Materials

2407 KiB

Open AccessArticle

Change in Dielectric Properties in the Microwave Frequency Region of Polypyrrole–Coated Textiles during Aging

by Eva Hakansson, Akif Kaynak and Abbas Kouzani

Materials 2016, 9(7), 609; https://doi.org/10.3390/ma9070609 - 22 Jul 2016

Cited by 5 | Viewed by 5416

Complex permittivity of conducting polypyrrole (PPy)-coated Nylon-Lycra textiles is measured using a free space transmission measurement technique over the frequency range of 1–18 GHz. The aging of microwave dielectric properties and reflection, transmission and absorption for a period of 18 months is demonstrated. [...] Read more.

Complex permittivity of conducting polypyrrole (PPy)-coated Nylon-Lycra textiles is measured using a free space transmission measurement technique over the frequency range of 1–18 GHz. The aging of microwave dielectric properties and reflection, transmission and absorption for a period of 18 months is demonstrated. PPy-coated fabrics are shown to be lossy over the full frequency range. The levels of absorption are shown to be higher than reflection in the tested samples. This is attributed to the relatively high resistivity of the PPy-coated fabrics. Both the dopant concentration and polymerisation time affect the total shielding effectiveness and microwave aging behaviour. Distinguishing either of these two factors as being exclusively the dominant mechanism of shielding effectiveness is shown to be difficult. It is observed that the PPy-coated Nylon-Lycra samples with a p-toluene sulfonic acid (pTSA) concentration of 0.015 M and polymerisation times of 60 min and 180 min have 37% and 26% decrease in total transmission loss, respectively, upon aging for 72 weeks at room temperature (20 °C, 65% Relative humidity (RH)). The concentration of the dopant also influences the microwave aging behaviour of the PPy-coated fabrics. The samples with a higher dopant concentration of 0.027 mol/L pTSA are shown to have a transmission loss of 32.6% and 16.5% for short and long polymerisation times, respectively, when aged for 72 weeks. The microwave properties exhibit better stability with high dopant concentration and/or longer polymerization times. High pTSA dopant concentrations and/or longer polymerisation times result in high microwave insertion loss and are more effective in reducing the transmission and also increasing the longevity of the electrical properties. Full article

(This article belongs to the Special Issue Electroactive Polymers)

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9084 KiB

Open AccessArticle

Design and Fabrication of a Precision Template for Spine Surgery Using Selective Laser Melting (SLM)

by Di Wang, Yimeng Wang, Jianhua Wang, Changhui Song, Yongqiang Yang, Zimian Zhang, Hui Lin, Yongqiang Zhen and Suixiang Liao

Materials 2016, 9(7), 608; https://doi.org/10.3390/ma9070608 - 22 Jul 2016

Cited by 40 | Viewed by 11204

Abstract

In order to meet the clinical requirements of spine surgery, this paper proposes the fabrication of the customized template for spine surgery through computer-aided design. A 3D metal printing-selective laser melting (SLM) technique was employed to directly fabricate the 316L stainless steel template, [...] Read more.

In order to meet the clinical requirements of spine surgery, this paper proposes the fabrication of the customized template for spine surgery through computer-aided design. A 3D metal printing-selective laser melting (SLM) technique was employed to directly fabricate the 316L stainless steel template, and the metal template with tiny locating holes was used as an auxiliary tool to insert spinal screws inside the patient’s body. To guarantee accurate fabrication of the template for cervical vertebra operation, the contact face was placed upwards to improve the joint quality between the template and the cervical vertebra. The joint surface of the printed template had a roughness of Ra = 13 ± 2 μm. After abrasive blasting, the surface roughness was Ra = 7 ± 0.5 μm. The surgical metal template was bound with the 3D-printed Acrylonitrile Butadiene Styrene (ABS) plastic model. The micro-hardness values determined at the cross-sections of SLM-processed samples varied from HV0.3 250 to HV0.3 280, and the measured tensile strength was in the range of 450 MPa to 560 MPa, which showed that the template had requisite strength. Finally, the metal template was clinically used in the patient’s surgical operation, and the screws were inserted precisely as the result of using the auxiliary template. The geometrical parameters of the template hole (e.g., diameter and wall thickness) were optimized, and measures were taken to optimize the key geometrical units (e.g., hole units) in metal 3D printing. Compared to the traditional technology of screw insertion, the use of the surgical metal template enabled the screws to be inserted more easily and accurately during spinal surgery. However, the design of the high-quality template should fully take into account the clinical demands of surgeons, as well as the advice of the designing engineers and operating technicians. Full article

(This article belongs to the Special Issue 3D Printing for Biomedical Engineering)

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6396 KiB

Open AccessArticle

Influence of Network Structure on Glass Transition Temperature of Elastomers

by Katarzyna Bandzierz, Louis Reuvekamp, Jerzy Dryzek, Wilma Dierkes, Anke Blume and Dariusz Bielinski

Materials 2016, 9(7), 607; https://doi.org/10.3390/ma9070607 - 22 Jul 2016

Cited by 93 | Viewed by 11927

Abstract

It is generally believed that only intermolecular, elastically-effective crosslinks influence elastomer properties. The role of the intramolecular modifications of the polymer chains is marginalized. The aim of our study was the characterization of the structural parameters of cured elastomers, and determination of their [...] Read more.

It is generally believed that only intermolecular, elastically-effective crosslinks influence elastomer properties. The role of the intramolecular modifications of the polymer chains is marginalized. The aim of our study was the characterization of the structural parameters of cured elastomers, and determination of their influence on the behavior of the polymer network. For this purpose, styrene-butadiene rubbers (SBR), cured with various curatives, such as DCP, TMTD, TBzTD, Vulcuren^®, DPG/S₈, CBS/S₈, MBTS/S₈ and ZDT/S₈, were investigated. In every series of samples a broad range of crosslink density was obtained, in addition to diverse crosslink structures, as determined by equilibrium swelling and thiol-amine analysis. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were used to study the glass transition process, and positron annihilation lifetime spectroscopy (PALS) to investigate the size of the free volumes. For all samples, the values of the glass transition temperature (T_g) increased with a rise in crosslink density. At the same time, the free volume size proportionally decreased. The changes in T_g and free volume size show significant differences between the series crosslinked with various curatives. These variations are explained on the basis of the curatives’ structure effect. Furthermore, basic structure-property relationships are provided. They enable the prediction of the effect of curatives on the structural parameters of the network, and some of the resulting properties. It is proved that the applied techniques—DSC, DMA, and PALS—can serve to provide information about the modifications to the polymer chains. Moreover, on the basis of the obtained results and considering the diversified curatives available nowadays, the usability of “part per hundred rubber” (phr) unit is questioned. Full article

(This article belongs to the Special Issue Advances in Research on Elastomers)

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2747 KiB

Open AccessFeature PaperArticle

Hydration of Hybrid Alkaline Cement Containing a Very Large Proportion of Fly Ash: A Descriptive Model

by Inés Garcia-Lodeiro, Shane Donatello, Ana Fernández-Jiménez and Ángel Palomo

Materials 2016, 9(7), 605; https://doi.org/10.3390/ma9070605 - 22 Jul 2016

Cited by 119 | Viewed by 10397

Abstract

In hybrid alkaline fly ash cements, a new generation of binders, hydration, is characterized by features found in both ordinary portland cement (OPC) hydration and the alkali activation of fly ash (AAFA). Hybrid alkaline fly ash cements typically have a high fly ash [...] Read more.

In hybrid alkaline fly ash cements, a new generation of binders, hydration, is characterized by features found in both ordinary portland cement (OPC) hydration and the alkali activation of fly ash (AAFA). Hybrid alkaline fly ash cements typically have a high fly ash (70 wt % to 80 wt %) and low clinker (20 wt % to 30 wt %) content. The clinker component favors curing at ambient temperature. A hydration mechanism is proposed based on the authors’ research on these hybrid binders over the last five years. The mechanisms for OPC hydration and FA alkaline activation are summarized by way of reference. In hybrid systems, fly ash activity is visible at very early ages, when two types of gel are formed: C–S–H from the OPC and N–A–S–H from the fly ash. In their mutual presence, these gels tend to evolve, respectively, into C–A–S–H and (N,C)–A–S–H. The use of activators with different degrees of alkalinity has a direct impact on reaction kinetics but does not modify the main final products, a mixture of C–A–S–H and (N,C)–A–S–H gels. The proportion of each gel in the mix does, however, depend on the alkalinity generated in the medium. Full article

(This article belongs to the Special Issue Advances in Geopolymers and Alkali-Activated Materials)

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Graphical abstract

Graphical abstract
Full article ">Figure 1
Position of hybrid alkaline fly ash cements on the pure Portland cement (PC)-pure alkali activation of fly ash (AAFA) spectrum, relative to pozzolanic fly ash cements. Full article ">Figure 2
Descriptive model for poly-phase OPC grain hydration and a typical calorimetric curve for OPC hydration (grain drawings adapted from Scrivener as quoted in Taylor (1997) [<a href="#B27-materials-09-00605" class="html-bibr">27</a>]). Full article ">Figure 3
Conceptual model for AAFA cement hydration. (a) Starting material; (b) initial alkaline attack on the ashes and early N–A–S–H gel formation; (c) gel polymerisation and positioning on the inner and outer surfaces of the exposed fly ash; and (d) mature and heterogeneous AAFA cement paste microstructure (adapted from Fernández and Palomo [<a href="#B7-materials-09-00605" class="html-bibr">7</a>]). Full article ">Figure 4
29Si MAS NMR spectra: (a) initial raw mix (30% clinker + 70% fly ash); (b) 28-day water-hydrated material; (c) 28-day NaOH-hydrated material; (d) 28-day NaOH + WG (waterglass)-hydrated material (adapted from Palomo et al. [<a href="#B18-materials-09-00605" class="html-bibr">18</a>]). Full article ">Figure 5
Changes in gel composition and microstructure of a hybrid alkaline cement with a very high fly ash content: (a) initial stage; (b) early age sample (min); (c) early age sample (h); (d) 7-day sample; (e) 28-day sample. Full article ">Figure 6
Nano-structural mechanism for gel formation in hybrid alkaline cements; (a) dissolution of ionic species from the source of alumino-and calcium silicates; (b) precipitation of aluminum-high (type I) N–A–S–H gels and C–S–H gels; (c) silica uptake by both gels with an increase in C–S–H gel mean chain length and the generation of silica-high type 2 N–A–S–H gels; (d) diffusion of aluminum and calcium in the matrix and their uptake, respectively, in C–S–H and N–A–S–H gels to form (N,C)–A–S–H gels; (e) distortion of the (N,C)–A–S–H gel due to the polarizing effect of calcium, leading to its rupture, while the C–A–S–H gel continues to take up aluminum species in bridging positions, favoring chain cross-linking and hence a more polymerized structure (hydrogen bonds omitted in this final stage). Full article ">

11275 KiB

Open AccessArticle

Transformation and Precipitation Reactions by Metal Active Gas Pulsed Welded Joints from X2CrNiMoN22-5-3 Duplex Stainless Steels

by Ion-Dragos Utu, Ion Mitelea, Sorin Dumitru Urlan and Corneliu Marius Crăciunescu

Materials 2016, 9(7), 606; https://doi.org/10.3390/ma9070606 - 21 Jul 2016

Cited by 12 | Viewed by 6123

Abstract

The high alloying degree of Duplex stainless steels makes them susceptible to the formation of intermetallic phases during their exposure to high temperatures. Precipitation of these phases can lead to a decreasing of the corrosion resistance and sometimes of the toughness. Starting from [...] Read more.

The high alloying degree of Duplex stainless steels makes them susceptible to the formation of intermetallic phases during their exposure to high temperatures. Precipitation of these phases can lead to a decreasing of the corrosion resistance and sometimes of the toughness. Starting from the advantages of the synergic Metal Active Gas (MAG) pulsed welding process, this paper analyses the structure formation particularities of homogeneous welded joints from Duplex stainless steel. The effect of linear welding energy on the structure morphology of the welded joints was revealed by macro- and micrographic examinations, X-ray energy dispersion analyses, measurements of ferrite proportion and X-ray diffraction analysis. The results obtained showed that the transformation of ferrite into austenite is associated with the chromium, nickel, molybdenum and nitrogen distribution between these two phases and their redistribution degree is closely linked to the overall heat cycle of the welding process. The adequate control of the energy inserted in the welded components provides an optimal balance between the two microstructural constituents (Austenite and Ferrite) and avoids the formation of undesirable intermetallic phases. Full article

(This article belongs to the Section Advanced Materials Characterization)

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4004 KiB

Open AccessArticle

Thermal Stability and Flammability of Styrene-Butadiene Rubber-Based (SBR) Ceramifiable Composites

by Rafał Anyszka, Dariusz M. Bieliński, Zbigniew Pędzich, Przemysław Rybiński, Mateusz Imiela, Mariusz Siciński, Magdalena Zarzecka-Napierała, Tomasz Gozdek and Paweł Rutkowski

Materials 2016, 9(7), 604; https://doi.org/10.3390/ma9070604 - 21 Jul 2016

Cited by 17 | Viewed by 9671

Abstract

Ceramifiable styrene-butadiene (SBR)-based composites containing low-softening-point-temperature glassy frit promoting ceramification, precipitated silica, one of four thermally stable refractory fillers (halloysite, calcined kaolin, mica or wollastonite) and a sulfur-based curing system were prepared. Kinetics of vulcanization and basic mechanical properties were analyzed and added [...] Read more.

Ceramifiable styrene-butadiene (SBR)-based composites containing low-softening-point-temperature glassy frit promoting ceramification, precipitated silica, one of four thermally stable refractory fillers (halloysite, calcined kaolin, mica or wollastonite) and a sulfur-based curing system were prepared. Kinetics of vulcanization and basic mechanical properties were analyzed and added as Supplementary Materials. Combustibility of the composites was measured by means of cone calorimetry. Their thermal properties were analyzed by means of thermogravimetry and specific heat capacity determination. Activation energy of thermal decomposition was calculated using the Flynn-Wall-Ozawa method. Finally, compression strength of the composites after ceramification was measured and their micromorphology was studied by scanning electron microscopy. The addition of a ceramification-facilitating system resulted in the lowering of combustibility and significant improvement of the thermal stability of the composites. Moreover, the compression strength of the mineral structure formed after ceramification is considerably high. The most promising refractory fillers for SBR-based ceramifiable composites are mica and halloysite. Full article

(This article belongs to the Special Issue Advances in Research on Elastomers)

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Figure 1
Cone calorimetry analysis of the vulcanizates: heat release rate (a); total heat released (b); averaged heat release rate (c) and mass loss (d). Full article ">Figure 2
Changes to specific heat capacity of the ceramifiable composites with the increase of temperature. Full article ">Figure 3
Thermogravimetric characteristic of the samples at heating rate of 5 K/min (a) and plots exhibiting Flynn-Wall-Ozawa approach toward estimation of decomposition energy for: SBR_pris (b); SBR_hal (c); SBR_kao (d); SBR_mic (e) and SBR_wol (f) vulcanizates. Full article ">Figure 3 Cont.
Thermogravimetric characteristic of the samples at heating rate of 5 K/min (a) and plots exhibiting Flynn-Wall-Ozawa approach toward estimation of decomposition energy for: SBR_pris (b); SBR_hal (c); SBR_kao (d); SBR_mic (e) and SBR_wol (f) vulcanizates. Full article ">Figure 4
SEM photographs of the ceramified composites’ cross-sections taken under different magnifications of 350× (a1, b1, c1 and d1) and 1000× (a2, b2, c2 and d2) for: SBR_hal (a); SBR_kao (b); SBR_mic (c) and SBR_wol (d). Full article ">Figure 4 Cont.
SEM photographs of the ceramified composites’ cross-sections taken under different magnifications of 350× (a1, b1, c1 and d1) and 1000× (a2, b2, c2 and d2) for: SBR_hal (a); SBR_kao (b); SBR_mic (c) and SBR_wol (d). Full article ">

7872 KiB

Open AccessArticle

Fiberglass Grids as Sustainable Reinforcement of Historic Masonry

by Luca Righetti, Vikki Edmondson, Marco Corradi and Antonio Borri

Materials 2016, 9(7), 603; https://doi.org/10.3390/ma9070603 - 21 Jul 2016

Cited by 16 | Viewed by 6366

Abstract

Fiber-reinforced composite (FRP) materials have gained an increasing success, mostly for strengthening, retrofitting and repair of existing historic masonry structures and may cause a significant enhancement of the mechanical properties of the reinforced members. This article summarizes the results of previous experimental activities [...] Read more.

Fiber-reinforced composite (FRP) materials have gained an increasing success, mostly for strengthening, retrofitting and repair of existing historic masonry structures and may cause a significant enhancement of the mechanical properties of the reinforced members. This article summarizes the results of previous experimental activities aimed at investigating the effectiveness of GFRP (Glass Fiber Reinforced Polymers) grids embedded into an inorganic mortar to reinforce historic masonry. The paper also presents innovative results on the relationship between the durability and the governing material properties of GFRP grids. Measurements of the tensile strength were made using specimens cut off from GFRP grids before and after ageing in aqueous solution. The tensile strength of a commercially available GFRP grid has been tested after up 450 days of storage in deionized water and NaCl solution. A degradation in tensile strength and Young’s modulus up to 30.2% and 13.2% was recorded, respectively. This degradation indicated that extended storage in a wet environment may cause a decrease in the mechanical properties. Full article

(This article belongs to the Section Advanced Composites)

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6489 KiB

Open AccessArticle

Guided Wave Based Crack Detection in the Rivet Hole Using Global Analytical with Local FEM Approach

by Md Yeasin Bhuiyan, Yanfeng Shen and Victor Giurgiutiu

Materials 2016, 9(7), 602; https://doi.org/10.3390/ma9070602 - 21 Jul 2016

Cited by 57 | Viewed by 6989

Abstract

In this article, ultrasonic guided wave propagation and interaction with the rivet hole cracks has been formulated using closed-form analytical solution while the local damage interaction, scattering, and mode conversion have been obtained from finite element analysis. The rivet hole cracks (damage) in [...] Read more.

In this article, ultrasonic guided wave propagation and interaction with the rivet hole cracks has been formulated using closed-form analytical solution while the local damage interaction, scattering, and mode conversion have been obtained from finite element analysis. The rivet hole cracks (damage) in the plate structure gives rise to the non-axisymmetric scattering of Lamb wave, as well as shear horizontal (SH) wave, although the incident Lamb wave source (primary source) is axisymmetric. The damage in the plate acts as a non-axisymmetric secondary source of Lamb wave and SH wave. The scattering of Lamb and SH waves are captured using wave damage interaction coefficient (WDIC). The scatter cubes of complex-valued WDIC are formed that can describe the 3D interaction (frequency, incident direction, and azimuth direction) of Lamb waves with the damage. The scatter cubes are fed into the exact analytical framework to produce the time domain signal. This analysis enables us to obtain the optimum design parameters for better detection of the cracks in a multiple-rivet-hole problem. The optimum parameters provide the guideline of the design of the sensor installation to obtain the most noticeable signals that represent the presence of cracks in the rivet hole. Full article

(This article belongs to the Special Issue Advances in Structural Health Monitoring for Aerospace Structures)

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2611 KiB

Open AccessArticle

Gas Diffusion Electrodes Manufactured by Casting Evaluation as Air Cathodes for Microbial Fuel Cells (MFC)

by Sandipam Srikanth, Deepak Pant, Xochitl Dominguez-Benetton, Inge Genné, Karolien Vanbroekhoven, Philippe Vermeiren and Yolanda Alvarez-Gallego

Materials 2016, 9(7), 601; https://doi.org/10.3390/ma9070601 - 21 Jul 2016

Cited by 23 | Viewed by 7548

Abstract

One of the most intriguing renewable energy production methods being explored currently is electrical power generation by microbial fuel cells (MFCs). However, to make MFC technology economically feasible, cost efficient electrode manufacturing processes need to be proposed and demonstrated. In this context, VITO [...] Read more.

One of the most intriguing renewable energy production methods being explored currently is electrical power generation by microbial fuel cells (MFCs). However, to make MFC technology economically feasible, cost efficient electrode manufacturing processes need to be proposed and demonstrated. In this context, VITO has developed an innovative electrode manufacturing process based on film casting and phase inversion. The screening and selection process of electrode compositions was done based on physicochemical properties of the active layer, which in turn maintained a close relation with their composition A dual hydrophilic-hydrophobic character in the active layer was achieved with values of ε_hydrophilic up to 10% while ε_TOTAL remained in the range 65 wt % to 75 wt %. Eventually, selected electrodes were tested as air cathodes for MFC in half cell and full cell modes. Reduction currents, up to −0.14 mA·cm²⁻ at −100 mV (vs. Ag/AgCl) were reached in long term experiments in the cathode half-cell. In full MFC, a maximum power density of 380 mW·m⁻² was observed at 100 Ω external load. Full article

(This article belongs to the Special Issue Advances in Renewable Energy Conversion Materials)

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4254 KiB

Open AccessArticle

Investigation of the Self-Healing Behaviors of Microcapsules/Bitumen Composites by a Repetitive Direct Tension Test

by Jun-Feng Su, Peng Yang, Ying-Yuan Wang, Shan Han, Ning-Xu Han and Wei Li

Materials 2016, 9(7), 600; https://doi.org/10.3390/ma9070600 - 21 Jul 2016

Cited by 33 | Viewed by 7734

Abstract

The aim of this work was to evaluate the self-healing behaviors of bitumen using microcapsules containing rejuvenator by a modified fracture healing–refracture method through a repetitive tension test. Microcapsules had mean size values of 10, 20 and 30 μm with a same core/shell [...] Read more.

The aim of this work was to evaluate the self-healing behaviors of bitumen using microcapsules containing rejuvenator by a modified fracture healing–refracture method through a repetitive tension test. Microcapsules had mean size values of 10, 20 and 30 μm with a same core/shell ratio of 1/1. Various microcapsules/bitumen samples were fabricated with microcapsule contents of 1.0, 3.0 and 5.0 wt. %, respectively. Tension strength values of microcapsules/bitumen samples were measured by a reparative fracture-healing process under different temperatures. It was found that these samples had tensile strength values larger than the data of pure bitumen samples under the same conditions after the four tensile fracture-healing cycles. Fracture morphology investigation and mechanism analysis indicated that the self-healing process was a process consisting of microcapsules being broken, penetrated and diffused. Moreover, the crack healing of bitumen can be considered as a viscosity driven process. The self-healing ability partly repaired the damage of bitumen during service life by comparing the properties of virgin and rejuvenated bitumen. Full article

(This article belongs to the Section Advanced Materials Characterization)

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3243 KiB

Open AccessFeature PaperReview

Recent Developments in Antimicrobial Polymers: A Review

by Madson R. E. Santos, Ana C. Fonseca, Patrícia V. Mendonça, Rita Branco, Arménio C. Serra, Paula V. Morais and Jorge F. J. Coelho

Materials 2016, 9(7), 599; https://doi.org/10.3390/ma9070599 - 20 Jul 2016

Cited by 163 | Viewed by 12910

Abstract

Antimicrobial polymers represent a very promising class of therapeutics with unique characteristics for fighting microbial infections. As the classic antibiotics exhibit an increasingly low capacity to effectively act on microorganisms, new solutions must be developed. The importance of this class of materials emerged [...] Read more.

Antimicrobial polymers represent a very promising class of therapeutics with unique characteristics for fighting microbial infections. As the classic antibiotics exhibit an increasingly low capacity to effectively act on microorganisms, new solutions must be developed. The importance of this class of materials emerged from the uncontrolled use of antibiotics, which led to the advent of multidrug-resistant microbes, being nowadays one of the most serious public health problems. This review presents a critical discussion of the latest developments involving the use of different classes of antimicrobial polymers. The synthesis pathways used to afford macromolecules with antimicrobial properties, as well as the relationship between the structure and performance of these materials are discussed. Full article

(This article belongs to the Special Issue Self-Cleaning and Antimicrobial Surfaces)

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2979 KiB

Open AccessArticle

Benefits of Sealed-Curing on Compressive Strength of Fly Ash-Based Geopolymers

by Sujeong Lee, Arie Van Riessen and Chul-Min Chon

Materials 2016, 9(7), 598; https://doi.org/10.3390/ma9070598 - 20 Jul 2016

Cited by 33 | Viewed by 5185

Abstract

There is no standardized procedure for producing geopolymers; therefore, many researchers develop their own procedures for mixing and curing to achieve good workability and strength development. The curing scheme adopted is important in achieving maximum performance of resultant geopolymers. In this study, we [...] Read more.

There is no standardized procedure for producing geopolymers; therefore, many researchers develop their own procedures for mixing and curing to achieve good workability and strength development. The curing scheme adopted is important in achieving maximum performance of resultant geopolymers. In this study, we evaluated the impact of sealed and unsealed curing on mechanical strength of geopolymers. Fly ash-based geopolymers cured in sealed and unsealed moulds clearly revealed that retention of water during curing resulted in superior strength development. The average compressive strength of sealed-cured geopolymers measured after 1 day of curing was a modest 50 MPa, while after 7 day curing the average compressive strength increased to 120~135 MPa. In the unsealed specimens the average compressive strength of geopolymers was lower; ranging from 60 to 90 MPa with a slight increase as the curing period increased. Microcracking caused by dehydration is postulated to cause the strength decrease in the unsealed cured samples. These results show that water is a crucial component for the evolution of high strength three-dimensional cross-linked networks in geopolymers. Full article

(This article belongs to the Special Issue Advances in Geopolymers and Alkali-Activated Materials)

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Particle size distribution for as-received ash and milled ash analysed by means of a laser diffractometer. Top cut (D97) particle size was reduced from 156.2 to 20.5 μm. Full article ">Figure 2
Percentage weight loss with temperatures ranging from room temperature to 900 °C. Full article ">Figure 3
Thermogravimetric curves for the sealed “A” and the unsealed “B” series geopolymer specimens. The sealed “A” series specimens lost relatively more moisture at low temperatures and the weight loss at high temperatures was reversed. Full article ">Figure 4
Compressive strength of geopolymers of the sealed “A” series specimens and the unsealed “B” series specimens. “A” series specimens in sealed moulds presented higher compressive strength at testing periods. Numbers in the bars are the apparent density (g/cm3). Full article ">Figure 5
BET surface area (♦) and BJH desorption average pore diameter (◦) of geopolymers. The sealed specimens presented higher BET surface area with smaller pore diameter compared with the unsealed specimens. Full article ">Figure 6
X-ray diffraction patterns of geopolymers described in <a href="#materials-09-00598-t001" class="html-table">Table 1</a>. XRD patterns do not show any differences that could explain the substantial strength difference between the “A” and “B” series specimens. Full article ">Figure 7
NMR spectra of samples A1, A7 and B7. Full article ">

7222 KiB

Open AccessArticle

A Numerical Study on the Effect of Debris Layer on Fretting Wear

by Tongyan Yue and Magd Abdel Wahab

Materials 2016, 9(7), 597; https://doi.org/10.3390/ma9070597 - 20 Jul 2016

Cited by 77 | Viewed by 7701

Abstract

Fretting wear is the material damage of two contact surfaces caused by micro relative displacement. Its characteristic is that debris is trapped on the contact surfaces. Depending on the material properties, the shapes of the debris, and the dominant wear mechanisms, debris can [...] Read more.

Fretting wear is the material damage of two contact surfaces caused by micro relative displacement. Its characteristic is that debris is trapped on the contact surfaces. Depending on the material properties, the shapes of the debris, and the dominant wear mechanisms, debris can play different roles that either protect or harm interfaces. Due to the micro scale of the debris, it is difficult to obtain instantaneous information and investigate debris behavior in experiments. The Finite Element Method (FEM) has been used to model the process of fretting wear and calculate contact variables, such as contact stress and relative slip during the fretting wear process. In this research, a 2D fretting wear model with a debris layer was developed to investigate the influence of debris on fretting wear. Effects of different factors such as thickness of the debris layer, Young’s modulus of the debris layer, and the time of importing the layer into the FE model were considered in this study. Based on FE results, here we report that: (a) the effect of Young’s modulus of the debris layer on the contact pressure is not significant; (b) the contact pressure between the debris layer and the flat specimen decreases with increasing thickness of the layer and (c) by importing the debris layer in different fretting wear cycles, the debris layer shows different roles in the wear process. At the beginning of the wear cycle, the debris layer protects the contact surfaces of the first bodies (cylindrical pad and flat specimen). However, in the final cycle, the wear volumes of the debris layers exhibit slightly higher damage compared to the model without the debris layer in all considered cases. Full article

(This article belongs to the Special Issue Numerical Analysis of Tribology Behavior of Materials)

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9456 KiB

Open AccessArticle

Microstructures and Mechanical Properties of Co-Cr Dental Alloys Fabricated by Three CAD/CAM-Based Processing Techniques

by Hae Ri Kim, Seong-Ho Jang, Young Kyung Kim, Jun Sik Son, Bong Ki Min, Kyo-Han Kim and Tae-Yub Kwon

Materials 2016, 9(7), 596; https://doi.org/10.3390/ma9070596 - 20 Jul 2016

Cited by 116 | Viewed by 8484

Abstract

The microstructures and mechanical properties of cobalt-chromium (Co-Cr) alloys produced by three CAD/CAM-based processing techniques were investigated in comparison with those produced by the traditional casting technique. Four groups of disc- (microstructures) or dumbbell- (mechanical properties) specimens made of Co-Cr alloys were prepared [...] Read more.

The microstructures and mechanical properties of cobalt-chromium (Co-Cr) alloys produced by three CAD/CAM-based processing techniques were investigated in comparison with those produced by the traditional casting technique. Four groups of disc- (microstructures) or dumbbell- (mechanical properties) specimens made of Co-Cr alloys were prepared using casting (CS), milling (ML), selective laser melting (SLM), and milling/post-sintering (ML/PS). For each technique, the corresponding commercial alloy material was used. The microstructures of the specimens were evaluated via X-ray diffractometry, optical and scanning electron microscopy with energy-dispersive X-ray spectroscopy, and electron backscattered diffraction pattern analysis. The mechanical properties were evaluated using a tensile test according to ISO 22674 (n = 6). The microstructure of the alloys was strongly influenced by the manufacturing processes. Overall, the SLM group showed superior mechanical properties, the ML/PS group being nearly comparable. The mechanical properties of the ML group were inferior to those of the CS group. The microstructures and mechanical properties of Co-Cr alloys were greatly dependent on the manufacturing technique as well as the chemical composition. The SLM and ML/PS techniques may be considered promising alternatives to the Co-Cr alloy casting process. Full article

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9507 KiB

Open AccessReview

Optimization of Layered Cathode Materials for Lithium-Ion Batteries

by Christian Julien, Alain Mauger, Karim Zaghib and Henri Groult

Materials 2016, 9(7), 595; https://doi.org/10.3390/ma9070595 - 19 Jul 2016

Cited by 93 | Viewed by 14300

Abstract

This review presents a survey of the literature on recent progress in lithium-ion batteries, with the active sub-micron-sized particles of the positive electrode chosen in the family of lamellar compounds LiMO₂, where M stands for a mixture of Ni, [...] Read more.

This review presents a survey of the literature on recent progress in lithium-ion batteries, with the active sub-micron-sized particles of the positive electrode chosen in the family of lamellar compounds LiMO₂, where M stands for a mixture of Ni, Mn, Co elements, and in the family of yLi₂MnO₃•(1 − y)LiNi_½Mn_½O₂ layered-layered integrated materials. The structural, physical, and chemical properties of these cathode elements are reported and discussed as a function of all the synthesis parameters, which include the choice of the precursors and of the chelating agent, and as a function of the relative concentrations of the M cations and composition y. Their electrochemical properties are also reported and discussed to determine the optimum compositions in order to obtain the best electrochemical performance while maintaining the structural integrity of the electrode lattice during cycling. Full article

(This article belongs to the Special Issue Advances in Renewable Energy Conversion Materials)

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1305 KiB

Open AccessArticle

Spectroscopic Study of Plasma Polymerized a-C:H Films Deposited by a Dielectric Barrier Discharge

by Thejaswini Halethimmanahally Chandrashekaraiah, Robert Bogdanowicz, Eckart Rühl, Vladimir Danilov, Jürgen Meichsner, Steffen Thierbach and Rainer Hippler

Materials 2016, 9(7), 594; https://doi.org/10.3390/ma9070594 - 19 Jul 2016

Cited by 10 | Viewed by 6049

Abstract

Plasma polymerized a-C:H thin films have been deposited on Si (100) and aluminum coated glass substrates by a dielectric barrier discharge (DBD) operated at medium pressure using C₂H_m/Ar (m = 2, 4, 6) gas mixtures. The deposited films [...] Read more.

Plasma polymerized a-C:H thin films have been deposited on Si (100) and aluminum coated glass substrates by a dielectric barrier discharge (DBD) operated at medium pressure using C₂H_m/Ar (m = 2, 4, 6) gas mixtures. The deposited films were characterized by Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS), Raman spectroscopy, and ellipsometry. FT-IRRAS revealed the presence of sp³ and sp² C–H stretching and C–H bending vibrations of bonds in the films. The presence of D and G bands was confirmed by Raman spectroscopy. Thin films obtained from C₂H₄/Ar and C₂H₆/Ar gas mixtures have I_D/I_G ratios of 0.45 and 0.3, respectively. The refractive indices were 2.8 and 3.1 for C₂H₄/Ar and C₂H₆/Ar films, respectively, at a photon energy of 2 eV. Full article

(This article belongs to the Special Issue Thin-Film Deposition, Characterization and Advanced Surface Engineering)

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Figure 1
Photographs of a-C:H films deposited on glass substrates in a dielectric barrier discharge (DBD) with different precursor gases C2Hm/Ar (m = 2, 4, 6). Full article ">Figure 2
(a) FTIR spectra of a-C:H films obtained from C2Hm/Ar (m = 2, 4, 6) gas mixture, (gas pressure = 300 mbar, gas ratio = 1:2, power = 4 W). To ease comparison, the data obtained with C2H6/Ar and C2H4/Ar are shifted upwards by 0.2 and 0.3, respectively; (b) Magnification of the FTIR region 3030–2820 cm−1. For comparison, the data obtained with C2H6/Ar and C2H4/Ar are shifted upwards. Full article ">Figure 3
Raman spectra of a-C:H films obtained from C2Hm/Ar (m = 4, 6) gas mixtures, (gas pressure p = 300 mbar, gas ratio 1:2, power p = 4 W). Full article ">Figure 4
Wavelength-dependence of the optical constants n(λ) and k(λ) for the films prepared with C2H4/Ar and C2H6/Ar gas mixtures. Full article ">Figure 5
Absorption coefficients for the films prepared with C2H4/Ar and C2H6/Ar gas mixtures obtained from spectroscopic ellipsometry. Full article ">

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Open AccessArticle

Binary Alkali-Metal Silicon Clathrates by Spark Plasma Sintering: Preparation and Characterization

by Igor Veremchuk, Matt Beekman, Iryna Antonyshyn, Walter Schnelle, Michael Baitinger, George S. Nolas and Yuri Grin

Materials 2016, 9(7), 593; https://doi.org/10.3390/ma9070593 - 19 Jul 2016

Cited by 9 | Viewed by 5454

Abstract

The binary intermetallic clathrates K_8-xSi₄₆ (x = 0.4; 1.2), Rb_6.2Si₄₆, Rb_11.5Si₁₃₆ and Cs_7.8Si₁₃₆ were prepared from M₄Si₄ (M = K, Rb, Cs) precursors by [...] Read more.

The binary intermetallic clathrates K_8-xSi₄₆ (x = 0.4; 1.2), Rb_6.2Si₄₆, Rb_11.5Si₁₃₆ and Cs_7.8Si₁₃₆ were prepared from M₄Si₄ (M = K, Rb, Cs) precursors by spark-plasma route (SPS) and structurally characterized by Rietveld refinement of PXRD data. The clathrate-II phase Rb_11.5Si₁₃₆ was synthesized for the first time. Partial crystallographic site occupancy of the alkali metals, particularly for the smaller Si₂₀ dodecahedra, was found in all compounds. SPS preparation of Na₂₄Si₁₃₆ with different SPS current polarities and tooling were performed in order to investigate the role of the electric field on clathrate formation. The electrical and thermal transport properties of K_7.6Si₄₆ and K_6.8Si₄₆ in the temperature range 4–700 K were investigated. Our findings demonstrate that SPS is a novel tool for the synthesis of intermetallic clathrate phases that are not easily accessible by conventional synthesis techniques. Full article

(This article belongs to the Special Issue Inorganic Clathrate Materials)

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Full article ">Figure 1
Crystal structures of (a) Na4Si4; (b) M4Si4 (M = K, Rb, or Cs); (c) M8-xSi46; and (d) M24-xSi136. Full article ">Figure 2
(a) SPS setup for the preparation of clathrates; (b–e) Location and relative amounts of the reaction products for different polarities and different die and punch materials; (f–h) Optical micrographs of the cross-sections from fractured post-reaction pellets illustrating the directional growth of clathrates at the anode. Full article ">Figure 3
Powder X-ray diffraction (PXRD) patterns of the SPS-prepared clathrates-I K7.6Si46 (550 °C) and K6.8Si46 (600 °C). Red asterisks mark the reflections of α-Si (<0.2 mass % in K7.6Si46 and 1.5 mass % in K6.8Si46). The experimental intensities are shown as a solid line. The difference between the experimental and calculated intensities is shown below the experimental data; the tick marks represent the positions of the diffraction reflections of the clathrate-I phase. Full article ">Figure 4
Powder X-ray diffraction (PXRD) patterns of the Rb6.2Si46 and Rb11.1Si136 mixed product and clathrate-II Cs7.8Si136. The experimental intensities are shown as a solid line. The differences between the experimental and calculated intensities are shown below the experimental data. The tick marks represent the positions of the diffraction reflections. Red asterisks mark the reflections from α-Si (<0.2 mass % in Rb6.2Si46 and Rb11.1Si136 and 2.5 mass % in Cs7.8Si136). Full article ">Figure 5
Lattice parameters versus M content, x for clathrate-I NaxSi136 (0 < x < 24) [<a href="#B28-materials-09-00593" class="html-bibr">28</a>], Na24Si136 [<a href="#B16-materials-09-00593" class="html-bibr">16</a>], K17.2Si136 [<a href="#B19-materials-09-00593" class="html-bibr">19</a>], Rb11.5Si136 (this work), and Cs7.8Si136 (this work), as well as ternary Rb8Na16Si136 and Cs8Na16Si136 [<a href="#B27-materials-09-00593" class="html-bibr">27</a>]. Full article ">Figure 6
Transport properties of polycrystalline K7.6Si46 and K6.8Si46. The low temperature, 2 to 300 K, and high temperature, 300 to 700 K, are described in the text. (Top) electrical resistivity ρ; (Middle) Seebeck coefficient S; (Bottom) thermal conductivity κ. Full article ">Figure 6 Cont.
Transport properties of polycrystalline K7.6Si46 and K6.8Si46. The low temperature, 2 to 300 K, and high temperature, 300 to 700 K, are described in the text. (Top) electrical resistivity ρ; (Middle) Seebeck coefficient S; (Bottom) thermal conductivity κ. Full article ">

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Open AccessArticle

Acoustic Performance of Resilient Materials Using Acrylic Polymer Emulsion Resin

by Haseog Kim, Sangki Park and Seahyun Lee

Materials 2016, 9(7), 592; https://doi.org/10.3390/ma9070592 - 19 Jul 2016

Cited by 4 | Viewed by 6189

Abstract

There have been frequent cases of civil complaints and disputes in relation to floor impact noises over the years. To solve these issues, a substantial amount of sound resilient material is installed between the concrete slab and the foamed concrete during construction. A [...] Read more.

There have been frequent cases of civil complaints and disputes in relation to floor impact noises over the years. To solve these issues, a substantial amount of sound resilient material is installed between the concrete slab and the foamed concrete during construction. A new place-type resilient material is made from cement, silica powder, sodium sulfate, expanded-polystyrene, anhydrite, fly ash, and acrylic polymer emulsion resin. Its physical characteristics such as density, compressive strength, dynamic stiffness, and remanent strain are analyzed to assess the acoustic performance of the material. The experimental results showed the density and the dynamic stiffness of the proposed resilient material is increased with proportional to the use of cement and silica powder due to the high contents of the raw materials. The remanent strain, related to the serviceability of a structure, is found to be inversely proportional to the density and strength. The amount of reduction in the heavyweight impact noise is significant in a material with high density, high strength, and low remanent strain. Finally, specimen no. R4, having the reduction level of 3 dB for impact ball and 1 dB for bang machine in the single number quantity level, respectively, is the best product to obtain overall acoustic performance. Full article

(This article belongs to the Section Advanced Composites)

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5460 KiB

Open AccessArticle

Stereocomplexation in Copolymer Networks Incorporating Enantiomeric Glycerol-Based 3-Armed Lactide Oligomers and a 2-Armed ɛ-Caprolactone Oligomer

by Ayaka Shibita, Seina Kawasaki, Toshiaki Shimasaki, Naozumi Teramoto and Mitsuhiro Shibata

Materials 2016, 9(7), 591; https://doi.org/10.3390/ma9070591 - 19 Jul 2016

Cited by 5 | Viewed by 4930

Abstract

The reactions of enantiomeric glycerol-based 3-armed lactide oligomers (H3DLAO and H3LLAO) and a diethylene glycol-based 2-armed ɛ-caprolactone oligomer (H2CLO) with hexamethylene diisocyanate (HDI) produced polyesterurethane copolymer networks (PEU-3scLAO/2CLOs 100/0, 75/25, 50/50, 25/75 and 0/100) with different feed ratios of stereocomplex (sc) lactide oligomer [...] Read more.

The reactions of enantiomeric glycerol-based 3-armed lactide oligomers (H3DLAO and H3LLAO) and a diethylene glycol-based 2-armed ɛ-caprolactone oligomer (H2CLO) with hexamethylene diisocyanate (HDI) produced polyesterurethane copolymer networks (PEU-3scLAO/2CLOs 100/0, 75/25, 50/50, 25/75 and 0/100) with different feed ratios of stereocomplex (sc) lactide oligomer (H3scLAO = H3DLAO + H3LLAO, H3DLAO/H3LLAO = 1/1) and H2CLO. Thermal and mechanical properties of the copolymer networks were compared with those of a simple homochiral (hc) network (PEU-3DLAO) produced by the reaction of H3DLAO and HDI. X-ray diffraction and differential scanning calorimetric analyses revealed that sc crystallites are formed without any hc crystallization for PEU-3scLAO/2CLOs, and that PEU-3DLAO is amorphous. The melting temperatures of sc crystallites for PEU-3scLAO/2CLOs were much higher than that of hc crystallites of H3DLAO. The polarized optical microscopic analysis revealed that the nucleation efficiency is enhanced with increasing feed of H3scLAO fraction, whereas the spherulite growth rate is accelerated with increasing feed H2CLO fraction over 100/0-50/50 networks. PEU-3scLAO/2CLO 100/0 (i.e., PEU-3scLAO) exhibited a higher tensile strength and modulus than PEU-3DLAO. The elongation at break and tensile toughness for PEU-3scLAO/2CLOs increased with an increasing feed amount of H2CLO. Full article

(This article belongs to the Section Biomaterials)

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8626 KiB

Open AccessArticle

Microscale Simulation on Mechanical Properties of Al/PTFE Composite Based on Real Microstructures

by Chao Ge, Yongxiang Dong and Wubuliaisan Maimaitituersun

Materials 2016, 9(7), 590; https://doi.org/10.3390/ma9070590 - 19 Jul 2016

Cited by 48 | Viewed by 7465

Abstract

A novel numerical method at the microscale for studying the mechanical behavior of an aluminum-particle-reinforced polytetrafluoroethylene (Al/PTFE) composite is proposed and validated experimentally in this paper. Two types of 2D representative volume elements (RVEs), real microstructure-based and simulated microstructures, are established by following [...] Read more.

A novel numerical method at the microscale for studying the mechanical behavior of an aluminum-particle-reinforced polytetrafluoroethylene (Al/PTFE) composite is proposed and validated experimentally in this paper. Two types of 2D representative volume elements (RVEs), real microstructure-based and simulated microstructures, are established by following a series of image processing procedures and on a statistical basis considering the geometry and the distribution of particles and microvoids, respectively. Moreover, 3D finite element modelling based on the same statistical information as the 2D simulated microstructure models is conducted to show the efficiency and effectiveness of the 2D models. The results of all simulations and experiments indicate that real microstructure-based models and simulated microstructure models are efficient methods to predict elastic and plastic constants of particle-reinforced composites. Full article

(This article belongs to the Section Advanced Materials Characterization)

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3153 KiB

Open AccessFeature PaperArticle

In Situ Thermal Generation of Silver Nanoparticles in 3D Printed Polymeric Structures

by Erika Fantino, Annalisa Chiappone, Flaviana Calignano, Marco Fontana, Fabrizio Pirri and Ignazio Roppolo

Materials 2016, 9(7), 589; https://doi.org/10.3390/ma9070589 - 19 Jul 2016

Cited by 80 | Viewed by 10467

Abstract

Polymer nanocomposites have always attracted the interest of researchers and industry because of their potential combination of properties from both the nanofillers and the hosting matrix. Gathering nanomaterials and 3D printing could offer clear advantages and numerous new opportunities in several application fields. [...] Read more.

Polymer nanocomposites have always attracted the interest of researchers and industry because of their potential combination of properties from both the nanofillers and the hosting matrix. Gathering nanomaterials and 3D printing could offer clear advantages and numerous new opportunities in several application fields. Embedding nanofillers in a polymeric matrix could improve the final material properties but usually the printing process gets more difficult. Considering this drawback, in this paper we propose a method to obtain polymer nanocomposites by in situ generation of nanoparticles after the printing process. 3D structures were fabricated through a Digital Light Processing (DLP) system by disolving metal salts in the starting liquid formulation. The 3D fabrication is followed by a thermal treatment in order to induce in situ generation of metal nanoparticles (NPs) in the polymer matrix. Comprehensive studies were systematically performed on the thermo-mechanical characteristics, morphology and electrical properties of the 3D printed nanocomposites. Full article

(This article belongs to the Special Issue Materials for Photolithography and 3D Printing)

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5291 KiB

Open AccessArticle

Novel Mg-Doped SrMoO₃ Perovskites Designed as Anode Materials for Solid Oxide Fuel Cells

by Vanessa Cascos, José Antonio Alonso and María Teresa Fernández-Díaz

Materials 2016, 9(7), 588; https://doi.org/10.3390/ma9070588 - 19 Jul 2016

Cited by 18 | Viewed by 6336

Abstract

SrMo_1−xM_xO_3−δ (M = Fe and Cr, x = 0.1 and 0.2) oxides have been recently described as excellent anode materials for solid oxide fuel cells at intermediate temperatures (IT-SOFC) with LSGM as the electrolyte. In this work, we [...] Read more.

SrMo_1−xM_xO_3−δ (M = Fe and Cr, x = 0.1 and 0.2) oxides have been recently described as excellent anode materials for solid oxide fuel cells at intermediate temperatures (IT-SOFC) with LSGM as the electrolyte. In this work, we have improved their properties by doping with aliovalent Mg ions at the B-site of the parent SrMoO₃ perovskite. SrMo_1−xMg_xO₃₋_δ (x = 0.1, 0.2) oxides have been prepared, characterized and tested as anode materials in single solid-oxide fuel cells, yielding output powers near 900 mW/cm⁻² at 850 °C using pure H₂ as fuel. We have studied its crystal structure with an “in situ” neutron power diffraction (NPD) experiment at temperatures as high as 800 °C, emulating the working conditions of an SOFC. Adequately high oxygen deficiencies, observed by NPD, together with elevated disk-shaped anisotropic displacement factors suggest a high ionic conductivity at the working temperatures. Furthermore, thermal expansion measurements, chemical compatibility with the LSGM electrolyte, electronic conductivity and reversibility upon cycling in oxidizing-reducing atmospheres have been carried out to find out the correlation between the excellent performance as an anode and the structural features. Full article

(This article belongs to the Special Issue Recent Advances in Materials for Solid Oxide Cells)

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4304 KiB

Open AccessFeature PaperArticle

Effect of Transition Metal Substitution on the Structure and Properties of a Clathrate-Like Compound Eu₇Cu₄₄As₂₃

by Igor V. Plokhikh, Dmitri O. Charkin, Valeriy Yu. Verchenko, Ivan A. Ignatyev, Sergey M. Kazakov, Alexey V. Sobolev, Igor A. Presniakov, Alexander A. Tsirlin and Andrei V. Shevelkov

Materials 2016, 9(7), 587; https://doi.org/10.3390/ma9070587 - 19 Jul 2016

Cited by 2 | Viewed by 5113

Abstract

A series of substitutional solid solutions—Eu₇Cu_44−xT_xAs₂₃ (T = Fe, Co, Ni)—based on a recently discovered clathrate-like compound (Eu₇Cu₄₄As₂₃) were synthesized from the elements at 800 °C. Almost up to [...] Read more.

A series of substitutional solid solutions—Eu₇Cu_44−xT_xAs₂₃ (T = Fe, Co, Ni)—based on a recently discovered clathrate-like compound (Eu₇Cu₄₄As₂₃) were synthesized from the elements at 800 °C. Almost up to 50% of Cu can be substituted by Ni, resulting in a linear decrease of the cubic unit cell parameter from a = 16.6707(1) Å for the ternary compound to a = 16.3719(1) Å for the sample with the nominal composition Eu₇Cu₂₄Ni₂₀As₂₃. In contrast, Co and Fe can only substitute less than 20% of Cu. Crystal structures of six samples of different composition were refined from powder diffraction data. Despite very small differences in scattering powers of Cu, Ni, Co, and Fe, we were able to propose a reasonable model of dopant distribution over copper sites based on the trends in interatomic distances as well as on Mössbauer spectra for the iron-substituted compound Eu₇Cu₃₆Fe₈As₂₃. Ni doping increases the Curie temperature to 25 K with respect to the parent compound, which is ferromagnetically ordered below 17.5 K, whereas Fe doping suppresses the ferromagnetic ordering in the Eu sublattice. Full article

(This article belongs to the Special Issue Inorganic Clathrate Materials)

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Projection of the Eu7Cu44As23 crystal structure onto the (001) plane. Full article ">Figure 2
Cubic unit cell parameters vs. x in solid solutions Eu7Cu44−xNixAs23. Markers cover standard deviations. Full article ">Figure 3
(a–d) Coordination polyhedra of Eu1, Cu1, Cu2, and Cu3, respectively, in the crystal structure of Eu7Cu44As23. Full article ">Figure 4
57Fe Mössbauer spectrum of Eu7Cu36Fe8As23 recorder at 15 K. Full article ">Figure 5
The (a) Cu/Ni–As; and (b) Cu/Ni–Cu/Ni distances in the structure of Eu7Cu44−xNixAs23 for different x. Cu/Ni denotes mixed sites of Cu and Ni. The standard deviations are below the sizes of experimental datapoints. The lines are drawn to guide the eye. Full article ">Figure 6
Magnetic susceptibility vs. temperature for (a) Eu7Cu36Fe8As23 and (b) Eu7Cu42Ni2As2; inverse magnetic susceptibility vs. temperature for (c) Eu7Cu36Fe8As23 and (d) Eu7Cu42Ni2As2; (e) magnetization vs. field for Eu7Cu36Fe8As23. Full article ">Figure 7
The Rietveld refinement plot for Eu7Cu36Ni8As23. Experimental profile, green; peak positions, black; differential profile, red. Full article ">

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Open AccessFeature PaperArticle

Solvent-Induced Polymorphism of Iron(II) Spin Crossover Complexes

by Ivan Šalitroš, Olaf Fuhr and Mario Ruben

Materials 2016, 9(7), 585; https://doi.org/10.3390/ma9070585 - 19 Jul 2016

Cited by 22 | Viewed by 5905

Abstract

Two new mononuclear iron(II) compounds (1) and (2) of the general formula [Fe(L)₂](BF₄)₂·nCH₃CN (L = 4-(2-bromoethyn-1-yl)-2,6-bis(pyrazol-1-yl)pyridine, n = 1 for (1) and n = 2 for [...] Read more.

Two new mononuclear iron(II) compounds (1) and (2) of the general formula [Fe(L)₂](BF₄)₂·nCH₃CN (L = 4-(2-bromoethyn-1-yl)-2,6-bis(pyrazol-1-yl)pyridine, n = 1 for (1) and n = 2 for compound (2)), were synthesized. The room temperature crystallization afforded concomitant formation of two different solvent analogues: compound (1) exhibiting triclinic P-1 and compound (2) monoclinic C2/c symmetry. Single-crystal X-ray studies confirmed the presence of the LS (low-spin) state for both compounds at 180 K and of the HS (high-spin) state for compound (2) at 293 K, in full agreement with the magnetic investigations for both solvent polymorphs. Compound (1) exhibits spin transition above 293 K followed by subsequent solvent liberation, while the spin transition of (2) takes already place at 237 K. After complete solvent removal from the crystal lattice, compound (1d) (the desolvated polymorph derived from (1)) exhibits spin transition centered at 342 K accompanied by a thermal hysteresis loop, while the analogous compound (2d) (the desolvated derivate of compound (2)) remains blocked in the HS state over all the investigated temperature range. Full article

(This article belongs to the Special Issue Advances in Molecular Magnets and related Phenomena)

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3885 KiB

Open AccessArticle

Superplasticizer Addition to Carbon Fly Ash Geopolymers Activated at Room Temperature

by Lorenza Carabba, Stefania Manzi and Maria Chiara Bignozzi

Materials 2016, 9(7), 586; https://doi.org/10.3390/ma9070586 - 18 Jul 2016

Cited by 31 | Viewed by 8766

Abstract

Present concerns about global warming due to the greenhouse emissions in the atmosphere have pushed the cement industry to research alternatives to ordinary Portland cement (OPC). Geopolymer binder may constitute a possible breakthrough in the development of sustainable materials: understanding the effectiveness and [...] Read more.

Present concerns about global warming due to the greenhouse emissions in the atmosphere have pushed the cement industry to research alternatives to ordinary Portland cement (OPC). Geopolymer binder may constitute a possible breakthrough in the development of sustainable materials: understanding the effectiveness and the influences of superplasticizers on geopolymer systems is one of the essential requirements for its large-scale implementation. This study aims to investigate the possibility of using commercially available chemical admixtures designed for OPC concrete, to improve fresh properties of fly ash-based geopolymers and mortars. A special emphasis is laid upon evaluating their influence on mechanical and microstructural characteristics of the hardened material realized under room-temperature curing conditions. Results indicate that the addition of a polycarboxylic ether-based superplasticizer, in the amount of 1.0 wt. % by mass of fly ash, promotes an improvement in workability without compromising the final strength of the hardened material. Moreover, the addition of the polycarboxylic ether- and acrylic-based superplasticizers induces a refinement in the pore structure of hardened mortar leading to a longer water saturation time. Full article

(This article belongs to the Special Issue Advances in Geopolymers and Alkali-Activated Materials)

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1196 KiB

Open AccessCommunication

Anomalous Halo Formation in an Arc-Melted ScNi-Sc₂Ni Off-Eutectic Binary Alloy

by Kai Wang, Ming Wei and Lijun Zhang

Materials 2016, 9(7), 584; https://doi.org/10.3390/ma9070584 - 18 Jul 2016

Cited by 7 | Viewed by 4512

Abstract

Diverse non-equilibrium eutectic structures have attracted numerous experimental and theoretical studies. One special type is the formation of a halo of one phase around a primary dendrite of another phase. In our experiments, it was occasionally observed that ScNi halos grow as dendritic [...] Read more.

Diverse non-equilibrium eutectic structures have attracted numerous experimental and theoretical studies. One special type is the formation of a halo of one phase around a primary dendrite of another phase. In our experiments, it was occasionally observed that ScNi halos grow as dendritic morphology around the primary Sc

_{2}

Ni dendrites in an arc-melted ScNi-Sc

_{2}

Ni off-eutectic binary alloy. The formation of this anomalous halo structure was then well reproduced by employing quantitative phase-field simulations. Based on the phase-field simulation, It was found that (i) the large undercooling and growth velocity of the ScNi phase during solidification causes the formation of halos; and (ii) the released latent heat induces the recalescence phenomenon, and changes the solidification sequence largely, resulting in the anomalous halo structure in the Sc-34 at % Ni alloy. Full article

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2107 KiB

Open AccessArticle

UV-Assisted 3D Printing of Glass and Carbon Fiber-Reinforced Dual-Cure Polymer Composites

by Marta Invernizzi, Gabriele Natale, Marinella Levi, Stefano Turri and Gianmarco Griffini

Materials 2016, 9(7), 583; https://doi.org/10.3390/ma9070583 - 16 Jul 2016

Cited by 133 | Viewed by 18888

Abstract

Glass (GFR) and carbon fiber-reinforced (CFR) dual-cure polymer composites fabricated by UV-assisted three-dimensional (UV-3D) printing are presented. The resin material combines an acrylic-based photocurable resin with a low temperature (140 °C) thermally-curable resin system based on bisphenol A diglycidyl ether as base component, [...] Read more.

Glass (GFR) and carbon fiber-reinforced (CFR) dual-cure polymer composites fabricated by UV-assisted three-dimensional (UV-3D) printing are presented. The resin material combines an acrylic-based photocurable resin with a low temperature (140 °C) thermally-curable resin system based on bisphenol A diglycidyl ether as base component, an aliphatic anhydride (hexahydro-4-methylphthalic anhydride) as hardener and (2,4,6,-tris(dimethylaminomethyl)phenol) as catalyst. A thorough rheological characterization of these formulations allowed us to define their 3D printability window. UV-3D printed macrostructures were successfully demonstrated, giving a clear indication of their potential use in real-life structural applications. Differential scanning calorimetry and dynamic mechanical analysis highlighted the good thermal stability and mechanical properties of the printed parts. In addition, uniaxial tensile tests were used to assess the fiber reinforcing effect on the UV-3D printed objects. Finally, an initial study was conducted on the use of a sizing treatment on carbon fibers to improve the fiber/matrix interfacial adhesion, giving preliminary indications on the potential of this approach to improve the mechanical properties of the 3D printed CFR components. Full article

(This article belongs to the Special Issue Materials for Photolithography and 3D Printing)

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Full article ">Figure 1
Viscosity curves of B33 formulations (dual-cure blend containing 33 wt. % of photocurable acrylic component) at increasing SiO2 content. Full article ">Figure 2
Viscosity curves of SiO2-containing B33, B50 and carbon fiber-reinforced (CFR) (B50C5) composite formulations. For comparison, CFR and glass fiber-reinforced (GFR) composite formulations without the addition of SiO2 are also presented (B50C5-noSiO2 and B33G5-noSiO2, respectively). Full article ">Figure 3
Representative stress–strain curves of UV-3D printed blend (B50) and composites incorporating untreated (B50C5) and sized (B50C5-S) carbon fibers. Full article ">Figure 4
SEM micrographs (2500× magnification) of (a) virgin untreated; and (b) sized carbon fibers; fracture surface of tensile CFR polymer composites incorporating (c) virgin untreated; and (d) sized carbon fibers. Full article ">Figure 5
Axonometric projection of the airfoil (a) and the propeller (b) 3D models used to demonstrate the printability of the GFR and CFR composite formulations developed in this work. UV-3D printed reproduction of the airfoil (c–f) and the propeller (g–j) 3D models based on the GFR (c,d,g,h) and CFR (e,f,i,j) polymer composite formulations developed in this work. Full article ">

6715 KiB

Open AccessArticle

Adsorption of Heavy Metals by Graphene Oxide/Cellulose Hydrogel Prepared from NaOH/Urea Aqueous Solution

by Xiong Chen, Sukun Zhou, Liming Zhang, Tingting You and Feng Xu

Materials 2016, 9(7), 582; https://doi.org/10.3390/ma9070582 - 16 Jul 2016

Cited by 127 | Viewed by 12009

Abstract

By taking advantage of cellulose, graphene oxide (GO), and the process for crosslinking using epichlorohydrin (ECH), we propose a simple and novel method to prepare GO/cellulose hydrogel with good potential to adsorb metal ions. GO nanosheets containing carboxyl and hydroxyl groups were introduced [...] Read more.

By taking advantage of cellulose, graphene oxide (GO), and the process for crosslinking using epichlorohydrin (ECH), we propose a simple and novel method to prepare GO/cellulose hydrogel with good potential to adsorb metal ions. GO nanosheets containing carboxyl and hydroxyl groups were introduced into the surface of the cellulose hydrogel with retention of the gel structure and its nanoporous property. Due to the introduction of GO, the GO/cellulose composite hydrogels exhibited good compressive strength. Adsorption capacity of Cu²⁺ significantly increases with an increase in the GO/cellulose ratio and GO/cellulose hydrogel showed high adsorption rates. The calculated adsorption capacities at equilibrium (

q_{e}^{cal}

) for GO/cellulose hydrogel (GO:cellulose = 20:100 in weight) was up to 94.34 mg·g⁻¹, which was much higher than that of the pristine cellulose hydrogels. Furthermore, GO/cellulose hydrogel exhibited high efficient regeneration and metal ion recovery, and high adsorption capacity for Zn²⁺, Fe³⁺, and Pb²⁺. Full article

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5971 KiB

Open AccessArticle

Mechanical Behavior of Dowel-Type Joints Made of Wood Scrimber Composite

by Minjuan He, Duo Tao, Zheng Li and Maolin Li

Materials 2016, 9(7), 581; https://doi.org/10.3390/ma9070581 - 15 Jul 2016

Cited by 16 | Viewed by 8551

Abstract

As a renewable building material with low embodied energy characteristics, wood has gained more and more attention in the green and sustainable building industry. In terms of material resource and physical properties, scrimber composite not only makes full use of fast-growing wood species, [...] Read more.

As a renewable building material with low embodied energy characteristics, wood has gained more and more attention in the green and sustainable building industry. In terms of material resource and physical properties, scrimber composite not only makes full use of fast-growing wood species, but also has better mechanical performance and less inherent variability than natural wood material. In this study, the mechanical behavior of bolted beam-to-column joints built with a kind of scrimber composite was investigated both experimentally and numerically. Two groups of specimens were tested under monotonic and low frequency cyclic loading protocols. The experimental results showed that the bolted joints built with scrimber composite performed well in initial stiffness, ductility, and energy dissipation. A three-dimensional (3D) non-linear finite element model (FEM) for the bolted beam-to-column joints was then developed and validated by experimental results. The validated model was further used to investigate the failure mechanism of the bolted joints through stress analysis. This study can contribute to the application of the proposed scrimber composite in structural engineering, and the developed FEM can serve as a useful tool to evaluate the mechanical behavior of such bolted beam-to-column joints with different configurations in future research. Full article

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7966 KiB

Open AccessArticle

Optimizing and Characterizing Geopolymers from Ternary Blend of Philippine Coal Fly Ash, Coal Bottom Ash and Rice Hull Ash

by Martin Ernesto Kalaw, Alvin Culaba, Hirofumi Hinode, Winarto Kurniawan, Susan Gallardo and Michael Angelo Promentilla

Materials 2016, 9(7), 580; https://doi.org/10.3390/ma9070580 - 15 Jul 2016

Cited by 41 | Viewed by 10905

Abstract

Geopolymers are inorganic polymers formed from the alkaline activation of amorphous alumino-silicate materials resulting in a three-dimensional polymeric network. As a class of materials, it is seen to have the potential of replacing ordinary Portland cement (OPC), which for more than a hundred [...] Read more.

Geopolymers are inorganic polymers formed from the alkaline activation of amorphous alumino-silicate materials resulting in a three-dimensional polymeric network. As a class of materials, it is seen to have the potential of replacing ordinary Portland cement (OPC), which for more than a hundred years has been the binder of choice for structural and building applications. Geopolymers have emerged as a sustainable option vis-à-vis OPC for three reasons: (1) their technical properties are comparable if not better; (2) they can be produced from industrial wastes; and (3) within reasonable constraints, their production requires less energy and emits significantly less CO₂. In the Philippines, the use of coal ash, as the alumina- and silica- rich geopolymer precursor, is being considered as one of the options for sustainable management of coal ash generation from coal-fired power plants. However, most geopolymer mixes (and the prevalent blended OPC) use only coal fly ash. The coal bottom ash, having very few applications, remains relegated to dumpsites. Rice hull ash, from biomass-fired plants, is another silica-rich geopolymer precursor material from another significantly produced waste in the country with only minimal utilization. In this study, geopolymer samples were formed from the mixture of coal ash, using both coal fly ash (CFA) and coal bottom ash (CBA), and rice hull ash (RHA). The raw materials used for the geopolymerization process were characterized using X-ray fluorescence spectroscopy (XRF) for elemental and X-ray diffraction (XRD) for mineralogical composition. The raw materials’ thermal stability and loss on ignition (LOI) were determined using thermogravimetric analysis (TGA) and reactivity via dissolution tests and inductively-coupled plasma mass spectrometry (ICP) analysis. The mechanical, thermal and microstructural properties of the geopolymers formed were analyzed using compression tests, Fourier transform infra-red spectroscopy (FTIR), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). Using a Scheffé-based mixture design, targeting applications with low thermal conductivity, light weight and moderate strength and allowing for a maximum of five percent by mass of rice hull ash in consideration of the waste utilization of all three components, it has been determined that an 85-10-5 by weight ratio of CFA-CBA-RHA activated with 80-20 by mass ratio of 12 M NaOH and sodium silicate (55% H₂O, modulus = 3) produced geopolymers with a compressive strength of 18.5 MPa, a volumetric weight of 1660 kg/m³ and a thermal conductivity of 0.457 W/m-°C at 28-day curing when pre-cured at 80 °C for 24 h. For this study, the estimates of embodied energy and CO₂ were all below 1.7 MJ/kg and 0.12 kg CO₂/kg, respectively. Full article

(This article belongs to the Special Issue Advances in Geopolymers and Alkali-Activated Materials)

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Materials, Volume 9, Issue 7 (July 2016) – 111 articles

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