Ralf Brüning

Low Temperature Sol-Gel Preparation of Nanocrystalline TiO2 Thin Films

TiO2 nanocrystalline thin films with varying degree of porosity have been prepared using a low temperature method. TiO2 films of the anatase form have been obtained by using a polyethylene glycol (PEG) modified sol-gel method. Densification and crystallization of the films was found to result from the thermal treatment of the dip coated films in boiling water. The films have been characterized by Raman, XRD, FTIR, AFM and optical methods. Highly transparent films with transmission in excess of 85% and porosity as high as 58% are formed predominantly of anatase crystallites of dimensions of the order of 5 nm. Initial results on lithium intercalation into these films resulting in an efficient optical modulation in the visible and near infrared regions demonstrate a good potential of these films for electrochromic applications.

Porous orthorhombic tungsten oxide thin films: synthesis, characterization, and application in electrochromic and photochromic devices

Porous orthorhombic tungsten oxide (o-WO3) thin films, stabilized by nanocrystalline anatase TiO2, are obtained by a sol–gel based two stage dip coating method and subsequent annealing at 600 °C. An Organically Modified Silicate (ORMOSIL) based templating strategy is adopted to achieve porosity. An asymmetric electrochromic device is constructed based on this porous o-WO3 layer. The intercalation/deintercalation of lithium ions into/from the o-WO3 layer of the device as a function of applied coloration/bleaching voltages have been studied. XRD measurements show systematic changes in the lattice parameters associated with structural phase transitions from o-WO3 to tetragonal LixWO3 (t-LixWO3) and a tendency to form cubic LixWO3 (c-LixWO3). These phase transitions, induced by the Li ions, are reversible, and the specific phase obtained depends on the quantity of intercalated/deintercalated Li. Raman spectroscopy data show the formation of t-LixWO3 for an applied potential of 1.0 V and the tendency of the system to transform to c-LixWO3 for higher coloration potentials. Optical measurements show excellent contrasts between colored and bleached states. An alternate photochromic device was constructed by sensitizing the o-WO3 layer with a ruthenium based dye. The nanocrystalline anatase TiO2 in the o-WO3 layer has led to an enhanced photochromic optical transmittance contrast of ∼51% in the near IR region. The combination of the photochromic and electrochromic properties of the synthesised o-WO3 layer stabilized by nanocrystalline anatase TiO2 opens up new vista for its application in energysaving smart windows.

Synthesis and Characterization of Macroporous Tungsten Oxide Films for Electrochromic Application

Macroporous tungsten oxide films have been prepared by combining a nonhydrolytic sol-gel method with a molecular assembly templating strategy. The material has been prepared by hydrolysis of an ethanolic solution of tungsten ethoxide in the presence of polyethylene glycol (PEG), followed by calcination of the dip-coated films. AFM images indicated that an important morphological diversity can be obtained by simply varying the amount of PEG in the coating solution and the conditions of the heat-treatment. The formation of nanostructures of controlled shapes and patterns (fibrils or striped phases) with relatively uniform channel spacings is accounted for by strong hydrogen bonding interactions between the PEG and the partially hydrolyzed tungsten oxide oligomers. XRD and FTIR data showed that PEG delays the crystallization of WO3. When compared to sol-gel prepared tungsten oxide fims prepared without PEG, the coloration efficiency of the macroporous films appears to be significantly improved especially in the near-infrared region.

Fragility of glass-forming systems and the width of the glass transition

Abstract The concept of fragility is now commonly used to classify glass-forming liquids. In the original description by Angell, the degree of fragility is most easily determined from the curvature of the viscosity on an Arrhenius plot. Other measures of fragility are discussed here which may be more accessible to experiments and computer simulations than viscosity data. Numerical values of different measures of fragility are related and experimental data are compared for a large number of systems. It is shown that the degree of fragility is independent of the glass transition temperature of the system, and that the glass transition temperature and the degree of fragility together are sufficient to specify the kinetics of the system in the glass transition regime. Based on an overview of different systems, varying degrees of fragility are tentatively explained in terms of the range of microscopic forces in the liquid.

Synthesis of hybrid silica materials with tunable pore structures and morphology and their application for heavy metal removal from drinking water

Porous silica materials S8, S12, S16, and SBA with controllable pore structures and morphology were synthesized by varying the type or alkyl chain length of the surfactant. Diverse amino-functionalized organic-inorganic hybrid porous materials were then prepared by post-grafting. Depending on the relation between the pore diameter of the porous silica materials and the size and content of the moiety to be grafted, the functionalized materials exhibited varying degrees of decline of structure properties, i.e. regular arrangement of pores, specific surface area, pore size, and pore volume. The hybrid silica materials have been employed as heavy metal ions adsorbents from simulated drinking water at room temperature. The results indicated that the diverse pore structures and different amino group densities influence the heavy metal ions adsorption of functionalized silicas significantly. The best adsorbent was found to be monoamino-functionalized silica S16-1N, which could effectively remove heavy metal Cd(II), Pb(II), Fe(III), as well as Mn(II). The good performance can be attributed to the accessibility of effective amino groups in the pores, as well as the suitable pore structure with high specific surface area of 728 m(2)/g and total pore volume of 0.34 cm(3)/g.

Study of the phase transition and the thermal nitridation of nanocrystalline sol–gel titania films

Abstract Nanocrystalline titania films were prepared by a complexing agent-assisted sol–gel method and converted to titanium nitride by a thermal nitridation process. The effect of acetylacetone (AcAc), diethanolamine (DEA) and acid catalysts (HCl and HNO 3 ) on the structure and morphology of the heat-treated titania films and on their nitridation products was examined by FTIR spectroscopy, X-ray diffraction (XRD) and atomic force microscopy (AFM). The carbothermal reduction of titania during the nitridation process with the formation of carboxynitrides has been considered. The results showed that the oxide to nitride transition strongly depends on the complexing agent used to prepare the titania films. The XRD results indicated the dependence of the lattice parameter of the nitridation product on the complexing agent or acid catalyst: AcAc and DEA lead to TiN x with a lattice parameter α close to the theoretical value, while with HCl the lattice parameter was found sensibly lower showing the presence of an oxynitride. These results are accounted for by the effect of complexing agents and acid catalysts on the size of both TiO 2 and TiN grains and the different reactivity of the anatase and rutile phases. The possibility of tailoring the composition and morphology of TiN films by using complexing agents is envisaged.

Reversible structural relaxation in Fe‐Ni‐B‐Si metallic glasses

Reversible structural relaxation has been studied in (Fe1−xNix)80B10Si10 by differential scanning calorimetry and x‐ray diffraction. It is found that surface crystallization plays a major role in inhibiting reversibility. In consequence, previously published data have presented a misleading picture about the composition dependence of the reversible effect in Fe‐Ni‐B‐Si, which simply increases monotonically with Ni composition. There is no evidence that chemical short‐range order plays any role in reversibility, and the data have been quantitatively modeled by simply assuming that reversible relaxation is caused by the thermal repopulation of excited structural states.

A comparison between the thermal properties of NiZr amorphous alloys obtained by mechanical alloying and melt-spinning☆

Abstract A comparison between amorphous NiZr alloys prepared by melt-spinning and mechanical alloying has been made through X-ray diffraction measurements, differential scanning calorimetry and measurements of the superconducting transition temperature. Oxygen impurities of 2–3 at.% in the mechanically alloyed samples were found to cause important differences in the crystallization behaviour and superconducting transition temperature, while the different ambient temperatures during amorphization strongly influence thermal structural relaxation. In spite of the differences found between the specific samples, the results confirm that the two methods of preparation yield, once the samples are thermally relaxed, essentially identical amorphous states.

Novel nanocomposite material consisting of poly[oxymethylene-(oxyethylene)] and molybdenum disulfide

Abstract Poly[oxymethylene-(oxyethylene)] has been inserted in the gallery space of MoS2 by using the exfoliation/re-stacking properties of LiMoS2. The resulting intercalation compound shows enhanced electronic conductivity due to a structural transformation of the MoS2 framework. The material was also characterized by powder X-ray diffraction and thermogravimetric analysis.

Glass transitions in one-, two-, three-, and four-dimensional binary Lennard-Jones systems

We investigate the calorimetric liquid-glass transition by performing simulations of a binary Lennard-Jones mixture in one through four dimensions. Starting at a high temperature, the systems are cooled to T = 0 and heated back to the ergodic liquid state at constant rates. Glass transitions are observed in two, three and four dimensions as a hysteresis between the cooling and heating curves. This hysteresis appears in the energy and pressure diagrams, and the scanning rate dependence of the area and height of the hysteresis can be described using power laws. The one-dimensional system does not experience a glass transition but its specific heat curve resembles the shape of the D≥2 results in the supercooled liquid regime above the glass transition. As D increases, the radial distribution functions reflect reduced geometric constraints. Nearest neighbor distances become smaller with increasing D due to interactions between nearest and next-nearest neighbors. Simulation data for the glasses are compared with crystal and melting data obtained with a Lennard-Jones system with only one type of particle and we find that with increasing D crystallization becomes increasingly more difficult.