Polymer precursor route for the synthesis of advanced ceramics has attracted main interest in recent years due to their exceptional high temperature stability, resistance to crystallization and thermal decomposition. Unfortunately, the field of ceramics corrosion is far less comprehensive than for metals. The corrosion and reliability performance of ceramics under extreme environment conditions has become increasingly important in chemical process environments where materials are subjected to increasingly higher temperatures, pressures, and aggressive chemical attack. Moreover, these polymer derived ceramics (PDCs) in acid environments and high temperature applications in salt environments are not well established. Therefore, corrosion behavior of PDCs was investigated under sodium, chlorine, sulfur and fluoride environments. Two oxides (Si-C-O and SiCHfNO) and two non-oxide PDCs (SiCN and SiBCN) were examined in this study. NaCl (mp 801 ºC) and Na2SO4 (mp 884 ºC) were used as sources of corrosive species of sodium, chlorine and sulphur at fixed temperature of 1000 ºC for 24 h. HF (azeotropic (38.26 % HF)) was used as sources of corrosive species of fluoride at fixed temperature of 90 ºC for 2 weeks. The corrosion mechanism is explained in line with the experimental discoveries.
Subcritical crack growth: Research investigates the subcritical crack growth (SCG) behavior of a rareearth oxide - gadolinium oxide/ gadolinia, sintered through pulsed sintering route and aided by stress-corrosion in saline water (3.5 wt% NaCl) and atmospheric vapor pressure (relative humidity 55-60%). Samples were subjected to highly saline and constant vapour pressure environments to understand the influence of chemistry in propagating the subcritical cracks, originating from a Vickers’ micro-indent. Crack lengths were observed to increase marginally with time, following the power law slope formulations, with crack velocities as low as 10-13 m/s and the relaxing residual stresses were quantified in terms of stress intensity factor. However, the total SCG contribution was constrained within the near-threshold range (< 10-9 m/s) even though there was a significant change in velocity for similar, non-induced cracks. Boron modified polyvinyl silazane was thermolysed at 1300 °C under argon atmosphere and subsequently spark plasma sintered individually at 1200 °C and 1600 °C in vacuum. The failure behaviour in chloride environment was compared with its behaviour in humid environment using the data extracted from Raman spectroscopy, the thermodynamic characteristics of the environment and the porosity parameter. The implications of the subcritical crack growth were derived by plotting v-KI graphs for each of the samples.
Deformation behavior of aerospace grade metallic materials under biaxial tension
- Nedunchezhian Srinivasan
Titanium and titanium alloys finds its applications in the aerospace industries because of its high strength to weight ratio in combination with excellent corrosion resistance. More often, such structural applications encounters complex stress state during its service. Hence, the objective of the present work is to understand the mechanical behavior of such aerospace grade materials under biaxial tension. Biaxial tensile testing rig of 250 kN capacity was designed to carry out equi-biaxial tensile testing of metallic materials till failure. The test rig consist of hydraulic power pack, double acting hydraulic cylinders and loading frames which can apply desired tensile force along X and Y directions respectively. Non-contact digital image correlation technique will be employed for the measurement of failure strain. X-ray diffraction and TEM analysis will be carried out in order to understand the operating deformation mechanisms, slip activity, texture evolution upon deformation.
Strain contour plot of gage section of uniaxial sample at various load steps (CP Titanium)
The main aim of the project is the synthesis and fabrication of porous titanium oxide/oxynitride/nitride nanocrystals in an amorphous silicon oxynitride matrix. Extensive structural characterisation as well as evaluation of photocatalytic activity and adsorbent capacity of the material will be carried out. The polymer derived ceramic route was chosen since it offers a promising route for the processing of new type of ceramics with excellent control on the chemistry and properties. The porosity induced in these materials acts as high density active centres for photocatalytic reactions and increases surface accessibility leading to increased photocatalytic reaction centres.Porous monolith was produced via spark plasma sintering and the mechanical stability of these materials will be evaluated.
Porous monolith obtained after pyrolysis and nanostructural features in as-pyrolysed sample
Thermodynamic modelling of transition metal containing silico-nitride ceramics coupled with ab initio calculations
- Soumya Sridar
This study involves development of multi-component thermodynamic database for the quaternary N-Si-Ti-Zr system. Extrapolation to higher order systems requires accurate thermodynamic descriptions for the constituent binary and ternary sub-systems. Re-assessment/assessment of the binary and ternary sub-systems using Thermo-calc software is the focal point of this study. The thermochemical data required for modelling different phases are evaluated using ab-initio calculations.
Biofouling is the undesired accumulation of living organisms on a surface – ranging from microbes like bacteria, algae to invertebrates like mussels, barnacles, etc. Marine biofouling impacts material and equipment life; disturbs the ecological balance and directly or indirectly leads to the loss of millions of revenue annually.
Marine biofouling increases the hydrodynamic drag on vessels by increasing hull roughness, which leads to higher fuel consumption and cost. The US Naval Sea Systems Command estimates that fuel costs increase 6 to 45% if the hull is fouled, depending on the size of the vessel. Further, it leads to the introduction and spread of non-indigenous marine species.
In this work, an attempt will be made to understand in detail - the process of marine biofouling, causes, effects and preventive measures.
In every system while doing some work, there is some energy wasted as heat. This waste heat can be recovered and converted into useful electrical energy with the help of thermoelectric materials. Thermoelectric polymers have shown the advantages such as low cost of fabrication, light weight, and flexibility. Lower thermal conductivity of polymeric materials helps to achieve temperature gradient which helps to increase the efficiency of energy conversion. Apart from that, oxidation and reduction of polymers can tune its electronic properties to desired level. The flexible nature of polymers decides the strain level under loads and corresponding structural change within the polymer affects the electrical and thermal conductivity of the polymer. Understanding the chemical modifications and mechanical loads on the thermoelectric polymers can helps to develop the different forms of thermoelectric polymers for various applications.
Sintering behaviour, microstructural charectirisation and thermal expansion properties Zr substituted SnMo2O8
Zirconium substituted SnMo2O8 is a material whose thermal expansion can be tuned by controlling the amount of Zr substitution. Previous studies have shown that this material can exhibit ultra low thermal expansion when an equimolar amount of Sn is substituted by Zr. This system belongs to a family of negatively expanding AM2O8 materials where A=Zr, Hf and M=W, Mo. These materials have a characteristic cubic structure consisting of AO6 octahedra and MO4 tetrahedra. In this project single phase cubic crystalline system Zr0.5Sn0.5Mo2O8 was synthesized and sintered at different temperatures. The microstructure of the material was studied using an electron microscope. The thermal expansion coefficient (CTE) was determined experimentally using a dilatometer. This CTE value was found to be on par with that of commercial glass ceramics. The dielectric properties of the material were analysed at room and elevated temperatures.
Schematic to show the rotation of polyhedra to accommodate thermal energy
Materials with Vickers hardness value greater 40 GPa are known as superhard materials. Few of the transition metal boride phases also falls into this category of materials. These materials have high hardness values, bulk moduli and good electronic conductivity which makes them promising candidate for many industrial applications. These materials have potential applicability as abrasives, cutting tools, wear resistant coatings and electrode materials in harsh environments. But synthesis of these materials often involves application of very high pressure of the order of gigapascals. In the synthesis process lighter element like boron is added to transition metals in order to obtain multi directional covalent bonding over metallic bonding present in transition metals, which results higher hardness. Efforts are made to synthesize nano-crystalline transition metal borides at low pressure condition aiming for increased hardness and better mechanical properties.
Mayenite, Ca12Al14O33, abbreviated as C12A7 (12CaO.7Al2O3), was basically used as a refractory oxide. It has a very interesting cage structure and most of the early research on Mayenite dealt with its unique crystal structure. The crystal structure of Ca12Al14O33 is made up of a 3D network of corner-sharing AlO4 tetrahedra together with seven-coordinated Ca ions, and six crystallographic cages of a subnanometer size (~0.4 nm in diameter). An additional oxygen ion, which is called ‘free oxygen’ or ‘extra-framework oxygen’, occupies one of those cages to maintain the charge neutrality of the entire crystal.
It was found later, that this compound exhibits high ionic conductivity, roughly an order less than YttrSZ. Also this compound transforms into an electride upon reduction. Also many other anions like (F-,Cl-, O2-, O-,OH-) can be incorporated into the structure of Mayenite. Hence physical properties like hardness, electrical and thermal conductivities can be altered by changing the chemistry of Mayenite. These special properties are attributed to its unusual crystal structure.
Mayenite was synthesized from Calcite and Alumina through thermal route. The mechanical properties, electrical and thermal conductivities of O-Mayenite and N-Mayenite will be investigated and the dependence of composition on these properties will be studied.