Reaction Sintering Research Articles

Reactive sintered dense carbon fiber reinforced high‐entropy boride composite using high‐entropy silicide as reactant

AbstractA novel reactive sintering strategy using high‐entropy disilicide, B4C, and carbon as initial materials is developed to fabricate dense carbon fiber reinforced high‐entropy diboride (HEB)‐based composite (Cf/HEBs) at a relatively low temperature. The Cf/(V0.2Nb0.2Cr0.2Mo0.2W0.2)B2–SiC composite (Cf/HEB–SiC) successfully achieves nearly full densification (with a relative density of 99.2%) at 1800°C. The reactive damage of carbon fibers can be effectively restrained by preassemble carbon coating during the preparation process of the composite. Lower preparation temperature and effective coating protection contribute to the exertion of carbon fibers toughening capacity, consequently noticeably elevating the critical crack size (αcr) from 27.7 µm for HEB–SiC to 110.4 µm for Cf/HEB–SiC. The current work provides a feasible way to substantially upgrade the damage tolerance of HEB–SiC and can be extended to other HEBs‐based ceramics.

Reactive spark plasma sintering and luminescence properties of Gd2O2S:Tb nanocrystalline phosphor

Gd2O2S:Tb is promising for energy-efficient lighting and display applications. This work explores the use of reactive spark plasma sintering (SPS) to synthesis Gd2O2S:Tb nanocrystalline phosphor. It allows to significantly simplify the technological process in comparison with traditional multi-stage liquid-phase synthesis. As-sintered Gd2O2S:Tb demonstrated average crystallite size of 26 nm and secondary particle size of 26 μm. Bright green emission occurs at 545 nm under UV excitation of 245 nm. Average fluorescence lifetime was 0.656 ms. Further consolidation of the Gd2O2S:Tb prepared via reactive SPS can also be expected to yield highly dense ceramics. The innovative reactive synthesis strategies provide valuable insights into the design and development of high-performance luminescent ceramics.

Reversible multimode modulations in photochromic transparent Eu3+doped K0.5Na0.5NbO3 based ceramics

Rare-earth doped transparent photochromic materials have attracted extensive attention in applications of optical information storage and optical switching, in which the high transparency and large fluorescence modulation ratio are required. In this work, 0.94 (K0.5Na0.5)NbO3-0.06 (Sr0.5Ba0.5) (Zn1/3Bi2/3)O3: x wt% Eu2O3 ceramics (KNN-SBZB: xEu) with x=0.1, 0.2, 0.3, 0.4 were prepared by traditional high temperature solid state sintering reaction method. High transmittance (T) was achieved due to the small grain size (less than 110 nm) and the highly symmetrical phase structure. High optical transmittance of 69.27 % at 780 nm and 78.24 % at 1100 nm were obtained in the KNN-SBZB: 0.3Eu ceramic, while large luminescence intensity modulation ratio (∆RL) ∼94.9 %, transmittance (∆RT) ∼5.76 % and diffuse reflectivity (∆ABS) ∼12.59 % were achieved in KNN-SBZB: 0.2Eu ceramic (T ∼62.92 % at 780 nm). Additionally, both the transmittance modulation and luminescence intensity modulation of KNN-SBZB: xEu ceramics show good stability after 10 cycles.

The influence of CeO2 on the oxidation resistance of TaC/Ni composites

The in-situ TaC/Ni composites with different CeO2 content (0 wt%, 1 wt%, 2 wt%) were prepared by reactive sintering method, and their oxidation behavior in air at 1073 K was analyzed by static cyclic oxidation method to investigate the influence mechanism of CeO2 on the oxidation resistance of composites. The results show that the CeO2 particles are distributed around the TaC particles in the Ni matrix, and the oxidation behavior of composites with different CeO2 content conforms to the linear kinetic law. The oxide film is mainly composed of NiTa2O6 and a small amount of NiO. The generation reactions of Ta2O5 and NiTa2O6 induce a high expansion and compressive stress in oxide scale, leading to the formation of nonprotective oxide film with pores and cracks on the surface. As the addition of CeO2 increases from 0 wt% to 2 wt%, the thickness of oxide scale on composites obviously reduces from 505 μm to 122 μm, and the Ni and Ta oxides content decreases, leading to the reduction of pores and cracks in oxide scale. The Ce ion generated from the dissolution of CeO2 particles mainly distributes in the Ta oxides during the oxidation process, which hinders the outward diffusion of Ni and Ta ions, resulted in the improved oxidation resistance of TaC/Ni composites.

Effect of Eu Ions Concentration in Y2O3-Based Transparent Ceramics on the Electron Irradiation Induced Luminescence and Damage

Eu3+-doped Y2O3-based luminescent materials can be used as a scintillator for electron or high energy β-ray irradiation, which are essential for applications such as electron microscopy and nuclear batteries. Therefore, it is essential to understand their defect mechanisms and to develop materials with excellent properties. In this paper, Y2O3-based transparent ceramics with different Eu3+ doping concentrations were prepared by solid-state reactive vacuum sintering. This series of transparent ceramic samples exhibits strong red emission under electron beam excitation at the keV level. However, color change appears after the high-energy electron irradiation due to the capture of electrons by the traps in the Y2O3 lattice. Optical transmittance, laser-excited luminescence, X-ray photoelectron spectroscopy (XPS), and other analyses indicated that the traps, or the color change, mainly originate from the residual oxygen vacancies, which can be suppressed by high Eu doping. Seen from the cathodoluminescence (CL) spectra, higher doping concentrations of Eu3+ showed stronger resistance to electron irradiation damage, but also resulted in lower emissions due to concentration quenching. Therefore, 10% doping of Eu was selected in this work to keep the high emission intensity and strong radiation resistance both. This work helps to enhance the understanding of defect formation mechanisms in the Y2O3 matrix and will be of benefit for the modification of scintillation properties for functional materials systems.

PMMA light channels facilitate the additive manufacturing of complex-structured SiC reflector mirrors

The rapid manufacturing of lightweight SiC reflector mirrors has always posed a formidable challenge. We have successfully solved the problem of SiC slurry’s difficulty in vat photopolymerization by using highly transparent organic particles to construct the light channels for the first time. Through the integration of digital light processing with reaction sintering, we manufactured complex-structured SiC/Si reflector mirrors. Utilizing the light channels constructed with PMMA, ultraviolet light can effectively penetrate SiC slurry, thereby significantly enhancing the curing depth. During subsequent reaction sintering, PMMA particles induce the formation of numerous spherical silicon through dissolution-precipitation mechanism, alleviating stress concentration in the matrix and enhancing the flexural strength of SiC/Si ceramics. The method achieves significantly higher curing depth for SiC slurry and greater flexural strength for ceramic compared to similar materials. Extending the success above further, we have demonstrated their potential as high-quality products for practical applications.

Effect of Cr content on the high temperature oxidation behavior of FeCoNiMnCrx porous high-entropy alloys

FeCoNiMnCr porous high-entropy alloys were prepared using the activation reaction sintering method with Fe, Co, Ni, Mn, and Cr as raw materials. The oxidation behaviors of FeCoNiMnCrx porous high-entropy alloys with different Cr contents, such as pore structure, the surface oxide film phase composition, structure, and morphology, were characterized by X-ray diffraction (XRD), electron probe micro analysis (EPMA), energy dispersive X-ray spectroscopy (EDS), and pore tester after high-temperature oxidation at 800–1000 °C. The results indicate that the initial porous high-entropy alloy comprises a single FCC solid solution phase. With the increase in Cr content, the average pore diameter, average porosity, and air permeability of the porous high-entropy alloy also increase. The oxidation kinetics of the porous high-entropy alloy after exposure to temperatures between 800 °C and 1000 °C follows a single-stage parabolic evolution law. At the same oxidation temperature, With the increase of Cr content, the oxidation gains gradually increased, at the same Cr content at different temperatures, the antioxidant performance of 1000 °C was the weakest, followed by 800 °C, and 900 °C showed excellent antioxidant performance. The oxidation products are mainly Mn2O3, Mn3O4, (Mn, Cr)3O4, Cr2O3and Fe2O3.

Microstructure and Mechanical Properties of Diamond–Ceramic Composites Fabricated via Reactive Spark Plasma Sintering

In order to prepare diamond composites with excellent mechanical properties under non-extreme conditions, in this study, a diamond–ceramic composite was successfully prepared via reactive spark plasma sintering using a diamond–Ti–Si powder mixture as the raw material. The microstructures and mechanical properties of the diamond–ceramic composite sintered at different temperatures were studied. When the sintering temperature was 1500 °C, the diamond–ceramic composite exhibited a volume density of 3.65 g/cm3, whereas the bending strength and fracture toughness were high at 366 MPa and 6.17 MPa·m1/2, respectively. In addition, variable-temperature sintering activated the chemical reaction at a higher temperature, whereas lowering the temperature prevented excessive graphitisation, which is conducive to optimising the microstructure and mechanical properties of the composite.

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.

Effect of processing parameters and carbon to metal ratio on microstructure and mechanical properties of (MoTaTiVW)Cx high entropy carbide produced by reactive spark plasma sintering

(MoTaTiVW)Cx high entropy ceramics with varying carbon to metal ratio in the range of 0.75-0.97 were synthesized by by reactive spark plasma sintering. The effect of ball milling time on particle size, phase evolution and carbon pick-up due to toluene decomposition were studied. Formation of single-phase high entropy carbide having face-centered cubic (FCC) crystal structure was confirmed by X-ray diffraction (XRD), electron backscattered diffraction (EBSD) and Selected area diffraction pattern (SAED) analysis of the sintered compacts. Transmission Kikuchi Diffraction images revealed presence of equiaxed nature of the grains. Transmission electron microscopy images revealed the presence of an intergranular phase at grain boundaries and triple junctions. 3D-Atom Probe Tomography studies showed uniform distribution of elements within the grains with some segregation of impurity elements Fe and Co to the grain boundaries indicating minor contribution of liquid phase sintering to densification. The grain size of single-phase sintered compacts ranged between 1.69 ± 0.60 μm to 4.6 ± 1.0 μm. A low sintering temperature and higher milling time resulted in finer grain size The microhardness was found to increase with milling time and C/M ratio. The microhardness, Young’s modulus and indentation fracture toughness lied between 2221 HV0.1 to 2732 HV0.1, 370 GPa to 473 GPa, 3.6 MPa.m1/2 to 4.4 MPa.m1/2, respectively. The highest microhardness and indentation fracture toughness was found to be 2732 ± 80 HV0.1 and 4.4 MPa.m1/2 on par with reported literature for other high entropy carbides.

Rapid fabrication of Ba1–xSrxTiO3 ceramics via reactive flash sintering

Ba1–xSrxTiO3 (BST) ceramics are promising dielectric materials. However, their fabrication is usually laborious, requiring long preparation stages and high temperatures, which are not favorable for tuning the grain size and dielectric properties. In this study, (Ba1–xSrx)TiO3 (x = 0.1, 0.3, 0.5) ceramics were produced via reactive flash sintering (RFS) of a mixture of BaCO3, SrCO3 and TiO2 powders at 900 °C and relatively short times (15–90 s). The electric field (E-field) with a strength of 400 V/cm and the current densities of 25–75 mA mm−2 were applied during RFS. Once the Sr2+ content increased, the incubation time for the RFS decreased, suggesting that the addition of Sr2+ facilitated the RFS process. At the RFS time above 15 s, a single-phase Ba0.6Sr0.4TiO3 perovskite structure was formed. When the time and the applied current density increased, both the densities and grain sizes within the ceramics increased. This increased the real part (ε’) of the complex permittivity compared to that of the conventionally sintered (CS) ceramic. Moreover, the enhancement of mass transport by E-field-induced oxygen vacancies was considered the predominant mechanism of RFS in the production of BST ceramics.

Sintering activation energy and low-temperature sinterable process of Boron-lanthanide glass/Li2Zn3Ti4O12 ceramic composite systems for LTCC technology

This work investigates the characteristics of wettability, activation energy for sintering, evolution of phase structure, and behavior during the low-temperature sintering process of B2O3-La2O3-MgO-TiO2 (BLMT) glass/Li2Zn3Ti4O12 ceramic composites for low temperature co-fried ceramics technology (LTCC). As the BLMT glass content increases from 0 to 10 wt%, the average sintering activation energy (Ea) of the composite decreases from 450 ± 8 to 356 ± 37 kJ/mol. There are two phases the primary Li2Zn3Ti4O12 phase and the secondary LaBO3 phase, the latter precipitates from the BLMT glass after sintering at 700 °C. This indicates that BLMT glass promotes the sintering of Li2Zn3Ti4O12 ceramic below 900 °C. In composites containing 20 wt% or more glass, sintering is governed by both liquid-phase sintering and reactive viscous sintering. As the BLMT glass content increases, the impact of viscous flow on the densification process becomes more pronounced. Simultaneously, when the glass content is greater than 20 wt%, the glass precipitates LaBO3, TiO2, and MgLaB5O10 phases between 700 and 750 °C. After 850 °C, the glass melts and reacts with Li2Zn3Ti4O12 phase to produce a new TiO2 phase. Accordingly, the activation energy (Ea) of sintering the composite material increases from 393 ± 17 to 667 ± 1 kJ/mol.

Structural, dielectric, impedance, ferroelectric, piezoelectric, energy harvesting, and optical properties of Ca1-xZnx(Fe0.5V0.5)O3 ceramics

The present work mainly describes the fabrication and characterization (structural, micro-structural, dielectric, piezoelectric, optical, and energy-harvesting) of Ca1-xZnx(Fe0.5V0.5)O3 with x = 0, 0.05, 0.10, and 0.15 complex perovskite. All four compounds are prepared by using the solid-state reaction method (calcination temperature = 1100 °C (6 h) and sintering temperature = 1150 °C (7 h)). The room temperature XRD data analysis confirms all four compounds' single-phase orthorhombic structures. Different modes of vibrations are confirmed by Raman spectroscopy. The grain size increases from 1.09 μm to 2.53 μm with increased zinc concentration. The variation of dielectric constant with frequency has been explained based on Maxwell-Wagner polarization. The effects of grain and grain boundary have been confirmed by impedance spectroscopy, which contributes to the conduction and relaxation mechanism of the compounds. The d33, g33, and energy harvesting performance gradually increase with an increase in zinc concentration, confirming the enhancement of the compounds' piezoelectric properties. The decrease in the band gap from 3.58 eV (x = 0) to 3.00 eV (x = 0.15) confirms the enhancement of optical properties.

Reaction-sintered highly transparent MgGa2O4 ceramics with enhanced dielectric properties

Reaction sintering of MgO and Ga2O3 powders was first used to prepare MgGa2O4 transparent ceramics. The microstructural evolution suggested that the negligible volume change resulting from the solid-phase reaction of MgO and Ga2O3, as well as the close and homogeneous arrangement of both fine particles, are the key factors in obtaining pre-sintered ceramics with an ideal microstructure at lower temperature. After a hot isostatic pressing (HIP) treatment, the fully dense ceramic exhibited improved in-transmittance (74.7%@600nm, and 85.4%@4401nm) and quality factor (Q×f=168000GHz). Due to the combination of high transparency, wide transmission range (6.22 μm at the 60% transmittance), ultra-low dielectric loss, and near-zero temperature coefficient of resonance frequency (–3.9 ppm/◦C), transparent MgGa2O4 ceramic is suggested to be an ideal optical-dielectric integration material.

Thermal conductivity enhancement of epoxy by a 3D Si3N4 crystal rod/grain skeleton fabricating from photovoltaic silicon waste

Silicon nitride (Si3N4) is one of the preferred fillers for the preparation of high thermal conductivity polymer composites because of its intrinsic high thermal conductivity and good electrical insulation. To maximize the thermal conductivity enhancement effect of the fillers in the composites, it is necessary to pre-construct a three-dimensional (3D) thermally conductive filler network. Herein, surfactant foaming was applied to pre-construct a 3D Si-containing framework from photovoltaic silicon waste (PSW), which was subsequently transformed into a porous skeleton consisting of Si3N4 rods and grains by in-situ nitridation reaction sintering. And finally, thermal conductive Si3N4/epoxy (EP) composites were fabricated after infiltrating EP into the network skeleton. When the filler content of Si3N4 was 41.00 vol%, the obtained Si3N4/EP composite showed the highest thermal conductivity of 2.99 W·m-1·K-1, which was 13.95 times higher than that of pure EP. Tests on running CPU and heating table showed that the Si3N4/EP substrate had excellent heat dissipation performance compared to the pure EP substrate. The 3D continuous network formed by "bridging" between two different morphologies of Si3N4 microcrystals contributed to providing effective multi-directional heat transfer paths and thereby improving the thermal conductivity of the composites. Overall, the low cost of raw silicon source originating from industrial waste as well as the simple and practical construction of heat-conducting filler network demonstrate the promising application of the Si3N4/EP composites fabricated in our work.

Enhancement of strength and thermal shock resistance of MgO‐MgAl2O4 ceramic filters with microporous MgO powder: Effect of α‐Al2O3 micro‐powder content

AbstractIn this work, MgO‐MgAl2O4 ceramic filters were fabricated from microporous MgO powder and α‐Al2O3 micro‐powder, in which microporous MgO powder was prepared using Mg(OH)2 as initial raw material. With the α‐Al2O3 micro‐powder content increased from 0 to 15 wt%, the tight packing and reaction sintering between MgO microparticles and Al2O3 microparticles promoted the formation and growth of magnesium aluminate spinel necks, reduced the apparent porosity of ceramic filter skeletons, decreased the pore size, and improved the strength of the filters. When the α‐Al2O3 micro‐powder content further raised to 20 wt%, the thermal mismatch between MgO microparticles and spinel microparticles caused a small amount of crack generation, which lowered the strength of the filter. Besides, after introducing the α‐Al2O3 micro‐powder, more spinel necks were produced due to a small amount of MgO solid dissolved in the spinel lattice during the thermal shock test, which enhanced the thermal shock resistance of the filters. When the α‐Al2O3 micro‐powder content was 15 wt%, the filter had excellent comprehensive performance, the bulk density and apparent porosity of the filter skeleton were 3.09 g/cm3 and 10.7%, and the compressive strength before and after thermal shock test was 3.48 and 3.70 MPa, respectively.

Characterization of (Zr,Ti)B2-SiC composites obtained by hot press sintering of ZrB2-SiC-TiO2 powder mixtures

The ability to enhance mechanical and oxidation properties for severe environmental applications has led to substantial academic interest in multiphase ultra-high temperature ceramics (UHTC). The purpose of this work is to study the in-situ solid solution formation of (Zr,Ti)B2 from ZrB2 and TiO2 in a ZrB2-SiC composite using hot pressing reaction sintering. For this, a mixture of 10, 20, and 30 % vol% SiC with ZrB2 was mixed with 2.0 wt% TiO2. Hot pressing sintering was performed with a load of 20 MPa at a final temperature of 1850 °C/30 min in an argon atmosphere. The microstructures, crystalline phases, densities, mechanical properties, and oxidation resistance of the composites were examined and compared with ZrB2-SiC samples lacking TiO2. In samples where TiO2 was added, the matrix grain size slightly decreased, the fracture mode was mainly intergranular, and the SiC grain morphology changed the aspect ratio to be more equiaxed. The solid solution (Zr,Ti)B2 was produced, and it was demonstrated by EDS elemental map images and the XRD analysis that Ti atoms incorporate into the ZrB2 crystalline structure. The development of solid solutions showed no impact on relative densities or Vickers hardness. However, the solid solution formation favored an improvement in fracture toughness, probably owing to the smaller matrix grain size and intergranular fracture mode. Samples exhibiting (Zr,Ti)B2 formation presented lower oxidation resistance than undoped samples in the same oxidizing condition.

Enhanced High-Temperature Thermoelectric Performance in Te-Doped Electronegative Element-Filled Skutterudites via Suppressing Bipolar Effects and Enhanced Phonon Scattering.

CoSb3-based skutterudites have great potential as midtemperature thermoelectric (TE) materials due to their low cost and excellent electrical and mechanical properties. Their application, however, is limited by the high thermal conductivity and the degradation of TE performance at elevated temperatures, attributed to the adverse effects of bipolar diffusion. Herein, a series of SeyCo4Sb12-xTex compounds were successfully synthesized by combining a solid-state reaction and spark plasma sintering techniques to mitigate these challenges. It was found that doping Te at the Sb sites effectively enhanced the carrier concentration and suppressed the bipolar effect to obtain a superior power factor of ∼43 μW cm-1 K-2. Furthermore, due to the low resonant frequency of Se, filling voids of CoSb3 with Se achieved a low lattice thermal conductivity of 1.55 W m-1 K-1. Nevertheless, Se filling introduced additional holes, reducing the carrier concentration without a significant detriment of the carrier mobility. As a result, a maximum figure of merit of 1.23 was achieved for Se0.1Co4Sb11.55Te0.45 at 773 K. This work provides a valuable guidance for selecting appropriate filling and doping components to achieve synergistic optimization of the acoustics and electronics of CoSb3-based skutterudites.

The effect of MnO2 additive on the microstructure and mechanical properties of magnesium aluminate spinel

AbstractIn this study, varying amounts of MnO2 up to 5 wt.% were added to magnesium aluminate spinel (MA) bodies using a solid‐state sintering method at 1200–1600°C. The effect of MnO2 addition on the phase composition, microstructure, distribution of elements, and ionic valence of MA was investigated via X‐ray diffraction, scanning electron microscopy, energy‐dispersive spectroscopy, and X‐ray photoelectron spectroscopy, respectively. The results showed that Mg2+ ions in MA crystals were replaced by Mn2+ ions, resulting in the formation of the (Mg1‐xMnx)Al2O4 solid solution. The distorted crystal structures promoted the sintering reactions, and the mechanical characteristics of MA were greatly improved by the solid solution strengthening process. When the additive amount of MnO2 was 5 wt.% and the sintered temperature reached at 1600°C, excess manganese ions hardly dissolved into the lattice of MA. And these ions were only distributed at the grain boundaries of MgAl2O4, forming a “barrier” that hindered the migration and diffusion of particles, thereby suppressing the sintering process and weakening the mechanical strength of MA.

The synergistic effect of cold sintering and reaction sintering on BST tape casting sheets

In this paper, the tape casting method was used to prepare BST (Barium Strontium Titanate) thin films and the effect of combining cold sintering and reaction sintering processes on the properties of BST sheets was explored. A comparison was made between cold sintering and direct BST sintering combined. The combined process yielded high-density BST sheets with significantly improved dielectric properties (the resulting BST sheets displayed a low dielectric loss of less than 0.02 with a dielectric constant of 720) at a reduced sintering temperature of 950 °C. The underlying mechanisms for this enhanced performance are explored through structural and sintering analysis.