Robert L. Parkhill

Passivation of metal alloys using sol–gel-derived materials — a review

Abstract The sol–gel method is being investigated as an environmentally compliant alternative for chromate-based conversion coatings currently in use. An overview of recent advances in the use of sol–gel derived coatings for improved corrosion resistance of aluminum and steel metal surfaces is given.

Optical limiting and nonlinear absorption of excited states in metalloporphyrin-doped sol gels

Optical limiting and upconverted luminescence have been studied in different chromophore-doped aluminosilicate gel materials and the effects of the dynamic processes from different central ions are discussed. Chromophores are metalloporphyrins of tetra-4-sulfonatophenylporphyrinato-copper(II) and tetra-4-sulfonatopfienylporphgrinato-zinc(II) (CuTPPS and ZnTPPS, respectively) which are embedded in aluminosilicate gel hosts. Photo-upconverted luminescence was observed and believed to result from the radiative recombination of the second excited singlet state. Saturated behavior of upconverted emission was found as a function of incident excitation intensity. An optical limiting phenomenon associated with reverse saturated absorption was studied. These effects are attributed to higher excited state, absorption. The observation of upconverted luminescence indicates that the upper excited state absorption occurs while nonlinear absorption is being observed. Dynamic analysis of excited state processes indicates that intersystem crossing and triplet-triplet (quartet-quartet) absorption dominates nonlinear processes and suggests that saturation absorption or reverse saturation absorption is dependent on the ratio of the effective absorption cross section for the excited states to that for the ground state. Absorption cross sections for the excited states are estimated by the data fitting to upconverted emission intensity and nonlinear absorption.

Optical limiting and upconverted luminescence in metalloporphyrin-doped sol-gels

In this paper, optical limiting and upconverted luminescence have been demonstrated in metalloporphyrin of ZnTPPS-doped aluminosilicate sol-gel hosts. The photo-upconverted radiative emission was observed and believed to result from the radiative recombination of the second excited singlet state. Intensity of the upconverted luminescence was found to be saturated as a function of excitation intensity. Optical limiting associated with reverse saturable absorption was observed. These effects are attributed to the higher excited state absorption. A method for determining absorption cross-section of excited states was developed by using dynamic calculation to fit the intensity dependence of upconverted intensity. The absorption cross sections of the excited singlet and triplet state were estimated.

Surface texturing of aluminum alloy 2024-T3 via femto- and nanosecond pulse excimer laser irradiation

Surface texturing effects of aluminum alloy 2024-T3 using femtosecond and nanosecond pulse laser irradiation were studied. The micrographs of the scanning electron microscopy (SEM) were characterized as a function of incident laser fluence. Results indicated that the surface features, ranging from nano to microdimension, can be developed through variation in laser fluence intensities. Two ablation regimes in the logarithmic fluence dependence of the ablated depth for the 500-fs-pulse irradiation were observed. The theoretical analysis for ablation processes is in good agreement with the experimental results.

Surface texturing of aluminum alloy 2024 via excimer laser irradiation

Excimer laser irradiation has been shown to yield unique results when used to process metallic surfaces. We have investigated surface texturing effects on 2024 aluminum alloy specimens. A pulsed XeCl excimer laser was used to generate irradiation fluences ranging from well below to substantially above the ablation threshold. Our results indicate that novel micron-dimensioned surface texturing is achieved over selected fluence regimes. At elevated fluences, texturing is lost due to the extensive surface- melting process. Under certain conditions, sub-micron dimensioned surface structures were generated.

Rapid prototyping of electronics via direct writing and laser processing

High-quality electronic circuit elements are being written directly onto substrates with relatively low temperature tolerances by means of a pen-dispensing process. Conductor, resistor, and dielectric paste formulations are deposited onto the substrate, then thermally treated with a laser to reach final form. The laser-treatment process is facilitated by using optical pyrometer temperature measurements as real-time feedback for laser power control. The paste deposition and laser processing technologies have been incorporated into the development of a machine tool for conducting rapid prototyping of electronic circuits deposited onto conformal substrates.High-quality electronic circuit elements are being written directly onto substrates with relatively low temperature tolerances by means of a pen-dispensing process. Conductor, resistor, and dielectric paste formulations are deposited onto the substrate, then thermally treated with a laser to reach final form. The laser-treatment process is facilitated by using optical pyrometer temperature measurements as real-time feedback for laser power control. The paste deposition and laser processing technologies have been incorporated into the development of a machine tool for conducting rapid prototyping of electronic circuits deposited onto conformal substrates.

CAD/CAM for MEMS and BioMEMS

Novel devices can be relatively simple in theory and modeling, but difficult and many times unfeasible to fabricate in a traditional cleanroom environment. We have developed a CAD/CAM tool capable of integrating multiple materials in the electronic, photonic, and biological regimes for applications in both MEMS and BioMEMS devices. Some materials are known and more fully characterized, such as thick film resistors or conductors, while other materials such as biodegradable scaffolding are new but showing promise to realize heterogenous tissue engineered constructs and drug delivery devices. The tool does not discriminate, but rather places these materials in specified locations with precision volumetric control, gently, conformally, and in 3-D. This paper will describe the enabling aspect of true 3-D maskless fabrication as well as describe multiple device structures and demonstrations.

Direct-write optical waveguides

Multimode optical waveguides have been fabricated by dispensing photopolymers onto various substrates using direct-write technologies. Fine-tuning is achieved by micromachining the waveguides using a femtosecond-regime pulsed laser. Propagation losses are determined using the cutback method. A 2 × 2 coupler and a 1 × 8 splitter are demonstrated.

Femtosecond studies of highly excited electrons and surface modification in metals

Interaction of highly excited electrons in nonequilibrium states with the lattice in metals has been studied using femtosecond pulse lasers. Two ablation regimes are identified as the optical and energy penetration by examining the fluence dependence of ablated depth per pulse. Surface structuring and surface compositioning of metal substrates using a femtosecond pulsed laser irradiation is characterized with the SEM images as a function of laser fluence. Laser ablation provides a mechanism to facilitate an achievement of a nano- and microstructuring on the metal surface. Based on our results, ultrashort pulse provides wide competitive range of applications in surface structuring and patterning from nano- to microdimensional scales.

Direct-Write Polymer Diodes

Conjugated polymer Schottky diodes have been directly written onto glass and alumina substrates. The rectifying contact was made between the p -type semiconductor poly(3,4-ethylene-dioxythiophene) doped with polystyrene sulfonic acid and Zn. The devices exhibit rectification ratios in the range of 10 2 :1 to 10 3 :1 at biases of ±1 V and ±10 V, respectively. The devices demonstrate rectification at frequencies up to 250 kHz. A full-wave bridge rectifier circuit written onto a glass substrate converts ac to dc up to 250 kHz and ±5 V.