Terahertz (THz) plays a pivotal part in numerous technology fields in modern times, including the system of the 6th generation wireless communication, imaging and elemental analysis. In addition to light sources and detectors, THz applications require quasi optics as lenses, waveguides, and reflectors for the design of a THz system. Three-dimensional (3D) printing has many advantages. However, 3D printing materials always have higher THz-wave absorption rates. In this study, we attempted to discover the optimum process parameters of the combustion temperature of rice husk ash (RHA) and mix RHA with acrylonitrile butadiene styrene (ABS). According to the THz spectrum, the mixture powder would be very useful in applications of 3D printing to increase the refractive index and decrease the absorption coefficient with RHA added into ABS. This improvement will benefit 3D printing technique applications in the far-infrared and THz range.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Terahertz (THz) refers to electromagnetic waves with frequencies between 0.1 to 10 THz and the wavelengths between 3 to 0.03 mm. The electromagnetic waves within the frequencies have a vast bright application prospect for interacting with materials. Hence, THz plays a pivotal part in numerous technology fields in modern times, especially in the sixth generation wireless communication . However, in addition to light sources and detectors, THz applications require guiding devices and quasi optics such as lenses [2–4], waveguides [5–7], reflectors [8–10] and fibers [11–13]. The traditional manufacturing techniques to fabricate the THz lens include turning , micro-electromechanical process  and ion etching . However, all above manufacturing methods require expensive equipment and higher making costs. Three-dimensional (3D) printing, which has been widely adopted, reshapes manufacturing processes with benefits of low costs, swiftness and convenience. Due to the low resolution of products, 3D printing techniques are not suitable for shorter wavelengths such as visible light but viable for longer wavelengths such as far-infrared and THz. Various research teams used 3D printing to produce THz optical components [17–19]. Common materials for 3D printing include acrylonitrile butadiene styrene (ABS) , polylactic acid (PLA) , and nylon . Nevertheless, the absorptivity of all the above-mentioned materials is higher than 5 cm−1 [23–25]. Although TOPAS polymer has a relatively low absorption and is a good candidate for printing THz optics, its refractive index is still lower than other polymers [26,27], which does not match the requirement of a high refractive index and a low absorption rate for producing lenses.
According to the Food and Agriculture Organization (FAO) in United Nations, international rice yield has increased from 530 million tons in 1992 to 750 million tons in 2017. The increasing rate is around 100 million tons every decade. After harvest and milling, 22% of rice will become waste, imposing a heavy burden on the environment. However, if the waste could be properly combusted, 75% of it will become volatile organic materials that could generate heat, and 25% of it will transform into rice husk ash (RHA) . According to Rosario et al., 95% of RHA is silicon dioxide (SiO2); thus, it could be applied to construction materials, insulators, and polymer compounds . Della et al. discovered that with higher temperature and longer combustion duration, more purified SiO2 could be produced after rice husks were combusted at different temperatures (between 400°C and 1,700°C) and for different periods (1, 3, and 6 hours) in experiments . Below 1.2 THz, the refractive index of SiO2 is higher than the value of 2, and the absorption coefficient of SiO2 is lower than 3 cm−1 . Hence, SiO2 is a suitable material for lenses due to its high refractive index and low THz-wave absorption rate.
In this study, we added RHA into ABS powder. For the reason to decrease the high absorption rate of ABS powder and to increase its refractive index. Due to the quality THz properties of SiO2, we burned rice husks to obtain SiO2 and mixed RHA with ABS powder. We burned rice husks at 1,100°C to obtain RHA, and multiple weight percentages of RHA were mixed with ABS powder to prove that the mixture’s THz properties are better than those of ABS powder alone. Moreover, we attempted to discover the optimum process parameters by shifting the combustion temperature of RHA. In order to find the optimal combustion temperature for RHA, we fixed the mass fraction of RHA within the mixture at 50 wt% and then compared the mixture’s THz spectrum. With the results of our study, we presented some feasible applications.
2. Experimental method
The SiO2 used in this experiment was produced from the burning of rice husks. In order to acquire more purified SiO2, a study of silicon-based materials preparation by Sun et al. was taken as a reference . Firstly, rice husks were washed with water and added into 3N HCL. Secondly, the 3N HCL with rice husks in it was heated to 110°C for an hour. Following the acid pickling, the rice husks were dried and put into a furnace tube. After a vacuum process, the rice husks were heated to 500°C for pyrolysis that lasted for one hour. Finally, the RHA was produced after the rice husks were burnt for 2 hours at different temperatures from 500°C to 1,200°C. The RHA was ground in a mortar, put into a ball mill (Retsch MM 400), and mixed with ABS powder, which served as a base with different weight percentages of 10%, 20%, 30%, 40%, and 50% in this experiment. In order to avoid agglomeration of ABS powder, the first grinding process carried out by a ball mill lasted for 30 seconds at a vibration frequency of 15Hz. After waiting for the temperature down, the second grinding process was conducted for 120 seconds at a vibration frequency of 5Hz. It is necessary to compress the mixed powder into the ingot-shaped tablet before conducting THz time-domain measurements. We put 0.3g mixture powder into the die and applied the force of 0.9ton on it. The ingots would be 13 mm in diameter, and 1.60 to 2.15 mm in thickness.
The composition, amorphous, and crystalline of rice husk silica were examined by X-ray Diffraction (XRD). The instrument of XRD measurements (Bruker, D8 discover) is with Cu-Kα radiation (k=0.1542 nm) at a voltage of 40 kV and a current of 40 mA. The scan range of two thetas was from 10 to 80, and the sampling step was 0.05°. The period between each sampling point would be 0.5 seconds. In this study, we observed the crystallization of RHA at different temperatures by XRD spectrum. In addition, the relation between crystallinity and THz absorption has been investigated in recent years [33–35]. According to its relation, it is very useful to analyze crystallography by THz-TDS spectrum. Therefore, the relation between RHA crystallinity and THz absorption is a promising research in the future.
In this study, we used an oscillator titanium sapphire laser (Vitara, Coherent) as the seed laser directly introduced into the laser amplifier (Legend Elite HE+ USp-5K-III, Coherent), to amplify the energy. The amplification process is called Chirped Pulse Amplification. The amplified laser can provide a pulse width of 40 fs, an average power of 8.2 W, and a repetition rate of 5 kHz at 800 nm. This high-power laser is rare in the world and is equipped with an optical parameter amplifier, which can generate difference frequency, sum frequency, and harmonic frequency with a nonlinear crystal to output full-band femtosecond laser beams.
In this work, we applied THz time-domain spectroscopy (THz-TDS), shown in Fig. 1(a), to determine the complex refractive indices of samples. The incident beam from the laser system was divided into the pump beam and the probe beam by the beam splitters. In the pump beam, we set a combination of a half-wave plate and a polarization beam splitter to change the polarization state of light to attain energy distribution and used a laser pump in a ZnTe crystal to generate THz radiation. The generated THz radiation was collimated and focused onto the sample by a pair of off-axis parabolic mirrors with focal lengths of 15 cm. The transmitted THz radiation was again collimated and focused onto a ZnTe crystal for free-space electro-optic sampling by another pair of parabolic mirrors with the same focal lengths as the previous pair's. The optical path of the probe beam must be as long as that of the pump beam. When the two beams overlapped, we acquired a self-coherent signal. By detecting the change of light in a ZnTe crystal, we could understand the THz signal’s modulation of the ZnTe crystal. In order to increase the signal-to-noise ratio (SNR), an optical chopper and a lock-in amplifier were used. The SNR of our system was about 106. It was estimated by dividing the signal power in the frequency domain by the noise. Figure 1(b) and Fig. 1(c) are THz field of samples.
If the thickness of the sample is thick enough, we can clearly distinguish the waveform generated by multiple reflections of the sample with THz-TDS and used analysis algorithm to consider the multiple reflection signals . Assuming a THz wave is a plane wave passing through air, and the electric field of the THz wave passing through the reference sample is given by
Similarly, the electric field of the THz wave that passed through the sample can be represented as
3. Results and discussion
Combustion temperature and time are important factors to define whether silica remains amorphous, as in RHA, or becomes crystalline . We produced RHA at various temperatures and set the burning time at 2 hours. The SiO2 composition of RHA was verified by X-ray diffraction (XRD), as shown in Fig. 2. For the samples of RHA burnt at 500°C, 600°C, 700°C, 900°C and 1,000°C, diffused peaks at about 2θ=18° to 22.5° were noticed, and these peaks in this range stood for amorphous cristobalite . For the pattern of RHA burnt at 1,100°C and 1,200°C, peaks of cristobalite were obvious at 2θ=21.8°. In general, in the condition of the higher burnt temperature, the behavior of crystallinity is getting more apparent.
We added 10 wt% to 50 wt% of RHA samples burnt at 1,100°C into ABS powder and then measured the THz spectrum. The analysis method is the result obtained by cross-analyzing a pair of samples and reference signals. The results are shown from Figs. 3(a) to 3(d), which are transmittances, refractive indices, extinction coefficients, and absorption coefficients, respectively. Compared with ABS powder from Fig. 3(d), the mixed powder had a lower absorption coefficient from 0.3 to 1 THz. It means transmittance is higher, which is plotted in Fig. 3(a). Concerning the variation of the refractive indices from Fig. 3(b), the refractive indices for adding RHA is higher than pure ABS. Moreover, the refractive indices increased with the RHA weight percentage, the refractive indices of the mixtures with 30 wt%, 40 wt% and 50 wt% of RHA, respectively, they were close to each other, but they were still higher than those of pure ABS powder and the mixtures with RHA’s weight percentages of 10 wt% and 20 wt%. This result can be predicted by the effective medium approximation (EMA) because it is the equivalent refractive indices contributed from the ABS and RHA.
The fitting lines of absorption coefficients are shown as Fig. 4(a). The absorption coefficient of the mixtures with 20 wt% is the lowest at 0.5 THz while the mixtures with 50 wt% is the lowest at 1.0 THz. Therefore, different proportions of RHA in the mixtures can change the optical properties. In Table 1, we calculated the rate of changes at 0.5 THz and 1THz by fitting line. In addition, the refractive index value of RHA was calculated from the effective refractive index in Fig. 4(b). A 50 wt% of RHA shows the lowest absorption coefficients at 1.0 THz, and there is a 32.1% decreased. A 20 wt% of RHA shows the lowest absorption coefficients at 0.5 THz, and there is a 36.7% decreased. The value of absorption is the mean value by the approach of linear regression. The standard error is the error value. The variety of the refractive indices and absorption coefficients at 1THz according to the increasing percentage of RHA are showed as Fig. 4(c) and Fig. 4(d), respectively. If we take into account the time cost, a 40 wt% of RHA is the optimum proportion for the mixture of ABS powder and RHA.
In order to explore the influence of the combustion temperature on THz properties, we mixed 50 wt% of RHA with ABS powder and measured the THz spectrum; the result was shown in Fig. 5. Figure 5(b) shows the refractive indices of the powders. The refractive indices of powders which added RHA is between 1.58 and 1.66. The mixture with the RHA burnt at 500°C had a higher refractive index ranging from 0.3 to 1.0 THz; the higher the RHA samples’ combustion temperatures were, the lower their refractive indexes became. However, when the RHA samples’ combustion temperatures were higher than 900°C, their refractive indexes became higher as well. In addition, the absorption coefficient is another parameter we concern, the result was shown in Fig. 5(d). Compared with ABS powder, the samples burnt at 500°C, 600°C, and 700°C had higher absorption coefficients, but the samples burnt at 900°C, 1,000°C, 1,100°C, and 1,200°C had lower absorption coefficients. Nevertheless, the absorption coefficients did not change significantly when the samples’ combustion temperatures were over 1,100°C.
Likewise, the absorption coefficients with the fitting line were shown in Fig. 6(a). RHA which is burnt at 1,200°C shows the lowest absorption coefficients, and there is a 23.6% decreased at 0.5 THz and 31.8% at 1.0 THz. According to the Table 2, the rate of changes were positive when the combustion temperature between 500°C and 1,000° C at 0.5 THz, and they were negative when the combustion temperature is higher than 1,100°C. In order to reduce the absorption of the samples, the combustion temperature of RHA must be higher than 1,000°C. Figure 6(b) and (c) are refractive indices and absorption coefficients at 1.0 THz of RHA burnt at various temperatures, respectively. The RHA is a porous material. Water can be easy to be adsorbed between pores. The size of pores rises in pace with the temperature, which will make water passes through the pores easier . Once the pores get large enough, it will cause destruction. This was the reason why the refractive indices had a turning point at 900°C and why the absorption coefficients decrease while the burnt temperature increases . It is notice that there are some obvious sine wave oscillations in the data. The reason is that the lacking of the short of purge dry air (e.g. nitrogen) in the system, which will cause the water vapor to be absorbed .
In summary, we have presented the optical properties of various mixtures of ABS powder and RHA at THz frequencies. The effect increases as the percentage weight of RHA increases, 40 wt% especially; however, the effect is not obvious when the percentage weight of RHA is smaller than 20 wt%. In terms of the comparison of the influences of various combustion temperatures of RHA on THz properties, the higher the combustion temperature an RHA sample was burnt at, the lower the mixture’s absorption coefficient was. The lowest absorption coefficient was observed as RHA burnt at 1,200°C because the higher burnt temperature shows the better crystallinity. Compared with the pure ABS powder and the other mixtures with RHA burnt at lower temperatures, the absorption coefficients of the mixtures with RHA burnt at 1,100°C and 1,200°C, respectively, were significantly lower. Compared with the traditional ABS powder, it would be very useful in applications of 3D printing to increase the refractive index and decrease the absorption coefficient of the composite with the components of RHA and ABS. In consideration of consolidate the powder of the mixture, more amount of ABS should be added. Furthermore, the RHA amount added in the composite materials for 3D printing could be further studied in the coming future.
Ministry of Science and Technology, Taiwan (107-2112-M-003-014-MY3, 107-2221-E-003-008, 109-2112-M-007-033-, 110-2112-M-003-012-MY3, 110-2923-E-007-006).
The authors would like to thank Prof. Tun-Ping Teng for lending us Ball Mill, and Yi-Sheng Cheng for optimizing the THz-TDS system
The authors declare no conflicts of interest.
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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