Electrostatic Levitation Lab

Iowa State University Department of Physics and Astronomy

Background on the ESL

The Chamber

The ESL setup is not mechanically complex. At the heart of the vacuum chamber is a set of electrodes; two aligned vertically and two for each horizontal direction. The bottom electrode is grounded, while the top electrode is held at a large negative potential, creating an electric field similar to that of a parallel plate capacitor. The sample is initially placed on a post in the bottom electrode, then charged by conduction and launched when the field is switched on. This method of levitation is flexible; processable materials are not restricted just to metals.

A statement known as Earnshaw's theroem describes that there can be no electrostatic equilibrium in a collection of point charges; in order to stably maintain the sample, a set of lateral electrodes are controlled by an LED positioning system. Several PSD measure the shadow cast by the LED's, and this shadow yields the position of the sample. This data is then sent to a computer running a control algorithm, which spits back corrections to the voltages in the electrodes. As can be seen from the video, the feedback gives excellent stability and control.

ESL @ ISU - Real Time Control

Real time control demonstration.

Once the sample is stabilized in the chamber, a dual fiber-heating laser is aligned towards the samples center. This laser provides up to 60 watts to melt materials in a controlled fashion. To control this process, we use programs written in LabView to heat or cool the samples at a variety of controlled rates. During the heating or cooling of the sample, A UV source provides an alternate charging mechanism via the photoelectric effect, to offset mass evaporation during this process. Two pyrometers measure the temperature; one in the low temperature regime and the other at the high. All while a charge-coupled device (CCD) camera captures video for density analysis and two cameras constantly take pictures to obtain information on density and viscosity.

And most recently, by collaborating with another group in the department, a non-contact method has been developed to measure the electrical properties of levitating samples througout the entire process.

The Science

A primary motivator of containerless processing is the study of phase transformations. Due to the energy associated with creating a solid-liquid interface, it is possible to maintain a system in the liquid state well below its freezing point. While there is an intrinsic and stochastic process of crystallization (known as homogenous nucleation), the process will crystallize at much higher rates if there are impurities or solid phases left in the melt, by heterogenous nucleation. By nature of the ESL, levitating samples in a vacuum, any heterogenous nucleation sites aside from those already present in the material are removed, allowing observation of deeply undercooled phases well below their equilibrium transition temperatures. These undercooled phases are interesting and, given the difficulties of maintaining materials in this state using older methods, relatively unexplored.

Additionally, when certain materials make the liquid - solid transition under the right conditions, they may undergo a glass transformation. Instead of the liquid attaining the ordered structure of a crystalline solid, it may "freeze out" into a disordered glass state. This kind of transition is not very well understood, and is of considerable interest considering the properties of metallic glasses. As ESL allows for controlled cooling rates as well as easy viscosity measurements, it is hoped ESL will give some more insight into how these materials form and why.

Another area that th ESL excels in is that of accurate measurement of thermophysical properties. For instance, by the very nature of the system the levitation of a sample is entirely decoupled from the heating mechanism, and it is possible to infer information about the heat capacity of a material just by observing various radiative cooling rates.

Current Challenges and Solutions

Materials have a property known as emissivity, which controls exactly how they absorb and emit radiation. The difficulty in this lies in the huge variation of emissivity across samples; as the emissivity is even dependent on the methods of processing the surface, even samples with the same chemical content can have different emissivities if they were treated differently. Emissivity can change across a temperature range and as a function of wavelength. As by nature measurements in a levitation chamber are non-contact, this can affect the accuracy of data.

To resolve this, there are plans to obtain a special pyrometer which, by measuring intensities across the entire spectrum, give not only accurate temperature data, but also information on the spectral emissivity.

Additionally, while the ESL offers a very simple method to measure viscosity, using an oscillating electric field to induce oscillations in the sample, the current CCD camera is too slow to measure this well. However, there are plans to test a high speed camera in the system which will allow not only accurate measurement of viscosity, but also comes with a bonus: crystallization velocity. By observing the solidification at high speeds, we can gain much more insight into the actual growth processes of the system.