Oxidation behaviour of Li–Na–Pb–P–O–N oxynitride phosphate glasses

https://doi.org/10.1016/j.jeurceramsoc.2005.02.019Get rights and content

Abstract

The oxidation behaviour of different oxynitride phosphate glasses, elaborated by thermal ammonolysis of oxide base glasses with composition (25  x/2)Li2O·(25  x/2)Na2xPbO·50P2O5 (0 < x < 50), has been studied in air by thermogravimetric analysis and differential scanning calorimetry. The oxidation of the nitrided powders proceeds first from a surface oxidation, followed by a bulk oxidation controlled by diffusion. As the nitrogen content increases, the sintering becomes slower and the surface oxidation is greater. The second step, which depends on the particular composition, takes place once the glass viscosity has decreased enough to allow oxygen diffusion inside the glass particles.

Introduction

Phosphate glasses have specific properties such as low glass transition and dilatometric softening temperatures, a high thermal expansion coefficient or UV transparency. These properties make them interesting for applications as low-temperature sealing materials,1, 2, 3, 4, 5, 6 vitrification of nuclear wastes,7, 8 as well as laser host matrices when doped with rare-earth elements.9, 10, 11, 12 However, their low chemical durability is a drawback which can limit their practical use.

Partial substitution of nitrogen for oxygen is one of the most effective ways to improve the chemical resistance of phosphate glasses. Since the first nitridation experiments reported by Marchand13 and Wilder et al.14 as early as 1983, many compositions have been studied from both fundamental and application viewpoints.

The higher bonding density generated by nitrogen rather than oxygen atoms along with the greater covalent character of Psingle bondN bonds compared to Psingle bondO bonds result in a greater toughness of the glass network with respect to corresponding oxide glasses. The consequence is that nitrogen incorporation produces an increase in the glass density, glass transition and softening temperatures, viscosity, hardness and mechanical properties, refractive index, electrical resistivity, as well as in the chemical durability.15, 16, 17, 18, 19, 20, 21, 22 In addition, nitrided glasses are more stable towards crystallization than corresponding oxide glasses.

Oxynitride phosphate glasses are commonly prepared by thermal ammonolysis of parent oxide glasses at temperatures ranging from 600 to 800 °C. Nitrogen atoms substitute for both bridging and non-bridging oxygens of the PO4 tetrahedra network. Nuclear magnetic resonance has evidenced formation of two new structural units, the PO3N and PO2N2 tetrahedra,23, 24, 25, 26 in which the nitrogen atoms are coordinated to two phosphorous, single bondNdouble bond (Nd), or to three phosphorous, single bondN2 bonds on the righthand side (Nt), as shown by X-ray photoelectron spectroscopy.26, 27

The presence of glassy phases in the grain boundaries of silicon nitride-based ceramics has led to study the formation conditions of such glass compositions and their properties as a function of the nitrogen content. At the same time, owing to a context of high temperature applications, systematic studies of the oxidation resistance of these SiAlON glasses have been carried out.28, 29

On the other hand, even though the structure and properties of oxynitride phosphate glasses have been widely studied, no references concern their oxidation behaviour. The application of oxynitride phosphate glasses as sealing materials requires the stability of the glass during the sealing process. The present work constitutes a first study of the behaviour of such glasses in an oxygenated environment. The oxidation behaviour in air of Li–Na–Pb–P–O–N oxynitride glasses has been studied by thermo gravimetric analysis (TG) and differential scanning calorimetry (DSC). The influence of the nitrogen content, the composition of the oxide base glass, and the particle size of the powdered glass samples have been particularly investigated.

Section snippets

Experimental

Metaphosphate glass compositions (25  x/2)Li2O·(25  x/2)Na2xPbO·50P2O5 (0 < x < 50), in mol%, were prepared from stoichiometric amounts of reagent grade Li2CO3, Na2CO3, Pb3O4 and H3PO4 (85 wt.%, d = 1.71 g cm−3) in a gas furnace. The batches were first calcined up to 450 °C in porcelain crucibles for 1 week, and then melted at 1100 °C for 1 h. The melts were poured in air over brass plates, after that the glasses were annealed for 30 min in an electrical furnace slightly above their glass transition

Results and discussion

Table 1 gathers the main thermal properties of some of the oxide and oxynitride glass compositions considered in this study, which are crucial information for their application as sealing glasses: the glass transition and dilatometric softening temperatures, and the thermal expansion coefficient. The maximum temperature to be used during the sealing process can be estimated reasonably as not higher than Ts + 100 °C. So, using oxynitride compositions, the temperature would not reach a limit of 500 

Conclusions

This study proves that oxynitride phosphate glasses (illustrated here with alkali lead glasses) are promising sealing materials, as low-melting materials with high thermal expansion coefficients, even if used in an oxygenated atmosphere.

From the recorded thermal curves, the other following conclusions can be given. Oxidation of oxynitride glass powders proceeds, in a first step, through a surface oxidation and, in a second one, by the bulk glass oxidation which is controlled by diffusion. This

Acknowledgements

This work has been sponsored by the CICYT of Spain through the project MAT (2003-05902-C02-01) and the Integrated Action CSIC-CNRS (HF2001-124).

References (31)

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