The origin of driving force for the formation of Sn whiskers at room temperature
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Sn whiskers can form at room temperature on the agglomerated flakes produced by mechanical alloying (MA) of Ti, Sn, and C powders, whether the flakes are stored in air or water. The Sn whiskers forming in air are tens of micrometers to several centimeters in length and 0.5 to ∼10 m in diameter. Whereas a large amount of Sn polyhedra forms on the flakes that are stored in water, a small amount of Sn whiskers forms on the polyhedra. The driving force for Sn whisker formation is the compressive stress induced by mechanical alloying (MA) and oxidation. The mechanism about the spontaneous growth of metal whiskers is discussed.
I. INTRODUCTION
Since the first reference concerning cadmium (Cd) whiskers was reported in 1946,1 spontaneous growth of low melting point metal whiskers at room temperature, such as tin (Sn), cadmium (Cd), zinc (Zn), and lead (Pb), as well as aluminum (Al) hillocks at elevated temperatures has attracted worldwide attention and has been extensively investigated.2–12 It is well known that the growth of whiskers is a surface relief phenomenon. During the past 60 years, many models and mechanisms, such as dislocation mechanism, internal stresses, and formation of intermetallics have been proposed to describe the spontaneous whisker growth.1–6 Recently, Barsoum and co-workers published a series of papers to explain the spontaneous metal whisker formation.10,13–15 The conclusion they drew is that the origin of the driving force for spontaneous growth of metal whiskers is essentially the reaction between oxygen and the sprouting metal and the volume expansion that creates a compressive stress to push the whiskers up.13–15 Although the driving force for spontaneous metal whiskers growth is thought to come from the compressive stress, there still remain some fundamental questions. For example, what are the kinetics of nucleation and growth of the whiskers? How much stress is needed to initiate a growth? How does the compressive stress form? Hence the origin of compressive stress is extensively studied. The above mechanisms cannot explain well the spon-
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0402 3226 J. Mater. Res., Vol. 22, No. 11, Nov 2007 http://journals.cambridge.org Downloaded: 18 Mar 2015
taneous soft metal growth, and there are still some fundamental questions and arguments due to the absence of any convincing experimental support. Here we investigate the growth of the Sn whisker at room temperature. The origin of the driving force for the formation of the Sn whisker is discussed. II. EXPERIMENTAL
Ti (average particle size: 48 m, >99% purity), Sn (average particle size: 75 m, >99% purity), and C (graphite, average particle size: 45 m, >99% purity) powders were used in the present study. Mechanical alloying (MA) was performed in a planetary ball mill (QM-1SP4) with stainless steel milling containers and balls. Ti, Sn, and C powders with a mole ratio of 2:1.1:1 were placed in 250 mL containers and then the containers were evacuated to ∼1
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