Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “sinter”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Effects of Post-sintering Annealing on (NdLa)-(FeCo)-B Magnets

A common limitation of La substitutions into (Nd, La) 2 Fe 14 B magnets is the reduction of the anisotropy field (H a ), which decreases magnet coercivity (H cj ). In this study, adding a small amount of Pr-Cu to a La-containing neo magnet may modify the grain boundary (GB), which can help recover a fraction of the degraded H cj . An optimal GB modification requires a carefully designed post-sinter heat treatment, which is the focus of this work on characterizing the effects of multi-step post-sintering annealing on the evolutions of microstructure and magnetic properties. We find that Nd and Cu concentrations at GBs and triple junctions (TJs) increase when the annealing temperatures are lowered. Annealing temperatures of 580°C and 480°C (near rare-earth-rich eutectic temperatures) enabled the development of a thick and continuous GB phase, which helps to magnetically decouple the grains, and, thus, enhance H cj . For the alloy with 25% Nd replaced by La, and with the addition of 7.5 wt.% of Pr 68 Cu 32 as GB modifier, the optimized multi-step post-sinter annealing improved the H cj from 4.8 kOe to 9 kOe.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Review of Solid State Consolidation Processing Techniques of ODS Steels (Hot Extrusion, Hot Isostatic Pressing, Spark Plasma Sintering, and Stir Friction Consolidation): Resulting Microstructures and Mechanical Properties

This paper reviews the solid-state consolidation processing techniques for Oxide Dispersion Strengthened steels and compares them in terms of the resulting microstructure and reported mechanical properties. Synthesis techniques include hot extrusion, hot isostatic pressing, spark plasma sintering and friction consolidation. The effects of process parameters on the resulting microstructures and performance are discussed for each technique. Average oxide size, average grain size, and porosity are compared across techniques, as well as mechanical strength and ductility. Across consolidation techniques where temperature can be controlled, a low sintering temperature with a high sintering pressure is favorable for keeping oxide sizes, grain sizes, and porosity low. The use of yttrium and titanium for oxide formation has been evidenced to reduce oxide sizes, and tungsten additions have been shown to improve strength. Current literature suggests that the more mature hot extrusion technique produces stronger steel with consistently fine oxides, while other alternative solid-state consolidation, while promising, offer margin for improvement.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Combined DSC/TGA Analysis to Rationalize the As-Sintered Microstructure in Binder Jet Additively Manufactured H13 Tool Steel

In this study, a combined DSC/TGA analysis has been used to rationalize the constant liquid channel width at higher sintering temperatures in H13 tool steel manufactured by binder jet additive manufacturing. Using enthalpy conservation along with coupled DSC + TGA analysis, the phase fractions of liquid and austenite phases during heating and cooling respectively were estimated. The estimates from the DSC/TGA analysis were compared with equilibrium phase fraction predictions from ThermoCalc. The resulting phase evolution was correlated to the as-sintered microstructure to rationalize the unexpected lack of change in liquid channel width at higher sintering temperatures.

Kannan, Rangasayee↗
DOE OSTI2021

Laser pre-sintering for denudation reduction in the laser powder bed fusion additive manufacturing of Ti-6Al-4V alloy

As a fundamentally different process for creation of complex parts, laser powder bed fusion (LPBF) additive manufacturing exhibits complex behavior at the melt pool that has a direct influence on the macroscopic properties of the final material. Powder denudation, a previously studied deleterious phenomenon, is the displacement of powder due to entrainment in a jet of rapidly vaporized metal created by the process laser. To minimize the effect of denudation and inspired by methods in adjacent fields, a laser pre-sintering (LPS) method was studied in an effort to reduce denudation. LPS was found to be most effective in the 1–4 J/mm3 volumetric energy density processing regimes, minimizing the common experimentally observed problems with the LPBF process. Scanning electron microscopy of the LPS-treated powder bed revealed insight into the accelerating effect morphological irregularities along the powder particle surface have in the pre-sintering process. To further understand the process, an estimation of the sintered neck sizes necessary to eliminate denudation in LPS is made, qualitatively affirming the low energy densities needed to prevent denudation. Here, the results presented in this work represent an initial step towards understanding and implementing LPS as a quality-enhancing step in LPBF.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Perovskite evolution on La modified Mn 1.5 Co 1.5 O 4 spinel through thermal ageing with enhanced oxidation activity: Is sintering always an issue?

Non-precious metal oxides have great potential in catalytic deep oxidation of emitted hydrocarbons to help address various environmental pollution concerns. However, sintering issue is a stumbling block in practical application upon long-term high-temperature operation or thermal shock experience. Herein, La was applied to modify Mn 1.5 Co 1.5 O 4 spinel to get a highly active oxidation catalyst with exceptional stability against high-temperature thermal ageing treatment. With thermal ageing at 750 °C for 100 h, the La modified Mn–Co composite reached 90 % conversion in toluene oxidation at 265 °C under the high WHSV of 120,000 mL g -1 h -1 , while this value increased to 312 °C over blank Mn–Co spinel. The addition of La not only inhibited the growth of spinel nanocrystals, but also brought about perovskite formation through sintering, generating perovskite-spinel interfaces, thus enhancing both activity and stability. Meanwhile, the La modified Mn–Co spinel exhibited superior performance in presence of water or sulfur dioxide after thermal ageing treatment. In conclusion, this work offers a versatile strategy to design anti-sintering and active oxidation nanocatalysts based on non-precious metal oxides for industrial applications.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Investigation of UO 2 doped with Fe 2 O 3 sintered under a reducing atmosphere

Nuclear fuel produced with trace amounts of transition metal additives is of potential interest for introducing intentional signatures for accelerating nuclear forensics. In this work, we investigate the effects on microstructure, grain size, crystal structure, and stoichiometry when trace amounts of Fe in the form of Fe 2 O 3 are added to UO 2 . Sintering of compacts with different concentrations, from 250 to 3000 ppmw, was performed under a reducing atmosphere at 1773 K. The persistence of the taggant during the fuel fabrication process as well as its impact on grain size, crystal lattice, and first-neighbor chemistry was evaluated using inductively coupled plasma–optical emission spectroscopy, powder X-ray diffraction (pXRD), Raman spectroscopy, scanning electron microscopy–backscatter electron spectroscopy, energy dispersive spectroscopy, and thermogravimetric analysis. We observed that a negligible amount of Fe was lost during sintering. Our results indicate that the feedstock, Fe 2 O 3 transforms into Fe and FeO under the test sintering conditions. For all compositions, metallic Fe precipitate was found in grain boundaries as a secondary phase. In conclusion, the potential incorporation of ionic Fe into the UO 2 unit cell was determined by pXRD and Raman spectra.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2020

Crystallization Mechanism in Spark Plasma Sintered Bulk Metallic Glass Analyzed using Small Angle Neutron Scattering

Understanding the thermal stability of metallic glasses is critical to determining their safe temperatures of service. In this paper, the crystallization mechanism in spark plasma sintered Fe 48 Cr 15 Mo 14 Y 2 C15B6 metallic glass is established by analyzing the crystal size distribution using x-ray diffraction, transmission electron microscopy and in-situ small angle neutron scattering. Isothermal annealing at 700 °C and 725 °C for 100 min resulted in the formation of (Fe,Cr) 23 C 6 crystals, measured from transmission electron micrographs, to be from 10 to 30 nm. The small angle neutron scattering intensity measured in-situ, over a Q-range of 0.02 to 0.3 Å -1 , during isothermal annealing of the sintered samples, confirmed the presence of (Fe,Cr) 23 C 6 crystals. The measured scattering intensity, fitted by the maximum entropy model, over the Q-range of 0.02 to 0.06 Å -1 , revealed that the crystals had radii ranging from 3 to 18 nm. The total volume fraction of crystals were estimated to be 0.13 and 0.22 upon isothermal annealing at 700 °C and 725 °C for 100 min respectively. The mechanism of crystallization in this spark plasma sintered iron based metallic glass was established to be from pre-existing nuclei as confirmed by Avrami exponents of 0.25 ± 0.01 and 0.39 ± 0.01 at the aforesaid temperatures.

42 ENGINEERING↗
DOE OSTI2025

Rheology improvement for silicon nitride and resin slurries for vat photopolymerization printing and sintering

This work presents the formulation, rheological characterization, and sintering of silicon nitride (Si₃N₄) slurries for vat photopolymerization (VPP) using digital light processing (DLP). Bimodal Si₃N₄ powder was dispersed into commercial photopolymer resin using two different dispersants with opposing effects on surface charge, resulting in varied slurry stability and flow behavior. Slurries were engineered to exhibit shear-thinning behavior suitable for VPP, and their flow properties were quantified using a power-law fluid model. The formulations achieved cure depths of approximately 40 µm and enabled printing of green bodies. Post-processing included thermal debinding and liquid-phase sintering, yielding primarily β-Si₃N₄ with a minor Y-Si-Al-O-N glass phase. The sintered parts reached ~85% of theoretical density and demonstrated a flexural strength of ~330 MPa. Microstructural analysis revealed closed porosity along with some defects related to powder agglomeration and interlayer adhesion. These findings provide insights into slurry formulation strategies for additive manufacturing of high-performance non-oxide ceramics.

Aerospace engineering↗
DOE OSTI2021

Onset of selective laser flash sintering of AlN

Abstract Flash sintering uses a combination of heating and electric fields to rapidly densify ceramics. Previously, it has been shown that a scanning laser can be used to initiate flash sintering in localized regions on an yttria‐stabilized zirconia (YSZ) sample in a process known as selective laser flash sintering (SLFS). In this work, we show using a combination of measurements of electric current flowing through the sample and observations of necks formed between powder particles that aluminum nitride (AlN) can also undergo SLFS. Scan conditions required to initiate SLFS are characterized over a range of laser powers and laser scan speeds in a dry nitrogen environment. It is shown that initiation of SLFS in AlN is governed by both the local input energy density per scan and heat dissipation and a numerical model is developed to predict temperatures during SLFS. Assuming the minimum temperature along the conductive path determines the onset of SLFS, the minimum temperature and time required is 450–670 K in 2–0.25 s for the pressed AlN pellets used in this study for laser scan speeds of 33–300 m/s, laser powers of 10–30 W, and an applied electric field of 3000 V/cm.

Gouws, Andre↗
DOE OSTI2021

The Impact of Sintering Atmosphere and Temperature on the Phase Evolution of High Surface Area LSCF Prepared by In Situ Carbon Templating

The thermochemical stability of lanthanum strontium cobalt ferrite (LSCF) processed between 1000 °C–1200 °C via the in situ carbon templating method was studied. This method generates high surface area ceramics at traditional solid oxide fuel cell (SOFC) sintering temperatures by generating a carbon template in situ and subsequently removing the template by oxidation at 700 °C. Argon processed samples produced an amorphous carbon template, whereas nitrogen tended to form graphitic carbon. Prior to the oxidation step, nitrogen samples comprised larger La 2 O 3 crystallites (22–40 nm) compared to argon (9–17 nm). Upon oxidation, argon samples resulted in a pure LSCF phase with surface areas in the 21–29 m 2 ·g −1 range, whereas nitrogen samples contained significant impurities. This demonstrates that the size of La 2 O 3 crystallites formed during inert processing limited the ability to produce a pure LSCF phase. Symmetrical cells comprising nano-LSCF electrodes generated by the templating method were compared to cells sintered directly in air. Impedance results suggest that nano-LSCF cells and cells processed in air were dominated by interfacial charge transfer resistance and gas diffusion, respectively. The results map out conditions for preparing and integrating high surface area, nanostructured LSCF into SOFC electrodes at traditional sintering temperatures. Strategies for improving the interfacial resistance of nano-LSCF electrodes are discussed.

Muhoza, Sixbert P. (ORCID:0000000299864241)↗
DOE OSTI2024

Embedding of Optical Fibers with Electric Field Assisted Sintering

The Electric Field Assisted Sintering (EFAS) technique was used for embedding the fibers, involving rapid heating via an electric current and pressure, reducing fiber exposure to high temperatures. Stainless steel guide tubes were inserted at the fiber-matrix junctions to prevent fiber breakage during the sintering process. The study varied key EFAS parameters—temperature (800°C to 980°C), pressure (40-50 MPa), and hold time (5-10 minutes)—to assess how they influenced fiber embedding. After embedding, the fibers were inspected using optical frequency domain reflectometry (OFDR) to evaluate optical losses and strain along the fiber length. The OFDR scans confirmed that the fibers remained intact, and subsequent transmission tests demonstrated no macroscopic fractures. The embedded samples were also analyzed using scanning electron microscopy (SEM) and X-ray computed tomography (CT) scans. These tests confirmed good bonding between the fiber and matrix with no cracking, though some porosity was present in samples fabricated at lower temperatures. At higher temperatures (980°C), the fiber-matrix bonding was continuous and defect-free, with metal coatings aiding in the bonding process. Elemental analysis showed interdiffusion between the fiber coatings and matrix materials, particularly between the gold coating and stainless steel. Helium leak tests revealed that lower sintering temperatures resulted in matrix porosity, causing leaks, but samples fabricated at 980°C were leak-tight.

46 - INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AN↗
DOE OSTI2023

Multi-scale modeling of the electric field assisted sintering process

The electric field assisted sintering (EFAS) process involves tightly coupled physics that influence microstructural evolution in the particles being compacted. It is also an inherently multi-scale phenomenon, with the microstructure of the compact influencing the subsequent engineering-scale response of the sintering system. To improve understanding of how processing parameters influence microstructural evolution, we have developed a multi-scale modeling approach that couples a continuum-level model of the sintering system with a phase-field model for microstructural evolution of particles within the compact. The phase-field model couples the effect of chemical and electrical driving forces on microstructural evolution and includes the effect of charged defect segregation to surfaces and grain boundaries; this segregation leads to enhanced defect transport and heat generation at these interfaces in response to applied electric field. The effect of enhanced heat generation on particle neck growth and the influence of microstructural evolution on the engineering-scale model are demonstrated.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Multi-scale characterization of supersolidus liquid phase sintered H13 tool steel manufactured via binder jet additive manufacturing

Additive manufacturing (AM) of H13 tool steel by binder jet 3D printing (BJ3DP) followed by pressureless supersolidus liquid phase sintering (SLPS) provides a low-cost alternative manufacturing method for components with intricate geometric features. However, the microstructure-mechanical property relationships for BJ3DP-SLPS produced H13 tool steel are not well understood, which makes it challenging to develop printing and post-processing methods that maximize part performance. In this work, we leverage atom probe tomography and transmission electron microscopy along with thermodynamic calculations to rationalize the microstructure-mechanical property relationships in as-sintered BJ3DP H13 tool steel. Here, we report for the first time, the presence of a continuous eutectic film-like carbide in H13 along with the more commonly observed cuboidal MX carbides in the prior liquid channels of the microstructure. Further, atom probe tomography revealed the interconnected nature of the MX carbides that appear to be discrete in two-dimensional micrographs. These continuous eutectic carbides and interconnected MX carbides result in brittle failure of the material. Characterization of these microstructural features will be critical in developing appropriate post-processing heat treatments for the improved mechanical performance of BJ3DP H13.

36 MATERIALS SCIENCE↗
DOE OSTI2025

High-temperature nanoindentation creep studies on castable and sintered nanostructured low-activation ferritic-martensitic alloys

In this article, we present the creep characteristics of two reduced activation ferritic-martensitic steels of identical starting compositions formed by different fabrication routes: a nanostructured ferritic alloy commonly referred to as a castable nanostructured alloy (CNA) and a sintered nanostructured alloy (SNA) variant. Through a series of nanoindentation experiments spanning a temperature range of 25 °C to 650 °C, with a maximum load of 100 mN, we find creep behaviors in the cast and sintered materials to be remarkably similar. The creep stress exponent (n) for CNA and SNA were found to be in the range of 8–35 and the activation volume was ∼14–42b 3 , underscoring a dominance of dislocation-mediated mechanisms in both alloys. Notably, we observed a decline in the creep stress exponent with increasing temperature, attributable to the heightened influence of thermally activated dislocations. This phenomenon suggests a potential transition in the deformation mechanism towards a thermally activated dislocation climb process, significantly impacting the observed creep behavior.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Modeling of flash sintering of ionic ceramics

A fundamental understanding of the influence of defects in ionic ceramics at the atomic, microstructural, and macroscopic levels, before, during, and after the flash sintering event is key to the development of ceramic processing operations that lead to fast, low cost, and environmentally safe fabrication of materials. The observed phenomenology of the flash process encompasses multiple time and length scales and have resulted on a wide variety of what sometimes appears to be contradictory explanations. Here, this article summarizes the latest developments on the modeling and simulation of flash sintering, specifically those related to the understanding of the equilibrium and kinetic properties and the corresponding microstructural evolution of ionic ceramics. Challenges and opportunities in the development of theoretical analyses that include unidentified multiphysical effects are discussed, as they pertain to the processing of technologically relevant ceramic materials for advanced structures and devices.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Pressureless Sintering of Pre-Alloyed Nanostructured Bainitic Steel Powders

A recent study shows that powder metallurgy and pressureless sintering are a viable pathway to realize full-density bulk components of nanostructure bainitic steels. A scalable, pressureless supersolidus liquid phase sintering (SLPS) approach to process nanostructured bainitic steel is evaluated. Furthermore, to address the oxidation issues associated with the mechanical alloying of elemental powders, powder atomization is used to produce pre-alloyed nanostructured bainitic steel powders.

mechanical alloying↗
DOE OSTI2021

Coupling of Spark Plasma Sintering with Advanced Modeling to Enable Process Scale-Up: LDRD Final Project Poster

Electric-field assisted sintering (EFAS) manufacturing techniques reduce energy usage requirements, fabricate near-net-shape parts, and produce bulk nanostructured materials through rapid heating and cooling rates achievable through Joule heating. Process parameters of pulsed DC voltage and constant pressure are applied through stainless steel rams and graphite tooling to the ceramic yttrium oxide powder to be sintered. Our high-fidelity modeling code application advances manufacturing for extreme environments by enabling process-informed design and improving microstructure consistency through multiphysics multiscale process-structure-property-performance correlation simulations.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Time-Resolved Evolution of the 3D Nanoporous Structure of Sintered Ag by X-Ray Nanotomography: Role of the Interface with a Copper Substrate

Here, the evolution of the nanoporous structure of cylindrical sintered silver samples during high temperature aging, ranging from 200 to 350 °C for 350 min, is studied through in situ computed X-ray tomography. Investigations are carried out for two types of specimens: pure sintered silver and specimens containing a silver-copper interface. It is shown that the overall pore evolution is driven by the evolution of very few large ones. The smaller pores, although being more numerous, do not really evolve before being absorbed by the few bigger ones. In pure silver, pore evolution is driven by diffusion (Ostwald ripening) but the presence of an interface promotes faster growth kinetics until the aging time reaches a threshold value, after which a deviation from Ostwald ripening occurs. The transition is a function of the aging temperature. This behavior is associated with the competition between elastic relaxation and surface energy minimization.

42 ENGINEERING↗