Heating In Argon Research Articles

A Comparison of Reducing and Oxidizing Heat Treatments of Hot-Pressed Silicon Nitride

The effects of postfabrication heat treatments in argon and air atmospheres at 1400°C on the microstructure of a hot-pressed silicon nitride are compared. Heating in argon leads to a disappearance of the intergranular glass phase (with the consequent formation of interconnected porosity) and a reduction in the content of silicon oxynitride, and results in a soft, easily abraded surface. On subsequent heating in air, an oxide scale, similar to that produced by oxidation, rich in impurity and additive cations, is formed on the surface. In addition, silicon oxynitride and silica are formed below the scale. On cooling, macroscopic cracking, attributed to the volume increase of SO2 on transformation to cristobalite, occurs. Material heated first in air and then in argon resembled that of material heated in argon alone.

Strong metal-support interactions in nickel/titania catalysts: The importance of interfacial phenomena

Nickel catalysts supported on silica and titania have been prepared by wet impregnation. The uncalcined catalysts, after an initial mild reduction at 623 K, have been treated by heating in hydrogen or argon at successively higher temperatures up to 923 K. The activity and selectivity of the catalysts have been determined under flow conditions at 1 bar pressure for the hydrogenolysis of n-hexane at 548 K, for the hydrogenolysis of ethane over the temperature range 478–673 K, and for the CO/hydrogen reaction at 548 K. Hydrogen adsorption isotherms have been measured and the specific metal surface areas determined using the Langmuir equation for dissociative adsorption to determine the monolayer coverage. Mean crystallite diameters have been determined by X-ray line broadening. Ni/titania catalysts exhibit normal hydrogen chemisorption properties after reduction at 723 K, although adsorption is suppressed after thermal treatment at 923 K. X-Ray line broadening measurements indicate a mean particle size of 8.0 nm after reduction at 723 K, and 13.3 nm after heating to 923 K. Activity measurements, after reduction at 723 K, show excellent agreement between the specific activities of the Ni/silica and Ni/titania catalysts for the hydrogenolysis of n-hexane and ethane. All these results are consistent with literature data for these reactions. However, for the CO/hydrogen reaction the specific activity of the Ni/titania catalyst is a factor of 50 higher than that of the Ni/silica catalyst. After heating at 923 K, there was a large decrease in the activity of the Ni/titania catalysts for all the reactions investigated. It is concluded that the enhanced activity of the Ni/titania catalysts for the CO/hydrogen reaction cannot be due to strong metal-support interactions (SMSI) as defined in the literature. A model of the Ni/titania system is presented which emphasizes the importance of considering the interface between the metal and the partially reduced support. It is proposed that new active sites created at the interface are responsible for the high specific activity of these catalysts for the CO/hydrogen reaction.

Correlation of the crystal structural and microstructural effects of the interfacial processes between gold and GaAs

The solid state interfacial reaction between gold and GaAs was found to result in small surface microstructural changes, whereas melting of the reaction product (tentatively identified as AuGa) and subsequent solidification to form the β phase resulted in aligned rectangular protrusions with sharp corners. The rectangular protrusions were found to be much finer in scale after heating in a vacuum than after heating in argon at 1 atm. High cooling rates ( e.g. 40°C min -1) during solidification resulted in the formation of both rectangular protrusions and irregularly shaped protrusions with jagged boundaries and topography. The irregular protrusions were associated with a phase tentatively identified as Au 2Ga. Rapid cooling at about 600°C min -1 during solidification resulted in the formation of the gold-rich terminal solid solution (α phase) which appeared as aligned rectangular protrusions with rounded corners.

Specific Heat of Ar‐Xe Solid Solutions at Low Temperatures

AbstractThe measured specific heat of argon with solid solutions of xenon containing 5.01 and 10.88 at% Xe are analyzed on the basis of low concentration Green's function theory. Maxima are observed at about 4 K in the relative variation ΔC(T)/C(T) of the solid mixture. The maxima are related to the appearance, in the solid mixture phonon spectrum, of quasilocal oscillations due to introduction of heavy Xe impurity atoms into the Ar lattice. The changes in mass at the impurity site as well as changes in the closest‐neighbour force constants are taken into account in performing the computations. The force constants for the interaction between the impurity atom and its nearest neighbours are larger than the atomic interaction constants in the initial lattice. For the Ar‐10.88 at% Xe system the low‐concentration theory is modified. Reasonably good agreement with the experimental data is obtained.

Correlation of the Crystal Structural and Microstructural Effects of the Interfacial Processes Between Gold and GaAs

The solid state interfacial reaction between gold and GaAs, was found to result in small surface microstructural changes, whereas melting of the reaction product (tentatively identified as AuGa) and subsequent solidification to form the β phase resulted in aligned rectangular protrusions with sharp corners. The rectangular protrusions were found to be much finer in scale after heating in a vacuum than after heating in argon at 1 atm. High cooling rates (e.g. 40°C min−1) during solidification resulted in the formation of both rectangular protrusions and irregularly shaped protrusions with jagged boundaries and topography. The irregular protrusions were associated with a phase tentatively identified as Au2Ga. Rapid cooling at about 600°C min−1 during solidification resulted in the formation of the gold-rich terminal solid solution (α phase) which appeared as aligned rectangular protrusions with rounded corners.

Qualitative Analysis by Photoacoustic Spectroscopy of Amorphous Uranium Tetrafluoride Deposits

Abstract Recent studies concerning uranium plasma chemistry have confirmed the generally complicated nature of uranium chemistry1. Uranium plasma, generated by either rf-heating or dc arc torch argon heating of uranium hexafluoride, tends to react with its environment to produce among other compounds both fluorides and oxy-fluorides. The products are characterized as amorphous, powdery deposits. While uranyl fluoride can be identified readily by its infra-red absorption band, uranium tetrafluoride is not easily analyzed by any routine, non-destructive technique. Procedures such as Ion Scattering Spectroscopy and Secondary Ion Mass Spectrometry are expensive and time consuming; while, X- ray and electron deffraction can not overcome the amorphous nature of the plasma deposits.

Calculation of thermodynamic averages of phonon spectra for fcc Lennard-Jones harmonic crystals

Based on moments of the frequency distribution, delta function representations for the phonon frequency distribution of an fcc Lennard-Jones harmonic crystal over a wide density range are given using the law of corresponding states. Their use and convergence is illustrated for the zero point energy and rms displacement. A comparison with a previous harmonic theory calculation is given for the specific heat of solid argon.

Numerical Calculation of Vibrational Relaxation and Dissociation for a Quantum Anharmonic Oscillator

The vibrational relaxation and dissociation of gaseous H2 highly diluted in an argon heat bath have been studied numerically at temperatures between 2500 and 15 000°K. Collisional transition probabilities between the vibrational levels of H2 were calculated by the method of Schwartz, Slawsky, and Herzfeld in a form modified for a Morse oscillator. The master equation was solved by numerical integration to give the time dependence of the vibrational level populations and of dissociated molecules in a simulated shock-wave experiment. The absolute values of the calculated rate coefficients for dissociation are in good agreement with experiment, but the predicted vibrational relaxation times are about a factor of 35 shorter than those determined experimentally. At all temperatures, the upper vibrational levels in the dissociating gas are substantially depleted below their equilibrium populations. The Arrhenius activation energy for dissociation is 102.5 kcal mole−1, slightly lower than the dissociation energy of 103.2 kcal but higher than the experimentally determined activation energies by about 10 kcal. An incubation period of about 1 to 1.5 vibrational relaxation times precedes the establishment of approximately steady dissociation. The computed incubation behavior is compared with that predicted from the diffusion theory model of Brau, Keck, and Carrier. The vibrational relaxation behavior of this anharmonic oscillator molecule has been examined. In the absence of dissociation, the relaxation time as defined by the Bethe–Teller equation differs from its SHO value by less than 10%, and is found to show variations of less than 10% with time. The present results are also used to examine some approximations made in theories of coupled vibration and dissociation.

The specific heat of liquid argon

The specific heat at constant volume of liquid argon has been measured within the temperature range from 87–150 K along the boiling line and at pressures up to 100 atm (10 MN m −2). By means of the measured values of the specific heat of the two—phase system liquid-gas, the specific heats at constant pressure and along the boiling line were calculated. The experimental results are in good agreement with the principle of corresponding states for the rare gases.

Lattice Dynamics with Three-Body Forces: Argon

The specific heat, thermal expansion, and bulk modulus of crystalline argon at low temperatures (up to 12 \ifmmode^\circ\else\textdegree\fi{}K) are calculated using pair potential functions known to be consistent with pair-interaction data, together with the Axilrod-Teller triple-dipole interaction. The methods of lattice dynamics are used and correction for anharmonicity is made. The potential of Barker and Pompe gives good agreement with experimental specific heats and a slight modification of this potential gives excellent agreement. The modified potential also gives excellent agreement with experimental thermal-expansion results, but the calculated bulk modulus is about 9% higher than current experimental estimates. Harmonic calculations are also made using the potential of Dymond and Alder. The resulting harmonic specific heats are in very poor agreement with experiment and there are very large anharmonic effects.

Vibrational Relaxation Measurements of Carbon Monoxide in a Shock-Tube Expansion Wave

A novel technique has been developed for use in the study of collisional energy transfer and chemical reaction rates in rapidly cooled gases. The principal feature is that the shock heated test gas sample is stationary during the cooling process so that the temporal behavior of a particular nonequilibrium property may be continuously monitored. The technique has been applied to the measurement of the vibrational de-excitation rate of carbon monoxide in an argon heat bath, using the fundamental infrared emission band of CO to follow its vibrational temperature throughout the expansion. In contrast to some recent investigations which indicate enhanced vibrational de-excitation rates, the present experiments show that when translation–vibration collisions control the relaxation process the characteristic vibrational relaxation time of carbon monoxide diluted in argon is the same no matter whether it is measured in a compression (shock wave) or expansion environment.

Structure and specific heat of liquid argon

A method is presented of estimating in a quantitative way the effect on the structure of a dense fluid of adding an attractive well to the repulsive core of the pair potential, and a calculation is carried through of the structure and specific heat of a model of a liquid with a pair potential having a rigid core and a 6-12 tail appropriate to argon. When compared with the structure of a fluid having a full 6-12 interaction the overall agreement is good. The constant volume specific heat Cv, however, is small. The importance of taking into account the softness of the core in a calculation of the specific heat is clearly shown for the case of a 6-12 fluid.

Photoemission Investigation of the Band Structure of Ferromagnetic Ni-Al Alloys

Photoemission measurements probing the electronic band structure of ferromagnetic Ni-Al alloys have been made with ultraviolet light, from 4.5 to 11.4 eV, to determine the optical density of electronic states down to about 6 eV below the Fermi level. Photoemission data were taken at room temperature on two Ni-Al alloys of compositions 92.0 and 95.4 at.% Ni. The results are interpreted in terms of the rigid-band model and scattering phenomena. The optical density of states is assumed to be the same for all the Ni-Al alloys investigated. Results obtained from a pure Ni sample cleaned by argon bombardment and heat treatment are the same as those of Blodgett from a pure Ni sample formed by evaporation onto a substrate.

On the preparation, optical properties and electrical behaviour of aluminium nitride

Methods are described for single crystal and film growth. The refractive index of crystals was found to be 2·16±0·08 and the fundamental optical absorption edge 5·9±0·2 eV. All crystals examined showed a weak absorption centred at 2·8 eV, which could be enhanced by the introduction of excess aluminium. The absorption edge of single crystals shifted to lower energies on heating in argon. Maximum photoconductivity occurred at 2·8 eV on illuminating the cathode. Thermal activation energies of 1·4 ± 0·1 eV and 0·5 ± 0·1 eV associated with electrical conductivity were observed. The log current-log voltage characteristics of crystals show an apparent Traps Filled Limit behaviour which is discussed in terms of a thin surface layer whose presence is indicated by experiments using current and potential probes. Thin films of A1N showed no apparent Traps Filled Limit behaviour.

The specific heat of solid argon

The atomic mobilities of Co, Mn, Pt and Re in face-centered cubic (fcc) Co–X ( X = Mn, Pt and Re) alloys have been assessed as a function of temperature and composition by CALculation of PHAse Diagram (CALPHAD) approach. To validate the literature data and provide new experimental data, we prepared two fcc Co–Mn diffusion couples, which were used to determine the composition-dependent inter-diffusivities at 1073 and 1273 K. Based on various kinds of experimental diffusivities available in the literature and the experimental results in this work, the atomic mobilities in fcc Co–X (X = Mn, Pt and Re) alloys were obtained. Comprehensive comparisons between the calculated and measured diffusivities show that most of the experimental data can be well reproduced by the presently obtained atomic mobilities.

Specific Heat of a Gas Near the Critical Point

Recent measurements of the constant-volume specific heat of argon (and of oxygen) at critical density and close to the critical point are compared with calculations based on the three-dimensional nearest-neighbor lattice gas models. It is argued that the configurational specific-heat density ${C}^{*}(T)=\frac{\ensuremath{\rho}{C}_{\mathrm{config}}(T)}{{\ensuremath{\rho}}_{max}}$ should be compared with the theoretical configurational specific heat per lattice site. On this basis the calculations above ${T}_{c}$ agree to within 10% with the observations for argon and are consistent with a divergence like ${(T\ensuremath{-}{T}_{c})}^{\ensuremath{-}\frac{1}{5}}$ over the range $\frac{(T\ensuremath{-}{T}_{c})}{{T}_{c}}={10}^{\ensuremath{-}1} \mathrm{to} {10}^{\ensuremath{-}4}$. Below ${T}_{c}$ the strength of the apparently logarithmic divergence of the specific heat for the fcc lattice agrees to within 10% with the experimental data but the theoretical magnitude of ${C}^{*}(T)$ is too small for $T>0.9{T}_{c}$ by an almost constant amount of $1.05k$ per site.

Vacancy contribution to the specific heat of solid argon

Abstract The specific heat of solid argon at constant volume is calculated on the basis of the anharmonic Einstein model in the temperature range 40–84°K, where anharmonic effects are important. The results are used to estimate the contribution made by thermal vacancies to the measured C v and from this the free energy of formation of a vacancy is determined. The value so obtained is in disagreement with theoretical values and the reasons for this are discussed.

The Specific Heats of Solid Argon and an Equimolar Argon-Krypton Mixture

The specific heat of argon has been measured over the temperature range 16° to 35°K. The results are used to calculate a value for the Debye θ, which is in excellent agreement with lattice dynamics. The specific heat of an equimolar argon-krypton mixture has been measured from 12°K to the melting point. No evidence of any phase separation can be found from the results.

The interaction of atoms and molecules with solid surfaces XII—Critical phenomena in a two-dimensional gas

In a paper recently published by Professor Lennard-Jones and the author (Lennard-Jones and Devonshire 1937) the equation of state of a gas at high concentrations has been calculated in terms of the interatomic fields. The equation found had the right kind of properties and, in particular, using the interatomic fields previously determined from the observed equation of state at low concentrations (Lennard-Jones 1931), the critical temperature was given correctly to within a few degrees for the inert gases. In this paper we shall apply the same method to determine the equation of state of a two-dimensional gas. Although such a gas cannot strictly be obtained in practice, an inert gas adsorbed on a surface (or in fact any gas held by van der Waals’ forces only) would probably behave very much like one, the fluctuations of the potential field over the surface not being of much importance. In confirmation of this it may be noted that the specific heat of argon adsorbed on charcoal was found by Simon (Simon 1935) to be equal to that of a perfect two-dimensional gas down to 60° K. A gas adsorbed on a liquid would be an even better representation of a two-dimensional one. Some measurements on the adsorption of krypton and xenon on liquid mercury have been made by Cassel and Neugebauer (Cassel and Neugebauer 1936), and they found no trace of any critical phenomena though they worked at temperatures considerably below the critical temperature of xenon. Our results are in agreement with this, for they show that the critical temperature of a two-dimensional gas should be about half that of the corresponding three-dimensional one.