A novel synthesis method of nanostructured MgO-coated hollow carbon nanofibers via CO decomposition over Mg/MgO catalyst

An easy and one-step synthesis method for preparation of oxide-coated carbon nanofibers (CNFs) by Mg/MgO catalyst has been reported. For this purpose, the mixture of Mg (metal) and MgO (fused magnesia) powders were heat treated at 1000 °C under reducing atmosphere of CO gas and the final product was comprehensively characterized by various analytical methods such as X-ray diffractometry, field emission scanning electron microscopy (FE-SEM), transition electron microscopy (TEM) and Raman spectroscopy. The results showed the formation of hollow CNFs with the average diameter and wall thickness of 100 nm and 25 nm, respectively. Furthermore, SEM micrographs have obviously revealed the presence of highly packed MgO nanoparticles (with average particle size of 30 nm) on the outer surface of fibers owing to catalytic reaction of Mg _(v) and CO gasses. Considering the superior physical properties of the synthesized oxide-coated material such as great packing, high uniformity as well as the absence of structural defects, oxide-coated CNFs showed considerable enhancement in the oxidation resistance compared to the conventional carbon nanofibers. The synthesized oxide coated CNFs showed a weight loss of less than 5 wt% after exposing to high temperature at oxidative atmosphere highlighting the significant effect of the oxide coating. This effect was confirmed by a thermoanalytical technique using differential scanning calorimeter equipped with the online gas analyzer.     

Pressureless sintering of carbon nanotube–Al2O3 composites

Alumina ceramics reinforced with 1, 3, or 5 vol.% multi-walled carbon nanotubes (CNTs) were densified by pressureless sintering. Commercial CNTs were purified by acid treatment and then dispersed in water at pH 12. The dispersed CNTs were mixed with Al₂O₃ powder, which was also dispersed in water at pH 12. The mixture was freeze dried to prevent segregation by differential sedimentation during solvent evaporation. Cylindrical pellets were formed by uniaxial pressing and then densified by heating in flowing argon. The resulting pellets had relative densities as high as ～99% after sintering at 1500 °C for 2 h. Higher temperatures or longer times resulted in lower densities and weight loss due to degradation of the CNTs by reaction with the Al₂O₃. A CNT/Al₂O₃ composite containing 1 vol.% CNT had a higher flexure strength (～540 MPa) than pure Al₂O₃ densified under similar conditions (～400 MPa). Improved fracture toughness of CNT–Al₂O₃ composites was attributed to CNT pullout. This study has shown, for the first time, that CNT/Al₂O₃ composites can be densified by pressureless sintering without damage to the CNTs.     

Properties of cement–sand-based piezoelectric composites with carbon nanotubes modification

Carbon nanotubes (CNTs) were dispersed in a cement–sand-based piezoelectric composite as conductive fillers to improve its poling efficiency. Specimens were prepared by mixing PZT powders, cement and sand with CNTs. The effect of CNTs ranging from 0 to 0.9 vol% on properties of the composite, including its piezoelectric coefficient, dielectric constant and loss, and sensing characteristic, were characterized. It was found that the addition of CNTs facilitated effective poling under a low electric field of 1 MV/m at room temperature and improved the piezoelectric and dielectric properties of the composite. The composite modified by CNTs achieved optimal properties when the CNTs content was 0.6 vol% and this was verified by the investigation of sensing effects of the composite through compressive tests.     

Microwave absorption study of carbon nano tubes dispersed hard/soft ferrite nanocomposite

Nickel substituted strontium hexaferrite, SrNi₂Fe₁₀O₁₉·(SrFe₁₂O₁₉/NiFe₂O₄) nanoparticles have been synthesized by low combustion method by citrate precursors using sol to gel (S–G) followed by gel to nano crystalline (G–N) conversion. The resulting ‘as-synthesized’ powder is heat treated (HT) at 800 and 1000 °C for 4 h in nitrogen atmosphere. The hysteresis loops show an increase in saturation magnetization from 27.443 to 63.706 emu/g with increasing HT temperatures. The multiwalled carbon nano tubes (CNTs) were synthesized by thermal decomposition of acetylene gas over iron-catalyst deposited on silicon wafer in the temperature range of 750–800 °C. A microwave absorbing medium is prepared by adding CNTs in the nickel substituted strontium hexaferrite nanoparticles. Addition of certain mass of CNTs improves the microwave absorption properties and wave band of SrFe₁₂O₁₉/NiFe₂O₄ absorbent. When 10 wt% CNTs is mixed with SrFe₁₂O₁₉/NiFe₂O₄ nanoparticles to fabricate a composite with 2 mm thickness, the maximum reflection loss reaches to ?36.817 dB at 9.292 GHz and ?10 dB bandwidth reaches 3.27 GHz.     

Effect of co-deposited SiC nanowires and carbon nanotubes on oxidation resistance for SiC-coated C/C composites

To protect carbon/carbon composites (C/Cs) against oxidation, SiC coating toughened by SiC nanowires (SiCNWs) and carbon nanotubes (CNTs) hybrid nano-reinforcements was prepared on C/Cs by a two-step technique involving electrophoretic co-deposition and reactive melt infiltration. Co-deposited SiCNWs and CNTs with different shapes including straight-line, fusiform, curved and bamboo dispersed uniformly on the surface of C/Cs forming three-dimensional networks, which efficiently refined the SiC grains and meanwhile suppressed the cracking deflection of the coating during the fabrication process. The presence of SiCNWs and CNTs contributed to the formation of continuous glass layer during oxidation, while toughed the coating by introducing toughing methods such as bridging effect, crack deflection and nanowire pull out. Results showed that after oxidation for 45 h at 1773 K, the weight loss percentage of SiC coated specimen was 1.35%, while the weight gain percentage of the SiCNWs/CNTs reinforced SiC coating was 0.03052% due to the formation of continuous glass layer. After being exposed for 100 h, the weight loss percentage of the SiCNWs/CNTs reinforced SiC coating was 1.08%, which is relatively low.     

Molten salt synthesis of LaAlO3 powder at low temperatures

Rhombohedral LaAlO₃ powder was synthesised by reacting equimolar La₂O₃ and Al₂O₃ in a molten KF–KCl eutectic salt for 3 h between 630 and 800 °C. The lowest synthesis temperature (630 °C) is about 1000 °C lower than that of conventional mixed oxide synthesis, and close to or lower than those used by most wet chemical methods. The LaAlO₃ particle size increased from <3 to 3–7 μm with increasing temperature from 630 to 700 °C, but changed little on further increasing temperature to 800 °C. On the other hand, it decreased with increasing salt/oxide weight ratio from 1:1 to 6:1. The “dissolution–precipitation” mechanism played a dominant role in the molten salt synthesis of LaAlO₃.     

Preparation of Bi4Ti3O12 nanopower by azeotropic co-precipitation and dielectric properties of the sintered ceramic

Bi₄Ti₃O₁₂ nanopowders were prepared by an azeotropic co-precipitation method and the phase evolution process, microstructure and sintering behavior were investigated. The results indicate that well dispersed and agglomerate-free nanocrystalline Bi₄Ti₃O₁₂ with average particle size of 21 nm can be obtained by calcinating the precursor at 750 °C, which is 50 °C lower than traditional solid reaction. The relative density of the ceramic reaches 96% at 1000 °C and shows no evident decrease until 1100 °C. The broadened sintering temperature range and the lower loss tangent of the ceramic show good sintering activity of the nanopowders.     

Processing and properties of sol gel derived alumina–carbon nano tube composites

High purity alumina–carbon nano tube (CNT) composites were prepared by an aqueous sol–gel processing route. CNTs were dispersed in alumina sol containing appropriate amount of MgO precursor. Aqueous slurry of alumina was seeded into the sol followed by gelation, drying and calcination at 1000 °C for 1 h. The calcined powder consisting of alumina-coated CNTs and alumina was milled, sieved, dried, pressed and pressureless sintered at 1400–1600 °C for 1 h in nitrogen atmosphere. Sintered samples were further isostatically hot pressed at 1300 °C and the properties were compared with the pressureless sintered samples. Phase formation was followed by XRD study, CNT retention was confirmed by Raman studies and the samples were further characterized for mechanical and microstructural properties.     

Mg-substituted hydroxyapatite nanopowders: Synthesis,thermal stability and sintering behaviour

This paper reports a systematic investigation on Mg-substituted hydroxyapatite (Ca_10?xMg_x(PO₄)₆(OH)₂) nanopowders produced by precipitation of Ca(NO₃)₂·4H₂O and Mg(NO₃)₂. The Mg content ranged between 0.6 and 2.4 wt%. Semicrystalline Mg-substituted hydroxyapatite powders made up of needle-like nanoparticles were obtained, the specific surface area ranged between 87 and 142 m²/g. Pure hydroxyapatite nanopowder decomposed around 1000 °C. Mg-substituted hydroxyapatites were thermally stable up to 660 °C (x = 1.0), 760 °C (x = 0.5) and 840 °C (x = 0.25) showing a distinct decreased thermal stability with respect to the pure sample.A relevant displacement of the sintering curve at lower temperature as a function of Mg content was observed, comparing to the behaviour of a pure HAp material, synthesized following the same procedure, and ascribed to the β-TCP formation.     

Synthesis of bamboo-like carbon nanotubes on a copper foil by catalytic chemical vapor deposition from ethanol

Bamboo-like carbon nanotubes (CNTs) were synthesized on a copper foil by catalytic chemical vapor deposition (CVD) from ethanol. The effects of temperature (700–1000 °C) and duration (5–60 min) on the growth of CNTs were investigated. Morphology and structure of the CNTs were characterized by scanning and transmission electron microscopy and Raman spectroscopy. The yield and size of the CNTs increased with temperature. Those prepared at 700 °C had a copper droplet tip and those at 800–900 °C had a copper nanoparticle inside. An amorphous carbon film consisted of a porous and non-porous layer was observed on the surface of the copper substrate, and the CNTs were really grown from this carbon film. The thickness of the carbon film increased from 187 to 900 nm when the duration increased from 5 to 60 min. It was also found that the copper foil became porous after ethanol CVD treatment. The growth mechanism of the CNTs, carbon film and motion of copper catalyst were discussed. It is proposed that a carbon film first deposited on the top surface of the copper foil while the top surface of the copper foil partially melted and migrated across the carbon film, where CNTs formed.     

CVD growth and field electron emission of aligned carbon nanotubes on oxidized Inconel plates without addition of catalyst

Direct growth of carbon nanotubes (CNTs) on Inconel 600 sheets was investigated using plasma enhanced hot filament chemical vapor deposition in a gas mixture of methane and hydrogen. The Inconel 600 sheets were oxidized at different temperatures (800 °C, 900 °C, 1000 °C, and 1100 °C) before CNT deposition. The structure and surface morphology of the pre-treated substrate sheets and the deposited CNTs were studied by scanning electron microscopy (SEM) and X-ray diffraction. The field electron emission (FEE) properties of the CNTs were also tested. The SEM results show that well aligned CNTs have been grown on the pre-treated Inconel sheets without addition of any catalysts and the higher treatment temperature resulted in CNTs with better uniformity, indicating that the oxidation pre-treatment of the substrate is effective to enhance the CNT growth. FEE testing shows that CNTs with better height uniformity exhibit better FEE characteristics.     

Electromagnetic wave absorption properties of graphene modified with carbon nanotube/poly(dimethyl siloxane) composites

By using a catalytic growth procedure, carbon nanotubes (CNTs) are in situ formed on reduced graphene oxide (RGO) sheet at 600 °C. CNTs growing on RGO planes through covalent C–C bond possess lower interfacial contact electrical resistance. As a hybrid structure, the CNTs/graphene (CNT/G) are well dispersed into poly (dimethyl siloxane). The hybrid combining electrically lossy CNTs and RGO, which disperses in electrically insulating matrix, constructs an electromagnetic wave (EM) absorbing material with ternary hierarchical architecture. The interfacial polarization in heterogeneous interface plays an important role in absorbing EM power. When the filler loading is 5 wt.% and thickness of absorber is 2.75 mm, the minimum value of reflection coefficient and the corresponding frequency are −55 dB and 10.1 GHz, and the effective absorption bandwidth reaches 3.5 GHz. Therefore, combining the CNTs and graphene sheet into three-dimensional structures produces CNT/G hybrids that can be considered as an effective route to design light weight and high-performance EM absorbing material, while the effective EM absorption frequency can be designed.     

Preparation and characterization of barium titanate nanofibers by electrospinning

PVP–BaTiO₃ composite nanofibers were successfully prepared by electrospinning and pure BaTiO₃ fibers were produced after calcination at 1000 °C. A homogeneous viscous solution of barium acetate + titanium acetate/titanium isopropoxide in poly vinyl pyrrolidone (PVP) was prepared by varying PVP concentration in the range of 8–12%. The above sols were electrospun at 9 kV DC by maintaining tip to collector distance (TCD) of 7 cm. The electrospun fibers were calcined at 1000 °C for 2 h. Thermo gravimetric analysis (TGA) of the fibers indicates the complete decomposition of organics below 700 °C with 45% weight loss. Scanning electron microscopy (SEM) study shows the fibers cylindrical, smooth with diameters in the range of 50–400 nm and the aspect ratio >1000. The average diameter of the fibers increases with the increase in PVP concentration. The calcined BaTiO₃ nanofibers were found to be coarse, brittle and diameter reduced by 12%. FT-IR study confirms the formation of metal oxide bond at higher temperature.     

Electrophoretic deposition of MnO2-coated carbon nanotubes on a graphite sheet as a flexible electrode for supercapacitors

A flexible electrode was prepared by microwave heating deposition of manganese oxide (MnO₂) on carbon nanotubes (CNTs) followed by electrophoretic deposition of the MnO₂-coated CNTs on a flexible graphite sheet (FGS). The prepared MnO₂-coated CNTs were characterized by scanning and transmission electron microscopy, and X-ray diffraction. A uniformly thin nano-scale MnO₂ coating was formed on the surface of the CNTs. The MnO₂-coated CNTs–FGS electrode showed highly capacitive behaviour in the 0.5 M Na₂SO₄ aqueous solution, with a specific capacitance of 442.9 F/g based on MnO₂ at 2 mV/s. It exhibited an excellent cycling stability with no more than 1.1% capacitance loss after 1000 cycles at 50 mV/s.     

Structural characterization of plasma sprayed basalt–SiC glass–ceramic coatings

In the present study, the effect of SiC addition on properties of basalt base glass–ceramic coating was investigated. SiC reinforced glass–ceramic coating was realized by atmospheric air plasma spray coating technique on AISI 1040 steel pre-coated with Ni + 5 wt.%Al bond coat. Composite powder mixture consisted of 10%, 20% and 30% SiC by weight were used for coating treatment. Controlled heat treatment for crystallization was realized on pre-coated samples in argon atmosphere at 800 °C, 900 °C and 1000 °C which determined by differential thermal analysis for 1–4 h in order to obtain to the glass–ceramic structure. Microstructural examination showed that the coating performed by plasma spray coating treatment and crystallized was crack free, homogeneous in macro-scale and good bonded. The hardness of the coated samples changed between 666 ± 27 and 873 ± 32 HV_0.01 depending on SiC addition and crystallization temperature. The more the SiC addition and the higher the treatment temperature, the harder the basalt base SiC reinforced glass–ceramic coating became. X-ray diffraction analysis showed that the coatings include augeite [(CaFeMg)–SiO₃], diopside [Ca(Mg_0.15Fe_0.85)(SiO₃)₂], albite [(Na,Ca)Al(Si,Al)₃O₈], andesine [Na_0.499Ca_0.492(Al_1.488Si_2.506O₈] and moissanite (SiC) phases. EDX analyses support the X-ray diffraction analysis.     

Microwave performance of thin film ferroelectric varactors in the wide temperature range of −223 °C to +227 °C

Parallel-plate Au(Pt)/Ba_0.25Sr_0.75TiO₃/(Pt)Au thin film varactors are fabricated on high resistive Si substrate and characterized at 1 MHz and microwave frequencies up to 45 GHz in the temperature range of −223 °C to +227 °C. The relative tunability of capacitance decreases with temperature, as capacitance does, from 80 to 90% at −193 °C down to 10% at +227 °C. The temperature coefficient of capacitance in the temperature range −55 to +125 °C is approximately 0.3% at 20 GHz and zero dc field. The temperature dependence of the varactor loss tangent, in general, follows that of the capacitance. The loss tangent at zero dc field and 20 GHz is less than 0.1 at −193 °C and less than 0.02 at +227 °C. The figure of merit of the varactor, taking into account both the tunability and the loss tangent, at 20 GHz is more than 1000 at −193°C and 50 at +227 °C.     

Carbon nanotube wires for high-temperature performance

We developed carbon nanotube wires (CNWs) and monitored in situ their electrical properties at high temperature conditions for the first time. The dominant type of CNTs present in the material and packing density of thereof proved to have a dominant effect on the thermal stability of CNWs. Furthermore, we showed that kinetics of CNW oxidation plays an important role and slow heating rates or prolonged heating times are essential for the proper determination of thermal stability of CNTs. To enhance the stability at high temperatures, we applied SiC coating onto the CNWs, what allowed a 300 °C improvement to the operational window, eventually reaching 700 °C in the long-haul. Correlation of the change in electrical properties with thermogravimetric response showed that the loss of electrical percolation takes place at 100 °C lower temperature than the last observed weight loss in CNTs content. Finally, we demonstrated feasibility of SiC-coated CNWs under high temperature conditions, by creating a heating device out of them. The presence of SiC layer gave rise to a significant improvement to the thermal stability of the CNT heaters, which now offer unprecedented range of operation reaching 700 °C, as compared to 400 °C when uncoated.     

Tribological properties of NiCr–ZrO2(Y2O3)–SrSO4 composites at elevated temperatures

The effect of SrSO₄ content on the tribological properties of NiCr–30wt%ZrO₂(Y₂O₃) (NC30Z) cermet was evaluated over a wide temperature range from room temperature to 1000 °C. The results indicated that the inclusion of SrSO₄ effectively improved the friction coefficients and wear rates of NC30Z cermet above 400 °C. NC30Z–5SrSO₄ composite against alumina ball exhibited satisfactory tribological performance, which was attributed to synergistic lubrication of pseudocubic-SrZrO₃ and NiCr₂O₄ between 400 °C and 800 °C and cubic-SrZrO₃, NiCr₂O₄, NiO and Cr₂O₃ at 1000 °C.     

The relationship between microstructure and flexural strength of pressureless liquid phase sintered SiC ceramics oxidized at elevated temperatures

The oxidation behavior of pressureless liquid phase sintered SiC ceramics with Al₂O₃ and Y₂O₃ as sintering additives was investigated in the temperature range from 1000 °C to 1600 °C at the interval of 100 °C for 5 h. The relationship between residual flexural strength and microstructure was analyzed in detail. It was found that the SiC specimens suffered from mild oxidation below 1300 °C. The flexural strength of SiC specimens after oxidation at 1100 °C was the highest (90% of the original strength) due to the formation of dendritic grains, which filled pores and healed cracks. And the flexural strength was almost above 80% of the original flexural strength when the oxidation temperature was below 1300 °C. Meanwhile, the weight of specimens underwent steady increase. However, when the oxidation temperature was elevated to above 1400 °C, the specimens began to suffer from severe oxidation, which resulted in a lot of through pores and cracks on the surface, bringing about the sharp decrease of flexural strength to 30% of original strength when the oxidation temperature of 1600 °C was reached. And the weight of the specimens after huge increase began to show downtrend when the oxidation temperature was elevated to 1600 °C due to the spalling of oxidation products.     

Synthesis of BaCeO3 and BaCe0.9Y0.1O3−δ from mixed oxalate precursors

An oxalate precipitation route is proposed for the synthesis of BaCe_1−xY_xO₃ (x = 0 and 0.1) after calcination at 1100 °C. The precipitation temperature (70 °C) was a determinant parameter for producing a pure perovskite phase after calcination at 1100 °C for 1 h. TG/DTA measurements showed that the co-precipitated (Ba, Ce and Y) oxalate had a different thermal behaviour from single oxalates. Despite a simple grinding procedure, sintered BaCe_0.9Y_0.1O_3−δ pellets (1400 °C, 48 h) presented 90.7% of relative density and preliminary impedance measurements showed an overall conductivity of around 2 × 10⁻⁴ S cm⁻¹ at 320 °C.