Microwave versus conventional sintering: A review of fundamentals, advantages and applications

Efforts to use microwaves in material processing are gradually increasing. However, the phenomena associated with the processing are less understood; popular mechanisms such as dipolar heating and conduction heating have been mostly explored. The current paper reviews most of the significant phenomena that cause heating during microwave–material interaction and heat transfer during microwave energy absorption in materials. Mechanisms involved during interaction of microwave with characteristically different materials – metals, non-metals and composites (metal matrix composites, ceramic matrix composites and polymer matrix composites) have been discussed using suitable illustrations. It was observed that while microwave heating of metal based materials is due to the magnetic field based loss effects, dipolar loss and conduction loss are the phenomena associated with the electric field effects in microwave heating of non-metals. Challenges in processing of advanced materials, particularly composites have been identified from the available literature; further research directions with possible benefits have been highlighted.

Microwave–material interaction phenomena: Heating mechanisms, challenges and opportunities in material processing

Plasma-facing materials and components in a fusion reactor are the interface between the 
plasma and the material part. The operational conditions in this environment are probably 
the most challenging parameters for any material: high power loads and large particle and 
neutron fluxes are simultaneously impinging at their surfaces. To realize fusion in a tokamak 
or stellarator reactor, given the proven geometries and technological solutions, requires an 
improvement of the thermo-mechanical capabilities of currently available materials. In its 
first part this article describes the requirements and needs for new, advanced materials for the 
plasma-facing components. Starting points are capabilities and limitations of tungsten-based 
alloys and structurally stabilized materials. Furthermore, material requirements from the 
fusion-specific loading scenarios of a divertor in a water-cooled configuration are described, 
defining directions for the material development. Finally, safety requirements for a fusion 
reactor with its specific accident scenarios and their potential environmental impact lead to 
the definition of inherently passive materials, avoiding release of radioactive material through 
intrinsic material properties. The second part of this article demonstrates current material 
development lines answering the fusion-specific requirements for high heat flux materials. 
New composite materials, in particular fiber-reinforced and laminated structures, as well 
as mechanically alloyed tungsten materials, allow the extension of the thermo-mechanical 
operation space towards regions of extreme steady-state and transient loads. Self-passivating

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Development of advanced high heat flux and plasma-facing materials

The sintering of nanosized particles is a scientific and technological topic that affects the manufacture of bulk nanocrystalline materials and the understanding of the stability of nanoparticles. Owing to their extremely small size and the high surface to volume ratio, nanoparticles during sintering exhibit a number of distinctively unique phenomena compared to the sintering of coarse powders. Particularly, it is generally found that the sintering temperatures of nanosized particles are dramatically lower than that of their micrometre or submicrometre sized counterparts. Research has also shown that the grain growth during nanosintering consists of an initial dynamic grain growth stage that occurs during heating up and the normal grain growth stage that occurs mostly during isothermal holding. For nanoparticles, the effect of the initial grain growth cannot be ignored because that it is sufficient to cause the material to lose nanocrystalline characteristics. This review aims to bring to focus th...

Densification and grain growth during sintering of nanosized particles

A new approach to joining of bulk copper using microwave energy

This study aims at microwave sintering of pure tungsten powder of as-received grade and tungsten powder activated by high-energy milling (HEM). Both the powder compacts are sintered under identical conditions and their sinterability is compared and analyzed for process optimization. Activated powder responds to microwaves and shows significant densification in a relatively shorter duration. The scope of the present work is to understand the effects of high-energy milling on the sinterability of tungsten in microwave and to make a comparative assessment between as-received (coarser) and activated (finer) tungsten powder in terms of their response to microwave sintering, densification and microstructure.

Microwave sintering of tungsten

The present investigation reports for the first time a dramatic decrease in the sintering temperature of elemental W from the conventional temperature of ≥2500 °C to the modest temperature range of 1700–1790 °C by making the W powder nanostructured through high-energy mechanical milling (MM) prior to sintering. The crystallite size of the initial W powder charge with a particle size of 3–4 μm could be brought down to 8 nm by MM for 5 h in WC grinding media. Further milling resulted in a high level of WC contamination, which apparently was due to work hardening and the grain refinement of W. A sintered density as high as 97.4% was achieved by sintering cold, isostatically pressed nanocrystalline (8 nm) W powder at 1790 °C for 900 min. The microstructure of the sintered rods showed the presence of deformation bands, but no cracks, within a large number of W grains. The mechanical properties, when compared with the hardness and elastic modulus, of the sintered nano-W specimen were somewhat superior to those reported for the conventional sintered W.

https://www.skpabi.com/documents/skp%20publications/87%29%20J.%20Mater.%20Res.,%2022%20%282007%29%201200-1206.pdf

On sinterability of nanostructured W produced by high-energy ball milling

MoSi2 coating on Mo substrate for short-term oxidation protection in air

Infiltration of liquid copper in partially sintered tungsten was undertaken to improve machinability of tungsten. Initial skeleton of tungsten was prepared by pressing commercial tungsten powder in cold isostatic press to near-net shapes. The green compacts are then subjected to controlled sintering to about 85% theoretical density. This leaves adequate amount of open channels which are filled subsequently with liquid copper by infiltration. The resulting composite material exhibits reasonable strength coupled with desired level of machinability. Some pilot samples were made with ∼99% density and were subjected to in-house characterization (e.g., density, shrinkage and porosity). Microstructural study has been carried out to compare the theoretically calculated porosity levels with the observed porosity level. X-ray diffraction studies revealed presence of elemental tungsten and copper with no mutual solubility. Mechanical properties (e.g., ultimate tensile strength, tensile elongation and hardness values) of the composite were also evaluated and reported.

Improvement of machinability of tungsten by copper infiltration technique

The present study attempts to investigate room temperature and high temperature flow behaviour of a tungsten heavy alloy (W-7.10Ni–1.65Fe–0.5Co–0.25Mo alloy). The alloy was deformed under compression at room temperature and elevated temperatures (400–700 °C) at different strain rates (1–0.0001 s −1 ) to observe plasticity under compression loading at these temperatures and strain rates. The alloy showed higher plasticity and positive strain rate sensitivity at room temperature. Samples hardness after 70% deformation at room temperature increased from 3.20 ± 0.14 to 5.08 ± 0.03 GPa. Barreling was observed in room temperature compression tested samples. Microstructure of the alloy after heavy compressive deformation at room temperature showed that severe deformation of W grains took place along a direction at 45° to the direction of applied stress. The alloy showed varying (positive and negative) strain rate sensitivity at elevated temperatures. Samples hardness after 70% deformation at elevated temperatures increased from 3.20 ± 0.14 to 4.60 ± 0.23 GPa. At 600 and 700 °C, the specimens failed by shear along the direction which is at an angle 45° to the direction of applied stress. Microstructural evidences indicate that the failure at these elevated temperatures seems to be triggered by the excessive void generation in the matrix and their coalescence under the influence of the applied stress. Room temperature deformation mechanism of tungsten heavy alloy was also studied by carrying out tensile testing at room temperature in strain rates ranging from 0.1 to 0.0001 s −1 . The values of strain rate sensitivity, and apparent activation volume with respect to different level of strain suggest that the deformation mechanism of the alloy is similar to that of bcc metals such as tungsten and molybdenum, i.e. Peierls mechanism controls the dislocation motion.

Bijoy Sarma

Papers

Microwave sintering of tungsten

On sinterability of nanostructured W produced by high-energy ball milling

MoSi2 coating on Mo substrate for short-term oxidation protection in air

Improvement of machinability of tungsten by copper infiltration technique

Deformation behaviour of a newer tungsten heavy alloy