Electric heating – Heating devices – Combined with container – enclosure – or support for material...
Reexamination Certificate
2001-02-01
2002-09-10
Walberg, Teresa (Department: 3742)
Electric heating
Heating devices
Combined with container, enclosure, or support for material...
C431S239000
Reexamination Certificate
active
06448539
ABSTRACT:
TECHNICAL FIELD AND BACKGROUND OF THE INVENTION
The invention relates to an electric heating element, more particularly for a radiant heater body of an electric range made of a semiconducting ceramic, and to a method for its production.
DE 296 19 758 U1 discloses an electric radiant heater including an electric resistance heating element comprising a changing temperature coefficient of the electrical resistance. In a first temperature range extending from 0° C. to at least 700° C. the temperature coefficient is negative. In a second temperature range following the first temperature range the temperature coefficient is required to be positive. In this way it is intended that the resistance heating element automatically adapts to the second temperature range or to automatically limit any further increase in temperature above a critical value after an initial very fast increase in temperature to red hot.
The radiant heater takes the shape of at least one elongated rod or strip, its cross-section preferably being rectangular and is supported by its narrow edge on a substrate, the resistance heating element being configured solid.
SUMMARY OF THE INVENTION
The invention is based on the object of providing a heating element as cited at the outset permitting fast heatup and good regulation and comprising in general good heating properties, particularly advantageous values for the thermal surface loading or heating radiance, as well as providing a method for the production thereof.
This object is achieved by a heating element as it reads from the features of claims
1
and
21
as well as by a method as it reads from the features of claims
12
and
25
. Advantageous aspects of the invention are the subject matter of the sub-claims.
In accordance with the invention the heating element may be made, on the one hand, of a semiconducting ceramic material which is open and/or porous at least in part or mostly and which more particularly in accordance with a first preferred embodiment may be foamy or spongy, particularly preferred cavitated. Foamy in this respect is to be understood as being a type of material in which the material comprises a large number of inclusions or chambers or pores preferably void. More particularly such a foamy ceramic material has the appearance of a sponge or a foam. Considered advantageous in this respect is a material having pores open to the environment to thus avoid gas release problems and the like.
Although the porosity may be selected over a wide range, a range of 10 to 50 ppi (pores per inch) is viewed advantageous, meaning that on a one inch line through the material 10 to 50 pores are cut or swept, corresponding roughly to 1 pore per mm. Values of approx. 30 ppi are considered to be particularly advantageous, thus making the material relatively finely porous.
A further advantageous aspect is the possibility of providing the ceramic material structured and limbed in the form of branchings which may result in, for example, a skeletonized structure in which the branching members are thin as compared to the cavitations. Advantageously the heat conducting material may be configured three-dimensionally meshed, more particularly similar to a three-dimensional textile material.
To cover both embodiments by one term the expression structured (foamed or skeletonized) ceramic is used in the following.
It is of advantage when in the course of the elongation of the heating element its electrically effective cross-section remains substantially the same to thus avoid in bent heating conductors the so-called hot paths of increased current flow, especially at the inner side of a curve. This is achieved to particular advantage by the skeletonized structure as described above, the limbs of which approach each other at such inner sides without changing their length or electrically effective conductor cross-section in each case.
The specific weight of the heating element or structured ceramic in both basic aspects may be in the range 0.1 to 3 g/cm
3
, preferably approx. 0.6 g/cm
3
for 30 ppi for foamed ceramic, it being obvious from this that the proportion of pores or open sections exceeds the proportion of ceramic material by far, i.e. more pores or cavitations or interspaces existing than ceramic material. A solid ceramic material has a specific weight in the range 3 to 4 g/cm
3
. Thus the volume of the pores or cavitations or open sections may be partly ten to twenty times higher than that of the actual ceramic material. It is particularly to be noted that by employing a skeletonized ceramic the so-called porosity is even higher.
One significant advantage afforded by such a structured ceramic is that it exhibits a highly favorable ratio of conductor cross-section to radiant surface to thus permit the resulting heat to be emitted particularly well whilst achieving a very fast red hot heatup of the heating element. Advantageously a structured ceramic is configured elongated or rod-shaped.
The thermal surface loading is preferably approx. 12 W/cm
2
at 1,200° C. and approx. 16 W/cm
2
at 1,300° C., surface in this respect being, however, the envelope of the surface of the heating element, not the surface of the pure ceramic material.
The specific resistance can be in the range of approx. 0.25 Ohm*cm (cold) at around 30 ppi to approx. 0.4 Ohm*cm (at approx. 800° C.). The value of the heating capacity can be set in one example embodiment with approx. 40% by weight silicon in the range of approx. 0.68 J/gK (cold) to approx. 1.15 J/gK (at approx. 1,000° C.). Analogous to the ppi specification for the foamed structured ceramic it is the number of meshes per volume that dicates the cavitation size or density for the skeletonized version thereof.
The material of the heating element is preferably formulated with silicon, more particularly it may be formulated with silicon carbide, further alternatives being SiSiC, RbSiC as well as SiN whereby aluminum oxide, zirconium oxide or AlN may be used instead of silicon. A material formulated with silicon may also be MoSi
2
commercially available as “kanthal super”, admixed to advantage with one of the aforementioned ceramics. Preferably the material of the heating element or this itself is sintered, The surface of the material may be coated with silicon oxide for surface protection, preferance being given particularly to silicon carbide doped with nitrogen or as an alternative reaction-bound silicon carbide to advantage. These procedures may take place to advantage in a reactive gas atmosphere.
The heating element may be formulated to advantage with Ti or TiN which is more particularly the electrically active material. The Ti material is coated on the outside to advantage with a protective coating which may be an oxide coating, for example SiO or Al
2
O
3
. Due to the mechanical properties of the compound or TiN it is preferably applied to a substrate, the material of which may be Al
2
O
3
since this exhibits a similar thermal coefficient of expansion. A substrate or substrate matrix may be configured as described above, for example, skeletonized or foamed. As an alternative the heating element may be fabricated in a sandwich configuration in which the heating layer of TiN is applied to a substrate which is then covered by a protective coating. Such a sandwich-type heating element is configured preferably flat, for example as a flat rod incorporating a plurality of limbs, where necessary.
Another preferred alternative is to admix the TiN with matrix material, for example Al
2
O
3
, lending itself to good sintering. The specific electrical resistance of the mix depends on the volume percentage of the TiN which should, however, exceed 15%, although percentages as high as 50% or even 60% are still possible, also as regards workability. Such TiN admixed ceramic mixtures likewise require a protective coating, for example Al
2
O
3
.
Another alternative provides for a foamed or skeletonized structured material comprising a siliconized coating. One such structured ceramic, especially of SiC comprises a highly favorable conductor cross-section
Essig Willi
Ose Lutz
Wilde Eugen
E.G.O. Elektro-Geraetebau GmbH
Fastovsky Leonid M
Senterfitt Akerman
Walberg Teresa
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