Fabrication and Analysis of Nanostructured Copper

1. Fabrication and Analysis of Nanostructured Copper Jonathon Shanks, Michigan State University Mentor: Dr. Ke Han, Magnet Science & Technology National High Magnetic Field Laboratory Research Experience for Undergraduates, Summer 2003

2. Copper • Copper is a “model material” • Very well known bulk properties • Many uses • Copper is used extensively in electromagnets • Normal copper is microstructured • Grain size is 1–100 microns • 1 million microns = 1 meter • Width of a human hair is about 50-100 microns

3. Nanoscience • Nanoscience deals with objects at the nanoscale level (1-100 nm) • 1 billion nm = 1 meter • 1000 nm = 1 micron • Visible light has wavelengths 400-700 nm • Nanoscience is a frontier • Inspiration comes from a talk given by Richard Feynman (1959) • Invention of STM in 1981 put nanoscale within reach • Today there is still a lot being found out about the world at nanoscale sizes

4. Nanostructured Materials • Materials with grain size 1-100 nm • Can have new physical properties not in bulk materials • Example: Quantization of conductance, discovered in 1987, showing that nanoelectronics is not just smaller but different • Two general methods of fabrication • Top down: Microcrystalline materials have their grain sizes reduced through mechanical processes such as ball milling and cold rolling • Bottom up: Deposition techniques such as sputtering

5. Nanostructured Copper • Research interest in combining the versatility of copper with the interesting properties observed in nanoscience • Good properties: Stronger and harder • Bad properties: More brittle, less electrically conductive • Research in this area affects nanoscience, materials science and fields that use copper (such as magnet construction) • Is there a nanograin size where copper can be strengthened with little loss of conductivity?

6. Increasing Copper Strength • Plastic deformation of copper introduces work-hardening (copper gets stronger) and reduces the grain size • Hall-Petch relation predicts materials get stronger as grain size decreases: y = 0 + KHPd-1/2 (Yield strength is inversely proportional to grain size)

7. Increasing Copper Strength • Depending on fabrication method, some observations of “reverse Hall-Petch behavior” (copper gets weaker) for grain sizes < 100 nm • Grain sizes < 1 nm may become amorphous • No crystalline structure • Unit FCC cells break down • Target layer thickness is 1-100 nm

8. Loss Of Conductivity • In nanostructured copper, grain size < the mean free path of an electron (about 40 nm) • Mean free path of electron is the distance traveled by an electron (on average) in a material before it collides with something else and scatters • At nanograin sizes, electron scattering off grain boundaries will become a dominant mechanism of resistivity • Much higher resistivity • Mayadas-Shatzkes model is used to describe the increasing resistivity

9. Cold Rolling Rolling Mill Repeated Rolling Process

10. Before and After Cold Rolling Same sample in both pictures Approximately 20x thinner and longer after rolling

11. Fabrication Problems • Poor binding between layers • Good binding needed to get rid of voids and other defects in the copper • Use more layers in stack to increase effective rolling compression • Compress sheath with large pneumatic press (400-750 MPa / 4.5-8.4 tons/cm2)

12. More Fabrication Problems • Not enough multilayer foil generated for successive packings • Not enough layers means poor binding • Larger initial sample • Sheath diameter close to sample width, longer sheath • The many thin films that compose copper multilayer are individually very fragile • Difficult to clean, cut and restack • Annealment stage proposed (1h at 150°C)

13. Product - Copper Multilayer 60m Photo of copper multilayer after three rollings (two packings) 8 layers x 19 layers = 152 layers (60 m / 152 layers = 395 nm / layer) Layer thickness is approx. one wavelength of purple light (400 nm) Good binding from first packing, but poor binding from second packing (distinct 8 layers) – not enough layers in second packing

14. Product – Copper Multilayer

15. Resistivity Model • Mayadas-Shatzkes model is accurate but complicated • Used a simpler model (Fuchs theory) to estimate resistivity of a sample with layer thickness 400 nm • Result: Sample is 1.6 times as resistive, much higher than hoped, however the model is relatively simple and does not take all aspects of the physical system into account

16. Strength Model • Hall-Petch relation was used (assuming no reverse Hall-Petch behavior)

17. Progression of Samples Width of human hair (50-100 m) One rolling 10,000x reduction in thickness of original 1mm thick sample is needed to reach nanoscale threshold Two rollings Three rollings Four rollings? Nanoscale threshold (100 nm)

18. Conclusions • Thin-film copper multilayers were produced by repeated cold rolling with average layer thickness 400 nm • Strength and resistivity were estimated using available models • Shown that it is feasible to fabricate nanostructured copper using cold rolling

19. The Next Step • Fourth rolling for nanostructured sample • Observe structure with TEM • Use Mayadas-Shatzkes model to estimate resistivity • Measure resistivity and strength, compare with normal copper

20. Many thanks to: • The National High Magnetic Field Laboratory • Florida State University • The National Science Foundation • Dr. Ke Han, Dr. Aferdita Ishmaku, Dr. Baozhi Cui in Magnet Science and Technology • Dr. Pat Dixon, Ms. Gina LaFrazza-Hickey and other staff in the Center for Integrating Research and Learning