Engineered Tungsten Armor for IFE Dry Chamber Walls

1. Engineered Tungsten Armor for IFE Dry Chamber Walls Scott O’Dell1 and Rene Raffray2 1Plasma Processes, Inc., Huntsville, AL 2University of California, San Diego, La Jolla, CA High Average Power Laser Program Workshop Oak Ridge National Laboratory

2. Introduction • Tungsten is an ideal material for armoring IFE dry chamber walls due to its high melting temperature, high thermal conductivity and inherent resistance to thermal erosion. • Vacuum Plasma Spray (VPS) has been identified as a promising technique for bonding tungsten armor to Low Activation Ferritic (LAF) steel. • PPI is currently working on a Phase II STTR to develop the techniques to produce VPS tungsten armor comprised of dense tungsten bonded to LAF steel with a thin outer layer of porous tungsten to prevent damage from helium entrapment. 2

3. VPS Tungsten Armor on LAF Steel Nanoporous W Layer Dense W Layer Low Activation Ferritic Steel • High temperature plasma enable processing of high melting materials. • Processing of oxygen sensitive materials due to processing in controlled atmosphere. • Use of reactive gases such as hydrogen to reduce impurities such as carbon and oxygen. • Robotic control enables deposition of uniform, repeatable deposits over complex geometries. • Ability to tailor the structure of the coating by changing the deposition parameters and use of multiple materials. 3

4. Approach • Modeling of the tungsten armor to determine the ideal architecture • Analysis of recent He implantation experiments by UNC/ORNL • Calculate the microstructure dimension for the IFE case • Development of the nanoporous layer to prevent helium build-up • Preliminary work used commercially available submicron powder (~500nm) • Techniques have been developed to produce finer W feedstock • Development of dense well adhered tungsten armor on LAF steel substrates • Determine the effect VPS processing parameters have on the coating • Evaluate the effects different size starting powders and alloys have on the properties of the tungsten armor 4

5. Modeling at UCSD • Activation energy results from the analysis of the UNC/ORNL experimental data has been used to predict He retention for an IFE case. The He dose (or concentration) per cycle in the IFE case would correspond to an activation energy of 3.1 eV for effective diffusion. • Quasi steady state was achieved for the cases with characteristic dimensions of 10, 50, and 100 nm, respectively. However, for the 1000 nm case, quasi steady state was not reached yet. • Clearly, on the basis of these results, the porous tungsten microstructure has to be much less than 1 µm since the atomic fraction of He retained is very high, ~0.1. • A microstructure dimension in the range of 50-100 nm would reduce the atomic fraction of He retained to ~2 x 10-4 - ~10-3, respectively. While a 10 nm microstructure dimension would reduce the atomic fraction retained to ~10-5. 5

6. Nanoporous Tungsten Development • Previous work using submicron W feedstock has demonstrated the ability of VPS forming techniques to produced nanoporous deposits with a microstructure dimension of ~500nm. • Porosity levels: 10-25% • He permeability tests have shown the porosity is interconnected. Nanoporous W deposit LAF steel substrate Submicron W feedstock Nanoporous W deposit on LAF TEM image showing microstructure dimension and interconnected pores. 6

7. Finer Tungsten Feedstock Powder • Difficult to find commercially available W powders with an average particle size less than 300nm • Two techniques have been developed to produce finer tungsten feedstock powders • High energy mechanical milling • Thermal plasma processing of tungsten precursor materials W powder produced by mechanical milling Of the two techniques, thermal plasma processing currently shows the most promise due to: 1) high throughput (kg/hr), 2) less potential for incorporation of impurities due to nucleation of the particles from the vapor phase and rapid quenching in an inert atmosphere, and 3) finer particles have been produced to date. W powder produced by thermal plasma processing 7

8. Characterization of Ultra-fine W Feedstock • Analysis of TEM images has shown the average particle size of different lots of thermal plasma processed tungsten is consistently less than 100nm (typically 36-43 nm). • XRD analysis has shown the material is pure tungsten. • Chemical analysis has shown carbon and oxygen levels can be controlled to less than 100 and 2000 ppm, respectively. • Work is currently underway to produce deposits for analysis. 8

9. Dense Tungsten Armor Develop • The focus of this portion of the investigation has been to optimize the tungsten armor beneath the thin porous layer to produce dense, well adhered W deposits. • One of the key properties of the dense W layer is its thermal conductivity. • To evaluate the effects of different Re additions (2 and 25wt%) and different size W feedstock powders on thermal conductivity of the dense W layer, thick deposits (2.5-3mm) were produced on 25mm x 25mm x 3mm LAF substrates. • Free standing thermal diffusivity (12.7mm dia. x 2mm) and specific heat (5.5mm dia. x 1.3mm) samples were electric discharge machined from the deposits. • Thermal diffusivity measurements were made from RT to 1400ºC and specific heat measurements were made from RT to 975ºC. • The densities of the free standing samples were determined using water immersion techniques. The thermal diffusivity, specific heat and density values were then used to determine the thermal conductivity of the different samples. 9

10. Rationale for Re additions in W • “Rhenium Effect” - an increase in the low-temperature ductility and improvement in the allied characteristics (decrease in the ductile-brittle transitions temperature, improvement in weldability, and decrease in tendency to longitudinal cracking) with simultaneous increase in strength at low and elevated temperatures.* Plot showing an increase in the UTS of tungsten-rhenium alloys as compared to pure tungsten as a function of temperature. Plot showing the change in DBTT for recrystallized and deformed W-Re alloys as a function of rhenium percentage. 10 * - B. Bryskin, Rhenium and Rhenium Alloys, TMS 1996

11. Effect of Re Additions on the Thermal Conductivity of VPS Tungsten VPS W and W-Re after Post Spray Heat Treatment (Produced on High M.P. Substrate) VPS W and W-Re in the As-Spayed Condition (Produced on LAF Steel Substrate) Typical densities: 99% of theoretical Typical densities: 88-92% of theoretical • VPS W deposits produced on high melting temperature substrates and given a post spray heat treatments result in a thermal conductivity equal to or exceeding wrought W material. • Both 2 and 5wt% Re additions reduce the RT thermal conductivity as compared to pure W. • However, at elevated temperatures, the thermal conductivity of the pure W approaches that of the W-2Re. • The thermal conductivities of the VPS W and W-Re deposits produced on the LAF steel substrates have a similar overall look as the VPS W deposits produced on high M.P. substrates and given a post spray heat treatment. • However, significant reductions in the RT thermal conductivities were observed (e.g. pure W is ~70%). • At elevated temperatures, the reduction in thermal conductivity is less pronounced (e.g., pure W is ~80%). 11

12. Microstructure Comparison VPS W after Post Spray Heat Treatment (Produced on High M.P. Substrate) VPS W-25Re in the As-Sprayed Condition (Produced on a LAF Substrate) • Micrograph of the VPS W deposit after post spray high temperature heat treatment shows the as-sprayed structure has been replace with a recrystallized, equiax grain structure. • By comparison, the VPS W-25Re deposit in the as-sprayed condition is comprised of a typical splat-type structure with some partially melted particles embedded in the deposit. • The cohesive strength between splats is typically less than can be achieved if recrystallization and grain growth can occur. • However, the lower melting temperature of the LAF steel substrate and the mismatch in CTE between the W deposit and the LAF steel limits the plasma spray parameters and the use of post-spray heat treatments. 12

13. Effect of Different W Feedstock Particle Sizes on Thermal Conductivity • W (-45/+20 µm) and W (5 µm avg.) were used to produce 2.5-3mm thick deposits on LAF substrates with densities of ~90% of theoretical. • A review of the literature* showed SNLs had produced W deposits using W feedstock with an average particle size of 13.5 µm on copper and stainless steel substrates. Essentially equivalent densities were achieved for the SNL deposits as were produced in the current study. • Particle size of the SNL feedstock fell between the two particle sizes currently being evaluated. Similarly, the RT thermal conductivity of the SNL fell between the thermal conductivities of the coarser and finer W feedstock materials. • At elevated temperatures (>500ºC), the thermal conductivity of the SNL material and the finer W feedstock (5 avg.) start to overlap. • However, the elevated thermal conductivity of the coarser W feedstock material is ~30% greater than both finer material at 1400ºC. 13 * - R.A. Neiser, et al, Proceedings of the 1993 National Thermal Spray Conference, Anaheim, CA, 7-11 June 1993, pp. 303-308.

14. Summary • Modeling results have shown the microstructure dimension for the porous tungsten layer should be 100nm or less to minimize the amount of He retained in the IFE case. • Manufacturing techniques have been developed to produce tungsten feedstock with an average particle size of less than 100nm. • Thermal conductivity results have shown the VPS processing parameters and W feedstock materials have a significant effect on the properties of the dense W layer. 14

15. Future Work • Techniques to enable the VPS of the ultra-fine tungsten powders are currently being evaluated. • Deposits produced with the ultra-fine tungsten powders should be produced with in the next couple of months. • Experiments are planned to evaluate the bond strength of the dense tungsten layer and the LAF steel substrate at ORNL. • He retention experiments will be conducted on both dense and porous VPS tungsten at UNC. 15