**3. Application of spark plasma sintering in NTC thermistor ceramics**

In the past decades, more advanced techniques such as microwave sintering [17, 18], and nitrogen atmosphere sintering [19] have been used for NTC ceramic powder consolidation. However, there are few reports focusing on the application of SPS technique to prepare dense ceramics for NTC thermistor applications. The advantages of spark plasma sintering against conventional sintering for high temperature NTC thermistor ceramics are reviewed as follows.

### **3.1. Brief introduction of NTC thermistors**

generated in the gaps between powder materials by electrical discharge at the beginning of ON-OFF DC pulse energizing [13]. In addition to have the Joule heating due to the electric current and plastic deformation produced by pressure, SPS also generates DC pulse voltage between the powder particles, and effectively makes use of the spontaneous heat generated by the discharge between powder particles, thus resulting in some special characteristics. Compared to the conventional sintering, SPS has two important characteristics [14, 15]: (1) SPS process can make high-energy pulse focus on the grain junction point, thus saving the energy; (2) A high energy, low voltage spark pulse current momentarily generates spark plasma and produces a high localized temperature from several to ten thousand ◦C between the particles and then resulting in optimum thermal diffusion and grain boundary migration, i.e. more material transfer can be intensified and thus high density ceramics can be obtained through spark plasma sintering with a low sintering temperature and a short sintering period. Fig. 1

**3. Application of spark plasma sintering in NTC thermistor ceramics**

In the past decades, more advanced techniques such as microwave sintering [17, 18], and nitrogen atmosphere sintering [19] have been used for NTC ceramic powder consolidation. However, there are few reports focusing on the application of SPS technique to prepare dense

shows the schematic of SPS furnace [16].

26 Sintering Techniques of Materials

**Figure 1.** Schematic of SPS furnace [16].

NTC thermistors are thermally sensitive resistors whose resistance decreases with increasing temperature. Their resistivity can be expressed by the following Arrhenius equation [20, 21]: *ρ*=*ρo*exp(*E*a/k*T*), where *ρo* is the resistivity of the material at infinite temperature, *T* is the absolute temperature, *E*a is the activation energy for electrical conduction, and *k* is the Boltzmann constant. They are mainly used in industrial areas as elements for temperature measurements, control, etc [22].

In the past few years, development of novel high temperature NTC thermistor materials has been motivated by the requirements of particle filters and catalytic converters in exhaust pipe for automotive motors [23, 24]. NTC thermistor ceramics composed of spinel structure (MMn2O4, where M=Ni, Co, Fe, Cu, Zn) show aging of the electrical properties and their application is commonly limited to temperatures below 300°C [25, 26]. The literature suggests that rare earth (Sm, Tb, Y, etc) perovskite oxides (ABO3) can be used for measurements from ambient to 1000◦C [23]. In particular, YCrO3 having an orthorhombic perovskite structure, has been considered as a candidate for high temperature NTC thermistor applications [19, 23, 27, 28]. However, the material shows poor sinterability and is difficult to densify under ambient atmospheric conditions or through pressureless sintering techniques [29, 30]. We have investigated the spark plasma sintering of YCr1-*x*Mn*x*O3 ceramics and MgAl2O4-YCr0.5Mn0.5O3 composite ceramics, and their NTC electrical properties.

## **3.2. Spark plasma sintering and electrical properties of YCr1-xMnxO3 NTC ceramics**

In the conventional sintering processes, extremely high sintering temperatures (up to 1600°C) and long holding time (several hours) in air are applied in the fabrication of YCrO3 ceramics to achieve the highest density and minimum porosity. The poor sinterability of YCrO3 material is attributed to the loss of Cr2O3 through its volatility during sintering process [30, 31]. In our previous work [32, 33], YCr1-*x*Mn*x*O3 (0≤*x*≤0.5) NTC ceramics with a high relative density have been obtained by combing the Pechini method synthesis and SPS. Fig.2 shows the flow chart for the fabrication of YCr1-*x*Mn*x*O3 thermistor powders by a Pechini method. The molar ratio of citric acid, ethylene glycol and metal ions was 1.5:1.5:1. The spark plasma sintering was carried out in vacuum (6 Pa) with an apparatus (FCT Systeme GmbH, FCT, Rauenstein, Germany). The SPS equipment used in the experiments is shown in Fig. 3. Fig.4 shows the time dependence of the temperature and applied pressure during SPS process. The sintering temperature was 1300 ◦C, and the dwell time was 10 min. Fig.5 shows the XRD patterns of the SPS-sintered YCr1-*x*Mn*x*O3 ceramics. All samples had a pure orthorhombic perovskite phase isomorphic to YCrO3 (JCPDF 34-0365) described by the space group *Pnma*, and no secondary phase occurred with the increase of Mn concentration. Fig.6 shows the SEM images of surface section of SPS-sintered YCr1-*x*Mn*x*O3 ceramics. It can be seen that as-sintered YCrO3 ceramics were highly dense, and had a bulk density of 5.6112 g/cm3 , corresponding to 97.6% of the theoretical density (5.751 g/cm3 ) [33]. One can observe that grain size decreased with the increase of Mn content. This result may be due to the dragging effect between Mn ions and grain boundary, which increases the energy for the movement of grain boundary and retards the grain growth [34, 35]. The YCr1-*x*Mn*x*O3 NTC thermistor over a wide temperature range of 25 to 300 ◦C showed a linear relationship between the logarithm of the resistivity and the reciprocal of the absolute temperature. And the resistivity increased at first and then decreased with increasing Mn contents, which had the same varying tendency with activation energy. This electrical conductivity anomaly has been revealed by using defect chemistry theory combination with X-ray photoelectron spectroscopy analysis [32]: The major carriers in YCrO3 are holes. Mn ions are acting as an n-type dope, and partly compensate for the effect of metal vacancies, thus leading to an increase in the resistivity. Mn4+ ions increase as Mn content increases from 0.2 to 0.5, which promote the rise in charge carriers and the electron hopping, thereby resulting in a decrease in the resistivity. Therefore, SPS has shown significant advan‐ tages against conventional sintering in the fabrication of high density YCr1-*x*Mn*x*O3 ceramics, and provides efficient and viable means for the study of the conduction mechanism of NTC thermistors.

**Figure 2.** Flow chart for the fabrication of YCr1-*x*Mn*x*O3 thermistor powders by a Pechini method.

Spark Plasma Sintering of Negative Temperature Coefficient Thermistor Ceramics http://dx.doi.org/10.5772/58496 29

**Figure 3.** Image of SPS equipment (From Alfred University).

increase of Mn content. This result may be due to the dragging effect between Mn ions and grain boundary, which increases the energy for the movement of grain boundary and retards the grain growth [34, 35]. The YCr1-*x*Mn*x*O3 NTC thermistor over a wide temperature range of 25 to 300 ◦C showed a linear relationship between the logarithm of the resistivity and the reciprocal of the absolute temperature. And the resistivity increased at first and then decreased with increasing Mn contents, which had the same varying tendency with activation energy. This electrical conductivity anomaly has been revealed by using defect chemistry theory combination with X-ray photoelectron spectroscopy analysis [32]: The major carriers in YCrO3 are holes. Mn ions are acting as an n-type dope, and partly compensate for the effect of metal vacancies, thus leading to an increase in the resistivity. Mn4+ ions increase as Mn content increases from 0.2 to 0.5, which promote the rise in charge carriers and the electron hopping, thereby resulting in a decrease in the resistivity. Therefore, SPS has shown significant advan‐ tages against conventional sintering in the fabrication of high density YCr1-*x*Mn*x*O3 ceramics, and provides efficient and viable means for the study of the conduction mechanism of NTC

**Figure 2.** Flow chart for the fabrication of YCr1-*x*Mn*x*O3 thermistor powders by a Pechini method.

thermistors.

28 Sintering Techniques of Materials

**Figure 4.** Time dependence of the temperature and applied pressure during SPS process [33].

**Figure 5.** XRD patterns of the SPS-sintered YCr1-*x*Mn*x*O3 ceramics [32].

**Figure 6.** SEM images of surface section of SPS-sintered YCr1-*x*Mn*x*O3 ceramics: (a) *x*=0; (b) *x*=0.2; (c) *x*=0.4; (d) *x*=0.5.

#### **3.3. Spark plasma sintering and electrical properties of MgAl2O4-YCr0.5Mn0.5O3 composite NTC ceramics**

Recently, there is an increasing interest in exploring NTC behavior in composite materials because of their high temperature potential for combining properties that are difficult to attain separately with the individual component [24]. We have designed and prepared *x*MgAl2O4- (1-*x*)YCr0.5Mn0.5O3 high temperature composite thermistor ceramics by associating a less resistive phase YCr0.5Mn0.5O3 with a high resistive MgAl2O4 combination with spark plasma sintering [12]. Fig.7 shows the time dependence of the temperature and applied pressure during SPS process. The sintering temperature was 1200◦C, and the dwell time was 20 min. The SPS-sintered composite ceramics consisted of a cubic spinel MgAl2O4 phase and an orthorhombic perovskite YCr0.5Mn0.5O3 phase isomorphic to YCrO3. Fig. 8 exhibits the micro‐ structures of the SPS-sintered samples. The SPS-sintered ceramics were highly dense, and their grain sizes were ranging from 0.5 to 2 μm. The relative densities were 95.5%, 97.4% and 94.1% of the theoretical density for *x*=0.1, 0.4, 0.6, respectively. The resistivity of composite ceramics decreased with increasing temperature from 25 to 1000 ◦C, indicative of NTC characteristics. The obtained *ρ25*, *B*25-150, *B*700-1000, *E*a25/150 and *E*a700/1000 of the SPS-sintered composite NTC thermistors were in the range of 1.53×106 -9.92×109 Ωcm, 3380-5172 K, 7239-9543 K, 0.291~0.446 eV, 0.624~0.823 eV, respectively. This result indicates that these values can be adjusted by changing MgAl2O4 content. Fig. 9 compares the temperature dependence of electrical resis‐ tivity *ρ* of the samples 0.4MgAl2O4-0.6YCr0.5Mn0.5O3sintered by conventional sintering (CS) and SPS. It can be seen that the samples from SPS-sintered ceramics possessed a higher resistivity than that from conventional sintered ceramics. there are two possible reasons for the increase in the resistivity of SPS-sintered samples [12]: (1) During the SPS process, the short sintering period is advantageous in reducing chromium volatilization, thus leading to a decrease in Cr4+ and Mn4+ ion concentration, thereby increasing the resistivity as a result; (2) SPS-sintered

**Figure 7.** Time dependence of the temperature and applied pressure during SPS process.

**Figure 5.** XRD patterns of the SPS-sintered YCr1-*x*Mn*x*O3 ceramics [32].

30 Sintering Techniques of Materials

**Figure 6.** SEM images of surface section of SPS-sintered YCr1-*x*Mn*x*O3 ceramics: (a) *x*=0; (b) *x*=0.2; (c) *x*=0.4; (d) *x*=0.5.

samples have a smaller grain size, resulting in a decrease in the time between electron scattering events of charge carriers and thus increasing the resistivity [36]. In conclusion to this, SPS has potential superiority on synthesis high temperature NTC ceramic materials.

**Figure 8.** SEM images of the SPS-sintered *x*MgAl2O4-(1-*x*)YCr0.5Mn0.5O3 composite ceramics: (a) *x*=0.1; (b) *x*=0.4; (c) *x*=0.6 [12].

**Figure 9.** Temperature dependence of electrical resistivity *ρ* of the samples 0.4MgAl2O4-0.6YCr0.5Mn0.5O3 sintered by conventional sintering (CS) and SPS [12].
