WEBVTT

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This is a crystal of sugar.

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If you press on it, it will actually
generate its own electricity.

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How can this simple crystal 
act like a tiny power source?

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Because sugar is piezoelectric.

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Piezoelectric materials 
turn mechanical stress,

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like pressure,

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sound waves,

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and other vibrations

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into electricity and vice versa.

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This odd phenomenon was first
discovered

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by the physicist Pierre Curie 
and his brother Jacques in 1880.

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They discovered that if they compressed
thin slices of certain crystals,

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positive and negative charges would appear
on opposite faces.

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This difference in charge, or voltage,

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meant that the compressed crystal
could drive current through a circuit,

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like a battery.

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And it worked the other way around, too.

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Running electricity through these crystals
made them change shape.

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Both of these results,

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turning mechanical energy into electrical,

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and electrical energy into mechanical,

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were remarkable.

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But the discovery went uncelebrated
for several decades.

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The first practical application
was in sonar instruments

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used to detect German submarines
during World War I.

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Piezoelectric quartz crystals 
in the sonar's transmitter

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vibrated when they were subjected
to alternating voltage.

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That sent ultrasound waves
through the water.

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Measuring how long it took these waves
to bounce back from an object

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revealed how far away it was.

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For the opposite transformation,

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converting mechanical energy 
to electrical,

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consider the lights that turn on
when you clap.

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Clapping your hands send sound vibrations
through the air

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and causes the piezo element to bend
back and forth.

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This creates a voltage that can drive
enough current to light up the LEDs,

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though it's conventional sources 
of electricity that keep them on.

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So what makes a material piezoelectric?

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The answer depends on two factors:

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the materials atomic structure,

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and how electric charge
is distributed within it.

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Many materials are crystalline,

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meaning they're made of atoms or ions

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arranged in an orderly 
three-dimensional pattern.

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That pattern has a building block
called a unit cell

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that repeats over and over.

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In most non-piezoelectric 
crystalline materials,

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the atoms in their unit cells
are distributed symmetrically

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around a central point.

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But some crystalline materials
don't possess a center of symmetry

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making them candidates 
for piezoelectricity.

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Let's look at quartz,

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a piezoelectric material 
made of silicon and oxygen.

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The oxygens have a slight negative charge
and silicons have a slight positive,

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creating a separation of charge,

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or a dipole along each bond.

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Normally, these dipoles 
cancel each other out,

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so there's no net separation of charge
in the unit cell.

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But if a quartz crystal is squeezed
along a certain direction,

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the atoms shift.

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Because of the resulting asymmetry
in charge distribution,

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the dipoles no longer cancel
each other out.

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The stretched cell ends up
with a net negative charge on one side

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and a net positive on the other.

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This charge imbalance is repeated
all the way through the material,

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and opposite charges collect
on opposite faces of the crystal.

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This results in a voltage that can
drive electricity through a circuit.

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Piezoelectric materials can 
have different structures.

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But what they all have in common is unit
cells which lack a center of symmetry.

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And the stronger the compression
on piezoelectric materials,

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the larger the voltage generated.

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Stretch the crystal, instead,
and the voltage will switch,

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making current flow the other way.

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More materials are piezoelectric
than you might think.

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DNA,

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bone,

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and silk

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all have this ability to turn
mechanical energy into electrical.

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Scientists have created a variety
of synthetic piezoelectric materials

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and found applications for them
in everything from medical imaging

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to ink jet printers.

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Piezoelectricity is responsible for 
the rhythmic oscillations

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of the quartz crystals
that keep watches running on time,

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the speakers of musical birthday cards,

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and the spark that ignites the gas 
in some barbecue grill lighters

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when you flick the switch.

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And piezoelectric devices may become
even more common

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since electricity is in high demand
and mechanical energy is abundant.

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There are already train stations
that use passengers' footsteps

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to power the ticket gates and displays

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and a dance club where piezoelectricity
helps power the lights.

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Could basketball players running back
and forth power the scoreboard?

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Or might walking down the street
charge your electronic devices?

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What's next for piezoelectricity?