Noble Alloys Research Articles

Electronic Structure of Noble-Metal-Noble-Metal Alloys

We describe optical studies of dilute noble-metal-noble-metal alloys. A differential technique compared the reflectance of the alloy with the reflectance of the pure metal and measured a quantity proportional to just the difference in reflectance. The method was particularly sensitive to small changes in reflectances and could be used to study very dilute alloys-the alloys here were between $\frac{1}{10}$- and 3-at.% impurity concentration. The measurements indicated that the noble alloys are of two types. In copper-gold and silver-gold alloys the electron energy bands shift smoothly as the concentration varies. The rates and directions of shifts are determined not only by differences in the host and impurity potentials, but also by the changes in lattice constant, and we emphasize the importance of the latter. In contrast, in copper-silver alloys separate host and impurity $d$ bands are formed.

Texture and Artifacts in SEM Observations Due to Vapor Coating

Specimen preparation techniques for scanning electron microscopy have not necessarily kept pace with the advances in instrumentation, especially in fields where quantitative measurements are important. Even though many operators have found that nonconducting surfaces can often be examined directly with low voltages and small beam currents, most commonly these surfaces are vapor-coated with 100 to 600 A of a noble metal alloy. More often than not, the coating procedures involve a bit of guesswork as to the final thickness and structure of the vapor coating. The purpose of this research was to illustrate the importance of close control on coating thickness and uniformity in order to minimize artifacts and distortion. Several noble metals and alloys were studied. The results for gold coatings are reported.Polystyrene spheres of statistically uniform diameters were dispersed on glass slides. The slides were rotated as they were coated by thermal deposition of Au from a W boat at a pressure of 2 x 10-5 torr in a standard oil diffusion pumped vapor coater.

The relations between the premartensitic instability of the ß-phase and the martensite structures and the shape-memory effect in the noble metal base alloys

A TEM study of the interphase boundary structure of 9R orthorhombic α1' martensite formed in β′ CuZn alloys shows that it consists of a single array of dislocations with Burgers vector parallel to 〈110〉β′ and spaced about 3.5 nm apart. This Burgers vector lies out of the interface plane; hence the interface dislocations are glissile. Unexpectedly, though, the Burgers vectors of these dislocations are not parallel when referenced to the matrix and the martensite lattices. This finding is rationalized on published hard sphere models as a consequence of relaxation of a resultant of the Bain strain and lattice invariant shear displacements within the matrix phase.Une étude par MET de la structure de l'interface interphase entre la martensite 9R orthorhombique α1′ et l'alliage CuZn β′ dans lequel elle se forme, montre qu'elle consiste en un seul ensemble de dislocations de vecteur de Burgers parallèle à 〈110〉β, espacées d'environ 3,5 nm. Le vecteur de Burgers n'est pas situé dans le plan de l'interface; les dislocations d'interface sont donc glissiles. Cependant, de manière inattendue, le vecteur de Burgers de ces dislocations n'est pas parallèle lorsqu'on se rapporte aux réseaux de la matrice et de la martensite. Nous expliquons ce fait à partir des modèles de sphères dures publiés antérieurement, comme une conséquence de la relaxation d'une résultante de la déformation de Bain et des déplacements de cisaillement de réseau invariant à l'intérieur de la matrice.Die Grenzfiächenstruktur des 9R orthorhombischen α1′-Martensits, der sich in β′-CuZn-Legierungen bildet, wurde elektronenmikroskopisch untersucht. Es zeigt sich, daß sie aus einem einzigen Netz von Fehlpassungsversetzungen im Abstand von etwa 3,5 nm mit einem Burgersvektor parallel zu 〈110〉gb′ besteht. Dieser Burgersvektor zeigt aus der Ebene der Grenzfläche heraus, folglich sind die Grenzflächenversetzungen gleitfähig. Unerwarteterweise ist der Burgers-vektor dieser Versetzungen im Gitter der Matrix und des Martensits nicht mit sich parallel. Dieser Befund wird erklärt anhand von den veröffentlichten Modellen mit harten Kugeln: er geht zurück auf eine Relaxation einer Resultierenden innerhalb der Matrix, die aus der Bainverzerrung und gitterinvarianten Scherverschiebungen herrührt.

The structure of glassy metallic alloys

It has been found possible to produce relatively stable amorphous metallic solids from many noble or transition metal-metalloid alloys in the vicinity of 80 at. % metal. Detailed consideration is given to a proposed structural model for these glasses. The structural model focuses on the role of composition in these alloys and successfully predicts the composition range for which glasses are most readily obtained. It also agrees well with experimentally determined radial distribution functions and densities and provides an understanding of the origin of the excess entropy of mixing of these alloys in the liquid state.

Relaxation time effects in the stacking fault energy of noble metal alloys

The stacking fault energy, γ, in noble metal alloys can be expressed as a sum of two terms: δufe, the contribution due to the conduction electrons; and δuc, the contribution due to thed electrons. For noble metals and alloys δuc 〉 δufe while for multivalent normal metals δufe 〉 δuc. The theory is first discussed in terms of recent calculations of the pseudopotential form factors of the noble metals (Moriarty) and some of the typical solutes (Shaw). The theory is then extended in a phenomenological fashion to include the effects of a finite relaxation time, τ, of the conduction electrons. It is shown that, for concentrated noble metal alloys with the electron-to-atom ratio,Z 〉 1.14 and multivalent normal metals, δufe and hence γ will be dependent on both temperature and deformation through their effects on τ. An increase in τ results in a decrease in the magnitude of δufe. In the case of concentrated noble metal alloys this results in an increase in γ with increasing t while for multivalent normal metals γ decreases with increasing τ.

The low-temperature specific-heats of several palladium-rich palladium-antimony alloys

The low-temperature specific-heat coefficients γ and the corresponding Debye temperatures θ were determined for four palladium-rich terminal solid-solution alloys containing 3, 6, 9 and 12 at.% antimony. The specific-heat results can be correlated with the room-temperature magnetic susceptability and the vacant d-states of palladium decrease as the solute concentration increases but at a reduced rate when compared with noble metal-palladium systems. Comparison with Mössbauer results obtained over the same concentration range of the palladium-antimony system and optical data on noble metal-palladium alloys suggest that a screening model, in which each atomic cell is electrically neutral, is a more appropriate description than a charge transfer model.

RESISTANCE MATERIALS FOR HIGH‐TEMPERATURE STRAIN MEASUREMENT

The performance of some commercially available base and noble metal resistance alloys usable as strain sensors in high temperature strain gauges is experimentally assessed with emphasis on the determination of post yield strain under these conditions.In order for Cu‐Ni resistance alloys to be used satisfactorily at 600 deg.C, gross surface oxidation must be prevented by an argon shield and the surface of the sensor must be pre oxidised to inhibit grain boundary attack from partial pressure of oxygen in the argon atmosphere.

Closure to “Discussion of ‘A Generalized Expression for the Tafel Slope and the Kinetics of Oxygen Reduction on Noble Metal and Alloys’ [D. S. Gnanamuthu and J. V. Petrocelli (pp. 1036–1041, Vol. 114, No. 10)

not Available.

Discussion of “A Generalized Expression for the Tafel Slope and the Kinetics of Oxygen Reduction on Noble Metal and Alloys” [D. S. Gnanamuthu and J. V. Petrocelli (pp. 1036–1041, Vol. 114, No. 10)

not Available.

Structure of martensites in β Au-Cd alloys

The crystal structures and metallographic features of martensites in the β phase region of Au-Cd alloys have been investigated by electron microscopy, the main emphasis being to investigate the origin of the stability of stacking variants of close packed structures which appear in martensites as compared to that of similar structures in equilibrium phases. Five types of martensite structures are found in the range of 45 at. % to 51 at. % Cd, and are designated with increasing Cd content as α′, β 2′, β 2′', γ 2′ and ζ 2′ among which the existence of two, γ 2′ and ζ 2′, had been known and been designated as β′ and β′' respectively. In addition, two structures, designated here as α 2 and α 20 are found to exist in the 45 % alloy as equilibrium phases, α′ has a disordered f.c.c. structure while β 2′ and γ 2′ are stacking variants, 3 R and 2 H respectively, of a CuAu type ordered structure. The β 2′' martensite is a distinct martensite phase but with a fine, regular lamellar mixture of β 2′ and γ 2′. The ζ 2′ martensite has an ordered hexagonal structure similar to ζ-AgZn. The composition dependence of the appearance of the crystal structures and their relative stability along with their metallographic characteristics are discussed in comparison with those in martensites in other noble metal base alloys as well as those structures found in noble metal alloys as stable states.

Superconductivity, Susceptibility, and Specific Heat in the Noble Transition Elements and Alloys. II. Comparison with Theory

The results of an experimental study of the superconducting transition temperature ${T}_{c}$, the magnetic susceptibility $\ensuremath{\chi}$, and the electronic specific heat $\ensuremath{\gamma}T$ in noble transition metals are discussed in terms of the existing theory for the interactions which affect these parameters. Particular attention is paid to the possible presence of virtual ferromagnetic spin fluctuations in these paramagnetic metals, using the recent theory of Berk and Schrieffer. The existing data do not prove that the spin fluctuations are suppressing the superconducting transition temperature; however, this seems to be the most reasonable explanation for the strong correlation between the decreasing ${T}_{c}$ and the increasing ratio of $\frac{\ensuremath{\chi}}{\ensuremath{\gamma}}$. We present calculated values for the variation of the bare density of states, the phonon interaction, the Coulomb interaction, and the spin susceptibility in these metals.

Superconductivity, Susceptibility, and Specific Heat in the Noble Transition Elements and Alloys. I. Experimental Results

We present experimental results of a study of superconductivity and magnetic susceptibility in face-centered-cubic alloys of Ir with Ru, Rh, Pd, Re, Os, and Pt, mostly on the Ir-rich side. The results show that the filling up of the $d$ band first depresses the superconducting transition temperature and then causes the susceptibility to increase rapidly. There is a correlation between these two experimental parameters in that metals with large (exchange enhanced) susceptibilities are not superconducting above our experimental limit (0.015\ifmmode^\circ\else\textdegree\fi{}K). We predict that Pd will not superconduct even at 0\ifmmode^\circ\else\textdegree\fi{}K and that superconductivity in Rh and Pt will occur far below ${10}^{\ensuremath{-}3}$ \ifmmode^\circ\else\textdegree\fi{}K, if at all.

A Generalized Expression for the Tafel Slope and the Kinetics of Oxygen Reduction on Noble Metals and Alloys

The usually employed expression for the Tafel slope is modified to take into account the potential which is effective in charge transfer within the double layer. The exchange currents and cathodic Tafel slopes are obtained on noble metal electrodes having different number of holes in the d‐band. Using the conventional and modified Tafel slopes, possible paths and rate‐determining steps are suggested for the cathodic reduction of oxygen.

Anodic oxidation of ethylene on noble metals and alloys. Parametric and isotopic examination of mechanisms

The anodic oxidation of ethylene has been studied on smooth noble metals and certain binary alloys, in aqueous solutions at 80°C, in the steady state. Reaction orders are derived from a parametric analysis and several types of mechanism are suggested and shown to occur. Studies in isotopic substitution serve to confirm these mechanisms. An experimentally derived “volcano” relationship between the heat of sublimation of the electrocatalytic substrate, and the rate of ethylene oxidation on it, is interpreted in terms of variable rates and variable rate-determining steps.

X-Ray Spectrometric Analysis of Noble Metal Dental Alloys.

The analysis of noble metal dental alloys for the various constituent elements is a difficult and tedious task by chemical or fire assay procedures. X-ray spectroscopy offered the possibility of increased speed, especially if solid metal samples were employed. This technique was investigated particularly with respect to the analysis of dental alloys having the nominal composition in percent, of gold 72, silver 12, copper 10, platinum 2, palladium 2, and zinc 2. Alloys were prepared by melting the component elements in a high frequency furnace and casting the metal into disk form. Compositions of the castings were determined by chemical analysis. Optimum procedures for casting the sample and for X-ray analysis were established, and analytical curves were developed relating concentrations to measured intensities of the X-ray lines Au Lβ, Ag Kα, Cu Kα, Pt Lα, Pd Kα, and Zn Kα. The observed typical coefficients of variation for the method were Au 0.34 percent, Ag 0.44 percent, Cu 2.2 percent, Pt 1.6 percent, Pd 1.2 percent, and Zn 0.72 percent. The results indicate that the method is sufficiently accurate and has marked advantages of speed and simplicity compared to chemical analysis.

Stacking Fault Probability of Noble Metal-Zinc Alloys

The stacking fault probability has been measured by x-ray means on a number of alloys of gold, silver, and copper with varying zinc content. By using these probabilities, the relative magnitudes of the stacking fault energies have been deduced.