B. Rambabu

X-ray absorption near edge structure (XANES) measurements of ceria-based solid electrolytes

The X-ray absorption near edge structure (XANES) spectra of a series of ceria samples doped with trivalent rare earth oxide have been measured at the L-III edges of the dopants and the cerium. These spectra give information on the valency of the excited atom and the geometric structure of the direct neighborhood of this atom. By comparing the XANES spectra of the dopants with those of the pure oxides, differences in the way the dopants are incorporated into the ceria lattice can be observed. These differences together with the mismatch of the ionic radii of the dopants compared to Ce, seem to be responsible for the decreasing ionic conductivity of the lighter rare earth dopants. For Pr, the spectra indicate a partial oxidation to Pr+4 leading to a reduced conductivity.

A.c. conductivity and dielectric studies of silver-based fast ion conducting glass system

Abstract Different dopant salt (AgI) compositions in the AgI–Ag 2 O–SeO 2 –V 2 O 5 (SSV) system were prepared by the melt quenching technique. All the prepared compositions of the SSV system were characterized by X-ray diffraction, IR and DSC. The electrical conductivity measurements were made on the various dopant salt contents of SSV glasses at 1 KHz as well as a function of frequency (5 Hz to 13 MHz) and temperature (295–323 K). The true bulk conductivity ( σ e =2.61×10 −2 S/cm) and the electrical behavior (equivalent circuit) of the SSV glassy system were obtained from impedance analysis. The variation of conductivity with the AgI content in the SSV system was explained using the diffusion path model. Also, frequency dependence of dielectric constant ( ϵ ) and conductivity ( σ ) were calculated using the impedance data, obtained for the various AgI contents of the SSV system. The variation of dielectric constant and conductivity with the frequency were explained by correlating the microscopic nature of the ionic conduction process in the SSV system.

Sol-gel synthesis and characterization of the Ag2O-SiO2 system

Abstract Binary silver silicate (SS) (XAg2O+(1–X) SiO2; X=0.1–0.9 in steps of 0.1) system was synthesized through sol–gel route for various compositions containing SiO2 (F=SiO2) and modifier (M=Ag2O) oxides. All the synthesized SS xerogel compositions were characterized by XRD, FTIR, EDS, WDS, SEM and impedance spectroscopy techniques. The XRD spectrum showed mixed (amorphous and crystalline) phases. The formation of the SiO2 network, and presence of Si–OH and NO3 were confirmed by FTIR. Chemical compositions of each element in the SS system were estimated from the EDS and WDS measurements. The SEM micrographs showed that the SS compounds have two phases and the assumption was further supported by EDS, XRD, and IR results. Electrical conductivity of the SS system was estimated from the impedance data and was found to be on the order of 10−3 cm−1 Ω−1. The role of the observed high ionic conductivity in the SS system is explained using the existing theoretical models.

Nanocrystalline LiCo1−xNixO2 (0≤x≤0.3) for Li-ion batteries

Nanocrystalline LiCo 1-x Ni x O 2 (0≤x≤0.3)-a promising cathode material for rechargeable lithium batteries has been successfully prepared by a novel soft chemical route. Both the formation of the metal-glycine complex and subsequent decomposition of the same at low temperatures under carefully controlled oxygen flow play a critical role in the formation of nanocrystalline material. The thermal history of the as-prepared gel is established by differential thermal analysis (DTA) and thermogravimetric analysis (TGA). Powder X-ray diffraction (XRD) and transmission electron microscopy (TEM) confirm the formation of layered α-NaFeO 2 structure at temperature as low as 330 °C. The exothermic combustion reaction of the organic precursors, which generates high temperature, should be avoided as it results in the spontaneous growth of large crystals. High-resolution transmission electron microscopy (HRTEM) investigation reveals that the particle size of LiCo 0.7 Ni 0.3 O 2 heated at 400 °C is in the range of 10- 15 nm. Substitution of nickel retards the crystal growth. Solid state 6 Li-Magic Angle Spinning (MAS) NMR investigation reveals that the micro-structural short range ordering of nickel ions in LiCo 1-x Ni x O 2 (0≤x≤0.3) is minimum at lower processing temperatures. Li-MAS NMR studies show that considerable amount of short range ordering of nickel ions is observed when the calcination temperature is raised beyond 800 °C indicating that the upper limit for processing temperature is around 750 °C. These materials were fabricated into thin electrodes using polyvinylidene fluoride (PVDF) as polymer binder and the electrochemical properties such as charge/discharge and impedance were evaluated. The electrodes cycled well with a coulombic efficiency of close to one.

Electrochemical Performance Measurements of PBI-Based High-Temperature PEMFCs

Acid-doped poly(2,2′-m-phenylene-5,5′-bibenzimidazole) membranes have been prepared and used to assemble membrane electrode assemblies (MEAs) with various contents of PBI (1–30 wt.%) in the gas diffusion electrode (GDE). The MEAs were tested in the temperature range of 140∘C–200∘C showing that the PBI content in the electrocatalyst layer influences strongly the electrochemical performance of the fuel cell. The MEAs were assembled using polyphosphoric acid doped PBI membranes having conductivities of 0.1 S cm−1 at 180∘C. The ionic resistance of the cathode decreased from 0.29 to 0.14 Ohm-cm2 (180∘C) when the content of PBI is varied from 1 to 10 wt.%. Similarly, the mass transfer resistance or Warburg impedance increased 2.5 times, reaching values of 6 Ohm-cm2. 5 wt.% PBI-based MEA showed the best performance. The electrochemical impedance measurements were in good agreement with the fuel cell polarization curves obtained, and the optimum performance was obtained when overall resistance was minimal.

Preparation, characterization and impedance studies of the superionic conducting AgI–Ag2O–CrO3–V2O5 glassy system

Abstract The quaternary superion conducting (SIC) 66.67%AgI–22.22%Ag 2 O–11.11% [( x )CrO 3 +(1− x )V 2 O 5 ] ( x =0.1–0.9 in steps of 0.1) (SCV) glassy system was prepared in various former (CrO 3 +V 2 O 5 ) compositions by melt quenching method. All the prepared compounds were characterized by XRD and IR techniques to find out the nature and the structure of the SCV compounds. The electrical conductivity was measured as function of frequency as well as temperature for the former compositions of the SCV compounds. The 66.67%AgI–22.22%Ag 2 O–11.11% [0.3CrO 3 +0.7V 2 O 5 ] composition of the SCV compound is found to have highest conductivity ( σ =1.01×10 −2 S/cm) at room temperature and it is well comparable to the bulk conductivity [ σ =1.01(±0.009)×10 −2 S/cm] obtained from the impedance measurements. The variation of conductivity with the former composition of the SCV system was explained using the random site model.

Transport properties and battery performance studies of AgI–Ag2O–Se2O–P2O5 glass

Abstract Various compositions of silver-based quaternary superionic conducting (SIC) [AgI–Ag2O–(SeO2+P2O5)(SSP)] glasses were prepared by melt quenching technique. The X-ray diffraction (XRD) and conductivity studies were carried out to identify the amorphous nature and to select high ionic conducting composition of the SSP system for the fabrication of batteries. Solid-state primary batteries were fabricated using the 66.67%AgI–23.07%Ag2O–10.26%(0.3SeO2+0.7P2O5) (66SSP37) glass with different cathode materials [I:C, (I+C):SE and {(I+C)+SE}:TAAI, where I is Iodine, C is graphite, SE is solid electrolyte (SSP glass) and TAAI is tetraalkyl (methyl and butyl) ammoniumiodide]. The open circuit voltage (OCV), polarization and discharge characteristics were measured to estimate the lifetime of the batteries made up of SSP glass.

Nanocrystalline bulk and thin films of La1−xSrxMnO3 (0≤x≤0.3)

Abstract Nanocrystalline La 1− x Sr x MnO 3 has been prepared by a new solution chemistry technique, and the processing parameters were optimized. The thermal history of the as-prepared gel is investigated by differential thermal (DTA) and thermogravimetric analysis (TGA). Formation of La 1− x Sr x MnO 3 starts at temperatures as low as 575 °C. Powder X-ray diffraction analysis was used to check the crystallographic phase purity of nanocrystalline La 1− x Sr x MnO 3 . Transmission electron microscopic investigation (TEM) reveals that the grain size of samples prepared at 575 °C were in the range of 20–30 nm. The Mn–K edge EXAFS measurements show the nanocrystalline LaMnO 3 and La 1− x Sr x MnO 3 indicate a distortion in the MnO 6 octahedra, yielding two discrete Mn–O distance in contrast to structural data obtained for bulk LaMnO 3 , in which there are three discrete Mn–O distances. Thin films of La 1− x Sr x MnO 3 were deposited by pulsed laser ablation technique on (100) LaAlO 3 , (1000) sapphire and polycrystalline alumina substrates. The deposition parameters were optimized to realize the growth of high-quality films. Films deposited on (100) LaAlO 3 were highly c -axis-oriented while the films on (1000) sapphire were highly a -axis-oriented. The morphology of the films as studied by atomic force microscopic analysis reveal that the films are highly granular with an average grain size in the range of 100–150 nm.

Effect of rapid thermally nitrided titanium films contacting silicided and nonsilicided junctions

The effect of rapid thermally nitrided titanium films contacting silicided (titanium disilicided) and nonsilicided junctions has been studied in the temperature range of 800 to 900°C. The rapid thermal nitridation of titanium films used as diffusion barriers between aluminum and silicon, has a major impact on shallow junction complementary metal oxide semiconductor technologies. During the process of rapid thermal nitridation, the dopants in the junctions undergo a redistribution and affect the electrical properties of shallow junction structures. This work focuses on using novel contact resistance structures to measure the variation in electrical parameters for rapid thermally nitrided titanium films annealed at different temperatures. The self-aligned silicide (salicide) junctions in this study were formed using rapid thermally annealed titanium films. Electrical contact resistance testers were used to measure the interface contact resistance between the salicide and silicon, as well as between the metal and the salicide. The results show that the interface contact resistance to the p− diffused salicided junctions increases with rapid thermal nitridation of the additional titanium film, whereas the interface contact resistance to the n− diffused salicided junction shows a decrease. Further, as a function of the rapid thermal annealing temperature (for fixed titanium thickness), the nonsalicided diffusions show an increase in the interface contact resistance. The boron profiles at the TiSi2/Si interface obtained using secondary ion mass spectroscopy show an excellent qualitative agreement with the electrical results for each of the conditions discussed. The films were also characterized using Rutherford back-scattering spectrometry and transmission electron microscopy and the results show good agreement with the measured variation in electrical parameters. These results also show that as the anneal temperature is increased, the TiN thickness increases, further the change in the silicide/silicon interface position with the nitridation of the additional titanium layer was verified.

FAST ION CONDUCTING GLASSES : EFFECT OF PREPARATION ON CONDUCTIVITY AND ITS CORRELATION WITH SURFACE ANALYSIS USING SEM

Glasses in the system Agl-Ag2O-As2O3 and Agl-Ag2O-As2O5 were prepared by three different methods and characterized by studying their transport and surface properties. A.c. conductivity studies performed on the samples with compositions (mol %) 66.6 Agl-33.3(xAg2O-yAs2O3) and 66.6 Agl-33.3 (xAg2O-yAs2O5) fory/x=0.20 to 5.0 revealed that the glasses are very good ionic conductors. The highest ionic conductivity was observed for the compositions 66.6Agl-16.66Ag2O-16.66As2O3 and 66.6Agl-6.66Ag2O-16.66As2O5, when prepared by the open-air crucible melting and rapid quenching method and 66.6Agl-16.66Ag2O-16.66As2O3 and 66.6Agl-22.2Ag2O-11.11As2O5, when prepared by the vacuum-sealed quartz tube melting and zone-controlled quenching method. The observation of high ionic conductivity for different glass former/glass modifier ratios in the Agl-Ag2O-As2O5 system and its dependence on the method of preparation was explained by correlating the observed surface properties. The contribution of electronic conductivity to the total conductivity was estimated by Wagners d.c. polarization technique. The surface of the bulk specimens belonging to the highest conducting composition in both the systems was studied using a scanning electron microscope to observe the formation of precipitated Β- and α-Agl in the form of colloids which are assumed to be responsible for high ionic conductivity.