COURSE: ADVANCED MANUFACTURING PROCESSES Module No. 5: OTHER PROCESSES Lecture No-3 Microwave Processing of Materials Microwave processing is a relatively new and emerging area in material processing. It has bright prospects as an unconventional manufacturing technique in the years to come. Application of microwaves in material processing is one of the significant developments in researches in material processing which is gaining popularity day by day. The use of microwaves to process absorbing materials was studied intensively in the 1970s and 1980s and has now been applied to a wide variety of materials. The metals were earlier considered not viable to process through microwaves owing to the fact that they primarily reflect microwaves at room temperature but recent research activities, however indicate that it is possible to process metals under certain conditions. The microwave processing of materials provides a new approach to improve the physical properties of materials. It offers an alternative for processing materials that are hard to process using conventional means. There are a number of processing advantages and environmental benefits that can be derived from the use of microwave processing. Some of these advantages include: Microwave processing is a green manufacturing process (environmentally safe), significantly fast and hence tends to be highly economical (energy saving). Microwaves have been effectively and efficiently used for processing of ceramics and composite materials which are otherwise difficult to process through conventional processes. In microwave processing, fine microstructures and improved mechanical properties are observed with reduced processing cycles. Conceived almost 50 years ago, microwave energy was developed primarily for communications; its application was later extended to some areas of processing such as cooking foods, tempering, thawing, and curing of wood, rubber products etc. However, a considerable
development has taken place in the last two decades. Today, microwaves are being extensively used not only in industrial applications but also in domestic appliances. In microwave processing, heat is generated internally within the material instead of originating from external sources. Heating is rapid as the material is heated by energy conversion rather than energy transfer. There is a 100% conversion of electromagnetic energy into heat. In conventional thermal processing, energy is transferred to the material through conduction, convection and radiation of heat from the surface of the material. On the other hand, microwave energy is delivered directly to materials through molecular interaction with the electromagnetic field. Significant advantages in material processing through microwaves have been observed in respect of savings in processing time, product uniformity, grain size control and consequent property enhancement etc. Microwave energy has been in use for variety of applications for over five decades, however, in the last few years more advancements and applications like ceramic sintering were noticed. Microwave heating/ processing of material depends on dielectric and magnetic properties. Microwaves are not form of heat; rather form of energy that is converted into heat energy. Microwaves are electromagnetic waves with wavelengths in the range of 10 mm to 300 mm. Microwaves generally refer to signals with frequency in the range of 0.3 GHz to 300 GHz. The most common microwave frequency used for materials research is 2.45 GHz. The Federal Communications Commission has issued the Industrial, Scientific and Medical (ISM) frequency bands used for industrial microwave heating. These frequencies are indicated in the Table 5.3.1. A frequency of 2.45 GHz is used for the domestic microwave ovens. Figure 5.3.1 shows the electromagnetic spectrum along with the range of microwaves. Table 5.3.1: Permitted frequencies and wavelength bands: Frequency (MHz) Wavelength (m) Area Permitted 896 0.32 Great Britain 915 0.33 North & South America 2450 0.122 Other parts, including India
SHF VHF MF VLF X-rays U.V IR EHF UHF HF LF Di-electric heating frequencies Millimeter waves MICROWAVES Radio frequencies Rear Bands K X S L 1cm 10 cm 1 m 10 m 100 m Wavelength 3 X 10 10 3 X 10 9 3 X 10 8 3 X 10 7 3 X 10 6 Frequency (Hz) 900 MHz 13.56 MHz 2.45 GHz 27.12 MHz 433.9 MHz 433.9 MHz Principal frequencies allotted for industrial use Fig. 5. 3.1 The electromagnetic spectrum Microwaves have unmatched application potential in wireless communication as well as in material processing. They can be reflected, absorbed and/or transmitted by materials. Reflection and absorption require interaction of the microwaves with the material. A brief comparison between conventional heating and microwave heating is illustrated in Table 5.3.2. Table 5.3.2: Conventional and microwave heating. Features Conventional Heating Microwave Heating Mechanism In the conventional processing, In microwave heating, energy is delivered of heating thermal energy is delivered to the directly to the material through molecular surface of the material by radiant interaction with the electromagnetic field. and/or convection heating and is transferred to the bulk of the material via conduction/convection. Here, heating is due to the transfer of electromagnetic energy to thermal energy and the mechanism is energy conversion
Volumetric heating Rate of heating Direction of heating Heating quality Rate of processing Here the heating is gradual and there is no volumetric heating concept. In conventional heating, slow heating rates are selected to reduce steep thermal gradient leading to processinduced stresses. Thus, there is a balance between processing time and product quality. From the outer surfaces into the core/center of the bulk. There is uneven heating on different areas, leading to uneven quality. The processing time is more. rather than heat transfer. The Microwave interaction is through either polarization or conduction process. Since microwaves can penetrate the material and supply energy, heat can be generated throughout the volume of the material resulting in volumetric heating. Hence, it is possible to achieve rapid and uniform heating of thick materials. Rate of heating is much faster. The thermal gradient is in the reverse fashion as that of conventional heating, that is the highest temperature is generally observed in the core/center of the bulk. During microwave processing, microwaves transfer energy throughout the whole volume of the material. This helps to reduce processing time and enhance the product quality. Relatively faster. Polarization and Conduction: There are two most widely accepted mode of interaction of microwaves with materialspolarization and conduction. The polarization involves short range displacement of the charges through formation and rotation of electric poles, while the conduction requires long range
transportt of the charges. Like lasers, microwaves are highly coherent and polarized. Microwaves obey the laws of optics. However, unlike laser heating, microwave heating is fundamentally different from conventional heating processes. The energy absorbed by the material during its interaction with microwaves is generally manifested ass heat. The electric field decreasess as a function of the distance from the surface of the material as energy is absorbed within the material,. Different types of Microwave-Material interactions are illustrated in the Figure 5.3.2. The power absorbed in microwave processing is given by: P=2πƒε E 2 (5.1) where, P = power absorbed in watts, ƒ = resonant frequency in hertz, = ε dielectric loss of material, and E= magnitude of electric field in volts/meter. Material Type TRANSPARENT (Low loss insulator) Penetration Total OPAQUE (Conductor) None (Reflected) ABSORBER (Lossy Insulator) Partial to Total ABSORBER (Mixed) Matrix= Low loss insulator Fiber/additives = absorbing materials Partial to total Fig. 5.3.2 Penetration of microwaves into differentt materials.
Unique benefits and distinctive features of Microwave Processing: Microwaves have the potential to process materials/ products that are difficult or impossible to produce reliably by conventional methods. In addition to being a cost effective, clean and environment friendly process, microwave processing has some unique benefits and features as below: Penetrating radiation which helps in controlling the electric field distribution, Precise, rapid and uniform internal (volumetric) heating, Differential coupling (selective heating) of materials, and Self-limiting reactions.