PTC heating element

Abstract

The invention relates to a PTC heating element having a three-dimensional main body (2) with at least two main surfaces (3, 4) facing away from each other, the distance of the main body (2) between the main surfaces (3, 4) being the thickness D of the main body (2), and wherein the resistance R of the PTC heating element (1) has a minimum value first as the temperature increases and has a resistance characteristic that continues to increase as the temperature continues to increase, the reference temperature Tref is the temperature at which the resistance has twice the minimum value at Tmin, and wherein for Tref: Tref = (40 * D / mm + 75) DEG C wherein the tolerance of Tref is + / -10 K, the condition of application: 0.625 mm < = D < = 2.5 mm wherein the tolerance of D is + / -0.25 mm, 100 DEG C < = Tref < = 175 DEG C.

Description

TECHNICAL FIELD

[0001] The present application relates to a PTC heating element, in particular for an electric heater, in particular for an electric heater of a motor vehicle, which PTC heating element is operated at an operating voltage level of 800 V. BACKGROUND

[0002] PTC heating elements are known in a wide variety in the prior art. They are typically made of a ceramic material, for example pressed into a cuboid, which typically has two substantially mutually parallel arranged faces, which are used for electrical contacting. The thickness of the PTC element in particular also determines the electrical resistance, the heat dissipation and the reference temperature, which is the temperature at which the reference resistance is reached, which is twice the minimum resistance of the resistance curve as a function of temperature.

[0003] Such PTC heating elements are typically used in electrically operated heaters, i.e. in electric heaters.

[0004] In the case of electric or hybrid vehicles in the field of passenger vehicles or commercial vehicles, electric heating or auxiliary heating is of particular importance, since there is no or less waste heat from an internal combustion engine. In electrically operated vehicles, an electric heater is a simple and feasible solution for heating a fluid, for example air, water or coolant.

[0005] Here, the electric heater is not only of crucial importance in the starting phase, as in a conventional internal combustion engine, but also in the driving operation in order to ensure the maintenance of the interior space temperature at cold ambient temperatures.

[0006] In principle, the possibility of an electric heater is provided in the electric heater, which constitutes a heater on the air side or an electric heater on the coolant side, in particular as a high-voltage heater (HV).

[0007] Since both the electric HV heater on the air side and on the coolant side are supplied by the drive battery of the vehicle, the current voltage range is set to 220 V to 850 V. The maximum operating voltage here is 480 V at an operating voltage level of 400 V. The maximum operating voltage at an operating voltage level of 800 V is 1000 V to 1250 V. The respective power level of these electric heaters is approximately in the range of 5 kW to 10 kW.

[0008] The PTC heating element has an electrical resistance as a function of temperature, i.e. a resistance-temperature characteristic (R-T), which first falls approximately linearly from lower temperatures to higher temperatures (NTC range), until a minimum is reached, and then, as the temperature continues to rise, the electrical resistance will rise sharply again (PTC range). The PTC heating element is operated at temperatures belonging to the PTC range of the R-T curve. The reference temperature Tref is here the temperature at which the electrical resistance R is twice as large as the electrical resistance minimum Rmin, with Tref > TRmin.

[0009] In a PTC heating element, the resistance-temperature characteristic R-T can be shifted to some extent in the vertical and horizontal direction. This shift can be achieved by changing various factors of the PTC heating element, such as the chemical composition, by changing the lead content, changing the thickness of the PTC heating element, etc.

[0010] A shift of the curve in the vertical direction changes the electrical resistance value. However, in steady-state operation, the electrical power generated and released can only be as much as the thermal power that can be released to the fluid to be warmed. Therefore, if the curve continues to shift downwards in the vertical direction, this power will generally not increase in the steady-state case, but the maximum start-up current will increase when "passing over" the electrical resistance minimum Rmin.

[0011] A shift of the curve in the horizontal direction changes the reference temperature to a lower or higher temperature value. By adding lead to the ceramic material, among other things, the curve will shift to the right to higher temperatures. From a purely thermal point of view, this will make the PTC heating element, and thus also the HV heater, perform better, since there is a greater driving temperature difference from the PTC heating element to the fluid to be warmed. However, the operating range of the PTC heating element is thereby shifted to higher temperatures. As a result, all components of the HV heater and the structural elements surrounding them will be subjected to a higher temperature load. Although the PTC-HV heater is dimensioned to limit at critical boundary conditions, the temperature can be too high. Also, as the characteristic line is shifted to higher temperatures, the PTC heating element also becomes less stable and the risk of voltage breakdown through the sintered ceramic PTC material increases. Increasing the material thickness of the PTC heating element remedies the reduced specific voltage strength. Since the PTC material has a relatively low thermal conductivity and the surface temperature Tsurf of the component is reduced in the case of an increased thickness, the thermal power output of the PTC heating element is also reduced.

[0012] The higher temperatures on the PTC heating element also increase the risk that the properties of the PTC heating element change during the service life. With increasing temperature, the catalytic behavior of the PTC heating element also increases, for example after the absorption of moisture, thereby producing protons, which accumulate at the material grain boundaries. The PTC heating element is subjected to accelerated aging, whereby the voltage strength is reduced.

[0013] Furthermore, lower operating temperatures will facilitate the possibility to use in the component a joining material with a possibly low melting point, such as a tin-based soft solder.

[0014] The operating range of the PTC heating element is such that the electrical energy is always equal to the heat energy released, i.e. Eel = Eheat. Thus, at the defined operating voltage U, the energy generated at the PTC heating element is equal to the energy released or able to be released to the fluid to be heated via the heat exchanger.

[0015] The greater the mass flow of the fluid and / or the greater the driving temperature difference between the PTC heating element surface Tsurf and the fluid temperature, the more power is released.

[0016] Exemplarily, if the mass flow of the fluid to be warmed decreases, the fluid becomes warmer at the set power / voltage. However, this will result in the PTC heating element also becoming warmer. Due to the strong warming of the PTC heating element, the electrical resistance increases along the R-T curve, and the power will decrease until the electrical power and the heat power are again automatically balanced.

[0017] At present, the series of PTC heating elements applied in the HV applications of the 800-volt operating voltage class in the prior art have a material thickness of between 2.8 mm and 3.5 mm and exhibit a continuing upward trend. Increasing the component thickness as a simple measure ensures the voltage strength at the HV operating voltage. The reason is that the voltage in the component falls at the grain boundaries of high ohms, and a greater component thickness will result in more grain boundaries, which will reduce the voltage per grain boundary. However, disadvantageously in these PTC heating elements is that the PTC heating element has a low thermal conductivity. The thermal conductivity is only about 2 W / (mK). This results in the PTC heating element being unevenly temperature-distributed over its thickness in the operating situation - the center within the core is the warmest, while the surface is significantly cooler than the core. With the increase in the PTC ceramic component thickness, the surface temperature Tsurf continuously decreases, thus the heat power that can be output continuously decreases. The reference temperature Tref of the commonly known PTC heating elements is essentially about 190°C to 230°C and higher.

[0018] In order to achieve an adequate surface temperature with the increase in the PTC component thickness at the HV operating voltage, the core temperature of the PTC ceramic must be increased. This can be achieved by increasing the Pb content, since with the increase in the Pb content, Tref also increases. However, increasing the Pb content of the PTC ceramic is not desirable in principle for environmental protection reasons and due to the possible increased burden in the component manufacture. SUMMARY

[0019] It is therefore the task to provide a PTC heating element which is improved compared to the prior art, which improves or avoids the disadvantages of the prior art, and which works at a lower operating temperature and reduces or completely avoids the proportion of lead.

[0020] This task is solved by the features of claim 1.

[0021] One embodiment of the PTC heating element according to the application relates to a PTC heating element having a three-dimensional base body having at least two mutually opposite main faces, wherein the spacing between the main faces of the base body is the thickness D of the base body, and wherein the electrical resistance R of the PTC heating element initially has a minimum value with increasing temperature and has a resistance characteristic which continues to increase with a continued increase in temperature, wherein the reference temperature Tref is the temperature at which the electrical resistance has twice the minimum value at Tmin, and wherein for Tref applies:

[0022] Tref = (40 * D / mm + 75) °C

[0023] wherein the tolerance for Tref is ±10 K,

[0024] The applicable conditions are:

[0025] 0.625 mm ≤ D ≤ 2.5 mm

[0026] wherein the tolerance for D is ±0.25 mm,

[0027] 100°C ≤ Tref ≤ 175°C.

[0028] This achieves a clear reduction in the reference temperature, while at the same time the thickness is reduced, so that the two counteracting effects mutually compensate and positively complement each other. The reduction in thickness thus leads to an increase in heating power as a result of the improved heat yield, and the reduction in the reference temperature combined therewith reduces the risk of breakdown as a result of the lower thickness of the PTC component.

[0029] It is particularly advantageous if the PTC heating element is designed for an operating voltage class of 800 V. Thereby, the above-mentioned effects can be utilized even at these high operating voltages of up to 1250 V.

[0030] It is also appropriate to determine the material thickness reduction amount Dred (in percentage) of thickness D based on a predefined starting value Dstart of 3 mm. This allows for the extraction of a metric that can be used for reference in other ways later. The starting value here is a commonly used predefined value, which is improved by the thickness reduction amount according to the present invention. The material thickness reduction amount Dred represents an improvement made as a percentage and is a measure of the reduced thickness, reduced material usage, etc.

[0031] Advantageously, based on a predetermined initial resistance value Rstart at a predetermined initial value Dstart for a predetermined thickness D, the resistance Rerh is increased by raising the specific resistance of the PTC heating element by a value equivalent to the material thickness reduction Dred, since the reduction in component thickness leads to a decrease in its resistance. This compensates for the resistance drop caused by the thickness reduction and restores the resistance to a higher value almost equivalent to that before the thickness reduction to value D. Therefore, this adjustment of the specific resistance can compensate for the resistance drop caused by the thickness reduction.

[0032] For improving the specific resistance Rspez,neu, the applicable conditions are:

[0033] Rspez,neu = (Dstart / D) * (Rstart / Dstart)

[0034] or

[0035] Rspez,neu = Rspez,alt * (Dstart / D)

[0036] Where Dstart is the initial value of the thickness, D is the thickness, Rstart is the initial value of the resistance, and Rspez,alt is the initial value of the specific resistance.

[0037] Suitable for the PTC heating element is a specific resistance of a low-ohm, medium-ohm, or high-ohm PTC grade. Specifically, the low-ohm PTC grade has a specific resistance of approximately 200 Ohm / mm at a thickness D of 2.2 mm, the medium-ohm PTC grade has approximately 300 Ohm / mm at a thickness D of 2.2 mm, and the high-ohm PTC grade has approximately 400 Ohm / mm at a thickness D of 2.2 mm. This allows for the adjustment and use of resistors that meet the above requirements. Specific resistances between these values ​​can also be used.

[0038] It is also expedient in further embodiments that the three-dimensional base body having at least two mutually opposing main faces is configured as a cuboid or as a cylinder, in particular as a flat cuboid or flat cylinder, in which the extension of the main faces in the plane is at least five times or more greater than the thickness D. Thereby thin but still planar PTC heating elements can be provided, which can be well electrically contacted.

[0039] It is also expedient in further embodiments that the base body is made of a ceramic material, in particular of sintered barium titanate, optionally mixed with further additives and / or unavoidable impurities. Thereby the electrical resistance can be set as desired. Typically, the ceramic material is mixed, added and / or doped barium titanate.

[0040] It is particularly advantageous that the base body is made with a small amount of lead and / or lead oxide added or without lead and / or lead oxide added, wherein the small amount of lead added is determined according to the following formula: Pb proportion in % = 0.045 * Tref - 4.5, wherein the limit for Pb in % is 3% to 0%. Here, the specification of Pb in % refers to the At.-% (atomic percent) relative to the total composition. Thus, the lead content is kept in a very low range, which is in principle desirable. The formula is understood to be an example for a chemical composition with about 3.5 at.-% Ca and 0.1 at.-% Sr incorporated. Here, elements such as Ca and Sr lower the temperature, for example Tref. The formula shows that with decreasing lead content the reference temperature will in principle decrease, however, wherein the given values 0.045 and 4.5 are not constants, but depend on the amount of elements such as Ca and Sr added, which lower Tref.

[0041] It is also advantageous that at least one main face of the base body or both main faces of the base body are provided with an electrically conductive coating, in particular a metal coating, such as a silver coating or an aluminum coating. Thereby a very stable electrical connection between the contact tabs of the adjacent arranged contacts is achieved.

[0042] It is also expedient that the PTC heating element is used in an electrically operated heater for air circulation and warming, which is arranged in an air conditioning device for air conditioning of an interior space of a motor vehicle. Thereby, by lowering the reference temperature of the PTC heating element, overheating of adjacent components and structural assemblies, in particular of plastic parts of the air conditioning facility, is avoided, so that the risk of fire or damage is reduced. BRIEF DESCRIPTION OF DRAWINGS

[0043] The application will be explained in detail below on the basis of embodiments in conjunction with the drawings.

[0044] wherein:

[0045] Figure 1 a schematic view of an embodiment of a PTC heating element according to the present application is shown;

[0046] Figure 2 a further view of a PTC heating element according to Figure 1 the present application is shown;

[0047] Figure 3 a further view of a PTC heating element according to Figure 1 the present application is shown;

[0048] Figure 4 a graph showing the relationship of the resistance characteristic R versus T of a PTC heating element is shown;

[0049] Figure 5 a graph showing the relationship of the resistance characteristic R versus T of a PTC heating element is shown;

[0050] Figure 6 a graph showing the relationship of the resistance characteristic R versus T of a PTC heating element is shown;

[0051] Figure 7 a graph showing the relationship of the reference temperature of a PTC heating element versus the thickness D of the PTC heating element is shown;

[0052] Figure 8 a graph showing the relationship of the specific resistance versus the thickness D of a PTC heating element is shown;

[0053] Figure 9 a graph showing the relationship of the specific breakdown strength UBD / mm versus the thickness D of a PTC heating element is shown;

[0054] Figure 10 a graph showing the relationship of the Pb content versus the reference temperature Tref of a PTC heating element is shown; and

[0055] Figure 11 a graph for representing the Pb, Ca and Sr concentration as a function of Tref is shown. DETAILED DESCRIPTION

[0056] Figure 1 , Figure 2 and Figure 3 a schematic view of a PTC heating element 1 is shown, the PTC element having a three-dimensional base body 2 with at least two main faces 3, 4 facing away from each other. The distance between the main faces 3, 4 of the base body 2 is the thickness D of the base body 2.

[0057] In an advantageous example, the PTC heating element 1 is configured as a cuboid or cylinder, in particular as a flat cuboid or flat cylinder, with its three-dimensional base body 2 having at least two main faces 3, 4 facing away from one another, wherein the extent in the plane of the main faces 3, 4 is at least five times or more greater than the thickness D.

[0058] Preferably, the base body 2 of the PTC heating element 1 is made of a ceramic material, in particular of sintered barium titanate, optionally mixed with further additives and / or unavoidable impurities. By mixing, it is possible, for example, to also influence the positioning of the resistance characteristic in the diagram.

[0059] Here, the base body 2 can also be produced without or with little addition of lead and / or lead oxide, which significantly improves environmental compatibility. Conventionally, for the production of PTC ceramics, Pb oxides are used in powder form, which are reduced to Pb in a so-called calcination production step and then diffuse into the BaTiO3 base body being formed.

[0060] Furthermore, at least one main face 3, 4 of the base body 2 or both main faces 3, 4 of the base body 2 can be provided with an electrically conductive coating, in particular a metal coating, such as a silver coating or an aluminum coating. This improves the electrical contact of the contact patch of the abutment to the heater.

[0061] The resistance R of the PTC heating element 1, see Figure 4 to Figure 6 has a minimum value at low temperatures and a resistance characteristic that increases with increasing temperature, wherein the reference temperature Tref is the temperature at which the resistance Rref has twice the minimum value at Tmin, i.e. Rref = 2 * Rmin.

[0062] The resistance characteristic R-T is influenced by the material selection and the thickness of the PTC heating element 1, as indicated in Figure 5 and Figure 6 can be moved both horizontally and vertically in the diagram, for example. In Figure 6 , the operating range is shown in the range indicated by the ellipse, which is arranged at the rising section of the resistance characteristic.

[0063] The PTC heating element 1 according to the application has a three-dimensional base body 2 with at least two main faces 3, 4 facing away from one another. The spacing of the base body 2 between the main faces 3, 4 is the thickness D of the base body 2.

[0064] The PTC heating element 1 is configured for an operating voltage class of 800 V.

[0065] The resistance R of the PTC heating element 1 has a minimum value at low temperatures and a resistance characteristic that increases with increasing temperature, see Figure 4 to Figure 6wherein the reference temperature Tref is the temperature at which the resistance has twice the minimum value at Tmin.

[0066] According to the idea according to the application, Tref applies:

[0067] Tref = (40 * D / mm + 75) °C

[0068] wherein the tolerance as deviation from the ideal straight line is ±10 K,

[0069] and the following values apply:

[0070] 0.625 mm ≤ D ≤ 2.5 mm, and

[0071] wherein the tolerance for D is ±0.25 mm,

[0072] 100 °C ≤ Tref ≤ 175 °C.

[0073] This limits the straight line as shown in Figure 7 to the upper right corner of the diagram of Figure 7 The individual points in the upper right corner of the diagram of

[0074] By simultaneously reducing the thickness D of the PTC heating element and the reference temperature Tref, a combination of three effects is obtained. By reducing the thickness D by, for example, 0.3 mm, the heating power can be increased via the increase in the surface temperature. Here, the relationship of the relatively low thermal conductivity of the PTC material is utilized. By reducing the reference temperature Tref by, for example, 12 K, a reduction in the risk of breakdown is caused. By this combination of the two effects, the respective disadvantages of the two measures are compensated equivalently.

[0075] That is, a corresponding increase in the risk of breakdown due to the reduction of the material thickness D by 0.3 mm can be correspondingly eliminated or compensated by the reduction of the reference temperature according to the application by 12 K. This also means that a corresponding reduction in the heating performance due to the reduction of the reference temperature by 12 K is correspondingly eliminated / compensated by the reduction of the material thickness according to the application by 0.3 mm.

[0076] Correspondingly, based on the comparison of the predefined starting value Dstart of 3 mm, the amount of reduction Dred of the material thickness of the actual thickness D in % is determined. It is thus possible to find a measure of how much the thickness D of the PTC heating element has been reduced compared to the predefined normal value as starting value.

[0077] In order to compensate for the resistance reduction due to the thickness reduction with the same material selection, the specific resistance Rerh of the PTC heating element 1 is increased by adjusting the material of the PTC heating element 1 accordingly.

[0078] Here, the increase of the specific resistance Rerh is performed by increasing the specific resistance of the PTC heating element 1 by a value corresponding to the inverse of the thickness reduction Dred = Dstart / D of the material, based on a predefined starting value Rstart of the resistance at a predefined starting value Dstart of the thickness D. Thus, the measure of the thickness reduction Dred also scales the increase of the specific resistance. Here, the specific resistance can be increased, for example, as shown in Figure 8 There, three different characteristic lines are shown, which have different specific resistances, respectively. Accordingly, a transformation between these characteristic lines can be performed, or intermediate values can also be taken as required.

[0079] Accordingly, for the amount of the increase of the specific resistance Rspez,neu, the following formula can apply:

[0080] Rspez,neu = (Dstart / D) * (Rstart / Dstart) or Rspez,neu = Rspez,alt *(Dstart / D)

[0081] where Dstart is the starting value of the thickness, D is the thickness, Rstart is the starting value of the resistance, and Rspez,alt is the starting value of the specific resistance.

[0082] The specific resistance Rspez,alt has to be increased by a factor of Dstart / D in order to obtain the same resistance R of the PTC ceramic as before the thickness reduction.

[0083] For example and as shown in Figure 8 The PTC heating element 1 can have a specific resistance belonging to a PTC class of low ohms, a specific resistance belonging to a PTC class of medium ohms, or a specific resistance belonging to a PTC class of high ohms. Therein, the PTC class of low ohms has a specific resistance of about 200 Ohm / mm at a thickness D of 2.2 mm, for example, the PTC class of medium ohms has a specific resistance of about 300 Ohm / mm at a thickness D of 2.2 mm, and the PTC class of high ohms has a specific resistance of about 400 Ohm / mm at a thickness D of 2.2 mm. Other values are possible depending on the material selection.

[0084] In order to increase the specific breakdown strength, the equivalent adjustment of the specific resistance in the reduction of the material thickness of the PTC heating element is defined as an inventive measure. That is to say, the corresponding reduction of the ohmic resistance of the PTC heating element due to the reduction of the material thickness or thickness D according to the application is eliminated or compensated by the increase of the specific resistance according to the application.

[0085] This also means that the corresponding equivalent increase of the heating power due to the reduction of the material thickness or thickness D and the accompanying increase of the starting current at the heating of the PTC heating element 1 is eliminated or compensated by the increase of the specific resistance according to the application.

[0086] Figure 9 A diagram for representing the specific breakdown strength EBD or the breakdown strength in the case of a change of the thickness D according to the application is shown. The diagram shows that, starting from a thickness D of 2.5 mm, the breakdown strength increases from 600 V / mm up to 1000 V / mm as the thickness D becomes smaller.

[0087] In order to increase the specific breakdown strength EBD of the PTC heating element, an equivalent adjustment of the specific ohmic resistance is combined.

[0088] That is to say, the corresponding reduction of the ohmic resistance of the PTC heating element 1 due to the exemplary reduction of the material thickness by 0.3 mm and the exemplary reduction of the reference temperature is eliminated / compensated by the increase of the specific resistance according to the application.

[0089] Furthermore, the corresponding increase of the starting current at the heating of the heating element PTC due to the exemplary reduction of the material thickness or thickness D by 0.3 mm and the exemplary reduction of the reference temperature by 12 K is eliminated or compensated by the increase of the specific resistance.

[0090] Figure 9 A change of the breakdown strength EBD as a material characteristic value with a material thickness of 1 mm according to the embodiment of the application is shown. The technical solution according to the application, in which the material thickness or thickness D is equivalently reduced in power adaptation while Tref is reduced and the resulting resistance change is additionally compensated by the increase of the specific resistance, leads to a continuous increase of the specific breakdown strength EBD. Here, the thermal performance of the PTC heating element 1 is not affected.

[0091] Preferably, the PTC heating element 1 of the three-dimensional base body 2 having at least two main faces 3, 4 facing away from each other is configured as a cuboid or a cylinder, in particular as a flat cuboid or a flat cylinder, in which the extension in the plane of the main faces 3, 4 is at least five times or more greater than the thickness D.

[0092] The PTC heating element 1 is here constructed in such a way that the base body 2 is made of a ceramic material, in particular of sintered barium titanate, optionally mixed with further additives and / or unavoidable impurities. The specific resistance can thus be set as desired.

[0093] Here, in principle, a small amount of lead can be added, wherein it is desirable to be able to dispense with lead. The base body 2 can be made with a small amount of lead and / or lead oxide added or without lead added, wherein the small amount of lead is determined as follows:

[0094] Pb% = 0.045 * Tref - 4.5

[0095] Pb in %

[0096] Tref in °C

[0097] wherein the limit (in %) for Pb is between 3% and 0% in atomic percentage with respect to the total composition. A small amount of lead is thus defined, which scales with the reference temperature and decreases in case the reference temperature decreases, until the lead content is zero.

[0098] Figure 10 The correlation of the lead content (Pb%) with the reference temperature Tref is shown.

[0099] In order to achieve the same thermal performance, heating power for each PTC heating element at the same or improved voltage resistance, the following embodiments are defined:

[0100] Pb% = 0.045 * Tref - 4.5

[0101] The effective range of the lead content in atomic percentage of the total sample is > 3% to 0% (± 1%) in the total sample.

[0102] In order to reduce the reference temperature Tref of the PTC heating element, the lead content in atomic percentage of the total sample is adjusted equivalently. This reduction is combined with the reduction of the reference temperature Tref as described to achieve an exemplary reduction of 12 K, i.e. 0.3 mm thickness reduction per 12 K reduction.

[0103] That is, the corresponding change of the PTC heating element 1 is produced by an exemplary reduction of the material thickness or thickness D by 0.3 mm and the combination of an exemplary reduction of the reference temperature Tref by reducing the lead percentage according to the invention. While reducing the Pb content, the amount of Ca, Sr, etc. added to the base body, which has a stronger Tref drop when the amount added is increased, can be adjusted and optimized.

[0104] Figure 11A diagram for representing the Pb, Ca and Sr concentration as a function of Tref is shown. Herein, experimental data are shown, in which Tref (in °C) is shown as x-axis, wherein the values of Tref are shown only from 50 °C to 300 °C, while different concentrations of lead concentration, strontium concentration and calcium concentration are plotted as y-axis.

[0105] From the measured data of the Pb concentration cPb, the Sr concentration cSr and the Ca concentration cCa, a linear function can be derived for x = Tref [°C]:

[0106] cPb = 0.036 (x) - 2.46 [At.-% / °C],

[0107] cCa = -0.012 (x) + 4.40 [At.-% / °C],

[0108] cSr = -0.003 (x) + 0.94 [At.-% / °C],

[0109] The plotted respective tolerance bands have a value of ± 20 %.

[0110] In the diagram of Figure 11 this is taken into account by multiplying the linear values by 1.2 (Max.) or 0.8 (Min.).

[0111] This means that, when correspondingly changing the PTC heating element 1 for an increase in the specific resistance, an increase in the specific pressure strength can also be produced by reducing the lead percentage according to the application. This can alternatively or additionally also be exhibited by adjusting the sintering parameters.

[0112] This also means that, when the limit value is reached, i.e. in the case of a reference temperature Tref below 120 °C, the doping with lead in the form of lead oxides during PTC production can be completely cancelled while maintaining the pressure strength and the heating power, and thus the PTC heating element 1 according to the application can be produced completely without harmful lead components under the prescribed use.

[0113] It is alternatively also possible that at least one main face 3, 4 of the base body 2 or both main faces 3, 4 of the base body 2 are provided with an electrically conductive coating, in particular a metal coating, such as a silver coating or an aluminum coating.

[0114] List of reference signs

[0115] 1 PTC heating element

[0116] 2 base body

[0117] 3 main face

[0118] 4 main face

Claims

1. PTC heating element (1) having a three-dimensional base body (2) with at least two mutually opposing main faces (3, 4), wherein The distance of the base body (2) between the main faces (3, 4) is the thickness D of the base body (2), and wherein the electrical resistance R of the PTC heating element (1) has a minimum value at first with increasing temperature and has a resistance characteristic which continues to increase with a continued increase in temperature, wherein the reference temperature Tref is the temperature at which the electrical resistance has twice the minimum value at Tmin, and wherein for Tref applies: Tref = (40 * D / mm + 75) °C, wherein the tolerance for Tref is ±10 K, Applicable conditions: 0.625 mm ≤ D ≤ 2.5 mm, wherein the tolerance for D is ±0.25 mm, 100 °C ≤ Tref≤ 175 °C.

2. The PTC heating element (1) according to claim 1, characterized in that The PTC heating element is configured for an operating voltage class of 800 V.

3. The PTC heating element (1) according to claim 1 or 2, characterized in that A material thickness reduction Dred in % of the thickness D is determined based on a predefined starting value Dstart of 3 mm.

4. The PTC heating element (1) according to claim 1, 2 or 3, characterized in that An increase in the specific resistance Rerh of the PTC heating element by a value corresponding to the material thickness reduction Dred = Dstart / D is performed based on a predefined starting value Rstart of the electrical resistance at the predefined starting value Dstart of the thickness D.

5. The PTC heating element (1) according to claim 4, characterized in that For the increase in the specific resistance Rspez,neu applies: Rspez,neu = (Dstart / D) * (Rstart / Dstart) or Rspez,neu = Rspez,alt * (Dstart / D) wherein Dstart is the starting value of the thickness, D is the thickness, Rstart is the starting value of the electrical resistance, and Rspez,alt is the starting value of the specific resistance.

6. The PTC heating element (1) according to any of the preceding claims, characterized in that The PTC heating element has a specific resistance which belongs to a low-ohmic PTC class having a specific resistance of about 200 Ohm / mm at a thickness D of 2.2 mm, a specific resistance which belongs to a medium-ohmic PTC class having a specific resistance of about 300 Ohm / mm at a thickness D of 2.2 mm, or a specific resistance which belongs to a high-ohmic PTC class having a specific resistance of about 400 Ohm / mm at a thickness D of 2.2 mm.

7. The PTC heating element (1) according to any of the preceding claims, characterized in that The three-dimensional base body (2) having at least two mutually opposite main faces (3, 4) is configured as a cuboid or as a cylinder, in particular as a flat cuboid or a flat cylinder, wherein the extension of the main faces (3, 4) in the plane is at least five times or more greater than the thickness D.

8. The PTC heating element (1) according to any of the preceding claims, characterized in that The base body (2) is made of a ceramic material, in particular of sintered barium titanate, optionally mixed with further additives and / or unavoidable impurities.

9. The PTC heating element (1) according to claim 8, characterized in that The substrate (2) is produced with a small amount of lead and / or lead oxide added or without the addition of lead and / or lead oxide, wherein the small amount of lead is determined according to the following formula: Pb proportion in % = 0.045 * Tref - 4.5, wherein the limits for Pb in % are 3% to 0%.

10. The PTC heating element (1) according to claim 9, characterized in that The amount of Ca, Sr, etc. added to the substrate (2) is adjusted and optimized in accordance with the reduction of lead.

11. The PTC heating element (1) according to any of the preceding claims, characterized in that Tref in °C can vary depending on the concentration of Pb, Sr and Ca, wherein the following applies: cPb = 0.036 (x) - 2.46 [At.-% / °C] cCa = -0.012 (x) + 4.40 [At.-% / °C] cSr = -0.003 (x) + 0.94 [At.-% / °C] wherein the tolerance bands are ± 20% each.

12. The PTC heating element (1) according to any of the preceding claims, characterized in that At least one main face (3, 4) of the substrate (2) or both main faces (3, 4) of the substrate (2) are provided with an electrically conductive coating, in particular a metallic coating, such as a silver coating or an aluminum coating, and, if necessary, also with additional functional layers, such as Cr, Ni, etc.

13. PTC heating element (1) according to any of the preceding claims, which is used in an electrically operated heater for circulating and warming air, which heater is arranged in an air conditioning device for air conditioning of an interior space of a motor vehicle.

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