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  • 1. Akansel, Serkan
    et al.
    Venugopal, Vijayaharan A.
    Kumar, Ankit
    Gupta, Rahul
    Brucas, Rimantas
    George, Sebastian
    Neagu, Alexandra
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Tai, Cheuk-Wai
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Gubbins, Mark
    Andersson, Gabriella
    Svedlindh, Peter
    Effect of seed layers on dynamic and static magnetic properties of Fe65Co35 thin films2018In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 51, no 30, article id 305001Article in journal (Refereed)
    Abstract [en]

    Fe65Co35 thin films have been deposited on SiO2 substrates using sputtering technique with different choices of seed layer; Ru, Ni82.5Fe17.5, Rh, Y and Zr. Best soft magnetic properties were observed with seed layers of Ru, Ni82.5Fe17.5 and Rh. Adding these seed layers, the coercivity of the Fe65Co35 films decreased to values of around 1.5 mT, which can be compared to the value of 12.5 mT obtained for films deposited without seed layer. Further investigations were performed on samples with these three seed layers in terms of dynamic magnetic properties, both on as prepared and annealed samples, using constant frequency cavity and broadband ferromagnetic resonance measurements. Damping parameters of around 8.0 x 10(-3) and 4.5 x 10(-3) were obtained from in-plane and out-of-plane measurements, respectively, for the as prepared samples, values that were reduced to about 6.5 x 10(-3) and 4.0 x 10(-3) for annealed samples.

  • 2.
    Neagu, Alexandra
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Local structure of perovskite ferroelectric ceramics as revealed by 3D electron diffuse scattering: A walk in between the Bragg peaks2018Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Local structural disorder in crystalline materials plays a crucial role in understanding their properties. The means to study structural disorder is diffuse scattering (DS). Even though DS was observed since the early days of X-ray diffraction the weak intensity and the sheer number of different kinds of disorder hindered the development of a unique solution strategy. However, the advent of X-ray and neutron synchrotron sources together with recent advances in automated electron diffraction techniques have unraveled a new world in between Bragg peaks where a wealth of information is available.

    In this thesis electron diffraction is used to explore the different kinds of DS in three-dimensions, for several perovskite ferroelectric ceramics. Based on the information presented in the reconstructed 3D reciprocal space volumes, disordered atomic structures were proposed and verified by the calculated electron diffraction patterns. A complex structural model for the local disorder in 85Na0.5Bi0.5TiO3-10K0.5Bi0.5TiO3-5BaTiO3 piezoceramic was developed by analyzing the morphology and intensity of electron DS in 3D. Next, the influence of potassium-content on the octahedral-tilt disorder for three different piezoceramics was studied by a combination of dark-field imaging and electron diffraction. Further on, the temperature-dependence of electron DS for 95Na0.5Bi0.5TiO3-5BaTiO3 piezoceramic revealed a local structural phase transition that was correlated with the depolarization mechanism. Lastly, strong electron DS was recorded from a Pb-based relaxor and simulations of disordered atomic structures showed that the local structure resembles a dipolar glass state. These demonstrate that electron diffraction is a powerful tool for the study of local structural disorder in crystalline materials, especially for ceramics. The major advantage is that we are able to record single-crystal electron diffraction data from individual grains. Moreover, since we are analyzing a 3D reciprocal volume several orientations can be studied simultaneously and we are not limited to zero-Laue zones. Finally, the models for local structural disorder provided valuable insight into how macroscopic properties are influenced by local structural disorder, in addition to the average structure.

  • 3.
    Neagu, Alexandra
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Tai, Cheuk-Wai
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    A TEM study of Bi0.5Na0.5TiO3-based ceramics2014In: 18th International Microscopy Congress, 2014Conference paper (Other academic)
  • 4.
    Neagu, Alexandra
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Tai, Cheuk-Wai
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Diffuse electron scattering in Bi0.5Na0.5TiO3-based ceramics2014In: Electroceramics XIV, 2014Conference paper (Other academic)
  • 5.
    Neagu, Alexandra
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Tai, Cheuk-Wai
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Investigation of local structural phase transitions in 95Na0.5Bi0.5TiO3-5BaTiO3 piezoceramics by mean of in-situ transmission electron microscopyManuscript (preprint) (Other academic)
    Abstract [en]

    The temperature-dependence of superstructure reflections, diffuse scattering intensity and ferroelectric domains morphology for 95Na0.5Bi0.5TiO3-5BaTiO3 ceramic was investigated by in-situ transmission electron microscopy. A subtle local structural phase transition was observed around 110°C. This local phase transition is characterized by a transformation in the dominant octahedral-tilt system from antiphase to in-phase tilting and was correlated with the depolarization mechanism common in this type of materials. Simulations of octahedral-tilt disorder were developed at different temperatures which enabled us to provide a comprehensive structural model that explains the temperature-dependence of superstructure reflections and electron diffuse scattering intensity. This model revealed that plate-like in-phase nanodomains tilted about three orthogonal axes coexist with an antiphase tilted matrix, even at room temperature. With increasing temperature, the concentration of in-phase nanodomains is increased till a critical volume fraction is reached. We propose that these plate-like in-phase nanodomains are an inherent characteristic of NBT-based compounds and that the local phase transition observed around 110°C leads to a disruption in the long-range ferroelectric order and is responsible for the depolarization mechanism.

  • 6.
    Neagu, Alexandra
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Tai, Cheuk-Wai
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Investigation of local structural phase transitions in 95Na(0.5)Bi(0.5)TiO(3)-5BaTiO(3) piezoceramics by means of in-situ transmission electron microscopy2018In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 123, no 24, article id 244105Article in journal (Refereed)
    Abstract [en]

    The temperature-dependence of superstructure reflections, diffuse scattering intensity, and ferroelectric domain morphology for 95Na(0.5)Bi(0.5)TiO(3)-5BaTiO(3) ceramics was investigated by in-situ transmission electron microscopy. A subtle local structural phase transition was observed around 110 degrees C. This local phase transition is characterized by a transformation in the dominant octahedral-tilt system from antiphase to in-phase tilting and was correlated with the depolarization mechanism common in this type of materials. Simulations of the octahedral-tilt disorder were developed at different temperatures which enabled us to provide a comprehensive structural model that explains the temperature-dependence of superstructure reflections and electron diffuse scattering intensity. This model revealed that plate-like in-phase nanodomains tilted about three orthogonal axes coexist with an antiphase tilted matrix, even at room temperature. With increasing temperature, the concentration of in-phase nanodomains is increased till a critical volume fraction is reached. We propose that these plate-like in-phase nanodomains are an inherent characteristic of NBT-based compounds and that the local phase transition observed around 110 degrees C leads to a disruption in the long-range ferroelectric order and is responsible for the depolarization mechanism.

  • 7.
    Neagu, Alexandra
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Tai, Cheuk-Wai
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Local disorder in Na0.5Bi0.5TiO3-piezoceramic determined by 3D electron diffuse scattering2017In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, article id 12519Article in journal (Refereed)
    Abstract [en]

    Local structural distortions in Na0.5Bi0.5TiO3-based solid solutions have been proved to play a crucial role in understanding and tuning their enhanced piezoelectric properties near the morphotropic phase boundary. In this work all local structural disorders in a lead-free ternary system, namely 85% Na0.5Bi0.5TiO3-10% Bi0.5K0.5TiO3-5% BaTiO3, were mapped in reciprocal space by 3D electron diffraction. Furthermore, a comprehensive model of the local disorder was developed by analysing the intensity and morphology of the observed weak diffuse scattering. We found that the studied ceramics consists of plate-like in-phase oxygen octahedral nanoscale domains randomly distributed in an antiphase tilted matrix. In addition, A-site chemical short-range order of Na/Bi and polar displacements contribute to different kinds of diffuse scattering. The proposed model explains all the observed diffraction features and offers insight into the ongoing controversy over the nature of local structural distortions in Na0.5Bi0.5TiO3-based solid solutions.

  • 8.
    Neagu, Alexandra
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Tai, Cheuk-Wai
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Study of local ordering in lead-free ferroelectrics2014Conference paper (Other academic)
  • 9.
    Neagu, Alexandra
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Tai, Cheuk-Wai
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Study of local structural disorder in 0.6PbIn½Nb½O3-0.4PbMgNbO3 ceramics as revealed by 3D electron diffuse scatteringManuscript (preprint) (Other academic)
    Abstract [en]

    The local structural disorder in 0.6PbIn½Nb½O3-0.4PbMgNbO3 ceramic was revealed by 3D electron diffraction. Rotation electron diffraction method was applied to reconstruct the 3D reciprocal space volume containing both diffuse scattering and Bragg reflections. The butterfly-shaped diffuse scattering in the vicinity of h00 type reflections and the ellipsoidal-shaped diffuse scattering in the vicinity of hk0 type reflections were detected. Moreover, sharp superstructure reflections and strong diffuse scattering sheets along <110>* directions were observed in thin-volume sections centered along several zone axes cut from the 3D reciprocal space volume. This indicates that the local structure deviates significantly from the average one. By analyzing the intensity and morphology of diffraction features in 3D a model of structural disorder was developed comprising of 1:1 chemical short-range order of B-site cations and local displacements of Pb atoms. The final local structure is more akin to a spin glass state and no polar nanoregions were observed. This model was able to explain all significant diffraction features observed in volume-sections centered along three different zone axes.

  • 10.
    Neagu, Alexandra
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Tai, Cheuk-Wai
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    The influence of potassium content on octahedral-tilt disorder in Na0.5Bi0.5TiO3-solid solutions near morphotropic phase boundary2018In: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 152, p. 49-54Article in journal (Refereed)
    Abstract [en]

    A semi-quantitative comparison of 3D electron diffraction data revealed structural differences at the nanoscale for three (95-x)Na0.5Bi0.5TiO3-xK0.5Bi0.5TiO3-5BaTiO3 solid solutions close to the morphotropic phase boundary, with x = 0, 5, 10. Using a novel rotation electron diffraction technique, diffuse scattering intensity was recorded in 3D as continuous rods along hk0.5 reciprocal-planes. By analyzing the different superstructure reflections and the intensity/morphology of diffuse scattering, a structural model of the local octahedral-tilt disorder for the different ceramics was developed. A good agreement was obtained between the experimental and simulated data with a model comprising of antiphase (aaa) and plate-like in-phase (a0a0c+) nanodomains.

  • 11.
    Nedumkandathil, Reji
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Jaworski, Aleksander
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Grins, Jekabs
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Bernin, Diana
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Karlsson, Maths
    Österberg, Carin
    Neagu, Alexandra
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Tai, Cheuk-Wai
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Pell, Andrew J.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Hydride Reduction of BaTiO3 – Oxyhydride vs O-Vacancy FormationManuscript (preprint) (Other academic)
    Abstract [en]

    We investigated the hydride reduction of tetragonal BaTiO3 using the metal hydrides CaH2, NaH, MgH2, NaBH4 and NaAlH4. The reactions employed molar BaTiO3:H ratios of up to 1.8 and temperatures near 600 °C. The air stable reduced products were characterized by powder X-ray diffraction (PXRD), transmission electron microscopy, thermogravimetric analysis (TGA) and solid-state 1H NMR spectroscopy. PXRD showed the formation of cubic products - indicative of the formation of BaTiO3-xHx - except for NaH. Lattice parameters were in a range between 4.005 Å (for NaBH4 reduces samples) and 4.033 Å (for MgH2 reduced samples). With increasing BaTiO3:H ratio, CaH2, NaAlH4 and MgH2 reduced samples were afforded as two-phase mixtures. TGA in air flow showed significant weight increase of up to 3.5 % for reduced BaTiO3, suggesting that metal hydride reduction yielded oxyhydrides BaTiO3-xHx with x values larger 0.5. 1H NMR, however, revealed rather low concentrations of H, and, thus a simultaneous presence of O vacancies in reduced BaTiO3. It has to be concluded that hydride reduction of BaTiO3 yields complex disordered materials BaTiO3-xHy(x-y) with x up to 0.6, y in a range 0.05 – 0.2 and (x-y) > y, rather than homogeneous solid solutions BaTiO3Hx. Resonances of (hydridic) H substituting O in the cubic perovskite structure appear in the -2 to -60 ppm spectral region. The large range of chemical shifts and breadth of the signals signifies the structural disorder in BaTiO3-xHy(x-y). Sintering of BaTiO3-xHy(x-y) in a gaseous H2 atmosphere resulted in more ordered materials as indicated by considerably sharper 1H resonances.

  • 12. Raudonyte-Svirbutaviciene, Eva
    et al.
    Neagu, Alexandra
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Vickackaite, Vida
    Jasulaitiene, Vitalija
    Zarkov, Aleksej
    Tai, Cheuk-Wai
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Katelnikovas, Arturas
    Two-step photochemical inorganic approach to the synthesis of Ag-CeO2 nanoheterostructures and their photocatalytic activity on tributyltin degradation2018In: Journal of Photochemistry and Photobiology A: Chemistry, ISSN 1010-6030, E-ISSN 1873-2666, Vol. 351, p. 29-41Article in journal (Refereed)
    Abstract [en]

    Herein, we report a simple, sustainable and low-cost approach to design Ag-CeO2 nanoheterostructures in pure aqueous and ethanol containing aqueous solutions via photochemical UV-light driven process with no capping agents nor stabilizers required. To this end, photochemically synthesized CeO2 nanoparticles were applied as photoactive compounds in order to generate formation of metallic silver nanoparticles. Irradiation of deaerated CeO2 suspensions in the presence of Ag+ resulted in the rise of a strong surface plasmon resonance band with a maximum at 393-422 nm in the absorption spectra of the solutions, indicating formation of small metallic silver particles. Faster formation of Ag nanoparticles with the lower amount of silver precursor being required was observed when ethanol was introduced to the reaction solution before the irradiation. This implies that oxidative reactions can be strongly suppressed in deaerated ethanol containing solutions with respect to the pure aqueous media. Not only was the overall efficiency of the process remarkably increased by the use of alcohol, but also smaller and more uniform silver nanoparticles with a size comparable to that of ceria nanoparticles (around 15 nm) were formed when compared to those synthesized without radical scavengers as revealed by TEM analysis. The proposed photochemical approach enables the production of silver-semiconductor system without employing organic stabilizers, thus resulting in formation of nanoparticles with clean, highly reactive metal surface. The as-synthesized silver-ceria nanoheterostructures demonstrated enhanced visible light driven photocatalytic activity on tributyltin (TBT) degradation if compared to pure ceria nanoparticles.

  • 13. Sun, Rui
    et al.
    Zhang, Peng
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Uppsala University, Sweden.
    Bajnócz, Éva G.
    Neagu, Alexandra
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Tai, Cheuk-Wai
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Persson, Ingmar
    Strømme, Maria
    Cheung, Ocean
    Amorphous Calcium Carbonate Constructed from Nanoparticle Aggregates with Unprecedented Surface Area and Mesoporosity2018In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 10, no 25, p. 21556-21564Article in journal (Refereed)
    Abstract [en]

    Amorphous calcium carbonate (ACC), with the highest reported specific surface area of all current forms of calcium carbonate (over 350 m(2) g(-1)), was synthesized using a surfactant-free, one-pot method. Electron microscopy, helium pycnometry, and nitrogen sorption analysis revealed that this highly mesoporous ACC, with a pore volume of similar to 0.86 cm(3) g(-1) and a pore-size distribution centered at 8-9 nm, is constructed from aggregated ACC nanoparticles with an estimated average diameter of 7.3 nm. The porous ACC remained amorphous and retained its high porosity for over 3 weeks under semi-air-tight storage conditions. Powder X-ray diffraction, large-angle X-ray scattering, infrared spectroscopy, and electron diffraction exposed that the porous ACC did not resemble any of the known CaCO3 structures. The atomic order of porous ACC diminished at interatomic distances over 8 angstrom. Porous ACC was evaluated as a potential drug carrier of poorly soluble substances in vitro. Itraconazole and celecoxib remained stable in their amorphous forms within the pores of the material. Drug release rates were significantly enhanced for both drugs (up to 65 times the dissolution rates for the crystalline forms), and supersaturation release of celecoxib was also demonstrated. Citric acid was used to enhance the stability of the ACC nanoparticles within the aggregates, which increased the surface area of the material to over 600 m(2) g(-1). This porous ACC has potential for use in various applications where surface area is important, including adsorption, catalysis, medication, and bone regeneration.

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