JPH05273020A - Resistance element - Google Patents

Abstract

PURPOSE:To obtain a resistance element which is prevented virtually from sticking of an extraneous substance such as dust in a fluid and of which the precision in responsiveness and an output characteristic is improved. CONSTITUTION:In regard to a resistance element 20 employed for a thermal type flow sensor and the like, the outer surface of a glass layer 16 protecting a ceramic pipe 10 and a metal thin film 11 is covered with a boron nitride layer 17. Thereby dust, oil and others are prevented virtually from sticking on the outer surface of the resistance element 20 and the thermal conductivity and the responsiveness of the resistance element 20 are improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a resistance element, and more particularly to a resistance element for detecting the flow rate of a fluid such as gas or liquid.

[0002]

2. Description of the Related Art Conventionally, a platinum thin film or a thin platinum wire is spirally wound around the outer surface of an alumina pipe as a resistance element for use in a thermal type flow rate sensor for detecting the intake flow rate of an automobile engine and the like. There is known a structure in which lead wires electrically connected to a thin film or a platinum wire are attached to both ends of an alumina pipe. In such a resistance element, a protective layer is provided on the surface of the resistance element in order to protect the platinum thin film or the platinum wire. As the protective layer, a glass layer having a smooth surface is generally used so that heat exchange between the resistance element and the fluid can be efficiently performed without disturbing the flow of fluid such as intake air.

[0003]

However, according to such a conventional resistance element, even if a glass layer having a smooth surface is formed as a protective film, dust contained in the air,
When dust, oil, water, etc. adhere to the protective layer in layers, this adherence acts as a heat insulating material, which deteriorates the heat exchange between the platinum thin film or platinum wire and the external air flow, and the air flow detection signal. There is a problem that not only the output characteristics are deteriorated, but also the response speed of the detection signal of the flow rate or the air temperature becomes slow.

It is also conceivable to use a fluororesin as a protective layer instead of glass, to which dust, oil, etc. are unlikely to adhere, but in this case, since the thermal conductivity of the fluororesin is low, the thickness of the protective layer is large. In the case, just like when dust adheres,
There is a problem that the response speed becomes slow. On the other hand, as another type of flow rate sensor, a hot wire type flow rate sensor using a single platinum wire as a hot wire is known. In this case, the method of removing the deposits such as dust is a method of burning and scattering the deposits by heating the heating wire.

When such a method of removing deposits is applied to a resistance element having a glass layer, the glass layer does not withstand high heat, and further, in a resistance element using a platinum thin film, heat is generated to burn and disperse deposits. When doing so, a problem such as disconnection of the platinum thin film occurs. Therefore, it is difficult to apply such a method to the resistance element for a flow sensor of the present invention.

The present invention has been made in order to solve such a problem, and provides a resistance element which makes it difficult for foreign matter such as dust in a fluid to adhere and improves the accuracy of response and output characteristics. The purpose is to provide.

[0007]

To this end, the resistance element of the present invention comprises a plate-shaped, cylindrical or columnar electrically insulating substrate.
A resistor formed on the outer surface of the electrically insulating substrate, a first lead wire and a second lead wire fixed to the electrically insulating substrate, the resistor, the first lead wire and the second lead wire. Is provided, and a coating layer made of a boron nitride-based material that covers the resistor is provided.

The thermal type flow sensor of the present invention is characterized by using the resistance element.

[0009]

According to the resistance element of the present invention, since the protective layer containing boron nitride is formed on the resistance element surface, dust, dust, oil and the like contained in the fluid such as air adheres to the resistance element surface. It is difficult, and even if it adheres, it peels off before the adhesion layer becomes thick. Therefore, a heat insulating layer due to these deposits is not formed, and a decrease in responsiveness of the resistance element and a change in output characteristics are prevented.

When a high-temperature melt such as glass or an organic substance such as oil or solvent is used as the fluid, boron nitride has heat resistance and does not easily get wet with these substances, so that a heat insulating layer is formed by the deposit. In addition, the performance degradation of the resistance element is prevented. Furthermore, since boron nitride has high thermal conductivity, even if the protective layer is thick, it has little effect on the resistance element performance.
It is possible to make a thick and durable film.

[0011]

FIG. 2 shows a schematic diagram of an electric circuit when the resistance element according to the present invention is applied to a thermal type flow sensor. As shown in FIG. 2, a heating resistor R _H and a temperature compensation resistor R _C are provided so as to come into contact with the air flow inside the air tube 1, and the resistor R ₁ and the resistor R ₁ and the resistor are provided outside the air flow path 2. R ₂ is connected. The heating resistor R _H is a flow rate detecting resistor, and the temperature compensating resistor R _C is a resistor that is maintained at the same temperature as the air temperature in the air flow path 2. In FIG. 2, 3 is a transistor, 4 is a comparator, and 5 is a terminal to which a sensor drive voltage is applied. When the air flow in the air flow path 2 increases, a large amount of heat is taken from the heating resistor element R _H, so that the temperature of the heating resistor R _H decreases and its resistance value decreases. The circuit of FIG. 2 corrects this decrease in resistance value by bridge balance, and controls the electric power of the heating resistor R _H so that the temperature of the heating resistor R _H is maintained at a predetermined temperature, for example, 100 ° C. The terminal 6 is an output terminal of the flow rate sensor.

The heat generating resistor R _H and the temperature compensating resistor R _C of the bridge constituting this electric circuit are shown in FIGS.
It consists of a resistance element 20 as shown in FIG. That is, a metal thin film 11 made of platinum or the like having a predetermined resistance value and spirally patterned is provided on the outer peripheral surface of a cylindrical ceramic pipe 10 made of alumina or the like.
Are electrically connected to the first lead wire 12 and the second lead wire 13 made of platinum through conductive paste 14 in which a conductor such as platinum is mixed with glass or the like at both ends of the ceramic pipe 10. The metal thin film 11 is formed to have a predetermined resistance value by heat-treating a thin film formed by a physical or chemical method such as sputtering, plating, CVT, vapor deposition, etc., and then spirally trimming by a laser trimming method. To be done. In this case, a resistance value in the range of, for example, about 10 to 50Ω is selected as the heating resistance element, and a resistance value in the range of, for example, about 100 to 1000Ω is selected as the temperature compensation element.

An insulating glass layer 16 made of glass or the like is formed on the outer surfaces of the ceramic pipe 10, the metal thin film 11 and the conductive paste 14 so as to have a predetermined thickness. Then, the boron nitride layer 17 is formed on the outer surface of the glass layer 16. Here, the boron nitride layer 17 is formed, for example, as shown in the following items. A mixture of boron nitride powder and water glass as a binder is directly applied onto the glass layer 16 and baked. In this case, the weight ratio of water glass to boron nitride powder 100 is preferably 3 to 40. Boron nitride powder added with phosphate as a binder is directly applied onto the glass layer 16 and baked. As the phosphate, for example, aluminum phosphate or the like may be used. In this case, a weight ratio of 5 to 30 of aluminum phosphate is preferable with respect to 100 of boron nitride powder. It is formed by mixing boron nitride powder and glass powder and melting and solidifying glass. The softening point of the glass powder is preferably 500 ° C. to 900 ° C., for example. The working temperature for forming the boron nitride layer is preferably determined from the viscosity of the glass. The viscosity range is, for example, 1 × 10 ² to
1 × 10 ⁸ poises are preferred, and more preferably 1 × 10 ⁴ to 1 × 10 ⁷ poises. When the viscosity is 100 poises or less, boron nitride does not wet the glass well, so that it becomes difficult to form a uniform protective layer. The amount of glass powder mixed is 3 to 30 with respect to 100 parts of boron nitride powder.
Is a preferable range. A mixture of boron nitride powder and resin is directly applied and baked. In this case, dust is less likely to adhere, and at the same time, the thermal conductivity of the protective layer is better than that of the resin alone. As the resin, a fluororesin or a polyimide resin may be used.
A volume ratio of 0-100 is preferred. The above-mentioned embodiments are only examples, and may be used in combination. Further, although the resistance element 20 according to the above-described embodiment has the two-layer structure of the glass layer and the boron nitride layer, it may have a single layer structure of only boron nitride. In this case, since boron nitride has better thermal conductivity than glass or the like, the response performance is improved as compared with the protective layer made of only glass.

According to the resistance element 20 according to the above embodiment,
Dust, dust, oil, melts, etc. are unlikely to adhere to the outer surface in contact with the fluid, and the heat insulating layer is difficult to form. Therefore, it is possible to significantly reduce fluctuations in response of the resistance element 20. Next, in order to investigate the response of the coating weight and the resistance element of the foreign matter in the air to the resistance element, the test was conducted for Examples 1 to 5 and Comparative Examples are shown below.

The resistance elements used in Examples 1 to 5 and Comparative Example were manufactured as follows. First, alumina purity 96%, inner diameter 0.25 mm, outer diameter 0.6 mm, length 2.
After washing the 5 mm alumina pipe, a platinum thin film is attached to the outer peripheral surface of the alumina pipe by sputtering. This platinum thin film is heat-treated together with the alumina pipe and laser-trimmed to a predetermined resistance value to obtain a spiral platinum thin film. Then, the outer diameter of the alumina pipe was 0.
A 2 mm stainless wire is baked and fixed through a mixed paste of platinum and glass, and the platinum wire is electrically connected to the stainless wire.

A protective layer was formed on the outer peripheral surface of the resistance element body thus obtained as follows. Example 1 Boron nitride powder (average particle size 2.3 μm, particle size distribution 0.3
˜15 μm) is dispersed in a water glass aqueous solution. The weight ratio of boron nitride to water glass solids is 80:20.
Next, the resistance element main body was dipped in this solution and pulled up, the water content was evaporated in a dryer, and then baked at 500 ° C. for 1 hour. The thickness of the protective layer was 35 μm on average.

Example 2 Boron nitride powder (average particle size 0.7 μm, particle size distribution 0.1 to 3 μm) is dispersed in an aluminum phosphate aqueous solution. The weight ratio of boron nitride to aluminum phosphate is 9
It is 0:10. This solution was applied to the body of the resistance element with a brush, dried, and baked at 700 ° C. for 15 minutes. The thickness of the protective layer was 20 μm on average.

Example 3 After applying glass paste to the outer periphery of the resistive element body, 8
It melted at 00 ° C. to form a glass protective layer having an average of 5 μm.
The glass paste is obtained by adding a predetermined solvent and a binder to glass powder. Then, a boron nitride layer was formed on the outer periphery of the resistance element body having the glass layer under the same conditions as in Example 1. The thickness of the protective layer together with the glass layer is 40 μm on average.

Example 4 A boron nitride layer was formed under the same conditions as in Example 2 on the outer periphery of a resistor element body having a glass layer obtained under the same conditions as in Example 3. The thickness of the protective layer, together with the glass layer, is 30 μm on average. Example 5 A mixed layer of boron nitride and glass was formed on the outer periphery of the resistance element body as follows. First, after mixing boron nitride powder (average particle size 2.3 μm, particle size distribution 0.3 to 15) and glass powder (average particle size 1.3 μm, particle size distribution 0.1 to 5) at a volume ratio of 80:20 , And was dispersed in an organic solution containing a binder. After immersing the resistive element body in this solution,
It was dried in a dryer and baked at 750 ° C. The thickness of the protective layer was 40 μm on average. The viscosity of the glass used here is 3 × 10 ⁴ poise at 750 ° C.

Comparative Example After applying glass paste to the outer periphery of the resistive element body, 8
It melted at 00 ° C. to obtain a glass layer having an average of 40 μm. Next, the test apparatus is shown in FIGS. 4 and 5. Test equipment 40
A test element mounting portion 34 and a blower 35 are provided in the middle of a main pipe 33 made of stainless steel and having an inner diameter of about 50 mm. A bypass pipe 36 branches between the test element mounting portion 34 and the blower 35. Main pipe 33
The shutter 37 that slides in the direction orthogonal to the bypass pipe 36 has openings 37a and 37b as shown in FIG.
In the state shown in (A), the bypass pipe 36 is closed and the main pipe 33 is opened. When the openings 37a and 37b are in the state shown in FIG. 4B, the main pipe 33 is closed and the bypass pipe 36 is opened.

As shown in FIG. 5, the test element 29 housed in the test element mounting portion 34 is fixed by welding the lead wire 31 to the stainless rod 30. Test element 29
The heating resistance element of the
Controlled to 00 ° C. The temperature compensation element is not energized. Test Example 1 Foreign Material Adhesion Test As shown in FIG.
The shutter 37 is fixed in the state shown in FIG. 4A so that the air flows to the main pipe 33 side. Next, air having a flow rate of 30 g / sec was circulated by the blower 35 in an atmosphere containing dust, dirt, oil mist, etc., and the presence or absence of foreign matter on the test element 29 after 50 hours and 300 hours was examined. The results are shown in Table 1.

[0022]

[Table 1]

Similar tests were carried out even when the resistance element was not heated. The result of this experiment was the same as when the resistance element was heated. Test Example 2 Response Test of Resistive Element As shown in FIG.
, And the shutter 37 in the state shown in FIG.
Is fixed, and the test element mounting portion 34 is kept in a windless state. At this time, the test element 29 is heated to about 250 ° C. by the external DC power supply.
Control to. Next, the shutter 37 is momentarily slid from the state shown in FIG. 4 (B) to the state shown in FIG. 4 (A), air is flown through the test element mounting portion 34, and the temperature of the test element 29 is cooled by the air flow. The response time until the temperature stabilized at a predetermined temperature was measured. The response time was the time required to reach 75% of the predetermined temperature. The temperature of the test element 29 was read from the resistance value of the test element 29. Table 2 is a summary of the results.

[0024]

[Table 2]

As described above, in Examples 1 to 5, foreign matter was less likely to adhere and the responsiveness was good as compared with Comparative Example.

[0026]

As described above, according to the resistance element and the thermal type flow sensor of the present invention, since the protective layer containing boron nitride is provided on the surface of the resistance element, dust, dust, oil, melt, etc. Since the adherence is reduced, it is difficult to form a heat insulating layer or the like on the surface of the resistance element, so that there is an effect that the responsiveness can be improved step by step.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing a resistance element according to an example of the present invention.

FIG. 2 is an electric circuit diagram of a thermal type flow sensor using a resistance element according to an embodiment of the present invention.

FIG. 3 is a perspective view showing a resistance element according to an exemplary embodiment of the present invention.

FIG. 4 shows a test apparatus according to an embodiment of the present invention,
(A) is a schematic diagram showing a state where air flows through the main pipe, and (B) is a schematic diagram showing a state where air flows through the bypass pipe.

5 is a cross section taken along line AA shown in FIG.

[Explanation of symbols]

 10 Ceramic Pipe (Electrically Insulating Substrate) 11 Metal Thin Film (Resistor) 12 First Lead Wire 13 Second Lead Wire 14 Conductive Paste (Connecting Part) 16 Glass Layer 17 Boron Nitride Layer (Coating Layer) 20 Resistive Element

Claims (2)

[Claims] 1. A plate-shaped, tubular or columnar electrically insulating substrate, a resistor formed on an outer surface of the electrically insulating substrate, a first lead wire and a second wire fixed to the electrically insulating substrate. A lead wire; a connecting portion for electrically connecting the resistor to the first lead wire and the second lead wire; and a coating layer made of a boron nitride-based material for coating the resistor. And a resistance element. 2. A thermal type flow sensor using the resistance element.