The interphase spacings lambda in the microstructures of most binary and pseudobinary ''normal'' metallic eutectics can roughly be quantified with regard to their respective solidification rates v according to a general relationshiplambda(2).v = constantthe constant being commonly between 10(-10) and 10(-11) cm(3) s(-1). The designation ''normal'' relates to similar degrees of undercooling of either phase on a plain-front solidification process at variance with anomaleous eutectics which solidify at different degrees of undercooling in an uncoupled or weakly-coupled manner. The ''constant'', however, is an individual value for each eutectic system. It is lowest for ''simple'' eutectics forming no intermetallic compounds and having low terminal solid solutions. The constant is raised for systems by 3 orders of magnitude or more if extensive terminal solid solutions are present. A closer fit for deliberate normal eutectics is presented in this pragmatic approach if the concentration differences between the terminal phases in eutectics Delta C-E [at%] are attributed to them as a square term according to lambda(2).v.Delta C-E(2) = constant. The data based on this relation still show some scatter but they are grouping to distinct material families e.g. to Pb-, Ag- or Al-based eutectics. The remanent differences are estimated to disappear as soon as the relation is extended by the interdiffusion coefficient D according tolambda(2).v.Delta C-E(2).D congruent to 1.9.10(-12) {cm(5).s(-2).at%)as is confirmed in cases where reliable D values are available. These findings are in contrast to the current published theories on eutectic solidification. Suggestions are given where the boundary conditions have to be altered in order to attain full accord between experiment and theory.
Fibrous rare-earth composite emitters were developed for heat-to-light conversion in a thermophotovoltaic (TPV) system to increase its conversion efficiency. This article demonstrates that a fibrous rare-earth oxide emitter, which is strengthened by alumina fibers, can also emit highly selective infrared light matching the bandgap (0.75eV) of a lattice-matched InGaAs photovoltaic cell. The emitter shows high selective efficiency and good mechanical properties. The unique material structure and processing technology that were developed for fabrication of the emitter are also described in this article. The infrared spectrum of the emitter, as well as its mechanical properties, were investigated. Experimental data indicate that using fibrous Er2O3 composite as a heat/light conversion emitter can significantly improve TPV conversion efficiency compared to that of using traditional blackbody emitters.
High-temperature molecular beam sampling in mass spectrometry may be disturbed by several phenomena that are related to the physico-chemical interactions of the evaporated species with their Knudsen cells and furnaces. To avoid the mass spectrometric detection of such parasitic vapors intermixed with the sampled beam, the principle of a restricted collimation is proposed and its transmission optimization is performed. The efficiency of such a device is compared to analogous classical Knudsen-cell mass spectrometric devices, which are usually four times more efficient. The advantages of the restricted collimation are discussed regarding the removing of spatial discrimination in ionization chambers, the influence on the second and third law calculation uncertainties, the mounting and use of shutters in case of permanent-gas detection, and finally the revaporizations in the ionization chamber.
Aluminum vapors were cocondensed with nitric oxide in excess argon and the interactions between Al and NO were probed using cryogenic matrix isolation techniques coupled with Fourier transform infrared spectroscopy. Spectra indicate that electron transfer to make the ion pair Al+NO- occurred. We also performed a series of ab initio and density functional theoretical calculations of various geometries and ground electronic states. Although there are some discrepancies in predicted minimum-energy geometry, the calculations suggest a triplet ground state.
The surfaces of high-purity Ti and Al foils and an extruded Ti-48Al titanium aluminide were examined by Auger electron spectroscopy. All three materials exhibited extensive contamination. The O in the Al foil is present in the form of Al2O3, whereas C is present as free carbon. The dominant oxide on the surface of the Ti foil is TiO2, which is a minimum of 30 nm thick. Sputtering for as long as 90 min does not eliminate it. In addition, the Ti-foil has C present in the form of free C as well as in the form of carbides. The surface of the TiAl alloy exhibits an abundance of both C and O. The C content is always larger in the two-phase alpha(2) + gamma region, whereas the O content is slightly larger in the single-phase gamma region. The Ti oxide formed on the surface of the gamma phase in the TiAl is primarily Ti2O3 lather than TiO2. On depth profiling, lower oxides of Ti are found. This can be attributed either to the process of Ar+ sputtering or the true existence of the lower oxidation states. Al2O3, on the other hand, is the only oxide of Al present on the surface of TiAl.
This article presents the results of the chemical vapor deposition of silicon carbide on carbon fibers with round and nonround (ribbon, C-shaped) cross-sections using methyltrichlorosilane (MTS) as a precursor and hydrogen as a carrier gas. In order to model the CVD deposition of silicon carbide on the fibers, the kinetic parameters for the chemical reaction on nonporous substrates were evaluated. The results show that the deposition process performed under reduced pressure (5-15 kPa) at 900 degrees C and MTS partial pressures ranging from 500 to 3000 Pa can be described by a first-order hyperbolic equation. Based on this information, a model was developed and used to predict the infiltration depth within fiber bundles of different geometry for uniform distribution of the SiC layer thickness. The predictions agreed with measured values. The use of this simple model allows one to predict the mean thickness of SiC layers deposited on carbon fibers with hollow, ribbon, or C-shaped cross-sections, if the kinetic parameters of the surface reaction, the molar diffusion coefficient, and the geometry of the fiber bundle are known.
The vaporization of Tl2Se was studied by the torsion method. The temperature dependence of the vapor pressure of this compound in the range 838-990 K is expressed by the equation:log[p(kPa)] = (7.78 +/- 0.27) - (9394 +/- 260)/[T(K)].By second-and third-law treatment of the pressure data, the selection sublimation enthalpy Delta(vap)H degrees(298) = 218 + 4 kJ/mol was derived.
Fibrous rare-earth composite emitters were developed for heat-to-light conversion in a thermophotovoltaic (TPV) system to increase its conversion efficiency. This article demonstrates that a fibrous rare-earth oxide emitter, which is strengthened by alumina fibers, can also emit highly selected infrared light matching the bandgap (0.75 eV) of a lattice-matched InGaAs photovoltaic cell. The emitter shows high selective efficiency and good mechanical properties. The unique material structure and processing technology that were developed for fabrication of the emitter are also described in this article. The infrared spectrum of the emitter, as well as its mechanical properties, were investigated. Experimental data indicate that using fibrous Er{sub 2}O{sub 3} composite as a heat/light conversion emitter can significantly improve TPV conversion efficiency compared to that of using traditional blackbody emitters.
The vaporization of Al{sub 2}Te{sub 3} was studied. The total vapor pressure was measured by the torsion-effusion method in the temperature range 1058--1163 K. The temperature dependence is given by the equation: log(p/kPa) = (11.14 {+-} 0.30)-(16900 {+-} 1000) (K/T). The standard enthalpy of the decomposition reaction: Al{sub 2}Te{sub 3}(s) = 2 Al(s) + 3/2 Te{sub 2}(g), {Delta}H{sub m}{degree}(298) = (505 {+-} 10) kJ/mol, an average of second- and third-law treatment of the results, was obtained. The derived heat of formation of Al{sub 2}Te{sub 3}, equal to {Delta}H{sub f}{degree}(298) = ({minus}260 {+-} 10) kJ/mol, lower than that quoted in literature, is discussed.
A laser ultrasonic method is described by which the microstructural evolution of a material may be monitored in situ during thermal processing. The method employs analysis of laser ultrasonic data that includes only those measurable quantities obtained from the ultrasonic data itself. Various microstructural changes, including the magnetic Curie transition and the dissolution of carbide structures in stainless-steel alloys, are investigated. These changes may be identified without specific knowledge of the sample temperature, thereby allowing for the possibility of direct microstructure analysis and control using laser-ultrasonic methods.
Radiative measurements using an InSb detector (lambda = 5.15 mu m) coupled to a photomultiplier (lambda = 0.36 mu m) were performed during melting and subsequent cooling on samples (pure Fe, Ni, Ti, and Ti-Al alloys and Nb-Ti-Al alloys) in an inductive cold crucible. The UV signal can be used to estimate the temperature, whereas the IR signal is subjected to a significant change related to the phase transformation. It is demonstrated that the transformation temperatures measured by this technique are closer to the thermodynamic values than those determined by the two-color pyrometer.
A review is given of the literature data for spectral emissivities at wavelengths (lambda) in the range 500-653 nm, the enthalpies, and heat capacities of the liquid phases of cobalt, titanium, and zirconium. Emissivity measurements were carried out by means of electromagnetic levitation at the solid-to-liquid transition with a partial-radiation pyrometer operating at lambda = 547 and 650 nm. Considering the sensitivity of the optical properties to surface impurities, investigations on the surfaces of several titanium and zirconium samples by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) were performed that confirmed a nitrogen-and oxygen-free process atmosphere during the measurements. Liquid phase normal emissivities obtained were epsilon(n,547) = 0.365 and epsilon(n,650) = 0.331 for cobalt, epsilon(n,547) = 0.409 and epsilon(n,650) = 0.393 for titanium, and epsilon(n,547) = 0.365 and epsilon(n,650) = 0.355 for zirconium. Enthalpy measurements on the liquid metals were carried out by means of levitation-drop calorimetry in the range 1591-2159 K for cobalt, 1847-2430 K for titanium, and 2025-2897 K for zirconium. The resulting heat capacities (values in J.mol(-1).K-1) obtained were 42.78 for cobalt, 43.79 for titanium, and 39.81 for zirconium.
The temperature dependence of the vapor pressure of InF3, measured by the torsion-effusion and Knudsen-effusion methods over the temperature range 1040-1249K, is represented by the selected linear equation: log p(kPa)=11.10 +/- 0.20-(16300 +/- 300)/T(K). From second- and third-law treatments of the results, the standard sublimation enthalpy of this compound, Delta(sub)H degrees(298)=330 +/- 4 kJ mol(-1), was derived.
A novel method for the synthesis of zirconia powder is presented in this paper. The formation of fine particles of zirconia takes place when metallic zirconium and hematite are heated in the presence of gaseous chlorine. The overall process, which can be described by the following reaction: 3 Zr(s) + 2 Fe2O3(s) --> 3 ZrO2(s) + 4 Fe(s), occurs by a mass-transport mechanism through the vapor phase between 723 and 1223 K. The vapor-mass transport among the solid species takes place by means of zirconium and iron chlorides. The fundamentals of synthesis are discussed on the basis of a detailed thermodynamic analysis of reactions involved in the process, as well as by a characterization of the solid phases formed at various temperatures by XRD and SEM examinations.
The vaporization of Al2Te3 was studied. The total vapor pressure was measured by the torsion-effusion method in the temperature range 1058-1163K. The temperature dependence is given by the equation: log(p/kPa) = (11.14 +/- 0.30)-(16900 +/- 1000) (K/T). The standard enthalpy of the decomposition reaction: Al2Te3(s) = 2 Al(s) + 3/2 Te-2(g), Delta H degrees(m)(298) = (505 +/- 10) kJ/mol, an average of second- and third-law treatment of the results, was obtained. The derived heat of formation of Al2Te3, equal to Delta H-f degrees(298) = (-260 +/- 10) kJ/mol, lower than that quoted in literature, is discussed.
Hot corrosion studies were conducted on Nimonic-75 superalloy in pure Na{sub 2}SO{sub 4} and 90% Na{sub 2}SO{sub 4} + 10% NaCl environments at 900 C for different time intervals. The results showed that Nimonic-75 exhibits good hot corrosion resistance in pure sodium sulfate, and the corrosion rate was enhanced considerably in the presence of 10% NaCl environment. It is also evident that the corrosion rate is time dependent and that the diffusion of corrosive species occurred toward the substrate. Based on these observations, an electrochemical model is proposed to show that the degradation of Nimonic-75 is an electrochemical phenomenon.
Thermodynamic constraints, together with Sieverts law and its modified form, are used to construct the equilibrium oxygen pressure-composition-temperature (p-C-T) spectrum over the Y-O system from 1000 to 2100 K where no direct measurements of P-O2 are available. The ensemble of the results is presented as the oxygen isobars superimposed on the phase diagram. The Gibbs energies of solution of oxygen dissolution in solid and in liquid yttrium are also calculated.
The distribution of gaseous product species resulting from the reaction of Mo(c) with Br{sub 2}(g) and with Br{sub 2}(g) + O{sub 2}(g) was studied by effusion-beam mass spectrometry over the range 1000--2000 K to aid in the chemical modeling of lamp transport cycles. Species identified included MoBr{sub 2}, MoBr{sub 3}, MoBr{sub 4}, MoOBr, MoOBr{sub 2}, MoOBr{sub 3}, MoO{sub 2}Br, and MoO{sub 2}Br{sub 2}. Enthalpies of formation of these species were derived from third-law analysis of the gas-solid reactions using partial pressures derived from measured ion intensities. Comparison of the measured gaseous product distributions with values calculated using a chemical equilibrium code and literature database aided significantly in the analysis. Results are compared with other information in the literature to give an internally consistent set of enthalpies of formation.
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Both powder and thin film CeO{sub 2}-ZrO{sub 2} ceramic materials have been successfully prepared via a sol-gel process. In this process, Zr and Ce hydroxide-mixed sols were prepared by hydrolysis of Zr oxychloride and Ce nitrate. The thin films were coated on glass or alumina plates and also alumina tubes by a spin- or dip-coating procedure. The structure and phase of the powder or thin-film CeO{sub 2}-ZrO{sub 2} have been characterized by X-ray diffraction (XRD). Electrical properties were measured by means of complex impedance analyses. The cyclic voltammogram measurements were performed for the ITO glass-supported CeO{sub 2}-ZrO{sub 2} thin films. Powder CeO{sub 2}-ZrO{sub 2} solid solutions having the tetragonal polymorph of zirconia are obtained for ceria contents up to 50 mol%, whereas for a ceria content more than 50 mol%, the CeO{sub 2}-ZrO{sub 2} is stabilized in the cubic fluorite structure. The thin-film CeO{sub 2}-ZrO{sub 2} usually has a cubic structural characteristic. Both powder and thin-film CeO{sub 2}-ZrO{sub 2} materials are nanostructured with an average grain size of 10 nm, and they have shown very different electrical properties. The CeO{sub 2}-ZrO{sub 2} thin films have great potential for applications in various electrochemical or optoionic devices, such as fuel cells, electrochromicmore » devices, and so forth.« less
