Electromagnetic and microwave absorbing properties of hollow carbon nanospheres

A mass of hollow carbon nanospheres (HCNSs) was fabricated by chemical vapour deposition of methane over Ni/Al₂O₃ catalyst at 600 °C. The products were characterized with high-resolution transmission electron microscope images, and the results showed that the external diameter of the HCNSs was 5–90 nm and the thickness of wall was about 10 nm. Microwave absorption of HCNSs/paraffin composites was mainly attributed to dielectric loss. The microwave-absorbing peaks of composites containing HCNSs shifts to low frequencies, and the bandwidth below ?10 dB and minimum RL decrease with increasing thickness of HCNSs/paraffin composites.     

The effects of different precipitant agents on the formation of alumina-magnesia composite powders as the magnesium aluminate spinel precursor

Al₂O₃/MgO composite powders were synthesized via a partially wet chemical method. The effects of precipitant agent on the morphology, size and chemical composition of the resultant powders were investigated. The structures of rod-like with polygonal prism surface, platelet-like and uniform spherical Mg-compound particles were successfully prepared by using ammonium hydrogen carbonate, sodium hydroxide and ammonia water as precipitant agents, respectively. Analysis results proved that using ammonium hydrogen carbonate as precipitant agent produced nesquehonite (MgCO₃·3H₂O) Mg-compounds but in the case of other two precipitant agents, Mg-compound particles with magnesium hydroxide (Mg(OH)₂) chemical composition were obtained. The morphological features of MgO particles in Al₂O₃/MgO composite powders were the same as individual Mg-compound particles. Furthermore, conversion of the Al₂O₃/Mg-compound precursor synthesized with ammonia water to pure magnesium aluminate spinel particles was studied. The precursor converted to pure magnesium aluminate spinel phase with 220?nm particle size at 1200?°C.     

Large-scale preparation of powder composites xNiO/(1-x)Al2O3 in reactions of solution combustion synthesis

The conditions for the production of composites by Solution Combustion Synthesis (SCS) from reaction solutions of aluminum and nickel nitrates of various concentrations with glycine have been studied. The concentration of reaction solutions increases the product yield and the productivity of SCS process. The effect of the presence of Al³⁺ cations in reaction solutions on the combustion reaction intensity reduction has been established. The SCS precursors contained metallic nickel, nickel oxide and amorphous alumina. The phase composition of the samples changed in the process of annealing: nickel oxidation, crystallization of γ-Al₂O₃ (600 °C) and formation of NiAl₂O₄ spinel (600–800 °C) were observed. The maximal specific surface area value recorded after annealing at 800 °C was 59–63 m²/g. In samples obtained with a lower fuel content, γ-Al₂O₃ or a mixture of γ-Al₂O₃ and NiAl₂O₄ crystallize on the surface of composite particles. The concentration of NiO on the surface of composites obtained from precursors synthesized from concentrated solutions at φ = 1.4 is close to the nominal composition.     

Synthesis of sodium β and β″ alumina

Sodium β and β″ alumina, with and without MgO doping, have been synthesized from Na₂CO₃, αAl₂O₃ and MgO. Experimental problems, such as segregation, have been emphasized. βAl₂O₃, with and without MgO, once formed is stable at least up to 1700°C. β″ Al₂O₃ decomposes at 1450°C, whereas MgO-stabilized β″ Al₂O₃ is stable at 1450°C but not at 1650°C.     

Dual growth of graphene nanoplatelets and carbon nanotubes hybrid structure via chemical vapor deposition of methane over Fe–MgO catalysts

Abstract

In this study, Fe–MgO catalyst substrates with various Fe and MgO combinations were evaluated for the growth of different types of carbon nanostructure materials (CNMs), particularly graphene nanoplatelets (GNPs) and carbon nanotubes (CNTs) via chemical vapor deposition using methane as a carbon source. The hydrogen yield was also determined as a valuable by-product in this process. Therefore, a set of Fe–MgO catalysts with different iron loadings (30, 80, 85, 90 and 100?wt %) were prepared by the combustion method to realize this target. The physicochemical properties of freshly calcined Fe–MgO catalysts were investigated by XRD, TPR and BET, while the as-grown CNMs were studied by HR-TEM, XRD and Raman spectroscopy. The results verified that the morphology of as-grown CNMs as well as the H₂ yield was directly correlated to the iron content in the catalyst composition. The XRD and TPR results showed that various FeMgO_x species with deferent levels of interactions were produced with the gradual incorporation of MgO content. TEM images indicated that GNPs were individually grown on the surface of high loaded iron-containing catalysts (90–100?wt %) due to the presence of highly aggregated iron particles. While multi-walled carbon nanotubes (MWCNTs) with uniform diameters were grown on the low iron-loaded catalyst (30%Fe/MgO) due to the formation of highly dispersed FeMgO_x particles. On the other hand, GNPs/MWCNTs hybrid materials were grown on the surface of 80%Fe and 85%Fe/MgO catalysts. This behavior can be interpreted by the co-existence of highly aggregated and highly dispersed Fe₂O₃ particles in the catalyst matrix. The results demonstrated that the catalyst composition has a notable effect on the nature of CNMs products and H₂ yield.     

Effect of in-situ formed MgO on the microstructure of thixomolded AZ91 magnesium alloy

ABSTRACT

In-situ magnesium matrix nano-composites were produced through a reaction of CO₂ with the AZ91 alloy at a semi-solid temperature range. The process was performed at 595°C, which corresponded to about 10% of solid fraction, under shear and mixing conditions generated by a screw-barrel system. As a result, nano-scale native MgO (30–50?nm) and a small amount of Al₄C₃ carbide within an eutectic consisting of α(Mg), β-Mg₁₇Al₁₂ were formed. Homogeneously distributed α(Mg) globular grains with volume 8–12% were visible. AZ91 composites revealed yield strength of 220?MPa at compression strength of 460?MPa and hardness 103?±?2?HV.     

Sintering, densification and crystallization of Ca–Al–B–Si–O glass/Al2O3 composites for LTCC application

Ca–Al–B–Si–O glass/Al₂O₃ composites were prepared based on the borosilicate glass powders (D₅₀ = 2.84) and Al₂O₃ ceramic powders (D₅₀ = 3.26), and the sintering, densification, crystallization of samples were investigated. The shrinkage of sample starts to have a sharp increase at 600 °C. The shrinkage of sample starts to have a further rapid increase after the glass softening temperature of about 713 °C. Glass/Al₂O₃ composites can be sintered at 875 °C/15 min and exhibit better properties of a relative density of 98.4 %, a λ value of 2.89 W/mK, a ε _r value of 7.82 and a tan δ value of 5.3 × 10^?4. The interface between glass and Al₂O₃ grains and the interface between anorthite and glass phase depicts a good compatibility according to transmission electron microcopy test. It is the low sintering temperature, high density and good compatibility with Ag electrodes that, guarantee borosilicate glass/Al₂O₃ composites suitable for low temperature co-fired ceramic materials.     

Synthesis and characterisation of nanostructured Al–Al3V and Al–(Al3V–Al2O3) composites by powder metallurgy

In this study, the formation and characterisation of Aluminium (Al)-based composites by mechanical alloying and hot extrusion were investigated. Initially, the vanadium trialuminide (Al₃V) particles with nanosized structure were successfully produced by mechanical alloying and heat treatment. Al₃V–Al₂O₃ reinforcement was synthesised by mechanochemical reduction during milling of V₂O₅ and Al powder mixture. In order to produce composite powders, reinforcement powders were added to pure Al powders and milled for 5?h. The composite powders were consolidated in an extrusion process. The results showed that nanostructured Al-10?wt-% Al₃V and Al-10?wt-% (Al₃V–Al₂O₃) composites have tensile strengths of 209 and 226?MPa, respectively, at room temperature. In addition, mechanical properties did not drop drastically at temperatures of up to 300°C.     

Microstructural development in Al/MgAl2O4 in situ metal matrix composite using value-added silica sources

Abstract

Al/MgAl₂O₄ in situ metal matrix composites have been synthesized using value-added silica sources (microsilica and rice husk ash) containing ～97% SiO₂ in Al-5 wt.% Mg alloy. The thermodynamics and kinetics of MgAl₂O₄ formation are discussed in detail. The MgO and MgAl₂O₄ phases were found to dominate in microsilica (MS) and rice husk ash (RHA) value-added composites, respectively, during the initial stage of holding the composites at 750 °C. A transition phase between MgO and MgAl₂O₄ was detected by the scanning electron microscopy and energy-dispersive spectroscopy (SEM–EDS) analysis of the particles extracted from the composite using 25% NaOH solution. This confirms that MgO is gradually transformed to MgAl₂O₄ by the reaction 3SiO_2(s)+2MgO_(s)+4Al_(l)→2MgAl₂O_4(s)+3Si_(l). The stoichiometry of MgAl₂O₄, n, computed by a new methodology is between 0.79 and 1.18. The reaction between the silica sources and the molten metal stopped after 55% of the silica source was consumed. A gradual increase in mean MgAl₂O₄ crystallite size, D, from 24 to 36 nm was observed in the samples held for 10 h.     

Mechanical properties of bovine hydroxyapatite (BHA) composites doped with SiO<Subscript>2</Subscript>, MgO,Al<Subscript>2</Subscript>O<Subscript>3</Subscript>, and ZrO<Subscript>2</Subscript>

Biologically derived hydroxyapatite from calcinated (at 850 °C) bovine bones (BHA) was doped with 5 wt% and 10 wt% of SiO₂, MgO, Al₂O₃ and ZrO₂ (stabilized with 8% Y₂O₃). The aim was to improve the sintering ability and the mechanical properties (compression strength and hardness) of the resultant BHA-composites. Cylindrical samples were sintered at several temperatures between 1,000 and 1,300 °C for 4 h in air. The experimental results showed that sintering generally occurs at 1,200 °C. The BHA–MgO composites showed the best sintering performance. In the BHA–SiO₂ composites, extended formation of glassy phase occurred at 1,300 °C, resulting in structural degradation of the resultant samples. No sound reinforcement was achieved in the case of doping with Al₂O₃ and zirconia probably due to the big gap between the optimum sintering temperatures of BHA and these two oxides.     

Major effects on microstructure and electrical properties of ZnO-based linear resistance ceramics with MgO changes

The ZnO-based linear resistance ceramics were fabricated from ZnO–Al₂O₃–MgO–TiO₂ at 1,340 °C. The effect of the different doping amounts of MgO on the microstructure and electrical properties were investigated in detail. The optimal sample with the MgO concentration of 7 wt% possesses an energy density of 812 J/cm³ and a resistance temperature coefficient reaches 0.4 × 10^?3/°C, which are improved by 8 and 103 %, respectively. Spontaneously, the nonlinear coefficient maintains about 1.15 and the resistivity reaches to 778 Ω cm, increased by 33 %.     

Preparation of high-yield multi-walled carbon nanotubes by catalytic decomposition of mixture of natural gas and propylene and their electrothermal properties

Abstract

Multi-walled carbon nanotubes (MWNTs) with high-yield were prepared by pyrolysis of mixture of natural gas (NG) and propylene (C₃H₆) over Fe-Ni/Al₂O₃-MgO catalyst. For C₃H₆/NG flow rate ratio ranging from 0 to 0.33, the carbon yield was increased from 903% to 4400%. The synthesized MWNTs after purification were dispersed by ball milling method and mixed with waterborne polyurethane to fabricate the electrothermal film. The mass fraction of CNT filler in the cured electrothermal film was controlled at 50%. The coating after drying was ca. 6?μm and the coating’s volume resistivity was 0.053 Ω·cm. The time-dependent temperature curves indicated that the heating rate of the electrothermal film was very fast under different low voltage and the steady-state temperatures were achieved within 100?s. The steady-state temperature reached 47.9?°C, 76.8?°C, and 102.8?°C, respectively at 10?V, 15?V, and 18?V.     

Effect of sintering temperatures on properties of BaO–B<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> glass/silica composites for CBGA package

BaO–B₂O₃–SiO₂–Al₂O₃ (BBSA) glass/silica composites synthesized by solid-state reaction method were developed for CBGA packages, and the effects of sintering temperature (900–950 °C) on the phase transformation, microstructure, thermal, mechanical and electrical properties were investigated. XRD results show that the major phases quartz and cristobalite, and the minor phase BaSi₂O₅ are detected in BBSA composites. Furthermore, it was found that the quartz phase transforms to cristobalite phase at 930–940 °C. The formation of cristobalite phase with higher coefficient of thermal expansion (CTE) led to the increase of CTE value of BBSA composites. However, excessive cristobalite phase content would degrade the mechanical properties and the linearity of thermal expansion of the ceramics. BBSA composites sintered at 920 °C exhibited excellent properties: low dielectric constant and loss (ε_r = 6.2, tanδ = 10^?4 at 1 MHz), high bending strength (179 MPa), high CTE (12.19 ppm/°C) as well as superior linearity of the thermal expansion.     

Preparation,characterization and photocatalytic activity of Bi2O3–MgO composites

A series of Bi₂O₃–MgO composites were synthesized by solvent-thermal method. It was found that the Bi₂O₃–MgO composites perform much better than TiO₂ (P25), Bi₂O₃ and MgO in the photocatalytic degradation of rhodamine B (RhB) in the presence of HCl and under irradiation of visible light (λ > 400 nm). The effects of Bi/Mg molar ratio, crystallization temperature of Bi₂O₃–MgO and reaction conditions on photocatalytic activity were studied. The best performance was observed over the composite with Bi:Mg molar ratio equal to 2:1 that had been subject to crystallization at 120 °C for 20 h. In addition, the photocatalytic efficiency of the composite can be significantly enhanced by the presence of hydrochloric acid. The prepared samples were characterized by XRD and UV–vis DRS techniques. The relationships between the structure and photocatalytic performance of the as-prepared Bi₂O₃–MgO samples were also investigated.     

The influence of nano/micro hybrid structure on the mechanical and self-sensing properties of carbon nanotube-microparticle reinforced epoxy matrix composite

Nano/micrometer hybrids are prepared by chemical vapor deposition growth of carbon nanotubes (CNTs) on SiC, Al₂O₃ and graphene nanoplatelet (GNP). The mechanical and self-sensing behaviors of the hybrids reinforced epoxy composites are found to be highly dependent on CNT aspect ratio (AR), organization and substrates. The CNT–GNP hybrids exhibit the most significant reinforcing effectiveness, among the three hybrids with AR1200. During tensile loading, the in situ electrical resistance of the CNT–GNP/epoxy and the CNT–SiC/epoxy composites gradually increases to a maximum value and then decreases, which is remarkably different from the monotonic increase in the CNT–Al₂O₃/epoxy composites. However, the CNT–Al₂O₃ with increased AR ⩾ 2000 endows the similar resistance change as the other two hybrids. Besides, when AR < 3200, the tensile modulus and strength of the CNT–Al₂O₃/epoxy composites gradually increase with AR. The interrelationship between the hybrid structure and the mechanical and self-sensing behaviors of the composites are analyzed.     

Effect of alumina particle size and thermal condition of casting on microstructure and mechanical properties of stir cast Al–Al2O3 composites

Abstract

Metal matrix composites are considered as a distinct category of the advanced materials, which have low weight, high strength, high modulus of elasticity, low thermal expansion coefficient and high wear resistance. Among them, Al–Al₂O₃ composites have achieved significant attention due to their desired properties. In the present research, Al–Al₂O₃ composites with 5 vol.-% alumina were produced by stir casting at a temperature of 800°C. Two different particle sizes of alumina were used as 53–63 and 90–105 μm. The microstructure of the samples was evaluated by SEM. In addition, the mechanical properties of the samples were measured, and hence, the optimum temperature and particle size of alumina to be added to the Al matrix were determined. The results demonstrated the positive effect of alumina on improving the properties of Al–Al₂O₃ composites.     

Studies of Ta,Al, and Carbon Sources on Combustion Synthesis of Alumina–Tantalum Carbide Composites

Alumina–tantalum carbide (Al₂O₃–TaC and Al₂O₃–Ta₂C) composites were synthesized by incorporating aluminothermic reduction into a self-propagating combustion process. The test specimens adopted were composed of Ta₂O₅, Al, Al₄C₃, and carbon powders. Experimental evidence showed that the use of Al₄C₃ to provide Al and carbon decreased combustion exothermicity and hindered Ta₂O₅ from being fully reduced. In contrast, for the formation of Al₂O₃–TaC and Al₂O₃–Ta₂C composites, Al₄C₃-free samples with molar ratios of Ta₂O₅:Al:C = 3:10:6 and 3:10:3, respectively, exhibited the highest combustion temperature and reaction rate in the self-propagating high-temperature synthesis process and yielded products with very few minor phases.     

Microstructure and dielectric properties of glass/Al2O3 composites with various low softening point borosilicate glasses

The effects of various low softening point borosilicate glasses on both the sintering behavior and dielectric properties of glass/Al₂O₃ composites were investigated by FTIR, DSC, XRD, SEM and EDS. Results show that the addition of alkali oxides and PbO can decrease the glass softening temperature. While, the addition of Al₂O₃ and more SiO₂ content in the glass can increase the continuity of glass network and further increase the glass softening temperature of samples. Glass with lower softening temperature has more time to flow to finish the densification of samples, and that can contribute to get better sintered composites. By contrast, CaO–PbO–B₂O₃–SiO₂–Na₂O–K₂O glass/Al₂O₃ composites sintered at 875?°C for 15?min exhibit better properties of a bulk density of 3.06?g/cm³, a porosity of 0.17?%, a λ value of 2.47?W/m?K, a ε _r value of 8.05 and a tan δ value of 8.8?×?10^?4 at 10?MHz.     

Simple method for synthesis of carbon nanotubes over Ni-Mo/Al2O3 catalyst via pyrolysis of polyethylene waste using a two-stage process

Synthesis of valuable multi-walled carbon nanotubes (MWCNTs) by thermal pyrolysis of low-density polyethylene (LDPE) waste was investigated via a two-stage process. The first stage was the thermal pyrolysis of LDPE to gaseous hydrocarbons, and the second stage was the catalytic decomposition of the pyrolysis gases over Ni-Mo/Al₂O₃ catalysts. Two catalysts with the compositions of 5.2%Ni-10.96%Mo/Al₂O₃ and 10%Ni-9.5%Mo/Al₂O₃ were tested for carbon nanotubes (CNTs) formation. The catalyst containing 10%Ni showed better activity in terms of CNTs production. Accordingly, the impact of either pyrolysis or decomposition temperatures was investigated using the 10%Ni-9.5%Mo/Al₂O₃ catalyst. TEM, XRD, Raman spectroscopy, TGA, TPR, and BET analysis tools were used to characterize the fresh catalysts as well as the obtained carbon nanomaterials. TEM images proved that MWCNTs with various morphological structures were obtained at all pyrolysis and decomposition temperatures. Moreover, cup-stacked carbon nanotubes (CS-CNTs) were observed at the decomposition temperature of 600°C. MWCNTs with the best quality were produced at decomposition temperature of 750°C. The optimum pyrolysis and decomposition temperatures in terms of CNTs production were at 700 and 650°C, respectively.     

Effect of 10Ce-TZP/Al2O3 nanocomposite particle amount and sintering temperature on the microstructure and mechanical properties of Al/(10Ce-TZP/Al2O3) nanocomposites

A zirconia/alumina nanocomposite stabilized with cerium oxide (Ce-TZP/Al₂O₃ nanocomposite) can be a good substitute as reinforcement in metal matrix composites. In the present study, the effect of the amount of 10Ce-TZP/Al₂O₃ particles on the microstructure and properties of Al/(10Ce-TZP/Al₂O₃) nanocomposites was investigated. For this purpose, aluminum powders with average size of 30 μm were ball-milled with 10Ce-TZP/Al₂O₃ nanocomposite powders (synthesized by aqueous combustion) in varying amounts of 1, 3, 5, 7, and 10 wt.%. Cylindrical-shape samples were prepared by pressing the powders at 600 MPa for 60 min while heating at 400–450 °C. The specimens were then characterized by scanning and transmission electron microscopy (SEM and TEM) in addition to different physical and mechanical testing methods in order to establish the optimal processing conditions. The highest compression strength was obtained in the composite with 7 wt.% (10Ce-TZP/Al₂O₃) sintered at 450 °C.