Patent Grant
8258915
This is a continuation-in-part of application Serial No. PCT/JP2007/068136, filed Sep. 19, 2007.
The present invention generally relates to NTC thermistor ceramics and in particular to NTC thermistor ceramics suitable for use in a NTC thermistor for suppressing inrush current generated when a power switch is turned ON, and a NTC thermistor.
NTC thermistors known in the art have been roughly categorized into two types depending on the usage, and temperature-compensating thermistors and inrush current-limiting thermistor. Among these, inrush current-limiting NTC thermistors are mainly built into power circuits and used for limiting the large inrush current that instantaneously flows when the capacitors in the circuits start charge accumulation upon turning on the power source.
One example of the above-described NTC thermistors known in the art is a multilayer NTC thermistor shown in
Various thermistor ceramic compositions that contain metal oxides containing manganese (Mn) and nickel (Ni) as main components have been known as the material for the ceramic element body.
For example, Japanese Unexamined Patent Application Publication No. 62-11202 (Patent Document 1) describes a thermistor composition including an oxide containing three elements, namely, manganese, nickel, and aluminum, in which the ratios of these elements are within the ranges of 20 to 85 mol % manganese, 5 to 70 mol % nickel, and 0.1 to 9 mol % aluminum, the total of the three elements being 100 mol %.
Another example, Japanese Patent No. 3430023 (Patent Document 2), describes a thermistor composition in which 0.01 to 20 wt % cobalt oxide, 5 to 20 wt % copper oxide, 0.01 to 20 wt % iron oxide, and 0.01 to 5.0 wt % zirconium oxide are added to a metal oxide, containing, in terms of the content of the metals only, 50 to 90 mol % manganese and 10 to 50 mol % nickel totaling to 100 mol %.
Another example is Japanese Unexamined Patent Application Publication No. 2005-150289 (Patent Document 3) which describes a thermistor composition containing a manganese oxide, a nickel oxide, an iron oxide, and a zirconium oxide, in which a mol % (wherein a is 45 to 95 excluding 45 and 95) manganese oxide in term of Mn and (100-a) mol % nickel oxide in terms of Ni are contained as main components, and per 100 wt % of these main components, the ratios of the respective components are 0 to 55 wt % (excluding 0 wt % and 55 wt %) iron oxide in terms of Fe2O3 and 0 to 15 wt % (excluding 0 wt % and 15 wt %) zirconium oxide in terms of ZrO2.
Meanwhile, COUDERC J. J., BRIEU M., FRITSCH S, and ROUSSET A., DOMAIN MICROSTRUCTURE IN HAUSMANNITE Mn3O4 AND IN NICKEL MANGANITE, THIRD EURO-CERAMICS, VOL. 1 (1993) pp. 763-768 (Non-Patent Document 1) reports a thermistor ceramic composition in which plate-shaped deposits which are generated by gradually cooling Mn3O4 from high temperature (cooling rate: 6° C./hr) but not when Mn3O4 is rapidly cooled from high temperature in air, giving instead a lamella structure (stripe-shaped contrast structure). In addition, this document also reports that NiO0.75Mn2.25O4 forms a spinel single phase when gradually cooled from high temperature (cooling rate: 6° C./hr) in which no plate-shaped deposits or lamella structures are observed, and forms a lamella structure but not plate-shaped deposits when rapidly cooled from high temperature in air.
When thermistor ceramic compositions proposed in the above-described documents are used to make inrush current-limiting NTC thermistors, the insufficient dispersion of raw materials results in inhomogeneous dispersion of the compounds forming the ceramic, and a variation in ceramic grain diameters of the raw materials results in local formation of low-resistance regions in the thermistor element bodies of the resulting NTC thermistors. If current, such as inrush current, flows in such NTC thermistor element bodies (
The documents described above report that different crystal structures can be derived from Mn3O4 and NiO0.75Mn2.25O4, i.e., the thermistor compositions, by changing the cooling rate from high temperature. However, the inventor of the present invention has found that none of the crystal structures of these compositions has sufficient voltage resistance.
An object of the present invention is to provide a NTC thermistor ceramic having excellent voltage resistance and a NTC thermistor.
In order to attain the object described above, the inventor assumed that the fracture mode caused by inrush current is attributable to the thermal melting of and cracks in the NTC thermistor element bodies, and studied various compositions and crystal structures. As a result, the inventor has found that the voltage resistance can be enhanced when a different phase having a relatively high electrical resistance and containing plate crystals is dispersed in the matrix. The present invention has been made on the basis of this finding.
A NTC thermistor ceramic of this invention includes a first phase, which is a matrix, and a second phase dispersed in the first phase, in which the second phase includes plate crystals and has an electrical resistance higher than that of the first phase.
According to the NTC thermistor ceramic of this invention, the second phase composed of plate crystals having a higher electrical resistance than the first phase exists in the first phase, i.e., the matrix. The present inventor conducted extensive investigations and found that even when regions having a low electrical resistance are locally formed in a NTC thermistor ceramic mainly composed of Mn, the potential gradient that occurs in the matrix as a result of concentration of electrical current in the low-resistance regions during application of inrush current can be moderated by the presence of a dispersed high-electrical-resistance phase having a higher resistance than the matrix. As a result, the electrical field concentration on the low-resistance regions can be moderated, and fracture caused by heat melting of the thermistor element body can be suppressed. Thus, the voltage resistance of a NTC thermistor using the NTC thermistor ceramic of the present invention can be further improved.
In the NTC thermistor ceramic of the present invention, preferably, the first and second phases contain manganese and the manganese content in the second phase is higher than that in the first phase.
In this manner, the electrical resistance of the second phase can be made higher than that of the first phase. Thus, fracture caused by heat melting can be suppressed, and the voltage resistance of the NTC thermistor ceramic can be improved. Furthermore, since the main components of the first and second phases are the same, no complicated synthetic process is needed in depositing plate crystals, and strains and cracks are not readily generated since the it is easy to bond the first phase to the second phase.
According a NTC thermistor ceramic according to one aspect of the present invention, preferably, the first phase has a spinel structure, the first and second phases contain manganese and nickel, the (manganese content)/(nickel content) ratio of the NTC thermistor ceramic as a whole is or more and 96/4 or less, and the NTC thermistor ceramic contains 0 at % to 15 at % copper, 0 at % to 10 at % aluminum, 0 at % to 10 at % iron, 0 at % to 15 at % cobalt, 0 at % to 5 at % titanium, and 0 at % to 1.5 at % zirconium.
According to this aspect, a structure in which a high-resistance phase having a higher electrical resistance than the matrix exists in the matrix can be achieved, the hardness of the NTC thermistor ceramic can be increased, and the toughness can be improved. As a result, not only fracture caused by heat melting is suppressed but also fracture attributable to cracks can be suppressed. Thus, the voltage resistance of the NTC thermistor ceramic can be further improved.
Incorporating 10 at % or less aluminum, 10 at % or less iron, 15 at % or less cobalt, and 5 at % or less titanium further improves the hardness or fracture toughness of the NTC thermistor ceramic. Thus, fracture attributable to cracks can be suppressed further and the voltage resistance can be further improved.
Incorporating 1.5 at % or less zirconium allows zirconium oxide to segregate in the grain boundaries of the ceramic crystal grains and thus improves mechanical properties of the grain boundaries of the ceramic crystal grains composed of the NTC thermistor ceramic. Thus, fracture attributable to cracks can be suppressed, and the voltage resistance can be further improved as a result.
According to a NTC thermistor ceramic of another aspect of the present invention, preferably, the first phase has a spinel structure, the first and second phases contain manganese and cobalt, the (manganese content)/(cobalt content) ratio of the NTC thermistor ceramic as a whole is 60/40 or more and 90/10 or less, and the NTC thermistor ceramic contains 0 at % to 22 at % copper, 0 at % to 15 at % aluminum, 0 at % to 15 at % iron, 0 at % to 15 at % nickel, and 0 at % to 1.5 at % zirconium.
According to this aspect, a structure in which a high-resistance phase having a higher electrical resistance than the matrix exists in the matrix can be achieved, the hardness of the NTC thermistor ceramic can be increased, and the toughness can be improved. As a result, not only fracture caused by heat melting is suppressed but also fracture attributable to cracks can be suppressed. Thus, the voltage resistance of the NTC thermistor ceramic can be further improved.
Incorporating 15 at % or less aluminum, 15 at % or less iron, and 15 at % or less nickel further improves the hardness or fracture toughness of the NTC thermistor ceramic. Thus, fracture attributable to cracks can be suppressed further and the voltage resistance can be further improved.
Incorporating 1.5 at % or less zirconium allows zirconium oxide to segregate in the grain boundaries of the ceramic crystal grains and thus improves mechanical properties of the grain boundaries of the ceramic crystal grains composed of the NTC thermistor ceramic. Thus, fracture attributable to cracks can be suppressed, and the voltage resistance can be further improved as a result.
The NTC thermistor ceramic of the present invention having any one of the features described above preferably further includes a third phase different from the second phase dispersed in the first phase, and the third phase preferably has an electrical resistance higher than that of the first phase.
In this manner, a third phase having an electrical resistance higher than that of the first phase exists in the first phase, i.e., in addition to the matrix and the second phase composed of plate crystals and having a higher electrical resistance than the first phase. Since another high-resistance phase different from the first high-resistance phase composed of plate crystals exists in the matrix, the potential gradient in the matrix can be decreased and local electrical field concentration can be moderated when excessive inrush current is applied. Thus, fracture caused by heat melting can be suppressed. The voltage resistance of the NTC thermistor ceramic can be increased.
Increasing the copper content in pursuing further improvements in voltage resistance sometimes generates cracks and the like during firing. However, the resistivity of the material at room temperature, at a low copper content, tends to be high. The invention having the above-described features can lower the resistivity at room temperature while maintaining high voltage resistance.
In such a case, the third phase preferably contains an alkaline earth element.
In the composition constituting the NTC thermistor ceramic of the present invention having the above-described features, preferably, the first phase has a spinel structure, the first and second phases contain manganese and nickel, the (manganese content)/(nickel content) ratio of the NTC thermistor ceramic as a whole is 87/13 or more and 96/4 or less, and the NTC thermistor ceramic contains 0 at % to at % copper, 0 at % to 10 at % aluminum, 0 at % to 10 at % iron, 0 at % to 15 at % cobalt, and 0 at % to 5 at % titanium, and further contains, as the alkaline earth metal, at least one element selected from the group consisting of calcium and strontium, the calcium content being 10 at % or less (excluding 0 at %) and the strontium content being 5 at % or less (excluding 0 at %).
In another composition constituting the NTC thermistor ceramic of the present invention having the above-described features, the first phase has a spinel structure, the first and second phases contain manganese and cobalt, the (manganese content)/(cobalt content) ratio of the NTC thermistor ceramic as a whole is 60/40 or more and 90/10 or less, and the NTC thermistor ceramic contains 0 at % to 22 at % or less copper, 0 at % to 15 at % aluminum, 0 at % to 15 at % iron, and 0 at % to 15 at % nickel, and further contains, as the alkaline earth element, at least one element selected from the group consisting of calcium and strontium, the calcium content being 5 at % or less (excluding 0 at %) and the strontium content being 5 at % or less (excluding 0 at %).
In this manner, the voltage resistance of the NTC thermistor ceramic can be further improved, and a structure having a low electrical resistivity at room temperature can be achieved.
A NTC thermistor according to the present invention includes a thermistor element body composed of the NTC thermistor ceramic having any of the features described above and an electrode disposed on a surface of the thermistor element body.
In this manner, a NTC thermistor with high voltage resistance suitable for limiting high inrush current can be achieved.
According to this invention, the voltage resistance of the NTC thermistor ceramic can be improved, and a NTC thermistor with high voltage resistance suitable for limiting high inrush current can be made using this NTC thermistor ceramic.
1: NTC thermistor, 11: internal electrode layer, 12: external electrode layer, 20: ceramic element body
The present inventor has made the following investigations on the reason why the voltage resistance of existing NTC thermistor ceramics is insufficient:
(1) First, the inventor assumed that the fracture mode caused by excessive inrush current is attributable to thermal melting as one of the reasons for insufficient voltage resistance. When the temperature of a NTC thermistor rises, its electrical resistance decreases. For example, when disintegration of the raw materials is insufficient and compounds forming the ceramic are dispersed inhomogeneously or when the ceramic grain diameters of the raw materials have a variation, the NTC thermistor ceramic may locally have portions with a low electrical resistance. When an inrush current is applied to such a NTC thermistor, the inrush current concentrates on portions with low electrical resistance, thereby raising the temperature of those portions. As a result, the electrical resistance of those portions becomes lower than the electrical resistance of other portions, and this promotes further concentration of electrical current. Consequently, electrical current concentrates on one region, further elevating the temperature and melting the ceramic constituting the thermistor element body, and the melted portion becomes a starting point of the fracture.
A NTC thermistor ceramic of the present invention contains, in its matrix, a phase composed of plate crystals and having a high electrical resistance relative to the matrix. Simulation results by finite element analysis have shown that according to this structure, the potential gradient in the matrix decreases when inrush current is applied. Based on these results, it has been found that presence of a high-resistance phase having a high resistance relative to the matrix moderates the local electrical field concentration in the matrix and suppresses fracture caused by thermal melting.
(2) Next, the inventor assumed that the fracture mode caused by inrush current is attributable to cracks as another reason for insufficient voltage resistance. The ceramic constituting a NTC thermistor ceramic undergoes thermal expansion with an increase in temperature. Thus, the ceramic is required to exhibit a strength that can withstand the thermal expansion in order to enhance the voltage resistance.
According to one embodiment of the present invention, the first phase has a spinel structure, the first and second phases contain manganese and nickel, and the (manganese content)/(nickel content) ratio of the NTC thermistor ceramic as a whole is 87/13 or more and 96/4 or less. The experiments conducted by the inventor have shown that a composition having a high hardness or a high fracture toughness can be obtained as the (manganese content)/(nickel content) ratio becomes higher. Based on these results, it is assumed that increasing the manganese content helps achieve a high hardness or a high fracture toughness and suppress fracture caused by cracks.
The first phase has a spinel structure, the first and second phases contain manganese and nickel, the (manganese content)/(nickel content) ratio of the NTC thermistor ceramic as a whole is 87/13 or more and 96/4 or less, the NTC thermistor ceramic contains 0 at % to 15 at % copper, 0 at % to 10 at % aluminum, 0 at % to 10 at % iron, 0 at % to 15 at % cobalt, 0 at % to 5 at % titanium, and 0 at % to 1.5 at % zirconium, and the manganese content in the second phase is higher than that of the first phase.
The basic structure of the NTC thermistor ceramic according to another preferred embodiment of the present invention includes a first phase which is a matrix having a spinel structure and a second phase dispersed in the first phase and composed of a plurality of plate crystals, in which the second phase shows a higher electrical resistance than the first phase, the first and second phases contain manganese and cobalt, the (manganese content)/(cobalt content) ratio of the NTC thermistor ceramic as a whole is or more and 90/10 or less, and the manganese content in the second phase is higher than that of the first phase.
The first phase has a spinel structure, the first and second phases contain manganese and cobalt, the (manganese content)/(cobalt content) ratio of the NTC thermistor ceramic as a whole is 60/40 or more and 90/10 or less, the NTC thermistor ceramic contains 0 at % to 22 at % copper, 0 at % to 15 at % aluminum, 0 at % to 15 at % iron, 0 at % to 15 at % nickel, and 0 at % to 1.5 at % zirconium, and the manganese content in the second phase is higher than that of the first phase.
A NTC thermistor ceramic of any embodiment of the present invention preferably further includes a third phase different from the second phase dispersed in the first phase, the third phase preferably has an electrical resistance higher than that of the first phase, and the third phase preferably contains an alkaline earth metal. In such a case, preferably, the NTC thermistor ceramic contains as an alkaline earth metal at least one element selected from the group consisting calcium and strontium, the calcium content is preferably in the range of 10 at % or less (excluding 0 at %) in a system containing manganese and nickel as main components or in the range of 5 at % or less (excluding 0 at %) in a system containing manganese and cobalt as main components, and the strontium content is preferably in the range of 5 at % or less (excluding 0 at %).
Although the first phase of the NTC thermistor ceramic according to the embodiment of the present invention described above has a spinel structure, compositions having structures other than the spinel structure can have structures that exhibit high voltage resistance. The first phase is thus not limited to one having a spinel structure. Furthermore, although the NTC thermistor ceramic of the embodiment of the present invention includes a second phase composed of plate crystals, the form of crystals is not limited. The second phase has an effect of increasing the voltage resistance if crystals having certain aspect ratios, such as plate and needle crystals, are dispersed in the first phase and the electrical resistance of the second phase is higher than that of the first phase. Such crystals have an average aspect ratio (long axis/short axis) of at least about 3:1 in the figure projected from three dimension to two dimension. Moreover, the NTC thermistor ceramic of the present invention may contain inevitable impurities such as sodium.
Examples of preparation of NTC thermistors of the present invention will now be described.
Manganese oxide (Mn3O4) and nickel oxide (NiO) were weighed and blended so that the atomic ratios (atom %) of the manganese (Mn) and nickel (Ni) after firing were adjusted to ratios indicated in Table 1. To the resulting mixture, poly(ammonium carboxylate) serving as a dispersant and pure water were added, and the resulting mixture was disintegrated by wet-mixing in a ball mill, i.e., a mixer and a disintegrator, for several hours. The resulting mixture powder was dried and calcined for 2 hours at a temperature of 650° C. to 1000° C. To the calcined powder, the dispersant and pure water were again added and the resulting mixture was disintegrated by wet-mixing in a ball mill for several hours. To the resulting mixture powder, a water-based binder resin, i.e., an acrylic resin, was added, and the resulting mixture was defoamed in a low vacuum of 500 to 1000 mHg to prepare a slurry. The slurry was formed by the doctor blade method on a carrier film constituted by a polyethylene terephthalate (PET) film and dried to prepare a green sheet 20 to 50 μm in thickness on the carrier film.
In the example described above, a ball mill was used as a mixer and an integrator. Alternatively, an attritor, a jet mill, and various other disintegrators may be used. For the method for forming the green sheet, pulling methods such as lip coating and roll coating may be used other than the doctor blade method.
The obtained green sheet was cut to a predetermined size, and a plurality of sheets were stacked to a certain thickness. Subsequently, the sheets were pressed at about 106 Pa to prepare a multilayer green sheet compact.
The compact was cut into a predetermined shape and heated at a temperature of 300° C. to 600° C. for 1 hour to remove the binder. Then the compact was fired in the firing step described below to prepare a ceramic element body that served as the NTC thermistor ceramic of the present invention.
The firing step included a temperature-elevating process, a high temperature-retaining process, and a temperature-decreasing process. In the high temperature-retaining process, a temperature of 1000° C. to 1200° C. was maintained for 2 hours, and the temperature-elevating rate was 200° C./hour. The rate of temperature-decreasing was also 200° C./hour except when the temperature was in the range of 500° C. to 800° C. when it was about ½ of that temperature-decreasing rate. Plate crystals mainly composed of manganese oxide constituting a high-resistance second phase of the NTC thermistor ceramic of the present invention can be produced by decreasing the temperature-decreasing rate when the temperature is in the range of 800° C. to 500° C. to a level lower than that in other temperature ranges in the firing step. X-ray diffraction analysis (XRD) has found that plate crystals mainly composed of manganese oxide start to form in the temperature range of 700° C. to 800° C. in the temperature-decreasing process, and the number of crystals produced increases during the temperature-decreasing process down to 500° C. Moreover, gradual cooling (6° C./hour, requiring about 8.3 days) described in the prior art documents is not needed in the present invention, and the temperature-decreasing time can be about several hours, which is efficient. The firing atmosphere was air. The firing atmosphere may be oxygen gas.
Silver (Ag) electrodes were applied on both surfaces of the NTC thermistor element body and baked at 700° C. to 800° C. The resulting product was diced into a 1 mm2 size to prepare a single plate-type NTC thermistor shown in
The electrical characteristics of each sample of the single plate-type NTC thermistor with electrodes were measured by a DC four-terminal method (Hewlett Packard 3458A multimeter).
In Table 1, “ρ25” indicates the resistivity (Ωcm) at a temperature of 25° C., calculated from the equation below where R25 (Ω) is the electrical resistance at 25° C. when current I (A) flows in the length direction of a sample having a width W (cm), a length L (cm), and a thickness T (cm) as shown in
ρ25=R25×W×T/L
“B25/50” (K) is calculated from the equation below,
where R25 (Ω) is the electrical resistance at a temperature of 25° C. and R50 (Ω) is the electrical resistance at a temperature of 50° C.:
B25/50=(log R25−log R50)/(1/(273.15+25)−1/(273.15+50))
The results of the measurements on the NTC thermistors having ceramic element bodies containing manganese and nickel are shown in Table 1.
The voltage resistance of each sample of the NTC thermistor that includes a ceramic element body containing manganese and nickel as main metal elements was evaluated as follows. After the ceramic element body formed as a single plate was mounted on a substrate, leads were attached to the electrodes on the ceramic element body and a predetermined voltage was applied thereto to supply inrush current. Changes in electrical resistance at that time were measured. An ISYS low-temperature voltage resistance tester (model IS-062) was used as the measurement instrument.
As the inrush current flows into the NTC thermistor, the electrical resistance starts to increase rapidly after a certain current value is attained. Having high voltage resistance means that the electrical resistance does not change until a high current value is reached. In this example, the rate of change in electrical resistance ΔR25 when 10 A current was supplied to a NTC thermistor having a thickness of 0.65±0.05 mm was calculated to evaluate voltage resistance.
In Table 1, “voltage resistance” (%) is calculated by the equation below where R025 (Ω) is the electrical resistance at a temperature of 25° C. before supplying the inrush current, and R125 (Ω) is the electrical resistance at 25° C. after supplying 10 A inrush current:
ΔR25=(R125/R025−1)×100
As shown in Table 1, it was confirmed that in all samples of single plate-type NTC thermistors having ceramic element bodies containing manganese and nickel as the main metal elements, plate crystals mainly composed of manganese oxide serving as the second phase having a high electrical resistance were dispersed in the first phase, i.e., the matrix having a high electrical resistance, when the atomic (manganese content)/(nickel content) ratio was in the range of 87/13 or more and 96/4 or less. In the “judgment” column of Table 1, samples in which generation of the second phase was observed are marked by circles and samples in which generation of the second phase was not observed are marked by X. It was found that sample Nos. 103 to 106 in which generation of the second phase was observed exhibited a “rate of change in electrical resistance ΔR25 after application of inrush current”, i.e., the indicator of the voltage resistance, of 10% or less and thus had high voltage resistance.
Manganese oxide (Mn3O4), nickel oxide (NiO), and copper oxide (CuO) were weighed and blended so that the atomic ratios (atom %) of the manganese (Mn), nickel (Ni), and copper (Cu) after firing were adjusted to ratios shown in Table 2. Then green sheets were prepared as in EXAMPLE 1A.
The resulting green sheets were stacked, pressed, and fired as in EXAMPLE 1A to prepare a ceramic element body that served as the NTC thermistor ceramic of the present invention. Electrodes were formed on the ceramic main body as in EXAMPLE 1A to obtain a NTC thermistor.
The voltage resistance of each sample of a single plate-type NTC thermistor including a ceramic element body containing manganese, nickel, and copper as main metal elements prepared as above was evaluated as follows. After the ceramic element body formed as a single plate was mounted on a substrate, leads were attached to the electrodes on the ceramic element body and a predetermined voltage was applied thereto to supply inrush current. Changes in electrical resistance at that time were measured. An ISYS low-temperature voltage resistance tester (model IS-062) was used as the measurement instrument.
As the inrush current flows into the NTC thermistor, the electrical resistance starts to increase rapidly after a certain current value. Having high voltage resistance means that the electrical resistance does not change until a high current value is reached. In this example, the rate of change in electrical resistance ΔR25 when 10 A current is supplied to a NTC thermistor having a thickness of 0.65±0.05 mm was calculated to evaluate voltage resistance.
In Table 2, “ΔR25 after application of inrush current” (%) is calculated by the equation below where R025 (Ω) is the electrical resistance at a temperature of 25° C. before supplying the inrush current, and R125 (Ω) is the electrical resistance at 25° C. after supplying 10 A inrush current:
ΔR25=(R125/R025−1)×100
In order to evaluate the reliability of the electrical resistance, the same type of NTC thermistor as above was used and the rate of change in electrical resistance ΔR25 after 100 cycles of heat test, each cycle including retaining at −55° C. for 30 minutes and at 125° C. for 30 minutes, was measured. The rate of change in electrical resistance ΔR25 is indicated as “reliability ΔR25” (%) in the table. The “reliability ΔR25” (%) is calculated by the following equation where R025 (Ω) is the electrical resistance at a temperature of 25° C. before the heat cycle test, and R225 (Ω) is the electrical resistance at 25° C. after the heat cycle test:
ΔR25=(R225/R025−1)×100
In the “judgment” column of Table 2, samples having “ΔR25 after application of inrush current” of 10% or less and “reliability ΔR25” of 20% or less are marked by circles while other samples are marked by X.
Vickers's hardness was measured with AKASHI MICRO HARDNESS TESTER (model MVK-E). In Table 2, Vickers's hardness Hv and fracture toughness KIc are indicated.
As shown in Table 2, it was confirmed that all samples that exhibited high voltage resistance, i.e., “ΔR25 after application of inrush current” of 10% or less, in evaluation of the voltage resistance had an atomic (manganese content)/(nickel content) ratio in the range of 87/13 or more and 96/4 or less.
These results indicate that when a NTC thermistor ceramic contains manganese and nickel and the (manganese content)/(nickel content) ratio is 87/13 or more and 96/4 or less, a structure is realized in which a high-resistance phase having a high resistance relative to a matrix is present in the matrix, and the hardness or the fracture toughness of the composition can be further enhanced. This not only moderates the electrical current concentration in the first phase and suppresses fracture caused by heat melting but also limits fracture caused by cracks. Thus, the voltage resistance of the NTC thermistor ceramic can be further improved. Moreover, it is shown that a NTC thermistor ceramic designed to contain 15 at % or less copper can realize a structure capable of improving the voltage resistance of the NTC thermistor ceramic.
Next, composition No. 116 was analyzed with a scanning ion microscope (SIM) and a scanning transmission electron microscope (STEM) to observe ceramic grains and conduct energy dispersive X-ray fluorescent spectrometry (EDX).
According to the results of energy dispersive X-ray fluorescent spectrometry, the first phase, i.e., the matrix, contained 68.8 to 75.5 at % manganese, 11.3 to 13.7 at % nickel, and 13.1 to 19.9 at % copper, and the second phase composed of plate crystals and having a high resistance contained 95.9 to 97.2 at % manganese, 0.6 to 1.2 at % nickel, and 2.1 to 3.0 at % copper. These results show that the manganese content in the second phase is higher than that in the first phase. Although this slightly depends on the contents of other additives, the results show that the second phase contains 1.2 times as much manganese as the first phase in terms of atomic percent.
The electrical resistance of the first and second phases was directly measured by analysis using a scanning probe microscope (SPM). As a result, it was found that the electrical resistance of the second phase was higher than that of the first phase and was at least 10 times larger than the electrical resistance of the first phase.
Manganese oxide (Mn3O4), nickel oxide (NiO), copper oxide (CuO), aluminum oxide (Al2O3), iron oxide (Fe2O3), cobalt oxide (CO3O4), and titanium oxide (TiO2) were weighed and blended so that the atomic ratios (atom %) of manganese (Mn), nickel (Ni), copper (Cu), aluminum (Al), iron (Fe), cobalt (Co), and titanium (Ti) after firing were adjusted to ratios shown in Table 3. Then green sheets were prepared as in EXAMPLE 1A.
The resulting green sheets were stacked, pressed, and fired as in EXAMPLE 1A to prepare a ceramic element body serving as the NTC thermistor ceramic of the present invention. Electrodes were formed on the ceramic main body as in EXAMPLE 1A to obtain a NTC thermistor.
The electrical characteristics, voltage resistance, and reliability of each sample of the single plate-type NTC thermistor were evaluated as in EXAMPLE 1B. The results are shown in Table 3.
As shown in Table 3, among all samples of NTC thermistors, composition Nos. 123 and 124 have an atomic (manganese content)/(nickel content) ratio of 85/15, which is less than 87/13, and thus the second phase having a high electrical resistance, i.e., plate crystals mainly composed of manganese oxide, was not observed. Composition Nos. 125 to 146 having an atomic ratio of 90/10 and composition No. 147 having an atomic ratio of 96/4 satisfy the range of 87/13 or more and 96/4 or less. When these samples contained 15 at % or less copper, and 10 at % or less aluminum, 10 at % or less iron, 15 at % or less cobalt, or 5 at % or less titanium, dispersion of plate-shaped manganese oxide crystals serving as the second phase having a high electrical resistance was confirmed in the first phase, i.e., the matrix having a low electrical resistance. Thus, not only the electrical current concentration in the first phase is moderated and fracture caused by heat melting is suppressed but also the hardness or fracture toughness of the NTC thermistor ceramic can be enhanced. Thus, fracture attributable to cracks can be suppressed, and the voltage resistance can be improved as a result.
Green sheets obtained in EXAMPLE 2A were punched out or cut into a particular size, and internal electrode pattern layers were formed on a predetermined number of sheets by a screen printing method. The electrode-forming paste used to form the internal electrode pattern layers could be a conductive paste mainly composed of a noble metal, such as silver, silver-palladium, gold, platinum, or the like, or a base metal, such as nickel. In this example, a silver-palladium conductive paste with a silver/palladium content ratio of 3/7 was used.
The green sheets with the internal electrode pattern layers formed thereon were stacked so that the internal electrode pattern layers were alternately exposed, and green sheets with no internal electrode pattern layers were provided as the outermost layers. The resulting green sheets were pressed to form a multilayer green sheet compact.
The compact was fired as in EXAMPLE 1A to form a ceramic element body which was the constitutional component of the NTC thermistor of the present invention.
Subsequently, the outer shape of the ceramic element body was finished by barrel polishing, and an external electrode-forming paste was applied on two side faces of the ceramic element body. The electrode-forming paste used could be a paste mainly composed of a noble metal, such as silver, silver-palladium, gold, platinum, or the like. In this example, a silver paste was used. The silver paste was applied and baked at 700° C. to 850° C. to form the external electrodes. Finally, nickel and tin were plated on the surfaces of the external electrodes to prepare a multilayer NTC thermistor.
In this example of the multilayer NTC thermistor shown in
The voltage resistance was evaluated by supplying inrush current to the multilayer NTC thermistor. The changes in electrical resistance after application of inrush current and the rate of change in electrical resistance ΔR25 were measured and calculated as in EXAMPLE 1B. From composition Nos. 126, 137, 139, and 145 in Table 3, multilayer NTC thermistors were prepared and inrush current was varied to measure changes in electrical resistance at that inrush current value and to calculate the rate of change in electrical resistance ΔR25. For comparative examination, multilayer NTC thermistors were prepared from composition Nos. 109 and 116 in Table 2, and the rate of change in electrical resistance ΔR25 at various inrush current values was calculated in the same fashion. The results are shown in Table 4.
Manganese oxide (Mn3O4), cobalt oxide (CO3O4), copper oxide (CuO), aluminum oxide (Al2O3), iron oxide (Fe2O3), and nickel oxide (NiO), were weighed and blended so that the atomic ratios (atom %) of manganese (Mn), cobalt (Co), copper (Cu), aluminum (Al), iron (Fe), and nickel (Ni) after firing were adjusted to ratios shown in Tables 4 and 5. Then green sheets were prepared as in EXAMPLE 1A.
The resulting green sheets were stacked, pressed, and fired as in EXAMPLE 1A to prepare a ceramic element body serving as the NTC thermistor ceramic of the present invention. Electrodes were formed on the ceramic main body as in EXAMPLE 1A to obtain a single plate-type NTC thermistor.
The electrical characteristics, voltage resistance, and reliability of each sample of the single plate-type NTC thermistor were evaluated as in Example 1B. The results are shown in Tables 4 and 5.
As shown in Tables 4 and 5, plate crystals mainly composed of manganese oxide and serving as the second phase having a high electrical resistance were not found in NTC thermistor samples prepared from composition Nos. 201 to 215 having an atomic (manganese content)/(cobalt content) ratio less than 60/40. For composition Nos. 216 to 266, when the atomic ratio is 60/40 or more and 90/10 or less, 22 at % or less copper is present, and 15 at % or less of aluminum, iron, or nickel is present, dispersion of plate crystals mainly composed of manganese oxide serving as the second phase having a high electrical resistance was observed in the first phase serving as the matrix having a low electrical resistance. Thus, not only the electrical current concentration on the first phase is moderated and fracture caused by heat melting is suppressed but also the hardness or fracture toughness of the NTC thermistor ceramic can be enhanced. Thus, fracture attributable to cracks can be suppressed, and voltage resistance can be improved as a result.
Green sheets obtained in EXAMPLE 3A were used to prepare a multilayer NTC thermistor shown in
The voltage resistance was evaluated by supplying inrush current to the multilayer NTC thermistor. The changes in electrical resistance after application of inrush current and the rate of change in electrical resistance ΔR25 were measured and calculated as in EXAMPLE 1B. From composition Nos. 210, 238, 242, 246, and 250 shown in Tables 4 and 5, multilayer NTC thermistors were prepared, and the inrush current value was varied to measure changes in electrical resistance at the inrush current value and to calculate the rate of change in electrical resistance ΔR25. The results are shown in
Manganese oxide (Mn3O4), nickel oxide (NiO), copper oxide (CuO), aluminum oxide (Al2O3), iron oxide, cobalt oxide (CO3O4), titanium oxide (TiO2), and zirconium oxide (ZrO2) were weighed and blended so that the atomic ratios (atom %) of manganese (Mn), nickel (Ni), copper (Cu), aluminum (Al), iron (Fe), cobalt (Co), titanium (Ti), and zirconium (Zr) after firing were adjusted to ratios shown in Table 7. Then green sheets were prepared as in EXAMPLE 1A.
The resulting green sheets were stacked, pressed, and fired as in EXAMPLE 1A to prepare a ceramic element body as the NTC thermistor ceramic of the present invention. Electrodes were formed on the ceramic main body as in EXAMPLE 1A to obtain a single plate-type NTC thermistor.
The electrical characteristics, voltage resistance, and reliability of each sample of the single plate-type NTC thermistor were evaluated as in Example 1B. The results are shown in Tables 6 and 7.
Tables 6 and 7 show that among all samples of NTC thermistors, composition Nos. 301 to 337, dispersion of plate crystals mainly composed of manganese oxide serving as the second phase having a high electrical resistance was observed in the first phase serving as the matrix having a high electrical resistance when the atomic (manganese content)/(nickel content) ratio was 87/13 or more and 96/4 or less, 15 at % or less copper was present, at least one of 10 at % or less aluminum, 10 at % or less iron, 15 at % or less cobalt, and 5 at % or less titanium was present, and 1.5 at % or less zirconium was contained. Thus, not only the electrical current concentration on the first phase is moderated and fracture caused by heat melting is suppressed but also the hardness or fracture toughness of the NTC thermistor ceramic can be enhanced. Thus, fracture attributable to cracks can be suppressed. Since segregation of zirconium oxide in the ceramic grain boundaries is observed, the hardness or fracture toughness of the NTC thermistor ceramic can be substantially retained at a high value, and thus the voltage resistance can be enhanced.
At a zirconium content exceeding 1.5 at %, e.g., 3 at %, the voltage resistance deteriorated. This is presumably because when a large amount of zirconium is present, the zirconium inhibits sinterability of the ceramic and increases the pore ratio in the ceramic element body.
Green sheets obtained in EXAMPLE 4A were used to prepare a multilayer NTC thermistor shown in
The voltage resistance was evaluated by supplying inrush current to the multilayer NTC thermistor. The changes in electrical resistance after application of inrush current and the rate of change in electrical resistance ΔR25 were measured and calculated as in EXAMPLE 1. From composition Nos. 306, 307, 310, 318, 319, 320, 323, 324, 325, 328, 329, 330, 333, 334, and 335 shown in Tables 6 and 7, multilayer NTC thermistors were prepared, and the inrush current value was varied to measure changes in electrical resistance at the inrush current value and to calculate the rate of change in electrical resistance ΔR25. The results are shown in
Similarly,
Similarly,
Likewise,
Similarly,
Manganese oxide (Mn3O4), nickel oxide (NiO), copper oxide (CuO), calcium carbonate (CaCO3), aluminum oxide (Al2O3), iron oxide (Fe2O3), cobalt oxide (CO3O4), and titanium oxide (TiO2) were weighed and blended so that the atomic ratios (atom %) of manganese (Mn), nickel (Ni), copper (Cu), calcium (Ca), aluminum (Al), iron (Fe), cobalt (Co), and titanium (Ti) after firing were adjusted to ratios shown in Tables 8 to 10. Then green sheets were prepared as in EXAMPLE 1A.
The resulting green sheets were stacked, pressed, and fired as in EXAMPLE 1A to prepare a ceramic element body as the NTC thermistor ceramic of the present invention. Electrodes were formed on the ceramic main body as in EXAMPLE 1A to obtain a single plate-type NTC thermistor.
The electrical characteristics, voltage resistance, and reliability of each sample of the single plate-type NTC thermistor were evaluated as in EXAMPLE 1. The results are shown in Tables 8 to 10.
As shown in Table 8, among all samples of NTC thermistors, for composition Nos. 401 to 440, when the atomic (manganese content)/(nickel content) ratio is 87/13 or more and 96/4 or less, 15 at % or less copper is present, and 10 at % or less (excluding 0 at %) calcium is further present, not only plate crystals mainly composed manganese oxide serving as the second phase having a high electrical resistance but also CaMn2O4 or CaMnO3 serving as a third phase having a high electrical resistance is dispersed in the first phase, i.e., the matrix having a low electrical resistance. Thus, the electrical current concentration on the first phase is moderated, fracture caused by heat melting is suppressed, and the voltage resistance can be improved further.
As shown in Tables 9 and 10, among all samples of NTC thermistors, for composition Nos. 441 to 482, when the atomic (manganese content)/(nickel content) ratio of 87/13 or more and 96/4 or less, 15 at % or less copper is present, and 10 at % or less aluminum, 10 at % or less iron, 15 at % or less cobalt, or 5 at % or less titanium is further present, and 10 at % or less (excluding 0 at %) calcium is yet further present, not only plate crystals mainly composed manganese oxide serving as the second phase having a high electrical resistance but also CaMn2O4 or CaMnO3 serving as a third phase having a high electrical resistance is dispersed in the first phase, i.e., a matrix having a low electrical resistance. Thus, the electrical current concentration on the first phase is moderated, fracture caused by heat melting is suppressed, and the hardness or fracture toughness of the NTC thermistor ceramic can be increased. Thus, fracture attributable to cracks can be suppressed, and the voltage resistance can be improved further.
Next, composition No. 421 was analyzed with a scanning ion microscope (SIM) and a scanning transmission electron microscope (STEM) to observe ceramic grains and conduct energy dispersive X-ray fluorescent spectrometry (EDX).
The electrical resistance of the first, second, and third phases was directly measured by analysis using a scanning probe microscope (SPM). As a result, it was found that the electrical resistance of the second phase and third phase was higher than that of the first phase, the electrical resistance of the second phase was at least 10 times larger than the electrical resistance of the first phase, and the electrical resistance of the third phase was at least 100 times larger than the electrical resistance of the first phase.
Green sheets obtained in EXAMPLE 5A were used to prepare a multilayer NTC thermistor shown in
The voltage resistance was evaluated by supplying inrush current to the multilayer NTC thermistor. The changes in electrical resistance after application of inrush current and the rate of change in electrical resistance ΔR25 were measured and calculated as in EXAMPLE 1B. From composition Nos. 420, 441, 442, 453, 454, 465, 466, 477, and 478 shown in Tables 8 and 10, multilayer NTC thermistors were prepared, and the inrush current value was varied to measure changes in electrical resistance at the inrush current value and to calculate the rate of change in electrical resistance ΔR25. The results are shown in
Similarly,
Likewise,
Similarly,
Manganese oxide (Mn3O4), nickel oxide (NiO), copper oxide (CuO), strontium carbonate (SrCO3), aluminum oxide (Al2O3), iron oxide (Fe2O3), cobalt oxide (CO3O4), and titanium oxide (TiO2) were weighed and blended so that the atomic ratios (atom %) of manganese (Mn), nickel (Ni), copper (Cu), strontium (Sr), aluminum (Al), iron (Fe), cobalt (Co), and titanium (Ti) after firing were adjusted to ratios shown in Tables 11 to 13. Then green sheets were prepared as in EXAMPLE 1A.
The resulting green sheets were stacked, pressed, and fired as in EXAMPLE 1A to prepare a ceramic element body as the NTC thermistor ceramic of the present invention. Electrodes were formed on the ceramic main body as in EXAMPLE 1A to obtain a single plate-type NTC thermistor.
The electrical characteristics, voltage resistance, and reliability of each sample of the single plate-type NTC thermistor were evaluated as in EXAMPLE 1B. The results are shown in Tables 11 to 13.
As shown in Table 11, among all samples of NTC thermistors, for composition Nos. 501 to 540, when the atomic (manganese content)/(nickel content) ratio is 87/13 or more and 96/4 or less, 15 at % or less copper is present, and 5 at % or less (excluding 0 at %) strontium is further present, not only plate crystals mainly composed of manganese oxide serving as the second phase having a high electrical resistance but also SrMnO3 that serves as a third phase having a high electrical resistance is dispersed in the first phase, i.e., the matrix showing a low electrical resistance. Thus, electrical current concentration on the first phase is moderated, fracture caused by heat melting is suppressed, and the voltage resistance can be enhanced.
As shown in Tables 12 and 13, among all samples of NTC thermistors, for composition Nos. 541 to 582, when the atomic (manganese content)/(nickel content) ratio is 87/13 or more and 96/4 or less, 15 at % or less copper is present, 10 at % or less aluminum, 10 at % or less iron, 15 at % or less cobalt, or 5 at % or less titanium is further present, and 5 at % or less (excluding 0 at %) strontium is yet further present, not only plate crystals mainly composed manganese oxide serving as the second phase having a high electrical resistance but also SrMnO3 serving as a third phase having a high electrical resistance is dispersed in the first phase, i.e., the matrix having a low electrical resistance. Thus, the electrical current concentration on the first phase is moderated, fracture caused by heat melting is suppressed, and the hardness or fracture toughness of the NTC thermistor ceramic can be improved. Thus, fracture attributable to cracks can be suppressed, and the voltage resistance can be further improved.
Green sheets obtained in EXAMPLE 6A were used to prepare a multilayer NTC thermistor shown in
The voltage resistance was evaluated by supplying inrush current to the multilayer NTC thermistor. The changes in electrical resistance after application of inrush current and the rate of change in electrical resistance ΔR25 were measured and calculated as in EXAMPLE 1B. From composition Nos. 520, 541, 542, 553, 554, 565, 566, 577, and 578 shown in Tables 11 and 13, multilayer NTC thermistors were prepared, and the inrush current value was varied to measure changes in electrical resistance at the inrush current value and to calculate the rate of change in electrical resistance ΔR25. The results are shown in
Similarly,
Likewise,
Similarly,
Manganese oxide (Mn3O4), cobalt oxide (CO3O4), copper oxide (CuO), aluminum oxide (Al2O3), iron oxide (Fe2O3), nickel oxide (NiO), and zirconium oxide (ZrO2) were weighed and blended so that the atomic ratios (atom %) of manganese (Mn), cobalt (Co), copper (Cu), aluminum (Al), iron (Fe), nickel (Ni), and zirconium (Zr) after firing were adjusted to ratios shown in Table 14. Then green sheets were prepared as in EXAMPLE 1A.
The resulting green sheets were stacked, pressed, and fired as in EXAMPLE 1A to prepare a ceramic element body as the NTC thermistor ceramic of the present invention. Electrodes were formed on the ceramic main body as in EXAMPLE 1A to obtain a single plate-type NTC thermistor.
The electrical characteristics, voltage resistance, and reliability of each sample of the single plate-type NTC thermistor were evaluated as in EXAMPLE 1B. The results are shown in Table 14.
As shown in Table 14, among all samples of NTC thermistors, for composition Nos. 601 to 637 and 639 to 643, when the atomic (manganese content)/(cobalt content) ratio is 60/40 or more and 90/10 or less, 17 at % or less copper is present, at least one of 15 at % or less aluminum, 15 at % or less iron, and 15 at % or less nickel is further present, and 1.5 at % or less (excluding 0%) zirconium is yet also present, plate crystals mainly composed of manganese oxide serving as the second phase having a high electrical resistance is dispersed in the first phase, i.e., the matrix showing a low electrical resistance. Thus, not only electrical current concentration on the first phase is moderated and fracture caused by heat melting is suppressed, but also the hardness or fracture toughness of the NTC thermistor ceramic can be enhanced. Thus, fracture attributable to cracks can be suppressed. Since segregation of zirconium oxide in the ceramic grain boundaries is observed, the hardness or fracture toughness of the NTC thermistor ceramic can be substantially retained at a high value, and thus the voltage resistance can be improved.
Green sheets obtained in EXAMPLE 7A were used to prepare a multilayer NTC thermistor shown in
The voltage resistance was evaluated by supplying inrush current to the multilayer NTC thermistor. The changes in electrical resistance after application of inrush current and the rate of change in electrical resistance ΔR25 were measured and calculated as in EXAMPLE 1B. From composition Nos. 613 and 616 shown in Table 14, multilayer NTC thermistors were prepared, and the inrush current value was varied to measure changes in electrical resistance at the inrush current value and to calculate the rate of change in electrical resistance ΔR25. The results are shown in
Manganese oxide (Mn3O4), cobalt oxide (CO3O4), copper oxide (CuO), calcium carbonate (CaCO3), aluminum oxide (Al2O3), iron oxide (Fe2O3), and nickel oxide (NiO) were weighed and blended so that the atomic ratios (atom %) of manganese (Mn), cobalt (Co), copper (Cu), calcium (Ca), aluminum (Al), iron (Fe), and nickel (Ni) after firing were adjusted to ratios shown in Tables 15 to 17. Then green sheets were prepared as in EXAMPLE 1A.
The resulting green sheets were stacked, pressed, and fired as in EXAMPLE 1A to prepare a ceramic element body serving as the NTC thermistor ceramic of the present invention. Electrodes were formed on the ceramic main body as in EXAMPLE 1A to obtain a single plate-type NTC thermistor.
The electrical characteristics, voltage resistance, and reliability of each sample of the single plate-type NTC thermistor were evaluated as in EXAMPLE 1B. The results are shown in Tables 15 to 17.
As shown in Tables 15 to 17, among all samples of NTC thermistors, for composition Nos. 701 to 703, 705 to 723, to 735, 737 to 749, 751 to 753, and 755 to 766, when the atomic (manganese content)/(cobalt content) ratio is 60/40 or more and 90/10 or less, 17 at % or less copper is present, at least one of 15 at % or less aluminum, 15 at % or less iron, and 15 at % or less nickel is further present, and 5 at % or less (excluding 0%) calcium is also present, not only plate crystals mainly composed of manganese oxide serving as the second phase having a high electrical resistance but also CaMn2O4 or CaMnO3 serving as a third phase having a high electrical resistance is dispersed in the first phase, i.e., the matrix having a low electrical resistance. Thus, the electrical current concentration on the first phase is moderated, fracture caused by heat melting is suppressed, and the voltage resistance can be improved further.
Green sheets obtained in EXAMPLE 8A were used to prepare a multilayer NTC thermistor shown in
The voltage resistance was evaluated by supplying inrush current to the multilayer NTC thermistor. The changes in electrical resistance after application of inrush current and the rate of change in electrical resistance ΔR25 were measured and calculated as in EXAMPLE 1B. From composition Nos. 716, 717, 718, and 719 shown in Table 16, multilayer NTC thermistors were prepared, and the inrush current value was varied to measure changes in electrical resistance at the inrush current value and to calculate the rate of change in electrical resistance ΔR25. The results are shown in
Manganese oxide (Mn3O4), cobalt oxide (CO3O4), copper oxide (CuO), strontium carbonate (SrCO3), aluminum oxide (Al2O3), iron oxide (Fe2O3), and nickel oxide (NiO) were weighed and blended so that the atomic ratios (atom %) of manganese (Mn), cobalt (Co), copper (Cu), strontium (Sr), aluminum (Al), iron (Fe), and nickel (Ni) after firing were adjusted to ratios shown in Table 18. Then green sheets were prepared as in EXAMPLE 1A.
The resulting green sheets were stacked, pressed, and fired as in EXAMPLE 1A to prepare a ceramic element body as the NTC thermistor ceramic of the present invention. Electrodes were formed on the ceramic main body as in EXAMPLE 1A to obtain a single plate-type NTC thermistor.
The electrical characteristics, voltage resistance, and reliability of each sample of the single plate-type NTC thermistor were evaluated as in EXAMPLE 1B. The results are shown in Table 18.
As shown in Table 18, among all samples of NTC thermistors, for composition Nos. 801 to 803, 805 to 809, 811, 812, 814, 816 to 819, 821 to 824, 826, 827, 829, 830, 832, 833, 835 to 838, 840, 841, and 843 to 845, when the atomic (manganese content)/(cobalt content) ratio is 60/40 or more and 90/10 or less, 22 at % or less copper is present, at least one of 15 at % or less aluminum, 15 at % or less iron, and 15 at % or less nickel is further present, and 5 at % or less (excluding 0%) strontium is also present, not only plate crystals mainly composed of manganese oxide serving as the second phase having a high electrical resistance but also SrMnO3 serving as a third phase having a high electrical resistance is dispersed in the first phase, i.e., the matrix having a low electrical resistance. Thus, the electrical current concentration on the first phase is moderated, fracture caused by heat melting is suppressed, and the voltage resistance can be improved further.
Green sheets obtained in EXAMPLE 9A were used to prepare a multilayer NTC thermistor shown in
The voltage resistance was evaluated by supplying inrush current to the multilayer NTC thermistor. The changes in electrical resistance after application of inrush current and the rate of change in electrical resistance ΔR25 were measured and calculated as in EXAMPLE 1B. From composition Nos. 817 and 819 shown in Table 18, multilayer NTC thermistors were prepared, and the inrush current value was varied to measure changes in electrical resistance at the inrush current value and to calculate the rate of change in electrical resistance ΔR25. The results are shown in
The embodiments and examples disclosed herein are merely examples and should not be construed as limiting in all aspects. The scope of the present invention is solely defined by the claims and not by the embodiments and examples described above, and includes equivalents to the terms of the claims and all modifications and alterations within the scope of the claims.
This invention is applicable to a NTC thermistor ceramic suitable for use in a NTC thermistor for limiting inrush current that occurs when a power switched is turned ON-OFF, and to a NTC thermistor. The invention can improve the voltage resistance of the NTC thermistor ceramic and provide an inrush current-limiting NTC thermistor including the NTC thermistor ceramic and having high voltage resistance.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-266976 | Sep 2006 | JP | national |
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| 63-315548 | Dec 1988 | JP |
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| Number | Date | Country | |
|---|---|---|---|
| 20090179732 A1 | Jul 2009 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2007/068136 | Sep 2007 | US |
| Child | 12414287 | US |
