PTC heating element

Abstract

The invention relates to a PTC heating element (1) with a three-dimensional base body (2) having at least two opposing main surfaces (3, 4), wherein the distance of the base body (2) between the main surfaces (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 at higher temperatures and a resistance characteristic that increases further at even higher temperatures, wherein the reference temperature Tref is the temperature at which the electrical resistance has twice the value at its minimum at Tmin, and wherein the following applies to Tref: Tref = ( 40 * D / mm + 107 ) * ° C with a tolerance of ±10 °C for 0.5 mm <= D <= 1.7 mm with a tolerance of D ± 0.25 mm 85 ° C < = Tref < = 175 ° C.

Description

[0001] The invention relates to a PTC heating element, in particular for an electric heater, especially for a motor vehicle, which is operated in a 400 V operating voltage class.

[0002] PTC heating elements are well-known in the prior art. They typically consist of a ceramic material, which is pressed into a cuboid, for example. This cuboid typically has two essentially parallel surfaces that serve for electrical contact. The thickness of the PTC element determines, among other things, its electrical resistance, heat dissipation, and the reference temperature, which is the temperature at which the reference resistance is reached. This reference resistance 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., electric heaters.

[0004] Electric heating or auxiliary heating is particularly important in electric vehicles or hybrid vehicles in the passenger car or commercial vehicle sector, as there is little to no waste heat from a combustion engine. In electrically powered vehicles, an electric heater is a simple systemic way to heat a fluid, such as air, water, or coolant.

[0005] The electric heater is not only of great importance during the start-up phase, as with a conventional combustion engine, but also during driving, in order to ensure the maintenance of the cabin interior temperature in cold ambient temperatures.

[0006] In principle, an electric heater can be designed as an air-side heater or a coolant-side electric heater, especially as a high-voltage heater (HV).

[0007] Since the electric high-voltage heaters are powered by the vehicle's traction battery on both the air and coolant sides, nominal voltage ranges of 220–850 V are currently provided. The maximum operating voltage in the 400 V operating voltage class is 480 V. The maximum operating voltage in the 800 V operating voltage class is between 1000 V and 1250 V. The respective power output of these electric heaters is approximately 5 to 10 kW.

[0008] The PTC heating element exhibits an electrical resistance as a function of temperature, i.e., the resistance-temperature characteristic RT. This characteristic initially decreases approximately linearly from low to high temperatures (NTC range) until a minimum is reached, and then increases sharply with further temperature increases (PTC range). The PTC heating element operates at temperatures that fall within the PTC range of the RT curve. The reference temperature Tref is the temperature at which the resistance R is twice as large as the resistance at the minimum Rmin, where Tref > TRmin.

[0009] The resistance-temperature characteristic RT of PTC heating elements can be shifted to a certain extent in both the vertical and horizontal directions. This shift can be achieved by changing various factors of the PTC heating element, such as its chemical composition, the lead content, the thickness of the PTC heating element, etc.

[0010] Shifting the curve vertically changes the electrical resistance. However, in steady-state operation, only as much electrical power can be generated and delivered as can be thermally transferred to the fluid being heated. Therefore, if the curve is shifted further downwards vertically, the power output in steady-state operation often does not increase, but the maximum switching current when passing through the minimum resistance value Rmin does.

[0011] Shifting the curve horizontally alters the reference temperature, shifting it towards lower or higher values. Adding lead to the ceramic material, among other materials, shifts the curve to the right towards higher temperatures. This makes the PTC heating element, and consequently the high-voltage heater, more efficient from a purely thermal perspective, as there is a greater driving temperature gradient from the PTC heating element to the fluid being heated. However, this also shifts the operating range of the PTC heating element to higher temperatures. As a result, all components of the high-voltage heater and surrounding components are subjected to higher temperatures. While the PTC high-voltage heater does regulate itself under critical conditions, this may occur at excessively high temperatures. Furthermore, shifting the characteristic curve to higher temperatures makes the PTC heating element less stable and increases the risk of stress breakdown through the sintered ceramic PTC material.The reduced specific dielectric strength can be remedied by increasing the material thickness of the PTC heating element. Since the PTC material has a comparatively low thermal conductivity, and the surface temperature (Tsurf) of the component decreases with increased thickness, the thermal power output of the PTC heating element is reduced.

[0012] Higher temperatures at the PTC heating element also increase the risk of its characteristics changing over its lifetime. With increasing temperature, the PTC heating element experiences an increase in catalytic activity, for example, after absorbing moisture, leading to the formation of protons that accumulate at the material grain boundaries. This accelerates the aging of the PTC heating element, reducing its dielectric strength.

[0013] Furthermore, lower operating temperatures contribute to the possibility of using low-melting-point joining materials such as tin-based soft solders in the component.

[0014] The operating range of the PTC heating element adjusts itself so that the electrical energy always equals the thermal energy output, i.e., Eelectrical = Ethermal. At a fixed operating voltage U, the PTC heating element generates as much energy as is transferred, or can be transferred, to the fluid being heated via the heat exchanger.

[0015] The greater the mass flow rate of the fluid and / or the driving temperature gradient between the temperature of the PTC heating element surface Tsurf and the temperature of the fluid, the more power can be delivered.

[0016] For example, if the mass flow rate of the fluid to be heated is reduced, the fluid will become warmer at a given power / voltage. However, this also causes the PTC heating element to heat up. Due to the increased heating of the PTC heating element, its electrical resistance increases along the temperature-time curve, and the power output decreases until an equilibrium between electrical and thermal power is automatically established.

[0017] Currently used PTC heating elements in series production for high-voltage (HV) applications in the 400-volt operating voltage class have a material thickness between 2.0 mm and 3.5 mm. The increased component thickness ensures sufficient dielectric strength at HV operating voltages as a simple measure. This is because the stress in the component drops at the high-resistance grain boundaries, and a greater component thickness results in more grain boundaries, thus reducing the stress per grain boundary. However, a disadvantage of these PTC heating elements is their low thermal conductivity. The thermal conductivity is only about 2 W / (mK). Consequently, the PTC heating element is not uniformly warm across its thickness during operation, but rather hottest in the center and significantly cooler at the surface than in the core. As the PTC ceramic component thickness increases, the surface temperature (Tsurf) decreases, thus continuously reducing the extractable thermal power.The reference temperature (Tref) of the usual, known PTC heating elements is essentially about 155 to 215 °C and more.

[0018] To achieve adequate surface temperatures with increasing PTC component thickness at high-voltage operating voltages, the core temperature of the PTC ceramic must be raised. This is achieved by increasing the lead content, since the thermistor (Tref) increases with increasing Pb content. However, increasing the Pb content of PTC ceramics is generally undesirable for environmental reasons and due to potentially increased stress during component manufacturing.

[0019] The task is therefore to create a PTC heating element that is improved compared to the state of the art, improves or avoids the disadvantages of the state of the art, operates at lower operating temperatures, and reduces or completely avoids the lead content.

[0020] This problem is solved using the features of claim 1.

[0021] One embodiment of the PTC heating element according to the invention relates to a PTC heating element with a three-dimensional base body having at least two opposing main surfaces, wherein the distance of the base body between the main surfaces is the thickness D of the base body, and wherein the electrical resistance R of the PTC heating element has a minimum at higher temperatures and a resistance characteristic that increases further at even higher temperatures, wherein the reference temperature Tref is the temperature at which the electrical resistance has twice the value of the minimum value at Tmin, and wherein the following applies to Tref: Tref=(40*D / mm+107)°C with a tolerance of ±10 K for 0.5<=D<=1.7 mm with a tolerance of D ± 0.25 mm. 85 °C<=Tref<=175 °C.

[0022] This results in a significant reduction in the reference temperature and a corresponding reduction in thickness, so that opposing effects compensate for and complement each other. The thickness reduction, through improved heat dissipation, leads to an increase in heating power, and the combined reduction in the reference temperature, in turn, reduces the risk of breakdown due to the reduced PTC component thickness.

[0023] It is particularly advantageous if the PTC heating element is designed for an operating voltage class of 400 V. This allows the effect described above to be utilized at these high operating voltages of up to 480 V or more.

[0024] It is also advantageous to determine a material thickness reduction, Dred, in %, from the thickness D compared to a predefined starting value, Dstart, of 2 mm. This allows for the extraction of a measure that can later be used for reference. The starting value is a predefined value that is commonly used and which is improved by the invention through the thickness reduction. The material thickness reduction, Dred, represents the percentage improvement and is a measure of the reduced thickness, the reduced material usage, etc.

[0025] It is also advantageous to increase the specific resistance Rspez,erh of the PTC heating element, starting from a predefined initial resistance Rstart at a predefined initial thickness Dstart, by a value corresponding to the reduction in material thickness Drred = Dstart / D. This is because a reduction in component thickness results in a reduction in its electrical resistance. This compensates for the resistance decrease caused by the thickness reduction and restores it to a higher value, essentially equivalent to the resistance value before the thickness reduction to D. This adjustment of the specific electrical resistance thus compensates for the reduction in electrical resistance due to the reduced thickness.

[0026] For the increase in specific resistance Rspez,neu, the following applies: Rspez,neu=(Dstart / D)*(Rstart / Dstart) or Rspez,neu=Rspez,al*(Dstart / D) with Dstart being the initial value of the thickness, D being the thickness, Rstart being the initial value of the electrical resistance and Rspez,alt being the initial value of the specific resistance.

[0027] It is also advantageous for the PTC heating element to have a specific electrical resistance in the low-resistance PTC class, the medium-resistance PTC class, or the high-resistance PTC class, where the low-resistance PTC class has a specific electrical resistance of approximately 150 ohms / mm at a thickness D of 1.4 mm, the medium-resistance PTC class has a specific electrical resistance of approximately 250 ohms / mm at a thickness D of 1.4 mm, and the high-resistance PTC class has a specific electrical resistance of approximately 350 ohms / mm at a thickness D of 1.4 mm. This allows for the selection and use of electrical resistances that meet the requirements described above. Specific resistance values ​​between these values ​​can also be used.

[0028] In another embodiment, it is also advantageous if the three-dimensional base body with the at least two opposing main surfaces is designed as a cuboid or a cylinder, in particular as a flat cuboid or a flat cylinder, in which the extent in the plane of the main surfaces is at least five times or more greater than the thickness D. This allows thin yet planar PTC heating elements to be created that are easily electrically contactable.

[0029] In another embodiment, it is also advantageous for the base body to be made of a ceramic material, in particular sintered barium titanate, optionally with the addition of further additives and / or unavoidable impurities. This allows the electrical resistance to be adjusted as desired. Typically, the ceramic material is barium titanate with additives, additions, and / or doping.

[0030] It is particularly advantageous if the base material is produced with a small addition or without the addition of lead and / or lead oxide, whereby the small addition of lead is determined according to the formula: Pb content in % = 0.045 * Tref - 4.5, which applies within the limits for Pb in % from 3% to 0%. The Pb percentage is to be understood as at% relative to the total composition. This keeps the lead content very low, which is generally desirable. The formula serves as an example for a chemical composition containing approximately 3.5 at% Ca and 0.1% at% Sr. It is known to those skilled in the art that elements such as Ca and Sr lower temperatures such as Tref. The formula shows that the reference temperature generally decreases with decreasing Pb content, whereby the stated numerical values ​​0.045 and 4.5 are naturally not constants, but depend on the amount of Tref-lowering elements such as Ca and Sr added.

[0031] According to an advantageous embodiment, it is also expedient if the amounts of Ca, Sr etc. added to the base body are adapted and optimized for Pb reduction.

[0032] It is also advantageous if Tref in [°C] varies depending on the concentrations c of Pb, Sr and Ca, where: cPb=0.036(x)−2.46[At.-% / °C], cCa=−0.012(x)+4.40[At.-% / °C], and cSr=−0.003(x)+0.94[At.-% / °C], with a tolerance band of + / - 20% in each case.

[0033] It is also advantageous if at least one main surface of the base body, or both main surfaces of the base body, is provided with an electrically conductive coating, in particular a metallic coating such as a silver or aluminum coating, and optionally additionally with functional layers such as Cr, Ni, etc. This ensures a very good, stable electrical connection to adjacent contact plates.

[0034] It is also advantageous when the PTC heating element is used in an electrically operated heater for the circulation and heating of air, such as in an air conditioning unit for the interior of a motor vehicle. By reducing the reference temperature of the PTC heating elements, the risk of overheating of adjacent components and assemblies is avoided, in particular overheating of plastic parts of the air conditioning system, thus reducing the risk of fire or damage.

[0035] The invention is explained in more detail below based on exemplary embodiments and the figures in the drawings.

[0036] They show: Fig. 1 a schematic view of an embodiment of a PTC heating element according to the invention, Fig. 2 another view of the PTC heating element after Fig. 1, Fig. 3 another view of the PTC heating element after Fig. 1, Fig. 4 a resistance characteristic R vs. T of a PTC heating element, Fig. 5 a resistance characteristic R vs. T of a PTC heating element, Fig. 6. A resistance characteristic R vs. T of a PTC heating element, Fig. 7 a diagram of the reference temperature of the PTC heating element vs. the thickness D of the PTC heating element, Fig. 8 a diagram of the specific electrical resistance vs. the thickness D of the PTC heating element, Fig. 9 a diagram of the specific dielectric strength UBD / mm vs. the thickness D of the PTC heating element, Fig. 10 a diagram of the Pb content vs. the reference temperature Tref of the PTC heating element, and Fig. 11 a diagram to show Pb, Ca and Sr concentrations as a function of Tref.

[0037] The Fig. 1, Fig. 2 and Fig. Figure 3 shows a schematic representation of a PTC heating element 1 with a three-dimensional base body 2 having at least two opposing main surfaces 3, 4. The distance of the base body 2 between the main surfaces 3, 4 is the thickness D of the base body 2.

[0038] In an advantageous example, the PTC heating element 1 with its three-dimensional base body 2 with the at least two opposing main surfaces 3, 4 is designed as a cuboid or as a cylinder, in particular as a flat cuboid or as a flat cylinder, in which the extent in the plane of the main surfaces 3, 4 is at least five times or more greater than the thickness D.

[0039] Preferably, the base body 2 of the PTC heating element 1 is made of a ceramic material, in particular of sintered barium titanate, optionally with the addition of further additives and / or unavoidable impurities. The additives can, for example, also influence the positioning of the resistance characteristic curve in the diagram.

[0040] For example, the base body 2 can also be produced without or with a reduced addition of lead and / or lead oxide, which significantly improves environmental compatibility. Typically, lead oxide in powder form is used to produce PTC ceramics. During the calcination step, this is reduced to lead, which then diffuses into the forming BaTiO3 base body.

[0041] Furthermore, at least one main surface 3, 4 of the base body 2, or both main surfaces 3, 4 of the base body 2, can be provided with an electrically conductive coating, in particular with a metallic coating, such as a silver or aluminum coating. This improves the electrical contact with an adjacent contact plate of a heater.

[0042] The electrical resistance R of the PTC heating element 1 is shown in the Fig. 4, Fig. 5 to Fig. 6, starting at low temperatures and progressing to higher temperatures, exhibits a minimum resistance and a resistance characteristic that increases again at further higher temperatures, where the reference temperature Tref is the temperature at which the electrical resistance Rref has twice the value as at its minimum at Tmin, i.e., Rref = 2*Rmin.

[0043] The resistance characteristic RT can be shifted both horizontally and vertically in the diagram by influencing the choice of material and the thickness of the PTC heating element 1, as shown in the Fig. 5 and Fig. 6 is indicated. In Fig. 6 shows the working area in an oval-shaped area, which is located on the rising branch of the resistance characteristic.

[0044] The PTC heating element 1 according to the invention has a three-dimensional base body 2 with at least two opposing main surfaces 3, 4. The distance of the base body 2 between the main surfaces 3, 4 is the thickness D of the base body 2.

[0045] The PTC heating element 1 is designed for an operating voltage class of 400 V.

[0046] The electrical resistance R of the PTC heating element 1 exhibits a minimum at higher temperatures and a resistance characteristic that increases again at even higher temperatures, see the Fig. 4, Fig. 5 to Fig. 6, where the reference temperature Tref is the temperature at which the electrical resistance is twice the value at its minimum at Tmin.

[0047] According to the inventive idea, the following applies to tref: Tref=(40*D / mm+107)°C with a tolerance of ± 10 K as a deviation from the ideal straight line and for the following values: 0.5 mm <= D <= 1.7 mm and with a tolerance of D ± 0.25 mm 85 °C<=Tref<=175 °C.

[0048] This defines a straight line, as it appears in Fig. Figure 7 is shown. According to the invention, a decreasing thickness D is combined with a decreasing reference temperature. The individual points at the top right of the diagram of the Fig. The 7 represent state-of-the-art values, randomly distributed.

[0049] By simultaneously reducing the thickness D and the reference temperature Tref of the PTC heating element, three effects are combined. Reducing the thickness D by, for example, 0.3 mm increases the heating power by raising the surface temperature. This utilizes the relationship with the relatively low thermal conductivity of the PTC material. Reducing the reference temperature Tref by, for example, 12 K reduces the risk of breakdown. The combination of these two effects effectively compensates for the respective disadvantages of each measure.

[0050] This means that the corresponding increase in the risk of puncture caused by reducing the material thickness D by 0.3 mm is correspondingly eliminated or compensated for by the reduction of the reference temperature by 12 K according to the invention. This also means that the corresponding reduction in heating capacity caused by reducing the reference temperature by 12 K is eliminated / compensated for by the reduction of the material thickness by 0.3 mm according to the invention.

[0051] Accordingly, the material thickness reduction Dred in % is determined by comparing the actual thickness D to a predefined starting value Dstart of 3 mm. This allows a measure to be found of how much the thickness D of a PTC heating element has been reduced compared to a given standard value as a starting value.

[0052] To compensate for the reduction in resistance caused by the reduction in thickness with the same choice of material, the specific resistance Rerh of the PTC heating element 1 is increased by adjusting the material of the PTC heating element 1 accordingly.

[0053] The specific resistance Rerh of the PTC heating element 1 is increased from a predefined initial value of electrical resistance Rstart at a predefined initial value Dstart of thickness D by a value corresponding to the inverse reduction of material thickness Dred = Dstart / D. The degree of thickness reduction Dred thus also scales the increase in specific resistance. For example, the specific resistance can be increased, as in the Fig. Figure 8 shows three different characteristic curves, each with a different specific resistance. It is possible to switch between these curves or, if necessary, to assume an intermediate value.

[0054] Accordingly, the following can apply to the increase in specific resistance Rspez,neu: Rspez,neu=(Dstart / D)*(Rstart / Dstart) or Rspez,neu=Rspez,al*(Dstart / D) with Dstart being the initial value of the thickness, D being the thickness, Rstart being the initial value of the electrical resistance, Rspez,alt being the initial value of the specific resistance.

[0055] This allows the specific resistance to be increased in the same way that reducing the thickness reduces the resistance.

[0056] For example, and how in Fig. As shown in Figure 8, the PTC heating element 1 can have a specific electrical resistance in the low-resistance PTC class, the medium-resistance PTC class, or the high-resistance PTC class, where the low-resistance PTC class has a specific electrical resistance of approximately 150 ohms / mm at a thickness D of 1.4 mm, the medium-resistance PTC class has a specific electrical resistance of approximately 250 ohms / mm at a thickness D of 1.4 mm, and the high-resistance PTC class has a specific electrical resistance of approximately 350 ohms / mm at a thickness D of 1.4 mm. Other values ​​are also possible depending on the choice of material.

[0057] To increase the specific dielectric strength, an equivalent adjustment of the specific ohmic resistance when reducing the material thickness of PTC heating elements is defined as an inventive measure. This means that the corresponding reduction in the ohmic resistance of a PTC heating element through a reduction of the material thickness D according to the invention is negated or compensated for by an increase in the specific ohmic resistance according to the invention.

[0058] This also means that the corresponding equivalent increase in heating power and thus the associated increase in the inlet current when heating a PTC heating element 1 resulting from the reduction of the material thickness D is eliminated or compensated by the increase in specific ohmic resistance according to the invention.

[0059] The Fig. Figure 9 shows a diagram illustrating the specific dielectric strength (breakdown strength) EBD with changes in thickness D according to the invention. The diagram shows that from a thickness D of 1.7 mm, the dielectric strength increases from 500 V / mm to over 700 V / mm for decreasing thickness D.

[0060] To increase the specific dielectric strength EBD of a PTC heating element, an equivalent adjustment of the specific ohmic resistance is combined.

[0061] This means that the corresponding reduction of the ohmic resistance of a PTC heating element 1 by, for example, reducing the material thickness by 0.3 mm and reducing the reference temperature, is canceled out / compensated by the increase in specific ohmic resistance according to the invention.

[0062] Furthermore, the corresponding increase in the inlet current when heating a PTC heating element is eliminated or compensated by the exemplary reduction of the material thickness D by 0.3 mm and exemplary reduction of the reference temperature by 12K by increasing the specific ohmic resistance.

[0063] In Fig. Figure 9 illustrates how the dielectric strength EBD, a material property value per 1 mm of material thickness, changes according to the inventive embodiments. The inventive reduction of the refractive index (Trf) combined with a performance-compliant equivalent reduction of the material thickness D and additional compensation of the resistance change by increasing the specific resistance, leads to a continuous increase in the specific dielectric strength EBD. This does not affect the thermal performance of the PTC heating element 1.

[0064] Preferably, the PTC heating element 1 is generally designed with a three-dimensional base body 2 with at least two opposing main surfaces 3, 4 as a cuboid or as a cylinder, in particular as a flat cuboid or as a flat cylinder, in which the extent in the plane of the main surfaces 3, 4 is at least five times or more greater than the thickness D.

[0065] The PTC heating element 1 is designed such that the base body 2 is made of a ceramic material, in particular sintered barium titanate, optionally with the addition of other additives and / or unavoidable impurities. This allows the specific resistance to be adjusted as desired.

[0066] Lead can be added in small quantities, although it is preferable to avoid its use. Base body 2 can be produced with or without a small addition of lead and / or lead oxide, the small addition of which is determined by the formula Proportion Pb=0.045*Tref−4.5 Pb in % This is determined within the limits for Pb in % from 3% to 0% in atomic percent based on the total composition. This defines a small amount of lead that scales with the reference temperature and decreases as the reference temperature decreases until the lead content is zero.

[0067] The Fig. Figure 10 shows a diagram illustrating the lead content (proportion of Pb) as a function of the reference temperature Tref.

[0068] To achieve the same thermal performance, the heating power per PTC heating element, with the same or improved dielectric strength, the following design is defined: Percentage of lead content Pb%=0.045×Tref-4.5 of the total sample in atomic percent with a validity of 3% to 0% percent lead content of the total sample in atomic percent (+ / - 1%).

[0069] To reduce the reference temperature (Tref) of a PTC heating element, an equivalent adjustment of the percentage of lead in atomic percent of the total sample is performed. This reduction is combined with the described reduction of the reference temperature (Tref) to achieve the example of 12 K by a 0.3 mm thickness reduction for every 12 K.

[0070] This means that the corresponding modification of a PTC heating element 1 is achieved by, for example, reducing the material thickness D by 0.3 mm and, in combination, by, for example, reducing the reference temperature Tref by reducing the percentage of lead according to the invention. Alongside the reduction of the Pb content, the amounts of Ca, Sr, etc. added to the base material, which have a stronger Tref-reducing effect when added in greater quantities, can be adjusted and optimized.

[0071] The Fig. Figure 11 shows a diagram representing Pb, Ca, and Sr concentrations as a function of Tref. Experimental data are presented, with Tref in °C shown as the x-axis (the range of Tref is only from 50°C to 300°C), and the different concentrations of Pb, Sr, and Ca plotted on the y-axis.

[0072] Linear functions can be derived from the measurement data for the Pb concentration cPb, the Sr concentration cSr and the Ca concentration cCa for x = Tref in [°C]: cPb=0.036(x)−2.46[At.-% / °C] cCa=−0.012(x)+4.40[At.-% / °C] cSr=−0.003(x)+0.94[At.-% / °C]

[0073] A value of + / - 20% is given for each tolerance band shown. In the diagram of the Fig. 11 takes this into account in the form of a multiplication of the linear value by 1.2 (Max.) or 0.8 (Min.).

[0074] This also means that, with a corresponding modification of a PTC heating element 1, the specific resistance and the specific dielectric strength can also be increased by reducing the percentage of lead according to the invention. This can alternatively or additionally be achieved by adjusting the sintering parameters.

[0075] This also means that if a limit value is reached, i.e., if a reference temperature Tref of < 120°C is undercut while simultaneously maintaining the dielectric strength and heating power, the addition of lead in the form of lead oxide during PTC production can be completely omitted, and thus a PTC heating element 1 according to the invention can be manufactured completely without harmful lead content for its intended use.

[0076] Optionally, it is also possible if at least one main surface 3, 4 of the base body 2 or both main surfaces 3, 4 of the base body 2 are provided with an electrically conductive coating, in particular with a metallic coating, such as a silver or aluminum coating. Reference symbol list 1 PTC heating element 2 basic shapes 3 Main area 4 Main area

Claims

[1] PTC heating element (1) with a three-dimensional base body (2) having at least two opposing main surfaces (3, 4), wherein the distance of the base body (2) between the main surfaces (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 at higher temperatures and a resistance characteristic that increases further at higher temperatures, wherein the reference temperature Tref is the temperature at which the electrical resistance has twice the value at its minimum at Tmin, and wherein the following applies to Tref: Tref=(40*D / mm+107)°C with a tolerance of ±10 K for 0.5 mm <= D <= 1.7 mm with a tolerance of D ± 0.25 mm 85°C<=Tref<=175°C. [2] PTC heating element (1) according to claim 1, characterized by that it is designed for the operating voltage class of 400 V. [3] PTC heating element (1) according to claim 1 or 2, characterized by , that a material thickness reduction Dred in % is determined from the thickness D compared to a predefined starting value Dstart of 3 mm. [4] PTC heating element (1) according to claim 1, 2 or 3, characterized by , that an increase Rerh of the specific resistance of the PTC heating element is made, starting from a predefined initial value of the electrical resistance Rstart at the predefined initial value Dstart of the thickness D by a value corresponding to the material thickness reduction Dred = Dstart / D. [5] PTC heating element (1) according to claim 4, characterized by , that for the increase in specific resistance Rspez,neu, the following applies: Rspez,neu=(Dstart / D)*(Rstart / Dstart) or Rspez,neu=Rspez,al*(Dstart / D) with Dstart being the initial value of the thickness, D being the thickness, Rstart being the initial value of the electrical resistance, Rspez,alt being the initial value of the specific resistance. [6] PTC heating element (1) according to any one of the preceding claims, characterized by , that the PTC heating element (1) has a specific electrical resistance in the low-resistance PTC class, in the medium-resistance PTC class or in the high-resistance PTC class, wherein the low-resistance PTC class has a specific electrical resistance of approximately 150 ohms / mm at a thickness D of 1.4 mm, the medium-resistance PTC class has a specific electrical resistance of approximately 250 ohms / mm at a thickness D of 1.4 mm and the high-resistance PTC class has a specific electrical resistance of approximately 350 ohms / mm at a thickness D of 1.4 mm. [7] PTC heating element (1) according to any one of the preceding claims, characterized by, that the three-dimensional basic body (2) with the at least two opposing principal surfaces (3, 4) is designed as a cuboid or as a cylinder, in particular as a flat cuboid or as a flat cylinder, in which the extent in the plane of the principal surfaces (3, 4) is at least five times or more greater than the thickness D. [8] PTC heating element (1) according to any one of the preceding claims, characterized by , that the base body (2) is made of a ceramic material, in particular of a sintered barium titanate, optionally with the addition of further additives and / or unavoidable impurities. [9] PTC heating element (1) according to claim 8, characterized by , that the base body (2) is produced with a small addition or without the addition of lead and / or lead oxide, the small addition of lead being determined according to the formula Pb content in % = 0.045 * Tref - 4.5, where this is within the limits for Pb in % of 3% to 0%. [10] PTC heating element (1) according to claim 9, characterized by , that the amounts of Ca, Sr etc. added to the base body (2) are adapted and optimized for Pb reduction. [11] PTC heating element (1) according to any one of the preceding claims, characterized by , that Tref in [°C] varies depending on the concentrations c of Pb, Sr and Ca, where: cPb=0.036(x)−2.46[At.-% / °C], cCa=−0.012(x)+4.40[At.-% / °C], and cSr=−0.003(x)−0.94[At.-% / °C], with a tolerance band of + / - 20% in each case. [12] PTC heating element (1) according to any one of the preceding claims, characterized by, that at least one main surface (3, 4) of the base body (2) or both main surfaces (3, 4) of the base body (2) is provided with an electrically conductive coating, in particular with a metallic coating, such as a silver or aluminum coating, and may additionally be provided with additional functional layers such as Cr, Ni etc. [13] PTC heating element (1) according to one of the preceding claims, which is used in an electrically operated heater for the flow and heating of air, which heater is arranged in an air conditioning unit for the air conditioning of the interior of a motor vehicle.

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