Sunday, 25 December 2016

Microwave Heating: From Cooking of Food to Materials Processing Technologies

Satnam Singh
Assistant Professor, 
NCU, Gurgaon

If you think that microwave oven can only be used for cooking potatoes or food, then you should read this article.

The first commercial application of microwaves was primarily in the field of communications, but later on the heating effects of microwaves was accidently discovered [1]. PL Spencer filed the patent for microwave heating of foodstuffs in 1945 [2]. Later on, the microwave heating effects were utilized by many researchers in processing of various materials but at lower temperatures. Applications of microwaves at that time were confined to the wood drying process, cooking food, rubber processing, enhanced chemical reactions etc. 

Microwave heating is totally different from the conventional heating techniques. In microwave heating, heat is generated at molecular level, by absorption of radiations. This causes heat generated from within the material moving towards the outer surface. Further, heating is volumetric in nature, means microwaves are absorbed throughout the volume of materials. In contrast to microwave heating, conventional heating starts from the surface of materials and progresses towards the core of the  material. Due to the volumetric heating nature of microwaves, heating is intense and rapid.  

Earlier it was thought that microwaves cannot be used for processing of metals due to reflection of radiations from the surface, but later it was discovered that microwaves can be effectively used in the processing of metallic based powders [3]. This allowed wide application of microwaves in the materials processing world. The developments in metallic materials processing by microwaves are shown in Figure 1, which depicts the chronological order of various research innovations in this novel field.  

Figure 1: Developments in microwave materials processing techniques [4]

The technology changed from microwave sintering of ceramics to efficient microwave sintering of metallic powders. This area was further targeted and microwaves were used for joining of bulk metallic steels [5]. Afterwards, researchers successfully developed the novel microwave cladding process at IIT Roorkee and developed claddings of metallic powders/ composites/ cermets on bulk steels [6]. In recent times, microwaves were successfully employed for the casting of metallic powders by using domestic ovens [7].

References:

[1] Singh, S, Gupta, D, Jain, V, & Sharma, AK, Microwave processing of materials and Applications in manufacturing industries: a review. Mater. Manuf. Process., 30, 1, 2015.
[2] Spencer, PL, Method of treating foodstuffs, US patent 2495429 A, assigned to Raytheon Mfg Co., 1945.
[3] Roy, R, Agrawal, D, Cheng, J, &Gedevanishvili, S, Full sintering of powdered-metal bodies in a microwave field. Nature, 399, 668-670, 1999.
[4] Mishra, RR, & Sharma, AK, A Review of Research Trends in Microwave Processing of Metal-Based Materials and Opportunities in Microwave Metal Casting. Crit. Rev. Solid State Mater.Sci. 41, 217–255, 2016.
[5] Srinath, MS, Sharma, AK, & Kumar, P, Investigation on microstructural and mechanical properties of microwave processed dissimilar joints. J. Manuf. Process. 13, 141–146, 2011.
[6] Gupta, D, & Sharma, AK, Development and microstructural characterization of microwave cladding on austenitic stainless steel. Surf.Coatings Technol. 205, 5147–5155, 2011.
[7] Singh, S, Gupta, D, & Jain, V, Novel microwave composite casting process: Theory, feasibility and characterization. Mater.Des.111, 51–59, 2016.


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