Hydrocarbon sensor

Details Hydrocarbon sensor Hydrocarbon sensor

1999-04-05

2001-05-29

Tung, T. (Department: 1741)

Chemistry: electrical and wave energy

Apparatus

Electrolytic

C204S425000, C204S426000, C204S292000, C204S293000, C205S784500, C205S787000

Reexamination Certificate

active

06238535

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a hydrocarbon sensor formed of a solid electrolyte for detecting hydrocarbon and for measuring the concentration thereof in an atmosphere in the temperature range of about 300° C. to 800° C.
PRIOR ART
Hydrocarbon sensors can be used to detect hydrocarbons in living environments and hydrocarbons included in exhaust discharged from vehicle engines, combustion heaters and catalytic converters, and to measure the concentration thereof. In particular, hydrocarbon sensors are used as lean-burn controlling sensors for vehicle engines and combustion apparatuses. As a conventional device for measuring or detecting hydrocarbons, a solid-electrolytic type sensor is known.
Since the solid-electrolytic hydrocarbon sensor is used in a high-temperature atmosphere, for example, in exhaust gas from combustion engines and the like, a proton based on an oxide capable of functioning at room temperature or more is used for the sensor. As the solid-electrolytic sensor, an electromotive-force type sensor and a limiting-current type sensor are known. For these sensors, a Ca—Zr-based oxide having a composition of CaZr
0.9
In
0.1
O
3−&agr;
has been developed as an oxide-based proton conductor, and an attempt is made to apply the oxide to hydrocarbon sensors.
For example, Proceedings of the 61st Conference (1994) pp99 of Electrochemical Society of Japan by Hibino, Tanaki and Iwahara have disclosed an electromotive-force type hydration sensor comprising Pd—Au alloyed electrodes as a hydrocarbon sensor formed of a Ca—Zr-based oxide electrolyte.
Furthermore, Inaba, Takahashi, Saji, Shiooka; Proceedings of the 21st Conference in 1995 Japan Association of Chemical Sensors pp145 have disclosed a limiting-current type hydrocarbon sensor having porous alumina as a diffusion rate determining layer.
However, the Ca—Zr-based oxide used as a solid electrolyte has a low proton conductivity of about 5×10
−4
S/cm at 600° C. In order to raise the sensitivity of the sensor, the operation temperature must be set at a high temperature of 700° C. or more in the case of the EMF type, or the electrolyte must be lowered in thickness to a thin layer in the case of the limiting-current type. Otherwise, it is difficult to use the sensor. For these reasons, solid-electrolytic materials having higher proton conductivity have been demanded.
Problems are also caused with respect to the detection mechanism and structure of the sensor formed of the Ca—Zr-based oxide. The EMF-type sensor cannot accurately detect hydrocarbons in an atmosphere where no oxygen is present or the concentration of oxygen changes significantly, since the sensor uses the catalytic function of electrodes. The limiting-current type sensor has difficulty in setting the electrolytic voltage of hydrocarbon, although the sensor uses porous alumina for its diffusion rate determining layer.
Accordingly, the inventors of the present invention have proposed a limiting-current type (or constant potential electrolytic type) hydrocarbon sensor formed of a Ba—Ce-based oxide having high proton conductivity (Japanese Laid-open Patent Publication No. 10-300718.
FIG. 19
shows a conventionally typical hydrocarbon sensor of limiting current type, which comprises a thin solid electrolyte layer
3
having high proton conductivity, two electrodes, anode
2
and cathode
4
, attached on the both main surfaces of the solid electrolyte layer
3
and a diffusion rate determining layer formed on the side of the anode on the solid electrolytic layer. In this case, the diffusion rate determining layer has a substrate
1
formed a space (i.e., anode chamber
20
) over the anode
2
and a through hole
61
between the substrate
1
and the electrolyte layer
3
. The through hole
61
transfers gas components containing hydrocarbon from a atmosphere to be measured to the anode chamber
20
by diffusion when the sensor is disposed in a atmosphere to be measured to determine the hydrocarbon therein. In use of such a sensor, current between the electrodes, under application of a constant voltage, is measured in a process of transferring protons, which are dissociated catalytically on the anode from hydrocarbon, through the solid electrolyte layer to the cathode, then obtaining concentration of hydrocarbon in the atmosphere.
This sensor using Ba—Ce-based oxide layer as a solid electrolyte layer
3
satisfactorily responds to hydrocarbon and in the absence of oxygen in the atmosphere to be measured, can linearly detect hydrocarbon in a wide range of the order of several ppm to the order of several percents.
However, in the case where the concentration of hydrocarbon is very low (10 ppm or less) and an oxygen-free condition is changed to an oxygen-mixed condition, the sensor causes a phenomenon in which its output current between its electrodes through the electrolyte increases. This is because the Ba—Ce-based oxide used as an electrolyte has a characteristic of conducting oxide ions, whereby oxygen in an atmosphere can dissociate and transfer the electrolyte to the anode.
In order that hydrocarbon sensors are used to detect hydrocarbon in living environments and to measure the concentration of hydrocarbon in combustion exhaust discharged from vehicle engines and combustion apparatuses, such as heaters and the like, the hydrocarbon sensors requires high selectivity for hydrocarbon without being affected by the concentration of oxygen, even in such atmospheres including oxygen, and also requires high sensitivity and reliability by virtue of the high selectivity. Furthermore, hydrocarbon sensors must be directly disposed in atmospheres to be measured in many cases. Therefore, hydrocarbon sensors are desired to be compact, easy to use and low in production cost.
In a conventional limiting-current type hydrocarbon sensor, a Ba—Ce-based oxide having high proton conductivity is used for a thin electrolytic layer, a pair of electrodes is formed on both sides of the electrolyte, one electrode on each side, so as to be opposite to each other. The pair of electrodes is usually made of platinum.
In the case where this type of hydrocarbon sensor is used in such an environment including oxygen as described above, the sensor satisfactorily responds to hydrocarbon in the atmosphere. However, at the same time, its output changes owing to the existence of oxygen in the atmosphere, in particular, owing to the concentration of the oxygen, thereby causing large errors in the actually measured values. This phenomenon occurs as described below. Oxygen is taken in from the atmosphere on the cathode side, and ionized by the cathode made of platinum. Oxide ions thus generated at the cathode may pass through such a thin electrolytic layer, thereby causing current to flow across the two electrodes.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a hydrocarbon sensor, formed of a solid electrolyte having high proton conductivity and also exhibiting conductivity of oxide ions, which is capable of highly accurately detecting only hydrocarbon included in an atmosphere to be measured without being affected by the existence of oxygen in the atmosphere.
Another object of the present invention is to provide a hydrocarbon sensor having high sensitivity, more particularly, a hydrocarbon sensor being insensitive to oxygen at the cathode on the solid electrolyte thereof and highly sensitive to only hydrocarbon at the anode.
First, generally speaking, the present invention provides a hydrocarbon sensor being insensitive to the concentration of oxygen in an atmosphere by using its cathode made of an electrode material being inactive with the oxygen in the atmosphere, instead of a conventional electrode made of platinum. The electrode material is selected to prevent the generation of oxygen ions on the surface of the electrode, thereby to prevent the entry of oxygen into the solid electrolyte.
The present invention thus provides a hydrocarbon sensor being insensitive to the concentration of oxygen in an atmosphere by providing a structur

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