Bulk Specimens Research Articles

Production of (Fe,Co)Si2 and (Fe.Mn)Si2 Thermoelectric Materials by Spark Plasma Sintering

In this study, FeSi2 bulk specimens were prepared by mechanical alloying, spark plasma sintering, and subsequent annealing. The annealed FeSi2 bulk specimens consisted of the β-FeSi2 phase and exhibited high Seebeck coefficient values. The maximum Seebeck coefficient of 356 μVK−1 was achieved in the FeSi2 bulk specimen annealed at 1173 K for 6 h. However, the power factor of the FeSi2 bulk specimen was quite small due to its high electrical resistivity, and a drastic improvement is required. Therefore, Mn- and Co-substituted specimens, Fe1−xMnxSi2 (x = 0.2–0.8) and Fe1−xCoxSi2 (x = 0.2–0.8), were produced, and their thermoelectric properties were evaluated. The Mn- and Co-substituted specimens exhibited lower electrical resistivity and a higher power factor than the FeSi2 bulk specimen. The Fe1−xMnxSi2 (x = 0.2–0.8) bulk specimens were p-type thermoelectric materials, and a Seebeck coefficient of 262 μVK−1 and a power factor of 339 μWm−1K−2 were achieved in the Fe0.94Mn0.06Si2 bulk specimen. On the other hand, the Fe1−xCoxSi2 (x = 0.2–0.8) bulk specimens were n-type thermoelectric materials, and a Seebeck coefficient of −180 μVK−1 and a power factor of 667 μWm−1K−2 were achieved in the Fe0.96Co0.04Si2 bulk specimen.

Designing size-controlled air cavities to reduce iron losses of 3D printed ferromagnetic parts: Modelling and experimental results

We studied the effect of cavity insertion to reduce iron losses into Fe-2.6wt.%Si ferromagnetic components fabricated by Laser Powder Bed Fusion (L-PBF) 3D printing process. Finite Element Method (FEM) simulation was used to understand the impact of various cavity configurations on eddy current losses and to identify design guidelines. Results highlight the dependence of losses on eddy current path length, which can be expressed by simple geometrical design parameters. Finally, L-PBF samples were magnetically characterized up to 500 Hz. A good correspondence between experimental data and modelling results is observed. A torus designed with six external cavities allowed the reduction of the losses by 60 % compared to a bulk specimen (0.5 T; 500 Hz).

Design and optimization of a conical electrostatic objective lens of a low-voltage scanning electron microscope for surface imaging and analysis in ultra-high-vacuum environment

Low-voltage scanning electron microscopy (LV-SEM) with landing energies below 5 keV has been widely used due to its advantages in mitigating the damage and charging effects to a specimen and enhancing surface information due to small interaction volume of electrons inside a specimen. Additionally, for elemental analysis of the surfaces of bulk specimens with Auger electron spectroscopy (AES) or electron energy loss spectroscopy (EELS), ultra-high-vacuum (UHV) environment is essential to maintain clean surfaces without the absorption of gas molecules during the electron beam irradiation for the acquisition of spectral data. In this study, we propose the optimal design and condition of a conical Electrostatic Objective Lens (EOL) for a UHV LV-SEM to achieve the high spatial resolution and secondary electron (SE) detection efficiency. The EOL is composed of only the three electrodes (retarding, focusing and booster electrodes) and the insulators, which is suitable for maintaining a UHV environment with less out-gassing. The cone angle of the EOL is determined as 60° to integrate a spectrometer in the UHV LV-SEM and in a large size and a higher tilt angle of the sample. Through the optimization with the simulations, the EOL achieves the minimized spherical and chromatic aberration coefficients of 0.05 and 0.03 mm at the sample side, respectively, at the landing energy of 50 eV and the shortest working distance (WD) of 1 mm for high-resolution imaging. In addition, the probe diameter of the optimized EOL is 2.3 nm at 1 keV and 5.7 nm at 50 eV with a WD of 1 mm and a probe current of 10 pA, which are comparable to previously studied compound objective lenses with magnetic and electrostatic lenses. Using a longer WD of 4 mm for analysis, the probe diameter was 5.4 nm at 1 keV and the SE detection efficiency was 83.3 % owing to the separated scintillator detector structure from the booster electrode.These results imply that the optimized EOL has the potential to be applied to a high-performance UHV LV-SEM for the surface imaging and analysis with a simple system configuration.

Red‐Emitting Cordierite Ceramic Enabling General Healthy Warm White Laser Lighting

AbstractLaser‐driven phosphor‐converted white‐laser diodes are supposed to be next‐generation high‐brightness lighting sources for their unparalleled photoelectric efficiency and light quality, yet a significant obstacle hitherto has resided in the shortage and serious laser‐induced luminescence saturation of red emitters. Here, a hot‐pressing synthesis route to obtain high‐performance red‐emitting cordierite (Mg2Al4Si5O18:Eu2+) ceramic is developed, which enables the in situ formation of bulk specimens with high crystallinity (>90%) from the counterpart glass powders. A preeminent saturation threshold of 21 W mm−2 is realized in a red‐emitting Mg2Al4Si5O18:Eu2+ ceramic film converter. More importantly, the luminous exitance reaches 209 lm mm−2, which is 5 times that of commercial red phosphor‐converted‐LED, indicating breakthrough progress in the comprehensive light‐emitting performance of red converters. This work innovates the solid‐state synthesis of laser bulk converters from glass to meet the demanding requirements of advanced warm white laser lighting.

Synthesis and microstructure of (Ce0.2Zr0.2La0.2Sm0.2Nd0.2)O2-δ high-entropy oxides characterized by fluorite structure

In this work, single-phase (Ce0.2Zr0.2La0.2Sm0.2Nd0.2)O2-δ high-entropy fluorite oxides (HEFOs) were successfully prepared by conventional solid-phase reaction method and crystal structure and microstructure were studied in-depth. Single-phase (Ce0.2Zr0.2La0.2Sm0.2Nd0.2)O2-δ is formed at 1600 °C for 12 h, and five cationic elements are uniformly distributed without any segregation at the micrometer and nanometer scales. And the XRD patterns with Rietveld refinement reveal the formation of single-phase fluorite structure of (Ce0.2Zr0.2La0.2Sm0.2Nd0.2)O2-δ. The SAED patterns on the [0 1‾ 1] band axes are similar to the diffraction patterns of face-centered cubic fluorite structure, and the standard deviation of the cation radius (coordination VIII) distribution of the specimens is 0.1277, which is again identified as forming a single-phase fluorite structure. The bulk specimens are solid compacts free of obvious cracks with randomly distribution porosity at a rare level, and the powder specimens are irregular fragmented shapes with an average particles size of 5.41 μm. The average cation radius and interplanar spacings becomes larger compared with standard fluorite structure of CeO2 due to the cation radius of La3+, Sm3+ and Nd3+ are larger than that of Ce4+, leading to an increase in lattice parameters of the HEFOs. In addition, the concentration of Ce3+ in the Ce cation is only 8.1 %, and the δ-value is calculated to be 0.3081 based on the content of all cations per mole of the substance, which indicates that the composition of the prepared specimen is (Ce0.2Zr0.2La0.2Sm0.2Nd0.2)O1.6919.

Anisotropic and elastoplastic mode‐I fracture toughnesses of three additively manufactured polymers fabricated via material extrusion and powder bed fusion

AbstractTwo types of additive manufacturing (AM) techniques, fused filament fabrication (FFF) and selective laser sintering (SLS), were employed to fabricate specimens from three widely used AM polymers: polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), and polyamide (PA): the first two using FFF and PA using SLS. The single‐edge notch bending (SENB) test was employed to measure the mode‐I fracture toughnesses of bulk and AM specimens in three directions relative to the printing direction. Unlike their bulk counterparts, AM polymers exhibited anisotropic fracture toughness. The degree of anisotropy in FFF‐PLA was much larger than FFF‐ABS and SLS‐PA. The smallest fracture toughness value for each AM polymer was achieved along the transverse printing direction. Different AM techniques affect fracture toughness differently. For instance, FFF is shown to increase PLA's plastic deformation, while SLS reduces PA's ductility. The decrease in fracture toughness is consistently observed when transitioning from bulk polymers to AM ones, reaching as much as 75%. This study provides insights into the fracture toughness of AM parts made via popular plastics, which are crucial for designing structural parts using FFF and SLS processes.

Reassessing chain tilt in the lamellar crystals of polyethylene

Semicrystalline polymers are extensively used in various forms, including fibres, films, and bottles. They exhibit remarkable properties, e.g., mechanical and thermal, that are governed by hierarchical structures comprising 10–20-nm-thick lamellar crystals. In 1957, Keller deduced that long polyethylene (PE) chains fold to form thin single lamellar crystals, with the molecular chains perpendicular to the flat faces of the crystals (the chain-folding model). Chains inclining to the perpendicular orientation in single crystals have since been reported, along with their effects on the physical properties of PE. For bulk specimens, the chain tilt angle (φ) has been investigated only for model samples with well-annealed internal structures. However, for briefly annealed specimens, the φ values of lamellae and their origins are controversial owing to the disordered lamellar morphology and orientation. Herein, we report the direct determination of molecular-chain orientations in the lamellar crystals of high-density PE using a state-of-the-art electron-diffraction-based imaging technique with nanometre-scale positional resolution and provide compelling evidence for the existence of lamellar crystals with different inner-chain orientations. Clarifying the nanoscale variation in lamellar crystals in PE can allow precise tuning of properties and expedite resource-saving material design.

Effect of Chloride Concentration on the Environment-Assisted Cracking Behavior of an Al-Mg Alloy in Atmospheric Environments

The effect of chloride concentration on the environment-assisted cracking (EAC) behavior of AA5083-H131 in atmospheric environments was investigated using high-fidelity fracture mechanics-based testing and concurrent electrochemical potential measurements. EAC susceptibility was found to increase across all environments as chloride concentration increased, denoted by a decrease in the threshold stress intensity and faster stage II crack growth rates. However, EAC susceptibility for a given chloride concentration decreased across all chloride concentrations as cathodic limitation due to solution geometry effects increased. These results are analyzed in the context of the proposed anodic dissolution-enabled hydrogen embrittlement mechanism for EAC in Al-Mg alloys. Specifically, the increase in EAC susceptibility noted at higher chloride concentrations is postulated to arise from an increased overpotential for hydrogen production at the crack tip. Conversely, the decrease in EAC susceptibility observed as the solution geometry becomes more restrictive is attributed to cathodic limitation at the bulk surface decreasing dissolution at the crack tip, resulting in a concomitant less aggressive crack chemistry, and thus lower levels of hydrogen production and uptake at the crack tip. A close correlation between the open-circuit potential on the bulk specimen surface and the crack growth kinetics was also observed across all environments and chloride concentrations, with higher chloride concentrations and cathodic limitations resulting in larger changes in electrochemical potential. This correlates well with known electrochemical potential-dependent EAC observations for these alloys.

Influence of sintering temperature on structural and optical properties of Cd0.5Cu0.5CrxFe2−xO4 ferrites

Two ferrite series were synthesized. One series has nanosize samples that have been prepared by the co-precipitation method, and the second series has the corresponding bulk samples that have been sintered at 1000 °C for 6 h. X-ray diffraction has been used to estimate the cubic spinel structure of both series. The crystallite size, theoretical density, and porosity of the nanomaterials are larger than those of the bulk materials. HRTEM analysis demonstrated the aggregation of nanoscale samples, including an average particle size of 9–22.5 nm. However, bulk specimens have a limited surface area. The agglomeration of the nanoparticles was seen in TEM images, in which the mean particle size was within the limit of the crystallite size (R) result and ranged from 14 to 20 nm. The appearance of the spinel phase in the samples was validated through Raman spectroscopy. Different cation occupation ratios in either tetrahedral or octahedral sites have been identified to be associated with an observable systematic shift and asymmetric flattening in Raman spectra with a variation in Cr3+ concentration. The optical characterization was performed using the UV/Vis methodology, and the results reveal that the absorption cutoff frequency declines as the chromium content rises. It was also estimated that the optical bandgap averaged 3.6 eV for nanosamples and 4.6 eV for overall bulk materials. The highest photoluminescence emission was seen at wavelengths between λem = 415 and 460 nm. The photoluminescence emission peaks of both bulk and nanoscale materials were red-shifted. These results accurately reflect the corresponding energy gap values for almost the same ranges. Sintering leads to a rise in photoluminescence.

Valence electron concentration as key parameter to control the fracture resistance of refractory high-entropy carbides.

Although high-entropy carbides (HECs) have hardness often superior to that of parent compounds, their brittleness-a problem shared with most ceramics-has severely limited their reliability. Refractory HECs in particular are attracting considerable interest due to their unique combination of mechanical and physical properties, tunable over a vast compositional space. Here, combining statistics of crack formation in bulk specimens subject to mild, moderate, and severe nanoindentation loading with ab initio molecular dynamics simulations of alloys under tension, we show that the resistance to fracture of cubic-B1 HECs correlates with their valence electron concentration (VEC). Electronic structure analyses show that VEC ≳ 9.4 electrons per formula unit enhances alloy fracture resistance due to a facile rehybridization of electronic metallic states, which activates transformation plasticity at the yield point. Our work demonstrates a reliable strategy for computationally guided and rule-based (i.e., VEC) engineering of deformation mechanisms in high entropy, solid solution, and doped ceramics.

A Study of the Microstructure and Mechanical and Electrochemical Properties of CoCrFeNi High-Entropy Alloys Additive-Manufactured Using Laser Metal Deposition

This work demonstrates the successful additive manufacturing of an in situ-alloyed CoCrFeNi HEA with a single phase (FCC) structure via the laser metal deposition (LMD) technique. In this work, bulk specimens of the CoCrFeNi high entropy alloy (HEA) of size 15 mm × 15 mm × 45 mm were additive-manufactured (AMed). An H320-type additive-subtractive manufacturing all-in-one system with a 2 kW fiber laser with a coaxial nozzle head integrated in a five-axis CNC machine was used. The effect of varying laser powers (1000 W, 1300 W, and 1600 W) on the microstructure and mechanical and electrochemical properties of the AMed HEA specimens was investigated. The AMed specimens were analyzed for their microstructure, elemental distributions, microhardness, and mechanical and electrochemical properties. An increase in the laser power led to a non-uniform cooling rate and non-steady solidification rates of the molten area during the AM process. As a result, the crystal constant decreased, and the microhardness fluctuated within a narrow range across the specimen. Among the three laser powers, the AMed CoCrFeNi HEA at 1300 W had the optimal mechanical properties and the best electrochemical behavior in 3.5 wt.% NaCl solution.

First-principle calculations and experimental investigations of thermal barrier coating on (AlxY1-x)TaO4 ceramics

In this paper, the crystal structure models of (AlxY1-x)TaO4 were constructed using the virtual crystal approximation method, and the crystal structure, electronic structure, and mechanical properties were calculated with the first-principle calculations. The bulk specimens were synthesized by hot-press sintering method, and their physical phase structures as well as mechanical properties were tested. Moreover, the calculated results were validated using the experimental data. Calculated results indicate that as Al content increases, the energy band gap of (AlxY1-x)TaO4 crystal decreases, the total density of states for Fermi energy level increases, the chemical stability of the compound decreases, and the chemical bonding between atoms diminishes. In accordance with the variation law of electronic structure, the Young's modulus, shear modulus, bulk modulus, and Vickers hardness of the compounds decrease as the Al content rises, whereas Poisson's ratio rises, indicating that the materials are more susceptible to deformation. In addition, the Young's modulus of (AlxY1-x)TaO4 (127–150 GPa) is lower than that of YSZ, and the hardness values (8.8–11.3 GPa) are comparable to those of YSZ, indicating that the material has high stress-strain tolerance.

Enhanced thermoelectric properties of NbCoSn half-Heuslers through in-situ nanocrystallization of amorphous precursors during the consolidation process

Tailoring nanostructures is a general approach used to obtain enhanced thermoelectric properties for half-Heusler compounds because the wide areas of grain and phase boundaries could be scattering centers that lower lattice thermal conductivity. However, a common fabrication method based on the sintering of crystalline precursors crushed from as-cast alloy ingots has limitations in obtaining a homogeneous microstructure without microsized impurity phases, owing to residual elemental segregation from casting. In this study, we used amorphous NbCoSn alloys as a precursor for the sintered specimen to obtain a homogeneous NbCoSn bulk specimen without microsized impurity phases and segregation, which led to the enhanced Seebeck coefficient due to the high purity of the half-Heusler phase after crystallization. Moreover, superplasticity originating from amorphous features enabled the powders to be largely deformed during the sintering process, even at a low sintering temperature (953 K). This resulted in less oxidation at both, the grain boundary and the interior, as the O diffusion pathway was blocked during the sintering process. As a result, the NbCoSn0.95Sb0.05 specimen using an amorphous precursor exhibited an enhanced zT of 0.7, due to the increase in the power factor and a decrease in lattice thermal conductivity compared to the specimen using a crystalline precursor.

Synthesis and characterization of bulk mechanical properties of a bio-based resin filled by graphene nanoplatelets and cellulose nanocrystals

In the present paper, a novel bio-based resin derived from epichlorohydrin was reinforced by Graphene NanoPlatelets (GNPs) and Cellulose NanoCrystals (CNCs) in different weight ratios and characterized experimentally through tension tests and fracture toughness tests on bulk specimens. The nanofillers were applied separately. The experimental results showed that the neat resin has a Young’s modulus of 3.29 GPa, a tensile strength of 45.15 MPa, a stress intensity factor of 0.61 MPa m1/2 and a critical strain energy release rate of 0.091 kJ/m2. The level of the properties reveal that the bio-based adhesive can be used for cosmetic and for some structural applications. The addition of cellulose nanocrystals in the epoxy resin didn’t improve the mechanical properties of the neat resin mainly due to the development of intense aggregation of cellulose nanocrystals. On the other hand, the addition of graphene has led to the increase of the Young’s modulus and the fracture toughness and to the decrease of the tension strength of the resin. Development of agglomerations of graphene were also present in this case. The contradictory findings on the mechanical properties of the reinforced resin gives a clear message about the need for optimizing the manufacturing process. Both nanocomposites have undergone a complete life-cycle analysis which has shown that they are far more environmentally friendly than a conventional epoxy resin.

Hollow Psychography: Toward Simultaneous 4D Scanning Transmission Electron Microscopy and Electron Energy Loss Spectroscopy.

With the recent development of high-acquisition-speed pixelated detectors, 4D scanning transmission electron microscopy (4D-STEM) is becoming routinely available in high-resolution electron microscopy. 4D-STEM acts as a "universal" method that provides local information on materials that is challenging to extract from bulk techniques. It extends conventional STEM imaging to include super-resolution techniques and to provide quantitative phase-based information, such as differential phase contrast, psychography, or Bloch wave phase retrieval. However, an important missing factor is the chemical and bonding information provided by electron energy loss spectroscopy (EELS). 4D-STEM and EELS cannot currently be acquired simultaneously due to the overlapping geometry of the detectors. Here, the feasibility of modifying the detector geometry to overcome this challenge for bulk specimens is demonstrated, and the use of a partial or defective detector for ptycholgaphic structural imaging is explored. Results show that structural information beyond the diffraction-limit and chemical information from the material can be extracted together, resulting in simultaneous multi-modal measurements, adding the additional dimensions of spectral information to 4D datasets.

Formula omitted]-irradiation effect on the mechanical properties of in situ specimens made of structural epoxy adhesive and comparison with adhesive bulk behavior

This work examines the effects of γ-irradiation (up to 500kGy) on the mechanical behavior of aluminum in situ specimens bonded with 3MTM Scotch-WeldTM EC-2216 B/A Gray, a commercial two-part epoxy adhesive. Shear tests were performed, from which failure data and incipient axial stiffness were obtained. The results were compared to those of dedicated finite element models that had been tuned with the previously obtained bulk cohesive properties of the adhesive. Analytic models to evaluate stress distribution in the adhesive and joints axial stiffness were applied to validate the developed models and to explain the relatively small stiffness variance (observed both in experimental and numerical analyses results) compared to that of the adhesive Young’s modulus.On the whole, the adhesive showed a good tolerance to radiation in the interval studied, as already observed for bulk specimens. Finite element models were found to be accurate in predicting of the stiffness for the entire deformative field, while they overestimated the failure loads and displacements also applying unconservative failure criteria. This is very likely due to the significant number of defects found on the fracture surfaces of bulk specimens.

Sintering of silicon carbide obtained by combustion synthesis

An activated combustion synthesis method was utilized to synthesize silicon carbide from the elementary mixture in the presence of polytetrafluoroethylene (PTFE) as a thermochemical promoter and product's morphology modifier. Thermodynamic calculations in the initial Si–C-(C2F4)n mixture revealed the influence of PTFE amount and external gas pressure on the adiabatic temperature and equilibrium composition of the products. Combustion parameters and phase composition of the product were sensitive to silicon particle size, PTFE amount, sample diameter and gas pressure. Optimum conditions for the preparation of a mixture of alpha and beta SiC with up to 200 nm in particle size were determined. After consolidation by the spark plasma sintering technique some physicomechanical characteristics of the bulk specimens were measured.

Exploring the Charge Density Wave Phase of 1T-TaSe_{2}: Mott or Charge-Transfer Gap?

1T-TaSe_{2} is widely believed to host a Mott metal-insulator transition in the charge density wave (CDW) phase according to the spectroscopic observation of a band gap that extends across all momentum space. Previous investigations inferred that the occurrence of the Mott phase is limited to the surface only of bulk specimens, but recent analysis on thin samples revealed that the Mott-like behavior, observed in the monolayer, is rapidly suppressed with increasing thickness. Here, we report combined time- and angle-resolved photoemission spectroscopy and theoretical investigations of the electronic structure of 1T-TaSe_{2}. Our experimental results confirm the existence of a state above E_{F}, previously ascribed to the upper Hubbard band, and an overall band gap of ∼0.7 eV at Γ[over ¯]. However, supported by density functional theory calculations, we demonstrate that the origin of this state and the gap rests on band structure modifications induced by the CDW phase alone, without the need for Mott correlation effects.

Effect of water on the mechanical and thermal properties of natural fibre reinforced hybrid composites

Research and development on natural fibre-reinforced composites (NFRCs) is constantly increasing and their use is expanding into new areas of applications. One of the main problems is the absorption of humidity due to the hydrophilic characteristic of the natural fibre, which leads to low interfacial adhesion between the fibre and the hydrophobic matrix and consequently, relatively low mechanical properties. One method to reduce the sensitivity of mechanical and thermal properties of NFRCs to water uptake is the hybridization by using synthetic fibres and/or fillers. This research investigates the effect of ageing on the mechanical and thermal properties of different natural fibre and hybrid composites. Pure jute, pure jute modified with two different types of fillers (micro glass and nano TiO2) and interlaminar hybrid jute/glass fibre-reinforced polymer composites were fabricated. Bulk specimens were used to measure the diffusion coefficient of water in the composites, and tensile and flexural specimens were used to measure the mechanical properties of the conditioned specimens. A thermogravimetric technique (TGA) analysis was used to measure and compare the degradation temperature of all composites studied as a function of ageing. Finally, a scanning electron microscopy (SEM) analysis was performed to examine the fracture surfaces of the specimens tested. It was found that the water uptake decreases by adding the synthetic fibres and the fillers to the NFRCs. The mechanical properties of the composites (the tensile and flexural strength and modulus) tend to decrease with the increase of water content, and they are partially recovered after desorption at the same temperature as the absorption. The thermal properties of natural fibre composites are affected by water absorption and by the hybridization. The hybridization of the composites increased the maximum degradation temperature when compared to the pure jute, while the addition of fillers improved the thermal properties of aged specimens (the initial degradation temperature values were higher than those for the pure jute composites).

An Exploratory Study on Determining and Modeling the Creep Behavior of an Acrylic Pressure-Sensitive Adhesive

In the present paper, an exploratory study on the creep behavior of a pressure sensitive adhesive (PSA) is performed. After the determination of the quasi-static behavior of the adhesive for bulk specimens and single lap joints (SLJ), SLJs were subjected to creep tests at 80%, 60%, and 30% of their respective failure load. It was verified that the durability of the joints increases under static creep conditions as the load level decreases, with the second phase of the creep curve becoming more pronounced, where the strain rate is close to zero. In addition, cyclic creep tests were performed for the 30% load level at a frequency of 0.04 Hz. Finally, an analytical model was applied to the experimental results in order to reproduce the values obtained for both static and cyclic tests. The model was found to be effective, reproducing the three phases of the curves which allowed for the characterization of the full creep curve, something not commonly found in the literature, especially for PSAs.