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The hydrothermal-alkaline treatment (HAT) is an efficient sludge reduction method. However, the roles of HAT in tannery sludge reduction are not very clear. In this study, the sludge reduction mechanism of HAT was explored. The results showed that HAT had good performance for the organic dissolution of tannery sludge. The reduction of the sludge mass was calculated by the mass of the separated wet sludge solid (4000 rpm, 10 min) before and after HAT. The HAT parameters for sludge reduction (SR) were optimized by the Box–Behnken, and the sludge reduction was 62.5% under optimal conditions (198 °C, 87 min and 8.7 g L−1 NaOH). The soluble organics, especially a polysaccharide (PS) and ammonia nitrogen (NH3–N), showed a close correspondence with the SR ratio. The rheological properties of the sludge indicated that the reticular structures in the sludge were destroyed after HAT. The tannery sludge solid became more pyknotic than raw sludge after the hydrothermal-alkaline treatment. The tannery sludge reduction can be attributed to the damage of reticular structures, organic dissolution and release of bound water.

Dyeing wastewater produced from textile industry has attracted considerable attention due to its pollution on environment. In this study, bio-char (BC), TiO2/bio-char (BC-T), TiO2/MgO/ZnO/bio-char composites (BC-TMZ) were prepared and utilized to remove methylene blue (MB). The surface morphology and composition of BC, BC-T and BC-TMZ were described by XRD, EDS, SEM, BET and TEM. Furthermore, the synergistic removal effects of as-prepared materials in MB solutions were explored under different bio-char dosage, initial MB concentration, reaction time and different light irradiation conditions. Finally, the residual MB solutions after treatment are characterized by LC-MS. The results indicated that the metal oxide particles (Fe2O3, ZnO, MgO and TiO2) have been loaded into pores of the BC-TMZ. Moreover, the BC-TMZ which act as both adsorbent and catalytic, possesses excellent reusability (reused for 4 times) to remove MB from solution. The adsorption behaviors of BC-TMZ were described by Pseudo-second order model and Langmuir model well. The maximum adsorption-catalytic capability of BC-TMZ (0.8 g/L) for MB (50 mg/L) was 62.5 mg/g with UV light irradiation (253.7 nm, 40 W) in 4 h. The MB molecules can be decomposed completely by BC-TMZ. The BC-TMZ is potential to be used as a reusable material for the removal of pollutants.

A reflective grating-coupled structure on the silicon substrate was designed to improve the detection efficiency of terahertz detectors for the frequency ranging from 0.26 THz to 0.36 THz. By using finite difference time domain (FDTD) solutions, the simulation and optimized design of the grating-coupled structure were carried out. The results showed that the signal was effectively reflected and diffracted by the reflective grating-coupled structure which significantly enhanced the electric field in the place of the detector. The maximum electric field can be increased by 2.8 times than that of the Fabry-Perot resonator. To verify the design results, the reflective grating-coupled structure was applied in the preparation of the Nb5N6 array detector chip and compared with the Nb5N6 array detector chip with the F-P resonator. The results showed that the maximum voltage responsivity of the Nb5N6 detector with the reflective grating-coupled structure was 2 times larger than the Nb5N6 detector with the F-P resonator. It indicates that the reflective grating-coupled structure can efficiently improve the detection efficiency of THz detectors.

Waste biomass and dye wastewater pollution have been the serious environmental problems. The interfacial solar-steam generation technology is an effective and sustainable method for the water purification. However, the complex preparation process, high economic cost and probably secondary environmental pollution of traditional photo-thermal materials restricted their practical large-scale application. Herein, the biochar cakes (BCs) deriving from waste biomass were prepared, and the granular and schistose MgO coatings were dressed on the surface of carbonized fiber to improve their hydrophilicity. The BCs with high solar absorbance and super-hydrophilicity were applied in the photo-thermal purification of dye wastewater with solar energy. The highest evaporation rate of dye wastewater with BCs reached 2.27 kg m−2 h−1, and the corresponding conversion efficiency of solar to steam generation was 78.98% under the simulated solar irradiation (1846.0 w/m2). The collected clean water from the solar-steam evaporators reached the emission standards of EU Water Framework Directive (91/271/EEC). Considering the simple and economical preparation method, this process made the practical large-scale application of photo-thermal BCs on dye wastewater treatment a reality, and also provided a cost-effective management strategy for the waste biomass.

Tannery sludge biochar (BC) has been widely studied and shown promising prospects in dye wastewater treatment. Dye wastewater is a complex mixture, including cationic and anionic dye pollutions. However, the unmodified BC usually exhibits low adsorption ability for anionic dye pollutants because of the electrostatic repulsion by negatively charged biochar surfaces. Herein, inspired by the great complexation of polyacrylamide flocculant for dye pollutants, the polar functional groups (cyano and acylamino groups) were introduced into BC via the secondary pyrolysis with melamine and KOH. The modified tannery sludge biochar (MBC) had excellent adsorption abilities for both cationic dye (cationic blue X-GRL) and anionic dyes (reactive red X-3B, direct yellow RS and acid blue 2GL) used in textile dyeing processes. Moreover, to separate the MBC from solution quickly, the iron oxide particles were in-situ loaded on MBC to obtain the magnetic MBC. Adsorption kinetic, isotherm and Gaussian calculation analyses demonstrated that the adsorption processes for dye pollutions were exothermic, spontaneous and chemical interacted. The excellent adsorption universality of magnetic MBC for both cationic and anionic dyes may benefit the large-scale application of tannery sludge biochar in dye wastewater treatment.

Resource utilization of organic matters in tannery sludge has drawn great attention. In this paper, the influences of sludge biochars (BCs) on volatile fatty acids (VFAs) production from the anaerobic digestion of sludge supernatant (SST) were investigated. Experimental results demonstrated that the VFAs yields improved in the presence of BCs with rich functional groups. The maximum yield of VFAs was 1037.5 mg/g SCOD with the addition of BC-1 biochar (zeta potential -50.42 mV). BCs decreased ammonia nitrogen concentration, thus reducing inhibition for bacteria during the anaerobic digestion. Microbial community analysis indicated that the BCs affected microbial community structures and contributed to a favorable environment for bacteria. Especially, the BC-1 biochar with rich functional groups enhanced the relative abundance of acid-forming bacteria (Clostridiales). A dual strategy was proposed to improve the resource utilization efficiency for tannery sludge.

The hollow catalytic microspheres (HCMs) with double-sided structures were fabricated, in which the external and internal sides are TiO2–Ag and MgO–ZnO respectively. For a hollow catalytic microsphere (HCM), the visible-light-driven (VLD) photocatalysts (TiO2–Ag) can receive light, while the non-light-driven catalysts (MgO–ZnO) are in shadow. The characterizations demonstrated that metal oxides were loaded on the biochar microspheres and the hollow microspheres were fabricated. Moreover, the adsorption and photocatalytic behaviors of HCMs for methylene blue (MB) were explored. The results indicate that the removal rate of MB (100 mL, 25 mg/L) and chemical oxygen demand (COD) by HCMs (0.04 g) were 96.9% and 92.9% under Xe-lamp light (150 W, 8 h) irradiation, respectively. Besides, the HCMs exhibited high catalytic efficiency and stable photocatalytic capability after being utilized for five times. The possible degradation pathway of MB by HCMs was proposed.

We designed a planar bow-tie antenna structure on the high-resistivity silicon substrate for the NbsN6 micro-bolometer at the frequency range from 0.55 THz to 0.75 THz. To effectively couple the high frequency signal, the Nb5N6 microbolometer is placed in the antenna gap. CST software was used to carry out numerical simulations for this antenna structure. The E-field and radio frequency (RF) current distribution show conspicuous resonance at about 0. 65 THz. Finally, the delicate design was verified by the experimental results.

Based on substrate cavity effect, an asymmetric coupled Fabry-Perot (FP) cavity is constituted by simply placing a movable metallic planar mirror in the back of the silicon substrate. The incident THz radiation onto the Nb 5 N 6 microbolometer can be effectively manipulated by changing the substrate-mirror distance to modulate the phase relation between the reflect wave and the incident wave. The distinct responsivity control can be observed and the experiments are well explained by numerically analyzing the radiation dynamics that highlights the role of FP cavity effect. All results discussed here can be extended to broad range of frequency and other type of THz detectors.

Flip-chip technology is a common way of connecting Terahertz (THz) array detectors with readout integrated circuits (ROICs). However, for our Nb 5 N 6 microbolometer THz array detectors, temperature and stress introduced from flip-chip process may adversely affect the performance of detectors. To evaluate this influence, firstly, we simulate the impact of solder bumps in a detector unit. Secondly, through flip-chip experiments between detectors and rewiring chips, we have characterized the performance of detectors before and after the process, which provide the technical support for optimizing this process and the interconnection between array detectors and ROICs.

We investigated the effect of metal reflector on responsivity of Nb 5 N 6 detector in the frequency range from 0.6 THz to 0.7 THz. The designed results showed that the response spectrum of the detector was in consistent with the signal intensity on the substrate surface whether it contained a metal reflector or not. Compared to the detector on the Si substrate with the resonant frequency at 0.648 THz, the resonant frequency of the detector on the Si substrate with a metal reflector shifted to 0.683 THz, meanwhile, appearing multiple peaks in the response spectrum. It distinctly reveals the effect of metal reflector on responsivity of Nb 5 N 6 THz detector.

Fabricating resonant cavities with conventional methods to improve the coupling efficiency of a detector in the terahertz (THz) region is difficult for the wavelength is too long. Here, we propose a solution by using the substrate cavity effect given that the substrate wavelength and thickness of the preparation device are in the same order. The planar dipole antenna-coupled Nb

We design and simulate planar antenna structure on the high- resistivity silicon substrate(ρ=1000Ω·cm) for the Nb5N6 micro- bolometer at the frequency range from 0.265 THz to 0.365 THz by CST Studio Suite. We have obtained the center frequency of the antenna at 0.3 THz by optimizing parameters of the antenna structure and the antenna has the very good radiation directivity. And the maximum directivity of the antenna is around 8.634 dBi at 0.3THz. The measured best voltage response of the Nb5N6 micro-bolometer detector is at 0.307 THz. The measured response frequency and the simulated S-parameter are in substantial agreement.

Nb 5 N 6 bolometer is a high sensitive terahertz detector at room temperature. The core part of traditional Nb 5 N 6 bolometer is microbridge with a typical size of 3 μm × 12 μm. We have recently proposed a method using electron beam lithography (EBL) instead of UV lithography on fabrication of Nb 5 N 6 micro bridge, which will reduce the size of the micro-bridge one to two orders of magnitude, the minimum up to 100 nm. We studied the relationship between the size of the micro bridge and the performance of the device, for further improving the sensitivity and the speed of the device and expanding the applications of this kind of detector.

In order to effectively improve the coupling efficiency of terahertz (THz) detectors, we design a grating-coupled structure on the high-resistivity silicon substrate for 0.2 THz to 0.35 THz band to enhance the ability of coupling terahertz signals. We simulated the electric field distribution of the grating-coupled structure in surface and inside by using the finite difference time domain (FDTD) method. The electric field in the central area of the silicon surface can be enhanced more than 4 times compared with the non-structure silicon substrate. We also simulated the Fabry-Perot cavity in the frequency range from 0.2 THz to 0.35 THz, and the electric field in the central area of the silicon surface can be improved one time compared with the non-structure silicon substrate. In addition, the electric field distribution on the silicon surface can be changed by adjusting parameters of the grating-coupled structure. When the period of the grating is 560 μm, the width of the gold is 187 μm, and the thickness of the silicon substrate is 720 μm, a 4.7 times electric field could be achieved compared with the non-structure silicon substrate at 0.27 THz and around. So, the simulation result shows that the grating-coupled structure has an obvious advantage compared with the Fabry-Perot cavity at THz coupling efficiency.

This Letter demonstrates tunable Nb5N6 microbolometers operating in the terahertz frequency range. An asymmetric-coupled Fabry-Perot cavity is constituted by simply placing a movable metallic planar mirror in the back of the silicon substrate. The incident THz radiation onto the Nb5N6 microbolometer is effectively manipulated by changing the air spacer gap to modulate the phase relation between the reflected wave and incident wave. The experimental measurements reveal that the detailed evolution of the resonance bands as a function of spacing is in excellent agreement with the analysis by using interference theory and simulation. The results detail the design of THz detectors wherein a wide degree of tunability or a variable number of detection bands is desirable.

Fabricating resonant cavities with conventional methods to improve the coupling efficiency of a detector in the terahertz (THz) region is difficult for the wavelength is too long. Here, we propose a solution by using the substrate cavity effect given that the substrate wavelength and thickness of the preparation device are in the same order. The planar dipole antenna-coupled Nb5N6 microbolometers with different substrate thicknesses were fabricated. The interference effect of the substrate cavity on the optical voltage response of the detector is analyzed experimentally and theoretically. The experimental results show that the optical response of the detector is determined by the length of the substrate cavity. Thus, the THz devices with different detection frequencies can be designed by changing the substrate cavity length. Furthermore, on the basis of this substrate cavity effect, an asymmetric coupled Fabry-Perot (FP) cavity is constituted by simply placing a movable metallic planar mirror at the backside of the Si substrate. The incident THz radiation on the Nb5N6 microbolometer can be effectively manipulated by changing the substrate-mirror distance to modulate the phase relation between the reflect wave and the incident wave. The distinct radiation control can be observed, and the experiments can be well explained by numerically analyzing the responsivity dynamics that highlights the role of the FP cavity effect during radiation. All of the results discussed here can be extended to a broad range of frequency and other type of THz detectors.

The reasons of some structural failures cannot be defined. In the reliability enhancement test of an airborne product, a simulation method has been proposed to calculate fatigue damage of random vibration power density spectrum of the fault point within frequency domain. The exact failure times can be calculated accurately and the most possible failure mechanism can be analyzed. In Sect. 5 (Analysis and Verification), the correctness of the results of the simulation analysis has been verified and an optimization design of the product structure has been proposed.