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At least 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Effect of the Fe/Ba Ratio and Sintering Temperature on Microstructure and Magnetic Properties of Barium Ferrites Prepared by Hydrothermal Method

Magnetoplumbite-type (M-type) hexagonal barium ferrite particles were synthesized by a hydrothermal method. The structure and magnetic properties of barium ferrites were investigated systematically, which indicated that the ratio of Fe/Ba (R{sub F/B}) and sintering temperature have an important effect on structure and performance. Phase composition, micro-morphology, and magnetic properties of the particles were investigated by X-ray powder diffraction (XRD) analysis, scanning electron microscopy (SEM), and vibrating sample magnetometry (VSM). When the ratio of Fe/Ba (R{sub F/B}) is even low or high, inferior images of XRD could be attained with certain impurities. When R{sub F/B} = 8, the single phase could be obtained without impurities. Micro-morphology of the particles exhibits the uniform plane hexagonal structures of nano-sized M-type ferrites when sintered at 800 °C. Saturation magnetization (M{sub s}) reaches the maximum value at R{sub F/B} = 7 (M{sub s} = 45.07 emu/g). The value of coercivity (H{sub c}) increases with the growth of R{sub F/B}, and H{sub c} is up to 6207 Oe when R{sub F/B} = 7. The effect of R{sub F/B} and temperature on magnetization (M{sub s}) and coercivity (H{sub c}) was studied in this paper.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Processing and Characterization of Fe-Mn-Cu-Sn-C Alloys Prepared by Ball Milling and Spark Plasma Sintering

In this work, Fe-Mn-Cu-Sn-C alloys were prepared by means of powder metallurgy (PM). Powder mixtures were ball-milled for 8, 30 and 120 h and densified to < 1% porosity using spark plasma sintering (SPS) at 900 °C and 35 MPa. After consolidation, all samples of the Fe alloys were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), hardness and flexural strength tests. Resistance to abrasive wear was evaluated in both three-body abrasion and two-body abrasion tests. The SEM observations revealed an evident dependence of grain size and microstructural homogeneity on milling time. The XRD analysis showed a marked increase in austenite content in the as-sintered specimens with milling time. Although the proportion of deformation-induced martensite was small, the strengthening effect of abrasion on the subsurface layer of the investigated alloys was clearly indicated by Knoop hardness measurements.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Study on Microstructures and Mechanical Properties of Foam Titanium Carbide Ceramics Fabricated by Reaction Sintering Process

Foam titanium carbide (TiC) ceramics with a three-dimensional network structure were fabricated by the reaction sintering process, in which polyurethane foam was taken as the template, and TiO{sub 2} and phenolic resin were used as the reactants. Phase, microstructures and fracture morphologies of foam TiC ceramics were characterized by x-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The results show that when the mass ratios of phenolic resin and TiO{sub 2} (F/T) are (0.8-1.2): 1, foam TiC ceramics with pure TiC phase can be formed. As the F/T ratios increase, crystal lattice parameters of fabricated foam TiC ceramics become bigger. When the value of F/T decreases from 1.2 to 0.8, grain size of TiC grows larger and microstructures get denser; meanwhile, the compressive strength increases from 0.10 to 1.05 MPa. Additionally, either raising the sintering temperatures or extending holding time can facilitate the completion of the reaction process and increase the compressive strength.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Phase Evolution and Mechanical Properties of AlCoCrFeNiSi{sub x} High-Entropy Alloys Synthesized by Mechanical Alloying and Spark Plasma Sintering

In the current investigation, AlCoCrFeNiSi{sub x} (x = 0, 0.3, 0.6 and 0.9 in atomic ratio) high-entropy alloy systems are prepared by mechanical alloying and subsequently consolidated by spark plasma sintering. The microstructural and mechanical properties were analyzed to understand the effect of Si addition in AlCoCrFeNi alloy. The x-ray diffraction analysis reveals the supersaturated solid solution of the body-centered cubic structure after 20 h of ball milling. However, the consolidation promotes the transformation of body-centered phases partially into the face-centered cubic structure and sigma phases. A recently proposed geometric model based on the atomic stress theory has been extended for the first time to classify single phase and multi-phases on the high-entropy alloys prepared by mechanical alloying and spark plasma sintering process. Improved microhardness and better wear resistance were achieved as the Si content increased from 0 to 0.9 in the present high-entropy alloy.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Sliding Wear of Spark Plasma Sintered CrFeCoNiCu High-Entropy Alloy Coatings: Effect of Aluminum Addition

High-entropy alloys (HEAs) are considered as attractive coating materials due to their high hardness, good wear and corrosion resistance. Al{sub x}CrFeCoNiCu (x = 0, 1, 2, 3) high-entropy alloy coatings were fabricated on carbon steel substrate using spark plasma sintering. The microstructure evolved from FCC to FCC–BCC mixed structure. Al{sub 3}CrFeCoNiCu coating possesses an average hardness of approximately 682 HV{sub 0.2}, which is the highest hardness in all of the HEA coatings. The dry sliding wear behavior of the HEA coatings was investigated using a ball-on-disk sliding tribo-meter with a silicon nitride ball. In all of the HEA coatings, Al{sub 3}CrFeCoNiCu presents the lowest wear rate 8.81 × 10{sup −5} mm{sup 3} m{sup −1} and Al{sub 2}CrFeCoNiCu has the lowest coefficient of friction 0.195 during 1000 m sliding distance at 0.1 m s{sup −1} sliding velocity under 20 N normal load. Comparing with AISI 52100 steel, spark plasma sintered Al{sub 2}CrFeCoNiCu and Al{sub 3}CrFeCoNiCu HEA coatings exhibit outstanding sliding wear resistance and extremely low friction coefficient. The wear resistance of Al{sub 3}CrFeCoNiCu HEA coating is approximately four times better than the bearing steel, showing a promising application as a wear-resistant material.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Strain Rate on the Plastic Deformation and Fracture of 90W-7Ni-3Fe Alloy Prepared by Liquid-Phase Sintering

In this study, the effect of the strain rates on the plastic deformation behavior and fracture mechanism of 90W-7Ni-3Fe heavy alloy prepared by liquid-phase sintering was investigated. Stress–strain tests were carried out in tensile and compression under quasi-static condition with the strain rate ranging from 10{sup −3} to 1 s{sup −1}. Dynamic test was examined using a compression split Hopkinson bar at strain rates ranging from 1000 to 1700 s{sup −1} in order to evaluate the effects of high strain rate on dynamic plastic deformation behavior. Quasi-static tests results show that the strength and hardness of this alloy increase with increasing strain rate. Results indicate that the as-sintered 90W-7Ni-3Fe heavy alloy has obvious strain-hardening characteristic under tensile or compression deformation under quasi-static condition. At this time, strain hardening is the main factor. Nevertheless, strain-hardening and strain-softening behaviors of the specimens occur simultaneously under dynamic compression condition. The effect of compressive stress on the work hardening is more obvious. A strong relevance of work-hardening effect on strain rate is also found. And the fracture mode of quasi-static and dynamic compression samples is also different.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Large-scale production of (GeTe) x (AgSbTe 2 ) 100$-$x (x=75, 80, 85, 90) with enhanced thermoelectric properties via gas-atomization and spark plasma sintering

(GeTe) x (AgSbTe 2 ) 100$-$x : TAGS thermoelectrics are an attractive class of materials due to their combination of non-toxicity and good conversion efficiency at mid-temperature ranges. Here in the present work, we have utilized energy and time efficient high-pressure gas atomization and spark-plasma sintering techniques for large-scale preparation of samples with varying composition (i.e., (GeTe) x (AgSbTe 2 ) 100$-$x where x = 75, 80, 85, and 90). High-temperature x-ray diffraction was used to understand the phase transformation mechanism of the as-atomized powders. Detailed high-resolution transmission electron microscopy of the sintered samples revealed the presence of nanoscale precipitates, antiphase, and twin boundaries. The nanoscale twins and antiphase boundaries serve as phonon scattering centers, leading to the reduction of total thermal conductivity in TAGS-80 and 90 samples. The maximum ZT obtained was 1.56 at 623 K for TAGS-90, which was ~94% improvement compared to values previously reported. The presence of the twin boundaries also resulted in a high fracture toughness (K IC ) of the TAGS-90 sample due to inhibition of dislocation movement at the twin boundary.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Sintering inhibition enables hierarchical porosity with extreme resistance to degradation during redox cycling of Fe-Mo foams

High-temperature (800 ºC) steam-hydrogen redox cycling, relevant to grid-scale energy storage, is studied for iron-based freeze-cast lamellar foams. In contrast to previously studied Fe, Fe-Ni, and Fe-Co foams that rapidly degrade, Fe-25Mo foams feature a much-enhanced structural damage resistance. Utilizing in-situ x-ray diffraction, microscopy, and x-ray tomography, strong sintering inhibition is observed in Fe-Mo foams, creating a hierarchically porous lamellar structure. This leads to (i) wide channels between lamellae, enabling high macroscopic porosity (~78%) which can accommodate gas flow as well as volumetric expansion without lamellar contact, and (ii) microporosity within lamellae, providing additional free volume to accommodate expansion during oxidation, limiting both swelling of the lamellae and the formation of Kirkendall pores. Finally, these combined effects enable a near-complete reversibility of the microstructure during cycling, preventing damage produced via internal lamellar buckling, cracking, contacting and sintering, with a remarkably high porosity (65%) remaining after 50 consecutive redox cycles.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Inkless, dry printing nanographene via in-Situ Coordinated laser ablation and sintering processes

Printing carbon, such as graphene and other carbon structures, for flexible and printed electronics currently relies on either ink-based printing, laser-induced forward transfer (LIFT), or laser-induced graphitization (LIG) to convert carbon-rich precursor materials, such as polymers, to graphene-like carbon structures. Liquid inks contain toxic solvents, surfactants, and stabilizing additives that degrade electrical conductivity and require high-temperature post-processing. On the other hand, LIG is limited by the substrate. Here, this study introduces an additive manufacturing method for dry-printing carbon nanomaterials, ranging from amorphous carbon to crystalline graphene-like structures, on various substrates. The system utilizes laser ablation of a solid graphite target to create pure carbon nanoparticles in situ and on demand. An inert gas carries the nanoparticles onto the substrate, where they can be deposited either as amorphous structures or laser-sintered in real time to form various graphitic structures. The study of laser processing parameters, specifically fluence and pulse repetition frequency, revealed three unique regimes of nanostructure evolution that influence the morphological and electrical properties of these printed structures. Raman spectroscopy confirmed graphitization with a resistivity slightly higher than that of the bulk graphite target. The conductivity/resistivity could be tuned as a function of sintering laser power. Scanning transmission electron microscopy (STEM) revealed that turbostratic nanographene formed with an interlayer spacing of 0.40 nm. Despite ink-based printing methods, such as screen printing, inkjet printing (IJP), and aerosol jet printing (AJP), this eco-friendly and green manufacturing technique could eliminate toxic chemicals, reduce environmental impact, and enable single-step fabrication of carbon-based devices for applications in wearable sensors, energy storage, flexible electronics, and Internet of Things (IoT) devices.

Additive nanomanufacturing↗
DOE OSTI2019

Reaction-bond composite synthesis of SiC-TiB 2 by spark plasma sintering/field-assisted sintering technology (SPS/FAST)

High-density SiC-TiB 2 composites were fabricated using the displacement reaction spark plasma sintering/field-assisted sintering technology (SPS/FAST) and SiC, B 4 C, TiC, and Si powders. The reaction process was performed in a narrow time frame compared hot pressing. The SiC-TiB 2 composites were sintered with precursor SiC at various pressures to determine the effects of processing with SPS/FAST. The composites completed synthesis during SPS/FAST processing, which occurs more quickly than hot pressing. SEM, STEM, and Raman spectroscopy are used to show the conversion and microstructure. Finally, the composite of 53.6 wt.% SiC and 46.4 wt.% TiB 2 has 99 % theoretical density, hardness of 26.4 GPa, and fracture toughness of 5.12 MPa m 1/2 .

36 MATERIALS SCIENCE↗
DOE OSTI2023

Reactive flash sintering of TiZrN and TiAlN ternary metal nitrides

Here, this study demonstrated the reactive flash sintering (RFS) for two powder mixtures: TiN-ZrN (both conducting) and AlN-TiN (TiN conducting but AlN insulating), targeting ternary metal nitrides (TMN) of Ti 0.5 Zr 0.5 N and Ti 0.5 Al 0.5 N, respectively. A constant volage and pressure (e.g., 8 V DC, ∼15 MPa) at room temperature triggered the flash (current density up to 27 A/mm 2 ) without pre-heating, and the entire RFS process finished in a few minutes. For TiZrN, the flash was instantaneous whereas for TiAlN, there was a long incubation before the flash followed by a quick and dramatic flash. Both conventional ex situ XRD and in situ synchrotron study had been carried out. They showed a uniform Ti 0.57 Zr 0.43 N solution formed in RFS and persisted upon cooling, while (Ti, Al) N solid solution formed at high temperature was not stable and likely went through a very quick phase separation in the cooling process. The final products from RFS had been characterized using SEM/EDS for microstructure. Both TiZrN and TiAlN were dense. Distribution of Ti, Zr, and N was uniform for TiZrN; for TiAlN, Ti and N distribution was uniform, while association of Al with oxygen was observed. TGA-DSC revealed the onset oxidation temperature for TiZrN was comparable to TiN and ZrN, while it was higher by ∼200 °C for TiAlN, likely due to the formation of Al 2 O 3 . In terms of mechanical properties such as hardness or fracture toughness, forming a single-phase solid solution (like TiZrN) does not offer obvious benefits. while large grain size from RFS seemed to be unfavorable. Future optimization of RFS condition and in-depth study by both experiments and simulation are needed to fully understand the composition-processing-structure-property relationships for such TMN from the reactive flash sintering process.

Flash Sintering↗
DOE OSTI2023

Toughening Sm–Co sintered magnets via microstructure modification with additives

We report the mechanical properties of the brittle Smsingle bondCo permanent magnets are of great practical significance. However, studies on the magnets have mostly focused on their magnetic properties. This paper reports the modified microstructure and refined unimodal grain size, enhanced flexural strength, and magnetic properties of Sm 2 (Co,Fe,Cu,Zr) 17 sintered magnets doped with La 2 O 3 , MgO, and CaF 2 fine particulates. The correlations between microstructure, phase composition, and mechanical and magnetic properties were studied. Doping of a small amount (e.g., 0.5–3 wt%) of La 2 O 3 , MgO, or CaF 2 fine particulates could significantly refine the unimodal grain sizes of the Smsingle bondCo magnets via the Zener pinning effect. Moreover, doping significantly improved the flexural strengths σ of the magnets. For example, the σ values of the magnets with 0.5 wt% MgO, 1 wt% La 2 O 3 , and 1 wt% CaF 2 were approximately 65%, 63%, and 42% higher than that of the reference magnet, respectively. Micromechanical simulations revealed that the fine particles of La 2 O 3 could deflect crack growth, while the CaF 2 particles could attract or arrest cracks during the fracture process. The mechanical strengthening effect was mainly due to grain size refinement. The Smsingle bondCo magnets with 0.5–1.5 wt% CaF2 and 0.5 wt% La 2 O 3 exhibited excellent magnetic properties while doping 1–3 wt% La2O3 and 0.5–3 wt% MgO deteriorated magnetic performance. The rational design of CaF 2 - or La 2 O 3 -doped microstructure can be an economical and effective method for producing toughened Sm–Co sintered magnets with high magnetic performance.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Sintering and characterization of SrBi{sub 2}Ta{sub 2}O{sub 9} obtained by high-pressure processing at low temperatures

High-pressure processing is a very attractive approach for the production of materials with new and/or improved properties. In this work, pressures in the order of 7.7 GPa and 2.5 GPa were induced in SrBi{sub 2}Ta{sub 2}O{sub 9} samples at different temperatures placed in a specific reaction cell and generated different effects on phase formation. The microstructural evolution during high-pressure processing was investigated by scanning electron microscopy in association with energy dispersion spectroscopy and with the support of an X-ray diffraction analyzer. Frequency response analysis was used to obtain the dielectric curves by electrochemical impedance spectroscopy. A SrBi{sub 2}Ta{sub 2}O{sub 9} single-phase sample, treated at 2.5 GPa and 900 °C, was used to evaluate the electrical properties, obtaining a dielectric response similar to SrBi{sub 2}Ta{sub 2}O{sub 9} samples sintered by conventional processes at temperatures above 1000 °C. In addition, by this method, it was possible to obtain ceramics with uniform microstructure and a relative density of 93%. - Highlights: • The first production of SrBi{sub 2}Ta{sub 2}O{sub 9} using the technique of high-pressure processing. • The ability to produce single-phase SrBi{sub 2}Ta{sub 2}O{sub 9} treated at 2.5 GPa and 900 °C. • The electrical properties are compatible with SBT sintered at high temperatures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Isothermal and cyclic oxidation behaviours of MoSi2 with additions of B at 1250 °C prepared by spark plasma sintering

Highlights: • A Mo-Si-B alloy is obtained when fine Mo-Si-B powder is sintered. • SPS Mo-Si-B alloy consists of fine MoSi{sub 2}, Mo{sub 2}B{sub 5}, MoB and SiO{sub 2} phases. • Increasing B content reduces incubation stage of oxide layer but increases mass gain. • SPS sample made from finer alloy powder has better oxidation resistance. • Spilling of SiO{sub 2} in SPS sample acts as a “Mechanical locking” on oxide layer. - Abstract: Mo-Si-B alloys were obtained by spark plasma sintering (SPS) using Mo-57Si-10B and Mo-52Si-15B alloy powders, and the effects of the B content and powder size on the isothermal and cyclic oxidation behaviours of the Mo-Si-B alloys at 1250 °C were investigated. A SiO{sub 2} phase was introduced during the SPS process and the Mo-Si-B alloys consisted of a fine MoSi{sub 2}/Mo{sub 2}B{sub 5}/MoB/SiO{sub 2} microstructure. Under isothermal and cyclic oxidation conditions, the kinetic curves of the SPS Mo-Si-B alloys exhibited parabolic time dependence on the weight gain. The increase in the nominal B caused a decrease in the incubation stage of the formation of continuously protective borosilicate layer but increased the mass gain due to the higher fluidity of the borosilicate layer. The SiO{sub 2} phase was found to supply the borosilicate layer with a “Mechanical locking” mechanism through the spilling of the SiO{sub 2} phase in the SPS samples, especially under the cyclic oxidation condition.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Graphene addition to MgB{sub 2} superconductor obtained by ex-situ spark plasma sintering technique

Highlights: • Graphene-added dense MgB{sub 2} was prepared by ex-situ spark plasma sintering. • There is a limited interaction between graphene and MgB{sub 2}. • Addition of graphene (G) shows a small enhancement of J{sub c} and μ{sub 0}H{sub irr}. • G is one of the least effective C-containing additions. - Abstract: Graphene nanopowder (G) with average thickness particle size of about 6–8 nm was added to MgB{sub 2} commercial powder. Starting composition was (MgB{sub 2}){sub (1−x)}(G){sub x}, x = 0.0125, 0.025, 0.05. Processing was performed by Spark Plasma Sintering (SPS) technique. All added samples have high density (above 95%). The critical temperature (T{sub c}) and the lattice parameter a (c-axis lattice parameter is constant) show a small variation suggesting that carbon substitution for boron is low. TEM observations show the presence of un-reacted graphene plates supporting the T{sub c} and structural results. It also indicates that G-addition does not modify the MgB{sub 2} microstructure. Despite this, there is an optimum doped sample (MgB{sub 2}){sub 0.9875}(G){sub 0.0125} for which the critical current density at temperatures below 25 K is slightly higher at high magnetic fields than for the pristine sample. The addition of G is found as one of the least effective C-source additions enhancing J{sub c}. We discuss results as being strongly related to variation of the residual stress.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Multicycle flash sintering of cubic Y 2 O 3 -stabilized ZrO 2 : An in situ energy dispersive synchrotron x-ray diffraction study with high temporal resolution

The current induced unit cell volume changes, (111) Bragg peak full width at half maximum (FWHM) and its integrated intensity in 8 % Y 2 O 3 stabilized ZrO 2 (8 %YSZ) solid state electrolyte was monitored during a triple-flash sintering experiment by in situ energy dispersive x-ray diffraction using a polychromatic synchrotron probe (max, photon energy 200 keV) with 2 second temporal resolution. The first spontaneous singularity in the unit cell volume (+0.54 %) was observed at 899 °C under 15 V/mm applied field intensity, which was associated with 13 mA/mm 2 current draw and an increase in density to 97 %. Following anelastic relaxation of the unit cell volume under open circuit conditions, the same applied field was applied twice in a row which resulted in additional induced singularities at 925 °C (+0.48 %) and 944 °C (+0.42 %). A floating baseline, which was above the thermal expansion baseline, was observed from 833 to 969 °C and was attributed to Joule heating. In conclusion, the singularity at 899 °C is associated with a sharp change in (111) FWHM and a 34 % decrease in integrated peak area that was attributed to changes in the distribution of oxygen vacancies and the changes in their concentration as induced by the applied field in the spontaneous transient stage of flash sintering.

36 MATERIALS SCIENCE↗
DOE OSTI2018

In situ X-ray characterization of uranium dioxide during flash sintering

The in situ response of stoichiometric and non-stoichiometric uranium dioxide during flash sintering is examined using high energy X-ray diffraction. Our results show that the onset of flash is driven by an increase in temperature and controlled by the applied field with no evidence of an accumulation of defects. The incubation time, that is the time after the application of the field before the flash occurs, is found to be material specific with hyper-stoichiometric samples requiring lower fields to flash. For low current/voltage fields we quantify very little change in the atomic and microstructure of the different uranium dioxide samples post-flash. Microstructural changes are identified for high fields and currents, where joule heating and sample temperatures are high, resulting in the complete transformation of the U 4 O 9 phase. Our results highlight the usefulness of high energy X-ray characterization in understanding the subtle structural changes that occur during the flash sintering process.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Revealing the Structural Evolution and Phase Transformation of O3-Type NaNi 1/3 Fe 1/3 Mn 1/3 O 2 Cathode Material on Sintering and Cycling Processes

O-type layered oxide cathode materials can be easily synthesized for a full sodium stoichiometry with high specific capacity, but they all suffer from a capacity fade on cycling. The sintering process control and optimization are critical to ensure a high quality and consistency of the prepared cathode materials with stable structure. Herein, in situ high-energy X-ray diffraction (HEXRD) was first employed to investigate the phase evolution of the oxides during the sintering process of O3-type NaNi 1/3 Fe 1/3 Mn 1/3 O 2 . The in situ HEXRD and both operando Bragg coherent diffraction (BCXD) and coherent multicrystal diffraction (CMCD) were utilized to investigate the phase transformation of the cathode materials during the sodiation/desodiation process. A composite structure comprised of O-type and P-type oxides was formed after the initial electrochemical activation of the cathode material, resulting in good structural and electrochemical stability. This finding implies that strain engineering can be a new design philosophy for the development of next generation high-performance sodium ion cathodes.

25 ENERGY STORAGE↗