Study on ultrasound velocities and elastic properties of sodium diboro-vanadate glasses

Glasses with general formula xNa2O-15V2O5-(85-x)B2O3 where x is varied from 5 to 50 mol% in steps of 5 mol%, are fabricated using melt quenching technique. Ultrasound velocities, elastic moduli and Poisson’s ratio exhibit composition dependent trends. Both ultrasound velocities and elastic moduli increases with the addition of Na2O mol% whereas Poisson’s ratio decreases. The introduction of V2O5 results in the formation of diboro-vanadate units similar to that of a diborate. Formation of these diboro-vanadate units leads to a monotonic increase in both sound velocities and elastic moduli.


Introduction
Among oxide glasses, borate glass continue to attract scientific investigations because of their superior physical, optical and mechanical properties and find applications in the fiber optic communication systems, nonlinear optical devices and lasers [1][2][3]. In addition these glasses possess wide range of applications which include glasses for lightening, laboratory, cookware, medical, LCD screens and optical glasses [4,5]. Glasses containing large quantity of transition metal oxide are known to exhibit electronic conductivity. The study of semiconducting glasses containing transition metal oxide (TMO) is very important because of their applications in memory switching devices and electrochemical batteries [6][7][8]. Many glasses containing V 2 O 5 exhibit semiconducting properties which arises due to the conduction mechanism taking place through hopping of electrons between different oxidation states V 5+ and V 4+ of vanadium [9].
Elastic properties of glasses are very important because they provide deep insight to understand the structure of glasses as well as provide important information about interatomic forces, nature of bonding and vibrational frequencies. Glasses exhibit perfect elasticity and it has been reported by Makishima and Mackenzie [10] that the bulk modulus of glasses are strongly dependent on packing density and bond dissociation energies. Yamane and Sakaino [11] estimated the Young's modulus as a function of melting temperature of constituent oxides which depends on the strength of cohesive forces in the network. The variation in the network connectivity as a result of modification in the structure can be observed in elastic properties. The variation of elastic properties of glasses as function of composition can provide important information about the role of network formers and modifiers [12][13][14][15][16][17][18][19][20][21]. The measurement of elastic moduli and Poissons's ratio in vanadium containing glasses [12][13][14][15][16], borate [17][18][19][20] and phosphate [21] as a function of composition were studied. The elastic properties of alkali borate [22], alkali molybdo-phosphate and alkali phosphate glasses [23] were reported to reflect in ultrasound velocities. Special feature of the present work is to introduce V 2 O 5 content into sodium-borate glass network and to study the role of V 2 O 5 and Na 2 O on the structure of borate glass as a function of elastic properties by performing ultrasound measurements.

Experimental
Glasses with general formula xNa 2 O-15V 2 O 5 -(85-x)B 2 O 3 are fabricated using melt quenching technique, where x is varied from 5 to 50 mol% in steps of 5 mol%. Analar grade sodium carbonate, vanadium pentoxide and boric acid (Sd fine chemicals) were used. The chemicals were taken in exact proportions ground together to uniform mixture and melted inside a muffle furnace maintained at a temperature around 1200 0 C. The hot melt is quenched to obtain cylindrical glass pieces. The obtained glass pieces are annealed for 2 hours at 200 0 C in a muffle furnace to remove thermal strains. The faces of the cylindrical glasses are polished using silicon carbide to obtain flat and smooth surface. Ultrasound velocity measurements were performed at room temperature of 298 K and at frequency 12 MHz using quartz transducers (X-Cut and Y-Cut). Glasses are bonded to transducers using phenyl salicylate.

Ultrasound velocities
Ultrasound velocities, longitudinal and transverse (V l and V t ) of sodium-vanado-borate glasses show composition dependence. Figure.1 gives the variation of V L and V t with Na 2 O mol%. A monotonic increase in ultrasound velocities unlike a similar variations seen lithium borate lasses. Binary alkali-borate glasses were investigated by Kodama [24]. Kodama [24] report reveals a non-liner variation in sound velocities as a function of alkali concentration, Surprisingly the present investigation reveals that the sound velocities increase monotonically as a function of modifier oxide concentration (Na 2 O) for the full range of composition. This can be attributed to structural changes occur due to the addition of alkali oxide. Details are given in the latter sections.
The modulus of elasticity and Poisson's ratio have been calculated using the density and sound velocities. The values are shown in the  The liberated oxide ion (O 2-) reach the basic structural group which has higher electronegativity and can participate in modification. Since both [BO 3/2 ] and [VOO 3/2 ] has the same electronegativity (χ = 2.79), the oxide has an equal a prior probability to modify and lead to the formation of boro-vanadate groups.
The diboro-vanadate groups of type (I) and type (II) are always formed in pairs. Further the formation of these groups are in proposition to the composition of the component oxides in the formula unit. Even though the concentration of modifier oxide (Na 2 O) increased from 5 to 50 mol%, the reconversion of tetrahedral boron into triagonal boron units is not initiated but these two units continue to form in pairs. Therefore  Poisson's ratio is an important parameter which provide good details about the cross link density of the glass network. It is known that a high cross link density network has a Poisson's ratio in the range of 0.1 to 0.2, while a low cross-link density has a Poisson's ratio  Fig.3. It can be observed from the Table.1, that the Poisson's ratio decreases with addition of Na 2 O mol%, indicating increase in cross link density as a result of network dimensionality.

Conclusions
Glasses with general formula xNa 2 O-15V 2 O 5 -(85-x)B 2 O 3 are fabricated using melt quenching technique. Ultrasound velocities, elastic moduli and Poisson's ratio exhibit composition dependent trends. Both ultrasound velocities and elastic moduli increases with the concentration of Na 2 O whereas Poisson's ratio decreases. The introduction of V 2 O 5 results in the formation of diboro-vanadate units similar to that of a diborate. Formation such units leads to a monotonic increase in both sound velocities and elastic moduli. Another special feature of the investigated glass system is that it exhibited good homogeneous glass formation over a wide range of composition.