A study of reaction between palladium,palladium silver alloy and silicon carbide ceramics at high temperature

The reactions between palladium (Pd), palladium-silver alloy (Pd-Ag) and silicon carbide (SiC) from 1200 °C to 1400 °C have been studied to investigate the impact of liquid phases on reaction products formation. The liquid phases were generated in Pd/SiC reactions at 1400 °C and in Pd-Ag/SiC reactions above 1300 °C. An increase in the amount of liquid associated with higher temperatures or Ag presence strongly affected the reaction mechanism and was responsible for the inhomogeneous dissolution of SiC grains and grain boundaries by Pd-rich phase at reaction front. This study helps understand Pd’s role on Ag release through SiC layer in Tri-structural isotropic (TRISO) fuel particles.     

Effect of in-situ formation of columnar mullite and pore structure refinement on high temperature properties of corundum casatbles

The effects of in-situ synthesis columnar mullite and pore structure on the hot modulus of rupture (HMOR), thermal shock resistance and corrosion resistance of corundum castables have been investigated in this paper. When 2% nano silica was added, the pore diameters of castables could be decreased to 15 nm (at 110 °C), 1 μm (1100 °C) and 6 μm (1500 °C), respectively. The corresponding reducing magnitude of pore size is 98.5%, 83.3% and 33.3%. The HMOR of castables fired at 1500 °C increased by 110% to 3.64 MPa. Furthermore, after three thermal shock cycles, the residual strength ratio of castables increased from 5.2% to 15.3%. A large amount of cross-distributed columnar mullite was formed between nano silica and α-Al₂O₃ by the two-dimensional nucleation mechanism, which remarkably enhanced the high temperature properties. The penetration index reduced from 30.86% to 19.88%, suggesting that smaller pore size and higher viscosity had a great influence to the penetration process.     

Fabrication of in situ elongated β-Sialon grains bonded to tungsten carbide via two-step spark plasma sintering

In order to reduce the difficulty of preparing binder-less cemented carbide and further broaden its application prospects, tungsten carbide toughened by in situ elongated β-Sialon grains was developed via sintering ball-milled WC and α-Si₃N₄ powders using Al₂O₃–ZrO₂ as a sintering aid and transformation additive. The two-step spark plasma sintering of the mixture at 1650 °C with dwelling at 1500 °C for 10 min was conducted under 30 MPa uniaxial pressure, and the densification behaviors, phase transformations, mechanical properties, and microstructures of the produced composites were investigated. The addition of Al₂O₃–ZrO₂ reduced the initial temperature of the densification process by approximately 100 °C and its final temperature by 200 °C (compared with the densification temperatures of pure WC and Si₃N₄ materials) and fully transformed α-Si₃N₄ to Sialon (Si–Al–O–N) phases. Microstructural characterization data showed that the WC matrix contained homogeneously distributed equiaxed and elongated β-Si₅AlON₇ grains. The WC composites containing in situ elongated β-Sialon grains exhibited an optimal hardness of 18.93 ± 0.03 GPa and enhanced fracture toughness of 10.43 ± 0.27 MPa m^1/2. The toughening mechanism of the β-Sialon phase involved the pull-out of elongated grains and crack bridging.     

Fracture mechanism of alumina-spinel castables containing ZrO2 by wedge splitting test and digital image correlation technique

To clarify the fracture mechanism of alumina-spinel castables, two kinds of alumina-spinel castables with or without fused zirconia-alumina (FZA) were prepared. The full-field strains and crack propagation process in the region of interest (ROI) of alumina-spinel castables were investigated by wedge splitting test (WST) and digital image correlation (DIC) technique. Fractographic methods were used to analyze the crack propagation path of the castables after the WST. The results indicated that the load-displacement curve of alumina-spinel castables containing FZA exhibits a non-linear fracture, demonstrated typical ductile fracture; while that of alumina-spinel castables without FZA is linear, showed typical brittle fracture. The characteristic length reaches to 258.9 mm in FZA containing castables, more than 4 times that of the castables without FZA. In contrast, castables containing FZA has longer and more tortuous crack propagation path, larger damage zone length, which results in the increase of the dissipated energy. Crack branching can be observed around the main crack in castables containing FZA, meaning that microcracks toughening is the main mechanism for flexibility improvement of the alumina-spinel castables containing FZA, formation of micro-cracks can be attributed to the martensitic transformation of zirconia.     

Pressureless-sintered boron nitride nanosheets/glass composite ceramics for excellent mechanical,dielectric and thermo-conductive performances

Because they have a high application potential in the thermal management of insulation environments, high-quality hexagonal boron nitride (h-BN)-based multiphase ceramics have been highly desired. However, so far, their synthesis is still full of challenges. Here, a kind of boron nitride nanosheets (BNNSs)/glass (GS) composite ceramics was prepared by a pressureless sintering method at a lower temperature of 900 °C. Due to a tightly bonded interaction between BNNSs and GS, the formed BNNSs/GS ceramics exhibit excellent multifunction performance. They have an outstanding compressive strength in the range of 19 ∼ 64 MPa and Vickers hardness ranging from 50 to 179 HV. For the BNNSs/GS ceramics with BNNS’s filling fraction of 90 wt%, their maximum side-surface TC values are 12.01 ± 0.18 W m⁻¹ K⁻¹ at 25 °C and 13.64 ± 0.37 W m⁻¹ K⁻¹ at 300 °C, respectively. In the ultra-high frequency range of 26.5 ∼ 40 GHz, the dielectric constant values of the BNNSs/GS ceramics are primarily between 2 and 3, and the corresponding loss tangent values are < 0.3. In addition, based on the remarkable integrity of their structure, these BNNSs/GS ceramics exhibit outstanding thermal-shock stability and prominent thermal management capacity during lots of heating/cooling-testing cycles. Therefore, we believe this kind of BNNSs/GS ceramic system will have great application potential in the new-generation thermal management and/or insulation packaging fields.     

Influence of the cold sintering process and post annealing on the microstructure and Li-ion conductivity of LiTa2PO8 solid electrolyte

Recently, LiTa₂PO₈ (LTPO) has attracted interest as a potential Li-ion solid electrolyte material because of its high bulk ionic conductivity and low grain boundary ionic conductivity. However, most ceramic-based solid electrolytes are fabricated via the high-temperature sintering process (typically above 1000 °C); such temperatures can cause the evaporation of Li from the compound. To replace high-temperature sintering of ceramics, the cold sintering process (CSP) was introduced; this process enables the densification of ceramics and composites at extremely low temperatures (below 300 °C). In this work, we investigate the effect of using the CSP and post annealing on the microstructure and Li-ion conductivity of LTPO pellets. It is found that the CSP pellets have an amorphous phase between particles. This intermediate amorphous phase creates a better contact between particles and is hypothesized to lead to more Li-ion migration paths. The CSP pellet is found to have a high density and high ionic conductivity of (1.19 × 10^?5 S/cm). The pellet obtained via the CSP has Li-ion conductivity similar to that of the pellet obtained via dry pressing after it has been annealed. The CSP pellet after post annealing shows good connections between particles and a high Li-ion conductivity of 1.05 × 10^?4 S/cm, which is comparable to the conductivity of a pellet obtained via high-temperature sintering. This work provides new evidence that the CSP is a promising alternative to high-temperature sintering for fabricating ceramic solid electrolytes.     

Mechanical properties and microstructure evolution of MgO–Al–C slide plate refractories in presence of Al powder-modified magnesia aggregates

MgO–Al–C slide plate refractories were fabricated using sintered magnesia and modified sintered magnesia as aggregates, fused magnesia aggregates and fines, Al powder and carbon black (N220) as fines, and thermosetting phenolic resin as the binder. Al powder-modified magnesia aggregates were prepared and characterized and were introduced into the MgO–Al–C slide plate refractories. The effects of the modified aggregates on the properties, phase composition, and microstructure were investigated. 1) The Al powder-modified magnesia aggregates exhibited considerably high bonding strengths and low Al powder shedding ratios, thus meeting the preparation requirements of MgO–Al–C slide plate refractories. 2) At high temperatures, more needle-like and fibrous Al₄C₃, AlN and octahedral MgAl₂O₄ were generated on the surface of the modified magnesia aggregates, which enhanced the bond between the matrix and the aggregates and increased the hot modulus of rupture of the material. 3) Non-oxide Al₄C₃ and AlN phases were formed in situ and had high thermal conductivity and low coefficient of expansion; this could relieve the internal thermal stress of the material and create a toughening effect, improving the thermal shock resistance of the material.     

A study on retention of MWCNT in robocasted MWCNT-HAP scaffold structures using vacuum sintering technique and their characteristics

Bioceramic scaffolds are being widely employed in bone tissue engineering applications for their ability to interact with host tissues without inducing any toxicity. Additionally, bioceramics possess good biocompatibility, osteointegration, osteoinduction, and biodegradation characteristics. Hydroxyapatite (HAP) is one such bioceramic known to exhibit closeness to natural bone in terms of chemical composition. The present reports additive manufacturing of HAP and Multiwalled carbon nanotubes (MWCNTs) reinforced HAP scaffold structures for bone tissue engineering applications using the Robocasting technique. Carboxymethyl Cellulose (CMC) was employed as the polymeric binder in this study to prepare the highly viscous HAP and CNT-HAP slurry ideal for robocasting of the scaffold structures. Different percentages of MWCNT (0.5, 1 and 2 wt%) incorporated into the developed CNT-HAP scaffold structures and were vacuum sintered at 1000 °C for 15 min. Vacuum sintering was found to effectively prevent oxidation of MWCNT which is subjected to decomposition at temperatures above 400 or 500 °C in Oxygen atmosphere as per literature. Further, the retention of MWCNTs in the developed CNT-HAP structures post sintering was confirmed using FESEM imaging. The mechanical characterizations revealed that 0.5CNT-HAP structures exhibited highest compression strength (3.36 ± 0.67 MPa) in comparison to 1CNT-HAP and 2CNT-HAP structures. Also, the in vitro biological characterizations demonstrated that the developed CNT-HAP scaffold structures were cytocompatible and remained stable for about 35 days at 37 °C.     

Low-temperature synthesis of nanometric apatite from biogenic sources

Nanosized calcium deficient carbonated apatite was synthesized from three different natural resources, namely chicken eggshells (calcite), cuttlefish bones (aragonite) and mussel shells (aragonite/calcite). The calcium precursors were ball-milled in (NH₄)₂HPO₄ or H₃PO₄ – containing aqueous medium for different times and then dried at temperatures ranging from 20 °C to 150 °C. The formation of hydroxyapatite (HA) is shown to be strongly affected by such treatment, the amount of synthesized HA increasing with temperature. Aragonite from cuttlebones and the aragonitic fraction of mussels is found to be transformed more easily with respect to the calcite from eggshells. The reaction is also favored by an acidic milling media with respect to basic one. The feasibility of the synthesis process at nearly room temperature described in the present work has interesting potentialities in the retention of the organic portions of the original biological resource in the produced nanometric inorganic compound.     

Migration,transformation and solidification/stabilization mechanisms of heavy metals in glass-ceramics made from MSWI fly ash and pickling sludge

In consideration of recycling solid waste to achieve high value-added products, glass-ceramics have been fabricated from municipal solid waste incineration (MSWI) fly ash, pickling sludge (PS), and waste glass (WG) by melting at 1450 °C firstly to achieve parent glass and then crystallizing at 850 °C. Results demonstrated that heavy metals have been well solidified in the prepared glass-ceramics, and relatively/extremely low leaching concentrations of heavy metals have been detected. The synthetic toxicity index of heavy metals has been greatly reduced from 7-18 to <3.2 after crystallization treatment, and the leaching concentrations of Cr, Ni, Zn, Cu, and Pb are 0.15, 0.05, 0.26, 0.12, 0.19 mg L^-1 respectively. Chemical morphology analysis, principal component analysis, TEM and EPMA were utilized to clarify the migration, transformation, and solidification mechanism of heavy metals from the as-received solid wastes. The major heavy metals, Cr and Ni which is responsible for the most toxicity, mainly exist in form of the oxidation state and residual state in parent glass, while the residual state in the glass-ceramics. The solidification performance was mostly positively correlated with the form of residue state, which the stability of heavy metals in glass-ceramics is improved. The solidification mechanism of heavy metals in glass-ceramics could be explained by the combination of chemical solidification/stabilization and physical coating. The TEM and EPMA confirmed that Cr and Ni mainly exist in the spinel crystalline (NiCr₂O₄, Fe_0.99Ni_0.01Fe_1.97Cr_0.03O₄) by solid solution or chemical substitution, and a small amount of Cr in the diopside phase. Pb, Cu, and Zn are homogenously dispersed in the glass-ceramics, which is considered as physical coating solidification.     

Microstructural and main mechanical properties of novalac based carbon–carbon composites as the pyrolysis heating rate

In this study, the effects of pyrolysis heating rate on microstructural and main mechanical properties of Novalac-based carbon/carbon composites were investigated by CHNS, optical microscope, FE-SEM, BET N₂ adsorption, XRD, Raman, FT-IR, wear analyzing, three-point bending test, tensile and Vickers micro-hardness tests. Firstly, PAN-derived carbon nanofibers (reinforcing agent) was synthesized using electrospinning followed by the functionalizing via the wet chemical oxidation to improve the strength of nanofiber bonding to the matrix of composites. Firstly, novalac resin (acting as a matrix), hexamethylenetetramine (hardener agent) and carbon nanofibers (reinforcing agent) were mixed and hot-pressed at 180 °C under the compression load of 40 kN to produce compressed CNFs-Novolac composites. Carbon/Carbon composites were obtained from the pyrolysis of CNFs-Novolac composites up to 1000 °C by the various heating rates under the compression press of 400 bar, finally. Structural and mechanical studies confirmed that the heating rates below or equal to 10 °C.min⁻¹ resulted in the production of low porosity (≤17%) carbon composite with high carbon content (>90 wt%), high fracture strength (≥270 MPa), high toughness (≥9 MPa m^1/2), high hardness (≥156 Hv), and low friction coefficient (<0.6).     

Sintering characteristics and microwave dielectric properties of ultralow-loss SrY2O4 ceramics

A SrY₂O₄ microwave dielectric ceramic suitable for 5G systems is synthesised via a solid-state reaction in a sintering temperature range of 1425–1525 °C. X-ray diffraction patterns and Rietveld refinement analysis show that the ceramic has an antispinel orthorhombic crystal structure belonging to the Pnma space group. Scanning electron microscopy images show that the ceramic particles are closely connected, the grain boundaries are clear, and the particles are uniform at the optimal sintering temperature of 1475 °C. The optimal microwave dielectric performances are ε_r = 14.78, Q × f = 84090 GHz, τ_? = ?14.98 ppm/°C. The relatively low dielectric constant, high Q × f value, low τ_? value, and easily available raw materials indicate that it is a good choice for 5G equipment.     

Thermal cycling behavior of plasma-sprayed yttria-stabilized zirconia thermal barrier coating with La0.8Ba0.2TiO3−δ top layer

In this study, a La_0.8Ba_0.2TiO_3?δ (LBT) upper layer was deposited on an yttria-stabilized zirconia (YSZ) thermal barrier coating (TBC) through atmospheric plasma spraying. The thermal cycling behaviors of the YSZ single-ceramic-layer and LBT–YSZ double-ceramic-layer coatings at 1000 °C were investigated through a water quenching method. Moreover, phases, microstructural evolution, and elemental distributions were studied through by X-ray diffraction and scanning electron microscopy–energy-dispersive X-ray spectroscopy. The results showed that the thermal cycling lifetime of the LBT–YSZ coating was 27% higher than that of conventional YSZ coating. The conventional YSZ coating failed after 251 cycles because of the joining of the continuous horizontal and vertical cracks caused by the formation of thermal growth oxides and the bending effect of the single-ceramic-layer structure. The thermal cycling behavior of the LBT–YSZ coating was different from that of the YSZ coating at the edge and center. Although the former was similar to the failure behavior of the YSZ coating, the cracks in the vertical direction were deflected as a result of the bending effect of the double-ceramic-layer structure during quenching. This deflection led to the formation of slope cracks with longer propagation paths and slope spallation zones. The latter showed small-debris spallation on top of the LBT upper layer due to the lower fracture toughness of the LBT, which protected the central coating from the structural damage of the ceramic coating. These two behaviors would either release the thermal stress or increase the crack-propagation energy requirement in the ceramic coating, consequently improving the thermal cycling lifetime of the LBT–YSZ coating. In summary, depositing an LBT upper layer could potentially improve the thermal cycling lifetimes of TBCs.     

Effect of microsilica addition on the properties of colloidal silica bonded bauxite-andalusite based castables

Colloidal silica bonded bauxite-andalusite based castables were prepared using homogenized bauxite and andalusite as aggregates, andalusite fines, corundum fines, ultrafine Al₂O₃ as matrixes and colloidal silica as binders. Effects of microsilica addition on the green strength, physical properties, hot strength and thermal shock resistance of castables were investigated. Moreover, phase composition and morphological evolution of specimens were characterized by XRD and SEM analysis. Green strength after demoulding, cold strength and hot strength as well as thermal shock resistance of the castables are enhanced with microsilica addition, which attribute to generating more chemical bond (–Si–O–Si–) after demoulding and heating at intermediate temperature (up to 1100 °C), and creating a stronger mullite bonding at higher temperature (1400 °C) compare to the specimens without microsilica.     

Evaluation of mechanical properties of YSZ TBCs doped by different ratios of Eu3+ ions after isothermal oxidation

Thermal barrier coatings (TBCs) are essential to improve the thermal insulation performance of high-temperature components. Rare earth element (Eu³⁺) doped yttrium stabilized zirconia (YSZ) TBCs have been proved to be an ideal solution for non-destructive testing of internal damages. Based on this theory, two types of coatings deposited by air plasma spray (APS) on Hastelloy-X were investigated: (1) Eu³⁺ doped YSZ (dopant ratios 1 mol%, 2 mol%, 4 mol%, respectively), (2) traditional undoped 8YSZ. Isothermal oxidation treatment at 1100 °C, in increments of 10h until the failure of the coatings are conducted to evaluate the mechanical properties of different coatings. The microscopic morphology and phase of the coatings were analyzed by scanning electron microscope (SEM) and X-ray diffraction (XRD) patterns, respectively. The indentation testing methods were used to study the apparent interfacial fracture toughness and the hardness of the ceramic top coat. Results show that the Vickers hardness of the top coat increases with the decrease of porosity in the early stage and then decreases with the heat treatment time increasing in the long-term stage. Simultaneously, compared with the undoped 8YSZ coating, the fracture toughness increased with the dopant of Eu³⁺ ions increasing, from 1 mol% to 2 mol%, nevertheless, that of 4 mol% Eu³⁺ doped YSZ decreased compared with in the undoped 8 YSZ. For all types of specimens, the interfacial fracture toughness decreases with the increase of isothermal oxidation time. Results also indicate that the content of Eu³⁺ doping does not affect the microstructure and interfacial morphology of the YSZ coating as well as the growth law of thermally grown oxides (TGO). Furthermore, EDS detection found that the Eu³⁺ ions almost do not diffuse inside the TBCs system after isothermal oxidation treatment.     

Ultra-high-temperature tantalum-hafnium carbonitride ceramics fabricated by combustion synthesis and spark plasma sintering

We report the fabrication of dense single-phase (Ta,Hf)CN carbonitride ceramics using a combination of combustion synthesis (CS) and spark plasma sintering (SPS). The ceramic powder was produced by high-energy ball milling of the reactants (Ta, Hf, C) in different atomic ratios followed by CS of the obtained nanostructured composites in a nitrogen atmosphere. X-ray diffraction analysis of the combustion products revealed the formation of (Ta,Hf)CN with cubic B1 structures as the dominant phases for all investigated compositions. The SPS of the as-synthesized powders allowed both homogenization of the composition and consolidation of the bulk single-phase carbonitride ceramics with a relative density of 98 ± 1 %. Ta₂₅Hf₇₅CN showed the highest hardness (19.4 ± 0.2 GPa) and fracture toughness (5.4 ± 0.4 MPa m^1/2) among the investigated composites and excellent oxidation resistance in air.     

Tuning the electronic and thermal transport behaviors of metal-dispersed Ti2O3 composites derived by metal–insulator transition

Thermal management requires an understanding of the relations among the thermal energy transfer, electronic properties, and structures of thermoconductive materials. Here, we enhanced the metal–insulator transition (MIT)-induced effect on the thermal conductivities of microstructure-controlled Ti₂O₃ composites containing W as a thermal conductive filler at approximately 450 K. To change the electronic and thermal transport properties, we varied the particle radii of the conductive phases in the raw material. The change in the calculated electronic thermal conductivity relative to the electrical conductivity of the W_x(Ti₂O₃)_1?x composite was enhanced by compounding the material. When x was reduced from 50 vol% to 20 vol% and the W particle diameter was reduced from 150 μm to 5 μm, the variation in the estimated electronic thermal conductivity of the W_x(Ti₂O₃)_1?x composite was increased by a factor of 2.01. The total thermal conductivity was also changed by the MIT. At x = 50 vol% and a W particle diameter of 5 μm, the maximum thermal conductivity change was 6.34 times larger than that of pure Ti₂O₃. The detailed relation between the MIT-induced changes in thermal transport and the microstructure were elucidated in classical effective medium approximations.     

Tetragonal multilayered ZnO/CuO composites derived from Zn- and Cu-containing metal-organic framework: Effect of calcination temperature on physicochemical properties and photocatalytic activity

Tetragonal multilayered ZnO/CuO composites prepared by the annealing of a Zn- and Cu-containing pillar-layered metal-organic framework were characterized by using instrumental techniques and investigated as catalysts for the degradation of 4-nitrophenol (4-NP) under irradiation with UV–vis light. The as synthesized samples contained p-n junctions, amorphous-crystalline heterojunctions, which benefitted light absorption and charge separation. The calcination temperature significantly influenced both the physicochemical properties and photocatalytic activities of these composites. The sample obtained at 400 °C (TL-ZC-400) exhibited the best photocatalytic performance, achieving a 4-NP degradation efficiency of 93.93% after 40 min of illumination. The TL-ZC-400 still showed high photodegradation ability (97.2%) after four times recycling. Furthermore, the recombination of ZnO and CuO adjusted the band gap structure of TL-ZC-400. Radical trapping experiments showed that the degradation of 4-NP was mainly mediated by hydroxyl radicals and holes. A possible photocatalytic mechanism was also proposed in this study.     

Effects of andalusite aggregate pre-fired at different temperatures on volume stability and oxidation resistance of Al2O3–SiC–C castables

In the present work, the effects of the pre-firing temperature of andalusite aggregates (5–3 mm) on the conversion of andalusite as well as the volume stability and oxidation resistance of the Al₂O₃–SiC–C castables were investigated. The phases of the andalusite aggregates were tested via X-ray diffraction, and their microstructures as well as those of the castables were characterized via scanning electron microscopy. The linear expansion of the castables decreased with increase in the pre-firing temperature of andalusite, as less residual andalusite in the pre-fired aggregates lowered its transformation in the castables during firing at high temperatures. Moreover, the higher amounts of SiO₂-rich glass produced by andalusite pre-fired at high temperatures and secondary mullite generated between andalusite and the matrixes were both favorable to the production of castables with denser structure. This prevented the diffusion of O₂ into the castables and improved their oxidation resistance.     

Improvement of the mechanical properties of SiC reticulated porous ceramics with optimized three-layered struts for porous media combustion

Silicon carbide reticulated porous ceramics (SiC RPCs) with three-layered struts were fabricated by polymer replica method, followed by infiltrating alumina slurries containing silicon (slurry-Si) and andalusite (slurry-An), respectively. The effects of composition of infiltration slurries on the strut structure, mechanical properties and thermal shock resistance of SiC RPCs were investigated. The results showed that the SiC RPCs infiltrated with slurry-Si and slurry-An exhibited better mechanical properties and thermal shock resistance in comparison with those of alumina slurry infiltration, even obtained the considerable strength at 1300 °C. In slurry-Si, silicon was oxidized into SiO₂ in the temperature range from 1300 °C to 1400 °C and it reacted with Al₂O₃ into mullite phase at 1450 °C. Meantime, the addition of silicon in slurry-Si could reduce SiC oxidation of SiC RPCs during firing process in contrast with alumina slurry. With regard to slurry-An, andalusite started to transform into mullite phase at 1300 °C and the secondary mullitization occurred at 1450 °C. The enhanced mechanical properties and thermal shock resistance of SiC RPCs infiltrated alumina slurries containing silicon and andalusite were attributed to the optimized microstructure and the triangular zone (inner layer of strut) with mullite bonded corundum via reaction sintering. In addition, the generation of residual compressive stress together with better interlocked needle-like mullite led to the crack-deflection in SiC skeleton, thus improving the thermal shock resistance of obtained SiC RPCs.