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

quite unclear. The only example of synthetic carbyne comes from synthesis in carbon nanotubes, when chains of several thousand sp. carbon atoms were linked to form polydisperse carbyne samples [33–35]. Among the attractive physical properties, the carbine is the unusual electrical transport of electrical charges (negative differential resistance [35, 36]).

Electrical conduction in carbonaceous materials has been studied by different authors and reviewed in a number of papers, such as those by [37, 38]. It is interesting to note that CNTs and graphene nanoribbons show ballistic conduction; that is, the transport of free electrons takes place over relatively long distances (longer than the active length of the medium in which the displacement of charge carriers occurs). Thus, these materials show practically zero resistivity, although they are not superconductors.

Carbon materials with current practical uses involving electrical conduction have sp2 hybridization, exhibit continuous conjugation, and can be classified as follows [32]:


composition, carbon fibers can also be included in this group, although structurally, they are more similar to graphite, and the precursor of vapor-grown carbon fiber (VGCF) is very similar to CNT [32].

Graphene, a monolayer (atomic single crystal) structure of covalently bonded sp2 hybridized carbon atoms, is currently the thinnest material known in the world. Due to its structure, graphene exhibits a series of interesting properties, distinguished by their values, compared to common materials [2, 41], such as: high mobility of charge carriers (electrical conductivity can reach 6000 S/cm, comparable to CNT), high thermal stability (2600 K [42]); very good thermal conductivity (5300 W/(mK) [42]); special mechanical properties: mechanical modulus ( 1 TPa) and breaking strength ( 125 GPa); gas barrier properties, high transparency, and high-specific surface area [41].

Therefore, graphene has great application prospects and market value in various fields such as electric current transport, high-frequency electronic devices, flexible display, sensors and biosensors, batteries, supercapacitors, aerospace, and biomedical technologies. Graphene is also an ideal nanofiller for reinforcing polymers (composites). Even a small amount of graphene added to the composite causes a spectacular increase in mechanical, electrical, and processing properties [41, 43].

The various known methods for obtaining (synthesizing) graphene are reviewed in the literature (see [43]). Since graphene is difficult to produce (requires a lot of energy and is difficult to structurally control), being consequently expensive, different forms of modified graphene are used in practice (**Figure 4**), such as graphene oxide (GO) and reduced graphene oxide (rGO), as well as graphene wafers, which represent more advantageous alternatives in terms of production costs (see [43, 44] for synthesis of GO and subsequent rGO). Like graphene, these materials can induce great enhancement when combined with polymers, resulting in low-density, corrosion-resistant, low-friction, and low-cost nanocomposites with highperformance and multifunctional properties. However, since the oxygenated groups of GO can significantly affect the properties of the graphene structure, the functionalization of GO is often practiced, whereby the influence of the oxygenated groups is blocked by physical bonds (noncovalent functionalization) or by chemical bonding (covalent functionalization); see more details within the review [43].

Graphene platelets (GnPs) consist of several graphene layers (3) overlapped. The assembly has an average thickness of 3.55 0.32 nm [2, 45], thus being considered a 2D nanomaterial [46]. It exhibits high mechanical strength and high electrical conductivity (1400 S/cm), as well as good compatibility with most polymers, as well as preparation of cost efficiency [46]. An interpretation of Raman spectroscopy data in

**Figure 4.** *Graphene, graphene oxide, and reduced graphene oxide.*

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

terms of confirming the structure and assessing its integrity [2], compared to GO and rGO, the use of graphene platelets is more efficient in terms of the costs associated with the preparation, as well as the lower concentration of defects compared to GO [45].

A simple method to prepare graphene platelets consists of a thermal shock applied to a graphitic intercalation compound (GIC), followed by an ultrasound treatment [2]. A more detailed description is given in the reference [45].

Polymer/graphene nanocomposites exhibit suitable properties for applications as sensors, and many other electronic and mechanical devices such as capacitors, electromagnetic shielding systems, transistors, electroluminescent devices, batteries, memory, gate dielectric devices, light-emitting diodes, devices with touch screen, and solar cells [41]. A recent review of applied methods for obtaining polymer composites with graphene and modified graphene is presented in the reference [43].

Since the first investigations on the PTC effect in MWCNT/high density polyethylene (HDPE) composites, it has been observed that the presence of CNTs in the polymer matrix can considerably improve the thermal stability of this type of composites due to the interpenetration of the molecular chains of CNTs and HDPE [47]. Carbon nanotubes (CNTs) are thus ideal for inducing electrical conductivity in insulating polymers [26].

The use of CNTs in HDPE matrix composites led to thermistors with significantly better properties (in terms of holding voltage, current through the sample, and response time to applied voltage) compared to commercial CB-based thermistors, with potential critical applications of high temperatures, intense currents, and high applied voltages [48]. However, a problem that arises in the case of using CNT as a conductive filler is the agglomeration of CNT particles, caused by the attraction through van der Waals forces, which results in a reduced value of the weight of CNT particles that effectively participate in electrical conduction (see §2). An effective way to counteract this effect is to modify the surface of the CNT particles. For example, Zhou & Lubineau [49] modified MWCNTs by coating them with a conductive polymer, poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate). After dispersion in a polycarbonate matrix, the electrical resistivity decreased by 11 orders of magnitude for a concentration of 1% (wt), compared to a decrease of only 8 orders for neat MWCNTs at the same loading.

Carbon black (CB) is the most used carbon material for making commercial CPCs, due to its low price, high availability, easy handling, as well as the good electrical conductivity imparted to the polymeric materials in which it is incorporated. However, a number of disadvantages have been identified that limit the exclusive use of CB in high-precision applications such as thermistors, as follows [48]:


To overcome some of these drawbacks, different solutions have been proposed to improve the properties of CPC with CB, such as: (i) radio-induced cross-linking, for

the stabilization of conductive paths [24]; (ii) the use of mixtures of fillers with a synergistic effect on electrical conductivity [50], in order to reduce the content of conductive charge; (iii) the use of binary, heterogeneous, or homogeneous polymer matrices, in order to obtain the percolation effect at low concentrations of conductive charge or, respectively, to increase electrical reproducibility and improve machinability and mechanical properties; (iv) the use of fillers of a different nature, such as graphite, graphene, or CNT, whose particles having a high aspect ratio and greater electrical conductivity and thermal stability than carbon black can ensure the easier formation and stabilization of conductive paths [48].

The general interest in using carbonaceous materials as conductive fillers in sensor construction is due to their special properties, such as high specific surface area, chemical and mechanical stability, adaptability and functionality, and in the case of resistive sensors, electrical conductivity close to that of metals, as well as good processability with different polymer matrices [10]. Although metal powders are intrinsically more conductive than some carbonaceous materials, such as carbon black, the latter (as well as carbonaceous materials in general) is much more used to obtain conductive composites due to its high chemical inertness. It is known that metal particles tend to undergo oxidation, covering themselves with insulating films of oxides, which results in a decrease in the electrical conductivity of the composite over time [25]. In addition, polymer composite films and sensors with carbonaceous materials (especially those with CNTs and graphene) have promising light weight, flexibility, elasticity, sensitivity, and durability compared to their counterparts prepared with metal nanoparticles. The structure-property relationships of polymer/ graphene and CNT nanocomposites and films have been analyzed in a number of previous works [22, 46, 51].

#### **2.3 The effect of temperature on the resistivity of CPCs**

Based on the temperature dependence of resistivity, CPC materials can fall into one of the following three categories [52]: (i) PTC—positive temperature coefficient (resistivity increases with temperature); (ii) NTC—negative temperature coefficient (resistivity decreases with temperature); and (iii) ZTC—zero temperature coefficient (resistivity does not depend on temperature). From the multitude of known CPC materials, this chapter deals only with those applicable as resistive temperature sensors with PTC effect (thermistors); more tangentially, certain aspects of NTC sensors will also be covered. Obviously, ZTC materials are not interesting for resistive sensors because their resistivity is independent of temperature.

It is worth to mention that many of the CPC materials that exhibit PTC effect at temperatures lower than the transition temperature may exhibit NTC effect at temperatures higher than the transition temperature [50]. Although PTC materials are often used as self-temperature-regulating heating elements or protective devices with self-limiting current (resettable fuses), micro-switch sensors [52], in reality, in all these applications the respective materials functioning as temperature sensors/ thermistors, causing the temporary interruption of the passage of electric current through a certain circuit, when a certain designed temperature is reached, or maintaining an element at a constant, specific temperature. Similar materials with PTC effect can also work as thermometers/temperature sensors, based on a calibration curve, taking into account that their resistance increases with temperature, unambiguously (**Figure 5**).

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

**Figure 5.** *Resistivity change with temperature for a composite showing PTC effect in solid.*

PTC polymer composites have been actively studied due to their high performance and low manufacturing cost. A large amount of research work was initially carried out on PTC materials composed of semicrystalline polyolefins and CB filler, due to the high availability of these materials, easy processability, as well as low resistivity at room temperature [53].
