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Space missions have unique requirements for payloads of electronics, sensors, instruments, and other components in terms of mass, footprint, power consumption, and resistance to various types of radiation. Nanomaterials offer the potential for future radiation-hardened or radiation-immune electronics. Gas-sensing needs in planetary exploration and crew-cabin air-quality monitoring are currently being met by bulky instruments. Routine health checkups of astronauts and testing of water in space habitats are being done on a delayed basis by bringing samples back to Earth. Instead, nanomaterials can be used to construct ultrasmall, postage-stamp-sized gas/vapor sensors with selective discrimination and also lab-on-a-chip biosensors for water-quality monitoring and crew health monitoring.
Vertically aligned graphene was grown by plasma-enhanced chemical vapor deposition using methane feedstock. Optical emission spectroscopy (OES) was used to monitor the plasma species, and Raman spectroscopy was used for characterizing the properties of as-grown vertically aligned graphene. OES-derived information on plasma species, such as C, C2, CH, and H, are correlated with the properties of the vertically aligned graphene. Graphene grown at 250 W and 15 sccm exhibited the lowest amount of defects. Although OES peak intensities occurred at the highest power and lowest flow conditions, the OES peak ratios of plasma species had a greater dependence on flow rate and exhibited a saddle point in the atomic C/H ratio corresponding to optimal growth involving the lowest amount of overall defects. Plasma diagnostics provides a valuable approach to optimize growth characteristics and material properties.
An array of 32 sensor elements with single-walled carbon nanotubes (SWCNTs) as the sensing medium has been fabricated. The microfabrication approach used allows reduction of the chip size and increases the number of sensor elements in a chip and is amenable for large wafer scale-up. The sensor array chip is designed as an electronic nose for use with the aid of a pattern recognition algorithm. The sensor chips were tested for NO2 sensing and interfering effects from humidity and a background of chlorine. The results indicate that NO2 can be detected at low concentration levels of 0.5 ppm in the presence of chlorine at 30 times higher concentrations. The sensor response is affected by humidity, which implies that the training data set for NO2 detection needs to be generated for multiple humidity levels for interpolation purposes during field use.
Objective: To conduct a study of patients presenting with perichondritis of the auricle and to analyse the possible aetiological factors responsible, the bacteriological agents involved, the treatment modalities and the complications of such infections.
Setting: Academic department of otolaryngology.
Design: A retrospective clinical study of patients treated over a five-year period.
Participants: Sixty-one patients with clinically proven perichondritis of the auricle, with or without diabetes mellitus (i.e. malignant otitis externa).
Results: Based on the severity of the disease, otherwise uncomplicated patients were assigned to group A and divided into three cohorts. Patients with perichondritis secondary to malignant otitis externa were analysed separately as group B. Men formed the majority of the patients and most were young (16–35 years). Trauma was the main cause (46 per cent) and Pseudomonas aeruginosa the most common micro-organism isolated. The condition was managed conservatively with antibiotics alone in 19 patients (31 per cent) and these cases had no residual deformity at follow up (group A, stage one). Incision and drainage was performed in a further 19 patients (31 per cent), resulting in minor residual deformity in one half (group A, stage two). Debridement was performed in 17 patients, and these patients had either gross (29 per cent) or minor residual deformity (71 per cent; group A, stage three). Six patients with perichondritis secondary to malignant otitis externa were managed by wound debridement via a post-auricular approach; all had minor residual deformities.
Conclusions: Perichondritis can be divided into two groups, depending on cartilage loss and on the presence or absence of malignant otitis externa. The treatment used and the residual deformity that will ensue are entirely dependent on the stage of disease.
The key hurdle in nanoscience and nanotechnology is the large-scale integration of nanoscale materials with micron scale electronics and structures to form functional devices and sensors. We have developed an innovative bottom-up wafer scale fabrication method that combines nanopatterning and nanomaterials synthesis with traditional silicon micromachining technologies. We have achieved nano-micro integration through catalyst nanopatterning and registration at wafer scale and through effective nanocatalyst protection and release before and after microfabrication. Our wafer scale fabrication process has produced 244 carbon nanotube (CNT) probes per 4-inch silicon wafer with control over the CNT location, diameter, length, orientation, and crystalline morphology. CNT probes with diameters of 40-80 nm and lengths of 2-6 μm are found to be functional nano probes for atomic force microscopy (AFM) imaging. In this paper, we will address our nano probe design and fabrication considerations in detail. CNT tip locations and diameters are defined by e-beam lithography. CNT length, orientation, and crystalline quality are controlled by the plasma enhanced chemical vapor deposition (PECVD) method. With effective catalyst protection schemes, this fabrication process is very similar to the conventional approach for fabricating wafer-scale silicon AFM probe tips. Process control is feasible and the overall yield is greatly improved. Our method and technology can be easily adapted to many other nanomaterials (nanotubes and nanowires) synthesis and processes for their rational design, fabrication, and integration in their applications.
State-of-the-art ICs for microprocessors routinely dissipate power densities
on the order of 50 W/cm2. This large power is due to the
localized heating of ICs operating at high frequencies, and must be managed
for future high-frequency microelectronic applications. Our approach
involves finding new and efficient thermally conductive materials.
Exploiting carbon nanotube (CNT) films and composites for their superior
axial thermal conductance properties has the potential for such an
application requiring efficient heat transfer. In this work, we present
thermal contact resistance measurement results for CNT and CNT-Cu composite
films. It is shown that Cu-filled CNT arrays enhance thermal conductance
when compared to as-grown CNT arrays. Furthermore, the CNT-Cu composite
material provides a mechanically robust alternative to current IC packaging
Carbon nanotube (CNT) related nanostructures possess remarkable electrical, mechanical, and thermal properties. To produce these nanostructures for real world applications, a large-scale controlled growth of carbon nanotubes is crucial for the integration and fabrication of nanodevices and nanosensors. We have taken the approach of integrating nanopatterning and nanomaterials synthesis with traditional silicon microfabrication techniques. This integration requires a catalyst or nanomaterial protection scheme. In this paper, we report our recent work on fabricating wafer-scale carbon nanotube AFM cantilever probe tips. We will address the design and fabrication considerations in detail, and present the preliminary scanning probe test results. This work may serve as an example of rational design, fabrication, and integration of nanomaterials for advanced nanodevice and nanosensor applications.
Poly (methyl methacrylate)/single walled carbon nanotube (PMMA/SWNT) composites were polymerized in the presence of carbon nanotubes via three methods: heat, uv radiation and ionizing radiation (gamma). Samples were solvent processed and cast into films. Thin films with varying degrees of transparency resulted from these composites. Differential Scanning Calorimetry (DSC) characterized glass transition temperatures. Ultraviolet-visible spectroscopy (UV-VIS) quantified the transparency of composites. The dielectric constant (ε') was obtained from Dielectric Analysis (DEA) and correlated to the refractive index values using Maxwell's Relationship. Scanning Electron Microscopy (SEM) provided images of the polymer- nanotube composite.
Single-wall carbon nanotube (SWNT)/poly(methyl methacrylate) (PMMA) composites were fabricated and exposed to ionizing radiation for a total dose of 5.9 Mrads. Neat nanotube paper and pure PMMA were also exposed for comparison, and nonirradiated samples served as controls. A concentration of 0.26 wt% SWNT increased the glass transition temperature (Tg), the Vickers hardness number, and modulus of the matrix. Irradiation of the composite did not significantly change the Tg, the Vickers hardness number, or the modulus; however, the real and imaginary parts of the complex permittivity increased after irradiation. The dielectric properties were found to be more labile to radiation effects than mechanical properties.
The purpose of this research was to probe nanotube-polymer composites for evidences of radiation induced chemistry at the interface of the host polymer and the nanotube structures. Single wall carbon nanotube (SWNT) / poly (methyl methacrylate) (PMMA) composites were fabricated and exposed to gamma radiation with a Co60 source at a dose rate of 1.28 X 106 rad/hour in an air environment for a total dose of 5.9 Mrads. Neat nanotube paper and neat PMMA were also exposed. Spun coat films of SWNT/PMMA were exposed to gamma radiation with a Ce157at a dose rate of 4.46 x 103 rad/hr for a total dose of 3.86 Mrads. Both irradiated and non-irradiated samples were compared. Glass transition temperatures were characterized by differential scanning calorimetry. Dynamic mechanical analysis and dielectric analysis evidenced changes in relaxations induced by irradiation. Irradiated composites exhibited radiation induced chemistry distinct from degradation effects noted in the pure polymer. Scanning electron microscopy provided images of the SWNTs and SWNT/PMMA interface before and after irradiation. This investigation imparts insight into the nature of radiation induced events in nanotubes and nanocomposites.
A simple analysis is provided to determine the characteristics of an electron cyclotron resonance (ECR) plasma source for the generation of active nitrogen species in the molecular beam epitaxy of III-V nitrides. The effects of reactor geometry, pressure, power, and flow rate on the dissociation efficiency and ion flux are presented. Pulsing the input power is proposed to reduce the ion flux.
Magnetron reactive ion etching is an attractive alternative to reactive ion etching since it has the potential for producing minimal surface damage while still retaining the advantages of reactive ion etching. We report here the results of a study of GaAs magnetron ion etching using Freon-12 and silicon tetrachloride etch gases. Differences are found in etch profiles and surface region characteristics of GaAs samples etched by the two gases. The relevant mechanisms are discussed.
Using a magnetic field to confine the plasma closer to the cathode has been shown to be advantageous in dry etching technology since this yields a high degree of ionization at low pressures. We report here the results of a study of magnetron reactive ion etching of GaAs using a freon discharge. Various characterization techniques have been employed to understand the etching process and identify the extent of surface damage. The results show that magnetron etching is capable of yielding high etch rates with low damage.
Simulation of if discharges through numerical solutions to the Moments of the Boltzmann Transport Equations (MBTE) is discussed. Continuity and momentum equations for the electrons and ions, and electron energy equation have been solved using an efficient finite difference scheme. Results for a 13.56 MHz argon discharge are presented.
Magnetron reactive ion etching has been receiving much attention since it offers low pressure, and low bias etching conditions with little damage. We have developed a process model for this process and present simulation results for boron trichloride etching of GaAs. The computed etch rates are uniform in the center with higher rates at the edges of the wafer. Flow rates and pressure can be optimized to improve uniformity. The etch rates with the aid of the magnetron are shown to be higher than the rates tor conventional reactive ion etching.
Gas bubble motion in a temperature gradient was observed in a sodium borate melt in a reduced gravity rocket experiment under the NASA SPAR program. Large bubbles tended to move faster than smaller ones, as predicted by theory. When the bubbles contacted a heated platinum strip, motion virtually ceased because the melt only imperfectly wets platinum. In some cases bubble diameter increased noticeably with time.
Theory and ground based studies of bubble behavior in a fluid in the presence of a temperature gradient strongly indicate the action of a thermocapillary force which causes the bubbles to move. This'phenomenon been considered in the traditional treatments of glass fining. To demonstrate that the observed motion conformed to theoretical prediction it was necessary to perform the experiment under low gravity conditions. NASA's SPAR program provided an excellent opportunity to do this.
A sodium borate melt containing a specific bubble array was subjected to a well defined temperature gradient for more than 4 minutes. The sample was contained in a platinum/ fused-silica cell which permitted photographic coverage of the experiment. Photographs were taken at one second intervals during the course of the experiment. They clearly show that the bubbles move toward the hot spot on the platinum heater strip. The observed motion is consistent with the theoretical predictions for the temperature gradients parallel and perpendicular to the heater strip.
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