Engineering topics

Diaz, Emmanuela

Publications and source records attributed to Diaz, Emmanuela.

4 records

DOE OSTI2022

Determining Infrared Optical Constants for Dolomite using Single Angle Reflectance and Spectroscopic Ellipsometry

The optical constants, namely the real (n) and imaginary (k) parts of the complex refractive index, are of particular interest to generate the infrared (IR) spectra of liquid and solid materials in different morphologies. To obtain n/k, however, most materials are typically not found in the monolithic forms necessary to easily measure n/k, and thus require the use of other methods such as pressing powders to form planar, specularly-reflective pellets. In this work dolomite crystals are measured using fixed-angle IR reflectance spectroscopy in both 1) a monolithic form with the crystal fixed in epoxy and polished, but data are also recorded from 2) pressed-pellet forms of powders of the same mineral using the same spectroscopic protocols. First results for the two methods are compared. It was found that the measured reflectance can vary by as much as a factor of two between the dolomite crystals and the pressed powder forms. For the two-sample preparation approaches the preliminary spectra are compared and the implications and limitations of each method for determining the optical constants of given materials are discussed. Comparison to literature data suggest that polarization effects likely account for the differing amplitude results for reflectance (and hence the k -vectors): Dolomite is known to be biaxial with significantly differing ordinary and extraordinary rays; the pressed pellet method measures an ensemble of microcrystals in randomly oriented positions, whereas the single crystal maintains just one orientation relative to the optical axis.

ellipsometry, dolomite, complex refractive index, ↗

DOE OSTI2020

Preliminary parameter studies to form pellets of hard materials for optical constants derived via single-angle reflectance spectroscopy

Infrared reflectance spectra can be influenced by many factors: the substrate and the thickness of the layer for liquids and the surface micromorphology, the form (powder, crystal) and the particle size for solids. All these parameters can play a role in the appearance of the measured spectrum. To avoid collecting multitudes of spectra to cover all such scenarios, the optical constants n and k, which are intrinsic properties of a material, can instead be used to model the reflectance spectrum. For solids, two techniques are often used to derive optical constants: ellipsometry and single-angle reflectance spectroscopy. For both methods, best results are usually obtained from single crystals. We have recently demonstrated for ammonium sulfate (a relatively soft material) that by optimizing certain conditions, high quality pellets with specularly reflective surfaces can be used in lieu of crystals. This was confirmed by the excellent agreement between the optical constants derived by these two methods. This work focuses on the possible extension of these methods to harder materials, starting with sodium sulfate. The first goal is to see if high quality pellet surfaces can be obtained as for ammonium sulfate. The reflectance values and the associated optical constants can also be obtained.

Diaz, Emmanuela↗

DOE OSTI2020

Infrared Optical Constants from Pressed Pellets of Powders: I. Improved n and k Values of (NH4)(2)SO(4)from Single-Angle Reflectance

In combination with other parameters, the real, n(?), and imaginary, k(?), components of the complex refractive index, = n + ik, can be used to simulate the optical properties of a material in different forms, e.g. its infrared spectra. But obtaining reliable values of the optical constants n/k for solid materials is challenging due to the lack of optical quality specimens, usually crystals, large enough to measure. An alternative to crystals is to press the powder into a uniform disk. We have produced pellets from ammonium sulfate, (NH4)2SO4, powder and derived the pellets’ n and k values via single-angle reflectance using a specular reflectance device in combination with a Fourier transform infrared spectrometer. The single-angle technique measures amplitude of light reflected from the material as a function of wavelength over a wide spectral domain; the optical constants are determined from the reflectance data using the Kramers-Kronig relationship. We investigate several parameters associated with the pellets and pellet formation and their effects upon delivering the most reliable n/k values. Parameters studied include pellet diameter, mass and density (void space), drying, grinding, sieving and particle size in the pellet formation, as well as pressing pressure and duration. Of these parameters, using size-selected mixtures of dried, small (< 100 µm) particles and pressing at = 10 tons for at least 30 minutes were found key to forming highly reflective samples. Comparison to two sets of previous literature n(?) and k(?) values obtained from crystalline (NH4)2SO4 both as crystal reflectance as well as extinction spectra of aerosols measured in a flow tube show reasonable agreement, but suggests the present values, as confirmed from two independent techniques, represent a substantial improvement for n/k values for (NH4)2SO4, also demonstrating promise to measure the optical constants of other materials.

Optical constants, index of refraction, single-ang↗

DOE OSTI2020

Impact of pellet preparation on reflectance measurements of ammonium sulfate by single-angle reflectance spectroscopy

The optical constants, n and k, are required to model the reflection, refraction and transmission of light at the first surface interface of a material, as well as its propagation through the material; here n corresponds to the real component and k the imaginary component of the refractive index. Several different spectroscopic methods have been used to determine the n and k values for different materials, including single angle reflectance spectroscopy and ellipsometry. The single-angle reflectance method quantitatively records the specular reflectance R(v) from a plane parallel face of the material and uses the Kramers-Kronig transform to extract the n and k values. For most compounds, however, it is difficult to obtain a single crystal or high-quality window of sufficient planarity and of the appropriate dimensions (several mm) to make the measurement. For this reason, we further investigate the use of pressed pellets of neat powdered substances to measure optical constants of these substances using the single-angle reflectance method. We have found that surface roughness can significantly influence the measured quantitative reflectance spectrum R(v) and, consequently, the derived n and k parameters. A collaborative study between Defence Research and Development Canada (DRDC) - Valcartier Research Center and Pacific Northwest National Laboratory (PNNL) has been carried out using different pellets of neat ammonium sulfate [(NH4)2SO4] to show how parameters such as the relative fraction of particle sizes for the component powder along with the grinding process of the pure compound all can affect the reflectance spectrum used to derive the optical constants. All pressed pellets were characterized by single-angle reflectance spectroscopy.

Diaz, Emmanuela↗