Microwave Processing of Materials and Microwave Active Materials: A Review
A review of the fundamental of microwaves (MWs) and its interaction with materials has been presented with emphasis of this application used for the synthesis of semiconducting materials. MW processing of materials was compared to conventional ways and the main benefits were found to be decreased processing times, cost savings and decreased energy usage. However, problems faced with using this method include the uncontrollable ‘microwave effects' like thermal runaways. The most important factor to consider in the end is whether the materials used for synthesis are compatible with MWs. Research was carried out on semiconducting materials which have been synthesized and compiled into a condensed table.
Microwave Processing of Materials and Microwave Active Materials: A Review
1 Literature Review
Microwaves (MWs) have been associated primarily with communications (e.g. mobile phones) and certain processing areas such as food preparation. However, microwaves have been extensively used at industrial scale for processes such as curing of wood and polymers and also the inclusion of waste materials into glass [].
In recent times, there has been an increase in the application of microwaves with regards to industry. Such examples include the processing of solutions, suspensions, drying of materials, sintering of ceramics and ceramic composites, preparation of specialty ceramics [], plasma processing and processing of polymers and polymer composites [].
The success of microwave ovens for domestic purposes has lead to the potential of translating this success to industry. Taking microwave ovens used for domestic purposes in to account, they have greatly reduced cooking times and have even accounted for saving energy. The promise shown by microwave ovens makes it a prospective candidate as an alternative way of processing materials. [], [].
This review aims to investigate the state-of-the-art in the field microwave processing of semiconductor materials. A summary of the fundamentals of microwaves, interaction of microwaves with materials, and microwave processing of materials in particular inorganic compounds will be reviewed. Furthermore, a comparison of advantages and disadvantages of MW processing to conventional methods is presented.
1.2 The Concepts of Microwaves
Microwaves are part of the electromagnetic spectrum with wavelengths ranging from 1mm to 1m with corresponding frequencies between 300MHz and 300GHz. As shown in Figure 1 it can be seen that microwaves lie between radio and infrared waves. However, the frequencies at which MW devices can operate have been legislated and standards have been published. The frequencies used specifically for industrial, scientific and medical (ISM) are 0.915GHz and 2.45GHz . The latter frequency is commonly used for household microwave ovens and the former for industrial purposes. There are two important reasons why these two frequencies are selected for microwave heating . The first reason being ISM set aside these frequencies for non-communication purposes. And finally the penetration depth of microwaves is greater at these low frequencies. The penetration depth is an important parameter. As the value increases, there is greater potential that heating of the whole material will occur. Despite this, the properties of the material must be taken into consideration as the internal field energy maybe low, so heating may not necessarily increase with decreasing frequency.
When it comes to processing materials, the use of variable frequency (0.9 - 18GHz) MW furnaces have been developed as a way to improve the heating of materials. Adjustment of frequency accordingly can improve the coupling of MW radiation with a material.
1.2.1 Electromagnetic theory
The theory of electromagnetic waves was first developed by Maxwell in the 1860's and later utilised successfully by Hertz []. An electromagnetic wave is formed by the coupling of an electric field with a magnetic field at high frequencies . Figure 2 shows when the two fields are coupled oscillation occurs perpendicular to each other. Depending on the frequency of the oscillations and wavelength of the wave, determines what type of electromagnetic wave it is (refer to Figure 1) .
There are three principal components to a microwave furnace:
1) The source:
High power and frequencies are required for microwave heating and in order to achieve this, vacuum tubes are used. The two consistently used sources for microwave heating are magnetrons and travelling wave tubes.
Magnetron tubes are used in domestic microwave ovens and due to their mass production are very cheap, easy to get hold of and are very efficient . However, magnetrons are only able to generate fixed frequency electromagnetic field unlike travelling wave tubes which are able to generate variable frequencies. It is due to the design of the wave tube which allows amplification of a wide band of microwave frequencies to be produced in the same tube .
2) Transmission lines
The transmission line delivers the electromagnetic radiation generated from the source to the applicator. For a microwave heating system (due to the high frequencies used), waveguides are the choice of transmission line. These are hollow tubes in which electromagnetic waves propagate.
There are two modes of microwave propagation: transverse electric (TE) and transverse magnetic (TM). The electric intensity in the propagation direction is zero when using TE mode. In TM mode, the magnetic intensity in the propagation direction is zero .
The electromagnetic energy delivered by the transmission lines from the source are either absorbed or reflected by the material in the applicator. There are three types of applicators: single mode; multimode; and travelling wave. The type of applicator used is very much dependent on the material being processed. Single mode and travelling wave applicators are used for processing materials with simple geometries. For industrial purposes (e.g. for polymer composites and ceramics), multimode applicators are better suited as they have the ability to process much larger, complex materials. However, despite greater uniformity being produced, penetration depth is sacrificed .
1.2.2 Dielectric Properties
A dielectric material is a material which comprises of either induced or permanent dipoles. When this is placed between two electrodes it acts as a capacitor, i.e. charge is stored. Rotation of dipoles or displacement of charge in an electric field leads to polarisation of the dielectric material. With regards to microwave heating, the polarisation that occurs at a molecular level is the physical rotation of molecular dipoles (Figure 5) .
Permittivity (ε) of a material defines the ability of the material to store charge, not taking dimensions into account. The dielectric constant (ε', also known as the relative permittivity) is the permittivity of a material relative to free space. Molecules with large permanent dipole moments, have large dielectric constants due to dielectric polarization being heavily dependent on their dipoles' ability to re-orientate when an electric field is applied , .
In microwave heating, the re-orientation of dipoles is equal to an electric current which is known as Maxwell displacement current. An ideal dielectric will have no lag between orientation of the molecules and the changes of the alternating voltage leads to the displacement current being 900 out of phase with regards to the oscillating field (refer to Figure 6) .
As shown in Figure 7(a), when the molecules of a dielectric material keeps up with the field change, no heating occurs. No part of the current is in phase with the electric field. As stated before there is a 900 phase lag between the field and the current, and therefore the product E x I is zero .
If the electromagnetic radiation frequency is increased to the microwave region (109 Hz), then rotations will begin to lag behind the oscillating electric field. As a result, phase displacement occurs (see Figure 7 (b)) in which component I x sinδ is acquired in phase with the electric field. This leads to resistive heating within the material - this is characterised as the dielectric loss in which energy is absorbed from the electric field. The lag is known as the relaxation time .
For the processing of complex and thick materials, it is vital to have knowledge of how the electric field decreases from the surface and the penetration depth. This is because if penetration depth is much less than the thickness of the material, then only the surface is heated and not the whole material.
Materials with low dielectric loss factors have large penetration depths and it means they absorb energy poorly and are therefore transparent to microwave energy. On the other hand, materials with high dielectric loss factors (e.g. metals) have penetrations depths which approach zero. The penetration depth can be calculated using equation (1) :
(1)where D is depth of penetration at which incident power is reduced by a half; λ is the incident wavelength; ε'r is the relative dielectric constant; and ε is in this case the permittivity of free space.
Permittivity dramatically falls at higher frequencies due to the dipoles being unable to follow the higher frequency electric field. When the loss angle (δ) greatly differs from 900 at certain frequencies, the material can have two functions: both as a dielectric and as a conductor. From Figure 7 (c), sin δ is an in phase current component and it gives the total relative permittivity a complex role (equation (2)).
(2) where ε' is the dielectric constant and ε'' is the loss factor (this indicates the conductance of the material). The complex permittivity measures the ability of a dielectric to absorb and to store electrical potential energy .
In relation to this (Figure 7 (c)) equation 3 is used to measure dielectric response:
(3) is known as the energy dissipation factor (loss tangent) and is significant for measuring the ability of a material to convert energy that is absorbed into heat. Loss tangent is a useful parameter for comparison of heating rates of compounds similar in characteristics (i.e. chemical and physical), though more technically challenging expressions (e.g. heat capacity; density etc.) are required to calculate consistent and reliable heating rates 
ε' and ε'' are highly dependent on frequency and it is the magnitude of the frequency which controls the amount to which coupling of MW radiation with a material can occur. Therefore, frequency is a very important parameter for interpretation of dielectric heating (see Figure 9).
Power absorbed (P) by a material per unit volume (equation 4), along with penetration depth and dielectric properties, is an important parameter in MW heating .
(4) Where E (V/m) is magnitude of internal field, σ is total effective conductivity (S/m), f is frequency (GHz), ε0 permittivity of free space (F/m), ε'r is relative dielectric constant, tan σ is loss tangent. The equation shows the P varies linearly with frequency, the square of the internal electric field, dielectric constant and loss tangent .
If we take a simple polar liquid (e.g. water), according to Debye, ‘single molecule with a small electric dipole is assumed to be at the centre of a spherical volume' . On application of an electromagnetic field, the dipoles will aim in the direction of the electromagnetic field (see Figure 5). However if no electromagnetic field is applied, then orientation of the dipoles is random. The relaxation time is the time measure for this random orientated state to be achieved .
Despite this, the Debye theory for an ideal liquid is simplified and in most cases cannot be applied to materials. The Debye model shows of only one relaxation time when in fact materials often exhibit more than one . However, an important factor in which Debye theory highlights is that structure of a material has a high dependence on the relaxation time. A direct relationship is forged between the capability of a material to heat and the orientation of dipoles in the electromagnetic field .
In addition to this, there are a few properties which aids the dielectric response with regards to materials: electronic polarization; atomic polarization; Maxwell-Wagner polarization; ionic conduction; and dipole polarisation. At microwave frequencies, the most important mechanism of energy transfer is thought to be dipole polarization. However, in particular for composite materials the heating mechanism of importance is Maxwell-Wagner polarization .
Figure 9 demonstrates how properties of the material affect it's interaction with MWs. Two interesting points are highlighted. Firstly, it highlights that due to low dielectric loss, SiO2 does not heat up. On the other hand, it shows NiO and Cr2O3 couples well with microwaves. The second interesting note is the sudden increase in temperature of Cr2O3 after 4 minutes. This is due to high dielectric loss at higher temperatures and this is known as 'thermal runaway' .
1.2.3 Fundamentals of Microwave heating
Material interaction with microwave field can be categorized in three general ways :
1) Transparent: microwaves pass through the material with little or no loss in intensity, i.e. low dielectric loss materials. For example alloys, such as brass are used to make microwave waveguides.
2) Conductors (also known as opaque): microwaves are reflected by the material and do not penetrate. Examples include Teflon and fused quartz. Because of their behaviour, these materials are used as containers for chemical reactions.
3) Absorbing: material absorbs microwaves based on the dielectric loss factor of the material i.e. high dielectric loss materials. These materials take up the energy produced by the microwave field and are heated up rapidly.
However, there is a fourth interaction which is known as a mixed absorber. In industrial applications, this interaction would be associated with ceramics and polymer composites. An important characteristic of microwave processing is selective heating (this is not possible in conventional heating). Mixed absorbers take advantage of selective heating as the component of high dielectric loss absorb the microwaves, passing through to the component with low dielectric loss and hence retaining the energy .
Principally for microwave processing, there are three very important parameters which must be considered when choosing the material to be processed: power absorbed (P); depth of penetration (D); and dielectric properties. These three factors govern the feasibility of processing a material by microwave . Heat is generated within a material by the microwave energy being able to enter the material and transmit it. Microwave heating is a result of dielectric relaxation , , [].
1.3 Microwave Processing of Semiconductor Materials
1.3.1 Comparison of conventional processing and microwave processing
There are fundamental advantages to using MW processing as opposed to conventional methods. Certainly for industry, using MW processing greatly increases cost and energy savings, but more importantly can reduce processing times and hence between the material and source of heat. MW energy directly interacts with the material via molecular interaction with electromagnetic field and the material itself generates the heat. On the other hand, in conventional heating, thermal energy interacts with the surface of the material and heat is transferred through conduction (see Figure 10). Therefore, microwave processing of materials is rapid and selective. This reinforces the advantage of cost reduction due to less heating time required as well as less energy being needed. As the material generates the heat, there is uniformity which leads to improvement in the quality of the product .
Despite these advantages, there are potential obstacles when considering using microwave processing for a certain material. A very important consideration is if the material is a good absorber of microwaves. Another consideration is controlling of accelerated heating, known as thermal runaway. Thermal runaway occurs specifically in the processing of ceramics in a non-uniform electromagnetic field. This arises from the increase of dielectric loss factor at higher temperatures. The temperature conditions vary within the material and if a local area reaches the critical temperature before the rest of the material, the temperature in the localized area begins to heat more rapidly leading to a ‘thermal runaway'. This can lead to great stress on the material and fracturing of the structure may occur. In addition to this, using inverted temperature as an advantage can be tricky, but has been done with processing of ceramics .
There is plenty of information on microwave synthesis of inorganic and organic compounds , , []. MW processing of semiconductors is potentially very viable. The production of semiconductors by conventional methods is a lot more inferior to the potential of MWs. By conventional methods, production costs are very expensive and annealing of materials is not only time consuming, but high energy power levels are required. As reported by Wei et al, wide band semiconductor nanostructures were synthesized on a large scale using MW means [].
Using MWs can increase production speed with better quality and can be scaled to industry. In addition to this, using MWs gives better control over homogeneity due to ability of selective heating. However, despite this potential, there has been a lack of published work specifically for semiconductor synthesis.
A semiconductor is simply a material with electrical conductivity found between an insulator and a conductor. An insulator exhibits very low conductivity whereas a conductor shows high conductivity.
Charge carriers are situated at different energy levels with a material []. The highest occupied energy level is known as the valence band and the lowest unoccupied energy level is known as the conduction band . In a conductor, the two bands overlap and hence electron movement occurs readily. In insulators, the two bands are too far apart and electrons cannot be promoted . In semiconductors, there is a forbidden energy gap in which no energy states can exist: this is known as the band gap []. Energy states are only allowed in the conduction band or the valence band . Therefore, given enough energy, electrons are able to be promoted. The band gap values of interest are measured between 0.5 and 3.5eV . When sufficient energy has been supplied and the electrons are promoted, leaving positively charged ‘holes' in the valance band.
126.96.36.199 Inorganic Semiconductors
There are two types of semiconductors: p-type and n-type. A p-type semiconductor occurs when the charge is carried by positive holes. An n-type semiconductor occurs when the charge is carried by negative electrons .
Some materials are naturally semiconductors (e.g. Ge, Si and Sn). However, it is possible to dope materials with another element. This introduces a new energy level between the valance and conduction bands .
An example of n-type doping is Germanium with Arsenic. Prior to doping Ge atoms share four valence electrons with four neighbouring atoms leading to formation of covalent bonds. The addition of As (which has five valence electrons), replaces a Ge atom and donates an electron to the lattice. This leads to the additional electron occupying the conduction band (refer to Figure 11) .
An example of p-type doping is Germanium with Indium. In is a trivalent atom. When In replaces a Ge atom, instead of donating an electron to Ge, it accepts an electron leaving a positive hole in the Ge lattice (refer to Figure 12) .
188.8.131.52 Organic Semiconductors
For organic semiconductors, there are two related classes: organic charge transfer and conducting polymers. The mechanism of conduction for organic charge transfer materials is similar to inorganic semiconductors i.e. the presence of band gaps between the two bands and presence of holes.
In relation to conducting polymers, there are three types: electron conducting; proton conducting; and ion conducting. There is great interest in conducting polymers due to their advantages over metals, the main being that they are more easily processed and do not break easily.
184.108.40.206 Microwave-Assisted Synthesis of Inorganic Materials
The potential of using microwave processing with regards to inorganic ceramics was investigated from the 1950's, but due to technological limitations was not really investigated properly until the 70's . In 1975, observations made by Sutton, highlighted the potential microwave processing had with in particular ceramics. Sutton noted while using microwave drying on high alumina castables, not only was water removed, but the microwave also heated the ceramic: temperatures of 14000C were obtained. This investigation was pivotal in that it demonstrated that ceramics could be processed with microwave energy rapidly, with similar if not better performance/properties and at a more cost effective price when compared to conventional heating .
Recently, the use of microwave processing for the synthesis of inorganic materials has rapidly increased. There are a number of methods adopted for synthesis , but two primary ways are by using direct or indirect methods. The direct method involves exposure of the required materials (which are MW active) to MWs. The indirect method utilizes susceptors which provide the initial heat required for a reaction to take place. They are not involved in the reaction directly.
From research, potential semiconducting materials have been purely synthesized. However, the applications of these synthesized materials have not been analyzed specifically. The research was gathered and materials synthesized have been placed in table format (Appendix 1). As some materials were purely synthesized, band gaps of some could not be found.
SnO, ZnTe and ZnO had been characterized for potential applications , ,  , . In addition to this, both SnO and ZnO have been synthesized on a large scale, so shows that synthesis is scalable to industry .
TiO2 has been thoroughly researched for its capabilities as a superconductor by processing with MWs , , , . The applications of TiO2 for semiconducting purposes using this method are as dye-sensitised solar cells. The viability of synthesizing using MWs as opposed to conventional methods was confirmed [], . TiO2 was synthesized in much shorter times and at lower heat than is required for conventional methods. This results in reduced synthesis times, lower energy requirements and costs. In addition to this, solar cell performance improved when processed by MWs .
Despite this, it has been reported by Hart et. al. of problems faced in producing TiO2 films by this method []. The main issue was cracking of the substrate which was down to temperature factors and thickness of TiO2 added (cracking occurred at ~4000C) .
For a material to be processed by MWs, it must fit a given criteria. The ideal conditions and properties are summarised in Figure 13:
All the properties are dependent on each other. In order to gain maximum tangent loss high dielectric loss and moderate dielectric constant are required. Interlinked to these properties are penetration depth and power. However, the material to be processed has to be taken into consideration. If the material is a large, thick ceramic slab, then a very high penetration depth would be required and hence the other properties required would have to be adjusted in order to accommodate this. If we take a contrasting material for example a thin film semiconductor, the opposite is required. A very low penetration depth is needed in order for processing to be feasible.
In some cases, a mixed absorbing material is an ideal candidate for processing. By adjustment of properties, this can be successfully achieved.
Comparing conventional method to MW processing, the advantages of processing by MWs far outweigh the disadvantages. In the context of industrial purposes, the decrease in processing times, energy savings and cost savings are such desired properties that it makes MW processing a very viable alternative. Potential semiconducting materials have been successfully synthesized, but not for a certain application.
Despite this, there is growing interest in using this method for large scale synthesis of semiconductors and there is promise that this method will be used to synthesise purely for application purposes.
Using MWs as a processing method does have it problems and hindrances. Nevertheless as more research is invested in this field more of these problems can be resolved and MW processing may be looked at as not an alternative way of processing, but maybe the best way.
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