This study focused on the highly efficient pyrolysis of rice husk (RH) for producing high-porosity biochar at above 450 °C under various microwave output powers (300–1000 W) and residence times (5–15 min). The findings showed that the maximal calorific value (i.e., 19.89 MJ/kg) can be obtained at the mildest microwave conditions of 300 W when holding for 5 min, giving a moderate enhancement factor (117.4%, or the ratio of 19.89 MJ/kg to 16.94 MJ/kg). However, the physical properties (i.e., surface area, pore volume, and pore size distribution) of the RH-based biochar products significantly increased as the microwave output power increased from 300 to 1000 W, but they declined at longer residence times of 5 min to 15 min when applying a microwave output power of 1000 W. In this work, it was concluded that the optimal microwave pyrolysis conditions for producing high-porosity biochar should be operated at 1000 W, holding for 5 min. The maximal pore properties (i.e., BET surface area of 172.04 m²/g and total pore volume of 0.1229 cm³/g) can be achieved in the resulting biochar products with both the microporous and the mesoporous features. On the other hand, the chemical characteristics of the RH-based biochar products were analyzed by using Fourier-transform infrared spectroscopy (FTIR) and energy-dispersive X-ray spectroscopy (EDS), displaying some functional complexes containing carbon–oxygen (C–O), carbon–hydrogen (C–H), and silicon–oxygen (Si–O) bonds on the surface of the RH-based biochar.

In this letter, a compact and high performance parallel coupled line wideband bandpass filter (PCL-WBF) with open stubs is designed and implemented on the commercial substrate, RT/Duroid 5880. The PCL-WBF is composed of a one-stage coupled line and two open stubs connected near the input/output ports. The design procedure and limit are discussed in detail. The designed filter was fabricated and measured, showing good characteristics of wide stopband and very high passband selectivity. Measured results also have a good agreement with the simulated results.

This paper presents a novel dual-band bandpass filter with narrow and wide passband simultaneously and a wide stopband. The dual-band bandpass filter is implemented using a steppedimpedance ring loaded resonator (SIRLR). The properties of the SIRLR are analyzed in detail based on the even-and odd-mode equivalent circuits. Resonant modes, excited by SIRLR, can be controlled and used to form a narrow passband at 2.4 GHz with bandwidth ratio of 8% and a wide passband from 3 to 5 GHz, with insertion losses less than 1.5 dB for both two passbands. Three transmission zeros are created near the passband edge to improve the passband selectivity and achieve a sharp cutoff skirt (over 60 dB/GHz). In addition, for wide stopband consideration, rectangular stub loaded resonators (RSLRs) are tapped on the input and output ports of above dual-band filter without destroying the dual-band performances. Thus, the proposed dual-band filter with a wide stopband region from 5 to 20 GHz with a rejection level of 25 dB is also achieved. The measured results show a good agreement with the simulated results.