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Papers by Gunnar Niklasson

Annual Technical Conference Proceedings

Glazing for energy efficient buildings comprises multi-pane units with surface coated glass. Acco... more Glazing for energy efficient buildings comprises multi-pane units with surface coated glass. According to today’s standards, glazing is characterized by optical measurements at normal light inciden ...

Bruggeman dielectric mixture expression is one of the widely used methods to characterize propert... more Bruggeman dielectric mixture expression is one of the widely used methods to characterize properties of heterogeneous materials. In this contribution, dielectric response of mixture systems generated with the Bruggeman effective mixture expression is studied to investigate the broadening of dielectric relaxation at various filler contents. The mixture is composed of two dielectric media. The studies are conducted below and above the percolation threshold for three-dimensional systems, where the percolation threshold qc is at 1/3. The percolating and non-percolating parts of the mixture are estimated with the spectral density representation using a non-parametric numerical analysis technique based on the Monte Carlo integration hypothesis. A conventional dielectric relaxation model is also applied to represent data.

Electrochromic materials have the ability to change their optical properties, gradually and rever... more Electrochromic materials have the ability to change their optical properties, gradually and reversibly, when an electrical potential is applied across them. Thin films of transition metal oxides impart electrochromic functionality to the emerging smart window technology. Together with transparent contacts and a solid or gel-like electrolyte they constitute the building blocks of electrochromic devices. A typical device consists of five layers between two substrates, or alternatively deposited on a single substrate. In the former configuration, each substrate is coated by a transparent conducting layer and subsequently by an electrochromic layer. The two sides of the device are laminated by an ion conductor, preferably a polymer-based one. The electrochromic layers are chosen to be complementary. One of them is cathodic (i.e. it colors under cation/electron insertion) and the other is anodic (coloring under cation/electron extraction).Some of the important performance parameters of an electrochromic device are the optical contrast between transparent and dark states, the charge capacity and the switching speed. Each of these has its roots in the fundamental physical and electrochemical properties of the materials. The optical properties depend on the nature of the electronic transitions and whether the electrons are free or localized. It appears that in many electrochromic materials intervalence transitions between localized states is the most important mechanism. Basically, a good description of the optical properties depends on a good understanding of the electronic band structure. The charge capacity of an electrochromic film depends on the magnitude of the electronic density of states in the reversible potential range. It should be noted that the achievable density of states may be less than the theoretically expected one, since intercalated ions may not be able to penetrate all parts of the material. The switching speed depends on the diffusion coefficient of ions in the electrochromic films, as well as in the electrolyte. The sheet resistance of the transparent conductors is also important in this respect.A number of electrochemical techniques are suitable for studying ionic transport in intercalation materials, such as for example electrochromic films in contact with an electrolyte. Impedance spectroscopy gives the most detailed characterization of the transport processes but measurements are time consuming, and in certain situations more simple techniques such as chronopotentiometry or choronoamperometry may be preferable. Electronic transport may be studied with conventional electrical measurements with the film samdwiched between two metal electrodes.In this contribution we will focus on the determination of the diffusion coefficient and the electronic density of states (DOS) by electrochemical methods, and how to use this knowledge to describe the optical properties. It is shown that the features observed in the “electrochemical density of states” often show good agreement with the theoretically computed DOS. However, the magnitude of the electrochemical DOS is always lower that the theoretical one. It seems that the whole film is not accessible to the ion intercalation process. We also address the question whether the electronic states are extended or localized and how to determine this experimentally. Theories for optical properties are reviewed based on the intervalence transfer concept. In some, but not all, cases the transitions may be identified as polaronic. We also comment on the existence of electrochromism due to free electrons and its possible limitations. Examples are given mainly from our work on WO3, but also results on SnO2, IrO2 and NiO-based films will be discussed.

Proceedings of SPIE, Aug 1, 1990

We introduce a model of the optical properties of nickel pigmented aluminium oxide selective sola... more We introduce a model of the optical properties of nickel pigmented aluminium oxide selective solar absorbers that can explain the degradation at elevated temperatures. The properties of the Ni-A1203 cermet layer are described by the Bruggeman effective medium theory. On top of this layer we assume one homogeneous and one graded aluminium oxide layer. Our optical model can qualitatively explain the change in opticai properties of the coatings upon heat treatments in air. The degradation mechanism appears to be oxidation of the nickel particles throughout the cermet layer. We also propose a novel theoretical model that can be used to account for optical degradation of solar collector coatings. The degradation is described in terms of an activation energy and a kinetic exponent. These quantities are determined for the Ni-A1203 coatings and are compared to data on the oxidation kinetics of nickel.

$Research paper thumbnail of Multifractal analysis of nanoparticle aggregates$

In this paper we assess the suitability of box-counting for determining the multifractal dimensio... more In this paper we assess the suitability of box-counting for determining the multifractal dimensions of nanoparticle aggregates. The algorithm was first tested on deterministic multifractal structures. The method was also applied to electron micrographs of gas evaporated cobalt particle aggregates. The finite fractal range due to a rather small correlation length complicated the analysis. Clear evidence of a multifractal structure was found for a sample consisting of superparamagnetic particles, which generally have diameters below 10 nm. No indications of multifractality was found for aggregates consisting of larger ferromagnetic particles

$Research paper thumbnail of Adsorption on fractal structures: applications to cement materials$

Cement and Concrete Research, Sep 1, 1993

Abstract Adsorption isotherm measurements are widely used to characterize the pore structure of c... more Abstract Adsorption isotherm measurements are widely used to characterize the pore structure of cement materials. Recent experimental evidence suggests that the cement gel displays a fractal structure. In this paper we investigate the possibility of observing effects of the fractal structure in sorption isotherms. Some water and nitrogen sorption measurements were analysed in terms of a recently proposed theory for capillary condensation in fractal pores. We discuss the advantages and shortcomings of this kind of analysis. At least in some cases the effective dimensions obtained from sorption isotherms show satisfactory agreement with fractal dimensions of cement materials as determined by other methods.

$Research paper thumbnail of The fractal dimension of music: Melodic contours and time series of pitch$

arXiv (Cornell University), Apr 6, 2020

Physics and Chemistry of Glasses: European Journal of Glass Science and Technology Part B, 2019

Four reflection loss correction methods have been investigated, theoretically and by analysing th... more Four reflection loss correction methods have been investigated, theoretically and by analysing the measured absorbance spectra for eight samples prepared from the same Cu-doped soda–lime–silica gla ...

Journal of Physics D: Applied Physics, 1995

Journal of Applied Physics, Feb 1, 1986

We present a method for analyzing current-voltage relationships to obtain information on the domi... more We present a method for analyzing current-voltage relationships to obtain information on the dominant conduction mechanisms. The method is based on the analysis of the derivative of the logarithmic conductivity with respect to inverse applied electrical field. To illustrate the method we apply it to the study of Co-Al2O3 and Au-Al2O3 composite films. The former material displays space-charge-limited conduction, which is due to a high density of trap states in the oxide matrix. On the other hand, Au-Al2O3 shows evidence of percolation and tunneling between metal particles.

Thin Solid Films, Dec 1, 2009

Electrochromic (EC) materials are able to change their optical properties such as transmission, a... more Electrochromic (EC) materials are able to change their optical properties such as transmission, absorption and reflection reversibly by application of an external voltage. EC metal oxides are divid ...

Journal of Applied Physics, Feb 1, 2021

Journal of physics, Oct 1, 2016

Journal of Applied Physics, Nov 1, 2001

Temperature dependent ac dielectric spectroscopy and room-temperature I–V characterization were p... more Temperature dependent ac dielectric spectroscopy and room-temperature I–V characterization were performed on atomic layer deposited (Ta1−xNbx)2O5 films. The high frequency permittivity, as well as the dc conductivity of the films, were found to increase with increasing Nb content. The conduction mechanism in the mixed Ta–Nb oxide films was of the Poole–Frenkel type, while the high field conduction in pure Ta2O5 was space-charge limited. The activation energy for dc conduction was higher in mixed Ta–Nb oxides compared to pure Ta2O5 and Nb2O5 films. Irreversible changes in the conduction mechanism took place upon heat treatment above a certain irreversibility temperature. This temperature was higher for the mixed oxides than for the binary ones.

Thin Solid Films, Oct 1, 1995

Abstract Titanium-oxide-based films were produced by reactive sputtering onto glass substrates or... more Abstract Titanium-oxide-based films were produced by reactive sputtering onto glass substrates oriented with a large angle between the surface normal and the direction of the incoming deposition species. The optical properties were angular selective with a higher transmittance for light incident from the same side of the normal (positive angles) as the deposition material than for light coming from negative angles. The selectivity was largest in the near-infrared wavelength range.