JPH1123337A - Measuring element for airflow rate measuring apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a measuring element for an airflow rate measuring apparatus in which the responsivity of another measuring element is enhanced. SOLUTION: An airflow rate measuring apparatus is formed in such a way that a measuring element 1 used to detect an air flow rate and another measuring element 1 for temperature correction are arranged inside an auxiliary air passage. A base body 2 which constitutes the measuring elements 1 is composed mainly of aluminum nitride, silicon oxide or boron nitride whose thermal conductivity is larger than that of alumina, and its surface is covered with an oxide- based ceramic such as mullite 2a, alumina or the like.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a measuring element of an air flow measuring device for detecting a flow of air taken into an engine, for example.

[0002]

2. Description of the Related Art Conventionally, as a resistor element for measuring a flow velocity of a fluid, a resistor is formed by a platinum thin wire on a cylindrical alumina substrate as described in JP-A-59-104513. A body coated with a protective coating glass for body protection is known, and is generally used in thermal air flow meters that control the heating of a measuring element to a constant temperature by utilizing the temperature dependence of the electrical resistance of platinum. ing.

[0003] In the case of controlling the heating of a measuring element having such a structure, various methods for increasing the response thereof have been studied. For example, in terms of structure, its responsiveness varies greatly depending on the length of the heat generating portion with respect to the entire length of the cylindrical alumina base,
It is known that, in an alumina substrate having the same length, a measurement element having a higher response can be obtained as the length of the heat generating portion becomes longer.

It is known that steatite, zirconia, mullite, or the like is used instead of an alumina substrate. This is to improve the responsiveness by preventing heat from escaping to the lead or the support by using a material having a relatively large heat insulating property instead of alumina.

On the other hand, when the leads and the support have sufficient insulating properties, on the other hand, the thermal conductivity of the base is improved so that the temperature distribution in the base is made uniform to achieve a high-speed response. It is possible.

[0006]

A commonly used material for a cylindrical ceramic substrate is alumina. This is inexpensive because it is manufactured in large quantities,
It has good compatibility with glass which is one of the constituent materials, does not react harmfully with glass, and has an appropriate hardness. In order to improve the response, it is necessary to use a material having better thermal conductivity than this alumina.

However, even if the thermal conductivity of the substrate is to be improved, alumina is a material having a relatively high thermal conductivity, and inorganic materials having a higher thermal conductivity are limited. For example, aluminum nitride has a high thermal conductivity, but is incompatible with the glass coating forming the resistor element. If heat treatment is performed for glass coating during the manufacturing process, the glass and aluminum nitride react to generate nitrogen gas. A normal glass coating cannot be formed. Although silicon carbide is also a high thermal conductor, it cannot be used simply as a substitute for an alumina substrate because it is a good conductor of electricity and has a small coefficient of thermal expansion close to silicon.

[0008] Boron nitride is a high thermal conductor having electrical insulation, but has a small thermal expansion, so that if it is used as it is in a resistor element, cracks occur due to the difference in thermal expansion between the resistor and lead made of platinum.

Except for its high thermal conductivity, beryllia has almost the same physical properties as alumina, so the above-mentioned problem does not occur. However, powdered beryllia in the manufacturing process may cause allergies when it enters the human body, so handling is a problem, and practical use is difficult due to manufacturing problems.

An object of the present invention is to provide an air flow measuring device having excellent responsiveness by manufacturing a measuring element made of a ceramic substrate having high thermal conductivity, which has been conventionally difficult to adapt by modifying the structure. It is in.

[0011]

In order to solve the above-mentioned problems, a cylindrical substrate is made of aluminum nitride or boron nitride having good thermal conductivity, and a mullite layer is formed thereon by barrel sputtering. Formed. Since mullite goes around the inside of the cylinder by forming the mullite layer by barrel sputtering, even if the lead wire is inserted and heated and bonded with glass, the glass does not directly contact with aluminum nitride or boron nitride. Stable bonding is possible. Next, a resistor was formed on the mullite layer and covered with glass.

Since aluminum nitride and boron nitride have a smaller thermal expansion coefficient than alumina, mullite having an intermediate thermal expansion coefficient of alumina is formed between the resistor and the glass coating to reduce thermal stress and crack. It is possible to manufacture a measuring element without any.

When the glass coating is heated in direct contact with aluminum nitride or boron nitride for glass coating, nitrogen gas is generated and remains in the glass coating and becomes brittle in appearance and strength. Since there is no reaction due to the presence of mullite, normal glass coating is possible.

A similar effect can be obtained by forming a cylindrical substrate of silicon carbide having good thermal conductivity and forming a mullite layer thereon by barrel sputtering.

In the resistor element manufactured in this way, even if the flow velocity changes, the temperature distribution is hardly generated in the base, so that the response can be made faster.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a view showing the structure of a measuring element 1 of an air flow measuring device according to one embodiment of the present invention. The measuring element 1 was made of aluminum nitride formed into a pipe shape having an outer diameter of 0.5 mm, an inner diameter of 0.3 mm and a length of 2 mm as a substrate 2. Mullite 2 a was coated on the surface of the substrate 2 with a thickness of about 5 μm by barrel sputtering. Filmed.

The barrel sputtering method is generally used when forming a metal resistor such as a metal film resistor.
The metal resistor can be formed uniformly on the surface of the adherend. In the case of the present invention, since barrel sputtering is performed on the hollow pipe-shaped substrate 2, mullite also adheres to the inner periphery thereof.

Next, the lead wire 4 is inserted into the pipe, and the lead wire 4 and the pipe are bonded to each other with a glass paste obtained by forming a glass powder into a paste with a solvent, and then fired. Thus, the pipe and the lead wire 4 are bonded with the bonding glass 5.

As the resistor, a platinum wire having an outer diameter of 0.02 mm is used.
After platinum wire is spot-welded to one lead wire, it is wound around the surface of the pipe, and the other lead wire is spot-welded to form a platinum resistor 6. Further, a glass paste is applied to the surface for the purpose of protecting the resistor, and baked to form a protective coating 3 of glass.

The features of this structure are that the pipe as the base 2 is made of aluminum nitride having a high thermal conductivity, and that a mullite layer 2a is formed on the surface thereof. That is, aluminum nitride makes the temperature distribution of the resistance portion of the heating resistor uniform, and the temperature distribution does not easily change even when the air flow rate changes, so that the responsiveness is improved.

On the other hand, aluminum nitride is a nitrogen compound, unlike oxides such as alumina and glass. Therefore, when firing is performed in a state of contact with glass, a part of the aluminum nitride is decomposed and nitrogen gas is generated.

For this reason, the heat treatment is performed in a state where the adhesive glass 5 for connecting the lead wire 4 and the pipe, which are the components of the measuring element 1, and the glass protective coating 3 for protecting the platinum resistor 6 are in direct contact with the aluminum nitride. Then, the nitrogen gas generated by the reaction remains in the glass and only forms an adhesive layer or a protective layer which is brittle and has an unstable shape.

The platinum wire having a thermal expansion coefficient of about 9 × 10 ^-6 /
Since the temperature is ° C, it is necessary to use a glass material, a lead wire 4 and a substrate 2 which are components of the measuring element 1 having a thermal expansion coefficient close to that of a platinum wire. However, aluminum nitride has a thermal expansion coefficient of about 4.5 × 10 ⁻⁶ / ° C., which is significantly different from platinum wire.

Therefore, when the cooling / heating cycle is repeated, abnormal stress is applied to the platinum wire, the glass material, and the lead wire, causing cracks in the glass and causing a change with time.

The reason for forming mullite on the surface of aluminum nitride is a means for solving the above two problems. Mullite is an oxidized compound of alumina and silica. Therefore, it has good compatibility with glass, and a stable glass adhesive layer or glass coating can be formed by firing. The coefficient of thermal expansion is about 5.3 × 10 ⁻⁶ / ° C., which is close to that of platinum, so that a change with time due to thermal stress can be prevented.

FIG. 2 shows the physical property values and characteristics of various ceramics. Ceramics having higher thermal conductivity than alumina are limited, and besides the above-mentioned aluminum nitride, there are only verilia, boron nitride, silicon carbide, diamond, sapphire and the like. Considering practical application,
Beryllia is highly toxic in powder form and causes allergic symptoms when inhaled. Therefore, it is necessary to design the production equipment specially so that fine particles of beryllia do not scatter.
Mass production is difficult due to equipment problems.

[0027] Diamond has a thermal conductivity of about 100 times that of alumina, but is expensive. Sapphire is the same as alumina in composition, but has different densities and crystal structures due to differences in purity and heat treatment conditions, and has only a slight improvement in thermal conductivity. Furthermore, since it is more expensive than alumina, practical application is difficult.

The present invention focuses on aluminum nitride, boron nitride, and silicon carbide, which are relatively easy to manufacture and inexpensive, and is characterized by having a structure that compensates for each of the disadvantages.

Boron nitride reacts with glass similarly to aluminum nitride to generate nitrogen gas. Further, since the thermal expansion coefficient is as small as about 2 × 10 ⁻⁶ / ° C., it cannot be used as a substrate as it is. Here, as in the case of aluminum nitride, it is possible to solve the problem of reaction with glass and improve responsiveness by forming a mullite layer on the surface.

Since silicon carbide has a thermal conductivity about 14 times as large as that of alumina, the effect of improving responsiveness is great if it can be adopted. Practical problems include electric conductivity, low coefficient of thermal expansion, and heat treatment in the glass bonding process and the glass coating process due to carbides, and carbon dioxide is generated, and part of silicon carbide is decomposed. Oxidation.

Therefore, these problems can be solved by forming a mullite layer on the surface, and a resistor element having a high responsiveness can be manufactured.

In the above description, an example using mullite has been shown. However, similar effects can be obtained by using other oxide-based ceramics such as alumina, sapphire, zirconia, zircon, magnesia spinel, and steatite. .

FIG. 3 shows another embodiment of the present invention. That is, two barrel sputtered films each having a thickness of about 5 μm are formed on the surface of a pipe-shaped substrate 2 made of aluminum nitride, boron nitride, silicon carbide or the like.

For example, on the surface of an aluminum nitride pipe, a mullite layer 2a is formed as a first ceramic layer, and an alumina layer 2b is formed as a second ceramic layer. The thermal expansion coefficient of aluminum nitride is about 4.5 × 10 ^-6
/ ℃, Mullite has a coefficient of thermal expansion of about 5.3 × 10 ^-6 / ℃
The thermal expansion coefficient of alumina is about 7.8 × 10 ⁻⁶ / ° C. By gradually bringing the coefficient of thermal expansion closer to that of platinum, a structure in which the thermal stress is gradually relaxed can be obtained, so that the reliability can be improved. In particular, since boron nitride has a small coefficient of thermal expansion of about 2 × 10 ⁻⁶ / ° C., there is a factor in which reliability is uncertain if it is used alone. Therefore, the first layer 2a of zircon is first applied to the surface of the pipe-shaped substrate 2 made of boron nitride.
, And a second layer 2b of mullite is formed thereon to relieve the thermal stress, so that a great improvement in reliability can be expected.

FIG. 4 shows another application example of the present invention. This figure shows a structure using a platinum thin film 7 instead of the platinum resistor 6. The difference in structure from the resistance element 1 using the platinum resistor 6 is that, in addition to the platinum thin film 7, noble metal particles such as platinum or gold are included in order to establish electrical conduction between the resistor and the lead wire 4. However, the basic structure is the same, for example, there is a point that the bonding is performed using a glass paste. Therefore, also in this case, the response can be improved by forming an oxide ceramic such as mullite on the surface of a pipe such as aluminum nitride and forming the platinum thin film 7 on the surface.

The measuring element 1 manufactured in this manner has a characteristic that the thermal expansion coefficient can be adjusted, so that cracks do not occur even when a repetitive stress such as a thermal cycle is applied.

Further, since a material having high thermal conductivity is used for the base 2, an air flow measuring device with improved responsiveness can be provided.

FIG. 6 shows one experimental data. The response time is shortened compared to the response time of the conventional product.

FIG. 5 shows an example of application of the present invention to a plate-like element. Aluminum nitride was used for the substrate 2, a mullite film 2a was formed on the surface thereof, a platinum thin film 7 was formed thereon, and then a protective coating 3 was formed of glass. In this structure, the response time can be reduced for the same reason as described above.

[0040]

According to the present invention, it is possible to provide a highly responsive air flow measuring device having excellent responsiveness.

[Brief description of the drawings]

FIG. 1 is a sectional view of a measuring element according to an embodiment of the present invention.

FIG. 2 is a data table showing physical property values and characteristics of various ceramics.

FIG. 3 is a partial cross-sectional view of a measuring element showing an application example of one embodiment of the present invention.

FIG. 4 is a partial cross-sectional view of a measuring element showing an application example of one embodiment of the present invention.

FIG. 5 is a partial cross-sectional view of a measuring element showing an application example of one embodiment of the present invention.

FIG. 6 is an experimental data diagram of one embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Measurement element, 2, 2a, 2b ... Substrate, 3 ... Protective coating, 4 ... Lead, 5 ... Adhesive glass, 6 ... Platinum resistor, 7 ... Platinum thin film.

Claims (6)

[Claims] 1. A cylindrical ceramic substrate, a platinum resistor or a platinum-based resistor provided on the substrate, and adhered and fixed to the substrate with an adhesive to electrically connect to the resistor. A flow meter detecting element, which is constituted by including a lead to be urged and entirely covered with a protective coating layer, characterized in that the ceramic substrate is made of aluminum nitride covered with a mullite film. Measuring element of air flow measuring device. 2. A cylindrical ceramic substrate, a platinum resistor or a platinum-based resistor provided on the substrate, and adhered and fixed to the substrate with an adhesive to electrically connect to the resistor. A flow meter detecting element, which is configured to include a lead to be urged and is entirely covered with a protective coating layer, wherein the ceramic base is characterized by a structure using silicon carbide covered with a mullite film. Measuring element of air flow measuring device. 3. A cylindrical ceramic substrate, a platinum resistor or a platinum-based resistor provided on the substrate, and adhered and fixed to the substrate with an adhesive to electrically connect to the resistor. A flowmeter detecting element, which is configured to include a lead to be applied, and is entirely covered with a protective coating layer, wherein the ceramic substrate has a structure using zircon and boron nitride covered with a mullite film. Characteristic measuring element of air flow measuring device. 4. A cylindrical ceramic substrate, a platinum resistor or a platinum-based resistor provided on the substrate, and adhered and fixed to the substrate with an adhesive to electrically connect to the resistor. A flow meter detecting element, which is configured to include a lead to be applied and is entirely covered with a protective coating layer, wherein the ceramic substrate has a structure in which two or more different types of ceramics are stacked, At least the main component of the constituent material is a ceramic having a lower thermal conductivity than alumina, and the coefficient of thermal expansion of the ceramic is configured to increase as approaching the protective coating layer. Measuring element. 5. The air flow rate of the ceramic according to claim 4, wherein at least the constituent material contains aluminum nitride, silicon carbide, or boron nitride having a lower thermal conductivity than alumina as a main component. A measuring element of a measuring device. 6. A measuring element of an air flow measuring device, wherein the aluminum nitride, silicon carbide, or boron nitride according to claim 5 is arranged so as not to directly contact with a protective coating layer and a resistor. .