Rahul Singh

Pyrolysis reaction networks for lignin model compounds: unraveling thermal deconstruction of β-O-4 and α-O-4 compounds

Although lignin is one of the main components of biomass, its pyrolysis chemistry is not well understood due to complex heterogeneity. To gain insights into this chemistry, the pyrolysis of seven lignin model compounds (five β-O-4 and two α-O-4 linked molecules) was investigated in a micropyrolyzer connected to GC-MS/FID. According to quantitative product mole balance for the reaction networks, concerted retro–ene fragmentation and homolytic dissociation were strongly suggested as the initial reaction step for β-O-4 compounds and α-O-4 compounds, respectively. The difference in reaction pathway between compounds with different linkages was believed to result from thermodynamics of the radical initiation. The rate constants for the different reaction pathways were predicted from ab initio density functional theory calculations and pre-exponential literature values. The computational findings were consistent with the experiment results, further supporting the different pyrolysis mechanisms for the β-ether linked and α-ether linked compounds. A combination of the two pathways from the dimeric model compounds was able to describe qualitatively the pyrolysis of a trimeric lignin model compound containing both β-O-4 and α-O-4 linkages.

Graph Fourier transform based on directed Laplacian

In this paper, we redefine the graph Fourier transform (GFT) under the DSPG framework. We consider the Jordan eigenvectors of the directed Laplacian matrix as graph harmonics and the corresponding eigenvalues as the graph frequencies. For this purpose, we propose a shift operator based on the directed Laplacian of a graph. Based on our shift operator, we then define total variation of graph signals, which is used for frequency ordering. We achieve natural frequency ordering as well as interpretation via the proposed definition of GFT. Moreover, we show that our proposed shift operator makes linear shift invariant (LSI) filters under DSPG to become polynomials in the directed Laplacian.

Impeding phonon transport through superlattices of organic–inorganic halide perovskites

Superlattice structures present a strategy to impede lattice thermal transport through organic–inorganic halide perovskites and improve their potential for thermoelectric applications. We investigate the phonon characteristics of such novel configurations and compare against predictions of the simple perovskite lattices using first principle calculations. Our results show the existence of structural instabilities due to distortions in the octahedral cage surrounding the methylammonium ion. In the superlattices, a strong phonon incoherence reduces the group velocities while interfacial resistance enhances scattering and limits the phonons impeding heat conduction. Although heat transfer is anisotropic in these perovskites, the interfaces in the superlattice obstruct phonon transport along all directions.

On spectral analysis of node centralities

In this paper, we study spectral properties of node centralities such as degree, closeness, and betweenness centralities, by utilizing graph Fourier transform. We consider node centralities as signals on various networks, namely, regular, random, small-world, and scale-free networks. The spectral analysis helps us to easily understand centrality patterns over various networks. We observe spectral patterns for different network models and subsequently classify networks from the spectra of node centralities.

ULTRASONIC EVALUATION OF TRANSIENT LIQUID PHASE BONDING IN SINGLE CRYSTAL SUPERALLOY CASTINGS

Transient liquid phase bonding (TLPB) is an effective means for joining high performance metal components. It differs from welding and conventional brazing in that it produces very little chemical segregation or microstructural demarcation at the bond-line. The method of transient liquid phase bonding was originally developed in the 1970’s [1] and has been used in the joining of titanium and nickel based superalloy components. In this method, a bonding alloy containing a melting point suppressing element is sandwiched between the parent metals to be joined. The temperature is raised to a point where the bonding alloy melts but the parent metals remain solid. The melting point suppressing element then diffuses away from the bondline, thus raising the melting point and solidifying the bond. Since the temperature never exceeds the melting point of the parent metal, single crystals may be joined without destroying their crystalline structure.