**1. Introduction**

Microwaves are electromagnetic waves with wavelengths ranging from 1m to 1 mm, which correspond to frequencies between 0.3 and 300 GHz. This frequency range lies just above radio waves and just below visible light on the electromagnetic spectrum (Katz, 1992). The possibility of processing ceramics by microwave heating was discussed over 50 years ago by Von Hippel (1954a), and experimental studies on microwave processing of ceramics were started in the mid 1960s by Tinga and Voss (Tinga & Voss, 1968). Since then, the results of many investigations into microwave sintering and joining of ceramics have been reported (Bykov et al., 2001). Activity in this field began to accelerate in the mid-1970s because of a shortage of natural gas, prompting the investigation of microwave heating and sintering of several ceramic materials in the late 1970s and 1980s.

While most of today's industrial microwave applications involve the relatively lowtemperature processing of food, wood, rubber, polymers, etc., interest in high-temperature microwave processing of materials has been growing. In recent years, microwave heating has been widely employed in the sintering and joining of ceramics (Bykov et al., 2001; Huang et al., 2009).

Most of the reposts published in the literature assert that microwave-driven processes are faster than conventional heating processes. This faster speed is manifested as a reduction in the densification time of ceramic powder compacts, often allied to lower sintering temperatures (Bykov et al., 2001). In general, the kinetics of synthesis and sintering reactions are reportedly augmented by two or three orders of magnitude or even more when conventional heating is substituted for microwave radiation (Oghbaei & Mirzaee, 2010).

Microwave heating is a process whereby microwaves couple to materials, which absorb the electromagnetic energy volumetrically and transform it into heat. This differs from conventional methods in which heat is transferred between objects through the mechanisms of conduction, radiation and convection. Because the material itself generates the heat, heating is more volumetric and can be very rapid and selective (Sutton, 1989). Thus, microwave sintering techniques allows for the application of high heating rates, markedly shortening the processing time.

Microwave Fast Sintering of Ceramic Materials 5

microwave absorption becomes sufficient to cause self-heating. This hybrid method can result in more uniform temperature gradients because the microwaves heat volumetrically, and the external heat source minimizes surface heat losses. Hybrid heating can be achieved by using either an independent heat source such, as a gas or electric furnace in combination with microwaves, or an external susceptor that couples with the microwaves. In the latter, the material is exposed simultaneously to radiant heat produced by the susceptor and to

There is growing evidence to support the use of microwave hybrid heating in ceramic sintering and to justify continued research and development for its use in many ceramics manufacturing processes. In this context, this chapter discusses microwave interactions with ceramic materials, dielectric properties of ceramics, and theoretical aspects of microwave sintering, as well as results that highlight the successful application of microwaves to the

Microwaves are electromagnetic waves in the frequency band of 300 MHz (3 × 108 cycles/second) to 300 GHz (3 × 1011 cycles/second), which correspond to a wavelength range of 1 m to 1 mm. Within this portion of the electromagnetic spectrum there are frequencies that are used for cellular phones, radar, and television satellite communications (Thostenson & Chou, 1999). Typical frequencies for materials processing are 915 MHz, 2.45 GHz, 5.8 GHz, 22.00 GHz, 24.12 GHz, 28GHz and 60GHz, but only 915 MHz and 2.45 GHz are widely applied ((Katz, 1992; Committee on Microwave Processing of Materials: An Emerging Industrial Technology et al., 1994). The advantages of higher frequencies are more uniform electric field distribution in the cavity and higher power dissipated in dielectric materials. However, microwave apparatus is available on a very limited basis and at

The interaction of an electric field with a material may elicit several responses, and microwaves can be reflected, absorbed and/or transmitted by the material. In a conductor, electrons move freely in the material in response to the electric field, resulting in electric current. Unless the material is a superconductor, the flow of electrons will heat the material through resistive heating. However, microwaves will be largely reflected from metallic conductors, and therefore such conductors are not effectively heated by microwaves. In insulators, electrons do not flow freely, but electronic reorientation or distortions of induced or permanent dipoles can give rise to heating (Committee on Microwave Processing of

Because microwaves generate rapidly changing electric fields, these dipoles change their orientations rapidly in response to the changing fields. If the electric field change takes place close to the natural frequency at which reorientation can occur, the maximum amount of energy is consumed, resulting in optimum heating. In microwave processing terminology,

It is known that in dielectric materials the external electric field causes a redistribution of internal bound charges, which results in the polarization of the material. A measure of the response of a material to an external electric field is its dielectric permittivity, *ε*. If the

microwaves (Clark & Sutton, 1996).

rapid sintering of ceramic materials.

extremely high cost.

**2. Fundamentals of microwave heating** 

Materials: An Emerging Industrial Technology et al., 1994).

this event is described by the term "well-coupled" material.

Microwave processing eliminates the need for spending energy to heat the walls of furnace or reactors, their massive components and heat carriers. Hence, the use of microwave processing methods significantly reduces energy consumption, particularly in hightemperature processes, since heat losses escalate considerably as processing temperatures increase. However, the advantages of using microwave energy in high-temperature processes are by no means limited to energy savings. In many cases, microwave processing can improve the product quality (Bykov et al., 2001).

High heating rates not only shorten processing time and reduce energy consumption. Many high-temperature processes involve a sequence of steps that follow each other as the temperature rises. These sequences occur in multistage thermally activated processes, in which separate stages are characterized by different values of activation energy. Some of these stages may have a negative effect on the properties of the final product. In such cases, rapid heating may be vital in reducing the effects of undesired intermediate stages of the process. An example of such a multistage process is the sintering of ceramics. Various diffusion processes, such as surface, grain boundary and bulk diffusion, determine mass transport in different sintering stages.

Harmer and Brook (1981) postulated that rapid sintering produces a finer grained microstructure. This theory applies to all rapid sintering techniques, including microwave sintering. The theory of rapid sintering is based on the assumption that densification and grain growth are thermally activated processes and that the activation energy for grain growth is lower than for densification.

In fast firing, the objective is to enhance the densification rate in detriment to the coarsening rate by a rapidly approaching to the sintering temperature. Because coarsening mechanisms (e.g., surface diffusion and vapor transport) usually prevail over densification mechanisms (e.g., lattice and grain-boundary diffusion) at lower temperatures, it has been suggested that rapid heating to higher temperatures can be beneficial to achieve high density allied to fine grain size. In this case, the shorter time spent at lower temperatures serves to reduce the extent of coarsening, while the driving force for densification is not decreased significantly (Menezes & Kiminami, 2010), resulting in high densification and fine microstructures, which is a factor of paramount importance in the sintering of nanostructured ceramic and composite materials.

However, various fundamental problems are usually encountered when sintering materials by direct microwave heating. Most of the research on material processing by microwaves is based on conventional low-frequency (2.45 GHz) microwave applicators. However, such applicators do not couple microwave power efficiently to many ceramics at room temperature, and poor microwave absorption characteristics make initial heating difficult. Thermal instabilities may occur, which can lead to the phenomenon of thermal runaway; i.e., the specimen overheats catastrophically. The temperature gradients inherent in volumetric heating can lead to severe temperature non-uniformities, which, at high heating rates, may cause non-uniform properties and cracking.

These problems have led researchers to develop hybrid heating techniques that combine direct microwave heating with infrared heat sources. Increasing the temperature (with radiant heat) is a common method used by many researchers to couple microwaves with poorly absorbing (low-loss) materials. Once a material is heated to its critical temperature,

Microwave processing eliminates the need for spending energy to heat the walls of furnace or reactors, their massive components and heat carriers. Hence, the use of microwave processing methods significantly reduces energy consumption, particularly in hightemperature processes, since heat losses escalate considerably as processing temperatures increase. However, the advantages of using microwave energy in high-temperature processes are by no means limited to energy savings. In many cases, microwave processing

High heating rates not only shorten processing time and reduce energy consumption. Many high-temperature processes involve a sequence of steps that follow each other as the temperature rises. These sequences occur in multistage thermally activated processes, in which separate stages are characterized by different values of activation energy. Some of these stages may have a negative effect on the properties of the final product. In such cases, rapid heating may be vital in reducing the effects of undesired intermediate stages of the process. An example of such a multistage process is the sintering of ceramics. Various diffusion processes, such as surface, grain boundary and bulk diffusion, determine mass

Harmer and Brook (1981) postulated that rapid sintering produces a finer grained microstructure. This theory applies to all rapid sintering techniques, including microwave sintering. The theory of rapid sintering is based on the assumption that densification and grain growth are thermally activated processes and that the activation energy for grain

In fast firing, the objective is to enhance the densification rate in detriment to the coarsening rate by a rapidly approaching to the sintering temperature. Because coarsening mechanisms (e.g., surface diffusion and vapor transport) usually prevail over densification mechanisms (e.g., lattice and grain-boundary diffusion) at lower temperatures, it has been suggested that rapid heating to higher temperatures can be beneficial to achieve high density allied to fine grain size. In this case, the shorter time spent at lower temperatures serves to reduce the extent of coarsening, while the driving force for densification is not decreased significantly (Menezes & Kiminami, 2010), resulting in high densification and fine microstructures, which is a factor of paramount importance in the sintering of nanostructured ceramic and

However, various fundamental problems are usually encountered when sintering materials by direct microwave heating. Most of the research on material processing by microwaves is based on conventional low-frequency (2.45 GHz) microwave applicators. However, such applicators do not couple microwave power efficiently to many ceramics at room temperature, and poor microwave absorption characteristics make initial heating difficult. Thermal instabilities may occur, which can lead to the phenomenon of thermal runaway; i.e., the specimen overheats catastrophically. The temperature gradients inherent in volumetric heating can lead to severe temperature non-uniformities, which, at high heating

These problems have led researchers to develop hybrid heating techniques that combine direct microwave heating with infrared heat sources. Increasing the temperature (with radiant heat) is a common method used by many researchers to couple microwaves with poorly absorbing (low-loss) materials. Once a material is heated to its critical temperature,

can improve the product quality (Bykov et al., 2001).

transport in different sintering stages.

growth is lower than for densification.

rates, may cause non-uniform properties and cracking.

composite materials.

microwave absorption becomes sufficient to cause self-heating. This hybrid method can result in more uniform temperature gradients because the microwaves heat volumetrically, and the external heat source minimizes surface heat losses. Hybrid heating can be achieved by using either an independent heat source such, as a gas or electric furnace in combination with microwaves, or an external susceptor that couples with the microwaves. In the latter, the material is exposed simultaneously to radiant heat produced by the susceptor and to microwaves (Clark & Sutton, 1996).

There is growing evidence to support the use of microwave hybrid heating in ceramic sintering and to justify continued research and development for its use in many ceramics manufacturing processes. In this context, this chapter discusses microwave interactions with ceramic materials, dielectric properties of ceramics, and theoretical aspects of microwave sintering, as well as results that highlight the successful application of microwaves to the rapid sintering of ceramic materials.
