Fabrication and electromagnetic performance of talc/NiTiO3 composite

In this study, the electromagnetic (EM) performance of talc/NiTiO3 composite was evaluated. The morphology of talc displayed a lamella structure; there were many nanoparticles of NiTiO3 coated on the talc lamella. Thermal destruction occurred, which increased the surface area from 2.51 m2 g−1 to 79.09 m2 g−1 at the calcined stage at 650°C. The presence of NiTiO3 increased dielectric loss and magnetic loss of talc. The calculation of EM wave absorption of talc/NiTiO3 obtained a maximum reflection loss of −11.94 dB at the thickness of 6.85 mm; the optimum thickness for microwave absorption is 6.3–7.3 mm. This study revealed a new approach for fabricating an EM absorber and broadening applications of both talc and NiTiO3 in EM absorption.


Introduction
Talc is a widely distributed layered silicate mineral [1][2][3] with a composite of Mg 3 Si 4 O 10 (OH) 2 . Compared with other silicate minerals, talc has good surface affinity, high chemical stability and high resistance to heat [4]. It is commonly used in the manufacture of paper, linoleum, ceramics, coatings, cosmetics, plastics and so on [5,6]. For example, Refaa et al. [7] melted polylactide (PLA) with talc and found that the addition of talc as a nucleating agent contributes to the crystallization kinetics without affecting the crystal structure of the PLA and lamellae thickness, which is an application of talc in polymer synthesis.
In recent years, considerable attention has been paid to super-paramagnetic materials such as Ni, along with titanate (Ti) with enhanced high surface area [25][26][27][28][29][30]. With a narrow band gap of −2.18 eV, NiTiO 3 possesses significant photocatalytic property and unique photo response in the range of visible light [31], which was usually fabricated as a photocatalyst, not EM wave absorber: Zeng et al. synthesized it for photocatalytic water splitting by harvesting solar energy; the rate of hydrogen generation was about threefold that of pure g-C 3 N 4 [32]. Pugazhenthiran et al. [30] prepared it for photocatalytic degradation of ceftiofur sodium (CFS); nearly 97% CFS was mineralized under direct sunlight irradiation. These studies broadened the NiTiO 3 applications in water treatment for its enhanced photocatalytic ability. Also some efforts were undertaken in the pursuit of exploring the EM performance of NiTiO 3 ; Lenin et al. reported that iron-doped nickel titanate (Fe 3+ /NiTiO 3 ) ferromagnetic nanoparticles displayed enhanced ferrodielectric behaviour at higher Fe content [25]. However, it still focused on EM absorption of Fe, not NiTiO 3 . Nevertheless, there have been no studies reported on the EM parameters of combined talc and NiTiO 3 , which will broaden applications of both talc and NiTiO 3. In this study, we present a straightforward method to fabricate a novel EM absorber. As the talc has amphiprotic property [33], talc sheets will be a good carrier for tetrabutyltitanate and nickel acetate to fabricate NiTiO 3 . The morphology and structure of talc/NiTiO 3 composite were examined using XRD and SEM. The complex permittivity and permeability of talc/NiTiO 3 composites were determined first, the reflection loss (RL) of talc/NiTiO 3 composite for EM waves at a frequency of 1-18 GHz was calculated as a function of thickness, and the optimum thickness and microwave frequency of the composite were obtained.

Materials
Talc powders with particle sizes of 500-700 nm and chemical composition of Mg 3 Si 4 O 10 (OH) 2 were obtained from Jiangxi Province, China. Tetrabutyltitanate (TNBT), nickel acetate (C 4 H 6 O 4 Ni·4H 2 O) and ethanol are of analytical reagent grade. They were purchased from Aladdin Industrial Co., Ltd, Shanghai, China and were used without further purification.

Preparation of talc/NiTiO 3 composite
In the typical procedure, 6 g talc and 6.8 g TNBT were dissolved in 10 ml of ethanol solvent, then ground until ethanol almost volatilized. In the second step, 5.8 g of C 4 H 6 O 4 Ni·4H 2 O was added with 10 ml of ethanol solvent, which was also then ground until ethanol almost volatilized. The obtained product was calcined in a muffle (Nabertherm N7/H, Germany) at 450, 550 and 650°C for 2 h at a heating rate of 2°C min −1 . The obtained talc/NiTiO 3 products were labelled TN 450, TN 550 and TN 650.

Scanning electron microscopy
The SEM images of talc and talc/NiTiO 3 composite are presented in figure 2. The surface feature of talc (figure 2a) exhibited a smooth lamella-shaped structure, and the widths of most lamellae are between 50 and 500 nm. When coated with NiTiO 3 , as shown in figure 2b-d, the resulting products (TN 450, TN 550 and TN 650) preserved the lamella-shaped structure, but the widths of most lamellae are less than 300 nm. This indicates that talc/NiTiO 3 was ashed due to the occurrence of thermal destruction [34]. It also can be seen that there were many more nanoparticles of less than 20 nm covering the surface of lamellae at higher calcined temperature. Figure 3 shows the dispersion of Si, Ni and Ti in these nanoparticles by element mapping in EDS measurements using the SEM system. The constituent with the most content in talc (Mg 3 Si 4 O 10 (OH) 2 ) is Si. It is found that Ni and Ti also were observed in the place of nanoparticles, indicating that NiTiO 3 was synthesized successfully and covered the surface of lamellae, resulting in the lamellae presenting less smooth surface features than talc. It was also verified that talc sheets were a good carrier to catch TNBT and nickel acetate for fabricating NiTiO 3 .

Brunauer-Emmett-Teller surface area and pore analyses
The BET surface area and pore size distribution may affect the EM wave absorption of talc/NiTiO 3 composite via the scattering effect [16], smaller size will benefit the impinging of EM radiation and higher BET surface area will increase permittivity. Their pore size distributions, specific surface areas (S BET ), total pore volumes (V t ) and average pore diameter (d) were also calculated (     range of between 2 and 16 nm. The BET surface area of TN 650 (79.05 m 2 g −1 ) was much larger than that of talc (2.51 m 2 g −1 ). The decreasing average pore size and increasing BET surface area resulted from two contradictory reasons: one is the thermal destruction that reduces the pore size [19], which occurs at higher temperatures; on the other hand, NiTiO 3 was not only coated on the surface of talc lamellae, but also intercalated between talc layers with subsequent formation of NiTiO 3 nanoparticles that increased porosity. Correspondingly, the total pore volume was also increased from 0.021 cm 3 g −1 (talc) to 0.139 cm 3 g −1 (TN 650); meanwhile the mesopore volume was also increased from 0.020 cm 3 g −1 (talc) to 0.130 cm 3 g −1 (TN 650) [35], indicating that a more porous structure of talc/NiTiO 3 composite had formed.

Permittivity and permeability
The frequency dependence of EM parameters of talc/NiTiO 3 composite was detected using a vector network analyser in the range of 1-18 GHz. As shown in figure 5a,b, the real (ε ) and imaginary permittivities (ε ) of talc are in the ranges 2.75-2.86 and 0.03-0.07, respectively. The ε kept gently with the increasing of frequency, while the ε displayed some fluctuations. Consequently, their dielectric loss tangents (tan δ e = ε /ε , figure 5c) were generally invariant in the range of 0-0. 025  talc/NiTiO 3 will increase the dependence of ε on frequency, which may increase the conductivity of the sample at higher temperature, resulting in talc/NiTiO 3 composite showing higher permittivity than talc. Figure 5d,e shows the frequency dependence of the relative complex permeability of talc/NiTiO 3 composite. Because of the absence of magnetic components, the real (μ ) and imaginary permeabilities (μ ) of talc and talc/NiTiO 3 composite are about 1.00-1.06 and 0.01-0.10, respectively. Four samples showed zigzag curves for μ and μ values. Nevertheless, the μ and μ values of talc/NiTiO 3 composite were slightly higher than those of talc and TN 650 exhibited the highest value; the maximum μ value is at 1-2 GHz. As a result, the magnetic loss tangent (tan δ m = μ /μ , figure 5f ) of talc and TN 650 is below 0.030 and 0.095, respectively. The magnetic loss of talc/NiTiO 3 composite in the GHz region can be a result of natural resonance or eddy current. The C = μ (μ ) −2 f −1 is introduced to determine the magnetic loss of talc/NiTiO 3 composite in this study [36]. As shown in figure 6, the C value exhibits a quicker decrease from 1 to 10 GHz at higher temperatures and maintains at about 0.001 between 10 and 18 GHz. It is indicated that the magnetic loss of talc/NiTiO 3 composite at 10-18 GHz is dominated by the eddy current effect. The resonance peak is not found because of the small μ values and larger measurement errors. The complex permittivity and permeability values of talc/NiTiO 3 composite suggest that the presence of NiTiO 3 will enhance the EM wave absorption, mainly induced by dielectric loss, and higher calcined temperature results in higher performances.

Electromagnetic wave absorption properties
According to transmission line theory, the RL of absorbers can be evaluated from the measured complex permittivity and permeability using the following equations [37]: and where Z in and Z 0 are the impedances of the absorber and air, respectively; and ε and μ are the complex permittivity and permeability of the absorber, respectively; f is the frequency of EM waves; d is the thickness of the absorber; and c is the velocity of light. As shown in figure 7a, the calculated RLs of  18.00 GHz, respectively. As discussed above, the values of permittivity and permeability of talc/NiTiO 3 are quite lower than those of conventional carbon-based nanocomposites, such as carbon nanotubes (CNTs). CNTs have special helical structures and chiral properties, which will result in special EM performance. However, the lamella structure of talc weakened the quantumlimiting effects, so the RL may have been mainly induced by the dielectric loss of NiTiO 3 . On the other hand, it showed NiTiO 3 with a morphology at nanoscale (figure 2b-d). It has been previously reported that nanoscale composites may cause a quantum tunnelling phenomenon. For the electrical conduction mechanism, the tunnelling transmission of the material can be calculated by the following equations: where E is the total energy of the spilled particle, E F is the Fermi energy of the fictitious medium, l is the dimer gap, V loc is the local dielectric response and T (V loc , E, l) is the tunnelling (transmission) probability between the nanoparticles. From this perspective and figure 2d, it can be seen that NiTiO 3 is dispersed on the talc lamella most uniformly, which will reduce the dimer gap to cause high V loc and then Fowler-Nordheim tunnelling [38]. The magnetic loss can also be explained by the tunnelling effect, but may be weak. This suggests a possible physical reason that the EM wave absorption of talc/NiTiO 3 may be affected by a combination of effects: the dielectric effect and the tunnelling effect.
The intrinsic wave impedances of these materials have been computed using the following equations: By inspection of measurements, it is worth noting that the relative permeability exhibits a real part very close (within the declared accuracy) to 1 and an imaginary part very close to 0. Therefore, we decided to neglect the effects on permeability, fixing μ = 1. Therefore, higher electrical permittivity resulted in lower intrinsic wave impedance values. From figure 8, TN 650 showed the lowest value of intrinsic wave impendance versus frequency. By the above results, the trend of the electrical permittivity with the concentration of NiTiO 3 was confirmed, with more NiTiO 3 content deposited on the talc lamella with increase in calcined temperature. Previously, it was found that the EM properties of these composites can be tailored by controlling the content of the lossy materials, so a gradual matching is achieved through a high percentage of the absorber at the appropriate concentration range. In this work, TN 650 showed the highest EM performance even though with lowest impendance value [39].
In addition, the RL of talc/NiTiO 3 composite as a function of thickness was analysed to obtain the optimum thickness and frequency for EM absorption. As shown in figure 9, the maximum RL of rsos.royalsocietypublishing.org R. Soc. open   It was also found that the maximum width of TN 550 was for RL < −5 dB at a value of 0.50 GHz at the thickness of 7.8-8.7 mm, and that of TN 450 was for RL < −5 dB at the thickness range of 0-10 mm. This verified that higher calcined temperature benefits talc/NiTiO 3 composite to show higher permittivity and permeability to absorb EM waves, so TN 650 exhibits the optimum value for an EM wave absorber. Because the increase in the thickness will significantly increase the weight of the absorber and thus restrict its applications, the optimum thickness of TN 650 for microwave absorption is 6.3-7.3 mm.
The corresponding frequency at this thickness is 13-18 GHz, with a maximum RL of -11.94 dB, and a bandwidth of 1.04-3.41 GHz for RL < −5 dB and 0-1.05 GHz for RL < −10 dB.
When compared with other EM wave absorbing materials (table 2), due to the absence of magnetic components, talc/NiTiO 3 composite showed lower absorption for EM waves than traditional EM wave absorbers, such as porous carbon fibre [40], porous Fe [41] and rice husk ash (SiC) [34]. However, the maximum RL, bandwidth < −5 dB and bandwidth < −10 dB are considerably higher than those for Fe pillared in halloysite [42] and montmorillonite blended with polymers [43]. In conclusion, talc/NiTiO 3 is a considerable potential EM wave absorber.