Field Assisted Sintering Technique Research Articles

Development and comparison of field assisted sintering techniques to densify CeO2 ceramics

Cerium dioxide (CeO2) was densified by conventional and field assisted sintering techniques in order to examine the effect of a range of sintering parameters on the resultant pellet microstructure, namely: temperature, hold time, atmosphere, electric field strength and polarity. CS at 1400 °C for 2 h in static air atmospheres provided the highest densities and grain sizes compared with an argon atmosphere, due to the retention of near stoichiometry maintaining sintering kinetics. SPS produced dense ceramics at similar sintering temperatures (1300 °C), but with greatly reduced sintering hold (5 min) and cycle times. However, microstructural inhomogeneity arose from the direct current polarity which led to oxygen ion diffusion toward the positive electrode. FS was performed with an alternating current electric field and produces samples of comparable density at sintering temperatures of ~1100 °C for a hold time of around 1 h with no inhomogeneity due to the alternating current employed.

A new MgB2 bulk ring fabrication technique for use in magnetic shielding or bench-top NMR systems

We report a new methodology in bulk MgB2 ring production for use in small-scale magnetic shielding or bench-top nuclear magnetic resonance systems. This process is a modified field-assisted sintering technique (mFAST) which enables direct formation of the rings without the need for machining or additives into the precursor powder. The shielding and trapped field capabilities of three mFAST MgB2 rings were determined using zero-field- and field-cooled magnetic experiments. Individual bulks trap magnetic fields up to 1.24 T at 20 K comparable to the highest published data for a ring sample. It is anticipated that for many applications, multiple rings will be stacked to form the required experimental structure. We find, for the three ring stack, a trapped field of 2.04 T and a maximum shielded field of 1.74 T at 20 K. The major factor limiting performance at low temperatures are flux jumps which cause rapid loss of the trapped field or shielding capability. Preliminary studies of magnetic field ramp rate dependence on flux jumps were conducted illustrating that even at very slow ramp rates (0.007 T min−1) they remain a significant issue. Despite this concern, we conclude that mFAST represents an exciting new fabrication methodology for bulk MgB2 rings.

Effect of cubic and hexagonal boron nitride additions on the microstructure and properties of bulk MgB2 superconductors

MgB2 is a promising material for intermediate temperature applications where conventional low temperature superconductors cannot be used, especially if the range of magnetic fields over which is has acceptable current carrying performance can be expanded. However, its applicability is limited by poor properties at elevated magnetic fields. Carbon-based dopants can be used to dramatically improve the high-field performance of MgB2, but at the cost of a reduction in the superconducting transition temperature (T c) that limits the operation temperature to 20 K or below. Here we report an enhancement of superconducting performance of MgB2 with the addition of cubic and hexagonal boron nitride (BN), without any significant reduction in T c. Ex-situ bulk samples of MgB2 with two forms of BN addition were manufactured by the field assisted sintering technique after high energy ball milling of powder mixtures. We find that hexagonal BN (hBN) nanoparticles mixed homogenously with MgB2 powder react much more easily to produce Mg–N–B impurities than larger cubic BN (cBN) particles (∼10 µm) under the same processing conditions. The addition of 1 wt% hBN or 5 wt% cBN combined with 6 h of milling has been demonstrated to improve the critical current density (J c) of MgB2 over the entire magnetic field range. It is proposed that the nano-sized Mg–N–B impurities, that typically reside at MgB2 grain boundaries, increase pinning strength by introducing additional flux pinning centres. In addition, excess Mg may benefit the low-field performance by improving the connectivity. This work shows the significance of microstructural characterization on inhomogeneous superconducting materials to analyse their performance.

Field‐Assisted Sintering of Nb–Al2O3 Composite Materials and Investigation of Electrical Conductivity

Field‐assisted sintering technique (FAST) is used for the preparation of Nb–Al2O3 composite materials. The electrical conductivity is investigated depending on the particle size of the used starting powders and under varying volume contents of the refractory metal in the starting powder mixture. The percolation threshold is investigated and found to be influenced not only by the metal fraction but also by the particle size of the alumina used for sample preparation. For the fine‐ and coarse‐grained alumina, a percolation threshold of 17.5 and 10 vol% Nb is estimated, respectively. Furthermore, the microstructure is investigated to gain a basic understanding of the dependency between microstructural features and the resulting material properties on the macroscopic scale. Also, the influence of the sintering process and the resulting microstructure–properties relationship is considered. It could be shown that the electrical properties are anisotropic because of anisotropy effects caused by the FAST process.

MulTi-FAST: A Machinability Assessment of Functionally Graded Titanium Billets Produced from Multiple Alloy Powders.

Technological developments in the area of functionally graded multi-material manufacture are poised to disrupt the aerospace industry, providing the means for step-change improvements in performance through tailored component design. However, the challenges faced during the downstream processing, i.e., machining of such functionally graded multi-materials are unclear. In this study, the challenges involved when face-turning billets consisting of multiple alloys are assessed. To achieve this, a cylindrical billet consisting of Ti-64, Ti-6242, Ti-5553 and Beta C alloys was manufactured from powder feedstock using field-assisted sintering technique (FAST) and termed MulTi-FAST billets. A detailed study of the structural integrity during machining at the diffusion bond interfaces of multiple titanium alloy bond pairings in the MulTi-FAST billet was conducted. The machining forces were measured during face-turning to investigate the impact and behaviour of different alloy pairings during a continuous machining operation. The results showed the significant differences in force machining response, surface topography and the type of surface damage was dependent on the direction the titanium alloy graded pairings were machined in. In terms of subsurface microstructural damage, regardless of the machining direction, no critical damage was found in the vicinity of the bonded alloys. The findings provide an insight into the deformation characteristics and challenges faced in the machining of functionally graded components with multiple titanium alloys.

Improving the connectivity of MgB2 bulk superconductors by a novel liquid phase sintering process

This work investigates a new processing method developed to improve the connectivity of ex-situ MgB2 bulks at low sintering temperatures. Mg additions (1–10 wt.%) were mixed to pre-synthesised MgB2 to make composite powders that were sintered at 900 ∘C by the field assisted sintering technique. Addition of 10 wt.% Mg resulted in a substantial increase in density from 68% to 79% and a dramatic reduction in MgB4 from 11 to ∼0 wt.%. Pressure and dilatometry data recorded in-situ during the sintering process revealed that Mg additions led to different sintering mechanisms depending on the Mg fraction. For large Mg fractions (6 and 10 wt.%) a Mg liquid phase was formed and led to significant density improvements, and all pre-existing MgB4 was transformed into MgB2. A small amount of residual Mg remained in the bulks after the sintering process. Connectivity was improved with Mg additions, increasing four fold in the 10 wt.% Mg-MgB2 sample compared to unmodified MgB2. J c values at low field were also significantly improved by Mg additions, in particular the 6 and 10 wt.% Mg-MgB2 specimens showed J c (20 K, 0 T) values 4–5 times higher than for unmodified MgB2.

Heat Treatment Influencing Porosity and Tensile Properties of Field Assisted Sintered AlSi7Mg0.6.

In this study, an attempt was made to improve the mechanical properties and in particular the strength of a precipitation-hardenable aluminum alloy while still maintaining high ductility. For this purpose, AlSi7Mg0.6 (A357) powder with an average particle diameter of d50 = 40 µm was consolidated using field assisted sintering technique (FAST), and two material conditions were compared: an as-sintered and an underaging heat treated condition (T61). Mechanical properties were determined using tensile tests and hardness measurements. In addition, the microstructure was investigated by optical microscopy. Further, porosity and density were analyzed after the different heat treatments. By the underaging heat treatment, the surface hardness was increased by 100% and the yield strength was increased by 80% compared to the as-sintered material. However, the elongation to failure dropped to one third of that of the as-sintered material. Presumably, this effect was a result of an increased porosity due to the heat treatment. It is assumed that the observed pores were generated by artefacts from the FAST process used to manufacture the samples. The internal gas pressure and equilibrium diffusion supported by heat treatment temperature, and the reduction in surface energy caused by coalescent micropores, led to the enlargement of previously undetectable inhomogeneities in the as-sintered material that resulted in pores in the heat-treated sintered alloy.

Manufacturing of biomedical Ti alloys with controlled oxygen content by blended elemental powder metallurgy

Biomedical Ti-xNb-7Zr-0.7O (wt%) alloys were manufactured from elemental powders and controlled addition of TiO2 using field assisted sintering technique (FAST). First, sintering parameters were optimized on the benchmark Ti-29Nb-7Zr-0.7O alloy. Second, alloys with various content of Nb (20, 23, 26 and 29 wt%) were manufactured. FAST is capable to produce homogeneous material with controlled amount of oxygen. In addition, FAST is a valuable method for fast sampling of alloys with different compositions. Detailed XRD and SEM study proved that oxygen prevents formation of α” martensite phase. Presence of α phase, α” phase and especially ω phase causes an increase of elastic modulus and microhardness. Single phase β Ti-29Nb-7Zr-0.7O alloy can be manufactured by FAST from elemental powders and subsequent solution treatment. The alloy is proposed for biomedical use due to its low elastic modulus.

Accessing the role of Joule heating on densification during flash sintering of YSZ

Flash sintering is a novel electric field and current assisted sintering technique which densify ceramics at moderate furnace temperature and in short time (few seconds) as compared to the conventional sintering techniques. The onset of flash encompasses three unique characteristics, (i) non-linear rise in conductivity, (ii) luminescence and (iii) rapid densification. the mechanism behind the flash phenomena is not fully understood yet. The present work experimentally demonstrated that the densification in flash sintering is primarily dominated by Joule heating. Herein, we distinguished the onset of flash and the degree of densification of cubic zirconia in sintering atmospheres with different oxygen partial pressures. In this study, we demonstrate that the flash onset temperature and densification are independently influenced by the sintering atmosphere during flash sintering. Furthermore, the extent of densification is not directly influenced by the current density rather, by the power dissipated in the sample.

Non-uniform He bubble formation in W/W2C composite: Experimental and ab-initio study

Tungsten-tungsten carbide (W/W2C) composites are considered as possible structural materials for future nuclear fusion reactors. Here, we report on the effect of helium (He) implantation on microstructure evolution of polycrystalline W/W2C composite consolidated by field-assisted sintering technique (FAST), homogenously implanted at room temperature with 1 MeV 4He+ ions at the fluence of 8 × 1016 ions cm−2 and annealed at 1873 K for 20 minutes. Samples were analysed by scanning and transmission electron microscopy to study the presence and size of He bubbles. Monomodal He bubbles in W (30-80 nm) are limited to point defects and grain boundaries, with a considerable void denuded zone (150 nm). Bubbles do not form in W2C, but at the W|W2C interface and are considerably larger (200-400 nm). The experimental observations on He behaviour and migration in W and W2C were assessed by density functional theory (DFT) calculations, suggesting He migration and accumulation in the composite are determined by the effective He-He binding in clusters, which will give rise to decohesion. In the presence of He clusters, the decohesion of bulk W into free surfaces is energetically highly favourable but not sufficient in the W2C; hence bubbles are only observed in W grains and interfaces and not within bulk W2C.

Flash Sintering Research Perspective: A Bibliometric Analysis.

Flash Sintering (FS), a relatively new Field-Assisted Sintering Technique (FAST) for ceramic processing, was proposed for the first time in 2010 by Prof. Rishi Raj’s group from the University of Colorado at Boulder. It quickly grabbed the attention of the scientific community and since then, the field has rapidly evolved, constituting a true milestone in materials processing with the number of publications growing year by year. Moreover, nowadays, there is already a scientific community devoted to FS. In this work, a general picture of the scientific landscape of FS is drawn by bibliometric analysis. The target sources, the most relevant documents, hot and trending topics as well as the social networking of FS are unveiled. A separate bibliometric analysis is also provided for Reaction or Reactive Flash Sintering (RFS), where not only the sintering, but also the synthesis is merged into a single step. To the best of our knowledge, this is the first study of this nature carried out in this field of research and it can constitute a useful tool for researchers to be quickly updated with FS as well as to strategize future research and publishing approaches.

Influence of Dwell Time and Pressure on SPS Process with Titanium Aluminides

Spark plasma sintering (SPS) or the field-assisted sintering technique (FAST) is commonly used to process powders that are difficult to consolidate, more efficiently than in the conventional powder metallurgy process route. During the process, holding time and applied holding pressure influence the product’s microstructure and subsequently its properties. In this study, in addition to the temperature impact, the influence of pressure and dwell time on the consolidation behaviour of titanium aluminide (TiAl) powders during the SPS process is investigated. Commercially available pre-alloyed TiAl48-2Cr-2Nb (GE48) and TiAl44-4Nb-0.7Mo-0.1B (TNM) powders were used, which have a high application potential in, for example, the aerospace industry. The results were evaluated based on microstructural analyses, hardness measurements and relative density calculations. It was shown that the investigated parameters significantly influence the sintering results, especially in the low temperature range. Depending on the temperature field in the sample, complete sintering is not achieved if the dwell time is too short in combination with too low a pressure. Above a certain temperature, the impact of holding pressure and holding time is significantly lower.

Fabrication and characterization of high entropy pyrochlore ceramics

High-entropy rare-earth (RE) zirconates with pyrochlore structure were successfully fabricated by pressureless and spark plasma sintering. RE2Zr2O7 compound with nominal composition (La0.2Y0.2Gd0.2Nd0.2Sm0.2)Zr2O7 was prepared by simple glycine nitrate procedure (GNP). GNP process yielded powders with low crystallinity and after subsequent calcination, well crystalline ceramics were formed. During calcination defective fluorite (F-RE2Zr2O7) and crystal pyrochlore (Py-RE2Zr2O7) structures coexist. Formation of pure crystalline pyrochlore occurs after sintering at 1450°C. High-density ceramics, free of any additives, were obtained after powders compaction and pressureless (PS), as well as field assisted sintering technique (FAST) at 1450°C. Theoretical investigations of the high-entropy RE2Zr2O7 pyrochlore systems were performed. Unit cell parameter of the obtained Py-RE2Zr2O7 is 10.5892(2)Å and 10.5999(2)Å for PS and FAST sintering, respectively, which is in good agreement with the results of Density Functional Theory (DFT) calculations. The thermal diffusivity of sintered samples at room temperature was ∼0.7mm2/s for both sintering methods.

Microstructure stability and hardening determination through heat treatment of sintered Co γ/γ′ based alloys

ABSTRACT Aiming to improve γ′ volume fraction through alloying elements and heat treatments, the influence of the combined effect of 2 Ti and 2 Ta addition (at%) to a nominal Co–12Al–10W at.% Co-based superalloy, and the heat treatments parameters have been investigated. The Co-based alloys were manufactured by PM using mechanical alloying for powder processing, and a field-assisted sintering technique to consolidate the material. The specific study of solution temperatures and ageing times to promote the γ/γ′ dual-phase was carried out through a complete microstructural analysis and micro and nano hardness measurements. The composition of the γ (fcc)-matrix and γ′ (L12)-precipitates, the nanoscale properties including γ′ volume fraction and γ/γ′ misfit were measured as a function of increasing solution and ageing time. In the end, Ti and Ta addition to the Co–12A–10W alloy increases γ’ volume fraction and thermal stability for long ageing time detecting slow coarsening.

Garnet-Based Composite Cathodes for All Solid-State Li Batteries

All-solid-state Li batteries are regarded as a highly promising system for future electrochemical energy storage. Oxide-ceramic based all-solid-state lithium batteries (ASB) can provide high intrinsic safety, extended operational temperature range and high energy density.As the first two are intrinsic to the materials system, one prerequisite to obtain high energy densities with such ASBs is the manufacturing of thick (approx. 150 µm), dual-conducting free-standing composite cathodes, analogous to that of conventional liquid electrolyte-based lithium batteries. However, the preparation of composite cathodes using oxide-ceramic electrolytes is challenging since high temperature sintering steps are necessary during electrode and cell manufacturing to achieve proper mechanical stability, contact between the individual phases and good ionic and electronic conductivity.[1, 2] For oxide-based ceramic electrolyte materials like Li7La3Zr2O12 (LLZ) or Li1+xAlxTi2-x(PO4)3 (LATP) a sintering temperature around 1000 °C is necessary. However, cathode active materials like spinels (e. g. Li2NiMn3O8) or layered materials (e. g. Li[Ni1-x-yCoxMny]O2 (NCM)) show thermal stability only to around 700 °C and the required sintering temperature for the electrolyte exceeds their thermal stability window.To mitigate these materials interactions, advanced sintering techniques like high-pressure Field Assisted Sintering Technique/ Spark Plasma Sintering (FAST/SPS) is required, as it allows low temperature consolidation.[3]Using high-pressure FAST/SPS we have developed a method to prepare thick mixed-conducting composite cathodes and half-cells at sintering temperatures as low as 700 °C. The analysis of these composite cathodes revealed well sintered pure phases and a homogenous distribution of cubic LLZ and cathode active material. Electrochemical tests showed promising electronic and ionic conductivity (0.4 mS cm-1) values and an increased capacity (4 mA cm-2) compared to cathodes with pure active material.FAST/SPS can, therefore, open new processing windows for ceramic-based ASSLBs to alleviate the problem of interphase formation.

Microstructure and mechanical properties of Ni-17Cr-xCo ternary alloys fabricated via field-assisted sintering

Herein, the microstructure and mechanical properties of Ni-17Cr-xCo, x = (2, 6, 10 wt%) ternary alloy system fabricated through field-assisted sintering technique for turbine disc applications were assessed. The sintered alloy’s average relative density reduces from 97.56% to 96.02%, with increasing cobalt content. The nanoindentation result shows that the hardness and elastic modulus of the alloys increases as the Co-content increases. The phase diagram calculations of the alloys via Thermo-Calc software predicted the stability of fcc crystal structure from 403 °C to 1406 °C for the compositions considered in this study.

Mechanical and structural properties of bulk magnesium materials prepared via spark plasma sintering

Field-assisted sintering is a modern approach to novel magnesium materials preparation; however, it is unclear whether it is better to sinter green compact or loose powder. This work focuses on preparing bulk materials from loose and cold-compacted magnesium powder through a field-assisted sintering technique – spark plasma sintering (SPS). Green compacts were prepared under a series of compacting pressures from 100 MPa to 500 MPa. SPS was performed at 400 °C, 500 °C, and 600 °C applying additional pressure of 100 MPa during the sintering process. Prepared materials were analysed regarding their microstructure, hardness, and microhardness and through the three-point bending test and fractography. The green compacts porosity decreased with increased cold-compaction pressure. The SPS positively affected porosity and mechanical properties only in loose powder and the lowest cold-compacted green compacts. Increasing cold compaction pressure of the green compacts above 200 MPa is therefore unfavourable for further SPS processing.

Influence of metal matrix powder size on the tensile strength of a SiCp/AlSi7Mg0,6 composite produced by field assisted sintering technique

In the present study, 35vol% SiCp/AlSi7Mg0,6 composites were prepared using field assisted sintering technique in order to investigate the effect of different particle fractions and size distributions of the AlSi7Mg matrix powder on the tensile properties of the produced composite material. In most usecases the size of the reinforcement phase is given by the application and is only variable within narrow limits (< 20pm particle size in this work). On the other hand, there is potential for optimization of the matrix powder. In this investigation, fine (d50 = 25 µm), coarse (d50 = 52 µm), bimodal (50wt% of fine + 50wt% of coarse, d50 = 36 µm) and as received (d50 = 40 µm) aluminum powder was used as the matrix powder. Using fine matrix powder has improved yield strength by 5 % and ultimate tensile strength by 7 % compared to the as received condition. This is largely due to the lower porosity of the composite produced under the use of the fine matrix powder ((0.07 ± 0.04) %) in contrast to the composite using the as received aluminum powder ((0.62 ± 0.35) %). At the same time, the consumed heating energy of the composite was decreased by almost a third when using the fine matrix powder in comparison to the use of the as-received matrix powder. This paper presents results of an optimization approach for mechanical properties of aluminum matrix composites without any changes of the sintering parameters.

Pulsed electric current sintering of TiB2-based ceramics using nitride additives

In this research, various types of nitride additives were incorporated into titanium diboride attaining dense TiB2-based ceramics by field-assisted sintering technique. The addition of different types of nitride additives, namely Si3N4, BN, AlN, and TiN, significantly improved the sinterability of TiB2, achieving near fully dense ceramics. The X-ray diffraction analysis and microstructural evaluation confirmed the presence of the h-BN compound in all specimens. In the TiB2-Si3N4 ceramic, Si3N4 additive reacted with B2O3 oxide, in-situ generating h-BN, and SiO2 phases. Although the h-BN phase was produced in the TiB2-AlN specimen, the main proportion of AlN remained in the sample as an unreacted ex-situ phase. In terms of the TiB2-TiN ceramic, some of the nitrogen and boron atoms could leave the TiN and TiB2 crystalline structures, contributing to the in-situ formation of h-BN.

Processing and oxidation response of Cr2AlC MAX-phase composites containing ceramic fibers

Three different ceramic matrix composites (CMCs) were produced using Cr2AlC as a matrix, and carbon, SiC and Al2O3 short fibers as a secondary phase. Cr2AlC powders were synthesized by solid-state reaction, followed by mixing with the fibers, and full densification using a field-assisted sintering technique. Of the three different fiber types, Carbon fibers reacted strongly with Cr2AlC, while the reaction with SiC fibers was more limited and alumina fibers didn’t show any reaction. Oxidation tests of the monolithic Cr2AlC and the composites were performed by thermogravimetric analysis. An alumina layer formed at 1000 ​°C on every sample, well attached and worked as a good oxidation barrier. Under realistic conditions using a burner rig for cyclic oxidation at 1200 ​°C for 500 cycles, the oxidation resistance of the alumina fiber CMC is good, as no defects or degradation are visible and the alumina layer is well attached.