Patent Application
20130319494
The present disclosure relates to thermocouples, and more specifically to thermocouples with high temperature endurance and improved corrosion resistance.
The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
A thermocouple is known to include a hot junction formed by bonding a pair of conductive wires of dissimilar metals. The hot junction is placed proximate an object to be measured. The other end of the conductive wires, known as cold junction or reference junction, is connected to a measuring system. The thermocouple generates an open-circuit voltage, which is proportional to the temperature difference between the hot and reference junctions. The temperature at the hot junction can be determined based on the generated voltage and the temperature of the reference junction.
Thermocouples are widely used because they are inexpensive, interchangeable and can measure a wide range of temperatures. One of the limitations with thermocouples is that the hot junction is susceptible to thermal and physical damage. It is known to use a metal sheath to surround and protect the hot junction. The metal sheath, however, affects heat transfer from the object to be measured to the hot junction and thus contributes to errors in the temperature measurements. In the absence of the metal sheath, however, the thermocouple can be easily damaged when used in elevated temperatures or corrosive environment.
In one form, a thermocouple includes a first thermocouple wire defining a distal end portion, and a second thermocouple wire defining a distal end portion. A hot junction is formed between the distal end portions of the first and second thermocouple wires. The hot junction defines a splice such that the first thermocouple wire and the second thermocouple wire are in direct contact at their distal end portions. A refractory coating is applied over the hot junction.
In another form, a thermocouple includes a first thermocouple wire defining a distal end portion and a second thermocouple wire defining a distal end portion. The first and second thermocouple wires each include a material selected from the group consisting of platinum and platinum-rhodium alloys. A hot junction is formed by laser welding the distal end portions of the first and second thermocouple wires to each other. A refractory coating is applied over the hot junction. The refractory coating is selected from the group consisting of Al2O3 and SiO2.
In another form, a method of manufacturing a thermocouple includes: placing a distal end portion of a first thermocouple wire into physical contact with a distal end portion of a second thermocouple wire to form a splice; laser welding the splice to form a hot junction; and coating the hot junction with a refractory material.
Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.
The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.
In order that the invention may be well understood, there will now be described an embodiment thereof, given by way of example, reference being made to the accompanying drawing, in which:
Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.
The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses.
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The first thermocouple wire 22 and the second thermocouple wire 24 comprise a material selected from the group consisting of platinum and platinum-rhodium alloys. It is understood that the first and second thermocouple wires 22 and 24 include dissimilar metals. Therefore, when one of the first and second thermocouple wires 22 and 24 includes platinum, the other one of the first and second thermocouple wires 22 and 24 includes platinum-rhodium alloys.
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The thermocouple assembly 50 further includes a refractory coating 70 applied over the hot junction 30 or 42. The refractory coating 70 may include ceramic materials or oxides materials. For example, the refractory coating 70 may include a material selected from the group consisting of alumina (Al2O3) and silica (SiO2). The refractory coating 70 is applied over the entire hot junction 30 or 42 and over at least a section of the distal end portions 26 and 28 of the first and second thermocouple wires 22 and 24. The refractory coating 70 may be applied by a process selected from the group consisting of physical vapor deposition, chemical vapor deposition, plasma enhanced chemical vapor deposition, plasma spray, and thick film. The refractory coating 70 has a continuous thickness between approximately 50 microns and approximately 150 microns.
The refractory coating 70 acts as a protection barrier against severe corrosion caused by, for example, silicon vapors at temperatures above 1450° C. With the protection of the refractory coating 70, the hot junction 30 or 42 is corrosion resistant, has prolonged life, and can be used in high temperature furnaces that are used to produce silicon ingots for the photovoltaic or semiconductor industries. Moreover, the life of the thermocouple further depends on the densification of the refractory coating 70. Therefore, to further prolong the life of a thermocouple for use in Si vapor environment, the densification of ceramic powder of the refractory coating 70 is made greater than 95% theoretical density to eliminate open porosity.
In addition, the refractory coating 70 also increases the mechanical strength of the thermocouple wires that includes Pt. Noble metals such as Pt have relatively low elastic modulus and low creep resistance. The refractory coating 70 of ceramic materials or oxides has relatively high creep resistance at high temperatures. When the refractory coating 70 is applied on a section of the thermocouple wires 22 and 24 that include Pt, the refractory coating 70 may protect the thermocouple wires 22 and 24 against gravity exerting on Pt wire, thereby reducing likelihood of tensile failure.
The thermocouples 20 and 40 or the thermocouple assembly 50 of the present disclosure have high temperature endurance, improved corrosion resistance, and prolonged life. The hot junction, which is formed by a lap weld or a butt weld, has low residual stress. The low-residual stress allows the refractory coating 70 to maintain its integrity without cracking and/or flaking off due to stress release. With the protection of the refractory coating 70, platinum and platinum-rhodium alloys, which would otherwise more susceptible to thermal and physical damage, may be used to form the first and second thermocouple wires, 22 and 24. Platinum and platinum-rhodium alloys result in a clean weld, thereby further prolonging the life of the thermocouple.
Further, the refractory coating 40 is applied on the entire surface of the hot junction 30 and a section of the first and second thermocouple wires 22 and 24. The refractory materials with low porosities not only have relatively high thermal conductivity to conduct heat from the object to be measured to the hot junction, but also prevents Si vapor from the surrounding environment from reacting with Pt in the underlying thermocouple wires.
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The description of the disclosure is merely exemplary in nature and, thus, variations that do not depart from the substance of the disclosure are intended to be within the scope of the disclosure. Such variations are not to be regarded as a departure from the spirit and scope of the disclosure.
