Ole Martin Løvvik

Model calculations on a flat-plate solar heat collector with integrated solar cells

A detailed physical model of a hybrid photovoltaic/thermal system is proposed, and algorithms for making quantitative predictions regarding the performance of the system are presented. The motivation for the present work is that solar cells act as good heat collectors and are fairly good selective absorbers. Additionally, most solar cells increase their efficiency when heat is drawn from the cells. The model is based on an analysis of energy transfers due to conduction, convection and radiation and predicts the amount of heat that can be drawn from the system as well as the (temperature-dependent) power output. Special emphasis is laid on the dependence of the fin width to tube diameter ratio. We attribute values to the model parameters, and show that hybrid devices are interesting concerning system efficiency as is also confirmed by previous experiments. Possible applications of such systems are also proposed.

The influence of electronic structure on hydrogen absorption in palladium alloys

The influence of the electronic structure and the lattice constant on hydrogen absorption in bulk Pd3M1 (M = Cd, Ag, Au, Pd, Cu, Ni, Pt, Pb, Sn, Fe, Rh, Ru) has been studied by density-functional calculations. We have assumed face-centred cubic structure for all the alloys, and hydrogen has been placed in the octahedral site surrounded by six Pd atoms. We have calculated the absorption energy of hydrogen in the alloys, and found that the influence of the electronic structure is much more important than that of the lattice constant. The results demonstrate that Miedemas empirical rule is also satisfied in this system, i.e., the higher the binding energy of the host alloy, the less stable the hydride. We have also calculated the detailed electronic structures of the alloys and their hydrides. We found that more stable hydrogen absorption is correlated with the hydrogen 1s electrons, palladium s electrons, palladium s-like electrons and the palladium d electrons moving higher in energy towards the Fermi level. The two latter relations have previously been described for bulk systems and surfaces respectively, while this study is apparently the first to point out the correlation between the position of the hydrogen band and the stability of the hydride, i.e., the deeper the hydrogen band, the less stable the hydride.

Au-Sn SLID Bonding—Properties and Possibilities

Au-Sn solid–liquid interdiffusion (SLID) bonding is a novel and promising interconnect technology for high-temperature applications. This article gives a review over previously published work on Au-Sn SLID bonding. An overview of the crystal phases and the thermomechanical properties of the Au-Sn phases relevant for Au-Sn SLID bonding is given. A summary of the bonding conditions used during Au-Sn SLID bonding is presented together with results from reliability tests. Additional challenges, possibilities, and recommendations for how a reliable high-temperature Au-Sn SLID bonding should be constructed are also discussed.

Density functional calculations on hydrogen in palladium–silver alloys

Periodic bulk calculations based on density functional theory within the generalized gradient approximation (GGA) have been used to calculate properties of the palladium–silver metals and alloys Pd4−nAgn (n=0–4) with and without absorbed hydrogen. For each of the metals and alloys, we have calculated the equilibrium lattice constant, the preferred hydrogen absorption site, the absorption energy as a function of hydrogen content (indicating the hydrogen solubility), and the equilibrium lattice constant of the resulting metal hydrides. Most of the results are qualitatively in good correspondence with experimental data, showing that periodic electronic structure calculations like this may be useful in understanding qualitative properties of disordered alloy–hydrogen systems. We are not able, however, to obtain satisfactory results for the cohesive energy and the absorption energy of hydrogen in pure silver. The former is a known failure of the GGA functional that we have used, while the latter could most probably be resolved by lattice relaxation.

A study of a polymer-based radiative cooling system

A radiative cooling system consisting of unglazed flat plate radiators, water as heat carrier and a reservoir is presented. The radiators are twin-wall sheets made of a modified PPO (polyphenylenoxid) resin, which are proposed as low cost roof integrated modules. The thermal performance of a system with a radiator aperture area of 5.3 m2 and reservoir volume of 280 l has been investigated in experiments for Oslo climate. A parameterisation for the cooling performance of a tilted radiator surface for clear and cloudy atmospheres is proposed and applied to model the experimental results. The impact of the tilt angle, the aperture area and the reservoir volume on the cooling performance has been studied in simulations. The feasibility of a radiative cooling system designed for a single-family house at southern latitudes has been modelled. Except for mid-summer ambient temperature and high relative humidity, the simulations show that the radiative cooling system seems to cover the demand.

High Temperature Interconnect and Die Attach Technology: Au–Sn SLID Bonding

Au-Sn solid-liquid interdiffusion (SLID) bonding is a novel and promising interconnect and die attach technology for high temperature (HT) applications. In combination with silicon carbide (SiC), Au-Sn SLID has the potential to be a key technology for the next generation of HT electronic devices. However, limited knowledge about Au-Sn SLID bonding for HT applications is a major restriction to fully realizing the HT potential of SiC devices. Two different processing techniques-electroplating of Au/Sn layers and sandwiching of eutectic Au-Sn preform between electroplated Au layers-have been studied in a simplified metallization system. The latter process was further investigated in two different Cu/Si3N4/Cu/Ni-P/Au-Sn/Ni/Ni2Si/SiC systems (different Au-layer thickness). Die shear tests and cross-sections have been performed on as-bonded, thermally cycled, and thermally aged samples to characterize the bonding properties associated with the different processing techniques, metallization schemes, and environmental stress tests. A uniform Au-rich bond interface was produced (the ζ phase with a melting point of 522°C). The importance of excess Au on both substrate and chip side in the final bond is demonstrated. It is shown that Au-Sn SLID can absorb thermo-mechanical stresses induced by large coefficient of thermal expansion mismatches (up to 12 ppm/K) in a packaging system during HT thermal cycling. The bonding strength of Au-Sn SLID is shown to be superb, exceeding 78 MPa. However, after HT thermal ageing, the ζ phase was first converted into the more Au-rich β phase. This created physical contact between the Sn and Ni atoms, resulting in brittle NixSny phases, reducing the bond strength. Density functional theory calculations have been performed to demonstrate that the formation of NixSny in preference to the Au-rich Au-Sn phases is energetically favorable.

Experimental studies of α-AlD3 and α′-AlD3versus first-principles modelling of the alane isomorphs

The thermal phase behaviour of cryomilled α′-AlD3 and α-AlD3 was investigated by in situsynchrotron powder X-ray diffraction (SR-PXD), differential scanning calorimetry and first principles atomic modelling. In situ measurements showed that α′-AlD3 decomposes directly into Al and D2 at around 80 °C during heating at 1 °C min−1. At higher temperatures the transformation of α′-AlD3 to α-AlD3 was observed by DSC measurements at 5 °C min−1, and tentatively by in situSR-PXD at 1 °C min−1. Atomic modelling was carried out to investigate possible structural relationships and transformation pathways between the α- and α′-phase. Group–subgroup relation analyses and direct method lattice dynamics were used to rule out a possible displacive transformation pathway between the α′- and α-phases. The likelihood of a reconstructive transformation was demonstrated by partial transformation of an interface between α′ and α domains during elevated temperature molecular dynamics. Such an α′- to α-phase transformation may be possible when kinetic barriers can be overcome at elevated temperatures or during long time periods. These insights are also relevant to the transformation mechanisms of the β-AlD3 and γ-AlD3 isomorphs to the α-phase.

Theoretical analysis of oxygen vacancies in layered sodium cobaltate, NaxCoO2−δ

Sodium cobaltate with high Na content is a promising thermoelectric material. It has recently been reported that oxygen vacancies can alter the material properties, reducing its figure of merit. However, experimental data concerning the oxygen stoichiometry are contradictory. We therefore studied the formation of oxygen vacancies in Na(x)CoO(2) with first principles calculations, focusing on x = 0.75. We show that a very low oxygen vacancy concentration is expected at the temperatures and partial pressures relevant for applications.

Enhancement of thermoelectric properties by energy filtering: Theoretical potential and experimental reality in nanostructured ZnSb

Energy filtering has been suggested by many authors as a means to improve thermoelectric properties. The idea is to filter away low-energy charge carriers in order to increase Seebeck coefficient without compromising electronic conductivity. This concept was investigated in the present paper for a specific material (ZnSb) by a combination of first-principles atomic-scale calculations, Boltzmann transport theory, and experimental studies of the same system. The potential of filtering in this material was first quantified, and it was as an example found that the power factor could be enhanced by an order of magnitude when the filter barrier height was 0.5 eV. Measured values of the Hall carrier concentration in bulk ZnSb were then used to calibrate the transport calculations, and nanostructured ZnSb with average grain size around 70 nm was processed to achieve filtering as suggested previously in the literature. Various scattering mechanisms were employed in the transport calculations and compared with the ...

Role of the self-interaction error in studying chemisorption on graphene from first-principles

Adsorption of gaseous species, and in particular of hydrogen atoms, on graphene is an important process for the chemistry of this material. At the equilibrium geometry, the H atom is covalently bonded to a carbon that puckers out from the surface plane. Nevertheless the flat graphene geometry becomes important when considering the full sticking dynamics. Here we show that GGA-DFT predicts the wrong spin state for this geometry, namely,