1,721,044 research outputs found
A transmission electron microscopy study of CoFe2O4 ferrite nanoparticles in silica aerogel matrix using HREM and STEM imaging and EDX spectroscopy and EELS
No full textMagnetic nanocomposite materials consisting of 5 and 10 wt% CoFe 2O4 nanoparticles in a silica aerogel matrix have been synthesized by the sol-gel method. For the CoFe2O4-10wt% sample, bright-field scanning transmission electron microscopy (BF STEM) and high-resolution transmission electron microscopy (HREM) images showed distinct, rounded CoFe2O4 nanoparticles, with typical diameters of roughly 8 nm. For the CoFe2O4-5wt% sample, BF STEM images and energy dispersive X-ray (EDX) measurements showed CoFe2O4 nanoparticles with diameters of roughly 3 ± 1 nm. EDX measurements indicate that all nanoparticles consist of stoichiometric CoFe2O4, and electron energy-loss spectroscopy measurements from lines crossing nanoparticles in the CoFe2O4-10wt% sample show a uniform composition within nanoparticles, with a precision of at best than ± 0.5 nm in analysis position. BF STEM images obtained for the CoFe2O 4-10wt% sample showed many "needle-like" nanostructures that typically have a length of ? 10 nm and a width of ? 1 nm, and frequently appear to be attached to nanoparticles. These needle-like nanostructures are observed to contain layers with interlayer spacing 0.33 ± 0.1 nm, which could be consistent with Co silicate hydroxide, a known precursor phase in these nanocomposite materials
Resistive switching mechanism in TiO<sub>2-x</sub> thin films: an X-ray absorption spectroscopy study
No full textMetal–insulator–metal (MIM) devices based on titanium dioxide thin films exhibit resistive switching behavior (RS); i.e., they have the ability to switch the electrical resistance between high-resistive states (HRS) and low-resistive states (LRS) by application of an appropriate voltage. This behavior makes titanium dioxide thin films extremely valuable for memory applications. The physical mechanism behind RS remains a controversial subject but it has been suggested that it could be interface-type, without accompanying structural changes of the oxide, or filament-type with formation of reduced titanium oxide phases in the film. In this work, X-ray absorption spectroscopy (XAS) at the Ti K-edge (4966 eV) was used to characterize the atomic-scale structure of a nonstoichiometric TiO2–x thin film before and after annealing and for the first time after inclusion in a MIM device based on a Cr/Pt/TiO2–x/Pt stack developed on an oxidized silicon wafer. The advantage of the XAS technique is that is element-specific. Therefore, by tuning the energy to the Ti K-edge absorption, contributions from the Pt, Cr, and Si in the stack are eliminated. In order to investigate the structure of the film after electrical switching, XAS analysis at the Ti K-edge was again performed for the first time on the Cr/Pt/TiO2–x/Pt stack in its virgin state and after switching to LRS by application of an appropriate bias. X-ray absorption near-edge structure (XANES) was employed to assess local coordination and oxidation state of the Ti and extended X-ray absorption fine structure (EXAFS) was used to assess bond distances, coordination numbers, and Debye–Waller factors. XAS analysis revealed that the as-deposited film is amorphous with a distorted local octahedral arrangement around Ti (average Ti–O distance of 1.95 Å and coordination number of 5.2) and has a majority oxidation state of Ti4+ with a slight content of Ti3+. The film remains amorphous upon insertion into the stack structure and after electrical switching but crystallizes as anatase upon annealing at 600 °C. These results do not give any indication of the appearance of conducting filaments upon switching and are more compatible with homogeneous interface mechanisms
X-ray absorption spectroscopy study of TiO<sub>2-x</sub> thin films for memory applications
No full textMetal-insulator-metal (MIM) devices based on titanium dioxide thin films exhibit resistive switching behaviour (RS) i.e. they have the ability to switch the electrical resistance between high-resistive states (HRS) and low-resistive states (LRS) by application of an appropriate voltage. This behaviour makes titanium dioxide thin films extremely valuable for memory applications. The physical mechanism behind RS remains a controversial subject but has been suggested that it could be interface-type, without accompanying structural changes of the oxide, or filament-type with formation of reduced titanium oxide phases in the film. In this work, X-ray absorption spectroscopy (XAS) at the Ti K-edge (4966 eV) was used to characterize the atomic-scale structure of a non-stoichiometric TiO2-x thin film before and after annealing and for the first time after inclusion in a MIM device based on a Cr/Pt/TiO2-x/Pt stack developed on an oxidised silicon wafer. The advantage of the XAS technique is that is element-specific. Therefore, by tuning the energy to the Ti K-edge absorption, contributions from the Pt, Cr and Si in the stack are eliminated. In order to investigate the structure of the film after electrical switching, XAS analysis at the Ti K-edge was again performed for the first time on the Cr/Pt/TiO2-x/Pt stack in its virgin state and after switching to LRS by application of an appropriate bias. X-ray absorption near-edge structure (XANES) was employed to assess local coordination and oxidation state of the Ti and X-ray absorption fine structure (EXAFS) was used to assess bond distances, coordination numbers and Debye-Waller factors. XAS analysis revealed that the as-deposited film is amorphous with a distorted local octahedral arrangement around Ti (average Ti-O distance of 1.95 Å and coordination number of 5.2) and has a majority oxidation state of Ti+4 with a slight content of Ti+3. The film remains amorphous upon insertion into the stack structure and after electrical switching but crystallizes as anatase upon annealing at 600ºC. These results do not give any indication of the appearance of conducting filaments upon switching and are more compatible with homogeneous interface mechanisms
Molecular Dynamics Modelling of Barium Silicate and Barium Fluorozirconate Glasses
Full text linkAdvancement in science and technology has profoundly depended on new types of glass innovation. The glasses that were studied in this project are binary barium silicate glasses, binary barium fluorozirconate glasses, ZBLAN glasses and ?Eu?^(3+) doped ZBLAN glass (the ZBLAN glasses are based on binary barium fluorozirconate glass). The high atomic number of barium in the barium silicate glasses provides high mass and high electron density providing its applications for heat and X-ray shielding. The phenomena such as phase separation in the barium silicate glass will affect its properties of durability and electrical conductivity. On the other hand, ZBLAN glasses have a broad infrared optical transmission window due to the weaker bonding/interaction of F^- ions. Due to the presence of lanthanum in the composition ZBLAN glass can be easily doped with rare-earth ions such as ?Eu?^(3+) giving it many optical applications such as optical amplifier and fibre lasers.
Hence, it's essential to study the structure of these glasses to understand their properties for applications. This thesis used the classical molecular dynamics modelling technique to study the static atomic structure of glass. Generally, fluoride glasses can be formed by totally replacing oxygen atoms in oxide glasses by fluorine atoms. The oxide silicate glasses are common glasses that follow the Zachriasen rules of glass formation but the fluorozirconate glasses do not and lack fixed structural units.
The structure analysis was performed at short-range order (e.g. coordination number, bond length and bond angle), medium-range order (e.g. network connectivity) and long- range order (e.g. phase separation). The related crystals were also simulated in similar conditions to the glasses to compare their atomic structure. Normally at short-range order glass
structure is similar to its related crystal but the differences between them starts from the position and number of next nearest neighbours and increases thereafter. Additionally, the
new methods such as rotational invariants and grid analysis were used to scrutinise structural units and phase separation respectively.
The model of barium silicate glass shows good agreement with experimental diffraction data. The typical bond length and coordination number for Ba were 2.97 Å and approximately 7 respectively. The model did not show any phase separation at low Ba content and hence for further investigation very large models of alkaline earth silicate glasses were studied to see how Ba, Ca and Mg are distributed in the glass. The grid analysis was used to see the distributions which show homogeneity for Ba and Ca and inhomogeneity for Mg cation.
The structural units of fluorozirconate glasses were carefully studied as they do not follow the Zachriasen glass model. The coordination number for Zr was mixture of 7 and 8. The rotational invariant analysis shows that the structural units of ZrF_n polyhedra for coordination number 7 and 8 were similar to Augmented Triangular Prism and Biaugmented Triangular Prism respectively. However, rotational invariant values for BaF_n polyhedra tend more towards random.
The large complex model of ?Eu?^(3+) doped ZBLAN glass was made as it is studied for optical applications. The initial analysis was to observe whether Zr and Ba has similar structural roles as in binary fluorozirconate glass system which they do. Considering the extra elements in ZBLAN glass, Al behaves like a network former and has octahedra structural units whereas La and Na behave like modifiers. In the glass Eu was uniformly distributed with predominantly coordination number of 8 and does not have well defined structural units
The Atomistic Structure of Amorphous Carbonate, Phosphate and Sulfate Biominerals
Full text linkBiominerals are key to all life, whether they make up exoskeletons of marine organisms (amorphous calcium carbonate), human bones (amorphous calcium phosphate) or being signs of extra-terrestrial life (amorphous iron sulfate) but little is know of their atomistic structures. How they behave could be determined by this structure and knowledge of this could lead to favouring certain crystallisation pathways or indeed speeding up the process, ie. If someone breaks their arm, can we induce faster healing? In the present study, amorphous biominerals including carbonates, phosphates and sulfates are synthesised (stabilised where necessary). The proton content is reduced either via heat-treatment or deuteratation. The deuteration method is the first of its kind and enabled the materials of study to be
examined at central facilities via x-ray and neutron diffraction. For the first time, neutron diffraction experiments have been conducted on amorphous calcium and magnesium carbonates. Also, a first synthesis of deuterated amorphous biominerals including amorphous calcium carbonate (ACC), amorphous magnesium carbonate (AMC) and amorphous calcium phosphate (ACP). Diffraction data from these materials are utilised by the empirical potential structure refinement (EPSR) algorithm to generate atomistic models using Reverse Monte Carlo (RMC). These models used well defined molecular units and yielded results showing the calcium distribution throughout ACC to be uniformed, contrary to former reports on the atomistic structure of ACC
Experimental and Modelling Study of Ionic Conductivity in Lithium Phosphate and Silicate Glasses
Full text linkThis study is an attempt to investigate ionic conductivity and structural components of lithium containing glasses for their use as possible electrolytes in solid state Li-ion batteries. 33.3Li 2 O-66.7SiO 2 , Li 2 O-P 2 O 5 (multiple compositions) and Li 3 Fe 2 (PO 4 ) 3 glasses were synthesised using conventional melt quenching and were characterised using various techniques including XRD, DSC, TGA, pycnometry and 31 P MAS-NMR. It was found a small (5mol%) addition of Nb 2 O 5 to the Li 3 Fe 2 (PO 4 ) 3 glass was necessary for the formation of a glass 37.5Li 2 O-20Fe 2 O 3 -5Nb 2 O 5 -37.5P 2 O 5 . The glasses conductivity was then measured using impedance spectroscopy over a frequency range of 200kHz – 100Hz and a temperature range of 300K – 525K. It was observed that an increasing temperature corresponded to an increasing conductivity, as E expected from the Arrhenious equation: ? = ? 0 exp (? k a T ). Conductivity values were B compared to published values and the first reported conductivity values for 37.5Li 2 O- 20Fe 2 O 3 -5Nb 2 O 5 -37.5P 2 O 5 glass were obtained. Activation energies were also calculated and compared to published data. MD models of lithium disilicate, lithium metaphosphate and 37.5Li 2 O-20Fe 2 O 3 -5Nb 2 O 5 - 37.5P 2 O 5 were made. These models were then analysed and compared to experimental diffraction results. It was found that the lithium disilicate and the lithium metaphosphate model structures compare well to experimental data (X-ray and Neutron diffraction). Conductivity was estimated from mean squared displacement. All models gave an overestimation of conductivity compared to experimental data, due to limitations of the simulation method. However, MD did predict that lithium disilicate and lithium metaphosphate glasses have a similar conductivity as observed experimentally. It was found that whilst the addition of Fe (and Nb) to the lithium phosphate glass improved chemical durability, its conductivity was reduced. In addition to this the conductivity for the 37.5Li 2 O-20Fe 2 O 3 -5Nb 2 O 5 -37.5P 2 O 5 glass was found to be significantly lower than its crystalline counterpart which will be a disadvantage for its use as a solid electrolyte
A Structural Investigation of Chlorine-Containing and Fluorine-Containing Oxide Glasses Using Molecular Dynamics, Neutron Diffraction, and X-ray Absorption Spectroscopy
Full text linkScientific developments have enabled glasses to fulfil an array of applications, from windows to bioactive glasses in regenerative medicine. To further exploit the capability of this versatile material, it is imperative that their structure is understood. In this thesis, the structure of three glass systems containing halides as anions were investigated. The first of these was the intermediate glass former ZnCl2 which was modelled computationally using classical molecular dynamics (MD). The addition of the adiabatic core-shell model was able to account for anion polarisability. This enabled the first fully tetrahedral model of ZnCl2 glass to be attained. While 86% of the ZnCl4 tetrahedral units were corner-sharing, 14% were found to be edge-sharing. The calculated total neutron and x-ray structure factors closely replicated those obtained experimentally in other works. The intermediate glass former ZnCl2 was later compared to the strong glass former SiO2. The main contribution in the first sharp diffraction peaks came from the cation-anion contribution, rather than the cation-cation contribution as previously reported.
Next to be investigated was a CaO-SiO2-CaCl2 glass series. This was to help elucidate the structure of more complex CaO-SiO2-P2O5-CaCl2 chlorine-containing bioactive glass compositions. A glass series was synthesised by collaborators, and compositional analysis in this work revealed that chlorine losses via chlorine volatilisation occurred as HCl. The glass series was studied experimentally using neutron diffraction (ND) and x-ray absorption spectroscopy (XAS) at the Ca and Cl K-edge. By probing the calcium environment using ND and XAS, generally good agreement between the Ca-O and Ca-Cl coordination numbers was achieved. The total correlation functions from neutron diffraction did not exhibit a noticeable contribution around 2.1Å which would have been expected for Si-Cl bonding. Computational modelling was performed using MD with the addition of the adiabatic core-shell model. No Si-Cl bonding was observed, and the calculated total neutron structure factors closely resembled those obtained experimentally. The glass models were found to become phase separated with increasing CaCl2 content to form a biphasic system of calcium silicate and calcium chloride phases. Interestingly, there was a tendency towards phase separation even in glass models containing small amounts of CaCl2.
The remaining glass system, CaO-SiO2-CaF2, was studied to help elucidate the structure of more complex CaO-SiO2-P2O5-CaF2 fluorine-containing bioactive glasses. Following the synthesis of the CaO-SiO2-CaF2 glass series, compositional analysis revealed that fluorine losses due to fluorine volatilisation occurred as HF. The calcium environment of the glasses was probed using ND and XAS at the Ca K-edge. Distinguishing the overlapping Ca-F and Ca-O paths around 2.3Å and 2.4Å respectively was challenging. The glass series was modelled computationally using MD with the addition of the adiabatic core-shell model. The calculated total neutron structure factors closely replicated those from experiment. The glass models also revealed that while fluorine ions overwhelmingly bond with calcium ions, small amounts of Si-F bonding are observed which conceivably cannot be resolved experimentally
Tuning Nanoparticles and Mesoporous Materials Using Simulated Acoustic Vibrations
No full textNanoparticles and mesoporous materials are commonly used in chemistry for catalysis, nano-medicine and photo-catalysis. These structures, specifically CeO2 and TiO2 excel at donating oxy-gen atoms, which is why they're commonly used in catalysis. Monitoring the rate at which these particles vibrate is quite difficult, although it can be seen and tested via simulation. Simulating en-ergy via irradiation allows the particle to vibrate, which emits a frequency, similar to how we hear sound. This frequency can be thought of as an acoustic wave, where one atom in the nanoparticle hits another creating a wave of energy giving off differing frequencies. These frequencies can then be used to describe the nanoparticle or mesoporous material via Fourier transform, the Fourier plot that has the most peaks shows more imperfections in the particle, which can be used to tune the particles or check to see if there are crystallographic defects which can then be changed or poten-tially modified. Smaller nanoparticles were seen to have a higher frequency; this shows it is easier to remove oxygen from smaller nanoparticles due to overall interactions between atoms, which could be due to weaker overall bonding, the displacement of bonds, coordination and surface bonding along the planes
Characterising nanoparticles by lattice vibration frequency
Full text linkIn this study, we simulate that irradiation of nanoceria has potential inducing (breathing mode) lattice vibrations. Irradiation therefore has potential to be used in increasing the catalytic activity of nanoceria structures. If irradiation can be implemented to vibrate atoms off their lattice sites, in a similar manner to temperature, this would enable surface atoms to be more easily extracted. Extracted surface oxygen has potential uses in oxidative catalysis or to modulate oxygen concentration in biological environments with nanoceria as a nanozyme. Here, Molecular Dynamics (MD) simulation was used to calculate vibration (breathing mode) frequencies of various ceria nanoparticles. Vibration was induced in polyhedral nanoparticles, 665 - 6708 cerium atoms in size, and compared to the vibrations induced in nanocylinders comprising 653 - 6721 cerium atoms. The simulations revealed that breathing mode frequencies decrease with increasing size for both polyhedral
and cylindrical nanoparticles in accord with experiment. The simulations also revealed that breathing mode frequencies depend upon the aspect ratio of nanocylinders. The simulations suggest that breathing mode vibrational spectra can be used as a fingerprint to identify the size, shape, and aspect ratio distribution of a ceria nanomaterial sample
Modelling the Structures and Melting of Mg-Zn Alloys using Molecular Dynamics with Lennard-Jones Potentials
No full textMolecular dynamics modelling has been used to study the Mg-Zn alloys in solid and molten states. GULP and DL_POLY simulation software simulated the crystal structure of Mg-Zn alloys at room temperature and highly disordered structures at melting temperatures.
The GULP has been used to validate the Lennard-Jones potential for the simulation. The GULP output energy minimisation showed almost the same values of cell parameters which are then used for input of the DL_POLY simulations. The Substitutional alloys have been simulated by creating a 4×4×4 supercell of 128 atoms of pure Mg using CrystalMaker, and substituting some Mg atoms with Zn atoms. The compression lattice strain caused by the substitution of dissimilar sizes of Mg and Zn has been studied.
The DL_POLY simulation is run in an NPT ensemble with varying temperatures. It was obtained that the simulated melting temperatures of Pure Mg, α - Mg, Pure Zn, Mg51Zn20, Mg21Zn25, Mg4Zn7, MgZn2, and Mg2Zn11 are obtained at 1860K, 1820K, 1520K, 1740K, 1780K, 1860K, 1480K, and 1380K respectively. The experimental melting points of Pure Mg, α - Mg, Pure Zn, Mg51Zn20, Mg21Zn25, Mg4Zn7, MgZn2, and Mg2Zn11 are 923K, 910.87K, 692.58K, 621.4K, 822.47K, 851.93K, 862.93K, and 727.13K respectively.
The investigation of cell parameters (a, b, c) for different Mg-Zn compounds have been done from 300K to 2300K. The simulated and computational values of densities at the molten state of Pure Mg, α - Mg, Pure Zn, Mg51Zn20, Mg21Zn25, Mg4Zn7, MgZn2, and Mg2Zn11 are calculated.
The simulation of metal glass is studied by heating the alloys to 2500K using DL_POLY "bath" and quenching at the rate of 1013 K/s using DL_POLY "quench" in NPT. The study of the simulated metal glass showed the amorphous glassy atomic structure at room temperature for Pure Mg, α - Mg, Pure Zn, Mg51Zn20, Mg21Zn25, Mg4Zn7, MgZn2, and Mg2Zn11.
The surface melting of the alloys is investigated using DL_POLY in the NVT ensemble. The surface melting temperatures of Pure Mg, α - Mg, Pure Zn, Mg21Zn25, Mg4Zn7, MgZn2, Mg2Zn11 and Mg51Zn20 are obtained at 760K, 765K, 910K, 850K, 930K, 1000K,730K, and 500K respectively
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