Joel L. Harringa

Nature of the cubic to rhombohedral structural transformation in (AgSbTe2)15(GeTe)85 thermoelectric material

The existence of a large thermoelectric figure of merit in (AgSbTe2)15(GeTe)85 has been known for many years. However, the nature of the crystallographic transformation in these materials from a high-temperature cubic to a low-temperature rhombohedral polymorph and its effect on electrical transport has not been clearly established. Transmission electron microscopy studies were performed that show extensive twinning in the low-temperature structure, resulting from lattice strain during the dilation along the (111) crystallographic direction. Analysis of differential scanning calorimetric studies indicates that the transformation is of second order, so that the high-temperature cubic phase is nonquenchable. High-temperature x-ray diffraction was performed to establish the transformation temperature, which was found to be complete upon heating at a temperature of 510K. Results of electrical conductivity measurements as a function of temperature on as-cast samples are discussed in terms of the observed twinning.

Sn-Ag-Cu solders and solder joints: Alloy development, microstructure, and properties

Slow cooling of Sn-Ag-Cu and Sn-Ag-Cu-X (X = Fe, Co) solder-joint specimens made by hand soldering simulated reflow in surface-mount assembly to achieve similar as-solidified joint microstructures for realistic shearstrength testing, using Sn-3.5Ag (wt.%) as a baseline. Minor substitutions of either cobalt or iron for copper in Sn-3.7Ag-0.9Cu refined the joint matrix microstructure, modified the Cu6Sn5 intermetallic phase at the copper substrate/solder interface, and increased the shear strength. At elevated (150°C) temperature, no significant difference in shear strength was found in all of the alloys studied. Ambient temperature shear strength was reduced by largescale tin dendrites in the joint microstructure, especially by the coarse dendrites in solute poor Sn-Ag-Cu.

Coefficient of thermal expansion of AlMgB14

Abstract The coefficient of thermal expansion (COTE) of AlMgB14 was measured by dilatometry and by high temperature X-ray diffraction using synchroton radiation. The COTE over the temperature range 298 K to 1373 K was determined to be 9×10−6 K−1, which is relatively close to the value of 8.3×10−6 K−1 for pure B. The anisotropy of the COTE appears to be relatively small.

Electrical properties of some (1, 1, 1) intermetallic compounds

Many intermetallic compounds form with a gap in their density of states at the Fermi level, giving rise to relatively large Seebeck coefficients, on the order of −150 to −300 μV/deg. at room temperature. Consequently, when combined with reasonable carrier mobilities in the range of 30 to 50 cm2/V-s, ‘half-Heusler’ compounds, such as MNiSn where M = (Ti, Zr, Hf), become attractive candidates for intermediate temperature (300°C to 600°C) thermoelectric applications. Samples of TiNiSn were prepared by arc melting and homogenized by various heat treatments. The temperature dependence of the electrical resistivity, Seebeck coefficient, and thermal diffusivity of these samples was characterized between 22°C and 900°C. The electrical resistivity and thermopower both decrease with increasing temperature, consistent with semiconducting behavior. The electrical power factor, defined as S2/ρ where S is the Seebeck coefficient and ρ is the resistivity, appears quite sensitive to the degree of homogenization in the microstructure and values in excess of 25 μW/cm-°C2 were observed in nearly single phase alloys within the 300 to 600°C temperature range. A brief survey of other selected ternary intermetallic compounds is also presented.

Al2MgO4, Fe3O4, and FeB impurities in AlMgB14

Abstract AlMgB 14 materials with Si and Ti additions produced by mechanical alloying/hot uniaxial pressing have recently been found to display hardness greater than 40 GPa. Al 2 MgO 4 and Fe 3 O 4 , and FeB are common impurities in AlMgB 14 produced in this manner. The characterization of these impurities and their effects on the hardness and electrical properties of AlMgB 14 without Si and Ti additions are presented in this study.

Effects of vial shape on the rate of mechanical alloying in Si80Ge20

The rate of mechanical alloying for doped Si80Ge20 alloys was studied using flat and concave ended hardened tool steel vials. Alloying was found to occur at significantly higher rates using a flat ended vial. It was also found by X-ray diffraction that the homogeneity of the alloy remained constant between 9 and 30 h for the flat ended vial and between 1.5 and 30 h for the round ended vial.

A new method for strengthening gold

Metal-metal composites were first produced in a copper matrix in the 1970’s, and they have since been produced in several other binary metal systems. This strengthening technique reinforces a ductile metal matrix with a ductile metal second phase. In some binary systems, this technique confers extraordinarily high strength and hardness while still maintaining low electrical resistivity. This article reports on the first gold matrix metal-metal composite, which was produced by deformation processing a 90%Au-10%Ag powder compact. The Au-Ag specimen studied had an ultimate tensile strength of 550 MPa and an electrical resistivity only 8% higher than that of pure Au at a deformation processing true strain of 5.6. The 590 nm average Ag filament thickness in this composite was relatively coarse compared to other deformation processed composites, which suggests that substantially higher strengths would be possible in a gold matrix metal-metal composite using deformation processing to higher true strains to reduce the filament thickness.

A Proof‐of‐Concept Study of the Use of Complex Borides for Disassembly of Decommissioned Nuclear Reactor Containment Vessels

Tool specimens of hot pressed AlMgB14 were employed in lathe turning tests cutting exterior surface material from 6061 aluminum, 304 stainless steel, Inconel, and concrete at various cutting rates. Performance was measured via analysis of mass change (removal rate), wear mechanisms, surface chemistry (reactivity), and fracture mechanisms. Preliminary results indicate that this new family of ultra‐hard materials exhibits good cutting performance against all four workpiece materials, while combining favorable toughness with an unusual absence of tool heating, leading to minimal wear and anticipation of long life in application for sectioning of ferrous‐based metals and structures such as reinforced concrete containing such metals. The potential value of these new materials for use in disassembly of decommissioned nuclear reactor pressure vessels is discussed.

Phase changes induced in hexagonal boron nitride by high energy mechanical milling

Six crystallographic forms of BN have been reported: hexagonal (h-BN), cubic (c-BN), rhombohedral, wurtzite, orthorhombic, and monoclinic. Only the first two of these have engineering applications. Cubic BN is prepared by a high-pressure, high-temperature process and is valued as an ultrahard material for cutting and grinding tools. Hexagonal BN is the most common form of the material and serves as a soft refractory material with high lubricity and high electrical resistivity. BN has also been prepared in an amorphous state (a-BN) [1, 2]. Bulk a-BN prepared by a process similar to that used to produce c-BN has been reported to be exceptionally hard [3]. The equilibrium phase diagram for BN indicates that c-BN is the thermodynamically stable phase under ambient conditions [4]; however, the transformation of h-BN to c-BN is difficult to achieve. This transformation is hindered by the sluggish kinetics of the process; commercial c-BN production uses high temperatures, high pressures, and catalysts to transform h-BN to cBN [5, 6]. Mechanical milling of powders can lead to formation of new equilibrium or metastable phases [7–9]. Previous work has shown that ball milling h-BN can transform it to c-BN [1, 2]. In one study, h-BN milled in a planetary mill transformed completely to c-BN at 1170 K without a catalyst; in other studies milled h-BN transformed to nano-crystalline h-BN, turbostratic BN, and a-BN [10]. In yet another investigation, an Ar ion beam created such high defect densities in an h-BN film that it transformed to c-BN [11, 12] without the high pressures and temperatures needed in commercial c-BN production. Although the h-BN to c-BN transformation has received considerable attention, less study has been devoted to formation and characterization of the a-BN phase. This project was designed to produce a-BN by high energy milling, measure its properties, and study its stability at elevated temperature. These experiments milled −325 mesh h-BN powder in Spex 8000 mills with hardened steel vials and balls. All Spex milling was done with a 5:1 ball-to-charge mass ratio. The powders were maintained in a dry, inert gas atmosphere throughout powder loading and milling. The Spex-milled powder specimens were analyzed by XRD to determine the degree of amorphization induced by the milling. XRD alone cannot provide a

Observations of multi-phase microstructures in R2(Fe1-xCox)14B where R = Nd or Dy

Abstract Multi-phase microstructures were observed in the psuedo-quaternary phase field of the 2–14–1 magnet materials Nd2Fe14B, Nd2Co14B, Dy2Fe14B, and Dy2Co14B. At equilibrium, (Nd1−yDyy)2(Fe1−xCox)14B had heretofore been widely assumed to be single phase where 1>x>0 and 1>y>0. In this study, three-phase microstructures were observed in (Nd1−yDyy)2(Fe1−xCox)14B when x>0.3 and y>0.5. The Curie temperatures and peritectic decomposition temperatures for Nd2(Fe1−xCox)14B are reported for several values of x in the range 1>x>0.