Optimization of CVD growth of CNT-based hybrids using the Taguchi method

Taguchi method, extensively applied for the optimization of multifactor processes in the most diverse fields, is for the first time applied to the synthesis of hybrids based on C nanotubes by iron-catalyzed chemical vapor deposition in 1:1 i-C₄H₁₀ + H₂ atmosphere. For this purpose, assumed synthesis-temperature (500 °C, 600 °C or 700 °C), support-material (alumina, magnesia or Na⁺-exchanged K10 montmorillonite), calcination (450 °C, 600 °C or 750 °C) and reduction (500 °C, 600 °C or 700 °C) temperature of the iron catalysts as the four factors of importance in the process, nine suitably designed experiments are conducted and the influence is evaluated of the four three-level factors on the issue of the process in terms of selectivity toward nanotubes, catalytic yield and content of carbonaceous and metallic impurities in the C nanotubes. By this procedure, the configurations giving optimal results are predicted, and tested by carrying out new experiments.     

A study on the synthesis of nanostructured WC–10 wt% Co particles from WO<Subscript>3</Subscript>, Co<Subscript>3</Subscript>O<Subscript>4</Subscript>, and graphite

The formation of the nanostructured WC–10 wt% Co powder from WO₃, Co₃O₄, and graphite is studied. The effects of the processing parameters of high-energy ball milling, reduction in H₂ atmosphere, and carburization in Ar/CO atmosphere are investigated. The crystallite size of the as-synthesized WC is 30–40 and 40–50 nm for 900 and 1000 °C carburized powders, respectively. The powder is agglomerated with the size of the primary particles ranging from 50 to 700 nm. High-energy ball milling of WO₃–Co₃O₄–C powder mixtures leads to finer particle and crystallite sizes with larger surface area. Such milled powders can be reduced to nanostructured W at 570 °C and carburized to form WC at temperatures as low as 900 °C. Crystal growth has taken place during carburization, particularly at 1000 °C, which results in the formation of truncated triangular prisms and nanoplates of WC at 1000 °C.     

Contact angles by the solid-phase grain boundary wetting (coverage) in the Co–Cu system

The microstructure of binary Co–13.6 wt% Cu and Cu–4.9 wt% Co alloys after long anneals (930–2,100 h) was studied between 880 and 1,085 °C. The contact angles between (Co) particles and (Cu)/(Cu) grain boundaries (GBs) in the Cu–4.9 wt% Co alloy are between 50° and 70°. In the Co–13.6 wt% Cu alloy, the transition from incomplete to complete wetting (coverage) of (Co)/(Co) GBs by the second solid phase (Cu) has been observed. The portion of completely wetted (Co)/(Co) GBs increases with increasing temperature beginning from T _wss = 970 ± 10 °C and reaches a maximum of 15% at 1,040 °C. This temperature is very close to the Curie point in the Co–Cu alloys (1,050 °C). Above 1,040 °C, the amount of completely wetted (Co)/(Co) GBs decreases with increasing temperature and reaches zero at T _wsf = 1,075 ± 5 °C. Such reversible transition from incomplete to complete wetting (coverage) of a GB by a second solid phase is observed for the first time.     

Deformation-driven formation of equilibrium phases in the Cu–Ni alloys

The homogeneous coarse-grained (CG) Cu–Ni alloys with nickel concentrations of 9, 26, 42, and 77 wt% were produced from as-cast ingots by homogenization at 850 °C followed by quenching. The subsequent high-pressure torsion (5 torsions at 5 GPa) leads to the grain refinement (grain size about 100 nm) and to the decomposition of the supersaturated solid solution in the alloys containing 42 and 77 wt% Ni. The lattice spacing of the fine Cu-rich regions in the Cu–77 wt% Ni alloy was measured by the X-ray diffraction (XRD). They contain 28 ± 5 wt% Ni. The amount of the fine Ni-rich ferromagnetic regions in the paramagnetic Cu–42 wt% Ni alloy was estimated by comparing its magnetization with that of fully ferromagnetic Cu–77 wt% Ni alloy. According to the lever rule, these Ni-rich ferromagnetic regions contain about 88 wt% Ni. It means that the high-pressure torsion of the supersaturated Cu–Ni solid solutions produces phases which correspond to the equilibrium solubility limit at 200 ± 40 °C (Cu–77 wt% Ni alloy) and 270 ± 20 °C (Cu–42 wt% Ni alloy). To explain this phenomenon, the concept of the effective temperature proposed by Martin (Phys Rev B 30:1424, 1984) for the irradiation-driven decomposition of supersaturated solid solutions was employed. It follows from this concept that the deformation-driven decomposition of supersaturated Cu–Ni solid solutions proceeds at the mean effective temperature T _eff = 235 ± 30 °C. The elevated effective temperature for the high-pressure torsion-driven decomposition of a supersaturated solid solution has been observed for the first time. Previously, only the T _eff equal to the room temperature was observed in the Al–Zn alloys.     

A new microwave dielectric ceramic for LTCC applications

A new low-sintering temperature microwave dielectric ceramic was found and investigated in the Li₂O–Nb₂O₅–TiO₂ (Li₂O:Nb₂O₅:TiO₂ = 5.7:1:14.7, by mole, abbreviated as LNT) system. This new microwave dielectric ceramic shows a relatively high permittivity (47), high Q × f values up to 17,800 GHz, and low temperature coefficients (57 ppm/°C), which were obtained via sintering at 1,125 °C. With the low-level doping of B₂O₃–CuO (BCu) (below 2 wt%), the sintering temperature of the LNT ceramic could be effectively reduced to 900 °C. The addition of BCu does not induce apparent degradation in the microwave properties but lowers the τ _f value. Typically, the 2.0 wt% BCu-doped ceramics sintered at 900 °C have better microwave dielectric properties of ε_r = 48.7, Q × f = 16,350 GHz, τ _f  = 32 ppm/°C, which suggest that the ceramics could be applied in multilayer microwave devices requiring low sintering temperatures.     

Fabrication and ferroelectric properties of sol–gel derived 1–1 intergrowth-superlattice-structured Bi<Subscript>3</Subscript>TiNbO<Subscript>9</Subscript>-Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> thin films

1–1 intergrowth-superlattice-structured Bi₃TiNbO₉–Bi₄Ti₃O₁₂ (BTN–BIT) ferroelectric thin films have been prepared on p-Si substrates by sol-gel processing. The precursor films are crystallized in the desired intergrown BTN–BIT superlattice structures by optimizing the processing conditions. Synthesized BTN–BIT thin films annealed below 750 °C are polycrystalline, uniform and crack-free, no pyrochlore phase or other second phase, and exhibited good ferroelectric properties. As the annealing temperature increases from 600 to 700 °C, both remanent polarization P _r and coercive electric field E _c of BTN–BIT thin films increase, but the pyrochlore phase in BTN–BIT films annealed above 750 °C will impair the ferroelectric properties. The BTN–BIT thin films annealed at 700 °C have a P _r value ~19.1μC/cm² and an E _c value ~135 kV/cm.     

Microstructures and high temperature oxidation resistance of alloys from Nb–Cr–Si system

Oxidation behavior of Nb–30Si–(10,20)Cr alloys have been evaluated in air from 700 to 1400 °C by heating for 24 h and furnace cooling them. The lower weight gain per unit area has been observed for 20Cr alloy at 1200, 1300, and 1400 °C. Pesting has been observed at lower temperatures (700, 800, 900 °C). Analysis indicates that the powder formation at 900, 100, 1100 °C may be associated with β form of Nb₂O₅ (base centered monoclinic form). However the m-monoclinic form of Nb₂O₅ evolves at temperatures above 900 °C while o-orthorhombic Nb₂O₅ forms at below this temperature. The phases in the alloys have been calculated using the Pandat^TM software program at different temperatures using calculated Nb–Cr–Si phase diagrams.     

Facile tailoring of titanate nanostructures at low alkaline concentration by a solvothermal route

Nanostructured titanates with different morphologies such as nanoflakes, nanotubes, and nanofibers have been selectively synthesized by a simple solvothermal treatment of commercial anatase TiO₂ using the mixed water–ethanol cosolvent at low alkaline concentration. The effects of solvothermal temperature, volume ratio of H₂O to C₂H₅OH, amount of NaOH and solvents on the formation of titanate nanostructures have been systematically studied through X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). At low concentration of NaOH solution (the actual concentration of OH⁻ in the solution is only 0.58 M), different titanate nanostructures are achieved by simply changing the volume ratio of H₂O to C₂H₅OH at 180 °C and titanate nanotubes can be synthesized between 100 and 180 °C. A probable formation mechanism is proposed based on XRD, SEM and TEM analysis. The influence of cosolvent on the transformation from anatase TiO₂ to titanate is also investigated.     

Preparation and thermoelectric properties of BP films on SOI and sapphire substrates

The chemical vapor deposited (CVD) BP films on Si(100) (190 nm)/SiO _x (370 nm)/Si(100) (625 μm) (SOI) and sapphire (R-plane) (600 μm) substrates were prepared by the thermal decomposition of the B₂H₆–PH₃–H₂ system in the temperature range of 800–1050 °C for the deposition time of 1.5 h. The BP films were epitaxially grown on the SOI substrate, but a two-step growth method, i.e., a buffer layer at lower temperature and sequent CVD process at 1000 °C for 1.5 h was effective for obtaining a smooth film on the sapphire substrate. The electrical conduction types and electrical properties of these films depended on the growth temperature, gases flow rates and substrates. The thermal conductivity of the film could be replaced by the substrate, so that the calculated thermoelectric figure-of-merit (Z) for the BP films on the SOI substrate was 10⁻⁴–10⁻³/K at 700–1000 K. Those on the sapphire substrate were 10⁻⁶–10⁻⁵/K for the direct growth and 10⁻⁵–10⁻⁴/K for the two-step growth at 700–900 K, indicating that the film on a sapphire by two-step growth would reduce the defect concentrations and promote the electrical conductivity.     

Effect of B2O3 and CuO additions on the sintering temperature and microwave dielectric properties of 3Li2O–Nb2O5–3TiO2 ceramics

The effects of B₂O₃–CuO (BCu, the weight ratio of B₂O₃ to CuO is 1:1) addition on the sintering behavior, microstructure, and the microwave dielectric properties of 3Li₂O–Nb₂O₅–3TiO₂ (LNT) ceramics have been investigated. The low-amount addition of BCu can effectively lower the sintering temperature of LNT ceramics from 1125 to 900 °C and induce no obvious degradation of the microwave dielectric properties. Typically, the 2 wt% BCu-added ceramic sintered at 900 °C has better microwave dielectric properties of ε _r  = 50.1, Q × f = 8300 GHz, τ _f  = 35 ppm/°C. Silver powders were cofired with the dielectric under air atmosphere at 900 °C. The SEM and EDS analysis showed no reaction between the dielectric ceramic and silver powders. This result shows that the LNT dielectric materials are good candidates for LTCC applications with silver electrode.     

Effect of H2S on Fe corrosion in CO2-saturated brine

The effect of H₂S at ppm level concentrations on iron corrosion in 3 wt% NaCl solutions saturated with CO₂ in the temperature range of 25–85 °C is examined using electrochemical and surface science techniques. Small H₂S concentrations (5 ppm) have an inhibiting effect on corrosion in the presence of CO₂ at temperatures from 25 to 55 °C. At 85 °C, however, 50 ppm H₂S is needed to provide significant corrosion inhibition. At higher H₂S concentrations, the corrosion rate increases rapidly, while still remaining below the rate for the H₂S-free solution. Characterization of the iron surfaces after corrosion was carried out using X-ray photoelectron spectroscopy and X-ray diffraction. A sulfur peak (S2p) is observed at a binding energy of 161.8 eV in all cases, attributable to disulfide (\textS₂^2-) ({\text{S}}_{2}^{2-}) formation. Corrosion protection in the temperature range 25–55 °C can be attributed to Fe(II) bonded to S and O. At 85 °C, protection of the iron surface is most likely due to FeS₂ formation. Morphological changes on the iron surface after exposure to H₂S containing solutions were observed by SEM. A thin protective film was seen after exposure to solutions containing 5 ppm H₂S at 25 °C, while at 85 °C, with the addition of 50 ppm H₂S to CO₂-saturated brine solution, a dense protective film was formed on the iron surface.     

Synthesis of boron nitride nanotubes employing mechanothermal process and its characterization

Multi-walled boron nitride (BN) nanotubes having cylindrical structure were synthesized employing the mechanothermal process. In this process hexagonal boron nitride powder (hBN) was first ball milled for 50–100 h using a high-energy ball mill and the ball-milled samples were annealed in N₂ atmosphere for about 10 h in the temperature range of 950–1300 °C. The BN nanotubes exhibited a well-crystallized hexagonal structure with about 25–40 nm in diameter and up to 1 μm length. These BN nanotubes were well characterized using various techniques, such as, XRD, SEM, TEM and Raman Spectroscopy.     

Mechanical properties and oxidation behaviors of carbon/carbon composites with C–TaC–C multi-interlayer

To improve the mechanical properties and oxidation-resistance properties, a C–TaC–C multi-interlayer structure was introduced in carbon/carbon (C/C) composites by chemical vapor infiltration. Compared with conventional C/C composites, a higher fracture toughness and strength have been achieved by using the C–TaC–C multi-interlayer. In addition, the composites also exhibit a higher preliminary oxidation temperature and a lower mass loss at high temperatures. The oxidation rate of the composites increases with temperature increasing in the range of 700–1300 °C, reaching a maximum value at 1300 °C, then decreases in 1300–1400 °C. A hexagonal structure of Ta₂O₅ phase is obtained when being oxidized at 700–800 °C, and it transforms to an orthorhombic phase at temperatures above 900 °C. The structures of C–TaC–C multi-interlayer are intact without cracks or porosities after being oxidized at 700–800 °C. In 900–1300 °C, the composites are oxidized uniformly with the formation of pores. At temperatures above 1300 °C, there are oxidation and non-oxidation regions with the oxidation process being controlled by diffusion.     

In situ high temperature X-ray diffraction study of UO2 nanoparticles

Nanocrystallites of UO₂ with a size of 3–5 nm were studied in situ with high temperature X-ray diffraction (HT-XRD), thermogravimetry (TGA), and differential thermal analysis. The evolution of the crystallite size, the lattice parameter, and the strain were determined from ambient temperature up to 1200 °C. Below 700 °C, a weak effect on the crystallite size occurs and it remains below 10 nm, while a strong expansion of the lattice parameter is measured. The strain decreases with temperature and is completely released at 700 °C. Above this temperature, begins the sintering of the nanocrystallites reaching a size of about 80 nm at 1200 °C. The weight loss curve observed in TGA is assigned to the desorption of water molecules and is correlated with the strain evolution observed by HT-XRD. The linear thermal expansion and the thermal expansion coefficient at 800 °C are 1.3% and 16.9 × 10⁻⁶ °C⁻¹, respectively.     

Microstructure and microwave dielectric characteristics of CaO–B2O3–SiO2 glass ceramics

CaO–B₂O₃–SiO₂ (CBS) glass powders are prepared by traditional glass melting method, whose properties and microstructures are characterized by Differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). It is found that the pure CBS glass ceramics possess excellent dielectric properties (ε _r = 6.5, tan δ = 5 × 10⁻³ at 10 GHz), but a higher sintering temperature (>900 °C) and a narrow sintering temperature range (about 10 °C). The addition of a low-melting-point CaO–B₂O₃–SiO₂ glass (LG) could greatly decrease the sintering temperature of CBS glass to 820 °C and significantly enlarge the sintering temperature range to 40 °C. The CBS glass ceramic with 30 wt% LG glass addition sintered at 840 °C exhibits better dielectric properties: ε _r ≈ 6, tan δ < 2 × 10⁻³ at 10 GHz, and the major phases of the sample are CaSiO₃, CaB₂O₄ and SiO₂.     

Facile fabrication of NiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>H<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> films and their unique electrochromic properties

The electrochromic (EC) NiO _x H _y films were fabricated through a facile sol–gel method. The formation of high quality NiO _x H _y films came from adding the xerogel back into the sol and prolonging the annealing time at gradually increasing temperature up to 250 °C. Scanning electron microscopy and atomic force microscopy characterizations indicated films were compact, homogenous, and smooth. Glance angle X-ray diffraction investigation testified NiO _x H _y films were of poor crystallization. The Fourier transform infrared, and thermogravimetry and differential thermal analysis showed that films contained the mixture of NiO, Ni(OH)₂, NiOOH, water, and organic substance. With the increasing of the xerogel ratio, the optical absorbance and reflectance of films had larger differences between the colored and bleached state, respectively. The film with the xerogel ratio of 1:5 showed excellent EC properties with a transmittance contrast as high as 60.88% at λ = 560 nm, which was higher than other sol–gel nickel oxide films reported.     

Investigation of reactive cerium-based oxides for H2 production by thermochemical two-step water-splitting

This study focuses on the use of cerium-based mixed oxides for hydrogen production by solar-driven thermochemical two-step water-splitting. Mixed cerium oxides are proposed in order to decrease the reduction temperature of ceria and to avoid material sublimation occurring above 2,000 °C during the high-temperature solar step. Ceria-based nanopowders were synthesized by soft chemistry methods including the modified Pechini method. The influence of the synthesis method, the type of cationic element mixed with cerium, and the content of this added element was investigated by comparing the reduction temperatures of the derived materials. The synthesized powders were characterized by X-ray diffraction, thermogravimetric analysis, SEM, and Raman spectroscopy. Results showed that the synthesized pure cerium oxide is more reactive toward reduction than a commercial powder. Among the different elements added to ceria that were screened, the addition of zirconium significantly improved the reduction of ceria at temperatures below 1,500 °C. Increasing zirconium content further favored cerium reduction yield up to 70%. Water-splitting tests were performed to demonstrate the reactivity of the developed materials for H₂ production. The amount of H₂ evolved was enhanced with a temperature increase, the maximum H₂ production from Ce_0.75Zr_0.25O_2−δ was 0.24 mmol/g at 1,045 °C, and the powder reactivity upon cycling was demonstrated via thermogravimetry through two successive reduction–hydrolysis reactions.     

Synthesis and properties of LaNi<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3−δ</Subscript> as cathode materials in SOFC

The synthesis of LaNi_{1− x} Fe _x O_3−δ (LNF) perovskites with x = 0.0–1.0, for use as cathode materials for an IT-SOFC, was investigated using four combustion methods, Water Citrate (WC), Modified Water Citrate (MWC), Nitric Citrate (NC), and Modified Nitric Citrate (MNC). The structures and homogeneities of the synthesized powders were examined using an XRD, and the particle sizes were examined using an SEM and a particle size analyzer. All four combustion methods gave the single phase perovskites with the same structure. The main difference was shown in a particle size that the smallest to the largest sizes were obtained from MNC, MWC, NC, and WC, respectively. In this LNF series, as x is 0–0.5, the crystal structure is cubic and rhombohedral at the calcination temperature of 700 and 900 °C, respectively. Further investigation indicated that the cubic structure changed to rhombohedral structure at 900 °C, and was stable up to 1200 °C. As x is 0.6–1.0, the crystal structure is in orthorhombic phase when calcined between 700 and 1000 °C. This orthorhombic phase decomposed above 1100 °C. From the XRD and SEM–EDX results, LaNi_0.6Fe_0.4O_3−δ (LNF64) has a good chemical compatibility with 8YSZ from room temperature up to 900 °C. In addition, its thermal expansion coefficient is 13.2 × 10⁻⁶ K⁻¹ close to that of 8 mol% Y₂O₃ (8YSZ). Therefore, LNF64 also has a good physical compatibility with 8YSZ.     

Wetting of grain boundaries in Al by the solid Al<Subscript>3</Subscript>Mg<Subscript>2</Subscript> phase

The microstructure of binary Al_100−x –Mg _x (x = 10, 15, 18 and 25 wt%) alloys after long anneals (600–4000 h) was studied between 210 and 440 °C. The transition from incomplete to complete wetting of Al/Al grain boundaries (GBs) by the second solid phase Al₃Mg₂ has been observed. The portion of completely wetted GBs increases with increasing temperature beginning from T _wsmin = 220 °C. Above T _wsmax = 410 °C all Al/Al GBs are completely wetted by the Al₃Mg₂ phase.     

Synthesis and variable temperature electrical conductivity studies of highly ordered TiO<Subscript>2</Subscript> nanotubes

Rafts of aligned, high aspect ratio TiO₂ nanotubes were fabricated by an electrochemical anodization method and their axial electrical conductivities were determined over the temperature range 225–400 °C. Length, outer diameter, and wall thickness of the nanotubes were approximately 60–80 μm, 160 nm, and 30 nm, respectively. Transmission electron microscopy studies confirmed that the TiO₂ nanotubes were initially amorphous, and became polycrystalline anatase after heat treatment at temperatures as low as 250 °C in air. The activation energy for conductivity over the temperature range 250–350 °C was found to be 0.87 eV. The conductivity values are comparable to those of nanocrystalline and nanoporous anatase thin films reported in literature.