Solid-state Sintering Research Articles

Microstructural evolution in polycrystals with faceted boundaries

Various types of grain growth behavior have been observed in polycrystals with faceted boundaries. The grain growth behavior observed in many different ceramic as well as metallic systems has been well explained by the mixed mechanism principle of grain growth and microstructural evolution. However, relatively few studies have considered repetitive grain growth in materials. To examine the generality of the microstructural evolution principle, two cases of repetitive grain growth during solid-state sintering are considered, one metallic (Ni) and one ceramic system (BaTiO3). The observed repetitive grain growth behavior with an increased sintering temperature (in the case of Ni) and sintering time (in the case of BaTiO3) supports well the generality of the mixed mechanism principle.

Microstructure, dielectric properties and phase transition of Y2O3-doped barium tin titanate ceramics

BaCO3, SnO2 and TiO2 et al were used as crude materials, and Y2O3 was used as dopant, BaSnxTi1-xO3 (BTS) ceramics were prepared by solid-state reaction sintering technology. The results show that the doping of Y element does not change the lattice structure of barium tin titanate obviously, and there is no new phase, however, owing to the Y3+ ion doping, the diffraction peak position is changed as the Y3+ doping amount increasing. Interestingly, Curie temperature of the sample increased from 35 o C (blank sample) to 50 o C (0.05 mol%), and then the Curie temperature moved to low temperature as the doping amount increased continuously, which is related to the doping mechanism of Y3+ ion. After doping Y2O3, the dielectric loss of the samples decreases, especially when the doping amount reaches 0.10 mol%, the specimen shows excellent temperature stability of dielectric loss, making it superior candidates for applications.

Broadening of the carbon window and the appearance of core-rim carbides in WC-Fe/Ni cemented carbides

Among several separate challenges, the major one for replacing cobalt in cemented carbides is the difficulty to obtain alternative binder materials with a C-window broad enough to be robustly processed under conventional industrial control on the C content. The C-window is defined as the C content range for which phases that are detrimental to the mechanical properties are avoided. The present paper has two main objectives: first, to show that the processing C-window of Fe-Ni based systems is in fact wider than what thermodynamic equilibrium calculations predict, and that its width can be controlled moderately by tweaking the initial WC grain size and the cooling rate used in the material’s processing. Secondly, in case those detrimental phases are not avoided, this work gives insight on how to make their appearance less detrimental for the mechanical properties. The morphology, volume fraction and particle size distribution of the detrimental phases, specifically η6-carbides at low C contents, are investigated to explore desirable combination of hardness and toughness of alternative binder cemented carbides. During this study it was also discovered that in samples with carbon contents below the low-C limit of the C window a carbide with hexagonal lattice known as κ, not commonly seen in cemented carbides, appeared and formed the core of a core-rim structure together with the more common η6-phase. It is believed that the κ-carbide form due to local high concentrations of tungsten during solid state sintering and that it has an impact on the precipitation characteristics of the η6-phase.

Developing novel high-lead equivalence transparent ceramic windows for effective γ-ray shielding

In response to the limitations of current transparent window materials in γ-ray shielding scenarios, herein, high-lead equivalent (0.865 mmpb) Yb2Hf2O7 transparent ceramics (YbHO-TCs) were prepared via a solid-state reaction and vacuum sintering. The fabricated YbHO-TC shows high transparency, where the in-line transmittance up to 78.9 % at 1200 nm, which is extremely close to their theoretical limit (79 %). Additionally, the linear absorption coefficient of YbHO-TC for γ-rays is 0.482, approximately twice that of the state-of-the-art lead glass materials widely employed in radiation-shielding windows. Remarkably, YbHO-TC displayed no discoloration, and its in-line transmittance exhibited little change even when exposed to a radiation dose as high as 800 kGy. The innovative developed YbHO-TC integrates high transparency and high-lead equivalent properties, not only rending them highly promising for deployment as γ-rays shielding windows but also inspiring the controllable design of high-performance optical windows tailored for use in radiation shielding applications.

Utilization of fused deposition modeling in the fabrication of lattice structural Al2O3 ceramics

This study focuses on fabricating lattice structural Al2O3 ceramics with lattice infill densities ranging from 25 % to 70 % using filament-based fused deposition modeling (FDM) technology. A specialized filament for FDM printing was developed, comprising 53 vol% Al2O3 powder and ethylene vinyl acetate (EVA) as the primary binder component. The filament demonstrated excellent flexibility as assessed by a self-designed three-point bending test. The impact of lattice infill density and sintering temperature on dimensional changes, microstructure evolution, compressive strength, and strength-to-density ratio was explored. It was found that with increasing sintering temperature, the samples became more compact, resulting in higher levels of shrinkage, bulk density, and compressive strength. At a sintering temperature of 1600 °C, the sintered parts reached their maximum density through the solid-state sintering process. The sample sintered at 1600 °C with 70 % infill density exhibited the maximum compressive strength of 31.47 MPa, with a corresponding strength-to-density ratio of 20.84 MPa/g·cm−3. These findings highlight the successful fabrication of lattice structural Al2O3 with low density and high compressive strength by FDM technology.

Structural, optical, and the effect of beta source (90Sr/90y) radiation on the thermoluminescence properties of the transition metal activated on lithium aluminum borate phosphor (Li3Al3(BO3)4:Mn)

The activation of Lithium aluminum borate material with varying concentrations of transition metal of manganese was studied. The synthesis of the phosphor samples was achieved by employing the solid-state sintering technique. The study aimed to determine the structural property, optical property, and the effect of beta source (90Sr/90y) radiation on the thermoluminescence properties of the phosphor material. The analyzed structure of both doped and undoped samples was obtained to be from 52.17 to 67.13 nm as the crystallite size and 67.00–120.00 nm as the particle size respectively. The optical studies showed an increase in mole concentration of the doped samples resulted in an increase in absorbance and a decrease in the energy band gap from 4.45 to 4.00 eV. The thermoluminescence report measured showed that the Mn-doped sample on lithium aluminum borate has the most prominent peak at 78 °C (major peak) and 294 °C (minor peak) when exposed to radiation dose (90Sr/90y beta source) of 50 Gy. The acquired activation energy was resolved using some of the models which include the initial rise, deconvolution, and the heat rate techniques. The dose response was determined to be linear from the Mn-doped lithium aluminum borate.

Microstructure, dielectric, ferroelectric and piezoelectric properties of K0.45Na0.45Li0.1NbO3 lead-free ceramics prepared via cold sintering with post-annealing

Via the method of cold sintering with post-annealing, the lead-free ceramics [Formula: see text][Formula: see text][Formula: see text]NbO3 were prepared. The cold sintered pellet without post-annealing shows a relative density ([Formula: see text] of 77.9%, which is larger than [Formula: see text] of the green pellet via the conventional solid-state sintering (SSS) method ([Formula: see text]). The cold sintered pellets were post-annealed at temperatures between [Formula: see text]C and [Formula: see text]C. The effect of post-annealing temperatures on microstructure, dielectric, ferroelectric and piezoelectric properties of the ceramics was studied in detail. After being post-annealed, the relative densities and grain sizes of the ceramics were increased. The ceramics post-annealed at [Formula: see text]C show excellent dielectric, ferroelectric and piezoelectric properties. Compared to the conventional SSS method, the method of cold sintering with post-annealing is successful in preparing dense [Formula: see text][Formula: see text][Formula: see text]NbO3 lead-free ceramics in a wide temperature range and at relatively low temperatures.

In-situ monitoring of sintering and analytical modeling of densification and shrinkage in binder jetted 316 L stainless steel

In binder jetting, shrinkage and deformation occur during the sintering step, both of which are affected by the green density of the binder jetted materials. The study innovatively introduces a cost-effective, practical, and in-process monitoring system for visualizing shrinkage and deformation on larger samples than conventionally observed using small-scale specimens in dillatometry equipment. The powder characteristics and binder jet printing process itself influence the initial green density. The comprehensive analysis of powder flowability and packing density, densification behavior, and shrinkage reveals that the consolidated parts using virgin powder (with a green density of 55 %) can achieve a relative density above 99.9 % with an anisotropic shrinkage in the Z > X > Y direction. In contrast, the used or recycled powder exhibits a lower green density of ∼48 %, higher shrinkage rate in all three dimensions, and a decreased degree of anisotropy. Using in-process imaging and experimental data on the grain size attained through optical microscopy and electron backscatered diffraction imaging, the material's shear and bulk viscosities were determined. The formation of delta-ferrite and its impact on densification were discussed in the context of solid-state and supersolidus liquid phase sintering. The model relied on the continuum sintering theory formulated by Skorohod and Olevsky. The strain evolution from the in-situ imaging of sintering process is correlated with porosity based on the used feedstock and applied sintering temperatures. The outcomes of this study offer valuable perspectives on anisotropic sintering mechanisms, bridging the knowledge gap regarding the relationships between structures produced through binder jetting and subsequent sintering of materials.

Large electrocaloric effect across the non-hysteretic electrostructural phase transition in lead-free (Ba0.88Ca0.12)(Ti0.94Sn0.06)O3 ceramics

Structural, dielectric, and electrocaloric properties were investigated in Pb-free ceramic (Ba0.88Ca0.12)(Ti0.94Sn0.06)O3 (BCST), synthesized using solid-state sintering method. Rietveld refinement of room temperature XRD data revealed the existence of pure perovskite tetragonal (P4mm) phase. Temperature-dependent XRD analyses confirmed a structural transition from tetragonal to cubic phase near 380 K. Temperature-dependent dielectric constant (εr) and specific heat capacity (Cp) confirmed the presence of thermal hysteresis in two phase transitions near 233 ± 10 K and 254 ± 10 K, and a non-hysteretic behavior at the Curie temperature (TC∼373 ± 10 K). A comprehensive Raman study corroborated the existence of phase transitions. Electrocaloric properties were determined using indirect method utilizing the non-hysteretic nature across TC. A large adiabatic temperature change of |∆T| = 0.52 K, isotropic entropy change |∆S| = 0.68 J/kg.K, electrocaloric coefficients, |∆T/∆E |= 0.311 K.mm/kV and |∆S/∆E| = 0.41 J.mm/K.kg.kV were obtained at 380 K for an electric field of 16.7 kV/cm. Obtained value of |∆T/∆E| is relatively good, reported among other BaTiO3-based ceramics.

Perovskite Nanocrystals In Situ Encapsulated in TiO2 Microspheres for Stable CO2 Photoreduction in Water.

Photoreduction of carbon dioxide (CO2) into fuels presents a promising approach to mitigate global warming and energy crises. Halide perovskite nanocrystals (NCs) with prominent optoelectronic properties have triggered substantial attention as photocatalysts but are limited by the charge recombination and instability. Here, we develop stable CsPbBr3/titania microspheres (TMs) by in situ growth of CsPbBr3 NCs inside mesoporous TMs through solid-state sintering, which significantly improves the stability of perovskite NCs, making them applicable in water with efficient CO2 photoreduction performance. Notably, the CsPbBr3/TMs demonstrates a 6.73- and 9.23-fold increase in the rate of CH4 production compared to TMs and CsPbBr3, respectively. The internal electric field facilitates S-scheme charge transfer, enhancing the separation of electron-hole pairs, as evidenced by X-ray photoelectron spectroscopy and electron paramagnetic resonance analysis, which is pivotal for the selective photoreduction of CO2. These insights pave the way for the design of CsPbBr3-based photocatalysts with superior efficiency and stability.

Flash viscous flow sintering of nanostructured amorphous ZrO2–Y2O3 composite ceramics starting from a partially amorphous precursor powder

Nanostructured amorphous composites are novel materials comprising nano-sized crystal phases embedded in an amorphous matrix, but often necessitating ultra-high-pressure molding for preparation. In this study, nanostructured amorphous ZrO2–Y2O3 composite ceramics were rapidly prepared for the first time using flash viscous flow sintering without the application of pressure. The relative density of the ZY ceramics increased with increasing current density, and ZY composite ceramics with relative density of 86%–97 % were prepared by FS in a short time of 60 s under 700 V/cm with 0.16–0.56 A/cm2. Furthermore, the sintering activities in FS were compared between partially amorphous powder and crystalline powder. The ceramics prepared by flash sintering of partially amorphous powder show a much higher relative density than that from crystalline powder, suggesting that partially amorphous powder achieves easier densification than crystalline powder in flash sintering under given current density and power dissipation. This phenomenon was attributable to the differing sintering mechanisms between the two materials, i.e., solid-state sintering for crystalline powder and viscous flow sintering for partially amorphous powder.

LiNbO3-induced phase structure evolution and improved piezoelectric properties in BiFeO3–BaTiO3 ceramics

Eco-friendly (0.73-x)Bi1.05FeO3-xLiNbO3-0.27BaTiO3 ternary ceramics were designed and prepared via a solid-state sintering route. Based on the X-ray diffraction results and Rietveld refinement, it was found that the structure of the ceramics transformed from rhombohedral to tetragonal symmetry with the addition of LiNbO3, and at x = 0.003, there existed a morphotropic phase boundary separating the two phases. Near the phase boundary, the largest values were achieved for the piezoelectric constant d33 and planar electromechanical coupling coefficient kp, being 180 pC/N and 0.27, respectively. Due to the addition of LiNbO3, the grain size tended to become smaller. Additionally, the ceramics were transitioned from normal to relaxor ferroelectrics with increasing x. Despite the fact that the LiNbO3 addition would result in a gradual decrease of Curie temperature, a relatively high Curie temperature (535 °C) was still maintained at x = 0.003, the phase boundary composition, which also showed good temperature stability of piezoelectric properties. These experimental results can provide a reference for the practical applications of high-temperature lead-free ceramics.

(Sb0.5Li0.5)TiO3-Doping Effect and Sintering Condition Tailoring in BaTiO3-Based Ceramics.

(1-x)(Ba0.75Sr0.1Bi0.1)(Ti0.9Zr0.1)O3-x(Sb0.5Li0.5)TiO3 (abbreviated as BSBiTZ-xSLT, x = 0.025, 0.05, 0.075, 0.1) ceramics were prepared via a conventional solid-state sintering method under different sintering temperatures. All BSBiTZ-xSLT ceramics have predominantly perovskite phase structures with the coexistence of tetragonal, rhombohedral and orthogonal phases, and present mainly spherical-like shaped grains relating to a liquid-phase sintering mechanism due to adding SLT and Bi2O3. By adjusting the sintering temperature, all compositions obtain the highest relative density and present densified micro-morphology, and doping SLT tends to promote the growth of grain size and the grain size distribution becomes nonuniform gradually. Due to the addition of heterovalent ions and SLT, typical relaxor ferroelectric characteristic is realized, dielectric performance stability is broadened to ~120 °C with variation less than 10%, and very long and slim hysteresis loops are obtained, which is especially beneficial for energy storage application. All samples show extremely fast discharge performance where the discharge time t0.9 (time for 90% discharge energy density) is less than 160 ns and the largest discharge current occurs at around 30 ns. The 1155 °C sintered BSBiTZ-0.025SLT ceramics exhibit rather large energy storage density, very high energy storage efficiency and excellent pulse charge-discharge performance, providing the possibility to develop novel BT-based dielectric ceramics for pulse energy storage applications.

Achieving high energy storage performance in PbHfO3-based antiferroelectric ceramics by Sr element doping

Antiferroelectric (AFE) ceramics are the most promising material system for energy storage in dielectric capacitors due to their fast charging-discharging rate, good chemical stability, and high energy storage density. However, it is exceedingly challenging to simultaneously achieve the excellent recoverable energy density (Wre) and high energy storage efficiency (η) at present. In this work, (Pb0.925-xSrxLa0.05)(Hf0.95Ti0.05)O3 AFE ceramics with various Sr contents are fabricated by solid-state sintering processing. It has been found that Sr-doping in the A-site can significantly affect the microstructure, including reducing grain size and modulation period, effectively strengthening the antiferroelectricity, which is responsible for the improved energy storage performance. The high recoverable energy storage density of 10.2 J/cm3 is obtained at 560 kV/cm with an ultra-high efficiency of 93.0% in (Pb0.875Sr0.05La0.05)(Hf0.95Ti0.05)O3 ceramics. These features suggest that Sr-doped PbHfO3-based AFE ceramics can serve as promising candidates for capacitor materials, offering significant potential in the realm of energy storage applications.

Tuning structural, dielectric, and ferroelectric properties of BTO-based ceramics through dual-site substitution

Different strategies for tuning the properties of ferroelectric materials have been employed for advancing electromechanical applications. Among several material synthesis techniques explored, site substitution is an effective approach. The present work focuses on Ba1-xCaxSnyTi1-yO3 (BCST) ceramics prepared by solid-state sintering route with different substitution levels (x = 0, 0.05 and y = 0, 0.09). The impact of site substitution on the properties of Barium Titanate (BTO), phase purity, structural analysis, chemical composition, dielectric, and ferroelectric properties have been studied. An ionic radii difference at A-site and B-site substitution is a key factor to achieve significant changes in physical and structural properties. X-ray diffraction patterns ensure the formation of pure phase perovskite structure in all the compositions. Raman spectroscopy indicates Ca substitution not only occurs at the A-site but also within the oxygen octahedra, adversely affecting the properties. Ca and Sn act as strong grain growth inhibitors, leading to a fine-grained and dense microstructure in BCST. While Ca substitution has a limited impact on the Curie temperature (TC), Sn substitution lowers TC in BST (54 °C) and BCST (50 °C) compared to BTO (126 °C). The Sn substitution results in the coexistence of multiple phases (rhombohedral, tetragonal and orthorhombic) in BST, enhancing its properties (ε' = 6118 at RT, Pr = 5.50 μC/cm2, Q11 = 0.060 m4/C2, d*33 = 526 pm/V) in comparison to BTO. These results acknowledge the role of substituents in lattice disruption, paving the way for chemistry-based materials design in the field of dielectric, actuator and energy storage applications.

Microwave dielectric properties and LTCC applications of new glass-free molybdate ceramics Li3Ba2Ln3(MoO4)8 (Ln=Gd, Tm)

The traditional solid-state sintering method was employed to prepare Li3Ba2Ln3(MoO4)8 (Ln = Gd, Tm) ceramics, and the impact of firing temperature on the structure, microstructure and microwave dielectric properties of the novel molybdate Li3Ba2Ln3(MoO4)8 (Ln = Gd, Tm) ceramics was examined. XRD and Rietveld refinement outcomes validate the monoclinic system with the C2/c space group for Li3Ba2Ln3(MoO4)8 (Ln = Gd, Tm). For the Li3Ba2Gd3(MoO4)8 ceramic sintered at 800 °C, a εr is 9.42, a Q × f is 16,988 GHz, and a τf is 4.57 ppm/°C. The Li3Ba2Tm3(MoO4)8 ceramic sintered at 800 °C has a εr of 9.74, a Q × f of 25,401 GHz, and a τf of 1.08 ppm/°C. XRD and EDS analyses prove the good chemical compatibility of the co-sintered Li3Ba2Gd3(MoO4)8 ceramic with the Ag electrode. Therefore, the Li3Ba2Gd3(MoO4)8 ceramic is suitable candidate material for LTCC applications.

Fabrication of Al2O3– Ce:(Y, Tb)3 (Al, Mn)5O12 composite ceramic phosphors for high color rendering white LED/LD illumination

Ce: YAG- Al2O3 composite ceramics are regarded as highly promising materials for laser illumination application due to their high luminous efficacy and thermal stability. Nevertheless, the Ce: YAG ceramics exhibit limited color rendering index (CRI) for indoor lighting due to their low red emission component. In this study, we synthesized high-performance Ce, Mn-doped (Y, Tb)3Al5O12–Al2O3 composite ceramic phosphors via a vacuum solid-state sintering method. It was observed that the introduction of Mn2+ ions created a second luminescence center with an emission peak centered at 580 nm. As the Mn2+ concentration increased, the emission peak redshifted from 580 nm to 591 nm, leading to a significant enhancement in the color rendering index of the ceramics. The cross-energy transfer between Tb3+ and Mn2+, Ce3+ ions augmented the absorption and conversion of blue light by the ceramics, thereby reducing the correlated color temperature greatly. Based on these findings, ceramics with a Ce3+ concentration of 3 at% and a Mn2+ concentration of 4 at% exhibited the highest CRI values (81.3 and 75.6) under both laser excitation and LED excitation, establishing them as strong candidates for achieving high color rendering in white LED/LD illumination applications.

Innovative synthesis technique for high-performance dielectric resonator antennas: laser-induced shockwave sintering of potassium sodium niobate (KNN)

This study explored the synthesis and sintering of potassium sodium niobate (KNN) nanoparticles, emphasizing morphology, crystal structure, and sintering methods. The as-synthesized KNN nanoparticles exhibited a spherical morphology below 200 nm. Solid state sintering (SSS) and laser-induced shockwave sintering (LISWS) were compared, with LISWS producing denser microstructures and improved grain growth. Raman spectroscopy and x-ray diffraction confirmed KNN perovskite structure, with LISWS demonstrating higher purity. High-resolution x-ray photoelectron spectroscopy spectra indicated increased binding energies in LISWS, reflecting enhanced density and crystallinity. Dielectric and loss tangent analyses showed temperature-dependent behavior, with LISWS-3 exhibiting superior properties. Antenna performance assessments revealed LISWS-3’s improved directivity and reduced sidelobe radiation compared to SSS, attributed to its denser microstructure. Overall, LISWS proved advantageous for enhancing KNN ceramics, particularly in antenna applications.

Investigation of a thermally robust orange-red MgO-YMASG: Ce composite phosphor ceramic for white light LED/LD illumination

With its distinctive ability to emit red light with a wavelength surpassing 600 nm, Mg2+-Si4+ co-doped Y3Al5O12: Ce (YMASG: Ce) ceramics emerge as superior candidates for achieving high color rendering and high power white light illumination compared to YAG ceramics. However, due to its higher Stokes energy loss, the orange-red phosphor ceramic YMASG: Ce encounters more pronounced thermal accumulation issues when excited by high-power blue light LEDs/LDs. In response to this challenge, we employed vacuum solid-state sintering technology to fabricate MgO-YMASG: Ce composite phosphor ceramics. As the MgO content increased from 0 to 30 wt%, the thermal conductivity of the samples rose from 5.13 W/(m∙K) to 8.96 W/(m∙K). Additionally, the lower refractive index of MgO effectively enhanced the light extraction efficiency of the ceramics. Consequently, even at a 30 wt% MgO content, the luminescent efficiency of the sample remained comparable to that of YMASG: Ce phosphor ceramics. This suggests that MgO-YMASG: Ce composite phosphor ceramics can serve as high thermal robust orange-red phosphor materials for white light LED/LD illumination.

Structural, spectroscopi c and laser properties of transparent Tm:YScO3 ceramic based on gas-phase synthesized nanoparticles

Owing to the combination of good thermo-optical properties and inhomogeneously broadened fluorescence spectra suitable for ultrashort laser pulse generation, rare-earth doped solid solutions of sesquioxides (Y,Lu,Sc)2O3 have attracted considerable interest in recent years. In this study, compositionally disordered Tm3+-doped YScO3 transparent ceramic was fabricated using solid-state vacuum sintering of nanopowder produced using laser ablation of solid target in air flow. The as-synthesized powder consisted of weakly aggregated spherical particles having 20 nm diameter and a mostly monoclinic (space group C2/m) crystal structure, whose symmetry was transformed into cubic (space group Ia-3) by calcination at 1050 °C. From high resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray analysis (EDX), we observed a homogeneous distribution of components both throughout individual nanoparticles and 100-nm-thick ceramic foil resulting in high structural and optical quality of the obtained samples. The sintered ceramic featured an optical transmission of 77% at 600 nm and above 81% near 2 μm, an average grain size of 24.2 μm and a content of scattering centers of 26.1 ppm. The absorption spectra at several temperatures, from 93 K to 293 K, were measured and the corresponding absorption coefficients were obtained by Lambert-Beer formula in the range from 600 nm to 900 nm and from 1400 nm to 2100 nm. In particular, the room temperature absorption cross section at 794 nm and 1634 nm were determined to be σabs = 4.25 × 10−21 cm2 and σabs = 5.01 × 10−21 cm2 respectively. Concerning the emission cross section it was calculated by Fuchtbauer-Ladenburg equation starting from the fluorescence spectrum acquired at the laser work temperature: at 1951 nm and 2106 nm corresponding to the main two emission peaks belonging the curve, it was found σem = 9.2 × 10−21 cm2 and σem = 4.9 × 10−21 cm2, respectively. Finally, it was tested the laser behavior of the ceramic: pumped at 793 nm in quasi-CW mode it delivered 1.27 W of output power with a slope efficiency of 10.33% at 2081 nm. The tunability curve spans from 1942 nm to 2110 nm (i.e. 168 nm).