Isothermal Sintering Research Articles

One-Step Molten-Salt-Assisted Approach for Direct Preparation and Regeneration of LiNi0.6Co0.2Mn0.2O2 Cathode.

Single-crystal lithium-nickel-manganese-cobalt-oxide (SC-NMC) is attracting increasing attention due to its excellent structural stability. However, its practical production faces challenges associated with complex precursor preparation processes and severe lithium-nickel cation mixing at high temperatures, which restricts its widespread application. Here, a molten-salt-assisted method is proposed using low-melting-point carbonates. This method obviates the necessity for precursor processes and simplified the synthetic procedure for SC-NMC down to a single isothermal sintering step. Multiple characterizations indicate that the acquired SC-LiNi0.6Mn0.2Co0.2O2 (SC-622) exhibits favorable structural capability against intra-granular fracture and suppressive Li+/Ni2+ cation mixing. Consequently, the SC-622 exhibits superior electrochemical performance with a high initial specific capacity (174 mAhg-1 at 0.1C, 3.0-4.3V) and excellent capacity retention (87.5% after 300 cycles at 1C). Moreover, this molten-salt-assisted method exhibits its effectiveness in directly regenerating SC-622 from spent NMC materials. The recovered material delivered a capacity of 125.4 mAhg-1 and retained 99.4% of the initial capacity after 250 cycles at 1 C. This work highlights the importance of understanding the process-structure-property relationships and can broadly guide the synthesis of other SC Ni-rich cathode materials.

Simulation on liquid phase sintering of CeO2-CoO ceramic by diffusional Monte Carlo Potts model

Difficulty in control of in-situ crystal growth of CeO2-CoO ceramics limits its catalyst application of solid oxide fuel cells. It is necessary to explore a method to simulate the microstructure growth process of this ceramics under high temperature. In this work, the diffusional Monte Carlo Potts model for the simulation on liquid phase sintering of CeO2-CoO ceramics is established. With geometric and concentration-dependent probability calculated, the phenomena of solidification and melting in liquid phase sintering process are simulated. Moreover, effects of diffusion and grain consolidation are also taken into consideration. The simulation on both isothermal sintering and cooling process is run. The properties of grain growth, grain size distribution and sintering neck evolution are studied. It is found that the grain growth exponent, temporal revolution of grain number and sintering neck obtained from simulation basically agree with relevant theories. In addition, grain size distribution basically remains constant and follows lognormal distribution during the sintering process. This work provides a method to study the in-situ crystal growth process of liquid phase sintered ceramics.

Influence of Sintering Time with Constant Temperature on the Development of the Tricalcium Silicate Crystallinity

The article investigates the influence of the sintering time with constant temperature on the development of the tricalcium silicate crystallinity using the Scherrer's and Rietveld method.One suitable raw material mixture consisting of only pure CaCO3 and SiO2 in their specific ratio of 73.6:26.3 wt.% was selected. This raw material powder was prepared using a wet milling process in the water medium in a PULVERISETTE 6 plan mill. Based on previous research in this area, a firing temperature of 1500 °C was chosen with sintering period of 5, 10, 15 and 20 minutes in an experimental furnace with a manipulator for isothermal sintering of laboratory samples developed by Dr. Chromy. This furnace requires special sample preparation, so a procedure for their preparation was designed and optimized. Thanks to this optimization, it is possible to repeatably prepare identical pellets with the exact parameters for this type of furnace. The resulting pellet weight is 73 mg at a volumetric weight of 2,285 g·cm-3.The results of the firings with different sintering times and constant temperatures showed that the sintering processes and mineral conversion occur very quickly, since after 5 minutes 30.81 wt.% of the mineral tricalcium silicate was identified and after 20 minutes the raw powder was converted to tricalcium silicate in a significant amount, i.e., 72.02 wt.%. At the same time, the crystallinity was monitored using Scherrer's method and after 20 minutes of sintering, the crystallite size of tricalcium silicate reached 104.889 nm. Thus, there was a relatively rapid development of the crystallinity of Alite.

Sintering kinetics of cerium-stabilised tetragonal zirconia

Zirconia ceramics stabilised by CeO2 (Ce-ZrO2) exhibit low strength owing to their coarse grain sizes, which are typically larger than 1–2 μm. A promising approach to enhance the mechanical properties of Ce-ZrO2 is refining of the grain size. This study focuses on achieving fully dense Ce-ZrO2 ceramics with sub-micron grain sizes by identifying a sintering kinetic window that facilitates active sintering while suppressing grain growth. Both non-isothermal and isothermal sintering techniques were systematically employed to investigate the sintering and grain growth behaviour of the Ce-ZrO2 ceramics. The activation energies for sintering and grain growth were also determined. The existence of a sintering kinetic window for the Ce-ZrO2 ceramics was confirmed by a comparative analysis of the calculated activation energies. Accordingly, we successfully prepared fully dense sub-micron Ce-ZrO2 ceramics (with relative density >98%) by sintering within this kinetic window, leading to an average grain size of 0.55 μm. This is a significant step towards tailoring the microstructure of Ce-ZrO2 ceramics to improve their mechanical performance.

Tetragonal phase stabilization and densification in AC flash-sintered 1.5 mol% yttria-stabilized zirconia polycrystals with high toughness

For the first time, isothermal pressureless flash sintering of 1.5 mol% yttria-stabilized zirconia (1.5YSZ) polycrystals designed for high toughness was performed in a 1-kHz alternating current field (120 V·cm−1). The parameters governing tetragonal phase stability and densification needed to maximize the toughness of 1.5YSZ were investigated by varying the applied current density limit and holding time. A sample with relative density exceeding 98% and indentation toughness of about 13 MPa·m0.5 was sintered at 1100°C by applying 40 mA·mm−2 to the green body for 2 min, where the average steady-state temperature during flash reached 1330°C. Unlike flash sintering of conventional oxide ceramics, the applied current density and densification were not positively correlated despite pore closure. While Joule heating intensified when applying higher current densities, sample densification was hindered as the accelerated grain growth resulted in intergranular cracks which were generated by spontaneous tetragonal-to-monoclinic martensitic phase transformation during cooling.

Densification kinetics and microstructural evolution of binder jet printed and sintered porous Ni-Mn-Ga magnetic shape-memory alloys

Binder Jet Printing (BJP) of polycrystalline magnetic shape-memory alloys (MSMAs) allows to create complex geometries from pre-alloyed powders, while avoiding elemental evaporation and internal stresses. However, to increase functionality, polycrystalline MSMAs require high porosity and large grains to decrease internal constraints to twin boundary motion. This study focuses on the microstructural evolution of BJPed porous Ni-Mn-Ga during isothermal sintering to understand the densification mechanisms taking place at different temperatures (1070, 1080 and 1090 ℃) over time (0–8 h) in order to tailor the microstructure to regimes of large, interconnected porosity and large grains. Stereology was used to determine the sintering mechanisms as defined by Coble's model for intermediate stage sintering. Densification and grain size increased over time for all temperatures at different rates, while pore size decreased to a minimum near 1 h sintering. Sintering at 1070 ℃ resulted in an interplay of densifying and coarsening mechanisms, while at 1080 ℃ the dominating mechanism was volume diffusion and at 1090 ℃ grain boundary diffusion. No changes in electron concentration occurred during sintering, and thus, transformation temperatures and magnetization remained nearly constant. Transformation enthalpy and Gibbs free energy showed an inverse behavior to pore size, indicating larger pores reduce transformation energy.

Grain coarsening during liquid phase sintering of zircon–glass composites

This study examined zircon grain growth during isothermal sintering of a zircon–glass composite with a zircon volume fraction of 0.525 zircon volume/solids volume, at a temperature of 1200°C, with different residence times, ranging from 5min to 1500min. Scanning electron microscopy and image analysis were used to determine zircon grain size and number in the fired test specimens. The residual glass composition was determined by energy dispersive X-ray spectroscopy. For each firing condition, the grain size number distribution was fitted to a lognormal function and to a Weibull function. In every case, the lognormal distribution described the results better. Grain size grew and distribution width decreased with sintering time in accordance with non-steady-state grain growth models. At high sintering times, the variation of average grain diameter and grain number density per unit solids cross-section fitted well to steady-state growth models. Moreover, when grain size distribution was scaled by the average size, it was practically time independent. In contrast, scaled grain distribution width was much wider than that predicted by these models. A model was developed that satisfactorily predicts zircon grain evanescence throughout the sintering process.

Sintering evolution monitoring of ultra‐high‐molecular‐weight polyethylene

AbstractUHMWPE nascent powders have been commercially processed by sintering for decades although key aspects of the mechanisms and kinetics of this process are not fully understood. Herein, the volume change during each stage of the sintering is monitored by in‐situ measurements with other characterization methods. Results indicate that the α‐transition plays a crucial role in the early stages of the sintering process. Without pressure during sintering, a large expansion is observed during heating through the α‐transition well before melting and this apparent volume change is also related to the residual stress resulting from the compaction. However, the application of a moderate pressure during heating leads to a significant densification. Specifically, five distinct processes are identified from the in‐situ measurements via customized pressure sintering apparatus. These include room temperature densification, subsequent densification while heating through the α‐transition, process of melt explosion that involves the enthalpic process of crystal melting and the entropic process density changes during the isothermal sintering of non‐equilibrium melt, and the subsequent recrystallization under pressure. The density evolution offers a comprehensive understanding of the intricate processing of UHMWPE nascent powder and this can be effectively employed to investigate the impact of various processing parameters on the sintering process.

Mechanical, thermophysical and electrochemical properties of dense BaCeO3 ceramics sintered from hydrazine-nitrate combustion products

The development of intermediate-temperature solid oxide fuel cells (IT-SOFCs) is currently a focus of research, as it has the potential to improve their operational characteristics, lower their cost, and enhance their stability. However, achieving high-performance functional materials at working temperatures of 600–800 °C poses a significant challenge. Barium cerate (BaCeO3) has been identified as a promising material for the solid electrolyte of IT-SOFCs due to its high proton conductivity among A2+B4+O3-type perovskites. Nevertheless, producing a single-phase and dense ceramic based on BaCeO3 with high conductivity in the intermediate-temperature range remains a challenge. This study employed the hydrazine-nitrate combustion method to obtain BaCeO3 precursor powder, which was then used to produce dense proton-conducting ceramics by isostatic pressing and subsequent isothermal sintering at temperatures of 1100–1500 °C. The resulting samples were characterized using various techniques, including XRD, SEM, Vickers microhardness, TMA, LFA, and impedance spectroscopy. The findings indicate that the proposed hydrazine-nitrate approach was successful in producing ceramic samples with an average crystallite size of 95–220 nm, an average grain size of 4.8–9.9 μm, and a relative density of 70.7–91.2% depending on the sintering conditions. The most densely sintered sample (91.2%) exhibited the highest mechanical and functional properties, with Vickers microhardness values of 3.0 GPa (R T.), a coefficient of thermal expansion of 8.7 × 10–6 (R T.-1000 °C), thermal conductivity of 1.51–1.06 W/m·°C (R.T.-1000 °C), and logarithmic conductivity of −3.37 to −6.35 S/cm (675–980 °C). Thus, the proposed hydrazine-nitrate approach shows promise for producing high-performance conducting ceramic materials for IT-SOFCs.

Densification kinetics and sintering behavior of UO2 and 0.5 wt.%MnO‐doped UO2

AbstractIn this work, the sintering kinetics of pure UO2 and 0.5 wt.%MnO‐doped UO2 was studied by a high‐temperature dilatometer heated up to 1500°C. In addition, the sintering behavior of pure UO2 and 0.5 wt.%MnO‐doped UO2 was studied by pressureless sintering technique. The results showed that MnO doping enhanced the grain boundary diffusion of UO2, which can effectively decrease the densification temperature and promote grain growth. The sintering temperature of UO2 was significantly reduced by about 200°C with the addition of 0.5 wt.%MnO. The microscopic morphology studies showed that there were still fine particles agglomerated, forming sintered spheres in the matrix even if no severe agglomeration and bimodal size distribution were observed in raw UO2 powder. The microstructure evolution of the sintered sphere and UO2 matrix during the densification process was studied by isothermal sintering. Finally, the present analyses indicated that the densification of UO2 matrix can be accelerated by adding MnO or increasing the sintering temperature, thus improving the densification inhomogeneity of UO2 matrix.

Influence of lattice vibrations and phonon interactions on the ion transport properties of grain boundary tailored Li1.3Al0.3Ti1.7(PO4)3 solid-state electrolyte ceramics

Lithium ions shuttle between electrodes through the ceramic solid electrolyte across the boundary regions in a solid-state Li-ion battery. This work demonstrates how phonon vibrations get altered by sintering conditions, and grain boundaries (GBs) could be useful in enhancing the ionic conductivity of solid electrolytes. GB engineered Li1.3Al0.3Ti1.7(PO4)3 (LATP) ceramics are prepared using a sol-gel process and performed sintering under different conditions, viz., spark plasma sintering (SPS) and conventional isothermal sintering (CIS). The former exhibits GB regions with amorphous characteristics, whereas the latter shows a sharp boundary between crystalline grains. LATP-SPS ceramic shows two orders of higher ionic conductivity (σ = 1.02 × 10−5 S/cm at 300 K and 100 Hz) than LATP-CIS. We investigate the interrelation between lattice vibration and lithium-ion migration by monitoring the changes in vibrational mode characteristics of LATP ceramics through temperature-dependent Raman spectroscopy. Raman modes of LATP-SPS exhibit a higher Raman shift (∼2 cm−1 at 123 K) due to increased defects, preferentially from grain boundary regions, compared to the LATP-CIS pellet. Most of the vibrational modes undergo a red shift (∼10 cm−1) with increasing temperature, except for the O–P–O bending mode [A1g(3)], which exhibits a blue shift (∼3 cm−1). These observations correlate with interstitial ionic migration in the LATP framework. Force constant of the observed Raman modes suggests that lithium-ion migration is assisted significantly by dynamic structural changes of the (PO4)3− sublattice. Anharmonicities observed from temperature-dependent changes in Raman profiles are explained using three-phonon and four-phonon scattering processes, which lower the migration barrier and, hence, contribute to higher ionic conductivity.

Sintering high-mechanical-properties microcrystalline PcBN by isothermal compression

Mechanical-performance of polycrystalline cubic boron nitride (PcBN) sintered using micron-sized powder particles might exceed that of samples produced from submicron crystals. However, no effective method has been found yet. In this study, commercial cubic boron nitride (cBN) powder (∼5 μm) was used to sinter high-performance PcBN without a binder. Two processes, isothermal compression (IC) and isothermal isobaric sintering (IIS), were used and compared. PcBN sintered with the former method possesses much more excellent mechanical performance, with the average Vickers hardness and indentation crack resistance of 73 GPa and 8.4 MPa·m1/2. The new temperature and pressure path adopted in the IC process can largely improve the hardness of PcBN sintered even with micro-sized powder. The quantitative analysis results of the defect structure in samples can be used as a reference for controlling the defect structure density to improve the material performance.

Novel Thermal Barrier Coatings with Phase Composite Structures for Extreme Environment Applications: Concept, Process, Evaluation and Performance

In this paper, a novel concept in the field of phase composite ceramics has been proposed and applied for creating the topcoats of durable thermal barrier coatings (TBCs), which is one of the most critical technologies for advanced high-efficiency gas turbine engines in extreme environments. The phase composite ceramic TBCs were designed to demonstrate superior and comprehensive performance-related merits, benefits, and advantages over conventional single-phase TBCs with a topcoat of 8YSZ or Gd2Zr2O7, including thermal phase stability, thermal shock durability, low thermal conductivity, and solid particle erosion resistance. In this paper, we review and summarize the development work conducted so far related to the phase composite ceramic concept, coatings processing, and experimental investigation into TBC behaviors at elevated temperatures (typically, ≥1250 °C) using different evaluation and characterization methods, including isothermal sintering, a burner rig test, a solid particle-impinging erosion test, and a CMAS corrosion test. Two-phase (t’+c) zirconia-based TBCs demonstrated improved thermal shock and erosion resistance in comparison to conventional single-phase (t’), 8YSZ TBC, and Gd2Zr2O7 TBC, when used separately. Additionally, a triple-phase (t’+c+YAG) TBC sample demonstrated superior CMAS resistance. The TBC’s damage modes and failure mechanisms for thermal phase stability, thermal cycling resistance, solid particle erosion behavior, and CMAS infiltration are also characterized and discussed in detail, in terms of microstructural characterization and performance evaluation.

Critical conditions of forced ignition in Ni-Al, Ti-Al powder systems under solid-phase synthesis

A theoretical study of the critical conditions of forced solid-phase ignition in Ti-Al, Ni-Al systems was carried out. Based on the analysis of the non-stationary diffusion equation with moving boundaries, the critical powers of the heating source and the corresponding critical temperatures were determined during the formation of NiAl, TiAl3 intermetallic compounds using the state diagrams of these binary systems. The diffusion activation energies, frequency factors, particle sizes, and heat removal conditions were considered as variable parameters. The diagrams of critical parameters which relate the corresponding specific heating powers with the indicated values were constructed. It has been established that for the same values of the parameters and under identical synthesis conditions, the critical temperatures in these systems differ by more than one hundred degrees. The latter is associated with a significant difference in the width of the solubility region for the corresponding compounds of the composition NiAl, TiAl3. The performed analysis allows separating the modes of isothermal sintering and thermal explosion which makes it possible to control the thermal and diffusion modes of synthesis. The comparison of the calculation results with the experimental data showed their satisfactory agreement.

Solid state recycling of Mg–Gd–Y–Zn–Zr alloy chips by isothermal sintering and equal channel angular pressing

The Mg-7Gd-4Y-2Zn-0.5Zr alloy chips were successfully recycled through isothermal sintering and equal channel angular pressing (ECAP). The mechanical properties and microstructure evolution of samples during the recycling process were studied in detail. The eutectic phases in the as-cast alloy transform into long period-stacking ordered (LPSO) phases after homogenization, which can improve the plasticity of the material. After isothermal sintering, the density of the sample is lower than that of the homogenized sample, and oxide films are formed adjacent to the bonding interface of the metal chips. Hence, the plasticity of the sintered sample is poor. Dense samples are fabricated after ECAP. Although the grains are not refined compared to the sintered sample, the microstructure becomes more uniform due to recrystallization. Fiber interdendritic LPSO phase and kinked 14H-LPSO phase are formed in the alloy due to the shear deformation during the ECAP process, which improves the strength and plasticity of the sample significantly. Furthermore, the basal texture is weakened due to the Bc route of the ECAP process, which can increase the Schmid factor of the basal slip system and improve the elongation of the sample. After 2 ECAP passes, the fully densified recycled billet shows superior mechanical properties with an ultimate tensile strength of 307.1 MPa and elongation of 11.1%.

A comparative study on mechanical properties of fully dense 420 stainless steel parts produced by modified binder jet printing

The mechanical properties of the parts produced by binder jet printing (BJP) are often considerably inferior to those of the parts produced by traditional manufacturing processes because BJP parts retain at best about 95% of theoretical density after full sintering. To address this deficiency of BJP, a new processing protocol was developed, which enables us to produce fully dense parts under an isothermal sintering condition. This paper compares the mechanical properties of the stainless steel 420 (SS420) parts produced by the several variations of this new protocol against commercially available cold rolled SS420 parts with and without the heat-treatment under the similar sintering condition used in the new protocol. The mechanical properties examined include elastic modulus and both static and fatigue strengths. The only minor drawback of the new protocol is the dimensional shrinkage occurring during sintering. However, this can be easily compensated by printing the part geometry about 15% larger in the linear dimension with the starting powder used in this study. The final results showed the excellent static and fatigue strengths of the SS420 specimens produced with the new BJP protocol, implicating the potential structural applications of the BJP parts.

Interface nucleation rate limited densification during sintering

In situ bicrystal sintering experiments performed in a transmission electron microscope indicate that the densification rate can follow interface nucleation rate limited kinetics. This work performs in situ sintering experiments on immiscible Al2O3-GdAlO3 interfaces enabling a statistical treatment of sintering at a single interface type as a function of time and temperature. The data provide a measure for the activation energy of the nucleation process, ≈4.8 eV. This large value is consistent with observations that the process is rate limiting. A new sintering model is developed based on the assumption of interface nucleation rate limited kinetics and is demonstrated to work well in predicting in situ bicrystal sintering data, fitting in situ multiparticle sintering data, predicting grain size versus density sintering trajectories, and fitting isothermal bulk sintering data.

Flash sintering behaviour of 8YSZ-NiO composites

Flash sintering is an electric field/current assisted sintering technique, which is reported to lower the furnace temperature and to reduce sintering time significantly. In this work, we have studied the processing of 8YSZ/NiO composites by flash sintering, for the first time. Two composites, with different amount of NiO (one below the percolation limit and another one above it) were processed in two different sintering atmospheres. Constant heating rate experiments were performed to know the minimum furnace temperature required to flash sinter the samples for a given applied electric field. Subsequently, isothermal flash sintering experiments were performed at different current densities. The flash onset temperature of the composites was lower in the reducing atmosphere compared to in air. The power dissipated in stage III of the flash was strongly influenced by the composite composition and the sintering atmosphere. The extent of densification in the composites was controlled by the current density. The composites were densified up to a relative density of ∼90% in 30 s when flash sintered in air. In reducing atmosphere, there was in-situ reduction of NiO to Ni. As a result, for composites containing NiO above the percolation limit, the current preferentially flew through the in-situ formed metallic phase and there was no densification in the composite in reducing atmosphere. Phase and microstructure evolution in the composites was studied through XRD, SEM and EDS. With proper control of the electrical parameters (electric field and current density), composites with controlled porosity can be processed through flash sintering which may have applications for solid oxide fuel cells.

Variation of photoluminescence intensity depending on the timing of electric field application during isothermal flash sintering for 3mol%Y2O3–ZrO2 polycrystal

The shrinkage process and the change in photoluminescence (PL) intensity during isothermal flash sintering were investigated for 3 mol% Y2O3–ZrO2 (3YSZ) polycrystals doped with 300 ppm Bi2O3 as a PL impurity centre. The PL intensity of 3YSZ is enhanced by isothermal flash sintering. However, the enhancement of the PL intensity was revealed to become smaller when flash-treatment was applied for thermally pre-sintered 3YSZ polycrystal. To maximize the efficiency of electric field on the enhancement of the PL intensity, the electric field needs to be applied throughout the sintering from the initial stage to the final densification. The necessity for continuing to apply the electric field until the densification is completed is due to the fact that the recovery of oxygen vacancies, the origin of the F+ band, must be suppressed.

Consolidation of complex-shape zircon compacts through agar gelation

Zirconium silicate (zircon, ZrSiO4) is a ceramic material with excellent thermal properties, such as low thermal conductivity and high thermal shock resistance. In this work, dense zircon complex shaped parts were obtained by aqueous gelcasting through agar gelation. The colloidal stability study was performed through zeta potential measurements as a function of pH and polyacrylic-based deflocculant concentration (PAA). The rheological study was carried out varying the solids content, the dispersant concentration, and the sonication mixing time, the best results being obtained for 0.1 wt.% PAA and 1 min sonication. The viscosity versus temperature curves were recorded on cooling for the zircon-agar mixtures showing a gelling temperature of approximately 35 °C. Better gelcasting performance was achieved with suspensions prepared at a solids loading of 40 vol.%. Dynamic sintering (25–1550 °C) and isothermal sintering tests (1550 °C and 1600 °C) were carried out, studying the shrinkage and densification of the material. Finally, the evolution of phases and the microstructure of the sintered parts were studied using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Relative densities of 94.5% TD were reached at 1600 °C/2 h, but increased to 96% with 8 h holding, with a small grain size increase from 1.8 to 2.4 µm.