A replacement of traditional insulation refractory brick by a waste-derived lightweight refractory castable

Lightweight insulation refractories are essential for high-temperature performance to reduce energy consumption. This study investigates a new insulation material, that is, solid waste rice husk ash (RHA) derived lightweight refractory castable, replacing traditional insulation refractory brick. The RHA is generated after the burning of rice husk as biomass fuel. The RHA is used as an aggregate and alkali-extracted silica sol from RHA as a binder to fabricate the insulation castable. The nanosilica containing (~30 wt%) sol is employed to synthesize the refractory castable by varying the sol amount (2.5-12.5 wt% silica from sol). The castable specimens are cast by a vibro-caster and fired at 900-1200°C in a muffle furnace. The physic-mechanical and thermal conductivity (κ) of the castable is investigated. At 1100°C with 10 wt% dry sol retaining sample shows an excellent apparent porosity (~65%), low bulk density (~ 0.8 g/cm³), and κ (0.136 W/m k) with sustainable compressive strength (6 MPa). The acquired results are a good match with the literature (other wastes-derived insulation materials) and industrial (silica insulation brick) obtained data. These promising outcomes may inspire the refractory industries for using RHA as an aggregate and RHA extracted sol as a binder for making insulation castable.     

Investigation of microstructure and mechanical properties of Cu/Ti–B–SiCp hybrid composites

In this study, Cu/Ti–B-SiC_p hybrid composite materials were produced by powder metallurgy method using three different sintering temperatures (950, 1000, 1050 °C). The optimum sintering temperature of Cu main matrix composites reinforced with Ti–B-SiC_p reinforcement materials at 2-4-6-8 wt.% were determined and their microstructure and mechanical properties were investigated. As a result of microstructure studies, it was determined that reinforcement elements have a homogeneous interface in the main matrix. The hardness of the produced composites was determined by the Brinell hardness method. The highest hardness value (77.74 HB) was determined in the sample with 6 wt% reinforcement ratio. In the tensile and three point bending tests, maximum strength values (112.96 MPa, 37.76 MPa) were found in samples with a reinforcement ratio of 4 wt%. It was determined that increasing reinforcement ratios and sintering temperature made a positive contribution to the hybrid composite materials produced.     

Alkali-activated refractory wastes exposed to high temperatures: development and characterization

The feasibility to prepare alkali-activated materials starting from refractory wastes and their properties after exposure to high temperatures (800, 1000, 1200 and 1400 °C) were investigated. Two different aluminosilicate wastes were used: chamotte (CH, mainly composed of corundum, mullite and andalusite) and alumina-zirconia-silica (AZS, composed by baddeleyite, corundum and amorphous silica). Very high mechanical properties were achieved in both cases (28-days compressive strength of approx. 70 and 60 MPa for CH- and AZS-based pastes, respectively). Then, alkali-activated pastes were exposed to high temperatures. For both kinds of samples, a sharp increase of mechanical properties was obtained after exposure to 800 and 1000 °C thanks to the matrix densification. Above 1000 °C viscous sintering occurs leading to a further increase of mechanical properties. AZS-based materials were able to withstand high temperatures up to 1400 °C while CH-based pastes mechanical properties decrease at 1400 °C due to andalusite decomposition.     

Porous hydroxyapatite‐bioactive glass hybrid scaffolds fabricated via ceramic honeycomb extrusion

The successful fabrication of hydroxyapatite‐bioactive glass scaffolds using honeycomb extrusion is presented herein. Hydroxyapatite was combined with either 10 wt% stoichiometric Bioglass^® (BG1), calcium‐excess Bioglass^® (BG2) or canasite (CAN). For all composite materials, glass‐induced partial phase transformation of the HA into the mechanically weaker β‐tricalcium phosphate (TCP) occurred but XRD data demonstrated that BG2 exhibited a lower volume fraction of TCP than BG1. Consequently, the maximum compressive strength observed for BG1 and BG2 were 30.3 ± 3.9 and 56.7 ± 6.9 MPa, respectively, for specimens sintered at 1300°C. CAN scaffolds, in contrast, collapsed when handled when sintered below 1300°C, and thus failed. The microstructure illustrated a morphology similar to that of BG1 sintered at 1200°C, and hence a comparable compressive strength (11.4 ± 3.1 MPa). The results highlight the great potential offered by honeycomb extrusion for fabricating high‐strength porous scaffolds. The compressive strengths exceed that of commercial scaffolds, and biological tests revealed an increase in cell viability over 7 days for all hybrid scaffolds. Thus it is expected that the incorporation of 10 wt% bioactive glass will provide the added advantage of enhanced bioactivity in concert with improved mechanical stability.     

Properties and characterization of alumina platelet reinforced geopolymer composites

Geopolymers are porous, amorphous, alkali-aluminosilicate hydrate materials formed at room temperature via a solution process. Geopolymer based on metakaolin had a relatively homogeneous microstructure that offered consistent behavior but suffered from dehydration cracking and large densification shrinkages when heated. It was found that by reinforcing a metakaolin geopolymer of composition (K₂O·Al₂O₃·4SiO₂·11H₂O) with 50 µm diameter alumina platelets, dehydration cracking could be prevented, and shrinkage could be reduced by an order of magnitude. Samples were reinforced with 30, 50, and 70 wt% of alumina platelets. Although the properties of the 30 and 50 wt% conditions were better than those of unreinforced geopolymer, those samples still showed warping, cracking, and strength losses on heating. The 70 wt% samples did not warp or crack when heated to temperatures of up to 1500°C. The room-temperature 4-point flexural strength of these samples remained at around 20 MPa regardless of heat treatments. The in situ measured flexural strength increased to almost 40 MPa at 600°C, and remained higher than 20 MPa until 1200°C. Samples subjected to propane-torch thermal shock heating and subsequent quenching did not crack or fragment. Dilatometry, X-ray diffraction, and scanning electron microscopy were used for additional characterization. Given these properties, this material showed promise as a castable refractory.     

Production of nepheline/quartz ceramics from geopolymer mortars

This research has investigated the mechanical properties and microstructure of metakaolin derived geopolymer mortars containing 50% by weight of silica sand, after exposure to temperatures up to 1200 °C. The compressive strength, porosity and microstructure of the geopolymer mortar samples were not significantly affected by temperatures up to 800 °C. Nepheline (NaAlSiO₄) and carnegieite (NaAlSiO₄) form at 900 °C in the geopolymer phase and after exposure to 1000 °C the mortar samples were transformed into polycrystalline nepheline/quartz ceramics with relatively high compressive strength (～275 MPa) and high Vickers hardness (～350 HV). Between 1000 and 1200 °C the samples soften with gas evolution causing the formation of closed porosity that reduced sample density and limited the mechanical properties.     

Strength of single-phase high-entropy carbide ceramics up to 2300°C

The mechanical properties of single-phase (Hf,Zr,Ti,Ta,Nb)C high-entropy carbide (HEC) ceramics were investigated. Ceramics with relative density >99% and an average grain size of 0.9 ± 0.3 µm were produced by a two-step process that involved carbothermal reduction at 1600°C and hot pressing at 1900°C. At room temperature, Vickers hardness was 25.0 ± 1.0 GPa at a load of 4.9 N, Young's modulus was 450 GPa, chevron notch fracture toughness was 3.5 ± 0.3 MPa·m^1/2, and four-point flexural strength was 421 ± 27 MPa. With increasing temperature, flexural strength stayed above ~400 MPa up to 1800°C, then decreased nearly linearly to 318 ± 21 MPa at 2000°C and to 93 ± 10 MPa at 2300°C. No significant changes in relative density or average grain size were noted after testing at elevated temperatures. The degradation of flexural strength above 1800°C was attributed to a decrease in dislocation density that was accompanied by an increase in dislocation motion. These are the first reported flexural strengths of HEC ceramics at elevated temperatures.     

High-strength,flexible and superhydrophobic graphene/aramid nanofiber nanocomposite films for electromagnetic interference shielding application

Electromagnetic interference (EMI) shielding composite materials exhibit many amazing characteristics, including low density, excellent flexibility and corrosion resistance, compared with metals. However, it is a long-standing challenge for EMI shielding materials to overcome inferior mechanical properties and limited hydrophobic characteristics. In this work, the high-strength, flexible and superhydrophobic graphene nanosheet/aramid nanofiber (GNS/ANF) nanocomposite films with layered microstructure were prepared by the sol-gel transformation and spray coating process. The resultant film exhibits excellent mechanical properties with a high tensile strength (131.17 ± 2.77 MPa), large fracture strain (9.58 ± 0.58%), and favorable toughness (8.84 ± 0.74 MJ m^?3), which are 26.2 times, 7.5 times and 221.0 times higher than those of pure GNS films. These results are attributed to synergistic effect between intensive stretching of three dimensional (3D) nanofiber network and extensive sliding of GNS produced effective energy dissipation. Moreover, the film with content of 70 wt% GNS has EMI shielding effectiveness of 31.3 dB, and its reflection coefficient is more than 0.89, revealing a reflection-dominant shielding mechanism. Meanwhile, the film possesses superhydrophobic property (158.7° ± 1.1°) and flame retardancy. The multilayered nanocomposite films have excellent potential for high-performance EMI shielding applications under some outdoor conditions.     

The influence of different SiC amounts on the microstructure,densification, and mechanical properties of hot-pressed Al2O3-SiC composites

The hot pressing process of monolithic Al₂O₃ and Al₂O₃-SiC composites with 0-25 wt% of submicrometer silicon carbide was done in this paper. The presence of SiC particles prohibited the grain growth of the Al₂O₃ matrix during sintering at the temperatures of 1450°C and 1550°C for 1 h and under the pressure of 30 MPa in vacuum. The effect of SiC reinforcement on the mechanical properties of composite specimens like fracture toughness, flexural strength, and hardness was discussed. The results showed that the maximum values of fracture toughness (5.9 ± 0.5 MPa.m^1/2) and hardness (20.8 ± 0.4 GPa) were obtained for the Al₂O₃-5 wt% SiC composite specimens. The significant improvement in fracture toughness of composite specimens in comparison with the monolithic alumina (3.1 ± 0.4 MPa.m^1/2) could be attributed to crack deflection as one of the toughening mechanisms with regard to the presence of SiC particles. In addition, the flexural strength was improved by increasing SiC value up to 25 wt% and reached 395 ± 1.4 MPa. The scanning electron microscopy (SEM) observations verified that the increasing of flexural strength was related to the fine-grained microstructure.     

High-temperature properties and interface evolution of silicon nitride fiber reinforced silica matrix wave-transparent composite materials

In order to improve the high-temperature performance of wave-transparent materials especially for the high-speed aircrafts application, filament winding combined with sol-gel method was adopted to the fabrication of unidirectional silicon nitride fiber reinforced silica matrix composites. The mechanical properties and the interface evolution at high temperatures were investigated. The results show that the composite sintered in N₂ maintains a flexural strength of 210MPa at up to 1200°C, while its counterpart prepared in air experiences a dramatic reduction to about 73MPa. The degradation is due to the partial oxidation of silicon nitride fibers at the fiber matrix interface. Besides, it is also notable that the bending strength of these two composites undergoes a similar growth from about 160 to 210MPa when tested under 900°C, which can be explained by the release of thermal stress on the silicon nitride fibers.     

Enhancing the physical and mechanical properties of pellet-shaped hydroxyapatite by controlling micron- and nano-sized powder ratios

Hydroxyapatite (HA) was fabricated in microns as its basic size. The particle size distribution was controlled by mixing micron- and nano-sized HA to obtain the optimum amount of mixture to improve its properties. HA powder with a size of 2.5 μm was mixed with that with a size of 200 nm, with a variety of concentrations of up to 20 wt%. A green body was fabricated using the uniaxial pressing method at a pressure of 200 MPa. The sintering process was conducted at a temperature of 1200 °C, heating rate of 3 °C/min, and holding time of 2 h in air. The physical characteristics of the HA sintered body were determined using X-ray diffraction, scanning electron microscopy, linear shrinkage, and density testing. The mechanical properties of the HA sintered body were tested using compressive strength testing. The test results indicated that the mechanical properties of the HA sintered body increased with the addition of nano-sized HA. The mechanism of the increasing strength occurred because nano-sized HA particles filled the gaps between the micron-sized particles. In this study, the highest mechanical properties were obtained by adding 20 wt% nano-sized HA. The compressive strength in the sample without added nano-sized HA was 132.2 MPa and increased significantly to 208.6 MPa with the addition of nano-sized HA of 20 wt%. No change in the phase in HA was observed within a sintering temperature of 1200 °C.     

Facile synthesis of phenolic-reinforced silica aerogel composites for thermal insulation under thermal-force coupling conditions

The objective of this research was to develop a reinforced silica aerogel composites with enhanced thermal insulation performance under thermal-force coupling conditions. Phenolic-reinforced silica aerogel composites (RAs) were synthesized via a sol-immersion-gel process based on the in-situ polymerization of resorcinol (R), formaldehyde (F), and triaminopropyltriethoxysilane (APTES). Ambient pressure drying (APD) was used to dry the gels. Samples with different carbon/silica ratios, RA12, RA13, and RA14, were synthesized by controlling the R/APTES molar ratio at 1/2, 1/3, and 1/4. The densities of the RA12, RA13 and RA14 samples were 0.32 ± 0.005, 0.34 ± 0.006, and 0.37 ± 0.003 g/cm³. The thermal conductivity of the RA12, RA13, and RA14 samples were 0.024, 0.026, and 0.027 W/(m·K). The existence of the phenolic network favored the mechanical strength of the RAs, RA14 showed compressive strength, tensile strength, and three-point bending strength of 4.3 MPa at 20% strain, 2.4 MPa, and 8.4 MPa at 1.45% strain. The RAs showed excellent thermal insulation performance on a customized apparatus, the back temperature was as low as 219.82 °C and 330.67 °C within 60 min in the environments of 600 °C −0.01 MPa and 600 °C −0.9 MPa. The excellent thermal-shock performance of RAs was also demonstrated under flame exposure from a butane torch, with a temperature difference of 878 °C within 30 min being reported. The excellent thermal insulation performance of RAs under thermal-force coupling conditions reveals a widespread application perspective in the field of new energy vehicles power battery thermal protection.     

Recycling of aluminum dross and silica fume wastes for production of mullite-containing ceramics: Powder preparation,sinterability and properties

The improper disposal of industrial wastes causes environmental pollution so their recycling for fabrication of new products became an interesting research issue. In this work, sintered mullite-containing ceramics were prepared from aluminum dross and silica fume (up to 40 wt%) waste materials after sintering up to 1500 °C. Before sintering, the starting waste materials were converted into nano powders by mechanical milling alloying method up to 15 h. The obtained waste nano powders were investigated using different techniques as X-ray diffraction (XRD), transmission electron microscope (TEM) and scanning electron microscope (SEM). On the other hand, phase identification by XRD, physical properties determination (bulk density and apparent porosity), microstructure by SEM, mechanical and electrical properties of sintered bodies were investigated. The results revealed that mullite phase was formed in higher amounts with increasing both sintering temperature (1500 °C) and silica fume content. At 1300 °C, amorphous mullite was formed in addition to the alumina phase. It is also noted that the apparent porosity and bulk density were reduced with increasing silica content. However, they exhibited opposite trend when the temperature increased from 1300 into 1500 °C. Moreover, with increasing the mullite content, the microhardness, compressive strength, Younges modulus and electrical conductivity were decreased and reached 10.2 GPa, 216.9 MPa, 119.7 GPa and 4.9 × 10 ^?12 S/m, respectively, for the sample that contained higher amount of mullite, while the fracture toughness was improved and reached to 3.44 MPa m^0.5.     

Amorphous self-glazed,chopped basalt fiber reinforced,geopolymer-based composites

Geopolymer composites reinforced with refractory, chopped basalt fibers, and low melting glass were fabricated and heat treated at higher temperatures. K₂O·Al₂O₃·4SiO₂·11H₂O was the stoichiometric composition of the potassium-based geopolymer which was produced from water glass (fumed silica, deionized water, potassium hydroxide), and metakaolin. Addition of low melting glass (T_m ~815°C) increased the flexure strength of the composites to ~5 MPa after heat treatment above 1000°C to 1200°C. A Weibull statistical analysis was performed exhibiting how the amorphous self-healing and self-glazing effect of the glass frit significantly improved the flexure strength of the geopolymer and ceramic composites after exposure for 1 hour to high temperatures. At 950-1000°C, the K-based geopolymer converted to primarily a crystalline leucite ceramic, but the basalt fiber remained intact, and the melted glass frit flowed out of the surface cracks and sealed them. 1150℃ was determined to be the optimum heat treatment temperature, as at ≤1200°C, the basalt fibers melt and the strength of the reinforcement in the composites is significantly reduced. The amorphous self-healing and amorphous self-glazing effects of the glass frit significantly improved the room temperature flexure strength of the heat-treated geopolymer and ceramic composites.     

Reactive hot pressing of TiC0.5 ceramic at low applied pressure with 1 wt% Ni additive

Densification in non-stoichiometric TiC_0.5 ceramic has been studied by reactive hot pressing (RHP) of Ti:0.5C composition at 4–40 MPa, 1200°C for 60 min. Incomplete reaction and 94% relative density (RD) at a pressure of 4 MPa changed to 99% RD and a negligible amount of residual Ti at 40 MPa. In contrast, the addition of 1 wt% Ni in the starting Ti-0.5C powder mixture resulted in full density at a lower pressure of 4 MPa, leading to comparable hardness. It is argued that both, reaction as well as densification, were improved by the formation of transient Ti-Ni liquid phase. The enhanced RD and residual metallic phase in the nickel-containing non-stoichiometric TiC_0.5 showed high flexural strength (537 ± 79 MPa), which is comparable to values obtained from materials processed at high temperature and pressure.     

Fabrication and characterization of silica ceramic compact prepared using Aloe-Vera mucilage as a binder

In this study, silica compacts were fabricated through a powder processing route at different compaction pressure, using Aloe-Vera (AV) mucilage as a binder. The silica compacts were prepared at 90, 100, and 110 MPa compaction pressure using 0%–16 wt% of AV binder. The optimum amount of AV binder was 14 wt% for both 90 and 100 MPa and 12 wt% for 110 MPa. The maximum achieved green density and green strength of silica compacts at the optimum binder amount were 62.3% and 4 MPa, respectively at 110 MPa compaction pressure. The green silica compacts prepared at 110 MPa compaction pressure exhibited a minimum porosity of 21% and maximum flexural strength of 15 MPa after sintering at 1400°C. The green silica compacts with the optimum amount of binder were strong enough for machining. The Fourier transform infrared spectroscopy analysis revealed the functional groups present in AV mucilage. The binder burnout characteristic of AV mucilage in the silica compact was determined by thermogravimetric analysis and differential thermal analysis. Additionally, AV gel acted as a binder and solvent simultaneously for ceramic compaction.     

Mechanical and chemical properties of metakaolin-based geopolymer exposed to elevated temperature

A method is presented to fabricate metakaolin-based geopolymers that are structurally and mechanically stable up to 600°C. The chemical environment of the geopolymers is characterized using thermogravimetric analysis and Fourier-transform infrared spectroscopy. Residual free water turned into steam and caused damage to the geopolymer when exposed to elevated temperatures. The curing temperature was increased from 80 to 120°C to remove water during the curing process. A correlation was drawn between the amount of Si-O-Al linkage formed and the position of fingerprint peaks in infrared spectra, providing a tool to evaluate the level of geopolymerization. Flexural and tensile properties of geopolymers fabricated using the optimized method were measured for no heat treatment and for exposure to elevated temperatures of 200, 400, and 600°C. The flexural strength was measured to be 10.80 ± 2.99 MPa at room temperature, 10.36 ± 0.64 MPa at 400°C, and 8.04 ± 1.60 MPa at 600°C. The flexural modulus is reported to be 13.09 ± 3.40 GPa at room temperature and 11.03 ± 0.53 GPa at 600°C. The flexural toughness decreased with increasing temperature. The tensile properties of the geopolymer were measured with direct tensile tests paired with an extensometer. The tensile strength decreased from 4.16 ± 2.08 MPa at room temperature to 3.13 ± 0.97 MPa at 400°C, and 2.75 ± 0.86 MPa at 600°C. The Young's modulus decreased from 45.38 ± 30.30 GPa at room temperature to 26.88 ± 6.65 GPa at 600°C. Both flexural and tensile tests have shown that the metakaolin-based geopolymers cured at 120°C is mechanically stable at temperatures up to 600°C.     

High-temperature mechanical properties of NextelTM 610 fiber reinforced silica matrix composites

Novel Nextel? 610 fiber reinforced silica (N610f/SiO₂) composites were fabricated via sol-gel process at a sintering temperature range of 800–1200?°C. The sintering-temperature dependent microstructures and mechanical properties of the N610f/SiO₂ composites were investigated comprehensively by X-ray diffraction, nanoindentation, three-point bending etc. The results suggested a thermally stable Nextel? 610 fiber whose properties were barely degraded after the harsh sol-gel process. A phase transition in the silica matrix was observed at a critical sintering temperature of 1200?°C, which led to a significant increase in the Young's modulus and hardness. Due to the weak fiber/matrix interfacial interaction, the 800?°C and 1000?°C fabricated N610f/SiO₂ composites exhibited quasi-ductile fracture behaviors. Specially, the latter possessed the highest flexural strength of ≈ 164.5?MPa among current SiO₂-matrix composites reinforced by fibers. The higher sintering-temperature at 1200?°C intensified the SiO₂ matrix, but strengthened the interface, thus resulting in a brittle fracture behavior of the N610f/SiO₂ composite. Finally, the mechanical properties of this novel composite presented good thermal stability at high temperatures up to 1000?°C.     

Nano-infiltration and transient eutectic (NITE) joining of SiC ceramics applied for the harsh environments

To expand the application of SiC/SiC joints under extreme conditions, Nano-Infiltration and Transient Eutectic (NITE) joining technology with AlN-Y₂O₃ as a sintering additive was successfully developed. The rheological properties of the slurry and the microstructure evolution of the joints were systematically characterized by rheometer, SEM, EDS, EBSD, and TEM, respectively. Both room-temperature and high-temperature flexural strength was measured to evaluate the mechanical properties of the joints. An immersion test with concentrated nitric acids was performed to evaluate the corrosion resistance of the joints. The defect-free joining layer was composed of a dense α-SiC phase, a small amount of YAG(Y₃Al₅O₁₂) distributed in the triangular grain boundary, and a Y-Al-O glass phase from AlN-Y₂O₃. The mechanism of NITE joining could be attributed to the incoherent growth of the newly generated α-SiC in the joining layer along the α-SiC substrate. The maximum room-temperature strength of the joints was 320.5 ± 37.6 MPa. When the test temperatures were 1000 °C, 1200 °C, and 1400 °C, the flexural strength reached 238.7 ± 33.1 MPa, 215.5 ± 52.5 MPa, and 166.9 ± 52.0 MPa, respectively. After immersing the joints in a concentrated HNO₃ for 168 h, the flexural strength was 173.3 ± 12.6 MPa. The joints' excellent mechanical properties and corrosion resistance reveal great application potential under extreme conditions.     

On the Use of Surfactant-Complexed Chitosan for Toughening 3D Printed Polymethacrylate Composites

This work reports a simple approach to prepare toughened 3D-printed polymethacrylate (PMA) composites using surfactant-modified chitosan (SMCS) particles at loadings between 2–10 wt%. Chitosan (CS) is modified with anionic surfactant, sodium dodecyl sulfate, via ionic complexation to facilitate compatibility and dispersion of CS to PMA matrix by non-covalent interactions between the components. The study successfully demonstrates high-accuracy 3D printing of composites with significant improvements in the overall mechanical properties. The composite with the best loading of 8 wt% SMCS shows a tensile modulus of 1.23 ± 0.05 GPa, a tensile strength at 49.8 ± 0.96 MPa, a yield stress at 33.3 ± 1.48 MPa, and a strain-at-failure 10.3 ± 0.61%, which are 45%, 40%, 32%, and 68% higher than neat PMA, respectively. This provides a significant improvement in toughness at 4.92 ± 0.55 MJ m⁻³ for the composite, 184% higher than that of neat PMA. The marked increase in toughness is due to enhanced filler-matrix interactions which improve the ability of the 3D printed composite to absorb energy under tensile load. The results from this work provide new understandings into the strategies for design and preparation of stereolithography 3D printed materials reinforced with toughening fillers from renewable resources.