A Hot Surface Igniter Is Made From Silicon Carbide

Hot surface ignitors are replacing spark ignition in modern furnaces. Constructed of silicon carbide or silicon nitride with ceramic bases to encase wire connections, 120-volt hot surface ignitors create red-hot glow before arcing over to light the gas in combustion cycles. Engineers who designed these units had some important decisions regarding where the ignitor should be positioned relative to the end of their burners.

Ignitor Type

Hot surface ignitors are used to ignite combustion gasses in various household and industrial appliances such as furnaces, kitchen ranges and clothing dryers. Made of ceramic material that heats when voltage is applied across its proximal ends, in order to ensure safe operation of hot surface ignitors it is essential that they be made from high quality materials such as silicon carbide or silicon nitride; manufacturing must adhere to stringent quality control standards; otherwise failing can result in property damage as well as injury or even death.

To meet their objectives, the inventors of this invention devised a method of producing sintered hot surface ignitors from silicon carbide fines. It begins by creating an unsintered body consisting of two regions along its length axis – one contains relatively resistive silicon carbide and aluminum oxide material while the other features more conductive portions – where one section contains relatively resistive resistors while both regions contain resistive silicon carbide material as well as aluminum oxide resistors and resistive portions, and another contains relatively resistive parts while another contains resistive regions comprised by resistive silicon carbide and aluminum oxide respectively while both regions contain resistive elements from silicon carbide fines fines, creating two unsintered bodies within this invention to produce sintered hot surface ignitors using silicon carbide fines as starting material to manufacture hot surface ignitors with aluminum oxide distal regions adjacent proximal regions adjacent distal sections comprised along their length axis that contain relatively resistive portions whereas distal regions contain relatively conductive sections respectively.

Unsintered body pieces are then sintered in a reducing atmosphere that is substantially devoid of nitrogen for its first sintering period, using noble gases such as helium or argon as fuel sources. Subsequently, this atmosphere may be partially-nitrogenated for additional sintering sessions.

Notching processes are then performed to produce two spaced apart sections that define the proximal end of the igniter body, with these connecting to conductors that lead to sources of electrical potential as well as being coated in nickel for easier connection.

Once the igniter’s proximal end has been connected to conductors, repeated sintering and notching processes are used to form the serpentine design shown in FIG. 2. Proximal leg ends 56 and 58 are then inserted into terminal block 24 so as to connect with conductors 26a and 26b; portions of proximal legs 56-58 may then be arc sprayed with nickel to an approximate length of 0.5 inches.

Notched proximal legs 102 and 104 can be adjusted in height relative to proximal portions of the body in order to control operating temperature and current draw, in order to optimize device performance according to particular applications.

Ignitor Material

At present, a hot surface igniter is produced using silicon carbide using methods as depicted in FIGS. 8-9 and 11. It consists of two legs 92 and 94 each equipped with distal ends 100 for current flow, and their respective proximal ends 96 and 98 spaced apart along width axis W; initially this spacing is achieved by notching billet to reduce overall thickness dimension (FIGS. 2 and 3). Later during fabrication, slots are adjusted dynamically by measuring room temperature resistance against high-temperature resistance (1100deg C.), to determine optimal length for optimal leg performance (FIGS. 2 and 3).

Sintered igniter bodies ideally consist of silicon carbide, aluminum, at least one transition metal and nitrogen. Their relatively resistive distal regions 206 and 208 should feature no more than 60 Ohm-cm room temperature resistivity to 1000deg C high temperature resistivity ratio with 40 being preferred as ideal. Furthermore, iron compounds should not be present within them although trace amounts of transition metals such as iron may still exist as impurities in their composition.

To produce the hot surface ignitor, an unsintered body composed of silicon carbide fines and coarses is sintered in two inert reducing atmospheres: one devoid of nitrogen entirely; then in another one partially nitrogenated but still devoid of nitrogen for another sintering period; this final one preferably comprises inert gases like helium or argon for optimal results.

Sintering allows for recrystallization of silicon carbide fines and incorporation of nitrogen into their lattice, creating an extremely resistant structure against oxidation with minimal surface area for electrical properties. Furthermore, by controlling how much nitrogen enters into their lattice structure they can improve heating properties while still remaining highly resistant to oxidation, and produce an ignitor that can operate under adverse conditions without suffering performance degradation.

Ignitor Voltage

Hot surface igniters are used to light gas furnaces, stoves and boilers in residential heating systems as well as some commercial and industrial applications. Their shape resembles that of artificial whetstones found on kitchen counters – only shorter-lived versions exist that break more often during operation than their ceramic counterparts do. Unfortunately they often cause no heat calls from technicians due to being extremely brittle; replacement should occur every five to ten years for optimal operation.

The igniter consists of a resistance element made of silicon carbide or silicon nitride in a ceramic base, mounted within a terminal block and connected to two wires. When activated with voltage of 80 to 240 volts across both wires, silicon carbide or silicon nitride begins glowing, creating a pilot flame in the heating system burner that is verified with thermocouples before gas flows to light the fire.

Sintered hot surface igniters produced according to this invention are generally free from transition metal compounds like aluminum oxide. Furthermore, their relatively conductive proximal portions may be doped with electron acceptors to decrease overall resistance of their igniter.

Under the present invention, sintered hot surface igniters prepared in accordance with this invention may include relatively resistive distal portions containing low concentrations of aluminum atoms and relatively conductive proximal and distal portions are configured so as to be separated by nonlinear regions.

Hot surface ignitors may be coated in nickel alloy for easier electrical connection with conductors 26a and 26b within terminal block 24. These conductors connect to opposite terminals of an unknown voltage source (not shown), with nonlinear region 214 in its central slot expanding distances between first and fourth leg proximal end regions while decreasing overall resistance of igniter.

Ignitor Lifespan

Hot surface igniters are relatively recent innovations that can be found in many heating systems and furnaces, including gas furnaces. To use hot surface igniters effectively, an M or fork-shaped piece made from silicon nitride or silicon carbide is attached with wires so the electric current running through it heats it to an incandescent glow that can then be detected by burner controls as a flame signal from which natural or propane gas flows freely into the system.

An igniter must withstand high temperatures and repeated cycling in a furnace environment; during each furnace cycle as set by its thermostat, multiple times is typically activated by its igniter, and over a year this could happen up to twenty or more times! Igniters made from various materials have different life expectancies with silicon nitride being more durable than silicon carbide for example.

Silicon carbide igniters are created through a process called “sintering.” In this method, raw materials are mixed into a slurry that is then formed into an igniter preform before sintered in an atmosphere with little or no molecular oxygen for initial sintering period. Sintering may also be altered to alter electrical properties by adding dopants such as electron acceptors and donors during initial sintering periods.

At times, sintering temperatures may be adjusted during the process to meet desired resistance and oxidation resistance goals. Furthermore, bath temperature can also be changed accordingly in order to meet these objectives.

When handling an igniter, be sure to wear gloves. Also consider placing it in a ceramic holder designed to protect the element from contamination such as dust, condensation, dirt, sealants or fiberglass insulation that might come into contact with it; even oil from hands could have the potential of polluting its elements.

High supply voltage can impede the lifespan of hot surface igniters. A voltage that exceeds 132 volts could cause it to burn out prematurely and require replacement – in such instances it would need to be found through other means.