Negative Temperature Coefficient Resistance (NTCR) Ceramic Thermistors: An Industrial Perspective Antonio Feteira w EPCOS OHG, Electronic Components and Parts, 43 Siemmensstrabe, A-8530 Deutschlandsberg, Austria Monitoring and control of temperature is of paramount impor- tance in every part of our daily life. Temperature sensors are ubiquitous not only in domestic and industrial activities but also in laboratory and medical procedures. An assortment of tem- perature sensors is commercially available for such purposes. They range from metallic thermocouples to resistive tempera- ture detectors and semiconductive ceramics, showing a negative temperature coefficient of resistance (NTCR). NTCR ceramic sensors occupy a respected market position, because they afford the best sensitivity and accuracy at the lowest price. Despite the enormous commercial success of NTCR thermistors, this area of advanced functional ceramics has not been recently reviewed. Nearly 100 years elapsed between the first report of NTCR be- havior and the fabrication of NTCR devices. The manufacture of the first NTCR ceramic thermistors was problematic, as often the devices suffered from poor stability and nonreproducibility. Before NTCR ceramics could be seriously considered for mass production of thermistors, it was necessary to devote a large amount of R&D effort to study the nature of their semiconduc- tivity and understand the influence of impurities/dopants and heat treatments on their electrical characteristics, particularly in their time dependence resistivity (aging). Simultaneously, from a technological viewpoint it was important to develop methods enabling reliable and permanent electrical contacts, and design suitable housing for ceramics, in order to preserve their electrical properties under conditions of variable oxygen partial pressure and humidity. These topics are reviewed in this article from an industrial perspective. Examples of common applications of NTCR thermistors and future challenges are also outlined. I. Introduction F OUR main types of sensors are used for temperature sensing in domestic, industrial, and medical applications. Those are categorized as follows: (i) thermocouples, (ii) resistance temper- ature detectors (RTDs), (iii) integrated circuit (IC) sensors, and (iv) thermistors. z The capabilities, advantages, and disadvan- tages offered by each type of sensor are summarized in Table I. Obviously, the choice of a particular sensor depends on the re- quired accuracy, speed of response, temperature range, thermal coupling, environment (chemical, electrical, or physical), and cost. Except for IC sensors, all the temperature sensors have nonlinear transfer functions, i.e. the temperature dependence of the physical parameter under scrutiny (e.g. resistance, voltage output, etc) is nonlinear. Thermocouples are widely used as temperature sensors be- cause they are small, robust, relatively inexpensive, easy to use, and cover the widest temperature range, as indicated in Table I. They are especially useful for making measurements at extremely high temperatures (up to 123001C). Nonetheless, they must be shielded from harsh atmospheres and liquids due to corrosion degradation. The most common metals used for the fabrication of thermocouples are iron, platinum, rhenium, tungsten, copper, alumel (Al–Ni alloy), cromel (Ni–Cr alloy), and constantan (Cu–Ni alloy). 1 The output given by thermocouples is only in the range of a few millivolts; therefore they require precision amplification for further information processing. Nevertheless, their main disadvantages are associated with lower sensitivity (microvolts per degree) and accuracy in comparison with therm- istors, and the need for a reference temperature. Thermocouples are characterized by a larger linearity than thermistors, but a smaller linearity than RTDs. Unlike thermocouples, RTDs, ICs, and negative temperature coefficient of resistance (NTCR) thermistors are passive sensors that require current excitation to produce a voltage output, which becomes larger than that given by thermocouples. Modern Si- and Ge-based semiconduc- tors are often integrated into multifunction ICs, which offer reasonable accuracy and high linearity over an operating tem- perature range of 551 to 1501C, as listed in Table I. In some industrial applications below 6001C, RTDs are re- placing thermocouples. Nevertheless, when compared with NTCR thermistors, platinum RTDs are less sensitive to small temperature changes, and have a much slower response time, as indicated in Table I. In fact, thermistors’ high sensitivity (typ- ically between 2%/1C and 6%/1C, at 251C), Fig. 1, allows the detection of minute variations in temperature, which some- times could not be observed with an RTD or thermocouple. The lack of interchangeability of thermistors was once one of the major factors against their widespread use, but nowadays thermistors can match manufacturer’s calibration curves to 70.11C. Thermistors’ accuracy can be as good as 70.0011C, Review D. J. Green—contributing editor z This term is actually a contraction of the words ‘‘thermal resistor’’, which was proposed by the Bell Telephone Laboratories in the early 1940s. Manuscript No. 25362. Received October 15, 2008; approved January 15, 2009. Antonio Feteira is a Visiting Lecturer in the Department of Engineering Materials at The University of Sheffield. w Author to whom correspondence should be addressed. e-mail: antonio.feteira @epcos.com; a.feteira@sheffield.ac.uk J ournal J. Am. Ceram. Soc., 92 [5] 967–983 (2009) DOI: 10.1111/j.1551-2916.2009.02990.x r 2009 The American Ceramic Society