We calculate the double resonant (DR) Raman spectrum of graphene, and determine the lines associated to both phonon-defect processes, and two-phonons ones. Phonon and electronic dispersions reproduce calculations based on density functional theory corrected with GW. Electron-light, -phonon, and -defect scattering matrix elements and the electronic linewidth are explicitly calculated. Defect-induced processes are simulated by considering different kind of idealized defects. For an excitation energy of $\epsilon_L=2.4$ eV, the agreement with measurements is very good and calculations reproduce: the relative intensities among phonon-defect or among two-phonon lines; the measured small widths of the D, $D'$, 2D and $2D'$ lines; the line shapes; the presence of small intensity lines in the 1800, 2000 cm$^{-1}$ range. We determine how the spectra depend on the excitation energy, on the light polarization, on the electronic linewidth, on the kind of defects and on their concentration. According to the present findings, the intensity ratio between the $2D'$ and 2D lines can be used to determine experimentally the electronic linewidth. The intensity ratio between the $D$ and $D'$ lines depends on the kind of model defect, suggesting that this ratio could possibly be used to identify the kind of defects present in actual samples. Charged impurities outside the graphene plane provide an almost undetectable contribution to the Raman signal.

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Theory of double-resonant Raman spectra in graphene: intensity and line shape of defect-induced and two-phonon bands

Applications of using nano material in concrete: A review

Freeze casting – A review of processing, microstructure and properties via the open data repository, FreezeCasting.net

Processing, properties and applications of highly porous geopolymers: A review

Journal of the European Ceramic Society

Incorporation of porosity into a monolithic material decreases the effective thermal conductivity. Porous ceramics were prepared by different methods to achieve pore volume fractions from 4 to 95%. A toolbox of analytical relations is proposed to describe the effective thermal conductivity as a function of solid phase thermal conductivity, pore thermal conductivity, and pore volume fraction (νp). For νp 0.65, the thermal conductivity of kaolin-based foams and calcium aluminate foams was well described by the Hashin Shtrikman upper bound and Russell’s relation. Finally, numerical simulation on artificially generated microstructures yields accurate predictions of thermal conductivity when fine detail of the spatial distribution of the phases needs to be accounted for, as demonstrated with a bio-aggregate material.

/pdf/thermal-conductivity-of-porous-materials-3rzzh1rqo7.pdf

Thermal conductivity of porous materials

Candidate materials for thermal insulation combine a solid phase of low thermal conductivity with porosity. Very porous ceramics based on a commercial clay containing 79% kaolinite were made by a direct foaming method using methylcellulose, drying at 70 °C before a consolidation at 1100 °C. The pore volume fraction ( v p ) was varied in the kaolin-based foam by modifying the amount of clay incorporated in the starting mixture. Measurements of the thermal conductivity with the hot disk method revealed a decrease from 0.23 W m−1 K−1 at v p  = 0.57 to 0.054 W m−1 K−1 at v p = 0.95 in close agreement to predictions by the Hashin–Shtrikman upper bound, a cubic pore model and numerical simulation of an artificial microstructure representing the polyhedral pore shape with Voronoi mosaics. The effective Young's modulus, obtained from mechanical compression tests, also decreases with pore volume fraction as described by Ashby's relation. However the mechanical strength was sufficient for handling even the more porous kaolin-based foams.

Effect of the pore volume fraction on the thermal conductivity and mechanical properties of kaolin-based foams

Two metakaolins exhibiting different reactivities were used to study geopolymers obtained via phosphoric acid activation. The study of consolidation of samples with various Si/Al and Al/P showed that the Al- and Si-rich and P-poor compositions were more likely to consolidate properly. The geopolymers formed at 20 or 70 °C kept the same crystalline impurities as the starting metakaolins, the samples remaining mostly amorphous. The microstructure of geopolymers formed with less reactive metakaolin was smooth and dense, while the geopolymers prepared from the most reactive one were more porous and rough. Finally, tests on water and fire resistances of all the samples were reported.

Composition and properties of phosphoric acid-based geopolymers

Industrial refractories can exhibit untypical Young's modulus behaviour versus temperature, characterized by hysteretic loops in the plot E(T). This paper shows how the use of simplified model materials helps in the understanding of such behaviours, obviously depending both on the properties of each component of the refractory material and on the presence of internal defects. Model materials constituted of a hot-pressed glass matrix surrounding spherical alumina inclusions have been studied. Glasses exhibiting various coefficients of thermal expansion (CTE) have been used as matrix in order to obtain three typical microstructural configurations (Δα   0, where Δα denotes the CTE difference between matrix and inclusions).

Untypical Young's modulus evolution of model refractories at high temperature

https://hal.archives-ouvertes.fr/hal-01549675/document

Nicolas Tessier-Doyen

Papers

Thermal conductivity of porous materials

Effect of the pore volume fraction on the thermal conductivity and mechanical properties of kaolin-based foams

Composition and properties of phosphoric acid-based geopolymers

Untypical Young's modulus evolution of model refractories at high temperature

A discrete element thermo-mechanical modelling of diffuse damage induced by thermal expansion mismatch of two-phase materials