Noble metal aerogels (NMAs), as the most important class of noble metal foams (NMFs), appear as emerging functional porous materials in the field of materials science. Combining the irreplaceable roles of noble metals in certain scenarios, as well as monolithic and porous features of aerogels, NMAs can potentially revolutionize diverse fields, such as catalysis, plasmonics, and biology. Despite profound progress, grand challenges remain in their fabrication process, including the efficient structure control, the comprehensive understanding of the formation mechanisms, and the generality of the fabrication strategies, thus inevitably retarding the material design and optimization. This Perspective focuses on the key progress, especially of the fabrication strategies for NMAs during the last two decades, while other NMFs are also succinctly introduced. Challenges and opportunities are summarized to highlight the unexploited space and future directions in expectation of stimulating the broad interest of interdisciplinary scientists.

Emerging Noble Metal Aerogels: State of the Art and a Look Forward

Summary Armed with merits of the metals (e.g., electrical conductivity, catalytic activity, and plasmonic properties) and aerogels (e.g., monolithic structure, porous network, and large specific surface area), metal aerogels (MAs) have stood out as a new class of porous materials in the last decade. With unparalleled potential in electrocatalysis, plasmonics, and sensing, they are envisaged to revolutionize the energy- and detection-related application fields. However, MA development is severely retarded by the lack of a sufficient material basis. Suffering from the ambiguous understanding of formation mechanisms, big challenges remain for tailoring MAs for task-specific applications. By surveying state-of-the-art developments, this review strives to summarize design principles and arouse interest in broad scientific communities. Moreover, critical challenges and opportunities are highlighted to provide a research roadmap for this young yet promising field.

A Roadmap for 3D Metal Aerogels: Materials Design and Application Attempts

Dynamics of localized current filaments is analyzed in electrostatic discharge protection devices of a smart power technology during microsecond long constant current pulses. Experiments performed by backside transient interferometric mapping technique and transmission line pulser stressing are correlated with 3D device simulation. Motion of the filament along the device width and its reflection at the device ends are observed. This behavior is related to the time evolution of the voltage waveform and is explained by the simulation. The filament motion is driven by the temperature gradient in the filament due to the negative temperature dependence of the impact ionization rates. A simplified analytical expression for the filament speed as a function of stress conditions and thermal characteristics is given.

Thermally-driven motion of current filaments in ESD protection devices

Helicity amplitudes for the simplest zero-entropy hadron amplitudes in topological particle theory are constructed. The kinematic singularities and the Regge asymptotic behavior of such amplitudes are studied in detail. The importance and effects of planar but nonzero-entropy corrections, which are expected to bring the zero-entropy amplitudes into reasonable agreement with the physical world, are discussed.

Helicity amplitudes and Regge behavior for simple planar hadron amplitudes

Bars of REBa 2 Cu 3 O y superconductor with three different rare earths were prepared by floating zone technique in air. Their nominal composition is (RE 0.33 Eu 0.33 Gd 0.33 )Ba 2 Cu 3 O y (RE=Y, La, Pr, Nd, Sm and Yb). The mixing of these three elements leads to a single phase formation. The physical properties of these systems and their microstructure were studied. All the compositions exhibit a superconducting transition excepted that one containing Pr and substitution of Ba by RE is avoided excepted in the case of RE=La. Only the phase with Nd shows a pronounced fishtail effect and the phase with Sm exhibits a very high J c value of 70 000 A/cm 2 in self-field and more than 30 000 A/cm 2 at 1.7 T.

RE0.33Eu0.33Gd0.33)Ba2Cu3Oy bars processed in air

“Quark-color” switching is given a smooth topological representation that leaves junction lines uncrossed by charge or momentum arcs. One of the 3 topological colors maintains a distinguished role, but color asymmetry is diminished with respect to an earlier nonsmooth switching proposal. It is shown that strong-interaction quantum surfaces now never become disconnected.

A smooth topological representation for “quark-color” switching

A topological representation for individual quark chirality is proposed which eliminates spin lines from the classical surface and provides “chiral entropy” through patch-boundary lines that correspond to chirality reversal. The effect on quark spin makes it appropriate to describe such a patch-boundary line as a “topological vector gluon”. The chiral entropy index is equal to the number of “vector gluons”.

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Topological “vector gluons”

Topological Particle Theory yields a unified particle description in terms of two topological constituents, which we call theI-triangle and theY-triangle. Each triangle is a charge doublet; theI-triangle has spin 1/2 while theY-triangle has spin 0. Leptons are\(\bar I \bar Y\); electroweak vector bosons are\(I \bar I\); hadrons are\(I \bar I\) (mesons),\(I I I \bar Y\) (baryons), or\(I I \bar Y Y \bar I \bar I\) (baryoniums). An as-yet-undiscovered singlet neutral electroweak scalar boson is predicted, corresponding to\(Y \bar Y\)

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Topological compositeness of quarks, leptons and electroweak bosons

We show that, under rather general assumptions, the ratio of the average multiplicity to the cross section (rather than the average multiplicity itself) is expected to grow as Ins. We see that this ratio is already linear in lns at presently-available energy, and we extrapolate this linear dependence to predict the value of this ratio at higher energy.

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On the scaling of the average multiplicity in hadron-hadron collisions

Recent modifications in the topology of chirality and “color” are incorporated into a representation of electroweak hadron interactions. It is shown that a connected oriented thickened momentum graph, housed in a single sheet of the classical surface, can represent a minimal electroweak hadronic vertex.

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J. Finkelstein

Papers

A smooth topological representation for “quark-color” switching

Topological “vector gluons”

Topological compositeness of quarks, leptons and electroweak bosons

On the scaling of the average multiplicity in hadron-hadron collisions

A single-sheet classical surface for electroweak hadronic currents