P. K. Khanna

Dielectric Anomaly in Mg Doped ZnO Thin Film Deposited by Sol–Gel Method

Magnesium doped ZnO Zn0.75Mg0.25O polycrystalline thin film was deposited on p-type Si by sol–gel method, and the same was annealed at 1000°C in oxygen rich environment. The magnesium doping in lattice structure of zinc oxide was examined by X-ray diffraction measurements. The dielectric phase transition observed at 378 K indicated the existence of ferroelectric properties in the deposited film. The ferroelectricity in the deposited film was confirmed by the polarization and electric field hysteresis curve. Maximum values of the spontaneous polarization and coercive field were found to be 0.29 C/cm2 and 18 kV/cm, respectively. The clockwise rotation in capacitance and voltage hysteresis loop of the deposited film on p-type Si indicated that the charge compensation on the Si surface was induced by the ferroelectric polarization. The grain size and microstructure were analyzed by atomic force microscopy.

Low-temperature nematic phase in asymmetrical 1,3,4-oxadiazole bent-core liquid crystals possessing lateral methoxy group

ABSTRACT Asymmetrical bent-core molecules based on 1,3,4-oxadiazole bent-core unit have been synthesised as a new design with a lateral methoxy group at outer phenyl ring of the molecule. These new asymmetrical bent-core molecules resemble hockey-stick shaped due to the presence of two different arms of different lengths. One arm of these molecules is elongated having two phenyl rings and possesses a 4-n-alkyloxy chain of a different number of carbon atoms (n = 4, 8, 12 and 18) and other arm is short and has one phenyl ring with fixed 4-n-octyloxy chain. The bent-core molecules possess a lateral polar methoxy group at the elongated arm of the molecule. These bent-core compounds exhibited fluorescence emission in the UV wavelength region (~377–386 nm) whereas in acetonitrile and dimethylformamide, solvent displays blue emission peak with a large stoke shift.The bent-core molecules with the number of carbon atoms (n = 4, 8 and 12) at the elongated arm exhibited monotropic nematic phase at low temperature, while the 4-n-octadecyloxy chain at the elongated arm displayed smectic A phase. Dielectric studies were performed in the nematic phase of the bent-core mesogens confirm the formation of the cybotactic cluster in the nematic mesophases. GRAPHICAL ABSTRACT

Design, simulation and modelling of LTCC based micro hotplate for gas sensor applications

Low temperature co-fired ceramics (LTCC)-technology, which is widely used for packaging and manufacturing high frequency RF components, is used for fabrication of micro-hotplates (MHP). Gas sensors produced using this technology leads to low power consumption (500 mW for 226 °C) due to low thermal conductivity (3–4 W/(m-K)). For small series, this technology is remarkably inexpensive compared to the expensive silicon technology. Thermal properties such as temperature distribution and power consumption have been investigated using FEM (Finite Element Method) simulations. Mathematical modeling of these hotplates is also done in accordance with the simulated results.

Alignment of smectic mesogens over engineered surfaces

The alignment of smectic C* liquid crystals (LCs) has been manipulated over and near different surfaces such as bare glass, indium tin oxide (ITO) coated glass, patterned glass, and near spacers. The LC sample cell is made of one ITO coated glass plate and other having striped ITO with a Mylar spacer maintaining a finite cell gap between them. Laser scribing is used to make striped ITO, while the scribed area results in the formation of the patterned glass surface. The geometry of the cell is such that overlapped ITO lie in the middle and spacers are placed in the two extreme corners of the cell. The alignment of LC is found to be homeotropic over the ITO coated glass, while it is planar near the spacer. Interestingly, a transition from homeotropic to planar is observed while moving away from the middle towards either corner of the cell. The origin of both types of alignment in the same cell has been explained by considering the difference in the surface energy of different surfaces. This work renders new...

Realization of High Packaging Density on Ceramics for High Temperature Applications

This study is focused on the high density packaging of devices working at high temperature gradient. At the first step isothermal solidification based interconnections are fabricated on the sample contact pads, printed on a ceramic substrate, using a low melting metal layer. At the second step the attachable device has been connected on the same pad connections, fabricated on the same area, using different interlayer metal. High melting intermetallic phases are formed during every process-step and the interconnect remains unaffected of the next bonding process-temperature. This paper reveals the possibility of high temperature stable Pb-free bonding in an overlapping area.

Study of the oxidation effects on isothermal solidification based high temperature stable Pt/In/Au and Pt/In/Ag thick film interconnections on LTCC substrate

The objective of the presented paper is to determine the oxidized phase compositions of indium lead-free solders during solidification at 190 ° C under room environment with the help of X-ray diffraction (XRD) and Energy dispersive spectroscopy (EDX). Many lead-free solders alloys available oxidizes and have poor wetting properties. The oxidation of pure indium solder foil, Au, Pt, and Ag alloys were identified and investigated, in the process of isothermal solidification based solder joints construction at room environment and humidity. Both EDX and XRD characterization techniques were performed to trace out the amount of oxide levels and variety of oxide formations at solder interface respectively. The paper also aims to report the isothermal solidification technique to provide interconnections to pads on Low temperature co-fired ceramic (LTCC) substrate. It also elaborates advantages of isothermal solidification over the other methods of interconnection. Scanning electron microscope (SEM) used to ident...

Bandgap Modulation in ZnO Through Doping By Sol‐Gel Method

Zinc oxide is a promising optical material for possible application in solid state lighting. Highly transparent and nanocrystalline Zinc Oxide thin films with different concentration of Cadmium and Magnesium are deposited on corning 7059 glass by sol‐gel spin coating method. The transparent and homogeneous sol of 0.2 mol/l concentration is prepared by dissolving zinc acetate dihydrate in 2‐methoxyethanol. Monoethanolamine is used as sol stabilizer. The calculated amounts of cadmium acetate dihydrarte and magnesium acetate tetrahydrarte are added for doping. All the samples are annealed at 550 °C in air ambient for an hour. The doping is confirmed by X‐ray diffraction and the bandgap is measured by UV‐Vis spectroscopy. The bandgap successfully modulated from near ultraviolet region (3.29 eV) to far ultraviolet region (3.83 eV).

Annealing Effects on Bi Doped Se-Te System

Annealing reduces the randomness and effects the adjustment of relative position between the layers. The annealing may provide a deeper insight into the mechanism of disorder and defect formation in the amorphous chalcogenides. A lot of research has been done on the study of the effect of thermal annealing on the optical properties of the semiconducting chalcogenide glasses, which has not been amply supported by corresponding experimental data on the effect of thermal annealing on the electrical properties. So, the effect of thermal annealing on the electrical properties and optical properties of Se–Te-Bi (2≤x≤8) chalcogenide glasses has been briefly discussed in the present paper. The annealing in the present study has been done below glass transition temperature.

Stable interconnections for LTCC micro-heater using isothermal solidification technique

Purpose The purpose of this work is to explore the forms of intermetallic phase compounds (IMPCs) in Pt/In/Au and Pt/In/Ag joints by using isothermal solidification. This lead-free technique leads to formation of IMPCs having high-temperature stable joints for platinum-based micro-heater gas sensor fabricated on low temperature co-fired ceramic (LTCC) substrate. Design/methodology/approach Proposed task is to make an interconnection for Pt micro-heater electrode pad to the silver and gold thick-films printed on LTCC substrate. Both Pt/In/Au and Pt/In/Ag configured joints with different interactive areas prepared at 190 and 220°C to study temperature and contact surface area effects on ultimate tensile strength of the joints, for a 20 s reaction time, at 0.2 MPa applied pressure. Those delaminated joint interfaces studied under SEM, EDAX and XRD. Findings IMPCs identified through material analysis using diffraction analysis of XRD data are InPt3, AgIn2, AgPt, AgPt3, Au9In4 and other stoichiometric compounds. The interactive surface area between thick-films and temperature increment shows improvement in the formations of IMPCs and mechanical stability of joints. These IMPCs-based joints have improved the mechanical stability to the joints to sustain even at high operating temperatures. Elemental mapping of the weak joint contact interface shows unwanted oxide formations also reported. Physical inter-locking followed by the diffusion phenomenon on the silver substrate strengthen the interconnection has been noticed. Research limitations/implications Inert gas environment creation inside the chamber to isolate the lead-free joint placed between heating stamp pads to avoid oxide formations at the interface while cooling which adds up to the cost of manufacturing. Most of the oxides at a joint-interface increase minute to moderate resistance with respect to the level of oxides took place. These oxides contributed heat certainly damage the micro-heater based gas sensors while functioning. Practical implications These isothermal solidification-based lead-free solder joints formation replace the existing lead-based packaging techniques. These lead-free interconnections on ceramic or LTCC substrate are reliable and durable, especially those designed to work for heavy-duty engines, even at severe environment conditions. Originality/value Platinum micro-heater-based gas sensors handles over a wide-range of temperatures about 300 to 500°C. The specific temperature level of different oxide films (SnO2) on the micro-heater is capable of detecting various specific gases. This feature of platinum based gas sensor demands durable and mechanically stable joints for continuous monitoring.

Deposition of ZnO ultrathin films by sol-gel route

Crack free, uniform and high quality ultrathin films (~ 2-3 nm) of zinc oxide were achieved by cost effective and simplest wet chemical sol-gel route. The films were deposited at different spinning speed (1500 to 6000 rpm) on the silicon substrates. The average thickness per layer for each sample was measured by using surface profiler. The thickness of the film is reduced linearly with the spinning speed but the uniformity of the films was degraded for deposition at higher RPM (≤ 3500). The uniformity of deposited layers was observed by Photoluminescence mapping and it was found that the cracks density enhanced for higher spinning. In AFM images for the deposited films the grains were appeared with outlying distribution and the average grain size was increased with spinning speed. The films possessed high uniformity with lowest thickness (~ 5 nm) at 3000 rpm. Further reduction in thickness was achieved by decreasing the concentration of zinc in the prepared sol. It was observed that the films were still smooth but the thickness extensively reduced up to 2 nm. The effect of substrate size on the quality of thin film also observed. To study this effect the films were deposited on 1 × 1, 2 × 2, 3 × 3 and 5 × 5 cm2 substrates at 3000 rpm. It was observed by PL mapping that for substrates with large area the density of cracks was enhanced.