Showing papers on "Sintering published in 2017"

Review of flash sintering: materials, mechanisms and modelling

Abstract: Flash sintering (FS) is an energy efficient sintering technique involving electrical Joule heating, which allows very rapid densification (<60 s) of particulate materials. Since the first publication on flash-sintered zirconia (3YSZ) in 2010, it has been intensively researched and applied to a wide range of materials. Going back more than a century ago, we have found a close similarity between FS of oxides and Nernst glowers developed in 1897. This review provides a comprehensive overview of FS and is based on a literature survey consisting of 88 papers and seven patents. It correlates processing parameters (i.e. electric field magnitude, current density, waveforms (AC, DC) and frequency, furnace temperature, electrode materials/configuration, externally applied pressure and sintering atmosphere) with microstructures and densification mechanisms. Theorised mechanisms driving the rapid densification are substantiated by modelling work, advanced in situ analysis techniques and by established theorie...

Direct selective laser sintering and melting of ceramics: a review

Abstract: Purpose This paper aims to provide a review on the process of additive manufacturing of ceramic materials, focusing on partial and full melting of ceramic powder by a high-energy laser beam without the use of binders. Design/methodology/approach Selective laser sintering or melting (SLS/SLM) techniques are first introduced, followed by analysis of results from silica (SiO2), zirconia (ZrO2) and ceramic-reinforced metal matrix composites processed by direct laser sintering and melting. Findings At the current state of technology, it is still a challenge to fabricate dense ceramic components directly using SLS/SLM. Critical challenges encountered during direct laser melting of ceramic will be discussed, including deposition of ceramic powder layer, interaction between laser and powder particles, dynamic melting and consolidation mechanism of the process and the presence of residual stresses in ceramics processed via SLS/SLM. Originality/value Despite the challenges, SLS/SLM still has the potential in fabrication of ceramics. Additional research is needed to understand and establish the optimal interaction between the laser beam and ceramic powder bed for full density part fabrication. Looking into the future, other melting-based techniques for ceramic and composites are presented, along with their potential applications.

Current understanding and future research directions at the onset of the next century of sintering science and technology

Abstract: Sintering and accompanying microstructural evolution is inarguably the most important step in the processing of ceramics and hard metals. In this process, an ensemble of particles is converted into a coherent object of controlled density and microstructure at an elevated temperature (but below the melting point) due to the thermodynamic tendency of the particle system to decrease its total surface and interfacial energy. Building on a long development history as a major technological process, sintering remains among the most viable methods of fabricating novel ceramics, including high surface area structures, nanopowder-based systems, and tailored structural and functional materials. Developing new and perfecting existing sintering techniques is crucial to meet ever-growing demand for a broad range of technologically significant systems including, for example, fuel and solar cell components, electronic packages and elements for computers and wireless devices, ceramic and metal-based bioimplants, thermoelectric materials, materials for thermal management, and materials for extreme environments. In this study, the current state of the science and technology of sintering is presented. This study is, however, not a comprehensive review of this extremely broad field. Furthermore, it only focuses on the sintering of ceramics. The fundamentals of sintering, including the thermodynamics and kinetics for solid-state- and liquid-phase-sintered systems are described. This study summarizes that the sintering of amorphous ceramics (glasses) is well understood and there is excellent agreement between theory and experiments. For crystalline materials, attention is drawn to the effect of the grain boundary and interface structure on sintering and microstructural evolution, areas that are expected to be significant for future studies. Considerable emphasis is placed on the topics of current research, including the sintering of composites, multilayered systems, microstructure-based models, multiscale models, sintering under external stresses, and innovative and novel sintering approaches, such as field-assisted sintering. This study includes the status of these subfields, the outstanding challenges and opportunities, and the outlook of progress in sintering research. Throughout the manuscript, we highlight the important lessons learned from sintering fundamentals and their implementation in practice.

Demonstration of the cold sintering process study for the densification and grain growth of ZnO ceramics

Abstract: With the cold sintering process (CSP), it was found that adding acetic acid to an aqueous solution dramatically changed both the densities and the grain microstructures of the ZnO ceramics Bulk densities >90% theoretical were realized below 100°C, and the average conductivity of CSP samples at around 300°C was similar to samples conventionally sintered at 1400°C Frequently, ZnO is also used as a model ceramic system for fundamental studies for sintering By the same procedure as the grain growth of the conventional sintering, the kinetic grain growth exponent of the CSP samples was determined as N = 3, and the calculated activated energy of grain growth was 43 kJ/mol, which is much lower than that reported using conventional sintering The evidence for grain growth under the CSP is important as it indicates that there is a genuine sintering process being activated at these low temperatures and it is beyond a pressurized densification process

Control of the γ-alumina to α-alumina phase transformation for an optimized alumina densification

Abstract: In this work, we studied the aptitude to sintering green bodies using γ-Al 2 O 3 transition alumina as raw powder. We focused on the influence of the heating rate on densification and microstructural evolution. Phase transformations from transition alumina γ → δ → θ → α-Al 2 O 3 were studied by in situ X-rays diffraction from the ambient to 1200 °C. XRD patterns revealed coexistence of various phase transformations during the heating cycle. DTA and dilatometry results showed that low heating rate leads to a significant reduction of the temperature of the α-Al 2 O 3 alumina formation. Around 1190, 1217 and 1240 °C were found when using 5, 10 and 20 °C/min of heating rate, respectively. The activation energy for θ-Al 2 O 3 → α-Al 2 O 3 transformation calculated by Kissinger and JMA equations using dilatometry method were 464.29 and 488.79 kJ/mol, respectively and by DTA method were 450.72 and 475.49 kJ/mol, respectively. In addition, the sintering of the green bodies with low heating rate promotes the rearrangement of the grains during θ-Al 2 O 3 → α-Al 2 O 3 transformation, enhancing the relative density to 95% and preventing the development of a vermicular structure.

Ultra-fast firing: Effect of heating rate on sintering of 3YSZ, with and without an electric field

Abstract: It has recently been reported that ceramics can be sintered in a few seconds with the aid of an electric field (“flash sintering”). This investigation tests the possibility that the accelerated sintering is a consequence of the rapid heating rate involved rather than a direct effect of the electric field on mass transport. The sintering of 3YSZ powder compacts at a temperature of ∼1300 °C was compared (i) in flash sintering, (ii) with rapid heating rates produced without the application of an electric field, and (iii) with conventional heating rates. The results show that rapid heating can accelerate sintering by over 2 orders of magnitude compared with heating to the same temperature at conventional rates, even without the application of an electric field. It is concluded that the rapid densification in flash sintering of 3YSZ is at least partly a consequence of the rapid heating involved. Possible explanations are discussed.

Powder metallurgical processing of equiatomic AlCoCrFeNi high entropy alloy: Microstructure and mechanical properties

Abstract: Phase formation, microstructural evolution and the mechanical properties of novel multi-component equiatomic AlCoCrFeNi high entropy alloy synthesized by high energy ball milling followed by spark plasma sintering have been reported here. The microstructure of the mechanically alloyed (MA) powder and sintered samples were studied using X-ray diffraction, scanning electron and transmission electron microscopy, whereas the detailed investigation of the mechanical properties of the sintered samples were measured using micro and nano hardness techniques. The fracture toughness measurements were performed by applying single edge V notch beam (SEVNB) technique. The MA powder shows the presence of FCC (tau) and BCC (kappa) solid solution phases. Extended ball milling (up to 60 h) does not change the phases present in MA powder. The sintered pellets show phase-separated microstructure consisting of Al-Ni rich L1(2) phase, alpha' and tetragonal Cr-Fe-Co based sigma phase along with Al-Ni-Co-Fe FCC solid solution phase (epsilon) for sample sintered from 973 to 1273 K. The experimental evidences indicate that BCC (kappa) solid solution undergoes eutectoid transformation during sintering leading to the formation of L1(2) ordered alpha' and sigma phases, whereas FCC (tau) phase remains unaltered with a slight change in the lattice parameter. The hardness of the sample increases with sintering temperature and a sudden rise in hardness is observed 1173 K. The sample sintered at 1273 K shows the highest hardness of similar to 8 GPa. The elastic modulus mapping clearly indicates the presence of three phases having elastic moduli of about 300, 220 and 160 GPa. The fracture toughness obtained using SEVNB test shows a maximum value of 3.9 MPa m(1/2), which is attributed to the presence of brittle nanosized sigma phase precipitates. It is proposed that significant increase in the fraction of sigma phase precipitates and eutectoid transformation of the tau phase contribute to increase in hardness along with better densification at higher sintering temperatures.

Foam-gelcasting preparation, microstructure and thermal insulation performance of porous diatomite ceramics with hierarchical pore structures

Abstract: Hierarchically pore-structured porous diatomite ceramics containing 82.9∼84.5% porosity were successfully prepared for the first time via foam-gelcasting using diatomite powder as the main raw material. Sizes of mesopores derived from the raw material and macropores formed mainly from foaming were 0.02∼0.1 μm and 109.7∼130.5 μm, respectively. The effect of sintering temperature, additive content and solid loading of slurry on pore size and distribution, and mechanical and thermal properties of as-prepared porous ceramics were investigated. Compressive strength of as-prepared porous ceramics increased with sintering temperature, and the one containing 82.9% porosity showed the highest compressive strength of 2.1 ± 0.14 MPa. In addition, the one containing 84.5% porosity and having compressive strength of 1.1 ± 0.07 MPa showed the lowest thermal conductivity of 0.097 ± 0.001 W/(m·K) at a test temperature of 200 C, suggesting that as-prepared porous ceramics could be potentially used as good thermal insulation materials.

Microstructure and mechanical properties of carbon nanotubes reinforced titanium matrix composites fabricated via spark plasma sintering

Abstract: The influence of various dispersion methods on the evolution of multi-walled carbon nanotubes (MWCNTs) in titanium (Ti) metal matrix composites (TMCs) prepared via spark plasma sintering (SPS) have been investigated. The synthesis procedures included sonication, high energy ball milling (HEBM), and rapid consolidation of powder mixtures at different sintering temperatures. The impact energy provided to the powder mixtures during HEBM process was optimized to disperse 0.5 wt% MWCNTs into Ti matrix in two controlled ball milling processes: with and without in-situ formation of TiC during HEBM. The interfacial reactions between MWCNTs and Ti matrix were controlled by retaining the crystallinity and sp2 carbon network of the MWCNTs even at high sintering temperature of 800 °C, which enhanced their compressive strength up to 1056 MPa with a compressive strain of 27.31%. The mechanical and tribological properties of the composites consolidated from the powder mixtures with in-situ TiC formation during HEBM and pre-sonicated MWCNTs were significantly enhanced as opposed to the composites consolidated from the powder mixtures without formation of TiC during HEBM.

Process development toward full-density stainless steel parts with binder jetting printing

Abstract: In the Additive Manufacturing (AM) community, the binder jet printing (BJP) process is known to produce parts not suitable for most structural applications due to the insufficient consolidation of the powder in the finished part. A new processing protocol for the BJP is presented to reach near full density and better surface finish for stainless steel (SS) parts. Two main modifications from the standard BJP processing are (1) the use of the mixtures of various powders and (2) the adaptation of a full sintering cycle in a vacuum furnace. Two distinct average particle sizes of SS powder were used to improve the packing density in the printing stage. Improving the packing density of the printed powder helps to consolidate the powder better and to reduce the shape distortion in the final parts. More importantly, an extremely small amount of the sintering additive was added to enhance the densification, which reduces the sintering time and temperature. In particular, up to 0.5 wt% of boron compounds as sintering additives were used to achieve a near full density in the final part. Thus, the starting powder, consisting of two distinct SS powders and sintering additive, is mixed before building a part in a layer-by-layer fashion. After completing the printing process with a binder phase, the printed powders are cured and the binder phase is burned out at 460 °C before sintering at 1250 °C for 6 h in a vacuum furnace to reach near-full densities (up to 99.6%). A subtle difference between SS 420 and SS 316 was evident because the enhanced oxidation during the binder burnout cycle on SS 316 due to a higher surface area of the SS 316 powder used in the experiment. The main contribution of this work is to provide the BJP process an important ability to fully consolidate the powders under an isothermal condition, which enable us to produce the final parts without residual stresses.

Lattice-Confined Sn (IV/II) Stabilizing Raft-Like Pt Clusters: High Selectivity and Durability in Propane Dehydrogenation

Abstract: Catalytic dehydrogenation of propane (DHP) to propene is highly endothermic, requiring a high reaction temperature. Under harsh conditions, it has been a great challenge to maintain excellent propene selectivity and suppress the irreversible deactivation caused by sintering of metallic active centers. This work reports a highly selective and durable Pt–Sn catalyst for DHP, in which metallic Pt centers are dispersed homogeneously in small raft-like clusters on Mg(Sn)(Al)O and form strong interactions with the SnIV/II sites confined in Mg(Al)O lattices. A propene selectivity of >99% at 550 °C with a conversion close to the equilibrium (specific rate of 0.96 s–1 for propene formation) and a propene selectivity of >98% (specific rate of 1.46 s–1) even under 600 °C have been produced by highly dispersed Pt sites in Pt/Mg(Sn)(Al)O. The Pt–Sn interactions and SnIV/II confinement were revealed to afford the catalyst with good durability. No visible sintering of Pt clusters was observed in the long-term DHP reaction.

Sintering behaviour and microwave dielectric properties of BaAl2−2x(ZnSi)xSi2O8 ceramics

Abstract: BaAl2−2x(ZnSi)xSi2O8 (x = 0.2–1.0) ceramics were prepared using the conventional solid-state reaction method. The sintering behaviour, phase composition and microwave dielectric properties of the prepared compositions were then investigated. All compositions showed a single phase except for x = 0.8. By substituting (Zn0.5Si0.5)3+ for Al3+ ions, the optimal sintering temperatures of the compositions decreased from 1475 °C (x = 0) to 1000 °C (x = 0.8), which then slightly increased to 1100 °C (x = 1.0). Moreover, the phase stability of BaAl2Si2O8 was improved. A novel BaZnSi3O8 microwave dielectric ceramic was obtained at the sintering temperature of 1100 °C. This ceramic possesses good microwave dielectric properties with er = 6.60, Q × f = 52401 GHz (at 15.4 GHz) and τf = −24.5 ppm/°C.

Effect of reinforcement concentration on the properties of hot extruded Al-Al2O3 composites synthesized through microwave sintering process

Abstract: In this study, Aluminum matrix composite reinforced with 5, 10 and 15 vol% Al2O3 particles were fabricated using microwave assisted rapid sintering technique followed by hot extrusion. The effect of concentration of Al2O3 reinforcement particles on physical, structural, mechanical and thermal behavior of the extruded Al–Al2O3 composites was investigated. Results showed that, as the content of hard ceramic particles increases, it improves overall mechanical properties including microhardness, 0.2% yield strength, ultimate compression/tensile strength, and Young's modulus while the ductility and co-efficient of thermal expansion decreases. XRD patterns and SEM/EDS mapping images show a uniform distribution of Al2O3 particles in the Al matrix. Owing to the existence of the reinforcement particles, the Al-15vol%Al2O3 composite attains an ultimate tensile strength and yield strength of 154±6 MPa and 139±8 MPa, respectively, representing an enhancement of 33% and 32.4% compared to the pure aluminum. In particular, the extruded Al-15vol%Al2O3 composite exhibited superior tensile strength at higher temperatures (~128±3 MPa at 100 °C and 110±4 MPa at 200 °C) when compared to Al matrix. The reduction of coefficient of thermal expansion (CTE) was ascribed to the strong interface bonding in the Al/Al2O3 composites. Findings presented are expected to pave the way to design, develop and synthesize aluminum based composites for weight critical applications at ambient and reasonable elevated temperatures.

Highly Enhanced Thermoelectric Properties of Bi/Bi2S3 Nanocomposites.

Abstract: Bismuth sulfide (Bi2S3) has been of high interest for thermoelectric applications due to the high abundance of sulfur on Earth. However, the low electrical conductivity of pristine Bi2S3 results in a low figure of merit (ZT). In this work, Bi2S3@Bi core–shell nanowires with different Bi shell thicknesses were prepared by a hydrothermal method. The core–shell nanowires were densified to Bi/Bi2S3 nanocomposite by spark plasma sintering (SPS), and the structure of the nanowire was maintained as the nanocomposite due to rapid SPS processing and low sintering temperature. The thermoelectric properties of bulk samples were investigated. The electrical conductivity of a bulk sample after sintering at 673 K for 5 min using Bi2S3@Bi nanowire powders prepared by treating Bi2S3 nanowires in a hydrazine solution for 3 h is 3 orders of magnitude greater than that of a pristine Bi2S3 sample. The nanocomposite possessed the highest ZT value of 0.36 at 623 K. This represents a new strategy for densifying core–shell powde...

Reliability of Ag Sintering for Power Semiconductor Die Attach in High-Temperature Applications

Abstract: Low-temperature Ag sintering provides a lead-free die attachment method that is compatible with high-temperature (300 °C) power electronics applications. The reliability of sintered Ag die attach for Si and SiC die has been studied on both thick film substrates for lower current power applications and direct bond copper (DBC) substrates for higher current power applications. Pressureless and low-pressure sintering were evaluated. Sintering with low pressure yielded lower porosity (15–17%) versus pressureless sintering (∼30%). Reliability was evaluated with thermal aging (300 °C) and thermal cycling (–55 °C to + 300 °C) tests. Reliable Ag sintered die attach was achieved with assemblies having Ag-bearing surface finishes on both the die and the substrate. In contrast, the shear strength after 300 °C aging was greatly reduced when Au metallization was used either on the die or on substrate surface. In some cases, low-pressure sintering delayed the failure of the sintered Ag die attach to Au surfaces when aged at 300 °C compared to the pressureless sintering. The reliability with Pd-containing substrate metallizations was intermediate between Ag and Au metallizations. The thermal cycle reliability on DBC substrates was limited by failure at the Cu-to-alumina interface over the wide temperature range, while on the thick film substrates high adhesion was maintained after 1000 thermal cycles.

Microstructural and mechanical characterization of in-situ TiC/Ti titanium matrix composites fabricated by graphene/Ti sintering reaction

Abstract: In-situ TiC/Ti titanium matrix composites were successfully fabricated through a novel approach, utilizing the reaction of graphene and Ti mixture powders in spark plasma sintering. Microstructure, hardness and compressive properties of such composites were investigated. The layered graphene nanosheets as the carbon source were dispersed in the powders, and subsequently formed uniform TiC particles in Ti matrix during the rapid sintering. The resulting TiC particles exhibited the micro- and nano-sized equiaxial structures. Such composites possessed the significantly enhanced hardness and room-temperature compressive strength comparing with the as-cast and as-sintered pure Ti. The ultimate and yield compressive strengths of as-prepared 7.0 vol% TiC incorporated composite respectively reached up to 2.64 GPa and 1.93 GPa, beyond some advanced Ti materials reported to date. In-situ micro- and nano-sized TiC particles were believed to be beneficial to harden and strengthen Ti matrix. The relevant strengthening mechanisms of such composites were discussed.