Spectral Reflectivity of Ceramics of Cubic Zirconium Oxide on the Wavelength of 0.63 μm under Intense Surface Heating above the Melting Point and Subsequent Cooling

The experimental data on the normally hemispherical reflectivity of zirconium oxide ceramics are obtained for the wavelength of 0.63 m in the course of the first cycle of heating by concentrated laser radiation up to temperatures above the melting point and free cooling in ambient air. The obtained data are compared with analogous results of the second and subsequent cycles of heating and cooling. Analysis is made of the effect of the temperature field in the heated layer on the reflectivity and on the effective temperature measured by a pyrometer.     

Temperature Dependence of the Density and Electrical Resistivity of Liquid Zirconium up to 4100 K

Experimental data on the temperature dependence of the density of liquid zirconium and its electrical resistivity are obtained under conditions of pulsed electric heating (for 5 to 10 s) of wire and foil zirconium samples. Samples in the form of a wedge-shaped blackbody model are prepared for use in temperature measurements. The thermal radiation from the model space is registered by a high-speed optical pyrometer at a wavelength of 855 nm. The temperature (up to 4100 K) is calculated by Planck's formula. The temperature plateau in the melting region of the blackbody model is used for calibration of the pyrometer. In so doing, it is assumed that this plateau has the equilibrium melting temperature of 2128 K. A digital oscilloscope is used for recording the current through the sample, the voltage across the sample, and the pyrometer signal. The density of zirconium is determined using the laser flash method while heating zirconium wires. The sample is illuminated by a Q-switched Nd-YAG laser. A CCD video camera is used for photographing. The experimental data on the density and electrical resistivity of liquid zirconium are essential for simulation of the behavior of nuclear power plants in the case of a serious emergency.     

Thermophysical Properties of the Zr–0.01Nb Alloy at Various Heating Rates and Repeated Cycles of Heating–Cooling

The results of an experimental study of the heat capacity, enthalpy, electrical resistivity, and spectral emissivity (for the wavelength of 0.65 m) of the Zr–0.01Nb alloy in the temperature range from 900 to 2000 K are presented. The study was carried out using subsecond pulse heating of the samples by passing electrical current through them. Experiments were conducted at different heating rates (10³ to 10⁴K·s^–1), and a series of experiments consisted of several cycles of pulse heating and subsequent cooling. The effect of these parameters on the temperature dependence of thermophysical properties in the region of the –transition was studied. With an increase in the heating rate, the temperature of the – transition, and the maximum in the heat capacity shifted to higher temperatures. There are significant differences in properties over the temperature range of the – transition for the various heating cycles.     

The Experimental Determination of the Melting Heat for E635 Zirconium Alloy

A procedure is developed for the determination of the melting heat of zirconium alloys which is based on the preparation of an ampoule in the form of a thin oxide film on the surface of a specimen during its rapid heating in air. Owing to its higher melting point, the zirconium oxide film holds the specimen at temperatures exceeding the melting point. The brightness (for the wavelength of 0.65 m) solidus and liquidus temperatures of E635 zirconium alloy are measured, and the respective true temperatures are calculated using the measured values of spectral emissivity. The data obtained on the melting heat and the heat of – transition are presented.     

Localized High-Tc Superconductivity on the Surface of Na-Doped WO3

WO₃ crystals were doped with Na on the surface to 7% nominal atomic concentration. Scanning tunneling microscopy and spectroscopy were employed, together with magnetization measurements, in the study of these crystals. Tunneling experiments reveal superconducting islands, 20–150 nm in size, covering about 10% of the surface, the rest of which is insulating. Magnetization measurements show that the superconducting phase formed at the surface has a critical temperature of 91 K, while tunneling spectroscopy yields superconducting gaps having values up to 2/kT _c 4. Presumably most of the sodium is concentrated in these islands and therefore they are metallic in nature, above 30% in atomic concentration. This material is therefore a noncuprate superconductor with a high critical temperature.     

Valence compensated perovskite oxide system Ca1−xLaxTi1−xCrxO3 Part II Electrical transport behavior

Electrical transport behaviour of the valence compensated system Ca_1–x La _x Ti_1–x Cr _x O ₃ (x 0.50) has been investigated by studying the Seebeck co-efficient, DC and AC conductivity as a function of temperature. Seebeck co-efficient, DC conductivity of different compositions has been measured in the temperature range 300 K–1000 K. AC conductivity for different compositions were determined in the temperature range 100–550 K and frequency range 10 Hz–10 MHz. Positive values of Seebeck co-efficient show that holes are the majority charge carriers. Conduction seems to occur by correlated barrier hopping of holes among Cr³⁺and Cr⁴⁺ions or V_O and V_O . Almost equal values of activation energies obtained for DC conductivity and dielectric relaxation process show that both the processes occur by the same mechanism.     

Simultaneous ac Susceptibility and Specific Heat Measurements of YBCO Ceramics

Simultaneous ac magnetic susceptibility and specific heat studies of YBCO ceramics permitted the correct correlation of the corresponding critical temperatures. The midpoint of the real component () and the maximum of the imaginary component () of ac susceptibility curves confirmed the T _c = 92 K critical temperature value determined by dc resistance measurements. The specific heat maximum's temperature (T _cc 93.6 K)—-taken as the calorimetric critical one—fell between T _c and T* (the pseudogap onset temperature). We suggest that T _cc signals the appearance of the first localized coherent pairs, while the temperature T _c marks their percolation. The existence of two different critical temperature values confirms the coexistence of partial coherent and coherent electron pairing phenomena in the region below T _cc of the superconducting phase diagram.     

Experiments on expanded liquid metals at high temperatures

The electrical resistivity of liquid tungsten was measured using electric pulse heating of the wires inside capillary tubes. Under fast heating (10 µs) or slow heating (50 µs), the wire expands and fills the inner cavity of the capillary. On the oscillogram traces of the voltage drop across the wire, one can see the phases solid, liquid, fast expansion, and then the moment when the cavity is filled with the metal. Using the voltage drop, current, and volume of the capillary cavity, one can calculate the electrical resistivity,, of the expanded metal. Tungsten densities from 7.5 to 1 g · cm^–3(3 x 10²² to 0.5 x 10²² atoms · cm^–3) were investigated at temperatures from 10 x 10³ to 14 x 10³ K. For these densities, the electrical resistivity increased from 0.5 to 5m·cm.     

Interlaminar Fracture Toughness of CF/PEI and GF/PEI Composites at Elevated Temperatures

An experimental study has been conducted to assess temperature effects on mode-I and mode-II interlaminar fracture toughness of carbon fibre/polyetherimide (CF/PEI) and glass fibre/polyetherimide (GF/PEI) thermoplastic composites. Mode-I double cantilever beam (DCB) and mode-II end notched flexure (ENF) tests were carried out in a temperature range from 25 to 130°C. For both composite systems, the initiation toughness, G _IC,ini and G _IIC,ini, of mode-I and mode-II interlaminar fracture decreased with an increase in temperature, while the propagation toughness, G _IC,prop and G _IIC,prop, displayed a reverse trend. Three main mechanisms were identified to contribute to the interlaminar fracture toughness, namely matrix deformation, fibre/matrix interfacial failure and fibre bridging during the delamination process. At delamination initiation, the weakened fibre/matrix interface at elevated temperatures plays an overriding role with the delamination growth initiating at the fibre/matrix interface, rather than from a blunt crack tip introduced by the insert film, leading to low values of G _IC,ini and G _IIC,ini. On the other hand, during delamination propagation, enhanced matrix deformation at elevated temperatures and fibre bridging promoted by weakened fibre/matrix interface result in greater G _IC,prop values. Meanwhile enhanced matrix toughness and ductility at elevated temperatures also increase the stability of mode-II crack growth.     

A transient (subsecond) technique for measuring heat of fusion of metals

A transient technique is described for measuring the heats of fusion of metals with melting temperatures above 1500 K. The specimen configuration consists of a strip of the metal under study sandwiched between two strips of another metal with a higher melting temperature. The basic method consists of rapidly heating the composite specimen by passing a subsecond-duration electrical current pulse through it and simultaneously measuring the radiance temperature of the containment metal surface, as well as the current through and voltage drop across the specimen. The melting of the metal under study is manifested by a plateau in the temperature versus time function for the containing metal surface. The time integral of the power absorbed by the specimen during melting yields the heat of fusion. Measurements on several tantalum-niobium-tantalum specimens yield a value of 31.5 kJ · mor^–1 for the heat of fusion of niobium, with an estimated maximum inaccuracy of ± 5%.     

Interferometric technique for the subsecond measurement of thermal expansion at high temperatures: Applications to refractory metals

A high-speed interferometric technique has been developed at the National Institute of Standards and Technology to measure thermal expansion of metals between room temperature and temperatures primarily in the range 1500 K to near their melting points. The basic method involves resistively heating the specimen from room temperature up to and through the temperature range of interest in less than 1 s by passing an electrical current pulse through it and simultaneously measuring, with submillisecond resolution, the specimen temperature by means of a high-speed photoelectric pyrometer and the shift in the fringe pattern produced by a Michelson-type interferometer. The polarized beam from a He-Ne laser in the interferometer is split into two components, one which undergoes successive reflections from highly polished flats on opposite sides of the specimen and one which serves as the reference beam. The linear thermal expansion of the specimen is determined from the cumulative fringe shift corresponding to each measured temperature. The technique is capable of measuring linear thermal expansion with a maximum estimated uncertainty which ranges from about 1% at 2000 K to approximately 2% at 3600 K. Measurements have been performed on the refractory metals, niobium, molybdenum, tantalum, and tungsten, yielding thermal expansion data in the temperature range 1500 K up to near their respective melting points. Also, the technique has been used to follow the rapid dimensional changes that occur during solid-solid phase transformations; in particular, the transformation in iron has been studied.Invited paper presented at the Tenth International Thermal Expansion Symposium, June 6–7, 1989, Boulder, Colorado, U.S.A.     

Solid state phase transformation of BaB2O4 during the isothermal annealing process

Solid-state phase transformation of BaB₂O₄ during the isothermal annealing process for both to and to were investigated using a platinum crucible. For the -phase crystal at the -phase stable temperature (> 925 °C), the phase transforms to the phase perfectly below the melting temperature of 1100 °C. Meanwhile, for the -phase crystal at the -phase stable temperature (< 925 °C), the phase transforms to the phase perfectly above 800 °C. There is some difference in phase transformation behaviour between bulk-shape crystals and the powder, caused by thermal stress.     

Complex of Programs for Imitative Simulation of Acceleration, Heating, and Melting of Particles in Gas‐Dynamic Channels of Technological Devices

This paper describes an interactive complex of programs designed for investigating the processes of acceleration, heating, and melting of particles in the gasdynamic channels of burners. To illustrate the efficiency of this method, we have investigated the possibility of melting of tungsten particles in the process of their acceleration and heating by combustion products of hydrocarbon fuel in the gasdynamic channel of the burner with the discharge method of acting on the flow.     

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     

Description of the Shape Memory Effect of Radiation‐Modified Polymers under Thermomechanical Action

The shape memory effect of crystallizing polymer materials is simulated. The polymer is considered to be an inhomogeneous medium with a moving boundary (temperaturedependent phase composition). Using a model based on the frozen strain hypothesis, the temperature dependences of stresses under isometric heating and cooling have been obtained. On the basis of the known data on the influence of irradiation on the thermomechanical characteristics the dependences of thermorelaxation and thermoshrinkage stresses on the absorbed dose for highdensity polyethylene have been found.     

Does Vortex Lattice of YBa2Cu3O6.94 in H⊥c Melt Below Intrinsic-Pinning Irreversibility Line?

Vortex lattice melting in the Hc configuration of an YBa ₂ Cu ₃ O _6.94 single crystal has been investigated by means of the ac susceptibility – i and the magnetic torque . The melting transition of vortex lattice occurs in Hc, too. Since the torque curve shows a sharp peak in the irreversible torque at _c 90° due to intrinsic pinning at lower temperatures, we can determine the irreversibility line for the intrinsic pinning. The melting transition in the Hc configuration appears at temperatures where the intrinsic-pinning peak is absent. We consider that the intrinsic pinning does not affect the nature of the vortex melting transition in the Hc configuration.     

Mechanical and Corrosion Characteristics of Zr + 2.5% Nb Zirconium Reactor Alloy

The results of structural and phase hardening of pipes made of Zr + 2.5% Nb alloy show that ultrahigh-frequency thermal treatment of pipes (fast heating to the temperature of existence of the -phase followed by sharp cooling and annealing in the high-temperature range of the -phase) destroys the texture and forms a fine-grained structure (the grain diameter is about 1 m) with numerous transitional twins and a high density of precipitations of the secondary -niobium phase ( 10¹⁶cm^–3). In this state, the alloy is rather strong and plastic (at room temperature, _u 650 MPa, _0.2 550 MPa, and 20% both in the longitudinal and transverse directions). The efficiency of hardening by ultrahigh-frequency thermal treatment is not reduced with increase in the temperature of testing up to 500°C. Corrosion tests of channel pipes made of Zr + 2.5% Nb alloy subjected to ultrahigh-frequency thermal treatment in water containing various amounts of oxygen (from 0.1–0.3 to 600 mg/kg) at temperatures of 285–350°C for 700–6600 h under static conditions and in reactor water of the Ignalina Nuclear Power Plant for 5000 h under dynamic conditions showed that the corrosion resistance of this alloy is on a par with the corrosion resistance of the material of assembly channels of high-power channel reactors subjected to a standard treatment.     

Microwave surface resistance of epitaxial YBa2Cu3O7 thin films at 18.7 GHz measured by a dielectric resonator technique

We used a dielectric resonator technique for highly sensitive measurements of the temperature dependence of the microwave surface resistanceR _s of 1×1 cm² superconducting films at 18.7 GHz. It consists of a sapphire disc positioned on the film under investigation within a copper cavity which is acting as a radiation shield. In the TE₀₁ oscillation mode the highly reproducible quality factor of about 10⁵ results in a sensitivity of ±50 forR _s measurements. The temperature dependence ofR _s can be measured up to values as high as 1 . We have investigated several YBa₂Cu₃O₇ thin films prepared by high oxygen pressure d.c. sputtering on LaAlO₃ and NdGaO₃. Our best films exhibit a pronounced nonlinear behavior of the d.c. resistivity(T) with(300K)/(100K) values of about 3.7. Those films show, besides the initial fall-off just belowT _c , a further strong decrease ofR _s at low temperatures. This was observed both at 18.7 GHz and 87 GHz, as measured by a conventional cavity end plate replacement technique. ForTT_c/2 these films exhibit an exp (–T _c/T) dependence ofR _s with-values around 0.4. These observations may be explained by a superconducting energy gap with 2/kT _c0.8 for charge carriers localized in the CuO chains for YBa₂Cu₃O₇.     

Particle size control of 1,3,5-triamino-2,4,6-trinitrobenzene by recrystallization from DMSO

Batches of up to 46g of the insensitive explosive 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) have been recrystallized from DMSO in an effort to prepare larger particle-size material for recycling previously-used TATB and also for use in special formulations. The first part of the study investigated the conditions required to shift the particle-size distribution maximum from 50–70 m to several hundred micrometres in diameter. Distributions peaking at 200–246 m were successfully produced by varying the cooldown rate and degree of agitation during cooling. The second part of the study emphasized regeneration of the standard 50–70 m distribution from submicron size (ultrafine TATB) particles. The distributions peaking at 76–88 m, 27–31 m, and 15–17 m, successfully bracketed the target particle sizes, were grown by changing the degree of solution saturation. The choice of saturation temperature for the TATB/DMSO solution was based on earlier small-scale recrystallization and solubility work.     

Laser-Pulse Melting of Nuclear Refractory Ceramics

An accurate method for melting-point measurement of refractory nuclear ceramics was developed, based on laser-pulse heating and thermal arrest detection. The temperature measurement is performed by a combined use of a brightness pyrometer and a high-speed spectrometer working in the range of 500 to 900 nm. This method provides both the true temperature and the spectral emissivity function of the examined materials. Pure sintered MgO and a Mg:Am mixed oxide were first measured. The resulting melting point of the former (2350±20 K) is significantly higher than that commonly recommended and decreases with the addition of americium. Furthermore, UO₂ irradiated to 37,000 MWd/t and submitted to a reactor loss-of-coolant test was investigated: the melting point decreases from 3120 K, in the as-fabricated state, to 2950 K. Both fresh Zr:U mixed oxides and corium lava from a reactor meltdown experiment were also investigated.