New developments?

[benf]

Not a mechanical engineer but this material caught my eye….Corning offers up Macor which seems to have a lot of useful properties and is machinable. Here’s a link to a New Jersey distributor that shows pricing on the website that seems within reach, although you’d still have to fabricate. Can it be made from a tube or hollowed out rod and a plate joined together, or does it need to be cast as one piece? Maybe the rubber o-ring isn’t sufficient to absorbing the shock and the insulator “crashes” against the steel plate or anode during the shots? I’m picturing the image of the broken view port from an earlier newsletter. How was that issue addressed?

Edit: Adding this spec sheet from another supplier comparing the material to aluminum silicate. If this material is too expensive maybe look for used pyrex or zerodur telescope mirror blanks? Probably hard to fabricate….

[pulser]

I just ordered Kapton film from ColePalmer last week to stand off a silicon wafer from a chuck (My arena is 30A, 1KV, 5ns pulses with 200ps rise times – different than DPF – but all fun to work with).

If one wants to use Kapton but is concerned about surface damage, then coat with a layer of SiO2, Al2O3, AlN, or Si3N3 by sputter or PECVD deposition.
Multiple layers of insulators interspersed with conductive layes can withstand higher fields).

Saphire can be purchased in wafer form, not sure what diameter is needed here. Much stronger than quartz or glass.
Gorilla glass (Corning) has substantial strength and resistance to damage. If a sheet can be used, this could be useful.

If you can forward me mechanical diagrams, pictures,engineering objectives/constraints, etc., I could make better recommendations.

[asymmetric_implosion]

benf: Macor is an interesting material but it is more prone to shattering than alumina. I’ve used it in other plasma facing applications and it doesn’t do as well alumina or alumina silicate.

pulser: A coating is probably not enough to protect against the plasma and UV. The shock from the breakdown can shatter the coating leaving the underlying material polymer exposed. NASA has looked into this problem for spacecraft that use Kapton as a protective coating. Cracking the tough outer coating leads to failure of the polymer underneath. Coating would likely enhance the lifetime of a Kapton insulator but I would guess the lifetime increase would be some fraction of the Kapton only lifetime instead of a factor of 10 or 100.

The insulator is generally one piece (machine or fired) to prevent any potential arcing at the joint. Sapphire would be ideal but it is difficult to make in a hat design. People make sapphire cylinders large enough in diameter and long enough but they cost ~$10K per unit (Saint Gobain) and they need to be made into a hat. Machining adds a great deal of stress to ceramics if not done properly. Few people have done it properly for me, but I might not be working with the right folks.

The insulator-metal-insulator solution is used a great deal for high voltage but the problem is mechanical. 🙁

[KeithPickering]

Physical strength, rather than dielectric strength, is the key. There are plenty of materials better than alumina from a dielectric standpoint, but you’d have to look long and hard to find anything stronger. And if breakage is the problem, that’s a tough nut to crack. In that regard it would be useful to know for sure the failure mode: flexural, tensile, or compressive. From the description above I’m guessing flexural, as the shots bend the “brim” of the hat (but I could be wrong). Macor, for example, has 3 times the dielectric strength as alumina, but only 1/4 the flexural strength. So if alumina is going to break under the stress, Macor will too.

In spite of asymmetric implosion’s comments, I’m thinking a zirconia ceramic may be the solution. Dynacer (http://www.dynacer.com/data_sheets.htm) has several grades of a zirconia ceramic they call Technox which has 3 to 4 times the flexural strength as alumina. Pure zirconia also beats alumina in dielectric resistance too. And like all ceramics it looks like it can be made into pretty much any shape.

[KeithPickering]

Just to follow up, here’s a comparison of various grades of Technox vs. various grades of alumina

	Material:                          Technox 2000   Technox 3000      96% AlO3    99.5% AlO3

	Flexural strength (Kpsi)                   145            203           50         55   

	Compressive strength (Kpsi)                290            290          305        377

	Modulus of elasticity (Mpsi)                30             30         43.5       54.4

	Poisson ratio:                             .30            .30          .21        .22

	Fracture toughness (KIC)                    10             10          3.5          4

	

Dynex doesn’t give dielectric constants in its materials sheet, but http://iopscience.iop.org/0508-3443/18/6/305 indicates that zirconia has a dielectric constant of 22, while that of alumina is about 4.

[pulser]

http://www.quartz.saint-gobain.com/MICAVER®.aspx

This looks like an interesting material for this application.

Sounds like a polyimide or other non-outgassing polymerizable organic and ceramic composite could be a good choice giving stiffness and robustness against shock.
The part could be coated by SiO2, AlN, etc. by sputtering or other deposition technique if no organic exposure is allowable.

[vansig]

Have you thought of using a liquid as the dielectric?

[Lerner]

Has anyone used technox or other zirconia in a DPF? Any disadvantages that people can see? It probably would not work in a generator because its proeprties change with temperature but we will only operate at 120 C in the experiment.

[asymmetric_implosion]

I’ve looked into Zirconia before but the problem was finding a vendor to make the part. I asked for quotes on a hat with dimension about ~1/10 scale that LPP would need and no one could make it near theoretical density (the key to getting the other properties). That was a couple years ago so a vendor might exist now. It has many desirable properties compared to alumina other than the sensitivity to temperature. The data I found was zirconia cost more than alumina as well.

A liquid dielectric could be used outside of vacuum to insulate the parallel plates that feed the anode and cathode. In vacuum, how would you maintain the sleeve shape? I guess you could make a water fall but a liquid is more volatile and less survivable than a solid in the presence of a plasma.

Pulser: How thick can you coat the polymers? Are we taking microns or hundreds of microns? It would probably take mm to protect the underlying polymer.

[benf]

Would Silicon Nitride be a possibility? It’s used in the Space Shuttle rocket motors. Here’s product description and spec sheet from Ceradyne and Wikipedia info.

[asymmetric_implosion]

The big problem with coating is the low temperature tolerance of the polymers. In most dense coating applications like you would need for a plasma focus insulator, you need to be coating at 300-400C. Kapton can’t survive it. CVD methods require high temp (>500C). PECVD can work at lower temp but you are coating a dielectric so charging is an issue. Energetic methods like cathodic arc can work, but the coating thickness is limited to ~100 um because of internal stress. ALD would do a beautiful job but it will take a lifetime to produce a coating of consequence. Other methods based upon atmoshperic pressure deposition are low density or require extreme temperature. Polymers are unlikely to survive explosive cladding. I can’t think of another type of coating method.

[willit]

insulator cracking caused by mechanical stresses can be relieved by increasing clearances and insulating the insulator with flesable element (O RING) or by decreasing the stress applied or by changing the location of the stress. much like applying a large force on a window pane well supported or applying the same force on a poorly supported pane will render a different result.
from what i understand the material must comply with several operating parameters.
1 capable of 100 to 200 KV dielectric strength at low frequency for now.
2 capable of exposure to high temperature plasma
3 capable of exposure to high intensity xrays/ gamma rays / and the occasional neutron bombardment.
4 mechanical stresses both loading and thermal.
5 inexpensive and readily available
it sounds as if the chosen material is suitable in many aspects but application needs reconsideration.
if all things are equal there shouldn’t be shifting of side loading of any part of the machine.
if there is a push to the side i think this may lead back to asymmetric firing of the filaments.
if the insulator is properly supported and equally loaded it should not break.
if the insulator is fracturing at the joint between the flat plane and the cylinder it seems likely that there is a side force acting on the inner cathode.
if the insulator is fracturing at the edge is is coming in contact with conductor plates and flexed out of plane. if the edge shows signs of thermal shock it is likely from flashover.

in my opinion keep the existing material and support it differently.

[KeithPickering]

benf wrote: Would Silicon Nitride be a possibility? It’s used in the Space Shuttle rocket motors. Here’s product description and spec sheet from Ceradyne and Wikipedia info.

Silicon nitride is a semiconductor, not a good choice for an insulator.

[asymmetric_implosion]

A high band gap semiconductor would be interesting if it was held in reverse bias. People tried it with barrier discharges and found some interesting results. It is purely speculative and probably not appropriate for the LPP experiments, but it could lead to some interesting results.

[benf]

@Keith: Ceradyne’s Ceralloy 147 is described in their brochure as an excellent insulator, 10^16 ohm resistance in the sintered version and is touted as superior to alumina in various measures of durability. Also Wikipedia states that silicon nitride is used as an insulator in integrated circuits. Links to Ceradyne’s spec sheets and brochure PDF’s don’t work from these posts, but here’s a link to their brochure section.

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