On the Mechanism of Microwave Flash Sintering of Ceramics

Technical Physics, 2018

We report on the results of the analysis of the effect of flash sintering, which is observed upon heating compacted powder materials by high-intensity microwave radiation. Ceramic samples of Y 2 O 3 , MgAl 2 O 4 , and Yb : (LaO) 2 O 3 were sintered to a density exceeding 98-99% of the theoretical value during 0.5-5 min without isothermal hold. The specific microwave power absorbed volumetrically in the samples was 20-400 W/cm 3. Based on the analysis of the experimental data (microwave radiation power and heating and cooling rates) and of the microstructure of the obtained materials, we propose a mechanism of flash sintering based on the evolution of the thermal instability and softening (melting) of the grain boundaries. The proposed mechanism also explains the flash sintering effect observed when a dc or a low-frequency ac voltage is applied to the samples. The microwave heating makes it possible to implement flash sintering without using electrodes for supplying energy to the articles being sintered.

IOP Conference Series: Materials Science and Engineering, 2016

In the present study, the potential of microwave irradiation as an innovative energyefficient alternative to conventional heating technologies in ceramic manufacturing is reviewed, addressing the advantages/disadvantages, while also commenting on future applications of possible commercial interest. Ceramic materials have been extensively studied and used due to several advantages they exhibit. Sintering ceramics using microwave radiation, a novel technology widely employed in various fields, can be an efficient, economic and environmentally-friendlier approach, to improve the consolidation efficiency and reduce the processing cycle-time, in order to attain substantial energy and cost savings. Microwave sintering provides efficient internal heating, as energy is supplied directly and penetrates the material. Since energy transfer occurs at a molecular level, heat is generated throughout the material, thus avoiding significant temperature gradients between the surface and the interior, which are frequently encountered at high heating rates upon conventional sintering. Thus, rapid, volumetric and uniform heating of various raw materials and secondary resources for ceramic production is possible, with limited grain coarsening, leading to accelerated densification, and uniform and fine-grained microstructures, with enhanced mechanical performance. This is particularly important for manufacturing large-size ceramic products of quality, and also for specialty ceramic materials such as bioceramics and electroceramics. Critical parameters for the process optimization, including the electromagnetic field distribution, microwave-material interaction, heat transfer mechanisms and material transformations, should be taken into consideration.

AIChE Journal, 1998

The microwave heating of ceramic materials has been analyzed by solving the equations for grain growth and porosity during the late stages of sintering, coupled with the heat conduction equation and electric field equations for 1 -D slabs. Microwave power absorption and heating profiles have been calculated for Al,O, and Sic in the absence of sintering, and calculations have been cam'ed out to study the effect of increasing dielectric loss of A1,03 as a function of temperature. A comparison of the densification and grain growth for Al,O, during microwave and conventional sintering indicates that within the ffamework of the present model, there is no difference between the two heating modes during the late stages of sintering.

Contemporary Engineering Sciences, 2015

Recent research has been significantly increased our fundamental understanding of microwave interactions with materials. Thermal absorption has been demonstrated to result from simultaneous action of multiple dissipation mechanisms during processing. In addition, it has been conclusively established that strong microwave fields exert a non-thermal driving force during sintering. This force acts as an additional driving force for atomic transport. For strong electric fields, the force can enhance diffusion rates during ceramic sintering. This paper describes recent research on microwave sintering of two oxide ceramics, a silica xerogel ceramic produced from rice husk ash (RHA) and a high purity alpha alumina. A millimeter waves (MMW) heating system with a 28 GHz gyrotron is applied for microwave sintering experiment. The ceramics were also sintered by using an electric furnace where served as comparison. Effect of microwave energy on the porosity reduction of the ceramics was investigated. Some possible physical mechanisms were discussed.

Microwave processing eliminates the need for spending energy to heat the walls of furnace or reactors, their massive components and heat carriers. Hence, the use of microwave processing methods significantly reduces energy consumption, particularly in hightemperature processes, since heat losses escalate considerably as processing temperatures increase. However, the advantages of using microwave energy in high-temperature processes are by no means limited to energy savings. In many cases, microwave processing can improve the product quality ).

Transactions of FAMENA

The present study examines the potential of microwave heating as an emerging and innovative energy-efficient alternative to conventional heating techniques used for different materials, with a focus on the processing of ceramic materials. Modern ceramics are studied extensively, and their use and different applications are wide due to many advantages of these materials. The most important factor in microwave sintering which differentiates it from conventional heating techniques is a unique heat transfer mechanism. Microwave energy is absorbed by the material, hence the transfer of energy takes place at the molecular level. This way, the heat is generated throughout the material, i.e. on the inside as well on the outside. This allows a very low temperature gradient throughout the material cross section. When conventional sintering is used, typically at high heating rates, high temperature gradients pose a problem. The accelerated microwave heating occurs through the whole volume, so the heating is uniform, which limits the grain growth and coarsening, and leads to a uniform and fine microstructure. The densification is accelerated as well during the unique heat transfer mechanism of microwave sintering, which enhances the mechanical properties of the sintered materials. This paper discusses the use of microwave sintering in the manufacturing of different modern technical materials, namely ceramics, composites, metals and alloys, and glasses. The improvement of different properties is described using the available literature.

Journal of the American Ceramic Society, 2006

A microwave/conventional hybrid furnace has been used to sinter three ceramics with different microwave absorption characteristics under pure conventional and a range of microwave/ conventional hybrid heating regimes. The precursor powder particle size was also varied for each material. In each case it was ensured that every sample within a series had an identical thermal history in terms of its temperature/time profile. An increase in both the onset of densification and the final density achieved was observed with an increasing fraction of microwave energy used during sintering, the effect being greatest for the materials that absorbed microwaves most readily. Twenty-three percent greater densification was observed for submicron zinc oxide powder, the material with the largest microwave absorption capability, when sintered using hybrid heating involving 1 kW of microwave power compared with pure conventional power under otherwise identical conditions. For the ceramic with the lowest microwave absorption characteristic, alumina, the increase in densification was extremely small; partially stabilized zirconia, a moderate microwave absorber, was intermediate between the two. Temperature gradients within the samples, a potential cause of the effect, were assessed using two different approaches and found to be too small to explain the results. Hence, it is believed that clear evidence has been found to support the existence of a genuine ''microwave effect.'' 1977

IOP Conference Series: Materials Science and Engineering, 2017

Samples of 3 % yttria-stabilized zirconia (3YSZ) ceramics have been sintered to near full density with no appreciable grain growth using an ultra-rapid microwave sintering process. The sintering experiments were carried out on a 24 GHz / 6 kW gyrotron system for microwave processing of materials with automatic process control. By varying the properties of the thermal insulation surrounding the samples it was possible to vary the microwave power required for heating. The final relative density of 3YSZ ceramic samples microwave heated at a rate of 50 °С/min to a temperature of 1400 °C without isothermal hold varied from 91.6 % when the specific absorbed microwave power was 4 W/cm 3 to 99.4 % when the specific absorbed microwave power was 90 W/cm 3. The specific absorbed power is therefore demonstrated to be the key parameter determining the achievable density in ultra-rapid field-assisted sintering processes.

Ceramics International, 2019

This work presents the dielectric properties of several oxides used as sintering aids for the manufacture of thermally stable ceramics in the radio frequency (RF) and microwave ranges. The oxides were prepared in pallets and cylinder geometries, with sintering close to their melting point for 4 h. The dielectric properties in the RF range were analysed by impedance spectroscopy, while the Hakki-Coleman and Silva-Fernandes-Sombra techniques were used for dielectric analysis in the microwave range. The temperature effect on the dielectric properties was also analysed in both frequency ranges. In the RF range, V 2 O 5 , Al 2 O 3 , ZnO and Nb 2 O 5 presented lower insulating properties than the other oxides studied due to their lower activation energies. In the microwave range, only CaTiO 3 , ZnO and TiO 2 presented positive temperature coefficients of resonant frequency (τ f) with CaTiO 3 , while TiO 2 presented the highest dielectric permittivity and Al 2 O 3 and B 2 O 3 presented the lowest dielectric permittivities. These dielectric properties are a feature that is relevant for future technological applications.

Journal of The American Ceramic Society, 2003

The sintering kinetics and microstructural evolution of alumina tubes (∼17 mm length, ∼9 mm inner diameter, and ∼11 mm outer diameter) were studied by conventional and microwave heating at 2.45 GHz. Temperature during microwave heating was measured with an infrared pyrometer and was calibrated to ±10°C. With no hold at sintering temperature, microwave-sintered samples reached 95% density at 1350°C versus 1600°C for conventionally heated samples. The activation energy for microwave sintering was 85 ± 10 kJ/mol, whereas the activation energy for conventionally sintered samples was 520 ± 14 kJ/mol. Despite the difference in temperature, grains grew from ∼1.0 μm at 86% density to ∼2.6 μm at 98% density for both conventionally sintered and microwave-sintered samples. The grain size/density trajectory was independent of the heating source. It is concluded that the enhanced densification with microwave heating is not a consequence of fast-firing and therefore is not a result in the change in the relative rates of surface and grain boundary diffusion in the presence of microwave energy.