| MadSci Network: Engineering |
Hi Gary!
I've taken apart an electric igniter before, but I don't know if it's exactly the same
as the one you have. It has a long red pushbutton mounted in a black plastic case. When you push it in, a
spring is compressed, and when the button is all the way in, there is a "thunk" sound, and a spark is
created between the tips of two attached wires.
The "active ingredient" in this device is a small, brownish-gray cylinder held between two aluminum parts. It
is composed of a ceramic material called "PZT", meaning "Lead Zirconate Titanate" (or sometimes Barium Titanate, BaTi,
is used instead.) As you suspect, this material is "piezoelectric", meaning that whenever we squeeze it, it can create high voltages and produce
a separated electric charge.
I don't really understand the mechanical parts of the sparker button. It appears that a spring is first
compressed and then suddenly released. The released spring somehow causes a hammer action, and this gives the
PZT ceramic cylinder a sharp whack. The PZT cylinder is briefly but strongly compressed, and the Piezoelectric Effect causes a large
electrical potential (high voltage) to appear between its ends. Two aluminum parts are held against the ends of the PZT
cylinder, and these parts are electrically connected to the outside wires. When the PZT cylinder is struck, a
positive imbalance of electric charge appears on one end of the cylinder, and negative one on the other end. The voltage
pulses appear on the wires, and this creates a spark at the place where the tips of the wires are held very
close together. The voltage pulse is large, but considering the 5mm length of the spark, it is probably below
5000 volts.
PZT is a particular type of substance which is not piezoelectric at the time
of its manufacture.
Each little bit of the PZT crystal does have a positive and a negative end, but the crystals are all scrambled up. On average, the positive and negative parts cancel out. To convert PZT into a piezoelectric material, it must be heated up and exposed to a strong voltage field (electrostatic field.) This causes the positive and negative electrical patterns in all the crystals to change. The patterns align themselves with the voltage field. The PZT is then cooled, and the alignment voltage is removed. Does this sound a bit like magnetizing a magnet? Yes, but we are dealing with electric fields here, not magnetic fields. Magnets are "magnetized," and they carry a magnetic field. PZT crystals are "poled," they are forced to take on an internal electrical alignment so that they
have positive and negative "poles," and they carry an electric field.
Here's a crude diagram of what happens when PZT is "poled":
+ - + - + - - + - + + - + - + - - + - +
- + - + - + + - + - - + - + - + + - + -
+ - + - + - - + - + + - + - + - - + - +
- + - + - + + - + - - + - + - + + - + -
One tiny PZT crystal before the polarizing voltage is applied
(Notice that the polarity at each end of the crystal is mixed up.)
+ - + - + - + - + - + - + - + - + - + -
+ - + - + - + - + - + - + - + - + - + -
+ - + - + - + - + - + - + - + - + - + -
+ - + - + - + - + - + - + - + - + - + -
The same tiny PZT crystal after it has been permanently "poled."
(Notice that one end of the crystal is positive, the other is negative.)
OK, so the PZT acts like it is permanently charged. What about the squeezing?
Good question. After PZT has been permanently polarized, opposite charges are always attracted
out of the environment, and they serve to cancel out the positive and negative charges on the two
ends of the ceramic.
The charges on the ends of the crystal seems to vanish. However, there is still a very strong
electrical pattern inside the material. If the ceramic is now squeezed, this changes the electrical
pattern slightly, and it becomes either weaker or stronger.
This causes the amounts of charge on the ends of the ceramic to be imbalanced once again. Imbalanced
charges are "static electricity." The regions of opposite charge will seek each other out and flow together again, and this
creates the small spark which ignites the gas grill.
PZT ceramic can also be used to convert electrical fields into vibration. In theory, if you could
apply a huge pulse of voltage to your electric grill sparker, the PZT cylinder would act like a
hammer and give your thumb a painful whack. This effect is used in many products: electronic
beepers in watches and cellphones, high-frequency "tweeter" loudspeakers, in the transducers
in ultrasonic cleaners, plastic welders, and "cool mist" humidifiers, and in all sonar systems
used on ships and submarines.
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