Showing papers on "Atmospheric temperature range published in 1970"

Equation of State of Polycrystalline and Single‐Crystal MgO to 8 Kilobars and 800°K

Abstract: MgO has been measured again. Most measurements of the elastic properties of materials, interesting to geophysics, have been made as a function of pressure at room temperature or as a function of temperature at atmospheric pressure. A lapped seal between a buffer rod and sample has made it possible to use ultrasonic interferometry to 1000°K and 10 kb. The elastic constants of poly crystalline and single-crystal MgO were measured in a gas high-pressure system over a temperature range from 300°K to the Debye temperature of MgO. Data from the polycrystalline specimen indicated large effects of temperature on the pressure derivatives. These data did not agree with the results obtained from single-crystal measurements. Upon remeasuring the ceramic sample it becomes apparent that the data are not reproducible after the sample has been cycled to high temperature and pressure. Additional sintering, deformation of the individual grains, and recrystallization take place, which change the properties of the sample. These problems and the problems of sintering isotropic aggregates of theoretical density limit the usefulness of this widely used procedure. Order from the American Geophysical Union, Suite 435, 2100 Pennsylvania Ave., N.W., Washington D.C 20037. Document J70-001; $1.00.

Temperature Dependence of the Transport Properties of Gallium Arsenide Determined by a Monte Carlo Method

Abstract: Transport properties of gallium arsenide in an electric field have been calculated in the temperature range 77°–500°K using a Monte Carlo technique. In intrinsic material it is found that the threshold field for the onset of negative differential mobility changes only slightly over this temperature range increasing from 3.1 kV/cm at 77°K to 3.7 kV/cm at 500°K, while the negative differential mobility reduces from 4200 to 1000 cm2/V sec over the same temperature rise. The influence of impurity scattering on the velocity field characteristic has been considered at 77°K for impurity concentrations of 1015 and 1017 cm−3 and at 300°K for concentrations for 1013, 1015, and 1017cm−3. The threshold field for the onset of negative differential mobility is found to change only slightly while the peak‐to‐valley ratio decreases significantly with increasing impurity concentration. The temperature dependence of the low‐field mobility has been calculated for impurity concentrations of 1015, 1016, and 1017 cm−3.

Studies of chemical exchange by nuclear magnetic relaxation in the rotating frame

Abstract: The exchange of protons between sites with different chemical shift is studied by means of N.M.R. relaxation time measurements (T 1ρ ) in the presence of an r.f. magnetic field. The system studied is chair-to-chair isomerization in cyclohexane. The exchange rate and chemical shift are measured in the temperature range 215°k to 250°k. The method is compared with other N.M.R. techniques for studying chemical exchange and is found to have advantages particularly at high exchange rates.

Carrier transport across metal-semiconductor barriers

Abstract: Carrier transport across metal-semiconductor barriers has been studied theoretically and experimentally to give a generalized and quantitative presentation. The thermionic and tunneling processes have been analyzed in terms of accurate quantum transmission coefficients. The effects of image-force lowering, temperature, and two-dimensional statistical variation of impurity concentration have also been incorporated in the theory. Theoretical results give a description of the current transport, due to combined effect of tunneling and thermionic emission over a temperature range from essentially absolute zero to the highest practical temperatures, and over doping densities from 1014 cm−3 to complete degeneracy. An interesting result of the analysis is the existence of a minimum in the saturation current density Js near 1016 cm−3; the current density rises slightly at lower dopings because of enhanced transmission coefficient for thermionic emission and increases drastically at higher dopings because of tunneling. For example for PtSiSi system at 300°K with a barrier height of 0.85 eV, Js is 80 nA/cm2 at 1014 cm−3, reaches a minimum of 60 nA/cm2 at 1016 cm−3, then rapidly increases to 103 A/cm2 at 1020 cm−3. In the high doping range the average saturation current density is considerably increased by the effect of two-dimensional impurity variation. The room-temperature transition doping for breakdown in metal-silicon systems occurs at 8×1017 cm−3; for lower dopings the breakdown is due to avalanche multiplication, and for higher dopings it is due to tunneling of carriers from the metal Fermi level to semiconductor bands. The metal-silicon diodes were fabricated by planar technology with guard-ring structures to eliminate edge effects. Extensive experimental studies, including current-voltage, capacitance-voltage, and photoelectric measurements covering the doping range from 1014 to 1020 cm−3 and the temperature range from 77°K to 373°K, gave good agreement with theoretical predictions.

Ferroelectric Properties of Pb(Zn1/3Nb2/3)O3

Abstract: The ferroelectric properties of a single crystal of Pb(Zn 1/3 Nb 2/3 )O 3 were investigated in the ferroelectric and paraelectric temperature regions The crystal undergoes a ferroelectric phase transition in a wide temperature range, changing from cubic perovskite structure to rhombohedral one A broad peak was observed in the dielectric constant vs temperature curve, being characteristic of the disordered perovskite ferroelectrics The spontaneous polarizations were obtained from the hysteresis loop and the pyroelectric current, and there was found a discrepancy in both measurements Without applying an electric field the optical anisotropy was not detected But with the field the domain and the birefringence were observed The effects of the dc field on the dielectric constant and the birefringence were observed over the wide temperature range A brief explanation of the character of the phase transition as well as dielectric and optical properties were given

Structural, electrical, and optical properties of amorphous germanium films

Abstract: Measurements of the ac and dc resistivity in the temperature range of 77-750\ifmmode^\circ\else\textdegree\fi{}K and of the optical transmission and reflectance have been made on amorphous Ge films of 1000-\AA{} to 7-\ensuremath{\mu} thickness prepared by thermal evaporation. The resistivity of amorphous Ge films at room temperature is \ensuremath{\sim}100\ensuremath{\Omega} cm, and increases with decreasing temperature. The conduction is thermally activated, with activation energy decreasing continuously from 0.5 eV at 600\ifmmode^\circ\else\textdegree\fi{}K down to 0.069 eV at 77\ifmmode^\circ\else\textdegree\fi{}K. At 450\ifmmode^\circ\else\textdegree\fi{}C, the amorphous \ensuremath{\rightarrow} crystalline transformation is observed and is accompanied by an irreversible decrease of resistivity by a factor of \ensuremath{\ge}20. The ac resistivity of amorphous films is found to decrease with frequency $\ensuremath{\omega}$. At high frequencies, the resistivity varies as ${\ensuremath{\omega}}^{\ensuremath{-}n}$, where $n$ lies between 0.5 and 1, depending on temperature. The resistivity decreases with applied field beyond the Ohmic regime. At 77\ifmmode^\circ\else\textdegree\fi{}K, the current increases exponentially with the square root of the field for fields $\stackrel{\mathrm{\ensuremath{\backsim}}}{g}$ 2.5\ifmmode\times\else\texttimes\fi{}${10}^{3}$ V/cm. The optical absorption coefficient is found to vary as exp ($\frac{h\ensuremath{ u}}{0.14}$ eV), in the energy range of 0.6-1.24 eV. Below 0.6 eV, the absorption coefficient falls rapidly down to a value of \ensuremath{\sim}60 ${\mathrm{cm}}^{\ensuremath{-}1}$ at 0.53 eV. The dc and ac electrical properties may be understood in terms of a mechanism of conduction by hopping of the carriers in the localized states (near the "fuzzy" band edges) caused by the fluctuation potential in the disordered lattice. The exponential decay of the opticalabsorption coefficient with energy near the absorption edge and the presence of a red-shifted absorption edge are consistent with the crystalline energy-band diagram modified by the presence of acceptor states due to vacancies, by tailing, and by localization of states near the band edges due to disorder. The sharpness (which is comparable to that in single-crystal Ge) of the amorphous absorption edge is, however, not compatible with the "tailing" concept.

Electrical Properties of Evaporated Molybdenum Oxide Films

Abstract: Thin film capacitors of MoO3 are found to be extremely temperature and frequency dependent Changes in capacitance are reported as high as 60:1 over a temperature range of 100°C (at constant frequency) and over a two‐decade frequency range (at constant temperature) At lower temperatures and higher frequencies the capacitance corresponds to the geometric capacitance, but at higher temperatures and lower frequencies the capacitance is independent of the film thickness Conductance and quality factor of the films are also observed to be extremely frequency and temperature sensitive The results are explained in terms of Schottky barriers existing at the metal‐insulator interfaces, which arise due to the autodoping of the insulator, occurring during deposition of the films, by excess molybdenum Excellent agreement is found to exist between the experimental data and the theory developed in the previous paper, and this correlation permits determination of the doping density (≃1018 cm−3), the donor depth (≃02

The diffusion of iron in aluminium

Abstract: The diffusion of radioactive 59Fe has been measured in both single crystals and polycrystalline specimens of aluminium in the temperature range 520 to 660°c. The material for the diffusion samples was between 99·99 and 99·999% pure aluminium. The temperature dependence of the diffusion coefficient for single crystal specimens could be expressed by in the temperature range 580 to 660°c. The results are not in agreement with values previously published for this system by Hirano, Agarwala and Cohen (1962). The earlier results are thought to be in error as a result of tracer hold-up at the surface-oxide film on the aluminium specimens. This effect was overcome in the present work by implanting the 59Fe tracer atoms below the surface-oxide layers using a 40 kev 59Fe ion beam. Analysis of the data indicates the activation energy for diffusion is dominated by an iron-vacancy repulsive interaction term. Calculations of the difference between the activation energies for self and solute diffusion in alumin...

The state of water in human and dog red cell membranes.

Abstract: The apparent activation energy for the water diffusion permeability coefficient, Pd, across the red cell membrane has been found to be 4.9 ± 0.3 kcal/mole in the dog and 6.0 ± 0.2 kcal/mole in the human being over the temperature range, 7° to 37°C. The apparent activation energy for the hydraulic conductivity, Lp, in dog red cells has been found to be 3.7 ± 0.4 kcal/mole and in human red cells, 3.3 ± 0.4 kcal/mole over the same temperature range. The product of Lp and the bulk viscosity of water, η, was independent of temperature for both dog and man which indicates that the geometry of the red cell membrane is not temperature-sensitive over our experimental temperature range in either species. In the case of the dog, the apparent activation energy for diffusion is the same as that for self-diffusion of water, 4.6–4.8 kcal/mole, which indicates that the process of water diffusion across the dog red cell membrane is the same as that in free solution. The slightly, but significantly, higher activation energy for water diffusion in human red cells is consonant with water-membrane interaction in the narrower equivalent pores characteristic of these cells. The observation that the apparent activation energy for hydraulic conductivity is less than that for water diffusion across the red cell membrane is characteristic of viscous flow and suggests that the flow of water across the membranes of these red cells under an osmotic pressure gradient is a viscous process.

Impurity conduction in synthetic semiconducting diamond

Abstract: Electrical conductivity and Hall effect measurements have been made on a series of synthetic p-type semiconducting diamonds in the temperature range 80 to 450K. The dominant conductivity mechanism at low temperatures is shown to be impurity conduction, and effects have been isolated due both to impurity-band conduction and to hopping transport between neutral and ionized acceptor centres. The activation energies associated with the latter processes are much lower than the acceptor ionization energy, and the variation with temperature of the onset of impurity conduction as a function of the neutral acceptor concentration accounts for the wide range of activation energies reported for synthetic diamonds by previous authors.

Temperature Dependence of Surface Acoustic Wave Velocity on α Quartz

Abstract: Measurements of the temperature dependence of surface acoustic wave velocity were performed for propagation on X, Y, AC, AT, and along the x axis of several other rotated Y cuts of quartz over the temperature range −25° to +75°C. A pulsed rf technique was employed. To facilitate the measurement of angular dependence of the temperature coefficient of velocity, surface wave delay lines were constructed in which the waves were excited on the quartz by means of an interdigital electrode structure on a glass substrate which was brought into contact with the quartz. Calculations of the temperature coefficients of velocity and delay time were performed using an iterative computer program based on the work of Coquin and Tiersten. The measurements and calculations are in good agreement in most cases, the largest discrepancy being approximately five percent. It is concluded that the orientation which best combines low‐temperature dependence of delay time, high coupling constant, and a minimum of deleterious side effects is 42½° rotated Y‐cut quartz with propagation along the x axis.

Elastic Constants of ZnTe and ZnSe between 77°–300°K

Abstract: The elastic constants of single crystals of ZnTe and ZnSe (zinc‐blende type) have been determined over the temperature range 77°–300°K by means of the ultrasonic pulse‐echo method. The velocity of the longitudinal and two transverse waves propagated in the [110] direction showed 1%–1.4% increase at 77°K from the room‐temperature values and as a result, the elastic constants of both crystals showed 2%–3.5% increase at 77°K from the room‐temperature values. From the low‐temperature elastic constants, the Debye temperature was calculated to be 225.3°K for ZnTe and 278.5°K for ZnSe, respectively.

Temperature Dependence of Elastic, Dielectric, and Piezoelectric Constants in TeO2 Single Crystals

Abstract: All of the elastic stiffness constants except c13 have been found to decrease almost linearly with temperature between −120° and 120°C. The constant c13 reaches the maximum in the vicinity of 0°C, above which it also decreases with temperature. On the other hand, the effective elastic constant (c11–c12)/2, which corresponds to the exceptionally slow shear wave propagating along [110], increases with temperature. The shear wave propagating in the (001) plane making angle of 35.9° with the X axis has zero temperature coefficient of velocity. Acoustic absorptions for several sound modes vary within 1 dB/cm for both the longitudinal wave (measured at 36 MHz) and shear wave (48 MHz) in the measured temperature range. Both real and imaginary parts of the dielectric constants increase monotonically with temperature, while the piezoelectric constant d14 decreases in the temperature range between −150° and 180°C.

Monoclinic–tetragonal phase transition in zirconia: mechanism, pretransformation and coexistence

Abstract: The high temperature X-ray diffraction study of the monoclinic tetragonal phase transition in ZrO2 showed that it is spread over a temperature range 930–1220°C. Anomalous intensity changes are observed in the pretransformation region 930–1100°C. Coexistence of phases through hybrid crystal formation in the region 1100–1220 °C and the mechanism of transition are discussed. The orientation relationship between the monoclinic (m) and the tetragonal (t) crystal structures consists in the parallelism of the (100)m plane to (110)t and of the bm axis to the ct axis. A drastic change in a small temperature range during the tetragonal–monoclinic transition is interpreted as a cooperative change in both short and long range interactions. The large thermal hysteresis is attributed to the difference in the mechanism of transition during heating and cooling.

Neutron-irradiated molybdenum: relationship of microstructure to irradiation temperature.

Abstract: The microstructural defects produced in molybdenum by neutron irradiation in the temperature range 50 °C to 800 °C have been characterized by transmission electron microscopy. At a low irradiation temperature, 50 °C, a large number of small dislocation loops, presumably interstitial, form within a complex dislocation network. At intermediate temperatures, 400–600 °C, small interstitial loops agglomerate into rafts. At high temperatures, 600–800 °C, the small loops which comprise a raft are sufficiently mobile to form large loops which interact with each other to produce a coarse dislocation network. The migration of small loops through the lattice by a combination of prismatic glide and conservative climb accounts for the observed microstructures. Vacancy loops are the only identifiable vacancy defects at temperatures ≥ 500. Between 575–650 °C, both vacancy loops and voids exist. Above 650 °C to at least 800 °C, voids are the exclusive vacancy defect. Information on the effect of impurities and g...

Segregation of Carbon to the (100) Surface of Nickel

Abstract: The reversible segregation of carbon to the (100) surface of nickel has been observed. The surface was studied at temperature through measurements of work function changes and observations of LEED patterns and Auger spectra. The work function increases with temperature by about 0.63 eV in the temperature range 500°–800°K. This difference is close to that obtained from reported values of the work function for Ni (100) and for carbon. Below about 650°K the diffraction pattern contains rings characteristic of a graphite layer with its basal plane parallel to the substrate. In contrast with previous LEED observations of graphite precipitation at single‐crystal surfaces, the segregation described here is reversible under temperature cycling.

Pressure Dependence of the Elastic Constants of NaCl at Low Temperatures

Abstract: The paper presents the pressure derivatives of single‐crystal NaCl in the temperature range of 180°–300°K. The data have been used to calculate the mode Gruneisen parameters. We have also predicted the shock‐velocity and the particle‐velocity relationship using the ultrasonic data at 300°K.

Kinetics and mechanism of llmenite reduction with graphite

Abstract: The reduction of synthetic ilmenite with graphite in the solid state has been studied by ther-mogravimetric analysis. The reaction has been observed to initiate near 860°C at the contact points between the reactants. Up to 1020°C solid state reduction appears to be the main reaction mechanism, while above this temperature a rate increase has been observed and has been attributed to a change of mechanism to gaseous reduction of ilmenite by regenerated CO. Microscopic examination and electron probe analysis of the reduced particles have indicated a tendency toward segregation of the products iron and TiO2. Iron particles as large as 80 μ can be obtained by keeping the reduced sample at 1025° to 1075°C for several hours. Reduction rate data under isothermal conditions were fitted to different rate equations and have been found to be well represented by the equation. 1 − 2x/3 − (1 −x)2/3 =K’ This equation is based upon diffusion of reactants through a product layer. CO is suggested as the diffusing species. The activation energy for the reaction in the temperature range 1075° to 1140°C has been calculated to be 64 ± 6 kcal per mole.

Temperature Dependences of the Rate Coefficients for the Reactions of Ar+ with O2, H2, and D2

Abstract: A series of measurements have been made using the flowing afterglow technique to investigate the temperature dependence of the rate coefficients for the reactions of Ar+ with O2, H2, and D2 in the temperature range 80–560°K. The rate coefficient for the reaction of Ar+ with O2 has been found to decrease with increasing temperature approximately as T−0.5. In contrast, the rate coefficients for the reactions of Ar+ with H2 and D2 increased with increasing temperature, and the rate coefficients for the reactions ArH+ + H2 and ArD+ + D2 were independent of temperature within experimental error. The rate coefficient for the reaction of Ar+ with N2 was measured at 280°K and found to be dependent on the Ar number density.

Temperature-dependence of self-diffusion for benzene and carbon tetrachloride

Abstract: Precise measurements have been made by the diaphragm-cell technique of the self-diffusion coefficients of benzene and carbon tetrachloride over the temperature range 15–50°C at atmospheric pressure. The data from these experiments have been used to test current phenomenological and statistical transport theories.

Oxygen diffusion in alpha and beta titanium in the temperature range of 932° to 1142°C

Abstract: An attempt is made to determine the diffusion coefficient of oxygen in α-Ti from oxidation studies of oxygen-unsaturated and oxygen-saturated β-Ti. Knowledge about the rates of movements of the α/β interface permits evaluation of oxygen diffusivity in α-Ti. The results conform to the relationship:D α=0.778 exp (−48,600/RT), sq cm per sec, for the temperature range of 932° to 1142°C. From microhardness measurements, the diffusion coefficient of oxygen in β-Ti can be expressed as:D β=3.30×102 exp (−58,800/RT), sq cm per soc, for the same temperature range.