• First steps: making big crystals

    Kelvin Ly • 09/16/2018 at 05:12 • 0 comments

    Right now, the most important task is making decently sized Rochelle salt (potassium sodium tartrate) crystals, because I want to make the xylophone bars out of solid chunks of the stuff. It needs to be crystalline because that's how the piezoelectricity works from my understanding; when the crystal structure's distorted, the charges move in a way so that their effects add up, instead of canceling out in materials with randomly oriented grains. Most of the popular ways to make Rochelle crystals use the old saturated solution trick; you mix a lot of the Rochelle salt into a heated volume of water, and then evaporation and the temperature drop cause the crystal to form. This works, but it seems really slow and hard to scale up, so I want to explore other means of creating the crystals. 

    The other obvious means to heating the salt above its melting point and slowly letting it cool so that it has time to form into large crystalline structures. Rochelle salt has a melting point of 75 C, and it decomposes at 220 C, so it's in a pretty convenient temperature range in terms of melting it in a workshop, and it's got a pretty decent margin for error as well. There's a couple of ways I want to explore, heating using steam, oil bath, and just chucking it in a toaster oven at a fairly low temperature. 

    With the steam approach, it'll have the advantage of having a very stable temperature, and it'll be very easy to make. However, according to Wikipedia, Rochelle salt is deliquescent, meaning it likes to absorb water,, even up to the point of dissolving itself. That means the salt has to be separated from the steam by some means, or else it'll tend to absorb the moisture and dissolve itself instead of forming nice crystals.

    The oil bath is about as easy as the steam approach. I could make a mold and float it on top of the oil after it's been heated to the right temperature. Getting it to the right temperature is going to be trickier, but this approach will let me use an open facing mold, which is going to be a lot more convenient than whatever I'd need to make for a steam-based heater.

    The oven approach is definitely simpler than both of the other ones, but I have a lot less control over the evenly heated the mold will be. This is definitely going to be the first approach I'm going to experiment with though.

    After a heating method's been decided, there's a few methods I want to look into to make orient the crystal's direction. I'm thinking it might be worth seeing if some variation on the Bridgeman-Stockbarger process can be used. This places the melted substance in a container that's held between two ovens of slightly different temperatures. The different temperatures of the ovens means a temperature gradient will naturally form, and the container is moved back and forth along the gradient so that part of the melt is cold enough to crystallize while the rest stays molten. By carefully controlling how the container's moved, you can reliably produce high quality crystals because of the good control you have over the temperature of the melt. This process is used to make crystalline ingots for mainly semiconductors, so it could potentially be scaled down to work for this project.

    I've already worked out my first ideas on the electronics too, I'll probably post that in another build log soon.