Flash sintering as a nucleation phenomenon and a model thereof

doi:10.1016/j.jeurceramsoc.2014.04.043

Journal of the European Ceramic Society

Volume 34, Issue 15, December 2014, Pages 4063-4067

https://doi.org/10.1016/j.jeurceramsoc.2014.04.043 Get rights and content

Abstract

Isothermal field-assisted sintering of two-phase, 50 vol% 3YSZ-alumina composites exhibit an incubation time for the onset of the flash event. Weaker applied fields and lower temperatures lengthen the incubation period. The effect is highly non-linear. For example at 1300 °C and 150 V cm⁻¹ the flash occurs nearly instantaneously (in 10 s), but extends to 2 h at 1275 °C and 65 V cm⁻¹. This behavior is reminiscent of nucleation and growth phenomena in chemically driven experiments involving phase transformations in the solid state. Here, a model for nucleation under electrical driving forces, based upon the growth of embryos of colossal permittivity is presented. Nucleation is the precursor to the onset of a flash. Therefore it occurs at the furnace temperature. Joule heating is a consequence of nucleation not the cause of it.

Introduction

Flash sintering is emerging as a distinct mechanism in field assisted sintering of ceramics. The “flash” is initiated above a threshold applied field at a given temperature, while the extent of densification is controlled by the limit on the current density placed at the power supply.¹ The method has been applied to several oxides,2, 3, 4, 5, 6, 7, 8 SOFCs9, 10 as well as to non-oxides.¹¹ The effect can be induced by both DC as well as AC electric fields.¹² Among the various mechanisms proposed in the literature, Joule heating remains of the greatest interest,13, 14 although the temperatures required for nearly instantaneous sintering are far above what can be reasonably well predicted in this way.

The other suggestion has been to say that a defect avalanche in the form of Frenkel pairs is precipitated which ionize into charge neutral defects and electron–hole pairs. The defects enhance diffusion while the e–h pairs induce high conductivity and photoemission.¹⁵ While, this suggestion does explain why diffusion and conductivity are simultaneously enhanced, a quantitative understanding of how it can happen remains obscure. Some support for the defect induced mechanism is found in residual effects of the flash on defect concentrations in MgO-doped alumina⁴ and in yttria stabilized zirconia.¹⁶

If indeed flash sintering is instigated by the “nucleation” of defects, then it should be accompanied by an incubation time in experiments carried out at isothermal furnace temperatures. Here we report these results, and show that they are related very non-linearly to the applied field. Tentatively, a model for the nucleation of dipole clusters, of abnormally large permittivity, is developed and analyzed to explain these results. Nucleation is a precursor to the onset of the conduction non-linearity, and, therefore, occurs at the furnace temperature. Joule heating is a result of and therefore is subsequent to nucleation.

Section snippets

Experiments

The samples were prepared from commercially available tetragonal zirconia (3 mol% yttria stabilized zirconia – 3YSZ) powder (TZ-3YS-E grade; Tosoh Corp., Shunan, Japan) with a particle size of 600 nm. The high purity α-alumina powder (purity >99.99%), was obtained from Taimicron TMDAR, Taimei Chemical Co., Ltd., Tokyo, Japan, and had a particle size of 100 nm. Two phase composites constituted from equal volume fractions (50/50) of 3YSZ and alumina composite was prepared by following the procedure

A model for nucleation

In isothermal furnace temperature experiments, the flash phenomenon occurs in three stages. The first is the onset of the non-linear conductivity at the applied field. This is the nucleation event, which occurs at the furnace temperature, and embodies an incubation time. The non-linear rise in conductivity is controlled by switching the power supply to current control; this is the second stage. The third stage is the quasi-steady state that is established under current control. Joule heating

Summary

The phenomenon of flash sintering has the characteristics of nucleation and growth. The nucleation event is the abrupt transition from insulating to conducting state in the ceramic. It is instigated when a critical field is applied while the specimen is held at a constant temperature, equal to that of the furnace. The nucleation event embodies an incubation time that lengthens from a few seconds to several thousand seconds as the applied field is reduced. This relationship of the incubation

Acknowledgements

The author would like to express great appreciation to Dr. John Francis, Mr. Shikhar Jha for their help and useful suggestions during the period of this study. The work was supported at the University of Colorado by the Basic Energy Sciences Division of the Department of Energy under Grant No. DE-FG02-07ER46403. KSN thanks the University of Trento, Italy for supporting his visit to Colorado.

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Short CommunicationFlash sintering as a nucleation phenomenon and a model thereof

Abstract

Introduction

Section snippets

Experiments

A model for nucleation

Summary

Acknowledgements

J Eur Ceram Soc

J Eur Ceram Soc

J Eur Ceram Soc

J Eur Ceram Soc

J Eur Ceram Soc

J Eur Ceram Soc

Scr Mater

J Eur Ceram Soc

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J Am Ceram Soc

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J Am Ceram Soc

Flash-sintering of cubic yttria-stabilized zirconia at 750 °C for possible use in SOFC manufacturing

J Am Ceram Soc

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J Am Ceram Soc

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J Am Ceram Soc

Short Communication
Flash sintering as a nucleation phenomenon and a model thereof