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3Y-TZP/3wt% Al2O3 powder was coated with varying amounts of BN using the urea and borate reaction sintering method, and then multiphase ceramics were prepared by hot pressing sintering. The micro-topography and the compositional analysis of synthesized ceramics were conducted through scanning electron microscopy, transmission electron microscopy and X-ray diffraction. A mechanical tester was used to analyze the Vickers hardness, fracture toughness, and bending strength of the synthesized ceramics. The results showed that the ceramic with a BN content of 12wt% showed the best processability, but had diminished mechanical properties (such as fracture toughness and bending strength). The ceramic with a BN content of 9wt% showed better processability than those with 3wt% and 6wt% BN. However, the fracture toughness was affected by the addition of 9wt% BN, making this ceramic only usable as a base material for a three-unit fixed bridge. In contrast, the ceramics with a BN content of 3wt% or 6wt% fulfilled the criteria for use in multi-unit restoration, but their low processability made them unsuitable for milling after sintering.
The evolution behavior of the γ″ phase of IN718 superalloy in a temperature/stress coupled field was investigated. Results showed that the coarsening rate of the γ′′ phase was significantly accelerated in the temperature/stress coupled field. Based on the detail microstructural and crystal defect analysis, it was found that the coarsening rate of the γ′′ phase with applied stress was significantly higher than that without stress. The main reasons for the increase in the coarsening rate of the γ′′ phase are as follows: the vacancy formation energy is decreased by the applied stress, which leads to an increase in the vacancy concentration; in the temperature/stress coupled field, the Nb atoms easily combine with vacancies to form complexes and diffuse with the complexes, resulting in a significant increase in the Nb atom diffusion coefficient; Nb atom diffusion is the key control factor for the coarsening of the γ′′ phase.
The optimized growth parameters of graphene with different morphologies, such as dendrites, rectangle, and hexagon, have been obtained by low-pressure chemical vapor deposition on polycrystalline copper substrates. The evolution of fractal graphene, which grew on the polycrystalline copper substrate, has also been observed. When the equilibrium growth state of graphene is disrupted, its intrinsic hexagonal symmetry structure will change into a non-hexagonal symmetry structure. Then, we present a systematic and comprehensive study of the evolution of graphene with different morphologies grown on solid copper as a function of the volume ratio of methane to hydrogen in a controllable manner. Moreover, the phenomena of stitching snow-like graphene together and stacking graphene with different angles was also observed.
Radioluminescence (RL) behaviour of erbium-doped yttria nanoparticles (Y2O3:Er3+ NPs) which were produced by sol–gel method was reported for future scintillator applications. NPs with dopant rates of 1at%, 5at%, 10at% and 20at% Er were produced and calcined at 800°C, and effect of increased calcination temperature (1100°C) on the RL behaviour was also reported. X-ray diffraction (XRD) results showed that all phosphors had the cubic Y2O3 bixbyite-type structure. The lattice parameters, crystallite sizes (CS), and lattice strain values were calculated by Cohen-Wagner (C-W) and Williamson-Hall (W-H) methods, respectively. Additionally, the optimum solubility value of the Er3+ dopant ion in the Y2O3 host lattice was calculated to be approximately 4at% according to Vegard’s law, which was experimentally obtained from the 5at% Er3+ ion containing solution. Both peak shifts in XRD patterns and X-ray photoelectron spectroscopy (XPS) analyses confirmed that Er3+ dopant ions were successfully incorporated into the Y2O3 host structure. High-resolution transmission electron microscopy (HRTEM) results verified the average CS values and agglomerated NPs morphologies were revealed. Scanning electron microscopy (SEM) results showed the neck formation between the particles due to increased calcination temperature. As a result of the RL measurements under a Cu Kα X-ray radiation (wavelength, λ = 0.154 nm) source with 50 kV and 10 mA beam current, it was determined that the highest RL emission belonged to 5at% Er doped sample. In the RL emission spectrum, the emission peaks were observed in the wavelength ranges of 510–575 nm (2H11/2, 4S3/2–4I15/2; green emission) and 645–690 nm (4F9/2–4I15/2; red emission). The emission peaks at 581, 583, 587, 593, 601, 611 and 632 nm wavelengths were also detected. It was found that both dopant rate and calcination temperature affected the RL emission intensity. The color shifted from red to green with increasing calcination temperature which was attributed to the increased crystallinity and reduced crystal defects.
Graphene oxide (GO) wrapped Fe3O4 nanoparticles (NPs) were prepared by coating the Fe3O4 NPs with a SiO2 layer, and then modifying by amino groups, which interact with the GO nanosheets to form covalent bonding. The SiO2 coating layer plays a key role in integrating the magnetic nanoparticles with the GO nanosheets. The effect of the amount of SiO2 on the morphology, structure, adsorption, and regenerability of the composites was studied in detail. An appropriate SiO2 layer can effectively induce the GO nanosheets to completely wrap the Fe3O4 NPs, forming a core-shell Fe3O4@SiO2@GO composite where Fe3O4@SiO2 NPs are firmly encapsulated by GO nanosheets. The optimized Fe3O4@SiO2@GO sample exhibits a high saturated adsorption capacity of 253 mg·g−1 Pb(II) cations from wastewater, and the adsorption process is well fitted by Langmuir adsorption model. Notably, the composite displays excellent regeneration, maintaining a ~90% adsorption capacity for five cycles, while other samples decrease their adsorption capacity rapidly. This work provides a theoretical guidance to improve the regeneration of the GO-based adsorbents.
Selective laser melting (SLM) technology plays an important role in the preparation of porous titanium (Ti) implants with complex structures and precise sizes. Unfortunately, the processing characteristics of this technology, which include rapid melting and solidification, lead to products with high residual stress. Herein, an in situ method was developed to restrain the residual stress and improve the mechanical strength of porous Ti alloys during laser additive manufacturing. In brief, porous Ti6Al4V was prepared by an SLM three-dimensional (3D) printer equipped with a double laser system that could rescan each layer immediately after solidification of the molten powder, thus reducing the temperature gradient and avoiding rapid melting and cooling. Results indicated that double scanning can provide stronger bonding conditions for the honeycomb structure and improve the yield strength and elastic modulus of the alloy. Rescanning with an energy density of 75% resulted in 33.5%–38.0% reductions in residual stress. The porosities of double-scanned specimens were 2%–4% lower than those of single-scanned specimens, and the differences noted increased with increasing sheet thickness. The rescanning laser power should be reduced during the preparation of porous Ti with thick cell walls to ensure dimensional accuracy.
Reaction-bonded B4C–SiC composites are highly promising materials for numerous advanced technological applications. However, their microstructure evolution mechanism remains unclear. Herein, B4C–SiC composites were fabricated through the Si-melt infiltration process. The influences of the sintering time and the B4C content on the mechanical properties, microstructure, and phase evolution were investigated. X-ray diffraction results showed the presence of SiC, boron silicon, boron silicon carbide, and boron carbide. Scanning electron microscopy results showed that with the increase in the boron carbide content, the Si content decreased and the unreacted B4C amount increased when the sintering temperature reached 1650°C and the sintering time reached 1 h. The unreacted B4C diminished with increasing sintering time and temperature when B4C content was lower than 35wt%. Further microstructure analysis showed a transition area between B4C and Si, with the C content marginally higher than in the Si area. This indicates that after the silicon infiltration, the diffusion mechanism was the primary sintering mechanism of the composites. As the diffusion process progressed, the hardness increased. The maximum values of the Vickers hardness, flexural strength, and fracture toughness of the reaction-bonded B4C–SiC ceramic composite with 12wt% B4C content sintered at 1600°C for 0.5 h were about HV 2400, 330 MPa, and 5.2 MPa·m0.5, respectively.
To investigate the oxidation behavior of a nickel-based superalloy with high hafnium content (1.34wt%), this study performed isothermal oxidation tests at 900, 1000, and 1100°C for up to 200 h. X-ray diffraction and scanning electron microscopy with energy-dispersive X-ray spectroscopy were applied to study the oxidation behavior. The weight gain of the high Hf nickel-based superalloy exhibited a parabola-like curve, and no spallation of the oxide scale was observed during the oxidation tests. The alloy presented excellent oxidation resistance, and no HfO2 was observed in the oxide scale at 900°C. With the increase of the oxidation temperature to 1000°C, HfO2 particles formed in the spinel phases of the scale, and “peg-like” HfO2 was observed within and beneath the inner layer of Al2O3 after 200 h. As the oxidation temperature rose to 1100°C, “peg-like” HfO2 was observed at the early stage of the oxidation test (within 25 h). The formation mechanism of HfO2 and its impact on oxidation resistance were investigated based on the analysis of the oxidation test results at different temperatures.
Wood-based panels containing urea-formaldehyde resin result in the long-term release of formaldehyde and threaten human health. In this study, inorganic aluminosilicate coatings prepared by combining metakaolin, silica fume, NaOH, and H2O were applied to the surfaces of wood-based panels to obstruct formaldehyde release. The Si/Al, Na/Al, and H2O/Na2O molar ratios of the coatings were regulated to investigate their effects on the structure and formaldehyde-resistant barrier properties of coatings. Results showed that the cracks in the coatings gradually disappeared and the formaldehyde resistance rates of the barrier increased as the Si/Al molar ratio was increased from 1.6 to 2.2. This value also increased as the Na/Al molar ratio was increased from 0.9 to 1.2 because of the improvement of the degree of polymerization. As the H2O/Na2O molar ratio was increased from 12 to 15, the thickness of the dry film decreased gradually and led to the reduction in the formaldehyde resistance rate. When the Si/Al, Na/Al, and H2O/Na2O molar ratios were 2.2, 1.2, and 12, respectively, the inorganic aluminosilicate coating showed good performance as a formaldehyde-resistant barrier and its formaldehyde resistance rate could reach up to 83.2%.
The aim of this investigation was to prepare geopolymeric precursor from vanadium tailing (VT) by thermal activation and modification. For activation, a homogeneous blend of VT and sodium hydroxide was calcinated at an elevated temperature and then modified with metakaolin to produce a geopolymeric precursor. During the thermal activation, the VT was corroded by sodium hydroxide and then sodium silicate formed on the particle surfaces. After water was added, the sodium silicate coating dissolved to release silicon species, which created an alkaline solution environment. The metakaolin then dissolved in the alkaline environment to generate aluminum species, which was followed by geopolymerization. The VT particles were connected by a gel produced during geopolymerization, which yielded a geopolymer with excellent mechanical performance. This investigation not only improves the feasibility of using geopolymer technology for large-scale and in-situ applications, but also promotes the utilization of VT and other silica-rich solid wastes.
Functionalized ionic liquids (FILs) as extractants were employed for the separation of tungsten and molybdenum from a sulfate solution for the first time. The effects of initial pH, extractant concentration, metal concentrations in the feed were comprehensively investigated. The results showed that tricaprylmethylammonium bis(2,4,4-trimethylpentyl)phosphinate ([A336][Cyanex272]) could selectively extract W over Mo at an initial pH value of 5.5; the best separation factor βW/Mo of 25.61 was obtained for a solution with low metal concentrations (WO3: 2.49 g/L, Mo: 1.04 g/L). The [A336][Cyanex272] system performed effectively for solutions of different W/Mo molar ratios and different metal ion concentrations in the feed. The chemical reaction between [A336][Cyanex272] and W followed the ion association mechanism, which was further proved by the Fourier-transform infrared (FTIR) spectra of loaded [A336][Cyanex272] and the free extractant. The stripping experiments indicated that 95.48% W and 100.00% Mo were stripped using a 0.20 mol/L sodium hydroxide solution. Finally, the selective extractions of W and Mo from two synthetic solutions of different high metal concentrations were obtained; the separation factor βW/Mo reached 23.24 and 17.59 for the first and second solutions, respectively. The results suggest the feasibility of [A336][Cyanex272] as an extractant for the separation of tungsten and molybdenum.
Hot compression tests were performed on AISI 321 austenitic stainless steel in the deformation temperature range of 800–1200°C and constant strain rates of 0.001, 0.01, 0.1, and 1 s−1. Hot flow curves were used to determine the strain hardening exponent and the strain rate sensitivity exponent, and to construct the processing maps. Variations of the strain hardening exponent with strain were used to predict the microstructural evolutions during the hot deformation. Four variations were distinguished reflecting the different microstructural changes. Based on the analysis of the strain hardening exponent versus strain curves, the microstructural evolutions were dynamic recovery, single and multiple peak dynamic recrystallization, and interactions between dynamic recrystallization and precipitation. The strain rate sensitivity variations at an applied strain of 0.8 and strain rate of 0.1 s−1 were compared with the microstructural evolutions. The results demonstrate the existence of a reliable correlation between the strain rate sensitivity values and evolved microstructures. Additionally, the power dissipation map at the applied strain of 0.8 was compared with the resultant microstructures at predetermined deformation conditions. The microstructural evolutions strongly correlated to the power dissipation ratio, and dynamic recrystallization occurred completely at lower power dissipation ratios.
In this work, sodium dicyanamide (SD) was used as a leaching reagent for gold recovery, and the effects of the SD dosage and solution pH on the gold-leaching performance were investigated. A gold recovery of 34.8% was obtained when SD was used as the sole leaching reagent at a dosage of 15 kg/t. In the presence of a certain amount of potassium ferrocyanide (PF) in the SD solution, the gold recovery was found to increase from 34.8% to 57.08%. Using the quartz crystal microbalance with dissipation (QCM-D) technique, the leaching kinetics of SD with and without PF were studied. The QCM-D results indicate that the gold-leaching rate increased from 4.03 to 39.99 ng·cm–2·min–1 when the SD concentration was increased from 0 to 0.17 mol/L, and increased from 39.99 to 272.62 ng·cm–2·min–1 when 0.1 mol/L of PF was used in combination with SD. The pregnant solution in the leaching tests was characterized by X-ray photoelectron spectroscopy and electrospray mass spectrometry, which indicated that Au and (N(CN)2)– in the SD solution formed a series of metal complex ions, [AuNax(N(CN)2)x+2]– (x = 1, 2, 3, or 4).
An explosion-welded technology was induced to manufacture the GH3535/316H bimetallic plates to provide a more cost-effective structural material for ultrahigh temperature, molten salt thermal storage systems. The microstructure of the bonding interfaces were extensively investigated by scanning electron microscopy, energy dispersive spectrometry, and an electron probe microanalyzer. The bonding interface possessed a periodic, wavy morphology and was adorned by peninsula- or island-like transition zones. At higher magnification, a matrix recrystallization region, fine grain region, columnar grain region, equiaxed grain region, and shrinkage porosity were observed in the transition zones and surrounding area. Electron backscattered diffraction demonstrated that the strain in the recrystallization region of the GH3535 matrix and transition zone was less than the substrate. Strain concentration occurred at the interface and the solidification defects in the transition zone. The dislocation substructure in 316H near the interface was characterized by electron channeling contrast imaging. A dislocation network was formed in the grains of 316H. The microhardness decreased as the distance from the welding interface increased and the lowest hardness was inside the transition zone.
The microstructural evolution and performance of diamond/Al composites during thermal cycling has rarely been investigated. In the present work, the thermal stability of diamond/Al composites during thermal cycling for up to 200 cycles was explored. Specifically, the thermal conductivity (λ) of the composites was measured and scanning electron microscopy of specific areas in the same samples was carried out to achieve quasi-in situ observations. The interface between the (100) plane of diamond and the Al matrix was well bonded with a zigzag morphology and abundant needle-like Al4C3 phases. By contrast, the interface between the (111) plane of diamond and the Al matrix showed weak bonding and debonded during thermal cycling. The debonding length increased rapidly over the first 100 thermal cycles and then increased slowly in the following 100 cycles. The λ of the diamond/Al composites decreased abruptly over the initial 20 cycles, increased afterward, and then decreased monotonously once more with increasing number of thermal cycles. Decreases in the λ of the Al matrix and the corresponding stress concentration at the diamond/Al interface caused by thermal mismatch, rather than interfacial debonding, may be the main factors influencing the decrease in λ of the diamond/Al composites, especially in the initial stages of thermal cycling.
Composite electrodes prepared by cation exchange resins and activated carbon (AC) were used to adsorb V(IV) in capacitive deionization (CDI). The electrode made of middle resin size (D860/AC M) had the largest specific surface area and mesoporous content than two other composite electrodes. Electrochemical analysis showed that D860/AC M presents higher specific capacitance and electrical double layer capacitor than the others, and significantly lower internal diffusion impedance. Thus, the highest adsorption capacity and rate of V(IV) are inhibited in the three electrodes. The intra-particle diffusion model fits well in the initial adsorption stage, while the liquid film diffusion model is more suitable for fitting at the later stage. The pseudo-second-order kinetic model is fit for the entire adsorption process. The adsorption of V(IV) on the composite electrode follows that of the Freundlich isotherm. Thermodynamic analysis indicates that this condition is an exothermic process with entropy reduction and the electric field force plays a dominant role in the CDI process. This work aims to improve our understanding of the ion adsorption behaviors and mechanisms on the composite electrodes in CDI.
In order to study the effect of continuous casting process parameters on the shape of slab solidification end under non-uniform cooling, a solidification model of a continuous-cast slab with non-uniform cooling condition was established with ProCAST software. The model was verified by the results of nail shooting tests and the infrared temperature measurement equipment. Four characteristic parameters were defined to evaluate the uniformity of the shape of slab solidification end. The results showed that the nonuniformity at the beginning and end of solidification, the solidification end length, and the solidification unevenness increased with the rise of casting speed. For each 10°C increase of superheat, the solidification unevenness increased by about 0.022. However, the effect of superheat on the solidification end length can be ignored. The secondary cooling strength showed minimal effect on the nonuniformity at the beginning and end of solidification. With the increase in secondary cooling intensity, the solidification end length decreased, but the solidification unevenness increased. In addition, the central segregation of the slab produced with and without the mechanical soft reduction (MSR) process was investigated. The transverse flow of molten steel with low solid fraction influenced the central segregation morphology under MSR.
Acid mine drainage presents an important threat to cementitious structures. This study is aimed at investigating the effect of cellulose nanocrystals (CNCs) on the acid resistance of cementitious composites. CNCs were added to mortar mixtures as additives at cement volume ratios of 0.2vol%, 0.4vol%, 1.0vol%, and 1.5vol%. After 28 d of standard curing, the samples were immersed in a sulfuric acid with a pH value of 2 for 75 d. The unconfined compressive strength (UCS) test, the density, water absorption, void volume test, and thermogravimetric analysis were carried out to investigate the properties of CNC mixtures before sulfuric acid immersion. It was found that the addition of CNC reduced the volume of permeable voids and increased the hydration degree and mechanical strength of the samples. Changes in mass and length were monitored during immersion to evaluate the acid resistance of mixtures. The mixture with 0.4vol% CNC showed a reduced mass change and length change indicating its improved acid resistance.
Bimetal materials derived from transition metals can be good catalysts in some reactions. When supported on graphene (GP), these catalysts feature remarkable performance in the hydrolysis of sodium borohydride. To obtain such catalysts easily and efficiently, a simple thermal reduction strategy was used in this study, and NixCo10−x series bimetal catalysts were prepared. Among all the catalysts, Ni1Co9 exhibited the best catalytic performance. The turnover frequency (TOF) related to the total number of atoms within the bimetallic nanoparticles reached 603.82 mL·mmol−1·min−1 at 303 K. Furthermore, graphene was introduced as a supporting frame. The Ni1Co9@Graphene (Ni1Co9@GP) had a large surface area and high TOF, 25534 mL·mmol−1·min−1, at 303 K. The Ni1Co9@GP exhibited efficient catalytic properties for H2 generation in alkaline solution because of its high specific surface area. Moreover, the high kinetic isotope effect observed in the kinetic studies suggests that using D2O led to the oxidative addition of an O–H bond of water in the rate-determining step.
Ti3AlC2-reinforced Ag-based composites, which are used as sliding current collectors, electrical contacts, and electrode materials, exhibit remarkable performances. However, the interfacial reactions between Ag and Ti3AlC2 significantly degrade the electrical and thermal properties of these composites. To diminish these interfacial reactions, we fabricated carbon-coated Ti3AlC2 particles (C@Ti3AlC2) as reinforcement and prepared Ag–10wt%C@Ti3AlC2 composites with carbon-layer thicknesses ranging from 50–200 nm. Compared with the uncoated Ag–Ti3AlC2 composite, Ag–C@Ti3AlC2 was found to have a better distribution of Ti3AlC2 particles. With increases in the carbon-layer thickness, the Vickers hardness value and relative density of Ag–C@Ti3AlC2 gradually decreases. With a carbon-layer thickness of 150 nm, we obtained the lowest resistivity of Ag–C@Ti3AlC2 of 29.4 135.5×10−9 Ω·m, which is half that of Ag–Ti3AlC2 (66.7 × 10−9 Ω·m). The thermal conductivity of Ag–C@Ti3AlC2 reached a maximum value of 135.5 W·m−1·K−1 with a 200-nm carbon coating (~1.8 times that of Ag–Ti3AlC2). These results indicate that the carbon-coating method is a feasible strategy for improving the performance of Ag–C@Ti3AlC2 composites.
In this study, Mg–9Al–1Si–1SiC (wt%) composites were processed by multi-pass equal-channel angular pressing (ECAP) at various temperatures, and their microstructure evolution and strengthening mechanism were explored. Results showed that the as-cast microstructure was composed of an α-Mg matrix, discontinuous Mg17Al12 phase, and Chinese script-shaped Mg2Si phase. After solution treatment, almost all of the Mg17Al12 phases were dissolved into the matrix, whereas the Mg2Si phases were not. The subsequent multi-pass ECAP at different temperatures promoted the dynamic recrystallization and uniform distribution of the Mg17Al12 precipitates when compared with the multi-pass ECAP at a constant temperature. A large number of precipitates can effectively improve the nucleation ratio of recrystallization through a particle-stimulated nucleation mechanism. In addition, the SiC nanoparticles were mainly distributed at grain boundaries, which effectively prevented dislocation movement. The excellent comprehensive mechanical properties can be attributed to grain boundary strengthening and Orowan strengthening.
A green method of super-gravity separation, which can enhance the filtration process of bismuth and copper phases, was investigated and discussed for the rapid removal of copper impurity from bismuth–copper alloy melts. After separation by the super-gravity field, the bismuth-rich liquid phases were mainly filtered from the alloy melt along the super-gravity direction, whereas most of the fine copper phases were retained in the opposite direction. With optimized conditions of separation temperature at 280°C, gravity coefficient at 450, and separation time at 200 s, the mass proportion of the separated bismuth from Bi–2wt%Cu and Bi–10wt%Cu alloys reached 96% and 85% respectively, which indicates the minimal loss of bismuth in the residual. Simultaneously, the removal ratio of impurity copper reached 88% and 98%. Furthermore, the separation process could be completed rapidly and is environmentally friendly and efficient.
Cobalt (Co)-modified brownmillerite KBiFe2O5 (KBFO; [KBiFe2(1−x)Co2xO5 (x = 0, 0.05)]) polycrystalline is synthesized following the solid-state reaction route. Rietveld refinement of X-ray diffraction data confirmed the phase purity of KBFO and KBiFe1.9Co0.1O5 (KBFCO). The optical bandgap energy (Eg) of KBFO decreased from 1.59 to 1.51 eV because of Co substitution. The decrease in bandgap can be attributed to the tilting of the Fe–O tetrahedral structure of KBFCO. The observed room-temperature Raman peaks of KBFCO shifted by 3 cm−1 toward a lower wavenumber than that of KBFO. The shift in Raman active modes can be attributed to the change in the bond angles and bond lengths of the Fe–O tetrahedral structure and modification in response to oxygen deficiency in KBFO because of Co doping. Compared with that of KBFO, the frequency-dependent dielectric constant and dielectric loss of KBFCO decrease at room temperature, which is a consequence of the reduction in oxygen migration and modification in response to vibrational modes present in the sample.