JES 3-4-05 1

Divinylbenzene (DVB) Shells

High Average Power Laser Program Workshop

Naval Research Lab

Washington, DC

March 3-4, 2005

Jon Streit, Diana Schroen

Schafer Corporation

Reny Paguio, Brian Vermillion

General Atomics

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Review

• Status at last review:

– Lowest average batch NC was 3 %, some shells < 2%.

– PVP coating alternatives proposed.

– Different agitation methods proposed.

Full Density Coating

DT & Low Density DVB Foam

DT Fuel

DT Gas

Parameter Value

Diameter 4 mm

Wall Thickness 300 μm

Density 100 mg/cc

Out Of Round <1% of radius

Overcoat Thickness

1 – 5 μm

Non-concentricity <1%

Surface Finish ~20 nm RMS

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Shell Production Status

Nonconcentricity

Out of 40 batches attempted since the last meeting, our best average batch NCs continue to be around 3% with some individual shells < 2%. We have continued to study density matching and gelation time, but suspect that agitation may have the greatest effect on NC as changing this variable has led to the greatest improvement in NC. We experimented with applying ultrasonic waves to improve NC, and we will propose some new agitation methods to lower NC.

Overcoat

The PVP overcoat continues to shrink upon drying. We are screening a variety of interfacial chemistries to try to find an alternative that will not shrink and provide the required surface smoothness. We have also tried a melamine formaldehyde coating followed by an interfacial coating.

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Ultrasonics to Improve NC

• To reduce NC we applied ultrasonic waves to the shell bath during gelation.

• The force of ultrasonics tended to emulsify the shells, so the sonics had to be applied in pulses.

• This method was attempted using a variety of conditions, pulse lengths and intervals.

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Results of Ultrasonic Disruption

Batch #Density

Offset (g/mL)Temperature

(ºC)

Pulse

(s)NC%

3JS10A 0.02 70 0.1 s / 59.9 s 3.6

3JS10B 0.02 70 0.1 s / 59.9 s 4.0

3JS14A 0.00 80 0.1 s / 9.9 s 4.0

3JS15B 0.00 80 0.1 s / 0.9 s 4.1

3JS16A -0.03 35 0.1 s / 59.9 s 4.8

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Mechanical Vibration to Reduce NC

• Since ultrasonics were too disruptive to be applied except in very short pulses, we are attempting to develop a centering force through mechanical vibrations.

• We have attached a vibration device to the rotavap that we use to gel the shells in which physically vibrates the flask as the shells gel.

• We will obtain results soon.

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Paddle Agitation to Reduce NC

• We are also planning experiments to provide additional agitation to the shells by using paddles in the flask during the gelation period.

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OC Chemistries Attempted

Aqueous Reactants Oil Reactant

PVP

Isophthaloyl Dichloride or Terephthaloyl

Dichloride

PVP/Tris(2-aminoethyl) amine

Tris(2-aminoethyl)amine

Ethylene Diamine

Diethylene Triamine

Resorcinol

Piperazine

2,2-Biphenol

4,4-Biphenol

Melamine-Formaldehyde None

• We have screened several different interfacial coating chemistries.

• Polymer has formed in all cases, but we are still trying to determine the chemistry or combination that will meet specifications.

• We have attempted MF followed by interfacial.

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Melamine Formaldehyde (MF) Overcoat

• Previously we coated shells in a one step process with MF.

• This process produced a relatively smooth coating, but the coating was thin and tended to have very small cracks when dried.

• Attempts were made to make the MF coating thicker, but the thickness appears to be reaction limited.

• Since MF adheres well to the shell, we attempted to use it as a base for interfacial overcoats as it might help the second overcoat adhere to the shell as well and could possibly help increase smoothness by slowing the interfacial reaction down.

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Diethylene Triamine (DET) Overcoat

• DET by itself does not tend to shrink but can have problems adhering to the shell and is rougher than PVP.

DET SEM 5000x DET Interferometer Image107 nm RMS

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DET over MF

• MF followed by an interfacial DET overcoat resulted in a coating that did not shrink , was smoother, but had some cracking effects.

DET over MF SEM 5000x DET over MF Interferometer Image38 nm RMS

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We have produced heater assemblies with an RF foam coating for experiments at LANL

• RF foam was chosen as it would be the most gentle to the epoxy covering the wires, but clearly did attack the copper.

50 micron wire

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Future Work

• Continue studying the effects of agitation to reduce nonconcentricity. In addition to the new agitation methods proposed, other methods are being considered in response to the recent paper concerning agitation from GA.

– We have attempted gelling in a coiled tube with periodic flow reversal.

– We are also considering using our current standard flask set up, but cycling the direction of rotation of the flask.

• Continue to investigate interfacial overcoating chemistries that will adhere to the shell and meet smoothness specifications.

• Continue to investigate MF followed by interfacial overcoats.