M. Subba Rao

Reactivity of rice husk ash

Effect of lime:silica ratio on the kinetics of the reaction of silica with saturated lime has been investigated. Below C/S=0.65 the reaction does not proceed to completion and even in the presence of a large excess of silica only 90% lime is consumed. A parameter, lime reactivity index, has been defined to quantity the reactive silica present in rice husk ash. The product of the reaction between rice husk ash and saturated lime is a calcium hydrosilicate, C---S---H(I)**. The fibrilar structure and the hollow tubular morphology of the fibres of C---S---H, have been explained by a growth mechanism, where the driving force is osmotic pressure.

Silica from rice husk through thermal decomposition

The thermal decomposition characteristics of rice husk have been investigated by dynamic thermoanalytical techniques: DTA, TG, DTG and isothermal heating. The observed thermal behaviour is explained on the basis of a superposition of the decomposition of cellulose and lignin, which are the major organic constituents of rice husk. Morphological features of silica in husk as well as the ash are examined by scanning electron microscopy. Silica in the residual ash has been characterised by X-ray diffraction and infrared spectroscopy. Controlled thermal decomposition of rice husk has been shown to be a convenient method for the liberation of silica.

Preparation, structure, and properties of strontium-doped lanthanum chromites: La1−xSrxCrO3

Strontium-doped lanthanum chromites, La1−xSrxCrO3, have been synthesised to investigate the effect of strontium doping on the stability and physico-chemical characteristics of the perovskite LaCrO3. Both microscopic and X-ray examinations show that the materials exist as single phase perovskite structure for all compositions up to 50 mole% strontium substitution. The materials have been further characterized by infrared and electron paramagnetic resonance spectra. These materials show a good sinterability even in air at 1773 K. Electrical conductivity of thse perovskites has been measured as a function of temperature. Electrical conductivity has been found to be a maximum at x=0.2. The observed electrical and magnetic properties are consistent with activated polaron transport as the mechanism for electrical conduction in these materials.

Reaction product of lime and silica from rice husk ash

Rice husk ash (about 95% silica) with known physical and chemical characteristics has been reacted with lime and water. The setting process for a lime-excess and a lime-deficient mixture has been investigated. The product of the reaction has been shown to be a calcium silicate hydrate, C-S-H(I)+ by a combination of thermal analysis, XRD and electron microscopy. Formation of C-S-H(I) accounts for the strength of lime-rice husk ash cement.

Thermal decomposition of titanyl oxalates—I: Barium titanyl oxalate

Thermal decomposition of barium titanyl oxalate tetrahydrate (BTO) has been investigated employing TGA, DTG and DTA techniques and gas and chemical analysis. The decomposition proceeds through five steps and is not affected much by the surrounding gas atmosphere. The first step which is the dehydration of the tetrahydrate is followed by a low-temperature decomposition of the oxalate groups. In the temperature range 190–250°C half a mole of carbon monoxide is evolved with the formation of a transient intermediate containing both oxalate and carbonate groups. The oxalate groups are completely destroyed in the range 250–450°C, resulting in the formation of a carbonate which retains free carbon dioxide in the matrix. The trapped carbon dioxide is released in the temperature range of 460–600°C. The final decomposition of the carbonate takes place between 600–750°C and yields barium titanate. The i.r. spectra, surface area measurements and X-ray, powder diffraction data support entrapment of carbon dioxide in the matrix.

Thermal decomposition of titanyl oxalates IV. Strontium and calcium titanyl oxalates

Abstract Thermal decomposition of strontium titanyl oxalate tetrahydrate and calcium titanyl oxalate hexahydrate have been studied employing TG, DTA, gas and chemical analysis. The decompositions proceed through three major steps: dehydration, decomposition of the oxalate to a carbonate and the decomposition of the carbonate to yield the final products, the metatitanates. The intermediates of the oxalate decomposition are found to be Sr 2 Ti 2 O 4+x (CO 3 ) 2 -x(CO 2 ) x and Ca 2 Ti 2 O 4 (CO 3 ) 2 , respectively. The entrapment of carbon dioxide in the former and the presence of non-equivalent carbonate groups in the latter are substantiated by their i.r. spectra. The penultimate solid residues are poorly crystalline Sr 2 Ti 2 O 5 CO 3 and amorphous Ca 2 Ti 2 O 5 CO 3 . Decompositions of these carbonates are accompanied by growth in particle size of the products, SrTiO 3 and CaTiO 3 , respectively.

Thermal decomposition of titanyl oxalates—II: Kinetics of decomposition of barium titanyl oxalate

Abstract Kinetics of the thermal decomposition of barium titanyl oxalate have been studied. Decomposition of the anhydrous oxalate is complex and deceleratory throughout. Kinetics of decomposition of the intermediate carbonate Ba2Ti2O5CO3 is greatly influenced by the thermal effects during its formation. The sigmoidal (α, t) curves obey a power law equation followed by first order decay. Presence of carbon in the vacuum prepared carbonate has a strong deactivating effect. Decomposition of the carbonate is accompanied by growth in particle size of the product, barium titanate.

Thermal decomposition of titanyl oxalates—III: Lead titanyl oxalate

Abstract Lead titanyl oxalate tetrahydrate (LTO) has been prepared using a chloride-free medium. The thermal decomposition of LTO has been investigated employing thermoanalytical and gas analysis techniques. The decomposition in air or oxygen is straight forward and proceeds through three main steps: dehydration, decomposition of the oxalate to a carbonate and decomposition of the carbonate to give lead metatitanate as the final product. This is complicated in a vacuum or non-oxidising atmosphere, by the partial reduction of Pb(II) to Pb(0) at the oxalate decomposition step. The formation of free metallic lead affects the stoichiometry of the intermediate carbonate, inhibits the evolution of carbon dioxide after the decomposition of the carbonate and yields a mixture of lead metatitanate and titanium rich PbTi3O7 as the final products. For the preparation of pure lead metatitanate by decomposing LTO, oxidising conditions should be maintained during the decomposition.

Rare-earth chromium citrates as precursors for rare-earth chromites: lanthanum biscitrato chromium(III) dihydrate, La[Cr(C6H5O7)2]·2H

Abstract The thermal decomposition of lanthanum biscitrato chromium(III) dihydrate has been studied in static air and dynamic argon atmospheres. The complex decomposes in four steps: dehydration, decomposition of the citrate to an intermediate oxycarbonate, formation of LaCrO 4 (V) from oxycarbonate, and finally decomposition of LaCrO 4 (V) to LaCrO 3 . Formation of LaCrCrO 4 (V) requires the presence of oxygen. The decomposition behaviour of a mechanical mixture of lanthanum citrate hydrate and chromium citrate hydrate was compared with that of the citrato complex. Both the starting material and the intermediates were characterized by X-ray diffraction, IR electronic and ESR spectroscopy, surface area and magnetic susceptibility measurements, as well as by chemical analysis. A scheme is proposed for the decomposition of lanthanum biscitrato chromium(III) dihydrate in air. LaCrO 3 can be obtained at temperatures as low as 875 K by isothermal decomposition of the complex.

Study of the thermal decomposition of lanthanum and chromium citrate hydrates

Abstract Lanthanum citrate tetrahydrate decomposes to give La 2 O 3 as an end product at 600°C via a three stage process. In addition to the final product, lanthanum sesquioxide, an anhydrous citrate and an oxycarbonate have been isolated by isothermal heating of the citrate. The thermal decomposition of chromium citrate pentahydrate is found to proceed through four steps. The lowest temperature for the formation of Cr 2 O 3 from chromium citrate has been found to be around 400°C. Based on the experimental results, decomposition schemes have been proposed for the decomposition of lanthanum citrate tetrahydrate and chromium citrate pentahydrate, respectively.