The influence of dissolved water on the internal friction of lithium metaphosphate glasses containing 1% potassium metaphosphate

A series of lithium metaphosphate glasses, containing different amounts of water, was prepared. The water content was determined from the weight-loss of specimens during heating in vacuum.It is found that dissolved water influences the internal friction and dielectric losses, in the same way as additions of dissimilar alkali ions do. With respect to the physical properties under discussions, one can consider water as an ‘alkali-oxide’.The intermediate temperature peak is increased in magnitude and shifted to lower temperatures with increasing water content. Extrapolation of the present results to the water-free composition shows that the intermediate temperature peak will be absent in the water-free glass.     

Internal friction of mixed alkali metaphosphate glasses: (II). Discussion

The internal friction and dielectric losses of mixed LiNa, LiK, LiCs, LiAg, NaK, NaCs and NaAg metaphosphate glasses are interpreted on the basis of explanation proposed for the mixed alkali effect. It was found that the magnitude of the mixed peak correlates better with size differences than with mass differences. The intermediate temperature peak is treated as a mixed proton-alkali peak. The large mechanical loss peak, appearing when the alkalis are mixed, was attributed to a coupled movement of dissimilar alkali ions and an explanation of the nature of this coupling is proposed. From this model it is predicted that mixed peaks can occur in any electrically insulating material containing dissimilar charge carriers.     

Internal friction of mixed alkali metaphosphate glasses: (I). Results

The internal friction of LiNa, LiK, LiCs, LiAg, NaCs and NaAg metaphosphate glasses was measured at 0.5 Hz and 2 kHz. The dielectric losses were also measured from 40 to 160°C, at frequencies of 300, 3 000 and 30 000 Hz. The densities of the glasses were determined and the molar volume of oxygen was calculated. In general, the mixed alkali behaviour of metaphosphate glasses is very similar to the mixed alkali behaviour of silicate glasses. Silver behaves in this respect like an alkali ion with approximately the same size as a sodium ion.     

Internal friction and dielectric losses of mixed alkalialkaline-earth metaphosphate glasses

Several series of glasses with the general formula xR₂O · (1 ? x)R′ O · P₂O₅ were prepared and the dielectric loss and internal friction data were taken. The dielectric losses of alkali metaphosphate glasses are greatly reduced by the substitution of alkaline-earth oxide for alkali oxide. Some of the investigated series show a weak minimum in the dielectric loss.The internal friction measurements reveal a high temperature peak, in addition to the single alkali peak and the intermediate temperature peak commonly observed in phosphate glasses. The origin of this high temperature peak is discussed.     

Relaxation processes in mixed alkali NaK methaphosphate glasses

The internal friction and the dielectric losses of NaK metaphosphate glasses have been investigated. A single alkali and a mixed alkali internal friction peak were observed. The single alkali peak shifted to higher temperatures with increasing concentration of the second alkali and eventually disappeared. The activation energy of this peak increased with the alkali mixing ratio. As in silicate and borate glasses, the single alkali peak correlates closely with the dielectric loss. The mixed peak showed a dramatic increase in size with the addition of the second alkali, but the activation energy was practically independent of the alkali mixing ratio. The large reduction in height of the mixed peak, observed on annealing, is discussed together with the influence of water.     

Anelastic spectroscopy in potassium aluminum metaphosphate glasses

Anelastic spectroscopy (internal friction and the dynamic modulus) was measured by means of a torsion pendulum at 3–12 Hz, in the range of 100–300 K, for a KAP metaphosphate glass. Two thermally activated internal friction peaks appeared at ∼190 and ∼250 K. These peaks were attributed to the behavior of potassium ions (high temperature) and to hydrogen (low temperature). Dynamic modulus showed a gradual decrease with increasing temperature in the range studied for all compositions.     

Mixed alkali glasses — Their properties and uses

The mixed alkali effect in glass refers to the large, orders of magnitude, changes in many properties when a second alkali oxide is added. Properties most affected are those associated with alkali ion movement such as electrical conductivity and loss, alkali diffusion, internal friction, viscosity and chemical durability. The compositional dependence of these properties is briefly reviewed and examples are given of the relevance of the mixed alkali effect to the manufacture of commercial glasses. Also reviewed are various theories for this scientifically interesting effect.     

Internal friction in three component metaphosphate glasses

The internal friction of three component metaphosphate glasses in the quasi-binary system MO·P₂O₅-M′O·P₂O₅ was measured as a function of temperature by a free torsional vibration method. The high-temperature peak of these glasses which include the various combinations of the modifying ions M and M′ where M or M′Ca, Sr and Ba appeared unanimously at ≈160°C. The peak height seemed to be dominated by a modifying ion of larger radius. The ionic radius is in the order Ba²⁺ > Sr²⁺ > Ca²⁺. For the glasses containing the modifying ions, M  Ca, Sr, Ba and M′  Mg, Zn, the internal friction appeared to be governed strongly by the ions which have a lower coordination number and a larger ionic radius. The MgO·P₂O₅ZnO·P₂O₅ glass exhibited a completely different behavior compared with the parent glasses MgO·P₂O₅ and ZnO·P₂O₅.     

Internal friction and dielectric losses of mixed alkali borate glasses

The internal friction and dielectric losses of NaK, LiNa, LiCs, LiK, NaCs, KCs, AgLi, AgNa, AgK and AgCs borate glasses were measured as functions of the temperature at various frequencies. In general, the behavior of mixed alkali borate glasses is very similar to the behavior of the comparable phosphate and silicate glasses. The magnitude of the mixed alkali peak was found to vary systematically with the size of the involved alkali ions. The silver-containing glasses also show the mixed alkali effect. The borate glasses are briefly compared with the silicate and the phosphate glasses and their behavior is found to be in agreement with the recent proposal that the mixed alkali peak is caused by an electro-mechanical cross effect.     

The influence of the crystallization process on the internal friction of some oxide glasses

The influence of phase separation and crystallization on the internal friction of some oxide glasses is reviewed and discussed. In alkali-containing glasses, the internal friction peak caused by stress-induced diffusion of alkali ions decreases in magnitude and shifts to higher temperature slightly due to phase separation. And in alkali-free glasses phase separation only exerts a minor decrease upon the background of internal friction curves, whereas crystallization influences the internal friction of these glasses more strongly. Because of crystallization, in alkali-containing glasses alkali ions might diffuse in a residual glass phase and a crystal phase, respectively. This might cause corresponding internal friction peaks. And in alkali-free glasses, no evident internal friction peak is observed. However, the author found a high and wide internal friction peak at about 100°C in the crystallized MgO·Al₂O₃·SiO₂·TiO₂ and ZnO·Al₂O₃·SiO₂·ZrO₂ glasses. The peak occurring in the two glasses studied is probably connected with glass crystallization and crystallized crystals.     

Internal friction of NaPO3 glasses containing water

The internal friction of sodium metaphosphate glasses containing from 0.016 to 0.330 wt% water has been investigated. Weight loss and infrared absorption measurements were used to determine the water content. Of the two internal friction peaks observed between ?100°C and ～250°C, the second peak occurring above room temperature has a pronounced dependence upon the water content; increasing water content causing the activation energy to decrease as the peak increased in size. A mechanism consisting of the cooperative motion of sodium ions and protons has been proposed for this peak. It is concluded that the second peak in the NaPO₃ glasses and the similar peak in alkali silicate glasses is not associated with the movement of the non-bridging oxygen ions.     

Multichannel conduction in alkali silicate glasses

The qualitative change of the electrical conduction of alkali silicate glasses with alkali mixing is discussed in connection with dielectric and mechanical relaxation. The observed conduction is understood as the additive effect of two kinds of independent diffusion processes — I and II. In single alkali glass, diffusion process-I is always observed as the dominant conduction process. However, when one alkali is progressively substituted by another, the dominant conduction process is altered from diffusion process-I to II. In the mixed alkali glass containing equal amounts of two alkali ions, diffusion process-I disappears, so that only diffusion process-II is observed. Consequently, it becomes evident that the mixed alkali effect is not only the successive change of conductivity, but the alternation of the dominant conduction process.     

Internal friction of alkaline-earth-iron-metaphosphate glasses

The internal frictions of Mg, Ca, Ba iron-metaphosphate glasses were studied from 120 to 700 K using the low-frequency torsion pendulum technique. The internal friction as a function of temperature revealed two distinct peaks. The low-temperature peak was absent in glass without iron oxide. Its activation energy compared favourably with that for dielectric loss and dc conductivity in similar glasses. This suggests the same mechanism for all three processes, probably connected with electron transfer between iron ions of different valence.     

Effect of aluminum ions on intrinsic sub-critical crack growth in metaphosphate glasses

Sub-critical crack growth in various kinds of metaphosphate glasses was investigated by using DCDC (Double Cleavage Drilled Compression) technique. The crack growth measurements were only being made in Region III, or in an inert environment. In order to evaluate intrinsic crack growth behaviors in Region III, crack propagation tests were performed in dehydrated heptane, and the crack velocity, v, was plotted as a function of the stress intensity factor, K_I. Fracture toughness of glass was also estimated from a stress intensity factor at a given crack velocity. For binary metaphosphate glasses (50MO · 50P₂O₅, M = Zn, Mg, Ca, Ba), fracture toughness increases in the order of Mg > Ca > Zn > Ba. However, the slope of K_I–v curve is almost unchanged. In the case of aluminum containing metaphosphate glasses, with increasing aluminum content, fracture toughness increases and the slope of K_I–v curve becomes smaller, regardless of the type of divalent cations in glass. It is concluded that an addition of aluminum ions into metaphosphate glass results in both high toughness and easy fatigue. In addition, the structural role of aluminum ions on the intrinsic sub-critical crack growth is discussed in terms of the models of atomistic bond rupture.     

Dielectric relaxation in vitreous metaphosphates at medium temperatures

Dielectric relaxation experiments were carried out on mixed alkali metaphosphate glasses between room temperature and about 300°C (f = 10³ s^?1). The observed relaxations appeared to be connected with the movement of the alkali ions from site to site. By using stainless steel electrodes it was possible to reveal a relaxation peak free from conduction losses and due only to the migration from site to site of the cations. When two cations of dissimilar sizes are simultaneously present a mixed alkali effect was observed, i.e. an unproportionally large reduction in the alkali ion mobilities. It is shown that this effect can be at least partly explained by considering the changes in the potential energy states upon mixing dissimilar cations due to polarizing forces and coulombic interactions.     

Mixed alkali glass spectra and structure

The far infrared and Raman spectra of several series of mixed alkali metaphosphate glasses have been investigated as a function of the mole fraction x of the network-modifying ionic oxides in xM₂O(1?x)M₂′O · P₂O₅. The frequencies of the cation-motion bands in the far infrared spectra, which correspond to cationsite vibrations, do not shift with x, indicating that the vibrationally significant local geometry and forces associated with a particular cation are unaffected by the introduction of the second cation into the glass structure. Each Raman-active band due to vibrations of the metaphosphate network occurs at a different frequency for each pure glass (x = 0 or 1), but for mixed alkali glasses only one band occurs for each type of mode and it varies linearly with x. This indicates that the cations in these mixed alkali glasses are homogeneously distributed, there is no significant molecular-level domain formation and the phosphate chains are associated with an averaged cation environment whose effect on the chain modes varies with x. A simple vibrational model is presented which shows that the cation-dependent shifts are due to small changes in network bond angles and variation of the cationsite forces.     

Structural properties of lithium metaphosphate glasses by ab initio molecular electronic structure calculations

Following a theoretical determination of cluster-models for lithium metaphosphate glass, a theoretical method for the prediction of structural data such as radial and bond distribution functions is presented. These are calculated and compared to experimental data for this particular glass. Useful conclusions are drawn regarding the general use of molecular electronic structure methods for the determination of the structure of glasses.     

The mixed alkali effect in the Na2OCs2OSiO2 glasses

As an approach to the mixed alkali effect in glass, the self-diffusion coefficients of sodium and cesium ions in Na₂OCs₂OSiO₂ glasses were measured at temperatures 350–550°C. Electrical conductivity of the glasses and the transport number for sodium ions were also measured. The substitution of the alkali ions in the glass by different alkali ions caused the mobility of each alkali ion to decrease pronouncedly and the activation energy for migration to increase rapidly. The increase of activation energy was attributed to an increase in alkali-oxygen bond strength resulting from the presence of two kinds of alkali ions. This is related to the expectation that the activity of the alkali ions decreases when two alkali ions are mixed.     

Activation volumes and site relaxation in mixed alkali glasses

Evidence is presented for site relaxations occurring in mixed alkali (cation) glasses based on activation volumes, V_A(σ) = −RT[d ln σ/dp]_T, which are determined for sodium aluminoborate glasses of varying Na₂O content, and for mixed alkali glasses where Na⁺ is partially replaced by Li⁺, K⁺ or Cs⁺ ions. In accordance with the ‘updated’ dynamic structure model, activation volumes are identified here with local expansions that accompany the opening up of C′ sites to admit incoming ions. ‘Anomalous’ increases in activation volume in mixed cation glasses correlate with the size of minority (guest) cations. This anomaly is interpreted in terms of a ‘leader follower’ mechanism that involves dynamic coupling between the faster (majority) and slower (minority) cations. Because of mismatch effects in mixed cation glasses this coupling requires the opening up of additional cation sites by the slower follower cations. The resulting disturbances in the glass network are responsible for many characteristic features of the mixed alkali effect, including the appearance of high temperature internal friction peaks and observed minima in glass transition temperatures and melt viscosities.     

Defect model for the mixed mobile ion effect

This paper presents a new defect model for the mixed mobile ion effect. The essential physical concept involved is that simultaneous migration of two unlike mobile ions in mixed ionic glass is accompanied by expansion or contraction of the guest-occupied sites with distortion of surrounding glass matrix; in many cases, an intensity of the local stresses in glass matrix surrounding ionic sites occupied by foreign ions is much greater than, or at least comparable to the glass network binding energy. Hence, when the stress exceeds the breaking threshold, relaxation occurs almost immediately via the rupture of the bonds in the nearest glass matrix with generation of pairs of intrinsic structural defects. The specificity of the mechanism of defect generation leads to the clustering of negatively charged defects, so that rearranged sites act as high energy anion traps in glass matrix. This results in the immobilization of almost all minority mobile species and part of majority mobile species, so mixed mobile ion glass behaves as single mobile ion glass of much lower concentration of charge carriers. Generation of defects leads also to the depolymerization of glass network, which in turn results in the reduction of the glass viscosity and T_g as well as in the compaction of glass structure (thermometer effect). In the spectra of mechanical losses of mixed alkali glasses it reflects as a shift of the maximum in mechanical losses corresponding to the glass transition to lower temperatures, and the dramatic increase of the maximum corresponding to the movement of non-bridging oxygens (so-called mixed alkali peak). The magnitude of the mixed mobile ion effect is defined by the size mismatch of unlike mobile ions, their total and relative concentrations, the binding energy of the glass-forming network, and temperature. Although the proposed model is based upon the exploration of alkali silicate glass-forming system, the approach developed here can be easily adopted to other mixed ionic systems such as crystalline and even liquid ionic conductors.