Mandyam Sriram

Chemical synthesis of the high-pressure cubic-spinel phase of ZnIn2S4

Chemical reactions conducted in solution are known to generate solid precursors containing molecular units that help in the formation of high-temperature phases. The structural units are created by controlling the molecular environments in solution, and as a result, phases that normally form and are stable at high temperatures can be synthesized at low or moderately elevated temperatures. However, the application of chemical approaches for synthesizing phases that normally form at high pressure are relatively unknown. In this work, a simple room-temperature aqueous chemical precipitation route has been used to synthesize the high-pressure cubic spinel modification of ZnIn2S4. A solution coordination model (SCM) has been proposed to explain the formation of the high-pressure phase. The crystallinity, phase purity and phase transformation characteristics of the cubic phase have been studied using X-ray diffraction (XRD) including Rietveld refinement, transmission electron microscopy (TEM), and Auger electron microscopy (AEM). Results of these studies are discussed in the light of a proposed solution coordination model (SCM).

The thio-sol-gel synthesis of titanium disulfide and niobium disulfide. Part 1.–Materials chemistry

Investigations pertaining to the synthesis of titanium disulfide (TiS 2 ) have so far been focused on solid state reactions, low temperature chemical techniques and vapor phase reactions using titanium tetrachloride (TiCl 4 ) as the starting material. In this paper, we have investigated the potential of titanium tetraalkoxides [Ti(OR) 4 ], which have been widely used for the synthesis of oxides by the sol-gel approach, for the synthesis of TiS 2 via the thio-sol-gel process. The mechanism of the reaction of titanium isopropoxide {Ti(OPr i ) 4 } with H 2 S in benzene has been studied using infrared spectroscopy (FTIR), gas chromatography (GC) and chemical analysis. Based on these studies, it has been determined that the precipitate obtained from the reaction forms following a thiolysis-condensation mechanism similar to the hydrolysis-condensation mechanism that operates in the oxide sol-gel process. The precipitate, which is an alkoxysulfide, can be converted to TiS 2 by heat treatments in flowing H 2 S. The influence of modifying agents, the role of solvents and the alkoxy group [in Ti(OR) 4 ] on the formation of the alkoxysulfide precipitate have also been presented and discussed. Finally, the applicability of this process for the synthesis of NbS 2 has also been demonstrated.

Novel low-temperature synthesis of glasses and glass-ceramics in the B2O3-SiO2-P2O5 system

In recent years considerable progress has been made in electronic packaging substrate technology. The future need of miniaturization of devices to increase the signal processing speed calls for an increase in the device density requiring the substrates to be designed for better thermal, mechanical and electrical efficiency. Fast signal propagation with minimum delay requires the substrate to possess very low dielectric constant. Several glasses and glassceramic materials have been identified over the years which show good promise as candidate substrate materials. Among these, borophosphate and borophosphosilicate glass-ceramics have been recently identified to have the lowest dielectric constant (3.8). Sol-gel processing has been used to synthesize borosilicate, borophosphosilicate and borophosphate glasses and glass-ceramics using inexpensive boron oxide and phosphorus pentoxide precursors. Preliminary results of the processing of these gels and the effect of volatility of boron alkoxide and its modification on the gel structure are described. X-ray diffraction, differential thermal analysis and Fourier transform-infrared spectroscopy have been used to characterize the as-as-prepared and heat-treated gels.

The thio-sol-gel synthesis of titanium disulfide and niobium disulfide. Part 2. : Morphology, defect structure and electrochemical characteristics of titanium disulfide

The thio-sol-gel process has been used to synthesize TiS 2 powders exhibiting different morphologies, from randomly agglomerated to radially oriented TiS 2 crystallites from a single precursor. It has been shown that different heat-treatment conditions can be utilized to simultaneously control the morphology of the sulfide powder and its defect concentration. Thus a variety of morphologies with crystallite sizes in the range of 2 µm to less than 0.5 µm and with defect concentrations ( x in Ti 1+x S 2 ) in the range of 0.003≤x≤0.03 have been synthesized. The crystallite sizes have been quantified using scanning electron microscopy and defect concentrations have been measured by determining the lattice parameter of the crystal. The performance of these materials as cathodes in secondary lithium batteries has also been evaluated and correlated to the crystallite size distributions and defect concentration.

Designing Ultra Low-k Dielectric Materials for Ease of Patterning

Organosilicate materials with dielectric constants (k) ranging from 3.0 to 2.2 are in production or under development for use as interlayer dielectric materials in advanced interconnect logic technology. The dielectric constant of these materials is lowered through the addition of porosity which lowers the film density, making the patterning of these materials difficult. The etching kinetics and surface roughening of a series of low-k dielectric materials with varying porosity and composition were investigated as a function of ion beam angle in a 7% C4F8/Ar chemistry in an inductively-coupled plasma reactor. A similar set of low-k samples were patterned in a single damascene scheme. With a basic understanding of the etching process, we will show that it is possible to proactively design a low-k material that is optimized for a given patterning. A case study will be used to illustrate this point.

New molecular routes for synthesizing titanium disulfide

Abstract Titanium isopropoxide was reacted with H2S to form amorphous alkoxy-sulfide precursors. Gas chromatography and infrared spectroscopy indicate that a thiolysis-condensation mechanism is responsible for the formation of the precursors. The alkoxide was also reacted with organic sulfidizing agents, such as dimethyldisulfide and hexamethyldisilthiane. The precursors are all amorphous and are comprised of spherical particles. On heat treatment in H2S, the spherical particles transform to the crystalline sulfide having unique morphologies and exhibiting random as well as preferred growth of sulfide platelets.

New Chemical Approaches to Synthesize Titanium Disulfide

The synthesis of oxides by the use of metal alkoxides is well known in the sol-gel process. However, little is known regarding the use of alkoxides as starting materials for nonoxides. In this work titanium isopropoxide has been reacted with different sulfidizing agents to form alkoxy sulfide precursors. In all the processes the transformation of the precursors to form TiS 2 has been studied and the morphologies of the sulfide powders have also been compared. Gas chromatography and infrared spectroscopy have been used to study the molecular processes that occur in one of the reactions.