Hot Surface Igniters

PRESENTED BY: Joe Barker Brent Blume Sam Alauddin

This is an example of the series of igniters developed by the Norton Corporation which incorporates an alumina base with a recrystallized silicon carbide heating element.

Ceramic Materials in the Igniter SiC Al 2 O 3

How the Igniter Works The electricity runs through the leads into the alumina base. The leads are soldered onto the SiC element. The contact point of the leads and SiC are designed to have a low resistance, keeping the base cool and avoids overheating. The current is then passed through the SiC body, which has a high resistivity, and is diminished by the time it reaches the center of the part. Once the maximum temperature is attained, the gas is released and lighted by the extremely hot surface of the igniter. The igniter then shuts off allowing for the elimination of a constant pilot light.

Design Specifications

Material Choices and Limitations »SiC The reason for choosing SiC is for its high resistance to chemical corrosion, high temperature resistance, and high thermal shock. The downside is the upper temperature limit is set by the oxidation of SiC at high temperatures given by the decomposition equation: SiC(s) + O 2 (g) -> SiO(g) + CO(g) »Al 2 O 3 Since the alumina serves only a minimal function as compared to the SiC, the major properties are high strength and temperature resistance, with its main function being served as a good electrical insulator and having low dielectric loss.

Of what material is the igniter composed? Silicon Carbide (SiC) is a common non-oxide ceramic heating element material. The ceramic crystal is not cubic (zinc blende) but hexagonal (Hemi-hedral class). SiC is known as a wide bandgap semiconductor Existing in many different polytypes. The difference between polytypes is the stacking order between succeeding double layers of carbon and silicon atoms. This affects all electronic &optical properties of the crystal. All polytypes are extremely hard (brittle), very inert, and possess high thermal conductivity.

Analysis Of Igniter Material Norton CRYSTAR Black Silicon Carbide Other properties

Why was SiC chosen for the igniter? Silicon carbide has exceptional thermal and electrical properties making it advantageous to use at varying voltage settings and high temperatures.

Why is SiC preferred over other materials? Property CRYSTAR Si/SiC CRYSTAR SiC Polysilicon Quartz Alumina Graphite Density Porosity (%) <1 16 < MOR (MPA) NA MOE (GPa) Thermal Conductivity (W/m oC) Thermal Expansion (x 10-6/oC) (RT to 1000oC) Electrical Resistivity(ohm oC 1100NA NA Emmisivity N/A 0.9 Specific Heat (J/kg oC) NA

How is SiC Fabricated and Purified? Modern industrial fabrication of SiC is done via Epitaxial CVD. CVD, also known as Chemical Vapor Deposition, consists of growing pure crystals from a substrate. In the case of SiC, a graphite substrate is generally used. In order to obtain the crystals, raw SiC (from silica sand and coke) is sublimated in a graphite chamber in a high temperature environment ( 1600 – 1800 C). The resulting gaseous phase, then cools and deposits on the substrate, leaving highly pure crystals. From this point, the SiC is now ready to be processed for engineering applications

From here powder is obtained and sized in order to hot press. Hot pressing allows the ceramic to be sintered and densified simultaneously. The igniter must be pressed into the near final shape due to material hardness. Finally the piece is machined for assembly. Electrical wire leads are soldered to the ends, then it is partially encased in Alumina for insulation. It is essential that process variables (temperature, pressure, cooling rate) be controlled to maintain purity, low porosity, etc…….this leads to desired thermal and electrical properties. Igniter Processing

Cost The cost of a single heating element from the Norton Corporation for a Model 201n is roughly 60$ - 80$. The estimated production cost is in the range of 30$ - 40$ per piece. The cost could be lowered if a different processing technique other than hot pressing was chosen, but the quality of the part would drastically decrease. Conclusions SiC is chosen for the application of hot surface igniters because of its unique properties such as Low coefficient of thermal expansion Optimal thermal and electrical conductivity Excellent resistance to wear and oxidation at high temperatures Design of the igniter allows for optimal heating and the top of the part and prevents overheating at the base. Hot pressing allows for high purity and most desirable properties, but with die material and wear costs the process is expensive.