Triboelectric effect

تاريخ التسجيل : 22/11/2009

The triboelectric effect (also known as triboelectric
charging) is a type of contact electrification in which certain
materials become electrically charged after they come into
contact with another different material and are then separated (such as through
rubbing). The polarity and strength of the charges produced
differ according to the materials, surface roughness, temperature, strain, and
other properties.

Thus, it is not very predictable, and only broad
generalizations can be made. Amber,
for example, can acquire an electric charge by contact and separation (or friction)
with a material like wool.
This property, first recorded by Thales
of Miletus, suggested the word "electricity"
(from William Gilbert's initial coinage,
"electra"), from the Greek
word for amber, ēlektron. Other examples of materials that can acquire a
significant charge when rubbed together include glass rubbed with silk, and hard rubber
rubbed with fur.

Triboelectric series

Materials are often listed in order of the
polarity of charge separation when they are touched with another object. A
material towards the bottom of the series, when touched to a material near the
top of the series, will attain a more negative charge, and vice versa. The
further away two materials are from each other on the series, the greater the
charge transferred. Materials near to each other on the series may not exchange
any charge, or may exchange the opposite of what is implied by the list. This
depends more on the presence of rubbing, the presence of contaminants or oxides, or upon properties
other than on the type of material. Lists vary somewhat as to the exact order
of some materials, since the charge also varies for nearby materials.

Cause

Although the word comes from the Greek for
"rubbing", τρίβω (τριβή: friction), the two materials only
need to come into contact and then separate for electrons to be exchanged.
After coming into contact, a chemical bond is formed between some parts of the
two surfaces, called adhesion, and charges move from one material to the other to
equalize their electrochemical potential. This is what
creates the net charge imbalance between the objects. When separated, some of
the bonded atoms have a tendency to keep extra electrons, and some a tendency
to give them away, though the imbalance will be partially destroyed by tunneling
or electrical breakdown (usually corona discharge).
In addition, some materials may exchange ions of differing mobility, or
exchange charged fragments of larger molecules.

The triboelectric effect is related to friction
only because they both involve adhesion. However, the effect is greatly enhanced by rubbing
the materials together, as they touch and separate many times. For surfaces
with differing geometry, rubbing may also lead to heating of protrusions,
causing pyroelectric charge separation which may add to the existing contact electrification, or which may oppose
the existing polarity. Surface nano-effects are not well understood, and the atomic force microscope has made sudden
progress possible in this field of physics.

Because the surface of the material is now
electrically charged, either negatively or positively, any contact with an
uncharged conductive object or with an object having substantially different
charge may cause an electrical discharge of the built-up static electricity; a spark.
A person simply walking across a carpet may build up a charge of many thousands
of volts, enough to cause a spark one centimeter long or more. Low relative
humidity in the ambient air increases the voltage at which electrical discharge
occurs by increasing the ability of the insulating material to hold charge and
by decreasing the conductivity of the air, making it difficult for the charge
build-up to dissipate gradually. Simply removing a nylon shirt or corset can
also create sparks, and car travel can lead to a build-up of charge on the
metal car body (which acts as a Faraday cage).
When the driver alights, sparks jump from frame to driver as he makes contact
with the ground.

This type of discharge is often harmless because
the energy
((V²
* C)/2)
of the spark is very small, being typically several tens of micro joules in cold dry
weather, and much less than that in humid conditions. However, such sparks can
ignite flammable vapours, see risks and counter-measures.

In aircraft and spacecraft

Aircraft flying in weather
will develop a static charge from air friction
on the airframe. The static can be discharged with static dischargers or static wicks.

NASA
follows what they call the Triboelectrification Rule whereby they will
cancel a launch if the launch vehicle is predicted to pass through certain
types of clouds. Flying through high-level clouds can generate “P-static” (P
for precipitation), which can create static around the launch vehicle that will
interfere with radio signals sent by or to the vehicle. This may prevent
transmitting of telemetry to the ground or, if the need arises, to send a
signal to the vehicle, particularly any critical signal for the flight
termination system. When a hold is put in place due to the triboelectrification
rule, it remains until Space Wing and observer personnel such as those
in reconnaissance aircraft indicate that the skies are clear.

Risks and counter-measures

Ignition

The effect is of considerable industrial
importance in terms of both safety and potential damage to manufactured goods.
Static discharge is a particular hazard in grain elevators
owing to the danger of a dust explosion. The spark produced is fully
able to ignite flammable vapours, for example, petrol, ether
fumes as well as methane
gas. For bulk fuel deliveries and aircraft fueling a grounding connection is
made between the vehicle and the receiving tank prior to opening the tanks.
When fueling vehicles at a retail station it is always good practice^{[citation needed]} to touch both
metal on the delivery nozzle and metal on the vehicle (simultaneously) before
inserting the nozzle into the tank spout.

Dust attraction

Means have to be found to discharge carts which
may carry such liquids in hospitals. Even where only a small charge is produced, this
can result in dust particles being attracted to the rubbed surface. In the case
of textile
manufacture this can lead to a permanent grimy mark where the cloth has been
charged. Dust attraction may be reduced by treating insulating surfaces with an
antistatic
cleaning agent.

Damage to electronics

Some electronic devices,
most notably CMOS integrated circuits and MOSFET transistors,
can be accidentally destroyed by high-voltage static discharge. Such components
are usually stored in a conductive foam
for protection. Grounding oneself by touching the workbench, others, or using a
special bracelet or anklet is standard practice while
handling unconnected integrated circuits. Another way of dissipating
charge is by using conducting materials such as carbon black
loaded rubber
mats in operating theatres, for example.

Devices containing sensitive components must be
protected from normal use, installation, and disconnection, accomplished by
designed-in protection at external connections where needed. Protection may be
through the use of more robust devices or protective countermeasures at the
device's external interfaces. These may be opto-isolators,
less sensitive types of transistors, and static bypass devices such as metal oxide
varistors.

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Triboelectric effect

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