A study of the mechanically milled h-BN-H system

Defective nanostructured h-BN, with different structural characteristics, can be prepared by mechanical milling under hydrogen and argon atmospheres. When h-BN was mechanically milled under a hydrogen atmosphere, hydrogen could be trapped by the B- and N-dangling bonds formed; the amount of which reached up to 2.6mass% after 80 h milling. The absorbed hydrogen can be released only as molecular hydrogen starting from about 570 K. As is clarified by a combination of TDS and IR measurements, the hydrogen detrapped from B–H and N–H bonds dominates the desorption at the lower and higher temperature ranges, respectively. After the nanostructured h-BN was heated to 1173 K, some of the hydrogen was still trapped by N–H bonds where, correspondingly, no recrystallization was detected. PACS 81.05.Tp; 61.46.+w; 81.20.Ev; 81.05.Zx     

Microstructure characterization of mechanosynthesized nanocrystalline NiFe2O4 by Rietveld's analysis

Nanocrystalline nickel ferrite (NiFe₂O₄) is synthesized at room temperature by high-energy ball milling the stoichiometric mixture of (1:1 mol%) of NiO and α-Fe₂O₃ powders. The structural and microstructural evolution of NiFe₂O₄ caused by milling is investigated by X-ray powder diffraction. The relative phase abundance, particle size, r.m.s. strain, lattice parameter changes of different phases have been estimated employing Rietveld structure refinement analysis of X-ray powder diffraction data. Particle size and content (wt%) of both NiO and α-Fe₂O₃ phases reduce rapidly with increasing milling time and a significant amount of nanocrystalline NiFe₂O₄ is formed within 1 h of ball milling. Particle sizes of all the phases reduce to ∼10 nm within 5 h of milling and remain almost unchanged with increasing milling time up to 20 h. Lattice parameter of cubic NiO decreases linearly with increasing milling time, following the Vegard's law of solid-solution alloy. A continuous decrease in lattice parameter of cubic NiFe₂O₄ phase clearly suggests that smaller Ni atoms have occupied some of the vacant oxygen sites of ferrite lattice. Cation distribution both in octahedral and tetrahedral sites changes continuously with milling time and the normal spinel lattice formed at early stage of milling, transforms to inverse spinel lattice in the course of milling. High-resolution transmission electron microscope (HRTEM) micrographs of 11 h milled sample corroborates the findings of X-ray profile analysis.     

Dynamical LEED analyses of the clean and the NO-adsorbed Ir(111) surface

The adsorption structure of nitric oxide (NO) on Ir(111) was studied by thermal desorption spectroscopy (TDS) and dynamical analyses of low-energy electron diffraction (LEED). At the saturation coverage at about 100 K, a 2 × 2 pattern was observed by LEED and two peaks appeared at 365 and 415 K in TDS. No change in the LEED I–V curves was observed by annealing at 280 K, which means that the NO-saturated surface was retained at this temperature. On the contrary, partial desorption and changes of the LEED I–V curves were observed by annealing at 360 K. Combined with previous vibrational studies, it is suggested that one adsorption species is not affected, while another species is partially desorbed and the rest of them are dissociated by annealing at 360 K. Dynamical analyses of LEED were performed for the 280 K-annealed and the 360 K-annealed surfaces, which correspond to the NO-saturated and the NO-dissociated Ir(111) surfaces, respectively. These revealed that NO occupies the atop, fcc-hollow and hcp-hollow sites (atop-NO + fcc-NO + hcp-NO) for the NO-saturated Ir(111) surface with the saturation coverage of 0.75 ML. For the 360 K-annealed surface, the atop-NO is not affected but the fcc-NO and the hcp-NO are partially desorbed as NO and partially dissociated to N and O, both of which occupy the fcc-hollow site on the surface.     

Nitrogen physisorption and site blocking on single-walled carbon nanotubes

N₂ physisorption on Single-Walled Carbon Nanotubes (SWCNTs) was investigated by cryogenic thermal-desorption spectroscopy (cryo-TDS). TDS spectra revealed a desorption peak at 48 K (α) for as-purified SWCNTs and an additional desorption peak at 73 K (β) for air-oxidized SWCNTs. When N₂ and H₂ were coadsorbed on SWCNTs, H₂ adsorption was blocked by the N₂ preadsorption. By comparing the TDS data with and without the N₂ preadsorption, the α and β peaks were attributed to N₂ adsorbed on the groove site and inside of SWCNTs, respectively.     

Behavior of decomposed ammonia borane at high pressure

High pressure behavior of ammonia borane after thermal decomposition was studied by Raman spectroscopy at high pressure up to 10 GPa using diamond anvil cell (DAC). The ammonia borane was decomposed at around 140 °C under the pressure at ∼0.7 GPa. Raman spectra show the hydrogen was desorbed within 1 h. The hydrogen was sealed in DAC well and cooled down to room temperature. Applying higher pressure up to ∼10 GPa indicates interactions between the products and loss of dihydrogen bonding. No rehydrogenation was detected in the pressure range investigated.     

Effect of milling time on the properties of Pb(Mg1/3Nb2/3)O3 ceramics using the starting precursors PbO and MgNb2O6

Lead magnesium niobate, Pb(Mg_1/3Nb_2/3)O₃ (PMN) ceramics were prepared from the columbite method using calcined powders of various milling time (24–96 h). The effects on the grain size and dielectric properties of the ceramics were investigated. The results show that dielectric properties of ceramics are strongly influenced by the milling time of the starting precursors. Higher percentage of perovskite phase was found in the ceramics that was milled longer and thus the dielectric constant was found to increase when compared to the conventional 24 h milled results. Moreover, milling time also affected the particle size of the starting precursors and that of PMN powders. Therefore, milling time did not only affect the particle size of PMN powders but also the resultant grain size and the formation of perovskite phase, consequently affecting the dielectric constant of the ceramics.     

The thermodynamic and kinetic properties of hydrogen dimers on graphene

The thermodynamic and kinetic properties of hydrogen adatoms on graphene are important to the materials and devices based on hydrogenated graphene. Hydrogen dimers on graphene with coverages varying from 0.040 to 0.111 ML (1.0 ML = 3.8 × 10¹⁵cm^? 2) were considered in this report. The thermodynamic and kinetic properties of H, D and T dimers were studied by ab initio simulations. The vibrational zero-point energy corrections were found to be not negligible in kinetics, varying from 0.038 (0.028, 0.017) to 0.257 (0.187, 0.157) eV for H (D, T) dimers. The isotope effect exhibits as that the kinetic mobility of a hydrogen dimer decreases with increasing the hydrogen mass. The simulated thermal desorption spectra with the heating rate α = 1.0 K/s were quite close to experimental measurements. The effect of the interaction between hydrogen dimers on their thermodynamic and kinetic properties was analyzed in detail.     

Ultrasonic assisted synthesis of Bikitaite zeolite: A potential material for hydrogen storage application

Li containing Bikitaite zeolite has been synthesized by an ultrasound-assisted method and used as a potential material for hydrogen storage application. The Sonication energy was varied from 150 W to 250 W and irradiation time from 3 h to 6 h. The Bikitaite nanoparticles were characterized by X-ray diffraction (XRD), infrared (IR) spectral analysis, and field-emission scanning electron microscopy (FESEM) thermo-gravimetrical analysis and differential thermal analysis (TGA, DTA). XRD and IR results showed that phase pure, nano crystalline Bikitaite zeolites were started forming after 3 h irradiation and 72 h of aging with a sonication energy of 150 W and nano crystalline Bikitaite zeolite with prominent peaks were obtained after 6 h irradiation of 250 W sonic energy. The Brunauer–Emmett–Teller (BET) surface area of the powder by N₂ adsorption–desorption measurements was found to be 209 m²/g. The TEM micrograph and elemental analysis showed that desired atomic ratio of the zeolite was obtained after 6 h irradiation. For comparison, sonochemical method, followed by the hydrothermal method, with same initial sol composition was studied. The effect of ultrasonic energy and irradiation time showed that with increasing sonication energy, and sonication time phase formation was almost completed. The FESEM images revealed that 50 nm zeolite crystals were formed at room temperature. However, agglomerated particles having woollen ball like structure was obtained by sonochemical method followed by hydrothermal treatment at 100 °C for 24 h. The hydrogen adsorption capacity of Bikitaite zeolite with different Li content, has been investigated. Experimental results indicated that the hydrogen adsorption capacities were dominantly related to their surface areas as well as total pore volume of the zeolite. The hydrogen adsorption capacity of 143.2 c.c/g was obtained at 77 K and ambient pressure of (0.11 MPa) for the Bikitaite zeolite with 100% Li, which was higher than the reported values for other zeolites. To the best of our knowledge, there is no report on the synthesis of a Bikitaite zeolite by sonochemical method for H₂ storage.     

Thermoexfoliated graphite as support for production of metal–graphite nanocomposites

The method of chemical deposition of metal salts from water–salt solutions with the subsequent thermal decomposition of salt to metal or metal oxide has been developed for thermoexfoliated graphite (TEG). The type of graphite support and character of metal salt thermal decomposition was shown to influence essentially the dispersity and morphology of metal particles being formed on graphite surface. Preliminary treatment of the initial dispersed graphite by H₂SO₄ leads to the formation of oxygen-containing groups on its surface, which are the exchange centres fixing the metal cations in graphite–metal (or graphite–metal oxide) composites. Thermal exfoliation of graphite, which is accompanied by the refinement of graphite structure, occurs as a result of oxidized graphite heat treatment. High porosity, defectiveness and chemical activity of TEG surface favour the effective impregnation of TEG by aqueous salt solution. Thermal treatment of the salt-impregnated graphite leads to formation of Co particles on graphite surface. These particles are 70–150 nm in size for TEG–Co at a relatively high metal content (up to 30 wt%) and 200 nm in size for oxidized dispersed graphite–Co at Co content ∼10 wt%.     

Disorder driven optical processes in nanocrystalline ZnO

A systematic study on the modification of optical properties in mechanically milled ZnO powder has been reported here. The average grain size of the powder becomes ～20 nm within 4 h of milling. Fluctuations of average grain size have been noticed at the initial stage of milling (within 15 min). Changes in grain morphology with milling have also been noticed in scanning electron micrographs of the samples. Room temperature optical absorption data shows a systematic red shift of absorption band edge (～3.25 eV). The band tail parameter (extracted from the optical absorption just below the band edge) follows a simple exponential relation with the inverse of the average grain size. Significant increase of the band tail parameter has been noticed at low grain size regime. It has been analyzed that high values of band tail parameter is a representative of V_Zn–V_O type divacancy clusters. Room temperature photoluminescence spectra show decrease (except for 120 min milling) of band edge emission intensity with increase of milling time. Subsequent decrease of sub-band edge emission is, however, less prominent. The variation of PL intensity ratio (intensity at band edge peak with that at 2.3 eV) follows simple exponential decrease with the increase of band tail parameter. This indeed shows that band edge emission in ZnO is related with the overall disorder in the system, not grain size induced only.     

Effects of Zr impurity on microscopic behavior of Hf metal

Hf metal with ∼ 3 wt% Zr impurity has been reinvestigated by perturbed angular correlation (PAC) spectroscopy using a LaBr₃(Ce)–BaF₂ detector set up to understand the microscopic behavior of this metal with temperature. From present measurements, five quadrupole interaction frequencies have been found at room temperature where both pure hcp fraction (∼33%) with 12 nearest neighbor Hf surrounding the probe ¹⁸¹Hf atom and the probe–impurity fraction (∼33%) corresponding to 11 nearest neighbor Hf plus one dissimilar Zr atom are clearly distinguished. At room temperature, the results for quadrupole frequency and asymmetry parameter are found to be ω_Q=51.6(4) Mrad/s, η=0.20(4) for the impurity fraction and ω_Q=46.8(2) Mrad/s, η=0 for the pure fraction with values of frequency distribution width δ=0 for both components. At 77 K, only 1 NN Zr impurity (∼93%) and pure hcp (∼7%) components have been found with a value of δ ∼ 10% for the impurity fraction. A drastic change in microstructural configuration of Hf metal is observed at 473 K where the impurity fraction increases to ∼ 50% and the pure hcp fraction reduces to ∼ 15% with abrupt changes in quadrupole frequencies for both components. The pure fraction then increases with temperature and enhances to ∼50% at 973 K. In the temperature range 473–973 K, quadrupole frequencies for both components are found to decrease slowly with temperature. Using the Arrhenius relation, binding energy (B) for the probe–impurity pair and the entropy of formation are measured from temperature dependent fractions of probe–impurity and pure hcp in the temperature range 473–773 K. The three other minor components found at different temperatures are attributed to crystalline defects.     

X-ray diffraction and Mössbauer studies of mechanically alloyed Fe–Ni nanostructured powders

Fe–10 wt% Ni and Fe–20 wt% Ni nanostructured alloys were prepared using a planetary ball mill P 4 vario mill from Fritsch. The Fe (Ni) BCC solid solution was identified by X-ray diffraction, allowing also to follow the size and shape of crystalline grains. The higher the shock power, the smaller the grain size. The Mössbauer spectra of the nanostructured powders recorded at 77 and 300 K differ according to the shock power and the friction energy component while the hyperfine structure gives relevant information on the local structure environment of Fe atoms in relation with the milling mode process (shock or friction mode).     

Preparation process and properties of exfoliated graphite nanoplatelets filled Bisphthalonitrile nanocomposites

Exfoliated graphite nanoplatelets (xGnP) filled 4,4'-Bis (3,4-dicyanophenoxy) biphenyl (BPh) nanocomposites were prepared by a resin transfer molding process. The rheological behavior of the BPh pre-polymer, and the morphology and electrical, mechanical and thermal properties of the xGnP/BPh nanocomposites were systematically investigated. The results showed that the xGnP/BPh pre-polymer possessed a higher complex viscosity and storage modulus than the pure BPh and that the xGnP could significantly enhance the mechanical and electrical properties of the resulted nanocomposites. The electrical percolation threshold of the xGnP/BPh nanocomposites was between 5 and 10 wt% xGnP. The flexural strength and modulus of the xGnP/BPh nanocomposites with 10 wt% xGnP exhibited maximum values and their thermal stabilities were greatly improved. Those novel xGnP/BPh nanocomposites could have advanced applications in areas like aerospace and military industry.     

Interaction of NO with the O-rich RuO2(1 1 0) surface at 300 K

The interaction of NO with the O-rich RuO₂(1 1 0) surface, exposing coordinatively unsaturated O-bridge, O-cus, and Ru-cus atoms, was studied at 300 K by thermal desorption spectroscopy (TDS) and high-resolution electron energy-loss spectroscopy (HREELS). The conclusions are validated by isotope substitution experiments with ¹⁸O. During exposure to NO an O···N–O surface group (NO₂-cus) is formed with O-cus. Additionally, a smaller number of empty Ru-cus sites are filled by NO-cus. If one warms the sample to 400 K, NO₂-cus does not desorb but decomposes into O and NO again, the latter being either released into gas phase or adsorbed as NO-cus. With O-bridge such a surface group is not stable at 300 K. Our experiments further prove that O-cus is more reactive than O-bridge.     

Microstructure and optical characterizations of mechanosynthesized nanocrystalline semiconducting ZrTiO4 compound

A ZrO₂–TiO₂ solid solution is obtained by high energy ball milling of equimolar mixture of monoclinic (m) ZrO₂ and anatase (a) TiO₂. Nanocrystalline orthorhombic ZrTiO₄ compound is initiated from the nucleation of TiO₂–ZrO₂ solid solution with isostructural s-TiO₂ (srilankite) base after 30 min of milling. After 12 h of milling, 95 mol% non-stoichiometric ZrTiO₄ phase is formed. Post-annealing of 12 h ball-milled powder mixture at 1073 K for 1 h in open air results in complete formation of stoichiometric ZrTiO₄ compound. Microstructures of all powder mixtures milled for different durations have been characterized by Rietveld's structure and microstructure refinement method using X-ray powder diffraction data. HRTEM images of 12 h milled and annealed samples provide direct evidence of the results obtained from the Rietveld analysis. Optical bandgaps of ball milled and annealed ZrTiO₄ compounds lie within the semiconducting region (~2.0 eV) and increases with increase in milling time.     

Evidence for non-thermalized vibrationally excited molecule production during azomethane pyrolysis in methane desorption from Cu(001)

A Cu(001) surface was exposed to products of an azomethane pyrolysis doser at varying temperatures. In addition to methyl radical adsorption, for certain doser conditions one or more doser emergent species can undergo an activated adsorption on the copper face. Directly after exposures, temperature programmed desorption between 170 K and 500 K was used to indicate the relative concentrations of adsorbed atomic hydrogen and methyl species, and thermally induced surface reactions. Two methane desorption features were invariably observed, indicating the presence of adsorbed methyl groups (CH₃) and transient adsorbed atomic hydrogen. The deduced relative surface concentrations levels of both H and CH₃ depend on the total exposures and the operating temperatures of the azomethane pyrolysis doser. The initial H concentrations apparent at surface temperatures between 275 K and 375 K are shown to arise from defect-related methyl decomposition and, at high operating doser temperatures, from the initial adsorption of one or more activated Cu incident species. It is proposed that the distributions of vibrational energies of emergent molecular hydrogen or methane species from higher temperature dosers are non-thermal. Hence, with doser temperatures of 800 °C or above, the effects of subsequent dissociative molecular adsorption on the copper surface can dominate over Cu defect chemistries.     

Thermal oxidation-grown vanadium dioxide thin films on FTO (Fluorine-doped tin oxide) substrates

By deposition of metallic vanadium on FTO substrate in Argon atmosphere at room temperature, the sample was then annealed in furnace for 2 h at the temperature of 410 °C in air ambient. (1 1 0) -orientated vanadium dioxide films were prepared on the FTO surface. A maximum transmittance of ∼40% happened at 900–1250 nm region at room temperature. The change of optical transmittance at this region was ∼25% between semiconducting and metallic states. In particular, vanadium dioxide thin films on FTO exhibit semiconductor–metal phase transition at ∼51 °C, the width of the hysteresis loop is ∼8 °C.     

Fast hydrogen sorption kinetics for ball-milled Mg2Ni alloys☆

The sorption kinetics of Mg₂Ni alloys are strongly improved using two different surface treatments. One consists of the preparation of tailor-made Mg₂Ni/C composites by ball-milling with previously ground carbons. The strong reducing character of carbon, allowing for the reduction of NiO initially present at the alloy surface, leads to the partial removal of the oxide layer that strongly hinders the hydrogen migration throughout the alloy surface on desorption, whereas resulting Ni particles act as catalysts during the absorption process. The second surface treatment deals with the deposition of Pd particles on the alloy surface using the polyol process. The catalytic effect of Pd is responsible for an important enhancement of the absorption kinetic, whereas Pd particles probably act as hydrogen pumps, during the desorption process, leading to a faster hydrogen release. By combining both techniques, desorption rates as high as 2.7 wt% in 60 min and 2.9 wt% in 30 min are obtained at 150 and 200 °C, respectively.     

Investigation of hydrogen storage in MWCNT–TiO2 composite

The hydrogen storage capacity of MWCNT–TiO₂ composite has been evaluated in the present work. The composite has been prepared by means of ultrasonication followed by drop casting on substrates. Morphology, structural and functional group studies of the prepared samples are carried out by transmission electron microscopy (TEM), scanning electron microscopy (SEM), powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. Then, the samples are hydrogenated in the hydrogenation chamber as a function of time. Hydrogen storage capacity of the composite sample is found to be 0.9 wt% at 100 °C. Hydrogen uptake of the composite is accounted for the spillover mechanism in CNTs–metal oxide composite. Desorption temperature range, activation energy of desorption, binding energy of hydrogen are determined from thermogravimetric (TG) analysis.     

CO adsorption on clean and oxidized Pt3Ti(111)

CO adsorption on clean and oxidized Pt₃Ti(111) surfaces has been investigated by means of Auger Electron Spectroscopy (AES), Thermal Desorption Spectroscopy (TDS), Low Energy Electron Diffraction (LEED) and High Resolution Electron Energy Loss Spectroscopy (HREELS). On clean Pt₃Ti(111) the LEED patterns after CO adsorption exhibit either a diffuse or a sharp c(4 × 2) structure (stable up to 300 K) depending on the adsorption temperature. Remarkably, the adsorption/desorption behavior of CO on clean Pt₃Ti(111) is similar to that on Pt(111) except that partial CO decomposition on Ti sites and partial CO oxidation have also been evidenced. Therefore, the clean surface cannot be terminated by a pure Pt plane. Partially oxidized Pt₃Ti(111) surfaces (< 135 L O₂ exposure at 1000 K) exhibit a CO adsorption/desorption behavior rather similar to that of the clean surface, showing again a c(4 × 2) structure (stable up to 250 K). Only the oxidation of CO is not detectable any more. These results indicate that some areas of the substrate remain non-oxidized upon low oxygen exposures. Heavily oxidized Pt₃Ti(111) surfaces (> 220 L O₂ exposure at 1000 K) allow no CO adsorption indicating that the titanium oxide film prepared under these conditions is completely closed.