High Energy Attrition Milling Research Articles

Synthesis of yttrium barium copper oxide-0.325Ag superconductors via intermediate precursor with overall composition Y:Ba:Cu:0=l:2:3:Y, Y>7 produced by high-energy attrition milling

A highly homogeneous composite precursor containing nano-scale particles was synthesized from the highenergy attrition milling of a mixture of metallic Y, metallic Cu and barium nitrate (Ba(N03)2) containing 5 wt.% silver. The particle size of the 20 hour attrition-milled precursor was in the range of 30~80 nm as estimated by the XRD technique and direct TEM observation. With the heat treatment of the attrition-milled precursor, an intermediate precursor with the overall composition Y:Ba:Cu:O=l:2:3:y, y>7 was synthesized. The analyzed Y:Ba:Cu:Ag:O molar ratio of the intermediate precursor by AES/AAS and iodimetric titration was 1:1.972:3.022:0.323:7.41, which was very close to the estimated composition: 0.5Y2O3+2BaCuO2.5+CuO +0.325Ag. Y123-Ag superconductors in powder or bulk forms were prepared using heat treatment or thermomechanical processing of the intermediate precursor containing BaCu02.5. The high oxygen orthorhombic Y123-Ag phase (y>7) was obtained in quenched samples which were held for 30 min or less at 950°C in ambient air. As the holding time extended to 40 min, the sample was still orthorhombic Y123-Ag (6.7<y <7). The transport critical current density (Jc) at 77 K and zero magnetic field for the quenched sample which was held for 40 min at 950°C was ~1.3xl03 A/cm2.

Microstructure and superconducting properties of attrition-milled Bi2Sr2CaCu2Ox

The microstructure and superconducting properties of Bi2Sr2CaCu2Ox (Bi-2212) during high-energy attrition milling were investigated in detail by a combination of x-ray diffraction, scanning electron microscopy, transmission electron microscopy, and magnetization techniques. The starting superconducting powder was milled in a standard laboratory attritor using yttria-stabilized ZrO2 balls and a stainless steel tank. After selected time increments, the milling was interrupted and a small quantity of milled powder was removed for further analysis. It was found that the deformation process rapidly refines Bi-2212 into nanometer-size crystallites, increases atomic-level strains, and changes the plate-like morphology of Bi-2212 to granular submicron clusters. At short milling times, the deformation seems localized at weakly linked Bi-O double layers, leading to twist/cleavage fractures along the {001} planes. The Bi-2212 phase decomposes into several bismuth-based oxides and an amorphous phase after excessive deformation. The superconducting transition is depressed by about 10 K in the early stages of milling and completely vanishes upon prolonged deformation. A deformation mechanism is proposed and correlated with the evolution of superconducting properties. The practical implications of these results are presented and discussed.

Mechano-Chemical Effects of High-Energy Attrition Milling On The Bi2Sr2CaCu2Ox Superconductor

ABSTRACTThe mechano-chemical effects of high-energy attrition milling on the microstructure evolution and superconducting properties of Bi2Sr2CaCu2Ox oxide powder were investigated by a combination of x-ray diffraction, scanning electron microscopy, and magnetization techniques. The powder was attrition-milled under an argon atmosphere, using a standard laboratory attritor with yittria-stabilized ZrO2 balls having a ball-to-powder weight ratio of ∼10:1. After selected time increments the mechanical attrition was interrupted and a small quantity of the milled powder was removed for analysis. The results indicate that (1) the crystal size decreases, (2) this decrease in crystal size is accompanied by degradation of crystallinity of the powder and accumulation of atomic-level strain, (3) the Bi-2212 phase decomposes under conditions of excessive deformation, and (4) the superconducting transition is depressed from 70 K to 60 K in the early stages of milling and completely vanishes upon prolonged deformation. The influence of defects (created during cold work) on the current carrying capacity is also presented and discussed.

The physical metallurgy and electrical contact resistance of mechanically alloyed CuRu composites

Mechanical alloying (high energy attrition milling) has been used to create a novel type of electrical contact material, a Cu-15vol.%Ru composite consisting of a fine (micron-scale) distribution of ruthenium in copper. Because these elements are mutually insoluble, mechanical alloying is one of the only ways of creating such a non-equilibrium microstructure. Consolidation of the composite powders into strip was carried out by warm and cold rolling, to preserve the metastable microstructure. The removal of surface copper from the consolidated strip gives rise to a “sandpaper” structure in which the hard, refractory and conductive ruthenium particles, now protruding from the surface, serve as the electrical contacts, whereas the copper matrix supports these particles and provides electrical continuity. The fact that this is so is borne out by the observation that in contact resistance versus exposure time tests, the Cu-15vol.%Ru composite behaves much more like pure ruthenium than like pure copper. Thus, we have developed a procedure for producing a base metal-ruthenium composite with electrical contact properties similar to those of ruthenium, without the difficulties (i.e. hot forging) or expense of working with pure ruthenium. This processing is applicable to an unlimited number of alloy systems.