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This work presents an LC resonant passive wireless gas sensor with a novel structure designed to mitigate the negative impact of substrate. The LC sensor antenna in the new structure, and the reader antenna, were designed and optimized utilizing HFSS software to improve the transfer efficiency. The superiority of the designed structure compared with general examples is highlighted and verified. The change in the substrate capacitance essentially makes no interference with the parameters of the LC sensor to be measured. The sensor for the new structure was prepared by combining etching and sputtering methods. The ZnO nanowires (NWs) were characterized to confirm their high purity and wurtzite crystal structure. The LC gas sensors demonstrated excellent wireless sensing performance, including a low detection limit of 0.5 ppm NO2, high response of 1.051 and outstanding stability at 180 °C. The newly developed sensor structure not only prevented interference from the substrate during gas sensing testing, but also expanded the choice of sensor substrates, playing a critical role in the development of sensors based on the LC resonance principle.

While metal oxides and metal sulfides have been extensively studied for gas sensing applications, there are no extensive reports on gas sensing properties of metal diborides. Here, for the first time, we have investigated the conductometric gas sensing behavior of HfB2 nanoparticles. The HfB2 nanoparticles is synthesized via a sol-gel method and characterized using X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The gas sensor is fabricated by drop casting the HfB2 nanoparticles on interdigitated Ag/Pd electrodes. The sensor exhibited a promising NO2 sensing performance at the operating temperature of 200 °C. Interestingly, it is found that resistance of the sensor decreases upon exposure to both oxidizing and reducing gases, which does not follow the gas sensing behaviors of the widely investigated metal oxides. In addition, the sensor response is minimally influenced by oxygen background concentration. These results highlight that HfB2 nanoparticles exhibit very unique sensing characteristics.

This article presents the design, fabrication, modeling, and preliminary tests of a bloodworm-inspired soft gripper for universal grasping. The gripper was designed and fabricated based on a toy called water snake wiggly (WSW). The toroidal WSW can evert itself inside-out or outside-in, just like a bloodworm everting its teeth outside to hunt and inside to feed. By driving a WSW rolling itself outside-in to wrap around the items, a bloodworm-inspired gripper was achieved with a flexible and passive form-fitting grasp. To enhance the capability of the gripper, two alternative detachable modules were added to the gripper-a vacuum suction cup for handling objects with smooth nonporous surfaces and an end-needle for taking in and expelling noncorrosive liquids like a syringe. We analyzed the working principles of the gripper and derived the relationship between the gripper's holding force and the objects' scale. Preliminary experiments with a motor-driven gripper prototype were conducted to verify its performance. The experimental results conform well with our theoretical analysis and also indicate the gripper's good universal grasping capacity and reliability in handling a wide range of objects with different surface shapes, geometric dimensions, and stiffness. In addition, the gripper has the unique abilities to pick more than one object during a maneuver, grasp multiple objects in a row without releasing the former ones, and even grasp powdered objects. These have presented a challenge for the existing robotic grippers.

This paper proposes a new kind of composite micro/nano structure composed of porous nanomaterials including titanium (Ti) gradient material and titania (TiO2) gradient material. The composite structure is fabricated via in-situ deposition of gradiently distributed and oxidized porous titanium nanoparticles on the surface of picosecond laser-induced micro/nano structures. The porous composite layer made up of Ti gradient material and TiO2 gradient material shows the best anti-reflectivity. The ability of TiO2 for absorbing –CH3 hydrophobic group and the air cushion effect of nanostructure endow the TiO2 gradient porous material with hydrophobic characteristics. Then, the period and height of the desired microstructure of the composite micro/nano structure with the best super-hydrophobicity and anti-reflectivity are confirmed to be about 10 μm and more than 10 μm, respectively. The designed composite micro/nano structures are fabricated using picosecond laser induction processing strategy with the etched depth of the materials being only about 12 μm. The reflectance of the designed composite micro/nano structure is around 2 % from the visible band to the near-infrared band and below 20 % from the ultraviolet to middle-infrared band. Moreover, its slip angle is just 4° and contact angle 167.

The effects of rare earth elements (Ce, Eu, and Er) doping on the microstructure and gas sensing properties of ZnO nanowires were investigated. The Ce, Eu, Er-doped ZnO nanowires and pristine ZnO nanowires were synthesized via a solvothermal route. The structure of the prepared samples was studied and compared by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron microscopy. The gas sensors were fabricated by coating the prepared samples on aluminum oxides tube-based Au sensing electrode, with Ni-Cr heating wire to control the operating temperature. The operating temperature of all sensors was determined to be 300 °C with consideration of ethanol response. At this temperature, all sensors showed good ethanol sensing performance, with the 1%Ce-doped ZnO nanowires-based sensor exhibited the highest ethanol response over the other sensors. The mechanism for the microstructure and gas sensing behavior difference of various samples was discussed.

A novel ternary Ag/TiO2/clinoptilolite (ATC) nanocomposite was synthesized via a two-step synthesis strategy by combining the hydrothermal and in-situ reduction process. The obtained samples were characterized by a series of methods of XRD, SEM, TEM, N2 adsorption-desorption isotherms, XPS, FTIR, and UV-vis DRS. It is revealed that Ag and TiO2 nanoparticles were tightly bonded with the clinoptilolite and well-dispersed on its surface. Xanthates were employed as the target pollutants to evaluate the photodegradation efficiency of the as-synthesized samples. Particularly, the ATC photocatalyst exhibited an improved visible-light-induced photodegradation performance towards xanthates, which was much superior to those of binary Ag/TiO2 and Ag/clinoptilolite nanocomposites. The key influencing factors were also investigated to obtain optimal degradation conditions. The excellent photocatalytic performance of the ATC nanocomposites mainly benefited from the enlarged adsorption capacity, visible-light absorption region, and charge separation efficiency. The possible photodegradation mechanism was proposed according to the results of real-time spectral evolution, trapping experiment, and UV-vis DRS.

Chemiresistive CO2 sensing is attractive due to low cost and ease of chip-level integration. Our previous studies (Yong Xia, 2021) showed the well-developed ZnO material fabricated by in-situ annealing exhibited good CO2 sensing performance. Here, we have expanded on those studies, including CO2 cyclic tests under both dry air and N2 background whereby a much higher response to CO2 in N2 background was observed. Detailed density functional theory calculations were conducted to understand the behavior. The results indicated nitrogen doping is mainly responsible for the observed response. In the presence of pre-adsorbed O2, N-doped ZnO can no longer interact with CO2, which agrees well with the observation of higher response in N2 background. Furthermore, density of states analysis showed N sp2 hybridized orbital and N 2p orbital of the N dopant mixed with sp2 hybridized orbital of C atom and 2p orbitals of C/O atoms in CO2 to form σ and π bonds, respectively. However, they mixed with O 2s/2p orbitals of O atom in O2 when pre-adsorbed O2 was present, hindering CO2 interaction with N-doped ZnO, and resulting in limited response in air. The illustrated mechanism does not only further the understanding of metal oxide-based CO2 sensing, but also guide the design of new functional materials for CO2 sensing or capture.

Ag- and Pt-doped WO3·0.33H2O nanorods with high response and selectivity to NH3 were synthesized from a tungsten-containing mineral of scheelite concentrate by a simple combined process, namely by a high pressure leaching method to obtain tungstate ions-containing leaching solution and followed by a hydrothermal method to prepare corresponding nanorods. The microstructure and NH3 sensing performance of the final products were investigated systematically. The microstructure characterization showed that the as-prepared WO3·0.33H2O nanorods had a hexagonal crystal structure, and Ag and Pt nanoparticles were uniformly distributed in the WO3·0.33H2O nanorods. Gas sensing measurements indicated that Ag and Pt nanoparticles not only could obviously enhance NH3 sensing properties in terms of response, selectivity as well as response/recovery time, but also could reduce the optimal operating temperature at which the highest response was achieved. The highest responses of 22.4 and 47.6 for Ag- and Pt-doped WO3·0.33H2O nanorods to 1000 ppm NH3 were obtained at 225 and 175 °C, respectively, which were about four and eight folds higher than that of pure one at 250 °C. The superior NH3 sensing properties are mainly ascribed to the catalytic activities of noble metals and the different work functions between noble metals and WO3·0.33H2O.

The exoskeleton robots to assist load-carrying have received much attention in recent years. However, their load capacity and comfort are still insufficient. In this paper, we have developed a novel parallel actuated lower limb exoskeleton(PALExo), that addresses these problems. The novelty is that each limb of the exoskeleton is a 2-SPU underactuated parallel mechanism. Owing to the innovative series elastic module (SEM) with multi-segment stiffness, PALExo is compliant and comfortable for the wearer. The high load characteristic of the parallel structure increases the exoskeleton load capacity, and it can accommodate wearers of different heights without manual adjustment. A dynamic model of the exoskeleton was established, and in order to increase the robustness of the system, we designed a force control method based on sliding mode control and disturbance observer. Finally, the rationality and load-bearing effect of the exoskeleton were verified through a series of experiments including swing following, squatting and walking with loads.

In order to meet the assist requirements of extravehicular activity (EVA) for astronauts, such as moving outside the international space station (ISS) or performing on-orbit tasks by a single astronaut, this paper proposes an astronaut robotic limbs system (AstroLimbs) for extravehicular activities assistance. This system has two robotic limbs that can be fixed on the backpack of the astronaut. Each limb is composed of several basic module units with identical structure and function, which makes it modularized and reconfigurable. The robotic limbs can work as extra arms of the astronaut to assist them outside the space station cabin. In this paper, the robotic limbs are designed and developed. The reinforcement learning method is introduced to achieve autonomous motion planning capacity for the robot, which makes the robot intelligent enough to assist the astronaut in unstructured environment. In the meantime, the movement of the robot is also planned to make it move smoothly. The structure scene of the ISS for extravehicular activities is modeled in a simulation environment, which verified the effectiveness of the proposed method.

The metakaolin-based porous substrates were used as the growth substrate to synthesize SnO2 nanowires (SNWs) gas sensing materials via the gold-catalyzed vapor-liquid-solid growth mechanism. The porous substrates were fabricated by a pore-forming method with poly methyl methacrylate (PMMA) microspheres as the pore former. The effect of the porous structure of different as-prepared substrates on their product morphologies were characterized using X-ray diffraction and scanning electron microscopy. The resulting structural and morphological properties showed that all SNWs had a tetragonal rutile structure, and the aspect ratio of the SNWs enhanced with the increase in the apparent porosity or total pore area of substrates. The growth mechanism of the SNWs was discussed with respect to the effect of temperature gradient inside and outside the pores. The SNWs synthesized on the porous substrates with the PMMA content of 30 wt% exhibited the highest sensing performance towards 0.5–10 ppm H2S at the operating temperature of 85 °C, accompanying with the SNWs synthesized on a dense substrate and the substrate with the PMMA content of 20 wt% for comparison. It demonstrates that the porous structure of the metakaolin-based substrates significantly improves the aspect ratio and gas sensing properties of the SNWs.

WO3 porous films (WPFs) were synthesized by a non-hydrolytic sol-gel (NHSG) method using WCl6, C2H5OH, and C3H7NO as the precursors. The polyethylene glycol (PEG) was employed as a porogen to prepare the porous structure. The microstructures of WPFs were characterized by means of X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). Nitrogen adsorption-desorption isotherms were used to characterize the specific surface areas of the samples. The results indicated that the honeycomb-like WPFs were assembled by numerous WO3 nanoparticles with the diameter of 20–60 nm. Gas sensing measurements showed that the peak sensor response to NO2 was obtained at the operating temperature of 100 °C. The sensor based on WPFs showed excellent reversibility and fast response to NO2 at various operating temperatures and NO2 concentrations. The sensor also exhibited a significant NO2 selectivity compared with NH3, SO2, CH3OH, and C2H5OH, demonstrating a promising application in detecting NO2. The porous structure formation process of the WPFs was analyzed and gas sensing mechanism based on electron depletion theory as well as first principle calculations were discussed.

One-dimensional (1D) ZnO nanorods (ZNRs) were synthesized by a facile and effective hydrothermal method using the mixture of sodium dodecyl sulfate (SDS) and polyethylene glycol 400 (PEG400) with a molar ratio of 1:1 as the complex surfactant. The microstructure and morphology were characterized using of X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The results demonstrated that the ZNRs are of a single crystal hexagonal wurtzite structure, having a larger length-to-diameter ratio with more regular surface morphology compared with the ZnO products obtained in the presence of only SDS or PEG400. A possible growth mechanism was proposed based the mediation reaction of the complex surfactant. Gas sensing measurements indicated that the ZNRs assisted by the complex surfactant demonstrated excellent ethanol sensing properties at an optimal operating temperature of 300 °C, which could be ascribed to their large length-to-diameter ratio, one-dimensional structure, and numerous surface defects of oxygen vacancies.

A novel sensing material of polyethyleneimine-mediated WO3 nanoparticles was prepared by a simple and efficient one-pot hydrothermal method. The structure and morphology characteristics of the as-prepared WO3 nanoparticles were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The results showed that the as-prepared WO3 nanomaterials were composed of highly dispersible WO3 nanoparticles, and these nanoparticles with the particle size in the range of 10–50 nm showed a monoclinic structure. NO2 sensing measurements demonstrated that WO3 nanoparticles-based gas sensor exhibited superior response, outstanding selectivity, excellent reversibility, and good long-term stability. The sensor response increased as NO2 concentration increased. The highest response value of 251.7 was achieved to 5 ppm NO2 at the optimal operating temperature of 100 °C. Especially, the sensor response could reach 3.2–50 ppb NO2. It also exhibited fast response and recovery times with a high sensor response even in a high-humidity environment. The excellent gas sensing properties of WO3 nanoparticles could be ascribed to their high effective surface areas as well as numerous oxygen vacancies, which foresee the great potential application for fast and effective detection of sub-ppm level NO2 under different humidity environments.

ZnO nanowires (ZNWs) and ZnO nanowires/Au nanoparticles hybrid (Au-ZNWs) with various Au concentrations were synthesized by a facile one-pot hydrothermal method and characterized by XRD, SEM, TEM, XPS, and FTIR. The structural characterization results exhibited that Au nanoparticles were self-assembled onto the surface of ZNWs and the c-axis growth of ZNWs is suppressed by the addition of HAuCl4 in the synthesis of Au-ZNWs hybrid. Gas sensing properties demonstrated the favorite sensing performance could be achieved for 1 mol% Au-ZNWs compared to pure ZNWs and Au-ZNWs with other Au concentrations. The maximum response of 1 mol% Au-ZNWs to 1 ppm NO2 was 31.4 at 150 °C, which was nearly 4 times higher than 8.2 of pure ZNWs. And the shortest response and recovery times could also be achieved by 1 mol% Au-ZNWs in a wide range of operating temperature. Au-ZNWs with various Au concentrations showed better selectivity to NO2 than pure ZNWs. The mechanism of enhanced NO2 sensing performance of Au-ZNWs was investigated by the combination of electronic and chemical sensitizations via Au nanoparticles functionalization.

As motivated by the driving force of the urgent demand for high-performance nitrogen dioxide (NO2) sensor, in this work, a novel structure of Au@WO3 core–shell nanospheres (CSNSs) was designed and successfully prepared for NO2 detection. The techniques of X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller measurement, and elemental mapping analysis were used for the characterizations of the obtained samples. The results demonstrated that Au nanoparticles with 25–50 nm in diameter were successfully encapsulated by WO3 shells with the thickness of 30–50 nm. The NO2 sensing performance of the Au@WO3 CSNSs as well as the pure WO3 nanospheres were systematically investigated. Compared with the pure WO3 nanospheres, Au@WO3 CNNSs exhibited overall enhanced NO2 sensing performances in terms of response, detection limit, and response/recovery times. At the optimal operating temperature of 100 °C, Au@WO3 CSNSs showed excellent NO2 selectivity and long-term stability. Notably, the excellent NO2 sensing performance of Au@WO3 CSNSs was slightly affected by humidity. The possible sensing mechanism of the enhanced NO2 sensing properties of the Au@WO3 CSNSs was discussed.

Cu-doped ZnFe2O4 nanoparticles (Cu-ZFNPs) with spinel structure were synthesized by a facile hydrothermal route without using any surfactant. The structure and morphology of pure ZFNPs and Cu-ZFNPs with different Cu concentrations were characterized by XRD, SEM, TEM, XPS, and FTIR measurements. Microstructural characterizations showed that the diameters of both pure ZFNPs and Cu-ZFNPs with spinel structure were approximately 50 nm, which was not significantly affected by Cu doping. Gas sensing measurements showed that Cu-ZFNPs with proper Cu concentrations were favorable for enhancing H2S sensing performance especially at low operating temperatures compared to pure ZFNPs. Gas sensors based on pure ZFNPs and Cu-ZFNPs showed reversible response and excellent selectivity to H2S at the operating temperature of room temperature to 175 °C. The maximum response of 37.9 was obtained for the Cu-ZFNPs to 5 ppm H2S at room temperature. The sensing mechanism was discussed in accordance with the electron depletion theory and catalytic effect of Cu doping. It indicates that the prepared Cu-ZFNPs have potential application in H2S detection at low operating temperatures.

The reaction kinetics for the leaching of low-grade scheelite concentrate was investigated in an autoclave with sodium hydroxide in the presence of phosphate. The effects of stirring speed (300−600 r/min), reaction temperature (353−383 K), sodium hydroxide concentration (1.69−6.76 mol/L) and phosphate concentration (0.68−1.69 mol/L) on the WO3 dissolution ratio were studied. The results showed that the WO3 dissolution ratio was practically independent of stirring speed, while it increased with increasing the reaction temperature, and the concentrations of sodium hydroxide and phosphate. The experimental data were consistent with the shrinking core model, with a surface chemical reaction as the leaching rate-determining step. The apparent activation energy was calculated as 49.56 kJ/mol, and the reaction orders with respect to the concentrations of sodium hydroxide and phosphate were determined as 0.27 and 0.67, respectively. The kinetics equation of the leaching process was established.

A pore-forming agent method was adopted to fabricate a novel diatomite-based porous substrate used for preparing SnO2 nanowires (SNWs) as gas sensing materials. The optimal porous substrate with a cristobalite structure was obtained at a graphite content of 40 wt%, and the SNWs were synthesized on the porous substrate by the thermal evaporation method. Structural characterizations by X-ray diffraction and scanning electron microscopy demonstrated that the SNWs grown on the porous substrate had diameters of 20–100 nm and lengths of several hundreds of micrometers, which were thinner, longer and denser than those grown on the dense substrate. The SNWs grown on the porous substrate were found to exhibit a higher SO2 response (Ra/Rg = 33.5–50 ppm) and faster response and recovery times (3 s and 6 s) at a low operating temperature of 85 °C. It revealed that the porous surface of the as-prepared substrate was favorable for the growth of SNWs with a large length-to-diameter ratio and subsequently enhanced their SO2 sensing properties.