Sintering Of Ceramics Research Articles

3D-Printed Bioceramic Scaffolds Reinforced by the In Situ Oriented Growth of Grains for Supercritical Bone Defect Reconstruction.

Porous calcium phosphate ceramics have attracted widespread attention owing to their excellent bioactivity. However, their poor mechanical properties severely limit their clinical applications. Significantly improving the mechanical strength of porous CaP ceramics while maintaining their bioactivity remains a major challenge. To address this issue, calcium sulfate is used to regulate the directional growth of hydroxyapatite grains during ceramic sintering. The in situ oriented grains can not only alleviate the stress concentration but also strengthen the bonding force between the ceramic grain boundaries. Calcium sulfate improves the release of active calcium ions from calcium phosphate ceramics, further enhancing their bioactivity and osteoinductivity in vivo. Transcriptome and proteome sequencing reveals that the in situ whisker-reinforced ceramics increase the expression of proteins related to calcium ion binding and promote the expression of osteogenesis-related proteins. In the supercritical bone defect repair model, repair of the defect is achieved within 3 months, with mechanical recovery reaching more than 70% of the autologous bone.

Enhanced density and microstructural integrity of binder jetting manufactured ceramics through cold isostatic pressing

Binder jetting (BJ) is a type of additive manufacturing (AM) process that enables fabrication of complex zirconia (ZrO2) ceramic components. However, there are significant challenges in fabricating ceramic parts with high relative density and superior performance. In this study, cold isostatic pressing (CIP) process was integrated into BJ, successfully producing high-performance ZrO2 ceramics. Comprehensive study was conducted on ZrO2 granules, binder, and their interactions. In addition, effects of CIP process on relative density and mechanical properties of sintered samples were investigated. Results indicate that CIP process significantly enhances green body density and reduces spacing between particles, thereby facilitating subsequent densification during ceramic sintering. Moreover, debinding followed by CIP treatment yields superior mechanical properties compared to the methods where CIP was performed prior to debinding, as it prevents defect formation during debinding. Subsequently, differences in performance between ZrO2 ceramics treated with and without CIP were compared at various sintering temperatures. After sintering at 1400 °C, relative density and flexural strength of ZrO2 ceramics reached 98.75 ± 0.19 % and 1047.80 ± 88.45 MPa, representing significant improvements of 53.24 % and 100-fold increase compared to those without CIP treatment, respectively. These findings indicate that proposed method is highly promising for fabrication of high-density and high-performance ZrO2 components.

Promoting transparency: Defect-driven sintering of Gd2Zr2O7 ceramics for advanced radiation applications

Transparent Gd2Zr2O7 (GZO) ceramics show significant potential for radiation detection and nuclear energy applications. However, scalable production through cost-effective solid-state vacuum sintering is challenging due to the material's high melting point and low thermal conductivity. This study introduces a novel approach by incorporating excess Gd to increase lattice defects, such as cation disorder and oxygen vacancies, which have low formation energy. This strategy lowers the energy barrier for ion diffusion, enhances material migration during sintering, and promotes the elimination of residual pores, leading to high densification. We examined the densification behavior of GZO compositions with varying Gd content (Gd1.8Zr2O6.7 to Gd2.5Zr2O7.75) during vacuum sintering at 1450°C to 1900°C, focusing on phase transformation, grain growth, and pore elimination. Notably, the optimal sample achieved 78.3 % transmittance, approaching the theoretical maximum. These findings offer a promising route for the large-scale production and commercialization of transparent GZO ceramics for advanced radiation applications.

Effect of pressure on the sintering mechanisms of tantalum carbide ceramics

AbstractHigh pressure sintering shows great superiority in promoting the densification of the hard‐to‐densify refractory ceramics, and the underlying mechanisms are thus of great interest. In the present work, the densification process of tantalum carbide (TaC) ceramics affected by pressure was discussed through comparing the microstructural characteristics and sintering kinetics. The TaC ceramics sintered at 30–250 MPa show dense and uniform microstructure without oxide impurities and high‐density dislocations. With the increase of sintering pressure, particle rearrangement becomes prominent at initial stage, whereas grain boundary diffusion rather than lattice diffusion dominants the final stage mechanism. Grain growth is enhanced under high pressure through the plastic deformation process. Our work should facilitate to understand the mass transport mechanisms during the high pressure sintering or deformation process of carbide ceramics.

Exploring the Potential of Cold Sintering for Proton-Conducting Ceramics: A Review.

Proton-conducting ceramic materials have emerged as effective candidates for improving the performance of solid oxide cells (SOCs) and electrolyzers (SOEs) at intermediate temperatures. BaCeO3 and BaZrO3 perovskites doped with rare-earth elements such as Y2O3 (BCZY) are well known for their high proton conductivity, low operating temperature, and chemical stability, which lead to SOCs' improved performance. However, the high sintering temperature and extended processing time needed to obtain dense BCZY-type electrolytes (typically > 1350 °C) to be used as SOC electrolytes can cause severe barium evaporation, altering the stoichiometry of the system and consequently reducing the performance of the final device. The cold sintering process (CSP) is a novel sintering technique that allows a drastic reduction in the sintering temperature needed to obtain dense ceramics. Using the CSP, materials can be sintered in a short time using an appropriate amount of a liquid phase at temperatures < 300 °C under a few hundred MPa of uniaxial pressure. For these reasons, cold sintering is considered one of the most promising ways to obtain ceramic proton conductors in mild conditions. This review aims to collect novel insights into the application of the CSP with a focus on BCZY-type materials, highlighting the opportunities and challenges and giving a vision of future trends and perspectives.

THz-TDS characterization of stress evolution of EB-PVD TBCs under thermal cycling

The failure of thermal barrier coatings (TBCs) during service is a critical issue affecting aeroengine efficiency. In this study, electron beam physical vapor deposition (EB-PVD) TBCs were subjected to thermal cycling at 1100 °C. Terahertz time-domain spectroscopy (THz-TDS) was employed to characterize the relationship between the evolution of characteristic frequency and the number of thermal cycles, along with the development of stress in the ceramic layer. In situ tensile test revealed a linear correlation between the characteristic frequency and micro strain derived from THz-TDS data, with a proportional coefficient of −9.94 between the characteristic frequency and the slope of the refractive index curve. The results show that the stress evolution can be effectively monitored by recording the trend of characteristic frequency. During the non-failure stage, the characteristic frequency followed a parabolic trend as thermal cycles increased, influenced by the growth of thermally grown oxide (TGO), ceramic layer sintering, and thermal mismatch. Approaching the failure stage, stress evolution was dominated by the expansion of vertical cracks in the ceramic layer and the widening of transverse cracks at the interface, which corresponded to a sudden change in the characteristic frequency.

Microwave dielectric properties of LiM(PO3)3 (M = Mg2+, Zn2+) ceramics

Microwave dielectric ceramics with low dielectric permittivity εr combined with low ceramic sintering temperatures have raised considerable research interest recently due to their potential application in LTCC technology for 5G wireless communications. Here in this study, ceramic processing, crystal structure, and microwave dielectric properties of LiM(PO3)3 (M = Mg2+, Zn2+) ceramics were comprehensively investigated. Optimized LTCC compatible ceramic sintering processes were developed at 725 °C and 625 °C for 10 hours, respectively, leading to high relative densities of 96.27% and 94.09%. Low microwave εr values of 5.74 and 6.30 and Q×f values of 16,660 GHz and 9,629 GHz (at 14 GHz) were obtained, where such values were found to be affected by the grain microstructure. LiMg(PO3)3 exhibited a small τf of -11 ppm/°C, while LiZn(PO3)3 showed -58 ppm/°C. Complementary impedance spectroscopy measurements indicated modest Li+-ion conductivity with a higher value of 2.92·10-7 (Ωcm)-1 at 560 K (287 °C) in LiZn(PO3)3. Both ceramics were found to be dielectrically homogeneous without any perceptible contributions from the grain boundaries.

Improved gyromagnetic properties of LiZnTi ferrites co-fired with Bi2O3-La2O3 additives at low temperature

In this work, low temperature-sintered Li0.43Zn0.27Ti0.13Fe2.17O4 ferrites mixed with x wt.% (x = 0.00–0.30 wt%) of La2O3 and 1.5 wt% of Bi2O3 were successfully prepared by the solid-state reaction method. The variations in phase composition, microstructure, and gyromagnetic properties of the as-prepared samples were studied in detail. XRD patterns and refinement results show that La2O3 didn't affect the formation of the spinel phase, but a small amount of La3+ might enter into the lattice structure. The micro-morphological analysis and grain size statistics demonstrate that the addition of La2O3 significantly suppressed the grain growth, and the average grain size decreased from 2.19 μm (x = 0.00 wt%) to 1.14 μm (x = 0.15 wt%). The uniformity of the grain size distribution was improved, and the degree of densification was significantly increased. Meanwhile, the saturation magnetization (4πMs) increased from 3921 Gs to 3989 Gs, the coercivity (Hc) decreased from 457.5 A/m to 312.5 A/m, and the ferromagnetic resonance linewidth (ΔH) decreased from 328.5 Oe to 220.7 Oe. Our study indicates that the Bi2O3-La2O3 mixture is an effective sintering additive for low-temperature sintering of LiZnTi ferrite ceramics, which will play an important role in the development of low-temperature co-fired ferrites.

Enhanced Sinterability and Mechanical Strength of Beta-type Tricalcium Phosphates Ceramics through Reaction Sintering with Hydroxyapatite

Beta-tricalcium phosphate (β-TCP) ceramics were fabricated via reaction sintering at 1100 °C, utilizing various phosphate salts and calcium compounds as raw materials. The sintering characteristics, reaction sintering behavior and mechanical properties of the resulting β-TCP ceramics were investigated. Reaction sintering with a Ca/P molar ratio of 1.50 consistently generated β-TCP. In addition, a notable variance in physical properties was observed contingent on the presence or absence of stoichiometric hydroxyapatite (HAp; Ca10(PO4)6(OH)2) in raw materials. Sintered bodies produced with HAp−β-Ca2P2O7 (β-CPP), HAp− CaHPO4·2H2O (DCPD) and HAp−(NH4)2H(PO4) all exhibited densification and enhanced mechanical strength, correlated with higher bulk densities. In contrast, conventional sintered bodies prepared from β-TCP and reaction sintered bodies prepared using β-CPP−Ca(OH)2 or β-CPP−CaCO3 were not as dense. Comprehensive assessments by thermogravimetry-differential thermal analysis, X-ray diffraction and scanning electron microscope together with the evaluation of shrinkage behavior were used to monitor the sintering of β-TCP with HAp. This process was found to involve thermal decomposition of HAp and crystal transitions leading to the formation and sintering of β-TCP with significant shrinkage. These findings underscore the efficacy of employing HAp as a raw material in reaction sintering. This technique allows the formation of sintered β-TCP bodies exhibiting exceptional sintering characteristics, superior bulk density and high mechanical strength.

Flash sintering of AlN ceramics with Y2O3 additive

This study demonstrates the fabrication of dense AlN-based ceramics through the flash sintering technique for the first time. Flash sintering was performed at a furnace temperature of 1500 °C with 10 wt% Y2O3 as a sintering aid in a nitrogen atmosphere. The effects of the current density and holding time on the microstructure evolution and mechanical properties were investigated. The results show that the relative density, grain size and Vickers hardness of specimens increase with increasing current density and holding time. The specimen showed the highest relative density of 98.6% and a Vickers hardness of 12.68 GPa when a current density of 100 mA/mm2 and a holding time of 60 min were used. Additionally, the grain sizes at the positive side of the electrodes were found to be larger than those at the negative side. These findings suggest that dense AlN-based ceramics can be fabricated in a rapid and efficient way through the flash sintering technique.

Porous titanium/hydroxyapatite interpenetrating phase composites with optimal mechanical and biological properties for personalized bone repair

This study introduces the first fabrication of porous titanium/hydroxyapatite interpenetrating phase composites achieved through an innovative processing method. The approach combines additive manufacturing of a customized titanium skeleton with the infiltration of an injectable hydroxyapatite foam, followed by in situ foam hardening at physiological temperature. This biomimetic process circumvents ceramic sintering and metal casting, effectively avoiding the formation of secondary phases that can impair mechanical performance. Hydroxyapatite foams, prepared using two foaming agents (polysorbate 80 and gelatine), significantly reinforce the titanium skeleton while preserving the microstructural characteristics essential for osteoinductive properties. The strengthening mechanisms rely on the conformation of the foams to the titanium surface, thereby enabling a stable mechanical interlocking and effective interfacial stress transfer. This, combined with the mechanical constriction of phases, enhances damage tolerance and mechanically reliability of the interpenetrating phase composites. In addition, the interpenetrating phase composites feature a network of concave pores with an optimal size for bone repair, support human osteoblasts proliferation, and exhibit mechanical properties compatible with bone, offering a promising solution for the efficient and personalized reconstruction of large bone defects. The results demonstrate a significant advancement in composite fabrication, integrating the benefits of additive manufacturing for bone repair with the osteogenic capacity of calcium phosphate ceramics.

Effects of Ni cation ratio and sintering temperature on microstructure and electrical properties of NiMnCoO4-based ceramics

Negative temperature coefficient (NTC) thermistors as temperature sensors play an important role in various electrical/electronic devices and are widely applied in the fields such as, automotive sensor modules, home appliances and control applications. The electrical properties of NTC thermistors can be effectively controlled by regulating the content of their chemical components or changing the microstructures. Therefore, this study investigated the effects of Ni cation ratio and sintering temperature on microstructures, crystal structures, and electrical properties of NiMnCoO4-based ceramics. With increasing Ni content, it was found that sinterability of NiMnCoO4-based ceramics was decreased with forming rock-salt phase of NiO, resulting in degrading resistivity. Furthermore, the spinel structures were detected by X-ray diffraction analysis regardless of sintering temperatures. Notably, values of average grain size for sintered samples were increased with increasing sintering temperature. Besides, it was clarified that electrical properties were significantly influenced by average grain size. The room temperature resistivity was increased from approximately 300 kΩ⋅cm for the sintered sample at 1100°C to about 600 kΩ⋅cm for for the sintered sample at 1300oC. On the other hand, the B(25/85) constant was not significantly influenced by the sintering temperature. Accordingly, it is expected that the results of this study are useful for controlling electrical properties of MnCoNiO4-based ceramics.

Study on liquid-phase sintering and magnetic properties of SLA-printed Mn-Zn ferrite ceramics

To obtain Mn-Zn ferrite magnetic ceramic parts with good densification and magnetic properties, a study was conducted. Mn-Zn ferrite magnetic ceramic parts with a solid particle content of 58 vol% were prepared using stereolithography (SLA). The SLA-cured ceramic blanks were degreased and sintered. The degreasing process for Mn-Zn ferrite parts was optimized using a direct heat degreasing method, resulting in a suitable process for the ferrite magnetic ceramic paste system. The method of liquid phase sintering is proposed for sintering the degreased ferrite parts. The study investigated the impact of the content of accelerant, sintering temperature, and holding time on the density and magnetic properties of Mn-Zn ferrite parts. The optimal parameters were found to be a firing aid content of 3 wt%, a sintering temperature of 900 °C, and a holding time of 90 min. Under these conditions, the parts exhibited a density of 4.43 g/cm3, densification of 91.4 %, saturation magnetization strength of 64 emu/g, and coercivity of 10 Oe. Finally, the sintering control mechanism of SLA-printed ferrite magnetic ceramics was analyzed. The study revealed the grain growth process and magnetization principle of Mn-Zn ferrite during liquid phase sintering. This research provides guidance for the subsequent photocuring printing and degreasing sintering of Mn-Zn ferrite ceramics. Additionally, Mn-Zn ferrite magnetic ceramic parts prepared by the SLA method also hold a wide application prospect in various precision electronic components.

Effect of Al2O3 coating on the properties of Si3N4 ceramics prepared by vat photopolymerization

The preparation of Si3N4 ceramics by vat photopolymerization (VPP) has motivated increasing research interest. However, it is challenging to prepare Si3N4 ceramics by VPP due to the high UV-light absorbance and refractive index of powder. In this paper, a method for Al2O3-coated Si3N4 powder was proposed. Combined with the boehmite-coated and high-temperature treatment, Al2O3 was successfully coated on the surface of Si3N4 powder. The effect of Al2O3 content on the properties of Si3N4 powders, slurry and the sintered Si3N4 samples were investigated. The Al2O3 coating layer not only improves the curing forming ability of Si3N4 slurries, but also can directly act as one of the sintering aids of Si3N4 ceramics. The bulk density of the samples decreases from 3.04 ± 0.02 g/cm3 to 2.97 ± 0.01 g/cm3 with the increase of coating content, while the porosity increases from 5.38 ± 0.89 % to 7.54 ± 0.63 %. The sample of 5 wt % Al2O3 coating content has the maximum flexural strength of 474.28 ± 16.38 MPa and the highest relative density of 94.62 ± 0.89 %. This work can not only obtain a great modification effect, but also promote the dense sintering of Si3N4 ceramics during subsequent stages, which provides a constructive method for modifying Si3N4 powders to achieve photopolymerization.

Sintering, microstructure and mechanical properties of anorthite-based ceramics prepared from iron-extracted steel slag

Steel slag is a promising source of ceramic materials, but it causes a waste of iron resources and inferior ceramic performance because the slag contains over 30 % iron oxides. To deal with this problem, we proposed a two-step utilization process for steel slag, including iron extraction by smelting reduction and ceramics preparation from residual slag. In this work, the effect of the reduced slag addition on phase constituents, microstructure, and properties of this novel ceramic was studied. The major crystalline phase is anorthite for all the ceramics. Although abundant liquid is expected theoretically in conventional ceramics at 1175 °C, it did not achieve vitrification until 1250 °C. The slag addition plays different roles in sintering under various slag contents. After introducing 15 wt% slag, the sintering and pore elimination are accelerated significantly because of the slag’s fluxing effect although the ceramic shows no change in the theoretical liquid content. The ceramics have no advantage in producing liquids with further increasing the slag content. Therefore, limited densification is observed with 30 wt% slag addition owing to the lack of liquid phase. Fortunately, solid solution diopside is largely generated in the anorthite matrix with 45 wt% slag addition, greatly enhancing the ion diffusion and grain growth. This effect contributes to a highly dense and uniform sintered structure with small rounded pores at 1175 °C, at which its flexural strength increases by 92 % compared to the conventional ones. However, a higher slag addition in the ceramic should be avoided to ensure satisfactory structure integrity.

Prediction and fabrication of textured Si3N4 ceramics via grain rotation model

The grain preferential orientation has a significant influence on the mechanical properties of silicon nitride (Si3N4) ceramics. Based on the Jeffery’s theory, a grain rotation model under uniaxial compression is created to predict the texture evolution mechanism during sintering of Si3N4 ceramics. With deformation, the grain orientation was predicted to rotate towards the plane perpendicular to the pressure direction, and large strain was favorable for the formation of strong texture. According to guidance of the model, the weakly textured microstructure prepared by normal hot pressing is attributed to the limited strain, due to the constrains of graphite die. Therefore, a new PHIP (pseudo-hot isostatic pressing) process with the large deformation is guided to fabricate the strongly textured ceramics by promoting the grain rotation and dynamic grain growth. Generally, the experimental results are well accordance with the model prediction. Furthermore, this model could be applied to guide the design and fabrication of other isotropic and anisotropic ceramic materials.

The Direct Cold Sintering of α-Al2O3 Ceramics in a Pure Water Medium

The Direct Cold Sintering of α-Al2O3 Ceramics in a Pure Water Medium

Structure, microstructure, mechanical and luminescent properties of (Y0.96Еu0.01Sm0.01Tb0.01Er0.01)NbO4 ceramics obtained by uniaxial hot pressing

Fine powders of yttrium orthoniobate activated by Eu, Sm, Tb, Er have been synthesized by the sol-gel. The synthesized powders have been calcined at different temperatures (1000, 1100 and 1180 °C). Luminescent ceramics (Y0.96Еu0.01Sm0.01Tb0.01Er0.01)NbO4 were prepared from fine powders by uniaxial hot pressing (UHP) via various technological modes. For the first time, the crystal structure, microstructure, spectral composition, photoluminescence properties of (Y0.96Еu0.01Sm0.01Tb0.01Er0.01)NbO4 ceramics have been studied depending on the temperature conditions of the synthesis of powders and UHP conditions during sintering of ceramics. The mechanical properties of these ceramics (Young's modulus, microhardness and critical stress intensity factor for the mode I KIC) have been evaluated.

A review of in-situ measurement and simulation technologies for ceramic sintering: towards a digital twin sintering system

A review of in-situ measurement and simulation technologies for ceramic sintering: towards a digital twin sintering system

Pushing the grain size limit of pressureless-sintered alumina nanocrystalline ceramics by non-oxidizing atmospheres

Nanocrystalline ceramics are of interest due to their refined grains and expected ductility, yet their pressureless sintering is challenging considering the large coarsening driving force and the coupled densification-coarsening kinetics. Of special interest is the pressureless sintering of alumina nanocrystalline ceramics, with fundamental significance to ceramic processing science and technology. Previously reported efforts offered dense alumina nanocrystalline ceramics with average grain size Gavg down to 34 nm and with a coarsening ratio β (defined as the ratio of average grain size after and before sintering) of 7.2 from nanoparticles to dense alumina ceramics, by synergistically optimized α-Al2O3 nanoparticles and pressureless two-step sintering in air. Here we further pushed Gavg down to 29 nm and β down to 5.8, by changing air to non-oxidizing atmosphere (e.g., N2, Ar, and vacuum). We identified pronounced difference in sintering kinetics in air versus in N2, Ar, or vacuum, which is unexpected for wide-bandgap, redox-insensitive oxide materials of alumina. With smaller initial nanoparticles (e.g., <3.5 nm) and better optimized sintering conditions, it may be possible to pressureless-sinter dense alumina ceramics with Gavg below 20 nm.