ISSN / EISSN : 1996-1944 / 1996-1944
Published by: MDPI (10.3390)
Total articles ≅ 22,831
Latest articles in this journal
Materials, Volume 14; https://doi.org/10.3390/ma14195599
Recently, nanomaterials have attracted attention in the field of pavement construction as modifiers to endure heavy loads and climate changes. In this study, conventional asphalt (bitumen) of penetration grade AC (60/70) was modified with graphene platelets (GnPs) at three different contents: 0.5%, 1.0%, and 1.5% by weight of asphalt content. Kinematic viscosity, softening point, penetration, and dynamic shear rheology tests were performed to evaluate the mechanical properties of modified binder. The results showed that adding GnPs improves the mechanical properties of asphalt binder; the kinematic viscosities, softening points, and rutting parameters increased but penetrations decreased with the contents of GnPs. Hot mix asphalt specimens with GnPs-modified asphalt were prepared and characterized with Marshall tests, thermal stress restrained specimen tests (TSRST), wheel tracking tests, and indirect tensile tests. Similar to the results of asphalt binder, the mechanical properties of asphalt mixture were improved by GnPs. Marshall stability increased by 21% and flow decreased by 24% with accepted value of 2.8 mm in penetration when the mixture was modified with 1.0 wt% of GnPs. At the same GnPs content, modified asphalt mixture led to lower failure temperature by 2 °C in comparison with unmodified asphalt mixture and the cryogenic failure stress was improved by 12%. The wheel tracking tests showed that GnPs-modified asphalt mixture has outstanding deformation resistance in comparison with unmodified asphalt mixtures: after 5000 cycles, 1.0 wt% of GnPs reduced the rut depth of asphalt mixture by 60%—the rut depth of unmodified asphalt mixture was 6.9 mm compared to 2.75 mm for modified asphalt mixture. After 10,000 cycles, the modified asphalt mixture showed rut depth of 3.24 mm in comparison with 8.12 mm in case of unmodified asphalt mixture. Addition of GnPs into asphalt mixture significantly improved the indirect tensile strength: 1.0 wt% of GnPs increased the indirect tensile strength of unmodified asphalt mixture from 0.79 to 1.1 MPa recording ~40% increment. The results of this study can confirm that graphene platelets enhance the mechanical properties of asphalt mixture and its performance.
Materials, Volume 14; https://doi.org/10.3390/ma14195577
A graphene oxide-based α-K6P2W18O62 (Dawson-type polyoxometalate) nanocomposite was formed by using two types of graphene oxide (GO) samples with different C/O compositions. Herein, based on the interaction of GO, polyoxometalates (POMs), and their nanocomposites with the Cs cation, quantitative data have been provided to explicate the morphology and Cs adsorption character. The morphology of the GO-POM nanocomposites was characterized by using TEM and SEM imaging. These results show that the POM particle successfully interacted above the surface of GO. The imaging also captured many small black spots on the surface of the nanocomposite after Cs adsorption. Furthermore, ICP-AES, the PXRD pattern, IR spectra, and Raman spectra all emphasized that the Cs adsorption occurred. The adsorption occurred by an aggregation process. Furthermore, the difference in the C/O ratio in each GO sample indicated that the ratio has significantly influenced the character of the GO-POM nanocomposite for the Cs adsorption. It was shown that the oxidized zone (sp2/sp3 hybrid carbon) of each nanocomposite sample was enlarged by forming the nanocomposite compared to the corresponding original GO sample. The Cs adsorption performance was also influenced after forming a composite. The present study also exhibited the fact that the sharp and intense diffractions in the PXRD were significantly reduced after the Cs adsorption. The result highlights that the interlayer distance was changed after Cs adsorption in all nanocomposite samples. This has a good correlation with the Raman spectra in which the second-order peaks changed after Cs adsorption.
Materials, Volume 14; https://doi.org/10.3390/ma14195578
Lithium-sulfur batteries are one of the most promising battery systems nowadays. However, this system is still not suitable for practical application because of the number of shortcomings that limit its cycle life. One of the main problems related to this system is the volumetric change during cycling. This deficiency can be compensated by using the appropriate binder. In this article, we present the influence of a water-soluble binder carrageenan on the electrochemical properties of the Li-S battery. The electrode with a carrageenan binder provides good stability during cycling and at high C-rates. Electrochemical testing was also carried out with a small prototype pouch cell with a capacity of 16 mAh. This prototype pouch cell with the water-based carrageenan binder showed lower self-discharge and low capacity drop. Capacity decreased by 7% after 70 cycles.
Materials, Volume 14; https://doi.org/10.3390/ma14195581
In this study, the shrinkage performance of recycled aggregate thermal insulation concrete (RATIC) with added glazed hollow beads (GHB) was investigated and a time-dependent shrinkage model was proposed. Two types of recycled fine aggregate (RFA) were used to replace natural fine aggregate in RATIC: RFA from waste concrete (RFA1) and waste clay brick (RFA2). Besides, the mechanical properties and thermal insulation performance of RATIC were also studied. Results showed that the pozzolanic reaction caused by RFA2 effectively improved the mechanical properties of RATIC; 75% was the optimal replacement ratio of RATIC prepared by RFA2. Added RFA decreased the thermal conductivity of thermal insulation concrete (TIC). The total shrinkage strain of RATIC increased with the increase of the replacement ratio of RFA. The 150d total shrinkage of RATIC prepared by RFA1 was 1.46 times that of TIC and the 150d total shrinkage of RATIC prepared by RFA2 was 1.23 times. The addition of GHBs led to the increase of early total shrinkage strain of concrete. Under the combined action of the higher elastic modulus of RFA2 and the pozzolanic components contained in RFA2, the total shrinkage strain of RATIC prepared by RFA2 with the same replacement ratio was smaller than that of RATIC prepared by RFA1. For example, the final total shrinkage strain of RATIC prepared by RFA2 at 100% replacement ratio was about 18.6% less than that of RATIC prepared by RFA1. A time-dependent shrinkage model considering the influence of the elastic modulus of RFA and the addition of GHB on the total shrinkage of RATIC was proposed and validated by the experimental results.
Materials, Volume 14; https://doi.org/10.3390/ma14195583
To enhance the sliding wear and corrosion behavior of steels with low carbon content, cermet composite coatings are usually deposited on their surface by various deposition processes. Laser cladding, compared to other deposition techniques such as electroplating, arc welding, and thermal spraying, has numerous advantages to produce such protective coatings. The paper presents the optimization of laser cladding deposition speed versus energy density in order to obtain WC-Co/NiCrBSi coatings with Ni-Al addition free of defects and reduced porosity deposited on low carbon steel substrate. The microstructure and chemical composition were investigated by SEM combined with EDX analysis while XRD was performed in order to examinate the phases within the coatings. In order to investigate the cladding speed influence on the coatings, hardness measurements, POD (pin on disk) wear tests and corrosion tests in 3.5% NaCl solution were carried out. The results showed that an optimal cladding speed has a crucial impact on the microstructure, composition, and hardness. It was found out that optimizing the cladding deposition speed proved to be effective in enhancing the sliding wear resistance and corrosion behavior by controlling the iron content within the coatings.
Materials, Volume 14; https://doi.org/10.3390/ma14195582
The hybrid process integrates two or more different processes, such as additive and subtractive manufacturing, which have gained appreciable consideration in recent years. The deformation of hybrid manufacturing is an essential factor affecting machining quality. The purpose of this paper is to study the effect of milling on stress release and surface deformation of additive manufacturing (AM) specimens in the process of additive and subtractive hybrid manufacturing (ASHM) of 316L stainless steel thin-walled parts, so as to effectively improve the forming quality of thin-walled parts manufactured by the combined processing of ASHM. To this end, a series of experiments were carried out to study the relationship between stress distribution and thermal stress deformation of 316L stainless steel thin-walled parts prepared by LMD, and the changes of stress and deformation of these thin-walled parts after subsequent milling. An infrared camera and laser distance sensor were used to record the temperature field data and deformation data to analyze the influence factors of temperature and stress on the machining results. Then, the finite element software was used to simulate the stress and deformation of the thin-walled parts in the additive manufacturing process and the subsequent milling process. Meanwhile, the model was verified through the experiments. In addition, the relationship between the milling force and the milling parameters of the AM parts was studied by orthogonal test and regression analysis.
Materials, Volume 14; https://doi.org/10.3390/ma14195585
Dry hydrated lime is an air binder often used in architectural injection grouts. This study compared the influences of three commercially available dry hydrated limes on the injection grouts’ workability and mechanical properties. The main differences between the limes were in their chemical and mineralogical composition and Blaine specific surface area. The grouts were composed of dry hydrated lime, finely ground limestone filler, water, and super plasticiser. Subsequent results obtained revealed that the Blaine specific surface area is not directly related to the fresh grout properties. Grain size distribution and shape of lime particles and their aggregates in the water suspension are key parameters influencing the following fresh grout properties: fluidity, injectability, the mixture’s stability, and water retention capacity. However, the lime injection grouts’ mechanical strengths were higher in relation to an increase in the content of portlandite and the Blaine specific surface area of the dry hydrate.
Materials, Volume 14; https://doi.org/10.3390/ma14195592
In this work, the suitability of natural raw materials with various modifications of SiO2—granite sawing waste (quartz) and opoka (a mixture of cristobalite, tridymite, quartz, and an amorphous part)—for the 1.13 nm tobermorite and xonotlite synthesis is examined, and their specific surface area, pore diameter and volume, and the predominant pores are determined. Hydrothermal syntheses were carried out at 200 °C for 12 and 72 h from mixtures with a molar ratio of CaO/SiO2 = 1.0. X-ray diffraction analysis, simultaneous thermal analysis, and scanning electronic microscopy were used, which showed that in the lime–calcined opoka mixture the formation of crystalline calcium silicate hydrates takes place much faster than in the lime–granite sawing waste mixture. The high reactivity of amorphous SiO2 results in the rapid formation of 1.13 nm tobermorite and xonotlite (12 h). According to Brunauer, Emmet and Taller (BET) analysis data, this product features a specific surface area of ~68 m2/g, a total pore volume of 245·10−3 cm3/g, and has dominating 1–2.5 nm and 5–20 nm diameter pores. This porosity of the material should provide good thermal insulation properties of the products made from it as no air convection occurs in the fine pores.
Materials, Volume 14; https://doi.org/10.3390/ma14195595
Functionally graded material (FGM) based on Inconel 625 and AISI 431 stainless steel powders was produced by applying the direct laser deposition (DLD) process. The FGM starts with layers of Inconel 625 and ends with layers of 431 stainless steel having three intermediate zones with the composition (100-X)% Inconel 625-X% 431 stainless steel, X = 25, 50, and 75, in that order. This FGM was deposited on a 42CrMo4 steel substrate, with and without preheating. Microstructures of these FGMs were evaluated, while considering the distribution of chemical composition and grain structure. Microstructures mainly consisted of columnar grains independent of preheating condition; epitaxial growth was observed. The application of a non-preheated substrate caused the formation of planar grains in the vicinity of the substrate. In addition, hardness maps were produced. The hardness distribution across these FGMs confirmed a smooth transition between deposited layers; however, the heat-affected zone was greatly influenced by the preheating condition. This study suggests that an optimum Inconel 625/AISI 431 FGM obtained by DLD should not exceed 50% AISI 431 stainless steel.
Materials, Volume 14; https://doi.org/10.3390/ma14195598
In this work, a kind of Gd/Cr codoped Bi3TiNbO9 Aurivillius phase ceramic with the formula of Bi2.8Gd0.2TiNbO9 + 0.2 wt% Cr2O3 (abbreviated as BGTN−0.2Cr) was prepared by a conventional solid-state reaction route. Microstructures and electrical conduction behaviors of the ceramic were investigated. XRD and SEM detection found that the BGTN−0.2Cr ceramic was crystallized in a pure Bi3TiNbO9 phase and composed of plate-like grains. A uniform element distribution involving Bi, Gd, Ti, Nb, Cr, and O was identified in the ceramic by EDS. Because of the frequency dependence of the conductivity between 300 and 650 °C, the electrical conduction mechanisms of the BGTN−0.2Cr ceramic were attributed to the jump of the charge carriers. Based on the correlated barrier hopping (CBH) model, the maximum barrier height WM, dc conduction activation energy Ec, and hopping conduction activation energy Ep were calculated with values of 0.63 eV, 1.09 eV, and 0.73 eV, respectively. Impedance spectrum analysis revealed that the contribution of grains to the conductance increased with rise in temperature; at high temperatures, the conductance behavior of grains deviated from the Debye relaxation model more than that of grain boundaries. Calculation of electrical modulus further suggested that the degree of interaction between charge carriers β tended to grow larger with rising temperature. In view of the approximate relaxation activation energy (~1 eV) calculated from Z’’ and M’’ peaks, the dielectric relaxation process of the BGTN−0.2Cr ceramic was suggested to be dominated by the thermally activated motion of oxygen vacancies as defect charge carriers. Finally, a high piezoelectricity of d33 = 18 pC/N as well as a high resistivity of ρdc = 1.52 × 105 Ω cm at 600 °C provided the BGTN−0.2Cr ceramic with promising applications in the piezoelectric sensors with operating temperature above 600 °C.