Paritosh Kundu

IR spectra and their application for evaluating physical properties of fluorophosphate glasses

Abstract Infrared (IR) absorbance spectra in the range 500–4000 cm −1 of the glasses in the system Ba(PO3)2–MgF2–CaF2–AlF3 have been studied with increasing fluoride content. Quantitative justification for assignment of some bands has been attempted. The spectra reveal that the glass structure changes with fluoride addition from metaphosphate, (PO3−)n to P2(O,F)7 and P(O,F)4 along with the formation of Al(O,F)6,[AlF6]3− and [AlF4]−. As a result of these structural changes density (d), refractive index ne) and IR band absorption coefficient at ∼2170 cm −1 (α IR ) of the glasses vary exhibiting maxima at ∼ 36 mol% fluoride. Statistical analyses show that d and ne are correlated with αIR by linear equations. Each of these equations gives a high correlation coefficient (r) and low residual standard deviation (RSD), and may, thus, be employed for the evaluation of physical properties of fluorophosphate glasses. Physical properties of some fluorophosphate glasses of this work and the published literature have been calculated by using these equations to test their validity. The experimental properties of the glasses present a very good fit of data within 95% confidence interval of the calculated mean values. These facts indicate the suitability of this IR spectroscopic method for the determination of physical properties of fluorophosphate glasses.

Preparation and thermal evolution of sol-gel derived transparent ZrO2 and MgO-ZrO2 gel monolith

Transparent gel monoliths of pure and MgO-doped zirconia having dopant concentrations in the range 0 to 15 mol % were prepared by chemical polymerization of zirconium n-propoxide and magnesium acetate tetrahydrate using 2-methoxy ethanol as solvent. The thermal evolution of amorphous gels was studied by differential thermal analysis, X-ray diffraction and transmission electron microscopy. The crystallization of pure and doped zirconia gels occurred in the temperature range 360 to 450° C. The first crystalline phase to appear is tetragonal for pure and 2 mol % doped zirconia, and cubic for 3 to 15 mol % doped samples. Both crystallization and decomposition temperatures are found to increase with increasing dopant concentration, approaching a saturation value for 10 mol % doped samples. It has been established that the transformation of the cubic to the monoclinic phase takes place via a metastable tetragonal phase. A linear relationship between the lattice parameter of cubic zirconia and MgO concentration has been established. X-ray diffraction studies have also revealed that the entire amount of MgO used in preparing doped zirconia gels remains in a single MgO-ZrO2 crystalline phase formed initially by thermal treatment.[/p]

Glass-Ceramic Sealants for Planar IT-SOFC: A Bilayered Approach for Joining Electrolyte and Metallic Interconnect

To develop suitable sealants for planar anode-supported intermediate temperature solid oxide fuel cells (IT-SOFC) operating at around 800 degrees C, several glass-ceramic compositions based on the RO-Al2O3-SiO2 (R=Ba,Ca) system have been prepared and investigated. Thermal, chemical, and electrical properties of the glass ceramics along with crystallization behavior and bonding characteristics of parent glasses with yttria-stabilized zirconia (YSZ) electrolyte and ferritic steel-based interconnect Crofer22APU are studied. The glass transition temperatures (T-g) of the developed glasses are within 600-635 degrees C. By varying the alkaline-earth metal content, the coefficient of thermal expansion (CTE) values can be tailored between that of YSZ and Crofer22APU. Based on the crystallization behavior, a single glass seal having matching CTE with Crofer22APU and a bilayer glass seal having different CTE values within the range of 10-13x10(-6)/K are found to be effective for metal-metal and metal-electrolyte sealing, respectively. The electrical resistivities of the sealants are quite high (rho(800 degrees C)> 10(5) Omega cm), and under sandwiched condition between two metals, the developed glasses are found to maintain this high resistivity even after 100 h of operation. A low helium leak rate value (< 10(-6) Pa m(2) s(-1)) has been observed for most of the developed glasses. (c) 2008 The Electrochemical Society.

UV transparency and structure of fluorophosphate glasses

Abstract Ultraviolet (UV) transparency and local structural environment of fluorophosphate glasses in the Ba(PO3)2–MgF2–CaF2–AlF3 system have been studied with increasing fluoride content (covering the whole compositional range) by infrared (IR) reflection spectroscopy (IRRS) in conjunction with 31P magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. UV cutoff of the glasses shifts towards longer wavelength with increasing fluoride up to ∼36 mol%. IRRS spectra indicate a gradual increase in nonbridging oxygens (NBOs) in this region. According to 31P MAS NMR spectra, it is due to the combined effects of gradual increase in Q1 sites and P–O–Al bridges (increase in optical basicity). Beyond ∼36 mol% fluoride content, the UV cutoff shifts towards shorter wavelength. It is attributed to decrease in NBOs or Q1 sites (decrease in optical basicity) as revealed in the IRRS and 31P MAS NMR spectra, respectively. In this region, true partial role of the metal fluorides in controlling UV transparency has been found to contribute significantly as well.

NIR emission and upconversion luminescence spectra of Nd3+:ZnO–SiO2–B2O3 glass

Abstract Zinc-borosilicate glass containing Nd 3+ ions has been prepared to study the NIR fluorescence and the violet upconversion emission properties. NIR photoluminescence spectrum has been recorded with the 514-nm line of Ar + laser and observed three emission transitions of 4 F 3/2 → 4 I 13/2,11/2, 9/2 . The decay curve of the prominent lasing transition 4 F 3/2 → 4 I 11/2 has been measured both at 514-nm line of an Ar + laser and at 808 nm of a pulsed laser diode. A bright violet color upconversion emission has been measured from a yellow light ( 4 I 15/2 → 4 G 5/2 at 576 nm) of a xenon arc lamp.

Effect of vacuum dehydroxylation on the UV transparency and structure of metaphosphate glasses

Abstract UV cutoff of calcium metaphosphate glass shifts towards shorter wavelengths in the initial stage dehydroxylation in vacuum. IR reflection spectral study reveals that it is due to the combined effects of polymerization of the phosphate glass network and formation of small metaphosphate rings caused by the decrease in OH content. This is supported by the decrease of Q 2 sites (decrease in optical basicity) in the 31 P MAS NMR spectra. On further dehydroxylation, the shifting of UV cutoff towards longer wavelengths is attributed to the increase of Q 1 and Q 2 sites (increase in optical basicity) associated with the formation of nonbridging oxygens (NBOs). It is caused by the preferential loss of P 2 O 5 and simultaneous decrease of OH groups during exhaustive dehydroxylation for a prolonged period of melting in vacuum.

Effect of hydroxyl content on the physical properties of calcium metaphosphate glasses

The hydroxyl (OH) content of calcium metaphosphate glasses has been controlled in the range 50–800 ppm by melting calcium dihydrogen phosphate in air, under vacuum and with fluoride addition. Density, refractive index and glass transition temperature of the glasses increase with decrease in OH content while the coefficient of thermal expansion remains almost unchanged. With gradual decrease in OH, the UV cutoff initially shifts towards shorter and finally towards longer wavelengths. IR spectroscopic study shows that the OH groups exist exclusively in the hydrogen bonded states. Correlations of the glass properties with OH content have been explained in terms of structural rearrangement leading to the change in P-O bond length and O-P-O/P-O-P bond angles of the PO4 tetrahedral units of (PO3−)n chains. These changes are caused due to conversion of non-bridging oxygens (NBOs) of the H-bonded OH groups into bridging oxygens (BOs) during progress of dehydroxylation.

Infrared Spectroscopic Method for Estimating Physical Properties of Metaphosphate Glasses

Calcium metaphosphate glass has been dehydroxylated under vacuum and by calcium fluoride addition. The first overtone band of Vss(OPO) and/or combinational band of Vas(OPO) + vas(POP) at around 2170 cm−1 in the infrared (IR) spectra of the glasses, not originally observed in high hydroxyl (OH) containing glasses, has been found to develop gradually with the progress of dehydroxylation. The absorption coefficient of this band has been statistically correlated with density, refractive index and glass transition temperature of low OH containing glasses in the form of equations. Each of these equations has high correlation coefficient and low residual standard deviation. Thus, they have been employed for the estimation of physical properties of high OH containing glasses. The experimental values of the properties of these glasses present a good fit with the calculated mean values within 95% confidence interval. This fact confirms the validity and utility of this IR spectroscopic approach for the estimation of physical properties of calcium metaphosphate glasses.

Glass-based Sealants for Application in Planar Solid Oxide Fuel Cell Stack

One of the major challenges for the commercialization of planar anode-supported solid oxide fuel cell (SOFC) is the development of suitable high temperature sealant. The sealants must maintain gas-tightness between the anode and the cathode compartments to avoid intermixing of fuel and oxidant at high operating temperature (700°-800°C), be an electrical insulator and chemically non-reactive with the stack components. Due to the high commercial importance of such sealants, very few sealing compositions are available in the open literature and have to be developed by the individual SOFC developer. In this context the present article gives an overview on such sealant development with particular emphasis on glass-ceramics based sealants. Other than glass-based sealants, very recent approaches such as compressive and self healing seals also show some potential as SOFC sealants. The review of literature is followed by description of the progress of work related to development of glass based sealant at our Institute for application in the developed planar anode-supported SOFC stack. Under this developmental activity, a number of glass systems having SiO2 and B2O3 as glass formers and BaO, MgO and CaO as the alkaline earth metal oxide content have been investigated and thoroughly characterized. Based on the findings, some of these glasses have been used for making seals between different types of SOFC components, e.g. ferritic stainless steel (Crofer22APU) (metallic interconnect) and 8 mol% yttria-stabilized zirconia (YSZ) (electrolyte) under various combinations, viz. metal-metal, metal-ceramic and ceramic-ceramic. A novel concept of “bi-layered sealing” consisting of two different glass compositions having graded CTE values has been implemented for the first time during actual SOFC stack operation. The performance of the developed sealants has been evaluated in several working SOFC short stacks (up to 6-cell) for a maximum period of 20 days. It is observed that the open circuit voltage (OCV) of these stacks is very near to its theoretical value and remains same during the stack testing period and thus ensured the potentiality of the developed sealant for SOFC application.

Tailor-made BaO-CaO-Al2O3-SiO2-based Glass Sealant for Anode-supported Planar SOFC

In the present investigation, several glasses based on the BaO-CaO-Al2O3-SiO2 (BCAS) system have been investigated to see their applicability as IT-SOFC sealants. The glass transition temperature (Tg) of the developed glasses are within 600-6350C. The CTE value can be tailored between that of YSZ electrolyte and ferritic steel (FS) interconnect, by varying the amount of alkaline-earth metal content. The electrical resistivities of the developed glasses are quite high (rho 800C ~ 105 Omega-cm) and thus suitable for IT-SOFC application. Based on the crystallization behavior, a single glass having matching CTE with FS and a bi-layer glass having different CTE values within the range of 10×10-6 K-1 to 13×10-6 K-1 are found to be very effective for the FS-FS and FS-YSZ sealing.