Preparation of Fe@Fe3O4/ZnFe2O4 Powders and Their Consolidation via Hybrid Cold-Sintering/Spark Plasma-Sintering.

Our study is focused on optimizing the synthesis conditions for the in situ oxidation of Fe particles to produce Fe@Fe3O4 core-shell powder and preparation via co-precipitation of ZnFe2O4 nanoparticles to produce Fe@Fe3O4/ZnFe2O4 soft magnetic composites (SMC) through a hybrid cold-sintering/spark plasma-sintering technique. XRD and FTIR measurements confirmed the formation of a nanocrystalline oxide layer on the surface of Fe powder and the nanosized nature of ZnFe2O4 nanoparticles. SEM-EDX investigations revealed that the oxidic phase of our composite was distributed on the surface of the Fe particles, forming a quasi-continuous matrix. The DC magnetic characteristics of the composite compact revealed a saturation induction of 0.8 T, coercivity of 590 A/m, and maximum relative permeability of 156. AC magnetic characterization indicated that for frequencies higher than 1 kHz and induction of 0.1 T, interparticle eddy current losses dominated due to ineffective electrical insulation between neighboring particles in the composite compact. Nevertheless, the magnetic characteristics obtained in both DC and AC magnetization regimes were comparable to those reported for cold-sintered Fe-based SMCs.

The Influence of Different Aggregates on the Physico-Mechanical Performance of Alkali-Activated Geopolymer Composites Produced Using Romanian Fly Ash.

In light of the urgent need to develop environmentally friendly materials that, at some point, will allow the reduction of concrete and, consequently, cement consumption-while at the same time allowing the reuse of waste and industrial by-products-alkali-activated fly ash (AAFA) geopolymer composite emerges as a material of great interest. The aim of this study was to investigate the physico-mechanical performance of composites based on AAFA binders and the effect of different types of aggregates on these properties. The experimental results indicate variations in flexural and compressive strength, which are influenced both by the nature and particle size distribution of aggregates and the binder-to-aggregate ratio. The analysis of the samples highlighted changes in porosity, both in distribution and pore size, depending on the nature of the aggregates. This supports the evolution of physico-mechanical performance indicators.

Structural, Microstructural and Compositional Changes of the AISI 314 Steel Used in the Sintering Furnace Belt Depending on the Operating Time.

The damage due to embrittlement of the sintering furnace belt and its replacement after a certain time of use represents a problem for the manufacturers of sintered parts. Finding out the reason for the damage could help to increase the duration of its operation. This research aimed to investigate the causes of embrittlement, considering both the temperatures and atmosphere of the sintering furnace to which the furnace belt is exposed during its operation. The furnace belt was made of AISI 314 stainless steel. Optical microscopy, scanning electron microscopy, combined with energy-dispersive X-ray analysis, X-ray diffraction and the Vickers hardness tests were used to analyze the microstructural, structural, compositional and hardness changes of the belt used for 45 weeks. Cr and Mn carbides, the oxides of Fe, Cr, Mn and Si were found to form at the edge of the furnace belt. The grains grew after 45 weeks of use, approximately 10 times, due to thermal cycles in an endothermic gas atmosphere to which the belt was exposed. Also, the hardness increased from 226 to 338 HV0.05, due to the formation of carbide and oxide-type compounds. All these results represent a starting point in optimizing the lifetime of the sintering furnace belt.

Morphological Analyses of W/Cu Functional Graded Materials Obtained by Conventional and Spark Plasma Sintering.

The paper presents the analysis of two compaction methods for obtaining W/Cu Functional Graded Materials (FGMs) consisting of three layers with the following compositions (% weight): first layer 80 W/20 Cu, second layer 75 W/25 Cu, and third layer 65 W/35 Cu. Each layer composition was obtained using powders obtained through mechanical milling. The two compaction methods were Spark Plasma Sintering (SPS) and Conventional Sintering (CS). The samples obtained after the SPS and CS were investigated from morphological (scanning electron microscopy-SEM) and compositional (energy dispersive X-ray spectroscopy-EDX) points of views. Additionally, the porosities and the densities of each layer in both cases were studied. It was found that the densities of the sample's layers obtained through SPS are superior to those obtained through CS. The research emphasizes that, from a morphological point of view, the SPS process is recommended for W/Cu-FGMs, having raw materials as fine-graded powders against the CS process.

Heat Treatment and Dynamic Mechanical Analysis Strain Sweep Effects on the Phase Structure and Morphology of an Fe-28Mn-6Si-5Cr Shape Memory Alloy.

Fe-Mn-Si-based shape memory alloys (SMAs) have been extensively investigated since 1982 for various useful properties that enhance the development of different applications such as anti-seismic dampers for very tall buildings, pipe joints, or rail fasteners. In particular, the Fe-28Mn-6Si-5Cr (mass. %) alloy has been mainly used in vibration mitigation or self-adjustable axial displacement applications. Dynamic mechanical analysis (DMA), performed by strain sweeps (SS), enables the monitoring of the evolution of storage modulus and internal friction variations with increasing strain amplitudes at different constant frequencies and temperatures. Thus, applying dynamic bending with various frequencies and amplitudes that actually represents an isothermal mechanical treatment. In the present paper, an Fe-28Mn-6 Si-5Cr (mass. %) SMA was cast by ingot metallurgy, hot-rolled, and water quenched in order to obtain thermally induced martensite and avoid the occurrence of cooling cracks. The influence of the holding time, between 2 and 10 h, at 1050 °C and the effects of DMA-SS performed at three different frequencies were analyzed by a differential scanning calorimetry, an X-ray diffraction, and a scanning electron and atomic force microscopy. The effects of the holding time and mechanical treatment on the structure and morphology of martensite plates were corroborated with the results of the thermal analysis.

Producing Soft Magnetic Composites by Spark Plasma Sintering of Pseudo Core-Shell Ni-Fe Alloy@Mn0.5Zn0.5Fe2O4 Powders.

Soft magnetic composite (SMC) cores have been obtained by Spark Plasma Sintering (SPS) using pseudo core-shell powders. Pseudo core-shell powders are formed by a core of soft magnetic particle (nanocrystalline permalloy or supermalloy) surrounded by a thin layer (shell) of nanosized soft ferrite (Mn0.5Zn0.5Fe2O4). Three compositions of pseudo core-shell powders were prepared, with 1, 2 and 3 wt.% of manganese-zinc mixt ferrite. The pseudo core-shell powders were compacted by SPS at temperatures between 500 and 700 °C, with a holding time ranging from 0 to 10 min. Several techniques have been used for characterization of the samples, both, powders and compacts X-ray diffraction (XRD, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), magnetic hysteresis measurements (DC and AC) and electrical resistivity. The electrical resistivity is in the order of 1 × 10-2 Ωm, 3-4 orders of magnitude higher than supermalloy electrical resistivity. The SPS at lower temperatures (500 °C) conserves the initial phases of the composite, but increasing the sintering temperature and/or sintering time produces a solid-state reaction between the alloy and ferrite phases, with negative consequence on the magnetic properties of the compacts. The initial relative permeability is around 40 and remains constant until to 2000 Hz. The power losses are lower than 2 W/kg until to 2000 Hz.

Frequency Properties of Polymer Bonded Compacts Obtained from Ball Milled Permalloy Powders with Mo and Cu Additions.

Nanocrystalline powders from the Permalloy family, Ni75Fe25, Ni79Fe16Mo5, and Ni77Fe14Cu5Mo4, were obtained by mechanical alloying starting from elemental powders. All compositions were milled for up to 24 h in a high-energy planetary ball mill. The powders were single phase and nanocrystalline as determined by X-ray diffraction studies, with larger flatted particle sizes for Ni75Fe25 (about 400 µm) and Ni77Fe14Cu5Mo4 (about 470 µm), and smaller particle sizes for Ni79Fe16Mo5 (about 170 µm). The homogeneity of the samples was verified by energy-dispersive X-ray spectroscopy (EDX). Soft magnetic composites were obtained by adding 3% of Araldite to the powders, followed by compaction at 700 MPa, and then polymerization. A very good powder covering by the polymer layer was proven by EDX elementals maps. The influence of composition change on the electrical resistivity of the compacts was studied. Hysteresis measurements in static and dynamic fields of up to 10 kHz were recorded, showing the influence of composition and particle size on the compact properties.

Easy and Affordable: A New Method for the Studying of Bacterial Biofilm Formation.

BACKGROUND: Bacterial biofilm formation (BBF) proves itself to be in the spotlight of microbiology research due to the wide variety of infections that it can be associated with, the involvement in food spoilage, industrial biofouling and perhaps sewage treatment. However, BBF remains difficult to study due to the lack of standardization of the existing methods and the expensive equipment needed. We aim to describe a new inexpensive and easy to reproduce protocol for a 3D-printed microfluidic device that can be used to study BBF in a dynamic manner. METHODS: We used the SolidWorks 3D CAD Software (EducationEdition 2019-2020, Dassault Systèmes, Vélizy-Villacoublay, France) to design the device and the Creality3D Ender 5 printer (Shenzhen Creality 3D Technology Co., Ltd., Shenzhen, China) for its manufacture. We cultivated strains of Enterococcus faecalis, Staphylococcus aureus, Klebsiella pneumoniae and Pseudomonas aeruginosa. For the biofilm evaluation we used optical coherence tomography (OCT), scanning electron microscopy (SEM), Fourier Transform Infrared (FTIR) spectroscopy and crystal violet staining technique. RESULTS: Based on the analysis, Enterococcus faecalis seems to produce more biofilm in the first hours while Pseudomonas aeruginosa started to take the lead on biofilm production after 24 h. CONCLUSIONS: With an estimated cost around €0.1285 for one microfluidic device, a relatively inexpensive and easy alternative for the study of BBF was developed.

Studies on Hot-Rolling Bonding of the Al-Cu Bimetallic Composite.

Through the approaches in this article, an attempt was made to analyze the bonding of Al-Cu bimetallic composite layers and the highlight of the diffusion at the boundary between the layers, by hot rolling. An aluminum alloy 6060 plate (EN-AW AlMgSi) and a Cu-ETP ½ hard (CW004A) plate were used. All of these layers of materials were TIG-welded, at both ends, into a heat-treated layered composite and subsequently subjected to the hot-rolling process. The Al-Cu composite material obtained was analyzed by scanning electronic microscopy (SEM) analysis, after being subjected to the tensile test, as well as energy-dispersive X-ray (EDX) analysis. The obtained results highlighted the diffusion at the boundary between the layers of the Al-Cu composite as well as its ductile breakage and the distribution of the amount of Al and Cu at the interface of the layers.

Structural, Chemical and Magnetic Characterization of Quartz Sand from Cluj Area, Romania for Future Beneficiation in the Glass Industry.

For use in crystal glass production, quartz sand must contain less than 0.09% iron. If the sand contains more than 0.09% Fe, the iron must be removed. In the present study, quartz sand from tailings ponds near the Cluj area of Romania is analyzed for potential use in the glass industry, after magnetic separation. The particle size distribution of raw sand was determined, and mineralogical analyses was realized. Using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX), morphology and elemental distribution maps on the particle was performed. The evolution of the iron content versus the particle size was analyzed. Using X-ray diffraction, the phases occurring in the sand were investigated in relation to the particle size. Magnetic separation with two magnets, having different energy, was performed for identifying the phases attached to the magnetic particles. Magnetic hysteresis measurements evidenced complex and multiple iron phase behavior.

Spark Plasma Sintered Soft Magnetic Composite Based on Fe-Si-Al Surface Oxidized Powders.

Soft magnetic composites (SMCs) need a stable matrix to apply heat treatments for enhancing their magnetic characteristics. A stable matrix can be offered by alumina, but the densification of the ferromagnetic particles covered by this oxide (by sintering) can be very difficult. This paper proposes a feasible synthesis route for obtaining alumina matrix SMCs. An Fe-Si-Al alloy with nominal composition Fe85Si9Al6 was obtained by mechanical alloying of elemental Fe, Si, and Al powders, and further, the as-milled powders were superficially oxidized by immersion in HCl solution. The oxide layer was composed of iron, silicon, and aluminum oxides, as the Fourier-transform infrared spectroscopy technique revealed. The Fe-Si-Al@oxide powder was densified by the spark plasma sintering technique-SPS. Upon sintering, a continuous matrix of oxide (mainly alumina) was formed by the reaction of the Fe-Si-Al powder coreswith their oxide layer. The main part of the composite compacts after sintering consisted of an Fe3Si-ordered phase dispersed in an oxide matrix. The DC and AC tests of magnetic composite compacts showed that upon increasing the sintering temperature, the density, magnetic induction, and magnetic permeability increased. The initial magnetic permeability was constant in the entire range of testing frequencies and the magnetic losses increased linearly. The stability of the magnetic characteristics in frequency is promising for developing further such types of magnetic composite.

Magnetic and Magnetocaloric Properties of Nano- and Polycrystalline Manganites La(0.7-x)EuxBa0.3MnO3.

Here, we report synthesis and investigations of bulk and nano-sized La(0.7-x)EuxBa0.3MnO3 (x ≤ 0.4) compounds. The study presents a comparison between the structural and magnetic properties of the nano- and polycrystalline manganites La(0.7-x)EuxBa0.3MnO3, which are potential magnetocaloric materials to be used in domestic magnetic refrigeration close to room temperature. The parent compound, La0.7Ba0.3MnO3, has Curie temperature TC = 340 K. The magnetocaloric effect is at its maximum around TC. To reduce this temperature below 300 K, we partially replaced the La ions with Eu ions. A solid-state reaction was used to prepare bulk polycrystalline materials, and a sol-gel method was used for the nanoparticles. X-ray diffraction was used for the structural characterization of the compounds. Transmission electron spectroscopy (TEM) evidenced nanoparticle sizes in the range of 40-80 nm. Iodometry and inductively coupled plasma optical emission spectrometry (ICP-OES) was used to investigate the oxygen content of the studied compounds. Critical exponents were calculated for all samples, with bulk samples being governed by tricritical mean field model and nanocrystalline samples governed by the 3D Heisenberg model. The bulk sample with x = 0.05 shows room temperature phase transition TC = 297 K, which decreases with increasing x for the other samples. All nano-sized compounds show lower TC values compared to the same bulk samples. The magnetocaloric effect in bulk samples revealed a greater magnetic entropy change in a relatively narrow temperature range, while nanoparticles show lower values, but in a temperature range several times larger. The relative cooling power for bulk and nano-sized samples exhibit approximately equal values for the same substitution level, and this fact can substantially contribute to applications in magnetic refrigeration near room temperature. By combining the magnetic properties of the nano- and polycrystalline manganites, better magnetocaloric materials can be obtained.

Invar/WC Composite Compacts Obtained by Spark Plasma Sintering from Mechanically Alloyed Powders.

The composite materials are used on an increasingly large scale in top fields, such as the automotive, aerospace, and nuclear industries, due to the combination of the specific properties of the composite components. Invar/WC nanocrystalline composite compacts were successfully obtained by spark plasma sintering from mechanical milled composite powder. The influence of the amount of tungsten carbide (WC) on sintering, coefficient of thermal expansion (CTE), and hardness has been investigated. The relative density and hardness of Invar/WC composite compacts increases with increasing the WC content up to 10 vol.%. At higher amount of WC (15% vol.), the relative density and hardness of the Invar/WC composite compacts decreases. The temperature up to which CTE remains at a low value (0.6-1) × 10-6 °C-1 is influenced by the WC content and decreases with the WC amount of increase.

3D Printed Microfluidic Bioreactors Used for the Preferential Growth of Bacterial Biofilms through Dielectrophoresis.

A realistic modelling of the way biofilms form and evolve in time requests a dynamic approach. In this study, the proposed route uses continuous-flow bioreactors under controlled flow rates and temperature in the culture medium containing bacteria or fungi. 3D printed, Polylactic acid (PLA), flow-based bioreactors with integrated copper electrodes were used to investigate the effect of dielectrophoresis on the formation and growth of Staphylococcus aureus ATCC 25923, Enterococcus faecalis ATCC 29212, Pseudomonas aeruginosa ATCC 27853, and Klebsiella pneumoniae ATCC 13883 biofilms. Bacterial suspensions of 1McF turbidity have been prepared and circulated through the bioreactors. At the same time, a 30 V potential difference was applied on the system. The effect of the non-uniform electric field induced upon the bacterial cells was determined using quantitative methods, such as an adjusted microtiter plate technique, as well as spectral domain optical coherence tomography (SD-OCT) images. The morphology and the surface quality of the biofilms were investigated using Scanning Electron Microscopy (SEM) images. The results show that the different bacterial cells present a positive dielectrophoretic behaviour, with the preferential formation of biofilms in the high field gradient region.

New Composite Materials Made from Rigid/Flexible Polyurethane Foams with Fir Sawdust: Acoustic and Thermal Behavior.

The aim of this work is to obtain new materials with improved sound absorbing and thermal properties, using rigid or flexible polyurethane foam reinforced with recycled fir sawdust from wood processing as well as by optimizing their mixing ratio. In this respect, we prepared and characterized samples by mixing rigid polyurethane foam (RPUF)/flexible polyurethane foam (FPUF) with 0, 35, 40, 45, and 50 wt% fir sawdust (FS) with grains size larger than 2 mm. The samples were evaluated by cell morphology analysis, sound absorption, and thermal insulation performance. The obtained composite materials containing 50% sawdust have superior acoustic properties compared to those with 100% FPUF in the range of 420-1250 Hz. The addition of 35% and 50% FS in the FPUF matrix led to improved thermal insulation properties and decreased thermal insulation properties in the case of RPUF. The results show that the use of FS-based composites with the FPUF/RPUF matrix for sound absorption and thermal insulation applications is a desirable choice and could be applied as an alternative to conventional synthetic fiber-based materials and as a recycling method of waste wood.

Experimental Investigation of the Tribological Behaviors of Carbon Fiber Reinforced Polymer Composites under Boundary Lubrication.

Friction and wear experiments were performed on carbon fiber-reinforced polymer (CFRP) composites, and the tribological behavior of these materials under boundary lubrication (based on the 5100 4T 10 W-30 engine oil with TiO2 Degussa P25 nanoparticles) was investigated. Experiments were carried out in two directions: one at a different normal load from 6 to 16 N and one at a low sliding speed of 110 mm/min under boundary lubrication conditions. The obtained results reveal the stick-slip effect and the static and dynamic coefficient of friction decreased slightly with increasing normal applied load on the carbon fiber reinforced polymer composite pairs. The second direction highlights through experimental tests on the pin on disc tribometer that the friction coefficient increases with the increase in normal load (20-80 N) and sliding velocity (0.4-2.4 m/s). On the other hand, it is found that the friction coefficient is slightly lower than in the stick-slip phase. During the running-in process, the friction coefficient of the CFRP pair increases steadily as the rubbing time increases, and after a certain rubbing period, it remains constant regardless of the material of the counter face. The obtained results show that for the observed interval, the influence of normal load and sliding velocity have relatively small fraction coefficients and low wear depths. A 3D analysis of the profile demonstrated the texture of wear marks and tracks of these engineering composite materials. Furthermore, the height variations of wear marks and the morphologies of the worn surfaces of specimens under boundary lubrication conditions were analyzed.

Analysis of Sheep Wool-Based Composites for Building Insulation.

The aim of this paper is to propose ecological thermal insulation materials that meet the goals of sustainability but also fulfill the imposed thermal performance requirements. This paper studies new composite materials based on sheep wool from the perspective of thermal conductivity. The composites were prepared using two types of binder: acrylic-polyurethane resin and natural rubber latex, which were applied to the wool fibres through different methods and percentages. Based on the obtained results of thermal conductivity, two types of samples were selected for further analysis, which aimed to determine the microstructure, chemical composition, water absorption, attack of microorganisms, water vapour permeability, hygrothermal adsorption characteristics and sound absorption of the samples. In order to analyse the variation of thermal conductivity, the following parameters were taken into account: thickness, density, type of binder and percentage of binder. Following the obtained results, it was observed that the value of the thermal conductivity of the samples varies between 0.0324 and 0.0436 W/mK. It was found that all the samples prepared and analysed in this study fulfil the national criteria for the thermal performance of thermal insulation material. After conducting the in-depth analysis of the two selected sample types, it was concluded that both materials have good sound absorption characteristics over the considered frequency range. In addition, as it was expected from the natural fibres, the samples had low resistance against the attack of microorganisms and water-related tests.

Combined Electrochemical, Raman Analysis and Machine Learning Assessments of the Inhibitive Properties of an 1,3,4-Oxadiazole-2-Thiol Derivative against Carbon Steel Corrosion in HCl Solution.

The inhibiting properties of 5-(4-pyridyl)-1,3,4-oxadiazole-2-thiol (PyODT) on the corrosion of carbon steel in 1.0 M HCl solution were investigated by potentiodynamic polarization, electrochemical impedance spectroscopy, Raman spectroscopy, and SEM-EDX analysis. An approach based on machine learning algorithms and Raman data was also applied to follow the carbon steel degradation in different experimental conditions. The electrochemical measurements revealed that PyODT behaves as a mixed-type corrosion inhibitor, reaching an efficiency of about 93.1% at a concentration of 5 mM, after 1 h exposure to 1.0 M HCl solution. Due to the molecular adsorption and structural organization of PyODT molecules on the C-steel surface, higher inhibitive effectiveness of about 97% was obtained at 24 h immersion. The surface analysis showed a significantly reduced degradation state of the carbon steel surface in the presence of PyODT due to the inhibitor adsorption revealed by Raman spectroscopy and the presence of N and S atoms in the EDX spectra. The combination of Raman spectroscopy and machine learning algorithms was proved to be a facile and reliable tool for an incipient identification of the corrosion sites on a metallic surface exposed to corrosive environments.

Effect of TiO2 nanoparticles on the tribological properties of lubricating oil: an experimental investigation.

Nano-lubricants offer improved tribological properties in many applications, such as machines and engines. The presence of nanoparticles in the lubricating oil affects its wear, friction, thermal, chemical and physical properties in many ways. Titanium dioxide (TiO2) is a promising lubricant additive for enhanced engine efficiency. This article reports the effect of 10 W-30 pure base engine oil suspended TiO2 nanoparticles. Four different volume concentrations (0.01%, 0.025%, 0.050% and 0.075%) of TiO2 nanoparticles in the base lubricating oil are used for the analysis. The tribological tests were performed at ambient temperature as well as at 75 °C using a four ball tribometer. Scanning electron microscope (SEM) and Alicona Inginite Focus G5 microscope were used to analyze the worn surface. The results show that the surface-modified TiO2 nanoparticles can remarkably improve the load-carrying capacity, the friction reducing, and anti-wear abilities of the additive oil. The diameter of the wear trace and the coefficient of friction are the tribological properties analyzed for the nano-lubricant prepared at different volume concentration (VC). It was found that the diameter of the wear scar and the coefficient of friction increase with increasing VC of TiO2 nanoparticles in the lubricating oil. The main objective of the paper is to present the recent progress and, consequently, to develop a comprehensive understanding of the tribological behavior of engine oil mixed with TiO2 nanoparticles.

Criteria to define a more relevant reference sample of titanium dioxide in the context of food: a multiscale approach.

Titanium dioxide (TiO2) is a transition metal oxide widely used as a white pigment in various applications, including food. Due to the classification of TiO2 nanoparticles by the International Agency for Research on Cancer as potentially harmful for humans by inhalation, the presence of nanoparticles in food products needed to be confirmed by a set of independent studies. Seven samples of food-grade TiO2 (E171) were extensively characterised for their size distribution, crystallinity and surface properties by the currently recommended methods. All investigated E171 samples contained a fraction of nanoparticles, however, below the threshold defining the labelling of nanomaterial. On the basis of these results and a statistical analysis, E171 food-grade TiO2 totally differs from the reference material P25, confirming the few published data on this kind of particle. Therefore, the reference material P25 does not appear to be the most suitable model to study the fate of food-grade TiO2 in the gastrointestinal tract. The criteria currently to obtain a representative food-grade sample of TiO2 are the following: (1) crystalline-phase anatase, (2) a powder with an isoelectric point very close to 4.1, (3) a fraction of nanoparticles comprised between 15% and 45%, and (4) a low specific surface area around 10 m2 g-1.