The electrical conductivity of graphite intercalated with superacid fluorides: experiments with antimony pentafluoride

The electrical conductivity in graphite measured normal to the crystallographic c-axis is observed to increase after intercalation with acid molecules which act as acceptors. This behaviour is regarded as the result of ionization of the acid molecule which, in turn, increases the positive current carriers in the host graphite. Since the carrier density depends on the degree of ionization of the acid, it follows that the stronger the acid the greater the increase in carrier concentration, and assuming no adverse mobility effects, the greater the electrical conductivity. The hydrogen fluoride-antimony pentafluoride system produces some of the strongest acid substances known. The experiments described here represent the initial examination of the electrical conductivity resulting from intercalation of this material into graphite. The experiments consisted of intercalating graphite powder with antimony pentafluoride in a copper tube and swaging the sheathed compound into wire. The measured conductivity of the graphite intercalation compound, when the copper conductivity is subtracted out and allowance is made for departure from ideal density, is about 1×10^{6 –1} cm^–1. This is approximately 40 times the conductivity of pristine graphite and more than one and a half times the conductivity of pure copper.     

An analysis of the impact of the loss of a primary care physician on a patient population

The authors describe two cases of giant middle cerebral artery aneurysms presenting as mass lesions. Angiograms in each case revealed a distinctive serpentine vascular channel surrounded by an avascular area causing a "mass effect." Both lesions were resected in toto with excellent clinical results. Similar lesions in the literature are noted and the pathophysiology and origin of this group of aneurysms are discussed.     

Rheological characterization and thermal degradation of PTFE

PTFE (¯M _n=5×10⁶), when heated near the melting temperature (335 to 337 C) while in contact with carbon black, is characterized by an effective viscosity and a thermal stability which are orders of magnitude lower than those found in the absence of the contacting high surface area material. The penetration of the PTFE into the porous carbon black occurs by the spreading of a very thin polymer film followed by a thickening of this film with time at temperature until a limiting concentration is reached. The lower the average molecular weight of the PTFE, the more rapidly it penetrates into the porous material. Similar phenomena have been observed with high molecular weight PTFE heated near the melting temperature while contacting high surface area metal blacks or porous sintered metals.     

The Influence of Glow and Afterglow Cold Plasma Treatment on Biochemistry,Morphology, and Physiology of Wheat Seeds

Cold plasma (CP) technology is a technique used to change chemical and morphological characteristics of the surface of various materials. It is a newly emerging technology in agriculture used for seed treatment with the potential of improving seed germination and yield of crops. Wheat seeds were treated with glow (direct) or afterglow (indirect) low-pressure radio-frequency oxygen plasma. Chemical characteristics of the seed surface were evaluated by XPS and FTIR analysis, changes in the morphology of the seed pericarp were analysed by SEM and AFM, and physiological characteristics of the seedlings were determined by germination tests, growth studies, and the evaluation of α-amylase activity. Changes in seed wettability were also studied, mainly in correlation with functionalization of the seed surface and oxidation of lipid molecules. Only prolonged direct CP treatment resulted in altered morphology of the seed pericarp and increased its roughness. The degree of functionalization is more evident in direct compared to indirect CP treatment. CP treatment slowed the germination of seedlings, decreased the activity of α-amylase in seeds after imbibition, and affected the root system of seedlings.     

Kristallin-flüssige Ketoximester

Liquid-crystallin Ketoxime Esters Disubstituted acetophenoneoximebenzoates 1a – q , 2a – j , a – d , 4a – b and 5a – b are nematic liquid crystals that exhibit remarkable thermal stability and, dependent on their terminal groups, relative low melting points. Binary and ternary mixtures, for which also the melting temperatures have been evaluated theoretically, are liquid far below room temperature and stable under the influence of a. c. fields. The substitution of CH₃ by C₂H₅ or n-C₃H₇ in 6a – d and 7 leads to pronounced depressions in the clearing points.     

Mobile satellite system propagation measurements at L-bandusing MARECS-B2

A mobile satellite system (MSS) propagation experiment at 1.5 GHz was performed near the east coast of the United States in central Maryland during December 1987 using the MARECS-B2 satellite as a transmitter platform. A receiving system in a vehicle measured signal fades caused by shadowing and multipath from roadside trees and utility poles. The propagation degradations were characterized for a system of three roads previously examined using a helicopter as the transmitter platform. The objectives of the MARECS-B2 MSS tests were to: (1) establish cumulative fade distributions for the particular satellite geometry for both rural and suburban roads; (2) validate the consistency of previous roadside tree measurements which employed a helicopter as the transmitter platform for the same system of roads; (3) obtain an additional set of fade levels at a lower angle hitherto not measured in central Maryland; and (4) combine the satellite-acquired data set with previous helicopter results and establish an analytic, empirically derived function describing the cumulative fade distributions for a family of path angles. An analysis of the satellite data has demonstrated the successful achievement of these objectives     

GSM phase 2+ general packet radio service GPRS: Architecture, protocols, and air interface

Selective packet dropping policies have been used to reduce congestion and transmission of traffic that would inevitably be retransmitted. For data applications using best-effort services, packet dropping policies (PDPs) are congestion management mechanisms implemented at each intermediate node that decide, reactively or proactively, to drop packets to reduce congestion and free up precious buffer space. While the primary goal of PDPs is to avoid or combat congestion, the individual PDP designs can significantly affect application throughput, network utilization, performance fairness, and synchronization problems with multiple Transmission Control Protocol (TCP) connections. Scalability and simplicity are also important design issues. This article surveys the most important selective packet dropping policies that have been designed for best-effort traffic in ATM and IP networks, providing a comprehensive comparison between the different mechanisms.     

Optimization of GaAsP/AlGaAs-based QW laser structures for high power 800 nm operation

We present a detailed study of the MOVPE growth of 800 nm diode laser structures based on the combination of a GaAsP quantum well with well-established AlGaAs waveguide structures. By optimizing the strain and thickness of the quantum well highly-reliable diode lasers with low threshold current and high efficiency were demonstrated. 100 μm aperture “broad area” devices mounted epi-side up achieve a CW output power of 8.9 W with a wall-plug efficiency of 50%. These output powers represent record values for diode lasers in this wavelength range. Reliability measurements at 1.5 W and 50°C ambient temperature suggest lifetimes >10 000 h.     

Design and measurements of a novel subharmonically pumpedmillimeter-wave mixer using two single planar Schottky-barrier diodes

A novel design of a subharmonically pumped millimeter-wave mixer operating at room temperature was developed and realized. The double sideband conversion loss and mixer noise temperature were measured to be 6.2 dB and 930 K, respectively, at a local oscillator frequency of 73 GHz and an IF of 1.5 GHz. These results are comparable to the best published results measured for subharmonically pumped mixers at similar frequencies. The mixer shows good performance even at IF's up to 9.5 GHz resulting in a useful RF range from 136 GHz to 156 GHz. For the first time a subharmonically pumped millimeter-wave mixer was designed without the use of any scale model measurements or other high-frequency measurements. The whole design process occurred on the basis of computer simulations. Two single low-capacitance planar air-bridge type Schottky-barrier diodes are used as the mixing elements