Reactive Spark Plasma Sintering Research Articles

Processing and microstructure development of reactive sintered (Ti-Zr-Nb-Hf-Ta)B2 + (Ti-Zr-Nb-Hf-Ta)C high-entropy ceramics

A novel dual-phase (Ti-Zr-Nb-Hf-Ta)B2 + (Ti-Zr-Nb-Hf-Ta)C high-entropy, compositionally complex composite was synthesized by reactive spark plasma sintering using commercial ZrB2, NbB2, HfB2, TaB2 and TiC powders as starting materials. The composite with high density, consists predominantly of boride (∼39.7 vol%) and carbide (∼56.4 vol%) phases and with residual amount of (Hf-Zr)O2. The microstructure is homogenous and relatively fine due to the reaction and solid solution coupling effect with average grain sizes of 3.4 μm and 2.2 μm of the boride and carbide phases, respectively. Large-area SEM analyses confirmed no microcracks or micropore formation and no presence of large grains or clusters. Investigation from micro to nano and atomic level, focused on local chemical composition and spatial distribution of constituent elements, revealed that Ti preferentially dissolves into the boride and all other elements to the carbide phase creating a (Ti0.82Zr0.04Nb0.08Hf0.03Ta0.03)B2 + (Ti0.49Zr0.12Nb0.13Hf0.11Ta0.15)C system. The analysis of grain and phase boundary interfaces revealed a continuous, <1.5 nm wide, segregation of impurity atoms of Fe and Co. Continuous (>20 nm) and atomically flat basal interfacial termination planes (001) decorated with monoatomic Zr enriched layer in the boride phase were observed. Triple points at grain and phase boundaries exhibit metal-rich composition due to impurities from the starting raw powders.

Kinetics-controlled reaction pathway and microstructure development of Ti3SiC2-TiC composite processed through reactive spark plasma sintering

Reactive spark plasma sintering of ternary Ti-Si-C system was performed using three different powder precursors systems 3Ti/Si/2C, 3Ti/SiC/C and 2Ti/TiC/Si, to explore the fundamental physics behind Ti3SiC2 MAX phase formation, its stability and microstructure development, and, finally linked with its hardening and contact induced damage tolerance. Phase evolution in Ti-Si-C system is a complex phenomenon, and, present experimental conditions never yield a phase pure Ti3SiC2 MAX phase, rather results in varying volume fractions of Ti3SiC2-(Tix,Si1-x)C solid solution due to non-equilibrium processing conditions exerted by SPS processing which restricts coherent site specific diffusional jumps and promotes the formation of (Ti, Si)C solid-solution instead of well reported non-stoichiometric TiCx. 3Ti/SiC/C precursor was the best candidate for processing composite with highest yields of Ti3SiC2. Phase evolution is guided by the free energy of formation of different phases and chemical affinity amongst the constituent elements rather than the equilibrium phase diagram of the Ti-Si-C system. Presence of free carbon, low temperature liquid phase and slow heating rate are the key requirements for forming phase pure Ti3SiC2, where excess free carbon reduces the stability of Ti3SiC2 via decarburization. Non-equilibrium processing conditions impart nano-precipitation of coherent hexagonal Ti3SiC2 precipitates within a cubic (Ti, Si)C matrix with a distinct orientation relation of (220)matrix ║(0004)precipitate and <114>matrix ║<2–1–10>precipitate that has never been reported, instead of growing highest density plane of hcp-on-fcc matrix. Coherency strain and fine interlocking microstructure of the as-processed composite experiences ≈36 % of enhancement in hardness followed by an improved contact damage for the as-processed composite.

Spark plasma sintering of hard wear‐resistant graphene nanoplatelet–reinforced TiB2 + SiC composites from TiC–B4C–SiC

AbstractGraphene nanoplatelet (GNP)–reinforced TiB2 + SiC composites were fabricated by reactive spark plasma sintering (SPS) from a TiC–B4C–SiC powder mixture with equal‐volume percentages, optimizing their SPS temperature and evaluating their unlubricated sliding wear against diamond. First, it is shown that during the heating ramp of the SPS cycle, TiC and B4C react according to the chemical reaction 2TiC + B4C → 2TiB2 + 3C, and that the thus‐formed C is graphenized as GNPs, leading to composites with microstructures consisting of a ceramic matrix of fine TiB2 and SiC grains with abundant randomly oriented GNPs at grain boundaries. It is also shown that this reactive SPS is optimal at 2000°C (under 75 MPa pressure and 5 min soaking), resulting in a very hard (∼28.5–29.9 GPa) and very tough (∼6.7(3) MPa m1/2) composite. And second, it is shown that these two properties and its proneness to develop an oxide tribolayer make this composite very resistant to unlubricated sliding wear against diamond (∼2.8(1)·108 (N m)/mm3), undergoing only very mild two‐body abrasion. Finally, opportunities for the fabrication of toughened and very hard ceramic composites for contact‐mechanical and tribological applications are discussed.

Phase and microstructure formation during reactive spark plasma sintering of AlxCoCrFeNi (x=0.3 and 1) high entropy alloys

This paper is one of the first studies focusing on phase and microstructure formation mechanisms during reactive sintering of AlxCoCrFeNi (x = 0.3, 1) high entropy alloys (HEAs). The alloys have been prepared by mechanical activation followed by spark plasma sintering (SPS). Both powders are subjected to gluing to the milling vials and the phenomenon intensifies with Al content, resulting in a more important mechanical alloying delay for the equiatomic composition. The temperature influence on phase and microstructure formation was investigated by SPS from 550 to 1100 °C. Al0.3CoCrFeNi reactive sintering led to a face-centered cubic (FCC) solid solution with a grain size around hundreds of nanometers with nanometric intergranular B2 precipitates which content and size reduced with the temperature increase. Grain growth was observed at prior particle boundaries due to dynamic recrystallization, a phenomenon that can be controlled through modification of the milling and sintering steps, leading thus to tailored grain sizes. For the AlCoCrFeNi composition, temperature increase favors secondary FCC formation and leads to sigma phase precipitation. A dual homogeneous-heterogeneous microstructure is obtained due to the important gluing leading to insufficient milling. The dynamic recrystallization at prior particle boundaries promotes also body-centered cubic (BCC) to FCC phase transition. Therefore, the proposed reactive sintering route allows to obtain, for the equiatomic composition, alloys exhibiting very fine microstructures, with higher FCC fraction content than by conventional metallurgy processes.

Correlative characterization of plasma etching resistance of various aluminum garnets

AbstractPlasma etching is a crucial step in semiconductor manufacturing. High cleanliness and wafer‐to‐wafer reproducibility in the etching chamber are essential in order to successfully achieve nanometer‐sized integrated functions on the wafer. The trend toward the application of more aggressive plasma compositions leads to higher demands on the plasma resistance of the materials used in the etching chamber. Due to its excellent etch resistance, yttrium aluminum garnet Y3Al5O12 (YAG) is starting to replace established materials like SiO2 or Al2O3 in this kind of application. In this study, reactive spark plasma sintering (SPS) was used to manufacture highly dense YAG ceramics from the respective oxides. In addition, yttrium was replaced with heavier lanthanoids (Er, Lu), intending to investigate the role of the A‐site cation in the garnet type structure on the plasma erosion behavior. The produced materials were exposed to fluorine‐based etching plasmas mimicking the conditions in the semiconductor manufacturing apparatus and the erosion behavior was characterized by atomic force microscopy (AFM), secondary ion mass spectrometry (SIMS), transmission electron microscopy (TEM), and profilometry. The induced chemical gradient in the samples is limited to a few nanometers below the surface, which makes its characterization challenging. For advanced analysis, we developed a correlative characterization method combining SIMS and scanning TEM (STEM)–energy‐dispersive spectroscopy (EDS) enabling us to examine the structural and chemical changes in the reaction layer locally resolved. In the case of lanthanoid aluminates, an altered reaction layer and reduced fluorine penetration compared to YAG were found. However, a correlation between the characteristics of the induced chemical gradient and the determined physical erosion rates was not evident.

On the potential of Transparent Rare-Earth-Free ZnAl2O4 Ceramics targeted at the UV-C to UV-B emission

Materials capable of emitting in the ultraviolet UV-C to UV-B spectral range have received considerable attention due to their potential applications in biophotonics, plant lighting, optical cleaning, sterilization, and disinfection. However, progress in the development of phosphors that emit in this short-wavelength range has been relatively sluggish. Most UV-C phosphors developed to date are based on hosts doped with the rare-earth cation Pr3+ due to its bright UV luminescence originating from the 4f15d1 → 4f2 interconfigurational transitions. In this study, we explore how the UV emission can be tailored to different wavelength ranges by developing rare-earth-free materials. Specifically, we demonstrate that UV-C to UV-B emission can be achieved in transparent ZnAl2O4 – based ceramics. For the first time, such transparent ceramics have been successfully produced with the help of reactive spark plasma sintering with SiO2 or LiF additives, which modified the process of grain growth resulting in an increase in the average grain size from 250 nm to 4 µm. Moreover, these additives also improved the optical transmission properties, with in-line transmission measurements at a wavelength of 550 nm reaching values of 50 % and nearly 65 % in the infrared region. Depending on the addition of SiO2 and LiF, the excitation by photons with energy exceeding 6.2 eV has generated the deep UV emission peaks in the range from 230 to 300 nm. Based on the analysis of the results of luminescence spectroscopy and EPR studies, the observed UV-C emission is suggested to originate from the excitons localised near oxygen vacancies. The number of vacancies is the highest in undoped ZnAl2O4 ceramics, whereas adding SiO2 and LiF suppresses their number and modifies their local environment due to changes in charge compensation, which in turn allows tuning the spectral position of the UV-C band. The quantum yield of the UV-C emission reached 5 % for undoped ZnAl2O4 ceramics. The sintered ZnAl2O4 transparent ceramic shows a versatile solution for next-generation rare-earth-free deep-UV devices.

The Microstructure, Mechanical, and Friction-Wear Properties of Boron Carbide-Based Composites with TiB2 and SiC Formed In Situ by Reactive Spark Plasma Sintering.

The paper presents the influence of the temperature of the sintering process on the microstructure and selected properties of boron carbide/TiB2/SiC composites obtained in situ by spark plasma sintering (SPS). The homogeneous mixture of boron carbide and 5% vol. Ti5Si3 micropowders were used as the initial material. Spark plasma sintering was conducted at 1700 °C, 1800 °C, and 1900 °C for 10 min after the initial pressing at 35 MPa. The heating and cooling rate was 200 °C/min. The obtained boron carbide composites were subjected to density measurement, an analysis of the chemical and phase composition, microstructure examination, and dry friction-wear tests in ball-on-disc geometry using WC as a counterpart material. The phase compositions of the produced composites differed from the composition of the initial powder mixture. Instead of titanium silicide, two new phases appeared: TiB2 and SiC. The complete disappearance of Ti5Si3 was accompanied by a decrease in the boron carbide content of the stoichiometry formula B13C2 and an increase in the content of TiB2, while the SiC content was almost constant. The relative density of the obtained boron carbide composites, as well as their hardness and resistance to wear, increased with the sintering temperature and TiB2 content. Unfortunately, the reactions occurring during sintering did not allow us to obtain composites with high density and hardness. The relative density was 76-85.2% of the theoretical one, while the Vickers hardness was in the range of 4-12 GPa. The mechanism wear of boron carbide composites tested in friction contact with WC was abrasive. The volumetric wear rate (Wv) of composites decreased with increasing sintering temperature and TiB2 content. The average value of coefficient of friction (CoF) was in the range of 0.54-0.61, i.e., it did not differ significantly from the value for B4C sinters.

Dual plate-like grains of (Ti, Zr)B2 and W2B5 toughened solid solution composites fabricated via in-situ reaction spark plasma sintering of (Ti, W)C and ZrB2 powders

Novel dual plate-like grains of (Ti, Zr)B2 and W2B5 toughened solid solution composites were fabricated by the in-situ reaction spark plasma sintering at 1800 °C using (Ti, W)C and ZrB2 powders. The composites were predominated and composed of the (Zr, Ti, W)C solid solutions and plate-like (Ti, Zr)B2. In the (Ti, W)C-ZrB2 reaction system, W promotes the anisotropic growth of (Ti, Zr)B2 grains, while Ti makes the formation reactions of WxBy thermodynamically feasible. When the addition of ZrB2 exceeds 40 mol%, W begins to be boronized into WxBy which is mainly composed of plate-like W2B5. The (Ti, W)C-50 mol% ZrB2 composite with dual plate-like grains of (Ti, Zr)B2 and W2B5 exhibited the best comprehensive mechanical properties of hardness of 26.5 GPa, flexural strength of 620 MPa, and fracture toughness of 6.6 MPa·m1/2, which provide a new approach for the enhanced toughness without the introducing of metal impurities.

B4C ceramics with increased dislocation density fabricated by reactive spark plasma sintering via carbon nanotubes–boron mixture

Boron carbide with high relative density was prepared in situ using spark plasma sintering. Research on the in-situ reaction of boron and carbon nanotubes reveals that the reaction exhibits high reactivity at 1200 °C. Beyond this temperature, increasing the external pressure was beneficial for particle rearrangement, gas release, and dislocation formation. The result of mechanical properties shows that, the flexural strength and fracture toughness of the boron carbides are 656 ± 9 MPa and 5.0 ± 0.3 MPa m1/2, respectively, when sintered at 1750 °C under continuous pressure ramping conditions–an increase of 64.1% and 46.6% compared to that prepared under constant pressure conditions. Increasing the magnitude of the external stress during the in-situ reaction process can also enhance the particle rearrangement efficiency, inducing the formation of twinning and stacking faults in boron carbide ceramics. High–density twinning and stacking faults dominated the toughening mechanisms of ceramic materials.

High-entropy boride-carbide based composite prepared by reactive spark plasma sintering

(Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C-(Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)B2-SiC (HEBC-SiC) and (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)B2-SiC (HEB-SiC) high-entropy boride-carbide composites were prepared by reactive spark plasma sintering with high-entropy carbides, B4C and Si as starting materials. Compared to monolithic (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C ceramics, both HEBC-SiC and HEB-SiC samples exhibited lower porosity and finer grain size due to mutual phase pinning effect. Also, the HEBC-SiC sample possessed superior mechanical properties compared to (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C ceramics, which demonstrated a hardness of 24.95 GPa (vs 20.18 GPa) and fracture toughness of 4.16 MPa·m1/2 (vs 2.94 MPa·m1/2). The present study provided a new approach to obtain dense (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C-(Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)B2-SiC composites with enhanced mechanical properties.

Structural and mechanical optimization of porous alumina structures fabricated by carbon sacrificial template

The Taguchi method is used to optimize the manufacture of porous alumina made through reactive spark plasma sintering and carbon sacrificial template. The goal is to design a new versatile procedure that allows the fabrication of porous alumina with tailored physical properties. The structural and mechanical properties taken as target parameters were the subtle combination of porosity and Young’s modulus of the human cortical bone: typical pore size ¿100μm, and Young’s modulus in the range of 3–30 GPa. The input factors of the Taguchi method are wt.% of carbon, sintering time, calcination heating rate, and final heat treatment. Hg porosimetry, electron microscopy, uniaxial compression and computer aided tomography were used for the characterization of the porosity, pore size distribution, pore interconnectivity, and Young’s modulus. Finally, according to the conclusions of the Taguchi analysis, the parameters of the process were changed for the fabrication of the new samples with optimized properties. Highly porous structures with 90% interconnectivity, Young modulus of 5.5 ± 1.1 GPa, and compression strength of 49 ± 20 MPa, were obtained, successfully emulating the targeted properties.

In-situ formation of CrB minor phase during reactive spark plasma sintering leads to the enhancement in the electrical transport performance of boron doped chromium disilicide

The present study focuses on the synergistic effect of substitution and in-situ composite formation of CrB in B-doped CrSi2. The samples of CrSi2.05-xBx have been synthesized in a single step employing reactive spark plasma sintering process (SPS) at 1423 K and 60 MPa. The diffusion process accelerates at high temperature and pressure and leads to the formation of the CrSi2 major phase and CrB minor phase. The X-ray diffraction technique was used to determine the phase purity of the synthesized samples. A field emission scanning electron microscope was used to examine the surface morphology of the CrSi1.85B0.20 sample. The elemental mapping confirms the presence of the CrB phase in the CrSi2 matrix. A significant enhanced power factor ≃ 1.95 × 10−3 W/mK2 at 473 K for CrSi1.85B0.20 was observed, which is primarily due to the formation of in-situ secondary metallic CrB phase which leads to the enhancement in the electrical conductivity. Further, the Vickers microhardness was also determined which is increasing with increasing B-concentration and exhibits a maximum value of ≃ 13 GPa for CrSi1.85B0.20. To comprehend the underlying physics of the enhancement of electronic transport characteristics of B-doped CrSi2, we employed density functional theory to calculate the projected density of states, electronic transport properties, and electronic band structure of B-doped CrSi2 at different temperatures and B-concentrations.

High-Entropy Diborides-Silicon Carbide Composites by Reactive and Non-Reactive Spark Plasma Sintering: A Comparative Study.

The reactive spark plasma sintering (R-SPS) method was compared in this work with the two-step SHS-SPS route, based on the combination of the self-propagating high-temperature synthesis (SHS) with the SPS process, for the fabrication of dense (Hf0.2Mo0.2Ti0.2Ta0.2Nb0.2)B2-SiC and (Hf0.2Mo0.2Ti0.2Ta0.2Zr0.2)B2-SiC ceramics. A multiphase and inhomogeneous product, containing various borides, was obtained at 2000 °C/20 min by R-SPS from transition metals, B4C, and Si. In contrast, if the same precursors were first reacted by SHS and then processed by SPS under the optimized condition of 1800 °C/20 min, the desired ceramics were successfully attained. The resulting sintered samples possessed relative densities above 97% and displayed uniform microstructures with residual oxide content <2.4 wt.%. The presence of SiC made the sintering temperature milder, i.e., 150 °C below that needed by the corresponding additive-free system. The fracture toughness was also markedly improved, particularly when considering the Nb-containing system processed at 1800 °C/20 min, whereas the fracture toughness progressively decreased (from 7.35 to 5.36 MPa m1/2) as the SPS conditions became more severe. SiC addition was found to inhibit the volatilization of metal oxides like MoO3 formed during oxidation experiments, thus avoiding mass loss in the ceramics. The benefits above also likely took advantage of the fact that the two composite constituents were synthesized in parallel, according to the SHS-SPS approach, rather than being produced separately and combined subsequently, so that strong interfaces between them were formed.

Novel (Ti, W)C–SiC–WSi2 ceramics fabricated via in situ reaction spark plasma sintering at 1600 °C

Novel (Ti, W)C–SiC–WSi2 ceramics were fabricated by in situ reaction spark plasma sintering (SPS) at 1600 °C using (Ti, W)C and Si powders. The effect of Si content on densification behavior, microstructures, and mechanical properties was investigated. The in situ reaction and the formation of carbon vacancies reduced the sintering temperature. The densification mechanism of (Ti, W)C-based ceramics transforms from reaction to grain-boundary sliding and diffusion. The WSi2 and primary SiC generated by in situ reactions are distributed along the grain boundaries and have a specific crystallographic orientation relationship. Intragranular secondary SiC formed because of rapid densification, grain-boundary diffusion, and (Ti, W1−y)C1−x particle migration. This multiscale particle-reinforced microstructure can improve the mechanical properties of (Ti, W)C ceramics, giving the TW10S (4.80 wt% SiC–5.01 wt% WSi2) good sintering performance, a fine-grained microstructure, high hardness (23.3 ± 1.6 GPa), and flexural strength (425 ± 24 MPa). The SiC and WSi2 produced through in situ reactions have exceptional intrinsic oxidation resistance, allowing the ceramics to achieve outstanding oxidation resistance.

Reactive spark plasma sintering of Si2N2O ceramic: Reaction process, microstructure, mechanical and dielectric properties

Dense Si2N2O ceramic was successfully prepared through the spark plasma sintering method at 1750 °C with nano-sized amorphous Si3N4 powder as raw material. The reaction sintering process, mechanical and dielectric properties of the Si2N2O ceramic were investigated in detail. The results revealed that the amorphous Si3N4 was initially crystallized to β-Si3N4, then oxidized to Si2N2O during the sintering process. With prolonging the dwell time, the grain orientation of Si2N2O gradually developed to the c-axis direction of the orthorhombic crystal. Meanwhile, the density of as-sintered ceramic was improved significantly, which greatly influenced the mechanical and dielectric properties of Si2N2O ceramic. In this work, the optimal dwell time of sintering process was 20 min, which is obviously shorter than the ordinary sintering methods. The Si2N2O ceramic fabricated at the above conditions exhibited the most favorable mechanical and dielectric properties with a bulk density of 2.80 ± 0.01 g cm−3, Vickers hardness of 13.1 ± 0.1 GPa, flexural strength of 214.1 ± 15.8 MPa, fracture toughness of 2.8 ± 0.3 MPa m1/2, and combined with low dielectric constant of around 3.5 to 4.5 at 8.2−12.4 GHz. Thus, the spark plasma sintering method is potentially regarded as a rapid way to prepare Si2N2O ceramic used as the structural and wave-transmitting functional material.

Microstructure, mechanical properties and ultra-high temperature thermal stability of GdB6 ceramics fabricated by reactive spark plasma sintering

In this study, a highly dense GdB6 bulk ceramic material was fabricated using one-step in-situ reaction spark plasma sintering technology. The sample with a relative density of 96.7 % and a purity of 96.9 wt% was successfully obtained by sintering at 1800 °C for 30 min under a uniaxial pressure of 50 MPa. The Vickers hardness, elastic modulus and fracture toughness of the as-obtained GdB6 bulk ceramics were 18.9 GPa, 209 GPa and 1.6 MPa·m1/2, respectively. The thermal conductivity of GdB6 at room temperature was 28.68 W·m−1·K−1, 70 % of which was mainly attributed to phonon vibration, while the remaining 30 % to the electrical thermal conductivity. In addition, the as-obtained GdB6 ceramics showed no phase transformation after heat treatment at a relatively high temperature of 2100 °C for 20 min in Ar atmosphere. The highly covalent octahedral boron framework played a critical role in maintaining the phase stability of the GdB6 crystal lattice. This suggested that GdB6 ceramics could be used as potential ultra-high temperature ceramics for aerospace applications and high-power vacuum electronic devices operated at high temperatures.

Reactive spark plasma sintering of an electrically conductive B4C-SiC-ZrB2 composite with enhanced mechanical properties

Ternary B4C-SiC-ZrB2 ceramics with eutectic compositions were prepared in situ by reactive spark plasma sintering at 1600–1800 °C. The two-step sintering, including an in situ reaction 2ZrSi2 + 5B4C+ 3 C= 4B4C+ 4SiC+ 2ZrB2 at 1600 °C and sintering at 1800 °C, is superior to combined one-step sintering at 1800 °C. The as-sintered 40B4C-40SiC-20ZrB2 composites (sample 16R18) had grain sizes of microns with a density over 97%. The Vickers hardness, bending strength and fracture toughness of the ternary ceramic reached 28.6 GPa, 631 MPa and 5.50 MPa m1/2 respectively. Crack deflection and branching were ascribed to improved fracture toughness. The elongated plate-like ZrB2 grains not only had toughening effects but also helped to lower the percolation threshold of the electrical conductivity. The 16R18 sample possessed an electrical conductivity of 5.38 × 104 S/m, owing to the tiny conductive ZrB2 grains with aspect ratios of 3–4, which is beneficial for the electro-discharge machining of the B4C-SiC based composites.

Reactive spark plasma sintering of high-entropy (La1/7Nd1/7Sm1/7Eu1/7Gd1/7Dy1/7Ho1/7)2Zr2O7 pyrochlore ceramic

In this work, an efficient method for the synthesis of high-entropy pyrochlore ceramic (La1/7Nd1/7Sm1/7Eu1/7Gd1/7Dy1/7Ho1/7)2Zr2O7 (7RE2Zr2O7) was developed via reactive spark plasma sintering. The sintering reactivity, phase composition, microstructure, and mechanical properties of 7RE2Zr2O7 were systematically investigated. With the aid of uniaxial pressure and direct current (DC) pulse voltage, the entire sintering process was greatly shortened to less than 1 h. Owing to the synergistic effects of the compositional complexity and severe lattice distortion, 7RE2Zr2O7 exhibited a reduced average grain size of only 1.45 μm, which is approximately 1/6 of that for Gd2Zr2O7 prepared using the same sintering process. Furthermore, 7RE2Zr2O7 possessed a high hardness of up to 13.86 GPa, while a low fracture toughness of 1.1 MPa m1/2, which can be mainly attributed to the severe internal stress and defects originated from the fast-sintering process. This work is expected to provide a guidance for the highly efficient and rapid preparation of high-entropy A2B2O7 oxides, especially for those compositions that are difficult to densify.

Perovskite/Pyrochlore Composite Mineral-like Ceramic Fabrication for 90Sr/90Y Immobilization Using SPS-RS Technique

A novel solid-phase synthetic approach was developed to produce a mineral-like composite ceramic based on strontium titanate (SrTiO3) and yttrium titanate (Y2Ti2O7) matrices for immobilizing radionuclides such as 90Sr and its daughter product 90Y, as well as lanthanides and actinides, via reactive spark plasma sintering technology (SPS-RS). Using XRD, SEM, and EDS analyses, the sintering kinetics of the initial mixed oxide reactants of composition YxSr1–1.5xTiO3 (x = 0.2, 0.4, 0.6 and 1) and structure-phase changes in the ceramics under SPS-RS conditions were investigated as a function of Y3+ content. In addition, a detailed study of phase transformation kinetics over time as a function of the heating temperature of the initial components (SrCO3, TiO2, and Y2O3) was conducted via in situ synchrotron XRD heating experiments. The composite ceramic achieved relatively high physicomechanical properties, including relative density between 4.92–4.64 g/cm3, Vickers microhardness of 500–800 HV, and compressive strength ranging from 95.5–272.4 MPa. An evaluation of hydrolytic stability and leaching rates of Sr2+ and Y3+ from the matrices was performed, demonstrating rates did not exceed 10−5–10−6 g·cm−2·day−1 in compliance with GOST R 50926-96 and ANSI/ANS 16.1 standards. The leaching mechanism of these components was studied, including the calculation of solution penetration depth in the ceramic bulk and ion diffusion coefficients in the solution. These findings show great promise for radioactive waste conditioning technologies and the manufacturing of radioisotope products.

Sintering mechanism of TiB2 reinforced Ti(C,N)-based cermet during reaction spark plasma sintering

TiB2 reinforced Ti(C,N)-based cermet was produced by the reaction spark plasma sintering of Co, Ni, Ti, C, BN, Mo2C, WC, and TaC mixtures. To understand the mechanism of reaction sintering, systematic investigation on the microstructure evolution and displacement variation rule was carried out. In addition, the influence of C/BN molar ratio on the microstructures and performances of cermet was investigated. Except an initially increased pressure, three densification patterns were found. The first method was the CoTi, TiNi, and Ti-BN diffusion reactions, resulting in a shrinkage of 22.06%. The second pattern was the viscous flow and dissolution-reprecipitation, which corresponded to the synthesis of Ti(C,N) and TiB2. In this process, the shrinkage ratio was increased by 35.62%. The final densification mainly originated from the viscous flow of metallic binder. It contributed to the transformation of macrovoids into microvoids, and an increased displacement of 30.10%. With the increasing of C/BN ratio, more C atoms diffused into Ti(C,N). Furthermore, core/rim structure-Ti(C,N) and coreless (W,Mo,Ta,Ti)(C,N) were fabricated. However, the microvoids in cermets enhanced. As a result, the hardness and fracture toughness first increased and then decreased. Results from this work are helpful to understand the reaction spark plasma sintering technique, and can enrich the preparation method of cermet.