Time-dependent contact behavior between diamond and a CNT turf.

The elastic and adhesive properties of nominally vertically aligned carbon nanotube (CNT) turfs have been measured using nanoindentation. The perceived stiffness of a CNT turf is dependent on the unloading rate, which decreases at slower unloading rates. Depth-controlled nanoindentation was used to examine adhesion effects. Adhesive loads between the turf and the probe tip increased as the time the tip is in contact with the turf increased. As these effects could be from either more tubes coming into contact with the tip due to relaxation and motion of CNTs relative to one another or each tube in contact increasing its adhesive behavior and sub-contact stiffness due to tube-tube interactions within the turf, electrical resistance measurements during nanoindentation were carried out. When the tip is held at a fixed nominal depth, the current remains constant while the contact load decreases, suggesting the number of tubes in contact with the tip stays constant with time while the relaxation mechanisms in the turf occur at positions lower than the contact surface. These observations, in conjunction with in situ TEM compression test of CNT arrays, are used to describe the relative effects the various length and time scales may have on the perceived properties measured during experiments, including elastic modulus and adhesion for gecko-like dry adhesives.

Adhesive properties of some fluoropolymer binders with the insensitive explosive 1,3,5-triamino-2,4,6-trinitrobenzene (TATB).

Adhesion between binders and explosive crystals is of critical importance for the mechanical performance of plastic-bonded explosives (PBXs). The surface properties of several prospective binders have been determined from static advancing contact angle measurements. The surface energies have been used to calculate theoretical work of adhesion to 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), a common insensitive high explosive. The fluorinated terpolymer Oxy-461™, and Kel-F™ chlorotrifluoroethylene-vinylidene fluoride copolymers show the greatest potential for wetting TATB surfaces, and should promote the best adhesion to TATB in PBX formulations. In general, none of the fluoropolymer binders investigated here exhibit markedly superior adhesion to TATB. Thus, bulk physical properties are likely to be more important when choosing a binder.

The transition between real and complex superconducting order parameter phases in UPt3.

Order parameter symmetry is one of the basic characteristics of a superconductor. The heavy fermion compound UPt3 provides a rich system for studying the competition between superconductivity and other forms of electronic order and exhibits two distinct superconducting phases that are characterized by different symmetries. We fabricated a series of Josephson tunnel junctions on the as-grown surfaces of UPt3 single crystals spanning the a-b plane. By measuring their critical current, we mapped out the magnitude of the superconducting order parameter as a function of the momentum-space direction and temperature. In the high-temperature phase, we observed a sharp node in the superconducting gap at 45 degrees with respect to the a axis; an out-of-phase component appeared in the low-temperature phase, creating a complex order parameter.

Evaluation of a thermal interface material fabricated using thermocompression bonding of carbon nanotube turf.

In this work a thermal interface material fabricated by thermocompression bonding of vertically aligned carbon nanotube turf (VACNT) to metallized substrates was characterized. The VACNT structure was fabricated onto silicon substrates using chemical vapor deposition. The structures were then transferred to metallized substrates using thermocompression bonding. The resulting structure consisted of VACNT turf sandwiched between two layers of Au. Two configurations of VACNT, full coverage and patterned, were fabricated and tested. In addition, the thermal interface resistance of structures at intermediate steps in the thermocompression bonding process were measured. For the full coverage turf a thermal interface resistance of 1.082 cm(2) degrees C W(-1) at an applied load of 1 N was measured, while a thermal interface resistance of 0.044 cm(2) degrees C W(-1) at a load of 1 N was measured for the patterned turf configuration.

Thermocompression bonding of vertically aligned carbon nanotube turfs to metalized substrates.

Vertically aligned carbon nanotube turfs (VACNTs), consisting of entwined, nominally vertical carbon nanotubes, are being proposed for use as electrical and thermal contact materials. Issues in their implementation include high contact resistance, the van der Waals interactions of carbon nanotubes, and a low temperature limit during processing. One route for circumventing the 750 degrees C temperatures required for VACNT growth using chemical vapor deposition is for the VACNTs to be grown separately, and then transferred to the device. A method of mechanical transfer, using thermocompression bonding, has been developed, allowing dry mechanical transfer of the VACNTs at 150 degrees C. This method can be used for the construction of both a thermal switch or a permanent conducting channel. The conductivity of the bonded structure is shown to be independent of the imposed strain, up to strains in excess of 100%.

The coordinated buckling of carbon nanotube turfs under uniform compression.

Complex structures consisting of intertwined, nominally vertical carbon nanotubes (CNTs), referred to as turfs, have unique properties that arise from their complex nanogeometry and interactions between individual CNT segments. For applications such as contact switches for electrical or thermal transfer it is necessary to understand the properties that arise from the collective behavior of an assemblage of CNTs rather than the properties of a single tube. In this study, the mechanical response of turfs bonded to substrates under compressive loading is demonstrated experimentally; coordinated alignment and buckling takes place under uniform loads. The mechanical response of turf structures provides some surprising results regarding parameters that control permanent deformation and buckling in assemblages of nanostructures; buckling of the turf structure is controlled by the height and effective modulus of the turf, but not the aspect ratio of the structure. We present and verify a model which describes the coordinated buckling phenomena relevant for applications such as CNT turfs for thermal transfer media.

Oxygen on Ganymede: laboratory studies.

To test proposals for the origin of oxygen absorption bands in the visible reflectance spectrum of Ganymede, the reflectance of condensed films of pure oxygen (O2) and O2-water mixtures and the evolution of O2 from the films as a function of temperature were determined. Absorption band shapes and positions for oxygen at 26 kelvin were similar to those reported for Ganymede, whereas those for the mixtures were slightly shifted. The band intensity dropped by more than two orders of magnitude when the ice mixture was warmed to 100 kelvin, although about 20 percent of the O2 remained trapped in the ice, which suggested that at these temperatures O2 molecules dissolve in the ice rather than aggregate in clusters or bubbles. The experiments suggest that the absorption bands in Ganymede's spectrum were not produced in the relatively warm surface of the satellite but in a much colder source. Solid O2 may exist in a cold subsurface layer or in an atmospheric haze.

[The crystallization behavior of chromatographically pure swine insulin]. / Untersuchungen zum Kristallisationsverhalten von chromatographisch gereinigtem Schweineinsulin.

Crystallization tests are performed with porcine insulin previously purified by single chromatography to produce Lente insulins. The following parameters are changed under the test: crystallization temperature, stirring rate, pH of crystallization, and the period from the beginning of insulin precipitation until the isoelectric point is reached. The effects of these parameters on crystallization kinetics, crystal size and crystal size distribution are being studied.

Epidural intracranial pressure monitoring: a new system.

An intracranial pressure (ICP) monitoring system has been developed, consisting of an epidural sensor acting as a pneumatic flow switch connected by tubing to a pneumatic system and microprocessor-based monitoring unit. The frequency response to the system is 8 Hz at 10 mmHg measured peak to peak. ICP and arterial pressure data are collected and maintained in memory. The ICP in memory can be scrolled across a 2-hour or 24-min graphic and digital video display. When memory is not being recalled, the previous 2 hours or 24 min of data are displayed on the screen in graphic form. High and low ICP, low cerebral perfusion pressure, and abnormal pressure waves trigger alarms. Calibration of the system is maintained automatically by periodic rezeroing of all transducers to air. Changes in volume within the pneumatic system indicate a leak, and airflow ceases. The software management and alarm systems, as well as available memory, represent the latest in computerized technology.

Membrane potentials in mitochondrial preparations as measured by means of a cyanine dye.

Changes in the fluorescent intensity of the dye 3,3'-dipropylthiodicarbocyanine iodide were measured in suspensions of hamster liver mitochondria upon the development of a K+ diffusion potential by the addition of valinomycin and upon the development of the energized state by the addition of succinate or ATP. The changes (large decreases) seen with the addition of succinate or ATP (inhibitable by NaCN and oligomycin respectively) were comparable to those recorded upon the addition of valinomycin to mitochondria suspended in media containing low concentrations of K+. The change observed with succinate was partially reversed by the addition of either 2,4-dinitrophenol or ADP. Oligomycin prevented the reversal seen with ADP. Decreases in fluorescent intensity were also recorded when succinate was added to suspensions of inner membranes (prepared from rat liver mitochondria) containing the dye. With submitochondrial particles (also from rat liver mitochondria), however, increases in fluorescent intensity were seen upon the addition of succinate or ATP. These observations are consistent with the idea that a large negative (internal) potential develops across the inner membrane of the mitochondrion during energization and with other aspects of the chemiosmotic hypothesis.