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The
igniter consists of two portions:
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the
base (20) |
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and
the heater (22) |
These
two portions form a cylindrical igniter heating element that is
very distinctive in the low voltage variants, being thicker in diameter
at the base tapering to a thinner diameter heater portion at the
tip. Because the heater portion has a lesser diameter than the base
portion there is a higher resistance in the heater portion, and,
consequently, a higher heat output. Thus, heating of the igniter
is concentrated in the heater portion.
The
base is formed of five layers of ceramic and conductive material
and the composition of the layers differs, particularly in the amount
of conductive material (MoSi2). In this way the electrical conductivity
of the different layers can be controlled.
Starting
at the centre, the base portion consists of an inner electrically
conductive core (24), an electrically conductive layer (26), an
electrically insulative layer (28), an electrically resistive layer
(30) and an outer electrically conductive layer (32).
The
base portion also includes a hole (34) that permits connection to
an electrical lead (not shown) when the igniter is assembled. For
the purposes of description, the conductive layer and resistive
layer are differentiated. However, these two layers have similar
characteristics, and any heating ascribed to the resistive layer
can be equally well accomplished in the conductive layer.
The
heater portion is formed of three layers of ceramic material. Beginning
again at the innermost layer, the heater portion consists of a conductive
layer (26), insulative layer (28) and resistive layer (30). The
distal end of the heater portion is formed into a tip (36) that
forms an electrical connection between the conductive layer (26)
and resistive layer (30).
Si3N4
is the ceramic material that forms the various layers. This non-conductive
ceramic material is doped with MoSi2. The percentage concentration
of the conductive component, in conjunction with the layer thickness,
determines the resulting conductivity of the ceramic material. A
sintering additive up to 10 percent by volume can also be included.
The sintering additive includes yttrium, magnesia, calcium and others
of the Lanthanide group of elements. Both conductive and non-conductive
components are supplied as finely ground particles. The particles
can range in size from about 0.2 to about 0.8 microns. These components
are mixed and suspended in a solvent, such as water, to form slurry.
A suitable deflocculant, such as ammonium polyacrylate, is also
added.
The
igniter is manufactured by slip casting. This process uses an absorbent,
tubular mold, open at both ends. The mold can is fabricated from
plaster of paris or any other suitable absorbent material.
Generally,
successive layers of the igniter are added to the mold from the
tip end. The method commences by laying down the outer layer, and
then forming the resistive layer. Next, the insulative layer (28)
is formed in the mold. Finally, the conductive layer (26) is formed.
The inner core (24) is then injected into the mold from the opposite
end so that it extends substantially the length of the base portion.
The connecting hole (34) is formed in the inner core (24) at this
time. To form an integral electrical connection between the conductive
layer (26) and the resistive layer (30), the tip of the green body
is reformed by applying low intensity vibrations from an ultrasonic
wand to the tip before the green body is removed from the mold.
These vibrations cause the particles at the apex to be blended into
an electrically conductive tip. Once the liquid phase has been substantially
absorbed through the walls of the mold, the green body is removed
from the mold and allowed to air dry. Sintering of the green ceramic
body takes place in vacuum furnaces using graphite heaters for 24
hours.
The
igniters are cleaned and polished and all sprues are removed using
diamond cutters. In operation, an electrical potential when applied
causes a current to flow through the conductive inner core to conductive
layer. The current then flows through resistive layer at the exterior
of heater portion, and returns along outer layer.
As
the current flows through the resistive layer in the region of the
heater portion, it heats to a temperature sufficient to ignite many
different fuels both gaseous and liquid. The igniter elements are
capable of repeated cycling to temperatures in the range of 1500C.
without failure. The high conductivity of outer layer results in
little current flow through the resistive layer in the base portion,
and therefore concentrates the heat in the resistive layer of the
heater portion.
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