**1. Introduction**

158 Sintering of Ceramics – New Emerging Techniques

[5] C. F. Hu, Y. Sakka, H. Tanaka, T. Nishimura, and S. Grasso, "Low Temperature Thermal

675-81 (2009).

Expansion, High Temperature Electrical Conductivity, and Mechanical Properties of Nb4AlC3 Ceramic Synthesized by Spark Plasma Sintering," *J. Alloys Compd*., 487,

> Sintering is one of the most frequently used processing techniques in the studies of material science that is used to produce density-controlled materials and components from metal or/and ceramic powders by applying thermal energy. Hence, sintering is categorized in the synthesis or processing element among the basic elements of materials science and engineering. As material synthesis and processing have become immensely vital in recent years for materials development, the importance of sintering is increasing as a material processing technology. Sintering is, in fact, one of the oldest technologies, originating in the prehistoric era with the firing of pottery. The production of tools from sponge iron was also made possible by sintering. Nevertheless, it was only after the 1940s that sintering was studied fundamentally and scientifically. Since then, remarkable developments in sintering science have been made. One of the most important and beneficial uses of sintering in the modern era is the fabrication of sintered parts of all kinds, including powder-metallurgical parts and bulk ceramic components (German, 1996).

> Sintering mostly aims to produce sintered parts with reproductive and, if possible, designed microstructure through control of sintering variables (Chiang et al., 1996; Green et al., 1989). Microstructural control means the control of grain size, sintered density, and size and distribution of other phases including pores. In most cases, the final goal of microstructural control is to prepare a fully dense body with a fine grain structure.

> Now in order to improvise the conditions of sintering, a number of researches have been carried out throughout the world. By sintering conditions, we mean the parameters involved in the process, namely, sintering time, temperature, rate of temperature increase, pressure involved in the process, or even the microstructural behavior in greater sense. This chapter focuses on one of these recently developed techniques called Magnetic Pulse Compaction, which may be used as an additional process on materials, prior to sintering. In special cases, based on commercial requirements, this process can stand alone, without even having the necessity of sintering.

> In addition to the detailed description of Magnetic Pulse Compaction, a number of material fields have also been tried to cover where this process can either be used, or has successful

Effect of Magnetic Pulsed Compaction (MPC) on Sintering Behavior of Materials 161

hammer. While almost any material can be compacted to full density using a sufficiently large impact pressure, the important benefits of magnetic powder compaction when

The basic working principle of the Magnetic Pulse Compaction process is shown in Figure 1. Powders are filled in a conductive container (armature) placed in the bore of a high field coil. The coil is pulsed with a high current to produce a magnetic field in the bore using the

compared to die pressing are higher green densities and higher aspect ratios.

Biot-Savart law, which in turn, induces currents in the armature using Ohm's law.

ε= −

( ) <sup>0</sup> ( )

μ

<sup>2</sup> 4 *I t dl r B t*

*dB dydx dt*

ε

( ) ( ) <sup>1</sup> *It V t*( ) *<sup>R</sup>* = −

where *V* is terminal voltage, *R* is resistance of external resistor and *ε* is the induction magnetic field. The induced currents interact with the applied magnetic field to produce an inwardly acting magnetic force that collapses the tube, thereby compacting the powder,

*F I t dx B* = × ( )

launched into the powders with a large kinetic energy within a few microseconds of the compaction cycle. The powders are pressed to full density via the transmitted impact

The process steps of MPC are similar to conventional PM compaction technique, and include tooling for compaction, powder filling, part extraction and sintering, as well as the optional steps of sizing and finishing. In most commercial applications, the powder filling and compaction are done in air medium at room temperature. The filling can also be carried

energy, with the entire compaction occurring in less than one millisecond.

are induction current and magnetic field, respectively. The armature is

π

where µ0 is the permeability of free space, r is the distance from the element to the point *P*, and *r*ˆ is a unit vector pointing from dl toward point *P*. We find the total field at *P* by integragrating this expression over the entire current distrubition. Now, Faraday's law of induction states that the emf (ε) induced in a circuit is directly proportional to the time rate

<sup>×</sup> <sup>=</sup>

*r*

(1)

(2)

(3)

(4)

Biot-Savart Law:

Faraday's Law:

Ohm's Law:

using Lorentz force.

Lorentz Force:

where I(t) and *B*

of change of magnetic flux through the circuit.

Hence, current (I) is induced using the Ohm's Law

contribution compared to other conventional methods. Several experimental results and reports have been discussed, especially on ceramics and hard-to-compress materials. In each case, we have assayed to delineate the differences that this process is introducing, the contributions it has, or the necessity to incorporate this in the chain of processes. Finally, we made some concluding remarks based on real term experimental results about the advantages of this process, which may aid certain streams of material research. Applications of MPC, both as a stand-alone process and prior to sintering have been highlighted throughout the entire chapter.
