Chapter 1 Introduction
1.2 Applications of ferrites as microwave materials
Initially, ferrites as microwave materials gained the interest of researchers due to the requirement for magnetic insulators in high-frequency inductor cores. By taking advantage of the dielectric response of ferromagnetic oxides, ferrites were used for various purposes such as directional coupler, phase shifter, isolator, circulator, etc., as reported earlier.
[20][21][22][23] Mainly the focus was laid on the development of magnetic loss of the materials and the ferromagnetic resonance (FMR) linewidths. Another crucial property of ferrites is magnetic anisotropy, which is usually used to bias the materials in microwave
regions. Depending upon the materials' performance and requirements, nowadays ferrites are used for various prospective.
1.2.1 Circulator, Isolator, and Phase shifter
The circulator is a multi-port, non-reciprocal passive device that exhibits low insertion loss in the forward direction and high loss in the reverse direction. The main purpose of this device is to control the flow of power. It has the power to regulate the direction of wireless as well as microwave signals where one antenna is needed instead of two for receiving and transmitting at duplex mode.[3] It comprises of a conductor placed over ferrite, which is usually biased by applying an external magnetic field which is perpendicular to the device plane. The graphical representation of a Y-junction circulator in stripline configuration and the magnitude of electric field is shown in Figure 1-1 (a) and (b). One layer stripline circuit with two ferrite discs make the sandwich structure. Port 1 is the input, whereas port 2 is the output port with simultaneously isolated port 3. The transmit-receive (T/R) modules of phased array radar systems are most often used in the Y junction circulator.
Figure 1-1. (a) Different parts of a stripline Y-junction circulator. (b) Low insertion loss in the forward direction of propagation (port I–port II). The finite element method is used to deduce the magnitude of the electric field in the stripline Y-junction circulator.[3]
The nonreciprocity behavior of the circulator can be used to separate received and transmitted waves in communication and radar systems. Initially, barium and strontium hexaferrite were mostly used ferrites for circulators due to their high magnetization,
anisotropy, and low loss. As soft ferrites like spinel ferrites render much beneficial low loss, the search for new ferrites for circulators is shifting towards spinel ferrites in recent times.
The isolator comes under two-port devices with nonprocity unidirectional transmission characteristics, as shown in Figure 1-2. [24] It is mainly used to block the high reflected power from the damage of the microwave sources. Let's take an example:
when plasma is ignited, the impedance of the system changes a lot. The change in impedance will lead to impedance mismatch that causes fatal reflection, which can damage the source. In such cases, an isolator can match or tune the network to absorb the reflected power through the ferrites. The isolators consist of similar essential elements as a circulator, which operates on different principles such as resonance absorption, field displacement, and Faraday rotation.
Figure 1-2. (a) The forward transmission is high from Port 1 to Port 2 : low transmission loss (S21), (b) Reverse transmission is low from Port 2 to Port 1 : high
isolation[24]
The phase shifters also belong to a two-port components system which renders different phase shifts by applying a different bias to the magnetic field, as shown in Figure 1-3.[24] The phase shifter is crucially used in phase array antenna to guide the antenna beam in space controlled electronically. The current phase shifter technology with semiconductor and ferroelectrics is suffering operational constraints like power handling
capacity and reliability, which limits their use in the military and commercial firms.[25] In contrast, the phase shifters with ferrites show superior microwave handling power and superior insertion loss. Apart from this, it is also rendering radiation-tolerant behavior and reliability. The spinel ferrites substituted with nonmagnetic ions like zinc, magnesium, and lithium are highly suitable for this application.[26][27]
Figure 1-3. (a)left side of the image refers to the empty wave guide and the right side shows the field strength with ferrites
1.2.2 Microwave absorber
Microwave absorbers have received wide attention in the last few years due to the increasing demand for telecommunication, wireless electronic devices, and stealth technology.[28][29] Besides, microwave absorbing materials (MAMs) are applied vastly to minimize electromagnetic (EM) reflections on huge bodies like military equipment, tanks, and planes.[30] Figure 1-4 shows the graphical illustration of the application of microwave absorbers. Majorly the absorbers can be classified into different types, such as dielectric type, resistor type, and magnetic type. In ferrites, the microwave absorption response of absorbent depends upon various factors such as complex permeability (interaction between absorbent and magnetic field), complex permittivity (interaction between absorbent and electric field), dielectric loss (defects), magnetic loss (eddy current effect and domain wall motion).[31][32] Apart from this, there should be effective blending between magnetic and dielectric loss (impedance matching) to achieve effective microwave
absorption.[33] Several ferrites such as Ni-Zn, [34][35] barium ferrites, strontium hexaferrite,[36][37] Mn-based spinel ferrites[38] are being used as microwave absorbers as reported earlier.[39]
Figure 1-4. Schematic illustration of the application of microwave absorbing material.
1.2.3 Electromagnetic interference shielding
In recent days, electromagnetic radiation in gigahertz (GHz) frequency has become an alarming danger for biological systems, commercial applications, high-quality information technology, etc.[10][40] When the EM waves interfere with the signals from different devices, they create noise known as electromagnetic interference (EMI) pollution, which is the consequence of the undesired outcome of modern technology and engineering.
Commercial appliances (microwave ovens) and communications devices (bluetooth devices, wi-fi routers, cell phones) are also part of EMI pollution.[41][42] Figure 1-5 illustrate the various source of EMI pollution around us. EMI pollution is hazardous for the smooth running of the devices and for normal human health, which causes sleeping disorders, headache, nausea, and heart attack.[43][44] The problem of this situation can be solved only by using appropriate EMI-shielded material. So, it is the urgent need of the hour for the whole human community to synthesize high-efficiency EMI shielded material.
The absorption mainly relies on magnetic and electric dipoles present in the shielded materials, whereas to have effective reflections mobile charge carriers are needed.[45][46]
The multiple reflections (reflection from various interfaces) are used to be neglected as the re-reflected waves are absorbed in the materials. Earlier, for EMI shielding, metal sheets were considered as an efficient material; however, their heavy weight, processing difficulty, and oxidation-prone nature limit their use in various practical applications.[47]
Then the hunt for efficient EMI-shielded material shifted to polymer and polymer-based composites.[48][49][50] But polymer-based composites also have certain limitations like temperature and chemical composition stability. So, currently, the research shifted to ceramics and ceramic-based composites with carbon and polymer.[51] [52][53][54] The shielding effectiveness (SE) of the material depends on several parameters like morphology, conductivity, permeability, permittivity, dielectric loss, and magnetic loss.
Figure 1-5. Various sources of EMI pollution/radiation.