Room-temperature ablation of a composite graphite-metal target, at 248 nm, in O2, produces large-channeled (50 - 150 nm) multiwalled carbon nanotubes. We find that the formation of these carbon nanotubes is dependent on the ambient gas employed during ablation. Such structures are not produced in inert atmospheres of Ar or in high vacuum. High-resolution, in-situ, time-resolved emission spectroscopy has been used to track the evolution of species (C2, C3, Ni/Co) in the ablation plume, in different ambient gas atmospheres. Spectral fits on low and high-resolution spectra reveal time-dependent vibrational-rotational temperatures for C2 that are different in O2 compared to Ar. Spectral modeling shows that the vibrational-rotational temperatures for C2 produced in O2 remain at ~ 5000 K for nearly 20 &mgr;sec, but drop rapidly in Ar. The key role of exothermic reactions occurring in the plume, and that of radiative cooling will be discussed.
Titanium carbide (TiC) thin films have very desirable properties that make them ideally suited for applications to MEMS devices. TiC has been one of the preferred coatings for improving the performance of macroscopic moving mechanical components due to its established wear-resistance. Pulsed laser deposition (PLD) has been an excellent method for the deposition of TiC because unlike any other deposition process for TiC, PLD offers the capability of producing high-quality films even at room-temperature. Using a patented PLD technique, especially designed and optimized for the deposition of high-hardness, particulate-free films, we have deposited TiC coatings on a variety of surfaces, including Si and several MEMS compatible film-layers. Our results have demonstrated that TiC coatings also offer a high wear-resistance to Si surfaces. This, together with the excellent chemical and mechanical properties, and thermal stability of our PLD TiC coatings, has led to our application of TiC to moving MEMS devices fabricated from Si. The fabrication of Si MEMS devices is quite well established, however, the reliability and performance of MEMS devices such as microgears, micromotors and microactuators, which involve sliding Si surfaces, remain an open question. The short functional life of these devices is attributed to the excessive wear rate of Si induced by high friction. The work presented here describes a hybrid process whereby PLD is used in conjunction with the Aerospace MEMS fabrication process (AIMMOS) for inserting TiC coatings into critical interfaces in MEMS devices that would involve sliding contact between two Si surfaces. The PLD of TiC, the spectroscopy of the plume, and the properties and applications of PLD-TiC for MEMS will be discussed.
Advanced materials are being designed and tested for use on ball bearings that have wide-ranging applications in almost any type of spacecraft. There has been considerable interest in 'hard' or wear-resistant coatings for protecting steel surfaces present in bearing components. Titanium carbide (TiC) has received serious consideration as a wear-resistant coating material that could be suitable for use in such applications. At present, the commercially available process for the deposition of TiC involves heating the steel substrates to fairly high temperatures. High-temperature coating deposition is not desirable for applications involving steel substrates as it results in a softening of the steels. This further necessitates post-deposition heat- treatments for re-hardening the steel and re-polishing the coating. This paper will describe the use of Pulsed Laser Deposition (PLD) to deposit high-quality thin films of TiC on bearing steels at room temperature. Such a process eliminates the problems associated with high temperature deposition, and the costs and complexities involved in the post-deposition heat treatment of steels. To develop an understanding of the deposition process, the plasma generated by laser ablation has been investigated using time-resolved emission spectroscopy. The PLD of TiC films on bearing steels, the material properties of these films, and the spectroscopy of the ablated plume will be discussed.
A fiber optic chemical dosimeter has been developed for use in the remote detection of vapors of toxic amine rocket fuels (hydrazine and its substituted derivatives) that are used at U.S. Air Force and civilian launch sites. The dosimeter employs a colorimetric indicating reagent immobilized in a porous sol-gel cladding on multimode fiber. This reagent reacts selectively with the fuel vapor to produce a strongly absorbing cladding that introduces light propagation losses in the fiber; these losses indicate the presence of hydrazine (N2H4) vapor. The absorption occurs over a broad spectral range ideally suited for interrogation by semiconductor diode lasers. We have shown that the dosimeter yields an average hydrazine detectivity of 2.3 ppb-hr with a standard deviation of 1 ppb-hr, a value that meets U.S. Air Force current detection requirements. Prolonged exposures of the dosimeter to laboratory air have not adversely affected the dosimeter. Additionally, its response to ammonia vapor has been determined to be 9200 times smaller than its response to hydrazine vapor.
Fiber optic chemical dosimeters are being developed for use in the remote detection of toxic rocket propellant vapors, (hydrazine and its derivatives, and nitrogen tetroxide) that are used at Air Force and civilian rocket launch sites. The dosimeters employ colorimetric indicators that react selectively and irreversibly with the propellant vapors to yield chemical compounds that absorb laser light launched into a fiber optic network. The dosimeters are fabricated by dispersing the reagent within either a porous cladding or a porous distal end coating, that is prepared by a low temperature sol-gel technique. Remote field- scale detection of hydrazine vapor in a few hundreds of ppb-min integrated dose regime has been demonstrated with a network that is approximately equals 1 kilometer in length and the use of a low power (10 mW) diode laser. We have also assembled a computer model of a multimode fiber optic dosimeter network for prediction of the operational capabilities of a multiplexed system containing 100 dosimeters. The model was encoded in both spreadsheet and BASIC formats. It was used to evaluate the performance of a field-scale, remote fiber optic detection system incorporating discrete chemical vapor dosimeters in serial, parallel, or hybrid serial/parallel topologies. Additionally, we have begun exploratory work utilizing chemical reagents that react reversibly with hydrazine vapor to develop hydrazine vapor concentration sensors that could be deployed in a similar fashion on a remote fiber optic network to detect hydrazine vapor in the ppb regime.
Excimer laser ablation is used as an effective surface cleaning technique for polyimide layers used in electronic packaging applications. Experiments are reported at 193 nm and 248 nm. At each of these wavelengths, ablations of the contaminated polyimide surface were carried out as a function of incident laser fluence. Quantitative determinations of contaminants present in the polyimide were made using surface analytical techniques. It is clearly demonstrated that excimer laser radiation at both 193 and 248 nm can be used to selectively remove trace metal contaminants from polyimide surfaces leaving intact the fine-line metallizations patterned on them. However, differences in the ablation thresholds, as well as in the ablation rates, are observed at these two wavelengths.
Excimer laser radiation at 193 and 248 nm has been used for the maskless and selective removal of chromium (Cr) contaminants present on polyimide surfaces with fine-line gold (Au) metallization features patterned on them. Excimer laser cleaning has been successful in restoring the dielectric performance of these polyimide surfaces. The differences in ablation thresholds and rates and the respective process windows, established at 193 and 248 nm, for the removal of Cr are discussed.
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