Natural rubber composites containing fillers of sol–gel glasses and glass–ceramics in the CaO–SiO2–P2O5 system

A number of properties of natural rubber-based composites containing fillers of sol–gel glasses and ceramics from the CaO–SiO₂–P₂O₅ system were investigated and compared. Its goals were to evaluate and select more suitable usages in flexible electronics, in particular as substrates and insulating layers in antennas for wireless communications. The fillers were characterized by XRD, SEM, adsorption–texture characteristics and particle size distribution. The composites were characterized by their vulcanization, physico-mechanical, dielectric and dynamic characteristics, as well as their volume resistivity and coefficient of thermal conductivity. The studies revealed that the specific features of the fillers, above all their texture characteristics (specific surface area, presence and amount of micro- and mesopores) have a significant impact upon all studied properties of the composites. The complex evaluation has demonstrated that the composites containing glass–ceramic fillers, especially at 20–50 phr, are more suitable as substrates and insulating layers in flexible antennas for wireless communications.

     

Preparation,structural and thermo-mechanical properties of lithium aluminum silicate glass–ceramics

Lithium aluminum silicate glasses of composition (wt%) 12.6Li₂O–71.7SiO₂–5.1Al₂O₃–4.9K₂O–3.2B₂O₃–2.5P₂O₅ were prepared by the melt quench technique. These glasses were converted to glass–ceramics based on DTA data. X-ray diffraction (XRD) and Fourier transform infra-red spectroscopy (FTIR) were used to discern the phases evolved in the glass–ceramics. Phase morphology was studied using scanning electron microscopy (SEM). Thermal expansion coefficient (TEC) and glass transition temperature (T_g) of all samples were measured using thermo-mechanical analyzer (TMA). It was found that 3 h dwell time at crystallization temperature yielded samples with good crystallinity with a TEC of 9.461 × 10⁻⁶ °C⁻¹. Glass–ceramic-to-metal compressive seal with SS-304 was fabricated using LAS glass–ceramic. The presence of metal housing and compressive stresses at the glass–ceramic-to-metal interface reduced average grain size and changed the overall microstructure.     

Elastic interlayer toughening of potassium titanate whiskers–nylon66 composites and their fractal research

Potassium titanate whiskers–nylon66 composites were effectively toughened with an elastic interlayer of epoxy resin. The optimal fraction of epoxy resin was 1.5 wt % of whiskers, which corresponded to an interlayer thickness of 3% of the radius of whiskers. SEM images showed that interfacial adhesion was improved with elastic interlay. The impact strength of the composite increased 132% compared with neat nylon66, whereas the bending and tensile strengths increased 55 and 48%, respectively. Digital image method was adopted to carry out fractal research of fracture surfaces of the composites. Fractal dimensional calculations were based on perimeter–area relation. It was found that with the sizes of measured slit islands increasing, calculated fractal dimensions increased first and became stable after area threshold reached about 500 η² (η was a yardstick). The toughness of composites increased with fractal dimensions, which was explained from the point of fracture mechanism. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 368–374, 2001     

Preparation of boron carbide–aluminum composites by non-aqueous gelcasting

Gelcasting is an attractive forming process to fabricate ceramic parts with near-net-shape. In the present work, non-aqueous gelcasting of boron carbide (B₄C)–aluminum (Al) composites was studied. A stable B₄C–Al slurry with solids loading up to 55 vol.% for gelcasting was prepared. The slurry was solidified in situ to green body with the mean value of relative density of 64% and flexural strength of 21 MPa. The SEM images showed that powders in green body compact closely by the connection of polymer networks. B₄C–Al samples were also obtained by the process of gelcasting and sintering at 1300 °C for 1 h in 0.1 MPa Ar atmosphere. The average bulk density of sintered body was 2.05 g cm⁻³.     

Crack healing in liquid-phase-pressureless-sintered silicon carbide–aluminum nitride composites

Crack healing in liquid-phase-pressureless-sintered SiC–AlN composites was investigated by introducing cracks into specimens and subsequently heat-treating the specimens. It was observed that cracks were healed and the strength was recovered. Cracks were filled with silica or mullite produced by the oxidation of the composites. It was shown that the healing temperature could be fixed in the range 1100–1300 °C and that large cracks up to about 300 μm could be healed completely. Our results imply that a simple oxidation heat-treatment can improve the reliability of silicon carbide–aluminum nitride components.     

Pore and microstructure change induced by SiC whiskers and particles in porous TiB2–TiC–Ti3SiC2 composites

TiB₂–TiC–Ti₃SiC₂ porous composites were prepared through a plasma heating reaction using powder mixtures of Ti, B₄C SiC whiskers (SiC_w) and SiC particles (SiC_p). The effects of the SiC_w and SiC_p content on pore structures, phase constituents, microstructure, and crystal morphology of TiC were studied. The results show that TiC, TiB, Ti₃B₄ phases are formed within the 5Ti+B₄C system. With the addition of SiC_w and SiC_p, the TiB and Ti₃B₄ phases are reduced, sometimes even disappeared. Interestingly, the content of TiB₂ and TiC increased, resulting in Ti₃SiC₂ and TiSi₂ being formed. The porosity of composites increases notably with the addition of SiC_w. However, with the increase of SiC_p, the porosity of the composites first decreases, followed by an increase. After adding the specified amount of SiC_w/SiC_p, the compressive strength of composites are improved significantly. Additionally, the pore size of the composites are decreased significantly with the addition of SiC_w/SiC_p. During the plasma heating process, some Si atoms will diffuse into the TiC lattice, which in turn made the cubic TiC grains into hexagonal lamellar TiC or Ti₃SiC₂ grains.     

Mechanical and thermal properties of hybrid carbon fibre–phenolic matrix composites containing graphene nanoplatelets and graphite powder

Carbon fibre–phenolic matrix (CF–P) composites containing graphene nanoplatelets (GNPs) were manufactured for improved mechanical and thermal properties. For comparison, micrometer-size pyrolytic graphite powder (GP) was also incorporated in CF–P composites. The loading of carbon fibres was kept constant at 60?wt-% while the quantity of GNPs was varied from 0.1?wt-% to 0.3?wt-% and GP from 1.0?wt-% to 3.0?wt-%. Only GNPs were functionalised by ultraviolet-ozone treatment to improve their dispersion in the matrix while all the composites were manufactured by hand layup method and characterised by scanning electron microscopy, impact, flexural, thermogravimetry and ablation tests. The composite containing 0.3?wt-% GNPs showed considerable improvement in ablation, flexural and impact testing as compared to CF-P composites containing GP. Finally, the ablation mechanisms of post-ablated composites were discussed in the light of available data in the literature.     

A review on the processing–morphology–property relationship in biodegradable polymer composites containing carbon nanotubes and nanofibers

Carbon nanofillers containing biodegradable polymer composites have become an emerging frontier in materials science and engineering because of their potential as environmentally friendly materials in multiple applications, from load-bearing to advanced packaging to biomedical applications. Herein, we present the effect of processing parameters on the final morphology and the resulting properties of the biodegradable polymer composites containing carbon nanotubes (CNTs) or carbon nanofibers (CNFs). Various strategies can be employed to develop such composites; however, the type of morphology, which results during processing, significantly affects the final properties of the obtained composites. Therefore, various processing strategies such as melt-blending, additive manufacturing, and electrospinning are critically reviewed, together with the potential applications in load-bearing, tissue engineering, electromagnetic shielding, gas sensing, and packaging. Finally, we discuss the existing challenges and future directions in designing CNTs/CNFs containing biodegradable polymer composites with desired properties.     

Glass–ceramics as oxidation resistant bond coat in thermal barrier coating system

Thermal barrier coating (TBC) system consisting of yttria stabilized zirconia (YSZ) top coat, glass–ceramic bond coat and nickel base superalloy substrate was subjected to static oxidation test at 1200 °C for 500 h in air. Oxidation resistance of this TBC system was compared with the conventional TBC system under identical heat treatment condition. Both the TBC systems were characterized by SEM as well as EDX analysis. No TGO layer was found between the bond coat and the top coat in the case of glass–ceramic bonded TBC system while the conventional TBC system exhibited a TGO layer of about 16 μm thickness at the bond coat-top coat interface region.     

Functional properties of glass–ceramic composites containing industrial inorganic waste and evaluation of their biological compatibility

A study has been carried out on the feasibility of using Latvian industrial waste (peat cool ash, fly ash, aluminium scrap metal processing waste, metallurgical slag and waste cullet glass) and raw mineral materials (limeless clay) to produce dense, frost resistant, chemically durable glass–ceramic materials by powder technology. Highly crystalline and dense products (density: 2.50–2.94 g/cm³, water uptake: 1.3–4.3%) were fabricated from different mixtures by sintering at temperatures in the range of 1060–1160 °C. Glass–ceramics were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and four point bending strength test. Chemical durability, soluble salt crystallization as well as biological tests were carried out in order to evaluate the environmental stability and possible toxicity of the materials. The novel glass–ceramics developed here can find applications as building materials, such as wall tiles and for manufacturing industrial floors.     

Formation of porous α-alumina from ammonium aluminum carbonate hydroxide whiskers

Ammonium aluminum carbonate hydroxide (AACH) whiskers prepared by hydrothermal technique were employed as precursor material for development of porous alumina. After compaction of AACH whiskers at 8 bars, calcination was performed at 650?°C followed by sintering at different temperatures. The sintered samples were characterized by XRD, FTIR, SEM and mercury intrusion porosimetry. Mechanical strength was determined by compression testing. At sintering temperatures of 1200?°C to 1400?°C, the % age porosity was around 80%. At 1500?°C, the percentage porosity decreased to 71%. The as-prepared AACH consisted of bundles of whiskers with diameters as thick as 0.7?µm, while an individual whisker had a diameter of about 100?nm with an aspect ratio of about 33. A two-phase mixture consisting of θ- and α-alumina was obtained at 1100?°C, while at 1200?°C and above, single phase α-alumina was formed. θ-alumina retained the bundle-like morphology. However, transformation to α-alumina was accompanied by formation of bead-like morphology. These beads were joined together through necks/stems within the whiskers as well as across the parallel-lying whiskers. These necks grew at 1300?°C to form aggregates with smooth surfaces. At 1400?°C, these aggregates started joining with each other by neck formation and at 1500?°C, a three-dimensional network was formed. For sintering temperatures of up to 1400?°C, pores with sizes around 260?nm were very stable. At 1500?°C, significant pore growth took place along with an overall densification. Therefore, number of pores with sizes of around 260?nm decreased and those with sizes around 10?µm, 1?µm and 5?nm increased. The compression strength of samples sintered at 1100?°C to 1300?°C was in the range of 3.4–4.3?MPa. At 1400?°C, the strength increased to 5.2?MPa, while at 1500?°C, it jumped to 10.8?MPa due to the formation of three-dimensional network.     

Characterisation and modelling of CNT–epoxy and CNT–fibre–epoxy composites

Abstract

This research presents an experimental and theoretical investigation on the effects of carbon nanotube (CNT) integration within neat epoxy resin (nanocomposites) and a carbon fabric–epoxy composite (multiscale composites). An approach is presented for the prediction of mechanical properties of multiscale composites. This approach combines woven fibre micromechanics (MESOTEX) with the Mori-Tanaka model which was used for the prediction of mechanical properties of nanocomposites in this research. Nanocomposite and multiscale composite samples were manufactured using cast moulding, resin infusion, and hand lay-up process. The CNT concentrations in the composite samples were from 0 to 5 wt-%. The samples were characterised using tensile, shear and flexural tests. The discrepancy between the theoretical predictions and the experimental observations was hypothesised to be due to dispersion and bonding issues and SEM images are presented in support of the hypothesis.     

Thermal aging on ethylene–propylene–diene rubber composites reinforced by samarium oxide,samarium borate and Sb in Sb doped SnO2

Abstract

Ethylene–propylene–diene monomer (EPDM) rubber composites reinforced with 50 phr samarium oxide (Sm₂O₃), samarium borate (SmBO₃) and Sb in antimony doped tin oxide (ATO) are aged at 150°C for different intervals. It is found that neutral Sm₂O₃ and alkaline SmBO₃ can retard the oxidative degradation of EPDM by blocking radical passage. The acidic ATO particles can accelerate the oxidative degradation of EPDM. The trend of tensile strength of EPDM composites is consistent with that of cross-link density of EPDM composites. SmBO₃ and ATO can retard the increase of dielectric loss until 10 days of aging, while Sm₂O₃ can keep the dielectric loss at low level until 14 days of aging. The increased surface charge of filler can make surface and volume resistivity decrease sharply. Antimony doped tin oxide can deteriorate the dielectric strength of EPDM, while SmBO₃ and Sm₂O₃ can keep the dielectric strength of EPDM at a constant level.     

Glass–Ceramic Foams from Borosilicate Glass Waste

In this study, we reported the studies on a glass–ceramic foam with wollastonite and cristobalite micrometric crystals prepared by sintering a borosilicate glass waste with organic binder as foaming agent. The waste glass, coming from the dismantling of washing machine, was characterized by high CaO content and low-temperature sinterability. The effect of the temperature on the sinter-crystallization ability of the borosilicate glass waste was followed with thermal analysis, heating microscopy, and electron scanning microscopy (ESEM) observations. Additionally, the effect of temperature on the evolution of crystalline phases and density variation was monitored with XRD and density measurements. The softening started at 800°C and crystallization at 845°C to be completed at 900°C with a linear expansion of 38–40% in the range 850–900°C. Wollastonite and cristobalite were identified as crystalline phases in variable proportions dependently upon temperature. No crack evidence was found at high ESEM magnification even though cristobalite crystals were present. The final products showed a total porosity around 78–79% and an apparent density of about 0.5 g/cm³, in line with common porous closed-cell glass foams used for thermal insulation.     

Synthesis and pyrolysis of polysilazane precursors containing linear–cyclic structures for Si/N/C-based ceramics

A new kind of polysilazane precursor containing linear–cyclic structure was synthesized from the de-lithium chloride condensation reaction of hexamethylcyclotrisilazane lithium salts (D) and 1,3-dichlorotetraorganodisilazanes [(ClR₁R₂Si)₂NH]. Pyrolyses of these precursors and some comparable linear polysilazanes were carried out at 900 °C in nitrogen. The results indicated that the existence of linear–cyclic structure in precursors provided relatively crosslinked or branched structures, which were helpful for improving ceramic yields. The precursor containing a linear–cyclic structure as well as reactive vinyl groups gave the highest pyrolytic yields. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2827–2831, 2001     

Influence of β-Si3N4 whiskers on sintering and crystallization of fused silica ceramics

Fused silica ceramics are widely applied for radome materials, crucibles, and vanes, but the mechanical properties were deteriorated due to the cristobalite crystallization. The fused silica ceramics added with by β-Si₃N₄ whiskers were prepared by a slip-casting method to retard the cristobalite crystallization. The influences of the sintering environments and the β-Si₃N₄ whiskers on the microstructure and phase structure were investigated. The silanol (Si-(OH)_n) and oxygen vacancies (V_O) in the fused silica in formed in different conditions were studied by Fourier Transform Infra-Red (FT-IR) and X-ray photoelectron spectroscopy (XPS), and the results indicated that the ball-milled produced a large amount of the silanol groups onto the surface of the fused silica particles. The fused silica heated in the vacuum created the maximum oxygen vacancies (24.2%) on the surfaces. Silanol groups reacted with the β-Si₃N₄ whiskers, and the O atoms in the silanol groups were fixed into the bulk materials. And the crystallization kinetics and the activation energy of Si₃N_4w/SiO₂ ceramics at the temperature ranging from 1200 to 1400°C were calculated based on the JMA(Johnson-Mehl-Avrami) model. The activation energy of the fused silica ceramics with the addition of the β-Si₃N₄ is 506.2 kJ/mol, increased by 23.6% than that of the pure fused silica ceramic.     

Sintering and microstructure of mullite–Mo composites

Mullite–Mo composites of different compositions (0–100 vol.% Mo) were sintered to near theoretical density by pulse electric current sintering (PECS). The densification behaviour and the microstructure of mullite–Mo composites as a function of Mo content were studied. The addition of 10 vol.% Mo significantly enhanced the strength and toughness of monolithic mullite to 556 MPa and 2.9 MPa m^1/2, respectively. SEM observations revealed the modification of discrete isolated Mo particles to continuosly interconnected network with the increase in the Mo content. Mo grains were located at the grain boundaries as well as inside the mullite grains. The addition of Mo to monolithic mullite led to a change in the fracture mode.     

Preparation of the organic–inorganic double-shell microencapsulated aluminum hypophosphite and its improved flame retardancy and mechanical properties of epoxy resin composites

To develop the functional particles with better flame-retardant and compatibility with epoxy resin (EP) matrix, organic–inorganic double-shell microencapsulated aluminum hypophosphite (MSiAHP) was prepared by situ polymerization. The water contact angles of MSiAHP (62.4°) is significantly larger than that of aluminum hypophosphite (34.4°), which shows that the organic shell material of MSiAHP endows excellent hydrophobicity and water resistance. With the incorporation of MSiAHP, EP/30%MSiAHP composite exhibits limiting oxygen index value of 27.3% and V-0 rating. Furthermore, the cone calorimetry test reveals that MSiAHP reduces the peak heat release rate, total heat release and total smoke release of EP matrix by 33.3%, 24.4% and 56.6%, respectively. Besides, due to the unique organic–inorganic double-shell structure of MSiAHP particles, EP/30%MSiAHP composite achieves greater thermal stability and higher char yields than pure EP. The investigation of the products in the gas and condensed phase demonstrates that MSiAHP is beneficial to the generation of a high-density and compact carbon layer structure with a high graphitization degree, and delay the generation time of pyrolysis products in the gas phase, which can improve the fire safety of EP composites effectively. Furthermore, preeminent dispersion and compatibility of MSiAHP lead to EP/MSiAHP composites with excellent mechanical properties.     

Impact of the aluminum sulfate 18-hydrate particle size on the coordination crosslinking behaviors of acrylonitrile–butadiene rubber–aluminum sulfate 18-hydrate composites

In this study, aluminum sulfate 18-hydrate [Al₂(SO₄)₃·18H₂O] particles of different sizes, which were obtained via high-energy ball-milling technology, were successfully compounded with acrylonitrile–butadiene rubber (NBR) to fabricate crosslinked rubber composites. The results suggest that high-energy ball milling had no significant change on the crystal structure of Al₂(SO₄)₃·18H₂O, but it significantly reduced the particle size. The effects of the particles size on the coordination crosslinking behaviors and mechanical properties of the NBR–Al₂(SO₄)₃·18H₂O composites were fully explored. The coordination crosslinking reaction was demonstrated to occur between the nitrile group (─CN) and Al(III). Moreover, with the decreasing particle size, the composites achieved a better interfacial adhesion and more crosslinking points, and this led to significant increases in the crosslinking density and the mechanical properties. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47717.     

Effect of SiC nanoparticles on in-situ synthesis of SiC whiskers in corundum–mullite–SiC composites obtained by carbothermal reduction

Corundum–mullite–SiC composites were synthesised using a carbothermal reduction method. The effects of SiC nanoparticles and sintering temperatures on the phase transformation of the composites and the synthesis of SiC whiskers were studied by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Results indicated that corundum, mullite, and SiC whiskers were produced as final products at 1600–1650 °C. SiC whiskers were formed through the vapor–solid mechanism. The added SiC nanoparticles worked as nucleating agents to facilitate the carbothermal reduction of aluminosilicates and formation of SiC whiskers. The sample with the added SiC nanoparticles exhibited a high yield of β-SiC of 17.1%. Furthermore, the SiC nanoparticles decreased the formation temperature of SiC whiskers from the original 1600 °C to 1500 °C, and the porosity of the composites was increased from 56.7% to 64.7% by increasing the partial pressure of SiO gas. This study provides an insight into the more efficient synthesis of composites with SiC whiskers through the carbothermal reduction of aluminosilicates.