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The Science of Materials Used in High Temperature Thermistors
Thermal Stability of Ceramic Oxides, Glass Body, and Metal Encasements
Heat-resistant thermistors utilize specific oxides of ceramic materials. Typically, positive temperature coefficients (PTC) thermistors use barium titanate, whereas negative temperature coefficients (NTC) thermistors use cubic manganese-nickel-cobalt spinel. What property do these materials possess that make them so useful? The materials allow a stable and repeatable change in resistance with temperature due to electrons moving between energy states and interacting with lattice structure vibrations. In terms of thermal stability, the sealing technique used is very important. Glass seals stop thermal destruction of elements at temperatures around 200 degrees Celsius and, there is no oxygen and water ingress. However, when temperatures surpass 300 degrees, metal (in this case, stainless or Inconel) encasements are necessary. These metals are structurally stable against rapid thermal cycles, mechanical assault, and aggressive corrosion. Although these metal covers are thermally conductive, they do allow the encased sensor to respond to the surrounding temperature, thereby enabling the sensor's thermal response.
When designing a system, one of the most important aspects is optimization of the thermal expansion coefficients for the encapsulation material and the ceramic components so cracking does not occur at the interface. Additionally, proper oxygen barrier layers and a specific range of porosity during sintering to withstand thermal stresses are essential to the system. These designs are preferable, if possible, in combination with passivated platinum electrodes, as the contact stability and oxidation protection improve the system's thermal performance. In field testing, these designs have been demonstrated to maintain stability with a drift of less than 0.5% over continuous operation for 1000 hours at 300degree C and response times have been less than 2 seconds in most cases. The reliability of the integrated materials in these systems enables the operation in severe environments that are not possible with conventional silicon sensors, such as within jet engine turbines or in molten aluminium, where traditional sensors fail.
Maximum Temperature Resistance: NTC vs PTC High Temperature Thermistor Limits
NTC Thermistors: Practical Upper Limits (Up to +300°C) and Derating Guidelines
NTC thermistors for niche uses should be able to go up to ~300°C before issues begin to occur. Problems include things like irreversible oxidation in metal oxides, and gain boundary depletion due to increased rates of evaporation. Above ~150 °C, the risk of thermal run away increases greatly, and power must be reduced overall. At 250 °C, power dissipation must be reduced between 40 to 60 % compared to room temperature to limit oneself to self-heating errors and changes due to resistance. Components that rest in the high resistance range of at least 100k Ω at 25 °C generally perform better at elevated temperatures. This behavior creates a challenge for engineers because they generally need to create specialized nonlinear control techniques to accurately regulate systems within less than one degree Celsius, like engine controls or feedback systems for industrial furnaces. These techniques include third-order corrections to the Steinhart Hart equation, for example.
Standard barium titanate PTC thermistors demonstrate a sharp increase in resistance at their Curie temperature which falls between 60 and 120 \textsuperscript{o}C. Due to this abrupt change in resistance, these models cannot be used in linear sensing applications above this temperature range. However, for aerospace and industrial applications, manufacturers engineer special versions of these thermistors that incorporate specific additives such as lead, strontium, or various rare earth oxides into the polycrystalline ceramic structures. These modifications can increase the Curie point and improve reliability and consistency of these devices so that they can be used at temperature ranges above 200textsuperscript{o}C. At 205textsuperscript{o}C, such thermistors have been shown to change their resistance from approximately 1 kΩ to more than 500 kΩ in less than 3 seconds, which is increasingly beneficial for fast-response applications such as safety cutoff systems in battery packs and power distribution systems in the aerospace industry. These materials also retain hysteresis, and testing has shown they can be cycled repeatedly thousands of times without failure in accordance with IEC 60738-1 and MIL-STD-202G testing.
Problems Related to Precision and Trustworthiness at Higher Temperatures
Issues Including Beta Drift, Calibration Drift, and Nonlinearity While Operating High-Temperature Thermistors
High temperatures, typically greater than two hundred degrees Celsius, create a number of problems relative to accurate data measurement. Three of these problems include beta drift, calibration drift, and increasing nonlinearity. One particular beta drift problem involves changing internal structures of a material. From about two hundred to three hundred degrees Celsius, a material's internal structure becomes stabilized, and if changes in the internal structure relax, the resistance may drift roughly five percent a year. Then, even after calibrating a device in a factory, the cal will become obsolete due to changes in resistance due to beta drift. Calibration drift problems are exacerbated by industrial heating and cooling cycles. It is not uncommon for a factory to have to calibrate its equipment once every six months in order to continue the production process. Furthermore, past three hundred degrees Celsius, the response of a device becomes very unpredictable, leading to ever increasing gaps in what's actually occurring in comparison to the reading of the device. Situations for which there is a need to adjust and flatten a curve become exceedingly rare in high temperature environments. For instance, in the 2021 volume of the Sensors and Actuators A, several non-appropriately adjusted thermistors in semiconductor manufacturing furnaces showed total errors between 2 and 8 percent. This is even larger than the +/- 1 degree constraint which is strictly enforced for heat control.
A real hardware and software twofold technique is necessary for alleviation. Ceramic materials composed of stabilized Mn-Ni-Co and lowered oxygen mobility were implemented in the hardware. Software establishes a real-time control system which uses embedded adaptive algorithms trained on expedited aging patterns. As a result, the device demonstrates curvature and offset changes which are dynamically corrected and ultimately result in a measurement uncertainty that meets
the metric of <% 0.3 °C at 300 °C.
How to Pick out the Best High Temperature Thermistor for Your Needs
To pick the best high temperature thermistor for your application, you need to consider five interrelated criteria and your system's physical and operational limitations:
Operating Temperature Range: Ensure the rated maximum temperature exceeds the peak process temperature by at least 25 - 50 °C. For instance, +250°C applications, a +300°C rated device would be a good fit due to self-heating and transient spikes.
Resistance Stability: For best results, pick components that specify a long-term, target temperature, drift ≤ 1% (per IEC 60738-1 Annex D). Avoid claims of “high temperature” that are unspecific.
Environmental Resilience: Ensure the encapsulation material you choose for your thermistor matches the environment you expect. For example, choose glass sealing for environments that are dry and oxidising and metal encapsulation (i.e. Inconel 600 or SS316) for those that are humid, sulfidizing, or molten salt environments over 300°C.
Response Dynamics: Select thermal time constant s that are less than 30 seconds for furnace zoning and less than 3 seconds for combustion monitoring, and less than 3 seconds for thermal time constant equal to or less than your control loop speed.
Physical Limitations: Ensure the dimensions will fit and the mounting style will mesh (i.e. threaded, flanged, surface mount) and that the lead wire insulation class (e.g. MgO filled, Teflon® coated) meets your assembly tolerances and electromagnetic interferences (EMIs) to the lead wire insulation class (e.g. MgO filled, Teflon® coated) meets assembly tolerances and electromagnetic interferences (EMIs) to the lead wire insulation class (e.g. MgO filled, Teflon® coated) meets assembly tolerances and electromagnetic interferences (EMIs) to the lead wire.
It is important to verify the distance from the manufacturer's data sheets to reports of independent testing not related to ratings for the performance of the test at ambient level. This is especially important for performance related to thermal cycling, vibration, and chemical, per MIL-STD-810H. The right choice must integrate the system's performance and the simple fact of physics and reliability.
FAQ
What are high temperature thermistors used for?
High temperature thermistors are used primarily for monitoring and controlling temperature in environments that are highly demanding. They are found in jet engine turbines, and molten aluminum baths. High temperature thermistors are also used in the manufacture of semiconductors, where traditional sensors would not be viable.
What are the differences in the applications of NTC and PTC thermistors?
For the most part, NTC thermistors are preferred for use in areas where temperatures can reach up to 300 degrees Celsius because of their thermally stable characteristics. On the other hand PTC thermistors are used in areas where at high temperatures, there are significant and rapid increases in resistance, as is the case in the safety cut off mechanisms in the battery system of electric vehicles.
What are the primary hurdles to the use of high temperature thermistors?
Quite high thermistors have a number of very serious challenges, amongst which are difficulties in calibration, response time heterodyning and loss of linearity in the response. Over time, the measurments do not hold true.
What are the methods of enhancing the precision of high temperature thermistors?
In order to enhance the precision of high temperature thermistors, use of designs that have dumb circuits, real time control with adaptive algorithms, and use of stable materials which leads to reduced mobility of oxygen.
