Sintering Step Research Articles

Additive Manufacturing of Solid Oxide Fuel Cell (AM-SOFC) Stacks

The Solid oxide fuel cell (SOFC) is an electrochemical energy-conversion device which offers tremendous promise for delivering high electrical efficiency and significant environmental benefits in terms of fuel flexibility, as well as clean and efficient electric power generation. However, the state-of-the-art SOFC cell currently involves expensive and time-consuming multi-step manufacturing processes including tape casting, screen printing, shaping, lamination, sintering, etc. In addition, SOFC stacks are usually assembled with pre-fabricated cells, metal interconnects, and glass-ceramic sealing. These indispensable components add extra layers of complexity to the stack structure’s fabrication and maintenance, significantly increasing the size and weight of the final product. These intrinsic limitations on the current planar stack cannot be easily removed by traditional manufacturing processes. However, as additive manufacturing (AM) technologies start to mature and show increasing market penetration, it lends itself admirably to breaking the limits on high-power-density SOFC stack manufacturing. Storagenergy’s SOFC project is developing an all-ceramic AM-SOFC by a high-speed AM process of multiple materials. The developed technology allows the fabrication of robust and compact AM-SOFC stacks with unique features such as monolithic architecture with embedded novel micro flow channels and thinner elements (anode, cathode, electrolyte, and interconnect) by only three production steps (3D printing, sintering, and catalyst infiltration) without the need for subsequent assembly of individual layers and indispensable components. This simplification of the fabrication process is expected to reduce the cost of the SOFC and increase SOFC design flexibility and manufacturing reliability.

Physical Characterization of Sintered NiMnGa Ferromagnetic Shape Memory Alloy.

The present work focused on the microstructural, thermal, electrical, and damping characterization of NiMnGa samples produced through a powder pressing and a sintering process; the effect of sintering times and of the starting powder size were evaluated. Moreover, an observation of the evolution of martensitic transformation typical of NiMnGa ferromagnetic shape memory alloy was conducted in comparison with the cast material behavior and in correlation with the material densification. The optimum powder size and sintering time for the process, i.e., 50 µm or lower and 72 h, were identified considering the investigated physical properties of the sintered samples in comparison to the cast material. The corresponding sample showed the best compromise between density, thermal and electrical properties, and damping and functional behaviour. In general, the outcomes of this study could be the basis of a useful tool for production processes that include a sintering step as well as being a starting point for the evaluation of an alternative low cost fabrication method of this alloy.

In Search of the Optimal Conditions to Process Shape Memory Alloys (NiTi) Using Fused Filament Fabrication (FFF).

This research was performed so as to investigate the additive manufacturing of NiTi shape memory alloys, which is associated with direct processes, such as selective laser melting. In addition to its expensive production costs, NiTi readily undergoes chemical and phase modifications, mainly as a result of Ni loss during processing as a result of high temperatures. This research explores the potential usefulness of NiTi as well as its limitations using indirect additive processes, such as fused filament fabrication (FFF). The first step was to evaluate the NiTi critical powder volume content (CPVC) needed to process high-quality filaments (via extrusion). A typical 3D printer can build a selected part/system/device layer-by-layer from the filaments, followed by debinding and sintering (SDS), in order to generate a near-net-shape object. The mixing, extruding (filament), printing (shaping), debinding, and sintering steps were extensively studied in order to optimize their parameters. Moreover, for the sintering step, two main targets should be met, namely: the reduction of contamination during the process in order to avoid the formation of secondary phases, and the decrease in sintering temperature, which also contributes to reducing the production costs. This study aims to demonstrate the possibility of using FFF as an additive manufacturing technology for processing NiTi.

Synthesis of bulk reactive Ni–Al composites using high pressure torsion

Self-propagating exothermic reactions, for instance in the nickel-aluminum (Ni–Al) system, have been widely studied to create high performance intermetallic compounds or for in-situ welding. Their easy ignition once the phase spacing is reduced below the micron scale, makes top-down methods like high-energy ball milling, ideal to fabricate such reactive nanostructures. A major drawback of ball milling is the need of a sintering step to form bulk pieces of the reactive material. However, this is not possible, as the targeted reactions would already proceed. Therefore, we investigate the ability of high pressure torsion as an alternative process, capable to produce bulk nanocomposites from powder mixtures. Severe straining of powder mixtures with a composition of 50 wt% Ni and 50 wt% Al enables fabrication of self-reactive bulk samples with microstructures similar to those obtained from ball milling or magnetron sputtering. Samples deformed at ambient temperature are highly reactive and can be ignited significantly below the Al melting point, finally predominantly consisting of Al3Ni2 and Al3Ni, independent of the applied strain. Although the reaction proceeds first at the edge of the disk, the strain gradient present in the disks does not prevent reaction of the whole sample.

Effect of MgO on physical and mechanical properties of dental porcelain

The aim of this work is to investigate the effect of MgO on physical and mechanical properties of Dental Porcelain. MgO was used in this work as particle reinforcement to improve these properties. 0.5, 1.0 and 1.5 weight percentages of MgO were reinforced in Dental Porcelain (75 wt% feldspar, 20 wt% quartz and 5 wt% kaolin). The samples were made by powder pressing process and sintered in one step sintering technique and sintered at three different temperatures (1050 °C, 1100 °C and 1150 °C). The prepared samples were tested by using X-Ray Fluorescence (XRF), Scanning Electron Microscope (SEM) and Differential Scanning Calorimeter (DSC), Universal Testing Machine (UTM) X-Ray Diffraction (XRD) and Vickers Hardness Tester to characterize the chemical composition, microstructure and phases present, fracture toughness, hardness respectively. Physical and mechanical properties such as density, hardness, diametral tensile strength, flexural strength and fracture toughness were determined. The addition of MgO up to 1% in 1100 °C influences all the physical and mechanical properties which was positive, where further addition of MgO showed adverse effect.

Enhanced thermal conductivity in Si3N4 ceramics prepared by using ZrH2 as an oxygen getter

This work reports the two-step sintering method and thermal conductivity of Si3N4 doped with 1.5 wt% MgO and varied contents of ZrH2 in the range of 0-5 wt%. The metal hydride reduction reaction in the first step sintering allows the in situ generations of ZrO2 and the elimination of native SiO2. A smaller amount of liquid phase with reduced oxygen content was produced during the second step sintering as a result of the replacement of ZrO2 by ZrH2. With the aid of ZrH2 as an oxygen getter, the native oxygen in those ZrH2-doped samples was removed via a route of SiO2 →ZrO2 →SiO(g). Consequently, the densified Si3N4 ceramics with larger volume percent of elongated purified β-Si3N4 grains and less glassy phase were obtained. Ultimately, the optimal thermal conductivity of 116.4 W m−1 K−1 was achieved after sintered at 1900 °C for 12 h by using 3 wt% ZrH2 and 1.5 wt% MgO as additives.

Hierarchically-porous metallic scaffolds via 3D extrusion and reduction of oxide particle inks with salt space-holders

3D ink-extrusion of powders followed by sintering is an emerging additive manufacturing method capable of creating metallic microlattices. Here, we study the creation of hierarchically porous Fe or Ni scaffolds by 3D extrusion of 0/90° lattices from inks consisting of fine oxide powders (Fe 2 O 3 or NiO, < 3 µm), coarse space-holder particles (CuSO 4, < 45 µm) and a polymer binder within a solvent. After space-holder leaching and debinding of the lattices, a sintering step densifies the metallic Fe or Ni powders created by oxide reduction with H 2 , while maintaining the larger pores templated by the space-holder particles within the printed struts. The resulting Fe or Ni lattices exhibit a hierarchical porosity: (i) open submillimeter channels within the lattice between the printed struts; (ii) open micropores (5–50 µm) within the struts, from leaching of space-holder particles; (iii) closed submicron pores within the micropore walls, from incomplete sintering of the metal microparticles created by oxide reduction. While the channel porosity is defined by the printed geometry, strut porosity is controlled by the sintering temperature, with the finest porosity removed first. Sintering at 873–1373 K for 1 h leads to strut porosities of 38–19% in Fe and 59–29% in Ni. In situ synchrotron X-ray tomography reveals the sintering kinetics and porosity evolution upon debinding, oxide reduction by H 2 and sintering of the porous metal, which, combined, lead to ~50% shrinkage of the scaffolds as compared to the printed state. The space-holder templated pores are not eliminated by sintering and remain in the struts, but they shrink upon reduction and sintering of the surrounding metal matrix (e.g., by 40% in Fe and by 30% in Ni upon sintering at 1173 K for 1 h).

Dielectric properties of barium titanium ceramics doped by lanthanum oxide

The method of oxalate co-precipitation assisted with microwave is used for the synthesis of pure barium titanate (BT) particle with a diameter of 30–50 nm by choosing tetrabutyl titanate Ti(OC4H9)4 (98.0%), barium nitrate Ba(NO3)2 (99.5%), oxalate H2C2O4 (99.5% purity) and nitric acid HNO3 (65 ∼ 68%) as starting materials. Lanthanum oxide is doped in barium titanate powders to prepare ceramics by pelleting, forming, dumping, sintering and other steps. The structure and morphology of ceramics are characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The dielectric constant and dielectric loss are obtained by dielectric frequency analyzer. The results show that grain size and tetragonality of the barium titanate ceramics doped by lanthanum oxide sintered at 1270 °C for 3 h are found to decrease with the increase of dopants concentration. Dielectric constant increases at first and then decreases as La2O3 concentrations grow, reaching the maximum value when the concentration is 0.3 mol%. The ceramic samples transform from normal ferroelectric to the ferroelectric with diffused phase transition.

Effect of three step sintering on piezoelectric properties of KNN-based lead free ceramics

Four different KNN-based piezoceramics were prepared using a three-step sintering method. The crystal structures prepared by this three-step sintering were compared to those from conventional sintering and the different tolerance factors for the dopant effects on the piezoelectric properties were evaluated. The piezoelectric properties for the 0.96 (K0.5Na0.5)0.95Li0.05Nb0.93Sb0.07O3-0.04CaZrO3 ternary system prepared with the three-step sintering method were obtained: the d33 = 420 pC/N, strain levels of 0.18% at 40 kV/cm, and the piezoelectric constant decreased with an increase in the tolerance factor.

Pure V2O5 high-electricity transmission properties at low temperatures by structural changes

There have been many studies concentrating on high-temperature thermoelectric properties of non-doping V2O5 thin films and doping ones, but high power factor (PF) of V2O5 thermoelectrics have been rarely reported up to now, particularly low temperature ones. In this work, the thermoelectric performance of pure V2O5 at low temperature is reported where V2O5 commercial powders are successfully prepared into sintered blocks by spark plasma sintering (SPS) and annealing steps. It is clear that the annealing temperature plays a major role on the electrical transport properties due to the microstructure changes. At annealing temperature (623 K), the maximum Seebeck coefficient (205 K) and PFmax (186 K) arrive at −618 μV/K and 2.17 × 10−4 W/K2m, respectively, based on growth of grains, the reduction of grain boundaries and defect concentration. The PF is 2 orders of magnitude higher than the annealed of V2O5 film reported so far (Adv Mater Technol 2016;1:1600077). This research may provide an alternative strategy for the preparation of high-performance V2O5 thermoelectric devices.

Porous asymmetric microfiltration membranes shaped by combined alumina freeze and tape casting

An assembled asymmetric alumina microfiltration membrane with high performance was prepared by combining freeze and tape casting techniques followed by two sintering steps. Freeze casting was used for manufacturing of the porous support layer with a highly interconnected pore network. Tape casting was applied on the top layer to form a pre-membrane with smaller pore size and controlled thickness, which was set on the sintered support. Morphology influences were investigated for different solid loadings, additives content and the assembled layer membrane structures. No delamination among the layers was observed. The assembled ceramic membrane had an average pore size between 30 and 50 μm together with a top surface layer around 0.35 μm, which is suitable to the microfiltration separation process. Porosity in the range of 26–50 % and water flux of 11–32 m3 m−2 h−1 bar−1 were reached for samples prepared with two sintering steps at 1600 and 1300 °C for 2 h.

Microstructure and mechanical properties of TiC steel-bonded carbides prepared by two-step sintering technique

In this study, a two-step sintering technique was applied to prepare TiC steel-bonded carbides. The first step sintering process was heating to a liquid sintering temperature of 1390 °C without soaking time for achieving a critical density, and then unstable micropores remained in the samples, in which the rapidly densification of sample primarily depended on liquid pore filling. The second step sintering was performed at a lower temperature of 1330 °C for 10 h to complete densification process due to the grain-boundary diffusion. Compared with traditional vacuum sintering method, the growth of TiC particle was inhibited significantly and the evaporation of metal binder was suppressed through the two-step sintering technique. Overall, the prepared samples possessed excellent mechanical properties with the TRS and hardness of 2093.3 MPa and 87.3 HRA, respectively.

Enhanced Thermoelectric Performance of Novel Reaction Condition-Induced Bi2S3-Bi Nanocomposites.

This is the first report on the enhanced thermoelectric (TE) properties of novel reaction-temperature (TRe) and duration-induced Bi2S3-Bi nanocomposites synthesized using a facile one-step polyol method. They are well characterized as nanorod composites of orthorhombic Bi2S3 and rhombohedral Bi phases in which the latter coats the former forming Bi2S3-Bi core-shell-like structures along with independent Bi nanoparticles. A very significant observation is the systematic reduction in electrical resistivity ρ with a whopping 7 orders of magnitude (∼107) with just reaction temperature and duration increase, revealing a promising approach for the reduction of ρ of this highly resistive chalcogenide and hence resolving the earlier obstacles for its thermoelectric application potentials in the past few decades. Most astonishingly, a TE power factor at 300 K of the highest Bi content nanocomposite pellet, made at 27 °C using ∼900 MPa pressure, is 3 orders of magnitude greater than that of hot-pressed Bi2S3. Its highest ZT at 325 K of 0.006 is over twice of that of similarly prepared CuS or Ag2S-based nanocomposites. A significantly improved TE performance potential near 300 K is demonstrated for these toxic-free and rare-earth element-free TE nanocomposites, making the present synthesis method as a pioneering approach for developing enhanced thermoelectric properties of Bi2S3-based materials without extra sintering steps.

Development of Novel Lightweight Metastable Metal-(Metal + Ceramic) Composites Using a New Powder Metallurgy Approach.

In the current study, metal–(metal + ceramic) composites composed of biocompatible elements, magnesium (Mg), zinc (Zn), calcium (Ca) and manganese (Mn) were synthesized using a sinter-less powder metallurgy method. The composite has a composition of Mg49Zn49Ca1Mn1 (wt.%) in which the compositional ratio between Mg and Zn was chosen to be near eutectic Mg-Zn composition. The synthesis method was designed to avoid/minimize intermetallic formation by using processing temperatures lower than the Mg-Zn binary eutectic temperature (~ 340 °C). The synthesis process involved extrusion of green compacts at two different temperatures, 150 °C and 200 °C, without sintering. Extrusion was performed directly on the green compacts as well as on the compacts soaked at temperatures of 150 °C and 200 °C, respectively. Microstructure and mechanical properties of the materials synthesized under various processing conditions were investigated. Effect of extrusion temperature as well as soaking temperature on the materials’ properties were also evaluated in details and different properties showed an optimum under different conditions. All the synthesized materials showed no evidence of intermetallic formation which was confirmed by SEM/EDS, XRD, and Differential Scanning Calorimetry (DSC) techniques. The study establishes development of unconventional metal–(metal + ceramic) eco-friendly composites and provides important insight into realizing certain properties without using sintering step thus to minimize the energy consumption of the process. The study also highlights the use of magnesium turnings (recyclability) to develop advanced materials.

Enhanced fracture toughness of ZrB2–SiCw ceramics with graphene nano-platelets

In this study, the spark plasma sintering technology was implemented to produce hybrid zirconium diboride ceramics comprising of silicon carbide whiskers (SiCw: 25 vol%) and various amounts of graphene nano-platelets (Gnp: 0–7.5 wt%). The effects of nano-graphene addition on the structural evolution, hardness, fracture toughness, and densification behavior of ZrB2–SiCw nanocomposites were investigated. Though all produced hybrid ceramics reached the full densification level, with relative densities of ≥99.9%, a negligible drop was observed in the relative density of the specimen reinforced with 7.5 wt% graphene nano-platelets. After the sintering step and due to the lack of any chemical interaction in the ZrB2–Gnp–SiCw system, no in-situ formed phases were detected by X-ray diffraction or electron microscopy investigations. The incorporation of nano-scale graphene additives resulted in grain coarsening inhibition significantly. The hardness slightly increased from 21.9 GPa for the specimen with no graphene to 22.2 GPa for the sample doped with 2.5 wt% graphene. However, with rising the nano-graphene amount, the hardness remarkably dropped to 16.6 GPa and 19.1 GPa for the samples doped with 7.5 wt% and 5 wt% nano-graphene, respectively. The fracture toughness remarkably enhanced, with a linear trend, by the addition of graphene nano-platelets from lower 5 MPa m½ in the carbon-free ceramic to above 6 MPa m½ in the specimen doped with 7.5 wt% nano-graphene. The identified dominant toughening mechanisms were crack bridging, crack arresting, and crack deflection.

Direct synthesis of p-type bulk BiCuSeO oxyselenides by reactive spark plasma sintering and related thermoelectric properties

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 for the preparation of oxyselenides.

Ambient flash sintering of reduced graphene oxide/zirconia composites: Role of reduced graphene oxide

Fabrication of graphene/ceramic composites commonly requires a high-temperature sintering step with long times as well as a vacuum or inert atmosphere, which not only results in property degradation but also significant equipment complexity and manufacturing costs. In this work, the ambient flash sintering behavior of reduced graphene oxide/3 mol% yttria-stabilized ZrO2 (rGO/3YSZ) composites utilizing rGO as both a composite component and a conductive additive is reported. When the sintering condition is carefully optimized, a dense and conductive composite can be achieved at room temperature and in the air within 20 s. The role of the rGO in the FS of the rGO/3YSZ composites is elucidated, especially with the assistance of a separate investigation on the thermal runaway behavior of the rGO. The work suggests a promising fabrication route for rGO/ceramic composites where the vacuum and furnace are not needed, which is of interest in terms of simplifying the fabrication equipment for energy and cost savings.

Comparison Study on Microstructure Properties of Kaolin Based Geopolymer Ceramics with Addition of UHMWPE under Different Sintering Condition

To better understand the structure-mechanical properties relation of additively fabrication of kaolin geopolymer ceramics with sintering method, a comparison study was performed. Kaolin based geopolymer ceramics were synthesized starting from the powders of kaolin based geopolymer, using powder metallurgy method. Typically, the sintering method used are one step (1200 °C – 5 min) and novel two step sintering processes (600 °C-5 min and 1200 °C – 5 min) for both kaolin based geopolymer ceramics with and without ultra high molecular weight polyethylene were applied and compared. The outcome revealed that there is no phase changes on the both sintering method pattern and the two step sintering method giving a smooth surface owing to the densification process during the preheat treatment.

Reversible Conductive Inkjet Printing of Healable and Recyclable Electrodes on Cardboard and Paper.

Conductive inkjet printing with metal nanoparticles is irreversible because the particles are sintered into a continuous metal film. The resulting structures are difficult to remove or repair and prone to cracking. Here, a hybrid ink is used to obviate the sintering step and print interconnected particle networks that become highly conductive immediately after drying. It is shown that reversible conductive printing is possible on low-cost cardboard samples after applying standard paper industry coats that are adapted in terms of surface energy and porosity. The conductivity of the printed films approaches that of sintered standard inks on the same substrate, but the mobility of the hybrid particle film makes them less sensitive to cracks during bending and folding of the substrate. Damages that occur can be partially repaired by wetting the film such that particle mobility is increased and particles move to bridge insulating gaps in the film. It is demonstrated that the conductive material can be recovered from the cardboard at the end of its life time and be redispersed to recycle the particles and reuse them in conductive inks.

Development of Metal Matrix Composites and Nanocomposites Via Double-Pressing Double-Sintering (DPDS) Method

Achieving high densification parameter is an essential factor for metal-based composites to represent excellent mechanical characteristics. Among various compaction methods, double-pressing double-sintering (DPDS) has gained the attraction of researchers to fabricate metal-matrix composites (MMCs) as an efficient technique over the last decade. In this process, the powder will be pre-pressed and then pre-sintered; it helps reduce work hardening and prepare the sample for better densification in the next pressing and sintering step. By employing the above-mentioned method, carbon nanotube (CNT) reinforced aluminum nanocomposites; ZrB2 reinforced copper nanocomposites; and CNT reinforced copper nanocomposites. Furthermore, very high-performance, self-lubricating Fe-based composites were fabricated as discussed in this review paper. The DPDS method enjoys the advantage of producing fine microstructures with near fully-dense parts, resulting in superior mechanical properties. However, the economic issue is the most concern in this fabrication route since it needs excess equipment as well as time for the second step compaction and heating.