R. S. Srivastava

Proton-conducting gel electrolyte

Abstract Proton conduction in solid state xerogels and polymeric gels are reported. Xerogels, doped with known proton conductors, were prepared by “sol–gel” method starting either from inorganic precursor sodium metasilicate (termed as hydrogel) or organic precursor tetraethyl orthosilicate (termed as silica or SiO 2 alcogel). The dopants chosen for the former were NH 4 BF 4 , NH 4 Cl, NH 4 H 2 PO 4 and N 2 H 6 SO 4 , while for the latter, the dopants used were H 3 PO 4 , NH 4 BF 4 , NH 4 H 2 PO 4 and KH 2 PO 4 . The SiO 2 :H 3 PO 4 alcogel gave the highest room temperature conductivity (∼10 −3 S cm −1 ). Some of the xerogels studied by us were stable even up to 300 °C. Another interesting group of proton-conducting materials discussed in this paper is polymeric gel which was prepared by dispersing PMMA in the liquid electrolyte obtained by dissolving o -, m -, p -hydroxybenzoic acid; o -, m -, p -nitrobenzoic acid and three dicarboxylic acids, viz., oxalic, malonic and succinic acid, in a high-dielectric constant organic solvent. The role of the dissociation constants of the dissolved acids and the interaction of the polymer were discussed. The addition of polymer, inspite of the increasing viscosity, was found to sometimes lead to an increase in the conductivity of liquid electrolyte, which was explained on the basis of a breathing polymer chain model.

Silica alcogel-based proton conducting solid-liquid electrolyte composite

Abstract Silica alcogels doped with proton conducting salts (NH 4 BF 4 or NH 4 H 2 PO 4 or KH 2 PO 4 ) have been prepared by sol–gel process starting from organic precursor tetraethyl orthosilicate (TEOS). Structural (IR, XRD, DSC) and electrical (transference number and conductivity) studies showed that the solutions of dopants salts remain entrapped in the porous SiO 2 –gel matrix giving a solid–liquid electrolyte composite material. An implication of the formation of solid–liquid electrolyte composite, apart from liquid-like conductivity, is that the slow drying of entrapped electrolyte solution on storage for 3–4 weeks results in the growth of whiskers of the dopant salt.

Ion transport and structural studies on a teos-precursor derived silica alcogel doped with acid H3PO4

Silica alcogels doped with proton conducting acid H 3 PO 4 have been prepared by the sol-gel process starting from organic precursor tetraethyl orthosilicate (TEOS). Structural (XRD, DTA/TGA, IR) and electrical (transference number, conductivity) studies showed that the solution of dopant remains entrapped in the porous SiO 2 -gel matrix and can be modelled as solid-liquid electrolyte composite. The H 3 PO 4 doped gels have been found to show about five orders higher protonic conductivity than the undoped SiO 2 -gel. The highest attainable room temperature conductivity 1.6 x 10 -2 S cm -1 is for the SiO 2 : H 3 PO 4 (40: 60) gel composite. The high liquid-like proton transport in this composite has been explained as a result of self dissociation of entrapped H 3 PO 4 or H 2 PO - 4 /HPO 2- 4 acid-base pair formation.

Studies on a Solid State Proton Conducting Gel

A new ionically conducting (primarily protonic) gel system xNa 2 SiO 3 + (100 - x)NH4BF4 has been prepared. The gelling took place in a limited compositional range with x = 15 to 50 mol%. The NH 4 BF 4 (which normally sublimes) gets stabilized in the gel matrix. Samples have been characterized by IR, DTA/TGA, impedance spectroscopy and transference number measurements. The temperature and humidity dependence of conductivity has also been studied.

Porous silica alcogel matrix entrapped with liquid electrolyte: Ionic conductivity and growth of whiskers

Abstract Highly porous silica alcogel, with pores entrapped with liquid electrolyte solutions of KH 2 PO 4 , has been successfully prepared by hydrolysis and polycondensation of tetraethylorthosilicate (TEOS) giving interesting “solid-liquid electrolyte composites”. The entrapped liquid electrolyte solution gives high liquid-like ionic conductivity (∼10 −5 S·cm −1 ). Further, on storage the entrapped solution slowly reaches the surface pore-heads where it evaporates resulting in the growth of long crystalline whiskers. Higher relative humidity (>60%) slows down the evaporation and has been found to be favorable for whisker growth.

Dielectric breakdown properties in silicon dioxide films

The time-dependent dielectric breakdown of thermally grown SiO2 films in M-O-S structures was studied using standard, PSG and HCl oxides. The time to breakdown was found to decrease exponentially with the increase of applied electric fields. The effect of mobile ion concentration and temperature on time to breakdown was also studied. Peeks law behaviour (Eαt-1/4) was verified for the contaminated samples. Contrary to previous reported results, the devices having comparatively low contamination level have been observed to obey Peeks law. The discrepancy in the result is attributed to the presence of comparatively large quantities of defects in our devices.

Stability of MOS capacitors prepared from oxide grown in O2-HCl gas

The standard bias-temperature test was performed to study the effect of HCl gettering on ion drift instability. It was found that though HCl gettered device was free of ion drift instability, it introduced another type of instability called trapping instability to the device. Lack of the reproducibility of the original curve after deplorization was also observed.