Modelling of Effective Thermal Conductivity of Composites Filled with Core-Shell Fillers

An effective model to calculate thermal conductivity of polymer composites using core-shell fillers is presented, wherein a core material of filler grains is covered by a layer of a high-thermal-conductivity (HTC) material. Such fillers can provide a significant increase of the composite thermal conductivity by an addition of a small amount of the HTC material. The model employs the Lewis-Nielsen formula describing filled systems. The effective thermal conductivity of the core-shell filler grains is calculated using the Russel model for porous materials. Modelling results are compared with recent measurements made on composites filled with cellulose microbeads coated with hexagonal boron nitride (h-BN) platelets and good agreement is demonstrated. Comparison with measurements made on epoxy composites, using silver-coated glass spheres as a filler, is also provided. It is demonstrated how the modelling procedure can improve understanding of properties of materials and structures used and mechanisms of thermal conduction within the composite.

Thin-Film Engineering of Mechanical Fragmentation Properties of Atomic-Layer-Deposited Metal Oxides

Mechanical fracture properties were studied for the common atomic-layer-deposited Al₂O₃, ZnO, TiO₂, ZrO₂, and Y₂O₃ thin films, and selected multilayer combinations via uniaxial tensile testing and Weibull statistics. The crack onset strains and interfacial shear strains were studied, and for crack onset strain, TiO₂/Al₂O₃ and ZrO₂/Al₂O₃ bilayer films exhibited the highest values. The films adhered well to the polyimide carrier substrates, as delamination of the films was not observed. For Al₂O₃ films, higher deposition temperatures resulted in higher crack onset strain and cohesive strain values, which was explained by the temperature dependence of the residual strain. Doping Y₂O₃ with Al or nanolaminating it with Al₂O₃ enabled control over the crystal size of Y₂O₃, and provided us with means for improving the mechanical properties of the Y₂O₃ films. Tensile fracture toughness and fracture energy are reported for Al₂O₃ films grown at 135 °C, 155 °C, and 220 °C. We present thin-film engineering via multilayering and residual-strain control in order to tailor the mechanical properties of thin-film systems for applications requiring mechanical stretchability and flexibility.

Focused Electron Beam-Induced Deposition and Post-Growth Purification Using the Heteroleptic Ru Complex (η3-C3H5)Ru(CO)3Br

Focused electron beam-induced deposition using the heteroleptic complex (η³-C₃H₅)Ru(CO)₃Br as a precursor resulted in deposition of material with Ru content of 23 at. %. Transmission electron microscopy images indicated a nanogranular structure of pure Ru nanocrystals, embedded into a matrix containing carbon, oxygen, and bromine. The deposits were purified by annealing in a reactive 98% N₂/2% H₂ atmosphere at 300 °C, resulting in a reduction of contaminants and an increase of the Ru content to 83 at. %. Although a significant volume loss of 79% was found, the shrinkage was observed mostly for vertical thickness (around 75%). The lateral dimensions decreased much less significantly (around 9%). Deposition results, in conjunction with previous gas-phase and condensed-phase surface studies on the electron-induced reactions of (η³-C₃H₅)Ru(CO)₃Br, provide insights into the behavior of allyl, carbonyl, and bromide ligands under identical electron beam irradiation.

Spin-Dependent Electronic Transport in IrMn–Co/Pd Multilayered Systems

Relation between magnetoresistance and magnetization reversal in thin multilayered IrMn–Co/Pd films deposited both on planar and nanoporous substrates is in focus of the present work. Magnetic anisotropy, exchange coupling and mechanisms of magnetic resistivity are discussed.

Direct Electron Beam Writing of Silver-Based Nanostructures

Direct writing utilizing a focused electron beam constitutes an interesting alternative to resist-based techniques, as it allows for precise and flexible growth onto any conductive substrate in a single-step process. One important challenge, however, is the identification of appropriate precursors which allow for deposition of the material of choice, e.g., for envisaged applications in nano-optics. In this regard the coinage metal silver is of particular interest since it shows a relatively high plasma frequency and, thus, excellent plasmonic properties in the visible range. By utilizing the precursor compound AgO₂Me₂Bu, direct writing of silver-based nanostructures via local electron beam induced deposition could be realized for the first time. Interestingly, the silver deposition was strongly dependent on electron dose; at low doses of 30 nC/μm² a dominant formation of pure silver crystals was observed, while at higher electron doses around 10⁴ nC/μm² large carbon contents were measured. A scheme for the enhanced silver deposition under low electron fluxes by an electronic activation of precursor dissociation below thermal CVD temperature is proposed and validated using material characterization techniques. Finally, the knowledge gained was employed to fabricate well-defined two-dimensional deposits with maximized silver content approaching 75 at. %, which was achieved by proper adjustment of the deposition parameters. The corresponding deposits consist of plasmonically active silver crystallites and demonstrate a pronounced Raman signal enhancement of the carbonaceous matrix.

Study of N-doped TiO2 thin films for photoelectrochemical hydrogen generation from water

The present work deals with nitrogen-doped stoichiometric TiO2:N and non-stoichiometric TiO2−x:N thin films deposited by means of dc-pulsed reactive sputtering for application as photoanodes for hydrogen generation from water, using solar energy. Stoichiometric thin films of TiO2 crystallize as a mixture of anatase and rutile while rutile phase predominates in TiO2:N at higher nitrogen flow rates as shown by X-ray diffraction at grazing incidence, XRD GID. Lack of bulk nitridation of stoichiometric TiO2:N is indicated by valence-to-core X-ray emission spectroscopy, XES, analysis. The energy band gap as well as flat band potential remain almost unaffected by increasing nitrogen flow rate in this case. In contrast to that, non-stoichiometric thin films of TiO2‑x:N demonstrate systematic evolution of the structural, morphological, optical and photolectrochemical properties upon increasing level of nitrogen doping. Pronounced changes in the growth pattern of non-stoichiometric TiO2-x:N upon varied nitrogen flow rate, demonstrated by scanning electron microscopy, SEM, can be easily correlated with the crystallographic properties studied by XRD GID. Relative positions of Kβ’’ XES lines of the TiO2-x:N thin films, which depend strongly on the nature of the ligands and their local coordination, change with the increasing nitrogen flow. Doping of nonstoichiometric titanium dioxide with nitrogen shifts the absorption spectrum towards the visible range and increases considerably the flat band potential which is beneficial for water photolysis.

Ca2.5Sr0.5GaMn2O8: Diamagnetic Ga in Control of the Structural and Electronic Properties of a Bilayered Manganate

The temperature dependence of the crystal structure and electronic properties of brownmillerite-like Ca(2.5)Sr(0.5)GaMn(2)O(8) has been studied by neutron powder diffraction and muSR spectroscopy. The results show that short-range 2D magnetic order begins to develop within the perovskite-like bilayers of MnO(6) octahedra approximately 50 K above the 3D Néel temperature of approximately 150 K. The bilayers show a structural response to the onset of magnetism throughout this temperature range whereas the GaO(4) layers that separate the bilayers only respond below the 3D ordering temperature. XANES spectroscopy shows that the sample contains Mn(3+) and Mn(4+) cations in a 1:1 ratio, and the behavior in the region of the Néel transition is interpreted as a local charge ordering. Electron diffraction and high-resolution electron microscopy have been used to show that the local microstructure is more complex than the average structure revealed by neutron diffraction, and that microdomains exist in which the GaO(4) tetrahedra show different orientations. It is argued that the bonding requirements of diamagnetic gallium control the electronic behavior within the perovskite-like bilayers.

NMR probe of phase segregation in electron doped mixed valence manganites

A 55Mn and 139La NMR study of La0.35Ca0.65MnO3 is reported. The zero field 55Mn spectra consist of two lines centered at 290 and 375 MHz. Their behavior under an applied magnetic field makes it possible to attribute them to regions of antiferromagnetically and ferromagnetically coupled Mn spins, respectively. This gives evidence for the existence of electronic phase segregation of microscopic ferromagnetic regions of double exchange coupled Mn spins over a charge ordered antiferromagnetic background. The behavior of these ferromagnetic regions in the applied magnetic field is related to the magnetoresistive properties of the manganites.