Patent Application
20130161311
Some types of high temperature gas sensors include a ceramic substrate element plated with a metal electrical conductor (typically platinum) as a high temperature heat source. These sensors are heated by the electrical conductor to a temperature higher than the temperature of the gas in the conduit, and the heated area of the sensor is exposed to the gas flow. These sensors, including oxygen, NOx, mass flow and specialty sensors, are used, for example, in the intake and exhaust conduits of heavy duty Diesel engines.
In internal combustion engine intake and exhaust conduits, liquids may be unintentionally present in the gas stream. For example, water from condensed humidity may be present during start-up or because of problems with the coolant, engine fuel because of fuel injector problems, engine oil because of engine seal or turbocharger issues, or coolant from cylinder sealing or exhaust gas recirculation cooler issues.
In current Diesel exhaust gas treatment systems, liquids are intentionally injected into the exhaust systems in areas where, as part of the treatment systems, high temperature gas sensors are used. In many cases liquids are injected periodically during regular use of the engine, for example, urea injection for a NOx reducing Selective Catalytic Reduction system, and hydrocarbon fuel for heating Diesel Particulate Filter systems,
A problem can occur with heated ceramic substrate elements if liquid is present in the gas stream. When a drop of liquid contacts the heated element, the ceramic substrate at the contact area is immediately cooled to the temperature of the liquid, and the temperature differential causes a stress in the material. Ceramic material is crystalline and brittle, and the temperature differential can cause the ceramic to crack, which eventually leads to failure of the substrate element and heating conductor.
For example, the heated substrate element may be operational at 700° C. If a drop of water, which is a liquid to 100° C., strikes the element, a localized area of the element is cooled to 100° C. A temperature differential across the boundary of the liquid contact area is about 600° C., creating temperature differential stresses in the ceramic substrate.
Typically, the control system for the heated substrate sensor senses the reduction of temperature and responds by supplying more power to the heater. However, the application of additional power adds to the imbalance of the cool spot to the rest of the heater temperature, which amplifies the differential temperature stress.
A crack in the ceramic substrate can produce strain in the metal electrical heater conductor. The portion of the conductor under strain may experience an increase in the electrical resistance, which can result in a temperature hot spot in the heater conductor, which can cause the metal to melt and the heater circuit to open.
Accordingly, solving the problem of liquid contact heat stress failures in sensors is important.
One approach to solving this problem is software control of the heated sensor. U.S. Pat. No. 7,084,378 discloses an algorithm for control of the heating cycle to prevent sensor body failures. However, software can respond only after the temperature change is detected.
The invention is an improvement in sensors that include a ceramic heater substrate plated with a metal (platinum) conductor.
The invention proposes providing, by plating or otherwise, a passive heat conductive layer on the opposite side of the high temperature heater on a ceramic substrate. The heat conductive layer, preferably a metallic material, is a better conductor of thermal energy than ceramic. The heat conducting layer is believed to act as a thermal damper by absorbing and distributing thermal energy, preventing the overly rapid heating of the ceramic substrate that causes fracture of the substrate. By “passive” is meant the metal layer has no electrical connection and no heat source or sink connections other than contact with the ceramic substrate.
The passive metal layer is also believed to lessen the cooling effect of moisture or liquid contacting the sensor substrate by transferring heat over a larger area of the substrate to the liquid contact area.
The passive metal layer is believed to distribute heat longitudinally of the ceramic substrate, which helps prevent site overheating as a failure mode.
According to one embodiment of the invention, the ceramic substrate is formed with a tip on which the heating element is disposed, the tip being a narrower, smaller cross section region, and a base, being a wider, larger cross section region. The plating extends from the tip onto a portion of the base, which facilitates distributing heat energy from a smaller mass area to a larger mass area of the substrate.
The metal layer is also believed to act to mechanically reinforce the ceramic substrate, maintaining integrity and minimizing the flexure of the substrate.
The passive metal coating may be formed with a curved or wavy end edge line on the base end to increase the effective heating distance in the lateral direction of the substrate.
The invention may also be embodied in a cylindrical substrate. According to one embodiment, the cylindrical substrate is a solid cylinder of ceramic material with a heating element provided on the surface at an end portion and a passive metal coating provided on an opposite side of the same end portion.
According to another embodiment, a sensor substrate is formed as a hollow cylinder with a heating element formed or deposited on an outer surface at an end portion and a passive metal coating provided on an interior surface opposite the heating element location. Alternatively the passive metal coating may comprise a metal core in the hollow cylinder.
The invention will be better understood by reference to the following Detailed Description read in conjunction with the appended drawings, in which:
A heated substrate element for a gas sensor according to a first embodiment of the invention is shown in
A heater 20 is formed on the first surface. The heater 20 may be a resistive film element, such as a platinum layer disposed on the first surface 12 by any convenient means such as deposition and etching or printing, for example. The heater 20 includes leads 22, 24 having terminals 26, 28, respectively, for connecting to a power source. The heater 20 includes a heating element 30, shown as a serpentine portion, formed at the tip 18.
The tip 18 and heating coil 30 are exposed to the gas when the heated substrate element 10 is in use.
A heat conducting layer 40 is formed on the second surface 14 at the tip 18. The heat conducting layer 40 is opposite the heating element 30, meaning it is located a shortest distance through the element from the heating element. The heat conducting layer 40 is formed of a material having high heat transfer properties, preferably a metal. The heating conducting layer 40 extends on the second surface 14 to cover at least the tip 18 and a portion of the base 16.
As known in the art, ceramic is a low heat conducting material and a ceramic substrate provides a thermal mass that can maintain a steady temperature. The heat conducting layer 40 of the invention provides a relatively high heat transfer layer that can quickly distribute heat across the ceramic substrate in the contact area. The heat conducting layer 40 is passive, that is, it is not connected to an external heat source or heat sink.
According to the invention, the heat conducting layer 40 extends onto the base 16 of the substrate element 10 to provide heat conduction between the tip 18 and the base. An edge of the heat conducting layer 40 at the base 16 is non-linear, that is, is formed as a wavy or curved line to provide an extended distance across the base.
In testing, a heated ceramic substrate sensor according to the invention experienced a 444% improvement in sensor service life when compared to a substrate without a heat conducting layer.
According to an alternative embodiment illustrated in
An additional alternative embodiment is shown in
The embodiments of
The invention is described in terms of illustrative embodiments and components, however, those skilled in the art will recognize that substitutions may be made without departing from the scope of the invention as defined by the appended claims.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US11/55921 | 10/12/2011 | WO | 00 | 3/1/2013 |
| Number | Date | Country | |
|---|---|---|---|
| 61392080 | Oct 2010 | US |
