Research Incentives

The above-mentioned analysis of the previous studies indicates that SPS is a rapid method for the production of dense ceramic materials. However, the SPS mechanisms of rapid densification have not been clearly identified, especially in terms of the effect of electric current, and mainly due to a number of obstacles, including: (i) the complexity of the deconvolution of the Joule heating and possible non-thermal field phenomena, (ii) the difficulty of the accurate experimental measurements of the specimen’s temperature and electric current parameters during SPS. At the same time, addressing the obstacles (i) and (ii) can provide a better understanding of SPS-specific factors distinguishing SPS from conventional powder consolidation techniques. To achieve this goal, a consistent research program should be focused on the following problems:

(i): Deconvolution of the electric current effect from the temperature effect is required. For electrically conductive powders, the source of the densification in SPS are the pressure, heat and electric current, while in HP those are pressure and heat by the external heating elements. Heating in HP is transferred by radiation and conduction, which induce certain spatial temperature distributions during the HP operation. However, the heat generation in SPS stems from the Joule heating of the graphite SPS tooling set and of the powder too. If the powder is electrically conductive, a significant fraction of the electric current can pass through it, so that the temperature of the powder becomes higher than that of the die. Usually, during SPS, the control temperature is measured at a selected point in the die wall, therefore special techniques are required to determine the real temperature of the powder for electrically conductive materials subjected to SPS.

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(ii): Quantification of the electric current density in conductive powders during SPS process is required. The electric current value recorded during SPS process is not the same as the value of the electric current directly flowing into the powder. The portion of the electric current flowing into the powder is mainly dependent on the electrical and thermal contact resistance, temperature, and the electric resistivity of each material in the die set. During SPS, the necks between particles evolve, so that the electric current density changes respectively.

For addressing the problem (i), various deconvolution techniques can be utilized enabling the comparison of the densification mechanisms with and without electric current under the same sintering conditions. For addressing the problem (ii), FEM simulations of the SPS specimen-tooling setup can be implemented for the assessment of the fraction of the electric current flowing directly into powders. Here the inter-particle neck area evolution should be taken into account for the correct estimation of the local electric current density evolution.

Based on the solution of the problems (i) and (ii), one can find whether the presence of electric current can change the densification mechanism of the powder. In the present study, we use the Olevsky’s approach utilizing the constitutive equation for high-temperature creep of a non-linear viscous porous material to find the densification mechanism of powder materials subjected to SPS.

We employ three strategies to analyze the electric current effects on the densification mechanism during the current-assisted powder consolidation.

First, the conventional constitutive equation describing the behavior of powders compacted by HP [84], is used to study the densification behavior of a conductive powder during SPS. If, as a result of the regression of the experimental data, based on the above-mentioned constitutive

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equation, the value of the power-law creep exponent (corresponding to the non-linearity of the material constitutive behavior) is found to be the same for the sintering with or without electric current (carried out under the same pressure and temperature conditions), then the densification mechanism is not changed by the presence of the electric current. Otherwise, if the value of the power-law creep exponent is found to be different for the pressure and temperature-identical sintering with or without electric current, then the densification mechanism is changed due to the presence of the electric current.

Second, following the results of the electroplasticity theory and experiments from Conrad [85-87] we assume that other power-law creep parameters like frequency factor or deformability of material can be changed due to the presence of the electric current.

Third, the presence of other dissipative mechanisms introducing extra terms in the SPS constitutive equation in addition to the traditional power-law creep expression due to the influence of electric current is assumed. The extra terms can be possible influences of additional dissipative mechanisms, such as electromigration, oxide reduction, defect generation, grain growth and grain orientation, etc. To support the theoretical assessment of the contributions of these phenomena into the intensity of mass transfer during SPS, additional experimental procedures have to be carried out including microscopic analysis of the structure and phase composition of the materials subjected to SPS.

Previously, MSC, B-G, and Olevsky’s methods analyzed the densification mechanisms by studying the porosity evolution curves. In particular, grain boundary sliding (GBS) is associated with grain rotation, resulting in the suppression of the grain orientation. In this connection, EBSD

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technique can give the experimental proof of the densification mechanism of tungsten powders during the sintering by assessing the degree of the grain orientation.

From the conducted literature survey, it can be concluded that SPS possibly can remove surface oxide layers or slower the grain growth. The influence of these electric current effects has been analyzed in the present study for the SPS of surface-oxidized Mo nanopowders.

One advantage of the SPS process is the fast heating rate which enabling the prevention of the grain growth. Using this advantage, for nuclear application, Mo metal mirror with submicron grain size can be fabricated by the Mo nanopowders compaction by SPS.

Flash sintering showed that increasing the electric voltage applied to an ionic conductive powder compact initiated the thermal runaway, and achieved the densification in less than 10 sec.

However, Todd and Zhang showed that flash sintering is simply a usual consequence of the negative temperature coefficient of electric resistivity. The results of our studies indicate that high electric current can change the densification mechanism, inducing the fast densification. Based on these facts, we developed a novel NFSPS (net-shape flash spark plasma sintering) process enabling the high electric current flow into a powder within the short time period. The high density of the electric current turns out to be beneficial in terms of decreasing the energy spent and fast densification.