Thermistor element and method for producing same

Abstract

Provided are a thermistor element including a conductive intermediate layer containing RuO.sub.2 which can have a lower resistance and a thinner profile, whereby the increase in resistance can be suppressed even when peeling of the electrode proceeds; and a method for producing the same. The thermistor element according to the present invention includes: a thermistor body 2 made of a thermistor material; a conductive intermediate layer 4 formed on the thermistor body; and an electrode layer 5 formed on the conductive intermediate layer, wherein the conductive intermediate layer has an aggregation structure of RuO.sub.2 particles that are in electrical contact with each other where SiO.sub.2 is placed in the gaps in the aggregation structure, and has a thickness of 100 to 1000 nm.

Claims

1. A thermistor element comprising: a thermistor body made of a thermistor material; a conductive intermediate layer formed on the thermistor body; and an electrode layer formed on the conductive intermediate layer, wherein the conductive intermediate layer has an aggregation structure of RuO.sub.2 particles that are in electrical contact with each other where SiO.sub.2 is placed in the gaps in the aggregation structure, and has a thickness of 100 to 1000 nm.

2. The thermistor element according to claim 1, wherein the rate of change in resistance at 25° C. is less than 2.5% before and after repeating a heat cycle test 50 times with one cycle consisting of a test conducted at −55° C. for 30 minutes and one at 200° C. for 30 minutes.

3. A method for producing a thermistor element comprising: an intermediate layer forming step for forming a conductive intermediate layer on a thermistor body made of a thermistor material; and an electrode forming step for forming an electrode layer on the conductive intermediate layer, wherein the intermediate layer forming step includes: applying a RuO.sub.2 dispersion containing RuO.sub.2 particles and an organic solvent on the thermistor body and drying it to form a RuO.sub.2 layer, and applying a silica sol-gel solution containing SiO.sub.2, an organic solvent, water, and an acid on the RuO.sub.2 layer and drying it with the silica sol-gel solution being penetrated into the RuO.sub.2 layer to form the conductive intermediate layer.

4. The method for producing a thermistor element according to claim 3, wherein the electrode forming step comprises: applying a noble metal paste containing a noble metal on the conductive intermediate layer; and heating the applied noble metal paste for baking to form the electrode layer of the noble metal.

5. The method for producing a thermistor element according to claim 3, wherein the thickness of the RuO.sub.2 layer is 100 to 1000 nm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1(a), FIG. 1(b) and FIG. 1(c) show cross-sectional views of a thermistor element in the order of steps according to one embodiment of a thermistor element and a method for producing the same of the present invention.

(2) FIG. 2 is a cross-sectional view of the thermistor element according to the present embodiment.

(3) FIG. 3 is a schematic enlarged cross-sectional view of the thermistor element according to the present embodiment.

(4) FIG. 4 is a SEM photograph of a cross section of a thermistor element produced according to the Example of a thermistor element and a method for producing the same of the present invention.

(5) FIG. 5 is a SEM photograph of a cross section of the thermistor element produced according the Example of the present invention before forming an electrode layer.

(6) FIG. 6 is a SEM photograph of the surface of the conductive intermediate layer produced according the Example of the present invention before forming an electrode layer.

(7) FIG. 7 is a graph showing heat cycle test results regarding the change in resistance (Δ25) with respect to the number of a heat cycle for the thermistor element according to the Example of the present invention.

DESCRIPTION OF THE EMBODIMENTS

(8) Hereinafter, a thermistor element and a method for producing the same according to one embodiment of the present invention will be described with reference to FIGS. 1 to 3. In the drawings used in the following description, the scale of each component is changed as appropriate so that each component is recognizable or is readily recognized.

(9) As shown in FIGS. 1 to 3, a thermistor element 1 according to the present embodiment includes a thermistor body 2 made of a thermistor material, a conductive intermediate layer 4 formed on the thermistor body 2, and an electrode layer 5 formed on the conductive intermediate layer 4.

(10) The conductive intermediate layer 4 has an aggregation structure of RuO.sub.2 particles 3a that are in electrical contact with each other where SiO.sub.2 is placed in the gaps in the aggregation structure, and has a thickness of 100 to 1000 nm. Specifically, the aggregation structure described above is constituted by the RuO.sub.2 particles that are in contact and electrical conduction with each other where SiO.sub.2 is placed in the gaps partially created in the aggregation structure.

(11) This thermistor element 1 exhibits a rate of change in resistance at 25° C. of less than 2.5% before and after repeating a heat cycle test 50 times with one cycle consisting of a test conducted at −55° C. for 30 minutes and one at 200° C. for 30 minutes.

(12) As shown in FIG. 1(a), FIG. 1(b) and FIG. 1(c), a method of producing the thermistor element 1 according to the present embodiment includes an intermediate layer forming step for forming the conductive intermediate layer 4 on the thermistor body 2 made of a thermistor material and an electrode forming step for forming the electrode layer 5 on the conductive intermediate layer 4.

(13) The intermediate layer forming step described above includes applying a RuO.sub.2 dispersion containing the RuO.sub.2 particles 3a and an organic solvent on the thermistor body 2 and drying it to form a RuO.sub.2 layer 3 as shown in FIG. 1(a), and applying a silica sol-gel solution containing SiO.sub.2, an organic solvent, water, and an acid on the RuO.sub.2 layer 3 and drying it with the silica sol-gel solution being penetrated into the RuO.sub.2 layer 3 to form the conductive intermediate layer 4 as shown in FIG. 1(b).

(14) The electrode forming step described above includes applying a noble metal paste containing a noble metal on the conductive intermediate layer 4 and heating the applied noble metal paste for baking to form an electrode layer 5 of a noble metal as shown in FIG. 1(c).

(15) In addition, the thickness of the RuO.sub.2 layer 3 is 100 to 1000 nm.

(16) For the thermistor body 2, Mn—Co—Fe, Mn—Co—Fe—Al, Mn—Co—Fe—Cu, or the like may be employed for example. The thickness of this thermistor body 2 is, for example, 200 μm.

(17) The RuO.sub.2 dispersion described above is consisted of, for example, a RuO.sub.2 ink made up by mixing the RuO.sub.2 particles 3a, and acetylacetone and ethanol as organic solvents.

(18) The RuO.sub.2 particles 3a having an average particle size of 10 to 100 nm may be used, but the particles having an average particle size of about 50 nm is preferred.

(19) The organic solvent may contain a dispersant, which is preferably a polymer type having a plurality of adsorbing groups.

(20) The silica sol-gel solution described above is a mixture of, for example, SiO.sub.2, ethanol, water, and nitric acid. In addition, other organic solvents except ethanol as described above may be used as the organic solvent in this silica sol-gel solution. In addition, the acid used in the silica sol-gel solution may function as a catalyst for facilitating hydrolysis, and other acids may also be used except nitric acid as described above.

(21) The noble metal paste described above is, for example, an Au paste containing glass frit.

(22) In the intermediate layer forming step, since the RuO.sub.2 dispersion containing the RuO.sub.2 particles 3a and an organic solvent is applied on the thermistor body 2 and it is dried to form the RuO.sub.2 layer 3, the RuO.sub.2 layer 3 is formed with many of the RuO.sub.2 particles 3a that are in close contact with each other at this stage.

(23) Specifically, when the RuO.sub.2 dispersion containing the RuO.sub.2 particles 3a is applied on the thermistor body 2 by spin-coating or the like and it is dried, for example, at 150° C. for 10 minutes, the acetylacetone and ethanol contained in the RuO.sub.2 dispersion are evaporated to form the RuO.sub.2 layer 3 with the RuO.sub.2 particles 3a being in contact with each other. This RuO.sub.2 layer 3 has fine gaps created in the area without containing the RuO.sub.2 particles 3a that are in a close contact each other.

(24) Next, when the silica sol-gel solution containing SiO.sub.2, an organic solvent, water, and an acid is applied on the RuO.sub.2 layer 3 and it is dried with the silica sol-gel solution being penetrated into the RuO.sub.2 layer 3 to form the conductive intermediate layer 4, the conductive intermediate layer 4 can have an aggregation structure of the RuO.sub.2 particles 3a that are in close contact with each other where the silica sol-gel solution is penetrated into the gaps therein so that the SiO.sub.2 is placed in the gaps after dried. Since the silica sol-gel solution can be cured when dried so as to give a high purity of SiO.sub.2, it can provide strength to the conductive intermediate layer 4 and serve to make the thermistor body 2 firmly adhered to the conductive intermediate layer 4.

(25) Specifically, when a silica sol-gel solution is applied on the RuO.sub.2 layer 3 by spin-coating or the like, the silica sol-gel solution penetrates into the fine gaps between the RuO.sub.2 particles 3a in the RuO.sub.2 layer 3. Then, it is dried, for example, at 150° C. for 10 minutes, the ethanol, water, and nitric acid are evaporated to leave only SiO.sub.2 in the gaps. The resulting SiO.sub.2 can function as a binder for the RuO.sub.2 particles 3a. In this way, the conductive intermediate layer 4 is formed with SiO.sub.2 being placed in the fine gaps between the RuO.sub.2 particles 3a that are in contact with each other.

(26) Next, when a noble metal paste is applied on the conductive intermediate layer 4 and it is baked, for example, at 850° C. for 10 minutes, the heating can make the contact of the RuO.sub.2 particles 3a very closer with each other. In addition, the melted glass frit can penetrate into the gaps between the RuO.sub.2 particles 3a that cannot be completely filled with the silica sol-gel solution.

(27) Thus, the thermistor element 1 is produced in which the electrode layer 5 made of Au is formed on the conductive intermediate layer 4, as shown in FIGS. 2 and 4.

(28) As described above, in the thermistor element 1 according to the present embodiment, since the conductive intermediate layer 4 has an aggregation structure of the RuO.sub.2 particles 3a that are in electrical contact with each other where SiO.sub.2 is placed in the gaps in the aggregation structure, and has a thickness of 100 to 1000 nm, the aggregation structure of the RuO.sub.2 particles 3a that are in contact with each other can assure enough electrical conductivity, while the SiO.sub.2 that is placed in the gaps in the porous structure can serve as a binder for the aggregation structure. Therefore, the thin conductive intermediate layer 4 can have a low resistance even if it is thin, whereby the increase in resistance can be suppressed in a heat cycle test or the like even when peeling between the conductive intermediate layer 4 and the electrode layer 5 proceeds.

(29) Moreover, with the thermistor element 1 according to the present embodiment, since the rate of change in resistance at 25° C. is less than 2.5% before and after repeating the heat cycle test described above, a temperature measurement can be stably performed with high reliability even under the environment where the temperature is greatly changed.

(30) In addition, in the method for producing a thermistor element according to the present embodiment, the RuO.sub.2 layer 3 is formed using a RuO.sub.2 dispersion containing no glass frit so that the RuO.sub.2 particles 3a are in close contact with each other in advance and then SiO.sub.2 is placed in the gaps between the RuO.sub.2 particles 3a so as to function as a binder. This configuration assures more area where the RuO.sub.2 particles 3a are in contact with each other, and does not allow the melted glass frit to get into the contact surface of the RuO.sub.2 particles 3a and then inhibit their contact so as not to increase the resistance, and thus the resistance of the conductive intermediate layer 4 can be lowered. On the other hand, in a conventional intermediate layer that is made from a RuO.sub.2 paste containing glass frit, the glass frit may inhibit the RuO.sub.2 particles 3a from being in sufficiently close contact with each other.

(31) In addition, in the method for producing a thermistor element according to the present embodiment, since the RuO.sub.2 dispersion to be applied has a lower viscosity than that of a paste, the conductive intermediate layer 4 can be made thinner than the one produced using a paste. Moreover, since the RuO.sub.2 layer 3 with many of the RuO.sub.2 particles 3a that are in close contact with each other is formed directly on the thermistor body 2 in advance, the conductive intermediate layer 4 can have a low resistance, whereby the increase in resistance can be suppressed in a heat cycle test or the like even when peeling of the electrode proceeds.

(32) In addition, since the method according to the present embodiment includes applying a noble metal paste containing a noble metal on the conductive intermediate layer 4 and heating the applied noble metal paste for baking to form the electrode layer 5 of a noble metal, baking of the noble metal paste can make the contact of the RuO.sub.2 particles 3a very closer with each other. In addition, since the melted SiO.sub.2 can penetrate into the gaps between the RuO.sub.2 particles 3a that cannot be completely filled with a silica sol-gel solution, it can serve as a binder for firmly binding the RuO.sub.2 particles 3a to each other so as to make the conductive intermediate layer 4 stable.

(33) Moreover, since the thickness of the RuO.sub.2 layer 3 is 100 to 1000 nm, the conductive intermediate layer 4 can be made thinner but have a sufficient resistance. If the thickness of the RuO.sub.2 layer 3 is less than 100 nm, the adherence thereof to the thermistor body 2 may become insufficient. As long as the RuO.sub.2 layer 3 has a thickness of up to 1000 nm, a sufficiently low resistance and enough adherence can be attained, but in order to obtain the RuO.sub.2 layer 3 having a thickness of more than 1000 nm, the amount of the RuO.sub.2 particles 3a to be used can be increased more than necessary, leading to an increase in cost.

Example 1

(34) FIG. 4 is a SEM photograph of a cross section of the thermistor element 1 produced according to the embodiment described above, and FIGS. 5 and 6 are SEM photographs of the cross section of the thermistor element 1 and the surface of the conductive intermediate layer respectively before forming an electrode layer.

(35) As can be seen from these photographs, the conductive intermediate layer is formed with the RuO.sub.2 particles being in contact and close contact with each other.

(36) The thermistor element 1 produced according to the Example was a chip thermistor having a size of 1.0×1.0×0.2 mm in a chip shape, that is, a whole size of 1.0×1.0 mm in a planar view with a thickness of 0.2 mm.

(37) This thermistor element 1 was mounted on a gold-metallized AlN substrate by an Au—Sn foil soldering in a N.sub.2 flow at 325° C. Then, the AlN substrate having this thermistor element mounted thereon was fixed by an adhesive on a printed circuit board on which wiring pattern is formed, and Au wire bonding was done to this board so as to produce an evaluation circuit as a sample for evaluation.

(38) Table 1 and FIG. 7 show the heat cycle test results regarding the rate of change in resistance at 25° C. before and after repeating a heat cycle test 25 and 50 times with one cycle consisting of a test conducted at −55° C. for 30 minutes and one at 200° C. for 30 minutes. In this heat cycle test, a test at a normal temperature (25° C.) for 3 minutes was performed between the test at −55° C. for 30 minutes and the one at 200° C. for 30 minutes.

(39) A thermistor element according to the Comparative Example was also produced, wherein an Au paste was directly applied on the thermistor body without using the conductive intermediate layer of the present invention and it was baked. Table 1 and FIG. 7 show the test results for the Comparative Example as well. Note that the test results are expressed as the mean value of measurements for 20 elements according to each of the Example and Comparative Example.

(40) As can be seen from these heat cycle test results, the resistance was significantly increased in all the elements according to the Comparative Example, whereas the rates of change in resistance were small for all the elements according to the Example employing the conductive intermediate layer produced according to the method of the present invention as described above. The following is believed to be the reason for the above results. In the Comparative Example, the resistance is significantly increased as peeling of the electrode is extended and the peeling rate of an electrode is increased by the heat cycle test because they have an intermediate layer having a high resistance, whereas in the Example of the present invention, the increase of the resistance can be suppressed even when peeling of the electrode is caused because the conductive intermediate layer has a low resistance. These test results are also consistent with the simulation results regarding the change in resistance associated with the change in the peeling rate of the electrode.

(41) TABLE-US-00001 TABLE 1 THICKNESS OF INITIAL BAKING INTERMEDIATE VALUE 25 cycs 50 cycs TIME [min] LAYER [nm] R25[Ω] R25[Ω] ΔR25 R25[Ω] ΔR25 COMPARATIVE 10 — 3772 4663 23.6% 5147 36.4% EXAMPLE 1 COMPARATIVE 30 — 3744 4113 9.9% 4291 14.6% EXAMPLE 2 COMPARATIVE 60 — 3728 4728 26.8% 5030 34.9% EXAMPLE 3 EXAMPLE 1 10 150 3699 3754 1.5% 3781 2.2% EXAMPLE 2 10 210 3756 3804 1.3% 3807 1.3% EXAMPLE 3 30 210 3690 3707 0.5% 3713 0.6% EXAMPLE 4 60 240 3672 3704 0.9% 3710 1.0% EXAMPLE 5 10 440 3672 3697 0.7% 3703 0.8% EXAMPLE 6 10 850 3675 3683 0.2% 3683 0.2%

(42) The technical scope of the present invention is not limited to the aforementioned embodiments and Example, but the present invention may be modified in various ways without departing from the scope or teaching of the present invention.

Reference Numerals

(43) 1: thermistor element, 2: thermistor body, 3: RuO.sub.2 layer, 3a: RuO.sub.2 particles, 4: conductive intermediate layer, 5: electrode layer