Sintering Aid Research Articles

Densification, microstructure, and wear properties of TiB2-TiC-GNP and TiB2-TiC-BN composites

In this study, TiB2–TiC, TiB2–TiC-GNP, and TiB2-TiC-BN composites were produced via spark plasma sintering (SPS). The densification and sintering behavior, phase and Raman spectroscopy analyses, microstructural properties, Vickers microhardness, and wear behavior of the binary and ternary composites were investigated. The addition of various amounts of TiC to TiB2 resulted in nearly complete densification. The TiB2–TiC-GNP specimens exhibited a relative density of more than 99 %; in contrast, the addition of BN did not contribute to the densification. The Vickers hardness of the GNP-added composites was ∼25 GPa, whereas hBN addition to the matrix decreased the hardness to ∼22 GPa (∼9 % decrease). According to the wear test, the addition of GNPs did not result in a significant change in the TiB2–TiC matrix; however, the addition of hBN increased the specific wear rate of the matrix by ∼100 %. The GNPs did not have a negative effect on the densification, microstructural, or mechanical properties, such as the Vickers hardness and wear resistance. Hence, they were determined to be a more suitable sintering aid for matrix composition than hBN.

Controlling low temperature sintering of UO2+x

UO2 nuclear fuel pellets are typically sintered at temperatures of approximately 1700 °C to achieve the high densities and large grain sizes necessary for safe reactor operation. Lowering this sintering temperature is desirable in order to decrease the energy input required for fuel manufacture. Hence, the effect of temperature, time, stoichiometry and ZrO2 doping on sintering efficacy have been investigated. ZrO2 doping, coupled with hyper-stochiometry, acted as a strong sintering aid, enabling higher densities and larger grain sizes when sintering at lower temperatures and for shorter periods compared to the undoped samples. Without ZrO2 doping, at 1500 °C sintering was strongly sensitive to hyper-stoichiometry and only weakly sensitive to sintering duration. Addition of 0.13 mol fraction ZrO2 increased theoretical density up to 10 % and the maximum grain size from 8 µm to 40 µm. Addition of 0.30 mol fraction ZrO2 resulted in even greater densification, reaching 98 % of maximum theoretical density, but also formation of a secondary phase that hindered grain growth.

Impact of microwave sintering and NiO additive on the densification and conductivity of BaCe0.2Zr0.7Y0.1O3-δ electrolyte for protonic ceramic fuel cell

This work investigated the influence of microwave sintering on the BaCe0.2Zr0.7Y0.1O3-δ phase, incorporating 2 mol% or 4 mol% NiO as sintering aid. X-ray diffraction analysis of perovskite phase detected a rhombohedral structure under both sintering heating rates (280 and 500 °C/min), also exhibiting the presence of a second phase deficient in Ba with Ni incorporated into the structure, replacing part of the Zr. Raman analysis indicated no distortions in the perovskite structure, independent of the rates and NiO concentrations. The efficacy of NiO addition as a sintering aid was demonstrated by high densification of the samples, exceeding 97 % relative density. Detailed analysis of structural, microstructural, and conductivity properties revealed that microwave sintering, particularly using a heating rate of 500 °C/min, yielded superior results compared to conventional sintering. The enhanced electrical response was attributed to a higher concentration of structural defects, oxygen vacancies, in the microwave-sintered samples, evidenced by the band at 630 cm−1 in the Raman spectrum. Among these, the BCZY sample with 2 mol% NiO, sintered under these conditions, stood out as the most promising, showcasing not only high density but also a uniform microstructure and appreciable electrical conductivity.

Enhanced Charge Transfer of TiO2 Photoanode for Low Temperature Dye Sensitized Solar Cell via Tin Sintering Aid Integration

Enhanced Charge Transfer of TiO2 Photoanode for Low Temperature Dye Sensitized Solar Cell via Tin Sintering Aid Integration

Synthesis of Si3N4 powder with a controllable alpha/beta ratio by sol-gel assisted carbothermal reduction and nitridation

The controllable α/β ratio of Si3N4 powder was synthesized using tetraethyl orthosilicate (TEOS), glucose, MgCl2 and YCl3·6H2O by sol-gel assisted carbothermal reduction and nitridation (CRN). This study aimed to investigate the impact of various parameters, including a H2O/TEOS mol ratio of the sol, a C/Si mol ratio, sintering aids, and the temperature of CRN on the phase transformation of α-Si3N4. The results demonstrated that the H2O/TEOS ratio significantly influenced the relative content of α and β phase in Si3N4 by changing the morphological characteristics of SiO2. When the H2O/TEOS ratio was 100, β-Si3N4 powder with an approximate β content over 88 wt% was synthesized at 1500 °C. This remarkable phase transformation was likely facilitated by the eutectic liquid phase consisting of MgSiO3/Mg2SiO4. The SEM image revealed rod-like whiskers with a length around 1.4 μm for the β-Si3N4 grains. Due to the presence of liquid phase, the synthesis of spherical-like α-Si3N4 nano-powder with an α-phase content of 91 wt% was achieved at a lower temperature of 1450 °C, with an H2O/TEOS ratio of 15 and a C/Si ratio of 4. The size of the particles was about 70 nm due to the reduced reaction temperature. Furthermore, without the addition of any sintering aids or Si3N4 seeds, high purity α-Si3N4 with an α phase content approaching 96 wt% could be synthesized at 1500 °C, using an H2O/TEOS ratio of 15 and a C/Si ratio of 4. The resulting α-Si3N4 powder exhibited an extremely regular hexagonal prism shape with a length of approximately 2 μm.

Comparing the effects of different sintering aids on spark plasma sintering of SiC ceramics

In the present work, to improve the mechanical properties of silicon carbide different sintering aids were used. 2.5 wt% B4C, 2.5 wt% AlN, and TiC in the 5 to 7 wt% range were selected to manufacture the SiC-based sample via spark plasma sintering at 1700 °C. The results show that the use of 2.5 wt% B4C-2.5 wt% AlN additives increases the strength (1206 MPa) of the composite through the compressive stress created in the grain boundaries and decreases its fracture toughness (5.13 MPa.m1/2). But in the case of TiC-doped SiC, the toughness (7.09 MPa.m1/2) and density (3.18 g/cm3) of the sample increases compared to the pure SiC sample.

Production of dense forsterite with ultra low dielectric loss using ZrO2 additive

Forsterite was synthesized with MgCO3 and SiO2 powders using a solid-state method at 1200–1300°C. The ceramic specimen with the highest forsterite phase content was sintered with the ZrO2 sintering aid at mass fractions of 0.5, 1.0, and 1.5 wt% at 1400°C for 4 h. The dielectric constant and loss tangent were also measured via the waveguide method at 8–12 GHz. The formation of enstatite and cristobalite was minimized through precise synthesis temperature control. Phase measurement by the Rietveld method showed that specimen F28 contained more than 99.8% forsterite. It was found that ZrO2 reduced the pore volume percent and increased the density even at low mass fractions. A liquid phase on the grain boundaries was demonstrated. The solid ceramic specimen F28-15Z showed a dielectric constant of 6.5 and high quality factor of 189000.

Mechanosynthesis of TaC-WC powders under environmental conditions and their consolidation via electric arc furnace

In this study, tantalum carbide (TaC) was synthesized using an innovative approach that synergistically integrates mechanosynthesis and electric arc furnace processes. By employing high-energy ball-milling (HEBM) for 50 min under environmental conditions, TaC-WC powders were successfully synthesized, using a powder mixture of tantalum and carbon in a 1:1 stoichiometric ratio. This method yielded a composition of 72.5 wt% TaC and 27.5 wt% WC, with an average particle size of 0.7 ± 0.3 μm. The use of an electric arc furnace led to the fabrication of a highly dense material with a relative density above 98%. Notably, WC derived from the mechanical milling material served as an effective sintering aid. x-ray photoelectron Spectroscopy (XPS) results indicated the formation of metal oxides on the surface of the sample, and despite the presence of these oxides, the density of the material remained uncompromised. Furthermore, x-ray diffraction (XRD) analysis after the electric arc furnace treatment demonstrated the preservation of the TaC and WC phases. Mechanical properties, including Vickers hardness, Young’s modulus and fracture toughness were 22.8 ± 0.5 GPa under an applied load of 9.8 N, 539 GPa and 6.6 MPa m1/2, respectively. The results underscore a novel and efficient synthesis route for TaC-WC with enhanced mechanical properties and high density, which are crucial aspects for applications in ultra-high temperature ceramics.

Flash sintering of silicon carbide at room temperature

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

WB2 ceramics prepared by combination of boro/carbothermal reduction process and spark plasma sintering: Effect of C content and sintering rate

Tungsten diboride ceramics with a relative density higher than 90 % without addition of sintering aid can be prepared by boron/carbothermal reduction method combined with spark plasma sintering. Among the raw materials,WO3, B4C, and C, C content is of vital importance to obtain WB2 bulk samples with high density and good mechanical properties. In the present work, we investigated the effects of C content in powder synthesis, as well as SPS sintering rate on phase composition, microstructure, and Vickers hardness of WB2 bulk samples.WB2 bulk sample with a relative density as high as ∼94 % was obtained with the optimal raw material ratio of WO3:B4C:C=2:1.075:4.8, which was then increased to ∼96 % with the decrease of sintering rate from 100 °C/min to 50 ℃/min. This represents the highest relative density of the pure WB2 ceramic publicly reported in the literatures.

Dual-template textured BNT-based ceramics with ultra-low electrostrain hysteresis

The biggest difficulty facing the grain growth (TGG) method is the compatibility between template and ceramic matrix. Different templates show great differences in texture degree. Consider the limitation of conventional single-template method, the strategy of dual-template textured ceramics and sintering aids were introduced in this work. With the addition of ST and NN templates into BNT system, the texture degree is higher than 90% and the electrostrain reach ∼ 0.5% with ultra-low hysteresis ∼ 15%, which cannot be realized in single template textured ceramics. The two templates and sintering aids play different roles in grain orientation growth, which is important for the compatibility among matrix and templates, leading to unique microstructures. Sintering processes clearly reveal the growth mechanism of textured ceramics under the joint action of two templates, and of course, the sintering aids are also indispensable. Regard to domain structure, the polar nanoregions can respond quickly under electric field, providing assistance for the realization of ultra-low hysteresis.

Printable silicate and RuO2 composite with wide-range linear PTC for high-temperature sensors

3D printing has revolutionised the design and manufacturing of high-temperature thin/thick-film sensors (TFSs). However, existing printable high-temperature materials face cost-performance trade-offs. This study proposes a silicate and RuO2 composite for the 3D printing of TFSs. A silicate compound (SiO2–Al2O3–CaO, SAC) was used as a sintering aid to reduce the sintering temperature of RuO2, forming a silicate glass phase that enhanced film density and substrate adhesion. The SAC also minimised RuO2 particle volatilisation at high temperatures, thus enhancing the stability of the SAC/RuO2 composite. The SAC/RuO2 TFSs demonstrated exceptional performance, with a positive temperature coefficient (502 ppm/°C) from room temperature to 800 °C, high linearity, and stability (resistance drift rate of 0.1 %/h at 800 °C), extending the application temperature of RuO2 by nearly 200 °C from the previous application temperature of 600 °C. Therefore, the SAC/RuO2 composite offers a new low-cost and high-performance solution for using 3D printing technology in harsh environmental sensors such as turbine blades.

Ablation behavior of Csf/SiC‐BN composites with polyborosilazane as sintering aids: High‐temperature damage mechanism

AbstractMicrostructural evolution and high‐temperature ablative damage mechanism of Csf/SiC‐BN composites with and without polyborosilazane (PBSZ) as sintering aids that can convert to polymer‐derived SiBCN ceramics (PDCs‐SiBCN) by cross‐linking and pyrolysis during sintering were studied. The density of Csf/SiC‐BN composites increases from 2.17 to 2.25 g/cm3 with 10 wt.% sintering aids because PBSZ promotes rearrangement and redistribution of raw materials by pyrolysis during the sintering process. With the increase of carbon fiber content, ablation performance (mass ablation rate: 15.25 mg/s; linear ablation rate: 0.100 mm/s) negligibly decreases, compared to counterparts with lower fiber content (mass ablation rate: 16.53 mg/s; linear ablation rate: 0.072 mm/s) because the presence of PDCs‐SiBCN partially compensates for the detrimental effect of carbon fibers on ablation performance. Csf/SiC‐BN composites with and without PBSZ show similar phase composition mainly containing quartz, amorphous SiO2, and matrix crystallization products and morphology after ablation because of similar phase composition before ablation. The ablation mechanism is oxidative corrosion and mechanical scouring and stripping.

Microstructure and mechanical properties of TiB2-TiNw ceramic composite fabricated by SPS

A TiN whisker toughening TiB2 based ceramic composite was fabricated by spark plasma sintering (SPS). The influence of sintering temperature and holding time on the microstructure and mechanical properties of the composite was investigated. The results indicate that, at a sintering temperature of 1500 °C and a holding time of 9 min, the composite exhibits a relative density of 99.24 %, flexural strength of 906 MPa, fracture toughness of 8.6 MPa m1/2, and Vickers hardness of 22.3 GPa. The sintering aid composed of Ni and Mo within the composite significantly enhanced atomic diffusivity in TiB2 grains, promoting a balanced grain growth rate. SPS technology promoted the rapid formation of numerous necks between TiB2 grains in a very short duration. Concurrently, the pinning effect exerted by TiN whiskers effectively inhibited excessively rapid migration of grain boundaries. These factors ensured that the composite could achieve optimized microstructures and good mechanical properties at a low sintering temperature.

Low temperature fabrication of Ba0.9Cs0.3Cr2.1Ti5.9O16 ceramic waste form for cesium immobilization using Bi2O3 as sintering aid

In this paper, we present a productive low-temperature fabrication of Cs-hollandite ceramics by adding Bi2O3 as a sintering aid to achieve high cesium retention. The effects of Bi2O3 addition on the phase composition, crystal structure and aqueous stability of the stoichiometric Ba0.9Cs0.3Cr2.1Ti5.9O16 ceramic waste form were investigated. It was found that adding Bi2O3 can effectively reduce the sintering temperature and promote densification of Cs-containing hollandite products. The Cs-hollandite ceramic with 2 wt% Bi2O3 sintered at 1000 °C showed a single phase with > 98 % cesium retention, which was ∼ 200 °C lower and ∼ 11 % retention rate higher than those of reported Cs-bearing hollandite. Moreover, the leached sample remained stable tetragonal structure, the LRCs was in the order of 10-3 g·m-2·d-1 after 28 days and the leaching indexes > 8, exhibiting good aqueous stability. This route can be considered as a promising solution to prevent cesium loss for incorporating the cesium waste in ceramic hosts.

Enhanced electrochemical performance of Ba0.5Sr0.5Co0.8Fe0.2O3-δ cathode by Zr4+, Sm3+ and Yb3+ tri-doped BaCeO3 compositing for intermediate-temperature thin-film fuel cells

In this study, Zr4+, Sm3+ and Yb3+ tri-doped BaCeO3 (BaCe0.7Zr0.1Sm0.1Yb0.1O3-α) was prepared using a citric acid-EDTA combined complexing sol-gel method. The influence of sintering aid (CuO) content on structure, sinter-ability and conductivity of BaCe0.7Zr0.1Sm0.1Yb0.1O3-α (BCZSY) was studied using XRD, SEM and AC impedance. XRD patterns of different sintering aid contents and temperatures showed that all samples formed a pure BaCeO3 orthorhombic phase without any impurities. The conductivity of 1500-BCZSY reached 1.8 × 10−2 S cm−1 and 1.9 × 10−2 S cm−1 in dry nitrogen and hydrogen atmospheres at 700 °C, respectively. The anode-supported membrane fuel cells of NiO-BCZSY | BCZSY (13 μm) | Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) and NiO-BCZSY | BCZSY (15 μm) | BCZSY-BSCF were fabricated, respectively. SEM results showed that the cathode, anode-support and electrolyte had good adhesion. The polarization resistance (Rp) of BCZSY-BSCF was significantly reduced by 34 %, 44 % and 55 % compared to BSCF at 500 °C, 550 °C and 600 °C, respectively. The maximum power density (Pmax) using BCZSY-BSCF as the cathode increased by 18 %, 17 % and 22 % compared to BSCF at 500 °C, 550 °C and 600 °C under the condition of slightly higher electrolyte film thickness, correspondingly.

Dual sinter-aids to fabricate high-entropy transition metal carbide ceramic and their properties

Due to excellent performances and easy adjustability, more and more researchers pay attention to high-entropy ceramics, which can be applied in many fields. In this work, a high-entropy transition metal carbide ceramic, (Hf0.2Ta0.2Zr0.2Nb0.2W0.2)C, was fabricated by pressureless sintering method. To obtain highly dense ceramic, dual sintering aids (Cr3C2 and C), are employed in the sintering process. Owing to suitable microstructure (grain size and defect density), the ceramics exhibit excellent electric conductivity(0.7040 MS/m) and flexural strength (378 ± 11.4 MPa). Meanwhile, the relationship between electromagnetic shielding effectiveness and different aid content in the X-band is also investigated. The P3 ceramic with a thickness of 1 mm can achieve an EMI shielding efficiency of 92 dB in the X-band as a result of the stronger interface polarization and relaxation effects. High-entropy ceramic obtained in this work would give a different way to fabricate shielding material and can promote the use of high-entropy ceramics as effective EMI shielding materials.

Fabrication of high-permeance SiC filters reinforced with in situ-grown mullite whiskers for gas/solid filtration

The pursuit of cost-efficiency, superior strength, and excellent gas permeance has driven the development of silicon carbide (SiC) membranes or filters for gas/solid filtration. In this study, SiC filters with high gas permeance reinforced with in situ-grown mullite whiskers were successfully prepared through reaction bonding. Low-cost kaolin and bauxite were used as sintering aids and raw materials for mullite whisker formation, and MoO3 was used as an additive to promote mullite whisker growth. The effects of different sintering temperatures and the addition of kaolin, bauxite, and MoO3 on filter performance were systematically investigated. When the sintering aid and MoO3 content additions were 6 and 1.5 wt%, respectively, and the sintering temperature was 1450 °C, the resulting ceramic filter exhibited a porosity of 45.5 %, a bending strength of 17.9 MPa, and an average pore size of 47.7 µm. Moreover, the filter exhibited a high Darcy’s permeability coefficient of 2.42 × 10−11 m2, satisfactory chemical stability, and thermal shock resistance. The addition of MoO3 facilitated the consumption of the molten phases and increased the anisotropic growth rate of the mullite whiskers, which contributed to the fabrication of the high-permeance filters. Finally, the ceramic filters demonstrated good performance for the filtration of dusty gases loaded with fly ash particles with a mean size of 15.3 µm at room temperature and 300 °C, and the removal rate reached > 99.9 %. The pressure drop was a little lower than that of our commercially available SiC filters. The SiC filters showed good prospects in the field of gas/solid filtration.

Design of highly-sinterable LATP-CNT composite powder by sequential particle assembly for fabrication of highly electrical-conductive composite electrodes

A sequential particle assembling process was developed to fabricate composite particles with multiple components. Cobalt sintering aids and carbon nanotube (CNT) conductive additives were sequentially and electrostatically absorbed on Li1.3Al0.3Ti1.7(PO4)3 (LATP) electrolyte particles under various dispersion conditions. The effect of the particle assembling conditions on the composite body's sintering ability and conductive properties was investigated. By optimizing the microstructure of the composite particle and its particle charging conditions, it became possible to obtain a sintered composite electrode with a finely connected path of each LATP and CNT phase.

Unraveling the conundrum of electronic leakage in protonic ceramic cells: Operation-specific insights and rational design strategies

Electronic conduction through proton-conducting electrolytes significantly impairs the efficiency of protonic ceramic cells (PCCs). In this study, we explore the electron and ion mixed transport properties of four common protonic ceramics, BaZr0.8Y0.2O3-δ (BZY82), BaZr0.7Ce0.2Y0.1O3-δ (BZCY721), BaZr04Ce0.4Y0.1Yb0.1O3-δ (BZCYYb4411), and BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb1711). It marks the first instance of investigating these properties under operation-specific scenarios: fuel side of electrolysis cell, air side of electrolysis cell, fuel side of fuel cell, and air side of the fuel cell. BZCYYb1711 exhibits the highest ionic conductivity, but two to three times higher electronic leakage when exposed to oxygen-containing environments than the others. BZY82 exhibits approximately two times higher electronic leakage in a hydrogen-containing environment. BZCY721 demonstrates excellent ion transport numbers (∼0.95) across these four operating conditions. BZCYYb4411 behaves quite similarly to BZCY721. The most challenging operating environment for all candidates is the air side of fuel cell mode. This mode leads to a high initial electronic leakage, followed by a significant increase with polarization. The probable cause for this behavior is a H2-free, polarization-induced reduction that leads to the formation of VO•. The electron small polaron associated with VO• is released by the electrical field due to the Poole-Frenkel effect. ZnO and NiO sintering aids are found to be detrimental to the ionic conductivity of the electrolytes. In particular, NiO substantially lowers the ion transport number. The correlation of the operation-specific electronic leakage to full cells is discussed. It is suggested that a rational PCC design should synergistically couple BZCYYb1711 at fuel side with BZCY4411 at air side to deliver a well-balanced performance and faradaic efficiency simultaneously, and the high temperature sintering process with a NiO fuel electrode should be shortened or replaced by ultra-fast sintering techniques or using a fuel electrode scaffold-infiltration fabrication strategy.