Gary L. Workman

Effects of Gravity on Processing Heavy Metal Fluoride Fibers

The effects of gravity on the crystal nucleation of heavy metal fluoride fibers have been studied in preliminary experiments utilizing NASAs KC-135 reduced gravity aircraft and a microgravity sounding rocket flight. Commercially produced fibers were heated to the crystallization temperature in normal and reduced gravity. The fibers processed in normal gravity showed complete crystallization while the fibers processed in reduced gravity did not show signs of crystallization.

Effects of Gravity on ZBLAN Glass Crystallization

Abstract: The effects of gravity on the crystallization of ZrF4‐BaF2‐LaF3‐AlF3‐NaF glasses have been studied using the NASA KC‐135 and a sounding rocket. Fibers and cylinders of ZBLAN glass were heated to the crystallization temperature in unit and reduced gravity. When processed in unit gravity the glass crystallized, but when processed in reduced gravity, crystallization was suppressed. A possible explanation involving shear thinning is presented to explain these results.

Effects of microgravity on ZBLAN optical fibers utilizing a sounding rocket

Samples of ZBLAN optical fiber were heated to the pulling and crystallization temperature in microgravity aboard a sounding rocket and on the ground at 1g. This was done in order to better understand the effects of gravity on the crystallization behavior of ZBLAN fibers. Samples heated in 1g at both temperatures crystallized. Samples heated to the crystallization temperature in microgravity were contaminated with water upon re-entry. Samples heated to the pulling temperature showed no evidence of crystallization in microgravity.

Effects of Microgravity on Crystallization of ZBLAN Optical Fibers

ZrF4-BaF2-LaF3-ALF3-NaF (ZBLAN) optical fiber was flown on board NASAs KC-135 aircraft to determine the effects of microgravity on crystal growth in this material. Fiber samples were placed in evacuated quartz ampoules and heated to the crystallization temperature in 0-g and on the ground in 1-9. The 1 -g samples had many regions of crystallites, while the 0-g samples showed no evidence of crystallization.

Study of the effect of gravity on ZBLAN glass as a commercial program

It is the intent of this study to utilize microgravity processing of glass materials composed of ZrF4-BaF2-LaF3-AlF3-NF (ZBLAN) to provide an infrared transmitting optical glass with reduced microcrystallites. Independent fiber annealing experiments using parabolic flights and sounding rockets performed by a Canadian group and our group in the United States have demonstrated that a reduction in nucleation does occur in reduced gravity. The commercial aspects of the ZBLAN program are to scale up to process preforms and to pull fibers in space. These experiments are in the developmental stage at this time. The end result will lead to the development of a process whereby ZBLAN fibers with improved optical transmission for commercial applications are achieved. Commercial participation includes Infra-red Fiber Systems, Galileo, Lucent Technologies (Bell Labs), and Corning Glass.

Study of SRM Critical Surfaces Using Near Infrared Optical Fiber Spectrometry

The measurement and control of cleanliness for critical bonding surfaces during manufacturing provides a unique challenge in the current thrust for the use of environmentally benign processes. Of particular interest has been work performed in maintaining quality in the production of bonds in aerospace propulsion systems and the identification of possible contaminants which are detrimental to the integrity of the bondline. This work requires an in-depth study of the possible sources of contamination, methodologies to identify contaminants, discrimination between contaminants and chemical species caused by environment, and the effect of particular contaminants on the bondline integrity of the critical bonds. Current research explores the use of near infrared (NIR) optical fiber spectrometry for process monitoring of materials used in aerospace systems. Characterization of contaminants on critical bondlines for aerospace materials such as D6AC steel, aluminum alloys and graphite epoxy composites will be presented. Experiments include quantitative measurement of silicone and Conoco HD2 grease, metal hydroxides, and tape residues on solid rocket motor surfaces.

Use of space to commercially produce ZBLAN optical fibers

Construction of the International Space Station Alpha (ISSA) will provide a platform not only for materials research but also a possible means to produce products in space which cannot be easily produced on the ground. Some products may even be superior to those now produced in 1g due to the lack of gravity induced convection effects. One such product is ZrF4‐BaF2‐LaF3‐AlF3‐NaF (ZBLAN) glass. At the present time this material is being produced on earth in fiber optic form for use in surgical lasers and fiber optic lasers. High attenuation coefficients, however, have kept this material from being used in other applications such as long haul data transmission links. The high attenuation is due to impurities which can be removed through improved processing techniques and crystals which can only be removed or prevented from forming by processing ZBLAN in a microgravity environment.

Commercial production of heavy metal fluoride glass fiber in space

International Space Station Alpha (ISSA) will provide a platform not only for materials research but also a possible means to produce products in space which cannot be easily produced on the ground. Some products may even be superior to those now produced in unit gravity due to the lack of gravity induced convection effects. Our research with ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN glass) has shown that gravity does indeed play a major role in the crystallization behavior of this material. At the present time ZBLAN is being produced on earth in fiber optic form for use in surgical lasers and fiber optic lasers among other applications. High attenuation coefficients, however, have kept this material from being used in other applications such as long haul data transmission links. The high attenuation coefficients are due to impurities which can be removed through improved processing techniques and crystals which can only be removed or prevented from forming by processing in a reduced gravity environment.

Inspection tools for aerospace critical surfaces

The measurement and control of cleanliness for critical surfaces during manufacturing and in service operations provides unique challenges in aerospace. For re-usable propulsion systems, such as the solid rocket motors, the current thrust for environmentally benign processes has had a major impact on programs designed for maintaining quality in the production of bondline surfaces. The major goal is to improve upon our ability to detect and identify possible contaminants which are detrimental to the integrity of the bondline. This effort requires an in-depth study of the possible sources of contamination, methodologies to detect and identify contaminants, discriminate between contaminants and chemical species caused by environmental conditions, and the effect of particular contaminants on the bondline integrity of the critical surfaces. This presentation will provide an overview of several optical methods used to detect and identify contamination on critical surfaces, currently being performed by the Surface Contamination and Analysis Team at Marshall Space Flight Center. The methods under development for contamination monitoring include FTIR and Near-IR SPectrometry, UV Fluorescence, and Variable Angle Spectroscopic Ellipsometry. Comparisons between these methods and the current primary tool, optical stimulation of electron emission for on-line inspection will be presented. Experiments include quantitative measurement of silicone and Conoco HD2 greases, metal hydroxides, tape residues, etc. on solid rocket motor surfaces.

Building better flaws for thermographic qualification and quantification

The concurrent inspection of calibrated test panels manufactured with artificially created, although realistically behaving flaws is essential to providing confidence in the thermographic inspection process of advanced composite structure. For honeycomb type composite structures of principle interest is identifying delamination and disbond type defects along the bondline between the core and faceplate, as well as within the faceplate itself. To ensure that these types of flaws will be caught during the inspection cycle of a structural component the test panels must have similar behaving artificial defects. A common practice for the manufacture of artificial flaws in test panels is the use of embedded Teflon tape, or other release agents, for force an unbond condition within the laminate. These procedures though, yield results that are questionable, since one is not sure whether or not the inspection process is identifying the unbond or the inserted materials. Several fabrication methods are compared and contrasted in this paper, for controlling the degree of disbond to simulate defects resulting from mishandling or manufacturing errors without the need for inserting foreign materials in the laminate. These results are also compared to those obtained by inspecting a composite inter-tank test structure which used Teflon tape as the means to simulate critically sized defects.