The chemistry of melting oxynitride phosphate glasses

Nitridation kinetics

At the usual nitridation temperatures the glass converts to the liquid state and the chemical reaction described above proceeds via the diffusion of ammonia within the melt. Thus, besides temperature and reaction time, the viscosity of the melt will determine how easy the nitrogen can substitute for oxygen in the structure. Nitridation at constant time showed that the incorporated nitrogen increases linearly with the reaction temperature (N/P = a + b⋅T), while, for a fixed temperature, the nitrogen content varies with the square root of time (N/P = c⋅√t). It is worth mentioning that higher temperatures, for the same reaction time, allows introducing higher nitrogen contents; however, all temperature dependent lines fall down onto a temperature of ca. 550 °C, below which no nitrogen is introduced as it was shown in alkali-lead metaphosphate glasses [13]. Decomposition of ammonia into H₂ and N₂ may start occurring at temperatures above 400 °C, although reaction with nitrogen or hydrogen/nitrogen mixtures does not produce nitridation of the glasses. Actually, the ammonia flow must always be kept above a minimum level within the furnaces to ensure that nitridation proceeds homogeneously. At 550 °C the alkali-lead metaphosphate glasses presented a viscosity of ca. 0.2 Pa.s, in Log units, that may be determine that the nitridation reaction can only take place in melts with lower viscosities. On the other hand, nitridation at higher temperatures is limited by the fact that above 800 °C phosphorus can be reduced to phosphine, giving rise to brownish colored glasses and even crystallizations within the glasses. So the temperature window for the ammonolysis of phosphate melts is relatively small, of about 200 °C, and the composition of the glasses must be chosen with care in order to facilitate their nitridation at those temperatures. Another relevant fact that shows how important the melt viscosity is for the nitridation is the one that the introduced nitrogen content is inversely proportional to the initial melt viscosity. This was very well shown in the nitrogen content introduced into alkali-lead metaphosphate glasses in which the viscosity was previously determined [13]. Furthermore, it is worth pointing out that for the same viscosity of the phosphate melt, the nitrogen content will also be determined by the composition throughout the ionic field strength of the modifier elements.

The nitridation reaction in phosphate melts was described assuming that both BO and NBO type oxygens of the network would be substituted by nitrogen. Thus, tri-coordinated nitrogen atoms (N _t ) appear after substitution of BO (N _t  = 3/2BO) while di-coordinated nitrogen (N _d ) after substitution of both BO and NBO (N _d  = NBO + 1/2BO) [14]. These two substitution rules would progressively give rise to an oxynitride network where the PO₃N groups are formed first when introducing nitrogen into the original PO₄ units; then, PO₂N₂ groups would appear as a consequence on substituting nitrogen for oxygen in the PO₃N formed before, giving rise to PO₂N₂ and new PO₃N [15]. The two reactions can be represented by the following equations:

The two reactions would respectively have their constant of reaction, k ₁ and k ₂, and were demonstrated to form a system of two pseudo-first order reactions with similar activation energies of ca. 150 kJ.mol⁻¹ in nitrided Li_0.25Na_0.25Pb_0.25PO_3−3x/2N _x glasses [16]. In these type of systems, there is a time, t _max, at which there is a maximum of formation of PO₃N groups from the PO₄ ones. Above t _max, PO₂N₂ start forming in a bigger amount from the joints PO₄–PO₃N if both reaction constants are similar. The reaction constants were determined in Li_0.25Na_0.25Pb_0.25PO_3−3x/2N _x glasses thanks to a study by Nuclear Magnetic Resonance spectroscopy of the contents of chemical groupings in the glasses obtained at temperatures between 600 and 700 °C and times up to 30 h. The results of speciation by NMR are similar in alkali or alkali-lead metaphosphates and so it is expected that reaction constants and activation energies are similar in all systems. However, it has been recently found that nitridation of LiPO₃ and NaPO₃ glasses gives rise to quite different results regarding the chemical speciation. Figs. 1 and 2 show the ³¹P MAS NMR spectra and % of P(O,N)₄ groups against the N/P ratio in LiPO_3−3x/2N _x and NaPO_3−3x/2N _x , respectively. NMR spectra show the three characteristic resonances attributed to the PO₄, PO₃N and PO₂N₂ groups of nitrided glasses, which relative content to total phosphorus can be represented against the nitrogen content; however, meanwhile the content of PO₄ groups decreases with introduced nitrogen similarly in both systems, the increase of PO₃N and PO₂N₂ presents a distinct behavior. The one in NaPO_3−3x/2N _x glasses (Fig. 2) resembles the one previously observed in Li_0.5Na_0.5PO_3−3x/2N _x as well as in Li_0.25Na_0.25Pb_0.25PO_3−3x/2N _x glasses but it can be clearly seen that the content of PO₂N₂ in LiPO_3−3x/2N _x glasses is always higher than the one of PO₃N. Despite no conclusive explanation has been yet given to this fact, it looks very likely that the value of the k ₂ reaction constant of reaction (2) adopts much higher values than k ₁ in the LiPO_3−3x/2N _x , so that once PO₃N groups are formed they are immediately converted to PO₂N₂ upon further nitrogen for oxygen substitution. A subtle structural interpretation must be at the origin of the behavior in the contents of nitrided groups with different alkali modifier. It has also been speculated that this would have occurred because of a kind of phase separation or segregation of nitrided regions within the phosphate glass matrix. However, no definitive answer has yet been obtained even after high temperature ³¹P MAS NMR experiments in both systems were performed that showed similar mixing of P(O,N)₄ groups with PO₄ ones independently of the modifier element [17].

Fig. 1:

³¹P MAS NMR spectra of LiPO_3−3x/2N _x glasses and content of P(O,N)₄ groups of oxynitride glasses against the N/P ratio.

Fig. 2:

³¹P MAS NMR spectra [18] of NaPO_3−3x/2N _x glasses and content of P(O,N)₄ groups of oxynitride glasses against the N/P ratio.

Influence of glass composition

The key factor that will determine the feasibility of nitridation is the viscosity of the melt, so the composition of the glass will need to be chosen to allow having very low viscosities at temperatures below 800 °C. Alkali phosphate glasses can be nitrided without problem, provided that they do not crystallize upon cooling, such it can happen in KPO₃ containing melts. Alkaline-earth phosphate would not be nitrided since their higher viscosity and so a combination of alkali and alkaline-earth element will be required. Other metals that can be incorporated into the composition and allow nitridation can be Zn, Pb and Sn. The last three are known to lower the viscosity of their respective melts and given that they are combined with alkalis, can produce compositions easily nitrided. The case of Sn is special, because in oxidative atmospheres Sn²⁺ can be oxidize to Sn⁴⁺ and precipitate as SnP₂O₇ crystals which are very refractory, producing inhomogeneous glasses. However, under the reducing atmosphere of the nitridation (N₂ + NH₃), Sn⁴⁺ is completely reduced to Sn²⁺, which does not crystallize in the form of phosphate, and can be completely dissolved within the glass network. This is related to the fact that the nitridation of phosphate glasses incorporating elements, notably transition metal elements, whose reduction potential is positive, give rise to precipitation of the elemental metals after the nitridation. However, those elements with negative reduction potentials, such as alkali and alkaline-earths, remain in their oxidize states after the nitridation. In the case of Sn, the reaction Sn⁴⁺ → Sn²⁺ has an E ⁰ = 0.15 V, while the E ⁰ for the reaction Sn⁴⁺ → Sn²⁺ stands for −0.14 V. The reducing character of oxynitride phosphate glasses was very well studied by Le Sauze et al. in Ag and Cu containing phosphate glasses [19]. Other transition metals like La or Ti could also be in the composition of the phosphate parent glass; however, even small contents of their oxides may provoke a quite big increase of the melt viscosity, thus limiting the possibilities of nitridation at relatively low temperatures.

Secondary glass former elements such as B or Si could be added to the glass composition although the nitrogen content that can be reached will be reduced with their content [20]. In general, the higher viscosities of B₂O₃ and SiO₂ containing phosphate glasses will diminish the introduction of nitrogen and for high boron contents, phase separation and crystallization of reduced boron in the form of BN can be produced. On the other hand, phosphate glasses containing other anionic species such as SO₄ ²⁻, S²⁻ and F- have been nitrided with very different results. Sulpho-phosphate glasses, or SO₃ containing phosphate glasses, were nitrided with unfortunate results because SO₄ ²⁻ species are reduced to volatile species and the drastic composition change of the glasses results in crystallized samples. On the other hand, S²⁻ containing glasses were also tested resulting also in inhomogeneous samples. However, introduction of S²⁻ anions bonded to phosphorus is possible if done on a previously nitrided glass. This was demonstrated by remelting a nitrided lithium metaphosphate glass with Li₂S under an Ar atmosphere [21]. The analysis of the composition and structure of the glasses revealed that S atoms are bonded to two neighboring phosphorus and are present simultaneously with the PO₃N and PO₂N₂ groups of the oxynitride network. The preparation of this kind of glasses was highlighted because one may see the effects of adding nitride as well as sulfide anions. On the one hand, nitrogen increases the chemical and mechanical stability of the glasses and, as seen above, in nitridation of lithium phosphate glasses produces an increase of the lithium ionic conductivity which is particularly relevant for the application as solid electrolytes. But, on the other hand, the fact of adding also S²⁻, which is a more polarizable anion, results in solids with even higher ionic conductivity than with N³⁻ alone, which has open up a new field of research in solid electrolytes for battery applications. LiF containing phosphate glasses are known to have much smaller glass transition temperatures, lower viscosity though poor chemical durability. The nitridation of fluoride containing phosphates was also investigated using a similar methodology than the one used in the processing of sulfide containing oxynitrides. Nitrided lithium phosphate glasses were obtained by melt and quenching followed by ammonolysis and the oxynitride glasses were then re-melted under a N₂ with increasing contents of LiF [22]. The results showed that the glass transition temperatures of the oxy-fluoro-nitride lithium phosphate glasses are higher than those of the fluoro-phosphate glasses, due to the increased cross-link density of the glass network through the P–N bonds. Furthermore, thanks to the effect of nitrogen on the melt viscosity, it was possible to incorporate higher contents of LiF within the oxynitride glasses without spontaneous crystallization upon cooling. This permitted to have glasses with even higher Li contents and thus higher ionic conductivities at the same time that higher chemical durability [23].

Homogeneity of oxynitride glasses

Because the main applications for which oxynitride glasses have been researched were focused on sealing glasses or solid electrolytes, the transparency and optical quality of oxynitride glasses has not been a key issue neither in phosphate glasses nor in silicate glasses in general. Ali et al. recently published a work analyzing the main issues of the preparation of transparent oxynitride silicate glasses [24], where they concluded that the advantages in the properties of oxynitride silicate glasses were not that higher considering the costs of the preparation and the much lower quality of the nitrogen containing glasses. Silicate glasses in particular have a main drawback related to the much higher temperatures required to run the nitridation via ammonolysis. Furthermore, the strong reducing atmosphere produces very frequently appearance of impurities in the form of elemental silicon or silicides that deteriorate much the optical quality of the final glasses.

Phosphate glasses, however, when nitrided at low temperatures do not present parallel reduction reactions and can be prepared without impurities. The major issue related to their preparation is the presence of bubbles that increases with the content of nitrogen due to the much higher viscosity of the nitrided melts than that of the non-nitrided phosphate one. Oxynitride phosphate glasses with relatively low nitrogen contents, typically below 4 wt%, can thus be prepared with very few defects and without important loss of their transparency. This last has been studied recently in Nd₂O₃ doped phosphate glasses, which can be nitrided and obtained with relatively good optical properties [25]. However, and because of the release of water as a product of the ammonolysis reaction, the oxynitride glasses will content some residual water that will affect negatively to the Nd luminescence as the nitrogen content in increased due to non-radiative quenching by OH ions in the glass. Fig. 3 shows the example of four nitrided Nd phosphate glasses with increased nitrogen contents from left to right. Despite no bubbles are appreciated, it is clear that stria are formed within the bulk glasses due mostly to the presence of water. In their study, Muñoz et al. did also observe that a further remelting of an oxynitride glass under N₂ flow for 3 h conducts to a new oxynitride glass with an increased transparency at ca. 3000 cm⁻¹ as observed by FTIR spectroscopy. This was attributed to a partial release of water from the melt that consequently carried out a small decrease of the nitrogen content. However, the good point is that the life time of neodymium luminescence was also increased. The same dehydroxylation procedure was later investigated in Nd₂O₃ doped phosphate glasses with very good results of their optical properties [26] and used to prepare an oxide glass to be subsequently nitrided [27]. The progressive nitridation of a homogeneous and water free phosphate glass produces the appearance of stria even for very low nitrogen contents, thus limiting their applicability as optical media for which differences of refractive index must be kept as low as possible. Despite the difficulties in the preparation of homogenous oxynitride phosphate glasses with adequate optical quality, several other investigations have been carried out in this topic, such as on the luminescence properties of ZnO containing phosphates that were prepared by common ammonolysis [28], or on that of Eu₂O₃-doped glasses that have been prepared by adding AlN to the phosphate batches [29].

Fig. 3:

Nitrided Nd-doped phosphate glasses with increased nitrogen contents from left to right.