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 two times the minimum value at Tmin, and wherein for Tref, it is applicable: Tref = (40 * D / mm + 107) DEG C wherein the tolerance is + / -10 K, the applicable condition: 0.5 mm < = D < = 1.7 mm wherein the tolerance of D is + / -0.25 mm, 85 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 400 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. an electrical resistance-temperature characteristic R-T, which initially falls approximately linearly from a lower temperature to a higher temperature (NTC range) until a minimum is reached and then rises sharply as the temperature continues to increase (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 electrical 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 is generally not increased in the steady-state case, but the maximum starting current is increased when "passing through" 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, in particular, 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 more powerful, 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 surrounding structural elements will be subjected to a higher temperature load. Although the PTC-HV heater is designed to be limited under critical boundary conditions, the temperature can be too high. Also, as a result of the characteristic line shifting 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 decreases with increasing thickness, the thermal power output of the PTC heating element also decreases.

[0012] A higher temperature on the PTC heating element also increases 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 absorbing moisture, thereby producing protons, which accumulate at the material grain boundaries. The PTC heating element is subjected to accelerated aging, thereby reducing the voltage strength.

[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 surface temperature Tsurf of the PTC heating element and the fluid temperature, the greater the power 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 prior art at the operating voltage level of 400 volts have a material thickness of between 2.0 mm and 3.5 mm. 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 distributed in temperature over its thickness in the operating situation - hottest in the center and significantly cooler at the surface than in the core. With an 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 155°C to 215°C and higher.

[0018] In order to achieve an adequate surface temperature at an 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 an 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 possible increased burdens in the manufacture of the component. SUMMARY

[0019] It is therefore the task to provide a PTC heating element which is improved compared to the prior art, which overcomes 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 opposing 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 as the temperature rises, and has a resistance characteristic which continues to rise as the temperature continues to rise, 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 + 107) °C

[0023] wherein the tolerance is ±10 K

[0024] The applicable conditions are:

[0025] 0.5 mm ≤ D ≤ 1.7 mm

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

[0027] 85°C ≤ Tref ≤ 175°C.

[0028] This achieves a significant 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, which is 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 400 V. Thereby, the above-mentioned effects can be utilized even at these high operating voltages of up to 480 V or more.

[0030] It is also expedient to determine the material thickness reduction Dred (in %) of the thickness D based on a predefined starting value Dstart of 2 mm. Thereby, a measure can be extracted which can be used in a later manner for the reference. The starting value is here a predefined value which is often used and is improved by the thickness reduction according to the application. The material thickness reduction Dred represents the improvement in percent and is a measure for the reduced thickness, the reduced material usage, etc.

[0031] It is also advantageous that the increase in the specific resistance Rerh is performed by increasing the specific resistance of the PTC heating element by a value corresponding to the amount of the thickness reduction Dred = Dstart / D of the material thickness at the starting value Dstart of the predefined thickness D, since the reduction of the thickness of the component leads to a reduction of its electrical resistance. Thereby, the resistance drop due to the thickness reduction can be compensated and the resistance is again brought to a higher value which almost corresponds to the resistance before the thickness was reduced to the value D. Thus, this setting of the specific resistance can compensate the resistance reduction due to the thickness reduction.

[0032] For the increase in the specific resistance Rspez,neu it applies that:

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

[0034] or

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

[0036] 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.

[0037] It is also suitable that the PTC heating element has a specific resistance which belongs to the PTC class of low ohms, to the PTC class of medium ohms or to the PTC class of high ohms, wherein the PTC class of low ohms has a specific resistance of about 150 Ohm / mm at a thickness D of 1.4 mm, the PTC class of medium ohms has a specific resistance of about 250 Ohm / mm at a thickness D of 1.4 mm and the PTC class of high ohms has a specific resistance of about 350 Ohm / mm at a thickness D of 1.4 mm. Thereby, a resistance can be set and used which meets the above requirements. It is also possible to use a specific resistance which is between the above values.

[0038] It is also suitable in further embodiments that the three-dimensional base body having at least two mutually opposed main faces is configured as a cuboid or as a cylinder, in particular as a flat cuboid or as a flat cylinder, wherein 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 if the base body is made with a small or no addition of lead and / or lead oxide, wherein the small addition of lead is determined according to the following formula: Pb 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) with respect 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.-% of Ca and 0.1 At.-% of Sr incorporated. It is known to the person skilled in the art that elements such as Ca and Sr lower the temperature, e.g. Tref. The formula shows that the reference temperature will in principle decrease with decreasing lead content, wherein the given values 0.045 and 4.5 are of course not constants, but depend on the amount of elements such as Ca and Sr added which lower Tref.

[0041] It is also expedient according to advantageous embodiments of the application that the amount of Ca, Sr, etc. added to the base body is adjusted and optimized according to the reduction of lead.

[0042] It is therefore also advantageous that Tref in °C can vary depending on the concentration c of Pb, Sr and Ca, wherein the following applies:

[0043] cPb = 0.036 (x) - 2.46 [At.-% / °C]

[0044] cCa = -0.012 (x) + 4.40 [At.-% / °C] and

[0045] cSr = -0.003 (x) + 0.94 [At.-% / °C]

[0046] wherein the tolerance bands are each ± 20%.

[0047] It is also advantageous if 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, and if necessary additionally with an additional functional layer, such as Cr, Ni, etc. Thereby a very stable electrical connection between the contact patches of the adjacently arranged contacts is achieved.

[0048] It is also expedient that the PTC heating element is used in an electrically operated heater for the circulation and warming of air, which is arranged in an air conditioning device for air conditioning of the interior of a motor vehicle. By lowering the reference temperature of the PTC heating element, overheating of adjacent components and structural assemblies is thus avoided, in particular overheating of plastic parts of the air conditioning installation, so that the risk of fire or damage is reduced. BRIEF DESCRIPTION OF DRAWINGS

[0049] The application will be explained in detail below on the basis of embodiments in conjunction with the drawings.

[0050] wherein:

[0051] Figure 1 A schematic view of an embodiment of a PTC heating element according to the application is shown;

[0052] Figure 2 A further view of a PTC heating element according to Figure 1 is shown;

[0053] Figure 3 A further view of a PTC heating element according to Figure 1 is shown;

[0054] Figure 4 A graph showing the relationship of the electrical resistance R of a PTC heating element to T is shown;

[0055] Figure 5 A graph showing the relationship of the electrical resistance R of a PTC heating element to T is shown;

[0056] Figure 6 A graph showing the relationship of the electrical resistance R of a PTC heating element to T is shown;

[0057] Figure 7 A graph showing the relationship of the reference temperature of a PTC heating element to the thickness D of the PTC heating element is shown;

[0058] Figure 8 A graph showing the relationship of the specific resistance of a PTC heating element to the thickness D is shown;

[0059] Figure 9 A graph showing the relationship of the specific pressure strength UBD / mm of a PTC heating element to the thickness D is shown;

[0060] Figure 10 A graph showing the relationship of the Pb content of a PTC heating element to the reference temperature Tref is shown; and

[0061] Figure 11 A graph for representing the Pb, Ca and Sr concentrations as a function of Tref is shown. DETAILED DESCRIPTION

[0062] Figure 1 、 Figure 2 and Figure 3 A schematic view of a PTC heating element 1 is shown, which has a three- dimensional base body 2 with at least two mutually opposing main faces 3, 4. The spacing between the main faces 3, 4 is the thickness D of the base body 2.

[0063] 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 mutually opposing main faces 3, 4, wherein the extension in the plane of the main faces 3, 4 is at least five times or more greater than the thickness D.

[0064] 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 a diagram.

[0065] 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. Typically, 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.

[0066] 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.

[0067] 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 has twice the minimum value at Tmin, i.e. Rref = 2 * Rmin.

[0068] 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 working range is shown in the range indicated by the ellipse, which is arranged at the rising section of the resistance characteristic.

[0069] The PTC heating element 1 according to the application has a three-dimensional base body 2 which has at least two mutually opposing main faces 3, 4. The spacing between the main faces 3, 4 of the base body 2 is the thickness D of the base body 2.

[0070] The PTC heating element 1 is designed for an operating voltage class of 400 V.

[0071] The electrical resistance R of the PTC heating element 1 initially has a minimum value as the temperature rises and has a resistance characteristic which rises as the temperature continues to rise, see Figure 4 to Figure 6 where the reference temperature Tref is the temperature at which the electrical resistance has twice the minimum value at Tmin.

[0072] According to the idea according to the application, Tref applies:

[0073] Tref = (40 * D / mm + 107) °C

[0074] where the tolerance as a deviation from the ideal straight line is ±10 K

[0075] and the following values apply:

[0076] 0.5 mm ≤ D ≤ 1.7 mm, and

[0077] where the tolerance for D is ±0.25 mm,

[0078] 85°C ≤ Tref ≤ 175°C.

[0079] This limits the straight line as shown in Figure 7 According to the application, the falling thickness D is combined with the falling reference temperature. Figure 7 The individual points in the upper right corner of the diagram of

[0080] By simultaneously reducing the thickness D of the PTC heating element and the reference temperature Tref, three effects are combined. 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 utilised. By reducing the reference temperature Tref by, for example, 12 K, a reduction in the risk of a breakdown is achieved. By these two effect combinations, the respective disadvantages of the two measures are compensated for equivalently.

[0081] That is, a corresponding increase in the risk of breakdown due to a reduction in the material thickness D by 0.3 mm can be eliminated or compensated for by a corresponding reduction in the reference temperature by 12 K according to the application. This also means that a corresponding reduction in the heating performance due to a reduction in the reference temperature by 12 K is eliminated / compensated for by a corresponding reduction in the material thickness by 0.3 mm according to the application.

[0082] Correspondingly, based on a comparison of the predefined starting value Dstart of 3 mm, the amount of reduction Dred in 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 a starting value.

[0083] In order to compensate for the reduction in the electrical resistance due to the reduction in the thickness with the same material selection, an increase in the specific resistance Rerh of the PTC heating element 1 is carried out by a corresponding adjustment of the material of the PTC heating element 1.

[0084] Here, based on a predefined starting value Rstart of the electrical resistance at the predefined thickness D of the starting value Dstart, the increase in the specific resistance Rerh is carried out by increasing the specific resistance of the PTC heating element 1 by a value corresponding to the inverse of the amount of reduction Dred = Dstart / D in the material thickness. Thus, the measure of the amount of reduction Dred also scales the increase in the specific resistance. Here, as shown in Figure 8 , the specific resistance can be increased, for example. There, three different characteristic lines are shown, which each have a different specific resistance. Correspondingly, a transformation can be made between these characteristic lines or intermediate values can also be taken as required.

[0085] Correspondingly, for the amount of increase Rspez,neu in the specific resistance, the following applies:

[0086] Rspez,neu = (Dstart / D) * (Rstart / Dstart) or Rspez,neu = Rspez,alt *(Dstart / D)

[0087] where Dstart is the thickness starting value, D is the thickness, Rstart is the starting value of the electrical resistance and Rspez,alt is the starting value of the specific resistance.

[0088] Thus, the specific resistance is increased to the same extent as the reduction in the electrical resistance due to the reduction in the thickness.

[0089] For example and as shown in Figure 8As shown in the middle, the PTC heating element 1 can have a specific resistance belonging to a low-ohmic PTC grade, a specific resistance belonging to a medium-ohmic PTC grade or a specific resistance belonging to a high-ohmic PTC grade. Therein, the low-ohmic PTC grade has a specific resistance of about 150 Ohm / mm at a thickness D of 1.4 mm, the medium-ohmic PTC grade has a specific resistance of about 250 Ohm / mm at a thickness D of 1.4 mm and the high-ohmic PTC grade has a specific resistance of about 350 Ohm / mm at a thickness D of 1.4 mm. Other values are possible depending on the material selection.

[0090] In order to increase the specific breakdown strength, the equivalent adjustment of the specific resistance when reducing the material thickness of the PTC heating element is defined as an inventive measure. That is, 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.

[0091] 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 when the PTC heating element 1 is heated is eliminated or compensated by the increase of the specific resistance according to the application.

[0092] Figure 9 A diagram for representing the specific breakdown strength (breakdown strength) EBD in case of a change of the thickness D according to the application is shown. The diagram shows that the breakdown strength increases from 500 V / mm up to more than 700 V / mm with decreasing thickness D starting from a thickness D of 1.7 mm.

[0093] In order to increase the specific breakdown strength EBD of the PTC heating element, an equivalent adjustment of the specific ohmic resistance is combined.

[0094] That is, 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.

[0095] Furthermore, the corresponding increase of the starting current when the heating element PTC is heated 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.

[0096] Figure 9The change in the breakdown strength EBD (as a material characteristic value in relation to a material thickness of 1 mm) is shown for PTC heating elements 1 according to embodiments of the application. The technical solution according to the application, in which the power is adapted to the equivalent reduction in material thickness or thickness D while reducing Tref, and the resulting change in resistance is additionally compensated by increasing the specific resistance, leads to a continuous increase in the specific breakdown strength EBD. Here, the thermal performance of the PTC heating element 1 is not affected.

[0097] Preferably, the PTC heating element 1 of the three-dimensional base body 2 having at least two main faces 3, 4 facing away from one another 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.

[0098] The PTC heating element 1 is here configured 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.

[0099] Here, a small amount of lead can be added in principle, wherein it is desirable that the lead can be dispensed with. The base body 2 can be produced with a small amount of added or without added lead and / or lead oxide, wherein the small amount of lead is determined as follows:

[0100] Pb% = 0.045 * Tref - 4.5

[0101] Pb in %

[0102] wherein the limits for Pb (in %) are 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 the event of a lower reference temperature until the lead content is zero.

[0103] Figure 10 The correlation of the lead content (Pb%) with the reference temperature Tref is shown.

[0104] In order to achieve the same thermal performance, heating power for each PTC heating element at the same or improved breakdown strength, the following embodiments are defined:

[0105] Pb% = 0.045 * Tref - 4.5

[0106] The effective range of the lead content in the total sample is 3% to 0% (±1%) in atomic percentage of the total sample.

[0107] To reduce the reference temperature Tref of the PTC heating element, the proportion of lead in terms of atomic percentage of the total sample is equivalently adjusted. This reduction measure is combined with the described reduction of the reference temperature Tref to achieve an exemplary reduction of 12 K, i.e. a reduction of 0.3 mm thickness per 12 K.

[0108] That is, the respective change of the PTC heating element 1 is produced by exemplarily reducing the material thickness or thickness D by 0.3 mm and combining this with an exemplary reduction of the reference temperature Tref by reducing the lead percentage according to the application. 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.

[0109] Figure 11 A graph is shown for representing the Pb, Ca and Sr concentrations as a function of Tref. Here, experimental data are shown, in which Tref (in °C) is shown as an x-axis, wherein the value range of Tref is shown only from 50 °C to 300 °C, while different concentrations of the lead concentration, the strontium concentration and the calcium concentration are plotted as a y-axis.

[0110] 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]:

[0111] cPb = 0.036 (x) - 2.46 [At.-% / °C],

[0112] cCa = -0.012 (x) + 4.40 [At.-% / °C],

[0113] cSr = -0.003 (x) + 0.94 [At.-% / °C],

[0114] The plotted respective tolerance bands have a value of ±20%.

[0115] In the graph of Figure 11 this is taken into account by multiplying the linear value by 1.2 (Max.) or 0.8 (Min.).

[0116] This means that, when the PTC heating element 1 is changed accordingly for an increase in the specific resistance, an increase in the specific pressure resistance 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.

[0117] This also means that, at the limit value, i.e. in the case of a reference temperature Tref below 120°C, the doping with lead oxide forms of lead during the manufacture of the PTC can be completely dispensed with while maintaining the pressure resistance and the heating power, and thus the PTC heating element 1 according to the application can be produced completely without harmful lead components in the prescribed use.

[0118] It is alternatively also possible for at least one main face 3, 4 of the base body 2 or both main faces 3, 4 of the base body 2 to be provided with an electrically conductive coating, in particular a metallic coating, such as a silver coating or an aluminum coating.

[0119] List of reference signs

[0120] 1 PTC heating element

[0121] 2 base body

[0122] 3 main face

[0123] 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 + 107) °C, wherein the tolerance is ±10 K, the applicable conditions: 0.5 mm ≤ D ≤ 1.7 mm, wherein the tolerance for D is ±0.25 mm, 85°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 400 V.

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

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

5. The PTC heating element (1) according to claim 4, characterized in that For the specific resistance increase 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 (1) has a specific resistance which belongs to a low-ohmic PTC class having a specific resistance of about 150 Ohm / mm at a thickness D of 1.4 mm, a specific resistance which belongs to a medium-ohmic PTC class having a specific resistance of about 250 Ohm / mm at a thickness D of 1.4 mm, or a specific resistance which belongs to a high-ohmic PTC class having a specific resistance of about 350 Ohm / mm at a thickness D of 1.4 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 opposing 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 base body (2) is produced 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 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%. The PTC heating element is configured for an operating voltage class of 400 V. Based on a predefined starting value Dstart of 3 mm, a material thickness reduction Dred in % of the thickness D is determined. Based on a predefined starting value Rstart of the electrical resistance at a predefined starting value Dstart of the thickness D, the specific resistance of the PTC heating element is increased by a value corresponding to the material thickness reduction Dred = Dstart / D. For the specific resistance increase 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. The PTC heating element (1) has a specific resistance which belongs to a low-ohmic PTC class having a specific resistance of about 150 Ohm / mm at a thickness D of 1.4 mm, a specific resistance which belongs to a medium-ohmic PTC class having a specific resistance of about 250 Ohm / mm at a thickness D of 1.4 mm, or a specific resistance which belongs to a high-ohmic PTC class having a specific resistance of about 350 Ohm / mm at a thickness D of 1.4 mm. The three-dimensional base body (2) having at least two mutually opposing 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. 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 base body (2) is produced 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 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%.

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 each ± 20%.

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.

Images