Low-temperature properties of ordered Cu<Subscript>3</Subscript>Pd-based alloys

The effect of atomic disordering and alloying with d elements (Fe, Pd, Cu) on the transport and magnetic properties of Cu₃Pd alloys has been investigated at low temperatures (T < 80 K) in strong magnetic fields (H ≤ 8 MA/m). The specific features of the crystal structure and temperature and field dependences of the electrical resistance, magnetoresistance, Hall effect, and magnetic susceptibility of Cu₇₂Pd₂₈, Cu₇₅Pd₂₅, Cu₈₀Pd₂₀ and Cu_74.5Pd_24.5Fe₁ alloys are discussed.     

中子辐照对Pd80Si20与Pd77.5Cu6Si16.5非晶态合金结构的影响

本工作用X射线衍射技术示差扫描量热法(DSC)和内耗测量等手段研究金属-类金属非晶态合金Pd₈₀Si₂₀和Pd_77.5Cu₆Si_16.5中子辐照前后的微观结构变化。结果表明,辐照在两种样品的对关联函数g(r)以及径向分布函数RDF(r)上都引起明显的变化;辐照后样品的晶化温度和晶化热有所提高,结构变得更加无序,Pd₈₀Si₂₀非晶态合金的内耗在T关键词：     

Photoemission (XPS,UPS) studies of Pd-Si metallic glasses

The valence bands of glassy $\text{Pd}{}_{\mathrm{100?x}}\text{Si}{}_{\mathrm{x}}\text{(15?x?21)}$ and pure Pd were studied by XPS and UPS. The valence band spectra of the alloys show a strongly reduced density of states at the Fermi energy E_F compared to Pd. From the measured relative photoelectric cross sections for the different excitation energies we conclude that the electron states near E_F of the glassy alloys have mainly d-character. This is in good agreement with recent measurements of the low-temperature specific heat, the magnetic susceptibility and the optical reflectivity.     

低扩散系数对Pd77Cu6Si17合金易非晶化的影响

采用激光熔凝技术,研究了Pd₇₇Cu₆Si₁₇合金快速凝固组织的演变规律.研究表明:随着生长速度的增大,Pd₇₇Cu₆Si₁₇合金晶体生长组织形貌发生了Pd₃Si枝晶+共晶-ξ枝晶+共晶-共晶团簇-规则共晶-非晶的转变,共晶失稳即非晶形成的临界生长速度为6 mm/s,最小层片间距为35 nm.通过与Al-25 wt% Sm,Al-32.7     

First-principles study of small palladium clusters on NiAl(1 1 0) alloy surface

Theoretical calculations focused on the geometry, stability, electronic and magnetic properties of small palladium clusters Pd_n (n=1–5) adsorbed on the NiAl(1 1 0) alloy surface were carried out within the framework of density functional theory (DFT). In agreement with the experimental observations, both Ni-bridge and Al-bridge sites are preferential for the adsorption of single palladium atom, with an adsorption energy difference of 0.04 eV. Among the possible structures considered for Pd_n (n=1–5) clusters adsorbed on NiAl(1 1 0) surface, Pd atoms tend to form one-dimensional (1D) chain structure at low coverage (from Pd₁ to Pd₃) and two-dimensional (2D) structures are more stable than three-dimensional (3D) structures for Pd₄ and Pd₅. Furthermore, metal-substrate bonding prevails over metal–metal bonding for Pd cluster adsorbed on NiAl(1 1 0) surface. The density of states for Pd atoms of Pd/NiAl(1 1 0) system are strongly affected by their chemical environment. The magnetic feature emerged upon the adsorption of Pd clusters on NiAl(1 1 0) surface was due to the charge transfer between Pd atoms and the substrate. These findings may shade light on the understanding of the growth of Pd metal clusters on alloy surface and the construction of nanoscale devices.     

Recovery of electrical resistivity in amorphous pdsi alloys after low temperature neutron irradiation

Measurements of electrical resistivity after low temperature fast neutron irradiation are made for amorphous Pd₈₀Si₂₀ and Pd₈₀Ni₂Sl₁₈ and then Pd₈₀Si₂₀ annealed at 230°C and 360°C, and the isochronal annealing curves are obtained. The resistivity increase of Pd₈₀Si₂₀ annealed at 360°C is about 10 times larger than that of amorphous alloys and no defined annealing stage is observed in amorphous alloys and Pd₈₀Si₂₀ annealed at 360°C. For amorphous Pd₈₀Si₂₀, about 60% of the resistivity increase by irradiation remains after annealing up to room temperature and these are discussed by the structural relaxation.     

Thermopower of some amorphous transition metal-alloys

Measurements of the thermopower for amorphous alloys are reported in the temperature range 77K to 320K. The alloys examined are Cu₅₀Ti₅₀, Pd₈₀Si₂₀, Pd₃₀Zr₇₀ and Cu₄₀Zr₆₀. The data are compared with values of similar systems, as far as available. In addition we analyse the temperature coefficient of the thermopower, applying a model, based on the Ziman formulation of the transport properties.     

Optical properties of metallic glasses

Optical reflection spectra of metallic glasses (Pd₈₁Si₁₉ and Pd₈₄Si₁₆) were measured in the spectral range 0.03 to 12 eV and the optical constants were determined by Kramers-Kronig analysis. The experimental reflectivity spectra of glassy Pd₁₈Si₁₉ are similar to the one obtained for Au₈₁Si₁₉ prepared by getter sputtering in argon. However, the energy ranges where intra- and interband optical transitions occur are quite different for the two alloys.     

Electrical resistivity of ternary glassy and liquid Pd-alloys

The electrical resistivity of liquid (Pd₁₀₀Cu_100?x)₈₀Ge₂₀ alloys has been measured as a function of temperature. For Cu-rich negative temperature coefficients and for Pd-rich alloys positive temperature coefficients of the electrical resistivity have been observed. This behavior is very similar to recent observed resistivity temperature curves of glassy (Pd₁₀₀Cu_100?x)₈₀P₂₀ alloys. An explanation of the resistivity behavior in terms of liquid metals theory is suggested.     

Positron lifetime measurements on electron irradiation damage in amorphous Pd80Si20 and Cu50Ti50 alloys

Positron lifetime measurements were performed on amorphous Pd₈₀Si₂₀ and Cu₅₀Ti₅₀ alloys irradiated with 3MeV electrons at 20K. The irradiation was found to increase the mean positron lifetime in both specimens indicating the presence of vacancy-like radiation damage. Isochronal annealing between 77 K and 300 K resulted in a continuous reduction of the positron lifetime, which suggests a gradual recovery of the irradiation induced defects.     

Critical phenomena in FINEMET alloy

The Fe_73.5Cu₁Nb₃Si_13.5B₉ FINEMET alloy has been prepared by the rapid solidification technique. The critical behaviour of this alloy in the amorphous as well as in its nanocrystalline states has been studied near their respective Curie temperatures. From the values of the critical exponents one can conclude that the alloy behaves like a 3D Heisenberg ferromagnet in the amorphous and nanocrystalline states. But there exists a slight increase in the value of β for the alloy annealed at 823 K (the nanocrystalline state) as observed in most of the amorphous alloys.     

Impurity Auger spectra: A probe of the local impurity density of state and the impurity electron—electron interactions

We show that the local impurity density of states and the impurity electron—electron interactions can be obtained from impurity Auger spectra. For a Ag_0.95Pd_0.05 alloy we find 11% Pd(d) character in the Ag d band and Pd(4d-4d) Coulomb interactions which are much larger than the virtual bound state widths.     

Ion implantation a powerful technique for the production of metastable superconducting alloys

Ion implantation at liquid helium temperatures is a new method of producing metastable alloys. The special features of this alloying technique are illustrated by two selected examples of dilute and concentrated alloys, respectively. Superconductors containing small amounts of paramagnetic (3d)-impurities such asHgMn, PbMn andSnMn exhibit strong pair breaking effects. In the case ofSnMn the Kondo effect is observed. Concentrated alloys of Pd and Pd-noble metal alloys with hydrogen (deuterium) are only superconducting in a very high concentration range, being unstable at room temperature. Such concentrations can be achieved by ion implantation at liquid He-temperatures. The superconducting transition temperature can thus be raised to a maximum value of 16.6 K in aH/Pd _0.55 Cu_0.45?0.7 alloy.     

Magnetic properties of metallic glasses of the type Fe80−x Pd x B20 and Fe80−x Pt x B20

Amorphous alloys of the type Fe_80???xPd_xB₂₀ and type Fe_80???xPt_xB₂₀ for 0?≤?x?≤?50 have been investigated by means of ⁵⁷Fe Mössbauer spectroscopy and magnetisation measurements in temperatures from 4.2 up to 300 K. Curie temperatures and crystallisation temperatures are found by DTMG-DTA method. Mössbauer spectroscopy magnetic field is observed to visible increase for x?=?1 and 1.5% at room temperature for Pd, while a decrease is observed for higher x values. Curie temperature for Pd alloys has a maximum at x?=?4 with T _C?=?753 K, which supports enforcing influence of Pd at low concentrations of Pd for magnetic interactions. We discuss different explanations for these measurements and compare with other findings for high Pd concentrations and alloys with Pt instead of Pd.     

The hall effect in PdSi-based amorphous alloys containing Co.

The Hall effect in amorphous Pd₈₀Si₂₀ and Pd_80–x Si₂₀Co _x , wherex=2, 4, 6 (at.% are implied throughout) alloys was investigated. Measurements were carried out at r.t. in fields up to 17·5 kG. Also the electrical conductivity was measured. The Hall effect was found negative in all alloys of the above composition. Observedx-dependence of the Hall constantR _H tends to change the sign of the effect and is interpreted on the assumption that an extraordinary Hall effect manifests itself besides the ordinary one in Co-containing alloys. The value ofR _H for the basal alloy should be looked upon as an evidence of electron transfer from glass-former (Si) to transition metal (Pd) empty d-states. The values ofR _H obtained for the alloys withx=0, 2, 4, 6 are respectively, –7·8; –8·7; –8·3; –5·2 (×10^–5 cm³/A. sec throughout).     

Disorder induced core photoelectron linewidth broadening in AgPd alloys

High resolution core level photoelectron spectra for a series of Ag_xPd_1−x alloys have been measured and reveal a large “disorder broadening” effect. A comparison of measurements of this effect for three alloy systems, Ag_xPd_1−x, Cu_xPd_1−x and Cu_xZn_1−x, with results of ab initio calculations is presented. Poor agreement is obtained and possible reasons for this observation are discussed.     

Low temperature Hall-effect measurements in some metallic glasses

We report on simultaneous measurements of the Hall-effect and resistivity in the metallic glasses Pd₈₀Si₂₀ and Ni₇₈Si₈B₁₄. Data were taken within a temperature range of 1.7 to 300 K and in magnetic fields up to 1.1 T. Whereas the Hall constant is nearly independent of temperature in case of Pd₈₀Si₂₀, it changes sign at low temperatures in Ni₇₈Si₈B₁₄. The implications of the low temperature results are discused with respect to the origin of a resistivity minimum, present in both alloys.     

Solubility of hydrogen (1 atm, 298 K) in some copper/palladium alloys

The solubility of hydrogen (1 atm, 298 K) decreases linearily with copper content in f.c.c. substitutional Cu/Pd alloys to an atom fraction of Cu = 0.20 and thereafter the solubility decreases more abruptly. The fall in solubility at X_Cu > 0.20 is attributed to the inability of alloys with X_Cu > 0.20 to form a second non-stoichiometric hydride phase at 1 atm (298 K). Thermodynamic data are reported for the absorption of hydrogen into the two-phase regions (H-saturated alloy + non-stoichiometric hydride) of these alloys.     

The adsorption and reaction of CO and O2 on PdAu alloy wires

Temperature programmed desorption (TPD) of CO and O₂ on PdAu alloy wires has been studied. The heat of adsorption, sticking coefficient and maximum coverage of CO were recorded for Pd, 83 Pd 17 Au, 60 Pd 40 Au. For Pd and Pd-rich alloys the heat of adsorption remained fairly constant but the maximum coverage fell markedly from 0.42 for Pd to less than 0.05 for bulk palladium atom fraction X_B^pd ? 0.83. The heat of adsorption, sticking coefficient and maximum coverage of O₂ were investigated for pure Pd. A very limited adsorption was recorded on 83 Pd 17 Au and none on the more Au-rich alloys. The adsorption data are used to discuss the CO + O₂ reaction. Activation energy and frequency factor are estimated on Pd, for the TPD conditions used here. Earlier rate constants (0.2 Torr, 150°C) for CO + O₂ on PdAu as a function of Au content correlates with the maximum coverage of chemisorbed CO, which in turn is correlated with the probability of finding a Pd_9±1 ensemble in the surface. Modern results on the d-band structure of the PdAu alloys suggest that the Pd₉ ensemble, i.e. a surface Pd atom without an Au atom in its coordination shell, would tend to optimise both the donor and acceptor actions of the Pd atoms involved in chemisorbing CO.     

Density and related thermophysical properties of metastable liquid Ni-Cu-Fe ternary alloys

The densities of liquid Ni-Cu-Fe ternary alloy system were investigated by molecular dynamics method in combination with a MEAM (Modified Embedded Atom Method) potential model. The temperature range is from 1000 to 2200 K, including both a broad superheating range and a large undercooled regime. The densities of six Ni100−2xCuxFex alloys (x=0,10,20,30,40,50) decrease linearly with the rise of temperature at the superheated and undercooled states, and increase with the enhancement of Ni content. Among the simulated alloys, the densities of only liquid pure Ni and Ni₆₀Cu₂₀Fe₂₀ alloy are available in literatures, which are in good agreement with the calculated values. According to the relationship between the excessive volume and the alloy composition, it can be deduced that Ni100−2xCuxFex alloys deviate from ideal solution. Moreover, a comparison was also performed between the calculated results and the approximated values by Neumann-Kopp's rule. Based on the obtained density data, the thermal expansion coefficients are also derived. It firstly decreases to a minimum value, and then displays a rise with the increase of Ni content.