Gunnar A. Niklasson

Effective medium models for the optical properties of inhomogeneous materials.

The Maxwell Garnett and Bruggeman effective medium theories are derived for the average dielectric permeability of heterogeneous materials from a unified theoretical approach. It starts by specifying two random unit cells which represent different microstructures. Requiring that these cells should not be detectable by electromagnetic radiation when embedded in an effective medium, we show from an extended optical theorem that the forward scattering amplitude must vanish. Setting the leading term in the expansion series of this quantity equal to zero yields the effective medium theories pertaining to the two microstructures. The remaining terms provide estimates of the accuracy of the approximations. This approach is then used in numerical computations for Co-AI(2)O(3) cermets.

Optical properties and solar selectivity of coevaporated Co‐Al2O3 composite films

Co‐Al2O3 composite films were produced by electron‐beam coevaporation in a system with elaborate process control. The deposits were analyzed by transmission electron microscopy, electron diffraction, Auger electron spectroscopy, secondary ion mass spectroscopy, Rutherford backscattering, field ion microscopy, mechanical stylus measurements, and electrical dc and ac measurements. A uniform separated‐grain structure with regular hcp Co particles embedded in Al2O3 was found for Co contents (fCos’) up to ∼30 vol %. The complex dielectric permeability e was evaluated in the 0.3≲λ≲40‐μm wavelength range for samples with 0.11≲ fCo ≲0.60 by carefully selected combinations of spectrophotometric transmittance and reflectance data. Numerical accuracy and internal consistency were investigated. Effective medium theories for e were derived by applying classical scattering theory to spherical random unit cells defined so as to properly represent a number of typical microgeometries. The formulations due to Maxwell Gar...

Recent advances in electrochromics for smart windows applications

Electrochromic smart windows are able to vary their throughput of radiant energy by low-voltage electrical pulses. This function is caused by reversible shuttling of electrons and charge balancing ions between an electrochromic thin film and a transparent counter electrode. The ion transport takes place via a solid electrolyte. Charge transport is evoked by a voltage applied between transparent electrical conductors surrounding the electrochromic film/electrolyte/counter electrode stack. This review summarizes recent progress concerning: (i) calculated optical properties of crystalline WO3, (ii) electrochromic properties of heavily disordered W oxide and oxyfluoride films produced by reactive magnetron bias sputtering, (iii) novel transparent reactively sputter-deposited Zr–Ce oxide counter electrodes and (iv) a new proton-conducting antimonic-acid-based polymer electrolyte. Special in depth presentations are given on elastic light scattering from W-oxide-based films and of electronic band structure effects affecting opto–chronopotentiometry data in Zr–Ce oxide. The review also contains some new device data for an electrochromic smart window capable of very high optical transmittance.

Mg doping of thermochromic VO2 films enhances the optical transmittance and decreases the metal-insulator transition temperature

Thermochromic films of MgxV1−xO2 were made by reactive dc magnetron sputtering onto heated glass. The metal-insulator transition temperature decreased by ∼3 K/at. %Mg, while the optical transmittance increased concomitantly. Specifically, the transmittance of visible light and of solar radiation was enhanced by ∼10% when the Mg content was ∼7 at. %. Our results point at the usefulness of these films for energy efficient fenestration.

Thermochromic VO2 films for energy-efficient windows

Abstract VO 2 films were produced by reactive e-beam evaporation followed by annealing post-treatment. Electrical measurements demonstrated a semiconductor-metal transition at τ c ∼ 60° C . Spectrophotometry showed that the near-infrared solar transmittance was reduced when τ c was exceeded while the luminous transmittance remained relatively unchanged. This thermochromism may be utilized for regulating the energy throughput of windows. Practical application hinges on improved transmittance and on τ c - depression . These goals can be accomplished to some extent by dielectric overlayers, as verified by measurements on SiO 2 -coated VO 2 films.

Nanothermochromics: Calculations for VO2 nanoparticles in dielectric hosts show much improved luminous transmittance and solar energy transmittance modulation

VO2-based films are thermochromic and show infrared reflectance above a “critical” temperature in the vicinity of room temperature. Implementations on energy efficient windows have been discussed for decades but have been severely curtailed since the luminous absorptance is undesirably large and the solar energy transmittance modulation is too small. Here we show by calculations based on effective medium theory that dilute composites with VO2 nanoparticles embedded in hosts with properties mimicking glass or polymer can yield significantly decreased luminous absorption jointly with much enhanced transmittance modulation of solar energy. These results demonstrate that VO2-based nanothermochromics opens new avenues toward energy efficient fenestration.

New approach to the origin of lognormal size distributions of nanoparticles

A new model for particle growth can predict the well known lognormal particle size distribution from first principles in a physically realistic way. The model is completely different from the usually applied coagulation models; it is based on a residence time approach, where the time available for the particles to grow determines the size distribution. The model is generally relevant in fields such as nanoparticle physics, aerosol science or environmental science, whenever particle growth occurs during transport through a growth zone due to diffusion and drift. Model predictions show excellent agreement with published experimental data obtained with the inert-gas evaporation technique.

Eliminating degradation and uncovering ion-trapping dynamics in electrochromic WO3 thin films

Amorphous WO3 thin films are of keen interest as cathodic electrodes in transmittance-modulating electrochromic devices. However, these films suffer from ion-trapping-induced degradation of optical modulation and reversibility upon extended Li+-ion exchange. Here, we demonstrate that ion-trapping-induced degradation, which is commonly believed to be irreversible, can be successfully eliminated by constant-current-driven de-trapping, i.e., WO3 films can be rejuvenated and regain their initial highly reversible electrochromic performance. Pronounced ion-trapping occurs when x exceeds ~0.65 in LixWO3 during ion insertion. We find two main kinds of Li+-ion trapping sites (intermediate and deep) in WO3, where the intermediate ones are most prevalent. Li+-ions can be completely removed from intermediate traps but are irreversibly bound in deep traps. Our results provide a general framework for developing and designing superior electrochromic materials and devices.

Noble‐metal‐based transparent infrared reflectors: Experiments and theoretical analyses for very thin gold films

Very thin gold films were prepared on glass by ion plating (IP) and by conventional evaporation (CE). Below a certain thickness—∼9 nm for IP and ∼15 nm for CE—the films comprised a metal network; above this thickness we found uniform films. Optical properties were recorded by spectrophotometry. Conspicuous near‐infrared transmittance plateaus were seen in network films. This effect is conducive to high solar transmission. The spectral features were explained from effective medium theories based on the film structure. The uniform films were consistent with the Drude theory, provided that an anomalously large frequency dependence of the relaxation energy was invoked. Significant induced transmission was found in calculations on dielectric/gold/dielectric coatings. Our results lead to improved noble‐metal‐based transparent infrared reflectors for potential use on energy efficient windows.Very thin gold films were prepared on glass by ion plating (IP) and by conventional evaporation (CE). Below a certain thickness—∼9 nm for IP and ∼15 nm for CE—the films comprised a metal network; above this thickness we found uniform films. Optical properties were recorded by spectrophotometry. Conspicuous near‐infrared transmittance plateaus were seen in network films. This effect is conducive to high solar transmission. The spectral features were explained from effective medium theories based on the film structure. The uniform films were consistent with the Drude theory, provided that an anomalously large frequency dependence of the relaxation energy was invoked. Significant induced transmission was found in calculations on dielectric/gold/dielectric coatings. Our results lead to improved noble‐metal‐based transparent infrared reflectors for potential use on energy efficient windows.

Surfaces for selective absorption of solar energy: an annotated bibliography 1955–1981

A comprehensive classified list of annotated references on surfaces which combine high solar absorptance with low thermal emittance is presented, covering the period 1955–1981. The list embraces papers printed in scientific journals and in published conference proceedings. An index of authors and on surface coatings studied is appended.