O.M. Sreedharan

A novel determination of thermodynamic activities of metals in an AISI 316 stainless steel by a metastable emf method

Abstract An emf technique was adopted for the first time for direct determination of thermodynamic activities of all major metallic components, namely, Fe, Cr, Ni and Mn in a commercial grade AISI 316 stainless steel in the temperature range of approximately 800 to 1200 K. The viability of this method was initially established in the case of chromium activity measurements which could be compared with literature values. For this purpose galvanic cells with M/MF2 and [M]316ss/MF2 (M = Fe, Cr, Ni or Mn) were used, employing single crystal CaF2 as the electrolyte. In addition, Mo activitiy in this alloy was measured using the emf of the galvanic cell, Pt, Mo, MoO2/7 YDT/MoO2, [Mo]316ss, Pt The activities determined by these galvanic cells could be represented as follows: log a Cr (±0.02) = −0.577 + 69.1/T , log a Ni (±0.02) = 0.589 − 800.31/T , log a Fe (±0.01) = 0.179 − 248.54/T , log a Mn (±0.01) = 0.742 − 2581.40/T , log a Mo (±0.05) = −4.548 + 3148.48/T . These activities were used to compute the threshold oxygen levels in Na(l)/AISI 316 stainless steel system for the formation of the corrosion products, viz., NaCrO2, Na4FeO3, MnO and NaMnO2. These data in conjunction with the carbon activity in this alloy reported in the literature, and initial composition of the M23C6, phase, could lead to the estimation of the Gibbs energy change for a typical reaction, 2.571Cr + 0.732Fe + 0.303Mo + 0.226Ni + C M 23/6 C , as follows: ΔG o f,T (M 23/6 C, s) (kJ) = −29.16 − 0.0522T (K) .

Manganese oxide–manganese tungstate composite humidity sensors

Experimental results on composites fabricated from different mole percent of MnWO4 and Mn2O3 for humidity sensing are described. Sintered polycrystalline disks of the composites were subjected to d.c. electrical conductivity measurements over the temperature range of 373–573 K in inert atmosphere from which the activation energies were determined. The terminal phases and the composites were subjected to d.c. resistance measurements as a function of relative humidity in the range of 5–98% RH, achieved by various water buffers at 298 K. The sensitivity factor, Sf (R5/R98) measured at 298 K revealed that the composite MWMO-82 has the highest sensitivity, greater than 4 � 10 3 . The response and recovery characteristics of MWMO82 were assessed. D 2002 Published by Elsevier Science B.V.

Microwave processing of sodium beta alumina

Abstract The conventional processing of sodium beta alumina (SBA) which includes synthesis and sintering in order to obtain dense bodies for use as a solid electrolyte usually requires high temperatures (∼1873 K) and long holding times. A substantial amount of Na2O may be lost during such high temperature heat treatment, the consequence of which may be deleterious when SBA is used for device applications. To overcome this problem, the use of microwave as heating source for the processing of SBA was attempted in the present process. Preliminary investigations on commercial Na-β-alumina bearing the composition NaAl11O17 were carried out and it was found that Na-β-alumina was a very good absorber of microwaves. Following this, a microwave active precursor of MgO-stabilized Na-β″-alumina was synthesized through a novel solution technique, which on compaction and exposure to microwaves yielded dense (94% theoretical) and phase-pure Na-β″-alumina body. This paper presents the results of a direct single-step synthesis cum sintering of MgO-stabilized Na-β″-alumina.

Microwave assisted synthesis and sintering of La0.8Sr0.2Ga0.83Mg0.17O2.815

Abstract A novel method to produce dense bodies of SrO- and MgO-doped LaGaO 3 (LSGM) ceramics by microwave assisted processing in a very short time of 10 min is reported here. The heating was carried out using Na-β″-alumina as the microwave susceptor. The phase purity and electrical properties of the microwave processed samples were determined and compared with a conventionally processed material. It was found that LSGM with a composition La 0.8 Sr 0.2 Ga 0.83 Mg 0.17 O 2.815 always yielded a two-phase mixture though the presence of second phase did not affect the conductivity significantly at lower temperatures. The microstructure was found to be more uniform in case of the microwave sintered material.

Comparative e.m.f. study of CaF2 and β-alumina cells with Ni/NiF2 and Fe/FeF2 or Cr/CrF2 electrodes

AbstractThe reliability of employing beta-alumina as electrolyte for fluorine potential measurement is examined by measuring the e.m.f.s of the galvanic cells with metal/metal fluoride electrodes and comparing with those obtained by using CaF2 as electrolyte under identical conditions. The results from both types of galvanic cell can be superimposed to give the following standard Gibbs energy of formation, ΔGf0, of FeF2 and CrF2 over extended ranges of temperature:

Direct measurement of thermodynamic activities of metals in an AISI 304 stainless steel by calcium fluoride EMF technique

\begin{gathered} \Delta G_f^0 (FeF_2 ) = - 702.0 + 0.125 20T (K) ( \pm 0.70) kJ mol^{ - 1} (506 - 1063K) \hfill \\ \Delta G_f^0 (CrF_2 ) = - 732.8 + 0.087 90T (K) ( \pm 0.64) kJ mol^{ - 1} (497 - 1063K) \hfill \\ \end{gathered}

Threshold oxygen levels in Na(I) for the formation of NaCrO2(s) on 18-8 stainless steels from accurate thermodynamic measurements

The absence of significant temperature-dependent errors in both these measurements are verified by a third law treatment of the data yielding values of −716.8 and −777.4 kJ mol−1 for ΔHf.2980 of FeF2 and CrF2, respectively.The feasibility of using beta-alumina electrolyte cells for e.m.f. measurements on other metal/metal fluoride systems is discussed in the light of the existence of a useful potential domain of beta-alumina. High sodium potential in the electrode system can lead to sodium depletion. Likewise, low sodium potential may result in oxidation of the metals in the electrodes. Both these limiting factors are also examined.

DTA studies on the liquidus temperatures of Cr complex with the addition of an anhydrous Ni complex

Abstract A polymeric gel cum combustion method, which is a hybrid of sol–gel and auto combustion methods, was employed for the synthesis of the analogous phase-pure inter-oxides of the type SrR 2 O 4 (R=Y, La, Sm, Eu, Gd, Er or Yb) at temperatures as low as 1120 K. The refined orthorhombic lattice parameters were computed and these crystal structures were found to conform to the space group Pnam 〈62〉. The values of unit cell volume of these oxides showed a linear dependence on the radius of the R 3+ cation except for La 3+ .