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Materials, Volume 13, Issue 12 (June-2 2020) – 213 articles

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Cover Story (view full-size image) The HA/SF-IPN (interpenetration network) (HS-IPN) hydrogels consist of SF of strain A, and high [...] Read more.
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Open AccessArticle
Theoretical and Experimental Studies on Thermal Properties of Polyester Nonwoven Fibrous Material
Materials 2020, 13(12), 2882; https://doi.org/10.3390/ma13122882 - 26 Jun 2020
Cited by 1 | Viewed by 438
Abstract
Polyester nonwoven fibrous material is widely used in construction and automobile industries for thermal insulation purposes. It is worthy and meaningful to understand the effect of structural parameters on the thermal property. Fiber orientation, as one of the most vital parameters, has a [...] Read more.
Polyester nonwoven fibrous material is widely used in construction and automobile industries for thermal insulation purposes. It is worthy and meaningful to understand the effect of structural parameters on the thermal property. Fiber orientation, as one of the most vital parameters, has a significant effect on thermal property. However, there has been little quantitative analysis focusing on this aspect. This paper theoretically and experimentally analyzes the thermal conductivity of samples with varying fiber orientation. Existing models were selected to predict the thermal conductivity of polyester nonwoven samples. Two different apparatus were applied to carry out the experimental measurements. The relative differences between the predicted and measured results were compared. One commonly used model was modified for accurate prediction. It was shown that some existing models under- or overestimate the thermal conductivity compared to the measured values. The results indicate that the modified model can accurately predict the thermal conductivity of polyester nonwoven materials within a 0.2% relative difference. Full article
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Open AccessReview
Metal-Organic Framework (MOF)/Epoxy Coatings: A Review
Materials 2020, 13(12), 2881; https://doi.org/10.3390/ma13122881 - 26 Jun 2020
Viewed by 521
Abstract
Epoxy coatings are developing fast in order to meet the requirements of advanced materials and systems. Progress in nanomaterial science and technology has opened a new era of engineering for tailoring the bulk and surface properties of organic coatings, e.g., adhesion to the [...] Read more.
Epoxy coatings are developing fast in order to meet the requirements of advanced materials and systems. Progress in nanomaterial science and technology has opened a new era of engineering for tailoring the bulk and surface properties of organic coatings, e.g., adhesion to the substrate, anti-corrosion, mechanical, flame-retardant, and self-healing characteristics. Metal-organic frameworks (MOFs), a subclass of coordinative polymers with porous microstructures, have been widely synthesized in recent years and applied in gas and energy storage, separation, sensing, environmental science and technology, and medicine. Nevertheless, less attention has been paid to their performance in coatings. Well-known as micro- and nanoporous materials, with a tailorable structure consisting of metal ions and organic linkers, MOFs have a huge loading capacity, which is essential for the delivery of corrosion inhibitors. This review paper attempts to highlight the importance of epoxy/MOF composites for coating applications. A particular emphasis was explicitly placed on the anti-corrosion, flame-retardant, mechanical, and dielectric properties of epoxy/MOF coatings. Full article
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Open AccessArticle
Cleaning and Conditioning of Contaminated Core Build-Up Material before Adhesive Bonding
Materials 2020, 13(12), 2880; https://doi.org/10.3390/ma13122880 - 26 Jun 2020
Viewed by 394
Abstract
The objective of this study was to evaluate the effects of different cleaning and conditioning procedures after contamination on the tensile bond strength (TBS) of a luting resin to a core build-up composite resin. Specimens (n = 384) made of a core [...] Read more.
The objective of this study was to evaluate the effects of different cleaning and conditioning procedures after contamination on the tensile bond strength (TBS) of a luting resin to a core build-up composite resin. Specimens (n = 384) made of a core build-up material were stored for 3 weeks in 37 °C water. Half of the specimens were contaminated with saliva and a disclosing silicone and then cleaned either using phosphoric acid, a pumice suspension, air-abrasion with alumina or polishing powder. Surface conditioning was performed by either using a dentin adhesive, a silane containing primer or a composite resin primer, which resulted in 24 unique combinations of 16 specimens per group. Before measuring TBS, half of the specimens of each group were stored in 37 °C water for 3d or were artificially aged for 150 days. Results show that cleaning with pumice or air-abrasion are superior methods compared to using a polishing powder or phosphoric acid. Silane is an inferior conditioning agent compared to composite or dentin primers. Ideally, after contamination, bonding surfaces should be cleaned with a pumice suspension and conditioned with a dentin adhesive. Those surfaces could also be cleaned and conditioned with air-abrasion with alumina particles and a composite resin primer. Full article
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Open AccessArticle
3D Printed Thermoelectric Polyurethane/Multiwalled Carbon Nanotube Nanocomposites: A Novel Approach towards the Fabrication of Flexible and Stretchable Organic Thermoelectrics
Materials 2020, 13(12), 2879; https://doi.org/10.3390/ma13122879 - 26 Jun 2020
Cited by 1 | Viewed by 443
Abstract
Three-dimensional (3D) printing of thermoelectric polymer nanocomposites is reported for the first time employing flexible, stretchable and electrically conductive 3D printable thermoplastic polyurethane (TPU)/multiwalled carbon nanotube (MWCNT) filaments. TPU/MWCNT conductive polymer composites (CPC) have been initially developed employing melt-mixing and extrusion processes. TPU [...] Read more.
Three-dimensional (3D) printing of thermoelectric polymer nanocomposites is reported for the first time employing flexible, stretchable and electrically conductive 3D printable thermoplastic polyurethane (TPU)/multiwalled carbon nanotube (MWCNT) filaments. TPU/MWCNT conductive polymer composites (CPC) have been initially developed employing melt-mixing and extrusion processes. TPU pellets and two different types of MWCNTs, namely the NC-7000 MWCNTs (NC-MWCNT) and Long MWCNTs (L-MWCNT) were used to manufacture TPU/MWCNT nanocomposite filaments with 1.0, 2.5 and 5.0 wt.%. 3D printed thermoelectric TPU/MWCNT nanocomposites were fabricated through a fused deposition modelling (FDM) process. Raman and scanning electron microscopy (SEM) revealed the graphitic nature and morphological characteristics of CNTs. SEM and transmission electron microscopy (TEM) exhibited an excellent CNT nanodispersion in the TPU matrix. Tensile tests showed no significant deterioration of the moduli and strengths for the 3D printed samples compared to the nanocomposites prepared by compression moulding, indicating an excellent interlayer adhesion and mechanical performance of the 3D printed nanocomposites. Electrical and thermoelectric investigations showed that L-MWCNT exhibits 19.8 ± 0.2 µV/K Seebeck coefficient (S) and 8.4 × 103 S/m electrical conductivity (σ), while TPU/L-MWCNT CPCs at 5.0 wt.% exhibited the highest thermoelectric performance (σ = 133.1 S/m, S = 19.8 ± 0.2 µV/K and PF = 0.04 μW/mK2) among TPU/CNT CPCs in the literature. All 3D printed samples exhibited an anisotropic electrical conductivity and the same Seebeck coefficient in the through- and cross-layer printing directions. TPU/MWCNT could act as excellent organic thermoelectric material towards 3D printed thermoelectric generators (TEGs) for potential large-scale energy harvesting applications. Full article
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Open AccessArticle
The Effect of Nanosizing on the Oxidation of Partially Oxidized Copper Nanoparticles
Materials 2020, 13(12), 2878; https://doi.org/10.3390/ma13122878 - 26 Jun 2020
Viewed by 364
Abstract
Copper nanoparticles are of great interest in various applications, such as catalysis, cooling fluids, conductive inks or for their antibacterial activity. In this paper, the thermal behavior of copper nanoparticles was studied using thermogravimetry, differential thermal analysis and differential scanning calorimetry. Original Cu [...] Read more.
Copper nanoparticles are of great interest in various applications, such as catalysis, cooling fluids, conductive inks or for their antibacterial activity. In this paper, the thermal behavior of copper nanoparticles was studied using thermogravimetry, differential thermal analysis and differential scanning calorimetry. Original Cu samples as well as the products of oxidation were analysed by X-ray diffraction, scanning/transmission electron microscopy and energy dispersive spectroscopy. A step-by-step oxidation mechanism during the oxidation of Cu nano-powders was observed. The Cu-nano oxidation starts slightly above 150 °C when bulk copper does not yet react. The dominant oxidation product in the first step is Cu2O while CuO was identified as the final state of oxidation. Our results confirm an easier oxidation process of Cu-nano than Cu-micro particles, which must be attributed to kinetic not thermodynamic aspects of oxidation reactions. Full article
(This article belongs to the Special Issue Synthesis and Properties of Nano-Based Composites)
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Open AccessArticle
An Insight into Ionic Conductivity of Polyaniline Thin Films
Materials 2020, 13(12), 2877; https://doi.org/10.3390/ma13122877 - 26 Jun 2020
Viewed by 358
Abstract
The work addresses an issue of the conductivity phenomenon in conductive polymer thin films. Polyaniline was chosen as a broadly used and thoroughly investigated conductive polymer in order to test and show capabilities of the developed original approach based on impedance spectra analysis. [...] Read more.
The work addresses an issue of the conductivity phenomenon in conductive polymer thin films. Polyaniline was chosen as a broadly used and thoroughly investigated conductive polymer in order to test and show capabilities of the developed original approach based on impedance spectra analysis. A number of films of different thickness were deposited onto a Pt electrode surface and consequently investigated in aqueous solution containing perchloric acid as an electrolyte. The processes that occur in polyaniline film were studied by cyclic voltammetry, electrochemical quartz crystal microgravimetry (EQCM) and electrochemical impedance spectroscopy (EIS). The role of incorporated ions as charge carriers was investigated with respect to the control of the conductivity properties of the film. Along with detailed polyaniline behavior study, the work makes up a fundamental scientific impact on theoretical electrochemistry and electroanalytical techniques. Full article
(This article belongs to the Special Issue Advances in Materials for Organic Optoelectronics and Photonics)
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Open AccessArticle
Parametric Study on Microwave-Assisted Pyrolysis Combined KOH Activation of Oil Palm Male Flowers Derived Nanoporous Carbons
Materials 2020, 13(12), 2876; https://doi.org/10.3390/ma13122876 - 26 Jun 2020
Viewed by 366
Abstract
Oil palm male flowers (PMFs), an abundant agricultural waste from oil palm plantation in Thailand, have been utilized as an alternative precursor to develop nanoporous carbons (NPCs) via microwave-assisted pyrolysis combined potassium hydroxide (KOH) activation. The influences of relevant processing variables, such as [...] Read more.
Oil palm male flowers (PMFs), an abundant agricultural waste from oil palm plantation in Thailand, have been utilized as an alternative precursor to develop nanoporous carbons (NPCs) via microwave-assisted pyrolysis combined potassium hydroxide (KOH) activation. The influences of relevant processing variables, such as activating agent ratio, microwave power, and activation time on the specific pore characteristics, surface morphology, and surface chemistry of PMFs derived nanoporous carbons (PMFCs) have been investigated to explore the optimum preparation condition. The optimum condition under a microwave radiation power of 700 W, activation holding time of 6 min, and activating agent ratio of 2:1 obtained the PMFC with the highest Brunauer–Emmett–Teller (BET) surface area and total pore volume approximately of 991 m2/g and 0.49 cm3/g, composed of a carbon content of 74.56%. Meanwhile, PMFCs have a highly microporous structure of about 71.12%. Moreover, activating agent ratio and microwave radiation power indicated a significant influence on the surface characteristics of PMFCs. This study revealed the potential of oil palm male flowers for the NPCs’ production via microwave-assisted KOH activation with a short operating-time condition. Full article
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Open AccessArticle
Molybdenum Oxide and Nickel Nitrate as Cooperative Sintering Aids for Yttria-Stabilized Zirconia
Materials 2020, 13(12), 2875; https://doi.org/10.3390/ma13122875 - 26 Jun 2020
Viewed by 324
Abstract
The entirely accidental observation of increased sintering performance of nickel-infiltrated yttria-stabilized zirconia (8YSZ) in a molybdenum and oxygen rich atmosphere was explored. Molybdenum and nickel were found to be synergistic sintering aids for 8YSZ. However, sintering had to take place in an atmosphere [...] Read more.
The entirely accidental observation of increased sintering performance of nickel-infiltrated yttria-stabilized zirconia (8YSZ) in a molybdenum and oxygen rich atmosphere was explored. Molybdenum and nickel were found to be synergistic sintering aids for 8YSZ. However, sintering had to take place in an atmosphere of flowing oxygen. Samples sintered in air consistently burst. The sintering performance, microstructure, and crystal structure of 8YSZ with additions of both Mo and Ni together are compared to the sintering performance, microstructure, and crystal structure of pure 8YSZ, 8YSZ with only Ni added as a sintering aid, and 8YSZ with only Mo added as a sintering aid. Enhanced densification and grain growth is observed in the Mo–Ni 8YSZ samples when compared to all other sintering samples. Order of magnitude sintering rate increases are observed in the Mo–Ni 8YSZ over that of pure 8YSZ. With a maximum sintering temperature of 1200 °C and a one-hour dwell, sintered densities of 85% theoretical density (5.02 g⁄cm3) are achieved with the Mo–Ni samples: a 57% increase in density over pure 8YSZ sintered with the same sintering profile. EIS results suggest conductivity may not be negatively impacted by the use of these two sintering aids at temperatures above 750 °C. Finally, the spontaneous generation of nickel-molybdenum nano-rods was observed on the 5, and 10 mol.% Mo–Ni infiltrated 8YSZ samples after being left under vacuum in a scanning electron microscope chamber, suggesting evaporation of a possible nickel–molybdenum compound from the sample fracture surfaces. Full article
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Open AccessArticle
A Nonenzymatic Glucose Sensor Platform Based on Specific Recognition and Conductive Polymer-Decorated CuCo2O4 Carbon Nanofibers
Materials 2020, 13(12), 2874; https://doi.org/10.3390/ma13122874 - 26 Jun 2020
Viewed by 334
Abstract
CuCo2O4 decoration carbon nanofibers (CNFs) as an enzyme-free glucose sensor were fabricated via electrospinning technology and carbonization treatment. The CNFs with advantages of abundant nitrogen amounts, porosity, large surface area, and superior electrical conductivity were used as an ideal matrix [...] Read more.
CuCo2O4 decoration carbon nanofibers (CNFs) as an enzyme-free glucose sensor were fabricated via electrospinning technology and carbonization treatment. The CNFs with advantages of abundant nitrogen amounts, porosity, large surface area, and superior electrical conductivity were used as an ideal matrix for CuCo2O4 decoration. The resultant CuCo2O4–CNF hybrids possessed favorable properties of unique three-dimensional architecture and good crystallinity, accompanied by the CuCo2O4 nanoparticles uniformly growing on the CNF skeleton. To further enhance the selective molecular recognition capacity of the developed sensor, a conductive film was synthesized through the electropolymerization of thiophene and thiophene-3-boronic acid (TBA). Based on the synergistic effects of the performances of CNFs, CuCo2O4 nanoparticles, and boronic acid-decorated polythiophene layer, the obtained poly(thiophene-3-boronic acid) (PTBA)/CuCo2O4–CNF-modified electrodes (PTBA/CuCo2O4–CNFs/glassy carbon electrode (GCE)) displayed prominent electrocatalytic activity toward electro-oxidation of glucose. The fabricated sensor presented an outstanding performance in the two linear ranges of 0.01–0.5 mM and 0.5–1.5 mM, with high selectivity of 2932 and 708 μA·mM−1·cm−2, respectively. The composite nanofibers also possessed good stability, repeatability, and excellent anti-interference selectivity toward the common interferences. All these results demonstrate that the proposed composite nanofibers hold great potential in the application of constructing an enzyme-free glucose sensing platform. Full article
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Open AccessArticle
Determination of Mortar Strength in Historical Brick Masonry Using the Penetrometer Test and Double Punch Test
Materials 2020, 13(12), 2873; https://doi.org/10.3390/ma13122873 - 26 Jun 2020
Viewed by 303
Abstract
This paper presents the results of the minor destructive testing of mortars in masonry structures of four buildings erected at the turn of the 19th and 20th centuries. The buildings were erected in the historical centre of Cracow. The objective of testing was [...] Read more.
This paper presents the results of the minor destructive testing of mortars in masonry structures of four buildings erected at the turn of the 19th and 20th centuries. The buildings were erected in the historical centre of Cracow. The objective of testing was to determine mortar compressive strength in masonry joints. The in situ tests were carried out with the use of a penetrometer RSM-15 with the standardised impact energy equalling 4.55 nm. Laboratory tests on mortar specimens taken from the structures were also performed. The double punch test method was used in the laboratory tests. On account of the specificity of the tested historical mortars, the typical procedures used in penetrometer and double punch tests were modified. For penetrometer tests, a new feature called “a surface disturbance zone” was introduced. Additionally, a procedure for determining a surface disturbance zone range was included. As confirmed in the paper, the consideration of the surface disturbance zone in the analysis of test results is crucial for the correct evaluation of mortar compressive strength. The thicknesses of bed joints in the tested historical masonry considerably exceeded the requirements included in the standard EN 1996-1-1. Thus, the thickness of the mortar specimens taken from historical masonry for the double punch tests clearly exceeded the thickness of specimens extracted from the typical structures erected nowadays. This article provides a method of considering a specimen thickness parameter in the analysis of double punch test results. The in situ test results with the use of penetrometer and double punch methods confirmed that the mortar strength in tested historical buildings ranged from 1.4 to 2.9 MPa. Mortar compressive strength values determined by both applied methods were similar. Full article
(This article belongs to the Special Issue Masonry Structures and Reinforced Concrete Structures)
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Open AccessArticle
Modal Damping Coefficient Estimation of Carbon-Fiber-Reinforced Plastic Material Considering Temperature Condition
Materials 2020, 13(12), 2872; https://doi.org/10.3390/ma13122872 - 26 Jun 2020
Viewed by 310
Abstract
Excellent mechanical properties of carbon-fiber-reinforced plastic material (CFRP) demonstrates many possibilities in industries using lightweight materials, but unlike isotropic materials, such as iron, aluminum, and magnesium, they show direction-sensitive properties, which makes it difficult to apply for them. The sensitivity of a modal [...] Read more.
Excellent mechanical properties of carbon-fiber-reinforced plastic material (CFRP) demonstrates many possibilities in industries using lightweight materials, but unlike isotropic materials, such as iron, aluminum, and magnesium, they show direction-sensitive properties, which makes it difficult to apply for them. The sensitivity of a modal damping coefficient of a CFRP material over the direction of carbon fiber was examined on spectral input patterns in recent research, but the effect of temperature was not considered up to now. To overcome this, uniaxial vibration tests were conducted using five simple specimens with different direction of carbon fiber in a CFRP specimen, the frequency response functions were experimentally determined and the modal damping coefficients were calculated. It was revealed that the resonance point and the modal damping of the specimen changed according to the change in temperature condition. Based on the experimental results, it was demonstrated that the theoretical frequency response function of the carbon composite material is a function of temperature, and it was confirmed that the nonlinear characteristic of the modal damping was the smallest under the 0 degree of direction of carbon fiber. Full article
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Open AccessArticle
Focused Ion Beam Milling of Single-Crystal Sapphire with A-, C-, and M-Orientations
Materials 2020, 13(12), 2871; https://doi.org/10.3390/ma13122871 - 26 Jun 2020
Viewed by 297
Abstract
Sapphire substrates with different crystal orientations are widely used in optoelectronic applications. In this work, focused ion beam (FIB) milling of single-crystal sapphire with A-, C-, and M-orientations was performed. The material removal rate (MRR) and surface roughness (Sa) of sapphire with the [...] Read more.
Sapphire substrates with different crystal orientations are widely used in optoelectronic applications. In this work, focused ion beam (FIB) milling of single-crystal sapphire with A-, C-, and M-orientations was performed. The material removal rate (MRR) and surface roughness (Sa) of sapphire with the three crystal orientations after FIB etching were derived. The experimental results show that: The MRR of A-plane sapphire is slightly higher than that of C-plane and M-plane sapphires; the Sa of A-plane sapphire after FIB treatment is the smallest among the three different crystal orientations. These results imply that A-plane sapphire allows easier material removal during FIB milling compared with C-plane and M-plane sapphires. Moreover, the surface quality of A-plane sapphire after FIB milling is better than that of C-plane and M-plane sapphires. The theoretical calculation results show that the removal energy of aluminum ions and oxygen ions per square nanometer on the outermost surface of A-plane sapphire is the smallest. This also implies that material is more easily removed from the surface of A-plane sapphire than the surface of C-plane and M-plane sapphires by FIB milling. In addition, it is also found that higher MRR leads to lower Sa and better surface quality of sapphire for FIB etching. Full article
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Open AccessArticle
Recyclability of Asphalt Mixtures with Crumb Rubber Incorporated by Dry Process: A Laboratory Investigation
Materials 2020, 13(12), 2870; https://doi.org/10.3390/ma13122870 - 26 Jun 2020
Viewed by 342
Abstract
Semi-Dense Asphalt (SDA) mixtures are nowadays recommended for the surface layer of low noise roads in urban areas due to their optimal functional characteristics. Moreover, the use of polymer-modified bitumen (PmB) in its design results in high mechanical performance. However, this type of [...] Read more.
Semi-Dense Asphalt (SDA) mixtures are nowadays recommended for the surface layer of low noise roads in urban areas due to their optimal functional characteristics. Moreover, the use of polymer-modified bitumen (PmB) in its design results in high mechanical performance. However, this type of highly modified bitumen implies significant economic and environmental disadvantages. The polymer modification increases the production cost, involves higher mixing temperatures, and makes the recycling process of the asphalt mixtures challenging. As a potential alternative to PmB in SDA mixtures, this experimental work analyses the dry process for the incorporation of crumb rubber (CR) from waste tires. Particularly, the main objective was to study the aging effect and the recyclability of asphalt mixtures prepared in the laboratory with two different types of CR. The volumetric properties and mechanical performance of the mixtures artificially aged and rejuvenated were evaluated. The results obtained show that mixtures with CR have adequate performance, being less susceptible to aging than a conventional polymer-modified mixture. Furthermore, the rheological response of asphalt binder samples recovered from the mixtures at different aging states was analyzed. It was observed that the effect of the rejuvenator depended on the CR type, but this fact did not negatively influence the performance of the recycled mixtures. Full article
(This article belongs to the Special Issue Novel Materials and Technologies for the Urban Roads of the Future)
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Open AccessArticle
Texture and Differential Stress Development in W/Ni-Co Composite after Rotary Swaging
Materials 2020, 13(12), 2869; https://doi.org/10.3390/ma13122869 - 26 Jun 2020
Viewed by 303
Abstract
Knowledge of texture and residual stresses in tungsten heavy pseudoalloys is substantial for the microstructure optimization. These characteristics were determined in cold and warm rotary swaged W/NiCo composite with help of neutron diffraction. The results were discussed in view of the observed microstructure [...] Read more.
Knowledge of texture and residual stresses in tungsten heavy pseudoalloys is substantial for the microstructure optimization. These characteristics were determined in cold and warm rotary swaged W/NiCo composite with help of neutron diffraction. The results were discussed in view of the observed microstructure and mechanical properties. The investigated bars consisted of tungsten agglomerates (bcc lattice) surrounded by NiCo-based matrix (fcc lattice). No preferential crystallographic orientation was found in the as-sintered bar. A strong texture was formed in both the tungsten agglomerates (<101> fiber texture parallel to the swaging axis) and in the NiCo-based matrix (<111> fiber texture) after rotary swaging. Although usually of double-fiber texture, the <001> fiber of the fcc structures was nearly missing in the matrix. Further, the cold-swaged bar exhibited substantially stronger texture for both phases which corresponds to the higher measured ultimate tensile strength. The residual stress differences were employed for characterization of the stress state of the bars. The largest residual stress difference (≈400 MPa) was found at the center of the bar deformed at room temperature. The hoop stresses were non-symmetrical with respect to the swaging axis, which was likely caused by the elliptical cross section of the as-sintered bar. Full article
(This article belongs to the Special Issue Structural Phenomena in Modern Metallic Materials)
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Open AccessArticle
Performance of Short Fiber Interlayered Reinforcement Thermoplastic Resin in Additive Manufacturing
Materials 2020, 13(12), 2868; https://doi.org/10.3390/ma13122868 - 26 Jun 2020
Cited by 1 | Viewed by 307
Abstract
To further improve the mechanical properties of thermoplastic resin in additive manufacturing (AM), this paper presents a novel method to directly and quantitatively place the short fibers (SFs) between two printing process of resin layers. The printed composite parts with SFs between the [...] Read more.
To further improve the mechanical properties of thermoplastic resin in additive manufacturing (AM), this paper presents a novel method to directly and quantitatively place the short fibers (SFs) between two printing process of resin layers. The printed composite parts with SFs between the layers was reinforced. The effects of single-layer fiber content, multi-layer fiber content and the length of fibers on the mechanical properties of printed specimens were studied. The distribution of fibers and quality of interlayer bonding were assessed using mechanical property testing and microstructure examination. The results showed that the tensile strength of the single-layered specimen with 0.5 wt% interlayered SFs increased by 18.82%. However, when the content of SFs continued to increase, the mechanical properties declined because of the increasing interlayered gap and the poor bonding quality. In addition, when the interlayered SFs length was 0.5–1 mm, the best reinforcement was obtained. To improve the interfacial bonding quality between the fiber and the resin, post-treatment and laser-assisted preheating printing was used. This method is effective for the enhancement of the interfacial bonding to obtain better mechanical properties. The research proves that adding SFs by placement can reduce the wear and breakage of the fibers compared to the conventional forming process. Therefore, the precise control of the length and content of SFs was realized in the specimen. In summary, SFs placement combined with post-treatment and laser-assisted preheating is a new method in AM to improve the performance of thermoplastic resin. Full article
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Open AccessArticle
Simple, Accurate and User-Friendly Differential Constitutive Model for the Rheology of Entangled Polymer Melts and Solutions from Nonequilibrium Thermodynamics
Materials 2020, 13(12), 2867; https://doi.org/10.3390/ma13122867 - 26 Jun 2020
Cited by 1 | Viewed by 284
Abstract
In a recent reformulation of the Marrucci-Ianniruberto constitutive equation for the rheology of entangled polymer melts in the context of nonequilibrium thermodynamics, rather large values of the convective constraint release parameter βccr had to be used in order for the model not [...] Read more.
In a recent reformulation of the Marrucci-Ianniruberto constitutive equation for the rheology of entangled polymer melts in the context of nonequilibrium thermodynamics, rather large values of the convective constraint release parameter βccr had to be used in order for the model not to violate the second law of thermodynamics. In this work, we present an appropriate modification of the model, which avoids the splitting of the evolution equation for the conformation tensor into an orientation and a stretching part. Then, thermodynamic admissibility simply dictates that βccr ≥ 0, thus allowing for more realistic values of βccr to be chosen. Moreover, and in view of recent experimental evidence for a transient stress undershoot (following the overshoot) at high shear rates, whose origin may be traced back to molecular tumbling, we have incorporated additional terms into the model accounting, at least in an approximate way, for non-affine deformation through a slip parameter ξ. Use of the new model to describe available experimental data for the transient and steady-state shear and elongational rheology of entangled polystyrene melts and concentrated solutions shows close agreement. Overall, the modified model proposed here combines simplicity with accuracy, which renders it an excellent choice for managing complex viscoelastic fluid flows in large-scale numerical calculations. Full article
(This article belongs to the Special Issue Rheology of Advanced Complex Fluids)
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Open AccessArticle
Modeling and Optimization of Bidirectional Clamping Forces in Drilling of Stacked Aluminum Alloy Plates
Materials 2020, 13(12), 2866; https://doi.org/10.3390/ma13122866 - 26 Jun 2020
Viewed by 293
Abstract
Interlayer burrs formation during drilling of stacked plates is a common problem in the field of aircraft assembly. Burrs elimination requires extra deburring operations which is time-consuming and costly. An effective way to inhibit interlayer burrs is to reduce the interlayer gap by [...] Read more.
Interlayer burrs formation during drilling of stacked plates is a common problem in the field of aircraft assembly. Burrs elimination requires extra deburring operations which is time-consuming and costly. An effective way to inhibit interlayer burrs is to reduce the interlayer gap by preloading clamping force. In this paper, based on the theory of plates and shells, a mathematical model of interlayer gap with bidirectional clamping forces was established. The relationship between the upper and lower clamping forces was investigated when the interlayer gap reaches zero. The optimization of the bidirectional clamping forces was performed to reduce the degree and non-uniformity of the deflections of the stacked plates. Then, the finite element simulation was conducted to verify the mathematical model. Finally, drilling experiments were carried out on 2024-T3 aluminum alloy stacked plates based on the dual-machine-based automatic drilling and riveting system. The experimental results show that the optimized bidirectional clamping forces can significantly reduce the burr heights. The work in this paper enables us to understand the effect of bidirectional clamping forces on the interlayer gap and paves the way for the practical application. Full article
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Open AccessArticle
Enhancement of Mechanical Properties and Porosity of Concrete Using Steel Slag Coarse Aggregate
Materials 2020, 13(12), 2865; https://doi.org/10.3390/ma13122865 - 26 Jun 2020
Viewed by 393
Abstract
This paper investigates the possibility of utilizing steel slags produced in the steelmaking industry as an alternative to burnt clay brick aggregate (BA) in concrete. Within this context, physical, mechanical (i.e., compressive and splitting tensile strength), length change, and durability (porosity) tests were [...] Read more.
This paper investigates the possibility of utilizing steel slags produced in the steelmaking industry as an alternative to burnt clay brick aggregate (BA) in concrete. Within this context, physical, mechanical (i.e., compressive and splitting tensile strength), length change, and durability (porosity) tests were conducted on concrete made with nine different percentage replacements (0%, 10%, 20%, 30%, 40%, 50%, 60%, 80%, and 100% by volume of BA) of BA by induction of furnace steel slag aggregate (SSA). In addition, the chemical composition of aggregate through X-ray fluorescence (XRF) analysis and microstructural analysis through scanning electron microscopy (SEM) of aggregates and concrete were performed. The experimental results show that the physical and mechanical properties of concrete made with SSA were significantly higher than that of concrete made with BA. The compressive and tensile strength increased by 73% when SSA fully replaced BA. The expansion of concrete made with SSA was a bit higher than the concrete made with BA. Furthermore, a significant lower porosity was observed for concrete made with SSA than BA, which decreased by 40% for 100% SSA concrete than 100% BA concrete. The relation between compressive and tensile strength with the porosity of concrete mixes are in agreement with the relationships presented in the literature. This study demonstrates that SSA can be used as a full replacement of BA, which is economical, conserves the natural aggregate, and is sustainable building material since burning brick produces a lot of CO2. Full article
(This article belongs to the Special Issue Recycled Materials for Concrete and Other Composites)
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Open AccessArticle
Geometric Effect on the Nonlinear Force-Displacement Relationship of Awl-Shaped Serpentine Microsprings for In-Plane Deformation
Materials 2020, 13(12), 2864; https://doi.org/10.3390/ma13122864 - 26 Jun 2020
Viewed by 318
Abstract
Even when made by brittle materials, awl-shaped serpentine microsprings (ASSMs) were found to have a nonlinear displacement–force relationship similar to springs made by ductile material. It is found that the nonlinear displacement–force relationship is due to the geometry and dimensions of the ASSMs. [...] Read more.
Even when made by brittle materials, awl-shaped serpentine microsprings (ASSMs) were found to have a nonlinear displacement–force relationship similar to springs made by ductile material. It is found that the nonlinear displacement–force relationship is due to the geometry and dimensions of the ASSMs. The geometric effect of the nonlinear force–displacement relationship of ASSMs for in-plane motion was investigated. A theoretical solution was derived to analyze this nonlinearity. By successfully fabricating and measuring an ASSM, the theoretical results agreed well with the experimental results. The results indicated that ASSMs have a nonlinear force–displacement relationship, which is similar to that of hardening springs. The taper angle has a significant effect on the nonlinear displacement of ASSMs. When the taper angle was small, no obvious effect appeared on the nonlinearity of the microsprings with different numbers of turns. When the beam length increased, the critical force for nonlinear deflection decreased. Full article
(This article belongs to the Section Electronic Materials)
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Open AccessArticle
Assessment of Rational Design of Self-Compacting Concrete Incorporating Fly Ash and Limestone Powder in Terms of Long-Term Durability
Materials 2020, 13(12), 2863; https://doi.org/10.3390/ma13122863 - 26 Jun 2020
Viewed by 346
Abstract
Self-compaction concrete (SCC) is ranked among the main technological innovations of the last decades. Hence, it introduces a suitable possibility for further utilization of supplementary cementitious materials (SCM) in terms of sustainable development. The aim of the work is the assessment of a [...] Read more.
Self-compaction concrete (SCC) is ranked among the main technological innovations of the last decades. Hence, it introduces a suitable possibility for further utilization of supplementary cementitious materials (SCM) in terms of sustainable development. The aim of the work is the assessment of a new approach to binder design, which takes into consideration the activity of the used mineral additive. The proposed approach, which allows a systematic design of a binding system with varied properties of the used mineral additive, was studied on ternary blends consisting of Portland cement (PC), limestone powder and fly ash (FA). The verification was conducted on SCC mixtures in terms of their workability, mechanical properties and the most attention was paid to long-term durability. The long-term durability was assessed on the basis of shrinkage measurement, freeze-thaw resistance and permeability tests including initial surface absorption, chloride migration, water penetration and an accelerated carbonation test, which was compared with the evolution of carbonation front in normal conditions. The durability of studied mixtures was evaluated by using durability loss index, which allow general assessment on the basis of multiple parameters. The carbonation resistance had a dominant importance on the final durability performance of studied mixtures. The experimental program revealed that the proposed design method is reliable only in terms of properties in fresh state and mechanical performance, which were similar with control mixture. Despite suitable results of freeze-thaw resistance and shrinkage, an increasing amount of fly ash in terms of the new design concept led to a fundamental increase of permeability and thus to decay of long-term durability. Acceptable properties were achieved for the lowest dosage of fly ash. Full article
(This article belongs to the Section Construction and Building Materials)
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Open AccessArticle
Harmony Search Optimisation of Dispersed Laminated Composite Plates
Materials 2020, 13(12), 2862; https://doi.org/10.3390/ma13122862 - 26 Jun 2020
Cited by 1 | Viewed by 327
Abstract
One of the major goals in the process of designing structural components is to achieve the highest possible buckling load of the structural component while keeping the cost and weight at a minimum. This paper illustrates the application of the harmony search algorithm [...] Read more.
One of the major goals in the process of designing structural components is to achieve the highest possible buckling load of the structural component while keeping the cost and weight at a minimum. This paper illustrates the application of the harmony search algorithm to the buckling load maximisation of dispersed laminated composite plates with rectangular geometry. The ply thicknesses and fiber orientation angles of the plies were chosen as the design variables. Besides the commonly used carbon fiber reinforced composites, boron/epoxy and glass/epoxy composite plates were also optimised using the harmony search algorithm. Furthermore, the optimisation algorithm was applied to plates with three different aspect ratios (ratio of the longer side length to the shorter side length of the plate). The buckling loads of the plates with optimised dispersed stacking sequences were compared to the buckling loads of plates with the commonly applied 0°, ±45°, and 90° fiber angle sequence and identical ply thicknesses. For all three aspect ratios and materials in this study, the dispersed stacking sequences performed better than the plates with regular stacking sequences. Full article
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Open AccessArticle
Effect of Strain Rate on Compressive Behavior of a Zr-Based Metallic Glass under a Wide Range of Strain Rates
Materials 2020, 13(12), 2861; https://doi.org/10.3390/ma13122861 - 25 Jun 2020
Viewed by 384
Abstract
The strain rate effect on the mechanical behavior of amorphous alloys has aroused general interest. Most studies in this area have focused on quasi-static and high strain-rate compressive deformations. However, experimental results have been few, or even non-existent, under a moderate strain-rate loading. [...] Read more.
The strain rate effect on the mechanical behavior of amorphous alloys has aroused general interest. Most studies in this area have focused on quasi-static and high strain-rate compressive deformations. However, experimental results have been few, or even non-existent, under a moderate strain-rate loading. This article extends the traditional split Hopkinson pressure bar (SHPB) technique to characterize compressive deformation of an amorphous alloy at medium strain rates. The compressive behavior of Zr65.25Cu21.75Al8Ni4Nb1 amorphous alloy shows a negative strain rate effect on the yield strength with a quasi-static, moderate to high strain-rate range, and the fracture angle increases from 44° at 10−5 s−1 to 60° at 4000 s−1 as strain rate increases. Herein, we introduce a modified cooperative shear model to describe the compressive behavior of the current amorphous alloy under a broad strain rate range. The model predicts that the normalized yield strength will linearly descend with logarithmic strain rate when the strain rate is less than a critical strain rate, however, which rapidly decreases linearly with the square of the strain rate at high strain rates. The predicted data of the model are highly consistent with the current experimental results. These findings provide support for future engineering applications of amorphous alloys. Full article
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Open AccessArticle
Effect of Heat Treatment on Cr2Nb Phase and Properties of Spark Plasma Sintered Cu-2Cr-1Nb Alloy
Materials 2020, 13(12), 2860; https://doi.org/10.3390/ma13122860 - 25 Jun 2020
Viewed by 387
Abstract
Achieving a good match between strength and conductivity is a challenge of the development of the high-performance Cu-Cr-Nb alloy for aerospace and fusion energy. The effect of heat treatment on Cr2Nb phase, strength and conductivity of spark plasma sintered (SPSed) Cu-2Cr-1Nb [...] Read more.
Achieving a good match between strength and conductivity is a challenge of the development of the high-performance Cu-Cr-Nb alloy for aerospace and fusion energy. The effect of heat treatment on Cr2Nb phase, strength and conductivity of spark plasma sintered (SPSed) Cu-2Cr-1Nb (at%) alloy was investigated. The results illustrated that Cr2Nb phase of Cu-2Cr-1Nb alloy can be regulated by heat treatment, multi-scale Cr2Nb phase with sizes of 0.10–0.50 μm, 30–100 nm and less than 30 nm was obtained, and the strength and conductivity were significantly increased after heat treatment at 500 °C for 2 h, the room temperature tensile strength and conductivity were 332 MPa and 86.7% IACS, 2.5% and 34.8% higher than those of as-SPSed alloy; the tensile strength at 700 °C was 76 MPa. Increasing heat treatment temperature and time, the tensile strength of the alloy was reduced by 1.5%, 4.3% and 12.3% after heat treatment at 500 °C, 700 °C and 950 °C for 72 h. The good match between strength and conductivity of Cu-Cr-Nb alloy was obtained by reducing the content of alloying elements (Cr and Nb) and microstructure regulation. This approach can be used to prepare structural/functional materials with excellent strength and conductivity. Full article
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Open AccessFeature PaperReview
Radio-Frequency/Microwave Gas Sensors Using Conducting Polymer
Materials 2020, 13(12), 2859; https://doi.org/10.3390/ma13122859 - 25 Jun 2020
Viewed by 421
Abstract
In this review, the advances in radio-frequency (RF) /microwave chemical gas sensors using conducting polymers are discussed. First, the introduction of various conducting polymers is described. Only polyaniline (PANi), polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT), which are mainly used for gas sensors in RF/microwave [...] Read more.
In this review, the advances in radio-frequency (RF) /microwave chemical gas sensors using conducting polymers are discussed. First, the introduction of various conducting polymers is described. Only polyaniline (PANi), polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT), which are mainly used for gas sensors in RF/microwave region, are focused in this review. Sensing mechanism of the three conducting polymers are presented. And the RF/microwave characteristics and RF/microwave applications of the three conducting polymers are discussed. Moreover, the gas sensors using conducting polymers in RF/microwave frequencies are described. Finally, the the challenges and the prospects of the next generation of the RF/microwave based chemical sensors for wireless applications are proposed. Full article
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Open AccessArticle
A Hybrid Model for Predicting Bone Healing around Dental Implants
Materials 2020, 13(12), 2858; https://doi.org/10.3390/ma13122858 - 25 Jun 2020
Viewed by 454
Abstract
Background: The effect of the short-term bone healing process is typically neglected in numerical models of bone remodeling for dental implants. In this study, a hybrid two-step algorithm was proposed to enable a more accurate prediction for the performance of dental implants. Methods: [...] Read more.
Background: The effect of the short-term bone healing process is typically neglected in numerical models of bone remodeling for dental implants. In this study, a hybrid two-step algorithm was proposed to enable a more accurate prediction for the performance of dental implants. Methods: A mechano-regulation algorithm was firstly used to simulate the tissue differentiation around a dental implant during the short-term bone healing. Then, the result was used as the initial state of the bone remodeling model to simulate the long-term healing of the bones. The algorithm was implemented by a 3D finite element model. Results: The current hybrid model reproduced several features which were discovered in the experiments, such as stress shielding effect, high strength bone connective tissue bands, and marginal bone loss. A reasonable location of bone resorptions and the stability of the dental implant is predicted, compared with those predicted by the conventional bone remodeling model. Conclusions: The hybrid model developed here predicted bone healing processes around dental implants more accurately. It can be used to study bone healing before implantation surgery and assist in the customization of dental implants. Full article
(This article belongs to the Special Issue Dental Implant Materials 2019)
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Open AccessArticle
Low Resistance TiO2/p-Si Heterojunction for Tandem Solar Cells
Materials 2020, 13(12), 2857; https://doi.org/10.3390/ma13122857 - 25 Jun 2020
Viewed by 345
Abstract
Niobium-doped titanium dioxide (Ti1−xNbxO2) films were grown on p-type Si substrates at low temperature (170 °C) using an atomic layer deposition technique. The as-deposited films were amorphous and showed low electrical conductivity. The films became electrically [...] Read more.
Niobium-doped titanium dioxide (Ti1−xNbxO2) films were grown on p-type Si substrates at low temperature (170 °C) using an atomic layer deposition technique. The as-deposited films were amorphous and showed low electrical conductivity. The films became electrically well-conducting and crystallized into the an anatase structure upon reductive post-deposition annealing at 600 °C in an H2 atmosphere for 30 min. It was shown that the Ti0.72Nb0.28O2/p+-Si heterojunction fabricated on low resistivity silicon (10−3 Ω cm) had linear current–voltage characteristic with a specific contact resistivity as low as 23 mΩ·cm2. As the resistance dependence on temperature revealed, the current across the Ti0.72Nb0.28O2/p+-Si heterojunction was mainly determined by the band-to-band charge carrier tunneling through the junction. Full article
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Open AccessArticle
Utilization of Polypropylene in the Production of Metal-Filled Polymer Composites: Development and Characteristics
Materials 2020, 13(12), 2856; https://doi.org/10.3390/ma13122856 - 25 Jun 2020
Viewed by 308
Abstract
Metal-filled composites based on polypropylene waste have been successfully obtained with an injection molding method of metalized polymer raw materials. Using the model polymer, the peculiarities of the formation of the copper layer in solutions of chemical metallization on the polypropylene surface were [...] Read more.
Metal-filled composites based on polypropylene waste have been successfully obtained with an injection molding method of metalized polymer raw materials. Using the model polymer, the peculiarities of the formation of the copper layer in solutions of chemical metallization on the polypropylene surface were investigated and the main factors influencing this process were established. The main influence on the rate of reduction of copper in solutions of chemical metallization has the concentration of copper sulfate, sodium hydroxide, and EDTA-Na2. It was shown that the efficiency of the copper plating process also strongly depends on polymer processing, which follows the activation. In case of the use of simple activation, it is not possible to obtain metalized raw materials with high efficiency. Additional processing of activated polymer raw materials is required to carry out the process with high efficiency. The amount of reduced copper on the polymer surface can be adjusted by changing the concentration of the components of the chemical metallization solution, as well as the degree of loading of the polymer raw material. Examination by electron scanning microscopy of the obtained metalized polypropylene showed that the copper coating on the polymer particles is formed with a high degree of surface coverage. The formed copper coating is free of copper oxides, which is confirmed by X-ray diffraction studies and analysis of the spectrum of characteristic X-rays. Metal-filled composites have been characterized by the effect of copper on mechanical and rheological (MFR) properties. The Differential Scanning Calorimetry (DSC) and Thermogravimetric (TG) methods show a certain effect of metal on the magnitude of thermal effects and the rate of weight loss. Full article
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Open AccessArticle
External Gas-Assisted Mold Temperature Control Improves Weld Line Quality in the Injection Molding Process
Materials 2020, 13(12), 2855; https://doi.org/10.3390/ma13122855 - 25 Jun 2020
Viewed by 329
Abstract
Simulations and experiments were conducted with gas temperatures of 200–400 °C to investigate the impact of external gas-assisted mold temperature control (Ex-GMTC) on the quality of weld line of molding products. In the heating step, the heating rate was 19.6 °C/s from 30 [...] Read more.
Simulations and experiments were conducted with gas temperatures of 200–400 °C to investigate the impact of external gas-assisted mold temperature control (Ex-GMTC) on the quality of weld line of molding products. In the heating step, the heating rate was 19.6 °C/s from 30 to 128.5 °C in the first 5 s in a 400 °C gas environment. When applied to heating the weld line area of an injection mold, Ex-GMTC improved the appearance of the weld line when the cavity temperature was preheated to 150 °C. For the tensile strength test, a melt flow simulation comparing the packing pressure of different mesh thicknesses revealed that Ex-GMTC helped maintain a high pressure in the weld line area in different packing periods. This was verified by an experiment where Ex-GMTC was applied with 400 °C gas to change the mesh area temperature. The result indicated that an increase in the weld line area temperature from 60 to 180 °C improves the tensile strength of all mesh thicknesses, which was more pronounced with thinner parts, especially at 0.4 mm. The simulations revealed that high temperature is concentrated in the weld line area of the cavity surface, thus reducing the energy wasted during heating. Full article
(This article belongs to the Special Issue Advances in Injection Molding for Polymer Processing)
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Open AccessArticle
Crossover from Ferroelectric to Relaxor Behavior in Ba1−xCaxTiO3 (x = 0.17) System
Materials 2020, 13(12), 2854; https://doi.org/10.3390/ma13122854 - 25 Jun 2020
Viewed by 319
Abstract
The dielectric properties of Ba1−xCaxTiO3 (x = 0.17) ceramics were studied in a wide frequency range of 20 Hz–53 GHz. Diffused ferroelectric phase transition was revealed close to 339 K in the dielectric properties of ceramics. [...] Read more.
The dielectric properties of Ba1−xCaxTiO3 (x = 0.17) ceramics were studied in a wide frequency range of 20 Hz–53 GHz. Diffused ferroelectric phase transition was revealed close to 339 K in the dielectric properties of ceramics. The behaviour of distributions of relaxation times in vicinity of the ferroelectric phase transition temperature is also typical for order-disorder ferroelectric phase transition. However, at lower temperatures (below 200 K), the most probable relaxation increased according to the Arrhenius law. At lower temperatures the maximum of the imaginary part of dielectric permittivity versus temperature strongly shifted to higher temperatures when the frequency increased (from 125 K at 1.21 kHz to 300 K at 33 GHz). This behaviour was attributed to the dynamics of Ti ions. The origin of the crossover from ferroelectric to relaxor behaviour of Ba1−xCaxTiO3 (x = 0.17) ceramics is discussed in the paper. Full article
(This article belongs to the Special Issue Ferroelectric Materials: From Fundamentals to Applications )
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Open AccessReview
Hydrogel Dressings for the Treatment of Burn Wounds: An Up-To-Date Overview
Materials 2020, 13(12), 2853; https://doi.org/10.3390/ma13122853 - 25 Jun 2020
Viewed by 391
Abstract
Globally, the fourth most prevalent devastating form of trauma are burn injuries. Ideal burn wound dressings are fundamental to facilitate the wound healing process and decrease pain in lower time intervals. Conventional dry dressing treatments, such as those using absorbent gauze and/or absorbent [...] Read more.
Globally, the fourth most prevalent devastating form of trauma are burn injuries. Ideal burn wound dressings are fundamental to facilitate the wound healing process and decrease pain in lower time intervals. Conventional dry dressing treatments, such as those using absorbent gauze and/or absorbent cotton, possess limited therapeutic effects and require repeated dressing changes, which further aggravate patients’ suffering. Contrariwise, hydrogels represent a promising alternative to improve healing by assuring a moisture balance at the burn site. Most studies consider hydrogels as ideal candidate materials for the synthesis of wound dressings because they exhibit a three-dimensional (3D) structure, which mimics the natural extracellular matrix (ECM) of skin in regard to the high-water amount, which assures a moist environment to the wound. There is a wide variety of polymers that have been used, either alone or blended, for the fabrication of hydrogels designed for biomedical applications focusing on treating burn injuries. The aim of this paper is to provide an up-to-date overview of hydrogels applied in burn wound dressings. Full article
(This article belongs to the Special Issue Nanomaterials for Drug Delivery)
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