Influence of alumina content and thermal treatment on the thermal conductivity of UPE/Al2O3 composite

Ultra high molecular weight polyethylene/alumina (UPE/Al₂O₃) microcomposites with high loading micro alumina (Al₂O₃, 20 to 100 phr) were prepared by casting method. The composites were thermal treated (cooled slowly) and then the thermal properties were studied at temperatures from 25 to 125°C. Thermogravimetric analysis (TGA) and scanning electron microscopic (SEM) proves the homodispersion of Al₂O₃ microparticles in UPE. TGA indicates that the temperature of 5% weight loss of UPE/Al₂O₃ (100 phr) composite is 467.0°C, 10.5°C higher than that of pure UPE. Differential scanning calorimetry (DSC) shows that the melting point and the real degree of crystal (X_rc) of treated UPE/Al₂O₃ composite (100 phr) are 141.4°C and 65.7%, respectively, all higher than that of untreated composite, which can be described by crystal bridge mechanism. The density of the composite is also be enhanced because of crystal volume shrinkage induced by thermal treatment. The thermal conductivity of the treated UPE/Al₂O₃ composite (100 phr) is 1.920 W (m K)^?1 at 25°C, 23.6% higher than that of the untreated composite. Crystal bridge thermal conduction mechanism is proposed. The thermal conductivity of UPE/Al₂O₃ composite has some dependency on the increasing Al₂O₃ content and also thermal treatment. These results can give some advice to design formulations for practical applications in pipe area and other wear area. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40528.     

High-temperature mechanical property and thermal shock resistance of Al2O3f/Al2O3 composites

Continuous alumina fiber–reinforced alumina matrix composites (Al₂O_3f/Al₂O₃ composites) were produced via sol–gel process, then the high-temperature mechanical property and thermal shock resistance of Al₂O_3f/Al₂O₃ composites were investigated. The results showed that the composites exhibited excellent high-temperature properties. The mechanical property of the composites was affected by heat treatment (prepared at 1100°C exhibited the most desirable mechanical property). The tensile strength of the composites abruptly decreased at higher temperatures. Although the mechanical property of the composites deteriorated after the thermal shock test was conducted at high temperatures, they exhibited excellent thermal shock resistance. After 50 thermal shock tests conducted at 1300 and 1500°C, the flexural strength of the composites was found to be 124.34 and 93.04 MPa, thus showing a decrease in strength with the increasing temperature.     

cBN-Al2O3 composites in NaCl environment under high pressure and high temperature conditions

A series of experiments of cubic boron nitride (cBN)-Al₂O₃ composites was conducted in NaCl environment under a pressure of 5 GPa at 1200–1650 °C using a Chinese multi-anvil high-pressure apparatus. The oxidation resistance of cBN-Al₂O₃ composites reached 1300 °C, which was 200 °C higher than that of raw cBN powder. The porosity was estimated by the content of NaCl impurities in cBN-Al₂O₃ composites. The content of NaCl impurity increases with increasing temperature and decreases with increasing Al₂O₃ level under high pressure and high temperature conditions. cBN+30 vol% Al₂O₃ sintered at 5 GPa and 1200 °C shows no NaCl impurity, and the Vickers hardness of the sample is 21.6 GPa which is half of cBN+10 vol% Al.     

Effect of Pluronic F-127 debinding temperature on the sintering and densification of alumina by direct ink writing

Hydrogel-based alumina (Al₂O₃) inks were prepared using Pluronic F-127 with 65 wt% of solid loading (Al₂O₃). The Al₂O₃ inks were deposited, and the freestanding samples were studied using TGA/DTA. Significant weight loss was observed between 180 and 360°C. A two-stage hydrogel debinding process of Al₂O₃ samples was carried out at 180 and 360°C with holding times of 30, 60, 90, 120, and 150 min. The Al₂O₃ samples were then sintered at 1600°C. X-ray diffraction was used for the phase analysis of the alumina inks, and a scanning electron microscope was used microstructural analysis. Based on the TGA/DTA analysis, a two-stage debinding process was adopted. Significant effect of hydrogel debinding temperature was observed on the sintering and densification behavior of alumina. It was observed that the porosities in the alumina samples were increasing when the debinding time was increased from 30 to 150 min, with the debinding temperature at 180 and 360°C. Moreover, the nature of the porosities was changing from closed porosities to interconnected porosities.     

Preparation of nano‐reinforced thermal conductive natural rubber composites

Natural rubber (NR) composites highly filled with nano‐α‐alumina (nano‐α‐Al₂O₃) modified in situ by the silane coupling agent bis‐(3‐triethoxysilylpropyl)‐tetrasulfide (Si69) were prepared. The effects of various modification conditions and filler loading on the properties of the nano‐α‐Al₂O₃/NR composites were investigated. The results indicated that the preparation conditions for optimum mechanical (both static and dynamic) properties and thermal conductivity were as follows: 100 phr of nano‐α‐Al₂O₃, 6 phr of Si69, heat‐treatment time of 5 min at 150°C. Furthermore, two other types of fillers were also investigated as thermally conductive reinforcing fillers for the NR systems: (1) hybrid fillers composed of 100 phr of nano‐α‐Al₂O₃ and various amounts of the carbon black (CB) N330 and (2) nano‐γ‐Al₂O₃, the particles of which are smaller than those of nano‐α‐Al₂O₃. The hybrid fillers had better mechanical properties and dynamic performance with higher thermal conductivity, which means that it can be expected to endow the rubber products serving under dynamic conditions with much longer service life. The smaller sized nano‐γ‐Al₂O₃ particles performed better than the larger‐sized nano‐α‐Al₂O₃ particles in reinforcing NR. However, the composites filled with nano‐γ‐Al₂O₃ had lower thermal conductivity than those filled with nano‐α‐Al₂O₃ and badly deteriorated dynamic properties at loadings higher than 50 phr, both indicating that nano‐γ‐Al₂O₃ is not a good candidate for novel thermally conductive reinforcing filler. POLYM. COMPOS., 37:771–781, 2016. © 2014 Society of Plastics Engineers     

Aluminum oxide particles/silicon carbide whiskers’ synergistic effect on thermal conductivity of high-density polyethylene composites

Aluminum oxide (Al₂O₃) particles and silicon carbide (SiC) whiskers improved the thermal conductivity of high-density polyethylene (HDPE). To improve the dispersion of inorganic fillers in the matrix, 5 wt% of maleic anhydride-modified polyethylene was added into HDPE as a compatibilizer, and the hybrid matrix was denoted as mHDPE. The thermal conductivity, heat resistance, and tensile properties of resulting HDPE composites were characterized. The results showed that the thermal conductivity reached its maximum value of 0.8876 W/(m K) at 1/4 weight ratio of Al₂O₃/SiC, which was 110.3, 54.8, and 8.8% higher than that of pure HDPE, mHDPE/Al₂O₃, and mHDPE/SiC composites, in the order given, indicating that hybrid fillers have synergistic effect on the thermal conductivity of HDPE composites. Moreover, they also have a synergistic effect on the heat resistance and Young’s modulus. As the SiC content increases, the heat resistance of the composites increases at first and then falls, and the maximum VST is reached at an Al₂O₃/SiC weight ratio of 3/2, which is 5.4 °C higher than that of HDPE. The maximum Young’s modulus of the composites (1160 MPa) is obtained at an Al₂O₃/SiC weight ratio of 1/4, and the yield strength increases gradually as the SiC whiskers’ content increases.     

Thermo-mechanical behaviors of epoxy resins reinforced with nano-Al2O3 particles

This study examined the thermo-mechanical behavior of epoxy resins/nano-Al₂O₃ composites including the curing behavior, thermal stability, dynamic mechanical properties and thermal mechanical properties. The DSC curve peak temperature of the composites was decreased by the addition of nano-Al₂O₃. The thermal stability of the composites was similar to that of the neat epoxy resins. Dynamic mechanical analysis (DMA) indicated the glass transition temperature of the composites to be approximately 11 °C higher than that of the neat epoxy resins. The coefficient of thermal expansion (CTE) of the composites decreased with increasing nano-Al₂O₃ content.     

Flexible and robust YAG-Al2O3 composite nanofibrous membranes enabled by a hybrid nanocrystalline-amorphous structure

A flexible and robust YAG-Al₂O₃ composite nanofibrous membrane was fabricated by a combination of sol-gel and electrospinning methods, then a sintering at 900 °C. The effects of Al₂O₃ on the microstructure and mechanical performance of YAG nanofibrous membranes were investigated. The YAG nanofibrous membrane is brittle but the composite membranes exhibit a brittle-to-flexible transformation as the Al₂O₃ content reaches 30 wt.%, which can be attributed to an optimized dense hybrid microstructure consisting of finer YAG grain size surrounded by amorphous Al₂O₃. The YAG-30 wt.% Al₂O₃ nanofibrous membrane sintered at 900 °C shows a tensile strength of 3.52±0.31 MPa, three times of that of pure Al₂O₃ sintered at the same temperature. The membrane still presents a decent flexibility with a tensile strength of 0.75±0.25 MPa after sintering at 1000 °C, which is at least 100 °C higher than the sintering temperature of most reported ceramic nanofibrous membranes.     

Thermal shock resistance of Al2O3/SiO2 composites by sol-gel

In this paper, the SiO₂ ceramic matrix composites were reinforced by the two-dimensional (2D) braided Al₂O₃ fibers by sol-gel. To develop the high performance aeroengine with excellent resistance to thermal shock for advanced aerospace application, two different thermal shock temperatures (1100?°C and 1300?°C) and three different thermal shock cycles (10, 20 and 30 cycles) were tested and compared in this paper; besides, the thermal shock resistance of Al₂O₃/SiO₂ composites was investigated in air. Our results suggested that, the flexural strength of the untreated composites was 78.157?MPa, while the residual strength of Al₂O₃/SiO₂ composites under diverse thermal shock cycles and temperatures had accounted for about 95% and 50% of the untreated composites, respectively. Meanwhile, the density and porosity of the composites were gradually increased with the increase in test temperature. Moreover, the changes in fracture morphology and micro-structural evolution of the composites were also observed. Our observations indicated that, the fracture morphology of the composites mainly exhibited ductile fracture at the thermal shock temperature of 1100?°C, whereas brittle fracture at the thermal shock temperature of 1300?°C. Additionally, Al₂O₃/SiO₂ composites belonged to the Oxide/Oxide CMCs, so no new phase was formed after thermal shock tests. Above all, findings of this paper showed that Al₂O₃/SiO₂ composites had displayed outstanding thermal shock resistance.     

Temperature-induced changes in morphology and microstructure of electrospun mullite nanofibers fabricated from a hybrid mullite sol

Mullite nanofibers with small diameter and high surface area are an ideal candidate as the reinforcements in composite materials, and have promising applications in the fields of catalysis, filtration, thermal storage and so forth. In this work, electrospun mullite nanofibers were successfully synthesized using a hybrid mullite sol. The morphology and microstructure of fibers calcined at different temperatures were investigated. The morphology of fibers synthesized at 900 °C is porous with coarse surface, and after crystallization it becomes compact with smooth surface. The densities of fibers increase with the increasing temperatures. At 1200 °C the surface of fibers becomes coarse again, as a result of the grain growth of mullite. The crystallization path of fibers was revealed that the Al-rich mullite (4Al₂O₃·SiO₂) together with amorphous silica formed at 1000 °C, changed into mullite with higher silica contents as temperature further increased, and finally transformed into a stable 3Al₂O₃·2SiO₂ phase at 1200 °C. During this crystallization process, the flow of amorphous silica phase and the formation of mullite crystal structure benefit the densification of fibers, leading to the resultant fibers with fine and compact microstructure. The present findings can provide a guideline for the preparation of the promising high-mechanical mullite nanofibers and the synthesized nanofibers display great potential as reinforcements in structural ceramic composites.     

Investigation of Characterization and Mechanical Performances of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and SiC Reinforced PA6 Hybrid Composites

In this study, the influence of different weight percentages of alumina oxide (Al₂O₃) and silicon carbide (SiC) reinforcement on the mechanical properties of Polyamide (PA6) composite is investigated. Test specimens of pure PA6, 85 wt% PA6 + 10 wt% Al₂O₃ + 5 wt% SiC and 85 wt% PA6 +10 wt% SiC + 5 wt% Al₂O₃ are prepared using an injection molding machine. To investigate the mechanical behaviors tensile test, impact test, flexural test, and hardness test were conducted in accordance with ASTM standards. Experimental results indicated that the mechanical properties, such as tensile, impact, hardness, and flexural strength were considerably higher than the pure PA6. The tensile fracture morphology and the characterization of PA6 hybrid composites were observed by scanning electron microscope and Fourier transform infrared spectroscopic method. Further, thermogravimetric analysis confirms the thermal stability of PA6 hybrid composites. The reinforcing effects of Al₂O₃ and SiC on the mechanical properties of PA6 hybrid composites were compared and interpreted in this paper. Improved mechanical and thermal characteristics were observed by the addition of small amount of Al₂O₃ and SiC simultaneously reinforced with the pure PA6.     

Enhanced thermal shock response of Al2O3–graphite composites through a layered architectural design

The use of ceramics such as alumina in moving components often requires the addition of low friction materials such as graphite. A new strategy for improving toughness, strength, and thermal-shock resistance of Al₂O₃–graphite self-lubricating composites was proposed in this study. Alumina layers embedded between Al₂O₃–graphite layers were fabricated and tested after thermal shock conditions ranging between 500 °C and 800 °C maximum temperature. Retained strength and apparent fracture toughness after the tests were compared to room temperature values. Results show that compressive residual stresses generated in the outer Al₂O₃–graphite layers during cooling down from sintering improve the failure resistance of the materials. The introduction of heat-resistant particles (Al₂O₃ particles) into graphite layers combined with a layered architecture can greatly decrease the oxidation degradation of the materials below 500 °C. In addition, the retained strength and toughness in the layered architectures after thermal shock between 550 °C and 800 °C remains constant, thus indicating that the new-developed Al₂O₃/Al₂O₃–graphite laminated composites may be reliable candidates for self–lubricating applications also for elevated temperatures.     

Properties of Ferrite-Filled Natural Rubber Composites

Nickel zinc ferrite (Ni-ZnFe₂O₄)-filled natural rubber (NR) composite was prepared at various loading of ferrite. The tensile properties included in this study were tensile strength, tensile modulus and elongation at break. The tensile strength and elongation at break of the composites increased up to 40 parts per hundred rubber (phr) of ferrite and then decreased at higher loading whereas the tensile modulus was increased gradually with increasing of ferrite loading. Scanning electron microscopy (SEM) was used to determine the wettability of filler in rubber matrix. From the observation, the increase of filler loading reduced the wettability of the filler. Thermal stability of the composites was conducted by using a thermogravimetry analyser (TGA). The incorporation of ferrite in NR composites enhanced the thermal stability of NR composites. The swelling test results indicate that the swelling percentage of the composites decreased by increasing of ferrite loading. The initial permeability, μ_i and quality factor, Q of magnetic properties of NR composites achieved maximum value at 60 phr of ferrite loading for frequency range between 5000–40,000 kHz. The maximum impedance, Z _max of the NR composites was at the highest value at 80 phr ferrite loading for frequency range between 200–800 MHz.     

Bi-metallic catalysts of mesoporous Al2O3 supported on Fe,Ni and Mn for methane decomposition: Effect of activation temperature

Methane decomposition reaction has been studied at three different activation temperatures (500 °C, 800 °C and 950 °C) over mesoporous alumina supported Ni–Fe and Mn–Fe based bimetallic catalysts. On co-impregnation of Ni on Fe/Al₂O₃ the activity of the catalyst was retained even at the high activation temperature at 950 °C and up to 180 min. The Ni promotion enhanced the reducibility of Fe/Al₂O₃ oxides showing higher catalytic activity with a hydrogen yield of 69%. The reactivity of bimetallic Mn and Fe over Al₂O₃ catalyst decreased at 800 °C and 950 °C activation temperatures. Regeneration studies revealed that the catalyst could be effectively recycled up to 9 times. The addition of O₂ (1 ml, 2 ml, 4 ml) in the feed enhanced substantially CH₄ conversion, the yield of hydrogen and the stability of the catalyst.     

High performance polymer composites on PEEK reinforced with aluminum oxide

A study on high performance poly(ether‐ether‐ketone) (PEEK) composites prepared by incorporating aluminum oxide (Al₂O₃), 0 to 50 wt % by hot compaction at 15 MPa and 350°C was described. Density, thermogravimetric analysis/differential scanning calorimetry, and scanning electron microscopy (SEM) were employed to evaluate their density, thermal stability, crystallinity, and morphology. Experimental density was found higher than theoretical density, which indicates that composite samples are sound. It was found that the addition of micron sized (< 15 μm) Al₂O₃ increased the peak crystallization temperature by 12°C when compared with neat PEEK with insignificant increase in melting temperature. Half‐time of crystallization is reduced from 2.05 min for the neat PEEK to 1.08 min for PEEK incorporated with 30 wt % Al₂O₃ because of the strong nucleation effect of Al₂O₃. The thermal stability of composites in air atmosphere was increased by 26°C. However, thermal stability in nitrogen atmosphere decreases at lower concentration of Al₂O₃ but increases above 20 wt % of Al₂O₃. Uniform dispersion of Al₂O₃ particles was observed in PEEK polymer matrix by SEM. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4623–4631, 2006     

Degradation behavior of SiO2f/Al2O3-SiO2 composites: Anti-oxidation behavior and high-temperature flexural strength

2.5-Dimensional SiO₂ fiber-reinforced Al₂O₃-SiO₂ (SiO_2f/Al₂O₃-SiO₂) composites were prepared by the sol-gel method, using diphasic SiO₂ sol as the precursor into which Al₂O₃ powders were added. Their antioxidative behaviors and flexural strengths at high temperature were tested and compared. In an oxidation atmosphere, the composites showed high oxidation resistance, with a flexural strength retention ratio of over 90.00% at 1200?°C. After oxidation at 1500?°C, the mass retention ratio and flexural strength were 97.49% and 65.0?MPa respectively. The oxidation resistance of SiO_2f/Al₂O₃-SiO₂ composites was higher than that of SiO_2f/SiO₂ composites. After high-temperature test, the flexural strength retention ratios of SiO_2f/SiO₂ and SiO_2f/Al₂O₃-SiO₂ composites were 86.18% and 94.80% respectively, and the latter had a flexural strength of 134.9?MPa. SiO_2?f/Al₂O₃-SiO₂ composites worked better than SiO_2f/SiO₂ composites did in the flexural strength test at 1200?°C. The mechanical performance degradation and mass variations of the composites during tests were closely associated with their microstructural evolutions.     

Evolution of phase composition and microstructure of commercial Al2O3 gel in different heat treatment condition

Evolutions of phase composition and microstructure of commercial Al₂O₃ gel in different heat treatment conditions (temperature, atmosphere and additives) were investigated. There was almost no effect of atmosphere and carbon additive on phase evolution of Al₂O₃ gel during heating, γ-Al₂O₃ formed at 800?°C, γ-Al₂O₃ and minor θ-Al₂O₃ co-existed at 1000 °C, and single phase of ɑ-Al₂O₃ occurred at heating temperature ≥1200?°C. Atmosphere and carbon had great effects on morphology and crystal size of Al₂O₃ particles. Crystal size of spherical-shape Al₂O₃ particles was 10–20?nm after heating at 800–1000?°C in air, afterwards, they rapidly grew into micro or macro scale when temperature was above 1200?°C, and sintering phenomena of worm-like Al₂O₃ particles were observed. In the presence of carbon, spherical-shape Al₂O₃ particles grew slightly from 10 to 20?nm to 50–60?nm with the temperature increasing from 800?°C to 1500?°C in reducing atmosphere, carbon inclusions in Al₂O₃ grain boundaries triggered a steric hindrance of Al₂O₃ particles growth. Al₂O₃ gel had a high reactive ability and could react with microsilica to form nano mullite crystals at relatively lower temperature.     

Effective thermal conductivity and thermal properties of phthalonitrile‐terminated poly(arylene ether nitriles) composites with hybrid functionalized alumina

A polymer‐based thermal conductive composite has been developed. It is based on a dispersion of micro‐ and nanosized alumina (Al₂O₃) in the phthalonitrile‐terminated poly (arylene ether nitriles) (PEN‐t‐ph) via solution casting method. The Al₂O₃ with different particle sizes were functionalized with phthalocyanine (Pc) which was used as coupling agent to improve the compatibility of Al₂O₃ and PEN‐t‐ph matrix. The content of microsized functionalized Al₂O₃ (m‐f‐Al₂O₃) maintained at 30 wt % to form the main thermally conductive path in the composites, and the nanosized functionalized Al₂O₃ (n‐f‐Al₂O₃) act as connection role to provide additional channels for the heat flow. The thermal conductivity of the f‐Al₂O₃/PEN‐t‐ph composites were investigated as a function of n‐f‐Al₂O₃ loading. Also, a remarkable improvement of the thermal conductivity from 0.206 to 0.467 W/mK was achieved at 30 wt % n‐f‐Al₂O₃ loading, which is nearly 2.7‐fold higher than that of pure PEN‐t‐ph polymer. Furthermore, the mechanical testing reveals that the tensile strength increased from 99 MPa for pure PEN‐t‐ph to 105 MPa for composites with 30 wt % m‐f‐Al₂O₃ filler loading. In addition, the PEN‐t‐ph composites possess excellent thermal properties with glass transition temperature (T_g) above 184°C, and initial degradation temperature (T_id) over 490°C. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41595.     

Flash sintering of 3YSZ/Al2O3-platelet composites

Preparation of 3YSZ/Al₂O₃-platelet composites always requires high temperature, long duration, and/or high pressure. Herein, 3YSZ/Al₂O₃-platelet composites are prepared at low temperature of 492°C-645°C in 30 seconds by flash sintering under the electric field of 300-800 V/cm. The influence of electric field and current limit on the densification and grain growth of composites is investigated. The onset temperature for flash sintering is determined by electric field, which is decreased with increasing the electric field. Under the constant electric field, the current limit has a great effect on the density and grain size of composite. The flash-sintered 3YSZ/Al₂O₃-platelet composites exhibit relatively high hardness and elastic modulus. Both Joule heating and defects generation are proposed to be responsible for the rapid densification in flash sintering. This work demonstrates the feasibility of employing the flash sintering to prepare ceramic composites with fine grain size.     

Effect of Al2O3 addition on thermo‐electrical properties of polymer composites: An experimental investigation

To develop a new class of composites with adequately high thermal conductivity and suitably controlled dielectric constant for electronic packages and printed circuit board applications, polymer composites are prepared with microsized Al₂O₃ particle as filler having an average particle size of 80–100 μm. Epoxy and polypropylene (PP) are chosen as matrix materials for this study. Fabrication of epoxy‐based composite is done by hand lay‐up technique and its counterpart PP‐based composite are fabricated by compression molding technique with filler content ranging from 2.5–25 vol%. Effects of filler loading on various thermal properties like effective thermal conductivity (k_eff), glass transition temperature (T_g), coefficient of thermal expansion (CTE) and electrical property like dielectric constant (ε_c) of composites are investigated experimentally. In addition, physical properties like density and void fraction of the composites along with there morphological features are also studied. The experimental findings obtained under controlled laboratory conditions are interpreted using appropriate theoretical models. Results show that with addition of 25 vol% of Al₂O₃, k_eff of epoxy and PP improve by 482% and 498% respectively, T_g of epoxy increases from 98°C to 116°C and that of PP increases from −14.9°C to 3.4°C. For maximum filler loading of 25 vol% the CTE decreases by 14.8% and 26.4% for epoxy and PP respectively whereas the dielectric constants of the composites get suitably controlled simultaneously. POLYM. COMPOS., 36:102–112, 2015. © 2014 Society of Plastics Engineers