Electronics > Beginners

dielectric constant for glycerin

<< < (3/5) > >>

Zero999:
Is the water a problem? As long as the glycerine doesn't contain any ionic salts, then I don't see the problem. It can soak up water from the air, but as long as the air isn't polluted with sulphur or nitrogen dioxide, then I don't see how it can be a problem.

How about adding silica gel to soak up the water?

Bud:
What is wrong with mineral oil? I just filled a dried 50 Ohm dummy load with it.

Zero999:

--- Quote from: Bud on March 21, 2019, 11:00:45 am ---What is wrong with mineral oil? I just filled a dried 50 Ohm dummy load with it.

--- End quote ---
Because glycerol has a higher dielectric constant, than mineral oil?

T3sl4co1l:
Dielectric constant doesn't mean much by itself, unfortunately. :(

Pure metals have a dielectric constant in the thousands.  That doesn't mean you can use them as a capacitor -- the phase of that displacement current is almost perfectly imaginary, which is to say, resistive rather than capacitive current flow.

Polyalcohols tend to have labile protons.  More simply, glycerin is chemically "wet": despite not itself being H2O, it can participate in many interactions the way H2O can.

Besides having huge affinity for water, it can also dissolve ionic salts.  Offhand, it looks that NaCl is soluble to the tune of 10%wt, so it is an effective ionic solvent.

I don't even know how you dry it -- can you dry it?  Concentrated sulfuric acid or P2O5 certainly isn't going to do (dehydration, condensation reactions), and anhydrous salts will simply dissolve in it (ruling out usual suspects like CaCl2).

Not only will you need to ensure it is very dry, but it will need to be free of ions, just as you'd use deionized water for an application like this.  The apparatus needs to be extremely clean, for the same reason; even then, ions simply dissolve from everything, everywhere, somehow or another, and the water needs to be cycled fairly often through an ion exchange resin, or replaced with fresh.

The biggest problem though is that alcohols tend to have strong dielectric losses to begin with, and I think glycerin will be one such case.  Typically, there is a relaxation time constant in the mere ~kHz, where the k goes from large to small, and while it's dropping off, it's very lossy (tan delta ~ 1).

Here's a study of PVA:
http://www.ufrgs.br/lapol/insulating_characteristics.pdf
PVA is alternating CH2s and CHOHs, while glycerin is HOH2C-CHOH-CH2OH (three CHOHs plus two H end caps, no CH2s), and it's also liquid (which means the OH protons are moving around constantly).  I would expect PVA to perform better, and clearly it doesn't perform all that well (depending on whatever they're doing there, different temperatures, and heat treatment I think?).  I believe water has a similar response?

Likewise, you can infer based on chemistry, what might perform actually well -- if those protons are locked down in bonds instead, say in esters, amides or hydrocarbons -- the dielectric constant will be a lot lower, true, but it will be a lot flatter with respect to frequency, and that's the important part.

PET (a polyester) for example, has a modest k, and reasonable enough losses (and various other features: stability, cost of raw materials, ease of manufacture..) that it's commercially important as a capacitor type already. :)  Still pretty lossy, enough that you can't use it for, say, resonant power supplies (or radio transmitters, CW Tesla coils, induction heaters..), but it can be okay for pulsed applications when the internal heating isn't enough to burn a hole through the thing.  (It may be hard to find caps that are constructed for pulsed application too, though -- heavy metallization, thicker leads, noninductive build.)

Downside of course being... interesting and all, but there aren't any esters in liquid form that are also cheap.  Well, except for esters of glycerin, oddly enough -- fats are triglycerides (three fatty acids stuck onto a glycerin backbone).  Not much k, they're mostly hydrocarbon -- but it satisfies everything else. :-+

Tim

Jwillis:
Would you be opposed to using something other than glycerin.Titanium Dioxide - Titania has much better dielectric strength at around 4kV /nano meter and a greater electron absorption or dielectric constant of around 85.It has a much better resistance to water absorption than glycerin/glycerol.You could significantly reduce the size of your construction with the same results.What are your parameters for voltage and charge?
Purification/distillation of glycerin/glycerol  can't be done simply by boiling it at atmospheric pressure because it will reabsorb as much water as it releases.It was a long time ago but I believe it needs a much lower atmospheric pressure then frozen to allow the crystals to precipitate.Then the crystals require washing with acetone.Seems a bit much for the kitchen.     

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod