M. Jayasimhadri

Enhanced thermoelectric figure-of-merit in spark plasma sintered nanostructured n-type SiGe alloys

We report a significant enhancement in the thermoelectric figure-of-merit of phosphorous doped nanostructured n-type Si80Ge20 alloys, which were synthesized employing high energy ball milling followed by rapid-heating using spark plasma sintering. The rapid-heating rates, used in spark plasma sintering, allow the achievement of near-theoretical density in the sintered alloys, while retaining the nanostructural features introduced by ball-milling. The nanostructured alloys display a low thermal conductivity (2.3 W/mK) and a high value of Seebeck coefficient (−290 μV/K) resulting in a significant enhancement in ZT to about 1.5 at 900 °C, which is so far the highest reported value for n-type Si80Ge20 alloys.

Host sensitized novel red phosphor CaZrSi2O7 : Eu3+ for near UV and blue LED-based white LEDs

A series of red phosphors Ca1?xZrSi2O7?:?Eux (x = 0.5,1,5,10,12?mol%) were prepared by a solid-state reaction technique at various temperatures and their structural and optical properties were investigated. The x-ray diffraction profiles showed that all peaks could be attributed to the monoclinic phase CaZrSi2O7 doped with Eu3+. SEM, FTIR, TG and DTA profiles have also been characterized to explore their structural properties. The luminescence properties of these resulting phosphors have been characterized by photoluminescence spectra. The host matrix itself has shown a strong blue emission which has its maximum intensity at 470?nm. The excitation spectra of CaZrSi2O7?:?Eu3+ revealed two excitation bands at 395 and 464?nm which correspond to the sharp 7F0?5L6 and 7F0?5D2 transitions of Eu3+ and matches well with the two popular emissions from n-UV/blue GaN-based LEDs. The prominent red emission was obtained at 615?nm by the excitation transitions 5L6, 5D2 of Eu3+ through the non-radiative energy transfer process from the host to the Eu3+ ion. The effects of charge compensation by monovalent ions on the luminescence behaviour of a red emitting phosphor CaZrSi2O7?:?Eu3+ were investigated. The high colour saturation and the low thermal quenching effect of these phosphors make it a potential red component for white light emitting diodes (w-LEDs).

Microstructure and mechanical properties of thermoelectric nanostructured n-type silicon-germanium alloys synthesized employing spark plasma sintering

Owing to their high thermoelectric (TE) figure-of-merit, nanostructured Si80Ge20 alloys are evolving as a potential replacement for their bulk counterparts in designing efficient radio-isotope TE generators. However, as the mechanical properties of these alloys are equally important in order to avoid in-service catastrophic failure of their TE modules, we report the strength, hardness, fracture toughness, and thermal shock resistance of nanostructured n-type Si80Ge20 alloys synthesized employing spark plasma sintering of mechanically alloyed nanopowders of its constituent elements. These mechanical properties show a significant enhancement, which has been correlated with the microstructural features at nano-scale, delineated by transmission electron microscopy.

Luminescent properties of orange emissive Sm3+-activated thermally stable phosphate phosphor for optical devices

Rare earth ion activated orthophosphates have a great deal of interest due to their thermal stability for white light emitting diodes. In this regard, thermally stable Sm3+ doped NaCaPO4 (NCP) phosphor was synthesized by conventional solid state reaction technique. The phase and the structure of the as prepared powders were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), FT-IR, emission and excitation properties were extensively investigated for NCP phosphors. X-ray diffraction analysis confirmed the formation of NaCaPO4 with orthorhombic structure. The excitation spectra indicate that this phosphor can be effectively excited by UV light from 350 to 500 nm. All the transitions in the excitation spectrum of Sm3+ start from the ground state 6H5/2 to various excited states. The emission spectra indicated that the emitted radiation was dominated by the emission peak wavelength at 599 nm originated from the transition of 4G5/2→6H7/2. The optimum concentration of Sm3+ is determined as 1.0 mol% based on the concentration dependent emission spectra. These results suggest that the NaCaPO4:Sm3+ phosphor is a promising orange emitting phosphor under 404 nm excitation with CIE coordinates of x=0.545, y=0.41, which might be used in the development of materials for LEDs and other optical devices in the visible region.

Concentration dependent luminescence characteristics of 5D4 and 5D3 excited states of Tb3+ ions in CFB glasses

Calcium fluoroborate (CFB) glasses doped with different concentrations of Tb3+ ions have been prepared with molar composition of (42-x) B2O3 + 20 CaF2 +15 CaO + 15 BaO + 8 Al2O3 + x TbF3 (x=0.05, 0.1, 0.5, 1.0, 2.0 and 4.0) by melt quenching method. Optical absorption spectrum for 1.0 mol% of Tb3+:CFB glass was recorded in UV-Vis-NIR regions. Radiative parameters such as radiative transition probabilities (AR), radiative lifetimes (τR), radiative branching ratios (βR) for the 5D3 and 5D4 excited states have been calculated by using the Judd-Ofelt parameters (Ωλ=2,4,6). The luminescence spectra recorded for different concentrations of Tb3+ ions in CFB glasses exhibit seven and four bands originating from 5D4 and 5D3 excited states respectively. As the concentration of Tb3+ ions increases the intensities of luminescence peaks originating from 5D4 state increases, whereas, the quenching of intensities has been observed for the emission peaks originating from 5D3 state due to energy transfer through cross-relaxation channel (5D3 : 7F6) → (5D4 : 7F0). The experimental lifetimes of 5D4 state for all the concentrations of Tb3+ ions are equal and exhibit single exponential decay with highest quantum efficiency which confirms that these glasses can be used for high intensity green emission even at high concentrations of Tb3+ ions.

Luminescence and advanced mass spectroscopic characterization of sodium zinc orthophosphate phosphor for low‐cost light‐emitting diodes

A new rare-earth-free NaZnPO4:Mn(2+) (NZP:Mn) phosphor powder has been developed by our group and investigated meticulously for the first time using secondary ion mass spectroscopy and chemical imaging techniques. The studies confirmed the effective incorporation of Mn(2+) into the host lattice, resulting in an enhancement of photoluminescence intensity. Phase purity has been verified and structure parameters have been determined successfully by Rietveld refinement studies. The NZP:Mn phosphor powder exhibits strong absorption bands in the ultraviolet and visible (300-470 nm) regions with a significant broad yellow-green (~543 nm) emission due to the characteristic spin forbidden d-d transition ((4)T1→(6)A1) of Mn(2+) ions, indicating weak crystal field strength at the zinc-replaced manganese site. The decay constants are a few milliseconds, which is a pre-requisite for applications in many display devices. The results obtained suggest that this new phosphor powder will find many interesting applications in semiconductor physics, as cost-effective light-emitting diodes (LEDs), as solar cells and in photo-physics.