Key Engineering Materials Vols. 510-511

Abstract: High-T_c superconductivity has been an emerging field for researchers since its discovery. Bismith based superconductors commonly called BSCCO have great importance among the superconducting family. It is divided into three phases among them 2223 phase is highly studied in order to investigate its superconducting properties by substitution of different elements. We have studied the substitution of cerium (Ce) on calcium site of bismuth based Bi (Pb)Sr (Ba)-2223 high-T_c superconductor. The nominal compositions of Bi_1.6Pb_0.4Sr_1.6Ba_0.4(Ca_1-xCe_x)₂Cu₃O_x ceramic superconductor were prepared by wet chemical method. Stoichiometric amounts of Bi₂O₃, PbO, Sr (NO₃)₂, BaCO₃, CaCO₃, CuO and CeO₂ were used as starting materials. Structural analysis was done by X-ray diffraction (XRD) at room temperature and different parameters were calculated. DC resistivity measurements for the transition temperature of synthesized superconducting samples were taken by the standard four-probe method, apparatus for which was developed in our laboratory. It is observed that with the substitution of cerium on calcium site the single high-T_c 2223 phase is obtained.

Abstract: A low alloy steel containing 0.2%C was heat treated with three cycles of heat treatments with the aim to acquire different morphologies of martensite in dual phase microstructure. Microscopic examination revealed that the morphologies consisting of grain boundary growth, scattered laths and bulk form of martensite were obtained. These morphologies have their distinct patterns of distribution in the matrix (ferrite). In tensile properties observations the dual phase steel with bulk morphology of martensite showed minimum of ductility but high tensile strength as compared to other two morphologies. This may be due to poor alignments of bulk martensite particles along tensile axes during deformation. Tempering was employed with various holding times at 550°C to induce ductility in the heat treated material. The tempering progressively increased the ductility by increasing holding time. However, tempering response to strengths and ductilities was different to all three morphologies of martensite.

Abstract: To overcome the naturally existing Schottky barrier problem between p-CdTe and any metal, an intermediate semiconductor thin buffer layer is a better choice prior to the final metallization for contact. Among many investigated back contact materials the ZnTe is suitable as a buffer layer. ZnTe thin films were deposited onto glass substrates by the thermal evaporation technique under vacuum ~2×10-5mbar. Undoped ZnTe thin films are highly resistive, extrinsic doping of Cu was made to improve the electrical conductivity. Films were doped by immersing in Cu N_O32.5H2O solutions for Cu doping. To optimize the growth parameters the prepared films were characterized using various techniques. The structural analysis of these films was performed by X-ray diffraction (XRD) technique and optical transmission. X-ray diffraction identified the phases present in these films and also observed that the prepared films were polycrystalline. Also the spectral dependence of absorption coefficient was determined from spectrophotometer. Energy band gap index were calculated from obtained optical measurements data.

Abstract: This paper presents an investigation on microstructure and mechanical properties of welded AA 6061-T6 plate using filler metal ER 4043 in the Gas Metal Arc Welding (GMAW) process. Double pass welding technique on both sides of 5 mm thick plate or more is required to provide sufficient weld pool in the joint. The weld metal of the first welding pass exhibits finer microstructure than the second welding pass. The size of Mg₂Si precipitations in the heat-affected zone (HAZ) region is larger than in the base metal due to the welding process that reheats the alloy from the T6 condition above the eutectic temperature. Rapid cooling of the first pass and moderate cooling rate for the second pass during weld metal solidification eventually resulted in significantly change the shape and size in the microstructure that had affected the hardness and mechanical properties. Comparisons made to the base metal on the hardness test results found that the hardness of first pass weld metal dropped by 15%, and by 37.5% for the second weld metal, while the hardness at the boundaries of the first and second weld metals dropped by 32.5%. The ultimate tensile strength and strain of the weld joint with ER 4043 also decreased by 48% and 94% respectively. Based on the findings of the study, it is concluded that even though the double sided welding technique is able to overcome shallow weld penetration to avoid stress concentration that leads to the fatigue failure, the metallurgical changes eventually contributes to degradation of mechanical properties.

Abstract: Martensitic transformations are first order solid state phase transitions and occur in the materials on cooling from high temperature. Shape memory effect is an unusual property exhibited by certain alloy systems, and based on martensitic transformation. The shape memory property is characterized by the recoverability of previously defined shape or dimension when they are subjected to variation of temperature. The shape memory effect is facilitated by martensitic transformation, and shape memory properties are intimately related to the microstructures of the materials. Martensitic transformations occur as martensite variant with the cooperative movement of atoms on {110}_β - type plane of austenite matrix. Martensitic transformations have diffusionless character, and the atomic movement is confined to interatomic lengths in the materials. The basic factors which govern the martensitic transformation are Bain distortion and homogeneous shears. Copper based alloys exhibit this property in metastable β-phase field.

Abstract: The effect of ageing on mechanical, shape memory and pseudoelastic properties of our recently developed Ti-5Cr-4Ag alloy was investigated for biomedical and engineering applications. The results are summarized as follows. (A) Solution-treated alloy exhibited ductile behavior (13% fracture strain), shape memory properties (57% shape recovery) and pseudoelastic response. (B) Ageing at 573K and 973K for 3.6ks has resulted into improvements of UTS and yield stress. However the fracture strain, shape recovery ratio and pseudoelastic response were decreased due to precipitation of α (hcp) phase and stabilization of remaining β (bcc) phase. (C) Ag addition suppressed ω (hexagonal) phase precipitation as ω phase could not be detected for solution-treated and aged conditions. It is concluded that Ti-5Cr-4Ag in solution-treated and 973K aged conditions has potential for biomedical and engineering applications due to good strength-ductility correspondence, shape memory and pseudoelastic properties and suppression of ω phase precipitation.

Abstract: A step-by-step, hierarchical approach is explored in the present work to purify and functionalize carbon nanotubes synthesized by chemical vapor deposition. Attempts are made to purify and functionalize CNTs without extinguishing their aspect ratios. The carbon impurities are removed by thermal oxidation, whilst the unprotected metallic catalyst particles are eliminated by wet oxidation, subsequently; CNT bundles are de-roped by surfactant assisted sonication. Finally, protected metallic catalyst particles are removed and functional groups (hydroxyl and carboxyl) are attached by acid treatment and wet oxidation, respectively. The derivate CNTs are characterized using zeta potential measurements, TGA, XRD, FTIR and SEM. The characterization showed that in optimum experimental conditions the catalytic particles are removed upto 80%, the carbon impurities are eliminated upto 95% and chemical functionalities of hydroxyl and carboxyl is occurred with noticeable de-roping of the CNT bundles.

Abstract: This paper presents the development of a laboratory scale setup for the production of CNTs by arc discharge process in an argon gas environment. The main problem with the existing lab scale production setups is the cleaning of the chamber and this problem is resolved in presented work by placing a circular metal sheet inside the chamber, which is removed after the synthesis, thus allowing easy cleaning and recovery of the produced CNTs. The most noteworthy feature of the presented setup is its low cost, hence offering for the first time the synthesis of economical CNTs on small scale. In addition to offering better control over operating conditions and hence the quality of CNTs, another unique feature of the developed setup is its capability to synthesize SWCNTs and MWCNTs with versatile range of diameters. Significant quantity of CNTs using the developed setup were produced and characterized by using Scanning Electron microscope (SEM) and few results are reported.

Abstract: We have grown a plenty of samples to investigate the effect of substrate temperature on the structural and electrical properties of grown films. X-ray diffraction analysis confirmed the hexagonal structure of ZnO with preferred c-orientation. As we increased the substrate temperature from 74 °C to 500 °C, the microstructure of films improved. The position of (002) peak remained constant but its intensity increased remarkably with increasing substrate temperature. The FWHM of films decreased and crystalline size increased with substrate temperature. We also observed strong dependence of electrical properties on the substrate temperature. The thickness, resistivity and growth rate are directly proportional to the substrate temperature. The possible reason of higher resistivity of thick films is that with improved microstructure, ZnO films have low density of interstitials defects, which acted as free carriers and therefore results in the increased of resistivity.

Abstract: A quaternary Ni₈₆Cr₇Si₄Fe₃ amorphous alloy was synthesized by melt spinning technique. Surface modification was done by electron beam melting (EBM), neutron irradiation and γ-rays. Microstructure of as cast, annealed and modified samples was examined by scanning electron microscope. Crystallization behavior was studied by annealing the samples in vacuum at different temperatures in the range 773-1073 K. Techniques of X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used for characterization. Differential scanning calorimetry (DSC) was conducted at various heating rates in the range 10-40 K/min. Thermal parameters like glass transition temperature T_g, crystallization temperature T_x, supercooled liquid region ΔT_x and reduced glass transition temperature T_rg were measured. The Ni₈₆Cr₇Si₄Fe₃ alloy exhibits wide supercooled liquid region of 60 K indicating good thermal stability. The activation energy was calculated to be 160±4 kJ/mol using Kissinger and Ozawa equations respectively which indicates high resistance against crystallization. The XRD results of the samples annealed at 773 K, 923 K, 973 K and 1073 K/20 min show nucleation of Ni₂Cr₃ and NiCrFe crystalline phases. Vickers microhardness of the as cast ribbon was measured to be 680.. About 30-50 % increase in hardness was achieved by applying EBM technique.