Experiments and Models for Flash Sintering of Ceramics
Abstract
The overall objective of this proposal is to identify the fundamental origin of the unusual phenomenon of flash sintering, where modest electrical fields can induce colossal rates of densification in ceramics. In sintering, nearly half the mass must be transported from grain boundaries to fill the adjacent pores; that this process can be accomplished in mere seconds by solid-state diffusion is truly remarkable [1]. Flash sintering presents a new paradigm in the science and technology of ceramics processing. New fundamental questions arise. In conventional models, the driving force for sintering is expressed by incorporating the “sintering pressure” into the description of the chemical potential gradient, while self or chemical diffusivity quantifies the kinetics [2,3]. How electrical fields can be included in these formulations by conceiving and identifying the underlying atomistic mechanisms represents a profound scientific enquiry [4]. The objective of this proposal is to find answers to this question. The breadth of the ceramic materials that respond to flash sintering is remarkable. Several oxides [5]*, silicon-carbide [6] boron carbide [7], and zirconium diboride [7,8] have all been shown to exhibit this phenomenon. Ceramics that are electronic conductors [9], ionic conductors [1] or insulators [10], have been flash sintered. This generality raises the question of an overarching explanation. Since flash sintering is always accompanied by an abrupt increase in conductivity, thermal runaway or Joule heating immediately springs to mind as a simple way of explaining the phenomenon [11-13]. Indeed the negative temperature coefficient for the resistivity of ceramics, expressed in the Arrhenius form, can be manipulated to analyze thermal runaway [11]. However, the highly non-linear time dependency of flash sintering on a time scale of fractions of a second, preceded by incubation times which can last several hours [14], poses both analytical and experimental challenges in ascertaining whether or not Joule heating is the underlying cause. Recent measurements of lattice expansion during in-situ experiments suggest thermal effects are not the sole cause of flash sintering [15,16]. Still, given the intricacies of the split-second experiments, and the uncertainties associated with modeling a highly non-linear phenomenon, greater certitude is needed to determine the extent of the role played by Joule heating in flash sintering. An avalanche-like nucleation of point defects has been proposed as an alternative explanation for flash sintering [4]. While plausible and supported circumstantially, devout experimental evidence and theoretical models that are consistent with this idea are missing. The Poole-Frenkel model [17] for electrical conduction in insulators requires fields that are far greater than employed in flash sintering experiments. Why is flash sintering seen in a variety of ceramics, ranging from carbides and borides to oxides? Carbides and borides are often electronic conductors. Thus the Poole-Frenkel model [16], which is used to explain the insulator to conductor transition, is inappropriate for covalently bonded semiconductors. We hypothesize that flash sintering in carbides and borides may have a different origin than in ionic and insulating ceramics. Non-oxides often have an oxide phase segregated to grain boundaries, which arises from the native oxide present on the surface of the powders from which they are processed. We propose to investigate the possibility that grain boundary chemistries, that contain oxides, may be a source of flash densification in non-oxides. Ceramics may be broadly classified into ionic and covalently bonded materials.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Aug 12, 2016
- Source ID
- N000141512505
Entities
People
- Rishi Raj
Organizations
- Office of Naval Research
- Regents of the University of Colorado
- United States Navy