Showing papers on "Atmospheric temperature range published in 1978"

Calculation of the Avrami parameters for heterogeneous solid state reactions using a modification of the Kissinger method

Abstract: Several isothermal experiments are generally needed to determine the parameters of the Avrami equation which describe most of the heterogeneous solid state reactions. Differential scanning calorimeters are suitable for such experiments. While most differential thermal analysis (DTA) apparatus cover a wider temperature range than DSC apparatus they cannot be used to perform isothermal determinations. However, Kissinger has already shown how activation energy and frequency factor can be calculated from DTA experiments for the case of homogeneous reactions following first order kinetics. We derive in this paper an extension of the Kissinger method and show its applicability to heterogeneous reactions described by an Avrami expression. The new method will allow the study of the kinetics of metallic reactions at the higher temperature range obtainable with DTA. The transformation kinetics of the metastable equiatomic tin-nickel alloy are given as an example.

Thermodynamic Properties of the Intermetallic Systems Lithium‐Antimony and Lithium‐Bismuth

Abstract: Galvanic cells of the type , were employed to determine the Gibbs free energy of formation , and the corresponding enthalpies , and entropies for the systems Li‐Sb and Li‐Bi as a function of composition from pure to the Li‐rich side of the phases. The Li/X ratio was changed systematically by use of the coulometric titration technique. values were obtained by integration of the coulometric titration curve from pure to the composition of interest. Thermodynamic data have been determined for the trilithium compounds as a function of the stoichiometry. and were calculated from the temperature dependence of . Data were obtained over the temperature range 355°–600°C. At 400°C the following values were found for the standard Gibbs free energy of formation: , , , and , all ±0.1 kJ mole−1. The standard enthalpies of formation are: ,, , , all ±0.7 kJ mole−1. The standard entropies of formation are: , , , and, all ±1 J mole−1K−1. The experimental results were also used to examine the phase diagrams for Li‐Sb and Li‐Bi. The range of stoichiometry of is very narrow , whereas exhibits a large deviation from the ideal stoichiometry , predominantly on the lithium‐deficit side. and have been found to be the only intermediate phases existing beside the trilithium phases in the temperature range investigated.

Structure and Stability of Low Pressure Chemically Vapor‐Deposited Silicon Films

Abstract: The structure of silicon films deposited by low pressure, chemical vapor deposition in the 600°C temperature range has been investigated by x‐ray diffraction and transmission electron microscopy. There is a critical temperature near 600°C, above which the deposited films are polycrystalline and below which amorphous films are obtained. This temperature is close to that used to deposit films for integrated circuit applications. When the films are polycrystalline, the texture dominates. The polycrystalline films are reasonably stable upon annealing to temperatures up to approximately 1100°C. The initially amorphous films, however, easily crystallize. Crystallization has been observed to be well advanced after annealing at 800°C. Large stresses are built into the amorphous films, while the stresses in the polycrystalline films are lower.

A new NMOS temperature-stable voltage reference

Abstract: An NMOS voltage reference has been developed that exhibits extremely low drift with temperature. The reference is based on the difference between the gate/source voltages of enhancement and depletion-mode NMOS transistors. The theoretical dependence of the reference voltage on both device and circuit parameters is analyzed and conditions for optimal performance are derived. The reference NMOS transistors are biased to the optimizing current levels by a unique feedback circuit. The measured output voltage drift in the integrated realization agrees well with theory and is less than 5 parts per million per degree Celsius over the temperature range -55/spl deg/ to +125/spl deg/C.

Ideal gas thermodynamic properties of methanoic and ethanoic acids

Abstract: The thermodynamic properties (H0−H00, (G0−H00)/T, (H0−H00)/T, S0, Cp0, ΔHf0, ΔGf0, and log Kf] for methanoic (formic) and ethanoic (acetic) acid monomers and dimers in the ideal gaseous state over the temperature range from 0 to 1500 K and 1 atm have been calculated by the statistical thermodynamic method using the most recent and reliable molecular and spectroscopic constants. The internal rotational contributions of −OH and −CH3 rotors to the thermodynamic properties were evaluated based on internal rotation partition functions formed by summation of calculated internal rotation energy levels. On an assumption that the vapor contains only monomers and dimers, the thermodynamic properties for the monomer‐dimer equilibrium mixture of methanoic and ethanoic acids in ideal gaseous state were derived. The results are in agreement with available experimental data.

Thermodynamic properties of a quasi‐ionic alloy from electromotive force measurements: The Li–Pb system

Abstract: Electromotive force measurements of the activity of lithium in lithium–lead liquid alloys are reported in the temperature range T=770–932 K. The partial molar excess Gibbs free energies are negative. Variations with composition exhibit an inflection point around an atomic fraction of lead equal to 0.2 which corresponds to the ratio of the valences of Li and Pb. The inflection point, as well as structural data, may be interpreted by assuming an at least partly ionic character of the alloy. A set of partial structure factors defined by Faber and Ziman are calculated as a function of the composition at the long‐wavelength limit. Their variations support the hypothesis of a local short‐range order around the above‐mentioned composition (0.2). Finally, a modeling approach based on an electron transfer from Li to Pb is used to explain some of the characteristics of this class of alloys.

Boron in Near‐Intrinsic and Silicon under Inert and Oxidizing Ambients—Diffusion and Segregation

Abstract: The diffusivity of boron in and silicon is experimentally determined under both inert and oxidizing (dry ) ambient conditions in the range of temperatures 850°–1200°C The boron is implanted at moderate dose and energy (70 keV) and subsequently activated by a moderate temperature anneal The resulting profile ensures near‐intrinsic silicon at the processing temperatures and serves as initial condition for subsequent processing Diffusivities and segregation coefficients are calculated as fitting parameters in numerical solution of the experiments A systematic fitting procedure is used and the target experimental parameters are sheet resistances and junction depths Inert ambient diffusivities agree well with previous measurements, thus demonstrating the integrity of newly published mobility data used in the simulations Diffusivities in oxidizing ambient are enhanced, more so in than in silicon The enhancement increases with decreasing temperature, being about 10 for at 850°C It is demonstrated that there is good agreement between the observed diffusivity enhancement and growth of oxidation stacking faults if an interstitialcy mechanism is invoked to explain both phenomena Observed segregation coefficients are different for the two silicon orientations but they obey the same activation energy over the temperature range

The Viscosity and Structural Relaxation Rate of Evaporated Amorphous Selenium.

Abstract: Certain properties of vacuum‐deposited thin films of a‐Se were measured and compared with those of melt‐quenched Se glass. After annealing in the temperature range 25–50 °C, the creep, heat capacities, and far‐infrared behaviors of the vapor‐deposited films were indistinguishable from those of melt‐quenched specimens. The shear‐viscosity η measurements on the films were extended into a temperature range well below that where such data on melt‐quenched samples are available. Over this temperature range, 25–35 °C, the T dependence of η of the equilibrated films was well fit by an Arrhenius relation with an activation energy of 130 kcal/mole. The stress and thermal‐relaxation rates for annealed films were also measured as functions of T. They are controlled by different sets of relaxation processes and are not directly proportional to the equilibrium η. However, they are in good agreement with each other, except that the thermal‐relaxation times are more widely dispersed over a span of about two orders of ma...

An Investigation of the Minimum Film Boiling Temperature on Horizontal Surfaces

Abstract: The frequency and magnitude of liquid-solid contacts in the film boiling of saturated ethanol and water on horizontal flat stainless steel and copper surfaces are examined with electrical conductance probes. It is observed that, at atmospheric pressure, contacts occur over a wide temperature range and are generally induced by hydrodynamic instabilities. In the ethanol system, these contacts can account for the entire heat transfer rate at the minimum film boiling temperature. The area and duration of contacts are strongly influenced by the dominant nucleation process, and thus depend on the interface temperature and wettability of the solid. At elevated pressures, direct liquid-solid contacts in film boiling are essentially nonexistent for stainless steel and the first major contact corresponds to quenching of the surface. Under these conditions, spontaneous nucleation upon contact seems to be the controlling mechanism for the minimum film boiling temperature. The minimum point appears to be determined either by a Taylor instability vapor removal limitation or by spontaneous nucleation. The governing mechanism is the one which is stable at the lowest wall temperature.

Thermal Oxidation Kinetics of Silicon in Pyrogenic H 2 O and 5% HCl / H 2 O Mixtures

Abstract: The kinetics of the thermal oxidation of silicon in and in in using a pyrogenic system have been investigated over the temperature range 900°–1100°C. Oxide thickness‐oxidation time data, rate constants, and activation energies have been obtained for (111) and (100) oriented, , n‐type silicon. Results obtained for oxidation in are very similar to those reported previously for water bubbler systems. The addition of to does not increase the silicon oxidation rate as with dry , but the rate actually decreases apparently because of the reduced vapor pressure. The also appears to reduce impurity contributions from the oxidation system.

Observation of the collapse of a polymer chain in poor solvent by small angle neutron scattering

Abstract: The variation of the radius of gyration R of a polymer chain in an extremely dilute solution is measured as a function of the temperature T below the Flory temperature ϑ. The data are deduced from small angle neutron scattering experiments. In the temperature range studied two domains appear: In the first one (the ϑ region) R is rather independent of the reduced temperature τ[τ= (T−ϑ)/ϑ]. In the second one the collapse of the chain is observed and the τ dependence of R is R∼‖τ‖−0.32±0.05. The temperature variation of the second virial coefficient is also given. These results are discussed with regards to a possible demixion of the solution.

Temperature variation of elastic constants of pyrope-almandine garnets.

Abstract: Elastic constants of three natural pyrope-almandine garnets (Fe/(Mg+Fe)=0.18, 0.48 and 0.58) were measured by an RPR method in the temperature range between -190° and 80°C. The temperature derivative of bulk modulus ∂Ks/∂T does not depend much on the composition, but those of shear constants ∂C44/∂T and∂Cs/∂T [Cs=(C11-C12)/2] increase considerably with increase in the Fe-content. The temperature derivatives of the elastic constants (kbar/deg) of pyrope and almandine at room temperature are determined by extrapolation: From the measured elastic parameters of garnet, the material of the high velocity layer (Vp=8.6km/s and Vs=4.9km/s) in the oceanic lithosphere is interpreted as eclogite or garnet peridotite constituted dominantly of garnet.

Rate constants for the reactions of the OH radical with (CH3)2NH, (CH3)3N, and C2H5NH2 over the temperature range 298–426 °K

Abstract: Rate constants for the reactions of the OH radical with (CH3)2NH, (CH3)3N, and C2H5NH2 have been determined over the temperature range 298–426°K by a flash photolysis–resonance fluorescence technique. The Arrhenius expressions obtained are with rate constants at room temperature of (6.54±0.66) ×10−11, (6.09±0.61) ×10−11, and (2.77±0.28) ×10−11 cm3 molecule−1 sec−1 for (CH3)2NH, (CH3)3N, and C2H5NH2, respectively. Possible mechanisms and atmospheric significance of these reactions are discussed.

Dielectric Properties of Polyimide Film. II. DC Properties

Abstract: Dielectric properties have been measured, in the frequency range 100 to 100 kHz and in the temperature range -200 to +150°C, for as-received polyimide and for polyimide annealed at 240°C/24 hr. Three loss peaks were found. The structural motions which give rise to these peaks have been determined through a comparison with other data on polyimide, as well as with data on the structurally simi lar polyamides and polyurethanes.

The electron capture cross section and energy level of the gold acceptor center in silicon

Abstract: The electron capture cross section of the gold acceptor center has been measured by a direct technique which measures the electron‐trapping rate. The value was found to be 9±2×10−17 cm2 over the temperature range 77–280 K. Using this value together with thermal emission data, the energy level of the center was calculated from the detailed balance equation. Assuming a degeneracy factor of 4, the calculated temperature variation of the energy level (as measured from the conduction band) was found to be very similar to the temperature variation of band gap itself, indicating that the acceptor level may be fixed to the valence band edge with a separation of 0.632±0.004 eV.

Mossbauer effect study of Fe3Sn2

Abstract: The intermetallic compound Fe3Sn2 has been studied by Mossbauer spectroscopy and magnetisation measurements. This compound is ferromagnetic from approximately 220K to Tc=657K. In this temperature range, the spin directions are close to or parallel to the c axis while at low temperature they are perpendicular to it. The mean magnetic moment is 2.1 mu B per iron atom at 4K. The spread in the 57Fe hyperfine fields is due to anisotropic contributions while the spread in the spin directions at low temperature is related to a weak basal plane anisotropy energy. The transferred fields at the 119Sn nucleus agree with the model deduced from 57Fe Mossbauer spectroscopy.

Low Temperature Sensitization of Welded Type 304 Stainless Steel

Abstract: It has been shown that the sensitization of Type 304 stainless steel occurs at temperatures well below the normal isothermal temperature range for sensitization. A prerequisite for this low temperature sensitization (LTS) is the presence of chromium carbide nuclei along grain boundaries. This paper shows that the thermal exposure during welding can nucleate the grain boundary carbides necessary for subsequent LTS. Quantitative transmission electron microscopy studies show that no new carbides nucleate during an LTS heat treatment at 400 C. However, carbides that were nucleated at the time of welding grow during the LTS heat treatment. These findings are consistent with the proposed nucleation and growth LTS model. Using an accelerated test for intergranular stress corrosion cracking (IGSCC) in high temperature, high purity, 8 ppm oxygen water, it has been found that the rate of LTS in a Type 304 stainless steel weld heat affected zone obeys an Arrhenius temperature dependence which predicts that ...

Thermal cycling-induced deformation of fibrous composites with particular reference to the tungsten-copper system

Abstract: Dimensional stability of fibrous composites under conditions of elevated temperature cycling has been examined with reference to the familiar model system, tungsten wirereinforced copper. Preferential growth of the matrix in the direction parallel to the reinforcing fibers, the amount of which increased with the number of cycles, was observed in specimens subjected to hundreds of repctitive thermal cycles in the temperature range between 0.35 and 0.8 of the matrix homologous temperature. The amount of growth per unit length after a given number of cycles was found to increase with increasing the holding time at the upper cycling temperature; it was also dependent on such composite variables as the fiber length, fiber diameter, and fiber volume fraction. This observation strongly suggests that interface sliding plays an important role in elevated temperature deformation of this class of material. On the basis of a model which assumes a viscous nature of the phase boundaries, the phenomenon observed is theoretically explained in terms of interfacial sliding-induced relaxation of the internal stress built up in the composite due to differential thermal expansion of the composite constituents.

Magnetic properties of rare‐earth magnesium compounds of the type RMg2

Abstract: The magnetic and crystallographic properties of rare earth compounds of the type RMg2 were studied. The compounds in which R represents La, Ce, Pr, Nd, Sm and Gd were found to be of the cubic Laves phase structure. The remainder of compounds of this series crystallize in the hexagonal Laves phase structure. For all compounds the lattice constants were determined. Most of these compounds order ferromagnetically at temperatures well below 100 K. Exceptions are LaMg2, YbMg2 and YMg2 which exhibit Pauli paramagnetism, whereas SmMg2 and EuMg2 order antiferromagnetically. From the magnetic properties and from the lattice constants it is concluded that Eu and Yb are both divalent in these compounds. A peculiar temperature dependence of the magnetization was observed in SmMg2. A sharp peak‐like rise in the magnetization occurs within a temperature range of 10 K only, the magnetization being nearly temperature‐independent otherwise. Details regarding the nature of the crystal fields in the cubic RMg2 compounds were derived from specific heat measurement performed on LaMg2, CeMg2 in the temperature range 1.5–40 K.

Single‐crystal elastic constants of Al2Cu

Abstract: The single‐crystal elastic constants of tetragonal Al2Cu have been measured over the temperature range 4.2–300 K by the pulse‐echo‐overlap technique. The directly measured constants show a normal monotonic decrease in magnitude with increasing temperature but with C44 showing the least change. The anisotropy ratios C33/C11, C12/C13, and C44/C66 show only small percentage changes between 4.2 and 300 K, but 2C44/(C11−C12) changes by nearly a factor of 2. Comparison of data for Zr2Ni with that for Al2Cu indicates some importance of atomic size effects. A Debye temperature of 402 K was evaluated from the elastic constants at 4.2 K.

A study of 'absorption currents' in low-density polyethylene

Abstract: A study of variations of charging and discharging transients in low-density (0.918 gm cm-3) polyethylene has been made over a wide range of fields and temperature and a limited range of sample thickness and electrode materials. In the temperature range 113-273K two broad isochronal (i.e. current at constant time) peaks were observed at 138K ( gamma -relaxation) and 203K respectively. The observed behaviour of both peaks may be explained by dipolar processes accompanying molecular relaxations in the bulk. Above 273K the 'absorption current' may be due to a charge carrier hopping mechanism.

Microstructural changes which occur during isochronal heat treatment of the nickel-base superalloy IN-738

Abstract: The microstructure of the cast superalloy IN-738 has been analysed after isochronal heat treatments (16 h) within the temperature range 773 to 1467 K. The effect of such heat treatments on the morphology, distribution and volume fraction of bothγ′ and carbide precipitates has been established and correlated with changes in the hardness. These measured hardness changes are discussed in relation to the contributions ofγ′ precipitation hardening and matrix solid solution hardening.