Measuring Morphological Changes in a Piezoelectric Crystal Damith Rozairo

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1 Measuring Morphological Changes in a Piezoelectric Crystal Damith Rozairo Abstract A Rochelle salt crystal (Piezo crystal) is prepared from a Super-saturated solution of Sodium-Potassium Tartrate (KNaC 4 H 4 O 6 ). The crystal structure can be deformed as a voltage is applied across the crystal. Using the atomic force microscope (AFM), the morphological changes on the surface of a piezoelectric crystal due to an electric potential was measured at a sub-micron level. I. INTRODUCTION The piezoelectric effect is the ability of certain crystals to deform their surface in response to an electric potential or to generate an electric potential in response to a mechanical stress. The Piezoelectric effect was first discovered in 1880 by the Curie brothers. Piezoelectricity is used much more widely than radioactivity, although it is not as well familiar. Some of its uses are, gas lighters, receivers/microphones, clocks/watches, beam scanners and computers. A piezo electric element can be found in any buzzing device or in gas lighters or in many other devices. Since the Piezoelectricity could not be imaged using a buzzing device, a Rochelle salt crystal was grown. As a reverse effect, morphological changes on the crystal surface can be measured when a voltage is applied across the surface. The goal is to see if the surface of the crystal changes in x-y direction or in the z height. These surface changes are measured using an AFM [1]. The AFM was used on static mode, where the tip is in contact with the sample surface throughout. II. Theory The morphological changes on the surface can be seen as either alternating or direct current changes. When an alternating current (AC) is applied the surface would oscillate according to the frequency, where as when a direct current (DC) is applied the surface would change its surface without oscillating. When 300 (DC) volts is placed across a Rochelle salt crystal one centimeter thick, it will change its dimensions only about percent [3]. Therefore, to see any change on the surface using the AFM, it was imaged at 100 nm pixel size resolution. III. EXPERIMENTAL METHODS Preparing a Piezo Crystal 120g of Sodium-Potassium Tartrate (KNaC 4 H 4 O 6 ) powder was mixed with 200ml of distilled water. This solution was stirred while heating using a hot pad (see figure 1). Figure 1: Supersaturated solution of KNaC 4 H 4 O 6. Experimental apparatus. As the solution was heated more KNaC 4 H 4 O 6 was able to dissolve. KNaC4H4O6 was added to the solution while heating, until it was not able to dissolve more. Once a supersaturated solution was prepared, the solution was removed from the hot pad and cooled to room temperature. Then a seed crystal was tied to a fishing line and dipped into the solution. The crystal was allowed to grow overnight.. Figure 2: Grown crystal along the fishing line

2 Testing for Piezoelectricity in the Crystal The main idea behind this experiment was to detect a generated voltage as the surface of the crystal was deformed. An oscilloscope was connected to opposite sides of the crystal using two pieces of Aluminum foil to make a better electrical connection (see figure 3). A small spherical ball-bearing BB was dropped right on top of the crystal from a 4.7cm, 9.8cm, 13.9cm and 18.8cm height. The generated electric potential was recorded on the oscilloscope and this process was repeated as the height of the BB drop was changed. crystal surface contracts and expands generating alternating current. Figure 5: Energy(mgh) vs. Generated potential, for the mass of 3.4 x 10-4 Kg, Height of the drop has an error of 0.004m and energy has an error of 1.33 x 10-5 J (Error bars are too small to be viewed). Potential energies of Figure 5 was calculated using mgh; m- mass of the BB, g-gravitational acceleration and h-height. Figure 3: Crystal and BB drop apparatus. A straw was used to keep the BB on a straight line and different sizes of straws were used to drop the BB from different heights mentioned above. Imaging the Crystal Surface using the AFM The crystal was connected to a voltage source (0-90V, Direct current) using the same Aluminum used in Figure 3. First the sample crystal was imaged without applying a voltage across the sample at 64.1 nm pixel size resolution. Then the sample was imaged with an applied voltage. Images were recorded at different voltages varying from 0-95V. IV. RESULTS Oscilloscope data of the marble drop When the crystal was tested for piezoelectricity using the oscilloscope, there were instant voltage differences as the BB was in contact with the crystal. Figure 4: Maximum Voltage generated vs. Time, when the BB was droped from 9.8cm. Reason for the tail: as the BB hits, the AFM Images of the Crystal All the images in appendix I were imaged at 64.1 nm pixel size resolution, to reflect expected changes on the surface. This is the highest resolution that could be achieved due to an uneven surface area. As it can be observed there is an increase in number of steps as the voltage across the sample is increased. These steps represent sudden valleys (dips) on the surface. Table 1 represents the number of steps across the sample and the depth of these valleys on the surface as the voltage was increased. Voltage Across (V) Number of Steps Across Maximum Z Height in Change(nm) Table 1: Applied voltage and different morphological changes on the surface. V. CONCLUSION When the crystal was tested for piezoelectricity, as expected there was a linear relationship between the generated voltage and the deformation of the sample. As the energy of the deformation (caused by the BB) is increased the generated voltage increases.

3 Up to 25+ 4V applied voltage across the sample, there were no detectable changes to the surface. As the voltage was increased to V there was a surface change of nm. The crystal size was 5 x x10-3 m, which gives a 0.001% change on the surface as 30V (DC) is applied across 0.5 cm thick crystal. This experimental value does not match with the theoretically calculated value 0.001% change as 300V is applied across 1cm thick crystal. This can be caused by many different reasons. VI. APPENDIX I These graphs represent AFM images (at different voltages) and the cross sectional z height change across one line of the same position of the crystal. Y-axis represents the Z height in nanometers and x- axis represents the distance across the sample in micrometers. 0 Volts Figure 6.a; AFM image, X distance across-16.4µm, maximum Y Change 120nm Additional Experimentation Points It is useful to see to see how the surface would change as the voltage across the sample is increased in smaller voltage increments (2-3V). Also it is curios how the surface steps would change when a negative voltage is applied. As an additional point, it is very useful to have a sample stage where one could move the sample at micrometer increments. Figure 5.a; AFM image, X distance across-16.4µm, maximum Y Change 160nm 24.9 Volts

4 Figure 5.b Cross sectional height change across the sample maximum Y Change 160nm. (no steps) Figure 8.a; AFM image, X distance across-16.4µm, maximum Y Change 225nm 60 Volts Figure 6.b; Cross sectional height change across the sample maximum Y Change 160nm. (no steps) 34 Volts Figure 9.a; AFM image, X distance across-16.4µm, maximum Y Change 240nm Figure 7.a; AFM image, X distance across-16.4µm, maximum Y Change 200nm 44 Volts Figure 7.b; Cross sectional height change across the sample maximum Y Change 200nm. (4 steps can be seen)

5 Figure 8.b; Cross sectional height change across the sample maximum Y Change 225nm. (5 steps can be seen) Figure 10.a AFM image, X distance across-16.4µm, maximum Y Change 220nm VII.ACKNOWLEDGMENTS I would like to thank and acknowledge my senior project committee; Dr. Steve Lindaas (Chair), Dr. Ananda Shastri and Dr. Russ Colson for the support and knowledge provided. Also I would like to thank Dr. Gary Edvenson and Dana Carlson for their guidance on making piezo crystals and the Physics Department of Minnesota State University Moorhead for the opportunity to use the Atomic Force Microscope and the lab space. VIII.REFERENCES Figure 9.b; Cross sectional height change across the sample maximum Y Change 240nm. (7 steps can be seen) 94 Volts [1] Atomic Force Microscope, Accessed May 2009, Last updated March 2009, [2] NnaoSurf easyscan 2 AFM operating instructions booklet, version 1.5, p , Oct [3] Alan Holden and Phylis Singer, Crystals and Crystal Growing 1 st ed, p , Columbus, Ohio [4] Growing Crystals, last updated July [5] Stephen Lindaas, X-ray Holography using a Scanning Force Microscope (Doctoral thesis), p , May [6] Walter Cady, Piezoelectricity, 1t ed, p , Vol 2, Dover Publications, NY

6 Figure 10.b; Cross sectional height change across the sample maximum Y Change 240nm. (14 steps can be seen)