Effect of dilute concentrations of Sm on the temperature‐dependent electrical and dielectric properties of ZnO

Compounds of undoped and samarium (Sm) doped ZnO have been prepared by standard solid‐state reaction method. X‐ray diffraction (XRD), Williamson‐Hall (W‐H) analysis, Transmission Electron Microscopy (TEM), temperature‐dependent electrical and dielectric studies have been done to characterize these materials. Inclusion of Sm as dopant in hexagonal wurtzite ZnO changes the lattice parameters to a small extent with some Sm aggregation at higher concentration. Also, the mean particle sizes of ZnO:Sm compounds showed an inter‐correlation with the Scherrer method, W‐H analysis as well as with TEM results. The electrical resistivity depicts an exponential decay and metal‐semiconductor transition (MST) at ~300 K for the pristine sample whereas there is large decrement in the resistivity with Sm doping. The analysis of σ_ac of ZnO suggests that the power law is obeyed and indicated an increase in the ac conductivity with Sm content. The mechanism behind this type of conductivity is elucidated by small polaron tunneling (SPT) model of conductivity. The dependence of ln dc on the temperature inverse shows that the traps of electrons are thermally activated such that low and high temperature activation energies confirm the presence of vacancies and interstitials of both O and Zn ions. Thus, a high value of dielectric constant makes these materials suitable for high frequency and charge storage device applications.     

In-situ growth of bamboo-shaped carbon nanotubes and helical carbon nanofibers on Ti3C2Tx MXene at ultra-low temperature for enhanced electromagnetic wave absorption properties

The potential of two-dimensional layered MXenes in electromagnetic wave (EMW) absorption needs further development. Herein, we carried out the in situ growth of carbon nanotubes (CNTs) on the surface of Ti₃C₂T_x MXene at ultra-low temperature via chemical vapor deposition. The obtained CNTs exhibited a bamboo-like structure and were accompanied by helical carbon nanofibers. The ultra-low temperature solved the problem that the high temperature required in the traditional CNT growth process would destroy the structural integrity of MXene. The lush CNT forest cross-linked the MXene layers, transforming the two-dimensional layered structure into a three-dimensional conductive network, providing abundant conductive channels for carriers, optimizing the impedance matching of the CNT/MXene hybrid, and resulting in a significant dielectric loss. The as-prepared CNT/MXene hybrid exhibited a minimal reflection loss of ?52.56 dB (99.9994% EMW absorption) in the X-band. This work proposes a new idea to enhance the EMW absorption properties of Ti₃C₂T_x MXene and fabricate high-performance MXene-based EMW absorbers.     

Fabrication of Ni/ZnO/C hollow microspheres decorated graphene composites towards high-efficiency electromagnetic wave absorption in the Ku-band

Developing light-weight, thin thickness and high-efficiency electromagnetic wave (EMW) absorbers is an effective strategy for dealing with the increasingly serious problem of electromagnetic radiation pollution. Herein, nickel/zinc oxide/carbon (Ni/ZnO/C) hollow microspheres decorated graphene composites were facilely prepared through the high-temperature pyrolysis of bimetallic NiZn metal-organic frameworks (MOFs) precursors. Morphological characterization results manifested that the Ni/ZnO/C microspheres with unique hollow structure were almost evenly anchored on the wrinkled surfaces of flake-like graphene. Moreover, the influences of additive amounts of graphene oxide (GO) in the MOFs precursors on the crystal structure, graphitization degree, micromorphology, magnetic properties, electromagnetic parameters and EMW absorption performance were investigated in detail. It was found that the superior EMW absorption performance could be achieved through facilely adjusting the additive amounts of GO in the precursors. As the additive amount of GO was equal to 60 mg, the obtained composite showed the comprehensive excellent EMW absorption performance. Notably, the optimal minimum reflection loss reached ?57.5 dB at 16.5 GHz in the Ku-band under an ultrathin matching thickness of merely 1.34 mm, and the broadest effective absorption bandwidth achieved 5.6 GHz (from 12.4 to 18 GHz) when the thickness was 1.5 mm. Furthermore, the underlying EMW absorption mechanisms of as-prepared composites were revealed. It was believed that our results could be valuable for the structural design and EMW absorption performance modulation for MOFs derived magnetic carbon composites.     

Au/ZnO and Au/Fe2O3 catalysts for CO oxidation at ambient temperature: comments on the effect of synthesis conditions on the preparation of high activity catalysts prepared by coprecipitation

The preparation of Au/ZnO and Au/Fe₂O₃ catalysts using two coprecipitation methods is investigated to determine the important factors that control the synthesis of high activity catalysts for the oxidation of carbon monoxide at ambient temperature. In particular, the factors involved in the preparation of catalysts that are active without the need for a calcination step are evaluated. The two preparation methods differ in the manner in which the pH is controlled during the precipitation, either constant pH throughout or variable pH in which the pH is raised from an initial low value to a defined end point. Non-calcined Au/ZnO catalysts prepared using both methods are very sensitive to pH and ageing time, and catalysts prepared at a maximum pH = 5 with a short ageing time (ca. 0–3 h) exhibit high activity. Catalysts prepared at higher pH give lower activity. However, all catalysts require a short operation period during which the oxidation activity increases. In contrast, the calcined catalysts are not particularly sensitive to the preparation conditions. Non-calcined Au/Fe₂O₃ catalysts exhibit high activity when prepared at pH ≥ 5. Calcined Au/Fe₂O₃ prepared using the controlled pH method retain high activity, whereas calcined catalysts prepared using the variable pH method are inactive. The study shows the immense sensitivity of the catalyst performance to the preparation methods. It is therefore not surprising that marked differences in the performance of supported Au catalysts for CO oxidation that are apparent in the extensive literature on this subject, particularly the effect of calcination, can be expected if the preparation parameters are not carefully controlled and reported.     

Wear behavior of SiC nanowire-reinforced SiC coating for C/C composites at elevated temperatures

To improve the wear resistance of SiC coating on carbon/carbon (C/C) composites, SiC nanowires (SiCNWs) were introduced into the SiC wear resistant coating. The dense SiC nanowire-reinforced SiC coating (SiCNW-SiC coating) was prepared on C/C composites using a two-step method consisting of chemical vapor deposition and pack cementation. The incorporation of SiCNWs improved the fracture toughness of SiC coating, which is an advantage in wear resistance. Wear behavior of the as-prepared coatings was investigated at elevated temperatures. The results show that the wear resistance of SiCNW-SiC coating was improved significantly by introducing SiC nanowires. It is worth noting that the wear rate of SiCNW-SiC coating was an order of magnitude lower than that of the SiC coating without SiCNWs at 800 °C. The wear mechanisms of SiCNW-SiC coating at 800 °C were abrasive wear and delamination. Pullout and breakage of SiC grains resulted in failure of SiC coating without SiCNWs at 800 °C.     

Interface adhesion properties of syndiotactic polypropylene/cellulose group composite: Relationship between chemical structure of coupling agent and reactivity for cellulose group

Reinforcement of interfacial adhesion between syndiotactic polypropylene (SPP) and fibrous pure cellulose (FC) was performed by the FC silanizations with 3‐aminopropyltrimethoxysilane (APTMS), with hexyltrimethoxysilane (HTMS) and by an addition of a SPP grafted with dimethyl itaconate (SPP‐g‐DMI: number‐average molecular weight = 3.3 × 10⁴, DMI cont. = 0.11 wt %), respectively. The adhesion and the morphology were improved by them, respectively. However, their behavior was considerably different. The hexyl group in the HTMS converted the FC surface into hydrophobicity, leading to the hindrance of the interhydrogen bonding in the FC. The silanized FC was embrittled with the increase of the degree of the silanization. In the case of the APTMS, the inter hydrogen bonding was kept because of the existence of the amino group. Whereas, the SPP‐g‐DMI was unable to penetrate into the cellulose because of its higher molecules, and the interhydrogen bonding was not hindered. The keeping the inter hydrogen bonding was important for the reinforcement of the tensile properties such as Young's modulus and tensile strength in the composite. Furthermore, the interfacial adhesion between the SPP and holocellulose (HC) was studied as well as the SPP/FC. Although the tensile properties were slightly improved by the APTMS silanization and by the addition of the SPP‐g‐DMI, respectively, the reactivity of the HC was much less than that of the FC. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Fatigue of a SiC/SiC ceramic composite with an ytterbium-disilicate environmental barrier coating at elevated temperature*

Tension-tension fatigue performance of a SiC/SiC composite with an ytterbium-disilicate environmental barrier coating (EBC) was investigated at 1200°C in air and steam. The composite is reinforced with Hi-Nicalon™ SiC fibers and has a melt-infiltrated matrix processed by chemical vapor infiltration of SiC with subsequent infiltration with SiC particulate slurry and molten silicon. The EBC consists of a Si bond coat and an Yb₂Si₂O₇ top coat applied via air plasma spraying. Tensile properties were evaluated at 1200°C. Tension-tension fatigue was examined for maximum stresses of 110-140 MPa. To assess the efficacy of EBC, experimental results obtained for the coated composite are compared to those for a control uncoated composite. Surface grit-blasting inherent in the EBC application process degrades tensile strength of the composite. However, the EBC effectively protects the composite from oxidation embrittlement during cyclic loading in air or steam. Fatigue runout set to 200 000 cycles (55.6 hours at a frequency of 1.0 Hz) was achieved at 110 MPa in air and steam. Retained properties of pre-fatigued specimens were characterized. Composite microstructure, along with damage and failure mechanisms were investigated. Damage and failure of the composite are attributed to the growth of cracks originating from numerous processing defects in the composite interior.     

Torsion and flexural-torsional coupled mechanical properties of different C/SiC torque tubes at room and elevated temperatures

In this paper, the torsion and flexural-torsional coupled mechanical properties of different C/SiC torque tubes were investigated for the testing condition at room and elevated temperatures. Effects of fiber types, fiber preforms, and small hole during fabrication process on torsion mechanical properties were investigated. Flexural-torsional coupled mechanical tests for C/SiC torque tubes with different external diameter and wall thickness were conducted at room and elevated temperatures. The torsion and flexural moments and corresponding shear and flexural strength were obtained. The fracture surface and cracks propagation path were observed and analyzed. The torque and shear strength in T300™-3k torque tube were much higher than those of T300™-1k torque tube. Among 3D needled (3DN), 2D plain-woven [0°/90°] and [±45°] C/SiC torque tubes, the density, torque, and shear strength of 3DN-C/SiC torque tube were the highest. For the C/SiC torque tubes with small hole, the small hole not only increased the densification and uniformity (axial and radial) of the torque tube, but also has the potential to make the damage cracks more zigzag, which improved the fracture toughness of the torque tubes.     

Using the TPS method for determining the thermal properties of concrete and wood at elevated temperature

The transient plane source (TPS) method is shown to be very promising for determining thermal properties of materials at room temperature as well as temperatures up to 700°C. To investigate the applicability of the method it has been used in the study for determining thermal properties of wood (spruce) and concrete. Conductivity (λ) and diffusivity (α) were determined simultaneously. The thermal properties thus obtained have been compared with some values found in literature. The paper also presents results where calculations using properties obtained with the TPS method are compared with fire test measurements. The results are very encouraging. Copyright © 2005 John Wiley & Sons, Ltd.     

Effect of MnO2 additive on the dielectric and electromagnetic interference shielding properties of sintered cement-based ceramics

Cement-based ceramic pellets were prepared and their properties were studied for electromagnetic interference (EMI) shielding applications. The shielding materials were made of Portland cement with the addition of different concentrations of manganese oxide (MnO₂) up to 10 wt%. The pellets were sintered at 850 °C for 5 h and then polished prior to characterizations of density, porosity, microstructures, dielectric properties, and EMI shielding effectiveness (SE). Results show that the MnO₂-cement pellets have good dielectric properties, i.e. high dielectric constant (∼300) and low dielectric loss (<0.3). The dielectric constant increased with increasing MnO₂ content in the cement matrix. The SE values of the MnO₂-cements fluctuated between 2 dB and 9 dB in the frequency range of 8-13 GHz. The sample with 10 wt% MnO₂ additive had SE values of up to 9 dB. Most of the samples with high additive concentrations produced SE exceeding 7 dB.     

Crystal structure,microwave dielectric properties and low temperature sintering of (Al0.5Nb0.5)4+ co-substitution for Ti4+ of LiNb0.6Ti0.5O3 ceramics

The phase composition, microstructure, microwave dielectric properties of (Al_0.5Nb_0.5)⁴⁺ co-substitution for Ti site in LiNb_0.6Ti_0.5O₃ ceramics and the low temperature sintering behaviors of Li₂O-B₂O₃-SiO₂ (LBS) glass were systematically discussed. XRD patterns and EDS analysis result confirmed that single phase of Li_1.075Nb_0.625Ti_0.45O₃ solid solution was formed in all component. The increase of dielectric constant (ε_r) is ascribed to the improvement of bulk density. The restricted growth of grain has a negative influence on quality factor (Q×f) value. The τ_f value could be continuously shifted to near zero as the doping content increases. Great microwave dielectric properties were obtained in LiNb_0.6Ti_(0.5-x)(Al_0.5Nb_0.5)_xO₃ ceramics (x?=?0.10) when sintered at 1100?℃ for 2?h: ε_r =?70.34, Q×f =?5144?GHz, τ_f =?4.8?ppm/℃. The sintering aid, LBS glass, can effectively reduce the temperature and remain satisfied microwave performance. Excellent microwave dielectric properties for x?=?0.10 were obtained with 1.0?wt% glass: ε_r =?70.16, Q×f =?4153?GHz (at 4?GHz), τ_f =?-0.65?ppm/℃ when sintered at 925?℃ for 2?h.     

The effect of a p‐phenylen‐bis‐maleamic acid grafted atactic polypropylene interface agent on the impact properties of iPP/mica composites

The present article is twofold. One of the purposes of this work lies in the study of the impact behavior of the polypropylene/mica system incorporating an interface modifier obtained from an industrial polymerization byproduct. The interface agent used was a p‐phenylen‐bis‐maleamic grafted atactic polypropylene (aPP‐pPBMA) obtained in our labs. The other objective is to obtain a mathematical model capable of forecasting the composite properties accurately. Consequently, this work has been undertaken by using a statistical Box‐Wilson experimental design in order to model the behavior of the composite system in terms of Charpy impact parameters. Two independent variables have been considered, the amount of mica particles and of interface agent. Impact strength, maximum force, and deformation were the dependent variables in the models. The existence of critical values in mica and interface agent optimizing the impact behavior is established. Additionally, an excellent correlation between the impact strength and the strength results coming from flexural and tensile tests is found. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44619.     

Enhanced dielectric properties of poly(vinylidene fluoride)–surface functionalized BiFeO3 composites using sodium dodecyl sulfate as a modulating agent for device applications

In this work, we prepared a series of poly(vinylidene fluoride) (PVDF)–surface functionalized BiFeO₃ (h‐BFO)–Sodium dodecyl sulfate (SDS) composite films by solvent casting method to investigate the effect of SDS in the composites. The X‐ray diffraction confirmed that the structure of h‐BFO significantly changed in the PVDF‐(h‐BFO)‐SDS composite in comparison with the rhombohedral structure of pure BiFeO₃. The microscopic study illustrated that the composite with a higher percentage of SDS content facilitated the dispersion as well as proper distribution of ceramic particles in the polymer matrix. The presence of different functionalities of respective polymer and the modified fillers was confirmed by FTIR Spectrophotometer. The dielectric and electrical study done by Impedance Analyzer revealed that the SDS treated surface functionalized composites showed relatively higher dielectric properties than that of two phase composites and pure polymer. Finally, the ferroelectric properties of the composite films done by P‐E loop tracer revealed that the SDS‐treated composites showed an enhanced remanent polarization. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45040.     

Synthesis of CoNi/SiO2 core-shell nanoparticles decorated reduced graphene oxide nanosheets for enhanced electromagnetic wave absorption properties

In this research, the nanocomposites, CoNi/SiO₂ core-shell nanoparticles decorated reduced graphene oxide (RGO) nanosheets, are successfully synthesized via liquid-phase reduction reactions combined with a sol-gel route. The structures, morphologies, chemical composition and magnetic properties of CoNi nanoparticles, CoNi/SiO₂ core-shell nanoparticles and RGO/CoNi/SiO₂ nanocomposites have been investigated in exhaustive detail. The electromagnetic (EM) parameters of RGO/CoNi/SiO₂ nanocomposites are measured using a vector network analyzer. The results reveal that the RGO/CoNi/SiO₂ nanocomposites display enhanced EM wave absorption properties with the maximum reflection loss (R_L) of ??46.3?dB at 6.2?GHz with a matching thickness of 4.2?mm. Additionally, the absorption bandwidth corresponding to the R_L less than ??10?dB is up to 14.3?GHz (3.7–18.0?GHz) with a matching thickness range of 2.0–5.0?mm. To comprehensively consider the absorption bandwidth and the maximum R_L, the integrational method which defines ΔS as the integration area of R_L (R_L < ??10?dB) and R_E as EM wave absorption efficiency is adopted to reveal that the RGO/CoNi/SiO₂ nanocomposites exhibit the excellent absorption properties with the matching thickness of only 2.0?mm. Accordingly, the as-prepared RGO/CoNi/SiO₂ nanocomposites could be applied as promising EM wave absorption materials.     

Effect of a coupling agent on the electromagnetic and mechanical properties of carbon black/acrylonitrile–butadiene–styrene composites

Titanate coupling agent (TCA) is widely used as a plasticizer in filled polymer processes. In this study, the effect of TCA with different contents (2 and 10 wt %) on the electrical conductivity, wave absorption, and mechanical properties of carbon black (CB)/acrylonitrile–butadiene–styrene (ABS) composites were investigated. The results indicate that with the addition of 2 wt % TCA to the filled CB, the electrical conductivity of CB/ABS composites improved greatly, but its wave absorption performance was weakened. In contrast, the addition of 10 wt % TCA to the filled CB improved the microwave absorption performance of the CB/ABS composites but led to poor electrical conductivity. However, TCA, regardless of the contents of 2 or 10 wt %, greatly improved the mechanical properties of the composites. The probable reasons for this are discussed on the basis of the fracture morphology of the sample, a chemical band between the filling and resin, and the physical coating between the filling and TCA. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1839–1843, 2006     

In situ synthesis,structure, and properties of a dendritic branched nano-thickening agent for high temperature fracturing fluid

The aim of this work was to prepare a novel dendritic branched nano-thickening agent by free radical polymerization of acrylamide (AM), acrylic acid (AA), sodium p-styrene sulfonate, dimethyl diallyl ammonium chloride, and multiwalled carbon nanotubes. The as-synthesized nano-thickening agent was characterized by Fourier transform infrared, Raman spectra, transmission electron microscope, ¹H-NMR, and thermogravimetric analysis (TGA). Compared with the pristine polymer and partially hydrolyzed polyacrylamide (HPAM), thickening capacity, temperature resistance, and salt tolerance properties of the nano-thickening agent considerably improved, and the viscosity of 0.5% nano-polymer solution was 126.5 mPa·s. Additionally, the properties of the nano-gel prepared by nano-thickener, such as temperature and shear tolerance, viscoelasticity, sand carrying capacity, and gel breaking performance, were evaluated showing the satisfactory performance of the nano-gel under high temperature condition. The results indicated that the nano-thickener has potential applications in the field of oil and gas production. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48446.     

Effects of prior aging at 288°C in argon environment on time‐dependent deformation behavior of a thermoset polymer at elevated temperature,part 1: Experiments

The inelastic deformation behavior of PMR‐15 neat resin, a high‐temperature thermoset polymer, aged at 288°C in argon environment for up to 2000 h was investigated. The experimental program was designed to explore the in?uence of prior isothermal aging on monotonic loading and unloading at various strain rates. In addition, the relaxation response and the creep behavior of specimens subjected to prior aging of various durations were evaluated. All tests were performed at 288°C. The time‐dependent mechanical behavior of the PMR‐15 polymer is strongly influenced by prior isothermal aging. The elastic modulus increased and the departure from quasi‐linear behavior was delayed with prior aging time. Stress levels in the region of inelastic flow increased with prior aging time. Furthermore, prior aging significantly decreased the polymer's capacity for inelastic straining, including the material's capacity to accumulate creep strain. Conversely, the relaxation response was not affected by the prior aging. © 2009 Wiley Periodicals, Inc.? J Appl Polym Sci, 2009     

In situ electrochemical oxidation of ethylenediamine on Co–B alloy electrode during cycling for improving its electrochemical properties at elevated temperature

A Co–B alloy/ethylenediamine (EDA) hybrid electrode system has been developed by adding EDA into electrolyte. Charge–discharge measurements show that the inorganic–organic hybrid electrode system exhibits high discharge capacity and long cycle life at elevated temperature (55 °C). Specifically, in the electrolyte containing 0.09 M EDA additive, the discharge capacity of Co–B alloy electrode after 100 cycles is still up to 601.7 m Ah g⁻¹ at 55 °C. However, for the electrode in EDA-free electrolyte, its discharge capacity is sharply decreased to only 138.5 m Ah g⁻¹ after 100 cycles. ICP-OES and IR measurements are used to clarify the reason of the improvement in the electrochemical properties. These results show that beneficial effect of EDA on electrochemical properties of Co–B alloy electrode can be attributed to in situ electrochemical oxidation of EDA during discharge cycles. First the oxidation of EDA contributes to part of the discharge capacity and secondly the oxidation products absorbed on the electrode may help to suppress the dissolution of Co at elevated temperature. Evidence is presented suggesting that the oxidation of Co can induce co-oxidation of EDA.     

Effects of prior aging at 288°C in argon environment on time‐dependent deformation behavior of a thermoset polymer at elevated temperature,Part 2: Modeling with viscoplasticity theory based on overstress

The viscoplasticity based on overstress (VBO) is augmented to model the effects of prior isothermal aging in an argon environment on the inelastic deformation behavior of PMR‐15 neat resin, a high‐temperature thermoset polymer. VBO is a unified state variable theory with growth laws for three state variables: the equilibrium stress, the kinematic stress and the isotropic stress. A systematic model characterization procedure based on a limited number of well defined experiments is employed to determine the VBO parameters. Experimental findings presented in Part I reveal the equilibrium stress and the kinematic stress to be affected by prior aging. Based on the experimental results, the isotropic stress is developed as a function of prior aging time. In addition, several VBO model parameters are made dependent on prior aging time. Comparison with experimental data demonstrates that the modified VBO successfully predicts the inelastic deformation behavior of the PMR‐15 polymer subjected to prior isothermal aging for up to 2000 h. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009