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

Abstract

An NTC composition based on manganese-cobalt oxide-spinel is provided, which has the molecular formula Mn 3-x-y-z Co x Al y Cu z O4 corresponds to where 0.80 ≤ x ≤ 1.40, 0.50 ≤ y ≤ 1.10 and 0.15 ≤ z ≤ 0.35, or the proportions of an addition of Al and Cu using the formulas B(T) = B max × tanhyp(3 T / T0) and T0 = (h × v0) / (2 k arcsinh γ) are determined, where the quantity proportions are set so that a value B max from 3400 to 4000 K and / or a lower limit temperature T0 ≤ 600 Kelvin is reached.

Description

[0001] The present application relates to an NTC composition, a thermistor comprising the same, a multilayer thermistor comprising the same, as well as a monolayer NTC thin-film thermistor and a multilayer thin-film thermistor comprising the same. Furthermore, the application relates to a method for producing a multilayer thermistor.

[0002] NTC ceramics are used in thermistors. Such thermistors are used in a variety of applications, for example, in electronics in general, but also in combustion-engine cars and electric vehicles. For some applications, the smallest possible components can be advantageous. Some applications require the widest possible temperature detection over a wide range. Still, some applications require temperature detection at low temperatures.

[0003] WO 2003 / 189 828 A1 and US 2012 / 0 154 105 A1 each disclose ceramic compositions for thermistors. US 2022 / 0 238 260 A1, in particular, shows a thin-film thermistor.

[0004] In view of the above requirements, an improved NTC composition and a thermistor comprising it shall be provided.

[0005] This object is at least partially achieved by an NTC composition according to claim 1. Further preferred embodiments are specified in the subclaims. Other subject matter is claimed in the subordinate claims or described below.

[0006] According to one embodiment, an NTC composition with a manganese-cobalt oxide spinel-based main component is described. This composition has aluminum with the element symbol Al and copper with the element symbol Cu as additives. The corresponding composition is represented by the molecular formula Mn 3-x-y-z Cox Al y Cu z O4. The composition may contain oxygen vacancies, but preferably has no oxygen vacancies.

[0007] In addition to the main component, which constitutes the majority of the NTC composition, the NTC composition can also contain a portion of a second phase as a so-called precipitate. Precipitations can form during sintering, particularly if sintering occurs at an excessively high sintering temperature. Preferably, the ceramic is selected with regard to its starting materials so that the entire NTC composition will ultimately consist of the main component as the main phase. However, during production, a precipitate phase or phases can be formed in addition to the main component, whereby portions of the ceramic still meet the target composition and then represent the main component. Preferably, the precipitate makes up a small portion of the composition. The main component preferably has a phase proportion of over 90%, i.e., in the case of the main component being represented by Mn 3-x-y-z Co x Al y Cu zO4, this spinel component has a phase fraction of over 90%. Even more preferably, it accounts for over 95%. With careful sintering, it can also be produced as a largely single-phase material, e.g., with a phase fraction of over 99%.

[0008] According to one embodiment, the proportions of the addition of Al and Cu can be determined using the formulas B(T) = B max × tanhyp(3 T / T0) and T0 = (h × v0) / (2 k arcsinh γ). Here, B(T) is the temperature-dependent B value of the ceramic, T is the temperature, B maxthe temperature-independent B value of the dominant small polaron hopping conduction, T0 the lower limit temperature of the dominant small polaron hopping conduction (SPH conduction for short), h the Planck constant, v0 the characteristic oscillation frequency of the crystal lattice, k the Boltzmann constant, and γ the coupling constant. The B value is a fundamentally known quantity in the NTC ceramic field, which represents a characteristic measure of the composition or of a thermistor manufactured with it, or can be used as a measure of the measurement sensitivity of a thermistor manufactured with it. Small polaron hopping conduction is the generally accepted dominant conduction mechanism in NTC thermistor ceramics. In addition to SPH conduction, a certain temperature-dependent proportion of the charge carriers is also transported by band semiconductors. Both mechanisms operate in parallel, resulting in a certain deviation from linear behavior.

[0009] In particular, the use of the formulas enables the determination of the otherwise difficult-to-access material parameters B max , T0, and γ can be determined easily, accurately, and reproducibly. This allows, for example, material optimization to be carried out in a short time.

[0010] According to one embodiment, proportions of ceramic components or in particular the proportions of Al and Cu are adjusted using the above-mentioned formulas so that a value B max 3400 to 4000 K. According to a preferred embodiment, proportions are adjusted so that a value B max 3500 to 3800 K. According to a particularly preferred embodiment, proportions are adjusted so that a value B max 3500 to 3600 K.

[0011] According to one embodiment, proportions of ceramic components, or in particular the proportions of Al and Cu, are adjusted using the above-mentioned formulas such that a value T0 of at most 600 K is achieved. According to a preferred embodiment, proportions are adjusted such that a value T0 of at most 550 K is achieved. According to a particularly preferred embodiment, proportions are adjusted such that a value T0 of at most 500 K is achieved.

[0012] A manganese-cobalt oxide spinel with Al and Cu as additives, which has a value B maxA thermistor with a B value of 3400 to 4000 K and / or a T0 value of max. 600 K may be suitable for use at temperatures below 0°C. For such a thermistor, for example, a B value can easily remain above a value of 3000 K at temperatures below 0°C. Furthermore, the linear range of the B value can be broader, extending, for example, to lower temperatures. Such a thermistor may therefore be particularly well-suited for extending the technically useful measuring range into temperature ranges below 0°C.

[0013] A manganese-cobalt oxide spinel with Al and Cu as additives, for example, can also be used effectively in a temperature range in which typical state-of-the-art thermistors based on a manganese-iron-nickel oxide spinel with Cu as an additive can no longer be used effectively. Such manganese-iron-nickel oxide spinels, such as Mn 3-x-y-z Fe x Ni y Cu z O4 with 0.80 ≤ × ≤ 0.85 and 0.60 ≤ y ≤ 0.65 and 0.001 ≤ z ≤ 0.10, have poor linearity and sensitivity of the measurement signal below 0°C and therefore cannot be used effectively in this measuring range.

[0014] According to one embodiment, the B value of the manganese-cobalt-oxide-spinel-based NTC composition can be adjusted via the Cu content. The addition of Al makes it possible to increase the specific resistance of the composition. For example, the Cu addition can increase the conductivity. The inventors have thus recognized that the composition can be optimized through the targeted interplay 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 balance out less preferred properties. Consequently, as an alternative and independent of the features mentioned above or below, an NTC composition with a main component based on manganese-cobalt-oxide-spinel with an addition of aluminum and copper is also generally disclosed. This can have further features of other embodiments.

[0015] According to one embodiment, the ceramic is adjusted to a specific resistance of 200 Ω·cm or more.

[0016] Preferably, a value of 250 Ω·cm or more, and even more preferably, a value of 400 Ω·cm or more. A value of 800 Ω·cm or higher is even more preferred. Higher resistivities allow thermistors to be made smaller or, in multilayer thermistors, to reduce layer thicknesses. In particular, a ceramic with the above-mentioned B values ​​and the resistivities described here is preferred.

[0017] Furthermore, the composition according to the invention is a ceramic NTC composition with a main component having the molecular formula Mn 3-x-y-z Co x Al y Cu z O4. In this case, 0.8 ≤ x ≤ 1.4, 0.6 < y ≤ 0.95 and 0.15 ≤ z ≤ 0.35 apply.

[0018] According to an embodiment not according to the invention due to the y-value, 0.90 ≤ x ≤ 1.30, 0.55 ≤ y ≤ 1.00 and 0.15 ≤ z ≤ 0.35.

[0019] The aforementioned features may be combined with each other and with the features mentioned below. In particular, a composition prepared using the above-mentioned formulas may exhibit the current properties or those set out below.

[0020] The inventors of the present invention have recognized that such a composition can achieve improved performance of the NTC composition or a thermistor formed therefrom. For example, the B value can exhibit better linearity, i.e., it can be linearly approximated over a different or wider range without significant errors. Furthermore, an excessive drop in the resistance of such a ceramic composition can be prevented. Furthermore, a thermistor can be provided that enables improved temperature measurement at low temperatures. In particular, the sensitivity at low temperatures can be improved.

[0021] According to a preferred embodiment, z is 0.15 ≤ z ≤ 0.30. More preferably, 0.15 ≤ z < 0.29, and even more preferably 0.15 ≤ z ≤ 0.28. According to an embodiment with certain properties, 0.15 ≤ z ≤ 0.27 may apply.

[0022] For z ≤ 0.35, it was recognized that this can prevent the specific resistance of the composition from decreasing too much. For values ​​of z ≤ 0.28 or even slightly lower, it was recognized that such an NTC composition preferably has a high specific resistance. In particular, using a composition for which x and y correspond to the values ​​stated above and z to those stated here, it is easier to obtain an NTC composition that has a high B value at low temperatures and, at the same time, has a high specific resistance. The specified lower limits for z have proven preferable, as they help to adjust the B value.

[0023] 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 ≤ × ≤ 1.10. Particularly preferred material properties have been identified for these value ranges.

[0024] For y, the range of 0.6 < y ≤ 0.95 is considered to be in accordance with the invention. A more preferred range may be 0 ≤ 0.60 < y ≤ 0.90 or 0.70 < y ≤ 0.95 or 0.75 ≤ y ≤ 0.85. Particularly preferred material properties have also been identified for these.

[0025] In particular, all material compositions in which two or, even better, three of x, y and z have a preferred value or a very preferred value were recognized as preferred.

[0026] According to a further embodiment, an NTC composition with a manganese-cobalt oxide spinel-based main component is described, to which aluminum and copper are added, and in which the aluminum to copper ratio is > 2.4. With such a ratio of Al to Cu, preferred properties regarding the B value and the resistivity can be achieved.

[0027] If an NTC composition with a main component with the molecular formula Mn 3-x-y-z Co x Al y Cu z O4 is considered as an embodiment of the NTC composition based on manganese-cobalt oxide spinel, then in the case of the previous embodiment, y:z > 2.4 applies. x can assume the values ​​described above. y can assume the values ​​described above and z can be determined according to the ratio of y:z. Alternatively, z can also assume the values ​​described above and y can be determined from the ratio of y:z. According to a preferred embodiment, the NTC composition with the empirical formula Mn 3-x-y-z Co x Al y Cu z O4 both the values ​​for x, y, and z described above and the ratio y:z > 2.4. The advantages can be particularly pronounced in this case.

[0028] According to one embodiment, the composition is free of nickel, except for unavoidable impurities.

[0029] Furthermore, a thermistor is disclosed. This thermistor comprises 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.

[0030] 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 and sintered from individual layers, which, however, unlike a multilayer thermistor, does not have internal electrodes.

[0031] According to a further embodiment, the thermistor can be a monolayer NTC thin-film thermistor or a multilayer thin-film thermistor. A monolayer NTC thin-film thermistor can mean either a component manufactured from a single thin-film layer or a component constructed from several individual thin-film layers stacked one above the other, even in successive manufacturing steps. However, unlike a multilayer thin-film thermistor, this component does not have internal electrodes.

[0032] The ceramic composition allows for small designs 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 mm 3According to a preferred embodiment, the volume of such a multilayer thermistor can be 0.3 to 5.5 mm 3 be.

[0033] According to one embodiment, a multilayer thermistor contains at least one ceramic layer containing or consisting of the NTC composition. A similar embodiment can also be implemented as a multilayer thin-film thermistor, wherein the multilayer thin-film thermistor contains a ceramic thin-film layer containing or consisting of the NTC composition.

[0034] According to one embodiment, a ceramic layer may have a thickness of 10 to 100 µm, and preferably 20 to 50 µm. According to one embodiment, a thin-film layer may have a thickness of 0.001 to 10 µm, and preferably 0.01 to 1 µm.

[0035] 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.

[0036] In particular, such a composition can be used for thermistors with nominal resistances at 25°C of 1 to 20 kΩ. In particular, a nominal resistance at 25°C can be 10 kΩ. The thermistors, and in particular the multilayer thermistors, can be designed as SMD components (surface-mounted device components). The thin-film thermistors, whether in multilayer construction or as a monolayer thin-film thermistor, can also be applied to or in substrates, be suitable for embedding (SESUB technology), or be intended as a component for a MEMS device.

[0037] Furthermore, the use of the formula B(T) = B max× tanhyp(3 T / T0) to determine the material properties of an NTC spinel ceramic. Here, B(T), B max , T and T0 correspond to the quantities described above.

[0038] Until now, NTC spinel ceramics, such as nickel-manganese spinels or nickel-cobalt spinels, have often been described exclusively using the linear approximation B(T1, T2) = ln(R2 / R1) / (1 / T2 - 1 / T1). Here, for example, T1 = 25°C and T2 = 100°C, where R1 and R2 are resistances at T1 and T2, respectively. Therefore, only the ideal RT characteristic of a thermistor ceramic was considered. However, this simplified view does not do justice to such ceramics, especially not at lower temperatures. In the linear range, the generally accepted conduction mechanism of "small polaron hopping" (SPH for short) dominates. This is particularly prevalent at higher temperatures, as sufficient thermal energy is available to excite the crystal oscillations. At room temperature and especially below, this is no longer entirely the case. In addition to SPH conduction, a smaller proportion of the charge carriers are also transported by band semiconductor conduction. 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, the lower the T1 is chosen, the lower the value. If a close-meshed measurement of T1 and T2 is chosen, a steady decrease in the B(T1,T2) value becomes apparent at lower temperatures.

[0039] To date, mechanisms and parameters have been captured only very imprecisely. The approaches published by Casado et al. (J. Phys.: Condens. Matter 6 (1994) 4685 - 4698) offer a physical description but only an inadequate basis for capturing material properties in technically relevant systems. According to Casado et al., the formula T0 = (h υ o ) / (2k arcsinh y) and B max = (γ·h·v o ) / (2k). Here, T0 is the lower limit temperature of the dominant SPH line, B max the constant B value of the dominant SPH line, v othe characteristic vibration frequency of the crystal lattice, h the Planck constant, k the Boltzmann constant, y the coupling constant of the vibrational energy with the electrical energy.

[0040] Formula B max =(γ·h·v o ) / (2k) contributes a lot to the understanding of the deviation from linearity at low temperatures, but the parameters T0, B max and y are difficult to determine with this method. Until now, a graphical method has generally been used, in which ln(R) is plotted against (1 / T) over the largest possible temperature range. In the range of smaller (1 / T) values, a straight line is fitted as an asymptote. This allows the values ​​T o (coincidence of the RT values ​​with the straight line) and B max (slope of the asymptote) can be estimated. However, the placement of the lines and their drawing is partly arbitrary and at least subject to considerable uncertainty.

[0041] The approach using the formula B(T) = B max × tanhyp(3 T / T0) offers an improved, simple, and accurate method. When analyzing the B(T1, T2) curve, the inventors realized 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 B. max .

[0042] Since B(T) asymptotically (T → ∞) adapts to B max According to one embodiment, it can be defined which approximation is considered sufficient. For practical application, T0 can be defined such that it is considered to be reached when 99.5% of the final value for B (i.e., B max ) can be achieved. The result is: B(T0) = tanhyp(3)·B max = 0.995·B max

[0043] Preferred are the support points for B(T2,T1) to which an adjustment according to the formula B(T) = B max × tanhyp(3 T / T0) is performed, the grid spacing is chosen to be narrow. Thus, a grid spacing with T2-T1 ≤ 10 K is preferred over a measurement range of -50°C to 200°C. However, T2-T1 can also be chosen more loosely. Accordingly, T2-T1 can be ≤ 25 K, while still achieving a good fit. This works particularly well when at least 9 grid points are available in the measurement range of -50°C to 150°C.

[0044] According to one embodiment, the coupling constant can be obtained via T0 = (h × v0) / (2 k arcsinh γ).

[0045] Using the described procedure, material properties can be determined quickly and easily. It particularly facilitates the evaluation of large data sets. Furthermore, the procedure can be applied automatically.

[0046] According to one embodiment, the use of the formula B(T) = Bmax × tanhyp(3 T / T0) for material property optimization of NTC spinel ceramics.

[0047] Since the procedure described above enables an improved recording of material properties, material development including material property optimization can be carried out in a targeted manner.

[0048] Furthermore, a method using the formula B max × tanhyp(3 T / T0). This can exhibit the characteristics described above.

[0049] According to one embodiment, the method is a method for optimizing the material properties of NTC spinel ceramics, wherein a curve of the B value of a starting material composition is applied using the formula B(T) = B max× tanhyp(3 T / T0) is applied. Together with the formula T0 = (h × v0) / (2 k arcsinh γ), the raw material properties of the raw material composition are determined. Based on these raw material properties, additives are selected that specifically influence the B value at a specific temperature, the T0 value, and / or the specific resistance.

[0050] In this way, material optimization can be carried out starting from a starting composition, whereby the advantages mentioned above take effect.

[0051] 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 by being subjected to the process sequence again as the starting material composition. In other words, the ceramic composition already optimized by additives is subjected to the described process sequence again at least once as the starting material composition.

[0052] Furthermore, a method for producing a multilayer thermistor is provided. According to this method, starting materials for a ceramic composition with the molecular formula Mn 3-x-y-z Co x Al y Cu z O4. These starting materials can be, for example, oxides, carbonates or hydroxides or similar to those in Mn 3-x-y-z Co x Al y Cu zO4 mentioned metal atoms. The starting materials are weighed in such a way that for the ceramic composition 0.8 ≤ x ≤ 1.4, 0.6 < y ≤ 1.1 and 0.15 ≤ z ≤ 0.35 is obtained. After weighing, the starting materials are ground and then calcined. The grinding can be wet grinding, for example. This produces an average grain size between 0.5 and 1.5 µm, and preferably between 0.7 and 1.0 µm. Calcining can take place at 800°C to 1000°C, e.g. at 825 to 925 °C. The calcinate obtained in this way is then ground, which can be referred to as re-grinding. The targeted grain sizes can correspond to those of the first grinding step. Green films are then produced using the ground calcinate. The calcinate can be mixed with additives such as binders, wetting agents, dispersants, or similar. This mixture can be applied to a carrier film. The green films are then stacked.Stacking occurs 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 several green sheets prior to stacking. One or more green thermistor components are cut from the resulting stack. This or these components are then debindered. The debindered 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 many main phases as possible are formed in the resulting ceramic. Decomposition can be reduced accordingly. A range of 1050°C to 1150°C is preferred in this regard. Furthermore, external electrodes are applied, each of which contacts the internal electrodes.

[0053] A monolithic thermistor can also be obtained in a similar way. For this purpose, the steps for forming internal electrodes, for example, are omitted.

[0054] Furthermore, the ceramic obtained by the process can have the above-mentioned properties.

[0055] 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 in terms of 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 acting 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 specifically described features of the exemplary embodiments. Fig. 1 shows schematically a first embodiment of an NTC thermistor in cross section. Fig. 2 shows a cross-sectional microscopy image of a second embodiment of an NTC thermistor. Fig.Figure 3 shows the curve B(T) for a first thermistor ceramic and a fitting curve. Fig. Figure 4 shows the curve B(T) for a second thermistor ceramic and a fitting curve. Fig. Figure 5 shows the curve B(T) for a third thermistor ceramic and a fitting curve. Fig. Figure 6 shows the curve B(T) for a fourth thermistor ceramic and a fitting curve. Fig. 7 shows a cross-sectional microscopy image of a third embodiment of an NTC thermistor. Fig. Figure 8 shows the comparison of the curves of B(T) for the first and fourth thermistor ceramics.

[0056] In Fig. 1 and Fig. 2 shows two embodiments of a multilayer thermistor 1. Fig. 1 shows a schematic cross-sectional view and Fig. Figure 2 shows a cross-sectional microscopy image taken with a scanning electron microscope.

[0057] Both embodiments of the multilayer thermistor 1 have several ceramic layers 2, which are alternately stacked with internal electrode layers 3. The ceramic layers 2 consist of a ceramic material, as described below. Fig. 4 to 6. The internal electrode layers 3 consist of a palladium-silver alloy. The ceramic layers 2 and the internal electrode layers 3 can be obtained by conventional multilayer technology. They can thus be formed from green films and by applying metallization. One or more green films can be laminated for a ceramic layer 2. In the stacking or layer direction, there are end regions 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 if the schematic Fig.Although no dimensions or proportions can be deduced from Figure 1, it can be seen that, depending on the design, any termination areas 5 can be selected with different thicknesses. Furthermore, both multilayer thermistors 1 have external electrodes 4, which are obtainable using conventional metallization processes.

[0058] 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.

[0059] Fig.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 NTC ceramic, as mentioned above and commonly used in the prior art, does not meet the desired requirements, particularly in the temperature range below 0°C. Sufficient data on the relevant parameters describing an NTC thermistor with regard to its properties are known for this ceramic in the prior art to also provide a basis for comparison for the improved determination method for these parameters according to the invention.

[0060] The composition, measured linearly with B(T1, T2) = ln(R2 / R1) / (1 / T2 - 1 / T1), has a B(25°C, 100°C) value of 3454 K and a specific resistance of 1700 Ω cm. A component with a nominal resistance of 10 kΩ was fabricated. Numerous sampling points (T2-T1 ≤ 10 K) were recorded.

[0061] The measured data were based on the function B(T) = B max × tanhyp(3 T / T0) was applied. 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 conduction (SPH conduction). Furthermore, the following approximation was used: T0 is reached when B(T0) = tanhyp (3) ·B max = 0.995·B max , i.e. when 99.5% of the final value has been reached.

[0062] B was able to max = 3870 K and T0 = 680 K (407°C).

[0063] Furthermore, using T0 = (h × v0) / (2 k arcsinh γ), the coupling constant γ was determined, which is 3.10. Here, h is the Planck constant, v0 is the characteristic vibration frequency of the crystal lattice, and k is the Boltzmann constant.

[0064] Thus, relevant material parameters can be obtained. The composition shows unsatisfactory linearity and unsatisfactory sensitivity at T < 0°C. B(T) decreases below 3000 K in the analyzed range. The analysis shows which parameters have potential for improvement: To achieve a lower T o To obtain this, both the coupling factor y should be increased towards 4 and B max be reduced, e.g., to approximately 3400 to 4000 K or, preferably, to 3500 - 3600 K. This should make it possible to achieve a B(T1, T2) value of > 3000 K at subzero temperatures and improve the linearity of the RT characteristic curve.

[0065] This approach can replace a previously used less reliable approach. Previously, material properties were determined based on the results of fundamental research by Casado et al. (J. Phys.: Condens. Matter 6 (1994) 4685 - 4698) using T0 = (h v o ) / (2k arcsinh y) and B max = (γ·h·v o ) / (2k). Here T0 is the lower limit temperature of the dominant SPH line, v o the characteristic vibration frequency of the crystal lattice, h the Planck constant, k the Boltzmann constant, y the coupling constant of the vibrational energy with the electrical energy. B max =(γ·h·v o ) / (2k) contributes a lot to the understanding of the deviation from linearity at low temperatures, but the parameters T o , B maxand γ are difficult to determine from this. A graphical method is used in the literature, where ln(R) is plotted against (1 / T) in a (largest possible) temperature range. In the range of smaller (1 / T) values, a straight line is fitted as an asymptote. This allows the values ​​T o (coincidence of the RT values ​​with the straight line) and B max (slope of the asymptote) can be estimated. But ultimately, the drawing of the straight line is arbitrary and subject to great uncertainty.

[0066] For this reason, the improved and more accurate method described above was developed, which is based on the analysis of the B(T1, T2) curve. It was recognized that the B-value curve ( Fig. 3) is very well represented by the hyperbolic tangent function. Furthermore, the hyperbolic tangent function has the physically correct behavior: as the temperature increases, the function value asymptotically approaches a constant final value.

[0067] In Fig. Figure 4 shows the B(T1, T2) curve for a ceramic composition and an approximation using the method described above. The composition follows the molecular formula Mn 3-x-y-z Co x Al y Cu z O4, where x = 1.18, y = 0.60, and z = 0.28, i.e., it contains a main phase with this composition. This composition was sintered at 1150°C. The composition is assigned sample number 15.

[0068] This composition shows improved values ​​compared to sample 2917, which corresponds to the state of the art. The specific resistance is 280 Ω·cm, the B(25,100) value is 3441 K, B max is 3549 K, T0 is 467 K and y is 3.7. Thus, compared to sample 2917, the coupling factor y was increased towards 4, and B maxto a value of 3500 - 3600 K. This has made it possible to achieve a B(T1, T2) value of > 3000 Kelvin at sub-zero temperatures and to improve the linearity of the RT characteristic, as shown in Fig. 4. An excessive drop in resistivity was also prevented. However, this could still be improved.

[0069] In Fig. Figure 5 shows the B(T1, T2) curve for a preferred ceramic composition according to the invention and an approximation using the method described above. The composition follows the empirical formula Mn 3-x-y-z Co x Al y Cu z O4, where x = 1.21, y = 0.90, and z = 0.22. The composition was sintered at 1100°C. The composition is assigned sample number 31.

[0070] This composition shows at least partially improved values ​​compared to sample 2917 and also to sample 15. The specific resistance is 1049 Ω·cm, the B(25,100) value is 3581 K, B max is between 3700 K and 3800 K, as can be approximately read from the figure, T0 is 543 K, and y is 3.5. Although the coupling factor is slightly reduced, the resistivity is significantly improved compared to sample 15. This allows a component with a nominal resistance of 10 kΩ to be easily achieved.

[0071] In Fig. Figure 6 shows the B(T1, T2) curve for a particularly preferred ceramic composition according to the invention and an approximation using the method described above. The composition follows the empirical formula Mn 3-x-y-z Co x Al y Cu zO4, where x = 1.02, y = 0.80, and z = 0.27. The composition was sintered at 1100°C. The composition is assigned sample number 49.

[0072] This composition shows at least partially improved properties compared to the previously described samples. The specific resistance is 832 Ω·cm, the B(25,100) value is 3446 K, B max is 3542 K, T0 is 457 K, and y is 3.8. Thus, the sample exhibits a very good balance of material properties.

[0073] The production process is also explained using sample 49.

[0074] First, a powder is prepared. The weight and ingredients used for an 80 kg powder batch can be found in Table 1. Table 1 component raw material Initial weight aluminum Al2O3, aluminum oxide 15.3 kg manganese Mn3O4, manganese oxide 26.0 kg cobalt Co3O4, cobalt oxide 30.6 kg copper CuO, copper oxide 8.1 kg Sum: 80 kg

[0075] The components were then mixed with water and ground in a stirred ball mill until a target particle size d(50%) of 0.7 to 1.0 µm was obtained. After the target particle size was achieved, the suspension was dried and sieved.

[0076] The powder is then calcined. For this purpose, the powder is filled into cordierite capsules and converted 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 of > 90%.

[0077] The reacted powder was then reground. The calcinate was first sieved dry, then mixed with water and reground until a target grain size d(50%) of 0.7 to 1.0 µm was obtained. After the target grain size was achieved, the suspension was dried and sieved again.

[0078] Green films were then produced. The powder is mixed with organic solvents and admixed with additives commonly used in this technology (binders, wetting agents, dispersants, etc.). After adjusting the appropriate viscosity and degassing the suspension, a ceramic film is produced on a carrier film using a film drawing machine. This film can be further processed using multilayer technology.

[0079] In the specific example, a film with a nominal thickness (“green thickness”) of 28.5 µm was produced.

[0080] The sintered body is then produced. This is first tested against the illustration in Fig.Figure 7 illustrates this, which shows a longitudinal section of a sintered component in a SEM image. In this exemplary embodiment, the so-called "tip design" was used. Two inner electrode layers 3 (electrode tips) protrude from each outer electrode 4 into the component, but without overlapping. The current therefore flows primarily from the electrode tips to the opposite electrode tips of the counter electrode. This minimizes the cross-section of the current flow. The further apart the electrode tips are from each other, the higher the resistance.

[0081] First, internal electrodes were created by printing the ceramic foil. For this purpose, an AgPd paste (60% Ag, 40% Pd) was applied to the designated areas using screen printing.

[0082] The printed and unprinted films are then stacked on top of each other in a defined sequence.

[0083] Stacking sequence based on the Fig. 7 illustrated embodiment: - 14 unprinted foils (form one of the final areas 5) - 1 printed foil (internal electrode tips of a level 1) - 5 unprinted foils (space to a level 2) - 1 printed foil (internal electrode tips of level 2) - 13 unprinted foils (form the top layer)

[0084] The stacked films are then pressed (laminated) to ensure stable adhesion of the individual layers to one another.

[0085] The stack was then cut into components ("green parts"). The dimensions of the unsintered components (green parts) are 1.8 x 0.9 mm (L × W).

[0086] The components are then debound. The binders required for the green processes are burned out before sintering. For this purpose, the components are slowly heated (< 1 K / min) to 450°C under air purge (holding time 6 hours).

[0087] The components are then sintered. The components were sintered 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

[0088] For the evaluation measurements, the two outer electrodes are applied. For this purpose, the caps are coated with silver paste and baked. The formation of the metallization caps is shown in the SEM image in Fig. 7 can be clearly seen.

[0089] Furthermore, the components of the exemplary embodiment were passivated on the surface (glass encapsulation) and nickel-plated and tin-plated on the caps by electroplating in order to be easy to solder.

[0090] Finally, in Fig.Figure 8 shows the comparison of the B(T) curves for samples 49 and 2917. Sample 49 exhibits significantly improved properties regarding the B value at low temperatures and linearity. Sample 49 thus has significantly improved properties compared to the NTC ceramics commonly used in the state of the art, corresponding to sample 2917, and can therefore extend the technically useful measuring range for NTC thermistors into the temperature range well below 0°C. List of reference symbols 1 multilayer thermistor 2 ceramic layers 3 Internal electrode layer 4 Outer electrode 5 Final area

Claims

[1] NTC composition having a main component with the molecular formula Mn 3-x-y-z Co x Al y Cu z O4, where 0.8 ≤ × ≤ 1.4, 0.6 < y ≤ 0.95 and 0.15 ≤ z ≤ 0.

35. [2] The NTC composition according to claim 1, wherein 0.15 ≤ z ≤ 0.

30. [3] The NTC composition according to claim 1, wherein 0.15 ≤ z ≤ 0.

28. [4] NTC composition according to any one of claims 1 to 3, wherein y:z > 2.

4. [5] NTC composition according to any one of claims 1 to 4, wherein 0.90 ≤ x ≤ 1.

30. [6] NTC composition according to any one of claims 1 to 4, wherein 0.95 < x ≤ 1.

25. [7] NTC composition according to any one of claims 1 to 6, wherein 0.70 < y ≤ 0.

95. [8] NTC composition according to any one of claims 1 to 6, wherein 0.75 ≤ y ≤ 0.

85. [9] NTC composition according to any one of claims 1 to 8, wherein the main component constitutes at least 90% of the phases of the NTC composition. [10] A thermistor comprising the NTC composition according to any one of claims 1 to 9. [11] Multilayer thermistor comprising a ceramic layer comprising the NTC composition according to any one of claims 1 to 9. [12] Multilayer thermistor according to claim 11, wherein it has a volume of 0.075 to 10 mm 3 has. [13] A multilayer thermistor according to claim 11 or 12, wherein the ceramic layer has a thickness of 10 to 100 µm and the thermistor is constructed from one or more layers. [14] Multilayer thermistor according to one of claims 11 to 13, which has a nominal resistance at 25 °C of 1 to 20 kΩ. [15] A multilayer thin film thermistor comprising a ceramic layer comprising the NTC composition according to any one of claims 1 to 9. [16] A monolayer NTC thin film thermistor comprising a ceramic layer comprising the NTC composition according to any one of claims 1 to 9. [17] A method for producing a multilayer thermistor, wherein Starting materials for a ceramic composition with the molecular formula Mn 3-x-y-z Co x Al y Cu z O4 are provided, the starting materials being weighed in such a way that the ceramic composition is 0.8 ≤ x ≤ 1.4, 0.6 < y ≤ 0.95 and 0.15 ≤ z ≤ 0.35, the raw materials are ground, the powder obtained after grinding the starting materials is calcined, the resulting calcinate is ground, Green foils are produced with the ground calcinate, the green foils are stacked together with starting materials for internal electrodes, green thermistor components are cut from the stack, the green thermistor components are debound, the debindered green thermistor components are sintered to sintered thermistor components at 1000 to 1200 °C, and external electrodes are applied to the sintered thermistor components. [18] The method according to claim 17, wherein the starting material for internal electrodes is a metal paste and this is printed on some green sheets before stacking. [19] A process according to claim 17 or 18, wherein the sintering is carried out at 1050°C to 1150°C.

Images