Ravi Kumar N V

Thermal shock resistance of precursor derived Si-Hf-C-N(O) foams at temperatures varying from 800o-1000oC subjected to multiple thermal cycles was investigated. The as-synthesized foams possessed interconnected pores with an average cell size of 1.09 mm. The X-ray diffractograms of the foams before and after thermal cycling showed that the amorphous nature of the foams was retained. FTIR spectra exhibited that there was no change in the bonding characteristics due to thermal shock. A damage parameter (DS) based on the compressive strength was used to quantify the extent of damage. Densification was expected to occur in the first thermal cycle and the strut structures did not show any sign of cracking. However, cracking of struts occurred in the third thermal cycle which caused severe damage.

Thermodynamic modelling of Ti-Zr-N system is performed using Calphad method coupled with ab initio calculations. The energies of formation of stable and metastable end-members of sublattice formulations of solid phases in Zr-N system and enthalpy of mixing of the mixed nitride (Ti, Zr)N (δ) are calculated using ab initio method. Phonon calculations are used to compute the Gibbs energies of stoichiometric ZrN and the mixed nitride δ. With the aid of experimental thermochemical and constitutional data from the literature along with the results of ab initio calculations, thermodynamic optimization is carried out to obtain the Gibbs energy model parameters.

The dielectric behavior of spark plasma sintered SiC(O)/HfCxN1-x nanocomposites synthesized through polymer derived ceramic route was investigated in the frequency range of 1 kHz to 1 MHz at room temperature. The nanostructural features revealed HfCxN1-x nanocrystals encapsulated in a nanometric thin layer of carbon dispersed uniformly in a SiC(O) matrix with segregated free carbon. The nanocomposites exhibited colossal permittivity values in the order of 103 at 1 kHz which reduced to 646 at 1 MHz. The interfacial polarization mechanism existing between complex nanostructural interfaces and the percolation of HfCxN1-x nanocrystals are believed to be responsible for the high permittivity values observed in the measured frequency range. The AC conductivity exemplified a frequency independent behavior at lower frequencies while at higher frequencies, the conductivity exhibited frequency dependence, indicating the existence of hopping type mechanism.

Water repellency, exemplified in nature, has fascinated the materials community. This behaviour is utilised to biomimic self-cleaning effects for technological applications. Tropical leaves of three species, namely Moringa oleifera, Euphorbia milii and Caesalpinia ferrea, are investigated in this study. Wettability measurements show that C. ferrea leaf was superhydrophobic with a contact angle greater than 150°, while the other two leaves were hydrophobic with contact angles greater than 140° but less than 150°. Surface morphological studies reveal a structure with irregular wax structures on all the three leaves, unlike the regular wax structures such as tubules or platelets typically observed in most of the superhydrophobic leaves. The surface features were quantified on C. ferrea leaf surface due to its reduced wettability among the three leaves. The parameters such as height, mid-width, and radius of curvature of the cell and thickness of the wax film were determined. This study will possibly provide a new insight into developing self-cleaning surfaces with irregular nanostructures as opposed to conventional regular nanostructure patterns.

Powder particles of gadolinia were spark plasma sintered at varied temperatures between 1400 °C and 1600 °C. High-density samples free of any sintering additives were obtained at the highest sintering temperature and the samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) including density measurements. The contact angle measurements and translucence studies implied the sintered samples to be hydrophobic and translucent respectively. The nanohardness values enhanced up to ∼88% with increase in sintering temperature from 1400 °C to 1600 °C. Young's modulii determined using nanoindentation varied from 126 GPa to 169 GPa for the samples sintered at the lowest and highest temperatures respectively. The theoretical Young's modulus was also determined using first principle calculations which were eventually used for fracture toughness calculations. The fracture toughness values of the sintered samples were calculated using the indentation crack length method (ICL) and compared with the crack opening displacement (COD) method.

Gibbs energy model parameters for Si-Zr-N system were obtained using Calphad approach coupled with ab initio calculations. The enthalpies of formation of α and βSi3N4 in Si-N system and Zr5Si3N end-member in Si-Zr-N system were calculated using density functional theory (DFT). The finite temperature thermodynamic properties were calculated using quasiharmonic approximation (QHA). The computed heat capacities were fitted to appropriate expressions valid down to 0 K. The ab initio thermochemical data obtained in the present work and the experimental thermochemical and constitutional data from literature were used for the thermodynamic optimization of Si-N and Si-Zr-N systems. The calculated phase equilibria and thermodynamic properties are in good agreement with the input data.

In this work we report the thermodynamic modelling of the Al-N, B-N and Al-B-N systems by the Calphad approach, making use of the ab initio calculations and the available experimental thermochemical and phase equilibria data. Gibbs energy description for the Al-B system is taken from the literature, with a modification for the model for the liquid phase. The enthalpy of formation and the heat capacity of various polymorphs of AlN and BN are estimated using the DFT and the phonon calculations, respectively. The enthalpy of mixing of the mixed nitride (Al,B)N in wurtzite and cubic structures are obtained using the SQS method. Gibbs energy of formation of the mixed nitride from the binary nitrides is also computed using the phonon calculations. These quantities along with the critically assessed thermochemical and constitutional data are used for obtaining the Gibbs energy functions of various phases in the Al-N, B-N, and Al-B-N systems. Calculated phase equilibria and thermochemical data are compared with the experimental data.

Polymer derived Si-Hf-C-N(O) foams were produced by the pyrolysis of polysilazane containing varying amounts of hafnium tetra n-butoxide. X-ray tomography was used to determine the cell size distribution. The ablation resistance of the foams was studied by subjecting it to different oxyacetylene flame temperatures. After ablation, fragmentation was not observed and the mass ablation rate was found to be minimal for the foams produced from highest vol% of hafnium tetra n-butoxide. The foams remained X-ray amorphous after ablation and the presence of [Formula presented] and [Formula presented] bonds were confirmed using ATR spectroscopy. For comparison, a reasonably dense spark plasma sintered pellet was also ablated and the evolution of monoclinic hafnia was confirmed using X-ray diffraction. The microstructural characterization exhibited three different zones on the ablated surface for sintered sample and the ablation mechanisms were understood using thermodynamic calculations.