Material and Structural Design: Why High Temperature Thermistors Withstand >150°C

Thermal Stability of Some Ceramic Materials and the Engineering of the Dopants

Certain thermistors demonstrate significant stability and operate efficiently above 150 degrees celsius, and is possible due to the invention of new ceramics. Materials such as Manganese, nickel, and cobalt are typical constituents of these thermistors, and the addition of rare earths such as yttrium or lanthanum is influential to develop new ion behaviour. The addition of these elements mitigates certain types of structural collapse during processing, which enhances the structural thermal integrity of the crystal lattice. Manufacturers refine the processing workflow to limit the formation of vacancies and occluded voids. Some practitioners use zirconia to limit ionic conduction and structural diffusion of oxygen during multiple thermal cycles. Materials of this nature are employed to ensure verbal minimal thermal hysteresis of the NTC. Standard thermistors, minimum 15 percent change in resistance is declared at 125 degrees celsius. High temperature thermistor's NTC only varies +/- 1 percent, and is considered to operate efficiently at 200 degrees celsius and beyond.

B-Value Linearity Breakdown in Standard Thermistors Above 125°C

In an NTC thermistor, the resistance value and the temperature are related by the equation R = R0  exp[B(1T - 1T0)] R0  is the resistance at the temperature T0, and B is the thermistor's B-value or ( beta ) parameter. B-value predicts the thermistor's usable range which for standard thermistors is -50°C to 125°C. Above and below these ranges, the thermistor's performance is affected by the following three processes:

1. Ionic Conductivity (Ionic Conduction):
Thermal energy causes ions to migrate and overwhelm the electronic conduction pathways.

2. Grain Boundary Relaxation:
Dopant segregation at grain boundaries relaxes the microstructure.

3. Material Decomposition:
This can include the partial reduction of a transition metal oxide which changes the stoichiometry and electron concentration.

These processes are responsible for B-values that deviate by textgreater 5%, and thus, one cannot rely on resistance predictions beyond such temperature. To improve such B-value predictions, high-temperature variants use other activation-energy ceramics and dopants that are designed to delay ionic conduction To improve such B-value predictions, high-temperature variants use other activation-energy ceramics and dopants that are designed to delay ionic conduction so that the mixed pathway is dominant at temperatures above 200°C. This extends the usable temperature range of these thermistors by 75°C.

Extreme Performance Reliability

Thermistors built for high temperatures can stay reliable for years due to their construction using ceramic and metal components with a seal and electrodes composed of precious metals (platinum or palladium alloys) that can withstand 200 degrees and higher without corrosion. In many thermal cycling applications (like jet engine monitoring) a sensor moisture intrusion is a common issue. Approximately three quarters of premature sensor failures can be attributed to moisture intrusion, but this design prevents moisture ingress and trap formation. The entire design can withstand thousands of thermal cycles and is necessary for operational accuracy in refineries that experience rapid thermal cycling (hundreds of degrees) every 24 hours. It is also useful in applications requiring a controlled environment where moisture and other gasses can alter performance, due to swift thermal cycling. Using alumina (an alumino-oxide) as base is utilized to reduce thermal and active oxygen gap formation, thus maintaining structural integrity.

In the case of geothermal wells, where humidity is persistently high at about 85% and is high in sulfuric acid, these upgrades mean that sensors can last decades instead of months, like regular sensors.

Operating Realities: Derating, Trade-offs in Accuracy, and the Lifetime of Systems at Elevated Temperatures

Derating and Accelerated Aging of Systems Beyond 125°C

Above a certain temperature, the lifespan of thermistors drops off steeply. In the case of most thermistors, each 10 degree Celsius increase beyond the rated maximum cuts the operational life by about 50%. For example, standard NTC thermistors reach 150 degrees Celsius and start to exhibit greater than 5% resistance drift within approximately 1000 hours of operation. The high temperature variants are able to last greater than 10,000 hours at the same conditions. In derating guides, these are the limits where a safe operating zone ends. Once these limits are exceeded, the material undergoes adverse permanent changes. Real life engineers must consider thermal inertia in the design of their systems. This means integrating an understanding of time constants and rates of heat transfer with a consideration of the state of the environment. Insufficient consideration of these aspects will result in the development of localized hot spots, which will reduce the measurement accuracy of the system over time.

The sensitivity temperature rating paradox in the design of high-temperature thermistors

In high temperature thermistors, the thermal sensitivity, often expressed as the alpha value of the thermistor, is reduced as the maximum operating temperature increases, which is an unavoidable design trade-off. While standard NTC thermistors achieve a thermal sensitivity of about –4% °C at room temperature, those designed for an operating temperature of 150 °C achieve only about –1.5% °C. Why does this happen? It relates to the choice of the doping material. While rare earth oxides improve the stability of the crystal structure, they also result in poorer mobility of the charge carriers. For temperatures above 150 °C, particularly for systems that require an accuracy of ±0.5 °C, a great deal of signal conditioning is required. This entails proper functioning of low noise amplifiers, the setting of multiple calibration points, and the application of algorithms to overcome B value shift. In addition, having redundant sensors is helpful in overcoming drift problems, which is particularly important in the presence of non-linear B values as they can compromise the stability of the control system.

Specification Driven Selection: When To Choose High Temperature Thermistors?  

Standard NTC sensors fail when there is thermal stress over long periods, rapid thermal cycling, and when exposed to aggressive chemicals. High-temperature thermistors can be specified for:  

Sustained ambient temperatures over 125°C, such as automotive exhaust manifolds, industrial furnace linings, and aerospace engine compartments;  

Environments with extreme thermal transients, such as power processors and rapid thermal processing semiconductors, in which ceramic formulations defeat microcracking and grain boundary slippage;  

High heat paired with moisture and aggressive chemicals, such as downhole sensors for oil and gas and medical sterilization autoclaves, where there is a combination of hermetic sealing and oxidation resistant metallization.

Standard thermistors are great for working below about 100 degrees celsius for example, household appliances, and heating systems that are zoned to heat different areas. In relation to these devices, it makes sense to try and determine their how long devices like these would survive under the conditions they are anticipated to be used. Industry data suggests that standard thermistors under 150 degrees fahrenheit wear out about 10 times faster than high temperature thermistors. This is due to multiple and often internal factors, such as chemical and physical material breakdown, internal movement, and moisture incursion. In cases where a temperature measurement is acceptable if it is within a range of +/- 3 degrees, Platinum Resistance Thermometers (PRTs) are a good middle of the road alternative. However, for high temperature applications thermistors have PRTs beat in virtually every category. They are smaller, faster, and more economical in almost every case, especially in high temperature applications where working space is limited. 

Common Questions about High Temperature Thermistors

Why are high temperature thermistors able to survive temperatures above 150 degrees celsius? This is because of the advanced ceramic construction of the thermistors, including stabilizing oxides of manganese, nickel, and cobalt, as well as rare earth elements like yttrium and lanthanum to reduce structural degradation.

Standard thermistors fail at temperatures above 125 °C due to a complete loss of thermistor function because of the over-dominance of ionic conduction, thermal degradation of grain boundaries, and thermal decomposition of materials.

How do thermistors for high temperatures survive extreme and high multi-cycling temperatures? These thermistors are fitted with a highly durable hermetic seal, moisture-impermeable and high-temperature oxidation-resistant metal barriers, and membranes that withstand thermal cycles, and do not significantly alter the calibration of the thermistor.

What are the design challenges of high temperature thermistors? The mechanical stability of the materials used at elevated temperatures results in a decrease in thermal sensitivity, requiring further design to achieve more accuracy.

When are high temperature thermistors needed? High temperature thermistors are warranted when the operating temperature is continuously above 125 °C, when multi-cycling severe thermal transients are present, or when the environment is moisture and chemically aggressive.