| 1. |
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| 2. |
- Berggren, Lars, et al.
(författare)
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Electrical Conductivity as a Function of Temperature in Amorphous Lithium Tungsten Oxide
- 2004
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Ingår i: Solar Energy Materials and Solar Cells. - : Elsevier BV. - 0927-0248 .- 1879-3398. ; 84:1, s. 329-336
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Tidskriftsartikel (refereegranskat)abstract
- Tungsten oxide is a widely used electrochromic material for smart windows. In order to study the charge carriers involved in the electrochromic process, it is important to characterize the electrical transport in tungsten oxide. Substoichiometric amorphous tungsten oxide films were prepared by DC-magnetron sputtering. The films were electrochemically intercalated with lithium. The Li/W intercalation ratios for the tungsten oxide films were in the range 0.15–0.53. Temperature dependent resistivity measurements were performed in the temperature range 77–300 K for samples at different lithium intercalation levels. It was found that the data are consistent with the variable range hopping model.
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| 3. |
- Berggren, Lars, et al.
(författare)
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Optical Absorption and Durability of Sputtered Amorphous Tungsten Oxide Films
- 2003
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Ingår i: Solid State Ionics. - : Elsevier BV. - 0167-2738 .- 1872-7689. ; 165:1-4, s. 51-58
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Tidskriftsartikel (refereegranskat)abstract
- Amorphous tungsten oxide films were made by sputtering onto glass substrates that were coated with conductive tin doped indium oxide (ITO). The films were deposited at different O2/Ar gas flow ratios and different substoichiometric compositions was determined by Elastic recoil detection analysis (ERDA). Substoichiometric as-deposited tungsten oxide is transparent above a particular oxygen content and is blue below that content. This indicates that there are at least two kinds of defects in the substoichiometric films. The oxygen vacancies may be coupled to W5+ sites, giving rise to strong absorption, or to (W–W)10+ complexes in the transparent films. Lithium ions were electrochemically intercalated at several charge levels. At each level the transmittance and reflectance were measured in the wavelength range between 0.3 and 2.5 μm. We show that as-deposited blue films and intercalated transparent films display similarly shaped optical absorption bands. Electrochromic devices were made by laminating the tungsten oxide films with sputtered Ni–V oxide deposited on ITO-coated plastic substrates. The durability under electrochemical cycling was best for the case of very substoichiometric WO2.63 films.
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| 4. |
- Berggren, Lars, 1967-
(författare)
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Optical Absorption and Electrical Conductivity in Lithium Intercalated Amorphous Tungsten Oxide Films
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Optical and electrical properties of electrochemically lithium ion intercalated thin films of x-ray amorphous tungsten oxide made by magnetron sputtering on glass substrates coated with a thin layer of conductive tin doped indium oxide, have been studied. The composition and the density of the films were characterized by the ion beam analysis methods Rutherford Backscattering Spectroscopy and Elastic Recoil Detection Analysis. The optical properties, transmittance and reflectance were investigated by spectophotometry in the wavelength range 300-2500 nm. The absorption coefficients were calculated at different lithium intercalation levels. It was found that the absorption coefficient in an as-deposited blue film has a similar asymmetric shape as for films intercalated to a Li/W ratio of ~0.03. It was possible to electrochemically bleach this film to transparency. Stoichiometric films show optical irreversibility between the bleached and the colored state in the first cycle. A polaron absorption model has been compared to the absorption coefficient for films of different intercalation levels. An increase in the Fermi level and in the polaron band width, and a nearly constant activation energy was found as the Li/W value increased. The radius of the polaron wavefunction for different lithium intercalation levels and film compositions has been estimated from electrical measurements. The total absorption coefficient has been compared to the site-saturation model. The model is good for films intercalated in the optically reversible region. A modified site-saturation model that could be applied also in the optically irreversible region and involves electron transitions between W6+, W5+ and W4+ sites, has also been compared to experimental values. It was found that the total absorption, optical density and the coloration efficiency is higher for the WO2.63 film than in the less oxygen deficient films and that this film is optically more durable in an electrochemically cyclic lifetime device test.
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| 5. |
- Berggren, Lars, et al.
(författare)
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Polaron Absorption in Amorphous Tungsten Oxide Films
- 2001
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 90, s. 1860-1863
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Tidskriftsartikel (refereegranskat)abstract
- Amorphous thin films of tungsten oxide were deposited by sputtering onto glass substrates covered by conductive indium–tin oxide. The density and stoichiometry were determined by Rutherford backscattering spectrometry. Lithium ions were intercalated electrochemically into the films. The optical reflectance and transmittance were measured in the wavelength range from 0.3 to 2.5 μm, at a number of intercalation levels. The polaron absorption peak becomes more symmetric and shifts to higher energies until an intercalation level of 0.25 to 0.3 Li+/W, where a saturation occurs. The shape of the polaron peak is in very good agreement with the theory of Bryksin [Fiz. Tverd. Tela 24, 1110 (1982)]. Within this model, the shift of the absorption peak is interpreted as an increase in the Fermi level of the material as more Li ions are inserted.
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| 6. |
- Frenning, Göran, et al.
(författare)
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Dielectric and Li transport properties of electron conducting and non-conducting sputtered amorphous Ta2O5 films
- 2001
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Ingår i: Electrochimica Acta. - 0013-4686 .- 1873-3859. ; 46:13-14, s. 2041-2046
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Tidskriftsartikel (refereegranskat)abstract
- Two types of sputtered thin film amorphous tantalum oxide (Ta2O5) were studied: one electron conducting Ta2O5 (ec-Ta2O5) and the other non-conducting Ta2O5 (nc-Ta2O5). The as-deposited films were characterized by impedance spectroscopy (IS) and isothermal transient ionic current (ITIC) measurements. From IS, the dc conductivity 2×10−14 S/cm was obtained for the ec-Ta2O5 film at an applied ac potential of 50 mV whereas a value ≤1×10−17 S/cm was obtained for the nc-Ta2O5 film. Li conducting properties were studied using the galvanostatic intermittent titration technique and ITIC measurements on the intercalated samples. Despite the very dissimilar dc conductivities of the as-deposited films, the two Ta2O5 samples showed surprisingly similar Li ion conducting properties for small Li/Ta2O5 ratios. The Li ion mobility was in the range 1.1×10−9–3.0×10−9 cm2/V s for both films. However, the Li storage behaviour as well as the chemical diffusion coefficient differed. For the nc-Ta2O5 film a plateau was observed in the equilibrium potential vs. composition curve for Li/Ta2O5 ratios between 7×10−5 and 2×10−3. This plateau was likely to have been caused by attractive interactions between the intercalated ions, possibly large enough to cause phase separation. The attractive interactions were shown to suppress the chemical diffusion coefficient in this composition range.
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| 7. |
- Frenning, Göran, et al.
(författare)
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Li conduction in sputtered amorphous Ta2O5
- 2001
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Ingår i: Journal of the Electrochemical Society. - : The Electrochemical Society. - 0013-4651 .- 1945-7111. ; 148:5, s. A418-A421
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Tidskriftsartikel (refereegranskat)abstract
- Electron and Li ion conducting properties of room temperature sputtered amorphous tantalum oxide (a-Ta2O5) films were studied in order to evaluate the feasibility of using a-Ta2O5 in electrochromic device applications. The films were investigated using the galvanostatic intermittent titration technique, impedance spectroscopy, and isothermal transient ionic current measurements. It was found that the a-Ta2O5 met two out of three requirements posed on a Li ion conductor in a WO3 based electrochromic device. There was a negligible intercalation in the potential window used in WO3-based electrochromic devices (above 2.4-2.5 V vs. Li/Li+). Furthermore, in this potential region, the chemical diffusion coefficient for Li was larger than the corresponding quantity in WO3. However, there was a nonzero electron conductivity in the a-Ta2O5 films, not observed in the chemical vapor deposition-made β-Ta2O5 investigated earlier. Still, the ionic conductivity was approximately one order of magnitude larger than the electronic one.
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| 8. |
- Jonsson, AnnaKarin, et al.
(författare)
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Dielectric study of thin films of Ta2O5 and ZrO2
- 2001
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Ingår i: IEEE transactions on dielectrics and electrical insulation. - : Institute of Electrical and Electronics Engineers (IEEE). - 1070-9878 .- 1558-4135. ; 8:4, s. 648-651
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Tidskriftsartikel (refereegranskat)abstract
- Electronic conduction in sputtered Ta2O5 and ZrO2 thin films have been studied using impedance spectroscopy, isothermal transient ionic current, and current-voltage measurements. The dielectric properties of Ta2O5 were shown to be sensitively dependent on deposition parameters with two different frequency responses: a flat loss behavior with very low DC conductivity, or a relaxation peak together with a somewhat higher DC conductivity. ZrO2 has different dielectric properties when fresh, i.e. newly deposited, or aged. A fresh sample arbitrarily can show two different behaviors, consisting of a DC conductivity with a relaxation peak superimposed on it. The DC conductivity shows either of two different values. The aged sample has a lower permittivity and DC conductivity, and the relaxation peak is found at much lower frequencies. Fresh samples of ZrO2 also show switching behavior
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| 9. |
- Larsson, Anna-Lena, 1971-
(författare)
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All-Thin-Film Electrochromic Devices for Optical and Thermal Modulation
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The optical properties of electrochromic materials can be changed by application of an electrical voltage. The conventional electrochromic device consists of several thin films of electrochromic materials and layers for electron- and ion conduction. The ion conductor in devices intended for applications using visible light is often a polymer electrolyte that is used to laminate two half-cells together. The miniaturization of satellites has led to reduced mass and volume available for systems to handle temperature variations onboard. The satellite will be submitted to large variations in the radiating environment in an earth bound orbit. An electrochromic device could provide adaptable radiation exchange due to its variable infrared optical properties. The polymer electrolyte is not a desirable component in the space environment, but it can be replaced by an inorganic thin film so that an all-thin-film (ATF) device is obtained. This thesis investigates the optical properties of amorphous and crystalline WO3, as well as the performance of ATF devices with sputtered ZrO2 as the ion conductor. The infrared reflectance for Li-intercalated WO3 has been measured in the wavelength range 2-50 mm. The near infrared absorption for low intercalation levels showed good agreement with large polaron theory. The infrared reflectance increased with higher intercalation levels and exhibited a free-electron behaviour. The infrared reflectance of a laminated device with polymer electrolyte was measured, and the calculated emittance varied between 0.56 and 0.65. The ATF device consisted of thin films of WO3, ZrO2 and NiVxOyHz, as well as evaporated Al top contacts. The substrates were commercial ITO on glass. The emittance for different device designs was calculated from reflectance measurements, and could be varied between 0.33 and 0.59. This makes them strong contenders to other contemporary emittance modulating devices.
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| 10. |
- Niklasson, Gunnar A., et al.
(författare)
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Surface roughness of pyrolytic tin dioxide films evaluated by different methods
- 2000
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Ingår i: Thin Solid Films. - 0040-6090 .- 1879-2731. ; 359:2, s. 203-209
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Tidskriftsartikel (refereegranskat)abstract
- The scaling of surface roughness in thin spray pyrolyzed fluorinated tin dioxide films of different thicknesses was obtained from atomic force microscopy. The data show that, within experimental uncertainties, the effective dimensionality of the surface is 2; hence no evidence of fractal surface roughness was found. Other methods – based upon light scattering and cyclic voltammetry – gave additional information on the surface topography. Cyclic voltammetry measurements show that the reaction sites on the surface are distributed in a fractal structure and may be identified with hillocks seen in surface reliefs.
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| 11. |
- Sotelo, Juan, et al.
(författare)
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Optical properties of polycrystalline metallic films
- 2003
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 67, s. 195106-
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Tidskriftsartikel (refereegranskat)abstract
- We extend the Mayadas and Shatkes’s approach @Phys. Rev. B 1, 1382 ~1970!# to study the optical propertiesof polycrystalline metal thick films in the visible and far infrared range of the spectrum. We show that in thisrange grain boundary scattering can account for the experimentally observed lowering of the film reflectivity asthe mean size of its constituent grains decreases.
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| 12. |
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| 13. |
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| 14. |
- Strömme, Maria, et al.
(författare)
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Densification-induced conductivity percolation in high-porosity pharmaceutical microcrystalline cellulose compacts
- 2003
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Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 82:4, s. 648-650
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Tidskriftsartikel (refereegranskat)abstract
- The percolation theory is established as a useful tool in the field of pharmaceutical materials science.It is shown that percolation theory, developed for analyzing insulator–conductor transitions, can beapplied to describe imperfect dc conduction in pharmaceutical microcrystalline cellulose duringdensification. The system, in fact, exactly reproduces the values of the percolation threshold andexponent estimated for a three-dimensional random continuum. Our data clearly show a crossoverfrom a power-law percolation theory region to a linear effective medium theory region at a celluloseporosity of ;0.7.
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| 15. |
- Strömme, Maria, et al.
(författare)
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Fractal dimension of cellulose powders analyzed by multilayer BET adsorption of water and nitrogen
- 2003
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 107:51, s. 14378-14382
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Tidskriftsartikel (refereegranskat)abstract
- The aim of this study was to show that multilayer fractal Brunauer-Emmett-Teller (mfBET) theory can be used as a tool to obtain information about the distribution of water in cellulose powder particles of varying crystallinity. Microcrystalline cellulose, agglomerated micronized cellulose, low-crystallinity cellulose, and cellulose powders from green and brown algae were characterized by scanning electron microscopy and mfBET analysis on water and nitrogen adsorption isotherms. The distribution of water in the cellulose materials was found to be characterized by a fractal dimension smaller than 1.5 for all powders. The results showed that for highly crystalline cellulose materials, such as Cladophora cellulose, the cellulose-water interactions take place mainly on cellulose fibril surfaces adjacent to open pores without causing any significant swelling of the material. For less ordered celluloses the water interaction was found to take place inside the bulk material and the water uptake process caused the pore volume to swell between 1 and 2 orders in magnitude. For the Cladophora cellulose, the thickness of the adsorbed water layer at the outer cut off of the fractal region was found to coincide very well with the average pore size obtained from nitrogen adsorption measurements. The multilayer fractal BET analysis on nitrogen adsorption isotherms showed that the particles could be characterized by fractal dimensions between 2.13 and 2.50. We conclude that water adsorption has the ability to alter the structure of the studied material and reveal a sorption-induced, "apparent" fractal structure over a relatively narrow length scale interval, while nitrogen adsorption probes the substrate morphology over a wide range of length scales and reveals the "true" fractal structure.
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| 16. |
- Strömme, Maria, et al.
(författare)
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(Ta1-xNbx)(2)O-5 films produced by atomic layer deposition : Temperature dependent dielectric spectroscopy and room-temperature I-V characteristics
- 2001
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 90:9, s. 4532-4542
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Tidskriftsartikel (refereegranskat)abstract
- Temperature dependent ac dielectric spectroscopy and room-temperature I–V characterization were performed on atomic layer deposited(Ta1−xNbx)2O5films. 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 Nb2O5films. 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.
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