Padma Nilaya Jonnalagadda

Titanium and its alloy Ti6Al4V have gained attention as advanced biomaterials in healthcare due to the presence of native amorphous titanium oxide layer on their surface. This inherent amorphous layer with thickness restricted to 3-7 nm imparts improve biocompatibility and corrosion resistance to the material. However, this limited thickness constrains its functional performance. This work is aimed at improving the functionality of Ti6Al4V by creating an oxide layer of suitable thickness on its surface through controlled heat treatment. Samples were oxidized at three different temperatures, 400 ºC, 600 ºC and 800 ºC for 1h and alteration in morphology, oxygen content, crystallization of oxide layer, surface energy, surface roughness and micro-harness of the samples were investigated. The electrochemical analysis revealed a systematic lowering in corrosion rate with increasing temperature. Notably, the corrosion rate of the sample heated at 800 ºC was 0.52 mil/y that is 1/4th times lower to pristine sample (2.04 mil/y). Tribology analysis showed a significant enhancement in wear resistance. The wear rate of pristine Ti6Al4V (7.56 x 10-3 mm3/Nm) reduced by two-orders for sample heat treated at 800 ºC (9.59 x 10-5 mm3/Nm) when tested against a stainless steel counterpart. Further, samples subjected to higher thermal oxidation exhibited superior bioactivity, as evidenced by increased apatite growth and calcium to phosphorus ratio closer to optimum value reported in litrature. The improved performance of thermally oxidized sample is attributed to an increase in rutile phase of crystalline titanium oxide on Ti6Al4V surface post heat treatment.

Small-signal gain defines the amplification potential of a laser medium before saturation and is crucial for optimizing resonator design and efficiency. We present the findings of small-signal gain measurements of a liquid nitrogen–cooled continuous-wave carbon monoxide (CO) laser operating with the CO ₂ laser gas mixture CO _₂ :He:N _₂ (1:3:1). Here, CO is generated in situ by electron-impact dissociation of CO _₂ , offering a safer alternative to direct CO handling. The oscillator employs a double-jacketed discharge tube. In contrast, a simplified single-jacketed tube serves as the amplifier, providing equivalent optical performance with easier fabrication. Under identical operating conditions, multiline gain measurements yielded a logarithmic gain coefficient of (0.37 ± 0.02) × 10 ⁻² cm ⁻¹ , with gain saturation occurring at an input power of ∼69 mW (saturation intensity: 0.98 W cm ⁻² ).

This study explores the synthesis of laser-induced graphene (LIG) on a polyimide substrate using a continuous-wave CO2 laser, focusing on optimizing laser power and scan speed to enhance the graphene quality. Raman spectroscopy of the laser-irradiated sample under ambient conditions confirmed the successful formation of graphene. Under optimized conditions of laser power (∼1 W) and scan speed (2 cm/s), defect density of ∼0.31 was estimated from the ratio of D-band to G-band intensity (ID/IG). Increasing laser power changed the LIG morphology from porous to fibrous structures. Numerical simulations using COMSOL Multiphysics provided an insight into time evolution of maximum surface temperature during laser interaction, underscoring the need to balance power and scan speed for controlled thermal decomposition. This work demonstrates a streamlined approach to LIG synthesis on polyimide, paving the way for applications in electronics, sensors, and energy storage.

Contact particle lens array (CPLA) technique has diverse applications in surface engineering such as encrypting laser-induced security features on metal surfaces. In this technique, when a monolayer of microparticles is irradiated with an ultrashort laser pulse, each particle acts as a microlens, concentrating the laser energy beneath it. This localized energy focus generates heat, resulting in material ablation and the formation of nanopits at the particle-substrate interface. This ability of CPLA has been used to generate unique microdimples on stainless steel such that each microdimple was embedded with periodic hexagonally arranged nanopits. For this, a self-assembled monolayer of polystyrene microparticle of sphere diameter 2 μm deposited on a stainless steel (SS) substrate was irradiated with a single shot of 30 ps pulsed laser and the laser-induced nanopits were characterized by scanning electron microscope images. With optimum laser power and microparticle size, various two-dimensional patterns were generated on SS such that each laser-induced microdimple contained ∼3000 numbers of nanopits. The dimension of the nanopits was controlled by varying laser powers. The simulation results obtained from coupled electromagnetic and thermal modelling techniques further verified the light intensity enhancement effect under particle-assisted focusing, providing theoretical support for the experimental results. Precise control over pitting dimension and ability of generating complex two-dimensional patterns demonstrates limitations on easy replication of CPLA-based inscription.

The lasing medium of cryo-cooled CW-CO laser operated with CO ₂ laser gain medium (CO ₂ –N ₂ –He mixture) resulting in 5 µ m emission is formed in-situ in the discharge and its composition, that is sensitively dependent on the discharge temperature, is not known a priori . The composition of the CO lasing gas mixture here was successfully arrived at by way of sampling the gas mixture at the exhaust and measuring the wavelength dependent absorbance by a Fourier Transform Infrared Spectrophotometer. This enabled us to determine the CO and CO ₂ concentrations in the lasing mixture accurately while the other major constituents were estimated from the initial partial pressures. The composition of the lasing gas mixture was studied for various discharge wall temperatures starting from no cooling condition (∼351 K) to LN ₂ cooled conditions (79 K). While the concentrations of both, CO ₂ and CO were found to decrease for temperatures below the freezing point of CO ₂ at the operating pressure of 11 mbar, the relative concentration of CO with respect to CO ₂ also increased with reducing temperature accompanied by a steady increase in the CO laser power output. The onset of lasing in 5 µ m region was found to occur at a discharge wall temperature of 212 K.