Ball Milling and Heating Ability of Iron Boride

(FeB) Nanoparticles for Magnetic Hyperthermia

Muhammad Sufian Sadiq

IntroductionMagnetic Hyperthermia:

The word Hyperthermia means increase in body temperature. Hyperthermia is the heating of certain organs or tissues for

cancer therapy. This thermotherapy is based on the fact that magnetic

nanoparticles can transform electromagnetic energy from an external high-frequency field to heat.

As a consequence, if magnetic nanoparticles are put inside a tumor and the whole patient is placed in an alternating magnetic field of well-chosen amplitude and frequency, only the tumor temperature would rise. The elevation of temperature may enhance tumor oxygenation , radio- and chemosensitivity, thus shrinking tumors.

Mechanisms of Heat Generation in Magnetic Nanoparticles: The magnetic losses occurring due to an applied

alternating magnetic field which contribute to the generation of heat, arises due to the different processes of magnetization reversal in the particle system. These are

1. Hysteresis Losses2. Relaxational Losses (Neel or Brown relaxation)

Hysteresis Losses: The energy lost as heat due to reversing the magnetization of the

material is known as hysteresis losses. It is a measure of the energy dissipated per unit cycle of the

magnetization reversal and is equal to the integration of the area of the hysteresis loop.

The area of the hysteresis loop depends on the coercively of the loop.

It is dependent on the amplitude of the field, the magnetic prehistory and as well as the size of the nanoparticles.

Figure1: Hysteresis losses

Neel and Brown Relaxation: When magnetic nanoparticles are exposed to AC magnetic field

there is a delay in the relaxation of the magnetic moment. These relaxations cause magnetization reversal which generates a large amount of heat during reversal. The delay can be due to two reasons

1. The rotation of the magnetic moment within the particle, called the Neel relaxation,

2. The rotation of the particle as a whole known as Brownian relaxation.

Equations of Neel and Brown relaxations

τ N = τ O e KVm / kT

τ B = 3ηVH / kT

τ = τ B τ N / τ B + τ N

where τN is the Neel relaxation time, τB is the Brown relaxation time, τ is the effective relaxation time if both effects occur at the same time, τ0 = 10-9 s, K is the anisotropy constant, VM is the volume of the particle, k is the Boltzmann constant, T is the temperature, η is the viscosity and VH is the hydrodynamic volume of the particle.

Structure of Iron Boride (FeB) Iron Boride is an inorganic compound with formula FexBy.

It is considered ferromagnetic at room temperature with a Curie temperature TC of 325oC..

Iron Boride (FeB) has a crystallization temperature of 1590oC and an orthorhombic crystal lattice with 8 atoms in an elementary cell. The lattice parameters of FeB are known to be a = 0.4061 nm, b = 0.5506 nm and c = 0.2952 nm.

Its crystal structure can be described as an arrangement of prisms with boron atoms at located in the center while iron atoms residing at the edges as shown in the figure below.

Ball Milling Ball milling is a process in which powder of a specific

material is placed in a high energy mill along with a suitable milling medium. The purpose of ball milling is to decrease the size of the particle .

The ball milling is used for the synthesis of different Nanomaterial's. The particle size is reduced by the impact of the ball on the powder.

As the kinetic energy of the balls depend on their mass and velocity, the size and size distribution of the balls should be optimized for a given mill.

During milling the temperature depends on the kinetic energy of the balls and the medium of the milling.

RF Induction Unit RF induction is the use of a radio frequency magnetic field

to transfer energy by means of electromagnetic induction in the near field.

Induction is a fast, efficient, precise, repeatable, non-contact method for heating metals or other electrically conductive materials

It uses high frequency electricity to heat materials that are electrically conductive. Since it is non-contact, the heating process does not contaminate the material being heated. It is also very efficient since the heat is actually generated inside the work piece.

According to the Joule effect, the movement of the electrons creating these currents dissipates the heat in the substance where they were generated.

Figure2: Rf induction

Experimental Procedure A 5 g powder of iron boride was taken. For the ball milling of this 5 g iron boride sample we took 25 g of balls (i.e. in 1:5) with 1 ml of isooctane as a milling media in one of the bowl. The isooctane was used to prevent the ball from oxidation. To equalize the weights of the bowls in the ball milling machine, we took a same amount i.e. 30 g of balls in the other bowl. Both of the bowls were placed in the ball milling machine and were milled for 20 minutes at a frequency of 500 rpm. After 20 minutes of milling the iron boride was collected and a 5 mg of sample was taken for XRD and heat measurements. The remaining iron boride was again taken in the bowl and was milled for 40 minutes at 500 rpm. The milled iron boride was again collected and a 5 mg of sample was taken for measurements. The same process was repeated at 500 rpm at 60 and 80 minutes. In this manner a total of 4 different samples of iron boride were collected. These samples were then taken to the RF Induction unit and their heat behaviors were studied at different frequencies and magnetic fields. First the samples were studied at a frequency of 518.7 kHz and a magnetic field of H = 23.89 mT and then at a frequency of 108.9 kHz and a magnetic field of H = 24.80 mT.

X Ray Diffraction patterns XRD Patterns of different samples are shown below .

Figure 3: XRD Pattern of 20 minutes ball milled FeB.

Figure 4: XRD pattern of 40 minutes Ball milled FeB

Figure 5: XRD Pattern of 60 minutes Ball milled FeB

Figure6: XRD pattern of 80 minutes Ball milled FeB.

Figure:8 Combined XRD pattern of all FeB samples.

X Ray Diffraction Results The data shows that as the milling time is increased the

particle size was reduced. The average particle size was calculated using the Debye-Scherer formula (D = kλ/(β CosӨ). For milling times of 20, 40, 60 and 80 minutes the particle size were calculated to be 44 nm, 32 nm, 19nm and 21nm respectively.

The particle sizes calculated by the Debye-Scherer formula confirmed that the particle size decreases as we increase the milling time. However we note that the particle size increases at 80 mints milling time which may be due the recombination of the particles. We also note a little shift in some of the peaks from the XRD data which is due to the strain produced in the nanoparticles.

RF Induction Measurement The samples were heated at frequencies of 518.7 kHz and 108.9 kHz with corresponding magnetics fields of 23.89 mT and 24.80 mT respectively. The heat measurement of the samples by RF induction unit is shown in the graphs

Figure 9 : RF induction measurements of FeB with different milling time at f = 108.9 kHz and H = 24.80 mT.

Figure10: RF induction measurements of FeB with different milling time at f = 518.7 kHz and H = 23.89 mT.

Figure 11: RF induction measurements of FeB for same milling time (20 min) and different frequencies.

Figure13: RF induction measurements of FeB for same milling time (60 min) and different frequencies.

Figure12: RF induction measurements of FeB for same milling time (40 min) and different frequencies.

Figure 14: RF induction measurements of FeB for same milling time (80 min) and different frequencies.

Conclusions

This data shows that particles of higher size produces more heat at lower frequencies. This is because as the particle size increases its corresponding frequency decreases thus its response at lower frequencies enhances generating more heat. Therefore at 20 mints milling time large amount of heat is produced. At higher milling time i.e. at 40, 60 and 80 minutes this trend is lost. It is due the fact that at higher milling time we have a broad distribution of particle size.