Compositions and properties of silicate glasses, as investigated by Wang et al.

Question

Compositions and properties of silicate glasses, as investigated by Wang et al. (2007), are shown in Tables I and II.

a. At 10 MHz, what polarization mechanisms are expected to play a role in contributing to the dielectric constant for these glasses?

b. Why does the glass made using Li+ (i.e., sample 11) exhibit the lowest dielectric constant?

c. Between samples 12 and 13 (i.e., samples containing sodium and potassium oxides), which sample has a higher total electronic polarizability and refractive index? Why?

d. Because the dielectric constant of samples 12 and 13 (at 10 MHz) is effectively the same, which sample and why has a higher ionic polarizability?

Table 1 Compositions of glasses (mol %) sio2 B203 Al203 Cao Li2O Na20 K2o 62 25 12 5.4 6.4- 62 25 12 5.4-6.4- 62 25 12 5.4-- sample number 12 13 6.4 Source: Wang, Z., Y. Hu, H. Lu, and F. Yu. 2008. J Non-Cryst Solids 354 (12):1128-32 Table Il Dielectric properties and resistivity of glasses Sample number Dielectric constant (at 10 MHz) Dielectric loss Resistivity (tan ) at 10 MHz (·cm) 5.58 6.03 6.05 4.97 x 10-2 4.5 x 10-2 4.41x 10-2 0.73 x1013 0.66 x 1013 1.17 x 1013 12 13 Source: Wang, Z., Y Hu, H. Lu and F. Yu. 2008. J Non-Cryst Solids 354(12):1128-32.

Explanation / Answer

a)

There is a reaction field due to the response of the medium to charges on the molecule and there is a local field due to polarization of the solvent in the applied field (10Mhz). This mechanism is important for relating the molecular polarizability to the bulk polarization. Here the role of the dielectric constant (also called the relative permittivity) as a factor that relates the polarization of an insulator to an applied electric field.

b)

Since r = 1 for vacuum and r > 1 for all dielectric media it is apparent that the local field is always larger than the applied field. Due to this simple consequence of the theory of dielectric polarization, we usually think of the dielectric constant as providing a screening of the applied field. Therefore, we might be inclined to think of a local field as smaller than the applied field. However, this naïve view ignores the role of the polarization of the dielectric itself. Inside the sphere we have carved out of the dielectric we observe the macroscopic (applied) field plus the field due to the polarization of the medium. The sum of these two contributions leads to a field that is always larger than the applied electric field. Based on this principles In glass making of Li+ and its dielectric constant properties.

c) d) The larger the dipole moment the greater the tendency of the solvent to respond to an applied field by reorientation of the microscopic dipoles. The dielectric constant as providing a screening of the applied field. molecular polarizability is frequency dependent. Electronic polarizability is present in all molecules and has a response time that is rapid (> 10 14 s -1 ). The high frequency response
can follow the undulations of electromagnetic radiation in the 10Mhz region and hence this response gives rise to refraction of light. No 12 has 6.03 and 13 has 6.05. so it has slightly more refractive index.

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