Okay, so here I am seeing that the traditional preparation of powder FTIR analysis is that it's put into some powdered KBr and mashed with a die, so
it makes like a clean lens that the IR beam passes through........
SO WHY DONT WE JUST MASH KBr INSTEAD OF USING CRYSTAL KBr IN OUR BEAMSPLITTERS?!!?!?!?!
Like........ it's not negatively affecting the spectrum or anything because it passes through there anyways.... theres definitely a setup in which the
Ge(germanium) can be combined with it, either still deposited onto it as thin film or deposited onto that optical plastic(glass is too opaque) and
have the two pressed together.Dr.Bob - 18-12-2014 at 12:17
I am not certain, but think the mashed and compressed KBr forms a glass, not a crystal. Glasses do not have a single crystal structure, so they may
not split the beam in the same manner that a single crystal might do. That is certainly the case for silicon, which is a much better semiconductor
in certain axises of the crystal, but not as good when melted into an amorphous solid (polysilicon). That is one of the main reasons that PV solar
cells are so expensive, as the simpler ones have much lower efficiencies than the ones cut from a single crystal. And complex integrated chips
definitely need single crystals.
I am an organic chemist, so my knowledge of solid state and optics is weak. but I know that single crystals are far different that glasses in many
ways. Plus I think there are other layers on a beam splitter, but not sure if chemrox - 18-12-2014 at 15:11
I'm not clear on the issue. I do not believe mashing KBr does anything except make the cryx smaller and less uniform. The beam splitter and the KBr
pellets do entirely different things. Making a KBr pellet dilutes the material so one can obtain a clear IR spectrum. The KBr doesn't give rise to a
spectrum of it's own within the range of the instrument and that's also the reason it can be employed as a beam splitter. I don't know the physics but
I believe the beam splitter is a crystal (??). Bob do you know? Or where to look?SupaVillain - 18-12-2014 at 15:26
Well the kbr is just a substrate and so the germanium is the actual beamsplitter, and KBr is opaque because the IR beam is passed through it before it
reaches the detector. Thing about crystal KBr is that it is a firm body for a thin film to be deposited onto. The powdered KBr pellet has the same IR
spectral transmission range as the crystal, but i think it's all much less structured and it's delicate since it was powder pressed into a disc. But
that makes me wonder if powdered germanium or a thin film could be applied to KBr powder and then mixed and stirred up and THEN finally pressed, and
come up as the same thing.... i mean i cant think of why that wouldnt workSupaVillain - 18-12-2014 at 15:33
Of course the beam splitter is a crystal, KBr is the substrate and Ge is the thing actually reflecting some and splitting the beam. But im trying to
see if i can make a beamsplitter for cheaper by just using a pellet disc of KBr instead of crystal, and then depositing the Ge film on that or mixing
it in to the powder previous to pressing to get the same effect. SupaVillain - 18-12-2014 at 18:08
Well here in this pic it shows its germanium, not KBr that actually splits the beam, the crystal KBr is only chosen for its ability to be polished
down to optical flat grade of flatness and because of its spectrum. Well KBr pellets have the same spectrum and can be pressed to optical flatness as
long as there's two optical flats on each side pressing it together.
This might actually be the limiting factor in the capability of making this happen because the top and the bottom flats both have to be level, but
maybe i can do some of my own "polishing" with this, that being, making it level to the point in which no point of it is some nm or micron amount
higher or lower than any other point
Marvin - 20-12-2014 at 14:26
If the KBr was pure, what would keep it as a disc, what stops it from falling apart?turd - 21-12-2014 at 01:04
I am not certain, but think the mashed and compressed KBr forms a glass, not a crystal.
Your ideas are intriguing to me and I want to subscribe to your newsletter.
Trivial answer: for optical components you want homogeneity. A perfectly homogeneous, anisotropic glass would probably be fine as well, but good luck
making one out of KBr.
Moreover, cubic KBr has well defined (100) and (110) crystal faces which conveniently happen to form an angle of exactly 45°.
Quote:
If the KBr was pure, what would keep it as a disc, what stops it from falling apart?
