Measuring Furnace Temperatures in Oxidizing Atmospheres

It is not easy to choose a temperature sensor: Thermistor, thermocouple or RTD? But measuring temperatures inside a furnace can put forth a number of challenges: temperature cycling, high temperatures and hostile atmospheres exceeding the limits of several measurement devices while others have significantly reduced lifetimes and poor accuracy. This article approaches mainly two particular challenges associated with temperature measurement in furnaces: types of oxidizing and reducing atmospheres in furnaces employed in microelectronics fabrication.

Furnaces Overview

The need to heat is common to a number of manufacturing processes. Adhesives and rubbers are cured, metals are annealed in order to adjust their properties and metallurgy, coatings are dried, metals are melted, and ceramics are vitrified or fired. Many 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 known as a furnace. A kiln is a specific type of furnace employed in ceramics. At high temperatures a number of materials begin to react with the surrounding atmosphere.

If that atmosphere is extremely short of oxygen, it may pull oxygen from the material that is being heated. Such an atmosphere is called “reducing”. Generally, gas heating 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 called “oxidizing.” This is the process executed in diffusion furnaces used in microelectronics fabrication in order to produce SiO₂. Electrical heating is more likely to generate an oxidizing atmosphere. Control of the atmosphere can be achieved in several ways. Gas may be piped into the chamber, which could be done to develop an inert atmosphere. Alternatively, a vacuum furnace could be used.

High Temperature Measurement 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 ideal devices for measuring temperatures more than 1000 °C (1832 °F).

Thermocouples

Thermocouples utilize the Seebeck effect, which is the difference in EMF between dissimilar metals, in order to produce a signal proportional to temperature. Nickelalumel and nickel-chromium are the metal pairs most frequently used in what is called the “Type K” thermocouple.

The Type K is low-priced and can be used across a temperature range from -200 to 1250 °C (-328 to 2282 °F). However, metallurgical changes at temperatures more than 1000 °C (1832 °F) decrease accuracy, and cycling via this temperature induces hysteresis effects, further reducing accuracy. Type K thermocouples are also vulnerable 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 an expensive and difficult undertaking. For this reason, it is common to include redundant thermocouples throughout the heating chamber.

IR Pyrometry

OS530E-DM E Series

A convenient contactless method of measuring high temperatures is presented by Infrared (IR) pyrometry. This technology takes advantage of Planks Law, whereby the intensity and wavelength of the IR radiation emitted from a surface is proportional to its temperature. A thermal imager or pyrometer detects this radiation, changing the signal to a temperature.

IR pyrometry functions well when the surface of the hot material is exposed, as in the case of molten metal in a ladle. Using it to measure temperatures inside a furnace is more complex, as it will have to be viewed through a window. It is necessary for this window to transmit 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, mainly between six and seven microns. Chalcogenide glass is developed particularly for IR transmission applications, but is limited to temperatures below around 370 °C (698 °F). Sapphire is considered to be an alternative window material capable of transmitting wavelengths up to four microns but is moderately soft and easily damaged.

A sapphire IR window should be designed without any projections that 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), making it unsuitable for furnace temperature measurement.

Emissivity is always considered to be an issue with pryometry: varied materials at the same temperature radiate different intensities of IR radiation and it is necessary for the sensor to be calibrated for this. The window will have an influence on the radiation transmitted.

High Temperature Thermocouples

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

Platinum-rhodium thermocouples, at times known as “noble metal thermocouples,” should be selected for oxidizing atmospheres. These are available as S, [maximum of 1450 °C (2642 °F)], Type R, [maximum of 1460 °C (2660 °F)] or B, [maximum of 1700 °C (3092 °F)]. They are even more expensive than base metal thermocouples.

Sheaths for Thermocouples

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

Consider using either molybdenum or tantalum sheaths for highest temperature capabilities. These will go up to 2315 °C (4199 °F) and 2200 °C (3992 °F) respectively, even though both are sensitive to oxidation, so should not be employed in oxidizing atmospheres.

The alternatives are ceramic sheaths capable of withstanding up to 1960 °C (3560 °F), platinum-rhodium alloy sheaths capable of withstanding 1650 °C (3002 °F), or Inconel^® 600, which goes up to 1150 °C (2102 °F). All of these have the potential to handle 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 in order to keep the wires from contacting the sides. It is essential for this insulation to have a temperature rating appropriate to the environment. Common materials for furnace temperatures are magnesia, alumina and hafnium oxide. Alumina comprises of 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 within furnaces. While the extensively-used “Type K” thermocouples will handle furnace temperatures, improved performance is provided by Types G, C and D and R, S and B.

The type of atmosphere used becomes a vital consideration at furnace temperatures. In particular, an oxidizing atmosphere, as employed in microelectronics fabrication, will cause a reaction with both Types G, C and D and the stainless steel sheaths frequently employed.

IR pyrometry is an alternative for measuring high temperatures, however, it needs a window or viewport to measure inside a furnace. For this reason, it is usually 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.