Sunday, November 15, 2020

Etching silicon wafers to make colorful Rugate optical filters (porous silicon)

Passing an electrical current through a silicon wafer in a special acid etchant will create a porous layer with a variable index of refraction. I describe how this process works, and how the Fourier transform relates filter design to electrical etch waveform and resulting spectral response.





Notes: 1. The Rugate filters look especially good in polarized light because the reflection from the silicon wafer is reduced, but the reflection from the filter remains strong. I noticed this while holding the wafer in front of my computer monitor. Later, when showing it to friends, the color intensity was poor. Their monitor must have been horizontally polarized, so holding the wafer low, and tilting it upward didn't work! Some monitors are horizontally polarized, and some are vertical. 2. The magnet used to hold down the PTFE cup to the wafer may have a very slight impact on the etch process. The dramatic shift in filter performance at the periphery is due to O-ring restricting the conductive etchant to a sharp edge, creating an electrical field concentration. Outstanding visual Fourier series tutorial:http://www.jezzamon.com/fourier/index.html LR-1 spectrometer: http://www.aseq-instruments.com/LR1.html p-type wafers on eBay. (You have to hunt around and check the photos for info on the label attached to the box of wafers. Be sure to get wafers less than 0.01 ohm-cm) eg https://www.ebay.com/itm/25-silicon-wafer-P-type-150mm-100-sumco/263441166009 n-type wafers are photosensitive during the etch process. Online graphing calculator page from this video: https://www.desmos.com/calculator/hiju8zdqfz Original Desmos page that I used: https://www.desmos.com/calculator/qpnz9celzf Code for Keithley 2450 control and processing.org waveform generation: https://github.com/benkrasnow/Porous_Silicon_Optics Fourier transform to understand optical coatings: http://www.willeyoptical.com/pdfs/92_180.pdf Porous silicon refs: https://www.rp-photonics.com/rugate_filters.html https://www.intechopen.com/books/porosity-process-technologies-and-applications/porous-silicon https://sci-hub.se/https://doi.org/10.1016/j.mee.2011.03.143 https://sci-hub.se/https://doi.org/10.1080/10408436.2010.495446 https://sci-hub.se/https://doi.org/10.1007/978-3-319-71381-6_2 https://sci-hub.se/https://doi.org/10.1364/AO.44.005415 https://sci-hub.se/10.1364/OE.16.015531 https://sci-hub.se/https://doi.org/10.1002/adma.19940061214 https://sci-hub.se/https://doi.org/10.1063/1.2906337

Sunday, September 20, 2020

Shooting an electron beam through air

 A special 100nm thick window allows 25 KeV electrons to pass from a vacuum tube to the atmosphere where they hit a fluorescent screen -- a CRT in air!





Shielded GoPro goes through a powerful electron beam: https://www.youtube.com/watch?v=yPMpAR9w-L0 More powerful amateur electron beam in air: https://fusor.net/board/viewtopic.php?t=7828 Deep technical resource on dielectric charging via electron bombardment: https://apps.dtic.mil/dtic/tr/fulltext/u2/a172204.pdf Tons of information on industrial e-beam processing: http://iiaglobal.com/uploads/documents/Industrial_Radiation_eBeam_Xray.pdf KF25 to glass tube quick adapter: https://www.idealvac.com/SWIFT-SEAL-KF-to-Compression-Port-Adapter/pp/P103772 100nm silicon nitride windows: https://www.tedpella.com/grids_html/silicon-nitride-x-ray-windows.htm More windows: http://www.temwindows.com/category_s/22.htm Tritium light sources (eBay removed most): https://usa.banggood.com/search/tritium.html E-beam crosslinking: https://ebeamservices.com/polymer-crosslinking/services/plastic-parts/ KF25 cross $18 on Amazon: https://www.amazon.com/gp/product/B07CKS3H99 Lightbulb sockets: https://www.amazon.com/gp/product/B07SSYN83Y/ Hysol 1C: https://www.amazon.com/Loctite-HL1373425-1373425-Hysol-1C/dp/B000B631G8 Applied Science on Patreon: https://www.patreon.com/AppliedScience

Tuesday, September 8, 2020

Sunday, August 30, 2020

Demo and teardown of an X-ray fluorescence gun (measures chemical composition)

I show how an X-ray fluorescence gun works, do a teardown and failure analysis, and discuss a new kind of XRF tech now "available" on eBay.


The XRF data can be transferred from the gun to a PC, and analyzed with http://pymca.sourceforge.net/ I did this for several materials, but ran out of time in this video to show the software usage, which is quite involved. Good overall description of XRF: https://phys.libretexts.org/Bookshelves/University_Physics/Book%3A_University_Physics_(OpenStax)/Map%3A_University_Physics_III_-_Optics_and_Modern_Physics_(OpenStax)/08%3A_Atomic_Structure/8.06%3A_Atomic_Spectra_and_X-rays Electron shell shielding described via roller derby ?! https://courses.lumenlearning.com/cheminter/chapter/electron-shielding/ XRF supplies: https://www.amptek.com/ MightyOhm geiger counter: https://mightyohm.com/blog/products/geiger-counter/ The two XRF guns in this video were generously donated by one of my viewers. Thank you! The Tribogenics tape-peel XRF gun on eBay: https://www.ebay.com/itm/New-Watson-XRF-Handheld-Metal-Alloy-Analyzer/123646159115 Applied Science on Patreon: https://www.patreon.com/AppliedScience

Sunday, July 19, 2020

Holograms on chocolate





DIY process to copy commercial holograms onto chocolate! Completely edible -- no dyes or inks used. Tech Ingredients' video: https://www.youtube.com/watch?v=SoTi0tM4yQ8 Samy Kamkar's diffractive chocolate: https://twitter.com/samykamkar/status/1259173832620830722 Hologram stickers: https://www.ebay.com/itm/100-Huge-1-9-16-Round-Adhesive-Hologram-Sticker-Labels/331178956157 Silver mirror process: https://sci-hub.tw/https://doi.org/10.1080/00202967.1941.11869396 Add a pinch of sodium lauryl sulfate to the stannous chloride primer solution to help the solution get into contact with the plastic surface. https://www.youtube.com/watch?v=y-4qqcCxD6g Morphotonix: https://www.npr.org/sections/thesalt/2014/06/14/321816570/holographic-chocolates-look-as-beautiful-as-they-taste http://www.morphotonix.com/products/ Caswell nickel electroplating kit: https://www.caswellplating.com/electroplating-anodizing/nickel-plating-kits/nickel-electroplating-kit-1-5-gal.html (Price was already pretty high years ago, and has gone up a lot since; and the kit has fewer components than it used to. But it does work well.) Holography Marketplace book -- lots of very practical information for making your own holograms, and good source of suppliers (though many are out-of-business now): https://www.amazon.com/Holography-MarketPlace-6th-Franz-Ross/dp/0894960997 Very good source for mold making supplies: https://shop.smooth-on.com/ Countermeasures against hologram copying. Holograms are often used as a security seal, so making copies as shown in the video could be a problem without countermeasures: http://www.nli-ltd.com/publications/countermeasures.php I was planning to talk more about different types of holograms in technical detail, but I think I will save it for a future, dedicated video on making original holograms. https://www.patreon.com/AppliedScience