MadSci Network: Physics
Query:

Re: What would be the effect(s) of putting quartz crystal(s) into a microwave?

Date: Wed Jun 18 21:59:24 2003
Posted By: Adrian E. Popa, Laboratory Director Emeritus
Area of science: Physics
ID: 1052322867.Ph
Message:


Greetings:

Reference: Reference Data For Radio Engineers,
Howard W. Sams & Co., NY, Any edition

Quartz is one of several forms of silicon dioxide molecules
(one atom of silicon bonded to two atoms of oxygen). The silicon
dioxide molecules form in two different types of structures,
amorphous and crystalline.

One of the most common forms of silicon dioxide is beach sand
which is composed of many small lumps of silicon dioxide fused
together in a random manner. If we heat sand to a temperature
above 1300 degrees Celsius, it will form a transparent glass.
In the glass the silicon dioxide molecules are attached to each
other in a random manner forming a three dimensional solid
structure that is technically called amorphous.

If we melt and cool silicon dioxide under very special conditions
in the laboratory we can grow a single crystalline form of
silicon dioxide that we call quartz. In quartz crystals all of
the molecules are aligned and bonded together in a regular three
dimensional tetrahedral structure forming a very hard, transparent
material with special electronic properties.

One of the properties that quartz crystals exhibit is called
the piezoelectric effect which is useful in many electronic
devices. Piezoelectric materials will generate a voltage across
the crystal if they are subjected to mechanical pressure (force).
Conversely, if a voltage is applied across a piezoelectric
crystal a mechanical movement of the crystal structure occurs.

A quartz crystal microphone reacts to the mechanical pressure
(force) from sound waves hitting a crystal. This mechanical force
causes a piezoelectric voltage to be generated across the crystal
that changes at the sound frequency.

In a quartz crystal tuned radio transmitter, a voltage from an
oscillator circuit is applied across the crystal causing the
quartz crystal to mechanically vibrate at a specific radio
frequency in a manner similar to a tuning fork used at audio
frequencies. However, the quartz crystal can vibrate at frequencies
up to one hundred million vibrations per second (one hundred megahertz).
The vibration frequency of quartz crystals are determined by the
physical dimensions of the crystal (length, width and thickness).
Quartz crystal frequency filters are also used in radio and television
amplifier circuits.

The voltages generated by quartz crystal microphones are very small
and are measured in millivolts (thousandths of a volt). Thus audio
frequency amplifiers are required to boost the microphone voltage up
to drive speakers so that we can hear the sound picked up microphone.

To damage a quartz crystal we would have to subject it to a great deal
of pressure or heat it to a very high temperature (greater than 1300
degrees centigrade). Radio waves, microwaves and laser waves pass through
quartz very efficiently so that a microwave oven would not heat a quartz
crystal enough to damage the crystal. However, the microwave oven could
damage itself with only a quartz crystal inside of it because the ovens
should always have some form of water in them to load the microwave
transmitter tube. Microwave oven manufactures suggest that you place
a cup of water in an oven when you want to test it.

Different minerals absorb microwaves differently; however, it is the water content
of the mineral that causes the greatest heating.

Caution: Many rocks and crystals have water trapped in pockets inside
of them and if they are placed in a microwave oven they might explode
from the steam pressure generated within them, just as popcorn pops.
However, exploding rock splinters can be much more dangerous to eyes
and skin and can also damage the microwave oven
.

Best regards, Your Mad Scientist
Adrian Popa


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