Oxide thermistor and method for manufacturing the same

Abstract

To provide an oxide thermistor which has a smaller change in resistance in a high temperature environment, and a method for manufacturing the same.SOLUTION: Provided is an oxide thermistor having Mn and Co as main ingredients, with Cu and M furthermore added thereto. The crystal structure of the oxide thermistor includes a cubic spinel phase 2 and a NaCl-type crystal phase 3, and the M represents at least one element selected from among Mg, Cr, Fe, Ni, Zn, Al and Ga. A method for manufacturing the oxide thermistor includes: a calcinating step for calcining a mixture in which Mn, Co, Cu, and M are mixed (where the M represents at least one element selected from among Mg, Cr, Fe, Ni, Zn, Al and Ga.); a molding step for molding the calcined mixture into a compact after the calcinating step; and a firing step for firing the compact and forming a sintered body, the firing step involving firing the compact at a temperature equal to or higher than the temperature at which the NaCl-type crystal phase precipitates, until the NaCl-type crystal phase precipitates.SELECTED DRAWING: Figure 1

Description

Technical Field

[0005] , , , ,

[0001] The present invention relates to an oxide thermistor with little change in resistance value even in a high-temperature environment and a method for manufacturing the same.

Background Art

[0002] Thermistors are widely used, such as in temperature sensors and protection circuits for electronic devices, because their resistance value changes with temperature and this change is very sensitive to temperature. In recent years, the operating temperatures of power devices, motors, etc. in in-vehicle devices for EVs have been increasing, and high-temperature performance is also required for thermistors as electronic components that control these electronic devices.

[0003] The high-temperature performance required for thermistor elements is that they can be measured with high precision from low to high temperatures, that is, they have a low temperature coefficient (B constant), and it is necessary to suppress characteristic fluctuations in a high-temperature environment. The thermistor materials used for these are spinel materials with Mn-Co as the main component and Cu added, and usually a cubic single-phase with excellent manufacturing stability in crystal structure is used, but problems of various reliability degradations have become apparent. In particular, when used for a long time in a high-temperature environment, a large change rate of resistance value over time becomes a problem.

[0004] Therefore, conventionally, for example, in Patent Document 1, a material with Mn-Co as the main component and Cu and Ti added has been proposed for the purpose of suppressing resistance value change at high temperatures. In addition, it is disclosed that by coexisting a cubic spinel phase and a NaCl-type (rock salt phase type) crystal phase (at least one or more phases) rather than a cubic single-phase in crystal structure, heat-resistant reliability is improved.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

[0006] The above conventional technologies still have the following challenges. In the above-mentioned Patent Document 1, simply having a cubic spinel phase and an NaCl-type crystalline phase coexist was insufficient because the resistance and B constant fluctuated as the NaCl-type crystalline phase precipitated. Therefore, with Ti, where the change in the B constant is small, it was difficult to accurately adjust the thermistor characteristics to the desired range while simultaneously achieving high reliability.

[0007] This invention has been made in view of the aforementioned problems, and aims to provide an oxide thermistor that exhibits less change in resistance under high-temperature conditions, and a method for manufacturing the same. [Means for solving the problem]

[0008] The inventors focused on oxide thermistors mainly composed of Mn and Co and diligently conducted research. They elucidated that the change in resistance under high-temperature conditions is caused by the absorption and desorption of oxygen. They discovered that by precipitating NaCl-type crystalline phase grains and balancing the oxidation-reduction between these NaCl-type crystalline phase grains and the cubic spinel phase, the resistance can be stabilized even under high-temperature conditions. In particular, they found that by adding specific elements other than Ti, a wide range of property adjustments can be made, and the change in resistance under high-temperature conditions can be stably suppressed. Therefore, the present invention is derived from the above findings, and the following configuration is adopted to solve the aforementioned problems.

[0009] In other words, the oxide thermistor according to the first invention is an oxide thermistor mainly composed of Mn and Co, with Cu and M added, wherein its crystal structure includes a cubic spinel phase and an NaCl-type crystal phase, and the M is at least one of Mg, Cr, Fe, Ni, Zn, Al, and Ga.

[0010] This oxide thermistor is mainly composed of Mn and Co, with Cu and M added, and its crystal structure includes a cubic spinel phase and an NaCl-type crystal phase, and since M is at least one of Mg, Cr, Fe, Ni, Zn, Al, and Ga, the properties can be adjusted over a wide range by adding elements other than Ti (at least one of Mg, Cr, Fe, Ni, Zn, Al, and Ga), and changes in resistance under high-temperature conditions can be stably suppressed. In this invention, "mainly composed of Mn and Co" means that the total number of Mn and Co atoms in the total metal elements of the oxide thermistor is 50 atomic percent or more.

[0011] In this invention, as an additive element that is effective in adjusting the resistance value and the B constant, which fluctuate with the precipitation of the NaCl-type crystalline phase, and that can achieve both resistance stability in high-temperature environments, we have adopted an element that readily dissolves in Mn-Co crystals, has the most stable valency of 2 or 3, and has an ionic radius (Shannon) of 0.5 to 0.7 angstroms when in 4-coordinate or 6-coordinate states, i.e., the element M (at least one of Mg, Cr, Fe, Ni, Zn, Al, and Ga). Furthermore, the changes in thermistor characteristics associated with the addition of each element M are as follows: for example, with Ni, there is no significant change in resistivity and the B constant decreases; with Fe, the resistance increases and the B constant decreases; and with Zn, Al, and Ga, both resistivity and the B constant increase.

[0012] Furthermore, it is more preferable that the crystal structure shows a precipitated cuprite-type crystalline phase. In other words, by including a cuprite-type crystalline phase containing monovalent Cu that is in contact with or embedded within the NaCl-type crystalline phase, the absorption and desorption (oxidation and reduction) of oxygen between the cuprite-type crystalline phase, the NaCl-type crystalline phase, and the cubic spinel phase, which have different valencies, is balanced, resulting in even greater resistance stability even in high-temperature environments.

[0013] The oxide thermistor according to the second invention is characterized in that, in the first invention, the amount of M added is 0.1 to 20 at.% when the total of Mn, Co, Cu, and M is 100 at.%. In other words, in this oxide thermistor, the amount of M added is 0.1 to 20 at.% when the total amount of Mn, Co, Cu, and M is 100 at.%. Therefore, the thermistor characteristics can be adjusted and sufficient resistance change suppression can be achieved while maintaining the cubic spinel crystal structure of the main phase. Furthermore, if the amount of M added is less than 0.1 at.% when the total amount of Mn, Co, Cu, and M is 100 at.%, the effect of adjusting the properties will be too small. Conversely, if it exceeds 20 at.%, the M will no longer dissolve, or the cubic spinel, which is the main phase, will undergo a phase transition to a tetragonal phase. Furthermore, the amount of M added may be 0.5 at.% to 15 at.% or 2.5 at.% to 10 at.% when the total amount of Mn, Co, Cu, and M is 100 at.%.

[0014] A method for manufacturing an oxide thermistor according to the third invention is a method for manufacturing an oxide thermistor according to the first or second invention, comprising: a calcination step of preparing a calcined mixture by calcining a mixture of Mn, Co, Cu, and M (wherein M represents at least one of Mg, Cr, Fe, Ni, Zn, Al, and Ga); a molding step of forming the calcined mixture into a molded body after the calcination step; and a firing step of firing the molded body to form a sintered body, wherein the firing step is characterized in that the molded body is fired at a temperature above the temperature at which the NaCl-type crystalline phase precipitates until the NaCl-type crystalline phase precipitates.

[0015] In other words, in this method for manufacturing oxide thermistors, the firing process involves firing the molded body at a temperature above the temperature at which the NaCl-type crystalline phase precipitates, until the NaCl-type crystalline phase precipitates. Therefore, the NaCl-type crystalline phase precipitates during firing, making it possible to produce oxide thermistors that suppress changes in resistance under high-temperature conditions. Furthermore, it is more preferable to continue the firing process until a cuprite-type crystalline phase precipitates. Furthermore, it is preferable that M is solid-dissolved in the cubic spinel phase during the calcination process.

[0016] A method for manufacturing an oxide thermistor according to the fourth invention is a method for manufacturing an oxide thermistor according to the first or second invention, comprising: a calcination step of preparing a calcined mixture by calcining a mixture of Mn, Co, Cu, and M (wherein M represents at least one of Mg, Cr, Fe, Ni, Zn, Al, and Ga); a molding step of forming the calcined mixture into a molded body after the calcination step; a firing step of firing the molded body to form a sintered body; and an annealing step of annealing the sintered body, wherein the firing step fires the molded body at a temperature below which the NaCl-type crystalline phase precipitates, and the annealing step anneals at a temperature above which the NaCl-type crystalline phase precipitates to precipitate the NaCl-type crystalline phase in the sintered body.

[0017] In other words, this method for manufacturing oxide thermistors includes an annealing step in which the NaCl-type crystalline phase is precipitated in the sintered body at a temperature above the temperature at which the NaCl-type crystalline phase precipitates. Therefore, by precipitating NaCl-type crystalline phase grains during annealing, the thermistor can be adjusted to the target composition, and an oxide thermistor can be produced that suppresses changes in resistance under high-temperature conditions. Furthermore, in the annealing process described above, it is more preferable to anneal until the cuprite-type crystalline phase precipitates. Furthermore, it is preferable that M is solid-dissolved in the cubic spinel phase during the calcination or firing process. [Effects of the Invention]

[0018] The present invention provides the following effects. That is, according to the oxide thermistor of the present invention, it is an oxide thermistor mainly composed of Mn and Co, and further added with Cu and M, and its crystal structure includes a cubic spinel phase and a NaCl-type crystal phase. Since the M is at least one of Mg, Cr, Fe, Ni, Zn, Al, and Ga, by adding and solidifying the element M (at least one of Mg, Cr, Fe, Ni, Zn, Al, and Ga) other than Ti, it is possible to adjust the characteristics over a wide range and stably suppress the change in the resistance value in a high-temperature environment. Further, according to the manufacturing method of the oxide thermistor of the present invention, since it is fired or annealed at a temperature above the temperature at which the NaCl-type crystal phase precipitates until the NaCl-type crystal phase precipitates, the NaCl-type crystal phase precipitates, and the thermistor can be adjusted to the target composition, and an oxide thermistor that suppresses the change in the resistance value in a high-temperature environment can be manufactured. Therefore, the oxide thermistor of the present invention is suitable for a thermistor used in a temperature sensor or a protection circuit that controls a power device, a motor, etc. in in-vehicle equipment for EVs with a high operating temperature.

Brief Description of the Drawings

[0019] [Figure 1] It is a schematic enlarged cross-sectional view showing an embodiment in the oxide thermistor and its manufacturing method according to the present invention. [Figure 2] It is a backscattered electron image showing Example 1 according to the present invention. [Figure 3] It is a backscattered electron image showing Example 2 according to the present invention. [Figure 4] It is a backscattered electron image showing Example 6 according to the present invention. [Figure 5] It is a backscattered electron image showing Example 9 according to the present invention. [Figure 6] It is a backscattered electron image showing Comparative Example 1 according to the present invention. [Figure 7] It is a backscattered electron image showing Comparative Example 2 according to the present invention. [Figure 8] It is a backscattered electron image showing Comparative Example 3 according to the present invention. [Modes for carrying out the invention]

[0020] Hereinafter, an embodiment of the oxide thermistor and its manufacturing method according to the present invention will be described with reference to Figure 1.

[0021] The oxide thermistor 1 of this embodiment is an oxide thermistor mainly composed of Mn and Co, with Cu and M added and containing oxygen, and as shown in Figure 1, its crystal structure includes a cubic spinel phase 2 and an NaCl-type crystal phase 3, and the M is at least one of Mg, Cr, Fe, Ni, Zn, Al, and Ga. Furthermore, the amount of M added is preferably 0.1 to 20 at.% when the total amount of Mn, Co, Cu, and M is set to 100 at.%.

[0022] Furthermore, the composition ranges of Mn, Co, and Cu are preferably within the following ranges. Mn:20~45at.%,Co:35~65at.%,Cu:1~20at.% Furthermore, the composition ranges of Mn, Co, and Cu are more preferably within the following ranges. Mn:30~45at.%,Co:35~60at.%,Cu:1~15at.%

[0023] Furthermore, it is more preferable that the crystal structure shows the presence of a cuprite-type crystalline phase 4. In other words, by including a cuprite-type crystal phase 4 containing monovalent Cu in contact with the NaCl-type crystal phase 3, the absorption and desorption (oxidation and reduction) of oxygen between the cuprite-type crystal phase 4, the NaCl-type crystal phase 3, and the cubic spinel phase 2, which have different valencies, is balanced, resulting in even greater resistance stability even in high-temperature environments.

[0024] The method for manufacturing the oxide thermistor 1 of this embodiment includes a calcination step of preparing a calcined mixture by calcining a mixture of Mn, Co, Cu, and M (wherein M represents at least one of Mg, Cr, Fe, Ni, Zn, Al, and Ga), a molding step of forming the calcined mixture into a molded body after the calcination step, and a firing step of firing the molded body to form a sintered body. The above firing process involves firing the molded body at a temperature above the temperature at which NaCl-type crystalline phase 3 precipitates, until NaCl-type crystalline phase 3 precipitates.

[0025] In other words, in the firing process, it is preferable to set the sintering temperature to a temperature at or above the temperature at which grains of NaCl-type crystalline phase 3 precipitate. Furthermore, the temperature at which grains of the NaCl-type crystalline phase 3 precipitate decreases as the content of Co and Cu, which mainly constitute the cubic spinel phase 2, increases. Furthermore, the molding process described above preferably involves pressurizing the mixture to form a molded body.

[0026] Alternatively, a different manufacturing method may be employed, comprising a calcination step of preparing a calcined mixture by calcining a mixture of Mn, Co, Cu, and M (wherein M represents at least one of Mg, Cr, Fe, Ni, Zn, Al, and Ga), a molding step of forming the calcined mixture into a molded body after the calcination step, a firing step of firing the molded body to form a dense sintered body, and an annealing step of annealing the sintered body, wherein the firing step fires the molded body at a temperature below the temperature at which the NaCl-type crystalline phase precipitates, and the annealing step anneals at a temperature above the temperature at which the NaCl-type crystalline phase precipitates to precipitate the NaCl-type crystalline phase in the sintered body. Regarding the annealing described above, controlling the atmosphere and finding that a lower oxygen concentration than in the atmosphere results in a greater precipitation effect.

[0027] In order to add the element M, during the calcination process, an oxide of M is added to the mixture as additive X. For example, if Mg is added, MgO is added; if Cr is added, Cr2O3 is added; if Fe is added, Fe2O3 is added; if Ni is added, NiO is added; if Zn is added, ZnO is added; if Al is added, Al2O3 is added; if Ga is added, Ga2O3 is added.

[0028] Thus, the oxide thermistor 1 of this embodiment is an oxide thermistor mainly composed of Mn and Co, with Cu and M added, and its crystal structure includes a cubic spinel phase 2 and an NaCl-type crystal phase 3, and since M is at least one of Mg, Cr, Fe, Ni, Zn, Al, and Ga, the properties can be adjusted over a wide range by adding the element M other than Ti (at least one of Mg, Cr, Fe, Ni, Zn, Al, and Ga), and changes in resistance under high-temperature conditions can be stably suppressed.

[0029] In other words, to effectively adjust the resistance value and the B constant, which fluctuate with the precipitation of NaCl-type crystalline phase 3, and to achieve both resistance stability under high-temperature conditions, we have adopted an additive element that readily dissolves in Mn-Co crystals, has the most stable valency of 2 or 3, and has an ionic radius (Shannon) of 0.5 to 0.7 angstroms when in 4-coordinate or 6-coordinate states, i.e., the element M (at least one of Mg, Cr, Fe, Ni, Zn, Al, and Ga).

[0030] Furthermore, since the amount of M added is 0.1 to 20 at.% when the total of Mn, Co, Cu, and M is 100 at.%, a sufficient effect of suppressing changes in resistance can be obtained, and a significant increase in resistance due to the addition can be suppressed. Furthermore, the changes in thermistor characteristics associated with the addition of each element M are as follows: for example, with Ni, there is no significant change in resistivity and the B constant decreases; with Fe, the resistance increases and the B constant decreases; and with Zn, Al, and Ga, both resistivity and the B constant increase.

[0031] In the manufacturing method of the oxide thermistor of this embodiment described above, the firing process involves firing the molded body at a temperature above the temperature at which the NaCl-type crystalline phase 3 precipitates, until the NaCl-type crystalline phase 3 precipitates. As a result, the NaCl-type crystalline phase 3 precipitates during firing, making it possible to produce an oxide thermistor 1 that suppresses changes in resistance under high-temperature conditions.

[0032] Furthermore, as another firing process, the process includes an annealing step in which the NaCl-type crystalline phase 3 is precipitated at a temperature above the temperature at which it precipitates after sintering, thereby precipitating the NaCl-type crystalline phase 3 in the sintered body. During annealing, grains of NaCl-type crystalline phase 3 are precipitated, allowing the thermistor to be adjusted to the target composition and producing an oxide thermistor that suppresses changes in resistance under high-temperature conditions. [Examples]

[0033] Based on the above embodiments, examples of the oxide thermistor of the present invention were fabricated under multiple conditions, their crystal structures were investigated, and the rate of change in resistance was measured. "Example 1" Example 1 of the oxide thermistor of the present invention was fabricated as follows. First, commercially available manganese carbonate, cobalt carbonate, copper oxide, and aluminum oxide (Al2O3: i.e., M is Al) as additive X were weighed out so that the metal atom ratio was Mn:Co:Cu:Al = 38:54.5:5:2.5, mixed in a ball mill for 16 hours, and then dehydrated and dried. Next, this mixture was calcined at 850°C for 2 hours, mixed again in a ball mill, and dehydrated and dried. 1% by weight of polyvinyl alcohol was added to the calcined raw material, and a cylindrical block (molded body) measuring 50 mm in diameter and 30 mm in size was produced by pressure molding using a mold.

[0034] This block was fired at 1125°C for 10 hours to obtain a sintered body. A wafer was cut from this sintered body by wire cutting and polished to a thickness of 0.4 mm. A commercially available glass frit-containing Ag paste was printed onto this wafer, baked onto it, and then cut into 0.2 mm × 0.2 mm × 0.4 mm pieces using a diamond blade to obtain thermistor chips for evaluation.

[0035] After measuring the resistance of the fabricated evaluation thermistor chip at 25°C, a heat resistance test was conducted at 125°C for 1000 hours, and the resistance was measured again after the test. Then, the rate of change "ΔR25 = (((Resistance after test - Initial resistance) / Initial resistance) - 1) × 100%" was calculated from the resistance values ​​before and after the test, and this was defined as the resistance change rate. As a result, the average value of the 20 thermistor chips used for evaluation in Example 1 was 0.3%, as shown in Table 1.

[0036] [Table 1]

[0037] Next, after embedding the evaluation thermistor chip from Example 1 in resin, a cross-section was prepared by mechanical polishing, and the backscattered electron image (BSE-COMPO image) observed with a scanning electron microscope (S-3400N) manufactured by Hitachi High-Technologies Corporation is shown in Figure 2. The presence or absence of the NaCl-type crystalline phase was determined by this cross-sectional SEM observation. In Figure 2, the brightest areas are the grains of the cuprite-type crystalline phase, the light gray areas are the grains of the NaCl-type crystalline phase, the dark gray areas are the cubic spinel phase, and the black areas are voids (pores).

[0038] Examples 2-8 Examples 2 to 8 of the present invention were prepared in the same manner as Example 1, according to the conditions in Table 1, and the results of evaluation are shown in Table 1. The crystal structures of Examples 2 and 6 are shown in Figures 3 and 4.

[0039] "Example 9" First, commercially available manganese carbonate, cobalt carbonate, copper oxide, and zinc oxide were used as starting materials. The metal atoms were weighed so that the ratio of Mn:Co:Cu:Zn was Mn:45:5:10, and the mixture was wet-mixed in a ball mill for 16 hours, followed by dehydration and drying. Next, this mixture was calcined at 850°C for 2 hours, then wet-ground in a ball mill, mixed, and dehydrated and dried. 1% by weight of polyvinyl alcohol was added to the calcined raw material, and a cylindrical block (molded body) measuring 50 mm in diameter and 30 mm in size was produced by pressure molding using a mold.

[0040] This block was fired at 1000°C for 5 hours to obtain a sintered body. From this sintered body, wafers with a thickness of approximately 0.5 mm were cut by wire cutting. These wafers were further annealed in air at 1100°C for 5 hours, and then polished on both sides to adjust the thickness to 0.4 mm. Subsequently, the thermistor chip for evaluation in Example 9 was fabricated in the same manner as in Example 1, and evaluation was performed. The evaluation results for this Example 9 are shown in Table 1, and the crystal structure after annealing is shown in Figure 5.

[0041] "Comparative Examples 1-3" For comparison, Comparative Examples 1 to 3 of the present invention were prepared in the same manner as Example 1, according to the conditions in Table 1, and evaluated. The results are shown in Table 1, and the crystal structures are shown in Figures 6 to 8. Comparative Example 1, on the other hand, does not contain element M, and the sintering temperature is set lower than the precipitation temperature of the NaCl-type crystalline phase. In addition, Comparative Example 2 uses Fe2O3 (i.e., M is Fe) as additive X, and Comparative Example 3 uses Al2O3 (i.e., M is Al) as additive X, but the sintering temperature is set lower than the precipitation temperature of the NaCl-type crystalline phase.

[0042] As can be seen from these results, in Comparative Examples 1-3, the crystal structure is simply due to the low sintering temperature causing voids (pores) to form in the cubic spinel phase. In contrast to these, the crystal structures of the embodiments of the present invention all show the formation of an NaCl-type crystal phase within a cubic spinel phase. Furthermore, while the resistance change rate for Comparative Examples 1-3 was large at 2.1% or more, the examples of the present invention, due to the addition of element M, all showed a small change of 0.9% or less, and stable resistance values ​​were obtained with little change even after the heat resistance test.

[0043] It should be noted that the technical scope of the present invention is not limited to the embodiments and examples described above, and various modifications can be made without departing from the spirit of the invention. [Explanation of Symbols]

[0044] 1…Oxide thermistor, 2…Cubic spinel phase, 3…NaCl-type crystalline phase

Claims

1. An oxide thermistor having Mn and Co as the main components, with Cu and M added, Its crystal structure is a cubic spinel phase, It includes a NaCl-type crystalline phase, An oxide thermistor characterized in that M is at least one of Mg, Zn, and Ga.

2. In the oxide thermistor according to claim 1, An oxide thermistor characterized in that the amount of M added is 0.1 to 20 at.%, when the total amount of Mn, Co, Cu, and M is 100 at.%.

3. A method for producing an oxide thermistor according to claim 1 or 2, A calcination step in which a mixture of Mn, Co, Cu, and M (wherein M represents at least one of Mg, Zn, and Ga) is calcined to produce a calcined mixture, A molding step in which the calcined mixture is formed into a molded body after the calcination step, The process includes a firing step of firing the molded body to form a sintered body, A method for producing an oxide thermistor, characterized in that the firing step involves firing the molded body at a temperature above the temperature at which the NaCl-type crystalline phase precipitates, until the NaCl-type crystalline phase precipitates.

4. A method for producing an oxide thermistor according to claim 3, A calcination step in which a mixture of Mn, Co, Cu, and M (wherein M represents at least one of Mg, Zn, and Ga) is calcined to produce a calcined mixture, A molding step in which the calcined mixture is formed into a molded body after the calcination step, A firing step in which the molded body is fired to form a sintered body, The process includes an annealing step for annealing the sintered body, The aforementioned firing step involves firing the molded body at a temperature below the temperature at which the NaCl-type crystalline phase precipitates. A method for manufacturing an oxide thermistor, characterized in that the annealing step is performed at a temperature above the temperature at which the NaCl-type crystalline phase precipitates, thereby precipitating the NaCl-type crystalline phase in the sintered body.

5. A method for producing an oxide thermistor having Mn and Co as the main components, with Cu and M added, wherein the crystal structure comprises a cubic spinel phase and an NaCl-type crystal phase, and the M is at least one of Mg, Cr, Fe, Ni, Zn, Al, and Ga, A calcination step is performed to prepare a calcined mixture by calcining a mixture of Mn, Co, Cu, and M (wherein M represents at least one of Mg, Cr, Fe, Ni, Zn, Al, and Ga), A molding step in which the calcined mixture is formed into a molded body after the calcination step, The process includes a firing step of firing the molded body to form a sintered body, The firing process involves firing the molded body at a firing temperature above the temperature at which the NaCl-type crystalline phase precipitates, for 5 to 10 hours until the NaCl-type crystalline phase precipitates. A method for producing an oxide thermistor, characterized in that the firing temperature is less than 1100°C.

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