Himansu Tripathi - Academia.edu (original) (raw)

Papers by Himansu Tripathi

Ceramics International, Jun 1, 2019

Bulk α-(Al,Cr) 2 O 3 (where, x = 0, 0.1, 0.3, 0.5) sintered compacts were obtained at 1600-1700°C... more Bulk α-(Al,Cr) 2 O 3 (where, x = 0, 0.1, 0.3, 0.5) sintered compacts were obtained at 1600-1700°C, whose thermo-mechanical behavior were observed upon reheating at 25-1200°C. Unlike bulk α-Al 2 O 3 sintered samples, all the bulk α-(Al,Cr) 2 O 3 sintered compacts showed a noticeable unusual flexural strength reduction at 600°C. However, thermal shock behavior of all the α-(Al 1-x Cr x) 2 O 3 samples (sintered at 1650°C) exhibited a similar trend with retention of 33-38% of initial strength after the 12th cycle. Selected sintered α-(Al 0.5 Cr 0.5) 2 O 3 samples were reheated and quenched to observe any changes (phase, structural) using XRD, XPS, and FT-IR. The phase and binding energy did not reveal any change. The characteristics bands of MO 6 units exhibited a lowering of wavelength by ∼2.5 cm −1 (original peaks at ∼592 and ∼648 cm −1) when reheated for 1 h and quenched from 600°C. But, on prolonging the reheating time at 600°C for 2 and 6 h, the band position shifted back to its original place. The force constant and bond energy are plausibly responsible for the fluctuational of molecular vibration to reach a time-dependent thermal equilibrium, particularly at ∼600°C in case of α-(Al,Cr) 2 O 3 .

Transactions of The Indian Ceramic Society, Oct 1, 2016

Indian magnesites from Salem and Almora regions were selected for the present study. Chemical ana... more Indian magnesites from Salem and Almora regions were selected for the present study. Chemical analysis of these magnesites confirmed that Salem region magnesite (SM) contains CaO and SiO 2 , whereas Almora origin magnesite (AM) shows higher amount of Fe 2 O 3 and CaO as major impurities. Crystalline phases developed in sintered Salem magnesite are periclase, forsterite and monticellite, while magnesioferrite and monticellite are found in sintered Almora magnesite apart from periclase as major phase. Microstructural analyses reveal that sintered SM has lower grain size than sintered AM. Although, no significant difference was noticed at room temperature flexural strengths of both the sintered samples, but sintered AM samples exhibit better flexural strength at elevated temperature (1200 o C).

$Research paper thumbnail of Refractories as Advanced Structural Materials for High Temperature Processing Industries$

Future Landscape of Structural Materials in India, 2022

Transactions of Nonferrous Metals Society of China, Sep 1, 2016

Mullite-zirconia composites containing 20% zirconia (mass fraction) were prepared by reaction sin... more Mullite-zirconia composites containing 20% zirconia (mass fraction) were prepared by reaction sintering route utilizing Indian coastal zircon flour and sillimanite beach sand. 4%−12% of CaO (mole fraction) with respect to zirconia was used as additive. The effect of additive on densification, microstructure as well as various mechanical and thermo-mechanical properties was studied. Incorporation of CaO reduced the densification temperature of the composites to 1550 °C compared to 1600 °C (for CaO free samples). CaO formed small amount of liquid phase (calcium aluminosilicate), which facilitated sintering. Average grain size of the composites decreased up to 4% CaO addition, afterwards grain size increased with further addition of CaO. Samples with 4% CaO exhibited ~225 MPa of flexural strength, ~6 MPa• m 1/2 of fracture toughness and significant improvement in thermal shock resistance. CaO stabilized the tetragonal zirconia phase and thus improved the mechanical properties.

Ceramics International, Mar 1, 2016

Abstract Magnesia rich magnesium aluminate spinel (MgO: Al 2 O 3 =2:1) was developed by reaction ... more Abstract Magnesia rich magnesium aluminate spinel (MgO: Al 2 O 3 =2:1) was developed by reaction sintering of Indian natural magnesite of Salem region as well as from synthetic caustic magnesia with calcined alumina. Dilatometric study of the green compacts was carried out to evaluate the spinelisation and sintering behaviour of both the samples. Green samples were heat treated between 1400 to 1600 °C and characterised in terms of densification behaviour, high temperature flexural strength, microstructure and phase development. Spinel and periclase are the major phases in both the samples, where as forsterite is found only in the sample developed from Indian magnesite due to presence of silica as impurities.

International Journal of Mineral Processing, Nov 1, 2015

Indian magnesite mineral has substantial amount of impurities like CaO, SiO 2 & Fe 2 O 3. During ... more Indian magnesite mineral has substantial amount of impurities like CaO, SiO 2 & Fe 2 O 3. During sintering at elevated temperature these impurities react to form low melting phases like monticellite (CMS), which can degrade the high temperature properties of magnesite. In the present study, ZrO 2 was added to reduce the formation of low melting phase in order to improve the hot strength. Amount of additive was varied between 2 to 6 wt % with respect to raw magnesite. It was observed that addition of ZrO 2 reduces the formation of low melting CMS at higher temperature and improves the flexural strength at 1200°C. Periclase grain shape also changed from rounded to subrounded in presence of zirconia.

International Journal of Applied Ceramic Technology, Jul 10, 2017

Small amount of Y 2 O 3 (0 to 3 wt%) was added into off−grade natural magnesite and its effects o... more Small amount of Y 2 O 3 (0 to 3 wt%) was added into off−grade natural magnesite and its effects on the phase assemblage, microstructural evolution in correlation with mechanical and thermo-mechanical properties were investigated. Presence of Y 2 O 3 facilitated the formation of calcium yttrium silicate and yttrialite phases, which prohibited the formation of detrimental phase monticellite, which was gradually reduced with the increase in Y 2 O 3 content. Y 2 O 3 promoted periclase grain growth and segregated the secondary phases at triple point junction of periclase grains. Also, high temperature (at 1200°C) flexural strength of the samples increased from 77.2 MPa (without Y 2 O 3) to 137.45 MPa with the addition of 2 wt% Y 2 O 3. Greater degree of direct bonding among periclase grains, compact microstructure and uniform grain size distribution in addition to reduced amount of monticellite were the responsible for the improvement in mechanical and thermo−mechanical properties.

Ceramics International, 2015

ABSTRACT Magnesia-rich sintered spinel (66% Al2O3 & 34% MgO) was prepared in the temperat... more ABSTRACT Magnesia-rich sintered spinel (66% Al2O3 & 34% MgO) was prepared in the temperature range of 1600–1700 °C by reaction sintering of caustic magnesia and calcined alumina with and without TiO2 additive in N2 atmosphere. Sintered products were characterised in terms of densification, phase assemblage, and microstructure. The role of TiO2 on spinelisation and densification was analysed using both dynamic and static heating regime. Although a static firing at 1700 °C with 2 h dwelling results in similar densification of spinel in presence and absence of titania, the specific role of titania was discerned only by critically analysing the dimensional change in dynamic heating regime. It was revealed that titania significantly increases only the densification rate of magnesia-alumina system with marginal effect on spinelisation. Better densification of titania-containing spinel, even at a relatively lower temperature, can be attributed to the incorporation of titania into the spinel structure and formation of magnesium aluminium titanate phase.

Ceramics International, May 1, 2016

The kinetic pathway for magnesia-excess magnesium aluminate spinel (MgAl 2 O 4) formation is dete... more The kinetic pathway for magnesia-excess magnesium aluminate spinel (MgAl 2 O 4) formation is determined from the dilation behaviour of MgO-Al 2 O 3 (34:66 w/w) oxide compacts during reaction sintering under dynamic heating regime at different constant heating rates (β). It is calculated from expansion values at different β, analysed through model-free and model-based kinetic methods to arrive at the most suitable and unambiguous kinetic pathway, where spinelisation is best described (r 2 ¼0.986) with Avrami model with exponent n ¼ 0.65 and activation energy (E a) 409 kJ mol À 1. To validate the kinetic model, an isothermal heat treatment of the oxide compact was done and % spinelisation was determined by Rietveld refinement of the XRD data. The results matched the predicted values very closely and validate our kinetic model. This method can be extended to several industrially important thermally activated solid state reactions which require thermal treatment in order to minimise the energy requirement by optimising the heating protocol.

$Research paper thumbnail of Development of Mullite Based Refractory Pot for High Lead Containing Glass Melting$

Transactions of the Indian Ceramic Society

Radiation shielding window (RSW) glass for nuclear reactor is primarily required for protecting t... more Radiation shielding window (RSW) glass for nuclear reactor is primarily required for protecting the operating personnel from harmful radioactive rays produced during nuclear energy generation in nuclear hot cells. In this respect, the production of high lead (>70% PbO) containing RSW glass is generally made using platinum pot through bottom pouring flow casting technique. However, owing to low capacity (40 L max.) of high cost platinum pot, the production of glass slab is limited to a maximum dimension of 400×400×100 mm3. Therefore, an alternate cost effective technology is highly required for making higher dimension glass slabs. To fulfill the requirement, low cost and higher volume refractory pot based tilt casting technology can be adopted. In this regard, the refractory pot is to be developed with adequate thermal, chemical and mechanical stabilities towards sustaining high corrosive lead oxide containing molten glass at 1100°-1200°C. For this purpose, a clay based alumino-silicate pot material enriched with mullite has been developed and characterized systematically. The rectangular bars have been fabricated by slip casting technique and the fabricated bars have been fired between 1450° and 1550° C. The fired materials have been characterized in terms of bulk density, apparent porosity, cold and hot modulus of rupture, etc. The RSW glass with more than 70% lead oxide content has successfully been melted in the pot. The quality of RSW glass produced using the pot has also been studied. In an optimized composition of the pot, the formation of higher content of mullite phase having high thermal shock and corrosion resistance with the desired mechanical stability is the key factor behind the stability of the refractory pot for making defect free RSW glass slabs. This cost effective refractory pot technology can substitute the platinum pot technology for producing larger dimension RSW glass slabs. GRAPHICAL ABSTRACT

$Research paper thumbnail of Research on refractory raw materials at CGCRI - a review$

Ceramics International, 2021

Abstract Magnesium aluminate spinel with an initial MgO: Al2O3 molar ratio of 2:1 was prepared fr... more Abstract Magnesium aluminate spinel with an initial MgO: Al2O3 molar ratio of 2:1 was prepared from its constituent oxides through a solid-state sintering process at temperatures ranging from 1550 to 1700 °C in a normal air atmosphere. The effect of varying amount (0.25–1.0 wt%) of TiO2 and Yb2O3 on densification, phase assemblage, mechanical, thermo-mechanical properties and microstructure of magnesia-rich spinel were investigated under static heating condition. The addition of TiO2 and Yb2O3 favours the densification of magnesia-rich spinel, which is discernible up to 1650 °C. This beneficial effect may be attributed to the development of the secondary phase and formation of solid solution due to the dissolution of the additive ions in the spinel structure. A marginal increase in the average grain size of the samples along with a narrower grain size distribution occurred with the incorporation of both the additives. Both the additives improved the mechanical properties of the magnesia-rich spinel; however, better room temperature flexural strength was achieved with Yb2O3 as compared to TiO2 addition. For the samples sintered at 1550 °C, 1.0 wt% Yb2O3 addition resulted in 30% increase in flexural strength; however, same amount of TiO2 addition increased the strength by 20%. In case of thermal shock resistance, 1.0 wt% TiO2 and 0.25 wt% Yb2O3 addition demonstrated promising result among all the samples.

Ceramics International, 2020

Obtaining a stable Cr 3+ phase at higher temperatures in the presence of hydraulic calcium alumin... more Obtaining a stable Cr 3+ phase at higher temperatures in the presence of hydraulic calcium aluminates (the phases found in calcium aluminate cement, CAC) in the Al 2 O 3-CaO-Cr 2 O 3 system remains a significant challenge in many application areas, as the formation of toxic, carcinogenic, and water-soluble Cr 6+ compounds often occur. To address the issue, recently, we synthesized a high-temperature stable Cr 3+ ternary compound (CaAl 2 Cr 2 O 7) with space group P3 (143). In the present work, we investigated the formation-stability of CaAl 2 Cr 2 O 7 with insitu calcium aluminate phases at 1500 • C under the CO 2 atmosphere in the Al 2 O 3-CaO-Cr 2 O 3 system through solid-oxide reactions route varying Cr 2 O 3 content (at constant Al 2 O 3 :CaO ratio). It co-existed with hydraulic calcium aluminates and other phases over the full investigated composition range of 2.76-68.3 mol% Cr 2 O 3 (5-80 wt%). Apart from CAC phases (CaAl 2 O 4 , CaAl 4 O 7), major Cr 3+-phases are CaAl 2 Cr 2 O 7 and (Al,Cr) 2 O 3 while (α,β)-CaCr 2 O 4 and Ca(Al,Cr) 12 O 19 formed as minor phases. At a constant Al 2 O 3 :CaO ratio of 5.6:4.4 mol% (7:3 wt%), the formation of the CaAl 2 Cr 2 O 7 phase increases with Cr 2 O 3 content (up to the investigated composition of 26.43 mol%) and then decreases gradually. The solid solubility of Al and Cr in the CaAl 2 Cr 2 O 7 phase limited over a narrow range, and presumably dependent more on heat treatment condition rather than composition as reflected from the lattice parameter calculations. Though XRD revealed the presence of only Cr 3+phases in the partial CO 2 atmosphere, however, traces of Cr 6+ could be detected using XPS and leaching tests. However, leachable Cr 6+ content (0.095-1.252 mg/L) were much below the United States Environmental Protection Agency (US-EPA) permissible limit of 5 mg/L. The formation mechanism of the CaAl 2 Cr 2 O 7 and other phases with plausible reactions were also discussed.

Transactions of the Indian Ceramic Society, 2004

The kinetics of decomposition of limestone was studied using both non-isothermal and isothermal m... more The kinetics of decomposition of limestone was studied using both non-isothermal and isothermal methods. The mechanism and kinetics of thermal decomposition were studied using Arrhenius equation applied to solid state reactions. It was found that in both non-isothermal and isothermal methods limestone seemed to be decomposed via a zero order reaction mechanism. The energy of activation for the non-isothermal and isothermal decompositions of limestone was 157.79 and 181.43 kJ.mol−1respectively. Similarly, In A values for non-isothermal and isothermal decomposition were found to be 14.2 and 13.2(s−1) respectively.

Ceramics International, 2016

The rationalization of selection of sintering additives for α-alumina was investigated using two ... more The rationalization of selection of sintering additives for α-alumina was investigated using two oxides (MgO and TiO 2) to discern their individual roles. Using both dynamic heating study in a thermomechanical analyzer and static heat treatment, the precise role of each oxide was established. Grain growth trajectory of different doped samples sintered at 1700°C revealed that MgO neither significantly affected densification nor facilitated grain growth upto 1700°C. MgO reacted with alumina to form spinel prior to the densification process. Thus it could not generate further extrinsic defects in corundum lattice during sintering, which usually facilitate densification. In contrast, TiO 2 significantly enhanced the densification and promoted grain growth in α-alumina. At 1700°C, the average grain size of titania doped samples were 7.7x larger than undoped ones and 10x larger than magnesia dopes samples. The sintered grains developed higher aspect ratio when TiO 2 was used which may be ascribed to preferred growth of the 012 and 024 planes of corundum. The nearly perfect junction of grain boundaries meeting at $ 120°indicates absence of liquid phase and that the entire sintering process most probably took place in solid state for both MgO and TiO 2 doped samples.

International Journal of Mineral Processing, 2015

$Research paper thumbnail of Hot corrosion behavior of Al2O3–Cr2O3 refractory by molten glass at 1200 °C under static condition$

Corrosion Science, 2016

Please cite this article in press as: M. Nath, et al., Hot corrosion behavior of Al 2 O 3-Cr 2 O ... more Please cite this article in press as: M. Nath, et al., Hot corrosion behavior of Al 2 O 3-Cr 2 O 3 refractory by molten glass at 1200 • C under static condition, Corros. Sci.