Heating element

A heating element with a ceramic body that has PTC properties is specified. The heating element has electrodes that are arranged on ceramic body. Both the ceramic body and the electrodes are lead-free.

BACKGROUND

Heating elements with ceramic PTC resistors are known, for example, from U.S. Pat. No. 4,899,032.

SUMMARY

In one aspect, the invention specifies an environmentally friendly heating element.

A heating element with a ceramic body that has PTC properties is disclosed. (PTC stands for “positive temperature coefficient”). The heating element has electrodes that are arranged on the ceramic body. Both the ceramic body and the electrodes are lead-free.

With the preferred heating element it is possible to essentially avoid environmental stressors connected with disposal of heavy metals.

The following list of reference symbols can be used in conjunction with the drawings:

2a,3aInner contact layer

2c,3cOuter contact layer

DETAILED DESCRIPTION

The heating element shown inFIG. 1includes a ceramic body1, a first electrode2, and a second electrode3. The electrode2is arranged on the lower primary surface and electrode3on the upper primary surface of body1. Both body1and electrodes2and3are lead-free.

FIG. 2shows a variation of the heating element shown inFIG. 1, in which each electrode includes a number of layers. The lower electrode has an inner contact layer2a, a diffusion barrier layer2b, and another contact layer2c. The upper electrode correspondingly has an inner contact layer3a, a diffusion barrier layer3band another contact layer3c.

The diffusion barrier layers2band3bare arranged between the contact layers2a,3aand2c, and3c. The inner contact layers2aand3aare arranged between the body1and the diffusion barrier layers2band3b.

Each of the layers2a,2b,2c,3a,3b,3cis lead-free.

The heating element can be used in motor vehicle applications in 12/24/42 V operation, preferably for heating of vehicle interiors, especially in the case of diesel vehicles (automobiles, trucks, commercial vehicles) as well as gasoline-powered vehicles. Preferably, several identical heating elements are arranged on a common carrier, electrically connected together and thus assembled into a heating system.

The ceramic body1is sintered. Ceramic raw materials without lead additives are used to make the ceramic body1. The ceramic raw material preferably contains BaTiO3. In one variation, the ceramic raw material contains an amount of SrTiO3(for example, in addition to the barium titanate). Alternatively, the body1can be free of SrTiO3.

The following ceramic compositions, for example, are considered to be advantageous: BaTiO350-85%, CaTiO33-15%, SrTiO3up to 50%, SiO21-2%.

The electrodes2,3or their partial layers2a-2c,3a-3care preferably produced in a metal deposition process. Examples are sputtering, evaporation, electrolytic deposition, and chemical deposition. However, the electrodes2,3can also be produced by baking on a metal paste. The thickness of the electrodes2,3can be between 2 μm and 25 μm, depending on the specific embodiment.

In an advantageous embodiment, the electrodes2,3can contain metallic Al as a base material. The base material of the electrodes2,3can be enriched with glass flux. The amount of glass flux is preferably about 5%. The thickness of an electrode2,3that contains Al as a base material and a glass flux as an additive is preferably 20 μm.

Alternatively, the glass flux can be omitted, so that the electrodes2,3are free of glass additives. The thickness of an Al electrode without glass flux is preferably 4 μm.

The electrodes2(3) can have a layer sequence that includes several partial layers2a-2c(3a-3c). The layer sequence can, in particular, have a base layer2a(3a) which functions as the inner contact layer, and a diffusion barrier layer2b(3b). The inner contact layer2a(3a) serves for ohmic contact with the ceramic body1. Aluminum, chromium or a zinc-containing layer, for example, is suitable as the contact layer2a(3a). A nickel layer can be applied directly to the ceramic body1or to the contact layer2a(3a) which depending on the embodiment, is suitable as a diffusion barrier layer. The layer sequence preferably also includes a conductive layer (outer contact layer2c(3c)), which has good electric conductivity that is higher than that of the underlying layers. For example, a silver layer or a silver-containing layer is suitable as the conductive layer2c(3c). Other layer sequences, not specified here, are also possibilities for the electrodes of the heating element.

The electrodes2,3produced in a bake-on process are produced with bake-on pastes that contain an amount of glass. In producing such electrodes, a metal paste with a glass additive that is lead-free is used. The metal paste also contains organic binders, which are preferably burned off completely when baking on the electrodes.

The heating element preferably has two main surfaces. In a preferred variation, the first electrode2is arranged on the first primary surface and the second electrode3is arranged on the second primary surface.

The heating element can be designed as a surface-mountable structural element. The specific resistance of the heating element can be set, for example, between about 10 and about 500 ohm·cm. However, the resistance value is not limited to this range.