Microwaves

Microwaves: a promising industrial tool Dr. S. S. VERMA; Department of Physics, S.L.I.E.T., Longowal; Distt.-Sangrur (Punjab)-148 106

An electromagnetic wave with a frequency in the range of 100 MHz to 30 GHz (lower than infrared but higher than other radio waves). Microwaves are a type of electromagnetic waves carrying very high energy with shorter wavelength (ranging from as long as one meter to as short as one millimeter). This high energy of microwaves has always fascinated scientists and engineers to make use of microwaves in different possible industrial applications. In many ways, Magnetron: Generation of microwaves microwaves act like light rays. They can be focused with lenses made of wax or paraffin. They can be refracted with prisms of these materials. They can be reflected from large, plane sheets of metal, as light is reflected from plane mirrors. Metal parabolas may be used to produce parallel beams. The waves can be diffracted by slits in metal surfaces. Interferometers can be constructed for their use On the other hand, microwaves will pass through dry wood, whereas light waves will not. The dielectric constant of pure water for 1-meter waves is around 80; it is around 1.3 for 1-centimeter radio waves and for light waves. With the understanding of microwaves properties and industrial need, microwaves have turned to be a useful industrial tool being applied in many applications with many benefits. The range of microwaves applications vary from microwave heating, chemical synthesis, microwave electronics, diagnostics, and power transmission etc. The write up attempts to throw light on what are microwaves, how they are being generated and what industrial importance they have.

Generation of microwaves Microwaves are generated naturally by many astronomical phenomena and are found in cosmic background radiation. It is an electromagnetic wave lying in the range of 3 X 10 8 - 3 X 1011 Hz, produced through devices mainly exemplified by magnetron, klystron & traveling wave tube. Microwave is generated in special type of electron tubes. These contain cathode, anode and grid inside an evacuated envelope. For generation of microwaves these should operate at very high frequency range (300 – 3000 MHz). Ordinary electron tubes can operate at frequencies up to about 30 MHz. So the tubes must be designed in a different manner, because the frequency is comparable to the electron transit time (it is the time needed for electrons to travel between electrodes). The vacuum tube for the microwave generation is Magnetron, while other tubes amplify the microwave signal. Examples of the later are Klystron and Traveling wave tube.

Magnetron: Magnetron, a thermionic diode, consists of an anode and a directly heated cathode.

As cathode gets up heated it releases electrons, which are then attracted towards the anode. The anode is made up of even number of small cavities, each of which acts as a tuned circuit, and the gap across each end cavity behaves to provide a desired capacitance. Anode, which is a series of 1

such circuits, is tuned to oscillate at a specific frequency. Path of electrons bends when they travel from cathode to anode, due to axially induced magnetic field at the anode assembly. The deflected electrons then pass through the cavity gaps where they induce a small charge into the tuned circuit resulting in the oscillations getting generated in the cavity. This continues until sufficiently high amplitude is generated. The waves with this high amplitude are taken out of the anode via an antenna.

Klystron: Klystron a cylindrical tube that is specialized vacuum tube (electron tube) capable of amplifying microwave signals. Electron gun at one end of the tube serves as the cathode, which generates an intense electron beam that is directed towards the collector. The beam traveling through the axis of the tube is focused by an electromagnet that surrounds the tube. One of the beams passing through open chamber is called the input buncher cavity. The input signal (microwave signal that has to amplify) is coupled to this cavity in such a way that it modulates the velocity of individual electrons in the beam, converting the initially steady current of the beam into a fluctuating current. One or more intermediate cascade cavities along the beam’s path encourage the formation of bunches of electrons by allowing faster electrons to catch up with slower ones. The fully modulated beam, which’s current of which is an amplified version of the incoming microwave signal, then enters an output wave-guide. This is the desired amplified output signal. The electron beam continues out of the end of the tube into a collector chamber.

This chamber may require water cooling, because the dissipating beam produces considerable heat. Klystrons can also be used as microwave oscillators if a portion of the output signal is fed back into the input cavity.

Traveling wave tube: A traveling wave tube is a specialized electron tube capable of amplifying a microwave signal. An electron gun at on end of the cylindrical tube generates electrons that are conveyed into a narrow beam by electromagnetic coil wound around the circumference of the tube. Microwave energy entering the tube near the electron gun travels by means of a waveguide along the path of the electron beam, where it modulates the beam by forcing the electrons in the beam to group into bunches. This amplified microwave energy then passes out of the far end of the tube. The electron beam leaves the tube through a collector. Traveling wave tube is quite efficient in amplifying microwave signals over a wide range of frequencies.

Industrial applications Microwaves are used in radar, radio transmission, cooking, and other applications. The main application of microwave energy is through its heating effect. The heating produced, have been found to be superior to the conventional in numerous situations. The application of microwave extends to thawing, drying, sterilization, production of ointments & sustained release dosage forms. The superiority of microwave lies in the uniform and efficient heating, and at times in localized and focused heating. Through the exhaustive search of literature it could be concluded that microwaves, when used with certain precaution, is a promising energy in pharmacy. Microwaves, which have gained recent popularity in the kitchen, has a great potential in pharmaceutical industry. Microwave is apparently heading for exhibiting good potential in the field of Pharmaceutical industry. The frequencies applicable to industrial, scientific, medical & domestic uses for heating purpose lie between 9.15 MHz to 2.45 GHz . However lots of obstacles coming in the way of microwave usage had to be overcome.

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i) Microwave heating Microwave processings involve dielectric materials. A dielectric is an electrical insulator that gets polarized by the action of an applied electrical field. In the influence of electric fields, electrons move freely through a conductor but in case of a dielectric these electric fields displace electrons only slightly from their normal positions. The electric field causes a separation of negative charges (electrons) from positive charges (proton in the atomic nuclei). Thus an electric dipole is created in the molecule and the material is said to be polarized. In polar substance, where each molecule posses a dipole, the material becomes polarized due to the potential rotation of each molecule so that it is aligned in the direction of the applied electric field.

Because this involves rotation of the complete molecule there is strong coupling to the lattice. In general, this coupling causes the material to exhibit a high dielectric constant and a high loss factor and makes its dielectric properties depend upon frequency and temperature. Microwave include following advantages, over the conventional heating.

• Uniform heating occurs throughout the material as opposed to surface and conventional heating process.

• Process speed is increased.

• Desirable chemical and physical effects are produced.

• The energy source is not hot.

• Floor space requirements are decreased.

• Better and more rapid process control is achieved • In certain cases selective heating occurs which may significantly increase efficiency and decrease operating cost.

Microwave heating can be used in many applications like: Microwave cooking: A microwave oven passes (non-ionizing) microwave radiation (at a frequency near 2.45 GHz) through food, causing dielectric heating primarily by absorption of the energy in water. Microwave ovens have become common kitchen appliances all over the world following the development of less expensive cavity magnetrons. Water in the liquid state possesses many molecular interactions that broaden the absorption peak. In the vapor phase, isolated water molecules absorb at around 22 GHz, almost ten times the frequency of the microwave oven.

Drying: Microwave heating is used in industrial processes for drying and curing products.

Microwave was conventionally and even today is mainly used in industry for drying purpose because of the advantages shown by the microwave dryer over the other types of dryer.

Continuous Wave (CW) Microwave energy offers efficient, cost-effective methods of drying, heating, curing, and sterilizing products in industrial manufacturing processes.

Sterilization: The need for new drugs has gone up; similarly to develop suitable methods for their sterilization. This is critical aspect for industry as well as regulatory authority. Every method will have its advantages and disadvantages. Advent of a new method i.e., microwave sterilization carries significance. The sterilization is brought about by microwave dielectric heating effect. The efficiency of microwave sterilizer was put to test via sterilization of two heat labile drugs, ascorbic acid & pyridoamine phosphate, both in solution form. The result showed that though reduction of bio-burden was equal to that of autoclaving, autoclaved drugs showed 3

certain deteriorations in quality, which was not observed in microwave sterilized drugs.

Therefore microwave was reported to be holding an upper hand over autoclave. iii) Chemical synthesis The microwaves do not create an environment of high temperature, but rather provide microwaves energy that the material being heated must absorb in order to attain the desired temperature. Properly equipped, microwaves setups can be used for high temperature processing include the sintering of ceramics and processing of other materials above 200 °C to a maximum of 1,800 °C. Microwaves are also being used for biorefining and biomass processing including pyrolysis and fluid bed processing. Microwaves for high temperature processing need to be carefully designed and built to insulate the electronics from the high temperatures that the sample reach within the processing cavity. Microwaves used for general processing may be used for heating laboratory samples, preparing solutions, drying, and heating samples or products. The range the microwave can be used for industrial, research, quality control processing is almost unlimited.

A number of different microwave models are used for general chemical analysis (ashing, digestion, dehydration, sample preparation and other chemical processing). Features of all of microwaves for chemical processing include powered cavity exhaust to remove corrosive or obnoxious fumes, corrosion resistant stainless steel cavities and housings and some of models have corrosion resistance, which adds coatings to the electronics to reduce damage to the electronics and seals the door and the important door choke from the corrosive chemicals to improve safety from microwave leakage as standard. Many semiconductor processing techniques use microwaves to generate plasma for such purposes as reactive ion etching and plasmaenhanced chemical vapor deposition (PECVD). Microwave frequencies typically ranging from 110 – 140 GHz are used in stellarators and more notably in tokamak experimental fusion reactors to help heat the fuel into a plasma state. The upcoming ITER Thermonuclear Reactor is expected to range from 110–170 GHz and will employ Electron Cyclotron Resonance Heating (ECRH). iv) Diagnostic applications: Less-than-lethal weaponry exists that uses millimeter waves to heat a thin layer of human skin to an intolerable temperature so as to make the targeted person move away. A two-second burst of the 95 GHz focused beam heats the skin to a temperature of 130 °F (54 °C) at a depth of 1/64th of an inch (0.4 mm).

Radar: Radar uses microwave radiation to detect the range, speed, and other characteristics of remote objects. Development of radar was accelerated during World War II due to its great military utility. Now radar is widely used for applications such as air traffic control, weather forecasting, navigation of ships, and speed limit enforcement. Microwaves cannot be carried with usable efficiency in ordinary transmission lines but require waveguide (metal pipe), like this example from an air traffic control radar.

Radio astronomy: Most radio astronomy uses microwaves. Usually the naturally-occurring microwave radiation is observed, but active radar experiments have also been done with objects in the solar system, such as determining the distance to the Moon or mapping the invisible surface of Venus through cloud cover.

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Navigation: Various Global Navigation Satellite Systems (GNSS) broadcast navigational signals in various bands between about 1.2 GHz and 1.6 GHz. v) Microwave electronics Microwave radio is used in broadcasting and telecommunication transmissions because, due to their short wavelength, highly directional antennas are smaller and therefore more practical than they would be at longer wavelengths (lower frequencies). There is also more bandwidth in the microwave spectrum than in the rest of the radio spectrum; the usable bandwidth below 300 MHz is less than 300 MHz while many GHz can be used above 300 MHz. Typically, microwaves are used in television news to transmit a signal from a remote location to a television station from a specially equipped van. Most satellite communications systems operate in the C, X, Ka, or Ku bands of the microwave spectrum. These frequencies allow large bandwidth while avoiding the crowded UHF frequencies and staying below the atmospheric absorption of EHF frequencies. Satellite TV either operates in the C band for the traditional large dish fixed satellite service or Ku band for direct-broadcast satellite. Military communications run primarily over X or Ku-band links, with Ka band.

Before the advent of fiber-optic transmission, most long-distance telephone calls were carried via networks of microwave radio relay links run by carriers such as AT&T Long Lines. Starting in the early 1950s, frequency division multiplex was used to send up to 5,400 telephone channels on each microwave radio channel, with as many as ten radio channels combined into one antenna for the hop to the next site, up to 70 km away. Microwave technology is extensively used for point-to-point telecommunications (i.e., non broadcast uses). Microwaves are especially suitable for this use since they are more easily focused into narrower beams than radio waves, allowing frequency reuse; their comparatively higher frequencies allow broad bandwidth and high data transmission rates, and antenna sizes are smaller than at lower frequencies because antenna size is inversely proportional to transmitted frequency. Microwaves are used in spacecraft communication, and much of the world's data, TV, and telephone communications are transmitted long distances by microwaves between ground stations and communications satellites. Microwaves are also employed in microwave ovens and in radar technology.

Metropolitan area network (MAN) protocols, such as WiMAX (Worldwide Interoperability for Microwave Access) are based on standards such as IEEE 802.16, designed to operate between 2 to 11 GHz. Commercial implementations are in the 2.3 GHz, 2.5 GHz, 3.5 GHz and 5.8 GHz ranges. Wireless LAN protocols, such as Bluetooth and the IEEE 802.11 specifications, also use microwaves in the 2.4 GHz ISM band, although 802.11a uses ISM band and U-NII frequencies in the 5 GHz range. Licensed long-range (up to about 25 km) Wireless Internet Access services have been used for almost a decade in many countries in the 3.5–4.0 GHz range. Dozens of service providers are securing or have already received licenses to operate in this band. The WIMAX service offerings that can be carried on the 3.65 GHz band will give business customers another option for connectivity. Some mobile phone networks, like GSM, use the lowmicrowave/high-UHF frequencies around 1.8 and 1.9 GHz. DVB-SH and S-DMB use 1.452 to 1.492 GHz, while proprietary/incompatible satellite radio in the U.S. uses around 2.3 GHz for DARS.

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vi) Power transmission: Microwaves can be used to transmit power over long distances, and post-World War II research was done to examine possibilities. NASA worked in the 1970s and early 1980s to research the possibilities of using solar power satellite (SPS) systems with large solar arrays that would beam power down to the Earth's surface via microwaves.

Acknowledgement: The use of information retrieved through various references/sources of internet in this article is highly acknowledged.

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