**2. Principle of microwave heating rocks**

*Microwave Heating - Electromagnetic Fields Causing Thermal and Non-Thermal Effects*

rock, breaking due to various advantages including high efficiency and having no dust or noise pollution [1–3]. Realizing microwave-assisted mechanical rock cutting (**Figure 1**) using the microwave-induced hard rock fracturing technique can prolong the mechanical life and improve the efficiency of rock breaking operations [4–7]. At present, tunnel boring machines (TBMs) and shield machines have been increasingly used in tunnel excavation. The shield machine is subjected to a series of problems such as deformation of the tool apron and severe cutter wear due to the presence of boulders [8–10]. During the tunneling of hard rocks by TBMs, the disc cutter is worn and frequently changed-out, thus increasing the cost of maintenance

The design of cutter heads of shield machines and TBMs is closely related to the properties of rocks and prevailing geological conditions [15, 16]. The mechanical strengths (uniaxial compressive strength, tensile strength, and point load strength, etc.) of rocks are important parameters influencing the service life and penetration of disc cutters on TBMs [17, 18]. Microwave treatment can significantly decrease the strength of rocks [3, 19–22], and thus can improve the penetration and life of disc cutters. Therefore, by introducing microwave heating technology into TBMs or shield machines, hard rocks or boulders can be pre-fractured through microwave irradiation. In this way, cutter wear can be reduced to increase efficiency in tunnel

Some scholars have carried out numerous experiments and numerical research into the mechanism governing the microwave-induced fracturing of rocks (or ores) and the influence of microwave treatment on the mechanical properties of rocks (or ores) [23–25]. Under the effect of microwave treatment, new intergranular and transgranular fractures are generated in rocks [26] to lead to the reduction of work index [27, 28] and strengths (including uniaxial strength, Brazilian splitting strength, and point load strength) of rocks [2, 29, 30]. More seriously, rocks are cracked and crushed or molten to cause rocks (or ores) to lose all of their bearing capacity [29, 31]. Hassani et al. [3, 6, 32] and Nekoovaght et al. [33, 34] studied the influence of microwave power and irradiation time on the strength of different kinds of rocks by using a frequency of 2450 MHz multi-mode cavity. In addition, they also studied the influence of the distance between the microwave antenna and the rock on the heating characteristics by experiments and numerical comparison.

and influencing construction progress [11–14].

*Schematic diagram of microwave-assisted mechanical rock cutting [4].*

**148**

excavation.

**Figure 1.**

When a dielectric material is subjected to an alternating current, it absorbs electrical energy, which is dissipated in the form of heat (the dielectric loss). The dielectric constant of the material consists of the real part and the imaginary part, as shown below

$$\boldsymbol{\varepsilon} = \boldsymbol{\varepsilon}\prime - j\boldsymbol{\varepsilon}\prime \tag{1}$$

where the real part (ε') is known as the dielectric constant. The imaginary part (ε") is known as the loss factor [41].

The loss tangent (tan δ) is the ratio of the imaginary part (ε") to the real part (ε'), i.e.

$$\tan \,\, \mathcal{O} = \frac{\mathcal{E}}{\mathcal{E}'} \tag{2}$$

It measures the ability of the dielectric to store energy and convert it into heat.

The microwave absorption capacity of electrolyte material is related to its dielectric properties. The microwave heating mechanism of minerals and rocks in the electromagnetic field is usually expressed by the power density, which can be expressed by the following Equation

$$P = 2\pi\varepsilon\_0\varepsilon''\mathbf{E}^2\mathbf{f} \tag{3}$$

where *P* is the loss power density deposited within the sample; *E* is the electric field and *f* is the microwave frequency; ε0 is the dielectric constant of free space (8.85 × 10−12 F/m) [42].

The temperature of the dielectric material increases when it absorbs microwave energy [41]. According to the laws of thermodynamics the amount of energy required to increase the temperature of a material to a given amount is calculated by the following equations

$$\mathbf{Q} = \mathbf{C}m\Delta\mathbf{T} \tag{4}$$

*Microwave Heating - Electromagnetic Fields Causing Thermal and Non-Thermal Effects*

$$\mathbf{P} = \rho \mathbf{C} \frac{\Delta \mathbf{T}}{\Delta \mathbf{t}} \tag{5}$$

where *Q* is the energy absorbed by the material; C is the specific heat capacity; *m* is the mass; ∆*T* is the temperature increase after absorbing energy; ∆*t* is the time difference.

By combining Eqs. (3) and (5), the rate of heating may be given by

$$\frac{\Delta \mathbf{T}}{\Delta \mathbf{t}} = \frac{2\pi \varepsilon\_0 \varepsilon'' \mathbf{E}^2 \mathbf{f}}{\rho \mathbf{C}} \tag{6}$$

An increase in material temperature causes the volume of the material to increase

$$\mathbf{V(T)} = V\_0 \left(\mathbf{1} + \alpha \Delta T\right) \tag{7}$$

**151**

**Figure 3.**

*Experimental Investigation on the Effect of Microwave Heating on Rock Cracking…*

of the magnetron. The magnetron converts direct-current electrical energy into microwave energy, thus providing continuous microwave power. The isolator is used for the unidirectional circular transmission of microwave energy. The function of the water cycle is to absorb the reflected microwave energy, thus protecting the magnetron from damage. The microwave applicator is used to emit microwave energy to the surface of rocks, where it is used to heat and crack rocks. The impedance tuner is used for impedance matching. Compared with the microwave source with a frequency of 915 MHz, the microwave source with a frequency of 2450 MHz has higher heating efficiency and smaller volume, which is conducive to the combination with the mechanical rock-breaking device. During testing, a metal net is used as shielding to avoid microwave interference with signal transmission to/from

*Temperature distribution on the surface of samples measured by infrared camera (ambient temperature at 13.5°C) [44]. Tmax = 92.0°C, Tave = 68.7°C Tmax = 90.8°C, Tave = 63.7°C Tmax = 91.9°C, Tave = 67.3°C (a) 5 kW, 10 s. Tmax = 158.2°C, Tave = 109.6°C Tmax = 169.3°C, Tave = 119.0°C Tmax = 166.8°C, Tave = 116.9°C (b) 5 kW, 20 s. Tmax = 240.8°C, Tave = 169.9°C Tmax = 225.9°C, Tave = 157.0°C Tmax = 218.0°C, Tave = 163.3°C. (c) 5 kW, 30 s.*

*DOI: http://dx.doi.org/10.5772/intechopen.95436*

the other apparatuses.

where, *V*0 is the volume at some reference temperature, and *α* is the coefficient of thermal expansion of the medium [43].

After being irradiated by microwaves, rocks absorb electromagnetic energy that is transformed into thermal energy, causing the temperature rising of rocks. After microwave irradiation, the temperature of the rocks is not uniform, resulting in uneven thermal expansion in the rocks. Different minerals within rocks have different dielectric properties, leading to different rocks have different microwave absorption capacities. Different minerals also have different thermal expansion coefficients, so the thermal expansion property is different after heating. Therefore, due to the different microwave sensitivity and thermal expansibility of different types of rocks, different types of rocks show different heating characteristics and fracturing effects.
