NTC composition, thermistor, multilayer thermistor, multilayer thin-film thermistor, method for manufacturing a multilayer thermistor

Abstract

NTC composition, which has a principal component with the sum formula Mn3-xy-zCoxAlyCuzO4, where 0.8 ≤ x ≤ 1.4, 0.6 < y ≤ 1.1 and 0.15 ≤ z ≤ 0.35.

Description

The present description relates to an NTC composition comprising a thermistor, a multilayer thermistor, and a multilayer thin-film thermistor. Furthermore, the application relates to the use of a formula for determining material properties, the use of the formula for material property optimization, a method for material property optimization, and a method for fabricating a multilayer thermistor. Ceramic compositions for thermistors are known from WO 2023 / 189 828 A1 and US 2012 / 0 154 105 A1. US 2022 / 0 238 260 A1 specifically discloses a thin-film thermistor. NTC ceramics are used in thermistors. These thermistors are used in a wide variety of applications, for example in electronics in general, but also in combustion engine and electric vehicles. For some applications, very small components can be advantageous. Some applications require the widest possible temperature sensing range. Others require temperature sensing at low temperatures. In view of the above-mentioned requirements, an improved NTC composition and a thermistor incorporating this composition will be provided. The problem is solved at least partially by an NTC composition according to claim 1 or claim 2. Further preferred embodiments are specified in dependent claims. Other items are claimed in collateral claims or described below. According to one embodiment, an NTC composition with a main component based on manganese-cobalt oxide spinel is described. This composition includes aluminum (with the element symbol Al) and copper (with the element symbol Cu) as additives. The corresponding composition can be described, for example, by the molecular formula Mn3-xy-zCoxAlyCuzO4. The composition may contain oxygen depletion sites, but preferably does not. The NTC composition can contain, in addition to the main component, which constitutes the majority of the NTC composition, a portion of a second phase known as a precipitate. Precipitates can form during sintering, particularly if the sintering temperature is too high. Preferably, the ceramic is selected with respect to its starting materials as if the entire NTC composition would ultimately consist of the main component as the main phase. However, during production, a precipitate phase or phases can form alongside the main component, with portions of the ceramic still fulfilling the target composition, which then constitute the main component. Preferably, the precipitate constitutes a small proportion of the composition. Preferably, the main component has a phase fraction of over 90%; thus, in the case of the main component being represented by Mn3-xy-zCoxAlyCuzO4, this spinel component has a phase fraction of over 90%.Even more preferably, it makes up a proportion of over 95%. With careful sintering, it can also be produced largely in a single phase, e.g., with a phase fraction of over 99%. According to one embodiment, the proportion of added Al and Cu can be determined using the formulas B(T) = Bmax × tanhyp(3 T / T0) and T0 = (h × ν0) / (2 k arcsinh γ). Here, B(T) is the temperature-dependent B-value of the ceramic, T is the temperature, Bmax is the temperature-independent B-value of the dominant small polaron hopping conduction, T0 is the lower limit temperature of the dominant small polaron hopping conduction (SPH conduction), h is the Planck constant, ν0 is the characteristic vibration frequency of the crystal lattice, k is the Boltzmann constant, and γ is the coupling constant. The B-value is a well-known quantity in the NTC ceramic field, representing a characteristic measure of the composition of a thermistor produced with it, or it can be used as a measure of the measurement sensitivity of such a thermistor.Small polaron hopping (SPH) conduction is the generally accepted dominant conduction mechanism in NTC thermistor ceramics. In addition to SPH conduction, a certain temperature-dependent fraction of charge carriers is also transported by band semiconduction. Both mechanisms operate in parallel, resulting in a slight deviation from linear behavior. In particular, the use of these formulas makes it possible to determine material parameters Bmax and Tund γ, which are otherwise difficult to access, simply, accurately, and reproducibly. This allows, for example, material optimization to be carried out quickly. According to one embodiment, the proportions of ceramic components, and in particular the proportions of Al and Cu, are adjusted using the formulas mentioned above such that the Bmax value is between 3400 and 4000 K. According to a preferred embodiment, the proportions are adjusted such that the Bmax value is between 3500 and 3800 K. According to a particularly preferred embodiment, the proportions are adjusted such that the Bmax value is between 3500 and 3600 K. According to one embodiment, the proportions of ceramic components, and in particular the proportions of Al and Cu, are adjusted using the formulas mentioned above such that the value T0 is at most 600 K. According to a preferred embodiment, the proportions are adjusted such that the value T0 is at most 550 K. According to a particularly preferred embodiment, the proportions are adjusted such that the value T0 is at most 500 K. A manganese-cobalt oxide spinel with aluminum and copper as additives, exhibiting a Bmax value of 3400 to 4000 K and / or a T0 value of max. 600 K, can be suitable for use at temperatures below 0°C. For such a thermistor, for example, the B value can more easily remain above 3000 K at temperatures below 0°C. Furthermore, the linear range of the B value can be more extended, for example, reaching lower temperatures. Such a thermistor can therefore be particularly well-suited to extending the technically useful measuring range into temperature ranges below 0°C. A manganese-cobalt oxide spinel with aluminum and copper as additives can be particularly useful in temperature ranges where typical state-of-the-art thermistors, such as those based on a manganese-iron-nickel oxide spinel with copper as an additive, are no longer suitable. Such state-of-the-art manganese-iron-nickel oxide spinels, for example, Mn3-xy-zFexNiyCuzO4 with 0.80 ≤ x ≤ 0.85 and 0.60 ≤ y ≤ 0.65 and 0.001 ≤ z ≤ 0.10, exhibit poor linearity and sensitivity of the measurement signal below 0°C and are therefore unsuitable for use in this measurement range. According to one embodiment, the B-value of the manganese-cobalt-spinel-based NTC composition can be adjusted via the copper content. The addition of aluminum allows the specific resistance of the composition to be increased. For example, the copper addition can increase the conductivity. The inventors have thus recognized that the composition can be optimized through the targeted interaction of these components, the proportions of which are adjusted, for example, using the formulas mentioned above. In this way, these two components can contribute preferred properties and mutually compensate for less preferred properties. Consequently, as an alternative and independent of the features mentioned before or after, a generalized NTC composition with a main component based on manganese-cobalt-spinel oxide with the addition of aluminum and copper is also disclosed. This composition may exhibit further features of other embodiments. According to one embodiment, the ceramic is adjusted to a specific resistance of 200 Ω·cm or more. Preferably to a value of 250 Ω·cm or more, and even more preferably to a value of 400 Ω·cm or more. A value of 800 Ω·cm or higher is still preferred. Higher specific resistances allow thermistors to be miniaturized or, in multilayer thermistors, layer thicknesses to be reduced. In particular, a ceramic with the aforementioned B-values ​​and the specific resistances described here is preferred. Furthermore, a ceramic NTC composition with a main component bearing the molecular formula Mn3-xy-zCoxAlyCuzO4 is described as another embodiment. In this embodiment, 0.8 ≤ x ≤ 1.4, 0.5 ≤ y ≤ 1.1, and 0.15 ≤ z ≤ 0.35 apply. According to a variant of this embodiment, the more precise parameter ranges 0.80 ≤ x ≤ 1.40, 0.50 ≤ y ≤ 1.10, and 0.15 ≤ z ≤ 0.35 can be achieved for the formula. According to one embodiment, 0.90 ≤ x ≤ 1.30, 0.55 ≤ y ≤ 1.00 and 0.15 ≤ z ≤ 0.35. The aforementioned characteristics can occur in combination with each other and also with characteristics mentioned below. In particular, a composition formulated using the formulas above can exhibit the properties described above or those described below. The inventors of the present invention have recognized that such a composition can achieve improved performance of the NTC composition or of a thermistor formed therefrom. For example, the B-value can exhibit better linearity, meaning it can be linearly approximated over a different or wider range without significant error. Furthermore, an excessive drop in the resistance of such a ceramic composition can be prevented. Additionally, a thermistor can be provided that enables improved temperature measurement at low temperatures. In particular, the sensitivity at low temperatures can be enhanced. According to a preferred embodiment, z ≤ 0.15 ≤ z ≤ 0.30. More preferably, z ≤ 0.15 ≤ 0.29, and even more preferably, z ≤ 0.15 ≤ 0.28. According to an embodiment with certain properties, z ≤ 0.15 ≤ 0.27. For z ≤ 0.35, it was found that this can prevent an excessive drop in the composition's resistivity. For values ​​of z ≤ 0.28 or even slightly lower, it was found that such an NTC composition exhibits a preferentially high resistivity. In particular, using a composition for which x and y correspond to the values ​​mentioned above and z to the values ​​given here, it is easier to obtain an NTC composition that exhibits a high B-value at low temperatures while simultaneously possessing a high resistivity. The specified lower limits for z have proven preferable because they aid in adjusting the B-value. Preferred ranges for x are 0.90 ≤ x ≤ 1.30, such as 0.95 ≤ x ≤ 1.25; even more preferred ranges are, for example, 1.00 ≤ x ≤ 1.10. Particularly desirable material properties were identified for these value ranges. For y, the range of 0.55 ≤ y ≤ 0.95 was classified as preferred. Even more preferred is a range of 0.60 ≤ y ≤ 0.95 or 0.60 ≤ y ≤ 0.90 or of 0.70 < y ≤ 0.95 or of 0.75 ≤ y ≤ 0.85. Particularly preferred material properties were also identified for these ranges. In particular, all material compositions were identified as preferred in which two or, even better, three of x, y and z have a preferred or highly preferred value. According to a further embodiment, an NTC composition is described with a main component based on manganese-cobalt oxide spinel, to which aluminum and copper are added, and in which the aluminum to copper ratio is > 2.4. With such an Al to Cu ratio, preferred properties with respect to the B-value as well as the resistivity can be achieved. If an NTC composition with a main component having the molecular formula Mn3-xy-zCoxAlyCuzO4 is considered as an embodiment of the manganese-cobalt-oxide-spinel-based NTC composition, then in the case of the preceding embodiment, y:z > 2.4. x can take on the values ​​described above. y can take on the values ​​described above, and z can be determined according to the ratio y:z. Alternatively, z can also take on the values ​​described above, and y can be determined from the ratio y:z. According to a preferred embodiment, the NTC composition with the molecular formula Mn3-xy-zCoxAlyCuzO4 satisfies both the values ​​for x, y, and z described above and the ratio y:z > 2.4. The advantages of this embodiment can be particularly pronounced. According to one embodiment, the composition is free of nickel, except for unavoidable impurities. Furthermore, a thermistor is disclosed. This thermistor has the aforementioned NTC composition in an NTC ceramic. This means that the ceramic NTC composition constitutes at least a portion or a partial volume of the ceramic of a thermistor. According to one embodiment, the thermistor can be a monolithic NTC thermistor or a multilayer thermistor. A monolithic NTC thermistor can be either a component sintered from a single piece or a component formed from individual layers and sintered, but unlike a multilayer thermistor, it does not have internal electrodes. According to another embodiment, the thermistor can be a monolayer NTC thin-film thermistor or a multilayer thin-film thermistor. A monolayer NTC thin-film thermistor can refer to a component manufactured from a single thin-film layer, or it can be a component built from several individual thin-film layers stacked on top of each other, even in successive manufacturing steps. However, unlike a multilayer thin-film thermistor, it does not have internal electrodes. The ceramic composition allows for small form factors for the aforementioned thermistors, as it enables a sufficiently high specific resistance. In particular, this allows for miniaturization of the components. For example, according to one embodiment, a multilayer thermistor with a volume between 0.075 and 10 mm³ can be obtained. According to a preferred embodiment, the volume of such a multilayer thermistor can range from 0.3 to 5.5 mm³. According to one embodiment, a multilayer thermistor contains at least one ceramic layer that contains or consists of the NTC composition. A similar embodiment can also be configured as a multilayer thin-film thermistor, wherein the multilayer thin-film thermistor contains a ceramic thin-film layer with or consisting of the NTC composition. According to one embodiment, a ceramic layer can have a thickness of 10 to 100 µm, and preferably 20 to 50 µm. According to another embodiment, a thin film layer can have a thickness of 0.001 to 10 µm, and preferably 0.01 to 1 µm. In particular, a ceramic composition with the above-mentioned values ​​for z, such as 0.20 ≤ z ≤ 0.30, has particularly suitable properties to ensure a sufficiently high resistance for corresponding layer thicknesses. In particular, such a composition can be used for thermistors with nominal resistances at 25 °C of 1 to 20 kΩ. Specifically, a nominal resistance at 25 °C can be 10 kΩ. The thermistors, and especially the multilayer thermistors, can be designed as SMD components (Surface Mounted Devices). The thin-film thermistors, whether multilayer or monolayer, can also be applied to or embedded in substrates, be suitable for embedding (SESUB technology), or be intended as a component for a MEMS device. Furthermore, the use of the formula B(T) = Bmax × tanhyp(3 T / T0) for determining the material properties of an NTC spinel ceramic is described. Here, B(T), Bmax, T, and T0 correspond to the quantities described above. Previously, NTC spinel ceramics, such as nickel-manganese spinels or nickel-cobalt spinels, were often described solely using the linear approximation B(T1, T2) = ln(R2 / R1) / (1 / T2 - 1 / T1). For example, T1 = 25°C and T2 = 100°C, where R1 and R2 are resistances at T1 and T2, respectively. Thus, only the ideal room-temperature characteristic of a thermistor ceramic was considered. However, this simplified view does not do justice to such ceramics, especially at lower temperatures. In the linear region, the generally accepted conduction mechanism of small polaron hopping (SPH) dominates. This is particularly dominant at higher temperatures, as sufficient thermal energy is available to excite crystal vibrations. At room temperature and especially below, this is no longer fully the case. In addition to SPH conduction, a smaller proportion of charge carriers are also transported by band semiconduction. Both mechanisms operate in parallel.This results in a deviation from the linear approximation. If the B(T1,T2) value is calculated in this temperature range, it is lower the lower T1 is chosen. If a close measurement interval for T1 and T2 is selected, a continuous decrease in the B(T1,T2) value is observed at lower temperatures. Until now, mechanisms and parameters have only been described with extreme imprecision. The approaches published by Casado et al. (J. Phys.: Condens. Matter 6 (1994) 4685-4698) offer a physical description but only an insufficient basis for determining material properties in technically relevant systems. According to Casado et al., the formulas T0 = (h·vo) / (2k arcsinh γ) and Bmax = (γ·h·vo) / (2k) are used. Here, T0 is the lower limit temperature of the dominant SPH conduction, Bmax is the constant B-value of the dominant SPH conduction, v is the characteristic vibrational frequency of the crystal lattice, h is the Planck constant, k is the Boltzmann constant, and γ is the coupling constant of the vibrational energy with the electrical energy. While the formula Bmax = (γ·h·vo) / (2k) contributes significantly to understanding the deviation from linearity at low temperatures, it makes it difficult to determine the parameters T0, Bmax, and y. Previously, a graphical method was typically used, plotting ln(R) against (1 / T) over the largest possible temperature range. In the region of small (1 / T) values, a straight line is fitted as an asymptote. This allows for estimations of the values ​​T0 (where the RT values ​​intersect the line) and Bmax (the slope of the asymptote). However, the placement and plotting of the line is sometimes arbitrary and, at the very least, subject to considerable uncertainty. The approach using the formula B(T) = Bmax × tanhyp(3T / T0) offers an improved, simpler, and more accurate method. The inventors discovered during their analysis of the B(T1, T2) curve that it is very well represented by the hyperbolic tangent function. Furthermore, the hyperbolic tangent function exhibits the physically correct behavior: as the temperature increases, the function value asymptotically approaches a constant final value. This final value corresponds to Bmax. Since B(T) asymptotically approaches Bmax (T → ∞), one embodiment allows us to define what approximation is considered sufficient. For practical application, T0 can be defined such that it is considered reached when 99.5% of the final value for B (i.e., Bmax) is attained. This results in: B(T0) = tanhyp (3) · Bmax = 0.995 · Bmax Preferably, the support points for B(T2,T1), to which an adjustment is made according to the formula B(T) = Bmax×tanhyp(3 T / T0), are chosen to be closely spaced. Thus, a support point spacing of T2-T1 ≤ 10 K is preferred over a measurement range of -50°C to 200°C. However, T2-T1 can also be chosen to be less widely spaced. Accordingly, T2-T1 can be ≤ 25 K, while still achieving a good fit. This works particularly well if at least 9 support points are available in the measurement range of -50°C to 150°C. According to one embodiment, the coupling constant can be obtained via T0= (h ×v0 / (2 k arcsinh γ). The described procedure allows material properties to be determined quickly and easily. In particular, it simplifies the analysis of large datasets. Furthermore, the procedure can be automated. According to one embodiment, the use of the formula B(T) = Bmax× tanhyp(3 T / T0) for material property optimization of NTC spinel ceramics is described. Since the above-described procedure enables improved recording of material properties, material development, including material property optimization, can be carried out in a targeted manner. Furthermore, a method using the formula Bmax× tanhyp(3 T / T0) is described. This method can exhibit the characteristics described above. According to one embodiment, the method is a method for optimizing the material properties of NTC spinel ceramics, whereby a fit is applied to a B-value curve of a starting material composition using the formula B(T) = Bmax × tanhyp(3 T / T0). Together with the formula T0 = (h × ν0) / (2 k arcsinh γ), the starting material properties of the starting material composition are determined. Based on these starting material properties, additives are selected that specifically influence the B-value at a certain temperature, the T0 value, and / or the resistivity. This allows for material optimization starting from an initial composition, thereby leveraging the aforementioned advantages. According to one embodiment, the process can be carried out as a multi-stage process. In this case, the composition already optimized by the process undergoes the process again, being subjected to the process sequence once more as the starting material composition. In other words, the ceramic composition, already optimized by additives, is subjected to the described process sequence at least once more as the starting material composition. Furthermore, a process for the fabrication of a multilayer thermistor is provided. According to this process, starting materials for a ceramic composition with the molecular formula Mn3-xy-zCoxAlyCuzO4 are first prepared. These starting materials can be, for example, oxides, carbonates, hydroxides, or similar compounds of the metal atoms named in Mn3-xy-zCoxAlyCuzO4. The starting materials are weighed out such that the ceramic composition is determined by the following proportions: 0.8 ≤ x ≤ 1.4, 0.5 ≤ y ≤ 1.1, and 0.15 ≤ z ≤ 0.35. After weighing, the starting materials are milled and then calcined. The milling can be, for example, wet milling. This allows for an average particle size between 0.5 and 1.5 µm, and preferably between 0.7 and 1.0 µm. Calcination can take place at temperatures between 800°C and 1000°C, for example, at 825°C to 925°C. The resulting calcinate is then ground, a process known as re-grinding.The targeted particle sizes can correspond to those of the first milling step. Subsequently, green films are produced using the milled calcinate. The calcinate can be mixed with additives such as binders, wetting agents, dispersants, or similar substances. This mixture can be applied to a carrier film. The green films are then stacked. This stacking is carried out together with starting materials for internal electrode layers. For example, the starting material for internal electrodes can be a metal paste, which can be printed onto some of the green films before stacking. One or more green thermistor components are cut from the resulting stack. These components are then debound. The debound green thermistor components are sintered. Sintering preferably takes place at 1000 to 1200 °C. At this temperature, as few secondary phases as possible and as much primary phase as possible are formed in the resulting ceramic.Decomposition can therefore be reduced. A temperature range of 1050°C to 1150°C is preferred in this regard. Furthermore, external electrodes are applied, each of which contacts internal electrodes. A monolithic thermistor can also be obtained in a similar way. For this, the steps for forming internal electrodes, for example, are omitted. Furthermore, the ceramic obtained through the process can exhibit the aforementioned properties. The following section lists embodiments, which are hereinafter referred to as "forms." These are numbered to better highlight the relationship between these embodiments and the relationship between their features. The numbered embodiments can also be modified by features of further features in this description, which may result in further embodiments. The invention is not limited to these embodiments. Form 1: NTC composition comprising a main component based on manganese-cobalt oxide-spinel, wherein the proportions of an addition of Al and Cu are determined using the formulas B(T) = Bmax × tanhyp(3 T / T0) and T0 = (h × ν0) / (2 k arcsinh γ), where B(T) is the B-value of the ceramic, T is the temperature, T0 is the lower limit temperature of the dominant small polaron hopping conduction, h is the Planck constant, ν0 is the characteristic vibrational frequency of the crystal lattice, k is the Boltzmann constant, and γ is the coupling constant, Bmax is the temperature-independent B-value of the dominant small polaron hopping conduction, and the proportions are adjusted such that a value of Bmax is between 3400 and 4000 K and / or a lower limit temperature T0 ≤ 600 Kelvin is achieved. Form 2: NTC composition having a principal component with the sum formula Mn3-xy-zCoxAlyCuzO4, where 0.8 ≤ x ≤ 1.4, 0.5 ≤ y ≤ 1.1 and 0.15 ≤ z ≤ 0.35. Form 3: NTC composition according to Form 2, where 0.15 ≤ z ≤ 0.30 or preferably 0.15 ≤ z ≤ 0.28 applies. Form 4: NTC composition according to Form 2 or 3, where y:z > 2.4. Form 5: NTC composition according to one of forms 2 to 4, where 0.90 ≤ x ≤ 1.30 or preferably 0.95 < x ≤ 1.25 applies. Form 6: NTC composition according to one of forms 2 to 5, where 0.55 ≤ y ≤ 0.95 or preferably 0.60 ≤ y ≤ 0.95 applies. Form 7: NTC composition according to one of forms 1 to 6, wherein the main component makes up at least 90% of the phases of the NTC composition. Form 8: Thermistor having the NTC composition according to one of forms 1 to 7. Form 9: Multilayer thermistor comprising a ceramic layer comprising the NTC composition according to one of forms 1 to 7. Form 10: Multilayer thermistor according to Form 9, wherein it has a volume of 0.075 to 10 mm3. Form 11: Multilayer thermistor according to Form 9 or Form 10, wherein the ceramic layer has a thickness of 10 to 100 µm and the thermistor is made up of one or more layers. Form 12: Multilayer thermistor according to one of forms 9 to 11, which has a nominal resistance at 25 °C of 1 to 20 kΩ. Form 13: Multilayer thin-film thermistor comprising a ceramic layer having the NTC composition according to one of forms 1 to 7. Form 14: Method for producing a multilayer thermistor, wherein starting materials for a ceramic composition with the molecular formula Mn3-xy-zCoxAlyCuzO4 are provided, wherein the starting materials are weighed such that the ceramic composition is obtained with the following properties: 0.8 ≤ x ≤ 1.4, 0.5 ≤ y ≤ 1.1 and 0.15 ≤ z ≤ 0.35, the starting materials are ground, the powder obtained after grinding the starting materials is calcined, the calcinate obtained is ground, green sheets are produced with the ground calcinate, the green sheets are stacked together with starting materials for inner electrodes, green thermistor components are cut from the stack, the green thermistor components are debound, the debound green thermistor components are sintered to form thermistor components at 1000°C. sintered at temperatures up to 1200 °C, and external electrodes are applied to the sintered thermistor components. Form 15: Method according to Form 14, wherein the starting material for internal electrodes is a metal paste and this is printed onto some green foils before stacking. Form 16: Method according to Form 14 or Form 15, wherein sintering is carried out at 1050°C to 1150°C. Form 17: Use of the formula B(T0) = Bmax× tanhyp(3 T / T0), where B(T) is the B-value of the ceramic, Bmax is the temperature-independent B-value of the dominant small polaron hopping conduction, T is the temperature, T0 is the lower limit temperature of the dominant small polaron hopping conduction, to determine the material properties of an NTC spinel ceramic. Form 18: Use of the formula B(T) = Bmax×tanhyp(3 T / T0), where B(T) is the B-value of the ceramic, Bmax is the temperature-independent B-value of the dominant small polaron hopping conduction, T is the temperature, T0 is the lower limit temperature of the dominant small polaron hopping conduction, for material property optimization of NTC spinel ceramics. Form 19: Use according to Form 17 or Form 18, where T0 is defined as reached when B(T) = tanhyp (3)·Bmax= 0.995·Bmax. Form 20: Method for optimizing the material properties of NTC spinel ceramics, wherein a curve of the B-value of a starting material composition is fitted using the formula B(T) = Bmax × tanhyp(3 T / T0), where B(T) is the B-value of the ceramic, Bmax is the temperature-independent B-value of the dominant small polaron hopping conductance, T is the temperature, T0 is the lower limit temperature of the dominant small polaron hopping conductance, and together with this, using the formula T0 = (h × v0) / (2 k arcsinh γ), where h is the Planck constant, v0 is the characteristic vibrational frequency of the crystal lattice, k is the Boltzmann constant, and γ is the coupling constant, a determination of starting material properties of the starting material composition is obtained, and based on these starting material properties, additives are selected that specifically influence the B-value at a certain temperature, the T0 value, and / or the influence specific resistance. Form 21: Process according to Form 20, wherein the process is carried out as a multi-stage process, wherein at least once the ceramic composition already optimized by additives is again subjected to the described process sequence as the starting material composition. The invention is described below with reference to exemplary embodiments and figures. Schematic representations of components are not to scale. Parts of these components may be distorted with respect to their size, length, or length ratio compared to other components. Accordingly, no sizes or ratios can be derived from the schematic drawings. Similar or similarly appearing components are provided with the same reference numerals. This does not imply that all features of these components are always identical or that the invention is limited to the specific features described in the exemplary embodiments. Fig. 1 schematically shows a cross-sectional view of a first embodiment of an NTC thermistor. Fig. 2 shows a cross-sectional microscopy image of a second embodiment of an NTC thermistor. Fig. 3 shows the B(T) curve for a first thermistor ceramic and a matching curve. Fig. 4 shows the B(T) curve for a second thermistor ceramic and a matching curve. Fig. 5 shows the B(T) curve for a third thermistor ceramic and a matching curve. Fig. 6 shows the B(T) curve for a fourth thermistor ceramic and a matching curve. Fig. 7 shows a cross-sectional microscopy image of a third embodiment of an NTC thermistor. Fig. 8 shows a comparison of the B(T) curves for the first and fourth thermistor ceramics. Figures 1 and 2 each show two embodiments of a multilayer thermistor 1. Figure 1 shows a schematic cross-sectional view and Figure 2 shows a cross-sectional microscopy image taken with a scanning electron microscope. Both embodiments of the multilayer thermistor 1 have several ceramic layers 2, which are stacked alternately with inner electrode layers 3. The ceramic layers 2 consist of a ceramic material as described below with reference to Figures 4, 5 to 6. The inner electrode layers 3 consist of a palladium-silver alloy. The ceramic layers 2 and the inner electrode layers 3 can be obtained using conventional multilayer technology. They can be formed from green foils and by applying metallizations. One or more green foils can be laminated to form a ceramic layer 2. In the stacking or layering direction, there are termination areas 5 at the top and bottom of the layer stack. These also comprise a ceramic material, which can be the same material as that of the ceramic layers 2. Even though the schematic figureAlthough no dimensions or size ratios may be inferred from the above, it can be seen that, depending on the embodiment, any termination areas 5 can be selected to have different thicknesses. Furthermore, both multilayer thermistors 1 have outer electrodes 4, which can be obtained by conventional metallization processes. The multilayer thermistors 1 according to Fig. 1 or Fig. 2 have dimensions of length × depth × height = (1.0 ± 0.1) mm × (0.5 ± 0.05) mm × 0.6 mm or of length × depth × height = (1.6 ± 0.15) mm × (0.8 ± 0.15) mm × 0.9 mm. The ceramic layers in Fig. 2 have a thickness of 30 µm. Figure 3 shows the curve for B(T1, T2) for a ceramic composition with sample number 2917. This is a manganese-iron-nickel-copper spinel sintered at 1070°C. This is a previously mentioned, generally accepted NTC ceramic, which does not meet the desired requirements, particularly in the temperature range below 0°C. Sufficient data on the relevant parameters describing the properties of an NTC thermistor are known in the prior art for this ceramic, providing a basis for comparison for the improved determination method for these parameters. The composition, approximated linearly by B(T1, T2) = ln(R2 / R1) / (1 / T2 - 1 / T1), exhibits a B(25°C, 100°C) value of 3454 K and a resistivity of 1700 Ω·cm. A component with a nominal resistance of 10 kΩ was successfully fabricated using this method. Numerous support points (T2-T1 ≤ 10 K) were identified. The measurement data were fitted using the function B(T) = Bmax × tanhyp(3T / T0). Here, B(T) is the B-value of the ceramic, T is the temperature, and T0 is the lower limit temperature of the dominant small polaron hopping line (SPH line). Furthermore, the following approximation was used: T0 is reached when B(T0) = tanhyp(3) × Bmax = 0.995 × Bmax, i.e., when 99.5% of the final value is reached. This allowed us to determine Bmax= 3870 K and T0= 680 K (407°C). Furthermore, the coupling constant γ, which is 3.10, could be determined using T0 = (h × v0) / (2 k arcsinh γ). Here, h is the Planck constant, v0 the characteristic vibrational frequency of the crystal lattice, and k the Boltzmann constant. Thus, relevant material parameters can be obtained. The composition shows unsatisfactory linearity and sensitivity at T < 0°C. B(T) drops below 3000 K in the analyzed range. The analysis reveals which parameters offer potential for improvement: To obtain a lower T0, both the coupling factor γ should be increased towards 4 and Bmax should be reduced, e.g., to approximately 3400 to 4000 K or preferably to 3500–3600 Kelvin. This should make it possible to obtain a B(T1, T2) value > 3000 Kelvin at sub-zero temperatures and improve the linearity of the RT characteristic. This approach can replace a previously used, less reliable method. Until now, material properties have been determined based on the results of fundamental research by Casado et al. (J. Phys.: Condens. Matter 6 (1994) 4685-4698) using T0 = (h·v0) / (2k arcsinh γ) and Bmax = (γ·h·v0) / (2k). Here, T0 is the lower limiting temperature of the dominant SPH conduction, v0 is the characteristic vibrational frequency of the crystal lattice, h is the Planck constant, k is the Boltzmann constant, and y is the coupling constant of the vibrational energy with the electrical energy. While Bmax = (γ·h·v0) / (2k) contributes significantly to understanding the deviation from linearity at low temperatures, the parameters T0, Bmax, and γ are difficult to determine from this formula. The literature employs a graphical method in which ln(R) is plotted against (1 / T) over a (preferably large) temperature range. In the region of small (1 / T) values, a straight line is fitted as the asymptote.This allows us to estimate the values ​​To (intersection of the RT values ​​with the line) and Bmax (slope of the asymptote). However, ultimately, drawing the line is arbitrary and subject to considerable uncertainty. For this reason, the improved and more accurate method described above was developed, based on the analysis of the B(T1, T2) curve. It was found that the B-value curve (Fig. 3) is very well represented by the hyperbolic tangent function. Furthermore, the hyperbolic tangent function exhibits the physically correct behavior: as the temperature increases, the function value asymptotically approaches a constant final value. Figure 4 shows the B(T1, T2) profile for a ceramic composition according to the invention and an approximation using the method described above. The composition follows the molecular formula Mn3-xy-zCoxAlyCuzO4, where x = 1.18, y = 0.60, and z = 0.28, i.e., it contains a main phase with this composition. The composition was sintered at 1150°C. Sample number 15 is assigned to this composition. This composition shows improved values ​​compared to sample 2917, which corresponds to the usual state of the art. The resistivity is 280 Ω·cm, the B(25,100) value is 3441 K, Bmax is 3549 K, T0 is 467 K, and γ is 3.7. Thus, compared to sample 2917, the coupling factor γ was increased towards 4, and Bmax was reduced to a value of 3500–3600 K. This made it possible to obtain a B(T1, T2) value of > 3000 Kelvin at sub-zero temperatures and to improve the linearity of the RT characteristic, as can be seen in Fig. 4. An excessive drop in resistivity was also prevented. However, it could still be improved further. Figure 5 shows the B(T1, T2) curve for a ceramic composition according to the invention and also a preferred one, and an approximation using the method described above. The composition follows the molecular formula Mn3-xy-zCoxAlyCuzO4, where x = 1.21, y = 0.90, and z = 0.22. The composition was sintered at 1100°C. Sample number 31 is assigned to this composition. This composition shows at least partially improved values ​​compared to sample 2917 and also to sample 15. The resistivity is 1049 Ω·cm, the B(25,100) value is 3581 K, Bmax is between 3700 K and 3800 K, as can be approximated from the figure, T0 is 543 K, and γ is 3.5. Although the coupling factor is slightly reduced, the resistivity is significantly improved compared to sample 15. A component with a nominal resistance of 10 kΩ can easily be achieved with this. Figure 6 shows the B(T1, T2) curve for a particularly preferred ceramic composition according to the invention and an approximation obtained using the method described above. The composition follows the molecular formula Mn3-xy-zCoxAlyCuzO4, where x = 1.02, y = 0.80, and z = 0.27. The composition was sintered at 1100°C. Sample number 49 is assigned to this composition. This composition exhibits further improved properties compared to the previously described samples, at least in some respects. The resistivity is 832 Ω·cm, the B(25,100) value is 3446 K, Bmax is 3542 K, T0 is 457 K, and γ is 3.8. Thus, the sample displays a very good balance of material properties. The manufacturing process is also explained using sample 49 as an example. First, a powder is prepared. The weight and substances used for an 80 kg batch of powder can be found in Table 1. Table 1 AluminumAl2O3, aluminum oxide15.3 kg ManganeseMn3O4, manganese oxide26.0 kg CobaltCo3O4, cobalt oxide30.6 kg Copper (CuO), copper oxide 8.1 kg Total: 80 kg The components were then mixed with water and milled in a stirred ball mill until a target particle size d(50%) of 0.7 to 1.0 µm was obtained. After achieving the target particle size, the suspension was dried and sieved. The powder is then calcined. For this purpose, the powder is filled into cordierite capsules and reacted in a stationary furnace. The heating rate was 5 K / min, the top temperature was 875°C, the holding time was 8 hours, and the cooling rate was -5 K / min. An XRD measurement showed a conversion rate > 90%. The reacted powder was then re-milled. The calcinate was first dry-sieved, then mixed with water and re-milled until a target particle size d(50%) of 0.7 to 1.0 µm was obtained. After achieving the target particle size, the suspension was dried and sieved again. Green films were then produced. The powder is mixed with organic solvents and additives typical for this technology (binders, wetting agents, dispersants, etc.). After adjusting the viscosity to the appropriate level and degassing the suspension, a ceramic film is produced on a carrier film using a film-drawing machine. This film can then be further processed using multi-layer technology. In this specific embodiment, a film with a nominal thickness (“green thickness”) of 28.5 µm was produced. The sintered body is then produced. This is first explained with reference to the illustration in Fig. 7, which shows a SEM image of a sintered component in longitudinal section. In this embodiment, the so-called "tip design" was used. Two inner electrode layers 3 (electrode tips) project from each outer electrode 4 into the component, but without overlapping. The current flow therefore occurs mainly from the electrode tips to the opposite electrode tips of the counter electrode. This minimizes the cross-sectional area of ​​the current flow. The further apart the electrode tips are, the higher the resistance. Initially, internal electrodes were created by printing on the ceramic foil. For this purpose, an AgPd paste (60% Ag, 40% Pd) is applied to the designated areas using screen printing. The printed and unprinted films are then stacked on top of each other in a defined sequence. Stacking sequence based on the embodiment shown in Fig. 7: - 14 unprinted films (form one of the lower end areas 5) - 1 printed film (inner electrode tips of a plane 1) - 5 unprinted films (space to a plane 2) - 1 printed film (inner electrode tips of plane 2) - 13 unprinted films (form the upper cover layer) The stacked films are then pressed (laminated) to ensure stable adhesion between the individual layers. The stack was then cut, producing components ("green parts"). The dimensions of the unsintered components (green parts) are 1.8 × 0.9 mm (L × W). The components are then debound. The binders required for the green processes are burned off before sintering. For this purpose, the components are slowly heated (< 1 K / min) to 450°C with air purging (holding time 6 hours). The components are then sintered. Sintering of the components took place in an air atmosphere under the following sintering conditions: Heating rate = 5 K / min Top temperature = 1100°C (1080°C - 1140°C possible) Holding time = 2 hours Cooling rate = -5 K / min For the evaluation measurements, the two outer electrodes are attached. For this purpose, the caps are coated with silver paste and baked on. The formation of the metallization caps is clearly visible in the SEM image in Fig. 7. Furthermore, the components of the exemplary embodiment were passivated on the surface (glass coating) and nickel-plated and tin-plated on the caps by means of electroplating to ensure they are easily solderable. Finally, Figure 8 shows a comparison of the B(T) curves for samples 49 and 2917. Sample 49 exhibits significantly improved properties with respect to the B-value at low temperatures and linearity. Sample 49 thus has significantly improved properties compared to the NTC ceramics typically used in the prior art, such as sample 2917, and can therefore extend the technically useful measuring range for NTC thermistors to well below 0°C. Reference symbol list 1 Multilayer thermistor 2 Ceramic layer 3 Inner electrode layer 4 Outer electrode 5 Termination area

Claims

NTC composition, which has a principal component with the sum formula Mn3-xy-zCoxAlyCuzO4, where 0.8 ≤ x ≤ 1.4, 0.6 < y ≤ 1.1 and 0.15 ≤ z ≤ 0.

35. NTC composition which has a principal component with the molecular formula Mn3-xy-zCoxAlyCuzO4, where 0.8 ≤ x ≤ 1.4, 0.5 ≤ y ≤ 1.1 and 0.22 ≤ z ≤ 0.3, and the principal component is a spinel component which makes up at least 90% of the phases of the NTC composition. NTC composition according to one of the preceding claims, wherein z ≤ 0.28 applies. NTC composition according to one of the preceding claims, wherein y:z > 2.4 applies. NTC composition according to one of the preceding claims, wherein 0.90 ≤ x ≤ 1.30 or preferably 0.95 < x ≤ 1.25 applies. NTC composition according to claim 2, wherein 0.55 ≤ y ≤ 0.95 or preferably 0.60 ≤ y ≤ 0.95 applies. NTC composition according to claim 1, wherein the main component comprises at least 90% of the phases of the NTC composition. NTC composition according to one of the preceding claims, wherein 0.60 < y ≤ 0.95 applies. Thermistor comprising the NTC composition according to one of the preceding claims. multilayer thermistor comprising a ceramic layer comprising the NTC composition according to any one of claims 1 to 8. Multilayer thermistor according to claim 10, wherein it has a volume of 0.075 to 10 mm3. Multilayer thermistor according to claim 10 or 11, wherein the ceramic layer has a thickness of 10 to 100 µm and the thermistor is composed of one or more layers. Multilayer thermistor according to one of claims 10 to 12, which has a nominal resistance at 25 °C of 1 to 20 kΩ. Multilayer thin-film thermistor comprising a ceramic layer and the ceramic layer comprising the NTC composition according to any one of claims 1 to 8. A method for producing a multilayer thermistor, wherein starting materials for a ceramic composition with the molecular formula Mn3-xy-zCoxAlyCuzO4 are provided, wherein the starting materials are weighed such that the ceramic composition is obtained with the following properties: 0.8 ≤ x ≤ 1.4, 0.5 ≤ y ≤ 1.1 and 0.22 ≤ z ≤ 0.30, the starting materials are ground, the powder obtained after grinding the starting materials is calcined, the calcinate obtained is ground, green sheets are produced with the ground calcinate, the green sheets are stacked together with starting materials for inner electrodes, green thermistor components are cut from the stack, the green thermistor components are debound, and the debound green thermistor components are heated at 1000 to 1200 °C. sintered thermistor components are sintered in such a way that a spinel main component in the ceramic composition has a phase fraction of over 90%,and external electrodes are applied to the sintered thermistor components. A process for the fabrication of a multilayer thermistor, wherein starting materials for a ceramic composition with the molecular formula Mn3-xy-zCoxAlyCuzO4 are provided, wherein the starting materials are weighed such that the ceramic composition has the following properties: 0.8 ≤ x ≤ 1.4, 0.6 < y ≤ 1.1 and 0.15 ≤ z ≤ 0.35, the starting materials are ground, the powder obtained after grinding the starting materials is calcined, the calcinate obtained is ground, green sheets are produced with the ground calcinate, the green sheets are stacked together with starting materials for inner electrodes, green thermistor components are cut from the stack, the green thermistor components are debound, and the debound green thermistor components are sintered at 1000 to 1200 °C. Thermistor components are sintered, and external electrodes are applied to the sintered thermistor components. Method according to claim 15 or 16, wherein the starting material for internal electrodes is a metal paste and this is printed onto some green foils before stacking. Method according to one of claims 15 to 17, wherein the sintering is carried out at 1050°C to 1150°C.

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