Thermosalient materials - example of N'-2-propylidene-4-hydoxybenzohydrazide

Termosalientni (termoodskočni) materijali, obično još zvani ”skačući kristali”, obećavajući su materijali za pretvaranje toplinske (ili svjetlosne) energije u mehanički rad na nanoskali. Prilikom grijanja ili hlađenja, ovi materijali prolaze kroz nagle i oštre polimorfne fazne prijelaze gdje je sačuvan integritet jediničnih kristala. Tokom tog prijelaza, mijenja se oblik kristala, kao i veličina jedinične ćelije, i ta je promjena toliko velika da izaziva skakanje kristala na visine nekoliko puta veće od njih samih. Ovaj efekt obično se dogodi u nekoliko mikrosekundi. U literaturi je objavljeno da se N’-2-propiliden-4-hidroksibenzohidrazid ponaša slično kao skačući kristali. Utvrđeno je da se ovaj sustav pojavljuje u tri polimorfna oblika (I, II i III) iste prostorne grupe Pna21, s topotaktičkim (očuvan integritet jediničnih kristala) faznim prijelazima I u II i III u II (i obrnuto). Također je utvrđeno da prilikom ireverzibilnog faznog prijelaza iz I u II dolazi do snažne kompresije (oko 15%) duž polarne osi; jedinični kristali faze I raspadnu se na manje kristale faze II (skakanjem), a kod reverzibilnog prijelaza iz faze III u II dolazi do produljenja polarne osi (oko 14%) te nije opaženo skakanje. Naša mjerenja pokazuju malo drugačije ponašanje. Tokom ireverzibilnog faznog prijelaza iz I u II kristali su pokazali tipično ponašanje skačućih kristala te su skakali u nasumičnim smjerovima nekoliko centimetara daleko. Jedan dio kristala raspao se na manje, ali veliki broj kristala ostao je očuvan. Također, tokom reverzibilnog faznog prijelaza II u III (i obrnuto), dolazilo je do skakanja kristala, iako slabijeg nego kod prijelaza I u II. To je u suprotnosti sa dosadašnjim rezultatima, zbog čega je potrebno novo objašnjenje termosalientnog efekta kod ovog spoja.Thermosalient materials, or colloquially commonly called “jumping crystals”, are promising materials for conversion of thermal (or light) energy into mechanical work on the nanoscale. These materials, when heated or cooled, undergo a sudden and sharp polymorphic single crystal to single crystal phase transition. During this transition the crystals experience a change in their shape, as well as in the size of the unit-cell, and this change is so large that it causes them to jump up to a height that is several times bigger than their size. This phenomenon usually happens in several microseconds. It was reported that N’-2-propylidene-4-hydroxybenzohydrazide behaved similarly as the jumping crystals. It was found that this system exhibited three polymorphic modifications (I, II and III), all having the same polar space group Pna21, with the phase transitions I to II and III to II reported as topotactic (single crystal to single crystal). It was also reported that during the irreversible phase transition from I to II, the periodicity along the polar axis underwent a strong compression (approximately 15%); single crystals of the phase I were violently disintegrated into single crystal fragments of the phase II (with jumping), while in the reversible phase transition III to II the periodicity along the polar axis expanded (approximately 14%) and no movement of crystals was observed. Our measurements showed a somewhat different behaviour. During the irreversible phase transition from I to II some of the crystals did indeed disintegrate into smaller fragments, but a large number remained intact and showed a typical jumping crystal behaviour – jumping all around over large distances (several cm). This is typical for other thermosalient materials. Also, during the reversible phase transition II to III (both ways), crystals exhibited the jumping behaviour, but somewhat weaker than during the phase transition I to II. This is in contrast to the statements reported previously and the new model for the thermosalient effect in this system has to be proposed