Field-Assisted Sintering

This chapter includes the general introduction to field-assisted sintering techniques. A brief historical overview of field-assisted powder consolidation methods is presented. The general categorization of field-assisted sintering technologies is introduced. The underlying physical mechanisms of field-assisted sintering are described in terms of thermal and nonthermal factors influencing mass transport.

This chapter introduces the basics of resistance sintering, which was an important step in the historical development of electric current-assisted sintering methods. Resistance sintering utilizes heat generated within the powder charge carrying a direct electric current or an alternating current of low frequency and is assisted by pressure when the initial state of the sample is a loose powder. As there is no heat coming from the die, only conductive materials can be sintered by resistance sintering. The die/plunger assemblies used for resistance sintering are described. The issue of temperature distribution within the sample during resistance sintering is discussed based on experiments and modeling.

In this chapter, the principles and physical mechanisms of high-voltage consolidation of powder materials are described. Several variations of this type of field-assisted sintering are introduced: high-voltage electric discharge consolidation, high-energy high-rate consolidation, pulse plasma sintering, and capacitor discharge sintering. Different stages of high-voltage powder consolidation are described. Various physical phenomena present during high-voltage consolidation, including local processes at inter-particle contacts, are analyzed. The equipment setups for different types of high-voltage consolidation are described. The results of processing by high-voltage electric pulse consolidation of selected examples of materials are discussed.

In this chapter, the principles and physical mechanisms of low-voltage electric pulse sintering (including spark plasma sintering (SPS)) are introduced. The low-voltage electric pulse sintering equipment is described. The experimental and theoretical analyses of the temperature distribution specifics and the possibilities of temperature control under SPS conditions are presented. Various physical phenomena of thermal and nonthermal (field) nature occurring at micro- and macro-level during SPS are discussed. The analyzed thermal factors include macroscopic temperature gradients, local temperature gradients at the inter-particle contacts, high heating rates, and thermal diffusion. The analyzed field factors include electromigration, the possibility of plasma formation, electroplasticity, and breakdown of oxide layers at the inter-particle contacts. The respective constitutive models of SPS are introduced. The results of coupled electromagnetic–thermal–mechanical finite element solutions of SPS problems are described. Various processing and testing methods developed using SPS equipment are analyzed. Selected examples of processes and materials developed using SPS, including SPS-based joining, are introduced. SPS-based surface engineering and processing of porous materials are introduced in addition to the description of SPS of dense bulk materials.

In this chapter, the principle of flash sintering is presented, and the underlying mechanisms of the phenomenon are discussed. Flash sintering has attracted significant attention as a sintering method offering energy saving and shortening of processing times of ceramics to full density. In its “traditional” format, flash sintering occurs when an electrical potential is applied to the pre-compacted specimen heated in a furnace. The characteristic field strength and power dissipation values in flash sintering are 100–100 V·cm⁻¹ and 10–1000 W·cm⁻³, respectively. From the viewpoint of sintering science, flash sintering is a remarkable phenomenon. It is currently agreed that “traditional” flash sintering is accompanied by a sudden increase in the conductivity of the sintered material, while the temperature instability plays a crucial role in the development of flash sintering. In the present chapter, initiation of flash sintering events by arc plasma and microwave radiation is also described. Possibilities of conducting flash sintering using sintering molds (including “flash spark plasma sintering”) are discussed. Microstructural evidence of grain-boundary melting in flash-sintered ceramics is provided. Possibilities to flash sinter all types of materials regardless of the way their electrical conductivity changes with temperature by forcing thermal runaway by applying a certain electric current pattern are presented. Application of flash sintering as a microstructure design method is exemplified by describing the origin and features of compositional and structural inhomogeneities arising in the flash-sintered materials due to melting of the material located at the grain boundaries. Examples of flash sintering of composite materials and accelerated phase homogenization during flash sintering of powder mixtures are provided. Successful applications of flash sintering for the production of functional materials and multilayered structures are discussed.

In this chapter, the principles and possible mechanisms of sintering in the constant electric field in the noncontact mode and sintering in the constant and pulsed magnetic fields are described.

In this chapter, the principles and mechanisms of microwave heating and sintering are described. The method of effective medium approximation for the determination of effective microwave dielectric properties is introduced. Principles of self-consistent electromagnetic and thermal modeling are described. Experimental evidence of microwave nonthermal effects and the respective models of microwave nonthermal effects are described. Models of microwave sintering taking into account the influence of ponderomotive forces are explained. Examples of fully coupled electromagnetic–thermal–mechanical finite element modeling of relative density and temperature fields during microwave sintering are presented. Grain growth during microwave sintering is discussed. Selected examples of materials consolidated by microwave sintering are presented.

In this chapter, the principle of induction heating sintering and the features of the microstructure development of induction sintered materials are presented. Direct induction heating can only be realized in conductive materials and is caused by eddy currents created by an alternating magnetic field. Two possible schemes of conducting induction sintering are described. In the first scheme, a conductive container or a die (susceptor) is heated by the eddy currents, while the powder in the die is heated through radiation and thermal conduction. In the second scheme, the eddy currents are induced directly in the pre-consolidated compact, which is placed inside a coil. While the first scheme is suitable for both conductive and non-conductive materials, the second scheme can be used only for conductive materials. Direct induction sintering requires powder compacts having high electrical conductivity in the beginning of the process. The existence of an incubation period at the beginning stages of induction heating due to poor coupling was reported. Both schemes of induction heating sintering provide fast heating and fast cooling (the latter happens when the coil current is switched off), dramatically shorten the sintering time relative to conventional sintering, and ensure energy efficiency. The suggested eddy current-induced effects responsible for fast densification during induction heating sintering are presented, and the influence of the heating rate on the microstructure development is discussed. Examples of the successful use of induction sintering for obtaining nanostructured and fine-grained metallic, ceramic, and composite materials are presented. Possibilities of preserving metastable phases in the materials processed by induction heating sintering are demonstrated.

In this chapter, the principles and equipment for magnetic pulse compaction (MPC) of powder materials are introduced. Modeling of uniaxial and radial MPC is described. Selected examples of application of MPC to different materials are presented.

In this chapter, the behavior of multicomponent and reacting powder systems in electromagnetic fields is discussed in view of the possibilities of the formation of dense materials as well as reaction products of different porosities and morphologies. General considerations regarding the process of reactive sintering and its driving forces are presented. Studies demonstrating the intensification of diffusion in the presence of the inter-particle contact heat sources are reviewed. Possibilities of reactive sintering during microwave treatment and sintering in constant magnetic field are presented. Initiation of chemical reactions by electric current, including high-voltage electric discharges, and mechanisms responsible for acceleration and deceleration of chemical reactions under applied electric field are discussed. It is shown that spark plasma sintering (SPS) has become a popular synthesis method in solid-state chemistry and a materials design tool at different length scales. The best scenario for obtaining a dense fine-grained material by reactive SPS is simultaneous reaction and densification: the reaction in the system should start at temperatures high enough to sinter the reaction product to high relative densities. Possible transformations of carbon allotropes under applied electric current are reviewed. Specifics of interaction of materials with carbon of graphite tooling and graphite foil and the mechanisms of carbon incorporation into materials of different chemical nature during SPS are discussed. Examples of materials with attractive mechanical and functional properties obtained by reactive SPS are presented. It is demonstrated that particles with core–shell morphology are interesting objects to be processed by SPS into bulk porous or dense solids. It is concluded that the successes of reactive SPS in synthesizing bulk materials can be further extended to the simultaneous synthesis and joining of different materials as well as manufacturing of coatings.

In this chapter, the use of different types of electromagnetic radiation (infrared (IR), visible, and ultraviolet (UV)) for sintering of particulate materials is described. The radiation is directed onto the layers of particulate materials to induce rapid heating and consolidation into porous or dense structures. IR radiation can be produced by specially designed emitters and lasers; it can also be harvested as part of the solar energy. Direct IR irradiation, treatment in solar furnaces, and laser treatment enable higher heating rates in comparison with furnace sintering. The IR radiation-assisted sintering and laser and photonic sintering methods are compatible with the roll-to-roll fabrication, which is a promising modern approach in the production of flexible electronics. The successful applications of photonic sintering for the fabrication of metallic and composite films on flexible substrates are reviewed. It was shown experimentally that the sintering efficiency of metal nanoparticles during photonic sintering depends on their size and size distribution; theoretical studies of the optical absorption as related to nanoparticle sintering are highlighted. For the purposes of powder sintering, UV light is used to initiate reduction of oxides of metals and induce decomposition of compounds to in situ synthesize nanoparticles of the target phase with a high propensity for sintering.