1. Introduction
Over recent years, the integration of different types of fillers in a polymer matrix has led to the development of high-performance and multifunctional composite materials with a wide range of industrial and technological applications. The incorporation of nanoparticles into polymer matrices enhances their performance in multiple ways. This enhancement depends on the filler’s characteristics, namely its size, aspect ratio, content, and processing conditions. The way the filler is incorporated into the polymer matrix is of paramount importance as it affects the overall dispersion and the subsequent properties of interest. Dispersion is further inhibited by the tendency of the filler particles to cluster in larger agglomerates, something that is mainly driven by the existence of the attractive interactions between the filler particles. Multifunctional materials represent a novel class of engineering materials, which can perform certain operations during exposure to an external stimulus or control signal. The challenge relies on the development of a composite system/device that can execute several functions while being easy to fabricate and remaining cost effective. Novel hybrid nanocomposites could be prepared to address these challenges [
1,
2,
3,
4,
5,
6,
7]. It is also of paramount importance to retain their structural integrity and attain a suitable thermal response.
Filled systems are essential for improving and inducing new functional properties otherwise unavailable in neat polymers. For a broad range of applications, suitable fillers can improve the mechanical performance of the matrix and enhance the dielectric, conductive and magnetic properties of the typically insulating polymers. Polymer hybrid nanocomposites can be used for the development of multifunctional and smart engineering systems [
1,
2,
3,
4,
5,
6,
7,
8,
9,
10]. Mechanical, electrical, and magnetic responses of these hybrid materials can be controlled by the employed type and amount of the ceramic inclusions. Modern electronic devices require novel high permittivity materials with enhanced dielectric strength operating at a wide temperature range. These materials should exhibit, at the same time, high permittivity and dielectric strength, low loss, improved mechanical performance, ease of processing and a relative low cost. Ceramic particles/polymer composites can be employed in a wide range of applications such as smart skins, acoustic emission sensors, interlayers in ceramic capacitive structures, microwave devices, portable energy storing systems, etc. In addition, by combining ferroelectric and/or piezoelectric and/or ferromagnetic fillers, smart/functional performance can be achieved, due to the varying electric polarization, conductance, and magnetic properties of the ceramic fillers. Smart performance refers to materials that can tune their behavior when responding to an external or internal stimulus. Several properties of these systems can be tuned in a controllable way, such as stiffness, shape, damping capacity, polarization, conductivity, energy storing efficiency, etc. [
2,
11]. Recently, there have been many studies focusing on the performance of hybrid ceramic particles/polymer composites [
9,
12,
13,
14,
15,
16,
17].
Ferroelectric materials such as barium titanate (BaTiO
3) undergo a disorder to order structural transition at a characteristic temperature (known as the Curie temperature,
TC). Below the Curie temperature, BaTiO
3 exhibits spontaneous polarization, being at the tetragonal/polar ferroelectric phase [
18,
19,
20,
21,
22,
23,
24,
25]. At this stage, ceramic barium titanate exhibits domains with randomly oriented spontaneous polarization. Above the characteristic temperature, at the cubic/non-polar paraelectric phase, BaTiO
3 is characterized by high structural symmetry and the spontaneous polarization is revoked. The application of an electric field upon cooling, while the ferroelectric material is in the paraelectric phase, results in the orientation of the dipole moments of all domains in the direction of the field, when
T <
TC. This procedure is called poling and the achieved polarization remains even after removing the applied field. Apparently thermally triggered, switching between these two phases leads to systems with tunable performance. Ferromagnetic oxides such as strontium ferrite (SrFe
12O
19) are attracting much interest lately due to their magnetic properties, since they have the ability to be magnetized while not being conductive. SrFe
12O
19 is a semiconductive material with bandgap ~1.89 eV [
26], thus under the influence of an external electric field it performs as dielectric. Therefore, dielectric analysis of such systems could provide additional insight into their electrical properties and molecular structure.
In this work, hybrid nanocomposites were developed by embedding ferroelectric (BaTiO3) and magnetic (SrFe12O19) particles in an epoxy resin. Structural properties, dielectric and thermomechanical response were investigated by employing several experimental techniques. The combination of both fillers allows us to achieve optimal performance, which is mainly related to the synergy of the physical mechanisms of both fillers. BaTiO3 induces the functional performance of the transition from the polar ferroelectric phase to the non-polar paraelectric phase, while the SrFe12O19 induces magnetic properties in the system. The dielectric analysis reveals several relaxation processes that are related to both the polymer matrix (glass-to-rubber transition and re-orientation of polar-side groups) and the presence of the reinforcing phase. High filler loading composites exhibit an intermediate relaxation phenomenon called IDE (Intermediate Dipolar Effect), which is attributed to the intrinsic interfacial polarization phenomena within SrFe12O19 grains. Strontium ferrite induces magnetic properties to the nanocomposites, which enhance with the magnetic filler content. The presence of ceramic filler strengthens the static and dynamic mechanical response of the nanocomposites, which results in higher thermal stability.
4. Discussion
There are several formalisms (i.e., dielectric permittivity, electric modulus, AC conductivity and complex impedance) that can be employed for the analysis of dielectric relaxation phenomena. All four formalisms can be used for the description of complex dielectric phenomena, which are present in polymer composites. However, there are cases in which one of them can be more suitable to reveal and analyze a physical mechanism. In the present study, the formalisms of dielectric permittivity, electric modulus, and AC conductivity were employed for the interpretation of dielectric data. Electric modulus is very effective in eliminating the parasitic effect of electrode polarization [
33,
34,
35]. Electric modulus is defined as the inverse quantity of complex permittivity (Equation (2)):
where
ε′,
ε″,
Μ′ and
Μ″ are the real and imaginary part of dielectric permittivity and electric modulus, respectively.
The dielectric reinforcing ability of the employed fillers is shown in
Figure 4, where the real part of dielectric permittivity as a function of frequency, at 30 °C, for all studied systems is depicted. Hybrid systems acquire a higher value of
ε′ compared to the epoxy resin in the whole frequency and temperature range.
Figure 5 and
Figure 6 present the frequency dependence of the imaginary part of electric modulus and loss tangent at various temperatures for the 10 phr SrFe
12O
19/10 phr BaTiO
3/epoxy and 40 phr SrFe
12O
19/10 phr BaTiO
3/epoxy nanocomposites, respectively. The presence of all relaxation processes becomes evident through the formation of loss peaks in the corresponding plots of
M″ and tan
δ. In all samples, at least two relaxation processes are evident: the α-relaxation process in the intermediate frequency range, and at high frequencies the β-mode is derived. It is very interesting to note that in high filler-loaded systems another relaxation process appears in the frequency range between the α and β-mode. This process is absent in the spectra of pure ER and low filler-loaded systems. Since this relaxation is observed between the slow α-mode process and the fast β-mode, it is referred to as the Intermediate Dipolar Effect (IDE), shown in
Figure 6 [
36]. IDE is attributed to the polarization effects taking place between grains of the ceramic fillers (SrFe
12O
19), which relax under the influence of the alternating electric field. IDE has been observed in other ceramic particles/polymer composites and it has been attributed to intrinsic interfacial polarization of the ceramic domains [
36,
37,
38,
39,
40,
41]. Previous studies [
22,
40] have shown the absence of IDE from the dielectric spectra of BaTiO
3/epoxy nanocomposites, and thus it is assigned to the strontium ferrite. Inside the grains of polycrystalline SrFe
12O
19 a number of charge carriers and possibly defects migrate under thermal activation. Their motion is restricted by potential barriers between adjacent crystal domains and thus they are accumulated at the interior interfaces, resulting in polarization effects. The occurring process is an intrinsic interfacial relaxation in the ceramic grains, and it cannot be confused with the IP (MWS) effect, which is present in heterogeneous systems and arises from the polarization effects at the boundaries (interface) between the various phases.
The variation of modulus loss index with temperature, at 1 MHz, is depicted in
Figure 7. The broad peak recorded at intermediate temperatures is assigned to β-relaxation mode and linked to the re-orientation of the polar-side groups of the polymer chains. In addition, a second peak is also observed at higher temperatures as the SrFe
12O
19 concentration increases. This peak is related to the IDE process, and it is evident in all nanocomposites with SrFe
12O
19 loading higher than 5 phr. Processes with higher relaxation times (i.e., α-relaxation and IP) are not shown in
Figure 7, since their peaks are formed at higher frequencies, because of the frequency–temperature superposition outside of the “window” of observation.
Cole–Cole plots for the 10 phr SrFe
12O
19/10 phr BaTiO
3/epoxy nanocomposite at various temperatures are presented in
Figure 8a, while in
Figure 8b Cole–Cole plots at 100 °C of all studied systems are shown. In the Cole–Cole presentation, relaxation mechanisms become evident via the formation of completed or even uncompleted semicircles. In the case of pure Debye processes, that is, relaxations characterized by a single relaxation time, perfect semicircles are formed with their center lying on the x-axis [
11,
30,
33]. However, it is well known that dielectric relaxations in polymers and polymer composites deviate significantly from pure Debye behavior, because of the distribution of relaxation times [
11,
30]. In condensed matter, such as polymer-based materials, interactions between polar and/or charged entities occur, leading to processes corresponding to symmetrical or non-symmetrical distribution of relaxation times, which could also superimpose [
11,
30,
42]. In
Figure 8a, α- and β-relaxation are shown. Formed curves deviate significantly from a perfect Debye process. The main process, indicated by the oblate semicircles, is attributed to the glass-to-rubber transition, followed by a weak tendency of forming a second semicircle assigned to β-relaxation. Although difficult to be observed, a slight change in the slope of the curves at the low-frequency edge could be considered as arising from the IP process. With the increase of temperature, data coincide at the origin of the graph, denoting that no other process is expected at lower frequencies [
33]. The area of the oblate semicircles seems to augment with temperature, representing the resulting increase in the capacitance of the system.
Relaxation dynamics can be studied by plotting the relaxation time as a function of reciprocal temperature for each system and for every relaxation process. The relaxation dynamics of all studied processes are presented in
Figure 9,
Figure 10 and
Figure 11. The β-relaxation and the IDE process can be described by the Arrhenius-type temperature dependence of Equation (3):
where
is a pre-exponential factor expressing the relaxation time at very high temperature and is considered temperature independent, and
EA is the activation energy of the process. A semilogarithmic plot of relaxation time versus (1000/
T) can be used for the determination of
EA. Although IP follows an Arrhenius dependence on temperature, a limited number of loss peaks could be extracted from the recorded data, leading to a non-reliable fitting procedure, and for this reason it is not presented here.
On the other hand, α-mode exhibits a Vogel–Fulcher–Tammann behavior described by Equation (4):
where
A is a parameter that is a measure of activation energy and
T0 is the Vogel temperature (also known as ideal glass transition temperature) [
11,
30,
42]. In the
Supplementary Material the detailed fitting procedure employed for the determination of the loss peak points for the data presented in
Figure 9,
Figure 10 and
Figure 11 is given. It is worth mentioning that in the case of β-mode and IDE relaxation the Havriliak–Negami model was used for the fittings, while in the case of α-relaxation the frequency of the loss peaks was extracted from the
M″ versus logf plots.
Determined values for the α-mode and β-mode via fitting data with Equations (3) and (4) are listed in
Table 1.
Dynamics for the α-relaxation process follow the VFT equation, as described above, since relaxation rate increases rapidly with increasing temperature because of the reduction of free volume. Glass-to-rubber transition is influenced by the presence of both fillers inside the polymer matrix and specifically by the particle–particle and the particle–polymer interactions.
Table 1 indicates that
T0 slightly decreases at low SrFe
12O
19 concentrations compared to epoxy. The 15 phr SrFe
12O
19/10 phr BaTiO
3/epoxy and the 40 phr SrFe
12O
19/10 phr BaTiO
3/epoxy nanocomposites acquire the highest
T0 values. Parameter
A increases upon the addition of SrFe
12O
19 at low to moderate concentrations (5 to 20 phr) indicating that the process is delayed, probably due to strong interaction among macromolecules and both types of particles. Then, a significant reduction of
A is observed at the higher SrFe
12O
19 loadings, implying that the process is facilitated. The decrease of the
A parameter indicates that particle–particle interaction of both types of fillers is dominant, thus facilitating the cooperative relaxation of the polymer chains. Obtained results are in accordance with the discussion of the effect of filler loading derived from the Cole–Cole plots of
Figure 8.
Activation energy reflects the occurring interactions within the nanocomposites and is a measure of the potential barriers exerted by the dipoles’ environment to their orientation. For the β-relaxation, EA initially decreases upon the addition of SrFe12O19 filler in the low filler loadings. This finding shows that nano-inclusions facilitate the orientation of the polar-side groups of the polymer matrix at low content, since particles are apart from each other and interact strongly with the main chains. In general, activation energy increases with filler content, because particles become closer and anchor on the main polymer chain. In the case of the 40 phr SrFe12O19/10 phr BaTiO3/epoxy system, the lowering of EA is attributed to the strong interactions between particles, which dominate upon macromolecule/particle interaction and do not exert spatial restrictions, since the generated clusters are small and scarce. Further increase in filler loading, causes an increment of activation energy, which reaches its highest value for the 50 phr SrFe12O19/10 phr BaTiO3/epoxy nanocomposite. The latter indicates that the presence of both fillers obstructs the orientation of the polar-side groups, due to spatial restrictions related to the formation of clusters, because of the strong interactions between SrFe12O19 and BaTiO3 particles.
Figure 11 indicates that IDE follows an unusual evolution with temperature. For the temperature range of 75–100 °C, it follows an Arrhenius-like behavior, while at a lower temperature range (45–70 °C), it follows a VFT behavior. The intrinsic interfacial polarization phenomena within SrFe
12O
19 particles are responsible for this response. At high temperatures, IDE, as an interfacial polarization effect, follows a typical Arrhenius behavior. However, at lower temperatures it seems that IDE is influenced by α-relaxation. The influence of α-relaxation upon IDE has been reported previously [
38] and has been attributed to the proximity of the temperature ranges where the two processes occur. The latter becomes evident by comparing the temperature ranges of
Figure 9 and
Figure 11. The value of activation energy obtained for its high-temperature part is 0.725 eV for the nanocomposite with 40 phr SrFe
12O
19, which is close to relative values of hybrid systems [
40] and deviates from values of binary composites reinforced with ZnO or TiO
2 particles [
38,
41]. The activation energy of IDE should be related to the type of filler (i.e., crystal structure) and the size of domains within the ceramic grains.
The magnetic hysteresis loops of the nanocomposites, at ambient temperature, are shown in
Figure 12a. The ferromagnetic behavior of strontium ferrite is induced into the nanocomposites and their magnetization increases with magnetic filler content, which is lower from the corresponding value of the monolithic SrFe
12O
19 because of the presence of the two non-magnetic phases (i.e., epoxy resin and BaTiO
3). Coercivity attains constant values in all nanocomposites (~4.5 kOe), indicating that the coercive field is an intrinsic property of the employed magnetic phase [
43]. The inset of
Figure 12a shows the virgin magnetization curves of the nanocomposites. From the slope of their linear parts at low magnetic fields and by taking account of the strontium ferrite density, magnetic susceptibility of nanocomposites can be determined. Obtained values are listed in
Table S1, and as expected, increase with filler content. Magnetic saturation (
Ms) and magnetic remanence (
Mr) of the nanocomposites as a function of magnetic phase content are shown in
Figure 12b. The observed linear relation of magnetization upon SrFe
12O
19 content can be considered as an indication for the fine dispersion of magnetic nanoparticles [
9,
43] and provides the effectiveness to tailor the magnetic response of the nanocomposites by controlling the amount of the employed magnetic nanoparticles.
Static mechanical properties were investigated via tensile tests at ambient temperatures.
Figure 13a summarizes the results. Young’s modulus initially decreases with filler content, which is afterwards followed by an augmentative response with SrFe
12O
19 nanoparticle content. Stiffness of nanocomposites appears to enhance with SrFe
12O
19 nanoparticles, providing an indirect indication for their good wetting and strong adhesion with the polymer matrix. Tensile strength and fracture toughness are also favorably influenced by the presence of fillers. Tensile strength increases significantly with respect to the unfilled matrix and retains almost constant values up to the system with the highest reinforcing phase content. Fracture toughness, which is an expression of the system ductility, increases up to the 10 phr SrFe
12O
19/10 phr BaTiO
3/epoxy nanocomposite. At higher concentrations, it follows a diminishing tendency that is more pronounced at the two higher SrFe
12O
19 concentrations. The possible existence of small clusters can be considered as responsible for this detrimental effect, since clusters and aggregates act as stress-raisers within the nanocomposites. However, from
Figure 13a it is evident that the systems’ mechanical integrity is improved by the presence of ceramic particles.
The dynamic mechanical response of the examined systems is depicted in
Figure 13b. Storage modulus increases with filler content in the glassy state, denoting the strengthening ability of the reinforcing phases. A step-like transition is observed in the temperature range of 40 to 60 °C, which is indicative of the glass-to-rubber transition of the thermosetting matrix. The transition range shifts to higher temperatures with filler content, denoting the good adhesion between the reinforcing particles and the matrix. The inset of
Figure 13b presents the variation of loss modulus with temperature for the same systems.
TGA thermographs were employed for studying the thermal degradation of the fabricated systems. In their spectra, two mass loss mechanisms are included, shown in
Figure S2.
The first process appears between 150 and 250 °C and is assigned probably to the breakdown of unreacted epoxy rings and to the possible existence of impurities. The second process related to the decomposition of the epoxy matrix is recorded between 300 and 400 °C. Nanocomposites’ thermal stability improves with ceramic particles because the first degradation mechanism shifts towards higher temperatures. The temperature where the 5% initial mass loss of the examined systems appears is listed in
Table S1.
Finally, DSC graphs were used for studying the thermal events occurring in the examined systems from ambient temperature up to 100 °C. Since the epoxy matrix is an amorphous thermosetting polymer and the reinforcing phases are ceramic crystalline particles with very high thermal stability, no other effect is expected to occur in this region besides the endothermic increase of specific heat capacity. The step-like increase of specific heat capacity corresponds to the glass-to-rubber transition of the matrix and can be exploited for the determination of glass transition temperature (
Tg). Glass transition temperature was evaluated via the point of inflexion of the step-like increase of specific heat capacity via suitable software provided by TA Instruments.
Figure 14 presents the DSC thermographs of all examined systems. The determined values, which do not vary significantly with filler content, are listed in
Table S1. It is well known that the values determined for
Tg via different experimental techniques do not coincide [
10,
11,
12]. However, the recorded variation tendency with filler content is in agreement with the previously discussed dielectric results.
The stored and retrieved energies were evaluated by integrating the time-dependent charging/discharging current functions, via Equation (5):
where
E is the stored or retrieved energy at the nanocomposite,
Q is the amount of charge,
I(
t) is the charging or discharging current and
C is the capacitance of the composite as derived by the BDS measurements [
10]. In
Figure 15a,b, the stored and retrieved energies for all studied systems and for charging voltage 100 V are presented as a function of time. Both energies increase with filler content since the integrated ceramic inclusions are acting as a distributed network of capacitors.
The applied voltage forces the charge carriers to migrate within the nanocomposites. However, the insulating matrix exerts potential barriers, diminishing the charge migration. Temperature could provide sufficient energy to carriers to overcome the barriers, and thus conductivity increases. At room temperature, a limited number of charges has the ability to overcome the potential barriers, so charge migration is restricted, and conductivity attains low values. Charges are accumulating at the phases’ interface and trapped. The application of external voltage causes a reduction of the potential barriers, and charges could migrate through the interfacial area executing a trapping/detrapping sequence. The relative coefficient of energy efficiency can be calculated via Equation (6):
where
Eretr,comp and
Eretr,matrix are the recovered energies from a nanocomposite and the matrix under the same charging voltage, respectively.
Figure 15c depicts the relative coefficient of energy efficiency as a function of time for the examined systems at 100 V charging voltage. Obviously, the ability of restoring energy is enhanced with filler loading.
From the presented results and the conducted analysis, it is apparent that the fabricated hybrid nanocomposites exhibit multifunctional performance since they simultaneously acquire improved dielectric, thermomechanical, and magnetic properties, and have energy storing/retrieving ability.