Negative characteristic thermistor

Abstract

This negative-characteristic thermistor is provided with: a ceramic substrate formed from a ceramic composition containing Mn, Ni, and Fe; and an external electrode provided at an end portion of the ceramic substrate, in which the external electrode includes a base layer covering the end portion of the ceramic substrate and including Cu and glass, and a plating layer covering the base layer, and the Ni content, the Mn content, and the F content in the ceramic substrate satisfy the following formulae (1) and (2): 26.4 mol% < = [Ni] < = 29.5 mol%, 1.65 < = [Mn] / [Fe] < = 1.90 (2). Wherein [Ni], [Mn], and [Fe] are the contents (mol%) of Ni, Mn, and Fe, respectively, when the total content of Mn, Ni, and Fe contained in the ceramic matrix is 100 mol%.

Description

TECHNICAL FIELD

[0001] The present application relates to a negative temperature coefficient (NTC) thermistor. BACKGROUND

[0002] A negative temperature coefficient (NTC) thermistor including a ceramic substrate formed of a ceramic composition of the Mn / Ni / Fe system is known (for example, Patent Documents 1 to 3). An external electrode is formed at an end portion of the ceramic substrate, and a base layer of the external electrode is formed by applying and sintering a conductive paste mainly containing Ag (Ag-based conductive paste).

[0003] PRIOR ART DOCUMENTS

[0004] PATENT DOCUMENTS

[0005] Patent Document 1: International Publication No. 2017 / 022373

[0006] Patent Document 2: Japanese Patent Application Publication No. 2016-54225

[0007] Patent Document 3: International Publication No. 2011 / 086850 SUMMARY

[0008] PROBLEMS TO BE SOLVED BY THE INVENTION

[0009] Ag is relatively expensive and is prone to migration. Therefore, the use of Cu, which is less expensive and less prone to migration, for the base layer is being studied. However, the present inventors and others have found that, if sintering is performed using a Cu-based conductive paste as the base layer, cracks of several tens of μm in depth are sometimes generated on the surface of the ceramic substrate.

[0010] If cracks are generated in the ceramic substrate, the mechanical strength of the ceramic substrate decreases, and thus the product is considered to be defective in appearance. In addition, if cracks are generated in the ceramic substrate, the cracks can propagate in the case of exposure to an environment of high humidity or a large temperature difference, and it can be impossible to achieve the required characteristics of the thermistor.

[0011] In the case where an Ag-based conductive paste is used in the formation of the base layer, no phenomenon of cracks occurring in the ceramic substrate has been confirmed, and thus no studies have been made on the occurrence and suppression of cracks up to now. In Patent Documents 1 to 3, the occurrence of cracks in the ceramic substrate when a Cu-based conductive paste is used in the formation of the base layer has not been recognized, and thus no studies have been made on the suppression of the occurrence of cracks.

[0012] In addition, NTC thermistors can be used in high-temperature environments, and it is important to develop an NTC thermistor that is less likely to deteriorate even if exposed to a high temperature (for example, 150°C) for a long period of time (i.e., has good high-temperature durability).

[0013] Therefore, an object of the present application is to provide a negative temperature coefficient thermistor in which a ceramic substrate is suppressed from cracking and which has good high-temperature durability, in a negative temperature coefficient thermistor in which a base layer of an external electrode is formed from a Cu-based conductive paste.

[0014] Solution to the problem

[0015] According to one aspect of the present application, there is provided a negative temperature coefficient thermistor,

[0016] which comprises: a ceramic substrate formed from a ceramic composition containing Mn, Ni, and Fe;

[0017] and an external electrode provided at an end portion of the ceramic substrate,

[0018] wherein the external electrode comprises: a base layer covering the end portion of the ceramic substrate and containing Cu and glass, and a plating layer covering the base layer,

[0019] the content of Ni, the content of Mn, and the content of F in the ceramic substrate satisfy the following formula (1) and formula (2),

[0020] 26.4 mol% ≤ [Ni] ≤ 29.5 mol% (1)

[0021] 1.65 ≤ [Mn] / [Fe] ≤ 1.90 (2)

[0022] wherein [Ni], [Mn], and [Fe] are the contents of Ni, Mn, and Fe (mol%) when the total content of Mn, Ni, and Fe contained in the ceramic substrate is taken as 100 mol%.

[0023] Effects of the invention

[0024] The negative temperature coefficient thermistor of the present application can form a base layer of an external electrode from a Cu-based conductive paste, while suppressing cracking of a ceramic substrate, and has good high-temperature durability. BRIEF DESCRIPTION OF DRAWINGS

[0025] [ Figure 1 ] Figure 1 is a perspective view of a negative temperature coefficient thermistor according to Embodiment 1.

[0026] [ Figure 2 ] Figure 2 is a cross-sectional view of a negative temperature coefficient thermistor according to Embodiment 1.

[0027] [ Figure 3 ] Figure 3 is a triangular graph showing the composition (Mn-Ni-Fe content) of the ceramic composition of the ceramic substrate produced in the Examples.

[0028] [ Figure 4 ] Figure 4 is a photograph of an optical microscope of the negative characteristic thermistor of Experimental Example No. 1.

[0029] [ Figure 5 ] Figure 5 is a photograph of an SEM image of a cross section of the negative characteristic thermistor of Experimental Example No. 1.

[0030] [ Figure 6 ] Figure 6 is a photograph of an SEM image of a cross section of the negative characteristic thermistor of Experimental Example No. 2.

[0031] [ Figure 7 ] Figure 7 is a graph showing the change in resistance before and after the high-temperature storage test in each experimental example. DETAILED DESCRIPTION

[0032] The present inventors et al. first found that when a base layer of an external electrode of an NTC thermistor is formed by applying a Cu-based conductive paste and sintering, cracks are generated on the surface of the ceramic base, and conducted an in-depth study on the cause thereof. As a result, it was found that there are two factors. The first factor is that the sintering temperature of the Cu-based conductive paste is 100°C or more higher than that of the conventional Ag-based conductive paste. The second factor is that the structure of the surface of the ceramic base is different from that of the inside, and thus a difference in thermal expansion rate is generated between the inside and the surface of the ceramic base. The present inventors first found that cracks are generated in the ceramic base due to the combination of the first factor and the second factor.

[0033] The present inventors et al. further conducted an in-depth study on the reason why the second factor (the difference in the structure between the surface and the inside of the ceramic base) occurs, and as a result, found that in the ceramic base formed of a Mn / Ni / Fe-based ceramic composition, NiO precipitates are confirmed in the inside of the ceramic base, but not in the surface of the ceramic base.

[0034] With regard to the behavior of the NiO precipitates, the present inventors et al. conjectured the following mechanism. In the sintering process of the ceramic base, although Ni is solid-solved in the ceramic composition, if the Ni content in the ceramic base becomes large, part of the Ni cannot be solid-solved and precipitates as NiO. This precipitation of NiO occurs in the entire ceramic base. However, in the process of cooling the ceramic base after maintaining it at a prescribed sintering temperature for a prescribed time (cooling process), the NiO reacts with oxygen in the atmosphere (re-oxidation reaction) and disappears. The re-oxidation reaction first occurs in the surface of the ceramic base and gradually proceeds to the inside, but the re-oxidation reaction does not progress to the deep inside of the ceramic base. Thus, the NiO disappears only in the vicinity of the surface of the ceramic base (for example, in the range from the surface to a depth of 50 μm). As a result, the ceramic base becomes one in which the NiO precipitates are present in the vicinity of the surface and the inside of the ceramic base is free of the NiO precipitates. In the ceramic base in which the NiO precipitates are present in the vicinity of the surface, the thermal expansion rate of the ceramic base is higher than that of the ceramic base in which the NiO precipitates are not present in the vicinity of the surface. Thus, the difference in the thermal expansion rate between the inside and the surface of the ceramic base is generated. Figure 5A1) does not have NiO precipitates, but has a non-uniform structure in which Figure 5 A2) has NiO precipitates.

[0035] The NiO precipitates are of a rock salt structure, which is a different crystal structure from the spinel structure of the Mn / Ni / Fe-based ceramic composition, and thus it is considered that the difference in thermal expansion rate between the inside and the surface of the ceramic substrate becomes significant, and as a result, becomes a cause of cracks on the surface of the ceramic substrate.

[0036] The present inventors et al. have first found that by limiting the Ni content to a prescribed amount (particularly, limiting the upper limit value), the precipitation of NiO in the inside of the ceramic substrate can be suppressed, and as a result, the difference in thermal expansion rate between the inside and the surface of the ceramic substrate can be suppressed.

[0037] Further, the present inventors et al. have further researched the composition of the ceramic substrate in order to suppress the occurrence of cracks and improve high-temperature durability. By increasing the Ni content, the high-temperature durability can be easily improved. However, when the upper limit value of the Ni content is limited in order to suppress the occurrence of cracks, the high-temperature durability becomes difficult to improve as a result. The present inventors et al. have found that by controlling the ratio of the Mn content with respect to the Fe content, the high-temperature durability can be improved, and thus completed the present application.

[0038] Hereinafter, an embodiment of the present application will be described with reference to the drawings.

[0039] [Embodiment 1]

[0040] Figure 1 is a perspective view of a negative temperature coefficient (NTC) thermistor 10 to which Embodiment 1 of the present application relates, Figure 2 is a cross-sectional view of the NTC thermistor 10.

[0041] The NTC thermistor 10 has a ceramic substrate 20 and external electrodes 30, 40 at the end portions of the ceramic substrate 20.

[0042] The external electrodes 30, 40 have a base layer 31, 41 and a plating layer (first plating layer) 34, 44. The external electrodes 30, 40 can further have a second electrode layer 33, 43 between the base layer 31, 41 and the first plating layer 34, 44, and can further have a second plating layer 35, 45 that covers the first plating layer 34, 44.

[0043] The internal electrodes 71, 72 are provided in the inside of the ceramic substrate 20 as shown in FIG. 1. Figure 2 The ceramic substrate 20 (a plurality of ceramic layers 200) and the internal electrodes 71, 72 are alternately laminated to become a laminate 80.

[0044] Hereinafter, each of the components will be described in detail.

[0045] (Ceramic base 20)

[0046] The ceramic base 20 is formed of a ceramic composition containing Mn, Ni, and Fe. In the NTC thermistor 10 related to Embodiment 1, when the total content of Mn, Ni, and Fe contained in the ceramic composition is set to 100 mol%, the molar ratio of Ni in the ceramic base 20 is 26.4 mol% or more and 29.5 mol% or less. That is, the ceramic composition satisfies the following formula (1),

[0047] 26.4 mol% ≤ [Ni] ≤ 29.5 mol% (1)

[0048] where [Ni] is the content of Ni (mol%) when the total content of Mn, Ni, and Fe contained in the ceramic base is set to 100 mol%.

[0049] If the content of Ni in the ceramic base 20 is 29.5 mol% or less, generation of a NiO phase can be suppressed, and as a result, the difference in thermal expansion rate between the inside of the ceramic base 20 and the surface 23 can be suppressed. Thus, even when the base layer 31, 41 is formed of a Cu-based conductive paste and heated to a high sintering temperature, cracking can be suppressed from occurring in the surface 23 of the ceramic base 20.

[0050] The content of Ni is preferably 29.0 mol% or less, more preferably 28.5 mol% or less, and further preferably 28.2 mol% or less.

[0051] If the content of Ni is 26.4 mol% or more, the solid solution amount of Ni in the ceramic composition increases, and thus a NTC thermistor 10 having a stable conductivity of the ceramic base 20, high reliability (particularly, good high-temperature durability) can be manufactured.

[0052] The content of Ni is preferably 26.7 mol% or more.

[0053] When the total content of Mn, Ni, and Fe contained in the ceramic composition is set to 100 mol%, the ratio of the content of Mn to the content of Fe (referred to as "Mn / Fe ratio") in the ceramic base 20 is 1.65 or more and 1.90 or less. That is, the ceramic composition satisfies the following formula (2),

[0054] 1.65 ≤ [Mn] / [Fe] ≤ 1.90 (2)

[0055] where [Mn] and [Fe] are the contents of Mn and Fe (mol%) when the total content of Mn, Ni, and Fe contained in the ceramic base is set to 100 mol%.

[0056] It is generally considered that if the Ni content in the ceramic composition is reduced, the high-temperature durability of the NTC thermistor decreases. The present inventors have found that in order to suppress the occurrence of cracks and obtain an NTC thermistor 10 that can satisfy the market demand for high-temperature durability, it is effective to control the Mn / Fe ratio in a preferable range.

[0057] By making the Mn / Fe ratio 1.65 or more and 1.90 or less, at the time of sintering of the ceramic base 20, the sintering of the ceramic composition is moderately promoted, and it is easy to become a crystal structure stable to heat. As a result, it is possible to manufacture an NTC thermistor 10 whose ceramic base 20 is stable in conductivity and high in reliability (in particular, has good high-temperature durability).

[0058] The Mn / Fe ratio is preferably 1.70 or more, more preferably 1.72 or more, particularly preferably 1.74 or more, and preferably 1.85 or less, more preferably 1.83 or less.

[0059] The contents of Mn and Fe are adjusted so that the Mn / Fe ratio satisfies formula (2).

[0060] When the total content of Mn, Ni, and Fe is set to 100 mol%, the molar ratio of Mn (i.e., [Mn]) can be set to 40.5 mol% or more and 50.0 mol% or less, for example. The [Mn] is preferably more than 45.6 mol%, and particularly preferably 46.0 mol% or more, and an NTC thermistor 10 having more excellent high-temperature durability can be obtained.

[0061] When the total content of Mn, Ni, and Fe is set to 100 mol%, the molar ratio of Fe (i.e., [Fe]) can be set to 20.5 mol% or more and 30.0 mol% or less, for example.

[0062] The ceramic composition contains Mn, Ni, and Fe as main components, but can contain Si, Na, K, Ca, Zr, Co, Ti, Al, Cu, and the like as impurities. These impurity elements can be contained in the raw material, and / or can be mixed in during the manufacturing process. It can be considered that if the amount of impurities present in the composition is 1000 ppm or less, at most around 5000 ppm or less, for each element, it will not adversely affect the characteristics of the NTC thermistor.

[0063] (Internal electrodes 71, 72)

[0064] Figure 2The illustrated NTC thermistor includes a laminate 80 in which internal electrodes 71, 72 and ceramic layers 200 are alternately stacked. Further, the laminate of the ceramic layers 200 corresponds to the ceramic base 20. The internal electrodes 71, 72 are exposed from any of the end faces 21, 22 of the ceramic base 20, and are electrically connected to the external electrodes 30, 40 (in Figure 2 the base layers 31, 41 in the present embodiment).

[0065] As the internal electrodes 71, 72, Ag, Pd, Pt, or an alloy containing at least one of them (for example, Ag-Pd) alone can be used.

[0066] (External electrodes 30, 40)

[0067] The external electrodes 30, 40 are provided at least one of the end portions of the ceramic base 20, and preferably at both end portions.

[0068] The external electrodes 30, 40 include: base layers 31, 41 that cover the end faces 21, 22 of the ceramic base 20 and a portion of the surface 23 adjacent to the end faces 21, 22; and plating layers (first plating layers 34, 44) that cover the base layers 31, 41. A second electrode layer 33, 43 can also be included between the base layers 31, 41 and the first plating layers 34, 44.

[0069] (Base layers 31, 41)

[0070] The base layers 31, 41 are formed by applying a Cu-based conductive paste and sintering. The conductive paste generally includes Cu powder as a metal component, a resin, a solvent, and glass powder. Since the resin and the solvent disappear by sintering, in the final product, the NTC thermistor 10, the base layers 31, 41 can be confirmed as a conductive film containing Cu and glass.

[0071] The fact that the base layers 31, 41 contain Cu and glass can be confirmed by SEM-EDX analysis. The cross section of the base layers 31, 41 is exposed, and SEM-EDX analysis is performed on the cross section, and mapping data of the Cu element and the Si element as a glass component are analyzed separately. By the presence of the Cu element in the region corresponding to the base layers 31, 41, it can be confirmed that Cu is contained, and by the presence of the Si element, it can be confirmed that glass is contained.

[0072] As described above, the main cause of the occurrence of cracks in the ceramic base 20 is that the base layers 31, 41 are formed by sintering a Cu-based conductive paste in a state in which NiO remains in the inside of the ceramic base 20. If exposed to high temperatures such as the sintering temperature of the Cu-based conductive paste, stress is generated in the surface 23 due to the difference in thermal expansion rates between the inside of the ceramic base 20 and the surface 23, and cracks are generated.

[0073] In Embodiment 1, by reducing the Ni content compared to conventional Mn-Ni-Fe ceramic compositions, NiO precipitation is suppressed. As a result, even when exposed to high temperatures such as those experienced during the sintering of Cu-based conductive pastes, the amount of expansion of the interior and surface 23 of the ceramic matrix 20 remains unchanged, thus preventing the generation of stress on the surface 23 of the ceramic matrix 20 that could lead to cracking.

[0074] (Second electrode layers 33, 43)

[0075] The second electrode layers 33 and 43 can be set arbitrarily.

[0076] The second electrode layers 33 and 43 are formed of a material that is conductive to and protects the base layers 31 and 41 and is capable of forming a coating on their surface. The second electrode layers 33 and 43 can be formed, for example, of at least one of a conductive resin layer, a sintered electrode layer, etc. The conductive resin layer is formed of a conductive resin material comprising resin and conductive powder.

[0077] (Coating layers (first coating layers 34, 44, second coating layers 35, 45))

[0078] The coating may be formed by a single coating (e.g., only the first coating 34, 44), or it may have a multilayer structure formed by multiple coatings (e.g., a two-layer structure formed by the first coating 34, 44 and the second coating 35, 45).

[0079] Specific examples of multilayer structures include two-layer structures such as Ni-Sn and Ni-Au, and three-layer structures such as Cu-Ni-Sn and Ni-Pd-Au.

[0080] exist Figure 2 In the example shown, a first plating layer 34, 44 covering the second electrode layers 33, 43 and a second plating layer 35, 45 covering the first plating layer 34, 44 are provided.

[0081] It should be noted that, in Figure 2 In the example, the first plating layers 34 and 44 cover the base layers 31 and 41 through the second electrode layers 33 and 43, but in the case where the second electrode layers 33 and 43 are not included as an arbitrary configuration, the first plating layers 34 and 44 directly cover the base layers 31 and 41.

[0082] [Manufacturing method of NTC thermistor 10]

[0083] The following is for reference Figure 2 An example of the manufacturing method of the NTC thermistor 10 according to Embodiment 1 will be described.

[0084] (Formation of ceramic matrix 20)

[0085] First, as raw materials of the ceramic composition constituting the ceramic base 20, Mn304, Fe203, and NiO are weighed in prescribed amounts. Note that it can be considered that the ratio of each metal element in the weighed raw materials is substantially the same as the ratio of each metal element in the ceramic composition constituting the ceramic base of the obtained NTC thermistor. The weighed raw materials are put into a ball mill containing a pulverizing medium such as zirconia balls, and wet pulverization is performed sufficiently, and then pre-sintering is performed at a prescribed temperature to produce a ceramic powder. An organic binder is added to the obtained ceramic powder, and mixing treatment is performed in a wet state to produce a slurry, and then a ceramic green sheet is produced by performing a forming process using a doctor blade method or the like. Note that in this embodiment, metal oxides such as Mn304, Fe203, and NiO are used as raw materials of the ceramic composition constituting the ceramic base, but carbonates, hydroxides, or the like of each element of Mn, Fe, and Ni can also be used as raw materials.

[0086] Next, a paste for internal electrodes, which has Ag-Pd or Pd as a main component, is applied to the ceramic green sheet to form an internal electrode pattern. The paste for internal electrodes can be applied, for example, by screen printing or the like. The ceramic green sheet to which the internal electrode pattern is applied in this way is stacked in a prescribed number, and then is sandwiched with a ceramic green sheet to which no internal electrode pattern is applied and is subjected to press bonding, whereby a laminate is produced. The laminate is cut to a prescribed size, and then is housed in a zirconia-made cartridge to be subjected to debinding treatment, and then is fired at a prescribed temperature (for example, 1100 to 1200°C), whereby a ceramic base 20 in which internal electrodes 71, 72 are internally disposed is formed.

[0087] (Formation of base layers 31, 41)

[0088] The base layers 31, 41 are formed in a manner so as to cover the end portions of the ceramic base 20 (the end faces 21, 22 and a part of the surface 23 of the ceramic base 20 in the example shown in the figure). Figure 2 The base layers 31, 41 can also be formed in a manner so as to cover only the end faces 21, 22 of the ceramic base 20.

[0089] In the NTC thermistor 10 related to Embodiment 1, as a material of the base layers 31, 41, a conductive material having Cu as a main component is used. Such a Cu-based base layer is advantageous in that it is inexpensive and is less likely to cause migration as compared with a conventional Ag-based base layer. The base layers 31, 41 are formed by a coating method (a Cu-based conductive paste is coated at a prescribed position and is subjected to sintering). The sintering is performed in a non-active gas atmosphere under conditions in which the maximum temperature is 800 to 900°C and the cumulative heat amount at 750°C or higher is 100000 to 150000°C-seconds.

[0090] Note that the sintering temperature of the Cu-based conductive paste is higher than that of the Ag-based conductive paste by about 100°C or more, and thus the amount of thermal expansion of the ceramic substrate becomes large. However, by controlling the composition of the ceramic substrate 20, even if heated to the sintering temperature of the Cu-based conductive paste, cracks can be suppressed from occurring on the surface of the ceramic substrate 20.

[0091] (Formation of second electrode layer 33, 43)

[0092] The second electrode layer 33, 43 can also be formed so as to cover the base layer 31, 41.

[0093] The material of the second electrode layer 33, 43 is not particularly limited as long as it is a material that is in conduction with the base layer 31, 41, protects the base layer 31, 41, and enables formation of a plating layer on the surface thereof. The second electrode layer 33, 43 can be formed of, for example, a conductive resin layer.

[0094] The conductive resin layer is provided by curing a resin electrode paste having fluidity. The resin electrode paste contains a conductive powder and a resin raw material. After the resin electrode paste is applied so as to cover the base layer 31, 41 at the end portion of the ceramic substrate 20, the resin raw material in the resin electrode paste is cured.

[0095] As the conductive powder contained in the resin electrode paste, a metal powder such as Ag, Au, Ni, Cu, Pt, Pd, and Al can be used.

[0096] As the resin raw material contained in the resin electrode paste, a resin raw material such as an epoxy resin, a phenol resin, a polyurethane resin, a silicone resin, a polyimide resin, or the like can be used.

[0097] (Formation of first plating layer 34, 44 and second plating layer 35, 45)

[0098] The plating layer (first plating layer 34 and second plating layer 35) is formed so as to cover the surface of the second electrode layer 33, 43. The plating layer preferably has a multilayer structure. As a specific example of the multilayer structure, a two-layer structure such as Ni-Sn, Ni-Au, a three-layer structure such as Cu-Ni-Sn, Ni-Pd-Au, or the like can be cited.

[0099] In the example of Figure 1 , the plating layer has a two-layer structure, the first plating layer 34, 44 covers the second electrode layer 33, 43, and the second plating layer 35, 45 covers the first plating layer 34, 44.

[0100] The first plating layer 34, 44 can be formed, for example, by electroplating at least one of Ni and Cu. The second plating layer 35, 45 can be formed, for example, by electroplating at least one of Sn and Au. Thus, a plating layer having a two-layer structure can be formed.

[0101] Further, a third plating layer can be formed between the first plating layer 34, 44 and the second plating layer 35, 45 to provide a three-layer structure. The third plating layer can be formed by electroplating at least one of Ni and Pd.

[0102] The first plating layer 34, 44, the second plating layer 35, 45 (and the third plating layer) can be formed by a publicly known plating method, for example, a barrel plating using balls.

[0103] Embodiment

[0104] The NTC thermistors of Experimental Examples No. 1 to 17 were produced in the following order. First, as raw materials of a ceramic composition constituting a ceramic base, powders of Mn304, Fe203, and NiO were prepared, and these powders were weighed so as to have the compositions shown in Table 1. Note that in Table 1, each column of "Mn", "Fe", and "Ni" indicates the molar ratio (mol%) of each element of Mn, Fe, and Ni when the total content of Mn element, Fe element, and Ni element in the raw materials is taken as 100 mol%.

[0105] Note that the values underlined in Table 1 indicate values deviating from the range of values defined in the embodiment of the present application.

[0106] The weighed raw materials were put into a ball mill containing a pulverizing medium such as zirconia balls, and wet pulverization was performed sufficiently, followed by pre-burning at 800°C for 2 hours, to produce a ceramic powder. An organic binder was added to the ceramic powder, and mixing treatment was performed in a wet state to produce a slurry. The slurry was formed into a green sheet by a doctor blade method.

[0107] Next, an internal electrode paste was screen-printed on the green sheet to form an internal electrode pattern. Note that, in Experimental Examples No. 1 to 17, an internal electrode paste containing a metal powder formed of an Ag-Pd alloy (blending ratio: Ag 30 wt%, Pd 70 wt%) as a main component was used. The green sheet to which the internal electrode pattern was applied was stacked in such a manner that each internal electrode pattern faced the green sheet of the other, and the upper and lower green sheets were sandwiched with a green sheet to which no internal electrode pattern was applied, and then, the stack was press-bonded, to produce a laminate. The laminate was cut into a size of 1.2 mm in length, 0.6 mm in width, and 0.6 mm in thickness, and then, debinding treatment was performed in a zirconia-made cartridge, followed by baking at a temperature of 1100 to 1200°C, to produce a ceramic base 20 in which internal electrodes 71, 72 were internally disposed.

[0108] Cu-based conductive paste containing glass, epoxy resin and alcohol-based organic solvent as main components was applied to both end portions of the obtained ceramic substrate 20, and sintering was performed at 900°C in a non-reactive gas atmosphere to form the base layers 31, 41. First plating layers 34, 44 formed of Ni were formed on the surfaces of the base layers 31, 41 by plating, and second plating layers 35, 45 formed of Sn were further formed thereon.

[0109] For the NTC thermistors of Experimental Examples No. 1 to 17 thus obtained, the tests (measurements) shown below were performed.

[0110] [SEM observation]

[0111] For the NTC thermistors of Experimental Examples No. 1, 2, 8, 9, 10, 16, 17, the central portion of the dimension in the W direction was cut in a manner that the TL surface was exposed (see Figure 2 ), and the cross section was exposed. The cross section (TL surface) was pretreated as an observation surface, and SEM observation was performed in the vicinity of the region C surrounded by the dotted line in Figure 5 . The various conditions were as follows.

[0112] • SEM device: Scanning Electron Microscope FlexSEM1000II (Hitachi High-Technologies)

[0113] • Magnification: 1000 times

[0114] • Pretreatment of observation surface: Carbon sputtering (30 nm thick)

[0115] • Electron image type: Reflection electron image

[0116] • Acceleration voltage: 15.0 kV

[0117] • WD (Working Distance): About 5 mm

[0118] • Field of view range: 127.0 μm x 95.3 μm

[0119] The SEM images of the NTC thermistors of Experimental Examples No. 1, 2 in the obtained SEM images are shown in Figure 6 and Figure 4 , respectively.

[0120] [Crack occurrence rate]

[0121] Whether cracks occurred in the NTC thermistors of Experimental Examples No. 1 to 17 was observed under the following conditions.

[0122] • Microscope: Inclined body type microscope SMZ745 (Nikon)

[0123] • Magnification: 100 times

[0124] • Number of observations: 17500

[0125] For each NTC thermistor, the entire surface (4 faces) of the ceramic base 20 not covered by the external electrodes was observed with a microscope to confirm the presence or absence of cracks. All cracks observed at the above magnification were identified as "cracks" without limitation to the orientation and length of the cracks, the face and position at which the cracks were observed.

[0126] Figure 7 is an optical microscope photograph of Example 1, and it can be confirmed that cracks occurred in the portion surrounded by the dotted line.

[0127] The number of NTC thermistors in which one or more cracks were observed was counted, and the crack occurrence rate (%) was calculated by dividing the number by the number of observations (17500). A crack occurrence rate of 0% was determined to be acceptable.

[0128] [High-temperature durability test]

[0129] For the NTC thermistors of Experimental Examples No. 1 to 17, the resistance change rate before and after the high-temperature durability test (1000 hours at a temperature of 150°C) was calculated. First, the resistance value at room temperature (25°C) of the NTC thermistor before the high-temperature durability test was measured by the four-terminal method (labeled as "resistance value R 25 (0h)" or simply "R 25 (0h)"). Next, the resistance value at room temperature (25°C) after 1000 hours at a temperature of 150°C was measured (labeled as "resistance value R 25 (1000h)" or simply "R 25 (1000h)"). The resistance change rate ΔR / R was calculated according to the following equation (5). Note that "0h" in parentheses refers to the property value of the NTC thermistor related to the time of the high-temperature durability test being 0 hours, i.e., before the high-temperature durability test, and "1000h" refers to the property value of the NTC thermistor related to the time of the high-temperature durability test being 1000 hours.

[0130] ΔR / R (%) = {R 25 (1000h) - R 25 (0h)} / R 25 (0h) x 100 (5)

[0131] The number of samples N was set to 80, and the arithmetic mean of the resistance change rates ΔR / R obtained was calculated. A case where the average value of ΔR / R deviated from the range of ±3.0% was determined to be "poor", a case where it was within ±3.0% was determined to be "good", and a case where it was within ±2.0% was determined to be "excellent".

[0132] The results of each measurement are shown in Table 2. Note that in Table 2, values judged to be "unqualified" in the crack occurrence rate test and values judged to be "poor" in the high-temperature durability test are underlined.

[0133] In addition, the results of the heat resistance test (resistance change rate before and after the high-temperature storage test) are shown in Figure 5 .

[0134] [Table 1]

[0135]

[0136] [Table 2]

[0137]

[0138] The results of Table 1 and Table 2 were examined.

[0139] (Regarding the crack occurrence rate)

[0140] The Ni content of the ceramic base 20 of the NTC thermistor of Experimental Examples No. 1, 16, and 17 exceeded the range specified in Embodiment 1. In these NTC thermistors, the occurrence of cracks was confirmed (i.e., the "crack occurrence rate" exceeded 0%).

[0141] As shown in Figure 6 , in the SEM image of the NTC thermistor of Experimental Example No. 1, no NiO phase was confirmed in the region Al on the surface side of the ceramic base 20, but a spotted white portion (NiO phase) was confirmed in the region A2 inside the region Al. As an example, several white portions are shown with arrows. In the SEM images of the NTC thermistors of Experimental Examples No. 16 and 17, the region Al (surface region without NiO phase) and the region A2 (internal region with NiO phase) were also observed.

[0142] On the other hand, the Ni content of the ceramic base 20 of the NTC thermistor of Experimental Examples No. 2 to 15 was within the range specified in Embodiment 1. In these NTC thermistors, the occurrence of cracks was not confirmed (i.e., the "crack occurrence rate" was 0%).

[0143] As shown in Figure 7 , in the SEM image of the NTC thermistor of Experimental Example No. 2, no NiO phase was confirmed on the entire ceramic base 20. The same was true for Experimental Examples No. 8, 9, and 10.

[0144] From the results of the Ni content, the SEM images, and the crack occurrence rate, it was confirmed that if the Ni content in the ceramic base 20 exceeds the specified range, a NiO phase exists in the interior of the ceramic base 20, and as a result, cracks occur on the surface of the ceramic base 20.

[0145] (about high-temperature durability)

[0146] In the NTC thermistors of Experimental Examples No. 11, 12, 13, and 14, the Ni content in the ceramic base 20 is less than the lower limit (26.4 mol%), and / or the Mn / Fe ratio is greater than the upper limit (1.90), and thus the electrical conductivity becomes unstable, and the high-temperature durability is determined to be "poor" (AR / R (%) deviates from the range of ±3.0%) (see Table 1). ​

[0147] In addition, the Mn / Fe ratio of the ceramic base 20 of the NTC thermistor of Experimental Example No. 1 is a value greater than the upper limit (1.90). However, it is considered that the high-temperature durability is determined to be "good" because the content of Ni, which improves the high-temperature durability, is large (exceeds the threshold value "29.5 mol%" at which the NiO phase is generated).

[0148] On the other hand, the Ni content in the ceramic base 20 of the NTC thermistors of Experimental Examples No. 2 to 10 and 15 to 17 is 26.4 mol% or more, and the Mn / Fe ratio is 1.90 or less. Thus, the high-temperature durability is determined to be "good" (AR / R (%) is within the range of ±3.0%).

[0149] In particular, the NTC thermistors of Experimental Examples No. 2 to 8, 10, 16, and 17 are determined to be "excellent" in high-temperature durability (AR / R (%) is within the range of ±2.0%) because the Mn content in the ceramic base 20 exceeds 45.6 mol%.

[0150] The present application can include the following modes.

[0151] (Mode 1)

[0152] A negative temperature coefficient thermistor comprising: a ceramic base formed of a ceramic composition containing Mn, Ni, and Fe;

[0153] and an external electrode provided at an end portion of the ceramic base,

[0154] wherein the external electrode contains: a base layer covering the end portion of the ceramic base and containing Cu and glass, and a plating layer covering the base layer,

[0155] the Ni content, the Mn content, and the F content in the ceramic base satisfy the following formula (1) and formula (2),

[0156] 26.4 mol% ≤ [Ni] ≤ 29.5 mol% (1)

[0157] 1.65 ≤ [Mn] / [Fe] ≤ 1.90 (2) ​

[0158] wherein [Ni], [Mn] and [Fe] are the contents of Ni, Mn and Fe (mol%) when the total content of Mn, Ni and Fe contained in the ceramic base is set to 100 mol%.

[0159] (Manner 2)

[0160] The negative characteristic thermistor according to Manner 1, wherein the plating layer is formed of a plurality of plating layers.

[0161] (Manner 3)

[0162] The negative characteristic thermistor according to Manner 1 or 2, wherein an internal electrode is further contained in the inside of the ceramic base.

[0163] This application claims priority based on Japanese Patent Application No. 2023-107328 filed in Japan on June 29, 2023, the entire contents of which are hereby incorporated by reference into the present specification.

[0164] Explanation of Reference Numerals

[0165] 10 Negative characteristic (NTC) thermistor

[0166] 20 Ceramic base

[0167] 21, 22 End surface of ceramic base

[0168] 23 Surface of ceramic base

[0169] 30, 40 External electrode

[0170] 31, 41 Base layer

[0171] 33, 43 Second electrode layer

[0172] 34, 44 First plating layer

[0173] 35, 45 Second plating layer

[0174] 71, 72 Internal electrode

Claims

1. A negative temperature coefficient thermistor comprising: a ceramic base formed of a ceramic composition containing Mn, Ni, and Fe; and an external electrode provided at an end portion of the ceramic base, wherein the external electrode comprises a base layer covering the end portion of the ceramic base and containing Cu and glass, and a plating layer covering the base layer, wherein the content of Ni, the content of Mn, and the content of F in the ceramic base satisfy the following formula (1) and formula (2), 26.4 mol% ≤ [Ni] ≤ 29.5 mol% (1) 1.65 ≤ [Mn] / [Fe] ≤ 1.90 (2) wherein [Ni], [Mn], and [Fe] are the contents (mol%) of Ni, Mn, and Fe, respectively, when the total content of Mn, Ni, and Fe contained in the ceramic base is taken as 100 mol%, and wherein the plating layer is formed of a plurality of plating layers.

2. The negative temperature coefficient thermistor according to claim 1, wherein the plating layer is formed of a plurality of plating layers. wherein 3. The negative temperature coefficient thermistor according to claim 1 or 2, wherein the ceramic base further contains an internal electrode in an internal portion of the ceramic base.

4. The negative temperature coefficient thermistor according to any one of claims 1 to 3, wherein the base layer contains 0.1 to 1.0 mol% of F.

5. The negative temperature coefficient thermistor according to any one of claims 1 to 4, wherein the base layer contains 0.1 to 1.0 mol% of Si.

6. The negative temperature coefficient thermistor according to any one of claims 1 to 5, wherein the base layer contains 0.1 to 1.0 mol% of B.

7. The negative temperature coefficient thermistor according to any one of claims 1 to 6, wherein the base layer contains 0.1 to 1.0 mol% of Mg.

2. The negative temperature coefficient thermistor according to claim 1, wherein 8. The negative temperature coefficient thermistor according to any one of claims 1 to 7, wherein the base layer contains 0.1 to 1.0 mol% of Ca.

3. The negative temperature coefficient thermistor according to claim 1 or 2, wherein 9. The negative temperature coefficient thermistor according to any one of claims 1 to 8, wherein the base layer contains 0.1 to 1.0 mol% of Sr.

10. The

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