Controlling Terahertz Dielectric Responses in Polymer Composites by Engineering α-Al₂O₃ Whisker Filler Distribution

Abstract

As the communication band gradually approaches the terahertz (THz) range, there is an urgent need to explore materials with ideal dielectric properties for THz communication devices. Nevertheless, most polymers present a low dielectric constant (Dk), and the regulation of their dielectric properties in the THz range has rarely been reported. In this work, the isotactic polypropylene (iPP)/α-Al₂O₃ whisker composites were synthesized and their THz dielectric parameters were optimized. The Dk values increased from 2.23 to 3.13 with filler (α-Al₂O₃ whisker) concentration, ranging from 0 to 20 vol%, but were almost independent of the test frequency. The loss tangent (Df) values presented an increasing tendency along with both filler concentrations and test frequency. All composites exhibited Df values of less than 4.0 × 10⁻³. Particularly, the dielectric properties of composites can be further regulated by adjusting the orientation direction of the whisker fillers. The orientation of the whisker fillers was adjusted via the injection molding method. Along the direction of the whisker orientation distribution, the composites exhibit higher Dk values and lower Df values. This work presented a schematic to design polymer composites with higher Dk but controlled Df in the THz range and is significant for the development of advanced materials-based THz devices.

1. Introduction

According to Edholm’s law of bandwidth, the bandwidth demands of communication systems double every 18 months. Projections indicate that by 2030, the peak rate of communication transmissions will reach the terabit-per-second (Tbit/s) range [1]. However, the microwave and millimeter-wave frequency bands utilized by fifth-generation (5G) mobile networks are insufficient to meet these massive bandwidth requirements, necessitating the evolution of sixth-generation (6G) systems toward higher-frequency regimes [2]. The terahertz (THz) band (0.1–10 THz), with its abundant spectrum resources and potential to enable Tbit/s transmission rates, has gained global recognition as both a key candidate frequency range and a foundational technology for 6G [3]. Dielectric materials are extensively used in communication devices such as dielectric resonators, antennas, filters, duplexers, and substrates, etc. These materials are critical components driving the advancement of 6G communication technology [4,5,6,7]. Particularly, the design and optimization of their THz dielectric properties are essential. In order to reduce the delay time of signal propagation and minimize the capacitive coupling effect, the dielectric material used in these devices is required to have a low dielectric constant (Dk). Meanwhile, the size of the electronic devices in a communication system is determined by the Dk value of the dielectric material (using a dielectric material with a high Dk value can effectively reduce the device size). Thus, to strike a balance between these two factors, the Dk value of the dielectric material must be optimized for each application [8]. In addition, reducing the loss tangent (Df) of the dielectric material can effectively reduce signal attenuation [9]. This is especially important for high-frequency devices in the THz band [10].

Various polymeric materials and inorganic materials are widely used in the microwave frequency band due to their excellent insulating properties [11,12,13]. Some non-polar polymers, due to the symmetry in their molecular chain structure, have lower Df values (on the order of about 10⁻⁴) because of their weaker ability to establish polarization. This gives them greater potential for applications at high frequencies, but it inevitably leads to low Dk values (on the order of about 2) for these non-polar polymers [14]. In contrast, some inorganic materials with single-crystal structures such as Al₂O₃ and MgO combine moderate Dk values (on the order of about 10) and low Df values (on the order of about 5 × 10⁻⁴) [15,16]. However, due to the difficulty of processing the inorganic material itself, it cannot be adapted to the complex manufacturing methods of current electronic devices [17]. Functional polymer composites have the ability to meet these needs. Various composites filled with inorganic fillers in a polymer matrix, such as polytetrafluoroethylene (PTFE)/MgO oxide, anisotropic polypropylene (iPP)/MgO whiskers, and cyclic olefin copolymers (COP)/Al₂O₃, have attracted much attention in microwave frequency band applications due to their excellent flexibility and tunable dielectric properties. Although the Dk values of these composites are suitable for the THz band, there is a need to further reduce the Df values [18,19,20].

α-Al₂O₃ is an ideal filler for THz dielectric composites due to its suitable Dk value and low Df value. The α-Al₂O₃ whiskers are rod-like crystals and grow along the c-axis orientation. Their anisotropic crystal structure results in the strong anisotropy of the dielectric properties, and the high aspect ratio of the filler makes it more conducive to the modulation of the dielectric properties of composites [5,21,22,23]. Therefore, it is an effective way to use α-Al₂O₃ whiskers as dielectric fillers to make full use of their anisotropy and adjust the dielectric properties of composites by controlling the orientation of fillers in the matrix [24]. However, the effect of the α-Al₂O₃ whisker filler concentration and orientation on the dielectric properties of iPP/α-Al₂O₃ whisker composites in the THz band remains unknown. Therefore, this study aims to develop iPP/α-Al₂O₃ whisker composites with suitable dielectric properties for THz devices. The α-Al₂O₃ whiskers were prepared via the hydrothermal method. The iPP/α-Al₂O₃ whisker composites were fabricated via the hot press molding and injection molding methods.

In this work, a scheme for designing polymer composites with high Dk but controllable Df in the THz range is proposed, and the effects of the concentration and orientation of α-Al₂O₃ whisker fillers on the THz dielectric properties of the composites are investigated in detail. The results of this study can provide a valuable reference for the development of dielectric composites in the THz frequency range, which is of great significance for the development of various types of composite-based THz devices.

2. Materials and Methods

Synthesis of α-Al₂O₃ whiskers: Aluminum sulfate (Al₂(SO₄)₃·18H₂O, Aladdin, Chengdu, China) and urea (CO(NH₂)₂, Aladdin, Chengdu, China) were used as the experimental raw materials for the hydrothermal preparation of ammonium aluminum carbonate hydroxide (AACH), which is a precursor of α-Al₂O₃ whiskers. First, 16 g of urea was added to 70 mL of stirred 0.15 mol/L aqueous Al₂(SO₄)₃ solution. The mixture was stirred thoroughly. Then, the solution was poured into a hydrothermal reactor (YIBEIER I&E Co., Ltd., Xi’an, China) with a capacity of 100 mL. Subsequently, the reactor was placed in a blast drying oven (Jinghong Experimental Equipment Co., Ltd., Shanghai, China) and heated at 150 °C for 12 h. A white precipitate was obtained via filtration. This precipitate was then washed with deionized water and ethanol, respectively. After that, it was dried at 80 °C in a drying oven for 8 h, thus obtaining the precursor powder. The dried precursor powder was loaded into a porcelain boat (Wanlian Ceramics Co., Ltd., Tangshan, China) and placed in a tube furnace (Kejing Materials Technology Co., Ltd., Hefei, China). Under a nitrogen atmosphere, the furnace was heated from room temperature to 300 °C at a heating rate of 5 °C per minute. Then, it was heated from 300 °C to 750 °C at a heating rate of 1 °C per minute, and then from 750 °C to 1200 °C at a heating rate of 5 °C per minute. After that, the temperature was held at 1200 °C for 2 h. Finally, the furnace was cooled down completely to obtain the α-Al₂O₃ whisker powder.

Fabrication of composites: The synthetically prepared α-Al₂O₃ whiskers were employed as fillers, and iPP (NOVATEC-iPP MA3, Japan Polypropylene Corp., Tokyo Metropolis, Japan) was used as the matrix polymer for the composites. IPP/α-Al₂O₃ whisker composites with α-Al₂O₃ concentrations ranging from 0 to 20 vol% were prepared by using an intermittent kneader (Kechuang Rubber and Plastic Machinery Equipment Co., Ltd., Shanghai, China) to add α-Al₂O₃ whisker powder to the iPP melt at 190 °C and knead for 6 min to obtain α-Al₂O₃ whisker-filled iPP-based composites.

Plate-shaped samples of the composites, with α-Al₂O₃ whisker concentrations ranging from 0 to 20 vol%, were prepared for testing the dielectric properties of the composites. These samples were prepared via the hot press molding (Labtech Engineering Co., Ltd., Samutprakarn, Thailand) and injection molding methods, respectively. In the hot-pressing process, the composite melt was molded into a circular plate with a diameter of 50 mm and a thickness of 2 mm using a hot press at 200 °C. In the injection molding process, the composite melt was molded into a 70 mm × 20 mm × 2 mm strip at 220 °C using an injection molding machine (Thermo Electron GmbH, Karlsruhe, Germany).

Characterizations: Phase compositions in the prepared alumina whisker powders were characterized using a Rigaku Ultima IV X-ray diffractometer (XRD, Rigaku Corp., Tokyo Metropolis, Japan) in steps of 0.02 degrees over a range of 20°–80°. The morphology of the prepared α-Al₂O₃ whiskers was studied using field emission scanning electron microscopy (SEM, Thermo Fisher Scientific, Waltham, MA, USA). Meanwhile, the average size of the prepared α-Al₂O₃ whisker samples was estimated by measuring the lengths of the long and short axes of multiple whiskers based on the scanning electron microscope images of the sample.

Testing of dielectric properties: A terahertz time-domain spectroscopy system (THz-TDS, Quenda Terahertz Technology Co., Ltd., Qingdao, China) was used to test the Dk values and Df values of the iPP/α-Al₂O₃ whisker composites in the 0.5–1.0 THz band. THz-TDS is mainly composed of a femtosecond laser, an optical delay line, a THz wave generator, and a THz wave detector. The laser generated by the laser is divided into two paths, one beam is called pump light, which is used to drive the THz wave generator to produce THz waves. The other beam of light, called detector light, passes through an optical delay line and then interacts with the terahertz wave pulse in a terahertz wave detector device to recognize the sample information carried by the terahertz wave. When a THz wave pulse passes over a sample, the sample absorbs, refracts, and scatters the THz wave, resulting in changes in the amplitude, phase, and waveform of the THz wave pulse. The transient spectrum of the sample is obtained by measuring these changes, and the Fourier transform is used to process the transient spectrum of the sample to obtain the THz spectral information of the sample, including absorption coefficients, refractive indices, and other optical parameters, so as to further analyze the dielectric properties of the sample. The broadband nature of the THz waves (0.1 THz to 10 THz) and their penetration into polymer materials, as well as the non-contact sample measurement capability of THz-TDS, make THz-TDS an excellent tool for comprehensive sample analysis.

3. Results and Discussion

3.1. Synthesis of α-Al₂O₃ Whiskers

In this study, highly crystalline alumina whiskers were prepared to serve as fillers for the composites. Figure 1 shows the XRD pattern of the prepared α-Al₂O₃ whiskers. It reveals a pure α-Al₂O₃ phase that precisely matches the standard PDF card (PDF#10-0173), with no extraneous peaks. In addition, the diffraction peaks in the image are narrow, indicating that the prepared α-Al₂O₃ whiskers possess a high degree of crystallinity. Significantly, the high crystallinity of the inorganic material is associated with a lower Df value [25]. Therefore, high crystallinity fillers will help to decrease the overall Df value of the composites.

The morphology of the precursor of the α-Al₂O₃ whiskers, AACH, is shown in Figure 2a. During the hydrothermal preparation of the precursor AACH, the growth of AACH crystals adheres to the polyhedral coordination growth laws in solution. The growth rate of each facet family is associated with the number of vertices of the coordination polyhedron exposed at the interface. Specifically, the facet family with the highest number of vertices of the coordination polyhedron exposed at the interface grows the fastest [26]. The crystal structure of AACH is layered. In this structure, aluminum–oxygen octahedra, composed of aluminum ions and oxygen ions, are arranged in chains parallel along the c-axis in a corner-topped manner. The prongs of the octahedra connect between the chains to form a wavy layered structure. Due to the characteristics of the AACH crystal structure, the octahedral vertices are exposed at the interfaces in the c-axis direction, while the octahedral prongs are exposed in the a- and b-axis directions. As a result, the AACH crystals grow faster along the c-axis direction [27]. This is demonstrated by the preferential growth of the grains along the c-axis direction, ultimately resulting in a long, rod-like structure. Meanwhile, as shown in Figure 2a, the AACH precursor has a smooth surface and good dispersion, with an average length of about 10 μm and a diameter of about 0.8 μm.

Compared with the precursor AACH, the α-Al₂O₃ whiskers shown in Figure 2b appear shorter and finer. This is due to the high-temperature calcination process of forming the α-Al₂O₃ whiskers; three reaction by-products, carbon dioxide, ammonia, and water vapor, escaped from AACH. This occurred due to the breakage of the chemical bonds, except for the strong aluminum–oxygen covalent bonds, ultimately resulting in the formation of the α-Al₂O₃ whiskers [28,29]. The average length of the prepared α-Al₂O₃ whiskers is about 6 μm, the diameter is about 0.3 μm, and the aspect ratio is 20.

3.2. Dielectric Properties of Composites

Figure 3 presents the SEM morphology of iPP/α-Al₂O₃ whisker composite samples containing 0–20 vol% filler after hot press molding, respectively. It can be seen that the whisker fillers are randomly oriented and diffusely distributed in the iPP matrix. Moreover, the morphology of the iPP/α-Al₂O₃ whisker composites does not change significantly with an increase in the filler volume fraction. From the morphologies, it can be observed that the combination between the α-Al₂O₃ whisker filler and the iPP matrix in the composites is tight and gapless. There are no obvious defects within the composites, except for the pits caused by whisker detachment during the preparation of the SEM samples. Therefore, in the following discussion, the effects of defects such as holes or uneven filler distribution, which could lead to the density change in the composites, on the dielectric properties of the composites will be disregarded.

Figure 4a depicts the composition and frequency-dependent behavior of the Dk values of the iPP/α-Al₂O₃ whisker composites. Evidently, the Dk values of the composites increase as the concentration of the added whisker filler rises. This is because the incorporation of whisker fillers introduces α-Al₂O₃-iPP interfaces in the matrix, which establish interfacial polarization leading to an increase in the Dk value of the composites. Over a filler volume fraction range from 0 to 20 vol%, the Dk values span from 2.23 to 3.14. The increased Dk would be more suitable for high-frequency device applications [24,30]. Furthermore, the Dk values of iPP/α-Al₂O₃ whisker composites are nearly independent of the test frequency in the frequency range of 0.5–1 THz. This phenomenon indicates that the main polarization mechanism affecting the Dk values of the composites in this study remained unchanged in the tested frequency range.

Figure 4b shows the composition and frequency dependence of the Df values of the iPP/α-Al₂O₃ whisker composites. As the test frequency increases, the motion of the charged particles in the composites intensifies, generating greater energy loss and increasing the Df values of the composites. Simultaneously, with the increase in filler content, more interfaces are inevitably introduced, thus establishing more interfacial polarization and leading to an increase in the Df value of the composite. Nevertheless, the dielectric properties of the composites containing 20 vol% whisker fillers (with a Dk value of 3.14 and a Df value of less than 0.002) are still superior to some of the polymer-based materials in the existing work [31,32,33].

3.3. Orientation of Filler in Composites of Dielectric Properties

The SEM morphologies of composites containing 20 vol% α-Al₂O₃ whisker filler (Figure 5) reveal the alignment of the filler within the injection-molded samples. The α-Al₂O₃ whiskers in the iPP/α-Al₂O₃ composite exhibit uniform orientation parallel to the injection direction, with the crystallographic c-axis of the whiskers aligning with this direction. This preferential orientation arises from transverse shear forces generated during extrusion of the molten composite through the injection port, which induce alignment of the whiskers along the flow direction.

The anisotropic dielectric properties of the injection-molded composites were tested as shown in Figure 6, using THz waves with the electric field polarization direction parallel to the injection direction (E ∥ c) and perpendicular to the injection direction (E ⊥ c), respectively.

Figure 7 illustrates the composition dependence of the Dk values for iPP/α-Al₂O₃ whisker composites measured perpendicular and parallel to the injection direction. As shown in Figure 4a, the Dk values of all composites exhibited minimal frequency-dependent variation within the 0.5–1.0 THz range. Consequently, the Dk value at each composition in Figure 7 represents the average response across the entire tested frequency band. Analysis of the Dk distribution reveals that, for a given composite, the Dk values along the parallel injection direction (Dk ∥ c) are larger than those along the perpendicular direction (Dk ⊥ c). This directional disparity becomes more pronounced at higher whisker filler concentrations, with the difference between Dk ∥ c and Dk ⊥ c increasing systematically. In contrast, pure iPP samples (without whisker fillers) display nearly identical Dk values in both orientations. These observations suggest that the anisotropic Dk behavior of the composites originates from the directional alignment of α-Al₂O₃ whiskers.

The intrinsic anisotropy of α-Al₂O₃, governed by its crystal structure, contributes directly to this phenomenon. Specifically, the Dk value parallel to the crystallographic c-axis (Dk ∥ c = 11.5) is significantly higher than that perpendicular to the c-axis (Dk ⊥ c = 9.3) [21]. Consequently, aligned whiskers preferentially enhance the composite’s Dk value along the c-axis orientation (i.e., the injection direction). To quantitatively assess the filler’s anisotropic contribution, the Bruggeman effective medium model—widely validated for predicting filler-concentration-dependent dielectric properties—was applied to estimate the Dk values for both principal orientations (parallel and perpendicular to the injection direction). The Bruggeman model’s predictions are expressed as follows [34]:

$$V_f{\displaystyle \frac{{Dk}_f-{Dk}_c}{{Dk}_f+{2Dk}_c}}+\left(1+V_f\right){\displaystyle \frac{{Dk}_m-{Dk}_c}{{Dk}_m+{2Dk}_c}}=0$$

where $V_f$ is the volume fraction of filler in the composite and Dk_f, Dk_c, and Dk_m represent the Dk value of the filler, matrix, and composite, respectively. The Dk values estimated using the Bruggeman model for Dk ∥ c and Dk ⊥ c are denoted in Figure 6 by a short-dashed line and dashed line, respectively. These curves represent the theoretical predictions for the dielectric constant parallel and perpendicular to the orientation of the α-Al₂O₃ whiskers. The experimental Dk values of the composites align closely with the model’s predictions, demonstrating that modulating the orientation distribution of α-Al₂O₃ whisker fillers effectively tunes the anisotropic dielectric properties of the composites.

The composition and frequency dependence of the Df values measured perpendicular (Df ⊥ c) and parallel (Df ∥ c) to the injection direction for the iPP/α-Al₂O₃ whisker composites are shown in Figure 8, respectively. For composites with the same content of whisker fillers, the Df ∥ c is always lower than the Df ⊥ c. The reason for this phenomenon is that the direction of THz wave electric field polarization is parallel to the orientation direction of the whisker fillers inside the composites during the test of Df ∥ c (shown as E ∥ c in Figure 6). Due to the large aspect ratio of the prepared α-Al₂O₃ whisker fillers and the highly oriented arrangement of the whisker fillers in the composites after the injection molding process, there are relatively fewer whisker filler–iPP matrix interfaces parallel to the whisker orientation direction. Therefore, when the electric field direction is parallel to this direction, it will result in less interface polarization and exhibit lower Df values. Similarly, during the test of Df ⊥ c, the electric field direction is perpendicular to the whisker filler orientation direction (shown as E ⊥ c in Figure 6), exhibiting higher Df values. The reason why the pattern that Df ∥ c is lower than Df ⊥ c becomes more prominent as the content of whisker filler increases is also that, after the filler content increases, the difference in the number of interfaces in the two oriented directions becomes greater. As a result, the difference in the interfacial polarization intensity between the two directions becomes larger, causing a greater difference in the Df values of the two.

4. Conclusions

In this work, single-crystal α-Al₂O₃ whiskers with an aspect ratio of 20 were prepared via a hydrothermal method, which were then mixed with the iPP matrix to prepare iPP/α-Al₂O₃ whisker composites. First, the dielectric properties of the composites were regulated by changing the filler concentration. The Dk values increased from 2.23 to 3.13 with the filler concentration ranging from 0 to 20 vol%, when the Df values kept at less than 4.0 × 10⁻³ were obtained in the frequency band of 0.5–1.0 THz. In particular, the dielectric properties of the composites can be further regulated by adjusting the orientation direction of the whisker fillers. The orientation of the whisker fillers in the composites was adjusted by the injection molding method, so that the whisker fillers exhibited a high degree of c-axis orientation along a direction parallel to the direction of injection molding. The dielectric properties of the injection-molded samples were significantly different in the perpendicular and parallel injection directions. Composites in the direction parallel to the injection direction (Dk ∥ c) usually present higher Dk values, while presenting lower Df values.