In the Classroom edited by
Tested Demonstrations
Ed Vitz Kutztown University Kutztown, PA 19530
Pyroelectric Effect of a Sucrose Monocrystal submitted by:
Metodija Najdoski,* Ljupˇ co Pejov, and Vladimir M. Petruˇ sevski Institut za Hemija, Prirodno-Matematiˇcki Fakultet, Arhimedova 5, Skopje, Macedonia
checked by:
Penny Snetsinger
Department of Chemistry, Sacred Heart University, Fairfield, CT 06432 Fred Juergens Department of Chemistry, University of Wisconsin, Madison, WI 53706
The generation of an electric polarization in crystals, or altering of the already existing one, as a consequence of temperature changes is called the pyroelectric effect (1). There are two types of pyroelectric effects. The first one, called the primary pyroelectric effect, occurs when the shape and size of the crystal are fixed during the temperature change. The other one is the secondary pyroelectric effect, which occurs with free expansion of crystal volume during the temperature change (2). If the temperature changes are uniform, the following equation is valid for relatively small temperature intervals (1): ∆P i = ∆T pi
stick to one end of the polarized pyroelectric crystal, giving it yellow color, while the positively charged Pb3O4 particles stick to the other end, coloring it red. Another method consists of the following. The thermostated pyroelectric crystal is placed on the glass surface and covered with a glass bell, under which a magnesium strip has just been combusted. The “precipitation” of the magnesium oxide particles occurs so that a generated electric field becomes visible.
(1)
where ∆Pi is the change in the i th component of the polarization vector, pi is the corresponding pyroelectric coefficient, and ∆T is the temperature change. Using vector notation, the previous equation takes the form
∆P x
px ∆P y = ∆T p y pz ∆P z
(2)
The existence of the polarization naturally induces an electric field between the charged boundaries of the monocrystal. The magnitude of the polarization is proportional to the length of the crystal and to the magnitude of generated charges. The generated polarization and the corresponding electric field are shown schematically in Figure 1. With the reversal of the sign of ∆T, an alternation of the electric field poles is observed. It can be shown theoretically, by symmetry arguments, that the following ten crystal classes may exhibit pyroelectricity:
Figure 1. Schematic drawing of the polarization in a pyroelectric crystal.
1 2 3 4 6 m mm2 3m 4mm 6mm The pyroelectric effect is closely related to another phenomenon, known as piezoelectric effect, in that the two phenomena can occur simultaneously. The appearance of the piezoelectric effect in a given crystal may result as a consequence of a nonuniform temperature change. Several methods for determination of pyroelectric effect have been proposed, such as the Kundt method (3, 4), which consists of a powdering of the crystal with a mixture of sulfur and Pb3O4 (minium). The negatively charged sulfur particles *Corresponding author. Email:
[email protected]. ukim.edu.mk.
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(a)
(b)
Figure 2. (a) Photograph of a sucrose monocrystal showing the pyroelectric effect. (b) Shematic drawing of the induced electric field.
Journal of Chemical Education • Vol. 76 No. 3 March 1999 • JChemEd.chem.wisc.edu
In the Classroom
Another simpler, yet remarkable, demonstration of the pyroelectric effect can be observed with certain crystals when they are cooled at very low temperature (liquid nitrogen may easily be used) and then left in air at ambient temperature. The water vapor from the air crystallizes on the opposite ends of the previously cooled pyroelectric monocrystal, so that a growth of ice crystals in the direction of the induced electric field lines of force can easily be seen. Experimental Procedure CAUTION: Safety goggles or a face shield must be worn while working with liquid nitrogen. The sucrose monocrystal can be grown from a supersaturated solution of sucrose (250 g/100 mL at 50 °C) at ambient temperature for a period of up to two months. (This, naturally, depends on the temperature and relative humidity of air.) Nucleation occurs in about one week. The best quality monocrystals are selected and are hung in the solution, near the surface, by means of thin fibers. It is desirable that the selected crystals are developed along one of the crystal axes. Owing to supersaturation, the solution is apt to form a crust on its surface. This crust should simply be broken by a glass rod and the polycrystalline conglomerate either removed or allowed to sink to the bottom of the beaker. In this way, monocrystals as large as ~ 0.5 cm3 can be obtained and successfully used in the experiment. A sucrose monocrystal should be immersed in a polystyrene beaker half filled with liquid nitrogen. This operation must be performed very carefully. To protect the monocrystal from temperature shocks (which will eventually crack it), it should be held near the liquid nitrogen surface for several minutes before immersing. The cooling then proceeds with the crystal dipped in the liquid nitrogen. After thermal equilibrium is reached, the monocrystal is placed on a sheet of black paper using plastic tweezers (to avoid both its mechanical destruction and discharge). A microscope is suitable (but not necessary) for observing a fast growth of ice crystals in the direction of the induced electric field lines of force, at the opposite ends of the sucrose monocrystal (see Fig. 2). To enable projection of the demonstration, one may use a video microscope. Since this equipment is expensive, an overhead projector may be used as an economical alternative. In this case, it is important to protect the crystal from the heat of the projector. We used colored glass filters from an old Lange colorimeter. The blue filters work best. Two pieces are placed on an overhead surface. Above them (at a distance
of ~2 cm) we place a piece of plain glass. The cooled sucrose crystal is then placed over the glass. The growth of oriented ice crystals may be observed (provided the overhead is at a proper distance to obtain an image of optimum size) for at least 20 s. To check whether the observed behavior is really due to the pyroelectric effect, we did some additional experiments. An alum crystal (cubic) was first cooled and then left in air. No preferential growth was observed (nor should it be, if the true cause for the phenomenon is the pyroelectric effect). Also, ice crystals were grown on a liquid-nitrogen-cooled standard microscope glass plate (~76 × 26 mm). The observed growth shows dramatic changes, depending on whether the glass was charged (the pattern is, in this case, analogous to that observed with the sucrose monocrystal) or not. A similar experiment was performed in a CO2 atmosphere, in which case there was no difference (between charged and uncharged glass plates) in the growth of dry ice crystals. The polarity of the water molecules is obviously of key importance. It is well known that the pyroelectric crystals can be applied in infrared (5) and thermal (6 ) detectors (pyrometers) where the generated charge is proportional to the temperature change. The pyroelectric properties are undesirable in substances that are used in preparation of explosives. Unfortunately, ammonium nitrate, TNT, picric acid, and α and β lead azides are pyroelectric. The static charge development in these types of materials has been investigated (7). Acknowledgments The technical assistance of Zoran Zdravkovski and Bogdan Bogdanov is sincerely appreciated. Literature Cited 1. Nye, J. F. Physical Properties of Crystals (Their Representation by Tensors and Matrices); Clarendon: Oxford, 1986; p 78. 2. Parker, S. P. McGraw-Hill Encyclopedia of Physics; McGraw-Hill: New York, 1983; p 886. 3. Patil, A. A.; Curtin, D. Y.; Paul, I. C. J. Am. Chem. Soc. 1985, 107, 726. 4. Buerger, M. J. Elementary Crystallography; Wiley: New York, 1956; p 186. 5. Abhai, M.; Kumar, A. A. Indian J. Pure Appl. Phys. 1991, 29, 657. 6. Robinson, M. K.; Shorrocks, N. M.; Bicknell, R. W.; Watson, P.; Pedder, D. J. Hybrid Circuits 1989, 18, 25. 7. Raha, K.; Chhabra, J. S. Defence Sci. J. 1991, 41(1), 21.
JChemEd.chem.wisc.edu • Vol. 76 No. 3 March 1999 • Journal of Chemical Education
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