Anette Gunnæs - Academia.edu (original) (raw)

Papers by Anette Gunnæs

Applied Physics Letters

In this work, we report on quite strong 1.54‐μm photoluminescence (PL) from an (Er, Ge) co-doped ... more In this work, we report on quite strong 1.54‐μm photoluminescence (PL) from an (Er, Ge) co-doped SiO2 film deposited by rf magnetron sputtering. The PL intensity reaches a maximum value after the film is annealed at 700 °C for 30 min in N2. High-resolution transmission electron microscopy observation, together with energy dispersive x-ray spectroscopy analysis, indicates that amorphous Ge-rich nanoclusters precipitate in the film after 700 °C annealing. X-ray diffraction shows the presence of Ge nanocrystals after 900 °C annealing, and increasing Ge nanocrystal size with increasing annealing temperature up to 1100 °C. The results suggest that the amorphous Ge-rich nanoclusters are more effective than Ge nanocrystals in exciting the Er3+ PL.

In catalysts for CO 2 hydrogenation, the interface between metal nanoparticles (NPs) and the supp... more In catalysts for CO 2 hydrogenation, the interface between metal nanoparticles (NPs) and the support material is of high importance for the activity and reaction selectivity. In Pt NPcontaining UiO Zr-metal−organic frameworks (MOFs), key intermediates in methanol formation are adsorbed at open Zrsites at the Pt−MOF interface. In this study, we investigate the dynamic role of the Zr-node and the influence of H 2 O on the CO 2 hydrogenation reaction at 170°C, through steady state and transient isotope exchange experiments, H 2 O cofeed measurements, and density functional theory (DFT) calculations. The study revealed that an increased number of Zr-node defects increase the formation rates to both methanol and methane. Transient experiments linked the increase to a higher number of surface intermediates for both products. Experiments involving either dehydrated or prehydrated Zr-nodes showed higher methanol and methane formation rates over the dehydrated Zr-node. Transient experiments suggested that the difference is related to competitive adsorption between methanol and water. DFT calculations and microkinetic modeling support this conclusion and give further insight into the equilibria involved in the competitive adsorption process. The calculations revealed weaker adsorption of methanol in defective or dehydrated nodes, in agreement with the larger gas phase concentration of methanol observed experimentally. The microkinetic model shows that [Zr 2 (μ-O) 2 ] 4+ and [Zr 2 (μ−OH)(μ-O)(OH)(H 2 O)] 4+ are the main surface species when the concentration of water is lower than the number of defect sites. Lastly, although addition of water was found to promote methanol desorption, water does not change the methanol steady state reaction rate, while it has a substantial inhibiting effect on CH 4 formation. These results indicate that water can be used to increase the reaction selectivity to methanol and encourages further detailed investigations of the catalyst system.

Quartz has been replaced by magnesium silicate hydrate cement at the Feragen ultramafic body in s... more Quartz has been replaced by magnesium silicate hydrate cement at the Feragen ultramafic body in southeast Norway. This occurs in deformed and recrystallized quartz grains deposited as glacial till covering part of the ultramafic body. Where the ultramafic body is exposed, weathering leads to high pH (~10), Mg-rich fluids. The dissolution rate of the quartz is about 3 orders of magnitude higher than experimentally derived rate equations suggest under the prevailing conditions. Quartz dissolution and cement precipitation starts at intergranular grain boundaries that act as fluid pathways through the recrystallized quartz. Etch pits are also extensively present at the quartz surfaces as result of preferential dissolution at dislocation sites. Transmission electron microscopy revealed an amorphous silica layer with a thickness of 100-200 nm around weathered quartz grains. We suggest that the amorphous silica is a product of interface-coupled dissolution-precipitation and that the amorphous silica subsequently reacts with the Mg-rich, high pH bulk fluid to precipitate magnesium silicate hydrate cement, allowing for further quartz dissolution and locally a complete replacement of quartz by cement. The cement is the natural equivalent of magnesium silicate hydrate cement (M-S-H), which is currently of interest for nuclear waste encapsulation or for environmentally friendly building cement, but not yet developed for commercial use. This study provides new insights that could potentially contribute in the further development of M-S-H cement.

RSC Advances

The heat treatment beneficially affects the performance of an all-oxide thermoelectric generator ... more The heat treatment beneficially affects the performance of an all-oxide thermoelectric generator through phase and element distribution within this p–n junction.

Catalysis Science & Technology

A series of zeolite ZSM-23 samples prepared with different organic structure-directing agents, ga... more A series of zeolite ZSM-23 samples prepared with different organic structure-directing agents, gave rise to crystalline phases, which show a large variety of particle dimensions. A detailed analysis of the...

CrystEngComm

A commercially feasible, nontoxic material that would convert heat into electricity in an efficie... more A commercially feasible, nontoxic material that would convert heat into electricity in an efficient and cheap way has not yet been identified. Half-Heusler compounds are one the most interesting candidates...

Acta Materialia

High resolution synchrotron radiation powder X-ray diffraction (SR-PXD) and scanning transmission... more High resolution synchrotron radiation powder X-ray diffraction (SR-PXD) and scanning transmission electron microscopy (STEM) have been employed for structural characterization of MNiSn, M 0.

Journal of Physics: Condensed Matter

Scientific reports, Jan 23, 2017

Thermoelectric application of half-Heusler compounds suffers from their fairly high thermal condu... more Thermoelectric application of half-Heusler compounds suffers from their fairly high thermal conductivities. Insight into how effective various scattering mechanisms are in reducing the thermal conductivity of fabricated XNiSn compounds (X = Hf, Zr, Ti, and mixtures thereof) is therefore crucial. Here, we show that such insight can be obtained through a concerted theory-experiment comparison of how the lattice thermal conductivity κ Lat(T) depends on temperature and crystallite size. Comparing theory and experiment for a range of Hf0.5Zr0.5NiSn and ZrNiSn samples reported in the literature and in the present paper revealed that grain boundary scattering plays the most important role in bringing down κ Lat, in particular so for unmixed compounds. Our concerted analysis approach was corroborated by a good qualitative agreement between the measured and calculated κ Lat of polycrystalline samples, where the experimental average crystallite size was used as an input parameter for the calc...

Journal of physics. Condensed matter : an Institute of Physics journal, Jan 22, 2017

The interface between ZnO and Cu2O has been predicted to be a good candida... more The interface between ZnO and Cu2O has been predicted to be a good candidate for use in thin film solar cells. However, the high predicted conversion efficiency has yet to be fully realized experimentally. To explore the underlying causes of this we investigate the interface between ZnO and Cu2O in magnetron sputtered samples. Two different sample geometries were made: In the first set thin layers of ZnO were deposited on Cu2O (type A), while in the second set the order was reversed (type B). Using x-ray photoelectron spectroscopy (XPS), an intermediate CuO layer was identified regardless of the order in which the Cu2O and ZnO layers were deposited. The presence of a CuO layer was supported by transmission electron microscopy (TEM) results. Changes in the electron hole screening conditions were observed in CuO near the interface with ZnO, manifested as changes in the relative peak-to-sat...

Frontiers of oral biology, 2009

The aim of this study was to explore further the preservation of tissues and the mineral distribu... more The aim of this study was to explore further the preservation of tissues and the mineral distribution in 1.6 million-year-old fossil hominin material from Koobi Fora, Kenya attributed to Paranthropus boisei (KNM-ER 1817). Bone, dentine and cementum microstructure were well preserved. Electron microprobe analysis of dentine and bone revealed an F-bearing apatite. Calcite now filled the original soft tissue spaces. The average Ca/P atomic ratio was 1.93, as compared to 1.67 in biological hydroxyapatite, indicating that the Ca-content had increased during fossilization. Analytical sums for mineral content were approximately 90 wt%. Some of the remaining 10 wt% may be preserved organic material. Demineralized dentine fragments showed irregularly distributed tubules encircled with a fibrous-like electron-dense material. A similar material was observed in demineralized dentine. Within this, structures resembling bacteria were seen. In demineralized bone an electron-dense material with a f...

2006 IEEE 4th World Conference on Photovoltaic Energy Conference, 2006

The quality and temperature stability of surface passivation of silicon by a double layer consist... more The quality and temperature stability of surface passivation of silicon by a double layer consisting of a hydrogenated amorphous silicon thin film capped by a silicon nitride anti-reflection coating are studied. It is established that the passivation effect of the double layer can be significantly enhanced after short annealing for temperatures up to about 500 °C, whereas annealing at higher temperatures results in degradation of the passivation properties. It is found that the increased effective recombination lifetime after annealing at temperatures below 500 ºC results from hydrogen redistribution in the interface region. Furthermore, presence of interfacial structural defects formed due to hydrogen release at temperatures around 600 ºC, is believed to be the cause of the lifetime decrease after heat treatments at higher temperatures.

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2003

The present paper reports on annealing of Ge implanted SiO2 films and emphasize the observation o... more The present paper reports on annealing of Ge implanted SiO2 films and emphasize the observation of voids and the mechanism behind their formation which is considered new. Samples were prepared by ion implanting fluences of 3×1016 and 1×1017 cm−2 respectively of 100 keV Ge into amorphous SiO2 films which were subsequently annealed up to 1000 °C in a N2 atmosphere.

Journal of Electronic Materials, 2015

ABSTRACT We report the preparation of nanosized ZnSb powder by cryo-milling. The effect of cryo-m... more ABSTRACT We report the preparation of nanosized ZnSb powder by cryo-milling. The effect of cryo-milling then hot-pressing of undoped ZnSb was investigated and compared with that of room temperature ball-milling and hot-pressing under different temperature conditions. ZnSb is a semiconductor with favorable thermoelectric properties when doped. We used undoped ZnSb to study the effect of nanostructuring on lattice thermal conductivity, and with little contribution at room temperature from electronic thermal conductivity. Grain growth was observed to occur during hot-pressing, as observed by transmission electron microscopy and x-ray diffraction. The thermal conductivity was lower for cryo-milled samples than for room-temperature ball-milled samples. The thermal conductivity also depended on hot-pressing conditions. The thermal conductivity could be varied by a factor of two by adjusting the process conditions and could be less than a third that of single-crystal ZnSb.

ABSTRACT Effects of sol–gel-based coatings on high-temperature oxidation of Ni were studied in th... more ABSTRACT Effects of sol–gel-based coatings on high-temperature oxidation of Ni were studied in the temperature range 700–1200C in air by thermogravimetry, SEM, and TEM. The oxidation kinetics are parabolic and lower than for uncoated Ni, which also shows subparabolic kinetics below 1000C. The temperature dependence of the parabolic rate constant for oxidation of silica-coated Ni corresponds to a single Arrhenius relation. The NiO scale formed on silica-coated Ni consists of three regions; an outer region with columnar grains and an inner region of equiaxed grains, separated by a fine-grain, Si-rich layer. For uncoated specimens only, the outer columnar and inner equiaxed regions were encountered. The beneficial effect of the silica on the rate of oxidation has been interpreted to reflect that Si segregated along grain boundaries in the intermediate layer decreases the outward Ni flux along grain boundaries.

The microstructure of laser-processed (Al 1 − x Ni x) 3 Ti with Ni content ranging from 5 to 15 a... more The microstructure of laser-processed (Al 1 − x Ni x) 3 Ti with Ni content ranging from 5 to 15 at. % has been studied by scanning-and transmission-electron microscopy (SEM and TEM). All samples contain Al-rich dendrites with Ni-rich interdendritic phases. The dendrites in the 5 at. % Ni alloy consist of the Al 3 Ti phase with the DO 22 type structure, whereas the alloys containing 8 to 15 at. % Ni consist of the Al 67 Ni 8 Ti 25 phase with the L1 2-type structure. The lattice parameter of the L1 2 type structure of Al 67 Ni 8 Ti 25 was determined by comparing the Higher Order Laue Zone (HOLZ) line patterns in experimental and calculated convergent beam electron diffraction disks. The lattice parameter was found to be a = 0.3935 ± 0.0003 nm. Within the L1 2 type regions of the alloy with 8 at. % Ni, precipitates with a new primitive tetragonal structure was found. The cell dimensions are a = 0.39 nm and c = 1.18 nm. AlNTi 2 with Ti : Al ratio equal to 2.0 ± 0.2 was found as needles in interdendritic regions. The cell dimensions are consistent with a = 0.299 nm and c = 1.361 nm and space group P6 3 /mmc.

Oxidation of Metals, 2003

ABSTRACT The effects of superficial (30–100 nm) La2O3 surface coatings on the oxidation kinetics ... more ABSTRACT The effects of superficial (30–100 nm) La2O3 surface coatings on the oxidation kinetics of Ni from 700 to 1100°C in air and the oxide morphology of the NiO scales have been investigated. The parabolic rate constant is lower than for uncoated Ni by a factor of 5 to 10. The oxide morphology changes with the La2O3 coatings: The oxide scale consists of an outer fine-grain layer with an inner region of coarser, but still equiaxed, grains. SIMS shows that the majority of the La remains at the surface where a highly oxygen-defective spinel, La2Ni4O7, was found by TEM. Two-stage oxidation followed by SIMS profiling reveals that the oxide growth occurs inside the scales.

Topics in Catalysis, 2013

Nanosheets of zeolite H-ZSM-5 were synthesized and characterized by X-ray diffraction, transmissi... more Nanosheets of zeolite H-ZSM-5 were synthesized and characterized by X-ray diffraction, transmission electron microscopy (TEM), N 2-physisorption, FT-IR spectroscopy, 27 Al and 29 Si MAS NMR spectroscopy in addition to catalytic testing in conversion of methanol to hydrocarbons (MTH). It was found that Rietveld analysis, involving anisotropic broadening parameters, gave average crystallite dimensions in good agreement with TEM images. The selectivities in MTH is intact in the mesoporous nanosheet H-ZSM-5 with the largest difference being a higher C 3 /C 2 ratio compared to regular H-ZSM-5.