**3. Resistive temperature sensors based on CPC materials**

Temperature sensors based on the change of some electrical properties can be of different types, such as *pyroelectric* (some ceramic materials or PVdF can generate a temporary voltage when the temperature changes due to the thermoinduced change of an internal polarization state), *RTD* (resistive temperature detector—change in the electrical resistance of a metal with temperature), or *thermistors* (see below) [6]; thermocouples (junction of two dissimilar metals that provide a temperaturedependent electrical voltage), and these are widely used in current practical applications but they appear to be less suitable for miniaturization and nanotechnologies than those already mentioned.

In a classical definition, thermistors were described as devices made of semiconductor materials whose resistance varies with temperature according to an exponential law, within certain limits. Inorganic thermistors were initially made by pressing oxide powders, or their mixtures, followed by sintering at characteristic temperatures. In general, the early thermistors were bulk materials, with NTC effect and allowed the detection of temperature variations of the order of 5<sup>10</sup>–<sup>4</sup> <sup>∘</sup> C. The characteristic temperature for inorganic thermistors depends on their type and is usually in the range of <sup>100</sup> … + 300 <sup>∘</sup> C. More recent literature indicates the use of ceramics (e.g., Ba0.5Bi0.5Fe0.9Sn0.1O3/alumina or Ni0.8Co0.2Mn2O4/alumina) deposited by screen printing processes resulting in thick film thermistors. This type of thermistors has a considerably smaller size than bulk thermistors, allowing integration in microelectrical circuits [54]. However, the current interest is especially related to organic or hybrid materials, which are more suitable for fine applications in advanced fields such as biomedical and robotics.

Newer materials are conductive polymer matrix composites (CPCs), where the filler can be a metal powder, a semiconducting ceramic, or a carbonaceous material [10, 50]. The operating principle of temperature sensors based on the thermoelectric behavior of CPCs is that, upon heating, the conductive paths of the CPCs change, consequently also changing the electrical resistance (or resistivity/conductivity) of the material [10].

The performance of polymer composites as resistive sensors is closely related to the conduction mechanisms of charge carriers in the conductive filler as such, as well as in the composite as a whole. In general, the key factors that determine the behavior of sensors based on composite materials are polymer properties, conductivity and structural characteristics of the nanofiller, dispersion quality, as well as processing conditions [2].

Since the electrical properties of CPCs depend on the state of the conductive paths in the CPC matrix, the slight change of this state under the action of an external stimulus, such as mechanical deformation, pressure, temperature, and the presence of some liquids (organic solvents), can lead to a significant (measurable and unequivocal) variation of an electrical output signal (resistance, conductivity, and current). Therefore, CPC materials can be designed as sensors for the detection and quantification of external stimuli of the type already mentioned [10].

#### **3.1 Temperature sensors obtained by hot forming (with thermoplastic matrix)**

#### *3.1.1 Generalities*

Polymer composites for thermistors are conductive materials consisting of a polymer matrix (a single polymer or a mixture of polymers) and a conductive filler (or a mixture of fillers), which give the material electrical conduction properties. Black carbon [10, 25, 50, 55, 56], graphite [50, 57], graphene platelets [28], carbon nanotubes [28], carbon fibers [58], as well as metal powders [59] are commonly used as fillers.

PTC effect thermistors made of polymer composite materials are resistors with positive temperature coefficient of resistivity, applicable as temperature sensors. Depending on how the resistance varies with temperature, two types of PTC thermistors are distinguished: linear and commutative [27]. Linear thermistors are usable as temperature sensors over a wide thermal range, being characterized by a slow, practically linear increase in resistivity with temperature. Switching PTC thermistors are characterized by a steep jump in resistivity at a characteristic temperature and are therefore used as high-sensitivity temperature sensors for narrow thermal ranges located in the vicinity of the characteristic temperature [27]. In applications as resettable fuses or heaters with thermal self-regulation, the respective materials are actually temperature sensors that ensure the functionality of the device as current/voltage protection or as a heating element with self-limiting power [50]. In this regard, the intensity and reproducibility of PTC effects are important factors for the application of temperature sensors [60]. The use of PTC materials as temperature sensors is based both on the slow increase of resistivity with temperature (on the low temperature portion of **Figure 5**) and on the abrupt and strong transition of resistivity at a predetermined temperature (related to a structural transition—melting in case of semicrystalline polymers, glass transition for resins).

In most cases, it is observed that the critical temperature Tc at which the conductor-insulator transition occurs is close to a transition temperature (melting or

#### *Novel PTC Composites for Temperature Sensors (and Related Applications) DOI: http://dx.doi.org/10.5772/intechopen.110358*

glass transition), at which the polymer matrix undergoes a sudden expansion, producing the interruption of the conductive paths [24]. However, for a number of CPC materials, such as those with metal fillers, PTC (resistivity spike) effects have been observed at temperatures significantly different from the transition temperatures of the respective matrices. Such examples can be CPCs with Ag-metallized glass beads filler and polymethyl methacrylate (PMMA) matrix [61], as well as CPCs with Nicoated graphite filler and polycarbonate (PC) + polycaprolactone (PC) matrix [62], the phenomenon being explained by the large difference between the coefficients of thermal expansion of the conductive filler and the polymer matrix. According to the results of Rybak et al. [63], in the case of nanocomposites with Ag and single matrices of HDPE, or polybutylene terephthalate (PBT), as well as similar nanocomposites with binary matrices HDPE+PBT or HDPE+ Poly(m-xylene adipamide) (MXD6), the Tc value increases substantially with the content of Ag nanoparticles, being between 45 and 180<sup>∘</sup> C. For example, for HDPE-xAg nanocomposites (x = volume percentage), Tc values of 44<sup>∘</sup> C for x = 18% and almost 100 <sup>∘</sup> C for x = 24% Ag. Similar effects were observed for PBT-xAg composites, but at higher temperatures, ranging from about 130 to 175<sup>∘</sup> C, depending on the Ag concentration. Such materials would allow interesting applications as temperature sensors, since the range of maximum sensitivity can be tuned by changing some compositional parameters, such as the nature of the polymer matrix and/or the nature and concentration of the conductive filler.

Despite the fact that composite materials (especially those with thermoplastic or elastomeric matrices) are suitable for industrial applications due to their easy processing, low density, flexibility, and toughness, their use as temperature sensors has been delayed by the poor reproducibility of the resistance values of the materials subjected to repeated heating-cooling cycles in which the temperature of reaching the maximum resistivity (which corresponds to the melting temperature of the polymer matrix of thermoplastic polymers) is exceeded. Thus, the electrical resistance values can differ significantly from one heating cycle to another [50], the phenomenon being explained by the random restoration of the conductive paths during the solidification of the melt. The phenomenon can be effectively countered by radio-induced crosslinking [24, 50], as well as by using polymer mixtures as polymer matrices and/or of mixtures of conductive phases [50]. It should be noted, however, that in the case of heating elements and overcurrent protections, this problem is not so serious, since in practice the material does not reach the melting temperature, the flow of current being practically cut off before the melting temperature is reached [50].

It is also worth to mention that, in the case of thermistors, the obtaining technology is not as simple as in the case of self-regulating heating elements, since even the production of "classical" thin films of uniform thickness (0.3 mm) is complicated by the high viscosity of composite material melts [24].

### *3.1.2 Thermistor performance evaluation*

Evaluation of the temperature sensing behavior of PTC sensors can be done simply by placing the sensor in a programmable temperature enclosure (which allows heating from ambient temperature to a specific temperature of interest using a heating schedule, usually linear, and which also allows cooling to ambient temperature). The temperature sensor is connected to an electrical resistance measuring instrument, which allows real-time measurements in the range 0 ohm … T ohm, or more (to be able to detect the maximum resistance at the critical transition temperature). A calibrated contact thermometer (or equivalent) measures the temperature on the sensor surface

(Tt). The dependence curve of the sensor signal (resistance and resistivity) is drawn as a function of the temperature Tt, which, in the case of the existence of a critical temperature in the scanned thermal domain, has the form of **Figure 5**.

To evaluate the performance of resistive temperature sensors, the normalized change in resistance (Rn) can be used, as well as the temperature coefficient of resistance (TCR), defined as follows [3]:

$$R\_n(\%) = \frac{R - R\_0}{R} \cdot 100\tag{2}$$

$$T\text{CR} = \frac{\text{R} - \text{R}\_0}{\text{R}\_0} \bullet \frac{\text{1}}{\Delta T} \tag{3}$$

where R and R0 are, respectively, the current resistance and the room temperature resistance and ΔT is the corresponding temperature interval.

It is observed that the signs of the magnitudes Rn and TCR also give the sense of resistance variation with temperature in the considered range: if Rn, TCR < 0, the resistance decreases with increasing temperature, and the material exhibits NTC effect; if Rn, TCR > 0, the material will be PTC.

#### *3.1.3 Examples*

Shafiei et al. [28] reported that the preparation of HDPE matrix composites with carbon black filler (18%) and graphene platelets (1%) showed a sudden increase in resistivity between 105 and 120<sup>∘</sup> C, good repeatability, and reproducibility, showing good potential for use as a thermometer, temperature sensor, and heating elements with self-temperature regulation.

Go et al. [60] reported the obtaining of PTC composites with ethylene-vinyl acetate (EVA) matrix and CB filler (0D filler) and exfoliated Gr (2D filler) exhibiting improved intensity and reproducibility at repeated thermal cycling through mobility control filling and thermal expansion due to the combination of fillings. Additionally, these composites exhibited a temperature sensitivity approximately 14 times higher than that reported in the literature for other temperature sensors. The PTC composite with the synergistic combination of 0D and 2D fillers can detect human skin temperature by real-time monitoring and exhibited an accuracy of 0.41°C, thus demonstrating the feasibility of the PTC temperature sensor in specific applications that require sensitivity and relatively high-temperature flexibility, such as monitoring human body temperature.

Polyvinylidene fluoride (PVdF) matrix composites filled with *in situ* thermally reduced graphene oxide (TrGO) and silver nanowires (AgNW) were prepared using solution mixing followed by coagulation and hot thermal pressing [64]. Binary TrGO/ PVdF nanocomposites exhibited a low percolation threshold of 0.12 vol % and a low electrical conductivity of about 10�<sup>7</sup> S/cm. Blending TrGO with silver nanowires led to a significant improvement in electrical conductivity due to the synergistic effect in conductivity of the two conductive materials (the bulk conductivity of TrGO + AgNW materials was higher than the combined conductivity of TrGO/PVdF and AgNW binary composites/PVdF at the same filler content). The hybrid composites showed an increase in resistivity with temperature (PTC), the jump in resistivity being observed at the melting temperature of PVdF. The 0.04 vol % TrGO/1 vol % AgNW/PVdF hybrid material exhibited pronounced PTC behavior, making this composite an interesting candidate for current limiting devices and temperature sensors.

An ingenious sensor is that described by [10], which consists of a poly(chlorinated propylene carbonate)-based polymer foam system filled with CB and cross-linked. This system removes the typical disadvantages related to the nonlinearity and nonmonotonicity of the resistance variation with temperature, specific to PTC composites obtained by randomly dispersing the nanofiller in the polymer matrix. During the heating, the gas bubbles in the closed pores of the foam expand, causing the reduction of the wall thickness and, implicitly, the decrease of the resistivity due to the decrease of the distance between the CB particles. The process of resistance decrease is linear with temperature, reproducible, and reversible (resistivity increases with decreasing temperature).

Lyashkov et al. [65] studied composites with tungsten oxide ceramic (WO3– 3.0MnO2–0.5Na2O5.0MoO3) and polyethylene matrix with ceramic filler volume fraction from 10 to43%. That ceramic was chosen among several WO3-based materials as having the most nonlinear I-V (*Intensity-Voltage)* characteristic. The obtained composites proved to be isotropic mixtures of filler grains in the polymer matrix and showed high values of the temperature coefficient of strength, in the range of 40–75°C, which depend on both the volume fraction of the filler and the intensity of the electric field. This dependence on the electric field can be explained by the nonlinearity of the I-V characteristic, typical for varistors. In the case of ordinary thermistors, the I-V characteristic is linear, the TCR being independent of the electric field. The dependence of the electrical conductivity of the composite on the volume fraction of the conducting ceramics can be described with a three-dimensional percolation model for a two-phase system.
