Chair: Georgios Stefanidis
MICROWAVE AND FLASH PROCESSING OF 3D PRINTED CERAMICS
Bala Vaidhyanathan, Annapoorani Ketharam, William Rowlands, Yumeng Chen, Pengxiang Zheng
Abstract: For highly demanding applications in electronics, energy, healthcare and defence sectors the usage of advanced ceramics and their hybridised products with metals and polymers is substantial, critical and irreplaceable. All these products however require densification/sintering, a high temperature process (e.g. 1000 – 2000oC) that in industry can take days. The amount of energy needed, and CO2 emitted, is therefore very significant; sometimes energy can account for 30% of costs. Further conventional processing technologies generally fall short in delivering multifunctional hybrid materials with the required performance and reliability. The fabrication of these functional devices/components are often plagued by interfacial issues, unwanted grain growth, migration of one component into another and limitations of co-firing dissimilar materials etc. Thus rapid and efficient sintering methods such as Spark Plasma Sintering (SPS), Microwave Assisted Sintering (MAS), and Flash Sintering (FS) are continuously being developed. These approaches are together referred as Field Assisted Sintering Techniques (FAST), and in all these cases application of electric, magnetic and/or electro-magnetic field were demonstrated to have a positive effect on ceramic densification. For example, the flash sintering method, for reasons that are far from fully understood, has yielded full densification in very short periods at very low temperatures, e.g. 5 s at 850oC for zirconia, and at a surprisingly low temperature of 325oC for Co2MnO4 spinel ceramics. Thus the associated time and energy advantage is estimated to be staggering, as well as the ability to tailor the required micro/nano structure and in turn the performance.
In this talk, we will have a closer look at two of the FAST methods, namely Microwave Sintering and Flash Sintering – one a well-established and the other a newly emerging densification method. The MAS method can be suitable for the processing of various simple and complex shaped engineering components, the early use of FS method was restricted to dog-bone shaped ceramic specimens – that are both difficult to make and do not have much industrial applicability. However, the recent developments have demonstrated that FS can also be used to sinter different sample shapes. At Loughborough we investigated the feasibility of sintering of 2D/3D printed ZnO-varistors, BaTiO3-capacitors, YSZ and ZTA components using both MS and FS methods along with simultaneous measurements of shrinkage, online thermal distribution mapping and atmospheric control. This talk will review these new developments on FS along with the operative mechanisms in comparison with microwave processing.
SURFACE CHARGE MOBILITY THEORY AN EXPLANATION FOR CERTAIN “MICROWAVE EFFECTS”
Abstract: 16th International Conference on Microwave and High Frequency Heating
AMPERE 2017, Delft, The Netherlands, September 18-21, 2017
SURFACE CHARGE MOBILITY THEORY
AN EXPLANATION FOR CERTAIN “MICROWAVE EFFECTS”
Y-12 NSC, Oak Ridge, TN
Keywords: microwave, microwave effects, electromagnetic frequency, enhanced diffusion.
Electrons in a metal system are delocalized which contributes to the properties of metals. Delocalized electrons are free to move in all directions throughout the metallic structure and give rise to properties such as conductivity.
Delocalized electrons at the surface of a metal cannot move in all directions, rather they can only remain on or near the surface, or return to the bulk. These surface electrons are exposed and are thus vulnerable to influences by outside forces, such as microwaves.
If an electromagnetic wave passes over the surface, it can cause a short range, localized disequilibrium of electrons to occur. Electrons are swept across the surface like sweeping water across a tennis court with a squeegee. In the area in front of the electromagnetic wave, the electrons pile up and are pushed across the surface. In the area behind the wave, there is momentarily, a localized absence of electrons.
With the sweeping of the surface electrons back and forth over the surface of the metal, it appears that the inter-atomic lattice spacing at the surface is being dilated and compressed.
This theory suggests several observable predictions for previously unexplainable “microwave effects”, and when understood, has practical applications related to diffusion.
1. E.B. Ripley, Y-12 Internal Correspondence April, 2001
2. E.B. Ripley, Lab Notebook (Y/NB 6013)Witnessed P.A. Eggleston, April, 2001