Ceramic heating elements

The present application claims the benefit of U.S. provisional application No. 61/009,381 filed Dec. 29, 2007, which is incorporated by reference herein in its entirety.

1. Field of the Invention

In one aspect, ceramic heating elements are provided that have a recessed portion for receiving an electrical lead. Such ceramic heating elements can more secure engagement of the heating element with an electrical lead. In a further aspect, ceramic heating elements are provided that have a conductive zone of substantially equal or increasing cross-section along a length of the element. The present heating elements are useful in a variety of application, including e.g. for fuel ignition for gas cooking appliances as well as vehicular glow plugs that have strict space constraints.

2. Background

Ceramic materials have enjoyed great success as heating elements (includes igniters) in e.g. gas-fired furnaces, stoves and clothes dryers. Ceramic heating element production includes constructing an electrical circuit through a ceramic component a portion of which is highly resistive and rises in temperature when electrified by a wire lead. See, for instance, U.S. Patent Publication 2006/0131295 and U.S. Pat. Nos. 6,028,292; 5,801,361; 5,405,237; and 5,191,508.

Typical igniters have been generally rectangular-shaped elements with a highly resistive “hot zone” at the heating element tip with one or more conductive “cold zones” providing to the hot zone from the opposing heating element end. One currently available igniter, the Mini-Heating element, available from Norton Igniter Products of Milford, N.H., is designed for 12 volt through 120 volt applications and has a composition comprising aluminum nitride (“AlN”), molybdenum disilicide (“MoSi₂”), and silicon carbide (“SiC”).

Since these heating elements are resistively heated, each of its ends must be electrically connected to a conductive lead, typically a copper wire lead. Ceramic heating elements have been connected to electrical contact by direct welding or brazing to wire or by brazing to an intermediate metal lead frame which is then welded or brazed to wire. See U.S. Pat. Nos. 7,241,975 and 6,933,471.

For heating elements that have cylindrical or other non-rectangular cross-section configurations, such attachment of electrical contacts can result in an increase in the diameter of the insulating section (where the electrical leads interface with the heating element). Such increased dimensions can be problematic for a number of applications, such as appliances or automotive environments where tight specifications may exists for the outer dimensions of the heating element block of the heating element. Additionally, separation of the electrical lead from the heating element can result in device failure.

It thus would be desirable to have new heating element systems. It would be particularly desirable to have new heating elements that have cylindrical or other non-rectangular cross-sectional configurations and that have comparatively narrow cross-sectional dimensions across regions that interface with electrical contacts. It would be further desirable to have new heating elements that have secure engagement of an electrical lead to the heating element.

In one aspect, ceramic heating elements are provided that have a recessed portion for receiving an electrical lead. Such ceramic heating elements can provide a reduced cross-sectional dimension across element regions that interface with electrical lead(s) as well as a more secure engagement of the lead(s) to the heating device. Consequently, heating elements can be highly useful in a variety of applications, including e.g. for fuel ignition for gas cooking appliances as well as vehicular glow plugs.

In a preferred aspect, a heating element may comprise at least one recess (e.g. hole) that can receive an electrical lead, where the recess is positioned at a bottom face of the heating element, although the recess also suitably may be situated in other regions of a heating element, such as a side portion of an element.

In certain aspects, the recess may be tapered, e.g. inwardly tapered (decreasing cross-sectional area), which can further secure an engagement of an electrical lead with the heating element.

Preferably, a conductive zone (i.e. region of relatively low resistivity) of the heating element forms at least a portion of the wall surface of the recess. As a consequence, power from an electrical lead nested within the recess can flow through the heating element via such conductive zone.

In a further aspect, ceramic heating elements are provided that have a conductive zone of substantially equal or increasing cross-section from a proximal end of the heating element along the element length. In particular, the cross-sectional dimension of the conductive zone that forms at least a portion of the wall surface of the recess for receiving an electrical lead will have a cross-sectional dimension at a portion that contacts the recess that is substantially equal to or greater than the cross-section dimension of that same conductive zone further along that conductive zone\'s length.

It has been found that such conductive zone configurations can avoid undesired warpage upon sintering of the heating element.

Preferred heating elements of the invention have an outer or substantially U-shaped or L-shaped electrical path, i.e. where the electrical path extends from (i) an outer conductive zone to (ii) an hot or ignition zone and then through (iii) a second outer conductive zone. Such an outer or U-shaped or L-shaped electrical path is different than and distinguished from a co-axial path that contains an interior first conductive zone that is encased by an outer conductive zone.

Particularly preferred heating elements of the invention may have cylindrical or other non-rectangular cross-section configurations. In a preferred aspect, preferred heating elements of the invention have a rounded cross-sectional shape along at least a portion of the heating element length (e.g., the length extending from where an electrical lead is affixed to the heating element to a resistive hot zone). More particularly, preferred heating elements may have a substantially oval, circular or other rounded cross-sectional shape for at least a portion of the heating element length, e.g. at least about 10 percent, 40 percent, 60 percent, 80 percent or 90 percent of the heating element length, or the entire heating element length. A substantially circular cross-sectional shape that provides a rod-shaped heating element is particularly preferred. The invention also provides heating elements that have non-rounded or non-circular cross-sectional shapes for at least a portion of the heating element length.

Preferred heating elements comprise multiple regions of differing electrical resistivity, i.e. preferred ceramic heating elements may comprise a first conductive zone, a resistive hot zone, and a second conductive zone, all in electrical sequence. Heating elements of the invention may have a variety of electrical configurations. As discussed, in preferred systems, the heating element may have a substantially U-shaped electrical path, e.g. where opposing conductive zones are separated by an interposed hot or ignition zone.

Ceramic heating elements of the invention can be employed at a wide variety of nominal voltages, including nominal voltages of 6, 8, 10, 12, 24, 120, 220, 230 and 240 volts.