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9 Aug 1999

Volume 75, Issue 6, pp. 745-878

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Acoustic waveguide properties of a thin film of nanoporous silica on silicon

John A. Rogers and Carlye Case

Appl. Phys. Lett. 75, 865 (1999); http://dx.doi.org/10.1063/1.124539 (3 pages) | Cited 5 times

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This letter reports picosecond laser-based measurements of acoustic waveguiding in a thin film of nanoporous silica (similar to silica aerogel or xerogel) on silicon for wavelengths between 2.5 and 25 μm. The large mismatch between the acoustic properties of the film and substrate in this system creates pronounced dispersion in the velocities and leads to unusual acoustic behavior: over a relatively large range of wavelengths, the group velocities of the lowest order Rayleigh mode and certain other modes are less than 50 m/s—nearly ten times slower than the intrinsic velocities of the nanoporous silica and more than one hundred times slower than those of silicon. An isotropic model of the waveguide reproduces these and other features. Nonlinear least-squares fitting of the data to this model determines the intrinsic acoustic velocities of the nanoporous glass. © 1999 American Institute of Physics.
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68.35.Gy Mechanical properties; surface strains
43.20.Mv Waveguides, wave propagation in tubes and ducts
43.35.Ud Thermoacoustics, high temperature acoustics, photoacoustic effect
62.65.+k Acoustical properties of solids

A 3×3 millimeter-wave micromachined imaging array with superconductor–insulator–superconductor mixers

Gert de Lange, Konstantinos Konistis, and Qing Hu

Appl. Phys. Lett. 75, 868 (1999); http://dx.doi.org/10.1063/1.124540 (3 pages) | Cited 1 time

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Results from a 3×3 micromachined millimeter-wave focal-plane imaging array with superconducting tunnel junctions as mixing elements are presented. The array operates in a frequency range of 170–210 GHz. The imaging array chip uses relatively large-area 9 μm2 and low-impedance (4–5 Ω) junctions. Integrated tuning structures are implemented to match the devices to the antenna impedance. Noise measurements yielded the lowest double-sideband noise temperature of 52 K (at 190 GHz) from the central element. The lowest noise temperatures from the off-axis elements are in the range of 60–100 K, with a uniform bandwidth of 30 GHz. Antenna beam patterns with an essentially Gaussian profile have been measured for on- and off-axis elements. © 1999 American Institute of Physics.
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85.25.Pb Superconducting infrared, submillimeter and millimeter wave detectors
07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors
84.30.Qi Modulators and demodulators; discriminators, comparators, mixers, limiters, and compressors
84.40.Ba Antennas: theory, components and accessories
84.40.Dc Microwave circuits

Electrical method of monitoring percolation and abrasion of conducting spheres due to shear flow of a dense suspension in a narrow gap

S. H. Mannan, D. A. Hutt, and D. C. Whalley

Appl. Phys. Lett. 75, 871 (1999); http://dx.doi.org/10.1063/1.124543 (2 pages) | Cited 3 times

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This letter describes a method for studying the behavior of rigid particles in a dense suspension when they are forced into contact during flow within a narrow gap. The particles form transient percolating networks spanning the boundary walls, and will be crushed together. The method involves measuring the dc electrical resistance across the gap. The suspension (e.g., solder paste) consists of electrically conducting particles suspended in an insulating fluid. The electrical resistance drops when the particles are in contact with each other and the walls, and the insulating films on the surface of the conductors have been broken through. The results show a dramatic change in behavior as the ratio of gap to particle diameter is varied. © 1999 American Institute of Physics.
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47.55.Kf Particle-laden flows
83.80.Hj Suspensions, dispersions, pastes, slurries, colloids
83.80.Iz Emulsions and foams
47.60.-i Flow phenomena in quasi-one-dimensional systems
73.40.Cg Contact resistance, contact potential
83.50.Ha Flow in channels
83.85.Cg Rheological measurements—rheometry

Large current density from carbon nanotube field emitters

W. Zhu, C. Bower, O. Zhou, G. Kochanski, and S. Jin

Appl. Phys. Lett. 75, 873 (1999); http://dx.doi.org/10.1063/1.124541 (3 pages) | Cited 274 times

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We observe that field emitters made from carbon nanotubes exhibit excellent macroscopic emission properties; they can operate at a very large current density, as high as 4 A/cm2. At electric fields as low as 4–7 V/μm, they emit technologically useful current densities of 10 mA/cm2. We show that the emission originates from nanotube ends with a characteristic structured ring pattern. The emission characteristics and durability of the carbon nanotube cold cathodes offer promising applications for vacuum microelectronic devices. © 1999 American Institute of Physics.
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85.45.Db Field emitters and arrays, cold electron emitters
79.70.+q Field emission, ionization, evaporation, and desorption
73.61.Wp Fullerenes and related materials
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