1. Introduction
Spark plasma sintering (SPS), also known as field assisted sintering technique (FAST), belongs to a class of sintering techniques in which densification is enhanced by the simultaneous application of axial pressure and elevated temperature generated by a high current flow [1, 2]. SPS has been successfully used in the preparation of functionally graded materials [3], ceramics [4-6], magnetic materials [7, 8], alloy [9], etc. It is believed that the SPS has a significant advantages in decreasing the sintering temperature and sintering periods, and also preparing high density ceramics [10]. Although SPS has been widely used to prepare dense transparent and structural ceramics, little reports focus on the application of this technique to prepare dense ceramics for negative temperature coefficient (NTC) thermistor applications [11,12]. So this chapter first provides a summary of fundamental theoretical aspects of spark plasma sintering, and then the application of spark plasma sintering in NTC thermistor ceramics are introduced emphatically. At the end, the future research and application of spark plasma sintering in the NTC thermistor ceramics are forecasted.
2. Theoretical aspects of spark plasma sintering
SPS is a novel effective sintering technique, which has been used in the research and development of various kinds of materials. However, there is no uniform understanding in the sintering mechanism of SPS. In general, SPS is a pressure sintering method based on the simultaneous application of axial pressure and high temperature plasma momentarily 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 shows the schematic of SPS furnace [16].
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
NTC thermistors are thermally sensitive resistors whose resistance decreases with increasing temperature. Their resistivity can be expressed by the following Arrhenius equation [20, 21]:
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-
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-
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
4. Summary
The fundamentals, applications of spark plasma sintering to thermistor ceramics are reviewed in this chapter. For thermistor ceramics, the advantages of spark plasma sintering against traditional sintering are as follows: (1) Faster heating rate decreases the sintering time by using spark plasma sintering, and thus saving energy than traditional sintering; (2) Spark plasma sintering can ensure ceramics with high density and small grain at a low sintering temperature and a short sintering period. What’s more, spark plasma sintering has shown significant advantages in the fabrication of YCrO3 perovskite-based thermistor ceramics and provides an efficient mean for the study of the conduction mechanism of NTC thermistors.
In the past years, there have had a significant developments and advances regarding to the spark plasma sintering of ceramics. However, the fundamental theory of spark plasma sintering is not fully understood, and also needs massive fundamental research and practical innovation to perfect. It can be forecasted that there is a great future for the successful commercialization of spark plasma sintering in ceramic preparation.
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