The Pb−Pu (Lead-Plutonium) system

This work was supported by the Department of Energy through the Joint Program on Critical Compilations of Physical and Chemical Data, coordinated through the Office of Standard Reference Data, National Bureau of Standards. Literature searched through 1984. Dr. Peterson is the ASM/NBS Data Program Category Editor for binary actinide alloys.     

Developments in the fabrication and performance of high-quality HgCdTe detectors grown on 4-in. Si substrates

We are continuing to develop our growth and processing capabilities for HgCdTe grown on 4-in. Si substrates by molecular beam epitaxy (MBE). Both short-wave and mid-wave infrared (SWIR and MWIR) double-layer hetero-junctions (DLHJs) have been fabricated. In order to improve the producibility of the material, we have implemented an in-situ growth composition-control system. We have explored dry etching the HgCdTe/Si wafers and seen promising results. No induced damage was observed in these samples. Detector results show that the HgCdTe/Si devices are state-of-the-art, following the diffusion-limited trend line established by other HgCdTe technologies. Focal-plane array (FPA) testing has been performed in order to assess the material over large areas. The FPA configurations range from 128×128 to 1,024×1,024, with unit cells as small as 20 μm. The MWIR responsivity and NEDT values are comparable to those of existing InSb FPAs. Pixel operabilities well in excess of 99% have been measured. We have also explored the role of growth macrodefects on diode performance and related their impact to FPA operability. The SWIR HgCdTe/Si shows similar results to the MWIR material. Short-wave IR FPA, median dark-current values of less than 0.1 e⁻/sec have been achieved.     

Thin film transmission matrix approach to fourier transform infrared analysis of HgCdTe multilayer heterostructures

The ability to achieve high-yield focal plane arrays from Hg_1−xCd_xTe molecular beam epitaxy material depends strongly on postgrowth wafer analysis. Nondestructive analysis that can determine layer thicknesses as well as alloy compositions is critical in providing run-to-run consistency. In this paper, we incorporate the use of a thin film transmission matrix model to analyze Fourier transform infrared (FTIR) transmission spectra. Our model uses a genetic algorithm along with a multidimensional, nonlinear minimization Nelder-Mead algorithm to determine the composition and thickness of each layer in the measured epitaxial structure. Once a solution has been found, the software is able to predict detector performance such as quantum efficiency and spectral response. We have verified our model by comparing detector spectral data to our predicted spectral data derived from the room-temperature FTIR transmission data. Furthermore, the model can be used to generate design curves for detectors with varying absorber thicknesses and/or different operating temperatures. The consequence of this are reduced cycle times and reduced design variations.     

Modeling Energy Recovery Using Thermoelectric Conversion Integrated with an Organic Rankine Bottoming Cycle

Engine and industrial waste heat are sources of high-grade thermal energy that can potentially be utilized. This paper describes a model system that employs thermoelectric conversion as a topping cycle integrated with an organic Rankine bottoming cycle. The model has many parameters that define combined system quantities such as overall output power and conversion efficiency. The model can identify the optimal performance points for both the thermoelectric and organic Rankine bottoming cycle. Key analysis results are presented showing the impact of critical design parameters on power output and system performance.     

Large-Format HgCdTe Dual-Band Long-Wavelength Infrared Focal-Plane Arrays

Raytheon Vision Systems (RVS) continues to further its capability to deliver state-of-the-art high-performance, large-format, HgCdTe focal-plane arrays (FPAs) for dual-band long-wavelength infrared (L/LWIR) detection. Specific improvements have recently been implemented at RVS in molecular-beam epitaxy (MBE) growth and wafer fabrication and are reported in this paper. The aim of the improvements is to establish producible processes for 512 × 512 30-μm-unit-cell L/LWIR FPAs, which has resulted in: the growth of triple-layer heterojunction (TLHJ) HgCdTe back-to-back photodiode detector designs on 6 cm × 6 cm CdZnTe substrates with 300-K Fourier-transform infrared (FTIR) cutoff wavelength uniformity of ±0.1 μm across the entire wafer; demonstration of detector dark-current performance for the longer-wavelength detector band approaching that of single-color liquid-phase epitaxy (LPE) LWIR detectors; and uniform, high-operability, 512 × 512 30-μm-unit-cell FPA performance in both LWIR bands.     

Population-based cohort study of microbial keratitis in Scotland: incidence and features.

OBJECTIVE: To investigate the incidence and features of bacterial, fungal and protozoal keratitis in Scotland. DESIGN: Prospective, population-based cohort study of all persons who developed culture proven microbial keratitis over an 8 month period. SETTING: West of Scotland, UK. SUBJECTS: Approximately 3,000,000 population. Main outcome measures: Incidence and risk factors for microbial keratitis. METHODS: All patients were included who had presumed microbial keratitis from which bacteria, fungi or Acanthamoeba was isolated from the corneal scraping by the hospital laboratory using a standardised protocol. In addition, contact lens wearing patients with pathognomonic features of Acanthamoeba keratitis, who yielded a negative culture result when referred on chlorhexidine therapy, were included if Acanthamoeba could be cultured from their lens storage case. RESULTS: The overall annual incidence of culture-proven microbial keratitis was 0.26 per 10,000 with a rate of 1.8 per 10,000 for contact lens wearers (all types, soft and rigid). Based on a previous pilot study of 'presumed' microbial keratitis in Glasgow, it was possible to estimate the incidence of expected 'presumed' microbial keratitis as 0.36 per 10,000 overall and 2.44 per 10,000 for contact lens wearers (all types). The incidence for Acanthamoeba keratitis was 1.49 per 10,000 soft contact lens wearers; this infection was not detected in the absence of contact lens wear nor with use of gas permeable or rigid contact lenses. CONCLUSIONS: 'Presumed' microbial keratitis from all causes, in the adult population, was approximately three times less common in the West of Scotland (0.36 per 10,000) than would be expected from a comparable retrospective study from Minnesota, USA for the years 1980-1988 (1.1 per 10,000). It was rare (approximately one case expected in 2 million per year) in the absence of pre-existing corneal disease, cosmetic contact lens wear or trauma. Ocular surface disease was the underlying cause predisposing to infection in 58% of cases, with an incidence of 'presumed' keratitis of 0.21 per 10,000 population; the highest incidence was found in the elderly population. Contact lens wear was responsible for 38% of cases, emphasising the importance of preventive hygiene and effective disinfection in this group. The estimated incidence of 'presumed' microbial keratitis in the West of Scotland associated with cosmetic wear (daily and extended use) of soft contact lenses was significantly less (P<0.05) than that expected from a prospective study in New England, America in 1985 (266 per 10,000, rather than 8.05 per 10,000). However, the estimated incidence for presumed microbial keratitis for the West of Scotland associated with wearing soft contact lenses for cosmetic purposes in the daily wear modality (266 per 10,000) was less, but not significantly less, than that found in the prospective American study (4.20 per 10,000). The daily wear mode for contact lenses is almost universal in the West of Scotland, where extended wear has never been recommended. Extended wear has been shown in the USA to be associated with an incidence of presumed microbial keratitis between five and ten times higher than that associated with daily wear. This explains the lower incidence we have observed and a difference with the US study for overall infection rates but not when associated with daily wear alone. The incidence of proven Acanthamoeba keratitis found in the Scottish study among wearers of soft contact lenses for daily wear cosmetic purposes was exceptionally high at 1.49 per 10,000.     

Cross-Sectional Study of Macrodefects in MBE Dual-Band HgCdTe on CdZnTe

HgCdTe dual-band mid-wave infrared/long-wave infrared focal-plane arrays on CdZnTe are a key component in advanced electrooptic sensor applications. Molecular beam epitaxy (MBE) has been used successfully for growth of dual-band layers on larger CdZnTe substrates. However, the macrodefect density, which is known to reduce the pixel operability and its run-to-run variation, is larger when compared with layers grown on Si substrate. This paper reports the macrodefect density versus size signature of a well-optimized MBE dual-band growth and a cross-sectional study of a macrodefect that represents the most prevalent class using focused ion beam, scanning transmission electron microscopy, and energy-dispersive x-ray spectroscopy. The results show that the macrodefect originates from a void, which in turn is associated with a pit on the CdZnTe substrate.     

Impact of Tellurium Precipitates in CdZnTe Substrates on MBE HgCdTe Deposition

State-of-the-art (112)B CdZnTe substrates were examined for near-surface tellurium precipitate-related defects. The Te precipitate density was observed to be fairly uniform throughout the bulk of the wafer, including the near-surface region. After a molecular beam epitaxy (MBE) preparation etch, exposed Te precipitates, small pits, and bumps on the (112)B surface of the CdZnTe wafer were observed. From near-infrared and dark field microscopy, the bumps and small pits on the CdZnTe surface are associated with strings of Te precipitates. Raised bumps are Te precipitates near the surface of the (112)B CdZnTe where the MBE preparation etch has not yet exposed the Te precipitate(s). An exposed Te precipitate sticking above the etched CdZnTe surface plane occurs when the MBE preparation etch rapidly undercuts a Te precipitate. Shallow surface pits are formed when the Te precipitate is completely undercut from the surrounding (112)B surface plane. The Te precipitate that was previously located at the center of the pit is liberated by the MBE preparation etch process.     

Two-dimensional singular vector elements for finite-elementanalysis

The finite-element method (FEM) exhibits a reduced convergence rate when used for the analysis of geometries containing sharp edges where the electromagnetic field is singular. The convergence of the method can be-improved by introducing singular elements that model analytically predicted singular behavior. A number of authors have developed singular elements that are compatible with the scalar FEM. In this paper, we propose a new singular element that is compatible with edge-based vector finite elements and can cope with any order of singularity while preserving the sparsity of the FEM equations. Edge-based singular elements more correctly model singular fields and thus require fewer unknowns, while avoiding the introduction of spurious modes in the numerical solution. Numerical results verify that the convergence of the FEM is significantly improved     

Higher-order vector finite elements for tetrahedral cells

Edge-based vector finite elements are widely used for two-dimensional (2-D) and three-dimensional (3-D) electromagnetic modeling. This paper seeks to extend these low-order elements to higher orders to improve the accuracy of numerical solutions. These elements have relaxed normal-component continuity to prohibit spurious modes, and also satisfy Nedelec's constraints to eliminate unnecessary degrees of freedom while remaining entirely local in character. Element matrix derivations are given for the first two vector finite element sets. Also, results of the application of these basis functions to cavity resonators demonstrate the superiority of the higher-order elements