Investigation of the dominant point defects in tetragonal YBa2Cu3Ox at elevated temperatures

The manner in which oxygen is incorporated into YBa₂Cu₃O _x (YBCO) at ～800°C for values ofx close to 6 is shown to be in the form of neutral oxygen interstitials, O _i ^x . The experimental data on which this conclusion is based are obtained from measurements of oxygen partial pressure,P(O₂), as a function of compositionx and temperatureT (5.99≤x≤ 6.35, 825≤T≤1120 K). The data are obtained by a solid-state electrochemical method. Other conclusions of this study include: (a) O _i ^x are noninteracting forx ? 6. (b) The stoichiometric composition of YBCO isx ? 6.0. (c) The reaction enthalpy of oxidation is 179 kJ/mol O₂. (d) The Fermi level changes by ? ?0.2 eV asx increases from 6.05 to 6.35.     

Radiance temperature (at 653 nm) of tungsten at its melting point

The radiance temperature (at 653 nm) of tungsten at its melting point was measured using a subsecond-duration pulse-heating technique. Specimens in the form of strips with initially different surface roughnesses were used. The results do not indicate any dependence of radiance temperature (at the melting point) on initial surface or system operational conditions. The average radiance temperature (at 653 nm) at the melting point for 23 tungsten specimens is 3208 K on IPTS-68, with a standard deviation of 0.8 K and a maximum absolute deviation of 1.9 K. The total error in the radiance temperature is estimated to be not more than ± 10 K.     

Effect of adding cobalt on mechanical and structural properties of Cu–2Be alloy aged below half its melting temperature

Abstract

The present study mainly covers the effect of adding cobalt on the mechanical and structural properties of Cu–2Be alloy. The stress–strain technique has been employed to investigate the effect of aging time and testing temperature on the work hardening characteristics of Cu–2Be and Cu–2Be–0·3Co alloys aged below half its melting temperature. The obtained results have been discussed based on the structural changes resulted during the aging process.     

Equilibrium concentration of interstitials in aluminum just below the melting temperature

A non-contact EMAT technique of ultrasonic generation and detection was used to make high temperature measurements of the C₄₄ deviation from perfect crystal behavior just below the melting temperature and a much smaller effect for C′, which is to be expected for interstitials in FCC crystals. The results are analyzed to give a detected interstitial concentration of 1.7±0.6×10⁻⁴ just below the melting temperature of 933 K. This corresponds to a reasonable range for the interstitial formation enthalpy and entropy, and supports a major prediction of the interstitialcy theory of condensed matter states.     

Investigation of Y1Ba2Cu3O6.9 and Er1Ba2Cu3O6.8 during cyclic temperature variations

Gravimetric investigations of the superconductive ceramics Y₁Ba₂Cu₃O_6.9, Y₁Ba₂Cu_2.7Al_0.3O_7.1 and Er₁ Ba₂Cu₃O_6.8 during cycling heating cooling in the interval 293–1323 K at partial pressure of the oxygen 0.02 and 0.2 × 10⁵ Nm^–2 are presented. Intervals of intense weight changes during heating and cooling are determined. Oxygen loss was shown to be the main reason for the weight changes. Structural characteristics and resistivity of the yttrium ceramics in the above-mentioned temperature interval are investigated. Experimental data are discussed within the framework of the models of the quasi-equilibrium and essentially nonequilibrium processes. Binding energy and diffusional parameters of the evolving gas are estimated.     

On the strength of a body at temperatures close to its melting point

On the example of frozen soils and ice existing at temperatures close to the melting point, the existence of two mechanisms (types) of destruction related to the phase transitions of water is shown. The first mechanism that operates near the melting point is plastic destruction, it gradually converts an ice body into a liquid-like state. The second mechanism operating far from the melting point is brittle destruction, disintegration into parts, with a slight preliminary deformation. For both types of destruction equations of long-term strength and of the behavior of deformation in time have been obtained.     

Radiance temperatures (at 658 and 898 nm) of niobium at its melting point

Radiance temperatures (at 658 and 898 nm) of niobium at its melting point were measured by a pulse-heating technique. A current pulse of subsecond duration was imparted to a niobium strip and the initial part of the melting plateau was measured by high-speed pyrometry. Experiments were performed with two techniques and the results do not indicate any dependence of radiance temperature (at the melting point) on initial surface or system operational conditions. The average radiance temperature at the melting point of niobium is 2420 K at 658 nm and 2288 K at 898 nm, with a standard deviation of 0.4 K at 658 nm and 0.3–0.6 K at 898 nm (depending on the technique used). The total uncertainty in radiance temperature is estimated to be not more than ±6 K. The results are in good agreement with earlier measurements at the National Institute of Standards and Technology (USA) and confirm that both radiance temperature and normal spectral emissivity (of niobium at its melting point) decrease with increasing wavelength in the region 500–900 nm.Paper presented at the Third Workshop on Subsecond Thermophysics, September 17–18, 1992, Graz, Austria.     

Measurement of the radiance temperature (at 655 nm) of melting graphite near its triple point by a pulse-heating technique

Measurements of the radiance temperature of graphite at 655 nm have been performed in the vicinity of its triple point by means of a rapid pulse-heating technique. The method is based on resistively heating the specimen in a pressurized gas environment from room temperature to its melting point in less than 20 ms by passing an electrical current pulse through it and simultaneously measuring the radiance temperature of the specimen surface every 120 s by means of a high-speed pyrometer. Results of experiments performed on two different grades of POCO graphite (AXM-5Q1 and DFP-1) at gas pressures of 14 and 20 MPa are in good agreement and yield a value of 4330±50 K for the radiance (or brightness) temperature (at 655 nm) of melting graphite near its triple point (triple-point pressure, 10 MPa). An estimate of the true (blackbody) temperature at the triple point is made on the basis of this result and literature data on the normal spectral emittance of graphite.Paper presented at the First Workshop on Subsecond Thermophysics, June 20–21, 1988, Gaithersburg, Maryland, U.S.A.Formerly National Bureau of Standards     

Oxygen deficiency dependence of transition temperature in (Sm, Er) Ba2Cu3O7−δ superconductors

We have investigated the effects of oxygen deficiency (δ) on the transition temperature (T _c) of (Sm, Er)Ba₂Cu₃O₇₋ δ superconductors by incorporating the effects of the two dimensional (2D) acoustic phonons and plasmons in the framework of strong coupling theory. The proposed approach for yttrium cuprates properly takes care of the double CuO₂ plane in a unit cell and has been found earlier to be successful in describing the pairing mechanism as well as the variation ofT _c withδ in Y Ba₂Cu₃O₇₋ δ system. The coupling strength (λ), the screening parameter (μ*) and the two dimensional acoustic phonon (plasmon) energyħω ₋(ω₊) as a function of oxygen deficiency is worked out. Finally, the transition temperature is evaluated and is found to be consistent with the earlier experimental data on yttrium cuprates. Thus, coupled phonon-plasmon mechanism is adequate to understand the nature of pairing mechanism and oxygen deficiency dependence of transition temperature in 90 K (Sm, Er)Ba₂Cu₃O₇₋ δ superconductors.     

Effects of pulling speed and hot zone temperature on the directional growth of YBa2Cu3Ox superconductor

The effects of pulling speed and hot zone temperature on the microstructure and the superconducting properties of YBa₂Cu₃O_x sample textured using directional growth of Y₂BaCuO₅, BaCuO₂, and CuO powder mixture were studied. The grain size, the alignment, and the critical current density of the sample were increased as the pulling speed decreased. The sample grown directionally at 1.5 mm h^–1 pulling speed consisted of a single grain. The zero resistivity temperature and the critical current density of the sample increased as the hot zone temperature increased up to 1120°C beyond which the sample consisted of Y₂O₃ and impurities and showed resistivity at 77 K. The sample grown directionally at 1.5 mm h^–1 pulling speed and at 1120 °C hot zone temperature showed sharp resistivity transition of 91 K zero resistivity and over 6000 Acm^–2. The sample showed well aligned microstructure. Compared to data from another study, the hot zone temperature required to produce maximum critical current density is lower due to low liquid forming temperature.     

Apparatus for realizing the ITS-90 in the temperature range from the triple point of argon to the melting point of gallium

Systems for realizing the fixed points of the ITS-90 for calibrating column and capsule standard platinum thermometers, namely, the triple points of argon and mercury and the melting point of gallium, are constructed and investigated. The errors of the values of the metrological characteristics of the systems obtained enable one, using platinum resistance thermometers, to reproduce and transfer the temperature scale in the 83.8–302.9 K range. The extended uncertainty in reproducing the temperatures of the fixed points does not exceed 0.4 mK. This paper has been prepared from the contributions presented at the 3rd All-Russia Conference “Temperature 2007”; see the selection of papers in Measurement Techniques, Nos. 8 and 9, 2007. __________ Translated from Izmeritel’naya Tekhnika, No. 11, pp. 26–31, November, 2007.     

Surface free energy of nickel and stainless steel at temperatures above the melting point

The surface tension (liquid-state surface free energy) of pure nickel and type 304 stainless steel was measured in a narrow temperature range above the melting point by the sessile drop method. The temperature coefficients of surface free energy in the liquid state were found to be –1.76 erg cm^–2 C^–1 for nickel and –2.48 erg cm^–2 C^–1 for stainless steel. These values are shown to be a factor of 2 larger than those previously determined for the solid surface free energies of nickel and stainless steel, but have the same sign. The latent heats of fusion of nickel and 304 stainless steel were determined by comparison of variations of solid and liquid-state surface energies with temperature and found to be 292 and 284 erg cm^–2 respectively. Measurement of the contact angle for a nickel sessile drop on thoria and a stainless steel sessile drop on alumina showed a decrease in the angle with an increase in temperature.     

Correlation between temperature dependences of thermal expansivity and heat capacity up to the melting point of tantalum

Following the course of previously published series, this work studies in detail the correlation of the volumetric thermal expansion coefficient β(T) and the heat capacity C(T) of refractory tantalum. It is demonstrated that a clear correlation β(C) takes place in the lower temperature range and remains up to the metal melting point inclusively. Significant deviation from lower temperature linear behavior of the β(C) dependence occurs when the heat capacity reaches the classical 3R Dulong–Petit limit. The temperature dependence of differential Grüneisen parameter γ' ~ (?β/?С) is estimated.     

Electrical conductivity,self-diffusion,and volume expansion of alkali halides at the melting point

In an earlier paper, Heisenberg's uncertainty principle was invoked at the melting point T _m of crystalline solids to provide fundamental justification for Lindemann's melting law and to compute diffusion coefficients of several alkali halides. The uncertainty principle defines breakdown of Debye zone boundary (ZB) phonons as valid collective excitations when phonon energies and line widths due to anharmonicity become comparable at T _m. Upon breakdown, random, high-frequency single-particle motion or partial decoupling of crystal ions sets in. Lifetimes of these single-particle ZB motions are determined from the minimum-uncertainty product inequality by assuming that it becomes an equality at T _m for ZB phonons. The present paper addresses improved formulation of that work and extended application to ionic electrical conductivities of 18 molten alkali halides at T _m. It is shown that use of the Debye model produces an approximate lower bound to the mean free time, not the unconstrained direct estimate previouslu implied. This feature is generally reflected in results for ionic conductivities and alkali halide diffusion coefficients for which comparison experimental data were found. However, in spite of this lower-bound formulation and the simple nature of the computation, the results compare favorably with experiment. A model of random single-particle harmonic motion superimposed on the lower-frequency collective motion is proposed to account for volume expansion accompanying the partial decoupling for hard-sphere ions. Experimental comparisons for 15 alkali halides show the decoupling volume change to account largely for the total volume change of melting (in the hard-sphere approximation), yielding a closer agreement with experiment than recent calculations aimed explicitly at the total volume change.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Radiance temperatures (in the wavelength range 519–906 nm) of tungsten at its melting point by a pulse-heating technique

Radiance temperatures (at six wavelengths in the range 519–906 nm) of tungsten at its melting point were measured by a pulse-heating technique. The method is based on rapid resistive self-heating of the specimen from room temperature to its melting point in less than 1 s; and on simultaneously measuring the specimen radiance temperatures every 0.5 ms with a high-speed six-wavelength pyrometer. Melting was manifested by a plateau in the radiance temperature versus time function for each wavelength. The melting-point radiance temperatures for a given specimen were determined by averaging the measured temperatures along the plateau at each wavelength. The melting-point radiance temperatures for tungsten were determined by averaging the results at each wavelength for 10 specimens (standard deviation in the range 0.5–1.1 K, depending on the wavelength) as follows: 3319 K at 519 nm, 3236 K at 615 nm, 3207 K at 652 nm, 3157 K at 707 nm, 3078 K at 808 nm, and 2995 K at 906 nm. Based on estimates of the random and systematic errors arising from pyrometry and specimen conditions, the total uncertainty in the reported values is about ±7 K at 653 nm and ± 8 K at the other wavelengths.Paper presented at the Third Workshop on Subsecond Thermophysics, September 17–18, 1992, Graz, Austria.     

Radiance temperatures (in the wavelength range 522–906 nm) of niobium at its melting point by a pulse-heating technique

Radiance temperatures (at six wavelengths in the range 522–906 nm) of niobium at its melting point were measured by a pulse-heating technique. The method is based on rapid resistive self-heating of the specimen from room temperature to its melting point in less than 1 s and on simultaneously measuring the specimen radiance temperatures every 0.5 ms with a high-speed multiwavelength pyrometer. Melting was manifested by a plateau in the radiance temperatureversus-time function for each wavelength. The melting-point radiance temperatures for a given specimen were determined by averaging the measured temperatures along the plateau at each wavelength (standard deviation of an individual temperature from the mean: 0.1–0.4 K). The melting-point radiance temperatures for niobium were determined, by averaging the results at each wavelength for 10 specimens (standard deviation: 0.3 K), as follows: 2497 K at 522 nm, 2445 K at 617 nm, 2422 K at 653 nm, 2393 K at 708 nm, 2337 K at 809 nm, and 2282 K at 906 nm. Based on estimates of the random and systematic errors arising from pyrometry and specimen conditions, the total error in the reported values is about 5 K at 653 nm and 6 K at the other wavelengths.Paper presented at the Second Workshop on Subsecond Thermophysics, September 20–21, 1990, Torino, Italy.     

Radiance temperatures (in the wavelength range 525 to 906 nm) of vanadium at its melting point by a pulse-heating technique

The radiance temperatures (at six wavelengths in the range 525 to 906 nm) of vanadium at its melting point were measured by a pulse-heating technique. The method is based on rapid resistive self-heating of the specimen from room temperature to its melting point in less than 1 s and on simultaneously measuring the specimen radiance temperatures every 0.5 ms with a high-speed six-wavelength pyrometer. Melting was manifested by a plateau in the radiance temperature-vs-time function for each wavelength. The melting-point radiance temperatures for a given specimen were determined by averaging the measured temperatures along the plateau at each wavelength. The melting-point radiance temperatures for vanadium as determined by averaging the results at each wavelength for 16 specimens (standard deviation in the range 0.3 to 0.4 K. depending on the wavelength) are 2030 K at 525 nm, 1998 K at 622 nm, 1988 K at 652 nm, 1968 K at 714 nm, 1935 K at 809 nm, and 1900 K at 906 nm. Based on estimates of the random and systematic errors that arise from pyrometry and specimen conditions, the resultant uncertainty (2 SD level) in the reported values is about ±7 K at each wavelength.     

Experimental determination of the volume change of pure salts and salt mixtures at their melting point

Measurements of the melting temperature as a function of pressure for some pure salts (NaNO₃, LiOH) and salt mixtures (Na₂SO₄-NaCl, NaNO₂-NaNO₃- KNO₃, Li₂SO₄-NaCl, NaCl-MgCl₂) are reported. The experiments were performed up to 250 MPa. From the melting curves, the slopes (dP/dT) _m ⁰ at atmospheric pressure were determined. Finally, using the Clapeyron equation, with the enthalpy of melting taken from the literature, the volume change on melting at atmospheric pressure was determined for each compound.