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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 ).
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.
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.
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.
MRS Proceedings, 1992
ABSTRACTTo make meaningful comparisons between conventional and microwave processing of materials, one must conduct experiments that are as similar as possible in the two environments. Particular attention must be given to thermal conditions, sample parameters, and furnace environment. Under thermal conditions, one must consider temperature measurement (pyrometer or thermocouple, sheath type, and arcing of thermocouples), thermal history (heating and cooling rates, thermal gradients), and exothermic reactions. Regarding sample parameters, one must. consider sample size, and packing powders and insulation systems. With respect to furnaces, one must consider differences in atmosphere, impurities, and uniformity of heating. Examples will be drawn from diffusion, grain growth, sintering, nitridation, and drying experiments conducted at the Oak Ridge National Laboratory (ORNL) over the past six years.
Metal Powder Report, 2008
Recent research on the ability of powdered metals to absorb and dissipate microwave radiation opened new opportunities for powder metallurgy. Microwave sintering can be an efficient, economic and valuable approach for the processing of some PM materials. There are some other advantages too…
International Journal of Refractory Metals & Hard Materials, 1998
Application of microwave radiation as a heat source for sintering of hardmetal is described. Sintering of hardmetal with microwaves leads to a finer microstructure because of lower sintering temperatures and shorter processing times. A further variant is the microwave reaction slntering of a powder mixture of metallic tungsten, carbon and cobalt to obtain finer mlcrostructures than by the conventional route. Moreover, this process offers a great potential for simplifying and shortening the process sequence in hardmetal production. The in-sttu formation of WC-platelets during microwave reaction sintering was observed.
MRS Proceedings, 1990
ABSTRACTProcessing oxide-based ceramics using microwave heating leads to a number of unexpected results, which can only be interpreted in terms of enhanced diffusion. Enhanced sintering has been observed in alumina and zirconia. Accelerated grain growth in dense, hot-pressed alumina has been demonstrated. Increased diffusion coefficients have been observed for diffusion of oxygen in sapphire. As yet, a satisfactory theory to account for these phenomena has not been developed. This paper reviews the experimental work conducted at the Oak Ridge National Laboratory during the past four years on the processing of oxides in both 2.45 and 28 GHz microwave furnaces.
Materials Today: Proceedings, 2018
In recent years microwave sintering has gained significant attention based on the improved mechanical properties as compared to the conventional material processing. Microwave sintering has found its applications for the processing of metal powder, metal matrix composites, ceramics and also in the processing of metal ores. This article reviews about basic processing aspects of microwaves, microwave sintering and some of its applications comparing it with the conventional processing.
… of Metals and Materials Volume 4: New, …, 2006
Microwave energy has been in use for variety of applications for over 50 years. These applications include communication, food processing, wood drying, rubber vulcanization, medical therapy, polymers, etc. In the last two decades microwave heating has been also applied very effectively and efficiently to heat and sinter ceramic materials. Microwave heating is recognized for its various advantages, such as: time and energy saving, very rapid heating rates, considerably reduced processing cycle time and temperature, fine microstructures and improved mechanical properties, better product performance, etc. The most recent application of microwaves has been in the field of metallic materials for sintering, brazing/joining and melting. Several common steel compositions, pure metals and refractory metals have been sintered in microwaves to nearly full density with improved mechanical properties. Many commercial powder-metal components of various alloy compositions including iron and steel, copper, aluminum, nickel, Mo, Co, Ti, W, Sn, etc., and their alloys have also been sintered in microwaves producing better properties than their conventional counterparts by using a 2.45 GHz multimode microwave system. This work has been further extended to join and braze bulk metal pieces, especially super alloy based turbine blades. Further, in a specially designed microwave cavity, even the bulk metals can be made to couple with the microwave field and melted. The implications of these findings are obvious in the field of powder metal technology.
Ceramics
Chemically and thermally stable ceramics are required for many applications. Many characteristics (electrochemical stability, high thermomechanical properties, etc.) directly or indirectly imply the use of refractory materials. Many devices require the association of different materials with variable melting/decomposition temperatures, which requires their co-firing at a common temperature, far from being the most efficient for materials prepared by conventional routes (materials having the stability lowest temperature determines the maximal firing temperature). We review here the different strategies that can be implemented to lower the sintering temperature by means of chemical preparation routes of oxides, (oxy)carbides, and (oxy)nitrides: wet chemical and sol–gel process, metal-organic precursors, control of heterogeneity and composition, transient liquid phase at the grain boundaries, microwave sintering, etc. Examples are chosen from fibers and ceramic matrix composites (CMCs)...
Materials, 2016
The results of a study of ultra-rapid (flash) sintering of oxide ceramic materials under microwave heating with high absorbed power per unit volume of material (10-500 W/cm 3) are presented. Ceramic samples of various compositions-Al 2 O 3 ; Y 2 O 3 ; MgAl 2 O 4 ; and Yb(LaO) 2 O 3-were sintered using a 24 GHz gyrotron system to a density above 0.98-0.99 of the theoretical value in 0.5-5 min without isothermal hold. An analysis of the experimental data (microwave power; heating and cooling rates) along with microstructure characterization provided an insight into the mechanism of flash sintering. Flash sintering occurs when the processing conditions-including the temperature of the sample; the properties of thermal insulation; and the intensity of microwave radiation-facilitate the development of thermal runaway due to an Arrhenius-type dependency of the material's effective conductivity on temperature. The proper control over the thermal runaway effect is provided by fast regulation of the microwave power. The elevated concentration of defects and impurities in the boundary regions of the grains leads to localized preferential absorption of microwave radiation and results in grain boundary softening/pre-melting. The rapid densification of the granular medium with a reduced viscosity of the grain boundary phase occurs via rotation and sliding of the grains which accommodate their shape due to fast diffusion mass transport through the (quasi-)liquid phase. The same mechanism based on a thermal runaway under volumetric heating can be relevant for the effect of flash sintering of various oxide ceramics under a dc/ac voltage applied to the sample.
Reference Module in Materials Science and Materials Engineering, 2017
Journal of Alloys and Compounds, 2010
Microwave sintering has emerged in recent years as a new method for sintering a variety of materials that has shown significant advantages against conventional sintering procedures. This review article first provides a summary of fundamental theoretical aspects of microwave and microwave hybrid sintering, and then advantages of microwave sintering against conventional methods are described. At the end, some applications of microwave sintering are mentioned which so far have manifested the advantages of this novel method.
Journal of the American Ceramic Society, 2006
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.
The present study investigates the effect of varying particle size and porosity on the heating behavior of a metallic particulate compact in a 2.45GHz multimode microwave furnace. Experiments on copper suggest that unlike monolithic (bulk) materials, metallic materials do couple with microwaves when they are in particulate form. The powder compacts having higher porosity and smaller particle sizes interact more effectively with microwaves and are heated more rapidly. A dynamic electromagnetic-thermal model was developed to simulate the temporal temperature distribution using a 2-D finite difference time domain (FDTD) approach. The model predicts the variation in temperature with time during heating of copper powder compacts. The simulated heating profiles correlate well with those observed from experiments.
Advances in Materials Science and Engineering, 2018
Over the years, microwave radiation has emerged as an efficient source of energy for material processing. This technology provides a rapid and a volumetric heating of material. However, the main issues that prevent microwave technology from being widespread in material processing are temperature control regulation and heating distribution within the sample. Most of the experimental works are usually manually monitored, and their reproducibility is rarely evaluated and discussed. In this work, an originally designed 915 MHz microwave single-mode applicator for high-temperature processing is presented. The overall microwave system is described in terms of an equivalent electrical circuit. This circuit has allowed to point out the different parameters which need to be adjusted to get a fully controlled heating process. The basic principle of regulation is then depicted in terms of a block function diagram. From it, the process has been developed and tested to sinter zirconia- and spine...
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.
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