Adriyan Milev

Morphological stability of hydroxyapatite precursor

Abstract Hydroxyapatite powders [Ca10(PO4)6(OH)2] (HAp) were produced via sol–gel method. The thermal crystallization behavior was studied by means of differential thermal analysis, differential scanning calorimetry (DSC), mass spectrometry (MS) and scanning electron microscopy. A variable heating rate approach was used to calculate the activation energy of crystallization of hydroxyapatite from amorphous precursor (235 kJ/mol). The plate-like morphology observed in the hydroxyapatite precursor at 150 °C resulted in plate-like hydroxyapatite.

31P NMR Studies of Diethyl Phosphite Derived Nanocrystalline Hydroxyapatite

Abstract31P nuclear magnetic resonance (NMR) spectroscopy was used to determine the structure of the intermediate species of sol derived from triethyl phosphite, calcium diethoxide and acetic acid. NMR spectral data revealed that the reaction proceeds via a dialkyl phosphite intermediate. The use of a dialkyl phosphite precursor (diethyl phosphite) with calcium diethoxide eliminated the aging time required in triethylphosphite method and offered an effective sol-gel procedure for monophasic hydroxyapatite.

Photoemission and absorption spectroscopy of carbon nanotube interfacial interaction.

Element-specific techniques including near edge X-ray absorption fine structure, extended X-ray absorption fine structure and X-ray photoemission spectroscopy for the characterization of the carbon nanotube interfacial interactions are reviewed. These techniques involve soft and hard X-rays from the laboratory-based and synchrotron radiation facilities. The results provided information of how the nano-particles of catalysts are involved in the initial stage of nanotube growth, the nanotube chemical properties after purification, functionalization, doping and composite formation.

Unoccupied electronic structure of ball-milled graphite

Changes in electronic and vibrational structure of well characterised macrocrystalline graphite milled by a planetary ball-mill are investigated by Raman spectroscopy and Near Edge X-ray Absorption Fine structure (NEXAFS) measurements at the C K-edge. The electronic structure changes at the surface and in the sub-surface of the particles are examined by comparing two-different NEXAFS detection modes: total fluorescence yield (TFY) and partial electron yield (PEY) respectively. When the in-plane crystallite sizes of graphite are decreased to nanosized (from approximately 160 nm to approximately 9 nm), a new spectral structure appears in TFY at 284.1 eV which is not present in the macrocrystalline graphite. This feature is assigned to electronic states associated with zigzag edges. Further the TFY shows a shift of the main graphite pi* band from 285.5 to 285.9 eV, attributed to breaking the conjugation and hence the electron localization effect during milling, The TFY spectra also show strong spectral features at 287.5 and 288.6 eV, which suggest that the local environment of carbon atoms changes from sp(2) to more sp(3) due to physical damage of the graphite sheets and formation of structures other than aromatic hexagons. Complementary Raman spectroscopic measurements demonstrate an up-shift of the graphite G band from 1575 to 1583 cm(-1)en route to nanosize. The changes in TFY NEXAFS and Raman spectra are attributed to modification of the sub-surface electronic structure due to the presence of defects in the graphite crystal produced during milling. The discovery of the strong spectral feature at 284.1 eV in nanographite and the 0.4 eV up-shift of the pi* band may open up possibilities to influence the electronic transport properties of graphite by manipulation of defects during the preparation of the nanographite.

Dispersion of silicate nano-plates within poly(acrylic acid) and their interfacial interactions

Abstract Dispersion of the silicate nano-plates of clays within polymers for nanocomposite formation generally involved interactions of the polymers with the plate surfaces. In this paper, we provide preliminary evidence showing dependency of the dispersion on the interactions of the plate lattices with polymers. Experiments have been carried out on the dispersion of the natural clay, montmorillonite (≈0.5 g) within the aqueous solutions of poly(acrylic acid) at varying temperatures and the products analysed by X-ray diffraction and related techniques. In the product from reaction at 60 ˚C, the silicate plates are dispersed with fully extended chains of poly(acrylic acid) intercalated within the interlayer spaces between unexfoliated plates. Photoemission spectroscopy showed that during the process sodium ions are removed from the silicate surface. In the product from reaction at 85 ˚C or above, the silicate plates are partly exfoliated. During the process, Fe2+ ions within the clay lattice are oxidised by the acidic poly(acrylic acid) solution, which suggests strong reaction of poly(acrylic acid) with the silicate plates facilitating exfoliation. Interestingly, the Al and Mg concentrations in the lattice remain virtually unchanged during the reactions.

Human Bone Derived Cell (HBDC) Behaviour of Sol-Gel Derived Carbonate Hydroxyapatite Coatings on Titanium Alloy Substrates

Poor cell adhesion to orthopaedic and dental implants results in implant failure. Establishing and maintaining mature bone at the bone/device interface is critical to the long-term success of the prostheses. Considerable effort has been devoted to alter the surface characteristics of these biomaterials in order to improve the initial interlocking of device and skeleton in the noncemented joint prosthesis. Previously we demonstrated that surface chemistry modification of bioceramics induced osteogenesis. In the present work, we investigate the effect of surface chemistry modification of titanium alloy (Ti-6Al-4V) with alkoxide-derived carbonate hydroxyapatite (CHAp) using sol-gel coating methods on human bone derived cell (HBDC)behaviour. Western blotting demonstrated that sol gel coating of Ti-6Al-4V with CHAp upregulated the expression of key signalling protein Shc isoforms (p46, p52, p66) and phosphorylated Erk1/2. CHAp-modification of Ti-6Al-4V is associated with signal transduction pathways involving the key signalling protein Shc and ERK1/2 which may lead to enhanced gene expression of extracellular matrix proteins at the skeletal tissue/device interface.

Influence of bond defects on coiling of graphite

Abstract The effect of annealing at 1400 ˚C in argon on the bond structure of graphite ball milled for 100 h at 400 rpm in polar (water) and in non-polar (n-dodecane) liquids was investigated primarily by near-edge X-ray absorption fine structure spectroscopy (NEXAFS) and transmission electron microscopy (TEM). Carbon K-edge NEXAFS allows the distortion of bonds in the hexagonal lattice to be investigated. It is shown that in-plane sp2 bonds are strained and distorted after ball milling because sp3 bonds are introduced. Not surprisingly, annealing of the milled product restores sp2 bonds but at the same time, coiling and formation of tube-like structures takes place. It is well established that graphite is not formed on annealing, and hence the results shown here demonstrate that the loss of sp3 carbons on annealing must proceed via a different mechanism by which they are formed by milling.

Sol-Gel Derived Nano-Coated Coralline Hydroxyapatite for Load Bearing Applications

Current bone graft materials are mainly produced from coralline hy drox apatite (HAp). Due to the nature of conversion process, commercial coralline HAp has retained coral or CaCO 3 and the structure possesses nanopores within the inter pore trabeculae re s ting in high dissolution rates. Under certain conditions these features reduce durability and strengt h respectively and are not utilised where high structural strength is required. To overcome thes limitations, a new-patented coral double-conversion technique has been developed. The current technique involves two-stage application route where in the first stage complete conversion of coral to pure HAp is achieved. In the second stage a sol-g el derived HAp nanocoating is directly applied to cover the micro and nano-pores within the in ra pore material, whilst maintaining the large pores. Biaxial strength was improved due to t his unique double treatment. This application is expected to result in enhanced durability and longevi ty due to monophasic hydroxyapatite structure and strength in the physiological environment. It is anticipated that this new material can be applied to load be ring bone graft applications where high strength requirements are pertinent.

Mechanism Of Silica Nano-Plate Formation From Lucentite

The mechanism of formation of silica nano-plates by exfoliation of a phyllosilicate magnesium containing clay, Lucentite, in an aqueous solution of poly(acrylic acid) has been studied. Fourier transform infrared spectroscopy and Mg K-edge near edge X-ray adsorption fine structure (NEXAFS) analysis shows that non-surface (bulk) Mg ions were not chemically involved in the poly(acrylic acid)/clay intercalation, but were substantially involved in the exfoliation resulting in the silica nano-plates. During intercalation, O K-edge NEXAFS shows that surface defects were formed which represent additional structural branches on the surface. During exfoliation, these increased significantly. Si L3,2-edge NEXAFS measurements shows that this occurred by migration of SiO4 groups within the exfoliated silica plates.

Molecular structure reorganisation of Fe-, Co-and Ni-phthalocyanines upon milling

A method for preparation of carbon nanotubes with relatively low concentration of defects requires milling and subsequent pyrolysis of metal-phthalocyanines (MPc, MC32H16N8). The molecular structure rearrangements in Fe-, Ni- and Co-phthalocyanines precursors upon milling in argon for 100 h have been investigated using short?range spectroscopic methods. Carbon K-edge near edge X-ray absorption fine structure (NEXAFS) spectra, acquired in bulk sensitive fluorescence-yield mode, show that packing order of the metal phthalocyanines has changed during the milling. The new packing shows similar order for the pyrrole subunits of the phthalocyanine macromolecule but less for the phenyl subunits. The increased energy potential due to loose packing of the milled phthalocyanines can explain the observed substantial reduction in the vaporisation temperature of the milled phthalocyanines.