Using Exotic Thermocouple Probes for use in Extreme Temperatures such as Furnaces

Thermocouple Elements

• Tungsten/Rhenium
• Platinum/Rhodium

XMO Sheath Materials

Insulation Materials

OMEGA^® Exotic Thermocouple Probes are designed for use in extreme temperatures, up to 2315 °C. Either Tungsten/Rhenium (types G, C, or D) or Platinum/Rhodium (types R, S, or B) elements are utilized by these probes, together with a variety of insulations and sheath materials. These probes can be used in inert, oxidizing, reducing or vacuum conditions based on the sheath material selected. The maximum temperature is based on the lowest maximum temperature of the insulation, element and sheath material. Five cold end probe terminations are available: transition joint with 2 m long lead wire, molybdenum-sheathed nylon connector, heavy-duty standard size, subminiature or standard size ceramic connector or stripped ends.

Furnace Temperature Measurement in Oxidizing Atmospheres

Measuring temperatures inside a furnace can present a number of challenges: temperature cycling, high temperatures and hostile atmospheres exceeding the limits of many measurement devices, while others have immensely reduced lifetimes and poor accuracy. This article focuses on two specific challenges related to temperature measurement in furnaces: types of oxidizing and reducing atmospheres in furnaces used in microelectronics fabrication.

Furnaces Overview

The need to heat is common to many manufacturing processes. Adhesives and rubbers are cured, metals are annealed to modify their properties and metallurgy, coatings are dried, metals are melted and ceramics are vitrified or fired. Most of these processes are performed in ovens, heated either by gas or electricity. An oven that can heat to above 1000 °C (1832 °F) is called a furnace. A kiln is a specific type of furnace used in ceramics. At high temperatures, many materials begin to react with the surrounding atmosphere. If that atmosphere is extremely short of oxygen, it may pull oxygen from the material being heated. Such an atmosphere is called “reducing”. Gas heating generally results in an oxygen-deficient atmosphere. The material being heated will capture a proportion, forming an oxide layer, if the atmosphere is oxygen-rich. Such an atmosphere is termed “oxidizing.” This is the process utilized in diffusion furnaces, used in microelectronics fabrication to produce SiO₂. Electrical heating may produce an oxidizing atmosphere. A number of ways can be used to control the atmosphere. Gas may be piped into the chamber, which might be done to produce an inert atmosphere. Alternatively, a vacuum furnace could be used.

High Temperature Measurements Options

XTA, XMO, XPA, XIN Series

The upper limit for thermistor devices is around 100 °C (212 °F) and RTDs are limited to around 750 °C (1382 °F). That leaves thermocouples and infrared pyrometers or imagers as the most appropriate devices for measuring temperatures above 1000 °C (1832 °F).

Thermocouples

Thermocouples produce a signal proportional to temperature by utilizing the Seebeck effect (the difference in EMF between dissimilar metals). Nickelalumel and nickel-chromium are the metal pairs most commonly used in what is called the “Type K” thermocouple.

The Type K is inexpensive and can be used across a temperature range from -200 to 1250 °C (-328 to 2282 °F). However, metallurgical changes at temperatures above 1000 °C (1832 °F) decrease accuracy, and cycling through this temperature induces hysteresis effects, further reducing accuracy. Type K thermocouples are also susceptible to corrosion in an oxidizing atmosphere.

Thermocouples can fail in-service or be damaged, requiring replacement. If this entails shutting down and cooling a continuous furnace, it can be a difficult and expensive task. For this reason, redundant thermocouples are mostly included throughout the heating chamber.

IR Pyrometry

Infrared (IR) pyrometry presents an easy contactless method of measuring high temperatures. This technology takes advantage Planks Law, whereby the intensity and wavelength of the IR radiation produced from a surface is proportional to its temperature. This radiation is detected by a pyrometer or thermal imager, converting the signal to a temperature.

OS530E-DM E Series

IR pyrometry functions well when the surface of the hot material is exposed, as with molten metal in a ladle. Using it to measure temperatures within a furnace is considered to be more complex, as it needs to be viewed through a window. This window must be capable of transmitting IR radiation of the wavelength corresponding to both the temperature being measured and the sensitivity of the detector.

Regular glass is opaque to some IR wavelengths, especially between six and seven microns. Chalcogenide glass is particularly manufactured for IR transmission applications but is limited to temperatures below 370 °C (698 °F). An alternative window material is sapphire, which has the potential to transmit wavelengths up to four microns but is relatively soft and easily damaged. A sapphire IR window should be designed without any projections as these would make it vulnerable to damage when it is used as a viewing port. Sapphire also has a temperature limit of around 450 °C (842 °F), and hence it is not suitable for measuring furnace temperature.

Emissivity is always an issue with pryometry: different intensities of IR radiation are radiated by different materials at the same temperature and the sensor must be calibrated for this. The window will have an effect on the radiation transmitted.

High Temperature Thermocouples

Two families of thermocouples are available, those using platinum-rhodium and those using tungsten-rhenium junctions. The tungsten-rhenium thermocouples, (Types G, C and D) work at temperatures as high as 2320 °C (4208 °F) but will not withstand an oxidizing atmosphere.

For oxidizing atmospheres, platinum-rhodium thermocouples, sometimes known as “noble metal thermocouples,” should be selected. These are available as Type R, [maximum of 1460 °C (2660 °F)] S, [maximum of 1450 °C (2642 °F)] or B, [maximum of 1700 °C (3092 °F)]. They are more expensive when compared to base metal thermocouples.

Sheaths of Thermocouples

It is common to protect thermocouple wires by placing them within a protective tube or sheath based on the installation. Stainless steel is extensively used as it resists corrosion and is inexpensive. However, it has limiting service temperature to under 1100 °C (2012 °F), a melting point of around 1400 °C (2552 °F) and reacts with oxidizing atmospheres.

Tantalum or molybdenum sheaths should be used for highest temperature capabilities. These will go up to 2315 °C (4199 °F) and 2200 °C (3992 °F) respectively, although both are sensitive to oxidation, and hence should not be used in oxidizing atmospheres. The alternatives are Inconel^® 600, which goes up to 1150 °C (2102 °F), platinum-rhodium alloy sheaths, which will survive 1650 °C (3002 °F), or ceramic sheaths, which will withstand up to 1960 °C (3560 °F). All of these are capable of handling oxidizing atmospheres.

Sheath Materials

*Refractory metals are extremely sensitive to any trace of oxygen above approximately 260 °C (500 °F).

Thermocouple Insulation

XC, XC4, and XS Insulation

Insulation is integrated into a thermocouple sheath to keep the wires from contacting the sides. It is essential for this insulation to have a temperature rating suitable for the environment. Hafnium oxide, magnesia and alumina are common materials for furnace temperatures. Alumina has a maximum temperature rating of 1540 °C (2804 °F), while magnesia and hafnium oxide will go to 1650 °C (3002 °F).

Takeaways

Thermocouples are considered to be a good option for measuring temperatures inside furnaces. While the widely-used “Type K” thermocouples are capable of handling furnace temperatures, Types G, C and D and R, S and B offer better performance. The type of atmosphere employed at furnace temperatures is an important consideration. In particular, an oxidizing atmosphere, as used in microelectronics fabrication, will promote a reaction with both Types G, C and D and the stainless steel sheaths often employed.

IR pyrometry is an alternative for measuring high temperatures, but needs a window or viewport to measure inside a furnace. For this reason, it is generally preferred when there is an uninterrupted line-of-sight.

This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.

For more information on this source, please visit OMEGA Engineering Ltd.