Tellurite And Fluorotellurite Glasses For Active And Passive
Tellurite And Fluorotellurite Glasses For Active And Passive Tellurite And Fluorotellurite Glasses For Active And Passive
4. Thermal properties and glass stability; MDO 89 has no fundamental meaning. The solidus and liquidus temperatures (Ts and Tl respectively) were obtained by the same interpolation method as Tg described above. Fig. (4.5) illustrates this. ←← Endothermic ∆∆T / °C Exothermic →→ T s 450 460 470 480 490 Temperature / °C Fig. (4.5): DTA trace of glass MOF005 (70TeO2-10Na2O-20ZnF2 (mol. %)), showing interpolation of the solidus temperature, Ts, and liquidus temperature, Tl. Transformations of this kind (melting) are reversible and obey Le Chatelier’s principle [4] which implies, for example, that ice-water will stay at 0°C until it fully transforms to ice or water. Any heat added to the system contributes to the phase change rather than a change in temperature, as long as both phases exist. For this reason melting transformations have a distinct shape in DTA traces. In a DTA scan at constant heating rate, where a solid sample fuses, the reference sample increases in temperature at the preprogrammed heating rate, while the sample temperature remains at the melting temperature until the transformation is complete. Therefore, when ∆T vs. reference temperature is plotted, a linear deviation from the T l
4. Thermal properties and glass stability; MDO 90 baseline is seen on the leading edge of the endotherm. The peak of the endotherm represents the temperature at which melting terminates and the transformation is complete. At this point the sample is at a lower temperature than its surroundings and it heats at an accelerated rate, returning to the temperature of its surroundings. This is seen on the DTA curve as a return to the baseline. The return portion of the curve follows an exponential decay, where the sample initially rapidly catches up with its surroundings and then more slowly as the sample and surroundings temperatures approach one another. Fig. (4.6) shows the difference in melting endotherm if reference or sample (ideal and actual) temperature is plotted on the x-axis against ∆T. Ideally the melting portion of the DTA curve of a single solid phase would correspond to a vertical line, as the sample temperature would not change until melting is complete. For thermocouple junctions immersed within the sample, this ideal case is observed. Most contemporary DTAs however (including the one used in this study), are designed with the thermocouple junction in contact with the sample holder, and this holder tends to increase in temperature to a certain degree under the influence of the surroundings, which are rising in temperature. For this set-up the DTA curve of the sample has a sharper rising slope and broader exponential drop-off than that of the reference. An important point to note is that although melting is complete at the peak of the endotherm, it is still the entire area under the peak which represents the latent heat of fusion. The enthalpy (H) of the reference increases during the time of the transformation since the reference temperature increases ( ∆H = C dT , where T is the temperature and Cp is the molar heat capacity at constant pressure) as dictated by the heating rate. However, during sample melting there is no enthalpy change for the sample due to ∫ T 0 p
- Page 51 and 52: 2. Literature review; MDO 38 near f
- Page 53 and 54: 2. Literature review; MDO 40 where
- Page 55 and 56: 2. Literature review; MDO 42 Transm
- Page 57 and 58: 2. Literature review; MDO 44 origin
- Page 59 and 60: 2. Literature review; MDO 46 4 α =
- Page 61 and 62: 2. Literature review; MDO 48 Bragli
- Page 63 and 64: 2. Literature review; MDO 50 It can
- Page 65 and 66: 2. Literature review; MDO 52 and su
- Page 67 and 68: 2. Literature review; MDO 54 be bro
- Page 69 and 70: 2. Literature review; MDO 56 2.5.2.
- Page 71 and 72: 2. Literature review; MDO 58 Table
- Page 73 and 74: 2. Literature review; MDO 60 2.5.2.
- Page 75 and 76: 2. Literature review; MDO 62 [20] J
- Page 77 and 78: 2. Literature review; MDO 64 [47] S
- Page 79 and 80: 3. Glass batching and melting; MDO
- Page 81 and 82: 3. Glass batching and melting; MDO
- Page 83 and 84: 3. Glass batching and melting; MDO
- Page 85 and 86: 3. Glass batching and melting; MDO
- Page 87 and 88: 3. Glass batching and melting; MDO
- Page 89 and 90: 3. Glass batching and melting; MDO
- Page 91 and 92: 3. Glass batching and melting; MDO
- Page 93 and 94: 3. Glass batching and melting; MDO
- Page 95 and 96: 4. Thermal properties and glass sta
- Page 97 and 98: 4. Thermal properties and glass sta
- Page 99 and 100: 4. Thermal properties and glass sta
- Page 101: 4. Thermal properties and glass sta
- Page 105 and 106: 4. Thermal properties and glass sta
- Page 107 and 108: 4. Thermal properties and glass sta
- Page 109 and 110: 4. Thermal properties and glass sta
- Page 111 and 112: 4. Thermal properties and glass sta
- Page 113 and 114: 4. Thermal properties and glass sta
- Page 115 and 116: 4. Thermal properties and glass sta
- Page 117 and 118: 4. Thermal properties and glass sta
- Page 119 and 120: 4. Thermal properties and glass sta
- Page 121 and 122: 4. Thermal properties and glass sta
- Page 123 and 124: 4. Thermal properties and glass sta
- Page 125 and 126: 4. Thermal properties and glass sta
- Page 127 and 128: 4. Thermal properties and glass sta
- Page 129 and 130: 4. Thermal properties and glass sta
- Page 131 and 132: 4. Thermal properties and glass sta
- Page 133 and 134: 4. Thermal properties and glass sta
- Page 135 and 136: 4. Thermal properties and glass sta
- Page 137 and 138: 4. Thermal properties and glass sta
- Page 139 and 140: 4. Thermal properties and glass sta
- Page 141 and 142: 4. Thermal properties and glass sta
- Page 143 and 144: 4. Thermal properties and glass sta
- Page 145 and 146: 4. Thermal properties and glass sta
- Page 147 and 148: 5. Crystallisation studies; MDO 134
- Page 149 and 150: 5. Crystallisation studies; MDO 136
- Page 151 and 152: 5. Crystallisation studies; MDO 138
4. Thermal properties and glass stability; MDO 90<br />
baseline is seen on the leading edge of the endotherm. The peak of the endotherm<br />
represents the temperature at which melting terminates and the transformation is<br />
complete. At this point the sample is at a lower temperature than its surroundings and it<br />
heats at an accelerated rate, returning to the temperature of its surroundings. This is seen<br />
on the DTA curve as a return to the baseline. The return portion of the curve follows an<br />
exponential decay, where the sample initially rapidly catches up with its surroundings<br />
and then more slowly as the sample and surroundings temperatures approach one another.<br />
Fig. (4.6) shows the difference in melting endotherm if reference or sample (ideal and<br />
actual) temperature is plotted on the x-axis against ∆T.<br />
Ideally the melting portion of the DTA curve of a single solid phase would correspond<br />
to a vertical line, as the sample temperature would not change until melting is complete.<br />
<strong>For</strong> thermocouple junctions immersed within the sample, this ideal case is observed.<br />
Most contemporary DTAs however (including the one used in this study), are designed<br />
with the thermocouple junction in contact with the sample holder, and this holder tends to<br />
increase in temperature to a certain degree under the influence of the surroundings, which<br />
are rising in temperature. <strong>For</strong> this set-up the DTA curve of the sample has a sharper rising<br />
slope and broader exponential drop-off than that of the reference.<br />
An important point to note is that although melting is complete at the peak of the<br />
endotherm, it is still the entire area under the peak which represents the latent heat of<br />
fusion. The enthalpy (H) of the reference increases during the time of the transformation<br />
since the reference temperature increases ( ∆H<br />
= C dT , where T is the temperature<br />
and Cp is the molar heat capacity at constant pressure) as dictated by the heating rate.<br />
However, during sample melting there is no enthalpy change for the sample due to<br />
∫<br />
T<br />
0<br />
p
Change language