摘要:

This paper is concerned with an investigation of the electrical,the galvanomagnetic, and the thermoelectric properties of pyrolyticgraphites whose morphological features are conditioned bythe deposition temperature, the heat treatment, and the dopinglevel.(1) Basal-plane magnetoresistance andc-direction specific resistanceof deposits prepared at temperatures ranging from 1900°to 2500°C point to a remarkable improvement of the crystallites'alignment with rising deposition temperature. In both crystallographicdirections the Seebeck coefficient closely follows semi-empiricalpredictions based on the two-dimensional model of the&pgr;-electron bands. The Fermi level of a standard deposit (2100°C)is at 0.025 eV below the valence-band edge and thus indicates thatcrystal defects trap about7.5×1018 electrons/cm3at room temperature;this figure is in accord with a Hall coefficient of0.33 cm3/C.The average in-plane mobility(930 cm2/V-sec)correspondsto a mean free path of the order of the crystallite diameter (250 Å).(2) Post-deposition treatment at temperatures above 2500°Cresults in (a) a rapid drop of the room-temperature basal-planeresistivity down to 50 &mgr;&OHgr;-cm or less, (b) a Hall effect conversionfromptontype in the early stages of graphitization, and (c) atrend toward negative Seebeck coefficients in the layer planes. Inconjunction with low-field magnetoresistance measurements theseresults can be described in terms ofsemimetallicconcepts, thesimultaneous presence of holes and electrons with equal concentrations(6×1018 cm−3at room temperature) stemming from aslight band overlap. Average mobilities imply that the carrier behaviorapproaches single-crystal characteristics ( ≈ 104 cm2/V-secat room temperature) after heat treatment above 3000°C. Normalto the layers, the specific resistance always exceeds 0.1 &OHgr;-cm, whichpoints to amolecularconduction process in this direction.(3) An incorporation of boron into the carbon-hexagon networkslowers the electrical resistance of graphite particularly inthecdirection (twenty-fold decrease at a composition of 0.6at.%B); concurrently the two temperature coefficients becomeapproximately equal to zero. In the rigid-lattice approximationband-population figures derived from the resistivity temperaturedependence reflect the Hall coefficient behavior, the peak occurringat an equivalent boron content of 0.04%. The ionization efficiencyis of the order of 50% with a Fermi level depressed by more than0.1 eV. Thermoelectric power measurements in thecdirectionaccord with the view that charge transport across the layer planesinvolves most of the excess holes, and reveal that boron enhancesthe Seebeck anisotropy of graphite.

ISSN:0021-8979    

DOI:10.1063/1.1931167

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

A considerable depolarization current was observed in rutile single crystals. It was ascribed to electronstrapped at the positive electrode in levels which probably exist only in superficial layers of the crystal. Thecharge piled up corresponds to, at minimum,1014/cm2elementary chargese. The dependence of the depolarizationcurrent on the depolarization time,I ∼ 1/t, could be explained by assuming that the time dependenceis governed by the probability of electron transitions from traps to the conduction band and these under theinfluence of the strong electric field of the space charge.Assuming that there are also1014/cm2ionized donors at the negative electrode, the observed current-voltagecharacteristics in thin flakes can be explained by taking into account that this gives rise to a field ofthe order of106 V/cmat the negative electrode. Electrons then can enter the crystal by field emission fromthe negative electrode, but the current will be space-charge-limited in the bulk of the crystal. This explainsthe observedI ∼ V2dependence.

ISSN:0021-8979    

DOI:10.1063/1.1931168

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

Asymmetries and jumps in amplitude of the resonance curves for lead are present in a region of strainamplitudes where the damping and modulus are rapidly varying with strain amplitude. This is due to thevibrations causing a change in the modulus of the specimen. This change takes place in a fraction of a secondat room temperature and anneals out in a few seconds when the amplitude of vibration is reduced.

ISSN:0021-8979    

DOI:10.1063/1.1931169

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

The anisotropy in the conductivity ofp-type germanium at fields of up to 3 kV/cm has been measuredat lattice temperatures of 77°K and 21°K for specimens with impurity concentrations from3.0×1014 cm−3to3.2×1015 cm−3. The electric field vector was found to deviate from the current vector towards the ⟨111⟩direction for specimens in the (110) plane and this behavior is opposite to that previously published bySasakiet al.The anisotropy decreased with increased impurity concentration and the reduction of temperaturefrom 77° to 21°K shifted the anisotropy-electric field curve to lower fields.

ISSN:0021-8979    

DOI:10.1063/1.1931170

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

The elastic moduli and characteristic lattice vibrational frequencies of semiconductors with diamond,zinc blende, and wurzite semiconductors are reduced to dimensionless parameters. The dimensionless elasticparameters exhibit a much smaller variation from compound to compound than the original parameters.Some generalizations can be made concerning the variation of the dimensionless parameters.

ISSN:0021-8979    

DOI:10.1063/1.1931171

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

Pure copper samples were electron bombarded at 4.2°K. The elastic modulus was observed to rise in atypically saturating manner during irradiation. Following irradiation, the modulus was observed to increasefurther in the temperature range from 35° to 50°K. The modulus increase is consistent with the Cottrell-Bilbymodel of stress-induced diffusion to dislocations.

ISSN:0021-8979    

DOI:10.1063/1.1931172

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

The influence of elastic strains, such as will arise around regions in a glass that have undergone an arbitraryvolume expansion, on the components of scattered light of various polarizations is calculated here. Theresults show that the effect of strains on the scattered intensities can be experimentally separated from theinfluence of size and shape of the scattering regions. Data on scattering from neutron-damaged vitreous silicaare analyzed. The strain scattering is well described by assigning a reasonable value to an optical parameterthat appears in the theory. Examination of that component of scattered light associated with the size andshape of the damaged regions reveals some peculiarities perhaps explicable in terms of a wide separation ofdamaged regions produced by a single primary recoil atom.

ISSN:0021-8979    

DOI:10.1063/1.1931173

出版商:AIP    

年代:1962

数据来源: AIP

ISSN:0021-8979    

DOI:10.1063/1.1931175

出版商:AIP    

年代:1962

数据来源: AIP

ISSN:0021-8979    

DOI:10.1063/1.1931176

出版商:AIP    

年代:1962

数据来源: AIP

ISSN:0021-8979    

DOI:10.1063/1.1931177

出版商:AIP    

年代:1962

数据来源: AIP