• Volume 24, Issue 1

February 2001,   pages  1-94

• Kinetics of pressure induced structural phase transitions—A review

The current status of experimental as well as theoretical advances in the understanding of kinetics of structural phase transitions is reviewed. A brief outline of the classification of phase transitions and classical ideas in the theory of kinetics of phase change is presented first. High pressure experimental techniques developed for studying the kinetics of structural transitions are reviewed and the salient features of each technique is brought out. The experimental technique using the diamond anvil cell (DAC) and image processing gets special mention as it promises to impart a new direction to this field. The usefulness of kinetic parameters in understanding the mechanism of a phase transition is examined. Typical examples from the literature are provided to give a flavour for these kind of studies. In conclusion, several open questions are raised which could pave way for future work in this area.

• On the origin of recrystallization textures

The development of recrystallization textures has been debated for the past 50 years. Essentially the rival theories of evolution of recrystallization textures i.e. oriented nucleation (ON) and oriented growth (OG) has been under dispute. In the ON model, it has been argued that a higher frequency of the special orientation (grains) than random occur, thus accounting for the texture. In the OG model, it has been argued that the specially oriented grains have a high mobility boundary and thus can migrate faster and grow to a larger size as compared to random orientations thus contributing to the final recrystallization texture.

In FCC metals and alloys like aluminium, cube orientation [(001) $\langle$100$\rangle$] is the recrystallization texture component. In the classic OG model, cube orientation is supposed to be misoriented from 𝑆-orientation [(123) $\langle$63$\bar{4}\rangle$] which is a deformation texture component by a 40° about $\langle$111$\rangle$ relationship which is supposed to be a high mobility boundary leading to faster growth of cube grains. Stereographic calculations and analytical calculations are presented in this paper to the effect that the 𝑆-orientation (123) $\langle$63$\bar{4}\rangle$ is not misoriented from cube (100) $\langle$001$\rangle$ by 40° (111) whereas another deformation texture component (123) $\langle$41$\bar{2}\rangle$ which is termed the 𝑅-component is misoriented from cube component by 40°$\ \langle$111$\rangle$ . 𝑅-component is also seen in deformation textures of aluminium and hence the classic OG model remains valid with respect to the 𝑅-component.

• Effect of high-energy heavy ion irradiation on the crystallization kinetics of Co-based metallic glasses

Differential scanning calorimeter (DSC) is employed to study the crystallization kinetics of irradiated (at three different fluences with high-energy heavy ion; Ni11+ of 150 MeV) specimens of two Co-based metallic glasses. It is found that the crystallization process in both the glasses is completed in two phases. The DSC data have been analysed in terms of kinetic parameters viz. activation energy (𝐸𝑐), Avrami exponent (𝑛), dimensionality of growth (𝑚), using two different theoretical models. The results obtained have been compared with that of virgin samples. The lower activation energy in case of second crystallization occurring at higher temperature indicates the easier nucleation of second phase. The abnormally high value of Avrami exponent in Co–Ni glass indicates very high nucleation rate during first crystallization.

• Effect of 80 keV Ar+ implantation on the properties of pulse laser deposited magnetite (Fe3O4) thin films

Highly oriented thin films of Fe3O4 were deposited on (100) LaAlO3 substrates by pulsed laser ablation. The structural quality of the films was confirmed by X-ray diffraction (XRD). The films showed a Verwey transition near 120 K. The films were subjected to 80 keV Ar+ implantation at different ion doses up to a maximum of 6 × 1014 ions/cm2. Ion beam induced modifications in the films were investigated using XRD and resistance vs temperature measurements. Implantation decreases the change in resistance at 120 K and this effect saturates beyond 3 × 1014 ions/cm2. The Verwey transition temperature, 𝑇𝑣, shifts towards lower temperatures with increase in ion dose.

• Synthesis of ferrite grade 𝛾-Fe2O3

Iron(II) carboxylato–hydrazinates: Ferrous fumarato–hydrazinate (FFH), FeC4H2O4.2N2H4; ferrous succinato–hydrazinate (FSH), FeC4H4O4.2N2H4;ferrous maleato–hydrazinate (FEH), FeC4H2O4.2N2H4;ferrous malato–hydrazinate (FLH), FeC4H4O5.2N2H4;ferrous malonato–hydrazinate (FMH), FeC3H2O4.1.5N2H4.H2O; and ferrous tartrato–hydrazinate (FTH), FeC4H4O6.N2H4.H2O are being synthesized for the first time. These decompose (autocatalytically) in an ordinary atmosphere to mainly 𝛾-Fe2O3, while the unhydrazinated iron(II) carboxylates in air yield 𝛼-Fe2O3, but the controlled atmosphere of moisture requires for the oxalates to stabilize the metastable 𝛾-Fe2O3. The hydrazine released during heating reacts with atmospheric oxygen liberating enormous energy, $$N_2H_4 + O_2 \rightarrow N_2 + H_2O;\Delta H_2O = – 621 kJ/mol,$$ which enables to oxidatively decompose the dehydrazinated complex to 𝛾-Fe2O3. The reaction products N2 + H2O provide the necessary atmosphere of moisture needed for the stabilization of the metastable oxide.

The synthesis, characterization and thermal decomposition (DTA/TG) of the iron(II) carboxylato–hydrazinates are discussed to explain the suitability of 𝛾-Fe2O3 in the ferrite synthesis.

• Structural and magnetic properties of CaMg2Fe16O27

A new compound, CaMg2Fe16O27, is synthesized for the first time, in polycrystalline form, using stoichiometric mixture of oxides with standard ceramic technique and characterized by X-ray diffraction. It is found to have a hexagonal W-type structure with lattice parameters 𝑎 = 5.850 Å and 𝑐 = 33.156 Å. Electrical studies show that the compound is a semiconductor with energy of activation, 𝛥 𝐸 = 0.56 eV. Electrical conductivity results show a transition in the conductivity vs temperature plot near the Curie temperature. The activation energy value obtained for the paramagnetic phase is found to be higher than that of the ferrimagnetic phase. The molar magnetic susceptibility was measured in the temperature range 300–850 K and the results show that the compound is ferrimagnetic at room temperature. The compound also shows hysteresis at 300 K. Paramagnetic nature of the sample above Curie temperature is also studied. The Curie molar constant 𝐶𝑀 calculated from the plot of 1/𝜒𝑀 vs 𝑇(𝐾) is found to be nearly in agreement with the expected value.

• Electrochemical behaviour of LiM𝑦Mn2–𝑦O4(M = Cu, Cr; 0 𝑦 0 4)

Spinel lithium manganese oxide, LiMn2O4, is beset with problems of capacity fade upon repeated cycling. The loss in capacity upon cycling is attributable to Jahn–Teller distortion and manganese dissolution in the electrolyte in the charged state. One way to circumvent this capacity fade is to introduce other 3𝑑 transition metal ions in the LiMn2O4 lattice. In this paper, we report on the effect of partial substitution of manganese in the LiMn2O4 phase with copper (II) and chromium (III) ions. It has been shown that the higher octahedral stabilization energy of trivalent chromium imparts greater structural stability to chromium-doped LiMn2O4 spinels. Both copper and chromium reduce the capacity of the spinel in the 4 V region. In terms of its good reversible capacity and ability to sustain cycling with minimal capacity fade, LiCr0.1Mn1.9O4 may be considered as a potential cathode material for lithium rechargeable cells.

• pH controlled dispersion and slip casting of Si3N4 in aqueous media

The dispersion characteristics of commercial Si3N4 powder in aqueous media (deionized water) was studied as a function of pH in the range 2–11. The slip was characterized for its dispersion quality by various experimental techniques like particle size analysis, sedimentation phenomena, viscosity and flow behaviour and zeta potential analysis. The optimum dispersion was found to be in the pH region 9–11 wherein the slurry displayed minimum sedimentation height, minimum viscosity, near Newtonian flow behaviour and maximum zeta potential. The slip is highly agglomerated in the pH range 2–8 as manifested by higher sedimentation height, higher viscosity, lower zeta potential and thixotropic non-Newtonian flow behaviour. The 72 wt% (44 vol.%) Si3N4 slips made at pH = 10 resulted in green bodies having 53–59% of theoretical density after casting into plaster molds.

• Acoustic investigations on PbO–Al2O3–B2O3 glasses doped with certain rare earth ions

Elastic moduli (𝑌, 𝜂), Poisson’s ratio (𝜎), microhardness (𝐻) and some thermodynamical parameters such as Debye temperature (𝜃𝐷), diffusion constant (𝐷𝑖), latent heat of melting (𝛥 𝐻𝑚) etc of PbO–Al2O3–B2O3 glasses doped with rare earth ions viz. Pr3+, Nd3+, Sm3+, Eu3+, Tb3+, Dy3+, Ho3+, Er3+ and Yb3+, are studied as functions of temperatures (in the temperature range 30–200°C) by ultrasonic techniques. All these parameters are found to increase with increasing atomic number 𝑍 of the rare earth ions and found to decrease with increasing temperature of measurement. From these results (together with IR spectra of these glasses), an attempt is made to throw some light on the mechanical strength of these glasses.

• Glass and glass–ceramic coatings, versatile materials for industrial and engineering applications

Among various coating systems for industrial and engineering applications, glass and glass–ceramic coatings have advantages of chemical inertness, high temperature stability and superior mechanical properties such as abrasion, impact etc as compared to other coating materials applied by thermal spraying in its different forms viz. PVD, CVD, plasma, etc. Besides imparting required functional properties such as heat, abrasion and corrosion resistance to suit particular end use requirements, the glass and glass–ceramic coatings in general also provide good adherence, defect free surface and refractoriness.

Systematic studies covering the basic science of glass and glass–ceramic coatings, the functional properties required for a particular end-use along with the various fields of application have been reviewed in this paper.

• Impact fatigue behaviour of carbon fibre-reinforced vinylester resin composites

Two types of unidirectional carbon fibre, one of high strength (DHMS) and another of medium strength (VLMS) reinforced vinylester resin composites have been examined for their impact fatigue behaviour over 104 impact cycles for the first time. The study was conducted using a pendulum type repeated impact apparatus specially designed and constructed for the purpose. A well-defined impact fatigue behaviour (S–N type curve) curve has been demonstrated. It showed a plateau region of 10–102 cycles immediately below the single cycle impact strength, followed by progressive endurance with decreasing impact loads, culminating in an endurance limit at about 71% and 85% of the single impact strength for DHMS-48 and VLMS-48, respectively. Analysis of the fractured surfaces revealed primary debonding, fibre breakage and pull-out at the tensile zone of the samples and a shear mode of fracture with breakage of fibre bundles at the compressive zone of the samples. The occurrence of a few major macrocracks in the matrix with fibre breakage at the high load–low endurance region and development of multiple microcracks in the matrix, coalescing and fibre breakage at the low-load–high endurance region have been inferred to explain the fatigue behaviour of the composites examined.

• Effects of moisture on the mechanical properties of glass fibre reinforced vinylester resin composites

Glass fibre reinforced vinylester resin composites incorporating varying amounts of fibres (63.5, 55.75, 48.48, 38.63 and 27.48 wt%) were characterized for their mechanical properties both as prepared and after treatment with boiling water for 2, 4, 6, 8 and 24 h. Weights of the samples were found to increase to a saturation at about 8 h with boiling water treatment.

In keeping with the composite principle, the mechanical properties improved with fibre loading. However, the properties were relatively inferior when treated with boiling water for longer hours attributing to ingress of moisture by capillary action through the interface between the fibre and the resin matrix. Considering the rates of moisture absorption and correlating with the mechanical properties, it was observed that the deteriorating effects were predominant up to 4 h treatment with boiling water. Estimation of defect concentrations for 63.5 wt% of nascent fibre reinforced composites as well as those composites treated with boiling water for 24 h were 56.93% and 64.16% respectively. Similarly, 27.48 wt% nascent fibre reinforced composites and those composites with boiling water treatment showed the estimation of defect concentrations of 39.94% and 50.55% respectively. SEM study of the fractured surfaces showed heavy fibre pull-out in the tensile zone whilst shear fracture of the fibre bundles was predominant at the compressive zone of the samples tested for flexural strength properties.

• # Bulletin of Materials Science

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