Future Directions in Sintering Research

From time to time, the gap between sintering science and technology is being attempted to be bridged, but there are still a number of unresolved issues in sintering. So far, only thermal energy was considered for accomplishing sintering of a powder compact. However, other sources of energy may be treated exclusively or in combination with others to achieve densification. The main goal is to tailor the material properties during sintering through microstructural modifications. But in doing so, the very chemical nature of the material subjected to sintering needs to be considered. It is at this stage that the role of electronic structure comes into picture. The present paper reviews this aspect and proposes how the studies on nano-crystalline materials are able to validate the very basic electronic processes occurring during sintering.


Introduction
Various attempts have been made from time to time to define sintering.Hausner [1] defined it as 'Bonding of particles in a mass of powder by molecular (or atomic) attraction in the solid state, by the application of heat, causing strengthening of the powder mass and possibly resulting in densification and recrystallization by transport of the material.'An updated definition according to the present author is as follows: ''Sintering is the bonding of particles consisting of one or more components in a mass of crystalline or amorphous powder, in a loose or green compact form, at atomic level by the application of energy, single or in combination with other, through external means, resulting in improvement in one or more properties due to the associated structural modifications.The system remains predominantly solid during sintering.' This definition takes care of the global aspect of structure-properties-processing relations and does not count heavily only on one factor densification.In many engineering applications, a set of optimized end properties may over-ride the single property i.e. densification.
It is gratifying that the gap in sintering science and technology has decreased appreciably in the last decades.The reason for this has been the rapid advances in sintering processing and research equipments.A burning example is the advent of field assisted sintering on a big scale, thanks to the active contributions of Japanese industries.New developments in structural characterization equipments have also helped in decreasing this gap.It is no wonder that Kuczynski [2] sixty years back had to select the big size copper spheres in his model sintering experiment to obtain accurate results.
Another point to be raised here is the classification of sintering into subsets like solid state and liquid phase sintering.There is need for a serious relook in this trend, as the commonality of microstructural features is pervading.Likewise, other subsets like activated solid state or liquid phase sintering do require a revisit [3].

Sintering Mechanisms
Some of the approaches presented for studying the sintering mechanisms can be categorized as [4]: • Analytical models In the analytical model, two schools of thoughts i.e. diffusion and plastic deformation are prevalent and the literature show how doggedly these two groups supported their theories.However it is worth mentioning that in essence they were not in dispute.Of late, the concept of kink in dislocations proposed by Seeger [5] is a good unifying approach.It is worth mentioning the study of Antsiferov [6] proposing the dissipative dislocation model for sintering.The last model i.e. electronic mechanism was proposed by Samsonov [7].According to him [8] the attention was given only to some separate stages of sintering that results from different theoretical presentations usually based on atomic level.This prevents the establishment of the general sintering theory that is based on uniform ideas, not mentioning at that, that modern sintering theories inadequately take care of chemical nature, viz. of characteristic properties of sintered materials.Perhaps, transition from the atomic theory to the electronic one should be examined for a uniform quantitative description of the fundamental phenomenon in diffusion and creep.One of the possibilities is the use of the activated volume, especially, because it allows in a definite way, simultaneous considerations of the problem concerning both the stress and material transport [16].Prydko et al. [17] discussed this in detail and showed how the electronic properties of oxide ceramics depend directly on the character of 'frozen' polarization, which in its own turn is related to the type and structure of defects formed during the sintering process.
Among metals, ionic solid, nonmetals, refractory compounds and covalent ceramics, metals with reasonably non-localized require lower homologous temperature for sintering as compared to the compounds.In compounds such as CaF 2 and MgO, possessing ionic bond, one requires a relatively higher homologous temperature.Refractory carbides with mixed bonds require still a higher temperature.The covalently bonded solids are followed by nonmetallic crystalline materials, like boron and silicon.
Roberts [18] gave a viewpoint of the electronic theory of sintering, which was contested by Savitskii [19].According to latter, sintering is based on the interaction of particles or particles of a powder mixture: that is taking into account the particles' nature, size, shape, relative position and many other factors.However, in the same breath Savitskii [17] did not rule out inter-atomic interaction during sintering, at far lower level for which the laws of electronic theory and quantum mechanics are the identifying characteristics.He justifies that some features of a phase diagram are defined by inter-atomic interactions, thus, making the connection with electronic theory an indirect one.
Kuczynski [9] in his last days was very active in the statistical model of sintering, which in a way was not far from the statistical concept proposed by Samsonov.With modern progress in sophisticated instrumentation, for example, scanning tunneling microscope, the concept proposed by Samsonov is justifiably amenable to verification.At present with the help of atomic force microscopy, it is possible to measure the inter-atomic potential for forcedistance spectroscopy of specific sites.
In the sintering definition by the author, mentioned earlier a point is made of the extrinsically activated atomic movement.Here the role of high pressure as the exclusive sintering route is worth mentioning.As compared to temperature, pressure induces more dramatic changes in the physical and chemical properties [10].Under the action of pressure, as the interatomic distances decrease, the overlap of the outer electron shells on the constituent atoms increases leading to an increase in the band widths, the extent of hybridization of the outer electronic orbital, shifting the energy bands as well as the Fermi energy.High pressure experiments have played a significant role in explaining the nature of felectron based lanthanide (4f) and actinide (5f) systems.High pressure induced sintering has great future for preparing high coercively rare-earth compound based permanent magnets.In f-electrons under pressure, an increase in delocalization of the f-electron states leading to an increase in e/a ratio as well as a decrease in the R a /R b ratio due to the contraction of f-electron orbital of the rare-earth atom take place.

Sintering of Nano-Crystalline Powder
Nano-scale particles are of great interest because new properties and unique behavior can be found when the size of object falls below ~100 nm.These new properties result because the ratio of surface area to volume is now so large that surface that surface effects become dominant.For example, a nano-particle 5nm in diameter has half of its atoms at the surface.Atomic diffusivities are many times different than for normal crystalline materials.For example diffusivity of silver atoms in copper in nanocrystalline state (10 nm average size) is enhanced up to twenty times as compared to single crystal copper [11].Moreover electronic and photonic enter a new level because electrons are confined within such a small volume that quantum effects now dominate electronic behavior.Under such circumstances nano-particles may be treated as ideal material for testing the applicability of electronic mechanism for sintering.The loose amorphous structure at the grain boundary in nanocrystalline material from the viewpoint of electronic structure can be exemplified by the sp 2 configuration i.e. graphite-like bond in case of nano-crystalline diamond confirmed after Raman spectroscopy measurement [12].
The main purpose of nano-powder sintering is retention of initial nano-crystalline structure in the final stage.The extremely fine grain size, many times, lead to inherent metastable structure of any powder to depart from equilibrium.This fine grain size may cause other deviations from the equilibrium such as alternate crystal structure, extended solubilities or changes in physical properties.In sintering of nano powders, it is necessary to define the conditions under which the metastability is lost.Mechanical milling (top-down process) is the conventional method for producing nano-sized powders where the decrease in the powder particle size with the concurrent increase in the specific surface area results.The steps involved are: • Deformation of powder creating a number of dislocations • Extremely dense dislocation structure resulting in grain sub-division • Emergence of nano-grained structure with high angle boundaries.
The main challenge with this method is to avoid contamination of the nano-powders by the materials used in the milling process.A second disadvantage of this process is that the particle sizes are not uniform.In a way this is not a major disadvantage when the nanopowder is going to be green compacted and sintered [13 ].
There is varying opinion whether the basic mechanisms for sintering of nanocrystalline powder is similar to conventional powders.The major question is whether the physics of sintering process in case of nano-scale particles is size-scale dependent.At lower and lower limit of size, the realization of electronic exchanges between atoms is more easily detectable.This can be easily demonstrated from the vapor pressure ratio of nano-scale metal particles of two elements zinc and gold, when a molten drop of the concerned bulk metal is in equilibrium over the flat surface of the same metal.Below a size of 3 nm, substantial increase in vapor pressure was noticed [13].It is interesting to note that for a particle size for say 100 nm, the values of the ratio for these two metals are practically similar, but below 3 nm the value for zinc is greater than that for gold.This difference in ratio gets widened with further decrease in the particle size.This means that a much stable electronic configuration s2 of zinc atoms as compared to those for gold is playing a vividly identifiable role, when the particle / drop size is in few nanometers range.
The description of nano-structured materials is incomplete without mentioning amorphous Melt Glasses.In glass, there is special atomic structure, which makes them stand between perfect liquid and crystalline solid.It is logical that these icosahedra are precursors for nano-crystals.Cohen and Grest [14] rightly termed the amorphous grain boundaries in nano-structured materials as liquid like material acting as the plasticity carrier.The connecting link between the sintering features of nano-structured materials and amorphous sintered solids must never be ignored.A knowledge in one sector is helpful in understanding the other.A good example of the existence of amorphous layer at grain boundary of nanostructured materials is sintered nano-crystalline diamond, where the loose structure of sp 2 graphite-like bond exists [12].It is, but necessary to get convinced that in nano-structured materials ,the bulk phase diagrams are no more valid, and one has to strive for 'grain boundary phase diagram.' Of late, a new term 'phase field' has come into being, which is defined as a region in space and time which is occupied by one single phase [15].In this definition the phase field variable plays the role of an indicator function making the local state of a system.The phase field theory of today addresses the complete picture of the evolution of microstructures in inorganic materials including metastable structures and history-dependent effects.

Conclusions
Based on the above highlighted considerations following conclusions can be drawn: 1.There is a need for a unifying approach rather than a divisive approach in understanding sintering mechanism.A master sintering recipe is the call of the day.
2. Future sintering processing shall not be dependent on only one type of energy i.e. thermal.

3.
A better awareness among the researchers in the area of nano-structured material is needed as far as the terminology is concerned.The 'nanostructure' designation ought to be assigned based on the final end product's structure, rather than on the initial particle size of the powder mass.
4. Atomic simulations are needed for bridging the length scale, although bridging the time scale is not easy.5.In the presence of practical difficulties encountered presently in consolidating the nanostructured monoliths, it is opportune to pay more attention on nano-structured composites.This shall be good precursor for understanding the sintering process of the former.