Sintering of Nanoparticles by QED?

PITTSBURGH - Jan. 15, 2016 - PRLog -- .
Introduction
NP self-assembly to superlattices of 3D polycrystalline close-packed atomic arrangements often exhibit defects such as random packed domains, grain boundaries, and vacancies that lack continuous pathways for electron transfer. Recently, PDA is thought [1] to form single crystal-like superlattices of gold NP arrays by high pressure-induced sintering at room temperature. PDA stands for pressure-directed assembly and NP for nanoparticle. PDA  is desirable compared to other methods as byproducts are removed without complicated chemical reactions and thermal processing.

PDA is depicted in the thumbnail. Initially, the NP assembly at room temperature is at point A. Upon increasing the pressure to 9 GPa, the d-spacing decreases to point B. Upon returning the pressure to ambient, the d-spacing is recovered which means A-B and B-A are reversible, i.e., compressive work is not irreversibly dissipated by heat. However, from B-C the pressure increases above 9 GPa, but the d-spacing somehow increases instead of decreasing as would be expected if sintering is caused by pressure alone. Upon returning to ambient from C-D, the d-spacing also somehow increases instead of returning to A. What this means beyond B, from B-C and C-A, the process is irreversible characterized by the generation of heat, but increased temperature is not considered in the nanoscale sample as the sintering is thought to occur at room temperature by pressure alone.

To explain the increased d-spacing with increasing pressure, MD simulations [2] were performed of gold NP assemblies under elevated pressures. MD stands for molecular dynamics. But the MD simulations assume a Nose-Hoover thermostat to hold the sample at room temperature, and therefore MD cannot reveal the mechanism of how irreversible deformation can charge the NP array at room temperature.

Problem
When the pressure is > 9 GPa, sintering of NPs is thought to occur, but this is questionable as the d-spacing increases from B-C instead of decreasing. Another mechanism is at play to explain the increased d-spacing under increased pressure.

Proposal
Heat generated in the irreversible process B-C cannot be conserved by increasing NP temperature because QM precludes the NP atoms under EM confinement from having the heat capacity necessary to change in temperature, a condition that simply cannot be simulated in classical MD. QM stands for quantum mechanics and EM for electromagnetic. Instead, conservation proceeds by the QED induced creation of EM radiation that ionizes the NPs to produce a collection of charged gold atoms that expand upon Coulomb repulsion even under high pressure thereby explaining the d-spacing increase in process B-C and the continued expansion upon returning to ambient in process C-D. Heat induced ionization of NPs without increased temperature is a natural consequences of QM, e.g., see diverse QED applications at http://www.nanoqed.org, 2010-2016.

Discussion
The sintering of the 3D gold networks thought driven by high pressure induced contact of NPs is instead caused by contact of positive charged NPs under the high pressure necessary to overcome Coulomb repulsion.

Surface-stabilized NPs by dodecanethiol monolayers prevent NPs from aggregation at ambient pressure. But at high pressure, the monolayers are promptly dissociated by QED induced EM radiation.

MD based on classical physics that assumes the NP atoms have heat capacity cannot explain the creation of charged NPs predicted by QM, although MD can be modified for QM.

Deviatoric stress produced by superposing external uniaxial pressure on the hydrostatic pressure field does indeed drive NPs together to sinter and form a skeleton, but only because the loose collection of charged NP atoms can readily move in the superlattice. Similarity is found with the flow of charged NP atoms in solids, i.e., see http://www.prlog.org/12245294-qed-charging-of-metal-nanoc...

Summary
PDA based on pressure alone cannot sinter an amorphous array of gold NPs with defects into a superlattice as evidenced by the increase in d-spacing upon increased pressure. However, once a threshold pressure is reached where the work of compression is no longer reversible, heat is generated in the amorphous NP array that cannot be conserved by an increase in temperature because the heat capacity of gold atoms in the NPs vanishes by QM. Conservation then proceeds by QED inducing the creation of EM radiation that ionizes the amorphous NP array to produce a collection of charged gold atoms that upon increased pressure sinter the gold atoms to form the superlattice.

Conclusion
The transformation of NP arrays into superlattices need not rely on the heat produced from irreversible compressive deformation to charge the NP atoms as external sources of heat may be supplied during PDA.

References
[1] H. Wu, et al., “Nanostructured Gold Architectures Formed through High Pressure-Driven
Sintering of Spherical Nanoparticle Arrays ,"J. Am. Chem. Soc., 132, 12826–12828, 2010.
[2] W. Li, et al., “Deviatoric Stress-Driven Fusion of Nanoparticle Superlattices,” Nano Lett. ,14, 4951−4958, 2014.