Electrospun Fe3O4-PVDF Nanofiber Composite Mats for Cryogenic Magnetic Sensor Applications

Magnetically responsive, mechanically stable and highly flexible iron (III) oxide-polyvinylidene fluoride (Fe3O4-PVDF) piezoelectric composite fiber mats were fabricated via one step electrospinning method for magnetic sensing at cryogenic temperature. The properties of Fe3O4-PVDF composite fiber mats were characterized using scanning electron microscopy (SEM), differential scanning calorimetry (DSC), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, d33 and magnetization test. The fiber diameter decreased as the concentration of Fe3O4 increased. The DSC results suggested a decrease in the crystallinity of the composite fiber mats after adding Fe3O4, and the XRD curves identified that the decrease in crystallinity took place in the β crystalline phases of the fibers. FT-IR results further confirmed the reduction of β phases of the composite fiber mats which dropped the piezoelectric response of the fiber mats by 38% for 5% Fe3O4-PVDF than PVDF fiber but still 400% higher than PVDF pellets. The magnetization test advocated a superparamagnetic state of the fiber at room temperature but a ferromagnetic behavior at a lower temperature. The coercivity values of the mats suggested a homogeneous dispersion of the Fe3O4 nanoparticles into the PVDF matrix. Young’s modulus (E) of the fibers remained the same before and after the magnetization test, indicating the mechanical stability of the fiber in the range of 5 K to 300 K. Its mechanical stability, superparamagnetic behavior at room temperature and ferromagnetic at low temperature could open up its application in spintronic devices at cryogenic temperature and cryogenic power electronic devices.


Introduction
Magnetically responsive flexible fabrics are a growing need for shielding electromagnetic interference especially in microelectronic applications to reduce growing electromagnetic pollution, sheaths for portable electronics and stealth weapon systems in military applications [1,2]. These flexible fabrics are also useful for different biomedical applications like magnetic cell separation membranes [3], magnetic resonance imaging contrast agents [4], etc. Their application could also be extended to spintronics and power electronic devices at extremely low temperatures [5,6]. Iron (III) oxide (Fe 3 O 4 ) nanoparticles are a valuable additive because of their electronic, magnetic, optical, and mechanical properties [7]. Composite nanofibers/thin films consisting of magnetic nanoparticles have been under intensive investigation in the last few years [7]. There remains a need to create flexible processable magnetically responsive and mechanically rugged films.
PVDF is a widely used piezoelectric semi-crystalline polymer with five different crystalline phases (α, β, δ, γ and ε) [8,9]. Its piezoelectric properties are directly related to its crystal phases [10]. Nonpolar α and polar β phase are the most common crystalline structures of PVDF where the β phase is responsible for the piezoelectric properties. Thereby, increasing β phase with eliminating α phase content in PVDF was a major area of focus

Preparation of Fe 3 O 4 -PVDF Fiber Mats
Firstly, 18 wt% (w/w) PVDF was dissolved in DMF/acetone (2:1) solution by stirring at 50 • C for 2 h using a magnetic stirrer in a 250 mL beaker at 200 rpm. After all the pellets dissolved, Fe 3 O 4 nanoparticles (1 wt%, 3 wt% and 5 wt% (w/w) with respect to polymer) were added into the solution. Then, ultrasonication was performed with Sonic Vibracell II (25% pulse amplitude) for 5 min to disperse the Fe 3 O 4 nanoparticles in the polymer solution. Next, 5 mL of this electrospinning solution was taken in a syringe and used for electrospinning. The electrospinning parameters were: voltage 15 kV, flow rate 0.05 mL/h, distance 15 cm, and the needle diameter of 0.038 in. At 400 rpm, a spinning roller with nonstick aluminum foil was capturing the fiber. The average thickness of the fabricated fiber mats was 0.1 mm.

Nanofiber Mats Characterization
Surface morphology of fiber mats was observed via FEI Nova Nano SEM 230 (Thermo Fisher Scientific, Hillsboro, OR, USA). All the samples were sputter coated to reduce the Textiles 2021, 1 229 charge development on their surface. Using a Perkin Elmer DSC6 differential scanning calorimeter, the thermal properties of PVDF and Fe 3 O 4 -PVDF mats were investigated. The scanning temperature range was 30 • C to 210 • C at a rate of 10 • C per minute. Crystal structures were studied using a Rigaku Ultima III X-ray Diffractometer (Rigaku Americas Corporation, The Woodlands, TX, USA). A Scientific Nicolet 6700 Fourier Transform Infrared (FT-IR) Spectroscopy (Thermo Scientific, Waltham, MA, USA) with an attenuated total reflectance (ATR) detector was used to characterize the functional groups in PVDF and Fe 3 O 4 -PVDF mats. A d 33 meter (APC International, Mackeyville, PA, USA) was used to measure the d 33 value of the fiber. The fiber mats were rolled to make them thick enough for d 33 testing. The d 33 testing was done on 5 different spots of the fiber mats and average values were taken. Magnetic properties of the fiber mats were explored by a DynaCool-14 (Quantum Design, San Diego, CA, USA) Physical Properties Measurement System (PPMS) machine. Like in d 33 testing, fiber mats were rolled and fitted into the coffee straws to do the magnetization testing. The weights of the samples used in the magnetization test were (0.1 ± 0.01) g.

Scanning Electron Microscopy (SEM)
Scanning electron microscopy images of the fibers as a function of iron oxide concentration were analyzed using ImageJ software to calculate the diameter of electrospun fibers. The fiber diameter reduced gradually as the concentration of the Fe 3 O 4 increased as shown in Figures 1 and 2. The average fiber diameter reduced up to 26% for 5% Fe 3 O 4 -PVDF fiber. Similar results were obtained from electrospun PLA and PMMA fibers loaded with Fe 3 O 4 nanomaterials [25]. At room temperature, Fe 3 O 4 is a conductor material and adding the Fe 3 O 4 particles into the PVDF solution increased the dielectric constant of the solution. A higher dielectric constant means higher polarity in the solution as well as a higher ability to stabilize the charges in the Taylor cone which leads to a decrease in nanofiber diameters [26]. O and Fe signals in the EDX chart of the fiber mats, shown in Figure 3, confirmed the presence of Fe 3 O 4 in PVDF fiber. Au and Pd picks came from the sputter coated thin conductive film over the samples.

Differential Scanning Calorimetry (DSC)
The thermal behavior and crystallinity of the fiber mats were studied using the DSC. The results are presented in Figure 4a, where PVDF and all composite electrospun fiber mats showed a similar melting temperature (T m ) at~160 • C, indicating no disruption in the PVDF thermal transitions. Furthermore, the melting enthalpy of the nanofiber mats was extracted from DSC curves and used to calculate the degree of crystallinity of the fiber mats based on Equation (1): where X c is the degree of crystallinity of the fiber mats; ∆H 0 is the melting enthalpy of 100% crystalline PVDF (105 J/g), and ∆H is the melting enthalpy of Fe 3 O 4 -PVDF meshes. ∆H was corrected by multiplying it with the mass fraction of PVDF in the composite mats. The results, presented in Table 1, indicated a reduction in crystallinity and enthalpy as the magnetite content increases. For 5% Fe 3 O 4 -PVDF, the crystallinity of the fiber mats decreased by up to 55%. Previous experiments on Fe 3 O 4 -PVDF films yielded similar findings [14,27]. This may be linked to the partial inhibition effect of Fe 3 O 4 addition on polymer crystal formation, similar to inorganic fillers like Al 2 O 3 , TiO 2 , c-LiAlO 2 , and Sm 2 O 3 decreased the crystalline phase of PEO-based polymer electrolyte systems [28][29][30][31][32].

X-ray Diffraction (XRD)
The X-ray diffraction (XRD) patterns of PVDF and Fe3O4-PVDF nanocomposite fiber mats exhibited strong diffraction peak for all samples at 2θ = 20.5° corresponding to β(110) and β(200) planes, as shown Figure 4b. Weaker diffraction peaks at 36° assigned to β(201) planes were also present [33]. These results indicated that electrospun nanofiber membranes contain mainly β-phase crystal structure of PVDF. Due to the high electric voltage (15 kV) used during the electrospinning method, the random electric dipoles in the PVDF solution aligned, resulting in the forming of the β phase crystal structure. Thereby, the piezoelectric property increased. Also, with increasing loading of the Fe3O4 in the polymer matrix, the intensity of the peak at 20.5° decreased (7155, 5634, 4684 and 3401 a.u. for PVDF, 1%, 3% and 5% Fe3O4-PVDF respectively). So, the addition of the Fe3O4 into PVDF did not promote the crystallization of the PVDF in the β phase. This has been ascribed to inhibition of crystallization by the inorganic particles during the solidification process, which decreases the volume fraction of the crystalline phase in the fiber as discussed in the DSC section [34].

X-ray Diffraction (XRD)
The X-ray diffraction (XRD) patterns of PVDF and Fe 3 O 4 -PVDF nanocomposite fiber mats exhibited strong diffraction peak for all samples at 2θ = 20.5 • corresponding to β(110) and β(200) planes, as shown Figure 4b. Weaker diffraction peaks at 36 • assigned to β(201) planes were also present [33]. These results indicated that electrospun nanofiber membranes contain mainly β-phase crystal structure of PVDF. Due to the high electric voltage (15 kV) used during the electrospinning method, the random electric dipoles in the PVDF solution aligned, resulting in the forming of the β phase crystal structure. Thereby, the piezoelectric property increased. Also, with increasing loading of the Fe 3 O 4 in the polymer matrix, the intensity of the peak at 20.5 • decreased (7155, 5634, 4684 and 3401 a.u. for PVDF, 1%, 3% and 5% Fe 3 O 4 -PVDF respectively). So, the addition of the Fe 3 O 4 into PVDF did not promote the crystallization of the PVDF in the β phase. This has been ascribed to inhibition of crystallization by the inorganic particles during the solidification process, which decreases the volume fraction of the crystalline phase in the fiber as discussed in the DSC section [34].
F(β) of the PVDF fiber increased significantly by 68% than PVDF pellets due to the electrospinning method as shown in Table 2. With the addition of Fe3O4, the β phase decreased slightly compared to the β phase of pure PVDF fiber due to the partial inhibition effect as described in the DSC result discussion section and it kept decreasing as the loading of Fe3O4 increasing. The 5% Fe3O4-PVDF had the lowest F(β) compare to all other electrospun fiber mats but still that value is higher than PVDF pellet by 35%. So, the 5% Fe3O4-PVDF should have higher piezoelectric properties than the PVDF pellets which is discussed in d33 section.

Piezoelectric Coefficient (d33) Test
The piezoelectric coefficient (d33) of a material specifies the intensity of its piezoelectric effect. The greater the d33 value, the more effective the piezoelectric effect [38]. Figure  5b represents the piezoelectric coefficient d33 of the PVDF fiber mats as a function of Fe3O4 mass fraction. The d33 of PVDF mats was 32 pC/N and it dropped gradually to 20 pC/N as Fe3O4 increased to a 5% mass ratio. This result was in an agreement with FTIR data. The piezoelectric coefficient d33 is directly related to the β phase of the fiber mats. The β phase of the fiber decreased after adding Fe3O4 as recorded from FTIR data, which led to the decrease in the d33 value. It was also notable that the d33 of PVDF pellet was 5 pC/N as recorded in Table 2, whereas the lowest d33 value of electrospun fiber mat (5% Fe3O4-PVDF) was 20 pC/N; four times higher than PVDF pellets. So, adding Fe3O4 might reduce F(β) of the PVDF fiber increased significantly by 68% than PVDF pellets due to the electrospinning method as shown in Table 2. With the addition of Fe 3 O 4 , the β phase decreased slightly compared to the β phase of pure PVDF fiber due to the partial inhibition effect as described in the DSC result discussion section and it kept decreasing as the loading of Fe 3 O 4 increasing. The 5% Fe 3 O 4 -PVDF had the lowest F(β) compare to all other electrospun fiber mats but still that value is higher than PVDF pellet by 35%. So, the 5% Fe 3 O 4 -PVDF should have higher piezoelectric properties than the PVDF pellets which is discussed in d 33 section.

Piezoelectric Coefficient (d 33 ) Test
The piezoelectric coefficient (d 33 ) of a material specifies the intensity of its piezoelectric effect. The greater the d 33 value, the more effective the piezoelectric effect [38]. Figure 5b represents the piezoelectric coefficient d 33 of the PVDF fiber mats as a function of Fe 3 O 4 mass fraction. The d 33 of PVDF mats was 32 pC/N and it dropped gradually to 20 pC/N as Fe 3 O 4 increased to a 5% mass ratio. This result was in an agreement with FTIR data. The piezoelectric coefficient d 33 is directly related to the β phase of the fiber mats. The β phase of the fiber decreased after adding Fe 3 O 4 as recorded from FTIR data, which led to the decrease in the d 33 value. It was also notable that the d 33 of PVDF pellet was 5 pC/N as recorded in Table 2, whereas the lowest d 33 value of electrospun fiber mat (5% Fe 3 O 4 -PVDF) was 20 pC/N; four times higher than PVDF pellets. So, adding Fe 3 O 4 might reduce the piezoelectric property of the fiber mats but it has still four times higher piezoelectricity than the PVDF pellet.

Magnetization Test
M-H measurements were done at 300 and 5 K with the magnetic field varying from −2 T to 2 T (1 T = 10,000 Oe; 1 Oe = 79.577 Am −1 ) as presented in Figure 6. At 300 K (Figure 6a,b), the samples showed zero coercivity, indicating that the nanofiber mats were in the superparamagnetic state. However, samples showed ferromagnetic behavior at 5 K with a coercivity of~370 Oe for all the samples as shown in Figure 6c,d. Superparamagnetic materials can fluctuate randomly by thermal fluctuation at high enough temperatures just as an atom spin in paramagnetic materials. When the temperature is low, this thermal energy decreases and the magnetic moments are blocked [39]. The temperature below this happens known as blocking temperature which is 290 K as shown in Figure 7. Below the blocking temperature, there is some net alignment of the particle spins, while above it, the spins are in random directions which caused the hysteresis loops to appear at 5 K and increased the magnetic saturation value as shown in Figure 6d and Table 3 [40]. Previous magnetization measurements also indicated that the Fe 3 O 4 particles are superparamagnetic at room temperature and ferromagnetic at low temperatures [41]. So, the composites made of Fe 3 O 4 particles show similar characteristics.
Textiles 2021, 1, FOR PEER REVIEW 8 the piezoelectric property of the fiber mats but it has still four times higher piezoelectricity than the PVDF pellet.

Magnetization Test
M-H measurements were done at 300 and 5 K with the magnetic field varying from −2 T to 2 T (1 T = 10,000 Oe; 1 Oe = 79.577 Am −1 ) as presented in Figure 6. At 300 K ( Figure  6a,b), the samples showed zero coercivity, indicating that the nanofiber mats were in the superparamagnetic state. However, samples showed ferromagnetic behavior at 5 K with a coercivity of ~370 Oe for all the samples as shown in Figure 6c,d. Superparamagnetic materials can fluctuate randomly by thermal fluctuation at high enough temperatures just as an atom spin in paramagnetic materials. When the temperature is low, this thermal energy decreases and the magnetic moments are blocked [39]. The temperature below this happens known as blocking temperature which is 290 K as shown in Figure 7. Below the blocking temperature, there is some net alignment of the particle spins, while above it, the spins are in random directions which caused the hysteresis loops to appear at 5 K and increased the magnetic saturation value as shown in Figure 6d and Table 3 [40]. Previous magnetization measurements also indicated that the Fe3O4 particles are superparamagnetic at room temperature and ferromagnetic at low temperatures [41]. So, the composites made of Fe3O4 particles show similar characteristics.   On the other hand, coercivity measures the resistance of a ferromagnetic material to become demagnetized. A similar coercivity for all the samples in our case indicated that the coercivity is independent of Fe3O4 nanoparticles loading, and a homogeneous distribution of nanoparticles in the polymer matrix [42]. PVDF fiber mats did not show any magnetic response at all on both the temperatures but the Fe3O4 nanoparticles loaded samples did. With the increase of Fe3O4 nanoparticles, the magnetic saturation value of the composites also increased (1.28, 3.29, 5.11 emu/g for 1%, 3% and 5% Fe3O4-PVDF respectively) which suggests that ferromagnetic behavior observed in the composites arose from the magnetic Fe3O4 nanoparticles.
The magnetic moment vs. temperature (M-T) graph as shown in Figure 7, the separation of zero-field-cooled (ZFC) and field-cooled (FC) curves occur at large temperatures (blocking temperature Tb~290 K) indicating thermal irreversibility of the samples below room temperature. In a superparamagnetic state around 5 K, when the samples were cooled at zero applied field, the magnetic moments were randomly oriented leading to the total magnetization to be around zero as shown in Figure 7. When the temperature started to increase, magnetic moments began to fluctuate, and the presence of an externally applied field aligned them that led to the increase in the magnetization. Similarly, in the case of the field-cooled sample, the magnetic moments were aligned at a higher applied field resulting in higher total magnetization at 5 K starting to decrease with heating. These observations are in agreement with previously reported studies on Fe3O4 nanoparticles incorporation in other materials, including ceramic matrices [43].

Dynamic Mechanical Analyzer (DMA)
To see the stability of the fiber mats, DMA was done for all the samples before and after the magnetization test. Since the magnetization test of fiber mats was done at cryogenic temperature (5 K), it was important to determine their mechanical stability after the test. Dynamic strain sweep was done for all the samples as shown in Figure 8. The Young's  On the other hand, coercivity measures the resistance of a ferromagnetic material to become demagnetized. A similar coercivity for all the samples in our case indicated that the coercivity is independent of Fe 3 O 4 nanoparticles loading, and a homogeneous distribution of nanoparticles in the polymer matrix [42]. PVDF fiber mats did not show any magnetic response at all on both the temperatures but the Fe 3 O 4 nanoparticles loaded samples did. With the increase of Fe 3 O 4 nanoparticles, the magnetic saturation value of the composites also increased (1.28, 3.29, 5.11 emu/g for 1%, 3% and 5% Fe 3 O 4 -PVDF respectively) which suggests that ferromagnetic behavior observed in the composites arose from the magnetic Fe 3 O 4 nanoparticles.
The magnetic moment vs. temperature (M-T) graph as shown in Figure 7, the separation of zero-field-cooled (ZFC) and field-cooled (FC) curves occur at large temperatures (blocking temperature T b~2 90 K) indicating thermal irreversibility of the samples below room temperature. In a superparamagnetic state around 5 K, when the samples were cooled at zero applied field, the magnetic moments were randomly oriented leading to the total magnetization to be around zero as shown in Figure 7. When the temperature started to increase, magnetic moments began to fluctuate, and the presence of an externally applied field aligned them that led to the increase in the magnetization. Similarly, in the case of the field-cooled sample, the magnetic moments were aligned at a higher applied field resulting in higher total magnetization at 5 K starting to decrease with heating. These observations are in agreement with previously reported studies on Fe 3 O 4 nanoparticles incorporation in other materials, including ceramic matrices [43].

Dynamic Mechanical Analyzer (DMA)
To see the stability of the fiber mats, DMA was done for all the samples before and after the magnetization test. Since the magnetization test of fiber mats was done at cryogenic temperature (5 K), it was important to determine their mechanical stability after the test. Dynamic strain sweep was done for all the samples as shown in Figure 8. The Young's modulus (E) calculated from the stress-strain curves are listed in Table 4. The retention of modulus following cryogenic temperature exposure in composite samples over the PVDF which shows a slight decrease indicates that Fe 3 O 4 helped the PVDF matrix to retain its mechanical property after being exposed to cryogenic temperature.
Textiles 2021, 1, FOR PEER REVIEW 10 modulus (E) calculated from the stress-strain curves are listed in Table 4. The retention of modulus following cryogenic temperature exposure in composite samples over the PVDF which shows a slight decrease indicates that Fe3O4 helped the PVDF matrix to retain its mechanical property after being exposed to cryogenic temperature.

Conclusions
Flexible and mechanically stable PVDF and Fe3O4-PVDF composite fiber mats were successfully prepared from one step electrospinning method. A decrease in the fiber diameter was revealed from SEM images as the mass fraction of Fe3O4 increased. DSC data suggested that the crystallinity of the composite fiber mats also decreased as a function of Fe3O4 concentration in the PVDF matrix. XRD curves identified the reduction in β crystalline phases of the fiber, causing the decrease detected in DSC results. Due to the reduction in the β crystalline phases, the piezoelectric response of the fiber mats dropped by 38% for the maximum Fe3O4 loading but that is still 4 times higher than PVDF pellets. The magnetization test advocated a superparamagnetic state of the fiber at room temperature but a ferromagnetic behavior at a lower temperature. The coercivity values of the mats suggested a homogeneous dispersion of the Fe3O4 nanoparticles into the PVDF matrix. Moreover, stable and reusable fibers mats were obtained from this process which was confirmed from DMA analysis. The overall results suggest the potential of using Fe3O4-PVDF fiber mats as a magnetically responsive textile in spintronics and power electronic devices at cryogenic conditions.

Conclusions
Flexible and mechanically stable PVDF and Fe 3 O 4 -PVDF composite fiber mats were successfully prepared from one step electrospinning method. A decrease in the fiber diameter was revealed from SEM images as the mass fraction of Fe 3 O 4 increased. DSC data suggested that the crystallinity of the composite fiber mats also decreased as a function of Fe 3 O 4 concentration in the PVDF matrix. XRD curves identified the reduction in β crystalline phases of the fiber, causing the decrease detected in DSC results. Due to the reduction in the β crystalline phases, the piezoelectric response of the fiber mats dropped by 38% for the maximum Fe 3 O 4 loading but that is still 4 times higher than PVDF pellets. The magnetization test advocated a superparamagnetic state of the fiber at room temperature but a ferromagnetic behavior at a lower temperature. The coercivity values of the mats suggested a homogeneous dispersion of the Fe 3 O 4 nanoparticles into the PVDF matrix. Moreover, stable and reusable fibers mats were obtained from this process which was confirmed from DMA analysis. The overall results suggest the potential of using Fe 3 O 4 -PVDF fiber mats as a magnetically responsive textile in spintronics and power electronic devices at cryogenic conditions.

Data Availability Statement:
The data presented in this study are available within the article.