Pd-Cr Alloys Research Articles

Nanoporous PdCr alloys as highly active electrocatalysts for oxygen reduction reaction.

A simple and convenient dealloying method is used to prepare nanoporous (NP) PdCr alloys with uniform ligament dimensions and controllable bimetallic ratio. The structural characterization methods demonstrate that the NP-PdCr alloy is comprised of a nanoscaled interconnected network skeleton and hollow channels extending in all three dimensions. Electrocatalytic measurements indicated that the as-made NP-Pd75Cr25 alloy exhibits superior specific and mass activities as well as higher catalytic stability toward oxygen reduction reaction compared with NP-Pd67Cr33, NP-Pd, and commercial Pt/C catalysts. X-ray photoelectron spectroscopy and density functional theory calculations both demonstrate that the weakened Pd-O bond and improved ORR performances depend on the downshifted d-band center of Pd due to the alloying of Pd with Cr (25 at%). It is expected that the as-made NP-PdCr alloy has prospective application as a cathode electrocatalyst in fuel-cell-related technologies with the advantages of superior overall ORR performances, unique structural stability, and easy preparation.

Formation of less-known structurally complex ζb and orthorhombic quasicrystalline approximant εn on solidification of selected Al–Pd–Cr alloys

The evolution of phases was investigated on cooling of Al71Pd24Cr5, Al73Pd20Cr7, and Al73Pd23Cr4 alloys from 1350°C down to ambient temperature with the rate of 10°C·min−1. To perform the investigation, differential thermal analysis, synchrotron X-ray powder diffraction, and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy were used. In all the investigated alloys structurally complex phases εn (ε6+ε28) and ζb, as well as the δ-phase were identified. Based on the results of differential thermal analysis, sequences of phase transformations were determined. The Al71Pd24Cr5 alloy started to solidify at 1031.4°C through β. Primary dendrites of ζb were observed in Al73Pd20Cr7 and Al73Pd23Cr4 alloys. In the second step of solidification δ and/or ζb were formed. The peritectic reaction, liquid+ζb+δ→εn+ζb+δ, leading to the formation of the quasicrystalline approximant εn (ε6+ε28) took place in the final step of solidification at approximately 792°C.

Transmission electron microscopy of hydrogen effects in internally oxidized palladium–chromium alloys

Hydrogen has been successfully used as a probe to characterize internally oxidized Pd alloys. Hydrogen isotherms indicated enhanced hydrogen solubilities in the dilute phase region for internally oxidized Pd–Cr alloys. The microstructures of internally oxidized Pd–Cr alloys have been studied by transmission electron microscopy. In the case of IOed Pd 0.98Cr 0.02 and Pd 0.96Cr 0.04 alloys, TEM revealed large chromia particles, in the size range 100–200 nm, on high angle grain boundaries. The size of loops and precipitates was larger and their number density smaller compared to those present in the grain interior where there was a uniform distribution of precipitates and dislocation loops throughout the matrix. After hydrogen cycling the internally oxidized Pd 0.96Cr 0.04 alloy, a high density of dislocations, in the form of dislocation cells, and sub-grain formation were observed. The microstructures were indicative of the high degree of cyclic stress induced during the hydrogen cycling process. The hydrogen solubility enhancement in internally oxidized Pd–Cr alloys have been co-related with the presence of dislocation loops.

X-ray absorption spectroscopy of Ag-Cr and Pd-Cr alloys formed by ion-beam-mixing

In this paper, we discuss X-ray absorption spectroscopy and Auger electron spectroscopy results that elucidate the local atomic arrangements and electronic structure change of ion-beam-mixed Ag-Cr and Pd-Cr alloys. Cr K-edgespectra were used to investigate the local crystal structure in these ion-beam-mixed thin film alloys. In ion-beam-mixed Ag-Cr thin films with a large positive value of heat of formation (ΔHm) the local crystal structure of the Cr site does not change, but in ion-beam-mixed Pd-Cr thin films with large negative value of ΔHm the structure of Cr site changes from a bcc to an fcc structure with increasing Pd concentration. In addition, Pd L3 X-ray absorption near-edge measurements show a decrease of the white line feature associated with the transition from 2p core level to 4d empty band in the Pd site of ion-beam-mixed Pd-Cr alloys. In contrast to the Pd-Cr system, the Ag L3 near-edge spectra remain unchanged in the ion-beam-mixed Ag-Cr system. It is thus concluded that the ion-beam-mixed Ag-Cr thin films consist of a mixture of elemental domains with no obvious indication of intermetallic alloying or compound formation as a consequence of the positive heat of mixing.

Spin-density-wave clusters inPd1−xCrxspin-glass alloys

Susceptibility and neutron scattering measurements were performed for ${\mathrm{Pd}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x}$ ($x=0.075,$ 0.11, 0.15) alloy single crystals. In neutron scattering measurements, satellite diffuse peaks were observed at $1\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta}$ 0 0 positions at low temperature. The satellite peak position varies with Cr concentration, but the peak linewidth is not sensitive to the solute concentration. Satellite peaks disappear far above the freezing temperature at which the cusp-type anomaly is observed in susceptibility measurement. Origin of the spin-glass-like behavior of PdCr alloys is ascribed to the fluctuating spin-density-wave clusters as is observed for the typical spin-glass alloys CuMn.

Hydrogen absorption characteristics of Pd[sbnd]Cr and Pd[sbnd]Mo solid solution alloys

Thermodynamic parameters for absorption of hydrogen by PdCr and PdMo solid solution alloys containing up to 10.0 at.% Cr(Mo) have been determined from measurements of pressure-composition isotherms at temperatures between 273 K and 433 K and hydrogen pressures up to 1000 torr. The lower and/or higher pressure solubilities of hydrogen in both alloys decrease with increasing Cr and Mo contents; especially the solubilities in high Mo content Pd-Mo alloys are markedly small. The plateau pressure of (α + β) two-phase mixture in PdCr and PdMo alloys increases with the solute metal content. The relative partial molar enthalpy at infinite dilution, ΔHH0, in both alloys becomes less exothermic with the solute metal content at the same alloy composition, the ΔHH0 values in PdCr alloys are less exothermic than those in PdMo alloys.

Precipitation and irradiation damage in proton-irradiated palladium-chromium alloys

Irradiation damage of Pd-Cr alloys containing 15, 20 and 25 at % Cr was studied over the temperature range 100 to 550° C, primarily in samples irradiated to a dose of 0.7 d.p.a. The solubility limit in this range of temperatures varies from 22 to 38% Cr, and precipitation of a phase having the L12 crystal structure was observed in unirradiated samples of the 25% Cr alloy aged at temperatures as low as 100° C. Octahedrally shaped voids, with faces parallel to {1 1 1}, were found only in the 25% Cr alloy irradiated from 350 to 550° C, but not in the other alloys at any temperature. The undersized chromium atoms migrate to point-defect sinks during irradiation, resulting in solute segregation and, eventually, precipitation under certain conditions. The precipitation of the L12 phase, which is the thermodynamically stable phase at higher chromium contents, was irradiation-induced at dislocation loops, voids and grain boundaries in the more concentrated undersaturated alloys. This irradiation-induced precipitation was observed in the mid-range and surface regions of the samples containing 20 and 25% Cr, but not in those containing 15% Cr. The behaviour of these alloys is compared with Pd-Fe and Ni-Si alloys, which have also been proton-irradiated in this laboratory, and the similarities and differences are noted and discussed.

Magnetisation and magnetoresistance of PdCr spin glasses

The authors present magnetic measurements on PdCr alloys which show that this system is a spin glass in the region 8 to 18 at.% Cr. In addition, alloys containing over 14 at.% Cr exhibit a resistance minimum of probable non-magnetic origin.

Anomalous temperature dependence associated with conduction electron scattering from isolated Cr impurities in Pd and Pt

The electrical resistivity of several dilute PtCr and PdCr alloys, containing between 0.05 and 0.3 at % Cr, has been measured from 1.4 to 300 K. The incremental resistivity Δp (T) appears to contain two contributions; the first arises from conventional Matthiessen’s Rule deviations, while the second contribution decreased monotonically with increasing temperature. With the conventional DMR assumed to be temperature-independent above about 60 K, it was found that the temperature dependence of the incremental resistivity Δρ (T≳60 K) in both systems was well represented by an expression of the form Δρ (T) =Ac+Bc ln (T2+T2)1/2. The single impurity characteristic temperature Ts was found to be concentration-independent in both systems with a value of Ts?45 K for PdCr and ?75 K for PtCr. The difference between the experimental values for Δρ (T) and the predicted form for Δρ (T) extended to low temperatures allowed a determination of the conventional DMR. The latter have been fitted to a ’’dirty limit’’ empirical model and are in good agreement with those deduced from previous measurements on nonmagnetic impurities in Pd and Pt. The latter result lends support to our assumption regarding the behaviour of the conventional DMR at high temperatures.

Comparison of the resistivity ofPdCr with canonical spin-glass systems

The electrical resistivity of several PdCr alloys, with between 11- and 18-at.% Cr, has been investigated between 1.4 and 300 K, and found to resemble that of canonical spin glasses (CSG). At low temperatures $\ensuremath{\Delta}\ensuremath{\rho}(T)\ensuremath{\propto}A{T}^{\frac{3}{2}}$, with $A\ensuremath{\propto}\ensuremath{-}logc$ over the concentration range investigated; the range of validity of this ${T}^{\frac{3}{2}}$ variation ($T<{T}_{1}$) is small in PdCr (where $A$ is comparatively large) as in AgMn. At higher temperatures $\ensuremath{\Delta}\ensuremath{\rho}(T)$ passes through an inflection point (at ${T}_{m}$) reaching a maximum (at ${T}_{\ensuremath{\mu}}$) above which it decreases with increasing temperature. The ratio $\frac{{T}_{m}}{{T}_{1}}$ is found to be roughly constant, as predicted by recent spin-glass theories, but as in CSG its magnitude is smaller than predicted. There are, however, several quantitative differences between PdCr and CSG; specifically both ${T}_{\ensuremath{\mu}}$ and $\ensuremath{\Delta}\ensuremath{\rho}({T}_{\ensuremath{\mu}})\ensuremath{-}\ensuremath{\Delta}\ensuremath{\rho}(0)$ are typically an order of magnitude smaller than in CSG of comparable concentration, whereas the slope of $\ensuremath{\Delta}\ensuremath{\rho}(T)$ above ${T}_{\ensuremath{\mu}}$ is about an order of magnitude larger than in CSG. This latter point in particular lends support to the assertion that such differences may arise from the fact that CSG are good moment systems (${T}_{K} or {T}_{S}\ensuremath{\ll}{T}_{m}$), but PdCr is not (${T}_{S}\ensuremath{\sim}{T}_{m}$).

Some low temperature properties of PdCr and PtCr alloys: A comparison with PdNi

Values of the low temperature magnetic susceptibility and specific heat of PdCr and PtCr are presented and compared with the data for PdNi. It is concluded that the properties of PdCr and PtCr can be understood in terms of localized spin fluctuations, like in PdNi.

Mictomagnetism in Pd–Cr and V–Mn Alloys

Face-centered cubic Pd–Cr alloys with 25%–35% Cr, quenched from 1200°C, have an average paramagnetic moment μ̄Cr of 0.5–1.2 μB. Below Tf=36°–76°K the moments are largely frozen, without long-range magnetic order, but ferromagnetic clusters of up to about 24 μB account for the thermomagnetic or isothermal remanence. These giant moments can be aligned in successive small thermally activated steps by cooling in a magnetic field. Once aligned, the giant moments can be turned collectively by much smaller fields. The bcc V–Mn alloys with 50%–76% Mn have similar magnetic properties, but with supergiant moments of more than 1000 μB, as quenched from 1200°C. For the Pd alloys with more than 25% Cr the minimum fields and temperatures necessary to align the moments are decreased considerably by annealing at 400°C, when the ferromagnetic clusters acquire dipole moments of up to 3600 μB, as measured in the superparamagnetic state, above Tf. The total moment of all supergiant clusters in the annealed 33% Cr alloy is equal to the thermomagnetic remanence of this alloy at 4.2°K. In the quenched as well as in the annealed condition the alloys with 29% and 33% Cr show the largest thermomagnetic remanence, presumably because the tendency for atomic ordering is largest at these compositions.

Exchange anisotropy in Pd-Cr alloys

Ferromagnetic properties are observed in concentrated Pd-Cr alloys. At low temperature (T < 70°K), these alloys display magnetic hysteresis characteristic of exchange anisotropy: large coercive field and asymmetric hysteresis loop.

Thermoelectric power of [formula omitted]Cr alloys

The thermoelectric power of four dilute Pd Cr alloys has been measured. They display an anomalously large positive value of the thermoelectric power due to a Kondo effect with a Kondo temperature of about 30°K.