Jatin Bhatt

Determination of Silica Activity Index and XRD, SEM and EDS Studies of Amorphous SiO2 Extracted from Rice Husk Ash

Rice husk ash (RHA) contains 20% SiO2 in hydrated amorphous form (Si–OH). On thermal treatment, the SiO2 converts to cristobalite, the crystalline form which is not reactive. However under controlled conditions, amorphous SiO2 with high reactivity is produced. Therefore rice husk has been one of the useful bio-mass. The silica activity index was determined to be equal to 97.73 which was used to determine the percentage of amorphous SiO2 in RHA. The values of the soluble fraction and silica activity index were also stated. Comparative study of amorphous and crystalline SiO2 done by X-ray diffraction revealed the total amorphous nature of SiO2. The scanning electron microscopy (SEM) images displayed the comparative morphological features of the rice husk and RHA. The energy dispersive spectroscopy analysis of rice husk was done to determine the presence of SiO2 on the upper portion of rice husk and to determine the percentage of SiO2 in RHA. The SiO2 particles in an agglomerated form was found to be of micron size when observed under SEM.

Critical evaluation of glass forming ability criteria

The available glass forming ability criteria have been examined by classifying them into four basic categories depending on critical temperatures, thermodynamic quantities, topological and kinetic aspects of glass forming alloys. A large number of glass forming alloys of widely varying natures and origin have been analysed with their experimentally measured properties to assess their glass forming ability. A novel approach using kinetic viscosity of glass forming alloys obtained by the Vogel–Fulcher–Tamman equation and the critical cooling rate calculated from the TTT diagram is demonstrated as an excellent universal glass forming ability criterion. Moreover, thermodynamic and topological modelling results through computation of a novel PHSS parameter for various alloy compositions spanning different alloy systems have rendered qualitative guidelines on propensity for glass formation in multicomponent alloy systems. Besides, the importance of kinetic interpretation of PHSS range observed for glass forming alloys is also elaborated.

Identification of Bulk Metallic Forming Compositions through Thermodynamic and Topological Models

This paper attempts to optimize the bulk metallic glass forming compositions using enthalpy of chemical mixing (DHchem) as thermodynamic, mismatch entropy (DSs/kB) as topological and configurational entropy (DSconfig/R) as statistical parameters. The product of DHchem and DSs/kB which is termed as PHS in the DSconfig/R range of 0.9 to 1.0 can be correlated strongly to glass forming ability. PHS being an important parameter has been used to design the quaternary and quinary Bulk Metallic Glass compositions from ternary compositions. This has been demonstrated for two Zr rich quaternary systems in Zr-Ti-Cu-Ni and Zr-Cu-Ni-Al based bulk metallic glasses. By weighing approach of PHS of four Zr rich quaternary systems and one non-Zr rich systems a new Zr rich quinary bulk metallic glass in Zr-Ti-Cu-Ni-Al system is designed. Mechanical alloying is used to prepare the bulk amorphous powder at the compositions predicted by the model in order to validate the model.

Compressive, tensile and wear behavior of ex situ Al/AlN metal matrix nanocomposites

The mechanical strength and wear behavior of Al–AlN metal matrix nanocomposites synthesized using melt metallurgy technique is investigated. Uniformly dispersed nanostructured AlN particulates in nanocomposites are found to improve the mechanical properties. Detailed study on compressive and tensile test is attempted in the present work to understand the mechanism of deformation in nanocomposites. Adhesive dry wear test results indicate that applied load has strong influence on wear behavior of nanocomposites compared to the sliding distance. Porosity and weak agglomeration of nanoparticles are found to significantly affect the wear behavior and mechanical strength of synthesized nanocomposites. It is revealed that the transfer of load from matrix to reinforcement particulate is not an instantaneous phenomenon but initiates after some incubation period depending on the fraction of porosity in nanocomposites during compressive and tensile testing.

Nucleation Criteria for the Formation of Aluminum Nitride in Aluminum Matrix by Nitridation

Synthesis of in situ metal matrix composites using liquid metallurgy technique involves processing the melt to generate ceramic particles and entrap them into the matrix during solidification. The process parameters such as temperature, holding time and buoyancy effect generated in the molten bath are crucial factors governing the size and distribution of ceramic particles into the matrix. Temperature being most important parameter and its role is easy to comprehend for in situ formation of ceramic particles using established nucleation criteria. In the present research paper, thermodynamic study is carried to understand the role of temperature on formation of aluminum nitride by nitridation of aluminum melt using ammonium chloride and calcium oxide. Mechanism of aluminum nitride formation by heterogeneous nucleation and its growth has also been discussed based on the nucleation theory. An attempt is also made to correlate nucleation theory for the formation of AlN in Al matrix at different temperatures. The mechanical behavior under compressive loading is carried out on synthesized composites to understand its capability to withstand static loading. Composite strength is analyzed and attempt is made to correlate nucleation dynamics of AlN particles.

Optimization of process parameter for synthesis of silicon quantum dots using low pressure chemical vapour deposition

Si quantum dots-based structures are studied recently for performance enhancement in electronic devices. This paper presents an attempt to get high density quantum dots (QDs) by low pressure chemical vapour deposition (LPCVD) on SiO2 substrate. Surface treatment, annealing and rapid thermal processing (RTP) are performed to study their effect on size and density of QDs. The samples are also studied using Fourier transformation infrared spectroscopy (FTIR), atomic force microscopy (AFM), scanning electron microscopy (SEM) and photoluminescence study (PL). The influence of Si–OH bonds formed due to surface treatment on the density of QDs is discussed. Present study also discusses the influence of surface treatment and annealing on QD formation.

Prediction of Bulk Metallic Glass Formation in Cu–Zr–Ag–Hf System by Thermodynamic and Topological Modeling

Cu based bulk metallic glasses (BMGs) are widely studied because of their high glass forming ability (GFA) and interesting combination of properties such as high strength coupled with good ductility and low cost. With these attributes, Cu based BMGs are being projected as promising materials for practical applications. The process of glass formation in metallic systems is a challenging task and alloys should be cooled from the liquid state at rates faster than a critical cooling rate (Rc) to resist crystallization. Interestingly, composition plays an important role in achieving easy glass formation, which is usually measured in terms of Rc. In the present work, attempt has been made to identify the composition for easy glass formation in Cu based quaternary system by theoretical approach. A GFA parameter PHS, which is a product of enthalpy of chemical mixing (∆Hchem) and mismatch entropy normalized with Boltzmann’s constant (∆Sσ/kB) is used to identify the best glass forming composition in Cu–Zr–Ag–Hf system. Further, a new parameter PHSS, which is a product of PHS and configurational entropy (ΔSconfig/R) is found to illustrate strong correlation with GFA. An attempt has also been made to correlate PHSS parameter with critical diameters and Rc using reported data in Cu–Zr–Ag–Hf system.

Thermodynamic model and synthesis of Bulk Metallic Glass in Cu-Zr-Ti system by Mechanical Alloying

Based on the thermodynamic and topological approach, Cu60Zr30Ti10 has been identified as the best bulk metallic glass forming composition in Cu-Zr-Ti system. Bulk metallic glass has been successfully produced using mechanical alloying of elemental blends and consolidation of the resulting glassy powders into pellets of 8 mm diameter. Dry sliding wear of glassy pellets at different annealed states showed that the relaxed metallic glass has excellent wear resistance.

Thermodynamic Basis for Glass Formation in Cu-Zr Rich Ternary Systems and Their Synthesis by Mechanical Alloying

Topological factors such as mismatch entropy and configurational entropy, along with thermodynamic entity such as enthalpy of chemical mixing, are found to control glass formation in metallic systems. Taking both these factors into consideration, a parameter called PHS was proposed to correlate glass forming ability successfully in the Cu-Zr-Ti system. The parameter PHS (=∆Hchem × ∆Sσ/kB) is a product of enthalpy of chemical mixing and mismatch entropy. Our study indicates that the more negative is the PHS value within the configurational entropy (∆Sconfig/R) range of 0.9 to 1.0, the higher is the stability of glassy phase resulting in a larger diameter of bulk metallic glass rods. Observed theoretical predictions are supported by experimental results in which the compositions with high negative PHS resulted in easy amorphous phase formation in comparison with less negative PHS compositions by mechanical alloying. This criterion was extended to Cu-Zr-Al and Cu-Zr-Ag systems as well, thus establishing a strong correlation between PHS and the glass forming ability of alloys. The role of size effect, probability of atomic arrangements, and heat of formation among constituent elements in obtaining a larger dimension bulk metallic glasses was addressed in this study.

Icosahedral Cluster Energetics in Zr60Cu10Al15Ni15 Bulk Metallic Glass and Their Role on Solidification Behavior

The energetics behind transformation of liquid structure into subsequent intermediate phases during solidification is expected to play a decisive role in glass/crystal formation. A great deal of experimental and simulation work on supercooled liquids has indicated that, there exists a close link between the liquid structure and icosahedral clusters, especially for bulk metallic glass forming liquids. Pertinently, icosahedral clusters are also found to be energetically favorable to form upon devitrification of Zr60Cu10Al15Ni15 glassy alloy. Such evolution of icosahedral clusters upon devitrification in this alloy invariably proves their manifestation at the intermediate stage during transition of supercooled liquid into glass. Hence understanding the energetics behind restructuring of these clusters into glass or crystal during solidification, aids in microstructure optimization of glass/crystal composites for structural and functional applications. In this paper, it has been attempted to investigate the energetics behind the evolution of Zr–Ni and Zr–Al binary intermetallic phases during crystallization of Zr60Cu10Al15Ni15 glassy alloy. Ascalaph Designer Molecular Modeling Suite is used to generate different models of clusters to understand the formation of Zr–Ni and Zr–Al phases. We propose solidification mechanism in this alloy via two steps, namely, formation of intermediate Zr–Cu icosahedral clusters which is structurally restricted process and precipitation of crystalline phases as thermodynamically favorable process.