A new flash (ultra-rapid) spark plasma sintering method applicable to various materials systems, regardless of their electrical resistivity, is developed. A number of powders ranging from metals to electrically insulative ceramics have been successfully densified resulting in homogeneous microstructures within sintering times of 8-35 s. A finite element simulation reveals that the developed method, providing an extraordinary fast and homogeneous heating concentrated in the sample's volume and punches, is applicable to all the different samples tested. The utilized uniquely controllable flash phenomenon is enabled by the combination of the electric current concentration around the sample and the confinement of the heat generated in this area by the lateral thermal contact resistance. The presented new method allows: extending flash sintering to nearly all materials, controlling sample shape by an added graphite die, and an energy efficient mass production of small and intermediate size objects. This approach represents also a potential venue for future investigations of flash sintering of complex shapes.
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.
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 …
Herein, we demonstrate that BiCuSeO compound can be formed in bulk directly from the raw materials through reactive spark plasma sintering (RSPS) followed by ball milling and a second short spark plasma sintering step. Compared to BiCuSeO samples obtained by a conventional solid-state reaction, the electrical transport properties of the RSPS bulk were moderately affected by the sintering technique, while the lattice thermal conductivity was almost unaffected, and the figure of merit zT attained a value at 773 K comparable to state-of-the-art BiCuSeO. The results indicate a new scalable method …
The modeling of powder compaction process, such as spark plasma sintering (SPS), requires the determination of the visco-plastic deformation behavior of the particle material including the viscosity moduli. The establishment of these parameters usually entails a long and difficult experimental campaign which in particular involves several hot isostatic pressing tests. A more straightforward method based on the coupled sinter-forging and die compaction tests, which can be easily carried out in a regular SPS device, is presented. Compared to classical creep mechanism studies, this comprehensive …
A perturbation analysis has been conducted to evaluate the generation of hotspots inside an electrical loaded ceramic sample, which is assumed to have an Arrhenius-type conductivity. The results identified a critical size, above which a small temperature perturbations will be magnified and hotspots will be generated. It provides an estimate for the largest sample size suitable for flash sintering, beyond which hotspots are likely to form, resulting in inhomogeneous heating and sintering.
The 3 heating modes are utilized to make ZrN powders have 3 different levels of the electric current density at the same temperature during spark plasma sintering (SPS). The constitutive equation of sintering for SPS is applied to the experimental porosity evolution of ZrN from three SPS modes, and this showed high electric current density increase the electric current assisted deformability value of ZrN pellets, resulting in a reduction of the flow stress. The electric current flow enhances the dislocation motion, which was experimentally proved and analyzed by modified Williamson-Hall equati…
Graphite creep has high importance for applications using high pressures (100 MPa) and temperatures close to 2000 {\textdegree}C. In particular, the new flash spark plasma sintering process (FSPS) is highly sensitive to graphite creep when applied to ultra-high temperature materials such as silicon carbide. In this flash process taking only a few seconds, the graphite tooling reaches temperatures higher than 2000 {\textdegree}C resulting in its irreversible deformation. The graphite tooling creep prevents the flash spark plasma sintering process from progressing further. In this study, a finit…
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…
DC flash sintering of both pure and 0.5 mol. % Bi2O3-doped ZnO at a relatively high activating field of 300 V/cm has been investigated. It is demonstrated that even high-purity ZnO single crystals can "flash" at 870C. In comparison, flash sintering occurs at a substantially lower onset temperature of 550C in ZnO powder specimens, indicating the important roles of surfaces and/or grain boundaries. A model has been developed to forecast the thermal runaway conditions and the predictions are in excellent agreements with the observed onset flash temperatures, attesting that the flash starts as a t…
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 …
A new medium-entropy diboride (MEDB) (W1/3Re1/3Ru1/3)B2 has been synthesized by spark plasma sintering of elemental powders at 1600 oC. Despite the dissimilar structures of WB2, ReB2 and RuB2, the sintered MEDB consists of a single hexagonal ReB2-type phase (space group P63/mmc) with a relative density of 94.2% and an average grain size of 6.8 $\pm$ 2.2 um. Structural refinement and electron microscopy measurements show that the W, Re, and Ru atoms occupy the same crystallographic site and are distributed uniformly in the lattice. The (W1/3Re1/3Ru1/3)B2 MEDB has Vickers hardnesses of 30.7 GPa …
The thermoelectric properties of AlMgB14-based materials prepared by spark plasma sintering were investigated. Al, Mg, and B powder were used as raw material powders. The raw powders were mixed using a V-shaped mixer, and then the mixture was sintered at 1673 K or 1773K. The mixture ratio of raw powders was varied around stoichiometric ratio of AlMgB14. X-ray diffraction patterns of samples showed that all samples consist of AlMgB14 and some by-products, MgAl2O4, B2O and AlB12. The Seebeck coefficient of the samples exhibited significant change depending on the varying mixture ratio. One sampl…
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…
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 …
Spark Plasma Sintering for high-speed diffusion welding of the ultrafine grained near-a Ti-5Al-2V alloy with high strength and corrosion resistance for nuclear engineering
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…
A novel and sustainable method to develop non-equiatomic CoCrFeNiMox high entropy alloys via spark plasma sintering using commercial commodity powders and evaluation of its mechanical behaviour
A mathematical model is presented for the Joule heating that occurs in a ceramic powder compact during the process of flash sintering. The ceramic is assumed to have an electrical conductivity that increases with temperature, and this leads to the possibility of runaway heating that could facilitate and explain the rapid sintering seen in experiments. We consider reduced models that are sufficiently simple to enable concrete conclusions to be drawn about the mathematical nature of their solutions. In particular we discuss how different local and non-local reaction terms, which arise from speci…
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…
Microwave Pressing is a promising way to reduce microwave sintering temperatures and stabilize microwave powder materials processing. A multi-physics simulation was conducted of the regulated pressure-assisted microwave cavity. This simulation took into consideration resonance phenomena and the nonlinear temperature-dependent material parameters of zirconia. The intrinsic behaviors of microwave systems and zirconia make the regulation of the microwave pressing difficult. However, the same phenomena can be used to activate flash sintering. Flash microwave sintering uses high electric fields of …
Although vanadium dioxide (VO2) exhibits the most abrupt metal to insulator transition (MIT) properties near room-temperature, the present regulation of their MIT functionalities is insufficient owing to the high complexity and susception associated with V4+. Herein, we demonstrate a spark plasma assisted reactive sintering (SPARS) approach to simultaneously achieve in situ doping and sintering of VO2 within largely short period (~10 minutes). This enables high convenience and flexibility in regulating the electronic structure of VO2 via dopant elements covering Ti, W, Nb, Mo, Cr and Fe, leadi…
Electric Charge Transport and Dielectric Properties of the Barium Titanate Ceramics Obtained by Spark-Plasma Sintering with Different Carbon Content
Institute of Physics of the National Academy of Sciences of Ukraine（乌克兰国家科学院物理研究所）；Educational Scientific Institute of High Technologies, Taras Shevchenko National University of Kyiv（基辅塔拉斯舍甫琴科国立大学高等技术教育科学研究所）；Frantsevich Institute for Problems in Materials Science, National Academy of Sciences of Ukraine（乌克兰国家科学院弗兰采维奇材料科学问题研究所）；Department of Chemistry, Stockholm University（斯德哥尔摩大学化学系）；Department of Physical Materials Science and Heat Treatment, National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute”（乌克兰国立技术大学“伊戈尔·西科尔斯基基辅理工学院”物理材料科学与热处理系）
Bulk p and n-type bismuth telluride were prepared using spark plasma texturization method. The texture development along the uniaxial load in the 001 direction is confirmed from both x-ray diffraction analysis and Electron Backscattering Diffraction measurements. Interestingly, those textured samples outperform the samples prepared by conventional spark plasma sintering (SPS) leading to a reduced thermal conductivity in the ab-plane. The textured samples of n- type BiTe2.7Se0.3 and p-type Bi0.5Sb1.5Te3 showed a 42% and 33% enhancement in figure of merit at room temperature respectively, as com…
Recent investigations have shown that highly oxygen defective cerium oxides generate non-classical electrostriction that is superior to lead-based ferroelectrics. In this work, we report the effect of field-assisted spark plasma sintering (SPS) on electro-chemo-mechanical properties on Ca-doped ceria (CDC). Nanometric powders of ca. 10 nm are rapidly consolidated to form polycrystalline nanostructures with a high degree of crystalline disorder. Remarkably, the resultant material demonstrates a large electromechanical strain without a frequency-related relaxation effect. We conclude that electr…
Due to the limited rare-earth elements resources, ferrite magnets need to be improved drastically. Ideally, for a true hard magnet, the coercive field should be larger than the saturation magnetization, which is not yet realized for ferrites. Thus, an alternative can be found in making very fine grain ferrite magnets, but it is usually impossible to get small grains and dense material together. In this paper, it is shown that the spark plasma sintering method is able to produce approximately 80% of dense material with crystallites smaller than 100 nm. The as-prepared bulk sintered anisotropic …
Textured alumina ceramics were obtained by Spark Plasma Sintering (SPS) of undoped commercial a-Al2O3 powders. Various parameters (density, grain growth, grain size distribution) of the alumina ceramics, sintered at two typical temperatures 1400°C and 1700°C, are investigated. Quantitative textural and structural analysis, carried out using a combination of Electron Back Scattering Diffraction (EBSD) and X-ray diffraction (XRD), are represented in the form of mapping, and pole figures. The mechanical properties of these textured alumina ceramics include high elastic modulus and hardness value …
We present the synthesis of D0$_{22}$ Mn$_{3 - δ}$Ga ($δ$ = 0, 1) Heusler alloys by Spark Plasma Sintering method. The single phase Mn$_3$Ga (T$_\mathrm{c}$ $\simeq$ 780 K) is synthesized, while Mn$_2$Ga (T$_\mathrm{c}$ $\simeq$ 710 K) is found to coexist with a near-stoichiometric room temperature paramagnetic Mn$_9$Ga$_5$~($\approx$ 15 \%) phase due to its lower formation energy, as confirmed from our density functional theory (DFT) calculations. The alloys show hard magnetic behavior with large room temperature spontaneous magnetization m$_s$(80 kOe) = 1.63 (0.83) $μ_\mathrm{B}$/f.u. and co…
Impact of Carbon Contamination on the Low-Temperature Electric Conduction of the Spark-Plasma Sintered Barium Titanate Ceramics
Institute of Physics of the National Academy of Sciences of Ukraine（乌克兰国家科学院物理研究所）；Frantsevich Institute for Problems in Materials Science, National Academy of Sciences of Ukraine（乌克兰国家科学院弗兰特谢维奇材料科学问题研究所）；V. Lashkarev Institute of Semiconductor Physics, National Academy of Sciences of Ukraine（乌克兰国家科学院瓦·拉舍列夫半导体物理研究所）
First thermoelectric properties measurements on bulk nanostructured Ba8Ga16Ge30 clathrate-I are presented. A sol-gel-calcination route was developed for preparing amorphous nanosized precursor oxides. The further reduction of the oxides led to quantitative yield of crystalline nanosized Ba8Ga16Ge30 clathrate-I. TEM investigations show the clathrate nanoparticles retain the size and morphology of the precursor oxides. The clathrate nanoparticles contain mainly thin plates (approx. 300 nm x 300 nm x 50 nm) and a small amount of nanospheres (diameter ~ 10 nm). SAED patterns confirm the clathrate-…
