In recent years, the demand for temperature sensors has been increasing in various fields. For instance, to measure the temperature distribution of the fuel cells mounted on fuel cell vehicles, there is a need for temperature sensors that are highly precise, compact, and have a long lifespan. The performance of electronic devices largely depends on the thermal conditions of the device itself or its operating environment. Therefore, temperature sensors require high measurement accuracy. Moreover, due to the diversification of devices utilizing temperature sensors, there is a need to consider sensors with high added value, such as ease of integration. This article not only discusses the types and characteristics of temperature-sensitive resistive bodies of metal temperature sensors but also explores the feasibility of realizing thin-film temperature sensors.
Firstly, regarding the use of metal temperature sensors, they primarily operate based on three principles: thermocouples, resistance temperature detectors (RTDs), and bimetal thermometers. Among these, resistance temperature detectors (RTDs) will become increasingly attractive in the future, especially in fields requiring miniaturization of electronic components.
Thermocouple (有熱電偶): It works by forming a closed circuit through the contact of two different metals. When one of them is heated, a temperature difference is created at the two contact points. This temperature difference generates a thermoelectric electromotive force inherent to the metals, allowing current to flow. This phenomenon is known as the Seebeck effect, and thermocouples are contact temperature sensors that utilize this effect.
Resistance Temperature Detectors (RTDs) (測溫電阻體): These are temperature sensors that utilize the metal resistance that varies according to temperature changes. Generally speaking, the resistance of a metal changes proportionally with temperature; it is very low at low temperatures and increases as the temperature rises. RTDs operate based on this principle. Compared to thermocouples, which are also contact temperature sensors, RTDs are characterized by their high sensitivity, excelling in measuring temperatures near room temperature and extremely low temperatures. Additionally, since the relationship between temperature and resistance is well understood, they are known for their high accuracy. On the other hand, their maximum operating temperature is about 500 degrees Celsius lower than that of thermocouples, and they are also vulnerable to mechanical shocks and vibrations.
-Metal Thermometer (雙金屬溫度計): This is a temperature sensor that involves bonding two thin metal plates with different thermal expansion coefficients together. Due to the differing thermal expansion rates, the bonded plates will bend towards the side with the lower thermal expansion when the temperature changes. The bi-metal thermometer measures temperature based on this displacement.
Types and Characteristics of Temperature Resistance Detectors (RTDs)
RTDs vary based on the characteristics of the metals used, making the choice of metal crucial. Initially, temperature sensors require the following characteristics:
A constant relationship between temperature and resistance over a wide temperature range.
Chemical stability and resistance to aging.
High intrinsic resistance.
Metals that satisfy these conditions include platinum (Pt), copper (Cu), nickel (Ni), and platinum-cobalt (Pt/Co), among others.
RTDs are structures that involve placing ultra-fine resistance wires in coil form into insulators like glass, covered with a metal casing. To miniaturize the temperature sensor, the resistance wire needs to be as thin as possible, but this thinning has its limitations, making it challenging to minimize RTD sensors.
In recent years, there has been a demand to embed them into small gaps and tiny components, a requirement that previous RTDs could not meet. However, with the increasing demand for temperature measurements in the small device market year by year, the industry is eagerly anticipating the advent of even smaller, thin-film RTDs.
II. Thin-Film Fabrication of Temperature Sensors
The thin-film fabrication of RTD temperature sensors is one of the focal points of recent industry development, targeting the needs in applications such as small electronic wearable products. The internal structure also varies based on the requirements and processing settings.
For instance, a concept of thin-film temperature sensors proposed in the paper "A Flexible Temperature Sensor Based on Reduced Graphene Oxide for Robot Skin Used in Internet of Things" published by Liu, Tan, Kou & Zhang in 2018:
Figure 1. Man exercising. Adapted from "A Flexible Temperature Sensor Based on Reduced Graphene Oxide for Robot Skin Used in Internet of Things," by Liu et al., Copyright 2018
Fabrication process of the temperature sensors:
(a) Cleaning polyethylene terephthalate (PET) with acetone, alcohol, and deionized (DI) water;
(b) O₂ plasma etching;
(c) Printing two conductive thin wires;
(d) Creating the sensitive layer using air-spray coating;
(e) The fabrication process of the insulating layer;
(f) Thin-film fabrication of temperature sensors.
In this context, C&T offers extremely low dk/df value .PET 或 PEN, pA, PI which are suitable for insulating layer ; which enable the realization of thin and highly flexible thin-film temperature sensors. For more details, feel free to reach out to C&T References:
Liu, Y., Tan, S., Kou, Z., & Zhang, D. (2018). A Flexible Temperature Sensor Based on Reduced Graphene Oxide for Robot Skin Used in Internet of Things. Retrieved from https://www.researchgate.net/figure/Fabrication-process-of-the-temperature-sensors-a-polyethylene-terephthalate-PET_fig1_324915482
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