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Optical fiber hydrogen sensor has become a research hotspot once proposed, since its unique properties of intrinsic safety. In the past three decades, varieties of optical fiber hydrogen sensors have been proposed, which could be categorized into five types, including interference type, micromirror type, evanescent field type, surface plasmon resonance type, and fiber Bragg grating type. This review focuses on the representative of the above five types of hydrogen sensors, based on Pd alloy or/and WO3 sensitive materials. Whereas, their structures, characteristics, and sensing performances are critically overviewed.

A process combining electrophoretic deposition (EPD) with hot pressing (HP) was developed to fabricate continuous carbon fiber-reinforced ZrB2-based composites (Cf/ZrB2-based composites). ZrB2-based ultra-high temperature ceramic (UHTC) particles were uniformly pre-coated on continuous carbon fibers via EPD. Then, the UHTC-coated carbon fibers were stacked and hot pressed to prepare the Cf/ZrB2-based composites. Microstructure observations revealed that almost no micro-pores were found in the inter-bundle and intra-bundle regions of fibers after HP. The flexural strength, fracture toughness and the work of fracture of the Cf/ZrB2-based composite were measured as 199 ± 26 MPa, 6.71 ± 1.29 MPa·m1/2, and 754 ± 58 J/m2, respectively. Based on the observations of non-brittle fracture behavior, fractured morphology and crack propagation, the enhanced fracture properties were mainly attributed to the multiple toughening mechanisms, such as fiber pull-out, fiber bridging, crack deflection and branching along the interfaces.

In order to improve the wettability and bonding performance of the interface between carbon fiber and aluminum matrix, nickel- and copper-coated carbon fiber-reinforced aluminum matrix composites were fabricated by the squeeze melt infiltration technique. The interface wettability, microstructure and mechanical properties of the composites were compared and investigated. Compared with the uncoated fiber-reinforced aluminum matrix composite, the microstructure analysis indicated that the coatings significantly improved the wettability and effectively inhibited the interface reaction between carbon fiber and aluminum matrix during the process. Under the same processing condition, aluminum melt was easy to infiltrate into the copper-coated fiber bundles. Furthermore, the inhibited interface reaction was more conducive to maintain the original strength of fiber and improve the fiber–matrix interface bonding performance. The mechanical properties were evaluated by uniaxial tensile test. The yield strength, ultimate tensile strength and elastic modulus of the copper-coated carbon fiber-reinforced aluminum matrix composite were about 124 MPa, 140 MPa and 82 GPa, respectively. In the case of nickel-coated carbon fiber-reinforced aluminum matrix composite, the yield strength, ultimate tensile strength and elastic modulus were about 60 MPa, 70 MPa and 79 GPa, respectively. The excellent mechanical properties for copper-coated fiber-reinforced composites are attributed to better compactness of the matrix and better fiber–matrix interface bonding, which favor the load transfer ability from aluminam matrix to carbon fiber under the loading state, giving full play to the bearing role of carbon fiber.

Fiber optic hydrogen gas sensors with low detection limit and high precision are highly demanding nowadays. Self-referenced fiber Bragg grating (FBG) hydrogen gas sensor offers promising advantages due to its improved systemic stability and compactness. Nevertheless, the low sensitivity and the low stability of hydrogen sensitive materials still greatly limit its detection capability in extremely low concentration range. Here we demonstrate that an improved self-referenced FBG hydrogen gas sensor design can overcome this limitation. It is based on an integrated architecture which combines a FBG structure coated with polyaniline modified WO3–PdPt composite film and an optical heating device using a high-power laser source. Due to the higher working temperature and the protective coating structure of the PdPt catalytic layer, the sensor shows faster response and higher measurement stability, which together results in a significantly low detection limit down to 4.5 ppm in air. These greatly improved sensing hydrogen gas performances highlights its promising applications in future.

ZrC nanoparticles were used to improve the microstructure and mechanical properties of C/C–SiC composites. It was found that the microstructural features and high temperature mechanical properties of C/C–SiC composites were dependent on the ZrC content. Microstructure analysis indicated that the introduction of ZrC nanoparticles effectively improved the distribution homogeneity of SiC matrix. With increasing the ZrC content, the distribution of SiC in the composites gradually transformed from a zonal distribution to a network structure distribution. The influence of ZrC content on the high temperature mechanical properties were investigated at elevated temperatures under ambient atmosphere. It was found that the introduction of ZrC nanoparticles could be an attractive way for improving the mechanical properties by the present processing route. The flexural strengths of all composites at RT increased with increasing the amount of ZrC. The CSZ10 exhibited the highest flexural strength at RT, which was about 196.1 MPa. Comparing with the CSZ0, it increased by 47%. However, at 1000 °C, the CSZ8 composites exhibited the highest flexural strength (55.2 MPa) and a better strength retention (~30%, compared with RT strength). The good strength retention could be attributed to a moderate fiber-matrix bonding strength and improved anti-oxidation resistance which were caused by the homogeneous distribution of SiC matrix.

Self-toughening ZrB2–SiC based composites are fabricated by in-situ reactive hot pressing. The effect of sintering additive content on the microstructure and mechanical properties of the composites is investigated. Microstructure observation found that the in-situ reactive hot pressing could promote the anisotropic growth of ZrB2 grains and the formation of interlocking microstructure. Such microstructure could improve the mechanical properties, especially, for the fracture toughness. The improved mechanical properties could be attributed to the self-toughening structure related to the ZrB2 platelets and the formed interlocking microstructure, which could trigger various toughening mechanisms such as grain pull-out, crack bridging, crack deflection, and crack branching, providing the main contribution to the high fracture toughness.

Effect of activated carbon and lithium chloride on polyimide-based fiber Bragg grating relative humidity sensor has been experimentally investigated. Through dip coating process, organic-inorganic hybrid coatings were deposited on fiber Bragg grating to form different relative humidity sensors. Experimental results show that the addition of activated carbon or lithium chloride alone could increase the humidity sensitivity (1.67 times and 1.61 times, respectively), but the structure which contains both lithium chloride and activated carbon increases the sensitivity most (3.9 times). In the 14-day stability test, the fluctuation of pure polyimide sensor is ±6.5%, while the ones of activated carbon sensor, lithium chloride sensor, activated carbon, and lithium chloride sensor are ±3%, ±8%, and ±1.6%.

In order to satisfy the requirements of structural and functional applications, tailoring microstructures and improving oxidation resistance of C/C-SiC is essential. In this study, microstructure tailoring is achieved by pre-treatment of fiber at 1500 °C in a nitrogen atmosphere. It is found that the pre-treatment of carbon fibers increases the mass fraction of the SiC matrix in composites, the distribution of the SiC matrix becomes homogenous, and the fine-grained SiC layer enwraps the fibers forming the dense shell. Thermo-gravimetric measurements indicate that microstructural tailoring by fiber treatment can significantly improve the oxidation resistance of C/C-SiC composites, attributed to the high mass fraction and the homogenous distribution of the SiC matrix.

A hydrogen and humidity simultaneous measurement system based on an integrated FBG sensor is proposed. Experimentally results indicate the sensor shows a high degree of hydrogen discrimination ranging from 100ppm-8000ppm and a high degree of humidity discrimination ranging from 20% - 80%, along with the high performance of anti-mutual interference of hydrogen and humidity.

In this paper, WO3-Pd2Pt-Pt nanocomposite films were deposited on a single mode fiber as the hydrogen sensing material, which changes its reflectivity under different hydrogen concentration. The reflectivity variation was probed and converted to an electric signal by a pair of balanced InGaAs photoelectric detectors. In addition, the performance of the WO3-Pd2Pt-Pt composite film was investigated under different optical powers, and the irrigating power was optimized at 5 mW. With the irrigation of this optical power, the hydrogen sensitive film exhibits quick response toward 100 ppm hydrogen in air atmosphere at a room temperature of 25 &deg, C. The experimental results demonstrate a high resolution at 5 parts per million (ppm) within a wide range from 100 to 5000 ppm in air. This simple and compact sensing system can detect hydrogen concentrations far below the explosion limit and provide early alert for hydrogen leakage, showing great potential in hydrogen-related applications.

We present an erratum to our Letter [Opt. Lett.45, 3601 (2020)OPLEDP0146-959210.1364/OL.395341]. Labeling errors in two figures and an incorrect sentence are revised. The corrections have no influence on the conclusions of the original Letter.

To improve the fracture properties of ZrB2-based composites, SiC nanowires (SiCnw) were introduced into the powder mixtures via an in-situ growth method and the composites were fabricated by hot pressing. Microstructure observations found that the SiCnw with a diameter of less than 100 nm presented twisted morphology and homogeneously distributed among ceramic particles. It was also found that the SiCnw could inhibit the grain growth of ZrB2-based composites. Compared to that of composites without SiCnw, the fracture toughness and the work of fracture for the composites with SiCnw were 7.17 MPa m1/2 and 205 J m−2, which increased by 61.1% and 91.6%, respectively. The main enhancing mechanisms could be associated with the obvious crack deflection, crack branching and pull out of SiCnw. The SiCnw could refrain the crack opening and reduce the stress intensity in front of crack tips, which absorbed more energy and improved the fracture properties.

α-MoO3 has been widely investigated as hydrogen sensing material due to its excellent electrical and optical properties. However, its irreversible structure change during reaction with hydrogen results in an obvious degradation of the sensor’s performance. In this paper, an ion intercalated method is proposed to improve the stability of fiber bragg grating (FBG) hydrogen sensor based on MoO3. Na+ and K+ ions are successfully intercalated into MoO3 nanobelts and the effect of the intercalated ions’ amount to sensing performance has been investigated. Compared to MoO3, ion intercalated MoO3 nanobelts exhibits significant improvement in stability and cycling performance. XRD and TEM analysis reveal that intercalated ions between the interlayers can reinforce the layer structure and inhibit the phase transition. This ion intercalation strategy may provide a new perspective to improve stability of sensors based on layer molybdenum trioxide and dichalcogenides.

Large quantities of multi-shaped SiC nanofibers were successfully synthesized on carbon fibers at elevated temperatures via a catalyst-free chemical vapor reaction method. FE-SEM and TEM investigations revealed that all SiC nanofibers grew along the [111] crystal orientation. The diameters of SiC nanofibers were in the range of 100–400 nm. The length of nanofibers was about several hundreds of micrometers. The vapor-solid mechanism was applied to understand the growth of the multi-shaped SiC nanofibers. The change in morphological shape of SiC nanofibers could be attributed to the temperature-dependent growth kinetics, which strongly associated with the partial pressure of SiO.

Several novel relative-humidity sensors based on fiber Bragg grating and various pore-foaming agent doped polyimide coating are proposed and demonstrated. Sensing characteristics of lithium chloride, acetone and methyl alcohol doped polyimide coated sensors were investigated, and all sensors show good stability and linear response during the experiment. The corresponding sensitivities are 1.71, 0.53, and 0.63 pm/%RH, respectively. Compared with the polyimide coated sensor (0.62 pm/%RH), the humidity sensor based on additive of lithium chloride can significantly enhance the sensitivity (2.76 times). Moreover, the proposed humidity sensors display linear response under different temperatures, which make it easier for the temperature compensation. Therefore, humidity sensor based on lithium chloride doped polymer sensor is promising to detect humidity fluctuation in some special facilities.

Optical fiber hydrogen sensor has been a research issue due to its intrinsic safety and anti-electromagnetic interference in recent years. However, its sensitivity and stability still need to be improved to meet the demand of application. Here we present a novel fiber-optic hydrogen platform with significantly enhanced performance, especially for low hydrogen concentrations. The hydrogen sensing system, which consists of two Bragg gratings paired with high-low reflectivities and employs WO3-Pd2Pt-Pt composite film as sensing element, can detect hydrogen concentration down 20 ppm in air at ambient temperature of 25 °C. And the hydrogen sensor exhibits quick response rate and good repeatbility. Moreover, the sensor has good anti-humidity interference and selectivity during hydrogen response. Our work proposes a new concept to develop hydrogen sensing technology with greatly improved performance, which can significantly promote its potential application in various fields.

High purity SiC nanostructures were grown in situ on the surfaces of C/SiC composites by the chemical vapor reaction route to improve the anti-oxidation capability of C/SiC composites. It was found that the SiC nanostructures formed a dense cluster with a thickness of about 20 μm. The nanostructures mainly included nanoparticles and nanorods. The oxidation tests found that the SiC nanostructures could significantly improve the anti-oxidation capability of C/SiC composites, particularly, at the temperature beyond 900 °C. The improvement of the anti-oxidation capability of the C/SiC composites was attributed to the formation of a silica layer.

We theoretically and experimentally investigated thin films based one-dimensional photonic crystal with detection of environmental humidity. The sensing elements are multilayer porous thin films deposited on fiber end face by e-beam evaporation. Theoretical simulation shows that the resonant mode of the photonic crystal shifts to longer wavelength due to the capillary condensation of thin films. Experimental results on humidity detection show that the wavelength shift reaches 21.6 nm when relative humidity ranges from 11 to 84%RH; especially the proposed sensor is more sensitive below the humidity of 57%RH.

A novel fiber Bragg grating (FBG) hydrogen sensor based on the Pd/Ni composite film and controllable laser heating system is proposed and demonstrated with enhanced sensitivity. The Pd/Ni composite film was sputtered around the greatly etched FBG, and the atomic ratio of Pd and Ni was controlled at 91:9 during the sputtering process. A 980-nm pump laser and proportion integration differentiation controller are employed to induce heating in the Pd/Ni composite film. Experimental results show that the stability of the hydrogen sensor is significantly improved with ±1 pm fluctuation at 1% hydrogen concentration. In addition, the proposed sensor exhibits higher sensitivity and quicker response rate during hydrogen exposure.

In order to improve the mechanical properties of the short carbon fiber-reinforced polymer (SCFRP) composites, SiC nw were introduced into the SCFRP composites through an in-situ grown method. It was found that the SiC nw were grown in the matrix and on the surface of carbon fibers. The in-situ grown SiC nw could effectively reinforce the SCFRP composites and lead to an enhanced mechanical properties. The SiC nw not only acted as the secondary reinforcement, but also increased the fiber-matrix interfacial bonding area and bonding strength.

Multi-walled carbon nanotubes (MWCNTs) were used to optimize the microstructure and improve the fracture properties of hot-pressed carbon fiber-reinforced ZrB 2 -based ultra-high temperature ceramic composites. Microstructure analysis indicated that the introduction of MWCNTs effectively reduced the carbon fiber degradation and prevented fiber-matrix interfacial reaction during processing. Due to the presence of MWCNTs, the matrix contained fine ZrB 2 grains and in-situ formed nano-sized SiC/ZrC grains. The fracture properties were evaluated using the single edge-notched beam (SENB) test. The fracture toughness and work of fracture of the C f /ZrB 2 -based composite with MWCNTs were 7.0±0.4 MPa m 1/2 and 379±34 J/m 2 , respectively, representing increases of 59% and 87% compared to those without MWCNTs. The excellent fracture properties are attributed to the moderate interfacial bonding between the fibers and matrix, which favour the toughening mechanisms, such as fiber bridging, fiber pull-out and crack deflection at interfaces.

In order to improve the mechanical properties of CBCFCs, the CBCFCs with in-situ grown SiC nanowires (SiCNW) were fabricated based on the chemical vapor reaction process and precursor impregnation and pyrolysis (PIP) process. The effect of multi-scaled reinforcements (micro-scaled carbon fibers and nano-scaled SiCNW) on the mechanical properties was investigated. The phase, microstructure and fracture surface of the composites were characterized by XRD, SEM and TEM. Microstructure characterization found that the in-situ grown SiCNW randomly distributed in the matrix and formed a network structure in the CBCFCs. The carbon fibers and the SiCNW formed the multi-scaled reinforcements can significantly improve the mechanical properties of CBCFCs. More importantly, the orientation anisotropy on mechanical properties of the composites is also obviously deceased due to the in-situ grown SiCNW. The improved mechanical properties and decreased orientation anisotropy could be attributed to the pull-out and bridging of carbon fibers and SiCNW, which formed multi-scaled reinforcements.

A simple, compact and highly sensitive hydrogen sensing device based on a polymer-filled hollow core fiber coated with Pt-loaded WO3/SiO2is demonstrated. The device is composed of a tiny segment of hollow core fiber (HCF) spliced to a standard single mode fiber (SMF) incorporated with a fiber Bragg grating (FBG). The HCF is filled with polymer and the inner air-gap near the interface between SMF and polymer forms a micro-cavity Fabry–Perot interferometer (FPI). With Pt-loaded WO3/SiO2 coating acting as the catalytic layer, hydrogen undergoes an exothermic reaction with oxygen in air and releases heat when the device is exposed to hydrogen, which induces local temperature change of FPI and hence leads to reflection spectrum shift of the proposed device. Here, the FBG is used to compensate the surrounding temperature effect and eliminate the temperature cross-sensitivity of the device. A wavelength shift of over 35 nm is obtained with a 242-μm-long polymer-filled HCF for 4% (vol%) H2 concentration, and the maximum sensitivity of 17.48 nm/% (vol%) H2 is achieved within the range of 0–4.0% (vol%) H2 in air. The sensor device also exhibits fast response time to hydrogen.

Novel hierarchical SiC–SiO2 heterostructures were synthesized on a graphite paper substrate via a catalyst free method. The morphology, microstructure and composition were characterized by SEM, HR-TEM, EDS and XRD. Results revealed that the hierarchy of SiC–SiO2 heterostructures consisted of nanochain structures and nanoweb structures located in different layers of the graphite paper substrate. The nanochain structures were SiC–SiO2 nanochains coated by an exterior SiO2 shell, in which the SiO2 beads were chained to the SiC nanostructure. The nanoweb structures showed that many intersecting SiC nanostructures were encapsulated in irregular SiO2 agglomerates. The formation of hierarchical SiC–SiO2 heterostructures was dependent on the synthesis temperature. The VS process followed by a modulation process was proposed to elucidate the formation mechanism of such hierarchical SiC–SiO2 heterostructures.

The SiC nanofibres with a length of few millimetres were synthesized successfully on carbon fabrics using a catalyst-free method. The Si and SiO2 powder mixtures functioned as a source for the generation of gaseous species. Following the chemical vapour reaction route, abundant SiC nanofibres with near stoichiometric composition and high crystallinity were grown on the surfaces of carbon fibres. The phase, morphology and crystal structure were characterized to reveal the nucleation and growth mechanism. SiC nanofibres presented a pagoda structure, which is a consecutive stacking of hexagonal segments with a high density of stacking faults. Combining the fabrication process with an analysis of the morphology and crystal structure, both the vapour–solid mechanism and the screw dislocation-driven mechanism are responsible for the nucleation and the growth of SiC nanofibres in the current study. The significant advantage of the present process is that it can be conveniently implemented and provides an approach to allow the fabrication of SiC nanostructures on a variety of carbonaceous substrates without the assistance of a catalyst, leading to less expensive, long length and high purity production of nanostructures.

&alpha, MoO3 has been used as a hydrogen sensing material due to its excellent properties and unique crystalline layer structure. However, the low repeatability of &alpha, MoO3 based hydrogen sensor restricts its practical application. In this paper, the effect of intercalated ion species and the amount in &alpha, MoO3 is experimentally investigated and discussed. It is concluded that the repeatability of the sensor depends on the radius of intercalated ions and amount of ionic bonds. The optimal ion species is Na+ and the optimal amount of precursor is 1 mmol.

The design of intrinsic safe, sensitive and reversible hydrogen sensor is still challenging. Employing new hydrogen sensitive film is an effective way to improve the performance of hydrogen sensor. In this work, a novel method was proposed to improve the sensitivity of Mg-Ti based fiber optic hydrogen sensor by sputtering Mg-Ti-Ni composite films on the end face of the fiber as hydrogen sensitive layer. Performances of two types of sensors were investigated under different hydrogen concentration conditions. The experimental results demonstrate that Mg-Ti-Ni based sensor displays better repeatability and stability than Mg-Ti based sensor, which brings new ideas for continuous monitoring of hydrogen concentration and its application in improving the performance of Mg based hydrogen sensors.

ZrC-based ceramic composites doped with 20 vol.% titanium (Ti) and 3 vol.% carbon nanotubes (CNTs) were prepared by hot pressing. The densification behavior, microstructural evolution and mechanical properties of the composites were studied. The results indicated that the sinterability of the composites was significantly improved, resulting in highly dense matrix (98.27%) at reduced sintering temperature (1600 °C). In addition to the main phase of (Zr, Ti)C, the composites also contained a certain amount of TiC, which was shown to be beneficial for the improvement of mechanical properties. As a result, the flexural strength of the composites was measured to be 415 MPa at room temperature and 647 MPa at 800 °C, respectively. Moreover, the composites showed a high fracture toughness of 5.27 MPa m1/2, which increased by 32% compared with the monolithic ZrC (4 MPa m1/2). The improved fracture toughness was attributed to crack deflection, crack bridging and crack branching. Keywords: ZrC, Ti, CNTs, Microstructure, Mechanical properties

ZrC-based composite doped with 20 vol.% niobium (Nb) and 3 vol.% carbon nanotubes (CNT) was prepared by hot pressing. Due to the addition of Nb and CNT, the sinterability of ZrC was significantly improved, resulting in highly dense matrix (99.28%) at reduced sintering temperature (1600 °C). The flexural strength was 590 MPa at room temperature and 563 MPa at 800 °C, respectively. The fracture toughness was 7.23 MPa·m1/2, which increased by 80% compared with the monolithic ZrC (4 MPa·m1/2). In addition, the work of fracture was calculated to be 267 J/m2. Based on the microstructure characterizations, the improved fracture properties could be attributed to crack deflection, crack bridging and crack branching, which consumed more fracture energy during the fracture process. Keywords: ZrC, Nb, CNT, Microstructure, Mechanical properties

We present an erratum to our Letter [Opt. Lett. 45, 3601 (2020)OPLEDP0146-959210.1364/OL.395341]. Labeling errors in two figures and an incorrect sentence are revised. The corrections have no influence on the conclusions of the original Letter.