The present work reports the effect of Ta on densification and microstructure evolution during non-isothermal and spark plasma sintering of nanocrystalline W. Nanocrystalline W-5wt.%Ta alloy powder was synthesized using mechanical alloying. The nanocrystalline powder was characterized thoroughly using X-ray diffraction line profile analysis. Furthermore, the shrinkage behavior of nanocrystalline powder was investigated during non-isothermal sintering using dilatometry. Subsequently, the alloy powder was consolidated using spark plasma sintering up to 1600 °C. The role of Ta on stabilizing the microstructure during spark plasma sintering of nanocrystalline W was investigated in detail using electron backscatter diffraction. The average grain size of spark plasma sintered W-5wt.%Ta alloy was observed as 1.73 micron.
The present work reports the role of different atomic mobility induced localized pore evolution on densification during spark plasma sintering of nanocrystalline W-Mo alloy powder. The shrinkage (or expansion) behavior of cold compacted milled powders was studied using dilatometry during non-isothermal sintering up to 1600 °C. Subsequently, the milled powders were densified to ~95% relative density using spark plasma sintering up to 1600 °C. The enhanced localized Joule heating due to dynamically evolved porous structure could be attributed for the densification during spark plasma sintering.
Zirconia (ZrO2) has emerged as a promising technical ceramic, both as high temperature structural material and thermal barrier coating material, etc. The traditional synthesis of ZrO2 with spark plasma sintering (SPS) usually requires a sintering temperature as high as 1200C. General interest in lowering the sintering temperature to reduce energy consumption and thermal stresses has led to research on two promising routes, cold sintering via temperature-dependent chemical reactivity and sintering aids, which facilitates mass transport and improves densification. Here we combine both by develop…
Spark plasma sintering (SPS) of MgAl2O4 powder was investigated at temperatures between 1200 and 1300°C. A significant grain growth was observed during densification. The densification rate always exhibits at least one strong minimum, and resumes after an incubation period. Transmission electron microscopy investigations performed on sintered samples never revealed extensive dislocation activity in the elemental grains. The densification mechanism involved during SPS was determined by anisothermal (investigation of the heating stage of a SPS run) and isothermal methods (investigation at given …
The effect of sintering temperatures (Ts) on the structural and magnetic properties of Ni0.6Zn0.4Fe2O4 (NZFO) ferrites synthesized by conventional double sintering method has been reported. The samples are sintered at 1200, 1250 and 1300 °C. The X-ray diffraction (XRD) analysis reveals the formation of a single phase cubic spinel structure of the sample. The magnetic parameters such as saturation magnetization, Ms; coercive field, Hc; remanent magnetization, Mr and Bohr magneton, μB are determined and well compared with reported values. The obtained values are found to be 71.94 emu/gm and 1.2 …
Nanocrystalline bulk materials are desirable for many applications as they combine mechanical strength and specific electronic transport properties. Our bottom up approach starts with tailored nanoparticles. Compaction and thermal treatment are crucial, but usually the final stage sintering is accompanied by rapid grain growth which spoils nanocristallinity. For electrically conducting nanoparticles, field activated sintering techniques overcome this problem. Small grain sizes have been maintained in spite of consolidation. Nevertheless, the underlying principles, which are of high practical i…
Nanocrystalline metals and ceramics with <100 nm grain sizes and superior properties (e.g., mechanical strength, hardness, fracture toughness and stored dielectric energy) are of great interest. Much has been discussed about achieving nano grains, but little is known about maintaining grain-size uniformity that is critical for material reliability. An especially intriguing question is whether it is possible to achieve a size distribution narrower than what Hillert[1] theoretically predicted for normal grain growth, a possibility suggested, for growth with a higher growth exponent, by the gener…
A cylindrical sintering method for more realistic grain boundaries in nanocrystalline thin films
Institute of Physics of Materials, The Czech Academy of Sciences（材料物理研究所，捷克科学院）；Central European Institute of Technology, Brno University of Technology（中欧技术研究所，布尔诺理工大学）；Charles University, Faculty of Mathematics and Physics（查理大学，数学与物理学院）
Direct and hybrid microwave sintering of 3Y-ZrO 2 are comparatively studied at frequency of 2.45 GHz. Using the continuum theory of sintering, a fully coupled electromagnetic-thermalmechanical (EMTM) finite element simulation is carried out to predict powder samples deformation during their microwave processing. Direct and hybrid heating configurations are computationally tested using advanced heat transfer simulation tools including the surface to surface thermal radiation boundary conditions and a numeric proportional-integral-derivative (PID) regulation. The developed modeling framework sho…
An energy efficient spark plasma sintering method enabling the densification of large size samples assisted by very low electric current levels is developed. In this method, the electric current is concentrated in the graphite foils around the sample. High energy dissipation is then achieved in this area enabling the heating and full densification of large (alumina) parts ({Ø} 40 mm) at relatively low currents (800 A). The electrothermal mechanical simulation reveals that the electric current needed to heat the large samples is 70 % lower in the energy efficient configuration compared to the t…
The microwave sintering homogeneity of large and complex shape specimens is analyzed. A new approach enabling the fabrication of complex shapes ceramics via 3D printing and microwave sintering is presented. The use of a dental microwave cavity is shown to enable a substantial level of densification of complex shape components while restricting the grain growth. The homogeneity of the processed samples during microwave sintering is studied by an electromagnetic-thermal-mechanical simulation. The realistic densification behavior, that phenomenologically takes into account the microwave effect, i…
This work addresses the two great challenges of the spark plasma sintering (SPS) process: the sintering of complex shapes and the simultaneous production of multiple parts. A new controllable interface method is employed to concurrently consolidate two nickel gear shapes by SPS. A graphite deformable sub-mold is specifically designed for the mutual densification of the both complex parts in a unique 40 mm powder deformation space. An energy efficient SPS configuration is developed to allow the sintering of a large-scale powder assembly under electric current lower than 900 A. The stability of …
Sintering, as a thermal process at elevated temperature below the melting point, is widely used to bond contacting particles into engineering products such as ceramics, metals, polymers, and cemented carbides. Modelling and simulation as important complement to experiments are essential for understanding the sintering mechanisms and for the optimization and design of sintering process. We share in this article a state-to-the-art review on the major methods and models for the simulation of sintering process at various length scales. It starts with molecular dynamics simulations deciphering atom…
One of the main challenges of the sintering of sterling silver is the phenomenon of swelling causing de-densification and a considerable reduction of the sintering kinetics. This swelling phenomenon opposes sintering and it needs to be addressed by a well-controlled processing atmosphere. In the present study, the pressure-less sintering behavior of sterling silver is investigated in air, argon, and vacuum. A specially modified spark plasma sintering mold is designed to study the pressure-less sintering of sterling silver in a high vacuum environment. The conducted analysis is extended to the …
The potential utilization of two-stage sintering for the production of highly dense and pure nickel diniobate (Ni4Nb2O9) ceramics with low firing temperature was demonstrated. Effects of designed sintering conditions on phase formation, densification and microstructure of the ceramics were characterized by X-ray diffraction (XRD), Archimedes method and scanning electron microscopy (SEM), respectively. It has been found that minor phase of columbite NiNb2O6 tended to form together with the desired Ni4Nb2O9 phase, depending on sintering conditions. The optimization of sintering conditions could …
Flash sintering phenomena are predominantly associated with ceramics due to thermal runaway of their electric conductivity noticeably represented in materials such as zirconia or silicon carbide. Because of their high electric conductivity, flash sintering of metals is nearly inexistent. In this work, an original metal powder flash sintering method based on a microwave approach is presented. Within the developed approach, an unusually fast (60 s) thermal and sintering runaway of Ti-6Al-4V powder is experimentally revealed under microwave illumination. This phenomenon is simulated based on an e…
Predicting the microstructure during selective laser sintering (SLS) is of great interests, which can compliment the current time and cost expensive trial-and-error principle with an efficient computational design tool. However, it still remains a great challenge to simulate the microstructure evolution during SLS due to the complex underlying physical phenomena. In this work, we present a three-dimensional finite element phase-field simulation of the SLS single scan, and revealed the process-microstructure relation. We use a thermodynamically consistent non-isothermal phase-field model includ…
Nanocrystalline metal alloys show great potential as structural materials, but are often only available in small volumes such as thin films or powders. However, recent research has suggested that dopant segregation and grain boundary structural transitions between states known as complexions can dramatically alter grain size stability and potentially enable activated sintering. In this study, we explore strategic consolidation routes for mechanically alloyed Cu-4 at.% Zr powders to capture the effects of amorphous complexion formation on the densification of bulk nanostructured metals. We obse…
The current work studies the effect of sintering conditions on the densification behavior, microstructure, and electrochemical performance of 6 mol% Sc2O3, 1 mol% CeO2 co-doped ZrO2 (6Sc1CeZr) for solid-state electrolyte applications. The specimen's physical properties and electrochemical performance sintered at different temperatures ranging from 1000 °C to 1700 °C for 6 hours are examined through diffraction, microscopy, and Raman and electrochemical impedance spectroscopy. The activation energy and pre-exponential conductivity factor of the oxygen ions are calculated using a linear fit to t…
Grain size engineering, particularly reducing grain size into the nanocrystalline regime, offers a promising pathway to further improve the strength-to-weight ratio of Al alloys. Unfortunately, the fabrication of nanocrystalline metals often requires non-equilibrium processing routes, which typically limit the specimen size and require large energy budgets. In this study, multiple dopant atoms in ternary Al alloys are deliberately selected to enable segregation to the grain boundary region and promote the formation of amorphous complexions. Three different fully dense bulk nanocrystalline Al a…
Spark plasma sintering (SPS) technique is applied in combination with hydrogen decrepitation process for the recycling of SmCo5 magnets. The SmCo5 magnets for recycling were first decrepitated by hydrogen gas of a pressure of 4 bar for 3 hours to produce decrepitated powder. This powder was then used to prepare isotropic sintered magnets using the SPS technique, by sintering at 800-1000 oC for 1 minute. Full densification of the SPS-ed magnets was possible at a temperature of 1000 oC. The sample sintered at 900 oC showed the best internal coercivity (jHc) of higher than 1500 kA/m with high rem…
In this contribution, the effect of P2O5 and SiO2 addition on the phase, microstructure, and electrical properties of KNbO3 was studied. Sample powders with the general formula (1-x)KNbO3.xP2O5 (x = 0.03, 0.05) and (1-x)KNbO3.xSiO2 (x = 0.1) were prepared via mixed-oxide route. The thermal behavior of the mixed-milled powder was investigated by TG/DTA which revealed an overall weight loss of 33.4 wt % in the temperature range of 30 < T < 1200 C and crystallization exotherm occurring at about 795 C. The present results indicated that P2O5 acted as a sintering aid and lowered the sintering tempe…
A Potts model and the Replica Exchange Wang-Landau algorithm are used to construct an energy landscape for a crystalline solid containing surfaces and grain boundaries. The energy landscape is applied to an equation of motion from the steepest-entropy-ascent quantum thermodynamic (SEAQT) framework to explore the kinetics of three distinct kinds of microstructural evolution: polycrystalline sintering, precipitate coarsening, and grain growth. The steepest entropy ascent postulate predicts unique kinetic paths for these non-equilibrium processes without needing any detailed information about the…
