![]() ![]() So bending the needle to the right, I expect the following from left to right: fixed & horizontal boundary nvex,concave,needlebase,convex,concave,fixed & horizontal boundary condition. But with glueing or clamping I expect the boundary condition to also fix the slope on the endpoints to horizontal. To me this seems correct in the case the boundary condition is rotation about a fixed point on the left most point of the cross section and the rightmost point of the cross section. concave function, and the right half as convex. I also wondered if one could mill the piezo disc to be shaped like a thin cross, to decrease the coupling with the fluid air?Īnother thing I thought about: in alexanders depiction of piezo disc bending, suppose the upwards pointing needle is deflected to the right, then the left cross-section of the disc is depicted as bulging upwards, i.e. I am under the impression you want stiffness for higher framerates? If we order the response times of the axes, then the highest bandwidth axis should be depth (in constant current mode at least), the second highest bandwidth axis should be the line axis (the one that retraces many times per frame), and the lower bandwidth axis the framerate axis, correct? the most common according to wikipedia is There are multiple kinds, some should be bidirectional transducers, others rely on resistance measurement. I wonder if part of phono cartridges could be used for 2 of the 3 positioning axes? That said, MLCC actuators do make for dirt-cheap, rigid nanopositioners, and you can always stack them to get displacements of several microns.Ĭongratulations on the STM, with interest I also read this page on mlcc piezo actuators. I might buy some piezo stacks instead, or look for a relatively cheap tube scanner. 0805 on the Z-axis is a possibility, but the required capacitance would still be fairly high. They give very little displacement for a given capacitance, but do have the advantages of rigidity and low-cost. Using a smaller package, i.e. I’m not sure if I’ll build a new scanner using these actuators. Not too bad, but I had hoped poling would make a bigger difference. I measured about ~3 nm/V for the unpoled actuator (or 300 nm over its 100 V range), and ~8 nm/V (800 nm over it’s 100 V range) for the poled actuator. Getting the displacement was then just a matter of reading off the amplitude from the trace/retrace plot during the scan (the Y-axis is in nm): I started scanning the copper tape surface and applied a small 10 Hz sine wave to the MLCC actuator from a function generator. I did this by sticking a small piece of copper tape the top of the actuator and applying the bias voltage (for tunneling) to the tape. I used my STM to measure the displacement of both an unpoled and a poled actuator. I then turned off the heater, let it cool, then turned off the 100 V supply. I heated the transistor up to ~150C, applied 100 V to the capacitor and let it sit for about 2 hrs. The Curie temperature for BaTiO3 (the ceramic used in MLCCs) is only 120C, so I used a metal-can transistor as a tiny hot plate: I built up a simple 555-based boost converter on a breadboard to supply the 100 V poling voltage. Sounds simple enough, so I gave it a try on a 2.2 uF, 100 V, 1812-sized capacitor. I found another article describing this process being used on MLCCs to increase their sensitivity to applied forces. Piezoelectric ceramics are ferroelectric, and can be poled by heating above the Curie temperature, applying a voltage to align the ferroelectric domains, and letting it cool with the voltage still applied, locking the domains in their aligned orientation. The displacement of MLCCs can be increased though by poling the ceramic dielectric. ![]() A buzzer, for comparison, has a capacitance around 10 nF. These capacitance values are quite high and would severely limit scanning speed. The authors were able to get displacements of ~500 – 1000 nm from unpoled X7R capacitors (10 – 100 uF). A quick google search turned up one article on the subject (journal article is here). I thought about using multilayer ceramic capacitors (MLCCs) as actuators, given that they’re piezoelectric. I’d like to replace the unimorph disk scanner in my STM with something more rigid. It does have one major disadvantage though: poor rigidity, especially in the Z-axis where it’s most important. This makes it highly vulnerable to vibrations and limits scan speed. The unimorph disk scanner is cheap, has low capacitance, and works at low voltages, making it great for a low-cost SPM. ![]()
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