We mainly focused on the magnetocapacitance effect of Fe3O4-PDMS nanocomposites. We also proposed the preparation method and measured microstructures, magnetic properties, and magnetocapacitance value of the nanocomposites. The magnetocapacitance measurement results show that the nanocomposites have magnetocapacitance property, the magnetocapacitance with magnetic field depends on the magnetic property, and the value at the same magnetic field is increasing with the volume fraction of Fe3O4 nanoparticles. The magnetocapacitance model is proposed to explain this phenomenon by analyzing the magnetic interaction between particles and the viscoelasticity of PDMS. We also calculated the theoretical capacitance value of all samples using the magnetization of nanoparticles and mechanical parameters of PDMS. From the theoretical values, it is concluded that the model we proposed can well explain the magnetocapacitance effect of Fe3O4-PDMS nanocomposites.
Materials with magnetocapacitance effect are promising for advanced applications in magnetic field sensors, data storage, and microwave communication devices [
Fe3O4 nanoparticles have wide applications because of their good electrical and magnetic characteristics, such as drug delivery and magnetic resonance imaging (MRI) [
In this study, we investigated the magnetocapacitance effect of Fe3O4 nanoparticles-PDMS composite at lower frequency (200 kHz). Firstly, the composite preparation method is introduced, and samples with different volume fraction of nanoparticles are prepared. Then. the variation of the composite magnetocapacitance dependence of magnetic field is studied. Finally, the model of composite magnetocapacitance effect is also analyzed.
In this experiment, Fe3O4 particles (200 nm, 99%) were used (Beijing DK Nanotechnology, China). Sylgard 184 Silicone from (Dow Corning, MI, USA) was chosen as PDMS polymer matrix. All materials were used as received.
The preparation procedure of Fe3O4-PDMS nanocomposite was as follows. First, Fe3O4 particles were weighted and mixed with alcohol, and then the suspension was sonicated for about 10 min. Amount of PDMS was added to the suspension. After a stirring of about 15 min, the mixture was dried at 100°C for 1 h in a vacuum for evaporating all alcohol. The curing agent was added with the ratio of 10 : 1 of PDMS to curing agent and stirred for 10 min. The prepolymer mixture was dropped into a square module with a size of 15 mm × 15 mm × 1.5 mm, degassed at ambient temperature under vacuum for 30 min to remove any air bubbles, and then cured for 2 h at 90°C in air atmosphere. After curing, the nanocomposite film was peeled off from the module. The detail experimental compositions are shown in Table
Experimental compositions and volume fractions of nanoparticles.
Sample number | Fe3O4 particle size (nm) | Particle content (vol%) |
---|---|---|
1 | 200 | 20 |
2 | 200 | 17 |
3 | 200 | 13 |
4 | 200 | 9 |
5 | 200 | 5 |
6 (Pure PDMS) | — | — |
Microstructures of the prepared samples were examined by scanning electron microscope (SEM, Quanta 250 FEG). Magnetic properties of particles and nanocomposites were investigated using superconducting quantum interference device (SQUID, MPMS-XL-7) magnetometry. Elastic modulus of PDMS was measured by dynamic thermomechanical analysis (DMA, SDTA861e) at ambient temperature. In order to determine the magnetocapacitance properties of the nanocomposite, the film samples were fabricated to be parallel plate capacitor with copper electrode and shielding shell. The magnetic field dependence of the capacitance was measured in the magnetic field range of −10 Gs to 10 Gs at the frequency of 200 kHz using an Agilent high-precision LCR meter (HP4284A). The magnetic field was applied by electromagnet (EMP3, East Changing Technologies). The scheme of the experimental setup is depicted in Figure
The scheme of the experimental setup: (a) nanocomposite; (b) Cu electrode; (c) Cu shielding shell; (d) electromagnet.
The SEM micrographs of sample number 1 are shown in Figure
Scanning electron microscope (SEM) photographs of sample number 1.
Variation of magnetization with applied magnetic field for 200 nm Fe3O4 nanoparticles (Nps) and sample number 1 at ambient temperature.
For analyzing the magnetocapacitance effect of nanocomposite, we have measured the magnetic field dependences of the capacitance of all samples at the frequency of 200 kHz. During the measurements, the magnetic field and the electric field are parallel. At one measurement, the magnetic field starts at 10 kGs and gradually decreases to −10 kGs (defined as decreasing cycle) and then gradually increases to 10 kGs back (defined as increasing cycle).
Figure
Variation of magnetocapacitance with applied magnetic field for all samples. Inset: an amplification of the curve for sample number 1.
The capacitance dependence of the magnetic field is mainly induced by the magnetostriction effect of Fe3O4-PDMS nanocomposite. We assumed that the particles are completely equally distributed in the composites. When the magnetic field is applied to the composites, the arrangement of particles and the attraction between particles are directed along magnetic field vector. Because of the similarity between the magnetic nanoparticles and the magnetic dipole, the strength of the magnetic attraction force is given by
Due to the viscoelasticity of PDMS which is the matrix of the nanocomposite, the motion of particles in the composites induced by applied magnetic field depends on elastic force, viscous force of PDMS, and the attraction force between two adjacent particles. The viscoelasticity of PDMS can be depicted as Kelvin model which contains parallel spring and damper. The particles are supposed as spherical. According to Stokes Law, the motion of dipoles with magnetic field can be described as follows:
If the particles are completely equally distributed in the composites, the mean mutual distance between the two adjacent particles at zero field is given by
The distance
Therefore, (
It is known that
If a stable compressive stress is applied, the variation of the strain with time for PDMS has the creep property and
Using the initial condition (
The number of each chained particle directed along the magnetic field on the sample thickness can be calculated by
When
On the other hand, the capacitance of the sample is
When the magnetization of the particles is zero, the capacity of the sample is
Equation (
Therefore, we can conclude from (
We can know that elastic modulus of PDMS is 6.72 Mpa and viscosity is 5.09 × 103 Pa·s by using DMA. We can also obtain the magnetic induction
Variation of experimental and theoretical capacitance with magnetic field for sample number 1. Left inset: an amplification of experimental data. Right inset: an amplification of theoretical data.
Variation of calculated magnetocapacitance with applied magnetic field for samples with various volume fraction.
In this paper, Fe3O4-PDMS nanocomposites are prepared. The morphology characteristics, magnetic property, and magnetocapacitance effect are investigated. By analyzing the viscoelasticity of PDMS and the magnetic interaction between particles, the magnetocapacitance model of nanocomposites is also proposed. The particles are equally distributed in the composite materials with the average size of 200 nm. The magnetic properties of nanocomposites depend on the magnetic properties of nanoparticles. The magnetocapacitance effects of nanocomposites are observed. The velocity, hysteresis, and saturation value of the variation of the magnetocapacitance with applied magnetic field depend on the magnetic property and the volume fraction of Fe3O4 nanoparticles. The magnetocapacitance model shows that the variation of magnetocapacitance also depends on the elastic module and the viscosity of PDMS. By comparing the calculated value with experimental value, we demonstrate that the model can well explain the magnetocapacitance effect in Fe3O4-PDMS nanocomposites.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was financially supported by the National Natural Science Foundation of China (Grant no. 51375463).