Jeffrey S. Hale

Infrared switching electrochromic devices based on tungsten oxide

Different types of electrochromic devices for thermal emittance modulation were developed in the spectral region from mid- to far-infrared (2–40 μm). In all devices polycrystalline and amorphous tungsten oxide have been used as electrochromic and ion storage layer, respectively. Two types of all-solid-state devices were designed, one with a metal grid for the top and bottom electrode deposited on a highly emissive glass substrate, and another with a top metal grid electrode and a highly reflecting bottom metal electrode layer. Tantalum oxide is used as an ion conductor in both device types. The third device type consists of a polymeric ion conductor. All solid-state constituent layers were grown by either reactive or nonreactive dc or rf magnetron sputtering in a high vacuum environment. Modulation of the emittance is accomplished by reversible insertion of Li ions into polycrystalline WO3 by applying and switching a small voltage across the structure. Spectrally dependent measured reflectance modulation ...

Visible and infrared optical constants of electrochromic materials for emissivity modulation applications

Electrochromic materials are being studied for applications involving infrared emissivity modulation. The materials being investigated are amorphous WO3 and poly-crystalline WO3, NiO, and Ta2O5. Hydrogen ions are intercalated into and deintercalated from the films leading to changes in the optical properties of the materials. Visible and infrared ellipsometry are used to measure the optical constants of these materials in various states of ion intercalation and to determine the reversibility of the reactions. The spectral range for the optical constants determination is from 0.3 to 14 μm. Simulations of electrochromic device structures using these optical constants show an emissivity modulation greater than 50% for wavelengths near the peak of the room temperature blackbody curve.

Modified Forouhi and Bloomer dispersion model for the optical constants of amorphous hydrogenated carbon thin films

Abstract The model of Forouhi and Bloomer (FB) for the optical properties of amorphous semiconductors is modified in order to describe more accurately the dispersion of the optical constants observed for amorphous carbon (a-C) and amorphous hydrogenated carbon (a-C:H) thin films. The FB model represents the optical absorption as the product of a lineshape function and a joint density of states function, which is derived by assuming the condition and valence bands to be parabolic and separated by an energy gap within which there are no allowed electronic states. Two modifications to this model are discussed to address the cases of non-parabolic bands and/or electron energy levels in the energy gap. These modified parametric models are then fit to a large number of a-C and a-C:H film optical constant spectra, and results are presented which indicate that non-parabolicity of the conduction and valence bands is the most important correction to the standard FB model required to describe a-C:H thin films. The modified model incorporating non-parabolic bands is shown to fit a broad range of both a-C and a-C:H spectra very well, and provides useful information about the optical absorption process and physical properties of the films.

All-solid-state electrochromic reflectance device for emittance modulation in the far-infrared spectral region

All-solid-state electrochromic reflectance devices for thermal emittance modulation were designed for operation in the spectral region from mid- to far-infrared wavelengths (2–40 μm). All device constituent layers were grown by magnetron sputtering. The electrochromic (polycrystalline WO3), ion conductor (Ta2O5), and Li+ ion-storage layer (amorphous WO3), optimized for their infrared (IR) optical thicknesses, are sandwiched between a highly IR reflecting Al mirror, and a 90% IR transmissive Al grid top electrode, thereby meeting the requirements for a reversible Li+ ion insertion electrochromic device to operate within the 300 K blackbody emission range. Multicycle optical switching and emittance modulation is demonstrated. The measured change in emissivity of the device is to 20%.

Infrared emittance modulation devices using electrochromic crystalline tungsten oxide, polymer conductor, and nickel oxide

Abstract A prototypical small area electrochromic device was fabricated, and emissivity was measured from 1 to 30 microns. The devices show change in emissivity from about 0.60 to about 0.68, that is a total modulation of 13%. The emittance performance was calculated, based on the reflectivity modulation. One difference between these devices and the more frequently explored visible light transmission devices is the utilization of crystalline tungsten oxide instead of highly disordered amorphous tungsten oxide. The crystalline tungsten oxide and nickel oxide charge storage films are characterized by IR transmission/reflection, and spectroscopic ellipsometry. A theoretical model has been developed which describes the device performance to within 10% of experimental results.

Optical constants of crystalline WO3 deposited by magnetron sputtering

Crystalline WO3−x is an infrared (IR) electrochromic material having possible applications in satellite thermal control and IR switches. Optical constants of electrochromic materials change upon ion intercalation, usually with H+ or Li+. Of primary concern for device design are the optical constants in both the intercalated and deintercalated states. In situ and ex situ ellipsometric data are used to characterize both the deposition process and the optical constants of the films. Ex situ data from a UV-Vis-NIR ellipsometer are combined with data from a mid-infrared Fourier-transform-infrared-based ellipsometer to provide optical constants over a spectral range of 0.031–6.1 eV.

In-situ ellipsometric characterization of the electrodeposition of metal films

The long-term objective of this work is to develop novel material structures and low-cost methods to prepare magneto-optic thin film materials. Ellipsometry offers diagnostics of the nucleation and growth processes on a nanometer-thickness scale. Magnetic Ni metal films were electrodeposited from sulfamate-based solutions using a galvanostatic technique. The deposition process was monitored in-situ using a spectroscopic ellipsometer measuring at 44 wavelengths simultaneously from 410 to 750 nm in real time. The spectral range for good data was limited by the transparency of the solution. Ellipsometric measurements were used to find the optical constants and growth rates for a series of depositions. Measurements of the charge passing through the sample during the growth were also used to monitor growth rates. These results were compared with the growth rates found from ellipsometric measurements. The comparison shows the two techniques to be in reasonable agreement and the advantages of each technique is discussed.

Thin Film Materials Exposure to Low Earth Orbit Aboard Space Shuttle

To study the effects of Atomic Oxygen on various thin film materials, fourteen thin film samples were exposed to the corrosive environment of low Earth orbit. Total exposure was 42 hours, resulting in a nominal atomic oxygen fluence of 2.2 X 1020 atoms/cm2. The films included aluminum, diamondlike carbon, diamond, and multilayer stacks. Included are experimental details of sample preparation, exposure, and post-flight results. Pre-flight characterization techniques included Variable Angle Spectroscopic Ellipsometry, optical reflectance and transmittance, Atomic Force Microscopy, and Raman scattering. Post-flight analysis repeated pre-flight characterization. Aluminum films resisted degradation. Surface contaminants were identified using Auger Electron Spectroscopy. Contaminants were SiC>2, fluorine, and sulfur which most likely result from degradation of cargo bay lining, waste water dumps, and outgassing. Diamondlike carbon films were completely etched away during exposure. Polycrystalline diamond films were extremely resistant to atomic oxygen degradation, showing no post-flight structural, compositional, or mass changes. Aluminum films 23.5 nm thick simultaneously protect silver reflecting layers from oxidation and increase the ultraviolet reflectance of the stack. Decreasing the aluminum thickness to 7.5 nm resulted in complete oxidation during exposure and failure as a protective coating.

Low Earth Orbit Effects on Indium Tin Oxide and Polyester and Comparison With Laboratory Simulations

Abstract Laboratory simulation of the low Earth orbit (LEO) environment using oxygen plasma ashers are discussed. Their effectiveness as space simulators are compared with LEO through analysis of indium tin oxide (ITO) thin films and bulk polyester exposed to both environments. Spectrophotometry and atomic force microscopy have been used to characterize optical and microstructural changes as a result of exposure to the simulated (oxygen plasma asher) and the actual space environment aboard shuttle flight STS-46. Results show that the low Earth orbit space environment is much harsher than the plasma asher on the optical properties of ITO as well as the surface roughness of polyester. On space-exposed samples, a significant shift in the ITO absorption edge is seen for fluences of 2 × 10 20 atoms cm -2 but not on films exposed in the asher. The surface roughness of polyester exposed in the asher increase by a factor of 5.5, while that of polyester exposed in space increases by a factor of 20 for the same atomic oxygen fluence. The directional nature and higher kinetic energy of atomic oxygen in LEO serves to erode polyester more than in the asher. The different results obtained in the asher for both ITO and polyester bring into question the suitability of using plasma ashers as space simulators for these materials.

Low Earth Simulation and Materials Characterization

Oxygen plasma ashers and an electron cyclotron resonance (ECR) sources are currently being used for low Earth orbit (LEO) simulation. The suitability of each of these simulation techniques is considered. Thin film coatings are characterized by optical techniques, including variable-angle spectroscopic ellipsometry, optical spectrophotometry, and laser light scatterometry. Atomic force microscopy (AFM) has been used to characterize the surface morphology of thin aluminum films as a function of substrate temperature during deposition. Results on diamondlike carbon (DLC) films show that DLC degrades with simulated atomic oxygen (AO) exposure at a rate comparable to Kapton polyimide. Since DLC is not as susceptible as Kapton to environmental factors such as moisture absorption, it could potentially provide more accurate measurements of AO fluence on short space flights.