Differential scanning calorimetry as a tool for monitoring mechanical alloying processing: application to the synthesis of (La,Ce,Nd,Pr)(Ni,Cu)5 intermetallic

P. LACOURT; M.R. ESQUIVEL; G. MEYER

Guarujá - Brasil

Congreso; VII Encontro SBPMat - 7th Brazilian MRS meeting; 2008

Sociedade Brasileira de Pesquisa em Materiais

Intermetallics synthesis and processing by mechanical alloying (MA) is a popular technique. The evolution of the metals/intermetallics/alloys is usually followed by standard methods such as X-ray diffraction, being successfully applied only when intrinsic crystalline species, in adecuate amounts, are present in the sample. Here, differential scanning calorimetry offers an alternative to determine both crystalline and intrinsic or extrinsic amorphous compounds. The thermal events observed with this technique are featured by specific heats of evolution and characteristics temperatures. These variables were used to monitor the evolution of the mechanical alloying of an AB5-based intermetallic.  Lumps of  LaO.25CeO.52.NdO.17Pr0.05  (Mm) and Ni and Cu powders were mechanically alloyed in a low energy mill.  The stages of milling were identified by combined scanning electron microscopy (SEM) and X-ray diffraction (XRD) observations. The process was also monitored by differential scanning calorimetry (DSC) and the presence of Cu, Ni and Mm were detected by the specific evolution of the thermal events. Initial stage was observed for integrated milling times (ti) between 0 and 15 h. Intermediate stage was observed at ti values between 30 h and 70 h. Final stage was detected between 70 and 120 h. Completion stage was identifed at 200 h. Samples obtained at selected ti values were used to analyze the advantage of a DSC-based method. The results for ti = 3h are shown as an example.  At Initial stage the chemical composition is not homogeneous. Only Cu and Ni were observed by XRD, but SEM images showed the presence of particles similar to initial lumps of Mm with morphology different of either Ni or Cu powders.  DSC measurements of individual Mm, Ni, Cu and the sample milled for 3 h were perfomed under air atmosphere . Ni is not oxidized in this temperature range [1]. Cu is slowly oxidized at temperatures higher than 200 ºC [2]. Mm slowly forms MmO2 (Ceria-based oxide) at room temperature and accelerates as temperature rises.  As observed, sample milled during 3h shows a thermal event that corresponds with the presence of free Mm in the sample, demonstrating that the proposed method is useful for phase identification. The technique was also used to determine the purity of LaO.25CeO.52.NdO.17Pr0.05(Ni,Cu)5 final samples, since this intermetallic is stable up to 200 ºC under O2 atmospheres and free Mm is oxidized at lower temperatures. The method sensitivity detects Mm as an impurity that could not be detected by other techniques.          References : [1] D.L. Cooke et. al, Vacuum, 79, (2005), 71-83. [2]. Z. Lockman, Physica C, 373,376,(2002), 831-834.