Unusual thermoelectric behavior of packed crystalline granular metals

Loosely packed granular materials are intensively studied nowadays. Electrical and thermal transport properties should reflect the granular structure as well as intrinsic properties. We have compacted crystalline $CaAl$ based metallic grains and studied the electrical resistivity and the thermoelectric power as a function of temperature ($T$) from 15 to 300K. Both properties show three regimes as a function of temperature. It should be pointed out : (i) The electrical resistivity continuously decreases between 15 and 235 K (ii) with various dependences, e.g. $\simeq$ $T^{-3/4}$ at low $T$, while (iii) the thermoelectric power (TEP) is positive, (iv) shows a bump near 60K, and (v) presents a rather unusual square root of temperature dependence at low temperature. It is argued that these three regimes indicate a competition between geometric and thermal processes, - for which a theory seems to be missing in the case of TEP. The microchemical analysis results are also reported indicating a complex microstructure inherent to the phase diagram peritectic intricacies of this binary alloy.Comment: to be published in J. Appl. Phys.22 pages, 8 figure

Local atomic order, electronic structure and electron transport properties of Cu-Zr metallic glasses

We studied atomic and electronic structures of binary Cu-Zr metallic glasses (MGs) using combined experimental and computational methods including X-ray absorption fine structure spectroscopy, electrical resistivity, thermoelectric power (TEP) measurements, molecular dynamics (MD) simulations, and ab-initio calculations. The results of MD simulations and extended X-ray absorption fine structure analysis indicate that atomic order of Cu-Zr MGs and can be described in terms of interpenetrating icosahedral-like clusters involving five-fold symmetry. MD configurations were used as an input for calculations of theoretical electronic density of states (DOS) functions which exhibits good agreement with the experimental X-ray absorption near-edge spectra. We found no indication of minimum of DOS at Fermi energy predicted by Mott's nearly free electron (NFE) model for glass-forming alloys. The theoretical DOS was subsequently used to test Mott's model describing the temperature variation of electrical resistivity and thermoelectric power of transition metal-based MGs. We demonstrate that the measured temperature variations of electrical resistivity and TEP remain in a contradiction with this model. On the other hand, the experimental temperature dependence of electrical resistivity can be explained by incipient localization of conduction electrons. It is shown that weak localization model works up to relatively high temperatures when localization is destroyed by phonons. Our results indicate that electron transport properties of Cu-Zr MGs are dominated by localization effects rather than by electronic structure. We suggest that NFE model fails to explain a relatively high glass-forming ability of binary Cu-Zr alloy

Structures of Cu-Based Glass Forming System Doped with Iron

The effects of iron additions on the structure of (Cu0.47Ti0.34Zr0.11Ni0.08)100−xFex (x = 0, 1.5, 3, 6) glass forming system are presented. The alloys were synthesized by arc melting of high purity elements and melt spun to form rapidly quenched ribbons. X-ray diffraction studies showed the amorphous structure of the base and Fe-containing alloys, confirmed by the presence of exothermic peaks on differential scanning calorimetry curves. Bulk samples in the form of rods (Φ3 × 55 mm) were cast using a suction casting unit attached to the arc melter. Structural investigations revealed partial crystallization of the base Cu47Ti34Zr11Ni8 alloy. Iron additions significantly decreased the glass forming ability of the alloy, leading to primary crystallization of dendrites

Isothermal Stability and Selected Mechanical Properties of Zr48Cu36Al8Ag8 Bulk Metallic Glass

The aim of this work was to investigate the influence of isothermal annealing on the amorphous structure stability of the Zr48Cu36Al8Ag8 alloy. A series of continuous heating examinations was performed on the differential scanning calorimeter in order to determine the temperature limits for isothermal annealing series where the time to crystallization was measured. The obtained results were calculated and a time-temperature-transformation diagram was created and discussed. Static compression test as well as microhardness measurements of the as-quenched samples gave a mechanical properties results supplement. The measured properties (σc = 1800 MPa and 614 HV0.05) are comparable to the literature results for this alloy. Fractographic observations with the scanning electron microscope were also performed in order to prove some plasticity observed during the strength tests

Isothermal Stability and Selected Mechanical Properties of Zr48Cu36Al8Ag8 Bulk Metallic Glass

The aim of this work was to investigate the influence of isothermal annealing on the amorphous structure stability of the Zr48Cu36Al8Ag8 alloy. A series of continuous heating examinations was performed on the differential scanning calorimeter in order to determine the temperature limits for isothermal annealing series where the time to crystallization was measured. The obtained results were calculated and a time-temperature-transformation diagram was created and discussed. Static compression test as well as microhardness measurements of the as-quenched samples gave a mechanical properties results supplement. The measured properties (σc = 1800 MPa and 614 HV0.05) are comparable to the literature results for this alloy. Fractographic observations with the scanning electron microscope were also performed in order to prove some plasticity observed during the strength tests

Formation and Structure of Nanocrystalline Al-Mm-Ni-Cu Alloys

This paper reports the results of a short investigation on the effect of Cu additions upon the nanocrystallization behaviour of an Al-Mm-Ni alloy. 2 at.%Cu added to a base alloy of Al$\text{}_{85}$Mm$\text{}_{10}$Ni$\text{}_{5}$ alloy by substitution for Mm (mischmetal). The control of cooling rate did not cause the formation of nanocrystals of fcc-Al phase. The nanocrystalline structure fcc-Al + amorphous phase in quarternary alloy was obtained by isothermal annealing and continuous heating method, but the last technique is more effective. The volume fraction, lattice parameter, and size of Al-phase were calculated

Isothermal Stability and Selected Mechanical Properties of Zr48Cu36Al8Ag8 Bulk Metallic Glass

The aim of this work was to investigate the influence of isothermal annealing on the amorphous structure stability of the Zr48 Cu36 Al8 Ag8 alloy. A series of continuous heating examinations was performed on the differential scanning calorimeter in order to determine the temperature limits for isothermal annealing series where the time to crystallization was measured. The obtained results were calculated and a time-temperature-transformation diagram was created and discussed. Static compression test as well as microhardness measurements of the as-quenched samples gave a mechanical properties results supplement. The measured properties (σc = 1800 MPa and 614 HV0.05) are comparable to the literature results for this alloy. Fractographic observations with the scanning electron microscope were also performed in order to prove some plasticity observed during the strength tests

Local Atomic Order in Al-Based Metallic Glasses Studied Using XAFS Method

Local atomic order in series of Al–(Y, Sm)–TM (TM = transition metal: Co, Ni) amorphous, melt-spun alloys was investigated by Sm$L_3$-edge and TM$K$-edge X-ray absorption fine structure (XAFS) method. The evaluated coordination number of Sm is approximately 16 for all investigated systems and TM coordination ranges form 5 to 6. A slight decrease in TM coordination along with increase of TM content is observed. The interatomic distances between Al and Sm atoms are equal to the sum of their metallic radii within the experimental error, while Al–TM bonds are shortened by 10%10%. The structural disorder around Sm atoms is significantly higher then around TM. We have found that only Al atoms are present in the first coordination shell of both Sm and TM species. The XAFS data show that the amorphous network coherency is lost at distances exceeding 4 Å. The results are discussed in a framework of Miracle’s structural model of metallic glasses suggesting that the average local structure around Sm atoms consists of 16 efficiently packed Al atoms in the first coordination shell of Sm and the TM atoms occupy the octahedral sites in the closely packed fcc cluster structure

Effect of Zr Purity and Oxygen Content on the Structure and Mechanical Properties of Melt-Spun and Suction-Cast Cu46Zr42Al7Y5 Alloy

The effect of oxygen content in zirconium on the structure and mechanical properties of the Cu46Zr42Al7Y5 alloy, in the form of melt-spun ribbons and suction-cast rods, was investigated. Two types of Zr, rod and crystal bar of different nominal purities and oxygen contents, were used to synthesize the alloy by arc melting. Rapidly solidified ribbons were produced by melt spinning and their amorphous structures were confirmed by X-ray diffractometry (XRD) and differential scanning calorimetry (DSC). Bulk samples in the form of rods were cast using a special water-cooled suction casting unit attached to the arc melting system. XRD and DSC studies proved the amorphous structure of the bulk alloy synthesized from low-oxygen Zr and partial crystallization of the same alloy for high-oxygen Zr. In both bulk samples, uniformly distributed crystalline particles were identified as yttrium oxides. Higher mean compressive strength of amorphous alloy was observed. The hardness of amorphous phase was close to 500 HV1 in both bulk alloys, while the hardness of crystalline dendritic areas, observed in the alloy synthesized from high oxygen Zr, was lower by about 50 HV1

Nanocrystallisation of Soft Magnetic Fe-Co-Zr-Cu-B Alloys

In the present study, Fe$\text{}_{41}$Co$\text{}_{41}$B$\text{}_{10}$Zr$\text{}_{7}$Cu$\text{}_{1}$ alloy has been investigated in order to evaluate its thermal stability and structure after heat treatment, as well as the impact of heat treatment on magnetic properties. X-ray diffractometry, differential scanning calorimetry, chemical composition microanalysis, transmission electron microscopy, and magnetic hysteresis loop measurement techniques were employed. The crystallisation temperature of the as-quenched alloy is 490ºC (continuous heating at 5ºC/min). The melt-spun ribbon having 27μm in thickness was annealed for 1 hour at temperatures from 400 to 700ºC. The alloy after treatment at about 550ºC underwent primary crystallisation, with the average size of crystals under 20 nm. This specimen shows the coercive field of 38 A/m, as compared to about 160 A/m reported for a similar alloy (Fe$\text{}_{44}$Co$\text{}_{44}$B$\text{}_{4}$Zr$\text{}_{7}$Cu$\text{}_{1}$) with a similar structure, annealed at 600ºC