Negative characteristic thermistor
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Filing Date
- 2025-12-09
- Publication Date
- 2026-03-12
AI Technical Summary
Negative characteristic thermistors using Cu-based conductive pastes for the underlayer face issues with cracking due to thermal expansion differences within the ceramic body, leading to mechanical weakness and potential failure in high humidity or temperature environments, which existing technologies do not adequately address.
A ceramic thermistor with a Cu-based conductive paste underlayer and a specific composition of Mn, Ni, and Fe, where the Ni content is limited to suppress NiO precipitation and the Mn/Fe ratio is controlled to reduce thermal expansion differences, ensuring the ceramic body's stability and high-temperature durability.
The solution effectively suppresses crack formation and enhances the high-temperature durability of the thermistor, maintaining mechanical strength and reliability even under extreme conditions.
Abstract
Description
Negative temperature coefficient thermistor
[0001] The present invention relates to a negative temperature coefficient thermistor.
[0002] Negative temperature coefficient (NTC) thermistors including a ceramic body made of a Mn / Ni / Fe ceramic composition are known (see, for example, Patent Documents 1 to 3). External electrodes are formed on the ends of the ceramic body, and the base layer of the external electrodes is formed by applying and baking a conductive paste whose main component is Ag (Ag-based conductive paste).
[0003] International Publication No. 2017 / 022373 Japanese Patent Application Laid-Open No. 2016-54225 International Publication No. 2011 / 086850
[0004] Ag is relatively expensive and prone to migration. Therefore, the use of Cu, which is less expensive and less prone to migration, for the underlayer has been considered. However, the inventors discovered that when a Cu-based conductive paste is used as the underlayer and baked, cracks of a depth of several tens of micrometers may occur on the surface of the ceramic body.
[0005] If a crack occurs in the ceramic element, the mechanical strength of the ceramic element decreases, resulting in the ceramic being judged as a defective product due to an appearance abnormality. Furthermore, if a crack occurs in the ceramic element, the crack may propagate if the ceramic element is exposed to a high humidity environment or an environment with large temperature differences, and the thermistor may no longer exhibit the characteristics required.
[0006] The phenomenon of cracks occurring in the ceramic body has not been observed when an Ag-based conductive paste is used to form the underlayer, and therefore no study has been conducted on the occurrence of cracks and how to prevent them. Patent Documents 1 to 3 also do not recognize that cracks may occur in the ceramic body when a Cu-based conductive paste is used to form the underlayer, and therefore no study has been conducted on how to prevent cracks from occurring.
[0007] Furthermore, since negative temperature coefficient thermistors may be used in high-temperature environments, it is important that the negative temperature coefficient thermistors exhibit minimal deterioration in characteristics (i.e., have good high-temperature durability) even when exposed to high temperatures (e.g., 150°C) for long periods of time.
[0008] Therefore, an object of the present invention is to provide a negative temperature coefficient thermistor in which the base layer of the external electrode is formed from a Cu-based conductive paste, which can suppress the occurrence of cracks in the ceramic body and has good high-temperature durability.
[0009] According to one aspect of the present invention, there is provided a NTC thermistor including a ceramic body made of a ceramic composition containing Mn, Ni, and Fe, and external electrodes provided on end portions of the ceramic body, wherein the external electrodes cover the end portions of the ceramic body and include an underlayer containing Cu and glass, and a plating layer covering the underlayer, and the Ni content, Mn content, and F content in the ceramic body satisfy the following formulas (1) and (2): 26.4 mol%≦[Ni]≦29.5 mol% (1) 1.65≦[Mn] / [Fe]≦1.90 (2) where [Ni], [Mn], and [Fe] are the contents (mol %) of Ni, Mn, and Fe, respectively, when the total content of Mn, Ni, and Fe in the ceramic body is taken as 100 mol %.
[0010] The negative temperature coefficient thermistor of the present invention has an external electrode underlayer formed from a Cu-based conductive paste, yet is capable of suppressing the occurrence of cracks in the ceramic body and has good high-temperature durability.
[0011] FIG. 1 is a schematic perspective view of a negative temperature coefficient thermistor according to embodiment 1. FIG. 2 is a schematic cross-sectional view of a negative temperature coefficient thermistor according to embodiment 1. FIG. 3 is a ternary diagram showing the ceramic composition (Mn—Ni—Fe content) of the ceramic body prepared in the example. FIG. 4 is an optical microscope photograph of the negative temperature coefficient thermistor of Experimental Example No. 1. FIG. 5 is an SEM image of a cross section of the negative temperature coefficient thermistor of Experimental Example No. 1. FIG. 6 is an SEM image of a cross section of the negative temperature coefficient thermistor of Experimental Example No. 2. FIG. 7 is a graph showing the resistance change rate before and after a high-temperature storage test for each experiment.
[0012] The present inventors first discovered that when the base layer of the external electrode of an NTC thermistor is formed by applying and baking a Cu-based conductive paste, cracks occur on the surface of the ceramic body, and they conducted extensive research to determine the cause. As a result, they discovered the following two factors. The first factor is that the baking temperature of the Cu-based conductive paste is more than 100°C higher than that of conventional Ag-based conductive paste. The second factor is that the surface and interior of the ceramic body have different structures, which results in a difference in thermal expansion coefficient between the interior and surface of the ceramic body. The present inventors first discovered that the cracks in the ceramic body were caused by a combination of the first and second factors.
[0013] The inventors further conducted extensive research into the reason for the second factor (the difference in structure between the surface and interior of the ceramic body) and found that, in a ceramic body made of a Mn / Ni / Fe-based ceramic composition, NiO precipitates are observed inside the ceramic body, but not on the surface of the ceramic body.
[0014] The inventors speculated that the behavior of NiO precipitates may be due to the following mechanism. During the sintering process of a ceramic body, Ni dissolves in the ceramic composition. However, when the Ni content in the ceramic body increases, some Ni does not dissolve and precipitates as NiO. This NiO precipitation occurs throughout the ceramic body. However, during the cooling process (temperature drop process) of the ceramic body after being held at a predetermined sintering temperature for a predetermined time, NiO reacts with oxygen in the atmosphere (reoxidation reaction) and disappears. The reoxidation reaction first occurs on the surface of the ceramic body and gradually progresses to the interior, but does not progress deep inside the ceramic body. Therefore, NiO disappears only near the surface of the ceramic body (e.g., within a depth of 50 μm from the surface). As a result, the ceramic body has a heterogeneous structure, with no NiO precipitates near the surface (region A1 in Figure 5 ) but present in the interior (region A2 in Figure 5 ).
[0015] The NiO precipitates have a rock salt structure, which is a different crystalline structure from the spinel structure of the Mn / Ni / Fe-based ceramic composition, and this is thought to result in a significant difference in the thermal expansion coefficient between the interior and the surface of the ceramic body, which is thought to be the cause of cracks occurring on the surface of the ceramic body.The present inventors have found for the first time that by limiting the Ni content to a predetermined amount (particularly by limiting the upper limit), it is possible to suppress the precipitation of NiO inside the ceramic body, and as a result, it is possible to suppress the difference in the thermal expansion coefficient between the interior and the surface of the ceramic body.
[0016] Furthermore, the present inventors further investigated the composition of the ceramic body in order to suppress cracking and improve high-temperature durability. High-temperature durability can be easily improved by increasing the Ni content. However, limiting the upper limit of the Ni content to suppress cracking makes it difficult to improve high-temperature durability. The present inventors discovered that high-temperature durability can be improved by controlling the ratio of the Mn content to the Fe content, and thus completed the present invention.
[0017] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
[0018] [Embodiment 1] Fig. 1 is a schematic perspective view of a negative temperature coefficient (NTC) thermistor 10 according to Embodiment 1 of the present invention, and Fig. 2 is a schematic cross-sectional view of the NTC thermistor 10. The NTC thermistor 10 includes a ceramic body 20 and external electrodes 30, 40 at the ends of the ceramic body 20. The external electrodes 30, 40 include base layers 31, 41 and plating layers (first plating layers) 34, 44. The external electrodes 30, 40 may further include second electrode layers 33, 43 between the base layers 31, 41 and the first plating layers 34, 44, or may further include second plating layers 35, 45 covering the first plating layers 34, 44. Internal electrodes 71, 72 are provided inside the ceramic body 20 shown in Fig. 2. The ceramic body 20 (plurality of ceramic layers 200) and the internal electrodes 71, 72 are stacked on top of each other to form a laminate 80.
[0019] Each component will be described in detail below.
[0020] (Ceramic element 20) The ceramic element 20 is made of a ceramic composition containing Mn, Ni, and Fe. In the NTC thermistor 10 according to the first embodiment, when the total content of Mn, Ni, and Fe in the ceramic composition is taken as 100 mol%, the molar ratio of Ni in the ceramic element 20 is 26.4 mol% or more and 29.5 mol% or less. In other words, the ceramic composition satisfies the following formula (1): 26.4 mol%≦[Ni]≦29.5 mol% (1), where [Ni] is the Ni content (mol%) when the total content of Mn, Ni, and Fe in the ceramic element is taken as 100 mol%.
[0021] When the Ni content in the ceramic body 20 is 29.5 mol% or less, the generation of a NiO phase can be suppressed, and as a result, the difference in thermal expansion coefficient between the interior of the ceramic body 20 and the surface 23 can be suppressed. This makes it possible to suppress the generation of cracks on the surface 23 of the ceramic body 20 even when heating is performed to a high baking temperature when forming the underlayers 31, 41 from the Cu-based conductive paste. The Ni content is preferably 29.0 mol% or less, more preferably 28.5 mol% or less, and even more preferably 28.2 mol% or less.
[0022] When the Ni content is 26.4 mol% or more, the amount of Ni dissolved in the ceramic composition is increased, which stabilizes the electrical conductivity of the ceramic body 20 and enables the production of a highly reliable NTC thermistor 10 (especially one with good high-temperature durability). The Ni content is preferably 26.7 mol% or more.
[0023] In the ceramic body 20, when the total content of Mn, Ni, and Fe in the ceramic composition is taken as 100 mol %, the ratio of the Mn content to the Fe content (referred to as the "Mn / Fe ratio") is 1.65 to 1.90. That is, the ceramic composition satisfies the following formula (2): 1.65≦[Mn] / [Fe]≦1.90 (2), where [Mn] and [Fe] are the contents (mol %) of Mn and Fe, respectively, when the total content of Mn, Ni, and Fe in the ceramic body is taken as 100 mol %.
[0024] It is generally believed that reducing the Ni content in a ceramic composition reduces the high-temperature durability of the NTC thermistor. The present inventors have discovered that controlling the Mn / Fe ratio within a preferred range is effective in obtaining an NTC thermistor 10 that suppresses cracking and satisfies market demands for high-temperature durability.
[0025] By setting the Mn / Fe ratio to be 1.65 or more and 1.90 or less, sintering of the ceramic composition is appropriately promoted during sintering of the ceramic body 20, and a crystal structure that is stable against heat is easily obtained. As a result, the electrical conductivity of the ceramic body 20 is stable, and an NTC thermistor 10 with high reliability (especially with good high-temperature durability) can be manufactured.
[0026] The Mn / Fe ratio is preferably 1.70 or more, more preferably 1.72 or more, particularly preferably 1.74 or more, and is preferably 1.85 or less, more preferably 1.83 or less.
[0027] The contents of Mn and Fe are adjusted so that the Mn / Fe ratio satisfies formula (2). When the total content of Mn, Ni, and Fe is taken as 100 mol%, the molar ratio of Mn (i.e., [Mn]) can be, for example, 40.5 mol% or more and 50.0 mol% or less. [Mn] is preferably greater than 45.6 mol%, and particularly preferably 46.0 mol% or more, resulting in an NTC thermistor 10 with superior high-temperature durability. When the total content of Mn, Ni, and Fe is taken as 100 mol%, the molar ratio of Fe (i.e., [Fe]) can be, for example, 20.5 mol% or more and 30.0 mol% or less.
[0028] The ceramic composition contains Mn, Ni, and Fe as its main components, but may also contain impurities such as Si, Na, K, Ca, Zr, Co, Ti, Al, and Cu. These impurity elements may be contained in the raw materials and / or may be mixed in during the manufacturing process. The amount of impurities present in the composition is thought to be 1000 ppm or less for each element, and at most 5000 ppm or less, so as not to adversely affect the characteristics of the NTC thermistor.
[0029] (Internal electrodes 71, 72) The NTC thermistor shown in Fig. 2 includes a laminate 80 in which internal electrodes 71, 72 and ceramic layers 200 are alternately stacked. The laminate of ceramic phases 200 corresponds to the ceramic body 20. The internal electrodes 71, 72 are exposed from either end face 21, 22 of the ceramic body 20 and are electrically connected to the external electrodes 30, 40 (underlayers 31, 41 in Fig. 2). The internal electrodes 71, 72 can be made of a single element such as Ag, Pd, or Pt, or an alloy containing at least one of these elements (e.g., Ag-Pd).
[0030] (External electrodes 30, 40) The external electrodes 30, 40 are provided on at least one end, preferably both end, of the ceramic body 20. The external electrodes 30, 40 include base layers 31, 41 that cover the end faces 21, 22 of the ceramic body 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 may be included between the base layer 31, 41 and the first plating layer 34, 44.
[0031] (Underlayers 31, 41) The underlayers 31, 41 are formed by applying and baking a Cu-based conductive paste. Conductive pastes generally contain Cu powder, a metal component, a resin, a solvent, and glass powder. Because the resin and solvent are removed by baking, the underlayers 31, 41 can be confirmed as conductive films containing Cu and glass in the final NTC thermistor 10. The presence of Cu and glass in the underlayers 31, 41 can be confirmed by SEM-EDX analysis. A cross section passing through the underlayers 31, 41 is exposed, and the cross section is subjected to SEM-EDX analysis to analyze mapping data for Cu element and Si element, a glass component. In the regions corresponding to the underlayers 31, 41, the presence of Cu element confirms the presence of Cu, and the presence of Si element confirms the presence of glass.
[0032] As described above, cracks occur in the ceramic body 20 because the Cu-based conductive paste is baked to form the underlayers 31 and 41 while NiO remains inside the ceramic body 20. When exposed to high temperatures such as the baking temperature of the Cu-based conductive paste, stress is generated in the surface 23 due to the difference in thermal expansion coefficient between the interior and surface 23 of the ceramic body 20, resulting in cracks. In the first embodiment, the Ni content is reduced compared to conventional Mn—Ni—Fe ceramic compositions, thereby suppressing the precipitation of NiO itself. As a result, even when exposed to high temperatures such as those used when baking the Cu-based conductive paste, the amount of expansion remains the same between the interior and surface 23 of the ceramic body 20, and therefore stress sufficient to cause cracks is not generated in the surface 23 of the ceramic body 20.
[0033] (Second electrode layer 33, 43) The second electrode layer 33, 43 can be provided as desired. The second electrode layer 33, 43 is electrically connected to the base layer 31, 41, protects the base layer 31, 41, and is formed from a material that allows a plating layer to be formed on its surface. The second electrode layer 33, 43 can be formed from at least one of, for example, a conductive resin layer, a baked electrode layer, etc. The conductive resin layer is formed from a conductive resin material containing resin and conductive powder.
[0034] (Plating Layers (First Plating Layers 34, 44, Second Plating Layers 35, 45)) The plating layers may be formed from a single plating layer (for example, only the first plating layer 34, 44), or may have a multilayer structure made up of a plurality of plating layers (for example, a two-layer structure made up of the first plating layer 34, 44 and the second plating layer 35, 45). 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.
[0035] 2, first plating layers 34, 44 are provided to cover the second electrode layers 33, 43, and second plating layers 35, 45 are provided to cover the first plating layers 34, 44. In the example of Fig. 2, the first plating layers 34, 44 cover the base layers 31, 41 via the second electrode layers 33, 43, but if the optional second electrode layers 33, 43 are not included, the first plating layers 34, 44 will directly cover the base layers 31, 41.
[0036] [Method for Manufacturing NTC Thermistor 10] An example of a method for manufacturing the NTC thermistor 10 according to the first embodiment will be described below with reference to FIG.
[0037] (Formation of ceramic body 20) First, as a raw material of the ceramic composition that constitutes the ceramic body 20, Mn ３ O ４ , Fe ２ O ３ , and NiO are weighed out in predetermined amounts. It is safe to assume 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 that constitutes the ceramic body of the resulting NTC thermistor. The weighed raw materials are placed in a ball mill containing grinding media such as zirconia balls and thoroughly wet-ground, and then calcined at a predetermined temperature to produce ceramic powder. An organic binder is added to the obtained ceramic powder, and the mixture is wet-mixed to form a slurry. This is then molded using a doctor blade method or the like to produce ceramic green sheets. In this embodiment, Mn is used as the raw material for the ceramic composition that constitutes the ceramic body. ３ O ４ , Fe ２ O ３ However, carbonates, hydroxides, etc. of the elements Mn, Fe, and Ni can also be used as raw materials.
[0038] Next, an internal electrode paste containing Ag—Pd or Pd as a main component is applied to the ceramic green sheets to form an internal electrode pattern. The internal electrode paste may be applied, for example, by screen printing or the like. A predetermined number of ceramic green sheets to which the internal electrode pattern has been applied are stacked, and then sandwiched and pressure-bonded between ceramic green sheets to which no internal electrode pattern has been applied, to produce a laminate. This laminate is then cut to a predetermined size, placed in, for example, a zirconia sack, subjected to a binder removal process, and then fired at a predetermined temperature (e.g., 1100 to 1200°C), to form a ceramic body 20 having internal electrodes 71 and 72 disposed therein.
[0039] (Formation of Base Layers 31, 41) The base layers 31, 41 are formed so as to cover the end portions of the ceramic body 20 (in the example shown in FIG. 2 , the end faces 21, 22 of the ceramic body 20 and part of the surface 23). Note that the base layers 31, 41 may be formed so as to cover only the end faces 21, 22 of the ceramic body 20.
[0040] In the NTC thermistor 10 according to the first embodiment, the base layers 31 and 41 are made of a conductive material primarily composed of Cu. Such Cu-based base layers are advantageous over conventional Ag-based base layers in that they are inexpensive and less susceptible to migration. The base layers 31 and 41 are formed using a coating method (a Cu-based conductive paste is applied to a predetermined location and then baked). Baking is performed in an inert gas atmosphere at a maximum temperature of 800 to 900°C, with an accumulated heat quantity of 100,000 to 150,000°C-seconds at 750°C or higher.
[0041] The baking temperature of Cu-based conductive paste is approximately 100° C. higher than that of Ag-based conductive paste, resulting in a larger amount of thermal expansion of the ceramic body. However, by controlling the composition of ceramic body 20, it is possible to prevent cracks from occurring on the surface of ceramic body 20 even when heated to the baking temperature of the Cu-based conductive paste.
[0042] (Formation of Second Electrode Layers 33, 43) The second electrode layers 33, 43 may be formed so as to cover the base layers 31, 41. The material of the second electrode layers 33, 43 is not particularly limited as long as it is a material that is electrically conductive with the base layers 31, 41, protects the base layers 31, 41, and allows a plating layer to be formed on the surface thereof. The second electrode layers 33, 43 can be formed, for example, from a conductive resin layer.
[0043] The conductive resin layer is formed by curing a fluid resin electrode paste. The resin electrode paste contains a conductive powder and a resin raw material. The resin electrode paste is applied to the ends of the ceramic body 20 so as to cover the base layers 31, 41, and then the resin raw material in the resin electrode paste is cured. The conductive powder contained in the resin electrode paste can be a metal powder such as Ag, Au, Ni, Cu, Pt, Pd, or Al. The resin raw material contained in the resin electrode paste can be, for example, an epoxy resin, a phenolic resin, a urethane resin, a silicone resin, or a polyimide resin.
[0044] (Formation of First Plating Layers 34, 44 and Second Plating Layers 35, 45) Plating layers (first plating layer 34 and second plating layer 35) are formed so as to cover the surfaces of the second electrode layers 33, 43. The plating layers preferably have a multilayer structure. 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. In the example of FIG. 2 , the plating layers have a two-layer structure, with the first plating layers 34, 44 covering the second electrode layers 33, 43, and the second plating layers 35, 45 covering the first plating layers 34, 44.
[0045] The first plating layers 34, 44 can be formed by electrolytic plating of at least one of Ni and Cu, for example. The second plating layers 35, 45 can be formed by electrolytic plating of at least one of Sn and Au, for example. This allows for the formation of a two-layer plating layer. Furthermore, a three-layer structure may be formed by forming a third plating layer between the first plating layer 34, 44 and the second plating layer 35, 45. The third plating layer can be formed by electrolytic plating of at least one of Ni and Pd. The first plating layers 34, 44 and the second plating layers 35, 45 (and the third plating layer) can be formed by known plating methods, such as barrel plating using a ball.
[0046] NTC thermistors of Experimental Examples 1 to 17 were manufactured according to the following procedure. First, Mn３ O ４ , Fe ２ O ３ Powders of Mn, Fe, and NiO were prepared and weighed to obtain the compositions shown in Table 1. In Table 1, the columns "Mn," "Fe," and "Ni" indicate the molar ratios (mol %) of the elements Mn, Fe, and Ni, respectively, when the total content of Mn, Fe, and Ni in the raw materials is taken as 100 mol %. In Table 1, underlined values indicate values outside the ranges specified in the embodiments of the present invention.
[0047] The weighed raw materials were placed in a ball mill containing grinding media such as zirconia balls, thoroughly wet-ground, and then calcined at 800°C for 2 hours to produce ceramic powder. An organic binder was added to this ceramic powder, and the mixture was wet-mixed to form a slurry. This slurry was then molded using a doctor blade method to produce ceramic green sheets.
[0048] Next, the internal electrode paste was screen-printed on the ceramic green sheets to form internal electrode patterns. For Experimental Examples 1 to 17, an internal electrode paste containing, as a main component, a metal powder made of an Ag-Pd alloy (compounding ratio: 30 wt % Ag, 70 wt % Pd) was used. A predetermined number of ceramic green sheets to which the internal electrode patterns were applied were stacked so that the internal electrode patterns faced each other via a ceramic green sheet. A laminate was then produced by sandwiching the top and bottom with ceramic green sheets to which no internal electrode patterns were applied and compressing them. This laminate was then cut to dimensions of 1.2 mm in length, 0.6 mm in width, and 0.6 mm in thickness, placed in a zirconia sack, subjected to a binder removal treatment, and then fired at a temperature of 1100 to 1200°C to produce a ceramic body 20 with internal electrodes 71 and 72 disposed therein.
[0049] A Cu-based conductive paste containing Cu as the main component, glass, epoxy resin, and an alcohol-based organic solvent was applied to both ends of the obtained ceramic body 20, and baked at 900°C in an inert gas atmosphere to form base layers 31, 41. First plating layers 34, 44 made of Ni were formed on the surfaces of the base layers 31, 41 by electrolytic plating, and second plating layers 35, 45 made of Sn were further formed thereon.
[0050] The NTC thermistors of Experimental Examples 1 to 17 thus obtained were subjected to the following tests (measurements).
[0051] [SEM Observation] The NTC thermistors of Experimental Examples 1, 2, 8, 9, 10, 16, and 17 were cut to near the center of the W-direction dimension (see Figure 1) to expose the TL plane, exposing the cross section. The cross section (TL plane) was pretreated as the observation surface, and SEM observation was performed near area C, surrounded by the dashed line in Figure 2. The various conditions were as follows: SEM device: Scanning electron microscope FlexSEM1000II (Hitachi High-Tech); Magnification: 1000x; Observation surface pretreatment: Carbon sputtering (30 nm thick); Electron image type: Backscattered electron image; Acceleration voltage: 15.0 kV; WD (working distance): approximately 5 mm; Field of view: 127.0 μm × 95.3 μm. Of the obtained SEM images, the SEM images of the NTC thermistors of Experimental Examples 1 and 2 are shown in Figures 5 and 6, respectively.
[0052] [Crack Occurrence Rate] The presence or absence of cracks in the NTC thermistors of Experimental Examples 1 to 17 was observed under the following conditions: Microscope: SMZ745 (Nikon) Oblique Stereo Microscope; Magnification: 100x; Number of Observations: 17,500. For each NTC thermistor, the entire surface (four sides) of the ceramic substrate 20 not covered with the external electrodes was observed under the microscope to confirm the presence or absence of cracks. All cracks observed at the above magnification were deemed to be "cracks," and there were no restrictions on the direction and length of the cracks, or the surface and position on which the cracks were observed. Figure 4 is an optical microscope photograph of Test Example 1, and cracks can be seen in the area surrounded by the dashed line.
[0053] The number of NTC thermistors in which one or more cracks were observed was counted and divided by the number of observed pieces (17,500 pieces) to calculate the crack occurrence rate (%). A crack occurrence rate of 0% was considered to be pass.
[0054] [High-Temperature Durability Test] The resistance change rate was determined for the NTC thermistors of Experimental Examples 1 to 17 before and after a high-temperature durability test (left at a temperature of 150°C for 1000 hours). First, the resistance value of the NTC thermistor at room temperature (25°C) before the high-temperature durability test (referred to as "resistance value R ２５ (0h)" or simply "R ２５ Next, the resistance value at room temperature (25°C) after leaving the sample at a temperature of 150°C for 1000 hours (hereinafter referred to as "resistance value R ２５ (1000h)" or simply "R ２５ The resistance change rate ΔR / R was calculated according to the following formula (5). The "0 h" in parentheses means that the high-temperature durability test time was 0 hours, i.e., the physical property value is for the NTC thermistor before the high-temperature durability test, and "1000 h" means that the physical property value is for the NTC thermistor after the high-temperature durability test was performed for 1000 hours. ΔR / R (%) = {R ２５ (1000h)-R ２５ (0h)} / R ２５ (0h)×100 (5)
[0055] The number of samples N was 80, and the obtained resistance change rates ΔR / R were arithmetically averaged. When the average value of ΔR / R was outside the range of ±3.0%, it was rated as "poor," when it was within ±3.0%, it was rated as "good," and when it was within ±2.0%, it was rated as "excellent."
[0056] The results of the measurements are shown in Table 2. In Table 2, the values judged as "fail" in the crack occurrence rate test and the values judged as "poor" in the high-temperature durability test are underlined. The results of the heat resistance test (the rate of change in resistance before and after the high-temperature storage test) are shown in Figure 7.
[0057]
[0058]
[0059] The results in Tables 1 and 2 are discussed below.
[0060] (Regarding the crack occurrence rate) In the NTC thermistors of Experimental Examples 1, 16, and 17, the Ni content of the ceramic body 20 exceeded the range specified in the first embodiment. The occurrence of cracks was confirmed in these NTC thermistors (i.e., the "crack occurrence rate" exceeded 0%). As shown in FIG. 5 , in the SEM image of the NTC thermistor of Experimental Example 1, no NiO phase was observed in region A1 on the surface side of the ceramic body 20, but spotted white areas (NiO phase) were observed in region A2, which was located inside region A1. Some of the white areas are indicated by arrows as examples. Region A1 (a surface region free of NiO phase) and region A2 (an internal region containing NiO phase) were also observed in the SEM images of the NTC thermistors of Experimental Examples 16 and 17.
[0061] On the other hand, the NTC thermistors of Experimental Examples 2 to 15 had ceramic body 20 with a Ni content within the range specified in the first embodiment. No cracks were observed in these NTC thermistors (i.e., the "crack occurrence rate" was 0%). As shown in FIG. 6 , the SEM image of the NTC thermistor of Experimental Example 2 did not reveal any NiO phase throughout the ceramic body 20. Similarly, no NiO phase was observed in Experimental Examples 8, 9, and 10.
[0062] From the measurement results of the Ni content, SEM images, and crack occurrence rate, it was confirmed that when the Ni content in the ceramic base 20 exceeds a predetermined range, a NiO phase is present inside the ceramic body 20, and as a result, cracks occur on the surface of the ceramic body 20.
[0063] (Regarding High-Temperature Durability) In the NTC thermistors of Experimental Examples 11, 12, 13, and 14, the Ni content in the ceramic substrate 20 was less than the lower limit (26.4 mol%) and / or the Mn / Fe ratio was greater than the upper limit (1.90), resulting in unstable electrical conductivity and a "poor" rating for high-temperature durability (ΔR / R (%) outside the range of ±3.0%) (see FIG. 7 ).
[0064] In the NTC thermistor of Experimental Example No. 1, the Mn / Fe ratio of the ceramic substrate 20 was greater than the upper limit (1.90). However, the Ni content, which improves high-temperature durability, was high (exceeding the threshold value of 29.5 mol % at which the NiO phase occurs), which is thought to be why the high-temperature durability was judged to be "good."
[0065] On the other hand, the NTC thermistors of Experimental Examples 2 to 10 and 15 to 17 had a Ni content in the ceramic base 20 that was equal to or greater than the lower limit (26.4 mol%) and a Mn / Fe ratio that was equal to or less than the upper limit (1.90). Therefore, the high-temperature durability was evaluated as "good" (ΔR / R (%) was within a range of ±3.0%). In particular, the NTC thermistors of Experimental Examples 2 to 8, 10, 16, and 17 had a Mn content in the ceramic base 20 that exceeded 45.6 mol%, so the high-temperature durability was evaluated as "excellent" (ΔR / R (%) was within a range of ±2.0%).
[0066] The present invention may include the following aspects: (Aspect 1) A negative temperature coefficient thermistor including a ceramic body made of a ceramic composition containing Mn, Ni, and Fe, and external electrodes provided on end portions of the ceramic body, wherein the external electrodes cover the end portions of the ceramic body and include an underlayer containing Cu and glass, and a plating layer covering the underlayer, and wherein the Ni content, Mn content, and F content in the ceramic body satisfy the following equations (1) and (2): 26.4 mol%≦[Ni]≦29.5 mol% (1) 1.65≦[Mn] / [Fe]≦1.90 (2) where [Ni], [Mn], and [Fe] are the contents (mol %) of Ni, Mn, and Fe, respectively, when the total content of Mn, Ni, and Fe in the ceramic body is taken as 100 mol %.
[0067] (Aspect 2) The NTC thermistor according to aspect 1, wherein the plating layer comprises a plurality of plating layers.
[0068] (Aspect 3) The NTC thermistor according to aspect 1 or 2, further comprising an internal electrode inside the ceramic body.
[0069] This application claims priority based on Japanese Patent Application No. 2023-107328, filed on June 29, 2023, the entire contents of which are incorporated herein by reference.
[0070] REFERENCE SIGNS LIST 10 Negative Temperature Coefficient (NTC) Thermistor 20 Ceramic body 21, 22 End faces of ceramic body 23 Surface of ceramic body 30, 40 External electrodes 31, 41 Underlayer 33, 43 Second electrode layer 34, 44 First plating layer 35, 45 Second plating layer 71, 72 Internal electrodes
Claims
1. a ceramic body made of a ceramic composition containing Mn, Ni, and Fe; and external electrodes provided at the ends of the ceramic body, the external electrodes cover the ends of the ceramic body and include a base layer containing Cu and glass, and a plating layer covering the base layer; A negative temperature coefficient thermistor, wherein the Ni content, Mn content, and Fe content in the ceramic body satisfy the following formulas (1) and (2): 26.4 mol%≦[Ni]≦29.5 mol% (1) 1.65≦[Mn] / [Fe]≦1.90 (2) Here, [Ni], [Mn], and [Fe] are the respective contents (mol %) of Ni, Mn, and Fe when the total content of Mn, Ni, and Fe contained in the ceramic body is 100 mol %.
2. 2. The NTC thermistor according to claim 1, wherein the plating layer comprises a plurality of plating layers.
3. 3. The NTC thermistor according to claim 1, further comprising an internal electrode within the ceramic body.