Dielectric, thermal, optical, and degradation characteristics of MgO/ZnO-containing phosphate glasses
Investigation was carried out on binary (MgO)xCP20S)I-x with x = 20, 25, 30, 35, 40, 45 and 50 mol % and ternary senes (ZnO)30(MgO)xCP20s)7o-x, and 20 mol % in order to determine the role of zinc and magnesium ions in phosphate glasses. All the samples were prepared by traditional melt quenching...
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| Format: | Thesis |
| Language: | English |
| Published: |
2011
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| Subjects: | |
| Online Access: | http://psasir.upm.edu.my/id/eprint/92438/1/FS%202011%2028%20IR.pdf http://psasir.upm.edu.my/id/eprint/92438/ |
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| Summary: | Investigation was carried out on binary (MgO)xCP20S)I-x with x = 20, 25, 30, 35, 40,
45 and 50 mol % and ternary senes (ZnO)30(MgO)xCP20s)7o-x,
and 20 mol % in order to determine the role of zinc and magnesium ions in
phosphate glasses. All the samples were prepared by traditional melt quenching
technique. X-ray diffraction (XRD) measurement confirmed that the samples were
amorphous. Dielectric spectroscopy, laser flash technique, differential thermal
analyzer (DTA) , UV -visible spectrophotometer, ellipsometry and inductively
coupled plasma-optical emission spectrometry (Iep-OES) have been used to
characterize the electrical, thermal, optical features and ion released concentration
respectively as well as to shed further light on the structure of the glasses. The
dielectric permittivity (E') and loss factor (En) Were measured in the frequency
range of 0.01 Hz to 1 MHz and in the temperature range between 303 and 573 K. The empirical data were sufficiently fitted and modeled with a superposition of
Harviliak-Negami (HN) dielectric relaxation functions and a conductivity term. The
results showed that the dielectric constant and dielectric loss factor decreased with
frequency and increased with temperature. These interesting variations have been
explained in the light of polarization and ionic interaction. At low frequencies, the
dielectric loss factor spectrum was dominated by de conduction which was
manifested by the 1/0) slope. Activation energy of dielectric relaxation (Em) was
in the range 0.05 to 0.14 eV, 0.40 to 0.51 eV, 0.05 to 0.11 eV and 0.06 to 0.09 eV
for binary glasses (MP) and ternary glasses with constant mole fraction of zinc (CZ),
mangnesium (CM) and phosphate (CP), respectively. Activation energy of de
conduction (Ecr) was in the range 1.00 to 1.15 eV, 1.04 to 1.16 eV, 0.92 to 1.07 eV
and 1.06 to 1.12 eV for MP, CZ, CM and CP glass systems, respectively. The values
of Ecr is higher than those for Em which suggest both the conduction and
relaxation processes are due to different mechanisms. Thermal diffusivity
measurements were carried out, in the temperature range of ambient to 573 K. The
values decreased from 0.32 to 0.23 mm2/s. The response was explained based on
phonon mean free path. The greater the network connectivity the greater the phonon
mean free path which makes it easier for the phonon to propagate and eventually
lead to higher values of thermal diffusivity. The glass transition temperature (TJ of
the glasses was measured by DTA from 25 DC to 700 DC and the values of Tg was
found in the range of 396 to 544 DC. The variation is proportional to the length of'
phosphate chain, cross-linking density and bonding strength of the structure. The decreases in Tg reflects the bond strength of the glass structure is weakened on
account of the rupture of phosphate cross-linked network. The UV spectra of the
glasses were measured in the wavelength range of 190 to 1100 nm at ambient
temperature. The Urbach rule was applied to evaluate the values of optical energy
band gap (Eopt) and Urbach energy (Eu) for all the samples from the absorption
spectrum. The Eopt was found to be in the range of3.64 to 3.78 eV, 3.36 to 3.44 eV,
3.47 to 3.79 eV and 3.54 to 3.81 eV for MP, CZ, CM and CP glass systems,
respectively. Meanwhile Eu was found to be in the range 0.26 to 0.28 eV, 0.29 to
0.47 eV, 0.27 to 0.32 eV and 0.27 to 0.45 eV for MP, CZ, CM and CP glass systems,
respectively. The behavior of both Eopt and Eu was correlated with structural
disorder in the sample. As the non-bridging oxygen sites increase in the glassy
matrix the valence bands were broadened resulting in a lower Eopt and higher Eu.
Refractive index of the glasses was measured at ambient temperature with
helium-neon laser of 632.8 nm wavelengths. The measured refractive index was
found varying in between 1.508 and 1.575 and was dependent on the amount of
non-bridging oxygen which has higher polarizability than bridging oxygen. This is
because the depolymerization effect brought about retardation of light propagating
through the phosphate network. The refractive index was found to vary
proportionally with density as well. Chemical durability of the studied glasses has
also been investigated in acidic, neutral and basic buffer solutions for 30 days to
express the resistance offered by a glass towards attack by aqueous solutions. In the
corrosion test, all the glass specimens experienced hydration, hydrolysis and precipitation steps. The dissolution rate (DR) of these glasses was in the range of 10-6
to 10-8 g/cm2 min and subjected to the relative concentration of Zn2+or Mg2+ions
and glass compositions. The binary compositions exhibit excellent chemical
durability which is comparable to window glass. The surface morphology of the CZ
glass system was found to be the most affected by the buffer solutions on account of
the formation of asymmetric bridging oxygen which tend to accelerate the
hydrolysis process once the surface is hydrated. The pH values of all the solutions
decreased as a function of time and this was attributed to the release of phosphate
species from the dissolving glasses and subsequently the formation of phosphoric
acid in the solution. Ion released measurement showed that all ions in the glass
structure leached out of the glass surface when reacted in an aqueous solution. In
addition, the ion leaching concentration strongly depended on the glass composition
and the pH of the aqueous solution. |
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