Hollow cathode sustained plasma microjets: Characterization and application to diamond deposition

Extending the principle of operation of hollow cathode microdischarges to a tube geometry has allowed the formation of stable, high-pressure plasma microjets in a variety of gases including Ar, He, and H2. Direct current discharges are ignited between stainless steel capillary tubes (d = 178 µm) which are operated as the cathode and a metal grid or plate that serves as the anode. Argon plasma microjets can be sustained in ambient air with plasma voltages as low as 260 V for cathode-anode gaps of 0.5 mm. At larger operating voltage, this gap can be extended up to several millimeters. Using a heated molybdenum substrate as the anode, plasma microjets in CH4/H2 mixtures have been used to deposit diamond crystals and polycrystalline films. Micro-Raman spectroscopy of these films shows mainly sp3 carbon content with slight shifting of the diamond peak due to internal stresses. Optical emission spectroscopy of the discharges used in the diamond growth experiments confirms the presence of atomic hydrogen and CH radicals

High-pressure micro-discharges in etching and deposition applications

High-pressure micro-discharges are promising sources of light, ions, and radicals and offer some advantages in materials processing applications as compared to other more conventional discharges. We review here results from etching experiments using stencil masks where the discharge is formed only in the pattern cutout. The mask consists of a thin metal-dielectric structure and is pressed against a Si wafer, which becomes part of the electric circuit. Pattern transfer takes place, albeit the profile shape appears to be limited by the expansion of the plasma into the etched hole at long etch times. We also review experiments on using micro-discharges as sources of radicals for materials deposition applications. In the latter case, the micro-discharges form in metal capillary tubes permitting incorporation of gas flow and a short reaction zone that can be controlled to favour production of specific radicals. We demonstrate these concepts by using CH4/H2 chemistry for diamond deposition on a heated Mo substrate. Good quality micro- and nano-diamond crystals could be produced

Argon excimer emission from high-pressure microdischarges in metal capillaries

We report on argon excimer emission from high-pressure microdischarges formed inside metal capillaries with or without gas flow. Excimer emission intensity from a single tube increases linearly with gas pressure between 400 and 1000 Torr. Higher discharge current also results in initial intensity gains until gas heating causes saturation or intensity drop. Argon flow through the discharge intensifies emission perhaps by gas cooling. Emission intensity was found to be additive in prealigned dual microdischarges, suggesting that an array of microdischarges could produce a high-intensity excimer source

Adaptive Pole Placement Control of Nonlinear Systems

Mechanical Engineerin

Magnetic tests for magnetosome chains in Martian meteorite ALH84001

Transmission electron microscopy studies have been used to argue that magnetite crystals in carbonate from Martian meteorite ALH84001 have a composition and morphology indistinguishable from that of magnetotactic bacteria. It has even been claimed from scanning electron microscopy imaging that some ALH84001 magnetite crystals are aligned in chains. Alignment of magnetosomes in chains is perhaps the most distinctive of the six crystallographic properties thought to be collectively unique to magnetofossils. Here we use three rock magnetic techniques, low-temperature cycling, the Moskowitz test, and ferromagnetic resonance, to sense the bulk composition and crystallography of millions of ALH84001 magnetite crystals. The magnetic data demonstrate that although the magnetite is unusually pure and fine-grained in a manner similar to terrestrial magnetofossils, most or all of the crystals are not arranged in chains

Synthesis of Blue Luminescent Si Nanoparticles Using Atmospheric-Pressure Microdischarges

Silicon nanoparticles are synthesized from a mixture of argon/silane in a continuous flow atmospheric-pressure microdischarge reactor. Particles nucleate and grow to a few nanometers (1−3 nm) in diameter before their growth is abruptly terminated in the short residence time microreactor. Narrow size distributions are obtained as inferred from size classification and imaging. As-grown Si nanoparticles collected in solution exhibit room-temperature photoluminescence that peaks at 420 nm with a quantum efficiency of 30%; the emission is stable for months in ambient air

Microplasmas for Advanced Materials and Devices

Microplasmas are low-temperature plasmas that feature microscale dimensions and a unique high-energy-density and a nonequilibrium reactive environment, which makes them promising for the fabrication of advanced nanomaterials and devices for diverse applications. Here, recent microplasma applications are examined, spanning from high-throughput, printing-technology-compatible synthesis of nanocrystalline particles of common materials types, to water purification and optoelectronic devices. Microplasmas combined with gaseous and/or liquid media at low temperatures and atmospheric pressure open new ways to form advanced functional materials and devices. Specific examples include gas-phase, substrate-free, plasma-liquid, and surface-supported synthesis of metallic, semiconducting, metal oxide, and carbon-based nanomaterials. Representative applications of microplasmas of particular importance to materials science and technology include light sources for multipurpose, efficient VUV/UV light sources for photochemical materials processing and spectroscopic materials analysis, surface disinfection, water purification, active electromagnetic devices based on artificial microplasma optical materials, and other devices and systems including the plasma transistor. The current limitations and future opportunities for microplasma applications in materials related fields are highlighted.</p

Formation of nanodiamonds at near-ambient conditions via microplasma dissociation of ethanol vapour

Clusters of diamond-phase carbon, known as nanodiamonds, exhibit novel mechanical, optical and biological properties that have elicited interest for a wide range of technological applications. Although diamond is predicted to be more stable than graphite at the nanoscale, extreme environments are typically used to produce nanodiamonds. Here we show that nanodiamonds can be stably formed in the gas phase at atmospheric pressure and neutral gas temperatures \u3c100 °C by dissociation of ethanol vapour in a novel microplasma process. Addition of hydrogen gas to the process allows in flight purification by selective etching of the non-diamond carbon and stabilization of the nanodiamonds. The nanodiamond particles are predominantly between 2 and 5 nm in diameter, and exhibit cubic diamond, n-diamond and lonsdaleite crystal structures, similar to nanodiamonds recovered from meteoritic residues. These results may help explain the origin of nanodiamonds in the cosmos, and offer a simple and inexpensive route for the production of high-purity nanodiamonds

Ferromagnetic resonance and low-temperature magnetic tests for biogenic magnetite

Magnetite is both a common inorganic rock-forming mineral and a biogenic product formed by a diversity of organisms. Magnetotactic bacteria produce intracellular magnetites of high purity and crystallinity (magnetosomes) arranged in linear chains of crystals. Magnetosomes and their fossils (magnetofossils) have been identified using transmission electron microscopy (TEM) in sediments dating back to ∼510–570 Ma, and possibly in 4 Ga carbonates in Martian meteorite ALH84001. We present the results from two rock magnetic analyses—the low-temperature Moskowitz test and ferromagnetic resonance (FMR)—applied to dozens of samples of magnetite and other materials. The magnetites in these samples are of diverse composition, size, shape, and origin: biologically induced (extracellular), biologically controlled (magnetosomes and chiton teeth), magnetofossil, synthetic, and natural inorganic. We confirm that the Moskowitz test is a distinctive indicator for magnetotactic bacteria and provide the first direct experimental evidence that this is accomplished via sensitivity to the magnetosome chain structure. We also demonstrate that the FMR spectra of four different strains of magnetotactic bacteria and a magnetofossil-bearing carbonate have a form distinct from all other samples measured in this study. We suggest that this signature also results from the magnetosomes' unique arrangement in chains. Because FMR can rapidly identify samples with large fractions of intact, isolated magnetosome chains, it could be a powerful tool for identifying magnetofossils in sediments