Sintering Step Research Articles

Fabrication and Characterization of Ice Templated Membrane Supports from Portland Cement.

Porous ceramic membranes for aqueous microfiltration and ultrafiltration processes suffer from the high-costs of material and processing. The latter is mainly due to the high-temperature sintering step. In this work, cement-based membrane supports from ultrafine Portland cement are studied as a low-cost alternative to traditional oxidic ceramic supports. An environmentally friendly freeze-casting fabrication route is applied for the fabrication of porous membrane supports. Cement membrane supports are becoming mechanically stabile after hydration reaction of cement with water, which does not require any high-temperature sintering step as in a conventional ceramic membrane fabrication process. This fabrication route, which is sintering-free, decreases the cost and environmental impact of the membrane fabrication process by eliminating extra energy consumption step during sintering. The Archimedes method, scanning electron microscopy (SEM), micro-computed tomographic (µCT), and mercury porosimetry characterize the membrane supports in respect to open porosity, pore size distribution, morphology, and connectivity. The flexural strength of the 3 mm thick membranes is in the range from 1 to 6 MPa, as obtained by the ring-on-ring tests. The obtained membrane supports possess porosity in the range between 48 and 73% depending on fabrication conditions (cooling rate and the solid content, as determined by Archimedes method enabling water flux in the range between 79 and 180 L/(h·m2) at 0.5 bar transmembrane pressure difference and 3 mm membrane thickness.

Microstructure and mechanical properties of Al2O3/ZrO2 composites by two‐step sintering

AbstractComposites of Al2O3/ZrO2 (containing 25, 50, and 75 vol% ZrO2) were prepared by mixing Al2O3 and ZrO2 suspensions. The microstructural control via two‐step sintering (TSS) was the main objective of this work. For this purpose, different sintering curves were constructed, aiming to achieve the best temperature combination for the sintering steps that provides higher density and finer microstructure. The results were compared with single‐step sintering (SSS). Furthermore, microhardness and fracture toughness were measured for the best TSS specimens under each composition. The results showed that the high densities were obtained, and the reduction of grain size was greater than 40% for two‐step sintered specimens, compared to SSS ones. Consequently, microhardness values increased. However, fracture toughness values remained unchanged.

Combined equivalent circuits and distribution of relaxation times analysis and interfacial effects of (La0.60Sr0.40)0.95Co0.20Fe0.80O3-x cathodes

In this work, symmetrical cells with a commercial (La0.60Sr0.40)0.95Co0.20Fe0.80O3-x, (LSCF) cathode and electrolyte have been fabricated and studied using a combination of complex non-linear least square fitting (CNLS), distribution of relaxation times (DRT), and high resolution SEM/EDS techniques. The cathode has been fired at different temperatures from 1050 °C to 1125 °C at 25 °C intervals. Electrochemical impedance spectroscopy (EIS) data indicate that at 1050 °C interfacial ionic resistances are dominant while chemical diffusion plays a larger role between 1075 °C and 1100 °C. At 1125 °C the cathode has over densified affecting both interfacial and diffusion resistances. Both CNLS and DRT have been used to identify the electrochemical processes; however, the resistance values obtained for the two methods are different in some cases. This problem is exacerbated when inductance is present in the data especially for resistance and constant phase elements equivalent circuits. In this case, CNLS analysis can give back the same values used for the simulation, while the DRT approach cannot produce the same values with negligible differences. SEM/EDS chemical maps show that no chemical interactions have occurred at the highest sintering step, reaffirming that gadolinium doped ceria (GDC) is a good barrier layer when fully dense.

High-performing electrolyte-supported symmetrical solid oxide electrolysis cells operating under steam electrolysis and co-electrolysis modes

Symmetrical solid oxide cells (s-SOC) present several advantages compared to typical configuration, as a reduction of sintering steps or a better thermomechanical compatibility between the electrodes and the electrolyte. Different mixed ionic-electronic conductors (MIEC) have been reported as suitable candidates for symmetrical configuration, allowing operations under steam electrolysis (SOEC) or co-electrolysis (co-SOEC) without the use of reducing safe gas (typically employed in SoA nickel based cells). In the present study, Sr2Fe1.5Mo0.5O6−δ (SFM) electrodes are deposited on both sides of YbScSZ tapes previously coated with a Ce1-xGdxO1.9 (GDC) barrier layer grown by PLD. Electrode sintering temperature is optimized and fixed at 1200 °C by means of electrochemical impedance spectroscopy (EIS) measurements in symmetrical atmosphere. The cell is then characterized at 900 °C in SOEC and co-SOEC modes without the use of any safe gas obtaining high current densities of 1.4 and 1.1 A cm−2 at 1.3 V respectively. Short-term reversibility is finally proven by switching the gas atmosphere between the cathode and anode sides while keeping the electrolysis conditions. Similar performances are obtained in both configurations.

One-Step Fabrication of Protonic Ceramic Fuel Cells Using a Convenient Tape Calendering Method

The present paper reports the preparation of multilayer protonic ceramic fuel cells (PCFCs) using a single sintering step. The success of this fabrication approach is due to two main factors: the rational choice of chemically and mechanically compatible components, as well as the selection of a convenient preparation (tape calendering) method. The PCFCs prepared in this manner consisted of a 30 µm BaCe0.5Zr0.3Dy0.2O3–δ (BCZD) electrolyte layer, a 500 μm Ni–BCZD supporting electrode layer and a 20 μm functional Pr1.9Ba0.1NiO4+δ (PBN)–BCZD cathode layer. These layers were jointly co-fired at 1350 °C for 5 h to reach excellent gas-tightness of the electrolyte and porous structures for the supported and functional electrodes. The adequate fuel cell performance of this PCFC design (400 mW cm−2 at 600 °C) demonstrates that the tape calendering method compares well with such conventional laboratory PCFC preparation techniques such as co-pressing and tape-casting.

Synthesis and Processing Effects on Microstructure and Mechanical Properties of Forsterite Ceramics

ABSTRACT Suitable sintering ability and excellent densification are important factors for forsterite (Mg2SiO4) ceramics and refractories when used in glass furnaces, continuous casting tundishes, non-ferrous metal smelters, etc. Here, a dense and fine-grained structure has been obtained in the forsterite ceramics prepared by two-step sintering process. Results show that combination of grinding and second step sintering can eliminate MgSiO3 secondary phase, reduce intergranular pores and increase sintering densification of forsterite. By substituting mechanical activation for grinding, grain refinement and microstructural uniformity can be further promoted, resulting in increased bulk density and enhanced cold crushing strength. This work suggests that combination of two-step sintering and mechanical activation is effective for the preparation of forsterite ceramics with excellent densification and microstructure used in the high temperature industrial field.

Morpho-Structural, Electrical and Magnetic Behavior of Chemically Synthesized Multiferroic GdMnO₃ Ceramic.

The observations of room temperature ferroelectricity and ferromagnetism for GdMnO₃ (GMO) ceramics have been reported in this paper. The nanocrystalline GMO powder was synthesized by chemical route and then sintered in the form of bulk pellet at 850 °C for 24 h by following slow step sintering schedule. Bulk GMO sample is observed to be polycrystalline in nature having orthorhombic perovskite structure and Pbnm space group. Well connected grains are observed with higher percentage of porosity in the micrographs obtained from field emission scanning electron microscope. The dielectric anomaly has been observed for the sintered sample at 338 K and a wide ferroelectric hysteresis loop has also been observed at room temperature. Room temperature dielectric constant (ε) value of the sample is high (~2736) at the frequency of 100 Hz. The zero field cooled (ZFC) and field cooled (FC) magnetizations shows the antiferromagnetic behavior around 12 K. Sample shows a weak ferromagnetic behavior at 3 K.

Tungsten-Doped Zinc Oxide and Indium-Zinc Oxide Films as High-Performance Electron-Transport Layers in N-I-P Perovskite Solar Cells.

Perovskite solar cells (PSCs) have attracted tremendous research attention due to their potential as a next-generation photovoltaic cell. Transition metal oxides in N–I–P structures have been widely used as electron-transporting materials but the need for a high-temperature sintering step is incompatible with flexible substrate materials and perovskite materials which cannot withstand elevated temperatures. In this work, novel metal oxides prepared by sputtering deposition were investigated as electron-transport layers in planar PSCs with the N–I–P structure. The incorporation of tungsten in the oxide layer led to a power conversion efficiency (PCE) increase from 8.23% to 16.05% due to the enhanced electron transfer and reduced back-recombination. Scanning electron microscope (SEM) images reveal that relatively large grain sizes in the perovskite phase with small grain boundaries were formed when the perovskite was deposited on tungsten-doped films. This study demonstrates that novel metal oxides can be used as in perovskite devices as electron transfer layers to improve the efficiency.

One-step synthesis of CuCo2O4-Sm0.2Ce0.8O1.9 nanofibers as high performance composite cathodes of intermediate-temperature solid oxide fuel cells

One-dimensional nanostructured CuCo2O4-Sm0.2Ce0.8O1.9 (SDC) nanofibers are prepared by the electrospinning method and one step sintering as a cathode with low polarization resistance for intermediate temperature solid oxide fuel cells (IT-SOFC). The CuCo2O4-SDC nanofibers cathodes form a porous network structure and have large triple-phase boundaries. Correspondingly, the electrochemical performance of the CuCo2O4-SDC nanofibers composite cathodes shows significantly improve, achieving the polarization resistance of 0.061 Ω cm2 and the maximum power densities of 976 mW·cm−2 at 750 °C. Thus, these results suggest that CuCo2O4-SDC nanofiber could be a highly active cathode material for IT-SOFCs.

Effects of various rare-earth additives on the sintering and transmittance of γ-AlON

Several rare earth elements (Sc, La, Pr, Sm, Gd, Dy, Er, and Yb) were evaluated to develop a sintering additive for transparent γ-AlON. After adding 0.2 wt. % of each of the rare earth additives to the α-Al2O3 and AlN starting material, hot pressing was performed at 1850 °C for 1 h under 20 MPa as a screening test, whereby the γ-AlON with Pr-nitrate showed the highest transmittance of 60.4 % at 632 nm. Two-step pressureless sintering was conducted as a separate test for the same starting material after adjusting the amount of Pr-nitrate to 0, 0.1, or 0.2 wt. % to enhance transmittance. The γ-AlON with 0.1 wt. % Pr-nitrate showed the highest transmittance of 80.36 % after 1st and 2nd sintering steps at 1610 and 1940 °C, respectively, indicating that Pr can be an alternative sintering additive to conventional Y2O3, MgO, and La2O3 for the fabrication of transparent γ-AlON.

One-shot wet chemical synthesis and physical properties of pyrochlore iridates A2Ir2O7, (A = Sm, Gd, Dy and Er)

Iridates of the pyrochlore A2Ir2O7 family are interesting materials whose low-temperature properties depend sensitively on the A/Ir ratio. Due to specific difficulties, namely: (i) the volatile nature of IrO2 above 1323 K, and (ii) slow reaction kinetics between IrO2 and A2O3 below 1323 K (where A is a members of the lanthanide series), it is challenging to prepare phase pure and stoichiometric A2Ir2O7 samples using the conventional solid state reaction (SSR) method in a reasonably short time. To overcome these limitations, we synthesized A2Ir2O7 with A = Sm, Gd, Dy, and Er using a wet chemical reaction (WCR) method in a single sintering step at T = 1273 K (a temperature well below the sublimation point of IrO2) for 12 h, and compared their electrical resistivity, magnetic susceptibility and specific heat with the SSR samples. We show that WCR samples exhibit metal-to-insulator & all-in all-out transitions in agreement with previous reports. The high density, low-sintering temperature, lack of IrO2 losses, and shorter preparation time are some of the attractive features of WCR method. This should be useful in exploring the true ground state properties of these materials in future.

Influence of sintering profile on the mechanical properties of manganese oxide doped 3Y-TZP

Zirconia ceramic has been identified as one of the advanced ceramic material with great mechanical properties which is used as engineering and implant materials. However, the great potential of the ceramic is hindered by low temperature degradation (LTD) where the ceramic experiencing t-m phase transformation that weakens the properties of 3Y-TZP in the presence of moisture. Two-step sintering was found to be effective in producing fully resistance 3Y-TZY but very long holding time is required. Manganese Oxide (MnO2) is reported as good densification aid at low sintering temperature for 3Y-TZP but the LTD issue is not fully resolved. In this study, the effects of different sintering profiles and dwell time on 3Y-TZP added with 0.5 wt.% of MnO2 were studied in order to improve the resistance towards LTD without affecting the intrinsic properties of the ceramic. The effects of adding 0.5 wt.% of manganese oxide (MnO2) into 3 mol% yttria stabilized tetragonal zirconia polycrystals (3Y-TZP) were studied using different sintering profiles and dwell time. The samples were sintered at 1400°C and 1250°C with varying dwell time of 1 min and 2 hours for single step sintering and a combination of 1 min and 30 min, 2 hours and 25 hours for two step sintering. The optimal sintering profile was Profile C2 where doped 3Y-TZP undergone two step sintering with dwell time of 1 minute at 1400°C and 1250°C for 30 minutes. At this optimal sintering conditions, the doped 3Y-TZP samples exhibited 98.5% relative density, with Young’s Modulus of 206.66 GPa and Vicker’s Hardness of 14.35 GPa.

Influence of tantalum pentoxide secondary phase on surface features and mechanical properties of hydroxyapatite coating on NiTi alloy produced by electrophoretic deposition

In the current research, the electrophoretic deposition (EPD) process has been applied to create the natural hydroxyapatite (nHA)-tantalum pentoxide (Ta2O5) coating on the NiTi substrate with the various contents of Ta2O5 of 0, 10, 15, and 20 wt.%. The subsequent sintering step has been conducted in the two temperatures of 850 and 800 °C to investigate the coating composition. The microstructure and roughness of sintered samples at 800 °C were examined using the field emission scanning electron microscope (FESEM) and optical profilometer respectively. The positive effect of Ta2O5 presence and its content augmentation in the nHA matrix on the corrosion resistance of NiTi was approved through determining the concentration of released Ni ions from the coated substrate in phosphate buffered saline (PBS). Then the effect of Ta2O5 secondary phase content on the mechanical performances of the nHA layer on the NiTi was interpreted by micro-indentation responses of samples. According to the achievements, the hardness of nHA coated NiTi was increased about 4, 12, and 19 times by the addition of 10, 15, and 20 wt.% of Ta2O5 respectively. Moreover, the elastic modulus of nHA was raised favorably by Ta2O5 enhancement as reached to 32.85 GPa in nHA-20 wt.% Ta2O5 coating, the closest value to that of human bones. At last the bonding behavior of coatings was assessed by considering the critical contact pressure parameter magnitude measured from nano-scratch data.

Modeling zirconia sintering trajectory for obtaining translucent submicronic ceramics for dental implant applications

Attaining high densification without grain growth is one of the main objectives of the sintering optimization in ceramic materials. For dental implant applications, achieving this objective has a decisive impact on the mechanical resistance, the duration and the translucency of the implant. To improve these sintering outcomes a long experimental explorative study is generally required. In this work, we developed a combined experimental/modeling approach allowing a rapid identification of the optimal sintering conditions. The determination of the model densification and grain growth kinetic constitutive parameters has been done experimentally. We found that the sintering/grain growth kinetics have a detrimental acceleration above a critical temperature level. The pressure-less sintering model able to predict the sintering stress, powder densification and grain growth has been used for the determination of the optimal sintering trajectory. We utilized the two step sintering method to approach the critical temperature without an undesirable grain growth. We obtained translucent sintered specimens with a very limited grain growth.

Influence of processing conditions on the ionic conductivity of holmium zirconate (Ho2Zr2O7)

Nanopowders of holmium zirconate (Ho2Zr2O7) synthesised through carbon neutral sol-gel method were pressed into pellets and individually sintered for 2 h in a single step sintering (SSS) process from 1100 °C to 1500 °C at 100 °C interval and in a two step sintering (TSS) process at (I) −1500 °C for 5 min followed by (II) - 1300 °C for 96 h. Relative density of each of the sintered pellet was determined using the Archimedes’ technique and the theoretical density was calculated from crystal structure data. Grain size was obtained from SEM micrographs using ImageJ. Pellets processed by TSS have been found to be denser (98 %) with less grain growth (1.29 μm) as compared to the pellets processed using SSS process. Ionic conductivity of Ho2Zr2O7 pellets sintered by two different processes was measured using ac impedance spectroscopy technique over the temperature range of 350 °C–750 °C in the frequency range of 100 mHz–100 MHz for both heating and cooling cycles. The temperature dependence of bulk (2.67⨯10−3 Scm−1) and grain boundary (2.50⨯10−3 Scm−1) conductivities of Ho2Zr2O7 prepared by TSS process are greater than those processed by SSS process suggesting the strong influence of processing conditions and grain size. Results of this study, indicates that the TSS is the preferable route for processing the holmium zirconate as it can be sintered to exceptionally high densities at lower temperature, exhibits less grain growth and enhanced ionic conductivity compared with the samples processed by SSS process. Hence, holmium zirconate can be considered as a promising new oxide ion conducting solid electrolyte for intermediate temperature SOFC applications between 350 °C and 750 °C temperature range.

Solid versus Liquid—A Bottom‐Up Calculation Model to Analyze the Manufacturing Cost of Future High‐Energy Batteries

All‐solid‐state batteries (ASSB) are promising candidates for future energy storage. However, only a little is known about the manufacturing costs for industrial production. Herein, a detailed bottom‐up calculation is performed to estimate the required investment and to facilitate comparison with conventional lithium‐ion batteries (LIB). Results indicate that sulfide‐based ASSBs can indeed be competitive if the material compatibility issues can be solved and production is successfully scaled. In contrast, oxide‐based ASSBs will probably not be able to compete if cost is the decisive factor. A sensitivity analysis with Monte Carlo simulation reveals that the inert gas atmosphere required for sulfide‐based ASSBs contributes little to the overall cell costs, whereas the sintering step for oxide‐based ASSBs is highly critical. The calculation also indicates that in‐house manufacturing of the lithium anode will be cheaper than purchasing the lithium foil externally if the cell producer has sufficient processing know‐how. Finally, the aerosol deposition method is investigated, revealing that a deposition rate far above 1000 mm3 min−1 would be required to make the technology economically feasible in ASSB production. The results of this study will help researchers and industry prioritize development efforts and push the scale‐up of future high‐energy batteries with improved performance.

Multi-material additive manufacturing of low sintering temperature Bi2Mo2O9 ceramics with Ag floating electrodes by selective laser burnout

ABSTRACTAdditive manufacturing (AM) of co-fired low temperature ceramics offers a unique route for fabrication of novel 3D radio frequency (RF) and microwave communication components, embedded electronics and sensors. This paper describes the first-ever direct 3D printing of low temperature co-fired ceramics/floating electrode 3D structures. Slurry-based AM and selective laser burnout (SLB) were used to fabricate bulk dielectric, Bi2Mo2O9 (BMO, sintering temperature = 620–650°C, εr = 38) with silver (Ag) internal floating electrodes. A printable BMO slurry was developed and the SLB optimised to improve edge definition and burn out the binder without damaging the ceramic. The SLB increased the green strength needed for shape retention, produced crack-free parts and prevented Ag leaching into the ceramic during co-firing. The green parts were sintered after SLB in a conventional furnace at 645°C for 4 h and achieved 94.5% density, compressive strength of 4097 MPa, a relative permittivity (εr) of 33.8 and a loss tangent (tan δ) of 0.0004 (8 GHz) for BMO. The feasibility of using SLB followed by a post-printing sintering step to create BMO/Ag 3D structures was thus demonstrated.

3D printing of ceramics and graphene circuits-on-ceramics by thermal bubble inkjet technology and high temperature sintering

Alumina ceramics and graphene circuits-on-ceramics were 3D printed using inkjet technology. Water-based binders for ceramics and graphene oxide inks were prepared and jetted through thermal inkjet printhead. To increase the density of the 3D printed ceramics, two steps sintering and one step infiltration were performed. The sintering temperature dependent structure properties of the printed ceramics were systematically studied. The 3D printed α-Al2O3 ceramic sintered at 1600 °C for 2 h has achieved maximum density of 95.2%, volume shrinkage rate of 32%, Vickers hardness of 291.6 Hv, surface roughness of 3.7 μm, and maximum compression strength and flexural strength of 89 MPa and 49.4 MPa, respectively. The average adhesion of reduced graphene oxide (rGO) to 3D printed α-Al2O3 ceramic is 7.5 N/m. The printed graphene circuits-on-ceramics has achieved conductivity of 5.34 × 102 S m−1 after annealing at 600 °C in nitrogen atmosphere.

First-time synthesis of a magnetoelectric core-shell composite via conventional solid-state reaction.

In recent years, multiferroics and magnetoelectrics have demonstrated their potential for a variety of applications. However, no magnetoelectric material has been translated to a real application yet. Here, we report for the first time that a magnetoelectric core-shell ceramic, is synthesized via a conventional solid-state reaction, where core-shell grains form during a single sintering step. The core consists of ferrimagnetic CoFe2O4, which is surrounded by a ferroelectric shell consisting of (BiFeO3)x-(Bi1/2K1/2TiO3)1-x. We establish the core-shell nature of these grains by transmission-electron microscopy (TEM) and find an epitaxial crystallographic relation between core and shell, with a lattice mismatch of 6 ± 0.7%. The core-shell grains exhibit exceptional magnetoelectric coupling effects that we attribute to the epitaxial connection between the magnetic and ferroelectric phase, which also leads to magnetic exchange coupling as demonstrated by neutron diffraction. Apparently, ferrimagnetic CoFe2O4 cores undergo a non-centrosymmetric distortion of the crystal structure upon epitaxial strain from the shell, which leads to simultaneous ferrimagnetism and piezoelectricity. We conclude that in situ core-shell ceramics offer a number of advantages over other magnetoelectric composites, such as lower leakage current, higher density and absence of substrate clamping effects. At the same time, the material is predestined for application, since its preparation is cost-effective and only requires a single sintering step. This discovery adds a promising new perspective for the application of magnetoelectric materials.

Direct Synthesis of <i>p</i>-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 modest enhancement of ~20% at 773 K. The synthesis time was reduced from a few days to less than 10 hours.