Elastic Behavior of Borate Glasses Containing Lead and Bismuth Oxides

PbO and Bi 2 O 3 binary borate glasses with different compositions, (MO) X (B 2 O 3 ) 1−X (M = Pb, Bi), have been characterized and ultrasonic velocity as well as density is taken into account. In addition, the results have been compared with those of Ag, K, and Li oxide borate glasses from others.The ultrasonic velocities (both longitudinal and transverse) and density for (PbO) X (B 2 O 3 ) 1−X and (Bi 2 O 3 ) X (B 2 O 3 ) 1−X have beenmeasured accurately and elastic moduli as well as hardness and Poisson’s ratio was determined. It has been demonstrated that density and ultrasonic velocities are enhanced by increasing PbO and Bi 2 O 3 molar fraction with different values for each borate glass composition. However, the enhancement of ultrasonic velocities did not carry on continuously and after reaching a maximum point, they fell down dramatically. Both PbO and Bi 2 O 3 showed almost similar glass improvement in case of density, ultrasonic velocity, and elastic moduli.


Introduction
Recently, great interest for electrochemical device applications, such as microbatteries, gas sensors, and electrochromics displays, has increased the attraction to glasses with high ionic conductivity [1,2].In comparison with crystalline materials, glasses show many advantages, where these isotropic materials do not have grain boundaries and fabrication is easy with low cost [3].Generally, boron (B), silicon (Si), and phosphorous (P) oxides are the typical glass forming oxides where borate glasses have high potential for technological applications.Metal oxides such as Bi 2 O 3 and Li 2 O play an active role in semiconducting glasses [4,5].They can be added to glass former like B 2 O 3 to enhance their properties [6].For example, Bi 2 O 3 containing glasses are good precursors for the preparation of ceramic high   superconductors [7][8][9].Glasses containing PbO show high refractive indices with low crystallization tendency, as well as lower melting point and glass transition temperatures [10].Although they do not form glass on their own, they modify a vitreous network to form glass when they combined with a second glass forming oxide such as B 2 O 3 [11].Several studies have proved that a preferred host glass for the addition of the above oxides has been B 2 O 3 [12,13].B 2 O 3 is an absolute glass former and is a primary component of many large volume industrial glasses [14][15][16].
Ultrasonic examinations are considered in understanding the structural characteristics of glass network [17].The properties of glass such as elastic properties are dependent on the interatomic forces and potential in lattice structure.The ultrasonic wave velocities as well as density of glass can be taken into account to discover the elastic properties of glass network which are strictly related to the glass structure [18].Therefore, changes in the glass structure due to modifier doping can be directly noted.
Young's, bulk, shear, and longitudinal modulus can be obtained from the density of the solid and longitudinal wave transmission velocity through the solid [19].
Despite so many studies that have been done on borate glasses, the role of different oxides on borate glasses is not understood clearly.The purpose of this work is to compare the effect of different modifier oxides on ultrasonic velocities as well as density of borate glasses containing lead and bismuth oxides with those of other borate glass systems.In Table 1: The room temperature physical and elastic properties of binary borate lead (PB) and bismuth (BB) glasses' density (), molar volume (  ), atomic volume (  ), ultrasonic longitudinal (  ) and transverse (  ) velocities, longitudinal modulus (), shear modulus (), bulk modulus (), Young's modulus (), Poisson's ratio (), and microhardness ().PB = (PbO)

Sample Preparation.
Bismuth and lead borate glasses were successfully synthesized by conventional melt quenching.Precursors with purity of bismuth oxide powder (purity 99.975%), lead oxide (purity 99.8%), and boron oxide (purity 99.8%) are used in the preparation of the glass samples.After mixing of B 2 O 3 with oxides and drying the mixture in 400 ∘ C, the mixture was melted for 1 hour in furnace at 1000 ∘ C.After that, the melt was quenched into the preheated metal mould to obtain transparent glass cylinder with 12 mm diameter.In order to relieve the residual stress, the glass samples were annealed in 350 ∘ C for 1 h.Finally, the cylindrical samples were cut and polished with 1-2 cm height for ultrasonic measurements.

Measurements.
Archimedes method was taken into account to obtain density of the glasses as described elsewhere [16].For ultrasonic velocity measurement in glass sample MATEC MBS 8000 was used.All measurements were taken at 5 MHz frequency and at room temperature.Elastic modulus (longitudinal, shear, bulk, and Young's) as well as Poisson's ratio and microhardness of bismuth and lead borate glasses with different contents has been determined from the measured ultrasonic velocities and density using the standard relations [20] (density (), molar volume (  ), ultrasonic longitudinal (  ) and transverse (  ) velocities, longitudinal , (1)

Results and Discussions
The codes, composition, density, molar volume, and elastic modulus are given in Table 1 for PbO and Bi 2 O 3 borate glasses (PB and BB).To explore the changes in the structure of glasses, the density is a powerful tool where the structural softening/compactness, change in geometrical configuration, coordination number, cross-link density, and dimension of interstitial spaces of all borate glasses affected chemical composition [21].It can be seen from Figure 1 Figure 1: (a) Density () and (b) molar volume (  ) of binary borate glasses as a function of modifier oxide concentrations of Pb and Bi oxides with those of Ag, K, and Li [22,23].molar volume increase with increasing of Bi 2 O 3 content due to expansion of the glass network by bismuth (Table 2).Bi ions go into the glass network interstitially; therefore, some network bonds of B 2 O 3 are ruptured and ionic bonds between Bi ions and individually bonded oxygen atoms replace them.Hence, if one assumed that only consequence of Bi cations addition was to break down the network bonds, then an increase in the molar volume with Bi 2 O 3 content would be expected for the total vitreous range of the studied glass system.
Figure 3 shows the longitudinal and transverse velocities (  ) and (  ).Both   and   increase with addition of modifier oxides that alkali borate glasses (LB, KB) have higher improvement than others.The highest belongs to LBs in which LB4 has value of 6702 and 3990 m/s for   and   , respectively.According to relations 1 to 4 elastic moduli are directly related to ultrasonic velocities (Figure 4).
The variations of ultrasonic velocities and elastic moduli can be explained on the basis of structural consideration of borate glassy networks which depend on bond strength, packing density, coordination number, and cross-linking of units.In all present glass systems shown in Figure 3, their ultrasonic velocities are enhanced by increasing of modifier oxide contents.However, in higher content of oxides they are decreasing after reaching peak points where it is known as boron anomaly.These points could be described as optimum ) Figure 2: Changes in density () of binary borate glasses as a function of modifier oxide concentrations of Pb and Bi oxides with those of Ag, K, and Li molar masses () [22,23].
compositions to obtain the best network structural modifications.The ultrasonic velocities and elastic moduli increase up to ∼0.3 molar fractions in alkali borate oxides K 2 O and Li 2 O and then decrease.These optimum compositions are shown in Table 3, in addition to nonlinear functions ( =  2 ++) of ultrasonic velocities with respect to molar fraction.
Figure 5: Correlation between transverse (  ) and longitudinal (  ) ultrasonic velocities in binary borate glasses containing Pb and Bi oxides, as well as oxides of Ag, K, and Li.[22,23].
It also shows that longitudinal (  ) and transverse (  ) ultrasonic velocities increase and decrease in the same pattern, however with different rates (Figure 5).  and   in borate glasses with bismuth oxides increase with the same rate.However, in ABs and KBs borate glasses rising of   is almost twice   .
Modulus of elasticity can be a characteristic of glass since glass is also considered as an elastic substance [24,25].This modulus increases as the expansion at a certain applied stress reduces.That would have happened if the glass structure was rigid and therefore contained the fewest possible nonbridging oxygen.When an oxide is introduced to borate, the field strength of the cation determines the strength of the structure.By increasing the oxides content in the borate glass, the structure becomes more rigid, and then density increases as well; hence, the elastic moduli increase [17,26].It may also be noted that the rate of change in elastic moduli is more evident in longitudinal modulus () than in case of transverse or shear modulus ().This designates resistance to deformation which is probably due to presence of large number of covalent bonds.
The increase in Young's modulus values with addition of modifier oxides has contributed to increase in rigidity of borate glassy network.This may be due to the increasing of the modifiers [27] which can increase Coulomb contribution to the lattice energy.This mechanism was discussed for the role of Bi and Pb [28,29].Shear and Young's moduli have a straight connection with bond bending (  ) and bond stretching () force constant, respectively [30].So, the increase in  values with modifier oxides can be attributed to the increase in , and the increase of  with oxide concentrations can indicate the increase in   bulk and Young's moduli.Compositional variation of experimental values of Poisson's ratio is demonstrated in Figure 4. Poisson's ratio decreases in full range but faster in Bi-rich.The crosslink density of two, one, and zero is related to Poisson's ratio of 0.15, 0.3, and 0.4, respectively [31].Poisson's ratio starts from 0.27 in Pb-rich and decreases to 0.21 in Bi-rich limit which means that the cross-link density increases from nearly one to two.
As mentioned the introduction of alkali oxides (Li 2 O and K 2 O) enhanced ultrasonic velocities and elastic moduli of borate glasses very well.It can be explained that, in alkali borate glasses, conversion of the trigonal borons (BO 3/2 units) to tetrahedral borons (BO 4/2 ' units) is by the coordination of O 2− to two trigonal borons: Modified borate glass structures by adding alkali oxides are shown in Figure 6.As long as maximum of 50% of B 3 changes to B 4 , the creation of B 4 proceeds [12].Then, by further increasing of the modifier concentration, the concentration of B 4 decreases quickly.In diborate composition B 3 and B 4 concentrations are equivalent, and the corresponding mole fraction of the alkali oxide is 0.33.In a case of lithium borate glasses, the diborate structure does not seem to favor B 4 -B 4 connections [32].Structure in borate glasses with less than 0.33 alkali oxide molar fractions takes rise to open by the connectivity of the tetrahedral and the diffuse spread of the negative charge together.Nevertheless, for alkali oxide mole fractions more than 0.33, B 4 units collapse and form  Consequently, network collapses and volume is utilized.Therefore, expansion of network occurs which contributes to network rigidity reduction.By decreasing the atomic bond rigidity ultrasonic velocities and elastic moduli reduce.In addition, rapid reversals occur in the variation of refractive indices,   , thermal expansivity, and so forth, as a function of alkali composition [12].All of these variations are directly or indirectly related to energy density and the observed variations are often referred to as borate anomaly.
Other borate oxides, PB, BB, and AB, are showing almost similar behavior in terms of ultrasonic velocities and elastic moduli.Modification of borate glasses by PbO can be explained by two approaches.The first one is the assumptions that cation ions of oxides, such as Pb 2+ in PBs, enter interstitially as a result of the addition of oxides, such as PbO, into the borate glass networks.Therefore, some types of modification of B-O-B linkages, which already exists in the glass, into B-O-Pb bonds will occur.The conversion of these linkages results in an increase in the molar volume and a decrease in the packing density which expand the glass network.Therefore, the reduction in the rigidity due to the formation of PBOs will contribute to the decrease in the ultrasonic velocity.
The second approach is the suggestion by Higazy and Bridge [28], where the longitudinal strain in a bond is directly dependent on the bond stretching force constant.In the studied glasses, the longitudinal strain in the main chains (B-O-B linkages) is affected by the modifying role of PbO, as these oxides decrease the overall stretching force constant (F B-O ) and consequently increase the bond length of these linkages, so the longitudinal wave velocity will decrease.On the other hand, the shear strain changes with the bond bending force constant (  ).Thus, the decrease of the shear wave velocity indicates that the addition of the modifiers has no bending effect on the behavior of bond bending force constant; that is, the increase in the modifiers cations did not contribute to filling the network interstices.

Conclusion
New binary borate base glasses added with bismuth and lead oxides were successfully prepared and their structures were analyzed by ultrasonic waves.Three other borate glasses including lithium, potassium, and silver oxides from others were compared.It has been demonstrated that density and ultrasonic velocities are enhanced by increasing PbO and Bi 2 O 3 molar fraction with different values for each borate glass composition.However, the enhancement of ultrasonic velocities did not carry on continuously and after reaching a maximum point, they fell down dramatically.
Density of borate glasses is enhanced uniformly by increasing of modifier oxides content where bismuth, lead, and silver oxides glasses demonstrated larger value than potassium and lithium oxides due to their larger molar masses.
The ultrasonic velocities of borate glasses added with modifier oxides increased as a result of higher glass network rigidity and vibration.Additionally, while the contents of all modifier oxides enhanced, the elastic moduli and microhardness were improved.However, Poisson's ratio illustrates a slight change.
Improving in ultrasound velocities as well as elastic moduli is much higher for Li 2 O borate glasses due to their higher compactness as a result of lower molar volume.
Oxide borate glasses demonstrated declining in ultrasonic velocities and elastic moduli in higher contents of modifier oxides which occurred due to reduction of glass network rigidity.In alkali borate glasses such as Li 2 O and K 2 O borate glasses, B 4 networks collapse into B-O-B linkages.However, in other borate glasses B 4 turns into B-O-M in which both address the decreasing of glass network rigidity.
In general comparison, both PbO and Bi 2 O 3 showed almost similar glass improvement in case of density and ultrasonic velocity; nevertheless Li 2 O in the other study pointed up the best improvement behavior due to lower density, higher ultrasonic velocity, and elastic moduli enhancements.

Figure 3 :
Figure 3: (a) Transverse (  ) and (b) longitudinal (  ) sound velocities of binary borate glasses as a function of modifier oxide contents of Pb and Bi oxides with those of Ag, K, and Li [22, 23].

Figure 4 :
Figure 4: (a) Young's modulus and (b) Poisson's ratio of binary borate glasses as a function of modifier oxides concentration of Pb and Bi oxides with those of Ag, K, and Li [22, 23].