Gasochromic response of nanocrystalline vanadium pentoxide films deposited from ethanol dispersions

Vanadium pentoxide (V₂O₅) nanocrystals having few tens nanometers average size, are obtained from ammonium metavanadate (NH₄VO₃) in the presence of oleic acid, and treating the reaction product at 400 °C. Nanocrystalline films, deposited from stable ethanol suspensions of the V₂O₅ nanopowder, adhere strongly to different kinds of substrates, without the need of any thermal post processing. At room temperature, the films are found to bleach when exposed to ammonia. Bleaching originates from the formation of ammonium metavandate, and is reversible, as after annealing in air at 350 °C, ammonium metavanadate converts back to V₂O₅. Formation of ammonium metavanadate, clearly evidenced by remarkable changes in infrared spectrum of V₂O₅ films exposed to ammonia, is a valuable detection mechanism, promising in view of developing highly selective ammonia sensors operating at room temperature.     

Phase formation of V2O5·xNb2O5 compounds via gels and freeze-dried precursors

An X-ray powder diffraction study of the phase formation in the system V₂O₅/Nb₂O₅ is performed. Freeze-dried ammonium vanadate and ammonium oxalato niobate, alkoxide-derived xerogels and a mixture of active oxides are used as precursors to compare the resulting phase composition. Thermal decomposition of the freeze-dried precursor is monitored with DTA/TG and mass spectrometry. In the quasi-binary system V₂O₅-Nb₂O₅ metastable VNbO₅, V₄Nb₁₈O₅₅, VNb₉O₂₅ and solid solutions of V₂O₅ in TT-Nb₂O₅ as also thermodynamically stable VNb₉O₂₅ exist. The thermal decomposition of freeze-dried vanadate-oxalatoniobate solution allows the synthesis of all these phases in a relative simple manner. Structural relationships between an intermediate phase and the product, or, in the case of solid-state reactions, between one of the starting oxide and the product, favour the desired reaction. Therefore, the structure of a former phase influences or directs the structure of the product similar to a topotactic reaction.     

Real structure of TiO2 in samples of the TiO2-V2O5 system

Measurements of electron spin resonance (ESR) spectra and X-ray structure analysis have been performed on samples of the TiO₂-V₂O₅ system, formed by anatase crystallites on the surface of which a thin layer of V₂O₅ was deposited by thermal decomposition of ammonium vanadate at 800°C. It was found that the deposited V₂O₅ gives rise to a change in the modification of TiO₂ (anatase goes over to rutile) and to a decrease in the volume of the TiO₂ elementary cell. Bands corresponding to Ti atoms of oxidation state +III were identified in the ESR spectrum of the initial TiO₂. The intensity of these bands decreases with increasing content of V₂O₅ in the TiO₂-V₂O₅ samples; the ESR spectra of the samples with high V₂O₅ content exhibit only one band corresponding to vanadium atoms in oxidation state +IV in the rutile lattice. The experimental results allow one to form an idea on the changes of the real structure of TiO₂ in connection with the deposition of V₂O₅ thin films on TiO₂ crystallites and with thermal treatment of these materials.     

Effects of alkali-metal ions on the thermal decomposition of ammonium metavanadate supported on alumina

The thermal decomposition of ammonium metavandate (AMV) supported on -alumina (11 mol) and treated with alkali-metal ions was investigated using differential thermal analysis (DTA), thermogravimetric analysis (TGA), infrared (i.r.) absorption spectroscopy and X-ray diffraction (XRD). The results revealed that the presence of alkali-metal ions did little to affect the thermal decomposition processes of AMV supported on alumina. However, these ions enhanced creation of V⁴⁺ ions by heating the samples in air at 400 °C. The oxides produced underwent a solid-solid interaction at 500–700 °C, forming alkali-metal vanadates. The formation of these spinels was found to be strongly dependent on the degree of diffusion and the concentration of the alkali-metal ions. Moreover, the addition of a higher concentration of alkali-metal ions inhibits the formation of aluminium vanadate while enhancing the crystallinity of - to -alumina.     

Thermal stability of hydronium and ammonium beta alumina at high pressure

The high pressure thermal stability of hydronium beta alumina and ammonium beta alumina have been investigated from 5 to 50 Kb pressure and from room temperature to 550°C. Both phases are stable at 400°C to 50 Kb pressure. At 50 Kb and 450°C, ammonium beta alumina decomposes into α-Al₂O₃ and hydronium beta alumina decomposes into H₂O.5Al₂O₃ (Tohdite) and α-Al₂O₃. A tentative phase diagram is suggested for the stability range of hydronium beta alumina in the Al₂O₃-H₂O system.     

Preparation and characterization of binary V2O5-Bi2O3 glasses

Characterization of the binary V₂O₅-Bi₂O₃ glasses prepared by rapidly quenching the melt has been made from the studies of X-ray diffraction, scanning electron microscopy, infrared absorption, differential thermal analysis, electron paramagnetic resonance, chemical analysis, density and electrical properties. Stable glasses are obtained for 95 to 75 mol % V₂O₅ by quenching on a stainless steel substrate, while quenching on a copper substrate extends the glass formation range from 95 to 70 mol % V₂O₅. The V-O bond vibration in the glasses occurs at 1020 cm^–1 and the V^5% ion exists in six-fold coordination as in crystalline V₂O₅. All the glasses appear to be in single phase. The spin concentration in the glasses is found to be independent of temperature. A second heat-treatment at 255° C develops crystalline phase in the glasses. Unlike infrared absorption, electron paramagnetic resonance, density and chemical compositions, the electrical and thermal (DTA) properties are found to be slightly sensitive to the thermal history of preparation of the glasses. The high-temperature (300 to 500 K) conduction in the glasses seems to be due to adiabatic hopping of polarons. The thermopower is observed to be independent of temperature and provides evidence for small polaron formation in the glasses.     

Effects of Ti and TiO2 on Combustion Synthesis of (Ti,V)2AlC/Al2O3 Solid Solution Composites

(Ti,V)₂AlC/Al₂O₃ solid solution composites were prepared by solid state combustion simultaneously incorporating reduction reactions of V₂O₅ and TiO₂/V₂O₅ with aluminum. Two reaction systems composed of Ti–V₂O₅–Al–Al₄C₃ and TiO₂–V₂O₅–Al–Al₄C₃ powder mixtures were studied. Combustion exothermicity was enhanced by increasing V₂O₅ and Al, which not only caused an increase in the combustion temperature and reaction front velocity, but also facilitated evolution of the (Ti,V)₂AlC phase. Between two reaction systems, the Ti-containing samples were more energetic and produced (Ti_1–xV_x)₂AlC/Al₂O₃ composites with x = 0.2–0.8. The degree of element substitution was reduced for the samples adopting TiO₂, which yielded Al₂O₃-added (Ti_1–yV_y)₂AlC with y = 0.4–0.8.     

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.     

Optical Properties of Solid Pentacene

We have prepared pentacene films on Si, Al₂O₃, and glass substrates by thermal evaporation and have investigated their optical properties at room temperature over a wide range of frequencies from infrared to ultraviolet (5 meV to 6.5 eV). A series of optical phonon modes and electronic transitions have been observed. The internal vibrational modes in the infrared region match well their molecular counterparts but the electronic transitions show substantial changes from those of a free pentacene molecule. The HOMO–LUMO gap energy of the pentacene films deposited on Al₂O₃ and glass substrates is 1.85 eV.     

A new chemical method for preparation of both pure and doped mullite

In order to prepare materials for a study of the properties of pure and intentionally doped mullite a new chemical synthetic method was developed. In this process a finely divided suspension is produced by introducing silicon acetate into a solution of aluminum nitrate and dopant compounds dissolved in alcohol. After evaporation this forms a gel, which undergoes thermal decomposition above 500°C to give an amorphous precursor of mullite. Mullites with well-defined x-ray diffraction patterns can then be obtained by heating this precursor at temperatures above 1300°C. Both pure Al₂O₃-SiO₂ mullite solid solutions and mullite solid solutions containing other oxides such as Fe₂O₃ or V₂O₅ can be made easily by this process.     

Comparative study of thermal barrier coatings for internal combustion engine

The paper presents the results of investigation into the thermal fatigue resistance of thermal barrier coatings (TBC). Two groups of double-layered thermal barrier coatings (TBC) were investigated: plasma sprayed with ZrO₂-8%Y₂O₃, Al₂O₃-40%TiO₂ or Al₂O₃-40%ZrO₂ top coats and powder flame sprayed with ZrO₂-30%CaO, Al₂O₃-40%TiO₂ and Al₂O₃-30%MgO. The extent of TBC deterioration experienced in thermal fatigue test was evaluated in the erosion test and SEM examinations. Flame sprayed coatings were found more prone to damage than plasma sprayed ones. The highest thermal fatigue resistance revealed TBC plasma sprayed with PSZ. Numerical calculation with Abaqus 6.7 finite element code was used to calculate temperature and stress variations in the coating throughout the test. Phase stability of plasma sprayed Al₂O₃-40%TiO₂ was evaluated by means of X-ray diffraction method. Thermal growth of oxides at the top coat/bond-coat interface and the decomposition of Al₂O₃-40%TiO₂ were found to be important degradation mechanisms leading to the spallation of coatings in the diesel engine and the petrol engine exploitation tests.     

PEG-directed hydrothermal synthesis of alumina nanorods with mesoporous structure via AACH nanorod precursors

Al₂O₃ nanorods with mesoporous structures are successfully synthesized from a hydrothermal and thermal decomposition process via the ammonium aluminum carbonate hydroxide (denoted as AACH) precursors. TEM images show that the average diameter of Al₂O₃ nanorods is about 60 nm, and the length is around 1–2 μm. The experimental results show that well-crystallized mesopores with hierarchically distributed pore sizes are embedded in the Al₂O₃ nanorods. The N₂ adsorption–desorption experiment indicates that the as-synthesized alumina nanorods have large surface area (ca. 176 m²/g) and narrow pore-size distributions. At the same time, the as-prepared Al₂O₃ nanorods exhibit strong photoluminescent properties at room temperature. A plausible surfactant-induced nanorod formation mechanism using the poly ethylene glycols as the template agent for the nanorod assembly is also proposed.     

Y3Al5O12:Tb phosphor particles prepared by spray pyrolysis from spray solution with polymeric precursors and ammonium fluoride flux

High brightness Y₃Al₅O₁₂:Tb phosphor particles with spherical shape and fine size were prepared by spray pyrolysis from spray solution with polymeric precursors and ammonium fluoride flux. The effect of ammonium fluoride flux on the characteristics of the Y₃Al₅O₁₂:Tb phosphor particles prepared by spray pyrolysis was investigated. Ammonium fluoride flux dissolved into spray solution improved the photoluminescence intensity of the Y₃Al₅O₁₂:Tb phosphor particles prepared by spray pyrolysis. The optimum addition amount of ammonium fluoride flux to produce the maximum photoluminescence intensity was 1 wt.% of the Y₃Al₅O₁₂:Tb phosphor particles. The spherical shape of the as-prepared particles obtained by spray pyrolysis from spray solution with polymeric precursors and small amount of ammonium fluoride flux had maintained after post-treatment below 1300 °C. The optimum photoluminescence intensity of the Y₃Al₅O₁₂:Tb phosphor particles prepared from polymeric precursors and ammonium fluoride flux was 156% of that of the phosphor particles prepared from pure aqueous solution.     

Preparation of carbon nanotube-neodymium oxide composite and research on its catalytic performance

Carbon Nanotube-Neodymium Oxide (CNT-Nd₂O₃) composite was prepared by using acid treated carbon nanotubes (CNTs) and neodymium nitrate in the presence of sodium dodecyl sulfate and ammonia liquid. Techniques of transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and differential thermal analysis (DTA) are used to characterize the morphology, structure, composition and catalytic property of the CNT-Nd₂O₃ composite. The experimental results show that the Nd₂O₃ nanoparticles, which have an average diameter of about 30-40 nm, are loaded on the surface of carbon nanotube. Compared with pure Nd₂O₃ nanorods, the CNT-Nd₂O₃ composite can catalyze the thermal decomposition of ammonium perchlorate more effectively. The sampling methods of the experimental samples made a difference on the catalytic experiment results, and the best catalytic result was obtained when de-ionized water served as the solvent of ammonium perchlorate.     

Use of microwave radiation for preparing uranium oxides from its compounds

The formation of uranium oxides by thermal decomposition of uranyl diaquadihydroxylaminate monohydrate, ammonium diuranate, ammonium tricarbonatouranylate, and uranium peroxide under the action of microwave (MW) radiation was studied. Uranium dioxide is formed by decomposition of these compounds in a reducing atmosphere at the MW radiation power of 600 W and treatment time of 5–10 min. In air, under the same conditions, U₃O₈ is formed. Under the action of MW radiation, substandard ceramic pellets of UO₂ fuel can be readily converted in air to powdered U₃O₈. The use of MW radiation for thermal decomposition of uranium compounds allows the power and time consumption to be considerably reduced relative to the process with electrical resistance furnaces. A quick method for gravimetric testing of the composition of uranium oxides (UO₂ or U₃O₈) using MW radiation was suggested.     

Highly mechanical strength and thermally conductive bismaleimide–triazine composites reinforced by Al2O3@polyimide hybrid fiber

Bismaleimide–triazine (BT) resins have received a great deal of attention in microelectronics due to its excellent thermal stability and good retention of mechanical properties. Thereafter, developing BT based composites with high mechanical strength, thermal conductivity and dielectric property simultaneously are highly desirable. In this study, one hybrid fiber of Al₂O₃ nanoparticle (200 nm) supported on polyimide fiber (Al₂O₃@PI) with core–shell structure was introduced into BT resin to prepare promising Al₂O₃@PI–BT composite. The results indicated that the resultant composites possessed high Young’s modulus of 4.06 GPa, low dielectric constant (3.38–3.50, 100 kHz) and dielectric loss (0.0102–0.0107, 100 kHz). The Al₂O₃@PI hybrid film was also conductive to improve thermal stability (T_d5% up to 371 °C), in-plane thermal conductivity (increased by 295% compared to that of the pure BT resin). Furthermore, the Al₂O₃@PI–BT composite were employed to fabricate a printed circuit substrate, on which a frequency “flasher” circuit and electrical components worked well.     

V2O5–polyimide hybrid material: synthesis,characterization, and sulfur removal properties in fuels

We describe the synthesis of a new sorbent material for desulfurization of gasoline, which is composed of polyimide (PI) and vanadium pentoxide via the solution direct-dispersing method. The highly porous PI–V₂O₅ hybrid materials, containing different concentrations of V₂O₅ ranging from 1 to 10 wt%, were prepared from pyromellitic dianhydride, 2,4,6-triaminopyrimidine, and vanadium pentoxide. The produced PI–V₂O₅ composites were investigated by X-ray diffraction analysis, Fourier transform infrared spectroscopy, scanning electron microscope, and thermal analysis techniques. The effect of V₂O₅ on the sulfur removal properties as well as the thermal stability and porous structure of composites were investigated. Then the obtained material was investigated to determine their sorption characteristics for sulfur compounds from fuels. By use of this material, 91 wt% of the sulfur content was removed from sulfur-containing standard oil. An economic sensitive and simple method for the removal and separation of sulfur in fuel samples, using an PI–V₂O₅ packed mini chromatographic column, was established.     

Compositional tailoring of the thermal expansion coefficient of tantalum (V) oxide

The effect of Al₂O₃ and Nb₂O₅ additions on the microstructure and thermal expansion behavior of tantalum (V) oxide has been studied. Both singly doped and co-doped compositions were examined. Compositions with 3 wt% Al₂O₃ (or greater), contained AlTaO₄ as a second phase. Complete solubility was observed for Nb₂O₅ contents in the range 1–5 wt%. It was found that doping with Al₂O₃ results in a decrease of the coefficient of thermal expansion (CTE), such that increasing amounts of Al₂O₃ (1–5 wt%) cause a successive decrease in the CTE value. In the case of Nb₂O₅ additions, the result is an increase in the CTE of the tantalum (V) oxide. However, in the range 1–5 wt% Nb₂O₅, the CTE value is relatively insensitive to the amount of Nb₂O₅ added. Due to the countervailing influences of these two additive oxides, it was demonstrated that co-doping with Al₂O₃ and Nb₂O₅ is an effective strategy for tailoring the CTE of tantalum (V) oxide.     

Effect of structural promoters on the catalytic performance of cobalt-based catalysts during natural gas decomposition to hydrogen and carbon nanotubes

Catalytic thermal decomposition of natural gas to CO/CO₂-free hydrogen production was studied over cobalt catalysts supported on Al₂O₃, MgO, and SiO₂. The physico-chemical properties of the fresh catalysts were investigated by X-ray diffraction (XRD), temperature-programmed reduction (TPR), and surface area. In addition, the morphological structure of as-deposited carbon over the spent catalysts was characterized by transmission electron microscope (TEM), Raman spectroscopy, thermogravimetric analysis, and XRD. The obtained results proved that the catalytic activity and longevity of the cobalt-based catalysts strongly depended on the nature of the applied support. Among the catalysts tested, the Co/Al₂O₃ catalyst exhibited the highest activity and stability due to the higher dispersion and stabilization of cobalt particles because of formation of CoAl₂O₃ spinel phase. The lower activity of Co/SiO₂ catalyst is mainly attributed to the aggregation of cobalt metal particles because of weak metal–support interaction. It was observed that both type and morphological structure of deposited carbon were strictly depended on the nature of the support. TEM images revealed that multi-walled carbon nanotubes were produced over Al₂O₃- and MgO-supported catalysts, whereas both carbon nanofibers and amorphous carbon were formed over the Co/SiO₂ catalyst.     

An investigation of phase stability in the Y2O3-Al2O3 system

The stability of the three known phases in the Y₂O₃-Al₂O₃ pseudo-binary system has been investigated. YAlO₃ (YAP) and Y₄Al₂O₉ (YAM) decompose at elevated temperatures, the products of the reaction being the third compound Y₃Al₅O₁₂ (YAG) and an unknown phase (designated X). The decomposition is most evident in powders but can also be initiated on the surface of bulk single crystals. X-ray diffraction studies have been performed in an attempt to identify the structure and composition of the unknown phase. The thermal decomposition has been found to be surface controlled and an optical and scanning electron microscope study of the associated morphological changes in YAlO₃ indicates that the reaction involves localized surface melting, probably with the loss of oxygen which effectively moves the composition off the binary join.