Sintering Atmosphere Research Articles

AlN/FeNi Microwave-Attenuating Ceramics with High-Efficiency Thermal Conductivity and Microwave Absorption

The integration, miniaturization, and high frequency of microwave vacuum electronics put forward higher requirements for heat-conducting and wave-absorbing integrated materials. However, these materials must balance the dispersion and isolation of wave-absorbing components to optimize absorption while maintaining the continuity of thermal conductivity pathways with low defect rates and minimal interfaces. This presents a significant challenge in achieving both high thermal conductivity and efficient wave absorption simultaneously. Here, AlN/FeNi microwave-attenuating ceramics were synthesized via non–pressure sintering in a nitrogen atmosphere. The influence of FeNi content (0–20 wt%) on the density, phase composition, microstructure, microwave-absorption properties and thermal conductivity of the composites was investigated. AlN/FeNi composites consist primarily of an AlN phase with FeNi0.0578, Fe, AlYO3, and Al5Y3O12 as secondary phases, and the microstructure is uniform and dense. As the FeNi content rises from 0 to 20 wt%, the density of the composites sintered at 1800 °C × 2 h increases from 3.3 to 3.7 g/cm3. Their X-band (2–18 GHz) dielectric constant goes up from 6.5 to 8.5, the dielectric loss factor rises from 0.1 to 0.9, and thermal conductivity diminishes from 130 to 123 W/m·K. Upon reaching an FeNi content of 20 wt%, the composite achieves a minimum reflection loss of −39.1 dB at 9.5 GHz, with over 90% absorption across an effective absorption bandwidth covering 2.5 GHz. It exhibits excellent impedance matching, electromagnetic wave-attenuation properties, a relative density of 98.6%, and a thermal conductivity of 123 W m−1 K−1. The prepared AlN/FeNi composites, with integrated outstanding microwave-absorption capabilities and thermal conductivity, holds great promise for applications in 5G communications, aerospace, and artificial intelligence.

Enabling Co-Firing of Li-Garnet-Electrolyte Tapes for Li-Metal Batteries

Garnet-type Li7La3Zr2O12 (LLZO) with its competitive ionic conductivity (~ 1 mS cm-1), wide electrochemical stability (above 5.0 V vs. Li/Li+), and compatibility with Li-metal, is a promising electrolyte for solid-state Li-metal batteries. Despite the promise, manufacturing all-solid-state battery architectures faces challenges, mainly due to missing robust contact and high interfacial resistance on the cathode-electrolyte interface due to the evolution of secondary phases during processing. Simultaneously achieving high cathode active material loadings and low interfacial impedances remain to be shown in devices with relevant architectures. In this project, we investigate the scalable powder-to-device fabrication for a cathode/electrolyte half- and full-cell with LiCoO2 as the active material and LLZO as the ion conducting electrolyte phase, combined with a Li-metal anode. We report the electrical and ionic resistance of LLZO-LiCoO2 composite cathodes being reduced by three orders of magnitude by manipulating the sintering atmosphere. Raman spectroscopy mapping reveals the secondary phase evolution inside the LLZO due to Cobalt diffusion being absent in composite cathodes sintered in a lithium-rich atmosphere as compared to ambient conditions. Further investigations lead to the conclusive assumption that the formation of secondary phases can be blocked by stopping lithium volatilization during high-temperature co-firing. Testing the dense composite cathodes in a liquid electrolyte battery, very low initial impedances of >30 Ω and high areal capacities of 3.5 mAh cm-2 with almost full utilizations can be achieved. Such batteries can be operated at 0.25 mA cm-2 at room temperature for 50 cycles without failing. Furthermore, we demonstrate tape-cast composite cathodes with thicknesses of < 100 µm being built into such battery setups, showcasing their mechanical integrity and density. Combining these results with recently developed dense-porous Li-garnet bilayers, we give an outlook to manufacture an all-solid-state, co-fired battery architecture that operates without the need of stack pressure.

Synthesis, Conductive Properties, and Crystal and Electronic Structures of Nasicon-Type Li1.5Al0.5Ge1.5-XMx(PO4)3[M=Mn, V, Mo]

Introduction All-solid-state lithium-ion batteries are expected to offer excellent stability and high energy density over a wide temperature range with a low risk of fire. Therefore, vigorous development efforts have been made in recent years, particularly with the increasing demand for electric vehicles. However, to improve the performance, novel solid electrolytes need to be discovered. Consequently, NASICON-type Li5Al0.5Ge1.5(PO4)3 materials have garnered attention because these materials exhibit higher chemical stability in air and water compared to other solid electrolytes and possess relatively high ion conductivity at room temperature. Nonetheless, further enhancement of conductivity is necessary for practical applications. In this study, Li1.5Al0.5Ge1.5-xMx(PO4)3 (M=Mn, V, Mo ; x=0.05, 0.1, 0.2, 0.3) was synthesized, and the composition-dependency of conductivity characteristics were investigated. Additionally, through average and electronic structure analyses using synchrotron X-ray and neutron diffraction, the correlation between composition, average/electronic structure, and conductivity was examined. Experiment Li5Al0.5Ge1.5-xMx(PO4)3 was synthesized by a solid-state reaction.1) Substitution at the Ge site was achieved using MnO2, V2O5, and MoO2. Phase identification was conducted through powder X-ray diffraction measurements, while the metallic composition was determined using an inductively coupled plasma optical emission spectrometer (ICP-OES). The sintering properties were evaluated using scanning electron microscopy (SEM, HITACHI, S-2600N) and true density measurements (Microtrac, BELPycno). The electrical conductivity characteristics were evaluated using AC impedance spectroscopy (SP-300, Bio-Logic). Additionally, the average structure was examined through Rietveld refinement (RIETAN-FP, Z-Code) using data obtained from synchrotron X-ray diffraction (BL19B2, SPring-8) and neutron diffraction (iMATERIA, J-PARC). The electronic structure was analyzed using the Maximum Entropy Method (MEM: Dysnomia). Results and DiscussionPowder X-ray diffraction measurements confirmed that most peaks of the synthesized Li5Al0.5Ge1.5-xMx(PO4)3 (x=0, 0.05, 0.1, 0.2, 0.3) were attributed to the R-3c space group, indicating a NASICON-type structure (Fig.1). In the case of Mn, V, and Mo substitutions (x=0.05, 0.1), no diffraction peaks from starting materials were observed, suggesting a successful substitution of Mn, V, and Mo for the Ge site. Furthermore, it was evident that the lattice constant of the a-axis and c-axis increased and decreased, respectively, due to the substitution. To assess the influence of the sintering process, a spark plasma sintering (SPS) was conducted in addition to a conventional atmospheric sintering, using finely ground powders obtained after the initial sintering. The results revealed that the density of the sintered bodies improved with the application of SPS. The temperature dependence of conductivity was measured for the obtained samples. It was found that as the substitution level of Mn, V, and Mo increased, the conductivity decreased, and the activation energy increased. Additionally, it was observed that some of the samples exhibited higher conductivity by SPS, likely due to the higher density of the sintered bodies. To investigate crystal and electronic structures, synchrotron X-ray diffraction measurements were conducted, followed by Rietveld refinement. The crystal structure was refined assuming the R-3c space group, with the substitution species M (Mn, V, Mo) occupying the 12c sites (Ge site). The results suggested that Li occupied two sites, 6b and 36f. Moreover, it was observed that substitution of metal (M) at the 12c sites led to reduced distortion of the polyhedra. This implies that the polyhedral distortion affects the conductivity. Regarding electronic structure, the bond strength between each metal and oxygen was evaluated based on electron density distributions obtained from MEM, and the correlation with lithium ion conductivity was examined.Reference1) J. Ock et al., ACS Omega, 6, 16187-16193 (2021). Figure 1

Effect of Carbon Black Content and Firing Atmosphere on the Properties and Microstructure of Al2O3-SiC-C Castables.

Al2O3-SiC-C (ASC) castables containing spherical asphalt are widely utilized in high-temperature metallurgical furnaces because of their good abrasive resistance and slag resistance; however, the release of hazardous benzopyrene during the pyrosis process in spherical asphalt is detrimental to the environment and to the health of furnace workers. Herein, nontoxic nano carbon black (CB) was selected as the carbon source for ASC castables, and the effects of the CB amount and sintering atmosphere on the properties of ASC castables were investigated in this work. The results show that on increasing CB from 0.5% to 2%, the cold strength of the samples after firing in the reducing atmosphere increased, the residual strength increased, and the slag penetrated depth decreased; the reasons can be ascribed to nano CB being able to fill the pores to reduce the apparent porosity of the castables. Furthermore, SiC whiskers were formed at elevated temperatures and generated a network structure, which was beneficial in improving their properties. When CB was 1%, the cold modulus of the rupture of the samples after firing in the oxidizing atmosphere and reducing atmosphere were higher (about 20 MPa), the retained strength ratio of the samples pre-fired in the reducing atmosphere was the highest (85.4%), the hot strength at 1400 °C of the samples tested in the oxidized atmosphere was the highest (5.3 MPa), and the slag resistance of the samples measured in the oxidizing atmosphere was the best. The castables heat-treated in the air atmosphere possessed higher hot strength and slag resistance; the reasons can be attributed to the formed SiO2 derived from the oxidation of SiC, which reacted with Al2O3 to form mullite, creating a strengthening effect and decreasing the porosity and increasing the viscosity of slag, thereby improving the hot strength and slag resistance.

Calcium vacancy enhanced self-reduction of Ce4+ for single-phase full-spectrum lighting and information encryption

Due to the wide and adjustable emission range, Ce3+ is an indispensable luminous center for full spectrum lighting. However, it needs to be sintered at high temperature in a reducing atmosphere, resulting in difficulty to coexisting with other multivalent activated ions (such as Eu3+, Tm3+), which greatly hinders the formation of full spectrum. In this study, a calcium vacancy enhanced self-reduction of Ce4+ is realized in CaNaSb2O6F (CNSOF) host under air atmosphere sintering, through which Ce3+, Tm3+ and Eu3+ coexisting in a single-phase full spectrum phosphor was prepared. Notably, the artificial introduction of a calcium vacancy was designed to verify this self-reduction mechanism. Moreover, the energy transfer kinetics among Tm3+, Ce3+ and Eu3+ were explored. Finally, combined with a 340 nm UV chip, a full spectrum phosphor-converted light-emitting diode (pc-LED) was fabricated, showing a broad emission range from 400 to 750 nm, Commission Internationale de I’Eclairage (CIE) of (0.3485, 0.3673), Ra of 92 and correlated color temperature (CCT) of 4933 K. Utilizing the variation in emission colors of this phosphor under different UV wavelengths, a dual encryption method combining point character code and fluorescent encryption technique is proposed. This work provides an effective path for Ce4+ self-reduction to apply in full spectrum pc-LED and information encryption.

Toughening Mechanism of CaAl12O19 in Red Mud–Al2O3 Composite Ceramics

The utilization of red mud in the production of ceramic products represents an efficient approach for harnessing red mud resources. Composite ceramics were prepared from Al2O3, red mud, and Cr2O3 by atmospheric pressure sintering, and the phase composition and microscopic morphology of the composite ceramics were investigated by XRD, SEM, and EDS. The flexural strength and fracture toughness of composite ceramics were measured by three-point bending and SENB methods. The results showed that the composite ceramics sintered at 1500 °C with the addition of 1.5 wt.% Cr2O3 had a flexural strength of 297.03 MPa, a hardness of 17.44 GPa, and a densification of 97.75% and fracture toughness of 6.57 MPa·m1/2. The addition of Cr2O3 helps to improve the low strength of red mud composite ceramic samples. The CaAl12O19 phase can form a similar “endo-crystalline” structure with Al2O3 grains, which changes the fracture mode of the ceramics and thus significantly improves the fracture toughness. The wettability tests conducted on Cu and RM–Al2O3 composite ceramic materials revealed that the composites exhibited non-wetting behavior towards Cu at elevated temperatures, while no interfacial reactions or elemental diffusion were observed. Composites have higher surface energy than Al2O3 ceramic at high temperatures. The present study provides a crucial foundation for enhancing the comprehensive utilization value of red mud and the application of red mud ceramics in the field of electronic packaging.

Li3V2(PO4)3 sintering atmosphere optimisation for its integration in all-solid-state batteries

Integration of active materials into the architecture of all-solid-state batteries represents a significant scientific inquiry. Li3V2(PO4)3 (LVP), a positive and negative electrode active material for Li-ion batteries, has been subject to extensive research to elucidate its electrochemical behaviour during cycling processes. However, the comprehensive analysis of its thermal behaviour under different sintering atmosphere conditions has remained underexplored, particularly in the context of its compatibility with solid electrolytes. This study presents a meticulous study of sintering process under different atmospheres with precise control over oxygen partial pressure. Interestingly, we were able to sinter LVP and obtain dense, pure ceramic phase under slightly oxidising atmosphere, characterized by conductivity properties analogous to those observed in samples sintered under Ar/H2 conditions. The findings of this investigation contribute to the understanding of the optimal conditions required for the sintering of LVP, paving the way for the co-sintering of this material with inorganic solid electrolyte unstable under reducing atmosphere, such as LATP.

Coke breeze combustion and emission behaviors of CO and NO during iron ore sintering

CO emission reduction, especially the coordinated emission reduction of CO and NO in sintering flue gas, has attracted widespread attention in the world. The effects of different sintering raw materials, temperature and atmosphere on coke breeze combustion and CO and NO emission behaviors were studied. The results show that, sintering raw materials significantly improved the combustion performance of coke breeze, and promoted the simultaneous reduction of CO and NO emissions. Among them, Dolomite had the strongest catalytic effect, which reduced the apparent activation energy of coke breeze combustion reaction from 99.35 kJ/mol to 56.11 kJ/mol, and the emission reductions of CO and NO were as high as 47.84 % and 73.25 %, respectively. The catalytic ability of the raw materials components was CaO ＞ MgO ＞ Fe2O3. However, during the combustion process of coke breeze without raw materials, the reduction of NO by CO was affected by the atmosphere of the boundary layer, the conversion of N element was negatively correlated with the content of CO. When the temperature increased from 600 ℃ to 1 050 ℃, the combustion performance was improved, the thickness of the combustion zone was reduced by 47.92 %, the total emission of CO was decreased by 65.39 %, and the total emission of NO was increased by 23.05 %. The O2 concentration decreased from 21 % to 10 %, the combustion rate decreased, the combustion zone thickened by 90 %, incomplete combustion was promoted, the total emission of CO increased by 101.15 %, and NO decreased by 21.09 %. The CO2 concentration increased from 5 % to 15 %, the gasification reaction was enhanced, the combustion time was extended, the combustion zone thickened by 45.9 %, the total emission of CO increased by 87.12 %, and the reducing atmosphere was enhanced, which was conductive to the reduction of NO by CO, and the total emission of NO decreased by 50.59 %.

Ultrafast high-temperature sintering of yttria-stabilized zirconia in reactive N2 atmosphere

So far, ultrafast high-temperature sintering (UHS) has always been carried out in an inert environment. In the present work, we investigated UHS of 3YSZ in nitrogen and argon atmosphere showing that “the atmosphere matters”. Highly densified samples can be obtained in both environments but densification and grain growth are significantly retarded in N2. Moreover, the phase evolution is strongly atmosphere-dependent with the samples treated in Ar remaining tetragonal and those treated under N2 progressively reducing their tetragonality, eventually converting into cubic zirconia and rock salt oxynitride. The results can be explained by the incorporation of nitrogen within the ZrO2 lattice. Electrochemical impedance spectroscopy demonstrates that while the ionic bulk conductivity are marginally influenced by the sintering atmosphere, the grain boundaries' capacitive behavior strongly changes. After UHS under 30 A, excellent ionic conductors were obtained without substantial grain boundary-blocking effects.

Decarburization and Gas Formation During Sintering of Alloyed PM Steel Components

This study investigates the delubrication, reduction, and decarburization processes of powder metallurgical steel alloys (CrM, CrL, AHC, Mo85, SintD 35) and an unalloyed steel during sintering in a pure hydrogen atmosphere. Utilizing in-situ FTIR gas phase analysis, components with ethylenebisstearamide (EBS) as a lubricant are analyzed. EBS decomposition in steel components yields CO, CO2, H2O, and CH4, with dominant CH groups observed in the 230 °C to 480 °C range. In the temperature range between 750 °C and 850 °C, where CO formation is expected due to the reduction of surface iron oxides, CH4 is present instead, indicating that an “internal getter effect” also occurs in pre-alloyed powders. In addition, with high carbon activity, the reduction of internal iron oxides and the reduction of chromium oxides also trigger an internal getter effect. Depending on the carbon potential, these processes cause a considerable reduction in the carbon content of the powder metallurgical components. The study therefore shows that the decarburization of powder metallurgical components during the heat treatment phases prior to sintering in a 100 pct hydrogen atmosphere is less due to the mechanism of delubrication, but rather to mechanisms of carbothermal reduction.

Study on the mechanism of potassium fluoride, magnesium fluoride and calcium fluoride on the crystal morphology and sintering performance of fluorapatite functional glass-ceramics

Fluorapatite glass-ceramics were prepared at lower temperatures by atmospheric pressure sintering method using KF, MgF2 and CaF2 as additives, respectively. Using XRD, SEM and mechanical property test to justify the effect of different fluorides on the sintering properties of fluorapatite composites. The results show that fluoride can be used as nucleating agent, react with wollastonite and phosphate glass to crystallize rod-like fluorapatite phase during heat treatment. The crystallization mechanism of fluorine-containing glass-ceramics was analyzed by crystallization thermodynamics and crystallization kinetics. KF with lower melting point is beneficial to promote the crystallization reaction and improve the morphology stability of the glass ceramic composite, but the volatile gas released during the reaction will affect the mechanical properties of it, resulting in a flexural strength of only 11.81 MPa at 900 °C. For the samples supplemented with MgF2 and CaF2, because of the formation of polycrystalline phase systems, the flexural strength of the composite at 900 °C increases to 62.79 and 52.6 MPa, respectively.

Impact of the atmosphere on the sintering capability and chemical durability of Nd-doped UO2+x mixed oxides

In this study, the influence of the working atmosphere on the sinterability and chemical durability of Nd-doped UO2 mixed oxides was investigated. To this end, the starting powder was first prepared by a hydroxide co-precipitation route, resulting in a nano-sized granulometry combined with a high specific surface area. The powders were then converted to oxides by heating and sintered in pellet form at 1600°C under an argon or reducing (Ar-4 %H2) atmosphere. The use of argon or reducing atmosphere resulted in very different densification pathways and final microstructures. The reducing sintering atmosphere hindered the uranium (IV) oxidation that could occur at high temperature, leading to the formation of U3O8, as was the case when working under argon atmosphere. Regarding the microstructure of the sintered pellets, the use of an argon sintering atmosphere resulted in an average grain size ten times larger than that of a reducing sintering atmosphere, while macroscopic properties such as relative density, porosity and homogeneity of cation distribution at the pellet scale remained the same. Nevertheless, a slight local enrichment of Nd at the grain boundaries was observed for the pellet sintered under Ar-4 %H2. In a second step, the study of the chemical durability of these sintered samples showed a significant influence of the sintering atmosphere on the dissolution kinetics and mechanism. These differences could be related to the microstructural properties of the pellets, i.e. the average grain size and the occurrence of grain boundaries. The cation distribution in the pellets could also influence their chemical durability, such as local Nd enrichment, the formation of defects in the oxygen sublattice and the presence of a different fraction of U(V) depending on the sintering atmosphere, as shown by HERFD-XANES measurements. The use of reducing or argon sintering atmospheres could even direct the charge compensation mechanisms that occur into the solid, thereby simultaneously affecting the sinterability and chemical durability of the samples.

Regulation of Intrinsic Defects of Self‐Activated MgGa2O4 Phosphors for Temperature Dynamic Anti‐Counterfeiting

AbstractPhotoluminescent (PL) anti‐counterfeiting plays a critical role in encrypted information and anti‐counterfeiting. Dynamic PL anti‐counterfeiting technology provides advanced security features compared with static counterparts. Yet, many current dynamic luminescent materials rely solely on an optical storage mechanism, leading to a lack of diversification and making them vulnerable to replication. Here, dynamic PL emission triggered by ambient temperature variations is addressed. By modifying the sintering atmospheres and non‐stoichiometric ratio of MgGa2O4 (MGO) samples, the concentration of intrinsic defects of the host are modified, resulting in temperature‐sensitive PL output dynamically manipulated from blue to green to red. Density‐functional theory (DFT) analysis elucidates the formation energies of intrinsic defects in the MGO samples, revealing that the self‐activated blue, green, and red emissions stem from lattice defects, [GaO6], antisite defects , and interstitial oxygen (), respectively. Additionally, by monitoring the luminescence lifetime, and the thermoluminescence (TL) curves of the MGO samples, it is shown that the dynamic PL emission is generated by a combination of thermally assisted carrier migration and thermal compensation from trapping centers. Herein, a dual‐channel encrypted dot matrix is designed to enhance information transfer security through the interference between the dual channels, showcasing the practical application of this dynamic anti‐counterfeiting technique.

Ultrafine-grained refractory high-entropy alloy with oxygen control and high mechanical performance

Grain boundary engineering plays a significant role in the improvement of strength and plasticity of alloys. However, in refractory high-entropy alloys, the susceptibility of grain boundaries to oxygen presents a bottleneck in achieving high mechanical performance. Creating a large number of clean grain boundaries in refractory high-entropy alloys is a challenge. In this study, an ultrafine-grained (UFG) NbMoTaW alloy with high grain-boundary cohesion was prepared by powder metallurgy, taking advantages of rapid hot-pressing sintering and full-process inert atmosphere protection from powder synthesis to sintering. By oxygen control and an increase in the proportion of grain boundaries, the segregation of oxygen and formation of oxides at grain boundaries were strongly mitigated, thus the intrinsic high cohesion of the interfaces was preserved. Compared to the coarse-grained alloys prepared by arc-melting and those sintered by traditional powder metallurgy methods, the UFG NbMoTaW alloy demonstrated simultaneously increased strength and plasticity at ambient temperature. The highly cohesive grain boundaries not only reduce brittle fractures effectively but also promote intragranular deformation. Consequently, the UFG NbMoTaW alloy achieved a high yield strength even at elevated temperatures, with a remarkable performance of 1117 MPa at 1200 °C. This work provides a feasible solution for producing refractory high-entropy alloys with low impurity content, refined microstructure, and excellent mechanical performance.

Qualitative study of carbides in liquid phase sintered M3:2 high speed steel

The microstructure of liquid phase sintered M3:2 high speed steel and the effect of adding carbon and silicon on the microstructure was characterized by scanning electron microscopy, dispersive spectrometry, and X-ray diffraction. Various types of carbides were formed depending on the added carbon and/or silicon, the sintering atmosphere and the cooling rate. The microstructure of sintered M3:2 high speed steel samples in vacuum conditions without the addition of Si and graphite is composed mainly of MC and M6C carbides inside cells and primarily at cell boundaries. The M2C eutectic carbides in these samples were formed at cell boundaries and their amounts and morphology depends on the cooling rate. Sintered samples in N2 atmosphere with added carbon and silicon, M6C eutectic and M7C3 eutectic carbides were dominantly formed while carbonitrides were formed in smaller amounts.

Influence of atmosphere-induction and synergistic effect on the dielectric property of rutile-type (Ge0.2Mn0.2Ti0.2Sn0.2Mo0.2)O2 high-entropy oxides

A new high-entropy oxide, (Ge0.2Mn0.2Ti0.2Sn0.2Mo0.2)O2 with rutile structure, was successfully synthesized using solid-state reaction method. The phase structure of the ceramic samples prepared forming a range of binary to quinary phase with increasing entropy were investigated, and the effects of atmosphere-induction and high-entropy synergistic behavior on the dielectric performance were studied. As the type of the cations increases, the temperature required to form a single rutile structure gradually decreases, while the dielectric properties of the ceramics gradually improved. The dielectric constant of the (Ge0.2Mn0.2Ti0.2Sn0.2Mo0.2)O2 high-entropy oxide ceramics sintered in the nitrogen atmosphere at 1350°C is nearly 20 times higher of the ceramics sintered in the air. This significant enhancement can be attributed to the occurrence of two rutile structures and the generation of excessive oxygen vacancies as a result of high-entropy synergistic behavior under the nitrogen sintering atmosphere.

From rapid prototyping to rapid firing: on the feasibility of high‐speed production for complex BaTiO3 components

AbstractDirect ink writing (DIW) is an attractive additive manufacturing (AM) technology because of its simplicity, production speed, and feedstock flexibility; in addition, the use of a limited amount of binder makes the subsequent thermal debinding process easy. Nevertheless, the conventional approach to debind and sinter AMed components remains extremely slow, representing a bottleneck in the manufacturing process. In order to address such limitation, we explored different rapid sintering strategies: ultrafast high‐temperature sintering (UHS), pressureless spark plasma sintering (P‐SPS), and fast firing (FF), for the densification of BaTiO3 components fabricated by DIW, one of the widely used lead‐free piezoceramics. All sintering technologies allow debinding and sintering of crack‐free components in a few minutes instead of several hours. The final density and microstructure are strongly dependent on the sintering atmosphere (inert for UHS and P‐SPS, air for FF) and a maximum relative density of only ≈72% was obtained when firing occurred in an inert environment, irrespective of the sintering technique (UHS and P‐SPS). An undesired phase transition from tetragonal to hexagonal BaTiO3 was also observed upon UHS and ‐PSPS. On the contrary, FF in air yielded a density of about 95% in a few minutes while maintaining the desired tetragonal polymorph. The results provide proof of feasibility for rapid processing of BaTiO3 components obtained by DIW.

Densification of zirconia-based ceramics by non-conventional sintering processes and chemical pathways

The present work reports an overview of the main results we have obtained from non-conventional sintering approaches towards the densification of 3mol.% yttria stabilized zirconia (3YSZ), the lessons learned from these results, and open questions that still need to be addressed. A wide range of techniques have been developed in the field of ceramic sintering, both in high temperature regime with extreme heating rates (>10 000 °C/min), as well as in very low temperatures (<400 °C) regime involving chemical reactivity to activate sintering mechanisms. In this context, we have recently explored several of these methods in order to understand and control the different sintering parameters and their impact on microstructure, crystallinity, grain boundaries and defect chemistry. Due to its complex chemistry, 3YSZ is a playground for the exploration of sintering capabilities and their influence on ceramic properties. Our recent studies in this field are presented in the present work, highlighting the effect of high heating rates through Flash Spark Plasma Sintering ( Flash-SPS) and Ultra High temperature Sintering (UHS) on sintering, but also the combination of low temperature Cold Sintering Process (CSP) and the associated precursors chemistry, with Spark Plasma Sintering (SPS). It also shows the possible densification by reactive hydrothermal sintering, using hydroxide precursors, and the effect of High Pressure SPS (HP-SPS) on 3YSZ nanopowders. All of these results touch upon current trends in the field of sintering, and highlight the possibilities in terms of chemical reactivity of precursors and physical parameter control. In particular, they show the importance of defect chemistry, related to the sintering atmospheres and their impact on both microstructures and properties.

Flash sintering of silicon carbide at room temperature

Although flash sintering (FS) is an effective method for rapidly preparing ceramics, FS of non-oxide ceramics at room temperature (RT) has not yet been achieved. In this study, we developed a feasible method for FS of SiC at room temperature (RT). A dog-bone-shape SiC sample prepared without using any sintering aid achieved a relative density of 96.5 % after sintering in a low-pressure Ar atmosphere for 60 s. Arc heating, which was constrained by the height of an alumina plate fixed above the sample, increased the conductivity of the sample, which then reached the stable stage of FS. The sintered sample exhibited a typical 6H–SiC crystal structure, which was consistent with that of the raw powder. The proposed FS method, based on constraining the arc using an alumina plate under a low-pressure inert gas atmosphere, provides a reference for developing further advanced RT FS strategies for non-oxide ceramics with low conductivities at RT.

Sintering of 3D-printed aluminum specimens from the slurry-based binder jetting process

This work investigates a novel method of producing complex-shaped aluminum parts by slurry-based binder jetting and sintering. In this process, a green body is built up by layer-wise deposition of an aqueous aluminum suspension and selective powder bonding by ink-jet printing. The powder bulk generated from the suspension shows an increased density compared to powder-based binder jetting and, thus, a high initial density for the subsequent densification step. This allows for higher final densities and reduced shrinkage. Aluminum is of special interest as it is widely available and of low density but challenging to sinter due to an oxide skin surrounding every particle. The research in this paper investigates the effects of the sintering atmosphere and sintering additives on the microstructure of powder compacts produced by slurry-based binder jetting. The incorporation of magnesium as an additive during the sintering process of aluminum has been found to substantially improve densification during sintering in an argon atmosphere.