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Abstract
Terahertz (THz) radiation is especially the frequency band that the developers of sixth-generation wireless communication networks aim to exploit. Thus, the research and development of relevant components have been an important issue. Studies have shown that ceramic materials demonstrate the properties of low optical loss and high refractive index when subjected to THz radiation. Furthermore, when a ceramic material is mixed into a polymeric material, it can effectively improve the latter’s mechanical strength. Therefore, in this study, we conducted heat treatment on the powders of four ceramic materials: Al2O3, SiO2, ZrSiO4, and quartz. Next, we respectively mixed each powder of the ceramic materials into each powder of acrylonitrile butadiene styrene (ABS). Then, we measured all the mixtures’ optic properties under THz radiation by observing their X-ray diffraction patterns. Measurement results indicated that SiO2 had a phase transition when it was calcined to 1100°C, and its optical coefficient also changed with the phase transition. After ABS was mixed with a ceramic material, the mixture’s effective refractive index increased as the mixture was subjected to THz radiation. Moreover, after ABS was mixed with Al2O3, quartz, heat-treated Al2O3, heat-treated quartz, and heat-treated SiO2, the mixtures’ effective refractive index increased, and their absorption coefficients decreased. Therefore, mixing a specific ceramic material into ABS, a common polymer, can not only improve the mechanical performance of ABS but also give ABS fine optical properties such as an increased effective refractive index and a decreased absorption coefficient under THz radiation.
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1. Introduction
Terahertz (THz) radiation is also termed submillimeter radiation; its region is between microwaves and far-infrared. Technologies concerning the THz region have drawn attention from many scientists due to the region’s potential applications in imaging [1–4], radars [5–7], sensing [8–11], and high-speed wireless communication [12–14]. In order to effectively manipulate the strengths and shapes of beams, THz components have been indispensable.
However, the manufacturing of THz components relies on microelectromechanical manufacturing techniques; the costs of the manufacturing process are high. Three-dimensional (3D) printing technologies have the advantages of low cost and high efficiency; thus, in recent years, many studies have begun to fabricate THz components such as lenses [15–17], optical gratings [18–20], and waveguides [21–24] using 3D printing technologies. Polymeric materials such as acrylonitrile butadiene styrene (ABS) [25], polylactic acid (PLA) [26], and nylon [27] are frequently used for 3D printing in order to attain the goal of rapid fabrication. Polymeric materials are light, insulating, and easy to transform; their strengths are weak, their melting points are low, and their absorption coefficients are higher when they are subjected to THz radiation [28].
Ceramic materials’ hardness and strength are high, their heat resistance and corrosion resistance are strong, their density is low, and their chemical stability is good. These advantages are the reason why ceramic materials have been widely applied in the aviation [29,30] and medical [31–33] industries. As the communication bandwidth widens, making the size of a THz component small enough and allowing the component to be interference-resistant are the biggest challenges for the developers of the next-generation THz wireless communication system. As previous research showed, we optimized the best conditions of progress of rice husk ash (RHA) to SiO2 reduction, and mixed it with ABS mixture, as well as investigating the best mixing ratio in order to improve the characteristic of high absorption coefficient and low refractive index of pure ABS under THz frequency domain [34]. In another paper, we individually mixed the common 3D printing materials, polyamide (PA), PLA and light curing resin with quartz, and concluded that all of above were able to reach the target of lowering absorption coefficient and elevating refractive index [35]. Synthesizing previous research findings, we find that polymeric composite materials mixed with ceramic powder are effective in changing the characteristic of pure polymer material under THz frequency domain. Therefore, this research aims to find which one of the ceramic powers that mixes with ABS powder is with the characteristic of higher effective refractive index and lower absorption coefficient. However, being highly brittle, fragile, and weak against impact makes ceramic materials hard to be formed through processing. Aluminum oxide (Al2O3) is a common ceramic material; its stationary phase (α-Al2O3) in a crystalline shape is termed sapphire. The mechanical strength and optical properties of Al2O3 are outstanding. When subjected to THz radiation, Al2O3 has a higher refractive index and a lower absorption coefficient compared with polymers [36]. It is worth noticing that Al2O3 crystals are uniaxial crystals with the property of optical anisotropy, and the phenomenon of double refraction evidently manifests in them as they are subjected to THz radiation [37]. Silicon dioxide (SiO2) is categorized into crystalline and amorphous. SiO2 crystals mainly exist in quartz ore and often appear in three phases: quartz, tridymite, and cristobalite. SiO2 is highly insulating against electricity, highly dispersive, and porous; due to its low refractive index, it is often applied to optical coatings. Amorphous SiO2 is often used to produce glass; when subjected to THz radiation, it has a very low absorption coefficient, which is about 0.5mm-1 when measured at 1 THz, and its refractive index is smaller than 2 [38]. Quartz has a low absorption coefficient and a high transmittance when subjected to near-infrared, visible light, and THz radiation [39]. Zircon (ZrSiO4) is a silicate mineral; it can also be synthesized with zirconia and silica. It is also a type of functional ceramic [40] that has a high refractive index. Al2O3, SiO2, and ZrSiO4 are also anisotropic hyperbolic materials [41]; they have different permittivity tensors and are suitable to be used for structures such as metasurfaces, hypercrystals, waveguides, and cavities [42].
The essential requirement for fabricating THz components is to reduce unnecessary optical loss. Thus, low-loss optical components such as optical fibers, lenses, and gratings are of crucial importance. When it comes to material selection, optical fibers are typically made of glass or plastic, with the latter often exhibiting a high absorption rate [43,44]. Many researchers have devoted themselves to the search for new materials to solve the problem of high absorption. In this study, we improved a common 3D printing material—ABS—by mixing a ceramic powder into it, which improved absorption coefficients, effective refractive index, and mechanical strength [45–47]. Since the refractive index of ceramic powder is higher than that of polymer materials, and the absorption coefficient is relatively low, the effective medium theory suggests that the performance of each component in the composite material is averaged to obtain the performance of the mixed material. Therefore, the addition of ceramic powder increases the effective refractive index of the overall mixture and decreases the absorption coefficient. Recently, there has been an increasing amount of research on ceramic materials in the THz frequency range. A.K. Klein et al. [36] studied Shapal Hi-M Soft, an aluminum nitride-based ceramic, by substituting ceramic processing for high-resistance silicon to create THz components. In order to apply ceramic materials to the THz communication band, two teams, Y. Chen [48] and P. S. Pálvölgyi [49], used different methods to increase porosity and reduce the dielectric constant. Q. Chapdelaine et al. employed two methods to combine PP and TiO2 powders for lens production in order to enhance the resolution of THz imaging. Changing the manufacturing technology can reduce production costs, but it results in high loss of TiO2 [50]. All of the above applications require materials with a low absorption rate for THz waves, making material selection and fabrication methods critical factors.
2. Experimental method
In order to analyze the variation in optical properties of ABS mixed with different kinds of ceramic powders when it is subjected to THz radiation as well as the influence of heated ceramic powders, we heated different kinds of commercial ceramic powders to 1100°C, kept their temperature at 1100°C for two hours, and left them in the stove until they cooled down to room temperature. It doesn't have to be heated for two hours, just to ensure that the excess moisture in the material can be evaporated. Next, we respectively mixed each of 50 wt% ceramic powders into each ABS powder. The particle size of ABS powder is about 150 µm ; SiO2 and quartz powder are about 45µm ; Al2O3 powder is about 20 nm ; ZrSiO4 powder is about 100 nm. To ensure thorough homogenization of the mixtures, we employed a ball mill (Retsch MM 400) with 10 stainless steel balls in the grinding jar. The high vibration frequency of the ball mill facilitated particle size reduction through collisions, aiming for a particle size smaller than the THz wave to prevent unnecessary dispersion. However, due to the thermoplastic nature of ABS, prolonged milling could cause the ABS powder to fuse and adhere to the stainless-steel balls. To address this, we divided the ball milling process into two phases. In Phase 1, the ball mill operated at 15 Hz for 30 seconds, followed by a cooling period to prevent excessive heat generation. In Phase 2, the ball mill operated at 5 Hz for three minutes, which helped further reduce the particle size and dislodge particles adhering to the stainless balls. After ball milling, the powders were compacted into ingots before conducting THz time-domain spectroscopy (THz-TDS) measurements. To achieve this, we used a stamping die with a cylindrical trough of 13 mm diameter and applied a consistent pressure of 9 tons to compact the powders separately at different times. The resulting samples had varying thicknesses, ranging from 1.89 mm to 4.60 mm, and were stored in a moisture-proof box before the measurements. It's worth noting that the analysis of the effective refractive indices and absorption coefficients considered the sample thickness, thereby mitigating the impact of differences in sample thickness on the results.
The THz-TDS system [34,51–53], we used split a laser beam into a pump beam and a probe beam with a beam splitter. The probe beam was delayed by the delay stage, creating an optical path difference between the pump and probe beams. Subsequently, the pump beam was directed onto a ZnTe crystal, generating THz radiation through a nonlinear phenomenon. The resulting THz beam produced by the crystal was focused onto a sample using a pair of off-axis parabolic mirrors. As the THz beam penetrated the sample, it was focused by another pair of off-axis parabolic mirrors on another ZnTe crystal. The probe beam was focused on the second ZnTe crystal for us to perform coherent detection. We acquired signals by electro-optical sampling that integrates a quarter-wave plate, a Wollaston prism, and a balanced detector. In order to reduce noise, we had applied an optical chopper and a lock-in amplifier in the system, and thus the system attained a signal-to-noise ratio (SNR) of 105. The SNR of the system is the power of a sample’s signal in a frequency domain divided by the power of a reference signal in the frequency domain. The effective measurement range of the THz-TDS system was 0.3-1 THz. Figure 1(a) and Fig. 1(b) show the waveforms and power spectra of the THz field of air and samples measured by the system. Based on the measurement results, we obtained the samples’ optical constants through the calculation of an analysis algorithm [34,51]. Due to limited environmental space, our THz-TDS system cannot add nitrogen to reduce water vapor. However, therefore we built dehumidification into the space to reduce the moisture content in the environment.
To understand more about the optical properties of mixtures of different powders, we used ABS powders as the base material and mixed them with various ceramic powders, including Al2O3, SiO2, ZrSiO4, and Quartz. Subsequently, we compacted the mixtures into ingots to create samples for testing. Firstly, we analyzed the crystal structure and phase of all the ceramic powders using X-ray diffraction (XRD). Next, we examined the optical properties of the mixtures using the THz-TDS system. Finally, based on the measurement results obtained from the system, we analyzed the variations in the optical properties of the samples when subjected to THz radiation.
3. Results and discussion
The X-ray diffractometer we used in the study was a Bruker D8 DISCOVER and our target material was Cu-α (1.54056 Å). The Al2O3, SiO2, ZrSiO4, and quartz powders before and after heat treatment were respectively measured with the X-ray diffractometer. The measuring angles ranged from 10° to 70°; the interval between two measuring angles was 0.05°; the measurement at each measuring point lasted one second. The results are listed in Fig. 2. The Al2O3 we used in this study was α-Al2O3, which is the stable phase of Al2O3; therefore, phase transitions would not occur in Al2O3 when it was calcined to 1100oC. The X-ray diffraction peaks of the Al2O3 before and after heat treatment were identical. Commercial SiO2 is amorphous, hence there was no obvious diffraction peak in its X-ray diffractogram. However, when it was calcined to 1100oC, many diffraction peaks appeared. Its crystal structure and crystal phase changed into tetragonal α-cristobalite and orthorhombic β-tridymite. The crystal structure of the ZrSiO4 sample we used is tetragonal, hence this sort of ZrSiO4 is very stable. When calcined to 1100oC at 1 atm of pressure, it will not change its phase or decompose. Thus, the X-ray diffractograms of the ZrSiO4 samples before and after heat treatment were the same. The quartz powder we used was β-quartz, whose crystal structure is hexagonal. This crystal structure takes shape during the calcination of α-quartz. Therefore, the phase of the quartz powder we used would no longer change when it was calcined again.
Fig. 2. (a) X-ray diffractograms and (b) SEM photographs of ceramic powder before and after heat treatment.
With THz-TDS, we obtained the transmittances, effective refractive indices, extinction coefficients, and absorption coefficients of our samples, as shown in Fig. 3. We cross-analyzed two sets of sample signals and two sets of reference signals, resulting in the data graphs presented in this study. Due to the small magnitude of the measurement errors, the error bars are so small that they overlap with the data points. According to the Ref. [37,54], the refractive index of the raw material Al2O3 is approximately 3.09, while SiO2 and Quartz are around 1.96 at 1 THz. As for ZrSiO4, there is limited information available, but based on the Ref. [41], it exhibits a remarkably high refractive index, with an approximate dielectric constant of nearly 20 at 30 THz. Figure 3(b) shows that the effective refractive indices of all the mixtures were higher than that of an ABS powder. The mixture of 50 wt% ZrSiO4 had the highest average effective refractive index, which reached up to 1.81 at 0.5THz. The order of the other mixtures’ effective refractive index from high to low is 1.75 for ABS mixed with 50 wt% ZrSiO4 (1100°C), 1.69 for ABS with 50 wt% Al2O3, 1.68 for ABS with 50 wt% Al2O3 (1100°C), 1.64 for ABS with 50 wt% Quartz (1100°C), 1.64 for ABS with 50 wt% SiO2 (1100°C), 1.62 for ABS with 50 wt% quartz, 1.61 for ABS with 50 wt% SiO2, and 1.56 for pure ABS. Regarding absorption coefficients, Fig. 3(d) shows that ABS mixed with 50wt% Quartz and ABS mixed with 50wt% Quartz (1100°C) had the smallest absorption coefficients. However, the absorption coefficients of mixtures of ABS and 50wt% SiO2, ABS and 50wt% ZrSiO4, and ABS and 50wt% ZrSiO4 (1100°C) were higher than that of pure ABS. Of all the samples in our experiment, ZrSiO4 had the highest refractive index. The effective refractive index of ABS mixed with 50wt% ZrSiO4 had a refractive index greater than that of pure ABS. However, this mixture was also influenced by the high absorption of ZrSiO4, hence its absorption coefficient increased. It can be observed from Refs. [55,56] that the thermal expansion coefficient of ZrSiO4 is higher than that of Al2O3. Consequently, when heated to 1100 °C, the structure of ZrSiO4 becomes less compact, resulting in a decrease in effective refractive index. This change is more pronounced compared to Al2O3. In addition, compared to ABS mixed with 50wt% SiO2, the effective refractive index of ABS mixed with 50wt% SiO2 (1100°C) increases obviously, while the absorption coefficient decreases massively. Because SiO2 is a sort of porous material, moisture can be easily trapped in its tiny holes. When SiO2 was calcined to 1100°C, its porous structure collapsed and thus could no longer adsorb moisture. This phenomenon gave the heat-treated SiO2 powder a higher refractive index and a lower absorption coefficient than those of the SiO2 powder before heat treatment [57]. Figure 3(a) shows that the mixture of ABS and 50wt% Al2O3 (calcined at 1100°C) had the lowest average absorption coefficient, but its average transmittance was also the lowest. The reason for both being the lowest is mainly due to transmittance being the ratio of incident electric field to emergent one. In the experiment, if an ingot was compacted too thin, it would easily disintegrate. Thus, the thicker an ingot was, the lower its transmittance would be. In this specific sample, the thickness was 4.6 mm, allowing us to directly predict from the shown data trend whether the sample's effective refractive index and absorption coefficient would increase or decrease.
Fig. 3. The (a) transmittances, (b) effective refractive indices, (c) extinction coefficients, and (d) absorption coefficients of the mixtures of ABS powders and different kinds of 50wt% ceramic powders
All the samples’ effective refractive index and absorption coefficient extracted from our data were organized in Tables 1 and 2. The mixture of ABS and 50wt% ZrSiO4 registered the highest refractive index, which were 1.81 at 0.5 THz and 1.80 at 1THz, and both indices were respectively 16.02% and 16.65% higher than those of pure ABS. The mixture of ABS and 50wt% Al2O3 (1100°C) registered the lowest absorption coefficient, which were 3.4 at 0.5 THz and 9.27 at 1THz, and both coefficients were respectively 26.3% and 27.44% lower than those of pure ABS. It is worth noting that although the mixture of ABS and 50wt% Al2O3 (at 1100°C) was not the sample that recorded the highest refractive index, its refractive index was measured as 1.68 at 0.5THz and 1.67 at 1THz, respectively. These values were 8.16% and 8.23% higher than those of pure ABS. If developers wish to improve 3D printing materials by incorporating ceramic materials, Al2O3 calcined at 1100°C would be the optimal choice.
Table 1. Effective refractive index of ABS powder blended with 50wt% different sorts of ceramic powder
Table 2. The absorption coefficient of the mixtures of ABS powders and 50wt% ceramic powders of different sorts
According to the Lens Maker's formula, 1/f = ((nsample - n0) / n0) / (1/R1 - 1/R2). When the effective refractive index of the material is larger, it allows for a reduction in the thickness of the lens. In this formula, f represents the focal length of the lens, and R1 and R2 are the radii of curvature of the two refractive surfaces of the lens. Moreover, n0 denotes the refractive index of air, while nsample represents the effective refractive index of the material used for the lens. Taking a plano-convex lens with a focal length of 10 cm as an example, Table 3 compares the thickness of the materials used in this study and terahertz lenses on the market. According to the table, while leaving out the edge thickness, no matter which materials we choose, the center thickness are thinner than the Thorlabs and Batop products. Among all the materials, ABS + 50% ZrSiO4 has the thinnest result (4 mm). If we take absorption coefficient of the materials into account, the Al2O3 calcined to 1100°C is the best choice, and the thickness of lens is only 4.8 mm. Therefore, we could decrease the possible optical aberration or decay caused by thickness for future application by selecting more advantaged materials to make THz lens.
Table 3. Specification comparison with commercially available lenses
4. Conclusion
In summary, we calcined commercial Al2O3, SiO2, ZrSiO4, and quartz powders to 1100oC and a change in the optical parameters of SiO2 was revealed during irreversible structural phase transition at 1100oC. In the range of 0.3 to 1 THz, the mixtures of ABS and 50wt% Al2O3, ABS and 50wt% quartz, ABS and 50wt% calcined Al2O3, ABS and 50wt% calcined quartz, and ABS and 50wt% calcined SiO2 had higher refractive indices and lower absorption coefficients compared with the pure ABS sample. Of all the samples, the mixture of ABS and 50wt% calcined quartz had the lowest average absorption coefficient, and the mixture of ABS and 50wt% ZrSiO4 had the highest average effective refractive index. Although the effective refractive index of ABS mixed with 50wt% ZrSiO4 is optimal, the mixture’s absorption coefficient is relatively high. The absorption coefficients of the mixtures of ABS and calcined ceramic powders were all lower than those of the mixtures of ABS and ceramic powders without heat treatment. In terms of the ceramic powders that we used for this study, calcining Al2O3 to 1100°C and then mixing it into ABS can give the mixture a greater effective refractive index and a much lower absorption coefficient in the frequency range of 0.3 to 1THz compared to pure ABS. Therefore, mixing Al2O3, quartz, and calcined SiO2 into polymers to improve 3D printing materials used for the fabrication of THz components can boost their effective refractive indices, and reduce their absorption coefficients.
Funding
National Science and Technology Council (110-2112-M-003 -012 -MY3, 111-2221-E-003 -011 -MY2, 111-2221-E-019 -010 -, 111-2224-E-006 -006 -).
Acknowledgments
The authors would like to thank National Taiwan Normal University (NTNU) within the framework of the Higher Education Sprout Project by the Ministry of Education(MOE) in Taiwan. The authors would like to thank Prof. Tun-Ping Teng for lending us Ball Mill, thank Y.-C. Hsu, Y.-S. Cheng, P.-J. Wu, and P.-H. Wu for the setup and optimization of the system, thank Prof. C.-L. Pan from Department of Physics, National Tsing Hua University, Hsinchu, Taiwan, for borrowing our Electro-optic detector and optics.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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