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68 The PCB Design Magazine • October 2016 a minimum of 80% of full strength and speci- fied properties. Gel time is the interval between mixing and the point at which the resin has just set and cannot flow. In this state, it is still rela- tively soft and can often be re-shaped by apply- ing slight pressure. Choice of hardener is the main way of alter- ing the speed of cure of two-part resin systems. While hardeners will have their own exother- mic effect, raising the temperature of the two- part mix, an external heat source may still be required to speed things up. It should be noted, however, that if heat is applied too early in the curing process, viscosity may reduce and the resin will flow more freely, leaking from holes and gaps. Well, hopefully the foregoing will help get you started as far as resin selection is concerned. Look out for next month's column, in which I will take a closer look at the main consider- ations relating to mixing resin. PCBDESIGN Alistair Little is global business/ technical director for the Resin Division of Electrolube. WHY ARE RESIN PROPERTIES SO IMPORTANT? Solid materials whose atoms are arranged in a well-ordered crystalline structure are usually bet- ter conductors of electricity than randomly structured, or amorphous, solids. Re- cently, however, A*STAR researchers found that iron-tellurium (FeTe) breaks this rule, displaying higher conductivity, and optical reflectivity, in the amor - phous phase. A recent study published in the journal Acta Ma- terialia describes their efforts to understand why FeTe's behavior is counterintuitive to expectations. FeTe is a phase-change material, with the abil- ity to rapidly switch its state from crystalline to amorphous and back again when it is heated or cooled, a property which makes it useful for data storage and memory applications. Conventional phase-change materials such as germanium-anti - mony-tellurium (GST), commonly used in rewrit- able DVDs, display higher optical reflectivity and electrical conductivity in their crystalline state be- cause the highly-ordered structuring of atoms in the crystal results in more electron vacancies, or holes, that act as charge carriers. The team prepared thin films of FeTe at room temperature to produce amorphous structures, and at 220 degrees Celsuis to acquire crystalline samples, and showed that the films could be flipped between the two states using a fast pulsing laser. They ana - lyzed the molecular struc- ture of the different films using X-ray spectroscopy, electron microscopy and first-principle calculations. The researchers confirmed the existence of lone- pair electrons in both the amorphous and crystal- line phases. In the crystalline phase, electrons were engaged in strong hybridization, meaning their orbitals overlapped and caused their electrons to localize. In contrast, when FeTe entered its amorphous phase, some Te atoms were orientated so that their lone-pair electrons delocalized from the atoms, re - sulting in holes that acted as charge carriers. "We are hopeful that FeTe could prove to be useful material for phase-change memory," says Bai. "It could also act as an effective thermo-electric material, generating electric current in response to temperature." Disorderly Conduct

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