Electrical conductivity and thermopower of (1-x) BiFeO3 - xBi(0.5)K(0.5)TiO(3) (x=0.1, 0.2) ceramics near the ferroelectric to paraelectric phase transition

Ferroelectric BiFeO3 has attractive properties such as high strain and polarization, but a wide range of applications of bulk BiFeO3 are hindered due to high leakage currents and a high coercive electric field. Here, we report on the thermal behaviour of the electrical conductivity and thermopower of BiFeO3 substituted with 10 and 20 mol% Bi0.5K0.5TiO3. A change from p-type to n-type conductivity in these semi-conducting materials was demonstrated by the change in the sign of the Seebeck coefficient and the change in the slope of the isothermal conductivity versus partial pressure of O. A minimum in the isothermal conductivity was observed at ~10−2 bar O2 partial pressure for both solid solutions. The strong dependence of the conductivity on the partial pressure of O2 was rationalized by a point defect model describing qualitatively the conductivity involving oxidation/reduction of Fe3+, the dominating oxidation state of Fe in stoichiometric BiFeO3. The ferroelectric to paraelectric phase transition of 80 and 90 mol% BiFeO3 was observed at 648 ± 15 and 723 ± 15 °C respectively by differential thermal analysis and confirmed by dielectric spectroscopy and high temperature powder X-ray diffraction.Author preprin

In situ characterisation for studying nucleation and growth of nanostructured materials and thin films during liquid-based synthesis

Knowledge about the nucleation, growth, and formation mechanisms during materials synthesis using sol-gel and solution-based methods is important to design a material with desired properties. We used aqueous chemical synthesis as an environmentally friendly and highly flexible route to tailored and reproducible synthesis of oxide nanomaterials and thin films. For studies of hydrothermal synthesis an in situ cell using synchrotron X-ray diffraction was used to investigate the formation mechanisms of SrxBa1-xNb2O6 piezoelectrics. Aqueous chemical solution deposition of phase pure oriented piezoelectric thin films demands strong control of processing parameters. An in situ cell for synchrotron X-ray diffraction studies of the annealing and crystallisation steps during aqueous chemical solution deposition was used to understand the nucleation and crystallisation of Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT). We discuss how the knowledge about nucleation and growth obtained by in situ characterisation can be used to design the optimal procedure for fabrication of oxide materials with desired properties

Spark Plasma Sintering and Hot Pressing of Hetero-Doped LaNbO4.

LaNbO4/La3NbO7 and LaNbO4/LaNb3O9 cer-cer composites were prepared by impregnating Ca-doped LaNbO4 powder, synthesized by spray pyrolysis, with La- or Nb-precursor solutions. The sintering of the calcined powders was investigated by dilatometry, and dense composites were prepared by conventional sintering, hot pressing, and spark plasma sintering. The particle size of the starting powders was about 50 nm, and the average grain size of the dense materials ranged from 100 nm and upwards, depending on the sintering temperature, sintering procedure, and the phase composition. The unit cell parameters of LaNbO4 showed a finite size effect and approached the cell parameters of tetragonal LaNbO4 with decreasing crystallite size, both for the single-phase material and the composites. The minority phase (La3NbO7 or LaNb3O9) were observed as isolated grains and accumulated at triple points and not along the grain boundaries, pointing to a large dihedral angle between the phases. The calcium-solubility in the minority phases was larger than in LaNbO4, which corresponds well with previous reports. The electrical conductivity of the heterodoped materials was similar to, or lower than, that for Ca-doped LaNbO4

Tuning the Thermoelectric Performance of CaMnO3-Based Ceramics by Controlled Exsolution and Microstructuring

The thermoelectric properties of CaMnO3-δ/CaMn2O4 composites were tuned via microstructuring and compositional adjustment. Single-phase rock-salt-structured CaO-MnO materials with Ca:Mn ratios larger than unity were produced in reducing atmosphere and subsequently densified by spark plasma sintering in vacuum. Annealing in air at 1340 °C between 1 and 24 h activated redox-driven exsolution and resulted in a variation in microstructure and CaMnO3-δ materials with 10 and 15 vol % CaMn2O4, respectively. The nature of the CaMnO3-δ/CaMn2O4 grain boundary was analyzed by transmission electron microscopy on short- and long-term annealed samples, and a sharp interface with no secondary phase formation was indicated in both cases. This was further complemented by density functional theory (DFT) calculations, which confirmed that the CaMnO3-δ indeed is a line compound. DFT calculations predict segregation of oxygen vacancies from the bulk of CaMnO3-δ to the interface between CaMnO3-δ and CaMn2O4, resulting in an enhanced electronic conductivity of the CaMnO3-δ phase. Samples with 15 vol % CaMn2O4 annealed for 24 h reached the highest electrical conductivity of 73 S·cm-1 at 900 °C. The lowest thermal conductivity was obtained for composites with 10 vol % CaMn2O4 annealed for 8 h, reaching 0.56 W·m-1K-1 at 700 °C. However, the highest thermoelectric figure-of-merit, zT, was obtained for samples with 15 vol % CaMn2O4 reaching 0.11 at temperatures between 800 and 900 °C, due to the enhanced power factor above 700 °C. This work represents an approach to boost the thermoelectric performance of CaMnO3-δ based composites

Effect of aging on silica aerogel properties

Silica aerogels’ unique physical and chemical properties make them fascinating materials for a wide variety of applications. In addition to hydrophobization by silylation, aging is very important in the synthesis of silica aerogel by ambient pressure drying. Here we systematically study the effect of aging on the physico-chemical properties of silica aerogel with emphasis on ambient dried materials. Silica gels were aged for different times and at different temperatures in their gelation liquid (without solvent exchange), hydrophobized in hexamethyldisiloxane and subsequently dried either at ambient pressure or from supercritical CO2. Dynamic oscillatory rheological measurements demonstrate that aging reinforces the alcogels, particularly at high strain. The specific surface area decreases with increasing aging time and temperature as a consequence of Ostwald ripening processes during aging. With increasing aging time and temperature, the linear shrinkage and bulk density decrease and the pore size and pore volume increase for the ambient dried gels, but remain nearly constant for supercritically dried gels. Small-Angle X-ray scattering does not detect significant structural changes at length scales smaller than about hundred nanometers, but hints at systematic variations at larger length scales. The findings of this study highlight the importance of aging to increase the ability of the gel particle network to withstand irreversible pore collapse during ambient pressure drying

Hydrothermal synthesis of SrxBa1–xNb2O6 on nanostructured substrates

I nyere tid har syntese av bly-frie ferroelektriske materialer tiltrukket seg stadig mer oppmerksomhet og et av de studerte materialene er strontium barium niobat (SBN). SBN har vist gode elektro-optiske egenskaper, samt at mange av egenskapene til materialet, inkludert Curie temperaturen, gitter parameterne og remanent polarisering, er justerbare ved å endre på forholdet mellom Sr og Ba i strukturen, noe som gir en økt fleksibilitet. Denne masteroppgaven har som fokus å utvikle en hydrotermisk syntesemetode for a produsere SBN tynnfilmer på et nanostrukturert (100) SrTiO3 (STO) substrat. Dette gjøres gjennom å studere effekten av ulike syntese parametere, inkludert pH og konsentrasjon til forløper løsningen, syntesetemperatur og substratplasseringen i autoklaven. Syntesen ble gjennomført ved 150-250 °C, ved bruk at en Nb-syre løsning, Sr- og Ba-nitrat som forløpere. Forløper løsningen hadde en varierende Nb-konsentrasjonen (0.125-0.5 M) og pH (9-10.8), men en fast støkiometri med et molart forhold på 0.4:0.60:2 mellom Sr:Ba:Nb. Fase renheten, teksturering og morfologi av filmene ble undersøkt ved bruk av XRD, SEM og AFM. Tynnfilmen bestod av SBN med indikasjoner om to ulike orienteringer av nanostrukturene, den ene med c-retningen vinkelrett på (100) STO substrat overflaten (001 orientering) og den andre med c-retningen parallell med overflaten (310 orientering). Strukturene hadde en gjennomsnittlig høyde på 53-219 nm og tilnærmet full dekningsgrad (95-99 %) for de fleste syntesene. Både morfologien og vekst av tynnfilmene ble observert til å være avhengig av pH og konsentrasjon i forløper løsningen, samt også syntesetemperatur og plassering av substratet i autoklaven. Vekst av nanostaver, i stedet for kontinuerlige tynnfilmer, ble observert ved syntese på lav temperatur (150 °C), samtidig som veksten var utlukkende på substrater og ingen nukleering ble sett i løsningen. Ved syntesene på høyere temperaturer (200-250 °C) ble det derimot observert en stor andel vekst av nanopartikler i løsningen i tillegg til tynnfilm veksten. En klassisk vekstmekanisme basert på 2D nukleering og delvis massetransportavhengig vekst, for høge konsentrasjoner og lav mobilitet, har blitt foreslått for tynnfilm veksten. Nanopartiklene som ble dannet i løsningen hadde en form som nanostaver eller -kuber med en lengde på 0.4-4 μm og et kvadratisk tverrsnitt med en bredde på omtrent 0.4 μm. Både strukturen og morfologien til nanopartiklene var avhengig av både pH og syntesetemperatur, men også aldringen av Nb-syre løsningen

1D oxide nanostructures from chemical solutions

Nanotechnology has motivated a tremendous effort to grow free standing or hierarchical nanomaterials such as nanowires and nanorods. Bottom-up approaches based on chemistry are an important approach to nanomaterials, and here the concepts of growing oxide 1D nanostructures from chemical solutions are reviewed. The thermodynamic and kinetic aspects of the nucleation and growth of oxide in solutions are presented with emphasis on hydrothermal and molten salt synthesis. The importance of solubility of precursors, the precursor chemistry, role of organic additives as well as the chemical complexity and dimensionality and symmetry of the crystal structure of the compound grown are highlighted

Nanopatterning and plasmonic properties of plasma sputtered gold on diatom frustules

Bio-silica nanostructures from diatoms (called frustules) featuring plasmonic gold nanoparticles (NPs) are elaborated using two methods based on plasma sputtering of gold. The first investigated method uses a thermal treatment to induce the thermal dewetting of a plasma sputtered gold layer on the diatom frustules. The second method first consists of coating the frustules with polyethylene glycol before sputtering gold on these frustules. For both methods, the amount of gold appears to be a key parameter regarding the final obtained layer, which can either be nanostructured by cavities or consist in individual gold NPs. For an amount of sputtered gold equivalent to form a 5 nm thick layer, both methods allow obtaining diatom frustules covered by gold NPs with a size around 20 nm and a narrow size distribution. The UV-visible characterization of the diatom frustules featuring gold NPs highlights a plasmon extinction band in agreement with individual gold NPs with a size below 25 nm