JPH04340452A - Gas sensor and manufacture thereof - Google Patents

Abstract

PURPOSE:To provide a gas sensor excellent in the stability of a heater resistance value by forming the shape within the main surface of the substrate of a heater so as to have a parallelogram or quadrilateral and applying a voltage pulse to the heater to adjust the resistance value thereof. CONSTITUTION:A heater is printed on an alumina substrate by screen printing in a parallelogram or quadrilateral and prepared by baking paste based on ruthenium oxide. At this time, the paste adjusted so that the resistance value of the heater 21 becomes higher than a resistance value for obtaining necessary temp. is used because the resistance value of the heater is lowered when high pulse like voltage is applied. The prepared heater 21 is connected to a voltage applying machine 22 through a switch and high pulse like voltage is applied to the heater. After the application of the voltage, the resistance value of the heater 21 is measured by a digital multimeter 23 through a switch 25. The applied voltage is 250-400V and the pulse width thereof is 5-10mus and, when the resistance value of the heater 21 does not reach a predetermined value by one application, the voltage is repeatedly applied.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas sensor used in a gas leak alarm and a method for manufacturing the same, and more particularly to a heater for a gas sensor and a method for adjusting its resistance value.

0002

[Prior Art] When a gas sensor using a metal oxide semiconductor as a gas sensitive body is used in a general household gas leak alarm, it detects methane, hydrogen, and isobutane, which are the target gases, and activates the gas leak alarm. In order to obtain the necessary output, the gas sensor must be constantly heated to 350-400°C.

FIG. 4 is a plan view showing a conventional thick film gas sensor. A ruthenium oxide paste is printed as a heater 11 on an alumina substrate 12 using a screen printing method, and after firing, the resistance value is adjusted to 350 to 40 by a predetermined voltage.
A temperature of 0°C is set. 13 and 14 are heater electrodes, and 15 and 16 are lead wires.

Conventionally, the resistance value has been adjusted by a so-called laser trimming method in which the heater is irradiated with a laser beam to evaporate a portion of the heater and make a cut 17 to increase the resistance value. .

[0005]

[Problems to be Solved by the Invention] However, in heaters based on the conventional laser trimming method, when the current is turned on for a long period of time, the resistance value gradually decreases, the heater power increases, and the temperature of the gas sensor increases. There was a problem.

[0006] The reason why the resistance value of the heater changes while it is being energized is not yet clear, but the cut is made because laser irradiation makes a cut in the resistor and changes the shape of the resistor. It is presumed that current concentration occurs around the area, causing local temperatures to rise and accelerating thermal deterioration.

Therefore, without changing the shape of the resistor,
If the resistance value can be adjusted, the problem of the resistance value changing during energized use, increasing the heater current, and increasing the temperature of the gas sensor can be solved.

SUMMARY OF THE INVENTION An object of the present invention is to provide a gas sensor and a method for manufacturing the same, in which the resistance value does not change over a long period of time and has excellent stability by adjusting the resistance value without changing the planar shape of the resistor.

[0009]

[Means for Solving the Problems] According to a first embodiment of the present invention, the above-mentioned object has a heater and two electrodes on a substrate,
The heater is a thick film of ruthenium oxide, which is laminated in contact with two electrodes, in which case the two sides that define the outer shape within the main surface of the substrate between the electrodes are parallel to each other, and the two opposing sides of the electrodes are parallel to each other. This is achieved by configuring them so that they are parallel to each other.

According to a second aspect of the present invention, there is provided a first step and a second step. The second step is achieved by applying a voltage pulse to the heater to adjust the heater resistance value.

[0011]

[Operation] Since the two opposing sides of the heater are parallel, the current density within the heater becomes uniform. Since the heater resistance value is adjusted by applying a voltage pulse to the heater, the resistance value can be adjusted without introducing a notch into the heater and without changing the shape of the heater.

0012

Embodiments Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a plan view showing a gas sensor according to an embodiment of the invention. In this figure, 1 is a heater whose main component is ruthenium oxide, and 2 is a square heater with a side of 2.5 mm and a thickness of 0.
5mm alumina substrate, 3 and 4 are platinum heater electrodes,
5 and 6 are lead wires.

FIG. 2 is a layout diagram showing a device for applying high voltage pulses to the heater. 21 is a heater whose resistance value is adjusted; 22 is an applied voltage machine that generates a pulsed high voltage;
23 is a digital multimeter for measuring the resistance value of the heater, and 24 and 25 are switches.

The applied voltage machine, digital multimeter, and switch are all controlled by computer 26. If the resistance value of the heater does not reach the predetermined value with one application of pulsed high voltage, repeat the same operation two or three times.

The applied voltage and pulse width of the pulsed high voltage are as follows:
With one pulse application, the resistance value changes by 0.5 to 1.
It is selected to be 5%. The voltage is 250~4
00 V, pulse width is 5-10 μs. If the change is small, it will take time to make the adjustment, and if the change is too large, it will be difficult to keep the resistance to a predetermined value.

The following procedure is used to adjust the resistance value of the heater by applying a high voltage in a pulsed manner. First, a heater paste containing ruthenium oxide as a main component, printed on an alumina substrate by screen printing, is fired at 850° C. for 15 minutes. At this time, a paste is used in which the resistance value of the heater is adjusted to be higher than the resistance value for obtaining the required temperature. This is because the resistance value of a heating resistor mainly composed of ruthenium oxide decreases when a pulsed high voltage is applied.

The heater 21, which has a resistance value higher than that required to obtain the required temperature, is connected to a voltage applying device 22 via a switch 24, and a pulsed high voltage is applied thereto. After applying the pulsed high voltage, the resistance value of the heater is measured with a digital multimeter via the switch 25.

FIG. 3 is a diagram showing the change over time in the heater power of the gas sensor according to the embodiment of the present invention in comparison with a conventional gas sensor. Each heater was manufactured as follows. First, in the heater according to the present invention, a ruthenium oxide paste having a heater resistance value of 22 kΩ after firing was printed and fired. Then, the voltage was 400 V,
A pulsed high voltage of 2 μs was applied, and the resistance value was adjusted to room temperature (
(25°C) and adjusted to 20 kΩ. The heater using the conventional laser trimming method has a heater resistance value of 18k after firing.
After the ruthenium oxide paste of Ω was printed and fired, the resistance value was adjusted to 20 kΩ by a laser trimming method.

When AC 100 V is applied to each heater, the heater power is 0.52 W. Heater manufactured by conventional laser trimming method (curve 32)
shows a tendency that the heater power increases as the energization period becomes longer. However, the heater made according to this invention (curve 31)
The heater power is constant and stable even if the energization period becomes long. The change in heater power is due to the change in resistance value of the heater, and the heater manufactured according to this invention has the following characteristics:
This shows that the resistance value does not change even when energized for a long period of time, indicating that it is stable.

[0020]

[Effects of the Invention] According to the present invention, a heater and two electrodes are provided on the substrate, and the heater is a thick film of ruthenium oxide.
Two electrodes are laminated in contact with each other, and in this case, the two sides that define the outer shape of the main surface of the substrate between the electrodes are parallel to each other, and the two opposite sides of the electrodes are parallel to each other, so the current of the heater is A gas sensor with uniform distribution, constant heater resistance, stable heater temperature, and excellent reliability can be obtained.

According to another invention, there are a first step and a second step. This is the process of installing a heater in contact with the two electrodes, and the second process is to adjust the heater resistance by applying a voltage pulse to the heater, so the heater resistance can be adjusted without introducing a cut into the heater. It is possible to prepare a heater that can be adjusted and has good uniformity of current distribution, the heater temperature is stabilized, and a gas sensor with excellent reliability can be obtained.

[Brief explanation of drawings]

FIG. 1 is a plan view showing a gas sensor according to an embodiment of the present invention.

[Figure 2] Layout diagram showing a device that applies high voltage pulses to the heater

FIG. 3 is a diagram showing the change in heater power over time of the gas sensor according to the embodiment of the present invention in comparison with that of a conventional gas sensor.

[Figure 4] Plan view showing a conventional thick film gas sensor

[Explanation of symbols]

1 Heater 2 Alumina substrate 3 Electrode 4 Electrode 5 Lead wire 6 Lead wire 21 Heater 22 Voltage application machine 23 Digital multimeter 24 Switch 25 Switch 26 Computer

Claims (3)

[Claims] Claim 1: A heater and two electrodes are provided on a substrate, the heater is a thick film of ruthenium oxide, and is laminated in contact with the two electrodes, defining the outer shape within the main surface of the substrate between the electrodes. A gas sensor characterized in that two sides are parallel to each other, and the electrodes have two opposite sides parallel to each other. 2. A first step and a second step, the first step is to form two electrodes on a substrate at a distance, and then a thick film heater made of ruthenium oxide is formed between the two electrodes. A method for manufacturing a gas sensor, characterized in that the second step is to apply a voltage pulse to the heater to adjust the heater resistance value. 3. In the manufacturing method according to claim 2, the voltage is 2
50 to 400 V, pulse width 5 to 10μ
A method for manufacturing a gas sensor, characterized in that: s.