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Silicone rubber is widely used as an insulating material. In this article, silicone rubber samples were prepared by varying the content of crosslinker (2,5-bis(tert-butyl-peroxy)-2,5-dimethylhexane, DBPMH), and the free volume holes in the samples were investigated by means of positron annihilation lifetime spectroscopy (PALS) measurement. The surface chemical structure, surface micromorphology and water diffusion of the samples after corona discharge treatment were studied by FTIR, SEM and EIS measurements, respectively. As the crosslinker weight ratio increased from 0.2 wt.% to 1.5 wt.%, the mean free volume hole size first decreased and then remained unchanged. However, the concentration of free volume holes did not vary as the crosslinker weight ratio increased. SEM morphologies show that surface cracks were produced on samples having high crosslinking levels after corona treatment. The water diffusion coefficient of samples after corona treatment increased from 3.13 × 10−10 cm2 s−1 to 17.68 × 10−10 cm2 s−1 in the initial immersion period, as the crosslinker weight ratio increased from 0.2 wt.% to 3.0 wt.%. The results indicated that deterioration of samples with high crosslinking levels were more serious and water repellency more easily lost. The corona resistance ability of low crosslinking level silicone rubber stems from internal low molecular weight molecules. [ABSTRACT FROM AUTHOR]

Fumed silica (FS)-filled polydimethylsiloxane (PDMS) composite is widely used as the external insulating material in power grid. The function of PDMS composite is to keep the core of an insulation device being dry by preventing the permeation of outside water. To study water diffusion in PDMS composite helps to shed light upon the failure mechanism of the insulation devices. In this paper, positron annihilation lifetime spectroscopy (PALS) and electrochemical impedance spectroscopy (EIS) were applied to study the FS/PDMS interface structure and water diffusion in PDMS composite. It is found that the FS has two basic existing states in PDMS matrix: dispersed state and percolated state, and the percolation threshold of FS is fitted to be 6.0 wt.%. In the dispersed state, the FS particles are randomly dispersed in PDMS matrix. The values of τ 3 remain stable, being approximately 1.0 ns, indicating no overlaps of the FS/PDMS interfaces. Moreover, water cannot diffuse through the sample after corona aging. In the percolated state, the FS particles are tightly packed in the PDMS matrix. The values of τ 3 decrease with the weight fraction of FS, suggesting the overlapping of the FS/PDMS interfaces. As the FS percolates in sample bulk, these overlapped FS/PDMS interfaces provide continuous diffusion tunnels for water, thus resulting in failure of the composite after corona aging. It is found that polarization and discharge of the electrons at FS/PDMS interface under corona discharge is the main mechanism that leads to the failure of the composite bulk. [ABSTRACT FROM AUTHOR]

Silicone rubber is widely used in power grids, electronics and aerospace, because of its hydrophobicity, high flexibility, etc. However, it is susceptible to some environmental chemical factors, such as NO2 and ozone, which can induce chain scission, the generation of defects and the ingress of water. In this work, we systematically studied the microstructure variations and water diffusion behavior of high-temperature vulcanized (HTV) silicone rubber after NO2-induced oxidative damage. Microstructure evolution in silicone rubber was investigated by positron annihilation lifetime spectroscopy (PALS). The results show that the oxidation of NO2 significantly degrades organic matrices, which induces the decrease of crosslinking degree and the formation of defects compared to virgin samples. The water transportation was evaluated by electrochemical impedance spectroscopy (EIS), which showed that the diffusion coefficient, under the NO2 concentration of 28.75 mg L−1, is 106 times that of virgin sample with a water uptake of 31%. Particularly, when the concentration reaches the critical value around 17.25 mg L−1, the diffusion coefficient and water uptake increase sharply, and the calculated average porosity of samples also dramatically increases by 4 orders of magnitude. These indicate the formation of more nano- and micron holes, serving as a pointer for percolation of defects in silicone rubber bulk. The study of microstructure variations and water transportation can help us to understand the aging mechanism, design reasonable composite polymer materials and prevent the damage of chemical contact. [ABSTRACT FROM AUTHOR]

ABSTRACT: Supertough polylactide (PLA)/polyurethane (PU) blends were prepared by reactive blending of PLA with polyester polyol and toluene‐2,4‐diisocyanate. The free volume and interfacial compatibility between the two polymers were investigated by positron annihilation lifetime spectroscopy. The PU particles dispersed homogeneously in the PLA matrix and self‐assembled into a subinclusion microstructure, resulting in fibrils and significant plastic deformation occurs during impact process. More phase interface and free volume cavities formed between PLA and PU boundary because of a good interfacial compatibility between the two polymers, leading to smaller sizes of free volume cavities, and the increasing of the number of these cavities. Therefore, the toughness of PLA was greatly improved by blending PU. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46596. [ABSTRACT FROM AUTHOR]

This article discusses a study which examined the influence of acrylic acid groups on the microstructure of high-density polyethylene (HDPE)/Bentonite (BT) and high-density polyethylene. The study used positron annihilation lifetime spectroscopy (PALS) in the examination. It found a similar mean free volume size in composites and HDPE matrices. It revealed a decrease in the ortho-positronium (o-Ps) intensity for HDPE-g-AA.

In this paper, gas chromatography–mass spectrometry (GC–MS) and positron annihilation lifetime spectroscopy (PALS) were used to probe the changes of oligomers and the polydimethylsiloxane (PDMS) network in silicone rubber, after different durations of UVA/UVB irradiation. At the early stage (<300 h) of UVA/UVB irradiation, the concentration of D 4 -D 9 decreases. The o-Ps intensity of the extracted silicone rubber increases in the stage after UVB irradiation. These results indicate the crosslinking of oligomers into the PDMS network. After a long duration (>300 h) of UVB irradiation, D 4 was generated and the lifetime of τ 3 also increased, indicating the rupture of the Si-O bond in the PDMS network. These two aging processes were termed the post curing process and the chain session process. The new finding was that UVA could only induce the post curing process; UVB causes the rupture of the chemical bond in silicone rubber. Photons of UVB could break the C-H bond, and then trigger the backbiting decomposition of PDMS, breaking the Si-O bond, while the photons of UVA cannot. The fact that D 4 was generated after UVB irradiation can be used to evaluate the UVB stability of silicone rubber in the future. [ABSTRACT FROM AUTHOR]

Vacancy defects have important impacts on the physical-chemical properties of metal oxides. In the present work, two types of Bi 2 O 3 powders with different morphologies were synthesized for supercapacitor electrodes by calcination (denoted as c-Bi 2 O 3) and two-step hydrothermal methods (donated as h-Bi 2 O 3). Vacancy clusters of V B i ‴ V O · · V B i ‴ with different concentrations were observed in these two samples by positron annihilation lifetime spectroscopy. The results of electrochemistry revealed that the charge storage behavior of h-Bi 2 O 3 with more V B i ‴ V O · · V B i ‴ defects obeyed both semi-infinite diffusion-controlled battery-type mechanism and surface-controlled pseudo-capacitance mechanism, while that of the calcined c-Bi 2 O 3 electrode with a lower concentration of V B i ‴ V O · · V B i ‴ defects followed only the semi-infinite diffusion-controlled battery-type mechanism. The pseudo-capacitance of h-Bi 2 O 3 could be attributed to the insertion/extraction process of more K+ ions in the V B i ‴ V O · · V B i ‴ surface defects. Due to the partial pseudo-capacitance and improved conductivity caused by more V B i ‴ V O · · V B i ‴ defects, the h-Bi 2 O 3 electrode had a larger capacitance (1043 F g-1 at 1 A g-1), a higher rate performance (560 F g-1 at 60 A g-1), and better cycle stability (93% retention at 50 A g-1 after 2000 cycles). Furthermore, because of the high-concentration of V B i ‴ V O · · V B i ‴ defects, the Ni/Co-MOF//h-Bi 2 O 3 asymmetric supercapacitor delivered a relatively higher specific energy density of 47 Wh kg-1 at 1125 W kg-1. Taken together, these results indicate that surface vacancy clusters play an important role in boosting the electrochemical performance of metal oxide electrodes. [ABSTRACT FROM AUTHOR]

Buffer layers of TiO 2 nanoparticles are introduced on the surface of the MIL-101-Cr, a kind of metal-organic frameworks (MOFs), to improve performances of MOFs/Nafion proton exchanging membranes. A series of Nafion composite membranes are fabricated by doping TiO 2 -wrapped MIL-101-Cr (TiO 2 @MIL-101-Cr). The water uptakes of composite membranes reasonably increase while their swelling ratios unexpectedly reduce in comparison to pristine Nafion. Nevertheless, the swelling ratios of composite membranes gradually increase with the increment of TiO 2 @MIL-101-Cr due to their enlarged free volumes and more amorphous phases. The introduction of additive facilitates water molecules diffusion as well as overall hydration of the composite membranes, resulting in a significant increment in the proton conductivity. As a result of the formation of extra proton conducting highways both around/in TiO 2 @MIL-101-Crs and the Nafion matrix, a composite membrane in single-cell possesses a maximum power density of 0.946 W/cm2. Furthermore, higher thermal and oxidative stabilities are observed for the composite membranes. This work demonstrates that the addition of encapsulated structural TiO 2 @MIL-101-Crs in Nafion is promising for fabricating proton exchanging membranes with superior electrochemical performance and high thermal, oxidative stabilities. [Display omitted] • TiO 2 @MIL-101-Cr is synthesized for preparation MOFs/Nafion composite membranes. • More amorphous phases and high water uptake for MOFs/Nafion membranes are found. • Extra highways for proton diffusion are formed in the composite membranes. • TiO 2 @MIL-101-Cr-5/Nafion membrane possesses a power density of 0.946 W/cm2. • Higher thermal and oxidative stabilities are observed for the composite membranes. [ABSTRACT FROM AUTHOR]

[Display omitted] • The yolk-shelled CoS 2 is successfully fabricated via self-sacrificing strategy. • Defect-riched CdS/CdCO 3 is firstly grown in-situ on CoS 2 via solubility product displacement method. • Introduction of wide-band CdCO 3 highly promotes H 2 evolution of CdS-CoS 2. • Mie's theory and monochromatic light HER indicates the possible Mie resonance. Yolk-shelled CoS 2 nanospheres are designed through Kirkendall Effect and subsequently converted into defect-rich CdS/CdCO 3 -CoS 2 photocatalysts via in-situ growth method. The optimal CdS/CdCO 3 -CoS 2 exhibits a significant hydrogen evolution rate of 64867.88 μmol h−1 g cat −1 that is 3.23 and 49.45 folds higher than CoS 2 -CdS and pure CdS. CO 2 reduction rate of CdS/CdCO 3 -CoS 2 is detected additionally (654.7 μmol h−1 g cat −1). Yolk-shelled CdS/CdCO 3 -CoS 2 induces the multi-scattering of incident light, possessing 1.52-fold H 2 production rate than that of full hollow ones. Enhanced photoexcited-carriers migration efficiency is attributed to the synergistic effect between possible Mie resonance at about λ = 450 nm in yolk-shelled architecture based on Mie's theory and monochromatic light HER test, and the formation of defect energy level within the wide-band gap of CdCO 3 in Schottky-type/type II heterojunction. Electron paramagnetic resonance, positron annihilation spectroscopy and density functional theory calculation etc. are employed to validate the presence of oxygen vacancies and the photocatalytic mechanism. [ABSTRACT FROM AUTHOR]

Proton exchange membrane (PEM) is a key component of proton exchange membrane fuel cells (PEMFCs) and provides proton conduction between fuel and oxidant, and it is vitally important and practical to develop PEMs with high proton conductivity through membranes. However, it is still a major challenge to develop such membranes with high through-plane proton conductivity. In this study, Nafion composite membranes with sulfonated graphene oxides (SGOs) were prepared from self-assembly sol-gel either in or not in a strong magnetic field. Microstructures, water uptake, and proton conductivity of the SGO/Nafion composite membranes have been investigated extensively. For the SGO/Nafion composite membranes recast without a magnetic field, it is found that the water uptake and in-plane proton conductivity are greatly increased with the increasing SGO content. Interestingly, SGO/Nafion composite membranes with a low SGO content of ∼ 0.5 wt% exhibit lower crystallinity and higher in-plane proton conductivity. For the SGO/Nafion composite membranes recast in a strong through-plane magnetic field, it is very appealing to find their through-plane proton conductivity is significantly higher than that of the composite membranes recast without a magnetic field. This is attributed to that the protons transport in the membranes preferentially along the water channels primarily interconnected by the aligned SGOs through-plane induced by the strong magnetic field, as confirmed by SEM. Applied in a single PEMFC, the resultant SGO/Nafion composite membrane with 1 wt% SGOs, recast in a strong magnetic field of 1.5 T, demonstrates a high power-density of 1.056 W cm-2, which is far superior to those of pristine Nafion and SGO/Nafion composite membranes recast without a magnetic field. The study provides a novel strategy for preparing proton exchanging membranes with high performances for new-generation fuel cells. [Display omitted] • The proton conductivity of sulfonated graphene oxide (SGO)/Nafion composite membranes was impacted by SGO contents. • Highly through-plane proton conductive membranes were prepared by recasting in a strong magnetic field. • A single fuel cell with optimal SGO/Nafion recast in a strong magnetic field shows a high power-density of 1.056 W cm-2. [ABSTRACT FROM AUTHOR]

Cyclic voltammetry and positronium (Ps) 3 γ -annihilation spectroscopy were applied to investigate pore interconnectivity/morphology of porous silica films fabricated with various loading of cetyltrimethyl ammonium bromide (CTAB). With increasing the ratio of CTAB up to 15 wt.%, the total charge Q , resulted from I − diffusion across the silica films, increased remarkably, indicative of formation of highly interconnected pores in the films prepared with more porogen. However, it decreased dramatically with further loading CTAB of 25 wt.%. Interestingly, 3 γ -annihilation fraction I 3 γ due to a triplet-state Ps ( ortho -positronium, o -Ps) emission from the silica films showed a similar behavior as a function of CTAB loading. The abnormal decrement in Q and I 3 γ in the film fabricated with 25 wt.% CTAB was well explained by formation of long nanochannels aligning parallel to the film surface. The results indicated that the total charge Q and Ps 3 γ -annihilation fraction were closely associated with I − and Ps diffusion governed by the pore interconnectivity/morphology of the silica films, which made cyclic voltammetry possible to be a feasible tool to characterize pore interconnectivity/morphology of porous thin films. [ABSTRACT FROM AUTHOR]

Highlights: [•] Decrements in S value and water uptake coefficient confirm the post-cure process. [•] Molecular chain scission leads to the formation of microporous structure. [•] The formation of an aged layer with high density is verified by EIS. [Copyright &y& Elsevier]

Abstract: Nano ZnO photocatalysts were prepared and characterized with XRD, BET, TEM, photocurrent measurement, and positron annihilation spectroscopy (PAS). It has been found that the photocatalytic activity for the gas-phase photocatalytic oxidation of acetone on the nano ZnO photocatalysts significantly increases with increasing reaction temperature. Temperature-dependent photocurrent measurement of the nano ZnO samples reveals that the significant enhancement of photocatalytic activity is attributed to a great enhancement in the separation efficiency of photogenerated charge carriers with increasing temperature. This enhancement is most probably attributed to the effects of the ionized impurity scattering and dislocation scattering, and the increased conductivity of the nano ZnO samples with the elevation of temperature. The PAS result indicates the presence of defects such as monovacancies and vacancy clusters in the nano ZnO samples, which probably leads to the temperature effect of the ionized impurity scattering and dislocation scattering. The novel strategy of improving photocatalytic activity of nano ZnO is facilely realized by coating nano ZnO on the surface of a high-pressure Hg lamp without using additional heater, under which nano ZnO was co-excited by both UV irradiation and thermal energy from the Hg lamp. Under such photothermocatalytic condition, nano ZnO exhibits an excellent durability for the gas-phase photodegradation of acetone, which is in striking contrast to the photoinstability of nano ZnO in liquid-phase photodegradation. [Copyright &y& Elsevier]

[Display omitted] • The functional g-C 3 N 4 nanosheets are synthesized by sol-gel method. • The NO 2 resistibility of nanocomposite is evaluated by water diffusion via EIS. • Successful grafting silanes reduces diffusion pathways for NO 2. • The effect of the g-C 3 N 4 on chemisorption to NO 2 is calculated by DFT. • The synergy of physical barrier and chemisorption is achieved. In this study, the functional graphitic carbon nitride (g-C 3 N 4) nanosheets are prepared to enhance the NO 2 resistibility of room temperature vulcanized (RTV) silicone rubber. After corrosive gas NO 2 oxidation for 24 h, the water barrier ability of RTV nanocomposite coatings was evaluated by electrochemical impedance spectroscopy (EIS). At the optimal filler content of 0.3 wt.%, the impedance module | Z | of functional g-C 3 N 4 /RTV is 3☓108 Ωcm2 with the best coating integrality, compared with 2☓107 Ωcm2 for the functional graphene oxide (f-GO)/RTV and 6.45☓106 Ωcm2 for the pure RTV silicone rubber. The microstructure evolution in nanocomposite was investigated by positron annihilation lifetime spectra (PALS). The results indicate that the functional g-C 3 N 4 nanosheets improve the interfacial compatibility and reinforce the RTV matrix, which decrease the free volume for NO 2 diffusion. The density functional theory (DFT) calculations reveal that the triazine structures in the g-C 3 N 4 surface can effectively interact with NO 2 molecules. Simultaneously, silanes grafting promotes exfoliation and good dispersity of g-C 3 N 4. Due to highly decentralized two-dimensional functionalized g-C 3 N 4 with high aspect ratios, the significant synergy effect between physical barrier and chemisorption is achieved. [ABSTRACT FROM AUTHOR]

• A flower-liked twin crystal Zn 0.2 Cd 0.8 S is synthesized via a simple hydrothermal method. • Theoretical calculations (DFT) reveal the reaction mechanism of photocatalytic hydrogen production. • Positron annihilation demonstrates the surface defect variation in the catalysts. • Synergistic effects of the heterojunction and Ni greatly improve the separation of the photo-generated carriers. • The catalyst exhibits 83-times hydrogen production rate increments comparing to Zn 0.2 Cd 0.8 S. The applications of sulfide in photocatalytic hydrogen production are limited due to the rapid recombination of the electron-hole pairs. In this paper, a kind of novel flower-like ternary Ni/NiS/Zn 0.2 Cd 0.8 S composite catalyst with twin crystals and heterojunction structure is synthesized by a multi-step method. Compared with pure Zn 0.2 Cd 0.8 S, the ternary Ni/NiS/Zn 0.2 Cd 0.8 S catalyst shows significantly a highly visible light absorption, photoelectric performance and hydrogen production, i.e., both transient photocurrent and hydrogen production rate are increased by 18 times and 83 times, respectively. Combining the theoretical calculations and experimental results, a possible photocatalytic reaction mechanism is proposed. In this catalyst, it is known that the twin crystal Zn 0.2 Cd 0.8 S belongs to the homojunction structure, Zn 0.2 Cd 0.8 S combines with NiS to form a heterojunction and simultaneously supports the excellent conductivity of Ni, Which therefore, the synergy of the nano-scaled NiS and Ni co-catalysts show a great improvement in separation efficiency of the photo-generated electron-hole pairs, and allows the Ni/NiS/Zn 0.2 Cd 0.8 S catalyst to have excellent hydrogen production performance in photocatalytic decomposition of water. [ABSTRACT FROM AUTHOR]

In this work, typical metal-organic frameworks (MOFs) MIL-101-Cr, MIL-53-Al and ZIF-8 with high specific surface areas were synthesized and applied in the removal of anionic dye methyl orange (MO) from water. Nitrogen adsorption and positron annihilation lifetime spectroscopy (PALS) were applied to study the pore characteristics of MOFs and their adsorption of MO, respectively. The specific surface areas of the synthesized MOFs are in the order MIL-101-Cr > ZIF-8>MIL-53-Al. The pore sizes of MOFs obtained from PALS are essentially the same as those from their crystal structure, suggesting that PALS is a useful probe to characterize the pore properties of MOFs. After MO adsorption in MOFs, the nominal mean pore sizes for MIL-101-Cr-MO and MIL-53-Al-MO from nitrogen adsorption are mainly attributed to the interspaces between grains because their inherent pores were occupied by MO molecules as revealed by PALS, while that of ZIF-8-MO represents the inherent pores and interspaces between grains. Langmuir adsorption of MO was found to occur in the MOFs, whereas the MO adsorption behaviors are different depending on the unique pore structures of specific MOFs. MIL-101-Cr and MIL-53-Al show the MO adsorption capacities of 196.08 and 100.0 mg/g due to their large specific surface areas and accessible pore windows, which can provide more accessible unsaturated metal sites. However, despite that the specific surface area of ZIF-8 is larger than that of MIL-53-Al, MO adsorption capacity in ZIF-8 is rather low because the size of its pore windows is smaller than MO dimension, which results in MO molecules inaccessible to the pores. Results showed that methyl orange anions (MO−) were electrostatically attracted by unsaturated metal sites in the cages/channel pores of MIL-101-Cr and MIL-53-Al in aqueous solutions, whereas MO was only adsorbed in the interspaces between grains in ZIF-8. Image 1 • MOFs with adsorbed MOFs were studied by PALS and nitrogen adsorption, respectively. • The pore sizes of MOFs obtained from PALS are essentially the same as those from ideal crystal structure. • MO adsorption in MOFs depends on both the inherent pore surface areas and the pore entry sizes. [ABSTRACT FROM AUTHOR]

• NiCo 2 S 4 /CdS showed excellent HER with lactic acid under visible light irradiation. • Ultrafast dynamics studies showed that NiCo 2 S 4 /CdS had more efficient charge separation. • The mechanism enhanced performance of the NiCo 2 S 4 /CdS nanocomposites were revealed. One-dimensional NiCo 2 S 4 /CdS nanocomposites are constructed using a two-step hydrothermal method and used for photocatalytic hydrogen generation with visible light. The nanocomposites are characterized using a combination of techniques including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), positron annihilation lifetime measurement, spectroscopy techniques, and femtosecond transient absorption (TA) spectroscopy. In comparison to bare CdS NWs, the NiCo 2 S 4 /CdS nanocomposites exhibit enhanced visible light hydrogen production, with an optimal rate reaching 20.0 mmol·h−1·g−1. The experimental results, in conjunction with theoretical electronic structure calculations, reveal that the enhanced hydrogen evolution reaction activities can be attributed to the formation of Schottky junction between NiCo 2 S 4 NPs and CdS NWs that facilitates photogenerated electron transfer from CdS to NiCo 2 S 4. Meanwhile, the NiCo 2 S 4 , as a metallic cocatalyst, increases the electron density and active sites desired for H 2 evolution. [ABSTRACT FROM AUTHOR]