Solid state potentiometric gaseous oxide sensor

Details Solid state potentiometric gaseous oxide sensor Solid state potentiometric gaseous oxide sensor

2001-09-28

2003-07-29

Argenbright, Tony M. (Department: 3747)

Internal-combustion engines

Charge forming device

Including exhaust gas condition responsive means

C204S426000, C204S429000, C205S781000

Reexamination Certificate

active

06598596

ABSTRACT:

FIELD OF THE INVENTION
The invention relates generally to measurement of components in a gas stream, and more particularly to an electrochemical apparatus and method for measuring the concentration of gaseous oxides in a gas mixture.
BACKGROUND
Various devices and methods have been described for determining the concentration of oxides of nitrogen (NO
x
, for example, N
2
O, NO and NO
2
), oxides of carbon (CO
x
for example, CO and CO
2
), oxides of sulfur (SO
x
for example, SO
2
and SO
3
), and other oxide compounds in a gas mixture. Such gases may include gaseous oxygen (O
2
), nitrogen (N
2
), other inert gases, as well as combustible gases such as H
2
and various hydrocarbons.
Most modern automobiles use an O
2
sensor which is disposed in the exhaust system together with an on board computer to control the amount of fuel injected for combustion. Usually, the computer only utilizes oxygen sensor data (“closed loop” mode) under cruise conditions to improve efficiency. The O
2
sensor outputs a voltage when the oxygen content of the exhaust gasses falls below the norm for the atmosphere. The voltage range is generally from 0 to 1 volt. The O
2
sensor is not sensitive to gases other than O
2
.
Oxygen in the air is consumed when fuel burns. Accordingly, increasing the amount of fuel for a given amount of air (a richer mixture) will deplete a greater part of the available oxygen. The O
2
sensor in the exhaust pipe responds to this condition by raising the output voltage. Thus, the O
2
sensor can help to maximize gas mileage and minimize the emission of pollutants. However, a typical O
2
sensor has poor sensitivity in the range needed for acceleration, where the typical air/fuel ratio used in most cars is 12.5:1. Conventional O
2
sensors are also sensitive to heat. Meaningful sensor output results only when exhaust temperatures are between approximately 360° C. and approximately 900° C.
The presence and concentration of gaseous oxide compounds have been measured using electrochemical sensing devices and methods which can generally be classified as either oxygen pumping sensors or potentiometric sensors. For example, U.S. Pat. No. 4,005,001 to Pebler, U.S. Pat. No. 4,770,760 to Noda et al., U.S. Pat. No. 4,927,517 to Mizutani et al., U.S. Pat. No. 4,950,380 to Kurosawa et al., U.S. Pat. No. 5,034,107 to Wang et al., and U.S. Pat. No. 5,034,112 to Murase et al and U.S. Pat. No. 5,217,588 to Wang disclose sensors for identifying presence and concentration of gaseous oxide compounds. Oxygen pumping sensors are amperometric sensors which “pump” O
2
through the cell at a rate proportional to electrical current induced in the pumping cell. However, most of the sensors referenced above are potentiometric sensors. Potentiometric sensors operate without “pumping” and generate a voltage rather than an output current.
For example, Wang discloses a sensor formed from two electrochemical cells on a zirconia electrolyte. One cell senses only oxygen gas and the other cell senses all the gases which contain oxygen, including the oxygen gas. Both electrochemical cells are exposed to the same gas mixture, and the differences between the sensed signals is a measure of the concentration of NO
x
in the gas mixture.
Murase et al. discloses a sensor in which a catalyst for reducing NO
x
is placed on an electrolyte adjacent to a pumping cell. A current is induced in the pumping cell to control the oxygen concentration in the environment around the pumping cell. When the oxygen concentration is depleted to a predetermined level, the catalyst supposedly begins to deplete NO
x
, and the oxygen concentration of NO
x
is determined by measuring the current supplied to the pumping cell.
While pumping type sensors can be used to pump O
2
from NO to form N
2
and O
2
, they cannot generally be used to pump O
2
from CO since C is not a gas and will deposit as a solid. Regarding potentiometric sensors such as the sensor disclosed by Wang, these sensors do not provide accurate measurement of CO or other oxide compounds in gas mixtures, because the electrodes used for the electrochemical cells are not sufficiently selective with respect to oxygen and oxide compounds, such as CO and NO. Moreover, if the gas mixture contains a relatively low oxide concentration compared with that of oxygen, an accurate determination of the oxide concentration is difficult. In exhaust gases or emissions produced by internal combustion engines or furnaces, the concentration of oxygen is typically much higher than the CO concentration. Thus, it is difficult to accurately measure the CO concentration in these gas mixtures using the typical pumping cell.
Another type of sensor described in U.S. Pat. No. 5,397,442 to Wachsman seeks to obviate this problem by providing a sensor including a chamber designed to receive a gas mixture in which two electrochemical cells are situated. Each cell is comprised of an electrode housed inside the chamber and an electrode outside the chamber, in which the internal and external electrodes are separated by an oxygen ion-conducting solid electrolyte. The first electrochemical cell is designed to consume oxygen by electrochemical reduction without appreciably consuming NO
x
, while the second electrochemical cell is relatively selective for the electrochemical reduction of NO
x
. A potential difference is applied across the first cell so that oxygen is removed from the chamber and then an electrical characteristic (voltage, current, power, etc.) of the second cell is measured that corresponds to the concentration of the oxide in the gas mixture. However, this system is somewhat complex and, because entry of gas into the chamber is diffusion limited, the response time of the sensor can be relatively slow.
SUMMARY
A solid state electrochemical cell for measuring the concentration of a component of a gas mixture includes a first semiconductor electrode and a second semiconductor electrode, the electrodes comprising first and second semiconductor materials, respectively. The electrode materials are selected so as to undergo a change in resistivity upon contacting the component. A change in resistivity of the electrode materials results in a change in voltage across the electrochemical cell. An electrolyte is disposed in contact with the first and second semiconductor electrodes. The electrochemical cell can include a reference electrode in contact with the electrolyte.
At least one metal layer can be disposed on a portion of the semiconductor electrodes. The electrochemical cell can also include a detector for measuring an electrical characteristic generated by the electrochemical cell.
The semiconductor materials can include a metal oxide. The metal oxide is preferably SnO
2
, TiO
2
, TYPd
5
, MoO
3
, ZnMoO
4
or WR3, where TYPd
5
and WR
3
are acronyms defined below. The acronym TYPd
5
is used herein to represent a composite prepared by selecting TiO
2
(titania), Y
2
O
3
(yttria) and Pd in a weight ratio of approximately 85:10:5. Anatase titania is mixed with yttria and Pd metal powder in the composition described above. The powder is then applied onto the solid electrolyte in a slurry, and then sintered at approximately 650° C. for 1 hr.
The acronym WR
3
will be used herein to represent a composite which can be formed from the decomposition of Rh
2
WO
6
at temperatures above approximately 1130° C. into WO
3
and metallic Rh. Oxygen is liberated in the decomposition reaction leaving a mixture of WO
3
and 2Rh.
By selecting a first semiconductor material that exhibits a voltage response opposite in slope direction, the response being a function of detected gas concentration, to that of the second semiconductor material, the resulting voltage signal measured across the electrodes is substantially equal to the sum of the absolute values of individual voltage responses of the electrodes. The gas component measured can include CO.
The electrolyte is preferably an oxygen ion-conducting electrolyte. The oxygen ion-conducting electrolyte can be based on ZrO
2
, Bi
2
O
3
or CeO
2
. Preferred oxyge

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