Beryllium oxide platelets and whiskers have been grown by heating beryllium metal in a silica furnace tube and boat with a hydrogen atmosphere for 16 hr at 1500°C. Three distinct types of whiskers formed: one type was parallel to the crystallographic c axis, another in the basal plane ([1̄010] direction), and the third in an intermediate direction (approximately the [3̄032] direction). The first type are called ``flagpole'' whiskers because they are topped by small balls of metallic appearance; they grew at the site of the original metal charge. These whiskers have pores along their axes, except in rare instances in which, instead of a pore, a whisker may have disconnected voids along its axis. Their axial growth rate was not observed directly, but was at least 0.02 μ/sec. The other two types have the more normal whisker appearance and grew at a distance of from 2 mm to 2 cm from the original metal charge. They have no pores or voids. Their growth rate was at least 0.2 μ/sec. The larger platelets, which grew in the vicinity of the normal whiskers, were long in [1̄010] direction and wide in the [0001] direction. Some of the very smallest platelets grew in the basal plane. A mechanism of growth for the flagpole whiskers is proposed. It is assumed that molten beryllium balls are splashed onto a BeO substrate, and then react with water vapor to form whiskers which push the balls away from the substrate. The normal whiskers and platelets grew at a distance from the beryllium metal, and therefore their growth involves a vapor transport mechanism.
Alumina whiskers have been grown on single-crystal alumina substrates by heating aluminum filings, located near the crystals, to 1400°C in a stream of wet hydrogen. The whiskers grew crystallographically coherent with the substrate, and had their axes either parallel to the c axis or in one of 12 equally spaced directions in the basal plane. These 12 directions divide into two distinct sets, a 〈112̄0〉 set and a 〈11̄00〉 set, each having sixfold symmetry, and with the directions of one set midway between the directions of the other. These growth directions are the screw-dislocation directions in alumina, hence it seems plausible that the whiskers grew coherently with the substrate at the site of emergent screw dislocations.
It has been shown previously that alumina (sapphire) whiskers grown on single-crystal alumina grow coherently with and in the directions of screw dislocations in the substrate. A possible explanation is that the whiskers grow by the screw-dislocation mechanism at the site of emergent screw dislocations on the substrate surface. The present study was undertaken to determine if the whiskers grow at the sites of substrate dislocations as revealed by etching techniques. To do this, whisker growth sites were compared with the positions of dislocation etch pits. This was done for the prism3tic [fllZo), (lioo)] and basal [(Oool), (1120)]slip systems on (llZO), ( l i O l ) , and (OOO1) crystal surfaces. The whiskers and etch pits did not form at the same places on the crystal surfaces. The reason may be that (1) the etch pits form at pure edge dislocations only, (2) surface nucleation generates screw dislocations at which the whiskers grow, or (3) screw dislocations play no part in the growth of these whiskers.
For an infinitely long iron rod of rectangular cross section with a [100] axis, and which has been magnetized in the [100] direction, theory predicts that an opposing field of about 565 Oe is required to reverse the direction of its magnetization, and that this reversing field increases linearly with axial stress. The proportionality constant relating the reversing field (nucleation field) to the stress is 3λ100/Ms, where λ100 is the saturation magnetostriction constant in the [100] direction and Ms is the saturation magnetization. Iron whiskers may be grown with a rectangular cross section and with the axis in the [100] direction. DeBlois and Bean studied the nucleation field at zero tension in such whiskers and found that in some parts of some whiskers the nucleation field approached the theoretical value. For convenience we call these ``ideal'' parts of the whiskers. We have studied the relationship between nucleation field and stress for ideal parts of whiskers and have found the proportionality constant to be 2.94 ± 0.32 Oe/kg/mm2, in reasonable agreement with the value obtained using the literature values of λ100 and Ms.
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