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Hadek, V.

Publications and source records attributed to Hadek, V..

17 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Far-infrared transmittance of boron-implanted germanium at liquid-helium temperatures

The transmission of far-infrared radiation of 100-microns-wavelength through Ge wafers ion implanted with B was measured at a temperature of approximately 4 K. Transmittance ranged from approximately 0.5 to 0.08 for doses from 3 x 10 to the 12th to 3 x 10 to the 15th/sq cm. The resistivity at temperatures between 4 and 1 K, and the impurity profiles of the implanted layers were also measured and correlated with the infrared transmittance.

Hadek, V.↗
NASA NTRS1985

Extension of long wavelength response by modulation doping in extrinsic germanium infrared detectors

A new concept for infrared detectors based on multilayer epitaxy and modulation doping has been investigated. This permits a high doping concentration and lower excitation energy in the photodetecting layer as is necessary for longer wavelength response, without incurring the detrimental effects of increased dark current and noise as would be the case with conventional detector designs. Germanium photodetectors using conventional materials and designs have a long wavelength cutoff in the infrared at 138 microns, which can only be extended through the inconvenient application of mechanical stress or magnetic fields. As a result of this approach which was arrived at from theoretical considerations and subsequently demonstrated experimentally, the long wavelength cutoff for germanium extrinsic detectors was extended beyond 200 microns, as determined by direct infrared optical measurements.

Hadek, V.↗
NASA NTRS1979

Antistatic coating for acrylics

After immersion in low molecular-weight solvents such as acetonitril or nitromethane, clear acrylic plastics dissipate up to 70% of induced electric charge within one minute, yet retain optical clarity.

Hadek, V.↗
NASA NTRS1978

Optical, spin-resonance, and magnetoresistance studies of /tetrathiatetracene/2/iodide/3 - The nature of the ground state

A recent investigation in which Isett and Perez-Albuerne (1977) found that (tetrathiatetracene)2(iodine)3 is a stable organic metal down to 3.3 K is of considerable interest. In view of the diversity of suggestions made regarding this compound, measurements, were made of its electrical, magnetic, and optical properties. The results obtained indicate a metallic state at high temperatures, but also support a nonmetallic state at temperatures below 30 K. At 20-30 K, a metal-to-insulator phase transition occurs. This is indicated by the onset of a large and positive magnetoresistance, the leveling off in the temperature dependence of the ESR linewidth, and by recent derivative analysis of the electrical conductivity in this region. The investigated compound represents an interesting one-dimensional electronic system in which it may be possible to study the combined effects of disorder and interchain coupling on charge transport processes.

Somoano, R. B.↗
NASA NTRS1977

Durable antistatic coating for polymethylmethacrylate

A durable antistatic coating is achieved on polymethylmethacrylate plastic without affecting its optical clarity by applying to the surface of the plastic a low molecular weight solvent having a high electron affinity and a high dipole moment, such as acentonitrile or nitromethane alone or in the presence of photopolymerizable monomer. The treated polymethylmethacrylate plastic dissipates most of the induced electrostatic charge and retains its optical clarity. The antistatic behavior persists after washing, rubbing and vacuum treatment.

Hadek, V.↗
NASA NTRS1977

Electrical, magnetic, and optical properties of the tetrathiafulvalene /TTF/ pseudohalides, /TTF/12/SCN/7 and /TTF/12/SeCN/7

The electrical, magnetic, and optical properties of charge-transfer salts containing tetrathiafulvalene (TTF) and the pseudohalides, thiocyanate (SCN) and selenocyanate (SeCN), have been investigated. These salts are quasi-one-dimensional compounds containing cation radicals only, in contrast to a cation-radical-anion-radical system, such as tetrathiafulvalene tetracyanoquinodimethane (TTF) (TCNQ). Measurements of electrical conductivity, thermoelectric power, and optical reflectivity of single crystals of the nonstoichiometric salts (TTF)12(SCN)7 and (TTF)12(SeCN)7 show metal-like characteristics above 200 K (high-temperature region). The conductivities at room temperature are approximately 750 mho/cm comparable to those found in (TTF) (TCNQ), and increase with decreasing temperature down to approximately 200 K. The thermoelectric power at room temperature is small and positive (approximately 9 microvolts/deg K), and decreases linearly with decreasing temperature in this region (as expected for metal-like hole conduction along the TTF chains). The ESR intensity, however, decreases with decreasing temperature above 200 K. At 170 K a metal-nonmetal transition occurs, and the transport and magnetic properties below this temperature are characteristic of a semiconducting state.

Somoano, R. B.↗
NASA NTRS1977

Electrical properties of /DEPE/ /TCNQ/sub 4

The electrical conductivity of two phases of (DEPE) (TCNQ)sub 4 is evaluated using four probe and Montgomery methods. The highest values (approximately 50 Ohms/cm) were found along the b sub 0 axis of the monoclinic phase (I) crystals at room temperature. The resistivities are described as semiconducting. Thermopower is observed to saturate at high temperatures to 60 microvolts/deg K, characteristic of a quarter-filled correlated band. It is pointed out that a stable metallic state for (DEPE) (TCNQ)sub 4 is theoretically possible, but none has thus far been found.

Somoano, R. B.↗
NASA NTRS1975

The electrical and magnetic properties of /TTF//I/-sub 0.71

The synthesis and transport properties of the salts (TTF) (iodine)-sub x have been investigated. These salts are quasi-one-dimensional compounds containing cation radicals only, in contrast to the cation radical-anion radical systems such as (TTF)(TCNQ). The phase diagram of (TTF)(I)-sub x is discussed, and found to consist of two nonconducting, stoichiometric phases, x = 2.0 and 3.0, and two conducting, nonstoichiometric phases, x = 0.71 and 2.3. The transport properties of (TTF)(I)-sub 0.71 are discussed in detail. This phase is highly conducting, undergoes a sharp transition to a less conducting state at 210 K, and exhibits an unusual hysteresis in some of the transport properties upon thermal cycling. The nature of the high-temperature ground state is examined.

Somoano, R. B.↗
NASA NTRS1975

The synthesis and transport properties of the complex salt /TMPD/ /TCNQ/2

The syntheses and transport properties of the complex salt /TMPD/ /TCNQ/2 are described. At high temperatures, the complex is a magnetic semiconductor with transport properties intermediate between those found in the highly conducting and poorly conducting TCNQ salts. The complex undergoes a transition below 50-60 K to a state exhibiting singlet-triplet behavior with weakly alternating exchange coupling.

Somoano, R.↗
NASA NTRS1975

The alkaline earth intercalates of molybdenum disulfide

Molybdenum disulfide has been intercalated with calcium and strontium by means of the liquid ammonia technique. Chemical, X-ray, and superconductivity data are presented. The X-ray data reveal a lowering of crystal symmetry and increase of complexity of the structure upon intercalation with the alkaline earth metals. The Ca and Sr intercalates start to superconduct at 4 and 5.6 K, respectively, and show considerable anisotropy regarding the critical magnetic field.

Somoano, R. B.↗
NASA NTRS1973

Apparatus and method for measuring the Seebeck coefficient and resistivity of materials

An apparatus for measuring the thermoelectric properties of materials under high pressure is described that includes a pair of force transmitting assemblies constructed of thermally and electrically conductive material positioned between the ram and anvil of a press. Each force transmitting assembly has a small diameter pressing portion for contacting a face of the sample so that the sample can be squeezed between them. Each assembly also includes a heat exchanger to maintain the sample face at a controlled temperature, and an electrical conductor to carry current generated by the sample. A sleeve of thermally and electrically insulative material closely surrounds the pressing portions of the two assemblies.

Hadek, V.↗
NASA NTRS1973

Alkali metal intercalates of molybdenum disulfide.

Study of some of the physicochemical properties of compounds obtained by subjecting natural molybdenite and single crystals of molybdenum disulfide grown by chemical vapor transport to intercalation with the alkali group of metals (Li, Na, K, Rb, and Cs) by means of the liquid ammonia technique. Reported data and results include: (1) the intercalation of the entire alkali metal group, (2) stoichiometries and X-ray data on all of the compounds, and (3) superconductivity data for all the intercalation compounds.

Somoano, R. B.↗
NASA NTRS1973

Apparatus for measuring electrical properties of materials

Resistance of sample is measured with aid of usual electrical test instruments applied to electrical contacts provided at ram and anvil assemblies. Temperature differential is established between ram and anvil for measurement of Seebeck coefficient. Voltage generated across sample is detected at electrical contacts.

Hadek, V.↗
NASA NTRS1972

Superconductivity in the alkali metal intercalates of molybdenum disulphide

The complete series of alkali metals, lithium through cesium, have been intercalated into molybdenum disulphide, using both the liquid ammonia and vapor techniques. All the intercalates with the exception of lithium yielded full superconducting transitions with onset temperatures of 6 K for AxMoS2(Ax=K,Rb,Cs) and 4 K for BxMoS2(Bx=Li,Na). The superconducting transition for lithium was incomplete down to 1.5 K. Stoichiometries and unit cell parameters have been determined for the intercalation compounds. Both rhombohedral and hexagonal polymorphs of MoS2 have been intercalated and found to exhibit the same superconductivity behavior. The nature of the extraneous superconducting transition of some intercalated samples on exposure to air was elucidated.

Somoano, R. B.↗
NASA NTRS1972

Superconductivity in intercalated molybdenum disulfide.

X-ray studies show the existence of two different types of expansions of the intercalated unit cell in both Na and K compounds. Two different phases are also indicated in the superconducting behavior of the K compound. All intercalated samples studied show a superconducting transition. K and Rb compounds become superconductors in the temperature range from 6.5 to 6.0 K. The Na compounds become superconductors at about 4.5 K. In all cases, the superconductivity disappears upon a short exposure of the sample to air. This phenomenon confirms that the superconductivity is due to the presence of the alkali metal.

Somoano, R.↗