JP2880009B2 - Detection element for flow meter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention is suitably used for a thermal flow meter or the like for detecting a flow rate or a flow velocity of a fluid in an internal combustion engine, for example, by utilizing the temperature dependence of an electric resistance value of a detecting element, particularly a resistor. The present invention relates to a detection element for a flow meter.

[0002]

2. Description of the Related Art Conventionally, as this type of detection element, a detection element having a cylindrical structure or a plate-like structure is known. For example, in the case of an element having the former structure, as shown in FIG. ,
On the outer surface of a ceramic pipe 2 made of alumina or the like and having an outer diameter of about 0.5 mm, a platinum thin film 8 is patterned so as to have a predetermined resistance value. A lead 4 made of platinum wire or the like having a diameter of about 2 mm is inserted, and the lead 4 is bonded to the ceramic pipe 2 by using a conductive adhesive such as a mixed paste of platinum and glass. And the lead 4 and the platinum thin film 8
The electrical connection 6 is formed with the ceramic pipe 2.
At both ends, the platinum thin film 8 and the lead 4 are electrically connected to each other. Further, in such a detecting element, a glass coating layer 10 for covering the platinum thin film 8 is formed in order to prevent a change in the resistance value of the resistor and improve the durability of the element. In this case, the lead 4 and the platinum thin film 8 are electrically connected together with the lead 4 by using a conductive adhesive, but instead, the lead 4 is connected to the ceramic pipe 2. Of the lead 4 and the platinum thin film 8 may be separately connected using a paste having excellent conductivity.

However, when such a detection element is subjected to a cooling / heating test (a heating operation by energization and a cooling operation by interrupting energization in an atmosphere of 0 ° C. or less are repeated), the resistance value changes. Is recognized. As a result of various studies of this phenomenon by the present inventors, it has been found that the inside of the electrical connection portion 6 made of a conductive adhesive or a conductive paste serving as a conductive means between the lead 4 and the platinum thin film 8 or such an electrical connection. It has been found that the cause is that moisture in the air enters the boundary between the electrical connection 6 and the lead 4 or the boundary between the electrical connection 6 and the platinum thin film 8. That is, by repeatedly repeating that the invading moisture freezes during cooling, expands in volume, and melts during heating, cracks are generated inside the electrical connection portion 6 or at the boundary portion, and the resistance value increases. It will be.

[0004] In particular, in order to improve the response of the detecting element, such as the detecting element disclosed in Japanese Patent Application Laid-Open No. 64-18023, an electric connection portion 6 made of a conductive adhesive is used.
In the case where the film has a porous structure, such a change in resistance value due to the intrusion of moisture becomes a more serious problem.

In addition, when the lead 4 and the electrical connection portion 6 are not well adapted to each other, a gap is easily formed between the lead 4 and the electrical connection portion 6, so that moisture easily enters, and as a result, a change in the resistance value of the detecting element is caused. It is easy. Specifically, lead 4
Is made of a base metal, when it is baked in an oxidizing atmosphere such as air, an oxide film is formed on the lead, and the electrical connection 6
And the expansion of the oxide film makes it easier to form a gap between the lead 4 and the electrical connection portion 6. When the lead 4 is made of a noble metal, the lead and the glass component contained in the adhesive / paste constituting the electrical connection 6 are not well adapted to each other. It is easy to do.

[0006]

The present invention has been made in view of such circumstances, and an object of the present invention is to provide an electric connection between a lead and a resistor in a detection element as described above. Preventing moisture in the air from entering the inside or the boundary between the electrical connection and the lead or the boundary between the electrical connection and the resistor, thereby preventing the resistance value of the detection element from changing. It is in.

[0007]

According to the present invention, in order to solve such a problem, a ceramic base, a resistor provided on the base, and an adhesive bonded and fixed to the base with an adhesive. A flow meter detecting element comprising a lead electrically connected to the resistor, wherein the detection element covers the entire outer surface of an electrical connection between the resistor and the lead, and The gist of the present invention is a flowmeter detection element characterized in that a coating made of a heat-resistant and air-tight substance is applied so as to cover at least the outer surfaces of the body and the lead on the connection portion side. .

[0008]

In the detecting element according to the present invention, the outer surface of the electrical connection between the resistor and the lead, the portion extending from the electrical connection to the resistor, and the The part that straddles the lead from the electrical connection part is covered with a heat-resistant and air-tight substance in an airtight manner, so that moisture in the air enters the electrical connection part, and the electrical connection part Moisture in the air is prevented from entering the boundary between the resistor and the boundary between the electrical connection and the lead. Therefore, cracking of the conductive portion due to freezing and thawing of moisture is well prevented, and a change in the resistance value of the element due to such cracking is well prevented. Therefore, the set resistance value can be maintained very stably as compared with the conventional detection element, and the durability of the detection element can be effectively improved.

[0009]

[Specific structure and embodiment] By the way, the detecting element according to the present invention has a structure except for an airtight covering portion made of a heat-resistant airtight substance.
It has a structure similar to that of a conventional element, and one example is shown in FIG. Here, a pipe-shaped bobbin 12 made of a known ceramic material such as alumina is used as a ceramic base, and a resistor thin film 18 made of platinum or the like is provided on the outer peripheral surface thereof in the same manner as in the related art. It is provided in a predetermined pattern. Also,
At both ends of the bobbin 12, leads 1 such as a platinum wire are provided.
The leads 4 and 14 are glued with a conductive adhesive in a state where they are inserted for a predetermined length, respectively, so that the leads 14 and 14 are fixed to the bobbin 12 and the resistor is formed with the adhesive. An electrical connection 16 connecting the thin film 18 and the lead 14 is formed.

As the conductive adhesive for providing the electrical connection portion 16, a conventionally known adhesive for bonding ceramic and metal can be used.
Platinum paste mixed with glass is selected to increase the bonding strength, whereby high bonding strength is achieved, and good electrical conduction between the lead 14 and the resistor thin film 18 is achieved. Become. In particular, even in a glass, it is desirable to blend the crystallized glass of the _{ZnO · B 2 O 3 · SiO} 2 type or the like can be improved thereby the strength effectively.

An airtight coating layer 20 made of glass is provided on such a detection element so as to cover the entire bobbin 12. That is, the coating layer 20
Covers the resistor thin film 18, further covers a portion of the outer surface of the electrical connection portion 16 between the resistor thin film 18 and the lead 14 from the electrical connection portion 16 to the resistor thin film 18, and Including a portion extending from the active connection portion 16 to the lead 14,
It is provided so as to cover integrally. Thus, the outer surface of the electrical connection 16, the boundary between the electrical connection 16 and the resistor thin film 18, and the electrical connection 16 and the lead 14
Are sealed with glass, and the intrusion of moisture in the air into the inside of the electrical connection portion 16 and the boundary portion is well prevented, and An increase in the resistance value of the detection element due to the crack is prevented. In other words, in this detection element, glass is used as a heat-resistant and air-tight substance so that the set resistance value can be stably maintained, and the durability is advantageously improved. Further, here, since the resistor thin film 18 is also covered with the coating layer 20, the oxidation and damage of the resistor thin film 18 can be prevented well.

Since the glass used as the heat-resistant gas-tight substance is not well compatible with precious metals such as platinum, when the lead 14 is made of a platinum wire or a wire coated with platinum, it is difficult to cover the glass. It is desirable to increase the contact length or the bonding length (indicated by t in FIG. 2) between the layer 20 and the lead 14 to enhance the adhesion. Further, a metal film is further formed at least at a boundary portion between the glass coating layer 20 and the lead 14, and the glass coating layer 20 and the lead 1 are formed.
4 may be supplemented.

The coating layer 20 can be formed of various heat-resistant and air-tight substances other than glass. For example, a heat-resistant metal such as platinum or a material such as polyimide is preferably used. Further, they can be used in combination as appropriate. For example, in the detection element shown in FIG. 3, the coating layer 24 made of a heat-resistant metal such as platinum is used.
Are formed at both ends of the bobbin 12, respectively, so that the outer surface of the electrical connection portion 16 and the boundary portion are airtightly covered. In such a detection element, since the adhesion between the metal coating layer 24 and the resistor thin film 18 or the lead 14 is high, excellent airtightness can be obtained. In such an element, when the resistor thin film 18 is made of platinum, it is preferable that the coating layer 24 is also formed of the same kind of platinum. Also, 22
A glass coating layer provided to protect the resistor thin film 18.

On the other hand, when glass or polyimide is used as the heat-resistant gas-tight substance, the glass or polyimide has a thermal expansion coefficient equal to or smaller than that of the substrate in order to prevent cracking due to thermal shock. It is desirable to select For example, when the substrate is alumina, 30 to 3
In the temperature range of 00 ° C., those having a thermal expansion coefficient of about 3 to 80 × 10 ⁻⁷ / ° C. are preferable. In particular, since polyimide generally has a large coefficient of thermal expansion, it is desirable to select a polyimide having low thermal expansion. In a temperature range of 30 to 300 ° C., a polyimide having a thermal expansion of 1 × 10 ⁻⁵ / ° C. or less is advantageously selected. You. In addition, as such a low thermal expansion polyimide, it is marketed under the name of Pyraline PI-2611D (trade name, manufactured by DuPont, USA), and any of them can be used.

Further, the structure for fixing the lead 14 and the bobbin 12 is not limited to the above-described embodiment, and can be appropriately changed as needed. For example, as in the detection element shown in FIG. 4, the electrical connection between the resistor thin film 18 and the lead 14 is made at an electrical connection portion 28 made of a conductive paste such as a glass-Pt mixed paste similar to the conductive adhesive. Take glass-N
The fixing strength between the lead 14 and the bobbin 12 may be increased in the fixing portion 26 made of an adhesive having a high fixing force (fixing force) such as i-mixed paste. Further, the electrical connection 1
It is also effective to make the electrical connection portions 16 and 28 porous in order to effectively reduce the thermal conductivity of the elements 6 and 28 and increase the responsiveness of the element. In such a case, the invasion of moisture in the air can be satisfactorily prevented, and the durability of the element can be improved.

Such excellent characteristics of the detecting element according to the present invention can be more easily recognized from the results of the thermal test shown below. In addition, the cooling test was -50 ° C.
The detection element is set in the atmosphere and energized for 10 minutes, and the element temperature is set to the ambient temperature (−50 ° C.) + 200 ° C. to 250 ° C.
, And the heating / cooling operation of cooling to ambient temperature (−50 ° C.) for 10 minutes was repeated for 120 hours. Then, the rate of change of the resistance value of the detection element before and after the test was determined. In Test Examples 1 to 4, the average value was n = 5, and in Test Example 5, the average value was n = 3.

(Test Example 1) First, in order to obtain a detecting element as shown in FIG. 4, an inner diameter: 0.3 mmφ and an outer diameter: 0.5 mm
An alumina bobbin 12 having a diameter of 2 mm and a length of 2 mm is used as a ceramic substrate, and a platinum thin film (thickness: 0.8 μm) is formed on its outer peripheral surface.
Is formed by sputtering, a spiral kerf is formed by laser trimming, and a resistance value of 10
The resistor thin film 18 was formed to have a resistance of 0Ω. On the other hand, a glass paste for bonding was prepared by blending 90 vol% of glass at 750 ° C. with 90 vol% of nickel powder and 10 vol% of nickel powder and further adding an organic binder and terpineol, and prepared a 0.2 mmφ stainless steel wire (SUS304). ) Are inserted into both ends of the alumina bobbin 12, fixed with the glass paste, dried, and then placed at 750 ° C. in a nitrogen atmosphere at a temperature of 750 ° C.
After baking for minutes, the leads 14 were bonded and fixed to both ends of the alumina bobbin 12 by bonding portions 26 made of glass paste.

Next, a conductive paste consisting of 50% by volume of platinum and 50% by volume of glass is applied to both ends of the alumina bobbin 12 and baked at a temperature of 700 ° C. for 5 minutes, so that the lead 14 and the resistor are connected to each other. An electrical connection portion 28 connected to the body thin film 18 was formed. Further, after that, glass is applied so as to cover the resistor thin film 18, the outer surface of the electrical connection portion 28, and the boundary portion between the electrical connection portion 28 and the resistor thin film 18 or the lead 14. 10 at ℃
By baking for a minute, the entire glass coating layer 20 was formed, and the target detection element was completed. The conductive paste had a porosity of about 20 vol% and was porous. The thermal expansion coefficient of the glass coating layer 20 is as follows:
40 × 10 ⁻⁷ / ° C., which is about 1 of the thermal expansion coefficient of alumina.
/ 2.

When the above-described cooling / heating test was performed on the obtained detecting element, the rate of change of the resistance value was 0.3.
% Or less, showing extremely excellent durability.

(Test Example 2) In order to obtain a detecting element as shown in FIG. 3, first, a ceramic substrate was used.
Using an alumina bobbin 12 having a diameter of 20 mmφ, an outer diameter of 0.45 mmφ and a length of 2.5 mm, a resistor thin film 18 made of a platinum thin film and having a resistance value of 100Ω was formed on its outer surface in the same manner as in Test Example 1. Formed. Then, the alumina bobbin 1
2 at 3 μm on the surface of a 40% Ni—Fe wire
A lead 14 having a thickness of 0.13 mmφ and plated with Pt is inserted. Pt: 60% by volume and glass: 40% by volume
Using a conductive paste consisting of
Baking was performed at 700 ° C. for 5 minutes in the air to form the electrical connection portion 16. And the alumina bobbin 12
Was baked in the air at 680 ° C. for 5 minutes to provide a glass coating layer 22 covering the resistor thin film 18. Thereafter, a platinum film having a thickness of about 1 mm is formed on both ends of the alumina bobbin 12 by vacuum deposition, and the outer surface of the electrical connection portion 16 and the electrical connection portion 16 and the resistor thin film 18 or A coating layer 24 for covering the boundary with the lead 14 was provided to obtain a target detection element.

When the above-described cooling / heating test was performed on the obtained detecting element, the rate of change of the resistance value was 0.3.
% Or less, showing extremely excellent durability.

(Test Example 3) In order to obtain a detecting element as shown in FIG.
Using an alumina bobbin 12 having a diameter of 20 mmφ, an outer diameter of 0.45 mmφ and a length of 2.5 mm, a resistor thin film 18 made of a platinum thin film and having a resistance value of 100Ω was formed on its outer surface in the same manner as in Test Example 1. Formed. Then, the alumina bobbin 1
2 at 3 μm on the surface of a 40% Ni—Fe wire
A lead 14 having a thickness of 0.13 mmφ and plated with Pt is inserted. Pt: 60% by volume and glass: 40% by volume
Using a conductive paste consisting of
The sintering was performed in air at 700 ° C. for 5 minutes to form the electrical connection portions 16. Thereafter, glass is applied so as to cover the resistor thin film 18, the outer surface of the electrical connection portion 16, and the boundary portion between the electrical connection portion 16 and the resistor thin film 18 or the lead 14, and the glass is applied in the air. Baking was performed at 680 ° C. for 5 minutes to form a glass coating layer 20, thereby obtaining a target detection element.

When the above-described cooling / heating test was performed on the obtained detection element, the rate of change of the resistance value was 0.3.
% Or less, showing extremely excellent durability.

Test Example 4 In the detection element of Test Example 3, the resistance thin film 18, the outer surface of the electrical connection portion 16,
A coating layer 20 for covering the electrical connection portion 16 and the boundary between the resistor thin film 18 and the lead 14 is formed of polyimide (DuPont, USA; trade name: Pyralin PI-2611D) instead of glass. Thus, a desired detection element was obtained. In addition, the coating layer (20) made of polyimide is formed by applying a polyimide material, and then, in an N ₂ atmosphere, at 400 ° C.
Formed by baking for 30 minutes.

When the above-described cooling / heating test was performed on the obtained detection element, the rate of change of the resistance value was 0.3.
% Or less, showing extremely excellent durability.

(Test Example 5 and Comparative Example) In the detection element of Test Example 2, only the glass coating layer 22 was provided, and the coating layer 24 covering the electrical connection portion 16 and the boundary portion was not provided. Was carried out, it was recognized that the rate of change in resistance varied between 1 and 3%, and that the durability was low and unstable.

In the above description, the detection element using the bobbin-shaped ceramic base has been described in detail, but it goes without saying that the present invention can be applied to a detection element using a plate-shaped base. . In addition, the present invention,
It should be understood that various changes, modifications, improvements, and the like can be made based on the knowledge of those skilled in the art without departing from the spirit of the present invention.

[Brief description of the drawings]

FIG. 1 is an explanatory sectional view showing an example of a conventional detection element.

FIG. 2 is an explanatory sectional view showing an example of a detection element according to the present invention.

FIG. 3 is an explanatory sectional view showing another example of the detection element according to the present invention.

FIG. 4 is an explanatory sectional view showing still another example of the detection element according to the present invention.

[Explanation of symbols]

Reference Signs List 12 bobbin 14 lead 16 electrical connection 18 resistor thin film 20 coating layer 22 glass coating layer 24 coating layer 26 bonding section 28 electrical connection section

Claims (3)

(57) [Claims] 1. A cylindrical ceramic substrate and an outer portion of the substrate
A thin film resistor provided on a peripheral surface , and an inner hole at an end of the base ;
Is inserted within, adhesively bonded, and fixed leads, a conductive adhesive as the adhesive used for said substrate
Or by using a conductive paste
Further, an electric device for electrically connecting the thin film resistor and the lead.
A flow meter for detecting element configured to include a connection portion, each of at least the said thin-film resistor and and the thin film resistor and the lead covers the entire surface of the electrical connection portion of the outer surface of the lead Same as the base so as to cover the outer surface on the connection side.
A detection element for a flow meter, which is provided with a coating made of a heat-resistant airtight substance having a coefficient of thermal expansion of one or less . 2. The detection element for a flow meter according to claim 1, wherein the heat-resistant airtight substance is glass. 3. The detection element for a flow meter according to claim 1, wherein the heat-resistant and air-tight substance is polyimide.