MadSci Network: Physics

Re: As a hobbyist where can I get 'cheap' Pyroelectric crystals?

Date: Fri Mar 30 12:12:00 2007
Posted By: Uncle Al Schwartz, Organic synthetic chemist
Area of science: Physics
ID: 1175233112.Ph

A pyroelectric crystal develops a surface voltage separation with changing
temperature.  The voltage difference can be large.  The crystal must be an
electrical insulator so the available amperage is small. The most accessible home-growable pyroelectric crystal is Rochelle salt (NaKC4H4O6.4H2O, sodium potassium tartrate tetrahydrate). Between its two Curie points of -18�C and +24�C, it is ferroelectric with a monoclinic point group 2. In the non-ferroelectric region it is in orthorhombic point group 222 and only shows piezoelectricity. Properties of Rochelle Salt Phys. Rev. 47(2) 175 (1935) Polymorphism of Rochelle Salt Phys. Rev. 51 43 (1937) Properties of Rochelle Salt. III Phys. Rev. 58(6) 565 (1940) WARNING! There are three different types of tartartic acid! L-(+)-(2R,3R)-Tartaric acid is the natural stuff from grape lees. D-(-)-(2S,3S)-Tartaric is the mirror image and rather expensive. DL-Tartaric acid (racemic tartaric acid) is a mixture of both *and will not work* to give pyroelectic crystals. If you must work with hot basic solutions, work rapidly. Grow crystals from warm solution H. Hope, "Apparatus for growth of crystals for X-ray diffraction study" J. Appl. Cryst. 4 333 (1971) Thiele-Dennis tube More reasonable: Take a beaker, stably mount it with a 20-30 degree rotation off vertical. Mostly fill with solvent (distilled water). Saturate with solute. Add more solute (signficant excess) to the bottommost corner and apply mild heat right there, toward the side. Seal top with plastic wrap and a pinhole (air pressure vs. temp). A concentrated convective plume will rise up the side, cool to supersaturation, and descend down the middle or so (you can see refractive wiggles by looking through the side). When everything is at equilibrium and there is still substantial solid at the bottom, suspend your perfect seed crystal in the descending plume. A very tiny drop of waterproof epoxy at the end of 2-lb test fishing line or a thick human hair will secure the seed crystal and controllably suspend it in the supersaturated descending plume. Rotate as necessary to keep growth symmetric. On what seed crystal face should you attach the filament?

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