Applications of microwave energy: Microwave heating

Microwaves are high-frequency radio waves with frequencies between 300 MHz and 300 GHz. From cell phones and microwave ovens to weather forecasting radars, microwaves play an important role in our everyday lives. RF (radio frequency) power amplifiers (power amplifiers and amplification modules), which strengthen microwave signals, make it possible for us to use microwaves in everyday applications.

Applications of microwave energy

Microwaves are one of many types of electromagnetic waves. Electromagnetic waves are waves that have both electric and magnetic fields, and travel through space at the speed of light. They can be described by two properties: wavelength and frequency. Electromagnetic waves are categorized based upon their frequencies. Visible light is a type of electromagnetic wave. Electromagnetic waves with frequencies less than 3,000 GHz are classified as radio waves. Within this category, radio waves with frequencies between 300 MHz and 300 GHz are known as microwaves.

Microwaves are used in many applications in daily life. In addition to wireless communications applications, the use of microwaves as an energy source and for energy transmission has gained attention in recent years.

Vacuum tubes called magnetrons are used to generate microwaves. Such devices have been used to heat food and for other uses for many years. Microwave ovens generate microwaves with a frequency of 2.45 GHz. This means that the electric field switches from positive to negative 2.45 billion times every second. Water molecules in the food absorb the microwave energy and heat up. The positive and negative poles of the  water molecules rotate due to this rapid switching, producing thermal energy and setting nearby molecules into similarly vigorous rotation. As more water molecules become excited, energy (heat) is disbursed throughout the food. Microwaves are being used in similar ways for material synthesis applications and in chemical reactions.

The use of microwaves for wireless power supply to IoT devices and electric vehicles has also progressed. In addition, research is currently underway on a power transmission system that is able to convert electricity collected by solar panels in outer space into microwaves that are transmitted to a receiving antenna (rectenna) on the Earth’s surface.

Advantages of heating solids with microwaves

Heating objects using microwaves has advantages compared to conventional heating methods.

Internal heating

The first advantage of microwaves compared to conventional heating methods is internal heating. Microwaves reach the target object at the speed of light and continue to travel through the object. As it travels through the object, the microwave energy is absorbed by the object. In this way, the object is heated from the inside. Conventional heating methods rely on conduction of heat from the outer surfaces to the inside. Therefore, heating effectiveness is greatly impacted by the heat transfer coefficient of the object and can lead to uneven heating. Since microwaves heat from the inside, no additional time and energy are required to conduct heat to the center of the object.

Easy temperature control

The second advantage is the ease of temperature control. When microwaves are transmitted, heating starts. When the transmission is stopped, heating stops. This is a very high level of control compared with conventional methods in which an oven is brought up to temperature and objects are placed inside. The power of the microwave can also be adjusted to control the heating rate.

Selective heating

The third advantage is selective heating. Because different materials absorb microwave energy at different rates, microwave frequencies and materials can be chosen to selectively heat the desired materials only. For example, containers that are designed to be used in microwave ovens are made out of materials that do not absorb microwave energy. Therefore, the container itself does become too hot and the energy goes to heating up the food. With conventional heating methods, everything placed in an oven becomes heated. Selective heating with microwaves is an efficient way to heat up just the target objects, saving both time and energy.

Lower environmental impact

The fourth advantage of using microwaves to heat things is the reduced impact to the environment compared to conventional heating methods. Microwaves are able to travel through space at the speed of light. They do not require wires or any other medium. They can even travel through a vacuum. They are able to heat target objects without affecting nearby objects, minimizing the impact to the surroundings. In addition, microwaves can be generated by electrical power. Unlike furnaces that use fire or heating elements to generate heat, microwaves can be generated and aimed precisely, reducing unwanted or unneeded heating of nearby objects. Microwaves are therefore considered to be more energy efficient than conventional heating methods.

Possibilities of super-small GaN microwave power amps

Power amplifiers that strengthen microwave signals are essential for the use of microwaves in modern applications. In particular, power amplifiers using gallium nitride (GaN) semiconductors are attracting attention in terms of miniaturization and efficiency improvement of microwave devices.

Vacuum tubes such as magnetrons are still used to generate microwaves in microwave ovens and similar devices. Magnetrons are large, have a short life, and cause uneven heating. Next-generation microwave heating appliances that use power amplifiers made of GaN semiconductors instead are under development. GaN microwave power amplifiers are capable of higher output than those that use gallium arsenide (GaAs) semiconductors, and can contribute to the miniaturization of equipment.

For example, 2.45 GHz microwaves are widely used in microwave ovens and other microwave heating devices in accordance with international standards. (The band of radio waves that can be used by industrial, scientific, and medical devices [ISM devices] is defined in the international standard CISPR11.) If 2.45 GHz microwave power amplifiers can be made smaller, the size of the microwave heating device itself can also be reduced. Ultra-compact power amplifiers that are about the size of a fingertip but have power output of 25 W or more have been developed. Such ultra-compact power amplifiers could be used to manufacture ultra-compact and ultra-portable microwave ovens for emergency use in the event of natural disasters or for commercial use such as portable ovens for mountain climbing. Other possible applications include small drying devices that could immediately dry printer ink or create dried food products, ultra-small medical devices that could cauterize the affected area from the inside, and small laboratory devices that could heat reagents in small reactors. Further development of compact microwave devices will make a wide range of industrial, scientific, medical, and commercial equipment possible in the near future.

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