EP1620864A1

EP1620864A1 - Electric heating element

Info

Publication number: EP1620864A1
Application number: EP04722845A
Authority: EP
Inventors: Hartwin Weber; Waldemar Döring
Original assignee: Vacuumschmelze GmbH and Co KG
Current assignee: Vacuumschmelze GmbH and Co KG
Priority date: 2003-03-28
Filing date: 2004-03-24
Publication date: 2006-02-01
Also published as: WO2004086420A1; DE10314218A1; US7230211B2; US20060065653A1

Abstract

The invention relates to an electric heating element provided with a durable PTC resistance made at least partially of an iron-based alloy, exhibiting stable characteristics to a temperature of 1500 DEG C and having a resistance/temperature characteristic curve which substentially increases in a linear manner within a temperature range between an ambient temperature and 1500 DEG C and can be used for a temperature control. Said PCT resistance is provided with an oxydation resistant metallisation or is gas-tightly enveloped by a jacket (2, 3, 12), a space between said jacket (2, 3, 12) and the PCT resistance (1,13) being filled with a powder or granules (14) which remove an important gas amount from said space.

Description

description

Electric heating element

The invention is in the field of electrical heating technology, where devices, materials or rooms are heated by converting electrical energy into thermal energy.

Electrical heating systems are used, for example, in household technology as hot plates in cookers, in hair dryers and fan heaters, in automotive technology to heat the charge air or the catalysts for gasoline or diesel fuel, and in chemical engineering on reaction columns. The temperature is usually measured and the measured variable is used by means of a control circuit to control heating elements in order to achieve a desired target temperature.

Heating conductors based on ferritic FeCr (AL) - or austenitic NiCr (Fe) - are usually used for heating conductors.

Alloys used. However, because of the low temperature dependence of their specific resistance, these substances are difficult to use for differentiated control of a temperature.

Co-alloys with additions of Fe, Ni and similar metals have also proven their worth in glow plugs for internal combustion engines. However, these are quite expensive due to the high proportion of Co. Such materials also have a very low specific resistance value, so that additional resistors have to be inserted in order to better adapt to the internal resistances of existing voltage sources.

From DE 3825012 AI a control element for a glow plug is known, which consists of a cobalt-iron alloy with 20-35% iron. This alloy goes into a face-centered cubic crystal structure above about 1000 ° Celsius about what leads to thermal fatigue of the material due to the frequent temperature changes and the associated phase transitions.

According to EP 0523062, cobalt-iron alloys with an iron content of 6-18% are also used, which avoid the effect of thermal fatigue. These alloys advantageously have a high temperature factor, but with the disadvantage of a relatively high price.

The use of pure Fe leads to a pronounced temperature dependence of the resistance, but problems arise due to phase transitions in the interesting temperature range above 900 ° Celsius. This leads to hysteresis which leads to an ambiguity in the temperature resistance characteristic and to thermal fatigue of the material which can lead to breakage.

In addition, the temperature dependency of a single resistor, which itself serves as a heating resistor, has hitherto been used at low temperatures and / or lower amounts of heat.

In order to largely eliminate the above-mentioned problems, DE 100 60 273, however, again proposes a two-part arrangement with a temperature sensor element which is separate from the heating element and serves as a series resistor, but which requires more effort because of the two-part nature.

The object of the present invention is to find a cost-effective heating element with alloys which are stable in the application area mentioned, so that applications at high temperatures and large amounts of heat are also possible. The object is achieved by a heating element according to claim 1. Refinements and developments of the inventive concept are the subject of dependent claims. The invention further comprises preferred applications of the inventive concept.

Specifically, an electrical heating element according to the invention comprises a PTC resistor at least partially, but preferably entirely from an iron-based alloy, which in operation has constant properties up to 1500 ° C and no crystallographic lattice conversions, i.e. shows no phase transitions, and which has a resistance-temperature characteristic curve rising in the temperature range between room temperature (RT) and 1500 ° C, the increase in the temperature range between RT and approx. 750 ° C is so steep that the temperature drops to a Target temperature can be regulated on the basis of the temperature behavior of the PTC resistor, the heating element being enclosed in a gas-tight manner by a casing and an intermediate space between the casing and the PTC resistor being filled by an electrically insulating material (e.g. powder or granulate), which displaced much of the gas inside the shell.

Advantageously, only one resistor (PTC resistor) is required, which is used for heating and control both as a heating resistor and as a control or temperature sensor resistor. It is also advantageous that it can also be used with hotplates, hair dryers and fan heaters.

The invention therefore relates to an electrical heating element with a PTC resistor (PTC = positive temperature coefficient) which can be operated not only below 900 ° Celsius, but also in the temperature range between 900 ° Celsius and 1500 ° Celsius with constant properties and a resistance resistance that rises sharply in the temperature range between 20 ° Celsius and 750 ° Celsius / Temperature characteristic (R / T characteristic). Between 750-1500 ° C it is sufficient if the resistance / temperature characteristic (R / T characteristic) rises slightly.

It has been found that substances that meet the above requirements are sensitive to the influence of surrounding gases, especially oxygen.

In order to counter this, the invention provides that the PTC resistor is enclosed in a gas-tight manner, that an intermediate space between the case and the PTC resistor is filled with a powder or granulate, which contains a large part of the gas inside displaces the casing, and that at least part of the powder / granulate consists of a material which binds the gas, in particular oxygen, or in an alternative embodiment, - that the PTC resistor has an oxidation-resistant, high-temperature-resistant metallic coating, preferably a nickel coating, is provided.

In addition to a nickel coating, nickel / chrome coatings or chrome coating or other metallizations are also conceivable.

Particularly when using Fe-containing alloys (for example iron-based alloys) there is a great risk of scaling (see Tables 1, 2, 3: Dz: scale layer thickness after 2h 1150 ° C in air) or internal oxidation due to the oxidation of iron , and / or the additives which, on the one hand, create a layer on the PTC resistor which does not allow any electrical current to flow, which therefore makes no contribution to heating and which partially thermally insulates the remaining metal part of the PTC resistor internal oxidation also conductive cross section of the PTC resistor and thus its electrical resistance changed. This makes it difficult to regulate the temperature on the basis of the R / T characteristic of the heating element, since the regulation is matched to a specific cross section and a specific length of the PTC resistor as a defined electrical conductor.

By displacing the gas inside the casing by the powder / granulate and binding the gas to the surface of a powder / granulate, the amount of gas inside the casing that can react with the PTC resistor is minimized.

The material that binds the gas can be such that the gas is physically sorbed on the surface of the material, the gas virtually adhering to the surface of the material in a molecular manner. For this purpose, the material can also be porous in order to increase the surface. On the other hand, the material can also be such that a chemisorption or a chemical reaction

(Gettern) with the gas that leads to gas binding. In any case, the resulting binding of the gas should, if possible, be stable even at high temperatures which occur in the heating element.

Fe-based alloys are characterized by somewhat different properties depending on the admixtures, but all have in common a relatively strong temperature dependence of the electrical resistance and a monotonic course of this dependence.

In the context of the invention, Fe-Ti alloys (iron-titanium alloys), Fe-V alloys (iron / vanadium alloys) and Fe-Mo alloys (iron / molybdenum alloys) have proven to be particularly advantageous , Iron / titanium alloys show the strongest temperature dependence of the resistance, iron / molybdenum alloys one compared to the other iron-based alloys, a reduced scaling tendency and iron / vanadium alloys the largest control range, that is, the largest range in which the steepness of the R / T characteristic is sufficient for active and reliable control. What these alloys have in common is that they essentially maintain a cubic, inner-centered lattice structure during operation.

The strong temperature dependence of such iron alloys is related to the ferromagnetic properties. The temperature dependence is extreme for alloys with the highest saturation magnetization. This is usually associated with a high Curie temperature (see the table below, T _c ). The Curie temperature determines the abnormal temperature range of the resistance.

Pure iron shows a phase transition from α-iron to γ-iron in a temperature range between 900 ° Celsius and 1400 ° Celsius, i. H. from a body-centered cubic to a face-centered cubic crystal structure. Since continuous phase transitions lead to thermal fatigue, other components are added to the iron (iron-based alloy), which can prevent a phase transition. Al or Cr, Ti or V, Mo and Si are particularly suitable for this. Binary alloys can also be used as alloys with more than two partners. Such alloys show an almost hysteresis-free course of the temperature coefficient during heating and subsequent cooling.

It has been shown that the alloys with aluminum and chromium or silicon had somewhat poorer properties than the variants with titanium, vanadium or molybdenum. This was due to the fact that aluminum showed a change in resistance that was too high and that the addition of chromium required too high amounts of chromium to meet the requirement to prevent phase transitions. In detail, the following alloys have been found to be advantageous: 2.0-4.0% by weight of Mo, remainder Fe or 1.00-2.50% by weight of V, remainder Fe or 0.75-2.0% by weight % Ti, balance Fe including the usual (melt-related) impurities.

Furthermore, the following alloys have been found to be particularly advantageous: 2.0-3.0% by weight of Mo, remainder Fe or 1.25-1.75% by weight of V, remainder Fe or 1.0-1.5% by weight % Ti, balance Fe including the usual (melt-related) impurities.

The alloys with molybdenum are particularly resistant to scaling and therefore place lower demands on the residual gas volume and leak rate, those with vanadium have particularly high melting points (approx. 1530 ° Celsius) and Curie temperatures and are therefore the most controllable. The Ti-alloyed variants show the highest slope between RT and 1000 ° C and thus the best control sensitivity (e.g. temperature factor> 6, for FeTi> 7)

In the case of an electrical heating element with a PTC resistor described above, it has proven to be advantageous to use ceramic material as a powder or granulate with which the spaces between the casing and the PTC resistor are filled. The ceramic material is sufficiently insulating to avoid a short circuit between the heating element and the possibly metallic shell, and is also temperature-stable so that it does not change its properties when the temperature rises.

However, the material in the gaps should be particularly good heat conductor, which is rather the exception with ceramic powder or ceramic granules, except for example with AlN. A tight packing of the powder or granulate helps here, that is, for example a compression or a

Shake in the manufacture of a heating element. This has the advantageous side effect that the gas closing spaces in the powder can be further reduced or reduced. This means that even less gas is available for reaction on the surface of the PTC resistor.

A mixture of different grain sizes down to the smallest manageable grain sizes can also be used as powder / granulate. Such mixtures of materials with different grain sizes make densest packs of pourable substances possible.

Furthermore, getter material (eg Al or Zr powder) can be provided in the interior of the casing. These materials bind oxygen very easily, so that scaling of the material of the PTC resistor is then further reduced. The materials aluminum or zirconium are electrically conductive and as such they must not form a conductive bridge between the PTC resistor and the sheath. However, they oxidize to non-conductive oxides, which are then harmless.

It can also be provided that the powder / granulate contains aluminum or zirconium powder. In this case, the aluminum or zirconium powder can, for example, be mixed with a ceramic powder in such a way that the overall electrical conductivity is that of an insulator. Nevertheless, there is enough aluminum or zircon at every point of the heating element to bind gas, especially oxygen.

In an advantageous embodiment of the electrical heating element according to the invention, the casing can comprise a metal tube. In this case, the sheath conducts the heat generated in the PTC resistor particularly well, since metals are known to be good heat conductors overall. The metal tube is made from an alloy that has a heat resistance and scale resistance that corresponds to the temperature value to be generated. It can also be glued, soldered or welded to an object to be heated. When the heating element according to the invention is used in this way, it can be positioned particularly well, with the physical connection to an object to be heated ensuring particularly good heat transfer.

The electrical heating element according to the invention can be connected particularly advantageously to a glass ceramic cooktop by virtue of an intermediate space between an upper cover plate and a lower cover plate of the glass ceramic cooktop, in which the PTC resistor is inserted, the largest possible part the space can be filled with a powder / granulate made of ceramic, and the space is sealed gas-tight. In this way, the cover of the heating element consists of the cover plates and the filling with the powder / granulate minimizes the amount of gas in the intermediate space, which could lead to scaling of the PTC resistor.

The electrical supply voltage is advantageously fed to the PTC resistor through a gas-tight glass bushing. This ensures that the closure of the space between the cover plates is actually gas-tight.

A further advantageous embodiment of the invention provides that at least one of the cover plates has groove-shaped recesses for receiving turns of the PTC resistor. In this case, on the one hand the position of the PTC resistor is precisely defined, on the other hand also the length of the PTC resistor to be installed, which corresponds to the heating power, so that with a known diameter of the PTC resistor and the length of the PTC specified by the groove-shaped recesses -Resistance whose resistance is reproducibly fixed and thus the temperature control is easy to achieve by specifying a certain supply voltage. This is particularly important when such heating elements have to be introduced as standard in glass ceramic cooktops, in which case no calibration of the individual heating elements is then necessary.

In addition, by specifying grooves in the cover plates, the overall space becomes smaller, so that the amount of gas that could react with the PTC resistor is further reduced.

The additional material in the form of a powder / granulate, which is arranged in the intermediate space for binding the gas, can be mixed into the ceramic powder, for example. However, it is also conceivable according to the invention that at least one separate pocket-like space is provided for receiving a gas-binding powder / granulate (getter powder), which is closed off in particular by a frit. In this case, the material (getter powder) can be exchanged in a targeted manner, for example when the PTC resistor is renewed, and replaced with new getter powder. The frit permits gas exchange within the intermediate space with the pocket-like space, so that the gas can be bound there, but dust is kept away from the getter powder. Here too, aluminum or zirconium powder can be used as the getter powder.

The invention is explained in more detail below with reference to the exemplary embodiments shown in the figures of the drawing. It shows:

Figure 1 shows schematically in cross section a glass ceramic hob and

Figure 2 in cross section a mineral insulated conductor. 1 shows in cross section an upper cover plate 2 and a lower cover plate 3, between which an intermediate space 10 is formed. This is sealed gas-tight in the area of the joints 4, for example by gluing. The lower cover plate 3 has a thickened area 3a where the PTC resistor 1 is provided. This is inserted spirally in grooves or a groove 3b of the lower cover plate 3. The PTC resistor 1 is connected to this by means of a glass bushing 5, which leads through the lower cover plate 3 to the outside to a voltage source 11. In addition, a center contact 6 is provided, to which the other end of the PTC resistor is connected and which is connected to the other pole of the voltage source 11.

The intermediate space 10 between the upper cover plate 2 and the lower cover plate 3 is provided with a pocket-like recess 9 which is covered by frits 8 and which contains an aluminum or zirconium powder as getter powder in the region 7.

In addition, the space 10, in particular in the area of the grooves 3b, is additionally largely filled with a ceramic powder (for example porcelain powder, glass ceramic powder, quartz sand) of different grain sizes.

FIG. 2 schematically shows a mineral-insulated conductor with a sheath 12 which is made of a temperature-resistant metal, a PTC resistor 13 made of an iron-based alloy according to the invention and between these a layer 14 made of a ceramic powder, some of which is aluminum or zirconium. Powder is added. The powder is stamped during manufacture of the mineral insulated conductor or the sheath 12 is pressed in order to achieve a higher packing density of the powder and thus to reduce the gas spaces within the sheath as much as possible. The following tables show some iron-based alloys that can be used as a material for a PTC resistor according to the invention. In addition, some materials are given for comparison which do not correspond to the invention. For the materials listed, specific resistance values are given at certain temperatures (Rho 1000 and Rho 20), as well as the temperature factor, which shows information about the slope of the R / T characteristic, and the melting temperature Tm and the Curie temperature Tc. Dz is the scale thickness after 2 hours in air at 1150 ° C. If it is not specified, the oxidation is not limited to the surface (internal oxidation).

Table 1.

Table 2

All alloy specifications include common, melt-related impurities, for example C, O, N, S, and deoxidation additives such as Mn and Si

Claims

claims

1.Electric heating element with a PTC resistor (1, 13) provided for heating and control, at least partially made of an iron-based alloy, which has constant properties in operation up to 1500 ° C and shows no crystallographic lattice conversions, one in the temperature range between room temperature and 1500 ° C increasing resistance / temperature characteristic curve, the increase in the temperature range between room temperature and about 750 ° C is so steep that the temperature of the PTC resistor can be regulated to a target temperature based on its own temperature behavior.

2. Electrical heating element according to claim 1, wherein the PTC resistor has a metallic coating, in particular a nickel coating.

3. Electric heating element according to claim 1, wherein the

PTC resistor (1, 13) is enclosed in a gas-tight manner by a casing (2, 3, 12) and an intermediate space between the casing (2, 3, 12) and the PTC resistor (1, 13) by an electrically insulating material (14) can be filled, which displaces a large part of the gas inside the shell (2, 3, 12).

4. Electrical heating element according to claim 3, wherein the electrically insulating material (14) is powder or granules.

5. An electrical heating element according to claim 4, wherein at least part of the powder / granulate consists of a material which binds the gas.

6. Electrical heating element according to one of the preceding claims, wherein the PTC resistor (1, 13) at least partially consists of one of the following Fe base alloys: Fe-Ti alloys, Fe-V alloys, Fe-Mo alloys.

7. Electrical heating element according to one of the preceding claims, in which the PTC resistor (1, 13) consists at least partially of an alloy of the following composition ranges including common impurities: 2.0-4.0% by weight Mo, balance Fe ; or 1.00-2.50% by weight V, balance Fe; or 0.75-2.00% by weight of Ti, remainder Fe.

8. Electrical heating element according to claim 7, wherein the PTC resistor (1, 13) consists at least partially of an alloy of the following composition ranges including common impurities: 2.0-3.0 wt.% Mo balance Fe or 1.25- 1.75% by weight V, balance Fe or 1.0-1.5% by weight Ti, balance Fe.

9. Electrical heating element according to one of claims 3 to 8, wherein the powder or granules contains electrically insulating ceramic material.

10. An electrical heating element according to claim 9, in which AlN is provided as the ceramic material.

11. Electrical heating element according to claim 9 or 10, wherein the powder / granulate comprises a mixture of different grain sizes down to the smallest manageable grain sizes.

12. Electric heating element according to one of claims 3 to 11, in which inside or entirely getter material is arranged in the casing.

13. An electrical heating element according to claim 12, wherein the getter material consists at least partially of Al or Zr powder.

14. Electrical heating element according to one of claims 3 to 13, wherein the casing is formed from a metal tube (12).

15. Electrical heating element according to one of claims 3 to 14, wherein the sheath (12) is glued, soldered or welded to an object to be heated.

16. Use of an electrical heating element according to one of the preceding claims in a glass ceramic cooktop, an intermediate space (10) being formed between an upper cover plate (2) and a lower cover plate (3), in the intermediate space (10) of the PTC resistor (1) is inserted, and the intermediate space (10) is sealed gas-tight.

17. Use of an electrical heating element according to claim 16, in which the intermediate space is at least partially filled by filler material.

18. Use of an electric heating element according to claim 17, in which powder / granulate made of ceramic is provided as the filling material.

19. Use of an electrical heating element according to one of claims 16 to 18, in which the electrical supply voltage to the PTC resistor is supplied through a gas-tight glass bushing (5).

20. Use of an electrical heating element according to one of claims 17 to 19, in which the PTC resistor (1) is at least partially formed in turns and at least one of the cover plates (2,3) groove-shaped recesses (3b) for receiving the turns of the PTC - Resistance (1).

21. Use of an electric heating element according to one of claims 16 to 18, in which at least one separate pocket-like space (9) is provided for receiving a gas-binding getter powder,

22. Use of an electric heating element according to claim 21, wherein the pocket-like space (9) is closed by a frit (8).