less tightly produce lower notes. In bells, chimes, or tuning forks, the pitch is controlled by the size of the instrument itself. For example, if you’ve ever listened to a bell choir, you know larger bells produce lower sounds.

 

Engineers have learned to produce a complete range of sounds with piezoelectric ceramics by carefully controlling the size and shape of the ceramic piece and the way electricity is applied to it. In one case, they’ve designed the ceramic to give off the jarring sound of an alarm clock to make sure we don’t roll over and go back to sleep, or the harsh sound of a smoke alarm, burglar alarm, or seatbelt buzzer. In another case, they’ve designed the ceramic to produce a clear, pure musical sound that can rival that from the best musical instruments. By making a separate ceramic piece for each note in the musical scale and having a separate electrical switch for each, designers can create the sound of a musical instrument or a whole orchestra. This idea has even been miniaturized to make possible musical greeting cards and toys that require only a small battery.

Piezoelectricity seems stranger than truth. Ceramics that can convert from electricity to pressure and vice versa have introduced the world to microphones and phonographs, to sonar and fish finders, and to a nondestructive way of seeing inside solid objects. They help us make integrated circuits, act as the timekeepers in our watches, and give us advanced warning before costly machines self-destruct .